Inner-sphere vs. outer-sphere reduction of uranyl supported by a redox-active, donor-expanded dipyrrin

Pankhurst, J. R., Bell, N. L. , Zegke, M., Platts, L. N., Lamfsus, C. A., Maron, L., Natrajan, L. S., Sproules, S. , Arnold, P. L. and Love, J. B. (2017) Inner-sphere vs. outer-sphere reduction of uranyl supported by a redox-active, donor-expanded dipyrrin. Chemical Science, 8(1), pp. 108-116. (doi: 10.1039/C6SC02912D) (PMID:28451154) (PMCID:PMC5304617)

Abstract

The uranyl(VI) complex UO2Cl(L) of the redox-active, acyclic diimino-dipyrrin anion, L− is reported and its reaction with inner- and outer-sphere reductants studied. Voltammetric, EPR-spectroscopic and X-ray crystallographic studies show that chemical reduction by the outer-sphere reagent CoCp2 initially reduces the ligand to a dipyrrin radical, and imply that a second equivalent of CoCp2 reduces the U(VI) centre to form U(V). Cyclic voltammetry indicates that further outer-sphere reduction to form the putative U(IV) trianion only occurs at strongly cathodic potentials. The initial reduction of the dipyrrin ligand is supported by emission spectra, X-ray crystallography, and DFT; the latter also shows that these outer-sphere reactions are exergonic and proceed through sequential, one-electron steps. Reduction by the inner-sphere reductant [TiCp2Cl]2 is also likely to result in ligand reduction in the first instance but, in contrast to the outer-sphere case, reduction of the uranium centre becomes much more favoured, allowing the formation of a crystallographically characterised, doubly-titanated U(IV) complex. In the case of inner-sphere reduction only, ligand-to-metal electron-transfer is thermodynamically driven by coordination of Lewis-acidic Ti(IV) to the uranyl oxo, and is energetically preferable over the disproportionation of U(V). Overall, the involvement of the redox-active dipyrrin ligand in the reduction chemistry of UO2Cl(L) is inherent to both inner- and outer-sphere reduction mechanisms, providing a new route to accessing a variety of U(VI), U(V), and U(IV) complexes.

• Item Type: Articles
• Additional Information: The authors thank the University of Edinburgh and the EPSRC for financial support. <br>A correction to this article is available at http://dx.doi.org/10.1039/c6sc90072k.</br>
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Sproules, Dr Stephen and Bell, Dr Nicola
• Authors: Pankhurst, J. R., Bell, N. L., Zegke, M., Platts, L. N., Lamfsus, C. A., Maron, L., Natrajan, L. S., Sproules, S., Arnold, P. L., and Love, J. B.
• College/School: College of Science and Engineering > School of Chemistry
• Journal Name: Chemical Science
• Publisher: Royal Society of Chemistry
• ISSN: 2041-6520
• ISSN (Online): 2041-6539
• Published Online: 28 October 2016
• Copyright Holders: Copyright © 2017 The Royal Society of Chemistry
• First Published: First published in Chemical Science 8(1):108-116
• Publisher Policy: Reproduced under a Creative Commons License

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