comprehensive inorganic chemistry ii || nitrogen activation

Co

8.14 Nitrogen ActivationDR Tyler, CG Balesdent, and AJ Kendall, University of Oregon, Eugene, OR, USA

ã 2013 Elsevier Ltd. All rights reserved.

8.14.1 Introduction 5258.14.2 Theory of Dinitrogen Activation 5268.14.2.1 Inertness of N2 5268.14.2.2 Coordination of N2 to Metal Complexes 5268.14.2.2.1 N2 bonding to a single metal center 5278.14.2.2.2 N2 bonding to two or more metal centers 5278.14.2.3 Measuring the Extent of N2 Activation 5298.14.3 Complexes with Coordinated Dinitrogen 5308.14.3.1 End-On Coordination of N2 5318.14.3.2 Side-On Coordination of N2 5328.14.3.2.1 Lanthanides 5328.14.3.2.2 Actinides 5338.14.3.2.3 Transition metals 5338.14.3.3 Side-On/End-On Coordination of N2 5348.14.4 Uses for Activated Dinitrogen-Containing Complexes: NH3 Formation 5348.14.4.1 Early Studies 5348.14.4.2 Molybdenum Systems 5358.14.4.3 Other Metals 5358.14.4.4 Iron Complexes 5378.14.4.5 Reduced Dinitrogen Complexes 5388.14.5 Incorporation of Nitrogen Derived from M–N2 Complexes into Organic Molecules 5398.14.5.1 Reactions of End-On-Bonded N2 Complexes 5418.14.5.1.1 NdC bond formation: dinitrogen to diazenido 5418.14.5.1.2 Ndheteroatom bond formation 5438.14.5.1.3 Alkylation of the coordinating nitrogen 5448.14.5.1.4 Reactions of diazenido complexes 5448.14.5.1.5 Hydrazido complexes from dinitrogen complexes 5448.14.5.1.6 Si functionalization 5458.14.5.2 NdC Bond Formation from Hydrazido Complexes 5458.14.5.2.1 Hydrazido condensations 5458.14.5.2.2 Hydrazido to amino 5468.14.5.3 Direct Cleavage of the N^N Bond: Nitride Complexes 5468.14.5.4 NdC Bond Formations Utilizing Multiple Metal Centers 5478.14.6 Biological Activation of N2: N2 Reduction and Nitrogenase Models and Mimics 5498.14.6.1 Nitrogenase 5498.14.6.2 Cubane Cluster Nitrogenase Models 5498.14.6.3 Vanadium-Nitrogenase Models 5508.14.7 Conclusion 550References 550

8.14.1 Introduction

Dinitrogen is an exceedingly unreactive molecule, and

chemists have long dreamed of finding a mild method for

converting this abundant molecule into useful compounds

(a process called ‘fixing’ nitrogen).1–5 The reason for fixing

dinitrogen, of course, is that fixed nitrogen is important for

the synthesis of nitrogen-containing molecules that are es-

sential to all life. The Haber–Bosch process for the produc-

tion of ammonia from N2 and H2 (eqn [1]) is the current

industrial process for fixing dinitrogen, and its discovery and

development have been described as arguably the most

mprehensive Inorganic Chemistry II http://dx.doi.org/10.1016/B978-0-08-09777

important invention of the twentieth century.2,3 In this pro-

cess, hydrogen gas, typically produced by steam reformation

of natural gas or partial oxidation of hydrocarbons, is reacted

with dinitrogen gas over a promoted iron catalyst (other

metals such as ruthenium are also commonly used) at high

temperature (>200 �C) and high pressure (>150 atm). Such

drastic reaction conditions, combined with the energy re-

quired to produce H2 for the process, account for the con-

sumption of �1015 J year�1 (which is equal to 1–2% of the

total annual global energy consumption) and for the output

of more than 3.3�108 M tons year�1 of CO2 (�2% of the

worldwide total).6,7 Due to the high energy input and high

4-4.00822-6 525

http://dx.doi.org/10.1016/B978-0-08-097774-4.00822-6

526 Nitrogen Activation

CO2 output, finding a more environmentally benign process

to fix N2 is a grand challenge in chemistry.

Fe2NH3N2 + 3H2

>200 oC, >150 atm

ΔHo = -11.0 kcal mol-1; ΔSo = -23.7cal mol-1 K-1 ½1�

Among inorganic chemists, finding new methods for fixing

dinitrogen nearly always starts with the synthesis of metal–N2

complexes and an exploration of their reactivity and proper-

ties. Bonding N2 to a metal ‘activates’ the nitrogen, sometimes

weakly and sometimes more strongly. In this chapter, we dis-

cuss the inorganic chemistry of dinitrogen activation. The dis-

cussion includes sections on how N2 is activated by bonding to

a metal; examples of N2 coordinated to a metal; reactions of

activated dinitrogen that yield ammonia; and finally, a discus-

sion of how activated dinitrogen can be directly incorporated

into organic molecules, bypassing the intermediate formation

of ammonia.

8.14.2 Theory of Dinitrogen Activation

8.14.2.1 Inertness of N2

The inertness of N2 is due in part to the strong N^N triple

bond (944 kJ mol�1).5,8 However, the strong bond is not

solely responsible for the inertness because other triply

bonded, small molecules with exceptionally strong bonds,

notably CO (1076 kJ mol�1), readily undergo a wide variety

of chemical transformations. Rather, as numerous authors

have pointed out, the inertness of N2 arises from the low

energy (�15.6 eV) of the highest occupied molecular orbital

(HOMO), and the high energy (7.3 eV) of the lowest unoccu-

pied molecular orbital (LUMO).5 The low and high energies

of the HOMO and LUMO, respectively, impede electron

transfer and oxidation–reduction reactions, as well as reactions

with nucleophiles and electrophiles. The absence of a dipole

moment in N2 also contributes to its lack of reactivity

with nucleophiles and electrophiles. In addition, the low

M N N

End-on

MN

N

Side-on

M N N M MN

NM

End-on/end-on Side-on/side-on

M N N

M

M

MM

M

N

Bonding modes to multiple meta

Figure 1 Dinitrogen bonding modes in monometallic, bimetallic, and highernot indicated because for a given bonding mode these can vary depending ondiscussed in the text.

polarizability of the N2 molecule contributes to its inertness

because it cannot form highly polar transition states that are

often involved in reactions with electrophiles and nucleo-

philes. Because N2 is inert, the general strategy that has

emerged for activating it for reaction is to coordinate it to a

metal in a complex.

8.14.2.2 Coordination of N2 to Metal Complexes

Despite the fact that N2 is isoelectronic to CO, N2 is a much

poorer ligand than CO. This is attributed to both the poorer

s-donating ability and the poorer p-accepting ability of N2

compared to CO. The poor s-donating ability of N2 is generally

attributed to its poor polarizability. Its poor p-accepting ability

is best explained by comparison to CO. Because O is more

electronegative than C, the p* orbital on CO is concentrated

on carbon, which makes for better p backbonding with filled

metal orbitals because of improved overlap. In contrast, the p*orbital on N2 is symmetric and therefore not concentrated on

one N atom. This results in poorer overlap of the p* orbital with

the metal d-orbitals in comparison to CO. Although dinitrogen

is both a poor s-donor and a poor p-acceptor (making it a poor

ligand), numerous dinitrogen complexes have been synthesized

with a variety of ancillary ligands. The common coordination

geometries of the N2 ligand to one metal center and to two

metal centers are shown in Figure 1. In addition, an (unusual)

bonding mode to three metal centers is shown, as is another

unusual geometry in which N2 is bonded to six metal centers.9

(The former structure is found in the [{(C10H8)Cp2Ti2}

{(C5H4)Cp3Ti2}(m3-N2)] molecule (1),10 which has an NdN

bond length of 1.30(1)A. The latter structure is found in

[(LAu)6(N2)][BF4]2 (2),11 where L is a phosphane; the N2 unit

bridges two Au3 clusters in this molecule.)

M

NN

M

MNN

M

End-on/side-on

NM

M

M

Side-on/side-on

l centers

-metallic complexes.12–14,9 The bond orders in the NdN bonds arethe metal and the other ligands. The various NdN bond orders are

Nitrogen Activation 527

Au

Au N

Au

L

L

L

Au

AuN

Au

L

L

L

L = a phosphane; d(N–N) =1.475(14) Å for L = PPh2

iPr

2+

2

Ti

Ti

N

Ti

N

Ti

d(N–N) = 1.301(12) Å

1

8.14.2.2.1 N2 bonding to a single metal centerThe end-on coordination of dinitrogen to a transition metal is

a common bonding mode and is described by the Dewar–

Chatt–Duncanson s-donor/p-acceptor model of ligand

bonding.5,15–17 In end-on-bonded dinitrogen complexes, the

filled dinitrogen s orbital (3sg) donates electron density to an

empty metal orbital forming a s bond, while back-donation

occurs from a filled metal orbital to the unfilled antibonding

p* orbital (1pg) of the dinitrogen ligand (Figure 2). As the

electron density of the metal increases, more back-donation

occurs and the NdN bond is activated (elongated and weak-

ened) due to increased population of the dinitrogen p* anti-

bonding orbital.

The valence bond description of end-on bonding is shown

in Figure 3with the two resonance structures.12 This resonance

depiction nicely illustrates that the effect of p-backbonding is

to increase the bond order of the MdN bond and to decrease

the bond order of the NdN bond. Note that some authors

denote resonance structure A as ‘weak’ activation (the N^N

triple bond is still intact) and resonance structure B as ‘strong’

activation (because the NdN bond order is less than three).8

-

- -

--

-

-

+

+

+

+

+

1pg

3sg

NM N

Figure 2 Dewar–Chatt–Duncanson model for the end-on coordinationof N2 to a single transition metal center. Red orbitals are filled, blueorbitals are unfilled.

M(n+2)+, N22-Mn+, N2

0

A B

Figure 3 Selected resonance structures depicting the bonding in theend-on coordination of N2 to a single transition metal center.

The side-on bonding of N2 to a single metal center is not

common. An interesting example is matrix-isolated OTi(N2).18

The low energy form of this molecule has an end-on-bonded

N2 ligand, but irradiation with light of wavelength 400–

580 nm converts it to a side-on-bonded complex. A similar

photochemical conversion of an end-on N2 to a side-on N2

was found on irradiation of [Os(NH3)5(N2)][PF6]2.19 In this

case, the side-on-bonded metastable molecule was crystallo-

graphically characterized using a ‘photocrystallographic’ tech-

nique at low temperature (Figure 4). A comparison of the

NdN bond distances in the end-on and side-on complexes

was of interest, but it was noted that the rather large estimated

standard deviations in the ‘photocrystallographic’ method pre-

vented such a comparison.

8.14.2.2.2 N2 bonding to two or more metal centersA number of researchers have investigated the side-on bonding

of N2 to twometal centers. In these molecules, the key bonding

interactions are between the N2 p and p* orbitals with unoc-

cupied and occupied metal d-orbitals, respectively, of appro-

priate symmetry. To illustrate, consider the bonding in the

hypothetical Co2(CO)6(N2) molecule,20 with an assumed

butterfly-shaped side-on/side-on geometry (sometimes called

an ‘edge-on’ geometry21). Goldberg and Hoffmann showed

that the Co2(CO)6 fragment orbitals shown in Figure 5 interact

with the N2 p* orbitals as indicated in the figure.20 In addition

to this interaction, the p orbitals of N2 participate in forward

donation with the appropriate dimetal orbitals. The net result

of this back-donation (into antibonding orbitals) and forward

donation (from bonding orbitals) is a weakening of the NdN

bond. A similar bonding scheme will apply for other bent side-

on/side-on molecules.

Fryzuk and coworkers probed the bonding in the end-on/

end-on-bridging geometry and in the planar side-on/side-on

bridging geometry for the purpose of comparing the relative

stabilities of these two bonding modes.22 The results of their

molecular orbital analyses are summarized in Figure 6. The key

to comparing the relative stabilities was in the bonding in-

teractions involving the N2 p* orbitals and, in particular, the

N(2)

N(1)

N(3A)

N(3) N(3C)

N(3B)

N(3MA)

N(3MB) N(3MC)

N(3M)

N(4M)

N(1MB) N(1M)

Os(1)Os(1M)

Figure 4 ORTEP drawings of the [Os(NH3)5(N2)]2þ ion in [Os

(NH3)5(N2)][PF6]2. Left: End-on-bonded N2. Right: Structure of thelight-induced metastable state with side-on coordination. Reprinted fromFomitchev, D. V.; Bagley, K. A.; Coppens, P. J. Am. Chem. Soc. 2000,122, 532–533, with permission. © (2000) American Chemical Society.


overlap between the empty N2 p* orbitals and the filled

d-orbitals of appropriate symmetry. In comparing the two

geometries, they found that the end-on/end-on-bridging

mode was preferred because two MdN2dM p-type bonds

are formed in this geometry, whereas one p-type bond and

y

z

x

L4M

dxy, dxz, dyz

dxz dxz

dyz dyz

dxy

L4M

L4M ML4N N

N2

p* p*

p

pppp

p

p*

y

z

x

L4M

dxy, dxz, dyy

dxy dxy

dyz dyz

dxz

L4M

L4M ML4

N

N

N2

p*

d* d*

p*

p

pppp

d d

p

p*

Figure 6 Molecular orbital diagrams showing the interactions of theN2 p and p* orbitals with the metal orbitals in end-on/end-on (Top) andplanar side-on/side-on (Bottom) geometries. Reprinted from Fryzuk, M.D.; Haddad, T. S.; Mylvaganam, M.; McConville, D. H.; Rettig, S. J. J. Am.Chem. Soc. 1993, 115, 2782–2792, with permission. © (1983) AmericanChemical Society.

Figure 5 Bonding interactions of the N2 p* orbitals with Co2(CO)6fragment orbitals of appropriate symmetry in the hypotheticalCo2(CO)6N2 molecule. Reprinted from Goldberg, K. I.; Hoffman, D. M.;Hoffmann, R. Inorg. Chem. 1982, 21, 3863–3868, with permission.© (1982) American Chemical Society.

one d-type bond were formed in the side-on/side-on bridging

geometry. They argued that the end-on/end-on-bridging mode

is preferred because a d bond is weaker than a p bond. In each

geometry, the p* orbitals of the N2 ligand are involved in

backbonding, and this leads to activation of the N2 bond.

The end-on/end-on geometry is very common and Fryzuk

suggested that the preference for this geometry indicates that

the distal nitrogen in the initially formed mononuclear N2

complex is more susceptible to attack by a coordinatively un-

saturated metal species than is N2 itself.13

The valence bond description of the bonding in the end-on/

end-on-bridging geometry is shown in Figure 7. With regard to

nomenclature, some authors denote resonance structure A as the

weak activation structure and resonance structure D as the

strong activation structure. Structures B and C would have in-

termediate activation status. As shown in the sections below,

some molecules have a structure best described by resonance

form A (i.e., they have a relatively unactivated N^N bond) and

other molecules have a structure best described by resonance

forms B–D (i.e., they have an NdN bond with a bond order

lower than three). Leigh points out that the progressive weaken-

ing of the NdN bond in the sequence A, B, C, D can continue to

a structure with a completely dissociated NdNbond (Figure 8),

the ultimate in an activated N2 ligand.4

On paper, the sequence of increasingly activated bonds in

structures A–E suggests a method for fixing nitrogen. Leigh

points out that this strategy was recognized early on by

Taube, who tried to use an osmium system to carry out such

a cleavage.23 What Taube was able to achieve, however, was the

opposite: starting with a nitrido complex, he formed an N2

complex (eqn [2]). This was a potentially interesting result

because mechanistic information could be gained about dini-

trogen cleavage by applying the principle of microscopic

reversibility to the study of nitride coupling. However, in

practice, any information gleaned from these studies is not

particularly useful because the formation of nitrides from N2

is generally so thermodynamically unfavorable that there are

no opportunities to apply the mechanistic principles to the

reverse reaction, that is, A! E in Figure 8.24–27

Several groups have reported reactions in which complete

cleavage of the N2 triple bond was achieved, analogous to

2[Osv(NH3)4(CO)N]2+ [(NH3)4(CO)OsIINNOsII(NH3)4(CO)]4+

½2�

the reverse of eqn [2].28,29 Reports by Cummins (Scheme 1),30

Floriani (Scheme 2),31 Cloke (Scheme 3),32,33,36 and Holland34

(Scheme 4) are noted here. These reactions tend to form very

stablemetal nitrides.Metal nitride complexes formedby the direct

cleavage of N2 have strong thermodynamic stability, and thus

require strong reductants to produce ammonia and regenerate

the reducedmetal complex. For example, the reaction inScheme1

is exothermic by 350 kJ mol�1 as a result of forming the strong,

but relatively unreactive Mo^N bonds (650 kJ mol�1).33

In the case of Scheme 231,35, note that the Nb(III)–calixarane

complex 3 reacts with N2 to give the end-on/end-on-bridged N2

complex 4. The NdN distance of 1.390(17)A suggests that the

N2 unit is the hydrazido form of dinitrogen, that is, the dinitro-

gen has been reduced by four electrons; the Nb centers are thus

Figure 8 Depiction of NdN bond cleavage in an end-on/end-oncoordinated molecule.

Figure 7 Resonance structures describing the bonding in the end-on/end-on coordination of N2 to two transition metal centers.


Nb(V). (The claim was made that this is the longest NdN

distance in an end-on/end-on complex.31) Reduction of com-

plex 4 with Na resulted in cleavage of the weak NdN bond to

form the bridged nitride dimer 5 (still containing Nb(V)).

An exciting system reported by Holland et al. (Scheme 4)

involves direct cleavage of N2 into two nitrido species, followed

by protonolysis or addition of H2 to generate ammonia. Though

the system is not catalytic and relies on a strong reductant, it

shows the considerable potential that lies in the development of

metal nitrides as a strategy to activate and reduce N2.

To conclude this section, selected valence bond resonance

structures for the bonding of N2 in a side-on/side-on bridging

geometry are shown in Figure 9. Note the extent of activation

fromweak (A) to strong (D), and also note that donation of N2

p (bonding) electron density into an empty metal orbital can

activate (weaken) the N2 bond by depletion of the NdN

bonding electron density.

8.14.2.3 Measuring the Extent of N2 Activation

As discussed above, a consequence of p-backbonding to the N2

ligand is that the NdN bond is weakened because the p*orbitals are occupied. This weakening of the N2 bond is re-

ferred to as ‘activation.’ The NdN distance typically increases

accordingly. The extent of N2 activation in metal–N2 com-

plexes is typically measured by the NdN bond length in

those molecules where a crystal structure has been obtained

or by infrared (IR) or by Raman spectroscopy (by examining

the N–N stretching frequency) in those complexes without

x-ray structural data. With many molecules, there is generally

a good linear relationship between the NdN bond length in

N2 complexes and n(N–N). For an example, see Figure 2 in the

work of Terrett et al.37 For comparison, the n(N–N) frequencies

and NdN bond lengths for some representative NdN and

N^N-bonded compounds and for N2 are shown in Table 1.13

In theory, 15N NMR may provide another method for de-

termining the extent of NdN bond activation, but not enough15N NMR chemical shifts have been reported for N2 complexes

to determine if there is a correlation between the chemical shift

and the extent of activation.38 (It is noteworthy that in N2

complexes there is no correlation between the 15N NMR reso-

nance frequencies and the bonding mode.22,13)

The extent of N2 activation, as measured by n(N–N) or the

NdN bond distance, is influenced by a number of factors. One

important factor is the metal. Generally, for identical dn electron

configurations, the extent of activation decreases left to right

across the periodic table, consistent with the more strongly

reducing transition metals on the left side of the periodic table.

An example of this effect is provided by the comparison of

Mo0(dppe)2(N2)2 (6) to FeII(depe)2(H)N2þ (7) (Figure 10).

Note, however, that p backbonding will decrease as the oxi-

dation state increases, so in many instances the decrease in acti-

vation (left to right in the periodic table) is likely also attributable

to the increase in oxidation state if the dn electronic configuration

is kept constant. An example showing the effect of oxidation state

is provided by the comparison of complex 7 (2091 cm�1; 1.07 A)

to Fe0(depe)2(N2) (1955 cm�1; 1.139 A). For additional exam-

ples showing that Fe(0) complexes are, in general, more activated

than Fe(II) complexes, the interested reader should consult the

extensive tables found in the literature.38,39

Coordination number also has a large effect on the activa-

tion of N2. Lower coordination numbers often show increased

activation of a coordinated N2 molecule, with the most acti-

vated N2 complexes to date having a coordination number of

only three. An example of such a strongly activated complex is

shown with the end-on/end-on Fe(I) complex 8.

Fe N N FeN

N

R

R

R = Me or tBu

N

N

R

R

1178 cm-1; 1.182 Å

8

It is tempting to correlate the extent of N2 activation with

the bonding mode of the N2 ligand. For example, many end-

on-bonded N2 ligands and end-on/end-on-bridging N2 ligands

are only weakly or moderately activated, and it is tempting

therefore to generalize that these bonding modes result in

weak activation. However, not all such bonding modes result

in a weakly activated N2 ligand. The strong activation in mol-

ecule 8 shows that the N2 ligand can be strongly activated in

these bonding modes; in this case, the low oxidation state of

the Fe centers (Fe(I)) and the low coordination number con-

tribute to the strong activation. In summary, the extent of the

activation is more dependent on the metal, the ligands, and the

oxidation state than on the bonding mode. Of course, these

parameters ultimately determine the N2-bonding mode and in

that sense the bonding mode is correlated to the extent of

activation.

Finally, the extent of activation will depend on the other

ligands in the complex. Straightforward examples of ligand

ArRN

ArRN

ArRN

ArRN

ArRN

MoNRAr

NRAr

NRAr

NRAr

NRAr

NRAr

NRAr

N2

ArRNMo

NRAr

NRAr

N

N

Mo

N

N

Mo

Mo

N

2

R = C(CH3)3; Ar = 3,5-C6H3(CH3)2

Scheme 1 Reaction sequence for the conversion of Mo(NRAr)3 to N^Mo(NRAr)3 in the presence of N2. Reproduced from Laplaza, C. E.;Cummins, C. C. Science 1995, 268, 861–863.

OO OO

Nb

N

N

Nb

O OO O

2-

N2

OO OO

Nb

Nb

O OO O

N NaLnN

OO OO

Nb

Nb

O OO O

NaLnNa LnNaNaLn

L = THF or DME

3 4 5

Scheme 2 Reaction of the Nb(III)–calixarene complex 3 with N2 to form a bridged hydrazido complex 4. Subsequent reduction by Na leads to NdNbond cleavage and the formation of the bridging nitrido complex 5. Reproduced from Zanotti-Gerosa, A.; Solari, E.; Giannini, L.; Floriani, C.;Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem. Soc. 1998, 120, 437–438. Caselli, A.; Solari, E.; Scopelliti, R.; Floriani, C.; Re, N.; Rizzoli, C.; Chiesi-Villa,A. J. Am. Chem. Soc. 2000, 122, 3652–3670.


effects are provided by the pairs of end-on-bonded Fe com-

plexes shown in Table 2. Note in each pair that the complex

with the more electron-donating ligands activates the NdN

bond more.

It is important to note that the term ‘activation’ refers only

to the lengthening of the NdN bond, to the decrease in the

N–N stretching frequency, and to any other feature that in-

dicates reduction of the N2 ligand (by electronic occupation of

the p* orbitals). The term does not necessarily apply to the

reactivity of the N2 ligand; as seen in the following sections,

strong activation is not necessarily a requirement to observe

reactivity of the coordinated N2 molecule.

8.14.3 Complexes with Coordinated Dinitrogen

The previous section showed only a small sample of the variety

of N2 complexes that have been synthesized. In this section,

additional examples are discussed, starting with the historically

important example of [Ru(NH3)5(N2)]2þ.


8.14.3.1 End-On Coordination of N2

The first example of N2 coordinated to a transition metal was

reported in 1965 by Allen and Senoff.48 The [Ru(NH3)5(N2)]

[X]2 complexes (where X¼Br�, I�, BF4�, or PF6

�) were charac-terized by elemental analysis and IR spectroscopy. Of historical

interest, Leigh pointed out that Allen’s interest in the IR spectra

of [Ru(NH3)x]2þ complexes probably led to the discovery

of the [Ru(NH3)5(N2)]2þ complex, which has a distinctive

n(N–N) band at 2130 cm�1; earlier researchers who stumbled

on N2 complexes may simply have been unable to identify

VCl3(THF)3+ Li[N{N99}2]THF

[V(N{N99}2)Cl]2

V

N V

N

RNN

RN

R

NR

N

N

R

KC8N2

V

N-

V

N

NR

N

RN

R

NRN

N

R

2KC8, N2

Ag[BPh4]

KC8

K+

RR

Scheme 3 Reductive cleavage of dinitrogen by a vanadiumdiamidoamine complex ([N{N0 0}2]

2�¼ [(Me3Si)N{CH2CH2N(SiMe3)}2]2�).

Reproduced from Clentsmith, G. K. B.; Bates, V. M. E.; Hitchcock, P. B.;Cloke, F. G. N. J. Am. Chem. Soc. 1999, 121, 10444–10445. Bates,V. M. E.; Clentsmith, G. K. B.; Cloke, F. G. N.; Green, J. C.;Jenkin, H. D. L. Chem. Commun. 2000, 927–928.

2 equiv.K reductant

Fe FeN

N

N

N

Cl

Cl N2

0.5

Scheme 4 Reductive cleavage of dinitrogen by a low-valent iron complex. ReScience 2011, 334, 780–783.

n+20 n+ (n+2)+

22- n+

Figure 9 Selected resonance structures of the bonding in the side-on/side-

them as such without the aid of IR spectroscopy.49 In the

years since the first N2 complex was reported, N2 has been

coordinated to almost every transition metal, and recently, to

many lanthanides and several actinides as well.

The coordination of N2 to Fe has always been of special

interest because the nitrogenase enzymes contain Fe and there

is, therefore, the strong possibility that N2 bonds to Fe during

the nitrogenase reduction of N2. The first report of N2 coordi-

nating to iron was by Sacco and Aresta in 1968.50 They found

that both solutions and solid samples of the hydrido–iron

phosphanes FeH2L2 (L¼PEtPh2 or PBuPh2) formed the dini-

trogen complexes FeH2N2L2 when reacted with N2 at room

temperature and atmospheric pressure. For all iron complexes

of N2, the N2 is bound in an end-on fashion. Most such

complexes are mononuclear, but a few display end-on/end-

on-bridging coordination.39 Almost all iron–N2 complexes

contain electron-donating phosphane ligands, which increases

the electron density on the metal center and which in turn

facilitates the necessary p-backbonding to N2. Although N2 is

typically not as activated in Fe complexes as it is in complexes

of other metals (because Fe lies to the right side of the transi-

tion metals, see Section 8.14.2.3), the coordinated N2 ligand is

still reactive. As discussed in that section, the most activated

Fe–N2 complexes are those where the metal center has a low

oxidation state and those with low coordination numbers.

In 1969, Hidai and coworkers synthesized the trans-[Mo

(N2)2(DPPE)2] complex (DPPE¼1,2-bis[bis(ethyl)phosphino]

ethane). Since that time, a large number of similar complexes

with the formula [trans-M(N2)2(L)4] (M¼Mo,W;L¼phosphane)

have been synthesized using the synthetic procedure pioneered by

Hidai.51,52 Many of these species display activated NdN bonds

and are reactive toward protonation and alkylation.

Fe N N FeN

N

N

N

Fe

Fe

K

K

Cl

Cl

N

N

N

N

Ar

Ar

+ KCl

Ar = 2,6-dimethylphenyl

produced from Rodriguez, M. M.; Bill, E.; Brennessel, W. W.; Holland, P. L.

(n+2)+ (n+2)+ (n+3)+ (n+3)+4- 3-2

on coordination of N2 to two transition metal centers.

Table 1 n(NdN) frequencies and NdN bond lengths for somerepresentative compounds

Compound NdN bond length (A) n(NdN) (cm�1)

N2(g) 1.0975 2331CH3N]NCH3 1.25(2)PhN]NPh 1.255 1442H2NdNH2 1.460 1111

Source: Fryzuk, M. D.; Johnson, S. A. Coord. Chem. Rev. 2000, 200–202, 379–409.

FeP

P P

P

N

H

N

RR

RR

RR R

R

+

R = Et

MoP

P P

P

N

N

N

RR

RR

RR R

RN

R = Ph

2091 cm-1; 1.07 Å2033 and 1980 cm-1; 1.12 Å

76

Figure 10 The extent of N2 activation decreases left to right across theperiodic table.

Table 2 n(N2) in selected end-on-bonded N2 complexes

Complex nNN (cm�1) References

Fe(PPh3)3(N2)(H)2 2074 40Fe(PMePh2)3(N2)(H)2 2058 41[Fe(DMPE)2(N2)] 1975 42[Fe(DEPE)2(N2)] 1955 43Fe(NHC)(N2)(C2H4) 2056 44Fe(NHC)(N2)(PMe3) 2032 44Fe(POiPr3)2(CO)2(N2) 2141 45Fe(PEt3)2(CO)2(N2) 2097 45[{Fe(Z5-C5H5)(DMPE)}2(N2)]

2þ 2054 46[{Fe(Z5-C5H5)(DPPE)}2(N2)]

2þ 2040 47

S

Ru

SN N

N PiPr3

N

S

Ru

SN

NN

PiPr3

N

S

Ru

NS

N

PiPr3

- N2

+ N2

Figure 11 Partial removal of the N2 atmosphere over [Ru(N2)(PiPr3)

(‘N2Me2S2’)] by addition of argon gas produced dinuclear [m-N2{Ru(PiPr3)(‘N2Me2S2’)}2] (right). Under higher N2 concentrations, themononuclear [Ru(N2)(P

iPr3)(‘N2Me2S2’)] formed (left).


Dinitrogen complexes with sulfur ligands are not as

common53 as those with phosphane ligands even though the

active site of the FeMo cofactor of nitrogenase contains multiple

S atoms (see the discussion in Section 8.14.6.1).54 To cite one

example, the Sellman group synthesized Ru thiolate complexes

that coordinate N2 (and also N2H2, N2H4, and NH3).55

Specifically, [Ru(N2)(PiPr3)(‘N2Me2S2’)] (‘N2Me2S2’¼ 1,2-

ethanediamine-N,N’-dimethyl-N,N’-bis(2-benzenethiolate)2�)was formed by displacement of the CH3CN ligand with N2 in

[Ru(NCCH3)(PiPr3)(‘N2Me2S2’)]. Under lower concentrations

of N2, the complex dimerized to form the bridged N2 complex

[m-N2{Ru(PiPr3)(‘N2Me2S2’)}2] (Figure 11).

Peters and coworkers demonstrated dimerization with the

first terminal Fe(IV)–nitride, [PhBPiPr3]FeIV^N ([PhBPiPr3]¼

[PhB(CH2PiPr2)3]

�).25 Removal of the N2 atmosphere over

this complex generated the bridged FeI–N2–FeI complex

{[PhBPiPr3]FeI}2(m-N2) (Figure 12). Similar to the Os complex

discussed in Section 8.14.2.2.1, this reaction is the micro-

scopic reverse of the NdN bond cleavage, which makes the

dimerization reaction potentially important when considering

possible mechanisms for N2 reduction. The FeIV–nitride

complex completes the beautiful series of tetrahedral Fe–N2

complexes in various oxidation states: [PhBPiPr3]Fe0N2

�,56

[PhBPiPr3]FeI–N2–Fe

I[PhBPiPr3], [PhBPiPr3]Fe

II–N2Me,56

[PhBPiPr3]FeIII^NAd,56 and [PhBPiPr3]Fe

IV^N. Note that the

lower-oxidation-state complexes form N2 complexes, while

the higher-oxidation-state complexes form nitride or imide

complexes, consistent with the p-acidity and p-basicity of the

N2 and nitride ligands, respectively.25

Although the nitrogenase cofactor (FeMoco) is a metal

sulfido cluster (see Section 8.14.6.1), there are few reports

of N2 bonding to molecules that are FeMoco structural mimics.

In one of the few examples where N2 binding does occur,

Mizobe and Hidai showed that N2 will coordinate to Ru

in the Ir3RuS4 cubane cluster [(Cp*Ir)3{Ru(N2)(TMEDA)}

(m3-S)4] (Figure 13(a)).57 Due to the strongly electron-

donating sulfido ligands, the n(N–N) band for this complex

is at 2019 cm�1, which is lower than the previously lowest

RuII–N2 value of 2055 cm�1.58 In fact, the n(N–N) band of

the cubane is even lower than that of certain Ru0 complexes.59

No explanation was offered for the particularly low stretching

frequency. Molybdenum and iron, metals relevant to the active

site of nitrogenase, have also been incorporated into cubanes

to form cores of the type Mo3MS4 and Ir3FeS4 (Figure 13(b)

and 13(c)).60 However, neither type of cluster has been suc-

cessful in binding N2 yet. Dinitrogen is not the only ligand of

interest in the study of nitrogenase mimics. Forms of reduced

N2 such as diazene, hydrazine, and their ionic counterparts

such as hydrazido and diazido are also of interest because these

species are logical intermediates in the overall reduction of N2.

Considerable effort, therefore, has been spent on synthesizing

molecules with these ligands. The Mo cubane containing a

hydrazido(2-) cluster (Figure 13(b)) is one such example.

It is noteworthy that this molecule can be reduced to form

N-methylaniline by reductive cleavage of the NdN bond.

8.14.3.2 Side-On Coordination of N2

8.14.3.2.1 Lanthanides(Cp*2Sm)2(m-Z

2:Z2-N2), discovered by Evans in 1988, was the

first truly side-on/side-on N2 complex (Figure 14).61 The x-ray

crystal structure revealed that the NdN bond length in this

complex (1.088 A) is actually shorter than in free N2

(1.0975 A). However, the SmdC(Cp*) bonds and 13C NMR

data support the assignment of Sm3þ to the metal center. Thus,

Fe

P

N

PP

B

Ph

FeP N

P

P

BPh Fe PN

P

P

B PhAr or vacuum

Figure 12 A N2 atmosphere favors the Fe–nitride (left), but exposing the complex to vacuum or to an argon atmosphere produced the bridged N2

complex (right).

S

Ir S

IrRu

S Ir

S

Cp*

Me2NNMe2

N

N

S

Ir S

Ir

Fe

S Ir

SCl

Cp*

S

M S

MoMo

S M

S

Cl

Cp* Cp* Cp*Cp*

Cp*

Cp*N

NPh Me

ClCl

DMF

Cl

M = Ir, Rh

A B C

Figure 13 Cubanes of Ru, Mo, and Fe, including the N2 complex of Ru.

Sm SmN

N

Figure 14 The first planar side-on/side-on complex of N2.


the N2 ligand is formally N22�. A large number of other side-

on/side-on N2 complexes have now been synthesized (in fact,

50 of them have been characterized crystallographically), and

most display greater reactivity than end-on N2 complexes.14 In

the case of the lanthanide metals, side-on/side-on complexes

are known14 for thulium,62,63 dysprosium,63,64 neodymium,65

gadolinium, holmium, terbium, yttrium, erbium, lutetium,

lanthanum,66,67 and praseodymium.68 Starting from Ln[N

(SiMe3)2]3, these complexes (Ln¼Tm, Dy, Nd, Gd, Ho, Tb, Y,

Er, Lu, La) are typically synthesized by reduction in THF under

N2 to yield products of the type {[(SiMe3)2N]2Ln(THF)}2(m-Z2:Z2-N2) (eqn [3]).66,67.

2Ln[N(SiMe3)2]3 + 2KC8N2

THF

N

NLnLn

(Me3Si)2N

(Me3Si)2N

N(SiMe3)2

N(SiMe3)2

O

O

½3�

8.14.3.2.2 ActinidesThe first actinideN2 complex was reported in 1998, 10 years after

(Cp*2Sm)2(m-Z2:Z2-N2) was first reported. Similar to the Sm

complex, the new [N(CH2CH2NSitBuMe2)3U]2(m-Z2:Z2-N2)

complex (Figure 15, left) did not display significant NdN

bond elongation.69 Density functional theory calculations on

the model complex [(NH2)3(NH3)U]2(m-Z2:Z2-N2)

70 showed

that p-backbonding was the most important U–N2 interaction,

a result which would seem to indicate that the NdN bond

should be longer than the experimentally determined distance

of 1.1097 A. One explanation for this discrepancy is that the

bulky ligand interferes with the overlap of the uranium f and

N2 p* orbitals, thereby decreasing the backbonding interaction.

However, a similar complex (Figure 15, right) did show elonga-

tion of theNdNbond to 1.232 A.71 The reasons for the differing

extents of activation remain unknown.14 (Note that the UdN

bond lengths are the same in the two complexes.) The other

N2–actinide complexes are U72–74 and Th75 compounds.

8.14.3.2.3 Transition metalsSide-on/side-on N2 bonding in the transition metals is espe-

cially intriguing because the bonding mode of N2 in nitroge-

nase is possibly side-on to the iron centers. Although several

examples of side-on/side-on complexes were discussed in

Section 8.14.2.2.2, additional examples are presented in this

section for the purpose of demonstrating the wide range of

metals and ligands that can form these complexes and the

wide range of structures that are possible. The first example

is the [Li(TMEDA)2]{[{(Me3Si)2N}2Ti]2(m-Z2:Z2-N2)2} com-

plex. Note that the molecule has two side-on/side-on N2

ligands (Figure 16).76 Zirconium77 and hafnium78 dimers

containing side-on N2 are also known.

U UN

N

N

NN

N

N

N

R

R

R

R

N

R

N

R

R = tBuMe2Si

U U

N

N

iPr3Si

iPr3Si

SiiPr3

SiiPr3

Figure 15 Left: The first actinide N2 complex. Right: A side-on/side-on U–N2 complex with an elongated NdN bond.

Ti TiN

N

N

N

N

N

SiMe3

SiMe3 SiMe3

SiMe3Me3Si

Me3Si

N

N SiMe3

SiMe3

Figure 16 Two Ti centers bind two molecules of N2 side-on.


In an interesting example of reactivity, the N2 ligand of a

bridged, end-on/end-on niobium dicalixarene [{[p-tBu–calix

[4]–(O)4]Nb}2(m-Z1:Z1-N2)][Na(diglyme)2]2 (4) was reduced

to yield the side-on/side-on N2 complex [{[p-tBu–calix[4]–

(O)4]Nb}2(m-Z2:Z2-N2)][Na(DME)]4(DME) (9). This reaction

is of conceptual mechanistic interest because the product mol-

ecule (9) is potentially an intermediate on the pathway to N2

bond cleavage. In fact, the molecule does indeed convert to the

bis(m-nitrido) complex (5) with heating (Scheme 5).35 Over-

all, the side-on/side-on!end-on/end-on!nitrido reaction

might represent a generalized scheme for N2 bond cleavage.

As discussed in Section 8.14.2.2.1, an unusual mononu-

clear side-on N2 complex, [(NH3)5Os(Z2-N2)][PF6]2, was char-

acterized by x-ray crystallography and IR spectroscopy. The

molecule was generated by irradiation of the corresponding

end-on complex, [(NH3)5Os(Z1-N2)][PF6]2.19 This type of re-

activity supports previous 15N NMR and IR evidence of N2

isomerization by way of a side-on/side-on intermediate in

CpRe(CO)L(Z1-N2) complexes (Scheme 6).79 Evidence for a

mononuclear side-on N2 complex also supports the possible

formation of a side-on intermediate in the isomerization of the

N2 ligand in [Ru(NH3)5N2]2þ.80

Finally, it is noted that Fryzuk developed an amidodipho-

sphane ligand (PNP)81 with both soft phosphane and hard

amino donors in order to form complexes with the full span of

the transition metals, as well as to allow for a range of oxida-

tion states in the coordinated metal (Figure 17).82 The side-on

N2 complexes of zirconium represent a significant degree of N2

activation, as demonstrated by the formation of hydrazine

when the complexes were protonated.83 This work has been

extensively reviewed.81

8.14.3.3 Side-On/End-On Coordination of N2

A fascinating side-on/end-on coordination of N2 was found in

the tantalum complex 10 (Figure 18), which contains both N2

and H2, the components necessary for NH3 formation.84 The

hydride ligands are coordinated to Ta by addition of H2 gas to a

solution of PhP(CH2SiMe2NPh)2TaMe3, forming the highly

reducing 11 without the use of a strong reducing agent. Note

that the substitution of N2 for H2 is a rare transformation for

early transition metals.14

8.14.4 Uses for Activated Dinitrogen-ContainingComplexes: NH3 Formation

Many of the complexes described in the preceding sections are

active toward nitrogen reduction. This section describes pro-

gress in using metal–dinitrogen complexes to form reduced

nitrogen products such as ammonia, hydrazine, and diazene.

8.14.4.1 Early Studies

Allen and Senoff’s [Ru(NH3)5(N2)]2þ complex48 did not react

with acid to form NH3, but the complex demonstrated the

weakly basic nature of the dinitrogen ligand (see Figure 3) by

reacting with itself to form the bridged [(NH3)5Ru(N2)Ru

(NH3)5]4þ complex.85 This was the first clue that coordinated

N2 might be able to react with protons to produce NH3.82 The

nucleophilic/basic nature of coordinated N2 was reinforced by

its reactions with electrophiles to form N–C (e.g., trans-[WCl

(N2COCH3)(DPPE)2])86 and NdH bonds (e.g., trans-[MX

(NNH2)(DPPE)2]X; M¼W, X¼Cl; M¼Mo, X¼Br); see

Section 8.14.5 for a full discussion of this reactivity.87,88 Pro-

tonation of coordinated N2 to form NH3 soon followed. Spe-

cifically, Chatt’s group protonated cis-[M(N2)2(PMe2Ph)4]

(M¼Mo or W) with H2SO4 to yield 2 equiv. of NH3 per

metal atom.89 Numerous mechanistic studies were carried

out on this and other ammonia-forming reactions by a number

of research groups, and some general mechanistic principles

began to emerge. In particular, two pathways came to be

regarded as likely. One is now known as the ‘Chatt cycle’

(Figure 19) and is characterized by an asymmetric protonation

pathway, that is, protonation of the distal N2 ligand produces

1 equiv. of NH3, followed by cleavage of the NdN bond,

leaving a nitride coordinated to the metal for further proton-

ation to form the second equivalent of NH3. In the early days

of these mechanistic studies, it was thought possible that ni-

trogenase could operate in a similar fashion, provided that the

active site of the enzyme was a single metal center. More recent

studies on nitrogenase, however, support a ‘symmetric’ proton-

ation pathway.54 The ‘symmetric’ pathway is characterized by

alternating protonation between the two nitrogen atoms. This

pathway is discussed in more detail later.

LnNa

LnNa NaLn

OO OO

Nb

N

N

Nb

O OO O

2-

N

Nb

O OO O

NbO

O

O

O

NNa

DME

Heat

OO OO

Nb

Nb

O OO O

N NaLnNLnNapy

4 9 5

Scheme 5 Side-on/side-on N2 complexes are intermediates in breaking the NdN bond in Nb calixarenes.

Re 15N N14 Re 14N N15Re

15N

14N

Re = CpRe(CO)L; L = CO, PR3

Scheme 6 Isomerization of CpRe(CO)L(Z1-N2) involves a side-onintermediate.

N

Me2Si

P

Me2Si

PR

R R

RM

Figure 17 General structure of Fryzuk’s PNP ligand.


8.14.4.2 Molybdenum Systems

Schrock’s low-valent molybdenum complex shown in

Figure 20 was the first complex to catalyze the reduction of

N2 to NH3 at room temperature and atmospheric pressure. The

system, however, required an external source of both protons

and electrons.90 Yields of ammonia up to 65% were obtained.

(The yields were calculated relative to the number of reducing

equivalents available; for comparison, FeMo nitrogenase oper-

ates at about 75% efficiency.) The catalyst degraded after about

four turnovers. Schrock and coworkers synthesized numerous

Chatt-cycle intermediates and performed calculations that ver-

ified that the Chatt cycle was energetically favorable for this

system. The efficiency of the system was susceptible to subtle

changes in the ligand environment and to the metal. For ex-

ample, switching the metal to vanadium significantly reduced

the efficiency. Overall, the system demonstrated the fine

balance between sterics, electronics, and activation that must

come together for an efficient conversion of N2 to NH3.91,92

The only other example of a successful catalyst for the reduc-

tion of N2 to NH3 under ambient conditions is a bridged mo-

lybdenum complex with PNP–pincer ligands recently developed

by Nishibayashi and coworkers (Figure 21).93 The catalyst was

able to produce up to 23 equiv. of NH3 per catalyst (12 equiv.

per Mo atom). However, the conversion was slow (20 h) and,

like the Schrock catalyst, used an external source of protons and

electrons. The proposed mechanism involves first breaking

the dimer to form a traditional end-on N2 Mo complex and a

Mo–Hnnþ complex. It was proposed that the Mo–N2 complex

then reacts to form NH3 by a traditional Chatt-type cycle.

Complexes that form or bind hydrazine (N2H4) or hydra-

zido(2�) (N2H22�) moieties are important to consider because

of their likely role in the formation of NH3. To investigate the

mechanism of how these molecules are reduced to NH3, George

and coworkers synthesized molybdenum–N2 complexes bound

to a polymer resin in order to isolate themetal centers from each

other (Figure 22).94 If the disproportionation of N2H4 toNH3 is

mediated by a single metal center, no change in NH3 formation

should be observed with the polymer-bound complex. On the

other hand, if the mechanism involves the interaction of two

metal centers, the production of NH3 would be completely shut

down because the polymer support limits the ability of the

metal centers to migrate toward each other. The formation of

NH3 was in fact shut down, and it was concluded that the

disproportionation mechanism involves the interaction of two

or more metal centers.

8.14.4.3 Other Metals

The reaction of N2 complexes with H2 typically leads to dis-

placement of the N2 ligand.14 Far more exciting would be the

formation of complexes with new NdH bonds, and in fact a

Ta TaHH

N

NN

N

Ph

Ph

Me2Si

P

Ph

Me2Si

Ph

SiMe2

P

SiMe2

PhPh

Ta TaHH

N

NN

NNN

Ph

Ph

Me2Si

P

Ph

Me2Si

Ph

SiMe2P

SiMe2

PhPh

HH

N2

-H2

10 11

Figure 18 Fryzuk’s side-on/end-on tantalum dimer (11), formed by displacement of H2 by N2.

MoNN

MoNNH2

MoNNH3+MoNH

MoNH

MoNH2

MoN2

MoNH3

NH3

H+, e-

H+, e-

H+, e-

H+, e-

H+, e- e-

H+

N2

Chatt cycle

Figure 19 Chatt cycle for nitrogen reduction to ammonia on amolybdenum center.

N

N

Mo N

N

N

N

HIPT

iPr

iPr

iPr

iPriPr

iPr

HIPT

HIPT

Figure 20 Schrock’s Mo catalyst for N2 reduction to NH3. The identityof the HIPT group is indicated in the drawing.

N N MoMoN

N

P

P

N

N

N

P = PtBu2

P

P

N

N

N

N

N

Figure 21 PNP Mo complex that catalytically produces NH3.

P

Ph

PPh

Ph

MoPPh2Me

PPh2Me

N2

N2

Figure 22 The polystyrene-supported Mo–N2 complex does notproduce ammonia upon protonation, but the analogous homogeneouscomplex Mo(N2)2(DPPE)2(PPh2Me)2 does produce NH3, likely throughN2H4 disproportionation.


few such reactions are now known. For example, Chirik and

coworkers showed that the zirconocene complex in Scheme 7

reacted with H2 to form the indicated diazenido (N2H22�)

complex.95 (This complex was the first diazenido complex of

a transition metal.) What makes this reaction so intriguing is

that the zirconocene with C5Me5 ligands does not display this

reactivity. This reaction is thus another example of the fine

balance between sterics, electronics, and activation that must

come together for an efficient conversion of N2 to NH3. Heat-

ing the diazenido complex in Scheme 7 cleaved the NdN

bond completely and dissociated H2.14 However, if the com-

plex is heated in a H2 atmosphere, NH3 is produced in 10–15%

yield (Figure 23).95 A particularly interesting feature of the

reactivity in Scheme 7 and Figure 23 is that H2 is the source

of protons and electrons in the reduction of N2. H2, of course,

is the reductant in the Haber–Bosch process, not Hþ and a

separate reducing agent. Efficient, homogeneously catalyzed

production of NH3 from N2 will need to use H2 as a feedstock

so the reactions in Scheme 7 and Figure 23 are important steps

forward in pursuit of this goal.

Shilov’s group extensively explored the reactivity of

vanadium–N2 complexes in aqueous solutions.96 V(II)–Mg

(II) hydroxides were freshly prepared, then reacted with N2 at

room temperature and ambient pressure to form hydrazine.97

Raising the temperature of the reaction favored NH3 produc-

tion, which could be a result of N2H4 reduction. It was later

shown that it is possible for N2H4 to be an intermediate in the

production of NH3, as long as it remained bound to the metal;

kinetics showed that free N2H4 cannot be a reduction interme-

diate. It was suggested that this system is significant because N2

can be reduced directly from the atmosphere (instead of pure

N2 gas) and the reduction is not inhibited by CO, as is the case

with nitrogenase. A tetranuclear V(II) cluster was proposed to

be the active complex in these systems (Scheme 8).

A mixed-metal system containing cis-[W(N2)2(PMe2Ph)4]

and trans-[RuCl(Z2-H2)(DPPP)2]X (DPPP¼1,3-bis(diphenyl-

phosphino)propane; X¼BF4�, PF6

�, or TfO�) generated NH3

under a H2 atmosphere at 55 �C (Figure 24).98 The Ru com-

plex was slowly generated in situ from [RuCl(DPPP)2]X and

1 atm of H2, so preformation of the Ru–H2 complex followed

by addition of cis-[W(N2)2(PMe2Ph)4] resulted in higher yields

Zr ZrN

N

H2 Zr ZrN

N

H

H

HH

85 �CZr Zr

N

NH2

Scheme 7 Chirik’s zirconocene dimer reacts with H2 to form a reduced diazenido complex, which can be heated to break the NdN bond.

Zr ZrN

N

H

H

HH

85 �C

H2Zr

H

H+ NH3

Figure 23 Heating [(Z5-C5Me4H)2Zr(H)]2(m-Z2:Z2-N2H2) in a H2

atmosphere produces NH3.


of ammonia (55% relative to W) than simply mixing all

the reagents together at the same time (22% NH3). The role

of the Ru–dihydrogen complex in this reaction is to act as

an acid to protonate the N2 ligand.99 The best Ru–H2 complex

to use in this reaction was also the most acidic: trans-[RuCl(Z2-

H2)(DPPE)2]X (pKa¼6.0; 79% NH3); trans-[RuH(Z2-H2)

(DPPP)2]X (pKa¼10.2; 0–6% NH3); trans-[RuH(Z2-H2)

(DPPE)2]X (pKa¼15.0; 0–6% NH3). The success of this system

demonstrated that it is possible to produce NH3 from N2 and

H2 under milder conditions than Haber–Bosch.100

8.14.4.4 Iron Complexes

With the discovery of vanadium–iron and iron-only

nitrogenases,101 the focus of research on dinitrogen reduction

to ammonia shifted toward iron. Leigh was the first to achieve

this goal using an Fe–DMPE system in 1991 (DMPE is 1,2-bis

[bis(methyl)phosphino]ethane).102 He showed that trans-[Fe

(DMPE)2(N2)H]þ could be formed by reacting trans-[Fe

(DMPE)2(H2)H]þ with N2. Though trans-[FeII(DMPE)2(N2)

H]þ did not react with acid to form NH3, deprotonation of

this complex to form the reactive five-coordinate trans-

Fe0(DMPE)2(N2) complex significantly activated the N2 ligand

(nNN¼1975 cm�1). Initial reports of 12% NH3 per mole of Fe

upon protonation were eventually improved by lowering the

temperature of the reaction to �40 �C, which produced up to

20% NH3.103 When HCl was used as the acid, Fe(DMPE)2Cl2

was regenerated, proving the feasibility (if not the actuality) of

a catalytic iron system (Figure 25).

Interestingly, Komiya showed that changing the DMPE

ligand to DEPE (1,2-bis[bis(ethyl)phosphino]ethane) in the

analogous iron complex resulted in no NH3.104 This difference

in reactivity might be explained by the fact that Fe0(DE-

PE)2(N2) was isolated before it was protonated to form am-

monia, whereas in the DMPE system, the Fe0(DMPE)2(N2)

complex was not isolated but simply generated in situ and

then protonated.102 Leigh suggested that interactions with

the various counterions that are present in the in situ sample

(e.g., Fe–NN!Liþ) might be crucial to the success of the

reduction.105

Peters showed that the iron–nitride complex [PhBPiPr3]

FeIV^N will form NH3 with added protons ([LutH][BPh4])

and electrons (CoCp2).25 However, no iron system has yet

been shown to generate ammonia catalytically. If such a

catalytic cycle were to follow the Chatt cycle proposed for

molybdenum, the iron must be able to accommodate a num-

ber of reduced nitrogen ligands in a variety of oxidation

states. Toward this end, Peters and coworkers studied an FeI/

FeIV system that forms stable FeIV–nitride complexes. Addi-

tionally, they developed and studied an Fe complex with a

tripodal ligand scaffold (Figure 26) that can accommodate

N2 in a variety of oxidation states (Fe0, FeI, and FeII).106 For

example, protonation of the FeI complex in Figure 26 pro-

duced 17% hydrazine (but no NH3).107 This yield was im-

proved to 47% N2H4 when an external reducing agent was

added. However, when the complex was reduced to Fe0(Si

(1,2-C6H4PPh2)3)N2 and then protonated, no hydrazine was

formed, even though the N2 molecule was more activated

than in the FeI complex. The decrease in hydrazine yield is

likely because the Fe0 complex is a stronger reducing agent

than the FeI complex and so favors Hþ reduction to H2 over

N2 reduction.

Holland has pioneered work with low-coordinate iron

complexes. These complexes react with N2 to afford molecules

with N2 in various degrees of reduction, depending on the

sterics of the ligand. One example is shown in Figure 27.34

In another example, the NdN bond is completely cleaved

to yield the fully reduced bis-nitride complex, a 4-Fe, 2-Kþ

structure 12 (Figure 28). This highly activated complex reacts

with excess hydrogen gas at room temperature to produce

2 equiv. of ammonia and two bridged hydride–iron dimers

(Figure 28, 13).

A homogeneous iron system has been reported that forms

NH3 at room temperature and ambient pressure using only N2,

H2, and acid–base chemistry.108 The system is a modification of

the Leigh cycle and is shown in Figure 29. The need for a hydride

source or reducing agent in the first step of the Leigh-type cycle

(see Figure 25) is eliminated because H2 is used directly to form

trans-[Fe(DMeOPrPE)2(H2)H]þ (DMeOPrPE¼1,2-bis[bis(meth-

oxypropyl)phosphino]ethane). Although the yields of NH3 for

this reaction are low (15% in terms of iron equivalents), the

DMeOPrPE ligands impart water solubility to the complex.

The potential use of water as a solvent along with cheap iron

makes this an energetically attractive system for the production

of ammonia.

The Field group is active in synthesizing tripodal-tetradentate-

phosphane ligands109 and their coordination complexes with

iron.110 This work resulted in amixed-valence (Fe0/FeII)-bridged

N2 complex from the deprotonation of [(FeH(PP3))2(m-N2)]2þ

(PP3¼P(CH2CH2PMe2)3).111 A second deprotonation yields

the Fe0/Fe0 dimer, (Fe(PP3))2(m-N2) (Figure 30).

In 1971, Shilov reported the first conversion of N2 into

N2H4 on an iron center.112 A dinuclear complex (Figure 31)

was proposed from the reaction of (Ph3P)2FeCl3,iPrMgCl, and

N2, which then reacted with HCl to make hydrazine. This

mechanism remains to be confirmed.38

O

VII

O

VII

O

Mg

O

VIIN

HO

HO

HO

HO

OH

OH

OH

OH

N

O

O

VII

O

VIII

O

VIII

O

Mg

O

VIIIN N

O

O

VIIIH2O

H+ OH-

H2ON2H4

Scheme 8 A V(II)–N2 cluster reacts with H2O to form N2H4.

W

N

N

P

P

P

P

N

N

RuP

P

P

P

H H

Cl

6+ 2NH3 + 6 Ru

HP

P

P

PCl

W(VI) ?+

P PDPPP=

55 �C

P = PMe2Ph

24 h

6 [RuCl(DPPP)2]X

H2

Figure 24 Reaction of cis-[W(N2)2(PMe2Ph)4] with trans-[RuCl(Z2-H2)(DPPP)2]þ to form NH3.

FeIICl2

FeII(N2)H+

FeII(N2)H+Fe0N2

H+,e-

N2

H2

HCl

BH4-

NH3

Leigh cycle

Figure 25 Leigh cycle for production of NH3 on a single iron center. Fein this figure represents Fe(DMPE)2.

FeI

Si

Fe0

N2

N2

Si

PPh2

PPh2

Ph2P

Ph2PPh2P

Ph2P

H+

H+

17% N2H4

0% N2H4

Figure 26 Experimental demonstration showing that strongeractivation of the N2 ligand does not necessarily lead to increasedreactivity.107 The N2 ligand in the Fe

0 complex (bottom) is more activatedthan in the FeI complex (top), but the Fe0 complex is a better reducingagent. The reaction of the Fe0 complex leads to Hþ reduction, limiting theformation of reduced N2 products.

R = Me or tBu

Fe N N FeN

N

R

R

K

K

N

N

R

R

FeN

N

R

R

Cl

excess K, N2

-KCl

Figure 27 Formation of an activated N2 ligand in a low-coordinate Fecomplex. The NdN bond distance is 1.24 A. The presence of the Kþ ionsnear the N2 ligand increases the charge transfer from the metal to the N2

ligand, thereby contributing to the activation of the N2.


In summary of the studies above using Fe complexes to

activate and react N2, the results indicate that Fe complexes

with bidentate ligands favor NH3 formation, whereas com-

plexes with tripodal (more rigid) ligands favor N2H4 forma-

tion. This tendency possibly provides insights into the

mechanism of NH3 formation. Perhaps the flexibility of the

bidentate ligands to accommodate Z2 ligands, such as reduced

forms of N2 like hydrazine and diazene, is necessary to reduce

N2 to NH3 because the pathway involves, perhaps, hydrazine

disproportionation. Another possibility is that the bidentate

ligands allow for trans-hydrides to form when the iron center is

protonated and the strong trans influence of the hydride acti-

vates bound intermediates, such as N2H4, toward dispropor-

tionation to form NH3.39

8.14.4.5 Reduced Dinitrogen Complexes

In both biological and synthetic systems that produce am-

monia, the mechanism of N2 reduction remains an unan-

swered question. The study of the coordination of reduced

dinitrogen species to metal centers and their subsequent

reactivity has provided some insight into the subject, giving

Fe N N FeN

N

N

N

Fe

Fe

K

K

Cl

Cl

N

N

N

N

Ar

Ar

xs H2

25 �CToluene

2 Fe FeN

N

N

N

H

H+ 2NH3 + 2KCl

12 13

Figure 28 Formation of NH3 from the fully reduced nitride complex 12 with H2.

FeII

Cl

ClP

P

P

FeII

H2

H

P

P

PFeII

H

P

P

P

P

P

P

+ +

Fe0

P

P

P

PN2

H2

N2N2

B-

B-

NH3

xs HCl

Fe

P

P

P

P

H

H

H-

H+

Figure 29 Modification of the Leigh cycle to give a homogeneous ironsystem that forms NH3 at room temperature and ambient pressure usingonly N2, H2, and acid–base chemistry.


way to two proposed mechanisms: the symmetric or alter-

nating pathway and the asymmetric or distal pathway

(Scheme 9).113 The intermediates in these pathways include

complexes of diazene (N2H2), hydrazine (N2H4), and re-

lated ligands, many of which are thought to be involved in

nitrogenase activity.

Diazene (N2H2) is a key reduction intermediate, as it rep-

resents the addition of two electrons and two protons to N2.

However, it is highly reactive and, therefore, few examples of

diazene coordination complexes exist.114,115 Most diazene

complexes are synthesized by oxidation of coordinated hydra-

zine using Pb(OAc)4, O2, or [FeCp2]þ. This method avoids the

problem of having free diazene in solution, which decomposes

(eqn [4]) or disproportionates (eqn [5])116,117 to produce

competing ligands.

N2H2 N2 + H2 ½4�

2N2H2 N2H4 + N2 ½5�

Sellmann et al. prepared Z1-bridged diazene complexes

with thiolate ligands by oxidation of the hydrazine complex

or by trapping N2H2 with a coordinatively unsaturated iron

complex (Scheme 10). In these examples, the diazene ligand is

stabilized by steric shielding, p-bonds between the iron

d-orbitals and the p system of diazene, and hydrogen bonding

between the diazene protons and the thiolate ligands.118,119

Schrock and coworkers reported a rare example of an

unsubstituted hydrazido(1-) ligand on a tungsten center,

Cp*WMe4(Z2-NHNH2).

120 The protons exhibit fluxional be-

havior, as evidenced by 1H and 15N NMR spectroscopy, which

is a combination of a 1,2-shift and rotation and inversion of

the NdN bond (Scheme 11). The Cp*WMe4(Z2-NHNH2)

complex can be protonated to yield the corresponding hydra-

zine complex, Cp*WMe4(Z2-NH2NH2), confirming the pres-

ence of a lone pair on one of the nitrogen atoms in the starting

hydrazido(1-) complex.

Recently, iron–phosphane scaffolds have been used to sta-

bilize a group of N2H4 derivatives, namely N2H3� and N2H2.

These Z2-N2Hx complexes can be interconverted using acid–

base chemistry (Scheme 12).121,122 End-on coordination of

the complete series of reduced N2 species (N2H2, N2H, N2H4,

and NH3) was also recently established using iron–phosphane

scaffolds with either a hydride–ligand trans to the reduced

nitrogenous ligand (Scheme 13)123 or bridging two iron cen-

ters with the ligand (Scheme 14).124

8.14.5 Incorporation of Nitrogen Derived from M–N2

Complexes into Organic Molecules

A wide range of commercial products – amines, amides, alky-

lammoniums, ureas, carbamates, isocyanates, nitriles, and

amino acids – are derived from ammonia.125 The direct con-

version of N2 into these complexes is an active area of research

because a successful process would bypass the need for ammo-

nia in the production of these materials, thus simplifying the

production of the organic molecules and saving energy and

producing fewer pollutants. As with the reduction of dinitro-

gen to ammonia, the hurdle to making viable processes is the

ongoing quest for mild conditions at a catalytic metal center. In

addition, successful processes will need to be selective and

produce high yields of the desired product. Although no sys-

tems currently meet these requirements, substantial progress

has been made, as detailed below.

P

P

N

P H

P

N

FeII FeII

FeII

P

P

P

H P

2+

Fe0 Fe0

Fe0

P

P

N

P P

N

P

P

P

H P

+

P

P

N

P P

N

P

P

P

P

B-

H+ H+

B-

Figure 30 Stepwise deprotonation of [(FeH(PP3))2(m-N2)]2þ to yield the mixed Fe0/FeII complex, [(FeH(PP3))(m-N2)(Fe(PP3))]

þ, followed by theFe0/Fe0 dimer, (Fe(PP3))2(m-N2).

N N Fe (PPh3)2

H

N2H4(Ph3P)2 FeCl3 (Ph3P)2 FeiPr MgCl

iPr iPr

N2

HCl

Figure 31 Formation of hydrazine from the reaction of (Ph3P)2FeCl3,iPrMgCl, and N2 with HCl.

M

N

N

M

N

NHM

N

NH2

M

N

NH3

M

NH

M

NH2 NH3

M

NH

NH

M

NH

NH2

M

NH2

NH2

NH3

Scheme 9 Two suggested pathways for the formation of ammonia from a metal–N2 complex. Top: Asymmetric/distal mechanism. Bottom:Symmetric/alternating mechanism.

Fe

S

PPr3

S

S

S

N2H4

Fe

S

PPr3

S

S

S

NH Fe

S

PPr3

S

S

S

HN

O2 N2H2

Fe

S

PPr3

S

S

S

Scheme 10 Two methods used by Sellmann et al. to prepare a bridged N2H2 iron complex.


Like dinitrogen fixation to ammonia, dinitrogen incor-

poration into organic molecules requires a reduction of

one electron per nitrogen to achieve a diazenido species

(M]N]NdR), two electrons per nitrogen to achieve a hydra-

zido species (M^NdNR2), and three electrons per nitrogen

atom to form amino (NR3) or nitrido species (M^N).

The trans-[M(N2)2(dppe)2] (dppe¼Ph2PCH2CH2PPh2 and

M¼Mo (14a), W (15b)), and cis-[M(N2)2(PMe2Ph)4]

(M¼Mo (15a), W (15b)) complexes (Figure 32) have been

extensively studied as potential catalysts for these reductions

because both Mo and W are stable in high-oxidation states.

This allows the M(0) metal centers to donate a significant


number of electrons to coordinated nitrogen and still exist as

stable high-oxidation-state species. Because the high-oxidation-

state metal centers are stable, the molecules are potentially able

to be reduced and the reaction repeated. This provides an attrac-

tive model in which the reduction of dinitrogen can be com-

pleted solely by the metal center(s) – avoiding exogenous

electron sources until after the reactive N-containing intermedi-

ates have been converted into products. For these reasons, con-

siderable research has gone into utilizing complexes 14 and 15

for organic transformations, and they are the initial focus of this

section. For the discussion that follows, it is important to re-

member that the distal nitrogen is basic/nucleophilic and there-

fore reactive toward appropriate electrophiles. In contrast, the

coordinating nitrogen is electrophilic and reactive toward ap-

propriate nucleophiles.126

N N

W

HA HC

HB

NN

W

HCHA

HB

N N

W

HA HB

HCNN

HB

HC

HA

W

Scheme 11 Proton exchange in Schrock’s Cp*WMe4(Z2-NHNH2)

complex. The methyl and Cp* ligands are not shown.

Fe

P

P

P

P

NH2

NH2

2+

Fe

P

P

P

P

NH

NHFe

P

P

P

P

N2H4

-H2N2

PP = DMPE or DMeOPrPE

KtBuO

Scheme 12 Conversion of a N2 complex (left) or N2H4 complex (right)to a N2H2 complex (center).

Fe

N2

H

P

P

P+

P

Fe

NH3

H

P

P

P

P

+

Fe

Cl

H

P

P

P

P

NH3

[NEt4][Cl] NN

N2H4

PP = DMeOPrPE

Scheme 13 Relationship between reduced N2 trans-hydrido complexes.

8.14.5.1 Reactions of End-On-Bonded N2 Complexes

8.14.5.1.1 NdC bond formation: dinitrogen to diazenidoThe reaction of M–N2 complexes to form diazenido complexes

is well known. Chatt and coworkers reacted 14a and 14b with

acetyl halides (RC(O)X) and aroyl halides (ArC(O)X) to pro-

duce either the corresponding organodiazenido with 1 equiv.

of electrophile or the organohydrazido with 2 equiv. of elec-

trophile (Scheme 15).86 This reactivity shows the nucleophilic

character of the distal nitrogen for strong electrophiles. The

organo-nitrogen diazenido complexes show a considerably

higher stability than their hydrogen diazenido analogs. For

instance, the aroyl organodiazenido complex trans-[MoCl

(N]NdC(O)Ph)(dppe)2] is stable enough to characterize in

the solid state, whereas the hydrogen analogs are typically not

stable enough to obtain x-ray data.127

The reactions of M–N2 complexes with acyl and aroyl ha-

lides are believed to proceed by an SN2 mechanism.128 In

contrast, the alkylations of dinitrogen with alkyl halides

(a slightly weaker electrophile) proceed primarily by a radical

mechanism. In the alkyl halide reactions, the rate-determining

step is dissociation of one N2 from the complex to form

[M(N2)(dppe)2]. The alkyl halide is believed to coordinate at

the open site, and this is followed by homolytic cleavage of the

carbondhalogen bond. The organic radical thus generated

attacks the distal nitrogen of the N2 ligand, ultimately forming

the organodiazenido complex.129

Chatt showed that [ReCl(N2)(PMe2Ph)4] (16) and [ReCl

(N2)(py)(PMe2Ph)3] (17) (py¼pyridine) could be acylated

but that these molecules showed no reactivity at the distal

nitrogen toward alkyl halides. The Os complex [OsCl(N2)

(PMe2Ph)4] (18) is unreactive toward even acyl chlorides at

the distal dinitrogen. Overall, these results show a trend of

lowered nucleophilicity at the distal nitrogen going across the

third row of the transition metals (W>Re>Os), and they

emphasize the importance of the metal in nitrogen activation

and reactivity.130

As might be expected, the reactivity of M–N2 complexes is

sensitive to the ligands as well as the metal center. Studies by

Pickett showed that decreasing the s-donating ability of the

phosphanes in 14 by substituting the phenyl rings with

electron-withdrawing groups significantly slowed the reactivity

of the distal nitrogen toward electrophiles. For completeness,

it is also noted that the coordination geometry of the

Fe

HN

H

P

P

P

P

+

Fe

H2N

H

P

P

P

P

+

NH

NH2

aBPh42H4 Fe

N

H

P

P

P

P

NH+

Fe

NH3

P

O

O

P

P

Fe

P P

OO

P

Fe NP

OO

PP

FeN P

O O

P P

FeH2NP

OO

PPFeN

H2

P

O O

P P

FeHNP

OO

PPFeN

HP

O O

P P

N2

N2H4

Scheme 14 Iron–phosphane complexes with bridged N2, N2H4, and N2H2 ligands and an NH3 complex with the same scaffold. For clarity, only thethree coordinating phosphorus atoms of the tridentate PhB(CH2P(CH2Cy)2)3

� ligand are shown.

M

N2

N2

N2

N2

P

P

P

PM

P

P

P

P

PP

= Ph2PCH2CH2PPh2 = dppe

M = Mo (14a), W (14b)

P = PMe2Ph

M = Mo (15a), W (15b)

14 15

Figure 32 Complexes 1 and 2 are well studied for the incorporation ofN2 into organic molecules.

M

N

N

PPPP

P

P

N2

R¢ XM

PP

PP

N

X

NR¢

MPP

PP

N

X

N

X R

O

O

R

Ar X or

No reaction

hv

M = W, MoR = alkyl, arylR¢ = alkyl

= dppe

Bn X

Scheme 15 Reactions of [ML(N2)2P4] with acyl, aroyl, alkyl, benzyl,and phenyl halides.


phosphanes has an effect on the reactivity of the distal nitrogen

with alkyl halides. Complex 14 is generally more reactive than

complex 15.131

Perhaps not surprisingly, none of the complexes (14–18)

are nucleophilic enough to react with aryl halides or benzyl

halides, which are less electrophilic than alkyl halides. In fact,

direct aryl functionalization of the distal nitrogen in dinitrogen

complexes is very limited. Several W–N2 complexes exhibit

nucleophilicity toward aryl halides. Thus, trans-[WX(N2)

(dppe)2][Bu4N] (X¼F�(19a), SCN�(19b)) reacts with

electron-deficient aryl fluorides to produce organodiazenido

trans-[W(N]NdAr)X(dppe)2] complexes (Scheme 16). This

reactivity occurs with only five known aryl fluorides that are

activated by metal coordination ([Ru(Z5-Cp)(Z6-p-C4H4FR)]

[PF6] (Cp¼C5H5�, R¼H, Me, OMe, COOMe) and [Cr

(CO)3(Z6-p-C4H4F(COOMe))]). The structures of the resulting

diazenido complexes were fully characterized by spectroscopic

and x-ray diffraction analysis, which showed that the N]N

bond lengths were substantially lengthened and the newly

formed NdC bonds were shortened. These bond lengths indi-

cate delocalization of the N]N bond over the aryl ring – a

possible driving force for the reaction. The reactions likely

proceed by a direct nucleophilic aromatic substitution mecha-

nism, rather than by the radical mechanism discussed above.

Interestingly, if Fe is used in place of Ru or a chloroarene in

place of a fluoroarene, no reaction occurs. These results em-

phasize the specificity of the reactivity.132

Despite the general unreactivity of [MN2L(P)4] complexes

with aryl halides, work by Ueda showed that a macrocyclic

tetrathioether Mo complex, trans-[Mo(N2)2(Me8[16]aneS4)]

(20), reacts readily with aryl halides (PhBr, PhI) and a benzyl

halide (PhCH2Br) with unprecedented reactivity (Scheme 17).

The reaction proceeds at room temperature without irradiation

to give MoII aryldiazenido and benzyldiazenido complexes,

Ru

F R+

WPP

PP

N

X

N-

Cr

F

COOC CO

O

OMe

WPP

PP

N

X

N Ru

R+

WPP

PP

N

X

N Cr CO

OC CO

O

OMeR = H, Me, OMe, COOMeX = NCS-, F-

W

N

N

PPPP

N2

[Bu4N]X

PP

= dppe

19

Scheme 16 Nucleophilic reactivity of trans-[WX(N2)(dppe)2][Bu4N] with electron-deficient benzyl fluorides and phenyl fluorides.

Mo

N

N

SS

SSN2

X R

O

SS

SS

R2–X

R1–XMo

N

N

SS

SSX

Mo

N

N

SS

SS

Mo

N

N

SS

SSX

X

R2

R1

R

O

Mo

N2

N2

hν

R = alkyl, arylR1= alkylR2= aryl, benzyl

20

Scheme 17 Nucleophilic reactivity of 7 with acyl, aroyl, alkyl, benzyl,and phenyl halides.

MP

P

P

P

N

Cl

NER3

M

N

N

PP

PPN2

R = alkylE = Si, Ge

R3EClNaI

Scheme 18 NdE bond formation by nucleophilic reactivity of 14 or 15with germanyl or silyl halides.

Fe

N

N

P

P

-

P

B

P

PP

B

Me3SiCl

=

PiPr2 PiPr2

PiPr2B

Fe

N

N

P

PP

B

SiMe3

21


respectively.133,134 This enhanced reactivity was assumed to be

caused by the greater p-interactions between Mo and N2 ac-

commodated by the tetradentate–thioether ligand.

Scheme 19 NdSi bond formation via nucleophilic reactivity of aniron–N2 complex with silyl halides.

8.14.5.1.2 Ndheteroatom bond formation8.14.5.1.2.1 NdSi, NdGe bond formation

Silyl halide electrophiles, SiClR3 (R¼Me, Et, OMe, Ph), react

with complexes 14 and 15 to form silyldiazeneido complexes

[MX(N]NdSiR3)(P)4] (Scheme 18). Although Me3SiCl does

not react directly with either 14 or 15, the addition of NaI salt

allows reactivity. Analogous treatment of 14 with a mixture of

R3GeCl and excess NaI gives the germylated complexes trans-

[WI(N]NdGeR3)(P)4]. This system can also be used to pro-

duce NE3-containing species catalytically (E¼GeR3, SiR3;

R¼alkyl). Thus, when a variety of chlorosilanes or Me3GeCl

were reacted with 1 equiv. of Na under dinitrogen in the

presence of a sub-stoichiometric amount of 15a, NE3 products

were catalytically produced.135

The reactivity of end-on N2 complexes of iron has also been

observed by Peters’ group. Thus, the anionic Fe–N2 complex

[(TPB)Fe(N2)][Na] (21) (TPB¼ tris[2-(diisopropylphosphino)

phenyl]borane) reacts with silicon electrophiles to yield the silyl-

diazenido complex [(TPB)Fe(N]NdSiMe3)] (Scheme 13). The

flexibility and nonclassical nature of the Fe!B bond was pro-

posed to play a key role in the reactivity of 21 (Scheme 19).136


8.14.5.1.2.2 NdB bond formation

Hadai and coworkers reacted 19bwith primary alkylboranes to

form the boryldiazenido complex trans-[W(NCS)(N]

NdBHR)(dppe)2] (BH2R¼1,1,2-trimethylborylpropane)

(Scheme 20). With a secondary alkyl borane (dicyclohexylbor-

ane), the trans-[W(NCS)(N]NdBCy2)(dppe)2] complex was

formed.137 Reactions with BH3 resulted in complex mixtures,

but tertiary boranes did not react. When 19b was reacted with

9-borabicyclo[3.3.1]nonane trifluoromethanesulfonate (9-

BBN OTf), an unusual linear diazene was formed [W(OTf)

(N]Nd(9-BBN))(dppe)2]. The WdN]NdB bonds are ef-

fectively linear (determined crystallographically), owing either

to the electronic influence of the OTf- on the metal’s interac-

tions with the diazenido ligand or to the steric influence of the

bicyclononane and the dppe phenyl groups.

P PN

NR

+H

8.14.5.1.3 Alkylation of the coordinating nitrogenNucleophiles can react with a coordinated N2 ligand

(Scheme 21), although it is much less common than the reaction

of the distal nitrogen with electrophiles. The resulting (reduc-

tion) product is typically trapped by methyl cations. Note that

the catalytic synthesis of azomethane (MedN]NdMe) from

N2 is possible in principle because the electrons used for the

reduction are supplied by the nucleophile, not the metal.138,139

Aryllithium reagents in the presence of Ti can react with N2, and

it is assumed to activate N2 through a similar mechanism,

though the details of the reaction are still not clear.140,141

W

N

N-

PPPP

NCS

H2B

W

N

N

PPPP

NCS

BH

BW N N

P

P

P

P

TfO

TfOB

Cy2BHW

N

N

PPPP

NCS

BCy2

Δ

19b

Scheme 20 Reactivity of 19b with primary boranes, secondaryboranes, and 9-BBN OTf.

MnOC

OCN

N

Li MeMn

OCOC

N N-

Me Li+

MnOC

OCN N

Me

CH3+

Me

Scheme 21 The electrophilic nature of the coordinated nitrogen in[CpMn(CO)2(N2)] allows for nucleophilic attack by a strong nucleophileand a novel route to azomethane.

8.14.5.1.4 Reactions of diazenido complexesMo andW organodiazenido complexes ([MX(N]NdR)(P)4])

are still nucleophilic enough at the distal nitrogen to react

further with electrophiles or protons to form organohydrazido

complexes (Scheme 22). This reactivity was first observed by

Chatt simply by adding 2 equiv. of electrophile per M–N2.130

The diazenido complexes can either be protonated (leading to

primary amine products upon NdN bond cleavage) or alky-

lated a second time (leading to an asymmetric secondary

amine upon NdN bond cleavage).

Diazenido complexes of the type trans-[MX(N]NdH)

(dppe)2] can generally be protonated further, producing a

versatile hydrazido complex trans-[MX(NdNH2)(P)4][X]. The

kinetics and additional details of such protonations have been

reviewed thoroughly elsewhere.128 These hydrazido complexes

provide a wealth of organic reactivity and are discussed inmore

detail in Section 8.14.5.2.

8.14.5.1.5 Hydrazido complexes from dinitrogencomplexesComplexes 14a and 14b can undergo reactions resulting di-

rectly in hydrazido complexes (Scheme 23). For example,

this reactivity can be achieved with a,o-dibromoalkanes,

MPP

PP

N

Br

N

MPP

PP

N

Br

N

(CH2)n+

Br–(CH2)n–Br

RR1

Br BrR1

R+

hv

MPP

PP

N

N2

N

M = Mo, Wn = 2–5

R,R1,R2= alkyl

= dppePP

R2–Li

MPP

PP

N

Br

N R1

RR2

No reactionH–X

Scheme 23 Direct formation of a metal hydrazido complex from themetal–N2 complex by a double halide displacement.

M

M = Mo, WR1, R = alkylX = halideP = tertiary phosphane

PP

PP

N

X

NR

R1–X

H–X MP PX

MPP

PP

N

X

NR

+R1

Scheme 22 Protonation or alkylation of a metal diazenido complex tothe corresponding hydrazido complex.


Br(CH2)nBr. Photochemical reaction of dibromoalkanes (with

two to five carbons) with 1 results in ring formation at the

distal nitrogen from the successive reactions with the a then

o-bromide.142 For longer dibromoalkanes (6–12 carbons)

two types of products can be isolated: dimers ([MBr

(dppe)2(N]Nd(CH2)ndN]N)(dppe)2BrM]) or diazenido

complexes of the type [MBr(dppe)2(N]Nd(CH2)ndBr)].143

Reactionof14bwith geminal-dibromidesproduces thenovel

hydrazone complexes trans-[WBr(NdN]CRR1)(dppe)2][Br].

These complexes do not react with protons, but the hydrazone

carbon is readily attacked by nucleophiles, such as LiR2

(R2¼alkyl), to yield diazenido complexes of the type trans-

[WBr(N]NdCRR1R2)(dppe)2].144

8.14.5.1.6 Si functionalizationAlkanes with terminal halosilanes (XdSiR2(CH2)nSiR2dX)

react with 15b to form a doubly silated hydrazido ring analo-

gous with a,o-dibromoalkanes (Scheme 24).145,146 This reac-

tivity was also observed by Peters with complex 21.136

M

N

N

(CH2)n

XSiR2–(CH2)n–R2SiXM

N

NSiR2R2Si

M = 15b ; X = Br, Cl ; n = 2,3M = 21 ; X = Cl ; n = 2

Scheme 24 Direct formation of a metal hydrazido complex with NdSibonds from the metal–N2 complex by a double silyl halide displacement.

MN

N 2HX

M

NN

H+

H RR

O

H+

HO

HO

NNM

O

MeO

M

O

MeO

MeO

NNM

O

NNM

O Isomerization

NNM

O

O

Cl

Cl

OO

Ph

C

Ph

O

MNN

HO

Ph

Ph

MN

N

P PP P

N2

M = Mo, WP = tertiary phosphane

Scheme 25 Reactivity of metal hydrazido complexes featuring versatile con

8.14.5.2 NdC Bond Formation from Hydrazido Complexes

8.14.5.2.1 Hydrazido condensationsAs mentioned earlier, hydrazido complexes of the type [MX

(NdNH2)(P)4][X] (M¼Mo, W) have a rich reactivity that can

be made use of for organic synthesis. In particular, work by

Hidai and coworkers showed off the versatility of these reac-

tions. The descriptions below primarily focus on the scope of

such reactions, as the fine points have been reviewed in detail

elsewhere.60,146–148

The distal nitrogen of the hydrazido complex is very nucle-

ophilic, allowing significant reactivity (Scheme 25). When

trans-[WF(NdNH2)(dppe)2][BF4] is reacted with succinyl

chloride, it produces a diacylhydrazido five-member ring.149

A reaction also occurred with phenylisocyanate; however, poor

characterization left the products in question.

Similar nucleophilic behavior was exhibited by the distal

hydrazido nitrogen when cis,mer-[MCl2(NdNH2)(PMe2Ph)3]

(M¼Mo, W) was reacted with diphenylketene. In this reaction,

the site of attack was the electron-deficient carbon and the

singly-acylated complexes cis,mer-[MCl2(NdNHdC(O)

CHPh2)(PMe2Ph)3] were formed.150

More interesting than the nucleophilic character of the

hydrazido complexes is their ability to perform amine-like

condensation reactions with ketones and aldehydes. This al-

lows for many diverse reactions to install nitrogen. For exam-

ple, the formation of a hydrazone complex ([MX(NdN]

CRR0)(P)4][X] (M¼Mo, W)) from a hydrazido complex and

a ketone is a simpler route than the analogous reaction with

the dinitrogen complex and a geminal-dihalide under irradia-

tion. The hydrazido complexes [MF(NdNH2)(dppe)2][BF4]

M

N

N

R

R

+

OOMe

N

+

NM

O+

N

+

N

ElectrophileNNM

E

+

densation reactions.


(M¼Mo, W) readily condense with a variety of aldehydes and

ketones to produce a new series of hydrazido or hydrazone-

type complexes [MF(NdN]CRR0)(dppe)2][BF4] in good

yields.151 Much like the analogous amine condensation, this

reactivity is remarkably accelerated in the presence of trace

amounts of acid, suggesting a similar mechanism to acid-

catalyzed amine condensations.152 The same reactivity is also

seen with [WX(NdNH2)(PMe2Ph)4][X] and (X¼Br)

[MoX2(NdNH2)(PMe2Ph)3] (X¼Cl, Br).153,154

Using this reactivity, the installment of nitrogen into many

ring systems is possible, as well as the formation of ring

systems utilizing the hydrazido-complex condensation reactiv-

ity. Some of the highlighted reactions include formation of

lactam precursors and oxonium–oxygen displacement to

form pyridine precursors. In this category, one of the most

intensely studied reaction is the synthesis of pyrrole from 2,5-

dimethoxytetrahydrofuran and [MX(NdNH2)(P)4][X], fol-

lowed by electrophilic functionalization of the ring.155 These

condensations have proven versatile and only suffer from the

need to use stoichiometric amounts of metal complex.

Another use for the metal hydrazido complexes is aryl

functionalization. The [WX(NdNH2)(dppe)2][BF4] complex

can react with a fluoroarene, providing a rare aryl functionali-

zation at the distal nitrogen (Scheme 26). This complex is the

only nonbimetallic complex that can achieve this reactivity

other than complex 20. Both the hydrazido complex and the

aryl ring (1-fluoro-2,4-dinitrobenzene) must be electronically

activated in order for this to occur. The result is a organodia-

zenidoaryl complex, for which the mechanism of formation is

still unknown.156

W

N

N2L

PP

PP

N

NR R1

PH H

P = PMe2Ph

NC bondformation

hv

2 H2

R R1

8.14.5.2.2 Hydrazido to aminoThus far, numerous methods have been discussed to functio-

nalize the distal nitrogen in a M–N2 complex up to the point

of a hydrazido complex. In order to cleave the final NdN bond

in a hydrazido complex ([MX(NdNRR1)(P)4][X]) a strong

reducing agent must be used. LiAlH4 (LAH) is sufficient, but

must be used in excess. The products are ammonia (formed by

reduction of the coordinating nitrogen), a secondary amine

(formed by reduction of the distal nitrogen), trace amounts

of hydrazine derivatives, and the tetrahydride metal complex

[M(H)4(P)4]. If a proton source is used instead of a hydride

source, the hydrazido complex releases hydrazines, hydra-

zones, or azines depending on the functionality of the hydra-

zido complex.153

K2CO3 aq.

F

O2N NO2

WPP

PP

N

X

N

WPP

PP

N

X

NNO2

NO2

H+

H

X = F, Br, CF3COOPP

= dppe

Scheme 26 Arylation of a W hydrazido complex using anelectron-deficient arylfluoride as the electrophile.

The metal tetrahydride complex can be isolated, and upon

exposure to light will reductively eliminate the hydrides as H2,

formally reducing the metal center and restoring its ability to

coordinate dinitrogen. Such a cycle has been successfully ac-

complished with pyrrole synthesis using [WX(N2)(PMe2Ph)4]

(X¼Cl, Br, I) (Scheme 27).157

Though these systems show an incredible diversity of ways

to form NdC bonds derived from N2, they rely on strong

reducing agents and stoichiometric amounts of metal. In addi-

tion, they release an equivalent of ammonia per equivalent of

amino product.

8.14.5.3 Direct Cleavage of the N^N Bond: NitrideComplexes

Several complexes of Mo with a low coordination number and

bulky ligands are able to directly form a nitride complex from

dinitrogen, fully reducing each nitrogen using three electrons

from themetal center (e.g., Scheme 1).30 This direct cleavage of

the dinitrogen triple bond provides a reactive nitride complex

that can undergo organic reactivity. Several reactions of the

[MoN(N0)3] complex (N0]N(tBu)(3,5-dimethylbenzene))

(22) have been developed that utilize the Mo^N moiety for

NdC bond formation. Henderickx and coworkers developed a

method to make trifluoroacetamide from 22 (Scheme 28).158

The reaction proceeds at room temperature over several mi-

nutes producing near quantitative trifluoroacetamide. Unfor-

tunately, it requires stoichiometric amounts of 22 and the N0

ligand is consumed in the reaction, producing a nonrecover-

able Mo(VI) species.

Another transformation that has been developed is the

synthesis of nitriles from 22 by Cummins (Scheme 29).159

Unlike the dinitrogen complexes 14a and 15a or their corre-

sponding diazenido or hydrazido complexes discussed earlier,

22 is a reluctant nucleophile and will not react directly with an

acyl chloride. This is partly attributed to the sterically hindered

nitrido nitrogen, caused by the three tert-butyl groups on the N0

W

N

L

PP

PP

WP

PP

H H

LiAlH4

N

NR R1

H H

NR R1

NH HH

H+ +

MinorMajor

H X

N

N

H H

Scheme 27 Cleavage of the MdN bond in metal hydrazido complexesto produce either hydrazine derivatives by protonolysis or ammoniaand amines with a hydride source.


ligands. The Me3SiOTf acts as a strong electrophile to produce

the silylimido species [Mo(NdSiMe3)(N0)3][OTf] that will

react with acyl chlorides (ClC(O)R, R¼Me, tBu, Ph) in the

presence of catalytic pyridine. Reducing the acylated complex

in the presence of Me3SiOTf affords a ketimide complex [Mo

(N]C)(OSiMe3)(R)(N0)3]. Treatment with ZnCl2 or SnCl2

yielded the nitrile and [MoCl(N0)3]. The molybdenum com-

plex can be reduced under N2 to regenerate 22. All of these

reactions proceeded with good yields and the starting complex

could be recovered and regenerated for multiple uses. Al-

though these syntheses required workup between each step,

stoichiometric metal complex (which can be recycled), and a

strong reducing agent (NaH), it does provide a novel route to

alkyl and aryl nitriles derived from N2. In similar work,

Mo

N

N¢N¢N¢

3+

N¢ =N

CD3

CD3

Me

F3C O

O

CF3

O

DMF F3C

O

NH2

22

Scheme 28 Novel synthesis of a trifluoroacetamide utilizing N2 as thenitrogen source.

Mo

N

N¢N¢N¢

N' =N

Mo

N

N¢N¢N¢

RO

Mo

N

N¢N¢N¢

OSiMe3

RMo N¢N¢N¢

Cl

MoN¢N¢N¢

N C R

SiOTf1)

2) R Cl

O

1) Mg0

2) SiOTf

1/2 SnCl2 or1 ZnCl2

Mg0

N2

NaHcat.

pyridine

R = Me, tBu, Ph

Scheme 29 Novel synthesis of nitriles using N2 as the nitrogen sourceand featuring a recyclable metal reactant.

Ph

O

ClN2 + TiCl4 + xs M¢

M' = Li0 or Mg0

1)

2) H2O H2

Scheme 30 Titanium-catalyzed N2 cleavage, NdC bond formation, and hyd

Cummins also showed that 22 reacted with methanol to

form cyanide compounds.160

Another system that can cleave N2 directly was developed

by Mori and coworkers using a Ti catalyst in the presence of

N2 and a reducing agent based on work done by Vol’pin

(Scheme 30).141 This catalyst performs a myriad of reactions

that have been thoroughly covered elsewhere.161 The details of

the reaction are that N2 is reduced using an alkali or alkali-

earth metal, producing a high-energy nitride complex. Argu-

ably, these conditions are not mild, but the applications are

among the most diverse and applicable of any system dis-

cussed for incorporation of N2 into organic molecules. Nota-

bly, this method has been used in several total syntheses.162

8.14.5.4 NdC Bond Formations Utilizing MultipleMetal Centers

The scope of organic transformations from multiple metal

centers is limited; however, this should be considered a rich

area for study. The discussion below focuses on some interest-

ing reactions that show off the emerging organic reactivity of

these activated N2 complexes.

Fryzuk has developed a bimetallic tantalumcomplex [((NPN)

Ta)2(m-H)2(m-Z2:Z1-N2)] (NPN]PhP(CH2SiMe2NPh)2)) (23)

that can activate dinitrogen as well as react to form NdC

bonds.163 The complex reacts readily with CO2, PhNCO, and

SCO; however, the resulting inseparable mixtures have yet to be

identified. Sulfur analogs of these reactants provide clean reactions

that are readily characterized. The reactions with carbon disulfide

and isothiocyanate result in the loss ofH2 anddisplacement of the

sulfur by a nitrogen, forming a m2-sulfido moiety and forming a

new N]C bond (Scheme 31). Addition of N,N0-diphenyl carbo-diimide to 23 results in a cycloaddition across the NdTa bond of

23 and the N]C bond of the carbodiimide. The resulting com-

plex has functionalized the Z1-nitrogen with a new NdC bond.

Chirik has developed bimetallic Hf and Zn systems that not

only activate N2 but that readily produce NdC bonds

(Scheme 32). With the [(Z5-C5Me4H)2Hf2]-(m2,Z2,Z2-N2)

complex (24-N2), up to 3 equiv. of isocyanate (PhNCO) can

be added, forming a hydrazido complex of dinitrogen.164 The

C]N bond in the isocyanate is believed to undergo cycload-

dition with the HfdN bond of 24-N2. This can occur multiple

times, resulting in a stable hydrazido complex.

Chirik showed that an analogous complex, [(Z5-C5H2-

1,2,4-Me)2Hf2](m2,Z2,Z2-N2) (25-N2) can be silated using

CySiH3, presumably by the cycloaddition of SidH to a HfdN

bond in 25-N2 (Scheme 33).165 After silation of the N2, the

complex can undergo several NdC bond-forming reactions.

The silated 25-N2 complex can isomerize via another cycload-

dition of Si–H across the N^Hf bond, coordinating both ni-

trogen atoms directly to the silicon. Upon addition of H2,

Ph

O

N Ph

O

NH

O

Ph+

Major Minor

+ TiCl4

rolysis to produce primary and secondary amines.

TaN

N

PTa

NN

PHH

HH

N2Ta

NN

PTa

NN

PHH

NN

PhN C NPh

X C S

Ta

NN

PTa

NN

PHH

NN

Ta

NN

P

N

C

Ph

NPh

TaN

N

P

N

SNC

X

NN

P=

PMe2Si

Me2Si

NN

PhPh

Ph

X = N–Ph, S

23

Scheme 31 NdC bond formation utilizing carbodiimide, isocyanate, or carbon disulfide.

NO

Ph

O

Hf N

Hf HfCp¢

Cp¢

N

N

HfN

N

Ph

OPhNCO

Hf N

NPhNCO

PhNCO

N Ph

Hf

Hf NN

NO

Ph

O

N Ph

Hf3PhNCO

O N

Ph

Cp¢

Cp¢Cp¢

Cp¢

Cp¢Cp¢

Cp¢

Cp¢ Cp¢

Cp¢

Cp¢

Cp¢ Cp¢

Cp¢

Cp¢ =

24-N2

Scheme 32 NdC bond formation utilizing an isocyanate and a bimetallic Hf–N2 complex.

Hf HfCp''

Cp''

N

N

Cp''

Cp''

CySiH3

Hf Hf

Cp''

Cp''

NN Cp''

Cp''H

SiH2

Cy

Cp'' =

Hf

HfCp''

Cp''

Cp''

Cp''

N

NH

Cp''

Cp''SiH

Cy

Cy

HH2

HCl

ΔHf Hf

Cp''

Cp''

Cp''

Cp''

H

NH

H

NH

Si

CyH

CO

Hf Hf

N Cp''

Cp''

SiH2

N

C OH

HfCp''

Cp'' Cl

ClNH4Cl

C

O

NH2H

25-N2

Scheme 33 NdC and NdSi bond formation utilizing the silane-activated nitrogen of a bimetallic Hf–N2 complex.


which adds across the NdHf bond, a novel complex is isolated

exhibiting a HfdNHdSiCyHdNHdHf unit. Alternatively,

the silated 25-N2 will form an imine in the presence of CO.

Protonolysis of the complex yields formamide and ammonia.

Chirik also showed that the Hf complex [Me2Si(Z5-C5Me4)

(Z5-C5H3-3-tBu)Hf]2(m

2,Z2,Z2-N2) (26-N2) will incorporate N2

into CO directly (Scheme 34).166 In the presence of CO, 26-N2

will form a bridged N2C2O2 moiety in which a CdC bond has

formed and each carbon is double bonded to a nitrogen. Proto-

nolysis of this complex yields oxime. If a mixture of gas (1:3 H2 to

CO) with 26-N2 is allowed to react, a Hf–isocyanate moiety

derived from CO and N2 can be isolated.

In summary, the activation of dinitrogen for incorporation

into organic molecules has seen much development since the

early work of Chatt and Vol’pin.86,141 The prospect of utilizing

N2 directly for organic transformations would bypass the need

for ammonia in organic syntheses, and introduce new strategies

for nitrogen incorporation. The viability of such a process re-

quires reactivity under mild conditions, a recyclable (or even

catalytic) metal, the use of a mild reductant (such as H2), and

reasonable selectivity and yields. Although some of these require-

ments have been met in the aforementioned systems, recyclable

metals (or catalytic metals) and the use of mild reductants stand

to see the most improvement. Promising systems that are

Hf Hf*Cp

Cp

N

N

CpCp*

Cp*

Cp= Si

tBu

MeMe

xs COHf Hf

Cp*

Cp

O

N O

N xs HCl

HfCp

C

C

O

H2N NH2

O

2

3 CO1 H2

Hf*Cp

*Cp

*Cp

Cp

Cp

N

NH

HfCp*

Cp

HC

O

26-N2

Scheme 34 NdC bond formation using CO and a bimetallic Hf–N2 complex to form either an isocyanate complex or an oxime.

S FeS

Fe S OC

-OO O-

O


emerging include the recent development of iron complexes and

exploitation of iron nitrides,167 the use of metathesis to react

metal nitrides derived from N2,168 and the continuing explora-

tion of polymetallic metal complexes, which show great promise

in both the activation and utilization of N2.

FeS S

S FeS

Fe S

Fe MoSFe OCysα275

N

S

CO

Hisα442

Figure 33 Structure of the FeMo cofactor in nitrogenase.

8.14.6 Biological Activation of N2: N2 Reduction andNitrogenase Models and Mimics

8.14.6.1 Nitrogenase

Nature uses nitrogenase enzymes to convert atmospheric N2 to

NH3.169–172 These enzymes, which are limited to certain bac-

terial species (100–200 species), are responsible for all biolog-

ically fixed nitrogen.173 Nitrogenase enzymes catalyze the

reduction and protonation of dinitrogen, and although break-

ing the dinitrogen bond is extremely energy intensive, these

enzymes are able to perform the conversion at biological tem-

peratures and atmospheric pressure (eqn [6]).

N2 + 8H+ + 8e- + 16MgATP 2NH3 + H2 + 16MgADP + 16Pi

½6�There are three types of nitrogenase enzymes, which vary

by the metal composition in the active site: iron and molyb-

denum, iron and vanadium, and iron only.101 The three

nitrogenases all have similar structures and reactivity, with

the latter two types typically produced only under

molybdenum-deficient conditions.174 The structure of the

iron–molybdenum nitrogenase enzyme, which is the most

efficient and most commonly studied type, consists of two

separate protein clusters: dinitrogenase reductase (an iron

containing dimer) that supplies electrons for the reduction

and dinitrogenase (an iron and molybdenum containing

tetramer) where dinitrogen binding and reduction occur.174

The active site of nitrogenase is the FeMo cofactor, which is

located within the dinitrogenase protein cluster. The FeMo

cofactor consists of seven iron atoms and one molybdenum

atom bridged by nine sulfur atoms (Figure 33). The FeMo

cofactor is ligated to the protein structure through a cysteine

residue (iron bound) and a histidine residue (molybdenum

bound). A homocitrate ligand completes the coordination

sphere of the molybdenum atom. Refined crystal structure

data of the FeMo cofactor revealed the existence of a central

carbon atom within the iron–sulfur cluster (Figure 33).175,176

The FeMo cofactor is accepted to be the site of N2 binding

and reduction based on a wide variety of evidence.169 In fact,

recent site-directed mutagenesis studies have suggested that two

of the ‘belt’ iron atoms of the FeMo cofactor are the site of N2

binding and reduction.177–180 The exact mechanism of N2 re-

duction mediated by nitrogenase remains unknown; however,

growing biochemical evidence supports a mechanism that pro-

ceeds through diazene and hydrazine intermediates en route to

ammonia formation.54 As discussed below, this proposedmech-

anism is in contrast to the mechanisms detailed by Chatt181 and

Schrock182 for synthetic Mo and W systems.

Because of the growing biochemical evidence suggesting

that iron is responsible for the reduction of N2 to ammonia

in nitrogenase, understanding the coordination chemistry of

iron with dinitrogen and reduced dinitrogen species, such as

diazene and hydrazine, is becoming increasingly important,

which is why there is a considerable growing literature dealing

with the Fe chemistry of N2 and its reduced forms such as N2H2

and N2H4.39,38

8.14.6.2 Cubane Cluster Nitrogenase Models

A number of research groups have explored the chemistry of

cubane clusters that are chemically and structurally related to

the nitrogenase cofactor, FeMoco, and a huge literature reports

VP

P P

P

N

N

N

RR

RR

RR R

RN

28

1-

+ 4H+NH4

+ + 1.5N2 + "V(II)"

Figure 34 A demonstration that vanadium–N2 complexes can react to form ammonia.


on the synthesis and reaction chemistry of these mimics (e.g.,

27).183,184 Unfortunately, these molecules exhibit only a lim-

ited reactivity with respect to N2 activation and fixation. Exten-

sive calculations (typically density functional theory

calculations) have been done on these model complexes.

Mo

Fe

S Fe

S

S

S

Fe

Cl

Cl

Cl

N

N

N

N

NN

BH

1-

27

8.14.6.3 Vanadium-Nitrogenase Models

Because one type of nitrogenase contains vanadium, several

studies have examined the ability of vanadium complexes to

activate and fix nitrogen. For example, complex 28 reacted with

acid to give NH4þ and a small amount of N2H5

þ (Figure 34).185

This reaction was presented as proof of concept that N2 coordi-

nated to a V metal center could be activated and reduced.

A number of other model complexes in which V is bonded to

N2-reduction intermediates (or, more correctly, derivatives of

N2-reduction intermediates) have been synthesized. One exam-

ple is molecule 29. This molecule is a catalyst for the conversion

of hydrazine to ammonia.186 The reactivity of this and related

complexes suggests that hydrazine, hydrazide, diazenide, and

ammonia can be bonded to and activated by vanadium centers.

S

V

Fe

Fe

SS

S

Fe

Cl

Cl

Cl

N N

1-

NH2 NHPh

29

8.14.7 Conclusion

In summary of this section, it is concluded that there is still a wide

gap between the structural and functional models of nitrogenase.

Beautiful structuralmodels exist for nitrogenase but, as of yet, the

N2-reduction chemistry of thesemolecules is limited. In contrast,

numerous complexes will activate N2 and reduce it. Yet, other

than containing Fe, Mo, or V, these complexes bear little resem-

blance to nitrogenase. As a result, the chemistry of nitrogenase

will likely remain a lively and active area of research. For a related

chapter in this Comprehensive, we refer to Chapter 9.23.

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