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.
Satcher
Joe H.
JH
Lawrence Livermore National Laboratory, Livermore, California 94550, and Departments of Chemistry, University of California Davis, Davis California, 95616, and The Catholic University of America, Washington, D.C. 20064.
Olmstead
Marilyn M.
MM
Droege
Michael W.
MW
Parkin
Sean R.
SR
Noll
Bruce C.
BC
May
Leopold
L
Balch
Alan L.
AL