Supramolecular Wiring of Benzo-1,3-chalcogenazoles through Programmed Chalcogen Bonding Interactions

Article abstract of DOI:10.1002/chem.201504328

The high-yielding synthesis of 2-substituted benzo-1,3-tellurazoles and benzo-1,3-selenazoles through a dehydrative cyclization reaction has been reported, giving access to a large variety of benzo-1,3-chalcogenazoles. Exceptionally, these aromatic heterocycles proved to be very stable and thus very handy to form controlled solid-state organizations in which wire-like polymeric structures are formed through secondary N?Y bonding interactions (SBIs) engaging the chalcogen (Y=Se or Te) and nitrogen atoms. In particular, it has been shown that the recognition properties of the chalcogen centre at the solid state could be programmed by selectively barring one of its σ-holes through a combination of electronic and steric effects exerted by the substituent at the 2-position. As predicted by the electrostatic potential surfaces calculated by quantum chemical modelling, the pyridyl groups revealed to be the stronger chalcogen bonding acceptors, and thus the best ligand candidate for programming the molecular organization at the solid state. In contrast, the thiophenyl group is an unsuitable substituent for establishing SBIs in this molecular system as it gives rise to chalcogen-chalcogen repulsion. The weaker chalcogen donor properties of the Se analogues trigger the formation of feeble N?Se contacts, which are manifested in similar solid-state polymers featuring longer nitrogen-chalcogen distances. Building benzo-1,3-chalcogenazoles: A versatile four-step synthesis of 2-substituted benzo-1,3-selenazoles and -tellurazoles starting from 2-bromoaniline is reported (see figure; R=alkyl, alkenyl, phenyl, furanyl, thiophenyl, pyridyl and ferrocenyl groups). The solid-state arrangement shows that, depending on the electronic and geometrical properties of the substituent at the 2-position, one can control the wiring organization through two- (X?Y) or three-atoms (X?Y?X) secondary bonding interactions.

Full text of DOI:10.1002/chem.201504328

DOI: 10.1002/chem.201504328
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&
Chalcogens |Hot Paper|
Supramolecular Wiring of Benzo-1,3-chalcogenazoles through
Programmed Chalcogen Bonding Interactions
Adrian Kremer,[a] Andrea Fermi,[a] Nicolas Biot,[a, c] Johan Wouters,[a] and
Davide Bonifazi*[a, b, c]
Abstract: The high-yielding synthesis of 2-substituted
benzo-1,3-tellurazoles and benzo-1,3-selenazoles through
a dehydrative cyclization reaction has been reported, giving
access to a large variety of benzo-1,3-chalcogenazoles. Ex-
ceptionally, these aromatic heterocycles proved to be very
stable and thus very handy to form controlled solid-state or-
ganizations in which wire-like polymeric structures are
formed through secondary N···Y bonding interactions (SBIs)
engaging the chalcogen (Y=Se or Te) and nitrogen atoms.
In particular, it has been shown that the recognition proper-
ties of the chalcogen centre at the solid state could be pro-
grammed by selectively barring one of its s-holes through
a combination of electronic and steric effects exerted by the
substituent at the 2-position. As predicted by the electro-
static potential surfaces calculated by quantum chemical
modelling, the pyridyl groups revealed to be the stronger
chalcogen bonding acceptors, and thus the best ligand can-
didate for programming the molecular organization at the
solid state. In contrast, the thiophenyl group is an unsuitable
substituent for establishing SBIs in this molecular system as
it gives rise to chalcogen–chalcogen repulsion. The weaker
chalcogen donor properties of the Se analogues trigger the
formation of feeble N···Se contacts, which are manifested in
similar solid-state polymers featuring longer nitrogen–chalc-
ogen distances.
Introduction
ducing the difference between the energy levels of the s(Y-
EWG) and s*(Y-EWG) molecular orbitals, thus favouring stron-
ger SBIs. Halogen bonding is certainly the most investigated
interactions of this class,[5–7] and its importance has been re-
cently demonstrated in different domains, ranging from mate-
rials to biological applications.[8–16]
The concept of secondary bonding interactions (SBIs) describes
interactions resulting from interatomic contacts that are longer
than the covalent single bonds but shorter than the sum of
van der Waals radii.[1,2] Depending on the context, these inter-
actions have been also called soft-soft, closed-shell, nonbond-
ing, semi-bonding, non-covalent, weakly bonding, or s-hole-in-
We can extend this type of interaction to all molecular struc-
tures in which an electron-deficient polarizable atom is pres-
ent. This includes electron-deficient chalcogen atoms, which
are known to form X···Y-EWG SBIs in the solid state[4,17–20] and
in solution.[21] Considering the experimental evidences and the
theoretical studies,[22–27] strong SBIs can be expected when de-
scending in the chalcogen group, with the Te-derived com-
pounds establishing the strongest interactions.[21] Among the
different chalcogenide derivatives, benzo-2,1,3-telluradiazoles
have certainly attracted the greatest interest because of their
unique ribbon-like self-organization in the solid state (Fig-
ure 1a) formed through strong N···Te SBIs (2.682(7)–
2.720(7) Š).[4,28–31] Although of great potential, these molecular
systems have remained confined to the fundamental research
because of their thermal instability and aptitude to undergo
hydrolysis under ambient conditions.
teractions.[3,4] Formally,
a secondary bond derives from
a n2(X)!s*(Y-electron withdrawing group (EWG)) donation, in
which the lone pair of an electro-donating atom X interacts
with an antibonding s* orbital of a Y-EWG bond, in which Y
and EWG stand for the polarizable atom and electron with-
drawing group, respectively. Typically, this is described by an
s-hole, a region of positive electrostatic potential located on
the Y atom at the opposite side of the Y-EWG bond. Descend-
ing the periodic table, the polarisability of atoms increases re-
[a] Dr. A. Kremer, Dr. A. Fermi, N. Biot, Prof. J. Wouters, Prof. Dr. D. Bonifazi
Namur Research College (NARC) and Department of Chemistry
University of Namur (UNamur), Rue de Bruxelles 61, Namur, 5000 (Belgium)
[b] Prof. Dr. D. Bonifazi
In contrast, although 2-substituted benzo-1,3-tellurazoles are
more chemically inert that their parent benzo-2,1,3-chalcoge-
nadiazoles, they have been only marginally studied for prepar-
ing self-organized functional materials. Only in the late
1980s[32] was the first and only X-ray structure of a 2-sustituted
benzo-1,3-tellurazole described. In the solid state, the molecule
organizes into wires held by intermolecular N···Te contacts (Fig-
Department of Pharmaceutical and Chemical Sciences
and INSTM UdR Trieste
University of Trieste, Piazzale Europa 1, Trieste 34127 (Italy)
[c] N. Biot, Prof. Dr. D. Bonifazi
School of Chemistry, Cardiff University, Park Place, CF10 3AT, Cardiff (UK)
E-mail: BonifaziD@cardiff.ac.uk
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504328.
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genadiazoles,[21] two electron deficient regions (s-holes) cen-
tred at the chalcogen Y atom, each situated at the terminus of
the CÀY bond, could be evidenced from the ESP surfaces. In
contrast to benzo-2,1,3-telluradiazoles, both benzo-1,3-selena-
zole and benzo-1,3-tellurazole features s-holes displaying dif-
ferent Vs,max values (Figure 2a).
Figure 1. Solid-state arrangements of a) benzo-2,1,3-telluradiazoles and
b) benzo-1,3-tellurazole in tapes and wires, respectively.
ure 1b).[32] A similar wire-like organization was also recently re-
ported by us with a phosphorescent 2-substituted ethynyl-
benzotellurazole.[33] Given these premises and considering the
ease of tuning their chemical and physical properties with
simple chemical modifications of the benzo ring, 2-substituted
benzo-1,3-chalcogenazoles can be privileged heterocyclic sys-
tems to engineer soft materials featuring wire-like arrangement
at the molecular scale. To shed further light on the use of X···Y
SBIs (Y=Se or Te) for mastering the supramolecular arrange-
ment at the solid state, we have developed a versatile protocol
to prepare 2-substituted benzo-1,3-selenazoles and benzo-1,3-
tellurazoles bearing alkyl, alkenyl, aryl and heteroaryl groups in
high yields. The solid-state arrangement shows that, depend-
ing on the electronic and geometrical properties of the sub-
stituent at the 2-position, one can control the wiring organiza-
tion through two- (X···Y) or three-atoms (X···Y···X) SBIs.
Figure 2. ESP (kcalmolÀ1, calculated using Gaussian 09 at B97-D3/def2-TZVP
level of theory) for a) unsubstituted chalcogenazoles displaying the two s-
holes (labelled as a and b, respectively) and b) furanyl, pyridyl and thiophen-
yl ligands. The red and blue indicate negative and positive charge densities.
To distinguish the s-holes, the following terminology will be
used throughout this manuscript: s-hole(a) and s-hole(b) for
describing positive electrostatic regions on the chalcogen
atom exposed on the side of the 4- and 2-positions, respec-
tively (Figure 2a). The calculated Vs,max values for benzo-1,3-se-
lenazole are +18.8 and +23.0 kcalmolÀ1 for the a and b s-
holes, respectively (Figure 2). Similar values were also obtained
for benzo-1,3-tellurazole, +21.3 and +24.5 kcal molÀ1 for the
a and b s-holes, respectively. This suggests that both benzo-
1,3-chalcogenazoles are suitable molecular synthons to under-
go SBI recognition. Although the differences in the Vs,max values
between the two s-holes are very small, the s-hole(b) in non-
substituted benzo-1,3-chalcogenazoles features slightly higher
electrostatic values, possibly anticipating selectivity in the rec-
ognition mode. Instead, the N atom of the chalcogenazole fea-
tures the most negative Vs,max values (À29.2 and À27.9 kcal
molÀ1, for the Se and Te derivatives, respectively), making it
the strongest basic site and thus a privileged acceptor atom
for establishing a chalcogen bonding interaction. This behav-
iour has been confirmed with 2-phenyl benzo-1,3-tellurazole at
the solid state.[32] Conversely, if we want to control the supra-
molecular architecture with SBIs engaging a different atom
than the N of the chalcogenazole, we have to use ligands fea-
turing basic sites with higher negative Vs,max values. Among the
different acceptors, we have focused our attention on the pyr-
idyl, furanyl and thiophenyl aromatic rings. Whereas the calcu-
lated Vs,max values are À18.2 and À35.5 kcalmolÀ1 for the O
and N atoms of the furane and pyridine structures, respectively
Results and Discussion
Programming the benzo-1,3-chalcogenazole synthons
Provided that a sufficiently basic heteroatom (X) is present in
the molecular structure of a substituted chalcogenazole, one
can expect to control the organization at the solid state of
functional molecular synthons through selected X···Y SBIs. In
particular, programmed supramolecular wire-like polymers
could be formed at the solid state, exploiting SBIs other than
those established through the chalcogenazole N atom. To ap-
praise the possibility of programming the benzo-1,3-chalcoge-
nazoles for controlling the solid-state arrangements through
chalcogen SBIs, we used electrostatic surface potentials (ESP).
As described recently by Taylor and co-workers,[21] it is well ac-
cepted that an estimation of the ability to make X···Y SBIs can
be obtained considering the magnitude of Vs,max at X and Y
atoms (i.e., the value of the electrostatic potential at the point
of the highest charge for both donor and acceptor
atoms),[21–27] although the chalcogen bonding interactions
cannot be considered as purely electrostatic interactions.[34–38]
Calculations have been firstly performed on non-substituted
benzo-1,3-chalcogenazoles using Gaussian 09 including the
D01 Revision, with the B97-D3/Def2-TZVP level of theory.[21]
After geometry optimization, the ESPs were mapped on the
van der Waals surface of each molecule up to an electron den-
sity of 0.001 electron bohrÀ3. Vs,max was determined with a classi-
cal method.[39] As previously observed for benzo-2,1,3-chalco-
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(Figure 2b), the thiophenyl ring displays an ESP distribution on
the S atom similar to that of the chalcogenazoles. Namely, two
s-hole electron-deficient regions centred on the S atom, each
situated at the termini of the SÀC bonds, are evident in the
ESP (Figure 2b). In particular, a calculated Vs,max value of
+14.4 kcalmolÀ1 has been obtained suggesting that the S
atom does not present the favourable electrostatic properties
for engaging the Se or Te atoms in an in-plane SBI. Therefore it
is evident that the chalcogen-bonding acceptor abilities that
can be inferred from the calculated Vs,max designates the pyridyl
moiety as the best ligand, followed by the chalcogenazole and
furanyl rings. Instead, the thiophenyl rings is expected to es-
tablish in-plane repulsive interactions.
rivatives bearing in 2-position an alkyl, aryl or heteroaryl sub-
stituent (Scheme 1). A first approach is the dehydrative cycliza-
tion of 2-selenoanilides reported in the 1980s by Christiaens
and co-workers, who prepared a few benzoselenazoles and tel-
lurazoles derivatives by cyclization of 2-chalcogenoanilides
with POCl3 (route a).[40]
Nevertheless, the extremely low yield (ꢀ1%, over the four
steps synthesis), the use of extremely evil-smelling and toxic
reagents (e.g., Et2Te2), the low chemical versatility, and contra-
dictory reports about reproducibility issues,[41] limited the
scope of the original protocol. Following reports by Minkin
and co-workers showed that 2-methyl- and 2-phenyl-substitut-
ed benzotellurazoles could be obtained by cyclization of 2-tel-
luroanilides in neat POCl3 with improved yields.[42,43] However,
the harsh acidic conditions severely narrow the compatibility
of the protocol to a limited family of substrates. Recently, two
new synthetic routes toward the preparation of alkyl-, aryl-
and heteroaryl-substituted derivatives have been developed
(routes b and d). In route b,[44–46] bis(2-aminophenyl)diselenide
is reacted with a carbonyl derivative in the presence of a reduc-
ing agent, whereas the second approach exploits Woollins’ re-
agent and 2-iodoanilides (route c).[47] Finally, a copper-catalysed
three-component one-pot synthesis, limited to the case of aryl
and heteroaryl derivatives, has been described very recently
(route d).[48] In the case of the benzotellurazoles analogues, the
synthetic protocols are scarcer.[40–43,49–51] To the best of our
knowledge, the developed methods to date (Scheme 2) are
limited to the preparation of 2-substituted alkyl and aryl deriv-
atives. This exploits the dehydrative cyclization of 2-telluroani-
lides with either phosphorus oxychloride,[40,42,43,49] phosphorus
trichloride[41] or hypophosphorus acid,[50,51] with the latter con-
ditions only compatible for the synthesis of alkyl and aryl 2-
substituted derivatives.
Indulging these computational results, we prepared benzo-
1,3-chalcogenazoles bearing different pyridyl, furanyl and thio-
phenyl moieties at the 2-position to tailor the recognition
properties of the chalcogen atom and to control the supra-
molecular organization at the solid state. Specifically, it is envis-
aged that the self-assembly behaviour of benzo-1,3-chalcoge-
nazoles can be mastered by programming the 2-position with
suitable functional groups through a) steric hindrance (Fig-
ure 3a), b) electrostatic stopping (Figure 3b), or c) intermolecu-
lar tethering (Figure 3c). This can result in either tuning the
recognition properties of the chalcogen atom (a and b) or gov-
erning the formation of different supramolecular arrangements
(c). As anticipated above, particular attention will be given to
the introduction of substituents at the 2-position with suitable
electronic and geometrical requirements to master the forma-
tion of supramolecular wires featured through two- (X···Y) or
three-atoms (X···Y···X) SBIs.
Synthesis
Inspired by the synthetic route developed by Christiaens
and co-workers,[40] we focused our attention on the dehydra-
Only a limited number of reports are described in the literature
about the preparation of 2-functionalised benzoselenazole de-
Figure 3. Programming the recognition properties of benzo-1,3-chalcogenazoles through a) hindering, b) stopping or c) tethering the s-holes.
Scheme 1. The different synthetic approaches toward the preparation of 2-functionalised benzoselenazole derivatives starting from: a) 2-selenoanilides
(R1 =Me or aryl; R2 =Me or Et);[40] b) bis(2-aminophenyl)diselenide (R1 =alkyl or aryl);[44, 45] c) 2-iodoanilides (R1 =alkyl, aryl or heteroaryl); and[47] d) 2-iodoaniline
(R1 =aryl or heteroaryl).[48]
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benzochalcogenazole derivatives, 18Se and 18Te, in modest
yields (Table 1, entries 15 and 16).
As anticipated above, the protocol is compatible with
a wide range of substituents in the 2-position. Indeed, alkyl
(Table 1, entries 1 and 2) and alkenyl (Table 1, entries 5 and 6)
moieties were successfully inserted, as well as aromatic
(Table 1, entries 3 and 4) and heterocyclic substituents, like fur-
anyl, thiophenyl, pyridyl and ferrocenyl moieties (Table 1, en-
Scheme 2. Synthetic approach for the preparation of 2-functionalised benzo-
tellurazole derivatives (R1 =alkyl or aryl).[40–43,49–51]
1
tive cyclization reaction to prepare the targeted 2-substituted
chalcogenazole derivatives. Starting from o-methylseleno- (1Se)
and o-methyltelluro-aniline (1Te), prepared in two steps from 2-
bromoaniline,[33] we have synthesized a series of selenoanilides
(2Se–10Se) and telluroanilides (2Te–10Te) with very good yields
(Table 1, first step) upon reaction with the appropriate acyl
chloride. Adapting the protocol by Minkin[42,43] to milder reac-
tion conditions, anilides 2Y–10Y could be converted into target-
ed benzoselenazoles and tellurazoles 11Y–19Y in very good to
excellent yields in less than three hours (Table 1, second step,
entries 1–14 and 17 and 18) using two equivalents of POCl3 in
the presence of Et3N heated at reflux in 1,4-dioxane.[42,43] The
only exception was restricted to the cases of picolinamide de-
rivatives 9Y, which were transformed into the 2-(pyridin-2-yl)-
tries 7–18). All structures were fully characterized by H- and
13C NMR spectroscopy and HR-Mass spectrometry. We also per-
formed complementary 125Te-NMR analyses on the tellurazoles.
All spectra display a singlet with similar chemical shifts in the
spectral region between d=852 and 892 ppm, with the 2-
phenyl and 2-i-butyl tellurazoles exhibiting the two limiting
values, respectively (Table 2). Because of the N atom in the 3-
Table 2. 125Te NMR chemical shifts in CDCl3 solution at 293 K (reference
taken with Ph2Te2).
Compounds
11Te
12Te
14Te
15Te
16Te
17Te 18Te
870 874
d (125Te, CDCl3) [ppm]
892
852
884
877
884
position, the 125Te chemical shifts are significantly de-
shielded with respect to reference Ph2Te2, in agree-
ment with the literature reports.[41,50,52,53] Considering
that the coordination of a Te atom can result in up-
field shifts as big as d=200 ppm,[52,53] the 125Te-NMR
measurements are not conclusive to support the ex-
istence of any inter- or intramolecular SBIs in solu-
tion.
Table 1. Two-step synthesis for preparing 2-substituted chalcogenazoles from 2-chal-
cogenoanilides by dehydrative cyclization. Y=Se or Te.
Entry
R
Y
a)
b)
Recently, the same protocol allowed us to prepare
2-ethynyl benzo-1,3-chalcogenazoles and bisbenzo-
1,3-chalcogenazoles.[33] To further show the synthetic
versatility of the approach, the preparation of trisben-
zo-1,3-selenazole 21Se and À1,3-tellurazole 21Te was
also attempted (Scheme 3). Treatment of anilines 1Y
with 1,3,5-benzenetricarbonyl trichloride in the pres-
ence of Et3N gave trisanilides 20Y. Subsequent cycliza-
tion of 20Y, using six equivalents of POCl3 in the pres-
ence of Et3N under reflux in 1,4-dioxane, afforded tar-
geted trisbenzo-1,3-chalcogenazoles 21Y in very good
yields. Both Se and Te trisbenzo-1,3-chalcogenazoles
were characterized by mass spectrometric data (see
Product Yield [%][a] t [h] Product Yield [%][a]
1
2
3
4
5
6
Se 2Se
Te 2Te
Se 3Se
Te 3Te
Se 4Se
Te 4Te
90
87
95
94
64
70
3
3
3
3
3
3
11Se
11Te
12Se
12Te
13Se
13Te
77
84
88
83
90
86
iBu
Ph
7
8
Se 5Se
Te 5Te
84
75
3
3
14Se
14Te
88
96
9
10
Se 6Se
Te 6Te
90
94
3
3
15Se
15Te
87
91
11
12
Se 7Se
Te 7Te
88
66
3
1.5
16Se
16Te
87
88
1
the Supporting Information), H NMR, and IR spectra.
Because of its poor solubility, only the 13C NMR spec-
trum of 21Te could be obtained. All attempts to
obtain suitable crystals for X-ray analysis revealed to
be fruitless.
13
14
Se 8Se
Te 8Te
98
93
2
1.5
17Se
17Te
66
80
15
16
Se 9Se
Te 9Te
98
90
8[b]
6[c]
18Se
18Te
39[b]
67[c]
Steady-state UV/Vis absorption and emission stud-
ies in solution
17
18
Se 10Se
Te 10Te
55
59
3
3
19Se
19Te
81
81
Comparative absorption spectra of compounds 11Y–
19Y in CH2Cl2 are shown in Figures 4, 5, and 6 (see
also the Supporting Information), with the key ab-
[a] Yield of the isolated product. [b] 83% conversion. [c] 92% conversion.
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Scheme 3. Synthesis of trisbenzo-1,3-chalcogenazoles 21Se and 21Te. Re-
agents and conditions: a) 1,3,5-benzenetricarbonyl trichloride, Et3N, CH2Cl2,
24 h, RT, Se: 82% of 20Se, Te: 57% of 20Te ; b) POCl3, Et3N, 1,4-dioxane, time,
RT to 1028C; Se from 20Se: 3 h, 72% of 21Se, Te from 20Te : 3 h, 75% of 21Te.
Figure 5. Comparison of the UV/Vis absorption spectra of 2-susbtituted
phenyl (12Y) and the pyridyl (16Y–18Y) benzoselenazoles (top) and benzotel-
lurazoles (bottom) in CH2Cl2 at 298 K.
Table 3. Absorption parameters of 11Y–19Y in CH2Cl2 at 298 K.
lmax [nm] (emax, 103 mÀ cmÀ1
)
M
Se
Te
11Y
12Y
13Y
14Y
15Y
16Y
17Y
18Y
19Y
260 (6.8), 292 (2.2), 302 (2.1)
238 (23.7), 298 (20.3)
272 (7.5), 327 (28.9)
238 (15.1), 315 (22.8)
242 (13.5), 257 (9.7), 325 (18.9)
300 (16.4)
241 (21.3), 260 (9.5), 316 (2.8)
255 (22.9), 308 (16.5), 365 (2.4)
265 (19.0), 335 (29.3), 395 (4.7)
256 (15.2), 320 (22.4), 370 (4.6)
258 (17.6), 329 (17.2), 375 (5.5)
253 (19.8), 310 (13.8), 365 (2.3)
253 (21.3), 310 (15.2), 365 (2.6)
301 (19.7)
237 (21.8), 305 (19.7), 315 (20.1) 255 (18.5), 318 (16.0), 369 (2.9)
304 (15.5), 464 (1.3) 248 (22.9), 309 (14.3), 467 (1.5)
Figure 4. UV/Vis absorption spectra of 2-susbtituted phenyl (12Y), furanyl
(14Y), thiophenyl (15Y) and pyridyl (18Y) benzoselenazoles (top) and benzo-
tellurazoles (bottom) in CH2Cl2 at 298 K.
this transition is slightly redshifted (Figure 4). In the region be-
tween 270 nm and 350 nm a broader band is detected for the
aryl-substituted benzoselenazoles and benzotellurazoles, show-
ing a vibrational structure only in the case of derivatives 14Te
and 15Te. Again, this band is redshifted with the progression of
the aryl substituents (phenyl>pyridyl>furanyl>thiophenyl) in
both chalcogenazole derivatives (Figure 4). Notably, among the
pyridyl-substituted chalcogenazoles, only the UV/Vis profiles of
18Se and 18Te are significantly redshifted with respect to the
phenyl analogues, probably because of the intramolecular
N···Te interaction that planarizes the two heterocyclic moieties
(see the X-ray structure in Figure 10). Most likely, the planar
conformation significantly increases the p-conjugation be-
tween the two aromatic cycles, thus favouring a bathochromic
sorption data resumed in Table 3. In general, absorption spec-
tra for compounds 11Se–18Se display a combination of two
main bands in the UV region, whereas in the case of the tellur-
azole analogues (11Te–18Te) an additional electronic transition
develops in the visible wavelengths.[33] Differently, 19Se and its
analogue 19Te show a broad and low intense absorption band
in the visible region, a typical electronic transition fingerprint-
ing the ferrocene unit (Figure 6). For both benzochalcogena-
zoles, a narrow band is recorded with e values between 15000
and 23000mÀ1 cmÀ1 at higher l (comprised between 235 and
265 nm), likely corresponding to the electronic transition cen-
tred on the phenyl unit. In the presence of an aryl substituent,
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shift of the electronic transitions. Finally, we can easily notice
that the spectra of the benzotellurazole derivatives show a low
intensity (between 2500 and 5500mÀ1 cmÀ1) unstructured band
(except for 13Te–15Te, in which it appears as a shoulder) that is
absent in the absorption profiles of the congener benzoselena-
zoles.[33] This can be attributed to the presence of the Te atom,
1
which most likely promotes additional n!p* electronic transi-
tion.[54–56]
Luminescence studies of air-equilibrated solutions of benzo-
tellurazoles 11Te–19Te in CH2Cl2 at room temperature showed
negligible or very low intense (F<10À3) emission profiles, ex-
hibiting no relevant enhancement of the F values in de-aerat-
ed solutions (after bubbling Ar in the medium for 25 min). On
the other hand, 2-pyridyl tellurazole 18Te displays appreciable
phosphorus emission only as a powder, with an unstructured
band centred at 535 nm (see the Supporting Information).
Emission lifetime analysis at 535 nm results in a ms timescale
decay (t=11 ms), which is compatible with a spin forbidden
triplet–singlet deactivation pathway typical of this kind of
system, as also recently observed by us with 2-ethynyl benzo-
1,3-chalcogenazole derivatives.[33]
Programmed supramolecular organization at the solid state
Figure 6. UV/Vis absorption spectra of 2-substituted alkyl- (11Y), phenyl (12Y)
and ferrocenyl (19Y) benzoselenazoles (top) and benzotellurazoles (bottom)
in CH2Cl2 at 298 K.
In this section we will describe the solid-state arrangement of
those benzo-1,3-chalcogenazoles that formed crystals suitable
for X-ray diffraction (Figures 7–12). In general, all the CÀSe, CÀ
Te and NÀC bonds belonging to the 1,3-chalcogenazole cores
Figure 7. ORTEP (left) and ball-stick representations of the crystal structures of a) 12Se and b) 12Te, from which one can evidence the supramolecular wiring
motif (the N···Y contacts are viewed with the space-filling model); c) intermolecular interactions between hydrogen and tellurium atoms (C-H···Y) are highlight-
ed with space-filling model representation for crystal structure of 12Te. Distances in Š between the chalcogen atom and the SBI donor atoms are highlighted
in the inset circles. Space groups: P21/c (12Se) and P21/a (12Te). Atom colours: blue N, ochre Te, yellow Se, grey C, light grey H.
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do not significantly differ between the different molecules
bearing the same chalcogen atom. This suggests that the crys-
tal organization does not alter the covalent molecular skeleton.
The recognition at the solid-state is discussed considering the
molecular conformation (Table 4) and the calculated electro-
static surface potentials (ESP) displayed in the Supporting In-
formation (Table S14). As general principle, one can anticipate
that the s-hole displaying the highest positive Vs,max value
(Table 5) will be preferentially engaged in the SBI with the
functional group bearing the most negative heteroatom.[21,57,58]
formed through chalcogen bonding interactions (dN···Te =
3.43 Š).
Colourless crystals of Se analogue 12Se could also be easily
obtained from slow evaporation of a CHCl3 solution (Fig-
ure 7a). As far as it concerns the chalcogenazole heterocycle,
molecule 12Se displays significantly shorter SeÀC bonds (ꢀ1.90
vs. ꢀ2.10 Š) and CÀNÀC angle (112.8 vs. 116.98), but a larger
CÀSeÀC angle (84.2 and 78.68) than those of Te-containing
molecule 12Te. These structural characteristics are similar in all
the analysed benzo-1,3-chalcogenazoles. As it was observed
for the Te derivative, molecule 12Se also arranges as rod-like
polymers in which the benzo-1,3-chalcogenazole units are con-
nected to each other through N···Se interactions (intermolecu-
lar dN···Se =3.36 Š), involving the chalcogen s-hole(a) and the
chalcogenazole N atom. In both cases, the 2-Ph substituent
adopts an out-of-plane conformation (conformation A, Table 4)
with respect to the benzo-1,3-chalcogenazole, featuring inter-
planar angles of about 258 and 318 for the Se and Te deriva-
tives, respectively. To shed further light on the structural prop-
erties of molecules 12Y, we compared the molecular conforma-
tion in the crystal with that deriving from DFT minimization in
the vacuum (Table 4). As one can clearly see, in all cases the
theoretical optimization feature a planar conformation as the
most stable, thus rejecting the initial hypothesis for which an
electrostatic repulsion could exist between the nearest phenyl
CÀH and the s-hole(b). However, a closer analysis of the crystal
structure suggests that the non-planar conformation likely
originates from the presence of double CÀH···Y interactions
(see Figure 7c for the Te case: dCÀH···Y =2.959 and 3.072 Š),
which clamps the chalcogen atoms along the supramolecular
wiring direction. The CÀH···Y interactions are established be-
tween positively charged hydrogen atoms and the negative
region of the electrostatic surface potential encircling the
chalcogen atom. These interactions are more evident with the
Te atom. Replacement of the phenyl ring by an alkyl moiety
did not alter the recognition properties of the benzo-1,3-chal-
cogenazole ring. In fact, X-ray analysis of a crystal of 2-isobu-
tyl-substituted benzotellurazole 11Te (see Figure S4 in the Sup-
porting Information) displays the polymeric arrangement held
by N···Te contacts (dN···Te =3.23 Š) through the s-hole(a).
Table 4. Experimental and theoretical conformational (A and B) proper-
ties of the 2-substituted benzo-1,3-tellurazoles. The theoretical and exper-
imental (from X-ray) distances d1À3 between the Te atom and the nearest
neighbouring atom of the 2-substituent is reported.
Conformation A
Conformation B
Derivatives
Distance [Š]
R
d1
d2
d3
Calcd.
Exptl. Calcd. Exptl. Calcd. Exptl.
11Te iBu
12Te Ph
3.278 3.192 3.033 3.072
3.307 3.187 2.945 2.753
–
–
–
–
14Te Furanyl
3.272 3.342
–
–
3.149 (X=O) 3.250 (X=O)
15Te Thiophenyl 3.266 3.341 3.072 3.145
–
–
–
–
–
–
16Te 4-Pyridyl
17Te 3-Pyridyl
18Te 2-Pyridyl
3.261 3.341 2.936 2.915
3.263 3.338 2.904 2.917
3.378 3.354
–
–
2.976 (X=N) 3.126 (X=N)
Table 5. ESP values for the benzo-1,3-chalcogenazoles derivatives. Y=Se
or Te ; X=O, N or S (heteroatom stopper).
Molecule
R
Y
VS,max [kcalmolÀ1
]
s-hole(a)
N
s-hole(b)
X
–
–
Se
Te
Se
Te
Se
Te
Se
Te
Se
Te
Se
Te
Se
Te
Se
Te
+18.8
+21.3
+14.1
+17.2
+15.7
+19.0
+16.4
+19.4
+17.2
+20.4
+20.5
+23.5
+19.1
+21.6
+14.1
+16.6
À29.2
À27.9
À28.9
À20.3
À22.7
À21.0
À25.8
À24.5
À27.0
À25.1
À18.5
À16.9
À21.1
À19.8
À22.3
À21.6
+23.0
+24.5
+16.6
+20.2
+14.4
+19.8
–
+10.3
+15.7
+22.0
+18.2
+24.1
+18.9
+22.6
–
–
–
–
–
–
–
H
11Se
11Te
12Se
12Te
14Se
14Te
15Se
15Te
16Se
16Te
17Se
17Te
18Se
18Te
iBu
It is clear that for this molecular series, the adopted confor-
mation (conformation A, Table 4) brings a CÀH bond in close
proximity to the chalcogen atom, hindering the access to the
s-hole(b). Thus, the CÀH bond on the a side being more dis-
tant from the chalcogen atom than that on b (DFT values: d1 =
3.192 and 3.187 Š and d2 =3.033 and 2.753 Š for 11Te and 12Te,
respectively, Table 4), the chalcogen recognition preferentially
occurs at the s-hole(a).
Ph
À9.4
À7.5
À15.7
À15.7
À33.9
À33.9
À33.9
À33.9
À23.2
À18.2
Furanyl
Thiophenyl
4-Pyridyl
3-Pyridyl
2-Pyridyl
–
Barring the s-hole through intramolecular SBIs: the s-hole
stopper
Colourless and beige crystals suitable for X-ray analysis were
instead obtained for 2-furanyl benzo-1,3-chalcogenazole 14Se
and 14Te by slow evaporation of a CHCl3 solution (Figure 8).
In both cases the molecule adopts a planar conformation
(conformation B, Table 4), in which the furanyl substituent is
Steric control of the s-hole recognition
As anticipated in the introductory section, this work was in-
spired by the solid-state organization of 2-phenyl-benzotellura-
zole 12Te (Figure 7b),[32] in which supramolecular wires are
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Figure 8. Ortep (left) and ball-stick/space-filling (right) representations of the crystal structure of a) 14Se and b) 14Te evidencing the supramolecular wiring
motif (the N···Y contacts are viewed with the space-filling model). Distances in Š between the chalcogen atom and the SBI donor atoms are highlighted in
the inset circles. Space groups: P21/c (14Se) and Pbca (14Te). Atom colours: blue N, red O, ochre Te, yellow Se, grey C.
essentially coplanar (interplanar angles: 1.45 and 6.988 for 14Se
and 14Te, respectively) with the bicyclic benzo-1,3-chalcogena-
zole. Detailed analysis of the crystal structure showed that
short intramolecular O…Y contacts are established (dO···Te =3.03
and 3.15 Š for 14Se and 14Te, respectively, Table 5) between the
furanyl O atom and the chalcogen s-hole(b). As visualized in
Table S14 (see the Supporting Information), this causes a com-
as those of the furanyl derivatives, namely, a co-planarity (con-
formation B, Table 4) between the pyridyl and the bicyclic
benzo-1,3-chalcogenazole moiety (interplanar angles for the
two crystallographically independent molecules: 3.2 and 5.58
for 18Se and 4.4 and 5.08 for 18Te). As one can clearly discern
from the X-ray crystal structure, an intramolecular N…Y contact
established between the N atom of the pyridyl ring and the
plete (14Se, Vs,max ꢀ 0) or partial (14Te, Vs,max = +10.3 kcalmolÀ1
)
chalcogen s-hole(b) is observed in both derivatives (dN···Te =
occlusion of the s-hole(b). As a result, molecules 14Se and 14Te
feature only one electron-deficient s-hole region on the a side
(Vs,max = +16.4 and +19.4 kcalmolÀ1, respectively, see Table 5).
It is thus through this s-hole(a) that molecules 14Y can de-
velop into supramolecular wires by N···Y SBIs (dN···Y =3.25 and
3.17 Š, for the Se and Te derivatives, respectively). Crystals suit-
able for X-ray analysis were also obtained for 2-thiophenyl-
benzochalcogenazole analogues 15Y (Figure 9). Although the
resolutions of the X-ray structures is slightly inferior if com-
pared to those of other derivatives, the data clearly display
that both molecules adopt a flat conformation A (Table 4), in
which the two chalcogen atoms are facing opposite sides,
likely governed by N···S contacts (dN···S =2.99–3.02 for 15Se and
2.92–3.01 for 15Te). The crystals of molecule 15Se only display
the presence of discrete trimeric clusters held together by
double N…Se interactions that, sandwiching the Se atom
(dN···Se =3.22 and 3.43 Š) through its two s-holes, form a three-
atom chalcogen bonding motif (e.g., N···(a)Se(b)···N). Although
the Te analogue (15Te) also forms three-atom chalcogen bond-
ing motifs (N···(a)Te(b)···N, dN···Te =3.11 and 3.37 Š, respectively),
2.93 and 3.02 Š for 18Se and 2.86 and 3.09 Š for 18Te), causing
the complete occlusion of the relevant s-hole (Vs,max =0,
Table 5 and Table S14 of the Supporting Information). As
a result, only the chalcogen s-hole(a) can engage intermolecu-
lar N···Y SBIs with the N atom (dN···Te =3.33 and 3.23 Š, for the
Se and Te derivatives, respectively) to form the characteristic
wire-like organization.
Triggering the wiring motif with a tethering moiety: Overcom-
ing the chalcogenazole nitrogen atom
In all the cases discussed in the previous sections, the polymer-
ic structures are triggered by intermolecular N···Y SBIs estab-
lished through the N atom of the chalcogenazoles. However,
when 2-pyrid-4’-yl benzotellurazole 16Te was crystallized, supra-
molecular wires featuring N···Te interactions (Figure 11) are
formed involving the pyridyl N atom and the Te s-hole(a)
(dN···Te =3.29 Š).
Again, no contacts through the s-hole(a) have been ob-
served, most likely due to the steric hindrance exerted by the
neighbouring CÀH function of the pyridyl ring (conformation
A, Table 4, dCÀH···Te =2.915 Š). Interestingly, no N···Se contacts
have been observed in the solid state for Se analogue 16Se.
Rather, only CÀH···p and p–p interactions seem to rule the
crystal organization of this molecule (Figure 11). The X-ray
structure of tellurazole 17Te shows the formation of supra-
molecular polymers, in which the Te atoms are engaged in
the clusters are bridged by an additional N···Te contact (dN···Te
=
3.42 Š), ultimately forming a supramolecular wire at the solid
state. Notably, because of the larger angles of the five-member
rings at the 2-position, the s-hole(b) is not barred by the furan-
yl CÀH bonds (dCÀH···Te =3.072 Š) and thus it results available for
establishing a SBI (Table 4). 2-Pyrid-2’-yl benzo-1,3-chalcogena-
zoles 18Y display similar conformational properties (Figure 10)
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Figure 9. Ortep (left) and ball-stick (right) representations of the crystal structure of a) 15Se and b) 15Te. Whereas disconnected trimeric clusters are formed at
the solid state for 15Se, molecule 15Te organizes into supramolecular polymers, alternating N···Te and N···Te···N contacts (the N···Y contacts are viewed with the
space-filling model). Distances in Š between the chalcogen atom and the SBI donor atoms are highlighted in the inset circles. Space groups: P21/c for both
structures. Atom colours: blue N, bright yellow S, ochre Te, yellow Se, grey C. Three crystallographically independent molecules are found the unit cell for
both benzochalcogenazoles.
Figure 10. Ortep (left) and ball-stick/space-filling (right) representations of the crystal structure of a) 18Se and b) 18Te. Both crystals evidence the wire-like or-
ganization and the conformational properties as exerted by the N···Y SBIs (the N···Y contacts are viewed with the space-filling model). Distances in Š between
the chalcogen atom and the SBI donor atoms are highlighted in the inset circles. Space groups: Pca21 for both structures. Atom colours: blue N, ochre Te,
yellow Se, grey C. Two crystallographically independent molecules are found the unit cell for both benzochalcogenazoles.
three-atom N···(a)Te(b)···N bonding motifs (Figure 12b) with
the pyridyl and tellurazole moieties at the s-hole(a) and s-
hole(b) sites, respectively (dN···Te =3.51 and 3.42 Š). Again, no
N···Se contacts have been found for Se analogue 17Se (Fig-
ure 12a).
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Figure 11. Ortep (left) and ball-stick/space-filling (right) representation of the crystal structure of a) 16Se and b) 16Te. Only molecule 16Te forms supramolecular
wires, this time tethered the pyridyl ring. Distances in Š between the chalcogen atom and the SBI donor atoms are highlighted in the inset circle. Space
groups: P21 (16Se) and Cc (16Te). Atom colours: blue N, ochre Te, yellow Se, grey C.
Figure 12. Ortep and ball-stick/space-filling representations of the crystal structure of a) 17Se and b) 17Te (the N···Y contacts are viewed with the space-filling
model only for 17Te). Distances in Š between the chalcogen atom and the SBI donor atoms are highlighted in the inset circle. Space group: P21/c for both
structures. Atom colours: blue N, ochre Te, yellow Se, grey C.
Conclusion
examples. In particular, by programming the recognition prop-
erties of the chalcogen s-hole sites and of the basic chalco-
gen-bonding acceptor atom through the selection of suitable
2-substituents, we gain control of the wiring arrangement of
the benzo-1,3-chalcogenazoles at the solid state. With such
building units, when the overall assembly process is symmetry-
conserved; the final supramolecular framework depends upon
the type of the chalcogen bonding donor and the symmetry
of its recognition s-hole sites. With this simple governing prin-
ciple and the large library of stable benzochalcogenazole mod-
ules that could be envisaged, it will be possible to engineer
a great variety of materials featuring a series of tunable organi-
We have developed a versatile four-step synthesis of 2-substi-
tuted benzo-1,3-selenazoles and tellurazoles starting from 2-
bromoaniline. The mild reaction conditions, simple procedure
and high yields make this a valuable approach for preparing
benzo-1,3-chalcogenazoles bearing a large variety of different
substituents (e.g., alkyl, alkenyl, phenyl, furanyl, thiophenyl,
pyridyl and ferrocenyl groups) at the 2-position. The principle
of exploiting the benzo-1,3-chalcogenazole as self-assembling
units to produce controlled organic networks at the solid-state
through SBIs has been demonstrated with a large number of
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Acknowledgements
D.B. gratefully acknowledges the FRS-FNRS, the Science Policy
Office of the Belgian Federal Government (BELSPO-IAP 7/05
project). A.K. thanks the FNRS for his research associate posi-
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Keywords: chalcogens
supramolecular chemistry · tellurium · UV/Vis spectroscopy
·
selenium
·
self-assembly
·
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Received: October 28, 2015
Published online on February 22, 2016
Chem. Eur. J. 2016, 22, 5665 – 5675
www.chemeurj.org
5675
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