Aktivierung von CO2 an Übergangsmetallzentren: Zum Ablauf der CO2-Reduktion an Nickel(0)-Fragmenten

Activation of CO₂ at Transition Metal Centres: The Route of the CO₂ Reduction at Nikel(0) Moieties A competing reaction in the catalytic cyclooligomerization of hex-3-yne and CO₂ at the (TMED)Ni(0)-fragment (TMED = N,N,N′,N′-tetramethylethylendiamine) is the formation of carbon monoxide and (TMED)Ni(CO₃). So it is possible to explain the generation of II (TMED)Ni(diethylmalicacidanhydride) and III (a nickel trimer with two (TMED)Ni(CO₃) units). Both complexes are characterized by X-ray analysis. The reduction of CO₂ to CO most likely proceeds via an intermediate in which two molecules of carbon dioxide are coupled head-to-tail to form a metallacycle. An ab initio scf geometry optimization supports the existence of such an intermediate.     

Oxidative Kopplung von Kohlendioxid mit substituierten Furfuryliden‐iminen an Nickel(0)‐Zentren: Struktur und Reaktivität der gebildeten Nickelacyclen

Furfurylidene‐imines as Components of the Oxidative Coupling with CO₂ at Nickel(0) centers: Influence of the Substituents on the Structure of the resulting Nickelacycles The oxidative coupling between benzaldehyd‐N‐furfurylidene‐imine, CO₂ und Ni(cod)₂ results in THF or 1.4‐dioxane in the formation of the organometallic macrocycle 1a , even if a large excess of Schiff base was used. 1a reacts with Ph₃P under partial elimination of CO₂ to form the tetranuclear complex 2 , which contains two nickela‐aziridine rings linked with two nickelacyclic carbamates. Surprisingly, the protolysis of 1 followed by elimination of CO₂ results in the formation of the organometallic product 3 which also contains a Ni‐C bond in a (protonated) nickelaaziridine ring. Para‐hydroxybenzaldehyd‐N‐furfurylidene‐imine ( B ), CO₂ und Ni(cod)₂ react under oxidative coupling to form the nickelacycle 4 , in which the monomeric metallacyclic units are connected by a hydrogen‐bonded network to form a polymeric supramolecular system. Two of the Schiff bases B coordinate as N‐donors at the Ni^II center, the third Schiff base acts as the substrate for CO₂. Ferrocene‐carbaldehyde‐N‐furfurylidene‐imine forms in the presence of bipy the monomeric nickelacycle 22 containing the intact ferrocenyl unit. The solid‐state structures of 2, 3, 4, and 22 were determinated by X‐ray crystallography. Comparison of CO valence frequencies allows to indicate the coordination mode in nickelacycles with other Schiff bases.     

The reactions of Cr(CO)6, Fe(CO)5, and Ni(CO)4 with O2 yield viable oxo‐metal carbonyls

Transition metal complexes with terminal oxo and dioxygen ligands exist in metal oxidation reactions, and many are key intermediates in various catalytic and biological processes. The prototypical oxo‐metal [(OC)₅Cr? O, (OC)₄Fe? O, and (OC)₃Ni? O] and dioxygen‐metal carbonyls [(OC)₅Cr? OO, (OC)₄Fe? OO, and (OC)₃Ni? OO] are studied theoretically. All three oxo‐metal carbonyls were found to have triplet ground states, with metal‐oxo bond dissociation energies of 77 (Cr? O), 74 (Fe? O), and 51 (Ni? O) kcal/mol. Natural bond orbital and quantum theory of atoms in molecules analyses predict metal‐oxo bond orders around 1.3. Their featured ν(MO, M = metal) vibrational frequencies all reflect very low IR intensities, suggesting Raman spectroscopy for experimental identification. The metal interactions with O₂ are much weaker [dissociation energies 13 (Cr? OO), 21 (Fe? OO), and 4 (Ni? OO) kcal/mol] for the dioxygen‐metal carbonyls. The classic parent compounds Cr(CO)₆, Fe(CO)₅, and Ni(CO)₄ all exhibit thermodynamic instability in the presence of O₂, driven to displacement of CO to form CO₂. The latter reactions are exothermic by 47 [Cr(CO)₆], 46 [Fe(CO)₅], and 35 [Ni(CO)₄] kcal/mol. However, the barrier heights for the three reactions are very large, 51 (Cr), 39 (Fe), and 40 (Ni) kcal/mol. Thus, the parent metal carbonyls should be kinetically stable in the presence of oxygen. © 2014 Wiley Periodicals, Inc.     

Trifluoromethyl substituted ring systems: synthesis,and reactivity towards metal carbonyls

Hexafluoro-but-2-yne and actafluoro-but-2-ene both readily add to cyclopentadiene. Similar Diels-Alder reactions occur between hexafluoro-but-2-yne and cycloheptatriene and cyclooctatetraene. 2,3-Bis(trifluoromethyl)bicyclo[2.2.1]hepta-2,5-diene reacts with chromium and molybdenum hexacarbonyls, and with enneacarbonyl di-iron to give metal complexes [M(diene)(CO)₄] (M = Cr, Mo) and [Fe(diene)(CO)₃], respectively. 6,7-Bis-(trifluoromethyl)tricyclo[3.2.2.0^2,4]nona-6,8-diene obtained from hexafluoro-but-2-yne and cycloheptatriene and 7,8-bis(trifluoromethyl)tricyclo[4.2.2.0^2,5]deca-3,7,9-triene formed from hexafluoro-but-2-yne and cyclooctatetraene also react with molybdenum hexacarbonyl to form complexes of molybdenum di- and tetracarbonyl groups, respectively. ¹H, ¹⁹F and ¹³C n.m.r. spectra of the compounds are described.     

The co-cyclotrimerization of hexynes and phenylacetylene with niobium(V) chloride as catalyst

Various alkynes were cyclotrimerized with NbCl₅ as catalyst and C₂H₅AlCl₂ as co-catalyst at an Al/Nb molar ratio = 2 in CCl₄ at room temperature and pressure. The tendency of the alkynes to cyclotrimerize decreased in the following order: phenylacetylene > hex-1-yne > hex-2-yne ~ hex-3-yne ~ oct-4-yne If phenylacetylene (X) was co-cyclotrimerized with another alkyne (Y), four different cyclotrimers were found, as represented by the following equation: X + Y → X₃ + X₂Y + XY₂ + Y₃ If the amount of cyclotrimers formed is plotted against the phenylacetylene mole fraction, a statistically random distribution of the products is observed. Differences with regard to the ideal random case are attributed to a polymerization reaction. During the cyclo-trimerization process there is therefore a statistically random coordination of the alkynes on the catalytic species, after which ring closure occurs to form the cyclotrimeric products.     

Relative reactivities of some dienes and acetylenes in copolymerization with sulfur dioxide

The reactivities of isoprene, piperylene,2,3-dimethylbutadiene, hex-1-yne, and phenylacetylene, at ?20°C, relative to that of cyclohexene, have been determined for the radical-initiated copolymerization with sulfur dioxide to form 1:1 polysulfones. The unsaturated hydrocarbons were copolymerized with sulfur dioxide in pairs and the composition of the terpolymers determined from the 100 MHz NMR spectra. The dienes react 11–15 times as fast as hex-1-ene, while hex-1-yne reacts 16 times more slowly. Phenylacetylene reacts 21 times as fast as hex-1-yne. The relative reactivities are interpreted mainly in terms of the effect of electron delocalization on the stability of the product radical.     

Cyclotrimerization of alkynes catalyzed by the naphthalene ruthenium complex [CpRu(C10H8)]+

The naphthalene ruthenium complex [CpRu(C₁₀H₈)]⁺ (in the presence of Cl^? ions) catalyzes the cyclotrimerization of 2,2-dimethyl-5,5-dipropargyl-1,3-dioxane-4,6-dione with alkynes (acetylene, hex-1-yne, hex-3-yne, oct-1-yne, phenylacetylene, trimetylsilylacetylene, octa-1,7-diyne, pent-1-yn-5-ol, methyl propargyl ether, and propargyl acetate) giving tricyclic aromatic compounds in 55–85% yields.     

(Fluoroalkyl)phosphine complexes of nickel(0) and cobalt(I)

The synthesis of a series of (fluoroalkyl)phosphine complexes of nickel is reported. Treatment of (cod)₂Ni with dfepe (dfepe=(C₂F₅)₂PCH₂CH₂P(C₂F₅)₂) yields (dfepe)Ni(cod) (1), which has been structurally characterized. Treatment of 1 with CO or bipy results in the formation of (dfepe)Ni(CO)₂ (2) and (dfepe)Ni(bipy) (3), respectively. Addition of excess dfepe to 1 results in incomplete cod displacement to form (dfepe)₂Ni (4). The homoleptic complex 4 may be independently prepared in high yield by reduction of (acac)₂Ni with (ⁱBu)₃Al in the presence of butadiene and excess dfepe. Solvation of (dfepe)Ni(cod) in acetonitrile gives a new complex tentatively identified as (dfepe)Ni(MeCN)₂ (6), whereas dissolution of (dfepe)₂Ni in acetonitrile leads to a mixture of 6 and the partial displacement product (dfepe)(η¹-dfepe)Ni(MeCN) (5). In contrast to (R₃P)₄Ni(0) phosphine and phosphite complexes, which undergo protonation by strong anhydrous acids such as HCl, H₂SO₄ and CF₃CO₂H to give (R₃P)₄Ni(H)⁺ products, Treatment of (dfepe)₂Ni with neat CF₃CO₂H or excess HOTf in dichloromethane gave no spectroscopic evidence for (dfepe)₂Ni(H)⁺. Exposure for extended periods leads to dfepe loss and decomposition to Ni(II) products. The synthesis of the first cobalt complex of dfepe, (dfepe)Co(CO)₂H, is also reported.     

CO2 reforming of methane over zirconia-supported nickel catalysts, II. Thermogravimetric analysis

Thermogravimetric analysis has been used to study carbon deposition during the CO₂ reforming of methane over Ni/ZrO₂ catalysts. The carbon deposits form on the reduced catalyst at a very fast rate during temperature-programmed surface reaction of reforming, and reach a steady state below 973 K. So, the amount of deposited carbon remains constant on the catalyst during the reaction at 973 K. A relationship between the amount of deposited carbon and the activity reveals that the initially formed carbon acts as a reaction intermediate and reacts with CO₂ to produce CO.     

Utilization of N,N‐Dialkylcarbamic Acid Derived from Secondary Amines and Supercritical Carbon Dioxide: Stereoselective Synthesis of Z Alkenyl Carbamates with a CO2‐Soluble Ruthenium–P(OC2H5)3 Catalyst

Reversible transformation of diethylamine ( 1 ) and supercritical carbon dioxide (scCO₂) into N,N‐diethylcarbamic acid ( 2 ) was confirmed by direct acquisition of ¹H and ¹³C{¹H} NMR spectra. The equilibrium between 1 +CO₂ and 2 is strongly influenced by conditions of the supercritical state. Low temperature favors formation of carbamic acid, whereas high temperature causes decarboxylation. On the basis of the spectroscopic results of carbamic acid formation under scCO₂ conditions, the ruthenium‐catalyzed formation of alkenyl carbamates from terminal alkynes, 1 , and carbon dioxide was investigated to demonstrate the useful transformation of elusive carbamic acids. Selectivity toward the CO₂‐fixation products over enynes obtained by dimerization of the alkyne substrates was improved by the use of scCO₂ as a reaction medium. In particular, a CO₂‐soluble ruthenium complex, trans‐[RuCl₂{P(OC₂H₅)₃}₄], was found to be effective in affording Z alkenyl carbamates with high stereoselectivity.     

O2 Inhibition of Ni‐Containing CO Dehydrogenase Is Partly Reversible

Ni‐containing CO dehydrogenases (CODHs) are very efficient metalloenzymes that catalyze the conversion between CO₂ and CO. They are a source of inspiration for designing CO₂‐reduction catalysts and can also find direct use in biotechnology. They are deemed extremely sensitive to O₂, but very little is known about this aspect of their reactivity. We investigated the reaction with O₂ of Carboxydothermus hydrogenoformans (Ch) CODH II and the homologous, recently characterized CODH from Desulfovibrio vulgaris (Dv) through protein film voltammetry and solution assays (in the oxidative direction). We found that O₂ reacts very quickly with the active site of CODHs, generating species that reactivate upon reduction—this was unexpected. We observed that distinct CODHs exhibit different behaviors: Dv CODH reacts half as fast with O₂ than Ch CODH, and only the former fully recovers the activity upon reduction. The results raise hope that fast CO/CO₂ biological conversion may be feasible under aerobic conditions.     

Synthesis of poly[2-(perfluorooctyl)ethyl acrylate-<Emphasis Type="Italic">co</Emphasis>-poly(ethylene glycol) methacrylate] and its control of enzyme activity in supercritical carbon dioxide

Poly[2-(perfluorooctyl)ethyl acrylate-co-poly(ethylene glycol) methacrylate], P(POA-co-PEGm) was prepared as a new surfactant for scCO₂. The random copolymer was obtained by the radical polymerization of 2-(perfluorooctyl)ethyl acrylate (POA) and poly(ethylene glycol) methacrylate (PEGm) in DMF. The molar ratio of the POA and PEGm units in the copolymer was POA/PEGm = 0.972/0.028 by ¹H NMR. The molecular weight and molecular weight distribution were estimated by size exclusion chromatography to be Mn = 133,000 and Mw/Mn = 8.25, respectively. It was suggested that the copolymer formed micellar aggregates with the cores of the PEGm chains in scCO₂, based on the analyses of the copolymer in hexafluorobenzene by ¹H NMR and dynamic light scattering. The copolymer was soluble in scCO₂ and had a cloud point at a much higher pressure than the critical pressure. It was found that the copolymer solubilized CO₂-insoluble proteins such as bovine serum albumin and subtilisin Carlsberg in scCO₂. The solubility of the copolymer was not influenced by the presence of the proteins; however, the solubility decreased in the presence of a small amount of water along with the protein. The activity of the subtilisin slightly decreased when only placed in scCO₂, whereas a marked decrease in the activity was observed for the subtilisin in the presence of the copolymer in scCO₂. The subtilisin activity decreased as the CO₂ pressure increased.     

Transmetalation of arylpalladium and platinum complexes. Mechanism and factors to control the reaction

This article reviews recent studies on intra- and intermolecular transfer of the aryl ligand bonded to Pd(II) and Pt(II). Cationic arylpalladium complexes with bpy and THF ligands undergo intermolecular aryl group transfer to produce biaryl via a diarylpalladium intermediate. This reaction is applied to cyclization of cationic dinuclear arylpalladium complexes, affording the crown ether derivative with biphenylene units. Analogous arylplatinum complexes do not form diaryl complexes via transmetalation, while they react with CO and phenylallene to cause replacement of the coordinated solvent and insertion of the small molecules into the Pt–C bond, respectively. Conproportionation of PtCl₂(cod) and PtPh₂(cod) produces PtCl(Ph)(cod), which is induced by dissociation of a Cl ligand from the former complex. PtCl₂(cod) reacts also with diarylplatinum complexes with bpy and dppe. Disproportionation of PtPh(CH₂COMe)(cod) and conproportionation of PtPh₂(cod) and Pt(CH₂COMe)₂(cod) take place at 50 °C, but the rates of apparently reversible reactions differ from each other. Addition of OH⁻ to a solution of PtI(Ph)(cod) causes intermolecular phenyl ligand transfer to produce PtPh₂(cod). The dinuclear intermediate complex with bridging OH ligand is prepared from an independent route and fully characterized. The complex causes transmetalation of aryl group of aryl boronic acid.     

傅立叶变换红外光谱技术在超临界CO2作用聚合物体系中的应用

超临界CO₂（scCO₂）作为一种物理化学性质优良、具有高扩散速率及优良溶解性能的溶剂,在科学研究及工业生产中广受青睐。将scCO₂应用于聚合物体系中,CO₂ 与聚合物间特殊的相互作用有利于CO₂分子在聚合物中的吸附与扩散。同时通过CO₂的吸附及其对聚合物的溶胀和塑化作用,聚合物所处微观化学环境以及整体结构性质会发生一定的变化。由于傅立叶变换红外光谱（FTIR）技术能够有效地考察化学环境变化对分子结构造成的影响,这一表征技术在超临界CO₂作用体系中广为应用。本文主要选取了近年来利用FTIR技术考察scCO₂作用于聚合物体系的一些实例,从CO₂-聚合物相互作用机理,scCO₂对聚合物或生物大分子的加工过程的影响两方面,阐述了利用红外光谱技术在scCO₂作用体系中的应用以及前景。     

Understanding the Origin of Highly Selective CO2 Electroreduction to CO on Ni,N‐doped Carbon Catalysts

Ni,N‐doped carbon catalysts have shown promising catalytic performance for CO₂ electroreduction (CO₂R) to CO; this activity has often been attributed to the presence of nitrogen‐coordinated, single Ni atom active sites. However, experimentally confirming Ni?N bonding and correlating CO₂ reduction (CO₂R) activity to these species has remained a fundamental challenge. We synthesized polyacrylonitrile‐derived Ni,N‐doped carbon electrocatalysts (Ni‐PACN) with a range of pyrolysis temperatures and Ni loadings and correlated their electrochemical activity with extensive physiochemical characterization to rigorously address the origin of activity in these materials. We found that the CO₂R to CO partial current density increased with increased Ni content before plateauing at 2 wt % which suggests a dispersed Ni active site. These dispersed active sites were investigated by hard and soft X‐ray spectroscopy, which revealed that pyrrolic nitrogen ligands selectively bind Ni atoms in a distorted square‐planar geometry that strongly resembles the active sites of molecular metal–porphyrin catalysts.     

Effect of supercritical CO2 on the copolymerization behavior of cyclohexene oxide/CO2 and copolymer properties with DMC/Salen‐Co(III) catalyst system

The copolymerization of cyclohexene oxide (CHO) and carbon dioxide (CO₂) was carried out under supercritical CO₂ (scCO₂) conditions to afford poly (cyclohexene carbonate) (PCHC) in high yield. The scCO₂ provided not only the C1 feedstock but also proved to be a very efficient solvent and processing aid for this copolymerization system. Double metal cyanide (DMC) and salen‐Co(III) catalysts were employed, demonstrating excellent CO₂/CHO copolymerization with high yield and high selectivity. Surprisingly, our use of scCO₂ was found to significantly enhance the copolymerization efficiency and the quality of the final polymer product. Thermally stable and high molecular weight (MW) copolymers were successfully obtained. Optimization led to excellent catalyst yield (656 wt/wt, polymer/catalyst) and selectivity (over 96% toward polycarbonate) that were significantly beyond what could be achieved in conventional solvents. Moreover, detailed thermal analyses demonstrated that the PCHC copolymer produced in scCO₂ exhibited higher glass transition temperatures (T_g ～ 114 °C) compared to polymer formed in dense phase CO₂ (T_g ～ 77 °C), and hence good thermal stability. Additionally, residual catalyst could be removed from the final polymer using scCO₂, pointing toward a green method that avoids the use of conventional volatile organic‐based solvents for both synthesis and work‐up. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2785–2793     

Water‐in‐Supercritical CO2 Microemulsion Stabilized by a Metal Complex

Herein we propose for the first time the utilization of a metal complex for forming water‐in‐supercritical CO₂ (scCO₂) microemulsions. The water solubility in the metal‐complex‐stabilized microemulsion is significantly improved compared with the conventional water‐in‐scCO₂ microemulsions stabilized by hydrocarbons. Such a microemulsion provides a promising route for the in situ CO₂ reduction catalyzed by a metal complex at the water/scCO₂ interface.     

Synthese und Reactivität von (2,6-iPr2Ph-dad)Ni(C2F4)

Upon warming the reaction mixture of Ni(cdt), C₂F₄, and 2,6-ⁱPr₂Ph-dad in THF from −78°C to room temperature the red-violet complex (2,6-ⁱPr₂Ph-dad)Ni(C₂F₄) (1) is obtained. 1 reacts with ethene already at −78°C by coupling of the olefinic ligands with the nickel atom to form the blue nickelatetrafluoro-cyclopentane compound (2,6-ⁱPr₂Ph-dad)Ni(C₂H₄C₂F₄) (2).     

Zur Kenntnis der sogenannten «Erdalkalicarbonyle»

The alkaline earth metals in liquid ammonia react with carbon monoxide to form the so-called ‘alkaline earth metal carbonyls’. These products which according to older literature are formulated as M(CO)₂(M = Ca, Sr, Ba) contain only 65–70% of the expected amount of carbon monoxide. As can be seen from the nitrogen content of the compounds the solvent ammonia participates in the reaction. By hydrolysis leading to a variety of compounds it is demonstrated that the ‘alkaline earth metal carbonyls’ are mixtures of substances such as metal acetylenediolates, metal methoxides, and ammonium carbonate. These mixtures are different from the ‘alkali carbonyls’ as for instance can be seen from the low acetylenediolate content of the ‘alkaline earth metal carbonyls’.     

Iridium Complexes of 2,2′‐Bidipyrrins

The reaction of [{Ir(cod)(μ‐Cl)}₂] and K₂CO₃ or of [{Ir(cod)(μ‐OMe)}₂] alone with the non‐natural tetrapyrrole 2,2′‐bidipyrrin (H₂BDP) yields, depending on the stoichiometry, the mononuclear complex [Ir(cod)(HBDP)] or the homodinuclear complex [{Ir(cod)}₂(BDP)]. Both complexes react readily with carbon monoxide to yield the species [Ir(CO)₂(HBDP)] and [{Ir(CO)₂}₂(BDP)], respectively. The results from NMR spectroscopy and X‐ray diffraction reveal different conformations for the tetrapyrrolic ligand in both complexes. The reaction of [{Ir(coe)₂(μ‐Cl)}₂] with H₂BDP proceeds differently and yields the macrocyclic [4e^?,2H⁺]‐oxidized product [IrCl₂(9‐Meic)] (9‐Meic = monoanion of 9‐methyl‐9,10‐isocorrole), which can be addressed as an iridium analog of cobalamin.