Molecular and electronic structure of terminal and alkali metal-capped uranium(V) nitride complexes.
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ABSTRACT:
SUBMITTER: King DM
PROVIDER: S-EPMC5187438 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:Treatment of [UIV(N3)(TrenTIPS)] (1, TrenTIPS = {N(CH2CH2NSiPri3)3}3-) with excess Li resulted in the isolation of [{UIV(μ-NLi2)(TrenTIPS)}2] (2), which exhibits a diuranium(IV) 'diamond-core' dinitride motif. Over-reduction of 1 produces [UIII(TrenTIPS)] (3), and together with known [{UV(μ-NLi)(TrenTIPS)}2] (4) an overall reduction sequence 1 → 4 → 2 → 3 is proposed. Attempts to produce an odd-electron nitride from 2 resulted in the formation of [{UIV(TrenTIPS)}2(μ-NH)(μ-NLi2)Li] (5). Use of heavier alkali metals did not result in the formation of analogues of 2, emphasising the role of the high charge-to-radius-ratio of lithium stabilising the charge build up at the nitride. Variable-temperature magnetic data for 2 and 5 reveal large low-temperature magnetic moments, suggesting doubly degenerate ground states, where the effective symmetry of the strong crystal field of the nitride dominates over the spin-orbit coupled nature of the ground multiplet of uranium(IV). Spin Hamiltonian modelling of the magnetic data for 2 and 5 suggest U⋯U anti-ferromagnetic coupling of -4.1 and -3.4 cm-1, respectively. The nature of the U⋯U electronic communication was probed computationally, revealing a borderline case where the prospect of direct uranium-uranium bonding was raised, but in-depth computational analysis reveals that if any uranium-uranium bonding is present it is weak, and instead the nitride centres dominate the mediation of U⋯U electronic communication. This highlights the importance of obtaining high-level ab initio insight when probing potential actinide-actinide electronic communication and bonding in weakly coupled systems. The computational analysis highlights analogies between the 'diamond-core' dinitride of 2 and matrix-isolated binary U2N2.
| S-EPMC9171730 | biostudies-literature
Project description:Uranium nitrides play important roles in dinitrogen activation and functionalization and in chemistry for nuclear fuels, but the synthesis and isolation of the highly reactive uranium(VI) nitrides remains challenging. Here, we report an example of transition metal (TM) stabilized U(VI) nitride complexes, which are generated by the photolysis of azide-bridged U(IV)-TM (TM = Rh, Ir) precursors. The U(V) nitride intermediates with bridged azide ligands are isolated successfully by careful control of the irradiation time, suggesting that the photolysis of azide-bridged U(IV)-TM precursors is a stepwise process. The presence of two U(VI) nitrides stabilized by three TMs is clearly demonstrated by an X-ray crystallographic study. These TM stabilized U(V) nitride intermediates and U(VI) nitride products exhibit excellent stability both in the solid-state and in THF solution under ambient light. Density functional theory calculations show that the photolysis necessary to break the N-N bond of the azide ligands implies excitation from uranium f-orbital to the lowest unoccupied molecular orbital (LUMO), as suggested by the strong antibonding N-(N2) character present in the latter.
| S-EPMC9249861 | biostudies-literature
Project description:Heterobimetallic complexes containing short uranium-group 10 metal bonds have been prepared from monometallic IU(IV)(OAr(P)-?(2)O,P)3 (2) {[Ar(P)O](-) = 2-tert-butyl-4-methyl-6-(diphenylphosphino)phenolate}. The U-M bond in IU(IV)(?-OAr(P)-1?(1)O,2?(1)P)3M(0), M = Ni (3-Ni), Pd (3-Pd), and Pt (3-Pt), has been investigated by experimental and DFT computational methods. Comparisons of 3-Ni with two further U-Ni complexes XU(IV)(?-OAr(P)-1?(1)O,2?(1)P)3Ni(0), X = Me3SiO (4) and F (5), was also possible via iodide substitution. All complexes were characterized by variable-temperature NMR spectroscopy, electrochemistry, and single crystal X-ray diffraction. The U-M bonds are significantly shorter than any other crystallographically characterized d-f-block bimetallic, even though the ligand flexes to allow a variable U-M separation. Excellent agreement is found between the experimental and computed structures for 3-Ni and 3-Pd. Natural population analysis and natural localized molecular orbital (NLMO) compositions indicate that U employs both 5f and 6d orbitals in covalent bonding to a significant extent. Quantum theory of atoms-in-molecules analysis reveals U-M bond critical point properties typical of metallic bonding and a larger delocalization index (bond order) for the less polar U-Ni bond than U-Pd. Electrochemical studies agree with the computational analyses and the X-ray structural data for the U-X adducts 3-Ni, 4, and 5. The data show a trend in uranium-metal bond strength that decreases from 3-Ni down to 3-Pt and suggest that exchanging the iodide for a fluoride strengthens the metal-metal bond. Despite short U-TM (transition metal) distances, four other computational approaches also suggest low U-TM bond orders, reflecting highly transition metal localized valence NLMOs. These are more so for 3-Pd than 3-Ni, consistent with slightly larger U-TM bond orders in the latter. Computational studies of the model systems (PH3)3MU(OH)3I (M = Ni, Pd) reveal longer and weaker unsupported U-TM bonds vs 3.
| S-EPMC4796865 | biostudies-literature
Project description:Molecular uranium nitride complexes were prepared to relate their small molecule reactivity to the nature of the U[double bond, length as m-dash]N[double bond, length as m-dash]U bonding imposed by the supporting ligand. The U4+-U4+ nitride complexes, [NBu4][{(( t BuO)3SiO)3U}2(?-N)], [NBu4]-1, and [NBu4][((Me3Si)2N)3U}2(?-N)], 2, were synthesised by reacting NBu4N3 with the U3+ complexes, [U(OSi(O t Bu)3)2(?-OSi(O t Bu)3)]2 and [U(N(SiMe3)2)3], respectively. Oxidation of 2 with AgBPh4 gave the U4+-U5+ analogue, [((Me3Si)2N)3U}2(?-N)], 4. The previously reported methylene-bridged U4+-U4+ nitride [Na(dme)3][((Me3Si)2)2U(?-N)(?-?2-C,N-CH2SiMe2NSiMe3)U(N(SiMe3)2)2] (dme = 1,2-dimethoxyethane), [Na(dme)3]-3, provided a versatile precursor for the synthesis of the mixed-ligand U4+-U4+ nitride complex, [Na(dme)3][((Me3Si)2N)3U(?-N)U(N(SiMe3)2)(OSi(O t Bu)3)], 5. The reactivity of the 1-5 complexes was assessed with CO2, CO, and H2. Complex [NBu4]-1 displays similar reactivity to the previously reported heterobimetallic complex, [Cs{(( t BuO)3SiO)3U}2(?-N)], [Cs]-1, whereas the amide complexes 2 and 4 are unreactive with these substrates. The mixed-ligand complexes 3 and 5 react with CO and CO2 but not H2. The nitride complexes [NBu4]-1, 2, 4, and 5 along with their small molecule activation products were structurally characterized. Magnetic data measured for the all-siloxide complexes [NBu4]-1 and [Cs]-1 show uncoupled uranium centers, while strong antiferromagnetic coupling was found in complexes containing amide ligands, namely 2 and 5 (with maxima in the ? versus T plot of 90 K and 55 K). Computational analysis indicates that the U(?-N) bond order decreases with the introduction of oxygen-based ligands effectively increasing the nucleophilicity of the bridging nitride.
| S-EPMC6853081 | biostudies-literature
Project description: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)-imidazolin-2-iminato) generates the sterically congested 3N-coordinate compound LArU(NImDipp) (1). Complex 1 reacts with 1 equiv. of Ph3CN3 to give the U(iv) azide LArU(N3)(NImDipp) (2). Structural analysis of 2 reveals inequivalent Nα-Nβ > Nβ-Nγ distances indicative of an activated azide moiety predisposed to N2 loss. Room-temperature photolysis of benzene solutions of 2 affords 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 3 occurs 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 2 in the presence of an isocyanide or PMe3 to give (LAr)U[NCN(C6H3Me2)](NImDipp) (5) and (N,C-LAr*)U(N[double bond, length as m-dash]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.
| S-EPMC8157337 | biostudies-literature
Project description:Despite their importance as mechanistic models for heterogeneous Haber Bosch ammonia synthesis from dinitrogen and dihydrogen, homogeneous molecular terminal metal-nitrides are notoriously unreactive towards dihydrogen, and only a few electron-rich, low-coordinate variants demonstrate any hydrogenolysis chemistry. Here, we report hydrogenolysis of a terminal uranium(V)-nitride under mild conditions even though it is electron-poor and not low-coordinate. Two divergent hydrogenolysis mechanisms are found; direct 1,2-dihydrogen addition across the uranium(V)-nitride then H-atom 1,1-migratory insertion to give a uranium(III)-amide, or with trimesitylborane a Frustrated Lewis Pair (FLP) route that produces a uranium(IV)-amide with sacrificial trimesitylborane radical anion. An isostructural uranium(VI)-nitride is inert to hydrogenolysis, suggesting the 5f1 electron of the uranium(V)-nitride is not purely non-bonding. Further FLP reactivity between the uranium(IV)-amide, dihydrogen, and triphenylborane is suggested by the formation of ammonia-triphenylborane. A reactivity cycle for ammonia synthesis is demonstrated, and this work establishes a unique marriage of actinide and FLP chemistries.
| S-EPMC6969212 | biostudies-literature
Project description:Addition of [UI2(THF)3(μ-OMe)]2·THF (2·THF) to THF solutions containing 6 equiv. of K[C14H10] generates the heteroleptic dimeric complexes [K(18-crown-6)(THF)2]2[U(η6-C14H10)(η4-C14H10)(μ-OMe)]2·4THF (118C6·4THF) and {[K(THF)3][U(η6-C14H10)(η4-C14H10)(μ-OMe)]}2 (1THF) upon crystallization of the products in THF in the presence or absence of 18-crown-6, respectively. Both 118C6·4THF and 1THF are thermally stable in the solid-state at room temperature; however, after crystallization, they become insoluble in THF or DME solutions and instead gradually decompose upon standing. X-ray diffraction analysis reveals 118C6·4THF and 1THF to be structurally similar, possessing uranium centres sandwiched between bent anthracenide ligands of mixed tetrahapto and hexahapto ligation modes. Yet, the two complexes are distinguished by the close contact potassium-arenide ion pairing that is seen in 1THF but absent in 118C6·4THF, which is observed to have a significant effect on the electronic characteristics of the two complexes. Structural analysis, SQUID magnetometry data, XANES spectral characterization, and computational analyses are generally consistent with U(iv) formal assignments for the metal centres in both 118C6·4THF and 1THF, though noticeable differences are detected between the two species. For instance, the effective magnetic moment of 1THF (3.74 μ B) is significantly lower than that of 118C6·4THF (4.40 μ B) at 300 K. Furthermore, the XANES data shows the U LIII-edge absorption energy for 1THF to be 0.9 eV higher than that of 118C6·4THF, suggestive of more oxidized metal centres in the former. Of note, CASSCF calculations on the model complex {[U(η6-C14H10)(η4-C14H10)(μ-OMe)]2}2- (1*) shows highly polarized uranium-arenide interactions defined by π-type bonds where the metal contributions are primarily comprised by the 6d-orbitals (7.3 ± 0.6%) with minor participation from the 5f-orbitals (1.5 ± 0.5%). These unique complexes provide new insights into actinide-arenide bonding interactions and show the sensitivity of the electronic structures of the uranium atoms to coordination sphere effects.
| S-EPMC8528047 | biostudies-literature
Project description:High-throughput computational screening has emerged as a critical component of materials discovery. Direct density functional theory (DFT) simulation of inorganic materials and molecular transition metal complexes is often used to describe subtle trends in inorganic bonding and spin-state ordering, but these calculations are computationally costly and properties are sensitive to the exchange-correlation functional employed. To begin to overcome these challenges, we trained artificial neural networks (ANNs) to predict quantum-mechanically-derived properties, including spin-state ordering, sensitivity to Hartree-Fock exchange, and spin-state specific bond lengths in transition metal complexes. Our ANN is trained on a small set of inorganic-chemistry-appropriate empirical inputs that are both maximally transferable and do not require precise three-dimensional structural information for prediction. Using these descriptors, our ANN predicts spin-state splittings of single-site transition metal complexes (i.e., Cr-Ni) at arbitrary amounts of Hartree-Fock exchange to within 3 kcal mol-1 accuracy of DFT calculations. Our exchange-sensitivity ANN enables improved predictions on a diverse test set of experimentally-characterized transition metal complexes by extrapolation from semi-local DFT to hybrid DFT. The ANN also outperforms other machine learning models (i.e., support vector regression and kernel ridge regression), demonstrating particularly improved performance in transferability, as measured by prediction errors on the diverse test set. We establish the value of new uncertainty quantification tools to estimate ANN prediction uncertainty in computational chemistry, and we provide additional heuristics for identification of when a compound of interest is likely to be poorly predicted by the ANN. The ANNs developed in this work provide a strategy for screening transition metal complexes both with direct ANN prediction and with improved structure generation for validation with first principles simulation.
| S-EPMC6100542 | biostudies-literature
Project description:Herein, we report the synthesis and characterization of a series of terminal uranium(iv) hydrosulfido and sulfido complexes, supported by the hexadentate, tacn-based ligand framework (Ad,MeArO)3tacn3- (= trianion of 1,4,7-tris(3-(1-adamantyl)-5-methyl-2-hydroxybenzyl)-1,4,7-triazacyclononane). The hydrosulfido complex [((Ad,MeArO)3tacn)U-SH] (2) is obtained from the reaction of H2S with the uranium(iii) starting material [((Ad,MeArO)3tacn)U] (1) in THF. Subsequent deprotonation with potassium bis(trimethylsilyl)amide yields the mononuclear uranium(iv) sulfido species in good yields. With the aid of dibenzo-18-crown-6 and 2.2.2-cryptand, it was possible to isolate a terminal sulfido species, capped by the potassium counter ion, and a "free" terminal sulfido species with a well separated cation/anion pair. Spectroscopic and computational analyses provided insights into the nature of the uranium-sulfur bond in these complexes.
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Project description:The electronic and molecular structures of metal-free tetrabenzoporphyrin (H2TBP) and its complexes with zinc, cadmium, aluminum, gallium and indium were investigated by density functional theory (DFT) calculations with a def2-TZVP basis set. A geometrical structure of ZnTBP and CdTBP was found to possess D4h symmetry; AlClTBP, GaClTBP and InClTBP were non-planar complexes with C4v symmetry. The molecular structure of H2TBP belonged to the point symmetry group of D2h. According to the results of the natural bond orbital (NBO) analysis, the M-N bonds had a substantial ionic character in the cases of the Zn(II) and Cd(II) complexes, with a noticeably increased covalent contribution for Al(III), Ga(III) and In(III) complexes with an axial -Cl ligand. The lowest excited states were computed with the use of time-dependent density functional theory (TDDFT) calculations. The model electronic absorption spectra indicated a weak influence of the nature of the metal on the Q-band position.
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