Construction and investigation of photo-switch property of azobenzene-bridged pillar[5]arene-based [3]rotaxanes

Series of azobenzene-bridged pillar[5]arene-based [3]rotaxanes with different alkyl chain length of guest molecules were constructed by threading-endcapping method with alkylenetriazole as axile and tetrahydrochromene as endcapping group.The encapsulation of pillar[5]arenes were proved by highresolution mass,¹ H NMR and NOESY spectra.The photo-responsive property were examined by irradiation of the synthesized [3]rotaxanes with 365 nm and blue light LED,which caused trans to cis and cis to trans isomerization,respectively.Irradiation of corresponding model guest compounds without pillar[5]arene encapsulation resulted in near completely trans to cis and cis to trans isomerization,indicating the existence of pillar[5]arenes is the determining factor for the comprised photo isomerization efficiency.     

Preparation and steric structures of saturated or partially saturated cyclopentane-and cyclohexane[b]pyrrolo[1,2-a][3,1]benzoxazine isomers [1]

The reactions of 2-ethoxycarbonylmethyleyclopentanone (1) with cis- (3a) and trans-2-hydroxymethylcyclohex-4-enyl-1-amine (3b) and of 2-ethoxycarbonylmethylcyclohexanone (2) with stereoisomeric 2-hydroxymethyl-1-cyclohexylamines (4a,b) yield the isomeric cyclopentane-(5a,b) and cyclohexane[b] pyrrolo[1,2-a][3,1]benzoxazinones (6a,b) with unsaturated (5) or saturated (6) ring A. The steric structures were elucidated by means ¹H, ¹³C NMR and X-ray measurements.     

Trans-cis isomerization of an azoxybenzene liquid crystal

Trans-cis isomerization was investigated in a room temperature liquid crystal mixture of two azoxybenzene compounds. Experiments were performed on isolated molecules in dilute solutions and on the liquid crystal phase composed of the pure compounds. The absorption spectra of the trans and cis isomers were found to be similar to those of azobenzene compounds, as were the birefringence and order parameter of the nematic liquid crystal phase. The photo-optic properties were also similar in that irradiation by ultraviolet light caused the conversion from trans to cis isomers, while short wavelength visible light incident on these compounds resulted in the conversion from cis to trans isomers. The activation energy for thermal relaxation from the cis to trans isomer in the liquid crystal phase was determined to be (66±7) kJ/mole, which is less than for azobenzene in solution. While a photostationary state in a dilute solution with approximately equal numbers of trans and cis isomers was achieved, the nematic-isotropic transition of the mixture of the pure compounds decreased from 70°C to room temperature with a cis concentration of only about 12%. One unusual finding was that the photostationary concentration of trans and cis isomers due to irradiation with light of a specific visible wavelength depended on the starting concentrations of the two isomers, indicating that there may be a molecular conformation that is not photo-responsive and relaxes only thermally.     

X-ray diffraction studies on cis- and trans-4'-cyanobiphenyl-4-yl 3-n-propylcyclobutane-1-carboxylate

X-ray diffraction has been used to study the nematic phases of the cis and trans isomers of an elongated molecule containing the cyclobutane group. It has been shown that these nematic phases consist of a mixture of single molecules and overlapping core dimers. It has been found that increasing the temperature of the cis isomer rapidly reduces the dimer population, whereas increasing the temperature of the trans isomer enhances the dimer population. This provides an explanation of the relatively high clearing temperature of the trans isomer. The different temperature dependences of the dimer population in the cis and trans isomers has been explained by a model involving inversion of the cylobutane group.     

Spectrophotometric evidence for the existence of rotamers in solutions of some ketocyanine dyes

The existence of rotational conformers in solutions of 1,5-bis[dialkylaminophenyl]-1,4-pentadien-3-one (I) was investigated by infrared, absorption and fluorescence spectroscopy. Comparison was made with 2,5-bis{[4-(dialkylamino)phenyl]-methylene}-cyclopentanone (II) and 2,6-bis{[4-(dialkylamino)phenyl]-methylene}-cyclohexanone (III) which may be considered as rigidified analogues of I. The study in organic solvents revealed that two different conformers coexisted in solutions of I. They differ by rotation about the carbonyl-carbon quasi-single bond and, according to spectroscopic data and AM1 calculations, could be cis, cis and cis, trans isomers, the latter being found in much larger proportions.     

Reactivity of ruthenium complexes with uninegative (X,S)-donor ligands (X = S,P)—II. Phosphine substitution in [Ru(S,S)2(PR3)2] derivatives. Crystal structure of trans-bis(O-ethyldithiocarbonate)bis(dimethylphenylphosphine)-ruthenium(II)

The complexes [Ru(S,S)₂(PPh₃)₂] [S,S = EtCOCS₂⁻, (CH₂)₄NCS₂⁻] react with a variety of tertiary phosphines with the substitution of triphenylphosphine and the formation of [Ru(S,S)₂(PR₃)₂]. The reaction occurs with the formation ofthe cis isomer, except for the complex with PMe₂Ph that gives rise to the trans isomer as the crystal structure shows. The effect of the different phosphines on the ruthenium complex is analysed in terms of the spectroscopic and electrochemical properties of the isolated compounds. The cyclic voltammetric studies of the cis complexes show that isomerization to the trans isomer occurs on oxidation. This isomerization is not observed in the trans-[Ru(S,S)₂(PMe₂Ph)₂] complexes that give rise to stable trans-ruthenium(II)/ruthenium(III) couples. In a similar way the diphosphine complexes afford a quasi-reversible cis-ruthenium(II)/ruthenium(III) process.     

Palladium(II) coordination and cyclometallated complexes derived from 1-tert-butylpyrazole

The synthesis and characterization of a number of new coordination compounds of Pd^II with the nitrogen donor 1-tert-butylpyrazole (^tBuPzH) are described. Compounds are trans-[Pd(^tBuPzH)₂Cl₂] and the cyclometallated structures [Pd₂(^tBuPz)₂(AcO)₂] and [Pd₃(^tBuPz)₂(AcO)₄]. All these complexes are mixtures of syn and anti isomers. Also, the chloro-bridged complex [Pd₂(^tBuPz)₂Cl₂] has been isolated as an equilibrium mixture of cis and trans isomers. The compounds have been studied by variable temperature ¹H- and ¹³C-NMR spectroscopy.     

Carbon-carbon double bond in pillar[5]arene cavity: Selective binding of cis/trans-olefin isomers

The binding behavior of pillar[5]arenes (P5As) towards a series of olefin guests ((E)-1,4-dichlorobut-2-ene (1E), (Z)-1,4-dichlorobut-2-ene (1Z), (E)-but-2-ene-1,4-diol (2E), and (Z)-but-2-ene-1,4-diol (2Z), as well as an alkyne derivative 1,4-dichlorobut-2-yne (3)) have been studied in organic solution. P5As exhibit considerable selectivities for the trans-olefin isomers (1E and 2E) over their cis-isomers (1Z and 2Z). The cis/trans-selective interactions hold the potential of utilizing P5As to separate olefin isomers.     

The manifestation of non-bonded attraction in the physical properties of cis and trans olefins

Non-bonded attraction is suggested to account for a host of differences in the physical properties of cis and trans olefins of the type XHC=CHX. The main predictions are: (i) The cis isomer is more stable than the trans isomer; (ii) The C=C bond is longer and the C-X bonds are shorter for the cis isomer; (iii) The π MO's orbital energies of the two isomers differ such that the trans isomer is a better electron donor and electron acceptor than the cis isomer. Ab initio calculations at the STO-3G and the 4-31G levels in support of the model are presented. The photoelectron spectra of cis and trans difluoro, dichloro and dibromoethylene are discussed, and found to be in accord with our qualitative model.     

N,N'-二-[3-羟基-4-(2-苯并噻唑)苯基]脲的光谱实验与密度泛函理论研究

利用实验观测与密度泛函理论(DFT)计算方法考察了新化合物N, N'-二-[3-羟基-4-(2-苯并噻唑)苯基]脲(4-DHBTU)的红外、核磁与紫外吸收光谱性质.与单体2-(4-氨基-2-羟苯基)苯并噻唑(4-AHBT)相比, 4-DHBTU的实验紫外吸收强度显著增强,最大吸收峰发生了明显红移,并呈现出双吸收峰特征.结合实验光谱数据与密度泛函理论计算分析表明, 4-DHBTU分子最稳定的基态异构体为cis-C₁₁和trans-C₁₁,而导致上述紫外光谱差异的主要原因是4-DHBTU样品中cis-C₁₁, trans-C₁₁, cis-C₂₂, trans-C₂₂等多种异构体共存.此外, 4-DHBTU与溶剂二甲基亚砜(DMSO)间氢键作用使得核磁实验中4-DHBTU的15H、16H氢谱化学位移显著增大.     

Conformational and NBO analysis on cis and trans isomers of methyl-1-(4-hydroxy-3-methoxyphenyl)-1,2,3,4-tetrahydro-9H-β-carboline-3-carboxylate

The cis and trans-methyl-1-(4-hydroxy-3-methoxyphenyl)-1,2,3,4-tetrahydro-9H-β-carboline-3-carboxylates were prepared and ¹H and ¹³C NMR spectra were recorded for both isomers. Conformational and NBO analysis were carried out for the cis and trans isomers. Conformer structures were pre-optimized using the hybrid method B3LYP along with the 6-311+G(d) basis set. Frequency calculations were employed to confirm the structures as minimum points. Potential energy surfaces (PES) were built at the same level of theory. Geometries obtained from DFT calculations were used to perform NBO analysis by the NBO 3.1 module in GAUSSIAN 03. The results obtained through theoretical calculations revealed that the shielding observed at C1 for the trans isomer can be attributed to carbomethoxy γ-effect, together with the hyperconjugative effect, while only hyperconjugative effects were found to explain the shielding of C3. The higher chemical shift value of C3 of the cis isomer was attributed to the carbonyl substituent, which plays an important role by capturing part of the electronic density in C3.     

The active catalytic species and its isomerisation in the catalytic carbonylation of methanol — a density functional study

The Monsanto acetic acid process is one of the most effective ways to produce acetic acid industrially. This process has been studied experimentally but theoretical investigations are so far sparse. In the current work the active catalytic species [Rh(CO)₂I₂]⁻ (1) and its isomerisation has been studied theoretically using the hybrid B3LYP exchange and correlation functional. Similar calculations has been performed for the iridium complex [Ir(CO)₂I₂]⁻ (2) that also is catalytically active in the methanol carbonylation. Experimental work has confirmed the existence of the cis forms of the active catalytic species, but they do not rule out the possibility of the trans isomers. Our gas phase results show that cis-1 has 4.95 kcal/mol lower free energy than trans-1, and cis-2 has 10.39 kcal/mol lower free energy than trans-2. In the case of rhodium, trans-1 can take part to the catalytic cycle but in case of iridium this is not very likely. We have also investigated the possible mechanisms of the cis to trans conversions. The ligand association mechanism gave free energy barrier of 13.7 kcal/mol for the rhodium complex and 19.8 kcal/mol for iridium. Thus the conversion for the rhodium complex is feasible whereas for iridium it is unlikely.     

Zur Komplexierung von enantiomerenreinen cis- und trans-3-Diphenylphosphino-4-hydroxy-tetrahydrofuranen mit [Rh(COD)2]BF4

The complexation behaviour of cis- und trans-3-diphenylphosphino-4-hydroxyl-tetrahydrofurans with [Rh(COD)₂]BF₄ is studied with the help of NMR and IR spectroscopy. In dependence of the spatial arrangement of hydroxyl and phosphino group the formation of different intra- and intermolecular bridged O-P complexes has been observed.

Zusammenfassung

Das Komplexierungsverhalten von cis- und trans-3-Diphenylphosphino-4-hydroxy-tetrahydrofuranen mit [Rh(COD)₂]BF₄ wird mit Hilfe von NMR- und IR-Spektroskopie studiert. In Abhängigkeit von der räumlichen Anordnung von Hydroxy- und Phosphinogruppe zueinander wird die Bildung von intra- bzw. intermolekular verbrückten O-P-Komplexen beobachtet.     

Palladium(II) coordination and cyclometallated complexes derived from 3- and 5-aryl-substituted pyrazoles

Palladium(II) coordination complexes of nine 3- or 5-arylpyrazoles (phenyl, 2-bromophenyl, or 3-methoxyphenyl), as well as of 3,5-diphenylpyrazole, are reported. A cis-trans mixture of [PdL₂Cl₂] isomers is found in the case of 3-aryl-1-methylpyrazoles, the cis-isomers being transformed into trans by heating. Only trans isomers are isolated with the other ligands. Cyclopalladation of 3-ary]-1-methylpyrazoles can be performed with palladium(II) acetate, and the resultant μ-acetate bridged dimers can be transformed into μ-chloro bridged dimers or acetylacetonate monomers. The structures of the complexes have been characterized by ¹H- and ¹³C-NMR spectroscopy.     

An unexpected [2+2]-cycloaddition reaction of 4-methyldithieno-[3,4-b:3′,2′-d]pyridinium iodide with dimethyl acetylenedicarboxylate

An unexpected [2+2]-cycloaddition occured in the reaction of 4-methyldithieno-[3,4-6:3′,2′-d]pyridinium iodide (3)with two equivalents of DMAD, giving 4-(trans-1,2-dicarbomethoxy-2- iodovinyl)-5-methyl-6,7-dicarbomethoxy-4,5-dihydrothieno [23-c]quinoline (4) in 54% yield. 4 is formed via 4-methyl-5-(trans-1,2-dicarbomethoxy-2-iodo-4,5-dihydrothieno [3,4-b:3′,2′-d]pyridine (16), followed by [2+2]-cycloaddition. The primary adduct rearranges via a thiepin to an episulfide which eliminates sulfur to give 4.     

Selectivity for the methoxycarbonylation of ethylene versus CO---ethylene copolymerization with catalysts based on C4-bridged bidentate phosphines and phospholes

The C₄-bridged phospholes 11,12-bis(2,3,4,5-tetramethylphospholylmethyl)-9,10-dihydro-9,10-ethano-anthracene (cis, 4a; trans, 4b) and diphosphines 11,12-bis(diphenylphosphinomethyl)-9,10-dihydro-9,10-ethano-anthracene (cis, 5a; trans, 5b) and their corresponding palladium complexes [(P---P)PdCl₂] (6a–d) have been prepared and characterized. Single-crystal X-ray analyses of 6a–d have been undertaken and they reveal that 4a and b and 5a and b coordinate in a bidentate manner forming seven-membered chelate rings with natural bite angles between 98.62 and 100.30°. The palladium-catalyzed carbonylation of ethylene has been studied using 4a and b and 5a and b. Catalyst mixtures generated from 4a and b, palladium acetate and methanesulfonic acid are selective for the copolymerization of ethylene with carbon monoxide, generating low molecular weight polymers. Surprisingly, the activity of catalyst systems based on cis-(4a) is markedly higher than that based on its trans-isomer, 4b. The marked influence of the nature of the four-carbon tether is highlighted by comparative catalyst testing with [{1,4-bis(2,3,4,5-tetramethylphospholyl)butane}Pd(OAc)₂], which rapidly decomposes under the conditions used for copolymerization. In contrast, under identical conditions catalyst mixtures formed from 5a and b show a marked dependence of the selectivity on the stereochemistry of the ethano-anthracene tether, the former generating a low molecular weight copolymer while the latter generate mainly methyl propanoate. Interestingly, polyketone generated from catalysts based on bisphosphole 4a has a markedly higher average molecular weight than that formed using its diphenylphosphino counterpart, 5a.     

Equilibrium constants and kinetics of carbon monoxide insertion in alkyl complexes of ruthenium(II)

The equilibrium constants of the reaction of cis, trans-[Ru(CO)₂(PMe₃)₂(CH₃)I] (Mc) with carbon monoxide to give cis, trans[Ru(CO)₂(PMe₃)₂ (COMe)i] (Ac) and trans, trans[Ru(CO)₂(PMe₃)₂(COMe)I] (At) were measured at various temperatures in toluene. The thermodynamic parameters are compared with those obtained for the isoelectronic complexes of iron, and the trend is discussed. The kinetics of the carbonylation reaction of Mc, as well as those of the inverse decarbonylation reaction of At were measured. The kinetics of the carbonylation of the new complex trans, trans-[Ru(CO)₂(PMe₃)₂(CH₃)I] (Mt) were also investigated. All the results afford further support to the previously proposed CO insertion mechanism occurring via methyl migration. The comparison of these kinetic results with those of isoelectronic complexes of iron indicates that ruthenium is more reactive than iron, which is reflected by its greater aptitude to act as catalyst in many processes.     

Coordination geometry of chromium(VI) species. The first example of cis-bridging chromate ions in helically structured catena(μ-CrO4-O,O′)[Ni(HIm)3H2O]

Nickel(II) chromate complex with imidazole (HIm) was isolated from the [Ni²⁺–HIm–CrO₄²⁻] system in various experimental conditions, i.e. reagent molar ratios and nickel(II) salts. The catena(μ-CrO₄-O,O′)[Ni(HIm)₃H₂O] (1) crystallizes in monoclinic crystal system—space group P2₁/n with cell parameters: a=11.784(2), b=8.899(2), c=13.934(3) (Å), β=95.19(3) (°). The unit cell contains two independent helixes, left- and right-handed, stabilized by intrahelical and interhelical hydrogen bonds (HB) and π–π interactions. The cis coordination of the CrO₄²⁻ anions and the HB systems appeared to be the main determinants of the helical architecture. To the best of our knowledge the cis-chromate coordination was observed for the first time. The cis coordination causes the distortion of the nickel octahedron, which was analysed by 4 K single crystal electronic spectra with D_4h symmetry approximation (gaussian resolution and crystal field parameters). This symmetry was also confirmed with the polarised electronic spectra. The magnetic properties of the complex suggest the occurrence of weak intrachain antiferromagnetic interactions between the magnetic Ni^II center. The computational DFT studies of complex 1 assuming three possible isomers mer[(HIm)₃]–cis[(CrO₄²⁻)₂], mer–trans and fac–cis suggested that the main contribution to the stability of 1 might have interhelical and intrahelical hydrogen bonds.     

Deuterium NMR studies of inositol liquid crystals [1]

Deuterium NMR investigations are presented on members of two new mesogenic series derived from the naturally occurring stereoisomers myo- and scW/o-inositol. Tetraethers of these two series exhibit thermotropic columnar phases in which the columns are apparently formed by stacked hydrogen bonded dimers of these molecules which chemically are vicinal diols. Deuterium NMR measurements were performed on the tetraoctyl homologues 2e (a cis diol) and 3e (a trans diol) of these series. We have investigated mixtures of these diols with small amounts of benzene-d₆ as probe molecules as well as samples of the neat diol compounds deuteriated at their hydroxyl groups. The results obtained show that the mesophases of both compounds are uniaxial and align partially in a magnetic field upon slow cooling from their isotropic liquids. The alignment is with the director parallel to the field direction indicating that the anisotropic magnetic susceptibility of this mesophase is positive. The deuterium quadrupole splitting of the benzene-d₆ probe in both systems is temperature dependent and in the trans diol 3e it even changes sign. This is interpreted in terms of a model in which the benzene-d₆ probe equilibrates rapidly between two (or more) solvation sites with quadrupole splittings of opposite signs The deuterium spectra of the neat deuterium labelled cis diol 2e exhibit two different signals due to the two deuterons which are located at the axial and equatorial hydroxyl groups. This indicates that there is no fast intra- or intermolecular exchange of the hydroxyl hydrogens. The overall quadrupole splittings of the hydroxyl deuterons in this compound are highly reduced compared to their static values and this is interpreted in terms of motional modes involving both reorientation of the hydroxyl deuterons about their C-O axis and overall reorientation of the molecules (or pairs of molecules) around the columnar axes. The corresponding spectra of the neat deuteriated trans diol 3e exhibit a single spectrum indicating that both hydroxyl deuterons in this compound are equivalent, or very nearly so. Within the mesophase region the spectrum undergoes gradual changes due to the increase in the molecular mobility, but the overall motional narrowing is less than in the cis isomer 2e. Apparently due to stronger hydrogen bonding in the trans isomer 3e the precession of the hydroxyl groups is hindered and a fast molecular reorientation is only possible at high temperatures.     

[2 + 2] Dimerization of norbornadiene and its derivatives in the presence of nickel complexes and zinc metal

Norbornadiene undergoes [2 + 2] reaction in THF in the presence of NiX₂ and zinc metal powder to give an exo-trans-exo dimer and an exo-trans-exo-trans-exo trimer. In these products, the norbornadiene molecules are linked to each other by forming cyclobutane rings with all cyclobutane carbons occupying exo positions relative to the bridging carbons on the two norbornadiene fragments. Polymerization of norbornadiene occurs if the catalyst NiX₂ is replaced by Ni(PPh₃)₂Cl₂. 1,4-Dihydro-1,4-epoxynaphthalene, 5,8-dimethoxy-1,4-dihydro-1,4-epoxynaphthalene, 5-methoxy-1,4-dihydro-1,4-epoxynaphathalene and methyl 1,4-dihydro-1,4-iminonaphthalene-9-carboxylate also dimerize to give exo-trans-exo products in excellent yields in toluene in the presence of Ni(PPh₃)₂Cl₂ and Zn powder. For the dimerization products of 5-methoxy-1,4-dihydro-1–4-epoxynaphthalene, cis and trans isomers with respect to the orientation of methoxy groups in about 1 : 1 ratio were oberved. Under similar reaction conditions for the dimerization of norbornadiene, norbornene undergoes reductive dimerization to afford a product which consists of two norbornyl groups. The structure of this product is also exo-trans-exo.