title: (&mgr;-Oxo)bis(&mgr;-carboxylato)bis(2,2'-bipyridyl)bis(X)diiron(III) Complexes, X = NCS(-), NCSe(-), and N(3)(-): Synthetic Models of Pseudohalide Derivatives of Carboxylate-Bridged Diiron Proteins.
abstract: The neutral (&mgr;-oxo)bis(&mgr;-carboxylato)diiron(III) complexes [Fe(2)O(&mgr;-XDK)(bpy)(2)(NCS)(2)] (1), [Fe(2)O(&mgr;-XDK)(bpy)(2)((15)NCS)(2)] (1a), [Fe(2)O(&mgr;-XDK)(bpy)(2)(NCSe)(2)] (2), [Fe(2)O(&mgr;-XDK)(bpy)(2)(N(3))(2)] (3), and [Fe(2)O(&mgr;-XDK)(bpy)(2)((15)NNN)(2)] (3a), where XDK is the doubly deprotonated form of m-xylylenediamine bis(Kemp's triacid imide) and bpy is 2,2'-bipyridyl, have been synthesized. The octahedrally coordinated iron centers are triply bridged by an oxo group and the two carboxylate groups of XDK. In the X-ray crystal structures of 1 and 2, the diiron centers have C(2) symmetry, whereas in 3 the coordinated azide ligands are also anti with respect to the Fe-O-Fe plane but in different rotational conformations with respect to the Fe-N(3) bond. The optical spectra of these highly colored compounds are characterized by broad ligand-field bands centered approximately 650 nm and a more intense combination of ligand-field and ligand-to-metal charge-transfer bands between 350 and 500 nm. Compounds 1-3 exhibit intense IR bands ( approximately 2050 cm(-)(1)) which are characteristic of terminally bound isothiocyanate, isoselenocyanate, and azide ligands, respectively. These bands shift to lower energies upon substitution with (15)NCS(-) in 1a (Deltanu = 24 cm(-)(1)) and (15)NNN(-) in 3a (Deltanu = 8 cm(-)(1)), and the failure of the latter to reveal two resolved features indicates a need for caution in interpreting analogous data from isotopically substituted protein adducts. Resonance Raman experiments on 1-3 revealed bands ( approximately 520 cm(-)(1)) attributable to the symmetric FeOFe stretching frequency. The Mössbauer spectra of 1-3 at 77 K display symmetric quadrupole doublets with isomer shifts ( approximately 0.5 mm s(-)(1)) and quadrupole splittings ( approximately 1.7 mm s(-)(1)) typical of (&mgr;-oxo)diiron(III) compounds. Crystal data: 1.2C(6)H(6), monoclinic, P2(1)/n, a = 19.6692(2) Å, b = 14.3069(1) Å, c = 22.6502(3) Å, beta = 96.922(1) degrees, V = 6327.4(1) Å(3), Z = 4, T = -90 degrees C; 2.2C(6)H(6), monoclinic, C2/c, a = 22.6834(3) Å, b = 14.4315(2) Å, c = 19.7013(4) Å, beta = 97.837(1) degrees, V = 6389.1(2) Å(3), Z = 4, T = -85 degrees C; 3.CH(2)Cl(2).C(6)H(3)(CH(3))(3), triclinic, P&onemacr;, a = 12.1645(2) Å, b = 14.9196(3) Å, c = 18.0690(3) Å, alpha = 79.688(1) degrees, beta = 78.611(1) degrees, gamma = 83.453(1) degrees, V = 3152.3(1) Å(3), Z = 2, T = -85 degrees C.
authors.affiliation: Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139.
authors.forename: Tadashi J.
authors.initials: TJ
authors.lastname: Mizoguchi
authors.forename: Stephen J.
authors.initials: SJ
authors.lastname: Lippard