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 [Fe(4)(&mgr;-O)(2)(&mgr;-BMDP)(2)(&mgr;-OAc)(2)](4+), which exhibits coordination number asymmetry. The structure of [Fe(4)(&mgr;-O)(2)(&mgr;-BMDP)(2)(&mgr;-OAc)(2)](NO(3))(3)(OH).12H(2)O has been determined by single-crystal X-ray diffraction. Each (&mgr;-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 N(2)O(3) donor set, while the other iron is six-coordinate with distorted octahedral geometry and an N(3)O(3) 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 &mgr;(B) per iron with J = -103.3 cm(-)(1), zJ' = -105.9 cm(-)(1). The properties of the unsymmetric cation [Fe(4)(&mgr;-O)(2)(&mgr;-BMDP)(2)(&mgr;-OAc)(2)](4+) are similar to those observed for binuclear iron proteins with comparable coordinative inequivalence. Efforts to increase the solubility of [Fe(4)(&mgr;-O)(2)(&mgr;-BMDP)(2)(&mgr;-OAc)(2)](4+) by metathesis with sodium tetrafluoroborate resulted in the isolation of crystals of a new octanuclear iron species, [Fe(8)(&mgr;-O)(4)(&mgr;-BMDP)(4)(OH)(4)(&mgr;-OAc)(4)](BF(4))(3)(OH).2CH(3)CN.8H(2)O( )()(2), which has also been characterized by single-crystal X-ray diffraction. The asymmetric unit consists of an Fe(2)(&mgr;-O)(&mgr;-BMDP)(&mgr;-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 N(2)O(4) donor set; the other has an N(3)O(3) donor set.