Secondary Sphere Effects on the Reactivity of Transition Metal Complexes

Transition metal catalysis is an important tool for chemical synthesis and industrial transformations. Utilizing secondary sphere effects and metal-ligand cooperativity has led to greatly improved catalysis in many different reactions, such as CO2 reduction or H2 evolution. There are generally two major strategies for these cooperative effects: redox activity and pendant acidic/basic sites. In order to expand strategies for secondary sphere effects, ligands were targeted that could transfer both protons and electrons as either H-atoms or H2 equivalents or exert an electric field over a substrate complex via distal charged groups. A 2,5-pyrrole pincer scaffold was particularly attractive for the former strategy, as it could be protonated on both arms and the central pyrrole ring can undergo two-electron reduction and oxidation to facilitate H2 gain and loss by the ligand. To study and quantify electric field effects, a distally anionic phosphine was synthesized, and the through-space and through-bond contributions of the charge were quantified through the Tolman Electronic Parameter and phosphorus-selenium coupling.