Pb, Sr and Ba calix[6]arene hexacarboxylic acid octahedral complexation: A dramatic effect of dealkylation

Article abstract of DOI:10.1080/10610278.2015.1078898

Calix[6]arene hexacarboxylic acid binds instantly and with low symmetry to Pb, Sr and Ba. Later a highly symmetric up-down alternating conformation emerges. The solution structures are identical to their p-tert-butylcalix[6]arene hexacarboxylic acid counterparts. With either receptor, an octahedral cage is formed around the metal. The transformation from low to high symmetry however proceeds at significantly faster rates for the de-t-butylated host.

Full text of DOI:10.1080/10610278.2015.1078898

Supramolecular Chemistry
ISSN: 1061-0278 (Print) 1029-0478 (Online) Journal homepage: http://www.tandfonline.com/loi/gsch20
Pb, Sr and Ba calix[6]arene hexacarboxylic acid
octahedral complexation: a dramatic effect of
dealkylation
Birendra Babu Adhikari, Cuong-Alexander To, Tetsuo Iwasawa & Michael P.
Schramm
To cite this article: Birendra Babu Adhikari, Cuong-Alexander To, Tetsuo Iwasawa & Michael P.
Schramm (2015) Pb, Sr and Ba calix[6]arene hexacarboxylic acid octahedral complexation: a
dramatic effect of dealkylation, Supramolecular Chemistry, 27:10, 724-730
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Date: 09 November 2015, At: 09:23

Supramolecular Chemistry, 2015
Vol. 27, No. 10, 724–730, http://dx.doi.org/10.1080/10610278.2015.1078898
Pb, Sr and Ba calix[6]arene hexacarboxylic acid octahedral complexation: a dramatic effect of
dealkylation
Birendra Babu Adhikaria, Cuong-Alexander Toa, Tetsuo Iwasawab and Michael P. Schramma*
aDepartment of Chemistry and Biochemistry, California State University Long Beach, Long Beach, CA, USA; bDepartment of Materials
Chemistry, Faculty of Science and Technology, Ryukoku University, Otsu, Japan
(Received 27 December 2014; accepted 27 July 2015)
Calix[6]arene hexacarboxylic acid binds instantly and with low symmetry to Pb, Sr and Ba. Later a highly symmetric up-
down alternating conformation emerges. The solution structures are identical to their p-tert-butylcalix[6]arene
hexacarboxylic acid counterparts. With either receptor, an octahedral cage is formed around the metal. The transformation
from low to high symmetry however proceeds at significantly faster rates for the de-t-butylated host.
Keywords: word; another word; lower case except names
Introduction
was replaced with ether, di-ether or ester, resulted in initial
low-symmetry binding, but only those hosts with acid
functional groups resulted in an octahedral cage for the
metal. This behaviour was attenuated when amine bases
were added, presumably due to ion capping (21). The use
of other bases to date has not had a marked impact on the
behaviour of these host–guest complexes, although our
studies have not been exhaustive. A variety of other metals
were tested and Pb, Sr and Ba are a short list of metals that
this host is selective for accommodation in an octahedral
environment.
t-Butyl calix[6]arene hexaesters and hexaamides have
enhanced selectivity towards the extraction and transport
of a variety of ions including Cs(I) and Sr(II) (1–5). Pb(II)
is effectively transported by the hexaester (6,7). t-Butyl
calix[6]arene hexathioamides (8) have been shown to be
efficient towards Cu(II) (9), Pb(II) (7) and Ag(I) (10).
Variation in selectivity changed when several of these
compounds lacked p-tert-butyl groups – and in many
cases binding affinity was drastically enhanced (11,12).
Calix[n]arene phosphine oxides (13) have also demon-
strated enhanced binding affinity for Th(IV) (14). Indeed,
work with calix[6]arenes towards nuclear waste separation
remains an ongoing area of active research (15).
The remaining variable of interest to us was the effect
of the t-butyl group (22). Is it possible that the t-butyl
group has an influence on either binding or the rate of
conversion from a low- to high-symmetry complex for
these three metals?
Very recently, we reported on the complexation of Pb
(II), Sr(II) and Ba(II) with p-tert-butylcalix[6]arene
hexacarboxylic acid 1 (Figure 1) (16,17). In these reports,
we found that calixarene hexacarboxylic acid binds
rapidly with all three metals and form low-symmetry
‘lozenge’ shaped complexes (18). These exhibit complex
NMR signals with multiple types of methylene bridges for
both the ether as well as the aryl linkages. Upon standing
something remarkable occurred – all three metal–host
complexes rearrange such that the individual phenyl rings
alternate up-down so that the carboxylates form an
octahedral cage with C3V symmetry around the metal
centre. NMR and X-ray measurements for all three
complexes confirm the solution and solid-state structure of
the host. The structures we reported have some semblance
to valinomycinzK (19) or Cram’s sphereands (20). With
these metals – differences in time to rearrange were
apparent. We demonstrated that several analogues of
calixarene hexacarboxylic acid, such as those where acid
Results and discussion
Calix[6]arene hexacarboxylic acid 2 is a known compound
(21). We prepared 2 through removal of tert-butyl group of
p-tert-butylcalix[6]arene by adopting a procedure reported
for p-tert-butylcalix[4]arene (23). Alkylation and sappo-
nification as for 1 (16) gave 2 (Figure 2).
When 2 was mixed with 1 equivalent of Pb(ClO4)2-
z3H2O in CDCl3/CD3CN immediate formation of one
major species was obvious (Figure 3), but some very small
peaks that can be attributed to low-symmetry ‘lozenge’
shaped species co-exist. After 30 min however the sample
appears to be at equilibrium with a high-symmetry
geometry that is identical in pattern to the analogous p-
tert-butyl host (Figure 4) (16). The complex remains
soluble in a mixture chloroform/acetonitrile and is highly
*Corresponding author. Email: michael.schramm@csulb.edu
q 2015 Taylor & Francis

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725
Figure 1. (Colour online) p-tert-Butyl calix[6]arene hexacarboxylic acid 1; its low-symmetry ‘lozenge’ solution structure with Pb, Sr
and Ba; and a truncated solution (and solid state) structure after adopting a high-symmetry C3V conformation (based on NMR and 3
X-ray structures).
likely to be neutral. The exact location of all protons isn’t
known, but we surmise that 2 negative charges and
4 protons are distributed amongst the 6 carboxylates and
are interchanging – indeed the symmetry of the solution-
state structure supports this assumption as well as exact
mass measurements (see ESI).The two sets of diaster-
otopic methylenes are split drastically. For the bridging
methylene, this isn’t surprising; in a cone conformation
calix[n]arenes place one H ‘down’ and the other ‘out’.
In our alternating up-down conformation, this relationship
remains. The second methylene group, a phenolic ether
also undergoes drastic splitting; for 1zPb (16) this result
was explained with NOESY and X-ray data, one hydrogen
points towards an adjacent aromatic proton, the other
Figure 2. Preparation of calix[6]arene hexacarboxylic acid 2.
Figure 3. (Colour online) H-NMR (400 MHz) of 2.5 mM2 in presenceof 1 eqPb(ClO4)2z3H2O in CDCl3/CD3CN (1:1). From bottom
to top: ligand 2(a), at the time of mixing with Pb (b), after 30 min (c), after 1 h (d), and 4 h (e), respectively. The NMR signals are
with reference to residual CH3CN signal at 1.94 ppm. Red arrows show faint traces of low-symmetry ‘lozenge’ conformation.

726
B.B. Adhikari et al.
3
Figure 4. (Colour online) Conformational reorganisation of 2 when mixed with Pb(II) with rendered model (MMFF) based on NMR
1zPb, some protons omitted for clarity.
data comparison to
away. The same is now true for 2zPb(II) (see ESI for
COSY, NOESY, HSQC spectra). In the instance of 2,
however, the transformation is complete in 30 min;
previously with the more hindered host 1, the transform-
ation takes approximately 15 h.
baseline resolution is achieved and we ascribe this to be an
approximate time to arrive at equilibrium. A small peak at
7.0 ppm remains that is part of the free ligand 2.
Finally, we conducted the same experiment with Ba
(ClO4)2z3H2O. Many sharp signals exist for 2zBa at the
time of mixing, consistent with a low-symmetry lozenge
(Figure 6). At 48 h after mixing, the sample is at
equilibrium. This last result is most surprising because the
high-symmetry 1zBa sample took 38 days of time to come
to equilibrium, both Pb and Sr were faster with host 1.
These results are summarised in Table 1 and we see that
the t-butyl group of 1 has a marked effect on the time it
takes complex 1 vs. 2 to arrive at equilibrium. In all cases,
equilibrium is arrived at an order of magnitude sooner.
Surprisingly though, the behaviour is not consistent – Ba
Given the drastic reduction in time for the formation of
the octahedral complex of 2zPb (1 h) vs. 1zPb (15 h), we
anticipated that the rearrangement of 2zSr would also
proceed more quickly than its t-butyl analogue 1zSr. 1zSr
took approximately 20 days to come to equilibrium. Sr
(ClO4)2z3H2O was again slow to entice host 2 into
octahedral geometry, upon mixing several low-concen-
tration species are present (Figure 5) and several major
broad peaks emerge. From 48 h to 72 h (latter not shown),
the signals remain poorly resolved and broad, but at 115 h
Figure 5. (Colour online) H-NMR (400 MHz) of 2.5 mM2 in presence of 1 eqSr(ClO4)2z3H2O in CDCl3/CD3CN (1:1). From bottom to
top: at the time of mixing with Sr (a), after 8 h (b), after 30 h (c), after 48 h (d), and after 115 h (e), respectively. The NMR signals
are with reference to residual CH3CN signal at 1.94 ppm. Red arrows show evidence of low-symmetry ‘lozenge’ conformation.

4
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727
Figure 6.
H-NMR (400 MHz) of 2.5 mM2 in presence of 1 eqBa(ClO4)2z3H2O in CDCl3/CD3CN (1:1). From bottom to
(Colour online)
top: at the time of mixing (a), and after 8 h (b), 20 h (c), 30 h (d), and 48 h (e), respectively.The NMRsignals arewith reference to residual
CH3CN signal at 1.94 ppm. Red arrows show some of the many peaks of low-symmetry ‘lozenge’ conformation.
and Sr switch their ranking as a function of host, always
slower than Pb. It remains unclear why Ba and Sr don’t
have the same relative order of time to reach equilibrium.
We added 10% v/v of D2O to test the stability of the
complex and found that they rapidly degraded. First
observations of NMR show a series of broadened peaks
that promptly disappear (see ESI). No evidence of NMR
signals could be found when 2 was directly mixed with Pb,
Sr or Ba in pure water.
mixing, the 2zPb complex is easily identified as the
predominant complex. Traces of 1zPb and the low-symmetry
complex of 1zPb can be identified. At the 16 h mark, the two
high-symmetry complexes are present and the t-butyl 1zPb
host is the major component. Surprisingly after 115 h
evidence of a roughly 1:1 mixture of 1zPb:2zPb was apparent.
Complex 2zPb more quickly converts from low to high
symmetry; the dynamics of the system remain under
investigation. Perhaps solubility differences are at work
after more than 16 h of resting while the system reverts to 1:1.
Next, we performed a competitive binding experiment
between host 2, Pb and Ba. Mixing the three components
in a 1:1:1 molar ratio, we see rapid formation of the high-
symmetry Pb complex after 4 h – the Pb complex is
readily identified. After 115 h, the Ba complex surpasses
the concentration of Pb (Figure 8).
Given the drastic change in the time it takes host 2 to
rearrange, we conducted an initial screening programme to
uncover other metals that might be accommodated in this
octahedral hole. As was the case with 1, 2 fails to show
evidence of complex formation with Ca(II), Y(III), Bi(III)
or Gd(III) (see ESI for details). While new complexes
weren’t found, it is clear that 2 remains a selective host.
Given the time differences to arrive at equilibrium, we
conducted a competitive binding experiment using a 1:1:1
ratio of 1, 2 and Pb(II) (Figure 7). At the initial time of
Host 1 and 2 now both serve as calix[6]arene hosts that
display a remarkable conformational change when binding
with Pb, Sr and Ba. In all combinations – initial low-
symmetry species are identified by NMR and given
enough time high-symmetry octahedral complexes are
formed. These complexes display a unique alternating
up-down arrangement of calix[6]arene phenyl groups so as
to surround the metal centre with 6 carboxylate groups.
We have clearly demonstrated the effect of removing the
t-butyl groups of 1; by using 2 the time to reorganisation
decreases by an order of magnitude (from weeks to days,
or 15 h to 30 min). The relative rates to equilibrium
however change with differential behaviour for Sr and Ba
Table 1. Times to arrive at high-symmetry octahedral
coordination for hosts 1 and 2 with Pb, Sr and Ba. All species
at 2.5 mM in CDCl3/CD3CN (1:1).
Host 1
Host 2
Pb(II)
Sr(II)
Ba(II)
15 h
20 days
38 days
30 min
115 h
48 h

728
B.B. Adhikari et al.
5
1
Figure 7. (Colour online) H-NMR (400 MHz, in CDCl3/CD3CN). Expanded stack plot from bottom to top:1zPb (a), 2zPb (b), an initially
mixed sample of 5 mM of 1 þ 2 þ Pb (1:1:1) (c), the mixed sample after 16 h (d), and 115 h (e).
1H-NMR (400 MHz, in CDCl3/CD3CN). Expanded stack plot from bottom to top: (a) 2zPb, (b) 2zBa,
(Colour online)
Figure 8.
(c) a sample of 2 þ Pb þ Ba (1:1:1) after 4 h, and (d) after115 h.
with 1 and 2. Additionally, initial competitive binding
experiments reveal some new details about stability and
dynamics. Other combinations are currently being
explored to thoroughly evaluate the competitive binding
affinity and kinetics of Pb, Sr and Ba for both hosts 1 and 2.
At the end of these studies, we hope to develop materials
suitable for deployment towards the separation of Pb or Sr
from complex waste streams.
Acknowledgements
We thank Japan Society of the Promotion of Science for an
Invitational Fellowship (long-term, L-15528, M.P.S.).
Disclosure statement
No potential conflict of interest was reported by the author(s).
Funding
The project was supported the National Cancer Institute [grant
number SC2CA167636]. NMR instrumentation was provided for
by the National Science Foundation (MRI CHE-1337559).
The content is solely the responsibility of the authors and does
not necessarily represent the views of the Japan Society for the
Experimental
All spectra described above are provided in the ESI along
with complete characterisation of all new compounds,
some experimental procedures are cited above.

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