'Cleft-form' electrochemical anion chemosensor with amide and triazole donor groups

Article abstract of DOI:10.1016/j.tetlet.2012.06.118

A novel 'cleft-form' electrochemical anion receptor bearing amide and triazole donor groups, 1, has been synthesized and characterized. Among various anions, 1 shows a significant anodic shift response for H₂PO4- and F^-, with its multiple N-H...anion and C-H...anion interactions, which is supported by theoretical calculation and NMR titration results.

Full text of DOI:10.1016/j.tetlet.2012.06.118

Tetrahedron Letters 53 (2012) 4917–4920
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
‘Cleft-form’ electrochemical anion chemosensor with amide and triazole
donor groups
Qian-Yong Cao a,b, , Tuhin Pradhan a,c, , Min Hee Lee a, Dong Hoon Choi a,c, Jong Seung Kim a,
⇑
a Department of Chemistry, Korea University, Seoul 136-701, South Korea
b Department of Chemistry, Nanchang University, Nanchang 330031, PR China
c Research Institute for Natural Sciences, Korea University, Seoul 136-701, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel ‘cleft-form’ electrochemical anion receptor bearing amide and triazole donor groups, 1, has been
synthesized and characterized. Among various anions, 1 shows a significant anodic shift response
Received 3 May 2012
Revised 18 June 2012
Accepted 26 June 2012
Available online 9 July 2012
for H2POꢀ and Fꢀ, with its multiple N–Hꢁ ꢁ ꢁanion and C–Hꢁ ꢁ ꢁanion interactions, which is supported by
4
theoretical calculation and NMR titration results.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Ferrocene
‘Cleft-form’ receptor
Anions sensing
There has been growing interest in anion recognition because
anions play important chemical and biological roles.1 Over the past
decade, various examples of N–Hꢁ ꢁ ꢁanion hydrogen bonding donors
(e.g., urea, thiourea, amine, amide, pyrrole) moieties have been
shown to be particularly effective anion receptors in organic sol-
vents.2 In addition, the (C–H)+ꢁ ꢁ ꢁanion hydrogen bonding donors
(e.g., imidazolium3 and triazolium4) moieties are often incorpo-
rated in these systems. More generally, however, neutral
C–Hꢁ ꢁ ꢁanion hydrogen bonding donors are less commonly recog-
nized on account of their weakness, even though the CH unit is
present in the majority (97%) of chemical compounds.5
N
N
N
N
N
N
O
N
NH
HN
N
H
N
O
O
N
Fe
Recently, the disubstituted aryl-1,2,3-triazoles have been em-
ployed in anion recognition, because the 1,2,3-triazole ring shows
large polarity (dipolemoment ꢂ5D), and that of the C5-H bond
creates an electropositive site that can function as an effective
C–Hꢁ ꢁ ꢁanions interaction.6 Some triazole-based receptors including
macrocycles,7 foldamers,8 and short flexible oligomers9 have been
prepared. Macrocycle [34] triazolophane has unexpectedly large
halide binding constants, because it takes an advantage of macrocy-
clic preorganization to direct four triazole C–H donors and four phe-
nylene C–H donors into the central cavity.7a In addition, the anion
binding capacity can be enhanced by incorporating more traditional
NH donors, such as pyrrole, into these systems.10
Fe
Fe
2
1
Scheme 1. Compounds 1 and 2 with crystal structure of 2.
element due to its strong p-donating ability and good reversibility
when it displays an one-electron oxidation at a desirable range.
Some ferrocene-based receptors for cations and anions have been
reported to reveal large shift in the redox potential of the ferro-
cene/ferrocenium redox couple upon the addition of target ions.12
Recently, we reported a ferrocene-appended aryl–triazole receptor
that can electrochemically respond to phosphate anions selectively
with a large cathodic shift due to pure C–Hꢁ ꢁ ꢁO interactions.11c
However, the binding constant of this receptor to anions is poor.
We envision that incorporating traditional NH donor amide into
this system may increase its binding ability. Therefore, we de-
signed here a ‘cleft-form’ ferrocene-based anion receptor with
We are interested in ferrocene-based receptors for anion/cation
recognition,11 since ferrocene is a good electrochemical response
⇑
Corresponding author.
E-mail address: jongskim@korea.ac.kr (J.S. Kim).
These two authors equally contributed to this work.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.06.118

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Q.-Y. Cao et al. / Tetrahedron Letters 53 (2012) 4917–4920
Figure 1. Optimized structures of ‘anti’ (left) and ‘syn’ (right) conformations of 1.
amide and triazole donor groups, 1 (Scheme 1). The ‘cleft-form’
frameworks possessing well-defined binding cavity of multiple
donor moieties to increase anion binding are well established.13
We find that this simple modification really improves its anion-
binding affinity via both sites, with amide N–Hꢁ ꢁ ꢁanion interac-
tions being more strongly than triazole (C–H)ꢁ ꢁ ꢁanion interactions.
However, for the control compound 2, only the amide donor takes
part strongly in N–Hꢁ ꢁ ꢁanion interaction especially for chloride and
fluoride anions.
separation of 9.95 Å (calcd) in ‘anti’ conformation. Though the amide
and triazole binding sites show an ‘anti’ conformation in the opti-
mized conformation, we found that it can easily change to its ‘syn’
conformation by the rotation of ꢂ129.12° (around C(H2)–N(H)
bond) with a low energy gap (ꢂ10.12 kcal/mol, calcd), which is more
favorable for binding anions via the chelate effect. The required en-
ergy (ꢂ10.12 kcal/mol) for the conversion of ‘anti’ to ‘syn’ conforma-
tion caneasily be compensated by thebindingenergy of anion with 1
With (chlorocarbonyl)ferrocene as starting material,14 we first
synthesized the key intermediate ferrocenecarboxylic propargyla-
mide by literature method.15 Then by ‘click’ reaction,16 receptor 1
was easily prepared in moderate yield (72%) by coupling 3,5-diazi-
do-1-tert-butylbenzene7a with ferrocenecarboxylic propargyla-
mide in toluene under reflux with (EtO)3PꢁCuI as catalyst. The
reference compound 2 was prepared by the similar method. Their
structures were confirmed by spectroscopic data (1H NMR, 13C
NMR, and MS), as well as by X-ray structure analysis in the case
of 2.17 The X-ray structure of 2 shows that the two binding units
are in the anti-conformation, and a one-dimensional supramolecu-
lar assembly is found in its packing structure by the intermolecular
hydrogen bond via amide group (N–Hꢁ ꢁ ꢁO 2.85 Å, symmetry code:
x, 0.5ꢀy, 0.5+z, Fig. S1).
Though we could not get the single crystal structure of 1, its
optimized conformation in the gas phase, calculated by density
functional theory (DFT) at B3LYP/6-31G⁄ level of theory, shows a
‘cleft-form’ conformation ( Fig. 1). The two triazole protons are
pointed inward the cavity, while the two amide protons are pointed
outward the cavity. The ferrocene moieties are almost parallel witha
3.2
16
a
b
c
2.8
2.4
2.0
1.6
1.2
0.8
0.4
0.0
0
1.0
1.5
2.0
12
8
μ
I
4
0
-4
-0.2 -0.1
0.0
0.1
E
0.2
0.3
0.4
0.5
0
1
2
3
4
5
6
Equiv
/V
Figure 2. CV titration profile of 1 (0.2 mM) upon addition of various amount of
Figure 3. (Top) Partial 1H NMR spectrum of 1 (CDCl3, 16 mM) and after addition
H2POꢀ in CH2Cl2 solution. Reference electrode = Ag/AgNO3; supporting electro-
2.0 equiv of Clꢀ, H2PO4ꢀ and Fꢀ. (Bottom) Chemical shift changes for several protons
4
lyte = [n-Bu4N]PF6 (0.1 M); scan rate = 100 mV Sꢀ1
.
of 1 upon addition of H2POꢀ
.
4

Q.-Y. Cao et al. / Tetrahedron Letters 53 (2012) 4917–4920
4919
second redox wave is positively shifted compared to the free recep-
tor. The larger potential shift and the ‘two wave behavior’ of 1 com-
pared with the ‘shifting behavior’ of 2 imply that 1 shows a higher
binding ability to anions than 2.
Both compounds show high energy (HE) absorption bands at k
<330 nm assigned to the
p–
p⁄ transitions of cyclopentadiene or
benzene rings, and a weaker low energy (LE) absorption in the
region 400–500 nm assigned to a ferrocenyl-based metal-to-ligand
(MLCT) band (Fig. S9). Upon addition of anions into the solution of
1 or 2, both rꢀeceptors show observable UV–vis spectral changes to
Fꢀ and H2PO4 . Since the spectral changes are small, their binding
properties were investigated by 1H NMR spectroscopy.
The binding ability of 1 and 2 for the selected anions (Fꢀ, Clꢀ,
and H2POꢀ was investigated by 1H NMR spectra in CDCl3 solution
4
(Fig 3). The titration of H2POꢀ to 1 produced a considerable shift in
4
signals of the amide protons (Ha), the triazole protons (Hb), and
the
a protons in the cyclopentadieneyl ring (Hc), demonstrating
that these protons participate in hydrogen bonding to the H2POꢀ
4
Figure 4. Partial 1H NMR spectrum of 2 (CDCl3, 27 mM) and after addition 2.0 equiv
ion. For example, 2 equiv of H2POꢀ induces these protons to be
of Clꢀ, H2POꢀ4 and Fꢀ.
4
downshifted up to 2.3, 0.8, and 0.3 ppm, respectively. Judging by
the changes in the NMR spectra, the amide donors bind the anion
more strongly than triazole groups, followed by ferrocene C–H,
which is in accordance with the sequence of their expected H acid-
ity.18 The titration curve of 1 and H2POꢀ4 is fitted to a 1:1 binding
model as confirmed by Job plot analysis (Fig. 3 and S11), and gen-
erates the binding constant of 2.15 (5) ꢃ 102 Mꢀ1 by using the
EQNMR program,19 which is much larger than that of the ferro-
cene-appended aryl-triazole receptor11c as a result of the incorpo-
ration of amide donors into this system.
(e.g. ꢂꢀ37.25 kcal/mol with chloride anion for example, calcd in gas
phase, BSSE corrected) in the ‘cleft-form’ of ‘syn’ conformation.
The recognition abilityꢀof 1 toward various monoanions (Fꢀ, Clꢀ,
Brꢀ, Iꢀ, AcOꢀ, NO3ꢀ, HSO4 and H2POꢀ) in the form of their corre-
4
sponding tetrabutylammonium salts (TBA+) was first investigated
by cyclic voltammetry (CV) and differential pulse voltammetry
(DPV) in CH2Cl2 solution containing 0.1 M TBAPF6 as a supporting
electrolyte. The free receptor 1 shows a reversible one-electron
redox wave with the half wave potential (E1/2) value of 270 mV
(versus Ag/AgNO3). Upon addition of 2.0 equiv of various anions
(Fig 2, Figs. S2–S4), 1 shows only marked electrochemical signal
The titration of Clꢀ or Fꢀ into 1 shows similar downfield shift
in signals of Ha, Hb, and Hc, demonstrating that these protons
are also involved in the ligand-anion binding. The changes of
these protons upon titration of Clꢀ are smaller than that upon
changes to Fꢀ and H2POꢀ, with a negative shift of 140 mV and
4
titration of H2POꢀ, which may be ascribed to its weaker basicity
130 mV respectively. The evolution of CVs and DPVs of 1 upon addi-
tion of both anions gives a ‘two wave behavior’, with decreasing ori-
ginal redox potential at 270 mV and concomitantly increasing new
redox potential band at a more negative position, which is obvi-
4
and spherical geometry. While the titration of strong basic
Fꢀ leads amide protons first downfield shift and then disappearing
quickly for deprotonation, which is often observed in some
amide-base anion receptors.20
ously attributed to the formation of 1ꢁH2POꢀ or 1ꢁFꢀ complex in
4
The anions (Fꢀ, Clꢀ and H2POꢀ4 ) binding behavior of 2 is different
from that of 1 (Fig. 4). Upon titration to 2, these anions produced
only a considerable shift in signals of the amide proton (Ha), and
little changes in Hb and Hc protons, indicating the presence of
the solution. The control compound 2 also shows electrochemical
sensing to Fꢀ and H2POꢀ4 (Figs. S5–S8), but with relative less poten-
tial shift (E1/2 ꢂꢀ50 mV). In addition, upon titration of Fꢀ or H2PO4ꢀ
to 2, its redox potential shows a ‘shifting behavior’, in which a
Figure 5. Calculated structure (B3LYP/6-31G⁄) of 1ꢁH2POꢀ (left) and 2ꢁH2POꢀ (right) complexes. Nitrogen, oxygen, phosphorous, carbon and hydrogen atoms are represented
4
4
as blue, red, orange, gray and white balls respectively. Dotted line indicates H-bonding. The calculated H-bond distances indicated by the symbols are respectively 2.17 Å (a),
1.92 Å (a0), 2.09 Å (b), 2.21 Å (b0), 2.41 Å (c), 2.33 Å (c0) and 2.35 Å (d).

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Q.-Y. Cao et al. / Tetrahedron Letters 53 (2012) 4917–4920
strong amide-anion interaction and very weak C–Hꢁ ꢁ ꢁanion inter-
actions between 2 and the anions in solutions.
References and notes
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4
4
of H2POꢀ, the host 1 prefers ‘syn’ conformation rather than ‘anti’
4
conformation for creating an electropositive cavity for the anions
by rotation of the amide group. According to the suggested geome-
try cutoffs for D–Hꢁ ꢁ ꢁA hydrogen bond definition21 (where D and A
represent H-bond donor and acceptor), the amide protons (Ha), the
triazole protons (Hb), and the cyclopentadienyl
a-protons (Hc) in
1ꢁH2POꢀ complex make H-bonding with H2POꢀ. The average dis-
4
4
tances (calcd), N–Haꢁ ꢁ ꢁO (2.04 Å) <C–Hbꢁ ꢁ ꢁO (2.15 Å) <N–Hcꢁ ꢁ ꢁO
(2.38 Å), indicate that amideꢁ ꢁ ꢁanion interaction is stronger than
CHꢁ ꢁ ꢁanion interaction, which is in accordance with the 1H NMR
titration results. However, in 2ꢁH2POꢀ complex, the triazole CH pro-
4
ton and amide proton may adopt either ‘anti’ or ‘syn’ conformation
(complex with ‘syn’ conformation is energetically more stable than
that with ‘anti’), but in contrast to 1, only one arm in 2 can partici-
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4
More interestingly, for Fꢀ and Clꢀ ions, despite the fact that the tri-
azole, cyclopentadienyl (a-protons), and amide protons of 1 partic-
ipate in binding with ions, only amide protons in 2 participate in
6. Hua, Y.; Flood, A. H. Chem. Soc. Rev. 2010, 39, 1262–1271.
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being a polyatomic ion, H2POꢀ has the bigger size with tetrahedral
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4
geometry, which facilitates to bind not only with amide protons but
also with other neighboring pꢀrotons in 2. On the other hand, Fꢀ and
Clꢀ, in comparison to H2PO4 , have much smaller volume with
spherical geometry, which preferably bind with the amide proton
of 2 due to its highest acidic nature among all the types (amide, tri-
azole, benzene and cyclopentadienyl protons). Besides, 2 with one
arm is unable to make ‘cleft-form’ geometry while 1 with ‘cleft-
form’ geometry provides the various protons additional chances
to come close to the ions simultaneously and to bind with the ions.
In conclusion, a neutral ‘cleft-form’ anion receptor (1) bearing
amide and triazole donors has been designed and synthesized. We
found that receptor 1 can bind anions via amide N–Hꢁ ꢁ ꢁanion inter-
action, triazole and ferrocene C–Hꢁ ꢁ ꢁanion interaction, with the
binding ability order: amide NH > triazole CH > ferrocene CH, which
is confirmed by theoretical calculation and NMR titration results. By
contrast, in its non-‘cleft-form’ system 2, only the amide group takes
part strongly in N–Hꢁ ꢁ ꢁanion interaction. In addition, 1 showed
12. (a) Beer, P. D.; Bayly, S. R. Top. Curr. Chem. 2005, 255, 125–162; (b) Molina, P.;
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marked electrochemical signal changes to H2POꢀ and Fꢀ over other
4
anions.
Acknowledgements
17. X-ray
data
for
2:
C
24H26FeN4O,
Mw = 442.34,
yellow,
size:
0
0.35 ꢃ 0.21 ꢃ 0.16 mm3, monoclinic, space group P21/c, a = 17.267(3) ÅA,
0
0
0
This work was supported by the CRI project (20120000243)
(J.S.K.). Q.-Y.C. acknowledges the funds from the National Nature
Science Foundation of China (No. 21162017). DHC and TP also
acknowledge Priority Research Centers Program through the NRF
funded by the Ministry of Education, Science and Technology
(NRF20120005860).
b = 12.767(3) ÅA, c = 10.038(2) ÅA, b = 99.688(12) o, V = 2181.3(7) ÅA3, T = 293(2)
K, Z = 4, D = 1.347 Mg/m3,
q
= 0.714 mmꢀ1, F(000) = 928; 17187 reflections
measured, of which 5326 were unique (Rint = 0.0401). 272 refined parameters,
final R1 = 0.0629 for reflections with
I >2r(I), wR2 = 0.1112 (all data),
0
GOF = 0.983. Final largest diffraction peak and hole: 0.248 and ꢀ0.280 e. ÅAꢀ3
18. Katritzky, A. R. Handbook of Heterocyclic Chemistry; Pergamon Press, 2000.
19. Hynes, M. J. J. Chem. Soc. Dalton Trans. 1993, 311–312.
.
20. Amendola, V.; Esteban-Gómez, D.; Fabbrizzi, L.; Licchelli, M. Acc. Chem. Res.
2006, 39, 343–353.
21. (a) Steiner, T.; Desiraju, G. R. Chem. Commun. 1998, 891–892; (b) Steiner, T.
Angew. Chem. Int. Ed. 2002, 41, 48–76.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.06.
118.