Syntheses, crystal structure, spectroscopic, redox and magnetic properties of oxo- and carboxylato-bridged polynuclear iron(III) complexes with phenolate- and pyridine-substituted benzimidazole ligands

摘要

Using the same trinuclear [Fe _3O] ~(+7) core having oxo and carboxylate bridges, two different types of polynuclear iron(III) complexes have been synthesized. The bidentate ligands, (2-hydroxyphenyl)benzimidazole (HL1) and (2-pyridyl)benzimidazole (L2), produce the dinuclear and tetranuclear complexes [Fe _2(L1) _4(C _6H _5COO)]NO _3·3H _2O (1) and [Fe _4O _2(C _6H _5COO) _7(L2) _2]NO _3· H _2O·CH _3CN (2·CH _3CN), respectively. Complex 1 crystallizes in the monoclinic space group P2(1)/c, while 2·CH _3CN crystallizes in the monoclinic space group C2/c. In 1, the two hexa-coordinated iron(III) centers are bridged by the two phenolate oxygens of the ligand HL1. The tetranuclear entity 2 consists of a [Fe _4(μ _3~(-O)) _2] ~(8+) unit comprising four Fe ~(III) centers with a "butterfly" arrangement. Each pair of iron(III) centers occupy the "body" or "hinge" and "wing-tip" sites, respectively. The Fe _2 ~(III)Fe _2 ~(III) complex 2 undergoes two stepwise one electron reductions at E _(1/2) = -0.625 V and -0.11 V, while 1 displays an irreversible reduction wave at E _(P·C) = -0.3 V. Variable-temperature magnetic susceptibility measurements have been carried out for 1 and 2 in the temperature range 2-300 K. Complex 1 exhibits a very weak Fe?Fe exchange interaction, J = -0.24(1) cm ~(-1) (H = -J(S _1·S _2)). In complex 2, moderate antiferromagnetic exchange interactions occur among the four high-spin Fe ~(III) centers. The exchange coupling constant J _(bb) (body-body interaction) is indeterminate due to the prevailing spin frustration, but the 'wing-body' antiferromagnetic interaction (J _(wb)) is evaluated as -73(3) cm ~(-1), using the spin Hamiltonion model H = -J _1(S _1·S _2 + S _2·S _3 + S _3·S _4 + S _4·S _1) - J _2(S _2·S _4).