Self-assembled meso-helicates of linear trinuclear nickel(ii)–radical complexes with triple pyrazolate bridges

Abstract

Complexation of 3-nitronyl-nitroxide-substituted pyrazolate (pzNN) and 3-imino-nitroxide-substituted pyrazolate (pzIN) with nickel(II) gave [Ni₃(pzNN)₆] and [Ni₃(pzIN)₆], respectively. They were practically isomorphous and characterized as a linear trinuclear structure with neighboring nickel ions triply bridged by pyrazolate moieties. The space groups were P2₁/n but the molecules have a pseudo three-fold axis. We found polymorphs in the crystals of [Ni₃(pzIN)₆] depending on the solvate molecules; another space group was a cubic Pa. The opposite chirality around the inversion center at the central nickel(II) ion leads to a meso-helical symmetry in the whole molecule. The radical oxygen atoms participate in the 6-membered chelation at the terminal nickel(II) ions. Antiferromagnetic couplings were observed in both complexes, which are ascribable to interactions between the nickel and radical spins and between the nickel spins across the pyrazolate bridges.