Mauro Micheloni

PROTON COORDINATION BY POLYAMINE COMPOUNDS IN AQUEOUS SOLUTION

Abstract The present article is concerned with proton transfer reactions in aqueous solution of open-chain, macrocyclic and macropolycyclic or cage compounds having nitrogen atoms as protonation sites in the molecular framework, although several compounds with additional different donors will be considered. The main purpose of this review is to collect some significant examples of proton transfer processes in order to show how the electronic properties and molecular topology of polyamines affect the thermodynamic parameters of their protonation equilibria.

A Macrocyclic Ligand as Receptor and ZnII‐Complex Receptor for Anions in Water: Binding Properties and Crystal Structures

Binding properties of 24,29-dimethyl-6,7,15,16-tetraoxotetracyclo[19.5.5.0(5,8).0(14,17)]-1,4,9,13,18,21,24,29-octaazaenatriaconta-Δ(5,8),Δ(14,17)-diene ligand L towards Zn(II) and anions, such as the halide series and inorganic oxoanions (phosphate (Pi), sulfate, pyrophosphate (PPi), and others), were investigated in aqueous solution; in addition, the Zn(II)/L system was tested as a metal-ion-based receptor for the halide series. Ligand L is a cryptand receptor incorporating two squaramide functions in an over-structured chain that connects two opposite nitrogen atoms of the Me(2)[12]aneN(4) polyaza macrocyclic base. It binds Zn(II) to form mononuclear species in which the metal ion, coordinated by the Me(2)[12]aneN(4) moiety, lodges inside the three-dimensional cavity. Zn(II)-containing species are able to bind chloride and fluoride at the physiologically important pH value of 7.4; the anion is coordinated to the metal center but the squaramide units play the key role in stabilizing the anion through a hydrogen-bonding network; two crystal structures reported here clearly show this aspect. Free L is able to bind fluoride, chloride, bromide, sulfate, Pi, and PPi in aqueous solution. The halides are bound at acidic pH, whereas the oxoanions are bound in a wide range of pH values ranging from acidic to basic. The cryptand cavity, abundant in hydrogen-bonding sites at all pH values, allows excellent selectivity towards Pi to be achieved mainly at physiological pH 7.4. By joining amine and squaramide moieties and using this preorganized topology, it was possible, with preservation of the solubility of the receptor, to achieve a very wide pH range in which oxoanions can be bound. The good selectivity towards Pi allows its discrimination in a manner not easily obtainable with nonmetallic systems in aqueous environment.

Small aza cages as “fast proton sponges” and strong lithium binders

The design and synthesis of new compounds in order to achieve expected or new chemical properties is one of the most fascinating aspects of modern synthetic chemistry. Among the many thousands of new compounds synthesized each year, macrocyclic compounds play an important role. The ever growing interest in this very active field stems from different points of view, including selective ion recognition, transport processes, reaction catalysis, industrial applications, model systems, and others [l-17]. Numerous books on macrocyclic chemistry have been published in the last few years [18-27]. Molecular topology and the nature of donor atoms are the two most important parameters influencing the chemical properties of macrocyclic compounds. Different classes of synthetic macrocyclic compounds have been developed and, in addition to the most studied crown-ether family, the aza-crowns are the next most studied class of synthetic macrocycles [ 19,251. These compounds could be considered to be derived from crown ethers by replacing the oxygen donor atoms with softer nitrogen donor atoms. The presence of this kind of donor atom makes these compounds water soluble bases and very prone to bind transition metal ions [28,29].

Supramolecular interaction between adenosine 5′-triphosphate (ATP) and polycharged tetraazamacrocycles. Thermodynamic and 31P NMR studies

Abstract The interaction of adenosine 5′-triphosphate (ATP) with the tetraammonium macrocyclic receptors 1,1,4,4,7,7,10,10-octamethyl-1,4,7,10-tetraazacyclododecane tetrakis(iodide) (L1·4I) and 1,6,11,16-tetraazacycloeicosane (L2) in its fully protonated form, has been studied by potentiometry and 31 P NMR in water at I = 0.15 mol dm −3 and 25 °C H 4 L2 4+ reacts both with ATP 4− and HATP 3− to produce H 4 L2·ATP and (H 4 L2·HATP) + whose equilibrium constants are 6.46 × 10 3 and 1.10 x 10 3 , respectively. In the case of L1, in which the quaternarization of the nitrogen atoms prevents the formation of hydrogen bonds, no detectable interactions arise with ATP. These results are consistent with the hypothesis that the formation of hydrogen bonds play a role of major importance in the interaction between ATP and tetrammonium receptors.

CRYPTAND LIGANDS FOR SELECTIVE LITHIUM COORDINATION

Abstract The binding properties, in aqueous solution, toward lithium ion of aza- and azaoxo- macrocycles with cage and cylindrical molecular topology are reviewed. The synthetic procedures and the acid–basic behavior are reported. Most of these compounds are able to selectively encapsulate the small lithium ion in aqueous solution. NMR spectroscopy, mainly 13C and 7Li, has been used to study the encapsulation equilibria. Few crystal structures of lithium complexes, indicating the lithium ion encapsulation have been reported.

Efficient fluorescent sensors based on 2,5-diphenyl[1,3,4]oxadiazole: a case of specific response to Zn(II) at physiological pH.

The coordination properties and photochemical responses of three fluorescent polyamine macrocycles, 9,12,15,24,25-pentaaza-26-oxatetracyclo[21.2.1.0(2,7).0(17,22)]hexaicosa-2,4,6,17,19,21,23,25(1)-octaene (L1), 9,12,15,18,27,28-hexaaza-29-oxatetracyclo[24.2.1.0(2,7).0(20,25)]enneicosa-2,4,6,20,22,24,26,28(1)-octaene (L2), and 9,12,15,18,21,30,31-heptaaza-32-oxatetracyclo[27.2.1.0(2,7).0(23,28)]diatriconta-2,4,6,23,25,27,29,31(1)-octaene (L3), toward Cu(II), Zn(II), Cd(II), and Pb(II) are reported. Each ligand contains the 2,5-diphenyl[1,3,4]oxadiazole (PPD) moiety inserted in a polyamine macrocycle skeleton. The stability constants were determined by means of potentiometric measurements in aqueous solution. L1 forms mononuclear complexes only with Cu(II). L2 and L3 form stable mononuclear species with all of the metals, while L3 is able to form dinuclear Cu(II) species. The fluorescence of all ligands was totally quenched by the presence of Cu(II). L2 behaves as an OFF-ON sensor for Zn(II) under physiological conditions, even in the presence of interfering species such as Cd(II) and Pb(II). This ligand combines selective binding of Zn(II) with a highly specific fluorescent response to Zn(II) due to the chelating enhancement of fluorescence (CHEF) effect. The interaction of Zn(II), Cd(II), and Pb(II) with L3 does not produce an appreciable enhancement of fluorescence at the same pH. The different behavior is attributed to the cavity size of the macrocycle and to the number of amine functions. L2 possesses the best arrangement of these two characteristics, allowing a full participation of all of the amine functions in metal coordination, as shown by the crystal structures of [CuL2(ClO(4))](ClO(4))·H(2)O and [ZnL2Br]Br·H(2)O species; this prevents the PET effect and supplies the higher CHEF effect. The interaction between L2 and Zn(II) can also be observed with the naked eye as an intense sky blue emission.

A new versatile solvatochromic amino-macrocycle. From metal ions to cell sensing in solution and in the solid state

A new fluorescent NBD-polyaza-macrocycle sensor (L) was synthesized. The coordination of Cu(ii) and Zn(ii) in acetonitrile switches on the fluorescence with different emission wavelengths. Cu(ii) complexes showed solid-state fluorescence. Both L and Cu-complex interact with human cell line (U937) highlighting the cell membrane by fluorescence microscopy.

Co-ordination tendency of [3k]aneNk polyazacycloalkanes. Thermodynamic study of solution equilibria

Equilibria between polyazacycloalkanes of the series [3k]aneNk(k= 3–11) and the bivalent metal ions of Mn through Zn have been studied. The stability constants (log K) for the complexes of Mn2+, Cu2+ and Fe2+ with 1,4,7,10,13,16-hexaazacyclooctadecane (L4), 1,4,7,10,13,16,19-heptaazacyclo-henicosane (L5) and 1,4,7,10,13,16,19,22-octaazacyclotetracosane (L6) have been determined potentiometrically in 0.15 mol dm–3 NaClO4 at 25°C: [MnL4]2+, 10.50; [MnL6]2+, 6.27; [CuL4]2+, 24.40; [CuL5]2+, 19.48; [Cu2L5]4+, 30.49; [Cu2L6]4+, 35.25; and [FeL5]2+ 12.09. The enthalpy changes (–ΔH°/kcal mol–1) for the formation of [CuL4]2+(23.9), [CuL5]2+(19.7) and [NiL5]2+(11.7) have been measured by means of microcalorimetric techniques. The results are discussed in terms of the increasing number of donor atoms and size of the macrocycles. As the size increases the co-ordination features of the metal compleses largely depend on the metal ion.

Di-and tri-palladium(II) polyazacycloalakane complexes. A case of deprotonated secondary nitrogen in solution and in solid state

Crystal structures of di- and tri-nuclear palladium(II) complexes with the azamacrocycles[18]aneN6 and [21]aneN7 have been solved by X-ray analysis; in the case of the trinuclear complex deprotonation of a secondary amino group occurs, both in solution and in the solid state.

(PdCl4)2– inclusion into the deca-charged polyammonium receptor (H10[30]aneN10)10+([30]aneN10= 1,4,7,10,13,16,19,22,25,28-deca-azacyclotriacontane)

The title decaprotonated deca-azamacrocycle encapsulates the tetrachloropalladate(II) anion forming a ‘super complex’ whose crystal structure has been determined.