Thorium(IV) alkyl synthesis from a thorium(III) cyclopentadienyl complex and an N-heterocyclic olefin

Synthetic Strategies of N-Heterocyclic Olefin (NHOs) and Their Recent Application of Organocatalytic Reactions and Beyond

N-heterocyclic olefin (NHO) derivatives have an electron-rich as well as highly polarized carabon-carbon (C=C) double bond because of the electron-donating nature of nitrogen and sulphur atoms. While NHOs have been developing as novel organocatalysts and ligands for transition-metal complexes in various organic compound syntheses, different research groups are currently interested in preparing imidazole and triazolium-based chiral NHO catalysts. Some of them have been used for enantioselective organic transformations, but were still elusive. N-heterocyclic olefins, the alkylidene derivatives of N-heterocyclic carbenes (NHC), have shown promising results as effective promoters for numerous organic syntheses such as asymmetric catalysis, hydroborylation, hydrosilylation, reduction, CO2 sequestration, alkylation, cycloaddition, polymerization and the ring-opening reaction of aziridine and epoxides, esterification, C-F bond functionalization, amine coupling, trifluoromethyl thiolation, amination etc. NHOs catalysts with suitable structures can serve as a novel class of Lewis/Bronsted bases with strong basicity and high nucleophilicity properties.These facts strongly suggest their enormous chemical potential as sustainable catalysts for a wide variety of reactions in synthetic chemistry. The synthesis of NHOs and their properties are briefly reviewed in this article, along with a summary of the imidazole and triazole core of NHOs' most recent catalytic uses.

Mesoionic Carbene Complexes of Uranium(IV) and Thorium(IV)

We report the synthesis and characterization of uranium(IV) and thorium(IV) mesoionic carbene complexes [An{N(SiMe3)2}2(CH2SiMe2NSiMe3){MIC}] (An = U, 4U and Th, 4Th; MIC = {CN(Me)C(Me)N(Me)CH}), which represent rare examples of actinide mesoionic carbene linkages and the first example of a thorium mesoionic carbene complex. Complexes 4U and 4Th were prepared via a C-H activation intramolecular cyclometallation reaction of actinide halides, with concomitant formal 1,4-proton migration of an N-heterocyclic olefin (NHO). Quantum chemical calculations suggest that the An-carbene bond comprises only a σ-component, in contrast to the uranium(III) analogue [U{N(SiMe3)2}3(MIC)] (1) where computational studies suggested that the 5f3 uranium(III) ion engages in a weak one-electron π-backbond to the MIC. This highlights the varying nature of actinide-MIC bonding as a function of actinide oxidation state. In solution, 4Th exists in equilibrium with the Th(IV) metallacycle [Th{N(SiMe3)2}2(CH2SiMe2NSiMe3)] (6Th) and free NHO (3). The thermodynamic parameters of this equilibrium were probed using variable-temperature NMR spectroscopy yielding an entropically favored but enthalpically endothermic process with an overall reaction free energy of ΔG 298.15K = 0.89 kcal mol-1. Energy decomposition analysis (EDA-NOCV) of the actinide-carbon bonds in 4U and 4Th reveals that the former is enthalpically stronger and more covalent than the latter, which accounts for the respective stabilities of these two complexes.

In search of tris(trimethylsilylcyclopentadienyl) thorium

Reduction of Cp'₃ThCl, Cp'₃ThBr, and Cp'₃ThI (Cp' = C₅H₄SiMe₃) with potassium graphite generates dark blue solutions with reactivity and spectroscopic properties consistent with the formation of Cp'₃Th. The EPR and UV-visible spectra of the solutions are similar to those of crystallographically-characterized tris(cyclopentadienyl) Th(iii) complexes: [C₅H₃(SiMe₃)₂]₃Th, (C₅Me₄H)₃Th, (C₅^tBu₂H₃)₃Th, and (C₅Me₅)₃Th. Density functional theory (DFT) analysis indicates that the UV-visible spectrum is consistent with Cp'₃Th and not [Cp'₃ThBr]^1-. Although single crystals of Cp'₃Th have not been isolated, the blue solution reacts with Me₃SiCl, I₂, and HC[triple bond, length as m-dash]CPh to afford products expected from Cp'₃Th, namely, Cp'₃ThCl, Cp'₃ThI, and Cp'₃Th(C[triple bond, length as m-dash]CPh), respectively. Reactions with MeI give mixtures of Cp'₃ThI and Cp'₃ThMe. Evidence for further reduction of the blue solutions to a Cp'-ligated Th(ii) complex has not been observed. The crystal structures of Cp'₃ThMe and (Cp'₃Th)₂(μ-O) were also determined as part of these studies.

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.