1,2-Addition of dihydrogen across rhodium(III)-OMe bonds

Samantha A Burgess; Deepa Devarajan; Tamara Bolaño; Daniel H Ess; T Brent Gunnoe; Michal Sabat; William H Myers

doi:10.1021/ic500636m

1,2-Addition of dihydrogen across rhodium(III)-OMe bonds

Inorg Chem. 2014 May 19;53(10):5328-40. doi: 10.1021/ic500636m. Epub 2014 May 7.

Authors

Samantha A Burgess¹, Deepa Devarajan, Tamara Bolaño, Daniel H Ess, T Brent Gunnoe, Michal Sabat, William H Myers

Affiliation

¹ Department of Chemistry, University of Virginia , Charlottesville, Virginia 22904, United States.

PMID: 24806731
DOI: 10.1021/ic500636m

Abstract

The Rh(III) complexes [((t)bpy)2Rh(OMe)(L)][X]n ((t)bpy = 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 Rh(III)-OMe bond. The bis(methoxide) complex [((t)bpy)2Rh(OMe)2][OTf] was synthesized by addition of CsOH·H2O in methanol to [((t)bpy)2Rh(OTf)2][OTf] in CH3CN. The addition of HTFA to [((t)bpy)2Rh(OMe)2][OTf] leads to the formation of [((t)bpy)2Rh(OMe)(MeOH)][OTf][TFA], which exists in equilibrium with [((t)bpy)2Rh(OMe)(TFA)][OTf]. The mixture of [((t)bpy)2Rh(OMe)(MeOH)][OTf][TFA] and [((t)bpy)2Rh(OMe)(TFA)][OTf] activates dihydrogen at 68 °C to give methanol and [((t)bpy)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 [((t)bpy)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.