Tuning reagent and catalyst concentrations is crucial in the development of efficient catalytic transformations. In enzyme-catalysed reactions the substrate is bound-often by multiple non-covalent interactions-in a well-defined pocket close to the active site of the enzyme; this pre-organization facilitates highly efficient transformations. Here we report an artificial system that co-encapsulates multiple catalysts and substrates within the confined space defined by an M12L24 nanosphere that contains 24 endohedral guanidinium-binding sites. Cooperative binding means that sulfonate guests are bound much more strongly than carboxylates. This difference has been used to fix gold-based catalysts firmly, with the remaining binding sites left to pre-organize substrates. This strategy was applied to a Au(I)-catalysed cyclization of acetylenic acid to enol lactone in which the pre-organization resulted in much higher reaction rates. We also found that the encapsulated sulfonate-containing Au(I) catalysts did not convert neutral (acid) substrates, and so could have potential in the development of substrate-selective catalysis and base-triggered on/off switching of catalysis.

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.

Registro de acceso restringido Este recurso no está disponible en acceso abierto por política de la editorial. No obstante, se puede acceder al texto completo desde la Universitat Jaume I o si el usuario cuenta con suscripción. Registre d'accés restringit Aquest recurs no està disponible en accés obert per política de l'editorial. No obstant això, es pot accedir al text complet des de la Universitat Jaume I o si l'usuari compta amb subscripció. Restricted access item This item isn't open access because of publisher's policy. The full--text version is only available from Jaume I University or if the user has a running suscription to the publisher's contents.

Comparative kinetic studies of a series of new ruthenium complexes provide a platform for understanding how strong trans effect ligands and redox-active ligands work together to enable rapid electrochemical CO 2 reduction at moderate overpotential. After synthesizing isomeric pairs of ruthenium complexes featuring 2′-picolinyl-methylbenzimidazol-2-ylidene (Mebim-pic) as a strong trans effect ligand and 2,2′:6′,2″-terpyridine (tpy) as a redox-active ligand, chemical and electrochemical kinetic studies examined how complex geometry and charge affect the individual steps and overall catalysis of CO 2 reduction. The relative trans effect of picoline vs the N-heterocyclic carbene (NHC) was quantified through a kinetic analysis of reductively triggered chloride dissociation, revealing that chloride loss is 1000 times faster in the isomer with the NHC trans to chloride. The kinetics of CO dissociation from a site trans to the NHC were examined in a systematic study of isostructural carbonyl complexes across four different overall charges. The rate constants for CO loss span 12 orders of magnitude and are fastest upon two-electron reduction, leading to a hypothesis that redox-active ligands play a key role in promoting reductive CO dissociation during catalysis. Analogous studies of complexes featuring the picoline ligand trans to the carbonyl reveal the importance of the trans effect of the CO ligand itself, with picoline ligand dissociation observed upon reduction. The complexes with NHC trans to the active site proved to be active electrocatalysts capable of selective CO 2 electroreduction to CO. In acidic solutions under a N 2 atmosphere, on the other hand, H 2 evolution proceeds via an intermediate that positions a hydride ligand trans to picoline. The mechanistic insight and quantitative kinetic parameters that arise from these studies help establish general principles for molecular electrocatalyst design.

Registro de acceso restringido Este recurso no está disponible en acceso abierto por política de la editorial. No obstante, se puede acceder al texto completo desde la Universitat Jaume I o si el usuario cuenta con suscripción. Registre d'accés restringit Aquest recurs no està disponible en accés obert per política de l'editorial. No obstant això, es pot accedir al text complet des de la Universitat Jaume I o si l'usuari compta amb subscripció. Restricted access item This item isn't open access because of publisher's policy. The full--text version is only available from Jaume I University or if the user has a running suscription to the publisher's contents.