Complexation studies of iodides of trivalent uranium and lanthanides (Ce and Nd) with 2,2'-bipyridine in anhydrous pyridine solutions

Influence of 1,2,4-Tri-tert-butylcyclopentadienyl Ligand on the Reactivity of the Thorium Bipyridyl Metallocene [η5-1,2,4-(Me3C)3C5H2]2Th(bipy)]

The thorium bipyridyl metallocene (Cp3tBu)2Th(bipy) (1; Cp3tBu = η5-1,2,4-(Me3C)3C5H2) shows a rich reactivity toward a series of small molecules. For example, complex 1 may act as a synthon for the (Cp3tBu)2Th(II) fragment as illustrated by its reactivity toward to CuI, hydrazine derivative (PhNH)2, Ph2E2 (E = S, Se), elemental sulfur (S8) and selenium (Se), organic azides, CS2, and isothiocyanates. Moreover, in the presence of polar multiple bonds, such as those in ketones Ph2CO and (CH2)5CO, aldehydes p-MePhCHO and p-ClPhCHO, seleno-ketone (p-MeOPh)2CSe, nitriles PhCN, Ph2CHCN, C6H11CN, and p-(NC)2Ph, and benzoyl cyanide PhCOCN, C-C coupling occurs to furnish (Cp3tBu)2Th[(bipy)(Ph2CO)] (10), (Cp3tBu)2Th[(bipy)((CH2)5CO)] (11), (Cp3tBu)2Th[(bipy)(p-MePhCHO)] (12), (Cp3tBu)2Th[(bipy)(p-ClPhCHO)] (13), (Cp3tBu)2Th[(bipy){(p-MeOPh)2CSe}] (14), (Cp3tBu)2Th[(bipy)(PhCN)] (16), (Cp3tBu)2Th[(bipy)(Ph2CHCN)] (17), (Cp3tBu)2Th[(bipy)(C6H11CN)] (18), [(Cp3tBu)2Th]2{μ-(bipy)[p-Ph(CN)2](bipy)} (20), and (Cp3tBu)2Th{(bipy)[PhC(CN)O]} (21), respectively. Nevertheless, ketazine (PhCH═N)2 or benzyl nitrile PhCH2CN forms the dimeric complexes [(Cp3tBu)Th]2[μ-NC(Ph)(bipy)]2 (15) and (Cp3tBu)2Th[(bipy){C(═CHPh)NH}] (19), respectively. In contrast, C-N bond cleavage and C-C coupling processes occur upon addition of isonitriles Me3CNC and C6H11NC to 1 to yield the thorium isocyanido amido complexes (Cp3tBu)2Th[4-(Me3C)bipy](NC) (22) and (Cp3tBu)2Th[4-(C6H11)bipy](NC) (23), respectively. Furthermore, a single-electron transfer (SET) process ensues when 1 equiv of CuI is added to 1 to yield the Th(VI) bipyridyl iodide complex (Cp3tBu)2Th(I)(bipy) (3).

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.

Small-Molecule Activation Mediated by [η5-1,3-(Me3Si)2C5H3]2U(bipy)

The uranium bipyridyl metallocene, [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U(bipy) (2), is readily accessible in good yield by adding potassium graphite (KC₈) to a mixture of [η⁵-1,3-(Me₃Si)₂C₅H₃]₂UCl₂ (1) and 2,2'-bipyridine. Compound 2 was fully characterized and employed for small-molecule activation. It has been demonstrated that 2 may serve as a synthon for [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U(II) fragment in the presence of Ph₂E₂ (E = S, Se), alkynes, and a variety of hetero-unsaturated molecules such as diazabutadienes, azine (Ph₂C═N)₂, o-benzoquinone, pyridine N-oxide, CS₂, isothiocyanates, and organic azides. However, upon exposure of 2 to thio-ketone Ph₂CS, aldehyde p-MePhCHO, ketone Ph₂CO, imine PhCH═NPh, azine (PhCH═N)₂, and nitrile PhCN, it may also promote C-C coupling reactions forming [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U[(bipy)(Ph₂CS)] (16), [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U[(bipy)(p-MePhCHO)] (17), [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U[(bipy)(Ph₂CO)] (18), [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U[(bipy)(PhCHNPh)] (19), [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U[(bipy)(PhCHNN═CHPh)] (20), and [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U[(N₂C₁₀H₇C(Ph)NH)] (22), respectively, in quantitative conversion. Furthermore, in the presence of CuI, a single-electron transfer (SET) process is observed to yield the uranium(III) iodide complex [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U(I)(bipy) (15).

Monomeric thorium chalcogenolates with bipyridine and terpyridine ligands

Thorium chalcogenolates Th(ER)4 react with 2,2'-bipyridine (bipy) to form complexes with the stoichiometry (bipy)2Th(ER)4 (E = S, Se; R = Ph, C6F5). All four compounds have been isolated and characterized by spectroscopic methods and low-temperature single crystal X-ray diffraction. Two of the products, (bipy)2Th(SC6F5)4 and (bipy)2Th(SeC6F5)4, crystallize with lattice solvent, (bipy)2Th(SPh)4 crystallizes with no lattice solvent, and the selenolate (bipy)2Th(SePh)4 crystallizes in two phases, with and without lattice solvent. In all four compounds the available volume for coordination bounded by the two bipy ligands is large enough to allow significant conformational flexibility of thiolate or selenolate ligands. 77Se NMR confirms that the structures of the selenolate products are the same in pyridine solution and in the solid state. Attempts to prepare analogous derivatives with 2,2',6',2''-terpyridine (terpy) were successful only in the isolation of (terpy)(py)Th(SPh)4, the first terpy compound of thorium. These materials are thermochromic, with color attributed to ligand-to-ligand charge transfer excitations.

Small-Scale Metal-Based Syntheses of Lanthanide Iodide, Amide, and Cyclopentadienyl Complexes as Analogues for Transuranic Reactions

Small-scale reactions of the Pu analogues La, Ce, and Nd have been explored in order to optimize reaction conditions for milligram scale reactions of radioactive plutonium starting from the metal. Oxidation of these lanthanide metals with iodine in ether and pyridine has been studied, and LnI₃(Et₂O)_x (1-Ln; x = 0.75-1.9) and LnI₃(py)₄ (2-Ln; py = pyridine, NC₅H₅) have been synthesized on scales ranging from 15 mg to 2 g. The THF adducts LnI₃(THF)₄ (3-Ln) were synthesized by dissolving 1-Ln in THF. The viability of these small-scale samples as starting materials for amide and cyclopentadienyl f-element complexes was tested by reacting KN(SiMe₃)₂, KCp' (Cp' = C₅H₄SiMe₃), KCp'' (Cp'' = C₅H₃(SiMe₃)₂-1,3), and KC₅Me₄H with 1-Ln generated in situ. These reactions produced Ln[N(SiMe₃)₂]₃ (4-Ln), Cp'₃Ln (5-Ln), Cp″₃Ln (6-Ln), and (C₅Me₄H)₃Ln (7-Ln), respectively. Small-scale samples of Cp'₃Ce (5-Ce) and Cp'₃Nd (5-Nd) were reduced with potassium graphite (KC₈) in the presence of 2.2.2-cryptand to check the viability of generating the crystallographically characterizable Ln²⁺ complexes [K(2.2.2-cryptand)][Cp'₃Ln] (8-Ln; Ln = Ce, Nd).

Radical anionic versus neutral 2,2'-bipyridyl coordination in uranium complexes supported by amide and ketimide ligands

The synthesis and characterization of (bipy)(2)U(N[t-Bu]Ar)(2) (1-(bipy)(2), bipy = 2,2'-bipyridyl, Ar = 3,5-C(6)H(3)Me(2)), (bipy)U(N[(1)Ad]Ar)(3) (2-bipy), (bipy)(2)U(NC[t-Bu]Mes)(3) (3-(bipy)(2), Mes = 2,4,6-C(6)H(2)Me(3)), and IU(bipy)(NC[t-Bu]Mes)(3) (3-I-bipy) are reported. X-ray crystallography studies indicate that bipy coordinates as a radical anion in 1-(bipy)(2) and 2-bipy, and as a neutral ligand in 3-I-bipy. In 3-(bipy)(2), one of the bipy ligands is best viewed as a radical anion, the other as a neutral ligand. The electronic structure assignments are supported by NMR spectroscopy studies of exchange experiments with 4,4'-dimethyl-2,2'-bipyridyl and also by optical spectroscopy. In all complexes, uranium was assigned a +4 formal oxidation state.

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.

Linear uranium metallocenes with polydentate aromatic nitrogen ligands

Treatment of [Cp*(2)U(NCMe)(5)]X(2) [Cp* = C(5)Me(5), X = BPh(4) (1) or I (1')] or Cp*(2)UI(2) in acetonitrile with the polydentate aromatic nitrogen bases phen, terpy and R(4)btbp led to the formation of the linear uranium metallocenes [Cp*(2)U(NCMe)(3)(phen)]X(2) [X = BPh(4) (2), I (2')], [Cp*(2)U(NCMe)(2)(terpy)][BPh(4)](2) (4), [Cp*(2)U(NCMe)(Me(4)btbp)][BPh(4)](2) (5) and [Cp*(2)U(NCMe)(CyMe(4)btbp)][X](2), [X = BPh(4) (6), I (6')], [phen = 1,10-phenanthroline, terpy = 2,2':6,2''-terpyridine, Me(4)btbp = 6,6'-bis-(3,3,6,6-tetramethyl-1,2,4-triazin-3-yl)-2,2'-bipyridine, CyMe(4)btbp = 6,6'-bis-(3,3,6,6-tetramethyl-cyclohexane-1,2,4-triazin-3-yl)-2,2'-bipyridine]. The bent metallocene [Cp*(2)U(phen)(2)][BPh(4)](2) (3) was isolated from the reaction of 1 and two molar equivalents of phen in THF. The X-ray crystal structures of 2.2MeCN, 3.2THF, and 6'.2MeCN were determined.

Structural Characterization of Methanol Substituted Lanthanum Halides

The first study into the alcohol solvation of lanthanum halide [LaX(3)] derivatives as a means to lower the processing temperature for the production of the LaBr(3) scintillators was undertaken using methanol (MeOH). Initially the de-hydration of {[La(micro-Br)(H(2)O)(7)](Br)(2)}(2) (1) was investigated through the simple room temperature dissolution of 1 in MeOH. The mixed solvate monomeric [La(H(2)O)(7)(MeOH)(2)](Br)(3) (2) compound was isolated where the La metal center retains its original 9-coordination through the binding of two additional MeOH solvents but necessitates the transfer of the innersphere Br to the outersphere. In an attempt to in situ dry the reaction mixture of 1 in MeOH over CaH(2), crystals of [Ca(MeOH)(6)](Br)(2) (3) were isolated. Compound 1 dissolved in MeOH at reflux temperatures led to the isolation of an unusual arrangement identified as the salt derivative {[LaBr(2.75)*5.25(MeOH)](+0.25) [LaBr(3.25)*4.75(MeOH)](-0.25)} (4). The fully substituted species was ultimately isolated through the dissolution of dried LaBr(3) in MeOH forming the 8-coordinated [LaBr(3)(MeOH)(5)] (5) complex. It was determined that the concentration of the crystallization solution directed the structure isolated (4 concentrated; 5 dilute) The other LaX(3) derivatives were isolated as [(MeOH)(4)(Cl)(2)La(micro-Cl)](2) (6) and [La(MeOH)(9)](I)(3)*MeOH (7). Beryllium Dome XRD analysis indicated that the bulk material for 5 appear to have multiple solvated species, 6 is consistent with the single crystal, and 7 was too broad to elucidate structural aspects. Multinuclear NMR ((139)La) indicated that these compounds do not retain their structure in MeOD. TGA/DTA data revealed that the de-solvation temperatures of the MeOH derivatives 4 - 6 were slightly higher in comparison to their hydrated counterparts.

First 5f-element complexes with the tetradentate BTBP ligand. Synthesis and crystal structure of uranyl(VI) compounds with CyMe4BTBP

Treatment of [UO2(OTf)2] or [UO2I2(thf)3] with 1 equiv. of CyMe4BTBP in anhydrous acetonitrile led to the formation of [UO2(CyMe4BTBP)(OTf)2] (1) and [UO2(CyMe4BTBP)I2] (2) which crystallized as the cationic forms [UO2(CyMe4BTBP)(py)][OTf]2 (3) and [UO2I(CyMe4BTBP)][I] (4) in pyridine and acetonitrile, respectively. These compounds are unique examples of structurally characterized actinide complexes with a BTBP molecule; this ligand adopts a planar conformation in the equatorial plane of the {UO2}2+ ion. In pyridine, 1 is dissociated into [UO2(OTf)2(py)3] and free CyMe4BTBP and the thermodynamic parameters (K, ΔH, ΔS) of this equilibrium have been determined by 1H NMR spectroscopy. The ethoxide derivative [UO2(OEt)(CyMe4BTBP)][OTf] (5) crystallized from a solution of 1 in a mixture of ethanol and acetone under air, and the dinuclear μ-oxo complex [{UO2(CyMe4BTBP)}2(μ-O)][I]2 (6) was obtained from [UO2I(thf)2.7] and CyMe4BTBP. The crystal structures of 6 and of the analogous derivatives [{UO2(py)4}2(μ-O)][I]2 (7) and [{UO2(TPTZ)(py)}2(μ-O)][I3]2 (8) exhibit a flexible [{UO2}–O–{UO2}]2+ moiety.

Lanthanide(III) and actinide(III) complexes [M(BH4)2 (THF)5][BPh4] and [M(BH4)2(18-crown-6)][BPh4] (M = Nd, Ce, U): synthesis, crystal structure, and density functional theory investigation of the covalent contribution to metal-borohydride bonding

Treatment of [M(BH4)3(THF)3] with NEt3HBPh4 in THF afforded the cationic complexes [M(BH4)2(THF)5][BPh4] [M = U (1), Nd (2), Ce (3)] which were transformed into [M(BH4)2(18-crown-6)][BPh4] [M = U (4), Nd (5), Ce (6)] in the presence of 18-crown-6; [U(BH4)2(18-thiacrown-6)][BPh4] (7) was obtained from 1 and 18-thiacrown-6 in tetrahydro-thiophene. Compounds 1, 3.C4H8S, 4.THF, 5, and 6.THF exhibit a penta- or hexagonal bipyramidal crystal structure with the two terdentate borohydride ligands in apical positions; the BH4 groups in the crystals of 7.C4H8S are in relative cis positions, and the thiacrown-ether presents a saddle shape, with two diametrically opposite sulfur atoms bound to uranium in trans positions. The crystal structures of these complexes, as well as those of previously reported [M(BH4)2(THF)5]+ cations, do not reveal any clear-cut lanthanide(III)/actinide(III) differentiation. The structural data obtained for [M(BH4)2(18-crown-6)]+ (M = U, Ce) by relativistic density functional theory (DFT) calculations are indicative of a small shortening of the U...B with respect to the Ce...B distance, which is accompanied by a lengthening of the U-Hb bonds and an opening of the Hb-B-Hb angle (Hb = bridging hydrogen atom of the eta3-BH4 ligand). The Mulliken population analysis and the natural bond orbital analysis indicate that the BH4 -->M(III) donation is greater for M = U than for M = Ce, as well as the overlap population of the M-Hb bond, thus showing a better interaction between the uranium 5f orbitals and the Hb atoms. The more covalent character of the B-H-U three-center two-electron bond was confirmed by the molecular orbital (MO) analysis. Three MOs represent the pi bonding interactions between U(III) and the three Hb atoms with significant 6d and 5f orbital contributions. These MOs in the cerium(III) complex exhibit a much lesser metallic weight with practically no participation of the 4f orbitals.

Analysis of Uranium Azide and Nitride Complexes by Atmospheric Pressure Chemical Ionization Mass Spectrometry

Atmospheric pressure chemical ionization mass spectrometry (APCI-MS) has been used to characterize the air-sensitive paramagnetic organouranium azide and nitride complexes [(C5Me5)2UN3(mu-N3)]3 and [(C5Me5)U(mu-I)2]3N, respectively. The trimetallic complex [(C5Me5)U(mu-I)2]3E had been identified by X-ray crystallography, but the data did not definitively identify E as N3- versus O2- or (OH)-, a common problem in heavy-element nitride complexes involving metals with variable oxidation states. A comparison of the 250 degrees C APCI-MS spectra of products made from NaN3 and Na15NNN showed mixed [M]+ and [M + H]+ envelopes at expected ion intensities for the 14N and 15N isotopomers. A compilation of U-C(C5Me5) and U-I bond distance data for U3+ and U4+ is also reported that shows that the ranges for the two oxidation states have significant overlap.

The vitality of uranium molecular chemistry at the dawn of the XXIst century

The intent of this Dalton Perspective is to highlight the recent advances in uranium molecular chemistry, with the results reported during the 2000-2006 period. This discipline is currently witnessing an impressive development, together with the theoretical chemistry and solid-state chemistry of the f-elements, and its face has profoundly changed, revealing unsuspected structural and reactivity features. This progress required and was facilitated by the use of new precursors. Studies of low-valent compounds gave a better insight into lanthanide(III)/actinide(III) differentiation and led to the discovery of unusual reactions, including activation of small molecules. A number of tetravalent uranium complexes, in particular polynuclear compounds, have been synthesized, which exhibit exciting structures and physicochemical properties. The potential of uranium(III) and uranium(IV) complexes in catalysis has been confirmed. The uranyl complexes, from mononuclear species to supramolecular assemblies, reveal a variety of novel structures, changing the generally accepted ideas on the coordination geometry and the stability of the UO2(2+) ion.

Lanthanide(III)/Actinide(III) Differentiation in the Cerium and Uranium Complexes [M(C5Me5)2(L)]0,+ (L=2,2′-Bipyridine, 2,2′:6′,2′′-Terpyridine): Structural, Magnetic, and Reactivity Studies

Treatment of [Ce(Cp*)(2)I] or [U(Cp*)(2)I(py)] with 1 mol equivalent of bipy (Cp*=C(5)Me(5); bipy=2,2'-bipyridine) in THF gave the adducts [M(Cp*)(2)I(bipy)] (M=Ce (1 a), M=U (1 b)), which were transformed into [M(Cp*)(2)(bipy)] (M=Ce (2 a), M=U (2 b)) by Na(Hg) reduction. The crystal structures of 1 a and 1 b show, by comparing the U-N and Ce-N distances and the variations in the C-C and C-N bond lengths within the bidentate ligand, that the extent of donation of electron density into the LUMO of bipy is more important in the actinide than in the lanthanide compound. Reaction of [Ce(Cp*)(2)I] or [U(Cp*)(2)I(py)] with 1 mol equivalent of terpy (terpy=2,2':6',2''-terpyridine) in THF afforded the adducts [M(Cp*)(2)(terpy)]I (M=Ce (3 a), M=U (3 b)), which were reduced to the neutral complexes [M(Cp*)(2)(terpy)] (M=Ce (4 a), M=U (4 b)) by sodium amalgam. The complexes [M(Cp*)(2)(terpy)][M(Cp*)(2)I(2)] (M=Ce (5 a), M=U (5 b)) were prepared from a 2:1 mixture of [M(Cp*)(2)I] and terpy. The rapid and reversible electron-transfer reactions between 3 and 4 in solution were revealed by (1)H NMR spectroscopy. The spectrum of 5 b is identical to that of the 1:1 mixture of [U(Cp*)(2)I(py)] and 3 b, or [U(Cp*)(2)I(2)] and 4 b. The magnetic data for 3 and 4 are consistent with trivalent cerium and uranium species, with the formulation [M(III)(Cp*)(2)(terpy(*-))] for 4 a and 4 b, in which spins on the individual units are uncoupled at 300 K and antiferromagnetically coupled at low temperature. Comparison of the crystal structures of 3 b, 4 b, and 5 b with those of 3 a and the previously reported ytterbium complex [Yb(Cp*)(2)(terpy)] shows that the U-N distances are much shorter, by 0.2 A, than those expected from a purely ionic bonding model. This difference should reflect the presence of stronger electron transfer between the metal and the terpy ligand in the actinide compounds. This feature is also supported by the small but systematic structural variations within the terdentate ligands, which strongly suggest that the LUMO of terpy is more filled in the actinide than in the lanthanide complexes and that the canonical forms [U(IV)(Cp*)(2)(terpy(*-))]I and [U(IV)(Cp*)(2)(terpy(2-))] contribute significantly to the true structures of 3 b and 4 b, respectively. This assumption was confirmed by the reactions of complexes 3 and 4 with the H(.) and H(+) donor reagents Ph(3)SnH and NEt(3)HBPh(4), which led to clear differentiation of the cerium and uranium complexes. No reaction was observed between 3 a and Ph(3)SnH, while the uranium counterpart 3 b was transformed in pyridine into the uranium(IV) compound [U(Cp*)(2){NC(5)H(4)(py)(2)}]I (6), where NC(5)H(4)(py)(2) is the 2,6-dipyridyl(hydro-4-pyridyl) ligand. Complex 6 was further hydrogenated to [U(Cp*)(2){NC(5)H(8)(py)(2)}]I (7) by an excess of Ph(3)SnH in refluxing pyridine. Treatment of 4 a with NEt(3)HBPh(4) led to oxidation of the terpy(*-) ligand and formation of [Ce(Cp*)(2)(terpy)]BPh(4), whereas similar reaction with 4 b afforded [U(Cp*)(2){NC(5)H(4)(py)(2)}]BPh(4) (6'). The crystal structures of 6, 6' and 7 were determined.

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.

The remarkable efficiency of N-heterocyclic carbenes in lanthanide(III)/actinide(III) differentiation

The capacity of NHC molecules to discriminate between trivalent lanthanide and actinide ions was revealed by competition reactions of analogous U(III) and Ce(III) compounds with C3Me4N2 and a comparison of the crystal structures of the corresponding carbene adducts.

The distinct affinity of cyclopentadienyl ligands towards trivalent uranium over lanthanide ions. Evidence for cooperative ligation and back-bonding in the actinide complexes

The mono and bis(cyclopentadienyl) compounds [M(C5H4Bu t)I2] and [M(C5H4Bu t)2I](M = U, La, Ce, Nd) were formed in thf by comproportionation reactions of [M(C5H4Bu t)3] and LnI3 or [UI3(L)4](L = thf or py) in the molar ratio of 1 : 2 and 2 : 1, respectively, while treatment of [UI(3)(py)(4)] or LnI(3)(Ln = La, Ce, Nd) with 1 or 2 mol equivalents of LiC5H4Bu t in thf afforded the [M(C5H4Bu t)I2] and [M(C5H4Bu t)2I2]- compounds, respectively. The X-ray crystal structures of [M(C5H4Bu t)I2(py)3](M = U, La, Ce, Nd), [{Ce(C5H4Bu t)2(mu-I)}2] and [M(C5H4Bu t)2I(py)2](M = U, Nd) have been determined; the differences between the average M-C distances in the mono(cyclopentadienyl) complexes correspond to the variation in the ionic radii of the trivalent uranium and lanthanide ions while the U-N and U-I bond lengths seem to be smaller than those predicted from a purely ionic bonding model. The distinct affinity of the cyclopentadienyl ligands towards Ln(III) and U(III) was revealed by two series of competing reactions: the ligand exchange reactions between [Ln(C5H4Bu t)(n')I(3-n')](Ln = La, Ce, Nd) and [U(C5H4Bu t)(n'')I(3-n'')] species (1 < or = n'+n'' =n < or = 5), and the addition of n mol equivalents of LiC(5)H(4)Bu(t)(1 [less-than-or-equal]n[less-than-or-equal] 5) to a 1 : 1 mixture of LnI3 and [UI3(thf)4] or [UI3(py)4]. The stability of the [M(C5H4Bu t)I2] species was found to vary in the order Nd > Ce > U > La, a trend which is in accord with an electrostatic bonding model. However, the bis and tris(cyclopentadienyl) complexes of uranium are more stable than their lanthanide analogues. This difference can be accounted for by a higher degree of covalency in the U-C5H4Bu t bond, resulting from the late appearance of back-bonding which would emerge only after the first cyclopentadienyl ligand is bound.