Redox switchable ligands suitable for transition metal ions: Protonation,complexation and electrochemical properties of a ferrocene-modified tetraamine diketone and its saturated analogue

Abstract

The new ferrocene-containing water-soluble ligands 1 and 2 were synthesized and their protonation and complexation properties toward Ni^II and Cu^II studied as a function of pH, by means of potentiometric titration experiments. Electrochemical measurements were performed in aqueous solution on pure 1 and 2 and in the presence of Ni^II and Cu^II cations, in the pH range 2–12, allowing us to determine the redox potential values relative to the ferrocene oxidation in the free ligands and in their Ni^II and Cu^II complexes. 1 and 2 behave as redox switchable ligands, the former enhancing, the latter decreasing its binding ability upon oxidation of the appended ferrocene function. Besides, the Cu^II complex of ligand 1 and the Ni^II complex of ligand 2 behave as two-centre two-electron redox systems, the complexed metal cation being subject to further oxidation to M^III.     

M and P double helical complexes of copper(I) with bis-imino bis-quinoline enantiomerically pure chiral ligands

The enantiomerically pure bis-imino bis-quinoline ligands R,R-ImQ and S,S-ImQ have been prepared by Schiff condensation of 2-quinoline carboxyaldehyde with the pure R,R and S,S enantiomers of trans-1,2-diaminocyclohexane. Both ligands form 2:2 helical complexes with CuI perchlorate, and the crystal and molecular structure of [Cu2(R,R-ImQ)2]ClO4.H2O have been determined by X-ray diffraction methods: the [Cu2(R,R-ImQ)2]2+ molecular cation is a chiral double helix of M handedness, in which the two ligands are entertwined in such an arrangement that half of each ligand is not equivalent to the other half of the same ligand. Coupled circular dichroism and 1H NMR studies reveal that in CH3CN solution a rearrangement takes place toward a more symmetric helical structure (in which the two halves of the same ligand become equivalent), which maintains the same handedness found in the solid state and is a pure M isomer. Solid state and CH3CN solution CD experiments confirm that [Cu2(S,S-ImQ)2]ClO4.H2O, both in solution and in the solid state, is a pure double helix of P handedness, i.e., the enantiomer of the species containing the R,R ligand.     

Three-component systems for conventional and window-shaped response fluorescent pH indicators

Three-component systems made of a tetradentate bis-amino bis-quinoline ligand, a transition metal cation (Ni2+ or Cu2+) and a fluorescent indicator (Coumarin 343) have been studied in a water-dioxane (1 : 4 v/v) mixture, through potentiometric, pH-spectrophotometric and pH-fluorimetric titrations. For the Cu2+ containing systems, an "on-off-on" variation of fluorescence intensity vs. pH has been observed, whereas in the presence of Ni2+ a simple "on-off" profile of the fluorescence intensity vs. pH was obtained. These ternary systems thus behave as window-shaped or conventional pH-indicators, depending whether Cu2+ or Ni2+ is used as the cation.     

Using micelles for a new approach to fluorescent sensors for metal cations

A new approach based on self-assembly inside micelles has been individuated to prepare a system behaving as a water-operating selective fluorescent sensor for Cu2+ and Ni2+.     

Synthesis and Ruthenium-Catalyzed Enantioselective Hydrogenation of 3-O-Substituted 1,3-Dihydroxypropan-2-ones. Part 2

A number of 3-O-substituted 1,3-dihydroxypropan-2-ones have been synthesized in view of their potential use as prochiral precursors of optically active glycerols. Indeed, the oxo-ethers have been reduced to the corresponding 3-O-substituted glycerols via chiral Ru complexes derived from (S)-binap, ( =(?)-(S)-2,2′-bis(diphenylphosphino)-1,1′-binaphthalene) with enantiomeric excesses up to 93%. The steric course of the catalytic reduction appears to be essentially dependent on the steric encumbrance of the substituents; indeed, a dramatic increase of the enantiomeric excess is observed when the bulky trityl group is substituted by the less encumbering benzyl or octadecyl groups.     

Micelles as Containers for Self‐Assembled Nanodevices: A Fluorescent Sensor for Lipophilicity

Potentiometric titrations, fluorescence versus pH titrations, dynamic light scattering and fluorescence polarization anisotropy studies demonstrate that inside the nanodimensioned Triton X‐100 micelles, 1‐pyrenecarboxylic acid, PCOO^?, forms an apical complex with the Zn²⁺ cation encircled by a lipophilic cyclen ligand and hugely increasing its fluorescence. The ability of the Zn²⁺‐cyclen‐PCOO^? complex plus its micellar container to act as a fluorescent sensor to evaluate the lipophilicity of molecular species is demonstrated on the fatty acid series CH₃(CH₂)_xCOOH (x=0–16). At pH 7.4 a decrease in fluorescence is observed on the addition of fatty acids that is directly related to their chain length, that is, to their tendency to enter the micellar containers, where they dislocate PCOO^? from the Zn²⁺ centre. The independent determination of fatty acid pK_a values in the presence of Triton X‐100 micelles confirms that our fluorescent micellar device is capable of sensing their lipophilicity.