Model Compounds for Iron Proteins. Structures and Magnetic, Spectroscopic, and Redox Properties of Fe(III)M(II) and [Co(III)Fe(III)](2)O Complexes with (&mgr;-Carboxylato)bis(&mgr;-phenoxo)dimetalate and (&mgr;-Oxo)diiron(III) Cores.

A series of heterobimetallic complexes of the type [Fe(III)M(II)L(&mgr;-OAc)(OAc)(H(2)O)](ClO(4)).nH(2)O (2-5) and [{Fe(III)Co(III)L(&mgr;-OAc)(OAc)}(2)(&mgr;-O)](ClO(4))(2).3H(2)O (6) where H(2)L is a tetraaminodiphenol macrocyclic ligand and M(II) = Zn(2), Ni(3), Co(4), and Mn(5) have been synthesized and characterized. The (1)H NMR spectrum of 6 exhibits all the resonances between 1 and 12 ppm. The IR and UV-vis spectra of 2-5 indicate that in all the cases the metal ions have similar coordination environments. A disordered crystal structure determined for 3 reveals the presence of a (&mgr;-acetate)bis(&mgr;-phenoxide)-Ni(II)Fe(III) core, in which the two metal ions have 6-fold coordination geometry and each have two amino nitrogens and two phenolate oxygens as the in-plane donors; aside from the axial bridging acetate, the sixth coordination site of nickel(II) is occupied by the unidentate acetate and that of iron(III) by a water molecule. The crystal structure determination of 6 shows that the two heterobinuclear Co(III)Fe(III) units are bound by an Fe-O-Fe linkage. 6 crystallizes in the orthorhombic space group Ibca with a = 17.577(4) Å, b = 27.282(7) Å, c = 28.647(6) Å, and Z = 8. The two iron(III) centers in 6 are strongly antiferromagnetically coupled, J = -100 cm(-1) (H = -2JS(1).S(2)), whereas the other two S(1) = S(2) = (5)/(2) systems, viz. [Fe(2)(III)(HL)(2)(&mgr;-OH)(2)](ClO(4))(2) (1) and the Fe(III)Mn(II) complex (5), exhibit weak antiferromagnetic exchange coupling with J = -4.5 cm(-1) (1) and -1.8 cm(-1) (5). The Fe(III)Ni(II) (3) and Fe(III)Co(II) (4) systems, however, exhibit weak ferromagnetic behavior with J = 1.7 cm(-1) (3) and 4.2 cm(-1) (4). The iron(III) center in 2-5 exhibits quasi-reversible redox behavior between -0.44 and -0.48 V vs Ag/AgCl associated with reduction to iron(II). The oxidation of cobalt(II) in 4 occurs quasi-reversibly at 0.74 V, while both nickel(II) and manganese(II) in 3 and 5 undergo irreversible oxidation at 0.85 V. The electrochemical reduction of 6 leads to the generation of 4.

Structural and Functional Models for the Dinuclear Copper Active Site in Catechol Oxidases: Syntheses, X-ray Crystal Structures, Magnetic and Spectral Properties, and X-ray Absorption Spectroscopic Studies in Solid State and in Solution.

Two novel tridentate dinucleating ligands containing benzimidazole were prepared, 1,3-bis(2-benzimidazolyl)-2-propanol (Hbbp, 1) and 1,5-bis(2-benzimidazolyl)-3-pentanol (Hbbpen, 2). Their complexing properties toward copper were studied in order to obtain structural and functional models for catechol oxidases. Syntheses and crystal structures of dinuclear Cu(II) complexes derived from these ligands are reported. [Cu(2)bbp(2)](ClO(4))(2).2MeOH, 3, crystallizes in the triclinic space group P&onemacr; with the following unit cell parameters: a = 7.702(3) Å, b = 10.973(6) Å, c = 12.396(6) Å, alpha = 100.59(4) degrees, beta = 99.02(4) degrees, gamma = 98.90(4) degrees, V = 998.7(8) Å(3), and Z = 1. [Cu(2)bbpen(2)](ClO(4))(2).3MeOH, 4, crystallizes in the orthorhombic space group Pccn, with the following unit cell parameters: a = 17.478(9) Å, b = 18.795(8) Å, c = 13.888(6) Å, V = 4562.2(4) Å(3), and Z = 4. Magnetic susceptibility measurements in the temperature ranges 4.6-459 K (3) and 4.6-425 K (4) indicate an antiferromagnetic coupling between the Cu(II) centers of both complexes. In order to determine the structures of the complexes in solution, XAS spectra (EXAFS and XANES) were recorded in the solid state and in solution. The interpretation of these data, including multiple scattering calculations, together with UV-vis titrations, shows that the complexes have the same structure in the crystalline state as well as in methanolic solution. Complex 4 is able to oxidize 3,5-di-tert-butylcatechol (3,5-DTBC) to the quinone (catecholase activity). This reaction was also studied by XAS and UV-vis spectroscopy. These measurements reveal the reduction of Cu(II) to Cu(I) accompanied by a decrease of the coordination number.

Macrocyclic Cu(II)(2), Cu(II)(4), Ni(II)(3), and Ni(II)(4) Complexes: Magnetic Properties of Tetranuclear Systems.

A binuclear tetraprotonated macrocyclic complex [Mg(2)(L(2)-H(4))(NO(3))(2)](NO(3))(2).6H(2)O (1) has been obtained by template condensation of 4-methyl-2,6-diformylphenol and 1,2-diaminoethane in the presence of magnesium acetate and nitrate. Complex 1 on reduction with NaBH(4), followed by the removal of magnesium, yields the 36-membered octaaminotetraphenol macrocyclic ligand H(4)L(1). The replacement of magnesium in 1 with copper(II) leads to the formation of the binuclear complex [Cu(2)L(3)(ClO(4))(2)] (2) derived from the [2+2] cyclization product of 4-methyl-2,6-diformylphenol and 1,2-diaminoethane. From H(4)L(1) a series of tetranuclear nickel(II) complexes 5-8 with the core cation [Ni(4)L(1)(&mgr;(2)-X)(2)(&mgr;(2)-H(2)O)(2)](2+) (X = NCS, N(3), OAc, or Cl) have been synthesized and characterized. The trinuclear complex [Ni(3)L(1)(acac)(2)(H(2)O)(2).2H(2)O (9), obtained by reacting nickel(II) acetylacetonate with H(4)L(1), on treatment with nickel(II) perchlorate produces the tetranuclear compound [Ni(4)L(1)(acac)(2)(H(2)O)(4)](ClO(4))(2) (10). Variable-temperature (4-300 K) magnetic susceptibility measurements have been carried out for the tetracopper(II) complex [Cu(4)L(1)(H(2)O)(4)](ClO(4))(4) (3) and the tetranickel(II) complexes [Ni(4)L(1)(&mgr;(3)-OH)(&mgr;(2)-H(2)O)(2)(ClO(4))](ClO(4))(2).2CH(3)COCH(3).H(2)O (4), [Ni(4)L(1)(&mgr;(2)-NCS)(2)(&mgr;(2)-H(2)O)(2)](ClO(4))(2).2CH(3)CN (5), [Ni(4)L(1)(&mgr;(2)-N(3))(2)(&mgr;(2)-H(2)O)(2)](ClO(4))(2).2CH(3)OH (6), [Ni(4)L(1)(&mgr;(2)-OAc)(2)(&mgr;(2)-H(2)O)(2)](ClO(4))(2).2H(2)O (7), and [Ni(4)L(1)(&mgr;(2)-Cl)(2)(&mgr;(2)-H(2)O)(2)]Cl(2).4H(2)O (8). The X-ray structure of 5 has been determined. The complex (C(50)H(70)N(12)O(14)Cl(2)S(2)Ni(4)) crystallizes in the triclinic space group P&onemacr; with a = 11.794(6) Å, b = 12.523(4) Å, c = 12.794(5) Å, alpha = 117.28(5) degrees, beta = 96.38(4) degrees, gamma = 109.65(3) degrees, and Z = 1. In the asymmetric unit each of the nickel(II) centers with distorted octahedral geometry is triply-bridged by a phenoxide group, a water molecule, and a N-bonded thiocyanate and these metal centers are further bridged to their symmetry-related counterparts by another phenoxide group. The experimental susceptibility data have been analyzed using appropriate Heisenberg spin coupling models (H = -2J(ij)()S(i)().S(j)()) and the best-fit spin exchange parameters obtained are as follows: J = -288(3) cm(-)(1) (3); J(1) = -8.1(2) cm(-)(1), J(2) = -10.2(2) cm(-)(1) (4); J(1) = -34.5(1.0) cm(-)(1), J(2) = -9.5(2.0) cm(-)(1) (5); J(1) = -34(1) cm(-)(1), J(2) = 11(2) cm(-)(1) (6); J(1) = -30(1) cm(-)(1), J(2) = -7.0(1.5) cm(-)(1) (7); J(1) = -32(1) cm(-)(1), J(2) = -4(1) cm(-)(1) (8).