Enantiopure pillar[5]arene active domains within a homochiral metal-organic framework

Article abstract of DOI:10.1039/c4cc02559h

Enantiopure struts containing pillar[5]arenes incorporating planar chirality have been linked together with Zn₄O clusters in order to create metal-organic frameworks that include homochiral active domains and so have the potential to act as a solid support in chiral chromatography. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02559h

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Enantiopure pillar[5]arene active domains within
a homochiral metal–organic framework†
Cite this: Chem. Commun., 2014,
50, 7455
Nathan L. Strutt, Huacheng Zhang and J. Fraser Stoddart*
Received 7th April 2014,
Accepted 19th May 2014
DOI: 10.1039/c4cc02559h
www.rsc.org/chemcomm
Enantiopure struts containing pillar[5]arenes incorporating planar
chirality have been linked together with Zn4O clusters in order to
create metal–organic frameworks that include homochiral active
domains and so have the potential to act as a solid support in chiral
chromatography.
Pillar[n]arenes,1 macrocycles composed of n hydroquinone rings
connected through their para-positions by methylene bridges, are a
new family of cavitands which have quickly become important
players in the field of host–guest chemistry2 since their initial
report by Ogoshi and co-workers.1a As a result of their novel
structures3 and the tunability of their pendant functional groups,4
pillar[n]arenes have been incorporated into many types of materials
including polymers,1c,5 nanoparticles6 and liquid crystals.7 Recently,
we have reported8 the incorporation of a rigid A1/A2-difunctionalised
pillar[5]arene strut rac-1 (Scheme 1) into the metal–organic frame-
work (MOF) rac-P5A-MOF-1. The pillar[5]arene cavities in rac-P5A-
MOF-1 served to create active domains9 within the porous
framework wherein an ordered distribution of guests can be
maintained through highly specific charge–transfer interactions
between the p electron-rich pillar[5]arene hosts and electron-poor
guests. These active domains have the ability to take up large
amounts of pyridinium cations and p-dinitrobenzene, and the
pillar[5]arenes were demonstrated to be selective towards guests
which are more electron deficient.
The substituents present in the pillar[n]arenes oblige them to exhibit
planar chirality.1e In 1,4-dimethoxypillar[5]arene (DMpillar[5]arene),1a
there are five planes of chirality which are coplanar with the
1,4-dimethoxybenzene rings and the two contiguous methylene
carbons at the 2/5 ring positions. The chiralities in the
two lowest energy enantiomeric conformational isomers of
DMpillar[5]arene can be described with Cahn–Ingold–Prelog
nomenclature as RpRpRpRpRp and SpSpSpSpSp which we have
Scheme 1 The organic strut rac-1 can be employed to synthesise rac-
P5A-MOF-1 (top) or it can be resolved to give (Sp)-1 (red) and (Rp)-1 (blue)
which can then be used in separate reactions to form homochiral
(Sp)-P5A-MOF-1 and (Rp)-P5A-MOF-1.
abbreviated here to Rp and Sp. The Rp and Sp conformational
isomers rapidly equilibrate as a result of ring rotations at room
temperature, making it impossible to isolate an enantio-
merically enriched sample of DMpillar[5]arene. The rate of
the inversion process can be curtailed in pillar[n]arene deriva-
tives such as in A1/A2-dihydroxy-pillar[5]arene,10 and arrested
completely in some derivatives, such as pillar[5]arenes with
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston,
IL 60208, USA. E-mail: stoddart@northwestern.edu
† Electronic supplementary information (ESI) available: Synthetic details, NMR,
UV-Vis and HPLC. CCDC 995695 and 995696. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c4cc02559h
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 7455--7458 | 7455

View Article Online
Communication
ChemComm
bulky substituents11 or pillar[5]arenes constituting mechani- After separation, the chiral auxiliaries can be removed to reveal
cally interlocked compounds.12
the enantiomerically pure struts (Sp)-1 and (Rp)-1.
In our previous work8 a racemic mixture of the organic strut
The synthetic protocol we have employed in order to obtain
rac-1, which cannot undergo stereochemical inversion on account enantiomerically pure struts (Sp)-1 and (Rp)-1 (Scheme 2) begins
of its rigid difunctionalisation, was connected through Zn4O with the previously reported8 pillar[5]arene ditriflate 2 which
secondary building units (SBUs) to create rac-P5A-MOF-1. The can be prepared in three steps from DMpillar[5]arene. Ditriflate
active domains in this MOF consist of pillar[5]arene recognition 2 can be reacted with the (S)-prolinol-substituted 4-carbonylphenyl-
sites with randomly distributed chiralities. Herein, we report the boronic acid pinacol ester 3 in a Pd-catalyzed Suzuki reaction to
resolution of rac-1 and the incorporation of the enantiopure struts produce the diastereoisomers (SSSp)-4 and (SSRp)-4. This reaction
(Rp)-1 and (Sp)-1 into the homochiral porous frameworks of proved to be slightly diastereoselective in favour of (SSRp)-4. The
(Rp)-P5A-MOF-1 and (Sp)-P5A-MOF-1 (Scheme 1).‡ Although there two diastereoisomers of 4 can be easily separated on the hundreds
have been many examples of homochiral MOFs reported13 with of milligrams scale by employing normal phase silica HPLC (see
components which have either stereogenic centres14 or axes15 of Fig. S1 in the ESI†). Single crystals of (SSSp)-4 and (SSRp)-4, suitable
chirality, the incorporation of planar chirality into homochiral for X-ray crystallography, were obtained from the slow evaporation
MOFs is a relatively unexplored phenomenon. Porous frameworks, of CH2Cl2. The solid-state structures§ can be used to assign the
which contain enantiopure active domains, are also an area of absolute stereochemistry of the pillar[5]arenes in (SSSp)-4 and
interest as a result of their potential application as highly engi- (SSRp)-4 since the absolute configuration of the (S)-prolinol
neered chiral stationary phases for carrying out the separation of is known. The (S)-prolinol auxiliaries in (SSSp)-4 and (SSRp)-4
enantiomers by high-performance liquid chromatography (HPLC). were easily removed by alkaline amide hydrolysis to give the
We have demonstrated8 previously that the methyl ester of strut enantiomerically pure pillar[5]arene struts (Sp)-1 and (Rp)-1.
rac-1 can be resolved on the analytical scale by chiral HPLC. In order
Circular dichroism (CD) spectroscopy of (Sp)-1 and (Rp)-1 (Fig. 1)
to produce homochiral versions of P5A-MOF-1 and assess the porous confirms that the conditions employed for amide hydrolysis do not
frameworks for their ability to separate small molecule racemates, lead to racemization of the strut. The positive Cotton effect observed
rac-1 must be resolved on a larger scale. While preparative scale for (Rp)-1 and the negative Cotton effect for (Sp)-1 agree well with the
chiral HPLC would likely be suitable for the resolution of rac-1, the Cotton effects witnessed for previously reported11a,16 enantio-
technique is cost-prohibitive and not widely available. Therefore, we merically pure pillar[5]arene derivatives. The optical purities of the
sought to develop a method to resolve rac-1 which would be operable methyl esters of (Sp)-1 and (Rp)-1 were assessed by analytical scale
on the gram scale without the use of chiral chromatography. In chiral HPLC¶ (see Fig. S2 in the ESI†) and samples of both
order to realize this objective, the enantiomers (Sp)-1 and (Rp)-1 can enantiomers were shown to have enantiomeric ratios of over 97 : 3.
be functionalised with an enantiomerically pure chiral auxiliary,
The homochiral frameworks (Sp)-P5A-MOF-1 and (Rp)-P5A-MOF-1
thereby converting the enantiomers into two diastereoisomers which were obtained from (Sp)-1 and (Rp)-1, respectively, and Zn(NO3)2Á6H2O
can then be separated using normal phase chromatography. in DMF at 100 1C (see the ESI†). Crystals of (Sp)-P5A-MOF-1 and
Scheme 2 Pure enantiomers of the chiral organic strut 1, (Sp)-1 and (Rp)-1 on account of planar chirality can be synthesised from the previously
reported8 ditriflate 2 using chiral auxiliaries in order to aid the separation of the diastereoisomers of intermediate 4, namely (SSSp)-4 and (SSRp)-4. The
solid-state structures of (SSSp)-4 and (SSRp)-4 are shown on the right (C atoms are grey, O atoms are white, N atoms are blue, H atoms have been
removed for the sake of clarity).
7456 | Chem. Commun., 2014, 50, 7455--7458
This journal is ©The Royal Society of Chemistry 2014

View Article Online
ChemComm
Communication
the solid-state structure. In common with rac-P5A-MOF-1, the disorder
within the homochiral frameworks is most likely a consequence of
the rotational freedom of the pillar[5]arene rings with respect to
the terphenylene linker.8 The powder X-ray diffraction (PXRD)
patterns of (Sp)-P5A-MOF-1 and (Rp)-P5A-MOF-1 are in almost
complete agreement (Fig. 2) and match well with the PXRD
pattern of the modelled version8 of rac-P5A-MOF-1 in a P1 space
group. The PXRD data indicates that the enantiopure
pillar[5]arene active domains in the homochiral frameworks do
not change significantly the overall structure of the MOF. CD
spectroscopy on samples of digested crystals of (Sp)-P5A-MOF-1
and (Rp)-P5A-MOF-1 (Fig. 1) provide evidence that racemization of
(Sp)-1 and (Rp)-1 does not occur at the elevated temperatures which
are required for the synthesis of the MOF.
The production of homochiral MOFs which contain enantio-
pure pillar[5]arene active domains has now been realized. These
homochiral MOFs are early examples of porous frameworks
incorporating active domains which possess planar chiralities.
In the preparation of these homochiral materials, an efficient
route to the large-scale resolution of racemic pillar[5]arene
derivatives has also been developed. Currently, efforts are
underway in our laboratory to investigate the enantioselectivities
of these homochiral porous frameworks towards small molecule
racemates which we expect to be bound enantioselectively within
the pillar[5]arene active domains.
This research is part (Project-34-943) of the Joint Center of
Excellence in Integrated Nano-Systems (JCIN) at the King Abdul-
Aziz City of Science and Technology (KACST) and Northwestern
University (NU). The authors would like to thank both KACST and
NU for their continued support of this research. N.L.S. is sup-
ported by a Graduate Research Fellowship from the National
Science Foundation under Grant No. DGE-0824162.
Fig. 1 Circular dichroism (CD) spectroscopy of (Sp)-1 (red) and (Rp)-1 (blue).
The solid line spectra were collected from (Sp)-1 and (Rp)-1 (MeCN, 0.5 mM)
as synthesised and the dashed line spectra were collected from digested
(Sp)-P5A-MOF-1 and (Rp)-P5A-MOF-1 dissolved (B0.5 mM) in MeCN.
(Rp)-P5A-MOF-1 were large and cubic with approximate average
volumes of 0.125 mm3 (see Fig. S7 in the ESI†). Single-crystal
X-ray diffraction analysis of the homochiral MOFs were found
to not be well enough resolved to discern individual atoms in
Notes and references
‡ Since pillar[5]arenes contain five planes of chirality, five stereochemical
designators can be used to describe their stereochemistry. We have opted
to abbreviate these multiple descriptors to only one, either Sp or Rp, in
cases where every plane of chirality has the same absolute stereochemistry,
i.e., SpSpSpSpSp-1 is abbreviated to Sp-1.
§ Crystal data for (SSSp)-4: [(C67H72N2O12)2ÁCH2Cl2]. Colourless plates
(0.015 Â 0.146 Â 0.237 mm). Monoclinic, P21, a = 12.0918(5), b =
21.3853(9), c = 26.0509(12) Å, a = 90.000, b = 92.817(3), g = 90.0001, V =
6728.3(5) Å3, Z = 2, T = 100(2) K, rcalc = 1.125 g cmÀ3, m = 0.972 mmÀ1
.
Of a total of 10 925 reflections which were collected, 10 925 were unique
(Rint = 0.0000). Final R1 (F2 4 2sF2) = 0.0824 and wR2 = 0.2421 (all data).
The SQUEEZE function17 of PLATON was used to remove the contribu-
tion of disordered solvent molecules. CCDC 995695. Crystal data for
(SSRp)-4: [C67H72N2O12]; colourless plates (0.011 Â 0.09 Â 0.173 mm);
monoclinic, P21, a = 12.9346(2), b = 11.7299(1), c = 21.685(3) Å, a =
90.000, b = 97.077(9), g = 90.0001, V = 3265.1(7) Å3, Z = 2, T = 100(2) K,
r
calc = 1.116 g cmÀ3, m = 0.616 mmÀ1; of a total of 12 576 reflections which
were collected, 6172 were unique (Rint = 0.0939); final R1 (F2 4 2sF2) =
0.0922 and wR2 = 0.2501; CCDC 995696.
¶ The diacids Sp-1 and Rp-1 are poorly separated using analytical chiral HPLC
and therefore the methyl esters of these compounds were prepared and used
to determine the enantiomeric ratio by chiral HPLC after resolution.
1 (a) T. Ogoshi, S. Kanai, S. Fujinami, T. Yamagishi and Y. Nakamoto,
J. Am. Chem. Soc., 2008, 130, 5022; (b) Y. Kou, H. Tao, D. Cao, Z. Fu,
D. Schollmeyer and H. Meier, Eur. J. Org. Chem., 2010, 6464;
(c) Z. Zhang, Y. Luo, J. Chen, S. Dong, Y. Yu, Z. Ma and F. Huang,
Angew. Chem., Int. Ed., 2011, 50, 1397; (d) N. L. Strutt, R. S. Forgan,
Fig. 2 The powder X-ray diffraction (PXRD) patterns of (Sp)-P5A-MOF-1 (red)
and (Rp)-P5A-MOF-1 (blue), shown above as graphical representations, match
closely those PXRD patterns of the modelled P5A-MOF-1 structure.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 7455--7458 | 7457

View Article Online
Communication
ChemComm
´
J. S. Spruell, Y. Y. Botros and J. F. Stoddart, J. Am. Chem. Soc., 2011,
133, 5668; (e) P. J. Cragg and K. Sharma, Chem. Soc. Rev., 2012,
41, 597; ( f ) M. Xue, Y. Yang, X. Chi, Z. Zhang and F. Huang, Acc.
Chem. Res., 2012, 45, 1294; (g) X.-B. Hu, Z. Chen, L. Chen, L. Zhang,
J.-L. Hou and Z.-T. Li, Chem. Commun., 2012, 48, 10999; (h) T. Ogoshi
and T.-a. Yamagishi, Eur. J. Org. Chem., 2013, 2961; (i) H. Zhang and
Y. Zhao, Chem. – Eur. J., 2013, 19, 16862; ( j) Z. Li, J. Yang, G. Yu,
J. He, Z. Abliz and F. Huang, Chem. Commun., 2014, 50, 2841;
(k) T. Ogoshi and T. Yamagishi, Chem. Commun., 2014, 50, 4776;
(l) T. Ogoshi, N. Ueshima, T. Akutsu, D. Yamafuji, T. Furuta,
F. Sakakibara and T.-a. Yamagishi, Chem. Commun., 2014,
S. Guerra, M. Holler, L. Karmazin-Brelot, J. Barbera, R. Deschenaux
and J.-F. Nierengarten, Eur. J. Org. Chem., 2013, 3675.
8 N. L. Strutt, D. Fairen-Jimenez, J. Iehl, M. B. Lalonde, R. Q. Snurr,
O. K. Farha, J. T. Hupp and J. F. Stoddart, J. Am. Chem. Soc., 2012,
134, 17436.
´
9 (a) Q. W. Li, W. Y. Zhang, O. S. Miljanic, C. H. Sue, Y. L. Zhao,
L. H. Liu, C. B. Knobler, J. F. Stoddart and O. M. Yaghi, Science, 2009,
325, 855; (b) C. Valente, E. Choi, M. E. Belowich, C. J. Doonan, Q. Li,
T. B. Gasa, Y. Y. Botros, O. M. Yaghi and J. F. Stoddart, Chem.
Commun., 2010, 46, 4911; (c) H. Deng, M. A. Olson, J. F. Stoddart and
O. M. Yaghi, Nat. Chem., 2010, 2, 439.
50, 5774; (m) X. Hou, C. Ke, C. Cheng, N. Song, A. K. Blackburn, 10 N. L. Strutt, S. T. Schneebeli and J. F. Stoddart, Supramol. Chem.,
A. A. Sarjeant, Y. Y. Botros, Y.-W. Yang and J. F. Stoddart, Chem. 2013, 25, 596.
Commun., 2014, 50, 6196; (n) Z. Li, J. Yang, G. Yu, J. He, Z. Abliz and 11 (a) T. Ogoshi, K. Masaki, R. Shiga, K. Kitajima and T.-a. Yamagishi,
F. Huang, Org. Lett., 2014, 16, 2066; (o) M. Pan and M. Xue, RSC Adv.,
2014, 4, 260; (p) Y. Guan, P. Liu, C. Deng, M. Ni, S. Xiong, C. Lin,
Org. Lett., 2011, 13, 1264; (b) I. Nierengarten, K. Buffet, M. Holler,
S. P. Vincent and J.-F. Nierengarten, Tetrahedron Lett., 2013, 54, 2398.
X.-Y. Hu, J. Ma and L. Wang, Org. Biomol. Chem., 2014, 12, 1079; 12 (a) T. Ogoshi, D. Yamafuji, T. Aoki, K. Kitajima, T.-a. Yamagishi,
(q) R. Manoni, P. Neviani, P. Franchi, E. Mezzina and M. Lucarini,
Eur. J. Org. Chem., 2014, 147; (r) Z.-Y. Li, Y. Zhang, C.-W. Zhang,
L.-J. Chen, C. Wang, H. Tan, Y. Yu, X. Li and H.-B. Yang, J. Am.
Chem. Soc., 2014, DOI: 10.1021/ja413047r.
2 (a) D. J. Cram and J. M. Cram, Science, 1974, 183, 803; (b) D. J. Cram,
Angew. Chem., Int. Ed., 1988, 27, 1009.
Y. Hayashi and S. Kawauchi, Chem. – Eur. J., 2012, 18, 7493; (b) C. Ke,
N. L. Strutt, H. Li, X. Hou, K. J. Hartlieb, P. R. McGonigal, Z. Ma,
J. Iehl, C. L. Stern, C. Cheng, Z. Zhu, N. A. Vermeulen, T. J. Meade,
Y. Y. Botros and J. F. Stoddart, J. Am. Chem. Soc., 2013, 135, 17019;
(c) K. Kitajima, T. Ogoshi and T.-a. Yamagishi, Chem. Commun.,
2014, 50, 2925.
3 (a) C. Han, F. Ma, Z. Zhang, B. Xia, Y. Yu and F. Huang, Org. Lett., 13 (a) J. S. Seo, D. Whang, H. Lee, S. I. Jun, J. Oh, Y. J. Jeon and K. Kim,
2010, 12, 4360; (b) T. Ogoshi, T. Aoki, K. Kitajima, S. Fujinami,
T.-a. Yamagishi and Y. Nakamoto, J. Org. Chem., 2011, 76, 328;
(c) Y. Ma, X. Chi, X. Yan, J. Liu, Y. Yao, W. Chen, F. Huang and
J.-L. Hou, Org. Lett., 2012, 14, 1532.
4 (a) T. Ogoshi, K. Demachi, K. Kitajima and T.-a. Yamagishi, Chem.
Commun., 2011, 47, 7164; (b) T. Ogoshi, R. Shiga, M. Hashizume and
T.-a. Yamagishi, Chem. Commun., 2011, 47, 6927; (c) T. Ogoshi,
D. Yamafuji, D. Kotera, T. Aoki, S. Fujinami and T.-a. Yamagishi,
J. Org. Chem., 2012, 77, 11146.
Nature, 2000, 404, 982; (b) L. Ma, C. Abney and W. Lin, Chem. Soc.
Rev., 2009, 38, 1248; (c) M. Yoon, R. Srirambalaji and K. Kim, Chem.
Rev., 2011, 112, 1196.
14 (a) J. Zhang, Y.-G. Yao and X. Bu, Chem. Mater., 2007, 19, 5083;
(b) Q. Ye, D.-W. Fu, H. Tian, R.-G. Xiong, P. W. H. Chan and
S. D. Huang, Inorg. Chem., 2008, 47, 772; (c) M. Banerjee, S. Das,
M. Yoon, H. J. Choi, M. H. Hyun, S. M. Park, G. Seo and K. Kim,
J. Am. Chem. Soc., 2009, 131, 7524; (d) D. Dang, P. Wu, C. He, Z. Xie
and C. Duan, J. Am. Chem. Soc., 2010, 132, 14321; (e) K. Gedrich,
¨
5 (a) N. L. Strutt, H. Zhang, M. A. Giesener, J. Lei and J. F. Stoddart,
Chem. Commun., 2012, 48, 1647; (b) T. Ogoshi, H. Kayama, D. Yamafuji,
T. Aoki and T.-a. Yamagishi, Chem. Sci., 2012, 3, 3221; (c) K. Wang,
C.-Y. Wang, Y. Wang, H. Li, C.-Y. Bao, J.-Y. Liu, S. X.-A. Zhang and
M. Heitbaum, A. Notzon, I. Senkovska, R. Frohlich, J. Getzschmann,
U. Mueller, F. Glorius and S. Kaskel, Chem. – Eur. J., 2011, 17, 2099;
( f ) Q. Li, S. M. Shin, D. Moon, K. S. Jeong and N. Jeong, CrystEngComm,
2013, 15, 10161.
Y.-W. Yang, Chem. Commun., 2013, 49, 10528; (d) L. Gao, Z. Zhang, 15 (a) O. R. Evans, D. R. Manke and W. Lin, Chem. Mater., 2002,
S. Dong and M. Xue, Macromol. Rapid Commun., 2014, 35, 987.
6 (a) Y. Yao, M. Xue, X. Chi, Y. Ma, J. He, Z. Abliz and F. Huang, Chem.
Commun., 2012, 48, 6505; (b) M.-M. Tian, D.-X. Chen, Y.-L. Sun,
Y.-W. Yang and Q. Jia, RSC Adv., 2013, 3, 22111; (c) Y. Yao, M. Xue,
Z. Zhang, M. Zhang, Y. Wang and F. Huang, Chem. Sci., 2013,
4, 3667; (d) Y.-L. Sun, Y.-W. Yang, D.-X. Chen, G. Wang, Y. Zhou,
C.-Y. Wang and J. F. Stoddart, Small, 2013, 9, 3224; (e) Y. Yao,
Y. Zhou, J. Dai, S. Yue and M. Xue, Chem. Commun., 2014, 50, 869;
( f ) Y. Yao, K. Jie, Y. Zhou and M. Xue, Tetrahedron Lett., 2014,
55, 3195; (g) Y. Yao, K. Jie, Y. Zhou and M. Xue, Chem. Commun.,
2014, 50, 5072.
14, 3866; (b) C.-D. Wu, A. Hu, L. Zhang and W. Lin, J. Am. Chem.
Soc., 2005, 127, 8940; (c) C.-D. Wu, L. Zhang and W. Lin, Inorg.
Chem., 2006, 45, 7278; (d) C.-D. Wu and W. Lin, Angew. Chem., Int.
Ed., 2007, 46, 1075; (e) L. Ma, J. M. Falkowski, C. Abney and W. Lin,
Nat. Chem., 2010, 2, 838; ( f ) Y. Peng, T. Gong and Y. Cui, Chem.
Commun., 2013, 49, 8253; (g) S. Regati, Y. He, M. Thimmaiah, P. Li,
S. Xiang, B. Chen and J. C.-G. Zhao, Chem. Commun., 2013, 49, 9836;
(h) P. Li, Y. He, J. Guang, L. Weng, J. C.-G. Zhao, S. Xiang and
B. Chen, J. Am. Chem. Soc., 2014, 136, 547; (i) K. Mo, Y. Yang and
Y. Cui, J. Am. Chem. Soc., 2014, 136, 1746.
16 T. Ogoshi, D. Yamafuji, T. Akutsu, M. Naito and T.-a. Yamagishi,
Chem. Commun., 2013, 49, 8782.
7 (a) I. Nierengarten, S. Guerra, M. Holler, J.-F. Nierengarten and
R. Deschenaux, Chem. Commun., 2012, 48, 8072; (b) I. Nierengarten, 17 A. L. Spek, Acta Crystallogr., Sect. D: Biol. Crystallogr., 2009, 65, 148.
7458 | Chem. Commun., 2014, 50, 7455--7458
This journal is ©The Royal Society of Chemistry 2014