Deconvoluting the Innocent vs. Non-innocent Behavior of N,N-diethylphenylazothioformamide Ligands with Copper Sources

Redox-active ligands lead to ambiguity in often clearly defined oxidation states of both the metal centre and the ligand. The arylazothioformamide (ATF) ligand class represents a redox-active ligand with three possible redox states (neutral, singly reduced, and doubly reduced). ATF-metal interactions result in strong colorimetric transitions allowing for the use of ATFs in metal detection and/or separations. While previous reports have discussed dissolution of zerovalent metals, the resulting oxidation states of coordination complexes have proved difficult to interpret through X-ray crystallographic analysis alone. This report describes the X-ray crystallographic analysis combined with computational modelling of the ATF ligand and metal complexes to deconvolute the metal and ligand oxidation state of metal-ATF complexes. Metal(ATF)₂ complexes that originated from zerovalent metals were found to exist as dicationic metal centers containing two singly reduced ATF ligands. When employing Cu(I) salts instead of Cu(0) to generate copper-ATF complexes, the resulting complexes remained Cu(I) and the ATF ligand remained "innocent", existing in its neutral state. Although the use of CuX (where X = Br or I) or [Cu(NCMe)₄]Y (where Y = BF₄ or PF₆) generated species of the type: [(ATF)Cu(μ-X)]₂ and [Cu(ATF)₂]Y, respectively, the ATF ligand remained in its neutral state for each species type.