• Reactivity of Copper(I) Complexes with Tripodal Ligands towards O2: Structures of a Precursor [L3CuI(NCCH3)](BF4), L3 = Tris(3-isopropyl-4,5-trimethylenepyrazolyl)methane and of its Oxidation Product [L3CuII(-OH)2CuIIL3](BF4)2 with Strong Antiferromagnetic Spin-Spin Coupling
    W. Kaim, C. Titze, T. Schurr, M. Sieger, L.M. Lawson Daku, J. Jordanov, D. Rojas, A.M. Garca and J. Manzur
    Zeitschrift fr anorganische und allgemeine Chemie, 631 (13-14) (2005), p2568-2574
    DOI:10.1002/zaac.200500053 | unige:3316 | Abstract | Article PDF
The molecular structure of the highly oxygen-sensitive complex [L3CuI(NCCH3)](BF4) (1) reveals approximately symmetrical coordination by the fac-tridentate (tripodal) ligand L3 = tris(3-isopropyl-4,5-trimethylenepyrazolyl)methane and a rather short CuI-N(acetonitrile) distance of 1.865(5) Å. In CH2Cl2 at -78 °C the colourless compound reacts with O2 to yield a labile purple intermediate (λmax 517 nm) - presumably a peroxodicopper(II) complex - which decomposes at -30 °C. No such intermediate was observed on reaction of the CuI complex of bis(2-pyridylmethyl)benzylamine with O2 at -80 °C. However, an EPR spectrum with g|| = 2.17 and g_|_ = 2.03 without 63,65Cu hyperfine splitting was observed at low temperatures. Exposure of the precursor 1 to air under ambient conditions yields dinuclear [L3Cu||(μ-OH)2CuIIL3](BF4)2 (2) which exhibits an EPR detectable dissociation into monomers in CH2Cl2 solution. The structure of the hexakis(dichloromethane) solvate of 2 with Cu-Cu and Cu-O distances of 3.055 and 1.94Å, respectively, is typical for dihydroxo-bridged dicopper compounds with square-pyramidal Cu|| configuration (τ = 0.03), adopting an anti arrangement. In agreement with the relatively wide Cu-O-Cu angles of 103.5° an analysis of the temperature dependence of the magnetic susceptibility revealed a rather strong (J = -633 cm-1) antiparallel spin-spin coupling. The effect is ascribed to the steric bulk of the ligand L3.
  • Investigation of the Reduced High-Potential Iron-Sulfur Protein from Chromatium vinosum and Relevant Model Compounds: A Unified Picture of the Electronic Structure of [Fe4S4]2+ Systems through Magnetic and Optical Studies
    L.M. Lawson Daku, J. Pcaut, A. Lenormand-Foucaut, B. Vieux-Melchior, P. Iveson and J. Jordanov
    Inorganic Chemistry, 42 (21) (2003), p6824-6850
    DOI:10.1021/ic034494n | unige:3717 | Abstract | Article HTML | Article PDF
Magnetization measurements and variable temperature optical spectroscopy have been used to investigate, within the 4−300 K temperature range, the electronic structure of the reduced high-potential iron protein (HiPIP) from Chromatium vinosum and the model compounds (Cat)2[Fe4S4(SR)4], where RS- = 2,4,6-triisopropylphenylthiolate (1), 2,6-diphenylphenylthiolate (2), diphenylmethylthiolate (3), 2,4,6-triisopropylbenzylthiolate (4, 4‘), 2,4,6-triphenylbenzylthiolate (5, 5‘), 2,4,6-tri-tert-butylbenzylthiolate (6), and Cat+ = +NEt4 (1, 2, 3, 4‘, 5‘, 6), +PPh4 (4, 5). The newly synthesized 22-, 32-, 52-, and 62- complexes are, as 12- and 42-, excellent models of the reduced HiPIPs:  they exhibit the [Fe4S4]3+/2+ redox couple, because of the presence of bulky ligands which stabilize the [Fe4S4]3+ oxidized core. Moreover, the presence of SCH2 groups in 42-, 52-, and 62-, as in the [Fe4S4] protein cores, makes them good biomimetic models of the HiPIPs. The X-ray structure of 2 is reported:  it crystallizes in the orthorhombic space group Pcca with no imposed symmetry and a D2d-distorted geometry of the [Fe4S4]2+ core. Fit of the magnetization data of the reduced HiPIP and of the 1, 2, 3, 4, 5, and 6 compounds within the exchange and double exchange theoretical framework leads to exchange coupling parameters J = 261−397 cm-1. A firm determination of the double exchange parameters B or, equivalently, the transfer integrals β = 5B could not be achieved that way. The obtained |B| values remain however high, attesting thus to the strength of the spin-dependent electronic delocalization which is responsible for lowest lying electronic states being characterized by delocalized mixed-valence pairs of maximum spin 9/2. Electronic properties of these systems are then accounted for by the population of a diamagnetic ground level and excited paramagnetic triplet and quintet levels, which are respectively J and 3J above the ground level. Optical studies of 1, 2, 4‘, 5‘, and 6 but also of (NEt4)2[Fe4S4(SCH2C6H5)4] and the isomorph (NEt4)2[Fe4S4(S-t-Bu)4] and (NEt4)2[Fe4Se4(S-t-Bu)4] compounds reveal two absorption bands in the near infrared region, at 705−760 nm and 1270−1430 nm, which appear to be characteristic of valence-delocalized and ferromagnetically coupled [Fe2X2]+ (X = S, Se) units. The |B| and |β| values can be directly determined from the location at 10|B| of the low-energy band, and are respectively of 699−787 and 3497−3937 cm-1. Both absorption bands are also present in the 77 K spectrum of the reduced HiPIP, at 700 and 1040 nm (Cerdonio, M.; Wang, R.-H.; Rawlings, J.; Gray, H. B. J. Am. Chem. Soc. 1974, 96, 6534−6535). The blue shift of the low-energy band is attributed to the inequivalent environments of the Fe sites in the protein, rather than to an increase of |β| when going from the models to the HiPIP. The small differences observed in known geometries of [Fe4S4]2+ clusters, especially in the Fe−Fe distances, cannot probably lead to drastic changes in the direct Fe−Fe interactions (parameter β) responsible for the delocalization phenomenon. These differences are however magnetostructurally significant as shown by the 261−397 cm-1 range spanned by J. The cluster's geometry, hence the efficiency of the Fe−μ3-S−Fe superexchange pathways, is proposed to be controlled by the more or less tight fit of the cluster within the cavity provided by its environment.

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