Abstract
Abstract The properties and applications of 2-methyl-(2H)-tetrazole-5-amino-saccharinate (2MTS) in catalysis and chelant-based chemotherapy stimulated investigations on its photostability. The photochemistry of monomeric 2MTS in solid argon (15 K) was compared with those of 2-methyl-(2H)-tetrazole-5-amine (2MT) and 1-methyl-(2H)-tetrazole-5-amine (1MT). Compounds were subjected to in situ narrowband UV-irradiation at different wavelengths. Reactions were followed by infrared spectroscopy, supported by B3LYP/6-311++G(d,p) calculations. Photochemical pathways for 2MT and 2MTS proved similar but photodegradation of 2MTS was 20× slower, unraveling the photostabilizing effect of the saccharyl moiety that extends into the nitrilimine formed from 2MTS and its antiaromatic 1H-diazirene isomer, which proved photostable at 290 nm, unlike the 1H-diazirene formed from 2MT. Analysis of the photochemistries of 2MTS/2MT (250 nm) and 1MT (222 nm), including energy trends calculated for the isomeric C2H5N3 species postulated/observed from photolysis and EPR results, enabled a deeper insight into the photodegradation mechanisms of 1,5-substituted and 2,5-substituted tetrazoles. We postulate a pivotal singlet state imidoylnitrene species, sN1, as common intermediate, which undergoes a Wolff-type isomerization to a stable carbodiimide. Photo-extrusion of N2 from 1,5-substituted tetrazoles generates sN1 directly but from 2,5-substituted tetrazoles it originates a nitrilimine, then a diazirene, which finally leads to sN1. Selective formation of cyanamide from 1MT requires photoisomerization between sN1 and sN2, accessible at 222 nm. EPR studies enabled the detection of methyl nitrene, arising from photolysis of 1H-diazirene intermediate.
Highlights
Tetrazoles exhibit excellent coordination abilities through their four nitrogen atoms, acting either as multidentate ligands or as bridging building blocks in supramolecular assemblies [1,2,3,4]
Following the reactivity patterns described above, two important observations result from recent studies regarding the effect of the ring substitution outline on the photofragmentation pathways of 5-amino tetrazoles [33]: (i) the 1H-diazirene was observed as a common intermediate from photolysis of both 1-methyland 2-methyl-5-aminotetrazole, subsequently isomerizing to carbodiimide as final photoproduct; (ii) upon irradiation at short wavelengths (222 nm), an amino cyanamide was obtained, together with the 1H-diazirene and carbodiimide, from photolysis of 1-methyl-5-aminotetrazole, unraveling a new reaction pathway; this cyanamide isomerizes to the carbodiimide upon irradiation at longer wavelengths (325 nm)
The potential applications of 2-methyl-(2H)-tetrazole-5-amino-saccharinate (2MTS; 4, Scheme 2) in catalysis and in chelant-based chemotherapy stimulated the study of its photostability
Summary
Tetrazoles exhibit excellent coordination abilities through their four nitrogen atoms, acting either as multidentate ligands or as bridging building blocks in supramolecular assemblies [1,2,3,4]. The results described based on molecular orbital calculations and the matrix-isolation technique coupled to FTIR and EPR spectroscopies, pave the way towards a better elucidation of the mechanistic pathways followed by 5-aminotetrazoles upon UV-irradiation and of the effect of the ring substitution pattern on the photoreactivity of these compounds.
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