9342 Chem. Commun., 2011, 47, 9342–9344 This journal is c The Royal Society of Chemistry 2011

Cite this: Chem. Commun., 2011, 47, 9342–9344

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 Cl�-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

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.

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

Fig. 1 (a) p-Conjugated acyclic anion receptor 1a and its Cl�-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.

College of Pharmaceutical Sciences, Institute of Science andEngineering, Ritsumeikan University, Kusatsu 525–8577, Japan.E-mail: maedahir@ph.ritsumei.ac.jp; Fax: +81-77-561-2659w Electronic supplementary information (ESI) available: Syntheticprocedures and analytical data, optimized structures, anion-bindingbehaviour and X-ray structural analysis of 3m2, 3p1, 3p2, 4m, 4p and4p�(TBACl)2. CCDC 819394 (4m), 819395 (4p), 819396 (4p�(TBACl)2).For ESI and crystallographic data in CIF or other electronic formatsee DOI: 10.1039/c1cc12822a

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This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 9342–9344 9343

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–

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 M�1

for Cl� in the case of 3m1) to those of a-unsubstituted 2a5b and

a-phenyl-substituted receptors 2b,c.5d

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 Cl� 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

acetone molecules as solvents. Dianionic 4p�Cl�2 interacts withTBA cations to form charge-by-charge stacking structures,

wherein the Cl�� � �Cl� 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 � 10�6 M) in CH2Cl2 at rt,

nearly non-cooperative [1+1]- and [1+2]-type binding modes

were observed; the apparent binding constants (Kapp)10 of each

receptor unit of 4m,p for, for example, Cl� were estimated at 6200

and 10000 M�1, respectively. On the other hand, 1H NMR

spectral changes of 4m (1� 10�3 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

4m2�Cl�2 (e.g., 10.95 and 10.92 ppm for two pyrrole NH and

7.23 ppm for bridging CH) was intermediate between 4m

(9.39/9.36 and 6.46 ppm) and 4m�Cl�2 (12.12/11.76 and

8.26 ppm). Similar to the [2+1]-type complex in 1a,5c

the addition of excess Cl� interfered with the formation

of a [2+2]-type complex and resulted in the formation of a

[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

complexes was suggested by DOSY of 4m, 4m2�Cl�2 and

4m�Cl�2, prepared upon the addition of appropriate amounts

of TBACl in CD2Cl2 (1 � 10�3 M, �50 1C), with diffusion

coefficients (10�10 m2 s�1) 4.64, 3.71 and 4.29, respectively.

Furthermore, it was supported by the AM1 optimization9

(Fig. 3b) and ESI-TOF-MS, exhibiting the formation of

dianionic species 4m2�Cl�2. The UV/vis absorption spectra

of 4m (1 � 10�3 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. Br� complexation exhibited a similar binding behaviour,

whereas I� showed the formation of [1+1]-type complexes.

Cl� 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 F�. 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 Cl�, 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.

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) 4p�(TBACl)2 ((i) top view of 4p�Cl�2 and (ii) charge-by-charge

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.

Fig. 3 (a) Selective 1H NMR spectral changes in CD2Cl2 (1.0 �10�3 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

with 1.1 equiv. of TBACl and (b) AM1 models of 4m, 4m2�Cl�2 and4m�Cl�2 (ethyl-substituents are omitted for facile calculations).

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9344 Chem. Commun., 2011, 47, 9342–9344 This journal is c The Royal Society of Chemistry 2011

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 Cl� were determined as 7100, 2700, 7500 and

7700 M�1, respectively. From the 1H NMR spectra in CD2Cl2

(1 � 10�3 M), it was observed that trans-bridged 5m,p showed

no [2+2]-type complexes with Cl� even at �50 1C, because of

the geometrical restrictions and sterical hindrance of PPh3.

Conversely, cis-bridged 6p constructed a [2+2]-type Cl�

complex as an intermediate species under appropriate

conditions, whereas 6m formed mainly a [1+1]-type Cl� 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

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, Cl�-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.

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.

Notes and references

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 cm�3, m(Mo-Ka) = 0.074 mm�1, 60 126reflections measured, 20 747 unique (Rint = 0.0731), R1 = 0.0663,wR2 = 0.1642, GOF = 0.916 (I > 2s(I)). CCDC 819394.

Crystal data for 4p (from acetone/hexane): C54H56B2F4N4O4�2acetone,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 cm�3, m(Mo-Ka) = 0.092 mm�1, 46 570 reflections

measured, 6093 unique (Rint = 0.0957), R1 = 0.0498, wR2 = 0.1131,GOF = 1.033 (I > 2s(I)). CCDC 819395.

Crystal data for 4p�(TBACl)2 (from CHCl3/hexane): C54H56B2F4N4O4�2C16H36NCl�0.74 water, Mw = 1490.4, triclinic, P%1 (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 cm�3, m(Mo-Ka) = 0.137 mm�1, 16 858 reflectionsmeasured, 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/4m2�Cl�2/4m�Cl�2 by addition of TBACl(1.78 equiv.), association constants defined as [4m2�Cl�2]/[4m]2[Cl�]2

(K22), [4m�Cl�2]/[4m][Cl�]2 (K12), and [4m�Cl�2]2/[4m2�Cl�2][Cl�]2(K12

2/K22) at �10, �30, �50 and �70 1C are 1.5 � 1010, 5.9 � 1010,1.2 � 1011 and 5.8 � 1011 M

�3 for K22, 4.7 � 106, 7.3 � 106, 7.9 � 106

and 1.6 � 107 M�2 for K12, and 1.5 � 103, 8.7 � 102, 5.1 � 102 and

4.4 � 102 M�1 for K12

2/K22, respectively.8 Stepwise oligomerization protocols based on mono-TIPS-substitutedderivatives 3m2 and 3p2, which are prepared from bis-TIPS precursors,give bis-TIPS dimers, which can be converted to discrete tetramers byanother single-deprotection and following homo-coupling reaction. Inaddition, 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 inSupramolecular Chemistry, ed. J.-P. Sauvage, John Wiley & Sons,Chichester, 1999; (b) Metal-Containing and MetallosupramolecularPolymers and Materials, ed. U. S. Schubert, G. R. Newkome andI. Manners, ACS Symposium Series 928, Washington DC, 2006.

2 A recent report on F�-assisted [2+2]-type complex in solutionstate: (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 solutionstate: ; (b) J. Sanchez-Queseda, C. Seel, P. Prados and J. deMendoza, J. Am. Chem. Soc., 1996, 118, 277.

3 Selected books for anion binding: (a) Supramolecular Chemistry ofAnions, ed. A. Bianchi, K. Bowman-James and E. Garcıa-Espana,Wiley-VCH, New York, 1997; (b) Anion Sensing, Topics in CurrentChemistry, ed. I. Stibor, Springer-Verlag, Berlin, 2005, vol. 255;(c) J. L. Sessler, P. A. Gale and W.-S. Cho, Anion ReceptorChemistry, RSC, Cambridge, 2006; (d) Anion Complexation inSupramolecualr 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 helicalstructures: 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.

8 (a) K. Onituska, M. Fujimoto, H. Kitajima, N. Ohshiro, F. Takei andS. Takahashi,Chem.–Eur. J., 2004, 10, 6433; (b) A. Godt, O. Unsal andM. 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 andD. F. Bocian,Chem.Mater., 2005, 17, 3728.

9 M. J. Frisch, et al., , Gaussian 03 (Revision C.01), Gaussian, Inc.,Wallingford, CT, 2004.

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.

12 Manuscript in preparation.

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