Cryptands proton cryptates

There is considerable interest in the properties of macrobicyclic cryptands, for example [78] to [81], and particularly in their ability to complex protons, metal ions, and small molecules (Lehn, 1978). In the proton cryptates there exists the possibility of intramolecular +N—H N hydrogen bonding as well as interaction of the proton with the oxygen atoms, and the properties are also strongly influenced by the size of the molecular cavity. In the [l.l.l]-cryptand [78] the molecular cavity is small (Cheney et al., 1978) and... 

For cryptands in which the molecular cavity is larger than in the case of the [l.l.l]-species [78], proton transfer in and out of the cavity can be observed more conveniently. Proton transfer from the inside-monoprotonated cryptands [2.1.1] [79], [2.2.1] [80], and [2.2.2] [81 ] to hydroxide ion in aqueous solution has been studied by the pressure-jump technique, using the conductance change accompanying the shift in equilibrium position after a pressure jump to follow the reaction (Cox et al., 1978). The temperature-jump technique has also been used to study the reactions. If an equilibrium, such as that given in equation (80), can be coupled with the faster acid-base equilibrium of an indicator, then proton transfer from the proton cryptate to hydroxide ion... 

Proton cryptates are obtained by internal protonation, the cavity concealing the protons very efficiently especially in the case of the protonated forms 11 and 12 of the smaller cryptand 6 [2.29,2.30]. 

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. 

The endo-endo conformation of cryptands can be internally protonated to form proton cryptates. With the small cryptands, e.g. [1.1.1]- and [2.1.1]-cryptand (15a and 15b), the two internal protons are so efficiently shielded from H2O and OH that deprotonation only very slowly occurs even in strong base (8UA6044). Alkali cation cryptates are able to stabilize unusual species as their counterions. Dye and coworkers have isolated several alkali metal anions by this method. The sodium species (Na [2.2.2]cryptand Na ) was obtained as gold metallic crystals and gave a Na NMR with a broad Na -cryptate resonance and a narrow, upheld Na resonance. The other alkali metals show similar behavior and an electride salt (Na [2.2.2]cryptand e l has even been isolated (B-79MI52105). Crystalline anionic clusters of the heavy post-transition metals (such as Sb7 , Pbs , Sng ) were first obtained with alkali metal cryptates as the counterions (75JA6267). 

Howard, S.T. (2000) Relationship between basicity, strain and intramolecular hydrogen-bond energy in proton sponges. Journal of the American Chemical Society, 122, 8238-8244. Smith, P.B., Dye, J.L., Cheney, J. and Lehn, J.-M. (1981) Proton cryptates. Kinetics and thermodynamics of protonation of the [ 1.1.1 ] macrobicyclic cryptand. Journal of the American Chemical Society, 103, 6044-6048. 

The small cryptand 11 (Fig. 11) was obtained with difficulty, because the major compound obtained was a macrotricyclic tetraamide resulting from a dimerization reaction. It was observed that stable proton cryptates of 11 can be obtained. The [1.1.1] bicycle binds one or two protons inside its intramolecular cavity. The diprotonated cryptate 11, 2H. has a high resistance to deprotonation. The monoprotonated cryptate may be obtained with difficulty but this latter species cannot be fully depro-tonated by base to afford the free cryptand. Smaller analogues of 11 containing only carbon atoms in the three chains were also described. They present similar behavior toward the proton. Another class of cryptands able to strongly bind the proton was developed more recently. 

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. 

Earlier discussion [12] of the structures of protonated cryptand/oxoanion assemblies was based on consideration of H-bonds between the encapsulated anion and the NH+ donors of the ciyptand. These interactions are assumed to be responsible for retention of the guest anion in the host ciyptand cavity, both in the sohd state and in solution. We have shown that, in all cases, anion cryptates exhibit at least three, and often more, direct H-bond NH+-Oanion contacts tethering the included oxoanion within the crypt. 

Abstract. A dimeric lanthanide cryptate was obtained by the addition of an excess of cryptand (2.2.1) to a slightly hydrated solution of the monomeric praseodymium (2.2.1) perchlorate complex in acetonitrile. This new lanthanide compound is centrosymmetric and displays the space group P2 /n. The encryptated metal ions are nine-coordinated, they are bonded to all the heteroatoms of a (2.2.1) ligand and they are linked to each other by two p-hydroxo bridges. The hydroxyl groups are relegating the cryptands to both end of the dimer and the praseodymium ions are less effectively accomodated in the macrocylic internal cavities than in the case of the monomeric Pr(2.2.1) complex. The formation of both the monomeric and the dimeric lanthanide complexes is readily observed by proton NMR. 

The formation of at least two lanthanide (2.2.1) species in acetonitrile is easily demonstrated by proton NMR spectroscopy. The proton NMR spectrum of monomeric Pr(2.2.1) cryptate displays eight peaks that are shifted from their diamagnetic position by the paramagnetism of the metal ion. The assignment of these peaks required [6] the synthesis of several partially deuterated derivatives of cryptand... 

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