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- Structural investigation of nitrogen-linked saccharinate–tetrazolePublication . Gómez-Zavaglia, A.; Ismael, Amin; Cabral, Lília; Kaczor, A.; Paixão, J. A.; Fausto, R.; Cristiano, Maria Lurdes SantosThe molecular structure of nitrogen-linked saccharinate–tetrazole, N-(1,1-dioxo-1,2-benzisothiazol-3-yl)-amine-1H-tetrazole (BAT), was investigated in the crystalline state using X-ray crystallography and infrared and Raman spectroscopies, and isolated in argon matrix by infrared spectroscopy. Interpretation of the experimental results was supported by quantum chemical calculations undertaken at the DFT(B3LYP)/6-311++G(3df,3pd) level of theory. In the neat crystalline solid (space group C2/c, a = 21.7493(3) Å, b = 8.85940(10) Å, c = 10.76900(10) Å, b = 103.3300(10) deg; Z = 8), BAT units exist in the (1H)-tetrazole aminosaccharin tautomeric form, with the NH spacer establishing a hydrogen bond to the nitrogen in position-4 of the tetrazole group of a neighbour molecule, and the NH group of the tetrazole fragment forming a bifurcated H-bond to the saccharyl nitrogen of the same molecule and to one of the oxygen atoms of a second neighbour molecule. On the other hand, according to both the matrix isolation infrared studies and the theoretical calculations, the isolated BAT molecule exists preferentially as the (1H)-tetrazole iminosaccharin tautomer, where the main stabilizing interaction is the intramolecular H-bond established between the NH group of the saccharyl ring and the tetrazole nitrogen atom in position 4. A detailed conformational analysis of the studied molecule and full assignment of the vibrational spectra for both the matrix-isolated compound and crystalline sample were undertaken.
- Photochemistry of 1-and 2-Methyl-5-aminotetrazoles: structural effects on reaction pathwaysPublication . Ismael, Amin; Fausto, R.; Cristiano, Maria Lurdes SantosThe 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.
- Molecular structure of nitrogen-linked methyltetrazole-saccharinatesPublication . Ismael, Amin; Paixão, J. A.; Fausto, R.; Cristiano, Maria Lurdes SantosThe molecular structures of nitrogen-linked 1- and 2-methyltetrazole-saccharinates, were investigated in the crystalline phase using X-ray crystallography and infrared and Raman spectroscopies, complemented by quantum chemical calculations performed at the DFT(B3LYP)/6-31++G(d,p) level of theory for the isolated molecules. In the neat crystalline solid (space group P 1, a = 6.9763 Å, b = 8.3097 Å, c = 10.0737 Å, a = 96.517 , b = 107.543 , c = 99.989 ; Z = 2), 1-methyltetrazole-saccharinate units assume the most stable configuration for the isolated molecule, (1H)-1-methytetrazole iminosaccharin tautomeric form (1MTIS), with the N@C spacer linking the two heterocycles. On the other hand, neat crystalline 2-methyl derivative units (space group P 1, a = 7.8010 Å, b = 8.6724 Å, c = 9.4984 Å, a = 114.083 , b = 107.823 , c = 93.080 ; Z = 2) exist in the (2H)-2-methytetrazole aminosaccharin tautomeric form (2MTAS), with the two heterocycles connected by an NH spacer. In both crystals, the structure consists of a packing of dimeric units, the dimers formed via hydrogen bonding involving either the NH group of the saccharyl system (1MTIS) or the spacer amine group (2MTAS). In the former, the hydrogen bond is bifurcated and the NH group acts as a donor both towards a neighbor molecule and an N atom of the tetrazole ring, forming an intramolecular hydrogen bond. The observed difference in the crystallographic basic units of the two compounds reveals the prevalence of the H-bond networks in determining the structural preferences of the tetrazole-saccharinates in the solid state. Such structural flexibility appears also to be of potential interest in the design of new ligands based on the tetrazole-saccharinate framework. The relative strengths of the H-bonds in the crystals of the two compounds were evaluated through inspection of their vibrational spectra and empirical correlations between spectroscopic data and the H-bond enthalpies and distances.
- Investigations into the Mechanism of Solvolysis of 3-aryloxybenzisothiazolesPublication . Ismael, Amin; Gago, David J. P.; Cabral, Lília; Fausto, Rui; Cristiano, Maria De LurdesThe solvolysis of selected 3-aryloxybenzisothiazoles (6a-c; Figure 1) in alcohols has been theoretically investigated. The geometries of ethers 6a-c were fully optimized at the DFT(O3LYP) level, with the 6-31++G(d,p) and 6-311++G(3df,3pd) basis sets. Calculations including solvation effects were performed with the 6-31++G(d,p) basis set. Overall, theoretical values for bond lengths and angles around the central ether linkage in ethers 6a-c are very close, for the isolated molecule and in methanol, and are also very close to those obtained by X-ray crystallography, revealing that the nature of the substituent on the aryl system has a negligible effect on geometric parameters around the ether linkage. The same applies to charge distributions, predicted using the NPA approach. However, measured rate constants for the solvolysis of the same compounds in alcohols show that the rate is affected by the electron-withdrawing/-donating characteristics of the substituent on the aryl ring and by the polarity of solvent. Two general pathways were considered for the solvolysis of ethers 6: associative (addition-elimination) or dissociative (fragmentation-recombination) mechanisms. Molecular orbital calculations by means of polarized continuum model (PCM) reaction field predicted that solvolysis of ethers 6 prefers an addition-elimination mechanism. Calculations show also that a dissociative mechanism for the solvolysis of ethers 6a-c is energetically much more demanding than its addition-elimination counterpart and is therefore a highly improbable pathway for the solvolysis. In addition, it was found that the putative cation intermediate formed during a dissociative process should easily convert into its 2-cyanobenzenesulfone cation isomer, via cleavage of the S-N bond.
- Matrix-isolation FTIR, theoretical structural analysis and reactivity of amino-saccharins: N-(1,1-dioxo-1,2-benzisothiazol-3-yl)-N-methyl amine and -N,N-dimethyl aminePublication . Almeida, R.; Gómez-Zavaglia, A.; Kaczor, A.; Ismael, Amin; Cristiano, Maria Lurdes Santos; Fausto, R.In this work, two novel amino-substituted derivatives of saccharin, N-(1,1-dioxo-1,2-benzisothiazol-3-yl)-N-methyl amine (MBAD) and N-(1,1-dioxo-1,2-benzisothiazol-3-yl)-N,N-dimethyl amine (DMBAD), were synthesized and characterized, and their molecular structure and vibrational properties were investigated by matrix-isolation FTIR spectroscopy and theoretical calculations undertaken using different levels of approximation. The calculations predicted the existence of two conformers of MBAD. The lowest energy form was predicted to be considerably more stable than the second conformer (DE > ca. 20 kJ mol 1) and was the sole form contributing to the infrared spectrum of the compound isolated in solid xenon. Both conformers have planar amine moieties. In the case of DMBAD, only one doubly-degenerated-by-symmetry conformer exists, with the amine nitrogen atom considerably pyramidalized. The effect of the electron-withdrawing saccharyl ring on the C–N bond lengths is discussed. The different structural preferences around the amine nitrogen atom in the two molecules were explained in terms of repulsive interactions involving the additional methyl group of DMBAD. Observed structural features are correlated with the reactivity exhibited by the two compounds towards nucleophiles. The experimentally obtained spectra of the matrix-isolated monomers of MBAD and DMBAD were fully assigned by comparison with the corresponding calculated spectra.
- Tautomer selective photochemistry in 1-(Tetrazol-5-yl)ethanolPublication . Ismael, Amin; Cristiano, Maria Lurdes Santos; Fausto, R.; Gómez-Zavaglia, A.A combined matrix isolation FTIR and theoretical DFT/B3LYP/6-311++G(d,p) study of the molecular structure and photochemistry of 1-(tetrazol-5-yl)ethanol [1-TE] was performed. The potential energy surface landscapes of the 1H and 2H tautomers of the compound were investigated and the theoretical results were used to help characterize the conformational mixture existing in equilibrium in the gas phase prior to deposition of the matrices, as well as the conformers trapped in the latter. In the gas phase, at room temperature, the compound exists as a mixture of 12 conformers (five of the 1H tautomer and seven of the 2H tautomer). Upon deposition of the compound in an argon matrix at 10 K, only three main forms survive, because the low barriers for conformational isomerization allow extensive conformational cooling during deposition. Deposition of the matrix at 30 K led to further simplification of the conformational mixture with only one conformer of each tautomer of 1-TE surviving. These conformers correspond to the most stable forms of each tautomer, which bear different types of intramolecular H-bonds: 1H-I has an NH· · ·O hydrogen bond, whereas 2H-I has an OH· · ·N hydrogen bond. Upon irradiating with UV light (λ > 200 nm), a matrix containing both 1H-I and 2H-I forms, an unprecedented tautomer selective photochemistry was observed, with the 2H tautomeric form undergoing unimolecular decomposition to azide + hydroxypropanenitrile and the 1H-tautomer being photostable.
- Photochemistry of 1-allyl-4-aryltetrazolones in solution; structural effects on photoproduct selectivityPublication . Ismael, Amin; Serpa, C.; Cristiano, Maria Lurdes SantosThe photochemistry of tetrazolones derived from the carbocyclic allylic alcohols cyclohex-2-enol and 3-methylcyclohex-2-enol and from the natural terpene alcohol nerol was investigated in solution with the aim of assessing the effect of solvent and of structural constraints imposed by bulky allylic moieties on photoproduct selectivity and stability. Photolysis of tetrazolones derived from nerol and cyclohex-2-enol afforded the corresponding pyrimidinones as major products through a pathway that appears to be similar to that proposed for other 1-allyl-4-phenyl-1,4-dihydro-5H-tetrazol-5-ones derived from acyclic and unhindered allylic alcohols previously investigated but photolysis of the tetrazolone derived from the bulkier 3–methylcyclohex-2-enol 4c leads to formation of a benzimidazolone, indicating that, in this case, cyclization of the biradical formed upon extrusion of N2 involves the phenyl substituent and not the allylic moiety. Theoretical calculations (DFT(B3LYP)/3-21G*) were conducted to support the interpretation of the experimental results and mechanistic proposals. Laser flash photolysis experiments were conducted with the aim of clarifying the nature of the intermediate involved in the primary photocleavage process.
- Structure and reactivity of novel tetrazole-saccharinates useful as multidentate nitrogen ligandsPublication . Ismael, Amin; Cristiano, Maria Lurdes SantosTetrazoles 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.
- Exploring saccharinate-tetrazoles as selective Cu(II) ligands: structure, magnetic properties and cytotoxicity of copper(II) complexes based on 5-(3-aminosaccharyl)-tetrazolesPublication . Ismael, Amin; Henriques, M. S. C.; Marques, C.; Rodrigues, M.; Barreira, Luísa; Paixao, J. A.; Fausto, R.; Cristiano, M. Lurdes S.The role of copper in the proliferation of cancer cells is under investigation and has been explored in the context of cancer chemotherapy. The evidence that proliferation of cancer cells requires a higher abundance of Cu(II) than their normal counterparts has prompted the development of new copper chelators that can avidly bind copper ions, forming redox active metal complexes that ultimately lead to harmful reactive oxygen species (ROS) in neoplasms. In this context, the mandatory properties of the chelators for medical applications are safety (neglectable cytotoxicity), high binding affinity and selectivity towards Cu(II). We report the synthesis, structure (calculations and single crystal X-ray diffraction), spectroscopic (IR; UV-Vis) and magnetic properties of two novel copper(II) complexes based on 5-(3-aminosaccharyl)-tetrazoles (TS and 2MTS), as well as their in vitro cytotoxicity against the human hepatic carcinoma cell line HepG2. Quite interestingly, we found that the saccharinate-tetrazoles tested exhibit strong binding selectivity to Cu(II), over Fe(II) and Ca(II). Additionally, the corresponding copper complexes have shown a huge increase in the in vitro cytotoxicity against tumoral cells, compared to the corresponding nontoxic ligands. Thus, the new ligands may be viewed as potential precursors of selective cytotoxic agents, acting as non-cytotoxic pro-drugs that can be activated inside neoplastic cells, known to be richer in Cu(II) than the corresponding normal cells.
- Sigmatropic rearrangements in 5-allyloxytetrazolesPublication . Frija, L.; Reva, I. D.; Ismael, Amin; Coelho, Daniela; Fausto, R.; Cristiano, Maria Lurdes SantosMechanisms of thermal isomerization of allyl tetrazolyl ethers derived from the carbocyclic allylic alcohols cyclohex-2-enol and 3-methylcyclohex-2-enol and from the natural terpene alcohol nerol were investigated. In the process of the syntheses of the three 1-aryl-5-allyloxytetrazoles, their rapid isomerization to the corresponding 1-aryl-4-allyltetrazol-5-ones occurred. The experiments showed that the imidates rearrange exclusively through a [3,3¢]-sigmatropic migration of the allylic system from O to N, with inversion. Mechanistic proposals are based on product analysis and extensive quantum chemical calculations at the DFT(B3LYP) and MP2 levels, on O-allyl and N-allyl isomers and on putative transition state structures for [1,3¢]- and [3,3¢]-sigmatropic migrations. The experimental observations could be only explained on the basis of the MP2/6-31G(d,p) calculations that favoured the [3,3¢]-sigmatropic migrations, yielding lower energies both for the transition states and for the final isomerization products.