Synthesis, Structure, and Magnetic Properties of Transition Metal Complexes of the Nitroxide 2,5-Dihydro-4,5,5-trimethyl-2,2-bis(2-pyridyl)imidazole-1-oxyl

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Abstract
With the radical 2,5-dihydro-4,5,5-trimethyl-2,2-bis(2-pyridyl)imidazole-1-oxyl (L) a series of transition metal complexes have been prepared: [ML2](SbF6)2 with M2+ = Mn2+ (1), Fe2+ (2), Co2+ (3), Ni2+ (4), Cu2+ (5), and Zn2+ (6), Cu(L)(Cl)2(MeOH) (7), and Cu(L)SO4·H2O (8). The structures of 1, 3, and 6 were determined by X-ray structural analyses. In these compounds the tridendate L is coordinated to the metal ion by the two pyridine nitrogen donors and by the oxygen atom of the nitroxide group. The N−O bond distances are 1.25(2) Å (1), 1.267(13) Å (3), and 1.260(11) Å (6). The M−O−N angles are 117.0(10)° (1), 114.5(8)° (3), and 114.2(7)° (6). Crystal data: space group P21/n, for 1, 3, and 6; compound 1, a = 10.806(3) Å, b = 14.101(6) Å, c = 14.253(4) Å, β = 108.82(2)°, V = 2055.7(12) Å3, Z = 2, R1 = 0.0677, wR2 = 0.1512. Compound 3, a = 10.761(4) Å, b = 14.253(6) Å, c = 14.108(5) Å, β = 111.16(3)°, V = 2017.9(13) Å3, Z = 2, R1 = 0.0702, wR2 = 0.1460; compound 6, a = 10.788(2) Å, b = 14.147(3) Å, c = 14.196(3) Å, β = 109.93(3)°, V = 2036.8(7) Å3, Z = 2, R1 = 0.0573, wR2 = 0.1194. Magnetic measurements of 1, 2, 5, and 8 show strong antiferromagnetic interaction between the spin of the metal ion and the spin of the radical which increases at lower temperatures. For 6 the magnetic moment corresponds to two noninteracting spins in the temperature range 60−300 K.

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