Low-valent Homobimetallic Rh Complexes: Influence of Ligands on the Structure and the Intramolecular Reactivity of Rh–H Intermediates

Supporting two metal binding sites by a tailored polydentate trop-based (trop = 5H-dibenzo[a,d]cyclohepten-5-yl) ligand yields highly unsymmetric homobimetallic rhodium(I) complexes. Their reaction with hydrogen rapidly forms Rh hydrides that undergo an intramolecular semihydrogenation of two CC bonds of the trop ligand. This reaction is chemoselective and converts CC bonds to a bridging carbene and an olefinic ligand in the first and the second semihydrogenation steps, respectively. Stabilization by a bridging diphosphine ligand allows characterization of a Rh hydride species by advanced NMR techniques and may provide insight into possible elementary steps of H2 activation by interfacial sites of heterogeneous Rh/C catalysts.