Solid- and solution-state studies of the novel mu-dicyanamide-bridged dinuclear spin-crossover system {[(Fe(bztpen)]2[mu-N(CN)2]}(PF6)3 x n H2O

The mononuclear diamagnetic compound {Fe(bztpen)[N(CN)2]}(PF6)CH3OH (1) (bztpen = N-benzyl-N,N',N'-tris(2-pyridylmethyl)ethylenediamine) has been synthesized and its crystal structure studied. Complex 1 can be considered to be the formal precursor of two new dinuclear, dicyanamide-bridged iron(II) complexes with the generic formula {[(Fe(bztpen)]2[mu-N(CN)2]}(PF6)3 x n H2O (n = 1 (2) or 0 (3)), which have been characterized in the solid state and in solution. In all three complexes, the iron atoms have a distorted [FeN6] octahedral coordination defined by a bztpen ligand and a terminal (1) or a bridging dicyanamide ligand (2 and 3). In the solid state, 2 and 3 can be considered to be molecular isomers that differ by the relative position of the phenyl ring of the two {Fe(bztpen)[N(CN)2]}+ halves (cis and trans, respectively). Depending on the texture of the sample, 2 exhibits paramagnetic behavior or displays a very incomplete spin transition at atmospheric pressure. Complex 3 undergoes a gradual two-step spin transition with no observed hysteresis in the solid state. Both steps are approximately 100 K wide, centered at approximately 200 K and approximately 350 K, with a plateau of approximately 80 K separating the transitions. The crystal structure of 3 has been determined in steps of approximately 50 K between 400 K and 90 K, which provides a fascinating insight into the structural behavior of the complex and the nature of the spin transition. Order-disorder transitions occur in the dicyanamide bridge and the PF6(-) ions simultaneously, with the spin-crossover behavior suggesting that these transitions may trigger the two-step character. In solution, 2 and 3 display very similar continuous spin conversions. Electrochemical studies of 2 and 3 show that the voltammograms are typical of dimeric systems with electronic coupling of the metals through the dicyanamide ligand.     

Formation and characterization of self-organized TiO2 nanotube arrays by pulse anodization

This paper describes TiO2 nanotube arrays prepared by anodic oxidation of Ti substrates using pulse voltage waveforms. Voltages were pulsed between 20 and -4 V or between 20 and 0 V with varying durations from 2 to 16 s at the lower limit of the pulse waveform. Ammonium fluoride or sodium fluoride (and mixtures of both) was used as the electrolyte with or without added medium modifier (glycerol, ethylene glycol, or poly (ethylene glycol) (PEG 400)) in these experiments. The pulse waveform was optimized to electrochemically grow TiO2 nanotubes and chemically etch their walls during its cathodic current flow regime. The resultant TiO2 nanotube arrays showed a higher quality of nanotube array morphology and photoresponse than samples grown via the conventional continuous anodization method. Films grown with a 20 V/-4 V pulse sequence and pulse duration of 2 s at its negative voltage limit afforded a superior photoresponse compared to other pulse durations. Specifically, the negative voltage limit of the pulse (-4 V) and its duration promote the adsorption of NH4+ species that in turn inhibits chemical attack of the growing oxide nanoarchitecture by the electrolyte F- species. The longer the period of the pulse at the negative voltage limit, the thicker the nanotube walls and the shorter the nanotube length. At variance, with 0 V as the low voltage limit, the longer the pulse duration, the thinner the oxide nanotube wall, suggesting that chemical attack by fluoride ions is not counterbalanced by NH3/NH4+ species adsorption, unlike the interfacial situation prevailing at -4 V. Finally, the results from this study provide useful evidence in support of existing mechanistic models for anodic growth and self-assembly of oxide nanotube arrays on the parent metal surface.     

Nanomicrointerface to read molecular potentials into current-voltage based electronics

Molecular potentials are unreadable and unaddressable by any present technology. It is known that the proper assembly of molecules can implement an entire numerical processing system based on digital or even analogical computation. In turn, the outputs of this molecular processing unit need to be amplified in order to be useful. We have developed a nanomicrointerface to read information encoded in molecular level potentials and to amplify this signal to microelectronic levels. The amplification is performed by making the output molecular potential slightly twist the torsional angle between two rings of a pyridazine, 3,6-bis(phenylethynyl) (aza-OPE) molecule, requiring only fractions of kcal/mol energies. In addition, even if the signal from the molecular potentials is not enough to turn the ring or even if the angles are the same for different combinations of outputs, still the current output yields results that resemble the device as a field effect transistor, providing the possibility to reduce channel lengths to the range of just 1 or 2 nm. The slight change in the torsional angle yields readable changes in the current through the aza-OPE biased by an external applied voltage. Using ab initio methods, we computationally demonstrate the amplification of molecular potential signals into currents that can be read by standard circuits.     

The role of monomers and dimers in the reduction of ruthenium(II) complexes of redox-active tetraazatetrapyridopentacene ligand

A combination of electrochemistry, spectroelectrochemistry, and 1H NMR has been used to study the reduction and solution speciation in acetonitrile of two mononuclear Ru complexes containing the redox-active 9,11,20,22-tetraazatetrapyrido [3,2-a:2',3'-c:3' ',2' '-l:2' ',3' '-n]pentacene (tatpp) ligand. These complexes, [(bpy)2Ru(tatpp)][PF6]2 (1[PF6]2), and [(phen)2Ru(tatpp)][PF6]2 (2[PF6]2) (where bpy is 2,2'-bipyridine and phen is 1,10-phenanthroline), form pi-pi stacked dimers (e.g., pi-{1}24+ and pi-{2}24+) in solution as determined by 1H NMR studies in an extended concentration range (90 - 5000 microM) as well as via simulation of the electrochemical data. The dimerization constant for 12+ in acetonitrile is 2 x 10(4) M(-1) as determined from the NMR data. Slightly higher dimerization constants (8 x 10(4) M(-1)) were obtained via simulation of the electrochemical data and are attributed to the presence of the supporting eletrolyte. Electrochemical and spectroelectrochemical data show that the pi-pi stacked dimers are electroreduced in two consecutive steps at -0.31 and -0.47 V vs Ag/AgCl, which is assigned to the uptake of one electron by each tatpp ligand in pi-{1}24+ to give first pi-{1}23+and then pi-{1}22+. At potentials negative of -0.6 V, the electrochemical data reveal two different reaction pathways depending on the complex concentration in solution. At low concentrations (< or =20 microM), the next electroreduction occurs on a monomeric species (e.g., [(bpy)2Ru(tatpp)]+/0) showing that the doubly reduced pi-pi dimer (pi-{1}22+ and pi-{2}22+) dissociates into monomers. At high concentrations (> or =100 microM), reduction of pi-{1}22+ or pi-{2}22+ induces another dimerization reaction, which we attribute to the formation of a sigma-bond between the radical tatpp ligands and is accompanied by the appearance of a new peak in the absorption spectrum at 535 nm. This new sigma-dimer can undergo one additional tatpp based reduction to form sigma-{1}20 or sigma-{2}20, in which the tatpp-bridged assembly is the site of all four reductions. Finally, potentials negative of -1.2 V result in the electroreduction of the bpy or phen ligands for complexes 12+ or 22+, respectively. For the latter complex 22+, this process is accompanied by the formation of an electrode adsorbed species.     

Synthesis,characterisation and chromatographic evaluation of polyether dendrimer stationary phases

Supercritical carbon dioxide has attracted attention as a potential replacement for traditional organic solvents due to its simplified workup procedures and reduced environmental impact—providing a green chemistry approach for organic solvent-free functionalisation. In addition to the environmental benefits, the enhanced diffusivity observed in supercritical solvents can often enhance reaction rates. We have applied these valuable features to the preparation of silica-bonded stationary phases and examined their potential in liquid chromatography. We report the successful preparation and characterisation of polyether silica based on Frechet dendrimers—this significantly enhances the range of stationary-phase chemistries that can be prepared in supercritical fluids. First- and second-generation polyether silicas were prepared, characterised, end-capped and evaluated for use as stationary phases for liquid chromatography. Figure SRM1649 on 2nd generation polyether silica Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.

Crystal Structure and Synthesis of Three New Steroidal Derivatives as Antiandrogens

Abstract  

The structures of 3β-cyclobutyl carbonyloxy-5-androsten-17-one (C₂₄H₃₄O₃), compound 1; 3β-cyclopentyl carbonyloxy-5-androsten-17-one (C₂₅H₃₆O₃), compound 2; and 3β-cyclohexyl carbonyloxy-5-androsten-17-one (C₂₆H₃₈O₃) compound 3 were established by spectral and X-ray diffraction studies. Steroidal derivatives 1–3 exhibited a high antiandrogenic effect and could be considered as potential drugs for the treatment of prostate cancer. Compound 1 crystallizes in the orthorhombic space group P2₁2₁2₁ with unit cell parameters a = 6.618(2), β = 14.167(3), c = 22.329(5) ?, Z = 4. Compound 2 crystallizes in the orthorhombic space group P2₁2₁2₁ with unit cell parameters a = 6.631(2), b = 13.865(4), c = 23.952(7) ?, Z = 4. Compound 3 crystallizes in the orthorhombic space group P2₁2₁2₁ with unit cell parameters a = 6.6100(9), b = 13.896(2), c = 24.491(3) ?, Z = 4. All three structures (1–3) were solved by direct methods and refined to R = 0.0708, 0.0750 and 0.0496, respectively. For compound 2, there is positional disorder of the side chain at C3. All the rings of both steroid skeletons are trans connected. For structures 1–3, the six-membered rings A, B and C have a deformed chair, half chair and deformed chair conformations. The five-membered rings D adopt an intermediate envelope and half-chair conformations. For structure 2, the five-membered rings E and EA have a deformed envelope and an intermediate envelope and half-chair conformations, respectively. For structure 3, the six-membered ring E adopts a deformed chair conformation. Cohesion of the crystals can be attributed to van der Waals and weak C–H⋯O interactions.     

Gas-phase oxidation of methyl crotonate and ethyl crotonate. kinetic study of their reactions toward OH radicals and Cl atoms

Rate coefficients for the reactions of hydroxyl radicals and chlorine atoms with methyl crotonate and ethyl crotonate have been determined at 298 K and atmospheric pressure. The decay of the organics was monitored using gas chromatography with flame ionization detection (GC-FID), and the rate constants were determined using the relative rate method with different reference compounds. Room temperature rate coeficcients were found to be (in cm(3) molecule(-1) s(-1)): k(1)(OH + CH(3)CH═CHC(O)OCH(3)) = (4.65 ± 0.65) × 10(-11), k(2)(Cl + CH(3)CH═CHC(O)OCH(3)) = (2.20 ± 0.55) × 10(-10), k(3)(OH + CH(3)CH═CHC(O)OCH(2)CH(3)) = (4.96 ± 0.61) × 10(-11), and k(4)(Cl + CH(3)CH═CHC(O)OCH(2)CH(3)) = (2.52 ± 0.62) × 10(-10) with uncertainties representing ±2σ. This is the first determination of k(1), k(3), and k(4) under atmospheric pressure. The rate coefficients are compared with previous determinations for other unsaturated and oxygenated VOCs and reactivity trends are presented. In addition, a comparison between the experimentally determined k(OH) with k(OH) predicted from k vs E(HOMO) relationships is presented. On the other hand, product identification under atmospheric conditions has been performed for the first time for these unsaturated esters by the GC-MS technique in NO(x)-free conditions. 2-Hydroxypropanal, acetaldehyde, formaldehyde, and formic acid were positively observed as degradation products in agreement with the addition of OH to C2 and C3 of the double bond, followed by decomposition of the 2,3- or 3,2-hydroxyalkoxy radicals formed. Atmospheric lifetimes, based on of the homogeneous sinks of the unsaturated esters studied, are estimated from the kinetic data obtained in the present work.     

Crystallization of zeolites from organo-silicic colloids

As shown recently, the networks of mesoporous high-surface-area silicates and zeolites undergo a deep depolymerization process in glycerol, near 200 degrees C. Within 1 h, X-ray diffraction analysis amorphous gels are obtained. However, some local ordering subsists as demonstrated by a striking similarity between the silicon and aluminum high-resolution solid-state NMR spectra before and after the reaction. The residual organization could be investigated indirectly in studying the recrystallization of these gels in the presence or absence of structure-directing agents. Were this attempt successful, the way should be opened for the synthesis of molecular sieves starting from gels obtained from naturally occurring zeolites. Here, it will be shown that an amorphous gel obtained from HZSM-5 recovers the initial long-range structure of the parent material in a few hours at 85 degrees C in the presence of an aqueous solution of tetrapropyl ammonium (TPA) or NH3. The recrystallization of HY requires the presence of tetramethylammonium, but about 25% of the crystallization is obtained rapidly (approximately = 1 day) at 80 degrees C with ammonia. Hypotheses about the preorganized structural units are presented. The value of the Si-O-Si angle in the silica cluster seems to be of paramount importance.