STRUCTURE AND REACTIVITY OF NOVEL TETRAZOLE-SACCHARINATES USEFUL AS MULTIDENTATE NITROGEN LIGANDS

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

Publication . Ismael, Amin; Abe, Manabu; Fausto, Rui; Cristiano, Maria De Lurdes

The properties and applications of 2-methyl-(211)-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 20x 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, (s)N1, as common intermediate, which undergoes a Wolff-type isomerization to a stable carbodiimide. Photo-extrusion of N-2 from 1,5-substituted tetrazoles generates (s)N1 directly but from 2,5-substituted tetrazoles it originates a nitrilimine, then a diazirene, which finally leads to (s)N1. Selective formation of cyanamide from 1MT requires photoisomerization between (s)N1 and (s)N2, accessible at 222 nm. EPR studies enabled the detection of methyl nitrene, arising from photolysis of 1H-diazirene intermediate.

2020Journal article

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Structure and reactivity of novel tetrazole-saccharinates useful as multidentate nitrogen ligands

Publication . Ismael, Amin; Cristiano, Maria Lurdes Santos

Tetrazoles have attracted increasing attention in recent years in the fields of coordination and supramolecular chemistry, due to the excellent coordination ability of the four nitrogen atoms, acting either as multidentate ligands or as bridging building blocks in supramolecular assemblies. Moreover, the structure of the coordination complex can be tailored by employing functional tetrazoles in the assembly process. Likewise, several studies on saccharin have shown that its coordination chemistry is very interesting, acting either as a bidentate or as bridging ligand. Inspired by the versatility of those heterocycles, the research described within this thesis focus on the synthesis, structure and reactivity of a library of new tetrazole-saccharyl conjugates with applications as multidentate nitrogen ligands. Ligands are the very body of coordination chemistry, therefore understanding the structural properties of these systems is crucial, in order to explore their properties and functions. As such, the structure of these new ligands has been investigated in detail, and their chelating capacity towards divalent cations of transition metals was evaluated. In parallel, the photochemistry of these ligands, isolated in solid argon, was investigated by means of infrared spectroscopy and quantum chemical calculations. The plethora of applications of saccharin derivatives often rely on the photochemical stability of the saccharyl system. On the other hand, tetrazoles are known for their rich photochemistry. As such, the photochemical stability of these conjugates, introduced by the saccharyl system into the photolabile tetrazole, was evaluated. In the course of those studies, important mechanistic questions regarding the effects of the ring substitution pattern, on the photochemical pathways of disubstituted tetrazoles, were addressed. Two novel copper(II) complexes based on selected tetrazole-saccharinates were prepared and their structural and spectroscopic properties investigated. Additionally, selected conjugates were tested as selective copper(II) chelators, and the complexes were evaluated for their in vitro cytotoxicity against human tumour cell lines.

2017-06-16Doctoral thesis

Open Access

Photochemistry of 1-and 2-Methyl-5-aminotetrazoles: structural effects on reaction pathways

Publication . Ismael, Amin; Fausto, R.; Cristiano, Maria Lurdes Santos

The influence of the position of the methyl substituent in 1- and 2-methyl-substituted 5-aminotetrazoles on the photochemistry of these molecules is evaluated. The two compounds were isolated in an argon matrix (15 K) and the matrix was subjected to in situ narrowband UV excitation at different wavelengths, which induce selectively photochemical transformations of different species (reactants and initially formed photoproducts). The progress of the reactions was followed by infrared spectroscopy, supported by quantum chemical calculations. It is shown that the photochemistries of the two isomers, 1-methyl-(1H)-tetrazole-5-amine (la) and 2-methyl-(2H)-tetrazole-5-amine (1b), although resulting in a common intermediate diazirine 3, which undergoes subsequent photoconversion into 1-amino-3-methylcarbodiimide (H2N-N=C=N-CH3), show marked differences: formation of the amino cyanamide 4 (H2N-N(CH3)-CE equivalent to N) is only observed from the photo cleavage of the isomer la, whereas formation of the nitrile imine 2 (H2N-C-=N+=N-CH3) is only obtained from photolysis of 1b. The exclusive formation of nitrile imine from the isomer lb points to the possibility that only the 2H-tetrazoles forms can give a direct access to nitrile imines, while observation of the amino cyanamide 4 represents a novel reaction pathway in the photochemistry of tetrazoles and seems to be characteristic of 1H-tetrazoles. The structural and vibrational characterization of both reactants and photoproducts has been undertaken.

2016-12Journal article

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