Multi-electron reduction of sulfur and carbon disulfide using binuclear uranium(iii) borohydride complexes

Arnold, P. L., Stevens, C. J., Bell, N. L. , Lord, R. M., Goldberg, J. M., Nichol, G. S. and Love, J. B. (2017) Multi-electron reduction of sulfur and carbon disulfide using binuclear uranium(iii) borohydride complexes. Chemical Science, 8(5), pp. 3609-3617. (doi: 10.1039/C7SC00382J) (PMID:30155206) (PMCID:PMC6094157)

Abstract

The first use of a dinuclear UIII/UIII complex in the activation of small molecules is reported. The octadentate Schiff-base pyrrole, anthracene-hinged ‘Pacman’ ligand LA combines two strongly reducing UIII centres and three borohydride ligands in [M(THF)4][{U(BH4)}2(μ-BH4)(LA)(THF)2] 1-M, (M = Li, Na, K). The two borohydride ligands bound to uranium outside the macrocyclic cleft are readily substituted by aryloxide ligands, resulting in a single, weakly-bound, encapsulated endo group 1 metal borohydride bridging the two UIII centres in [{U(OAr)}2(μ-MBH4)(LA)(THF)2] 2-M (OAr = OC6H2tBu3-2,4,6, M = Na, K). X-ray crystallographic analysis shows that, for 2-K, in addition to the endo-BH4 ligand the potassium counter-cation is also incorporated into the cleft through η5-interactions with the pyrrolides instead of extraneous donor solvent. As such, 2-K has a significantly higher solubility in non-polar solvents and a wider U–U separation compared to the ‘ate’ complex 1. The cooperative reducing capability of the two UIII centres now enforced by the large and relatively flexible macrocycle is compared for the two complexes, recognising that the borohydrides can provide additional reducing capability, and that the aryloxide-capped 2-K is constrained to reactions within the cleft. The reaction between 1-Na and S8 affords an insoluble, presumably polymeric paramagnetic complex with bridging uranium sulfides, while that with CS2 results in oxidation of each UIII to the notably high UV oxidation state, forming the unusual trithiocarbonate (CS3)2− as a ligand in [{U(CS3)}2(μ-κ2:κ2-CS3)(LA)] (4). The reaction between 2-K and S8 results in quantitative substitution of the endo-KBH4 by a bridging persulfido (S2)2− group and oxidation of each UIII to UIV, yielding [{U(OAr)}2(μ-κ2:κ2-S2)(LA)] (5). The reaction of 2-K with CS2 affords a thermally unstable adduct which is tentatively assigned as containing a carbon disulfido (CS2)2− ligand bridging the two U centres (6a), but only the mono-bridged sulfido (S)2− complex [{U(OAr)}2(μ-S)(LA)] (6) is isolated. The persulfido complex (5) can also be synthesised from the mono-bridged sulfido complex (6) by the addition of another equivalent of sulfur.

• Item Type: Articles
• Additional Information: We thank EaStCHEM, the University of Edinburgh and the Engineering and Physical Sciences Research Council EPSRC, grants EP/H004823/1 and EP/M010554/1, and the European COST network CM1205. We thank Dr Markus Zegke for additional X-ray crystallographic analysis. We thank the NSF CCI Center for Enabling New Technologies through Catalysis (CENTC, CHE-1205189) and the EPSRC Centre for Doctoral Training in Critical Resource Catalysis (CRITICAT, Grant code EP/L016419/1) for funding the researcher exchange visit (JMG). PLA also thanks the Technische Universit¨at Munchen ¨ – Institute for Advanced Study, funded by the German Excellence Initiative.
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Bell, Dr Nicola
• Authors: Arnold, P. L., Stevens, C. J., Bell, N. L., Lord, R. M., Goldberg, J. M., Nichol, G. S., 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: 10 March 2017
• Copyright Holders: Copyright © 2017 The Royal Society of Chemistry
• First Published: First published in Chemical Science 8(5): 3609-2617
• Publisher Policy: Reproduced under a Creative Commons License