Publications (26) View all
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Article: Mixed Copper, Silver, and Gold Cyanides, (M(x)M'(1-x))CN: Tailoring Chain Structures To Influence Physical Properties.
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ABSTRACT: Binary mixed-metal variants of the one-dimensional MCN compounds (M = Cu, Ag, and Au) have been prepared and characterized using powder X-ray diffraction, vibrational spectroscopy, and total neutron diffraction. A solid solution with the AgCN structure exists in the (Cu(x)Ag(1-x))CN system over the range (0 ≤ x ≤ 1). Line phases with compositions (Cu(1/2)Au(1/2))CN, (Cu(7/12)Au(5/12))CN, (Cu(2/3)Au(1/3))CN, and (Ag(1/2)Au(1/2))CN, all of which have the AuCN structure, are found in the gold-containing systems. Infrared and Raman spectroscopies show that complete ordering of the type [M-C≡N-M'-N≡C-](n) occurs only in (Cu(1/2)Au(1/2))CN and (Ag(1/2)Au(1/2))CN. The sense of the cyanide bonding was determined by total neutron diffraction to be [Ag-NC-Au-CN-](n) in (Ag(1/2)Au(1/2))CN and [Cu-NC-Au-CN-](n) in (Cu(1/2)Au(1/2))CN. In contrast, in (Cu(0.50)Ag(0.50))CN, metal ordering is incomplete, and strict alternation of metals does not occur. However, there is a distinct preference (85%) for the N end of the cyanide ligand to be bonded to copper and for Ag-CN-Cu links to predominate. Contrary to expectation, aurophilic bonding does not appear to be the controlling factor which leads to (Cu(1/2)Au(1/2))CN and (Ag(1/2)Au(1/2))CN adopting the AuCN structure. The diffuse reflectance, photoluminescence, and 1-D negative thermal expansion (NTE) behaviors of all three systems are reported and compared with those of the parent cyanide compounds. The photophysical properties are strongly influenced both by the composition of the individual chains and by how such chains pack together. The NTE behavior is also controlled by structure type: the gold-containing mixed-metal cyanides with the AuCN structure show the smallest contraction along the chain length on heating.Journal of the American Chemical Society 09/2012; 134(39):16387-400. · 9.91 Impact Factor -
Article: Unraveling the electronic structures of low-valent naphthalene and anthracene iron complexes: X-ray, spectroscopic, and density functional theory studies.
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ABSTRACT: Naphthalene and anthracene transition metalates are potent reagents, but their electronic structures have remained poorly explored. A study of four Cp*-substituted iron complexes (Cp* = pentamethylcyclopentadienyl) now gives rare insight into the bonding features of such species. The highly oxygen- and water-sensitive compounds [K(18-crown-6){Cp*Fe(η(4)-C(10)H(8))}] (K1), [K(18-crown-6){Cp*Fe(η(4)-C(14)H(10))}] (K2), [Cp*Fe(η(4)-C(10)H(8))] (1), and [Cp*Fe(η(4)-C(14)H(10))] (2) were synthesized and characterized by NMR, UV-vis, and (57)Fe Mössbauer spectroscopy. The paramagnetic complexes 1 and 2 were additionally characterized by electron paramagnetic resonance (EPR) spectroscopy and magnetic susceptibility measurements. The molecular structures of complexes K1, K2, and 2 were determined by single-crystal X-ray crystallography. Cyclic voltammetry of 1 and 2 and spectroelectrochemical experiments revealed the redox properties of these complexes, which are reversibly reduced to the monoanions [Cp*Fe(η(4)-C(10)H(8))](-) (1(-)) and [Cp*Fe(η(4)-C(14)H(10))](-) (2(-)) and reversibly oxidized to the cations [Cp*Fe(η(6)-C(10)H(8))](+) (1(+)) and [Cp*Fe(η(6)-C(14)H(10))](+) (2(+)). Reduced orbital charges and spin densities of the naphthalene complexes 1(-/0/+) and the anthracene derivatives 2(-/0/+) were obtained by density functional theory (DFT) methods. Analysis of these data suggests that the electronic structures of the anions 1(-) and 2(-) are best represented by low-spin Fe(II) ions coordinated by anionic Cp* and dianionic naphthalene and anthracene ligands. The electronic structures of the neutral complexes 1 and 2 may be described by a superposition of two resonance configurations which, on the one hand, involve a low-spin Fe(I) ion coordinated by the neutral naphthalene or anthracene ligand L, and, on the other hand, a low-spin Fe(II) ion coordinated to a ligand radical L(•-). Our study thus reveals the redox noninnocent character of the naphthalene and anthracene ligands, which effectively stabilize the iron atoms in a low formal, but significantly higher spectroscopic oxidation state.Inorganic Chemistry 05/2012; 51(12):6719-30. · 4.60 Impact Factor -
Article: Spectro-electrochemical and DFT studies of a planar Cu (II)-phenolate complex active in the aerobic oxidation of primary alcohols
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ABSTRACT: Dedicated to Prof. Dr. Wolfgang Kaim for his significant contribution to coordination chemistry and spectro-electrochemistry Keywords: Copper(II) Galactose oxidase Primary alcohol oxidation Magnetic susceptibility Spectro-electrochemistry DFT a b s t r a c t A square-planar compound [Cu(pyrimol)Cl] (pyrimol = 4-methyl-2-N-(2-pyridylmethylene)aminopheno-late) abbreviated as CuL–Cl) is described as a biomimetic model of the enzyme galactose oxidase (GOase). This copper(II) compound is capable of stoichiometric aerobic oxidation of activated primary alcohols in acetonitrile/water to the corresponding aldehydes. It can be obtained either from Hpyrimol (HL) or its reduced/hydrogenated form Hpyramol (4-methyl-2-N-(2-pyridylmethyl)aminophenol; H 2 L) readily con-verting to pyrimol (L À) on coordination to the copper(II) ion. Crystalline CuL–Cl and its bromide deriva-tive exhibit a perfect square-planar geometry with Cu–O(phenolate) bond lengths of 1.944(2) and 1.938(2) Å. The cyclic voltammogram of CuL–Cl exhibits an irreversible anodic wave at +0.50 and +0.57 V versus ferrocene/ferrocenium (Fc/Fc +) in dry dichloromethane and acetonitrile, respectively, cor-responding to oxidation of the phenolate ligand to the corresponding phenoxyl radical. In the strongly donating acetonitrile the oxidation path involves reversible solvent coordination at the Cu(II) centre. The presence of the dominant Cu II –L Å chromophore in the electrochemically and chemically oxidised spe-cies is evident from a new fairly intense electronic absorption at 400–480 nm ascribed to a several elec-tronic transitions having a mixed p ? p ⁄ (L Å) intraligand and Cu–Cl ? L Å charge transfer character. The EPR signal of CuL–Cl disappears on oxidation due to strong intramolecular antiferromagnetic exchange cou-pling between the phenoxyl radical ligand (L Å) and the copper(II) centre, giving rise to a singlet ground state (S = 0). The key step in the mechanism of the primary alcohol oxidation by CuL–Cl is probably the a-hydrogen abstraction from the equatorially bound alcoholate by the phenoxyl moiety in the oxidised pyrimol ligand, Cu–L Å , through a five-membered cyclic transition state. Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.04/2011; -
Article: Homoleptic diphosphacyclobutadiene complexes [M(η(4)-P2C2R2)2]x- (M = Fe, Co; x = 0, 1).
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ABSTRACT: The preparation and comprehensive characterization of a series of homoleptic sandwich complexes containing diphosphacyclobutadiene ligands are reported. Compounds [K([18]crown-6)(thf)(2)][Fe(η(4)-P(2)C(2)tBu(2))(2)] (K1), [K([18]crown-6)(thf)(2)][Co(η(4)-P(2)C(2)tBu(2))(2)] (K2), and [K([18]crown-6)(thf)(2)][Co(η(4)-P(2)C(2)Ad(2))(2)] (K3, Ad = adamantyl) were obtained from reactions of [K([18]crown-6)(thf)(2)][M(η(4)-C(14)H(10))(2)] (M = Fe, Co) with tBuC[triple bond]P (1, 2), or with AdC[triple bond]P (3). Neutral sandwiches [M(η(4)-P(2)C(2)tBu(2))(2)] (4: M = Fe 5: M = Co) were obtained by oxidizing 1 and 2 with [Cp(2)Fe]PF(6). Cyclic voltammetry and spectro-electrochemistry indicate that the two [M(η(4)-P(2)C(2)tBu(2))(2)](-)/[M(η(4)-P(2)C(2)tBu(2))(2)] moieties can be reversibly interconverted by one electron oxidation and reduction, respectively. Complexes 1-5 were characterized by multinuclear NMR, EPR (1 and 5), UV/Vis, and Mössbauer spectroscopies (1 and 4), mass spectrometry (4 and 5), and microanalysis (1-3). The molecular structures of 1-5 were determined by using X-ray crystallography. Essentially D(2d)-symmetric structures were found for all five complexes, which show the two 1,3-diphosphacyclobutadiene rings in a staggered orientation. Density functional theory calculations revealed the importance of covalent metal-ligand π bonding in 1-5. Possible oxidation state assignments for the metal ions are discussed.Chemistry 12/2010; 16(48):14322-34. · 5.93 Impact Factor -
Article: Role of an electron‐transfer chain reaction in the unusual photochemical formation of five‐coordinated anions [Mn(CO)3(α‐diimine)]− from fac‐[Mn(X)(CO)3(α‐diimine)] (X = halide) at low temperatures
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ABSTRACT: The 16e− radicals [Mn(CO)3(α-diimine)] are the key transients in the complex mechanism of formation of five-coordinated anions [Mn(CO)3(α-diimine)]− in 2-MeTHF at 135K by visible excitation of the complexes fac-[Mn(X)(CO)3(α-diimine)] [X = halide; α-diimine = 2,2′- bipyridine (bpy), pyridine-2-carbaldehyde N-isopropylimine (iPr-PyCa), and pyridine-2-carbaldehyde N-p-tolylimine (pTol-PyCa)] into their low-energy MLCT/XLCT transitions. This article describes the as yet unresolved electron-transfer step in the mechanism which converts the radicals [Mn(CO)3(α-diimine)] into the corresponding anions. Cyclic voltammetry and IR spectroelectro-chemistry in optically transparent thin-layer electrochemical cells were employed at variable temperatures in order to study the temperature-dependent stability of the radicals [Mn(CO)3(α-diimine)], their redox properties and their interaction with a coordinating solvent (Sv) to give the 18e− adducts fac-[Mn(Sv)(CO)3(α-diimine)]. The latter radicals are strong reductors capable of electron transfer to their five-coordinated precursors which are quite stable at temperatures below 220K. This behaviour was demonstrated in the complex fac-[Mn(Cl)(CO)3(bpy)] and the related complexes fac-[Mn(Br)(CO)3(iPr-DAB)] and fac-[Mn(PrCN)(CO)3(iPr-DAB)]+(PrCN = n- butyronitrile; iPr-DAB = N, N′-diisopropyl-1,4-diaza-1,3-butadiene† which were used as reference. The (spectro)electrochemical results have revealed that the formation of [Mn(CO)3(α-diimine)]− in the course of the above photoreaction is most probably accompanied by parallel generation of the cations fac-[Mn(Sv)(CO)3(α-diimine)]+. The cations in turn react back with the free halide to give the starting complex fac-[Mn(X)(CO)3(α-diimine)], thereby closing the catalytic cycle. The overall mechanism of the formation of [Mn(CO)3(α-diimine)]− from the neutral Mn(halide) complexes can therefore be viewed as a photo-assisted/ETC reaction. No anions [Mn(CO)3(α-diimine)]− are formed in the presence of excess PR3 which is reasonably ascribed to a complete conversion of the 16e− transients [Mn(CO)3(α-diimine)] into the stable 18e− adducts fac-[Mn(PR3)(CO)3(α-diimine)], the final photoproducts observed in this case.Recueil des Travaux Chimiques des Pays-Bas. 09/2010; 114(11‐12):565 - 570.
