Limiting nuclearity in formation of polynuclear metal complexes through [2 + 3] cycloaddition: synthesis and magnetic properties of tri- and pentanuclear metal complexes

Abstract

A tridentate ligand p-chloro-2-{(2-(dimethylamino)ethylimino)methyl}phenol (HL) was used to generate an octahedral nickel complex [Ni(L)Cl(H₂O)₂] 1 which was further converted into a square-planar nickel complex [Ni(L)(N₃)] 2. The [2 + 3] cycloaddition reaction between metal coordinated azide 2 and different organonitriles under microwave irradiation afforded tri- and pentanuclear nickel(II) complexes 4a–4c. Reaction with benzonitrile and 3-cyano pyridine furnished the trinuclear species [Ni₃L₂(5-phenyltetrazolato)₄(DMF)₂] 4a and [Ni₃L₂{5-(3-pyridyl)-tetrazolato}₄(DMF)₂]·2H₂O 4b, respectively. The nickel centers were found to be linearly disposed to each other and the complex is formed by a 2,3-tetrazolate bridge and a phenoxo bridge between central and terminal nickel atoms. Compound 2 when treated with 1,2-dicyanobenzene under identical conditions furnished a pentanuclear complex [Ni₅L₄{5-(2-cyanophenyl)-tetrazolato}₄(OH)₂(H₂O)₂]·3H₂O·DMF 4c. In this pentanuclear compound two dimeric nickel units are connected to the central nickel center by a μ₃-hydroxo bridge and a tetrazolate ligand operating via a relatively rare 1,2,3-bridging mode. The compounds were characterized by IR, elemental analysis, thermogravimetric analysis and single crystal X-ray crystallography. The magnetic susceptibility data for compounds 4a–4c show dominant antiferromagnetic interactions between the nickel centers for all the complexes. DFT calculations were performed to investigate the magnetic parameter in one of the complexes 4b by a broken symmetry approach.