Cryptands proton binding

Quinone type carriers perform the cotransport of two protons and two electrons (2e, 2H+ symport) [6.48, 6.49] and take part in mitochondrial and photosynthetic electron transport. Cation receptor sites such as crown ethers or cryptands bearing a quinone [6.50a] or a ferrocene [6.50b] group (see also Section 8.3.1), bind and carry cations with redox coupling through switching between a low affinity state (quinone, ferricinium) and a high affinity state (reduced quinone, ferrocene). 

Macrocycles such as cryptands and azacrowns can readily bind anions upon protonation. 

Menif, R. Reibenspies, J. Martell, A. E. Synthesis, protonation constants, and copper(II) and cobalt(II) binding constants of a new octaaza macrobicylic cryptand (MX)3crystal structures of the cryptand and of the carbonato-bridged dinuclear copper(II) cryptate, Inorg. Chem. 1991, 30, 3446-3454. 

NMR titrations (of anion into ligand at fixed pH) and pH-potentiometric titrations (of pH at fixed anion ligand ratios) provide comparable values of the stability constants for binding of mononegative oxoanions by protonated R3Bm, R3F, and R3P hosts [15,20,21] Table 2. The weak complexation at hexaprotonated levels for tetrahedral monoanionic oxoanions makes it difficult to obtain reliable data for protonation levels below 5. This has however been achieved for nitrate with the cleft binding host R3P as well as for Re O4 with the most basic cryptand R3Bm. 

A derivative of the (bpy.bpy.bpy) cryptand, obtained by modifying one of the chains, Lbpy, forms a di-protonated cryptate with EuCb in water at acidic pH, [EuCl3(H2Lbpy)]2+ in which the metal ion is coordinated to the four bipyridyl and two bridgehead nitrogen atoms, and to the three chlorine ions (Fig. 4.25). The polyamine chain is not involved in the metal ion coordination, due to the binding of the two acidic protons within this triamine subunit. In solution, when chlorides are replaced by perchlorate ions, two water molecules coordinate onto the Eu(III) ion at low pH and one at neutral pH, a pH at which de-protonation of the amine chain occurs, allowing it to coordinate to the metal ion. As a result, the intensity of the luminescence emitted by Eu(III) is pH dependent since water molecules deactivate the metal ion in a non-radiative way. Henceforth, this system can be used as a pH sensor. Several other europium cryptates have been developed as luminescent labels for microscopy. 

---