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
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 NiII 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 NiII 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 NiII for Oh over D5h symmetry. The results are discussed in relation to the structures and reactions of complexes of related multidentate ligands.