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Intramolecular Alkyl Phosphine Dehydrogenation in Cationic Rhodium Complexes of Tris(cyclopentylphosphine)



[Rh(nbd)(PCyp3)2][BArF4] (1) [nbd = norbornadiene, ArF = C6H3(CF3)2, PCyp3 = tris(cyclopentylphosphine)] spontaneously undergoes dehydrogenation of each PCyp3 ligand in CH2Cl2 solution to form an equilibrium mixture of cis-[Rh{PCyp22-C5H7)}2][BArF4] (2 a) and trans-[Rh{PCyp22-C5H7)}2][BArF4] (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 H2 to 2 a/b hydrogenates the phosphine–alkene ligand and forms the bisdihydrogen/dihydride complex [Rh(PCyp3)2(H)22-H2)2][BArF4] (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 PCyp3 ligand, which can be trapped by addition of MeCN to form trans-[Rh{PCyp22-C5H7)}(PCyp3)(NCMe)][BArF4] (6). Dehydrogenation of a PCyp3 ligand also occurs on addition of Na[BArF4] to [RhCl(nbd)(PCyp3)] in presence of arene (benzene, fluorobenzene) to give [Rh(η6-C6H5X){PCyp22-C5H7)}][BArF4] (7: X = F, 8: X = H). The related complex [Rh(nbd){PCyp22-C5H7)}][BArF4] 9 is also reported. Rapid (≈5 minutes) acceptorless dehydrogenation occurs on treatment of [RhCl(dppe)(PCyp3)] with Na[BArF4] to give [Rh(dppe){PCyp22-C5H7)}][BArF4] (10), which reacts with H2 to afford the dihydride/dihydrogen complex [Rh(dppe)(PCyp3)(H)22-H2)][BArF4] (11). Competition experiments using the new mixed alkyl phosphine ligand PCy2(Cyp) show that [RhCl(nbd){PCy2(Cyp)}] undergoes dehydrogenation exclusively at the cyclopentyl group to give [Rh(η6-C6H5X){PCy22-C5H7)}][BArF4] (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.

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