Self-Organized Porphyrin Dimer as a Highly Specific Receptor for Pyrazine Derivatives

Abstract

Complex formation between pyrazine derivatives and dimeric self-assembly of αααα-isomer of meso-tetrakis(2-carboxy-4-nonylphenyl)porphyrin Zn complex (1·Zn) was investigated. Thermal atropisomerization of 1·Zn in a nonpolar solvent such as toluene gives the αααα-atropisomer exclusively. Vapor pressure osmometry for the resultant αααα-atropisomer solubilized in CHCl3 shows molecular weight of 2650 ± 200, indicating formation of dimeric self-assembly of 1·Zn. 1H NMR and UV/vis titration experiments for this dimeric assembly with pyrazine derivatives show highly specific 1:2 complex formation of pyrazine and 1·Zn. The equilibrium constants for 1·Zn dimer/pyrazine complex formation are estimated to be over 107 M-1. The most characteristic feature of the present ternary system, (1·Zn)2·pyrazine, is that the pyrazine derivative, having a large side moiety such as benzoyl group, can coordinate two zinc atoms inside the dimer cavity by sticking the side moiety out of a window formed between hydrogen-bond pillars of the complex.