Synthesis and characterization of [Rh(PP)(PP)]X complexes (PP = DPPE or DPPP, X = Cl− or BF4-). Phosphine exchange and reactivity in transfer hydrogenation conditions

The synthesis of heteroleptic monomeric cationic Rh(I) bis-diphosphine complex [Rh(dppe)(dppp)]Cl (dppe = 1,2-bis-(diphenylphosphino)ethane and dppp = 1,3-bis-(diphenylphosphino)propane) was achieved by reaction between the neutral dimeric complex [Rh 2 (dppe) 2 (μ 2 -Cl) 2 ] and the dppp ligand. The corresponding complex [Rh(dppe)(dppp)]BF 4 was instead obtained from reaction between [Rh(dppe)(nbd)]BF 4 (nbd = bicyclo-[2.2.1]-hepta-2,5-diene) and the dppp ligand. Previous protocols had extended to the synthesis of the analogous homoleptic complexes containing two dppe or two dppp ligands. The structural analysis of such complexes conducted in the solid state by single-crystal x-ray crystallography and in solution by 31 P and 1 H NMR revealed a distorted square planar geometry where the anion and the cation are not bonded each other. Nevertheless, important differences in reactivity can be found if same complexes with different anions are compared, as in the case of the phosphine exchange reaction between [Rh(dppp) 2 ]X (X = Cl − , BF 4 − ) and the dppe ligand or in the case of the employment of such complexes as catalytic precursors for the transfer hydrogenation reaction from 2-propanol to acetophenone.