Lecture 15a

Metallocenes

Ferrocene I

• Ferrocene • It was discovered by two research groups by serendipity in 1951

• P. Pauson: Fe(III) salts and cyclopentadiene• S. A. Miller: Iron metal and cyclopentadiene at 300 oC

• It is an orange solid• Thermodynamically very stable due to its 18 VE configuration

• Cobaltocene (19 VE) and Nickelocene (20 VE) (and their derivatives) on the other side are very sensitive towards oxidation because they have electrons in anti-bonding orbitals

• Ferrocene can be oxidized electrochemically or by silver nitrate to form the blue ferrocenium ion (FeCp2

+)

Ferrocene II• Pauson proposed a structure containing two cyclopentadiene rings that

are connected to the iron atom via s-bonds

• The diene should undergo Diels-Alder reaction, but ferrocene does not! Instead it undergoes aromatic substitution i.e., Friedel-Crafts acylation

• During the following year, G. Wilkinson (NP 1973) determined that it actually possesses sandwich structure, which was not known at this point• The molecule exhibits D5d-symmetry, but is highly distorted in the solid state

because of the low rotational barrier around the Fe-Cp bond (~4 kJ/mol)

• All carbon atoms have the same distance to the Fe-atom (204 pm)

• The two Cp-rings have a distance of 332 pm (ruthenocene: 368 pm)

Ferrocene III

• In solution, a fast rotation is observed due to the low rotational barrier around the Fe-Cp axis:• One signal is observed in the 1H-NMR spectrum (d=4.15

ppm) • One signal in the 13C-NMR spectrum (d=67.8 ppm)• Compared to benzene the signals in ferrocene are shifted

upfield • This is due to the increased p-electron density (1.2 p-electrons

per carbon atom in ferrocene vs. 1 p-electron per carbon atom in benzene)• The higher electron-density causes an increased shielding of the hydrogen

atoms and carbon atoms in ferrocene• The shielding is larger compared to the free cyclopentadienide ligand

(NaCp: dH=5.60 ppm (THF), dC=103.3 ppm)

Ferrocene IV

• Cyclopentadiene• It tends to dimerize (and even polymerize) at room temperature via a Diels-

Alder reaction• It is obtained from the commercially available dimer by cracking, which is a

Retro-Diels-Alder reaction (DHo= 77 kJ/mol, DSo= 142.3 J/mol*K, DGo = 34.6 kJ/mol, Keq(25 oC)=8.6*10-7, Keq(180 oC)=3.6*10-2)

• The monomer is isolated by fractionated distillation (b.p.=40 oC vs. 170 oC (dimer)) and kept at T= -78 oC prior to its use

• Note that cyclopentadiene is very flammable, forms explosive peroxides and also a suspected carcinogen

O

O

Ferrocene V

• Acidity of Cyclopentadiene• Cyclopentadiene is much more acidic (pKa=15) than other

hydrocarbon compounds i.e., cyclopentene (pKa=40) or cyclopentane (pKa=45)

• The higher acidity is due to the resonance stabilized anion formed in the reaction

• The cyclopentadienide ion is aromatic (planar, cyclic, conjugated, possesses 6 p-electrons)

HHH

+ OH-

-H2O

Ferrocene VI

• The high acidity implies that cyclopentadiene can be deprotonated with comparably weak bases already i.e., OH-, OR-

• Potassium cyclopentadienide is ionic and only dissolves well in polar aprotic solvents i.e., DMSO, DME, THF, etc.

• The reaction has to be carried out under the exclusion of air because KCp is oxidized easily, which is accompanied by a color change from white over pink to dark brown

H H

+ KOH K + H2O

Ferrocene VII

• The actual synthesis of ferrocene is carried out in DMSO because FeCl2 is ionic as well

• The non-polar ferrocene precipitates from the polar solution while potassium chloride remains dissolved in this solvent

• If a less polar solvent was used (i.e., THF, DME), the potassium chloride would precipitate while the ferrocene would remain in solution

FeFeCl2 + 2 K +Cp- + 2 KCl

Ferrocene VIII

• Infrared spectrum• n(CH, sp2)= 3085 cm-1

• n(C=C)= 1411 cm-1

• asym. ring breathing: = n 1108 cm-1

• C-H in plane bending: = n 1002 cm-1

• C-H out of plane bending: =n 811 cm-1

• asym. ring tilt: = n 492 cm-1 (E1u)

• sym. ring metal stretch: = n 478 cm-1 (A2u)

• Despite the large number of atoms (21 total=57 modes total), there are only very few peaks observed in the infrared spectrum….why?

• Point group: D5d: 4 A1g, 2 A1u, 1 A2g, 4 A2u, 5 E1g, 6 E1u, 6 E2g, 6 E2u

• Only the modes highlighted in bold red are infrared active!

n(CH, sp2) n(C=C)

asym. ring breathing

Synthesis I• Alkali metal cyclopentadienides

• Alkali metals dissolve in liquid ammonia with a dark blue color due to solvated electrons that are trapped in a solvent cage

• The addition of the cyclopentadiene to this solution causes the color of the solution to disappear as soon as the alkali metal is consumed (‘titration’)

• Magnesium • It is less reactive than sodium or potassium because it possesses

often a thick oxide layer and does not dissolve well in liquid ammonia

• Its lower reactivity compared to alkali metals demands elevatedtemperatures (like iron) to react with cyclopentadiene

• The THF adduct of MgCp2 displays one cyclopentadienide ligandbonded via a s-bond (h1) and the second one via pentahapto (h5)

M + C 5 H 6 NH 3 (l) M C 5 H 5 + 1/2 H 2 M=Li, Na, K

M + 2 C 5 H 6 500 o C

M ( C 5 H 5 ) 2 + H 2 M=Mg, Fe

Synthesis II

• Transition metals are generally not reactive enough for the direct reaction except when very high temperatures are used i.e., iron (see original ferrocene synthesis)

• A metathesis reaction is often employed here• The reaction of an anhydrous metal chloride with an alkali metal

cyclopentadienide• The reaction can lead to a complete or a partial exchange• The choice of solvent determines, which product precipitates

I

MCl 2 + 2 NaC 5 H 5 Solvent

M ( C 5 H 5 ) 2 + 2 NaCl M=V, Cr, Mn, Fe, Co, Ni Solvent= THF, DME, NH 3 (l)

FeCl 2 + C 5 H 6 + 2 Et 2 NH F e ( C 5 H 5 ) 2 + 2 [ E t 2 N H 2 ] C l

M Cl 4 + 2 NaC 5 H 5 T o l u e n e

M= Ti, Zr (C5H5)2MCl2 + 2 NaCl

Synthesis III

• Problem: Most chlorides are hydrates, which react with the Cp-anion in an acid-base reaction • The acid strength of the aqua ion depends on the metal and its charge

• The smaller the metal ion and the higher its charge, the more acidic the aqua complex is

• All of these aquo complexes have higher Ka-values than CpH itself (Ka=1.0*10-16), which means that they are stronger acids

Aqua complex Ka

[Fe(H2O)6]2+ 3.2*10-10 (~hydrocyanic acid)[Fe(H2O)6]3+ 6.3*10-3 (~phosphoric acid)[Co(H2O)6]2+ 1.3*10-9 (~hypobromous acid)[Ni(H2O)6]2+ 2.5*10-11 (~hypoiodous acid)[Al(H2O)6]3+ 1.4*10-5 (~ acetic acid)

Synthesis IV• Anhydrous metal chlorides can be obtained from various

commercial sources but their quality is often questionable • They can be obtained by direct chlorination of metals at elevated

temperatures (200-1000 oC)• The dehydrating of the metal chloride hydrates with thionyl chloride

or dimethyl acetal to consume the water in a chemical reaction

• Problems:• Accessibility of thionyl chloride (restricted substance because it used in the

illicit drug synthesis) • Production of noxious gases (SO2 and HCl) which requires a hood

• Very difficult to free the product entirely from SO2

• Anhydrous metal chlorides are often not very soluble

CoCl2*6 H2O + 6 SOCl2 CoCl2 + 6 SO2 + 12 HCl

Synthesis V

• The hexammine route circumvents the problem of the conversion of the hydrated to the anhydrous forms of the metal halide

• The reaction of ammonia with the metal hexaqua complexes affords the hexammine compounds

• Color change: dark-red to pink (Co), green to purple (Ni)• Advantages:

• A higher solubility in some organic solvents • The ammine complexes are less acidic than aqua complexes because ammonia

itself is less acidic than water! • They introduce an additional driving force for the reaction

• Disadvantage:• [Co(NH3)6]Cl2 is very air-sensitive because it is a 19 VE

system. It changes to [Co(NH3)6]Cl3 (orange) upon exposure to air

[M(H2O)6]Cl2 + 6 NH3 [M(NH3)6]Cl2 + 6 H2O (M=Co, Ni)

Synthesis VI

• The synthesis of the metallocene uses the ammine complex

• The solvent determines which compound precipitates• THF: the metallocene usually remains in solution, while sodium

chloride precipitates• DMSO: the metallocene often times precipitates, while sodium

chloride remains dissolved

• The reactions are often accompanied by distinct color changes i.e., CoCp2: dark-brown, NiCp2: dark-green

• Ammonia gas is released from the reaction mixture, which makes the reaction irreversible and highly entropy driven

[M(NH3)6]Cl2 + 2 NaCp MCp2 + 2 NaCl + 6 NH3(g)

Properties I

• Alkali metal cyclopentadienides are ionic i.e., LiCp, NaCp, KCp, etc.

• They are soluble in many polar solvents like THF, DMSO, etc. but they are insoluble in non-polar solvents like hexane, pentane, etc.

• They react readily with protic solvents like water and alcohols (in some cases very violently)

• Many of them react with chlorinated solvents as well because of their redox properties

KCp

LiCp, NaCp

Properties II

• Many divalent transition metals form sandwich complexes i.e., ferrocene, cobaltocene, etc.• They are non-polar• They are often soluble in non-polar or low polarity solvents like hexane,

pentane, diethyl ether, dichloromethane, etc. but are usually poorly soluble in polar solvents

• Their reactivity towards chlorinated solvents varies greatly because of their redox properties

• The M-C bond distances differ with the number of total valence electrons (i.e., FeCp2: ~204 pm, FeCp2

+: ~207 pm; CoCp2: ~210 pm, CoCp2+: ~203

pm, NiCp2: ~214 pm, NiCp2+: ~206 pm )

• Many of the sandwich complexes can also be sublimed because they are non-polar i.e., ferrocene can be sublimed at ~80 oC in vacuo (MCp2: DHsubl.= ~72 kJ/mol (M=Fe, Co, Ni)

Properties III

• Cobaltocene is a fairly strong reducing reagent (E0= -1.33 V vs. FeCp2)• 19 valence electron system with the highest electron in an anti-bonding

orbital• The oxidation with iodine leads to the light-green cobaltocenium ion, which

is often used as counter ion to crystallize large anions

• The reducing power can be increased by substitution on the Cp-ring i.e., Co(CpMe5)2: (E0= -1.94 V vs. FeCp2)

• Cobaltocene is paramagnetic while the cobaltocenium ion is diamagnetic

• Placing electron-accepting groups on the Cp-ring make the reduction potential more positive i.e., acetylferrocene (E0= 0.24 V vs. FeCp2), cyanoferrocene (E0= 0.36 V vs. FeCp2)

2 CoCp2 + I2 2 CoCp2+ + 2 I-

Properties IV

• HgCp2 can be obtained as a yellow solid fromaqueous solution, but is sensitive to heat and light

• The X-ray structure displays two s-bonds between the mercury atom and one carbon atom of each ring

• HgCp2 does undergo Diels-Alder reactions as well as aromatic substitution (i.e., coupling with Pd-catalyst)

• In solution, HgCp2 only exhibits one signal in the 1H-NMR and the 13C-NMR spectrum (d=5.8 ppm,116 ppm), which in indicative of 1, 5-bonding

• A similar mode is found in BeCp2 and Zn(CpMe5)2

HgCl2 + 2 TlCp HgCp2 + 2 TlClH2O

Hg

Applications I

• Cp2TiCl2

• Used to prepare the Tebbe reagent (used to convert keto to alkene functions)

• Used to prepare Cp2TiS5, which is a precursor to cyclic sulfur allotropes (i.e., S6, S7, S9-15, S18, S20)

• Used to prepare CpTiCl3, which is used for the syndiotactic polymerization of styrene

Na

TiCl4+2 Ti

Cl

+ 2 NaCl

Cl

TiCl4

Ti

S S S

SS

Li(HBEt3)/S8

S

S S S

SS

SCl2

TiCl

Cl

ClS2Cl2

SS

S

SSSS

+ 2 Al(CH3)3- CH4- Al(CH3)2Cl

Ti

Cl

CH2

AlCH3

CH3

Applications II• Cyclopentadiene compounds of early transition metals i.e., titanium,

zirconium, etc. are Lewis acids because of the incomplete valence shell i.e., Cp2ZrCl2 (16 VE)

• Due to their Lewis acidity they were used as catalyst in the Ziegler-Natta reaction (Polymerization of ethylene or propylene)

• Of particular interest for polymerization reactions are ansa-metallocenes because the bridge locks the Cp-rings and also changes the reactivity of the metal center based on X

• Variations of the cyclopentadiene moiety leads to the formation of catalyst that yield different forms of polypropylene (atatic, isotactic, syndiotactic)

Applications III

• Mechanism of Ziegler-Natta polymerization of ethylene

Applications IV

• Metallocenes are used to prepare thin films of metals or metal oxides via OMCVD (Organometallic Chemical Vapor Deposition)• Pyrolysis (ansa-Zr and Hf metallocenes)

• Reduction with hydrogen (i.e., NiCp2, CoCp2)

• Catalysis of formation of carbon nanotubes (i.e., FeCp2)

• Petasis Reagent (Cp2Ti(CH3)2) and Tebbe Reagent (Cp2TiCH2AlCl(CH3)2) are used to convert carbonyl groups to alkenes via a titanium carbene complex (Cp2Ti(=CH2))