21-Telluraporphyrins. 1. Impact of 21,23-Heteroatom Interactions on Electrochemical Redox Potentials, 125Te NMR Spectra, and Absorption Spectra

meso -Tetraphenyl-21-chalcogenaporphyrins 4 − 6 (S, Se, and Te as 21-chalcogen atoms, respectively) and meso -tetraphenyl-21,23-dichalcogenaporphyrins 7 − 10 [(S,S), (Se,S), (Se,Se), and (Te,S) combinations as 21,23-chalcogen atoms, respectively] were prepared by condensation of the appropriate 2,5-bis(phenylhydroxymethyl)chalcogenophene 11 with (1) benzaldehyde, pyrrole, tetrachlorobenzoquinone (TCBQ), and boron trifluoride etherate for the preparation of 4 − 6 or (2) the appropriate 2,5-bis(1-phenyl-1-pyrrolomethyl)chalcogenophene 13, TCBQ, and boron trifluoride etherate for the preparation of 7 − 10 . Electrochemical oxidation and reduction potentials were measured by cyclic voltammetry for 4 − 10 and indicated that oxidation of 21-telluraporphyrins 6 and 10 was more facile (more cathodic) than for the other analogues in the series and 10 was more readily oxidized than 6 . The band I absorption maxima of 21-telluraporphyrins 6 and 10 were at shorter wavelengths than those of the corresponding analogues containing only sulfur and/or selenium chalcogen atoms. The extinction coefficients, ∈, of the Soret bands of 6 and 10 were 7.6 × 10 4 and 7.2 × 10 4 M - 1 cm - 1, respectively, which is significantly smaller than analogues 4, 5, and 7 − 9, which have corresponding values of >2 × 10 5 M - 1 cm - 1 . The 125 Te NMR spectrum of 6 gave a chemical shift of δ 834. Oxidation of 6 to oxotelluraporphyrin 12 gave a 125 Te NMR chemical shift of δ 1045. 21-Tellura-23-thiaporphyrin 10 gave a 125 Te NMR chemical shift of δ 1039, perhaps reflecting deshielding of the Te nucleus by the less than van der Waals contact with the S nucleus.