Intramolecular ligand rearrangement in the isomerisation of a seven-co-ordinate to a six-co-ordinate nickel(II) complex of a multidentate Schiff base. The crystal and molecular structures of the six-co-ordinate product

Abstract

The pentagonal-bipyramidal nickel(II) complex [NiL¹][BPh₄]₂, containing the heptadentate ligand L¹ derived from the condensation of one molecule of 2,6-diacetylpyridine with two molecules of diethylenetriamine, isomerises in acetonitrile solution to the distorted octahedral complex [NiL²][BPh₄]₂·MeCN of an isomeric hexadentate ligand L². Crystals of [NiL²][BPh₄]₂·MeCN are triclinic with a= 11.738(10), b= 14.219(12), c= 18.865(14)Å, α= 83.0(1), β= 109.7(1), γ= 89.2(1)°, Z= 2, space group P. 2 362 Reflections above background were measured by diffractometer and refined by full-matrix least squares to R 0.093. The Ni^II ion is bound to six nitrogen atoms of the rearranged ligand L², derived from L¹ by nucleophilic addition of one secondary amine group across the neighbouring azomethine bond with concomitant formation of a five-membered imidazolidine ring. The coordination geometry of the Ni^II ion is approximately octahedral with the Ni–N bonds 2.003(20)–2.319(18)Å. The thermodynamic driving force for the ligand isomerisation is attributed to the stereochemical preference of Ni^II for O_h over D_5h symmetry. The results are discussed in relation to the structures and reactions of complexes of related multidentate ligands.