Extractive fluorimetric determination of ultratraces of lead with cryptand 2.2.2 and eosin

A new speetrofluorimetric method for determination of ultratraces of lead is based on solvent extraction into chloroform of the ion-pair formed between the positively-charged cryptate of lead with cryptand 2.2.2 and the eosinate anion. The detection limit for lead is 1 ng ml , and the linear working range is from the detection limit to 250 ng ml . The relative standard deviation is 3.7% at the 100 ng ml level. The method is highly selective for the extraction and determination of lead in the presence of other cations, and has been tested for direct determination of lead contamination in soft drinks. Aggregation of the extracted ion-pair in the organic phase has been demonstrated in fundamental extraction studies.     

Cyclodextrin chemistry. Selective modification of all primary hydroxyl groups of α- and β-cyclodextrins

Two efficient methods are described for the selective modification of all six primary hydroxyl groups of α-cyclodextrin (α-CD, 1 1 ). One, using an indirect strategy, involves protection of all 18 hydroxyl functions as benzoate esters, followed by selective deprotection of the six primary alcohol groups. The other, using a direct strategy, involves selective activation of the primary hydroxyl groups via a bulky triphenylphosphonium salt, which is then substituted by azide anion as the reaction proceeds. A number of modified α-cyclodextrin derivatives have been prepared and fully characterized, among which are: the useful intermediate α-cyclodextrin-dodeca (2, 3) benzoate ( 3 ); hexakis (6-amino-6-deoxy)-α-cyclodextrin hexahydrochloride ( 7 ); hexakis (6-amino-6-deoxy)-dodeca (2, 3)-O-methyl-α-cyclodextrin hexahydrochloride ( 9 ), hexa (6)-O-methyl-α-cyclodextrin ( 13 ). The direct substitution is shown to be even more efficient for β-cyclodextrin ( 16 ), giving the heptakis (6-azido-6-deoxy)-β-CD-tetradeca (2, 3)acetate ( 17 ), while the indirect strategy fails. The compounds are characterized by extensive use of ¹³C- and ¹H-NMR. spectroscopy. The steric and statistical problems of selective polysubstitution reactions for the cyclodextrins are discussed, and possible reasons for the observed differences in reactivity between α- and β-cyclodextrins are examined. The dodecabenzoate 3 presents a very marked solvent effect on physical properties (IR. and NMR. spectra, optical rotation); the effects observed may be ascribed to an unusually strong intramolecular network of hydrogen bonds which severely distorts the α-cyclodextrin ring and lowers the symmetry from six-fold to three-fold.     

Cryptates Sphériques. Synthèse et Complexes d'Inclusion de Ligands Macrotricycliques Sphériques

Spherical Cryptates. Synthesis and Inclusion-Complexes of Spherical Macrotricyclic Ligands A general strategy for the synthesis of spherical macrotricyclic ligands has been developed. Four spherical cryptands, SC - 24 , SC - 25 , SC - 26 and SC - 27 have been obtained by this route. The synthesis and cation-complexing properties of these compounds are described in detail. Stability constants and cation exchange rates of the spherical cryptates obtained with alkali and alkaline-earth cations have been determined. Highly stable complexes are formed by SC - 24 ; the Rb⁺ and Cs⁺ cryptates of SC - 24 are the most stable complexes of these cations known to date. The size of the intramolecular cavity affects the complexation selectivity. The cation exchange rates are very slow, and the corresponding free energies of activation are even larger than, for macrobicyclic cryptates of similar stability. Both the high complex stabilities and the high activation energies required for cation exchange indicate a marked ‘spherical cryptate effect’ resulting from the highly connected nature of the molecular architecture of spherical macrotricyclic ligands.     

CRYPTATES XIX. Ligands polyaza-polyoxa-macrobicycliques: Synthèse et complexes métalliques

Polyaza-polyoxa macrobicyclic ligands: its synthesis and metal complexes. The synthesis of the polyaza-polyoxa macrobicyclic ligands 1–4 is described. They form complexes with a variety of metal cations, transition metal cations as well as alkali and alkaline-earth cations. These complexes may be formulated as cation inclusion complexes, cryptates, in which the cation is contained in the intramolecular cavity. The properties of the complexes are described. An especially interesting feature is that these ligands, polymines of macrobicyclic topology, provide a means of trapping transition metal cations inside a molecular cavity; thus they impose coordination geometries and may modify the spectral and redox properties of the cations.     

Accroissement de la reactivite des phosphorines en tant que dienes et philodienes par complexation du phosphore

The 2-Phenyl-4,5-dimethylphosphorin P-W(CO)₅ complex reacts easily as a dienophile with 2,3-dimethylbutadiene through its 1,6-positions and as a diene with N-phenylmaleimide, dimethyl acetylenedicarboxylate and cyclopentadiene, through its 1,4-positions.     

Anion-receptor molecules: Macrocyclic and macrobicyclic effects on anion binding by polyammonium receptor molecules

The stability constants for anion binding by the acyclic hexaamine 1 , its macrocyclic analogue 2 , and the bicyclic compound 3 in their protonated forms are reported. Compound 3 forms stable and selective complexes with halide ions, the stability sequence being I^? > Br^? > Cl^?. Compound 2 forms more stable complexes with sulfate, oxalate, and malonate dianions than its acyclic analogue 1 and shows a better selectivity pattern. Compound 3 forms stronger complexes with oxalate^2? than 2 and shows a remarkably high binding selectivity between oxalate^2? and malonate^2?. The comparison of the ability of 1–3 to complex anions demonstrates the macrocyclic and macrobicyclic effects on anion binding stability and selectivity.     

Ab initio calculations of all-trans octatetraene and eight isoelectronic conjugated chains: Semiempirical heats of formation and stabilization energies

In the framework of our study of the changes of the electric polarizability upon substituting heteroatomic linkages in conjugated hydrocarbon backbones, we have been led to examine the relative stability of eight compounds isoelectronic to the all-trans octatetraene using the Cox and Pilcher concept of stabilization energy (SE ), SE = ΔH_a – ΣN_ABE_AB, where ΔH_a is the heat of atomization of the species under consideration and the E_AB's are bond-energy terms. Full geometry optimizations at the 4-31G level have been performed to obtain the equilibrium geometries needed to deduce reliable semiempirical heats of formation from appropriate isodesmic processes. Some of the compounds containing the ? CH?N? and ? N?N ? groups have electric polarizabilities and stabilization energies comparable to octatetrene.     

Channel-Type Molecular Structures. Part 2. Synthesis of Bouquet-Shaped Molecules Based on a β-Cyclodextrin Core

A new series of channel-type molecules presenting the features of potential transmembrane structures is described. They result from the grafting of amphiphilic side chains on a β-cyclodextrin derivative 3 that constitutes the organizing core. They belong to the ‘bouquet’ family (B). Compounds bearing poly(oxyethylene) side chains, B 16 and 17 , and their polymethylene analogues B 18 and 19 , were synthesized. The properties investigated emphasize the suitability of such molecules to be incorporated into lipid bilayer membranes.     

Separation and structural analysis by 1H-NMR of the telomers of methyl methacrylate with thiophenol

The stereoselectivity of the addition step and the transfer step of the radical telomerization of methyl methacrylate with thiophenol has been studied by determining the structure and the amount of each isomer by ¹H-NMR. We found, in this case, that the addition step gave preferentially a diad r and that the transfer step gave preferentially a diad m. On the other hand, the tacticity observed for higher telomers is preferentially syndiotactic, similar to that of the poly(methyl methacrylate) prepared by radical initiation. The propagation process for radical telomerization obeys, as for the radical polymerization, Bernoulli's statistics.     

Mixed-valence, mixed-spin-state, and heterometallic [2x2] grid-type arrays based on heteroditopic hydrazone ligands: synthesis and electrochemical features

An extended family of heterometallic [(M1)2(M2)2(L-)4](n+) [2x2] grid-type arrays 1-9 has been prepared. The three-tiered synthetic route encompasses regioselective, redox and enantioselective features and is based on the stepwise construction of heteroditopic hydrazone ligands A-C. These ligands contain ionisable NH and nonionisable NMe hydrazone units, which allows the metal redox properties to be controlled according to the charge on the ligand binding pocket. The 2-pyrimidine (R) and 6-pyridine (R') substituents have a significant effect on complex geometry and influence both the electrochemical and magnetic behaviour of the system. 1H NMR spectroscopic studies show that the Fe(II) ions in the grid can be low spin, high spin or spin crossover depending on the steric effect of substituents R and R'. This steric effect has been manipulated to construct an unusual array possessing two low-spin and two spin-crossover Fe(II) centres (grid 8). Electrochemical studies were performed for the grid-type arrays 1-9 and their respective mononuclear precursor complexes 10-13. The grids function as electron reservoirs and display up to eight monoelectronic, reversible reduction steps. These processes generally occur in pairs and are assigned to ligand-based reductions and to the Co(III)/Co(II) redox couple. Individual metal ions in the heterometallic grid motif can be selectively addressed electrochemically (e.g., either the Co(III) or Fe(II) ions can be targeted in grids 2 and 5). The Fe(II) oxidation potential is governed by the charge on the ligand binding unit, rather than the spin state, thus permitting facile electrochemical discrimination between the two types of Fe(II) centre in 7 or in 8. Such multistable heterometallic [2x2] gridlike arrays are of great interest for future supramolecular devices incorporating multilevel redox activity.