Intramolecular alkyl phosphine dehydrogenation in cationic rhodium complexes of tris(cyclopentylphosphine)

Rh(nbd)(PCyp(3))(2)]BAr(F) (4)] (1) nbd = norbornadiene, Ar(F) = C(6)H(3)(CF(3))(2), PCyp(3) = tris(cyclopentylphosphine)] spontaneously undergoes dehydrogenation of each PCyp(3) ligand in CH(2)Cl(2) solution to form an equilibrium mixture of cis-Rh{PCyp(2)(eta(2)-C(5)H(7))}(2)]BAr(F) (4)] (2 a) and trans-Rh{PCyp(2)(eta(2)-C(5)H(7))}(2)]BAr(F) (4)] (2 b), which have hybrid phosphine-alkene ligands. In this reaction nbd acts as a sequential acceptor of hydrogen to eventually give norbornane. Complex 2 b is distorted in the solid-state away from square planar. DFT calculations have been used to rationalise this distortion. Addition of H(2) to 2 a/b hydrogenates the phosphine-alkene ligand and forms the bisdihydrogen/dihydride complex Rh(PCyp(3))(2)(H)(2)(eta(2)-H(2))(2)]BAr(F) (4)] (5) which has been identified spectroscopically. Addition of the hydrogen acceptor tert-butylethene (tbe) to 5 eventually regenerates 2 a/b, passing through an intermediate which has undergone dehydrogenation of only one PCyp(3) ligand, which can be trapped by addition of MeCN to form trans-Rh{PCyp(2)(eta(2)-C(5)H(7))}(PCyp(3))(NCMe)]BAr(F) (4)] (6). Dehydrogenation of a PCyp(3) ligand also occurs on addition of NaBAr(F) (4)] to RhCl(nbd)(PCyp(3))] in presence of arene (benzene, fluorobenzene) to give Rh(eta(6)-C(6)H(5)X){PCyp(2)(eta(2)-C(5)H(7))}]BAr(F) (4)] (7: X = F, 8: X = H). The related complex Rh(nbd){PCyp(2)(eta(2)-C(5)H(7))}]BAr(F) (4)] 9 is also reported. Rapid ( approximately 5 minutes) acceptorless dehydrogenation occurs on treatment of RhCl(dppe)(PCyp(3))] with NaBAr(F) (4)] to give Rh(dppe){PCyp(2)(eta(2)-C(5)H(7))}]BAr(F) (4)] (10), which reacts with H(2) to afford the dihydride/dihydrogen complex Rh(dppe)(PCyp(3))(H)(2)(eta(2)-H(2))]BAr(F) (4)] (11). Competition experiments using the new mixed alkyl phosphine ligand PCy(2)(Cyp) show that RhCl(nbd){PCy(2)(Cyp)}] undergoes dehydrogenation exclusively at the cyclopentyl group to give Rh(eta(6)-C(6)H(5)X){PCy(2)(eta(2)-C(5)H(7))}]BAr(F) (4)] (17: X = F, 18: X = H). The underlying reasons behind this preference have been probed using DFT calculations. All the complexes have been characterised by multinuclear NMR spectroscopy, and for 2 a/b, 4, 6, 7, 8, 9 and 17 also by single crystal X-ray diffraction.