Spectroscopic and Computational Investigation of Low-Spin MnIII Bis(scorpionate) Complexes

Six-coordinate MnIII complexes are typically high-spin (S = 2), however, the scorpionate ligand, both in its traditional, hydridotris(pyrazolyl)borate form, Tp– and Tp*– (the latter with 3,5-dimethylpyrazole substituents) and in an aryltris(carbene)borate (i.e., N-heterocyclic carbene, NHC) form, [Ph(MeIm)3B]–, (MeIm = 3-methylimidazole) lead to formation of bis(scorpionate) complexes of MnIII with spin triplet ground states; three of which were investigated herein: [Tp2Mn]SbF6 (1SbF6), [Tp*2Mn]SbF6 (2SbF6), and [{Ph(MeIm)3B}2Mn]CF3SO3 (3CF3SO3). These trigonally symmetric complexes were studied experimentally by magnetic circular dichroism (MCD) spectroscopy (the propensity of 3 to oxidize to MnIV precluded collection of useful MCD data) including variable temperatures and fields (VTVH-MCD) and computationally by ab initio CASSCF/NEVPT2 methods. These combined experimental and theoretical techniques establish the 3A2g electronic ground state for the three complexes, and provide information on the energy of the “conventional” high-spin excited state (5Eg) and other, triplet excited states. These results show the electronic effect of pyrazole ring substituents in comparing 1 and 2. The tunability of the scorpionate ligand, even by perhaps the simplest change (from pyrazole in 1 to 3,5-dimethylpyrazole in 2) is quantitatively manifested through perturbations in ligand-field excited-state energies that impact ground-state zero-field splittings. The comparison with the NHC donor is much more dramatic. In 3, the stronger σ-donor properties of the NHC lead to a quantitatively different electronic structure, so that the lowest lying spin triplet excited state, 3Eg, is much closer in energy to the ground state than in 1 or 2. The zero-field splitting (zfs) parameters of the three complexes were calculated and in the case of 1 and 2 compare closely to experiment (lower by < 10 %, < 2 cm–1 in absolute terms); for 3 the large magnitude zfs is reproduced, although there is ambiguity about its sign. The comprehensive picture obtained for these bis(scorpionate) MnIII complexes provides quantitative insight into the role played by the scorpionate ligand in stabilizing unusual electronic structures

Structural Differences between the Streptococcus agalactiae Housekeeping and Pilus-Specific Sortases: SrtA and SrtC1

The assembly of pili on the cell wall of Gram-positive bacteria requires transpeptidase enzymes called sortases. In Streptococcus agalactiae, the PI-1 pilus island of strain 2603V/R encodes two pilus-specific sortases (SrtC1 and SrtC2) and three pilins (GBS80, GBS52 and GBS104). Although either pilus-specific sortase is sufficient for the polymerization of the major pilin, GBS80, incorporation of the minor pilins GBS52 and GBS104 into the pilus structure requires SrtC1 and SrtC2, respectively. The S. agalactiae housekeeping sortase, SrtA, whose gene is present at a different location and does not catalyze pilus polymerization, was shown to be involved in cell wall anchoring of pilus polymers. To understand the structural basis of sortases involved in such diverse functions, we determined the crystal structures of S. agalactiae SrtC1 and SrtA. Both enzymes are made of an eight-stranded beta-barrel core with variations in their active site architecture. SrtA exhibits a catalytic triad arrangement similar to that in Streptococcus pyogenes SrtA but different from that in Staphylococcus aureus SrtA. In contrast, the SrtC1 enzyme contains an N-terminal helical domain and a ‘lid’ in its putative active site, which is similar to that seen in Streptococcus pneumoniae pilus-specific sortases, although with subtle differences in positioning and composition. To understand the effect of such differences on substrate recognition, we have also determined the crystal structure of a SrtC1 mutant, in which the conserved DP(W/F/Y) motif was replaced with the sorting signal motif of GBS80, IPNTG. By comparing the structures of WT wild type SrtA and SrtC1 and the ‘lid’ mutant of SrtC1, we propose that structural elements within the active site and the lid may be important for defining the role of specific sortase in pili biogenesis

Global patient outcomes after elective surgery: prospective cohort study in 27 low-, middle- and high-income countries.

BACKGROUND: As global initiatives increase patient access to surgical treatments, there remains a need to understand the adverse effects of surgery and define appropriate levels of perioperative care. METHODS: We designed a prospective international 7-day cohort study of outcomes following elective adult inpatient surgery in 27 countries. The primary outcome was in-hospital complications. Secondary outcomes were death following a complication (failure to rescue) and death in hospital. Process measures were admission to critical care immediately after surgery or to treat a complication and duration of hospital stay. A single definition of critical care was used for all countries. RESULTS: A total of 474 hospitals in 19 high-, 7 middle- and 1 low-income country were included in the primary analysis. Data included 44 814 patients with a median hospital stay of 4 (range 2-7) days. A total of 7508 patients (16.8%) developed one or more postoperative complication and 207 died (0.5%). The overall mortality among patients who developed complications was 2.8%. Mortality following complications ranged from 2.4% for pulmonary embolism to 43.9% for cardiac arrest. A total of 4360 (9.7%) patients were admitted to a critical care unit as routine immediately after surgery, of whom 2198 (50.4%) developed a complication, with 105 (2.4%) deaths. A total of 1233 patients (16.4%) were admitted to a critical care unit to treat complications, with 119 (9.7%) deaths. Despite lower baseline risk, outcomes were similar in low- and middle-income compared with high-income countries. CONCLUSIONS: Poor patient outcomes are common after inpatient surgery. Global initiatives to increase access to surgical treatments should also address the need for safe perioperative care. STUDY REGISTRATION: ISRCTN5181700

Visible-light photoredox catalysis: selective reduction of carbon dioxide to carbon monoxide by a nickel N-heterocyclic carbene-isoquinoline complex.

The solar-driven reduction of carbon dioxide to value-added chemical fuels is a longstanding challenge in the fields of catalysis, energy science, and green chemistry. In order to develop effective CO2 fixation, several key considerations must be balanced, including (1) catalyst selectivity for promoting CO2 reduction over competing hydrogen generation from proton reduction, (2) visible-light harvesting that matches the solar spectrum, and (3) the use of cheap and earth-abundant catalytic components. In this report, we present the synthesis and characterization of a new family of earth-abundant nickel complexes supported by N-heterocyclic carbene-amine ligands that exhibit high selectivity and activity for the electrocatalytic and photocatalytic conversion of CO2 to CO. Systematic changes in the carbene and amine donors of the ligand have been surveyed, and [Ni((Pr)bimiq1)](2+) (1c, where (Pr)bimiq1 = bis(3-(imidazolyl)isoquinolinyl)propane) emerges as a catalyst for electrochemical reduction of CO2 with the lowest cathodic onset potential (E(cat) = -1.2 V vs SCE). Using this earth-abundant catalyst with Ir(ppy)3 (where ppy = 2-phenylpyridine) and an electron donor, we have developed a visible-light photoredox system for the catalytic conversion of CO2 to CO that proceeds with high selectivity and activity and achieves turnover numbers and turnover frequencies reaching 98,000 and 3.9 s(-1), respectively. Further studies reveal that the overall efficiency of this solar-to-fuel cycle may be limited by the formation of the active Ni catalyst and/or the chemical reduction of CO2 to CO at the reduced nickel center and provide a starting point for improved photoredox systems for sustainable carbon-neutral energy conversion

Carbon Dioxide Reduction by Iron Hangman Porphyrins

Iron hangman porphyrins with phenol, guanidinium, and sulfonic acid proton donor groups placed above the Fe porphyrin platform reduce CO₂ to CO with Faradaic efficiencies >93%. Computations show that the activation of CO₂ at the Fe center is enhanced by the hanging group. Intramolecular hydrogen bonding from the phenol and guanidinium groups results in a 2.1–6.6 kcal/mol stabilization of CO₂ within the hangman pocket; the hanging sulfonate group is deprotonated, thus resulting in destabilization of the CO₂ adduct due to unfavorable electrostatic interactions. Electrochemical studies show that Fe hangman porphyrins exhibit canonical S-curve character; together with computation results, the apparent rate constant for CO₂ reduction is found to be governed by CO₂ binding within the hangman cleft

Carbon Dioxide Reduction by Iron Hangman Porphyrins

Iron hangman porphyrins with phenol, guanidinium, and sulfonic acid proton donor groups placed above the Fe porphyrin platform reduce CO₂ to CO with Faradaic efficiencies >93%. Computations show that the activation of CO₂ at the Fe center is enhanced by the hanging group. Intramolecular hydrogen bonding from the phenol and guanidinium groups results in a 2.1–6.6 kcal/mol stabilization of CO₂ within the hangman pocket; the hanging sulfonate group is deprotonated, thus resulting in destabilization of the CO₂ adduct due to unfavorable electrostatic interactions. Electrochemical studies show that Fe hangman porphyrins exhibit canonical S-curve character; together with computation results, the apparent rate constant for CO₂ reduction is found to be governed by CO₂ binding within the hangman cleft

Visible-Light Photoredox Catalysis: Selective Reduction of Carbon Dioxide to Carbon Monoxide by a Nickel <i>N</i>‑Heterocyclic Carbene–Isoquinoline Complex

The solar-driven reduction of carbon dioxide to value-added chemical fuels is a longstanding challenge in the fields of catalysis, energy science, and green chemistry. In order to develop effective CO₂ fixation, several key considerations must be balanced, including (1) catalyst selectivity for promoting CO₂ reduction over competing hydrogen generation from proton reduction, (2) visible-light harvesting that matches the solar spectrum, and (3) the use of cheap and earth-abundant catalytic components. In this report, we present the synthesis and characterization of a new family of earth-abundant nickel complexes supported by <i>N</i>-heterocyclic carbene–amine ligands that exhibit high selectivity and activity for the electrocatalytic and photocatalytic conversion of CO₂ to CO. Systematic changes in the carbene and amine donors of the ligand have been surveyed, and [Ni­(^Prbimiq1)]²⁺ (<b>1c</b>, where ^Prbimiq1 = bis­(3-(imidazolyl)­isoquinolinyl)­propane) emerges as a catalyst for electrochemical reduction of CO₂ with the lowest cathodic onset potential (<i>E</i>_cat = −1.2 V vs SCE). Using this earth-abundant catalyst with Ir­(ppy)₃ (where ppy = 2-phenylpyridine) and an electron donor, we have developed a visible-light photoredox system for the catalytic conversion of CO₂ to CO that proceeds with high selectivity and activity and achieves turnover numbers and turnover frequencies reaching 98,000 and 3.9 s^–1, respectively. Further studies reveal that the overall efficiency of this solar-to-fuel cycle may be limited by the formation of the active Ni catalyst and/or the chemical reduction of CO₂ to CO at the reduced nickel center and provide a starting point for improved photoredox systems for sustainable carbon-neutral energy conversion

Tris(carbene)borate Ligands Featuring Imidazole-2-ylidene, Benzimidazol-2-ylidene, and 1,3,4-Triazol-2-ylidene Donors. Evaluation of Donor Properties in Four-Coordinate {NiNO}¹⁰ Complexes

The synthesis and characterization of new tris­(carbene)­borate ligand precursors containing substituted benzimidazol-2-ylidene and 1,3,4-triazol-2-ylidene donor groups, as well as a new tris­(imidazol-2-ylidene)­borate ligand precursor are reported. The relative donor strengths of the tris­(carbene)­borate ligands have been evaluated by the position of ν­(NO) in four-coordinate {NiNO}¹⁰ complexes, and follow the order: imidazol-2-ylidene > benzimidazol-2-ylidene > 1,3,4-triazol-2-ylidene. There is a large variation in ν­(NO), suggesting these ligands to have a wide range of donor strengths while maintaining a consistent ligand topology. All ligands are stronger donors than Tp* and Cp*

N–O Bond Homolysis of an Iron(II) TEMPO Complex Yields an Iron(III) Oxo Intermediate

The reaction of TEMPO with the iron­(I) synthon PhB­(MesIm)₃Fe­(COE) leads to formation of the κ¹-TEMPO complex PhB­(MesIm)₃Fe­(TEMPO). Structural and spectroscopic data establish the complex contains divalent iron bound to a nitroxido anion and is isoelectronic to an iron­(II) peroxo complex. Thermolysis of the complex results in N–O bond homolysis, leading to the formation of an iron­(III) oxo intermediate. The oxo intermediate is active in oxygen atom transfer reactions and can be trapped by the triphenylmethyl radical to give the iron­(II) alkoxo complex PhB­(MesIm)₃Fe­(OCPh₃)

N–O Bond Homolysis of an Iron(II) TEMPO Complex Yields an Iron(III) Oxo Intermediate

The reaction of TEMPO with the iron­(I) synthon PhB­(MesIm)₃Fe­(COE) leads to formation of the κ¹-TEMPO complex PhB­(MesIm)₃Fe­(TEMPO). Structural and spectroscopic data establish the complex contains divalent iron bound to a nitroxido anion and is isoelectronic to an iron­(II) peroxo complex. Thermolysis of the complex results in N–O bond homolysis, leading to the formation of an iron­(III) oxo intermediate. The oxo intermediate is active in oxygen atom transfer reactions and can be trapped by the triphenylmethyl radical to give the iron­(II) alkoxo complex PhB­(MesIm)₃Fe­(OCPh₃)