Self-organized porphyrin dimer as a highly specific receptor for pyrazine derivatives

Complex formation between pyrazine derivatives and dimeric self-assembly of alpha alpha alpha alpha-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 alpha alpha alpha alpha-atropisomer exclusively. Vapor pressure osmometry for the resultant alpha alpha alpha alpha-atropisomer solubilized in CHCl3 shows molecular weight of 2650 +/- 200, indicating formation of dimeric self-assembly of 1 . Zn. H-1 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 10(7) 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.