Verdazyl radicals as oligopyridine mimics: Structures and magnetic properties of M(II) complexes of 1,5-Dimethyl-3-(2,2'-bipyridin-6-yl)-6-oxoverdazyI (M = Mn, Ni, Cu, Zn)

The verdazyl radical 1,5-dimethyl-3-(2,2'-bipyridin-6-yl)-6-oxoverdazyl (3) was prepared, and its homoleptic metal complexes M(3)(2)(2+).2X(-) (5, M = Mn(II); 6, M = Ni(II); 7, M = Cu(II); 8, M = Zn(II); X = CIO4, PF6) were characterized by single-crystal X-ray diffraction and variable-temperature magnetic susceptibility measurements. Relevant crystallographic parameters are as follows: 5, monoclinic space group Pna2(1), a = 18.755(4) Angstrom, b = 11.154(3) Angstrom, c = 16,594(4) Angstrom, alpha = 90.00degrees, beta = 90.00degrees, gamma = 90.00degrees, V = 3471.4(13) Angstrom(3), and Z = 4; 7, triclinic space group P1, a = 9.4638(18) Angstrom, b 9.8442(19) Angstrom, c 18.769(4) Angstrom, alpha = 103.746(3)degrees, beta = 92.925(3)degrees, gamma = 94.869(3)degrees, V = 1687.8(6) Angstrom(3), and Z = 2; 8, triclinic space group P1, a = 9.4858(14) Angstrom, b = 9.7919(14) Angstrom, c = 18.889(3) Angstrom, alpha = 104.196(3)degrees, beta = 92.855(3)degrees, gamma = 94.216(3)degrees, V = 1692.1(4) Angstrom(3), and Z = 2. In all cases, the two verdazyl-based ligands bind almost perpendicular to each other in meridional positions, yielding pseucdooctahedral metal complexes whose general structural features are strongly reminiscent of metal bis(terpyridine) complexes. The intramolecular metal-verdazyl magnetic exchange coupling is strongly ferromagnetic in 6 (J(Ni-vd) = +240 cm(-1)), and strongly antiferromagnetic in 5 (J(Mn-vd) = -93 cm(-1)). Complex 7 exhibits weak ferromagnetic coupling (J(Cu-vd) = -4.5 cm(-1)). Intramolecular radical-radical coupling in the zinc complex 8 was found to be weakly antiferromagnetic (J(vd-vd) = -8 cm(-1)). Intramolecular radical-radical exchange was generally weak in the four metal complexes, ranging from -10 cm(-1) (for 5) to +2 cm(-1) (for 7). The low-temperature magnetic behavior of 7 and 8 is complex, possibly arising from a combination of intra- and intermolecular interactions.