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Why, on Interaction of Urea Based Receptors with Fluoride, Beautiful Colors Develop.
Full text linkUrea-based receptors, containing electron-withdrawing chromogenic
substituents, in a DMSO solution, in the presence
of varying excess of fluoride, do not form H-bond complexes,
but undergo stepwise deprotonation of the two N-H fragments,
an event which is signaled by the development of
vivid colors. Double deprotonation is also observed in the
presence of hydroxide. Less basic anions (CH3COO-, H2PO4
-) induce deprotonation of only one N-H
What Anions Do to N-H-Containing Receptors.
No full textMolecules containing polarized N-H fragments behave as H-bond donors toward anions and are widely used as receptors for recognition and sensing purposes in aprotic solvents (CHCl3, MeCN, and DMSO). We present examples of receptors containing pyrrole and urea subunits, and we discuss the stability of their H-bond complexes with a variety of anions. It is demonstrated that the stability of the 1:1 complexes is strictly related to the acidic tendencies of the receptor and to the basic properties of the anion. It may happen also that more basic anions induce the deproto- nation of the receptor, if equipped with electron-withdrawing substituents. This is typically observed on interaction with fluoride, due to the formation of the very stable [HF2]- self-complex. For urea-based receptors armed with chromogenic substituents, the addition of a large excess of the anion (F-, OH-) may induce the consecutive deprotonation of both N-H fragments, processes signaled by the development of vivid colors
Urea vs. thiourea in anion recognition
Full text linkNeutral anion receptors (LH) form stable 1 : 1 H-bond [LH · · · X]− complexes with carboxylates, halides and
phosphate (X−). Some of the [LH· · · X]− complexes, in presence of an excess of X−, release an HX fragment, with
formation of [HX2]− and the deprotonated receptor L−. The tendency towards deprotonation increases with the
acidity of the receptor and with the stability of the [HX2]− self-complex. Thus, the more acidic thiourea containing
receptor deprotonates in the presence all the investigated anions except chloride, whereas the less acidic urea
containing receptor undergoes deprotonation only in the presence of fluoride, due to the high stability of [HF2]−
A two-channel chemosensor for the optical detection of carboxylic acids, including cholic acid.
No full textA neutral receptor, in which a urea fragment has been equipped with two naphthaleneimide subunits, on interaction with acetate, in a DMSO solution, undergoes deprotonation of one of the N–H fragments; an event which is signalled by a yellow-to-red colour change and by the quenching of the blue fluorescence of the naphthalneimide subunit, with no competition by a number of anions (phosphate, nitrate, sulfate, chloride or bromide). This procedure can be employed for the visual and spectroscopic detection of cholic acid, even in presence of the other competing bile acids, such as glycocholic and taurocholic
Some guidelines for the design of anion receptors
No full textAnions can be recognized by either pos. charged or neutral artificial receptors. Pos. charges within a receptor's cavity can be provided by transition metal ions (e.g. CuII) which offer a binding site to one donor atom of the anion. Dicopper(II) bis(tren) cryptates are ideal receptors for ambidentate anions capable of bridging the two CuII centers. The size of the ellipsoidal cavity and consequent inclusion selectivity can be modulated by varying the length of the spacers linking the two tren subunits. Examples of the selective recognition of halides, polyat. anions and arom. and aliph. dicarboxylates are discussed. Among neutral receptors attention is centered on systems contg. the urea subunit. Urea behaves as a bifurcate H-bond donor towards oxoanions. On the basis of equil. studies in aprotic solvents (mainly MeCN and DMSO) it is shown that the energy of the hydrogen bonding interaction and the selectivity are solely related to the acidic tendencies of the receptor and to the basic properties of the anion. In particular, the H-bond interaction can be conveniently viewed as an advanced (and "frozen") proton transfer from the -N-H fragment of urea and the oxygen atom of the anion. Addn. of excess fluoride may induce deprotonation of the -NH fragment due to the unique stability of the [HF2]- species which forms
Nature of Urea-Fluoride Interaction: Incipient and Definitive Proton Transfer.
No full text1,3-bis(4-nitrophenyl)urea (1) interacts through hydrogen bonding with a variety of oxoanions in
an MeCN solution to give bright yellow 1:1 complexes, whose stability decreases with the decreasing basicity
of the anion (CH3COO- > C6H5COO- > H2PO4- > NO2- > HSO4- > NO3-). The [Bu4N][1.CH3COO]
complex salt has been isolated as a crystalline solid and its molecular structure determined, showing the
formation of a discrete adduct held together by two N-H...O hydrogen bonds of moderate strength. On
the other hand, the F- ion first establishes a hydrogen-bonding interaction with 1 to give the most stable
1:1 complex, and then on addition of a second equivalent, induces urea deprotonation, due to the formation
of HF2-. The orange-red deprotonated urea solution uptakes carbon dioxide from air to give the
tetrabutylammonium salt of the hydrogencarbonate H-bond complex, [Bu4N][1.HCO3], whose crystal and
molecular structures have been determined
Chiral receptors for phosphate ions
No full textThe binding tendencies of the enantiomeric forms, R,R and S,S, of the neutral receptor 1 towards anions were investigated through UV-vis and 1H NMR titrn. expts. in DMSO. Both enantiomers form stable H-bond complexes with carboxylates and phosphates. In particular, receptor 1 strongly binds two H2PO4- ions according two stepwise equil., in which logK2 is higher than logK1. Such an unusual cooperativity effect is to be ascribed to the formation of strong H-bond interactions between the two H2PO4- anions, when bound to the two urea subunits of the receptor, as demonstrated by the crystal and mol. structures of the 1 : 2 complex salt: [Bu4N]2[R,R-1•••(H2PO4)2]. The S,S enantiomer forms an H-bond complex with the biol. relevant d-2,3-diphosphoglycerate anion, whose assocn. const. is twice that of the R,R complex. Such an effect is ascribed to the different structural features of the two diastereomeric complexes in soln., as shown by 31P NMR studies
Protonated macrobicyclic hosts containing pyridine head units for anion recognition.
No full textIn this paper, we report two macrobicyclic receptors containing pyridine head units derived from 1,10-diaza-15-crown[5] (L1) or 4,13-diaza-18-crown[6] (L2) that can be protonated in MeCN and used for anion recognition. The interaction of these protonated lateral macrobicycles with different anions has been investigated by means of spectrophotometric titrations in MeCN. The association constants for the complexes of halide anions with the protonated macrobicycles follow the sequences Cl->Br->I->F- (L1) and Cl->F->I->Br- (L2), whereby an increase of more than two logarithmic units is observed from F- to Cl- for the binding constants of the receptor derived from L1. The association constants also indicate an important degree of selectivity of these macrobicyclic receptors for Cl- over Br- or I-. The X-ray crystal structure analyses of the chloride and bromide complexes confirms the formation of the envisaged supramolecular complexes. Moreover, the binding constants indicate that these receptors present a high sulfate-to-nitrate binding selectivity. The stability trend observed for the recognition of halide anions by the macrobicycles presented herein as well as the sulfate-to-nitrate binding selectivity have been rationalised by means of DFT calculations at the B3LYP/LanL2DZ level. These studies indicate that the especially high binding selectivity for Cl- is the result of the optimum fit between the protonated macrobicyclic cavity and the size of the anion, whereas the sulfate-to-nitrate selectivity results from shape complementarity between the hydrogen-binding acceptor sites on sulfate and the hydrogen-bond donors of the macrobicycle
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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