Active Sites of Cobalt Phthalocyanine in Electrocatalytic CO2 Reduction to Methanol

Many metal coordination compounds catalyze CO₂ electroreduction to CO, but cobalt phthalocyanine hybridized with conductive carbon such as carbon nanotubes is currently the only one that can generate methanol. The underlying structure–reactivity correlation and reaction mechanism desperately demand elucidation. Here we report the first in situ X-ray absorption spectroscopy characterization, combined with ex situ spectroscopic and electrocatalytic measurements, to study CoPc-catalyzed CO₂ reduction to methanol. Molecular dispersion of CoPc on CNT surfaces, as evidenced by the observed electronic interaction between the two, is crucial to fast electron transfer to the active sites and multi-electron CO₂ reduction. CO, the key intermediate in the CO₂-to-methanol pathway, is found to be labile on the active site, which necessitates a high local concentration in the microenvironment to compete with CO₂ for active sites and promote methanol production. A comparison of the electrocatalytic performance of structurally related porphyrins indicates that the bridging aza-N atoms of the Pc macrocycle are critical components of the CoPc active site that produces methanol. In situ X-ray absorption spectroscopy identifies the active site as Co(I) and supports an increasingly non-centrosymmetric Co coordination environment at negative applied potential, likely due to the formation of a Co−CO adduct during the catalysis.     

Synthesis of the quorum sensing molecule Diffusible Signal Factor using the alkyne zipper reaction

The development of a concise synthesis of the quorum sensing molecule Diffusible Signal Factor is described. This route exploits an alkyne zipper reaction to form a key terminal alkyne intermediate. The chemistry outlined here may also be applied to the preparation of cis-unsaturated analogues of Diffusible Signal Factor.     

Nonobtuse triangulation of polygons

We show how to triangulate a polygon without using any obtuse triangles. Such triangulations can be used to discretize partial differential equations in a way that guarantees that the resulting matrix is Stieltjes, a desirable property both for computation and for theoretical analysis.A simple divide-and-conquer approach would fail because adjacent subproblems cannot be solved independently, but this can be overcome by careful subdivision. Overlay a square grid on the polygon, preferably with the polygon vertices at grid points. Choose boundary cells so they can be triangulated without propagating irregular points to adjacent cells. The remaining interior is rectangular and easily triangulated. Small angles can also be avoided in these constructions.     

Phosphinoborylenes as stable sources of fleeting borylenes

Base-stabilised borylenes that mimic the ability of transition metals to bind and activate inert substrates have attracted significant attention in recent years. However, such species are typically highly reactive and fleeting, and often cannot be isolated at ambient temperature. Herein, we describe a readily accessible trimethylphosphine-stabilised borylborylene which was found to possess a labile P–B bond that reversibly cleaves upon gentle heating. Exchange of the labile phosphine with other nucleophiles (CO, isocyanide, 4-dimethylaminopyridine) was investigated, and the binding strength of a range of potential borylene “ligands” has been evaluated computationally. The room-temperature-stable PMe₃-bound borylenes were subsequently applied to novel bond activations including [2 + 2] cycloaddition with carbodiimides and the reduction of dichalcogenides, revealing that PMe₃-stabilised borylenes can effectively behave as stable sources of the analogous fleeting dicoordinate species under mild conditions.

A room-temperature stable phosphinoborylene provides a source of a reactive two-coordinate borylene via dissociation of a labile phosphine upon gentle heating. Ligand exchange, the capture of unsaturated molecules, and oxidation have been explored.     

Combination of nitrogen dioxide radicals with 8-oxo-7,8-dihydroguanine and guanine radicals in DNA: oxidation and nitration end-products

The oxidation and nitration reactions in DNA associated with the combination of nitrogen dioxide radicals with 8-oxo-7,8-dihydroguanine (8-oxoGua) and guanine radicals were explored by kinetic laser spectroscopy and mass spectrometry methods. The oxidation/nitration processes were triggered by photoexcitation of 2-aminopurine (2AP) residues site-specifically positioned in the 2'-deoxyribooligonucleotide 5'-d(CC[2AP]TC[X]CTACC) sequences (X = 8-oxoGua or G), by intense 308 nm excimer laser pulses. The photoionization products, 2AP radicals, rapidly oxidize either 8-oxoGua or G residues positioned within the same oligonucleotide but separated by a TC dinucleotide step on the 3'-side of 2AP. The two-photon ionization of the 2AP residue also generates hydrated electrons that are trapped by nitrate anions thus forming nitrogen dioxide radicals. The combination of nitrogen dioxide radicals with the 8-oxoGua and G radicals occurs with similar rate constants (approximately 4.3 x 10(8) M(-1) s(-1)) in both single- and double-stranded DNA. In the case of 8-oxoGua, the major end-products of this bimolecular radical-radical addition are spiroiminodihydantoin lesions, the products of 8-oxoGua oxidation. Oxygen-18 isotope labeling experiments reveal that the O-atom in the spiroiminodihydantoin lesion originates from water molecules, not from nitrogen dioxide radicals. In contrast, combination of nitrogen dioxide and guanine neutral radicals generated under the same conditions results in the formation of the nitro products, 5-guanidino-4-nitroimidazole and 8-nitroguanine adducts. The mechanistic aspects of the oxidation/nitration processes and their biological implications are discussed.     

A synthetic glycan array containing Cryptococcus neoformans glucuronoxylomannan capsular polysaccharide fragments allows the mapping of protective epitopes

A convergent synthetic strategy to Cryptococcus neoformans glucuronoxylomannan (GXM) capsular polysaccharide part structures was developed based on di-, tri-, tetra-, penta- and hexasaccharide thioglycoside building blocks. The approach permitted the synthesis of a library of spacer-containing serotype A and D related GXM oligosaccharide structures, ranging from di- to octadecasaccharides. Ten deprotected GXM compounds (mono- to decasaccharide) were printed onto microarray plates and screened with seventeen mouse monoclonal antibodies (mAbs) to GXM. For the first time a GXM oligosaccharide structure (a serotype A decasaccharide), capable of being recognized by neutralizing forms of these GXM-specific mAbs, has been identified, offering insight into the binding epitopes of a range of protective monoclonal antibodies and furthering our efforts to develop semi-synthetic conjugate vaccine candidates against C. neoformans.

A library of Cryptococcus neoformans glucuronoxylomannan (GXM) oligosaccharides was synthesized using a thioglycoside building block strategy. The first GXM microarray was printed, allowing mapping of epitopes of antibodies directed towards GXM.     

Development of a micro-fluidic manifold for copper monitoring utilising chemiluminescence detection

The progressive development of a micro-fluidic manifold for the chemiluminescent detection of copper in water samples, based on the measurement of light emitted from the Cu(ii) catalysed oxidation of 1,10-phenanthroline by hydrogen peroxide, is reported. Micro-fluidic manifolds were designed and manufactured from polymethylmethacrylate (PMMA) using three micro-fabrication techniques, namely hot embossing, laser ablation and direct micro-milling. The final laser ablated design incorporated a reagent mixing channel of dimensions 7.3 cm in length and 250 x 250 microm in width and depth (triangular cross section), and a detection channel of 2.1 cm in length and 250 x 250 microm in width and depth (total approx. volume of between 16 to 22 microL). Optimised reagents conditions were found to be 0.07 mM 1,10-phenanthroline, containing 0.10 M cetyltrimethylammonium bromide and 0.075 M sodium hydroxide (reagent 1 delivered at 0.025 mL min(-1)) and 5% hydrogen peroxide (reagent 2 delivered at 0.025 mL min(-1)). The sample stream was mixed with reagent 1 in the mixing channel and subsequently mixed with reagent 2 at the start of the detection channel. The laser ablated manifold was found to give a linear response (R(2) = 0.998) over the concentration ranges 0-150 microg L(-1) and be reproducible (% RSD = 3.4 for five repeat injections of a 75 microg L(-1) std). Detection limits for Cu(ii) were found to be 20 microg L(-1). Selectivity was investigated using a copper selective mini-chelating column, which showed common cations found in drinking waters did not cause interference with the detection of Cu(ii). Finally the optimised system was successfully used for trace Cu(ii) determinations in a standard reference freshwater sample (SRM 1640).     

Mercury binding by ferrocenoyl peptides with sulfur-containing side chains: Electrochemical, spectroscopic and structural studies

Ferrocenoyl peptides incorporating amino acids derived from either l-methionine, l-cysteine or dl-homocysteine have been synthesised and investigated as agents for heavy metal binding and detection. Heavy metal-peptide interactions have been characterised using cyclic voltammetry to follow changes in the potential of the Fe(II)/Fe(III) redox couple, revealing that these systems interact with mercury(II) ions more strongly than with other thiophilic heavy metals such as cadmium(II), silver(I) and lead(II). Proton NMR experiments have demonstrated 1:1 peptide:mercury binding and enabled quantitative characterisation of this binding interaction. Crystal structures for two of these ferrocenoyl peptide derivatives have been elucidated, revealing that these compounds adopt a P-1,3′ open solid state conformation in the absence of mercury; this arrangement precludes intramolecular hydrogen bonding between chains, while extensive intermolecular hydrogen bonding is evident. The particular affinity of these systems for mercury(II) opens the possibility of incorporating them in new, biologically inspired sensors for detecting this toxic pollutant.     

Reduction of dipyrido-[3,2-a:2',3'-c]-phenazine (dppz) by photolysis in ethanol solution

Photolysis of dipyrido-[3,2-a:2',3'-c]-phenazine (dppz) (1) in ethanol solution leads to the formation of 9,14-dihydrodipyridophenazine (2), which has been characterised by detailed NMR analysis, UV/VIS absorption spectroscopy, and theoretical calculations which reveal that its red colour is due to a low-lying intramolecular charge transfer state.     

Pentacarbonyl(4‐phenylpyridine)­tungsten(0) and pentacarbonyl(2‐phenylpyridine)chromium(0)

In penta­carbonyl(4‐phenyl­pyridine)­tungsten(0), [W­(C₁₁H₉N)(CO)₅], the mol­ecules have mm site symmetry and the pyridine ligand, with m symmetry, is completely planar. In penta­carbonyl(2‐phenyl­pyridine)­chromium(0), [Cr(C₁₁­H₉N)(CO)₅], the mol­ecules are in general positions and the phenyl and pyridine rings of the ligand are twisted by 67.7 (3)° with respect to one another by rotation about the C—C bond joining them. In both compounds, the axial M—C_carbonyl bond trans to the M—N_ligand bond is significantly shorter than the equatorial M—C_carbonyl bonds.