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

Reaction of [(XA 2 )U(CH 2 SiMe 3 ) 2 ] (1; XA 2 = 4,5-bis(2,6-diisopropylanilido)-2,7-di- tert -butyl-9,9-dimethylxanthene) with 1 equivalent of [Ph 3 C][B(C 6 F 5 ) 4 ] in arene solvents afforded the arene-coordinated uranium alkyl cations, [(XA 2 )U(CH 2 SiMe 3 )(η n -arene)][B(C 6 F 5 ) 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 [(XA 2 )U(CH 2 SiMe 3 )] + cation follow the order: toluene ≈ benzene > bromobenzene > fluorobenzene. Compounds 2-4 generated in C 6 H 5 R (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 [(XA 2 )An(CH 2 SiMe 3 ) 2 ] {M = U (1) or Th (1-Th)} with [Ph 3 C][B(C 6 F 5 ) 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'.