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

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posted on 28.12.1998, 00:00 by Joe H. Satcher,, Marilyn M. Olmstead, Michael W. Droege, Sean R. Parkin, Bruce C. Noll, Leopold May, Alan L. Balch
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.