Organoplatinum(IV) compounds : II. Preparation and characterization of trimethylplatinum(IV) compounds with chelating nitrogen donor ligands. The crystal and molecular structure of iodotrimethyl[bis(3,5-dimethyl-1-pyrazolyl)methane]platinum(IV)

Compounds of the types Me₃PtX(NN) (where X = Cl, I, OAc, NO₃; NN = bis(1-pyrazolyl)methane (pz₂CH₂), bis(3,5-dimethyl-1-pyrazolyl)methane ((Me₂pz)₂CH₂), or bis(2-pyridyl)methane (py₂CH₂)), [Me₃Pt(NNN)][PF₆] (where NNN=tris(1-pyrazolyl)methane (pz₃CH), or tris(2-pyridyl)methane (py₃CH)), and [Me₃Pt((Me₂pz)₂CH₂(py)][PF₆] have been prepared and characterized by elemental analyses and ¹H and ¹³C NMR spectroscopy; the structure of Me₆PtI[(Me₂pz)₂CH₂] (1) has also been determined by X-ray crystallography. Crystals of 1 are orthorhombic, space group Pcmn with four molecules in a unit cell of dimensions a 11.936(5), b 14.462(4), c 10.138(5) Å. The structure was solved by the heavy-atom method and refined by full-matrix least-squares calculations to R = 0.022 for 1719 observed data. The molecule has crystallographic mirror symmetry. The Pt atom has octahedral geometry with one methyl group trans to iodine and two methyl groups trans to the N atoms of the bidentate ligand (Pt---I 2.843(1), Pt---N, 2.236(4), Pt---C 2.077(6) (trans to I) and 2.032(5) Å (trans to N)).     

Syntheses, crystal structures and electronic properties of a series of copper(II) complexes with 3,5-halogen-substituted Schiff base ligands and their solutions

We have systematically investigated the structural features, electronic properties, thermally-induced structural phase transitions and absorption spectra depending on the solvent for ten Cu(II) complexes with 3,5-halogen-substituted Schiff base ligands. Structural characterization of two new complexes, bis(N-R-1-phenylethyl- and N-R,S-2-butyl-5-bromosalicydenaminato-κ²N,O)copper(II), reveals that they afford a compressed tetrahedral trans-[CuN₂O₂] coordination geometry with trans-N–Cu–N = 159.4(2)° and trans-O–Cu–O = 151.7(3)° for the 1-phenylethyl complex and trans-N–Cu–N = 157.9(3)° and trans-O–Cu–O = 151.0(3)° for the 2-butyl one. All the complexes exhibit a structural phase transition by heating in the solid state regardless of their structures at room temperature. The absorption spectra of a series of ten complexes exhibit a slight shift of the d–d band at 16 000–20 000 cm⁻¹ and remarkable shift of the π–π* band at 24 000–28 000 cm⁻¹, which suggests that the dipole moment of the solvents presumably affects the conformation of the π-conjugated moieties of the ligands rather than the coordination environment. We have also attempted ‘photochromic solute-induced solvatochromism’ by a system of bis(N-R-1-phenylethyl-3,5-dichlorosalicydenaminato-κ²N,O)copper(II) and photochromic 4-hydroxyazobenzene in chloroform solution. We successfully observed a change of the d–d and π–π* bands of the complex in the absorption spectra caused by cis–trans photoisomerization of 4-hydroxyazobenzene.     

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.     

Synthesis and structure of novel heterocyclic aminotelluranes

A series of novel 6-alkoxy-12H, 14H-[1,2,3]benzoxatellurazino[2,3-b]-[1,2,3]benzoxatellurazines 3 was synthesized from tetraalkoxytelluranes and bis(2-hydroxybenzyl)amines. ¹H, ¹³C and ¹²⁵Te NMR studies suggest that these aminotelluranes, unlike their orthoamidoester analogs, have a rigid trans-fused conformation. A preferential trans-fused conformation was also deduced from ab initio MO calculations of a model oxatellurazinooxatellurazine 7, using the STO-3G and SBK basis sets, and the Gaussian-92 program, which imply that the trans-fused aminotellurane is more stable than the corresponding cis isomer by 10.3 kcal/mol.     

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.     

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.     

Structure of trans-4-(trans-4-n-pentylcyclohexyl)cyclohexylcarbonitrile (CCH5) in the isotropic and nematic phases: a computer simulation study

We have studied the single particle structural properties of the nematogen trans-4-(trans-4-n-pentylcyclohexyl)cyclohexylcarbonitrile (CCH5) by molecular dynamics simulation using realistic atom-atom potentials. On going from the isotropic phase at 390 K to the nematic phase at 350 K, the molecules become significantly longer and thinner, as indicated by the equivalent molecular moment-of-inertia spheroid and the distribution of trans and gauche bonds. This change is only partly accounted for by the lowering of the temperature, there being a significant quenching effect due to the change in the molecular environment. This quenching effect is also apparent in the distribution of molecular shapes seen in molecular width-breadth contour maps. In the nematic phase, at 350K, the distributions of alkyl tail bond orientations with respect to the director show a pronounced odd-even effect, with peaks in the distributions occurring alternately parallel to, and at an angle to, the director.     

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.     

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.     

Conformational study of CH3OP(O)Cl2 and CD3OP(O)Cl2 : Part 2. Gas phase spectra and profile sinulation

The gas phase infrared- and far infrared spectra of the normal and totally deuterated molecules have been recorded and are briefly discussed.

Rigid-rotor asymmetric top profiles for gauche and trans conformers were used to simulate the experimental gas phase infrared absorption profiles of four conformational doublets of CH₃OP(O)Cl₂. From this, a consistent assignment of the more intense component of each doublet to the gauche, and the less intense component to the trans conformer, is deduced.     

Photodynamics of azobenzene in a hindering environment

We have run trajectory surface hopping simulations of the trans → cis photoisomerization of azobenzene, subject to a pulling force. The model mimics two situations: a trans-azobenzene derivative with bulky substituents that may not be easily displaced, and a recent experiment by Gaub’s group [T. Hugel, N.B. Holland, A. Cattani, L. Moroder, M. Seitz, H.E. Gaub, Science 296 (2002) 1103; N.B. Holland, T. Hugel, G. Neuert, A. Cattani-Scholz, C. Renner, D. Oesterhelt, L. Moroder, M. Seitz, H.E. Gaub, Macromolecules 36 (2003) 2015; G. Neuert, T. Hugel, R.R. Netz, H.E. Gaub, Macromolecules 39 (2005) 789], in which a polymer with azobenzene units was stretched in an atomic force microscope. In both cases, the shortening of the azobenzene moiety in going from the trans to the cis form is opposed by a pulling force. Our simulations show that the trans → cis photoconversion is only partially suppressed by considerably large forces (≈500 pN or more). However, the cis isomer reverts to trans in the ground state, with the help of the pulling force and using the vibrational energy that is available in the first 1–2 ps. The lowering of the quantum yields is therefore the combined result of hindering of the excited state process and of the hot ground state back reaction.     

Potential constants and Coriolis coupling constants of perhalogenated ethylenes: Part 3. trans-ClFC=CFCl, trans-ClBrC=CBrCl and trans-FBrC=CBrF

Based on vibrational frequencies a calculation of a general quadratic force field has been carried out for trans-ClFC=CFCl, trans-ClBrC=CBrCl and trans-FBrC=CBrF via an iterative method. The resulting transformation L matrices were used to evaluate the Coriolis coupling constants and these were used to calculate inertial defects. The calculated values of the inertial defects are lower than those of the respective cis-ethylene-type molecules.     

Bis(2-bromoethyl) selenium dibromide as the selenium-introducing reagent: One-pot preparation of 2,5-bis(alkoxymethyl)tetrahydroselenophenes by the cyclization of 1,5-hexadiene

The reaction of bis(2-bromoethyl)selenium dibromide (1a) with 1,5-hexadiene (2) in methanol or ethanol affords 2,5-bis(alkoxymethyl)tetrahydroselenophene-1,1-dibromides (R = CH₃ (3b), R = C₂H₅ (3c)) via 2,5-bis(bromomethyl)tetrahydroselenophene-1,1-dibromide (3a). The reaction of 1a with 2 in 1-propanol, 2-methyl-1-propanol or 1-butanol in the presence of sodium carbonate gave 2,5-bis(alkoxymethyl)tetrahydroselenophene (R = C₃H₇ (4a), R = (CH₃)₂CHCH₂ (4b) and R = C₄H₉ (4c)) via 3a. The ratios of the trans and cis isomers of 3a–3c are 3:2. In addition, the structure of trans-2,5-bis(methoxymethyl)tetrahydroselenophene-1,1-dibromide (trans-3b) was determined by X-ray crystallography.     

Structure and stability of thioureate anions with water, DFT calculations

Structure and stability of thioureate anion with the water clusters CSNH₂NH⁻-(H₂O)_n = 1–7, are studied, using density functional theory. Molecular structures and the stability of the clusters are discussed based on the calculation results of the stable conformers and their relative energies. All the clusters are stable thermodynamically in gas phase with respect to separate monomers. The clusters are stabilized progressively with an increasing number of water molecules, as indicated by the increasing of the binding energies. The binding energies of CSNH₂NH⁻ and a water molecule are 14.34 and 16.36 kcal/mol for cis CSNH₂NH⁻ and trans CSNH₂NH⁻, respectively. As the reaction in aqueous solution progresses, the CS bond distance increases monotonically, indicating that the CS bond of the thioureate anion unit in the clusters is de-stabilized with an increasing number of water molecules.     

The structure of 2,4-dibromomenthone

A three-dimensional X-ray diffraction study has shown that the product obtained by bromination of - or -menthone is 2,4-dibromomenthone [2(a), 6(e)-dibromo-2(e)-isopropyl-5(e)methyl cyclohexanone]. The two Br atoms are trans, and the isopropyl and methyl groups are also trans. There are four molecules in the unit cell, which has symmetry P2₁2₁2₁ and dimensions a = 13·58, b = 13·81 and c = 6·25 Å. One intermolecular Br … Br contact of 3·52 ± 0·01 Å occurs, a distance which is about 0·4 Å shorter than the van der Waals distance.     

Stereochemical studies of optically active 2-hydroxy-5-alkyl-δ-valerolactone derivatives as chiral dopant for ferroelectric liquid crystals

Both (2S, 5R)- and (2R, 5R)-2-hydroxy-5-alkyl-δ-valerolactone derivatives, cis and trans, respectively, show almost the same magnitude of spontaneous polarization (P_s) when added to a non-chiral smectic C mixture. The stereochemistry of these chiral dopants was studied using ¹H NMR. Trans derivatives seem to have a half-chair conformation with the 2, 5-diequatorial substituents and the cis derivatives have rather a flat conformation in solution. However in the liquid-crystalline phase, the cis and trans derivatives appear to change their conformation or the distribution of their conformations as the alkyl chain length is varied. The difference in the effect as a chiral dopant depends upon lateral interactions between chiral molecules through the solvent liquid crystal phase.     

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.     

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.     

Square planar diphosphinoazine rhodium(I) amido carbonyl complexes with an unsymmetrical PNP′ pincer-type coordination

A series of novel diphosphinoazine rhodium amido carbonyl complexes [{R₂PCHC(Bu^t)–NNC(Bu^t)CH₂PR₂}Rh(CO)] (R = Ph, Prⁱ, c-C₆H₁₁, Bu^t) was prepared by deprotonation of cationic diphosphinoazine rhodium amino carbonyl complexes. The complexes were characterized by NMR as were also their precursors. The crystal structures of two cationic and one neutral deprotonated complex were determined by X-ray diffraction showing the complexes to be essentially planar with mutual trans arrangement of phosphine groups and nitrogens trans to carbonyl ligands. Measurement of valence vibration frequencies of carbonyl groups in the complexes allowed to estimate the electron density on the rhodium centre. The ene-hydrazone ligand backbone (nitrogen covalently bonded) is more electron donating than the azine backbone (nitrogen bonded by electron pair donation) as expected. In the neutral series of complexes electron donation increases with phosphine substitution in the order Ph < Prⁱ = c-C₆H₁₁ < Bu^t with the corresponding decrease of carbonyl valence vibration frequency. The tert-butyl cationic complex undergoes in a low yield an unusual diphosphinoazine bond cleavage with simultaneous oxidation of the metal resulting in a binuclear bis(iminophosphine)dirhodium complex [{(Bu^t)₂PCH₂C(Bu^t)NH}Rh(Cl)₂(μ-Cl)]₂ the structure of which was also determined by X-ray diffraction.     

X-ray study of the hetero ring flexibility in trans-5,6- trimethylene- and tetramethylene-5,6-dihydro 2-phenyl-[4H]-1,3-thiazines

The structures to two 1,3-thiazine derivatives differing only in the number of CH₂ groups in their trans fused hydrocarbon ring (_n = 3 for I and n = 4 for II) have been established by X-ray crystallography from diffractometer data. Crystals of I (trans-5,6- trimethylene-5,6-dihydro-2-phenyl-[4H] - 1,3-thiazine) are triclinic, space group P with a = 7.661(1), b = 8.282(1), c = 9.566(2) Å, = 91.75(1), β = 100.72(1), γ = 105.45(1)° Z = 2, D_c = 1.260 g cm^-3. Crystals of II (trans-5,6-tetramethylene-5,6-dihydro-2-phenyl [4H]-1,3-thiazine) are monoclinic, space group P2₁/c with a = 7.914(2), b = 19.362(13), c = 8.440(1) Å, β = 109.16(2)°C Z = 4, D_c = 1.258 g cm^-3. The structures determined by Patterson (I) and direct (II) methods were refined to R = 0.050 for 1330 reflections of I and R = 0.082 for 1012 reflections of II. The proper treatment of the positional disorder of the carbon atoms (C(5) and C(6)) forming the trans ring junction in I discovered two discrete conformations with a ratio of 1:2. The opposite chirality of atoms C(51) and C(52), and C(61) and C(62), indicates a simultaneous configurational disorder with a pattern of total disorder: A A . The puckering parameters of the hetero rings in the same enantiomers of molecules IA, IB and II indicate a connection between the conformers: ⁵E(II)→⁵H₆(IB)→E₆IA) via pseudorotation. Their relationship is discussed and compared with the conformational freedom of the analogous 1,3-oxazine derivatives.