Nickel(II) complexes of the isomeric tetraazamacrocyclic ligands 1,11- and 1,8-bis(2-pyridylmethyl)-cyclam and of a structurally constrained N4, N8-methylene bridged analogue

Abstract

The nickel(II) complexes of 1,8-bis(2′-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane (L 1) and of the isomeric ligand 1,11-bis(2′-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane (L 2) have been isolated. The X-ray crystal structure of a mixed acetate/chloride salt of [NiL 1] 2+ revealed an octahedral Ni coordination by two N atoms of the pyridine groups (in cis-related positions) and four of the macrocycle, which is folded along the 1,8-direction and bound in the cis-V configuration. The results of UV–Visible absorption spectroscopy and cyclic voltammetry in solution and the effective magnetic moments in the solid state are also consistent with octahedral N 6-coordination of high-spin nickel(II). The intermediate in the synthesis of ligand L 2, a bis-(2-pyridylmethyl) substituted formamidinium cation (L 3) +, reacted with nickel(II) perchlorate to form a [Ni(H 2L 3)Cl] 2+ complex incorporating a methylene bridge between the non-pyridylmethylated N atoms ( N 4 and N 8) of the macrocycle, which was characterised by X-ray crystallography. The Ni atom has a fac-octahedral coordination by the two pyridine and three macrocyclic nitrogens (the fourth one, N 4, is protonated) and one Cl atom.