CARBON CARBON BOND FORMATION VIA CARBYNE CARBONYL MIGRATORY COUPLING PROMOTED BY H- OR OR- ADDITION TO [FE2(MU-CSR)(MU-CO)(CO)2(CP)2]+

The complexes [Fe2(mu-CSR) (mu-CO) (CO)2(CP)2] + [R = Me (1a), Et (1b)] react with LiHBEt3 to form FeFe[mu-C(CHO)SR](mu-CO)(CO)(Cp)2 (2a,b), together with smaller amounts of Fe2[mu-C(CHO)H](mu-CO)(CO)(Cp)2 (3). The Major Product the aldehyde 2, is proposed to result from H- addition at the terminally coordinated CO ligand in 1 and subsequent CHO migration to the bridging carbyne carbon atom. The corresponding reactions of 1a,b with RO- (R = Me, Et) afford the alkoxycarbonyls Fe2(mu-CSR)(mu-CO)(CO)(COOR')(CP)2 (4), which in turn, upon standing in CH2Cl2 solution, rearrange to form the esters FeFe[mu-C(COOR')SR](mu-CO)(CO)(Cp)2 (5). The relevance of these reactions is discussed in the general context of C-C bond formation involving transition metal complexes. In addition to spectroscopic characterization three compounds [2b, 3, 5a (R = R' = Me)] have been fully characterized by X-ray crystallography. These molecules contain the bonded Fe-Fe unit supporting a bridging CO and a bridging carbene unit. The sulfur atom is also coordinated to the iron in 2a and 5a, in the place of a terminal CO ligand. The carbene atoms bear a CHO substituent in 2b and 3 and a C(O)OMe substituent in 5a. The eta-C5H5 groups occupy mutually cis coordination sites.