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
[ET(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.