U(III)-CN versus U(IV)-NC coordination in tris(silylamide) complexes

Accessing a Highly Reducing Uranium(III) Complex through Cyclometalation

U(IV) cyclometalated complexes have shown rich reactivity, but their low oxidation state analogues still remain rare. Herein, we report the isolation of [K(2.2.2-cryptand)][UIII{N(SiMe3)2}2(κ2-C,N-CH2SiMe2NSiMe3)], 1, from the reduction of [UIII{N(SiMe)2}3] with KC8 and 2.2.2-cryptand at room temperature. Cyclic voltammetry studies demonstrate that 1 has a reduction potential similar to that of the previously reported [K(2.2.2-cryptand)][UII{N(SiMe)2}3] (Epc = -2.6 V versus Fc+/0 and Epc = -2.8 V versus Fc+/0, respectively). Complex 1, indeed, shows similar reducing abilities upon reactions with 4,4'-bipyridine, 2,2'-bipyridine, and 1-azidoadamantane. Interestingly, 1 was also found to be the first example of a mononuclear U(III) complex that is capable of reducing pyridine. In addition, it is shown that a wide variety of substrates can be inserted into the U-C bond, forming new U(III) metallacycles. These results highlight that cyclometalated U(III) complexes can serve as versatile precursors for a broad range of reactivity and for assembling a variety of novel chemical architectures.

Uranium versus Thorium: A Case Study on a Base-Free Terminal Uranium Imido Metallocene

The structure of and bonding in two base-free terminal actinide imido metallocenes, [η5-1,2,4-(Me3C)3C5H2]2An═N(p-tolyl) (An = U (1), Th (1')) are compared and connected to their individual reactivity. While structurally rather similar, the U(IV) derivative 1 is slightly more sterically crowded. Furthermore, density functional theory (DFT) studies imply that the 5f orbital contribution to the bonding within the individual actinide imido An═N(p-tolyl) moieties is significantly larger for 1 than for 1', which makes the bonds between the [η5-1,2,4-(Me3C)3C5H2]2U2+ and [(p-tolyl)N]2- fragments more covalent. Therefore, steric and electronic factors impact the reactivity of these imido complexes. For example, complex 1 is inert toward internal alkynes, but it readily forms Lewis base adducts [η5-1,2,4-(Me3C)3C5H2]2U═N(p-tolyl)(L) (L = OPMe3 (6), dmap (9), PhCN (14), and 2,6-Me2PhNC (17)) with Me3PO, 4-dimethylaminopyridine (dmap), nitrile, PhCN, or isonitrile 2,6-Me2PhNC. It may also react as a nucleophile or undergo a [2 + 2] cycloaddition with CS2, isothiocyanates, thio-ketones, ketones, lactides, and acyl nitriles, forming the four- or five-membered metallaheteroacycles, terminal sulfido, or oxido complexes, and cyanide amidate complexes, respectively. In contrast, after the addition of aldehyde p-tolylCHO, the tetranuclear complex [η5-1,2,4-(Me3C)3C5H2]4[OCH(p-tolyl)CH(p-tolyl)O]2U4O4 (10) is isolated. However, while 1 is unreactive toward dicyclohexylcarbodiimide (DCC), an equilibrium exists in benzene solution between N,N'-diisopropylcarbodiimide (DIC), 1, and the four-membered metallaheterocycle [η5-1,2,4-(Me3C)3C5H2]2U[N(p-tolyl)C(═NiPr)N(iPr)] (12). Furthermore, 1 may also engage in single- and two-electron transfer processes. It is singly oxidized by Ph3CN3, CuI, Ph2S2, and Ph2Se2, yielding the uranium(V) imido complexes [η5-1,2,4-(Me3C)3C5H2]2U═N(p-tolyl)(X) (X = N3 (20), I (22), PhS (23), and PhSe (24)), or is doubly oxidized by organic azides (RN3) and 9-diazofluorene, forming the uranium(VI) bis-imido metallocenes [η5-1,2,4-(Me3C)3C5H2]2U═N(p-tolyl)(=NR) (R = p-tolyl (18), mesityl (19)) and [η5-1,2,4-(Me3C)3C5H2]2U=N(p-tolyl)[=NN=(9-C13H8)] (21), respectively.

Synthesis and Structure of [η5-1,2,4-(Me3Si)3C5H2]2Th(bipy) and Its Reactivity toward Small Molecules

Halide exchange of (Cp3tms)2ThCl2 (1; Cp3tms = η5-1,2,4-(Me3Si)3C5H2) with Me3SiI furnishes (Cp3tms)2ThI2 (2), which is then reduced with potassium graphite (KC8) in the presence of 2,2'-bipyridine to give the thorium bipyridyl metallocene (Cp3tms)2Th(bipy) (3) in good yield. Complex 3 was fully characterized and readily reacted with various small molecules. For example, 3 may serve as a synthetic equivalent for the (Cp3tms)2Th(II) fragment when exposed to CuI, Ph2S2, organic azides, and CS2. Moreover, upon the addition of thiobenzophenone Ph2CS, p-methylbenzaldehyde (p-MeC6H4)CHO, benzophenone Ph2CO, amidate PhCONH(p-tolyl), seleno-ketone (p,p'-dimethoxy), selenobenzophenone (p-MeOPh)2CSe, di(p-tolyl)methanimine (p-tolyl)2C═NH, 1,2-di(benzylidene)hydrazine (PhCH═N)2, and nitriles PhCN, PhCH2CN, and Ph2CHCN C-C coupling results to give (Cp3tms)2Th[(bipy)(Ph2CS)] (8), (Cp3tms)2Th[(bipy)(p-MePhCHO)] (9), (Cp3tms)2Th[(bipy)(Ph2CO)] (10), (Cp3tms)2Th[(bipy){(p-tolylNH)(Ph)CO}] (11), (Cp3tms)2Th[(bipy){(p-MeOPh)2CSe}] (12), (Cp3tms)2Th[(bipy){(p-tolyl)2CNH}] (13), (Cp3tms)2Th[(bipy)(PhCHNN═CHPh)] (14), (Cp3tms)2Th[(bipy)(PhCN)] (16), (Cp3tms)2Th[(bipy)(PhCH2CN)] (17), and (Cp3tms)2Th[(bipy)(Ph2CHCN)] (18), respectively. However, when thiazole is added to 3, the dimeric sulfido complex [(Cp3tms)2Th]2[μ-(bipy)CH2NCHCHS]2 (15) can be isolated. Moreover, the addition of isonitriles such as Me3CNC and PhCH2NC to 3 results in C-N bond cleavage and C-C coupling processes to form the thorium isocyanido amido complexes (Cp3tms)2Th[4-(Me3C)bipy](NC) (19) and (Cp3tms)2Th[4-(PhCH2)bipy](NC) (20), respectively. Nevertheless, upon exposure of 3 to (trimethylsilyl)diazomethane Me3SiCHN2, the bis-amido complex (Cp3tms)2Th[5,6-(Me3SiCH)bipy] (21), concomitant with N2 release, is isolated.

A Comprehensive Study Concerning the Synthesis, Structure, and Reactivity of Terminal Uranium Oxido, Sulfido, and Selenido Metallocenes

Terminal uranium oxido, sulfido, and selenido metallocenes were synthesized, and their reactivity was comprehensively studied. Heating of an equimolar mixture of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂UMe₂ (2) and [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(NH-p-tolyl)₂ (3) in the presence of 4-dimethylaminopyridine (dmap) in refluxing toluene forms [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U═N(p-tolyl)(dmap) (4), which is a useful precursor for the preparation of the terminal uranium oxido, sulfido, and selenido metallocenes [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U═E(dmap) (E = O (5), S (6), Se (7)) employing a cycloaddition-elimination methodology with Ph₂C═E (E = O, S) or (p-MeOPh)₂CSe, respectively. Metallocenes 5-7 are inert toward alkynes, but they act as nucleophiles in the presence of alkylsilyl halides. The oxido and sulfido metallocenes 5 and 6 undergo [2 + 2] cycloadditions with isothiocyanate PhNCS or CS₂, while the selenido derivative 7 does not. The experimental studies are complemented by density functional theory (DFT) computations.

A rare isocyanide derived from an unprecedented neutral yttrium(ii) bis(amide) complex

A room temperature stable complex formulated as Y(NHAr*)₂ has been prepared, where Ar* = 2,6-(2,4,6-(ⁱPr)₃C₆H₂)C₆H₃, by KC₈ reduction of ClY(NHAr*)₂. Based on EPR evidence, Y(NHAr*)₂ is an example of a d¹ Y(ii) complex with significant delocalization of the unpaired electron density from the metal to the ligand. The isolation of molecular divalent metal complexes is challenging for rare earth elements such as yttrium. In fact, stabilization of the divalent state requires judicious ligand design that allows the metal center to be coordinatively saturated. Divalent rare earth elements tend to be reactive towards various substrates. Interestingly, Y(NHAr*)₂ reacts as a radical donor towards ^t BuNC to generate an unusual yttrium isocyanide complex, CNY(NHAr*)₂, based on spectroscopic evidence and single-crystal X-ray diffraction data.

In silico design criteria for high blocking barrier uranium (III) SIMs

A combination of DFT and ab initio CASSCF/PT2 calculations on U(III) fictitious models and numerous reported X-ray structures unveils several geometries from coordination number 1 to 12 that can be targeted to design potential U(III) SIMs with attractive barrier heights. Among the geometries studied, the T-shaped and capped pentagonal antiprism geometries yield values exceeding 1500 cm^-1 - a value that is elusive for any uranium SIMs.

UCN@Cs(6)-C82: An Encapsulated Triangular UCN Cluster with Ambiguous U Oxidation State [U(III) versus U(I)]

Understanding the chemical behavior of actinide elements is essential for the effective management and use of actinide materials. In this study, we report an unprecedented η² (side-on) coordination of U by a cyanide in a UCN cluster, which was stabilized inside a C₈₂ fullerene cage. UCN@C_s(6)-C₈₂ was successfully synthesized and fully characterized by mass spectrometry, single crystal X-ray crystallography, cyclic voltammetry, spectroscopy, and theoretical calculations. The bonding analysis demonstrates significant donation bonding between CN^- and uranium, and covalent interactions between uranium and the carbon cage. These effects correlate with an observed elongated cyanide C-N bond, resulting in a rare case where the oxidation state of uranium shows ambiguity between U(III) and U(I). The discovery of this unprecedented triangular configuration of the uranium cyanide cluster provides a new insight in coordination chemistry and highlights the large variety of bonding situations that uranium can have.

Expanding the Nonaqueous Chemistry of Neptunium: Synthesis and Structural Characterization of [Np(NR2)3Cl], [Np(NR2)3Cl]-, and [Np{N(R)(SiMe2CH2)}2(NR2)]- (R = SiMe3)

Reaction of 3 equiv of NaNR₂ (R = SiMe₃) with NpCl₄(DME)₂ in THF afforded the Np(IV) silylamide complex, [Np(NR₂)₃Cl] (1), in good yield. Reaction of 1 with 1.5 equiv of KC₈ in THF, in the presence of 1 equiv of dibenzo-18-crown-6, resulted in formation of [{K(DB-18-C-6)(THF)}₃(μ₃-Cl)][Np(NR₂)₃Cl]₂ (4), also in good yield. Complex 4 represents the first structurally characterized Np(III) amide. Finally, reaction of NpCl₄(DME)₂ with 5 equiv of NaNR₂ and 1 equiv of dibenzo-18-crown-6 afforded the Np(IV) bis(metallacycle), [{Na(DB-18-C-6)(Et₂O)_0.62(κ¹-DME)_0.38}₂(μ-DME)][Np{N(R)(SiMe₂CH₂)}₂(NR₂)]₂ (8), in moderate yield. Complex 8 was characterized by ¹H NMR spectroscopy and X-ray crystallography and represents a rare example of a structurally characterized neptunium-hydrocarbyl complex. To support these studies, we also synthesized the uranium analogues of 4 and 8, namely, [K(2,2,2-cryptand)][U(NR₂)₃Cl] (2), [K(DB-18-C-6)(THF)₂][U(NR₂)₃Cl] (3), [Na(DME)₃][U{N(R)(SiMe₂CH₂)}₂(NR₂)] (6), and [{Na(DB-18-C-6)(Et₂O)_0.5(κ¹-DME)_0.5}₂(μ-DME)][U{N(R)(SiMe₂CH₂)}₂(NR₂)]₂ (7). Complexes 2, 3, 6, and 7 were characterized by a number of techniques, including NMR spectroscopy and X-ray crystallography.

A spatially separated [KBr6]5− anion in the cyanido-bridged uranium(IV) compound [U2(CN)3(NH3)14]5+[KBr6]5−·NH3

The reaction of uranium tetrabromide with potassium cyanide in anhydrous liquid ammonia at room temperature leads to the formation of brown crystals of [U2(CN)3(NH3)14]5+ [KBr6]5− · NH3. We determined the crystal structure of the compound by single crystal X-ray diffraction. To the best of our knowledge it contains the unprecedented spatially separated [KBr6]5− anion and presents the first uranium(IV) cyanide compound which forms a layer structure. The compound crystallizes in the trigonal space group P3̅m1 (No. 164) with a = 10.3246(13), c = 8.4255(17) Å, V = 777.8(3) Å3, Z = 1 at T = 100 K and is well described with the Niggli formula [ U ( CN ) 3 2 ( NH 3 ) 7 1 ] ∞ 2 2 [ KBr 6 1 ] . $\mathop {} \limits_{\infty}^{2}{\left[ {{\rm{U}}{{({\rm{CN}})}_{{3 \over 2}}}{{({\rm{N}}{{\rm{H}}_3})}_{{7 \over 1}}}} \right]_2}\left[ {{\rm{KB}}{{\rm{r}}_{{6 \over 1}}}} \right].$

Formation of Cerium and Neodymium Isocyanides in the Reactions of Cyanogen with Ce and Nd Atoms in Argon Matrices

Laser ablation of metallic Ce and Nd reacting with cyanogen in excess argon during codeposition at 4 K forms Ce(NC)_x and Nd(NC)_x for x = 1-3, which are identified from their matrix infrared spectra using cyanogen substituted with ¹³C and ¹⁵N. The electronic structure calculations were performed for isocyano and cyano Cd and Nd compounds for up to n = 4. The frequencies were calculated at the density functional theory level with three different functionals as well as correlated molecular orbital theory (MP2) and are consistent with the experimental assignments and the corresponding ¹²C/¹³C isotopic frequency ratios for the isocyano species. The computed frequencies for the analogous cyanide complexes are significantly higher than those for the isocyano isomers, and they are not observed in the spectra. The high spin isocyano complexes are the lowest energy structures. On the basis of the natural population analysis results, the bonding in ⁴CeNC and ⁶NdNC is essentially purely ionic with the Ce/Nd in the +I-oxidation state. The bonding for disocyano (³Ce(NC)₂ and ⁵Nd(NC)₂) and triisocyano (²Ce(NC)₃ and ⁴Nd(NC)₃) complexes is still quite ionic with the lanthanide in the +II and +III formal oxidation states, respectively. For ¹Ce(NC)₄, the oxidation state is best described as being between +III and +IV. Formation of ⁵Nd(NC)₄ does not really change the electron configuration on the Nd from that in ⁴Nd(NC)₃ and the oxidation state on the Nd remains at +III. Although Nd compounds with up to 3 NC^- groups have more ionic binding than do the corresponding Ce compounds, Ce(NC)₄ has more ionic binding than does Nd(NC)₄. The ionic nature of isocyano Ce and Nd complexes decreases as the number of isocyano groups increases. The energetics of formation of the isocyano Ce and Nd complexes using cyanogen or CN radicals are calculated to be mostly due to exothermic processes, with the exothermicity decreasing as the number of isocyano groups increases.

Theoretical Investigation of the Electronic Structure and Magnetic Properties of Oxo-Bridged Uranyl(V) Dinuclear and Trinuclear Complexes

The uranyl(V) complexes [UO₂(dbm)₂K(18C6)]₂ (dbm = dibenzoylmethanate) and [UO₂(L)]₃(L = 2-(4-tolyl)-1,3-bis(quinolyl)malondiiminate), exhibiting diamond-shaped U₂O₂ and triangular-shaped U₃O₃ cores respectively with 5f¹-5f¹ and 5f¹-5f¹-5f¹ configurations, have been investigated using relativistic density functional theory (DFT). The bond order and QTAIM analyses reveal that the covalent contribution to the bonding within the oxo cores is slightly more important for U₃O₃ than for U₂O₂, in line with the shorter U-O distances existing in the trinuclear complex in comparison to those in the binuclear complex. Using the broken symmetry (BS) approach combined with the B3LYP functional for the calculation of the magnetic exchange coupling constants (J) between the magnetic centers, the antiferromagnetic (AF) character of these complexes was confirmed, the estimated J values being respectively equal to -24.1 and -7.2 cm^-1 for the dioxo and trioxo species. It was found that the magnetic exchange is more sensitive to small variations of the core geometry of the dioxo species in comparison to the trioxo species. Although the robust AF exchange coupling within the U_xO_x cores is generally maintained when small variations of the UOU angle are applied, a weak ferromagnetic character appears in the dioxo species when this angle is higher than 114°, its value for the actual structure being equal to 105.9°. The electronic factors driving the magnetic coupling are discussed.

Facile N-functionalization and strong magnetic communication in a diuranium(v) bis-nitride complex

Uranium nitride complexes are of high interest because of their ability to effect dinitrogen reduction and functionalization and to promote magnetic communication, but studies of their properties and reactivity remain rare. Here we have prepared in 73% yield the diuranium(v) bis-nitride complex [K₂{[U(OSi(O ^t Bu)₃)₃]₂(μ-N)₂}], 4, from the thermal decomposition of the nitride-, azide-bridged diuranium(iv) complex [K₂{[U(OSi(O ^t Bu)₃)₃]₂(μ-N)(μ-N₃)}], 3. The bis-nitride 4 reacts in ambient conditions with 1 equiv. of CS₂ and 1 equiv. of CO₂ resulting in N-C bond formation to afford the diuranium(v) complexes [K₂{[U(OSi(O ^t Bu)₃)₃]₂(μ-N)(μ-S)(μ-NCS)}], 5 and [K₂{[U(OSi(O ^t Bu)₃)₃]₂(μ-N)(μ-O)(μ-NCO)}], 6, respectively. Both nitrides in 4 react with CO resulting in oxidative addition of CO to one nitride and CO cleavage by the second nitride to afford the diuranium(iv) complex [K₂{[U(OSi(O ^t Bu)₃)₃]₂(μ-CN)(μ-O)(μ-NCO)}], 7. Complex 4 also effects the remarkable oxidative cleavage of H₂ in mild conditions to afford the bis-imido bridged diuranium(iv) complex [K₂{[U(OSi(O ^t Bu)₃)₃]₂(μ-NH)₂}], 8 that can be further protonated to afford ammonia in 73% yield. Complex 8 provides a good model for hydrogen cleavage by metal nitrides in the Haber-Bosch process. The measured magnetic data show an unusually strong antiferromagnetic coupling between uranium(v) ions in the complexes 4 and 6 with Neel temperatures of 77 K and 60 K respectively, demonstrating that nitrides are attractives linkers for promoting magnetic communication in uranium 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 U^III [N(SiMe₃ )₂ ]₃ resulting in uranium(IV) products. Reduction of benzophenone lead to U^IV [OC⋅Ph₂ )][N(SiMe₃ )₂ ]₃ , (1.1) which forms the dinuclear complex, [N(SiMe₃ )₂ ]₃ U^IV (OCPhPh-CPh₂ O)U^IV [N(SiMe₃ )₂ ]₃ (1.2), through coupling of the ketyl radical species upon crystallization. Reaction of N,N-dimethylbenzamide with U^III [N(SiMe₃ )₂ ]₃ resulted in U^IV [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 U^IV (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 U^V (=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 U^III complexes and demonstrate a strategy for isolating novel metal-stabilized radicals.

[UCl4(HCN)4] – a hydrogen cyanide complex of uranium tetrachloride

The reaction of uranium tetrachloride with anhydrous liquid hydrogen cyanide yields a turquoise microcrystalline powder of tetrachloridotetraformonitrileuranium(iv), [UCl₄(HCN)₄].

Advances in f-element cyanide chemistry

By using the cyanide ligand, actinide compounds with unprecedented structures, U^III–CN vs. Ce^III–NC and U^III–CN vs. U^IV–NC coordination modes, and novel high-valent uranium complexes were revealed.