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

Abstract

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 = Mg^II, Zn^II, Cu^II).