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-xylenedia mine-bis(Kemp's triacid)imide (XDK), in the self-assembly synthesis of (mu-oxo)bis(mu-carboxylato)diiron(III) complexes as models for the ac tive site of nonheme diiron protein cores resulted in exclusive format ion 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 t erminal positions. The geometry of this complex, [Fe2O(XDK)(CH3OH)(5)( H2O)](NO3)(2).4(2)O), was determined by X-ray crystallography, and Mos sbauer spectroscopy demonstrated th e persistence of its dinuclear str ucture in frozen methanol solution. Compound 1 reacted cleanly with N- donor ligands such as 2,2'-dipyridyl (bpy) to afford terminally substi tuted products, for example [Fe2O(XDK)(bpy)(2)(CH3OH)(2)](2+) (2). The reactions were reactions were readily monitored by changes in the opt ical spectroscopic features of the complexes, and a stopped-flow kinet ics 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, on e on each iron of the dinuclear center, proceeds at the same rate, exc ept for a statistical factor of 2 favoring binding of the first equiva lent, 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 iso lation procedure used. Crystallization from a methanol/diethyl ether m ixture 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 sy n-4 (X = Y = NO3-, n = 0), the X-ray structure of which is reported. R eaction of 1 (NO3)(2) with 1-methylimidazole (1-MeIm) in methanol solu tion resulted in successive replacement of each of the terminal solven t ligands, affording [Fe2O(XDK)(1-MeIm)(6)](2+) (5). Formation of this complex was monitored spectroscopically with the use of UV/visible an d resonance Raman spectroscopy, and its solid state structure was dete rmined by X-ray crystallographic analysis of the tetraphenylborate sal t.