Alkyne Dicobalt Carbonyl Complexes with Sulfide Ligands. Synthesis, Crystal Structure, and Dynamic Behavior

Hexacarbonyl dicobalt complex of bis(tert-butylsulfonylethyne) [Co2(μ-ButSO2C)2(CO) 26, 3] experiences a thermally induced ligand exchange process with methyl p-tolyl sulfide, dibenzyl sulfide, and diethyl sulfide to give the corresponding stable sulfide complexes [Co2(μ-But-S02C)2(CO) 5SR2] 4, 5, and 6, respectively, in good yield (59-65%). The reaction with tetrahydrothiophene gives a disubstituted complex 7 in 74% yield. Oxathiane 9, derived from (+)-(2R)-10-mercaptoisoborneol, also reacts with 3 to generate a chiral sulfide complex 8 (58%). The solid-state structures of 5 and 8 have been established by X-ray crystallography and reveal the preference of the incoming sulfur ligand to occupy an equatorial coordination site. Further structural studies on 5 have been performed by low-temperature 1H NMR analysis and by theoretical procedures at the PM3(tm) level of theory. Analysis of the low-temperature 1H NMR spectrum of 5 shows a signal splitting 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 lower in energy than the most stable axially coordinated one, in agreement with 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.