Uranium(IV) Alkyl Complexes of a Rigid Dianionic NON-Donor Ligand: Synthesis and Quantitative Alkyl Exchange Reactions with Alkyllithium Reagents

A Synthetic, Structural, Spectroscopic, and Computational Study of Alkali Metal-Thioether, -Selenoether, and -Telluroether Interactions

The rigid thioether- and selenoether-containing pro-ligands, 4,5-bis(phenylsulfido)-2,7,9,9-tetramethylacridan (H[AS2Ph2] (1)) and 4,5-bis(phenylselenido)-2,7,9,9-tetramethylacridan (H[ASe2Ph2] (2)), were deprotonated with one equiv of nBuLi to afford dimeric lithium complexes [Li(AE2Ph2)]2 (E = S (3), Se (4)) or with one equiv of KCH2Ph to afford the previously reported potassium complexes [K(AS2Ph2)(dme)]x (5) and [K(ASe2Ph2)(dme)2] (6). Attempts to prepare a direct telluroether analogue of compounds 1-2 were unsuccessful. However, the bulky selenoether- and telluroether-containing pro-ligands 4,5-bis(2,4,6-triisopropylphenylselenido)-2,7,9,9-tetramethylacridan (H[ASe2Tripp2] (7)) and 4,5-bis(2,4,6-triisopropylphenyltellurido)-2,7,9,9-tetramethylacridan (H[ATe2Tripp2] (8)) were accessed via the reaction of 4,5-dibromo-2,7,9,9-tetramethylacridan with three equiv of nBuLi, followed by the addition of two equiv of the corresponding diaryl dichalcogenide and quenching with dilute HCl(aq). The new selenoether- and telluroether-containing pro-ligands were subsequently deprotonated using KCH2Ph to afford [K(AE2Tripp2)(dme)2] (E = Se (9), Te (10)). Compounds 1-10 were characterized by 1H, 13C{1H}, 77Se{1H}, 125Te{1H}, and 7Li NMR spectroscopy, where applicable, and single-crystal X-ray structures were obtained for all lithium and potassium complexes (3-6 and 9-10). DFT calculations were also performed to assess the nature of bonding between the hard group 1 cations and the soft chalcogenoethers.

Synthesis, Characterization, and Reduction of Thorium Pyridinediimine Complexes

A family of thorium complexes featuring the redox-noninnocent pyridinediimine ligand MesPDIMe was synthesized, including (MesPDIMe)ThCl4 (1-Th), (MesPDIMe)ThCl3(THF) (2-Th), (MesPDIMe)ThCl2(THF)2 (3-Th) and [(MesPDIMe)Th(THF)]2 (5-Th) Full characterization of these species shows that these complexes feature MesPDIMe in four different oxidation states. The electronic structures of these complexes have been explored using 1H NMR and electronic absorption spectroscopies, X-ray crystallography, and SQUID magnetometry where appropriate.

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'.

Synthesis and characterization of group 13 dichloride (M = Ga, In), dimethyl (M = Al) and cationic methyl aluminum complexes supported by monoanionic NNN-pincer ligands

A series of monoanionic NNN-pincer ligands effectively stabilize five-coordinate gallium and indium dichloride complexes, as well as neutral dimethyl aluminum species, and organometallic cations thereof.

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

A series of mono-anionic non-carbocyclic ligands, including bidentate benzamidinate [PhC(NDipp)2] (Dipp = C6H3-2,6-iPr2), iminophosphinamide [Ph2P(NDipp)2] and phosphinoamide [Ph2PNDipp], and tridentate hydrotris(3,5-dimethyl-1-pyrazolyl)borate (TpMe2) were used to stabilize the corresponding thorium(iv) trialkyl complexes [PhC(NDipp)2]Th(CH2SiMe3)3 (1), [Ph2P(NDipp)2]Th(CH2SiMe3)3 (2), [Ph2P(NDipp)]Th(p-CH2-C6H4-Me)3 (3) and (TpMe2)Th(CH2SiMe3)3 (4), which were characterized by NMR spectroscopy and single-crystal X-ray analysis. Complexes 1-4 in combination with [Ph3C][B(C6F5)4] and AliBu3 form non-Cp-ligated actinide catalyst systems to show high activity and high cis-1,4-selectivity (89.9%) or trans-1,4-selectivity (91.4%) for the polymerization of isoprene. The reaction rate and selectivity of complexes 1 and 2 were controlled by the crowded space around the thorium centre, corroborated by the kinetics of the polymerization and the steric maps.

Thorium(iv) alkyl and allyl complexes of a rigid NON-donor pincer ligand with flanking 1-adamantyl substituents

A rigid new pincer pro-ligand, 4,5-bis(1-adamantylamino)-2,7-di-tert-butyl-9,9-dimethylxanthene, is described, with deprotonation and complexation to afford thorium(iv) chloro, alkyl, and allyl derivatives.

Cyclometalation and coupling of a rigid 4,5-bis(imino)acridanide pincer ligand on yttrium

An extremely rigid NNN-donor proligand, 4,5-bis{(diphenylmethylene)amino}-2,7,9,9-tetramethylacridan, H[AIm2] was prepared in five steps starting from 5-methyl-2-aminobenzoic acid and 4-bromotoluene. Reaction of intensely orange H[AIm2] with LiCH2SiMe3 formed deep blue Li(x)[AIm2]x (x = 2 in the solid state), while reaction with [Y(CH2SiMe3)3(THF)2] (0.5 equiv.) afforded deep blue [Y(AIm2)(AIm)] (1; AIm = an AIm2 ligand cyclometalated at the ortho-position of one of the phenyl rings). Compound 1 slowly isomerizes to form green-brown 2, which contains a single trianionic, hexadentate ligand that features one amine, two imine, and three amido donors. The acridanide backbone and one imine group in each of the original AIm2 ligands is intact, but the two acridanide backbones are now linked by an isoindoline heterocycle. Yttrium in 2 is coordinated to six nitrogen donors and the ortho carbon of an isoindoline phenyl substituent. The intense colours of H[AIm2], Li(x)[AIm2]x and 1 were shown by TD-DFT calculations to arise from charge transfer transitions from the HOMO, which is localized on the acridanide ligand backbone, to the LUMO and LUMO+1, which are localized on the imine substituents. The conversion of 1 to 2 was studied by UV-Visible absorption spectroscopy and is first-order with a half-life of 7.8 hours at room temperature.

Achievements in uranium alkyl chemistry: celebrating sixty years of synthetic pursuits

This Perspective highlights the synthesis, characterization, and reactivity of significant organouranium complexes with σ-bonded alkyl ligands.