Dicationic salt of β-alkyl substituted sapphyrin-type porphyrinoid: Acid-basic properties, thermal stability and quantum-chemical calculations

Abstract

The UV-Vis spectra of the dicationic salt of 2,3,7,13,17,18,18,22,23-octamethyl-8,12-diethylsapphyrin {H5β-Me8Et2Sap[Formula: see text](Cl[Formula: see text]} in solutions with different polarity and acid-basic properties have been investigated. The tautomeric and acid-base equilibria of H5β-Me8Et2Sap[Formula: see text](Cl[Formula: see text] in the presence of organic bases have been examined by spectrophotometric titration. It was demonstrated that an addition of small amounts of 1,8-diazabicyclo[5.4.0]undecene (DBU) and tetramethylammonium hydroxide (Me4NOH) dissolved in methylene chloride (DCM) and MeOH, respectively results in a stepwise detachment of two protons from the basic nitrogen atoms of the macrocycle, with no anionic forms of sapphyrin being registered. The results obtained were supported using quantum-chemical calculations (DFT, B3LYP, cc-pVDZ including NBO analysis). The interactions of 2,3,7,13,17,18,18,22,23-octamethyl-8,12-diethylsapphyrin (H3β-Me8Et2Sap) with a series of compounds of different acidity and basicity were studied and the formation of very weak hydrogen bond of H3β-Me8Et2Sap as NH-acid with N,N-dimethylformamide (DMF) molecule with the value of the interaction energy (E[Formula: see text] equal -16.40 kJ/mol was revealed. The low acidity of bonds NH in the sapphyrin molecule was confirmed on the base of thermogravimetric data obtained for the solid samples of H5β-Me8Et2Sap[Formula: see text](Cl[Formula: see text] evaporated from DMF and DCM concentrated solutions.