Alkoxy Oxygen Research Articles

Octanuclear {Ln(III)8}(Ln = Gd, Tb, Dy, Ho) Macrocyclic Complexes in a Cyclooctadiene-like Conformation: Manifestation of Slow Relaxation of Magnetization in the Dy(III) Derivative

The synthesis of a series of macrocyclic, isostructural octanuclear lanthanide complexes [Gd8 (LH2)4 (μ-Piv)4 (η(2)-Piv)4 (μ-OMe)4]·6CH3OH·2H2O (1), [Tb8 (LH2)4 (μ-Piv)4 (η(2)-Piv)4 (μ-OMe)4]4CH3OH·4H2O (2), [Dy8(LH2)4 (μ-Piv)4 (η(2)-Piv)4 (μ-OMe)4]·8CH3OH (3), and [Ho8(LH2)4(μ-Piv)4 (η(2)-Piv)4 (μ-OMe)4]·CH3OH·4H2O (4) have been achieved, using Ln(III) nitrate salts, pivalic acid, and a new multidentate chelating ligand (2E,N'E)-N'-(3-((bis(2- hydroxyethyl)amino)methyl)-2-hydroxy-5-methylbenzylidene)-2-(hydroxyimino) propane hydrazide (LH5), containing two unsymmetrically disposed arms; one side of the phenol unit is decorated with a diethanolamine group while the other side is a hydrazone that has been built by the condensation reaction involving 2-hydroxyiminopropanehydrazide. All the compounds, 1-4, are neutral and are held by the four [LH2](3-) triply deprotonated chelating ligands. In these complexes all the lanthanide ions are doubly or triply bridged via phenolate, alkoxy, and pivalate oxygens. The metal centers are distributed over the 8 vertices of an octagon, resembling a cyclooctadiene ring core. The details of magnetochemical analysis for complexes 1-4 shows that they exhibit antiferromagnetic interactions between the Ln(3+) ions through the phenoxo, alkoxo, and pivalato bridging groups. None of the compounds exhibits slow relaxation of the magnetization at zero applied direct current (dc) magnetic field, which could be due to the existence of a fast quantum tunneling relaxation of the magnetization (QTM). In the case of 3, the application of a small dc field is enough as to fully or partly suppress the fast and efficient zero-field QTM allowing the observation of slow relaxation above 2 K.

Influence of Terminal Acryloyl Arms on the Coordination Chemistry of a Ditopic Pyrimidine–Hydrazone Ligand: Comparison of Pb(II), Zn(II), Cu(II), and Ag(I) Complexes

A new ditopic pyrimidine-hydrazone ligand, 6-hydroxymethylacryloyl-2-pyridinecarboxaldehyde, 2,2'-[2,2'-(2-methyl-4,6-pyrimidinediyl)bis(1-methylhydrazone)] (L2), was synthesized with terminal acryloyl functional groups to allow incorporation into copolymer gel actuators. NMR spectroscopy was used to show that L2 adopted a horseshoe shape with transoid-transoid pym-hyz-py linkages. Metal complexation studies were performed with L2 and salts of Pb(II), Zn(II), Cu(II), and Ag(I) ions in CH3CN in a variety of metal to ligand ratios. Reacting L2 with an excess amount of any of the metal ions resulted in linear complexes where the pym-hyz-py linkages were rotated to a cisoid-cisoid conformation. NMR spectroscopy showed that the acryloyl arms of L2 did not interact with the bound metal ions in solution. Seven of the linear complexes (1-7) were crystallized and analyzed by X-ray diffraction. Most of these complexes (4-7) also showed no coordination between the acryloyl arms and the metal ions; however, complexes 1-3 showed some interactions. Both of the acryloyl arms were coordinated to Pb(II) ions in [Pb2L2(SO3CF3)4] (1), one through the carbonyl oxygen donor and the other through the alkoxy oxygen donor. One of the acryloyl arms of [Cu2L2(CH3CN)3](SO3CF3)4 (2) was coordinated to one of the Cu(II) ions through the carbonyl oxygen donor. There appeared to be a weak association between the alkoxy donors of the acryloyl arms and the Pb(II) ions of [Pb2L2(ClO4)4]·CH3CN (3). Reaction of excess AgSO3CF3 with L2 was repeated in CD3NO2, resulting in crystals of {[Ag7(L2)2(SO3CF3)6(H2O)2] SO3CF3}∞ (8), the polymeric structure of which resulted from coordination between the carbonyl donors of the acryloyl arms and the Ag(I) ions. In all cases the coordination and steric effects of the acryloyl arms did not inhibit isomerization of the pym-hyz bonds of L2 or the core shape of the linear complexes.

Layered transition metal carboxylates: synthesis, structural aspects and observation of multi-step magnetic transition through phase diagram

Two new layered transition metal carboxylate frameworks, [Co3(L)2(H2O)6]·2H2O () and [Ni3(L)2(H2O)6]·2H2O () (L = tartronate anion or hydroxymalonic acid), have been synthesized and characterized by X-ray single crystal analysis. Both compounds have similar 2D structures. In both compounds there are two types of metal centers where one center is doubly bridged by the alkoxy oxygen atoms through μ2-O bridging to form a 1D infinite chain parallel to the crystallographic b-axis with the corners shared between the metal polyhedra. Magnetic susceptibility measurements revealed the existence of antiferromagnetic short range correlations between Co(Ni) intra-chain metal centers (with exchange constants JCo = -22.6 and JNi = -35.4 K). At low temperatures, long range order is observed in both compounds at Néel temperatures of 11 (for ) and 16 (for ) K, revealing that other exchange interactions, rather than the intra-chain ones, play a role in these systems. Whereas compound has an antiferromagnetic ground state, compound exhibits a ferromagnetic component, probably due to spin canting. Isothermal magnetization data unveiled a rich phase diagram with three metamagnetic phase transitions below 8 K in compound .

Hydrogen-bonded complexes of trimethyl phosphate and fluoroform: A matrix isolation infrared and ab initio study

Trimethyl phosphate (TMP) and fluoroform (CHF3) were co-deposited in a nitrogen matrix and complexes of these species were identified using infrared spectroscopy. Formation of the complexes was evidenced by the shifts in the vibrational frequencies of the modes involving the TMP and CHF3 submolecule. The structures of these complexes, energies and the vibrational frequencies were computed at the B3LYP/6-31++G(d,p) level. Both the experimental and computational studies indicated two types of TMPCHF3 complexes; one in which the hydrogen of CHF3 was bonded to the phosphoryl oxygen and another in which the bonding was at the alkoxy oxygen of the phosphate. In addition to the primary hydrogen-bonded interaction at the phosphoryl and alkoxy oxygen, these complexes also appeared to be stabilized by a secondary and weaker interaction between the methyl hydrogen in TMP and the fluorine in CHF3. The computed vibrational frequencies in the complexes agreed well with the observed frequencies. The stabilization energies of these complexes, corrected separately for zero-point energies and basis set superposition error were ≈−5.0 and −5.24kcal/mol for the phosphoryl complexes and ≈−2.8 and −0.57kcal/mol for the alkoxy complexes respectively. The nature of the bonding in this complex was also studied using the atom-in-molecule theory, which clearly showed a (3,−1) bond critical point between the two submolecules.

A quantum chemical study on the polycondensation reaction of polyesters: The mechanism of catalysis in the polycondensation reaction

We carried out a theoretical study on the mechanism of catalysis in the poly(ethylene terephthalate) (PET) polycondensation reaction. Transesterification reaction of diethylterephthalate with ethanol is investigated as a model system by using the B3LYP level of theory, and Sb(OEt) 3, Ge(OEt) 4 and Ti(OEt) 4 are adopted as model catalysts. We found that the metal center of metal alkoxides coordinates to the carbonyl oxygen atom of the ester, and the alkoxy oxygen atom of alkoxy ligands attacks to the carbonyl carbon atom of the ester to form the four-centered transition state. The activation energy for tetraethoxy titanium catalyzed reaction in vacuo is 15.47 kcal/mol; this is comparable to the experimental result of 11.2 kcal/mol for poly(butylene terephthalate)/Ti(OBu) 4. Because the other mechanisms gave much higher activation energies, this is the most convincing mechanism of PET polycondensation catalysis by antimony, germanium and titanium alkoxides.

Hydrophobic side-chain interactions in a family of dimeric amide foldamers-potential alpha-helix mimetics

A series of new alpha-helix mimetics based on a benzamide scaffold and potentially able to disrupt protein-protein interactions have been synthesized and characterized by X-ray analysis. Inspection of the solid state structures of aromatic amide dimers confirmed that the molecules adopt a curved conformation with intramolecular H-bonding between the amide NH and the alkoxy oxygen of the neighboring aromatic fragment (d NH… O ∼ 2 Å). Adjacent dimer molecules are prone to form supramolecular assemblies due to both hydrophobic alkyl side-chain/side-chain interactions and intermolecular H-bonding.

17O NMR study of ortho and alkyl substituent effects in substituted phenyl and alkyl esters of benzoic acids

17O NMR spectra for 44ortho-,meta- andpara-substituted phenyl and alkyl benzoates (C6H5CO2C6H4-X, C6H5CO2R) at natural abundance in acetonitrile were recorded. Substituent effects on the17O NMR chemical shifts, δ(17O), of the carbonyl oxygen and the single-bonded phenoxy (OPh) and alkoxy (OR) oxygens have been studied. The δ(17O) values of the carbonyl oxygen forparaderivatives showed a good correlation with the σ° constants. The δ(17O) values of carbonyl oxygen fororthoderivatives were found to be described well with the Charton equation containing the inductive, σI, resonance, σ°R, and steric,EsB, substituent constants in case the data treatment was performed separately for electron-donating +Rand electron-attracting –Rsubstituents. The electron-donating +Rorthoandparasubstituents in substituted benzoates caused shielding and the electron-withdrawing –Rsubstituents produced deshielding of the O signal. The steric interaction oforthosubstituents with the ester group decreased the electron density at the carbonyl oxygen. In alkyl benzoates the δ(17O) values were found to be described well with the inductive, σI, and the steric,EsB, substituent constants.

Use of73Ge NMR Spectroscopy and X-ray Crystallography for the Study of Electronic Interactions in Substituted Tetrakis(phenyl)-, -(phenoxy)-, and -(thiophenoxy)germanes

NMR chemical shifts of 1H, 13C, and 73Ge, molecular modeling, and single-crystal X-ray diffraction results are reported for a series of substituted tris- and tetrakis(phenyl)germanes of the type (XC6H4)3GeY and (XC6H4)4Ge, where X = o-, m-, and p-OCH3, o-, m-, and p-OC2H5, m- and p-CF3, H, p-C(CH3)3, p-Cl; and Y = Cl and H. Chemical shifts and X-ray data are also reported for o-CH3 and o-OCH3 tetrakis(phenoxy)- ((XC6H4O)4Ge) and thiophenoxygermanes ((XC6H4S)4Ge). For tetrakis derivatives, 73Ge resonances are observed for all but the o-methoxyphenoxy compound, for which the inability to detect a resonance is attributed to rapid quadrupolar relaxation caused by intramolecular interactions of the methoxy oxygen with the central atom. The observation of a relatively broad, slightly upfield 73Ge resonance in the analogous phenyl and thiophenoxy derivatives suggests, as do the results of molecular modeling, that in these compounds there is some hypercoordination. The solid-state structures show bond angles at the aromatic carbon bearing the alkoxy group that suggest an interaction of the alkoxy oxygen with germanium. Oxygen−germanium bond distances are about 17% shorter than the sum of the van der Waals radii.

Transformations of N-Confused Porphyrin Triggered by Insertion of Silicon(IV)

N-confused porphyrin, 5,10,15,20-tetraaryl-2-aza-21-carbaporphyrin, dissolved in triethylamine reacts with dichloromethylsilane yielding the methylsilicon(IV) complex of 5,10,15,20-tetraaryl-2-aza-21-hydroxy-21-carbaporphyrin. Addition of aldehydes or ketones (acetone, acetaldehyde acetophenone, butanone, propanal, benzaldehyde, p-methylbenzaldehyde, p-methoxybenzaldehyde, terephthaldehyde) into the insertion mixture triggered the profound transformation of N-confused porphyrin to form the methylsilicon(IV) complex of N-fused porphyrin derivative substituted at the inner C(9) position by a hydroxyalkyl moiety derived from aldehyde or ketone. The macrocyle is structurally related to an aromatic N-fused inner phlorin while the coordination polyhedron of bound silicon resembles the trigonal bipyramid. The macrocyclic ligand coordinates in the facial mode as the three pyrrolic nitrogen donors lie at the vertices of the single trigonal face. The meridional positions of the trigonal bipyramid are occupied by two pyrrolic nitrogen donors and a sigma-methyl ligand. The coordination sphere is completed by apical coordination of the alkoxy oxygen atom derived from alkanal or alkonone. The incorporation of aldehydes and ketones is stereoselective. Acidic desililation of alkanal compounds yields two aromatic N-confused porphyrin derivatives, that is, 3-(1-hydroxyalkyl)-5,10,15,20-tetraaryl-2-aza-21-carbaporphyrin and its oxidation product 3-alkanoyl-5,10,15,20-tetraaryl-2-aza-21-carbaporphyrin. The acid triggered desililation of ketone derivatives produces the equimolar amounts of N-confused porphyrin and ketone. The first spectroscopically identified step involves the protonation of the C(7) position affording the non-aromatic silicon(IV) complex. The density functional theory (DFT) has been applied to model the molecular and electronic structure of all species identified in the course of silicon insertion into the N-confused and N-fused porphyrin.

Ferromagnetic exchange coupling in a new bis(μ-chloro)-bridged copper(II) Schiff base complex: Synthesis, structure, magnetic properties and catalytic oxidation of cycloalkanes

Abstract A new (μ-chloro)-bridged complex [Cu(HL)Cl]2 · H2O (1) with the Schiff base ligand H2L, [2-((E)-(2-hydroxyethylimino)methyl)-4-bromophenol], has been synthesized and characterized by elemental analysis, IR, UV–Vis and EPR spectroscopic studies. X-ray diffraction studies show that 1 is a binuclear CuII complex with a pair of chlorine atoms bridging the copper atoms in a central Cu2Cl2 core. Each copper atom in 1 adopts a distorted square-pyramidal geometry with the imine nitrogen atom, alkoxy and phenoxy oxygen atoms from the Schiff base ligand and a bridging chlorine atom constructing the basal plane, while the apical position of the pyramid is occupied by the other bridging chlorine atom. Variable temperature susceptibility measurements show that complex 1 presents the highest ferromagnetic coupling [J = +43.2(5) cm−1] reported till date in any doubly chloro-bridged CuII dimer, and a weak interdimer antiferromagnetic coupling [J′ = -0.276(8) cm−1]. The complex also exhibits high catalytic activity towards the oxidation of hydrocarbons using H2O2 as terminal oxidant.

DFT Study on the Selectivity of Complexation of Metal Cations with a Dioxadithia Crown Ether Ligand

The interactions of a dioxadithia crown ether ligand with Li(+), Na(+), K(+), Mg(2+), Ca(2+), and Zn(2+) cations were investigated using density functional theory (DFT) modeling. The modeling was undertaken to gain insight into the mechanism of the selective complexation of the mono- and dications observed with this ligand experimentally. Two types of conformationally different complexes were located with both mono- and dications. In the first conformer, the cation is bonded to the ether oxygens; in the second conformer, the cation is bonded to the alkoxy and suger oxygens. In general, the complexes formed with dications were found to be more stable than those with monocations, with the stability decreasing with the period number within a given periodic table group of elements. The highest stability was observed for the complexes formed with zinc. The complex formed with lithium was the most stable among those involving monovalent cations. The system interaction energy was decomposed into electrostatic (ES), polarization (P), charge-transfer (CT), exchange (EX), and geometry-deformation (DEF) contributions using the self-consistent charge and configuration method for subsystems (SCCCMS). The stabilizing energy components (ES, P, and CT) exhibit the same trend as the total interaction energy, whereas the destabilizing contributions (EX and DEF) exhibit the opposite trend. It was found that the main contributions responsible for stabilization of the dicationic systems are the P and ES energies; in the monocationic systems, the CT stabilization is equally important. The gas-phase preferences changed when the solvent effect was included. The dioxadithia crown ether ligand preserved its selectivity toward Zn(2+), but the selectivity sequence toward monovalent cations was reversed.

1,5-Anti Stereocontrol in the Boron-Mediated Aldol Reactions of β-Alkoxy Methyl Ketones: The Role of the Formyl Hydrogen Bond

The boron-mediated aldol reactions of certain types of beta-alkoxy methyl ketone show remarkably high levels of stereoinduction with achiral aldehydes, leading preferentially to 1,5-anti related stereocenters. Given the low levels of asymmetric induction usually observed in acetate aldol reactions, this is of great synthetic utility and has been used successfully in the total synthesis of a number of polyketide natural products. We have investigated the effects of the alkoxy protecting group (OMe, OPMB, PMP acetal, tetrahydropyran, and OTBS) present in the boron enolate on the level and sense of remote 1,5-stereoinduction, using density functional theory calculations (B3LYP/6-31G**). Our predictions of diastereoselectivity from comparison of the competing aldol transition structures are in excellent qualitative and quantitative agreement with experimentally reported values. We conclude that the boron aldol reactions of unsubstituted boron enolates proceed via boat-shaped transition structures in which a stabilizing formyl hydrogen bond exists between the alkoxy oxygen and the aldehyde proton. It is this interaction that leads to preferential formation of the 1,5-anti adduct, by minimizing steric interactions between the beta-alkyl group and one of the ligands on boron. In the case of silyl ethers, the preference for this internal hydrogen bond is not observed due to the size of the protecting group and the electron-poor oxygen atom that donates electron density into the adjacent silicon atom. We show that this stereochemical model is also applicable in rationalizing the 1,4-syn stereoselectivity of boron aldol reactions involving certain alpha-chiral methyl ketones. These detailed results may be summarized as a conformational diagram that can be used to predict the sense of stereoinduction.

Factors determining the pattern of a hydrogen‐bonding network in the diastereomeric salts of 1‐arylethylamines with enantiopure P‐chiral acids

In order to clarify factor(s) determining the pattern of a hydrogen-bonding network in the diastereomeric salts of 1-arylethylamines (1) with enantiopure P-chiral acids, three kinds of enantiopure P-chiral alkylphenylphosphinothioic acids (3) were synthesized, and their chiral recognition abilities for racemic 1 were examined. The characteristics of the diastereomeric salt crystals of 1 with 3 were then studied on the basis of their X-ray crystallographic analyses. Statistical analysis on the molecular conformations observed in the diastereomeric salts with 3 as well as those with P-chiral O-alkyl arylphosphonothioic acids (2), which have a chemical structure similar to that of 3, and molecular orbital calculations for 2 and 3 in a gas phase revealed that the torsion angle between the aromatic plane and the P--O(alkoxy in 2) or P--C(alkyl in 3) plays an important role in determining the pattern of a hydrogen-bonding network in the diastereomeric salts, either a closed globular cluster or an infinite 2(1) column type. The calculations also indicated that there is a hydrogen-bonding-like interaction between the ammonium hydrogen atom of 1-arylethylammonium cations and the P--O(alkoxy) oxygen atom of phosphonothioate anions in the clusters.

Indução assimétrica 1,5-Anti na adição de enolatos de boro de metilcetonas beta-oxigenadas a aldeídos

High levels of substrate-based 1,5-stereoinduction are obtained in the boron-mediated aldol reactions of b-oxygenated methyl ketones with achiral and chiral aldehydes. Remote induction from the boron enolates gives the 1,5-anti adducts, with the enolate p-facial selectivity critically dependent upon the nature of the b-alkoxy protecting group. This 1,5-anti aldol methodology has been strategically employed in the total synthesis of several natural products. At present, the origin of the high level of 1,5-anti induction obtained with the boron enolates is unclear, although a model based on a hydrogen bonding between the alkoxy oxygen and the formyl hydrogen has been recently proposed.

Design principles to tune the optical properties of 1,3,4-oxadiazole-containing molecules

We have synthesized a series of oxadiazole compounds and ethynylene analogs. Our data reveal that the ring is both optically transparent in the visible range and fully conjugating while we have also discovered the presence of a non-radiative mechanism active in molecules containing common para-dialkoxy substituents adjacent to the oxadiazole ring(s). This structure leads to a greatly reduced quantum yield, in our example dropping from 95.0% to 48.0%. Through our thorough study we have revealed evidence that this is the result of a repulsive interaction between the oxadiazole and the adjacent alkoxy oxygen atom, which we believe prevents excited-state planarity. This quantum yield reduction is preventable through the design principles presented here.

Understanding the Origins of Remote Asymmetric Induction in the Boron Aldol Reactions of β-Alkoxy Methyl Ketones

We report theoretical studies into the remote 1,5-stereoinduction shown by certain types of beta-alkoxy methyl ketones in boron-mediated aldol reactions with achiral aldehydes. For a range of common alkoxy groups, our calculations are in excellent agreement with experimentally observed diastereoselectivities. In the aldol transition structures, a stabilizing hydrogen bond between the alkoxy oxygen and formyl proton leads to preferential formation of the 1,5-adduct, by minimizing steric interactions between the beta-alkyl group and one of the ligands on boron.

A comparison of the mechanisms of hydrolysis of benzimidates, esters, and amides in sulfuric acid media

The mechanisms given in textbooks for both ester and amide hydrolysis in acid media are in need of revision. To illustrate this, benzimidates were chosen as model compounds for oxygen protonated benzamides. In aqueous sulfuric acid media they hydrolyze either by a mechanism involving attack of two water molecules at the carbonyl carbon to give a neutral tetrahedral intermediate directly, as in ester hydrolysis, or by an SN2 attack of two water molecules at the alkyl group of the alkoxy oxygen to form the corresponding amide, or by both mechanisms, depending on the structure of the benzimidate. The major line of evidence leading to these conclusions is the behavior of the excess acidity plots resulting from the rate constants obtained for the hydrolyses as functions of acid concentration and temperature. The first of these mechanisms is in fact very similar to one found for the hydrolysis of benzamides, as inferred from: (1) similar excess acidity plot behaviour; and (2) the observed solvent isotope effects for amide hydrolysis, which are fully consistent with the involvement of two water molecules, but not with one or with three (or more). This mechanism starts out as essentially the same one as that found for ester hydrolysis under the same conditions. Differences arise because the neutral tetrahedral intermediate, formed directly as a result of the protonated substrate being attacked by two water molecules (not one), possesses an easily protonated nitrogen in the amide and benzimidate cases, explaining both the lack of 18O exchange observed for amide hydrolysis and the irreversibility of the reaction. Protonated tetrahedral intermediates are too unstable to exist in the reaction media; in fact, protonation of an sp3 hybridized oxygen to put a full positive charge on it is extremely difficult. (This means that individual protonated alcohol or ether species are unlikely to exist in these media either.) Thus, the reaction of the intermediate going to product or exchanged reactant is a general-acid-catalyzed process for esters. For amide hydrolysis, the situation is complicated by the fact that another, different, mechanism takes over in more strongly acidic media, according to the excess acidity plots. Some possibilities for this are given.Key words: esters, amides, benzimidates, hydrolysis, excess acidity, mechanism, acid media.

A molecular dynamics simulation study on the crystallization of 22,8-polyurethane

By means of the molecular dynamics (MD) simulation, the crystallization mechanism of 22,8-polyurethane which contains hydrogen-bond units is investigated and the results show that the crystallization process at a fixed temperature can be characterized by three stages: (1) The extended chain collapses to a globular random coil; (2) The random coil reorganizes into an ordered lamellar structure; (3) Accompanied with the segments clustering due to the hydrogen-bond formation, the lamellar develops with local defects. Two kinds of hydrogen-bond, which are formed between NH group and C O group (N–H⋯O C), and between NH group and urethane alkoxy oxygen (N–H⋯O), respectively, are found to play an important role in the crystallization process of 22,8-polyurethane. Furthermore, the effect of temperature on the crystallization is also studied by selecting three temperatures 200, 300 and 400 K. The lower the crystal temperature is, the slower the crystallization rate is and the stronger the hydrogen-bonding interactions are presented. This is in harmony with the experimental results.

Synthesis and structural studies of heteroleptic complexes of ytterbium(III) involving aryloxy- or alkoxy- and cyclopentadienyl ligands

Treatment of tris-cyclopentadienyl-ytterbium in thf with one equivalent of 2,6-di( tert-butyl)phenol, N, N-dimethyl-2-aminoethanol or N, N-diethyl-2-aminoethanol resulted in substitution of one cyclopentadienyl ligand and formation of [YbCp 2(O-C 6H 3 t Bu-2,6)(thf)] ( 1), [{YbCp 2(μ-OCH 2CH 2NMe 2)} 2] ( 2) or [{YbCp 2(μ-OCH 2CH 2NEt 2)} 2] · (thf) 2 ( 3), respectively. All compounds were characterised by spectroscopic and X-ray crystallographic techniques, the latter two also being studied by variable temperature 1H NMR spectroscopy. Compound ( 1) is mononuclear with the Yb centre bound by two η 5-cyclopentadienyl ligands one O-bound thf and an O-bound phenoxy ligand. Compounds ( 2) and ( 3) are centrosymmetric dimers with the Yb centre bound by two η 5-cyclopentadienyl ligands, while the bidentate ligands chelate the metal centre and also bridge to the adjacent Yb through the alkoxy oxygen atom. Variable temperature 1H NMR studies on compounds ( 2) and ( 3) show a solution-state equilibrium between the dimeric solid-state structure and one with the nitrogen atoms non-bound to Yb.

Mechanism and structure–reactivity correlation in the homogeneous, unimolecular elimination kinetics of 2‐substituted ethyl methylcarbonates in the gas phase

AbstractThe gas‐phase elimination kinetics of 2‐substituted ethyl methylcarbonates were determined in a static reaction system over the temperature range of 323–435°C and pressure range 28.5–242 Torr. The reactions are homogeneous, unimolecular and follow a first‐order rate law. The kinetic and thermodynamic parameters are reported. The 2‐substituents of the ethyl methylcarbonate (CH3OCOOCH2CH2Z, Z=substituent) give an approximate linear correlation when using the Taft–Topsom method, log(kZ/kH)=−(0.57±0.19)σα+(1.34±0.49)σR− (r=0.9256; SD=0.16) at 400°C. This result implies the elimination process to be sensitive to steric factors, while the electronic effect is unimportant. However, the resonance factor has the greatest influence for a favorable abstraction of the β‐hydrogen of the Cβ—H bond by the oxygen carbonyl. Because ρα is significant, a good correlation of the alkyl substituents of carbonates with Hancock's steric parameters was obtained: log(kR/kH) versus ESC for CH3OCOOCH2CH2R at 400°C, R=alkyl, δ=−0.17 (r=0.9993, SD=0.01). An approximate straight line was obtained on plotting these data with the reported Hancock's correlation of 2‐alkyl ethylacetates. This result leads to evidence for the β‐hydrogen abstraction by the oxygen carbonyl and not by the alkoxy oxygen at the opposite side of the carbonate. The carbonate decompostion is best described in terms of a concerted six‐membered cyclic transition state type of mechanism. Copyright © 2003 John Wiley & Sons, Ltd.