METAL ALKOXIDES - MODELS FOR METAL-OXIDES .17. REDUCTIVE CLEAVAGE OF CARBON-MONOXIDE TO CARBIDE AND OXIDE BY DITUNGSTEN AND TETRATUNGSTEN ALKOXIDES - CRYSTAL AND MOLECULAR-STRUCTURES OF W4(MU-4-C)(OCH2-C-PEN)14, W4(MU-4-C)(O)(OCH2-T-BU)12, AND W4(MU-4-C)(O)(O-I-PR)12

Two synthetic strategies for the reductive cleavage of C=O to carbide and oxide ligands have been explored. Both were successful but both had competing side reactions. (1) Addition of CO (1-3 equiv) to W4(OCH2-C-Pen)12, where Pen = pentyl, leads to the formation of three carbonyl derived prodUCtS, W4(mu-4-C)(OCH2-C-Pen)14 (1), W4(mu-4-CO)(CO)2-(OCH2-c-Pen)12 (II), and an oxo-tungsten alkoxide. The formation of I occurs by C-O bond cleavage even at -30-degrees-C in toluene-d8. Reactions employing labeled CO indicate the C-O bond cleavage occurs prior to oxo/alkoxide exchange in the formation of I. The formation of II is favored in the presence of an excess of CO, and the isobutoxide analogue, W4(mu-4-CO)(CO)2(OCH2-i-Pr)12, has been structurally characterized and shown to contain a bridging carbonyl ligand that is carbon bonded to three tungsten atoms and oxygen bonded to one. Compounds II (R = CH2-C-Pen, CH2-i-Pr) are not directly involved in the formation of carbides such as I. (2) The reaction between W2(mu-CO)(O-t-Bu)6 and W2(O-t-Bu)6 in the presence of an excess of alcohol yields W4(mu-4-C)(O)(OR)12 where R = CH2-t-Bu (III) and i-Pr (IV). Competing with the formation of the carbido compounds, III and IV, are reactions leading to W4(mu-CO)2(OR)12 and W4(OR)12. The compounds I, III, and IV have been characterized by single-crystal X-ray crystallography and are closely related in having a common W4(mu-4-C)14+ template. The structures differ only in the identity of R and the position of the mu-2-oxo group or its replacement by a pair of OR ligands. These reactions are the first examples of the cleavage of carbon monoxide to carbide and oxide ligands in a molecular system. These findings are compared to other reactions that are known to yield products derived from cleavage of C=O and, in particular, to the cleavage of C=O on certain metal surfaces that generates surface bound carbide and oxide as in the Fischer-Tropsch reaction.