organometallic chemistry

Organometallic Chemistry Organometallic Chemistry

Kenneth D. KarlinDepartment of Chemistry, Johns Hopkins University

[email protected] http://www.jhu.edu/~chem/karlin/

JHU Course 030.442Prof. Kenneth D. Karlin

Spring, 2009

JHU Course 030.442Prof. Kenneth D. Karlin

Spring, 2009

Organometallic Chemistry030.442 Prof. Kenneth D. Karlin Spring, 2009

Class Meetings: TTh, 12:00 – 1:15 pm

Textbook – The Organometallic Chemistry of the Transition Metals”4th Ed., R. H. Crabtree

Course Construction: Homeworks, Midterm Exams (1 or 2), Oral Presentations

Rough Syllabus Most or all of these topics• Introduction, History of the field

• Transition Metals, d-electrons

• Bonding, 18 e– Rule (EAN Rule)

• Ligand Types / Complexes

• Types of Compounds

M-carbonyls, M-alkyls/hydrides

M-olefins/arenes

M-carbenes (alkylidenes alkylidynes)

Other

• Reaction Types Oxidative Addition

Reductive elimination Insertion – Elimination Nucleophilic/electrophilic Rxs.

• Catalysis – Processes Wacker oxidation Monsanto acetic acid synthesis

Hydroformylation Polymerization- Olefin metathesis

Water gas-shift reaction Fischer-Tropsch reaction

p. 1

Reaction Examplesp. 3

• Oxidative Addition

Reductive Elimination

• Carbonyl Migratory Insertion

• Reaction of Coordinated Ligands

CH3Mn(CO)5 CH3CMn(CO)5

OCO

Vaska’s complex

(Iron pentacarbonyl) (CO)4Fe– + :OH– ––––> (CO)4Fe

––––––> (CO)4Fe–H + CO2

O

O

HC O

Reaction Examples - continuedp. 4

• Wacker Oxidation C2H4 (ethylene) + ½ O2 –––> CH3CH(O) (acetaldehyde)

Pd catalyst, Cu (co-catalyst)

• Monsanto Acetic Acid Synthesis CH3OH (methanol) + CO –––> CH3C(O)OH (acetic acid) (Rh catalyst)

• Ziegler-Natta catalysts – Stereoregular polymerization of 1-alkenes (-olefins)

1963 Nobel Prize

Catalyst: Ti compounds and organometalllic Al compound (e.g., (C2H5)3Al )

• Olefin metathesis – variety of metal complexes

2005 Nobel Prize – Yves Chauvin, Robert H. Grubbs, Richard R. Schrock

n CH2=CHR –––> –[CH2-CHR]n–

Organo-transition Metal Chemistry History-Timelinep. 5

• Main-group Organometallics

1760 - Cacodyl – tetramethyldiarsine, from Co-mineral with arsenic

1899 –> 1912 Nobel Prize: Grignard reagents (RMgX)

• 1827 – “Zeise’s salt” - K+ [(C2H4)PtCl3]–

n-Butyl-lithium

Synthesis: PtCl4 + PtCl2 in EtOH, reflux, add KCl Bonding- Dewar-Chatt-Duncanson model

Organo-transition Metal Chemistry History-Timeline (cont.)p. 6

1863 - 1st metal-carbonyl, [PtCl2(CO)2]

1890 – L. Mond, (impure) Ni + xs CO –––> Ni(CO)4 (highly toxic)

1900 – M catalysts; organic hydrogenation (---> food industry, margerine)

1930 – Lithium cuprates, Gilman regent, formally R2Cu–Li+

1951 – Ferrocene discovered. 1952 -- Sandwich structure proposed

Ferrocene was first prepared unintentionally. Pauson and Kealy, cyclopentadieny-MgBr and FeCl3 (goal was to prepare fulvalene) But, they obtained a light orange powder of "remarkable stability.”, later accorded to the aromatic character of Cp– groups. The sandwich compound structure was described later; this led to new metallocenes chemistry (1973 Nobel prize, Wilkinson & Fischer). The Fe atom is assigned to the +2 oxidation state (Mössbauer spectroscopy).

The bonding nature in (Cp)2Fe allows the Cp rings to freely rotate, as observed by NMR spectroscopy and Scanning Tunneling Microscopy. ----> Fluxional behavior. (Note: Fe-C bond distances are 2.04 Å).

(Cp)2Fe

Cp = cyclopentadienyl anion)

(h5-C5H5)2Fe

(pentahapto) Solid-statestructure

1955 - Cotton and Wilkinson (of the Text) discover organometallic-complex fluxional behavior (stereochemical non-rigidity)

The capability of a molecule to undergo fast and reversible intramolecular isomerization, the energy barrier to which is lower than that allowing for the preparative isolation of the individual isomers at room temperature. It is conventional to assign to the stereochemically non-rigid systems those compounds whose molecules rearrange rapidly enough to influence NMR line shapes at temperatures within the practical range (from –100 °C to +200 °C ) of experimentation. The energy barriers to thus defined rearrangements fall into the range of 5-20 kcal/mol (21-85 kJ/mol).

Organo-transition Metal Chemistry History-Timeline (cont.)

Aside:Oxidation State18-electron Rule

p. 7

Fluxional behavior; stereochemical non-rigidity (cont.)

Butadiene iron-tricarbonyl

Xray- 2 CO’s equiv, one diff., If retained in solution, expect, 2:1 for 13-C NMR. But, see only 1 peak at RT. Cooling causes a change to the 2:1 ratio expected. Two possible explanations: (1)Dissociation and re-association or (2) rotation of the Fe(CO)3 moiety so that CO’s become equiv.

Former seems not right, because for example addition of PPh3 does NOT result in substitution to give (diene)M(CO)2PPh3.

Note: You can substitute PPh3 for CO, but that requireseither high T or hv. So, the equivalency of the CO groups is due to rotation without bond rupture, pseudorotation.

13C-NMR spectraCO region, only

p. 8

Pseudorotation: Ligands 2 and 3 move from axial to equatorial positions in the trigonal bipyramid whilst ligands 4 and 5 move from equatorial to axial positions. Ligand 1 does not move and acts as a pivot. At the midway point (transition state) ligands 2,3,4,5 are equivalent, forming the base of a square pyramid. The motion is equivalent to a 90° rotation about the M-L1 axis. Molecular examples could be PF5 or Fe(CO)5.

Berry Pseudorotationp. 9

The Berry mechanism, or Berry pseudorotation mechanism, is a type of vibration causing molecules of certain geometries to isomerize by exchanging the two axial ligands for two of the equatorial ones. It is the most widely accepted mechanism for pseudorotation. It most commonly occurs in trigonal bipyramidal molecules, such as PF5, though it can also occur in molecules with a square pyramidal geometry. The process of pseudorotation occurs when the two axial ligands close like a pair of scissors pushing their way in between two of the equatorial groups which scissor out to accommodate them. This forms a square based pyramid where the base is the four interchanging ligands and the tip is the pivot ligand, which has not moved. The two originally equatorial ligands then open out until they are 180 degrees apart, becoming axial groups perpendicular to where the axial groups were before the pseudorotation.

p. 10


1961 – D. Hodgkin, X-ray structure – Coenzyme Vitamin B12 (see other page) Oldest organometallic complex (because biological) (see other page)

1963 - Ziegler/Natta Nobel Prize, polymerization catalysts

1964 - Fischer, 1st Metal-carbene complex

1965 – Cyclobutadieneiron tricarbonyl, (C4H4)Fe(CO)3– theory before experiment

(C4H4) is anti-aromatic (4 -electrons)With -Fe(CO)3, C4H4 behaves as aromatic

1965 – Wilkinson hydrogenation catalyst, Rh(PPh3)3Cl

1971 – Monsanto Co. – Rh catalyzed acetic acid synthesis

C CHH

R HC C

HH

H RCatalysisof 1,2-shifts(mutases)

orHomocysteine

methylation

Methylmalonyl-CoA––> Succinyl-CoA(CoA = coenzyme A)

(homocysteine) RSH RSCH3 (methionine)

[B12CoIII-CH3]+ [B12CoI]–

Vitamin B-12 is a water soluble vitamin, one of the eight B vitamins. It is normally involved in the metabolism of every cell of the body, especially affecting DNA synthesis and regulation, but also fatty acid synthesis and energy production. Vitamin B-12 is the name for a class of chemically-related compounds, all of which have vitamin activity. It is structurally the most complicated vitamin. A common synthetic form of the vitamin, cyanocobalamin (R = CN), does not occur in nature, but is used in many pharmaceuticals, supplements and as food additive, due to its stability and lower cost. In the body it is converted to the physiological forms, methylcobalamin (R = CH3) and adenosylcobalamin, leaving behind the cyanide.

Vitamin B-12 Co-enzyme

p.12

5-deoxyadenosyl group


1973 – Commercial synthesis of L-Dopa (Parkinson’s drug)asymmetric catalytic hydrogenation

2001 Nobel Prize – catalytic asymmetric synthesis, W. S. Knowles (Monsanto Co.)

R. Noyori,, (Nagoya, Japan), K. B. Sharpless (Scripps, USA)

1982, 1983 – Saturated hydrocarbon oxidative addition, including methane

1983 – Agostic interactions (structures)

AGOSTIC INTERACTIONS:

Agostic – derived from Greek word for "to hold on to oneself”C-H bond on a ligand that undergoes an interaction with the metal complex resembles the transition state of an oxidative addition or reductive elimination reaction.

Detected by NMR spectroscopy, X-ray diffraction

Compound above: Mo–H = 2.1 angstroms, IR bands were observed at 2704 and 2664 cm–1 and the agostic proton was observed at –3.8 ppm. The two hydrogens on the agostic methylene are rapidly switchingbetween terminal and agostic on the NMR time scale.

p. 14

Organometallic ChemistryDefinition: Definition of an organometallic compound Anything with M–R bond R = C, H (hydride)

Metal (of course) Periodic Table – down & left electropositive element (easily loses electrons)

NOT: • Complex which binds ligands via, N, O, S, other

M-carboxylates, ethylenediamine, water• M–X where complex has organometallic behavior, reactivity patterns

e.g., low-valent

Oxidation StateCharge left on central metal as the ligands are removed in their ‘usual’ closed shell configuration (examples to follow).

d n for compounds of transition elements

N d < (N+1) s or (N+1) p in compounds

e.g., 3 d < 4 s or 4 p

M –NR'

R''

p. 15

d n computation – very important in transition metal chemistry

d n zero oxidation state of M in M-complex has a configuration d n where n is the group #.

Examples: Mo(CO)6 Mo(0) d n = d 6 (CO, neutral)

HCo(CO)4 H is hydride, H–, --> --> Co(I), d n = d 8 Group 5 Group 6 Group 7

V(CO)6– Cr(CO)6 Mn(CO)6

+

V(–1) Cr(0) Mn(+1)d 6 d 6 d 6

Isoelectronic and isostructural compounds (importance of d n)

Effective Atomic # Rule; 18-Electron Rule (Noble gas formalism)# of electrons in next inert gas =

# Metal valence electrons + (sigma) electrons from ligands

Rule: For diamagnetic (spin-paired) mononuclear complexes in organotransition metal compounds, one never exceeds the E.A.N.

p. 16

Cr(CO)6 Cr ---> d6 6 electrons (CO)6 e– - pairs from 6 ligands 12 electrons

––> to [Ar] configuration 18 electrons

(will see more in M.O. diagram)

Consequence of EAN Rule:leads to prediction of maximum in coordination #

Max coordination # = (18 – n) / 2 n is from d n .d n 10 8 6 4 2 0

Max Coord # 4 5 6 7 8 9– Change in 2-electrons results in change of only one in Coord. #– Any Coord. # less than Max # ---> “coordinatively unsaturated”

Fe(CO)42– Fe(CO)5

18 e– 18 e–

Fe(–2) Fe(0)d 10 d 8

4-coord 5-coord both Coord. Saturated

p. 17

–2e– +CO

2e– –CO

[ReH9]2– e.g., as Ba2+ salt

Re(VII), (Mn,Tc, Re triad)

d 0, 9 hydride ligands; CN = 9

Geometry: Face capped trigonal prism

p. 18

A compound not obeying an rules

Fe5(CO)15CIron-carbonyl carbide

Eighteen-Electron Rule - Examples

Co(NH3)63+ Cr(CO)6

Obey 18-electron rule for different reasons

Carbonyl Compounds in Metal-Metal Bonded Complexesless straightforward

Fe2(CO)9 [-Cp)Cr(CO)3]2 Co2(CO)8 (2 isomers)

p. 19

Δo

Δt

eg

t2g

t2

e

M+

M+

M+

M+

Free ionspherical

Free ionspherical

six point chargesspherically distributed

four point chargesspherically distributed

octahedralligand field

tetrahedralligand field

d6 Octahedral maximum of 6 coordinate

p. 20

L

L

L

L

L

LM

s orbital

spherical field of 6 charges

dz2, dx2-y2 (e2g)(destabilized)

dxy, dxz, dyz (t2g)(stabilized)

10Dq or Δo

Oh

lower case letters for orbital

Picture of Octahedral Complex

Various representations

(ignore “s orbital”

p. 21

The five d-orbitals form a set of two bonding molecular orbitals (eg set with the dz2 and the dx2-y2), and a set of three non-bonding orbitals (t2g set with the dxy, dxz, and the dyz orbitals).

L

L L

L

L

L

L

L

L

L

dz2

dx2-y2eg set

bonding

L

L

L

L

dxy, dxz, dyz

All are non-bonding

t2g set eg orbitals point at ligands (antibonding)

appropriate symmetry for -bonds to ligands

-bonds will be six d2sp3 hybridsndz2, ndx2-y2, (n+1)s, (n+1)px,py,pz

t2g orbital set left as non-bonding

p. 22

__ __ __

__

__

__

eg__ __

__ __ __

__ __

__ __ __

eg*

t2g

t1u

t1u*

o

a1g

a1g*

____

________

six ligand orbitals

3d

4s

4p

metal-based orbitals

BondingMO's

Anti-BondingMO's

__________

______

Molecular Orbitals

non-bonding

Standard MO diagram forOctahedral ML6 complexes

with -donor ligands

e.g., [Co(NH3)6]3+ (18 e–)e.g., W(Me)6 (12 e–)

Case IElectron-configuration unrelated to 18–-Rule

1st Row-Complexes with “weak ligands”

o small or relatively small, eg* only weakly antibonding

No restriction on # of d-electrons –– 12 to 22 electrons

p. 24

Case II Compounds which follow rule insofar as theynever exceed the 18-e– rule

• Metal in high oxidation state o is large(r) (for a given ligand)

radius is small –-> ligands approach closely ––> stronger bonding

• 2nd or 3rd Row Metal - 4d, 5d

o is large(er) (for a given ligand); d-orbitals larger, more diffuse.

Complex d n Total e–Complex d n Total e–

ZrF62– 0 12 OsCl62– 4 16

ZrF73– 0 14 W(CN)8

3– 1 17

Zr(C2O4)44– 0 16 W(CN)6

4– 2 18

WCl6 0 12 PtF6 4 16

WCl6– 1 13 PtF6– 5 17

WCl62– 2 14 PtF62– 6 18

TcF62– 3 15 PtCl42– 8 16

Less than 18 e–, but rarely exceed 18 e–

p. 26

Similar Result if ligands are high in Spectrochemical Seriese.g., CN– o is larger

V(CN)63– d2

Cr(CN)63– d3

Mn(CN)63– d4 Less than or equal to 6 d-electrons

Fe(CN)63– d5 eg* not occupied

Fe(CN)63– d6

Co(CN)63– d6

however Co(II) d7 ––> Co(CN)53–

Ni(II) d8 ––> Ni(CN)42– and

Ni(CN)53–

Can have less than maximum # of non-bonding (t2g) electrons, because they

are nonbonding. Addition or removal of e– has little effect on complex stability

p. 27

o can get (or is) very small with -donor ligandsF– example (could be Cl–, H2O, OH–, etc.)

a) Filled p-orbitals are the only orbitals capable of -interactions

• 1 lone pair used in -bonding• Other lone pairs -bond

• The filled p-orbitals are lower in energy than the metal t2g set

• Bonding Interactiona) 3 new bonding MO’s filled by Fluorine electrons

b) 3 new antibonding MO’s form t2g* set contain d-electrons

c) o is decreased (weak field)

i) Ligand to metal (L M) -bonding• Weak field, -donors: F, Cl, H2O• Favors high spin complexes

p. 28

T1g,T2g

T1u,T2u

Eg

T2g

A1g

T1u

A1g

T1u

Egeg ( )σ

eg ( *)σ

t2g ( )π

t2g ( *)π

3d

4s

4p

π-orbitalspx, py

σ-orbitalpz

MetalOrbital

s

MolecularOrbitals

LigandOrbitals

focus on this part only

both sets of d orbitals are driven ↑ in energy due to lower lying ligand orbitalsΔo

p. 29

Δo ΔoΔo

eg (σ*)

eg ( *)σ eg ( *)σ

t2g ( )πt2g (n.b.)

t2g ( *)π

M-L bondingnon-bonding

both are antibonding

π-acceptor

largest separationbetween sets of d-orbitals

σ-donor

intermediate separation

π-donor

smallest separation

antibonding

Have discussed -donor and -donor – now -acceptor

p. 30

T2g

Eg

T1g, T2g

T1u, T2u

A1g

T1u

Eg

eg ( σM-L)

eg (σ* M-L)

t2g ( )π

t2g ( *)π

4d

Δo

π* orbitals on CO(6 x 2 each - orthogonal)

σ orbitals on CO(6 x 1 each)

Mo(CO)6Metal Orbitals(only consider the d orbitals – 4s and 4p orbitals not included

in the analysis)

Molecular Orbitals

Ligand Orbitals

CASE III

L high in spectrochemical series:CO, NO, CN–, PR3, CNR

-acid ligands – -acceptorsCan form strong -bonds

18 e– rule followed rigorously

Orbitals on M used in such -bonding are just those which are non-bonding

Result: Increase in o

Imperative to notHave electrons in eg* orbitals

Want to maximize occupation of t2g

because they are stabilizing

p. 31

Implications of 18e– Rule for Complexes with -accepting ligands

In octahedral geometry almost always have 6 d-electrons 12 electrons from ligands

Other cases: # d-electrons and coordination # complementary

• Coordination # exactly determined by electron-configuration and vice-versa(see previous notes)

BrMn(CO)5 (d ?) I2Fe(CO)4 (d ?) Fe(CO)5 (d ?) Ni(PF3)4 (d ?) All 18-electron

When M has odd electron ––––> metal-metal bond (often bridging CO’s)Mn2(CO)10 Co2(CO)8

Some 17 electron species known: V(CO)6 d 5 Mo(CO)2(diphos)2]+ d 5

See MO diagram: Want to fill stable MO’s’ there is a large gap to LUMO

p. 33

eg

t2g dxz

dyz

dxy

dz2

dx2-y2

Δo

(degenerate )

ML6 ML4

Major Exception: d 8 square-planar complexes

As one goes across periodic table, d and p orbital energyLevel splitting gets larger – hard to use p orbitals for -bonding

Common to have 4-coordinate SP complexes – dsp2 hybridization

Common for:Rh(I), Ir(I)

Pd(II), Pt(II)

Rationalize d-orbital splittingslook at d-orbital pictures/axes

Which d-orbitals?

p. 34

Again, examples of complexes:

dn C.N. Coord. Geom. Example(s)

d10 4 Td Ni(CO)4, Cu(py)41+

d10 3 Trig.planar Pt(PPh3)3

d10 2 Linear (PPh3)AuX, Cu(py)2+

d8 5 TBP Fe(PF3)5

d8 4 (square) planar Rh(PPh3)2(CO)Cl (trans)

d4 7 capped octahedral Mo(CO)5X2

d2 8 sq. antiprism ReH5(PMePh2)3, Mo(CN)84–

d0 9 D3h symmetry [ReH9]2–

tricapped trig. prism

p. 36

LIGANDS in Organometallic Chemistry:

Ligands, charge, coordination # (i.e., denticity)

X SnCl3 H (hydride) CH3 (alkyl, perfluoroalkyl)

Ar RC(O) (acyl) R3E (E = P, As, Sb, N) R2P

CO RNC (isonitrile, isocyanide) R2C (cabenoid, carbene)

R2N N2 C2H4 (olefin, alkene) R2C2 (acetylene)

C4H4 (cyclobutadiene) C3H5– (-allyl) CH=CH-CH2

– (-allyl)

benzene (arenes) -C5H5 (-Cp) -C7H7 (tropylium)

-C3H3 (cyclopropenium, +) O (O-atom; oxide) NO (nitrosyl)

ArN2+ (diazonium)

p. 37

Carbon Monoxide – exceedingly important ligand CO-derivatives known for all transition metals

Structurally interesting, important industrially, catalytic RxsSource of pure metal: Ni (Mond); Fe contaminated with Cu, purify via Fe(CO)5

Fe & Ni only metals that directly react with COSource of oxygen in organics: RC(O)H, RC(O)OH, esters Processes: hydroformylation, MeOH ––> acetic acid

double insertion into olefins, hydroquinone synthesis (acetylene + CO;

Ru catalyst), acrylic acid synthesis (acetylene, CO, Ni catalyst)

Fischer Tropsch Rx: CO + H2 ––> ––> CnH2n+2 + H2O

Most of these involve CO “insertion”

p. 38

Metal-Carbonyl Synthesis:Reduction of available (in our O2-environment) metal salts,

e.g., MX2, M’X3, other (e.g., carbonates)

M-carbonyls generally in low-valent oxidation states ––––> “Reductive Carbonylation”

Reductants: CO itself ( ––> CO2), H2, Na-dithionite

Some Reactions: WMe6 + xs CO –––> W(CO)6 + 3 Me2CO

NiO + H2 (400 °C) + CO ––> Ni(CO)4

Re2O7 + xs CO ––> (OC)5Re–Re(CO)5 + 7 CO2

RhCl3 + CO + pressure + (Cu, Ag, Cd, Zn) –––> Rh4(CO)12 or Rh6(CO)16

Structures Possible: X-ray diffraction, Infrared spectroscopyNi(CO)4 Fe(CO)5 M(CO)6

Td D3h Oh

2058 cm–1 2013, 2034 cm–1 2000 cm–1

Cl– acceptor/reductant

p. 39

13C NMR spectroscopy of M-CO fragments: 180 – 250 ppmUseful to use 13C enriched carbon monoxide

Can be useful to observed “coupling” to other spin active nuclei,e.g., 103Rh or 13P

H3B–CO = 2164 cm–1

no backbonding possible

p. 40

Metal-Carbonyl Structures (cont.): Polynuclear Metal-Carbonyls

p. 41

The backbonding between the metal and the CO ligand, where the metal donates electron density to the CO ligand forms a dynamic synergism between the metal and ligand, which gives unusual stability to these compounds.

C O

Dynamic synergism bonding

Valence Bond formalism: M–C O: M=C=O– +

p. 45

C–O stretching frequencies, (C-O)

Put more electron density on metal– by charge– by ligands which cannot -accept

Remaining CO’s have to take up the charge (e–-density) on the metalSee effects on (C-O).

Ni(CO)4 [Co(CO)4]– Fe(CO)42–

2057 cm–1 1886 cm–1 1786 cm–1

–––––––> –––––––> more –ve charge

Mn(dien)(CO)3+ ~ 2020, 1900 cm–1

Cr(dien)(CO)3 ~1900, 1760 cm–1 (dien not -acceptor)

p. 46

organometallic chemistry

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