Reactions of hydrogen or deuterium molecule with a rhodium(II) metalloradical: kinetic evidence for a four-centered transition state

Abstract

Metal-catalyzed hydrogenation of substrates involves addition of hydrogen to the metal center as an obligatory step. The importance of this class of reactions has stimulated efforts to understand the scope of mechanistic pathways operative in reactions of H{sub 2} with metal complexes. Formation of dihydrides by addition to a single metal center and monohydrides by reaction with two metal sites are both frequently observed. Concerted addition of H{sub 2} to form dihydrides through a three-centered triangular transition state has been thoroughly documented by kinetic studies, observation of {eta}{sup 2}-dihydrogen complexes, and theoretical considerations. Formation of monohydrides by addition of H{sub 2} to two metal centers is less fully investigated, but both multistep heterolytic and concerted homolytic cleavage of dihydrogen have been recognized. This article describes the reaction of H{sub 2} (D{sub 2}) with (tetramesitylporphyrinato)rhodium(II), (TMP)Rh{sm bullet}, to form (TMP)Rh-H through a four-centered transition state.