The Effects Of Cationic Charges In Metal-Ligand Cooperativity

The utility of metal ligand cooperativity has demonstrated great promise in the ability to activate strong bonds under mild conditions. The work described below endeavors to contribute to this field through two thrusts: 1. Exploiting a chelating phosphinimine ligand designed to take advantage of the electron density at the phosphinimine nitrogens, caused by zwitterionic character of the phosphinimine, to aid in catalytic applications. 2. The development of ligands that can pre-form heterobimetallic clusters for combined action toward strong-bond activation chemistry. To this end, two ligand designs were studied. The first is a set of novel tren-based tris(phosphinimine) ligands (P3tren; tren = tri(2 aminoethyl)amine). The P3tren ligands were designed to maintain the primary coordination sphere observed in other tetradentate ligands such as tren and tmpa, while providing a convenient means for varying the donor strength of the terminal nitrogens. Poor π-overlap between nitrogen and phosphorous results in significant ylidic character, thereby enhancing the basicity of the nitrogen donor atom while maintaining an overall neutral charge. The second was through a collaboration with the Walsh group, where Chen Wu experimentally found that DavePhos(Pd) can activate aryl fluorides in similar yields and rates as aryl chlorides for the coupling with phenyl Grignard. DFT calculations were performed to probe the possible reaction mechanism, suggesting the key factor was the coordination of a magnesium cation by the biphenyl system of DavePhos. The magnesium ion was proposed to aid aryl halide activation through a heterobimetallic pathway.