Slow anion exchange, conformational equilibria, and fluorescent sensing in venus flytrap aminopyridinium-based anion hosts

Published on Web 07/17/2003

Slow Anion Exchange, Conformational Equilibria, and

Fluorescent Sensing in Venus Flytrap Aminopyridinium-Based

Anion Hosts

Karl J. Wallace, Warwick J. Belcher, David R. Turner, Kauser F. Syed, and

Jonathan W. Steed*

Contribution from the Department of Chemistry, King’s College London, Strand,

London WC2R 2LS, U.K.

Received February 28, 2003; E-mail: jon.steed@kcl.ac.uk

Abstract: The synthesis, anion binding, and conformational properties of a series of 3-aminopyridinium-

based, tripodal, tricationic hosts for anions are described. Slow anion and conformational exchange on the

1

H NMR time scale at low temperature, coupled with NMR titration, results in a high level of understanding

of the anion-binding properties of the compounds, particularly with respect to significant conformational

-

change resulting from induced fit complexation. Peak selectivity for halides, particularly Cl , is observed.

The approach has been extended to dipodal and tripodal podands based on 3-aminopyridinium “arms”

containing photoactive anthracenyl moieties. The 1,3,5-tripodal host shows a remarkable selectivity for

acetate over other anions, in contrast to the analogous unsubstituted tris(3-aminopyridinium) analogue,

1

despite the fact that low-temperature H NMR experiments reveal a total of four acetate-binding

conformations. Photodimerization of anthracene units results in the formation of potential fluorescent anion

sensors.

1

Introduction

switches or switchable sensing devices. Significant recent work

has focused on tripodal species containing a trisubstituted

The design and synthesis of flexible “podand” type host

molecules capable of selective guest recognition is highly

1

,2,4-6,15,16

triethylbenzene core.

molecules exhibit a preference of some 10-15 kJ mol for an

The resulting hexasubstituted

-

1

topical¹

-13

and has been recently reviewed.

14,15

In comparison

alternating (3-up, 3-down) conformation about the central aryl

to more rigid cyclic systems, podands are frequently more

readily synthesized, can display rapid complexation/decom-

plexation kinetics, and may undergo significant conformational

changes on binding, which may form the basis of molecular

2

,5,15

ring,

allowing the preparation of partially preorganized

tripodal hosts in which the three guest-binding arms are all

orientated in the same direction and are thus able to chelate

their target substrate. Anslyn et al. have used guanidinium and

boronic acid derivatives, for example, as selective hosts and

sensors for a variety of carboxylic acid conjugate anions found

(

1) Abouderbala, L. O.; Belcher, W. J.; Boutelle, M. G.; Cragg, P. J.; Steed,

J. W.; Turner, D. R.; Wallace, K. J. Proc. Natl. Acad. Sci. U.S.A. 2002,

9

9, 5001-5006.

4,6

in beverages such as Gatorade and scotch.

2) Abouderbala, L. O.; Belcher, W. J.; Boutelle, M. G.; Cragg, P. J.; Fabre,

M.; Dhaliwal, J.; Steed, J. W.; Turner, D. R.; Wallace, K. J. Chem. Commun.

In the development of molecular sensors, fluorescence

spectroscopy has become an important analytical technique in

2

002, 358-359.

3) Lavigne, J. J.; Anslyn, E. V. Angew. Chem., Int. Ed. 1999, 38, 3666-

1

7

3

669.

recent years. There have been numerous receptors containing

chromophores for the detection of cationic species, and the topic

(

4) Wiskur, S. L.; Anslyn, E. V. J. Am. Chem. Soc. 2001, 123, 10109-10110.

(

5) Metzger, A.; Lynch, V. M.; Anslyn, E. V. Angew. Chem., Int. Ed. Engl.

1

8

1

997, 36, 862-865.

has been eloquently reviewed by de Silva. Photoactive

molecular sensors for the detection of anions have been studied

(6) Cabell, L. A.; Best, M. D.; Lavigne, J. J.; Schneider, S. E.; Perreault, D.

M.; Katherine, M.-K.; Anslyn, E. V. J. Chem. Soc., Perkin Trans. 2 2001,

1

9-22

3

15-323.

less, but are of significant and growing interest.

The most

(

7) Hartshorn, C. M.; Steel, P. J. Angew. Chem., Int. Ed. Engl. 1996, 35, 2655-

common mechanism used for fluorescence studies in host-guest

chemistry is via a photoinduced electron-transfer strategy (PET).

2

657.

8) Choi, H. J.; Park, Y. S.; Yun, S. H.; Kim, H. S.; Cho, C. S.; Ko, K.; Ahn,

K. H. Org. Lett. 2002, 4, 795-798.

9) Sun, W.-Y.; Xie, J.; Okamura, T.-A.; Huang, C.-K.; Ueyama, N. Chem.

Commun. 2000, 1429-1430.

(16) O’Leary, B. M.; Szabo, T.; Svenstrup, N.; Schalley, C. A.; L u¨ tzen, A.;

Sch a¨ fer, M.; Rebek, J., Jr. J. Am. Chem. Soc. 2001, 123, 11519-11533.

(17) Kubo, J.; Tsukahara, M.; Ishitara, S.; Tokita, S. Chem. Commun. 2000,

653-654.

(18) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.;

McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97, 1515-

1566.

(19) Gale, P. A. Coord. Chem. ReV. 2001, 213, 79-128.

(20) Gale, P. A. Coord. Chem. ReV. 2000, 199, 181-233.

(21) Beer, P. D.; Gale, P. A. Angew. Chem., Int. Ed. 2001, 40, 487-516.

(22) Schmidtchen, F. P.; Berger, M. Chem. ReV. 1997, 97, 1609-1646.

(

10) Christofi, A. M.; Garratt, P. J.; Hogarth, G. Tetrahedron 2001, 57, 751-

7

59.

(

11) Garratt, P. J.; Ng, Y. F.; Steed, J. W. Tetrahedron 2000, 56, 4501-4509.

12) Chin, J.; Walsdorff, C.; Stranix, B.; Oh, J.; Chung, H. J.; Park, S.-M.; Kim,

K. Angew. Chem., Int. Ed. 1999, 38, 2756-2759.

(

13) Walsdorff, C.; Saak, W.; Pohl, S. J. Chem. Res., Synop. 1996, 282-283.

14) Steed, J. W.; Wallace, K. J. In AdVances in Supramolecular Chemistry;

Gokel, G. W., Ed.; Cerberus: New York, 2003; Vol. 9, Ch. 5, pp 221-

(

2

62.

(15) Hennrich, G.; Anslyn, E. V. Chem.-Eur. J. 2002, 8, 2219-2224.

J. AM. CHEM. SOC. 2003, 125, 9699-9715

9

9699

A R T I C L E S

Wallace et al.

Scheme 1. Synthesis of Aminopyridine-Based Hosts

by treatment with borohydride. Reaction of 11, 12, and 13 with

1

followed by anion metathesis gave substituted hosts 15, 16,

and 17 in unoptimized overall yields of 62%, 72%, and 51%,

Scheme 2. The (N-ethylamino)pyridinium host 14 was isolated

as the bromide salt, but counterion metathesis did not proceed

cleanly and the compound was not explored further. The

analogous 2- and 4-pyridyl isomers of 9-11 could be prepared,

but clean analogues of 12-14 could not be isolated. For

comparison, the bis(anthracenyl) molecular clips 18a-c and the

single-arm N-benzyl model compound 19 were also prepared,

in good yield.

X-ray Crystallography. Compounds 3, 2b, and 2c were all

characterized by X-ray crystallography as bromide salts, and

the hosts along with their immediate environments are shown

in Figures 1-3, respectively. The model compound 3, which

does not contain a strong hydrogen bond acid group (i.e., an

amine), exists as two pseudopolymorphs depending on crystal-

lization conditions. The host species in these two forms of 3

exhibit radically different conformations with the orthorhombic

3‚3Br‚EtOH (from ethanol/hexane) displaying a cone conforma-

tion with all three pyridinium arms orientated in the same

direction (3-up or “3u”, Figure 1a), while the triclinic 3‚3Br‚

4

.5MeOH (from methanol/triethylamine) exhibits two crystal-

lographically independent molecules both in the “2u, 1d” form

d ) down). The orthorhombic form is able to incorporate a

The host is made up of a fluorophore site, a receptor site, and

a spacer. The PET signaling system shows either an “on-off”

or an “off-on” fluorescence signaling as guest binding results

in either fluorescence quenching or inhibition of a quenching

(

bromide anion within the molecular cavity formed by the three

pyridinium groups. Crystal packing effects (formation of an edge

to face π-π interaction) also result in two methyl groups being

orientated in the same way as the pyridinium groups despite

the preference for alternation about the hexasubstituted core.

The host interacts with the bromide anion via a six-fold array

of pyridinium charge-assisted CH‚‚‚Br hydrogen bonds with

C‚‚‚Br distances in the range 3.42-3.70 Å. Related CH‚‚‚halide

interactions have been shown to be important in pyridinium-

and imidazolium-based hosts recently, and a growing weight

of evidence suggests that they play an integral role in the present

complexes (vide infra). The host adopts a T-shaped conforma-

tion rather than a symmetric C3 geometry. This conformation

allows the close approach to the bromide anion of an additional

CH group from an adjacent host molecule. The coordination

sphere of the intracavity bromide anion is completed by a

hydrogen bond from enclathrated ethanol.

The “2u, 1d” conformation of the triclinic form is surprising

and suggests that electrostatic interactions between pyridinium

rings destabilize the usual 3u conformation in comparison to

related systems,

mational templating. One of the two independent molecules in

23,24

effect innate to the host.

Much less explored is the concept

of a conformational switching process in which induced fit guest

binding results in a marked change in host geometry that, in

turn, influences signal intensity. We have applied this concept

in the case of electrochemical sensors via a series of bis- and

-

_t_r_i_s₍_f_e_r_r_o_c_e_n_y_l₎_p_o_d_a_n_d_s_.1

,2,25

As part of a versatile “modular”

approach to di- and tripodal pyridinium derivatives capable of

2

1,2

selectively binding and sensing anions using both organic and

2

6

_i_n_o_r_g_a_n_i_c₂5 core moieties, we now describe the full characteriza-

tion and remarkable dynamic and photochemical behavior of a

simple, but highly effective, series of tripodal aminopyridinium-

derived anion-binding hosts.

Results and Discussion

By adopting a modular approach to the preparation of anion

host compounds, we have been able to prepare a versatile range

of tripodal materials based on aminopyridines. Thus, reaction

of 2-, 3-, or 4-aminopyridine with tri(bromomethyl)triethylben-

13

zene 1 results in the high-yield synthesis of compounds 2a-c

as bromide salts. Anion metathesis cleanly converts the com-

pounds to the hexafluorophosphate analogues, Scheme 1. As a

control, the analogous tris(pyridinium) species 3 and 4 were

also prepared in a similar manner from 1 or tri(bromomethyl)-

benzene, respectively. Functionalized derivatives were prepared

via a procedure similar to that reported by us for the synthesis

4

,5,15

in the absence of specific anion confor-

-

the structure binds bromide via CH‚‚‚Br interactions between

two pyridinium rings in a fashion comparable to the ortho-

rhombic analogue (Figure 1b). The second independent mol-

ecule, however, binds methanol in a similar position and does

not exhibit a specific anion-binding pocket. The structures imply

that in the absence of strong hydrogen bonding functionality

there is a fine balance between a variety of conformations and

anion-binding modes. In the solid state, the host molecules in

the triclinic form are arranged in offset dimers, linked by pairs

of bromide anions binding to two pyridinium rings from one

host and one from another, Figure 1b.

1

of the tris(ferrocenyl) compound 5. Thus, condensation of

3

-aminopyridine with ethanal, benzaldehyde, 4-pentylbenzal-

dehyde, and anthracene-9-carbaldehyde gave imines 6-9. These

compounds were reduced to the corresponding amines 10-13

(

23) Gawley, R. E.; Zhang, Q.; Higgs, P. I.; Wang, S.; Leblane, R. M.

Tetrahedron Lett. 1999, 40, 5461-5465.

24) Gunnlaugsson, T.; Davies, A. P.; Glynn, M. Chem. Commun. 2001, 2556-

2

775.

25) Wallace, K. J.; Daari, R.; Belcher, W. J.; Abouderbala, L. O.; Boutelle,

(26) Ihm, H.; Yun, S.; Kim, H. G.; Kim, J. K.; Kim, K. S. Org. Lett. 2002, 4,

2897-2900.

M. G.; Steed, J. W. J. Organomet. Chem. 2003, 666, 63-74.

9700 J. AM. CHEM. SOC.

9

VOL. 125, NO. 32, 2003

Aminopyridinium-Based Anion Hosts

A R T I C L E S

Scheme 2. Synthesis of Substituted 3-Aminopyridinium-Based Hosts

The bromide salt of compound 2b was also found to exist in

two pseudopolymorphic forms in the solid state, differing in

the nature of the included solvent, although the conformation

of the host and the anion-binding mode are very similar in both

structures. The X-ray crystal structure of the monoclinic

disorder of the lattice methanol solvent, and in both orientations

the “arm” is able to maintain CH‚‚‚Br interactions as in the

-

previous examples. It is noteworthy that the arms that chelate

-

the intracavity anion via NH‚‚‚Br interactions are not subject

to disorder.

-

dichloromethane solvate of 2b‚3Br (Figure 2) shows a remark-

Overall, it is clear from both structures that the host is highly

complementary to bromide and to smaller halides, in general,

and binds via multiple interactions in a chelating manner. This

is in marked contrast to the structure of the analogous 4-ami-

nopyridine derivative 2c (Figure 3).

able encapsulation of an intracavity bromide anion by a six-

-

fold array of NH‚‚‚Br and CH‚‚‚Br interactions. Two of the

three NH groups are involved in interactions to bromide, which

appears to be just slightly too large for the cavity (N‚‚‚Br

distances 3.35 and 3.40 Å). The remaining NH protons all

interact with lattice bromide anions. The fact that not all three

-

The 4-aminopyridinium derivative 2c‚3Br adopts a partial

cone conformation that is radically different from that observed

for 2b with a molecular cavity defined by two essentially

coplanar pyridinium groups (reminiscent of two of the pyri-

dinium rings in 3) and two methyl substituents. The third

pyridinium group is on the opposite face of the molecule. The

molecular cavity is occupied by a molecule of methanol solvent

which hydrogen bonds to a bromide anion sandwiched on the

edge of the cavity between the coplanar pyridinium rings and

-

NH2 groups interact with the intracavity Br anion is thought

-

to arise from the presence of three Br anions in the structure

and their effect on crystal packing. NMR and molecular

2

modeling results on this and comparable systems suggest that

the solution structure, particularly in the presence of less than

3

equiv of halide, is dominated by the C3 symmetric form (vide

-

infra). The CH‚‚‚Br interactions with ortho-pyridyl CH groups

C(6) and C(18) are longer than those in 3, consistent with the

dominance of the NH‚‚‚Br hydrogen bonds. The bond to the

hydrogen atom of the ortho-pyridyl CH group of the amino-

pyridinium arm that has its NH2 group orientated away from

-

held in place entirely by CH‚‚‚Br interactions. This very

different structure for 2c as compared to 2b highlights the

importance of the anion chelation in the case of the 3-amino-

pyridine derivative.

-

the intracavity Br , C(8), is shorter and similar to those involved

For comparison, host 2b was also structurally characterized

as the hexafluorophosphate salt (Figure 4). In the presence of

in 3.

-

The X-ray crystal structure of the orthorhombic methanol

PF6 , host 2b adopts a “2u, 1d” conformation in stark contrast

-

solvate of 2b‚3Br is essentially identical to the monoclinic form

to the analogous bromide and is more reminiscent of 2c‚3Br.

Unlike the structure of 2c shown in Figure 3, however, Figure

4 reveals that 2b‚3PF6 possesses two partial cavities, each of

except that it involves a two-fold disorder of the one amino-

-

pyridinium ring that does not engage in NH‚‚‚Br interactions

-

with the intracavity anion. This disorder is apparently linked to

which includes a PF6 anion. Each anion interacts with one NH

J. AM. CHEM. SOC.

9

VOL. 125, NO. 32, 2003 9701

A R T I C L E S

Wallace et al.

Figure 3. X-ray crystal structure of the tris(4-aminopyridinium) species

2

c. The host adopts a partial cone conformation. The host conformation

-

dominated by the crystal packing which maximizes NH‚‚‚Br interactions

to the remaining anions.

Figure 1. X-ray crystal structure of the model tris(pyridinium) species (a)

3

‚Br3‚EtOH, (b) 3‚Br3‚4.5MeOH.

Figure 4. X-ray crystal structure of the tris(3-aminopyridinium) species

2b as the hexafluorophosphate salt showing the two intracavity anion

environments.

anthracene arms are splayed out, Figure 5, and do not converge

on a central binding cavity.

We have termed conformations involving an NH group on

the outside of the central cavity as “out” or “o”; hence the

present system may be described as “3u, 3o” and contrasts to

the bromide salt of the unsubstituted tris(3-aminopyridinium)

receptor 2b‚3Br which adopts a “3u, 2i, 1o” conformation in

both pseudopolymorphs (i ) “in”), Scheme 3b. It is character-

istic of “out” pyridinium rings that they engage in CH‚‚‚anion

Figure 2. X-ray crystal structure of the tris(3-aminopyridinium) species

b as the bromide salt showing the chelation of a bromide anion by two of

the three amino groups, supported by CH‚‚‚Br interactions. In solution, a

symmetric “3u, 3i” conformation predominates at low temperature.

2

-

functionality of a particular host. The anion also forms a number

of additional intra- and intermolecular CH‚‚‚F interactions.

The striking contrast of the nearly symmetrical chelate

-

interactions. In this case, there is a single PF6 anion which

sits in the central part of the cavity and interacts with the

-

structure of 2b‚3Br as compared to 2b‚3PF6 and the partial

cone (2u, 1d) 2c clearly indicates a great deal of structural

complementarity between 2b and halide anions.

pyridinium groups by an array of CH‚‚‚F interactions. Two of

-

the NH groups interact with the other two PF6 anions, while

the final NH functionality interacts with the only methanol

molecule enclathrated in the crystal structure. This difference

makes the receptor unsymmetrical. The fact that only two of

The anthracenyl receptor 17 was characterized by X-ray

-

crystallography as the PF6 salt. The structure of 17‚3PF6‚

-

MeOH‚2MeCN shows that the cationic host adopts the 3u cone

the NH groups are binding to two of the PF6 anions and the

1

5

conformation expected for the hexasubstituted core with

third is found in the cavity is rather interesting. It suggests that

alternation about the central arene ring. The NH groups and

the CH‚‚‚F hydrogen bonds, together with the electrostatic

9702 J. AM. CHEM. SOC.

9

VOL. 125, NO. 32, 2003

Aminopyridinium-Based Anion Hosts

A R T I C L E S

Figure 5. X-ray structure of receptor 17‚MeOH‚2MeCN showing cooperative anion binding of the central PF6^-anion by weak hydrogen bonds (hydrogen

atoms removed for clarity). Selected hydrogen bond lengths, N(6)‚‚‚F(16) 2.964(10), N(2)‚‚‚O(1S) 2.926(10), N(4)‚‚‚F(11) 3.239(14), C‚‚‚F 3.022(15)-

3

.417(15) Å.

Scheme 3. Conformational Equilibria in 3-Aminopyridinium-Based

Hosts (a) “3u” and “2u, 1d” Exchange, (b) In-Out Exchange (X )

H, CH2Ph, CH2C6H4(CH2)4CH3)

O’Hagan have also conducted a database search (up to 1996)

to investigate the importance of fluorine as a hydrogen bond

acceptor when the fluorine is covalently connected to carbon.²

They found that there are few examples of C-F‚‚‚H-X contacts

that are below 2.35 Å. This conclusion is also supported by

work carried out by Dunitz and Taylor who have carried out

database analysis and theoretical calculations that show that

contacts of type C-F‚‚‚H-X (X ) O, N, and C) do not

8,29

3

0

form hydrogen bonds. The cutoff distance for a C-H‚‚‚F

interaction is greater than 2.6 Å (the van der Waals radius),

but there are many examples in which the hydrogen bonding

distance is below the van der Waals radius. Grepioni and

Braga have also shown that most of the shortest distances for

C-H‚‚‚F-P interactions are found when there is a metal

coordinated to either cyclopentadienyl or arene ligands. Dunitz

and Taylor also confirmed that when fluorine is part of an anion,

-

for example, PF6 , the fluorine becomes a rather good hydrogen

3

0

acceptor.

Receptor 17 contains three PF6^-anions. The anion in the

middle of the cavity is bonded by five CH‚‚‚F interactions with

the distances ranging from 2.34 to 2.66 Å (CH bond length

normalized to 0.96 Å) in good agreement with the distances

2

7

-

obtained by Grepioni and Braga. The two other PF6 ions are

hydrogen bonding to the NH groups with hydrogen bonding

distances of 2.11 and 2.26 Å, which represent relatively strong

hydrogen bonds for these types of interactions.

interactions, are more significant than the NH‚‚‚F interaction

alone, and hence the PF6 anion is found sitting in the central

anion-binding pocket of the host, despite the lack of strong

hydrogen bonds.

-

NMR Titrations. The anion-binding ability of hosts of

1

type 2 and compounds 3, 4, and 15-17 was investigated by H

NMR titration at room temperature in MeCN-d3. The binding

constants for the first anion derived from a model incorporating

a final host:guest stoichiometry of 1:3 are summarized in

Grepioni and Braga have shown that in the solid state different

organometallic compounds containing PF6 as the counterion

_f_o_r_m_C_-_H_‚_‚_‚_F_-_P_h_y_d_r_o_g_e_n_b_o_n_d_i_n_g_i_n_t_e_r_a_c_t_i_o_n_s₂7 that play a

-

significant role when assisted by the difference in charge

between the anions and the cation. In contrast, Howard and

(

28) Howard, J. A. K.; Hoy, V. J.; O’Hagan, D.; Smith, G. T. Tetrahedron 1996,

52, 12613.

(29) Borwick, S. J.; Howard, J. A. K.; Lehmann, C. W.; O’Hagan, D. Acta

Crystallogr., Sect. C 1997, 53, 124.

(

27) Grepioni, F.; Cojazzi, G.; Draper, S. M.; Scully, N.; Braga, D. Organo-

metallics 1998, 17, 296.

(30) Dunitz, J. D.; Taylor, R. Chem.-Eur. J. 1997, 3, 89-98.

J. AM. CHEM. SOC.

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VOL. 125, NO. 32, 2003 9703

A R T I C L E S

Wallace et al.

1

Table 1. Binding Constants (K1) Determined by H NMR Titration

for the Interaction of the New Hosts with Various Anions^a

-

1

host K11 (M )

anion

Cl^-

2b

3

4

15

16

17

5²

>100 000

>100 000 6452

5270 17 380

-

Br

13 800 850 17

3953 2330

486

355

617

2950

1860

1410

3680

4

-

I

675

200

198

246

-

NO3

467

-

CH3CO3

10 500

2511 1389 49 000

-

ReO4

112

7

4

-

CF3SO3

a

Anions as n-Bu4N⁺salts in MeCN-d3, host NH, ortho-pyridyl protons,

3

2,53

and CH2 groups fit simultaneously with HypNMR 2000.

Errors in K1

are <10%. In some cases, precipitation occurred during the latter parts of

the titration, and data so affected were discarded. Equilibria were modeled

according to the binding of up to three anions by each host. Binding of the

second and third anions was generally found to be weaker than the first,

although it was nonnegligible and in the region of 10-100 M^-¹

.

Figure 6. Chemical shift of the ortho-pyridyl CH resonance of 2b (pink

square) and 2c (blue diamond) on the addition of NBu4Br in DMSO-d6 at

297 K.

Table 1. Data for the previously reported 5 are included for

2

comparison. The analysis was carried out with the aid of

3

1,32

HypNMR 2000.

3

The unsubstituted tris(pyridinium) species

gives K11 ) 850 M^-for Br which represents the effect of

cooperative electrostatic and CH‚‚‚Br interactions. Removal

1

-

is these protons that interact most with the anion in the solid

state.

-

of the preorganization imparted by the three ethyl functionalities

Attempts were made to compare the anion-binding ability of

2b with its isomers 2a and 2c. Unfortunately, in acetonitrile,

addition of small quantities of halide anions to both 2a and 2c

caused immediate precipitation. In contrast, in DMSO-d6, only

-1

(

host 4) reduced the binding to 17 M , corresponding to a pre-

-1

organization contribution of ca. 10 kJ mol for 3 and implying

a significant degree of cooperativity between the three arms in

binding the first anion. The fact that a significant interaction is

found between 3 and Br is interesting evidence in support of

the importance of CH‚‚‚Br interactions, particularly because

-

very weak binding of Cl was observed for both 2b and 2c

-

-1

(K11 ) 15 and 8 M , respectively), consistent with the highly

-

competitive nature of the solvent and relatively nonacidic nature

of the primary amine donors when compared to amides or ureas,

this affinity essentially disappears upon examination of the

nonethyl control compound 4.

2

1,33,34

for example.

In a mixture of DMSO/acetonitrile (50% v/v),

-

Host 2b proved to be an extremely effective anion receptor

2b‚3PF6 bound chloride extremely strongly (K11 ) ca. 82 000

M ). The analogous figure for 2c‚3PF6 in the DMSO/

acetonitrile solvent mixture was ca. 3000 M , although

-

-1

-

with an affinity for Cl too high to measure in the chosen

-

1

-

-1

medium (K11 > 100 000 M ). The Br anion was also bound

strongly, as was the highly basic acetate. Significantly, halides

such as Br are much more strongly bound by 2b than by 3,

solubility constraints rendered this measurement rather impre-

cise. The greater effect upon addition of anions to 2b as

compared to 2c in DMSO-d6 is highlighted by the titration plots,

however, Figure 6.

-

consistent with the involvement of the more acidic NH

functionalities and confirming the implications from the X-ray

-

crystal structures of 2b and 3 that NH‚‚‚Br interactions are

The substituted 3-aminopyridine hosts 15 and 16 showed

behavior similar to that of 2b, although the presence of the bulky

benzyl groups results in a reduction of the affinity for halide

anions. This inhibition is attributed to unfavorable steric

interactions between the N-substituents when folded into the

C3 symmetric “3-up, 3-in” (“3u, 3i”) conformation that is

necessary to bind spherical anions (Scheme 3). These steric

interactions could potentially contribute to the observation of

alternative conformers despite the alternation preference of the

hexasubstituted core, and the involvement of non-C3 conformers

-

stronger than the analogous pyridinium CH‚‚‚Br hydrogen

bonds. The position of the NH group in 2b is also more

-

complementary to Br binding than the meta-pyridinium CH

-

in 3. It is also clear evidence for structural selectivity that Cl

-

and even Br are bound more strongly than acetate. Acetate is

a highly basic anion that can form a two-point interaction with

NH and CH groups on a single “arm” or with two NH groups

on adjacent arms, and hence it would be expected to form

intrinsically more stable complexes with the aminopyridinium

functionalities of hosts of type 2b. However, the acetate complex

cannot adopt a tris(chelate) C3 symmetric conformation simul-

taneously maximizing interactions to all three arms. Similarly,

-

in binding nonspherical anions such as PF6 is clearly indicated

1

by variable-temperature H NMR spectroscopy (vide infra).

Examination of the shape of the titration curves shows that

nonspherical anions are bound very differently to halides. Thus,

-

host 2b is also clearly sterically noncomplementary to NO3

-

(which is also intrinsically less basic than acetate) and hence is

in the case of Br binding by the tris(benzyl) host 15, the

weakly bound.

titration curve for both NH and singlet pyridyl CH protons

essentially reaches saturation at a host:anion ratio of 1:1. On

titrating 15 with acetate, however, clear evidence is observed

of significant interactions of both the NH and the ortho-pyridyl

CH protons with a second anion in a fashion consistent with

At room temperature, all spectra display C3 symmetry in

contrast to the X-ray crystal structure of 2b‚3Br . The largest

chemical shift changes (∆δ) on addition of Cl are noted for

the NH protons and pyridinium ortho CH singlet signal, and it

-

(

31) Frassineti, C.; Ghelli, S.; Gans, P.; Sabatini, A.; Moruzzi, M. S.; Vacca,

A. Anal. Biochem. 1995, 231, 374-382.

(33) Beer, P. D.; Cadman, J. Coord. Chem. ReV. 2000, 205, 131-155.

(34) Berrocal, M. J.; Cruz, A.; Badr, I. H. A.; Bachas, L. G. Anal. Chem. 2000,

72, 5295-5299.

(

32) Gans, P. HypNMR 2000, University of Leeds: Leeds, 2000.

9704 J. AM. CHEM. SOC.

9

VOL. 125, NO. 32, 2003

Aminopyridinium-Based Anion Hosts

A R T I C L E S

Figure 8. Proposed binding modes between receptor 17 and MeCO2^-(a)

first anion via NH‚‚‚O interactions and (b) second anion via NH‚‚‚O and

CH(s)‚‚‚O interactions.

Table 2. The Maximum Chemical Shift Changes (ppm) Observed

on the Addition of 0.7 equiv of NBu4X, Added to the Molecular

Clips 18a-c

∆

δ/ppm

Br^-

NO

3

-

MeCO ^-

2

compound

NH

PyH

NH

PyH

NH

PyH

1

18c

8a

8b

0.23

0.12

0.15

0.25

0.13

0.11

0.19

0.22

0.26

0.11

0.08

0.10

2.00

2.28

1.90

0.31

0.66

0.23

Figure 7. ¹H NMR titration plot of compound 17 and NBu4MeCO2 at

room temperature in MeCN-d6 (a) following the NH proton and (b) the

pyridinium singlet, H(s). Some points are missing due to overlap of

resonances.

close mutual proximity. This proposal is supported by variable-

temperature NMR studies (vide infra), which show evidence

for acetate complexation by up to four conformers. In hosts

where steric interactions are less important, the acetate-selective

out conformations are less prevalent, consistent with the

variable-temperature NMR results for 2b. Also, a low binding

constant is obtained on titrating acetate with the model

compound 19, for which the binding affinity was calculated to

that of nonconvergent binding sites. This effect becomes

extremely pronounced in the very bulky tris(anthracenyl)

analogue 17.

-

Compound 17 is able to bind halide anions in the order Cl

-

>

Br > I ; however, the affinity for halides is greatly reduced

(by at least an order of magnitude), even more than for 5, 15,

and 16. Because the most effective halide binding occurs via a

C3 symmetric “3u, 3i” conformation in which the halide is

chelated by all three NH groups, halide affinity is lowered as a

consequence of unfavorable steric interactions between the bulky

anthracenyl arms in this convergent geometry. This destabiliza-

tion of the “3u, 3i” geometry remarkably increases the affinity

of 17 for the less symmetric acetate anion, and indeed 17 is

highly acetate selective with a binding constant almost too high

-1

be 7 M . Hence, the highly sterically hindered nature of

receptor 17 may predispose it to adopt a series of conformations

preorganized for chelate acetate binding at the expense of the

halide selective “3u, 3i” geometry.

Unfortunately, accurate binding constants for the molecular

clips 18a-c could not be obtained for comparison due to

solubility constraints. However, on the addition of 0.7 equiv of

NBu4X (X ) Br, NO3, and MeCO2) to a solution of MeCN-d3

containing the receptors, small shifts were observed for the NH

protons and the ortho CH proton on the pyridinium ring. The

largest chemical shift change seen for the NH proton was

obtained for the meta isomer on the addition of acetate, Table

. This result suggests that a chelate effect occurs more for the

meta isomer than the ortho and para isomers, which is in good

agreement to the analogous ferrocenyl dipodal hosts.

Variable-Temperature H NMR Spectroscopy. In the case

of 2b, addition of substoichiometric amounts of chloride in

acetonitrile results in an immediate and dramatic broadening

of the H NMR spectrum, clearly indicative of a dynamic

process occurring at a rate comparable to the H NMR

spectroscopic time scale. Because of the high freezing point of

acetonitrile, the H NMR spectrum of 2b as a function of

temperature (297-183 K) was examined in acetone-d6 solution

a) as the hexafluorophosphate salt, (b) in the presence of

-

1

to measure by NMR (49 000 M ), comparable to that recently

observed by Beer et al. for a rigidly preorganized calixarene-

3

5

based host system. The acetate anion also remarkably gives a

large chemical shift change, attributed to its strong binding and

high basicity (∆δ 3.07 ppm for the NH protons in 17). Close

examination of the behavior of various nuclei as a function of

added anion suggests that acetate binding by 17 occurs in a

way different from the binding of halides. The very gradual

change in the chemical shift behavior of the pyridyl singlet

resonance, H(s), on addition of acetate is in marked contrast to

the near saturation of the chemical shift change in this resonance

after the addition of 1 equiv of halide anion (Figure 7) and

suggests that this hydrogen atom is not directly involved in

binding the first acetate anion. Addition of more than 1 equiv

of acetate results in more significant changes to this resonance.

These observations could be interpreted as binding of the first

acetate anion in a chelating fashion by two NH moieties from

adjacent arms and binding of the second to the NH and CH

moieties of the remaining arm, Figure 8. This binding mode

may just as readily occur in both “3u, 2i, 1o” and “3u, 1i, 2o”

conformers in which the anthracene units are not forced into

2

1

(

0

.4 equiv of NBu4Cl, and (c) in the presence of 0.8 equiv of

NBu4Cl (precipitation occurred after the addition of 1 equiv).

The resulting spectra are shown in Figure 9. Before addition of

chloride (Figure 9a), the spectrum is essentially constant down

to 223 K at which temperature significant broadening occurs,

with coalescence at 203 K. At 183 K, two sets of resonances

are evident with splitting particularly noticeable for the signals

(

35) Beer, P. D.; Drew, M. G. B.; Hesek, D.; Nam, K. C. Organometallics 1999,

1

8, 3933.

J. AM. CHEM. SOC.

9

VOL. 125, NO. 32, 2003 9705

A R T I C L E S

Wallace et al.

1

Figure 9. Variable-temperature H NMR spectra of 2b in acetone-d6 (a) as the hexafluorophosphate salt, (b) after addition of 0.4 equiv of NBu4Cl, and (c)

after addition of 0.8 equiv of NBu4Cl. # NBu4Cl, / ethyl CH3 (Ha), b ethyl CH2 (Hb), and [ NH (Hc).

associated with the ethyl groups of the triethylbenzene-derived

core. The CH3 resonance of the ethyl groups at δ 1.03 ppm is

characteristically split into two new signals in the ratio 4:1 at

This postulate is confirmed by the freezing out of additional

minor conformers in more bulky systems (vide infra).

Addition of 0.4 equiv of NBu4Cl to the sample and repeating

the variable-temperature experiment reveals an addition fluxional

process, that coalesces at much higher temperature, Figure 9b.

Broadening is immediately noted in the room-temperature

spectrum, with coalescence occurring at 253 K. Below this

temperature, two sets of resonances in a ratio of 3:2 are observed

down to 223 K at which point one set of resonances (the major

set) undergoes further broadening along the pattern outlined

above for 2b‚3PF6. The other signals remain sharp down to 183

K. The slightly lower intensity of the sharp resonances as

compared to those that broaden at 223 K allows us to

unambiguously assign them to the chloride complex 2b‚Cl‚2PF6

1

.32 and 0.55 ppm. The high field chemical shift of the latter

signal is attributed to a host conformation in which the ethyl

group enters the shielding region of one of the pyridinium rings.

Such features are evident in all of the X-ray crystal structures

discussed above, particularly Figure 1b, and it is likely that the

fluxionality of 2b‚3PF6 is associated with a conformational

equilibrium of six substituents around the hexasubstituted

aromatic core. This fluxionality could take the form of a minor

conformational change involving rotation of the ethyl groups

or a much more substantial rotation of the pyridinium substit-

uents, Scheme 3a. Electrostatic arguments would tend to suggest

that it arises from the mutual repulsion of the pyridinium cations

to give a 2-up, 1-down isomer, and indeed the spectrum of the

minor conformer is less symmetrical than the major species.

1

which is conformationally rigid on the H NMR time scale, C3

symmetric, and coexists with the remaining 2b‚3PF6 responsible

for the major set.

9706 J. AM. CHEM. SOC.

9

VOL. 125, NO. 32, 2003

Aminopyridinium-Based Anion Hosts

A R T I C L E S

In a final VT experiment, a total of 0.8 equiv of Cl^-was

added to host 2b. This experiment showed a spectrum that

remained essentially constant down to 183 K with some

broadening as a result of the residual 2b‚3PF6 and solvent

viscosity effects. Differentiation of the NH signal into two

resonances was also observed, consistent with the preferential

more evident for the PF6 salts (essentially 50% of the sample)

consistent with the destabilizing of the C3 “3-up” isomer by

steric clashes between the aryl substituents. Second, a large

number of signals are noted for the protons of the ethyl

substituents consistent with the freezing out of a number of

isomers arising from the in-out pyridinium rotation (Scheme

3a and 3b), not observed in the case of 2b (we have observed

fluxionality of 3-aminopyridyl derived ligands in related metal-

-

interaction of the Cl anion with the NH proton pointing into

the cavity. Thus, the second, high-temperature fluxional process

is attributed to the freezing of the anion exchange equilibrium

between 2b‚Cl‚2PF6 and 2b‚3PF6. This is extremely unusual

2

5

based anion hosts ). For example, the single peak at 0.55 ppm

assigned to the shielded methyl protons in the “2-up, 1-down”

conformation in 2b appears as four separate signals from 0.1

to 0.6 ppm in 15 and 16 at 183 K. Coalescence of the methyl

signals is also observed at a slightly higher temperature in 15

and 16 (233 K, as compared to 223 K in 2b).

in podand anion-binding hosts, which generally display fast

anion exchange kinetics.³

6-39

We note, however, that slow

exchange of NMe4Cl has recently been reported by Atwood

and Szumna for an elegant resorcarene-based tetrapodal mo-

4

0

-

lecular capsule. Even more remarkably, the fact that 2b‚Cl‚

PF6 is conformationally rigid and exhibits, for example, only

Addition of 0.4 equiv of Cl to both 15 and 16 results in an

2

extremely broad room-temperature spectrum, and the coales-

a single peak for the terminal CH3- group of the ethyl moieties

at all temperatures, and is C3 symmetric, clearly indicates that

binding of Cl is by a C3 symmetric “3u, 3i” conformation and

cence temperature for the CH resonance is 283 K (cf., 253 K

in 2b). As in 2b, moving to lower temperature results in the

initial observation of two sets of resonances consistent with time-

3

-

that conformation is stabilized as compared to the “2u, 1d”

partial cone conformer observed for the PF6 salt (cf., Figure 4

and Scheme 3a). Moreover, the unhindered nature of 2b results

averaged C symmetry for the chloride complex, but at 233 K

3

-

one set of resonances, assigned to 15‚3PF₆(or the 16 analogue),

-

broadensandsplitsfurther.Theothersetassignedto15‚Cl ‚2PF

6

-

in essentially all of the sample being present as a 3i isomer

or 16‚Cl ‚2PF remains generally sharp and C symmetric

6

3

(

Scheme 3b) in contrast to the X-ray structure of 2b‚3Br^-

Figure 2). Thus, 2b‚3PF6 exists as an equilibrium mixture of

down to the solvent limit at 183 K, although other signals of

low intensity are detected at very low temperature. In the

-

cone and partial cone conformers; 2b‚Cl‚2PF6 is purely a cone

isomer consistent with the strong binding observed by NMR

titration. This observation represents a nice example of induced

fit binding in an entirely artificial, anion-binding system. The

strong binding of chloride by 2b and slow exchange kinetics

may be explained by the encapsulation of the anion in a highly

complementary trigonal prismatic array of hydrogen bonds from

both NH and pyridinium CH moieties of the host, broadly

consistent with Figure 2 and modeling studies on related

ferrocenyl derivatives.²

presence of 0.8 equiv of Cl , the signals assigned to the

hexafluorophosphate salt are markedly diminished in intensity,

and the spectrum is almost temperature invariant. Additional

resonances corresponding to ca. 10% of the sample are observed,

however, and these are assigned to alternative 3u isomers with

an out conformation of the pyridinium rings, as in Scheme 3b.

1

Similar VT H NMR experiments on 2b and 15 were repeated

-

with Br , I , CH3CO3 , and NO3 . For the bulky compound

5, the spectra in each case remain invariant down to ca. 233

1

K, at which point a single equilibrium process apparently freezes

-

Given that the high field peak at 0.55 ppm observed in Figure

out. While slow anion exchange as in the case of Cl cannot

9

a is absent in Figure 9c at 183 K, we can confidently assign

be ruled out, the spectra may be rationalized by a model

involving fast anion exchange even at low temperature coupled

with slow conformational exchange of the type shown in

Scheme 3b. All of the anions apparently stabilize the “3-up”

conformation relative to “2-up, 1-down”; however, the propor-

tion of out conformers of 15 increases going down the halides

it to a partial cone isomer of 2b‚3PF6 which is likely to have

two of the three methyl groups in the shielding region of the

pyridinium rings, as demonstrated structurally in Figures 1a and

4

NMR spectrum allows the calculation of an equilibrium constant

at 183 K for the two conformations of the PF6 salt of 2.0 M ,

suggesting that the energy difference between them is minimal

1

. Integrating the two resonances in the low-temperature H

-

-1

-

and reaches almost 50% for I . Acetate, which is strongly

bound, is also effective at stabilizing the “3u, 3i” conformer,

-1

(ca. 1 kJ mol ). This is in contrast to values of 10-15 kJ mol

and the complex has time-averaged C symmetry, although the

3

-

suggested for the stabilization of the “3-up” conformer by a

hexasubstituted aromatic core in related systems and probably

static symmetry must be lower. The more weakly bound NO₃

is less effective, and even in the presence of 1.1 equiv of NO₃

5

-

arises from the destabilization of the “3-up” conformer by

(no precipitation), a significant amount of out conformer is

evident. For the less hindered 2b, no evidence is observed for

in and out conformers as a result of the absence of steric

interactions disfavoring the 3i form.

-

binding the bulky PF6 , as suggested by molecular modeling

2

in the case of the tris(ferrocenyl) appended analogue.

1

The variable-temperature H NMR spectra of the bulky hosts

1

5 and 16 as the hexafluorophosphate salts display behavior

In view of the interesting VT results obtained for 2b, 15,

and 16, the H NMR spectrum of the previously reported

1

1,2,41

essentially similar to that of 2b except for two key differences:

first, the unsymmetrical “2-up, 1-down” conformer is much

tris(ferrocenyl) compound 5 was also examined in the presence

of 0.4 equiv of Cl at various temperatures. As with 15 and

-

(

36) Renard, S. L.; Kilner, C. A.; Fisher, J.; Halcrow, M. A. J. Chem. Soc.,

Dalton Trans. 2002, 4206-4212.

16, the room-temperature spectrum proved to be extremely

broad, and at 283 K two sets of resonances could be discerned

assigned to 5‚Cl‚2PF6 and 5‚3PF6, respectively. Lowering the

(37) Hettche, F.; Reiss, P.; Hoffmann, R. W. Chem.-Eur. J. 2002, 8, 4946-

4

956.

(

38) Causey, C. P.; Allen, W. E. J. Org. Chem. 2002, 67, 5963-5968.

39) Denuault, G.; Gale, P. A.; Hursthouse, M. B.; Light, M. E.; Warriner, C.

N. New J. Chem. 2002, 26, 811-813.

(

(41) Zhang, J.; Bond, A. M.; Wallace, K. J.; Belcher, W. J.; Steed, J. W. J.

Phys. Chem. B 2003, 107, 5777-5786.

(

40) Atwood, J. L.; Szumna, A. J. Am. Chem. Soc. 2002, 124, 10646-10647.

J. AM. CHEM. SOC.

9

VOL. 125, NO. 32, 2003 9707

A R T I C L E S

Wallace et al.

temperature to 243 K resulted in the dramatic broadening of

one set of resonances. At 183 K, a relatively sharp spectrum

was observed which could be assigned to a static mixture of

predominantly 3u, 3i-5‚Cl‚2PF6 and a mixture of 3u and 2u,

coalescence of the anion exchange equilibrium at 243 K. In the

presence of 0.8 equiv, an extremely unsymmetrical spectrum

is observed at 183 K in stark contrast to 2b, indicative of a

number of conformers, although crucially none display the

characteristic signals at ca. 0.9 ppm for the “2u, 1d” species. A

total of four signals are observed which could be assigned to

the acetate CH3 group at -0.80, -0.91, -1.34, and -1.58 ppm,

although they could also arise from highly shielded host ethyl

groups. These signals are tentatively assigned to acetate binding

by four isomers of the “3u” conformer arising from pyridinium

group rotation, “3u, 3i”, “3u, 2i, 1o”, “3u, 1i, 2o”, and “3u,

1d conformers of 5‚3PF6. Small amounts of out conformers were

observed for both species consistent with the bulk of the

ferrocenyl group.

1

As with the previous hosts, the H NMR spectrum of 17 as

a function of temperature (297-183 K) was examined in

acetone-d6 solution (a) as the hexafluorophosphate salt, (b) in

the presence of 0.4 equiv of NBu4Cl, and (c) in the presence of

3

o”. The chemical shifts of these resonances indicate that the

0

.8 equiv of NBu4Cl (precipitation occurred after the addition

1

anions all fall within the shielding region of the anthracenyl or

pyridinium rings.

of 1 equiv). The H NMR spectrum for 17‚3PF6 is sharp at

room temperature. Cooling brings about an immediate broaden-

ing in the resonances associated with the pyridinium rings and

ethyl substituents. Coalescence occurs at 233 K, and at 183 K

the spectrum is relatively sharp. The resonance assigned to the

CH3 group of the ethyl substituents splits into two groups of

three peaks (six in all) centered on 1.4 and 0.9 ppm. The

presence of a high field signal under 1 ppm is indicative of a

CH3 group that is in the shielding region of one of the

pyridinium rings and is a characteristic of a “2u, 1d” conforma-

tion (cf., 2b). The intensity distribution suggests that, as for

the unsubstituted 3-aminopyridinium analogue, an approximately

UV-Vis Titration. The UV-vis spectra of receptors 15 and

16 as the hexafluorophosphate salts were measured in aceto-

-

5

nitrile at a concentration of 2.5 × 10 M at room temperature.

The spectra show a broad band at λmax ) 348 and 350 nm for

15 and 16, respectively, assigned to the π f π* transition of

42

the pyridinium rings. A strong band appears at approximately

200 nm, which is a characteristic band of benzene derivatives.⁴²

On titration of 16 with NBu X (X ) Cl, Br, I, and NO )

4

3

from 0 to 10 equiv, there is a small but significant bathochromic

shift of 3 nm in all cases, possibly indicative of a stronger

hydrogen bond interaction between the anions studied in

2:1 mixture of “3u” and “2u, 1d” conformations is present. In

-

compounds such as 2b, we only observe two peaks of this type.

The fact that six are observed in this case suggests that additional

conformational equilibria have been frozen out. Similarly, the

resonance assigned to the CH2 group bonded to the pyridinium

nitrogen atom splits into four signals rather than the usual two.

The remainder of the spectrum is likewise unsymmetrical, Figure

comparison to the poor hydrogen bond acceptor, PF₆anion.

The observation of isosbestic points at λ ) 347, 341, 331, and

-

341 nm for Cl , Br , I , and NO , respectively, means that

3

there are only two species present under these conditions. For

the three halides studied, the isosbestic point moves in the

hypsochromic direction descending the series, an indication of

the stability of the host-guest complex, which is becoming less

1

0. Interestingly, the protons assigned to the anthracene groups

1

remain relatively sharp, suggesting that the anthracene moieties

are conformationally mobile and independent from the core.

Hence, it is likely that the additional peaks are due to slow

exchange between in and out conformers of the pyridinium

rings.

stable going from chloride to iodide, in agreement with the H

NMR studies.

UV spectroscopy was also employed to provide further

evidence for the conformational behavior of 17. The UV

spectrum of 17 shows two peaks at 248 and 255 nm assigned

to a second electronic transition of the anthracenyl rings. Work

carried out by Desvergne and Tucker has shown that the shape

of the second electronic absorption band is related to the

characteristic mutual interaction between two anthracene units.⁴

In 17, the band at 256 nm dominates in the free host situation.

The overall intensity of the UV spectrum decreased on the

addition of acetate, with an isosbestic point observed at 272

nm; however, the second electronic band at 248 nm increases

somewhat in relative intensity, Figure 11b, indicating that the

anthracene units are mutually interacting and that this interaction

increases on the addition of acetate (cf., Figure 7). In contrast

to these results, the UV spectrum of model 19 that only contains

one anthracene moiety shows only a single band at 252 nm,

assigned to the second electronic transition. There is a small

shoulder that can be assigned to intermolecular interactions

between two molecules of 19 in solution, whereas the band at

248 nm for 17 is due to the intramolecular interactions of the

On addition of substoichiometric amounts of chloride (0.4

1

equiv), there is immediate broadening of the H NMR spectrum

at 297 K, as observed for 5, 15, and 16. Coalescence occurs at

3,44

2

73 K, and, for example, the peaks for the CH2 groups bonded

to the nitrogen on the pyridinium ring split into two distinct

peaks in a 3:2 ratio as high as 253 K. This is consistent with

the freezing out of the anion exchange equilibrium. On repeating

-

the experiment on the addition of 0.8 equiv of Cl , the spectrum

remains generally sharp to 263 K and then becomes broad. At

low temperature (183 K), the spectrum is again unsymmetrical;

however, unlike the low-temperature spectrum of 17‚3PF3, the

high field peaks between 0 and 1 ppm indicative of a “2u, 1d”

conformation are absent, indicating that the host binds the anion

in a “3u” cone conformation, probably of the “3u, 3i” type.

The spectrum is consistent with the presence of one major

isomer as indicated by a dominant pyridinium CH2 signal at

6

.07 ppm, along with a number of other minor conformers

-

tentatively assigned to out isomers arising from rotation of the

pyridinium rings.

anthracene moieties. On addition of MeCO2 to compound 19,

_T_h_e_e_x_p_e_r_i_m_e_n_t_s_w_i_t_h_C_l- were repeated with acetate, an

anion lacking spherical symmetry and therefore likely to disfavor

the C3 symmetric “3u, 3i” conformation. The low-temperature

spectra recorded in the presence of 0.4 equiv of anion show the

(42) Lambert, J. B.; Shurvell, H. F.; Verbit, L.; Stout, G. H. Organic Structural

Analysis; Macmillian Publishing: New York, 1976.

(43) Marquis, D.; Desvergne, J. P.; Bouaslaurent, H. J. Org. Chem. 1995, 60,

7

984-7996.

(

44) Tucker, J. H. R.; Bouas-Laurent, H.; Marsau, P.; Riley, S. W.; Desvergne,

J.-P. Chem. Commun. 1997, 1165-1166.

9708 J. AM. CHEM. SOC.

9

VOL. 125, NO. 32, 2003

Aminopyridinium-Based Anion Hosts

A R T I C L E S

1

Figure 10. Partial variable-temperature H NMR spectra in acetone-d6 (a) as 17‚3PF6, (b) 17‚3PF6 plus 0.8 equiv of NBu4Cl, and (c) 17‚3PF6 plus 0.8 equiv

of NBu4MeCO2. Note the splitting of the acetate methyl resonance at 0.45 ppm into four peaks between -0.7 and -1.8 ppm. # NBu4X, and solvent peaks,

/

ethyl CH3 (Ha), b ethyl CH2 (Hb), and [ CH2 (Hc) adjacent to the pyridinium nitrogen.

there is only a slight decrease in absorbance intensity, consistent

with the weak binding observed by NMR. The experiment was

repeated on the addition of NBu4Cl to 19. Again, only one band

is observed at 254 nm. On the addition of Cl to receptor 17,

the relative intensity of the band at 248 nm is unchanged, Figure

photoinduced electron-transfer (PET) sensor. However, the

presence of more than one anthracene group, coupled with

significant anion-dependent conformational changes on anion

binding, suggests that additional effects may be observed arising

from the anion-induced mutual interaction of anthracene moi-

eties. An excited anthracene unit can associate with the ground

state of a second fluorophore and produce an intramolecular

excimer (anthracene-anthracene). Also, an excited-state an-

thracene-pyridinium complex (exciplex) could also be envis-

aged. The observation of such excimer or exciplex systems is

-

1

1a, and suggests that there is little change in the anthracene

mutual interaction.

Fluorescence Studies. Compound 17 falls into the category

of the fluorophore-spacer-receptor model proposed by de

1

8

Silva, and therefore the compound could act as a simple

J. AM. CHEM. SOC.

9

VOL. 125, NO. 32, 2003 9709

A R T I C L E S

Wallace et al.

Figure 11. UV spectra of receptor 17 (1.0 × 10^-⁶M) on the addition of (a) NBu4Cl and (b) NBu4MeCO2, at room temperature.

often dependent on the local concentration, which has been

4

5,46

found to play an important role in fluorescence.

-

5

A 1 × 10 M solution of receptor 17 was made up in a 2

3

cm volume cell in MeCN and excited at 366 nm. The

fluorescence emission spectrum was recorded straight away and

displays monomer (single anthracene) emissions at 396, 416,

and 440 nm. The fluorescence intensity of 17 was very low,

approximately 100 (arbitrary units), when the sample was fresh;

that is, fluorescence is quenched. However, it was observed that

the fluorescence intensity increased when the solution was

exposed to ambient light. Also, in some cases, the fluorescence

spectrum of 17 showed evidence of the presence of an additional

broad band at 540 nm possibly attributable to excimer or

exciplex emission suggesting interaction of the arms with one

another.

Figure 12. Plot of fluorescence intensity against time for 17‚3PF6 (blue

diamond) and 19‚PF (pink square), irradiated at 254 nm at 2 min intervals.

6

to UV irradiation for 30 min resulted in the appearance of two

small resonances at 4.51 and 4.52 ppm. The sample was run

A simple experiment was carried out to see how much the

fluorescence intensity increased over time. A fresh sample of

receptor 17 and control 19 were subjected to UV irradiation

1

again after another 30 min, and the H NMR spectrum was

repeated and the intensities of these peaks became larger.

(254 nm) over a 12 min period with spectra being recorded

1

Leaving the sample in the dark for 7 days and repeating the H

every 2 min. In both cases, the fluorescence intensity was

observed to increase markedly, Figure 12. This result suggests

that a more fluorescent species is being formed in solution. It

is possible that this species is a result of 4π + 4π cycloaddition

of two anthracene units. The fact that the fluorescence intensity

of the control compound 19 is also increased implies that the

cycloaddition between the anthracene moieties can occur in an

intermolecular as well as an intramolecular fashion. Formation

of addition products under UV irradiation is further supported

NMR measurement resulted in a reduction of the intensity of

these signals, but they did not completely disappear, suggesting

that the photoadduct is relatively long-lived. The appearance

of two resonances in the NMR spectrum either can be assigned

to cis and trans isomers of the photodimer or may represent the

formation of both intra- and intermolecular addition species,

particularly given the relatively high concentration of the sample

-

3

in the NMR tube (20 mmol dm ). The appearance of one of

the resonances is consistent with work carried out by Desverge

and Tucker who designed an anthracene-derived receptor that

can bind cations, in particular, sodium, in which the receptor

1

by H NMR spectroscopy. Exposure of an NMR sample of 17

(

45) Bouas-Laurent, H.; Casellan, A.; Desvergne, J.-P.; Lapouyade, R. Chem.

Soc. ReV. 2000, 29, 43-55.

47

can undergo a 4π + 4π cycloaddition reaction. Evidence of

(

46) Bouas-Laurent, H.; Castellan, A.; Desvergne, J.-P.; Lapouyade, R. Chem.

Soc. ReV. 2001, 30, 248-263.

this reaction has been obtained from UV spectroscopy and NMR

9710 J. AM. CHEM. SOC.

9

VOL. 125, NO. 32, 2003

Aminopyridinium-Based Anion Hosts

A R T I C L E S

Figure 13. Fluorescence titration of receptor 17‚3PF6 with (a) Cl^-and (b) I^-(MeCN, 2.5 × 10^-⁵M, addition of 20 equiv).

1

studies. In the H NMR spectrum, the characteristic resonances

of the tertiary bridgehead protons appear as a singlet at δ 4.56

ppm in CHCl3,⁴⁸in good agreement with our findings.

Titrations were carried out by adding 20 equiv aliquots of

NBu4X (X ) Cl, Br, I, and MeCO2) to a fresh sample of receptor

binding occurs via an equilibrating mixture of conformers with

the first anion binding to a pair of pyridinium arms, resulting

in an increase in the anthracene-anthracene mutual interaction.

The high affinity for acetate is a result of the destabilization of

the alternative 3-up, 3-in conformer that is more complementary

to halides. The bulk of the anthracene units results in slow

rotation of the pyridinium groups at low temperature. While

receptor 17 is not an effective fluorescent sensor, it undergoes

a 4π + 4π cycloaddition reaction to produce a more effective

anion sensing species.

1

7, but no significant change in fluorescence intensity was

observed. If the sample was allowed to stand for an hour or so,

however, the fluorescence intensity was observed to increase

due to the formation of the cycloadduct. On addition of anions

to these samples, the fluorescence intensity decreased in all

cases; that is, quenching is observed. Emission is switched off

-

by at least 30% on the addition of Cl and 50% for I , Figure

3.

Experimental Section

1

General Considerations. Mass spectra were run at King’s College

London on a JEOL AX505W spectrometer in FAB mode in a

thioglycerol or NBA matrix. NMR spectra were recorded on a Bruker

AV-400 or AV-500 spectrometer operating at 400 or 500 MHz,

respectively. IR spectra were recorded on a PE Paragon 100 FTIR

spectrometer as Nujol mulls. Microanalyses were performed at London

Metropolitan University. All reactions were carried out under nitrogen,

although the products showed no oxygen or moisture sensitivity. The

fluorescence spectra of the anthracene derivatives were made up in a

Conclusions

A series of aminopyridinium-based receptors have been

successfully synthesized in good yields. The binding ability of

these hosts depends crucially on their ability to chelate the anion,

and in the case of Cl^-extremely strong binding is observed

along with slow anion exchange kinetics even at room temper-

ature. Such strong non-Hoffmann selectivity is surprising in a

host that operates via relatively poorly acidic NH‚‚‚anion and

CH‚‚‚anion hydrogen bonding interactions.

The ethyl groups situated in the 2, 4, and 6 positions on the

core introduce a degree of preorganization into the molecular

scaffold forcing the pendant sidearms to point in the same

direction as part of a “3-up, 3-down” or cone geometry;

however, repulsions between the pyridinium rings destabilize

4

concentration of 10 µM in MeCN. The NBu X salts were also dissolved

in MeCN and added in aliquots of 10 µL to a 2.0 mL solution of the

host, whereupon they were excited at 368 nm. The excitation

wavelength was chosen from the absorption spectra. The spectra were

recorded using a Perkin-Elmer luminescence spectrometer LS50B under

PC control (FL Win Lab). UV/vis spectra were recorded in MeCN at

ambient temperature using a Hewlett-Packard HP8453 diode-array

spectrophotometer operating under a PC controlled using Hewlett-

Packard software. The hosts were made up in 0.25 mmol solutions,

4-6,15

this conformation in contrast to analogous neutral systems.

4

and aliquots of NBu X salts were added to the 2.0 mL solution of the

The 3-up conformer is further destabilized by unfavorable steric

interactions in the case of compounds such as 15-17, which

have bulky substituents on the amino group. Anion binding

results in a conformational switching process from 2-up, 1-down

host in a cell of path length 1.0 cm.

NMR Titrations. A solution of the host (0.0500 M) was made up

in a single NMR tube in the desired deuterated solvent (0.500 mL).

Solutions of anions (0.2500 M) as the tetrabutylammonium salts were

made up in a 2 mL volumetric flask, with the desired deuterated solvent.

Ten microliter aliquots of the guest were added to the NMR tube with

thorough mixing, and the spectra were recorded after each addition.

Materials. The preparation of 1 was carried out in accordance to

the literature procedure.¹³

General Procedure for Counterion Metathesis. Method A: The

relevant host bromide salt was dissolved in methanol (75 mL), and a

methanolic solution of sodium hexafluorophosphate, in a 3-fold excess,

was added to the bromide salt solution and left to stir at room

1

to 3-up that can be monitored by VT H NMR spectroscopy.

Solubility problems have been encountered, which have

rendered some of the binding constants imprecise. This problem

was overcome by placing alkyl chains on the pendant sidearms

in the successful synthesis of receptor 16.

It has been shown by ¹H NMR titration and variable-

temperature NMR work that receptor 17 is able to selectively

bind acetate over spherical anions such as chloride. Acetate

6

temperature for 2 h. Over this time, the PF salt precipitated. The solid

was filtered, washed twice with methanol and then twice with diethyl

ether, and dried in air for 24 h.

(

47) McSkimming, G.; Tucker, J. H. R.; Bouas-Laurent, H.; Desvergne, J. P.

Angew. Chem., Int. Ed. 2000, 39, 2167-2169.

(

48) Wilson, D. T.; Selinger, B. K. Photochem. Photobiol. 1969, 9, 171.

J. AM. CHEM. SOC.

9

VOL. 125, NO. 32, 2003 9711

A R T I C L E S

Wallace et al.

), 6.28 (s, 6H, CH ),

Method B: The appropriate host bromide salt was dissolved in

dichloromethane (75 mL), and an aqueous solution of sodium hexa-

fluorophosphate (75 mL), in a 3-fold excess, was added to the bromide

salt. The resulting biphasic solution was stirred for 2 h. The organic

layer was separated, washed several times with water (100 mL), dried

over magnesium sulfate, and filtered through Celite. The solvent was

removed under reduced pressure, and the product was recrystallized

as described below.

(t, 9H, J ) 7.4, CH

3

), 2.83 (q, 6H, J ) 7.4, CH

2

8.34 (m, 6H, PyH), 8.76 (t, 3H, J ) 7.7, PyH), 9.30 (d, 6H, J ) 7.7,

+

PyH). FAB-MS: m/z ) 598 [M - Br] . Anal. Calcd for C30

H36Br

3 3

N ‚

2

2H O: C, 50.44; H, 5.64; N, 5.88. Found: C, 50.48; H, 5.52; N, 5.72.

Counterion metathesis to give the hexafluorophosphate was achieved

by method A described above. The white solid was isolated by filtration

and washed several times with methanol and ether to yield the desired

1

PF

δ/ppm): 1.103 (t, 9H, J ) 7.6, CH

(s, 6H, CH ), 8.20 (m, 6H, PyH), 8.70 (m, 9H, PyH). FAB-MS: m/z

) 729 [M - PF

‚2H

H, 4.37; N, 4.65.

6

salt. Yield: 78 mg, 0.9 mmol, 76%. H NMR (MeCN-d

3

, J/Hz,

Syntheses. 1,3,5-Tris(2-aminopyridiniummethyl)-2,4,6-triethyl-

benzene Bromide (2a). 1,3,5-Tribromomethyl-2,4,6-triethylbenzene

3

), 2.66 (q, 6H, J ) 7.6, CH

2

), 5.99

2

+

-1

(

1.00 g, 2.27 mmol) and 2-aminopyridine (0.640 g, 6.80 mmol) were

dissolved in CH Cl (50 mL) and stirred at reflux for 12 h. During this

time, a white precipitate formed. This was filtered off and washed with

6

] . IR (ν/cm ): 830 s (PF ). Anal. Calcd for

C

30

H

36

F N

18 3

P

3

2

O: C, 39.62; H, 4.43; N, 4.62. Found: C, 39.84;

2

CH

2

Cl

2

(3 × 50 mL) to give the desired product as a white powder

1,3,5-Tris(pyridiniummethyl)benzene Hexafluorophosphate (4).

1,3,5-Tribromomethylbenzene (1.00 g, 2.80 mmol) was dissolved in

1

(0.87 g, 53%). H NMR (400 MHz, MeOH-d

4

, J/Hz, δ/ppm): 1.03 (t,

J ) 7.4, 9H, CH

3

), 2.47 (q, J ) 7.4, 6H, CH

2

CH

3

), 5.25 (s, 6H, -CH

2

-

2 2

CH Cl (50 mL), and pyridine (1.00 mL, 12.3 mmol) was added

dropwise and with continued stirring. A white precipitate formed almost

immediately. The suspension was stirred for an hour and then filtered

+

Py ), 6.82 (t, J ) 7.0, 3H, PyHp), 7.15 (d, J ) 8.0, 3H, PyHm), 7.31

(

6

d, J ) 6.9, 3H, PyHo), 7.81 (t, J ) 7.9, 3H, PyHm). FAB-MS: m/z

41 [M - Br] . Anal. Calcd for C30

+

H39Br

3

N

6

: C, 49.81; H, 5.43; N,

to give the desired product as a white powder. Yield: 1.13 g, 1.90

1

1.62. Found: C, 49.45; H, 5.88; N, 11.61.

,3,5-Tris(3-aminopyridiniummethyl)-2,4,6-triethylbenzene Hexa-

fluorophosphate (2b). 1,3,5-Tri(bromomethyl)-2,4,6-triethylbenzene

1.00 g, 2.3 mmol) and 3-aminopyridine (640 mg, 6.8 mmol) were

mmol, 68%. H NMR (400 MHz, MeCN-d

3

): 5.82 (s, 6H, -CH -

2

+

1

Py ), 7.87 (s, 3H, ArH), 8.02 (m, 6H, PyHm), 8.50 (t, J ) 7.8 Hz,

3H, PyHp), 9.11 (d, J ) 7.8 Hz, 6H, PyHo). The bromide salt

(

(0.80 g, 1.35 mmol) was dissolved in CH

of tetrabutylammonium hexafluorophosphate (5.30 g, 13.5 mmol) in

CH Cl (30 mL) was added dropwise with stirring. A white precipitate

2 2

Cl (50 mL), and a solution

refluxed for 6 h in chloroform (100 mL). The solvent was then removed

under reduced pressure to produce the desired product as the tribromide

2

1

formed almost immediately. The suspension was stirred for an hour

and then filtered to give the desired product as a white powder. Yield:

salt (1.5 g, 1.3 mmol, 92%). H NMR (MeOH-d

4

, J/Hz, δ/ppm): 1.07

), 5.92 (s, 6H, CH ), 7.6

m, 3H, ArH), 7.73 (m, 3H, ArH), 8.26 (d, 3H, J ) 6.4, ArH), 8.34 (s,

(

t, 9H, J ) 7.5, CH

3

), 2.68 (q, 6H, 7.6, CH

2

0.89 g, 1.13 mmol, 84%. ¹H NMR (400 MHz, MeCN-d

, J/Hz,

), 7.44 (s, 3H, ArH), 8.01 (m, 6H,

PyHm), 8.51 (t, J ) 7.9, 3H, PyHp), 8.628 (d, J ) 7.9, 6H, PyHo).

3

+

δ/ppm): 5.64 (s, 6H, -CH -Py

+

3

H, ArH). FAB-MS: m/z ) 641 [M - Br] , 561 [M - 2Br] . Anal.

: C, 39.23; H, 4.28; N, 9.15. Found: C, 39.35;

2

3 6

Calcd for C30H39Br N

+

-

6 24 18 3 3

]. Anal. Calcd for C24H F N P ‚

H, 4.02; N, 8.80. Counterion metathesis to give the hexafluorophosphate

was achieved by method B described above. The solvent was removed

under reduced pressure to produce the desired product, which was

FAB-MS: m/z 789 [M - PF

2H O: C, 34.92; H, 3.42; N, 5.09. Found: C, 34.62; H, 3.67; N, 4.90.

2

Ethylidenepyridin-3-ylamine (6). Acetyl aldehyde (2.34 g, 53

mmol) was added to a solution of 3-aminopyridine (5.0 g, 53 mmol)

in dichloromethane (100 mL) and refluxed for 6 h, over which time a

pale orange color formed. The solvent was removed under reduced

recrystallized from methanol and acetonitrile. Yield: 1.8 g, 2.0 mmol,

1

9

0%. H NMR (MeCN-d

3

, J/Hz, δ/ppm): 0.93 (t, 9H, J ) 7.5 Hz,

), 5.47 (s, br, 3H, NH) disappears on D

), 7.64 (m, 6H, ArH), 7.65 (m, 3H, ArH), 7.81

CH

shake, 5.69 (s, 6H, CH

s, 3H, ArH). FAB-MS: m/z ) 774 [M - PF

m, 3412 m (NH ), 837 s (PF br). Anal. Calcd for C30

Cl : C, 37.11; H, 3.92; N, 8.38. Found: C, 36.64; H, 4.05; N, 8.37.

,3,5-Tris(4-aminopyridiniummethyl)-2,4,6-triethylbenzene Hexa-

fluorophosphate (2c). 1,3,5-Tris(bromomethyl)-2,4,6-triethylbenzene

1.00 g, 2.3 mmol) and 4-aminopyridine (640 mg, 6.8 mmol) were

refluxed for 6 h in chloroform (100 mL). The white precipitate that

formed over this period was isolated by filtration and washed with CH

Cl , to produce the desired product as a white solid. Yield: 1.5 g, 2.1

mmol, 91%. H NMR (MeOH-d

CH ), 2.62 (q, 6H, 7.4, CH ), 5.50 (s, 6H, CH

3

), 2.60 (q, 6H, 7.6, CH

2

O

2

pressure to produce the desired imine as a yellow oil. Yield: 5.0 g, 42

+

-1

1

(

6

] . IR (ν/cm ): 3508

‚CH

mmol, 79%. H NMR (CHCl

3

-d, J/Hz, δ/ppm): 1.1 (d, 3H, J ) 4.0,

2

6

H

39

F

18

6

N P

3

2

-

CH

3

), 6.90 (m, 1H, PyH), 7.21 (m, 1H, PyH), 7.64 (m, 1H, PyH), 8.55

+

2

(m, 1H, PyH), 8.15 (m, 1H, CHdN). FAB-MS: m/z ) 121 [M + H] .

IR (ν/cm ): 1700 s (CHdN).

-

1

Benzylidenepyridin-3-ylamine (7). Benzaldehyde (5.69 g, 54 mmol)

was added to a solution of 3-aminopyridine (5.0 g, 54 mmol) in

dichloromethane (100 mL) containing molecular sieves and refluxed

(

2

-

2

for 6 h, under N . The solvent was removed under reduced pressure to

produce the desired imine, as a colorless oil. Yield: 6.0 g, 33 mmol,

2

1

, J/Hz, δ/ppm): 0.87 (t, 9H, J ) 7.4,

3

61%. H NMR (CHCl -d, J/Hz, δ/ppm): 7.08 (m, 1H, ArH), 7.28 (m,

4

), 6.80 (d, 6H, J ) 7.3,

5H, ArH + PyH), (dd, J ) 1.6, 7.3, 1H, ArH), 8.20 (s, 1H, CHdN),

8.30 (dd, 1H, J ) 1.5, 5.0, PyH), 8.34 (d, 1H, J ) 2, PyH). FAB-MS:

3

2

+

PyH), 7.80 (d, 6H, J ) 7.3, PyH). FAB-MS: m/z ) 641 [M - Br] .

Anal. Calcd for C30

+

-1

H39Br N

3 6

‚4H

2

O: C, 40.43; H, 4.96; N, 11.82.

m/z ) 182 [M] . IR (ν/cm ): 1712 w (CHdN).

Found: C, 41.06; H, 5.38; N, 12.16. Counterion metathesis to give the

(4-Pentylbenzylidene)pyridin-3-ylamine (8). Benzaldehyde (5.69

g, 54 mmol) was added to a solution of 3-aminopyridine (5.0 g, 54

mmol) in dichloromethane (100 mL) containing molecular sieves and

refluxed for 24 h, under N . The solvent was removed under reduced

2

pressure to produce the desired imine, as a white oil. Yield: 9.0 g, 36

hexafluorophosphate was achieved by method A described above.

1

Yield: 1.8 g, 1.9 mmol, 90%. H NMR (DMSO-d

6

, J/Hz, δ/ppm): 0.77

), 5.47 (s, 6H, CH ),

(

t, 9H, J ) 7.2, CH

3

), 2.55 (q, 6H, J ) 1.0, CH

2

6

.85, 8.03 (AA′ BB′, 12H, J ) 7.3, ArH), 8.15 (br, s, 6H, NH)

+

1

disappears on D

2PF

]⁺. IR (ν/cm^-¹): 3443 m, 3311 m, 3204 m (NH

PF ). Anal. Calcd for C30 : C, 39.23; H, 4.28; N, 9.15.

Found: C, 39.14; H, 4.32; N, 9.18.

,3,5-Tris(pyridiniummethyl)-2,4,6-triethylbenzene Hexafluoro-

phosphate (3). 1,3,5-Tri(bromomethyl)-2,4,6-triethylbenzene (1.00 g,

.3 mmol) and pyridine (550 µL, 6.8 mmol) were dissolved in

2

O shake. FAB-MS: m/z ) 774 [M - PF

6

] , 628 [M

mmol, 66%. H NMR (CHCl

3

-d, J/Hz, δ/ppm): 0.84 (t, 3H, J ) 6.7,

), 1.27 (sp, 4H, J ) 3.4, 5.1, 9.8, CH ), 1.57 (q, 2H, J ) 7.5,

), 2.62 (t, 2H, J ) 7.0), 7.33, 8.50 (AA′ BB′, 4H, J ) 8.1, ArH),

-

(

6

2

); 842 s, br

CH

3

2

6

39 18 6 3

H F N P

2

7.41 (ddd, 1H, J ) 1.0, 2.5, 9.0, PyH), 6.65 (dd, 1H, J ) 4.7, 8.2,

PyH), 8.45 (dd, 1H, J ) 4.5, 1.0, PyH), 8.50 (d, 1H, J ) 2.7, PyH),

1

+

-1

8.63 (s, 1H, CHdN). FAB-MS: m/z ) 252 [M] . IR (ν/cm ): 1725

s (CHdN).

2

chloroform (70 mL), and the mixture was refluxed for 4 h. During this

time, a solid white precipitate formed, which was filtered, washed with

Anthracen-9-ylmethylenepyridin-3-ylamine (9). Anthracene-9-

carbaldehyde (5.00 g, 24 mmol) and 3-aminopyridine (2.28 g, 24 mmol)

were dissolved in dichloromethane (200 mL) and refluxed for 24 h

under a nitrogen atmosphere. The solvent was removed under reduced

chloroform/ether, and dried in air for 24 h to give the bromide salt.

1

Yield: 1.4 g, 2.0 mmol, 92%. H NMR (MeOH-d

4

, J/Hz, δ/ppm): 1.17

9712 J. AM. CHEM. SOC.

9

VOL. 125, NO. 32, 2003

Aminopyridinium-Based Anion Hosts

A R T I C L E S

+

pressure to produce dark yellow oil, which solidified after 24 h. Yield:

AnH), 8.52 (s, 1H, PyH). FAB-MS: m/z ) 284 [M] . IR (ν/cm^-¹):

1

6

.0 g, 21 mmol, 88%. H NMR (CHCl

3

-d, J/Hz, δ/ppm): 7.74 (m,

3215 w (NH). Anal. Calcd for C20

Found: C, 84.78; H, 5.52; N, 9.59.

,3,5-Tris(3-(N-ethylamino)pyridiniummethyl)-2,4,6-triethylben-

16 2

H N : C, 84.48; H, 5.67; N, 9.85.

1

H, PyH), 7.46 (m, 1H, PyH), 7.65-7.72 (m, 4H, AnH), 8.09 (m, 2H,

AnH), 8.64 (m, 1H, PyH), 8.73 (s, 1H, CH), 8.75 (m, 1H, PyH), 8.83

1

+

(

m, 2H, AnH), 9.76 (s, 1H, AnH). FAB-MS: m/z ) 282 [M] . IR

zene Bromide (14). Ethylpyridin-3-ylamine (10, 2.0 g, 16.0 mmol)

and 1,3,5-tris(bromomethyl)-2,4,6-triethylbenzene (2.4 g, 5.4 mmol)

were dissolved in chloroform (80 mL) and refluxed for 24 h. The solvent

was removed under reduced pressure, and the resulting oil was placed

under high vacuum, producing a white solid identified as the bromide

salt. Yield:₂_.₅_g_,₃_.₁_m_m_o_l_,₅₇_%_.1H NMR (MeOH-d

δ/ppm): 0.93 (t, 9H, J ) 7.3, CH ), 1.10 (t, 9H, J ) 7.0, CH

q, 6H, 7.6, CH ), 2.69 (q, 6H, 7.6, CH ), 4.46 (s, 6H, CH ), 7.34-

.40 (m, 3H, ArH), 7.69 (m, 3H, J ) 2.0, 9.0, ArH), 7.70 (m, 3H,

-1

(ν/cm ): 1750 s (CdN). Anal. Calcd for C20

14 2

H N : C, 85.08; H, 5.00;

N, 9.92. Found: C, 85.14; H, 4.92; N, 9.85.

Ethylpyridin-3-ylamine (10). Ethylidenepyridin-3-ylamine (6, 3.0

g, 25 mmol) was dissolved in methanol (70 mL), to which a slight

excess of sodium borohydride (1.13 g, 30 mmol) was added slowly as

a solid to the methanolic solution, and the resulting solution was stirred

at room temperature for 45 min, during which time the orange solution

turned yellow. 2 M hydrochloric acid was then added to destroy the

excess sodium borohydride. Once the effervescence had stopped, 2 M

sodium hydroxide was added until pH 9 was obtained. The yellow

solution was then extracted into dichloromethane, washed three times

with water (150 mL), separated, dried over magnesium sulfate, and

filtered through Celite before the solvent was removed under reduced

4

, J/Hz,

), 2.59

3

(

2

7

ArH), 8.22 (d, 3H, J ) 6.0), 8.62 (s, 3H, ArH). FAB-MS: m/z ) 727

+

[

M-Br] . Anal. Calcd for C36

H51Br

N

3 6

: C, 53.54; H, 6.37; N, 10.41.

Found: C, 53.00; H, 6.35; N, 10.28. Attempts to prepare a clean sample

of the corresponding hexafluorophosphate salt were unsuccessful.

1

,3,5-Tris(3-(N-benzylamino)pyridiniummethyl)-2,4,6-triethyl-

benzene Hexafluorophosphate (15). 3-(N-Benzylamino)pyridine (11,

.5 g, 8.0 mmol) and 1,3,5-tri(bromomethyl)-2,4,6-triethylbenzene (1.2

g, 3 mmol) were dissolved in chloroform (80 mL), and the mixture

was refluxed for 24 h. The solvent was removed under reduced pressure,

and the resulting oil was placed under high vacuum, producing a pale

1

pressure, producing a yellow oil. Yield: 2.5 g, 20 mmol, 82%. H NMR

(

7

CHCl

.2, 6.3, CH

PyH), 8.23 (m, 1H, PyH), 8.50 (s, 1H, PyH). EI-MS: m/z ) 123 [M] .

3

-d, J/Hz, δ/ppm): 1.23 (t, 3H, J ) 7.2, CH

3

), 3.10 (q, 2H, J )

1

2

), 4.50 (br, s, 1H, NH), 7.26 (m, 1H, PyH), 7.40 (m, 1H,

+

-

1

IR (ν/cm ): 2370 w (NH). Anal. Calcd for C12

12 2

H N : C, 68.82; H,

8

.27; N, 22.93. Found: C, 68.34; H, 8.11; N, 22.21.

orange solid identified as the bromide salt. Yield: 2.5 g, 2.5 mmol,

Benzylpyridin-3-ylamine (11). Benzylidenepyridin-3-ylamine (7,

1

9

3%. H NMR (MeOH-d

4

, J/Hz, δ/ppm): 0.93 (t, 9H, J ) 7.3 Hz,

), 4.46 (s, 6H, CH ), 5.92 (s, 6H, CH ),

.24-7.40 (m, 15H, ArH), 7.59 (dd, 3H, J ) 2.0, 9.0, ArH), 7.70 (m,

9.5 g, 52 mmol) and sodium borohydride (4.0 g, 105 mmol) were added

CH

3

), 2.64 (q, 6H, J ) 7.6, CH

2

to methanol (100 mL), and the procedure was carried out as described

for 10. The solvent was removed under reduced pressure, and the crude

7

3

9

H, ArH), 8.22 (d, 3H, J ) 6.0), 8.38 (s, 3H, ArH). FAB-MS: m/z )

product was recrystallized from CH

desired product as a pale pink solid. Yield: 7.7 g, 42 mmol, 80%. H

NMR (CHCl -d , J/Hz, δ/ppm): 4.33 (s, 2H, CH ), 4.40 (s, br, 1H,

NH) disappears on D O shake, 6.87 (m, 1H, ArH), 7.20 (dd, 1H, J )

.0, 8.0, ArH), 7.36 (m, 5H, ArH), 7.96 (dd, 1H, J ) 1.0, 5.0, ArH),

2 2

Cl and n-hexane to produce the

+

14 [M - Br] . Anal. Calcd for C30

H39Br N

3 6

2

‚4H O: C, 40.43; H, 4.96;

1

N, 11.82. Found: C, 41.06; H, 5.38; N, 12.16. Counterion metathesis

to give the hexafluorophosphate was achieved by method A as described

above, producing a pale blue solid which was filtered, washed with

methanol/ether, and dried in air to produce the desired hexafluoro-

phosphate salt. Yield: 800 mg, 0.7 mmol, 67%. H NMR (MeCN-d

), 2.46 (q, 6H, J ) 7.6, CH

), 5.61 (s, 6H, CH ), 6.35 (br, s, 3H, NH),

.29-7.37 (m, 15H, ArH + PyH), 7.57-7.61 (m, 9H, ArH + PyH),

3

2

5

8

3

+

-1

.07 (d, 1H, J ) 3.0, PyH). EI-MS: m/z ) 184 [M] . IR (ν/cm ):

248 w (NH). Anal. Calcd for C12 : C, 78.23; H, 6.57; N, 15.21.

1

3

,

12 2

H N

J/Hz, δ/ppm): 0.79 (t, 9H, J ) 7.3, CH

3

2

),

Found: C, 78.10; H, 6.51; N, 15.18.

4-Pentylbenzyl)pyridin-3-ylamine (12). (4-Pentylbenzylidene)-

pyridin-3-ylamine (8, 9.5 g, 38 mmol) and sodium borohydride (4.3 g,

13 mmol) were added to methanol (100 mL), and the procedure was

carried out as described for 9. The solvent was removed under reduced

pressure, and the crude product was recrystallized from CH Cl and

n-hexane to produce the desired product as a white solid. Yield: 8.5

4

7

2

.41 (d, 6H, J ) 5.9, CH

2

(

+

.72 (s, 3H, PyH). FAB-MS: m/z ) 1045 [M - PF

6

] , 901 [M -

1

+

-1

PF

6

] . IR (ν/cm ): 3225 m, 840 s, br (PF

6

). Anal. Calcd for

: C, 51.52; H, 4.83; N, 7.07. Found: C, 51.63; H, 4.80;

C

51

H

57

F N

18 6

P

3

2

N, 7.01.

1

1,3,5-Tris(3-(N-(4-pentyl)benzylamino)pyridiniummethyl)-2,4,6-

triethylbenzene Hexafluorophosphate (16). 3-(N-(4-Pentylbenzyl)-

amino)pyridine (12, 1.5 g, 6.0 mmol) and 1,3,5-tri(bromomethyl)-2,4,6-

triethylbenzene (868 mg, 2.0 mmol) were dissolved in chloroform (80

mL) and refluxed for 24 h. The solvent was removed under reduced

pressure, and the resulting oil was placed under high vacuum, producing

g, 42 mmol, 86. H NMR (CHCl

.8, CH ), 1.26 (sp, 4H, J ) 3.5, 5.0, 9.6, CH

), 2.51 (t, 2H, J ) 7.6, CH ), 4.04 (br, s, 1H, NH), 4.21 (s, 2H,

), 6.80 (dd, 1H, J ) 4.7, 8.2, PyH), 6.98 (ddd, 1H, J ) 1.0, 2.7,

3

-d, J/Hz, δ/ppm): 0.83 (t, 3H, J )

6

CH

3

2

), 1.53 (qt, 2H, J ) 7.6,

2

9

4

.4, PyH), 7.08, 7.16 (AA′ BB′, 4H, J ) 8.0, ArH), 7.89 (dd, 1H, J )

.5, 1.0, PyH), 7.99 (d, 1H, J ) 2.7, PyH). EI-MS: m/z ) 255 [M +

+

-1

H] . IR (ν/cm ): 3215 w (NH). Anal. Calcd for C12

12 2

H N : C, 80.27;

a pale orange solid which was identified as the bromide salt. Yield:

1

H, 8.72; N, 11.01. Found: C, 80.46; H, 9.00; N, 11.61.

6.0 g, 5.0 mmol, 83%. H NMR (MeOH-d

4

, J/Hz, δ/ppm): 0.96 (m,

), 1.63 (m, 6H, CH ), 2.52 (m, 6H, CH ),

), 4.27 (s, 6H, CH ), 5.68 (s, 6H, CH ), 7.05, 7.21 (d,

1

8H, CH

3

), 1.50 (m, 12H, CH

2

Anthracen-9-ylmethylpyridin-3-ylamine (13). Sodium borohydride

1.2 g, 32 mmol) was added to a methanolic solution of 9 (3.0 g, 11

2

.61 (m, 6H, CH

2

(

AA′ BB′ 12H, J ) 8.0, ArH), 7.40 (m, 3H, PyH), 7.66 (m, 3H, PyH),

.14 (d, 3H, J ) 6.0 PyH), 8.62 (s, 3H, PyH). FAB-MS: m/z ) 1124

[M - Br] , 1004 [M - 2Br] . Anal. Calcd for C66H87Br N : C, 55.83;

3 6

mmol), and the mixture was stirred at room temperature for 2 h. Over

this time period, the solution turned a light yellow. The solution was

made acidic (pH 5) by the addition of 2 M HCl and then basic (pH 9)

8

+

with 2 M NaOH. The product was extracted into CH

washed several times with water (100 mL), dried over MgSO

filtered to produce a yellow solution. The solvent was removed under

pressure to produce a dark yellow solid. This solid was chromato-

2

Cl

2

(100 mL),

, and

H, 7.28; N, 6.98. Found: C, 55.60; H, 7.30; N, 6.23. Counterion

metathesis to give the hexafluorophosphate was achieved by method

A described in the General Procedure for Counterion Metathesis section,

to produce a pale blue solid which was filtered, washed with methanol/

4

graphed on silica gel with CH

The solvent was removed, and the residue was recrystallized from CH

Cl and hexane, to yield the product as a yellow solid. Yield: 2.5 g,

.0 mmol, 83%. H NMR (CHCl

NH), 5.17 (d, 2H, J ) 4.3, CH ), 7.11-7.13 (m, 1H, PyH), 7.21-7.24

m, 1H, PyH), 7.50-7.59 (m, 4H, ArH), 8.07 (d, 2H, J ) 8.9, AnH),

2

Cl

2

:MeOH (97:3) solution as the eluent.

ether, and dried in air to produce the desired hexafluorophosphate salt.

1

2

-

Yield: 2.0 g, 1.4 mmol, 87%. H NMR (MeCN-d

3

, J/Hz, δ/ppm): 0.88

), 1.57 (m, 6H, CH ), 2.48 (m, 6H,

), 4.36 (s, 6H, CH ), 5.62 (s, 6H, CH ), 6.38

(m, 18H, CH

3

), 1.30 (m, 12H, CH

2

1

9

3

-d, J/Hz, δ/ppm): 3.89 (s, br, 1H,

CH ), 2.57 (m, 6H, CH

2

(br, s, 3H, NH), 7.10-7.22 (m, 12H, ArH + PyH), 7.50-7.60 (m, 9H,

ArH + PyH), 7.83 (br, s, 3H, PyH). FAB-MS: m/z ) 1256 [M -

(

+

-1

8

.10 (m, 1H, PyH), 8.20 (d, 1H, J ) 2.7, PyH), 8.25 (d, 2H, J ) 8.7,

PF

6

] , 1112 [M - 2PF

6

] . IR (ν/cm ): 3220 m, 837 s, br (PF

6

). Anal.

J. AM. CHEM. SOC.

9

VOL. 125, NO. 32, 2003 9713

A R T I C L E S

Wallace et al.

Calcd for C66

H, 6.30; N, 5.60.

,3,5-Tris(3-(9-anthracenylmethylamino)pyridinium)-2,4,6-trieth-

ylbenzene Hexafluorophosphate (17). 1,3,5-Tri(bromomethyl)-2,4,6-

triethylbenzene (400 mg, 0.9 mmol) and compound 13 (773 mg, 2.7

H

87

F

18

N

6

P

3

: C, 56.65; H, 6.27; N, 6.01. Found: C, 56.27;

3160 w (NH). Anal. Calcd for C48

N, 6.45. Found: C, 66.60; H, 4.65; N, 6.22. Counterion metathesis to

give the hexafluorophosphate salt was achieved by stirring a solution

H

40Br

2

N

4

‚2H

2

O: C, 66.37; H, 5.11;

1

of the bromide salt (100 mg, 0.12 mmol) in CH

2 2

Cl (50 mL) with a

solution of NaPF (121 mg, 0.7 mmol) in water (50 mL). After 1 h, a

6

mmol) were refluxed in CHCl

removed under reduced pressure, to produce the desired product as the

3

(70 mL) for 18 h. The solvent was

precipitate formed that was filtered, washed with methanol and diethyl

ether, and dried in air for 24 h (109 mg, 0.11 mmol, 92%). H NMR

1

bromide salt. Yield: 90 mg, 0.9 mmol, 77%. H NMR (MeOH-d

δ/ppm): 1.04 (t, 9H, J ) 7.3, CH ), 3.3 (q, 6H, J ) 7.1, CH ), 5.25 (s,

H, CH ), 5.97 (s, 6H, CH ), 7.49-7.60 (m, 12H, AnH), 7.61 (m, 3H,

4

, J/Hz,

3

(MeCN-d , J/Hz, δ/ppm): 5.18 (d, 4H, J ) 4.2, CH

2

), 5.71 (s, 4H,

CH ), 5.99 (br, 2H, NH) disappears on D

2

O shake, 7.30-7.32 (m, 2H,

3

2

6

2

PyH), 7.50-7.59 (m, 10H, AnH + BzH + PyH), 7.67-7.69 (m, 4H,

BzH + PyH), 7.70 (m, 2H, BzH), 8.02 (s, 2H, AnH), 8.11 (d, 4H, J )

8.0, AnH), 8.23 (d, 4H, J ) 8.0, AnH), 8.62 (s, 2H, PyH). FAB-MS:

PyH), 7.69 (m, 3H, PyH), 8.01 (m, 9H, AnH + PyH), 8.25 (m, 6H,

AnH), 8.51 (s, 3H, PyH), 8.56 (s, 3H, AnH). FAB-MS: m/z ) 954 [M

+

-1

+

-1

-

Br] , 810 [M - Br] . IR (ν/cm ): 3207 w (NH). Anal. Calcd for

‚H O: C, 68.65; H, 5.45; N, 6.40. Found: C, 68.30; H,

.96; N, 5.86. Counterion metathesis to give the hexafluorophosphate

m/z ) 962 [M] , 817 [M - PF

3411 w, 3422 w (NH), 844 s (PF

59.88; H, 4.19; N, 5.82. Found: C, 59.72; H, 4.06; N, 5.72.

6

] , 671 [M - 2PF

6

] . IR (ν/cm ):

C

75

H

69Br

N

3 6

2

6

). Anal. Calcd for C48

40 12 4 2

H F N P

: C,

4

salt was achieved as described for the bis(anthracenyl) species. The

yellow solid was filtered and washed with MeOH and ether and dried

in air for 24 h, to produce the desired hexafluorophosphate salt. Yield:

r,r′-Bis(3-(9-anthracenylmethylamino)pyridinium)-para-xy-

lene Hexafluorophosphate (18c). The procedure was carried out as

described above. The pale orange/yellow solid obtained after the

metathesis was filtered, washed with ether, and dried in air for 24 h.

1

9

CH

7

.0 g, 0.7 mmol, 71%. H NMR (MeCN-d

3

, J/Hz, δ/ppm): 1.05 (t,

), 5.24 (d, 6H, J ) 4.0,

), 6.01 (br, 3H, NH) disappears on D O shake,

.54-7.68 (m, 21H, AnH + PyH), 8.10 (s, 3H, AnH), 8.16 (d, 6H, J

1

H, J ) 7.3, CH

3

), 2.67 (q, 6H, J ) 7.1, CH

2

Yield: 98 mg, 0.1 mmol, 85%. H NMR (MeOH-d , J/Hz, δ/ppm):

4

2

), 5.80 (s, 6H, CH

2

5.27 (s, 4H, CH ), 5.78 (s, 4H, CH ), 7.48-7.57 (m, 8H, AnH + PyH),

2

7.62 (s, 4H, BzH), 7.67-7.84 (m, 4H, AnH + PyH), 8.11 (m, 4H,

AnH), 8.50 (m, 6H, AnH + PyH), 8.42 (s, 2H, PyH), 8.58 (s, 2H,

)

7.7, AnH), 8.28 (b, 6H, J ) 8.4, PyH), 8.67 (s, 3H, PyH). FAB-

+

-1

+

MS: m/z ) 1343 [M - PF

6

] , 1197 [M - 2PF

6

] . IR (ν/cm ): 3392

‚H O: C, 59.76;

H, 4.75; N, 5.58. Found: C, 59.61; H, 4.21; N, 5.31.

AnH). FAB-MS: m/z ) 753 [M - Br] , 671 [M - 2Br] . IR (ν/

-1

m (NH), 854 s (PF

6

). Anal. Calcd for C75

H N

69 6

F

18

P

3

2

48 40 2 4

cm ): 3196 w (NH). Anal. Calcd for C H Br N : C, 69.24; H, 4.84;

N, 6.73. Anal. Calcd for C H Br N ‚2H O: C, 66.37; H, 5.11; N,

48

40

2

4

2

General Procedure for the Synthesis of Compounds 18a-c. The

three bis(anthracenyl) isomers were synthesized in the same way. The

appropriate dibromoxylene (0.500 g, 1.9 mmol) and anthracen-9-yl

6.45. Found: C, 66.32; H, 5.29; N, 6.28. Counterion metathesis to give

the hexafluorophosphate salt was achieved by stirring a solution of the

bromide salt (100 mg, 0.12 mmol) in CH

of NaPF (121 mg, 0.7 mmol) in water (50 mL). After 1 h, a precipitate

formed that was filtered, washed with methanol and diethyl ether, and

2 2

Cl (50 mL) with a solution

methylpyridin-3-ylamine (1.08 g, 3.8 mmol) were dissolved in CH

100 mL) and refluxed for 18 h. During this time, a precipitate appeared

in all three cases. The solid was filtered, washed with CH Cl and ether,

and dried in air for 24 h.

Cl

2 2

6

(

1

2

dried in air for 24 h (86 mg, 0.09 mmol, 75%). H NMR (MeCN-d

J/Hz, δ/ppm): 5.21 (d, 4H, J ) 4.3, CH ), 5.65 (s, 4H, CH ), 5.96 (br,

O shake, 7.49-7.61 (m, 12H, AnH + PyH),

.72 (s, 2H, AnH), 7.73 (d, 2H, J ) 1.2, PyH), 8.04 (d, 2H, J ) 1.5,

3

,

2

7

H, NH) disappears on D

2

r,r′-Bis(3-(9-anthracenylmethylamino)pyridinium)-ortho-xy-

lene Hexafluorophosphate (18a). The procedure was carried out as

described above. The pale yellow solid obtained after the metathesis

was filtered, washed with ether, and dried in air for 24 h (100 mg, 0.1

BzH), 8.11 (d, 2H, J ) 1.5, BzH), 8.13 (d, 4H, J ) 8.6, AnH), 8.25

+

(

d, 4H, J ) 8.6, AnH), 8.62 (s, 2H, PyH). FAB-MS: m/z ) 961 [M] ,

+

6

-1

1

817 [M - PF ] , 671 [M - 2PF ] . IR (ν/cm ): 3424 w (NH), 838

mmol, 87%). H NMR (MeOH-d

.00 (s, 4H, CH ), 6.05-7.3 (m, 2H, PyH), 7.46-7.56 (m, 10H, AnH

BzH), 7.66-7.76 (m, 4H, BzH + PyH), 8.07 (m, 6H, AnH + PyH),

.26 (d, J ) 8.0, 4H, AnH), 8.38 (s, 2H, AnH), 8.56 (s, 2H, PyH).

4

, J/Hz, δ/ppm): 5.24 (s, 4H, CH

2

),

6

6 48 12 4 2

s (PF ). Anal. Calcd for C40H F N P : C, 59.88; H, 4.19; N, 5.82.

6

+

8

2

Found: C, 59.71; H, 4.29; N, 5.79.

N-Benzyl-3-(9-anthracenylmethylamino)pyridinium Hexafluoro-

phosphate (19). Benzyl bromide (500 mg, 2.9 mmol) and compound

+

-1

FAB-MS: m/z ) 753.1 [M - Br] , 671.1 [M - 2Br] . IR (ν/cm ):

384 w (NH). Anal. Calcd for C48 ‚2H O: C, 66.37; H, 5.11;

3

H40Br

N

2 4

2

13 (833 mg, 2.9 mmol) were refluxed in CHCl

solvent was removed under reduced pressure, to produce the desired

product as the bromide salt; this was converted to the PF salt as

described for 17. Yield: 150 mg, 2.8 mmol, 88%. H NMR (MeOH-

, J/Hz, δ/ppm): 5.32 (s, 2H, CH ), 5.61 (s, 2H, CH ), 7.47-7.52

3

(70 mL) for 18 h. The

N, 6.45. Found: C, 66.56; H, 5.05; N, 6.55. Counterion metathesis to

give the hexafluorophosphate salt was achieved by stirring a solution

6

1

of the bromide salt (100 mg, 0.12 mmol) in CH

2 2

Cl (50 mL) with a

solution of NaPF (121 mg, 0.7 mmol) in water (50 mL). After 1 h, a

6

d

4

2

precipitate formed that was filtered, washed with methanol and diethyl

ether, and dried in air for 24 h (103 mg, 0.10 mmol, 89%). H NMR

(m, 4H, BzH), 7.52-7.64 (m, 5H, AnH + PyH), 7.72-7.75 (m, 2H,

PyH), 8.03 (m, 1H, BzH), 7.47-7.52 (m, 3H, BzH), 8.33 (m, 2H, BzH),

1

+

-1

3

(MeCN-d , J/Hz, δ/ppm): 5.19 (d, 4H, J ) 4.2, CH

2

), 5.72 (s, 4H,

8.66 (s, 1H, AnH). FAB-MS: m/z ) 375 [M - Br] . IR (ν/cm ):

3330 m (NH). Anal. Calcd for C27 Br: C, 71.21; H, 5.09; N, 6.15.

CH ), 5.97 (br, 2H, NH) disappears on D

2

O shake, 7.46-7.56 (m, 12H,

23 2

H N

AnH + BzH + PyH), 7.66-7.76 (m, 4H, BzH + PyH), 8.07 (m, 6H,

AnH + PyH), 8.26 (d, J ) 8.0, 4H, AnH), 8.38 (s, 2H, AnH), 8.56 (s,

Found: C, 70.83; H, 4.52; N, 6.38. Counterion metathesis to give the

hexafluorophosphate salt was achieved by the same procedure described

+

1

2

H, PyH). FAB-MS: m/z ) 962 [M] , 817 [M - PF

6

] , 672 [M -

]. IR (ν/cm ): 3656 w (NH), 3561 w (NH), 833 s (PF ). Anal.

: C, 59.88; H, 4.19; N, 5.82. Found: C, 59.58;

for compounds 18. H NMR (MeCN-d

3

, J/Hz, δ/ppm): 5.24 (d, 2H, J

-

1

PF

6

) 4.0, CH

2

), 5.61 (s, 2H, CH

2

), 5.92 (br, 1H, NH) disappears on D O

2

Calcd for C48

H F N P

40 12 4 2

shake, 7.47-7.52 (m, 4H, BzH), 7.52-7.64 (m, 5H, AnH + PyH),

7.72-7.75 (m, 2H, PyH), 8.03 (m, 1H, BzH), 7.47-7.52 (m, 3H, BzH),

8.33 (m, 2H, BzH), 8.66 (s, 1H, AnH). FAB-MS: m/z ) 375

H, 4.07; N, 5.83.

r,r′-Bis(3-(9-anthracenylmethylamino)pyridinium)-meta-xy-

lene Hexafluorophosphate (18b). The procedure was carried out as

described above. The pale orange/yellow solid obtained after the

+

-1

[M - PF

6

] . IR (ν/cm ): 3433 m, 831 s (PF

6

). Anal. Calcd for

27 23 2 6

C H N F P: C, 62.31; H, 4.64; N, 5.37. Found: C, 61.71; H, 4.45;

metathesis was filtered, washed with ether, and dried in air for 24 h.

N, 5.38.

X-ray Crystallography. Details of general crystallographic proce-

dures in our laboratory have been published previously. Structures

1

Yield: 98 mg, 0.1 mmol, 85%. H NMR (MeOH-d

4

, J/Hz, δ/ppm):

), 7.34-7.43 (m, 8H, AnH + PyH

BzH), 7.48 (m, 4H, PyH), 7.85 (m, 4H, PyH), 7.61 (m, 1H, BzH),

.95 (m, 4H, AnH), 8.14 (m, 5H, AnH + BzH), 8.44 (m, 4H, AnH +

4

9

5

.14 (s, 4H, CH

2

), 5.68 (s, 4H, CH

2

+

7

(

49) Calleja, M.; Johnson, K.; Belcher, W. J.; Steed, J. W. Inorg. Chem. 2001,

+

-1

PyH). FAB-MS: m/z ) 753 [M - Br] , 671 [M - 2Br] . IR (ν/cm ):

40, 4978-4985.

9

714 J. AM. CHEM. SOC. VOL. 125, NO. 32, 2003

9

Aminopyridinium-Based Anion Hosts

were solved and refined using the programs SHELXS-97 and

SHELXL-97, respectively. The program X-Seed was used as an

interface to the SHELX programs and to prepare the figures.

A R T I C L E S

50

0.1098, R indices based on 6786 reflections with I > 2σ(I) (refinement

on F ), 357 parameters, 0 restraints. Lp and absorption corrections

applied, µ ) 3.612 mm . Absolute structure parameter ) 0.459(13),

racemic twin model adopted.

Crystal data for 3‚3Br‚4.5MeOH: C₃₄_.₅H Br N O₄_.₅₀, M ) 818.53,

triclinic, space group P-1, a ) 11.8589(2), b ) 16.2547(3), c )

19.7964(4) Å, R ) 84.524(8), â ) 77.220(9), γ ) 86.894(8)°, V )

3702.34(12) Å , Z ) 4, D ) 1.468 g/cm , F ) 1682, T ) 120(2)

5

1

52

2

-

1

Crystal data for 2b‚3PF

OP , M ) 1028.54, monoclinic, space group P2

1.801(4), b ) 11.741(2), c ) 18.512(4) Å, â ) 113.781(3)°, V )

6

‚CH

2

Cl

2

‚0.5H

2

O‚0.5MeOH: C31.5

2 18 6

H44Cl F N -

3

1

/c (No. 14), a )

53

3

2

4

3

336.3(15) Å , Z ) 4, D ) 1.575 g/cm , F000 ) 2096, Nonius Kappa

c

3

CCD diffractometer, Mo KR radiation, λ ) 0.71073 Å, T ) 120(2) K,

max ) 50.0°, 16 725 reflections collected, 7471 unique (Rint ) 0.1009).

Final GOF ) 1.138, R1 ) 0.1348, wR2 ) 0.2582, R indices based on

c

000

2θ

K, 2θ_m_a_x) 50.0°, 23 784 reflections collected, 19 297 unique (R

)

int

2

0.0959). Final GOF ) 1.017, R [F > 2σ(F )] 0.0632, wR (all data)

1

2

4

561 reflections with I > 2σ(I) (refinement on F ), 564 parameters, 0

0.1785, R index based on 8579 reflections with I > 2σ(I) (refinement

on F ), 782 parameters, 0 restraints. Lp and absorption corrections

1

-

1

2

restraints. Lp and absorption corrections applied, µ ) 0.374 mm

.

-

1

Crystal data for 2b‚3Br‚CH

monoclinic, space group P2 /c (No. 14), a ) 21.047(4), b ) 12.394(3),

c ) 13.721(3) Å, â ) 101.324(3)°, V ) 3509.4(12) Å , Z ) 4, D

2

Cl

2

: C31H41Br

3

Cl

2

N

6

, M ) 808.33,

applied, µ ) 3.309 mm

.

1

Crystal data for 17‚MeOH‚2MeCN: C H F N OP , M 1601.41 g

80

77 18

8

3

-1

c

)

mol , triclinic, space group P-1, a ) 9.890(2), b ) 17.600(4), c )

22.450(5) Å, R ) 104.40(3), â ) 97.60(3), γ ) 91.30(3)°, V )

3

1

0

6

.530 g/cm , F000 ) 1632, Nonius Kappa CCD, Mo KR radiation, λ )

.71073 Å, T ) 120(2) K, 2θmax ) 52.0°, 11 460 reflections collected,

179 unique (Rint ) 0.1060). Final GOF ) 1.009, R1 ) 0.0654, wR2

3745.4(13) Å

3

, Z ) 2, µ ) 0.177 mm-1, T ) 120 K, reflections

2

measured 17 493, unique data 10 301, parameters 984, R [F

1

> 2σ(F )]

)

0.1286, R indices based on 3990 reflections with I > 2σ(I)

0.1881, wR (all data) 0.3269. Crystals proved to be very difficult to

2

(refinement on F ), 385 parameters, 0 restraints. Lp and absorption

obtain, and the best sample proved to be extremely weakly diffracting

and showed evidence of twinning. As a result, the overall precision of

the structure is extremely poor; however, the location of the anions

and overall molecular conformation are unambiguous. Loose ISOR

-

1

corrections applied, µ ) 3.631 mm

.

Crystal data for 2c‚3Br‚3MeOH: C33

3 6 3

H51Br N O , M ) 819.53,

monoclinic, space group P2 /c (No. 14), a ) 9.9283(3), b )

1

3

2

3.4328(8), c ) 16.0910(3) Å, â ) 95.029(2)°, V ) 3729.13(18) Å ,

restraints (0.02 Å ) were applied to C and N atom anisotropic

3

Z ) 4, D

c

) 1.460 g/cm , F000 ) 1680, Nonius Kappa CCD

displacement parameters to stabilize the refinement.

diffractometer, Mo KR radiation, λ ) 0.71073 Å, T ) 100(2) K, 2θmax

55.0°, 26 967 reflections collected, 8506 unique (Rint ) 0.0598). Final

)

Acknowledgment. This work was supported by EPSRC

studentships (to K.J.W. and D.R.T.) and a King’s College

postdoctoral fellowship (W.J.B.). We thank the EPSRC and

King’s College London for funding of the diffractometer system,

GOF ) 1.021, R1 ) 0.0371, wR2 ) 0.0675, R indices based on 6662

2

reflections with I > 2σ(I) (refinement on F ), 440 parameters, 0

-

1

restraints. Lp and absorption corrections applied, µ ) 3.285 mm

Crystal data for 3‚3Br‚EtOH: C32 O, M ) 724.42, ortho-

rhombic, space group P2 (No. 19), a ) 10.1837(3), b )

.

3 3

H42Br N

5

2

Dr. L. J. Barbour for the program X-Seed used in the X-ray

structure determinations, and Dr. Peter Gans for the program

1 1 1

2 2

3

1

4.8317(5), c ) 22.35450(10) Å, V ) 3376.46(15) Å , Z ) 4, D )

.425 g/cm , F000 ) 1472, Kappa CCD, Mo KR radiation, λ ) 0.71073

c

3

2

3

HypNMR used in the binding constant analyses.

Å, T ) 100(2) K, 2θmax ) 55.0°, 18 283 reflections collected, 7505

unique (Rint ) 0.0739). Final GOF ) 1.050, R1 ) 0.0502, wR2 )

Supporting Information Available: CIF files for 2b‚3PF6‚CH2-

Cl2‚0.5H2O‚0.5MeOH, 2b‚3Br‚CH2Cl2, 2c‚3Br‚3MeOH, 3‚3Br‚

EtOH, 3‚3Br‚4.5MeOH, 17‚MeOH‚2MeCN. This material is

available free of charge via the Internet at http://pubs.acs.org.

(

50) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467.

51) Sheldrick, G. M. University of G o¨ ttingen, 1997.

52) Barbour, L. J. J. Supramol. Chem. 2001, 1, 189-191.

53) Frassineti, C.; Ghelli, S.; Gans, P.; Sabatini, A.; Moruzzi, M. S.; Vacca,

A. Anal. Biochem. 1995, 34, 374-382.

JA034921W

J. AM. CHEM. SOC.

9

VOL. 125, NO. 32, 2003 9715

Article Doi

Slow anion exchange, conformational equilibria, and fluorescent sensing in venus flytrap aminopyridinium-based anion hosts

DOI: 10.1021/ja034921w

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Article abstract of DOI:10.1021/ja034921w

Full text of DOI:10.1021/ja034921w

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