Basicity properties of two paracyclophane receptors. Their ability in ATP and ADP recognition in aqueous solution

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Abstract
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) Å, α = 102.80(3)°, β = 93.43(5)°, γ = 101.41(6)°, V = 2656(3) Å3, 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.

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