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

• Published in: Dalton transactions : an international journal of inorganic chemistry 2018, Vol.47 (32), p.1953-1964
• Main Authors: Pienkos, Jared A, Webster, Alexander B, Piechota, Eric J, Agakidou, A. Danai, McMillen, Colin D, Pritchett, David Y, Meyer, Gerald J, Wagenknecht, Paul S
• Format: Article
• Language: English
• Subjects: Absorption spectra; Charge transfer; Couples; Dependence; Dichloromethane; Electrochemical analysis; Electrons; Organic chemistry; Oxidation; Spectroscopy
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c8dt01853g

The synthesis, spectroscopic, and electrochemical characterization of oxidatively stable D-π-A compounds of the form (Me 2 CpC 2 Fc) 2 TiCl 2 and R Cp 2 Ti(C 2 Fc) 2 CuX (where Fc = ferrocenyl) are reported. Oxidative stability enabled by the addition of CuX is evidenced by voltammagrams of the R Cp 2 Ti(C 2 Fc) 2 CuX 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 CH 2 Cl 2 /[ n -Bu 4 N][PF 6 ], demonstrated that the redox potential difference between the two 1e − Fe III/II couples (Δ E 1/2 ) is between 112 mV and 146 mV, being most pronounced with the electron rich Cp* 2 Ti(C 2 Fc) 2 CuBr. The Δ E 1/2 values were unaffected by solvent (THF) and displayed only a small dependence on the identity of the counterion, either PF 6 − or B(C 6 F 5 ) 4 − . For each complex with a measurable Δ E 1/2 value, spectroelectrochemical experiments were performed in CH 2 Cl 2 /[ n -Bu 4 N][PF 6 ] 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 TMS Cp 2 Ti(C 2 Fc)(C 2 Ph)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 Δ E 1/2 from inductive effects and a superexchange mechanism are discussed. Coordination of Cu I into R Cp 2 Ti(C 2 Fc) 2 compounds results in well-separated Fe III/II couples and mixed-valent states with distinct spectroscopic signatures.