Search

The heterotrinuclear linear arrangement Ln–Pd–Ln has short Ln–Pd distances: does it imply a covalent bond?

By taking inspiration from the structures and reactivities of its past, organolanthanide chemistry has managed to reinvent itself for the challenges of today and the future.

International audience; The article reports the synthesis and structural characterization of a series of Ln(C9H9)(3) complexes with the cyclononatetraenyl (Cnt, C9H9) ligand (Ln = Y, Gd, Tb, Dy, Ho, Er, Tm). The Yb and Sm complexes were not obtained, and the reaction of the potassium salt of the Cnt ligand with trivalent halide salts of the corresponding metals led to the known bis-Cnt sandwich compounds Ln(C9H9)(2). The X-ray diffraction studies on the trivalent complexes show that the Cnt ligand is significantly bent in order to accommodate the large size of the ligand while it maintains its aromaticity. When the size of the lanthanide ion decreases, the ligand does not switch away but swings over the metal ion in order to maximize the electrostatic interactions. H-1 NMR and UV-visible spectra, as well as the solid-state magnetism, were recorded. UV-visible spectroscopy highlights a remarkable charge-transfer band in the Tm complex, while ligand-based transitions are principally observed with all other metal ions. The magnetic behavior of the series agrees with trivalent lanthanide ions, and the computations at the CASSCF level confirm the trivalent electronic structure.

Solvation of [(CNT)Ln(η$^8$-COT)] (Ln=La, Ce, Nd, Tb, Er; CNT=cyclononatetraenyl, i.e., C$_9$H$_9$−; COT=cyclooctatetraendiid, i.e., C$_8$H$_8$$^2−$) complexes with tetrahydrofuran (THF) gives rise to neutral [(η$^4$-CNT)Ln(thf)$_2$(η$^8$-COT)] (Ln=La, Ce) and ionic [Ln(thf)$_x$(η$^8$-COT)][CNT] (x=4 (Ce, Nd, Tb), 3 (Er)) species in a solid-to-solid transformation. Due to the severe distortion of the ligand sphere upon solvation, these species act as switchable luminophores and single-molecule magnets. The desolvation of the coordinated solvents can be triggered by applying a dynamic vacuum, as well as a temperature gradient stimulus. Raman spectroscopic investigations revealed fast and fully reversible solvation and desolvation processes. Moreover, we also show that a Nd:YAG laser can induce the necessary temperature gradient for a self-sufficient switching process of the Ce(III) analogue in a spatially resolved manner.

Simple

The divalent thulium complex [Tm(Cp

International audience; An alternative synthesis for M(κ2-bipy)4 (M = La, Ce) and [Li(thf)4][M(κ2-bipy)4] (M = Tb, Dy) and the crystal structures for M = La, Ce, and Tb are described. The isomorphous and isostructural neutral molecules, M = La and Ce, are polymeric in the solid-state, as are those of M = Sm and Eu, which were reported in earlier work. The polymeric network is built from eight coordinate units whose geometry in all four cases is that of a square prism. The known molecules, M = Yb and Lu, are also polymeric, but the eight coordinate units have dodecahedral geometries. The structure of the anions in the separated ion pair, [Li(thf)4][M(κ2-bipy)4], in which Tb is reported in this work and Lu is known, are monomeric with geometries that are between that of a square antiprism and a dodecahdron. The electronic structure, from CASSCF multireference quantum mechanical calculations, shows that the electronic ground states for M = La and Lu are multiconfigurational spin doublets and those for the M = Ce and Yb are multiconfigurational spin triplets. This is confirmed by magnetic susceptibility studies as a function of temperature that are consistent with the metals (La, Ce, Sm, Tb, Dy, Yb, and Lu) being trivalent, as are the LIII-edge XANES spectra (Ce, Yb), and divalent for Eu. The multiconfigurational nature of the ground states, developed from CASSCF molecular orbital calculations, renders a single Lewis structure and a single reference molecular orbital representation misleading. The results from the multireference calculations are extended to the other lanthanide molecules and are the genesis of a new model for understanding the magnetic properties of these molecules.

Sandwich complexes of lanthanides have recently attracted a considerable amount of interest due to their applications as Single Molecule Magnet (SMM). Herein, a comprehensive series of heteroleptic lanthanide sandwich complexes ligated by the cyclononatetraenyl (Cnt) and the cyclooctatetraenyl (Cot) ligand [Ln(Cot)(Cnt)] (Ln=Tb, Dy, Er, Ho, Yb, and Lu) is reported. The coordination behavior of the Cnt ligand has been investigated along the series and shows different coordination patterns in the solid‐state depending on the size of the corresponding lanthanide ion without altering its overall anisotropy. Besides the characterization in the solid state by single‐crystal X‐ray diffraction and in solution by 1H NMR, static magnetic studies and ab initio computational studies were performed., A series of trivalent lanthanide heteroleptic sandwich complexes with the cyclooctatetraenyl (Cot) and cyclononatetraenyl (Cnt) ligands shows a significant bending of the Cnt ligand when the size of the ion decreases, implying a size‐controlled hapticity switch.

Over more than 50 years, intermediate valence states in lanthanide compounds have often resulted in unexpected or puzzling spectroscopic and magnetic properties. Such experimental singularities could not be rationalised until new theoretical models involving multiconfigurational electronic ground states were established. In this minireview, the different singularities that have been observed among lanthanide complexes are highlighted, the models used to rationalise them are detailed and how such electronic effects may be adjusted depending on energy and symmetry considerations is considered. Understanding and tuning the ground-state multiconfigurational behaviour in lanthanide complexes may open new doors to modular and unusual reactivities.

The use of stoechiometric SmI

Divalent lanthanide organometallics are well known highly reducing compounds usually used for single electron transfer reactivity and small molecule activation. Thus, their very reactive nature prevented for many years the study of their physical properties, such as magnetic studies on a reliable basis. In this article, the access to rare organometallic sandwich compounds of TmII with the cyclooctatetraenyl (Cot) ligand impacts on the use of divalent organolanthanide compounds as an additional strategy for the design of performing Single Molecule Magnets (SMM). Herein, the first divalent thulium sandwich complex with f13 configuration behaving as a Single Molecule Magnet in absence of DC field is highlighted.

The divalent samarium triflate salt does not react with CO2 or water, but does react with traces of O2 or N2O to form a tetrameric bis-oxo samarium motif. The reaction with O2 is a 4e− reductive cleavage where the electrons are coming from four different samarium centers. This highlights a rare synergistic effect for cleaving O2, which has no precedent in divalent lanthanide complexes. Additionally, the addition of CO2 to the tetrameric bis-oxo intermediate leads to the formation of a tetrameric bis-carbonate samarium triflate. Thus, the concomitant reaction of CO2 with traces of O2 leads to the same bis-carbonate tetrameric assembly.

The base-free divalent samarium complex Cptt2Sm (1; Cptt = 1,3-(tBu)2(C5H3)) has been synthesized in diethyl ether by salt metathesis of SmI2. Crystals of 1 suitable for X-ray study have been obtained by sublimation at 116 °C under reduced pressure. The dissolution of 1 in thf and pyridine solution leads to the solvent adducts Cptt2Sm(thf)2 (3) and Cptt2Sm(py) (4), respectively, while drying 3 under reduced pressure yields CpttSm(thf) (5). The reaction of CO2 with the base-free divalent samarium complexes Cptt2Sm (1) and Cpttt2Sm (2; Cpttt =1,2,4-(tBu)3(C5H2)) leads to the clean formation of bridged carbonate samarium dimers [Cpttt2Sm]2(μ-CO3) (7) and [Cptt2Sm]2(μ-CO3) (8). This is indicative of the reductive disproportionation of CO2 in both cases with release of CO. This contrasts with the formation of the oxalate-bridged samarium dimer reported from the reaction of CO2 with the Cp*2Sm(thf)2 complex. Otherwise, the reaction with CO does not proceed with the bulky complexes, while traces of O2 have led t...

International audience; A series of six hybrid polymers based on the mixed-valent {ε-PMoV8MoVI4O40Zn4} (εZn) Keggin unit have been synthesized under hydrothermal conditions using tritopic (1,3,5-benzenetricarboxylate (trim) or 1,3,5-benzenetribenzoate (BTB)) or ditopic (4,4′-biphenyldicarboxylate (biphen)) linkers and [M(bpy)3]2+ (M = Co, Ru) complexes as charge-compensating cations. (TBA)2[Co(C10H8N2)3][PMo12O37(OH)3Zn4](C27H15O6)4/3·1.5C27H18O6·24H2O (Co-ε(BTB)4/3) has a three-dimensional (3D) framework with two interpenetrated networks and is isostructural to (TBA)4[PMo12O37(OH)3Zn4](C27H15O6)4/3·1.5C27H18O6·8H2O (ε(BTB)4/3). In Co-ε(BTB)4/3, two tetrabutylammonium (TBA+) cations over the four present in ε(BTB)4/3 are replaced by one [Co(bpy)3]2+ complex. [Co(C10H8N2)3][PMo12O37(OH)3Zn4](C9H3O6)Co(C10H8N2)4(H2O)·16H2O (Co-ε(trim) (bpy)2) is a 1D coordination polymer with two types of CoII-containing complexes, one covalently attached to the 1D chains and the other located in the voids as the counterion. [Ru(C10H8N2)3]4[PMo12O38(OH)2Zn4]2(C9H3O6)2·42H2O (Ru-ε2(trim)2) and [Ru(C10H8N2)3]3[PMo12O37(OH)3Zn4Cl]2(C14H8O4)2·24H2O (Ru-ε2(biphen)2) contain dimeric (εZn)2 units linked by dicarboxylate linkers, and both have [Ru(bpy)3]2+ countercations. Ru-ε2(trim)2 has a 3D framework, while Ru-ε2(biphen)2 is only 2D because of the presence of chloride ions on one-fourth of the ZnII ions. [P(C6H5)4]6[PMo12O37(OH)3Zn4]2(C9H3O6)2·18H2O (PPh4-ε2(trim)2) is isostructural to Ru-ε2(trim)2. These insoluble compounds entrapped in carbon-paste electrodes exhibit electrocatalytic activity for the hydrogen evolution reaction. The effects of their structure and the nature of the counterions on the activity have been studied. For the first time, different POM-based coordination polymers are compared for catalytic H2 production using controlled-potential electrolysis. This study shows that the nature of the countercation has a strong effect on the electrocatalytic activity of the compound.

The Article presents the synthesis, structure, and bonding of a series of neutral and linear sandwich compounds with the cyclononatetraenyl (Cnt) ligand and divalent lanthanides. These compounds account for the emergence of the lanthanidocene series in reference to the ferrocene and uranocene. The synthetic strategy uses the solubility difference between two conformational isomers of the ligand, as well as the isomerization of the compounds induced by solvent coordination, yielding the isomorphous and isostructural neutral and rigorously linear sandwich complexes. The molecular structures feature a Cnt–Ln–Cnt angle of 180° and a ring size close to the Cnt–Cnt(centroid) distance. A qualitative molecular orbital diagram is provided, in D9d symmetry, and DFT calculations enforce the bonding model.

Author(s): Halbach, Robert L; Nocton, Gregory; Booth, Corwin H; Maron, Laurent; Andersen, Richard A | Abstract: Page 7295. In the Summary section of the article, the text should read "The energy of the closed-shell singlet that corresponds to the traditional Lewis structures used to represent these molecules is 400-700 cm-1 above the openshell triplet and singlet." instead of "The energy of the openshell singlet that corresponds to the traditional Lewis structures used to represent these molecules is 400-700 cm-1 above the open-shell triplet."

The divalent samarium triflate salt does not react with CO2 or water, but does react with traces of O2 or N2O to form a tetrameric bis-oxo samarium motif. The reaction with O2 is a 4e- reductive cleavage where the electrons are coming from four different samarium centers. This highlights a rare synergistic effect for cleaving O2, which has no precedent in divalent lanthanide complexes. Additionally, the addition of CO2 to the tetrameric bis-oxo intermediate leads to the formation of a tetrameric bis-carbonate samarium triflate. Thus, the concomitant reaction of CO2 with traces of O2 leads to the same bis-carbonate tetrameric assembly.

A new synthesis of cerium tetrakis(tropolonate), Ce(trop)4, where trop is deprotonated 2-hydroxy-2,4,6-cycloheptatrienone) or Ce(O2C7H5)4, is developed that results in dark-purple crystals whose X-ray crystal structure shows that the geometry of the eight-coordinate compound closely resembles a D2 d dodecahedron, based on shape parameters. The magnetic susceptibility as a function of the temperature (4-300 K) shows that it is a temperature-independent paramagnet, χ = 1.2(3) × 10-4 emu/mol, and the LIII-edge X-ray absorption near-edge structure spectrum shows that the molecule is multiconfigurational, comprised of a f1:f0 configuration mixture in a 50:50 ratio. Ce(acac)4 and Ce(tmtaa)2 (where acac is acetylacetonate and tmtaaH2 is tetramethyldibenzotetraaza[14]annulene) have similar physical properties, as does the solid-state compound CeO2. The concept is advanced that trop-, acac-, tmtaa2-, cot2-, and O2- are redox-active ligands that function as electron donors, rendering the classification of these compounds according to their oxidation numbers misleading because their magnetic susceptibilities, χ, are positive and their effective magnetic moments, μeff, lie in the range of 0.1-0.7 μB at 300 K.

This paper reports the synthesis of a series of complexes based on the bis(pentamethylcyclopentadienyl)ytterbium(II) (1; Cp*2Yb) and bis(tetramethylphospholyl)ytterbium(II) (2; Tmp2Yb) fragments bearing an additional neutral bis(methyliminophosphoranyl)pyridine ligand (L) on which the steric demand is modulated at the phosphorus position (triethyl, LEt; triphenyl, LPh; tricyclohexyl, LCy) to yield the original complexes Cp*2YbLEt (1-LEt), Cp*2YbLPh (1-LPh), Tmp2YbLEt (2-LEt), Tmp2YbLPh (2-LPh), and Tmp2YbLCy (2-LCy), while no reaction occurs between 1 and LCy. The crystal structures of these sterically crowded complexes are reported as well as room-temperature NMR data for all the complexes. The solid-state coordination mode of LR differs depending on the nature of the fragments 1 and 2 and on the steric bulk of LR. The crystal structure of the divalent Tmp2Yb(py)2 (3) is also reported for structural and spectroscopic comparisons. Interestingly, in both 2-LEt and 2-LCy, one of the two Tmp ligands coordina...

Reaction of the neutral diniobium benzene complex {[Nb(BDI)N t Bu]2(μ-C6H6)} (BDI = N,N'-diisopropylbenzene-β-diketiminate) with Ag[B(C6F5)4] results in a single electron oxidation to produce a cationic diniobium arene complex, {[Nb(BDI)N t Bu]2(μ-C6H6)}{B(C6F5)4}. Investigation of the solid state and solution phase structure using single-crystal X-ray diffraction, cyclic voltammetry, magnetic susceptibility, and multinuclear NMR spectroscopy indicates that the oxidation results in an asymmetric molecule with two chemically inequivalent Nb atoms. Further characterization using density functional theory (DFT) calculations, UV-visible, Nb L3,2-edge X-ray absorption near-edge structure (XANES), and EPR spectroscopies supports assignment of a diniobium complex, in which one Nb atom carries a single unpaired electron that is not largely delocalized on the second Nb atom. During the oxidative transformation, one electron is removed from the δ-bonding HOMO, which causes a destabilization of the molecule and formation of an asymmetric product. Subsequent reactivity studies indicate that the oxidized product allows access to metal-based chemistry with substrates that did not exhibit reactivity with the starting neutral complex.

International audience; Non‐innocent ligands render the determination of the electronic structure in metal complexes difficult. As such, a combination of experimental techniques and quantum chemistry are required to correctly elucidate them. This paper deals with the one‐electron oxidation of copper(II) and nickel(II) complexes featuring a phosphasalen ligand (Psalen), which differs from salen analogues by the presence of iminophosphorane groups (P=N) instead of imines. Various experimental techniques (X‐ray diffraction, cyclic voltammetry, NMR, EPR, and UV/Vis spectroscopies, and magnetic measurements) as well as quantum chemical calculations were used to define the electronic structure of the oxidized complexes. These can be modified by a small change in the ligand structure, that is, the replacement of a tert‐butyl group by a methoxy on the phenoxide ring. The different techniques have allowed quantifying the amount of spin density located on the metal center and on the Psalen ligands. All complexes were found to possess a multi‐configurational ground state, in which the ratio of the +II versus +III oxidation state of the metal center, and therefore the phenolate versus phenoxyl radical ligand character, varies upon the substituents. The tert‐butyl group favors a strong localization on the metal center whereas with the methoxy group the metallic configurations decrease and the ligand configurations increase. The importance of the geometrical considerations compared with the electronic substituent effect is highlighted by the differences observed between the solid‐state (EPR, magnetic measurements) and solution characterizations (EPR and NMR data).

The oxidation of a Ni(II) complex bearing a tetradentate phosphasalen ligand, which differs from salen by the presence of an iminophosphorane (PN) in place of an imine unit, was easily achieved by addition of a silver salt. The site of this oxidation was investigated with a combination of techniques (NMR, EPR, UV/Vis spectroscopy, X-ray diffraction, magnetic measurements) as well as DFT calculations. All data are in agreement with a high-valent Ni(III) center concentrating the spin density. This markedly differs from precedents in the salen series for which oxidation on the metal was only observed at low temperature or in the presence of additional ligands or anions. Therefore, thanks to the good electron-donating properties of the phosphasalen ligand, [Ni(Psalen)](+) represents a rare example of a tetracoordinated high-valent nickel complex in presence of a phenoxide ligand.

International audience; The synthesis and X-ray crystal structures of Cp*2Yb(3,3′-Me2bipy) and [Cp*2Yb(3,3′-Me2bipy)][Cp*2YbCl1.6I0.4]*CH2Cl2 are described. In both complexes, the NCCN torsion angles are approximately 40°. The temperature-independent value of nf of 0.17 shows that the valence of ytterbium in the neutral adduct is multiconfigurational, in reasonable agreement with a CASSCF calculation that yields a nf value of 0.27; that is, the two configurations in the wave function are f13(π*1)1 and f14(π*1)0 in a ratio of 0.27:0.73, respectively, and the open-shell singlet lies 0.28 eV below the triplet state (nf accounts for f-hole occupancy; that is, nf = 1 when the configuration is f13 and nf = 0 when the configuration is f14). A correlation is outlined between the value of nf and the individual ytterbocene and bipyridine fragments such that, as the reduction potentials of the ytterbocene cation and the free x,x′-R2-bipy ligands approach each other, the value of nf and therefore the f13:f14 ratio reaches a maximum; conversely, the ratio is minimized as the disparity increases.

Reference EPFL-ARTICLE-203035doi:10.1002/anie.200702374View record in Web of Science Record created on 2014-11-07, modified on 2016-08-10

The reaction of a series of organolanthanide fragments of samarium and thulium with phenanthroline is reported. All adducts couple in the 4-position of the phenanthroline ligand to yield the 4–4′ dimers when they crystallize. The analysis of the solution structure revealed a thermally reversible C–C coupling in all cases.

Although several ruthenium complexes of the type Ru(acac)(2)(L)(x), where x is 1 for a bidentate or 2 for a monodentate ligand, are known, the tmed complex is conspicuously absent. This article describes the synthesis of this complex from trans-RuCl(2)(tmed)(2), acetylacetone, and triethylamine in toluene. A new synthesis of trans-RuCl(2)(tmed)(2) is also described. Some physical properties and the X-ray crystal structure of Ru(acac)(2)(tmed) are provided.

Three new cation-cation complexes of pentavalent uranyl, stable with respect to the disproportionation reaction, have been prepared from the reaction of the precursor [(UO 2py 5)(KI 2py 2)] n (1) with the Schiff base ligands salen 2-, acacen 2-, and salophen 2- (H 2salen=N,N′- ethylene-bis(salicylideneimine), H 2acacen=N,N′- ethylenebis(acetylacetoneimine), H 2salophen=N,N′-phenylene- bis(salicylideneimine)). The preparation of stable complexes requires a careful choice of counter ions and reaction conditions. Notably the reaction of 1 with salophen 2- in pyridine leads to immediate disproportionation, but in the presence of [18]crown-6 ([18]C-6) a stable complex forms. The solid-state structure of the four tetranuclear complexes, [UO 2(acacen)] 4[μ 8-] 2[K([18]C-6)(py)] 2 (3) and [UO 2(acacen)] 4[μ 8-]·2[K([222])(py) ] (4), [UO 2(salophen)] 4[μ 8-K] 2[μ 5-KI] 2[(K([18]C-6)]·2[K([18]C-6) (thf) 2]·2I (5), and [UO 2(salen) 4][μ 8-Rb] 2[Rb([18]C-6)] 2 (9) ([222]=[222]cryptand, py=pyridine), presenting a T-shaped cation-cation interaction has been determined by X-ray crystallographic studies. NMR spectroscopic and UV/Vis studies show that the tetranuclear structure is maintained in pyridine solution for the salen and acacen complexes. Stable mononuclear complexes of pentavalent uranyl are also obtained by reduction of the hexavalent uranyl Schiff base complexes with cobaltocene in pyridine in the absence of coordinating cations. The reactivity of the complex [U VO 2(salen)(py)][Cp 2Co] with different alkali ions demonstrates the crucial effect of coordinating cations on the stability of cation-cation complexes. The nature of the cation plays a key role in the preparation of stable cation-cation complexes. Stable tetranuclear complexes form in the presence of K + and Rb +, whereas Li + leads to disproportionation. A new uranyl-oxo cluster was isolated from this reaction. The reaction of [U VO 2(salen)(py)][Cp 2Co] (Cp=pentamethylcyclopentadienyl) with its U VI analogue yields the oxo-functionalized dimer [UO 2(salen)(py)] 2[Cp 2Co] (8). The reaction of the [UO 2(salen) 4][μ 8-K] 2[K([18]C-6)] 2 tetramer with protons leads to disproportionation to U IV and U VI species and H 2O confirming the crucial role of the proton in the U V disproportionation. Size matters: Large alkali ions promote the assembly of Schiff base complexes of pentavalent uranyl into three new stable polynuclear cation-cation clusters, while small cations assemble unstable intermediates, resulting in disproportionation (see figure). Protons also promote disproportionation. Copyright © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Dibenzoylmethanate replaces the bridging triflate ligands in uranium triflate polyoxo-clusters and cleaves the U12O20 core yielding the new [U6O4(OH)4(η-dbm) 12] dibenzoylmethanate (dbm-) cluster which slowly dissociates into a monomeric complex. This reactivity demonstrates the importance of bridging ligands in stabilizing uranium polyoxo-clusters. © 2010 The Royal Society of Chemistry.

A high energy resolution x-ray absorption near-edge structure (XANES) spectroscopy study on ${\text{U}}^{4+}$ $({\text{UO}}_{2})$, ${\text{U}}^{5+}$ $([{\text{UO}}_{2}{\text{Py}}_{5}][{\text{KI}}_{2}{\text{Py}}_{2}])$, and ${\text{U}}^{6+}$ $[{\text{UO}}_{2}{({\text{NO}}_{3})}_{2}{({\text{H}}_{2}\text{O})}_{6}]$ demonstrates the potential of this experimental technique for qualitative/semiqualitative and quantitative actinide speciation investigations. We observe a pre-edge feature with quadrupole nature in a $\text{U}\text{ }{L}_{3}$ edge partial fluorescence yield-XANES spectrum. This feature is a tool for characterizing the participation of $5f$ orbitals in U-O bonding. The feature origin is explained by performing calculations with the finite difference method near-edge structure code based on the multiple-scattering theory and the finite difference method.

Reference EPFL-ARTICLE-203028doi:10.1002/anie.200903457View record in Web of Science Record created on 2014-11-07, modified on 2017-05-12

A pentavalent uranyl complex supported by a bulky dianionic tetradentate (ONNO)-type salan ligand has been prepared by direct synthesis from the iodide precursor {[UO(2)Py(5)][KI(2)Py(2)]}(n), and displays high stability towards disproportionation and ligand dissociation but reactivity towards oxidizing substrates.

Reaction of {[UO2Pys][mu-KI2Py2]}n (1) with 2 equiv of potassium dibenzoylmethanate (Kdbm) in pyridine or acetonitrile affords, respectively, the corresponding tetranuclear complexes of pentavalent uranyl ([UO2(dbm),2]2[mu-K(Py)2]2[mu8-K(Py)]}2I2 x Py2 (2) (in 70% yield) and {[UO2(dbm)2]2[mu-K(MeCN)2][mu8-K]}2 (3) (in 40% yield) in which four UO2+ are mutually coordinated (T-shaped "cation-cation" interaction). The X-ray structures of 2 and 3 show also the presence of, respectively, six and four potassium cations involved in UO2+...K+ interactions. Reaction of 2 with an excess of 18-crown-6 (18C6) affords the dimeric complex [UO2(dbm)2K(18C6)]2 (4) presenting a diamond-shaped interaction between two UO2+ groups, in 45% yield. 1H and PFGSTE diffusion NMR spectroscopy of 2 and 3 in pyridine show unambiguously the presence of UO2+...UO2+ and UO2+...K+ interactions (tetrametallic species) in solution, which leads to a rapid (7 days) disproportionation of pentavalent uranyl to afford [U(dbm)4] and [UO2(dbm)2] species. The UO2+...K+ interaction plays an important synergistic role in the stabilization of the UO2+...UO2+ interactions. Accordingly, the lower affinity of (K(18C6))+ for the uranyl(V) oxygen in complex 4 results in a lower number of coordinated K+ and therefore in a weakened UP2+...UO2+ interaction. The UO24+...UO2+ interactions is completely disrupted in dmso or in the presence of Kdbm, preventing disproportionation of pentavalent uranyl. Solid-state variable-temperature magnetic susceptibility studies showed the unambiguous presence of antiferromagnetic coupling between the two oxo-bridged uranium centers of complex 4, with the appearance of a maximum in chi versus T at approximately 5 K. The different behavior of the tetrameric complex 3, which probably involves a magnetic coupling occurring at lower temperature, can be ascribed to the different geometric arrangement of the interacting uranyl(V) groups.

Reference EPFL-ARTICLE-203040doi:10.1002/anie.200705742View record in Web of Science Record created on 2014-11-07, modified on 2017-05-12

Reference EPFL-ARTICLE-203040doi:10.1002/anie.200705742View record in Web of Science Record created on 2014-11-07, modified on 2017-05-12