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

Using the same trinuclear [Fe3O]+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 [Fe2(L1)4(C6H5COO)]NO3·3H2O (1) and [Fe4O2(C6H5COO)7(L2)2]NO3·H2O·CH3CN (2·CH3CN), respectively. Complex 1 crystallizes in the monoclinic space group P2(1)/c, while 2·CH3CN 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 [Fe4(μ3-O)2]8+ unit comprising four FeIII centers with a “butterfly” arrangement. Each pair of iron(III) centers occupy the “body” or “hinge” and “wing-tip” sites, respectively. The Fe2IIIFe2III complex 2 undergoes two stepwise one electron reductions at E1/2 = −0.625 V and −0.11 V, while 1 displays an irreversible reduction wave at EP·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(S1·S2)). In complex 2, moderate antiferromagnetic exchange interactions occur among the four high-spin FeIII centers. The exchange coupling constant Jbb (body–body interaction) is indeterminate due to the prevailing spin frustration, but the ‘wing-body’ antiferromagnetic interaction (Jwb) is evaluated as −73(3) cm−1, using the spin Hamiltonion model H = −J1(S1·S2 + S2·S3 + S3·S4 + S4·S1) − J2(S2·S4).

Graphical abstractTwo different oxo- and carboxylato-bridged polynuclear iron(III) complexes, [Fe2(L1)4(C6H5COO)]NO3·3H2O and [Fe4O2(C6H5COO)7(L2)2]NO3·H2O·CH3CN where HL1 and HL2 are ON/NN donor bidentate ligands having a benzimidazole moiety, have been synthesized and characterized by X-ray crystallography, UV–Vis, IR spectroscopic and cyclic voltammetric techniques. Variable-temperature magnetic susceptibilities were measured in the temperature range 2–300 K, and the results indicate antiferromagnetic exchange interactions amongst the high-spin FeIII ions.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Polynuclear oxo-/carboxylato-bridged iron(III) complexes synthesized from a [Fe3O]+7core. ► In the binuclear complex, two FeIII centers are bridged by the two phenolate oxygens of the ligand. ► The tetra-nuclear entity consists of a [Fe4(μ3-O)2]8+ “bent-butterfly” unit. ► The [Fe4(μ3-O)2]8+ unit undergoes two stepwise one electron quasi-reversible reductions. ► Antiferromagnetic exchange interactions are observed between the high-spin FeIII ions.