Proton coordination by polyamine compounds in aqueous solution
Introduction
‘The most general and important reaction in chemistry’ [1] has been defined as the reaction which involves transfer of a proton from one atom to another. This elementary reaction plays a fundamental role in innumerable processes including acid–base neutralization, electrophilic addition, etc. [2], [3]. The proton occupies a special position as a promoter in chemical reactions occurring in solution. It is well known that because of its small radius, proton does not exist as an elementary particle in water, but binds to a molecule of water forming a covalent hydronium ion H3O+ (primary hydration). The last species through hydrogen bond agglomerates other water molecules forming more hydrated species like H9O4+ (secondary hydration) [4]. In spite of the complexity of the proton–water system, we will use the name ‘proton’ with the meaning of ‘proton in aqueous solution’.
Although the literature including thermodynamic data on protonation equilibria in aqueous solution is very vast [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], there was not a comprehensive review centered on this topic. 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. These compounds have been widely employed as receptors for different types of substrates like metal ions and anions. Since polyamines are bases in aqueous solution they give rise to competition between their protonation and complexation reactions. Therefore, the basicity behavior of such compounds in aqueous solution has to be investigated preliminary to any complexation study.
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.
Section snippets
Determination of thermodynamic parameters
The determination of protonation constants of many organic molecules received important consideration during about the first half of the present century owing to the increasing interest in their application as ligands in the new coordination chemistry area. Experimental data obtained in those studies were subjected to graphical analysis [16], [17]. More recently, the development in instrumentation, the introduction of glass electrodes for precise and quick measurements of H+ equilibrium
Open-chain polyamines
It is well known that in the gas-phase the basicity of ammonia and its methyl derivatives follows the sequence N(CH3)3>NH(CH3)2>NH2(CH3)>NH3 [36], [37], [38]. Since alkyl groups are electron donating towards electronegative atoms, the replacement of hydrogen atoms by methyl groups in ammonia yields a regular increase in basicity in the gas phase. In aqueous solution, however, the basicity sequence is rather different [39], [40]. Reported values for the logarithms of the stepwise basicity
Macrocyclic polyamines (polyazacycloalkanes)
This section deals with polyamines having a cyclic structure. Such molecules are commonly referred to as macrocycles. Their proton transfer properties are strictly connected with the number of amino groups in the molecular skeleton, the type of amino groups (secondary, tertiary), the presence of different donor atoms, the length of the spacers connecting the amino groups and their nature (aliphatic, aromatic), the rigidity and the overall structure of the molecule. All these aspects will be
Cryptands
Proton transfer is usually very fast, but can be considerably affected by structural factors, slow proton transfer has been found in N-alkylated derivatives of 1,8-diaminonaphthalene [195], [196] where the proton is held by a tight hydrogen bond and even slower rate of protonation/deprotonation can be found when the proton is bound in an intramolecular cavity [197]. Such a case is also the smallest [1.1.1] macrobicyclic cryptand 4,10,15-trioxa-1,7-diaza-bicyclo[5.5.5]heptadecane (100) whose
Acknowledgements
We thank the Spanish DGICYT project no. PB96-0792-CO2 and the Italian Ministero dell’Università e della Ricerca Scientifica (MURST) and Consiglio Nazionale delle Ricerche (CNR) for financial support. We would specially like to acknowledge the meticulous task of one of the reviewers who has significantly contributed to improve the review.
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