No abstract

Reduction of CO 2 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 CO 2 reduction. Stoichiometric studies show that the palladium hydride complexes react rapidly with CO 2 , although the resulting palladium formate complexes can lose CO 2 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 CH 3 OBcat. This process is also accompanied by the formation of palladium bis(catecholato)borate complexes, which diverts some of the HBcat to yield B 2 (cat) 3 and B 2 H 6 . 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 CO 2 , HBcat also undergoes H 2 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 CO 2 with the borane.