Designation and Exploration of HalideâAnion Recognition Based on

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Designation and Exploration of Halide-Anion Recognition Based on Cooperative Noncovalent Interactions Including Hydrogen Bonds and Anion-# Yan-Zhi Liu, Kun Yuan, LingLing Lv, Yuancheng Zhu, and Zhao Yuan J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b02952 • Publication Date (Web): 30 Apr 2015 Downloaded from http://pubs.acs.org on May 10, 2015

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The Journal of Physical Chemistry

Designation and Exploration of Halide-Anion Recognition Based on Cooperative Noncovalent Interactions Including Hydrogen Bonds and Anion-π

Yan-Zhi Liua, *, Kun Yuana,b, Ling-Ling Lva, Yuan-Cheng Zhua,*, Zhao Yuanc a

College of Chemical Engineering and Technology, Key Laboratory for New Molecule Materials Design and Function of Gansu Universities, Tianshui Normal University, Tianshui, 741001, China b Institute for Chemical Physics & Department of Chemistry, State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an, 710049, China c Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida, 32306, USA ABSTRACT: A novel urea-based anion receptor with an electron-deficient-aromatic structural unit, N-p-nitrophenyl-N-(4-vinyl-2-five-fluoro-benzoic

acid

benzyl

ester)-phenyl-urea

(FUR), was designed to probe the potential for halide-anion recognition through cooperation of two distinct noncovalent interactions including hydrogen bond and anion-π in this work. The nature of the recognition interactions between halide-anion and the designed receptor were theoretically investigated at the molecular level. The geometric features of the hydrogen bond and anion-π of the FUR@X- (X=F, Cl, Br and I) systems were deeply investigated. The binding energies and thermodynamic information of the halide-anion recognitions show that the present designed FUR maybe selectively recognizes the anion F- based on the cooperation of N-H…Fhydrogen bond and anion-π interactions both in vacuum and in solvents. IR and UV-visible spectrums of the free FUR and the FUR@F- have been simulated and discussed qualitatively, which may be helpful for further experimental investigation in future. Additionally, electronic properties and behaviors of the FUR@X- systems were discussed according to the calculations on the natural bond orbital (NBO) data, *

corresponding author, e-mail: [email protected]; [email protected]

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molecular electrostatic potential (MEP) and weak interactions regions. Keywords: Anion recognition; Receptor design; Hydrogen bond; Anion-π interaction; Theoretical calculation

■ Introduction Either in biological processes or in artificial molecular systems, anion recognition event is considered to play important and sometimes crucial roles for DNA recognition, environmental detection, catalysis, medical and chloride trans-membrane transport.1-5 Despite the great importance and significant progress of anion recognition and sensing, 6-16 the molecular design and synthesis of anion receptors still remain a challenging field of chemistry. Especially, the efficient and selective recognition of anions by synthetic novel and promising receptors is an issue that continues to fascinate chemists in recent years. Among many artificial anion receptors, the amide group is one of the most widely utilized and promising anion binding sites because of the existing of N-H fragments which can bind anion via hydrogen bond. In fact, the most versatile and frequently used -(CO)NH fragment is that of urea derivatives. The success of the urea sub-unit as a receptor may depend upon the fact that it possess two close N-H fragments, which can chelate a spherical ion or establish two parallel H-bonds with two oxygen atoms of a carboxylate or of an inorganic oxoanion.17 Nonetheless, anion receptors that rely upon other noncovalent forces, including halogen bond18-20 and anion-π, etc.21-26, have also been well developed, with considerable success. These studies have provided new strategies to achieve high selectivity and affinity of anion recognition. In particular, a number of studies about anion–π have been published

27-32

since it was

introduced on the basis of theoretical calculations for designating an attractive force between an electron-deficient aromatic π system and an anion.33 Currently, the noncovalent interactions of anion–π are well employed in the design of anion receptors or selective hosts for anion recognition. However, only few reports on cooperative weak interactions involving more than one noncovalent force in the


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designation of the anion receptors have been presented, and to the best of our knowledge, the cooperative noncovalent interactions including hydrogen bond and anion-π have rarely been reported. Excitedly, M. S. Taylor’ group successfully synthesized a series of urea-based anion receptors composed of both -(CO)NH- and -C6H4-X (X=F, I) groups, which employ hydrogen- and halogen-bond interactions cooperatively in molecular recognition, and modulate the anion selectivity with combination of the above two distinct noncovalent interactions.34 Subsequently, hydroxyl

functionalized

tetraoxacalix[2]arene[2]triazine

host

molecule

was

synthesized,35 and its infinite self-assemblies under the directing of cooperative anion–π, lone-pair electron–π interactions and intermolecular hydrogen bond were obtained. Very recently, M. Albrecht’s group36 prepared a novel biaryl substituted quinoline based anion receptors, the binding behavior of halide anions in chloroform shows that, besides strong N-H···Cl hydrogen bonds, either some nonclassical CH···Cl hydrogen bonding or anion−π interactions possibly contribute to the anion binding.

Scheme 1. The chemical structure of the present designed anion receptor (FUR) Here,

we

describe

a

novel

receptor

(N-p-nitrophenyl-N-(4-vinyl-2-five-fluoro-benzoic acid benzyl ester)-phenyl-urea, donated as FUR, see Scheme 1) capable of well binding of halide anions through double-dentate hydrogen bond and anion-π weak interactions. This represents the first system in which the cooperative actions of multiple hydrogen-bond and anion-π are employed to expectedly achieve high-anion-affinity binding behavior. In addition, the



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nature of the interactions between halide-anion and the designed receptor are theoretically investigated at a molecular level. The selectivity of these receptors maybe differing substantially from those of related receptors based on only hydrogen bond will be investigated later by our group. Moreover, the –C=CH2 group is contained in the present designed anion receptor. Therefore, it can form macromolecule receptor via polymerization, which is of certain importance for anion recognition with high sensitivity and selectivity. Generally, the approach for obtaining a synthetic molecule consists of employing methods of computational chemistry to design a desired functional system followed by experimental optimization of its synthesis.37 It is expected that the theoretical exploration of this work from a molecular level would be useful for the future experimental study and realizable in laboratory.

■ Computational methods In this work, the density functional of Turhlar’s M06-2X38 and Grimme’s DFT-D339 were employed for the study of anion-receptor@X- systems (X=F, Cl, Br and I). M06-2X functional has been designed to include medium-range correlation energy. Additionally, the DFT-D3 method provides an empirical dispersion correction for DFT39, 40. The ability of these new density functionals to predict and explain the supramolecular chemistry at van der Waals distances is very encouraging because density functional theory can be used conveniently for supramolecular systems. 41, 42 All the geometric configurations were optimized at the M06-2X/6-31+G(D,P) and DFT(B3LYP)-D3/6-31+G(D,P) levels, while for I atom, the basis set of MIDIX 43 was used, which is considered as a well-balanced and economical double-ζ basis set that gives reasonably good geometries and partial atomic charges. No symmetry constraints were applied during optimizations. Harmonic frequency analyses were performed at the same level to confirm that these structures were local minima or transition state on the potential energy surfaces. The binding energies (∆Ecp) with basis set superposition errors (BSSE) corrected were calculated by the counterpoise


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method44. Additionally, a visual study of intermolecular noncovalent interaction between host and guest was performed via calculating the reduced density gradient (RDG),45 coming from the electron density (ρ(r)) and its ﬁrst derivative (RDG(r)=1/(2(3π2)1/3)|∇ρ(r)|/ρ(r)4/3), and the second largest eigenvalue of Hessian matrix of electron density (λ2) functions by using Multiwfn program46,47. Natural bond orbital (NBO) theoretical48 calculation carried out with the NBO 5.0 package49 was employed to analyze the electronic behaviors. All other calculations were performed with the Gaussian 09 program.50

■ Results and discussion Geometrics and binding energies

Figure 1. The geometric configurations of the receptor@halide-anion systems. As having been mentioned above, the binding of halide anions by designed FUR through double-dentate hydrogen bond and anion-π weak interactions. Figure 1 shows



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the optimized geometric configurations of the receptor@halide-anion systems. It is known that the anion-π interactions are termed as favorable noncovalent contacts between an electron deficient (π-acidic) aromatic system and an anion.22 Here, -five-fluoro

benzoic

acid

benzyl

ester

group

is

considered

to

offer

electron-deficient-aromatic unit. As Figure 1 (a) showing, the halide-anions lie above the plane of –five-fluoro benzoic structural unit, and the two –N-H groups are toward halide-anions. Therefore, there has an ideal geometric feature to form cooperative noncovalent interactions including hydrogen bond and anion-π between FUR and halide-anions. In order to manifest the necessary and the key role of introduction of –five-fluoro

substitutes,

the

complex

formed

with

N-p-nitrophenyl-N-(4-vinyl-2-benzoic acid benzyl ester)-phenyl-urea (HUR) and Fwas also optimized. It distinctly showed that the F- is far away from the center of benzoic acid group and nearly lies on the same plane with benzoic acid group (Figure 1 (b)), suggesting that anion-π interaction in HUR@halide-anion systems decreased significantly. Table 1 The key geometry parameters (defined as Figure 1 showing) of the FUR, HUR, FUR@X and HUR@X (X=F-, Cl-, Br- and I-) at M06-2X/6-31+G(D,P) l1/Å

l2/Å

d1/Å

d2/Å

θ1/°

θ2/°

dpenetration/Å b

HUR

1.012

1.009

–

–

–

–

–

FUR

1.012

1.013

–

–

–

–

–

HUR@F-

1.056

1.047

1.581

1.614

156.95

158.09

1.31

FUR@F-

1.051

1.048

1.612

1.609

156.18

159.21

1.31

FUR@Cl-

1.032

1.026

2.124

2.270

165.21

159.86

1.11

FUR@Br-

1.030

1.025

2.282

2.463

167.48

159.94

1.18

1.026

1.025

2.630

2.666

166.10

164.36

1.08

a

FUR@I-

a

MIDIX basis set used for I atom. b Here, the dpenetration, mutual penetration distance

between halide-anion and H atom, is defined as sum of the van der Waals radii of H atom

and

halide-anion

substrate

the

average

of

dpenetration=∑vdW-radii ̶ 1/2(d1+d2).


d1

and

d2,

namely

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Table 1 lists the key geometry parameters (defined as Figure 1 showing) of the FUR, HUR, FUR@X and HUR@X (X=F-, Cl-, Br- and I-) at M06-2X/6-31+G(D,P). It is clearly seen that the bond length of N-H increased obviously after the formations of the complexes. Van der Waals radii is an important parameter for investigating the structure of the noncovalent system. It is known that the van der Waals radii of H, F-, Cl-, Br- and I- are 1.20, 1.71, 2.11, 2.35

51

and 2.53 Å,52 respectively. The N-H…X-

distances, d1 and d2, are both distinctly shorter than sum of the van der Waals radii of X- and H atom, indicating that the van der Waals surfaces of H atoms and halide-anions are drastically penetrated into each other. The mutual penetration distances, dpenetration (defined as the footnote of Table 1), of the four FUR@X (X=F-, Cl-, Br- and I-) complexes are 1.31, 1.11, 1.18 and 1.08 Å, respectively. So, it is can be inferred that the strong hydrogen bond formed between halide-anions and -N-H groups, and the N-H…F- might be the strongest among the four N-H…X- hydrogen bond structures. Moreover, the distances between H atom of CH2 (which is connected with phenyl) and X- are also shorter than sum of the van der Waals radii of X- and H atom, meaning that C-H…X- hydrogen bond also exists in the present systems, Meanwhile, C-H is commonly very weak hydrogen bond donor, so the C-H…Xinteraction is not to be mainly focused here. Additionally, it is noted that the d1 and d2, θ1 and θ2 are not equal, respectively, suggesting that the two N-H…X- of the double-dentate hydrogen bond in each complex are not utterly symmetry either in structure or in strength. It is evidenced that in many of the X-ray structures exhibiting anion-π contact, the anion is not located exactly over the center of the ring.

51

Instead, it is displaced with

respect to the center of the ring. Since most aromatic rings are asymmetrically substituted, the more favourable location is probably not above the center of the ring. For the present theoretical designed systems, the value of α (defined in Figure 2) in FUR@F- is 23.53°, which is obviously larger than those in FUR@Cl-, FUR@Br- and FUR@I- (Table 2), which means that the anion F- is much less located above the center of the aromatic ring than Cl-, Br- and I-. Conversely, the anions Cl-, Br- and Iare not significantly far away from just above the center of the aromatic ring owing to



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the small corresponding values of α (4.00~6.40°). Therefore, the specific positions of the anion above the aromatic ring are dependent not only on the substitutions of the aromatic ring but also on the anions themselves.

Figure 2. The anion-π structures and the definitions of the related geometry parameters of the receptor@halide-anion systems.

In addition, it should be mentioned that it is difficult to clearly define and establish a criterion which allows classifying a given anion-aromatic contact as an anion-π interaction. According to Frontera’s recommendation,51 the geometry structure of the anion–π noncovalent interaction could be characterized by the parameters of dcentroid, dplane and doff-set, which are defined as Figure 2 showing. And a probably unrestrictive criterion but more realistic is to consider an anion-π contact when the anion is located at any place over the ring since the π-system covers the entire ring at distances ≤∑vdW radii + r, where r = 0.7/cosα, and 0.7 is an empirical constant. However, this is not a very easy understandable criterion owing to the ambiguous definition of the distance between the anion and the involving aromatic ring. Moreover, it is also a relatively loose criterion, which maybe over estimates the maximum allowable anion-aromatic contact distance. For clarity and simplicity, we would like to define the mutual penetration distances for the anion-π interaction, dpenetration (defined in the


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footnote of Table 2), to describe the real anion-π contact. When the dpenetration > 0, the van der Waals surfaces of atoms involving anion-π interaction have been penetrated into each other, that is, the anion-π interaction would be existent. The larger the dpenetration is, the stronger the anion-π interaction will be. As shown in Table 2, it is found that the dpenetration between X- and involving aromatic ring of the four complex are in the range of 0.4~0.7 Å, indicating that the existences of X--π interactions. The dpenetration in FUR@F- is 0.1~ 0.3 Å which is smaller than those in FUR@Cl-, FUR@Br- and FUR@I-, implying that F--π interaction is relatively weaker than Cl--, Br-- and I--π interactions in the present study. This is probably due to the larger value of α in FUR@F- than in others. Although the anion-π interaction in FUR@F- is relatively weaker, it is still worth pointing that the stability of FUR@F- is not necessarily weaker than ther others, because the whole structures of the present investigated FUR@X- systems are the results of the competition balance of N-H…Xhydrogen bond and X--π interactions. On the other hand, if the distances between the anion and the involving aromatic ring are regard as dcentroid (defined in Figure 2) together with the fact that the van der Waals radii of C atom is 1.70 Å, it is easily found that they also well meet the criterion of the anion-π contact given by Frontera.51

Table 2 The key geometry parameters of the FUR, HUR, FUR@X and HUR@X (X=F-, Cl-, Br- and I-) at M06-2X/6-31+G(D,P) b

dpenetration/Å

dcentroid/Å

dplane/Å

doff-set/Å

α/°

FUR@F-

3.01

2.76

1.20

23.53

0.40

FUR@Cl-

3.23

3.22

0.22

4.00

0.58

FUR@Br-

3.37

3.36

0.28

4.82

0.68

a

3.72

3.70

0.42

6.40

0.51c

FUR@I-

a

MIDIX basis set used for I atom. b Here, the dpenetration, mutual penetration distance between halide-anion and -C6F5 arene π system, is defined as sum of the van der Waals radii of C atom and halide-anion substrate dcentroid, namely dpenetration=∑vdW-radii ̶ dcentroid. c Obtained based on calculated value of van der Waals radii of I-.



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Binding energy is a valid way of measuring the stability and strength of an anion-π interaction. The values of the BSSE corrected binding energies, ∆ECP, both at M06-2X/6-31+G(D,P) and DFT(B3LYP)-D3/6-31+G(D,P) levels of theory were tabulated in Table 1. The ∆ECP obtained at M06-2X/6-31+G(D,P) are very close to those at DFT(B3LYP)-D3/6-31+G(D,P), and the relative orders of the four complexes at above two levels of theory are completely same. Therefore, the relative stabilities of the complexes should be reliable. For the FUR@F- system, the ∆ECP is -385.14 kJ·mol-1 at DFT(B3LYP)-D3/6-31+G(D,P) level of theory, which is distinctly larger than those in FUR@Cl-, FUR@Br- and FUR@I- by 112.23, 132.59 and 154.20 kJ·mol-1, respectively. The relative stability of the four complexes increases in the order FUR@I-< FUR@Br-< FUR@Cl-

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