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

Abstract Using the same trinuclear [Fe 3 O] +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 6 H 5 COO)]NO 3 ·3H 2 O ( 1 ) and [Fe 4 O 2 (C 6 H 5 COO) 7 (L2) 2 ]NO 3 ·H 2 O·CH 3 CN ( 2 ·CH 3 CN), respectively. Complex 1 crystallizes in the monoclinic space group P 2(1)/ c , while 2 ·CH 3 CN crystallizes in the monoclinic space group C 2/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 ).