| The reactivity of the dihapto zirconium acyl complexes Cp(,2)Zr{lcub}(eta)('2)-C(O)R{rcub}R (R = methyl, phenyl; Cp = ((eta)('5)-C(,5)H(,5))) with some transition metal hydrides has been examined. Basic hydrides of Groups VIB and VIIB, such as Cp(,2)MoH(,2), Cp(,2)WH(,2) and Cp(,2)ReH, effect hydride transfer to the acyl carbon of the zirconium complexes. With Cp(,2)WH(,2) and Cp(,2)ReH, bimetallic products arising from a single hydride transfer to the acyl carbon are isolable and the crystal structure of Cp(,2)ReCH(CH(,3))OZr(CH(,3))Cp(,2) is reported. With Cp(,2)MoH(,2), much faster reaction occurs and Cp-ring protons are also transferred. Ultimate products of these hydride transfers include alkoxide, olefin and carbene complexes; thus, resulting in a formal reduction of the acyl carbon. This carbon is originally derived from carbon monoxide, hence, the net reactions represent stoichiometric reduction of carbon monoxide to olefinic and alcoholic oxidation states.; The ease with which the reductions occur is noteworthy in light of the very weak hydridic character of the metal hydrides. An explanation involving the oxy-carbene character of the d(DEGREES) acyl complexes is presented. The crystal structure of Cp(,2)W = CH(C(,6)H(,5)) is also discussed.; With CpMo(CO)(,3)H (an acidic hydride) and Cp(,2)Zr{lcub}(eta)('2)-C(O)-CH(,3){rcub}CH(,3), methane elimination occurs to give a bimetallic acyl complex, Cp(,2){lcub}(eta)('2)-C(O)CH(,3){rcub}Zr((mu)-OC)Mo(CO)(,2)Cp, which contains a bridging carbonyl O-bound to zirconium and C-bound to molybdenum. This complex is formed more cleanly by carbonylation of {lcub}Cp(,2)Zr(CH(,3))Mo(CO)(,3)Cp{rcub}. The acyl complex decarbonylates slowly, but does not reform the alkyl bimetallic complex. Instead, yet another bimetallic acyl complex is obtained. This complex, Cp(,2)Zr{lcub}(mu)-OC(CH(,3)){rcub}{lcub}(mu):(eta)('2)(OC){rcub}-Mo(CO)Cp contains both a bridging acetyl group (O-bound to Zr and C-bound to Mo) and a four-electron-bridging carbonyl. The crystal structures of the two bimetallic acyl complexes are discussed, as is their relevance to CO activation.; Finally, spectroscopic studies of some Fischer-Tropsch model systems are presented. It was found that {lcub}((eta)('5)-C(,5)-(CH(,3))(,5){rcub}(,2)ZrH(,2)(CO), an intermediate in the reduction of CO with {lcub}((eta)('5)-C(,5)(CH(,3))(,5){rcub}(,2)ZrH(,2), could be observed by low temperature infrared spectroscopy. Despite the fact that no d electrons are available for back-bonding, the carbonyl complex exhibits a significant lowering of (nu)(CO) to 2040 cm('-1) from 2141 cm('-1) in free CO. In addition, MN(,2)((mu):(eta)('2)-CO)(CO)(,4)(dpm)(,2), the first complex reported to contain a four-electron-bridging carbonyl ligand, was examined by low temperature ('13)C NMR spectroscopy. The carbonyl ligand was found to be fluxional (Ea = 17 kcal/mol), exchanging with one of the terminal carbonyl ligands. The fluxional process does not, however, lead to all of the carbonyls becoming equivalent. Surprisingly, the chemical shift of the bridging carbonyl falls in the normal region for manganese carbonyls. |
