Abstract: Pincer compounds are organometallic complexes with intriguing tunable reactivities, thus applicable to a wide range of potential applications. In this work we explore their unique properties and reactivities, with a focus on the PN3P pincer platform, through spectroscopic and computational investigations.
We conducted a computational study on pincer complexes with stereogenic phosphine arms and found that an energy difference as high as 16.8 kcal/mol could exist between the highest and lowest energy conformer, and that high energy conformers have bulky groups adjacent, causing steric clash. The full set of conformers for reactant, transition state and product were evaluated in a reaction energy profile of a CO2 reduction by a pincer nickel hydride, and we found that this reaction could be found either favorable or unfavorable depending on the choice of conformer.
The basicity of three PN3P* nickel pincer complexes have been determined in THF using the spectroscopic vverlapping indicator method. The relative basicity was found to be tBu(PN3P*)NiH > cPe(PN3P*)NiH > tBu(PN3P*)NiCl. This concludes that the imine arms of the PN3P* nickel hydrides are more basic than PN3P* nickel chloride, and changing the alkyl group on the phosphine arms had a smaller impact on the basicity compared to changing from chloride to hydride ligand on the nickel metal center.
Finally, we explored the reactivity between a PN3P* rhodium carbonyl pincer complex and dioxygen, at room temperature in solution, and at 180-200 degrees Celsius in the solid state. The solution reaction affords oxidation on pyridine moeity on the ligand backbone. This reaction between the singlet PN3P* rhodium carbonyl complex and triplet dioxygen is found to be possible due to the ligands ability to transfer an electron to dioxygen, creating a superoxide radical anion and a ligand-centered radical cation. This reaction is an example of the pincer ligand being redox non-innocent. The solid state reaction was studied with in situ rhodium K-edge x-ray absorption spectroscopy and an isobestic point was observed, indicating a clean reaction and a significant change occuring at the rhodium metal center. In situ FTIR studies revealed that the oxidation on the ligand also occurs in the solid state, but additionally the PN3P rhodium carbonyl complex facilitates the reaction between dioxygen and carbon monoxide to produce CO2.