Anion-responsive covalently linked and metal-bridged oligomers

Article abstract of DOI:10.1039/c1cc12822a

Ethynyl-substituted acyclic anion receptors were synthesized for discrete covalently linked and metal-bridged dimers, which form various anionic complexes including double helical structures as the first examples of Cl ^--bridged [2+2]-type comp

Full text of DOI:10.1039/c1cc12822a

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COMMUNICATION
Anion-responsive covalently linked and metal-bridged oligomersw
Hiromitsu Maeda,* Kanako Kitaguchi and Yohei Haketa
Received 13th May 2011, Accepted 6th July 2011
DOI: 10.1039/c1cc12822a
Ethynyl-substituted acyclic anion receptors were synthesized for
discrete covalently linked and metal-bridged dimers, which form
various anionic complexes including double helical structures as
the first examples of Clꢀ-bridged [2+2]-type complexes that are
stable in the solution state.
The appropriate arrangement of multiple receptor units is
an essential fabrication strategy of functional assembled
architectures such as helicates and grid-like structures comprising
multiple building components. Ion binding is an effective
protocol to tightly connect multiple building units using a
small number of noncovalent bonds. In sharp contrast to the
numerous reports on metal cations as the glue connecting
p-conjugated species,1 there have only been a few reports on
halide anions that construct [n+n]-type discrete assemblies in
the solution state.2 This is because of the fact that there have
been no appropriate binding motifs to form such anion-driven
assembled structures. Furthermore, complexation of electronically
neutral p-conjugated receptor molecules with anions would
result in the formation of negatively charged assembled
structures, which are promising as electron-rich p-spaces. As
electroneutral p-conjugated units that efficiently bind anions,3
BF2 complexes of 1,3-dipyrrolyl-1,3-propanediones (e.g., 1a,
2a–c, Fig. 1a,b) have exhibited anion-driven dynamic
conformation changes to afford [1+1]-type planar receptor–anion
complexes (Fig. 1a).4–6 Under adequate conditions, receptor
1a forms a [2 (receptors)+1 (anion)]-type complex (Fig. 1a),5c
which serves as a connecting point for the formation of
assembled structures on the basis of receptor oligomers.6 We
focused on an ethynyl group, which can be converted to
various utility moieties using coupling reactions and metal
complexation, as a unit that forms covalent linkages and
related bridges between receptors.7 In this Communication,
we report the synthesis of ethynyl-substituted acyclic anion
Fig. 1 (a) p-Conjugated acyclic anion receptor 1a and its Clꢀ-binding
mode, (b) anion receptors 2a–c, 3m1,2 and 3p1,2 and (c) covalently
linked 4m,p and 4m0 and metal-bridged 5m,p and 6m,p.
receptors 3m1,2 and 3p1,2 (Fig. 1b) as the building subunits
for anion-responsive oligomer systems such as dimers 4–6
(Fig. 1c). It is noteworthy that some of the dimers form
moderately stable [2+2]-type assemblies comprising halide
anions in the solution state.
College of Pharmaceutical Sciences, Institute of Science and
Engineering, Ritsumeikan University, Kusatsu 525–8577, Japan.
E-mail: maedahir@ph.ritsumei.ac.jp; Fax: +81-77-561-2659
w Electronic supplementary information (ESI) available: Synthetic
procedures and analytical data, optimized structures, anion-binding
behaviour and X-ray structural analysis of 3m2, 3p1, 3p2, 4m, 4p and
4pꢁ(TBACl)2. CCDC 819394 (4m), 819395 (4p), 819396 (4pꢁ(TBACl)2).
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c1cc12822a
Following the reported synthetic procedures for 2b,c,5d
ethynyl-substituted derivatives 3m1,2 and 3p1,2 were obtained
in good yields by Suzuki coupling reactions of mono- or
bis-iodinated derivatives of b-ethyl-substituted 2a5b and corres-
ponding triisopropylsilylethynyl-substituted benzene boronates8
followed by silyl deprotection. Single-crystal X-ray analysis of
c
9342 Chem. Commun., 2011, 47, 9342–9344
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Fig. 2 Single-crystal X-ray structures of (a) 4m (top and side view;
one of the two independent structures), (b) 4p (top and side view) and
(c) 4pꢁ(TBACl)2 ((i) top view of 4pꢁClꢀ and (ii) charge-by-charge
2
structure with TBA cations). TBA cations are omitted for clarity in
(c)(i). Atom colour code: brown, pink, yellow, green, blue, red and
yellow green refer to carbon, hydrogen, boron, fluorine, nitrogen,
oxygen and chlorine, respectively.
3m2, 3p1 and 3p2 elucidated the exact structures in the solid
state.z UV/vis absorption spectra of 3m1,2 and 3p1,2 in
CH2Cl2 are observed at 475, 498, 480 and 508 nm, respectively,
and are correlated to the order of DFT-estimated HOMO–
Fig. 3 (a) Selective 1H NMR spectral changes in CD2Cl2 (1.0 ꢂ
10ꢀ3 M) of (i) 4m at ꢀ50 1C upon the addition of TBACl (0, 1.0, 2.0
and 5.0 equiv. from top to bottom), (ii) 4m at rt with 1.0 equiv. of
TBACl, (iii) 4m0 at ꢀ50 1C with 1.1 equiv. of TBACl, and (iv) 4m0 at rt
LUMO gaps.9 On the other hand, fluorescence emissions (lem
)
(and quantum yields FF) excited at each lmax in CH2Cl2 are
observed at 507 (0.83) (3m1), 535 (0.91) (3m2), 516 (0.88) (3p1)
and 550 (0.87) nm (3p2). Further, the binding constants (Ka) of
3m1,2 and 3p1,2 for anions as tetrabutylammonium (TBA)
salts in CH2Cl2 (ESIw, Table S2) are comparable (e.g., 5100 Mꢀ1
for Clꢀ in the case of 3m1) to those of a-unsubstituted 2a5b and
a-phenyl-substituted receptors 2b,c.5d
with 1.1 equiv. of TBACl and (b) AM1 models of 4m, 4m2ꢁClꢀ and
2
4mꢁClꢀ (ethyl-substituents are omitted for facile calculations).
2
[1+2]-type complex (Fig. 3a (i)).z The [2+2]-type complex
along with the [1+1]-type complex was not observed at rt
(Fig. 3a (ii)), presumably due to the fast transitions between
the [1+1]- and the [2+2]-type complexes. Interestingly,
a-phenyl-substituted 4m0 formed a similar [2+2]-type complex
at ꢀ50 1C (Fig. 3a (iii)) as well as at rt (Fig. 3a (iv)). This result
suggests that terminal aryl moieties stabilize [2+2]-type
complexes. The formation of double helical [2+2]-type
Homo-coupling reactions of mono-ethynyl 3m1 and 3p1
afforded dimers 4m and 4p in 37% and 47% yields, respectively.
a-Phenyl-substituted 4m0 was prepared in a low yield by further
bisiodination of 4m followed by a coupling reaction with phenyl-
boronic acid. The lmax and lem values (FF) of the dimers are
477 and 507 (0.87) nm for 4m and 497 and 529 (0.85) nm for 4p.
Single-crystal X-ray analyses of the dimers 4m and 4p and the
[1+2]-type Clꢀ complex of 4p as a TBA salt elucidated the solid-
state structures (Fig. 2).y 4m forms a wire-like 1-D chain using two
p-planes, whereas 4p has completely parallel receptor units that
form no specific assemblies, possibly due to the existence of
complexes was suggested by DOSY of 4m, 4m2ꢁClꢀ and
2
4mꢁClꢀ2, prepared upon the addition of appropriate amounts
of TBACl in CD2Cl2 (1 ꢂ 10ꢀ3 M, ꢀ50 1C), with diffusion
coefficients (10ꢀ10 m2 ꢀ1) 4.64, 3.71 and 4.29, respectively.
s
Furthermore, it was supported by the AM1 optimization9
(Fig. 3b) and ESI-TOF-MS, exhibiting the formation of
dianionic species 4m2ꢁClꢀ2. The UV/vis absorption spectra
of 4m (1 ꢂ 10ꢀ3 M) with 1.0 and 2.0 equiv. for the formation of
[2+2]- and [1+2]-type complexes as major species, respectively,
showed different absorbance changes with decreasing tempera-
tures. Brꢀ complexation exhibited a similar binding behaviour,
whereas Iꢀ showed the formation of [1+1]-type complexes.
Clꢀ in this case is suitable for the formation of [2+2]-type
complexes due to the appropriate size and affinity to be bound
by the receptor units. Such anion-driven double helical struc-
tures, which are stable in solution, are very rare,2 and, to the
best of our knowledge, this is the first example of halide-assisted
[2+2]-type assemblies with the exception of Fꢀ. Conversely,
under similar conditions, 4p did not form any [2+2]-type
complexes but resulted in a [1+1]-type complex as an inter-
mediate to the [1+2]-type complex with Clꢀ, possibly due to the
differences in the geometries of the spacer units. This observa-
tion suggests that the stable intermediates were significantly
affected by the relative orientations of the receptor units.
acetone molecules as solvents. Dianionic 4pꢁClꢀ interacts with
2
TBA cations to form charge-by-charge stacking structures,
wherein the Clꢀꢁ ꢁ ꢁClꢀ distance in the same complex and that
aligned in the stacking columns are 15.63 and 9.60 A, respectively.
The anion-binding behaviour of the dimers in solution was also
examined. Under dilute conditions (5 ꢂ 10ꢀ6 M) in CH2Cl2 at rt,
nearly non-cooperative [1+1]- and [1+2]-type binding modes
10
were observed; the apparent binding constants (Kapp
)
of each
receptor unit of 4m,p for, for example, Clꢀ were estimated at 6200
and 10 000
ꢀ1, respectively. On the other hand, 1H NMR
M
spectral changes of 4m (1 ꢂ 10ꢀ3 M) upon the addition of 1 equiv.
of TBACl in CD2Cl2 at ꢀ50 1C suggested the formation of a
[2+2]-type complex as a major species. The chemical shift of
4m2ꢁClꢀ (e.g., 10.95 and 10.92 ppm for two pyrrole NH and
2
7.23 ppm for bridging CH) was intermediate between 4m
(9.39/9.36 and 6.46 ppm) and 4mꢁClꢀ (12.12/11.76 and
2
8.26 ppm). Similar to the [2+1]-type complex in 1a,5c
the addition of excess Clꢀ interfered with the formation
of a [2+2]-type complex and resulted in the formation of a
c
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measured, 6093 unique (Rint = 0.0957), R1 = 0.0498, wR2 = 0.1131,
GOF = 1.033 (I > 2s(I)). CCDC 819395.
Mono-ethynyl 3m1 and 3p1 were also transformed to
PtII-bridged dimers 5m,p in 64% and 61% yields, respectively,
by treatment with PtCl2(PPh3)2.11 Trans-bridged 5m,p were
converted to cis-PtII-bridged 6m,p in 54% and 57% yields,
respectively, by ligand exchange using 1,2-bis(diphenylphosphino)-
ethene (dppee). Dimers 5m,p and 6m,p show lmax values in
CH2Cl2 at 481, 503, 479 and 493 nm, respectively, and they
exhibit significantly weak fluorescence of lem at 517, 560, 514
and 547 nm, respectively. Compared to diethynyl-bridged
4m,p, PtII-bridged 5m,p and 6m,p show larger Stokes shifts,
for example, 0.25 eV in 5p presumably due to the presence of
heavy metal ions. The Kapp values of receptor units in 5m,p
and 6m,p for Clꢀ were determined as 7100, 2700, 7500 and
7700 Mꢀ1, respectively. From the 1H NMR spectra in CD2Cl2
(1 ꢂ 10ꢀ3 M), it was observed that trans-bridged 5m,p showed
no [2+2]-type complexes with Clꢀ even at ꢀ50 1C, because of
the geometrical restrictions and sterical hindrance of PPh3.
Conversely, cis-bridged 6p constructed a [2+2]-type Clꢀ
complex as an intermediate species under appropriate
conditions, whereas 6m formed mainly a [1+1]-type Clꢀ complex
whose binding-site signals in 1H NMR were also observed
between those of the anion-free receptor and a [1+2]-type
complex. The geometry-dependent formation of [2+2]- or
[1+1]-type complexes was also confirmed by PM6 calculations,
which provided optimized structures of helical anionic
structures in cis-bridged dimers. Introduction of chiral phosphine
ligands to cis-PtII-bridged dimers exhibited anion-responsive
augmentation of CD signals.12
Crystal data for 4pꢁ(TBACl)2 (from CHCl3/hexane): C54H56B2F4N4O4ꢁ
%
2C16H36NClꢁ0.74 water, Mw = 1490.4, triclinic, P1 (no. 2), a =
9.681(7), b = 9.695(7), c = 22.899(14) A, a = 95.21(2), b =
91.74(2), g = 92.92(3)1, V = 2136(3) A3, T = 123(2) K, Z = 1,
Dc = 1.159 g cmꢀ3, m(Mo-Ka) = 0.137 mmꢀ1, 16 858 reflections
measured, 9234 unique (Rint = 0.0727), R1 = 0.0698, wR2 = 0.1809,
GOF = 0.958 (I > 2s(I)). CCDC 819396.
z From the ratios of 4m/4m2ꢁClꢀ2/4mꢁClꢀ by addition of TBACl
2
(1.78 equiv.), association constants defined as [4m2ꢁClꢀ2]/[4m]2[Clꢀ]2
(K22), [4mꢁClꢀ2]/[4m][Clꢀ]2 (K12), and [4mꢁClꢀ2]2/[4m2ꢁClꢀ2][Clꢀ]2
(K122/K22) at ꢀ10, ꢀ30, ꢀ50 and ꢀ70 1C are 1.5 ꢂ 1010, 5.9 ꢂ 1010
,
1.2 ꢂ 1011 and 5.ꢀ82ꢂ 1011
M
ꢀ3 for K22, 4.7 ꢂ 106, 7.3 ꢂ 106, 7.9 ꢂ 106
and 1.6 ꢂ 1ꢀ017 for K12, and 1.5 ꢂ 103, 8.7 ꢂ 102, 5.1 ꢂ 102 and
for K122/K22, respectively.
M
4.4 ꢂ 102
M
8 Stepwise oligomerization protocols based on mono-TIPS-substituted
derivatives 3m2 and 3p2, which are prepared from bis-TIPS precursors,
give bis-TIPS dimers, which can be converted to discrete tetramers by
another single-deprotection and following homo-coupling reaction. In
addition, 3m2 and 3p2 can be directly transformed to dispersed polymers,
which form film-like amorphous matters revealed by XRD (BL40B2,
SPring-8).
1 Selected books for metal-assisted assemblies: (a) Transition Metals in
Supramolecular Chemistry, ed. J.-P. Sauvage, John Wiley & Sons,
Chichester, 1999; (b) Metal-Containing and Metallosupramolecular
Polymers and Materials, ed. U. S. Schubert, G. R. Newkome and
I. Manners, ACS Symposium Series 928, Washington DC, 2006.
2 A recent report on Fꢀ-assisted [2+2]-type complex in solution
state: (a) C.-Y. Chen, T.-P. Lin, C.-K. Chen, S.-C. Lin,
M.-C. Tseng, Y.-S. Wen and S.-S. Sun, J. Org. Chem., 2008,
73, 900. As an oxoanion-driven [2+2]-type assemblies in solution
state: ; (b) J. Sanchez-Queseda, C. Seel, P. Prados and J. de
Mendoza, J. Am. Chem. Soc., 1996, 118, 277.
3 Selected books for anion binding: (a) Supramolecular Chemistry of
Anions, ed. A. Bianchi, K. Bowman-James and E. Garcıa-Espana,
´
In summary, ethynyl-substituted acyclic anion receptors are
used to form discrete covalently linked and metal-bridged
dimers, which form various anionic assemblies including
double helical structures depending on the linkages. In parti-
cular, Clꢀ-driven [2+2]-type helical assemblies in solution
have not been reported thus far. Furthermore, various func-
tional oligomer systems can be prepared from ethynyl-
substituted receptors not only by using homo-coupling8 but
also by cross coupling and click chemistry, which are currently
under investigation.
Wiley-VCH, New York, 1997; (b) Anion Sensing, Topics in Current
Chemistry, ed. I. Stibor, Springer-Verlag, Berlin, 2005, vol. 255;
(c) J. L. Sessler, P. A. Gale and W.-S. Cho, Anion Receptor
Chemistry, RSC, Cambridge, 2006; (d) Anion Complexation in
Supramolecualr Chemistry, Topics in Heterocyclic Chemistry, ed.
P. A. Gale and W. Dehaen, Springer-Verlag, Berlin, 2010, vol. 24.
4 As a recent review: H. Maeda, in Handbook of Porphyrin Science,
ed. K. M. Kadish, K. M. Smith and R. Guilard, World Scientific,
New Jersey, 2010, vol. 8, ch. 38.
5 (a) H. Maeda and Y. Kusunose, Chem.–Eur. J., 2005, 11, 5661;
(b) H. Maeda, Y. Kusunose, Y. Mihashi and T. Mizoguchi, J. Org.
Chem., 2007, 72, 2612; (c) H. Maeda, Y. Haketa and T. Nakanishi,
J. Am. Chem. Soc., 2007, 129, 13661; (d) H. Maeda and Y. Haketa,
Org. Biomol. Chem., 2008, 6, 3191; (e) Y. Haketa, S. Sasaki,
N. Ohta, H. Masunaga, H. Ogawa, N. Mizuno, F. Araoka,
H. Takezoe and H. Maeda, Angew. Chem., Int. Ed., 2010,
49, 10079; (f) H. Maeda, K. Naritani, Y. Honsho and S. Seki,
J. Am. Chem. Soc., 2011, 133, 8896; (g) H. Maeda, Y. Bando,
K. Shimomura, I. Yamada, M. Naito, K. Nobusawa,
H. Tsumatori and T. Kawai, J. Am. Chem. Soc., 2011, 133, 9266.
6 Meta-phenylene-bridged oligomers form various anion-driven helical
structures: Y. Haketa and H. Maeda, Chem.–Eur. J., 2011, 17, 1485.
7 Acetylene Chemistry: Chemistry, Biology, and Material Science, ed.
F. Diederich, P. J. Stang and R. R. Tykwinski, Wiley-VCH,
Weinheim, 2005.
This work was supported by PRESTO/JST (2007–2011),
Grants-in-Aid for Young Scientists (B) (No. 21750155) and
(A) (No. 23685032) from the MEXT and Ritsumeikan
R-GIRO project (2008–2013). We thank Prof. Atsuhiro
Osuka, Dr Shohei Saito, Mr Eiji Tsurumaki and Mr Sumito
Tokuji, Kyoto University, for X-ray analysis, Prof. Hiroshi
Shinokubo and Dr Satoru Hiroto, Nagoya University, for
ESI-TOF-MS and Prof. Hitoshi Tamiaki, Ritsumeikan
University, for various measurements. Y.H. thanks JSPS for
a Research Fellowship for Young Scientists.
8 (a) K. Onituska, M. Fujimoto, H. Kitajima, N. Ohshiro, F. Takei and
Notes and references
¨
¨
S. Takahashi, Chem.–Eur. J., 2004, 10, 6433; (b) A. Godt, O. Unsal and
M. Roos, J. Org. Chem., 2000, 65, 2837; (c) Z. Liu, I. Schmidt,
P. Thamyongkit, R. S. Loewe, D. Syomin, J. R. Diers, Q. Zhao,
V. Misra, J. S. Lindsey and D. F. Bocian, Chem. Mater., 2005, 17, 3728.
9 M. J. Frisch, et al., , Gaussian 03 (Revision C.01), Gaussian, Inc.,
Wallingford, CT, 2004.
z See the ESI for the crystal data for 3m2, 3p1 and 3p2.
y Crystal data for 4m (from CH2Cl2/hexane): C54H56B2F4N4O4,
Mw = 922.65, monoclinic, P21/c (no. 14), a = 29.692(5), b =
23.863(4), c = 16.660(3) A, b = 93.025(3)1, V = 11787(4) A3, T =
123(2) K, Z = 8, Dc = 1.04 g cmꢀ3, m(Mo-Ka) = 0.074 mmꢀ1, 60 126
reflections measured, 20 747 unique (Rint = 0.0731), R1 = 0.0663,
wR2 = 0.1642, GOF = 0.916 (I > 2s(I)). CCDC 819394.
10 D. Aldakov and P. Anzenbacher Jr., J. Am. Chem. Soc., 2004,
126, 4752.
11 K. Campbell, C. A. Johnson II, R. McDonald, M. J. Ferguson,
M. M. Haley and R. R. Tykwinski, Angew. Chem., Int. Ed., 2004,
43, 5967.
Crystal data for 4p (from acetone/hexane): C54H56B2F4N4O4ꢁ2acetone,
Mw = 1038.80, orthorhombic, Pbca (no. 61), a = 14.226(2), b =
19.130(3), c = 19.557(4) A, V = 5322.4(16) A3, T = 123(2) K, Z = 4,
Dc = 1.296 g cmꢀ3, m(Mo-Ka) = 0.092 mmꢀ1, 46 570 reflections
12 Manuscript in preparation.
c
9344 Chem. Commun., 2011, 47, 9342–9344
This journal is The Royal Society of Chemistry 2011