Pyridine-base complexes of the tetrachlorophosphonium ion

Abstract

Complex formation between the tetrachlorophosphonium ion, usually in the form of [PCl₄][SbCl₆], and some pyridine bases has been investigated in solution by means of ³¹ P n.m.r. spectroscopy. Bidentate pyridines L′ form stable 1 : 1 ionic complexes [PCl₄L′][SbCl₆]. Sufficiently basic and sterically unhindered unidentate pyridines L form 2 : 1 complexes [PCl₄L₂][SbCl₆], but these take part in a solution equilibrium with the molecular species PCl₅·L and SbCl₅·L. Unusual behaviour was observed with 3-cyano- and 2-cyano-pyridine, the latter probably forming both ionic and molecular complexes co-ordinated via the cyano-group. 2-Substituted methylpyridines undergo halogenation, with simultaneous formatjon of PCl₃. Very weak or sterically hindered bases other than 2-cyanopyridine appear not to take part in complex formation. Several of the complexes have been isolated as solids, in which state they are comparatively stable, sometimes showing remarkable resistance to hydrolysis. The structures have been confirmed by analysis and ³¹P n.m.r., i.r., and ³⁵Cl n.q.r. spectroscopy. A cis configuration is suggested for some of the [PCl₄L₂]⁺ ions from the n.q.r. data, although the structure of the pyridine complex itself remains uncertain.