Synthesis and Electrochemistry of Formazan(ate) Re(I) Complexes: Ligand-Based Reactivity toward CO ₂

Re(I) tricarbonyl complexes of the type <i>fac</i>-[ReX(CO)₃(L)]^<i>n</i> (<i>n</i> = -1, 0, +1) (X = Br^-, MeCN), furnished with the redox-active formazan (L = Ph-N(R)-N═CH-N═N-Ph; R = H, (<b>H5</b>^{<b><i>Br</i></b>}); R = Me, (<b>Me5</b>^{<b><i>X</i></b>})) or formazanate (L = [Ph-N═N-C(-Ph-4-R¹)═N-N-Ph]^-; R¹ = H <b>[1</b>^{<b><i>Br</i></b>}<b>]</b>^<b>-</b>, Me <b>[2</b>^{<b><i>Br</i></b>}<b>]</b>^<b>-</b>, MeO <b>[3</b>^{<b><i>Br</i></b>}<b>]</b>^<b>-</b>, F <b>[4</b>^{<b><i>X</i></b>}<b>]</b>^<b>-</b>; L = [Ph-N═N-C(-H)═N-N-Ph]^-, Py = pyridine <b>5</b>^{<b><i>Py</i></b>}) ligands were prepared and characterized by spectroscopy and electrochemistry. <i>In situ</i> characterization of the reduced species by (spectroelectro)chemical and computational methods revealed that the redox-active scaffold behaves as a two-electron sink, allowing two consecutive one-electron reductions to take place at the ligand. The reactivity of the reduced formazan(ate) rhenium complexes toward CO₂ was explored. (Spectroelectro)chemical experiments along with DFT calculations suggested that CO₂ reacts with the reduced formazanate Re(I) complexes at low overpotentials forming a carbamate-type adduct. This ligand-based reactivity provides a thermodynamic sink for CO₂ binding and hinders catalytic turnover via metal-centered CO₂ activation. These findings provide new insights into the advantages and limitations of using catalysts with redox-active ligands to activate and convert small molecules.