Computational mechanistic study of Ru-catalyzed CO2 reduction by pinacolborane revealing the σ–π coupling mechanism for CO2 decarbonylation

Abstract

It has been reported that RuH₂(η²-H₂)₂(PCy₃)₂ (1) could mediate CO₂ reduction by pinacolborane (HBpin), affording pinBOBpin (7), pinBOCH₃ (8), pinBOCHO (9), pinBOCH₂OBpin (10), and an unprecedented C2 species pinBOCH₂OCHO (11), which meanwhile is converted to the Ru complexes, including the transient 3 (RuH(κ²-O₂CH)(CO)(PCy₃)₂) and 5 (RuH{(μ-H)₂Bpin}(CO)(PCy₃)₂), and the persistent 4 (RuH(κ²-O₂CH)(CO)₂(PCy₃)₂) and 6 (RuH₂(CO)₂(PCy₃)₂). To gain an insight into the catalysis, a DFT study was carried out. The study identified the key active catalyst to be the hydride 13 (RuH₂(CO)(PCy₃)₂) and characterized the mechanisms leading to the experimentally observed species (3–11). By investigating the experimental system, we learned a new mechanism called σ–π coupling for CO₂ decarbonylation. Under this mechanism, CO₂ and HBpin first co-coordinate to the Ru center of 13, then σ–π coupling takes place, forming a B–O bond between CO₂ and HBpin, Ru–H and Ru–C bonds, and simultaneously breaking the H–Bpin bond, followed by -OBpin group migration to the Ru center, completing the CO₂ decarbonylation. An interesting feature regarding the Ru catalysis was the involvement of η¹-H⋯η¹-H → η²-H₂ and η¹-H⋯η¹-Bpin → η²-HBpin reductions, which facilitated the oxidative H–Bpin addition or the coordination mode change of CO₂ from η¹-O to η²-CO for CO₂ activation or σ–π coupling. The facilitation effects could be attributed to the reductions enhancing the electron donations from the Ru center to the antibonding orbitals of the activating bonds.