Copolymerization of ethylene and isoprene via silicon bridge metallocene [rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2] catalyst: A new way to control the composition and microstructure of copolymers

The copolymerization of ethylene (E) with isoprene (Ip) was performed catalyzed by a symmetrical catalyst exhibiting a silicon bridge [rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2 with the combination of borate/TIBA activator. The effect of cocatalyst, Ip concentration, and polymerization temperature on the activity, molecular weight (Mw), distribution (MWD), comonomer composition, chain structure (regio- and stereoselectivity), and resulting side reactions were logically addressed. Gel-permeation chromatography (GPC) was used to characterize the Mw and polydispersity, while nuclear magnetic resonance (NMR) was employed for the chain structure of the polymers. The catalytic activity was significantly lower by increasing the Ip concentration in the feed, and the isoprene content in resulting polymers was lower under the reaction condition, leading to higher activity. Insertion of isoprene units in polymer structure demonstrates the higher regioselectivity for the 3,4 connections than the 1,4 connections and is expected to be a high-resistance polymer against acids. The MWD presented monomodal even with a higher concentration (1.44 mol/L) and did not appear as low Mw peaks of Ip. The Mw was higher with a broader MWD when purely TIBA was used as a cocatalyst, and it significantly reduced and presented a narrowed MWD with TEA in the cocatalyst. The higher efficiency of the catalyst for the higher insertion of Ip (C=C double bond) effectively modifies the polymer backbone. It is expected to be a promising candidate for easily degradable and favorable solutions for solving environmental problems caused by PE. wastes.

Catalytic regeneration of metal-hydrides from their corresponding metal-alkoxides via the hydroboration of carbonates to obtain methanol and diols

Thorium complexes decorated with 5-, 6-, and 7-membered N-heterocyclic iminato ligands containing mesityl wingtip substitutions have been synthesized and fully characterized. These complexes were found to be efficient in the hydroboration of cyclic and linear organic carbonates with HBpin or 9-BBN promoting their decarbonylation and producing the corresponding boronated diols and methanol. In addition, the hydroboration of CO2 breaks the molecule into "CO" and "O" forming boronated methanol and pinBOBpin. Moreover, the demanding depolymerization of polycarbonates to the corresponding boronated diols and methanol opens the possibility of recycling polymers for energy sources. Increasing the core ring size of the ligands allows a better performance of the complexes. The reaction proceeds with high yields under mild reaction conditions, with low catalyst loading, and short reaction times, and shows a broad applicability scope. The reaction is achieved via the recycling of a high-energy Th-H moiety from a stable Th-OR motif. Experimental evidence and DFT calculations corroborate the formation of the thorium hydride species and the reduction of the carbonate with HBpin to the corresponding Bpin-protected alcohols and H3COBpin through the formate and acetal intermediates.

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'.

A Sacrificial Iminato Ligand in the Catalytic Cyanosilylation of Ketones Promoted by Organoactinide Complexes

Four new complexes containing the bis(pentamethylcyclopentadienyl)thorium(IV) moiety, Cp*₂Th(L1)(Me) (Th2), Cp*₂Th(L2)(Me) (Th3), Cp*₂Th(L1)Cl (Th5), and Cp*₂Th(L2)Cl (Th6), were synthesized in quantitative yields via the protonolysis reaction of the metallocene precursor complexes Cp*₂Th(Me)₂ (Th1) and Cp*₂Th(Me)Cl (Th4) and the respective six- and seven-membered N-heterocyclic neutral imine ligands L1H and L2H. The molecular structures of all the complexes were established by single-crystal X-ray structure analyses. The synthesized complexes along with the precursor complexes were employed as catalysts for the cyanosilylation reaction of ketones with trimethylsilyl cyanide (Me₃SiCN). The removal of the iminato ligand is necessary to trigger the reaction, allowing the formation of the active catalyst.

Recent developments in highly basic N-heterocyclic iminato ligands in actinide chemistry

In the last decade, major conceptual advances in the chemistry of actinide molecules and materials have been made to demonstrate their distinct reactivity profiles as compared to lanthanide and transition metal compounds, but some difficult questions remain concerning the intriguing stability of low-valent actinide complexes, and the importance of the 5f-orbitals in reactivity and bonding. The imidazolin-2-iminato moiety has been extensively used in ligands for the advancement of actinide chemistry owing to its unique capability of stabilizing the reactive and highly electrophilic metal ions by virtue of its strong electron donation and steric tunability. The current review article describes recent developments in the chemistry of light actinide metal ions (thorium and uranium) bearing these N-heterocyclic iminato moieties as supporting ligands. In addition, the effect of ring expansion of the N-heterocycle on the catalytic aptitude of the organoactinides is also described herein. The synthesis and reactivity of actinide complexes bearing N-heterocyclic iminato ligands are presented, and promising apposite applications are also presented. The current review focuses on addressing the catalytic behavior of actinide complexes with oxygen-containing substrates such as in the Tishchenko reaction, hydroelementation processes, and polymerization reactions. Actinide complexes have also found new catalytic applications, as demonstrated by the potent chemoselective carbonyl hydroboration and tandem proton-transfer esterification (TPTE) reaction, featuring coupling between an aldehyde and alcohol.

Catalytic regeneration of a Th-H bond from a Th-O bond through a mild and chemoselective carbonyl hydroboration

Here we present an unprecedented chemoselective hydroboration for aldehydes and ketones catalysed by actinides. The reaction features a very low catalyst loading (0.1-0.004 mol%) and quantitative product formation in less than 15 minutes, at room temperature. Thermodynamic and kinetic studies including stoichiometric and labeling studies with deuterated pinacolborane allow us to propose a plausible mechanism for this remarkable catalytic regeneration of a Th-H bond via carbonyl hydroboration.

Organoactinides in catalytic transformations: scope, mechanisms and Quo Vadis

The last decade has witnessed brilliant and remarkable advances in the chemistry of the early actinides in stoichiometric and in challenging catalytic processes. This canvas of knowledge allows the design of chemical reactivities reaching a high level of sophistication. This review highlights the latest results obtained since 2008 on those catalytic processes.

Thorium complexes possessing expanded ring N-heterocyclic iminato ligands: synthesis and applications

Novel thorium complexes containing six- and seven-membered rings iminato moieties are disclosed. The complexes are highly active for the Tishchenko and cross-Tishchenko reaction.

Dimethylsilyl bis(amidinate)actinide complexes: synthesis and reactivity towards oxygen containing substrates

Ligand1reacts with ThCl₄and UCl₄yielding complexes2and4, respectively. Complex3is obtained from complex2displaying extremely short Th–OH bond distances.