The interaction of Lewis acidic boron derivatives with Re(CO)5−nH(PMe3)n complexes

Lewis acid adducts of the hydrides cis- and trans-Re(CO)(PMe₃)₄H (1) and (2), mer-Re(CO)₂(PMe₃)₃H (3), fac-Re(CO)₂(PMe₃)₃H (4) and trans-Re(CO)₃(PMe₃)₂H (5) were studied with BH₃ and 9-borabicyclo[3,3,1] norbonane (BBNH). Using BH₃·THF and (BBNH)₂ 1 and 2 afforded Re(CO)(PMe₃)₃(η²-BH₄) (6) and Re(CO)(PMe₃)₃(η²-BBNH₂) (7) as stable and isolable products. VT IR studies established for the reaction to 7 that BBNH first attaches in a pre-equilibrium to the O_CO atom of 1 or 2. At higher temperatures ReH adduct formation occurs with instantaneous transformation to 7 and elimination of PMe₃·BBNH. In a similar way, the hydrides 3 and 4 were converted with BH₃·THF and (BBNH)₂ to yield the stable complexes Re(CO)₂(PMe₃)₂(η²-BH₄) (8) and Re(CO)₂(PMe₃)₂(η²-BBNH₂) (9). The intermediacy of the η¹-BH₄ adducts mer-/fac-Re(CO)₂(PMe₃)₃(η¹-BH₄) was confirmed by VT ¹H, ³¹P NMR and VT IR experiments. The conversion of 5 with BH₃·THF led to equilibria with adducts at the O_CO terminus in trans position to H and with H_Re as revealed by VT IR studies. Temperature dependent ³¹P equilibrium studies allowed to calculate ΔH=−4.9 kcal mol⁻¹ and ΔS=+0.034 e.u. for this reaction. These adducts could not be isolated. Compound 5 does not react with (BBNH)₂ even at elevated temperatures. DFT calculations were carried out to support the structures of the BH₃ adducts of 5. In addition a vibrational analysis helped to unravel the IR band assignments of the involved compounds. DFT calculations on 8 confirmed its C_2v structure. X-ray diffraction studies were carried out on single crystals of 6 and 7.     

Dioxo-molybdenum(VI) and -tungsten(VI) BINOL and alkoxide complexes: Synthesis and catalysis in sulfoxidation, olefin epoxidation and hydrosilylation of carbonyl groups

The reaction of MoO₂(mes)₂ with S-H₂BINOL (mes = 2,4,6-Me₃C₆H₂; H₂BINOL = 1,1′-bi-2-naphthol) and (CF₃)₂MeCOH in THF yielded the novel dioxo-molybdenum(VI) complexes MoO₂(S-BINOL)(THF)₂ and MoO₂[OCMe(CF₃)₂]₂(THF), respectively. Similar tungsten derivatives WO₂(S-BINOL)(THF) and WO₂[OCMe(CF₃)₂]₂(THF) have been prepared by the reaction of WO₂Cl₂(DME) with the corresponding lithium salts of BINOL and 1,1,1,3,3,3-hexafluoro-2-methyl-2-propanol, respectively. Catalytic experiments have shown that MoO₂(S-BINOL)(THF)₂ is an active catalyst in the sulfoxidation of methyl phenyl sulfide and in the epoxidation of cis-cyclooctene with tert-butylhydroperoxide, under mild conditions. The BINOL complex was, however, not found to be enantioselective. In addition, the catalytic activity of the molybdenum species MoO₂(S-BINOL)(THF)₂ and MoO₂[OCMe(CF₃)₂]₂(THF) in the hydrosilylation of carbonyl groups has been explored.     

Cis- and trans-titanium complexes with doubly silyl-bridged dicyclopentadienyl ligands: molecular structure of [(TiCl)2(μ-O){(SiMe2)2(η-C5H3)2}]2(μ-O)2

The reaction of TiCl₄ with Li₂[(SiMe₂)₂(η⁵-C₅H₃)₂] in toluene at room temperature afforded a mixture of cis- and trans-[(TiCl₃)₂{(SiMe₂)₂(η⁵-C₅H₃)₂}] in a molar ratio of 1/2 after recrystallization. The complex trans-[(TiCl₃)₂{(SiMe₂)₂(η⁵-C₅H₃)₂}] was hydrolyzed immediately by the addition of water to THF solutions to give trans-[(TiCl₂)₂(μ-O){(SiMe₂)₂(η⁵-C₅H₃)₂}] as a solid insoluble in all organic solvents, whereas hydrolysis of cis-[(TiCl₃)₂{(SiMe₂)₂(η⁵-C₅H₃)₂}] under different conditions led to the dinuclear μ-oxo complex cis-[(TiCl₂)₂)(μ-O){(SiMe₂)₂(η⁵-C₅H₃)₂}] and two oxo complexes of the same stoichiometry [(TiCl)₂(μ-O){(SiMe₂)₂(η⁵-C₅H₃)₂}]₂(μ-O)₂ as crystalline solids. Alkylation of cis- and trans-[(TiCl₃)₂{(SiMe₂)₂(η⁵-C₅H₃)₂}] with MgCIMe led respectively to the partially alkylated cis-[(TiMe₂Cl)₂{(SiMe₂)₂(η⁵-C₅H₃)₂}] and the totally alkylated trans-[(TiMe₃)₂{(SiMe₂)₂(η⁵-C₅H₃)₂}] compounds. The crystal and molecular structure of the tetranuclear oxo complex [(TiCl)₂(μ-O){(SiMe₂)₂(η⁵-C₅H₃)₂}]₂(μ-O)₂ was determined by X-ray diffraction.     

Structures and magnetism of rubidium and cesium salts of dimeric oxovanadium(IV) tartrate(4−) complexes

Complexes of type A₄[VO(tart)]₂·nH₂O, where A = Rb or Cs and tart =d,l-tartrate(4−) (n = 2) or d,d-tartrate(4−) (n = 2 for Rb and n = 3 for Cs), were prepared from an aqueous mixture of V₂O₅, AOH and H₄tart. These complexes were studied by single-crystal X-ray diffraction methods: Rb₄[VO(d,l-tart)]₂·2H₂O, space group P1 with a = 8.156(1),b = 8.246(1),c = 8.719(1)Å, = 66.09(1)°, β = 65.07(1)°, γ = 82.40(1)°,Z = 2, 1917 observed reflections, and final R_w = 0.035; Cs₄[VO(d,l-tart)]₂·2H₂O, space group P₂₁/c with a = 9.350(1),b = 13.728(2),c = 8.479(1)Å, β = 106.77(1)°,Z = 4, 2235 observed reflections, and final R_w = 0.054; Rb₄[VO(d,d-tart)]₂·2H₂O, space group P₄₁₂₂ with a = 8.072(1),c = 32.006(3)Å,Z = 8, 1014 observed reflections and final R_w = 0.038; Cs₄[VO(d,d-tart)]₂·3H₂O, space group P₁₂₂ with a = 8.184(1),c = 33.680(5)Å,Z = 8, 1310 observed reflections, and final R_w = 0.063. Bulk magnetic susceptibility data (1.5–300 K) for these compounds and A₄[VOl,l-tart)]₂·nH₂O (A = Rb, Cs) were obtained on polycrystalline samples. These data were analyzed in terms of a Van Vleck exchange coupled S = 1/2 model which was modified to include an interdimer exchange parameters Θ. Analysis of the low-temperature (1.5–20 K) susceptibility data gave 2J = +1.30 cm⁻¹ and Θ = −1.86 K for Rb₄[VO(d,l-tart)]₂·2H₂O, 2J = +1.16 cm⁻¹ and Θ = −1.69 K for Cs₄[VO(d,l-tart)]₂·2H₂O, 2J = +1.90 cm⁻¹ and Θ = −0.82 K for Rb₄[VO(d,d-tart)]₂·2H₂O, 2J = +2.04 cm⁻¹ and Θ = −0.80 K for Rb₄[VO(l,l-tart)]₂·2H₂O, 2J = +1.52 cm⁻¹ and Θ = −0.25 K for Cs₄[VO(d,d-tart)]₂·3H₂O, and 2J = +1.64 cm⁻¹ and Θ = −0.31 K for Cs₄[VO(l,l-tart)]₂·3H₂O. These results suggest the magnitudes of intradimer (ferromagnetic and interdimer (antiferromagnetic) exchange interactions are similar in these complexes, as observed for the analogous Na salts.     

Optimization of culture conditions for production of cis-epoxysuccinic acid hydrolase using response surface methodology

Response surface methodology, which allows for rapid identification of important factors and optimization of them to enhance enzyme production, was employed here to optimize culture conditions for the production of cis-epoxysuccinic acid hydrolase from Bordetella sp. strain 1–3. In the first step, a Plackett–Burman design was used to evaluate the effects of nine variables (yeast extract, cis-epoxysuccinic acid, KH₂PO₄, K₂HPO₄ · 3H₂O, MgSO₄ · 7H₂O, trace minerals solution, culture volume, initial pH and incubation time) on the enzyme production. Yeast extract, cis-epoxysuccinic acid and KH₂PO₄ had significant influences on cis-epoxysuccinic acid hydrolase production and their concentrations were further optimized using central composite design and response surface analysis. A combination of adjusting the concentration of yeast extract to 7.8 g/l, cis-epoxysuccinic acid to 9.8 g/l, and KH₂PO₄ to 1.12 g/l would favor maximum cis-epoxysuccinic acid hydrolase production. An enhancement of cis-epoxysuccinic acid hydrolase production from 5.6 U/ml to 9.27 U/ml was gained after optimization.     

η and η olefin complexation by S, S-[Ru(Me2edda)]: a structural change to accommodate bidentate dienes

[Ru^II(Me₂edda)(H₂O)₂] (1), Me₂edda²⁻ = N,N′-dimethylethylenediaminediacetate, exhibits a sterically-controlled molecular recognition in forming η² and η⁴ olefin complexes. 1 exists with an N₂O₂ in-plane set of chelate donors and axial H₂O ligands. The two CH₃ functionalities of Me₂edda²⁻ are poised above and below the N₂O₂ plane of the glycinato rings. Studies herein of the 2,2′-bipyridine complex, [Ru^II(Me₂edda)(bpy)], with bidentate bpy chelation as established via ¹H NMR and electrochemical methods show 1 to be ligated in the S,S configuration with the glycinato rings in-plane as a cis-O form. 1 is sterically discriminating in forming η² complexes with smaller olefins (ethylene, 2-propene, cis-2-butene, methyl vinyl ketone and 3-cyclohexene-1-methanol), but rejects larger decorated ring structures and branched olefins (1,2-dimethyluracil, cyclohexene-1-one 2-methyl-2-propene). η² complexes of 1 have characteristic Ru^II/III DPP waves near 0.55 V which vary slightly with olefin structure. Potentially bidendate dienes (1,3-butadiene, 1,3-cyclohexadiene and 2,5-norbornadiene (nbd) form η⁴ complexes as shown by Ru^II/III waves between 0.94 and 1.30 V, indicate of a highly stabilized Ru^II center by π-backboning. An η²η⁴ ‘equilibrium’ with apparent K = 22 at 25 °C is observed for nbd coordinated to 1. (The η² and η⁴ distribution may be a kinetic one and not a thermodynamic one). To allow formation of the cis η⁴ complexes, 1 must undergo a shift of one or both glycinato donors from the N₂O₂ plane into the axial site away from the dimethyl functionalities. η⁴ chelation by 1,3-butadiene has been confirmed by ¹H NMR spectral assignments of two [Ru^II(Me₂edda)] isomers, one in the axial rans-O glycinato configuration, e.g. 1,3-butadiene is bidentate in the original N₂O₂ plane and a second unsymmetrical glycinato arrangement with in-plane and axial glycinato as well as in-plane and axial η⁴-1,3-butadiene coordination. [Ru^II(hedta)(H₂O)]⁻ (2), hedta³⁻ = N-hydrpxyethylenediaminetriacetate, is less discriminating for olefin structures, forming η² complexes with all eleven olefins and dienes mentioned for studies with 1. However, 2 does not undergo displacement of a carboxylate donor by the second olefin unit of a diene [Ru^II(hedta)(diene)]⁻ complexes possess a pendant non-coordinated olefin and on η²-bound olefin in the complex, indicated by a normal Ru^II(pac)(olefin)Ru^II/III wave near 0.55 V.     

Structural determinations, magnetic and EPR studies of complexes involving the Cr(OH)2Cr unit

Five heterometallic compounds with formulae [Ba(H₂O)₄Cr₂(μ-OH)₂(nta)₂] · 3H₂O (I), [M(bpy)₂(H₂O)₂] [Cr₂(OH)₂(nta)₂] · 7H₂O, where M²⁺ = Zn, (II); Ni, (III); Co, (IV) and [Mn(H₂O)₃(bpy)Cr₂(OH)₂(nta)₂] · (bpy) · 5H₂O (V); bpy = 2,2′-bipyridine, (nta = nitrilotriacetate ion) have been prepared by reaction of I with the corresponding M^II-sulfates in the presence of 2,2′-bipyridine. Substances I–V have been characterized by magnetic susceptibility measurements, EPR and X-ray determinations. I represents a 2D coordination polymer formed by coordination of centrosymmetrical dimeric chromium(III) units and Barium cations. The 10-coordinate Ba polyhedron is completed by four water molecules. Compounds II–IV are isostructural and consist of non-centrosymmetric dimeric anions [Cr₂(μ-OH)₂(nta)₂]²⁻, complex cations [M^II(bpy)₂(H₂O)₂]²⁺ and solvate water molecules. The octahedral coordination of chromium atoms implies four donor atoms of the nta³⁻ ligands and two bridging OH groups. Multiple hydrogen bonds of coordinated and solvate water molecules link anions and cations in a 3D network. A similar [Cr₂(μ-OH)₂(nta)₂]²⁻ unit is found in V. The bridging function is performed by a carboxylate oxygen atom of the nta ligand that leads to the formation of a trinuclear complex [Mn(bpy)(H₂O)₂Cr₂(μ-OH)₂(nta)₂]. Experimental and calculated frequency and temperature dependences of EPR spectra of these compounds are presented. The fine structure appearing on the EPR spectra of compound V is analyzed in detail at different temperatures. It is established that the main part of the EPR signals is due to the transitions in the spin states of a spin multiplet with S = 2. Analyses of experimental and calculated spectra confirm the absence of interaction between metal ions (M^II) and Cr-dimers in complexes III and IV and the presence of weak Mn–Cr interactions in V. The temperature dependence of magnetic susceptibilities for I–V was fitted on the basis of the expression derived from isotropic Hamiltonian including a bi-quadratic exchange term.     

Reactivity of {(Ph3P)Pt[μ-η-H---SiH(Ar)]}2 (Ar=2-isopropyl-6-methylphenyl) with phosphines. X-ray crystal structure of trans-{(dppe)Pt[μ-SiH(Ar)]}2

The dinuclear Pt---Si complex {(Ph₃P)Pt{μ-η²-H---SiH(IMP)]}₂ (trans-1a–cis-1b=3:1; IMP=2-isopropyl-6-methylphenyl) reacted with basic phosphines such as 1,2-bis(diphenylphosphino)ethane (dppe) and dimethylphenylphosphine (PMe₂Ph) to afford different dinuclear Pt---Si complexes with loss of H₂, {(P)₂Pt[μ-SiH(IMP)]}₂ [P=dppe, trans-2a (major), cis-2b (trace); PMe₂Ph, 3 (trans only)]. Complexes 2 and 3 were characterized by multinuclear NMR spectroscopy and X-ray crystallography (2a). In contrast, the reaction of 1a,b with the sterically demanding tricyclohexylphosphine (PCy₃) afforded {(Cy₃P)Pt{μ-η²-H---SiH(IMP)]}₂ (trans-4a–cis-4b 2:1) analogous to 1a,b where the central Pt₂Si₂(μ-H)₂ core remains intact but the PPh₃ ligands have been replaced by PCy₃. Complexes 4a and 4b was characterized by multinuclear NMR and IR spectroscopies.     

Fractional and structural characterization of hemicelluloses isolated by alkali and alkaline peroxide from barley straw

Eight hemicellulosic fractions were obtained by sequential treatment of dewaxed barley straw with 0.1 M NaOH at 45 °C for 3 h, 0.25, 0.5, 1.0, 1.5, 2.0, and 3.0% H₂O₂ at 45 °C for 3 h at pH 11.5, and 10% KOH–1% Na₂B₄O₇·10H₂O at 28 °C for 15 h under continuous agitation. The yields of the fractions were 8.0, 3.1, 3.3, 3.3, 2.2, 2.0, 2.0, and 9.9%, respectively, of the initial amount of barley straw, corresponding to the dissolution of 21.6, 8.4, 8.9, 8.9, 5.9, 5.4, 5.4, and 26.7% of the original hemicelluloses. Meanwhile, the successive treatment also solubilized 29.1, 15.8, 14.6, 10.8, 4.5, 3.2, 2.7, and 3.7% of the original lignin, respectively. This sequential extraction together resulted in dissolution of 91.1% of the original hemicelluloses and 84.8% of the original lignin. The 0.1 M NaOH-soluble hemicellulosic fraction contained mainly xylose, glucose, and arabinose, 44.2, 15.7, and 15.2%, respectively, while the 10% KOH–1% Na₂B₄O₇·10H₂O-soluble fraction predominated in xylose, 75.0%. The six alkaline peroxide-soluble fractions were composed of 50.3–54.4% xylose, 14.7–16.9% arabinose, 6.8–10.7% glucose, 6.8–8.5% glucuronic acid or 4-O-methyl- -glucuronic acid, 0.4–1.5% mannose, and 0.3–1.2% rhamnose. All the hemicellulosic fractions contained substantial amounts of glucuronoarabinoxylans and noticeable quantities of β-glucans. In comparison, the six hemicellulosic fractions, isolated with alkaline peroxide, had much higher molecular weights (56,890–63,810 g mol⁻¹) than those of the two hemicellulosic preparations (28,000–29,080 g mol⁻¹), isolated with alkali in the absence of hydrogen peroxide. The thermal stability of the hemicelluloses increased with an increment of their molar mass.     

Two new multi-cobalt-containing heteropolyoxotungstates

Two new multi-cobalt-containing polyoxotungstates K₄Na₆Co₂(H₂O)₁₂{Co(H₂O)₄[Co₂(H₂O)₁₀Co₄(H₂O)₂(B--SiW₉O₃₄)₂]₂} · 40H₂O (1) and K₁₀Na₂[Co₄(H₂O)₂(GeW₉O₃₄)₂] · 20H₂O (2) have been obtained by the routine synthetic reactions in aqueous solution. The polyoxoanion framework of 1 consists of two sandwich-type polyoxoanions [Co₄(H₂O)₂(B--SiW₉O₃₄)₂]¹²⁻ connected together by a [CoO₂(H₂O)₄] cluster to constitute the sandwich dimer, and then, four isolated Co(H₂O)₅ cations coordinate to the dimer through four μ₂-O atoms. The polyoxoanion 2 is isomorphic to the sandwich-type polyoxoanion [Co₄(H₂O)₂(B--SiW₉O₃₄)₂]¹²⁻ in 1. The magnetic property of compound 1 has been studied by measuring its magnetic susceptibility in the temperature range 2.0–300.0 K, indicating the existence of intramolecular ferromagnetic Co–Co interactions, and, the electrochemical properties of 1 and 2 are detected in the pH 4 buffer solution.     

Diplatinum(III) complexes with bridging 1-methylcytosinate ligands and variable axial ligands, including guanine nucleobases

A series of diplatinum(III) complexes derived from cis-(NH₃)₂Pt^II and the model nucleobase 1-methylcytosine (1-MeC) has been prepared and X-ray structurally characterized, all of which contain two anionic base ligands (1-MeC⁻) in a head–tail (ht) arrangement: ht-cis-[(ONO₂)(NH₃)₂Pt(1-MeC⁻-N3,N4)₂Pt(NH₃)₂(ONO₂)](NO₃)₂·HNO₃·3H₂O (2b), ht-cis-[(NO₂) (NH₃)₂ Pt(1-MeC⁻-N3,N4)₂Pt(NH₃)₂(OH₂)](ClO₄)₃·3.5H₂O (3), ht-cis-[(OH₂)(NH₃)₂Pt(1-MeC⁻-N3,N4)₂Pt(NH₃)₂(OH₂)](ClO₄)₄·H₂O (4b), and ht-cis-[(9-EtGH-N7)(NH₃)₂Pt(1-MeC⁻-N3,N4)₂Pt (NH₃)₂(9-EtGH-N7)](NO₃)₄·9H₂O (7b) (9-EtGH=9-ethylguanine). Several other compounds, differing in the nature of the axial ligands, have been isolated and or observed in solution by ¹H and ¹⁹⁵Pt NMR spectroscopy. The chemistry of these diplatinum(III) compounds is dominated by facile substitution reactions of the axial ligands. Of particular interest in this context is the ready reaction of 2b or 3 with guanine nucleobases. Since similar compounds are not obtained with any of the other common nucleobases, 2b and 3 can be considered guanine-specific chemical probes.     

Slippage of η-allenyl/propargyl to an η structure and tautomerization of η-propargyl to η-allenyl in ligand addition and substitution reactions of platinum(II) complexes

Reactions of [(PPh₃)₂Pt(η³-CH₂CCPh)]OTf with each of PMe₃, CO and Br⁻ result in the addition of these species to the metal and a change in hapticity of the η³-CH₂CCPh to η¹-CH₂CCPh or η¹-C(Ph)=C=CH₂. Thus, PMe₃ affords [(PMe₃)₃Pt(η¹-C(Ph)=C=CH₂)]⁺, CO gives both [trans-(PPh₃)₂Pt(CO)(η¹-CH₂CCPh)]⁺ and [trans-(PPh₃)₂Pt(CO)(η¹-C(Ph)=C=CH₂)]⁺, and LiBr yields cis-(PPh₃)₂PtBr(η¹-CH₂CCPh), which undergoes isomerization to trans-(PPh₃)₂PtBr(η¹-CH₂CCPh). Substitution reactions of cis- and trans-(PPh₃)₂PtBr(η¹-CH₂CCPh) each lead to tautomerization of η¹-CH₂CCPh to η¹-C(Ph)=C=CH₂, with trans-(PPh₃)₂PtBr(η¹-CH₂CCPh) affording [(PMe₃)₃Pt(η¹-C(Ph)=C=CH₂)]⁺ at ambient temperature and the slower reacting cis isomer giving [trans-(PPh₃)(PMe₃)₂Pt(η¹-C(Ph)=C=CH₂)]⁺ at 54 °C . All new complexes were characterized by a combination of elemental analysis, FAB mas spectrometry and IR and NMR (¹H, ¹³C{¹H} and ³¹P{¹H}) spectroscopy. The structure of [(PMe₃)₃Pt(η¹-C(Ph)=C=CH₂)]BPh₄·0.5MeOH was determined by single-crystal X-ray diffraction analysis.     

Syntheses and structural characterization of rhenium-bis-hydrazinopyrimidine core complexes with thiolate and Schiff base coligands

The reaction of perrhenate with 2-hydrazinopyrimidine in MeOH–HCl yields [ReCl₃(η¹-NNC₄H₃N₂H)(η²-HNNC₄H₃N₂)] (1). The analogous reaction with Na₂MoO₄ yields [MoCl₃(η¹-NNC₄H₃N₂H)(η²-HNNHC₄H₃N₂)] (1a). The reaction of 1 with pyrimidine-2-thiol and triethylamine produces [Re(η¹-C₄H₃N₂S)(η²-C₄H₃N₂S)(η¹-NNC₄H₃N₂)(η²-HNNC₄H₃N₂)] (2), while reaction of 1 with the Schiff base HSC₆H₄N=C(H)C₆H₄OH provides [Re(η³-SC₆H₄N=C(H)C₆H₄O)(η¹-NNC₄H₃N₂)(η²-HNNC₄H₃N₂)]·0.6CH₂Cl₂ (3·0.6CH₂Cl₂). The analogous hydrazinopyridine complex of the Schiff base, [Re(η³-SC₆H₄N=C(H)C₆H₄O)(η¹-NNC₅H₄N)(η²-HNNC₅H₄N)] (4), was also synthesized by reacting [ReCl₃(η¹-NNC₅H₄NH)(η²-HNNC₅H₄N)] with HSC₆H₄N=C(H)C₆H₄OH. The crystal structures of 1–4 have been determined.     

Structural studies on products of reduction of a nitro-Ni1C3N2-metallocene

The reduction of 12-nitro-(1,1,2,8,9,9-hexamethyl-3,7,10-14-tetraaza-4,6-oxa-5-hydra-tetradeca-2,7,10-12-tetrene)nickel(II) (Nioyl-NO₂), with Zn(s) and NaOH or HCl solution or utilizing Pd-H₂ under most conditions produces an intensely purplee complex ion ε_(max) at 552 nm which is not the expected amine. This product was found to be a conjugated dimer ion with two Nioyls multiply bonded to a single nitrogen atom. It was shown that the initial reduction produces the amine or amine hydrochloride which oxidizes rapidly in the presence of traces of O₂ under low acidity conditions to the dimer. Under high acidity conditions the amine salt is isolated. The X-ray crystal structures of three complexes are described: [(Nioyl)₂NH](ClO₄)₂·2.5CCl₄, [(Nioyl-NH₃)₂H]ZnCl₄Cl·3H₂O, [Nioyl-NH₃]H_0.5(ClO₄)_1.5·2CH₃CN·2H₂O and structural differences are discussed. The 2e⁻ reduction of [(Nioyl)₂N]⁺ with dithionite ion reversibly gives the yellow [(Nioyl)₂NH]⁺ which is extremely sensitive to air oxidation. A postulated reaction sequence is presented and discussed to explain the formation of the highly stable conjugated dimeric purple product.     

Influence of chelators and iron ions on the production and degradation of H2O2 by beta-amyloid-copper complexes

β-Amyloid peptide (Aβ) 1–42, involved in the pathogenesis of Alzheimer’s disease, binds copper ions to form Aβ · Cu_n complexes that are able to generate H₂O₂ in the presence of a reductant and O₂. The production of H₂O₂ can be stopped with chelators. More reactive than H₂O₂ itself, hydroxyl radicals HO (generated when a reduced redox active metal complex interacts with H₂O₂) are also probably involved in the oxidative stress that creates brain damage during the disease. We report in the present work a method to monitor the effect of chelating agents on the production of hydrogen peroxide by metallo-amyloid peptides. The addition of H₂O₂ associated to a pre-incubation step between ascorbate and Aβ · Cu_n allows to study the formation of H₂O₂ but also, at the same time, its transformation by the copper complexes. Aβ · Cu_n peptides produce but do not efficiently degrade H₂O₂. The reported analytic method, associated to precipitation experiments of copper-containing amyloid peptides, allows to study the inhibition of H₂O₂ production by chelators. The action of a ligand such as EDTA is probably due to the removal of the copper ions from Aβ · Cu_n, whereas bidentate ligands such as 8-hydroxyquinolines probably act via the formation of ternary complexes with Aβ · Cu_n. The redox activity of these bidentate ligands can be modulated by the incorporation or the modification of substituents on the quinoline heterocycle.     

One-dimensional copper phosphonates containing μ-halide, μ-pyridyl N-oxide and phosphonate bridging ligands

This paper reports the syntheses and characterization of four copper phosphonates with chain structures based on (2-pyridyl-N-oxide)phosphonate, namely, [Cu₂X₂(C₅H₄NOPO₃)₂][Cu(H₂O)₆] · 2H₂O [X = Cl (1), Br (2)] and CuX(C₅H₄NOPO₃H) · H₂O [X = Cl (3), Br (4)]. Compounds 1 and 2 are isostructural and show a chain structure where Cu(1) and Cu(2) are triply bridged by halide, oxygen donor of the pyridyl N-oxide and O–P–O group. The [Cu(H₂O)₆]²⁺ serves as a charge-balancing cation and locate between the chains together with the water molecules. Compounds 3 and 4 are also isostructural. In these cases, one of the three phosphonate oxygen atoms is protonated, thus leading to a neutral chain structure which is very similar to the anionic chains in compounds 1 and 2. Magnetic studies of compounds 1–4 reveal that antiferromagnetic interactions are mediated between the copper ions.     

Synthesis, structure, and heterogeneous catalytic activity of tungsten chalcogenide cubane cluster containing alkaline earth metal complex

A new compound containing a cubane tungsten chalcogenide cluster [W₄(μ₃-Te)₄(CN)₁₂]⁶⁻ and Ca²⁺ complex units has been prepared by the reaction of aqueous solution of K₆[W₄(μ₃-Te)₄(CN)₁₂] · 5H₂O with the solution of a Ca(NO₃)₂ and phen(1,10-phenanthroline) (1:2 molar ratio) in a solvent mixture of H₂O/EtOH. The structure of [{Ca(phen)₂(H₂O)}{Ca(phen)(H₂O)₄}{Ca(phen)₂(H₂O)₃}][W₄Te₄(CN)₁₂] · 5H₂O 1 has been determined by X-ray crystallography. Compound 1 contains [{Ca(phen)(H₂O)₄}{Ca(phen)₂(H₂O)₃}][W₄(μ₃- Te)₄(CN)₁₂] units bridged by {Ca(phen)₂(H₂O)}²⁺ units to form an one-dimensional zigzag chain structure. Interestingly, compound 1 showed a heterogeneous catalytic activity in the transesterification of a range of esters with methanol under the mild conditions. Moreover, it can be reused without any loss of activity through 10 runs with ester.     

The stereoisomerism of bacterial, reaction-center-bound carotenoids revisited: An electronic absorption, resonance Raman and H-NMR study

In order to solve discrepancies between earlier assignments we have reinvestigated the stereoisomerism of the spheroidene molecule bound to reaction centers (RC) of Rhodobacter sphaeroides. A stable cis isomer could be extracted and purified from the reaction centres by working at very low ambient light. Resonance Raman spectroscopy showed that this cis isomer assumed the same configuration as that of the RC-bound molecule. Proton-NMR spectroscopy of the extracted isomer permitted to assign it the 15–15′ mono cis configuration. Comparisons between resonance Raman spectra of the native form and of the 15 cis extract showed that, in the reaction center, 15 cis spheroidene is in addition twisted into a non-planar conformation. Comparisons of extraction-induced changes in relative intensities of Raman bands of the 760–1060 cm⁻¹ regions, which largely correspond to out-of-plane modes, further indicated that the out-of-plane twist of RC-bound spheroidene should predominantly affect C₈–C₁₂ and/or C_8′–C_12′ regions of the molecule rather than the central region. Comparisons between difference electronic absorption spectra of RC-bound spheroidene and of RC-bound methoxyneurosporene showed that the out-of-plane twisting of both these native forms results in a drastic weakening of their ¹C ← ¹A electronic transitions, compared with those of the planar, 15 cis forms. Finally, it is proposed, on the basis of their resonance Raman spectra, that spirilloxanthin bound to RCs of Rhodospirillum rubrum as well as dihydroneurosporene or dihydrolycopene bound to RCs of Rhodopseudomonas viridis shares 15 cis configurations and out-of-plane twisting with carotenoids bound to RCs of various strains of Rb. sphaeroides.     

Synthesis and characterisation of copper(II), zinc(II) and cobalt(II) complexes of salicylglycine, a metabolite of aspirin

Copper(II), nickel(II) and cobalt(II) complexes of the aspirin metabolite salicylglycine (H₂L), of stoichiometry M(HL)₂·solvate, have been prepared and characterised. In these complexes salicylglycinate is coordinated to the metal via its carboxylato group and possibly also its amide oxygen in the copper(II) complex. Under basic conditions copper(II) forms the complex Cu(LH₋₁)·2H₂O·MeOH, in which the ligand is coordinated to the metal via its carboxylate and phenolate oxygen atoms and the deprotonated peptide nitrogen atom.     

Reactivity study of cyclopentadienyl osmium(II) bisphosphine azido complexes with activated alkynes and nitriles: Isolation of osmium triazolato and tetrazolato complexes by 1,3-dipolar addition

The cyclopentadienyl osmium(II) complexes [(η⁵-C₅H₅)Os(PPh₃)₂X] [X = Br (1), CH₃CN (2)] reacts with sodium azide (NaN₃) to yield the corresponding azido complex [(η⁵-C₅H₅)Os(PPh₃)₂N₃] (3). This undergoes [3+2] dipolar cycloaddition reaction with activated alkynes like dimethyl and diethyl acetylenedicarboxylate to yield triazolato complexes [(η⁵-C₅H₅)Os(PPh₃)₂{N₃C₂(CO₂R)₂}] [R = –CH₂CH₃ (4) and –CH₃ (5)]. The complex 3 also reacts with nitriles such as tetracyanoethylene (TCE), fumaronitrile and p-nitrobenzonitrile to yield complexes of the type [(η⁵-C₅H₅)Os(PPh₃)₂{N₄C₂(CN)C(CN)₂}] (6), [(η⁵-C₅H₅)Os(PPh₃)₂{N₃C₂HCN}] (7) and [(η⁵-C₅H₅)Os(PPh₃)₂{N₄C(C₆H₄–p-NO₂)}] (8). These complexes were fully characterized on the basis of microanalyses, FT-IR and NMR spectroscopic data. The molecular structure of the representative complex [(η⁵-C₅H₅)Os(PPh₃)₂{N₃C₂(CO₂CH₂CH₃)₂}] (4) was determined by single crystal X-ray analysis.