Unique uranium(III) cations anchored by polydentate sulfur-based ligands: synthesis and structure of [U[H(R)B(tim(Me))(2)](2)(THF)(3)][BPh(4)] (R = H, Ph)

Dehydration of UO2Cl2·3H2O and Nd(NO3)3·6H2O with a Soft Donor Ligand and Comparison of Their Interactions through X-ray Diffraction and Theoretical Investigation

We investigated whether the relatively Lewis basic imidazole-2-thiones could be used to substitute water ligands bound to f-element cations and generate f-element soft donor complexes. Reactions of 1,3-diethylimidazole-2-thione (C₂C₂ImT) with Nd(NO₃)₃·6H₂O and UO₂Cl₂·3H₂O led to the isolation of the anhydrous thione complexes Nd(NO₃)₃(C₂C₂ImT)₃ and UO₂Cl₂(C₂C₂ImT)₂, characterized by single crystal X-ray diffraction. Differences in the strength of metal-thione interactions have been examined by means of the crystal structure analysis and density functional theory (DFT) calculations. The C₂C₂ImT ligands were found to be affected by both coordination and noncovalent interactions, making it impossible to deconvolute the effects of one from the other. Calculated partial atomic charges indicated greater ligand-to-metal charge transfer in the [UO₂]²⁺ complex, indicative of a stronger interaction. The reactivity of C₂C₂ImT demonstrates its usefulness in the preparation of f-element soft donor complexes from readily available hydrates that could be useful intermediates for promoting the coordination and studying the effects of soft donor anions.

How important is the coordinating atom in controlling magnetic anisotropy in uranium(iii) single-ion magnets? A theoretical perspective

Theoretical investigation of actinide based nanomagnets is of paramount interest in the field of molecular magnetism as they offer remarkable properties compared to their lanthanide counterparts. Unlike lanthanides, the magnetic properties of actinides can be fine-tuned by modulating the ligand field as they possess a large metal-ligand covalency. In this regard, two complexes reported earlier have gained attention: [U(BcMe)₃] (1) has been found to show Single-ion Magnet (SIM) characteristics whereas isomeric [U(BpMe)₃] (2) does not exhibit any SIM behaviour. To unravel the origin of the differences observed in magnetic anisotropy, a detailed ab initio CASSCF study has been undertaken on the X-ray structure of complexes 1 and 2. Since actinide compounds exhibit strong covalency, the desired active space needs to be benchmarked to address this issue. Here, we have enlarged the active space systematically from CAS(3,7) to CAS(3,12) where all 5f electrons in 5f orbitals are sequentially expanded to include five formally empty 6d orbitals. Our calculations reveal that the incorporation of the 6d_z² orbital is vital in reproducing many experimental observables such as temperature dependent susceptibility, g-factors, ground state m_J level, and ground-state-excited-state gap. Inclusion of this orbital in the reference space is found to describe better the UH-BH agostic interactions leading to significant variations in the computed parameters. Gaining from this understanding, we have carried out extensive bonding analysis within the DFT framework using tools such as Natural Bond Orbital (NBO) and Atoms In Molecule (AIM) to further probe these weak agostic interactions. Also, predictions to enhance the U-ligand covalency using U-sulphur bonds and the role of the U-C distance and C-U-C bite angles in the nature of anisotropy have been studied, and relevant magneto-structural correlations have been developed. Thus our results for the first time provide a comprehensive understanding of uranium based SMMs and offer ways to fine tune the anisotropy for experimental chemists.

Uranium(iii) complexes supported by hydrobis(mercaptoimidazolyl)borates: synthesis and oxidation chemistry

The reaction of [UI3(thf)4] with the sodium or lithium salts of hydrobis(2-mercapto-1-methylimidazolyl)borate ligands ([H(R)B(timMe)2]-) in a 1 : 2 ratio, in tetrahydrofuran, gave the U(iii) complexes [UI{κ3-H,S,S'-H(R)B(timMe)2}2(thf)2] (R = H (1), Ph (2)) in good yields. Crystals of [UI{κ3-H,S,S'-H(Ph)B(timMe)2}2(thf)2] (2) were obtained by recrystallization from a tetrahydrofuran/acetonitrile solution, and the ion-separated uranium complex [U{κ3-H,S,S'-H(Ph)B(timMe)2}2(CH3CN)3][I] (3-I) was obtained by dissolution of 2 in acetonitrile followed by recrystallization. One-electron oxidation of 2 with AgBPh4 or I2 resulted in the formation of the cationic U(iv) complexes [U{κ3-H,S,S'-H(Ph)B(timMe)2}3][X] (X = BPh4 (6-BPh4), I (6-I)), due to a ligand redistribution process. These complexes are the first examples of homoleptic poly(azolyl)borate U(iv) complexes. Treatment of complex 2 with azobenzene led to the isolation of crystals of the U(iv) compound [UI{κ3-H(Ph)B(timMe)2}2(κ2-timMe)] (7). Treatment of 2 with pyridine-N oxide (pyNO) led to the formation of the uranyl complex [UO2{κ2-S,S'-H(Ph)B(timMe)2}2] (8) and of complex 6-I, while from the reaction of [U{κ3-H(Ph)B(timMe)2}2(thf)3][BPh4] (5) with pyNO, the oxo-bridged U(iv) complex [{U{κ3-H(Ph)B(timMe)2}2(pyNO)}2(μ-O)][BPh4]2 (9) was also obtained. In the U(iii) and U(iv) complexes, the bis(azolyl)borate ligands bind to the uranium center in a κ3-H,S,S' coordination mode, while in the U(vi) complex the ligands bind to the metal in a κ2-S,S' mode. The presence of UH-B interactions in the solid-state, for the nine-coordinate complexes 1, 2, 3, 6 and 7 and for the eight-coordinate complex 9, was supported by IR spectroscopy and/or X-ray diffraction analysis.

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.

Molecular structures of tris(2-mercapto-1-tert-butylimidazolyl)hydroborato and tris(2-mercapto-1-adamantylimidazolyl)hydroborato sodium complexes: analysis of [Tm(R)] ligand coordination modes and conformations

The tris(mercaptoimidazolyl)hydroborato complexes, [κ(3)-S2H-Tm(Bu(t))]Na(THF)3 and [κ(3)-S2H-Tm(Ad)]Na(THF)3, which feature t-butyl and adamantyl substituents, have been synthesized via the reactions of the respective 1-R-1,3-dihydro-2H-imidazole-2-thiones with NaBH4 in THF (R = Bu(t), 1-Ad). X-ray diffraction studies indicate that the compounds are monomeric and that the [Tm(R)] ligands coordinate to the metal in a κ(3)-S2H manner via two of the sulfur donors and the hydrogen attached to boron, a combination that is unprecedented for sodium derivatives. Analysis of the tris(mercaptoimidazolyl)hydroborato compounds that are listed in the Cambridge Structural Database has allowed for the formulation of a set of criteria that enables κ(x)-S(x) and κ(x+1)-S(x)H coordination modes to be identified. Furthermore, the various κ(x)-S(x) and κ(x+1)-S(x)H coordination modes have also been analyzed with respect to the conformations of the [Tm(R)] ligands, which differ by rotation of the imidazolethione moieties about the B-N bond.

Synthesis and structural characterization of bis and tris(2-mercapto-1-methylbenzimidazolyl)hydroborato complexes: benzannulation promotes κ³-coordination

The benzannulated bis and tris(mercaptoimidazolyl)borohydride compounds, [BmMeBenz]Na and [TmMeBenz]Na, have been synthesized via the reactions of NaBH4 with two and three equivalents of 1-methyl-1,3-dihydro-2H-benzimidazole-2-thione, respectively. X-ray diffraction studies on the THF adducts, {μ-[BmMeBenz]Na(THF)₂}₂ and {[TmMeBenz]Na}₂(μ-THF)₃, indicate that both compounds are dinuclear but differ according to the nature of the bridging ligand. Specifically, {μ-[BmMeBenz]Na(THF)₂}₂ possesses bridging [BmMeBenz] ligands and terminal THF ligands, while {[TmMeBenz]Na}₂(μ-THF)₃ possesses terminal [TmMeBenz] ligands and bridging THF ligands. The tris(mercaptoimidazolyl)borohydride ligand of {[TmMeBenz]Na}₂(μ-THF)₃ coordinates in a κ³-manner, which is in marked contrast to the κ²-, κ¹- and κ⁰-modes that have been reported for various [TmMe]Na derivatives. Density functional theory (DFT) geometry optimization calculations of the anions [TmMeBenz]⁻ and [TmMe]⁻ in the gas phase indicate that the conformation required for κ³-S₃ coordination, i.e. one in which the three sulfur donors point away from the B-H group, is relatively more stable for [TmMeBenz]⁻ than for [TmMe]⁻, and thus provides a rationalization for the observation that benzannulation enables κ³-coordination of tris(mercaptoimidazolyl)borohydride ligand in {[TmMeBenz]Na}₂(μ-THF)₃. Furthermore, comparison of the molecular structure and IR spectroscopic properties of [TmMeBenz]Re(CO)₃ with those of [TmMe]Re(CO)₃ indicates that benzannulation reduces the electron donating properties of the ligand, but has little effect on its steric properties. {μ-[BmMeBenz]Na(THF)₂}₂ and {[TmMeBenz]Na}₂(μ-THF)₃ react with [Me₃PCuCl]₄ to give [BmMeBenz]CuPMe₃ and [TmMeBenz]CuPMe₃, the first pair of structurally related bis and tris(mercaptoimidazolyl)hydroborato copper(I) compounds.

Applications of Tripodal [S(3)] and [Se(3)] L(2)X Donor Ligands to Zinc, Cadmium and Mercury Chemistry: Organometallic and Bioinorganic Perspectives

The tripodal tris(2-mercapto-1-R-imidazolyl)hydroborato ligand system, [Tm(R)], and its selenium counterpart, [Tse(R)], provide useful platforms for investigating organometallic and bioinorganic aspects of the chemistry of zinc, cadmium and mercury in sulfur-rich and selenium-rich coordination environments. For example, the tridentate [Tm(R)] ligand provides an [S(3)] donor array that is of use for mimicking aspects of zinc enzymes and proteins that have sulfur-rich active sites, such as the Ada DNA repair protein. With respect to mercury, an interesting application of the [Tm(Bu(t) )] ligand is the synthesis of the mercury alkyl compounds [Tm(Bu(t) )]HgR (R = Me, Et) that react with PhSH to yield [Tm(Bu(t) )]HgSPh and RH, a reaction that emulates mercury detoxification by the organomercurial lyase, MerB. In addition to the tridentate [Tm(R)] and [Tse(R)] ligands, applications of the bidentate counterparts, [Bm(R)] and [Bse(R)] are also described.

Bis- and tris(2-seleno-1-methylimidazolyl)hydroborato complexes, {[BseMe]ZnX}2(X = Cl, I), [BseMe]2Zn and [TseMe]Re(CO)3: Structural evidence that the [BseMe] ligand is not merely a “heavier” version of the sulfur counterpart, [BmMe]

New bidentate and tridentate ligands that feature selenium donors, namely the bis- and tris(2-seleno-1-methylimidazolyl)hydroborato ligands [BseMe] and [TseMe], have been constructed via the reaction of MBH4(M=Na, K) with 1-methylimidazole-2-selone. Comparison of the structure of {[BseMe]ZnI}2 with its sulfur counterpart, [BmMe]ZnI, demonstrates that the seleno ligand exhibits a greater tendency than the mercapto ligand to bridge two metal centers, while comparison of [TseMe]Re(CO)3 and [TseMes]Re(CO)3 indicates that the [TseMe] ligand is more electron donating and less sterically demanding than the [TseMes] ligand.

Practical Synthetic Routes to Solvates of U(OTf)3: X-ray Crystal Structure of [U(OTf)3(MeCN)3]n, a Unique U(III) Coordination Polymer

The reaction of UH3 or U metal with triflic acid results in the formation of a mixture of species including U(OTf)4 and leads to the reproducible isolation of the mononuclear U(IV) hydroxo complex [U(OTf)3(OH)(py)4] (1) and the U(IV) dinuclear mu-oxo-complex [{U(OTf)2(py)3}2{mu-O}{mu-OTf}2] (2). The X-ray crystal structures of these complexes have been determined. Analytically pure complex 1 can be prepared in a 17-27% yield providing a good precursor for the synthesis and study of the reactivity of the hydroxo complexes with different coordination environments. Two practical synthetic methods for the preparation of Lewis base adducts of U(OTf)3 are described. Analytically pure [U(OTf)3(py)4] (4) was easily and reproducibly prepared (50-60% yield) by protonolysis of the amide U{N(SiMe3)2}3 with pyridinium triflate in pyridine. Salt metathesis of UI3(thf)4 with potassium triflate in acetonitrile resulted in the complete substitution of the iodide counterions by triflate producing the acetonitrile solvate [U(OTf)3(MeCN)3]n (3). The solid-state structure of 3 shows the formation of a unique U(III) coordination polymer in which the metal ions are connected by three triflates acting as bidentate bridging ligands to form a 1D chain.

Homoleptic uranium(iii) imidodiphosphinochalcogenides including the first structurally characterised molecular trivalent actinide–Se bond

Preparation of U[N(EPPh2)2]3 (E = S or Se) by treating U[N(SiMe3)2]3 with three equivalents of NH(EPPh2)2 is described together with presentation of a hitherto unreported molecular crystal structure containing An(III)-Se bonds (An = actinide).

Caveats for poly(methimazolyl)borate chemistry: the novel inorganic heterocycles [H2C(mt)2BR2]Cl (mt = methimazolyl; BR2 = BH2, BH(mt), 9-BBN)

Whilst frequently used for reactions of poly(methimazolyl)borates, dichloromethane is not an innocent solvent, but rather slowly forms heterocyclic salts [H(2)C(mt)(2)BR(2)]Cl, three examples of which (BR(2) = BH(2), BH(mt), 9-borabicyclononyl) have been structurally characterised to confirm the unprecedented B(NCS)(2)C connectivity.

Oxidation chemistry of uranium(III) complexes of Tpa: synthesis and structural studies of oxo, hydroxo, and alkoxo complexes of uranium(IV)

The crystal structure of the complex [U(tpa)(2)]I(3), 1 (tpa = tris[(2-pyridyl)methyl]amine), has been elucidated. The complex exists as only one enantiomer in the crystal leading to the chiral space group P2(1)2(1)2(1). The coordination geometry of the metal can be described as a distorted cube. Accidental oxidation of [U(tpa)(2)]I(3) led to the isolation of the unusual mononuclear bishydroxo complex of uranium(IV) [U(tpa)(2)(OH)(2)]I(2).3CH(3)CN, 2, which was structurally characterized. The controlled reaction of [U(tpa)(2)]I(3) with water resulted in the oxidation of the metal center and led to the formation of protonated tpa and of the trinuclear U(IV) oxo complex ([U(tpa)(mu-O)I](3)(mu(3)-I))I(2), 3. The solid state and solution structures of this trimer are reported. The pathway suggested for the formation of this complex is the oxidation of the [U(tpa)(2)]I(3) complex by H(2)O to form a U(IV) hydroxo complex which then decomposes, eliminating mono-protonated tpa. The comparison with the reported reaction with water of cyclopentadienyl derivatives points to a higher reactivity toward water reduction of the bis(tpa) complex with respect to the cyclopentadienyl derivatives. The reaction of U(III) with methanol in the presence of the supporting ligand tpa leads to formation of alkoxo complexes similarly to what is found for amide or cyclopentadienyl derivatives. The monomethoxide complex [U(tpa)I(3)(OMe)], 4, has been prepared in good yield by alcoholysis of the U(III) mono(tpa) complex. The crystal structure of this complex has been determined. The reaction of [U(tpa)(2)]I(3) with 2 equiv of methanol in acetonitrile allows the isolation of the bismethoxo complex of U(IV) [U(tpa)I(2)(OMe)(2)], 5, in 35-47% yield, which has been fully characterized. To account for the oxidation of U(III) to U(IV) the suggested mechanism assumes that hydrogen is evolved in both reactions.

Synthesis and structural characterization of unprecedented bis-asymmetric heteroscorpionate U(III) complexes: [U(kappa3-H2B(pztBu,Me)(pzMe,tBu))2I] and [U(kappa3-H2B(pztBu,Me)(pzMe2))2I]

[UI(3)(THF)(4)] reacts at room temperature with 2 equiv of KBp(tBu,Me), in toluene, yielding [U(kappa(3)-H(mu-H)B(pz(tBu,Me))(pz(Me,tBu)))(2)I] (1). This unprecedented complex, stabilized by two asymmetric heteroscorpionate ligands, is formed due to an isomerization process promoted in situ by the metal center. To find a general method for preparing this type of compound, we synthesized the novel asymmetric K[H(2)B(pz(tBu,Me))(pz(Me2))], and by a straightforward salt metathesis with [UI(3)(THF)(4)] the novel bis-asymmetric complex [U(kappa(3)-H(mu-H)B(pz(tBu,Me))(pz(Me2)))(2)I] (2) was isolated and characterized in the solid state and in solution. As indicated by X-ray crystallographic analysis, the U(III) in 1 and 2 is seven-coordinated by two tridentate asymmetric dihydrobis(pyrazoly)borates and by an iodide. In both cases, the coordination geometry around the metal is very distorted, the pentagonal bipyramid being the one which better describes the arrangement of the atoms around the U(III). An approximate C(2) axis can be defined in the solid state, and is maintained in solution as indicated by the (1)H NMR spectrum of 1 and 2. In the course of attempting to crystallize some of the compounds, monocrystals of the dimer [U(kappa(3)-Bp(tBu,Me))(Hpz(tBu,Me))I(mu-I)](2) (3) were isolated. In this compound each U(III) atom is seven-coordinated by one kappa(3)-Bp(tBu,Me), by one terminal and by two bridging iodide ligands, and by a monodentate Hpz(tBu,Me), exhibiting a distorted 4:3 tetragonal base-trigonal geometry.

Homoleptic group 12 metal bis(mercaptoimidazolyl)borate complexes M(Bm(R))2 (M = Zn, Cd, Hg)

The sodium salt of the bis(2-mercapto-1-methylimidazolyl)borate anion [Bm(Me)](-) and those of the new bis(2-mercapto-1-alkylimidazolyl)borates [Bm(R)](-) (R = Bz, Bu(t), p-Tol) have been readily obtained from NaBH(4) and the appropriate 2-mercapto-1-alkylimidazoles. To contrast the binding preferences of the group 12 metals in a sulfur-rich environment, the four complete series of homoleptic complexes M[Bm(R)](2) (M = Zn, Cd, Hg), including the first bis(mercaptoimidazolyl)borate derivatives of cadmium and mercury, have been prepared. X-ray diffraction studies of Cd[Bm(Me)](2) and M[Bm(tBu)](2) (M = Zn, Cd, Hg) show the presence of distorted tetrahedral [MS(4)] central cores supplemented by two weak vicinal M.H-B bonds, interactions which appear to be a common feature in the coordination chemistry of Bm(R) ligands. In the case of zinc, it has been found that only in the presence of bulky ligands, as in Zn[Bm(tBu)](2), may an unexpected expansion in the coordination number from four to six be induced. This observation suggests the viability of octahedral intermediates in the processes whereby certain zinc enzymes transfer or exchange metal ions.