Interplay of electronic and geometric structure on Cu phenanthroline, bipyridine and derivative complexes, synthesis, characterization, and reactivity towards oxygen

Copper complexes are highly versatile species that find promising uses in broad fields such as catalysis, drug design, and bioinorganic applications. However, copper complexes display a rich chemical behavior in solution due to their oxidation-state dependent geometrical rearrangements that make it daunting to the inexperienced. This review summarizes the advances in the synthesis of copper complexes containing phenanthroline- and bipyridine-like ligands covering from the electronic aspects of Cu(I) and Cu(II) metal ions to the intricate mechanisms of helicate formation and generation of reactive oxygen species (ROS). To illustrate the complexity of Cu complexes and strategies to address it, we will focus on three main aspects of their chemistry: 1) their electronic/geometric structure relationships, 2) the formation and behavior of helicates, and 3) oxygen reaction chemistry with copper complexes. In the first case, g values from EPR spectroscopy are discussed as a powerful method to elucidate the molecular structure of different copper complexes using crystal field approximations and the R parameter. Also, we correlate these with the geometries and the electronic properties observed for different copper species. In the second case, we cover structural changes observed in helicates from the synthesis of tetracoordinate ligands to trimetallic species. We consider relevant equilibria in the helicates, and we discuss the chiral separation of diastereosimers and their properties. In addition, we summarize oxygen reduction by copper(I) complexes and the oxygen adducts determined for hydroxyl and peroxyl radicals in different solvents. Their water oxidation chemistry and the concept of "autoreduction" that many copper complexes have is addressed. Lastly, we revised photoinduced charge transfer activates the electron transfer mechanisms proposed for copper complexes onto support TiO2 receiving charge injection from copper photosensitizers for use in solar cells. We describe the generation of reactive oxygen species and their detection by EPR. While the reader may find different challenges in their research on coordination chemistry, these three illustrations provide solid examples of how ligand field theory, crystal structure, and spin characterization are used to solve concrete problems posed by Cu chemistry. We hope this review will help jumpstart the novice chemist into the fascinating chemistry of Cu complexes and provide new discussion and conceptual tools to more advanced researchers.