Chinese Chemical Letters xxx (2014) xxx–xxx

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CCLET-2903; No. of Pages 7

Original article

Neutral C–H bond vs. electron pair of N(sp2): A binding site effect studyof macrocycle anion receptor

Chuan-Cai Fan a, Li-Jin Xu a,*, Han-Yuan Gong b,*a Department of Chemistry, Renmin University of China, Beijing 100872, Chinab College of Chemistry, Beijing Normal University, Beijing 100875, China

A R T I C L E I N F O

Article history:

Received 7 January 2014

Received in revised form 12 February 2014

Accepted 21 February 2014

Available online xxx

Keywords:

Macrocycle

Anion binding

Neutral C–H bond

N(sp2)

Hydrogen bond

A B S T R A C T

To evaluate the effect of neutral C–H bond or electron pair of nitrogen atom with sp2 hybridization

(N(sp2)) involving into the same chemical environment for anion binding, two analogous tetracationic

imidazolium macrocycles, namely cyclo[2](2,6-bis-(1H-imidazol-1-yl)pyridine) [2](1,3-dimethylene-

benzene) (14+), and cyclo[2](2,6-bis-(1H-imidazol-1-yl)pyridine)[2](2,6-di methylenepyridine) (24+)

were studied in detail as small inorganic anion receptors. The guest anions with different shapes are Cl�,

N3�, NO3

�, and H2PO4�. The host–guest interactions were characterized via 1H NMR spectroscopy,

electrospray ionization mass spectrometry (ESI-MS) and single crystal X-ray crystallography. The results

implied that macrocyclic hosts with similar backbone but two distinct binding sites (14+ with neutral C–

H vs. 24+ with N (sp2)) vary markedly in their response to anions, including the binding modes and

association constants. The finding will serve to the construction of new anion receptors, even improve

insights into the anion binding process in biology.

� 2014 Li-Jin Xu and Han-Yuan Gong. Published by Elsevier B.V. on behalf of Chinese Chemical

Society. All rights reserved.

Contents lists available at ScienceDirect

Chinese Chemical Letters

jo u rn al h om epag e: ww w.els evier .c o m/lo cat e/cc le t

1. Introduction

As the foundation elements of natural world, anions playimportant roles in a range of chemical, biological and environ-mental processes [1–3]. In the past a few decades, supramolecularchemists have made significant advances toward anion recogni-tion [4].

Until recent five years, the property of neutral C–H site(s) hasproved to effectively bind with anion via intermolecular hydrogenbond(s) both in experimental and computational investigations[5,6]. The finding attracts the interests both in chemistry andbiology fields because C–H bond is exist in the majority (97%) ofchemical compounds [7]. On the other hand, the nitrogen atomwith sp2 hybridization (N(sp2)) also received significant attentionof chemists and biologists for so long time with its metal bindingand intermolecular hydrogen acceptor properties, but have rarelybeen evaluated in anion complexation [8,9].

Recently, we reported the facile synthesis of two noveltetraimidazolium macrocycles, cyclo[2](2,6-bis-(1H-imidazol-1-yl)-pyridine)[2](1,3-dimethylenebenzene) (14+), and cyclo[2]

* Corresponding authors.

E-mail addresses: xulj@chem.ruc.edu.cn (L.-J. Xu), hanyuangong@bnu.edu.cn

(H.-Y. Gong).

Please cite this article in press as: C.-C. Fan, et al., Neutral C–H bond vs

anion receptor, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j

http://dx.doi.org/10.1016/j.cclet.2014.03.019

1001-8417/� 2014 Li-Jin Xu and Han-Yuan Gong. Published by Elsevier B.V. on behalf

(2,6-bis-(1H-imidazol-1-yl)pyridine)[2](2,6-dimethylenepyridine)(24+), with the names as ‘‘Texas-sized’’ molecular boxes [10]. Thesmall differences between them locate on two sites of the bridgedgroups toward the center of the macrocycle (X sites in Fig. 1).More specifically, macrocycle 14+ has neutral C–H bonds whereas24+ instead with N(sp2) groups. These two structures provide amodel that evaluate the effect of neutral C–H bond vs. N(sp2) foranion binding in similar chemical environment. For the purpose,the interactions between host 14+ or 24+ and four anion guestswith different shapes (i.e. Cl�, N3

�, NO3� and H2PO4

�) werestudied in detail to investigate the influence of neutral C–H bondor N(sp2) group in anion complexations. As shown below, thesupramolecular complexes formed between macrocycle 14+ or 24+

and anions were distinct in stoichiometries, association con-stants, and binding modes. The results develop the knowledge ofthe different effects between two wide-spread active sites,namely neutral C–H vs. N(sp2), and their roles in anion bindingeven in biology.

2. Experimental

2.1. Instruments and reagents

Deuterated dimethyl sulfoxide (DMSO-d6) was purchased fromCambridge Isotope Laboratory (Andover, MA). All the other

. electron pair of N(sp2): A binding site effect study of macrocycle.cclet.2014.03.019

of Chinese Chemical Society. All rights reserved.

Fig. 1. Structures of ‘‘Texas-sized’’ molecular box 14+ and 24+.

C.-C. Fan et al. / Chinese Chemical Letters xxx (2014) xxx–xxx2

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CCLET-2903; No. of Pages 7

solvents and chemicals were purchased commercially (Aldrich,Acros, or Fisher) and used without further purification. All theguest anions were studied as their tetrabutylammonium (TBA+)salts (i.e. TBA+�Cl�, TBA+�N3

�, TBA+�NO3� and TBA+�H2PO4

�). 1HNMR spectra were recorded in DMSO-d6 on a Bruker AdvancedInstrument (600 MHz). The chemical shifts of the proton signalsare reported relative to tetramethylsilane (TMS). For all the anionbinding studies involving 14+ or 24+, the starting counter anionswere always hexafluorophosphate (i.e. PF6

�), which is obtained asreference [10].

The crystallographic data were collected on a Rigaku Mercury2(2 � 2 bin mode) or a Saturn724 + (2 � 2 bin mode) CCDdiffractormeter using a graphite monochromator with Mo Karadiation (l = 0.71073 A). The data were collected using w-scanswith a scan range of 18 at low temperature using a Rigaku Tek50low-temperature device on the Rigaku Mercury2 (2 � 2 bin mode),or a Rigaku XStream low-temperature device on theSaturn724 + (2 � 2 bin mode) diffractometer.

The compound for electrospray ionization mass spectrometry(ESI-MS) analysis were obtained with anion exchange process via

adding 10 molar equiv. of TBA+�A� (A� = Cl�, N3�, NO3

� andH2PO4

�) in the acetonitrile solution of 14+�4PF6� or 24+�4PF6

� withconcentration as 5.00 � 10�3 mol/L. White precipitate productswere collected and washed with acetonitrile for ESI-MS spectros-copy study. The ESI-MS data were collected via infusion on a liquidchromatograph mass spectrometer (LC–MS-2010) operating inpositive ion model.

2.2. Experimental procedure

The Job-plots experiment: As described in Ref. [11,12], thewhole concentration of host and guest remain constant during theprocess as below.

Fig. 2. Job-plots corresponding to the binding between host 14+ ([host] + [guest] = 5.00

with different representation symbol (Cl� , N3� , NO3

� , H2PO4�&) constructed

Please cite this article in press as: C.-C. Fan, et al., Neutral C–H bond vs

anion receptor, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j

The titration experiment: Following the procedure shown inRef. [12], in the process of every titration, the concentration of hostremains stable as 5.00 � 10�4 mol/L.

2.3. The single crystal X-ray structure study

2.3.1. Crystal cultivation

Diffraction grade crystals were obtained by slow evaporationfrom solution in the mixture of water and N,N-dimethylformamide(DMF) as described below.

2.3.2. Data refinement

Data reduction was performed using CrystalClear [13]. Thestructures were solved by direct methods using SIR97 [14] andrefined by full-matrix least-squares on F2 with anisotropicdisplacement parameters for the non-H atoms using SHELXL-97[15]. The hydrogen atoms were calculated in ideal positions withisotropic displacement parameters set to 1.2 � Uequiv. of theattached atom (1.5 � Uequiv. for methyl hydrogen atoms). Theutility ROTAX [16] in the program WinGX [17] was used to look forpossible twins. The function, w(jFoj2 � jFcj2), was minimized.Neutral atom scattering factors and values used to calculate thelinear absorption coefficient are from the International Tables forX-ray Crystallography (1992). All ellipsoid figures were generatedusing SHELXTL/PC [18]. Tables of positional and thermal param-eters, bond lengths and angles, torsion angles, figures and lists ofobserved and calculated structure factors are located in the cifdocuments available from the Cambridge Crystallographic Centervia quoting reference numbers 842438, 979059 and 979060.

3. Results and discussion

3.1. 1H NMR Job-plots

To confirm the binding stoichiometry between macrocycle host14+ or 24+ and anion guests with different shapes (namely Cl�

(ball), N3� (linear), NO3

� (triangle) or H2PO4� (tetrahedron)), Job-

plots were performed for every host/guest pair respectively. TheJob-plots of 14+ and Cl� or N3

� displayed maximum values at 0.5,which are consistent with a 1:1 (host: guest) binding stoichiome-try; meanwhile the data of 14+ and guest anion NO3

� or H2PO4�

peaked at 0.67, indicating the complexation stoichiometry both are1:2 (host: guest) (Fig. 2a). Differently, host 24+ bind every kind ofguest (i.e. Cl�, N3

�, NO3� or H2PO4

�) with the same 2:3 (host:guest) stoichiometry, which were characterized with all themaximum values at 0.6 in their Job-plots (Fig. 2b). Obviously,the small difference between two macrocycle anion receptors(neutral C–H on 14+ vs. N(sp2) on 24+) with analogous skeleton

� 10�4 mol/L) (a), or 24+ ([host] + [guest] = 1.00 � 10�3 mol/L) (b) and anion guests

from 1H NMR (600 MHz) spectral data.

. electron pair of N(sp2): A binding site effect study of macrocycle.cclet.2014.03.019

Fig. 3. 1H NMR (600 MHz) binding isotherms of H(1), H(8) on 14+, H(10) on 24+ (a), or H(3), H(6) on 14+, H(30), H(70) on 24+ (b) with additional Cl� in DMSO-d6 at 300 K. The red

dash lines show the non-linear curve fit of the experiment data to the appropriate equation. (For interpretation of the references to color in this figure legend, the reader is

referred to the web version of this article.).

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CCLET-2903; No. of Pages 7

causes their distinct binding modes with same anion, resulting indifferent equilibrium. It is noted that one macrocyclic host has thepotential to bind more than one small inorganic anionic guest inthe solution. The multi-anion binding property was also supportedvia the additional evidences from electrospray ionization massspectrometry (ESI-MS) study in gas phase and single crystal X-raycrystallography study shown below. There are two possiblereasons of the fact: (1) One host with big cavity can bind smallanionic guests both inside and outside of its cavity; (2) Macrocyclehosts contains tetracationic charge so that it could combine withmultiple small single charge anion.

3.2. 1H NMR titration

In order to complete the binding study in solution besidesstoichiometry and obtain the associate constants between 14+ or24+ and every anion guest (namely Cl�, N3

�, NO3� or H2PO4

�),corresponding titration experiments were carried out respective-ly. As Fig. 3a shows below, the titration results betweenmacrocycle host 14+ or 24+ and Cl� have been discussed in detailto explain the different effect of neutral C–H or the electron pair ofN(sp2) on analogue anion receptor backbones. The large low fieldchemical shift change of H(8) on 14+ with increasing Cl� suggestedthat the neutral C–H� � �Cl intermolecular hydrogen bond(s) playan important role in the complexation. It is observed that thesignal of H(1) on 14+ shows less low field chemical shift, even theprotons has more acidity than H(8) (Fig. 3a). The possible reason ofthe phenomenon is the influence of intermolecular anion–p

Fig. 4. 1H NMR spectroscopic titration of 14+ (5.00 � 10�4 mol/L) (a), and 24+ (5

Please cite this article in press as: C.-C. Fan, et al., Neutral C–H bond vs

anion receptor, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j

interactions. Further supporting evidence of the deduction camefrom the signal of H(3) on 14+ moving to the high field withadditional Cl�. Meanwhile the signal of H(6) on 14+ shifts to lowfield, which implied that anion–p interactions is mainly suppliedvia pyridinyl moieties other than bridged benzene rings on 14+

(Figs. 3b and 4a).As the titration process between 14+ and Cl�, the low field

chemical shift change of H(10) on 24+ with increasing Cl�gave out theevidence of intermolecular hydrogen bonds (Fig. 4b). Differently,when bridged benzene rings of 14+ were replaced with pyridinemoieties (i.e. 24+), the chemical shift of H(30), H(70) on 24+ took an up-down change tendency relative to 14+. More specifically, the lowfield chemical shift change of H(30) and the high field chemical shiftchange of H(70) on 24+ co-supported the suggestion that the anion–pinteraction between 24+ and Cl�was primarily provided via bridgedpyridine ring (the pyridinyl fragments with proton H(7’)), instead ofpyridine plane on 2,6-bis-(1H-imidazol-1-yl) pyridine with protonH(30), which efficiently support the anion–p interactions between14+ and Cl� as discussed before. Besides Cl� (as the representative ofanion with ball shape), what were explored in the study included theinteractions between macrocycle anion receptor 14+ or 24+ and theanion guests with different shapes (i.e. N3

� (linear), NO3� (triangle),

or H2PO4� (tetrahedron)). Surprisingly, the chemical shift change

tendencies of the proton signals on 14+ or 24+ were similar as they didwith additional Cl�, no matter what shape the anion guest wasintroduced. The results suggested that for all the studied smallinorganic anion guests, the macrocycle anion receptor 14+ or 24+

adopted analogical binding mode as it does for Cl�.

.00 � 10�4 mol/L) (b) with increasing Cl� in DMSO-d6 at 300 K (600 MHz).

. electron pair of N(sp2): A binding site effect study of macrocycle.cclet.2014.03.019

Table 1Summary of equilibrium and the calculated association constants Ka.

Guest (shape) Host Equilibrium Ka

Cl� (ball) 14+ ½H� þ ½G� ) *K1 ½H � G� K1 = (1.3 � 0.1) � 103 (mol/L)�1

24+ ½H� þ ½G� ) *K1 ½H � G� K1 = (1.3 � 0.1) � 102 (mol/L)�1

2½H � G� þ ½G� ) *K2 ½H2 � G3� K2 = (2.2 � 0.1) � 104 (mol/L)�2

N3� (line) 14+ ½H� þ ½G� ) *

K1 ½H � G� K1 = (3.7 � 0.2) � 103 (mol/L)�1

24+ ½H� þ ½G� ) *K1 ½H � G� K1 = (3.5 � 0.2) � 103 (mol/L)�1

2½H � G� þ ½G� ) *K2 ½H2 � G3� K2 = (1.5 � 0.1) � 105 (mol/L)�2

NO3�(triangle) 14+ ½H� þ ½G� ) *

K1 ½H � G� K1 = (1.9 � 0.1) � 103 (mol/L)�1

½H � G� þ ½G� ) *K2 ½H � G2� K2 = (4.2 � 0.2) � 102 (mol/L)�1

24+ ½H� þ ½G� ) *K1 ½H � G� K1 = (2.5 � 0.1) � 102 (mol/L) �1

2½H � G� þ ½G� ) *K2 ½H2 � G3� K2 = (6.7 � 0.3) � 100 (mol/L)�2

H2PO4� (tetrahedron) 14+ ½H� þ ½G� ) *

K1 ½H � G� K1 = (3.8 � 0.2) � 105 (mol/L) �1

½H � G� þ ½G� ) *K2 ½H � G2� K2 = (8.7 � 0.4) � 102 (mol/L)�1

24+ ½H� þ ½G� ) *K1 ½H � G� K1 = (1.2 � 0.1) � 103 (mol/L)�1

2½H � G� þ ½G� ) *K2 ½H2 � G3� K2 = (1.3 � 0.1) � 108 (mol/L)�2

Table 2Summary of ESI-MS results.

Compound Observed m/z value Peak assignment

14+�4Cl� 227.5 [14+ + Cl + Na-6H]3+�

14+�4N3� 224.3 [14+ + N3]3+

14+�4NO3� 231.1

396.4

[14+ + NO3]3+

[14+ + 2NO3 + K-H]2+

14+�4H2PO4� 363.4

436.3

[14+ + H2PO4-H]2+

[14+ + 2H2PO4 + 2Na + 2H]2+�

24+�4Cl� 222.0

374.4

[24+ + Cl-2H]3+�

[(24+)2 + 3Cl + 2Na + 2K + 2H]4+�

24+�4N3� 363.5

231.4

[(24+)2 + 3N3 + Na + K]4+�

[24+ + NO3]3+

24+�4NO3� 378.2

363.9

[(24+)2 + 3NO3 + Na + K]4+�

[24+ + H2PO4-2H]2+�

24+�4H2PO4� 526.2 [(24+)2 + 3H2PO4 + Na-H]3+�

C.-C. Fan et al. / Chinese Chemical Letters xxx (2014) xxx–xxx4

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Based on every binding stoichiometry between each host-guestpair detected via Job-plots, the functional relationship between thechemical shift change of proton signals on the host and thecorresponding concentration of anion guest was investigated via

Hyperquad2003 program. Finally, the values of K1 and/or K2 of anioncomplexation were obtained by non-linear curve fit [19,20]. The

Table 3X-ray crystallographic data of [14+�4NO3

�4H2O], [14+�4Cl�2DMF 4H2O] a

[14+ 4NO3��4H2O]

CCDC No. 842438

Description Block

Color Colorless

Solution Water/DMF

Empirical formula C38H42N14O16

Mr 950.86

Crystal size (mm3) 0.284 � 0.280 � 0.144

Crystal system Monoclinic

Space group P2(1)/c

a [A] 11.295(2)

b [A] 25.244(5)

c [A] 14.836(3)

a [deg] 90.00

b [deg] 99.61(3)

g[deg] 90.00

V/[A3] 4170.7(15)

d/[g/cm3] 1.514

Z 4

T [K] 173(2)

R1, wR2 I > 2o(I) 0.0560, 0.1298

R1, wR2 (all data) 0.0612, 0.1345

Quality of fit 1.000

Please cite this article in press as: C.-C. Fan, et al., Neutral C–H bond vs

anion receptor, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j

results of 1H NMR Job-plot and titration study has been summed upin Table 1. Herein, K1 of macrocycle 14+ is much larger than 24+ forevery anion guest binding, which also further support that neutralC–H on benzene rings of 14+ stabilize the anion complexation via

intermolecular hydrogen bond formation. On the other hand, theN(sp2) electron pair of bridged pyridinyl moieties with H(7) on 24+

tends to form possible intramolecular hydrogen bond(s). Bothfactors were suggested to cause the larger K1 values of 14+ than 24+

for the same anion complexation as shown in Table 1.

3.3. Details of ESI-MS study

ESI-MS experiments were also carried out to detect the host/guest interactions. The results listed in Table 2 lead us to suggestthat the host/guest complexes may also exist in the gas phase.

3.4. Single crystal X-ray structure analysis

The data collection and structure refinement statistics of singlecrystal X-ray structures, namely [14+ 4Cl� 2DMF�4H2O] and [24+

2PF6��2NO3

�] involving into this work are summarized in Table 3.The single crystal structure of [14+ 4NO3

��4H2O] reported before[10] was displayed here for the following discussion.

nd [24+�2PF6�2NO3

�].

[14+ 4Cl� 2DMF�4H2O] [24+ 2PF6��2NO3

�]

979059 979060

Prism Block

Colorless Colorless

Water/DMF Water/DMF

C44H60Cl4N12O8 C36H32F12N14O6P2

1026.84 1046.70

0.48 � 0.15 � 0.10 0.20 � 0.14 � 0.08

Triclinic Triclinic

P-1 P-1

9.4978(19) 7.9364(7)

16.910(3) 11.1651(12)

17.332(4) 11.9799(11)

70.07(3) 76.686(4)

82.72(3) 87.191(3)

79.55(3) 80.387(4)

2567.1(9) 1018.45(17)

1.328 1.707

2 1

153(2) 153(2)

0.0561, 0.1261 0.0490, 0.1227

0.0717, 0.1361 0.0652, 0.1346

1.002 1.004

. electron pair of N(sp2): A binding site effect study of macrocycle.cclet.2014.03.019

Fig. 5. Top view with the ellipsoid form (a), as well as the side view (b), top view (c)

and front view (d) with the stick form of the ‘‘clip like’’ conformation of 14+ with a

insert NO3� anion present in 14+ 4NO3

��4H2O. Displacement ellipsoids are scaled to

the 25% probability level. All the other molecules have been omitted for clarity.

Fig. 6. Top view with ellipsoid form (a), as well as the top view (b), front view (c) and

side view (d) showing 14+ in stick form, along with the two co-bound NO3� anions

(in space filling form) present in 14+ 4NO3��4H2O. Displacement ellipsoids are

scaled to the 25% probability level. All the other molecules have been omitted for

clarity.

Fig. 7. Top view with ellipsoid form (a), as well as the side view (b), top view (c) and

front view (d) showing the ‘‘clip like’’ conformation of 14+ in stick form, along with

the two co-bound DMF and one water molecules (in space filling form) present in

[14+ 4Cl� 2DMF�4H2O]. Displacement ellipsoids are scaled to the 25% probability

level. All the other molecules have been omitted for clarity.

C.-C. Fan et al. / Chinese Chemical Letters xxx (2014) xxx–xxx 5

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CCLET-2903; No. of Pages 7

The single crystal X-ray structure of [14+ 4NO3��4H2O] has been

studied to show the flexibility of 14+ [10]. But the effect of neutralC–H bond for anion binding via intermolecular hydrogen bonds hasnot been discussed. In this structure, 14+ contains one NO3

� anionin its cavity. The ‘‘clip’’ conformation of macrocycle with NO3

�

result in a molecular ‘‘sandwich’’ structure, which is stabilized via

intermolecular hydrogen bond between NO3� and neutral C–H on

C(18) (characterized with short distance between O(4) on NO3�

and C(18) less than 3.4 A), as well as anion-p interactions betweenthe plane of NO3

� anion and the neighbor pyridinyl moieties on 14+

with distance less than 3.3 A (Fig. 5). Select bond distances are asfollows: selected interatomic hydrogen bonding distances [A]:O(4). . .C(18) 3.393, O(6). . .C(37) 3.730. Selected interatomicangles: C(18)–H(18A). . .O(4) 141.98, C(37)–H(37A). . .O(6) 131.08.Selected anion. . .p distances [A]: O(5). . .N(3) 3.757, O(5). . .N(8)3.294, O(5). . .C(23) 3.250.

Meanwhile, it is noted that two other neighbor NO3� anion bind

with 14+ mainly via anion–p interactions with ‘‘outside’’ mode,which is characterized with the short distances (less than 3.0 A)between the plane of NO3

� anion and the neighbor pyridinylmoieties of 2,6-bis-(1H-imidazol-1-yl)pyridine on 14+, as thesolution study implied before (Fig. 6). Select bond distances areas follows: selected anion. . .p distances [A]: N(11). . .C(27) 3.285,N(11). . .N(8) 3.763, N(11). . .N(9) 3.502, N(11). . .C(26) 3.416,O(1). . .C(26) 3.618, O(1). . .C(27) 3.618, O(1). . .N(9) 3.522,O(1). . .C(29) 3.491, O(2). . .C(26) 3.638, O(3). . .N(8) 3.140,O(3). . .N(9) 3.071, O(3). . .C(27) 2.993, O(3). . .C(28) 3.223;N(13). . .C(8) 3.374, N(13). . .N(3) 3.741, N(13). . .N(4) 3.399,N(13). . .C(7) 3.754, O(7). . .C(9) 3.652, O(7). . .C(10) 3.653,O(7). . .N(4) 3.452, O(8). . .C(9) 3.020, O(8). . .N(4) 3.153,O(8). . .C(8) 3.141, O(8). . .N(3) 3.116, O(9). . .C(7) 3.758,O(9). . .C(8) 3.748.

A new single crystal X-ray structure, the pure chloride salt of 14+

was obtained by slow evaporation from solution using mixtures ofwater and DMF. The structure of [14+ 4Cl� 2DMF�4H2O], shows theneutral C–H on aromatic ring (e.g. benzene) can act as strongintermolecular hydrogen bond donor for not only anionic species,but also neutral molecules binding (e.g. DMF and H2O). As listedbelow, 14+ bind with DMF and H2O mainly via intermolecularhydrogen bond between O(1) and C(32) with distance less than3.6 A, and between O(6WA) and C(32) with distance less than 3.7 A.p–p donor-acceptor interactions also appeared between the planeof DMF molecules and the neighbor 2,6-bis-(1H-imidazol-1-yl)

Please cite this article in press as: C.-C. Fan, et al., Neutral C–H bond vs

anion receptor, Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/j

pyridinyl moieties on 14+ with distance less than 3.5 A (Fig. 7). Selectbond distances are as follows: selected interatomic hydrogenbonding distances [A]: O(1). . .C(32) 3.522, O(6WA). . .C(32) 3.603.Selected interatomic angles: C(32)-H(32A). . .O(1) 113.58, C(32)-H(32A). . .O(6WA) 169.28. Selected p. . .p distances [A]: O(1). . .C(1)3.042, O(1). . .C(28) 3.060, O(2). . .C(9) 3.365, O(2). . .C(20) 3.365,C(39). . .C(1) 3.558, C(39). . .C(27) 3.268, C(42). . .N(4) 3.314,C(42). . .N(7) 3.363, N(11). . .C(27) 3.475, N(11). . .C(4) 3.557,N(12). . .C(10) 3.549, N(12). . .C(22) 3.519.

With the similar procedure to obtain [14+ 4NO3��4H2O], but

instead 14+ 4PF6� with 24+ 4PF6

�, only half PF6� in 24+ 4PF6

� wassubstituted by NO3

� and gave out the single crystal structure of[24+ 2PF6

��2NO3�]. The possible reason is the weaker interaction of

24+ than 14+ for NO3� binding. It is suggested that the

intramolecular hydrogen bond formation between hydrogenbond-acceptor (N(sp2)) on bridged pyridine and its neighboringcationic C–H on imidazolium ring. Herein, 24+ bind with NO3

�

mainly via anion–p interaction between NO3� and neighbor

. electron pair of N(sp2): A binding site effect study of macrocycle.cclet.2014.03.019

Fig. 8. Top view with ellipsoid form (a), as well as the top view (b), side view (c) and

front view (d) showing the host 24+ in stick form, along with the two co-bound NO3�

anions (in space filling form) present in [24+ 2PF6��2NO3

�]. Displacement ellipsoids

are scaled to the 25% probability level. All the other molecules have been omitted

for clarity.

C.-C. Fan et al. / Chinese Chemical Letters xxx (2014) xxx–xxx6

G Model

CCLET-2903; No. of Pages 7

imidazolium ring with N(1), which is characterized with the shortdistance (less than 3.4 A) (Fig. 8). Select bond distances are asfollows: selected anion. . .p distances [A]: O(1). . .C(2) 3.249,O(1). . .N(1) 3.329, O(2). . .N(1) 3.629, O(2). . .N(2) 3.279,O(2). . .C(2) 3.337, O(2). . .C(4) 3.577, O(2). . .C(5) 3.599;N(1B). . .N(1) 3.410, N(1B). . .N(2) 3.336, N(1B). . .C(2) 2.963,O(3). . .N(2) 3.353, O(8). . .C(2) 3.034, O(3). . .C(5) 3.309,O(3). . .C(6) 3.234.

These findings in single crystal analysis also support theconclusion from studies in solution and gas phase described above.

4. Conclusion

In summary, the interactions between different artificialmacrocycle anion receptor (14+ or 24+) and four anion guests withvarious shapes (i.e. Cl�, N3

�, NO3�, H2PO4

�) were studied in detailvia 1H NMR spectroscopy studies in DMSO-d6, ESI-MS studies, andsingle crystal X-ray diffraction analyses. Neutral C–H vs. N(sp2)present in the similar chemical environment for anion bindingproperty comparison. The results have shown that neutral C–Hbonds on 14+ effectively stabilize anion combination via strongC–H� � �Cl hydrogen bond(s). Differently, N(sp2) sites on 24+ withintramolecular hydrogen bond potential weaken the interactionwith anion species. The present findings may not only provide theguidance for design of artificial anion receptor, but also help toimprove understanding of anion binding mode in biology system.

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (No. 21202199 to Han-Yuan Gong,and No. 21372258 to Li-Jin Xu), The Young One-Thousand-TalentsScheme, and Beijing Normal University. Thanks also go to Dr. Jun-Feng Xiang for his assistance with the NMR spectroscopic analyses.

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