Novel Square Arrangements in Tetranuclear and Octanuclear Iron(III) Complexes with Asymmetric Iron Environments Created by the Unsymmetric Bridging Ligand <i>N</i>,<i>N</i>,<i>N</i>‘-Tris((<i>N</i>-methyl)-2-benzimidazolylmethyl)-<i>N</i>‘-methyl-1,3-diamino-2-propanol

The synthesis and characterization of novel tetranuclear and octanuclear iron(III) complexes with structures based on a nearly square arrangement of four iron ions are reported. Reaction of ferric nitrate, sodium acetate, and the unsymmetrical binucleating ligand HBMDP, where HBMDP is N,N,N‘-tris((N-methyl)-2-benzimidazolylmethyl)-N‘-methyl-1,3-diamino-2-propanol, in acetone/water yields the tetranuclear iron complex [Fe4(μ-O)2(μ-BMDP)2(μ-OAc)2]4+, which exhibits coordination number asymmetry. The structure of [Fe4(μ-O)2(μ-BMDP)2(μ-OAc)2](NO3)3(OH)·12H2O has been determined by single-crystal X-ray diffraction. Each (μ-BMDP) ligand spans two iron(III) ions and causes these ions to become structurally distinct. Within this binuclear unit one iron atom is five-coordinate with distorted square pyramidal geometry and an N2O3 donor set, while the other iron is six-coordinate with distorted octahedral geometry and an N3O3 donor set. Two of these binuclear units are linked through a pair of oxo and acetato bridges to form the centrosymmetric tetranuclear complex. The coordinatively nonequivalent iron atoms exhibit distinct Mössbauer spectroscopic parameters and produce a pair of doublets at 80 K. The iron(III) centers are coupled antiferromagnetically with a room-temperature moment of 1.9 μB per iron with J = −103.3 cm-1, zJ‘ = −105.9 cm-1. The properties of the unsymmetric cation [Fe4(μ-O)2(μ-BMDP)2(μ-OAc)2]4+ are similar to those observed for binuclear iron proteins with comparable coordinative inequivalence. Efforts to increase the solubility of [Fe4(μ-O)2(μ-BMDP)2(μ-OAc)2]4+ by metathesis with sodium tetrafluoroborate resulted in the isolation of crystals of a new octanuclear iron species, [Fe8(μ-O)4(μ-BMDP)4(OH)4(μ-OAc)4](BF4)3(OH)·2CH3CN·8H2O(2), which has also been characterized by single-crystal X-ray diffraction. The asymmetric unit consists of an Fe2(μ-O)(μ-BMDP)(μ-OAc)(OH) group which is externally bridged via the oxo ions to form a molecular square with four of the eight iron ions at the corners. Both iron sites are six-coordinate with distorted octahedral geometry. One has an N2O4 donor set; the other has an N3O3 donor set.