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

Rh­(I) and Rh­(III) complexes of tricyclopentylphosphine
(PCyp3), or its dehydrogenated variant PCyp2(η2-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 PCyp3 ligand. The complexes fac-[Rh­(κ3-P,O,P-L)­{PCyp2(η2-C5H7)}]­[BArF4] (L = DPEphos, Xantphos)
show pseudo-trigonal-bipyramidal structures in which the dehydrogenated
phosphine alkene ligand acts in a chelating manner. Addition of H2 to fac-[Rh­(κ3-P,O,P-DPEphos)­{PCyp2(η2-C5H7)}]­[BArF4] resulted in an equilibrium mixture of hydride
and hydride-dihydrogen complexes, fac-[Rh­(κ3-P,O,P-DPEphos)­(H)2(PCyp3)]­[BArF4] and [Rh­(κ2-P,P-DPEphos)­(η2-H2)­(H)2(PCyp3)]­[BArF4], in which the DPEphos acts as a hemilabile ligand.
For the more rigid Xantphos ligand two dihydride isomers, fac-[Rh­(κ3-P,O,P-Xantphos)­(H)2(PCyp3)]­[BArF4] and mer-[Rh­(κ3-P,O,P-Xantphos)­(H)2(PCyp3)]­[BArF4], 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 fac-geometry being entropic. Addition of MeCN to these hydrido
complexes results in the central oxygen atom being displaced to form
[Rh­(κ2-P,P-L)­(PCyp3)­(H)2(MeCN)]­[BArF4], while removal of H2 from the hydrido
complexes (under vacuum or on addition of a hydrogen acceptor) forms
the Rh­(I) complexes [Rh­(κ3-P,O,P-L)­(PCyp3)]­[BArF4], 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 re-form the phosphine–alkene ligands
does not occur, even under forcing conditions.