Alkoxide Species Research Articles

Bioinorganic Chemistry of Group 12 Complexes Supported by Tetradentate Tripodal Ligands Having Internal Hydrogen‐Bond Donors

AbstractInspired by the proposed role of secondary hydrogen‐bonding interactions in modulating the chemistry of mononuclear zinc centers in metalloenzymes, zinc complexes supported by tetradentate tripodal ligands having one or more internal hydrogen‐bond donors have been prepared, characterized, and investigated for biologically relevant reactivity. Complexes of this class have been examined in terms of water activation and CO2 reactivity, alcohol/alkoxide coordination, and amide methanolysis and phosphate ester hydrolysis reactivity. The results of these studies indicate that the presence of internal hydrogen‐bond donors will lower the pKa of a zinc‐bound water molecule, stabilize zinc alkoxide species with respect to hydrolysis, and enhance the phosphate ester cleavage reactivity of mononuclear zinc complexes. In addition, use of tripodal ligands having a single internal hydrogen‐bond donor has enabled the isolation of a novel cadmium hydroxide complex and examination of its CO2 chemistry, as well as the identification of a novel amide cleavage reaction which proceeds by initial formation of a deprotonated amide intermediate. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Oxygen- and Temperature-Dependent Kinetic Isotope Effects in Choline Oxidase: Correlating Reversible Hydride Transfer with Environmentally Enhanced Tunneling

Choline oxidase catalyzes the flavin-linked oxidation of choline to glycine betaine, with betaine aldehyde as intermediate and oxygen as electron acceptor. Here, the effects of oxygen concentration and temperature on the kinetic isotope effects with deuterated choline have been investigated. The D(kcat/Km) and Dkcat values with 1,2-[(2)H4]-choline were pH-independent at saturating oxygen concentrations, whereas they decreased at high pH to limiting values that depended on oxygen concentration at < or = 0.97 mM oxygen. The kcat/Km and kcat pH profiles had similar patterns reaching plateaus at high pH. Both the limiting kcat/Km at high pH and the pKa values were perturbed to lower values with choline and < or = 0.25 mM oxygen. These data suggest that oxygen availability modulates whether the reduced enzyme-betaine aldehyde complex partitions forward to catalysis rather then reverting to the oxidized enzyme-choline alkoxide species. At saturating oxygen concentrations, the D(kcat/Km) was 10.6 +/- 0.6 and temperature independent, and the isotope effect on the preexponential factors (A(H)'/A(D)') was 14 +/- 3, ruling out a classical over-the-barrier behavior for hydride transfer. Similar enthalpies of activation (deltaH(double dagger)) with values of 18 +/- 2 and 18 +/- 5 kJ mol(-1) were determined with choline and 1,2-[(2)H4]-choline. These data suggest that the hydride transfer reaction in which choline is oxidized by choline oxidase occurs quantum mechanically within a preorganized active site, with the reactive configuration for hydride tunneling being minimally affected by environmental vibrations of the reaction coordinate other than those affecting the distance between the donor and acceptor of the hydride.

Investigation of the two-photon polymerisation of a Zr-based inorganic–organic hybrid material system

Abstract Two-photon polymerisation of photo-sensitive materials allows the fabrication of three dimensional micro- and nano-structures for photonic, electronic and micro-system applications. However the usable process window and the applicability of this fabrication technique is significantly determined by the properties of the photo-sensitive material employed. In this study investigation of a custom inorganic–organic hybrid system, cross-linked by a two-photon induced process, is described. The material was produced by sol–gel synthesis using a silicon alkoxide species that also possessed methacrylate functionality. Stabilized zirconium alkoxide precursors were added to the precursor solution in order to reduce drying times and impart enhanced mechanical stability to deposited films. This enabled dry films to be used in the polymerisation process. A structural, optical and mechanical analysis of the optimised sol–gel material is presented. A Ti:sapphire laser with 80 MHz repetition rate, 100 fs pulse duration and 795 nm is used. The influence of both material system and laser processing parameters including: laser power, photo-initiator concentration and zirconium loading, on achievable micro-structure and size is presented.

Microkinetic modeling of propane aromatization over HZSM-5

Reaction kinetic studies of propane conversion to aromatics were conducted on an HZSM-5 zeolite at a pressure of 1 atm, temperatures in the range 793–823 K, and different space times (0–12 g cat h/mol). The rates of production of methane, ethane, ethene, propene, propane, butane, butene, benzene, toluene, and xylene are reported. A kinetic model has been postulated that considers surface species as neutral alkoxides, reactions of these alkoxide species by carbenium ion-like transition states, and alkane activation by carbonium ion-like transition states. The associated elementary steps, categorized within the reaction types adsorption, desorption, unimolecular protolytic cracking and dehydrogenation, β-scission, oligomerization, hydride transfer, alkylation, dealkylation, and cyclization, were parsed into reaction families based on an equal reactivity assumption. A total of 311 reaction steps were grouped into 37 reaction families, and the number of unknown parameters was reduced to 25 using adsorption parameters for n-alkanes and relative rates for β-scission and hydride transfer from the literature. It is proposed that this kinetic model describes the reaction behavior over an HZSM-5 catalyst in terms of relevant rate and equilibrium constants and activation energies.

Studies on the transglucosylation reactions of cassava and Thai rosewood β-glucosidases using 2-deoxy-2-fluoro-glycosyl-enzyme intermediates

β-Glucosidases from cassava and Thai rosewood can synthesize a variety of alkyl glucosides using various alcohols as glucosyl acceptors for transglucosylation. Both enzymes were inactivated by 2-deoxy-2-fluoro-sugar analogues to form the covalent glycosyl-enzyme intermediates, indicating that the reaction mechanism was of the double-replacement type. The trapped enzyme intermediates were used for investigating transglucosylation specificity, by measuring the rate of reactivation by various alcohols. The glucosyl-enzyme intermediate from the cassava enzyme showed a 20- to 120-fold higher rate of glucose transfer to alcohols than the glucosyl-enzyme intermediate from the Thai rosewood enzyme. Kinetic analysis indicated that the aglycone binding site of the cassava enzyme was hydrophobic, since the enzyme bound better to more hydrophobic alcohols and showed poor transfer of glucose to hydrophilic sugars. With butanol, transglucosylation was faster with the primary alcohols than with the secondary or tertiary alcohol. Studies with ethanol and chloro-substituted ethanols indicated that the rate of transglucosylation was significantly faster with alcohols with lower p K a values, where the reactive alkoxide was more readily generated, indicating that the formation of the alkoxide species was a major step governing the formation of the transition state in the cassava enzyme.

Stabilities of C 3–C 5 alkoxide species inside H-FER zeolite: a hybrid QM/MM study

Adsorption ( π-complex formation) and chemisorption (alkoxide formation) of ethene, propene, 1-butene, and 1-pentene, as well as of isobutene and 2-methyl-1-butene, in zeolite H-FER are examined by a hybrid quantum mechanics/molecular mechanics (QM/MM) method that takes van der Waals (dispersion) interactions into account. The predicted heats of adsorption (36, 51, 45, 64, 41, 47 kJ/mol, respectively) fall into energy ranges inferred from experimental values for alkanes in H-zeolites. Vibrational frequencies are calculated for adsorbed butenes and butyl alkoxide species and compared with observed spectra for adsorbed 1-butene and isobutene. Primary and secondary alkoxide species are found to be significantly more stable than the adsorption complexes with chemisorption energies between 137 and 205 kJ/mol. The stability increases with the length of the alkyl chain. Geometrical constrictions close to the active site result in reduced stabilities of bulky alkoxide isomers. The tert-butyl and 1,1-dimethylpropyl species are less stable (chemisorption energies of 62 and 78 kJ/mol, respectively) or even nonexistent at sterically hindered framework positions. The implications of relatively stable alkoxide intermediates for hydrocarbon transformation reactions over H-zeolites are discussed.

Mononuclear, five-coordinate lanthanide amido and aryloxide complexes bearing tetradentate (N2O2) Schiff bases.

Two monomeric, five-coordinate lanthanide complexes, [bis-5,5'-(1,3-propanediyldiimino)-2,2-dimethyl-4-hexene-3-onato]samarium[2,6-bis(tert-butyl)-4-methylphenoxide] and [bis-5,5'-(1,3-propanediyldiimino)-2,2-dimethyl-4-hexene-3-onato]erbium[2,6-bis(tert-butyl)-4-methylphenoxide], were isolated from the reactions of 2,6-bis(tert-butyl)-4-methylphenol with [bis-5,5'-(1,3-propanediyldiimino)-2,2-dimethyl-4-hexene-3-onato]lanthanide[bis(trimethylsilyl)amido] (lanthanide = Er(3+) and Sm(3+)). The purified phenoxides were recovered in excellent yields and analytical purity, and the reactions proceeded cleanly without Schiff-base degradation or cluster formation. Analogously, [bis-3,3'-(1,3-propanediyldiimino)-1-phenyl-2-butene-1-onato]erbium[bis(trimethylsilyl)amido] was also directly converted to [bis-3,3'-(1,3-propanediyldiimino)-1-phenyl-2-butene-1-onato]erbium[2,6-bis(tert-butyl)-4-methylphenoxide]; however, a less sterically demanding alcohol (i.e., ethanol) yielded a neutral trinuclear oxo alkoxide species with each dianionic Schiff base asymmetrically bridging through micro-oxo interactions. In this polynuclear cluster, each symmetry-related, seven-coordinate erbium(III) ion exhibits monocapped trigonal prismatic geometry, which assembles by sharing triangular capped faces. Single-crystal X-ray diffraction revealed square-pyramidal metal coordination in each five-coordinate lanthanide ion with varied S(4) ruffling of the "square base" donor atoms and the six-membered propylene diamine chelate ring adopting the boat conformation. To contrast the effect of subtle ligand changes, we also report the synthesis and characterization of [bis-5,5'-(2,2-dimethyl-1,3-propanediyldiimino)-2,2-dimethyl-4-hexene-3-onato]samarium[bis(trimethylsilyl)amido], having gem-dimethyl substituents appended to the propylene bridge central carbon. The six-membered diamine chelate ring in this compound adopts the chair conformation without metal-hydrocarbon interaction. Also presented are qualitative activity observations and polymerization data for the polymerization of rac-lactide and epsilon-caprolactone using the five-coordinate lanthanide amidos and phenoxides.

Ruthenium‐Catalyzed Hydrogenation of Carbon Dioxide to Formic Acid in Alcohols

AbstractCatalytic hydrogenation of CO2 to formic acid with the (solvento)metal hydride complex, TpRu(PPh3)(CH3CN)H [Tp = hydrotris(pyrazolyl)borate], in various alcohols was studied. High‐pressure NMR monitoring of the catalytic reaction in non‐acidic methanol shows that the observable intermediate is a formate complex resulting from CO2 insertion into the Ru−H bond and is stabilized by the hydrogen‐bonding interaction between the formato ligand and a methanol molecule. However, in the case of the acidic alcohol, CF3CH2OH, the observable intermediates are [TpRu(PPh3)(CH3CN)2]+CF3CH2O− and the alkyl carbonate complex, TpRu(PPh3)(η2‐O2COCH2CF3), which are formed by the reaction of CO2 with the alkoxide species, TpRu(PPh3)(CH3CN)(OCH2CF3), generated by a very facile reaction between TpRu(PPh3)(CH3CN)H and CF3CH2OH. We propose that the productive catalytic cycles of the reactions conducted in a variety of alcohols are similar to the one we formulated for the catalytic hydrogenation of CO2 in hydrous THF. The formic acid is produced by the transfer of a hydride and a proton from the transient alcohol hydride intermediate, TpRu(PPh3)(ROH)H, to an approaching CO2 molecule. The activity of TpRu(PPh3)(CH3CN)H is higher in CF3CH2OH than in methanol and other non‐acidic alcohols and it is probably due to the enhanced electrophilicity of the carbon atom of CO2, which results from the strong interaction between the proton of the highly acidic alcohol in TpRu(PPh3)(CF3CH2OH)H and an oxygen atom of CO2. The electrophilic carbon atom of CO2 could in turn abstract the hydride from Ru−H in a more facile manner. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2004)

In‐depth characterization of granular starch‐graft‐polyester compositions as obtained by in situ polymerization of lactones from the starch surface

AbstractThe synthesis of poly(ε‐caprolactone) (PCL)‐grafted granular starch was carried out either in bulk (without solvent) or in toluene suspension by a two‐step procedure. First step relied upon the activation of the hydroxyl groups available at the starch surface by alkylaluminum derivatives like AlEt3 and removal of non‐surface‐grafted organo‐aluminum active species. The latter species were made free in solution by reaction with the remaining water molecules still contaminating the polymerization medium despite intensive drying of the starch granules. In the second step, ε‐caprolactone was polymerized via a coordination‐insertion ring‐opening polymerization as initiated by the surface‐grafted aluminum alkoxide species. The present contribution aims at investigating various parameters such as nature of the alkyl aluminum activator and monomer (δ‐valerolactone was studied as well), temperature, concentration, and addition of a solvent (polymerization in toluene suspension), reaction time, and also the experimental procedure used to recover the polyester chains and measure the grafting efficiency. It turns out that, under the studied conditions, dialkylaluminum alkoxides surface‐grafted onto the starch granules were more likely generated and promote a fast polymerization reaction with the formation of grafted PCL chains with a molecular weight that can be as high as 225 000 (Mn value) for polymerization carried out in toluene suspension. Copyright © 2004 Society of Chemical Industry

Active phases of supported cobalt catalysts for 2,3-dihydrofuran synthesis

Dehydrogenation of 1,4-butanediol to 2,3-dihydrofuran over kaolin-supported Co and Co–Au catalysts has been investigated. Catalytic test and XRD analysis show that the presence of metallic cobalt with hexagonal structure is favourable for selectivity to 2,3-dihydrofuran of the reaction studied. It was established by XPS method that an optimal for the reaction selectivity Co0/Co2+ ratio in Co/kaolin and Co–Au/kaolin catalysts exists. Quantum chemical calculations suggest that the initial step of 1,4-butanediol dehydrogenation on cobalt catalyst surface may be the cleavage of OH bond to form alkoxide species on Co2+ ion. Based on the effect of metallic and ionic cobalt on the catalyst selectivity, it could be presumed that both cobalt species are involved in the rate-determining step in dehydrogenation of 1,4-butanediol into 4-hydroxybutanal intermediate.

Direct Observation of Surface Intermediates Formed by Selective Oxidation of Alcohols on Silica-Supported Molybdenum Oxide

The behavior of surface species, formed from the adsorption of methanol and isobutyl alcohol (2-methyl-1-propanol) on MoO3/SiO2, was studied in an oxygen-containing atmosphere by in situ Fourier transform infrared spectroscopy (FTIR). At 250 °C, methanol adsorption resulted in the formation of surface methoxide species (CH3O−), whereas isobutyl alcohol adsorption resulted in surface isobutoxide species ((CH3)2CHCH2O−). Kinetic studies indicated that the rate-determining step for the alcohol oxidation was the cleavage of the α-C−H bond in the alkoxide intermediates. Although the α-C is a primary carbon in the case of methanol (CH3OH) and a secondary carbon in the case of isobutyl alcohol ((CH3)2CHCH2OH), the activation energies for the rate-determining step were found to be similar. Theoretical molecular orbital calculations confirmed these experimental results by also predicting similar activation energies for both alkoxide species. During oxidation of the alkoxide species in the absence of alcohol in the gas phase, new bands were observed, which were assigned to the CO bands of adsorbed acyl species. In the case of these species, further reaction with oxygen produced CO2. The acyl species are therefore likely not to be intermediates that lead to aldehyde products in alcohol oxidation.

Theoretical Investigation of the Dimerization of Linear Alkenes Catalyzed by Acidic Zeolites

The zeolite-catalyzed dimerization of ethene, propene, 1-butene, and trans-2-butene has been modeled using quantum chemical methods. Reactants, transition states, and products have been investigated. A cluster model consisting of four tetrahedrally coordinated atoms (T-atoms) has been used to represent the catalyst. Two different mechanism types have been evaluated: concerted and stepwise. In the concerted pathway, protonation and C−C bond formation occur simultaneously. The stepwise mechanism proceeds via alkoxide formation followed by C−C bond formation. The order of reactivity among the different alkene reactants has been assessed. Quantum chemistry predicts that the activation energy of the concerted mechanism lies between the two barriers of the stepwise mechanism. More detailed knowledge concerning the stability of alkoxide species relative to physisorbed alkenes will be necessary for discrimination between the two mechanistic proposals. Implications for the reverse reaction, the β-scission of alkenes, are briefly discussed.

Auto-ionization in lutetium iodide complexes: effect of the Iioic radius on lanthanide-iodide binding.

Reaction of lutetium metal with 1.5 equiv of elemental iodine in 2-propanol leads to the isolation of LuI(3)(HO(i)Pr)(4) (1). An X-ray crystal structure reveals an ionic structure with well-separated [LuI(2)(HO(i)Pr)(4)] cations and [I] anions. Dissolution of 1 in pyridine generates the unusual alkoxide species [LuI(O(i)Pr)(py)(5)][I] (2) with the elimination of HI. An X-ray crystal structure of 2 confirmed the ionic nature of the compound, with the cationic portion of the complex exhibiting a seven-coordinated lutetium center with trans-disposed iodo and alkoxide ligands and five pyridine molecules equally displaced within the equatorial plane. Exposure of 2 to iodotrimethylsilane yields the expected triiodide species [LuI(2)(py)(5)][I] (3), which may also be prepared by refluxing commercially available LuI(3) in THF, followed by crystallization from a THF/pyridine mixture. The solid-state structure of 3 is similar to that of 2, with the alkoxide ligand having been replaced by an iodide. The formation of ionic structures 1-3 as opposed to the higher-coordinated neutral species may be traced to the small lutetium center and the presence of relatively strong Lewis bases within the coordination sphere of the metal.

DFT-Based Reaction Pathway Analysis of Hexadiene Cyclization via Carbenium Ion Intermediates: Mechanistic Study of Light Alkane Aromatization Catalysis

We conducted density functional theory calculations to identify the complete cyclization mechanism (ring formation and ring expansion) for protonated hexadiene in the gas-phase as a precursory means of studying aromatization of light alkanes in acidic zeolite catalysts. We identify the rate-determining step to consist of ring expansion from a methylcyclopenta carbenium precursor to a stable cyclohexa carbenium intermediate, exhibiting an activation barrier of 9.6 kcal/mol and proceeding through a bicyclic intermediate starting from a secondary cyclopentyl carbocation. This pathway for ring closure was preferred over tertiary precursor expansion and direct cyclization to a cyclohexyl carbocation. Expecting carbocation intermediates to be represented by alkoxide species near Brønsted acid sites, we calculated the relative stability of primary, secondary, and tertiary alkoxide analogues to cyclopentyl carbocation intermediates involved in ring expansion and notice a reversal of stability relative to gas-phase carbocation stabilities (i.e., primary > secondary > tertiary stability). However, on the basis of the notion that transition state stability depends heavily on carbocation character of the transition state, even in alkoxide-based chemistry, we conclude that ring expansion would nevertheless constitute the rate-determining step in a zeolite-catalyzed mechanism.

Methyl methacrylate polymerization at samarium(II)-grafted MCM-41

Sm(II)-modified periodic mesoporous silica (PMS), Sm[N(SiHMe2)2]2(THF)x@MCM-41, was used for the synthesis of Sm(II) alkyl, alkoxide, and indenyl surface species via secondary ligand exchange. The performance of the novel Sm(II)-based organometallic–inorganic hybrid materials as initiators for the graft polymerization of methyl methacrylate (MMA) is reported. All of the Sm(II) hybrid materials including the new PMMA–PMS composites were characterized via N2 physisorption, elemental analysis, FTIR spectroscopy, and scanning electron microscopy (SEM). The organic–inorganic composites revealed complete pore blockage as well as enrichment and strong adhesion of the polymer at the exterior of the porous silica material.

Interaction of some divalent metal acetylacetonates with Al, Ti, Nb and Ta isopropoxides. Factors influencing the formation and stability of heterometallic alkoxide complexesElectronic supplementary information (ESI) available: synthesis details for 5–7; microanalysis data for 1–9; tables of selected bond lengths and angles for 1–5 and 7–9; variable temperature 1H NMR spectra for 7 and 8; UV-Vis spectrum of 6 in

Interaction of Co(acac)2 with 2 eq. of Ti(OiPr)4 in hexane leads to an equimolar mixture of the new heterometallic complex [Co2Ti2(acac)2(OiPr)10] (1) and the known heteroleptic alkoxide Ti2(acac)2(OiPr)6. When redissolved in iPrOH 1 is destroyed, forming [Co2(acac)4(iPrOH)2] (2) as the only isolable crystalline product. Reaction of Co(acac)2 with 3 eq. of Ta(OiPr)5 gives [Co2Ta2(OiPr)14] (3) together with Ta(acac)(OiPr)4. Reaction of Ni(acac)2 with 4 eq. of Nb(OiPr)5 produces the heterometallic complex [NiNb2(OiPr)12] (4). Reactions of M(acac)2 (M = Co, Ni, Mg and Zn) with Al(OiPr)3 in 1 ∶ 2 ratios provides access to [MAl2(OiPr)4(acac)4] [M = Co (5), Ni (6), Mg (7)] and [Zn2Al2(OiPr)6(acac)4] (8) respectively. When redissolved in iPrOH, [NiAl2(OiPr)4(acac)4] (6) is destroyed, forming a mixture of [Ni2(acac)4(iPrOH)2] (9), Al(OiPr)3 and Al(acac)3. Interaction of M(acac)2, (M = Co, Ni) with metal isopropoxides in iPrOH as well as their recrystallization from iPrOH give 2 and 9 respectively. 5–7 are unstable in the hydrocarbon solution on heating, decomposing with formation of [M2Al2(OiPr)6(acac)4], along with Al(acac)3 and heterometallic alkoxide species. Mg(acac)2 was found to recrystallize unchanged from the solutions of Ti(OiPr)4 in toluene. The solution structure and stability of the heterometallic complexes has been investigated by UV-Vis spectroscopy and NMR, and the details of their decomposition on sublimation by mass-spectroscopy. The factors influencing the formation and stability of heterometallic complexes originating from the reactions of divalent metal acetylacetonates with metal isopropoxides are discussed.

Deprotonation of zinc(II)-water and zinc(II)-alcohol and nucleophilicity of the resultant zinc(II) hydroxide and zinc(II) alkoxide in double-functionalized complexes: theoretical studies on models for hydrolytic zinc enzymes.

The factors governing the deprotonation ability of zinc(II)-water and zinc(II)-alcohol and nucleophilicity of the resultant zinc(II) hydroxide and zinc(II) alkoxide as complex models for zinc enzymes have been investigated through Hartree-Fock and density-functional theory methods with the 6-311++G(d,p) basis set. Our calculations showed that in these double-functionalized complexes (i.e., zinc complexes having both a zinc(II)-alcohol motif and a zinc(II)-water motif) zinc(II)-alcohol is preferred in deprotonation over zinc(II)-water (i.e., zinc(II)-alcohol has a much lower pK(a) than zinc-coordinated water in the same molecule). Natural bond orbital analysis revealed that zinc(II) alkoxides are more nucleophilic than their respective counterparts zinc(II) hydroxides. The analysis of the transition state in the transformation reaction from zinc(II) hydroxide species to zinc(II) alkoxide species indicates that zinc(II) alkoxides are the preferred deprotonated species not only thermodynamically but also kinetically. Further examination of the proposed mechanisms of the zinc(II) alkoxide-promoted transesterification path and the zinc(II) hydroxide-promoted hydrolysis path revealed the structures of the intermediates and energy diagrams in the reactions. These results, entitled double-functionalized complexes, for the first time, put a firm theoretical foundation of why the zinc(II)-alcoholic OH is a better model for hydrolytic zinc enzymes (having both stronger acidity and better nucleophilicity).

Role of Defects in the Adsorption of Aliphatic Alcohols on the TiO2(110) Surface

The role of defects in the adsorption of aliphatic alcohols on TiO2(110) was studied by means of TPRS and XPS. Point defects (oxygen vacancies) were created by electron bombardment in order to minimize the structural damage inflicted to the surface. Ethanol, n-propanol, and 2-propanol adsorbed dissociatively on the TiO2(110) surfaces at room temperature, forming alkoxide and hydroxide groups. Adsorbed alkoxides evolved from the surface in two different reaction channels. About half of the alkoxide species recombined at 345 K with hydroxyl groups and desorbed as the parent alcohols. The rest of the alkoxide groups underwent reactions to produce the corresponding aldehydes, alkenes, and alcohols at temperatures above 550 K. The creation of point defects increased the alkoxide coverage and shifted the product distribution toward the high-temperature channel, favoring alkoxide decomposition over recombination. Within the decomposition products, formation of the dehydration product (alkene) was promoted at the...

Mononuclear nitrogen/sulfur-ligated zinc methoxide and hydroxide complexes: investigating ligand effects on the hydrolytic stability of zinc alkoxide species.

The synthesis and properties of mononuclear zinc methoxide ([(ebnpa)Zn-OCH3]ClO4) (1) and hydroxide ([(ebnpa)Zn-OH]ClO4) (2) complexes of a new mixed nitrogen/sulfur ligand (ebnpa = N-2-(ethylthio)ethyl-N,N-bis(6-neopentylamino-2-pyridylmethyl)amine) are reported. The structures of 1 and 2 were determined by X-ray diffraction. Each possesses a single zinc-coordinated anion (methoxide or hydroxide) and exhibits an overall trigonal bipyramidal geometry. Structural and spectroscopic studies indicate the presence of two hydrogen-bonding interactions involving the oxygen atom of the zinc-bound anion in each complex. Treatment of [(ebnpa)Zn-OH]ClO4 with CH3OH results in the formation of an equilibrium mixture of 1 and 2. 1H NMR spectroscopic methods were used to examine the equilibrium as a function of temperature, yielding KMe (304 K) = 0.30(8), DeltaHMe = -0.9(1) kcal/mol, and DeltaSMe = -5(1) eu. The negative enthalpy indicates that spontaneous zinc alkoxide formation from a hydroxide precursor occurs in this system at low temperature. Using the experimentally determined DeltaHMe value, we found the homolytic Zn-O bond dissociation energy (BDE) in the Zn-OCH3 unit to be approximately -14 kcal/mol relative to the Zn-O BDE in the Zn-OH unit.

New homo- and hetero-alkali metal alkoxide cages; crystal structures of [Me2N(CH2)2OLi]8 and [{Me2N(CH2)2O}12Li8K6

The reaction of N,N′-dimethylaminoethanol (DMAEH), (Me)2N(CH2)2OH, with nBuLi and nBuLi/nBuK respectively results in the formation of the homo- and hetero-metallic alkoxides [Me2N(CH2)2OLi]8, 1, and [{Me2N(CH2)2O}12Li8K6]O, 2, rather than mixed alkyl–alkoxide species. From single crystal X-ray diffraction studies the two complexes are shown to be highly aggregated cage structures. Complex 1 (monoclinic, C2/c) adopts a rare octameric structure constructed from a ring stacked hexamer fused with a (LiO)2 dimer creating a sixteen vertex Li–O cluster. Complex 2, which crystallises (rhombohedral, R) with thf and cyclohexane molecules in the lattice, forms a highly symmetrical, almost spherical, cage with an unprecedented Li ∶ K ratio of 8 ∶ 6.