Bis-o-semiquinonato nickel complexes with pyridine and pyridine modified by nitronyl-nitroxide moiety

Abstract

Four coordinated bis-o-semiquinonato nickel complexes interact with pyridine forming five- or six-coordinated compounds. Coordination number and structure of the product depends on the nature of o-quinonato ligand and synthetic conditions. (Py)2Ni(3,6-DBSQ)2·Tol has trans-arrangement of pyridine ligands whereas in it analog – (Py)2Ni(4,5-N,N′-pipe-3,6-DBSQ)2 pyridine molecules occupy cis-positions (3,6-DBSQ – anion-radical of 3,6-di-tert-butyl-o-benzoquinone; 4,5-N,N′-pipe-3,6-DBSQ – tricyclic 5,8-di(tert-butyl)-2,3-dihydro-1,4-ethanoquinoxaline-6,7-dione). The structure of six-coordinated compounds is close to octahedral which promotes high values of effective magnetic moments and the predomination of ferromagnetic exchange interactions. In spite of different geometries complexes demonstrate very close magnetic behavior. Using 1:1 stoichiometric ratio allows to obtain five-coordinated nonsolvated square-pyramidal complex (Py)Ni(3,6-DBSQ)2 with disordered position of pyridine. The last compound has essentially lower magnetic moment compared to six coordinated analogs. Complex reveals strong metal–ligand antiferromagnetic coupling due to deviation of nickel atom out of the bis-o-semiquinonato plane. Pyridine modified in 4-position by nitronyl-nitroxide radical (4-NIT-Py – 2-(4-pyridinyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) also coordinate to bis-o-semiquinonato nickel precursors forming (4-NIT-Py)2Ni(3,6-DBSQ)2 and (4-NIT-Py)2Ni(4,5-N,N′-pipe-3,6-DBSQ)2. Nickel complexes containing different types of free radical ligands were obtained for the first time. However two additional spins do not produce significant changes in their magnetic behavior.