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Douglas TM, Brayshaw SK, Dallanegra R, Kociok-Kohn G, Macgregor SA, Moxham GL, Weller AS, Wondimagegn T, Vadivelu P: Intramolecular alkyl phosphine dehydrogenation in cationic rhodium complexes of tris (cyclopentylphosphine). Chemistry. 2008;14(3):1004-22.
[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 Na [BAr (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 Na [BAr (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. |
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