Formation of chiral ionic liquids and imidazol-2-ylidene metal complexes from the proteinogenic aminoacid l-histidine

Treatment of the amino acid derivative Bz-His-OMe with excess n-propyl bromide gave the corresponding histidinium salt [Bz-His(n-propyl)₂-OMe⁺Br⁻]. It features a melting point of 39 °C and may serve as a useful readily available optically active ionic liquid. Its subsequent treatment with silver oxide gave the corresponding l-histidine derived chiral N-heterocyclic carbene complex [“(carbene)₂Ag · AgBr₂”]. Transmetallation by treatment with Pd(CH₃CN)₂Cl₂ or [Rh(cod)Cl]₂ led to the formation of the respective chiral late metal imidazol-2-ylidene complexes [“(carbene)₂PdCl₂”] and [“(carbene)RhCl(cod)”], respectively. Four diastereomers of the square planar palladium system were observed. Due to the additional chirality center in the l-histidine-derived “Arduengo-carbene ligand” two diastereomers of the rhodium carbene complex were formed.     

The reaction of amidinato(pyridine) complexes of molybdenum and tungsten with triethylborane adduct of N-heterocyclic carbene (NHC · BEt3): Investigation on the reactivity of NHC · BEt3 as carbene precursor toward transition metal complexes

The reaction of triethylborane adduct of N-heterocyclic carbene, NHC · BEt₃, (NHC = IⁱPr = 1,3-diisopropylimidazol-2-ylidene (IⁱPr · BEt₃; 1a), NHC = IMes = 1,3-dimesitylimidazol-2-ylidene (IMes · BEt₃; 1b)), which was prepared by the reaction of the corresponding imidazolium salt with one equivalent of LiBEt₃H, with amidinato(pyridine) complex, [M(η³-allyl){η²-(NPh)₂CH}(CO)₂(NC₅H₅)] (M = Mo; 2-Mo M = W; 2-W), was investigated. The reaction of compound 1 with complex 2 under toluene-reflux conditions resulted in the formation of carbene complex [M(η³-allyl){η²-(NPh)₂CH}(CO)₂(NHC)] (M = Mo, NHC = IⁱPr; 3a-Mo, M = Mo, NHC = IMes; 3b-Mo, M = W, NHC = IⁱPr; 3a-W, M = W, NHC = IMes; 3b-W). These complexes were characterized spectroscopically as well as by X-ray analyses. Complex 3a-Mo was formed in various solvents such as 1,2-dimethoxyethane (DME), 1,2-dichloroethane, and acetonitrile under refluxing conditions for 3 h. In toluene, 3a-Mo was obtained in a good yield by heating at 70 °C for only 20 min. Employment of NHC · BEt₃ (1) was found to afford convenient route for the introduction of the carbene ligand to the transition metal complexes.     

Metal–metal bond cleavage of carbene complexes by halogens: The crystal and molecular structures of ax-[Mn2(CO)9{C(OEt)2-thienyl}], [Mn(CO)4(I){C(OEt)2-thienyl}] and eq-[Mn2(CO)9{C(NH2)2-thienyl}]

The metal–metal bond in [M₂(CO)₉{C(OEt)R}] (M = Mn (1), Re (2), R = 2-thienyl (a), 2-bithienyl (b)) is readily cleaved with halogens to afford cis-[M(CO)₄(X){C(OEt)R}] (M = Mn (3), X = I; M = Re (4), X = Br). In the binuclear manganese complex, the carbene ligand is found in an axial position due to steric reasons, whereas the electronically favoured equatorial position is found for the carbene ligands in the corresponding rhenium complexes and in [Mn₂(CO)₉{C(NH₂)thienyl}] (5a), containing a sterically less demanding NH₂-substituent.     

Divergent reactivity of alk-5-ynylidenecyclopropanes in the presence of the 1st or the 2nd generation Grubbs’ catalysts

Alk-5-ynylidenecyclopropanes, by virtue of being equipped with a strained cyclopropane system, can be divergently elaborated into bicyclo[3.3.0]octenes or exocyclopropylidenecycloalkenes depending on whether they react with the first or the second generation Grubbs’ ruthenium carbenes. While the highly reactive second generation system catalyses the formation of ring-closing metathesis products, the less [metathesis] active first generation carbene promotes an intramolecular [3 + 2] cycloaddition to give the bicarbocyclic skeletons.     

Fast transfer hydrogenation using a highly active orthometalated heterocyclic carbene ruthenium catalyst

The free carbene 1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene reacts with trans,cis-RuHCl(PPh₃)₂(ampy) (ampy = 2-(aminomethyl)pyridine) affording an orthometalated N-heterocyclic carbene complex characterized by an X-ray diffraction study. This compound in presence of NaOH shows very high catalytic activity for the transfer hydrogenation of several ketones to alcohols using 2-propanol as hydrogen source, affording TOF values up to 120,000 h⁻¹ (at 50% conversion).     

Functionalized N-heterocyclic carbene iridium complexes: Synthesis,structure and addition polymerization of norbornene

Picolyl, pyridine, and methyl functionalized N-heterocyclic carbene iridium complexes [Cp¹Ir(C^N)Cl]Cl (4, C^N = 3-Methyl-1-picolyimidazol-2-ylidene), [Cp¹Ir(C^N)Cl][Cp¹IrCl₃] (5), [Cp¹Ir(C-N)Cl]Cl (6, C-N = 3-Methyl-1-pyridylimidazol-2-ylidene) and [Cp¹Ir(L)Cl₂] (7, L = 1,3-dimethylimidazol-2-ylidene) have been synthesized by transmetallation from Ag(I) carbene species, and characterized by ¹H NMR, ¹³C NMR spectra and elemental analyses. The molecular structures of 5–7 have been confirmed by X-ray single-crystal analyses. The iridium carbene complexes 4 and 6 show moderate catalytic activities (3.03 × 10⁵ g PNB (mol Ir)^?1 h^?1 and 1.70 × 10⁶ g PNB (mol Ir)^?1 h^?1) for the addition polymerization of norbornene in the presence of methylaluminoxane (MAO) as co-catalyst. The produced polynorbornene have been characterized by IR, ¹H NMR and ¹³C NMR spectra, showing it follows the vinyl-addition-type of polymerization.     

Photochemical generation of carbonate radicals and their reactivity with phenol

The reaction of carbonate radical with phenol in aqueous solution has been investigated in systems in which carbonate radicals were generated by UV irradiation of an aqueous solution of [Co(NH₃)₅CO₃]⁺ (pH 8.0 phosphate buffer). Both steady state and time resolved photolysis experiments were performed. Upon continuous irradiation of complex phenol mixtures, phenol was converted into benzoquinone and dihydroxybenzenes. Benzoquinone was the major by-product in the early stages of the reaction. Laser flash excitation (266 and 355 nm) of the cobalt complex clearly showed the formation of the carbonate radical. When phenol was added to the solution of the complex, a second species was observed which was assigned to the phenoxyl radical. The second-order rate constant of reaction between phenol and carbonate radical was found to be equal to 1.6 × 10⁷ M⁻¹ s⁻¹, in agreement with literature data of 2.2 × 10⁷ M⁻¹ s⁻¹.     

Neptunium(III) application in extraction chromatography

This paper describes a novel strategy for actinide separation by extraction chromatography with Np(III) valence adjustment. Neptunium(IV) was reduced to Np(III) using Cr(II) and then selectively separated from uranium (IV) on a TEVA resin. After elution, Np(III) was retained on a DGA resin in order to remove any detrimental chromium impurities. Neptunium(III) formation was demonstrated by the complete and selective elution of Np from TEVA resin (99 ± 7%) in less than 12 mL of 9 M HCl from U(IV) (0.7 ± 0.7%). It was determined by UV–visible and kinetic studies that Cr(II) was the only species responsible for the elution of Np(IV) as Np(III) and that the Cr(II) solution could be prepared from 2 to 30 min before its use without the need of complex degassing systems to prevent the oxidation of Np(III) by oxygen. The methodology proposed here with TEVA/DGA resins provides removal of Cr(III) impurities produced at high decontamination factors (2.8 × 10³ and 7.3 × 10⁴ respectively).     

A mass spectrometric investigation of the vaporisation behaviour in the (U + Pu + O) system

The vaporisation behaviour of (U, Pu)O₂ mixed oxides (Pu/M = 0.25, 0.50 and 0.75, with M = U + Pu) was studied by means of mass spectrometry. Hyperstoichiometric samples were heated in a Knudsen cell up to T = 2300 K. The evolution of the uranium and plutonium bearing gaseous species was studied as a function of time in order to evaluate the congruent vapour composition. Ionisation efficiency measurements were performed and the partial pressures of the gaseous species involved in the vaporisation process were determined. The vapour pressure has also been calculated using a thermochemical model for the (U + Pu + O) system. A quasi congruent composition with respect to the O/M ratio has been assumed, in agreement with the experiments. Nevertheless, the evaluation of all the experimental and calculated results shows that a total congruent composition exists for a single composition of the mixed oxide (MOX) samples with a Pu/M content slightly lower than 0.50. A good agreement is obtained between the calculated and experimental vapour pressure data, as well as the quasi congruent vaporisation compositions.     

Uranium and Plutonium Extraction by N,N-dialkylamides Using Multistage Mixer-settler Extractors

N,N-Dialkylamides (monoamides) are known as extractants for U and Pu, and many studies have been carried out mainly by single-stage batch method. We have focused on two monoamides: N,N-di(2-ethylhexyl)-2,2-dimethylpropanamide (DEHDMPA) and N,N-di(2-ethylhexyl)butanamide (DEHBA), and proposed a multistage extraction process for recovering U and Pu by these monoamides. A continuous counter-current experiment was carried out to demonstrate the validity of this process. This process consisted of two cycles, and the 1st cycle and the 2nd cycle employed DEHDMPA and DEHBA as extractants, respectively. The feed solution for the 1st cycle was 5.1 mol/dm³ (M) nitric acid containing 0.92 M U, 1.6 mM Pu, and 0.6 mM Np. The raffinate collected in the 1st cycle was used as the feed for the 2nd cycle. The ratios of U recovered in the U fraction and U-Pu fraction were 99.1% and 0.8%, respectively, and the ratios of U in the used solvents were <0.04%. The ratio of Pu recovered in the U-Pu fraction was 99.7%, and the ratio of Pu in the used solvents was in the order of 10^–3–10^–4%. The concentration ratio of U with respect to Pu in the U-Pu fraction was 9, and this indicated that Pu was not isolated. The decontamination factor of U with respect to Pu in the U fraction was obtained as 4.5×10⁵. These results supported the validity of the proposed process.     

Excited state properties of [Pd0(NHC)(quinone)]2 with NHC = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene and quinone = 1,4-naphthoquinone

The carbene complex [Pd⁰(NHC)(quinone)]₂with NHC = 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene and quinone = 1,4-naphthoquinone shows two long-wavelength absorptions at 312 and 399 nm which are assigned to (NHC→quinone) LLCT and (Pd⁰ → quinone) MLCT transitions. The MLCT state is not reactive, but emissive (λ_max = 564 nm at 77 K). At r.t., the complex undergoes a photoredox decomposition which is initiated by the LLCT state.     

Toward more efficient photochemical CO2 reduction: Use of scCO2 or photogenerated hydrides

Rhenium(I) and ruthenium(II) complexes have been successfully used for photochemical CO₂ reduction to CO or formate. However, a typical turnover frequency for such reactions is <20 h^?1 and the formation of reduced species beyond CO or formate is very limited. In the case of the rhenium(I) bipyridyl tricarbonyl system, the key intermediate has been shown to decay with a first-order dependence on [CO₂] to produce CO, which is the rate-determining step. The limited concentration of dissolved CO₂ in organic solvents results in extremely slow CO₂ reduction. To improve the reaction rate, we prepared new CO₂-soluble rhenium(I) bipyridine complexes bearing fluorinated alkyl ligands and investigated their photophysical properties in CH₃CN and supercritical CO₂. We also investigated the properties of a metal complex with an NAD⁺ model ligand, [Ru(bpy)₂(pbn)]²⁺ (bpy = 2,2′-bipyridine, pbn = 2-(2-pyridyl)-benzo[b]-1,5-naphthyridine), and prepared the corresponding NADH-like complex [Ru(bpy)₂(pbnHH)]²⁺ upon MLCT excitation followed by reductive quenching. This species can be used as a renewable hydride donor. The electrochemical and photochemical properties, and the reactivity of these species toward CO₂ reduction were investigated.     

Influence of the structure of medium-sized aromatic precursors on the reactivity of their dications towards rare gases

The gas-phase reactivity of dications generated by dissociative electron ionization of several aromatic C_mH_nN_o precursors with 4 ≤ m ≤ 13, 4 ≤ n ≤ 21, and 0 ≤ o ≤ 2 with rare gases is investigated. Whereas most of these reactions lead to monocations via simple electron transfer, proton transfer, or Coulomb explosion, the formation of organo rare-gas dications is observed in a few cases. Specifically, dications generated from 2,4,6-trimethylpyridine react with krypton and xenon to form organo rare-gas species as major products and under maintenance of the two-fold positive charge. Such a reactivity is not observed in the presence of lighter rare gases. The formation of organo rare-gas dications are also observed for dications generated from 3-vinylpyridine, N,N-dimethylaniline, isopropylbenzene, and 4-ethyltoluene as neutral precursors. In some cases, isomeric dications are characterized by very different reactivity toward rare gases, suggesting that the structure of the precursors is crucial and that electron ionization does not lead to a total scrambling of the structures of the doubly charged ions obtained.     

X-ray absorption LIII and MV edges of hexavalent lower actinides

X-ray absorption spectroscopy is used to probe the electronic and structural features of the actinides. To better understand the effect of the effective charge and polyhedron on the edge structure, we discuss the L_III and M_V edges in AnO₂(NO₃)₂(TBP)₂ (where the actinide is shortly linked to the ‘yl’ oxygens) and in the perovskite Ba₂ZnAnO₆ (where the hexavalent actinide is in the cubic structure as An⁶⁺), An=U, Np, Pu. Using FEFF8 simulations, both features and position of the L_III and M_V edges are explained within a crystal field splitting of the orbitals picture and a multiple scattering picture. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SASXANES / uranium / neptunium / plutonium / perovskite     

Silver alkoxide and amino N-heterocyclic carbenes; syntheses and crystal structures

Silver(I) complexes of heterobidentate ligands that incorporate one or two N-heterocyclic carbene moieties coupled with an alcohol or amine group have been made by direct deprotonation of ligands of the form [HOCR¹R²CH₂(1-HC{NCHCHNR})][X], H₂L¹X (X = Br, I), [H₂NR¹CHR²CHR²(1-HC{NCHCHNR})][Br]₂ H₃L²X₂ (X = Cl, Br), and [H₂N{CH₂CH₂(1-HC[NCHCHNMes])}₂][X]₃ H₄L³X₃ (X = Cl, Br). Silver(I) oxide is sufficiently basic to deprotonate both the imidazolium and the alcohol functional groups of all but one of the L¹ ligand precursors, to afford rare examples of silver alkoxide complexes [Ag(L¹)], stabilised by the soft donor carbene. Another complex of L¹ is characterised as the carbene alcohol adduct [Ag(HL¹)₂I]. The analogous reactions of silver(I) oxide with the amino imidazolium precursors afford silver amino-carbenes [Ag(HL²)Br] with the potentially bidentate L² ligand, and [Ag(HL³)X] (X = Cl, Br) with the potentially tridentate L³ ligand. A single crystal X-ray diffraction study of the latter complex confirms that the neutral amine of the potentially tridentate L³ ligand is unco-ordinated; instead the structure contains discrete chains of T-shaped silver bis(carbene) halide moieties that bridge to form a zig-zag 2-connected polymer. Protonolysis of two of the silver alkoxide and amino adducts, [Ag(L^1a)] and [Ag(HL^2a)Br], affords imidazolium complexes salts [H₂L^1a][AgCl₂] and [Ag(H₂L^2a)Br][AgBr₂] that retain the Ag(I) centre as complex counterions. The single crystal X-ray structures of these salts have been determined and show the silver(I) cations are now incorporated into ladders or chains as silver(I) halo-anions, and a silver amine dative bond is present in the latter complex.     

Synthesis of ZnS nanoparticles from pyridine adducts of zinc(II) dithiocarbamates

Zn(thqdtc)₂, Zn(thqdtc)₂(py) and Zn(thiqdtc)₂(py) (where thqdtc = 1,2,3,4-tetrahydroquinolinecarbodithioate, thiqdtc = 1,2,3,4-tetrahydroisoquinolinecarbodithioate and py = pyridine) have been used as single source precursors for the synthesis of ZnS nanoparticles. The formation of ZnS nanoparticles was achieved by thermal decomposition of the complex under heating in presence of triethylenetetraamine. Transmission electron microscopy, energy dispersive X-ray analysis (EDAX) and powder X-ray diffraction studies were carried out to study the structure and morphology of the nanoparticles. The optical properties of the ZnS nanoparticles were studied by UV–visible and fluorescence emission spectral studies. UV–visible absorption spectral studies indicate a blue shift in the absorption maxima due to the quantum size effect. A single crystal X-ray analysis was carried out for a precursor [Zn(thqdtc)₂].     

New tripodal N-heterocyclic carbene chelators for small molecule activation

In an effort to develop new tripodal N-heterocyclic carbene (NHC) ligands for small molecule activation, two new classes of tripodal NHC ligands TIME^R and TIMEN^R have been synthesized. The carbon-anchored tris(carbene) ligand system TIME^R (R = Me, t-Bu) forms bi- or polynuclear metal complexes. While the methyl derivative exclusively forms trinuclear 3:2 complexes [(TIME^Me)₂M₃]³⁺ with group 11 metal ions, the tert-butyl derivative yields a dinuclear 2:2 complex [(TIME^t-Bu)₂Cu₂]²⁺ with copper(I). The latter complex shows both “normal” and “abnormal” carbene binding modes and accordingly, is best formulated as a bis(carbene)alkenyl complex. The nitrogen-anchored tris(carbene) ligands TIMEN^R (R = alkyl, aryl) bind to a variety of first-row transition metal ions in 1:1 stoichiometry, affording monomeric complexes with a protected reactivity cavity at the coordinated metal center. Complexes of TIMEN^R with Cu(I)/(II), Ni(0)/(I), and Co(I)/(II)/(III) have been synthesized. The cobalt(I) complexes with the aryl-substituted TIMEN^R (R = mesityl, xylyl) ligands show great potential for small molecule activation. These complexes activate for instance dioxygen to form cobalt(III) peroxo complexes that, upon reaction with electrophilic organic substrates, transfer an oxygen atom. The cobalt(I) complexes are also precursors for terminal cobalt(III) imido complexes. These imido complexes were found to undergo unprecedented intra-molecular imido insertion reactions to form cobalt(II) imine species. The molecular and electronic structures of some representative metal NHC complexes as well as the nature of the metal–carbene bond of these metal NHC complexes was elucidated by X-ray and DFT computational methods and are discussed briefly. In contrast to the common assumption that NHCs are pure σ-donors, our studies revealed non-negligible and even significant π-backbonding in electron-rich metal NHC complexes.     

Multinuclear NMR and theoretical investigation on interactions between diperoxovanadate complex and 4-picoline-like ligands

To understand the 4-substituting group effects of organic ligands in pyridine ring on the reaction equilibrium, the interactions between a series of 4-picoline-like ligands and [OV(O₂)₂(D₂O)]^?/[OV(O₂)₂(HOD)]^? in solution were explored by the combined use of multinuclear (¹H, ¹³C, and ⁵¹V) magnetic resonance, DOSY, and variable-temperature NMR in 0.15 mol/L NaCl ionic medium for mimicking the physiological condition. Some direct NMR data are given for the first time. The reactivity among the 4-picoline-like ligands is 4-picoline > isonicotinate > isonicotinamide > ethyl isonicotinate. The competitive coordination results in the formation of a series of new six-coordinated peroxovanadate species [OV(O₂)₂L]^n? (L = 4-picoline-like ligands, n = 1 or 2). The results of density functional calculations provide a reasonable explanation on the relative reactivity of the 4-picoline-like ligands. Solvation effects play an important role in these reactions.     

The effect of copper content on the reactivity of Cu/Co6Al2 solids in the catalytic steam reforming of methane reaction

The steam reforming of methane over Cu/Co₆Al₂ mixed oxides with different copper contents was studied. The Co₆Al₂ support was prepared via the hydrotalcite route. It was thermally stabilized at 500 °C, impregnated with 5 wt.%, 15 wt.% or 25 wt.% copper using copper (II) nitrate Cu(NO₃)₂·3H₂O precursor and then calcined again at 500 °C under an air flow. The impregnation of copper enhanced significantly the reactivity of the solids in the considered reaction. The 5Cu/Co₆Al₂ solid was the most reactive one, with a methane conversion of 96% at 650 °C. The selectivities of H₂ and CO₂ were also better for the catalyst containing 5 wt.% copper compared to higher copper loadings. The decrease in the catalytic reactivity with increasing the copper content was attributed to the formation of agglomerated and less reactive CuO species, which were detected by XRD and TPR analyses.     

First examples of dipyrido[1,2-c:2′,1′-e]imidazolin-7-ylidenes serving as NHC-ligands: Synthesis,properties and structural features of their chromium and tungsten pentacarbonyl complexes

The first use of dipyridocarbenes as Arduengo–Wanzlick type carbene ligands for transition metal complexes is reported. The complexes M(CO)₅L (L = dipyridoimidazolinylidene, di-tert-butyldipyridoimidazolinylidene, M = Cr, W) were synthesized and their spectroscopic and structural properties compared with the literature known N-heterocyclic carbene (NHC) group 6 metal pentacarbonyl complexes. This reveals that the ¹³C NMR carbene signals of theses complexes with dipyrido carbene ligands show the strongest high-field shift ever observed for M(CO)₅(NHC) (M = Cr, W) complexes. The structural characterization shows alternating single and double bonds in the conjugated dipyrido moiety of the ligand.