Carbon Dioxide Insertion into Rhenium Hydrides as a Probe for the Impact of Solvent on Linear Free Energy Relationships between Thermodynamic and Kinetic Hydricity

The kinetics of CO2 insertion into electronically different Re­(Rbpy)­(CO)3H (Rbpy = 4,4′-R-2,2′-bipyridine; R = OMe, tBu, Me, H, Br, COOMe, or CF3) complexes to form Re­(Rbpy)­(CO)3{OC­(O)­H} compounds were determined in acetone, dimethylacetamide, dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and 3-methoxypropionitrile and compared with previous data in acetonitrile (MeCN). The rate of CO2 insertion for any one complex of the type Re­(Rbpy)­(CO)3H in different solvents correlates with the Dimroth–Reichardt [E T(30)] solvent parameter. Hammett plots in each solvent indicate that insertion reactions are faster for bpy ligands with electron-donating groups. There is, however, no correlation between the slope of the Hammett plot in different solvents and those of any common solvent parameter. Similarly, the enthalpies and entropies of activation and kinetic isotope effects associated with insertion of CO2 into Re­(bpy)­(CO)3H in different solvents do not correlate with any common solvent parameters. Theoretical calculations were used to determine the relative thermodynamic hydricities of Re­(Rbpy)­(CO)3H type complexes in MeCN, acetone, DMF, and DMSO, and in each solvent, complexes with more electron-donating substituents on the bpy ligand are stronger hydride donors. Linear free energy relationships (LFERs) between calculated thermodynamic and experimental kinetic hydricity, as measured through the CO2 insertion reactions, were observed in MeCN, acetone, DMF, and DMSO. The slopes of the LFERs correlate with the dielectric constant of the solvent. Overall, this work provides fundamental information about the thermodynamics and kinetics of hydride transfer reactions in different solvents, which is valuable for catalyst design.