Magnetic and electronic properties in novel terpyridine-based nitroxide complexes: Strong radical-metal interaction via a pyridyl ring

Transition metal complexes of 2-[4'- (2,2':6',2 " -terpyridyl)] -(4,4,5,5 -tetramethylimidazolinyl-3-oxide-1-oxyl) (terpy-NIT) and 2-[4'-(2,2':6',2 " -terpyridyl)]-(4,4,5,5-tetramethylimidazolinyl-1-oxyl) (terpy-IM) have been prepared. Whereas the pyridyl fragments of the free ligands are in an anti conformation, the complexes are obtained by coordination of two terpyridines in a syn conformation, forming a distorted octahedron around the metal center: [M(terpy-NIT)(2)](ClO4)(2) (M = Ni(II) 1, Zn(II) 2, Cu(II) 3) and [M(terpy-IM)(2)](ClO4)(2) (M = Ni(II) 4, Zn(II) 5). The ligands and their complexes have been characterized by FAB-MS, UV-vis, Fr-IR spectroscopies, elemental analysis, and by EPR spectroscopy and susceptibility measurements. Single-crystal X-ray diffraction have been performed on the ter t,,py-NIT ligand and on complexes 1, 4, and 5 giving following crystal data: terpy-NIT, monoclinic, P2(1/c), Z = 4, a = 14.2186(5), b = 12.9129(6), c = 11.704(1) Angstrom, beta = 108.615(4)degrees; 1, orthorhombic, P(n a 2(1)), Z = 4, a = 23.6367(6), b 8.7836(1), c = 24.2748(7) Angstrom; 4, monoclinic, P2(1), Z = 1, a = 8,738(1), b = 25.010(1), c 11.704(1) Angstrom, beta 102.849(3)degrees; 5, monoclinic, P2(1), Z = 1, a = 8.7463(2), b = 25.0833(5), c = 11.8168(3) Angstrom,beta = 102.757(3)degrees. For complexes 1 and 3, an antiferromagnetic behavior has been found and parametrized by considering a symmetric magnetic trimer, highlighting a strong intramolecular coupling between the metal and the radicals (average values 2J(M-NIT) = -19.6 K for M = Ni and -22.8 K for M = Cu). In the case of compound 4, an asymmetric magnetic trimer has been used to model the antiferromagnetic interactions (2J(Ni-IM1) = -13.0 K, 2J(Ni-IM2) = -5.6 K). The shape of the EPR spectra of complexes 2, 3, and 5 in solution indicates the intermediate exchange limit, of the order of a few mK, between the two nitroxide radicals through the pyridyl-metal-pyridyl fragment.