Iron-Gallium and Cobalt-Gallium Tetraphosphido Complexes

The synthesis and characterization of two heterobimetallic complexes [K([18]crown-6){(η⁴-C₁₄H₁₀)Fe(μ-η⁴:η²-P₄)Ga(nacnac)}] ( 1 ) (C₁₄H₁₀ = anthracene) and [K(dme)₂{(η⁴-C₁₄H₁₀)Co(μ-η⁴:η²-P₄)Ga(nacnac)}] ( 2 ) with strongly reduced P₄ units is reported. Compounds 1 and 2 are prepared by reaction of the gallium(III) complex [(nacnac)Ga(η²-P₄)] (nacnac = CH[CMeN(2,6-iPr₂C₆H₃)]₂) with bis(anthracene)ferrate(1–) and -cobaltate(1–) salts. The molecular structures of 1 and 2 were determined by X-ray crystallography and feature a P₄ chain which binds to the transition metal atom via all four P atoms and to the gallium atom via the terminal P atoms. Multinuclear NMR studies on 2 suggest that the molecular structure is preserved in solution.     

Synthesis and Characterization of a Magnesium Boryl and a Beryllium‐Substituted Diazaborole

Reaction of a lithium boryl, [(THF)₂Li{B(DAB)}] (DAB=[(DipNCH)₂]^2?, Dip=2,6‐diisopropylphenyl), with a dinuclear magnesium(I) compound [{(^MesNacnac)Mg}₂] (^MesNacnac=[HC(MeCNMes)₂]^?, Mes=mesityl) unexpectedly afforded a rare example of a terminal magnesium boryl species, [(^MesNacnac)(THF)Mg{B(DAB)}]. Attempts to prepare the magnesium boryl via a salt metathesis reaction between the lithium boryl and a β‐diketiminato magnesium iodide compound, instead led to an intractable mixture of products. Similarly, reaction of the lithium boryl with a β‐diketiminato beryllium bromide precursor, [(^DepNacnac)BeBr] (Dep=2,6‐diethylphenyl) did not give a beryllium boryl, but instead afforded an unprecedented example of a beryllium substituted diazaborole heterocycle, [{(^DepNacnac)Be(4‐DAB^?H)}BBr]. For sake of comparison, the same group 2 halide precursor compounds were treated with a potassium gallyl analogue of the lithium boryl, viz. [(tmeda)K{:Ga(DAB)}] (tmeda=N,N,N’,N’‐tetramethylethylenediamine), but no reactions were observed.     

Ruthenium(II) Tris-Pyrazolylmethane Complexes in Transfer Hydrogenation Reactions

While ruthenium(II) arene complexes have been widely investigated for their potential in catalytic transfer hydrogenation, studies on homologous compounds replacing the arene ligand with the six-electron donor tris(1-pyrazolyl)methane (tpm) are almost absent in the literature. The reactions of [RuCl(κ³-tpm)(PPh₃)₂]Cl, 1 , with a series of nitrogen ligands (L) proceeded with selective PPh₃ mono-substitution, affording the novel complexes [RuCl(κ³-tpm)(PPh₃)(L)]Cl (L=NCMe, 2 ; NCPh, 3 ; imidazole, 4 ) in almost quantitative yields. Products 2 – 4 were fully characterized by IR and multinuclear NMR spectroscopy, moreover the molecular structure of 4 was ascertained by single crystal X-ray diffraction. Compounds 2 – 4 were evaluated as catalytic precursors in the transfer hydrogenation of a series of ketones with isopropanol as the hydrogen source, and 2 exhibited the highest activity. Extensive NMR experiments and DFT calculations allowed to elucidate the mechanism of the transfer hydrogenation process, suggesting the crucial role played by the tpm ligand, reversibly switching from tri- to bidentate coordination during the catalytic cycle.     

Calcium–Amidoborane–Ammine Complexes: Thermal Decomposition of Model Systems

Hydrocarbon‐soluble model systems for the calcium–amidoborane–ammine complex Ca(NH₂BH₃)₂ ? (NH₃)₂ were prepared and structurally characterized. The following complexes were obtained by the reaction of RNH₂BH₃ (R=H, Me, iPr, DIPP; DIPP=2,6‐diisopropylphenyl) with Ca(DIPP‐nacnac)(NH₂) ? (NH₃)₂ (DIPP‐nacnac=DIPP? NC(Me)CHC(Me)N? DIPP): Ca(DIPP‐nacnac)(NH₂BH₃) ? (NH₃)₂, Ca(DIPP‐nacnac)(NH₂BH₃) ? (NH₃)₃, Ca(DIPP‐nacnac)[NH(Me)BH₃] ? (NH₃)₂, Ca(DIPP‐nacnac)[NH(iPr)BH₃] ? (NH₃)₂, and Ca(DIPP‐nacnac)[NH(DIPP)BH₃] ? NH₃. The crystal structure of Ca(DIPP‐nacnac)(NH₂BH₃) ? (NH₃)₃ showed a NH₂BH₃^? unit that was fully embedded in a network of BH???HN interactions (range: 1.97(4)–2.39(4) Å) that were mainly found between NH₃ ligands and BH₃ groups. In addition, there were N? H???C interactions between NH₃ ligands and the central carbon atom in the ligand. Solutions of these calcium–amidoborane–ammine complexes in benzene were heated stepwise to 60 °C and thermally decomposed. The following main conclusions can be drawn: 1) Competing protonation of the DIPP‐nacnac anion by NH₃ was observed; 2) The NH₃ ligands were bound loosely to the Ca²⁺ ions and were partially eliminated upon heating. Crystal structures of [Ca(DIPP‐nacnac)(NH₂BH₃) ? (NH₃)]_∞, Ca(DIPP‐nacnac)(NH₂BH₃) ? (NH₃) ? (THF), and [Ca(DIPP‐nacnac){NH(iPr)BH₃}]₂ were obtained. 3) Independent of the nature of the substituent R in NH(R)BH₃, the formation of H₂ was observed at around 50 °C. 4) In all cases, the complex [Ca(DIPP‐nacnac)(NH₂)]₂ was formed as a major product of thermal decomposition, and its dimeric nature was confirmed by single‐crystal analysis. We proposed that thermal decomposition of calcium–amidoborane–ammine complexes goes through an intermediate calcium–hydride–ammine complex which eliminates hydrogen and [Ca(DIPP‐nacnac)(NH₂)]₂. It is likely that the formation of metal amides is also an important reaction pathway for the decomposition of metal–amidoborane–ammine complexes in the solid state.     

Highly Efficient Transfer Hydrogenation Catalysis with Tailored Pyridylidene Amide Pincer Ruthenium Complexes

The rational optimization of homogeneous catalysts requires ligand platforms that are easily tailored to improve catalytic performance. Here, it is demonstrated that pyridylidene amides (PYAs) provide such a platform to custom-shape transfer hydrogenation catalysts with exceptional activity. Specifically, a series of meta-PYA pincer ligands with differently substituted PYA units has been synthezised and coordinated to ruthenium(II) centres to form bench-stable tris-acetonitrile complexes [Ru(R-PYA-pincer)(MeCN)₃](PF₆)₂ (R=OMe, Me, H, Cl, CF₃). Analytic studies including ¹H NMR spectroscopy, cyclic voltammetry, and X-ray crystallography reveal a direct influence of the substituents on the electronic properties of the ruthenium center. The complexes are active in the catalytic transfer hydrogenation of ketones, with activities directly encoded by the PYA substitution pattern. Their perfomance improves further upon exchange of an ancillary MeCN ligand with PPh₃. While complexes [Ru(R-PYA-pincer)(PPh₃)(MeCN)₂](PF₆)₂ were only isolated for R=H, Me, an in situ protocol was developed to generate these complexes in situ for R=OMe, Cl, CF₃ by using a 1:2 ratio of the complexes and PPh₃. This in situ protocol together with a short catalyst pre-activation provided highly active catalytic systems. The most active pre-catalyst featured the methoxy-substituted PYA ligand and reached turnover frenquencies of 210 000 h⁻¹ under an exceptionally low catalyst loading of 25 ppm for the benchmark substrate benzophenone, representing one of the most active transfer hydrogenation systems known to date.     

Kinetics and mechanisms of homogeneous catalytic reactions. Part 5. Regioselective reduction of heteroaromatic nitrogen compounds catalysed by [OsH(CO)(NCMe)2(PPh3)2]BF4

The [OsH(CO)(NCMe)₂(PPh₃)₂]BF₄ complex (1) is an efficient and regioselective precatalyst for the hydrogenation of the nitrogen-containing ring of quinoline (Q), isoquinoline (iQ), 5,6- and 7,8-benzoquinoline (BQ), and acridine (A) under mild reaction conditions (125 °C and 4 atm H₂). Kinetic studies of the hydrogenation of Q and iQ to give tetrahydroquinoline (THQ) and tetrahydroisoquinoline (THiQ), respectively, lead to the rate law r = K ₁ k ₂/(1 + K ₁[H₂])[Os][H₂]², which becomes r = K ₁ k ₂[Os][H₂]², at low hydrogen concentrations (below 1 atm H₂); the catalytically active species is of the type [OsH(CO)(L)( ¹-N)(PPh₃)₂]BF₄ [(2a): L = NCMe, N = Q; (2b): L = N = iQ]. The generic mechanisms involve a rapid and partial hydrogenation of the coordinated substrate (N) of complex (2) to yield the corresponding dihydroderivative (DHN) species [OsH(CO)(L)( ¹-DHN)(PPh₃)₂]BF₄ [(3a): L = NCMe, DHN = DHQ; (3b): L = iQ or THiQ, DHN = DHiQ], followed by the rate-determining second hydrogenation of the DHN ligand, which yield [OsH(CO)(L)( ¹-THN)(PPh₃)₂]BF₄ [(4a): L = NCMe, THN = THQ; (4b): L = iQ or THiQ, THN = THiQ]; substitution of the THN ligand by a new molecule of the respective substrate regenerates the active species and restarts the catalytic cycle. For the hydrogenation of acridine to give 9,10-dihidroacridine (acridane), the rate law was r = k ₁[Os][H₂]; the mechanism involves the hydrogenation of the active species [OsH(CO)(NCMe)( ¹-A)(PPh₃)₂]BF₄ (2c) to yield acridane and the unsaturated species [OsH(CO)(NCMe)(PPh₃)₂]BF₄ as the rate-determining step.     

Cationic Aluminium Complexes as Catalysts for Imine Hydrogenation

Strongly Lewis acidic cationic aluminium complexes, stabilized by β–diketiminate (BDI) ligands and free of Lewis bases, have been prepared as their B(C₆F₅)₄⁻ salts and were investigated for catalytic activity in imine hydrogenation. The backbone (R1) and N (R2) substituents on the ^R1,R2BDI ligand (^R1,R2BDI=HC[C(R1)N(R2)]₂) influence sterics and Lewis acidity. Ligand bulk increases along the row ^Me,DIPPBDI<^Me,DIPePBDI≈^tBu,DIPPBDI<^tBu,DIPePBDI; DIPP=2,6-C(H)Me₂-phenyl, DIPeP=2,6-C(H)Et₂-phenyl. The Gutmann-Beckett test showed acceptor numbers of: (^tBu,DIPPBDI)AlMe⁺ 85.6, (^tBu,DIPePBDI)AlMe⁺ 85.9, (^Me,DIPPBDI)AlMe⁺ 89.7, (^Me,DIPePBDI)AlMe⁺ 90.8, (^Me,DIPPBDI)AlH⁺ 95.3. Steric and electronic factors need to be balanced for catalytic activity in imine hydrogenation. Open, highly Lewis acidic, cations strongly coordinate imine rendering it inactive as a Frustrated Lewis Pair (FLP). The bulkiest cations do not coordinate imine but its combination is also not an active catalyst. The cation (^tBu,DIPPBDI)AlMe⁺ shows the best catalytic activity for various imines and is also an active catalyst for the Tishchenko reaction of benzaldehyde to benzylbenzoate. DFT calculations on the mechanism of imine hydrogenation catalysed by cationic Al complexes reveal two interconnected catalytic cycles operating in concert. Hydrogen is activated either by FLP reactivity of an Al⋅⋅⋅imine couple or, after formation of significant quantities of amine, by reaction with an Al⋅⋅⋅amine couple. The latter autocatalytic Al⋅⋅⋅amine cycle is energetically favoured.     

Mechanistic Studies on the Bismuth-Catalyzed Transfer Hydrogenation of Azoarenes

Organobismuth-catalyzed transfer hydrogenation has recently been disclosed as an example of low-valent Bi redox catalysis. However, its mechanistic details have remained speculative. Herein, we report experimental and computational studies that provide mechanistic insights into a Bi-catalyzed transfer hydrogenation of azoarenes using p-trifluoromethylphenol ( 4 ) and pinacolborane ( 5 ) as hydrogen sources. A kinetic analysis elucidated the rate orders in all components in the catalytic reaction and determined that 1 a (2,6-bis[N-(tert-butyl)iminomethyl]phenylbismuth) is the resting state. In the transfer hydrogenation of azobenzene using 1 a and 4 , an equilibrium between 1 a and 1 a ⋅ [OAr]₂ (Ar=p-CF₃−C₆H₄) is observed, and its thermodynamic parameters are established through variable-temperature NMR studies. Additionally, pK_a-gated reactivity is observed, validating the proton-coupled nature of the transformation. The ensuing 1 a ⋅ [OAr]₂ is crystallographically characterized, and shown to be rapidly reduced to 1 a in the presence of 5 . DFT calculations indicate a rate-limiting transition state in which the initial N−H bond is formed via concerted proton transfer upon nucleophilic addition of 1 a to a hydrogen-bonded adduct of azobenzene and 4 . These studies guided the discovery of a second-generation Bi catalyst, the rate-limiting transition state of which is lower in energy, leading to catalytic transfer hydrogenation at lower catalyst loadings and at cryogenic temperature.     

Synthesis of a Hexameric Magnesium 4-pyridyl Complex with Cyclohexane-like Ring Structure via Reductive C-N Activation

The reaction of [{(^Arnacnac)Mg}₂] (^Arnacnac = HC{MeC(NAr)}₂, Ar = 2,6-diisopropylphenyl, Dip, or 2,6-diethylphenyl, Dep) with 4-dimethylaminopyridine (DMAP) at elevated temperatures afforded the hexameric magnesium 4-pyridyl complex [{(^Arnacnac)Mg(4-C₅H₄N)}₆] via reductive cleavage of the DMAP C-N bond. The title compound contains a large s-block organometallic cyclohexane-like ring structure comprising tetrahedral (^Arnacnac)Mg nodes and linked by linear 4-pyridyl bridging ligands, and the structure is compared with other ring systems. [(^Dipnacnac)Mg(DMAP)(NMe₂)] was structurally characterised as a by-product.     

Synthese,Struktur und Eigenschaften von [nacnac]MX3‐Verbindungen (M = Ge,Sn; X = Cl,Br, I)

Synthesis, Structure, and Properties of [nacnac]MX₃ Compounds (M = Ge, Sn; X = Cl, Br, I) Reactions of [nacnac]Li [(2,6‐ⁱPr₂C₆H₃)NC(Me)C(H)C(Me)N(2,6‐ⁱPr₂C₆H₃)]Li ( 1 ) with SnX₄ (X = Cl, Br, I) and GeCl₄ in Et₂O resulted in metallacyclic compounds with different structural moieties. In the [nacnac]SnX₃ compounds (X = Cl 2 , Br 3 , I 4 ) the tin atom is five coordinated and part of a six‐membered ring. The Sn–N‐bond length of 3 is 2.163(4) Å and 2.176(5) Å of 4 . The five coordinated germanium of the [nacnac]GeCl₃ compound 5 shows in addition to the three chlorine atoms further bonds to a carbon and to a nitrogen atom. In contrast to the known compounds with the [nacnac] ligand the afore mentioned reaction creates a carbon–metal‐bond (1.971(3) Å) forming a four‐membered ring. The Ge–N bond length (2.419(2) Å) indicates the formation of a weakly coordinating bond.     

ZnSO4和La2O3作共修饰剂单金属Ru催化剂上苯选择加氢制环己烯

用沉淀法制备了单金属纳米Ru（0）催化剂,考察了ZnSO₄和La₂O₃作共修饰剂对该催化剂催化苯选择加氢制环己烯性能的影响,并用X射线衍射（XRD）、X射线荧光（XRF）光谱、X射线光电子能谱（XPS）、俄歇电子能谱（AES）、透射电镜（TEM）和N₂物理吸附等手段对加氢前后催化剂进行了表征. 结果表明,在ZnSO₄存在下,随着添加碱性La₂O₃量的增加,ZnSO₄水解生成的（Zn（OH）₂）₃（ZnSO₄）（H₂O）_x（x=1,3）盐量增加,催化剂活性单调降低,环己烯选择性单调升高. 当La₂O₃/Ru 物质的量比为0.075 时,Ru催化剂上苯转化率为77.6%,环己烯选择性和收率分别为75.2%和58.4%. 且该催化体系具有良好的重复使用性能. 传质计算结果表明,苯、环己烯和氢气的液-固扩散限制和孔内扩散限制都可忽略. 因此,高环己烯选择性和收率的获得不能简单归结为物理效应,而与催化剂的结构和催化体系密切相关. 根据实验结果,我们推测在化学吸附有（Zn（OH）₂）₃（ZnSO₄）（H₂O）_x（x=1,3）盐的Ru（0）催化剂有两种活化苯的活性位：Ru⁰和Zn²⁺. 因为Zn²⁺将部分电子转移给了Ru,Zn²⁺活化苯的能力比Ru⁰弱. 同时由于Ru和Zn²⁺的原子半径接近,Zn²⁺可以覆盖一部分Ru⁰活性位,导致解离H₂的Ru⁰活性位减少. 这导致了Zn²⁺上活化的苯只能加氢生成环己烯和Ru（0）催化剂活性的降低. 本文利用双活性位模型来解释Ru基催化剂上的苯加氢反应,并用Hückel分子轨道理论说明了该模型的合理性.     

Generation and Stabilization of a Dinickel Catalyst in a Metal-Organic Framework for Selective Hydrogenation Reactions

Although many monometallic active sites have been installed in metal–organic frameworks (MOFs) for catalytic reactions, there are no effective strategies to generate bimetallic catalysts in MOFs. Here we report the synthesis of a robust, efficient, and reusable MOF catalyst, MOF-NiH, by adaptively generating and stabilizing dinickel active sites using the bipyridine groups in MOF-253 with the formula of Al(OH)(2,2′-bipyridine-5,5′-dicarboxylate) for Z-selective semihydrogenation of alkynes and selective hydrogenation of C=C bonds in α,β-unsaturated aldehydes and ketones. Spectroscopic studies established the dinickel complex (bpy⋅⁻)Ni^II(μ₂-H)₂Ni^II(bpy⋅⁻) as the active catalyst. MOF-NiH efficiently catalyzed selective hydrogenation reactions with turnover numbers of up to 192 and could be used in five cycles of hydrogenation reactions without catalyst leaching or significant decrease of catalytic activities. The present work uncovers a synthetic strategy toward solution-inaccessible Earth-abundant bimetallic MOF catalysts for sustainable catalysis.     

Understanding the Deactivation Pathways of Iridium(III) Pyridine-Carboxiamide Catalysts for Formic Acid Dehydrogenation

The degradation pathways of highly active [Cp*Ir(κ²-N,N-R-pica)Cl] catalysts (pica=picolinamidate; 1 R=H, 2 R=Me) for formic acid (FA) dehydrogenation were investigated by NMR spectroscopy and DFT calculations. Under acidic conditions (1 equiv. of HNO₃), 2 undergoes partial protonation of the amide moiety, inducing rapid κ²-N,N to κ²-N,O ligand isomerization. Consistently, DFT modeling on the simpler complex 1 showed that the κ²-N,N key intermediate of FA dehydrogenation ( I_NH ), bearing a N-protonated pica, can easily transform into the κ²-N,O analogue ( I_NH2 ; ΔG^≠≈11 kcal mol⁻¹, ΔG ≈−5 kcal mol⁻¹). Intramolecular hydrogen liberation from I_NH2 is predicted to be rather prohibitive (ΔG^≠≈26 kcal mol⁻¹, ΔG≈23 kcal mol⁻¹), indicating that FA dehydrogenation should involve mostly κ²-N,N intermediates, at least at relatively high pH. Under FA dehydrogenation conditions, 2 was progressively consumed, and the vast majority of the Ir centers (58 %) were eventually found in the form of Cp*-complexes with a pyridine-amine ligand. This likely derived from hydrogenation of the pyridine-carboxiamide via a hemiaminal intermediate, which could also be detected. Clear evidence for ligand hydrogenation being the main degradation pathway also for 1 was obtained, as further confirmed by spectroscopic and catalytic tests on the independently synthesized degradation product 1 c . DFT calculations confirmed that this side reaction is kinetically and thermodynamically accessible.     

A Neutral Tetraphosphacyclobutadiene Ligand in Cobalt(I) Complexes

The unusual reactivity of the newly synthesized β‐diketiminato cobalt(I) complexes, [(L^DepCo)₂] ( 2 a , L^Dep=CH[C(Me)N(2,6‐Et₂C₆H₃)]₂) and [L^DippCo ? toluene] ( 2 b , L^Dipp=CH[CHN(2,6‐ⁱPr₂C₆H₃)]₂), toward white phosphorus was investigated, affording the first cobalt(I) complexes [(L^DepCo)₂(μ₂:η⁴,η⁴‐P₄)] ( 3 a ) and [(L^DippCo)₂(μ₂:η⁴,η⁴‐P₄)] ( 3 b ) bearing the neutral cyclo‐P₄ ligand with a rectangular‐planar structure. The redox chemistry of 3 a and 3 b was studied by cyclic voltammetry and their chemical reduction with one molar equivalent of potassium graphite led to the isolation of [(L^DepCo)₂(μ₂:η⁴,η⁴‐P₄)][K(dme)₄] ( 4 a ) and [(L^DippCo)₂(μ₂:η⁴,η⁴‐P₄)][K(dme)₄] ( 4 b ). Unexpectedly, the monoanionic Co₂P₄ core in 4 a and 4 b , respectively, contains the two‐electron‐reduced cyclo‐P₄^2? ligand with a square‐planar structure and mixed‐valent cobalt(I,II) sites. The electronic structures of 3 a , 3 b , 4 a , and 4 b were elucidated by NMR and EPR spectroscopy as well as magnetic measurements and are in agreement with results of broken‐symmetry DFT calculations.     

Amine-Borane Dehydrogenation and Transfer Hydrogenation Catalyzed by α-Diimine Cobaltates

Anionic α-diimine cobalt complexes, such as [K(thf)_1.5{(^DippBIAN)Co(η⁴-cod)}] ( 1 ; Dipp=2,6-diisopropylphenyl, cod=1,5-cyclooctadiene), catalyze the dehydrogenation of several amine-boranes. Based on the excellent catalytic properties, an especially effective transfer hydrogenation protocol for challenging olefins, imines, and N-heteroarenes was developed. NH₃BH₃ was used as a dihydrogen surrogate, which transferred up to two equivalents of H₂ per NH₃BH₃. Detailed spectroscopic and mechanistic studies are presented, which document the rate determination by acidic protons in the amine-borane.     

Catalytic Transfer Hydrogenation with a Methandiide‐Based Carbene Complex: An Experimental and Computational Study

The transfer hydrogenation (TH) reaction of ketones with catalytic systems based on a methandiide‐derived ruthenium carbene complex was investigated and optimised. The complex itself makes use of the noninnocent behaviour of the carbene ligand (M?CR₂→MH?C(H)R₂), but showed only moderate activity, thus requiring long reaction times to achieve sufficient conversion. DFT studies on the reaction mechanism revealed high reaction barriers for both the dehydrogenation of iPrOH and the hydrogen transfer. A considerable improvement of the catalytic activity could be achieved by employing triphenylphosphine as additive. Mechanistic studies on the role of PPh₃ in the catalytic cycle revealed the formation of a cyclometalated complex upon phosphine coordination. This ruthenacycle was revealed to be the active species under the reaction conditions. The use of the isolated complex resulted in high catalytic activities in the TH of aromatic as well as aliphatic ketones. The complex was also found to be active under base‐free conditions, suggesting that the cyclometalation is crucial for the enhanced activity.     

Highly Selective Ortho-Directed Dicarboxylation of Cyclopentadiene by Methylcarbonates and CO2 or COS – First Insight into Co-ordination Chemistry of New Ambident Ligands

This research presents the highly regioselective syntheses of 1,2-dicarboxylated cyclopentadienide salts [Cat]₂[C₅H₃(CO₂)₂H] by reaction of a variety of organic cation methylcarbonate salts [Cat]OCO₂Me (Cat=NR₄⁺, PR₄⁺, Im⁺) with cyclopentadiene (CpH) or by simply reacting organic cation cyclopentadienides Cat[Cp] (Cat=NR₄⁺, PR₄⁺, Im⁺) with CO₂. One characteristic feature of these dianionic ligands is the acidic proton delocalized in an intramolecular hydrogen bridge (IHB) between the two carboxyl groups, as studied by ¹H NMR spectroscopy and XRD analyses. The reaction cannot be stopped after the first carboxylation. Therefore, we propose a Kolbe-Schmitt phenol-carboxylation related mechanism where the acidic proton of the monocarboxylic acid intermediate plays an ortho-directing and CO₂ activating role for the second kinetically accelerated CO₂ addition step exclusively in ortho position. The same and related thiocarboxylates [Cat]₂[C₅H₃(COS)₂H] are obtained by reaction of COS with Cat[Cp] (Cat=NR₄⁺, PR₄⁺, Im⁺). A preliminary study on [Cat]₂[C₅H₃(CO₂)₂H] reveals, that its soft and hard coordination sites can selectively be addressed by soft Lewis acids (Mo⁰, Ru²⁺) and hard Lewis acids (Al³⁺, La³⁺).     

Zincocene and Dizincocene N‐Heterocyclic Carbene Complexes and Catalytic Hydrogenation of Imines and Ketones

The N‐heterocyclic carbene (NHC) adducts Zn(Cp^R)₂(NHC)] (Cp^R=C₅HMe₄, C₅H₄SiMe₃; NHC=ItBu, IDipp (Dipp=2,6‐diisopropylphenyl), IMes (Mes=mesityl), SIMes) were prepared and shown to be active catalysts for the hydrogenation of imines, whereas decamethylzincocene [ZnCp*₂] is highly active for the hydrogenation of ketones in the presence of noncoordinating NHCs. The abnormal carbene complex [Zn(OCHPh₂)₂(aItBu)]₂ was formed from spontaneous rearrangement of the ItBu ligand during incomplete hydrogenation of benzophenone. Two isolated Zn^I adducts [Zn₂Cp*₂(NHC)] (NHC=ItBu, SIMes) are presented and characterized as weak adducts on the basis of ¹³C NMR spectroscopic and X‐ray diffraction experiments. A mechanistic proposal for the reduction of [ZnCp*₂] with H₂ to give [Zn₂Cp*₂] is discussed.     

Oxidation of Ethylbenzene with Dioxygen in the Presence of Iron(III) Tris(acetylacetonate)-Based Catalytic Systems: 1. Mechanism of Ethylbenzene Oxidation with Dioxygen Catalyzed by Fe(III)(acac)<Subscript>3</Subscript>

The catalytic activity of Fe(III)(acac)₃ (Cat) in ethylbenzene oxidation with dioxygen is studied. 1-Phenylethyl hydroperoxide (PEHP), acetophenone (AP), and methyl phenyl carbinol (MPC) are the main products of the process throughout the Cat concentration range examined. Phenol (Ph) is formed in much smaller amounts. The highest PEHP selectivity, S _PEHP = 65%, is observed at an ethylbenzene conversion of C ≈ 2% at low [Cat] values. PEHP and the other main oxidation products (AP and MPC) form by parallel reactions at any Cat concentration. Depending on [Cat], AP and MPC form by parallel or consecutive reactions. When [Cat] is high enough, AP results from MPC oxidation. At the initial stages of the reaction, the MPC selectivity (S _MPC = 50%) exceeds the PEHP selectivity (S _PEHP = 25–30%). The mechanism of ethylbenzene oxidation catalyzed by Fe(III)(acac)₃ and the role of active complexes in its steps are considered.__________Translated from Kinetika i Kataliz, Vol. 46, No. 3, 2005, pp. 354–359.Original Russian Text Copyright © 2005 by Matienko, Mosolova.     

Synthesis and Structures of Two Dinuclear Transition Metal Complexes and Their Catalytic Applications in Hydrogenation of Ketones

The complexes [Ni₂(L)₂]₂ · H₂O ( 1 ) and [Cu₂(L)₂(H₂O)] · 2CH₃OH ( 2 ) were prepared by reaction of the chiral Schiff base ligand N‐[(1R,2S)‐2‐hydroxy‐1,2‐diphenyl]‐acetylacetonimine (H₂L) with Ni^II and Cu^II ions, respectively, aiming to develop economically and environmentally‐friendly catalysts for the hydrogenation of ketones. They have a dinuclear skeleton with axial vacant sites. The catalytic effects of the two complexes for hydrogenation of ketones were tested using dihydrogen gas as hydrogen source. They present some catalytic effects in hydrogenation of acetophenone, which has a dependence on the temperature and base used in these reactions. However, no apparent catalytic effects were found for the two complexes in hydrogenation of 4‐nitroacetophenone and 4‐methylacetophenone. Although the catalytic conversion in these hydrogenation reactions is low, they do represent a kind of cheap and environmentally‐friendly hydrogenation catalyst.