Molecular Metamagnet [Ni(4ImNNH)2(NO3)2] (4ImNNH = 4-Imidazolyl Nitronyl Nitroxide) and the Related Compounds Showing Supramolecular H-Bonding Interactions

A new chelating radical ligand 4ImNNH (2-(4-imidazolyl)-4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide) was prepared, and complexation with divalent transition metal salts gave complexes, [M(4ImNNH)2X2], which showed intermolecular ferromagnetic interaction in high probability (7 out of 10 paramagnetic compounds investigated here). The nitrate complexes (X = NO3; M = Mn (1), Co (2), Ni (3), Cu (4)) crystallize isomorphously in monoclinic space group P21/a. The equatorial positions are occupied with two 4ImNNH chelates and the nitrate oxygen atoms are located at the axial positions. Magnetic measurements revealed that the intramolecular exchange couplings in 1, 2, and 4 were antiferromagnetic, while that in 3 was ferromagnetic with 2J/kB = +85 K, where the spin Hamiltonian is defined as H = -2J(S1·S2 + S2·S3) based on the molecular structures determined as the linear radical-metal-radical triads. The intramolecular ferromagnetic interaction in 3 is interpreted in terms of orthogonality between the radical (pi)* and metal d (sigma) orbitals. Compounds 1-3 exhibited intermolecular ferromagnetic interaction ascribable to a two-dimensional hydrogen bond network parallel to the crystallographic ab plane. Complex 3 became an antiferromagnet below 3.4 K and exhibited a metamagnetic transition on applying a magnetic field of 5.5 kOe at 1.8 K. The complexes prepared from metal halides, [M(4ImNNH)2X2] (X = Cl, Br; M = Mn, Co, Ni, Cu), showed intramolecular antiferromagnetic interactions, which are successfully analyzed based on the radical-metal-radical system. The crystal structures determined here on 1-4, [Mn(4ImNNH)2Cl2], and [Cu(4ImNNH)2Br2] always have intermolecular hydrogen bonds of H (imidazole)···X(axial ligand)-M, where X = NO3, Cl, Br. This interaction seems to play an important role in molecular packing and presumably also in magnetic coupling.