Dehydrogenation of the alkane chain of 1,3-bis[2-(diphenylphosphino)phenyl]-propane on a rhodium(I) centre. Complexes of the resulting olefinic ligand with rhodium(I), iridium(I), rhodium(III) and nickel(II)

Abstract

The title compound, o-Ph₂PC₆H₄(CH₂)₃C₆H₄PPh₂-o (5), has been synthesized in three steps, starting with the Fe₂(CO)₉-promoted conversion of 2- bromobenzyl bromide into 1,3-di(2-bromo-phenyl)propan-2-one, o- BrC₆H₄CH₂COCH₂C₆H₄Br-o. Compound (5) undergoes dehydrogenation on reaction with the cycloocta-1,5-diene (cod) complex [RhCl(cod)]₂ to give a rhodium(1) complex [RhCl((E)-o-Ph₂PC₆H₄CH=CHCH₂C₆H₄PPh₂-o)] (9), in which the olefin and mutually trans phosphorus atoms are bound to the metal. The tridentate ligand (E)-o-Ph₂PC₆H₄CH=CHCH₂C_6- H₄PPh_2-o (4) can be liberated from (9) by treatment with sodium cyanide, the overall yield based on 2-bromobenzyl bromide being about 17 %. On reaction with the chlorides of rhodium(III) or nickel(II), (4) loses a methylene proton to give chelate η³-allylic complexes RhCl₂(o-Ph₂PC₆H₄CH=CH=CHC_6- H₄PPh₂-o) (11) and NiCl(o-Ph₂PC₆H₄CH=CH=CHC₆H₄PPh_2-o) (13) respectively. The proposed structures for the new complexes are based on i.r. and N.M.R. (¹H, ³¹P) data, and also on a comparison with similar complexes formed by the known tridentate ligand (E)-o-Ph₂PC₆H₄CH=CHCHMeC_6- H₄PPh_2-o (2). Conformational isomers of the rhodium(I) complex (9) and its iridium(I) analogue (10) formed from ligand (4) cannot be detected by n.m.r, spectroscopy, in contrast with the corresponding complexes formed by ligand (2).