Thorium(iv) trialkyl complexes of non-carbocyclic ligands as highly active isoprene polymerisation catalysts

N-Aryloxide-Amidinate Thorium Complexes

An N-aryloxide-amidine ligand (1), [ONNO] ligand, integrating phenoxide (PhO-) and amidine ligands through methylene linkers, was employed in actinide chemistry. Upon reaction of the deprotonated ligand with ThCl4(DME)2 in ether, the corresponding dimer complex 2 was obtained. Upon treatment of 2 with KCp* (Cp* = Cp(Me)5) in tetrahydrofuran, the corresponding {[ONNO]ThIVCp*(LiCl)}2 (4) was obtained. In complex 2, the two ArO- arms bonded from the same ligand to different ThIV centers. In contrast, both ArO- arms coordinated to the same metal center in 4. Notably, when a mixture of 2 and bipyridine was treated with one or two equiv of KC8, the [ONNO]ThIV-bipyridyl•̅ radical dimer complex (5) and [ONNO]ThIV-bipyridyl2- dianionic dimer species (6) were obtained, respectively.

Rare earth dialkyl cations and monoalkyl dications supported by a rigid neutral pincer ligand: synthesis and ethylene polymerization

A palladium-catalyzed coupling reaction between 4,5-dibromo-2,7-di-tert-butyl-9,9-dimethylxanthene and 2 equiv. of 1,3-diisopropylimidazolin-2-imine afforded the rigid neutral 2,7-di-tert-butyl-4,5-bis(1,3-diisopropylimidazolin-2-imino)-9,9-dimethylxanthene (XII2) pincer ligand. Reaction of XII2 with YCl3(THF)3.5 provided [(XII2)YCl3] (1). However, compound 1 failed to react cleanly with 3 equiv. of LiCH2SiMe3, and the reaction of XII2 with [Y(CH2SiMe3)3(THF)2] afforded a complex mixture of products. To access group 3 alkyl complexes without the intermediacy of [(XII2)M(CH2SiMe3)3], the XII2 ligand was protonated using [H(OEt2)2][B(C6F5)4] to form [H(XII2)][B(C6F5)4], and subsequent reaction with [M(CH2SiMe3)3(THF)2] (M = Y, Sc) directly afforded the cationic scandium and yttrium dialkyl complexes [(XII2)M(CH2SiMe3)2][B(C6F5)4] {M = Y (2) and Sc (3)}. Reaction of 3 with B(C6F5)3 in C6D5Br afforded dicationic [(XII2)Sc(CH2SiMe2CH2SiMe3)][MeB(C6F5)3][B(C6F5)4] (4) featuring a CH2SiMe2CH2SiMe3 ligand, formed as a result of methyl anion abstraction from silicon, with concomitant migration of the neighbouring CH2SiMe3 group from scandium to silicon. The MeB(C6F5)3 anion in 4 forms a contact ion pair. By contrast, reaction of 1 with [CPh3][B(C6F5)3] in C6D5Br/toluene or o-C6H4F2/toluene afforded dicationic [(XII2)Sc(CH2SiMe3)(ηx-toluene)n][B(C6F5)4]2 (5). Compounds 2-4 showed negligible ethylene polymerization activity, whereas 5 is highly active (up to 870 kg mol-1 h-1 atm-1 in o-C6H4F2/toluene under 1 atm of ethylene at room temperature).

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 [(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)(η ⁿ -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'.