Tetrakis(1,2,5-thiadiazolo)porphyrazines. 9. Synthesis and spectral and theoretical studies of the lithium(i) complex and its unusual behaviour in aprotic solvents in the presence of acids

The template cyclotetramerization of 1,2,5-thiadiazolo-3,4-dicarbonitrile in the presence of lithium n-butoxide in n-butanol leads to the Li(I) complex of tetrakis(1,2,5-thiadiazolo)porphyrazine. Various possible structures of dilithium and monolithium complexes have been considered by DFT/B3LYP molecular modelling using the cc-pvtz basis set, and their theoretical IR and UV-VIS spectra have been calculated. The experimental 7Li NMR, IR and UV-VIS spectra measurements show that the complex contains two inequivalent lithium atoms – one is coordinated to the macrocyclic dianion to form the anionic lithate complex [TTDPaLi]−, while the other forms the solvated countercation [Li(Solv)4]+. The lithate complex is stable in protic solvents, such as methanol, and is soluble in water to give aggregated solutions. Its demetallation occurs in the presence of acids (CH3COOH, CF3COOH, H2SO4). In aprotic solvents (DMF, DMSO), the acid-catalyzed formation of the [TTDPa]2− dianion is observed which is followed by the formation of the meso-protonated form {H[TTDPa]}− at higher acid concentrations. Both processes can be reversed by the addition of a lithium salt excess or neutralization of the acid. The fluorescence quantum yield for the lithate complex [TTDPaLi]− is much higher than that for the [TTDPa]2− dianion (0.34 and 0.01 in DMSO), and this can be used for detecting low concentrations of acids and Li+ in aprotic solvents (10−6–10−5 M). The first reversible reduction of the macrocycle in the anionic lithate complex (−0.94 V vs. SCE in DMSO) is ∼0.5 V more difficult than that in the complexes with divalent metals [TTDPaM] (M = MgII, ZnII, CuII).