Proton Tunneling Distances for Metal Hydrides Formation Manage the Selectivity of Electrochemical CO2 Reduction Reaction

A series of manganese polypyridine complexes were prepared as CO2 reduction electrocatalysts. In these catalysts, the intramolecular proton tunneling distance for metal hydride formation (PTD-MH) vary from 2.400 to 2.696 Å while the structural, energetic and electronic factors remain essentially close. The experimental and theoretical results revealed that the selectivity of CO2 reduction reaction (CO2RR) is dominated by the intramolecular PTD-MH within a difference of ca. 0.3 Å in PTD-MH. Specifically, the catalyst functionalized with a pendent phenol group featuring a slightly longer PTD-MH prefers the binding of proton to the [Mn-CO2] adduct rather than the Mn center and results in ca. 100% selectivity for CO product. By contrast, decreasing the PTD-MH by attaching a dangling tertiary amine in the same catalyst skeleton facilitates the proton binding on the Mn center and switches the product from CO to HCOOH with a selectivity of 86%.