Recognition of Amines and Diamines

The most thoroughly studied host-guest system with a porphyrin-based host is the complex formed between amines and metalloporphyrins. Tire stoichiometry of the host-guest complex and the binding affinity will depend on the nature of the metal, the basicity of the amine, the reaction media and the pH in the case of water. Porphyiin-amine equilibria in water were studied by Pasternack et u/. [5.10,15,20-tetrakis(/V-methylpyridinium-4-yl)porphyrina-tolCo(IIl) (27) binds pyridine in water, with the binding constants and the rale constants of complex formation depending upon pH. At acidic pH, pH 4, the first replacement of water by pyridine. 

For the binding in organic solvents, zinc porphyrins are used in a number of studies, where a 1 1 complex is predominantly formed and a single equilibrium can describe the host-guest system. 

Hereafter the values of AG A// and A5 refer to the standard state of 1 M of each species. We also use notation, A-B, to represent a host-guest complex between compound A and compound B. Table I summarizes the binding free energy for complex formation for various amine-metallo-porphyrin complexes. Monomeric porphyrins bind amines with — AG values in the range of 10-25 kJ/mol, while dimeric porphyrins bind ditopic guests with a larger — AG of 30-43 kJ/mol. 

Porphyrin tweezer 51 was used to determine the absolute configuration of chiral diamines by measuring the CD spectrum of the diamine-51 complex. - Upon com-plexation, the two porphyrins adopt a chiral configuration, which leads to a coupled CD. The absolute configuration of amino acids and amino alcohols can be similarly detennined after derivatization to diamines. Ji and coworkers also reported CD smdies of the binding of diamine to a zinc porphyrin dimer.  

Entry Host Guest AC /kl-mol Solvent Temperature (K) Method Reference 

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