RESUMO
Reversible catalysis is a hallmark of energy-efficient chemical transformations, but can only be achieved if the changes in free energy of intermediate steps are minimized and the catalytic cycle is devoid of high transition-state barriers. Using these criteria, we demonstrate reversible CO2 /HCO2 - conversion catalyzed by [Pt(depe)2 ]2+ (depe=1,2-bis(diethylphosphino)ethane). Direct measurement of the free energies associated with each catalytic step correctly predicts a slight bias towards CO2 reduction. We demonstrate how the experimentally measured free energy of each step directly contributes to the <50â mV overpotential. We also find that for CO2 reduction, H2 evolution is negligible and the Faradaic efficiency for HCO2 - production is nearly quantitative. A free-energy analysis reveals H2 evolution is endergonic, providing a thermodynamic basis for highly selective CO2 reduction.
RESUMO
The sensitivity provided by fluorescence microscopy enabled the observation of surface intermediates in the synthesis of soluble organozinc reagents by direct insertion of alkyl iodides to commercial zinc powder. Five hypotheses were examined for the mechanistic role of lithium chloride in enabling this direct insertion. The data are consistent with lithium chloride solubilizing organozinc reagents from the surface of the zinc after oxidative addition.
RESUMO
Millions of tons of acetyl derivatives such as acetic acid and acetic anhydride are produced each year. These building blocks of chemical industry are elaborated into esters, amides, and eventually polymer materials, pharmaceuticals, and other consumer products. Most acetyls are produced industrially using homogeneous precious metal catalysts, principally rhodium and iridium complexes. We report here that abundant nickel can be paired with imidazole-derived carbenes or the corresponding salts to catalyze methyl ester carbonylation with turnover frequency (TOF) exceeding 150 hour-1 and turnover number (TON) exceeding 1600, benchmarks that invite comparisons to state-of-the-art rhodium-based systems and considerably surpass known triphenylphosphine-based nickel catalysts, which operate with TOF ~7 hour-1 and TON ~100 under the same conditions.
RESUMO
[Pt(depe)2](PF6)2 electrocatalyzes the reversible conversion between CO2 and HCO2- with high selectivity and low overpotential but low rates. A comprehensive kinetic analysis indicates the rate determining step for CO2 reduction is the reactivity of a Pt hydride intermediate to produce HCO2-. To accelerate catalysis, the use of cationic and hydrogen-bond donor additives are explored.
RESUMO
Flexible ligands that can adapt their donor strength have enabled unique reactivity in a wide range of inorganic complexes. Most examples are composed of flexible multi-dentate ligands containing a donor that can vary its interaction through its distance to the metal center. We describe an alternative type of adaptable ligand interaction in the neutral multi-dentate ligand tris(2-pyridylmethyl)-azaphosphatrane (TPAP), which contains a proazaphosphatrane unit. Prozaphosphatranes are intrinsically strong phosphorus donors; upon coordination to more Lewis acidic atoms the phosphorus can accept additional electron density from a tertiary nitrogen to form a transannular bond, increasing its donor strength. An experimental and computational investigation of the varying degree of transannular interaction in TPAP when coordinated to late transition metals is reported. The synthesis and characterization of the complexes M(TPAP), where M = Co(i)Cl, Ni(0)(1,5-cyclooctadiene), Ni(ii)(CH3CN)(BF4)2, Pd(ii)(CH3CN)(BF4)2, or Pt(ii)Cl(PF6) is described. Structural characterization and density functional theory calculation of these complexes, along with the previously reported [Co(ii)(TPAP)(CH3CN)](BF4)2 establish significant increases in the degree of transannular interaction of the proazaphosphatrane unit when coordinated to more electron deficient metal ions.