Credit ligands for copper-complex chemistry

Copper complexes play important roles in catalysis and biological chemistry, which are often explained by the metal’s reduction from the formal oxidation state Cu(III) to Cu(I). But for decades, chemists have argued over whether those oxidation numbers really reflect the complexes’ electronic environments. Kyle M. Lancaster of Cornell University and colleagues have opened a new front in the so-called copper wars, using spectroscopy and density functional theory to show that the chemistry of more than a dozen formally Cu(III) complexes relies less on metal oxidation and reduction and more on the ligands’ electronic characteristics (J. Am. Chem. Soc. 2019, DOI: 10.1021/jacs.9b09016). Their analysis found that the molecular orbitals involved in the reactions mediated by these complexes take their character mostly from the ligands. Lancaster says these ligand contributions can help explain why Cu(phen)(CF₃)₃ is unreactive, but alkyl or imido copper complexes are reactive. He hopes that better understanding these complexes’ electronic structures will enable chemists to tune the species’ reactivity through ligand choice. Lancaster’s results are unlikely to be the last word in this debate, but his group is already moving on to nickel and cobalt species.