Progress in the chemistry of molecular actinide-nitride compounds

The chemistry of actinide-nitrides has witnessed significant advances in the last ten years with a large focus on uranium and a few breakthroughs with thorium. Following the early discovery of the first terminal and bridging nitride complexes, various synthetic routes to uranium nitrides have since been identified, although the range of ligands capable of stabilizing uranium nitrides still remains scarce. In particular, both terminal- and bridging-nitrides possess attractive advantages for potential reactivity, especially in light of the recent development of uranium complexes for dinitrogen reduction and functionalization. The first molecular thorium bridged-nitride complexes have also been recently identified, anticipating the possibility of expanding nitride chemistry not only to low-valent thorium, but also to the transuranic elements.

Stabilization of uranyl(v) by dipicolinic acid in aqueous medium

Preparation of a stable U(v) complex in an aqueous medium is a challenging task owing to its disproportionation nature (conversion into more stable U(vi) and U(iv) species) and sensitivity to atmospheric oxygen. The stable uranyl (UO₂²⁺)/dipicolinic acid (DPA) complex ([U^(VI)O₂(DPA)(OH)(H₂O)]^-) was formed at pH 10.5-12.0, which was confirmed by potentiometric and spectrophotometric titrations, and NMR, ESI-MS and EXAFS spectroscopy. The complex [U^(VI)O₂(DPA)(OH)(H₂O)]^- can be electrochemically reduced on the Pt electrode at -0.9 eV (vs. Ag/AgCl) to [U^(V)O₂(DPA)(OH)(H₂O)]^2- in aqueous medium under an anaerobic environment. According to cyclic voltammetric analysis, a pair of oxidation and reduction waves at E'₀ = -0.592 V corresponds to the [U^(VI)O₂(DPA)(OH)(H₂O)]^-/[U^(V)O₂(DPA)(OH)(H₂O)]^2- redox couple and the formation of [U^(V)O₂(DPA)(OH)(H₂O)]^2- was confirmed by the electron stoichiometry (n = 0.97 ± 0.05) of the reduction reaction of [U^(VI)O₂(DPA)(OH)(H₂O)]^-. The pentavalent uranyl complex [U^(V)O₂(DPA)(OH)(H₂O)]^2- was further characterized via UV-vis-NIR absorption spectrophotometry and X-ray absorption (XANES and EXAFS) spectroscopy. The [U^(V)O₂(DPA)(OH)(H₂O)]^2- complex is stable at pH 10.5-12.0 in anaerobic water for a few days. DFT calculation shows the strong complexing ability of DPA stabilizing the unstable oxidation state U(v) in aqueous medium.

Enantiomerically Pure Tetravalent Neptunium Amidinates: Synthesis and Characterization

The synthesis of a tetravalent neptunium amidinate [NpCl((S)-PEBA)3 ] (1) ((S)-PEBA=(S,S)-N,N'-bis-(1-phenylethyl)-benzamidinate) is reported. This complex represents the first structurally characterized enantiopure transuranic compound. Reactivity studies with halide/pseudohalides yielding [NpX((S)-PEBA)3 ] (X=F (2), Br (3), N3 (4)) have shown that the chirality-at-metal is preserved for all compounds in the solid state. Furthermore, they represent an unprecedented example of a structurally characterized metal-organic Np complex featuring a Np-Br (3) bond. In addition, 4 is the only reported tetravalent transuranic azide. All compounds were additionally characterized in solution using para-magnetic NMR spectroscopy showing an expected C3 -symmetry at low temperatures.

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 L^ArUI(DME) ((L^Ar)²⁻ = 2,2′′-bis(Dippanilide)-p-terphenyl; Dipp = 2,6-diisopropylphenyl) with LiNIm^Dipp ((NIm^Dipp)⁻ = 1,3-bis(Dipp)-imidazolin-2-iminato) generates the sterically congested 3N-coordinate compound L^ArU(NIm^Dipp) (1). Complex 1 reacts with 1 equiv. of Ph₃CN₃ to give the U(iv) azide L^ArU(N₃)(NIm^Dipp) (2). Structural analysis of 2 reveals inequivalent N_α–N_β > N_β–N_γ 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-L^Ar)U(NIm^Dipp) (3) via intramolecular N-atom insertion into the benzylic C–H bond of a pendant isopropyl group of the (L^Ar)²⁻ ligand. The formation of 3 occurs as a result of the intramolecular interception of the intermediately generated, terminal uranium nitride (L^Ar)U(N)(NIm^Dipp) (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 (L^Ar)U[NCN(C₆H₃Me₂)](NIm^Dipp) (5) and (N,C-L^Ar*)U(N Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> PMe₃)(NIm^Dipp) (6), respectively. These results expand upon the limited reactivity studies of terminal uranium–nitride moieties and provide new insights into their chemical properties.

Photolysis of the U(iv) azide L^ArU(NIm^Dipp) generates a reactive uranium nitride intermediate that can be intercepted by nucleophilic substrates – the first example of intermolecular chemistry of a rare photochemically generated uranium nitride.

Effect of the coordination of π-acceptor 4-cyanopyridine ligand on the structural and electronic properties ofmeso-tetra(para-methoxy) andmeso-tetra(para-chlorophenyl) porphyrin cobalt(ii) coordination compounds. Application in the catalytic degradation of methylene blue dye

To examine the influence of both the important π-acceptor character of the 4-cyanopyridine ligand and the nature of the para-substituted phenyls of meso-porphyrins on the electronic, electrochemical and structural properties of cobaltous metalloporphyrins, we prepared and fully characterized two coordination compounds: the (4-cyanopyridine)[meso-tetra(para-methoxyphenyl)porphyrinato]cobalt(ii) and the (4-cyanopyridine)[meso-tetra(para-chlorophenyl)porphyrinato]cobalt(ii) with the [Co^II(TMPP)(4-CNpy)] and [Co^II(TClPP)(4-CNpy)] formulas (complexes 1–2). The solution structures of compounds 1–2 were confirmed by ¹H NMR spectroscopy and mass spectrometry methods. They were further characterized by cyclic voltammetry and photoluminescence studies. The X-ray molecular structure data show that the Co-TClPP-4-NCpy derivative (2) exhibits high ruffling deformation compared to that of the Co-TMPP-4-CNpy species (1). Notably, the crystal packing of complex 1 shows the formation of Co⋯Co supramolecular dimers with a distance of 5.663 Å. As an application of our two cobaltous compounds, an investigation involving complexes 1–2 in the degradation of the methylene blue dye in the presence and absence of H₂O₂ in aqueous solutions was carried out. These promising results show that 1–2 can be used as catalysts in the degradation processes of dyes.

Preparation and UV/vis, IR, MS, ¹H NMR, cyclic voltammetry and molecular structures of two new Co(ii) complexes with para-methoxy-phenyl and para-chloro meso-porphyrins and 4-cyanopyridine ligand (1–2). Catalytic oxidation data of MB dye using 1–2.

Thorium- and uranium-azide reductions: a transient dithorium-nitride versus isolable diuranium-nitrides

Molecular uranium-nitrides are now well known, but there are no isolable molecular thorium-nitrides outside of cryogenic matrix isolation experiments. We report that treatment of [M(Tren^DMBS)(I)] (M = U, 1; Th, 2; Tren^DMBS = {N(CH₂CH₂NSiMe₂Bu ^t )₃}^3-) with NaN₃ or KN₃, respectively, affords very rare examples of actinide molecular square and triangle complexes [{M(Tren^DMBS)(μ-N₃)} _n ] (M = U, n = 4, 3; Th, n = 3, 4). Chemical reduction of 3 produces [{U(Tren^DMBS)}₂(μ-N)][K(THF)₆] (5) and [{U(Tren^DMBS)}₂(μ-N)] (6), whereas photolysis produces exclusively 6. Complexes 5 and 6 can be reversibly inter-converted by oxidation and reduction, respectively, showing that these UNU cores are robust with no evidence for any C-H bond activations being observed. In contrast, reductions of 4 in arene or ethereal solvents gives [{Th(Tren^DMBS)}₂(μ-NH)] (7) or [{Th(Tren^DMBS)}{Th(N[CH₂CH₂NSiMe₂Bu ^t ]₂CH₂CH₂NSi[μ-CH₂]MeBu ^t )}(μ-NH)][K(DME)₄] (8), respectively, providing evidence unprecedented outside of matrix isolation for a transient dithorium-nitride. This suggests that thorium-nitrides are intrinsically much more reactive than uranium-nitrides, since they consistently activate C-H bonds to form rare examples of Th-N(H)-Th linkages with alkyl by-products. The conversion here of a bridging thorium(iv)-nitride to imido-alkyl combination by 1,2-addition parallels the reactivity of transient terminal uranium(iv)-nitrides, but contrasts to terminal uranium(vi)-nitrides that produce alkyl-amides by 1,1-insertion, suggesting a systematic general pattern of C-H activation chemistry for metal(iv)- vs. metal(vi)-nitrides. Surprisingly, computational studies reveal a σ > π energy ordering for all these bridging nitride bonds, a phenomenon for actinides only observed before in terminal uranium nitrides and uranyl with very short U-N or U-O distances.

Schiff base thorium(iv) and uranium(iv) chloro complexes: synthesis, substitution and oxidation chemistry

Schiff base chloro complexes of U(iv) and Th(iv) are prepared and provide access to rare pseudo trans diazide species, and a facile pathway to uranyl complexes through oxidation with NaNO₂.

New Twists and Turns for Actinide Chemistry: Organometallic Infinite Coordination Polymers of Thorium Diazide

Two organometallic 1D infinite coordination polymers and two organometallic monometallic complexes of thorium diazide have been synthesized and characterized. Steric control of these self-assembled arrays, which are dense in thorium and nitrogen, has also been demonstrated: infinite chains can be circumvented by using steric bulk either at the metallocene or with a donor ligand in the wedge.

Uranium(III) redox chemistry assisted by a hemilabile bis(phenolate) cyclam ligand: uranium-nitrogen multiple bond formation comprising a trans-{RN═U(VI)═NR}(2+) complex

A new monoiodide U(III) complex anchored on a hexadentate dianionic 1,4,8,11-tetraazacyclotetradecane-based bis(phenolate) ligand, [U(κ(6)-{((tBu2)ArO)2Me2-cyclam})I] (1), was synthesized from the reaction of [UI3(THF)4] (THF = tetrahydrofuran) and the respective potassium salt K2((tBu2)ArO)2Me2-cyclam and structurally characterized. Reactivity of 1 toward one-, two-, and four-electron oxidants was studied to explore the reductive chemistry of this new U(III) complex. Complex 1 reacts with one-electron oxidizers, such as iodine and TlBPh4, to form the seven-coordinate cationic uranium(IV) complexes [U(κ(6)-{((tBu2)ArO)2Me2-cyclam})I][X] (X = I (2-I), BPh4 (2-BPh4)). The new uranium(III) complex reacts with inorganic azides to yield the pseudohalide uranium(IV) complex [U(κ(6)-{((tBu2)ArO)2Me2-cyclam})(N3)2] (4) and the nitride-bridged diuranium(IV/IV) complex [(κ(4)-{((tBu2)ArO)2Me2-cyclam})(N3)U(μ-N)U(κ(5)-{((tBu2)ArO)2Me2-cyclam})] (5). Two equivalents of [U(κ(6)-{((tBu2)ArO)2Me2-cyclam})I] (1) effect the four-electron reduction of 1 equiv of PhN═NPh to form the bis(imido) complex [U(κ(4)-{((tBu2)ArO)2Me2-cyclam})(NPh)2] (6) and the U(IV) species 2-I. Moreover, the hemilability of the hexadentate ancillary ligand ((tBu2)ArO)2Me2-cyclam(2-) allows to perform the reductive cleavage of azobenzene with an unprecedented formation of a trans-bis(imido) complex. The complexes were characterized by NMR spectroscopy, and all the new uranium complexes were structurally authenticated by single-crystal X-ray diffraction.

CO2 conversion to isocyanate via multiple N–Si bond cleavage at a bulky uranium(iii) complex

CO₂ fixation by a bulky uranium(iii) tetrasilylamido complex leads to CO₂ insertion into the U–N bond affording a U(iv) isocyanate.

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.

Bent thorocene complexes with the cyanide, azide and hydride ligands

Reaction of the linear thorocene with NC(-), N3(-) and H(-) led to the bent derivatives [(Cot)2Th(X)](-) (X = CN, N3) and the bimetallic [{(Cot)2Th}2(μ-H)](-), whereas only [(Cot)2U(CN)](-) could be formed from (Cot)2U.

Iodide, azide, and cyanide complexes of (N,C), (N,N), and (N,O) metallacycles of tetra- and pentavalent uranium

In contrast to the neutral macrocycle [UN*(2)(N,C)] (1) [N* = N(SiMe(3))(3); N,C = CH(2)SiMe(2)N(SiMe(3))] which was quite inert toward I(2), the anionic bismetallacycle [NaUN*(N,C)(2)] (2) was readily transformed into the enlarged monometallacycle [UN*(N,N)I] (4) [N,N = (Me(3)Si)NSiMe(2)CH(2)CH(2)SiMe(2)N(SiMe(3))] resulting from C-C coupling of the two CH(2) groups, and [NaUN*(N,O)(2)] (3) [N,O = OC(═CH(2))SiMe(2)N(SiMe(3))], which is devoid of any U-C bond, was oxidized into the U(V) bismetallacycle [Na{UN*(N,O)(2)}(2)(μ-I)] (5). Sodium amalgam reduction of 4 gave the U(III) compound [UN*(N,N)] (6). Addition of MN(3) or MCN to the (N,C), (N,N), and (N,O) metallacycles 1, 4, and 5 led to the formation of the anionic azide or cyanide derivatives M[UN*(2)(N,C)(N(3))] [M = Na, 7a or Na(15-crown-5), 7b], M[UN*(2)(N,C)(CN)] [M = NEt(4), 8a or Na(15-crown-5), 8b or K(18-crown-6), 8c], M[UN*(N,N)(N(3))(2)] [M = Na, 9a or Na(THF)(4), 9b], [NEt(4)][UN*(N,N)(CN)(2)] (10), M[UN*(N,O)(2)(N(3))] [M = Na, 11a or Na(15-crown-5), 11b], M[UN*(N,O)(2)(CN)] [M = NEt(4), 12a or Na(15-crown-5), 12b]. In the presence of excess iodine in THF, the cyanide 12a was converted back into the iodide 5, while the azide 11a was transformed into the neutral U(V) complex [U(N{SiMe(3)}SiMe(2)C{CHI}O)(2)I(THF)] (13). The X-ray crystal structures of 4, 7b, 8a-c, 9b, 10, 12b, and 13 were determined.