1,721,064 research outputs found
Molecular motions driven by transition metal redox couples: ion translocation and assembling-disassembling of dinuclear double-strand helicates
No full textThere exists a current interest in the design of molecular systems capable of converting chemical energy into an intramolecular movement, thus producing mechanical work. The energy necessary for promoting the motion is provided by an ancillary reaction: when the reaction has a redox nature, we have the molecular equivalent of the electrical machines of the macroscopic world
Three component systems for conventional and window-shaped response pH indicators,
No full textThree-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 catio
Anion receptors that contain metals as structural units
No full textTransition metals can be used as structural elements to build up anion receptors, enhancing H-bond
donor tendencies, favouring the assembling of the molecular framework, inducing the formation of
a cage. The versatile spectroscopic and electrochemical properties of the metal ion can provide a
convenient signalling mechanism to communicate the occurrence of anion recognition
A ditopic tetradentate pyridyl amine ligand containing an anthracene fragment: fluorescence intensity and ‘closed’ vs. ‘open’ species formation in the presence of Cu2+, as a function of pH
No full textThe compounds 9,10-bis(2-pyridylmethylaminoethyl)anthracene L1 and 9-(2-pyridylmethylaminomethyl)anthracene L2, based on nitrogen donors of the amine and pyridine type, were prepared and their properties examined in aqueous solution (water–1,4-dioxane 1∶4 v/v). The protonation constants of both have been determined by means of potentiometric titrations and the obtained species vs. pH distribution diagrams superimposed on the fluorescence intensity vs. pH profiles. This revealed an ‘off-on-offhair space’ window behaviour: the fluorescence intensity is high only in the pH range in which both free pyridine and protonated amines exist. Moreover, the formation constants of the complex species which form in the presence of 1 or 0.5 (in the case of L1 and L2, respectively) equivalent of CuII have been determined by means of potentiometric titrations. In particular, in the case of the tetradentate ditopic ligand L1 the ‘closed’ two ligand–two metal species [Cu2L12]4+ and [Cu2L12(OH)]3+ form in basic solution, in addition to two ligand–one metal partially protonated ‘open’ complex species. Moreover, a strong variation of the fluorescence intensity of the anthracene fragment signals the formation of the ‘closed’ species with respect to the ‘open’ ones, while a more subtle variation of the fluorescence intensity also allows one to distinguish between [Cu2L12]4+ and [Cu2L12(OH)]3+
The Interaction of Fluoride with Fluorogenic Ureas: An ON1–OFF–ON2Response
No full textThe anion binding tendencies of the two fluorogenic ureas L1H and L2H, containing the 2-anthracenyl and 1-pyrenyl moieties as signaling units, respectively, have been investigated in MeCN and DMSO by absorption, emission, and 1H NMR spectroscopies. The formation of stable 1:1 receptor:anion H-bond complexes has been confirmed by structural studies on the crystalline [Bu4N][L1···Cl] and [Bu4N][L2H···CH3COO] salts. Complexation induces significant variations of the emission properties of L1H and L2H according to a multifaceted behavior, which depends upon the fluorogenic substituent, the solvent, and the basicity of the anion. Poorly basic anions (Cl–, Br–) cause a red shift of the emission band(s). Carboxylates (CH3COO–, C6H5COO–) induce fluorescence quenching due to the occurrence of an electron-transfer process taking place in the locally excited complex [*L-H···X]−. However, this excited complex may undergo an intracomplex proton transfer from one urea N–H fragment to the anion, to give the tautomeric excited complex [L···H–X]−*, which emits at higher wavelength. F– displays a unique behavior: It forms with L1H a stable [L–H···F]− complex which in the excited state undergoes intracomplex proton transfer, to give the poorly emissive excited tautomer [L···H–F]−*. With L2H, on moderate addition of F–, the 1:1 H-bond complex forms, and the blue fluorescence of pyrene is quenched. Large excess addition of F– promotes deprotonation of the ground-state complex, according to the equilibrium [L2H···F]− + F– [L2]− + HF2–. The deprotonated receptor [L2]− is distinctly emissive (yellow fluorescence), which generates the fluorimetric response ON1–OFF–ON2 of receptor L2H with respect to F–
Bistren cryptands and cryptates: Versatile receptors for anion inclusion and recognition in water
No full textBistren cryptands can be easily synthesised through the Schiff base condensation of two molecules of tren and three molecules of a dialdehyde, followed by hydrogenation of the six C=N double bonds to give octamine cages, whose ellipsoidal cavity can be varied at will, by choosing the appropriate dialdehyde, in order to include substrates of varying sizes and shapes. Bistrens can operate as effective anion receptors in two ways: (i) in their protonated form, providing six secondary ammonium groups capable of establishing hydrogen bonding interactions with the anion; (ii) as dicopper(II) cryptates, in which the two coordinatively unsaturated metal centres can be bridged by an ambidentate anion. Representative examples of the two approaches, as well as the design of an anion molecular dispenser, in which a dicopper(II) bistren cryptate acts as a bottle will be illustrated
The squaramide versus urea contest for anion recognition
No full textThe interaction of a neutral squaramide-based receptor, equipped with two 4-nitrophenyl substituents, with halides and oxoanions has been studied in MeCN. UV/Vis and 1H NMR spectroscopy titration experiments clearly indicated the formation of 1:1 hydrogen bonding complexes with all the investigated anions. X-ray diffraction studies on the chloride and bromide complex salts confirmed the 1:1 stoichiometry and indicated the establishment of bifurcated hydrogen-bond interactions between the squaramide-based receptor and the halide anion that involved both 1) amide NH and 2) aryl proximate CH fragments, for a total of four bonds. Probably due to the contribution of CH fragments, complexes of squaramide with halides are 1 to 2 orders of magnitude more stable than the corresponding ones with the analogous urea-based receptor that contains two 4-nitrophenyl substituents. In the case of oxoanions, squaramide forms complexes, the stability of which decreases with the decreasing basicity of the anion, and is comparable to that of complexes of the urea-based receptor. Such a behaviour is ascribed to the predominance of different contributions: electrostatic interaction for halides, acid-to-base ‘frozen’ proton transfer for oxoanions. Finally, with the strongly basic anions F− and CH3COO−, squaramide first gives genuine hydrogen-bond complexes of 1:1 stoichiometry; then, upon addition of a second anion equivalent, it undergoes deprotonation of one NH fragment, with the simultaneous formation of the dianion hydrogen-bond complexes. In the case of the urea-based derivative, deprotonation takes place with fluoride but not with acetate. The apparently higher Brønsted acidity of squaramide with respect to urea reflects the capability of the squaramide receptor to delocalise the negative charge formed on NH deprotonation over the cyclobutene-1,2-dione ring and the entire molecular framewor
Octahedral Copper(II) and Tetrahedral Copper(I) Double-Strand Helicates: Chiral Self-Recognition and Redox Behavior
No full textThe racemic form of the linear multidentate ligand L (RRL þ SSL) gives dinuclear complexes of 2:2 stoichiometry both with CuII, acting as a bisterdentate
ligand, and with CuI, acting as a bis-bidentate ligand. Single crystal X-ray diffraction studies have shown that
the CuII complex exists as double-strand homochiral helicate molecules: P,P-[Cu2
II(RRL)2]4þ and M,M-[Cu2
II-
(SSL)2]4þ; in which the two trans-1,2-cyclohexanediamine subunits have the same chirality for of the two strands.
Each CuII metal center is six-coordinated according to a cis-octahedral geometry and interacts with a NNO donor
subunit of each strand. The CuI complex, when crystallized from THF in the presence of racL, gives a double-strand
homochiral helicate complex and in the solid state forms a racemic mixture of the homochiral metal complexes M,
M-[Cu2
I(RRL)2]2þ and P,P-[Cu2
I(SSL)2]2þ. When crystallizing from a MeCN solution, CuI and racL give rise to the
heterochiral nonhelicate dimeric complex [Cu2
I(RRL)(SSL)]2þ, in which the two strands of the dimer have inverse
configuration of the trans-1,2-cyclohexanediamine subunits and are assembled side-by-side. In both structural
architectures, the CuI centers are four-coordinated by two nitrogen atoms from each strand, according to a distorted
tetrahedral geometry. In MeCN solution, the dinuclear CuII complex disassembles to give the mononuclear species,
which, on reduction at a platinum electrode in a cyclic voltammetry experiment, gives two CuI mononuclear complexes
that quickly assemble to give the dinuclear CuI complex. This complex undergoes two consecutive one-electron
oxidation processes, but the dinuclear CuII species that forms decomposes in less than 1 s. On the contrary, the
[Cu2
I(racL)2]2þ complex is stable in MeCN solution and undergoes two one-electron oxidation processes to give a form
of dinuclear CuII complex that lasts in solution for more than 20 s
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