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

Abstract

We report the synthesis and characterization of uranium(IV) and thorium(IV) mesoionic carbene complexes [An{N(SiMe 3 ) 2 } 2 (CH 2 SiMe 2 NSiMe 3 ){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(SiMe 3 ) 2 } 3 (MIC)] ( 1 ) where computational studies suggested that the 5f 3 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(SiMe 3 ) 2 } 2 (CH 2 SiMe 2 NSiMe 3 )] ( 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 Δ G 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.

Synthesis and Characterization of a Series of U(IV) trans-bis(Wittig) Adducts Across the UX4 Halide Series

“…2.637(9) (NHO)An(NR2)(CH2Me2SiNSiMe3) 13 2.62(1) Cp * 2U=NMes(CH2PPh3) 19 2.617(5) [(Ph3P)2C]UCl4 14 2.411(3) paramagnetic nature of the 5f 2 uranium ion, the halides clearly play a major role in the polarization of the U-CH2=PAr3 bonds that manifests in the observed nuclear shielding effects. This supports the notion that Lewis acidity of the uranium center directly impacts the U-CWittig interaction as the complex with the most Lewis acidic metal center, UI4(CH2PAr3)2, displays the most deshielded chemical shifts values for the -CH2…”

“…Cp*2U(O)(IMe4) (U-CNHC = 2.637(9) Å), 9 (η 5 -C5H4 t Bu)3U(IMe4) (U-CNHC = 2.768(5) Å), 10 Cp*2UI(IMe4) (U-CNHC = 2.687(5) Å), 10 (IPr)2UCl4 (U-CNHC = 2.675(7) -2.687(7) Å), 11 and 13 but longer than the uranium-carbone [(Ph3P)2C]UCl4 (2.411(3) Å) that is described as possessing double-dative U=C bonding character (Table 2). 14 The comparison to other uranium-Wittig and uranium-NHC complexes (Table 2) reveal that the uraniumcarbon bonds in the bis(Wittig) adducts UX4(CH2PAr3)2 are the shortest among this class of molecules, with a general correlation between the π-donor strength of the halide ligand and the decrease in the length of the uranium-carbon bond.…”

“…Liddle and coworkers reported that the bis(NHC) complex (IPr)2UCl4 (IPr = 1,3-Dippimidazole-2-ylidene) is easily accessed from the addition of IPr to UCl4 in THF solutions. 11 Reactions with more exotic uranium C-donor complexes have been recently reported, including carbodicarbenes, 12 N-heterocyclic olefins (NHOs), 13 and carbones or carbodiphosphoranes. 14 Bart and coworkers showed that treatment of THF solutions of UO2X2(THF)3 (X = Cl, OTf) with 2 equiv.…”

f-Element Zintl Chemistry: Actinide-Mediated Dehydrocoupling of H2Sb1– Affords the Trithorium and Triuranium Undeca-Antimontriide Zintl Clusters [{An(TrenTIPS)}3(μ3-Sb11)] (An = Th, U; TrenTIPS = {N(CH2CH2NSiiPr3)3}3–)

“…In all cases, An–Sb 3,–1 bond critical points were located, and these were found to be polar, indeed rather ionic, in nature with low ρ and H terms. The ε values for the An–Sb bonds reflect that the An–Sb bonds are “slipped”, that is the antimony σ-bonding orbitals are not perfectly aligned to the An centers, and also the dative π-donations. , …”

“…The ε values for the An−Sb bonds reflect that the An−Sb bonds are "slipped", that is the antimony σ-bonding orbitals are not perfectly aligned to the An centers, and also the dative π-donations. 52,53 ■ CONCLUSIONS To conclude, although cesium antimonide 1 does not undergo protonolysis reactions with triamidoamine cyclometallate thorium and uranium complexes, it does engage in salt elimination with triamidoamine thorium and uranium separated ion pair complexes to afford the triactinide undecaantimontriide Zintl clusters 3Th and 3U. Clusters 3Th and 3U clearly form via dehydrocoupling, providing two new, structurally authenticated, examples of the Sb 11 3− Zintl trianion with only seven structurally authenticated examples reported previously.…”