Elisabeth Bouwman

Electrocatalytic CO2 conversion to oxalate by a copper complex.

Oxalate from Air In light of increasing concerns about the consequences of excessive atmospheric carbon dioxide, there is demand for methods to use carbon dioxide in the preparation of more elaborate compounds. Though reactions with hydroxide salts to form carbonates tend to proceed fairly cleanly, reductive processes to form carboxylic acids, esters, and alcohols are often rather unselective. Angamuthu et al. (p. 313; see the news story by Service) discovered that a copper complex exhibited remarkable selectivity in reductively coupling carbon dioxide to form oxalate through coordinative electron transfer, even in the presence of excess oxygen, normally a much more potent electron acceptor. Precipitation of the oxalate as a lithium salt and electrochemical re-reduction of the copper produced a preliminary catalytic cycle, demonstrated through six turnovers. A copper complex can reductively couple carbon dioxide, even in the presence of oxygen. Global warming concern has dramatically increased interest in using CO2 as a feedstock for preparation of value-added compounds, thereby helping to reduce its atmospheric concentration. Here, we describe a dinuclear copper(I) complex that is oxidized in air by CO2 rather than O2; the product is a tetranuclear copper(II) complex containing two bridging CO2-derived oxalate groups. Treatment of the copper(II) oxalate complex in acetonitrile with a soluble lithium salt results in quantitative precipitation of lithium oxalate. The copper(II) complex can then be nearly quantitatively electrochemically reduced at a relatively accessible potential, regenerating the initial dinuclear copper(I) compound. Preliminary results demonstrate six turnovers (producing 12 equivalents of oxalate) during 7 hours of catalysis at an applied potential of –0.03 volts versus the normal hydrogen electrode.

Homogeneously catalysed isomerisation of allylic alcohols to carbonyl compounds

Abstract Isomerisation of allylic alcohols forms an elegant shortcut to carbonyl compounds in a completely atom-economical process that offers several useful applications in natural-product synthesis and in bulk chemical processes. This review focuses on the heart of isomerisation catalysis: the catalyst. Combinations of transition metals (from Group 4 to 10), ligands and reaction conditions are compared with respect to yield, turnovers, rate and selectivity. A selected number of clever solutions to synthetic problems are highlighted, such as the synthesis of enols and enolates, chiral carbonyl compounds and silyl substituted ketones. Furthermore, a general overview of the mechanisms proposed for the isomerisation of allylic alcohols is given while some catalyst systems are singled out to discuss mechanistic research.

Mixed-Lanthanoid Metal–Organic Framework for Ratiometric Cryogenic Temperature Sensing

A ratiometric thermometer based on a mixed-metal Ln(III) metal-organic framework is reported that has good sensitivity in a wide temperature range from 4 to 290 K and a quantum yield of 22% at room temperature. The sensing mechanism in the europium-doped compound Tb0.95Eu0.05HL (H4L = 5-hydroxy-1,2,4-benzenetricarboxylic acid) is based not only on phonon-assisted energy transfer from Tb(III) to Eu(III) centers, but also on phonon-assisted energy migration between neighboring Tb(III) ions. It shows good performance in a wide temperature range, especially in the range 4-50 K, reaching a sensitivity up to 31% K(-1) at 4 K.

Selective Conversion of Hydrocarbons with H2O2 Using Biomimetic Non-heme Iron and Manganese Oxidation Catalysts

Publisher Summary This chapter discusses selective conversion of hydrocarbons with H2O2 using biomimetic non-heme iron and manganese oxidation catalysts. Oxidation catalysis is particularly an important technological area that lies at the heart of a variety of processes for producing bulk and fine chemicals and for eliminating pollution. In technical chemistry, oxidation processes play a central role and serve as a basis for the generation of a number of major intermediates and final products. More than 20% of all industrial organic materials are obtained by catalytic oxidation. In nature, catalysis is of vital importance in a wide variety of biochemical processes catalyzed by metalloenzymes. The functions performed by the metalloenzymes and their significance have inspired a range of biomimetic studies, and intense efforts have been concentrated on the synthesis of low molecular weight complexes to model the spectroscopic features of such enzymes. Various biomimetic manganese complexes are identified to perform catalytic hydrocarbon oxidation in combination with dihydrogen peroxide.

Synthesis, characterization and crystal structures of nickel complexes with dissymmetric tetradentate ligands containing a mixed-donor sphere

Abstract Four new nickel(II) complexes of dissymmetric tetradentate ligands, containing mixed-ligand donor sets of NSNS or NSNO, have been synthesized. These complexes were prepared by facile template reactions of the appropriate aldehyde and amine in the presence of [Ni(H 2 O) 6 ](BF 4 ) 2 , resulting directly in the desired nickel compounds. The nickel compounds were characterized by analytical, spectroscopic and electrochemical methods. The structures of [Ni(pyzs)]BF 4 , [Ni(pyrs)]BF 4 and [Ni(pyzo)]BF 4 (see Scheme 1 ) have been determined by single-crystal X-ray crystallography, showing the geometry of the nickel ion to be square-planar. Vis–NIR spectra show that the phenolate-containing complexes [Ni(pyzo)]BF 4 and [Ni(pyro)]BF 4 (see Scheme 1 ) are essentially square-planar in nitromethane, but tetragonal octahedral in methanol, whereas the thiophenolate-containing compounds [Ni(pyzs)]BF 4 and [Ni(pyrs)]BF 4 remain square-planar in both solvents. Titration of the thiophenolate-containing complexes with 1-methylimidazole results in diamagnetic five-coordinated complexes. Electrochemistry shows quasi-reversible reductions to Ni(I) to occur for [Ni(pyzo)]BF 4 , [Ni(pyrs)]BF 4 and [Ni(pyro)]BF 4 .

N-donor functionalized N-heterocyclic carbene nickel(II) complexes in the Kumada coupling

The synthesis and characterization of novel nickel(II) complexes bearing two bidentate N-heterocyclic carbene ligands functionalized with anionic N-donor moieties are described. Two different N-donor groups are employed, namely amido and benzimidazolato moieties. The solid-state structures of three of these complexes have been determined by X-ray crystallography. The amido-functionalized low-spin, square-planar Ni(II) complexes exhibit a cis geometry around the metal centre, while the benzimidazolato-functionalized complex crystallizes as the trans isomer. The activity of these novel complexes in the Kumada cross-coupling of phenylmagnesium chloride with 4-chloroanisole and 4-fluoroanisole was investigated. One of the benzimidazolato-functionalized complexes shows the highest activity in this reaction reported to date, yielding the desired product in quantitative yields within 30 min (4-chloroanisole), or 150 min (4-fluoroanisole) with only 1 mol% catalyst.

Two reactions of allylic alcohols catalysed by ruthenium cyclopentadienyl complexes with didentate phosphine ligands: isomerisation and ether formation

The activity of chloro(η5-cyclopentadienyl)(didentate phosphine)ruthenium(II) complexes in the catalysis of two types of reaction with allylic alcohols is described. The isomerisation of 3-buten-2-ol to butanone (MEK) and allyl alcohol to propanal proceeds at high rates. A trend in catalytic activity is observed upon variation of the carbon-chain length in the didentate phosphine ligand: dppm<dppe<dppp<dppb. Complexes with rigid didentate phosphine ligands like cis-dppv and dppph show no activity. The second type of reaction constitutes the first example of a ruthenium-catalysed ether formation directly from allylic alcohols. Homo-coupled ethers like di-allyl ether (DAE) are easily formed as well as ethers from heterocoupling of allyl alcohol with aromatic and aliphatic alcohols. In fact, the ruthenium complexes achieve much higher turnover frequencies and turnover numbers than reported before in palladium-catalysed ether formation. Complexes with dppe and dppp in the presence of a conjugated diene switch from isomerisation to ether formation, but the new complex [RuClCp(o-MeO-dppe)] (Cp=η5-cyclopentadienyl) (3) has proven to be very active in ether formation, even in the absence of a diene. The mechanisms of the reactions have been studied by using both deuterium-labelled substrates and 31P nuclear magnetic resonance (NMR).

Catalytic and electrochemical properties of new manganese(III) compounds of 2-(2′-hydroxyphenyl)-oxazoline (Hphox or HClphox). Molecular structures of [Mn(Clphox)2(MeOH)2](ClO4) and [Mn(phox)2(MeOH)2][Mn(phox)2(ClO4)2](H2O)2

Abstract New manganese(III) complexes of Hphox (2-(2′-hydroxyphenyl)-oxazoline) and HClphox (2-(5′-chloro-2′-hydroxyphenyl)-oxazoline) have been synthesised. The X-ray structures of [Mn(phox) 2 (MeOH) 2 ][Mn(phox) 2 (ClO 4 ) 2 ](H 2 O) 2 and [Mn(Clphox) 2 (MeOH) 2 ](ClO 4 ) show the manganese(III) ions to be octahedrally coordinated with methanol or perchlorate at the axial coordination sites. The cyclic voltammograms of the complexes, with the exception of [Mn(phox) 2 (acac)] (Hacac=2,4-pentanedione), show an irreversible reduction wave of manganese(III) to manganese(II). After addition of an excess of 1-methylimidazole (1-Meim), the reduction process shifts towards lower potentials and becomes (quasi-) reversible, indicating that the presence of 1-Meim affects the catalytic efficiency of the complexes. The complexes catalyse the epoxidation of styrene by dihydrogen peroxide. The cumulative turnover numbers towards styrene oxide obtained after 15 min. vary from 16 for [Mn(Clphox) 2 (MeOH) 2 ](ClO 4 ) to 26 for [Mn(phox) 2 (acac)]. Ligand degradation appears to be the limiting factor for obtaining higher turnover numbers.

Mechanistic implications of the active species involved in the oxidation of hydrocarbons by Iron complexes of pyrazine-2-carboxylic acid

The reactivity towards H(2)O(2) of the complexes [Fe(pca)(2)(py)(2)].py (1) and Na(2){[Fe(pca(3))](2)O}.2H(2)O.CH(3)CN (2) (where pca(-) is pyrazine-2-carboxylate) and their catalytic activity in the oxidation of hydrocarbons is reported. Addition of H(2)O(2) to 1 results in the formation of a dinuclear Fe(III)-(mu-O)-Fe(III) species characterized spectroscopically and by cyclic voltammetry. By contrast, treatment of 2 with H(2)O(2) results in the formation of mononuclear iron(II) complexes, [Fe(pca)(2)(solvent)(2)]. The experimental results indicate that the catalytic activity of the starting complexes 1 and 2 is strongly dependent on the species formed in solution.

Ruthenium-Catalyzed Isomerization of Allylic Alcohols: Oxidation State Determines Resistance Against Diene Inhibition

The novel complex mer-[RuCl3(dmso)(phen)] (1) has been prepared and characterized by X-ray diffraction. The ruthenium center is in a distorted octahedral environment with the three chloride ions coordinated in a mer-fashion and an S-bonded dmso ligand trans to one of the phen nitrogen atoms. Electrochemical experiments show two reversible waves in acetonitrile solution, corresponding to the couples RuIII/II (0.11 V) and Ru IV/III (1.73 V). The analogous Ru II complex cis,cis-[RuCl2(dmso)2(phen)] (2) shows one reversible wave at 1.08 V, corresponding to the Ru III/II couple. Both complexes exhibit high catalytic activity in the isomerization of 3-buten-2-ol to butanone. The initial turnover frequency (TOF) for 1 in diglyme/water at 130 °C is 295 h −1 with a firstorder kini of 0.6 h −1 , while 2 reaches an initial TOF of 260 h −1 and a kini of 0.5 h −1 . A cumulative turnover number (TON) of 1025 has been obtained with 1 as a catalyst precursor. The activity of 1 and 2 has been compared to that of in situ mixtures with both Ru III and Ru II precursors. All Ru II complexes are deactivated before 100% conversion has been attained.