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

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 μ_B 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.