Alkyne dicobalt carbonyl complexes with sulfide ligands. Synthesis, crystal structure, and dynamic behavior

Hexacarbonyl dicobalt complex of bis(tert-butylsulfonylethyne) [Co-2(mu-(Bu SO2C)-S-t)(2)(CO)(6), 3] experiences a thermally induced ligand exchange pr ocess with methyl p-tolyl sulfide, dibenzyl sulfide, and diethyl sulfide to give the corresponding stable sulfide complexes [Co-2(mu-Bu-t-SO2C)(2)(CO) (5)SR2] 4, 5, and 6, respectively, in good yield (59-65%). The reaction wit h tetrahydrothiophene gives a disubstituted complex 7 in 74% yield. Oxathia ne 9, derived from (+)-(2R)-10-mercaptoisoborneol, also reacts with 3 to ge nerate a chiral sulfide complex 8 (58%). The solid-state structures of 5 an d 8 have been established by X-ray crystallography and reveal the preferenc e of the incoming sulfur ligand to occupy an equatorial coordination site. Further structural studies on 5 have been performed by low-temperature H-1 NMR analysis and by theoretical procedures at the PM3(tm) level of theory. Analysis of the low-temperature H-1 NMR spectrum of 5 shows a signal splitt ing consistent with the freezing of an equilibrium between two equatorially coordinated sulfides, and the computational study of the different isomers of 5 shows that the equatorially coordinated complex is 3.9 kcal mol(-1) l ower in energy than the most stable axially coordinated one, in agreement w ith solid-state and solution studies. Finally, ligand exchange experiments have been performed in order to provide an explanation for the Pauson-Khand reactivity of alkynes containing ancillary sulfide ligands and were found to support the experimentally observed rate enhancements.