Localized Class-II Mixed Valency in Zwitterionic Diiron Complexes Containing [Cp*(dppe)Fe]n+ Units and an Internal Carboxylate: Experimental Validation and Theoretical Insights into Solvent-Stabilized Charge Trapping

Zwitterionic mixed-valence (MV) complexes offer a compelling strategy for designing charge-neutral molecular components for quantum-dot cellular automata (QCA). In this work, organometallic zwitterions 1a and 1b featuring two [Cp*­(dppe)­Fe] redox centers linked by a meta-phenylene ethynylene bridge and an internal carboxylate were generated and characterized. A comprehensive investigation combining cyclic voltammetry, IR, and vis–NIR spectroscopies unambiguously establishes that both 1a and 1b behave as charge-localized Robin–Day class-II MV systems. This crucial finding demonstrates that the covalently tethered counterion does not significantly alter the weak electronic communication inherent to the meta-substituted bridge. To rationalize these experimental results, density functional theory (DFT) calculations were performed. It is shown that while simplified gas-phase models inadequately predict a delocalized state, calculations incorporating a polarizable continuum model (PCM–CH2Cl2) successfully reproduce the experimentally observed charge localization. The agreement between the experimental data and the solvated theoretical model provides robust validation for the class-II description and confirms this zwitterionic design as a viable approach for creating charge-neutral molecular wires.