Thermal Stability of Tris(3,5-dimethylpyrazolyl)hydridoboratorhenium(V)(oxo)- (1,2-dithiolate) and -(1,2-monothiodiolate) Complexes and DFT Studies of C−S Bond Cleavage

Rhenium(V) complexes are prepared by reductive cyclocondensation of alkane-1,2-dithiols or alkane-1-thio-1,2-diols with Tp‘ReO3 (Tp‘ = tris(3,5-dimethylpyrazolyl)hydridoborate) and PPh3. One complex, Tp‘Re(O)(SCH2CH2S), was characterized by X-ray crystallography. Unlike the corresponding 1,2-diolato compounds, these sulfur analogues are thermally stable in hydrocarbon solvents for more than one week at 120 °C. To evaluate whether the barrier to cycloreversion was thermodynamic or kinetic in origin, DFT calculations were performed at the B3LYP/LACVP** level. Comparison of calculated reaction energies reveals that substitution of each sulfur raises the predicted ΔE rxn for alkene cycloreversion by about 10 kcal/mol.