Valeria Amendola

Anion recognition by dimetallic cryptates

Abstract Bis-tren cryptands (i.e. octamine cages consisting of two tripodal tetramine subunits covalently linked by given spacers) are able to incorporate first two metal ions, then an ambidentate anion, according to a cascade mechanism. In particular, dicopper(II) cryptates behave as effective receptors for anions, which fill the empty cavity of the cage and place their donor atoms in the two axial sites left available by each Cu(II) centre (which adopts a trigonal bipyramidal stereochemistry). Anion encapsulation by dicopper(II) cryptates often induces the development of a rather intense anion-to-metal charge transfer absorption band in the visible region, so that the recognition process is signalled by the appearance of a bright colour. Two examples are considered in detail: (i) that of a rigid bis-tren cryptate containing 1,3-xylyl spacers, which does not recognise the shape, but the bite of the polyatomic anion (i.e. the distance between two consecutive donor atoms); and (ii) that of the flexible cryptate containing 2,5-furanyl spacers, which is able to include also monoatomic anions, in particular halides, displaying peak selectivity in favour of Cl − .

Molecular events switched by transition metals

Transition metals can typically give rise to two (or more) distinct states of comparable stability (two consecutive oxidation states; two different stereochemical arrangements). In a multicomponent system, the conversion of one state to the other can modify a given property of a nearby subunit or can induce drastic changes in the system topology. In this sense, the metal behaves as a switch, which can be operated through an external input (the variation of the pH or of the redox potential). Recent examples of molecular switching by transition metals are reviewed: (i) the quenching/enhancement of the emission of a luminescent fragment effected by a nearby metal centred redox couple (e.g. NiII/NiIII); (ii) the pH driven motion of an aminoalkyl side chain in a NiII scorpionate complex, which is signalled by the variation of the light emission intensity of an appended anthracenyl fragment; (iii) the pH controlled translocation of a NiII ion within a multidentate ligand containing two compartments of different coordinating tendencies and (iv) the intramolecular translocation of a Cl− anion between two pre-positioned metal centres CuII and NiII, within a ditopic receptor, electrochemically driven through the NiII/NiIII redox change.

Recognition and sensing of nucleoside monophosphates by a dicopper(II) cryptate.

The dimetallic cryptate [Cu(2)(II)(1)](4+) selectively recognizes guanosine monophosphate with respect to other nucleoside monophosphates (NMPs) in a MeOH/water solution at pH 7. Recognition is efficiently signaled through the displacement of the indicator 6-carboxyfluorescein bound to the receptor, monitoring its yellow fluorescent emission. Titration experiments evidenced the occurrence of several simultaneous equilibria involving 1:1 and 2:1 receptor/NMP and receptor/indicator complexes. It was demonstrated that the added NMP displaces the indicator from the 2:1 receptor/indicator complex, forming the 1:1 receptor/analyte inclusion complex. Recognition selectivity is thus ascribed to the nature of nucleotide donor atoms involved in the coordination and their ability to encompass the Cu(II)-Cu(II) distance within the cryptate.

The Interaction of Fluoride with Fluorogenic Ureas: An ON1–OFF–ON2 Response

The anion binding tendencies of the two fluorogenic ureas L(1)H and L(2)H, containing the 2-anthracenyl and 1-pyrenyl moieties as signaling units, respectively, have been investigated in MeCN and DMSO by absorption, emission, and (1)H NMR spectroscopies. The formation of stable 1:1 receptor:anion H-bond complexes has been confirmed by structural studies on the crystalline [Bu4N][L(1)···Cl] and [Bu4N][L(2)H···CH3COO] salts. Complexation induces significant variations of the emission properties of L(1)H and L(2)H 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 L(1)H 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 L(2)H, 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 [L(2)H···F](-) + F(-) ⇆ [L(2)](-) + HF2(-). The deprotonated receptor [L(2)](-) is distinctly emissive (yellow fluorescence), which generates the fluorimetric response ON(1)-OFF-ON(2) of receptor L(2)H with respect to F(-).

Urea‐, Squaramide‐, and Sulfonamide‐Based Anion Receptors: A Thermodynamic Study

In this work, we compare the anion-binding capabilities of receptors 1-5, characterized by similar structures, but possessing different hydrogen-bond-donor moieties (urea, squaramide, and sulfonamide). The presence of chromophoric substituents on the receptors skeleton allowed the determination of association constants by performing UV/Vis titrations with the investigated anions on solutions of the receptors in pure acetonitrile. Additional quantitative studies of the anion-binding properties of receptors 1-5 were performed by isothermal titration calorimetry (ITC). The experimental results indicated that 1 and 2 formed 1:1 hydrogen-bonded complexes with most of the anions investigated. In the case of receptors 3-5, the formation of the 1:1 adduct was observed only with anions of low basicity (i.e., chloride, bromide, iodide, and hydrogen sulfate). With more basic anions (i.e., acetate and dihydrogen phosphate), both spectrophotometric and ITC titrations accounted for the deprotonation of the sulfonamide group, involving the formation of the conjugated base of the receptor.

Chiral receptors for phosphate ions

The 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 titration experiments 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 equilibria, 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 molecular 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 biologically relevant D-2,3-diphosphoglycerate anion, whose association constant is twice that of the R,R complex. Such an effect is ascribed to the different structural features of the two diastereomeric complexes in solution, as shown by 31P NMR studies.

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+.

Electrochemically Controlled Assembling/Disassembling Processes with a Bis‐imine Bis‐quinoline Ligand and the CuII/CuI Couple

The easily attainable quadridentate ligand bis-N,N′-(2-methylquinoline)-1,2-trans-cyclohexanediimine forms double helical dimeric complexes with CuI and monomeric complexes with CuII, which are air- and moisture-stable and whose structures have been determined through X-ray diffraction. In an acetonitrile solution, it is possible to cycle electrochemically between the two forms, through a fast process which can be repeated at will (see diagram).

99TcO4−: Selective Recognition and Trapping in Aqueous Solution

Too hot to handle: Unprecedented affinity and specificity for (99)TcO(4)(-) in aqueous solution was shown with the p-xylyl azacryptand in the hexaprotonated form. A crystal structure of the complex reveals how the anion fits within the cavity of the cage, and the formation of multiple H-bond interactions with protonated amino groups stabilize the adduct.

Redox Active Cage for the Electrochemical Sensing of Anions

The tripodal system [1]3+ forms a 1:1 complex with CoII in which the metal is octahedrally coordinated by three bpy fragments. The [CoII(1)]5+ complex provides a cavity suitable for solvent or anion inclusion. X-ray diffraction studies on the crystalline complex salt of formula [CoII(1)...H2O]Cl(PF6)(4).2MeCN have shown that a water molecule is included in the cavity and the water oxygen atom receives six H-bonds from the C-H fragments of the three imidazolium subunits and of the three proximate pyridine rings, according to a slightly distorted trigonal prismatic geometry. Anion inclusion in an aqueous MeCN solution induces a distinct cathodic shift of the potential of the CoIII/CoII couple, whose magnitude decreases along the series: Cl->Br- approximately NCO->I- approximately NCS-, which reflects anion tendencies to receive H-bonds from the receptor. The variation of the water content in the MeCN solution (from 0 to 20%) induces a gradual change of the voltammetric response to anion titration: from two well distinguished peaks at a fixed potential to a single peak progressively shifted to a more cathodic potential. Such a behavior parallels the gradual decrease of the equilibrium constant for anion inclusion into the [CoII(1)]5+ receptor.