Elucidation of Marcus Relationships for Hydride Transfer Reactions Involving Transition Metal Hydrides

The rate of hydride transfer from three Ir hydride complexes
of
the type Cp*Ir(Rbpy)H+ (Cp* = C5Me5; Rbpy = 4,4′-R-2,2′-bipyridine,
R = OMe, H, CO2Me) to six N-methylacridinium
(RAcr+) acceptors with electronically different
substituents in the 2- or 2,7-positions were measured. Using the thermodynamic
hydricity of the donors and the hydride affinity of the acceptors
the thermodynamic driving forces for hydride transfer were determined.
Brønsted plots, which correlate kinetic and thermodynamic hydricity,
demonstrate distinct linear free energy relationships for each complex,
with different Brønsted α values. Thus, at the same driving
force hydride transfer from Cp*Ir(OMebpy)H+ is
faster than for Cp*Ir(bpy)H+ or Cp*Ir(CO2Mebpy)H+. Experimental and computational analyses are consistent with
a concerted hydride transfer mechanism for all Ir complexes. As the
thermodynamic driving force increases an earlier transition state
is observed and all transition states also include π-stacking
interactions between the donor and acceptor, which likely contribute
to the different α values. The experimental data fits well to
the Marcus model, enabling the determination of reorganization energies
(λ) that range from 58 to 69 kcal mol–1. These
are lower than λ values for hydride transfer reactions involving
organic donors and acceptors. This work provides a rare example of
the correlation of kinetic and thermodynamic hydricity using only
experimental data and shows that hydride transfer reactions involving
metal hydrides can follow Marcus theory. The findings offer insight
into controlling metal-catalyzed hydride transfer reactions, which
is valuable for designing improved systems for a range of transformations.