On the role of noncovalent interactions in electrocatalysis : two cases of mediated reductive dehalogenation

Two cases of mediated electron transfer are presented: chloroform reduction catalysed by MoII/I alkoxy scorpionates and debromination of hexabromocyclododecane (HBCD) in the presence of free-base tetraphenylporphyrin (H2TPP). Although H2TPP should act as a typical outer-sphere mediator, it is not active towards analogous dehalogenation of 1,2-dibromocyclododecane. The observed phenomena can be rationalised by considering the catalytically relevant transient adducts formed owing to noncovalent interactions (Csingle bondH hydrogen bonds and dispersive Csingle bondhalogen⋯π interactions or directional halogen bonding), which warrants the close and prolonged contact between the catalyst and its substrate, thus increases the probability of electron transfer, and decisively accelerates the reaction. Crucial for this action is thermodynamic stability of the adducts, which can only be explained if dispersive van der Waals interactions are properly accounted for, e.g., as by dispersion-corrected density functional theory (DFT-D) calculations. The structures involving strong and anisotropic interactions, like the surprisingly short Csingle bondH⋯Oalkoxide H-bonding in the MoI–chloroform adduct, may be reasonably well described by standard DFT calculations and the energy needs only be corrected for dispersion without the need for structure re-optimisation at the DFT-D level. The latter is, however, a method of choice for the prediction of supramolecular structures chiefly controlled by weak non-directional van der Waals forces

The effect of C−H···O bonding and Cl···π interactions in electrocatalytic dehalogenation of C2C_2 chlorides containing an acidic hydrogen

A tungsten alkoxy scorpionate shows activity in the electrocatalytic reductive dehalogenation of pentachloroethane and trichloroethylene owing to the formation of hydrogen and halogen bonded adducts with the substrates, which is further reinforced by dispersive interactions. The ensuing proximity between the substrate molecule and the metal centre promotes dechlorination in a concerted process. Two-electron reduction of pentachloroethane yields trichloroethylene that undergoes further, non-catalysed, reactions that ultimately give acetylenes. Interestingly, pentachloroethane proved to be a highly efficient proton donor for the transient anions, in extremely exothermic and rapid proton transfer concerted with chloride anion abstraction, which yields perchloroethylene. The total process and the mechanism thereof were verified based on DFT and coupled cluster (CC) calculations. The calculations evaluated feasibility of various pathways in the mechanism. Standard redox potentials for the environmentally relevant species, participating in the studied reactions, were accurately computed employing the explicitly correlated CCSD(T)-F12 method that provides an improved C-Cl bond energy, of essential importance to the dissociative potentials