Revised ionic radii of lanthanoid(III) ions in aqueous solution

A new set of ionic radii in aqueous solution has been derived for lanthanoid(III) cations starting from a very accurate experimental determination of the ion-water distances obtained from extended X-ray absorption fine structure (EXAFS) data. At variance with previous results, a very regular trend has been obtained, as expected for this series of elements. A general procedure to compute ionic radii in solution by combining the EXAFS technique and molecular dynamics (MD) structural data has been developed. This method can be applied to other ions allowing one to determine ionic radii in solution with an accuracy comparable to that of the Shannon crystal ionic radii.     

Structure of solvated metal ions in nitrate and chloride solutions of erbium(III) and yttrium(III) in dimethyl sulfoxide

X-ray diffraction measurements on 1M yttrium(III) and erbium(III) nitrate and chloride solutions in dimethyl sulfoxide (DMSO) have shown that Er(III) and Y(III) solutions of equal compositions are isostructural. The intensity difference functions can then be used to derive the detailed structure of the coordination sphere around the metal ions. The DMSO molecules are coordinated over oxygen with average M-O-S bond angles of about 130°. Two different conformations, corresponding to different relative orientations of the M-O and O-S bonds, seem to be prevalent. In the nitrate solutions an average of about 1.5 nitrate ions are coordinated as bidentate ligands to each metal ion. In the chloride solutions about 1.3 chloride ions belong to the inner-coordination sphere.On leave from the Department of Inorganic Chemistry, Royal Institute of Technology, S-10044 Stockholm, Sweden     

Lanthanide luminescence efficiency in eight- and nine-coordinate complexes: Role of the radiative lifetime

The photophysical parameters for the sensitization of metal-centred luminescence are analyzed in two series of complexes with tridentate and hexadentate ligands having N_xO_y chelating units. In particular, the radiative lifetime τ_rad is experimentally estimated for 29 nine-coordinate Eu^III complexes and 10 eight- and nine-coordinate Yb^III complexes. The known dependence of τ_rad on refractive index is substantiated by comparing data for solid-state samples and solutions. Moreover, a clear dependence of τ_rad with the coordination environment is evidenced and in the case of Eu^III, a comparison between τ_rad and the nephelauxetic effect generated by the ligands is attempted. Altogether, this extensive analysis points to the importance of having a handle on τ_rad when designing ligands for highly luminescent lanthanide-containing molecular edifices. This, in turn, should stimulate initiating theoretical considerations to unravel a reliable relationship between τ_rad and the electronic structure of the ligands.     

Co-extraction of lanthanoid(III) with various metal ions in the chelate extraction system with acetylacetone.

The co-extraction phenomenon was found in a typical chelate extraction system, in which the extraction of lanthanoid ion (Ln3+) with acetylacetone (Hacac) was highly enhanced by various metal ions (M(n+)) such as Cu2+, Al3+, and Zr4+. This phenomenon was ascribed to the formation of the 1:1 adduct between Ln(acac)3 and the M(acac),, extracted into the organic phase. The co-extraction occurred more readily for La3+ than that for Lu3+, and increased in the order of Cu2+ < Al3+ < Zr4+. This work elucidated that the co-extraction due to the adduct formation is a rather common phenomenon in the chelate extraction.     

Crystallographic and vibrational spectroscopic studies of octakis(DMSO)lanthanoid(III) iodides

The octakis(DMSO) (DMSO = dimethylsulfoxide) neodymium(III), samarium(III), gadolinium(III), dysprosium(III), erbium(III), and lutetium(III) iodides crystallize in the monoclinic space group P21/n (No. 14) with Z = 4, while the octakis(DMSO) iodides of the larger lanthanum(III), cerium(III), and praseodymium(III) ions crystallize in the orthorhombic space group Pbca (No. 61), Z = 8. In all [Ln(OS(Me2)8]I3 compounds the lanthanoid(III) ions coordinate eight DMSO oxygen atoms in a distorted square antiprism. Up to three of the DMSO ligands were found to be disordered and were described by two alternative configurations related by a twist around the metal-oxygen (Ln-O) bond. To resolve the atomic positions and achieve reliable Ln-O bond distances, complete semirigid DMSO molecules with restrained geometry and partial occupancy were refined for the alternative sites. This disorder model was also applied on previously collected data for the monoclinic octakis(DMSO)yttrium(III) iodide. At ambient temperature, the eight Ln-O bond distances are distributed over a range of about 0.1 A. The average value increases from Ln-O 2.30, 2.34, 2.34, 2.36, 2.38, 2.40 to 2.43 A (Ln = Lu, Er, Y, Dy, Gd, Sm, and Nd) for the monoclinic [Ln(OSMe2)8]I3 structures, and from 2.44, 2.47 to 2.49 A (Ln = Pr, Ce, and La) for the orthorhombic structures, respectively. The average of the La-O and Nd-O bond distances remained unchanged at 100 K, 2.49 and 2.43 A, respectively. Despite longer bond distances and larger Ln-O-S angles, the cell volumes are smaller for the orthorhombic structures (Ln = Pr, Ce, and La) than for the monoclinic structure with Ln = Nd, showing a more efficient packing arrangement. Raman and IR absorption spectra for the [Ln(OS(CH3)2)8]I3 (Ln = La, Ce, Pr, Nd, Gd, Tb, Dy, Er, Lu, and Y) compounds, also deuterated for La and Y, have been recorded and analyzed by means of normal coordinate methods. The force constants for the Ln-O and S-O stretching modes in the complexes increase with decreasing Ln-O bond distance and show increasing polarization of the bonds for the smaller and heavier lanthanoid(III) ions.     

4f-4f absorption spectra of nine-coordinate Pr (III) and Nd (III) complexes in different environments

Absorption spectroscopic studies on the mixed-ligand complexes of Pr (III) and Nd (III) with 2,2'-bipyridyl and thiocyanate in pyridine, DMSO, DMF and methanol are presented. The oscillator strengths for the hypersensitive and non-hypersensitive transitions have been calculated and variation in the oscillator strength and band shapes with respect to solvent type is rationalized in terms of solvent structures and coordinating properties. A comparison is made with 1,10-phenanthroline complexes and has been shown that bpy is a weaker ligand than phen for these ions. Pyridine has been found most effective in promoting 4f-4f intensity and the increase in the oscillator strength in this solvent is due to dynamic ligand polarization mechanism.     

Yttrium(III) bonding to organic ligands: A comparison with the lanthanoid(III) cations

Summary The yttrium(III) bonding to organic substrates (oximes, -diketonates and (poly)amino-(poly)carboxylates) has been compared with that of the lanthanoid(III) cations. The complexation constants of Y³⁺ with the examined organic ligands are similar to those of some cations of the first half of the lanthanoid series, in contrast with the fact that the Y³⁺ ionic dimensions are similar to those of Ho³⁺. This has been explained by correlating the formation constants of the Y³⁺ and the lanthanoids(III) complexes by the equation logK ₁=C _AC_B+E _AE_B, where the parametersC andE indicate the tendency of each Lewis acidA and Lewis baseB to undergo covalent or ionic bonding, and where the ratioH=E/C indicates the charge control on the bond formation tendency of each speciesA orB. The results are commented in terms of the utility of Y³⁺ in assisting organic reactions.

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Bindung von Yttrium(III) an organische Liganden: Vergleich mit Lanthanoid(III)-Kationen

Zusammenfassung Es wurde die Bindung von Yttrium(III) an organische Substanzen [Oxime, -Diketonate und (Poly)Amino(poly)carboxylate] im Vergleich mit Lanthanoid(III)-Kationen behandelt. Die Komplexierungskonstanten von Y³⁺ sind ähnlich denen einiger Kationen der ersten Hälfte der Lanthanoidenserie; dies steht im Gegensatz zur Tatsache, daß die Dimensionen des Y³⁺-Ions denen des Ho³⁺ entsprechen. Die Erklärung wurde mittels der für die Bildungskonstanten der Y³⁺- und Lanthanoid(III)-Komplexe gültigen Gleichung logK ₁=C _AC_B+E _AE_B gefunden, wobeiC undE Parameter sind, die die Tendenz der Lewis-SäurenA und der Lewis-BasenB zum Eingehen von kovalenten oder ionischen Bindungen charakterisieren und wo das VerhältnisH=E/C den Steuerungseffekt der Ladung auf die Bindungstendenz der SpeziesA oderB beschreibt. Die Ergebnisse werden im Hinblick auf den Nutzen von Y³⁺ zur Unterstützung organischer Reaktionen diskutiert.

     

Photoelectron detachment and solvated electron dynamics of the cobalt(III) and iron(III) oxalato complexes

The photochemical redox reaction kinetics of [M(III)(C(2)O(4))(3)](3-) [M = Co, Fe] complexes have been reexamined and studied by time-resolved spectroscopy. The redox mechanisms of the two systems, Co and Fe, were found to be similar to each other, and solvated electrons were observed immediately after 266/267 nm photoexcitation. A reaction mechanism is proposed that involves photoelectron detachment as a primary process. The charge-transfer bands for both complexes, which had been attributed to ligand to metal charge-transfer (LMCT) transitions previously, are reassigned to charge transfer to solvent (CTTS) transitions.     

Hydrated structures and dehydration processes of lanthanoid(III) chloranilates studied by EXAFS

The local structures of lanthanoid(III) chloranilate complexes of Pr(III), Nd(III), Tb(III) and Er(III) have been studied by EXAFS (extended X-ray absorption fine structure). Hydrated structures of the lanthanoid(III) ions in these complexes have been investigated with respect to their coordination numbers and interatomic distances. Six or four water molecules coordinate to the lanthanoid(III) ion of Pr(III) or Nd(III), respectively, just after preparation of the complexes. The temperature dependence of the first coordinated structures has been studied in order to reveal the behavior of the coordinated water molecules in dehydration process. The coordination number around the central lanthanoid(III) ion decreases stepwise as temperature increases, depending on the type of central lanthanoid(III) ion present. The interatomic distance between the central lanthanoid(III) ion and oxygen atoms in the first shell decreases, accompanying the decrease of the coordination numbers. A parameter representing proportion shows the reduction of interatomic distance as one coordinated water molecule removes from the central ion, depending on the type of lanthanoid(III) ions.     

4f-4f absorption spectra and hypersensitivity in nine-coordinate Ho(III) and Er(III) complexes in different environments

The effect of change in the environment upon 4f-4f absorption spectra of nine-coordinate Ho(III) and Er(III) complexes with thiocyanate and 2,2'-bipyridyl in methanol, DMSO, DMF and pyridine have been investigated. The oscillator strength for hypersensitive and non-hypersensitive transitions have been calculated and variation in the intensity and band shape with respect to solvent type is rationalized in terms of solvent structure and coordinating properties. A comparison with 1,10-phenanthroline complexes is investigated and has been found that phen has a larger impact on the transition intensities for these ions. Pyridine has been found most effective in promoting 4f-4f intensity. The results indicate that it is important to consider both the static and dynamic coupling mechanism while analysing the oscillator strength of hypersensitive transition.     

An ethanol‐solvated centrosymmetric dimer of bismuth(III) and thiosaccharinate resulting from `semicoordination' contacts

In the title compound, bis(μ‐1,1‐dioxo‐1,2‐benzothiazole‐3‐thiolato)‐κ³N,S:S;κ³S:N,S‐bis[(1,1‐dioxo‐1,2‐benzothiazole‐3‐thiolato‐κ²N,S)(ethanol‐κO)bismuth(III)] ethanol hemisolvate, [Bi₂(C₇H₄NO₂S₂)₆(C₂H₅OH)₂]·0.5C₂H₅OH, three independent thiosaccharinate (tsac) anions chelate the metal centre through the endocyclic N and exocyclic S atoms. The complex also presnts two `semicoordination' contacts, one from a pendant ethanol solvent molecule and a second one from an S atom of a centrosymmetrically related molecule. This latter interaction complements two π–π interactions between tsac rings to form a dimeric entity which is the elemental unit that builds up the crystal structure. These dinuclear units are connected to each other via a second type of π–π interaction, generating chains along [11]. Two ethanol molecules, one of them of full occupancy at a general position and semicoordinated to the central cation, and a second one depleted and disordered around a symmetry centre, stabilize the structure. The complex was studied theoretically and the vibrational assignations were confirmed by employing theoretical density functional theory (DFT) methods.     

Thermodynamic data for lanthanoid(III) sequestration by phytate at different temperatures

Abstract  

The results of an investigation on the interactions between phytate ion (myo-inositol hexaphosphate, Phy) and some lanthanoid cations (La³⁺, Nd³⁺, Sm³⁺, Dy³⁺, and Yb³⁺) are reported. The stability constants of various LnH _j Phy species (Ln = generic lanthanoid) were determined by potentiometry (ISE-H⁺ glass electrode) in NaCl_aq at I = 0.15 mol dm⁻³ and t = 25 °C, and the corresponding formation enthalpies by calorimetric titration. The thermodynamic data obtained were used to provide a speciation scheme for the lanthanoid(III)–phytate systems at different temperatures. The sequestering ability of this ligand toward Ln³⁺ was also evaluated by calculation of pL₅₀ values (the total concentration of ligand necessary to bind 50% of a cation present in trace amounts) under different conditions, and equations were formulated to model their dependence on temperature and pH.     

Synergistic effect of tris(4-isopropyltropolonato)cobalt(III) on the extraction of lanthanoid(III) with 2-thenoyltrifluoroacetone.

Synergistic enhancement of the extraction of lanthanoid(III) (Ln) with 2-thenoyltrifluoroacetone (Htta) in benzene has been found by the addition of tris(4-isopropyltropolonato)cobalt(III) (Co(ipt)3). The synergistic effect of Co(ipt)3 was ascribed to the formation of a 1:1 adduct of Ln(tta)3 with Co(ipt)3, i.e., a binuclear complex, in the organic phase. The adduct formation constant (beta s,1) determined by the extraction equilibrium analysis was reasonably consistent with that determined by spectrophotometry. The beta s,1 values decreased with increase in the atomic number of Ln and showed a large difference between light and heavy Ln. Spectroscopic studies were performed to explain the difference in the beta s,1 values. Electronic absorption spectra showed that the change in the structure of Co(ipt)3 complexed with the light Ln chelate is larger than that with the heavy Ln. The IR spectra showed the displacement of the coordinated water molecules of the light Ln chelates with Co(ipt)3. On the other hand, the adduct formation of heavy Ln was caused by the hydrogen bonding between Co(ipt)3 and the coordinated water of the Ln chelate.     

The first enantiomerically pure helical noncovalent tripod for assembling nine-coordinate lanthanide(III) podates

Decomplexation of the trivalent lanthanide, Ln(III), from the racemic bimetallic triple-stranded helicates [LnCr(L8)(3)](6+) provides the inert chiral tripodal nonadentate receptor [Cr(L8)(3)](3+). Elution of the latter podand with Na(2)Sb(2)[(+)-C(4)O(6)H(2)](2).5H(2)O through a cation exchange column allows its separation into its inert helical enantiomers M-(+)(589)-[Cr(L8)(3)](3+) and P-(-)(589)-[Cr(L8)(3)](3+), whose absolute configurations are assigned by using CD spectroscopy and exciton theory. Recombination with Ln(III) restores the original triple-stranded helicates [LnCr(L8)(3)](6+), and the associated thermodynamic parameters unravel the contribution of electrostatic repulsion and preorganization to the complexation process. Combining M-(+)(589)-[Cr(L8)(3)](3+) with Eu(III) produces the enantiomerically pure d-f helicate MM-(-)(589)-[EuCr(L8)(3)](CF(3)SO(3))(6).4CH(3)CN, whose X-ray crystal structure (EuCrC(113)H(111)N(25)O(21)S(6)F(18), monoclinic, P2(1), Z = 2) unambiguously confirms the absolute left-handed configuration for the final helix. The associated ligand-centered and metal-centered chiro-optical properties recorded for the complexes MM-[LnCr(L8)(3)](6+) and PP-[LnCr(L8)(3)](6+) (Ln = Eu, Gd, Tb) show a strong effect of helicity on specific rotary dispersions, CD and CPL spectra.     

Synergic extraction of lanthanoid(III) with 2-thenoyltrifluoroacetone and triphenylarsine oxide in benzene

The synergic extraction of La(III), Eu(III), and Lu(III) with 2-thenoyltrifluoroacetone (Htta) and triphenylarsine oxide (tpao) in benzene has been studied. The extractability of lanthanoids, Ln(III), is significantly affected by the association of Htta with tpao in the organic phase. The associated species is Htta·tpao and the association constant is determined as 10^1.63. The intrinsic extraction equilibrium of Ln(III) is analyzed using the free concentration of Htta and tpao. The synergic enhancement is ascribable to the formation of the adduct complexes shown as Ln(tta)₃tpao and Ln(tta)₃(tpao)₂ in the present extraction system. The adduct formation constants determined are very large as expected from the high basicity of tpao.     

Comparative structural studies of iodide complexes of uranium(III) and lanthanide(III) with hexadentate tetrapodal neutral N-donor ligands

The syntheses, the solution structures, and the crystal structures of the two new tetrapodal N-donor ligands N,N,N',N'-tetrakis(2-pyrazylmethyl)-1,3-trimethylenediamine (tpztn), 1, and N,N,N',N'-tetrakis(2-pyrazylmethyl)-trans-1,2-cyclohexanediamine (tpzcn), 2, are described. Two different geometric isomers of the cation [La(tpztn)I(2)](+) were isolated in which the ligand adopts two different conformations leading to strong differences in the metal-ligand bond distances. The crystal structure of isostructural complexes of La, U, Ce, and Nd were determined by X-ray diffraction studies for the ligands tpztn and tpzcn. In both series of complexes the two methylpyrazyl arms and the diamine spacer (trimethylene or cyclohexane) around each aliphatic nitrogen adopt the same helical configuration. The complexes crystallize as a racemic mixture of Lambda,Lambda and Delta,Delta enantiomers with distorted square antiprism geometries. In these complexes the M-N(pyrazine) distances show a decrease from La to Ce and from La to Nd which corresponds well to the decrease in ionic radius as expected in a purely ionic bonding model. Conversely the mean value of the U-N(pyrazine) distances is shorter (0.043(3) A for tpztn and 0.054(11) A for tpzcn) than the mean value of the La-N(pyrazine) distances. These differences are significantly larger than the decrease expected from the variation of the ionic radii and can be interpreted in terms of a stronger M-N interaction for U(III). Previously reported extraction studies have shown that while the tripod tris[(2-pyrazyl)methyl]amine (tpza) containing three pyrazyl nitrogens extracts An(III) preferentially to Ln(III), tpztn and tpzcn display no selectivity despite the presence of four pyrazyl groups connected to a different spacer. The structural studies described here show that despite the lack of selectivity observed in the extraction conditions, the arrangement of pyrazyl nitrogens in the tetrapodal architectures of tpztn and tpzcn allows for metal-ligand interaction similar to that observed for tpza.     

Hydrated hexacyanometallate(III) salts of triaqua(18‐crown‐6)lanthanoid(III) and tetraaqua(18‐crown‐6)lanthanoid(III) cations containing nine‐ and ten‐coordinate lanthanoids

Tetraaqua(18‐crown‐6)cerium(III) hexacyanoferrate(III) dihydrate, [Ce(C₁₂H₂₄O₆)(H₂O)₄][Fe(CN)₆]·2H₂O, and tetraaqua(18‐crown‐6)neodymium(III) hexacyanoferrate(III) dihydrate, [Nd(C₁₂H₂₄O₆)(H₂O)₄][Fe(CN)₆]·2H₂O, are isomorphous and isostructural in the C2/c space group, where the cations, which contain ten‐coordinate lanthanoid centres, lie across twofold rotation axes and the anions lie across inversion centres. In these compounds, an extensive series of O—H...O and O—H...N hydrogen bonds links the components into a continuous three‐dimensional framework. Triaqua(18‐crown‐6)lanthanoid(III) hexacyanoferrate(III) dihydrate, [Ln(C₁₂H₂₄O₆)(H₂O)₃][Fe(CN)₆]·2H₂O, where Ln = Sm, Eu, Gd or Tb, are all isomorphous and isostructural in the P space group, as are triaqua(18‐crown‐6)gadolinium(III) hexacyanochromate(III) dihydrate, [Gd(C₁₂H₂₄O₆)(H₂O)₃][Cr(CN)₆]·2H₂O, and triaqua(18‐crown‐6)gadolinium(III) hexacyanocobaltate(III) dihydrate, [Gd(C₁₂H₂₄O₆)(H₂O)₃][Co(CN)₆]·2H₂O. In these compounds, there are two independent anions, both lying across inversion centres, and the lanthanoid centres exhibit nine‐coordination; in the crystal structures, an extensive series of hydrogen bonds links the components into a three‐dimensional framework.