Reactions of Cp(2)Ta(CH2)(CH3) with substituted and unsubstituted metal carbonyls (groups 7, 8, and 9). Evidence for intermediates involved in the carbon-carbon bond-forming steps of the reduction and deoxygenation of CO

The mechanisms of surface-catalyzed reactions that deoxygenate carbon monoxide (CO) and convert it into longer chain hydrocarbons are not well understood. Homogeneous models involving soluble, well-characterized organometallic complexes would be helpful in developing an understanding of these reactions. Reported here are transformations in which CH2, CO, alkyl, and aryl fragments incorporated in soluble metal complexes undergo a variety of changes that lead to new multicarbon ligands. In one example, treatment of the tantalum methylene complex Cp(2)Ta(CH2)(CH3) (1, Cp = eta(5)-C5H5) With the methyl- or phenylrhenium pentacarbonyl complexes R-Re(co) (CO)(5) (R = CH3 (2a), Ph (2b)) above 0 degrees C leads to >90% yields of the bridging oxo complexes Cp(2)(CH3)Ta(mu-O)Re(CR=CH2)(CO)(4) (R = Me (3a) and Ph (3b)). Low-temperature NMR monitoring and use of a perfluoroalkyl ligand has provided information about the initial steps in these transformations. These demonstrate the first observation of ''Wittig-like'' attack of a metal alkylidene group on a CO ligand to give a zwitterion (e.g., fully characterized 16) followed by cleavage to oxometal and vinylidene complexes. In another example, the tantalum-methylene complex 1 reacts with the dinuclear metal carbonyls Co-2(CO)(8) and Fe-2(CO)(9) to yield new complexes (17 and 18) that incorporate a C3H2O2 ligand bridging three metal centers. Reaction of the tantalum-methylene complex with Re-2(CO)(10), Mn-2-(CO)(10), or Fe(CO)(5) leads to 19, 20, and 21, requiring even more deep-seated changes in which extensive rearrangement along with three-carbon coupling occurs. In this process, an oxygen atom is removed from one CO group, leading to the oxotantalum compound Cp(2)(CH3)Ta = O. The carbon atom from the transformed CO couples with two CH2 groups initially bound to tantalum, and the CH2 hydrogens are simultaneously rearranged to produce a CH3-C=C- ligand. This C-3 fragment is stabilized by binding to a tantalum-late-metal chain. These products also contain the first examples of tantalum-carbon monoxide bridges. A reaction between 1 and Ru-3(CO)(12) that results in CO deoxygenation along with coupling of the CO carbon to methylidene groups and other CO carbons to yield the cluster complex Cp(2)(CH3)Ta(mu-O)Ru-3(C4H4)(CO)(9) (23) and the unstable free tantalum oxo species Cp(2)Ta(=O)-(CH3) is also reported. The TaRu3 product contains a 4-carbon cumulene ligand that bridges the three late-metal centers. The crystal structures of complexes 3b, 16, 17, 18, 19, 20, 21, and 23 are reported.