Subphthalocyanine basicity: reversible protonation at the azomethine bridge

Abstract

International audienceSubphthalocyanine (SubPc) could be reversibly protonated at the azomethine bridge. This phenomenon was examined by addressing the pKa of the acid (TFA, MSA, TMSA) and the SubPc electron-withdrawing properties of the peripheral isoindolic substituents (F12 vs. H12 and NO2), which tunes the basic character of the azomethine moiety. The protonation of up to three azomethines was suggested and monitored spectrophotometrically with the appearance of new absorption bands at 610, 630 nm and 660 nm, together with marked colour changes from purple to blue to green. Evidence of such a protonation was also shown by significant changes in the 1H-NMR spectrum and new bands in the infrared spectrum

Versatile Synthesis of Siloxy Silicon Tetrabenzotriazacorroles and Insight into the Mode of Macrocycle Formation

“…Honda et al 36,37 has shown this for peripherally phenylated Pc derivatives. Our group 38 and Decreáu et al 39 have shown the same for boron subphthalocyanines. Therefore, the protonation and nitrogen atom loss are likely to be intimately associated.…”

Recent advances in subphthalocyanines and related subporphyrinoids

“…In 19 F-NMR spectra of SubPc-F derivatives, a quartet centered between −160 and −155 ppm is originated by the axial fluorine substituent due to the coupling with the boron atom. 49,67,102,159…”

“…The appearance of the N–H and N–D stretching bands of the iminium species formed upon protonation and deuteration of the SubPc azomethine bridges was suggested by the authors as a possible rationale for this experimental finding. 67…”

“…at 610, 631 and 665 nm for H 12 SubPc-Cl) along with a decrease of the intensity of the SubPc Q absorption, probably as a consequence of the protonation of the azomethine bridges. 64,67…”

“…1,65,66 Recently, the reversible protonation of the azomethine bridge of the SubPc macrocycle in the presence of organic Brønsted acids was studied by Decre ´au and co-workers through UV-vis, NMR and IR spectroscopic assays. 67 In particular, the number of protonated sites was shown to depend on the pK a of the acid employed as well as on the peripheral substitution pattern of the SubPc, which was argued to regulate the basicity of the meso-nitrogen atoms.…”