Syntheses of new P–N ligands containing an imidazolyl group and their co-ordination behaviors toward nickel(II), cobalt-(II) and -(III)

Two new bidentate imidazolylphosphine P–N chelate ligands 2-(diphenylphosphinomethyl)-1-methylimidazole (PN^I) and 2-(diisopropylphosphinomethyl)-1-methylimidazole (PN^II) have been synthesized in good yields by treating R₂PCl (R = Ph or Prⁱ) with 1-methyl-2-trimethylsilylmethylimidazole. Various types of nickel(II), cobalt-(II) and -(III) complexes have been prepared with these two ligands and characterized by physico-chemical techniques. The structures of PN^I·HBr and two nickel(II) complexes, [Ni(PN^I)₂][BF₄]₂ 2 and [Ni(PN^II)₂][BF₄]₂·MeCN 4·MeCN, have been studied by single crystal X-ray analyses. Both take planar structures with different configurations. In 2 the two phosphorus atoms are in cis position whereas in 4·MeCN they are trans to each other. The complex [NiCl₂(PN^I)] 1 adopts a five-co-ordinate dimeric structure with chloride bridges in the solid state and a monomeric square-planar structure, [Ni(PN^I)(solv)₂]Cl₂, in aqueous and in methanolic solutions. However, complex [NiCl₂(PN^II)] 3 is diamagnetic and takes square-planar geometry around the metal ions both in the solid and solution states. The structures of cobalt(II), [CoX₂(L)] (where X = Cl or Br), and cobalt(III) complexes, [Co(acac)₂(L)]ClO₄ (where acac = acetylacetonate, L = PN^I or PN^II), are tetrahedral and octahedral, respectively. A variable temperature ¹H and ³¹P NMR study of complexes 1 and 2 demonstrated the presence of dynamic motion of the PN-chelate ring(s). On the other hand, in the cobalt(III) complexes [Co(acac)₂(PN^I)]ClO₄ and [Co(acac)₂(PN^II)]ClO₄ two protons of the backbone methylene group of the PN ligands are magnetically inequivalent.