DEOXYGENATIVE REDUCTION OF DIMOLYBDENUM CARBONYL-COMPLEXES MO-2(ETA(5)-C(5)H(4)R')(2)(CO)(4) BY HYDROSILANE GIVING TRINUCLEAR MU(3)-ETHYLIDYNE AND MU(3)-ETHYLIDYNE COMPLEXES, (MU(3)-CR)[MO(ETA(5)-C(5)H(4)R')(CO)(2)](3)(R,R'=H,CH3), AND ANALYSIS OF THE ROTATIONAL PROCESSES OF THE MO(ETA(5)-C(5)M(4)R')(CO)(2) MOIETIES

Photolysis of the dimolybdenum carbonyl complex (eta(5)-C(5)H(4)R')(2)Mo-2(CO)(4) 1 (a-series: R' = H; b-series: R' = CH3) in the presence of an excess amount of HSiMe(2)Ph produces the trinuclear mu(3)-methylidyne complex (mu(3)-CH)[Mo(eta(5)-C(5)H(4)R')(CO)(2)](3) 2a,b and the mu(3)-ethylidyne complex (mu(3)-C-CH3)[Mo(eta(5)-C(5)H(4)R')(CO)(2)](3) 3a,b along with siloxane O(SiMe(2)Ph)(2) 4. The structure with the tetrahedral CMo3 core and fluxional properties of 2 and 3 have been characterized by means of X-ray crystallography and NMR techniques. All of the three eta(5)-C(5)H(4)R' rings in the mu(3)-methylidyne complexes 2a,b project to the same side as does the mu(3)-CH ligand (conformer A), whereas one of the three eta(5)-C(5)H(4)R' rings in the mu(3)-ethylidyne complexes 3a,b projects to the side distal from the mu(3)-C-CH3 (conformer B). The C-3-symmetrical structure (A) of 2 is supported by the three strong semibridging interactions of CO ligands with the adjacent Mo centers, but 3 with the unsymmetrical structure B contains less strong semibridging interactions. In solutions, 2 exists as an equilibrated mixture of two isomers A and B as revealed by variable temperature H-1- and C-13-NMR measurements. The complicated dynamic behavior of 2 can be analyzed as a combination of rotation of one of the three Mo(eta(5)-C(5)H(4)R')(CO)(2) units of A leading to B (local rotation) and concerted rotation of all the three metal fragments operating for B (gearlike rotation), which are associated with cleavage and regeneration processes of the semibridging interaction. The fluxional behavior of the mu(3)-ethylidyne complexes 3 can be explained by the gearlike rotation mechanism. Labeling experiments using 1-(CO)-C-13 and DSiMe(2)Ph verify that all the hydrogen and carbon atoms of the ys-alkylidyne parts in 2 and 3 come from HSiMe(2)Ph and CO in 1, respectively. Therefore the present system can be viewed as a model system for a crucial step in Fischer-Tropsch mechanism, i.e. deoxygenative reduction of CO giving CH and CCH3 species. The deoxygenative reduction has been realized by using hydrosilane with high oxygenophilicity as an equivalent for H-2.