SYNTHESIS, STRUCTURE, AND REACTIVITY OF (MU-OXO)BIS(MU-CARBOXYLATO)DIIRON(III) COMPLEXES OF A DINUCLEATING DICARBOXYLATE LIGAND, STABLE MODELS FOR NONHEME DIIRON PROTEIN CORES

Use of a pre-organized, cleft-shaped dicarboxylate ligand, m-xylenediamine-bis(Kemp's triacid)imide (XDK), in the self-assembly synthesis of (mu-oxo)bis(mu-carboxylato)diiron(III) complexes as models for the active site of nonheme diiron protein cores resulted in exclusive formation of dinuclear complexes. Reaction of H(2)XDK with Fe-(NO3)(3).9H(2)O in methanol solution afforded multigram quantities of a (mu-oxo)bis(mu-carboxylato)diiron(III) complex having six solvent ligands in the terminal positions. The geometry of this complex, [Fe2O(XDK)(CH3OH)(5)(H2O)](NO3)(2).4(2)O), was determined by X-ray crystallography, and Mossbauer spectroscopy demonstrated th e persistence of its dinuclear structure in frozen methanol solution. Compound 1 reacted cleanly with N-donor ligands such as 2,2'-dipyridyl (bpy) to afford terminally substituted products, for example [Fe2O(XDK)(bpy)(2)(CH3OH)(2)](2+) (2). The reactions were reactions were readily monitored by changes in the optical spectroscopic features of the complexes, and a stopped-flow kinetics analysis of the ligand exchange reaction revealed the second order rate constant k to be (1.51 +/- 0.04) x 10(3) M(-1) s(-1). This value is quite similar to the rate constants for ligand substitution at the terminal site in hemerythrin. The substitution by two bpy ligands, one on each iron of the dinuclear center, proceeds at the same rate, except for a statistical factor of 2 favoring binding of the first equivalent, so that k = k(2) = 0.5k(1). Various solid products were obtained from this reaction, [Fe2O(XDK)(bpy)(2)XY](n+), depending upon the isolation procedure used. Crystallization from a methanol/diethyl ether mixture gave the unsymmetrical complex 3 (X = CH3OH, Y = NO3-, n = 1); recrystallization from CH2Cl2/toluene gave the symmetrical anti-4 (X = Y = NO3-, n = 0); and recrystallization from acetonitrile afforded syn-4 (X = Y = NO3-, n = 0), the X-ray structure of which is reported. Reaction of 1 (NO3)(2) with 1-methylimidazole (1-MeIm) in methanol solution resulted in successive replacement of each of the terminal solvent ligands, affording [Fe2O(XDK)(1-MeIm)(6)](2+) (5). Formation of this complex was monitored spectroscopically with the use of UV/visible and resonance Raman spectroscopy, and its solid state structure was determined by X-ray crystallographic analysis of the tetraphenylborate salt.