Comparative study of CO2 insertion into pincer supported palladium alkyl and aryl complexes

Abstract

The insertion of CO₂ into metal alkyl bonds is a crucial elementary step in transition metal-catalyzed processes for CO₂ utilization. Here, we synthesize pincer-supported palladium complexes of the type (^tBuPBP)Pd(alkyl) (^tBuPBP = B(NCH₂P^tBu₂)₂C₆H₄⁻; alkyl = CH₂CH₃, CH₂CH₂CH_3, CH₂C₆H₅, and CH₂-4-OMe-C₆H₄) and (^tBuPBP)Pd(C₆H₅) and compare the rates of CO₂ insertion into the palladium alkyl bonds to form metal carboxylate complexes. Although, the rate constant for CO₂ insertion into (^tBuPBP)Pd(CH₂CH₃) is more than double the rate constant we previously measured for insertion into the palladium methyl complex (^tBuPBP)Pd(CH₃), insertion into (^tBuPBP)Pd(CH₂CH₂CH₃) occurs approximately one order of magnitude slower than (^tBuPBP)Pd(CH₃). CO₂ insertion into the benzyl complexes (^tBuPBP)Pd(CH₂C₆H₅) and (^tBuPBP)Pd(CH₂-4-OMe-C₆H₄) is significantly slower than any of the n-alkyl complexes, and CO₂ does not insert into the palladium phenyl bond of (^tBuPBP)Pd(C₆H₅). While (^tBuPBP)Pd(CH₂CH₃) and (^tBuPBP)Pd(CH₂CH₂CH₃) are resistant to β-hydride elimination, we were unable to synthesize complexes with n-butyl, iso-propyl, and tert-butyl ligands due to β-hydride elimination and an unusual reductive coupling, which involves the formation of new C–B bonds. This reductive process also occurred for (^tBuPBP)Pd(CH₂C₆H₅) at elevated temperature and a related process involving the formation of a new H–B bond prevented the isolation of (^tBuPBP)PdH. DFT calculations provide insight into the relative rates of CO₂ insertion and indicate that steric factors are critical. Overall, this work is one of the first comparative studies of the rates of CO₂ insertion into different metal alkyl bonds and provides fundamental information that may be important for the development of new catalysts for CO₂ utilization.