Mn(III) O

Polyurethanes are synthesized on industrial scale by the reaction of diisocyanates with diols in the presence of catalysts which are commonly based on tin complexes and amines. However, due to the toxicity and volatility of these tin catalysts and amines, there is the need to develop new catalysts that are more environmentally benign. Herein, we report the synthesis of O^N^O pincer-ligated Mn(III) and Fe(III) complexes that serve as suitable catalysts for urethane formation and are stable to hydrolysis as predicted by computations and observed experimentally. The O^N^O pincer scaffold is vital to the activity of these catalysts, simultaneously ensuring increased solubility in the reaction medium as well as providing a stable framework upon dissociation of co-ligands in the catalytic cycle. In silico mechanistic investigations for urethane formation show that the stabilization of active species in square-planar geometries enabled by these O^N^O ligands permit the simultaneous coordination of alcohol and isocyanate in suitable configuration at the metal center.

Chiral-at-Metal: Iridium(III) Tetrazole Complexes With Proton-Responsive P-OH Groups for CO2 Hydrogenation.

A rise in atmospheric CO2 levels, following years of burning fossil fuels, has brought about increase in global temperatures and climate change due to the greenhouse effect. As such, recent efforts in addressing this problem have been directed to the use of CO2 as a non-expensive and non-toxic single carbon, C1, source for making chemical products. Herein, we report on the use of tetrazolyl complexes as catalyst precursors for hydrogenation of CO2. Specifically, tetrazolyl compounds bearing P-S bonds have been synthesized with the view of using these as P∧N bidentate tetrazolyl ligands (1-3) that can coordinate to iridium(III), thereby forming heteroatomic five-member complexes. Interestingly, reacting the P,N'-bidentate tetrazolyl ligands with [Ir(C5 Me 5)Cl 2]2 led to serendipitous isolation of chiral-at-metal iridium(III) half-sandwich complexes (7-9) instead. Complexes 7-9 were obtained via prior formation of non-chiral iridium(III) half-sandwich complexes (4-6). The complexes undergo prior P-S bond heterolysis of the precursor ligands, which then ultimately results in new half-sandwich iridium(III) complexes featuring monodentate phosphine co-ligands with proton-responsive P-OH groups. Conditions necessary to significantly affect the rate of P-S bond heterolysis in the precursor ligand and the subsequent coordination to iridium have been reported. The complexes served as catalyst precursors and exhibited activity in CO2 and bicarbonate hydrogenation in excellent catalytic activity, at low catalyst loadings (1 µmol or 0.07 mol% with respect to base), producing concentrated formate solutions (ca 180 mM) exclusively. Catalyst precursors with proton-responsive P-OH groups were found to influence catalytic activity when present as racemates, while ease of dissociation of the ligand from the iridium center was observed to influence activity in spite of the presence of electron-donating ligands. A test for homogeneity indicated that hydrogenation of CO2 proceeded by homogeneous means. Subsequently, the mechanism of the reaction by the iridium(III) catalyst precursors was studied using 1H NMR techniques. This revealed that a chiral-at-metal iridium hydride species generated in situ served as the active catalyst.

Synthesis, characterization and evaluation of bulky bis(pyrazolyl)palladium complexes in Suzuki-Miyaura cross-coupling reactions.

Pyrazole-containing compounds have been used in recent times as ligands to stabilize metal complexes used as pre-catalysts in cross-coupling reactions. With various substituents at various positions in the pyrazole ring, the overall electrophilic and steric properties of the metal complexes can be fine-tuned. Herein, we report the synthesis of bulky pyrazole-based ligands by condensation of methyl 4-(bromomethyl)benzoate or benzyl bromide with various substituted pyrazole compounds. These ligands were utilised in the synthesis of bis(pyrazolyl)palladium(ii) complexes. The complexes' catalytic activity in Suzuki-Miyaura cross-coupling reactions was evaluated. Phenyl bearing pre-catalyst 7, at a catalyst loading of 0.33 mol%, successfully converted 98% of bromobenzene and phenylboronic acid to biphenyl in 4 h at 140 °C, while the tertiary butyl bearing pre-catalyst 8 converted up to 81% of the same substrates to biphenyl. An increase in conversion was seen for all pre-catalysts when an electron-withdrawing substituent was present on the aryl halide substrate, and the opposite was observed when the electron-withdrawing group was present on the phenyl boronic acid.

Antimicrobial and antioxidant properties of the crude peptide extracts of Galatea paradoxa and Patella rustica.

This study evaluated the antimicrobial and antioxidant activities of crude peptide extracted from Galatea paradoxa (G. paradoxa) and Patella rustica (P. rustica). The extracts were tested against eight strains of bacteria (Escherichia coli, Staphylococcus aureus, Bacillus subtilis, Salmonella typhi, Enterococcus feacalis, Klebseilla pneumoniae, Streptococcus pneumoniae, Pseudomonas aeruginosa) and one strain of fungi (Candida albicans) using agar well diffusion and broth dilution assays. The extracts from G. paradoxa demonstrated a high degree of activity against the bacteria strains but were inactive towards the fungus. P. rustica, however, showed a markedly higher antifungal activity but little antibacterial effect. The minimum inhibitory concentrations (MIC) of the extracts determined by the broth tube dilution assay were 17 mg/mL of G. paradoxa against the entire spectrum of microorganisms tested except for C. albicans which was 20 mg/mL. The MIC of the extracts of P. rustica was 13 mg/mL against all the strains of microorganisms tested except for E. feacalis (17 mg/mL), K. pneumoniae (17 mg/mL) and C. albicans (13 mg/mL). Antioxidant activity using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay showed scavenging ability on the DPPH radical was 56.77 % at 0.39 mg/mL for G. paradoxa and 79.77 % at 0.39 mg/mL for P. rustica. The study indicates that the crude peptide extracts from the two molluscs have promising antimicrobial and antioxidant activities that can be harnessed as leads for potential bioactive compounds.