Metal Complexes of a Picolinate-Based Nitronyl Nitroxide Free Radical

A nitronyl nitroxide free radical containing an appended picolinate moiety was synthesized. The resulting tridentate ligand picNN forms neutral mononuclear metal complexes of formula [M(picNN)(2)]center dot 3H(2)O (M = Mn, Co, Ni, Zn). These compounds are isostructural and crystallize in the orthorhombic Pnna space group. The metal complexes have a C-2 symmetric structure, with the metal centers lying on the binary axis and surrounded by two equivalent picNN radicals. The magnetic properties of this family of compounds indicate the presence of very strong metal-radical exchange interactions, ranging from J(Ni-rad) = -193 cm(-1) to J(Mn-rad) = -98 cm(-1). Relatively weak (J(rad-rad) = -15 cm(-1)) through-space magnetic interactions between free radicals coordinated to the same metal center were also evidenced in the study of the diamagnetic zinc(II) complex. Complexation with Cu2+ affords the carboxylate-bridged tetranuclear copper(II) complex [Cu-4(picNN)(4)(H2O)(4)](ClO4)(4) center dot 4H(2)O having eight interacting S = 1/2 spins in a cyclic topology. The antiferromagnetic copper-radical exchange interaction (J(Cu-rad) = -268 cm(-1)) is one of the largest ever reported.