Hairong Guan, PhD , University of Cincinnati
For the second year of the project, we have devoted much of our efforts to improving the catalytic performance of nickel bis(phosphinite) pincer complexes in the reduction of CO2 to methanol derivatives. We have compared the catalytic activity of a series of nickel pincer hydride complexes bearing different sizes of alkyl groups on the phosphorous donor atoms. For the reaction of CO2 with catecholborane, the most efficient catalyst involves the complex with the most sterically hindered hydride ligand. We have investigated other boranes and silanes as the reducing reagents. 9-borabicyclo[3.3.1]nonane behaves similarly as HBcat, while the reaction of pinacolborane stops at the formate stage rather than the methoxy stage. In contrast, phenylsilane is not a viable reducing reagent for nickel-catalyzed hydrosilylation of CO2.
We have also explored other catalytic applications of nickel bis(phosphinite) pincer complexes. We have shown that [2,6-(Ph2PO)2C6H3]NiCl catalyzes cross-coupling of aryl iodides and aryl thiols. The optimal catalytic conditions involve 1 mol% of the nickel catalyst and 2 equiv of KOH (with respect to aryl thiols) in DMF at 80 °C, and tolerate a variety of functional groups in the substrates. Mechanistic studies have suggested that the pincer ligand framework in the nickel complex is destroyed by KOH via the cleavage of P-O bonds to release Ph2POK, and further decomposition leads to Ph3P and other phosphorus-containing products. The cross-coupling reactions are more effectively catalyzed by Ni(COD)2/Ph2P(O)H.
We have extended our studies beyond the chemistry of nickel. As our initial efforts on iron catalysis, we have synthesized iron bis(phosphinite) pincer hydride complexes via the C-H bond activation of resorcinol-derived ligands 1,3-(R2PO)2C6H4 (R = iPr and Ph) with Fe(PMe3)4. These new iron POCOP-pincer hydride complexes catalyze the hydrosilylation of aldehydes and ketones with different functional groups. Isotopic labeling experiments rule out the hydride ligand being directly involved in the reduction. The hydrosilylation reactions are more likely to proceed via the activation of silanes or carbonyl substrates after ligand dissociation from the iron center.
This ACS PRF grant has made a significant impact on the career of PI as well as the students that were supported by the grant. The grant provided the seed money that eventually led to a successful NSF career award. Two undergraduate students working on the project have co-authored a paper and one of them is currently a graduate student in a chemistry PhD program.