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

Nicholas Andreychuk (1) , Balamurugan Vidjayacoumar (1) , Jeffrey Price (1) , Sophie Kervazo (2, 1) , Craig Peeples (1) , David Emslie (1) , Valérie Vallet (2) , André Severo Pereira Gomes (2) , Florent Réal (2) , Georg Schreckenbach (3) , Paul Ayers (1) , Ignacio Vargas-Baca (1) , Hilary Jenkins (1) , James Britten (1)

1 Department of Chemistry, McMaster University
2 PCMT - Physico-Chimie Moléculaire Théorique
3 University of Manitoba [Winnipeg]

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)(hn-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'.

Domaines: Chimie de coordination; Catalyse; Cristallographie; Polymères; Chimie théorique et/ou physique

Nicholas Andreychuk, Balamurugan Vidjayacoumar, Jeffrey Price, Sophie Kervazo, Craig Peeples, et al.. Uranium(IV) alkyl cations: synthesis, structures, comparison with thorium(IV) analogues, and the influence of arene-coordination on thermal stability and ethylene polymerization activity. Chemical Science, 2022, 13, pp.13748-13763. ⟨10.1039/D2SC04302E⟩. ⟨hal-03847721⟩