Pitfalls for POCOP-Type Palladium Pincer Complexes in Catalytic Reduction of CO2 with Catecholborane

Abstract

Reduction of CO2 with catecholborane (HBcat) is known to be catalyzed by nickel hydride complexes supported by a bis(phosphinite)-based or POCOP-type pincer ligand. The objective of this research is to examine how changing the metal center from nickel to palladium would impact the outcome of CO2 reduction. Stoichiometric studies show that the palladium hydride complexes react rapidly with CO2, although the resulting palladium formate complexes can lose CO2 under reduced pressure or in chloroform. In the presence of HBcat, the formate complexes are converted back to the palladium hydride species while the formate group is reduced to CH3OBcat. This process is also accompanied by the formation of palladium bis(catecholato)borate complexes, which diverts some of the HBcat to yield B2(cat)3 and B2H6. For palladium hydrides stabilized by a POCOP-pincer ligand with relatively small phosphorus substituents (e.g., isopropyl or cyclopentyl groups), HBcat cleaves the P–O bonds in the ligand backbone to degrade the catalysts to secondary phosphine complexes. Without CO2, HBcat also undergoes H2 elimination with the Pd–H moiety to form palladium boryl complexes. These side reactions with HBcat and the palladium pincer complexes play profound roles in the catalytic reduction of CO2 with the borane.