The influence of pseudohalide ligands on the SIM behaviour of four-coordinate benzylimidazole-containing cobalt(ii) complexes.

Three, mononuclear complexes of the formula [Co(bmim)(2)(SCN)(2)] (1), [Co(bmim)(2)(NCO)(2)] (2) and [Co(bmim)(2)(N-3)(2)] (3) [bmim = 1-benzyl-2-methylimidazole] were prepared and structurally analyzed by single-crystal X-ray crystallography. The cobalt(ii) ions in 1-3 are tetrahedrally coordinated with two bmim molecules and two pseudohalide anions. The angular distortion parameter was calculated and the SHAPE program (based on the CShM concept) was used for 1-3 to estimate the angular distortion from an ideal tetrahedron. The molecules of 1-3 are effectively separated, and the values of the shortest distance of cobalt-cobalt are 8.442(6) and 6.774(8) angstrom for 1, 10.349(8) and 10.716(8) angstrom for 2 and 6.778(1) and 9.232(1) angstrom for 3. Direct current (dc) magnetic susceptibility measurements on the crushed crystals of 1-3 were carried out in the temperature range 1.9-295 K. The variable-temperature magnetic data of 1-3 mainly obey the zero-field splitting effect (D) of the (4)A(2) ground term of the tetrahedral cobalt(ii) complexes (2D being the energy gap between the vertical bar +/- 1/2 > and vertical bar +/- 3/2 > levels of the spin). The analysis of their magnetic data through the Hamiltonian H = D[S-7(2) - S(S + 1)/3] + E(S-x(2) - S-y(2)) + gHS led to the following best-fit parameters: g = 2.29, D = -7.5 cm(-1) and E/D = 0.106 (1), g = 2.28, D = + 6.3 cm(-1) and E/D = 0.007 (2) and g = 2.36, D = + 6.7 cm(-1) and E/D = 0.090 (3). The signs of D for 1-3 were confirmed by Q-band EPR spectra on powdered samples in the temperature range 4.0-20 K. Field-induced SIM behaviour was observed for 1-3 below 4.0 K, and the frequency-dependent maxima of chi "(M) were observed for 1 and only incipient signals of chi "(M) occurred for 2 and 3. The values of the exponential factor (tau(0)) and activation energy (E-a) for 1-3 which were obtained from the Arrhenius plot suggest a single relaxation process characteristic of an Orbach mechanism.