2,4,5-Trimethylimidazolium Scaffold for Anion Recognition Receptors Acting Through Charge-Assisted Aliphatic and Aromatic C-H Interactions

Article abstract of DOI:10.1021/acs.joc.6b00468

A series of two-armed 2,4,5-trimethylimidazolium-based oxoanion receptors, which incorporate two end-capped photoactive anthracene rings, being the central core an aromatic or heteroaromatic ring, has been designed. In the presence of HP₂O₇^3-, H₂PO₄^-, and SO₄^2- anions, ¹H- and ³¹P NMR spectroscopical data clearly indicate the simultaneous occurrence of several charge-assisted aliphatic and aromatic C-H noncovalent interactions, i.e., significant downfield shifts were observed for the imidazolium C(2)-CH₃ protons, the methylene N-CH₂ protons, and the inner aromatic proton or the outer heteroaromatic protons. Density functional theory calculations confirm the occurrence of these noncovalent interaction and suggest that the interaction between the anions and the receptors is mainly electrostatic in nature.

Full text of DOI:10.1021/acs.joc.6b00468

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Article
2,4,5-Trimethylimidazolium Scaffold for Anion Recognition Receptors
Acting Through Charge-Assisted Aliphatic and Aromatic C–H Interactions.
Paula Sabater, Fabiola Zapata, Antonio Caballero, Israel
Fernandez, Carmen Ramirez de Arellano, and Pedro Molina
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b00468 • Publication Date (Web): 14 Apr 2016
Downloaded from http://pubs.acs.org on April 17, 2016
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The Journal of Organic Chemistry
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2,4,5-Trimethylimidazolium Scaffold for Anion Recognition Re-
ceptors Acting Through Charge-Assisted Aliphatic and Aromatic
C–H Interactions.
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Paula Sabater, † Fabiola Zapata,† Antonio Caballero,* † Israel Fernández,‡ Carmen Ramirez de Arellano,
# Pedro Molina*†
† Departamento de Química Orgánica, Universidad de Murcia, Campus de Espinardo, 30100, Murcia, Spain.
‡ Departamento de Química Orgánica I, Facultad de Ciencias Químicas, Universidad Complutense, E-28040 Madrid, Spain.
# Departamento de Química Orgánica, Universidad de Valencia, E-46100 Valencia, Spain
ABSTRACT: A series of two-armed 2,4,5-trimethylimidazolium-based oxoanion receptors, which incorporate two end-capped
photoactive anthracene rings, being the central core an aromatic or heteroaromatic ring, has been designed. In the presence of
HP2O73–, H2PO4– and SO42– anions, 1H- and 31P-NMR spectroscopical data clearly indicate the simultaneous occurrence of several
charge-assisted aliphatic and aromatic C-H noncovalent interactions: i.e. significant downfield shifts were observed for the imid-
azolium C(2)-CH3 protons, the methylene N-CH2 protons and the inner aromatic proton, or the outer heteroaromatic protons. Densi-
ty Functional Theory calculations confirm the occurrence of these noncovalent interaction and suggest that the interaction between
the anions and the receptors is mainly electrostatic in nature.
INTRODUCTION
Given the importance of anions in the natural world, it is not surprising that considerable effort within the area of supramolecular
chemistry has been devoted to the design and synthesis of receptors able to recognize selectively anionic species. In this context, a
wide variety of NH donor groups have been used for hydrogen bonding-based anion recognition receptors.1 Recently, the supramo-
lecular arsenal has been reinforced by new items, namely receptors based either on neutral or cationic CH hydrogen bond donors
groups,2 halogen bonding (XB), which refers to a noncovalent interaction operating between an electrophilic region developed of a
halogen atom and Lewis bases,3 and anion-π interactions, an tractive force between an electron-deficient π aromatic moiety and an
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anion.4 The synthesis of selective anion receptors based on these types of interactions represents a significant advance in the field of
supramolecular chemistry.
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The venerable imidazole ring has played a multifaceted key role in the rational design of structural motifs for anion recognition.
This simple ring system acts as an excellent hydrogen bond donor unit in synthetic anion receptor systems. In addition, the acidity
of the NH proton can be tuned by changing the electronic nature of the imidazole substituents.5 Moreover, imidazolium cations
provide both a charge and relatively C(2)-H hydrogen bond donor groups to bind anionic species,6 whereas 2-haloimidazolium-
based receptors have been used for obtaining charged bipodal, imidazoliophane systems, interlocked rotaxane anion receptors, and
sensing systems.7 Recently, Beer and co-workers have explored the 2-methylimidazolium and 4,5-dimethylimidazolium derivatives
in anion templated pseudorotaxanes and rotaxanes. However, despite the solid state evidences, no interactions between the different
methyl protons of the imidazolium ring with anions were detected in solution phase.8
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In this context, considering the great versatility showed by the imidazole ring to act as anion receptor under different decoration
patterns (Figure 1), and also the relatively few examples reported on interactions between anions and aliphatic CH hydrogen-bond
donors,9 we wish to report a series of two-armed 2,4,5-trimethylimidazolium-based anion which provide a suitable geometric ar-
rangement that allows four converging C(sp3)-H and one C(sp2)-H moieties to support oxoanion binding.
Figure 1. Different decorated imidazole rings as anion recognition binding site.
RESULTS AND DISCUSSION
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Synthetic Strategy. Preparation of the target receptors 102+·2PF6 to 132+·2PF6 was achieved starting from 2,4,5-trimethyl-1H-
imidazole 1, which was obtained in grams scale following an improved procedure.10 Subsequent alkylation with 1,3-
bis(bromomethyl)benzene
bis(bromomethyl)pyridine
2,
2,7-bis(bromomethyl)naphthalene
3,
1,4-bis(bromomethyl)benzene
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or
2,6-
5
afforded the bis(imidazole) compounds 6-9 respectively, which by reaction with 9-
(bromomethyl)anthracene provided the desired bis-imidazolium receptors as bromide salts in moderate to good yields (22-61%).
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Further counteranion exchange upon addition of aqueous NH4PF6 produced the desired 102+·2PF6 to 132+·2PF6 receptors (Scheme
1).
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Scheme 1. Synthesis of the bis-imidazolium receptors 102+·2PF6 -132+·2PF6 . Reagents and conditions: a) NaOH in H2O and
1,3-bis(bromomethyl)benzene 2, 2,7-bis(bromomethyl)naphthalene 3, 1,4-bis(bromomethyl)benzene or 2,6-
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bis(bromomethyl)pyridine 5 in acetonitrile; b) 9-(bromomethyl)anthracene (22-61%); c) wash with saturated NH4PF6 (aq).
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The structure of compound 132+·2PF6 ·(CH3CN) was determined by single crystal X-ray diffraction (Figure 2). In the structure,
the receptor is folded showing a dihedral angle between both anthryl groups of 88.68(6)º and both triazolyl groups being almost
parallel to every second anthryl group (dihedral angles of 9.08(9)º and 10.62(12). The receptor is connected to the [PF6]- anions
through C-H··F hydrogen bonds with the pyridine, methyl and methylene groups acting as hydrogen bond donors. The stronger
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interactions found are C(3)-H(3)...F(11) [H(3)···F(11) 2.54 Å, C(3)···F(11) 3.374(4) Å, C(3)-H(3)...F(11) 146.3º], C(20)-
H(20A)...F(16)i [i: -x+3/2, y+1/2, -z+1/2, H(20A)···F(16)i 2.47 Å, C(20)···F(16)i 3.351(5) Å, C(20)-H(20A)...F(16)i 148.5º], C(20)-
H(20B)...F(4)ii [ii: x, y+1, z, H(20B)···F(4)ii 2.56 Å, C(20)···F(4)ii 3.439(4) Å, C(20)-H(20B)...F(4)ii 147.5º], C(26)-H(26B)...F(4)ii
[H(26B)···F(4)ii 2.50 Å, C(26)···F(4)ii 3.472(4) Å, C(26)-H(26B)...F(4)ii 171.6º] and C(27)-H(27C)...F(3)iii [iii: x-1, y+1, z,
H(27C)···F(3)iii 2.61 Å, C(27)···F(3)iii 3.544(4) Å, C(27)-H(27C)...F(3)iii 158.5º]. The receptor interacts with the acetonitrile solvent
trhough a the methyl group forming a C-H···N hydrogen bond [C(16)-H(16B)...N(51)iv, iv: -x+1/2, y-1/2, H(16B)···N(51)iv 2.52 Å,
C(16)···N(51)iv 3.286(5) Å, C(16)-H(16B)...N(51)iv 135.2º].
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Figure 2. Ellipsoid plot (50% probability level) for the receptor 132+, with the labeling scheme.
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Anion Binding Studies. The anion binding properties of the receptors 102+·2PF6 to 132+·2PF6 were investigated initially by H
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NMR titration experiments with the following set of anions as tetrabutylammonium salts: HP2O73–, H2PO4 , SO42–, HSO4 , NO3 , F–,
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Cl–, Br–, I–, AcO–, ClO4 and C6H5CO2 in CD3CN, but a precipitate was observed during the titration of some anion, this issue was
solved by utilization of the solvent mixture CD3CN/CD3OD (95:5 v/v).
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The 1H NMR spectra of receptors 102+·2PF6 to 132+·2PF6 in CD3CN/CD3OD (95:5 v/v) display the following characteristic sig-
nals: protons of the methyl group at C-2 (Hm), C-4 (Hk) and C-5(Hl) position of the imidazolium ring appear as a clear and sharp
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singlet around δ = 1.90, 2.10, and 2.05 ppm, respectively, with the exception of receptor 102+·2PF6 where the methyl groups at C-4
(Hk) and C-5(Hl) appear as two overlapped signals in the form of a broad singlet at δ = 2.04 ppm. Additionally, the methylene pro-
tons of the two arms N+-CH2-anthracene (Hj) and the N-CH2-aryl (Hi) appear as two different singlets around δ = 5.20 ppm and δ =
6.25 ppm, respectively. Finally, the aromatic C–H proton (Hh) of the phenyl or naphthyl rings appears as a sharp singlet around δ =
7.00 ppm. The rest of the protons attached to the different aryl fragments (Hf-g and Ha-e) appear within the typical aromatic region at
the δ = 6.77–8.73 ppm range.
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Stepwise addition of the above-mentioned set of anions to a solution of the receptors 102+·2PF6 to 132+·2PF6 ( c = 5 x 10-3 M ) in
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CD3CN/CD3OD (95:5 v/v) showed that the selectivity of the receptors towards the set of anions tested did depend on the spacer
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used. Thus, the receptors 102+·2PF6 and 112+·2PF6 bearing a meta-substituted phenyl ring and 2,7-substituted naphthyl moiety,
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respectively, showed that the addition of HP2O73–, H2PO4– and SO42– anions induced significant perturbations in their corresponding
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1H NMR spectra. At variance, the H NMR spectra of the receptors 122+·2PF6 and 132+·2PF6 , bearing a para-substituted phenyl
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ring and 2,6-substituted pyridyl moiety, respectively, were only perturbed with the addition of HP2O73–and SO42– anions.
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1
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The presence of the recognized anions caused similar perturbations in the H NMR spectra of the receptors 102+·2PF6 to
132+·2PF6 in CD3CN/CD3OD (95:5 v/v). Thus, addition of increasing amounts of the anions to a solution of these receptors pro-
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moted a significant downfield shift in the signal corresponding to the methyl group protons (Hm) at C-2 of the imidazolium ring,
(Δδ = 0.50-0.28 ppm for HP2O73–, Δδ = 0.39-0.28 ppm for H2PO4 and Δδ = 0.20-0.05 ppm for SO42–). Interestingly, remarkable
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downfield shifts were also observed in the C(sp3)-H methylene protons Hj (N+-CH2-anthracene, Δδ = 0.12-0.07 ppm) and Hj (N-
CH2-Ph, Δδ = 0.34-0.18 ppm). On the other hand, the signal corresponding to the methyl group (Hl) at C-4 of the imidazolium ring
was considerably upfield shifted in the presence of the anions, while the signal due to the other methyl group (Hk) was also upfield
shifted although in a lesser extent (Figure 3). The nature of such upfield shifts can be reasonably ascribed to an increase in the elec-
tron density on the imidazolium ring upon interaction with negatively charged species.7b
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Figure 3. 1H NMR spectral changes observed for the receptor 132+·2PF6– ( c = 5 x 10-3 M ) in CD3CN/CD3OD (95:5 v/v) during the addi-
tion of HP2O73– ions (up to 2 equiv).
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Additionally, in the receptors 102+·2PF6 and 112+·2PF6 , a higher downfield shift of the more acidic inner C(sp2)-H proton (Hh) of
the central core aryl ring (Δδ = 0.65-0.40 ppm for HP2O73–, Δδ = 0.64-0.46 ppm for H2PO4– and Δδ = 0.36-0.11 ppm for SO42–) was
1
observed. These changes caused in the H NMR spectra after addition of the mentioned anions clearly suggest the occurrence of
strong interactions between the anions and the receptors through neutral C(sp2)-H and C(sp3)-H donor groups (Figure 4).
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Figure 4. 1H NMR spectral changes observed in the aromatic region of the receptor 102+·2PF6– ( c = 5 x 10-3 M ) in CD3CN/CD3OD (95:5
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v/v) during the addition of SO42– ions (up to 2 equiv).
3–
Despite the similarities observed after the addition of these anions, a remarkable difference was detected. In the case of HP2O7
and SO42– anions the maximum shift of the signals was reached after the addition of one equiv of the anions while for H2PO4 anions
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at least two equiv were required to produce a similar effect.
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Job plot analysis of the titration data revealed a 1:1 anion-binding stoichiometry for HP2O7 and SO4 anions and 1:2 (recep-
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tor/anion) for H2PO4 anions (see Supporting Information). The stoichiometry obtained for H2PO4 anions could be due to the ability
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of the monodentate H2PO4 anions to form stable dimers in solution in the presence of dicationic receptors.11 The calculated associa-
tion constants were obtained by fitting the titration data to the corresponding binding mode using the WinEQNMR program12 and
are gathered in Table 1.
Table 1. Association Constants for Receptors 102+·2PF6 to 132+·2PF6 with different anions in CD3CN/CD3OD (95:5 v/v)
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obtained from monitoring the methyl protons at C-2 of the imidazolium ring.
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2–
Receptor
HP2O7
H2PO4
SO4
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102+·2PF6
112+·2PF6
122+·2PF6
1267 ± 130
6542 ± 427
2234 ± 147
K11 = 1828 ± 87
K12 = 656 ± 32
K11 = 4662 ± 458
K12 = 258 ± 15
5744 ± 334
3840 ± 242
868 ± 6
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[a]
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-
[a]
132+·2PF6
853 ± 98
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3935 ± 341
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[a] No detectable evidence of binding observed
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The analysis of the obtained association constants reveal that the bis-imidazolium receptors 102+·2PF6 and 132+·2PF6 , bearing
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the meta-substituted phenyl ring and the analogous 2,6-substituted pyridyl moiety, respectively, exhibit a remarkable binding affini-
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ty for SO4 ions: Ka = 5744 ± 334 M-1 for receptor 102+·2PF6 and Ka = 3935 ± 341 M-1 for receptor 132+·2PF6 which are almost
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five times higher than those calculated for HP2O73– (Ka 102+·2PF6 = 1267 ± 130 M-1 and Ka 132+·2PF6 = 853 ± 98 M-1). Interestingly
the utilization of a larger spacer such as naphthalene and p-phenyl units in receptors 112+·2PF6– and 122+·2PF6 , respectively, caused
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an inversion in the selectivity, where the higher association constants were observed for HP2O73– anions instead. The key role of the
aromatic C(sp2)-H protons pointing into the cavity is illustrated by the fact that the receptors 102+·2PF6 and 112+·2PF6 display
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moderate association constants for H2PO4– anions whereas in receptors 122+·2PF6– and 132+·2PF6– where these protons are absent no
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binding for H2PO4 anions was detected. On the other hand and for comparison purposed only it is important to note that values of
the association constant for H2PO4 anions with 2-iodoimidazolium (halogen bonding) derivatives7b in DMSO and 4,5-
–
dimethylimidazolium (hydrogen bonding) derivative8 in CDCl3 were found to be Ka = 1100 ± 300 M-1 and Ka = 4301 ± 572 M-1
respectively.
31P NMR spectral changes of HP2O73– and H2PO4 anions were also studied in the presence of 1 and 0.5 equiv of the receptors, re-
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spectively. The addition of either 1 equiv of the receptors 102+·2PF6 to 132+·2PF6 to a solution of HP2O73– in CD3CN/CD3OD (95:5
–
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v/v) or 0.5 equiv of the receptors 102+·2PF6 to 112+·2PF6 to a solution of H2PO4– in CD3CN/CD3OD (95:5 v/v) promotes an upfield
–
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shift in the 31P NMR spectra of the HP2O7 or H2PO4 anion around Δδ = -1.00 ppm and Δδ = -0.50 ppm, respectively (Figure 5).
3–
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Upfield shifts of δ about -3 ppm have been reported for hydrogenpyrophosphate trianion with other receptors.13
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Figure 5. Changes in the 31P NMR spectra of a) HP2O73- (CD3CN/CD3OD 95:5) and b) H2PO4- in CD3CN/CD3OD 95:5 after the addition
1
of different receptors.
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The emission spectra of receptors 102+·2PF6 to 132+·2PF6 in CH3CN (c = 2.5·10-6 M) when excited at λexc = 370 nm, display the
6
7
typical anthracene monomeric emission bands at λ = 397, 419, 443 and 469 nm, with high quantum yields (Φ = 0.64-0.51).
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–
−
–
The emission binding studies show that the addition of HSO4 , NO3 , Cl–, Br–, I–, ClO4 , and BF4– anions does not modify signifi-
cantly the intensity of the emission bands of the receptors 102+·2PF6 to 132+·2PF6 ( c = 2.5 x 10-6 M in CH3CN) while the presence
–
–
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–
of HP2O73– (Figure 6), H2PO4 or SO42– anions leads to fluorescence quenching.
Figure 6. Changes in the fluorescence spectra of receptor 112+·2PF6– ( c = 2.5 x 10-6 M in CH3CN) upon addition of HP2O73– anions at 20
°C. Inset: changes of emission at λ = 419 nm of receptor 112+·2PF6– upon addition of HP2O73– anions.
3–
The decrease of the emission bands strongly depends on the nature of the anion added. Thus, the presence of HP2O7 anions
caused the highest decrease in the fluorescent emission bands on all the receptors 102+·2PF6 to 132+·2PF6 (88-55%), followed by
–
–
H2PO4 (51-36%) and SO4 (39-26%) anions. Receptors 112+·2PF6 and 122+·2PF6 having the larger spacers (naphthalene and p-
–
2–
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phenyl) showed less selectivity than the receptors 102+·2PF6 and 132+·2PF6 and also the presence of F–, AcO– and PhCOO– anions
–
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caused a moderate decrease in the intensity of their emission bands (Figure 7). The decrease in the intensity of the emission bands
of the receptors 102+·2PF6 to 132+·2PF6 in the presence of these anions could be attributed to a photo-induced electron transfer
–
–
(PET) mechanism.14
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Figure 7. Graphical representation of the decrease (in percentage) in the emission intensity of the receptors 102+·2PF6– to 132+·2PF6– ( c =
2.5 x 10-6 M in CH3CN) after the addition of excess (up to 3 equivalents) of different anion.
–
–
The absorption spectra of the receptors 102+·2PF6 -132+·2PF6 in CH3CN are very similar and show a strong absorption band at λ
= 255 nm (ε ~ 242000 M-1·cm-1) along with the characteristic absorption bands attributed to the anthracene moieties at λ = 338 nm
(ε ~ 6000 M-1·cm-1), 353 nm (ε ~ 11200 M-1·cm-1), 370 nm (ε ~ 16100 M-1·cm-1) and 390 nm (ε ~ 14000 M-1·cm-1). The addition of
–
–
the above-mentioned set of anions to a solution of the receptors 102+·2PF6 -132+·2PF6 ( c = 1 × 10–5 M ) in CH3CN demonstrates
–
that only the addition of H2PO4 anions induces remarkable changes in the absorption spectra in all receptors. Thus, addition of
–
H2PO4 anions promotes the decreasing of the high-energy band at λ = 255 nm while the anthracene absorption bands were red
shifted by Δλ = 4 nm. The presence of well defined isosbestic points at λ = 262, 364, 373, and 392 nm was also observed during the
titration process (Figure 8).
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–
–
Figure 8. Changes in the absorption spectra of receptor 112+·2PF6 ( c = 1 × 10–5 M ) in CH3CN upon addition of H2PO4 anions up to 4
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equiv. at 20 °C. Arrows indicate the absorptions that increase or decrease during the titration.
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Computational Study: Density Functional Theory (DFT) calculations were carried out to gain more insight into the nature of
the interactions between receptors 102+ and 132+ and anions. To this end, the complexes formed upon the addition of HP2O7 and
3–
SO4 anions to the model dicationic receptors 10M2+ and 13M2+, where the non-interacting anthracene groups were replaced by
2–
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phenyl groups, were analyzed in detail by means of the Atoms in Molecules (AIM) and Energy Decomposition Analysis (EDA)
methods.15
Figure 9. Optimized geometries of complexes [10M-HP2O7]–, [10M-SO4], [13M-HP2O7]– and [13M-SO4]. Bond distances are
given in angstroms. All data have been computed at the PCM(MeCN)-M06-2X/6-31+G(d,p) level.
The optimized geometries of complexes [10M-HP2O7]–, [10M-SO4], [13M-HP2O7]– and [13M-SO4] (PCM(MeCN)-M06-2X/6-
31+G(d,p) level, Figure 9) clearly show short contacts (i.e. shorter than the sum of the van der Waals radii, 2.72 Å)16 between the
oxygen atoms of the anions and the hydrogen atoms of the receptor (mainly, Hi, and Hm, according to the numbering in Figures 3
and 4). Moreover, complexes derived from 10M2+ present an additional interaction involving the aromatic hydrogen Hh, not possi-
ble in complexes derived from 13M2+ due to the replacement of this CHh fragment by a nitrogen atom, which is in part responsible
for the higher association constants calculated for receptor 102+ (see above). This CH/N replacement provokes a significant change
in the equilibrium geometry of the complexes derived from 13M2+, namely the relative orientation of the pyridyl moiety with the
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respect to the anion, in order to diminish the electronic repulsion between the lone-pairs of the nitrogen and oxygen atoms. Despite
that, it can be concluded that both type of receptors behave similarly in terms of anion recognition.
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4
In addition, the AIM method reveals the occurrence of bond critical points (BCPs) and bond paths (BPs) running between the
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6
3–
2–
oxygen atoms of HP2O7 and SO4 and these particular hydrogen atoms for both type of receptors (Figure 10), hence confirming
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the interactions established between the anions and the trimethylimidazolium scaffolds. Therefore, it can be suggested that the
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1
above described downfield shifts in the H NMR titration experiments find their origin in these intermolecular O···H–C(sp3) and
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O···H–C(sp2) interactions.
Further quantitative insight into the bonding situation of the above complexes can be gained by means of the EDA-NOCV meth-
od. To this end, we focused on the interaction between 10M2+ and 13M2+ with the anion SO4 in the complexes [10M-SO4] and
2–
[13M-SO4], respectively. Table 2 gathers the EDA-NOCV data computed at the dispersion corrected BP86-D3/TZ2P level using
the above described PCM(MeCN)-M06-2X/6-31+G(d,p) optimized geometries (Figure 9).
From the data in Table 2, it becomes clear that the interaction between the receptors 10M2+ and 13M2+ with SO42– is mainly elec-
trostatic, as viewed from the computed high ∆Eelstat value which contributes ca. 76% to the total attractions. This finding is not sur-
prising if we take into account that the interacting fragments are highly charged species. Despite that, the orbital interaction is not
2–
negligible as the ∆Eorb term contributes ca. 18% to the total interaction energy between the cationic receptor and SO4 fragments.
Among the different orbital interactions, the EDA-NOCV method identifies the donation from the lone-pairs of the oxygen atoms
2–
of SO4 to the vacant σ*(C–H) molecular orbitals of the methyl C–Hm, methylene C–Hi and aromatic C–Hh groups as the main
orbital contributions to the total ∆Eorb term (∆E = –7.6, –8.5 and –5.9 kcal/mol, respectively, see Figure SI34 in the Supporting
Information). Finally, the contribution coming from dispersion forces can be considered as negligible as the corresponding ∆Edisp
term only amounts ca. 5% to the total bonding. In addition, our EDA-NOCV calculations also reveal that the interaction between
SO4 and the pyridyl-derived receptor 13M2+ is slightly lower than that computed for 10M2+ as a consequence of the lack of the
2–
aryl
CH···O
interaction
present
in
10M2+.
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Figure 10. AIM diagrams computed for complexes [10M-HP2O7]–, [10M-SO4], [13M-HP2O7]– and [13M-SO4]. The lines con-
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necting the atomic nuclei are the bond paths, while the small red spheres indicate the corresponding bond critical points.
Table 2. EDA-NOCV values (in kcal/mol) for complexes [10M-SO4] and [13M-SO4].a
Complex
∆Eint
∆EPauli
∆Eelstat
∆Eorb
∆Edisp
[10M-SO4]
-330.6
52.8
-292.9
-70.2
-20.2
(76.4%)
(18.3%
)
(5.3%
)
[13M-SO4]
-323.7
46.8
-282.9
-67.6
-20.1
(76.4%)
(18.2%
)
(5.4%
)
a Fragments: 11M2+ or 13M2+ and SO42– computed as close-shell singlets. The percentage values in parentheses give the contribution to the
total attractive interactions, ∆Eelstat + ∆Eorb + ∆Edisp. All data have been computed at the BP86-D3/TZ2P//PCM(MeCN)-M06-2X/6-
31+G(d,p) level.
CONCLUSION
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–
–
In conclusion, the two-armed 2,4,5- trimethylimidazolium-based oxoanion receptors 102+·2PF6 to 132+·2PF6 have proved to be
1
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interesting benchmarks for exploring the potential of several aliphatic and aromatic C-H noncovalent interactions. The importance
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4
–
2–
of these interactions is highlighted by the fact that the receptors are able to selectively interact with HP2O73–, H2PO4 and SO4
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anions mainly via O···H–C(sp3) and O···H–C(sp2) interactions. According to our DFT calculations, it can be concluded that the
interaction between the anion and the receptors is mainly electrostatic in nature. Despite that, the orbital interaction is not negligible
and also contributes, although in a lesser extent, to the total attraction between the receptor and the anion.
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EXPERIMENTAL SECTION
General Comments. All reactions were carried out using solvents that were dried by routine procedures. All melting points were
determined by means of a hot-plate melting-point apparatus and are uncorrected. Solution H, 13C, and 31P NMR spectra were rec-
1
orded with 200, 300, 400, or 600 MHz spectrometers. The following abbreviations have been used to state the multiplicity of the
signals: s (singlet), m (multiplet), and q (quaternary carbon atom). Chemical shifts (δ) in the 1H and 13C NMR spectra are referenced
to tetramethylsilane (TMS). Fluorescence spectra were carried out in the solvents and concentrations stated in the text and in the
corresponding figure captions, using a dissolution cell with 10 mm path length, and they were recorded with the spectra background
corrected before and after sequential additions of different aliquots of anions. Quantum yield values were measured with respect to
anthracene as the standard (Φ = 0.27 ± 0.01) using the equation Φx/Φs = (Sx/Ss) [(1 − 10−As)/(1 − 10−Ax)](ns2/nx ), where x and s indi-
2
cate the unknown and standard solution, respectively, Φ is the quantum yield, S is the area under the emission curve, A is the ab-
sorbance at the excitation wavelength and n is the refractive index. Mass spectra were recorded in FAB+, mass spectra were carried
out with 3-nitrobenzylalcohol as a matrix.
2,4,5-trimethyl-1H-imidazole 1. Yield 1.19 g (15 %). Acetaldehyde (3.168 g, 0.072 mol), butane-2,3-dione (6.192 g, 0.072
mol), and ammonium acetate (54.15 g, 0.7 mol) were dissolved in 90 mL of acetic acid and stirred during 24 h at room temperature.
The reaction mixture was added dropwise to a solution of concentrated NH4OH (300 mL) at 0 °C, diluted with 400 ml of water and
extracted with AcOEt (4 x 200 mL). The organic phase was collected and the AcOEt was removed under reduced pressure to give
an orange oil, which was washed with Et2O to give the desired 2,4,5-trimethyl-1H-imidazole as a white solid.1H NMR (400 MHz,
CDCl3) δ 2.32 (3H, s), 2.11 (6H, s); 13C NMR (100 MHz, DMSO-d6) δ 142.5, 126.7, 14.9, 11.55 ppm.; MS (ESI): m/z calc. for [M +
H]+ 111.08, found 111.08; mp = 127-129ºC; Anal. Calcd for C6H10N2: C, 65.42; H, 9.15; N, 25.43. Found: C, 65.50; H, 9.27; N,
25.30.
General procedure for the synthesis of the Bis-imidazoles 6-9.
To a solution of 2,4,5-trimethyl-1H-imidazole (0.230 g, 2.08 mmol) in acetonitrile (15 ml) was added dropwise a solution of 1M
NaOH (3.0 mmol) and stirred during 10 min. with the corresponding dibromo derivative (0.25 g, 0.95 mmol) was added in one
portion and the resultant mixture was heated at reflux overnight. The solvent was removed and the resultant residue dissolved in
chloroform (100 mL), which was washed with 1 M NaOH solution (50 mL) and water (2 x 25 mL). The aqueous layers were re-
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extracted with 25 mL chloroform, and the combined organic layers dried over magnesium sulphate, filtered and dried in vacuo,
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giving a pure colourless oil
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6
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1,3-Bis((2,4,5-trimethylimidazol-1-yl)methyl)benzene 6. Yield 0.157 g (50%), 1H NMR (400 MHz, CD3CN) δ 7.29 (1H, t, J =
8 Hz), 6.92 (2H, d, J = 8 Hz), 6.45 (1H, s), 4.95 (4H, s), 2.13 (6H, s), 2.03 (6H, s), 1.92 (6H, s) ppm; 13C NMR (100 MHz, CD3CN)
δ 143.2, 139.3, 132.0, 130.0, 125.9, 124.5, 122.6, 47.2, 13.2, 12.6, 8.9 ppm; MS (ESI): m/z calc. for [M + H]+ 323.22, found 323.22.
9
2,7-Bis((2,4,5-trimethylimidazol-1-yl)methyl)naphthalene 7. Yield 0.231 g (63%), 1H NMR (300 MHz, CDCl3) δ 7.79 (2H, d,
J = 9 Hz), 7.14 (2H, dd, J = 9 Hz, J = 1 Hz), 7.12 (2H, s), 5.10 (4H, s), 2.32 (6H, s), 2.19 (6H, s), 2.03 (6H, s) ppm; 13C NMR (75
MHz, CDCl3) δ 143.7, 135.3, 134.3, 133.1, 131.3, 129.8, 125.1, 125.0, 123.7, 48.1, 13.8, 12.9, 9.8 ppm; MS (ESI): m/z calc. for [M
+ H]+ 373.23, found 373.23.
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1,4-Bis((2,4,5-trimethylimidazol-1-yl)methyl)benzene 8. Yield 0.130 g (41%), 1H NMR (300 MHz, CDCl3) δ 6.79 (4H, s), 4.84
(4H, s), 2.16 (6H, s), 2.03 (6H, s), 1.89 (6H, s) ppm; 13C NMR (75 MHz, CDCl3) δ 142.5, 136.1, 131.5, 126.2, 121.8, 46.4, 13.1,
12.4, 8.8 ppm; MS (ESI): m/z calc. for [M + H]+ 323.22, found 323.22.
2,6-Bis((2,4,5-trimethylimidazol-1-yl)methyl)pyridine 9. Yield 0.128 g (40%), 1H NMR (300 MHz, CDCl3) δ 7.51 (1H, t, J = 8
Hz), 6.51 (2H, d, J = 8 Hz), 5.00 (4H, s), 2.25 (6H, s), 2.11 (6H, s), 1.98 (6H, s) ppm; 13C NMR (75 MHz, CDCl3) δ 157.8, 143.9,
139.6, 132.8, 122.9, 119.8, 49.7, 14.3, 13.5, 9.9 ppm; MS (ESI): m/z calc. for [M + H]+ 324.44, found 324.44.
General procedure for the synthesis of the Bis-imidazolium receptors 102+-132+ as bromide salt.
To a solution of the Bis-imidazole 6-9 (0.78 mmol) in acetonitrile (25 mL) was added dropwise 40 mL a solution of 9-
(bromomethyl)anthracene in acetonitrile (1.56 mmol) and the resultant mixture was heated at reflux temperature during 72 h. The
volume of solvent was concentrated to 50 mL and then cooled to 0ºC. The resulting precipitate was collected and washed with
diethylether, giving the desired bis-imidazolium receptor as bromide salt.
1
Bis-imidazolium receptor 102+·2Br-. Yield 0.408 g (61%), H NMR (400 MHz, DMSO-d6) δ 8.81 (2H, s), 8.20 (4H, d, J = 8
Hz), 8.14 (4H, d, J = 8 Hz), 7.65 (4H, t, J = 8 Hz), 7.58 (4H, t, J = 8 Hz), 7.33 (1H, t, J = 8 Hz), 7.05 (1H, s), 6.85 (2H, d, J = 8
Hz), 6.48 (4H, s), 5.40 (4H, s), 2.30 (6H, s), 2.02 (6H, s), 1.77 (6H, s) ppm; 13C NMR (75 MHz, DMSO-d6) δ 145.2, 136.7, 132.3,
131.9, 131.3, 131.1, 131.0, 129.1, 127.9, 127.3, 127.1, 127.0, 126.6, 125.0, 124.7, 124.6, 48.8, 45.9, 12.4, 10.7, 9.7 ppm; MS (ESI):
m/z calc. for [M2++Br-]+ 783.31, found 783.31; mp = 188-190 ºC; Anal. Calcd for C50H48Br2N4: C, 69.45; H, 5.59; N, 6.48. Found:
C, 69.60; H, 5.50; N, 6.39.
Bis-imidazolium receptor 112+·2Br-. Yield 0.135 g (22%), 1H NMR (400 MHz, CD3CN/ CD3OD (95/5 v/v)) δ 8.58 (2H, s), 8.29
(4H, d, J = 8 Hz), 8.06 (4H, d, J = 8 Hz), 7.84 (2H, d, J = 8 Hz), 7.74 (2H, s), 7.62 (4H, t, J = 6 Hz), 7.50 (4H, t, J = 6 Hz), 7.30
(2H, dd, J = 8 Hz, J = 1 Hz), 6.49 (4H, s), 5.37 (4H, s), 2.24 (6H, s), 1.96 (6H, s), 1.86 (6H, s) ppm; 13C NMR (100 MHz, CD3CN/
CD3OD (95/5 v/v)) δ 144.6, 134.5, 133.1, 133.0, 132.1, 137.1, 130.9, 130.4, 129.3, 128.6, 128.1, 126.9, 126.4, 125.5, 125.2, 124.5,
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124.3, 48.9, 46.2, 12.1, 10.3, 8.9 ppm; MS (ESI): m/z calc. for [M2++Br-]+ 833.32, found 833.32; mp = 216-218 ºC; Anal. Calcd for
1
2
C54H50Br2N4: C, 70.90; H, 5.51; N, 6.12. Found: C, 71.02; H, 5.45; N, 6.25.
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9
Bis-imidazolium receptor 122+·2Br-. Yield 0.38 g (38%), 1H NMR (400 MHz, CD3CN/ CD3OD (95/5 v/v)) δ 8.82 (2H, s), 8.20
(4H, d, J =8 Hz), 8.15 (4H, d, J = 8 Hz), 7.65 (4H, t, J =8 Hz ), 7.56 (4H, t, J =8 Hz ), 7.03 (4H,s), 6.49 (4H, s), 5.43 (4H, s), 2.34
(6H, s), 2.04 (6H, s), 1.79 (6H, s) ppm; 13C NMR (100 MHz, CD3CN/ CD3OD (95/5 v/v)) δ 145.0, 135.6, 132.2, 131.8, 131.3,
131.0, 129.0, 128.2, 127.8, 127.2, 126.8, 124.9, 124.5, 48.5, 45.7, 12.2, 10.6, 9.6 ppm; MS (ESI): m/z calc. for [M2++Br-]+ 783.3,
found 783.3; mp = 206-208 ºC; Anal. Calcd for C50H48Br2N4: C, 69.45; H, 5.59; N, 6.48. Found: C, 69.38; H, 5.48; N, 6.60.
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1
Bis-imidazolium receptor 132+·2Br-. Yield 0.227 g (34%), H NMR (400 MHz, DMSO-d6) δ 8.77 (2H, s), 8.15 (4H, d, J = 8
Hz), 8.07 (4H, d, J = 8 Hz), 7.90 (1H, t, J = 8 Hz), 7.60 (4H, t, J = 7 Hz), 7.54 (4H, t, J = 7 Hz), 7.36 (2H, d, J = 8 Hz), 6.30 (4H,
s), 5.32 (4H, s), 2.09 (6H, s), 1.85 (6H, s), 1.65 (6H, s) ppm; 13C NMR (100 MHz, DMSO-d6) δ 154.8, 145.1, 140.3, 132.1, 131.7,
131.3, 130.9, 129.0, 127.4, 127.2, 126.8, 124.6, 124.5, 123.2, 50.1, 45.5, 11.9, 10.4, 9.3 ppm; MS (ESI): m/z calc. for [M2++Br-]+
785.83, found 785.83; mp = 194-196 ºC; Anal. Calcd for C49H47Br2N5: C, 67.98; H, 5.47; N, 8.09. Found: C, 67.88; H, 5.39; N,
8.05.
General procedure for the synthesis of the Bis-imidazolium receptors 102+-132+ as hexafluorophosphate salt.
A solution of bis-imidazolium receptor as bromide salt 102+·2Br--132+·2Br- (0.100g, 0.1 mmol) in CH2Cl2 (20 mL) was washed
with a saturated solution of NH4PF6 in H2O and stirred during 20 min (5 x 20 mL). The organic solvent was collected and dried with
anhydrous Na2SO4. The solid was separated by filtration and the solvent was removed under reduced pressure, to give the PF6- salt
in quantitative yields.
Bis-imidazolium receptor 102+·2PF6 . Yield 0.112 g (97%), H NMR (600 MHz, CD3CN/ CD3OD (95/5 v/v)) δ 8.73 (2H, s),
8.15 (4H, d, J = 7.6 Hz), 8.03 (4H, d, J = 7.6 Hz), 7.65 (4H, t, J = 7.6 Hz), 7.58 (4H, t, J = 7.6 Hz), 7.25 (1H, t, J = 7.8 Hz), 6.76
(2H, dd, J = 1.8 Hz, J = 7.8 Hz), 6.66 (1H, s), 6.29 (4H, s), 5.09 (4H, s), 2.04 (12H, s), 1.85 (6H, s); 13C NMR (100 MHz, CD3CN)
δ 144.4, 135.7, 132.2, 131.5, 131.3, 130.8, 130.7, 128.8, 128.0, 127.4, 126.5, 126.4, 125.2, 123.6, 123.6, 48.5, 45.5, 11.4, 9.9, 8.9
-
1
ppm; MS (ESI): m/z calc. for [M2++PF6 ]+ 849.35, found 849.35; mp = 167-169 ºC; Anal. Calcd for C50H48F12N4P2: C, 60.36; H,
-
4.86; N, 5.63. Found: C, 60.20; H, 4.75; N, 5.60.
Bis-imidazolium receptor 112+·2PF6 . Yield 0.112 g (98%), 1H NMR (600 MHz, CD3CN/ CD3OD (95/5 v/v)) δ 8.74 (2H, s),
-
8.17 (4H, d, J = 8 Hz), 8.06 (4H, d, J = 8 Hz), 7.86 (2H, d, J = 8 Hz), 7.66 (4H, t, J = 6 Hz), 7.59 (4H, t, J = 6 Hz), 7.32 (2H, s),
7.08 (2H, dd, J = 8 Hz, J = 2 Hz), 6.33 (4H, s), 5.36 (4H, s), 2.15 (6H, s), 2.06 (6H, s), 2.02 (6H, s); 13C NMR (100 MHz, CD3CN)
δ 144.4, 133.7, 133.3, 133.2, 132.2, 131.6, 131.3, 130.7, 129.8, 128.8, 128.0, 127.5, 126.4, 125.8, 125.2, 123.6, 49.0, 45.5, 11.6,
9.9, 9.0 ppm; MS (ESI): m/z calc. for [M2++PF6 ]+ 899.37, found 899.37; mp = 181-183 ºC; Anal. Calcd for C54H50F12N4P2: C, 62.07;
-
H, 4.82; N, 5.36. Found: C, 62.21; H, 4.87; N, 5.47.
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Bis-imidazolium receptor 122+·2PF6 . Yield 0.111 g (96%), 1H NMR (400 MHz, CD3CN/ CD3OD (95/5 v/v)) δ 8.74 (2H, s),
8.16 (4H, d, J = 8 Hz), 8.02 (4H, d, J = 8 Hz), 7.64 (4H, t, J = 6 Hz), 7.57 (4H, t, J = 6 Hz), 6.89 (4H, s), 6.30 (4H, s), 5.17 (4H, s),
2.07 (6H, s), 2.03 (6H, s), 1.93 (6H, s) ppm; 13C NMR (100 MHz, CD3CN) δ 144.3, 134.9, 132.2, 131.5, 131.2, 130.7, 128.8, 128.0,
-
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-
127.6, 127.4, 126.4, 123.6, 48.5, 45.5, 11.5, 9.9, 8.9 ppm; MS (ESI): m/z calc. for [M2++PF6 ]+ 849.35, found 849.35; mp = 167-169
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ºC; Anal. Calcd for C50H48F12N4P2: C, 60.36; H, 4.86; N, 5.63. Found: C, 60.42; H, 4.73; N, 5.69.
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Bis-imidazolium receptor 132+·2PF6 . Yield 0.113 g (98%), 1H NMR (600 MHz, CD3CN/ CD3OD (95/5 v/v)) δ 8.67 (2H, s),
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8.11 (4H, d, J = 8 Hz), 7.94 (4H, d, J = 8 Hz), 7.78 (1H, t, J = 8 Hz), 7.59 (4H, t, J = 7 Hz), 7.53 (4H, t, J = 7 Hz), 7.21 (2H, d, J =
8 Hz), 6.15 (4H, s), 4.99 (4H, s), 1.87 (6H, s), 1.80 (6H, s), 1.72 (6H, s) ppm; 13C NMR (100 MHz, CD3CN/ CD3OD (95/5 v/v)) δ
154.0, 144.5, 139.8, 132.1, 131.5, 131.3, 130.7, 128.8, 127.4, 127.3, 126.4, 123.6, 123.4, 122.7, 49.9, 45.2, 11.2, 9.7, 8.7 ppm; MS
-
(ESI): m/z calc. for [M2++PF6 ]+ 850.3, found 850.3; mp = 158-160 ºC; Anal. Calcd for C49H47F12N5P2: C, 59.10; H, 4.76; N, 7.03.
Found: C, 59.25; H, 4.67; N, 6.93.
X-ray crystal structure analysis. Compound [132+][PF6]2·(CH3CN): weakly diffracting crystals were obtained by re-
crystallization from CH3CN/CH3OH/iPr2O solution. C51H50F12N6P2, M = 1036.91, monoclinic, a = 11.094(2), b = 13.316(3), c =
31.782(6) Å, β = 94.03(3)º, U = 4683.1(16) Å3, space group P2(1)/n, Z = 4, T = 120(2) K, 34775 reflections measured, 13251
unique (Rint = 0.0902). Methyl group hydrogen atoms refined as rigid, others riding. R1 (I > 2σ(I)) = 0.0674 and wR2 (all data) =
0.2260.17
ASSOCIATED CONTENT
1
Supporting Information Available: H- and 13C-NMR spectra, Job plot experiments, 31P NMR, fluorescence and Uv-Vis anion binding
studies, computational details and Cartesian coordinates and total energies of all species discussed in the text, Ellipsoid plot for com-
pound [132+][PF6]2·(CH3CN), crystallographic information file (CIF) containing X-ray data of compound [132+][PF6]2·(CH3CN). This
material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
* Email: pmolina@um.es; antocaba@um.es.
ACKNOWLEDGMENT
This work was funded by European Commission FP7-PEOPLE-2012-CIG Nº 321716, Ministerio de Economía y Competitividad Govern-
ment of Spain and European FEDER (Projects CTQ2011-27175, CTQ2013-46096-P, CTQ2013-44303-P and CTQ2014-51912-REDC),
Fundación Séneca Región de Murcia (CARM) Projects 18948/JLI/13 and 19337/PI/14. F.Z. and AC acknowledge to the Government of
Spain for a Juan de la Cierva and Ramon y Cajal contracts, respectively.
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15 See computational details in the Supporting Information.
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16 Bondi, A. J. Phys. Chem. A, 1964, 68, 441-451.
17 CCDC-1456591 contains the supplementary
(www.ccdc.cam.ac.uk/data_request/cif).
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