1,2,4,6-Thiatriazinyl radicals and dimers: Structural and electronic tuning through heteroaromatic substituent modification

Article abstract of DOI:10.1021/acs.cgd.5b00296

The functionalization of thiatriazinyl (TTA) radicals with pyridyl and thienyl moieties is described, and their influence at both the molecular and solid-state level has been investigated. Comparative electron paramagnetic resonance studies of 3,5-bis-(2-pyridyl)-1,2,4,6-thiatriazinyl (Py₂TTA) and 3,5-bis-(2-thienyl)-1,2,4,6-thiatriazinyl (Th₂TTA) reveal the impact of heteroaromatic substitution on the electronic structure, which is supported by density functional theory calculations. Single crystal X-ray analysis emphasizes the importance of intermolecular contacts on the crystal packing and demonstrates the potential for structural control through S - -N′ and N - -HC′ interactions, which are enhanced in Py₂TTA and Th₂TTA by the presence of the pyridyl and thienyl groups, respectively. This work represents a fundamental study of heterocyclic aromatic substitution in thiazyl-based radicals and investigates how varying these functional groups influences the molecular properties and long-range order within the supramolecular structure.

Full text of DOI:10.1021/acs.cgd.5b00296

Article
pubs.acs.org/crystal
1,2,4,6-Thiatriazinyl Radicals and Dimers: Structural and Electronic
Tuning through Heteroaromatic Substituent Modification
Nathan J. Yutronkie,†,‡ Alicea A. Leitch,†,‡ Ilia Korobkov,† and Jaclyn L. Brusso*,†,‡
‡
†Department of Chemistry and Centre for Catalysis Research and Innovation, University of Ottawa, Ottawa, Ontario K1N 6N5,
Canada
S
* Supporting Information
ABSTRACT: The functionalization of thiatriazinyl (TTA)
radicals with pyridyl and thienyl moieties is described, and their
influence at both the molecular and solid-state level has been
investigated. Comparative electron paramagnetic resonance
studies of 3,5-bis-(2-pyridyl)-1,2,4,6-thiatriazinyl (Py2TTA)
and 3,5-bis-(2-thienyl)-1,2,4,6-thiatriazinyl (Th2TTA) reveal
the impact of heteroaromatic substitution on the electronic
structure, which is supported by density functional theory
calculations. Single crystal X-ray analysis emphasizes the
importance of intermolecular contacts on the crystal packing
and demonstrates the potential for structural control through
S---N′ and N---HC′ interactions, which are enhanced in
Py2TTA and Th2TTA by the presence of the pyridyl and
thienyl groups, respectively. This work represents a fundamental study of heterocyclic aromatic substitution in thiazyl-based
radicals and investigates how varying these functional groups influences the molecular properties and long-range order within the
supramolecular structure.
ligands. Furthermore, variation of the aromatic substituents,
from phenyl groups to heterocycles containing nitrogen,
oxygen, or sulfur, facilitates the development of chelating
paramagnetic ligands, thus enabling coordination to a
potentially diverse range of transition metals and lanthanides.
The first report of a TTA radical was described by Markovskii
et al. using electron paramagnetic resonance (EPR) spectro-
scopy;22 however, it was not until 1984 that Oakley and co-
workers first isolated and structurally characterized 3,5-bis-
(phenyl)-1,2,4,6-thiatriazinyl (Ph2TTA, Chart 1).23 Since then,
both symmetrically and asymmetrically substituted TTA
INTRODUCTION
■
For some time stable radicals have been recognized as attractive
building blocks in the development of molecular conductors
and magnetic materials, in addition to their use as spin bearing
ligands in transition metal complexes.1−10 The exploration of
organic and organo-main group radicals in the aforementioned
applications represents a challenging yet fruitful research area,
which drives the design and development of new open-shell
materials. In that regard, heterocyclic thiazyl radicals are
perhaps the most promising candidates as the S−N unit not
only affords an unpaired electron, but the incorporation of
heavy heteroatoms into planar π-conjugated frameworks also
provides the potential for structural control through S---N′ and
N---HC′ interactions and better opportunities for enhanced
magnetic and electronic interactions by virtue of greater orbital
overlap.11−13 These features have attracted researchers in the
direction of incorporating S−N moieties into π-conjugated
frameworks, and a number of heterocyclic thiazyl radicals have
been shown to crystallize as discrete radicals demonstrating
interesting charge transport14,15 and magnetic15−18 properties.
Others still have been reported to exhibit heat- and light-
induced dimer-to-radical interconversions with potential
applications in magnetic switches.19,20
Chart 1
Concerning their potential as spin bearing ligands, multi-
dentate thiazyl radicals have proven to be an attractive building
block.21 In this regard, the thiatriazinyl radical (TTA)
represents an archetypal system, as the general structure is
ideal for the development of N-coordinate paramagnetic
Received: March 2, 2015
Revised: March 30, 2015
© XXXX American Chemical Society
A
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radicals have been reported, most of which are partially
functionalized by a phenyl group.23−26 With respect to metal
coordination, Boere and co-workers have explored the reactivity
́
aqueous, basic, and thermal conditions. Its only instability
appears to be in acidic conditions, in which the thiatriazine ring
opens and regenerates an N-bridgehead-1,2,5-thiadiazonium
salt.30
of symmetric (Ph2TTA) and asymmetric (3-phenyl-5-trifluor-
omethyl-TTA) derivatives with [Cr(Cp)(CO3)2].27,28 As may
be expected, the substituents played a role in the coordination
as Ph2TTA afforded an η1-adduct bonded through the sulfur
atom, whereas the asymmetric ligand resulted in an η2 SN
linkage. Interestingly, in both cases, coordination through the
central nitrogen of the TTA ring did not occur. It is anticipated
that heteroaromatic substituents, such as in Py2TTA and
Th2TTA (Chart 1), would promote such coordination through
the chelating effect rendering the N−S−N portion of the
heterocyclic ring devoid of substituents. This would therefore
permit these heteroatoms to engage in supramolecular
interactions, which has been shown to direct crystal packing
in the solid-state and facilitate conductive or magnetic exchange
pathways in thiazyl based radical complexes.21
Since deprotonation was not feasible, treatment with a
relatively strong oxidant such as bromine was considered in
order to prepare 3,5-bis-(2-pyridyl)-1-bromo-1,2,4,6-thiatria-
zine. This was not only expected to eliminate the problem
associated with the proton on the central TTA ring, but also to
provide a compound that can be treated with a suitable
reductant to afford Py2TTA, similar to the route employed for
the phenyl-based derivatives.23 However, while reaction with
bromine formed the desired 1-bromo-1,2,4,6-thiatriazine, the
generation of HBr led to protonation of the pyridyl nitrogen
atoms affording the 3,5-bis-(2-pyridinium)-1-bromo-1,2,4,6-
thiatriazine dication, isolated as a mixed bromide/tribromide
salt (3, Scheme 1). Crystallization of this material from
As a first step toward the development of heteroaromatic
functionalized TTA, we recently reported the synthesis of
Py2TTAH, a task which involved the intermediacy of the
mono- and bis-N-bridgehead-1,2,5-thiadiazolium salts 1 and 2.
This unusual noninnocent behavior of pyridyl nitrogens has
only rarely been observed;29 thus the apparent proclivity of the
pyridyl ligands to form N-bridgehead-heterocycles represents
an important finding and holds potential in the design of novel
open and closed shell heterocyclic compounds. In our initial
communication, the Py2TTA radical was generated in situ and
characterized by EPR spectroscopy. We have now successfully
isolated and characterized Py2TTA in the solid-state. In
addition, the synthesis and characterization of the bis-thienyl
derivative Th2TTA are also reported herein. Comparison of the
EPR spectra of the two radicals, along with computational
studies, reveals the impact of heteroaromatic substitution on
the electronic structure of TTA radicals. Moreover, crystal
analysis emphasizes the importance of intermolecular contacts
on the crystal packing and demonstrates the potential for
structural control through S---N′ and N---HC′ interactions,
which are significantly influenced by the presence of the pyridyl
and thienyl groups in Py2TTA and Th2TTA, respectively. This
work therefore represents a fundamental study of heterocyclic
aromatic substitution in thiazyl-based radicals and investigates
how varying these functional groups influences the molecular
properties and long-range order within the supramolecular
structure.
Scheme 1. Synthesis of the Bis-(2-pyridyl)-1,2,4,6-
thiatriazinyl Radical
a
a
Reagents and conditions: (a) NH3, MeCN; (b) S2Cl2, MeCN, RT;
RESULTS AND DISCUSSION
■
(c) S2Cl2, MeCN, reflux; (d) PhCl, reflux; (e) Ph3Sb, MeCN; (f) Br2,
MeCN, RT; (g) Br2, MeCN, DMAP, reflux; (h) NCS, DMAP, MeCN,
RT.
Synthesis of Py2TTA. Our initial synthetic work en route to
the closed-shell 3,5-bis-(2-pyridyl)-4-hydro-1,2,4,6-thiatriazine
(Py2TTAH) elucidated the preparation and structural charac-
terization of two unusual N-bridgehead-1,2,5-thiadiazolium salts
1 and 2.30 While the in situ preparation of Py2TTA through
oxidation of Py2TTAH with half an equivalent of iodine in the
presence of 4-dimethylaminopyridine (DMAP) was successfully
achieved, isolation of the radical in the solid state proved
difficult due to the formation of intractable mixtures of DMAP
salts. Initial attempts to overcome this problem included
deprotonation of Py2TTAH prior to oxidation in order to
isolate the anionic derivative; however, the proton affinity of
the central nitrogen on the TTA ring is quite high and,
regardless of the base used (e.g., DMAP, proton sponge, KOH,
etc.), generation of anionic Py2TTA− was unsuccessful. In fact,
Py2TTAH is surprisingly robust with a high tolerance toward
dichloroethane (DCE) afforded yellow needles suitable for X-
ray analysis confirming the structural identity of 3 (Supporting
Information, Figure S1).
To avoid protonation of the pyridyl nitrogen atoms,
Py2TTAH was treated with bromine in the presence of
DMAP, affording the desired compound 3,5-bis-(2-pyridyl)-1-
bromo-1,2,4,6-thiatriazine (4). However, while this confirmed
that DMAP successfully inhibits protonation, the product 4
cocrystallized with DMAP·HBr3 (Supporting Information,
Figure S2), and attempts to reduce this mixed salt with
triphenylantimony resulted in the regeneration of Py2TTAH.
In order to isolate the Py2TTA radical, we therefore turned
to the use of oxidants, which would not generate protons as a
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side-product. Oxygen, of course, also had to be avoided due to
the proclivity of TTA radicals to form N-hydro-1-oxo-1,2,4,6-
thiatriazines (Supporting Information, Figure S3). Various mild
oxidants, including 2,3-dichloro-5,6-dicyanobenzoquinone
(DDQ), N-bromosuccinimide (NBS), and N-chlorosuccini-
mide (NCS) were explored, and of these NCS proved to be the
most effective affording Py2TTA as a microcrystalline copper
colored solid. Recrystallization of Py2TTA from hot acetonitrile
(MeCN) afforded crystals suitable for X-ray analysis (vide
infra).
Scheme 2. Synthesis of the Bis-(2-thienyl)-1,2,4,6-
thiatriazinyl Radical
a
Synthesis of Th2TTA. In regard to Th2TTA, the absence of
basic sites on the thienyl residue suggested that the synthesis
would resemble that of the phenyl derivatives more closely than
that of the pyridyl derivative. Nonetheless, regardless of the
reaction pathway the most successful preparative routes include
condensation of imidoylamidines with excess sulfur hal-
ides,26,30,31 which therefore requires N-2-thienylimidoyl-2-
thienylamidine (5) as a starting point. Although several
preparative routes have been reported, unsubstituted N-
imidoylamidines remain a synthetic challenge and have received
relatively little attention in the literature. The classical route
involves the reaction between an N-thioimidoylamidine with an
amidine,32 whereas an alternative avenue includes ring opening
of triazines with lithium bis(trimethylsilyl)amide.33 More
recently, however, nitriles activated through inductive effects
have been shown to undergo nucleophilic addition reactions
with ammonia or amidine.26,30,34 Our initial attempts to
prepare 5 therefore focused on the reaction of 2-cyanothio-
phene with NH3(g), which proved successful for the pyridyl
derivative N-2-pyridylimidoyl-2-pyridylamidine.30 Unfortu-
nately, negligible yields of the desired product were obtained
regardless of the temperature or pressure conditions used. This
may be attributed to the electron donating character of the
thiophene moiety, which deactivates the nitrile to nucleophilic
addition. Similar challenges were encountered when effort was
directed toward the reaction between the free-base 2-
thienylamidine with 2-cyanothiophene. This was overcome by
strengthening the nucleophilicity of the amidine through
deprotonation via lithium diisopropylamide (LDA) affording
5 as a brown oil. Although this route can be used to prepare 5,
isolation of N-2-thienylimidoyl-2-thienylamidine was problem-
atic leading to relatively poor purified yields. This was alleviated
by changing the base to sodium hydride and employing a very
polar aprotic solvent (e.g., DMSO),35 affording 5 in much
higher yields and greater purity. Subsequent condensation of 5
with excess sulfur monochloride afforded 3,5-bis-(2-thienyl)-1-
chloro-1,2,4,6-thiatriazine (6) as a bright yellow crystalline
material (Scheme 2). Because of its limited stability, 6 was used
directly without further purification in the reduction reaction
with triphenylantimony resulting in a dark green microcrystal-
line solid. Crystallization from hot DCE afforded Th2TTA as
dark red platelets suitable for single crystal X-ray analysis.
EPR Spectroscopy. We previously reported the in situ EPR
spectrum of Py2TTA in dichloromethane (DCM), which
consisted of a complex multiplet that was simulated using a
model based on hyperfine coupling to two equivalent and one
a
Reagents and Conditions: (a) (i) NaOMe, MeOH, RT; (ii) NH4Cl;
(b) NaH, 2-thiophenecarbonitrile, DMSO, RT; (c) S2Cl2, MeCN,
reflux; (d) Ph3Sb, MeCN, RT.
hot toluene (Figure 1), which was necessary to provide the
elevated temperatures required to dissolve Py2TTA. By
Figure 1. Experimental (top) and simulated (middle) EPR spectrum
of (a) Py2TTA in toluene collected at 40 °C (g = 2.0068; SW = 3.5
mT; LW = 0.024 mT) and (b) Th2TTA in DCM collected at RT (g =
2.0066; SW = 3.5 mT; LW = 0.019 mT). (bottom) Experimentally
derived and UB3LYP/EPR-II/6-31G(d)//UB3LYP/6-311G(d,p) cal-
culated (in parentheses) coupling constants aN (in mT).
contrast, the X-band EPR spectrum of Th2TTA recorded in
DCM at room temperature was much more easily obtained,
indicating that dissolution and/or dissociation of Th2TTA is
clearly more extensive than in the case of Py2TTA. The EPR
for Th2TTA exhibits a seven-line pattern that is consistent with
the DFT calculated spin distribution for a TTA radical. It was
simulated using a model based on hyperfine coupling to three
quasi-equivalent 14N nuclei (aN = 0.392 mT and 0.386 mT),
unique 14N nuclei (experimentally derived constants: aN
=
0.372 mT; aN = 0.427 mT; calculated coupling constants: aN =
0.347 mT; aN = 0.420 mT).30 Interestingly, when recrystallized
Py2TTA was dissolved in degassed DCM, the solution
remained EPR silent. While this finding was initially surprising,
an EPR spectrum consistent with that obtained from in situ
preparation of Py2TTA was eventually achieved by switching to
which may be compared with DFT calculated values (aN
=
0.377 mT and 0.367 mT). The near equivalence of the aN
values here is in contrast to Py2TTA (see Figure 1) and is
consistent with the expected electronic differences between the
thienyl and pyridyl ligands, the latter being a more potent
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electron-withdrawing group. As a result, the Py2TTA spin
density is more extensively polarized away from the N−S−N
region of the TTA core. Similar trends have been observed in
the EPR spectra of p-substituted aryl TTA radicals.25,36 For
example, spin density in TTA systems is equally distributed
over the three nitrogen atoms when phenyl or 4-MeOC6H4
substituents are employed, but becomes strongly polarized
toward the central nitrogen atom (in the TTA ring) when
electron-withdrawing 4-NO2C6H4 groups are present (see
Supporting Information Table S8).25 Thus, comparison
between the EPR spectra of Py2TTA and Th2TTA confirms
the expectation that heterocyclic aromatic substitution
influences the electronic structure of TTA radicals. Further-
more, this study also speaks to the solubility and, accordingly,
the strength of intermolecular interactions observed in the
pyridyl derivative compared to that of Th2TTA, which is
further exemplified by inspection of the crystal structures (vide
infra).
Py2TTA and Th2TTA, the two TTA rings that form the
cofacial dimer exhibit shallow boat-like configurations due to a
slight displacement of the nitrogen and sulfur atoms that reside
along the center of the molecular framework (i.e., the sulfur and
nitrogen atom para to it). It is interesting to note that similar
structural features (e.g., cofacial dimer, boat-like configuration)
were described for the phenyl derivative Ph2TTA,23 indicative
of minimal structural variation of the TTA ring and cofacial
dimers upon heterocyclic substitution.
While a number of similarities exist between the structures of
Py2TTA and Th2TTA, the impact of heteroaromatic
substitution becomes apparent when looking at the supra-
molecular contacts, which direct packing in the solid state. For
example, within the asymmetric unit of Py2TTA, which belongs
to the triclinic space group P1 and contains two molecules,
̅
many unique close contacts that are within or nominally larger
than the sum of the van der Waals separation exist,38,41 whereas
crystals of Th2TTA belong to the monoclinic space group P21/
n and consists of fewer intermolecular interactions (see
Supporting Information, Tables S3−S6). Although both
compounds form tetramers in the solid state, in Py2TTA
these head-to-head dimer-of-dimers are held together by
numerous S---N′ interactions, whereas only one S---S′ and
three unique S---N′ contacts exist in the structure of Th2TTA
(Figure 2 and Supporting Information, Table S4). Furthermore,
the tetramers of Py2TTA form a brick-like array held together
by a series of close π-contacts between the carbon atoms on the
pyridyl rings and either the pyridyl or TTA ring on adjacent
tetramers (Figure 3). A bird’s-eye view of the Py2TTA brick-
like array reveals additional close contacts, many of which stem
from hydrogen bonding between adjacent pyridyl rings, holding
the molecules together in a 2D fashion (Figure 3c).
X-ray Crystallography. To explore the impact of
heteroaromatic substitution (i.e., pyridyl vs thienyl) on crystal
packing, single crystals of Py2TTA and Th2TTA were analyzed
by X-ray diffraction (Table 1). Two views of the solid-state
Table 1. Crystal Data for Py2TTA and Th2TTA
Py2TTA
Th2TTA
formula
fw
C24H16N10S2
508.59
C20H12N6S6
528.72
crystal system
space group
a (Å)
triclinic
monoclinic
P21/n
P1
̅
8.3797(3)
10.5524(3)
13.0609(4)
81.4929(12)
84.0073(13)
75.1622(12)
1101.45(6)
2
11.3840(6)
10.7358(6)
17.9318(9)
90
b (Å)
c (Å)
Similar to Py2TTA, tetramers of Th2TTA also form a brick-
like arrangement held together primarily through π−π
interactions (e.g., C---C′ and C---S′); however, contrary to
Py2TTA, the structure of Th2TTA consists of alternating layers
of π-stacked tetramers that are rotated with respect to one
another by 51.60(7)° forming a cross-braced configuration
(Figure 4a). These layers are held together through lateral N---
H′, S---H′, and S---C′ interactions (Figure 4b). While a number
of contacts exist within the thienyl derivative, there are fewer
than that of the pyridyl derivative. Such structural features may
explain the enhanced solubility and presence of an EPR signal
for Th2TTA compared with Py2TTA, which is EPR silent at
room temperature. While the electron-withdrawing vs donating
substituents have little structural impact on the molecular
framework of the thiatriazinyl rings, heteroaromatic substitution
(i.e., pyridyl vs thienyl) significantly influences the long-range
molecular order and, therefore, the supramolecular architec-
tures. This finding is further supported by comparing the crystal
packing of the phenyl derivative (Ph2TTA) with that of the
pyridyl (Py2TTA) and thienyl (Th2TTA) analogues, where
even fewer supramolecular contacts exist.23
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
98.461(3)
90
2167.7(2)
4
Dcalc (mg/m3)
1.533
1.620
T (K)
μ (mm−1
200
200
)
0.281
0.993
2θmax (deg)
28.304
24.812
no. of total reflections
no. of unique reflections
Rint
15900
25114
5341
3671
0.0229
0.0629
R1, wR2 (on F2)
0.0412, 0.1024
0.0462, 0.0996
structures, that of the cofacial π-dimer and dimer-of-dimers
(i.e., tetramers), are shown in Figure 2. Focusing on the
molecular framework, a number of similarities exist between the
two compounds. For example, crystals of Py2TTA and
Th2TTA consist of discrete pairs of TTA rings linked in a
cofacial manner containing short S---S contacts of 2.592(6) Å
and 2.647(1), respectively. While these distances are longer
than a disulfide bond (2.06 Å in S8),37 they are substantially
shorter than the sum of the van der Waals radii (3.6 Å)38 and
are comparable to the transannular S---S interactions often
observed in binary sulfur nitrides, which range from 2.43 to
2.65 Å (e.g., 2.43 Å for bis-(Me2N)2-DTTA;39 2.58 Å for
S4N4).40 As well, since the two TTA rings are perfectly eclipsed,
numerous close S---N, N---N, and C---C interactions exist
within the dimer as a result of the close S---S contact (see
Supporting Information Table S1). In the structure of both
SUMMARY AND CONCLUSIONS
■
The isolation and solid-state characterization of Py2TTA has
now been achieved. In addition, the thienyl derivative Th2TTA
was prepared and structurally characterized. From a synthetic
standpoint, the nature of the heteroatom in the aromatic
substituents plays a pivotal role in the preparative pathway. On
the basis of the results described here, the generation of a wide
range of heteroaromatic TTA derivatives can be anticipated.
Comparison of the EPR spectra along with computational
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Figure 2. Two views of the dimer (a, b) and dimer-of-dimers (c, d) for Py2TTA (a, c) and Th2TTA (b, d) are shown with close intermolecular
contacts highlighted by dashed lines. S---S′ contacts are shown in red; C---C′ in black; N---N′ in blue; S---N′ in green; C---N′ in purple.
studies demonstrates the effect of heteroaromatic substitution
on the electronic structure, while crystal analysis emphasizes
the importance of intermolecular contacts on the crystal
packing. In this regard, although structural studies reveal several
similarities between Py2TTA and Th2TTA, the presence of
heteroatoms in the aromatic substituents significantly influen-
ces the crystal packing and, hence, the supramolecular
architectures. The enhanced intermolecular interactions
observed in Py2TTA and Th2TTA (compared to Ph2TTA)
result in more tightly bound structures, the consequence of
which was particularly noted in the EPR studies of Py2TTA, in
which a purified sample requires hot toluene to produce an
EPR signal.
Inclusion of heteroatoms in the aromatic substituents of
TTA not only influences the molecular and solid-state
properties of the discrete molecules, but it also facilitates
coordination to a variety of metals. Not only is this anticipated
to stabilize the unpaired electron thus preventing dimerization,
but it also provides an avenue to generate coordination
complexes that can take advantage of the unencumbered N−
S−N portion of the TTA ring, which can engage in
supramolecular interactions that are known to direct crystal
packing in thiazyl based ligands and provide conductive and
magnetic exchange pathways; such studies are currently being
pursued. In regard to coordination chemistry, a logical next step
would involve the preparation of the pyrimidyl derivative
Pm2TTA (Chart 2). Isolation of Pm2TTA would not only
facilitate multimetallic coordination, but the design of such a
ligand has significant potential toward the development of
extended coordination polymers and networks that contain
spin-bearing ligands. In regard to the thienyl derivative, the
availability of reactive α-sites enable extension of the thienyl
moieties such that oligothiophene functionalized TTA radicals
may be realized. Accordingly, the current Th2TTA system can
act as a prototype for the construction of a large series of
(Thn)2TTA derivatives. Preparation of the aforementioned
pyrimidine and oligothiophene functionalized TTA radicals is
currently underway.
EXPERIMENTAL METHODS
■
General Procedures and Starting Materials. The reagents
sulfur monochloride (Sigma-Aldrich) and triphenylantimony (TCI
America) were obtained commercially and used as received. Sodium
hydride (60% dispersion in mineral oil, Alfa Aesar) was washed with
hexanes prior to use. Acetonitrile (MeCN) and 1,2-dichloroethane
(DCE) were dried by distillation over P2O5 and stored over 4 Å
molecular sieves. Dimethyl sulfoxide (DMSO) was dried and stored
over 4 Å molecular sieves. 3,5-bis-(2-pyridyl)-4-hydro-1,2,4,6-thiatria-
zine (Py2TTAH),30 2-thiophenecarbonitrile42 and 2-thiophenecarbox-
amidine43 were prepared as outlined in the literature. All reactions
were performed under an atmosphere of dry nitrogen. Melting points
1
are uncorrected. H NMR and 13C NMR spectra were run in CDCl3
solutions at room temperature on the Bruker Avance 400 MHz and
Bruker Avance 300 MHz spectrometers. IR spectra of solid samples
were recorded on an Agilent Technologies Cary 630 FT-IR
spectrometer. Elemental analyses were performed by MHW
Laboratories, Phoenix, AZ 85018.
EPR Spectroscopy. The X-band EPR spectra of Th2TTA was
recorded at ambient temperature using a Bruker EMX-200
spectrometer on samples dissolved in degassed dichloromethane
and/or toluene. Hyperfine coupling constants were obtained by
spectral simulation using PEST Winsim and Bruker WinEPR
Simfonia.44
Crystal Growth. Yellow needles of 3 suitable for X-ray analysis
were grown by slowly cooling a saturated DCE solution. Orange
E
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Figure 3. Two views of the packing arrangement exhibited by crystals of Py2TTA demonstrating the (a) brick-like array and (b) superimposed π-
stacked tetramers with π−π interactions between adjacent tetramers highlighted by blue dashed lines. (c) Bird’s-eye view of the packing arrangement
of Py2TTA illustrating the N---H′ contacts (blue dashed lines) holding the molecules together in a 2D network. S---S dimer contacts are shown in
red; tetramer contacts in green.
blocks of 4 suitable for X-ray analysis were grown by slowly cooling a
saturated MeCN solution. Black cubes of 3,5-bis-(pyridyl)-4-hydro-1-
oxo-1,2,4,6-thiatriazine and transparent needles 3,5-bis-(thienyl)-2-
hydro-1-oxo-1,2,4,6-thiatriazine suitable for X-ray analysis were grown
by slowly cooling saturated MeCN solutions. Black blocks of Py2TTA
suitable for X-ray analysis were grown by slowly cooling a saturated
MeCN solution. Red platelets of Th2TTA suitable for X-ray analysis
were grown by slowly cooling a saturated DCE solution.
Crystallography. The data collection results for compounds 3, 4,
3,5-bis-(pyridyl)-4-hydro-1-oxo-1,2,4,6-thiatriazine, 3,5-bis-(thienyl)-2-
hydro-1-oxo-1,2,4,6-thiatriazine, Py2TTA and Th2TTA represent the
best data sets obtained in several trials for each sample. The crystals
were mounted on thin glass fibers using paraffin oil. Prior to data
collection, crystals were cooled to 200.15 K. The data were collected
on a Bruker AXS KAPPA single crystal diffractometer equipped with a
sealed Mo tube source (wavelength 0.71073 Å) and APEX II CCD
detector. The raw data collection and processing were performed with
the APEX II software package from BRUKER AXS.45 The diffraction
data for crystals of 3, 3,5-bis-(thienyl)-2-hydro-1-oxo-1,2,4,6-thiatria-
zine and Th2TTA were collected with a sequence of 0.3° ω scans at 0,
120, and 240° in φ. Because of the lower unit cell symmetry, in order
to ensure adequate data redundancy, diffraction data for 4, 3,5-bis-
(thienyl)-2-hydro-1-oxo-1,2,4,6-thiatriazine and Py2TTA were col-
lected with a sequence of 0.3° ω scans at 0, 90, 180, and 270° in φ.
The initial unit cell parameters were determined from 60 data frames
with 0.3° ω scan each, collected at the different sections of the Ewald
sphere. Semiempirical absorption corrections based on equivalent
reflections were applied.46 Systematic absences in the diffraction data
and unit cell parameters were consistent with triclinic P1 (No. 2) for
̅
compound 4, 3,5-bis-(pyridyl)-4-hydro-1-oxo-1,2,4,6-thiatriazine and
Py2TTA, monoclinic P21/c (No. 14) for compound 3, monoclinic P21
(No. 4) for 3,5-bis-(thienyl)-2-hydro-1-oxo-1,2,4,6-thiatriazine, and
monoclinic P21/n (No. 14, alternative settings) for Th2TTA. Solutions
in the centrosymmetric space groups for all the compounds except 3,5-
bis-(thienyl)-2-hydro-1-oxo-1,2,4,6-thiatriazine yielded chemically rea-
sonable and computationally stable results of refinement. Meanwhile,
F
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Article
Figure 4. (a) The cross-braced arrangement found for Th2TTA, which consists of alternating layers of π-stacked tetramers that are rotated with
respect to one another by 51.60(7)°. (b) A 90° rotated view of (a) illustrating the lateral N---H′, S---H′, and S---C′ interactions (blue dashed lines)
between the cross-braced layers. S−S dimer contacts are shown in red.
of the units (S(1)−S(2) = 2.59 Å). Both molecules are located in the
general positions in the unit cell.
Chart 2
The asymmetric unit for the structure of 3,5-bis-(thienyl)-2-hydro-
1-oxo-1,2,4,6-thiatriazine contains one target molecule located in a
general position. On the final stages of refinement, the numbers
suggested the presence of merohedral twinning in the structure. To
take this into account, simple TWIN and BASF instructions were
introduced, and the twinning parameter was refined to 0.0687. After
introduction of anisotropic refinement parameters for all the non-
hydrogen atoms, it was discovered that one of the five-member rings
(S(3), C(7)−C(9)) of the structure is disordered by 180°-rotation
around the C(2)−C(7) carbon−carbon bond. The atomic positions of
this fragment were split to model the disorder, and occupancy was
allowed to refine. On the latest stages of refinement, occupancy was
fixed at 61% − 39% providing satisfactory anisotropic thermal motion
parameters. To ensure proper geometry of the disordered moiety, a set
of geometric restraints (SAME) were introduced.
The structural model for Th2TTA demonstrates two molecules of
the target compound located in the general position of the unit cell.
Similar to the structure of Py2TTA, these two molecules reveal close
contact between two sulfur atoms (S(1)−S(4) = 2.65 Å). The
anisotropic refinement of the structure revealed possible disorder by
180° rotation around the carbon bonds for all four of the five-member
rings. To model the disorder, the positions of sulfur atoms within
these five-member rings and the positions of carbon atoms opposite to
sulfur were split, and the occupancy coefficients were allowed to refine.
On the later stages of refinement, occupancy coefficients were fixed at
the following values: S(2), C(3)−C(6) ring is rotated around C(1)−
C(3) bond with partial occupancies 90%: 10%; S(3), C(7)−C(10)
ring is rotated around C(2)−C(7) bond with partial occupancies 75%:
25%; S(5), C(13)−C(16) ring is rotated around C(11)−C(13) bond
with partial occupancies 80%: 20%; S(6), C(17)−C(20) ring is rotated
around C(12)−C(17) bond with partial occupancies 65%: 35%. To
ensure proper fragments, geometries and acceptable anisotropic
thermal displacement coefficients for two sets of geometry (SADI)
and thermal motions (EADP) restraints were used during the
refinement.
diffraction data for 3,5-bis-(thienyl)-2-hydro-1-oxo-1,2,4,6-thiatriazine
suggested non-centrosymmetric space groups for the structural model.
The structures were solved by direct methods, completed with
difference Fourier synthesis, and refined with full-matrix least-squares
procedures based on F2.
The structural model for 3,5-bis-(pyridyl)-4-hydro-1-oxo-1,2,4,6-
thiatriazine contains one target compound molecule located in general
positions. An initial unit cell determination procedure suggested that
the crystal mounted on the machine was non-merohedrally twinned.
Refinement of the structure confirmed the presence of the second
independent crystallographic domain. In order to find independent
orientation matrices 5423 reflections were collected from 3 sets of 50
frames each in the different sections of the Ewald sphere. The
collected reflection data were processed with CELL_NOW software47
and produced two independent orientation matrices. The data set was
reintegrated with two obtained matrices, treated for twinning
absorption corrections, and consecutive model refinement was
performed against HKLF 5 reflection data file. Twinning domain
ratio coefficient (BASF) was refined to 0.4102.
The asymmetric unit for compound 4 includes two target molecules
and one molecule of para-amino pyridine, all located in general
−
positions. The structure also incorporates two Br3 anions located in
the two independent inversion centers.
The diffraction data for the crystal of the complex 3 were collected
to 0.75 Å resolution. The absorption correction stage, that both R(int)
and R(sigma) exceed 35% for the data below 0.85 Å resolution. On the
basis of the R(sigma) value, data were truncated to 0.80 Å resolution
for refinement. The asymmetric unit for this crystallographic model of
3 consists of one target compound molecule, one Br3− anion, and one
separate Br− atom. All the fragments are located in the general
positions.
For all six compounds, positions of all hydrogen atoms were
calculated based on the geometry of related non-hydrogen atoms. All
hydrogen atoms were treated as idealized contributions during the
refinement. All scattering factors are contained in several versions of
The structural model of Py2TTA consists of two aligned molecules
of the target compound with close contact between two sulfur atoms
G
DOI: 10.1021/acs.cgd.5b00296
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Article
the SHELXTL program library, with the latest version used being
v.6.12.48
AUTHOR INFORMATION
■
Corresponding Author
Preparation of 3,5-Bis-(2-pyridyl)-1,2,4,6-thiatriazinyl (Py2TTA).
Under an inert atmosphere, Py2TTAH (0.500 g, 1.96 mmol), 4-
dimethylaminopyridine (0.136 g, 1.11 mmol) and N-chlorosuccini-
mide (0.134 g, 1.00 mmol) were combined and 5 mL of degassed
MeCN (3 freeze−pump−thaw cycles) was added affording a maroon
slurry. After 2 h, a fine brown solid was filtered in vacuo and washed
twice with 3 mL of MeCN. Crude yield = 0.175 g (0.688 mmol, 35%).
Recrystallization from MeCN afforded black-brown microcrystalline
material. IR: υmax (cm−1): 3050 (w), 3004 (w), 1641 (w), 1571 (w),
1530 (m), 1510 (m), 1489 (m), 1476 (m), 1461 (m), 1422 (m), 1396
(m), 1380 (s), 1345 (m), 1254 (m), 1231 (m), 1160 (w), 1112 (m),
1042 (w), 994 (m), 974 (w), 912 (w), 843 (w), 814 (w), 768 (s), 757
(m), 739 (s), 726 (s), 692 (m), 668 (w). Anal. Calcd for C12H8N5S: C,
56.68; H, 3.17; N, 27.54. Found: C, 56.90; H, 3.37; N, 27.66.
Preparation of N-2-Thienylimidoyl-2-thienylamidine (3). Sodium
hydride (1.71 g, 71.3 mmol) was added to a DMSO (86 mL) solution
of 2-thiophenecarboxamidine (6.00 g, 47.6 mmol) and 2-thiophene-
carbonitrile (5.20 g, 47.6 mmol), initially releasing hydrogen gas. After
stirring for 56 h at room temperature, the resulting dark brown
solution was quenched with 200 mL of water and extracted with
DCM. The organic phase was washed with water, dried over potassium
carbonate, and then filtered, and the solvent was removed under
reduced pressure affording 3 as a brown solid (9.26 g, 39.4 mmol,
83%). Pale yellow needles of 3 were obtained by recrystallization in
hexanes. mp 98−99 °C, 1H NMR (δ, CDCl3): 7.51 (2H, dd, J = 1.63,
Ar), 7.46 (2H, dd, J = 2.06, Ar), 7.09 (2H, dd, J = 2.94, Ar), 13C NMR
(δ, CDCl3): 162.09, 142.75, 130.10, 127.73, 126.43, IR νmax (cm−1):
3434.7 (m), 3263.3 (m), 1588.3 (s), 1522.5 (s), 1481.0 (s), 1422.6 (s),
1381.5 (s), 1319.7 (m), 1241.3 (w), 1167.1 (s), 1115.6 (w), 1046.8
(m), 978.2 (m), 910.2 (w), 856.2 (m), 826.8 (m), Anal. Calcd for
C10H9N3S2: C, 51.04; H, 3.86; N, 17.86, Found: C, 51.16; H, 3.73; N,
17.74.
*E-mail: jbrusso@uottawa.ca.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors wish to thank the University of Ottawa, the
Canadian Foundation for Innovation, and the National
Sciences and Engineering Council of Canada for financial
support. N.J.Y. acknowledges support through an Ontario
Graduate Scholarship.
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were dissolved in dry, degassed MeCN (three freeze−pump−thaw
cycles) resulting in a maroon slurry. After stirring for 1 h, a dark purple
solid was filtered in vacuo. Crude yield 1.66 g (3.14 mmol, 91%).
Recrystallization from DCE afforded Th2TTA as a dark red
microcrystalline solid. dec >135 °C, IR νmax (cm−1): 3089.3 (w),
1519.2 (m), 1464.0 (m), 1418.1 (s), 1403.8 (s), 1369.9 (br, s), 1340
(s), 1328.1 (s), 1235.8 (m), 1195.7 (br, s), 1142.1 (w), 1126.3 (w),
1079.9 (m), 1052.0 (m), 1031.1 (s), 1015.4 (s), 1000.9 (s), 939.5 (m),
910.1 (w), 862.8 (m), 852.7 (m), 798.3 (br, m), 764.3 (s), 747.2 (s),
736.7 (s), 710.4 (br, s). Anal. Calcd for C10H6N3S3: C, 45.43; H, 2.29;
N, 15,90, Found: C, 45.60; H, 2.47; N, 15.97.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallography, EPR spectroscopy, and computational studies.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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