Redox-active zwitterionic ZCp ligands for stable RhI/IrI complexes and hyperpolarisation applications

We report the synthesis and characterisation of stable RhI and IrI 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 by single-crystal X-ray diffraction. Electrochemical studies revealed a reversible one-electron reduction for the Ir complex and DFT analysis demonstrated substantial spin delocalisation on the IZCp ligand, evidencing its non-innocent redox character. Beyond fundamental redox chemistry, we evaluated the Ir complex for parahydrogen-based nuclear spin hyperpolarisation. Parahydrogen-induced polarisation (PHIP) through catalytic hydrogenation of styrene was demonstrated, showing that ZCp–Ir complexes can serve as functional platforms for parahydrogen-based hyperpolarisation. These findings establish zwitterionic Cp ligands as a new platform for stabilising low-valent group 9 metallocenes and for tuning their redox-responsive electronic structures.