Issue 46, 2022

Uranium(iv) alkyl cations: synthesis, structures, comparison with thorium(iv) analogues, and the influence of arene-coordination on thermal stability and ethylene polymerization activity

Abstract

Reaction of [(XA2)U(CH2SiMe3)2] (1; XA2 = 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) with 1 equivalent of [Ph3C][B(C6F5)4] in arene solvents afforded the arene-coordinated uranium alkyl cations, [(XA2)U(CH2SiMe3)(ηn-arene)][B(C6F5)4] {arene = benzene (2), toluene (3), bromobenzene (4) and fluorobenzene (5)}. Compounds 2, 3, and 5 were crystallographically characterized, and in all cases the arene is π-coordinated. Solution NMR studies of 2–5 suggest that the binding preferences of the [(XA2)U(CH2SiMe3)]+ cation follow the order: toluene ≈ benzene > bromobenzene > fluorobenzene. Compounds 2–4 generated in C6H5R (R = H, Me or Br, respectively) showed no polymerization activity under 1 atm of ethylene. By contrast, 5 and 5-Th (the thorium analogue of 5) in fluorobenzene at 20 and 70 °C achieved ethylene polymerization activities between 16 800 and 139 200 g mol−1 h−1 atm−1, highlighting the extent to which common arene solvents such as toluene can suppress ethylene polymerization activity in sterically open f-element complexes. However, activation of [(XA2)An(CH2SiMe3)2] {M = U (1) or Th (1-Th)} with [Ph3C][B(C6F5)4] in n-alkane solvents did not afford an active polymerization catalyst due to catalyst decomposition, illustrating the critical role of PhX (X = H, Me, Br or F) coordination for alkyl cation stabilization. Gas phase DFT calculations, including fragment interaction calculations with energy decomposition and ETS-NOCV analysis, were carried out on the cationic portion of 2′-Th, 2′, 3′ and 5′ (analogues of 2-Th, 2, 3 and 5 with hydrogen atoms in place of ligand backbone methyl and tert-butyl groups), providing insight into the nature of actinide–arene bonding, which decreases in strength in the order 2′-Th > 2′3′ > 5′.

Graphical abstract: Uranium(iv) alkyl cations: synthesis, structures, comparison with thorium(iv) analogues, and the influence of arene-coordination on thermal stability and ethylene polymerization activity

Supplementary files

Article information

Article type
Edge Article
Submitted
02 août 2022
Accepted
31 oct. 2022
First published
10 nov. 2022
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY-NC license

Chem. Sci., 2022,13, 13748-13763

Uranium(IV) alkyl cations: synthesis, structures, comparison with thorium(IV) analogues, and the influence of arene-coordination on thermal stability and ethylene polymerization activity

N. R. Andreychuk, B. Vidjayacoumar, J. S. Price, S. Kervazo, C. A. Peeples, D. J. H. Emslie, V. Vallet, A. S. P. Gomes, F. Réal, G. Schreckenbach, P. W. Ayers, I. Vargas-Baca, H. A. Jenkins and J. F. Britten, Chem. Sci., 2022, 13, 13748 DOI: 10.1039/D2SC04302E

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