Novel rectangular [Fe4(mu4-OHO)(mu-OH)(2)](7+) versus "butterfly" [Fe4(mu3-O2](8+) core topology in the Fe(III)/RCO2(-)/phen reaction systems (R = Me,Ph; phen = 1,10-phenanthroline): preparation and properties of [Fe4(OHO)(OH)(2)(O2CMe)(4)(phen)(4)](ClO4)(3), [Fe4O2(O2CPh)(7)(phen)(2)](ClO4), and [Fe4O2(O2CPh)(8)(phen)(2)

The preparations, X-ray structures, and detailed physical characterizations are presented for three new tetranuclear Fe(III)/RCO(2)(-)/phen complexes, where phen = 1,10-phenanthroline: [Fe(4)(OHO)(OH)(2)(O(2)CMe)(4)(phen)(4)](ClO(4))(3).4.4MeCN.H(2)O (1.4.4MeCN.H(2)O); [Fe(4)O(2)(O(2)CPh)(7)(phen)(2)](ClO(4)).2MeCN (2.2MeCN); [Fe(4)O(2)(O(2)CPh)(8)(phen)(2)].2H(2)O (3.2H(2)O). Complex 1.4.4MeCN.H(2)O crystallizes in space group P2(1)/n, with a = 18.162(9) A, b = 39.016(19) A, c = 13.054(7) A, beta = 104.29(2) degrees, Z = 4, and V = 8963.7 A(3). Complex 2.2MeCN crystallizes in space group P2(1)/n, with a = 18.532(2) A, b = 35.908(3) A, c = 11.591(1) A, beta = 96.42(1) degrees, Z = 4, and V = 7665(1) A(3). Complex 3.2H(2)O crystallizes in space group I2/a, with a = 18.79(1) A, b = 22.80(1) A, c = 20.74(1) A, beta = 113.21(2) degrees, Z = 4, and V = 8166(1) A(3). The cation of 1 contains the novel [Fe(4)(mu(4)-OHO)(mu-OH)(2)](7+) core. The core structure of 2 and 3 consists of a tetranuclear bis(mu(3)-O) cluster disposed in a "butterfly" arrangement. Magnetic susceptibility data were collected on 1-3 in the 2-300 K range. For the rectangular complex 1, fitting the data to the appropriate theoretical chi(M) vs T expression gave J(1) = -75.4 cm(-1), J(2) = -21.4 cm(-1), and g = 2.0(1), where J(1) and J(2) refer to the Fe(III)O(O(2)CMe)(2)Fe(III) and Fe(III)(OH)Fe(III) pairwise exchange interactions, respectively. The S = 0 ground state of 1 was confirmed by 2 K magnetization data. The data for 2 and 3 reveal a diamagnetic ground state with antiferromagnetic exchange interactions among the four high-spin Fe(III) ions. The exchange coupling constant J(bb) ("body-body" interaction) is indeterminate due to prevailing spin frustration, but the "wing-body" antiferromagnetic interaction (J(wb)) was evaluated to be -77.6 and -65.7 cm(-1) for 2 and 3, respectively, using the appropriate spin Hamiltonian approach. M?ssbauer spectra of 1-3 are consistent with high-spin Fe(III) ions. The data indicated asymmetry of the Fe(4) core of 1 at 80 K, which is not detected at room temperature due to thermal motion of the core. The spectra of 2 and 3 analyze as two quadrupole-split doublets which were assigned to the body and wing-tip pairs of metal ions. (1)H NMR spectra are reported for 1-3 with assignment of the main resonances.     

Metalloradical complexes of manganese and chromium featuring an oxidatively rearranged ligand

Redox events involving both metal and ligand sites are receiving increased attention since a number of biological processes direct redox equivalents toward functional residues. Metalloradical synthetic analogues remain scarce and require better definition of their mode of formation and subsequent operation. The trisamido-amine ligand [(RNC6H4)3N]3-, where R is the electron-rich 4-t-Bu Ph, is employed in this study to generate redox active residues in manganese and chromium complexes. Solutions of [(L1)Mn(II)-THF]- in THF are oxidized by dioxygen to afford [(L1re-1)Mn(III)-(O)2-Mn(III)(L1 re-1)]2-as the major product. The rare dinuclear manganese (III,III) core is stabilized by a rearranged ligand that has undergone an one-electron oxidative transformation, followed by retention of the oxidation equivalent as a pi radical in ano-diiminobenzosemiquinonate moiety. Magnetic studies indicate that the ligand-centered radical is stabilized by means of extended antiferromagnetic coupling between the S ) 1/2 radical and the adjacent S ) 2 Mn(III) site, as well as between the two Mn(III) centers via the dioxo bridge. Electrochemical and EPR data suggest that this system can store higher levels of oxidation potency. Entry to the corresponding Cr(III) chemistry is achieved by employing CrCl3 to access both[(L1)Cr(III)-THF] and [(L1re-1)Cr(III)-THF(Cl)], featuring the intact and the oxidatively rearranged ligands, respectively. The latter is generated by ligand-centered oxidation of the former compound. The rearranged ligand is perceived to be the product of an one-electron oxidation of the intact ligand to afford a metal-bound aminyl radical that subsequently mediates a radical 1,4-(N-to-N) aryl migration.     

Ising-type magnetic anisotropy in a cobalt(II) nitronyl nitroxide compound: a key to understanding the formation of molecular magnetic nanowires

The compound [Co(hfac)2-(NITPhOMe)2] (2) (hfac = hexafluoroacetylacetonate, NITPhOMe = 4'-methoxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) crystallizes in the triclinic P1 space group, a= 10.870(5), b = 11.520(5), c = 19.749(5) A, alpha = 78.05(5), beta = 84.20(5), gamma = 64.51(5) degrees, Z = 2. It can be considered a model system for studying the nature of the magnetic anisotropy of [Co(hfac)2(NITPhOMe)] (1), which was recently reported to behave as a molecular magnetic wire. The magnetic anisotropy of 2 was investigated by EPR spectroscopy and SQUID magnetometry both in the polycrystalline powder and in a single crystal. The experimental magnetic anisotropy was related to the anisotropy of the central ion and to the exchange interaction between the cobalt(II) ion and the radicals.     

Hydrothermal synthesis and characterization of 2D M(II)-Quinate (M = Co,Zn) metal-organic lattice assemblies: solid-state solution structure correlation in M(II)-hydroxycarboxylate systems

Co(II) and Zn(II) ions exhibit variable reactivity toward O-containing ligands in aqueous media, affording isolable materials with distinct solid-state lattice properties. d-(-)-quinic acid is a cellular α-hydroxycarboxylate metal ion binder, which reacts with Co(II) and Zn(II) under pH-specific hydrothermal conditions, leading to the isolation of two new species [Co(2)(C(7)H(11)O(6))(4)](n)·nH(2)O (1) and [Zn(3)(C(7)H(11)O(6))(6)](n)·nH(2)O (2). Compound 1 was characterized by elemental analysis, spectroscopic techniques (FT-IR, UV-visible, EPR), magnetic studies, and X-ray crystallography. Compound 2 was characterized by elemental analysis, spectroscopic techniques (FT-IR, ESI-MS), and X-ray crystallography. The 2D molecular lattices in 1 and 2 reveal the presence of octahedral M(II) units bound exclusively to quinate in a distinct fashion, thereby projecting a unique chemical reactivity in each investigated system. The magnetic susceptibility and solid-state/frozen solution EPR data on 1 support the presence of a high-spin octahedral Co(II) in an oxygen environment, having a ground state with an effective spin of S = 1/2. Concurrent aqueous speciation studies on the binary Zn(II)-quinate system unravel the nature and properties of species arising from Zn(II)-quinate interactions as a function of pH and molar ratio. The physicochemical profiles of 1 and 2, in the solid state and in solution, earmark the importance of (a) select synthetic hydrothermal reactivity conditions, affording new well-defined lattice dimensionality and nuclearity M(II)-quinate materials, (b) structural speciation approaches delineating solid state-aqueous solution correlations in the binary M(II)-quinate systems, and (c) pH-specific chemical reactivity in binary M(II)-quinate systems reflecting structurally unique associations of simple aqueous complexes into distinctly assembled 2D crystalline lattices.     

Synthesis,spectroscopic, structural and magnetic studies of new binary Cr(III)–citrate pH-specific structural variants from aqueous media

In an attempt to understand the aqueous interactions of Cr(III) with the low molecular mass physiological ligand citric acid, the pH-specific synthesis in the binary Cr(III)–citrate system was pursued, leading to the new complexes Na₃[Cr(C₆H₅O₇)₂]·8.5H₂O (1) and (Hdmphen)₆[Cr(C₆H₅O₇)₂]·(NO₃)₃·14H₂O (2). Complexes 1 and 2 were characterized by elemental analysis, spectroscopic, structural, thermal, and magnetic susceptibility studies. The structures of 1 and 2 reveal a mononuclear octahedral complex of Cr(III) with two citrate ligands bound to it. Albeit of the same deprotonation state, the disposition of the two citrate ligands with respect to Cr(III) differs between 1 and 2 in the solid state, thus reflecting the presence of pH-structural variants in the requisite binary system. This conformational difference is lifted in aqueous solution, thus providing (a) comparative information on the distribution and diversity of species in the binary Cr(III)–citrate system, and (b) insight into the nature of interactions developing in the binary Cr(III)–hydroxycarboxylate systems in abiotic and biological applications.