Correlating Thermodynamic and Kinetic Hydricities of Rhenium Hydrides

The kinetics of hydride transfer from Re(Rbpy)(CO)3H (bpy = 4,4′-R-2,2′-bipyridine;
R = OMe, tBu, Me, H, Br, COOMe, CF3) to CO2 and
seven different cationic N-heterocycles were determined. Additionally,
the thermodynamic hydricities of complexes of the type Re(Rbpy)(CO)3H were established primarily using computational
methods. Linear free-energy relationships (LFERs) derived by correlating
thermodynamic and kinetic hydricities indicate that, in general, the
rate of hydride transfer increases as the thermodynamic driving force
for the reaction increases. Kinetic isotope effects range from inverse
for hydride transfer reactions with a small driving force to normal
for reactions with a large driving force. Hammett analysis indicates
that hydride transfer reactions with greater thermodynamic driving
force are less sensitive to changes in the electronic properties of
the metal hydride, presumably because there is less buildup of charge
in the increasingly early transition state. Bronsted α values
were obtained for a range of hydride transfer reactions and along
with DFT calculations suggest the reactions are concerted, which enables
the use of Marcus theory to analyze hydride transfer reactions involving
transition metal hydrides. It is notable, however, that even slight
perturbations in the steric properties of the Re hydride or the hydride
acceptor result in large deviations in the predicted rate of hydride
transfer based on thermodynamic driving forces. This indicates that
thermodynamic considerations alone cannot be used to predict the rate
of hydride transfer, which has implications for catalyst design.