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.

Is Indenyl a Stronger or Weaker Electron Donor Ligand than Cyclopentadienyl? Opposing Effects of Indenyl Electron Density and Ring Slipping on Electrochemical Potentials

“…As has been observed previously for R Cp 2 Ti(C 2 Fc) 2 complexes, 16 coordination of CuBr to Ind 2 Ti(C 2 Fc) 2 stabilizes the complex toward oxidation, resulting in the observation of two separate, chemically reversible, 1e − oxidations in the cyclic voltammograms (Figure 8, Table 1). The absence of an anodic feature in the analogous voltammogram for Cp 2 Ti-(C 2 Ph) 2 CuBr 16 precludes ascribing either feature in Ind 2 Ti-(C 2 Fc) 2 CuBr to a Cu II/I redox couple. Thus, these are ascribed to two separate Fe III/II couples, indicating some degree of electronic communication between the two redox-active sites.…”

“…As with other complexes in this series, the coordination of CuBr results in a slight redshift and attenuation of the molar absorptivity. 16 Similar behavior is observed in THF (Supporting Information, Figure S5).…”

“…The majority of features observed for Ind 2 Ti(C 2 Fc) 2 and Ind 2 Ti(C 2 Fc) 2 CuBr have been observed for other complexes in the R Cp 2 Ti(C 2 Fc) 2 series. 15,16 These are briefly discussed, followed by a focus on the peculiar nature of the redox potentials of the first oxidation and reduction of the parent Ind 2 Ti(C 2 Fc) 2 . Like all of the complexes in this series, oxidation of Ind 2 Ti(C 2 Fc) 2 is followed by rapid decomposition on the electrochemical time scale to give the electrochemically active FcC 4 Fc.…”

“…Previous reports have interpreted the 2e − wave for these previously characterized R Cp 2 Ti(C 2 Fc) 2 complexes in terms of there being no electronic communication between the two Fc termini, and no intermediate decomposition step. 15,16,23,24 Regardless of the nature of this difference in behavior, the potentials of the first oxidation step for each complex can be compared. For the Cp*, 15 Ind, and Cp 15 complexes, the Fe III/II potentials are −0.22, −0.21, and −0.08 V, respectively.…”

Synthesis and characterization of a tert-butyl ester-substituted titanocene dichloride: t-BuOOCCp2TiCl2

“…One architecture used in compounds of this type is the Donor-bridge-Acceptor (D--A) architecture, where absorption of a photon results in the transfer of charge from an electron-rich donor portion of the molecule to an electron-poor acceptor portion through a conjugatedlinkage (Ji et al, 2018). Alkynyl titanocenes utilizing titanocene acceptors and ferrocenyl or arylamine donors are promising candidates for sensitizers in DSSCs (Turlington et al, 2016;Pienkos et al, 2016Pienkos et al, , 2018Livshits et al, 2019). In photovoltaic technologies, the sensitizer must be attached to a semiconductor substrate, commonly TiO 2 , using an anchoring group exhibiting a high binding affinity for the substrate (Zhang & Cole, 2015;Kalyanasundaram & Grä tzel, 1998).…”