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

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 a fforded (DDQ)(2)H2T(4-X)PP molecular complexes as the sole products. The ve ry close correspondence between the optical spectra of these 2∶1 mole cular complexes and their related diprotonated porphyrins, and particularly the remarkable agreement of their corresponding H-1, C-13 NMR resonances w ith those of tetraphenylporphyrin dication, H4TPP2+, strongly suggest simil ar distorted porphyrin core structures in all these compounds. C-13 NMR and IR spectra of the complexed DDQs appear to be consistent with the interact ion of one of their empty CN pi* 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 occ urrence of intramolecular H-bondings. It is proposed that coordination of D DQs in (DDQ)(2)H2T(4-X)PP complexes occurs from above and below the "plane" of the porphyrins, and they occupy the same positions as HCls in the struc ture of [H4TPP](2+), 2Cl(-) species.