P-tert-Butylcalix[6]arene hexacarboxylic acid conformational switching and octahedral coordination with Pb(ii) and Sr(ii)

Article abstract of DOI:10.1039/c3cc48465c

p-tert-Butylcalix[6]arene hexaacetic acid is in a symmetric cone conformation in CHCl₃, but it becomes conformationally flexible in CHCl₃/CH₃CN (1:1). In this mixture the host has a strong binding affinity towards Pb(ii) and instantly forms a complex of low symmetry-shortly thereafter structural reorganization occurs resulting in a high symmetry complex of Pb(ii) in an octahedral cage of carboxylates. Sr(ii) and Ba(ii) display similar behavior over a longer period of time.

Full text of DOI:10.1039/c3cc48465c

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p-tert-Butylcalix[6]arene hexacarboxylic acid
conformational switching and octahedral
coordination with Pb(II) and Sr(II)†
Cite this: Chem. Commun., 2014,
0, 1903
5
Received 5th November 2013,
Accepted 24th December 2013
ab
b
a
Birendra Babu Adhikari, Keisuke Ohto and Michael P. Schramm*
DOI: 10.1039/c3cc48465c
www.rsc.org/chemcomm
p-tert-Butylcalix[6]arene hexaacetic acid is in a symmetric cone
conformation in CHCl , but it becomes conformationally flexible in
CHCl /CH CN (1 : 1). In this mixture the host has a strong binding affinity
3
3
3
towards Pb(II) and instantly forms a complex of low symmetry – shortly
thereafter structural reorganization occurs resulting in a high symmetry
complex of Pb(II) in an octahedral cage of carboxylates. Sr(II) and Ba(II)
display similar behavior over a longer period of time.
Scheme 1 Synthesis of p-tert-butylcalix[6]arene hexacarboxylic acid 3.
Recent attention has been devoted to the functionalization of
calix[4]arenes resulting in supramolecular species with diverse func- a fixed cone conformation of the complex. Additionally, positive
tional properties. Cation binding of calix[4]arene derivatives is one allosteric effects were observed as the host extracts two Pb(II) ions
1
9
area that has been well developed. Unlike calix[4]arene whose in stepwise. We became interested in the behavior of the higher order
thermodynamic preference is for a rigid C4v cone conformation, p-tert-butylcalix[6]arene hexacarboxylic acid ligand 3 (Scheme 1).
calix[6]arene is flexible and of lower symmetry with phenyl groups
The synthesis of p-tert-butylcalix[6]arene 1 and its ethoxy-
1
0
alternating orientation in solution presenting challenges to character- carbonylmethyl derivative 2a has been described. Compound
2
ization. Only a few reports are available regarding the conformational 2b was prepared in high yield following analogous methods.
behavior and solution structure of lower rim modified non-bridged Alkaline hydrolysis of 2a with aq KOH in THF and acidification
3–7
10
calix[6]arene derivatives and their metal complexes.
The tert-butyl ester of p-tert-butylcalix[6]arene (2, where R =
afforded the corresponding hexacarboxylic acid 3 in 93% yield.
1
The H NMR of 3 in CDCl
3
suggests a regular cone conformation
t-butyl, Scheme 1) adopts a 1,2,3-alternate conformation in solution (distinct doublets at 3.41 and 4.49 ppm). Presumably this conforma-
+
but changes to C6v cone conformation upon complexation with K
tion is due to hydrogen bond interactions among six carboxylic acid
+
5
11
and Cs . For the corresponding ethyl ester (2a), no identifiable groups. In polar solvents loss of resolution and broadening occur
conformation was observed even at low temperatures, but again (see ESI† for spectra in several polar solvents).
this changed upon the addition of alkali metal cations. The p-tert-
6
5
3 3
We titrated host 3 in CDCl /CD CN (Fig. 1a) with Pb(II) and
butylcalix[6]arene hexaamide adopts a 1,2,3-alternate conformation observed changes to the spectra. With 0.25 equivalents a new series
+
7
in solution, two Na ions fix the conformation as a cone.
of small peaks emerged (Fig. 1b). An additional 0.25 equivalents
The specific role of carboxylic acid groups in the extraction provided a more clear, yet complex spectra with numerous sharp
8
of Pb(II) with calix[4]arene derivatives has been well addressed.
Recently, we reported on the very strong binding of p-tert-butylcalix[5]- doublets for ArH protons with 1 : 1 : 1 : 1 : 1 : 1 integration, (b) six pairs
arene pentacarboxylic acid with Pb(II). The size-match effect resulted in of doublets for ArOCH and ArCH Ar, (c) three tert-butyl peaks with
:1:1 integration (see ESI†). These patterns, in particular the
resonances (Fig. 1c). The following were notable: (a) three pairs of
2
2
1
+
a
splitting of ArH protons, are consistent with the observed K
complexed 2a ‘‘lozenge’’ conformation. The new signals coexist
with those of the unbound ligand in a 1 : 1 ratio, indicating a strong
Department of Chemistry and Biochemistry, California State University
Long Beach, 1250 Bellflower Blvd., Long Beach, 90840, CA, USA.
E-mail: michael.schramm@csulb.edu; Fax: +1-562-985-8557
5
b
Department of Chemistry and Applied Chemistry, Saga University, Honjo-1,
1
: 1 host guest complex. Addition of 1.0 equivalent Pb(II) resulted in
8
40-8502, Saga, Japan
the disappearance of the free ligand.
†
Electronic supplementary information (ESI) available: All preparative proce-
A few new, weak signals appeared (Fig. 1d) and at 1.5 equivalents
(Fig. 1e) these new peaks were easier to identify.
dures, titrations and CIF data are provided. CCDC 970239. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/c3cc48465c
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1
Fig. 1 H NMR titration of 3 (5 mM) with various equivalents of Pb(ClO
4
)
2
in
CN = 1/1 at 300 MHz. (a) Free host and (b)–(e) in presence of 0.25,
.5, 1.0 and 1.5 equivalents Pb(II), respectively. Residual solvent peaks CHCl
7.26 ppm), CH CN (1.73 ppm), H O from Pb(ClO O (2.4 ppm, very broad).
ꢀ3H
Fig. 3 Partial COSY NMR of 3 (5 mM) with 1 equivalent of Pb(ClO
at equilibrium in CDCl /CD CN at 300 MHz.
4
)
2
ꢀ3H
2
O
CDCl
3
/CD
3
3
3
0
3
(
3
2
4
)
2
2
Furthermore, the six aryl carbons were non-equivalent as
1
3
indicated by six C signals and only one signal was observed
Curious, we next acquired H NMR spectrum of 3 in the presence for ArCH Ar carbon at around 33 ppm.
1
2
of one equivalent Pb(II) over a period of one day and observed the shift
2
The appearance of two distinct AB doublets for calixarene ArCH Ar
from the initial spectra (Fig. 2a) to a new spectra comprised exclusively protons was consistent with a highly C symmetrical cone conforma-
6
of the small peaks we initially observed in Fig. 1d. After 15 hours this tion, but the diastereotopic ArOCH protons and the meta coupling of
2
new, sole species is of significantly higher symmetry than the initial ArH posed a problem with assignment of such a simple structure.
lozenge (Fig. 2d); the ArH region exhibits two doublets with meta COSY and NOSEY afforded a complete assignment of protons for the
coupling, the ArCH Ar protons exist as a pair of sharp doublets 3ꢀPb(II) complex – Hb and Hc were assigned as the ArCH Ar protons
2
2
indicative of a rigid conformation and the ArOCH protons also exist (Fig. 3). Of the pair, Hb exhibits a more intense coupling to the
2
0
as a pair of sharp diastereotopic doublets. The tert-butyl groups aromatic Ha/Ha protons, consistent with the illustrated dihedral angle;
0
required 26 hours to come to equilibrium as one sharp signal and the Ha/Ha to Hc cross peak was less intense. NOSEY on the other
1
0
the H NMR spectra remained the same after 15 days.
hand displayed a through space correlation between Ha/Ha and Hc
The carboxylic acid groups play a crucial role in this reorganization – but not Hb. Hd and Hd have a significantly large chemical shift
H NMR titrations of Pb(II) with ester 2b exhibited a strong 1 : 1 difference for diastereotopic nuclei (J = 16 Hz, Dd = 1.37 ppm) and NOE
host: guest complex at 1 equivalent of Pb(II) with similar spectral cross peaks between Hd and one of the Ha/Ha nuclei were observed.
features as Fig. 1d – but no further changes were observed with A small opposite phase peak that could indicate chemical exchange
0
1
0
0
0
additional Pb(II) or time (see ESI†).
between d/d coexists within a larger NOE signal. No NOE cross peaks
0
0
Some similarities exist for 3ꢀPb(II) with the site exchange of two between Hd and neither Ha or Ha were observed, thus d and d are in
identical complexes adopting C3v symmetry of 2aꢀCs(I) complex at distinct chemical environments (ESI† contains full COSY and NOESY).
5
ꢁ
30 1C. However, in our case, the tert-butyl signal does not split into
High accuracy electrospray mass data of a 1 : 1 host : guest
two and the spectrum remained the same in the temperature range of complex at equilibrium gave a base peak at m/z 3054.2599 corres-
20 1C to 60 1C (ESI†) indicating that all the phenyl units are ponding to a dimeric structure [C156H189O Pb (dimer + H ) =
Ar 3054.2491]. The expected molecular ion was observed with a m/z
protons in our case are separated by 0.71 ppm, far smaller than the 1527.6349 [C H O Pb (host:guest + H ) = 1527.6285] with 40%
+
ꢁ
36
2
arranged identically with respect to the metal ion. The ArCH
2
+
78 95 18
12
separation of 1.15 ppm in C flattened cone conformation but close intensity of the base peak. Our initial thought was to explore the
3v
5
to the separation by 0.87 ppm for C symmetrical cone conformation.
possibility of the formation of a dimeric solution structure with time.
The H and 2D NMR evidences along with changes to simple
6
1
13
features such as the carboxylic acid resonance in conjunction with
MS data gave some support for this idea. Indeed similar NMR
features are observed in other H-bonded dimeric structure at
equilibrium including splitting patterns for a assemblies of
calix[4]arene urea hydrogen bond dimers
dimer formed by hydrogen bonded interactions of leucine amino
14,15
and a calix[6]arene
16
acid groups introduced at the lower rim.
We attempted to confirm this solution structure with DOSY NMR.
A sample of 3 in the presence of 0.5 equivalents Pb(II) was allowed to
come to equilibrium resulting in a 1 : 1 mixture of free 3 and the
3
ꢀPb(II) complex. The free ligand 3 had an average diffusion coefficient
1
Fig. 2 H NMR of 3 (5 mM) with 1 equivalents of Pb(ClO
)
4 2
ꢀ3H
CN = 1/1 at 300 MHz. (a) At the time of mixing and (b)–(e)
after 4, 12, 15, 24 hours respectively. Residual solvent peaks CHCl
7.26 ppm) and CH CN (1.73 ppm). The broad signal at around 2 ppm is
due to H O from Pb(ClO O.
ꢀ3H
2
O in
ꢁ10 2 ꢁ1
of 4.823 ꢂ 10 m s (average of 3 signals), while the signals from
3 3
CDCl /CD
ꢁ10
2
ꢁ1
the host–guest complex had an average of 4.716 ꢂ 10
m s
3
ꢁ
9
2
ꢁ1
(average of 5) (uncorrected CDCl
3
= 2.324 ꢂ 10 m s ) (see ESI†).
(
3
2
4
)
2
2
Based on the observed molecular weight differences these results were
1904 | Chem. Commun., 2014, 50, 1903--1905
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Remediation of heavy metals and radionuclides is one such
ongoing challenge – a highly selective host may provide new
opportunities to extract and/or transport those cations. In this
regard, 3 holds great potential for applications to the environmen-
90
tally malevolent Pb(II) as well as Sr(II) polluted nuclear wastes. The
events of conformational change with time and this newly observed
octahedral cage geometry for calix[6]arene add to an already rich
field of supramolecular science but may also hold promise for the
development of novel metal organic frameworks.
We gratefully acknowledge Dr Xiang Zhao for X-ray acquisition
and refinement and Prof. James D. Crowley for helpful comments.
Fig. 4 Rendered X-ray crystal structure of equilibrium host 3–Pb(II) guest
complex. Cropped top side views.
Notes and references
not consistent with a dimeric structure. Curious we reran the high
accuracy MS experiment at time of mixing and then again at
‡
Single-crystal X-ray analysis was performed on a Bruker Smart APEX II
CCD area diffractometer using MoKa radiation (l = 0.71073 Å), operating
24 hours and observed the same MS data at both time points – a
in the o and j scan mode over a range of 1.02 r y r 25.01, SADABS was
dimeric MS structure prevailed in both experiments and the used and the structure was solved by direct methods, all non-hydrogen
atoms were refined anisotropically and disordered solvent was resolved
molecular ion for the monomer was 40% the intensity of the base
peak (see ESI†). The observed dimer is likely an artifact of the
19
2
with SQUEEZE and ascribed to one H O per unit formula, SHELXTL 97
2
96
was used for final full-matrix refinements were against F . C78H O18Pb
3
ꢁ
1
measurement and unrelated to structure as we observed two (H O), 1546.65 g mol , cubic, 28.3035(7) Å, 90.001, 22673.6(17) Å ,
2
1
%
su max = 0.000, su mean = 0.000, 150 K, Pn3n, Z = 8, independent
reflections = 3367, R = 0.0424, wR = 0.1243 and S = 1.080.
distinctly different species by H NMR in solution at time of mixing
Fig. 2a) and after 24 hours only one (Fig. 2d).
(
Single crystal X-ray difraction data (Fig. 4)‡ resolved the incongruous
NMR and MS data – a newly observed octahedral cage of carboxylates
formed around Pb(II)! The aromatic rings of calix[6]arene 3 alternate up
and down with respect to the plane of the metal center – the carboxylate
oxygens poised perfectly at 2.725 Å to provide an octahedral coordina-
tion environment for Pb(II). The six ethers also point inward with
oxygen–Pb distances of 2.887 Å (see ESI†). Careful inspection of the
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5
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0
the aromatic Ha and Ha resonances become non-equivalent due to an
2
in vs. out twist in the solid state structure. The ArCH Ar are sharp and
6
7
8
S. Ahn, J. W. Lee and S.-K. Chang, J. Chem. Soc., Perkin Trans. 2,
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1996, 79.
0
ArOCH
2
along with NOSEY correlation between Hd and the aromatic
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0
protons Ha/Ha is now easily understood.
This newly observed geometry and behavior for calix[6]arene is the
result of an initially fast and strong binding event that gives rise to a low
symmetry structure; this slowly undergoes conformational switching to
17
9 B. B. Adhikari, M. Gurung, H. Kawakita and K. Ohto, Analyst, 2011,
36, 3758.
a very high symmetry and stable structure. Hexacoordinate Pb(II) has
an ionic radius of 133 pm and we found that hexacoordinate Sr(II) with
a radius of 132 pm undergoes the same initial binding event, but
coerces calix[6]arene into an octahedral cage rather slowly – the NMR
solution structure after 360 hours has all the same H NMR features that
1
1
0 C. D. Gutsche, B. Dhawan, K. W. No and R. Muthukrishnan, J. Am.
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1
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1
1 K. Shimojo, T. Oshima and M. Goto, Anal. Chim. Acta, 2004, 521, 163.
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000, 56, 3365.
2
data reveals an identical space group and unit cell (see ESI†). The larger
149 pm) hexacoordinate Ba(II) again binds strongly to 3, but undergoes
1
3 While initially broad and weak, the signal due to carboxyl proton
becomes prominent over the time course.
(
conformational reorganization even slower than the 3ꢀSr(II) complex. 14 K. D. Shimizu and J. Rebek Jr., Proc. Natl. Acad. Sci. U. S. A., 1995,
9
2, 12403; B. C. Hamann, K. D. Shimizu and J. Rebek Jr., Angew.
After 360 hours there exists a 1 : 1 mixture of the initially formed low
symmetry complex and the high symmetry cage structure (see ESI†).
These effects are likely a combination of both size of the cation
Chem., Int. Ed. Engl., 1996, 35, 1326.
1
5 O. Mogck, M. Pons, V. B o¨ hmer and W. Vogt, J. Am. Chem. Soc., 1997,
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6 A. M. Rinc ´o n, P. Prados and J. de Mendoza, Eur. J. Org. Chem.,
2002, 640.
18
and their affinity for carboxylates. No evidences of strong
binding of 3 with Ca(II), Cu(II), Bi(III), Y(III), Gd(III) and UO (II) in
1
2
CDCl
3
/CD
3
CN exists at this time (see ESI†). Work with these 17 The resulted octahedral cage complex survives polar protic solvent
such as methanol for 48 hours and then disintegrates very slowly.
cations in other solvent systems is ongoing.
1
8 D. L. Melton, D. G. VanDerveer and R. D. Hancock, Inorg. Chem.,
The application of calixarenes to separation and extraction
2006, 45, 9306.
sciences can be used to address environmental challenges. 19 P. van der Sluis and A. L. Spek, Acta Crystallogr., Sect. A, 1990, 46, 194.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 1903--1905 | 1905