1,2-Addition of Dihydrogen across Rhodium(III)–OMe Bonds

The
Rh­(III) complexes [(tbpy)2Rh­(OMe)­(L)]­[X]n (tbpy = 4,4′-di-tert-butyl-2,2′-bipyridyl; L = MeOH, n = 2, X = OTf (OTf = trifluoromethanesulfonate), TFA (TFA = trifluoroacetate);
L = TFA, n = 1, X = OTf) have been shown to activate
dihydrogen via net 1,2-addition of the H–H bond across the
RhIII–OMe bond. The bis­(methoxide) complex [(tbpy)2Rh­(OMe)2]­[OTf] was synthesized
by addition of CsOH·H2O in methanol to [(tbpy)2Rh­(OTf)2]­[OTf] in CH3CN. The
addition of HTFA to [(tbpy)2Rh­(OMe)2]­[OTf] leads to the formation of [(tbpy)2Rh­(OMe)­(MeOH)]­[OTf]­[TFA],
which exists in equilibrium with [(tbpy)2Rh­(OMe)­(TFA)]­[OTf].
The mixture of [(tbpy)2Rh­(OMe)­(MeOH)]­[OTf]­[TFA]
and [(tbpy)2Rh­(OMe)­(TFA)]­[OTf] activates dihydrogen
at 68 °C to give methanol and [(tbpy)2Rh­(H)­(TFA)]­[OTf].
Studies indicate that the activation of dihydrogen has a first-order
dependence on the Rh­(III) methoxide complex and a dependence on hydrogen
that is between zero and first order. Combined experimental and computational
studies have led to a proposed mechanism for hydrogen activation by
[(tbpy)2Rh­(OMe)­(MeOH)]­[OTf]­[TFA] that involves
dissociation of MeOH, coordination of hydrogen, and 1,2-addition of
hydrogen across the Rh–OMe bond. DFT calculations indicate
that there is a substantial energy penalty for MeOH dissociation and
a relatively flat energy surface for subsequent hydrogen coordination
and activation.