Exclusive 2∶1 molecular complexation of 2,3-dichloro-5,6-dicyanobenzoquinone and para-substituted meso-tetraphenylporphyrins: spectral analogues for diprotonated meso-tetraphenylporphyrin

Abstract

Various molar ratios of 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) and para-substituted meso-tetraphenylporphyrins (H2T(4-X)PP, X = H, Cl, CH3, CH(CH3)2, OCH3) in dichloromethane at room temperature in several days always afforded (DDQ)2H2T(4-X)PP molecular complexes as the sole products. The very close correspondence between the optical spectra of these 2∶1 molecular complexes and their related diprotonated porphyrins, and particularly the remarkable agreement of their corresponding 1H, 13C NMR resonances with those of tetraphenylporphyrin dication, H4TPP2+, strongly suggest similar distorted porphyrin core structures in all these compounds. 13C NMR and IR spectra of the complexed DDQs appear to be consistent with the interaction of one of their empty CN π* orbitals with a lone pair of a pyrrolenine nitrogen of the porphyrin. Also the loss of pyrrolic NH stretchings of the porphyrins in the IR spectrum of the molecular complexes indicates the occurrence of intramolecular H-bondings. It is proposed that coordination of DDQs in (DDQ)2H2T(4-X)PP complexes occurs from above and below the “plane” of the porphyrins, and they occupy the same positions as HCls in the structure of [H4TPP]2+, 2Cl− species.