Anion and cation binding by a new indole/pyridine/amine-based ion-pair receptor

Article abstract of DOI:10.1016/j.tetlet.2010.10.012

A new ion-pair receptor bis(3-bromoindol-2-ylmethyl)(2-pyridylmethyl)amine (1) was synthesized and studied for its anion and cation binding behavior using ESI-MS and ¹H NMR spectroscopy. Among halides, 1 exhibits the strongest binding with Cl^- to form a 1:1 adduct (K_a= 1042 ± 21 in CD₃CN). Among alkali metal ions, Li⁺ and Na⁺ showed the strongest binding in the formation of a 1·M⁺ complex. The simultaneous binding of Cl^- and Li⁺ to 1 was confirmed by ¹H NMR titration of a 1:1 mixture of 1 and Cl^- with LiPF₆ in 83:17 v/v mixture of CDCl₃ and DMSO-d₆. DFT-optimized structures of 1·Cl^-, 1·Li⁺, and 1·Li ⁺·Cl^- are consistent with the chemical shift changes observed in ¹H NMR studies.

Full text of DOI:10.1016/j.tetlet.2010.10.012

Tetrahedron Letters 51 (2010) 6516–6520
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Anion and cation binding by a new indole/pyridine/amine-based
ion-pair receptor
Kristin N. Skala, Katelyn G. Perkins, Amna Ali, Robert Kutlik, Addie M. Summitt,
⇑
⇑
Satyanarayana Swamy-Mruthinti, Farooq A. Khan , Megumi Fujita
Department of Chemistry and Department of Biology, University of West Georgia, 1601 Maple St., Carrollton, GA 30118, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 26 July 2010
Revised 30 September 2010
Accepted 3 October 2010
Available online 28 October 2010
A new ion-pair receptor bis(3-bromoindol-2-ylmethyl)(2-pyridylmethyl)amine (1) was synthesized and
studied for its anion and cation binding behavior using ESI-MS and H NMR spectroscopy. Among halides,
1
À
1
exhibits the strongest binding with Cl to form a 1:1 adduct (K
a
= 1042 ± 21 in CD
3
CN). Among alkali
+
+
+
metal ions, Li and Na showed the strongest binding in the formation of a 1ÁM complex. The simulta-
neous binding of Cl and Li to 1 was confirmed by 1H NMR titration of a 1:1 mixture of 1 and Cl with
À
+
À
À
+
+
À
LiPF
6
in 83:17 v/v mixture of CDCl
3
and DMSO-d
6
. DFT-optimized structures of 1ÁCl , 1ÁLi , and 1ÁLi ÁCl
Keywords:
Indole
Ion-pair receptor
Cation binding
Anion binding
ESI-MS
1
are consistent with the chemical shift changes observed in H NMR studies.
Ó 2010 Elsevier Ltd. All rights reserved.
1
. Introduction
contains two indole N–H groups that can potentially enable inter-
action with an anion through hydrogen bonding, and two amine/
pyridine N atoms to allow a cation to bind. While it is a common
approach to synthesize an ion-pair receptor by functionalizing
well-known cation-binding macrocycles such as crown ethers,
Ion-pair receptors are molecules that are capable of binding
1
both an anion and a cation simultaneously. Such systems have
drawn much attention in recent years, and the number of publica-
tions on this topic has grown rapidly in the past decade. Ion-pair
receptors have potential applications in solubilization and extrac-
tion of salts from organic solvents or effluents, in the transfer of
salts through lipophilic membranes, and as sensors for biological
and environmental systems.1
The use of indole groups in anion or ion-pair receptors, along
with related carbazoles, biindoles, and indolo[2,3-a]carbazoles, is
fairly new. The work from 2004 to 2008 on anion receptors with
indole motifs is well summarized by Gale, and more recent works
are cited in Jeong’s recent article on new indolocarbazole-based
anion receptors. However, the number of indole-based ion-pair
receptors reported to date is still limited. Jeong and co-workers
have synthesized a biindole-based ion-pair receptor by coupling
biindole with a diazacrown ether, which binds an alkali metal
ion and a halide anion cooperatively.4a Ito and co-workers reported
1
a,e
calixarenes or cholapods,
receptor 1 is acyclic and contains a
minimal number of cation-binding heteroatoms. The Br groups in
the indolyl moieties of 1 are not directly involved in ion binding,
yet were included in the synthesis to allow the possibility to sub-
stitute one or both with other groups to tune the molecule’s elec-
tronic properties or to enhance colorimetric responses to ion
binding.
2
The receptor 1 was synthesized in four steps starting from 2-
methylindole 2 (Scheme 1). The first two steps, N-protection and
dibromination by NBS, were modified from Nagarathnam’s
2
3
5
procedure. The dibrominated compound 4 was coupled with
4
2-(aminomethyl)pyridine in 2:1 ratio to give 5 in 59 % yield. After
N-deprotection of 5, the final product 1 was obtained (48%).
We first examined the halide-binding behavior of 1 in methanol
using ESI-MS. A series of methanol solutions of 1 with 5 equiv of
4
b
+
À
À
À
À
À
À
a series of indolylmethanes that bind anions and ion-pairs.
4
Bu N X (X = F , Cl , Br and I ) was used for individual halide-
Herein we report a newly synthesized receptor bis(3-bromoin-
dol-2-ylmethyl)(2-pyridylmethyl)amine (1, Scheme 1) that is
capable of binding both an anion and a cation. This molecule
binding assessment. A representative negative-ion mode spectrum
À
with Cl is shown in Figure 1. This spectrum shows the 1:1 adduct
À
1ÁCl along with signals corresponding to the mono-deprotonated
+
À
+
À
À
2
receptor [1ÀH ] and [Bu
4
N Á(Cl )
] . Analogous peaks were also
À
À
À
À
found with Br and I . With F , no 1:1 adduct was observed but in-
⇑
Corresponding authors. Tel.: +1 678 839 6024; fax: +1 678 839 6551 (M.F.).
E-mail address: mfujita@westga.edu (M. Fujita).
stead the 2:1 adduct (1)
small intensity. The signal intensities of the 1ÁX species are in
2
ÁF (m/z = 1067) was detected with a
À
0
040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.10.012

K. N. Skala et al. / Tetrahedron Letters 51 (2010) 6516–6520
6517
(
1)LiHMDS/THF
-78°C to r.t.
O
O
O
O
H
N
2.2NBS/CCl4
r.t.
N
N
Br
(
2)(MeO)CO
78°C to r.t.
-
Br
2
9
4%
3
8
1%
4
4
3
2
5
1
/2
N
6
N
N1
2
NH2NH2·HCl
NH2
Br
Br
Br
Br
N
N
3
KOH
MeOH
0°C to r.t.
Na CO
RN
4
2
3
NR
NH
HN
CH3CN
reflux
1
5
7
5
6
5
9%
48%
1
R = C(O)OCH3
Scheme 1. Synthesis of 1.
determine the relative order were made using 1H NMR as dis-
5
58.92
1
00
+
•Cl-
cussed later. A complex of 1 with Rb was not observed when
1
+
-
was used a Rb source, but when RbF was used as a Rb+
+
Bu N (Cl )
RbBF
4
4
2
8
6
4
2
0
0
0
0
0
+
source a signal of 1ÁRb was found, with the intensity comparable
+
À
to 1ÁK . It is not clear why the BF salt prevented the formation of
4
+
6
1
– H+
22.92
1ÁRb .
1H NMR studies of ion-binding behavior of 1 was carried out in
5
three deuterated solvents: CD
volume) of CDCl and DMSO-d
N Cl in these solvents, notable downfield shift of the NH signal
3
CN, CDCl
3
, and an 83:17 mixture (by
3
12.17
7
6
.
When 1 was titrated with
(* impurity)
600
3
*
3
39.08
+
À
Bu
was observed. This indicates that 1 binds Cl through the NHÁ Á ÁCl
hydrogen bonding. The titration curves in these solvents all fit to
4
À
À
200
400
m/z
the formation of a 1:1 complex. The association constant K
a
À1
À
(
M
) for the formation of 1ÁCl in CD
3 3 3
CN, CDCl , and CDCl /
À
Figure 1. Negative-ion ESI mass spectrum of 1:5 mole ratio of 1:Cl in methanol.
DMSO-d
6
(83:17) was calculated as 1042 ± 21, 440 ± 23, and
8
1
77 ± 18, respectively, using a program WINEQNMR2. Association
À
À
À
constants in CD CN with other halides were also determined
3
À
the order of Cl > Br > I , though for more quantitative compari-
1
(Table 1). With F , Ka could not be calculated since the chemical
son we used H NMR titrations as discussed later.
We also examined the Group 1A cation binding to 1 in methanol
by ESI-MS. A series of solutions of 1 with 5 equiv of MPF
À
shift changes caused by F were so small (
D
0.15 ppm change of
+
À
the NH signal upon addition of 8 equiv Bu
4
N F ). The general order
6
(M = Li,
À
À
À
À
of the K
a
values is Cl > Br > I > F , which is consistent with the
Na and K) or RbBF were used for individual cation-binding assess-
4
+
ESI-MS observation.
ment. In the positive-ion mode, signals of a 1:1 adduct 1ÁM and
+
Figure 3 shows the movement of chemical shifts of NH and aro-
the protonated receptor (1 + H ) were observed. A sample posi-
+
À
+
4 3
matic protons of 1 upon titration with Bu N Cl in CD CN. Upon
tive-mode spectrum of a mixture of 1 and Na is shown in Figure 2.
À
+
addition of 6.8 equiv Cl , the NH signal had the most significant
downfield shift by +2.35 ppm. The notable downfield shifts were
also observed with the pyridyl proton at the 3rd position (Py 3)
and the indolyl proton at the 7th position (In 7). These peaks
Among the first three cations, the 1ÁM signal intensities were
+
+
+
roughly in the order of Li ꢀ Na > K though further attempts to
moved by +0.85 and +0.24 ppm, respectively, upon addition of
525.25
100
À
6
.8 equiv Cl , while other aromatic protons hardly moved or only
slightly upfield shifted. Similar trend was observed in CDCl
3:17 CDCl /DMSO-d as well. This indicates a possibility of weak
hydrogen bonding-like interaction of Py 3 and In 7 protons with
3
and
9
0
0
0
0
0
0
0
0
0
8
3
6
8
7
6
5
4
3
2
1
À
À
the Cl . The DFT-optimized structure of 1ÁCl (Figure 4), obtained
1
by the program GAUSSIAN 09, is consistent with the H NMR observa-
À
tion. The Cl ion is bound by the two indolyl NH groups of 1 in a
À
propeller-like conformation. The NHÁ Á ÁCl
distances are
9
2
.14–2.15 Å, clearly within a range of hydrogen bonding. The
Py 3 HÁ Á ÁCl and In 7Á Á ÁCl distances are 2.49 Å and 3.4–3.6 Å,
5
47.17
À
À
Table 1
À1
The association constants K
Bu
CD
a
(M ) of the formation of 1ÁXÀ adducts from 1 and
4 4
NX (X = F, Cl, Br and I) based on titrations of 5 mM of 1 with 50 mM of Bu NX in
0
3
CN.
5
00
520
540
560
580
600
À
À
À
À
m/z
Halide
F
Cl
1042 ± 21
Br
157 ± 9
I
K
a
—
12 ± 1
Figure 2. Positive-ion ESI mass spectrum of 1:5 mole ratio of 1:NaPF
6
in methanol.

6
518
K. N. Skala et al. / Tetrahedron Letters 51 (2010) 6516–6520
Table 2
Chemical shift changes of proton signals
LiPF or NaPF
D
3
d (ppm) of 1 in CDCl upon saturation with
6
6
.
NH
Py 3
Py 4
Py 5
Py 6
LiPF
NaPF
6
+0.78
+0.78
+0.08
+0.01
+0.15
+0.11
À0.25
À0.21
À0.38
À0.11
CH Py
2
6
6
In 4
In 5
In 6
In 7
CH
2
In
LiPF
NaPF
6
À0.11
À0.06
À0.04
À0.11
À0.09
À0.09
À0.06
+0.32
+0.14
+0.17
+0.06
À0.002
+
NH more electron-deficient. The DFT-optimization of 1ÁLi indeed
resulted in both indolyl N atoms in the coordination sphere of
+
Li , in addition to the expected pyridyl N and the amine N
(
Figure 5).
1
Ion-pair binding behavior of 1 was also examined by H NMR. In
CD
3
CN, addition of 4.5 equiv TBACl and 4.5 equiv LiPF
6
resulted in
À
+
the precipitate formation (LiCl from excess Cl and Li ), and the
resulting solution matched the spectrum of 1:1 mixture of 1 and
À
Cl from the titration experiment. Therefore 3.5 equiv of LiCl must
À
À
+
have precipitated out, and 1 equiv Cl and 1 equiv Li must have
Figure 3. Chemical shift movement over titration of 1 with Bu
4
NCl in CD
3
CN. The
remained in solution, in which the Cl ions were interacting with
pyridyl and indolyl proton signals are shown separately for clarity. See Scheme 1 for
numbering of the aromatic protons.
+
1
. But apparently Li in solution does not have any influence on
either 1 or the 1ÁCl complex. Therefore in CD
À
3
CN, there was no
À
cation binding even by 1ÁCl .
On the other hand, we detected evidences of simultaneous
À
+
3 6
binding of Cl and Li in 83:17 CDCl /DMSO-d . In Figure 6 the
chemical shift changes of proton signals of 1 upon addition of (b)
+
À
À
1
0 equiv Li , (c) 10 equiv Cl , and (d) 10 equiv Cl and 10 equiv
+
+
Li are shown. The changes from (a) to (b), upon addition of Li ,
were relatively small, with most notable changes in the two meth-
ylene peaks, CH
2
In and CH
2
Py, by +0.14 and +0.17 ppm. The large
À
changes from (a) to (c) upon addition of 10 Cl in the signals of
NH (+1.00 ppm), Py 3 (+0.41 ppm), and In 7 (+0.16 ppm) were con-
sistent with the strong (NH) and weak (Py 3 and In 7) hydrogen
À
bondings to Cl as discussed earlier. When both 10 equiv Bu
4
NCl
were added (in either order) to 1 in 83:17
, no precipitation occurred, therefore the solution
and 10 equiv LiPF
CDCl /DMSO-d
6
À
Figure 4. Two views of a DFT-optimized structure of 1ÁCl . Color scheme: Gray = C,
3
6
white = H, blue = N, scarlet = Br, and green = Cl. Some hydrogen atoms are omitted
for clarity.
+
À
contained 10 equiv each of Li and Cl . The proton spectrum of
the resulting solution (d) was quite different from 1 only, 1 plus
+
À
+
Li , and 1 plus Cl . The effect of addition of Li to the mixture of
À
1
+ 10 Cl (c vs d) is obviously larger compared to the minimal ef-
À
respectively. The former is well in the range of reported CHÁ Á ÁCl
+
À
fect of Li to free 1 (a vs b), indicating that the complex 1ÁCl has a
hydrogen bonding distances.10 Though aromatic CH bonds do not
+
+
significant interaction with Li , possibly an inclusion of Li .
However, in the presence of excess of both ions, we cannot
À
usually form hydrogen bonds, in this case the geometry of 1ÁCl
À
brings these protons in proximity of Cl ion, within a possible
À
+
ascertain the stoichiometry of 1:Cl :Li in the ternary complex.
In order to find the stoichiometry, We titrated a 1:1 mixture of 1
range of some interaction.
The interaction between 1 and alkali metal cations from MPF
M = Li, Na and K) was also examined in CD CN, 83:17 CDCl
DMSO-d , and CDCl . In the first two solvents, addition of 5 equiv
MPF caused very minimal changes (no more than 0.15 ppm,
most peaks moved less than 0.01 ppm) in the H NMR signals of
. Strong coordination of solvent molecules to the cations may be
6
and Bu
4
NCl with LiPF
6
in 83:17 CDCl
3
/DMSO-d
6
. We have already
(
3
3
/
À
established the 1:1 stoichiometry of the 1ÁCl complex based on
6
3
the titration as discussed earlier. In a 1:1 mixture of 1 and Bu
4
NCl
6
D
À
a mixture of free 1 and a complex 1ÁCl exist, but since we know
1
+
that the interaction between free 1 and Li is insignificant, any
1
+
preventing a direct interaction between 1 and M .
In CDCl , due to poor solubility of MPF , we could not carry out
a titration or quantitative addition of the cation source to 1. We did
a coarse experiment of saturating a 5 mM solution of 1 in CDCl
with MPF by adding excess solid MPF and shaking it vigorously.
The H NMR signals of 1 were affected by saturation with LiPF and
NaPF , but not by KPF . The chemical shift changes of 1 in CDCl
upon saturation with LiPF and NaPF are summarized in Table 2.
Interestingly, the NH peak had a significant downfield shift upon
3
6
3
6
6
1
6
6
6
3
6
6
addition of LiPF
6
and NaPF
6
. This is not due to the hydrogen bond-
À
ing of the NH groups to the anion PF6 , since in a separate exper-
iment, Bu
4
NPF
6
did not cause any changes in proton signals of 1.
+
Figure 5. Two views of a DFT-optimized structure of 1ÁLi . Color scheme: Gray = C,
The possible reason of the downfield shift of the NH protons is
the coordination of the indolyl N atoms to the cation, making the
white = H, blue = N, and scarlet = Br, and pink = Li. Some hydrogen atoms are
omitted for clarity.

K. N. Skala et al. / Tetrahedron Letters 51 (2010) 6516–6520
6519
+
‘
bending’ at around 1 equiv point of the Li addition. As discussed
earlier, the NH and Py 3 signals were the most sensitive to the
À
À
Cl binding environment due to their hydrogen bonding to Cl .
The bending of the titration curves of these signals is an indication
À
that the nature of the interaction between the 1ÁCl complex and
+
Li changed at the bending point. The Py 3 peak upfield-shifted
up to around 1 equiv point, and then leveled off. The Py 3 peak’s
À
upfield shift suggests that the Py 3 HÁ Á ÁCl hydrogen bonding is
+
weakened by incoming Li . The level-off of this upfield shift after
À
the 1 equiv point suggests that the 1ÁCl complex is now saturated
+
with 1 equiv of Li . This supports the formation of an ion-pair com-
À
+
8
plex 1ÁCl ÁLi with 1:1:1 ratio. Using WINEQNMR2, we confirmed that
the titration curve up to the 2.5 equiv point fit to the 1:1 equilib-
À
+
À
+
rium equation of [1ÁCl ] + Li ꢀ [1ÁCl ÁLi ], and the association
À1
constant K
a
was calculated as 189 ± 10 M
.
The NH signal, on the other hand, had a two-step shape: it
downfield-shifted up to 1 equiv, leveled off, and after around
2
equiv point it started to downfield-shift again. The first down-
field-shift leveled off at 1 equiv point, but the second downfield-
shift was a continuous steady shift up to 15 equiv point (beyond
the range shown in the graph). We propose that the first downfield
+
shift is due to incorporation of 1 equiv of Li to form a ternary com-
À
+
plex 1ÁCl ÁLi , as proposed earlier using the Py 3 titration curve. The
curve up to 1.7 equiv point reasonably fit to the 1:1 equilibrium
À
+
À
+
equation of [1ÁCl ] + Li ꢀ [1ÁCl ÁLi ] on the WINEQNMR2 program’s
curve fitting. The second downfield-shift is not specific to a certain
stoichiometry, and it may be attributed to the excess Li having
+
À
+
less specific interaction with 1ÁCl ÁLi , such as cation-aromatic
p
electron interaction. The association constant K
a
based on the NH
À1
shift was calculated as 28 ± 13 (M ). This is sevenfold smaller than
Figure 6. 1H NMR spectra in 83:17 CDCl
/DMSO-d
6
NCl + 10 LiPF .
3
6 6
with (a) 1 only, (b) 1 + 10 LiPF ,
the K
the 0–1.7 equiv region used for this K
affected by the secondary non-specific interaction with excess Li .
value calculated based on the Py 3 proton, but the tale end of
a
(
c) 1 + 10 Bu
4
NCl, and (d) 1 + 10 Bu
4
a
calculation was likely to be
+
1
À
+
changes in the H NMR spectrum can be attributed to the interac-
Two proposed forms of the ion-pair complex 1ÁCl ÁLi are
À
+
À
tion of the 1ÁCl complex with Li . If the 1ÁCl complex incorpo-
depicted in Figure 8. Both structures were DFT-optimized. The first
+
À
+
À
+
rates Li to form an ion-pair complex 1ÁCl ÁLi , the equilibrium of
À
is an intact ion-pair complex (Figure 8a), in which Cl and Li are
held in proximity, literally an ‘ion-pair’ binding by 1. After DFT
geometry optimization the resulting structure had a covalent bond
À
À
À
1
ÁCl formation (1 + Cl ꢀ 1ÁCl ) would shift toward the 1ÁCl
+
which would become available for further incorporation of Li .
+
À
The resulting titration curves are presented in Figure 7. The
curves of two signals, the NH and the Py 3 proton, had a subtle
between Li and Cl with an interatomic distance of 2.24 Å, which
is a good match to the calculated and experimental bond distance
Figure 7. Chemical shift movement of 1H NMR signals over titration of [1 + 1 equiv Bu
NCl] mixture with LiPF in 83:17 CDCl /DMSO-d .
4 6 3 6

6
520
K. N. Skala et al. / Tetrahedron Letters 51 (2010) 6516–6520
this research (Fujita: 45505-GB3; Khan: 50147-URG). Authors are
also grateful for NSF MRI grants 0521238 (LC–MS (ESI)) and
0821504 (400 MHz NMR), and the University of West Georgia
0
Technology Fee Grant (GAUSSIAN 09). The authors are grateful to Pro-
fessor Michael Hynes for his help with the software WINEQNMR2.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.10.012.
À
+
Figure 8. DFT-optimized structures of two possible modes of 1ÁCl ÁLi . Color
scheme: Gray = C, white = H, blue = N, and scarlet = Br, and pink = Li. Some hydro-
gen atoms are omitted for clarity.
References and notes
1.
For selected reviews, see: (a) Katayev, E. A.; Melfi, P. J.; Sessler, J. L. In Modern
Supramolecular Chemistry: Strategies for Macrocycle Synthesis; Diederich, F.,
Stang, P. J., Tykwinski, R. R., Eds.; Wiley-VCH: Weinheim, Germany, 2008; pp
315–347; (b) Gale, P. A.; Garcia-Garrido, S. E.; Garric, J. Chem. Soc. Rev. 2008, 37,
1
1
of a molecule of LiCl, reported as 2.02 Å by Dixon and co-workers.
À
+
The slightly longer LiÁ Á ÁCl bond distance in 1ÁCl ÁLi is expected
since both Li+ and Cl ions have other interactions outside of the
LiCl core. While both intact and isolated forms are consistent with
151–190; (c) Itsikson, N. A.; Geide, I. V.; Morzherin, Y. Yu.; Matern, A. I.;
À
Chupakhin, O. N. Heterocycles 2007, 72, 53–77; (d) Smith, B. D. In Ion-pair
Recognition by Ditopic Receptors, Macrocyclic Chemistry: Current Trends and
Future; Gloe, K., Antonioli, B., Eds.; Kluwer: London, UK, 2005; pp 137–152; (e)
Kirkovits, G. J.; Shriver, J. A.; Gale, P. A.; Sessler, J. L. J. Incl. Phenom. Macrocycl.
Chem. 2001, 41, 69–75; (f) Sutherland, I. O. Adv. Supramol. Chem. 1990, 1, 65–
108.
1
the H NMR observation, the former is probably more likely due to
the extra stabilization energy from the formation of the LiÁ Á ÁCl
bond. Calculations of energies using DFT show that the intact com-
plex is more stable than the isolated complex by 25 kcal/mol
2
3
4
.
.
.
Gale, P. A. Chem. Commun. 2008, 4525–4540.
Lee, G. W.; Kim, N.-K.; Jeong, K.-S. Org. Lett. 2010, 12, 2634–2637.
(a) Chae, M. K.; Lee, J.-I.; Kim, N.-K.; Jeong, K.-S. Tetrahedron Lett. 2007, 48,
6624–6627; (b) Nishiki, M.; Oi, W.; Ito, K. J. Incl. Phenom. Macrocycl. Chem. 2008,
(
104 kJ/mol).
In conclusion, we have synthesized a new receptor 1 and dem-
6
1, 61–69.
onstrated its ability to bind an anion and a cation separately or
simultaneously. Anion binding was generally stronger than cation
binding, and among halides the chloride ion had the strongest
5
6
.
.
Nagarathnam, D. Synthesis 1992, 743–745.
À
BF4 could potentially receive hydrogen bonding from 1 but we did not
À
observe any signal corresponding to 1ÁBF
.
4
7. CD OD and pure DMSO-d were tested as well, but the 1H NMR signals of 1
1
affinity to 1. H NMR titrations were used to show the stoichiom-
3
6
À
+
4
were not affected upon addition of Bu NCl in these solvents. In the former, the
NH peak disappeared upon H/D exchange.
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etry of the ternary complex 1ÁCl ÁLi .
8
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Acknowledgments
1
690.
Acknowledgment is made to the Donors of the American
Chemical Society Petroleum Research Fund for partial support of
11. Vasiliu, M.; Li, S.; Peterson, K. A.; Feller, D.; Gole, J. L.; Dixon, D. A. J. Phys. Chem.
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