Redox-Active Zwitterionic ZCp Ligands for Stable Rh(I)/Ir(I) Complexes and Hyperpolarization Applications

We report the synthesis and characterization of stable Rh(I) and Ir(I) half-sandwich complexes supported by redox-active zwitterionic cyclopentadienyl (ZCp) ligands. Using imidazolium-substituted ZCp (IZCp), we isolated well-defined complexes, [ZCpM(COD)]BF4 (M = Rh, Ir), and confirmed their structures via single-crystal X-ray diffraction. Electrochemical studies revealed a reversible one-electron reduction for the Ir complex, and DFT analysis demonstrated substantial spin delocalization on the IZCp ligand, evidencing its non-innocent redox character. Beyond fundamental redox chemistry, we evaluated the Ir complex for parahydrogen-based nuclear spin hyperpolarization. Parahydrogen-induced polarization (PHIP) through catalytic hydrogenation of styrene was demonstrated, showing that ZCp–Ir complexes can serve as functional platforms for parahydrogen-based hyperpolarization. These findings establish zwitterionic Cp ligands as a new platform for stabilizing low-valent group 9 metallocenes and for tuning their redox-responsive electronic structures.