Acid-base properties of tetrapyrazinoporphyrazines. 1. Deprotonation of octaethyltetrapyrazinoporphyrazine in CH 2 Cl 2 , THF, DMSO and pyridine. The crucial role of water

Autor: Pavel A. Tarakanov, Pavel A. Stuzhin, Alena S. Malyasova, Elena Kokareva, Oskar I. Koifman, V. B. Sheinin
Rok vydání: 2017
Předmět:
Zdroj: Dyes and Pigments. 139:509-516
ISSN: 0143-7208
Popis: Combination of experimental and theoretical methods were used to study NH-acidity and related spectral properties of tetrapyrazinoporphyrazines – octaazaanalogues of phtalocyanines. The theoretical values of deprotonation enthalpy (DPE) in gaseous phase indicate that substitution of benzene rings in phthalocyanine by pyrazine fragments strongly enhances the NH-acidity of the macrocycle which can be further tuned by introduction of substituents in pyrazine rings. Spectrophotometric titration study of octaethyl substituted tetrapyrazinoporphyrazine, [Et 8 TPyzPAH 2 ], in organic solvents (CH 2 Cl 2 , THF and DMSO) has revealed that it is easily deprotonated not only in the presence of strong bases such as tetrabutylammonium hydroxide, but also in such proton-acceptor solvent as DMSO. The thermodynamic acidity constants (pK a1 = 7.77 and pK a2 = 7.80) determined for [Et 8 TPyzPAH 2 ] in DMSO solution using spectropotentiometric titration indicate that TPyzPA macrocycle has ∼ 1000 times stronger NH-acidity than phthalocyanine. Deprotonation of [Et 8 TPyzPAH 2 ] in pyridine solution is only possible in the presence of water playing a crucial role in formation of the dianion, which can be considered as a sensitive spectral sensor for water traces. Theoretical DFT B3LYB/631++G(p,d) study has confirmed that the dianion is only stabilized as bisaqua complex [TPyzPA(H 2 O) 2 ] 2− with complementary H-bonds between water molecule and N-atoms of two opposite pyrrole rings (N…HOH…N).
Databáze: OpenAIRE
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