Fluoride-selective host based on anion-π interactions, ion pairing, and hydrogen bonding: Synthesis and fluoride-ion sandwich complex

DOI: 10.1002/anie.200704005

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Angewandte Chemie - International Edition p. 8782 - 8784 (2007)

Update date:2022-08-11

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Authors:

Mascal, Mark

Yakovlev, Ilya

Nikitin, Edward B.

Fettinger, James C.

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Article abstract of DOI:10.1002/anie.200704005

A fluoride sandwich: A cylindrophane macrocycle framed with cyanuric acid rings selectively complexes fluoride ion by a combination of anion-π interactions and ion-pair-reinforced hydrogen bonds. This compound is the first example of a purpose-designed host that exploits anion-π bonding. (Chemical Equation Presented).

Full text of DOI:10.1002/anie.200704005

Communications

DOI: 10.1002/anie.200704005

Anion–p Interactions

Fluoride-Selective Host Based on Anion–p Interactions, Ion Pairing,

and Hydrogen Bonding: Synthesis and Fluoride-Ion Sandwich

Complex**

Mark Mascal,* Ilya Yakovlev, Edward B. Nikitin, and James C. Fettinger

The theoretical description of the anion–p interaction by

Frontera, Deyà, and co-workers;^[1]Alkorta and co-workers;^[2]

and our group^[3]has, in the five years since the publication of

these studies, given rise to a large body of work describing

physical observations of close anion–p contacts as well as

further theoretical investigations.^[4]Such reports are often

accompanied by the suggestion that this phenomenon could

form the basis for anion recognition and binding, and indeed,

this is turning out to be the case.^[5]For example, a synthetic

halide-ion channel composed of electron-deficient aromatic

rings has recently been reported,^[6]as well as a chloride–p

complex formed in a calix-like cavity within a dendrimer.^[7]

We ourselves described a hypothetical series of purpose-

designed fluoride receptors that took advantage of the

cylindrophane effect, that is, the high conformational stability

and limited vertical flexibility of [1,3,5]cyclophanes bridged

by chains with odd numbers of atoms, such as 1 (Scheme 1).

addition to acute steric issues associated with accommodating

the larger chloride anion in the cavity, led to a calculated

23.5 kcalmol^À1difference in binding energy between the two

halides in aqueous solution.^[9]Herein, we present the proof of

principle behind the design of 2.

The synthesis of a cyanuric acid based cylindrophane is

shown in Scheme 2. Although 2 itself was useful for modeling

purposes, it was anticipated that substitution at the chain

nitrogen atoms would be required to eventually be able to

work with the tricationic receptor in common organic

solvents. Thus, commercially available diethanolamine (3)

was alkylated with n-hexyl bromide to give 4. Statistical

monoprotection of 4 as the triisopropylsilyl (TIPS) ether and

subsequent conversion of the remaining hydroxy group to

chloride gave 6. Installation of the first cyanuric acid ring

could be accomplished in good yield by simply heating the

components together in DMF in the presence of the base 1,8-

diazabicyclo[5.4.0]undec-7-ene (DBU). Desilylation of 7 and

subsequent chlorination gave the cage precursor 9. Heating

trichloride 9 with cyanuric acid in the presence of DBU at

high dilution gave the macrocyclic product 10 in good yield

for this type of reaction.

Cylindrophane 10 is freely soluble in dichloromethane but

only poorly so in methanol and crystallizes from a mixture of

these solvents. An X-ray crystal structure of the empty cage is

shown in Figure 1. Unexpectedly, the conformation of one of

the three bridging chains is gauche-anti-gauche-gauche, devi-

ating from the gauche-anti-anti-gauche conformation of the

other chains. There are two molecules in the asymmetric unit

of the crystal (Z’ = 2), and this distortion appears in both of

them, causing the cyanuric acid rings to tilt relative to each

other such that the height of the cylinder varies from 4.30 to

4.91 .^[10]In one of the structures (not shown), one of the

three hexyl sidechains also has a gauche kink.

The cage was armed for the binding of fluoride by

protonation. Thus, 10 was suspended in methanol, and an

excess of concentrated aqueous hydrochloric acid was added

to give a clear solution, the evaporation of which gave

[H₃10]Cl₃as a white solid. The corresponding tetrafluorobo-

rate salt could similarly be prepared by the use of aqueous

HBF₄. Redissolution of either [H₃10]³⁺salt in methanol and

treatment with an excess of aqueous HF gave a white solid

after evaporation of the solvent. The first evidence of the

presence of a fluoride complex in this material came from the

electrospray mass spectrum, which showed a base peak at

m/z 370, corresponding to the [H₃10F]²⁺ion.^[12]Also present

in the spectrum were signals corresponding to [H₂10]²⁺

(m/z 360), [H10]⁺(m/z 718), and [H₂10F]⁺(m/z 738). No

Scheme 1. Structures of cation host 1 and the proposed anion host 2.

While 1 has been shown to be a selective silver(I) and

copper(I) host,^[8]it was demonstrated that an inversion of its

electronic character from electron-rich to electron-poor, such

as in 2, would produce an anion host.^[9]In the theoretical

treatment of this receptor, the complexation of fluoride ion

was shown to benefit from intrinsically stronger p···X^À···p and

⁺NH···X^Àinteractions than for chloride. This difference, in

[*] Prof. M. Mascal, I. Yakovlev, Dr. E. B. Nikitin, Dr. J. C. Fettinger

Department of Chemistry

University of California, Davis

Davis, CA 95616 (USA)

Fax: (+1)530-752-8995

E-mail: mascal@chem.ucdavis.edu

[**] The authors gratefully acknowledge Prof. Marilyn Olmstead and Dr.

Martyn Jevric for assistance with X-ray crystallography and mass

spectrometry, respectively.

Supporting information for this article is available on the WWW

under http://www.angewandte.org or from the author.

8782

ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Angew. Chem. Int. Ed. 2007, 46, 8782 –8784

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Angewandte

Chemie

Scheme 2. Reagents and conditions: a) n-C₆H₁₃Br, Na₂CO₃, MeCN, reflux, 48 h (Hx=n-hexyl); b) NaH, THF, RT, 30 min; TIPSCl, 10 h; c) SOCl₂,

CH₂Cl₂, RT, 16 h; d) cyanuric acid, DBU, DMF, 708C, 16 h; e) [Bu₄N]F, THF, RT, 4 h; f) cyanuric acid, DBU, DMF, 908C, slow addition over 10 h,

then 16 h.

Figure 1. Views of the X-ray crystal structure of 10.^[11]Only one of the

two molecules in the asymmetric unit is shown. Thermal ellipsoids are

shown at the 50% probability level. N blue, O red, C gray.

Figure 2. Views of the X-ray crystal structure of [H₃10F][BF₄]₂.^[13]Only

one of the two molecules in the asymmetric unit is shown. Counter-

ions, sidechain disorder components, and solvate molecules are

omitted for clarity. Thermal ellipsoids are shown at the 50% probability

peaks containing chloride were ever observed, even in the

spectrum of the [H₃10]Cl₃salt.

Slow diffusion of dichloromethane into a solution of

[H₃10F](BF₄)₂in acetonitrile produced good-quality crystals.

level. F green, N blue, O red, C gray.

As can be seen in the X-ray crystal structure (Figure 2), the

fluoride ion occupies the cavity. As in the crystal of 10, the

asymmetric unit contains two independent structures which

Table 1 and show that the design of the cage is nearly optimal

for fluoride inclusion. Although [H₃10F]²⁺lacks the

C_3hsymmetry of the [2F]²⁺model, the fluoride ion in the

former compound is located less than 0.1 from the center of

the trigonal plane described by the ammonium nitrogen

atoms and is equidistant between the two cyanuric acid rings

to within 0.01 (distance from F^Àto the least-squares ring

planes). The data for the [2X]²⁺complexes are included in

Table 1 for comparison purposes. As can be seen, the distance

between the centroid of a trimethylcyanuric acid model and

the fluoride ion is 0.14 less than that within 2. The free

hydrogen-bond length between a trimethylammonium model

and fluoride ion is likewise 0.10 less than the corresponding

differ in the conformations of the n-hexyl chains. One of the

structures is associated with two well-ordered BF₄counter-

ions, while the other has two multiply disordered BF₄ions.

Interestingly, both of the disordered BF₄sites also include

centrally located, partial-occupancy chloride anions. The

observation of this adventitious chloride is attributed to the

presence of a trace amount of [H₃10F]Cl₂in the crystallization

sample.

The relationship of the fluoride ion to its host in [H₃10F]²⁺

is nearly identical to that of the modeled complex [2F]²⁺,

which appeared in the theoretical study that preceded this

work.^[9]The key structural parameters are presented in

Angew. Chem. Int. Ed. 2007, 46, 8782 –8784

ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.angewandte.org

8783

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Communications

Table 1: Selected structural parameters from the X-ray crystal structure

Keywords: anion–p interactions · cyclophanes · fluorides ·

host–guest systems · hydrogen bonds

of the [H₃10F]²⁺complex, the modeled [2X]²⁺complexes, trimethylcya-

nuric acid (Me₃CA)–halide sandwich complexes, and tris(trimethylam-

monium)–halide complexes.^[a]

p···X^À···p []^[b]

⁺NH···X^À[]^[c]

N H···X^À[8]

[1] a) D. Quiæonero, C. Garau, A. Frontera, P. Ballester, A. Costa,

P. M. Deyà, Chem. Phys. Lett. 2002, 359, 486; b) D. Quiæonero,

C. Garau, C. Rother, A. Frontera, P. Ballester, A. Costa, P. M.

Deyà, Angew. Chem. 2002, 114, 3539; Angew. Chem. Int. Ed.

2002, 41, 3389.

[2] I. Alkorta, I. Rozas, J. Elguero, J. Am. Chem. Soc. 2002 , 124 ,

8593.

[3] M. Mascal, A. Armstrong, M. D. Bartberger, J. Am. Chem. Soc.

2002 , 124 , 6274.

Complex

[H₃10F]²⁺

2.68Æ0.0091^[d]2.79Æ0.038^[d]178.3Æ0.47^[d]

2.69Æ0.0042^[e]2.80Æ0.053^[e]175.6Æ0.78^[e]

[2F]²⁺

2.62

2.48

–

2.80

–

2.70

3.03

–

176.2

–

179.7

164.1

–

Me₃CA···F^À···Me₃CA

[(Me₃NH⁺)₃···F]²⁺

[2Cl]²⁺

2.72

Me₃CA···Cl^À···Me₃CA 3.15

[(Me₃NH⁺)₃···Cl]²⁺

–

3.24

179.7

[4] P. Gamez, T. J. Mooibroek, S. J. Teat, J. Reedijk, Acc. Chem. Res.

2007 , 40 , 435.

[a] Modeled complexes optimized at B3LYP/6-31+G(d,p), see refer-

ence [9] for a full description of the computational methods. [b] Distance

from the centroid of the cyanuric acid ring to the anion. [c] Distance from

nitrogen to the anion. [d] Structure 1 of the asymmetric unit. [e] Struc-

ture 2 of the asymmetric unit.

[5] The participation of anion–p bonding in synthetic anion hosts

containing electron-deficient rings is also being recognized

retrospectively. See A. Frontera, F. Saczewski, M. Gdaniec, E.

Dziemidowicz-Borys, A. Kurland, P. M. Deyà, D. Quiæonero, C.

Garau, Chem. Eur. J. 2005 , 11 , 6560, as it regards the work of

Hoffmann: a) F. Hettche, P. Reiss, R. W. Hoffmann, Chem. Eur.

J. 2002 , 8 , 4946; b) F. Hettche, R. W. Hoffmann, New J. Chem.

2003 , 27 , 172.

[6] V. Gorteau, G. Bollot, J. Mareda, A. Perez-Velasco, S. Matile, J.

Am. Chem. Soc. 2006 , 128 , 14788.

[7] P. de Hoog, P. Gamez, H. Mutikainen, U. Turpeinen, J. Reedijk,

Angew. Chem. 2004 , 116 , 5939; Angew. Chem. Int. Ed. 2004 , 43 ,

5815.

[8] M. Mascal, J.-L. Kerdelhue, A. J. Blake, P. A. Cooke, Angew.

Chem. 1999 , 111 , 2094; Angew. Chem. Int. Ed. 1999 , 38 , 1968.

[9] M. Mascal, Angew. Chem. 2006 , 118 , 2956; Angew. Chem. Int.

Ed. 2006 , 45 , 2890.

[10] Defined as the distance from a ring atom of one cyanuric acid

ring to the corresponding ring atom of the opposite cyanuric acid

ring.

[11] Crystal data for 10 is given in the Supporting Information.

CCDC-659002 contains the supplementary crystallographic data

for this compound. These data can be obtained free of charge

from The Cambridge Crystallographic Data Centre via www.

ccdc.cam.ac.uk/data_request/cif.

distance in 2, and the hydrogen-bond angle 3.58 less acute.

The fit of chloride in 2 is much inferior to that of fluoride, with

the preferred trimethylcyanuric acid–Cl^Àdistance 0.43

greater than that within [2Cl]²⁺and the hydrogen-bond

length and attendant angle 0.21 and 15.68 less than optimal,

respectively. With such violations of the unconstrained bond

lengths and angles in [2Cl]²⁺, it is no surprise that [H₃10Cl]²⁺is

not observed.

In conclusion, we report herein the first practical appli-

cation of anion–p bonding in a purpose-designed macrocyclic

anion host. The cyanuric acid based cylindrophane 10 can be

synthesized in seven steps from cheap, commercially available

diethanolamine. The triprotonated receptor [H₃10]³⁺forms

an inclusion complex with fluoride by a combination of

anion–p interactions and ion-pair-reinforced hydrogen bond-

ing. [H₃10]³⁺shows no affinity for chloride in the electrospray

mass spectrum and indeed has been predicted by comparative

binding energetics in models to be completely fluoride-

selective. Although a number of halide complexing agents

have been described,^[14]cage 10 introduces a new genre of

anion binding, wherein anion–p interactions operate along-

side conventional ion pairing, hydrogen bonding, and the

classic “preorganization” effect.^[15]An X-ray crystal structure

of the [H₃10F]²⁺sandwich complex shows the fluoride ion

occupying the center of the cavity, in very close agreement

with published theory.^[9]A detailed study of the solution-state

binding of fluoride by [H₃10]³⁺and some related receptors is

being undertaken and will be the subject of a later report.

[12] HRMS (ESI) m/z calcd for C₃₆H₆₆FN₉O₆369.75599; found

369.75555.

[13] Crystal data for the [H₃10F]²⁺complex is given in the Supporting

Information. CCDC-659003 contains the supplementary crystal-

lographic data for this compound. These data can be obtained

free of charge from The Cambridge Crystallographic Data

Centre via www.ccdc.cam.ac.uk/data_request/cif.

[14] Reviews: a) K. Bowman-James, Acc. Chem. Res. 2005 , 38 , 671;

b) P. D. Beer, P. A. Gale, Angew. Chem. 2001 , 113 , 502; Angew.

Chem. Int. Ed. 2001 , 40 , 486; c) J. W. Steed, J. L. Atwood in

Supramolecular Chemistry, Wiley, New York, 2000, pp. 198 –

249; d) Supramolecular Chemistry of Anions (Eds.: A. Bianchi,

K. Bowman-James, E. Garcia-Espaæa), Wiley-VCH, New York,

1997.

[15] D. J. Cram, Angew. Chem. 1986 , 98 , 1041; Angew. Chem. Int. Ed.

Engl. 1986 , 25 , 1039.

Received: August 30, 2007

Published online: October 12, 2007

8784 www.angewandte.org

ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Angew. Chem. Int. Ed. 2007, 46, 8782 –8784

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Article Doi

Fluoride-selective host based on anion-π interactions, ion pairing, and hydrogen bonding: Synthesis and fluoride-ion sandwich complex

DOI: 10.1002/anie.200704005

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Authors:

Article abstract of DOI:10.1002/anie.200704005

Full text of DOI:10.1002/anie.200704005

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