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.