New Pentacoordinated Rhodium Species as Unexpected Products during the In Situ Generation of Dimeric Diphosphine-Rhodium Neutral Catalysts

Dimeric rhodium complexes of the type [Rh(PP)((2)-Cl)](2) (PP=diphosphine) are often used as precatalysts and are generated in situ from the corresponding diolefin complexes by exchange of the diene with the desired diphosphine. Herein, we report that the in situ procedure also leads to unexpected monomeric pentacoordinated neutral complexes of the type [RhCl(PP)(diolefin)], for the first time herein characterized by NMR spectroscopy and X-ray crystallography for the ligands 1,4-bis(diphenylphosphino)propane (DPPP), 1,4-bis(diphenylphosphino)butane (DPPB), and 2,2-bis(diphenylphosphino)-1,1-binaphthyl (BINAP). The pentacoordinated complexes are in equilibrium with the dimeric target compound [Rh(PP)((2)-Cl)](2). The equilibrium is influenced by the rhodium-diolefin precursor, the solvent and the temperature. Based on the results of NMR and UV/Vis spectroscopic analysis (kinetics) it could be shown that the pentacoordinated complex [RhCl(PP)(diolefin)] may arise both from the in situ-generated neutral complex [Rh(PP)((2)-Cl)] by reaction with the free diolefin and, more surprisingly, directly from [Rh(diolefin)((2)-Cl)](2) and the diphosphine.