IZCp and PZCp: Redox Non-innocent Cyclopentadienyl Lig-ands as Electron Reservoirs for Sandwich Complexes

Numerous developments in the electronic and steric modification of cyclopentadienyl (Cp) ligands have led to a corresponding growth in their applications. Herein, we present two new redox non-innocent Cp compounds, imidazolium- and pyrrolinium-substituted zwitterionic Cps (IZCp 1a and PZCp 1b). The Cps were obtained in high yields and could be handled easily owing to their high stability in air and moisture, unlike other known Cps. The redox non-innocent behavior of the Cp ligands was demonstrated via experimental and computational analyses of their half-sandwich and sandwich complexes. During cyclic voltammetry, the molybdenum half-sandwich (3a, 3b) and ruthenium sandwich ([4a]PF6, [4b]PF6) complexes of IZCp and PZCp showed reversible one-electron reduction at a low potential (E1/2 ranging from −1.7 to −2.7 V vs. Fc/Fc+). This phenomenon has not been studied previously because of the high instability of the reduced complexes. DFT calculations for the reduced complexes ([3a]‾, [3b]‾, 4a, and 4b) confirmed that their spin density was mainly delocalized over the ligand center (70%–90%). EPR analysis of the isolated K[3b] and 4b indicated their ligand-centered radical structures. Furthermore, the CO stretching frequencies of K[3b] (νCO 1871.35, 1748.10, and 1698.80 cm−1) revealed that the redox non-innocent [PZCp]‾ ligand is the strongest electron-donating Cp ligand among previously reported CpMo(CO)3 derivatives (νCO > 1746 cm−1).