A convenient synthetic route to partial- cone p-Carboxylatocalix[4]arenes

Article abstract of DOI:10.2533/chimia.2015.516

p-Carboxylatocalix[n]arenes have emerged as useful building blocks for the construction of a diverse range of supramolecular assemblies. A convenient route to a p-carboxylatocalix[4]arene that is locked in a partial-cone conformation is presented. The conformation gives the molecule markedly different topological directionality relative to those previously used in self- And metal-directed assembly studies.

Full text of DOI:10.2533/chimia.2015.516

516 CHIMIA 2015, 69, No. 9
doi:10.2533/chimia.2015.516
Supramolecular chemiStry
Chimia 69 (2015) 516–519 © Schweizerische Chemische Gesellschaft
A Convenient Synthetic Route to Partial-
Cone p-Carboxylatocalix[4]arenes
Robyn E. Fairbairna, Simon J. Teatb, Kevin J. Gagnonb, and Scott J. Dalgarnoa*
Abstract: p-Carboxylatocalix[n]arenes have emerged as useful building blocks for the construction of a diverse
range of supramolecular assemblies. A convenient route to a p-carboxylatocalix[4]arene that is locked in a
partial-cone conformation is presented. The conformation gives the molecule markedly different topological
directionality relative to those previously used in self- and metal-directed assembly studies.
Keywords: p-Carboxylatocalix[4]arene · Supramolecular assemblies
The calix[4]arene framework (C[4],
non-covalent nanotubes and bi-layers,[5]
Given our interest in p-CO2[4]-based
Fig. 1A) represents one of the most widely and b) the metal-directed assembly of co- self- and metal-directed assembly we re-
modifiablesyntheticplatformsin supramo- ordination polymers and nanometre-scale cently began to investigate the introduction
lecular chemistry.[1] In this regard signifi- capsules.[6] The positioning and number of
of a variety of groups to the lower-rim of
cant effort has been invested in controlling CO2H groups around the upper-rim of the the general p-CO2[4] framework. As many
the conformation of these molecules for a C[4] framework is important in controlling of the groups we wished to introduce
variety of reasons.[2] A good example of assembly propertiesand, although they ap- would not survive the conditions required
this can be seen when considering OH (or pear simple from a synthetic perspective, for C[4] upper-rim functionalisation, it was
pairs of distal OH/alkoxy) groups at the isolation of these molecules can be chal- necessary to find an alternative synthetic
lower-rim, with the molecules adopting a lenging at times. As shown in Scheme 1, strategy to these new building blocks. As a
cone conformation as shown in Fig. 1A. p-carboxylatocalix[4]arene (1) is accessed starting point we explored esterification of
In this conformation, C[4]s with varying through an upper-rim two-step formylation compound 1 prior to lower-rim modifica-
upper-rim functionality (e.g. sulfonate,[3] via a bromomethyl intermediate, followed tion, the results of which are presented in
phosphonate[4] andcarboxylate[5,6])possess by oxidation.[7,8] Calix[4]arene lower-rim this contribution.
molecular clefts that are perfectly suited to tetra-O-alkylation (with alkyl chains great-
Esterification of compound 1 was car-
act as hosts towards a wide range of both er or equal in length to propyl) is known to ried out by acid catalysis in ethanol to af-
organic and inorganic guests. Changes in lock the C[4] framework in a pinched-cone ford p-carboxyethylcalix[4]arene (3) as a
lower-rim functionalisation, for example conformation, preventing rotation through brown solid in 72% yield (Scheme 2).[10]
1
tetra-O-alkylation in the presence of dif- the annulus. Compound 2 (Scheme 1) is Analysis by H NMR spectroscopy con-
ferent bases, provide access to the entire accessed via lower-rim tetra-O-alkylation, firmed that 3 was in a cone conformation
range of C[4] conformations shown in upper-rim formylation and subsequent ox- as expected, which is driven by the pres-
Fig. 1. Through synthetic control one can
idation to afford the corresponding carbox- ence of lower-rim OH groups, all of which
thus a) control the topological directional- ylic acid.[8]
hydrogen bond in concert to stabilise the
ity of this platform and b) tune the nature
of any resulting molecular cleft by careful
selection of the group to be introduced at
the C[4] lower-rim.
p-Carboxylatocalix[4]arenes(pCO [4]s)
have emerged as versatile building b2locks
for a) the template-directed assembly of
Fig. 1. A) calix[4]arene (C[4]) in the cone conformation. B) Lower-rim altered C[4]s in the other
three general conformations found for this molecular framework (R = alkyl).
*Correspondence: Dr. S. J. Dalgarnoa
E-mail: S.J.Dalgarno@hw.ac.uk
aInstitute of Chemical Sciences
Heriot-Watt University
Scheme 1. Synthetic routes to p-carboxylatocalix[4]arene (1) and the related tetra-O-propyl ana-
logue (2). Conditions as follows: i) paraformaldehyde / HBr / acetic acid; ii) HMTA / glacial acetic
acid; iii) NaClO2 / NaH PO4 / dmso; iv) NaH / iodopropane / dmf; v) HMTA / trifluoroacetic acid; vi)
Riccarton, Edinburgh, UK.
bAdvanced Light Source
Lawrence Berkeley National Laboratory Berkeley
CA 94720, USA
2 [7–9]
NaClO2 / sulfamic acid.

Supramolecular chemiStry
CHIMIA 2015, 69, No. 9 517
arrangement. Compound 3 was recrystal-
lised from a number of solvents in order
to obtain supporting structural data. Slow
evaporation of a pyridine solution of 3 af-
forded colourless single crystals that were
suitable for diffraction studies.
Structure analysis showed the crystals
to be in a monoclinic cell and structure
solution was performed in the space group
Scheme 2. Synthetic routes to p-carboxyethylcalix[4]arene (3) and related partial-cone tet-
ra-O-propyl derivatives 4 and 5. Conditions as follows: vii) TsOH / EtOH; viii) NaH / iodopropane /
P21/n. The asymmetric unit revealed for-
mation of a pyridinium complex as shown
in Fig. 2, with corresponding formula of
[10]
dmf; ix) NaOH / THF / MeOH / H2O.
occurs through two crystallographically
[PyH][3-H]. Protonation of the pyridine as a yellow solid in 24% yield. Attempts
occurs with simultaneous deprotonation of were made to recrystallise 5 from a number
the lower-rim O(1) hydroxyl group in 3. of different solvents, and slow evaporation
unique OCOH…OCOH hydrogen bonding
interactions with respective H[8O]…O[12]
and H[11O]…O[7] distances of 1.820 and
The pyridinium cation is found to reside
1.805 Å. The second dmf is disordered
over two positions and was modelled in
each at 50% occupancy. This also forms
in the C[4] cavity, which is stabilised by
concerted hydrogen bonding interactions;
this occurs with three OH...O distances ly-
hydrogen bonding interactions with the
ing in the range of 1.667–1.994 Å. There
are two CH…π interactions observed be-
CO2H group of the splayed aromatic ring
with O[13]…H[5O] and O[6]…H[45] dis-
tances of 1.787 and 3.230 Å respectively.
tween the ortho and para H atoms on the
[PyH]+ guest and the aromatic rings of [3-
H]–, and these occur with H[43]…centroid
Examination of the extended structure
and H[45]…centroid distances of 2.901 and
reveals that molecules/dimers pack in an
anti-parallel bi-layer array as is often ob-
2.494 Å respectively (Fig. 2). In addition
to this, the pyridinium cation forms one
served for such systems (Fig. 5C).[5]
Fig. 2. Selectively labelled asymmetric unit in
the single crystal X-ray structure of [PyH][3-H].
Hydrogen bonding interactions are shown as
split-colour dashed lines and CH...π interac-
tions are shown as red dashed lines. Hydrogen
atoms (apart from those involved in hydrogen
bonding and CH...π interactions) omitted for
clarity.
NH…OCOH and one CH…OCOH hydro-
To conclude, we have presented the
synthesis of an unexpected partial-cone
pCO2[4] and have confirmed structural
gen bonding interaction with symmetry
equivalent (s.e.) [3-H]– anions to build up
an extended H-bonded array; this occurs
changes throughout the synthetic pathway
with respective H[1N]…O[12] s.e. and
via single crystal X-ray studies. Through
H[42]…O[8] s.e. distances of 1.864 and
comparison with the synthetic route for
2.307 Å (Fig. 3).
Following successful esterification
of 1 (to give 3) we attempted lower-rim
alkylation under analogous conditions
to those used to alkylate C[4] in the syn-
Fig. 3. Extended
structure in [PyH]
[3-H] showing
H-bonding and
thetic pathway to compound 2 (Scheme
...
π π interactions as
1).[8] Reaction of 3 with iodopropane in
split-colour and red-
dashed lines respec-
tively. Hydrogen at-
oms (apart from those
involved in hydrogen
the presence of NaH as a base in dmf, fol-
lowed by recrystallisation from acetone,
afforded colourless single crystals in 68%
yield. It was immediately obvious from 1H
...
bonding and π π in-
NMR spectroscopy that the product was
not in a cone conformation as desired,
but rather that it had reacted in an alterna-
teractions) have been
omitted for clarity.
tive manner and become locked in a par-
tial-cone arrangement. Structural analysis
confirming this found single crystals of of a dmf solution did afford colourless
tetra-O-propoxy-p-carboxyethylcalix[4] single crystals that were suitable for dif-
arene (4) to be in an orthorhombic cell fraction studies. Structural analysis found
and structure solution was performed in the crystals to be in a monoclinic cell and
the space group Pnma. The asymmetric
structure solution was performed in space
unit comprises one half of the molecule group C2/c. The asymmetric unit compris-
and symmetry expansion reveals the par- es one molecule of 5 and two dmf of crys-
tial-cone conformation in 4 as shown in tallisation as shown in Fig. 5A. One dmf
Fig. 4. Following this observation we at- forms hydrogen bonding interactions with
tempted to isolate a sodium complex of 3 the CO2H group of the rotated aromatic
but were unsuccessful. We can only there- ring with O[14]…H[10O] and O[9]…H[48]
fore conclude that there is an influencing distances of 1.777 and 2.423 Å respec-
interaction between the sodium ion and the tively. Symmetry expansion results in the
upper-rim CO2Et group rotated out of the formation of a hydrogen-bonded head-
cone prior to O-alkylation.
to-head dimer, a motif seen for other
With formation of partial-cone 4 con- pCO2[4]s[5] in the absence of a solvent/spe-
Fig. 4. Symmetry expanded single crystal X-ray
crystal structure of compound 4 in the par-
tial-cone conformation.
firmed, subsequent saponification afforded cies capable of interrupting the common
the carboxylic acid derivative 5 (Scheme 2) CO2H…CO2H homosynthon (Fig. 5B); this

518 CHIMIA 2015, 69, No. 9
Supramolecular chemiStry
(q, J = 7.0 Hz, 2H, OCH CH3), 3.15 (d,
J = 12.0 Hz, 2H, ArCH2A2r), 2.12 (q, J =
7.0 Hz, 2H, OCH CH CH ), 1.93 (q, J =
6.8 Hz, 2H, OCH22CH22CH33), 1.41 (m, 5H,
overlap of OCH CH and OCH CH CH ),
1.29 (t, J = 7.42Hz,3 6H, OCH22CH22CH33),
1.13 (m, 10H, overlap of 3 × OCH CH2CH
and OCH CH ), 0.64 (t, J = 7.42 Hz, 3H3,
OCH2CH22CH33). 13C NMR (300 MHz, 25
°C, CDCl3): δ = 166.78, 166.65, 166.07,
161.56, 160.90, 159.47, 136.34, 133.77,
133.32, 132.37, 131.47, 131.30, 130.91,
130.25, 124.82, 123.97, 123.92, 76.42,
75.72, 74.84, 60.78, 60.49, 60.21, 35.73,
30.93, 30.37, 23.71, 21.86, 14.45, 14.38,
14.20, 10.90, 10.73, 9.05. MS m/z ob-
served 898.4740, theoretical 898.4736 [M
+ NH4]+. IR (solid phase, ν cm–1) = 3663w,
2969m, 1714s, 1600m, 1283s, 1187s.
Crystal data for 4 (CCDC 1404297):
C52H64O12, M = 881.03, Colourless block,
0.40 × 0.25 × 0.20 mm3, orthorhombic,
space group Pnma, a = 17.0322(13), b
= 16.7264(11), c = 17.3355(13) Å, V =
4938.7(6) Å3, Z = 4, Bruker X8 Apex II
CCD diffractometer, MoKα radiation, λ =
0.71073 Å, T = 100(2)K, 2θmax = 49.5°,
26905 reflections collected, 4394 unique
(Rint = 0.0818). Final GooF = 1.630, R =
0.1277, wR2 = 0.2703, R indices based1on
Fig. 5. A) Selectively labelled asymmetric unit found in the single crystal X-ray structure of 5. B)
Symmetry expanded structure showing a hydrogen-bonded head-to-head dimer. C) Extended
structure showing the bilayer array. Hydrogen bonding interactions are shown as split-colour
lines. Hydrogen atoms (apart from those involved in hydrogen bonding) have been omitted for
clarity. DMF of crystallisation omitted for clarity in C.
124.89, 60.88, 31.42, 14.37. MS m/z ob-
served 730.2859, theoretical 730.2858
[M + NH ]+. IR (solid phase, ν cm–1) =
3192m, 24978m, 2928m, 2360w, 1711s,
1607m, 1307s, 1281s, 1187s. Compound
3 was crystallised by slow evaporation
from pyridine. Crystal data for [PyH][3-
H] (CCDC 1404296): C45H N1O12, M =
791.82, Colourless lath, 0.194×5 0.10 × 0.02
mm3, monoclinic, space group P2 /n, a =
cone-shaped tetra-O-propyl-p-carboxy-
latocalix[4]arene, it is clear that the pres-
ence of upper-rim ester functionality has
a dramatic effect on the prevailing confor-
mation. Future work will focus on the use
of partial-cone pCO2[4]s in the formation
of both non-covalent and metal-directed
assemblies with a view to exploring the
effects of this unusual conformation cou-
pled with topological directionality. These
3429 reflections with I >2sigma(I) (refine-
1
9.3836(3), b = 20.4700(7), c = 20.3370(7)
results will be reported in due course.
ment on F2).
Å, β = 98.885(2)°, V = 3859.5(2) Å3, Z = 4,
Bruker D8 diffractometer operating with a
Synthesis of Partial-cone Tetra-
PHOTON 100 detector, synchrotron radi-
Experimental
O-propoxy-p-carboxylatocalix[4]
ation, λ = 0.77490 Å, T = 100(2)K, 2θmax
= 62.3°, 58367 reflections collected, 9613
unique (Rint = 0.0490). Final GooF = 1.101,
R1 = 0.0618, wR = 0.1546, R indices based
on 7285 reflecti2ons with I >2sigma(I) (re-
finement on F2).
arene, 5
Compound 4 (0.31 g, 0.35 mmol)
and NaOH (0.17 g, 4.18 mmol) were dis-
p-Carboxylatocalix[4]arene (1) was
synthesised according to literature proce-
dures.[7] CCDC 1404296 – 1404298 con-
tain the supplementary crystallographic
solved in a mixture of THF, MeOH and
water (2:1:1, 5 mL) and the mixture was
data for this paper. These data can be ob- heated at 50°C for 3 days. The solution
tained free of charge from The Cambridge
was acidified with 1M HCl, diluted with
ethyl acetate and washed with water (2 ×
50 mL) followed by brine (2 × 50 mL).
Synthesis of Partial-cone Tetra-
O-propoxy-p-carboxyethylcalix[4]
arene, 4
Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
The solution was dried over MgSO4 and
A solution of 3 (1.00 g, 1.40 mmol) dis-
solved in 50 mL DMF was placed in a 250
mL round-bottom flask. Sodium hydride
(60%, 0.48 g, 11.90 mmol) was added
Synthesis of p-Carboxyethyl-
25,26,27,28-tetrahydroxycalix[4]
arene, 3
A suspension of 1 (4.00 g, 6.66 mmol)
and p-toluenesulfonic acid (12.70 g, 0.07
mol) in ethanol (250 mL) was added to a
500 mL round-bottomed flask and the mix-
the solvents evaporated to afford pure 5 as
1
a yellow solid (63.0 mg, 24%). H NMR
(300 MHz, 25°C, DMSO-d6): δ = 12.23
(bs, 4H, COOH), 7.92 (s, 2H, ArH), 7.87
and the mixture allowed to stir at RT
for
30 minutes. After this time 1-iodopropane
(1.16 mL, 0.01 mol) was added and the
(s, 2H, ArH), 7.59 (s, 2H, ArH), 6.96 (s,
2H, ArH), 4.01 (m, 2H, OCH2CH2CH3),
3.94 (d, J = 12.0 Hz, 2H, ArCH2Ar), 3.74
solution was heated at 80°C for 1 h. The
ture was heated at reflux for one week. The (m, 10H, overlap of OCH2CH2CH3 and
reaction was quenched by careful addition
of MeOH and the solvents removed under
reduced pressure to afford a yellow solid.
solvent was removed under reduced pres-
sure, and the residue dissolved in DCM
(300 mL) and washed twice with saturated
NaHCO3 (150 mL) and water (150 mL).
ArCH2Ar), 3.42 (m, 2H, OCH2CH2CH3),
3.20 (d, J = 12.0 Hz, 2H, ArCH Ar), 1.91
(m, 2H, OCH2CH2CH3), 1.742 (m, 4H,
The crude product was washed with wa-
OCH CH CH ), 1.08 (t, J = 7.4 Hz, 6H,
The organic layer was dried over MgSO4
OCH2CH2CH3), 0.92 (t, J = 7.2 Hz, 3H,
ter and filtered before being recrystallised
and the solvent removed under reduced
pressure to afford 3 as a brown solid
(3.44 g, 72%). 1H NMR (300 MHz, 25°C,
CDCl3): δ = 10.17 (s, 4H, OH), 7.85 (s,
8H, ArH), 4.34 (q, 8H, OCH CH3), 4.26
(broad s, 4H, ArCH Ar), 3.752 (broad s,
4H, ArCH2Ar), 1.38 2(t, J = 4.2 Hz, 12H,
OCH2CH ). 13C NMR (300 MHz, 25°C,
CDCl3):δ3=165.70,152.57,131.16,127.48,
OCH2CH2CH3), 0.55 (t, J = 7.2 Hz, 3H,
OCH22CH22CH33). 13C NMR (300 MHz, 25
°C, DMSO-d6): δ = 167.46, 166.78, 160.99,
159.09, 136.17, 133.27, 132.81, 132.52,
131.60, 131.16, 130.43, 23.04, 22.88,
10.67, 10.22, 8.90. IR (solid phase, ν cm–1)
= 2961m, 2931m, 2875m, 1711s, 1676s,
1599m, 1424m, 1308s, 1283s, 1186s.
Compound5wascrystallisedbyslowevap-
from hot acetone to afford colourless sin-
1
gle crystals (0.84 g, 68%). H NMR (300
MHz, 25 °C, CDCl3): δ = 8.0 (s, 2H, ArH),
7.88 (s, 2H, ArH), 7.67 (s, 2H, ArH), 7.01
(s, 2H, ArH), 4.42 (m, 4H, OCH2CH3),
4.23 (m, 6H, OCH2CH3), 4.08 (d, J = 12.0
Hz, 2H,ArCH Ar), 3.81 (q, J = 6.8 Hz, 4H,
OCH2CH2CH32), 3.75 (s, 4H, ArCH Ar),
3.59 (q, J = 6.8 Hz, 2H, OCH2CH3),23.24

Supramolecular chemiStry
CHIMIA 2015, 69, No. 9 519
c) S. Kennedy, I. E. Dodgson, C. M. Beavers,
S. J. Teat, S. J. Dalgarno, Cryst. Growth Des.
2012, 12, 688; d) S. Kennedy, C. M. Beavers,
S. J. Teat, S. J. Dalgarno, New J. Chem. 2011,
oration of a dmf solution. MS m/z observed
769.8901, theoretical 769.8896 [M + H]+.
Crystal data for 5·2dmf (CCDC 1404298):
C50H62N2O14, M = 915.02, Colourless
plate, 0.17 × 0.13 × 0.02 mm3, monoclin-
ic, space group C2/c, a = 37.3004(15),
b = 14.2873(6), c = 18.2486(8) Å, β =
93.557(2)°, V = 9706.3(7) Å3, Z = 8,
Bruker D8 Diffractometer operating with
[1] For calix[4]arene modification see: I. Thondorf,
A. Shivanyuk, V. Böhmer, ‘Calixarenes’, Chap.
2, Kluwer Academic Publishers, Dordrecht,
2001.
35, 28; e) S. Kennedy, S. J. Dalgarno, Chem.
[2] For calixarene conformations see: I. Thondorf,
Commun. 2009, 5275.
‘Conformations and Stereodynamics’, in
‘Calixarenes’, Chap. 15, Kluwer Academic
Publishers, Dordrecht, 2001.
[6] For metal-organic pCO2[4]-based assemblies
see: a) P. P. Cholewa, S. J. Dalgarno,
CrystEngComm 2014, 16, 3655; b) P. P.
[3] a) I. Ling, A. N. Sobolev, C. L. Raston,
Cholewa, C. M. Beavers, S. J. Teat, S. J.
Dalgarno, Cryst. Growth Des. 2013, 13, 5165;
CrystEngComm 2015, 17, 1526; b) A. Husain,
C. L. Oliver, CrystEngComm 2014, 16, 3749;
c) I. Ling, Y. Alias, A. N. Sobolev, C. L. Raston,
Cryst. Growth Des. 2009, 9, 4497; d) S. J.
c) P. P. Cholewa, C. M. Beavers, S. J. Teat,
a PHOTON 100 detector, synchrotron radi-
S. J. Dalgarno, Cryst. Growth Des. 2013, 13,
ation, λ = 0.77490 Å, T = 100(2)K, 2θmax
= 52.4°, 54147 reflections collected, 7495
unique (Rint = 0.0512). Final GooF = 2.978,
R1 = 0.1240, wR = 0.3633, R indices based
on 5993 reflecti2ons with I >2sigma(I) (re-
finement on F2).
2703; d) P. P. Cholewa, C. M. Beavers, S. J.
Teat, S. J. Dalgarno, Chem. Commun. 2013, 49,
3203; e) S. Pasquale, S. Sattin, E. C. Escudero-
Adán, M. Martinez-Belmonte, J. de Mendoza,
Dalgarno, J. L. Atwood, C. L. Raston, Chem.
Commun. 2006, 4567; e) J. L. Atwood, L. J.
Barbour, S. J. Dalgarno, M. J. Hardie, C. L.
Raston, H. R. Webb, J. Am. Chem. Soc. 2004,
126, 13170; f) J. L. Atwood, L. J. Barbour, M.
J. Hardie, C. L. Raston, Coord. Chem. Rev.
2001, 222, 3; g) G. W. Orr, L. J. Barbour, J. L.
Atwood, Science 1999, 285, 1049;
Nature Comms. 2012, 3, 785; f) S. P. Bew,
A. D. Burrows, T. Düren, M. F. Mahon, P. Z.
Moghadam, V. M. Sebestyen, S. Thurston,
Chem. Commun. 2012, 48, 4824; g) S. Kennedy,
G. Karotsis, C. M. Beavers, S. J. Teat, E. K.
Brechin, S. J. Dalgarno, Angew. Chem. Int.
Acknowledgments
[4] a) I. Ling, I. K. Swaminathan, C. S. Bond, A. N.
Sobolev, Y. Alias, C. L. Raston, CrystEngComm
We thank Heriot-Watt University for finan-
cial support of this work through the James-
Watt Scholarship Scheme (studentship for
REF). Mass spectrometry data was acquired at
the EPSRC UK National Mass Spectrometry
Ed. 2010, 49, 4205; h) S. J. Dalgarno, K. M.
2012, 14, 8541; b) T. E. Clark, M. Makha, A.
Claudio-Bosque, J. E. Warren, T. E. Glass, J. L.
N. Sobolev, D. Su, H. Rohrs, M. L. Gross, C. L.
Atwood, Chem. Commun. 2008, 1410.
Raston, Cryst. Growth Des. 2009, 9, 3575; c) T.
[7] M. Yilmaz, U. S. Vural, Synth. React. Inorg.
E. Clark, M. Makha, A. N. Sobolev, H. Rohrs,
J. L. Atwood, C. L. Raston, Chem. Eur. J. 2008,
14, 3931.
Met.-Org. Chem. 1991, 21, 1231.
Facility at Swansea University. The Advanced
[8] C. D. Gutsche, B. Dhawan, J. A. Levine, K. H.
Light Source is supported by the Director,
Office of Science, Office of Basic Energy
Sciences, of the US Department of Energy un-
der contract no. DE-AC02-05CH11231.
No, L. J. Bauer, Tetrahedron 1983, 39, 409.
[5] For non-covalent pCO2[4] assemblies see: a)
[9] Q. Lin, A. D. Hamilton, C. R. Chimie 2002, 5,
S. Kennedy, C. M. Beavers, S. J. Teat, S. J.
441.
Dalgarno, CrystEngComm 2014, 16, 3712; b)
[10] P. G. M. Wuts, T. W. Greene, ‘Green’s Protective
S. Kennedy, P. P. Cholewa, R. D. McIntosh, S.
Groups in Organic Synthesis’, Chap. 5, J Wiley
J. Dalgarno, CrystEngComm 2013, 15, 1520;
& Sons, Hoboken, New Jersey, 2007.
Received: June 25, 2015