Computational Studies Explain the Importance of Two Different Substituents on the Chelating Bis(amido) Ligand for Transfer Hydrogenation by Bifunctional Cp*Rh(III) Catalysts

A computational approach (DFT-B3PW91)
is used to address previous
experimental studies (Chem. Commun. 2009, 6801) that showed that transfer hydrogenation of a cyclic imine
by Et3N·HCO2H in dichloromethane catalyzed
by 16-electron bifunctional Cp*RhIII­(XNC6H4NX′) is faster when XNC6H4NX′ = TsNC6H4NH than when XNC6H4NX′ = HNC6H4NH or TsNC6H4NTs (Cp* = η5-C5Me5, Ts = toluenesulfonyl). The computational study also considers
the role of the formate complex observed experimentally at low temperature.
Using a model of the experimental complex in which Cp* is replaced
by Cp and Ts by benzenesulfonyl (Bs), the calculations for the systems
in gas phase reveal that dehydrogenation of formic acid generates
CpRhIIIH­(XNC6H4NX′H)
via an outer-sphere mechanism. The 16-electron Rh complex + formic
acid are shown to be at equilibrium with the formate complex, but
the latter lies outside the pathway for dehydrogenation. The calculations
reproduce the experimental observation that the transfer hydrogenation
reaction is fastest for the nonsymmetrically substituted complex CpRhIII­(XNC6H4NX′) (X = Bs and
X′ = H). The effect of the linker between the two N atoms on
the pathway is also considered. The Gibbs energy barrier for dehydrogenation
of formic acid is calculated to be much lower for CpRhIII­(XNCHPhCHPhNX′) than for CpRhIII­(XNC6H4NX′) for all combinations of X and X′.
The energy barrier for hydrogenation of the imine by the rhodium hydride
complex is much higher than the barrier for hydride transfer to the
corresponding iminium ion, in agreement with mechanisms proposed for
related systems on the basis of experimental data. Interpretation
of the results by MO and NBO analyses shows that the most reactive
catalyst for dehydrogenation of formic acid contains a localized Rh–NH
π-bond that is associated with the shortest Rh–N distance
in the corresponding 16-electron complex. The asymmetric complex CpRhIII(BsNC6H4NH) is shown to generate a
good bifunctional catalyst for transfer hydrogenation because it combines
an electrophilic metal center and a nucleophilic NH group.