Allosteric effects in binuclear homo- and heterometallic triple-stranded lanthanide podates.

This work illustrates a simple approach for deciphering and exploiting the various free energy contributions to the global complexation process leading to the binuclear triple-stranded podates [Ln(2)(L9)](6+) (Ln is a trivalent lanthanide). Despite the larger microscopic affinities exhibited by the binding sites for small Ln(3+), the stability constants measured for [Ln(2)(L9)](6+) decrease along the lanthanide series; a phenomenon which can be ascribed to the severe enthalpic penalty accompanying the intramolecular cyclization around small Ln(III), combined with increasing anticooperative allosteric interligand interactions. Altogether, the microscopic thermodynamic characteristics predict ß(1,1,1)(La,Lu,L9)/ß(1,1,1)(Lu,La,L9) = 145 for the ratio of the formation constants of the target heterobimetallic [LaLu(L9)](6+) and [LuLa(L9)](6+) microspecies, a value in line with the quantitative preparation (>90%) of [LaLu(L9)](6+) at millimolar concentrations. Preliminary NMR titrations indeed confirm the rare thermodynamic programming of a pure heterometallic f-f' complex.

Silver baits for the "miraculous draught" of amphiphilic lanthanide helicates.

The axial connection of flexible thioalkyls chains of variable length (n=1-12) within the segmental bis-tridentate 2-benzimidazole-8-hydroxyquinoline ligands [L12(Cn) -2 H](2-) provides amphiphilic receptors designed for the synthesis of neutral dinuclear lanthanides helicates. However, the stoichiometric mixing of metals and ligands in basic media only yields intricate mixtures of poorly soluble aggregates. The addition of Ag(I) in solution restores classical helicate architectures for n=3, with the quantitative formation of the discrete D(3) -symmetrical [Ln(2) Ag2(L12(C3) -2 H)(3) ](2+) complexes at millimolar concentration (Ln=La, Eu, Lu). The X-ray crystal structure supports the formation of [La(2) Ag(2) (L12(C3) -2 H)(3) ][OTf](2) , which exists in the solid state as infinite linear polymers bridged by S-Ag-S bonds. In contrast, molecular dynamics (MD) simulations in the gas phase and in solution confirm the experimental diffusion measurements, which imply the formation of discrete molecular entities in these media, in which the sulfur atoms of each lipophilic ligand are rapidly exchanged within the Ag(I) coordination sphere. Turned as a predictive tool, MD suggests that this Ag(I) templating effect is efficient only for n=1-3, while for n>3 very loose interactions occur between Ag(I) and the thioalkyl residues. The subsequent experimental demonstration that only 25 % of the total ligand speciation contributes to the formation of [Ln(2) Ag(2) (L12(C12) -2 H)(3) ](2+) in solution puts the bases for a rational approach for the design of amphiphilic helical complexes with predetermined molecular interfaces.

Planned failures from the principle of maximum site occupancy in lanthanide helicates.

Despite the recent emergence of a toolbox fitted with microscopic thermodynamic descriptors for predicting the stabilities and speciations of polynuclear complexes in solution, the discovery of novel or unusual type of metal-ligand assemblies in metallosupramolecular chemistry still often relies on serendipity. In order to highlight the novel perspectives offered by a rational exploitation of these thermodynamic parameters, the segmental bis-tridentate ligands L7 and L8 have been designed for providing effective molarities upon reaction with trivalent lanthanides, Ln(III), so small that the saturated binuclear triple-stranded helicates [Ln(2)(Lk)(3)](6+), which obey the well-respected principle of maximum site occupancy, cannot be detected in solution because of their deliberately planned instabilities. The hierarchical evolution of the effective molarities with an increasing number of ligand strands in these complexes indeed favors the formation of the alternative unsaturated single-stranded [Ln(2)(Lk)](6+) and double-stranded [Ln(2)(Lk)(2)](6+) complexes, whose relative speciations in solution depend on the nature of the binding sites introduced into the segmental ligand.

A simple chemical tuning of the effective concentration: selection of single-, double-, and triple-stranded binuclear lanthanide helicates.

The replacement of terminal 2-benzimidazol-6-carboxypyridine (two internal rotational degrees of freedom) with 2-benzimidazol-8-hydroxyquinoline (one internal rotational degree of freedom) into segmental bis-tridentate ligands in going from L2 and [L3-2 H](2-) to [L12 b-2 H](2-) does not significantly affect the structures of the resulting binuclear lanthanide triple-stranded helical complexes [Ln(2)(L2)(3)](6+), [Ln(2)(L3-2 H)(3)], and [Ln(2)(L12 b-2 H)(3)] (palindromic helices, intermetallic contact distance approximately 9 A, helical pitch approximately 1.4 nm per turn). However, their thermodynamic assemblies are completely different in solution, as evidenced by the spectacular decrease of the effective concentrations by two orders of magnitude for [L12 b-2 H](2-). This key parameter in the [Ln(2)(L12 b-2 H)(n)] (n=2, 3) complexes is further abruptly modulated along the lanthanide series (Ln=La to Lu), which provides an unprecedented tool for 1) tuning the number of ligand strands in the final helicates, 2) selectively coordinating lanthanides in the various complexes, and 3) controlling the ratio of lanthanide-containing polymers over discrete assemblies.

182tungsten Mössbauer spectroscopy of heteropolytungstates.

The tungsten-182 Mössbauer spectra of a series of Keggin structure heteropolytungstates, [EW12O40]n- are reported. There is a very considerable variation in quadrupole coupling at the tungsten nucleus indicating considerable asymmetry in the electron distribution for the more electronegative elements E. The quadrupole coupling correlates well with the structural data, in particular with the distance between the tungsten and the oxygen atom of the EO4 group. These compounds may be regarded as rigid W12O36 cages interacting more or less strongly with an EO4n- host. The spectra of salts of metatungstate [H2W12O40]6- and [W6O19]2- are also given.

Encoding calamitic mesomorphism in thermotropic lanthanidomesogens.

Peripheral cyanobiphenyl dendrimers impose a microphase organization compatible with smectic mesomorphism, in which the bulky nine-coordinate lanthanide core is located between the decoupled mesogenic sublayers made up of parallel cyanobiphenyl groups.

The first self-assembled trimetallic lanthanide helicates driven by positive cooperativity.

The segmental tris-tridentate ligand L7 reacts with stoichiometric quantities of Ln(III) (Ln=La-Lu) in acetonitrile to give the complexes [Ln(2)(L7)(3)](6+) and [Ln(3)(L7)(3)](9+). Formation constants point to negligible size-discriminating effects along the lanthanide series, but Scatchard plots suggest that the self-assembly of the trimetallic triple-stranded helicates [Ln(3)(L7)(3)](9+) is driven to completion by positive cooperativity, despite strong intermetallic electrostatic repulsions. Crystallization provides quantitatively [Ln(3)(L7)(3)](CF(3)SO(3))(9) (Ln=La, Eu, Gd, Tb, Lu) and the X-ray crystal structure of [Eu(3)(L7)(3)](CF(3)SO(3))(9).(CH(3)CN)(9).(H(2)O)(2) (Eu(3)C(216)H(226)N(48)O(35)F(27)S(9), triclinic, P1, Z=2) shows the three ligand strands wrapped around a pseudo-threefold axis defined by the three metal ions rigidly held at about 9 A. Each metal ion is coordinated by nine donor atoms in a pseudo-trigonal prismatic arrangement, but the existence of terminal carboxamide units in the ligand strands differentiates the electronic properties of the terminal and the central metallic sites. Photophysical data confirm that the three coordination sites possess comparable pseudo-trigonal symmetries in the solid state and in solution. High-resolution luminescence analyses evidence a low-lying LMCT state affecting the central EuN(9) site, so that multi-metal-centered luminescence is essentially dominated by the emission from the two terminal EuN(6)O(3) sites in [Eu(3)(L7)(3)](9+). New multicenter equations have been developed for investigating the solution structure of [Ln(3)(L7)(3)](9+) by paramagnetic NMR spectroscopy and linear correlations for Ln=Ce-Tb imply isostructurality for these larger lanthanides. NMR spectra point to the triple helical structure being maintained in solution, but an inversion of the magnitude of the second-rank crystal-field parameters, obtained by LIS analysis, for the LnN(6)O(3) and LnN(9) sites with respect to the parameters extracted for Eu(III) from luminescence data, suggests that the geometry of the central LnN(9) site is somewhat relaxed in solution.

A novel extended covalent tripod for assembling nine-coordinate lanthanide(III) podates: a delicate balance between flexibility and rigidity.

The introduction of long semirigid spacers between the capping carbon atom of the tripod and the unsymmetrical tridentate binding units provides the novel, extended covalent podand tris-[2-[2-(6-diethylcarbamoylpyridin-2-yl)-1-ethyl-1H-benzoimidazol-5-yl-methoxy]ethyl]methane (L(15)). Reaction of L(15) with lanthanide(III) in acetonitrile produces stable podates [Ln(L(15))](3+) (Ln=La-Lu) in which three tridentate binding units are facially organized. These wrap around the nine-coordinate pseudo-tricapped trigonal-prismatic metal ions. The crystal structure of [La(L(15))](ClO(4))(3) (18, LaC(67)H(82)N(12)O(18)Cl(3), trigonal, R3c, Z=6) reveals the formation of a C(3)-symmetrical triple-helical podate. Two slightly different arrangements of the flexible ethylenoxy parts of the spacer are observed in the solid state in agreement with the formation of two conformational isomers (M:m) in a 4:1 ratio. A qualitative analysis of the aromatic diamagnetic anisotropies affecting the NMR signals of [Ln(L(15))](3+) (Ln=La, Y, Lu) in solution, combined with the quantitative determination of electron-induced relaxation in the paramagnetic complex [Nd(L(15))](3+), demonstrate that the solid state structure is maintained in solution. This leads to a mixture of two triple-helical conformers of similar stabilities and that do not interconvert on the NMR timescale between 243 and 343 K. Particular attention has been given to the structural programming of extended covalent tripods for facially organizing unsymmetrical tridentate binding units around Ln(III). Photophysical measurements show that L(15) efficiently protects the metallic coordination spheres and sensitizes Eu(III) and Tb(III) upon UV irradiation.

The first self-assembled trimetallic lanthanide helicate: different coordination sites in symmetrical molecular architectures.

A tris-tridentate segmental ligand has been designed for the self-assembly of homotrimetallic triple-stranded lanthanide helicates possessing different coordination sites along the threefold axis.

Analysis of paramagnetic NMR spectra of triple-helical lanthanide complexes with 2,6-dipicolinic acid revisited: a new assignment of structural changes and crystal-field effects 25 years later.

Variable-temperature (1)H and (13)C NMR measurements of the D(3)-symmetrical triple-helical complexes [Ln(L1-2H)(3)](3)(-) (L1 = pyridine-2,6-dicarboxylic acid; Ln = La-Lu) show evidence of dynamic intermolecular ligand-exchange processes whose activation energies depend on the size of the metal ion. At 298 K, the use of diastereotopic probes in [Ln(L3-2H)(3)](3)(-) (L3 = 4-ethyl-pyridine-2,6-dicarboxylic acid) shows that fast intramolecular P <==> M interconversion between the helical enantiomers occurs on the NMR time scale. Detailed analyses of the paramagnetic NMR hyperfine shifts according to crystal-field independent techniques demonstrate the existence of two different helical structures, one for large lanthanides (Ln = La-Eu) and one for small lanthanides (Ln = Tb-Lu), in complete contrast with the isostructurality proposed 25 years ago. A careful reconsideration of the original crystal-field-dependent analysis shows that an abrupt variation of the axial crystal-field parameter A(0)2 parallels the structural change leading to some accidental compensation effects that prevent the detection of structural variations according to the classical one-nucleus method. Crystal structures in the solid state and density functional theory calculations in the gas phase provide structural models that rationalize the paramagnetic NMR data. A regular triple-helical structure is found for small lanthanides (Ln = Tb-Lu) in which the terdentate chelating ligands are rigidly tricoordinated to the metals. A flexible and distorted structure is evidenced for Ln = La-Eu in which the central pyridine rings interact poorly with the metal ion. The origin of the simultaneous variation of structural parameters and crystal-field and hyperfine constants near the middle of the lanthanide series is discussed together with the use of crystal-field-independent techniques for the interpretation of paramagnetic NMR spectra in axial lanthanide complexes.

Reactivity of the Coordinated Hydroperoxo Ligand.

The reactivity of the hydroperoxo complex [Co(CN)(5)OOH](3)(-) has been studied in aqueous solution. The complex undergoes acid-catalyzed aquation (k = 1.89(5) x 10(-)(2) s(-)(1), pK(a) = 5.21(4), T = 20 degrees C, I = 0.1 M). Assuming an I(d) mechanism, this allows the relative affinity for Co(III) to be deduced as H(2)O(2) < H(2)O < HO(2)(-) and implies H(2)O(2) to be a very weak ligand. At neutral pH the hydroperoxo complex effects efficient oxygen atom transfer to L-methionine to give an intermediate identified as [Co(CN)(5)(L-methionine S-oxide)](2)(-), which then dissociates to [Co(CN)(5)OH(2)](2)(-) and L-methionine S-oxide. The reaction is acid catalyzed and is proposed to take place via nucleophilic attack of sulfur on the proximal oxygen of the hydroperoxo ligand with concerted loss of water. The significance of these results for the interaction of hydrogen peroxide with labile metal ions is discussed.