Iridium dihydride complexes supported by PCP-type pincer ligands rapidly insert CO(2) to yield κ(2)-formate monohydride products in THF. In acetonitrile/water mixtures, these complexes become efficient and selective catalysts for electrocatalytic reduction of CO(2) to formate. Electrochemical and NMR spectroscopic studies have provided mechanistic details and structures of key intermediates.

Numerous transition metal-mediated reactions, including hydrogenations, hydrosilations, and alkane functionalizations, result in the cleavage of strong sigma bonds. Key intermediates in these reactions often involve coordination of the sigma bond of dihydrogen, silanes (Si-H), or alkanes (C-H) to the metal center without full scission of the bond. These sigma complexes have been characterized to varying degrees in solid state and solution. However, a sigma complex of the simplest hydrocarbon, methane, has eluded full solution characterization. Here, we report nuclear magnetic resonance spectra of a rhodium(I) sigma-methane complex obtained by protonation of a rhodium-methyl precursor in CDCl2F solvent at -110 degrees C. The sigma-methane complex is shown to be more stable than the corresponding rhodium(III) methyl hydride complex. Even at -110 degrees C, methane rapidly tumbles in the coordination sphere of rhodium, exchanging free and bound hydrogens. Kinetic studies reveal a half-life of about 83 minutes at -87 degrees C for dissociation of methane (free energy of activation is 14.5 kilocalories per mole).

A comprehensive mechanistic study of electrocatalytic CO2 reduction by ruthenium 2,2′:6′,2″-terpyridine (tpy) pyridyl-carbene catalysts reveals the importance of stereochemical control to locate the strongly donating N-heterocyclic carbene ligand trans to the site of CO2 activation. Computational studies were undertaken to predict the most stable isomer for a range of reasonable intermediates in CO2 reduction, suggesting that the ligand trans to the reaction site plays a key role in dictating the energetic profile of the catalytic reaction. A new isomer of [Ru­(tpy)­(Mebim-py)­(NCCH3)]2+ (Mebim-py is 1-methylbenzimidazol-2-ylidene-3-(2′-pyridine)) and both isomers of the catalytic intermediate [Ru­(tpy)­(Mebim-py)­(CO)]2+ were synthesized and characterized. Experimental studies demonstrate that both isomeric precatalysts facilitate electroreduction of CO2 to CO in 95/5 MeCN/H2O with high activity and high selectivity. Cyclic voltammetry, infrared spectroelectrochemistry, and NMR spectroscopy studies provide a detailed mechanistic picture demonstrating an essential isomerization step in which the N-trans catalyst converts in situ to the C-trans variant. Insight into molecular electrocatalyst design principles emerge from this study. First, the use of an asymmetric ligand that places a strongly electron-donating ligand trans to the site of CO2 binding and activation is critical to high activity. Second, stereochemical control to maintain the desired isomer structure during catalysis is critical to performance. Finally, pairing the strongly donating pyridyl-carbene ligand with the redox-active tpy ligand proves to be useful in boosting activity without sacrificing overpotential. These design principles are considered in the context of surface-immobilized electrocatalysis.

No room for side‐on: In the complex shown, Et3SiH is bound to the cationic IrIII center in an unprecedented end‐on fashion through the SiH bond with no appreciable metal–silicon interaction (white H, green Si, pink Ir, red O, orange P, gray C). The long Ir⋅⋅⋅Si distance of 3.346(1) Å is 0.97 Å greater than the sum of the covalent radii of Ir and Si. DFT studies indicate that the tBu substituents on the bidentate phosphorus ligand dictate the coordination mode of the silane.

Accurate predictions of molecular acidity using ab initio and density functional approaches are still a daunting task. Using electronic and reactivity properties, one can quantitatively estimate pKa values of acids. In a recent paper ͓S. B. Liu and L. G. Pedersen, J. Phys. Chem. A 113, 3648 ͑2009͔͒, we employed the molecular electrostatic potential ͑MEP͒ on the nucleus and the sum of valence natural atomic orbital ͑NAO͒ energies for the purpose. In this work, we reformulate these relationships on the basis of conceptual density functional theory and compare the results with those from the thermodynamic cycle method. We show that MEP and NAO properties of the dissociating proton of an acid should satisfy the same relationships with experimental pKa data. We employ 27 main groups and first to third row transition metal-water complexes as illustrative examples to numerically verify the validity of these strong linear correlations. Results also show that the accuracy of our approach and that of the conventional method through the thermodynamic cycle are statistically similar.