Structure and magnetic properties of a ring of four spins formed by manganese(II) and a pyridine substituted nitronyl nitroxide

A compound of formula [Mn(hfac)2NITpPy]2, where hfac = hexafluoroacetylacetonato and NITpPy = (2-(4-pyridyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide), was synthesized. The X-ray analysis showed that it crystallizes in the monoclinic P2(1)/n group with a = 10.995(2), b = 15.378(2), c = 17.990(3) angstrom, beta = 106.58(1)-degrees, and Z = 2. The manganese ions are octahedrally coordinated by the four oxygen atoms of the hfac ligands, an oxygen of a nitronyl nitroxide radical, and the pyridine nitrogen of another radical, related by the inversion center to the first one, thus yielding a dinuclear structure. The uncoordinated NO groups are at short distances from the ones translated along the a axis. The magnetic data suggested that in the four spins system the antiferromagnetic interaction between manganese and radical through the NO group dominates and that a weak ferromagnetic interaction through the pyridine might be responsible for the diamagnetic ground state. The analysis however is not unambiguous due to the presence of short contacts between radicals.