Oxidative electrochemical switching in dithienylcyclopentenes, part 1: effect of electronic perturbation on the efficiency and direction of molecular switching

The electro- and spectroelectrochemical properties of dithienylhexahydro- and dithienyhexafluorocyclopentenes are reported. The large effect of variation in the central cyclopentene moieties on the redox properties of the dithienylcyclopentenes is in striking contrast to the minor effect on their photochemical properties. The electronic properties of the oxidised compounds in the +1 and +2 oxidation state are reported, and the possibility of electrochemical cyclisation and cycloreversion were explored by UV/Vis spectroelectrochemistry. The efficiency of electrochemical switching is found to be dependent both on the central cyclopentene unit and on the nature of the substituents at C5 of the thienyl rings. For the hexahydrocyclopentene-based compounds oxidative ring closure of the ring-open form is observed, while for the hexafluorocyclopentene-based compounds oxidative ring opening of the ring-closed form is observed. However, the introduction of electroactive groups such as methoxyphenyl allows oxidative ring closure to occur in the hexafluoro compounds. The effect of electrolyte, solvent and temperature on the spectroelectrochemical properties were examined, and the switching process was found to be sensitive to the donor properties of the solvent/electrolyte system employed. In addition, thermally activated reversible isomerisation of the dicationic closed form was observed. The driving force for electrochemical ring opening and closure appears to be dependent on the relative stabilisation of the dicationic ring-open and ring-closed states. This study provides insight into the factors which determine the direction of cyclisation.     

Asymmetric Covalent Triazine Framework for Enhanced Visible‐Light Photoredox Catalysis via Energy Transfer Cascade

Complex multiple‐component semiconductor photocatalysts can be constructed that display enhanced catalytic efficiency via multiple charge and energy transfer, mimicking photosystems in nature. In contrast, the efficiency of single‐component semiconductor photocatalysts is usually limited due to the fast recombination of the photogenerated excitons. Here, we report the design of an asymmetric covalent triazine framework as an efficient organic single‐component semiconductor photocatalyst. Four different molecular donor–acceptor domains are obtained within the network, leading to enhanced photogenerated charge separation via an intramolecular energy transfer cascade. The photocatalytic efficiency of the asymmetric covalent triazine framework is superior to that of its symmetric counterparts; this was demonstrated by the visible‐light‐driven formation of benzophosphole oxides from diphenylphosphine oxide and diphenylacetylene.     

Asymmetric Synthesis of (R)‐1‐Alkyl‐Substituted Tetrahydro‐ß‐carbolines Catalyzed by Strictosidine Synthases

Stereoselective methods for the synthesis of tetrahydro‐ß‐carbolines are of significant interest due to the broad spectrum of biological activity of the target molecules. In the plant kingdom, strictosidine synthases catalyze the C?C coupling through a Pictet–Spengler reaction of tryptamine and secologanin to exclusively form the (S)‐configured tetrahydro‐ß‐carboline (S)‐strictosidine. Investigating the biocatalytic Pictet–Spengler reaction of tryptamine with small‐molecular‐weight aliphatic aldehydes revealed that the strictosidine synthases give unexpectedly access to the (R)‐configured product. Developing an efficient expression method for the enzyme allowed the preparative transformation of various aldehydes, giving the products with up to >98 % ee. With this tool in hand, a chemoenzymatic two‐step synthesis of (R)‐harmicine was achieved, giving (R)‐harmicine in 67 % overall yield in optically pure form.     

Culturing and investigation of stress-induced lipid accumulation in microalgae using a microfluidic device

There is increasing interest in using microalgae as a lipid feedstock for the production of biofuels. Lipids used for these purposes are triacylglycerols that can be converted to fatty acid methyl esters (biodiesel) or decarboxylated to “green diesel.” Lipid accumulation in most microalgal species is dependent on environmental stress and culturing conditions, and these conditions are currently optimized using slow, labor-intensive screening processes. Increasing the screening throughput would help reduce the development cost and time to commercial production. Here, we demonstrated an initial step towards this goal in the development of a glass/poly(dimethylsiloxane) (PDMS) microfluidic device capable of screening microalgal culturing and stress conditions. The device contained power-free valves to isolate microalgae in a microfluidic growth chamber for culturing and stress experiments. Initial experiments involved determining the biocompatibility and culturing capability of the device using the microalga Tetraselmis chuii. With this device, T. chuii could be successfully cultured for up to 3 weeks on-chip. Following these experiments, the device was used to investigate lipid accumulation in the microalga Neochloris oleabundans. It was shown that this microalga could be stressed to accumulate cytosolic lipids in a microfluidic environment, as evidenced with fluorescence lipid staining. This work represents the first example of microalgal culturing in a microfluidic device and signifies an important expansion of microfluidics into the biofuels research arena.     

Influence of the dielectric medium on the carbonyl infrared absorption peak of acetylferrocene

The solvent effect on the position of the carbonyl vibrational stretching of acetylferrocene in aprotic media was studied in this work. The solvent-induced shifts in this organometallic compound were interpreted in terms of the alternative reaction field model(SCRF-MO) proposed by Kolling. In contrast to the established trends for carbonyl groups in organic systems, the results suggest that the continuum models for the reaction field are not adequate and that the influence of dipolarity-polarizability described by an inhomogeneous coupling function theta(epsilon )L(n(2)) that assumes optical dielectric saturation is responsible for the carbonyl band shift and, there is empirical evidence that the effect of field-induced intermolecular interaction on band shift, interpreted in terms of the van der Waals forces from the solvent, have a important contribution to this phenomena.     

Cooperative copper-squaramide catalysis for the enantioselective N–H insertion reaction with sulfoxonium ylides

The first examples of a highly efficient and enantioselective carbene-mediated insertion reaction, from a sulfur ylide, are described. By way of a catalytic asymmetric insertion reaction into N–H bonds from carbonyl sulfoxonium ylides and anilines, using a copper-bifunctional squaramide cooperative catalysis approach, thirty-seven α-arylglycine esters were synthesized in enantiomeric ratios up to 92 : 8 (99 : 1 after a single recrystallization) and reaction yields ranging between 49–96%. Furthermore, the protocol benefits from quick reaction times and is conducted in a straightforward manner.

The first examples of a highly efficient and enantioselective carbene-mediated insertion reaction, from a sulfur ylide, are described.     

Multinuclear Magnetic Resonance Study of Zinc(II) and Cadmium(II) Complexing with Isothiocyanate

A study of zinc(II) and cadmium(II) complexes with isothiocyanate ion has been completed, using a low-temperature, multinuclear magnetic resonance technique that permits the observation of separate resonance signals for bound and free ligand, and Cd(II) metal ion. The Zn²⁺–NCS^– complexes were studied by ¹H, ¹³C, and ¹⁵N NMR spectroscopy. In the ¹H spectra, the intensity of the coordinated water signal, corresponding to a Zn(II) hydration number of six in the absence of NCS^–, decreases dramatically as this anion is added, indicating the complexing process involves more than a simple 1:1 ligand replacement. The ¹³C and ¹⁵N NMR spectra reveal signals for four species, most reasonably assigned to a series of tetrahedrally coordinated Zn²⁺–NCS^– complexes. In the Cd²⁺–NCS^– solution spectra, the ¹³C and ¹⁵N signals for four complexes also are observed and they are three line patterns, corresponding to a doublet from ¹¹³Cd J-coupling, and a dominant central peak, resulting from bonding to magnetically inactive Cd isotopes. The ¹¹³Cd spectra, showing signals for four complexes, correlate well in all respects with the ¹³C and ¹⁵N results, including coupling in specific cases. The spectral results for both metal ions reflect binding at the nitrogen atom of NCS^–, with the complexes changing from an octahedral to a tetrahedral configuration when doing so. Confirming evidence for these conclusions also was provided by several infrared measurements of these metal–ion systems.     

Effect of protonation and deprotonation on surface charge density of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>

The protonation and deprotonation of the Nb₂O₅ surface has been followed in order to understand the reactions of surface of this catalyst. The simultaneous potentiometric and conductometric titrations had been carried by using 50 mL of water suspension of Nb₂O₅ 40 g L⁻¹. The oxide was entirely deprotonated when adding 0.4 mL NaOH 1 mol L⁻¹, and later titrated with 0.1 mol L⁻¹. The titration had supplied K ₁ and K ₂ and the obtained values were 3.24 × 10⁻³ and 4.17 × 10⁻⁸, respectively. The zero point charge was pH_pcz = 4.94. The thermodynamic studies were carried out by using 50 mL of a 40 g/L Nb₂O₅ aqueous suspension with the pH adjusted to pH_PZC value. The suspension was titrated with 0.5 mol/L of HNO₃ or NaOH for protonation or deprotonation studies, respectively, in an isoperibol calorimeter CSC ISC-4300. Thus, the obtained thermodynamic values of the protonation and deprotonation of Nb₂O₅ were Δ_dp G = −37.60 kJ/mol, Δ_dp H = −23.72 kJ/mol and Δ_dpS = 47 J/(mol K).     

Application of manganese (II) phthalocyanine synthesized in situ in the SiO2/SnO2 mixed oxide matrix for determination of dissolved oxygen by electrochemical techniques

This work describes the in situ immobilization of Mn(II) phthalocyanine (MnPc) in a porous SiO₂/SnO₂ mixed oxide matrix obtained by the sol gel processing method. The chemically modified matrix SiO₂/SnO₂/MnPc, possessing an estimated amount of 8 × 10⁻¹⁰ mol cm⁻² of MnPc on the surface, was used to prepare an electrode to analyze dissolved oxygen in water by an electrochemical technique. The electrode was prepared by mixing the material with ultrapure graphite and evaluated using differential pulse voltammetry. Dissolved O₂ was reduced at −0.31 V with a limit of detection (LOD) equal to 7.0 × 10⁻⁴ mmol L⁻¹. A mechanism involving four electrons in O₂ reduction was determined by the rotating disk electrode technique.     

X-ray and neutron powder diffraction studies of Ba(NdxY2−x)CuO5

Ba(R,R′)₂CuO₅ (R,R′=lanthanides and Y) plays an important role as a flux-pinning agent in enhancing the superconducting properties of the Ba₂(R,R′)Cu₃O_6+x (R,R′=lanthanides and Y) coated conductors. Using X-ray diffraction and neutron diffraction, we found that the Ba(Nd_xY_2−x)CuO₅ solid solution adopts two structure types. In the Nd-rich region (1.8?x?2.0), the materials are of brown color (commonly referred to as the ‘brown phase’), and the structure is tetragonal with space group I4/mbm (no. 127). In the Y-rich region (0.0?x?1.4), the materials are green (commonly referred to as the ‘green phase’) and the structure is orthorhombic with space group Pnma (no. 62). A two-phase region (1.4<x<1.8) exists between the orthorhombic and tetragonal phases. The crystal chemistry and crystallography of the orthorhombic ‘green phase’ series, Ba(Nd_xY_2−x)CuO₅ (isostructural to BaY₂CuO₅), are discussed in this paper.