Solvent-dependent tautomerism of the inverted isomer of meso-tetraphenylporphine: Effect of the polarity of the medium

It is found that aza-imino tautomerism (a ? b) of the inverted porphyrinoids and its mechanism are, along with the stability of tautomeric forms in the Solv–B system, determined by the nature of a base B and the polarity of a solvent Solv. It is shown that the transition from the C₆H₆–B system to MeCN–B is characterized by an approximate doubling of stability constants K_T of the imine form (b), and by a change of the number of molecules B involved in the process (from two to one). According to quantum-chemical data (DFT, B3LYP, CC-pVDZ) and the results from spectral measurements (electronic absorption spectra, EAS), the stability of tautomer b (imino form) falls in the series of solvents DMF > Py ~ Et₂NH > MeCN > Me₂CO, and tautomer a is to a lesser extent stabilized in the given media by electron donors through the formation of hydrogen bonds (except for Me₂CO: DMF > Py ? Me₂CO ? MeCN, Et₂NH).