Michel Meyer

Conformations and coordination schemes of carboxylate and carbamoyl derivatives of the tetraazamacrocycles cyclen and cyclam, and the relation to their protonation states

Abstract This paper discusses and rationalizes the metal coordination pattern of 12- and 14-membered tetraazamacrocyclic carboxylate and carbamoyl ligands based on the cyclen and cyclam framework in light of their acido–basic properties. Structural and protonation data are reviewed in order to illustrate the influence of the free ligands protonation state and of the pH conditions during the complexation reaction on the final coordination mode of the metallic cations.

Oxygen Reduction Catalyzed by a Fluorinated Tetraphenylporphyrin Free Base at Liquid/Liquid Interfaces

The diprotonated form of a fluorinated free base porphyrin, namely 5-(p-aminophenyl)-10,15,20-tris(pentafluorophenyl)porphyrin (H(2)FAP), can catalyze the reduction of oxygen by a weak electron donor, namely ferrocene (Fc). At a water/1,2-dichloroethane interface, the interfacial formation of H(4)FAP(2+) is observed by UV-vis spectroscopy and ion-transfer voltammetry, due to the double protonation of H(2)FAP at the imino nitrogen atoms in the tetrapyrrole ring. H(4)FAP(2+) is shown to bind oxygen, and the complex in the organic phase can easily be reduced by Fc to produce hydrogen peroxide as studied by two-phase reactions with the Galvani potential difference between the two phases being controlled by the partition of a common ion. Spectrophotometric measurements performed in 1,2-dichloroethane solutions clearly evidence that reduction of oxygen by Fc catalyzed by H(4)FAP(2+) only occurs in the presence of the tetrakis(pentafluorophenyl)borate (TB(-)) counteranion in the organic phase. Finally, ab initio computations support the catalytic activation of H(4)FAP(2+) on oxygen.

Factors affecting copper(II) binding to multiarmed cyclam-grafted mesoporous silica in aqueous solution.

Single- as well as multi-anchored cyclam-functionalized silica samples have been prepared by grafting amorphous silica gel (K60) and mesostructured silica (SBA-15) with silylated cyclam precursors bearing one, two, or four triethoxysilyl groups, respectively ascribed to cyclam-mono, cyclam-di, and cyclam-tetra. Their reactivity toward copper(II) has been thoroughly investigated in aqueous solution and discussed with respect to the number of arms tethering the ligand to the silica surface and the structural ordering of the adsorbent in terms of capacity, long-term stability, and speed of access to the binding sites. Less-than-complete metal ion uptake was always observed, even in excess of cyclam groups with respect to solution-phase Cu(II), suggesting lower stability of immobilized complexes relative to those in solution. Therefore, the number of arms attaching cyclam moieties to the silica walls (one, two, or four) was found to dramatically affect the binding properties of these hybrids toward copper(II), revealing significantly larger capacities when reducing the number of arms (less rigidity constraints in the macrocycle). In parallel, multiarm tethering resulted in better chemical resistance toward degradation as evidenced by UV-visible monitoring of Cu-cyclam complexes in solution (i.e., more ligand leaching from the adsorbent for singly tethered cyclam). On the other hand, electron spin resonance (ESR) experiments did not evidence significant differences between complexes bearing one, two, or four alkyl arms, since all Cu(II)-cyclam surface complexes were found to be hexacoordinated with a strong equatorial ligand field. Comparison of amorphous gels and mesostructured materials indicates that the binding properties of the adsorbents were hardly influenced by their level of ordering, suggesting that accessibility to the binding sites was not the limiting factor. Some advantage belonging to mesostructured adsorbents was however observed with respect to the rate of access to the active centers at pH values close to neutrality (due to faster mass transport), but this was no more the case when operating at lower pH values where the formation of the Cu-cyclam complex became the rate-determining step, as pointed out by electrochemistry.

Colorimetric Hg2+ sensing in water: from molecules toward low-cost solid devices.

A new colorimetric molecular sensor allowing for cheap, fast, sensitive, and highly selective naked-eye detection of Hg(2+) in water is described. This molecule combines a 1,8-diaminoanthraquinone signaling subunit and phosphonic acid esters that confer the water solubility to the dye (R = H). A ready-to-use colorimetric solid sensor was obtained by incorporating an amphiphilic analog (R = OC(12)H(25)) exhibiting similar binding properties and optical responses in an agarose film.

Study of a laminar falling film flowing over a wavy wall column: Part I. Numerical investigation of the flow pattern and the coupled heat and mass transfer

Abstract Flow pattern and heat and mass transfer characteristics for a film flowing over a vertical wavy column are numerically investigated in a laminar flow regime. In our approach, the heat and mass transfer coefficients are avoided in order to include hydrodynamics directly in the heat and mass transfer rates. As a consequence the numerical model is decomposed into two steps. Firstly, the flow pattern for a film with a free interface is developed. Secondly, heat and mass transfer are investigated with the incorporation of velocity fields. The heat and mass transfer coefficients increase in laminar flow.

Study of a laminar falling film flowing over a wavy wall column: Part II. Experimental validation of hydrodynamic model

Abstract The interface position of a film flowing over a wavy wall column is experimentally studied by an optical method composed of a charge coupled device (CCD) video camera. The results are compared with theoretical calculations and show a good agreement between results for both the film thickness and the vortex position. However, there exist some discrepancies because the interface is travelled by waves not accounted for in the mathematical model as it is supposed to be flat. Some characteristics of the waves are experimentally noted. Furthermore, the influence of the viscosity on the film thickness is established as well.

Synthesis, characterization and X-ray crystal structures of cyclam derivatives. Part VI. Proton binding studies of a pyridine -strapped 5,12-dioxocyclam based macrobicycle

The 14-membered cyclic diamide 1,4,8,11-tetraazacyclotetradecane-5,12-dione (5,12-dioxocyclam) can be considered as a trans-autodiprotected tetraazamacrocycle and provides a convenient starting material for the preparation of macrobicyclic receptors. As an example, the secondary amine nitrogen atoms located at the 1 and 8 positions were cross-bridged with a 1,3-pyridyl strap, affording the constrained ansa-dioxocyclam ligand 1,9,12,18,22-pentaazatricyclo[7.6.6.13,7]docosa-3,5,7(22)-triene-13,19-dione (L1). The proton binding properties of this cage-type compound, which possesses a hemispherical cavity, were fully investigated by spectroscopic (IR, NMR, UV, MALDI-TOF MS), quantum chemical, and potentiometric methods. While both bridgehead tertiary amines have their free lone pairs oriented inside the cavity, intramolecular hydrogen bonding was found to play a key role in determining the structural features of the free base and its protonated forms. L1 behaves as a diprotic base in water with log K011 = 8.94(1) and log K012 = 2.32(9), but most interestingly shows slow proton-transfer rates on the NMR timescale.

On the Physicochemical Properties of Pyridohelicenes

A comprehensive study on the physicochemical properties of a series of mono- and diaza[5]helicenes as well as mono- and diaza[6]helicenes is reported. Through the use of both computational and experimental methods, these helically chiral pyridohelicenes with the nitrogen atom(s) in various positions are characterised according to their inversion barriers, protonation constants and redox potentials. By using DFT calculations, kinetic measurements, UV/Vis titrations, cyclic voltammetry and EPR spectroscopy, a self-contained picture of their behaviour under conventional treatment by heat, acids and oxidising/reducing agents is provided.

Capillary electrophoresis–inductively coupled plasma-mass spectrometry hyphenation for the determination at the nanogram scale of metal affinities and binding constants of phosphorylated ligands

A screening strategy based on hyphenated capillary electrophoresis and inductively coupled plasma mass spectrometry (CE-ICP-MS) was developed to classify phosphorylated ligands according to their europium(III) binding affinity in a hydro-organic medium (sodium formate, pH 3.7, H(2)O/MeOH 90:10, v/v). Taking advantage of the high sensibility of ICP-MS for detecting phosphorus, this method enabled to assess the affinity of a variety of phosphorylated compounds, including phosphine oxides, thiophosphines, phosphonates, and phosphinates, in less than 1h and using less than 5 ng of substance. By varying the total europium concentration, complexation constants could be determined according to a sequential multiple run strategy, which proved to be in excellent agreement with the values obtained by UV-Vis absorption spectrophotometric titrations.

Some Factors Affecting the Removal of Lead(II) Ions from Aqueous Solution by Porous Calcium Hydroxyapatite: Relationships between Surface and Adsorption Properties

A porous hydroxyapatite (p-HAp) was prepared and employed for the removal of lead(II) ions at different concentrations from aqueous solution to determine the adsorption properties of p-HAp and compare them with those of a commercial hydroxyapatite (CAp) sample. The kinetic data obtained indicated that the adsorption performances of the adsorbents depended both on their specific surface area and crystallinity. Complexation of the Pb(II) ion on the adsorbent surface favoured the dissolution of hydroxyapatites characterized by a Ca/Pb molar ratio in the 0.85–1.5 range. The maximum adsorption capacity of p-Hap for Pb(II) ions at 30 ± 2°C was 2.30 mmol/g relative to 1.38 mmol/g for the commercial compound Cap at the same temperature. The higher capacity of p-HAp was explained in terms of its porosity and crystallinity. The Pb(II) ions sorption results could be modelled by the Langmuir and Freundlich isotherms.