Cationic Rhodium-BINAP Complexes: Full Characterization of Solvate- and Arene-Bridged Dimeric Species

In the asymmetric hydrogenation promoted by rhodium-diphosphine complexes in coordinating solvents such as methanol, the solvate complex is regarded as the catalytically active species. Here, we present for the first time the X-ray structures of rhodium-diphosphine solvate complexes [Rh((S)-BINAP)(MeOH)2]BF4, [Rh((R)-BINAP)(acetone)2]BF4, and [Rh((S)-BINAP)(THF)2]BF4. In noncoordinating solvents such as dichloromethane or dichlorethane hydrogenation of the diolefin in cationic precatalysts of the type [Rh(BINAP)(diolefin)]X (X = noncoordinating anion) results in the formation of the dimeric arene-bridged species [Rh(BINAP)]2(BF4)2. The X-ray structure of the dinuclear species reveals that each BINAP acts as both a chelating and bridging ligand that coordinates a second unsaturated rhodium center through one of the phosphorus phenyl substituents.