The development of palladium catalysis has been influential in a wide range of organic transformations, in particular C-C coupling, C-Heteroatom coupling and C-H functionalization reactions. These catalytic transformations normally proceed through the Pd0/II catalytic cycle. These reactions are remarkably useful, however, they suffer from two major problems: -hydride elimination and palladium black deposition. To circumvent these problems, recent reports have been focused on developing novel organic transformations proceeding through high-valent palladium and nickel catalytic cycles.
To this point, we recently reported the isolation and characterization of various mononuclear PdIII complexes using the tetradentate ligands, N,N-di-alkyl-2,11-diaza[3.3](2,6)pyridinophane (RN4, R = tBu, iPr, Me). As a result, the ligand effects on the stability and reactivity of the corresponding high-valent palladium complexes were studied. Herein, we report our continued effects to improve our overall understanding of these high-valent complexes and the roles they may play in catalytic transformations.
First, we investigated the synthesis and utilization of an asymmetric N4 ligand, resulting in the destabilization of the ensuing PdIII complexes. Then we synthesized a series of arene-substituted nickel complexes to study the effect of altering the electronics and sterics of the ligand on the nickel complexes stability and reactivity. The synthesis and reactivity of the first reported dialkyl NiIII complexes was investigated next. These studies along with additional synthesis and characterization of a NiIV complex provided strong evidence for the involvement of NiIV intermediates during nickel catalyzed cross-coupling reactions. Lastly, we investigated the deprotonation of the N4 ligand system in hopes of facilitating various C-H activation reactions. All told, these studies gave us valuable insights into high-valent palladium and nickel complexes relevant to C-C coupling, C-Heteroatom coupling and C-H functionalization reactions. By continuing to improve our understanding of these valuable organic transformations, we can continue to develop more effective catalytic systems
