Disulfide ligands and sulfur-bridging carbonyls: Remarkable examples in manganese carbonyl chemistry

Abstract

An important recent development in metal disulfide chemistry is the 2001 synthesis by Adams et al. of Mn2S2(μ-CO)(CO)6 containing both a bridging disulfide ligand and a bridging carbonyl group. During the ensuing decade this manganese complex has proven to be a key intermediate for the synthesis of diverse manganese carbonyl sulfur derivatives, including heterometallic complexes, phosphine, arsine, and thioether derivatives. The related series of Mn2S2(CO)n complexes (n=10, 9, 8, 7, 6) has now been studied by density functional theory. The lowest energy Mn2S2(CO)10 structures have two Mn(CO)5 units bridged by a symmetrical two-electron donor disulfide ligand without a metal–metal bond. This structure may be derived from dimethyl disulfide by replacement of both methyl groups with isolobal Mn(CO)5 groups. Decarbonylation of this Mn2S2(CO)10 structure gives the lowest energy Mn2S2(CO)9 structure containing an unprecedented unsymmetrical four-electron donor bridging disulfide ligand and still no metal–metal bond. However, this Mn2S2(CO)9 structure is predicted to be thermodynamically unstable with respect to disproportionation into Mn2S2(CO)10+Mn2S2(CO)8. The lowest energy Mn2S2(CO)8 structure has a sulfur-bridging carbonyl group as well as a carbonyl group bridging an Mn–Mn bond. Removal of this sulfur-bridging carbonyl group gives the lowest energy Mn2S2(CO)7 structure. This structure corresponds to the experimental Mn2S2(μ-CO)(CO)6 structure synthesized by Adams et al., which has a carbonyl-bridged Mn–Mn single bond. For Mn2S2(CO)6, one of the two low-energy structures can be derived from this Mn2S2(μ-CO)(CO)6 structure by loss of the bridging carbonyl group. The other low-energy Mn2S2(CO)6 structure has two Mn(CO)3 groups joined by an Mn–Mn bond, which is bridged by two separate sulfide (S) ligands.