13C NMR Shifts as an Indicator of U-C Bond Covalency in Uranium(VI) Acetylide Complexes: An Experimental and Computational Study.

A series of uranium(VI)-acetylide complexes of the general formula UVI(O)(C≡C-C6H4-R)[N(SiMe3)2]3, with variation of the para substituent (R = NMe2, OMe, Me, Ph, H, Cl) on the aryl(acetylide) ring, was prepared. These compounds were analyzed by 13C NMR spectroscopy, which showed that the acetylide carbon bound to the uranium(VI) center, U- C≡C-Ar, was shifted strongly downfield, with δ(13C) values ranging from 392.1 to 409.7 ppm for Cl and NMe2 substituted complexes, respectively. These extreme high-frequency 13C resonances are attributed to large negative paramagnetic (σpara) and relativistic spin-orbit (σSO) shielding contributions, associated with extensive U(5f) and C(2s) orbital contributions to the U-C bonding in title complexes. The trend in the 13C chemical shift of the terminal acetylide carbon is opposite that observed in the series of parent (aryl)acetylenes, due to shielding effects of the para substituent. The 13C chemical shifts of the acetylide carbon instead correlate with DFT computed U-C bond lengths and corresponding QTAIM delocalization indices or Wiberg bond orders. SQUID magnetic susceptibility measurements were indicative of the Van Vleck temperature independent paramagnetism (TIP) of the uranium(VI) complexes, suggesting a magnetic field-induced mixing of the singlet ground-state (f0) of the U(VI) ion with low-lying (thermally inaccessible) paramagnetic excited states (involved also in the perturbation-theoretical treatment of the unusually large paramagnetic and SO contributions to the 13C shifts). Thus, together with reported data, we demonstrate that the sensitive 13C NMR shifts serve as a direct, simple, and accessible measure of uranium(VI)-carbon bond covalency.

C-H Bond Addition across a Transient Uranium-Nitrido Moiety and Formation of a Parent Uranium Imido Complex.

Uranium complexes in the +3 and +4 oxidation states were prepared using the anionic PN- (PN- = ( N-(2-(diisopropylphosphino)-4-methylphenyl)-2,4,6-trimethylanilide) ligand framework. New complexes include the halide starting materials, (PN)2UIIII (1) and (PN)2UIVCl2 (2), which both yield (PN)2UIV(N3)2 (3) by reaction with NaN3. Compound 3 was reduced with potassium graphite to produce a putative, transient uranium-nitrido moiety that underwent an intramolecular C-H activation to form a rare example of a parent imido complex, [K(THF)3][(PN)UIV(═NH)[ iPr2P(C6H3Me)N(C6H2Me2CH2)]] (4). Calculated reaction energy profiles strongly suggest that a C-H insertion becomes unfavorable when a reductant is present, offering a distinctively different reaction pathway than previously observed for other uranium nitride complexes.

Reduction of Carbonyl Groups by Uranium(III) and Formation of a Stable Amide Radical Anion.

Methyl benzoate, N,N-dimethylbenzamide, and benzophenone were reduced by UIII [N(SiMe3 )2 ]3 resulting in uranium(IV) products. Reduction of benzophenone lead to UIV [OC⋅Ph2 )][N(SiMe3 )2 ]3 , (1.1) which forms the dinuclear complex, [N(SiMe3 )2 ]3 UIV (OCPhPh-CPh2 O)UIV [N(SiMe3 )2 ]3 (1.2), through coupling of the ketyl radical species upon crystallization. Reaction of N,N-dimethylbenzamide with UIII [N(SiMe3 )2 ]3 resulted in UIV [OC⋅(Ph)(NMe2 )][N(SiMe3 )2 ]3 (2), a uranium(IV) compound and the first example of a charge-separated amide radical. In the case of methyl benzoate, the reduction resulted in UIV (OMe)[N(SiMe3 )2 ]3 (3) and benzaldehyde as the reduced organic fragment. Compound 2 showed the ability to act as a uranium(III) synthon in its reactivity with trimethylsilyl azide, a reaction that yielded UV (=NSiMe3 )[N(SiMe3 )2 ]3 . Additionally, 2 was reduced with potassium graphite resulting in [U(µ-O)[O=C(NMe2 )(Ph)][N(SiMe3 )2 ]2 ]2 (4), a dinuclear uranium compound bridged by oxo ligands. Reduction of 2 in the presence of 15-crown-5 afforded isolation of the mono-oxo compound, [(15-crown-5)2 K][UO[N(SiMe3 )2 ]3 ] (5). The results expand the reduction capabilities of UIII complexes and demonstrate a strategy for isolating novel metal-stabilized radicals.

Redox-Active vs Redox-Innocent: A Comparison of Uranium Complexes Containing Diamine Ligands.

Uranium complexes (MesDAE)2U(THF) (1-DAE) and Cp2U(MesDAE) (2-DAE) (MesDAE = [ArN-CH2CH2-NAr]; Ar = 2,4,6-trimethylphenyl (Mes)), bearing redox-innocent diamide ligands, have been synthesized and characterized for a full comparison with previously published, redox-active diimine complexes, (MesDABMe)2U(THF) (1-DAB) and Cp2U(MesDABMe) (2-DAB) (MesDABMe = [ArN═C(Me)C(Me)═NAr]; Ar = Mes). These redox-innocent analogues maintain an analogous steric environment to their redox-active ligand counterparts to facilitate a study aimed at determining the differing electronic behavior around the uranium center. Structural analysis by X-ray crystallography showed 1-DAE and 2-DAE have a structural environment very similar to 1-DAB and 2-DAB, respectively. The main difference occurs with coordination of the ene-backbone to the uranium center in the latter species. Electronic absorption spectroscopy reveals these new DAE complexes are nearly identical to each other. X-ray absorption spectroscopy suggests all four species contain +4 uranium ions. The data also indicates that there is an electronic difference between the bis(diamide)-THF uranium complexes as opposed to those that only contain one diamide and two cyclopentadienyl rings. Finally, magnetic measurements reveal that all complexes display temperature-dependent behavior consistent with uranium(IV) ions that do not include ligand radicals. Overall, this study determines that there is no significant bonding difference between the redox-innocent and redox-active ligand frameworks on uranium. Furthermore, there are no data to suggest covalent bonding character using the latter ligand framework on uranium, despite what is known for transition metals.

Synthesis and Reduction of Uranium(V) Imido Complexes with Redox-Active Substituents.

Organic azides that contain naphthyl functional groups were used to prepare uranium(V) imido complexes UV [=NC(2-naph)Ph2 ][N(SiMe3 )2 ]3 (2), UV [=NC(2-naph)3 ][N(SiMe3 )2 ]3 (3), and UV [=N(2-naph)][N(SiMe3 )2 ]3 (4), and their properties were compared with UV [=NCPh3 ][N(SiMe3 )2 ]3 (1). The electronic structures of these compounds were investigated by solution electrochemistry studies, which revealed accessible UV/VI , UIV/V , and naphthalene0 /naphthalene-1 couples. The uranium(V) naphthylimido complexes were reduced by potassium graphite to yield their uranium(IV) congeners K[UIV [=NC(2-naph)Ph2 ][N(SiMe3 )2 ]3 ] (2-K), K[UIV [=NC(2-naph)3 ][N(SiMe3 )2 ]3 ] (3-K), and K[UIV [=N(2-naph)][N(SiMe3 )2 ]3 ] (4-K). The electronic structure of the dianionic compounds were investigated by DFT calculations, and this revealed that the second reduction was ligand-based, which opens the possibility of accomplishing multi-electron redox chemistry by using a tailored multiply-bonded ligand.

A 1,2-Addition Pathway for C(sp2 )-H Activation at a Dinickel Imide.

A dinickel imido complex was synthesized using a redox-active naphthyridine-diimine supporting ligand. Upon coordination of an external ligand, the Ni2 core was disrupted, triggering an aromatic C-H activation reaction to generate a Ni2 (µ-NHAr)(Ar) species. This intermediate is capable of liberating free carbazole and phenanthridine products upon heating or treatment with excess tBuNC. Collectively, these studies establish a kinetically facile 1,2-addition mechanism for C(sp2 )-H activation, taking advantage of cooperative reactivity between two Ni centers.

The Hexachlorocerate(III) Anion: A Potent, Benchtop Stable, and Readily Available Ultraviolet A Photosensitizer for Aryl Chlorides.

The hexachlorocerate(III) anion, [CeIIICl6]3-, was found to be a potent photoreductant in acetonitrile solution with an estimated excited-state reduction potential of -3.45 V versus Cp2Fe0/+. Despite a short lifetime of 22.1(1) ns, the anion exhibited a photoluminescence quantum yield of 0.61(4) and fast quenching kinetics toward organohalogens allowing for its application in the photocatalytic reduction of aryl chloride substrates.

Anomalous one-electron processes in the chemistry of uranium nitrogen multiple bonds.

Novel reaction pathways are illustrated in the synthesis of uranium(IV), uranium(V), and uranium(VI) monoimido complexes. In contrast to the straightforward preparation of U(V)(═NSiMe3)[N(SiMe3)2]3 (1), the synthesis of a uranium(V) tritylimido complex, U(V)(═NCPh3)[N(SiMe3)2]3 (4), from U(III)[N(SiMe3)2]3 and Ph3CN3 was found to proceed through multiple one-electron steps. Whereas the oxidation of 1 with copper(II) salts produced the uranium(VI) monoimido complexes U(VI)(═NSiMe3)X[N(SiMe3)2]3 (X = Cl, Br), the reaction of 4 with CuBr2 undergoes sterically induced reduction to form the uranium(VI) monoimido complex U(VI)(═NCPh3)Br2[N(SiMe3)2]2, demonstrating a striking difference in reactivity based on imido substituent. The facile reduction of compounds 1 and 4 with KC8 allowed for the synthesis of the uranium(IV) monoimido derivatives, K[U(IV)(═NSiMe3)[N(SiMe3)2]3] (1-K) and K[U(IV)(═NCPh3)[N(SiMe3)2]3] (4-K), respectively. In contrast, an analogous uranium(IV) monoimido complex, K[U(IV)(═NPh(F))[N(SiMe3)Ph(F)]], Ph(F) = -pentafluorophenyl (6), was prepared through a loss of N(SiMe3)2Ph(F) concomitant with one-electron oxidation of a uranium(III) center. The uranium(IV) monoimido complexes were found to be reactive toward electrophiles, demonstrating N-C and N-Si single bond formation. One-electron reduction of nitrite provided a route to the uranium(VI) oxo/imido complex, [Ph4P][U(VI)O(═NSiMe3)[N(SiMe3)2]3]. The energetics and electrochemical processes involved in the various oxidation reactions are discussed. Finally, comparison of the U(VI)(═NSiMe3)X[N(SiMe3)2]3, X = Cl, Br, complexes with the previously reported U(VI)OX[N(SiMe3)2]3, X = Cl, Br, complexes suggested that the donor strength of the trimethylsilylimido ligand is comparable to the oxo ligand.

The inverse trans influence in a family of pentavalent uranium complexes.

Systematic ligand variation in a structurally conserved framework of pentavalent uranium complexes of the formulas U(V)X2[N(SiMe3)2]3 (X = F, Cl, Br, N3, NCS, 2-naphthoxide) and U(V)OX[N(SiMe3)2]3(-) (X = -CCPh, -CN) allowed an investigation into the role of the inverse trans influence in pentavalent uranium complexes. The -CCPh and -CN derivatives were only stable in the presence of the trans-U═O multiple bond, implicating the inverse trans influence in stabilizing these complexes. Spectroscopic, structural, and density functional theory calculated electronic structural data are explored. Near-IR data of all complexes is presented, displaying vibronic coupling of 5f(1) electronic transitions along the primary axis. Electrochemical characterization allowed assessment of the relative donating ability of the various axial ligands in this framework. Electron paramagnetic resonance data presented display axial spectra, with hyperfine coupling along the primary axis.