Uranium(IV) and Thorium(IV) Coordination Polymers Based on Tritopic Carboxylic Acids.

Nine new coordination polymers based on U(IV) and Th(IV) were synthesized solvothermally utilizing four different trianionic carboxylates (H3BHTC = biphenyl-3,4',5-tricarboxylic acid, H3NTB = 4,4',4″-nitrilotribenzoic acid, H3BTB = 4,4',4″-benzene-1,3,5-triyl-tris(benzoic acid), H3BTE = 4,4',4″-(1,3,5-benzenetriyltri-2,1-ethynediyl)trisbenzoic acid). The influence of the ligand architecture, the pH, the stoichiometry, the nature of the metal, and the concentration on the structure and dimensionality of the final actinide assembly is discussed. The H3BHTC ligand allowed the synthesis of a cationic three-dimensional (3D) framework [U(BHTC)(DMF)3]I (1), which is the first example of a cationic U(IV) polymer. The H3NTB ligand yielded the 3D neutral polymer [U3(NTB)4] (2) or the two-dimensional (2D) cationic polymer [U(NTB)(NMP)3]I (3), depending on the solvent. When conditions leading to (2) were used with a Th(IV) precursor, the 2D neutral polymer [Th(NTB)(DMF)3Cl] (4) was obtained. The ligand H3BTB allowed the synthesis of two 3D cationic networks [U(BTB)(DMF)2]I (5) and [U(BTB)(DMF)3]I (7) or the neutral 3D analogue [U3(BTB)4] (6), depending on the precursor's oxidation state and the acidity of the reaction mixture. The ligand H3BTE allowed the synthesis of the anionic 3D [(CH3)2NH2][U2(BTE)3] (8) framework featuring large accessible pores, and under the same conditions, an isostructural Th(IV) was also obtained [(CH3)2NH2][Th2(BTE)3] (8-Th). All isolated coordination polymers were characterized by single-crystal X-ray diffraction (SCXRD). The Langmuir surface areas of the U(IV) polymers (2), (7), and (8) increased from 140 to 310 m2/g owing to the increasing size of the linker, with polymer (8) showing a value that is comparable to the highest surface area reported to date. The effect of the postsynthetic solvent substitution was also studied, revealing a crystal-to-crystal transformation of the cationic framework (7) to the neutral framework [U(BTB)(THF)I] (7c).

Structure and Reactivity of Polynuclear Divalent Lanthanide Disiloxanediolate Complexes.

Trinuclear molecular complexes of europium (II) and ytterbium(II) [Ln3{(Ph2SiO)2O}3(THF)6], 1-Ln3L3 (Ln = Eu and Yb), supported by the dianionic tetraphenyl disiloxanediolate ligand, were synthesized via protonolysis of the [Ln{N(SiMe3)2}2(THF)2] complexes. In contrast, the reaction of [Sm{N(SiMe3)2}2(THF)2] with the (Ph2SiOH)2O ligand led to the isolation of the mixed-valent Sm(II)/Sm(III) complex [Sm3{(Ph2SiO)2O}3{N(SiMe3)2}(THF)4], 2-Sm3L3, which was crystallographically characterized. The Eu(II) complex 1-Eu3L3 displays weak ferromagnetic coupling between the Eu(II) metal centers (J = 0.1035 cm-1). The addition of 3 equiv of (Ph2SiOK)2O to 1-Eu3L3 resulted in the formation of the polynuclear Eu(II) dimer of dimers [K4Eu2{(Ph2SiO)2O}4(Et2O)2]2, 3-Eu2L4. Complexes 1-Ln3L3 (Ln = Eu and Yb) are stable in solution at room temperature, while 3-Eu2L4 shows higher reactivity and rapidly decomposes to give the mixed-valent Eu(II)/Eu(III) species [K3Eu2{(Ph2SiO)2O}4], 4-Eu2L4. Complex 1-Yb3L3 affects the slow reductive disproportionation of carbon dioxide, but 1-Eu3L3 does not display any reactivity toward CO2. However, the presence of one additional (Ph2SiO-)2O per Eu(II) metal center in 3-Eu2L4 increases dramatically the reductive ability of the Eu(II) metal centers, affording the first example of carbon dioxide activation by an isolated divalent europium complex. The reduction of CO2 by 3-Eu2L4 is immediate, and carbonate is formed selectively after the addition of a stoichiometric amount of CO2.

Structure, reactivity and luminescence studies of triphenylsiloxide complexes of tetravalent lanthanides.

Among the 14 lanthanide elements (Ce-Lu), until recently, the tetravalent oxidation state was readily accessible in solution only for cerium while Pr(iv), Nd(iv), Dy(iv) and Tb(iv) had only been detected in the solid state. The triphenylsiloxide ligand recently allowed the isolation of molecular complexes of Tb(iv) and Pr(iv) providing an unique opportunity of investigating the luminescent properties of Ln(iv) ions. Here we have expanded the coordination studies of the triphenylsiloxide ligand with Ln(iii) and Ln(iv) ions and we report the first observed luminescence emission spectra of Pr(iv) complexes which are assigned to a ligand-based emission on the basis of the measured lifetime and computational studies. Binding of the ligand to the Pr(iv) ion leads to an unprecedented large shift of the ligand triplet state which is relevant for future applications in materials science.

Structure and small molecule activation reactivity of a metallasilsesquioxane of divalent ytterbium.

The first metallasilsesquioxane of a divalent lanthanide, [Yb{Cy7Si7O11(OSiMe3)}(THF)]2, 1, was synthesized and structurally characterized. The Cy7Si7O11(OSiMe3)2- ligands in 1 bind two Yb(ii) ions in a bridging mode. The dinuclear complex effects the two-electron reduction of azobenzene yielding the Yb(iii) complex [{Yb(Cy7Si7O11(OSiMe3))(THF)2}2(PhNNPh)], 2, and the CO2 reduction to CO and carbonate.

Accessing the +IV Oxidation State in Molecular Complexes of Praseodymium.

Out of the 14 lanthanide (Ln) ions, molecular complexes of Ln(IV) were known only for cerium and more recently terbium. Here we demonstrate that the +IV oxidation state is also accessible for the large praseodymium (Pr) cation. The oxidation of the tetrakis(triphenysiloxide) Pr(III) ate complex, [KPr(OSiPh3)4(THF)3], 1-PrPh, with [N(C6H4Br)3][SbCl6], affords the Pr(IV) complex [Pr(OSiPh3)4(MeCN)2], 2-PrPh, which is stable once isolated. The solid state structure, UV-visible spectroscopy, magnetometry, and cyclic voltammetry data along with the DFT computations of the 2-PrPh complex unambiguously confirm the presence of Pr(IV).

Stabilization of the Oxidation State +IV in Siloxide-Supported Terbium Compounds.

The synthesis of lanthanides other than cerium in the oxidation state +IV has remained a desirable but unmet target until recently, when two examples of TbIV with saturated coordination spheres were isolated. Here we report the third example of an isolated molecular complex of terbium(IV), where the supporting siloxide ligands do not saturate the coordination sphere. The fully characterized six-coordinate complex [TbIV (OSiPh3 )4 (MeCN)2 ], 2-TbPh , shows high stability and the labile MeCN ligands can be replaced by phosphinoxide ligands. Computational studies suggest that the stability is due to a strong π(O-Tb) interaction which is stronger than in the previously reported TbIV complexes. Cyclic-voltammetry experiments demonstrate that non-binding counterions contribute to the stability of TbIV in solution by destabilizing the +III oxidation state, while alkali ions promote TbIV /TbIII electron transfer.

CS2 Reductive Coupling to Acetylenedithiolate by a Dinuclear Ytterbium(II) Complex.

The activation of CS2 is of interest in a broad range of fields and, more particularly, in the context of creating new C-C bonds. The reaction of the dinuclear ytterbium(II) complex [Yb2 L4 ], 1, [L=(OtBu)3 SiO- ] with carbon disulfide led to the isolation of unprecedented reduction products. In particular, the crystallographic characterization of complex [Yb2 L4 (µ-C2 S2 )], 2, provided the first example of an acetylenedithiolate ligand formed from metal reduction of CS2 . Computational studies indicated that this unprecedented reactivity can be ascribed to the unusual binding mode of CS2 2- in the isolated "key intermediate" [Yb2 L4 (µ-CS2 )], 3, which results from the dinuclear nature of 1.

Carbon dioxide reduction by dinuclear Yb(ii) and Sm(ii) complexes supported by siloxide ligands.

Two dinuclear homoleptic complexes of lanthanides(ii) supported by the polydentate tris(tertbutoxy) siloxide ligand ([Yb2L4], 1-Yb and [Sm2L4], 1-Sm, (L = (OtBu)3SiO-)) were synthesized in 70-80% yield and 1-Sm was crystallographically characterized. 1-Yb and 1-Sm are stable in solution at -40 °C but cleave the DME C-O bond over time at room temperature affording the crystal of [Yb2L4(µ-OMe)2(DME)2], 2. The 1-Yb and 1-Sm complexes effect the reduction of CO2 under ambient conditions leading to carbonate and oxalate formation. The selectivity of the reduction towards oxalate or carbonate changes depend on the solvent polarity and on the nature of the ion. For both the lanthanides, carbonate formation is favoured but oxalate formation increases if a non-polar solvent is used. Computational studies suggest that the formation of oxalate is favoured with respect to carbonate formation in the reaction of the dimeric lanthanide complexes with CO2. Crystals of the tetranuclear mixed-valence oxalate intermediate [Yb4L8(C2O4)], 3 were isolated from hexane and the presence of a C2O42- ligand bridging two [YbIIL2YbIIIL2] dinuclear moieties was shown. Crystals of the tetranuclear di-carbonate product [Sm4L8(µ3-CO3-κ4-O,O',O'')2], 4 were isolated from hexane. The structures of 3 and 4 suggest that the CO2 activation in non-polar solvents involves the interaction of two dimers with CO2 molecules at least to some extent. Such a cooperative interaction results in both oxalate and carbonate formation.