Structure and photochemistry of a saccharyl thiotetrazole

Article abstract of DOI:10.1021/jo502419u

The molecular structure and photochemistry of 5-thiosaccharyl-1-methyltetrazole (TSMT) were studied by means of matrix-isolation FTIR spectroscopy, X-ray crystallography, and theoretical calculations. The calculations predicted two conformers of TSMT that differ in energy by more than 15 kJ mol^-1. The infrared spectrum of TSMT isolated in solid argon was fully assigned on the basis of the spectrum calculated (O3LYP/6-311++G(3df,3pd)) for the most stable conformer. In the crystal, TSMT molecules were found to assume the same conformation as for the isolated molecule, with each molecule forming four hydrogen bonds with three neighboring molecules, leading to a network of TSMT oligomers. Upon UV (λ = 265 nm) irradiation of the matrix-isolated TSMT, two photodegradation pathways were observed, both arising from cleavage of the tetrazolyl ring. Pathway a involves cleavage of the N₁-N₂ and N₃-N₄ bonds with extrusion of N₂, leading to photostable diazirine and thiocarbodiimide derivatives. The photostability of the photoproduced diazirine under the conditions used precluded its rearrangement to the nitrile imine, as reported for 5-phenyltetrazole by Bégué et al. (J. Am. Chem. Soc. 2012, 134, 5339). Pathway b involves cleavage of the C₅-N₁ and N₄-N₃ bonds, leading to a thiocyanate and methyl azide, the latter undergoing subsequent fragmentation to give CNH.

Full text of DOI:10.1021/jo502419u

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Article
Structure and photochemistry of a saccharyl thiotetrazole
Amin Ismael, Ana Borba, Marta Sofia C. Henriques, José A.
Paixão, Rui Fausto, and Maria de Lurdes Santos Cristiano
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/jo502419u • Publication Date (Web): 01 Dec 2014
Downloaded from http://pubs.acs.org on December 6, 2014
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Structure and photochemistry of a saccharyl thiotetrazole
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A. Ismael,†,‡ A. Borba,‡ M.S.C. Henriques,§ J. A. Paixão,§ R. Fausto‡,* and M. L. S. Cristiano†,*
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† CCMAR and Department of Chemistry and Pharmacy, F.C.T., University of Algarve, Pꢀ8005ꢀ039 Faro (Portugal)
‡Department of Chemistry, University of Coimbra, Pꢀ3004ꢀ535 Coimbra (Portugal)
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§ CEMDRX, Department of Physics, University of Coimbra, Pꢀ3004ꢀ516 Coimbra (Portugal)
Eꢀmail: mcristi@ualg.pt; rfausto@ci.uc.pt
Keywords: Tetrazoles, Benzisothiazoles, TetrazoleꢀSaccharinates, Matrix isolation, Photocleavage mechanisms, Diazirines,
Thiocarbodiimides, FTIR Spectroscopy, Xꢀray Crystallography
Abstract
The molecular structure and photochemistry of 5ꢀthiosaccharylꢀ1ꢀmethyltetrazole (TSMT) were
studied by means of matrix isolation FTIR spectroscopy, Xꢀray crystallography and theoretical
calculations.
Calculations predict two conformers of TSMT, differing in energy by more than 15 kJ mol–1. The
infrared spectrum of TSMT isolated in solid argon was fully assigned based on the spectrum calculated
(O3LYP/6ꢀ311++G(3df,3pd)) for the most stable conformer. In the crystal, TSMT molecules were found
to assume the same conformation as for the isolated molecule, with each molecule forming four hydrogen
bonds with three neighboring molecules, leading to a network of TSMT oligomers.
Upon UV (λ= 265 nm) irradiation of the matrix isolated TSMT, two photodegradation pathways
were observed, both arising from cleavage of the tetrazolyl ring. Pathway (a) involves cleavage of the N1–
N2 and N3–N4 bonds, with extrusion of N2, leading to photostable diazirine and thiocarbodiimide
derivatives. The photostability of the photoproduced diazirine under the conditions used precludes its
rearrangement to nitrile imine, as reported by Bégué et al (J. Am. Chem. Soc., 134 (2012) 5339) for 5ꢀ
phenyltetrazole. Pathway (b) involves cleavage of the C5–N1 and N4–N3 bonds, leading to a thiocyanate
and methyl azide, the last undergoing subsequent fragmentation to CNH.
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Introduction
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Derivatives of the heterocycles tetrazole and saccharin are widely applied in coordination chemistry,
as ligands 1,2. Tetrazole can coordinate through four nitrogen electronꢀdonating atoms and may therefore
act as a multidentate ligand or as a bridging building block in supramolecular assemblies. It has potential
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to participate in seven distinct types of coordination modes with metal ions and, as such, tetrazoleꢀbased
3ꢀ6
coordination compounds are used in catalysis and in the construction of metalꢀorganic frameworks
.
Similarly, saccharin and the saccharinate anion (1,2ꢀbenzisothiazoleꢀ3ꢀone 1,1ꢀdioxide anion;
deprotonated saccharin) interact with metal centers in various ways, generating relatively strong
interactions in crystalline environments, mostly through hydrogen bonding. As a polyfunctional ligand,
saccharin can be engaged in N, O(C=O) or O(SO2)ꢀmetal coordination, and can also act as a bidentate
amidatoꢀlike bridging agent 7–9. Based on the structural features and coordination capabilities of both
heterocycles we envisaged investigating the potential of saccharylꢀtetrazole conjugates (Figure 1) as
bridging ligands.
Figure 1. General structure of the saccharylꢀtetrazole conjugates.
Synthetic routes to a small library of these target compounds were devised and optimized 10, in view of
exploring their application as coordination ligands for transition metals.
Understanding the relevance of different tautomeric forms and preferred conformations of chemical
systems is instrumental in predicting and exploring their properties and function. As such, the structure of
selected saccharylꢀtetrazole conjugates was investigated in detail, both in the isolated molecule situation,
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by matrix isolation infrared spectroscopy and contemporary methods of quantum chemistry, and in the
neat crystalline phase, by Xꢀray crystallography and infrared and Raman spectroscopies 11,12. Among the
conjugates investigated, nitrogenꢀlinked saccharinate tetrazoles (Figure 1; X= NH) proved especially
challenging due to the possibility of tautomerism involving the saccharyl system 11,12. For these
compounds, two forms are possible, the iminosaccharin and aminosaccharin tautomers. If, additionally,
the tetrazole ring is unsubstituted (Figure 1; X= NH; R= H), as is the case for Nꢀ(1,1ꢀdioxoꢀ1,2ꢀ
benzisothiazolꢀ3ꢀyl)ꢀamineꢀ1Hꢀtetrazole 11, then four tautomers become possible, two belonging to the
iminosaccharin group and other two to the aminosaccharin group. We found that the aminoꢀlinked
conjugates may exhibit different structure in the crystalline and gaseous phases 11,12. On the contrary, for a
thioꢀlinked saccharyl tetrazole, as is the case of the title compound, the structure simplifies, due the
absence of tautomerism involving the saccharyl system. In addition, the possibility of tautomerism
involving the tetrazole ring may be quenched through the introduction of a suitable substituent. We now
describe the synthesis and detailed structural analysis of 1,2ꢀbenzisothiazole 1,1ꢀdioxo 3ꢀ(5ꢀthioꢀ1ꢀ
methyl) tetrazole (5ꢀthiosaccharyl 1ꢀmethyltetrazole, TSMT) (3: Scheme 1), in the crystalline state and in
solid argon.
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Scheme 1. Synthetic approach to 5ꢀthiosaccharylꢀ1ꢀmethyltetrazole (TSMT)
Cl
N
N
N
S
O2
N
S
2
N
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HS
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O2
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3
When considering applications of saccharylꢀtetrazole conjugates, assessment of photostability may be
a relevant aspect. In general, the bensizothiazole moiety is known to be rather photostable, compared to
the tetrazole moiety. The photochemistry of monomeric saccharin isolated in solid argon was recently
investigated 13. A search in the literature also yields some data regarding the photochemistry of some
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benzisothiazoles in solution 14ꢀ23, indicating that photoreactivity is determined by the structure of the
saccharyl ring, the nature of substituents and reaction media. By their turn, tetrazoles are known for their
rich photochemistry, both in solution and isolated in cryogenic matrices 24,25. Generally, the
photochemistry of tetrazole derivatives is influenced by the chemical nature and conformational
flexibility of substituents and, when the molecule exhibits tautomerism, tautomerꢀselective
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photochemistry may take place 25,26
.
The fact that tetrazoleꢀsaccharyl conjugates combine heterocyclic moieties with dissimilar intrinsic
photoreactivity is challenging, and yet another reason to explore the photoreactivity of benzisothiazolylꢀ
tetrazole conjugates by matrix isolation FTIR spectroscopy. This method offers several advantages for
studying the photochemistry of monomeric compounds (i.e.: molecular diffusion is inhibited leading ꢀin
principleꢀ only to the observation of unimolecular reactions and enabling the stabilization, detection and
characterization of products and, in some cases, of intermediates) 25.
We report here on the UVꢀinduced photochemistry of TSMT in solid argon. As mentioned above, in
this saccharylꢀtetrazole conjugate the methyl substituent on the tetrazole ring quenches the possibility of
tautomerism involving the tetrazole, and the thioether linkage restricts tautomerism involving the
saccharyl system, thus reducing the number of structures present on the matrix and simplifying the
interpretation of the photochemical reactions. Interpretation of the experimental results is assisted by
quantum chemical calculations undertaken at different levels of approximation.
Results and Discussion
Geometries and energies of TSMT
TSMT has two intramolecular rotational degrees of freedom, represented by arrows in Figure 2,
which may result in different conformers. These are related with the orientations of the saccharyl and
tetrazolyl rings relatively to the C–S–C plane (dihedral angles A and B, respectively).
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To identify the minimum energy conformations of TSMT, a systematic investigation on the
potential energy surface of the molecule was undertaken using the B3LYP/6ꢀ31++G(d,p) method. The
location of the minima on the bidimensional potential energy surface (PES) map obtained by performing
relaxed scans on the PES is shown in Figure 3. In building this map, dihedral angles A (N8–C9–S16–C17)
and B (C9–S16–C17–N18) were varied in increments of 30 degrees, while all other geometric parameters
were let free during the geometry optimization. According to the calculations, TSMT may exist in two
different conformers (I and II, Figure 2). The full geometry optimization of the two conformers was
carried at the DFT(O3LYP)/6ꢀ311++G(3df,dpd) level of theory. The O3LYP hybrid functional was
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selected for these calculations because it has been shown to constitute an improvement to the B3LYP one
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Geometrical parameters are provided as Supporting Information (“Computational data”).
.
Figure 2. Optimized structures of the theoretically predicted conformers of TSMT. The atom numbering
adopted is shown. Arrows A and B relate to the two conformationally relevant degrees of freedom.
In the most stable conformer I, the tetrazole ring is considerably rotated in relation to the C–S–C
plane (i.e., B is equal to ±115º in the two symmetryꢀequivalent minima corresponding to this conformer;
see Figure 3), while the dihedral angle A is close to 0º (i.e., the plane of the saccharyl moiety is nearly
aligned with the C–S–C plane, with the C=N bond eclipsing the S16–C17 bond). Note that the Cs symmetry
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structures with the tetrazolyl and saccharyl groups coꢀplanar (A= 0º, B= 0º and A= 0º, B= 180º) are
destabilized by steric repulsions between the closely located N18 and N8 loneꢀelectron pairs or CH3 group
and N8 loneꢀpair, respectively. In conformer I, a stabilizing interaction occurs between the nitrogen atom
of the saccharyl moiety and the hydrogen of the methyl group, because in the minimum energy geometry
the distance between these atoms and their relative orientation fit well the requirements for an attractive
interaction (weak C–H…N hydrogen bond like interaction). In addition, electronic delocalization
involving the loneꢀpairs of electrons on the sulphur atom and the π systems of the rings shall also
contribute to determine this conformational preference.
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0
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E / kJ mol -1
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N8C9-S16C17 dihedral angle / º (A)
Figure 3. Potential energy surface map of TSMT calculated at the B3LYP/631++G(d,p) level of theory,
as function of the dihedral angles C9S16C17N18 and N8C9S16C17. These two dihedral angles were
incrementally fixed with a step of 30 degrees while the remaining parameters were let free during
geometry optimization. The position of both conformers are shown in the graph by their numbers, I and
II. Each of these minima belongs to the C1 symmetry point group and has a symmetry equivalent
counterpart, also presented in the graph with quotes, I’ and II’. The electronic energy of form I is chosen
as relative zero. See Figure 2 for structures and atom numbering.
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The energy of conformer II was predicted to be 15 kJ molꢀ1 higher than that of conformer I.
Conformer II differs from conformer I by internal rotation about the C9ꢀS16 bond (i.e., A is equal to ±
137º). In this conformer, a weak stabilizing intramolecular interaction of the CꢀH…N type is present,
between the nitrogen atom N18 of the tetrazole ring and the hydrogen atom H13 of the phenyl ring (see
Figure 2). The calculated barrier for isomerization of the higher energy conformer II into the most stable
conformer I is 6.1 kJ molꢀ1 (19.4 kJ molꢀ1 in the reverse direction; for coordinates of the transition state
structure, see Supporting Information, Section “Computational Data”). According to the calculated
energy difference between II and I, the population of conformer II in the gas phase, at room temperature
(25º C), is only 0.2%; at the temperature used to prepare the matrices of the compound in the present
investigation (100 ºC), it is still below 1%. One can then anticipate that only conformer I might be trapped
from the gas phase into the deposited cryogenic matrices.
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Crystal structure of TSMT and IR spectrum of the crystal
The structure of crystalline TSMT, determined by Xꢀray, is provided in Figure 4 (packing
diagram) and Figure S1 (structure of the monomeric unit in the crystal). Atomic coordinates, bond
lengths, valence angles, dihedral angles, and other crystallographic data were deposited at the Cambridge
Crystallographic Data Center, with the reference CCDC 1025233.
In the crystal, TSMT exists in the most stable conformation predicted for the isolated molecule
(compare Figures 2 and S1). Each molecule is involved in four weak hydrogen bonds with three neighbor
molecules. The methyl substituent on the tetrazolyl ring establishes two intermolecular hydrogen bonds
with two neighbor molecules, one with a nitrogen atom in position 4 of the tetrazole ring (N18) and
another with an oxygen atom O14 of the SO2 group of the saccharyl system from a second molecule, as
acceptors. This same molecule establishes a third intermolecular hydrogen bond through the oxygen atom
O14 of the SO2 group with the methyl substituent of the previous “second” neighbor, and a fourth
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hydrogen bond with a third neighbor molecule, where the nitrogen atom N18 acts as acceptor. In addition
to these four intermolecular hydrogen bonds, a weak intramolecular interaction can also be considered
between the third H atom of the methyl group and the S atom.
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Figure 4. Hydrogen bonding network in the crystal of TSMT. The ORTEPII plot showing the geometry
of the TSMT molecule in the crystal is provided in Figure S1 and closely resembles the theoretically
predicted conformer I.
The hydrogenꢀbonding scheme is shown in Figure 4, and the symmetry unique short contacts are
a
b
C22H24ꢁꢁꢁꢁN18 : 3.486(2) Å; C22H25ꢁꢁꢁꢁO14 ; 3.345(2) Å; C22H23ꢁꢁꢁꢁS16: 3.323(2) Å, where the superscripts “a”
or “b” designate atoms belonging to neighboring molecules generated by the symmetry operators a: x,1/2ꢀ
y,1/2+z; b:1ꢀx,1ꢀy,1ꢀz. This hydrogen bond pattern leads to the formation of TSMT oligomers. The
oligomeric structure is formed from dimers of TSMT. In the dimers, the two molecules of TSMT are
disposed in a headꢀtoꢀtail arrangement, with the tetrazole moiety of one molecule overlapping the
thioimino moiety of the other, at the inner part of the dimer, while the carbocyclic aromatic rings are
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directed to the extremities. The oligomeric structure results from an intricate network of weak hydrogen
bonds formed between consecutive dimers (with involvement of both molecules of the dimer) and
adjacent dimers. Weak πꢀπ interactions between tetrazole rings and thioimino moieties, as well as SꢀO…π
interactions (short contacts of 4.65 Å and 3.01 Å, respectively), may also play a role in the stabilization of
the observed supramolecular structure.
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A comparison between the geometrical parameters of the TSMT monomer I, calculated at the
O3LYP/6ꢀ311++G(3df,3pd) level of theory, and those determined for the compound in the crystal (Table
S1), reveals an excellent correlation between the two sets of data, although in the crystal the dihedral
angles A and B (see Figure 2) appear slightly rotated (i.e., the A dihedral is equal to –10.5º and B to
–108.8º) compared to those obtained from theory for the isolated molecule (A dihedral equal to –3.3º and
B to –115.3º), as a result of the intermolecular interactions.
The infrared spectrum of neat crystalline TSMT at room temperature is provided as Supporting
Information (Figure S2). The calculated spectrum of conformer I is also shown in this figure, for
comparison. Proposed assignments for the spectra are given in Table S2. The good general description of
the experimental spectra of the crystal by the calculated spectra of the isolated monomer is an indication
that the intermolecular interactions present in the solid do not perturb substantially the intramolecular
potential.
Infrared spectra of matrix-isolated TSMT
As described in the experimental section, a sample of crystalline TSMT was sublimated in vacuo
(at ~100 ºC), the vapors of the compound were mixed with argon (ca. 1:1000 molar ratio), and the
mixture was deposited onto a substrate kept at 15 K. Figure 5 shows the spectrum of TSMT isolated in an
argon matrix prepared by this way.
The detailed analysis of the infrared spectrum for matrixꢀisolated TSMT doubtlessly shows that
only conformer I is present in the matrix, as anticipated taken into account the predicted relative energies
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of the two conformers and their estimated populations in the gas phase equilibrium before deposition.
This is particularly noticeable in the low frequency region where, according to the calculations, the
spectra of the two conformers are clearly differentiable (see Figure 6).
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0.20
Ar Matrix (15K)
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Calculated
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Wavenumber/ cm-1
Figure 5. Infrared spectrum of matrix isolated TSMT (Ar matrix; 15 K) and O3LYP/6ꢀ311++G(3df,3pd)
calculated infrared spectrum for the most stable conformer. The calculated spectra were simulated by
Lorentzian functions centered at the calculated frequency scaled by 0.992 and with FWHM (full width at
half maximum) equal to 2 cmꢀ1. Please note also that the peak intensities in the calculated spectrum differ
from the calculated intensity values; in the figure the calculated intensities correspond to the areas of the
peaks.
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Conformer I
Conformer II
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Wavenumber/ cm-1
Figure 6. Lowꢀfrequency region of the infrared spectrum of matrix isolated TSMT (Ar matrix; 15 K) and
O3LYP/6ꢀ311++G(3df,3pd) calculated infrared spectra for the two conformers of the compound in the
same spectral range. The calculated spectra were simulated by Lorentzian functions centered at the
calculated frequency scaled by 0.992 and with FWHM (full width at half maximum) equal to 2 cmꢀ1.
Please note also that the peak intensities in the calculated spectrum differ from the calculated intensity
values; in the figure the calculated intensities correspond to the areas of the peaks.
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UV-induced photochemistry of matrix-isolated TSMT
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In order to investigate the photochemical reactions of TSMT, the deposited matrix was irradiated
with UV light, as described in experimental section. The sample was monitored after each irradiation, by
recording its infrared spectrum. The wavelengths used for irradiation have been chosen according to the
UVꢀVis spectrum of TSMT in ethanol, at room temperature, which shows the absorption maximum at
263 nm.
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Upon irradiation at 265 nm, the bands due to TSMT decreased in intensity, indicating that the
compound was being consumed and, simultaneously, new bands appeared in the spectrum, due to
photoproduced species. The changes are particularly noticeable in the 2300ꢀ1700 cmꢀ1 spectral region,
presented in Figure 7, where the results of irradiation at λ= 265 nm for 1 h are shown. In this figure, the
calculated spectra of the proposed photoproducts (see also Scheme 2), in the same spectral range, are also
shown for comparison. The observation that the changes in the intensities of the various new bands in the
2300ꢀ1700 cmꢀ1 spectral range follow different patterns with time of irradiation indicates that these bands
originate in different products.
For the interpretation of the photochemical results, it must be noticed that TSMT is a molecule
composed of 25 atoms, 15 of them belonging to the rigid benzisothiazole ring. Thus, 39 out of 69
vibrational modes are related with this benzisothiazole ring. This ring is unaffected by irradiation under
the experimental conditions used, which results in an extensive overlap of the bands of TSMT with those
of the photoproducts, complicating interpretation of the data. The strong overlap between the spectra of
some of the proposed photoproducts and between these and the spectrum of TSMT can be clearly noticed
in Figure S3, where the calculated infrared spectra for the relevant species are shown. As it can be seen in
that figure, such superposition is particularly noticeable in the case of the most intense bands appearing in
the low frequency spectral range (below 1700 cmꢀ1), where most of the bands due to the benzisothiazole
moiety appear. However, the most characteristic bands of the photoproducts are observed in the 2300ꢀ
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1700 cmꢀ1 spectral region, which is a clean spectroscopic window; this allowed us to doubtlessly identify
these species, as described in detail below.
6
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0.05
2139
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Irradiated
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Calculated
B3LYP/6311++G(3df,3pd)
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Wavenumber/ cm-1
Figure 7. Top: IR spectra in spectral region 2300ꢀ1700 cmꢀ1 of TSMT after irradiation of the matrix with
= 265 nm) for 1 h, showing peaks due to the photolysis products. The peak at 1754 cmꢀ1 is
UV light (
λ
assigned to the N=C stretching vibration in diazirine 5, the peak at 2139 cmꢀ1 is assigned to the N=C=N
antiꢀsymmetrical stretching of the thiocarbodiimide 6, the peak at 2090 cmꢀ1 is assigned to the N=N=N
antiꢀsymmetric stretching of methyl azide 8, the peak at 2050 cmꢀ1 is assigned to the CN stretching of
CNH 9, and the peak at 2265 cmꢀ1 is assigned to the C N stretching of the thiocyanate 7. Bottom:
Theoretical spectra obtained for the observed photoproducts. The intensities of the individual spectra
were multiplied by different factors to obtain a better simulation of the experimental spectra. Theoretical
wavenumbers were scaled by a factor of 0.978 and bands were broadened by Lorentzian functions with
FWHM equal to 4 cmꢀ1.
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The band appearing at 2139 cmꢀ1 (Figure 7) fits nicely the predicted frequency for the NCN antiꢀ
symmetric stretching vibration of the thiocarbodiimide 6 (Scheme 2). The observation of carbodiimides,
resulting from photolysis of matrixꢀisolated tetrazole derivatives has been reported 28–30. Additionally, the
bands appearing around 1754 cmꢀ1 fit well the predicted frequencies for the νC=N stretching vibration of
diazirine 5. Formation of diazirines (including of 1,3ꢀsubstituted diazirines) from photochemically
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induced cleavage of the tetrazole ring has also been reported 25,29
.
The identification of bands assigned to thiocarbodiimide 6 and diazirine 5 suggests photocleavage
of the N18–N19 and N20–N21 bonds (see Figure 2 for atom numbering) with extrusion of molecular
nitrogen. Note that the broad profile of the band around 1754 cmꢀ1 may be due to the presence in the
matrix of different conformers of diazirine 5. Actually, this molecule has two rotational axes, S–C(saccharin)
and S–C(diazirine) and calculations predict the existence of two conformers with relative energies differing
only by 2.9 kJ molꢀ1. The two conformers have similar IR spectra, but the calculated frequencies for the
νC=N mode are different enough (1722 cmꢀ1 and 1740 cmꢀ1) to result in the observed broad band.
Note also that in the case of the carbodiimide 6, the calculated spectrum does not coincide so
much with those of the reactant molecule and other photoproducts. As seen in Figure 8, several bands
observed in the low frequency range in the spectra of the photolysed matrix doubtlessly confirm this
species as one of the dominant photoproducts of TSMT. Moreover, it shall also be noticed that the
carbodiimide 6, as well as the diazirine 5, are formed since the very beginning of the irradiation, being
already present in significant amounts after only 30 s of irradiation (see Figure 8).
The band observed around 2100 cmꢀ1 is likely to be due to methyl azide 8 (νNNN antiꢀsymmetric
stretching), which can be formed by photocleavage of the N17–N21 and N18–N19 bonds of the tetrazole ring
of TSMT upon irradiation, concomitantly with the thiocyanate 7. Under the used irradiation conditions,
methyl azide has been found to partially react to yield, as final main product, CNH 31 (together with H2
and N2), in a process where methylenimine (CH2=NH) acts as intermediate 31ꢀ34. Indeed, the
photochemistry of methyl azide 5 in an argon matrix has been studied in the past and, for example, the
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Figure 8. (Top; violet): infrared difference spectrum obtained by subtracting the spectrum of the
deposited TSMT matrix from that resulting from irradiation of the matrix (λ = 265 nm) during 30 s;
(Bottom): DFT calculated (wavenumbers scaled) spectra for TSMT (blue trace, bands pointing down),
thiocarboidiimide 6 (red trace) and diazirine 5 (magenta trace).
32
production of radicals such as CN has been safely excluded . According to the available knowledge on
the photochemistry of methyl azide 31ꢀ34, one can then expect that formation of this compound from
photolysis of TSMT should essentially result in the accumulation of CNH. In consonance with such
expectations, a product band was observed around 2050 cmꢀ1, which shall correspond to the νCN
31
stretching mode of CNH, observed by Milligan and Jacox upon photolysis of matrixꢀisolated methyl
azide at nearly this frequency (2032 cmꢀ1). In the present study, CNH is formed together with other
products and is, with all probability, interacting in the matrix cage with other species. Because of that, the
frequencies may shift somewhat. Nevertheless, the assignment of the band at 2050 cmꢀ1 to CNH seems to
be doubtless. It is unfortunate that the other two additional bands due to CNH reported by Milligan and
31
Jacox (at ca. 535 and 3583 cmꢀ1) could not be ascribed with certainty in the present study, though a
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product band at 536 cmꢀ1 (almost coincident with a band of TSMT) seems also to be present in the spectra
of the photolysed matrix. However, the large spectral noise below ca. 600 cmꢀ1 in the spectra of the
photolysed matrix makes this observation uncertain. On the other hand, the band observed by Milligan
and Jacox 31 at 3583 cmꢀ1 is due to the NH stretching mode of CNH and can be expected to be extensively
broadened due to intermolecular interactions (Hꢀbonding) between CNH and other species present in the
matrix cage (several bumps in the baseline are present in the spectra of the photolysed TSMT matrix in
the high frequency region; however, no band could be safely assign to this mode). It shall also be noticed
that no spectroscopic evidence of presence of methylenimine in the photolysed matrix could be found,
specifically in the CH and C=N stretching spectral regions, where putative bands due to this compound
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appearing in a relatively less congested spectral region by bands due to other species should be observed..
However, in the present experiments this species can be expected to be present only in very low amounts,
31-34
due to its expected prompt fragmentation into CNH and H2
and, since its infrared spectrum is
characterized by bands of very low intensity 32, its successful observation would be very improbable.
The bands observed around 2265 cmꢀ1 correspond to typical frequency values for C≡N stretching
modes 35,36 and can be assigned to the νC≡N vibrational mode of thiocyanate 7, which is the photoproduct
that can be expected to be formed together with methyl azide 5 (see Scheme 2). The calculated intensity
of this band for the thiocyanate in gas phase is small, but it is possible that it intensifies in some extent in
the matrix due to polarization effects induced by the medium. Very unfortunately, most of the other
thiocyanate bands with significant predicted intensity are expected to be superimposed with bands due to
the diazirine 5 and TSMT (see Figure S3). Since the direct evidence for identification of the thiocyanate 7
is mainly in the observation of the feature in the ~2265 cmꢀ1 region, it must be considered as tentative.
However, this assignment is strongly suggested from the mechanistic point of view, since, as already
mentioned, the thiocyanate is the product that can be expected to be formed simultaneously with methyl
azide.
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Scheme 2. Proposed reaction pathways resulting from irradiation of TSMT monomer isolated in argon
5
6
matrix.
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A schematic drawing summarizing the proposed photochemistry of matrixꢀisolated TSMT is
presented in Scheme 2. As mentioned above, according to the identified photoproducts, two
photodegradation pathways can be postulated, both arising from cleavage of the tetrazolyl ring: the
dominant Pathway a involves photoinduced cleavage of the N18–N19 and N20–N21 bonds, with extrusion
of molecular nitrogen, leading to production of diazirine 5 and thiocarbodiimide 6. A nitreneꢀtype
28,29
intermediate can be postulated to be involved in this processes,
though we could not experimentally
observe it. The concentrations of both the thiocarbodiimide 6 and diazirine 5 increased continuously with
time, with no apparent sign of their further chemical transformations. The minor Pathway b involves
photoinduced cleavage of the C17–N21 and N18–N19 bonds of the tetrazole ring, leading to formation of
thiocyanate 7 and methyl azide 8; the thiocyanate remains photostable, while methyl azide undergoes
subsequent decomposition into CNH 9.
An additional note shall here be made in relation with the observed photostability of the diazirine
5. The recent study of Nunes et al. on the photolysis of the predominant 2H tautomeric form of matrix
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isolated 5ꢀmethyltetrazole, with light of λ= 222 and ~328 nm, demonstrated that elimination of molecular
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nitrogen leads to the initial production of a nitrile imine, after a few seconds of irradiation, which can then
7
8
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isomerize to a 1Hꢀdiazirine, this species undergoing subsequent photoconversion into a carbodiimide .
9
In the present work, no evidence of formation of a nitrile imine was found, while the photoproduced
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diazirine revealed to be photostable even after 1 h of irradiation (λ= 265 nm). This might indicate, as
suggested by Begué et al. 28, that the photochemistries of 1Hꢀ and 2Hꢀ tautomers of substituted tetrazoles
might be different. Indeed, the present results on TSMT shall be compared more directly with those
28
reported by the last authors on 5ꢀphenylꢀ1Hꢀtetrazole, since TSMT is also a 1ꢀsubstituted tetrazole
derivative. Begué et al. observed that, upon photolysis (λ= 254 nm) of 5ꢀphenylꢀ1Hꢀtetrazole, this
tautomer did not produce the nitrile imine directly, contrarily to what was observed for the 2Hꢀtautomer 28
(and also in the case of the 2Hꢀtautomer of 5ꢀmethyltetrazole 29, as mentioned above). The suggested
route for production of the nitrile imine starting from the 1H-tautomer of 5ꢀphenyltetrazole considered by
Begué et al. implies the initial production of a diazirine intermediate, despite they were not able to
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observe this species (see Scheme S1) . The present observation of diazirine 5 as a stable photoproduct
seems to confirm the proposition of Begué et al. 28. The photostability of 5 is, probably, due to both the
stabilizing effect of the electron rich sulphur substituent of the tetrazole ring and presence of the methyl
substituent, which preclude the photoinduced rearrangement of the diazirine 5 under the experimental
conditions used.
Conclusions
A combined matrix isolation FTIR and theoretical study of the molecular structure and
photochemistry of 5ꢀthiosaccharyl 1ꢀmethyltetrazole was performed. The conformational preferences of
TSMT in both matrix isolated situation and in the crystalline phase were also investigated.
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According to the performed DFT calculations, TSMT has two conformers, I and II. However, the
higher energy form (II) has a relative energy of ca. 15 kJ molꢀ1 and only conformer I is significantly
populated in the gas phase. As it could be expected, this conformer was found to be the only detectable in
the cryomatrices (15 K) prepared from the vapor of the compound at 100 ºC. The infrared spectrum of
matrix isolated TSMT was fully assigned based on the O3LYP/6ꢀ311++G(3df,3pd) spectra calculated for
conformer I.
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In the crystal, TSMT was also found to exist in form I. In the crystalline phase, each molecule of
TSMT forms four hydrogen bonds with three neighbor molecules, and a network of TSMT dimers is
clearly observed. In the dimers, the two molecules of TSMT are disposed in a headꢀtoꢀtail arrangement,
with the tetrazole moiety of one molecule overlapping the thioimino moiety of the other (and vice versa),
at the inner part of the dimer, while the aromatic rings are directed to the extremities. Formation of dimers
is possibly due to πꢀ π interactions between complementary tetrazole ring and thioimino moieties. The
oligomeric structure results from an intricate network of weak hydrogen bonds formed between
consecutive dimers (with involvement of both molecules of the dimer) and adjacent dimers.
Upon irradiation of the matrix isolated compound with UV light (λ= 265 nm), two
photodegradation pathways were observed, both arising from cleavage of the tetrazolyl ring: Pathway (a)
involves photoinduced cleavage of the N1ꢀN2 and N3ꢀN4 bonds, with extrusion of molecular nitrogen,
leading to production of diazirine 5 and thiocarbodiimide 6. The concentration of these two photoproducts
increases with time, and there is no apparent sign of their photodegradation, even after almost complete
consumption of the reagent (TSMT). The observed photostability of the photoproduct diazirine 5 under
the conditions used is possibly ascribable to a stabilizing effect of the electronꢀrich sulphur substituent
and the presence of the 1ꢀmethyl substituent. Pathway (b) involves photoinduced cleavage of the C5ꢀN1
and N4ꢀN3 bonds, leading to thiocyanate 7 and methyl azide 8 as primary photoproducts. Thiocyanate 7
remains photostable while methyl azide 8 originates CNH 10, in a secondary photoreaction.
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Experimental Section
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General Methods
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A convergent synthetic approach was followed to prepare TSMT, as depicted in Scheme 1. Solvents
for extraction were of technical grade. When required, solvents were freshly distilled from appropriate
drying agents before use. Melting points were recorded and are uncorrected. Mass spectra were obtained
by electron ionization (EI) at 70 eV. NMR (400 MHz) spectra were measured using TMS as the internal
reference (δ = 0.0 ppm).
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Preparation of 3-chloro-1,2-benzisothiazole 1,1-dioxide
Saccharyl chloride (3ꢀchloroꢀ1,2ꢀbenzisothiazoleꢀ1,1ꢀdioxide, 2) was obtained from saccharin (10.2 g;
56 mmol) and phosphorus pentachloride (14.0 g; 66 mmol), heated at 180 ºC, using a procedure reported
37
1
previously : colorless needles (7.00 g; 63% yield); H NMR (400 MHz, CDCl3): δ 7.95ꢀ7.82 (m, 4H);
elemental analysis: C, 41.5%, H, 2.0%, N, 6.9% (found), C, 41.7%, H, 2.0%, N, 7.0% (calculated for
C7H4NO2SCl).
Preparation of 1,2-benzisothiazole 1,1-dioxo 3-(5-thio-1-methyl) tetrazole (TSMT)
A mixture of anhydrous 5ꢀmercaptoꢀ1ꢀmethyltetrazole 1 (0.30 g; 2.56 mmol) and 3ꢀchloroꢀ1,2ꢀ
benzisothiazole 1,1ꢀdioxide 2 (0.53 g, 2.56 mmol) in dry THF (20 mL) was stirred at 60 ºC, under a
nitrogen atmosphere. The reaction was monitored by TLC (toluene/acetone; 5:3). After 24 h, the solvent
was evaporated under reduced pressure and the remaining solid was dried under vacuum at room
temperature. Crystallization from chloroform/THF (2:1) yielded the required product as colorless crystals
1
(0.6 g; 83% yield), m.p. 222–224 ºC. H NMR (400 MHz, CDCl3):
δ 7.98–7.96 (m, 1H), 7.91–7.79 (m,
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3H), 4.22 (s, 3H); 13C NMR (101 MHz, CDCl3)
123.34, 122.94, 35.26; MS (EI): m/z 281 [M]+.
δ 170.35, 165.77, 139.44, 135.13, 134.35, 128.84,
6
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Matrix preparation, infrared spectroscopy and irradiation experiments
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The matrix was prepared by coꢀdeposition, onto the cooled CsI substrate of the cryostat, of the matrix
gas (argon 99.9998%, obtained from Air Liquid) and vapors of TSMT produced by sublimation in a
specially designed temperature variable miniꢀoven assembled inside the cryostat. The temperature of the
miniꢀoven used for sublimation of TSMT was ca. 100 ºC. The cryogenic system was based on a closeꢀ
cycle helium refrigeration system with a DEꢀ202A expander. The temperature of the CsI substrate during
deposition was 15 K.
The infrared spectra of matrixꢀisolated TSMT were obtained using a Fourier transform infrared
spectrometer equipped with a deuterated triglycinesulphate (DTGS) detector and a Ge/KBr beam splitter,
with 0.5 cmꢀ1 spectral resolution. The roomꢀtemperature IR spectrum (4000ꢀ400 cmꢀ1) of solid
polycrystalline TSMT in a KBr pellet was obtained using a Fourier Transform spectrometer equipped
with a DTGS detector and Zn/Se optics. Data collection was performed with 128 scans and 4 cmꢀ1
spectral resolution.
The matrix was irradiated through the outer quartz window of the cryostat, with the frequencyꢀdoubled
signal beam of the QuantaꢀRay MOPOꢀSL pulsed (10 ns) optical parametric oscillator (FWHM ~0.2 cmꢀ1,
repetition rate 10 Hz, pulse energy ~5.8 mJ) pumped with a pulsed Nd:YAG laser.
Crystallografic analysis
Xꢀray diffraction data were collected using a small single crystal. The crystallographic structure was
solved by direct methods using SHELXSꢀ97 38. Refinements were carried out with the SHELXLꢀ97
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package . All refinements were made by fullꢀmatrix leastꢀsquares on F2 with anisotropic displacement
parameters for all nonꢀhydrogen atoms (see Table S3 for details). All hydrogen atoms could be located on
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a difference Fourier synthesis; their positions were refined as riding on parent atoms with an isotropic
temperature factor using SHELXLꢀ97 defaults that constrain these atoms to idealized positions. Xꢀray
data collection and processing parameters for TSMT are presented in Table S3.
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Computational methods
Quantum chemical calculations were performed at the DFT level of theory using either the valence
doubleꢀζ polarized 6ꢀ31++G(d,p) or the extended valence tripleꢀζ polarized 6ꢀ311++G(3df,3pd) basis set
40–43 and the B3LYP or O3LYP functionals 44,45. Inclusion of both diffuse and polarization functions in the
basis sets is required for a more accurate approximation to the calculated infrared spectra, since
vibrational modes involving hypervalent S atoms (in particular the >SO2 stretching and bending modes)
are known not to be correctly predicted at a lower level of approximation 46,47. The use of the
DFT/B3LYP or O3LYP method with the 6ꢀ311++G(3df,3dp) basis set was found to be appropriate to
attain reliable results at moderate computational effort 12,13,46,47. Geometries were optimized using the
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Direct Inversion in the Iterative Subspace (DIIS) method . The transition state was located using the
synchronous transit quasiꢀNewton method (QST3 implementation)49,50. The optimization of geometries
was followed by harmonic vibrational calculations undertaken at the same theory level. The nature of the
obtained stationary points was checked through analysis of the corresponding Hessian matrix. The
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calculated harmonic vibrational frequencies (scaled by the factor 0.978 for the B3LYP functional and
by the factor 0.992 for the O3LYP functional 13) were used to assist the analysis of the experimental
spectra. All calculations were performed with the Gaussian 09 suite of programs 52.
Associated Content
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Supporting Information: Figures showing H NMR and C NMR spectra of TSMT, the Xꢀray and IR
spectrum of the TSMT crystal and calculated infrared spectra for TSMT and relevant photoproducts, and
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Tables with experimental (Xꢀray, single crystal) details of the crystallographic studies and calculated
geometrical parameters of TSMT, including Cartesian coordinates, electronic energies and calculated
infrared spectra for all optimized structures, at the B3LYP/6ꢀ311++G(3df,3pd) and O3LYP/6ꢀ
311++G(3df,3pd) levels are provided as supporting information. This material is available free of charge
via the Internet at http://pubs.acs.org.
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Acknowledgements
The authors gratefully acknowledge Fundação para a Ciência e a Tecnologia, Portugal (Project PEstꢀ
C/MAR/LA0015/2013), coꢀfunded by QRENꢀCOMPETEꢀUE, and CCMAR for generous financial
support. The Coimbra Chemistry Centre (CQC) and CEMDRX are also supported by FCT through
projects PEstꢀOE/QUI/UI0313/2014 and PEstꢀOE/FIS/UI0036/2014, respectively. A.I. acknowledges
FCT for the award of a doctoral Grant (SFRH/BD/90435/2012). A.B. acknowledges FCT for the award of
a PostꢀDoctoral Grant (SFRH/BPD/66154/2009).
Author Information
Corresponding Author
E-mail: mcristi@ualg.pt
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