Rim-functionalized cryptophane-111 derivatives via heterocapping, and their xenon complexes

Article abstract of DOI:10.1039/c4cc08001g

Capping of cyclotriphenolene (3a) by the more available cyclotriguaiacylene (3c) or trisbromocyclotriphenolene (3b) gives the first rim-functionalized cryptophane-111 derivatives. Crystal structures of the xenon complexes reveal high cavity packing coeffi

Full text of DOI:10.1039/c4cc08001g

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Rim-functionalized cryptophane-111 derivatives
via heterocapping, and their xenon complexes†
Cite this: Chem. Commun., 2014,
50, 15905
Akil I. Joseph,‡a Gracia El-Ayle,‡a Celine Boutin,b Estelle Leonce,b Patrick Berthaultb
´
´
and K. Travis Holman*a
Received 9th October 2014,
Accepted 1st November 2014
DOI: 10.1039/c4cc08001g
www.rsc.org/chemcomm
Capping of cyclotriphenolene (3a) by the more available cyclo- affinity. The room temperature xenon binding constant of 111 is
triguaiacylene (3c) or trisbromocyclotriphenolene (3b) gives the more than three times that of 222 in organic solvents under similar
first rim-functionalized cryptophane-111 derivatives. Crystal struc- conditions (Ka E 104
M
À1).10 The cage of the 111 core is also
tures of the xenon complexes reveal high cavity packing coeffi- insusceptible to collapse.11 To date, however, only a few derivatives
cients and unprecedentedly short XeÁ Á ÁC contacts.
of 111 have appeared, limited in part due to the low yield (6%)
synthesis of the requisite cyclotriphenolene (CTP, Scheme 1a) pre-
Cryptophanes (Scheme 1) are cage-like container molecules con- cursor.12,13 Nonetheless, the first water soluble 111 derivative,
structed by the covalent linking of two concave cyclotribenzylenes namely the pentamethylcyclopentadienyl ruthenium functionalized
(CTBs), most commonly by alkyldioxy linkers.1 They typically possess salt [(Cp*Ru)6(111)]Cl6,12 was found to exhibit a high room tempera-
electron rich hydrophobic cavities that strongly and selectively bind ture xenon binding constant (Ka = 2.9(2) Â 104 MÀ1 in D2O, by 129Xe
complementary small molecules, gases, cations,2 or even anions.3 NMR) comparable to the best water soluble 222 derivatives.8a We
One of the most promising potential applications of cryptophanes is report here the synthesis and xenon binding properties of two new
related to the smaller ones being the highest known affinity hosts rim-substituted cryptophane-111 derivatives achieved by a hetero-
for xenon, allowing development of as-low-as picomolar4,5 detection capping synthetic approach that exploits the greater availability of
limit 129Xe NMR based indirect sensors or imaging/contrast agents.6 methoxy or, ostensibly, bromine functionalized cyclotriphenols 3b
To date, essentially all such sensors—e.g., pH, temperature, protein, and 3c (Scheme 1). The surprising crystal structures of their xenon
nucleotide, peptide, and Zn2+ ion sensors, to name a few—are
derived from the (Æ)-cryptophane-222 core (222; R = OMe, n = 2).
The 222 core features ethylenedioxy linkers that provide a flexible
cavity ranging from B85–119 Å3 in volume (Vc)7 and methoxy or
other8 substituents amenable to synthetic manipulation for the
installment of water solubilizing and/or substrate binding sites for
sensing applications. Although the parent 222 exhibits a high xenon
binding constant in organic solvents (Ka E 3000 MÀ1 at 278 K in
(CDCl2)2),9 the core 222 cavity appears to be somewhat large for
xenon (VXe = 42 Å3), even in its most contracted conformation (Vc =
89 Å, for Xe@222). The smaller, also flexible, (Æ)-cryptophane-111
(111, R = H, n = 1, Vc E 32–72 Å3), however, is thought to possibly be
a better core platform for xenon, at least in terms of its xenon
a Department of Chemistry, Georgetown University, Washington, D.C., USA 20057.
E-mail: kth7@georgetown.edu; Fax: +1 202-687-6209
b
´
CEA, IRAMIS, NIMBE, Laboratoire Structure et Dynamique par Resonance
´
Magnetique, UMR CEA/CNRS 3299, 91191 Gif-sur-Yvette, France
Scheme 1 (a) General cryptophane structure. (b) Synthesis of rim-
substituted cryptophane-111 derivatives (MeO)3-111 and Br3-111 by the
heterocapping method. (i) P2O5, Et2O or CH2Cl2, reflux, (ii) 60% HClO4,
(iii) BBr3, CH2Cl2, À78 1C, (iv) 10% Pd/C, 1,4-dioxane, (v) Cs2CO3, DMF,
80 1C, BrCH2Cl. All chiral compounds are isolated as racemates.
† Electronic supplementary information (ESI) available: Syntheses, mass spectra,
NMR spectra, crystallographic details. CCDC 935199 and 1027974–1027977. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4cc08001g
‡ These authors contributed equally to this work.
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complexes reveal extremely compact xenon complexes with unpre- 2-bromo-5-methoxybenzyl alcohol regioisomer. 1b was con-
cedentedly short XeÁÁÁC contacts. sequently synthesized from 3-hydroxybenzoic acid as reported
All reported 111 derivatives to date12,13—none of them rim- in the literature.†17
functionalized—were obtained by post-synthetic modification of Preliminary evidence for the binding of xenon by (MeO)3-111
1
111, which itself is best synthesized by the SN2-mediated dimeriza- and Br3-111 was obtained by room temperature H and hyper-
tion of two units of cyclotriphenolene 3a using excess bromochloro- polarized (HP) 129Xe NMR spectroscopy in CDCl3 and nitroben-
methane (Scheme 1b, 46% optimized yield).14 Unfortunately, zene-d5 (or CD2Cl2), respectively. The solvents are too large to
despite recent progress,15 the availability of 3a remains limited by enter the 111 cavity and essentially cannot compete with xenon.
the low yield (6–14%) synthesis of its methylated cyclotrianisylene The degassed, xenon-free 1H spectra (Fig. 1) are indicative of C3
precursor (2a) from 3-methoxybenzylalcohol (1a). We reasoned that symmetric cryptophanes. After saturation of the solutions with
synthesis of rim-functionalized 111 derivatives might be achieved xenon, the host resonances in the 1H spectra of (MeO)3-111 and
more directly by the heterocapping of 3a with a pre-functionalized Br3-111 split into two signals. Under similar conditions, the HP
cyclotriphenolene, such as trisbromocyclotriphenolene (3b) or cyclo- 129Xe spectra simultaneously show the appearance of reso-
triguaiacylene (3c). Considering that homodimeric 111-core crypto- nances corresponding to Xe@(MeO)3-111 and Xe@Br3-111
phanes of 3b or 3c should, at best, occur only in low yield due to (in CD2Cl2) at 39.3 and 80.7 ppm (Fig. S12 and S13, ESI†),
steric crowding at opposing CTB rims,16 the heterocapping respectively. The data demonstrate, as expected, slow exchange
approach could, in principal, alleviate up to half of the demand of the xenon between bound and free states on both spectral
for precious 3a while also directly providing functionalized 111 deriva- timescales, but time-averaged C3-symmetry host conforma-
tives (MeO)3-111 or Br3-111. Like 222, which is rim-functionalized with tions. The Xe@(MeO)3-111 resonance is significantly downfield
methoxy (or other) substituents, the new rim-functionalized 111 from the Xe@111 resonance (31.1 ppm)10 and that of a hexa-
derivatives ought to be amenable to further modification. More- phenolic Xe@(OH)6-111 derivative (31 ppm);13a this may suggest
over, trisphenol 3c is readily available in many-gram quantities there is less space available to xenon within this rim-functionalized
and trisphenol 3b is obtained from 1b in two steps in reasonable 111 derivative. Preliminary data (not provided) also shows that
yield.17 Also, 3b ought to be more accessible due to a somewhat guest in–out exchange is slower for Br3-111 and (MeO)3-111 than
recent report of the high yield, regioselective bromination of 1a for 111, reflecting the presence of substituents at the cavity
to give 1b,18 removing two steps from the overall synthesis of 3b. windows. Though further study is needed to extract accurate
We also hypothesized that the introduction of substituents (e.g., binding constants, the Ka values are similar for both cryptophanes
methoxy or bromo) on one of the inner rims of 111 might enhance and are lower than expected (Ka o 100 MÀ1, estimated). It is not
the binding affinity of the cryptophane toward small gases due to: (i) yet known whether the lower xenon binding constants are due to
increased host–guest dispersion interactions resulting from the entropic or enthalpic issues. In any case, xenon binding affinity is
introduction of heavy atoms—and, particularly, polarizable atoms
like Br—at the surface of the binding site; (ii) the presence of a
permanent dipole in the host, also thought to likely enhance host–
guest dispersion interactions, and (iii) the bulk of the rim substi-
tuents prohibiting the 111 core from achieving the most contracted,
small-cavity-volume conformation. It was thought that inhibiting
contracted conformations may effectively pre-organize the crypto-
phane toward the more expanded, xenon-accommodating confor-
mations and counter possible entropic consequences of substrate
binding.
Reaction of 3a with excess 3b or 3c under conditions similar to
those used for synthesis of 111,14 was found to give the expected
functionalized 111 derivatives (MeO)3-111 or Br3-111 in 18% and
17% yield based on 3a (Scheme 1). The anti-stereochemistry of the
products was confirmed by crystallography and there was no
evidence for the presence of syn diastereomers in the products.
Notably, as anticipated, only very small amounts of the homo-
dimeric hexamethoxy- or hexabromo-111 derivatives are observed.
The latter (Br6-111) remains as a minor impurity (o2%) in the
Br3-111 product after purification. The results suggest that a
heterocapping approach may also be successful if applied to a
recent attempted synthesis of functionalized cryptophane-000
Fig. 1 Selected portions of room temperature 1H spectra before (top) and
after (bottom) saturation of the solvent with xenon. (a) (MeO)3-111 in CDCl3,
derivatives.16 Unfortunately, the proposed shorter approach to
3b did not proceed as intended; the previous report18 of the
(b) Br3-111 in nitrobenzene-d5. Open and filled circles represent signals of
regioselective bromination of 1a to give the necessary 1b was
concluded to be erroneous, yielding instead the unproductive dimeric hexabromo derivative (Br6-111), found as an impurity (B2%).
the free and xenon-occupied hosts, respectively. *Represents the homo-
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expected to increase considerably in aqueous solution should these of the ArOCH2 connections (t = 4(4)1), an 8.6 Å end-to-end length (l),
hosts be further modified to provide water soluble derivatives. a minimal twist angle (y = 18(1)1) and a cavity volume (Vc) that
Single crystals of (MeO)3-111Á1.5DCE, the corresponding measures 69 Å3. In contrast the conformation of the empty 111 core
xenon complex, 0.92Xe@(MeO)3-111Á1.5DCE, (MeO)3-111Á2/ of [(Cp*Ru)6(111)][CF3SO3]6ÁxNO2Me is seemingly as-contracted-as-
3NO2Me, Br3-111, and its xenon complex, 0.96Xe@Br3-111, possible (l = 7.4 Å), being highly twisted (y = 611) with all six ArOCH2
were grown at room temperature from 1,2-dichloroethane connections in an antiperiplanar (t = 177(2)1) arrangement and a
(DCE), NO2Me, and NO2C6H5, respectively, and were analysed minimized cavity volume (Vc E 32 Å3). In the absence of xenon,
by X-ray diffraction (Fig. 2).† Crystals of the xenon complexes (MeO)3-111 and Br3-111 are also found to be empty in the solid
were obtained from vessels pressurized with xenon (14 or 17 bar), state.† Both (MeO)3-111 and Br3-111 exhibit conformations in
ensuring nearly 100% xenon occupancy.
between the fully contracted and fully expanded forms exhibited
The conformation of 111 and its derivatives can be described by empty and water-occupied 111. In short, though there is some
by several structural parameters: the length of the cryptophane cryptophane conformational disorder in (MeO)3-111Á1.5DCE, 2.35 of
(l), defined by the CH2 carbons of the CTB units, the relative the three crystallographically unique cryptophanes observed in the
twist angle between the CTB units (y), and the torsion angles two crystal structures of empty (MeO)3-111 have only three of their
about the Ar–O bonds involving the methylenedioxy linkers six ArOCH2 connections (t) residing in the antiperiplanar ‘‘closed’’
(t, defines in Scheme 1). The cryptophane cavity volume (Vc) conformation (0.65/3 show four of the six connections in this
can also be quantified and the reported crystal structures of conformation) whereas the remaining connections are all ‘‘gauche’’
0.75H2O@111Á2CHCl3 and metalated [(Cp*Ru)6(111)][CF3SO3]6Á (synclinal or anticlinal). Similarly, in the structure of empty Br3-111,
xNO2Me serve as useful comparisons (Fig. 2a and b).12 In the only two of the six ArOCH2 connections (t) are antiperiplanar while
former compound, 111 is found to have scavenged water from four are gauche. The net result of these linker conformations are
CHCl3 solution and the cryptophane adopts a fully expanded less twisted (Fig. 2) cryptophane conformations and larger cavity
conformation characterized by six synperiplanar conformations volumes as compared to empty 111 core of [(Cp*Ru)6(111)]6+
(Vc = 42 and 46 Å3 for (MeO)3-111 and Br3-111, respectively). The
observation suggests that, as hypothesized, the steric bulk imposed
by the rim substituents prevents the empty cryptophanes from
contracting as much as is possible with 111, and that the range of
achievable conformations (and cavity volumes) is narrower for the
rim-functionalized 111 derivatives. Further conformational details
are available in the ESI† (Table S3, Fig. S14–S19).
Surprisingly, the crystal structures of the xenon-occupied
cryptophanes—0.92Xe@(MeO)3-111Á1.5DCE and 0.96Xe@Br3-
111—are isostructural and nearly identical to their empty
crystal forms, except that xenon is found within the crypto-
phane cavities at nearly 100% occupancy. The unit cell volume
of 0.96Xe@Br3-111, for instance, is only 24 Å3 greater than
Br3-111—almost completely attributable to the slight (5 Å3, 11%)
expansion of the Br3-111 cavities—despite the introduction of
4 Â 42 Å3 of atomic xenon. The xenon atom is centered within
the Br3-111 cavity (Fig. S21, ESI†) and the 36 closest heavy atoms
to the xenon are the arene carbon atoms of the host. In fact,
several of the XeÁ Á ÁC(arene) distances are the shortest ever
measured for a complex of atomic xenon,19 ranging from 3.66–
4.20 Å (avg. = 3.89(16) Å), with more than 15 contacts being
shorter than the sum of the van der Waals radii (3.86 Å). The
XeÁ Á Ácentroid(arene) distances average 3.63(4) Å, considerably
shorter than that measured for the XeÁ Á ÁC6H6 complex in the gas
phase (3.77 Å).20 The result is clearly a very tightly enshrouded
xenon atom; the packing coefficient (PC, VXe/Vc) of xenon within
the Br3-111 cavity measures 0.82, extremely high for supra-
Fig. 2 Thermal ellipsoid plots of (a) the contracted 111 core of the
reported empty [(Cp*Ru)6111]6+ cation,12 (b) the expanded 111 conforma-
tion from the reported structure of 0.75H2O@111Á2CHCl3,12 (c) empty
(MeO)3-111 from the crystal structure of (MeO)3-111Á1.5DCE, (d) the
Xe@(MeO)3-111 complex from the crystal structure of Xe@(MeO)3-111Á
1.5DCE, (e) empty Br3-111, and (f) Xe@Br3-111. The twist angles, y, and
cavity volumes (Vc, depicted in orange) are provided. Only the major
occupancy positions of disordered species are shown. Xenon is depicted
as semi-transparent blue spheres.
molecular complexes governed by dispersion forces (typically
0.55 Æ 0.09) and particularly so for gas complexes.21 Notably, this
is the highest PC reported to date for any structurally characterized
neutral guest@cryptophane complex.22 In comparison, the Xe@111
complex exhibits a cavity volume of 70 Å3 (PC = 0.62) and signifi-
cantly longer, likely more optimal, XeÁÁÁC(arene) contacts, with
XeÁÁÁC(arene) distances averaging 4.01(9) Å (range: 3.86–4.20 Å)
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and XeÁÁÁcentroid(arene) distances measuring 3.77(3) Å.23 The
xenon thermal parameters (at 100 K) are also noticeably larger for
the Xe@111 complex than for Xe@Br3-111. Similarly, the xenon
atoms of the Xe@222 complex is even less crowded, exhibiting
longer XeÁ Á ÁC(arene) contacts and a packing coefficient of 0.47.
Interestingly, single crystals of 0.96Xe@Br3-111 appear to be
indefinitely stable under ambient conditions; over a period of
months, no loss of xenon can be detected by X-ray diffraction,
despite the otherwise volatile nature of the guest. We note that
gas-encapsulating molecules such as these may have materials
applications related to gas confinement or separations.24
Crystals of 0.92Xe@(MeO)3-111Á1.5DCE are similarly iso-
structural to the empty crystal form (MeO)3-111Á1.5DCE, except that
the (65 : 35) conformational disorder of the cryptophane observed in
the empty structure is not present in 0.92Xe@(MeO)3-111Á1.5DCE.
Only the more open of the two conformers is observed (Fig. S14 and
S20, ESI†), yet, like Xe@Br3-111, the xenon is centered and highly
crowded within an intermediate cryptophane-111 core conforma-
tion (PC = 0.79, Vc = 53 Å3). Similarly also, close XeÁÁÁC(arene)
intermolecular contacts are observed for the Xe@(MeO)3-111
complex, ranging from 3.64–4.35 Å (avg. = 3.91(19) Å) and exhibiting
XeÁÁÁcentroid(arene) distances averaging 3.65(14) Å.
4 (a) M. M. Spence, S. M. Rubin, I. E. Dimitrov, E. J. Ruiz, D. E.
Wemmer, A. Pines, S. Q. Yao, F. Tian and P. G. Schultz, Proc. Natl.
Acad. Sci. U. S. A., 2001, 98, 10654–10657; (b) L. Schroder, T. J.
Lowery, C. Hilty, D. E. Wemmer and A. Pines, Science, 2006, 314,
446–449.
5 (a) Y. Bai, P. A. Hill and I. J. Dmochowski, Anal. Chem., 2012, 84,
9935–9941; (b) K. K. Palaniappan, R. M. Ramirez, V. S. Bajaj,
D. E. Wemmer, A. Pines and M. B. Francis, Angew. Chem., Int. Ed.,
2013, 52, 4849–4853.
6 (a) K. K. Palaniappan, M. B. Francis, A. Pines and D. E. Wemmer,
¨
Isr. J. Chem., 2014, 54, 104–112; (b) L. Schroder, Physica Medica,
2013, 29, 3–16; (c) O. Taratula and I. J. Dmochowski, Curr. Opin.
Chem. Biol., 2010, 14, 97–104; (d) P. Berthault, G. Huber and
H. Desvaux, Prog. Nucl. Magn. Reson. Spectrosc., 2009, 55, 35–60.
7 (a) D. Cavagnat, T. Brotin, J.-L. Bruneel, J.-P. Dutasta, A. Thozet,
M. Perrin and F. Guillaume, J. Phys. Chem. B, 2004, 108, 5572–5581;
(b) O. Taratula, P. A. Hill, N. S. Khan, P. J. Carroll and I. J. Dmochowski,
Nat. Commun., 2010, 1, 148–154.
´
8 (a) D. R. Jacobson, N. S. Khana, R. Colle, R. Fitzgerald, L. Laureano-
´
Perez, Y. Baia and I. J. Dmochowski, Proc. Natl. Acad. Sci. U. S. A.,
´
2011, 108, 10969–10973; (b) L. Delacour, N. Kotera, T. Traore,
S. Garcia-Argote, C. Puente, F. Leteurtre, E. Gravel, N. Tassali,
´
C. Boutin, E. Leonce, Y. Boulard, P. Berthault and B. Rousseau,
Chem. – Eur. J., 2013, 19, 6089–6093.
9 K. Bartik, M. Luhmer, J.-P. Dutasta, A. Collet and J. Reisse, J. Am.
Chem. Soc., 1998, 120, 784–791.
10 (a) H. A. Fogarty, P. Berthault, T. Brotin, G. Huber, H. Desvaux and
J. P. Dutasta, J. Am. Chem. Soc., 2007, 129, 10332–10333; (b) K. E.
Chaffee, H. A. Fogarty, T. Brotin, B. M. Goodson and J.-P. Dutasta,
J. Phys. Chem. A, 2009, 113, 13675–13684.
The first rim-functionalized derivatives of cryptophane-111
were synthesized by a heterocapping synthetic approach. As
observed by crystallography and 1H and 129Xe NMR spectro-
scopy, (MeO)3-111 and Br3-111 bind xenon in organic solvents,
albeit more weakly than expected. The crystallographically
characterized xenon complexes exhibit the shortest known
XeÁ Á ÁC intermolecular contacts. At this time, we do not have a
definitive explanation for the crowded xenon complexes. It is
possible that crystal packing forces dictate that the complexes
maintain somewhat contracted conformations. It is more
likely, however, that the rim-positioned functional groups
may prevent the cryptophanes from adopting the synperiplanar
ArOCH2 conformations (t) characteristic of the most expanded
111 core conformation. Rim-functionalization thus appears to
significantly limit the range of achievable conformations of the
111 core and suggests that such 111 derivatives may be better
hosts than 111 for smaller gases such as N2, O2, etc.
11 S. T. Mough, J. C. Goeltz and K. T. Holman, Angew. Chem., Int. Ed.,
2004, 43, 5631–5635.
12 R. M. Fairchild, A. I. Joseph, K. T. Holman, H. A. Fogarty, T. Brotin,
J.-P. Dutasta, C. Boutin, G. Huber and P. Berthault, J. Am. Chem. Soc.,
2010, 132, 15505–15507.
13 (a) E. Dubost, J.-P. Dognon, B. Rousseau, G. Milanole, C. Dugave,
´
Y. Boulard, E. Leonce, C. Boutin and P. Berthault, Angew. Chem., Int.
Ed., 2014, 53, 9837–9840; (b) E. Dubost, N. Kotera, S. Garcia-Argote,
´
Y. Boulard, E. Leonce, C. Boutin, P. Berthault, C. Dugave and
´
´
B. Rousseau, Org. Lett., 2013, 15, 2866–2868; (c) T. Traore, G. Clave,
L. Delacour, N. Kotera, P.-Y. Renard, A. Romieu, P. Berthault,
C. Boutin, N. Tassali and B. Rousseau, Chem. Commun., 2011, 47,
9702–9704.
´
14 T. Traore, L. Delacour, S. Garcia-Argote, P. Berthault, J.-C. Cintrat
and B. Rousseau, Org. Lett., 2010, 12, 960–962.
´
15 (a) T. Traore, L. Delacour, N. Kotera, G. Merer, D.-A. Buisson,
C. Dupont and B. Rousseau, Org. Process Res. Dev., 2011, 15,
435–437; (b) M. A. Little, J. Donkin, J. Fisher, M. A. Halcrow,
J. Loder and M. J. Hardie, Angew. Chem., Int. Ed., 2012, 51, 764–766.
16 M. A. Little, M. A. Halcrow and M. J. Hardie, Chem. Commun., 2013,
49, 1512–1514.
17 (a) D. J. Cram, M. E. Tanner, S. J. Keipert and C. B. Knobler, J. Am.
Chem. Soc., 1991, 113, 8909–8916; (b) J. Canceill and A. Collet, New
J. Chem., 1986, 10, 17–23.
18 A. Speicher, T. Backes and S. Grosse, Tetrahedron, 2005, 61, 11692–11696.
19 Y. Miyahara, K. Abe and T. Inazu, Angew. Chem., Int. Ed., 2002, 41,
3020–3023.
20 T. Brupbacher, J. Makarewicz and A. Bauder, J. Chem. Phys., 1994,
101, 9736–9746.
This work was partially supported by Georgetown University, the
U.S. National Science Foundation (NSF; DMR-1106266, CHE-
1337975), and the IMI Program under DMR-0843934. Support from
the French Ministry of Research (project ANR-12-BSV5-0003) and
´
from the Foundation pour la Recherche Medicale (project
DCM20111223065) is acknowledged. We thank John C. Sherman
21 S. Mecozzi and J. J. Rebek, Chem. – Eur. J., 1998, 4, 1016–1022.
for helpful discussions regarding 1b/2b. We thank Leonard 22 Though the CDCl3@222 complex was originally estimated (a) to
have a PC of 0.89, our analysis of the crystal structures of the
complex (b) reveal that the value is closer to 0.61. (a) G. Laurent,
D. Jean-Pierre and A. Collet, Angew. Chem., Int. Ed. Engl., 1993, 32,
J. Barbour for assisting with the cavity volume calculations and
providing the apparatus for crystal growth under pressurized xenon.
1169–1171; (b) D. Cavagnat, T. Brotin, J.-L. Bruneel, J.-P. Dutasta,
A. Thzoet, M. Perrin and F. Guillaume, J. Phys. Chem. B, 2004, 108,
5572–5581.
Notes and references
1 T. Brotin and J. P. Dutasta, Chem. Rev., 2009, 109, 88–130.
2 T. Brotin, S. Goncalves, P. Berthault, D. Cavagnat and T. Buffeteau,
J. Phys. Chem. B, 2013, 117, 12593–12601.
23 A. I. Joseph, S. H. Lapidus, C. M. Kane and K. T. Holman, Angew.
Chem., Int. Ed., 2014, accepted.
24 L. Chen, P. S. Reiss and S. Y. Chong, et al., Nat. Mater., 2014, 13,
954–960.
3 R. M. Fairchild and K. T. Holman, J. Am. Chem. Soc., 2005, 127, 16364–16365.
15908 | Chem. Commun., 2014, 50, 15905--15908
This journal is ©The Royal Society of Chemistry 2014