An Isolable Silicon Dicarbonate Complex from Carbon Dioxide Activation with a Silylone

The first isolable molecular silicon dicarbonate complex (bis‐NHC)Si(CO₃)₂ 2 (bis‐NHC=H₂C[{NC(H)=C(H)N(Dipp)}C:]₂, Dipp=2,6‐iPr₂C₆H₃) was synthesized by facile reaction of the bis‐N‐heterocyclic carbene stabilized silylone (bis‐NHC)Si 1 , bearing a zero‐valent silicon atom, with carbon dioxide. The monomeric silicon dioxide complex (bis‐NHC)SiO₂ 3 supported by the bis‐NHC ligand was proposed as a key intermediate resulting from double oxygenation of the zero‐valent silicon atom in 1 by two molar equivalents of CO₂ under liberation of CO; its subsequent Lewis acid–base reaction with CO₂ leads to 2 which has been fully characterized including an single‐crystal X‐ray diffraction analysis. Its electronic structure, spectroscopic data and the thermochemistry of the formation have been studied quantum‐chemically.     

Elaboration and characterization of hybrid organic-inorganic gels obtained by reaction of 1,4-butanediol on tetramethoxysilane

New hybrid organic-inorganic gels have been obtained by reaction of 1,4-butanediol, on tetramethoxysilane Si(OMe)₄ dissolved in CCl₄. This reaction does not require water and leads to the formation of polymeric transparent materials.Infrared, ²⁹Si and ¹³C NMR spectroscopy shows that interchange reactions between OMe groups of alkoxide and -O-(CH₂)₄-O of 1,4-butanediol occurred, leading to the monolithic transparent gels in which both organic (Si-O-(CH₂)₄-O-Si) and inorganic (Si-O-Si) bridges are formed.     

Bis(organyldimethoxysilyl)acetylenes

Conclusions Bis(trimethoxysilyl)-, bis(methyldimethoxysilyl)-, and bis(vinyldimethoxysilyl)acetylenes were obtained by the reaction of bis(bromomagnesium)acetylene with Si(OMe)₄ and RSi(OMe)₃. Their reaction with MeMgl led to replacement of the methoxy groups by methyl ones at both silicon atoms.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 851–852, April, 1988.     

Synthesis and Cyclohexene Oxide/Carbon Dioxide Copolymerizations of Zinc Acetate Complexes Bearing Bidentate Pyridine‐Alkoxide Ligands

Summary: The reaction of 2‐lithio‐6‐methylpyridine or 2‐lithiopyridine and the appropriate diaryl ketone followed by hydrolysis yields 6‐Me‐pyCAr₂OH pyridine alcohols or pyCAr₂OH pyridine alcohols. The reactions of zinc acetate with 1 equiv. of the lithiated products of the ligands proceed rapidly to afford LiOAc salt and mono‐ligand complexes (6‐Me‐pyCAr₂O)Zn(OAc) and (pyCAr₂O)Zn(OAc), respectively, in high yield. The copolymerizations of carbon dioxide with cyclohexene oxide were investigated. The (6‐Me‐pyCAr₂O)Zn(OAc) showed moderate yield and CO₂ incorporation. The [6‐Me‐pyC(4‐Cl‐C₆H₄)₂O]Zn(OAc) complex gave large polymers with high proportions of carbonate linkage (>60%) and narrow polydispersity, indicating single active sites.

The monoligated Zn complexes synthesized and used here as catalysts for the copolymerization of cyclohexene oxide and carbon dioxide.     

Amorphous Silicon: New Insights into an Old Material

Amorphous silicon is synthesized by treating the tetrahalosilanes SiX₄ (X=Cl, F) with molten sodium in high boiling polar and non‐polar solvents such as diglyme or nonane to give a brown or a black solid showing different reactivities towards suitable reagents. With regards to their technical relevance, their stability towards oxygen, air, moisture, chlorine‐containing reaction partners RCl (R=H, Cl, Me) and alcohols is investigated. In particular, reactions with methanol are a versatile tool to deliver important products. Besides tetramethoxysilane formation, methanolysis of silicon releases hydrogen gas under ambient conditions and is thus suitable for a decentralized hydrogen production; competitive insertion into the MeO?H versus the Me?OH bond either yields H‐ and/or methyl‐substituted methoxy functional silanes. Moreover, compounds, such as Me_nSi(OMe)_4?n (n=0–3) are simply accessible in more than 75 % yield from thermolysis of, for example, tetramethoxysilane over molten sodium. Based on our systematic investigations we identified reaction conditions to produce the methoxysilanes Me_nSi(OMe)_4?n in excellent (n=0:100 %) to acceptable yields (n=1:51 %; n=2:27 %); the yield of HSi(OMe)₃ is about 85 %. Thus, the methoxysilanes formed might possibly open the door for future routes to silicon‐based products.     

Influence of the nature of organic groups on the properties of organically modified silica aerogels

Organically modified aerogels were prepared by NH₄OH-catalyzed hydrolysis and condensation of RSi(OMe)₃ (R = Me, Pr ⁿ , Ph, Bu ⁱ )/Si(OMe)₄ or (MeO)₃Si-Y-Si(OMe)₃/Si(OMe)₄ (Y = C₂H₄, p-C₆H₄, C₆H₁₂) mixtures, followed by supercritical drying of the alcogels with methanol. Starting from 1:4 mixtures of RSi(OMe)₃ and Si(OMe)₄, hydrophobic aerogels with nearly no residual Si-OH or Si-OMe groups were obtained. These aerogels were therefore insensitive towards moisture. Their elastic constant was distinctly lower than that of unmodified silica aerogels. Aerogels similarly prepared from 1:8 mixtures of (MeO)₃Si-Y-Si(OMe)₃ and Si(OMe)₄ had a rather high concentration of residual Si-OMe groups, and therefore they were hydrophilic. Their elasticity was about the same as that of unmodified silica aerogels. The difference between the two types of aerogels suggests different microstructures, depending on the nature of the organic groups.     

Reactions of some sterically-hindered organosilicon hydrides and iodides with halogens

The reaction of TsiSiMe₂H (I) (Tsi = (Me₃Si)₃C) with I₂ or with a molar equivalent of ICl gives the iodide TsiSiMe₂I (II) in hydroxylic media (MeOH, CH₃CO₂H, CF₃CO₂H) as it does in CCl₄. The reaction with I₂ is very fast in CF₃CO₂H, but in MeOH is only about as fast as in CCl₄. The iodide II reacts with ICl in MeOH to give a mixture of TsiSiMe₂OMe (III) and TsiSiMe₂Cl (IV), but the reaction is markedly slower than that in CCl₄ (in which IV is formed). The hydride I also reacts with INO₃ in MeOH to give II, and the latter reacts with INO₃ to give III. The reactions of TsiSiPh₂H (V) and TsiSiPh₂I (VI) with ICl in MeOH are markedly slower than those of I and II; even with one equivalent of ICl in MeOH, V gives a mixture of VI and the (rearranged) methoxide (Me₃Si)₂C(SiPh₂Me)(SiMe₂OMe) (VII). Reaction of VI with ICl in MeOH gives VII and the rearranged chloride (Me₃Si)₂C(SiPh₂Me)(SiMe₂Cl). The formation of methoxides in the reactions of the iodides II and VI with ICl in MeOH, and the rearrangements observed in the case of VI, are consistent with a mechanism involving an intermediate silicocation. Other mechanistic aspects are discussed.     

Dual zinc catalysts for L‐lactide polymerization and reaction of CO2 with cyclohexene oxide

A series of zinc complexes, [ L ^X ZnEt] ( 1–5 ) and [ L ^X Zn ₂ (OAc) ₃ ] (6–9) , associated with NNO‐tridentate Schiff base ligands (2‐(((2‐((cyclohexyl[methyl]amino)methyl)phenyl)imino)methyl)phenolate (CAP) derivatives), were synthesized, and their activity toward ring‐opening polymerization (ROP) of L‐lactide (LA) and the reaction of CO₂ with cyclohexene oxide were also investigated. All of [ L ^X ZnEt] revealed excellent catalytic activity to ring‐opening polymerization (ROP) of LA in the presence of benzyl alcohol. Among them, [ L ^H ZnEt] (1) showed the highest activity with 82% conversation within 45 s. In contrast, [L ^X Zn ₂ (OAc) ₃ ] (6–9) were inactive in ROP of L‐lactide. In addition, all of these Zn complexes demonstrated moderate activity in the reaction of CO₂ with cyclohexene oxide in the presence of Bu₄NCl.     

Preparation of tris(trimethylsilyl)methyl (“trisyl”) derivatives of silicon

Treatment of (Me₃Si)₃CLi (“trisyl”lithium, TsiLi) with appropriate silicon halides has given a range of compounds of the type (Me₃Si)₃CSiRR′X; e.g., TsiSiCl₃, TsiSiMeCl₂, TsiSiMe₂X (X = Cl, OMe), TsiSiPh₂X (X = F, Cl, OMe), and TsiSiPhMeH. The trisyl group causes very large steric hindrance to nucleophilic displacements at the silicon to which it is attached, so that (unless one or more hydride ligands are present) most of the common displacements at silicon do not occur. However, halides can be reduced to hydrides by LiAlH₄, and the hydrides can be converted into halides in electrophilic displacements by hallogens. The presence of even one hydride ligand markedly reduces the hindrance, so that, for example, TsiSiPhHI reacts with refluxing methanol to give TsiSiPhH(OMe).     

Computational Exploration of Mechanistic Avenues in Metal-Free CO2 Reduction to CO by Disilyne Bisphosphine Adduct and Phosphonium Silaylide

Recent years have seen a growing interest in metal-free CO₂ activation by silylenes, silylones, and silanones. However, compared to mononuclear silicon species, CO₂ reduction mediated by dinuclear silicon compounds, especially disilynes, has been less explored. We have carried out extensive computational investigations to explore the mechanistic avenues in CO₂ reduction to CO by donor-stabilized disilyne bisphosphine adduct ( R1^M ) and phosphonium silaylide ( R2 ) using density functional theory calculations. Theoretical calculations suggest that R1^M exhibits donor-stabilized bis(silylene) bonding features with unusual Si−Si multiple bonding. Various modes of CO₂ coordination to R1^M have been investigated and the coordination of CO₂ by the carbon center to R1^M is found to be kinetically more facile than that by oxygen involving only one or both the silicon centers. Both the theoretically predicted reaction mechanisms of R1^M and R2 -mediated CO₂ reduction reveal the crucial role of silicon-centered lone pairs in CO₂ activations and generation of key intermediates possessing enormous strain in the Si−C−O ring, which plays the pivotal role in CO extrusion.     

Mechanistic study on iron(II)-mediated direct arylation of benzene with chlorobenzene

Density functional theory calculations on the reaction mechanisms of the direct arylation of benzene with chlorobenzene mediated by a series of low-valent iron complexes, in which the Fe(II) center is surrounded by different electron-donor ligands (acetate anion (OAc), baphophenanthroline (baph), 1,10-phenanthroline (phen), and redox active ligand amidophenolate (ap)) using density functional theory. Fe(II) models, 1b Fe^II(baph), 1p Fe^II(phen), 1d Fe^II(diimine), 2o Fe^II(OAc)₂, 2po Fe^II(OAc)(phen), 2p Fe^II(phen)₂ as well as 2a Fe^II(ap)₂ were established. According to our calculations, 1b and 2a are promising candidates for the direct arylation transformation. The complexes under different ligands show their unique mechanism characteristics. Furthermore, a correlation has been established among the activation barriers, the energy gaps of frontier orbitals, the distortion energies, as well as the reaction enthalpies. The knowledge obtained herein not only deepens our mechanistic understanding of iron-mediated direct arylation but may also provide guidance for the rational design of catalysts.     

Complexes of Cu2(μ‐OAc)4(H2O)2 with 1,10‐Phenanthroline and Comparison with those of 2,2′‐Bipyridine. Crystal Structure of the Dimer [Cu(OAc)2(phen)](μ‐H2O)

The two complexes of composition Cu₂(OAc)₄(phen)(H₂O)₂ ( 1 ) andCu₂(OAc)₄(phen)₂(H₂O) ( 2 ) have been synthesized and characterized by chemical analysis and IR and electronic spectroscopies. Compound 2 has the structure of a dimer with a phenanthroline molecule and two monodentate acetate groups coordinated to each copper atom and a water molecule as the only bridging ligand between them. Each copper atom has a distorted square‐planar pyramidal coordination, determined by two oxygen atoms at 1.94(3) and 1.959(3) Å, two nitrogen atoms at 2.023(4) Å and the oxygen atom of the bridging water molecule at 2.289(2) Å. The distance between the two copper atoms is of 4.29 Å and the angle Cu(1)‐O(3)‐Cu(1A) 139.2(2)°. The water molecule is involved in two intramolecular hydrogen bonds with non coordinated oxygen atoms. The distance between the molecules of phenanthroline is 3.75 Å. Magnetic and EPR results for Cu₂(OAc)₄(phen)(H₂O)₂ ( 1 ), Cu₂(OAc)₄(phen)₂(H₂O) ( 2 ), Cu₂(OAc)₄(bipy) ( 3 ) and Cu₂(OAc)₄(bipy)₂(H₂O)₂ ( 4 ) have been analysed and compared. For 1 and 3 an antiferromagnetic dimer unit [Cu₂(μ‐OAc)₄] with 2J = ?325 and ?292 cm^?1, respectively, and other two copper atoms without significant magnetic interaction are present. Triplet signals are detected in the EPR spectra. In 2 and 4 there is no practically magnetic exchange and the orthorhombic signals are observed in the EPR spectra.     

Preparation of high homogeneity BaO-TiO2-SiO2 gel

In BaO-TiO₂-SiO₂ system, crystallization from various gel by heating depended on the sol-gel processes and gel homogeneity. Through the condensation reaction of Si(OAc)₄ and TiAcAc(O ⁱ Pr)₃ in tetrahydrofuran solvent, homogeneous TiO₂-SiO₂ sol with oligomers of relatively large molecular weight was obtained. The gel prepared by mixing the binary sol, Ba(OAc)₂, and Si(OMe)₄ was the most homogeneous in term of suppression of crystallization. By heating the above gel, only Ba₂TiSi₂O₈ crystal appeared, which was observed in a melt quenched glass. In the case of the gels made by other sol-gel processes, TiO₂ or BaTiO₃ crystal was first observed from the heated gels prior to the precipitation of Ba₂TiSi₂O₈.     

A Solvent Switch for the Stabilization of Multiple Hemiacetals on an Inorganic Platform: Role of Supramolecular Interactions

Reaction of Zn(OAc)₂ ? 2 H₂O with 2,6‐diisopropylphenyl phosphate (dippH₂) in the presence of pyridine‐4‐carboxaldehyde (Py‐4‐CHO) in methanol resulted in the isolation of a tetrameric zinc phosphate cluster [Zn(dipp)(Py‐4‐CH(OH)(OMe))]₄ ? 4 MeOH ( 1 ) with four hemiacetal moieties stabilized on the double‐4‐ring inorganic cubane cluster. The change of solvent from methanol to acetonitrile leads to the formation of [Zn(dipp)(Py‐4‐CHO)]₄ ( 2 ), in which the coordinated Py‐4‐CHO retains its aldehydic form. Dissolution of 1 in CD₃CN readily converts it to the aldehydic form and yields 2 . Similarly 2 , which exists in the aldehyde form in CD₃CN, readily converts to the hemiacetal form in CD₃OD/CH₃OH. Compound 1 is an unprecedented example in which four hemiacetals have been stabilized on a single molecule in the solid state retaining its stability in solution as revealed by its ¹H NMR spectrum in CD₃OD. The solution stability of 1 and 2 has further been confirmed by ESI‐MS studies. To generalize the stabilization of multiple hemiacetals on a single double‐four‐ring platform, pyridine‐2‐carboxaldehyde (Py‐2‐CHO) was used as the auxiliary ligand in the reaction between zinc acetate and dippH₂, leading to isolation of [Zn(dipp)(Py‐2‐CH(OH)(OMe))]₄ ( 3 ). Understandably, recrystallization of 3 from acetonitrile yields the parent aldehydic form, [Zn(dipp)(Py‐2‐CHO)]₄ ( 4 ). Single‐crystal X‐ray diffraction studies reveal that supramolecular bonding, aided by hydrogen‐bonding interactions involving the hemiacetal functionalities (C?OH, C?OMe, and C?H), are responsible for the observed stabilization. The hemiacetal/aldehyde groups in 1 and 2 readily react with p‐toluidine, 2,6‐dimethylaniline, and 4‐bromoaniline to yield the corresponding tetra‐Schiff base ligands, [Zn(dipp)(L)]₄ (L=4‐methyl‐N‐(pyridin‐4‐ylmethylidene)aniline ( 5 ), 2,6‐dimethyl‐N‐(pyridin‐4‐ylmethylene)‐aniline ( 6 ), and 4‐bromo‐N‐(pyridin‐4‐ylmethylene)aniline ( 7 )). Isolation of 5 – 7 opens up further possibilities of using 1 and 2 as new supramolecular synthons and ligands.     

Transition Metal Complexes with Pyrazole-based Ligands. Part 8. Characterization and thermal decomposition of zinc(II) complexes with di- and trisubstituted pyrazoles

We report the synthesis and the characterization (elemental analysis, FT-IR spectroscopy, thermal methods and molar conductivity measurements) of the mixed complexes of zinc with acetate and 3-amino-5-methylpyrazole, HL ¹, [Zn(OAc)₂(HL¹)₂], or 3-amino-5-phenylpyrazole, HL ² [Zn(OAc)₂(HL²)₂], or 4-acetyl-3-amino-5-methylpyrazole, HL ³, [Zn(OAc)(L³)(HL³)]₂, with isothiocyanate and HL ² [Zn(SCN)₂(HL²)₂], or HL ³ [Zn(SCN)₂(HL³)₂], and with nitrate, isothiocyanate and 3,5-dimethyl-1-carboxamidinepyrazole, HL ⁴ [Zn(NO₃)(NCS)(HL⁴)₂]. The thermal decomposition of the complexes is generally continuous resulting zinc oxide as end product,except [Zn(OAc)(L³)(HL³)]₂ in which case a well-defined intermediate was observed between 570–620 K. On the basis of the IR spectra and elemental analysis data of the intermediate a decomposition scheme is proposed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Activation of Carbon Dioxide by Silyl Triflate‐Based Frustrated Lewis Pairs

Silyl triflates of the form R_4?nSi(OTf)_n (n=1, 2; OTf=OSO₃CF₃) are shown to activate carbon dioxide when paired with bulky alkyl‐substituted Group 15 bases. Combinations of silyl triflates and 2,2,6,6‐tetramethylpiperidine react with CO₂ to afford silyl carbamates via a frustrated Lewis pair‐type mechanism. With trialkylphosphines, the silyl triflates R₃Si(OTf) reversibly bind CO₂ affording [R′₃P(CO₂)SiR₃][OTf] whereas when Ph₂Si(OTf)₂ is used one or two molecules of CO₂ can be sequestered. The latter bis‐CO₂ product is favoured at low temperatures and by excess phosphine.     

Nickel‐Catalyzed CO2 Rearrangement of Enol Metal Carbonates for the Efficient Synthesis of β‐Ketocarboxylic Acids

4‐Methylene‐1,3‐dioxolan‐2‐ones underwent oxidative addition of a Ni⁰ catalyst in the presence of Me₂Al(OMe), followed by a coupling reaction with alkynes, to form δ,ϵ‐unsaturated β‐ketocarboxylic acids with high regio‐ and stereoselectivity. The reaction proceeds by [1,3] rearrangement of an enol metal carbonate intermediate and the formal reinsertion of CO₂.     

Zn(OAc)2 catalyzed microwave irradiated synthesis of substituted thieno[2,3-d]pyrimidin-4(3H)-one

Different Lewis acids were screened to catalyze the reaction of 2-amino-thiophene-3-carboxylate, orthoformate and aryl amine to form 2-substituted thieno[2,3-d]pyrimidin-4(3H)-one. Zn(OAc)₂ was demonstrated to efficiently catalyze the reaction. 20 substituted thieno[2,3-d]pyrimidines were synthesized by adding 0.5% mol Zn(OAc)₂ as catalyst under microwave irradiation.     

The solid state reactions of o-aminobenzoic acid with Zn(Ⅱ), Cu(Ⅱ), Ni(Ⅱ), Mn(Ⅱ) acetate hydrate at room temperature

The solid-solid state reactions of o-aminobenzoic acid with Zn(OAc)2.2H2O, Cu(OAc)2 .H2O, Ni(OAc)2.4H2O and Mn(OAc)2.4H2O result in the formation of corresponding complexes M(OAB)2 (M = Zn(Ⅱ), Cu(Ⅱ), Ni(Ⅱ), Mn(IⅡ)). XRD, IR and elemental analysis methods have been used to characterize the solid products. The activation energies of these reactions, which are calculated from the kinetic data obtained by means of the isothermal electrical conductivity measurement method, have been found to increase in the order: Cu(OAc)2.H2O(37.7 kJ.mol-1)~Mn(OAc)2.4H2O (39.7kJ.mol-1) < Zn(OAc)2.2H2O (56.3 kJ.mol-1) < Ni(OAc)2.4H2O (85.2 kJ.mol-1). The trend is related to their crystal structures.     

Bifunctional Catalysts Containing Zn(II) and Imidazolium Salt Ionic Liquids for Chemical Fixation of Carbon Dioxide

Zn(II) can efficiently promote the catalytic performance of imidazolium salt ionic liquids (imi-ILs) for the chemical fixation of CO₂ into epoxides. To obtain sustainability, immobilized bifunctional catalysts containing both imi-ILs and Zn(II) were prepared using bimodal mesoporous silica (BMMs) as carrier, through grafting of Zn(OAc)₂ and 1-(trimethoxysilyl)propyl-3-methylimidazolium chloride (Si-imi) separately in the nanopores. The catalysts, named as BMMs−Zn&ILs, were identified as efficient catalysts for cycloaddition reaction of CO₂ into epoxides under solvent-free conditions. BMMs−Zn&ILs showed good catalytic activity, which increased with the increase of the molar ratio of Zn(II) to Si-imi. As a comparison, different catalytic systems including homogeneous imi-IL, BMMs-ILs and BMMs−Zn were studied to demonstrate different cooperation behaviors. Furthermore, the kinetics studies of homogeneous and heterogeneous bifunctional catalysts were employed to confirm the differences, as well as to support the proposed cooperative catalysis mechanism in the nanopores.