Hybrid magnetic materials based on nitroxide free radicals and extended oxalato-bridged bimetallic networks

A series of hybrid organic-inorganic magnets of formula p-rad[(MCr)-Cr-II(ox)(3)] [M = Mn (1), Co (2), Ni (3), Zn (4)] and m-rad[(MCr)-Cr-II(ox)(3)] [M = Mn (5), Co (6)], in which N-methylpyridinium cations bearing a nitronyl nitroxide moiety in positions 3 (m-rad) or 4 (p-rad) of the pyridine ring coexist with the 2D honeycomb-like oxalato-bridged bimetallic lattice, has been prepared and studied by AC and DC magnetic susceptibility measurements and EPR spectroscopy. In general, the physical properties of these magnets are not altered significantly by the insertion of the nitronyl nitroxide radicals although these paramagnetic molecules seem to interact weakly with the inorganic network as demonstrated by EPR spectroscopy. Some differences can also be observed between the p-rad and m-rad series, i.e. m-rad derivatives have smaller values for the critical temperatures and coercive fields. We also report on the X-ray crystal structures and magnetic properties of p-rad[Mn(H2O)Cr(ox)(3)].2H(2)O (7) and m-rad[Mn(H2O)(2)Cr(OX)(3)]-2H(2)O (8), two extended oxalato-bridged compounds with new topologies. Compound 7 is antiferromagnetic and its structure is a 3D achiral lattice in which zigzag ferromagnetic MnCr chains (J/k = +0.8 K) are interconnected to form hellicoidal hexagonal channels with the cationic free radicals residing in the free space. Compound 8, however, exhibits a ladder-like structural pattern with competing magnetic interactions and paramagnetic behaviour down to low temperatures. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)