Organosulphur–transition-metal chemistry. Part 7. Sulphur inversion in dimolybdenum complexes: crystal and molecular structure of [Mo2(CO)2{P(OMe)3}(µ-SBut)3(η-C7H7)]

Abstract

Thiolato-bridged dimolybdenum complexes [Mo₂(CO)₃(µ-SR)₃(η-C₇H₇)] are formed by treating [Mol(CO)₂(η-C₇H₇)] with RSH (R = Me, Et, Prⁱ, or Bu^t) in the presence of NEt₃. Infrared and variable-temperature n.m.r. spectroscopy reveal that the complexes exist as two isomers, arising from different relative orientations of the R groups on bridging sulphur. At ambient temperatures and above, these isomers interconvert rapidly, attributed to a low-energy sulphur inversion process, such that complete averaging of R group environments is achieved on the n.m.r. time scale. Intercoverting isomers are also present for the monosubstituted derivatives [Mo₂(CO)₂(PR′₃)(µ-SBu^t)₃(η-C₇H₇)][R′₃= P(OMe)₃, PMePh₂, PMe₂Ph, or PMe₃], but for these only a limited averaging is observed, generating a time-averaged mirror plane of molecular symmetry. This results from the presence of a phosphorus ligand trans to one bridging sulphur and of CO trans to the other two, shown in an X-ray diffraction study of [Mo₂(CO)₂{P(OMe)₃}(µ-SBu^t)₃(η-C₇H₇)]. Crystals are monoclinic, space group P2₁/n, with four molecules in a unit cell of dimensions a= 10.186(7), b= 15.677(7), c= 19.821(15)Å, and β= 96.31 (5)°. The structure has been solved by conventional techniques and refined by least squares to R 0.046 for 5 149 diffractometer-measured reflection intensities. Three bridging SBu^t groups display an asymmetric stereochemistry about a Mo–Mo bond [2.9460(15)Å], rationalised on the basis of intramolecular terminal ligand–bridging ligand contacts. One molybdenum is η⁷ co-ordinated to a cycloheptatrienyl ring while the other carries the two CO groups and a P(OMe)₃ ligand. Metal–sulphur distances range over 2.411(2)–2.549(2)Å and Mo–P is 2.445(2)Å. Free energies of activation for sulphur inversion as low as 46kJ mol^–1 have been derived from coalescence temperatures for several complexes.