Metalloradical complex Co–C˙Ph3 catalyzes the CO2 reduction in gas phase: a theoretical study

Abstract

Metal-stabilized radicals have been increasingly exploited in modern organic synthesis. Here, we theoretically designed a metalloradical complex Co–C˙Ph₃ with the triplet characters through the transition metal cobalt (Co⁰) coordinating a triphenylmethyl radical. The potential catalytic role of this novel metalloradical in the CO₂ reduction with H₂/CH₄ in the gas phase was explored via density functional theory (DFT) calculations. For the CO₂ reduction reaction with H₂, there are two possible pathways: one (path A) is the activation of CO₂ by Co–C˙Ph₃, followed by the hydrogenation of CO₂. The other (path B) starts from the splitting of the H–H bond by Co–C˙Ph₃, leading to the transition-metal hydride complex CoH–H, which can reduce CO₂. DFT computations show that path B is more favorable than path A as their rate-determining free energy barriers are 18.3 and 27.2 kcal mol⁻¹, respectively. However, for the reduction of CO₂ by CH₄ two different products, CH₃COOH and HCOOCH₃, can be generated following different reaction routes. Both routes begin with one CH₄ molecule approaching the metalloradical Co–C˙Ph₃ to form the intermediate CoH–CH₃. This intermediate can evolve following two different pathways, depending on whether the H bonded to Co is transferred to the O (pathway PO) or the C (pathway PC) of CO₂. Comparing their rate-determining steps, we identified that the PO route is more favorable for the reduction of CO₂ by CH₄ to CH₃COOH with the reaction barrier 24.5 kcal mol⁻¹. Thus, the present Co⁰-based metalloradical system represents a viable catalytic protocol that can contribute to the effective utilization of small molecules (H₂ and CH₄) to reduce CO₂, and provides an alternative strategy for the exploration of CO₂ conversion.