Borderline of hydrogen bonding of silanols

Two new silanols bearing very bulky silyl groups, (i-Pr₃ Si)₃SiOH and (t − BuMe₂Si)₃SiOH were prepared by peracidoxidation of their respective silanes. The X − ray crystallographic analysis revealed that (t − BuMe₂Si)₃ SiOH forms a dimeric structure with hydrogen bonding, while (i − Pr₃ Si)₃ SiOH exists as a monomer in the crystal. The effects of the size of the substituents as well as the reactivity of these silanols are discussed.     

Synthesis,Crystal Structures,and Thermal Properties of New Zinc(II) Thiocyanato Coordination Compounds

Reaction of zinc(II) thiocyanate with pyrazine, pyrimidine, pyridazine, and pyridine leads to the formation of new zinc(II) thiocyanato coordination compounds. In bis(isothiocyanato‐N)‐bis(μ₂‐pyrazine‐N,N) zinc(II) ( 1 ) and bis(isothiocyanato‐N)‐bis(μ₂‐pyrimidine‐N,N) zinc(II) ( 2 ) the zinc atoms are coordinated by four nitrogen atoms of the diazine ligands and two nitrogen atoms of the isothiocyanato anions within slightly distorted octahedra. The zinc atoms are connected by the diazine ligands into layers, which are further linked by weak intermolecular S ··· S interactions in 1 and by weak intermolecular C–H ··· S hydrogen bonding in 2 . In bis(isothiocyanato‐N)‐bis(pyridazine‐N) ( 3 ) discrete complexes are found, in which the zinc atoms are coordinated by two nitrogen atoms of the isothiocyanato ligands and two nitrogen atoms of the pyridazine ligands. The crystal structure of bis(isothiocyanato‐N)‐tetrakis(pyridine‐N) ( 4 ) is known and consists of discrete complexes, in which the zinc atoms are octahedrally coordinated by two thiocyanato anions and four pyridine molecules. Investigations using simultaneous differential thermoanalysis and thermogravimetry, X‐ray powder diffraction and IR spectroscopy prove that on heating, the ligand‐rich compounds 1 , 2 , and 3 decompose without the formation of ligand‐deficient intermediate phases. In contrast, compound 4 looses the pyridine ligands in two different steps, leading to the formation of the literature known ligand‐deficient compound bis(isothiocyanato‐N)‐bis(pyridine‐N) ( 5 ) as an intermediate. The crystal structure of compound 5 consists of tetrahedrally coordinated zinc atoms which are surrounded by two isothiocyanato anions and two pyridine ligands. The structures and the thermal reactivity are discussed and compared with this of related transition metal isothiocyanates with pyrazine, pyrimidine, pyridazine, and pyridine.     

Understanding the Synthesis of Supported Vanadium Oxide Catalysts Using Chemical Grafting

The complexity of variables during incipient wetness impregnation synthesis of supported metal oxides precludes an in-depth understanding of the chemical reactions governing the formation of the dispersed oxide sites. This contribution describes the use of vapor phase deposition chemistry (also known as grafting) as a tool to systematically investigate the influence of isopropanol solvent on VO(OⁱPr)₃ anchoring during synthesis of vanadium oxide on silica. The availability of anchoring sites on silica was found to depend not only on the pretreatment of the silica but also on the solvent present. H-bond donors can reduce the reactivity of isolated silanols whereas disruption of silanol nests by H-bond acceptors can turn unreactive H-bonded silanols into reactive anchoring sites. The model suggested here can inform improved syntheses with increased dispersion of metal oxides on silica.     

Substituent Reactivity and Tautomerism of Isoguanosine and Related Nucleosides

The substituent reactivity and tautomerism of isoguanine nucleosides is studied. Benzoylation or tosylation of isoguanine nucleosides (pyridine, room temperature) yields the 2-benzoyl derivatives 7c, 11 , and 12 or the 2-tosyl compounds 13 and 14 . The isobutyrylation of the 6-amino group which did not occur under these conditions was induced in the presence of Me₃SiCl. In the absence of Me₃SiCl, the reactivity of isoguanine substituents decreases in the order from 2-oxo → 5′-OH → 3′-OH → 6-NH₂. From isoguanine nucleosides, the N¹-( 2b ), N³-( 17 ), N⁶-( 15a,b ), and 2-O-alkylated ( 3b ) derivatives were prepared. Their pK_a values were determined and the UV and ¹³C-NMR spectra compared with regard to the alkylation position. Also the tautomeric forms of isoguanine nucleosides were determined UV-spectrophotometrically in aqueous and nonaqueous solution. Isoguanosine ( 1a ), its 2′-deoxy analogue 1b as well as the N⁶-methyl- and 8-substituted derivatives form lactam tautomers in aqueous solution, whereas the lactim form is present in dioxane.     

Zinc–Organic Battery with a Wide Operation‐Temperature Window from −70 to 150 °C

Aqueous zinc (Zn) batteries have been considered as promising candidates for grid‐scale energy storage. However, their cycle stability is generally limited by the structure collapse of cathode materials and dendrite formation coupled with undesired hydrogen evolution on the Zn anode. Herein we propose a zinc–organic battery with a phenanthrenequinone macrocyclic trimer (PQ‐MCT) cathode, a zinc‐foil anode, and a non‐aqueous electrolyte of a N,N‐dimethylformamide (DMF) solution containing Zn²⁺. The non‐aqueous nature of the system and the formation of a Zn²⁺–DMF complex can efficiently eliminate undesired hydrogen evolution and dendrite growth on the Zn anode, respectively. Furthermore, the organic cathode can store Zn²⁺ ions through a reversible coordination reaction with fast kinetics. Therefore, this battery can be cycled 20 000 times with negligible capacity fading. Surprisingly, this battery can even be operated in a wide temperature range from ?70 to 150 °C.     

Gem-diol Generation in Copper and Zinc N-methyl-2-imidazolecarboxaldehyde Complexes: Solid-state NMR,EPR and Single-Crystal X-ray Studies

Novel coordination compounds, mono- ( 1 ) and binuclear copper complexes ( 2 ) and a zinc complex ( 3 ) were synthesized and studied through single-crystal X-ray crystallography, solid-state NMR and EPR techniques to determine the chemical functionalization of the carbonyl group in N-methyl-2-imidazolecarboxaldehyde ligand. Particularly, molecules containing carbonyl groups are versatile ligands that give rise to a wide range of new materials due to the high reactivity of the carbonyl group. However, the chemical identification of the functional group present in these coordination compounds is a challenge because the copper ion affects the NMR signals. In this sense, X-ray crystallography becomes an indispensable tool for the analysis. The imidazole ligands in copper complexes 1 and 2 were found to be the aldehyde and the gem-diol forms, respectively. Furthermore, the gem-diol and carboxylate moieties were detected in the crystal lattice for the zinc complex 3 and studied by solution- and solid-state NMR.     

Mechanism of coating by electropolymerization on metal cathodes from zinc chloride solutions of acrylamide

Electrocoating by polymerization of aqueous zinc chloride solutions of acrylamide and methylenebisacrylamide has been reexamined. The original investigators of this system proposed a mechanism in which the initiation was formed by electrochemical reduction of a zinc complex of molecular oxygen. That mechanism is not compatible with the present findings that polymerization occurs when oxygen is excluded by nitrogen bubbling. Evidence is presented for the intervention of a zinc complex with monomer. The pH dependence of electrocoating indicates that the species involved is ZnOH⁺. Failure to achieve coating in acidic solutions of monomers in the absence of zinc points to a multisite attchment of the metal species to monomer in a ring structure. The interpretation is consistent with the observed electropolymerization of N,N-dimethylacrylamide and with the correlation of electropolymerizability with shifts detected in laser Raman spectra of selected acrylic monomers.     

Chemical structure of networks resulting from curing of N,N-diglycidylaniline-type resins with aromatic amines. IV. Characterization of TGDDM/DDS and TGDDM/DDM networks by high-resolution solid-state 13C-NMR

Cured N,N,N′,N′-tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM) based epoxy resins were investigated by high-resolution solid-state ¹³C-NMR spectroscopy. Associated hardeners were the most commonly used low reactivity 4,4--diaminodiphenylsulphone (DDS), as well as, for comparisons reasons, the higher reactivity 4,4′-diaminodiphenylmethane (DDM) with, in each case, a1 to 1 or 1 to 0.6 epoxy/NH ratio. In order to interpret the spectra, the poorly resolved aliphatic region was decomposed into elementary lines, the structural assignments of which were made using solution ¹³C-NMR data resulting from a previous model compound study. The main structural feature of all investigated systems is the predominance of small cyclic units resulting from intramolecular reactions of N,N-diglycidylaniline groups. The resins are therefore far less crosslinked that it could be anticipated from the functionality of the reactants. Using the low reactivity DDS still increases this effect, due to a higher proportion of residual non reacted secondary amines. Reducing the initial ratio of hardener could on the contrary lead to a higher proportion of reacted amine function, and thus to a higher crosslinking degree. A qualitative picture of such networks is given at the end.     

Solid-state polymerization of acrylamide and its derivatives complexed with some lewis acids. II. Radiation-induced in-source polymerization

Radiation-induced solid-state polymerizations of complexes of N-tert-butylacrylamide, N-tert-amylacrylamide, and N-tert-hexylacrylamide with zinc chloride and zinc bromide have been studied. An accelerating effect of temperature and an inhibiting effect of oxygen on the polymerization process were observed. The activation energies have been established. The influence of monomer structure as well as the halide used on the polymerization rate have been discussed and some regularities have been pointed out. The polymers obtained show good solubilities in common solvents, which proves that they are not crosslinked.     

Water-soluble copolymers. IX. Copolymers of acrylamide with N-(1,1-dimethyl-3-oxybutyl)acrylamide and N,N-dimethylacrylamide: Synthesis and characterization

The copolymerizations of acrylamide (AM) with N-(1,1-dimethyl-3-oxybutyl)acrylamide (DAAM) and with N,N-dimethylacrylamide (DMAM) have been studied. The values of r₁,r₂ have been determined to be 0.75 for the AM-DAAM pair and 0.86 for the AM-DMAM pair. The molecular weights of the copolymers were found to decrease with an increase in the feed composition of DAAM or DMAM. The microstructure was predicted for a wide range of feed compositions through a knowledge of reactivity ratios. These model structures are utilized for assessment of structure/dilute solution relationships reported in a subsequent paper in this series.     

Zinc–Organic Battery with a Wide Operation-Temperature Window from −70 to 150 °C

Aqueous zinc (Zn) batteries have been considered as promising candidates for grid-scale energy storage. However, their cycle stability is generally limited by the structure collapse of cathode materials and dendrite formation coupled with undesired hydrogen evolution on the Zn anode. Herein we propose a zinc–organic battery with a phenanthrenequinone macrocyclic trimer (PQ-MCT) cathode, a zinc-foil anode, and a non-aqueous electrolyte of a N,N-dimethylformamide (DMF) solution containing Zn²⁺. The non-aqueous nature of the system and the formation of a Zn²⁺–DMF complex can efficiently eliminate undesired hydrogen evolution and dendrite growth on the Zn anode, respectively. Furthermore, the organic cathode can store Zn²⁺ ions through a reversible coordination reaction with fast kinetics. Therefore, this battery can be cycled 20 000 times with negligible capacity fading. Surprisingly, this battery can even be operated in a wide temperature range from −70 to 150 °C.     

Influence of supporting ligand microenvironment on the aqueous stability and visible light-induced CO-release reactivity of zinc flavonolato species

The visible light-induced CO-release reactivity of the zinc flavonolato complex [(6-Ph₂TPA)Zn(3-Hfl)]ClO₄ (1) has been investigated in 1?:?1 H₂O?:?DMSO. Additionally, the effect of ligand secondary microenvironment on the aqueous stability and visible light-induced CO-release reactivity of zinc flavonolato species has been evaluated through the preparation, characterization, and examination of the photochemistry of compounds supported by chelate ligands with differing secondary appendages, [(TPA)Zn(3-Hfl)]ClO₄ (3; TPA = tris-2-(pyridylmethyl)amine) and [(bnpapa)Zn(3-Hfl)]ClO₄ (4; bnpapa = N,N-bis((6-neopentylamino-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine)). Compound 3 undergoes reaction in 1?:?1 H₂O?:?DMSO resulting in the release of the free neutral flavonol. Irradiation of acetonitrile solutions of 3 and 4 at 419 nm under aerobic conditions results in quantitative, photoinduced CO-release. However, the reaction quantum yields under these conditions are lower than that exhibited by 1, with 4 exhibiting an especially low quantum yield. Overall, the results of this study indicate that positioning a zinc flavonolato moiety within a hydrophobic microenvironment is an important design strategy toward further developing such compounds as CO-release agents for use in biological systems.     

Light‐Controlled Orthogonal Covalent Bond Formation at Two Different Wavelengths

We report light‐induced reactions in a two‐chromophore system capable of sequence‐independent λ‐orthogonal reactivity relying solely on the choice of wavelength and solvent. In a solution of water and acetonitrile, LED irradiation at λ_max=285 nm leads to full conversion of 2,5‐diphenyltetrazoles with N‐ethylmaleimide to the pyrazoline ligation products. Simultaneously present o‐methylbenzaldehyde thioethers are retained. Conversely, LED irradiation at λ_max=382 nm is used to induce ligation of the o‐methylbenzaldehydes in acetonitrile with N‐ethylmaleimide via o‐quinodimethanes, while 2,5‐diphenyltetrazoles also present are retained. This unprecedented photochemical selectivity is achieved through control of the number and wavelength of incident photons as well as favorable optical properties and quantum yields of the reactants in their environment.     

Reactivity characteristics of squid β-chitin as compared with those of shrimp chitin: High potentials of squid chitin as a starting material for facile chemical modifications

Chemical reactivity of β-chitin isolated from squid pens has been examined in various reactions to elucidate the possibility of facile modifications in simple manners leading to the preparation of derivatives with well-defined structures. β-Chitin swelled in common solvents such as methanol and pyridine unlike the ordinary α-chitin and exhibited much higher reactivity than β-chitin. Free amino groups present in β-chitin were easily and selectively acetylated with acetic anhydride in methanol to give chitin with a uniform structure, poly(N-acetyl-D-glucosamine). When acetylation reaction was carried out in pyridine, O-acetylation proceeded smoothly besides N-acetylation. In the presence of 4-dimethylaminopyridine as the catalyst, even full acetylation was achieved under mild conditions. Tosylation was also quite efficient in pyridine without side reactions such as N-deacetylation which is unavoidable in the tosylation of α-chitin. β-Chitin also enabled direct tritylation in pyridine in the presence of 4-dimethylaminopyridine. All these reactions were quite sluggish with β-chitin, and no reactions or only very low extents of substitution were observed, indicating the high potential of β-chitin as a versatile starting material for facile modification reactions. © 1994 John Wiley & Sons, Inc.     

Reactivity and catalytic activity of cationic lonomers for the nucleophilic substitution reactions

Polystyrene-based crosslinked cationic ionomers containing ammonium or phosphonium chlorides (AxRCI and PxBuCI) were reacted with decyl methanesulfonate. The kinetic data were correlated with the swelling behavior of the ionomers and the solution viscosity of the corresponding linear ionomers. The reactivity of the ionomers was independent of the particle size of the ionomer beads, indicating no diffusion control of the reaction. The solvent and the ion content of the ionomers greatly affect the reactivity. In nonpolar solvents with a low acceptor number, A_N, such as toluene, the aggregation of ionic groups with an increasing ion content reduces the reactivity. A solvent with a high value of A_N, such as chloroform, led a very low reactivity independent of the ion content. Aprotic polar solvent, such as acetonitrila, promoted the dissociation of the ionic groups and furnished a relatively high reactivity independent of the ion content. Several catalytic substitution reactions were carried out under liquid-solid-solid triphase conditions. The kinetic results were accounted for in terms of slow nucleophile transport and fast chemical reaction within the ionomer particles. © 1994 John Wiley & Sons, Inc.     

An Efficient Zinc‐Catalyzed Dehydration of Primary Amides to Nitriles

In the present study, the zinc‐catalyzed dehydration of a variety of amides with N‐methyl‐N‐(trimethylsilyl)trifluoroacetamide (MSTFA) as a dehydration reagent into the corresponding nitriles has been examined in detail. With the straightforward and commercially available zinc(II)triflate as the precatalyst and MSTFA, an excellent system has been established to afford nitriles in excellent yields and chemoselectivities. After investigation of reaction conditions and the scope and limitations, several efforts were carried out to understand the reaction mechanism.     

Molecular Weight Development in Free‐Radical Polymerization with Polyfunctional Chain‐Transfer Agents, 2. Substitution Effects

The analytic expression for the weight‐average molecular weight development proposed in the first part of this series^[1] is extended to account for the substitution effects of the functional groups in a polyfunctional chain‐transfer agent. The obtained functional form of the matrix formula, P_w = W { D _w + ( I + T ) SPE ( I – TSPE )^–1 D _f } is essentially the same as for equal reactivity model, but the NxN square matrixes, S and P in the equal reactivity model is now expanded to NxfN and fNxfN matrixes, respectively, to account for the different reactivities of f functional groups. The number of chain types, N is extrapolated to infinity to obtain the P_w‐value for free‐radical polymerization. Practically, such extrapolation can be conducted with calculated P_w‐values for only three different N‐values. Illustrative calculations show that the obtained results agree with those from the Monte Carlo method. The present approach provides newer insight into the branched structure formation during complex polymerization reactions.     

Crystallographic report: A three‐dimensional coordination polymer: poly‐[µ7‐1,2,4,5‐benzenetetracarboxylato‐bis(N,N‐dimethylformamide)dizinc(II)]

The structure of the title compound, [Zn₂(btc)(DMF)₂]_n (btc = 1,2,4,5‐benzenetetracarboxylate, DMF = N,N‐dimethylformamide), shows that the octahedrally and tetrahedrally coordinated zinc atoms (present in the ratio 1:1) are linked by btc ligands to form a three‐dimensional network comprising cross‐linked channels occupied by coordinated DMF molecules. Copyright © 2004 John Wiley & Sons, Ltd.     

Bis[N-alkyl-NN-di(2-pyridylmethyl)amine]zinc(II) perchlorates display cis-facial stereochemistry in solid state and solution

N-Alkyl-N,N-di(2-pyridylmethyl)amines are ligands commonly used by supramolecular chemists in molecular recognition and sensing applications. The metal coordination complexes of these ligands, in particular those with 2:1 (ligand:metal) molar ratio, have not been sufficiently characterised in solution. In this work, bis[N-alkyl-N,N-di(2-pyridylmethyl)amine]zinc(II) perchlorates are characterised in both solid and solution phases, using X-ray crystallography and NMR spectroscopy, respectively. Only the cis-facial stereoisomer is observed. Density functional theory calculations support the thermodynamic preference for this stereochemistry, as in one representative case the gas phase energy of the cis-facial configuration is lower than those of the trans-facial and meridional configurations by 4.0 and 4.5 kcal/mol, respectively.