Synthesis and Electrochemistry of Formazan(ate) Re(I) Complexes: Ligand-Based Reactivity toward CO₂

Re(I) tricarbonyl complexes of the type fac-[ReX(CO)₃(L)]ⁿ (n = -1, 0, +1) (X = Br^-, MeCN), furnished with the redox-active formazan (L = Ph-N(R)-N═CH-N═N-Ph; R = H, (H5^Br); R = Me, (Me5^X)) or formazanate (L = [Ph-N═N-C(-Ph-4-R¹)═N-N-Ph]^-; R¹ = H [1^Br]^-, Me [2^Br]^-, MeO [3^Br]^-, F [4^X]^-; L = [Ph-N═N-C(-H)═N-N-Ph]^-, Py = pyridine 5^Py) ligands were prepared and characterized by spectroscopy and electrochemistry. In situ 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.