Rhodium Cyclopentyl Phosphine Complexes of Wide-Bite-Angle Ligands DPEphos and Xantphos

Rh(I) and Rh(III) complexes of tricyclopentylphosphine (PCyp(3)), or its dehydrogenated variant PCyp(2)(eta(2)-C5H7), partnered with wide-bite-angle chelating diphosphine ligands DPEphos and Xantphos have been prepared and characterized in solution and the solid state with the aim of studying their potential for reversible dehydrogenation of the PCyp(3) ligand. The complexes fac-[Rh(kappa(3)-P,O,P-L){PCyp2(eta(2)-C5H7)}][BAr4F] (L = DPEphos, Xantphos) show pseudo-trigonal-bipyramidal structures in which the dehydrogenated phosphine alkene ligand acts in a chelating manner. Addition of H-2 to fac-[Rh(id-P,O,P-DPEphos){PCyp2(eta(2)-C5H7)}][BAr4F] resulted in an equilibrium mixture of hydride and hydride-dihydrogen complexes, fac-[Rh(kappa(3)-P,O,P-DPEphos)(H)(2)(PCyp(3))][BAr4F] and [Rh(kappa(2)-P,P-DPEphos)(eta(2)-H-2)(H)(2)(PCyp(3))][BAr4F], in which the DPEphos acts as a hemilabile ligand. For the more rigid Xantphos ligand two dihydride isomers, fac-[Rh(kappa(3)-P,O,P-Xantphos)(H)(2)(PCyp(3))][BAr4F] and mer-[Rh(kappa(3)-P,O,P-Xantphos)(H)(2)(PCyp(3))][BAr4F], are formed, which are also in equilibrium with one another. A van't Hoff analysis of this mixture shows that enthalpically there is very little difference between the two geometries for this system, with the driving force for the preferred far-geometry being entropic. Addition of MeCN to these hydrido complexes results in the central oxygen atom being displaced to form [Rh(kappa(2)-P,P-L)(PCyp(3))(H)(2)(MeCN)][BAr4F], while removal of H-2 from the hydrido complexes (under vacuum or on addition of a hydrogen acceptor) forms the Rh(I) complexes [Rh(kappa(3)-P,O,P-L)(PCyp(3))][BAr4F], which are characterized as having square-planar geometries with meridonial coordination of the respective chelating phosphines. Dehydrogenation of the PCyp3 ligand in these complexes to reform the phosphine-alkene ligands does not occur, even under forcing conditions.