Oxidatively stable ferrocenyl-π-bridge-titanocene D–π-A complexes: an electrochemical and spectroscopic investigation of the mixed-valent states

Abstract

The synthesis, spectroscopic, and electrochemical characterization of oxidatively stable D–π-A compounds of the form (Me₂CpC₂Fc)₂TiCl₂ and ^RCp₂Ti(C₂Fc)₂CuX (where Fc = ferrocenyl) are reported. Oxidative stability enabled by the addition of CuX is evidenced by voltammagrams of the ^RCp₂Ti(C₂Fc)₂CuX compounds which all display two chemically-reversible 1e⁻ Fe^III/II couples, indicative of electronic communication between the Fc- termini. Differential pulse voltammetry (DPV) in CH₂Cl₂/[n-Bu₄N][PF₆], demonstrated that the redox potential difference between the two 1e⁻ Fe^III/II couples (ΔE_1/2) is between 112 mV and 146 mV, being most pronounced with the electron rich Cp*₂Ti(C₂Fc)₂CuBr. The ΔE_1/2 values were unaffected by solvent (THF) and displayed only a small dependence on the identity of the counterion, either PF₆⁻ or B(C₆F₅)₄⁻. For each complex with a measurable ΔE_1/2 value, spectroelectrochemical experiments were performed in CH₂Cl₂/[n-Bu₄N][PF₆] and gave clear evidence of both the one-electron oxidized mixed-valent (MV) state and the two-electron oxidized state, each with distinct spectroscopic signatures. The MV states of these complexes showed absorbance between 820 and 940 nm which were replaced with a higher energy feature following a second oxidation. A very similar absorption band was also observed in the one-electron oxidized state of an analogue with only a single Fc substituent, namely ^TMSCp₂Ti(C₂Fc)(C₂Ph)CuBr, suggesting this feature is not an Fe^II/Fe^III intravalence charge-transfer (IVCT) band. Despite DFT calculations suggesting a pathway exists for electronic coupling, NIR spectroscopy on the MV states gave no evidence of an Fe^II/Fe^III IVCT. Possible contributions to ΔE_1/2 from inductive effects and a superexchange mechanism are discussed.