Magnetic properties and electronic structure of five- and six-coordinate manganese(II)2,6-bis(benzimidazol-2-yl)pyridine complexes

Two manganese(II) complexes with the terdentate ligand 2,6-bis(benzimidazol-2-yl)pyridine, namely [Mn(bzimpy)(2)(ClO4)(2)], 1 and [Mn(bzimpy)Cl-2]. 0.5MeOH, 2 have been investigated by magnetic susceptibility measurements in the temperature range 18 to 300 K. Electron spin resonance has been measured in solid state and frozen solution at 77 K. Parameters for the magnetic contributions to the Hamiltonian obtained by these techniques confirm axial magnetic anisotropy. ACS data are: g(av) = 1.92 +/- 0.04, \D/hc\ = 0.9 +/- 1.3 cm(-1) and g(av) = 1.89, \D/hc\ = 1.4 cm(-1) for 1 and 2, respectively. The three found ESR resonances correspond to g(eff)(1) = 1.99, g(eff)(2) = 3.3 and g(eff)(3) = 4.3. The highest field resonance exhibits a hyperfine sextet splitting of \A(av)/hc\ = 92 x 10(-4) cm(-1) for both complexes. Well resolved forbidden transitions allow for an estimation of the zero-field splitting parameter being D/hc = -0.09 and 0.13 cm(-1) for the two complexes, respectively. Ab initio MO-LCAO-SCF calculations on a double-zeta basis set indicate that complex 2 should be more stable than 1. The calculations also yielded information on electronic structure, bond-strengths and charge distribution within the complexes.