Pentamethylcyclopentadienyl-zirconium

The (pentamethylcyclopentadienyl)zirconium amidinate unit also served as a platform for the synthesis and characterization of remarkable cationic and zwitterionic allyl zirconium complexes derived from trimethylenemethane (TMM). A direct synthetic route to the neutral precursors was found in the... 

Effective catalysts have recently been developed for the addition of trialkyl-aluminum reagents to alkenes (carboalumination). 6 -(Pentamethylcyclopentadienyl) zirconium dimethylide activated by fra-(pentafluorophenyl)boron promotes the addition of trimethylaluminum to terminal alkenes.221... 

Bis(i7-pentamethylcyclopentadienyl)zirconium was proposed as an intermediate in the permethylzirconocene hydride promoted reduction of coordinated carbon monoxide by Bercaw and co-workers (65,67-69). In hopes of intercepting such an intermediate, CO was allowed to diffuse... 

In none of the cases discussed above is molecular hydrogen involved. The first report of the stoichiometric reduction of coordinated carbon monoxide by molecular hydrogen is that published by Bercaw et al. (35, 36). They reported that mononuclear carbonyl and hydride complexes of bis(pentamethylcyclopentadienyl)zirconium are capable of promoting stoichiometric H2 reduction of CO to methoxide under mild conditions. Thus, treatment of the dicarbonyl complex (rj5-C5Me5)2Zr(CO)2 with... 

The CO reductions generally could likely proceed through formyl intermediates, probably at a multinuclear site (420) hydride migration to a coordinated CO [e.g., as in the hypothetical scheme outlined in Eq. (72)] has not yet been observed, although metal formyl complexes have been synthesized via other methods (422-425). A ir-bonded formyl also seems plausible (426), since 7r-bonded acyl groups have been demonstrated (427). A stoichiometric hydrogen reduction of CO to methanol under mild conditions via a bis(pentamethylcyclopentadienyl)zirconium complex is considered to go through a formyl intermediate (428, 429) ... 

Bis(pentamethylcyclopentadienyl)zirconium dichloride is prepared readily in 50-60% yield following Reactions 4 and 5. Although (ry5-CsMes Zrb (7) can be obtained by treating ( -CsMes ZrC with Li+fBH Hs ]- or with Na+[AlH2(0CH2CH20CH3)2] in THF or benzene, respectively, its isolation from these reaction mixtures is extremely difficult. We therefore used an indirect synthesis (Reactions 6 and 7). Compound K -CsMes Zr fe (8) is obtained as a pure crystalline material in 60% yield its structure is reported elsewhere (13). Structure 8 reacts quantitatively (NMR) with H2 at 0°C according to Reaction 7. 

The results implicate the mechanism shown in Scheme 6. Thus initial migratory insertion of CO into the Zr-C bond is followed by intramolecular insertion of the oxycarbene into the Zr-H bond to produce what could be considered an aldehyde adduct of bis(pentamethylcyclopentadienyl)zirconium(II). The final step involves simple /J-hydride abstraction to give the enolate hydride, 24. 

Resconi, L., Piemontesi, F., Franciscono, G., Abis, L. and Fiorani, T., Olefin Polymerization at Bis(pentamethylcyclopentadienyl)zirconium and -hafnium Centers Chain-transfer Mechanisms , J. Am. Chem. Soc., 114, 1025-1032 (1992). 

The bis(i7-pentamethylcyclopentadienyl)zirconium dinitrogen complex [tj-C5(CH3)5kZrj(N2)3 (22), in solution reversibly releases its dinitrogen in vacuo over several hours (Section III,E). The solutions remaining are said to consist mainly of [T7-C5(CH3)5][T7-C5(CH3)4CH2]ZrH (23) (33). Permethylzirconocene [tj-C5(CH3 )5l,Zr (24) may exist in these solutions in equilibrium with this hydride tautomer (as in the case of titanium). However, thus far permethylzirconocene has not been isolated as a discrete compound or otherwise characterized. It is very likely that permethylzirconocene is less stable and more prone to form complex metallocene hydrides than permethyltitanocene (16). 

In his first article on soluble metallocene/methylalumoxane catalyst systems producing isotactic polypropylene,112 Ewen reported the 13C NMR analysis of the end-groups of the atactic polypropylene formed with bis(cyclopentadienyl)zirconium dichloride or bis(pentamethylcyclopentadienyl) zirconium and methylalumoxane. The polymer formed at 50°C contained M-propyl and vinylidene end-groups in 1 1 ratio, consistent with /3-hydrogen elimination from the metal-primary carbon bond and subsequent reinitiation via primary insertion ... 

The reaction of bis-(pentamethylcyclopentadienyl) zirconium(IV) dichloride, 110, with dilithio-acetylene or dilithiodiacetylene, 111, formed zirconocene acetylene polymers (Scheme 2.32).197 Polymers 112 showed limited solubility in organic solvents other than n-hexane and possessed weight-average molecular weights between 55,000 and 68,000. 

Dicarbonylbis(-n -pentamethylcyclopentadienyl)zirconium, unlike its titanium analog, is highly air-sensitive. It is easily soluble in both aliphatic and aromatic solvents. The H NMR spectrum (CftO ) exhibits a singlet at 8 1.73 ppm (external TMS), and the IR spectrum displays two metal carbonyl stretching vibrations at 1945 and 1852 cm (hexane). The mass spectrum shows a parent ion at mie 416. 

When sodium cyclopentadienide 469 was treated with trichloro(pentamethylcyclopentadienyl)zirconium-(IV) (474), unchelated 475 was obtained in 71% yield (Scheme 89). This compound was characterized by an X-ray structure analysis. 

Resconi, L. Abis, L. Franciscono, G. 1-Olefin polymerization at bis(pentamethylcyclopentadienyl) zirconium and -hafnium centers Enantioface selectivity. Macwmolecules 1992, 25, 6814—6817. 

Scheme 35 shows the formation of zirconocene acetylene polymers from the reaction of bis(pentamethylcyclopentadienyl)zirconium (I VO dichloride 118 with dilithiodiacetylene 119. Polymer 120 displayed a weight-average molecular weight of 68,000. 

FIG. 13 (a) Oscillating metallocene catalysts. (From Ref. 6.) (b) Ethylene bis(pentamethylcyclopentadienyl) zirconium dichloride. 

McAlister DR, Erwin DK, Bercaw JE. Reductive elimination of isobutane from an isobutyl hydride derivative ofbis(pentamethylcyclopentadienyl)zirconium. J Am Chem... 

Similarly conjugated zirconocene acetylene polymers (51) were prepared by reaction of Z)w-pentamethylcyclopentadienyl zirconium(IV) dichloride (49) with dilithioacetylene or dilithiodiacetylene (50) (Scheme 13). These relatively high MW polymers (M =55,000-68,000) displayed limited solubility in organic solvents other than n-hexane. 

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