Hemispherand-Strapped Calix[4]pyrrole: An Ion-pair Receptor for the Recognition and Extraction of Lithium Nitrite

Article abstract of DOI:10.1021/jacs.6b05713

The hemispherand-strapped calix[4]pyrrole (1) acts as an ion pair receptor that exhibits selectivity for lithium salts. In organic media (CD₂Cl₂ and CD₃OD, v/v, 9:1), receptor 1 binds LiCl with high preference relative to NaCl, KCl, and RbCl. DFT calculations provided support for the observed selectivity. Single crystal structures of five different lithium ion-pair complexes of 1 were obtained. In the case of LiCl, a single bridging water molecule between the lithium cation and chloride anion was observed, while tight contact ion pairs were observed in the case of the LiBr, LiI, LiNO₃, and LiNO₂ salts. Receptor 1 proved effective as an extractant for LiNO₂ under both model solid-liquid and liquid-liquid extraction conditions.

Full text of DOI:10.1021/jacs.6b05713

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Communication
A Hemispherand-Strapped Calix[4]pyrrole: An Ion-pair
Receptor for the Recognition and Extraction of Lithium Nitrite
Qing He, Zhan Zhang, James T. Brewster, Vincent M. Lynch, Sung Kuk Kim, and Jonathan L. Sessler
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b05713 • Publication Date (Web): 21 Jul 2016
Downloaded from http://pubs.acs.org on July 22, 2016
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A Hemispherand-Strapped Calix[4]pyrrole: An Ion-pair Receptor
for the Recognition and Extraction of Lithium Nitrite
10
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Qing He†, Zhan Zhang‡, James T. Brewster†, Vincent M. Lynch†, Sung Kuk Kim*§, and Jonathan L.
†
Sessler*
† Department of Chemistry, The University of Texas at Austin, 105 East 24th, StreetꢀStop A5300, Austin, Texas 78712ꢀ
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1224, United States
‡ Institute for Supramolecular and Catalytic Chemistry, Shanghai University, 99 Shangda Road, Shanghai 200444, China
§ Department of Chemistry and Research Institute of National Science, Gyeongsang National University, Jinju, 660ꢀ701,
Korea
Supporting Information Placeholder
ing the rather hydrophobic picrate anion as the counter ion.9 In
ABSTRACT: The hemispherandꢀstrapped calix[4]pyrrole (1)
2004 Smith disclosed a metalꢀfree ditopic receptor that bound
lithium chloride in the form of a waterꢀbridged complex and
showed that it could be used to effect the solidꢀliquid (chloroꢀ
form) extraction of LiCl and LiBr.10 These seminal reports not
withstanding, to our knowledge, no system has been reported that
is capable of promoting the extraction of simple lithium salts unꢀ
der aqueousꢀorganic LLE or transport conditions. We thought that
by using a lithium coordinating subunit to strap a calix[4]pyrrole
anion recognizing core we would be able to create ion pair recepꢀ
tors capable of recognizing and extracting lithium salts under both
solidꢀliquid and liquidꢀliquid conditions. Of particular interest
would be systems capable of extracting lithium nitrite because of
the importance of the nitrite anion in the environment, its use in
the construction industry, and its presence at high levels in U.S.
radioactive tank waste. Such considerations led to the design of
receptor 1.
acts as an ion pair receptor that exhibits selectivity for lithium
salts. In organic media (CD2Cl2 and CD3OD, v/v, 9:1), receptor 1
binds LiCl with high preference relative to NaCl, KCl, and RbCl.
DFT calculations provided support for the observed selectivity.
Single crystal structures of five different lithium ion pair comꢀ
plexes of 1 were obtained. In the case of LiCl, a single bridging
water molecule between the lithium cation and chloride anion was
observed, while tight contact ion pairs were observed in the case
of the LiBr, LiI, LiNO3 and LiNO2 salts. Receptor 1 proved effecꢀ
tive as an extractant for LiNO2 under both model solidꢀliquid and
liquidꢀliquid extraction conditions.
The lithium cation is of considerable commercial interest. It
plays a central role in modern battery technology, is important in
lubricants, and is used therapeutically for the treatment of depresꢀ
sion.1 Salt flats in Bolivia provide much of the lithium currently in
use, although other more limited reserves remain throughout the
world.2 Although not representing an imminent crisis, there are
growing concerns regarding the dwindling supply of available
lithium.3 An ability to recognize and purify this cation through,
e.g., solidꢀliquid extraction (SLE) or liquidꢀliquid extraction
(LLE) protocols may help alleviate these concerns.
Soꢀcalled ionꢀpair receptors, species containing disparate bindꢀ
ing sites for cations and anions, offer considerable advantages in
terms of affinity and selectivity as compared to analogous single
ion receptors.4 In recent years, they have attracted attention in
areas as diverse as ion extraction, recognition, throughꢀmembrane
transport, salt solubilization, ion sensing, and logic gate design,
among other applications.4c,5 Our own interest has centered
around calix[4]areneꢀstrapped calix[4]pyrroles. These polytopic
receptors were found to effective for cesium salt recognition,
extraction, and ionꢀtriggered release.4c However, lithium selective
salt recognition could not be achieved. In fact, more broadly, reꢀ
ceptors capable of recognizing the Li+ cation7 or lithium ion pairs8
are rare. Those capable of extracting simple lithium salts are all
but unknown. In classic early work, Cram demonstrated that his
spherands could be used to extract the lithium cation from an
aqueous phase into an organic phase. This extraction was not
effected in the form of an ion pair; rather, it was achieved by usꢀ
Scheme 1. Synthesis of receptor 1
The synthesis of compound 1 is shown in Scheme 1. Precursor 2
was
prepared
in
63%
yield
via
a
tetrakis(triphenylphosphine)palladium(0)ꢀcatalyzed
Suzukiꢀ
Miyaura crossꢀcoupling reaction using 1,3ꢀdibromoꢀ2ꢀ
methoxybenzene and 6ꢀbromopyridinꢀ2ꢀamine as the starting
materials. Dipyrromethane 3 was prepared in 19% yield by conꢀ
desning levulinic acid with pyrrole in the presence of excess meꢀ
thanesulfonic acid (See Supporting Information (SI)). Condensaꢀ
tion of
2
and
3
in the presence of NꢀEthylꢀN'ꢀ(3ꢀ
dimethylaminopropyl)carboꢀdiimide hydrochloride (EDCI) and
pyridine in CH2Cl2 then yielded the key intermediate 4 in 81%
yield. The resulting species was condensed with acetone in the
presence of excess BF3·OEt2 to give 1 in 19% yield. Compound 1
was characterized by NMR spectroscopy and mass spectrometry
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(See SI), as well as via Xꢀray diffraction analyses of single crysꢀ
tals grown under three different conditions (cf. Figure S1)
signals were seen when receptor 1 was treated with excess NaCl,
1
2
3
4
5
6
7
8
KCl and RbCl as compared to the free receptor (Figure 2). Expoꢀ
sure to LiClO4 was found to induce changes in the aromatic proꢀ
ton signals of the hemispherand moiety analogous to those seen
with LiCl (Figure S3). In contrast, treatment with NaClO4 (up to
20 equiv) failed to induce any significant change in these signals
(Figure S4). On this basis we conclude that the lithium cation is
selectively bound to the hemispherand subunit and that the high
selectivity of receptor 1 for LiCl over other alkali metal salts origꢀ
inates at least in part from the cation binding site. Further eviꢀ
dence of this selectivity came from the finding, confirmed by a
single crystal diffraction analysis, that a 1:1 complex of 1•LiCl
Initial evidence that compound 1 could act as an ion pair recepꢀ
tor for lithium salts came from a singleꢀcrystal Xꢀray diffraction
analysis of the LiCl complex. Suitable crystals were obtained via
the slow evaporation of a CHCl3/CH3CN/CH3OH solution of
receptor 1 in the presence of excess lithium chloride. The resultꢀ
ing structure revealed a 1:1 LiCl complex (Figure 1). The Li+ ion
resides on one side of the cavity in a cleft formed by the hemiꢀ
spherand and the amido group. It is bound to a methanol and waꢀ
ter molecule in addition to the receptor. Key distances observed in
this structure include 1.89 Å (Li+···O1), 2.12 Å (Li+···N6) 3.22 Å
(Li+···O2), and 4.30 Å (Li+···N7). The Clꢀ ion is hydrogenꢀ
bonded to the NH groups of the calix[4]pyrrole subunit (with
N···Clꢀ distances in the range of 3.34ꢀ3.40 Å), as well as to the
water molecule bound to the Li+ cation. The distance between the
Li+ cation and the Cl– anion was 4.34 Å, as would be expected for
a solventꢀbridged ion pair complex. A similar singleꢀcrystal strucꢀ
ture was also obtained with the Li+ cation included in the other
side of the pocket formed by the hemispherand and the amido
groups (Figure S2). This finding led us to consider that in solution
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was obtained when
a solution of receptor 1 in CH3Cl
/CH3OH/CH3CN containing an excess of LiCl, NaCl, KCl and
RbCl was subject to slow evaporation (Figure S2).
1
Figure 2. Partial H NMR spectra of a 5.0 mM solution of (a) 1
only, 1 with (b) excess LiCl, (c) excess NaCl, (d) excess KCl and
(e) excess RbCl in 10% CD3OD in CD2Cl2 (v/v).
Theoretical support for the conclusion that receptor 1 was highꢀ
ly selective for LiCl over NaCl and KCl came from Density Funcꢀ
tional Theory (DFT) calculations carried out in the gas phase at
the b3lyp/6ꢀ31g*//b3lyp/6ꢀ31g* level (Figure S5ꢀS7). Two possiꢀ
ble limiting complexation modes for 1•MCl (M = Li, Na, K),
namely without and with hydration, were considered (Table S1).
In the absence of water, the calculated binding energies of comꢀ
plexes, 1•MCl, were ꢀ220.49, ꢀ198.06, and ꢀ174.65 kcal molꢀ1 for
LiCl, NaCl, and KCl, respectively. In contrast, for the putative
hydrates, the energies for the corresponding complexes,
1•MCl•H2O, were ꢀ244.27, ꢀ217.44, and ꢀ188.65 kcal molꢀ1 for
LiCl, NaCl, and KCl, respectively. We thus conclude that forꢀ
mation of the lithium chloride complexes is favored independent
of the model used.
Figure 1. A) Front view and B) side view of the single crystal
Xꢀray diffraction structure of the lithium complex
[1·LiCl·H2O·MeOH].
the Li+ cation might possibly shuttle rapidly between the two subꢀ
cavities defined by the central methoxy group.
The ability of 1 to bind lithium chloride in solution was probed
via 1H NMR spectroscopy using a mixture of CD3OD and CD2Cl2
(1:9, v/v) as the solvent (Figure 2). Spectroscopic analysis of
compound 1 revealed only one set of resonances. Upon exposure
to excess LiCl, the singlet associated with the NH proton seen at
8.48 ppm in free 1 underwent a shift to 10.50 ppm, a change atꢀ
tributed to interactions between the bound Cl– and the NH protons
of the calix[4]pyrrole subunit. Significant downfield shifts were
also observed for the aromatic hydrogen atoms signals, except
proton e. The upfield shift for proton e is ascribed to the loss of an
intramolecular hydrogen bond (highlighted in light red in Figure
2) resulting from the conformational changes associated with Li+
complexation. The signals for the methoxy protons attached to the
benzene ring (h) and the methylene groups (f) connected to the
amide subunits underwent considerable downfield shifts in the
presence of LiCl. These changes are rationalized in terms of the
deshielding that results from the lithium cationꢀoxygen atom inꢀ
teractions and the structural changes induced upon lithium cation
complexation. Taken together, these findings are consistent with
the expectation that Li+ and Cl– are coꢀbound by receptor 1.
Moreover, the observation of only one set of signals in the comꢀ
plex supports the proposal that the Li+ cation moves rapidly withꢀ
in the large hemispherand cavity on the NMR timescale and is not
constrained to one binding site.
To obtain further insights into ionꢀpair complexation of 1, sevꢀ
eral lithium salts with larger anions, e.g. Br–, I–, NO2 and NO3 ,
were screened. Evidence for ionꢀpair recognition and following
1:1 ionꢀpair complexation of 1 in the solid state came from the
single crystal Xꢀray structures of the LiBr, LiI, LiNO2. and LiNO3
complexes (Figure 3). The resulting crystal structures revealed
that these four lithium salts were encapsulated by receptor 1 as
tight contact ion pairs (as opposed to hydrated species) with
Li+···Aꢀ distances of 3.040, 3.064, 2.077, and 2.609 Å for the
LiBr, LiI, LiNO2, and LiNO3 complexes, respectively. In all four
structures, the lithium cation is bound to one side the hemiꢀ
spherand strap with contact distances ranging from 1.863 to 2.219
Å. All counter anions were hydrogenꢀbonded to the NH groups of
the calix[4]pyrrole subunits with average N···A– distances of
3.59, 3.80, 3.05, and 3.05 Å for LiBr, LiI, LiNO2, and LiNO3,
respectively.
–
–
DFT calculations were also carried out in an effort to estimate
the selectivity of receptor 1 for LiCl, LiBr, LiNO2, and LiNO3
(Figures S5B and S8, Table S2). The initial input coordinates
were obtained from the crystal structures discussed above. The
Under the same 1H NMR spectroscopic conditions as employed
above, no appreciable changes in any of the receptorꢀbased proton
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resulting binding energies for the complexes were calculated to be
of an analogous CD2Cl2 solution receptor 1 after exposure to exꢀ
ꢀ244.27 kcal molꢀ1 for LiCl•H2O, ꢀ236.72 kcal molꢀ1 for LiBr, ꢀ
237.91 kcal molꢀ1 for LiNO2 and ꢀ230.24 kcal molꢀ1 for LiNO3,
leading us to infer no appreciable selectivity between these lithiꢀ
um salts. Experimental analyses of intraꢀlithium salt selectivity
came from 1H NMR spectroscopic titrations (Figures S9ꢀS13) and
fitting the data to a 1:1 binding mode. The resulting binding conꢀ
stants were found to be 45±1 Mꢀ1, 74±8 Mꢀ1, 108±7 Mꢀ1, and
263±13 Mꢀ1 for LiBr, LiI, LiNO2, and LiNO3, respectively.
cess NaNO2 and KNO2, even after 20 min under sonication (Figꢀ
ures 4c and 4d). Competition experiments, involving the use of
LiNO2, NaNO2, and KNO2, produced spectral changes identical to
those seen in the presence of LiNO2 alone (cf. Figures 4e and 4b).
We thus conclude that receptors 1 is able to extract LiNO2 effiꢀ
ciently and with high selectivity over NaNO2 and KNO2 under
solidꢀliquid extraction conditions as illustrated schematically in
Figure 4f.
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1
Figure 4. Partial H NMR spectra of a 5.0 mM solution of (a) 1
only, 1 with (b) excess LiNO2, (c) excess NaNO2, (d) excess
KNO2 and (e) excess LiNO2+NaNO2+KNO2 in CD2Cl2. (f) Carꢀ
toon illustration of the solidꢀliquid extraction of LiNO2.
In order to test whether receptor 1 could extract LiNO2 from an
aqueous phase to an organic phase, model liquidꢀliquid extraction
studies were carried out using a 10 mM solution of 1 in CDCl3
1
and various saturated D2O salt solutions. A comparison of the H
NMR spectrum of free 1 in D2O saturated CDCl3 revealed considꢀ
erable changes in the resonance signals after contacting with a
saturated D2O solution of LiNO2 (Figures 5a and 5b). In contrast,
almost no appreciable chemical shift changes were observed in
1
Figure 3. Single crystal structures of complexes of receptor 1
with A) LiBr; B) LiI and C) LiNO2 and D) LiNO3.
the H NMR spectrum of the CDCl3 layer containing receptor 1
after it was washed thoroughly with saturated D2O solutions of
either NaNO2 or KNO2. We thus conclude that receptor 1 is able
to bind and extract LiNO2 from D2O phase into a CDCl3 phase
and do so selectively under these initial test conditions.
Due to the relatively low hydration energy of the nitrite anion (ꢀ
330 kJ molꢀ1 for NO2 vs. ꢀ475 kJ molꢀ1 for Li+),11 which should
–
favor liquidꢀliquid extraction, and its presence at high levels in
U.S. radioactive tank waste (2ꢀ4 M in many instances),12 addiꢀ
tional efforts were devoted towards understanding the determiꢀ
nants of lithium nitrite recognition in the case of 1. As a first step
in this effort, 1H NMR spectroscopic titration studies were carried
out using TBANO2. Even after the addition of 20 equiv, no appreꢀ
ciable chemical shifts in the signals for receptor 1 were observed
–
(Figure S15), leading us to infer that 1 does not bind the NO2
anion effectively in the absence of the lithium cation. However,
upon addition of either lithium tetraphenylborate or LiClO4 to a
solution of 1 in a mixture of CD2Cl2 and CD3OD (9:1, v/v) in the
absence of a nitrite anion source, the aromatic protons correꢀ
sponding to the benzene and pyridine rings underwent shifts analꢀ
ogous to those seen in the case of 1•LiCl above (cf. Figure S16).
Subsequent addition of TBANO2 resulted in a significant shift of
both the NH and aromatic protons. Taken in concert, these findꢀ
ings lead us to suggest that the recognition of nitrite by receptor 1
is facilitated when the lithium cation is preꢀbound as illustrated in
Figure S17.
While a number of receptors capable of complexing other nitrite
anion salts are known,13 to the best of our knowledge, the hemiꢀ
spherandꢀcalix[4]pyrrole 1 is the first system capable of recognizꢀ
ing LiNO2 as an ion pair. Its ability to act as an extractant was
thus probed in detail.
Figure 5. Partial 1H NMR spectra of a 10 mM solution of 1 in
CDCl3 exposed to (a) D2O, (b) saturated LiNO2 in D2O, (c) satuꢀ
rated NaNO2 in D2O and (d) saturated KNO2 in D2O. (e) Cartoon
illustration of the liquidꢀliquid extraction of LiNO2 inferred on the
basis of these studies.
In summary, a new ion pair receptor, the hemispherandꢀ
strapped calix[4]pyrrole 1, was synthesized and characterized by
standard spectroscopic protocols as well as by single Xꢀray crystal
diffraction analysis. Receptor 1 was shown to form 1:1 ion pair
complexes with several lithium salts (e.g., LiCl, LiBr, LiI, LiNO2
and LiNO3) with high selectivity over the corresponding sodium
and potassium salts both in the solid state and in organic media.
These experimental findings were supported by DFT calculations.
To the best of our knowledge, compound 1 is the first ion pair
receptor capable of recognizing and extracting lithium nitrite unꢀ
der both solidꢀliquid and liquidꢀliquid conditions and to do so
As a first test, solidꢀliquid extraction studies were carried out
1
using H NMR spectroscopy (Figure 4). Exposing receptor 1 in
CD2Cl2 to an excess of microcrystalline LiNO2 engendered disꢀ
tinctive changes in the 1H NMR spectrum (Figure 4b). In contrast,
1
no appreciable changes were observed in the H NMR spectrum
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(4) (a) Smith, B.; Gloe, K., Antonioli, B., Eds.; Kluwer: , Ed.; Springer
with high selectivity relative to sodium nitrite and potassium niꢀ
trite.
Netherlands: London, U.K., 2005, p 137. (b) Kim, S. K.; Sessler, J. L.
Chem. Soc. Rev. 2010, 39, 3784. (c) Kim, S. K.; Sessler, J. L. Acc. Chem.
Res. 2014, 47, 2525. (d) McConnell, A. J.; Beer, P. D. Angew. Chem. Int.
Ed. 2012, 51, 5052. (e) Kirkovits, G. J.; Shriver, J. A.; Gale, P. A.;
Sessler, J. L. J. Incl. Phenom. Macro. 2001, 41, 69. (f) Gale, P. A. Coord.
Chem. Rev. 2003, 240, 191. (g) Steed, J. W.; Atwood, J. L. In
Supramolecular Chemistry; John Wiley & Sons, Ltd: Hoboken, NJ, USA,
2009, p 285. (h) Dalla Cort, A. In Supramolecular Chemistry; John Wiley
& Sons, Ltd: Hoboken, NJ, USA, 2012.
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Parsons, S.; Coles, S.; Tasker, P. A. Chem. Commun. 1999, 2077. (c)
Picot, S. C.; Mullaney, B. R.; Beer, P. D. Chem. Eur. –J. 2012, 18, 6230.
(d) Custelcean, R.; Delmau, L. H.; Moyer, B. A.; Sessler, J. L.; Cho, W.
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Ballester, P. J. Am. Chem. Soc. 2015, 137, 2047.
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ASSOCIATED CONTENT
Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org.
Synthetic details, NMR and procedure and additional data for
extraction, calculation data for optimized geometry of the comꢀ
plexes with receptor 1, and Xꢀray structural data for free 1 and
complexes (PDF)
Xꢀray crystallographic data for 1ꢀLiBr (CIF); 1ꢀLiI (CIF); 1ꢀ
LiNO2 (CIF); 1ꢀLiNO3 (CIF); 1ꢀLiClꢀright (CIF); 1ꢀLiClꢀleft
(CIF); 1-CH3CN (CIF); 1-2H2O (CIF); 1-2CH3OH (CIF)
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(6) Park, I. W.; Yoo, J.; Adhikari, S.; Park, J. S.; Sessler, J. L.; Lee, C. H.
Chem.-Eur. J. 2012, 18, 15073.
AUTHOR INFORMATION
Corresponding Author
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Chem. Soc. 1979, 101, 6752. (b) Cram, D. J.; Kaneda, T.; Helgeson, R. C.;
Brown, S. B.; Knobler, C. B.; Maverick, E.; Trueblood, K. N. J. Am.
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Tarraga, A.; Veciana, J.; Molina, P. Chem.-Eur. J. 2010, 16, 1532.
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Savannah River Site (SRS) high level waste tanks – comparison of SRS
and Hanford modeling predictions. Savannah River National Laboratory,
Aiken, SC, U.S. Department of Energy report WSRCꢀTRꢀ2004ꢀ00468
Revision 0; August 2004.
*sessler@mail.utexas.edu
*sungkukkim@gnu.ac.kr
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
The work in Austin was supported by the Office of Basic Enerꢀ
gy Sciences, U.S. Department of Energy (DOE) (grant DEꢀFG02ꢀ
01ER15186 to J.L.S.). S.K.K. acknowledges the Basic Science
Research Program of the National Research Foundation of Korea
(NRF) funded by the Ministry of Science, ICT & Future Planning
(2015R1C1A1A02037578).
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