Novel hydrogen- and halogen-bonding anion receptors based on 3-iodopyridinium units

Article abstract of DOI:10.1039/c6ra14703h

Novel tripodal 3-iodopyridinium-based receptors were investigated through (i) UV-vis and NMR titrations with anions in solution, (ii) theoretical calculations, and (iii) X-ray diffraction studies. Their anion binding properties were compared to those of the monobranched model and/or non-halogenated model systems. Investigations in acetonitrile pointed out that the iodine atom in the meta position to pyridinium enhances anion affinity. According to computational studies, this effect seemed to depend on the electron-withdrawing nature of the iodine-substituents. Notably, 1:1 adducts were observed to form in solution with all the investigated anions. The strong de-shielding effect observed on the receptors' protons upon anion binding indicated their participation in hydrogen-bonds with the coordinated anion. This result was supported by theoretical calculations and, in the solid state, by X-ray diffraction studies on the complexes with nitrate and bromide. In the crystalline state, the pyridinium arms of the tripodal receptor assume a 2-up, 1-down conformation. Both nitrate and bromide anions are included in the receptor's cavity, forming two hydrogen-bonding interactions with the protons of the 2-up arms, and one halogen-bonding interaction with the C-I group of a second molecular cation. The combination of hydrogen and halogen bonds leads to supramolecular chains in the crystals.

Full text of DOI:10.1039/c6ra14703h

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Novel hydrogen- and halogen-bonding anion receptors based on
3-iodopyridinium units
Receive d 00th January 20xx,
Accepted 00th January 20xx
Valeria Amendola,a* Greta Bergamaschi,a Massimo Boiocchi,b Nadia Fusco,a Mario Vincenzo La
Rocca,c Laura Linati,b Eliana Lo Presti,a Massimo Mella,c* Pierangelo Metrangolod* and Ana
Miljkovica
DOI: 10.1039/x0xx00000x
www.rsc.org/
Abstract. Novel tripodal 3-iodopyridinium-based receptors were investigated through (i) UV-vis and NMR titrations with
anions in solution, (ii) theoretical calculations, and (iii) X-ray diffraction studies. Their anion binding properties were
compared to those of the monobranched model and/or non-halogenated model systems. Investigations in acetonitrile
pointed out that the iodine atom in the meta position to pyridinium enhances anion affinity. According to computational
studies, this effect seemed to depend on the electron-withdrawing nature of the iodine-substituents. Notably, 1:1 adducts
were observed to form in solution with all the investigated anions. The strong de-shielding effect observed on the
receptors’ protons upon anion binding indicated their participation to hydrogen-bonds with the coordinated anion. This
result was supported by theoretical calculations and, in the solid state, by X-ray diffraction studies on the complexes with
nitrate and bromide. In the crystalline state, the pyridinium arms of the tripodal receptor assume a “2-up, 1-down”
conformation. Both nitrate and bromide anions are included into the receptor’s cavity, forming two hydrogen-bonding
interactions with the protons of the “2-up” arms, and one halogen-bonding interaction with the C−I group of a second
molecular cation. The combination of hydrogen and halogen bonds leads to supramolecular chains in the crystals.
recognized.4
Thanks to the pioneering work by Metrangolo and Resnati,
another type of non-covalent interaction, i.e., halogen
bonding, has become popular among supramolecular
Introduction
Anion recognition has been an important issue in
supramolecular chemistry for four decades, still attracting
attention as shown, for instance, by the number of reviews
published on the topic in 2015.1 In particular, most purely
organic receptors have been setting up their interactions with
anions on electrostatic forces and/or hydrogen bonding (HB).2
The latter, in particular, has attracted the specialists in the
field.3 This is not surprising for several reasons: first of all,
chemists feel inspired by Nature, and Nature mostly bases self-
assembling and recognition processes on HB. Moreover, HB
can be effective in polar solvents, allowing to achieve highly
selective recognition of anionic species even in water. In the
field of anion recognition, the importance of the so-called
“non-conventional” HB interactions based on “weak” H-bond
donors, e.g. nucleophilic aromatic C atoms, has also been
chemists, and in the last few years it has been having a
significant impact on the supramolecular world.5
Halogen bonding (XB) was proved to have significant
similarities with HB, allowing to build sophisticated
supramolecular architectures and functional materials, and
leading to selective anion recognition in competing media, as
shown by Beer6 and others.7 HB and XB can be considered as
the most relevant among non-covalent interactions.8-9 They
both are characterised by high directionality and strong
attraction, leading to contact distances shorter than the sum
of the Van der Waals radii of the involved atoms. For both HB
and XB, interaction involves an electrophilic species [i.e., H and
X atoms for HB and XB, respectively] and a nucleophilic atom
therefore binding has a dominant electrostatic contribution.
However, recent theoretical and experimental studies have
shown that polarization, charge transfer, and dispersion forces
also play an important role.10
Investigating a series of halopyridinium and haloanilinium
salts, Rissanen11 pointed out how the balance between HB and
XB is fundamental in determining the structure of these
compounds in the solid state. This balance is also central in
solution, and can be exploited in the design of anion receptors.
The similar features of HB and XB have stimulated chemists to
compare and contrast (supra)molecular systems based on
either one or the other type of interactions, with a special
regard to the field of anion recognition in solution.12
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In the last fifteen years, the research interests of our Details are reported in SI. The interaction with anions of all
group in Pavia have been oriented towards the synthesis of receptors was investigated by NMR and UV-vis experiments in
selective anion receptors, containing positively charged and/or organic solvents (acetonitrile, DMSO). Due to the low solubility
HB-donor fragments.13 Experience has taught us that stronger of
2(PF6)3 and 3(PF6)3 in acetonitrile, NMR titrations on these
anion binding is obtained when several HB-donor groups two receptors were performed in DMSO/acetonitrile mixture.
converge towards the anionic guest, better if within a well-
defined cavity.14 Moreover, positive charges close to HB-donor Single-branched pyridinium-based systems
groups in a receptor have a synergistic effect, thus increasing In the case of the model system 1a+, the formation constants
the interaction. As an example, the concave receptor of the 1:1 complexes with chloride, bromide, and iodide could
containing three 9H-
halide anions, thanks to the concerted efforts of the positive Table 1). Spectra and profiles are shown in the SI (Figs. S1-S3).
β
-carbolin-2-ium units strongly binds be calculated from the fitting of the 1H-NMR titration data (see
charges and the H-donor groups of 9H-β
-carbolin-2-ium.15-16 1H-NMR titrations with halides evidenced the preference of
Before us, Steed had observed similar effects in a series of 3- 1a+ for the chloride anion, followed by bromide and iodide
aminopyridinium based receptors.17 Steed also pointed out (see Figure 1).‡ This is not surprising, as it is the common trend
that, in the absence of the amino N−H donor groups, the anion observed in pyridinium systems.
binding capabilities of these systems strongly decrease.
Our goal is now to move the filed forward by studying the
synergic effect of positioning a number of positive charges, XB
donors and non-classical HB donor groups (i.e. C−H) around
the bowl-shaped cavity of a tripodal host. To do this, we
synthesized new receptors, containing three 3-iodopyridinium
arms appended to trialkylbenzene platforms, and we
investigated their anion binding capabilities through NMR and
UV-vis spectroscopies. We also compared our results to those
obtained for the N-benzyl-3-iodopyridinium, as
a model
compound, and to those already available for analogous
tripodal receptors based only on electrostatic and/or HB
interactions. A computational study was carried out as a way
to gauge the relative contribution provided by HB and XB to
the stability of the complexes. For this purpose, isomeric “ion
1
pair dissociation energies” (IPDE) and H-NMR chemical shifts
were computed and helped us in rationalising the
experimental titration results.
Figure 1. 1H-NMR titrations of 1a+ with halides in CD3CN. Variation of the
chemical shift of H upon anion (as TBA salt) addition (symbols: red, chloride;
α
blue, bromide; black, iodide). The experimental profiles are superimposed to the
formation curves of the 1:1 adducts with the anions, calculated according to the
constants reported in Table 1.
Table 1. Affinity constants determined by 1H-NMR titrations with halides as TBA salts
(in CD3CN, T = 25°C). a Constants obtained through UV-vis titrations in CH3CN (25°C). In
parenthesis, the uncertainties on the last figures are reported.b See ref. 14.
Anion
Cl−
LogK11/1a+
LogK11/1b+
2.06(1)
n.d.
LogK11/1c+b
[3.20(1)]a
[2.48(1)]a
n.d.
2.30(3) [2.27(1)]a
1.98(4) [2.08(1)]a
1.70(6)
Br−
I−
n.d.
Upon anion addition, protons in the ortho positions to the
nitrogen, i.e. H and H , are the most affected. Therefore, the
corresponding signals undergo a significant downfield shift,
e.g., ꢀδ +0.53 ppm and +0.43 ppm for and
respectively (see Fig.s S1-S2). Notably, also protons H
in 1b+
Scheme 1. Pyridinium-based anion receptors studied in this work.
α
δ
=
H
α
Hδ,
α
Results and discussion
are de-shielded even if to a lower extent (+0.40 ppm up to 20
eqv. of chloride, see Figure S3). In principle, the
iodopyridinium-based model compound 1a is capable of
The iodopyridinium-based receptors (see Scheme 1) were
synthesized modifying a procedure, already applied by our
group in the preparation of polypyridinium systems.15-16
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binding halide anions via different modes. XB is one, but also this may be due to entropic effects due to the lower
HB may be present, either in a mono- or bi-dentate fashion population of low lying isomers as in the iodide case.Our
(Scheme 1). Anion···π
interactions may also play a role.18 The theoretical results suggest, in fact, that the halogen-bonded
observed downfield shifts are more indicative of HB rather species (d) shown in Fig. 2 lies at least 0.9 kcal/mol above the
than XB, for which upfield shifts would be expected.19 other stable conformers found for 1a+/Cl− and 1a+/Br− (i.e., Fig.
However, disentangling the different contributions in solution 2, (a) and (c), respectively), and has a lower IPDE, which
is difficult as different binding modes may occur indicate that it is not the most relevant species occurring in
simultaneously. The binding constants of 1a+ and 1b+, shown in solution. Interestingly, the structures shown in Fig.s 2 and S7
Table 1, point out that anion affinity is higher for 1a+ than for also fully support the NMR assignments presented in Fig.s S1-
the simple N-benzyl pyridinium analogue. We can thus S3, justifying the incremental shifts of the hydrogen atoms
conclude that the iodine-substituent has a positive effect on involved in the interaction with the anions. Chemical shifts
the anion binding capabilities of pyridinium receptors, as a computed at the B3LYP/6-31+G(d,p)/GIAO level, in fact,
likely consequence of its electron withdrawing effect on the suggest that H
pyridine hydrogen atoms. substantially shifted downfield (ꢀδ=1.0-3.3 ppm) for the
The interaction of 1a+ with chloride and bromide was also species shown in Fig. 2, (a) and (b). On the other hand, H
and
studied by UV-vis titrations in acetonitrile. The molecular should remain mostly unchanged. Only minor shifts are
cation displays band at 290 nm (2.1
103 M-1cm-1), instead predicted for the X-bonding species [i.e., Fig. 2(d)].
attributable to a charge transfer that involves the iodine As for the size of the computed chemical shifts, these
α, Hδ and the methylene protons should all be
β
Hγ
a
×
substituent. Upon anion addition, this band broadens. From appear larger than the experimental data. Such apparent
the fitting of the profiles, the affinity constants for both discrepancy can be readily rationalized, considering that the
chloride and bromide could be determined [2.27(1) and measured shifts represent the average of all possible
2.08(1) Log units, respectively], confirming the NMR titration structures accessible within the time scale of the NMR
results. Both UV-vis spectra and the distribution diagrams are measurement. In this respect, the small energy differences
shown in the SI (see Fig.s S4-S6). Interestingly, the obtained reported in Table 2 suggest that the ion pairs are highly
affinity constants are lower than those determined in the fluxional, so that the structures in Fig.s 2 and S7 represent only
same conditions by our group15 for the 9H-
system (see 1c+ in Table 1). This suggests that the NH group in shown in Fig. S9. The fluxionality also explains the presence in
9H- -carbolin-2-ium may have a stronger impact on the the NMR spectrum of a singlet for CH2, instead of the double
affinity towards anions than the iodine-substitution in the doublet expected on symmetry considerations (i.e., the
studied pyridinium-based receptors. symmetry-breaking induced by the interaction with the anion).
β-carbolin-2-ium limiting cases. This is corroborated by the energy profiles
β
To understand the origin of the higher anion affinity for 1a+ Albeit lower in magnitude, Counterpoise corrected IPDE (see
compared to 1b+, we computationally studied the two model ESI) support our conclusions.
systems in presence of Cl− and Br−. Several low-lying solution
Table 2. Ion pair dissociation energies, IPDE=E(S+/X-)-E(S+)-(X-), where S+ is the
conformers were optimized. The corresponding IPDE values
pyridinium cation and X- is the anion. Solvent effects are introduced via the PCM
model. The letters refer to the isomers shown in Fig. 2 for 1a+, and in Fig. S7 for 1b+.
are within 1.8 kcal/mol (see Table 2), although the conformers
show different “modes of interaction” with the anion (see Fig.s
2 and S7, for 1a+ and 1b+ with Cl–, respectively).
Counterpoise corrected energies are given in the ESI.
Comp. IPDE (kcal/mol) X=Cl- IPDE (kcal/mol) X=Br- IPDE (kcal/mol) X=I-
1a+
7.06(a); 6.67(b); 6.64(c)
5.86(d)
;
6.48(a); 6.13(b); 6.84(c)
5.58(d)
;
6.04(a); 5.68(b)
7.14(c); 5.34(d)
n.d.
;
1b+
1c+
6.50(a) ; 5.62 (b)
6.00(a) ; 5.78(b)
9.83
8.75
n.d.
By slow diffusion of diethyl ether into acetonitrile solutions of
1a(PF6), in both the absence and presence of TBAI, single
crystals suitable for X-Ray diffraction analysis were obtained.
In the solid state, for both 1a(PF6) and 1a(I), the coexistence of
HB and XB interactions could be observed. In the case of
1a(PF6), the interactions between the molecular cation 1a+ and
–
the PF6 anion are weak (see Tables 3-4, and the SI for details).
On the other hand, significant XB interactions could be
detected in 1a(I), where two similar but not symmetrically
equivalent 1a+ molecular cations interact with the same I– ion
via two C-I···I– halogen bonds. In particular, the I···I− distances
[i.e., 3.541(1) and 3.575(2) Å, see Table 4] were much shorter
than the value of 4.18 Å, obtained by summing the van der
Figure 2. Geometries of four possible conformers for the binding of Cl– by 1a.
Regardless, the computational results strongly support the
experimental data as far as the relative stability of the
complexes is concerned (see also Fig. S8 for 1c+/Cl–), even if
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Waals radius of iodine (1.98 Å)20 and the ionic radius of iodide
Table 4. Geometrical features for the C-I···Y halogen bonds in the crystal structures of
this work. The normalized R value is defined as: dI···Y/(rI+rY), with rI = 1.98 Å and rY = 1.47
Å for F[PF6]-, 1.52 Å for O[NO3]-, 2.20 Å for II-, 1.96 Å for BrBr-.
(2.20 Å)21. The normalized
parameters [defined as the ratio
R
between the observed I···I– separation and the sum of the
proper radii of the involved species] are 0.85 for both
interactions, thus confirming the strength of the two halogen
XB donor C-I (Å)
group
I···Y (Å)
R
C-I···Y XB acceptor
atom
bonds. As further proof, the two C
linear [178.4(3) and 171.1(3)°, respectively], the closest to 180° 1a(I)
−
I···I− angles are almost
1a(PF6)
C(1)-I(1) 2.089(7) 3.181(6) 0.92 166.4(3)
C(1)-I(1) 2.093(10) 3.575(2) 0.85 171.1(3)
C(13)-I(2) 2.100(10) 3.541(1) 0.85 178.4(3)
F(1)PF6-
I(3)I-
I(3)I-
1a(I)
corresponding to a well-established XB interaction (Table 4).
Notably, similar features were found by Rissanen,10 in the
crystal structure of an ethanol-clathrate hydrate crystal,
containing two independent 1a+ molecular cations interacting
with a chloride anion. In that case, two short halogen bonds
2(NO3)2(PF6) C(9)-I(1) 2.091(10) 2.954(10) 0.84 178.4(4) O(3) NO3
-
2(NO3)2(PF6)C(16)-I(2) 2.071(11) 3.057(20) 0.87 159.0(4) O(5a) NO3
-
2(NO3)2(PF6)C(23)-I(3) 2.096(11) 3.332(10) 0.97 150.6(4)
2(Br)(PF6)2 C(16)-I(2) 2.110(8) 3.368(1) 0.85 171.9(3)
F(3)PF6
-
Br(1)Br-
were observed, involving the C
molecular cations and two independent Cl– anions. The
calculated values, 0.83 and 0.85, as well as the observed C-
−
I XB-donor groups of two 1a+
R
In the 1a(I) crystal, weak HB interactions could also be
observed. The shortest C −H
H···I– distances involve two C
I···Cl− angles, 174.1(1) and 174.6(1)°, are similar to those
−
bonds of the iodopyridinum moiety and two I– anions [i.e.
3.72(1) and 3.77(1) Å, see Table 3]. These separations are
shorter than 3.90 Å, obtained by summing the Van der Waals
radius of C (1.70 Å)20 and the ionic radius of I− (2.20 Å).21
measured in our system.
Table 3. Geometrical features for the C-H···A hydrogen-bond interactions in the crystal
structures of this work. The reported contacts have D···A separations shorter than the
sum of the Van der Waals radii of the involved atom (ionic radii are used for I- and Br-).
Symmetry code: (') = -1+x, 1/2-y, -1/2+z; ('') =1+x, y, z; (''') -1+x, y, z.
Tripodal 3-iodopyridinium-based receptors
Comp.
D donor
group
H···A (Å)
D···A (Å) D-H···A A acceptor
atom
Bowl-shaped positively charged systems, such as those obtained by
appending three pyridinium groups to a tris(alkyl)benzene scaffold,
are known to form stable complexes with anions in acetonitrile
solution. Studies performed by Steed,17 and independently by our
group,15-16 demonstrated that anion affinity is strongly influenced
by i) the receptor pre-organisation imparted by the alkyl chains on
the platform, and depends on ii) the presence of HB donor groups
on the pyridinium arms.
1a(PF6)
1a(I)
C(3)-H(3)
2.526(5)
3.133(9) 123.2(5)
F(4)'PF6-
C(15)-H(15) 3.326(13) 3.773(13) 112.0(9)
C(17)-H(17) 3.221(12) 3.719(12) 115.7(9)
I(4)''I-
I(3)''I-
1a(I)
2(NO3)2(PF6) C(12)-H(12) 2.306(14) 3.104(14) 143.5(9) O(4)a'''NO3
2(NO3)2(PF6) C(19)-H(19) 2.637(22) 2.971(22) 101.8(8) O(6)a'''NO3
-
-
2(Br)(PF6)2 C(12)-H(12) 2.781(10) 3.565(10) 142.5(6)
2(Br)(PF6)2 C(19)-H(19) 2.707(9) 3.522(9) 146.7(6)
Br(1)Br-
Br(1)Br-
In order to shed light on how appending XB-donor groups on
pyridinium-based tripodal receptors affects their anion binding
capabilities, we synthesised 2(PF6)3 and 3(PF6)3. These molecular
systems were obtained by reacting an excess of 3-iodopyridine with
1,3,5-tribromomethyl mesitylene and 1,3,5–tribromomethyl–2,4,6–
triethylbenzene, respectively (see SI for details). The two chosen
platforms are expected to exert
a different degree of pre-
organisation on the tripodal receptors, thus affecting anion affinity.
Anion binding studies were performed by UV-vis and NMR
titrations both in pure acetonitrile and in the presence of 10%
DMSO. Due to the low solubility of the receptors in 100%
acetonitrile, UV-vis titrations were performed at 2.0× 10-5
M
concentration. Both 23+ and 33+ display an absorption band at about
295 nm. Upon anion addition (as the TBA salt), an hyper-chromic
effect was observed (UV-vis spectra, profiles and the Job plot for
chloride are available in the SI).
The fitting of the titration profiles suggests the presence of a
single equilibrium, leading to the formation of a 1:1 complex with
all anions. The binding constants are shown in Table 5. The affinity
trend is similar in the two receptors (i.e., Cl– >> Br– > CH3COO–
>
HSO4 , NO3 > I–). However, stronger binding was observed for 33+
with spherical anions, Cl– and Br– in particular. This might be due to
the higher pre-organisation imparted by triethyl arms to the bowl-
–
–
Figure 3. A simplified sketch of the crystal structure of 1a(I). Atom names are shown
only for atoms involved in C-I···Y halogen bonds (drawn with dotted lines) and weak C-
shaped receptor, compared to the methyl groups of 23+
.
H···A hydrogen bonds (drawn with dashed lines). The normalized
R
values
=
dI···Y/(rI+rY) and the C-I···Y bond angles are reported. Simmetry code: (') = -1+x, 1/2-y, -
1/2+z; ('') =1+x, y, z.
In the case of chloride, UV-vis titrations were also performed in
CH3CN/DMSO (10% DMSO) mixture. Notably, better solubility
allowed us to work at higher concentrations (2.0 × 10-4 M) than in
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pure acetonitrile. A general lower affinity was observed in the carbolin-2-ium species has to be attributed to the acidity of the
mixture as a consequence of the competing effect of DMSO on carbazole-like NH group, and to its good HB donor tendencies.
anion binding. However, these experiments confirmed the
formation of 1:1 complexes, with stronger binding capabilities for solution was obtained through the 1H-NMR titration with TBACl (see
Further information on the interaction of 33+ with anions in
3
3+ (see Table 5) compared to 23+.
Figs. 4 and 5; the family of spectra is shown in the SI). Protons Hδ, in
the para position to iodine, seem to be directly involved in the
binding, undergoing a down-field shift of ꢀδ = +0.70 ppm upon
chloride addition (vs. +0.43 ppm for 1a+). Protons Hα are also
affected, even if to a significantly lower extent (ꢀδ = +0.14 ppm for
Table 5: Affinity constants determined through UV-vis. titrations of 23+ and 33+ with
anions (as the TBA salts) in 100% CH3CN, and a in the presence of 10% DMSO (25°C).
bConstants obtained through 1H-NMR titrations in CD3CN/d6-DMSO mixture (10% d6-
DMSO). The uncertainties on the last figures are reported in parentheses.
3
3+ vs. + 0.54 ppm for 1a+). The slight shielding of protons Hβ can be
attributed to the increase of the electron density on the receptor
framework upon anion binding. These results indicate that the
interaction with the chloride anion mainly involves the ortho
protons of pyridinium groups. Notably, in most examples in the
literature and in the mono-branched compound 1a+, the ortho-
protons are the most affected by anion binding, due to the direct
participation of C-H bonds in the interaction (as HB-donor
groups).15-16 By treating the experimental data obtained through
UV-vis and 1H-NMR titrations, we could determine the affinity
constants reported in Table 5. The distribution diagram of the
species, with the superimposed titration profiles, is shown in Figure
5. A good correspondence of the results obtained by the two
techniques can be observed.
Anion
Cl−
LogK11/23+
4.65(1) [3.70(2)]a
4.46(1)
LogK11/33+
5.16(1) [4.07(2)a; 4.04(3)b]
4.91(2)
Br−
I−
3.59(1)
3.71(1)
CH3COO–
4.45(1)
4.40(2)
–
HSO4
4.13(1)
4.08(2)
The interpretation of the experimental data is fully supported
by the theoretical analysis of the 23+/Cl– or 23+/Br– complexes (Table
6). As in the case of 1a+, various conformers can be formed. Their
structures differ in the relative position of the pyridinium groups
with respect to the plane of the phenyl ring (“3-up” or “2-up, 1-
down”). Differences are also observed in the position of the Hα and
Hδ atoms pointing towards the anion. The four lowest lying species
with Cl− are shown in Fig. 6; these low energy conformers are all
within 3.7 kcal/mol, a slightly wider interval than the one seen in
the case of 1a+. Notably, the substitution of Hδ with Hα (belonging
to the same pyridinium ring) in the interaction with Cl− raises the
energy by 0.8 kcal/mol for the “3-up” isomer, its two conformers (a
and b in Fig. 6) being the most stable species in solution.
Figure 4: 1H-NMR spectra of 33+ in CD3CN/d6-DMSO mixture (10% d6-DMSO), taken
before (a) and after addition of excess TBACl (b). The family of spectra is shown in the
SI.
Very interestingly, the affinity constants obtained for 33+ in
acetonitrile are significantly higher than those determined by Steed,
for
a
tris(3-aminopyridinium) receptor based on the
tris(ethyl)benzene scaffold [Br– , logK11= 4.14; CH3COO–, logK11
=
4.02].17 This result indicates that the iodine atom in the meta
position of pyridinium has a stronger effect on anion binding than
an amino (HB) group.
However, the very good anion binding properties of 33+ are in
turn exceeded by the 9H-β-carbolin-2-ium based tripodal receptor,
studied by our group some years ago.16 This trifurcate system,
Figure 5: 1H-NMR and UV-vis profiles for the titration of 33+ with TBACl in
acetonitrile/DMSO mixture. The distribution diagram was obtained, by
considering a 1:1 binding constant of 4.04 Log units (see Table 5). For details see
the text; further material is available in SI (see Fig. S15 for the family of 1H NMR
spectra).
consisting of three 9H-β-carbolin-2-ium arms appended to
a
mesitylene platform, showed association constants one/two order
of magnitude higher [e.g. Br– , logK11= 6.65] than those reported for
both 23+ and 33+ in Table 5. As already mentioned for the mono-
branched system 1c+, the high anion affinity observed for 9H-β-
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Journal Name
bonds in the para position to the iodine atoms, belonging to
the “2-up” arms, point towards the centre of the cavity and
interact with the bromide ion through HB interactions.
Notably, the involved H atoms correspond to H
NMR titration.
δ protons in the
The bromide anion is located almost above the centre of
the trimethylbenzene ring, and profits of a XB interaction with
the C-I group of a second 23+ cation (see Fig. 7). In the solid,
the combination of these HB and XB interactions gives origin to
supramolecular chains in both
2(NO3)2(PF6) and 2(Br)(PF6)2
crystals. Geometrical features are reported in Tables 3 and 4;
details on the crystal structure of
the SI.
2(NO3)2(PF6) are available in
Figure 6. Lowest conformers for 23+/Cl–.Note that top and bottom conformers differ
due to the rotation of an iodo-pyridinium group in the up position around the CH2-N
bond. Such rotation substitutes Hα to Hδ in the contact with the anion.
Table 6. Ionic pair dissociation energies. IDPE=E(S+/X-)-E(S+)-(X-), where S+ is the
ligand molecule and X– is the anion. All the energies take into account the effect of
appropriate solvent via SCRF models. Letters refer to the conformer labels in Figure 6.
Counterpoise corrected energies for the complexes are given in the ESI.
Comp.
23+
IPDE (kcal/mol) X=Cl–
8.34(a); 7.55(b); 6.05(c); 5.67(d)
IPDE (kcal/mol) X=Br–
9.05(a); 8.97(d)
As the population of the remaining two species (c and d, Fig. 6)
is expected to be low, the energy data rationalize the smaller
change in the chemical shift of Hα observed upon NMR titration
with chloride (see Fig. 4). The Counterpoise correction supports this
conclusion (see SI).
Figure 7. A simplified sketch of the crystal structure of 2(Br)(PF6)2 (PF6− counter ions are
not shown for simplicity). Names are shown only for atoms involved in C-I···Y halogen
bonds (drawn with dotted lines) and C-H···A hydrogen bonds (drawn with dashed lines).
The slow diffusion of diethyl ether into acetonitrile
solutions of
the TBA salts) allowed us to isolate crystals of
(Br)(PF6)2 salts suitable for X-ray diffraction studies.
The conformation of the 23+ molecular cation, as well as
2(PF6)3 in the presence of nitrate and bromide (as
2
(NO3)2(PF6) and The normalized R values
= dI···Y/(rI+rY) and the C-I···Y bond angles are reported.
Symmetry code: ('') x, -1+y, z; (''') -1+x, y, z.
2
the arrangement of HB and XB interactions, are very similar in
the nitrate and bromide salts (compare Fig.s 7 and S16). In
both crystals, one of the three iodopyridinium arms is turned
out of the receptor’s cavity, and it is placed on the other side
of the trimethylbenzene’s plane (i.e., “anti” conformation).
Conclusions
In this work, 3-iodo-pyridinium units in tripodal systems have
been used for the first time to selectively bind anions in
competing media (e.g., acetonitrile with 10% DMSO). Thanks
to 3-iodopyridinium, tripodal cavities were obtained in which
different types of interactions cooperate in the binding of the
included anionic guest: (i) electrostatic forces; (ii) halogen-
bonding and (iii) non-conventional hydrogen bonding
interactions (i.e. with the receptor’s C-H donor groups). Our
studies in solution through UV-vis. and NMR titrations pointed
out that iodine atoms effectively enhance the anion binding
tendencies of our pyridinium-based systems. Computational
investigations on the model compound 1a+ suggested that this
finding may depend more on the electron withdrawing effect
of iodine [on the coordinating pyridyl hydrogens] rather than
on the occurrence of relevant halogen bonding in solution.
Such
a “2-up, 1-down” conformation has already been
reported by Steed for other tripodal pyridinium-based systems
in the solid state.17 Notably, it also corresponds to one of the
low energy conformers obtained for this receptor in the
theoretical calculations.
The presence of ethyl substituents on the platform, such as
in 3
3+, is expected to enhance the receptor’s preorganization,
thus forcing the pyridinium arms to point in the same
direction. Unfortunately, all attempts to obtain single crystals
suitable for diffraction studies have been unsuccessful.
In the crystal structure of 2(Br)(PF6)2, all the iodine atoms
are oriented out of the receptor’s cavity, probably to comply
with steric requirements. On the other hand, the two C-H
6 | J. Name., 2012, 00, 1-3
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ARTICLE
0.17 x 0.10 x 0.08 mm3) crystals were collected on a Bruker-
AXS diffractometer equipped with the SMART-APEX CCD
detector. Both instruments work at room temperature with
MoKα X-radiation (λ = 0.71073 Å).
However, this latter interaction dominates the binding of
anions in the solid state.
In the case of the tripodal receptors, stable 1:1 inclusion
complexes were obtained in solution for all the investigated
anions. X-ray diffraction studies on 2(NO3)2(PF6) and 2(Br)(PF6)2
Table 7. Crystal data for the studied compounds.
disclosed that the pyridinium arms assume a “2-up, 1-down”
conformation with respect to the mesitylene platform. In both
adducts the anion lies within the receptor’s cavity, forming (i)
two H-bonding interactions with the C-Hδ protons of the “2-
up” arms, and (ii) one halogen-bonding interaction with the C-I
group of a second 23+ cation. This combination of halogen- and
hydrogen-bonds (i.e. with iodine atoms and C-H groups,
respectively) leads to the formation of supramolecular chains
1a(PF6)
C12H11F6INP
441.09
1a(I)
2(NO3)2(PF6)
2(Br)(PF6)2
Formula
M
C24H22I4N2 C27H27F6I3N5O6PC27H27BrF12I3N3P2
846.04
1043.21
triclinic
1144.06
monoclinic
C2/c (no. 15)
24.163(2)
11.441(1)
27.559(2)
90
crystal system
space group
a [Å]
monoclinic orthorhombic
P21/c (no. 14) Pc21b (no. 29) P-1 (no. 2)
10.256(1)
9.723(2)
15.285(1)
90
7.526(3)
11.789(4)
30.882(6)
90
11.352(2)
13.844(3)
14.366(3)
118.339(4)
101.442(4)
99.631(4)
1856.0(7)
2
b [Å]
c [Å]
in the crystals of both 2(NO3)2(PF6) and 2(Br)(PF6)2.
α
β
γ
[°]
[°]
[°]
96.674(9)
90
90
97.977(1)
90
Notably, in the endo-coordination of the included anion,
HB interactions are preferred over XBs. This may depend on
the fact that all of the iodine atoms are oriented out of the
cavity, due to steric congestion.
90
V [ų]
1513.8(3)
4
2740.1(15)
4
7545.1(10)
8
Z
ρ
calcd [g cm-3]
1.935
2.276
ω
2.051
4.562
ω
1.867
2.014
In conclusion, the combination of multiple interactions
within a single receptor brought about strong anion binding in
solution, even in a competing medium. Moreover, this also led
to an unexpected outcome: the formation of supramolecular
chains in the crystals. The results exposed herein represent a
significant advance in the field of anion recognition based on
HB and XB, provide valuable tips for those working in the field
and encourage researchers to continue along this path, i.e.
using multiple and diverse interactions within receptor cavities
in order to obtain a higher selectivity.
µ MoKα [mm-1]
2.643
3.713
Scan type
ω
ω
θ
range [°]
2-27
2-27
2-25
2-25
measured refl.
unique refl.
Rint
Strong dataa
Refined param.
R1, wR2a
3500
7009
3899
0.0338
3387
271
19857
6663
31513
6669
3289
0.0189
2298
0.0386
4891
0.0447
4435
190
469
436
0.0604, 0.1314 0.0323, 0.0630 0.0730, 0.2343 0.0651, 0.1877
0.0874, 0.1538 0.0403, 0.0677 0.0926, 0.2561 0.0920, 0.2127
R1, wR2b
GOF
residuals [eÅ-3]
1.088
1.081
1.071
1.064
0.53, -0.72
0.68, -0.51
1.60, -1.14
1.95, -1.35
Experimental
a strong data = IO>2σ(IO), b all data
Crystal data were reported on Table 7. Data reductions
(including intensity integration, background, Lorentz and
polarization corrections) for intensities collected with the
conventional diffractometer were performed with the WinGX
package;25 absorption effects were evaluated with the psi-scan
method26 and absorption correction was applied to the data.
Frames collected by the CCD-based system were processed
with the SAINT software27 and intensities were corrected for
Lorentz and polarization effects; absorption effects were
Materials and methods
All reagents for syntheses were purchased from Sigma-Aldrich and
used without further purification. All reactions were performed
under dinitrogen. Mass spectra were acquired on a Thermo
Finnigan ion trap LCQ Advantage Max instrument equipped with an
1
ESI source. H- and 13C-NMR spectra were recorded on a Bruker
ADVANCE 400 spectrometer (operating at 9.37 T, 400 MHz). UV-vis.
spectra were run on a Varian Cary 50 SCAN spectrophotometer,
with quartz cuvettes of the appropriate path length (1 or 0.1 cm) at
25.0 ± 0.1°C under inert conditions. Solvents were dried by common
methods. Solvents were dried by common methods. Syntheses are
reported in the Supplementary; 1b(PF6) was prepared according to
a known procedure.22 The experimental procedures of NMR and
UV-vis. titrations have been described elsewhere.23 Titration data
were processed with the Hyperquad package24 to determine the
equilibrium constants.
empirically evaluated by the SADABS software28
and
absorption correction was applied to the data. All crystal
structures were solved by direct methods (SIR 97)29 and
refined by full-matrix least-squares procedures on F2 using all
reflections
(SHELXL-2014).30 Anisotropic
displacement
parameters were used for all non-hydrogen atoms. Hydrogens
have been placed at calculated positions and their positions
refined according to a riding model.
Positional disorder affected a nitrate counter ion in the
–
(NO3)2(PF6) crystal and the NO3 anion resulted placed over
Crystal structure analysis
2
Diffraction data for 1a(PF6) (colourless, 0.43 x 0.28 x 0.07 mm3)
and 1a(I) (pale yellow, 0.45 x 0.14 x 0.08 mm3) crystals were
collected by means of an Enraf-Nonius CAD4 four circle
two alternative positions half populated. The X-ray diffraction
quality of the (NO3)2(PF6) crystal did not allow a full
2
unconstrained structure refinement and the geometries of the
disordered nitrate counter ions, as well as those of the
diffractometer, whereas diffraction data for
(colourless, 0.23 x 0.07 x 0.05 mm3) and
2(NO3)2(PF6)
2
(Br)(PF6)2 (colourless,
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5
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hexafluorophosphate ion, were restrained to the expected
ones using soft DFIX and DANG restraints.
CCDC 1476623-1476624-1476625-1476626 contain the
supplementary crystallographic data for this paper.
Computational details
All calculations were carried out with the Gaussian 09
package.31 Conformation analysis and geometry optimizations
,
were carried out at the MP2 level for complex 1a+/X– 1b+/X–
and 1c+/X–. For the complex 23+/X–, structural optimizations
were carried out using the B3LYP functional due to the larger
species size.32 MP2 single point energies were subsequently
6
7
obtained
employing
the
B3LYP
geometries.
A
polarized/augmented double zeta basis set (6-31+G(d,p) for
light atoms and he LANL2DZ basis set augmented with the
diffuse function from the aug-cc-pVDZ set for the halogen
atoms) was used in all the calculations; effective-core
potentials (LANL) were also used for Cl, Br, and I to reduce
computational costs. Solvent effects were evaluated using the
PCM model and different solvents were selected in order to
reproduce the experimental conditions.33 Basis set
superposition errors were estimated via the Counterpoise
approach at the MP2 level in all cases. The calculation of
chemical shifts for the hydrogen atoms was carried out
employing the GIAO procedure as implemented in Gaussian
09.34
Metrangolo and M. Fujita, Angew. Chem. Int. Ed., 2015, 54
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Text: Iodopyridinium-based tripodal receptors strongly bind anionic species in their
DOI: 10.1039/C6RA14703H
bowl-shaped cavity through the synergistic effect of hydrogen- and halogen-bonding
interactions
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