Comparative study of CO 2 insertion into pincer supported palladium alkyl and aryl complexes.

The insertion of CO 2 into metal alkyl bonds is a crucial elementary step in transition metal-catalyzed processes for CO 2 utilization. Here, we synthesize pincer-supported palladium complexes of the type ( t Bu PBP)Pd(alkyl) ( t Bu PBP = B(NCH 2 P t Bu 2 ) 2 C 6 H 4 - ; alkyl = CH 2 CH 3 , CH 2 CH 2 CH 3, CH 2 C 6 H 5 , and CH 2 -4-OMe-C 6 H 4 ) and ( t Bu PBP)Pd(C 6 H 5 ) and compare the rates of CO 2 insertion into the palladium alkyl bonds to form metal carboxylate complexes. Although, the rate constant for CO 2 insertion into ( t Bu PBP)Pd(CH 2 CH 3 ) is more than double the rate constant we previously measured for insertion into the palladium methyl complex ( t Bu PBP)Pd(CH 3 ), insertion into ( t Bu PBP)Pd(CH 2 CH 2 CH 3 ) occurs approximately one order of magnitude slower than ( t Bu PBP)Pd(CH 3 ). CO 2 insertion into the benzyl complexes ( t Bu PBP)Pd(CH 2 C 6 H 5 ) and ( t Bu PBP)Pd(CH 2 -4-OMe-C 6 H 4 ) is significantly slower than any of the n-alkyl complexes, and CO 2 does not insert into the palladium phenyl bond of ( t Bu PBP)Pd(C 6 H 5 ). While ( t Bu PBP)Pd(CH 2 CH 3 ) and ( t Bu PBP)Pd(CH 2 CH 2 CH 3 ) 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 ( t Bu PBP)Pd(CH 2 C 6 H 5 ) at elevated temperature and a related process involving the formation of a new H-B bond prevented the isolation of ( t Bu PBP)PdH. DFT calculations provide insight into the relative rates of CO 2 insertion and indicate that steric factors are critical. Overall, this work is one of the first comparative studies of the rates of CO 2 insertion into different metal alkyl bonds and provides fundamental information that may be important for the development of new catalysts for CO 2 utilization.