Insights into the photochemistry of 5-aminotetrazole derivatives with applications in coordination chemistry. Effect of the saccharyl moiety on the photostability

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.