Hydride pentamethylcyclopentadienyl complexes

Watson s investigations of mixed alkyl/pentamethylcyclopentadienyl complexes of ytterbium and lutetium have shown that the polymerization occurs due to the successive insertion of olefine into the Ln-C bond. The process includes the p-hydride or p-alkyl decomposition of Ln-R intermediates. The splitting-off of the coordinative bonded ligand from REM precedes the above mentioned reactions (see Scheme XIII.l). 

The highly syndiospecific-living polymerization of methyl methacrylate has been initiated by the neutral bis(pentamethylcyclopentadienyl)lanthanide-alkyl or -hydride complexes [215,216]. The plausible reaction mechanism is shown in Scheme XI. 

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) ... 

Systems which fulfil these conditions are tris(2,2 -bipyridyl)rhodium complexes [63] and, more effectively, substituted or unsubstituted (2,2 -bipyridyl) (pentamethylcyclopentadienyl)-rhodium complexes [64], Electrochemical reduction of these complexes at potentials between — 680 mV and — 840 mV vs SCE leads to the formation of rhodium hydride complexes. Strong catalytic effects observed in cyclic voltammetry and preparative electrolyses are... 

In 1976 Bercaw and co-workers reported the stoichiometric reduction of CO to methanol using derivatives of bis(pentamethylcyclopentadienyl)zir-conium (78). Bercaw had shown previously (79) that permethylated cyclo-pentadienyl ligands greatly enhance the stability of these complexes, permitting their isolation and/or in situ identification. In their study, these workers observed and partially characterized the novel hydride carbonyl species (13) which reacts with the dihydride complex Zr(C5Me5)2H2 at room temperature to form the methoxy species (14). 

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). 

Bi(pentamethylcyclopentadienyl) lanthanide hydride and methyl complexes [(C5Me5)2Ln(p.-R)]2 (R = H, CH3) were the first to be reported as excellent initiators for the highly... 

Jeske, G, Lauke, H., Mauermann, H. etal. (1985) Highly reactive organolanthanides. Systematic routes to and olefin chemistry of early and late bis(pentamethylcyclopentadienyl) 4f hydrocarbyl and hydride complexes. Journal of the American Chemical Society, 107, 8091. 

The dissociation of the dimer [Cp 2Y(/i-H)]2 to the Cp 2YH monomer is an important process in the reactions of the dimer with alkenes. The kinetics and formation of yttrium alkyl complexes from [Cp 2Y(//-I I)]2 and alkenes have been investigated.587 In situ prepared dimeric bis(pentamethylcyclopentadienyl)yttrium hydride reacted rapidly with 3,3-dimethyl-l,4-pentadiene in methylcylohexane-r/ at — 78 °C and formed a bright yellow solution of the t/°-yttrium(m)pentenyl chelate complex Cp 2Y[7/,7/2-CI I2CI I2CMc2CI I=CH2] in 98% yield (Scheme 130). This pentenyl chelate complex was also prepared in toluene-// in 84% yield. The chelate complex was stable for about 2 weeks at — 78 °C but decomposed after a few hours at —50 °C. The complex was characterized without isolation by 1H and 13C NMR spectroscopy at —100 °C such complexes can be regarded as models for the coordination of alkenes to lanthanide and isoelectronic grouop 4 polymerization catalysts.588 Agostic interactions in yttrium alkyls of the type... 

Many metal hydrides protonate to give H2 complexes, but kinetic protonation can take place on M-H to give an M-(H2) complex, even when protonation at the metal is thermodynamically favoured. Protonation of [FeH(dppe)Cp ] (Cp = pentamethylcyclopentadienyl) gave the dihydrogen complex at — 80 °C, followed by rearrangement to the dihydride at 25 Kinetic protonation by... 

The )j -(pentamethylcyclopentadienyl)-j -arene-Ir(III) and -lr(III) complexes such as XV undergo hydride attack from the exo face to give jj -cyclohexadienyl ligands (XVI) with an endo-methyl substituent . The preference for addition to an unsubstituted position is clear from the corresponding reaction of the )j -pentamethylbenzene ligand (in XVII) where a simple product is formed (XVIII) ... 

The complex, formally derived from a double oxidative addition of benzene to two Ir centers , is in contrast to the mononuclear product obtained with the pentamethylcyclopentadienyl derivative . In the absence of light there is no reaction presumably photolysis displaces one carbonyl ligand to give a coordinatively unsaturated precursor. An apparent intermediate in the photolysis has H NMR resonances in the aromatic, cyclopentadienyl and hydride regions and may be IrGp-j/ (GO)H(G6H5), but its nature remains uncertain. 

The lanthanides and actinides are also active in electrophilic attack on arenes. Bis(pentamethylcyclopentadienyl)lutetium methyl or hydride complexes react readily with benzene to give phenyl complexes and methane or dihydrogen . Similar reactions are observed for Sc and Th In a remarkable reaction benzene can be dimetallated by Lu, yielding II ... 

The pentamethylcyclopentadienyl niobocene (f complexes [Cp 2NbH(CH2CHR)], in which the alkene is coordinated to the metal center cis to the metal hydride, with a planar arrangement of the alkene carbon-carbon bond and the metal-hydrogen bond, are ideally suited for insertion without interference of the need for alkene rotation, and have been studied thoroughly [46]. In solution only the endo hydrido-alkene tautomer (Scheme 6.9) is observed by... 

The binuclear complexes derived from the dimerization of Cp (q -allyl)hydride iridium fragments (Cp = r] -pentamethylcyclopentadienyl) reported by Bergman provide noteworthy examples of CSp -H bond activation (Scheme 20) [105]. 

Cp = pentamethylcyclopentadienyl, bpy = 2,2 -bipyridine) as the catalyst (43). The reaction takes place at ambient temperature, and no CO was detected in the H2-CO2 mixture. A rhodium hydride complex was identified in the catalytic cycle, which undergoes a rapid H/D exchange in presence of D2O. This result is consistent with the proposed rate-determining step, which is y3-hydrogen elimination from the formate complex (Fig. 1). 

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