Issue 32, 2018

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 FeIII/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 FeIII/II couples (ΔE1/2) is between 112 mV and 146 mV, being most pronounced with the electron rich Cp*2Ti(C2Fc)2CuBr. The ΔE1/2 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 ΔE1/2 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 FeII/FeIII 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 FeII/FeIII IVCT. Possible contributions to ΔE1/2 from inductive effects and a superexchange mechanism are discussed.

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

Supplementary files

Article information

Article type
Paper
Submitted
08 May 2018
Accepted
10 Jul 2018
First published
11 Jul 2018

Dalton Trans., 2018,47, 10953-10964

Author version available

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

J. A. Pienkos, A. B. Webster, E. J. Piechota, A. D. Agakidou, C. D. McMillen, D. Y. Pritchett, G. J. Meyer and P. S. Wagenknecht, Dalton Trans., 2018, 47, 10953 DOI: 10.1039/C8DT01853G

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