Selenacalix[3]triazines: Anion versus proton association

Article abstract of DOI:10.1002/ejoc.201201548

Selenacalix[3]triazines, cyclotrimeric metacyclophanes with direct Se linkages between the heteroaryl constituents, were shown to associate with various guest species. The preorganization of three electron deficient triazine rings allows for anions to bin

Full text of DOI:10.1002/ejoc.201201548

FULL PAPER
DOI: 10.1002/ejoc.201201548
Selenacalix[3]triazines: Anion Versus Proton Association
Wim Van Rossom,*[a,b] Joice Thomas,[a] Tatyana G. Terentyeva,[c] Wouter Maes,[a,d] and
Wim Dehaen*[a]
Keywords: Calixarenes / Selenium / Host–guest systems / Anions / Protonation
Selenacalix[3]triazines, cyclotrimeric metacyclophanes with
direct Se linkages between the heteroaryl constituents, were
shown to associate with various guest species. The pre-
organization of three electron deficient triazine rings allows
for anions to bind through anion-π interactions, and align-
ment of the central nitrogen lone pairs and the well-defined
size of the macroring enable association with a single proton.
Extended UV/Vis titration studies indicated a clear differ-
ence in complexation behavior depending on the outer-rim
substitution pattern. The host–guest properties of analogous
selena- and thiacalix[3]triazines were found to be notably
different.
the synthesis and host–guest properties of selenacalix[3]-
triazines was published by our group.[5] The presence of
bridging Se atoms results in an increased cavity size (owing
to its large atomic radius) and may impose different elec-
tronic and structural characteristics relative to those of
N-, O- or S-bridged macrocycles, as was recently shown for
analogous homothia- and homoselenacalix[n]arenes
(CH2SeCH2 linkages).[6] Organoselenium compounds are
also interesting targets from a more general biological and
pharmaceutical point of view.[7]
In recent years, anion–π interactions have been explored
as an appealing type of noncovalent bonding.[8] In particu-
lar, application of these weak attractive forces to the design
of highly selective anion receptors has shown potential im-
pact in the field of molecular recognition. Anion binding is
of great importance owing to the high abundance of dif-
ferent types of anions in chemical, biological and environ-
mental processes.[9] Calixarenes (including heteracalixar-
enes) have proven to be suitable macrocyclic scaffolds upon
which multiple anion binding moieties [amides, (thio)ureas,
triazoles, etc.] can easily be inserted and oriented in space,
depending on the conformational preferences of the respec-
tive calixarenoids.[3r,3s,10] More recently, it has been demon-
strated that heterocalixarenes, heterocyclic analogues of
classical calixarenes, can be used themselves as the actual
host molecules, even in the absence of specifically designed
anionophoric substituents, when constructed from electron
deficient 1,3,5-triazines. Triazine rings embedded in a highly
preorganized 1,3-alternate oxacalixarene skeleton were
shown to bind Cl– and F– ions through noncovalent halide–
π interactions in combination with lone pair–π interac-
tions.[1g,1j,3e,3k,3m,3q] The effect of the peripheral substituents
on the binding behavior was also recognized.[3e,5]
Introduction
Heteracalixarenes, in which the methylene linkages of
traditional calixarenes are replaced by heteroatoms, have
gained considerable attention in recent years for their po-
tential value in supramolecular chemistry.[1] The hetero-
atom bridges allow tuning of the macrocycle size, the elec-
tron density on the arene building blocks and the preferred
conformation, and provide additional binding sites towards
a perfect (induced) fit of a desirable guest molecule. Among
these heterametacyclophanes, the thia analogues or thia-
calixarenes have been studied most intensively and they are
widely recognized as effective receptors for small organic
compounds and heavy/transition metals.[1,2] The fields of
oxa- and azacalixarenes have also steadily grown,[1,3,4] both
in synthetic scope and supramolecular applications. How-
ever, extension of heteracalixarene chemistry to the larger
group through chalcogen elements (Se/Te) was noticeably
absent from literature until a very recent communication on
[a] Molecular Design and Synthesis, Department of Chemistry,
KU Leuven,
Celestijnenlaan 200F, 3001 Leuven, Belgium
Fax: +32-16327990
E-mail: wim.dehaen@chem.kuleuven.be
Homepage: chem.kuleuven.be
[b] World Premier International (WPI) Research Center for
Materials Nanoarchitectonics (MANA), National Institute of
Materials Science (NIMS),
Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
E-mail: wimvanrossom@gmail.com
Homepage: www.nims.go.jp
[c] Molecular Imaging and Photonics, Department of Chemistry,
KU Leuven,
Celestijnenlaan 200F, 3001 Leuven, Belgium
[d] Design
& Synthesis of Organic Semiconductors (DSOS),
Institute for Materials Research (IMO-IMOMEC), Hasselt
University,
Universitaire Campus, Agoralaan 1 – Building D,
3590 Diepenbeek, Belgium
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201548.
We envisioned that the π-electron deficient N-tridentate
cavity of selenacalix[3]triazines should be capable of both
metal and anion recognition. Therefore, a preliminary in-
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W. Van Rossom, W. Dehaen et al.
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vestigation into their affinity for metal salts and a small try of the complex was identified by a Job plot analysis.
–
variety of putative anionic guests was performed.[5] The Titration with the less acidic H2PO4 anion (pKa = 7.2) re-
macrocyclic oligotriazines were shown to bind both CuIBr sulted in a weaker association (Kass = 1.2ϫ104 m–1),
–
and CuIIBr2 and interact with various anions. The complex whereas with HCO3 (pKa = 10.3) no visible changes were
pattern observed among the association constants sug- observed (Table 1). Also a range of other anions lacking an
–
–
gested more than one interaction mechanism. In line with acidic hydrogen atom (AcO–, Cl–, Br–, NO3 , NCS–, PF6
–
these preliminary results, we now report on an extended and CH3SO3 ) did not cause any change in the UV/Vis pro-
investigation of the association properties, the specific bind- file (see the Supporting Information). In previous work by
ing mechanisms and the effect of the peripheral substituents Wang and co-workers, a similar lack of anion affinity was
for an enlarged series of selenacalix[3]arenes.
observed for a dimethylamino-substituted oxacalix[2]-
arene[2]triazine macrocyclic host.[3e] The observed trend for
macrocycle 2a hints towards protonation rather than anion
binding, as previously reported for analogous heteracalix-
[3]arenes. A remarkably high collective basicity or “proton
sponge” behavior was reported for azacalix[3]pyridines, as-
sembling the three pyridine constituents in a specific macro-
ring structure with a defined size, and aligning the central
chelating nitrogen atoms.[12d–12f,12h] The three nitrogen lone
pairs point towards the center and efficiently trap a single
proton. This type of neutral organic superbase is profitable
in a number of organic transformations and have some ad-
vantages relative to their inorganic counterparts.[12f] Even
though the triazine moieties in our systems are considerably
less basic, the additional electron donating substituents
(and the Se bridges) might create a system capable of pro-
ton abstraction. For that reason an additional experiment
Results and Discussion
Selenacalix[3]triazines 2a–2e were synthesized by using a
straightforward one-pot nucleophilic aromatic substitution
reaction (SNAr) procedure starting from sodium hydrosele-
nide (NaSeH) and readily available 2-substituted 1,3,5-tri-
azine building blocks 1a–1e (Scheme 1),[5] of which electron
deficient features ensure high reactivity in the SNAr reac-
tions.[11] The optimum macrocyclization conditions were
(slightly) dependent on the electronic nature of the ap-
pended substituents. Notably, only heteracalix[3]arenes[12]
could be isolated without any trace of the larger cyclooligo-
mers, which suggests a thermodynamically controlled reac-
tion.
–
with NH4PF6 (pKa = 9.2) was performed. The PF6 anion
was independently (through the tetrabutylammonium salt)
shown to have no effect on the UV/Vis spectrum of 2a (Fig-
ure S5).
Table 1. Association constants Kass [m–1] for the 1:1 complexation
between hosts 2a–2e and 3, and ionic guests in dry MeCN.[a]
–
–
–
+
Compound
HSO4
H2PO4
HCO3
AcO–
NH4
2a (Se, NEt2) Ͼ 1ϫ106 [b] 1.2ϫ104 0[c]
0[c]
0[c]
1.6ϫ104
[d]
2b (Se, NHEt) 5.3ϫ105
0[c]
0[c]
–
[d]
2c (Se, nBu)
2d (Se, tBu)
2e (Se, OPh)
3 (S, OPh)
0[c]
0[c]
0[c]
3.2ϫ101 5.4ϫ102 3.2ϫ103
–
[d]
1.2ϫ103 8.5ϫ102
–
Scheme 1. Synthetic protocol towards selenacalix[3]triazines 2a–2e.
5.7
9.5ϫ102 5.2ϫ103 8.6ϫ104
–
[d]
Ͼ 1ϫ106 [b] 1.2ϫ104 1.7ϫ104 3.6ϫ105
–
[d]
The combination of direct Ar–Se–Ar bonds and elec-
tron-deficient triazine heterocycles results in cyclotrimeric
macrocycles with high π-acidity and high polarizability. In
preliminary studies, we observed that the π-electron-de-
ficient cavity expressed some affinity for a small variety of
[a] Calculated on the basis of UV/Vis titration data using Hyp-
Spec.[13] Values are the average of at least three separate λ’s and are
considered reproducible to 15%. [b] This value is Ն 1ϫ106 m–1. As
such the stability constant is at the upper limit of what can be
determined by this technique and should be treated with caution.
[c] No notable spectral changes. [d] Not determined.
–
–
putative anionic guests (HSO4 , H2PO4 , HCO3– and AcO–,
added as tetrabutyl- or tetraethyl-ammonium salts), by
means of UV/Vis titrations in acetonitrile (MeCN;
1ϫ10-5 m, 293 K).[5] The difference in the UV/Vis absorp-
For the combination 2a-NH4+, a bathochromic (13 nm)
tion profiles upon titration suggested the presence of two and hyperchromic shift were observed (Figure 1), which
different mechanisms, working in parallel. This supramolec- were nearly identical to the shifts in absorption after ad-
– [5]
ular behavior has now been explored in more detail.
dition of HSO4 , giving a clear indication that 2a acts as
For host molecule 2a (R = NEt2), decorated with strong a proton scavenger rather than an anion receptor.
electron-donating diethylamine groups giving rise to polar- Moreover, the observed changes are very similar to the
ization of the ligand towards the N tridentate center (creat- shifts detected upon complexation of (cationic) Cu spe-
ing an electron-rich pocket), a small red shift (from 260 to cies.[5] The red shift upon protonation reflects the stabiliza-
275 nm; isosbestic point at 263 nm) and strong hyperchro- tion of the charge-transfer excited state, corresponding to
mic shift were observed (Kass = Ͼ1ϫ106 m–1; Table 1) upon the efficient propagation of the cationic resonance with the
gradual addition of HSO4– (pKa = 2.0).[5] A 1:1 stoichiome- amino junctions on the outer rim (Figure 2).[14] The central
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Selenacalix[3]triazines: Anion Versus Proton Association
Figure 3. UV/Vis titration of 2c with tetra-n-butylammonium acet-
ate in MeCN. Inset: variation of absorbance at λ = 337 nm ver-
sus equiv. acetate added.
Figure 1. UV/Vis titration of 2a with NH4PF6 in MeCN. Inset:
+
variation of absorbance at λ = 272 nm versus equiv. NH4 added.
larization of the ligand upon complex formation. Gradual
nitrogen atoms form a synergistic hydrogen bond with the
proton, which gives rise to a substantial increase in the di-
pole with which the optical transition takes place and is
observable as a red shift.
–
–
addition of HCO3 and H2PO4 resulted in similar shifts
(Kass = 5.4ϫ102 and 3.2ϫ101 m–1, respectively), whereas
for Cl– no spectral changes were observed (see Table 1 and
the Supporting Information). The interaction between the
ligand and the anions seems to follow the basicity order of
the guests, whereas in the previous pattern the association
aligned itself with guest acidity.
The reduced electron density of 2c allowed interaction
with anionic species. Also for macrocycle 2d (R = tBu), a
new band at 330 nm was formed, increasing in intensity
upon slow titration with anions (Table 1 and the Support-
ing Information). Strong interactions were also observed
for cyclotrimer 2e (R = OPh), following the basicity order
Figure 2. Schematic representation of the hydrogen bond of the
1,3,5-triazine inner nitrogen atom, causing an increased dipole and
the observed bathochromic shift.
–
–
of the anions AcO– Ͼ HCO3 Ͼ H2PO4 (Kass = 8.6ϫ104,
5.2ϫ103 and 9.5ϫ102 m–1, respectively; Table 1).
For comparison, an analogous thiacalix[3]triazine was
synthesized. Reaction of 2,4-dichloro-6-phenoxy-1,3,5-tri-
azine (1e) with NaSH in THF for 3 days resulted in thiaca-
lix[3]triazine 3 with peripheral phenol substituents in 28%
yield (Scheme 2).[15] UV/Vis titrations of receptor 3 with
different anions resulted in spectral changes that somewhat
Similar proton-binding behavior was observed for selena-
calix[3]triazine 2b (R = NHEt). The somewhat smaller elec-
tron-donating character of the ethylamine group results in
a lower molecular polarizability and consequently a lower
–
affinity towards protons (Kass = 5.3ϫ105 m–1 for HSO4 ;
1:1 stoichiometry). A substantial shift was seen upon ti-
–
tration with HSO4 , whereas no shift was observed for the
less acidic H2PO4– and HCO3– anions (nor for AcO– or Cl–;
see Table 1 and the Supporting Information).
A completely different UV/Vis titration profile was ob-
tained for the remaining selenacalix[3]triazine ligands dec-
orated with fewer electron-donating substituents. Titration
–
of macrocycle 2c (R = nBu) with HSO4 had no noticeable
effect on the UV/Vis spectrum. However, slow addition of
the aprotic AcO– anion resulted in the appearance of a new
band centered at 337 nm (Kass = 3.2ϫ103 m–1, Table 1) with
a simultaneous hypochromic shifting of the initial band as
the concentration of free ligand decreased (Figure 3).
Because protonation can be excluded in this case, the
most probable mechanism involved is pure anion binding.
The new redshifted absorption band can be ascribed to po- Scheme 2. Synthesis of thiacalix[3]triazine 3.
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W. Van Rossom, W. Dehaen et al.
FULL PAPER
deviate from the above. A decrease in binding constant
along the acidity series was observed, with exception of
HSO4 (Table 1). Addition of more basic anions caused a
new absorbance peak around 290 nm to emerge, corre-
sponding to the anion complex formed (by analogy to the
selenacalix[3]triazine case; Figure 4).
Conclusions
A series of selenacalix[3]triazines was analyzed for its
anion-binding character. The peripheral substituents were
shown to have a major impact on the host–guest behavior
of the macrocycles. Strongly electron-donating substituents
(Et2N and EtNH) resulted in preferential proton binding,
whereas anion binding was favored for substituents like
nBu, tBu and PhO. For the analogous thiacalix[3]triazine
derivative (R = PhO) both mechanisms were observed, de-
pending on the acidity of the guest.
–
Experimental Section
General: Tetrabutyl- and tetraethylammonium salts were purchased
from Sigma–Aldrich and dried overnight under vacuum at 40 °C
prior to the recording of the spectrophotometric measurements.
UV/Vis spectra were recorded by using an ultraviolet–visible spec-
trophotometer Perkin–Elmer Lambda40 with quartz cuvettes (path
length 1 cm) at 293 K. A stock solution of 1.0ϫ10–5 m of the host
compounds 2a–2e and 3 was prepared in acetonitrile (dried with
molecular sieves 4 Å). Solutions of 1.0ϫ10–3 m of the tetrabutyl-
or tetraethylammonium salts of the respective anions were pre-
pared with the host stock solution to maintain a constant host
concentration throughout the titration experiments.
Selenacalix[3]arenes 2a–2e: These compounds were prepared ac-
cording to the procedure previously reported by Dehaen et al.[5]
Material identity and purity were confirmed by MS, 1H and 13C
NMR spectroscopy.
Figure 4. UV/Vis titration of thiacalix[3]arene 3 with tetra-n-ethyl-
ammonium hydrogen carbonate in MeCN. Inset: variation of ab-
sorbance at λ = 262 nm versus equiv. hydrogen carbonate added.
2,4-Dichloro-6-phenoxy-1,3,5-triazine (1e): This compound was pre-
–
pared according to the procedure reported by Götz et al.[16] Mate-
Titration with HSO4 , however, resulted in a small red
1
rial identity and purity were confirmed by MS, H and 13C NMR
(5 nm) and hyperchromic shift, in agreement with the pro-
tonation mechanism (Figure 5). Thiacalixarene 3 seems to
interact according to the anion association mechanism with
non-protic or less-acidic protic anions, whereas it follows
the protonation mechanism with more-acidic protic anions.
spectroscopy.
4,6,10,12,16,18,19,20,21-Nonaaza-5,11,17-triphenoxy-2,8,14-tri-
thiacalix[3]arene (3): 2,4-Dichloro-6-phenoxy-1,3,5-triazine (1e;
300 mg, 1.23 mmol, 1 equiv.) was dissolved in dry THF (30 mL)
and the solution was purged with argon for 10 min. NaSH
(0.0764 g, 1.36 mmol, 1.1 equiv.) was added and the resulting reac-
tion mixture was stirred at room temp. for 3 d and subsequently
evaporated to dryness. The crude residue was dissolved in a mixture
of CH2Cl2 and water (20:20 mL). The organic fraction was sepa-
rated, washed with water, dried with MgSO4, filtered and the sol-
vents evaporated to dryness. Purification by column chromatog-
raphy (silica, CH2Cl2/heptane/ethyl acetate, 46:46:8) afforded ca-
lix[3]arene 3 as a pure off-white solid (71 mg, 28%). M.p. 120–
122 °C. 1H NMR (300 MHz, CDCl3): δ = 7.43–7.40 (d, J = 7.3 Hz,
6 H), 7.33–7.26 (m, 3 H), 7.17–7.14 (d, J = 7.9 Hz, 6 H) ppm. 13C
NMR (75 MHz, CDCl3): δ = 178.5, 168.3, 151.3, 129.9 (CH), 126.7
(CH), 121.3 (CH) ppm. MS (ESI+): m/z = 610.1 [M + H]+. HRMS:
calcd. for C27H16N9O3S3 [M + H]+ 610.0538; found 610.0540.
Supporting Information (see footnote on the first page of this arti-
cle): 1H and 13C NMR spectra of macrocycle 3 and additional data
on the anion titrations.
Acknowledgments
The authors thank the Fund for Scientific Research – Flanders
(FWO), the KU Leuven, the Ministerie voor Wetenschapsbeleid
and the Japanese Society for the Promotion of Science (JSPS) for
continuing financial support.
Figure 5. UV/Vis titration of thiacalix[3]arene 3 with tetra-n-butyl-
ammonium hydrogen sulfate in MeCN. Inset: variation of ab-
sorbance at λ = 268 nm versus equiv. hydrogen sulfate added.
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Selenacalix[3]triazines: Anion Versus Proton Association
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Published Online: March 8, 2013
2090
www.eurjoc.org
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2013, 2085–2090