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, (H5Br); R = Me, (Me5X)) or formazanate (L = [Ph-N═N-C(-Ph-4-R1)═N-N-Ph]-; R1 = H [1Br]-, Me [2Br]-, MeO [3Br]-, F [4X]-; L = [Ph-N═N-C(-H)═N-N-Ph]-, Py = pyridine 5Py) 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.