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In this tutorial review, we will discuss the most significant achievements in the structure-based design of cyclic receptors containing mainly aliphatic amine groups as metal-free hosts for phosphate-based guests in aqueous environment; in particular, inorganic phosphates and mononucleotide anions. The bulk of our attention will be devoted to show how this class of receptors has structurally evolved following the attempts by chemists to optimize all factors indicated by Nature as necessary to achieve the best possible binding of phosphates via non-covalent interactions.

The synthesis and characterization of the small, new benzyl aza-cage 4-benzyl-10,15-dimethyl-1,4,7,10,15-pentaazabicyclo[5.5.5]heptadecane (L) are reported. The basicity constants have been determined in aqueous solution by potentiometry (25 °C, [NaCl] 0.15 mol dm–3). L behaves as a biprotic base: log K1= 11.8, log K2= 8.3. The crystal structure of the monoprotonated salt [HL][BPh4] has been determined by single-crystal X-ray analysis. The compound crystallizes in an orthorhombic unit cell (space group Pc21n, Z= 4) with lattice constants a= 11.400(4), b= 16.893(3), c= 20.537(6)A. Least-squares refinement converged at R= 0.078 (Rw= 0.056) for 1290 unique reflections with I > 3σ(I). The structure of [HL]+ shows the five nitrogens atoms in an endo configuration, indicating that the proton is inside the cage cavity. The cage L encapsulate Li+ and the complex formation equilibrium for [LiL]+ has been investigated by potentiometry [log K= 3.0(1)] and 7Li NMR techniques.

The synthesis and characterization of the new azamacrocycle 4,10,15-trimethyl-1,4,7,10,15-pentaazabicyclo[5.5.5]heptadecane (L) are reported. The stepwise basicity constants have been determined by potentiometry (25C, NaCl 0.15 mol dm[sup [minus]3]). L behaves as biprotic base: log K[sub 1] = 11.8; log K[sub 2] = 10.0. The molecular structure of the monoprotonated salt [HL][BPh[sub 4]] has been determined by single-crystal X-ray analysis. The structure of [HL]+ shows the five nitrogen atoms in endo configuration, indicating that the proton is inside the cage cavity. Geometrical considerations indicate that the proton is bound to the bridgehead nitrogen atom N(2). The cage L selectivity encapsulates Li+, and the equilibrium for the formation of the inclusion complex [LiL]+ has been investigated by potentiometry (log K = 5.5) and [sup 7]Li NMR techniques. The molecular structure of the complex [LiL][BPh[sub 4]] has been determined by single-crystal X-ray analysis. Li+ is wholly enclosed in the cage cavity adopting a regular bipyramidal geometry with the Li-N distances for the 2.01-2.08-[angstrom] range. Cu(II) is also encapsulated by L; the electronic spectra of the complex show essentially the same features in the solid state and in solution and are diagnostic of a five-coordinate trigonal-bipyramidal structure.

The synthesis and characterization of the new macrotricycle 10,22,28,32-tetraoxa-1,4,7,13,16,18-hexaazatricyclo[17.5.5.57,13]tetratriacontane L is reported. Its basicity in aqueous solution has been investigated by potentiometric and spectroscopic (1H, 13C NMR) techniques. The basicity constants have been determined (25 °C, I= 0.15 mol dm–3) in two ionic media. Macrotricycle L behaves as a hexaprotic base: log K1= 10.27(3), log K2= 9.52(3), log K3= 7.12(3), log K4= 4.62(3), log K5= 2.80(3), log K6= 2.27(3) in NaClO4 and log K1= 10.09(3), log K2= 9.27(3), log K3= 6.54(3), log K4= 4.88(3), log K5= 3.75(3), log K6= 3.07(3) in NaCl. The constants relative to the equilibrium Cl–+ HnLn+⇌ HnLCl(n–1)+ have been measured: n= 5, log K= 2.48(3); n= 6, log K= 3.59(3). The crystal structure of L has been determined by single-crystal X-ray analysis. The compound crystallizes in a triclinic unit cell (space group P, Z= 2) with lattice constants a= 8.984(5), b= 12.122(2), c= 14.667(3)A, α= 109.52(2), β= 105.47(3), γ= 95.24(3)°. Least-squares refinement converged at R= 0.088 (R′= 0.075) for 3198 unique reflections with I > 4σ(I). The structure shows that the molecule has a skewed cylindrical shape, the bases being formed by the two N2O2 macrocycles with the N(CH2CH2)2 chains constituting the walls. The cylindrane L binds two Li+ ions and the resulting complex has been studied by 7Li NMR spectroscopy. Copper(II) and cadmium(II) complexes with L have been investigated in aqueous solution and their stability constants determined by potentiometric techniques. For both metals the following species have been found: [ML]2+, [MLH]3+, [ML(OH)]+ and [ML(OH)2].

The interaction of adenosine 5'-triphosphate, pyrophosphate, and hexacyanoferrate(II) anions with the cyclic polyamines 1,4,7,10,13,16-hexaazacyclooctadecane ([18]aneN 6 ) and 1,4,7,13-tetramethyl-1,4,7,10,13,16-hexaazacyclooctadecane (Me 4 [18]aneN 6 ) and with the open-chain polyamine 1,14-bis(methylamino)-3,6,9,12-tetraazatetradecane (Mezpentaen) has been studied in the pH range 3-10 by potentiometry and cyclic voltammetry

The synthesis of a giant-size polyazamacrocycle 4,34-dimethyl-1,4,7,10,13,16,19,22,25,28,31,34,37,40,43,46,49,52,55,58-icosaazapentacyclo[56.2.2.27,10.228,31.237,40]octahexacontane (L) is reported together with the crystal structure of its dinuclear cadmium(II) complex [Cd2(H2L)][ClO4]65H2O.

The protonation behaviour of the aza-cyclophane receptor 6,13,15,18-tetramethyl-6,13,15,18-tetraaza-1(1,4),11(1,4)-dibenzena-3(1 ,4),9(1,4)-dipiperazina-cyclodecanonaphane (L2) has been studied in aqueous solution by means of potentiometric and 1H and 13C NMR techniques. L2 behaves as a hexaprotic base and NMR experiments allow the determination of the stepwise protonation sites. Considering the [H5L2]5+ species, the acidic protons occupy alternate positions in the macrocycle, separated by an unprotonated amino group or by a p-phenylene moiety. The crystal structure of [H5L2](ClO4)5·1.5H2O (space group P, a = 8.87(1), b = 15.388(6), c = 20.476(7) A, α = 102.80(3)°, β = 93.43(5)°, γ = 101.41(6)°, V = 2656(3) A3, Z = 2, R = 0.0803) confirms the NMR data, showing one proton located on the N(1) methylated nitrogen and four protons on the amino groups adjacent to the aromatic rings. These features are analogous to those found in the paracyclophane L1, which shows a similar molecular architecture. Binding of ATP and ADP by L1 and L2 was studied by means of potentiometry and 31P NMR in aqueous solution. Although the [H4L1]4+ and the [H5L2]5+ species show a similar charge distribution, [H4L1]4+ forms stable complexes with ATP and ADP, while [H5L2]5+ does not bind such nucleotides. These results may be explained considering the different orientation of the N–H+ bonds in the two receptors.

Coordination/detachment of a pendent functionality in the Zn(II) complex with a macrocyclic ligand L gives rise to on/off switching of exciplex emission, defining an elementary molecular machine whose movements are driven by both pH and light.

The synthesis and characterization of the new aza-cage 12,17-dimethyl-1,9,12,17-tetra-azabicyclo-[7.5.5]nonadecane (L) are described. The basicity behaviour in both aqueous and water–DMSO (50 : 50, mol : mol) solutions has been investigated by potentiometric and microcalorimetric (25 °C, 0.15 mol dm–3 NaCl) techniques. L behaves, in aqueous solution, as a fairly strong base (log K1= 12.00; ΔH1⊖=–54.0 kJ mol–1; TΔS1⊖= 14.5 kJ mol–1) in the first protonation step and as a moderate base in the second step (log K2= 7.86 ΔH2⊖=–44.8 kJ mol–1; TΔS2⊖ca. 0). The enthalpic and entropic contributions are discussed in terms of molecular topology and translational entropy. The CuII ion is encapsulated by L, the electronic spectra being diagnostic of a square-planar structure. The molecular structures of the diprotonated salts [H2L][CoCl4] and [H2L1][CoCl4](L1 = 5,12,17-trimethyl-1,5,9,12,17-penta-azabicyclo[7.5.5]nonadecane) have been determined by single-crystal X-ray analysis.Crystals of [H2L][CoCl4] are orthorhombic, space group pbn21, with a= 9.311 (3)A, b= 15.480(2)A, c = 16.636(6)A, and Z= 4. Final R values of 0.043 (Rw= 0.037) for 1 358 observed reflections. X-Ray analysis shows that the protonation occurs on the N–CH3 groups. Each hydrogen atom of the NH+–CH3 interacts with the two bridgehead nitrogen atoms.The compound [H2L1][CoCl4] crystallizes in a monoclinic unit cell (space group P21) with lattice constants a= 8.877(3)A, b= 14.088(8)A, c= 10.122(5)A, β= 105.51 (4)°, and Z= 2. Final R values of 0.064 (Rw= 0.049) for 2 374 observed reflections. In this case the conformation of the diprotonated cage is different and the protonation probably occurs at one of the two methylated nitrogens and at the apical nitrogen atom.

Constantes de stabilite des especes [Cu 3 L] 6+ . Formation egalement de [Cu 2 L] 4+ , [Cu 2 LH] 5+ , [Cu 2 LH 2 ] 6+ , [Cu 3 LOH] 5+ et [Cu 3 L(OH) 2 ] 4+

Potentiometric and calorimetric studies on the equilibria between Cd(II) and large polyazacycloalkanes (I) are carried out at 25 °C. Mononuclear complexes are formed with (Ia) and dinuclear complexes with (Ib).