Relaxation Dynamics in See-Saw Shaped Dy(III) Single-Molecule Magnets

Utilizing a terphenyl bisanilide ligand, two Dy(III) compounds [K(DME)n][LArDy(X)2] (LAr = {C6H4[(2,6-iPrC6H3)NC6H4]2}2−), X = Cl (1) and X = I (2) were synthesized. The ligand imposes an unusual see-saw shaped molecular geometry leading to a coordinatively unsaturated metal complex with near-linear N–Dy–N (avg. 159.9° for 1 and avg. 160.4° for 2) angles. These compounds exhibit single-molecule magnet (SMM) behavior with significant uniaxial magnetic anisotropy as a result of the transverse coordination of the bisanilide ligand which yields high energy barriers to magnetic spin reversal of Ueff = 1334 K/927 cm−1 (1) and 1278 K/888 cm−1 (2) in zero field. Ab initio calculations reveal that the dominant crystal field of the bisanilide ligand controls the orientation of the main magnetic axis which runs nearly parallel to the N–Dy–N bonds, despite the identity of the halide ligand. Analysis of the relaxation dynamics reveals a ca. 14-fold decrease in the rate of quantum tunneling of the magnetisation when X = I (2). Most notably, the relaxation times were on average 5.6× longer at zero field when the heavier group 17 congener was employed. However, no direct evidence of a heavy atom effect on the Orbach relaxation was obtained as the height of the barrier is defined by the dominant bisanilide ligand.<br/

A diuranium carbide cluster stabilized inside a C80 fullerene cage.

Unsupported non-bridged uranium-carbon double bonds have long been sought after in actinide chemistry as fundamental synthetic targets in the study of actinide-ligand multiple bonding. Here we report that, utilizing Ih(7)-C80 fullerenes as nanocontainers, a diuranium carbide cluster, U=C=U, has been encapsulated and stabilized in the form of UCU@Ih(7)-C80. This endohedral fullerene was prepared utilizing the Krätschmer-Huffman arc discharge method, and was then co-crystallized with nickel(II) octaethylporphyrin (NiII-OEP) to produce UCU@Ih(7)-C80·[NiII-OEP] as single crystals. X-ray diffraction analysis reveals a cage-stabilized, carbide-bridged, bent UCU cluster with unexpectedly short uranium-carbon distances (2.03 Å) indicative of covalent U=C double-bond character. The quantum-chemical results suggest that both U atoms in the UCU unit have formal oxidation state of +5. The structural features of UCU@Ih(7)-C80 and the covalent nature of the U(f1)=C double bonds were further affirmed through various spectroscopic and theoretical analyses

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

Addition of 2 equiv. of the Wittig reagents CH2PAr3 (Ar = Ph; 3,5-di-tert-butylphenyl (tBuAr)) to the tetravalent uranium halides, UX4(solvent)n (X = Cl, n = 0; X = Br, solvent = THF, n = 2; X = I, solvent = 1,4-dioxane, n = 2), generates the trans-bis(Wittig) adducts UX4(CH2PAr3)2 (Ar = Ph, X = Cl (1Ph-Cl), I (1Ph-I); Ar = tBuAr, X = Cl (2tBu-Cl), Br (2tBu-Br), I (2tBu-I)) in low to good yields. Complexes 1Ph-X exhibit poor solubility in aromatic solvents but are partially soluble in 1,2-difluorobenzene, while 2tBu-X possesses greatly improved solubility in these solvents. In all cases, 1Ph-X and 2tBu-X are sensitive to polar coordinating solvents, such as THF or DME, decomposing in these solutions to intractable products. The solid-state molecular structures of 1Ph-XꞏC7H8 and 2tBu-Xꞏn(o-DFB), reveal U-CWittig bonds that range from = 2.506(3) – 2.58(1) Å, generally shorter than those found in other uranium-Wittig (2.60(1) – 2.71(1) Å) and untethered, monodentate U-CNHC (2.62(1) – 2.79(1) Å) complexes. Notably, a general contraction of the U-CWittig bond is observed in the UX4(CH2PAr3)2 as the halide series is descended, which may be attributable to the poorer π-donation of the heavier halides that gives rise to increased Lewis acidity at the uranium center that results in contraction of the U-CWittig bond. Attempts to oxidize 2tBu-Cl with Ag+ or Fc+ salts leads to complicated product mixtures from which a few crystals of {[(tBuAr)3PCH2]UCl3(μ-Cl)}2·C7H8 can be isolated, whereas addition of the reductant Cp*2Co to 2tBu-I, in the presence of an extra equiv. of CH2P(tBuAr)3, leads to the formation of the highly encumbered U(III) tris(Wittig) adduct UI3[CH2P(tBuAr)3]3 (3tBu-I). Preliminary experiments show these complexes are amenable to substitution reactions as treatment of 2tBu-Cl with 2 equiv. of LiCH2SiMe3 generates the thermally sensitive bis(alkyl) bis(Wittig) complex trans-UCl2[CH2P(tBuAr)3]2(CH2SiMe3)2 (4tBu-TMS), a mixed ylide-alkyl system featuring distinct U-CWittig and U-Calkyl σ-bonds. This chemistry demonstrates that untethered, neutral Wittig ligands, when coordinated to uranium, are compatible with redox transformations and metathesis reactions. The report of these UX4(CH2PAr3)2 complexes significantly expands the library of known uranium-Wittig compounds and contributes to the relatively small collection of uranium complexes featuring neutral C-donor ligands

Synthesis of an Arenide-Masked Scandium Complex Accom-panied by Reductively Induced C-H Activation

Reduction of 3N-supported ScCl(ketguan)(NImDipp) (ScCl) with K(C10H8) generates the naphthalenide-masked species [(18-c-6)K(μ-η6:η4-C10H8)Sc(ketguan)(NImDipp)] (Scnaph) and cyclometallated [K(18-c-6)(Et2O)][Sc{(DippN)[2-iPr-6-(CMe2)C6H3N]C(NCHtBu2)}(NImDipp)(THF)] (ScC-H·Et2O), the latter formed from a rare instance of oxidative addition of a low valent scandium center across an unactivated C(sp3)-H bond. Moreover, ScC-H displays solid-to-solution phase dependent tautomerism within the moiety of the scandium metallacyle. Finally, a safe and convenient method is described for the dehydration of ScCl3·6H2O

Intra- and Intermolecular Interception of a Photochemically Generated Terminal Uranium Nitride

The photochemically generated synthesis of a terminal uranium nitride species is here reported and an examination of its intra- and intermolecular chemistry is presented. Treatment of the U(III) complex LArUI(DME) ((LAr)2-= 2,2”-bis(Dippanilide)-p-terphenyl; Dipp = 2,6-diisopropylphenyl) with LiNImDipp ((NImDipp)–= 1,3-bis(Dipp)-imidaozolin-2-iminato) generates the sterically congested 3N-coordinate compound LArU(NImDipp) (1). Complex 1reacts with 1 equiv of Ph3CN3to give the U(IV) azide LArU(N3)(NImDipp) (2). Structural analysis of 2reveals inequivalent Nα-Nβ> Nβ-Nγdistances indicative of an activated azide moiety predisposed to N2loss. Room-temperature photolysis of benzene solutions of 2affords the U(IV) amide (N-LAr)U(NImDipp) (3) via intramolecular N-atom insertion into the benzylic C-H bond of a pendant isopropyl group of the (LAr)2- ligand. The formation of 3occurs as a result of the intramolecular interception of the intermediately generated, terminal uranium nitride (LAr)U(N)(NImDipp) (3’). Evidence for the formation of 3’is further bolstered by its intermolecular capture, accomplished by photolyzing solutions of 2in the presence of an isocyanide or PMe3to give (LAr)U[NCN(C6H3Me2)](NImDipp) (5) and (N,C-LAr*)U(N=PMe3)(NImDipp) (6), respectively. These results expand upon the limited reactivity studies of terminal uranium-nitride moieties and provide new insights into their chemical properties. </p

Probing the Reactivity and Electronic Structure of a Uranium(V) Terminal Oxo Complex

Treatment of the U(III)-ylide adduct U-(CH2PPh3)(NR2)(3) (1, R = SiMe3) with TEMPO generates the U(V) oxo metallacycle [Ph3PCH3][U(O)(CH2SiMe2-NSiMe3)(NR2)(2)] (2) via O-atom transfer, in good yield. Oxidation of 2 with 0.85 equiv of AgOTf affords the neutral U(VI) species U(O)(CH2SiMe2NSiMe3)(NR2)(2) (3). The electronic structures of 2 and 3 are investigated by DFT analysis. Additionally, the nudeophilicity of the oxo ligands in 2 and 3 toward Me3SiI is explored

Synthesis of a “Super Bulky” Guanidinate Possessing an Expandable Coordination Pocket

Friedel–Crafts alkylation of 4-tert-butylaniline with 2 equiv of benzhydrol affords bulky 2,6-bis(diphenylmethyl)-4-tert-butylaniline (Ar*NH2) in good yield, which can be readily synthesized on a tens of grams scale. The reaction of 6 equiv of Ar*NH2 with triphosgene generates the symmetric urea (Ar*NH)2CO, which, upon dehydration with a P2O5/Al2O3 slurry in pyridine, produces the sterically encumbered carbodiimide (Ar*N)2C as an air-stable white solid. The treatment of (Ar*N)2C with LiN═CtBu2 in tetrahydrofuran cleanly gives the monomeric lithium guanidinate Li[Ar*ketguan], free of coordinating solvent, in 85% yield. Protonation of Li[Ar*ketguan] with lutidinium chloride produces the guanidine Ar*ketguanH (MW = 1112.60 g/mol), which is easily derivatized to give the monomeric alkali metal complexes M[Ar*ketguan] (M = K, Cs) in 94% and 51% yield, respectively. The solid-state molecular structures of M[Ar*ketguan] (M = Li, K, Cs) show formally two-coordinate alkali metal cations encapsulated within a hydrophobic coordination pocket formed by the peripheral diphenylmethyl substituents of the guanidinate. Remarkably, percent buried volume analyses (% VBur) of M[Ar*ketguan] [M = Li (94.8% VBur), K (92.1% VBur), Cs (81.7% VBur)] reveal a coordination cavity that adjusts to individually accommodate the variously sized metal ions despite the highly encumbering nature of the ligand. This demonstrates a flexible ligand framework that is able to stabilize low-coordinate metal centers within a “super bulky” coordination environment