Mechanism, selectivity, and what prevents closure of the catalytic cycle for H2 reduction of CO2 promoted by a heterodinuclear zirconium–iridium complex

Abstract

Dinuclear metal–metal cooperative effects hold promise for activation and functionalization of small molecules. Perhaps one of the most important targets of transition metal heterodinuclear bond activation and functionalization is the reaction between CO₂ and H₂. Bergman and coworkers reported a heterodinuclear Zr–Ir complex that promotes the activation and functionalization of CO₂ with H₂ at low temperature and pressure. Here we report the use of density functional theory calculations to determine the reaction mechanism, reactivity, and selectivity of this heterodinuclear promoted reaction. The Zr–Ir complex has surprisingly low barriers for activation of both CO₂ and H₂, and molecular dynamics trajectories show that CO₂ activation is a concerted, one-step process. The H₂ activation transition state is lower in energy than CO₂ activation and therefore when H₂ is added first or when they are added together formate is generated through a reactive Zr–H intermediate. The lack of catalytic turnover is only the result of unfavorable thermodynamics, not kinetic barriers, where the metal bound formate and Ir–H is favored compared to the release of formic acid and reforming the Zr–Ir dinuclear complex. Overall, these calculations point towards possible system changes that could enable catalysis.