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

Thomas M. Douglas, Simon K. Brayshaw, Romaeo Dallanegra, Gabriele Kociok-Köhn, Stuart A. Macgregor, Gemma L. Moxham, Andrew S. Weller, Tebikie Wondimagegn, Hand Prabha Vadivelu

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Abstract

[Rh(nbd)(PCyp3)2][BArF4] (1) [nbd = norbornadiene, ArF = C6H3(CF 3)2, PCyp3 = tris (cyclopentyl-phosphine)] spontaneously undergoes dehydrogenation of each PCyp3 ligand in CH2Cl2 solution to form an equilibrium mixture of cis-[Rh{PCyp2(?2-C5H7)} 2][BArF4] (2a) and trans-[Rh-{PCyp 2(?2-C5H7)}2] [BArF4] (2b), which have hybrid phosphine-alkene Iigands. In this reaction nbd acts as a sequential acceptor of hydrogen to eventually give norbornane. Complex 2b is distorted in the solid-state away from square planar. DFT calculations have been used to rationalise this distortion. Addition of H2 to 2a/b hydrogenates the phosphine-alkene ligand and forms the bisdihydrogen/dihydride complex [Rh(PCyp3)2(H) 2(?2-H2)2][BArF4] (5) which has been identified spectroscopically. Addition of the hydrogen acceptor tert-butylethene (tbe) to 5 eventually regenerates 2a/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-{PCyp2(?2-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(?6-C6H5X) {PCyp2(?2-C5H7)}][BAr F4] (7: X = F, 8: X = H). The related complex [Rh(nbd){PCyp2-(?2-C5H7)}] [BArF4] 9 is also reported. Rapid (˜5 minutes) acceptorless dehydrogenation occurs on treatment of [RhCl(dppe)(PCyp 3)] with Na[BArF4] to give [Rh(dppe){PCyp 2(?2-C5H7)}]-[BAr F4] (10), which reacts with H2 to afford the dihydride/dihydrogen complex [Rh(dppe)(PCyp3)(H)2(? 2-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(?6-C6H 5X) [PCy2(?2-C5H 7)}][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 2a/b, 4, 6, 7, 8, 9 and 17 also by single crystal X-ray diffraction. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.

Original languageEnglish
Pages (from-to)1004-1022
Number of pages19
JournalChemistry - A European Journal
Volume14
Issue number3
DOIs
Publication statusPublished - 2008

Keywords

  • C-H activation hydrides
  • Hydrogenation
  • Phosphines
  • Rhodium

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    Douglas, T. M., Brayshaw, S. K., Dallanegra, R., Kociok-Köhn, G., Macgregor, S. A., Moxham, G. L., Weller, A. S., Wondimagegn, T., & Vadivelu, H. P. (2008). Intramolecular alkyl phosphine dehydrogenation in cationic rhodium complexes of tris(cyclopentylphosphine). Chemistry - A European Journal, 14(3), 1004-1022. https://doi.org/10.1002/chem.200700954