Pincer ligands have been extensively investigated due to the ease by which their steric and electronic properties can be tuned. One of the most important features of pincer metal complexes is their robustness, which renders them excellent candidates for a wide variety of applications such as catalysis, medicinal chemistry, and chemical sensing. The increased robustness of pincer complexes is not associated with a loss of reactivity at the metal center, which renders them good ancillary ligands. Proper tuning of the electron donating properties of pincer ligands has allowed for some of their metal complexes to display unprecedented reactivity.
The objective of the research described herein was the synthesis and characterization of novel pincer ligand architectures, with a particular interest in ligands with strong electron donating moieties at the central position. Two PBP pincer ligand precursors with a 2-chloro-1,3,2-diazaborane central moiety were synthesized. The coordination of the ligand precursors to palladium via a B-Cl bond activation was investigated. The synthesized PBP ligands represent two of the first examples of pincer ligands with a boryl at the central donor position, and their palladium complexes are the first ever (PBP)Pd complexes. These complexes displayed good thermal stability but no catalytic activity under Heck cross-coupling reaction conditions. The synthesis of PBP and NBN pincer ligand precursors with an acyclic backbone was also explored.
PCP pincer ligand precursors with six and five membered N-heterocyclic carbene (NHC) moieties at the central donor position, which upon coordination would generate two five membered metallacycles, were synthesized and characterized. Double C-H bond activation by rhodium in PCP-carbene precursors yielded the desired compounds with six membered NHC backbones. These are among the rare examples where a double C-H bond activation leads to the formation of an NHC complex. The rhodium complexes displayed good thermal stability and promising reactivity. The analogous PCP-carbene precursor with a five membered NHC backbone could be readily deprotonated to the free carbene, allowing for the synthesis of rhodium, palladium, nickel and molybdenum complexes. These complexes have some of the shortest M-CNHC bonds reported for each metal, suggesting a strong interaction between the metal center and the ligand.