Porphyrin Boxes: Rationally Designed Porous Organic Cages

Article abstract of DOI:10.1002/anie.201505531

The porphyrin boxes (PB-1 and PB-2), which are rationally designed porous organic cages with a large cavity using well-defined and rigid 3-connected triangular and 4-connected square shaped building units are reported. PB-1 has a cavity as large as 1.95 n

Full text of DOI:10.1002/anie.201505531

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201505531
German Edition: DOI: 10.1002/ange.201505531
Organic Cages
Porphyrin Boxes: Rationally Designed Porous Organic Cages
Soonsang Hong, Md. Rumum Rohman, Jiangtao Jia, Youngkook Kim, Dohyun Moon,
Yonghwi Kim, Young Ho Ko, Eunsung Lee, and Kimoon Kim*
Dedicated to Professor Stephen J. Lippard on the occasion of his 75th birthday
Abstract: The porphyrin boxes (PB-1 and PB-2), which are
rationally designed porous organic cages with a large cavity
using well-defined and rigid 3-connected triangular and 4-
connected square shaped building units are reported. PB-1 has
a cavity as large as 1.95 nm in diameter and shows high
chemical stability in a broad pH range (4.8 to 13) in aqueous
media. The crystalline nature as well as cavity structure of the
shape-persistent organic cage crystals were intact even after
complete removal of guest molecules, leading to one of the
highest surface areas (1370 m2gÀ1) among the known porous
organic molecular solids. The size of the cavities and windows
of the porous organic cages can be modulated using different
sized building units while maintaining the topology of the
cages, as illustrated with PB-2. Interestingly, PB-2 crystals
showed unusual N2 sorption isotherms as well as high
selectivity for CO2 over N2 and CH4 (201 and 47.9, respectively
at 273 K at 1 bar).
usually packed efficiently with minimal void space, and their
supramolecular networks formed by noncovalent interactions
often collapse upon guest removal.[4] In fact, there are only
a limited number of organic molecular materials showing
permanent porosity with reversible gas sorption behavior.[5]
Since it is still extremely difficult to rationally design
supramolecular networks using molecular building blocks,[6]
it is highly advantageous to use covalent cage molecules with
prefabricated cavities for the synthesis of porous organic
solids with permanent porosity. Since the elegant work of
Cooper and co-workers,[7] increasing attention has been
drawn towards porous organic cages, and significant progress
has been made in synthesizing various cage compounds to
generate new porous organic solids.[8] However, there are only
a few organic cages with a relatively large cavity (cavity
diameter > 1.5 nm); furthermore, most of them become
nonporous solids during the drying process.[9] This is mainly
due to the lack of structural rigidity, which is essential for the
maintenance of a permanent cavity.[3b, 9b,10] Although the
recent work of Mastalerz and co-workers led to the successful
synthesis of large organic cages with high surface area,[11] the
cages formed by boronic ester bond are chemically unstable
and thus unsuitable for practical applications. Therefore,
a general and rational approach for the synthesis of shape-
persistent organic cages with a large cavity and high chemical
stability is still needed.
Porous organic cages may be rationally designed and
synthesized by combining two differently shaped building
units as the components of Archimedean solids. To date, most
porous organic cages have been synthesized by a combination
of 2-connected linear (or bent) and 3-connected triangular
shaped building units.[12] Although the combination of 3-
connected triangular and 4-connected square shaped building
units may generate even larger porous organic cages, it has
not been utilized yet for the synthesis of porous organic
cages.[13] Herein, we report the porphyrin boxes (PB-1 and
PB-2), which are rationally designed porous organic cages
with a large cavity by combination of well-defined and rigid 3-
connected triangular and 4-connected square shaped building
units (Scheme 1). The crystallinity of the shape-persistent
organic cage crystals was maintained even after desolvation,
leading to one of the highest surface areas (1370 m2gÀ1)
among the reported organic molecular porous materials. The
size of the cavities and windows of the organic cages can be
modulated using different sized building units while main-
taining the topology of the cages. Interestingly, PB-1 exhibits
excellent chemical stability in a wide pH range in aqueous
media and PB-2 crystals showed unusual N2 sorption iso-
T
he search for new porous materials is a subject of intense
research, as they offer a wide range of applications including
gas storage and separation. Extended porous frameworks
such as metal–organic frameworks (MOFs),[1] and covalent
organic frameworks (COFs)[2] have been extensively inves-
tigated for these purposes during the last decade. However,
relatively little attention has been paid to organic molecular
porous materials, even though they have potential benefits
over extended frameworks such as processability and easy
functionalization.[3] It may be due to the fact that they are
[*] S. Hong, Dr. M. R. Rohman, Dr. J. Jia, Dr. Y. Kim, Dr. Y. H. Ko,
Prof. Dr. E. Lee, Prof. Dr. K. Kim
Center for Self-assembly and Complexity (CSC)
Institute for Basic Science (IBS)
Pohang 790-784 (Republic of Korea)
E-mail: kkim@postech.ac.kr
Homepage: http://csc.ibs.re.kr/
S. Hong, Dr. Y. Kim, Prof. Dr. E. Lee, Prof. Dr. K. Kim
Department of Chemistry
Pohang University of Science and Technology
Pohang 790-784 (Republic of Korea)
Prof. Dr. K. Kim
Division of Advanced Materials Science
Pohang University of Science and Technology
Pohang 790-784 (Republic of Korea)
Dr. D. Moon
Beamline Department, Pohang University of Science and Technology
Pohang 790-784 (Republic of Korea)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201505531.
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Scheme 1. (a) Design and (b) synthesis of the covalent porphyrin box
(PB-1).
therms as well as highly selective sorption behavior of CO2
over other gases such as N2, CO, and CH4.
Figure 1. Crystal structure of PB-1: a) wireframe and b) space filling
model (C gray, N blue, O red). Hydrogen atoms are omitted for clarity.
c) N2 sorption isotherm at 77 K for PB-1 (solid circles: adsorption;
open circles: desorption). d) Pore size distribution profile.
As described above, our strategy for the synthesis of
porous organic cages involves the combination of 3-connected
triangular and 4-connected square shaped building units that
are the components of Archimedean solids (Scheme 1a). This
strategy is expected to afford a rhombicuboctahedron-like
geometry linked by six 4-connected square and eight 3-
connected triangular faces.[14] In this context, rigid porphyrin
molecules are good candidates as building units because of
their size, shape, and stability along with easy functionaliza-
tion and photophysical properties. Square-shaped meso-
tetra(p-formylphenyl)porphyrin (1) and triangular-shaped
(2,4,6-tributoxybenzene-1,3,5-triyl)trimethanamine (2) were
designed for the synthesis of cage compounds.
analysis confirmed that PB-1 has a rhombicuboctahedron
structure with square faces of 1 and triangular faces of 2
(Figure 1a,b). It has a large cavity of 1.95 nm in diameter and
twelve accessible windows with a dimension of 6.60 ” 8.50 Š
(Figure S13). In the solid-state structure, each single cage is
surrounded closely by six neighboring cages to form an
efficient packing without significant external voids (Figur-
es S14,S15). However, the cage cavities are connected with
each other through the windows to form open channels along
the a, b axes and [110] direction with a dimension of 6.60 ”
8.50 Š (Figure S16). The solvent-accessible volume of PB-1 is
estimated to be 48.2%.[16]
To demonstrate the generality of our synthetic approach,
we extended this strategy to synthesize another porphyrin
box, PB-2, using another triamine linker module, tris(2-
aminoethyl)amine (TREN). Heating a 1,2-dichlorobenzene
solution of 1 and TREN at 808C for 5 days and slowly cooling
down to room temperature afforded the crystalline product
PB-2 in 74.7% yield. The resulting cage compound was also
characterized by various analytical techniques including
elemental analysis (EA), mass-spectrometry, and NMR
spectroscopy (Figures S7, S17–S21). The solid-state structure
of PB-2 was determined by single-crystal X-ray crystallog-
raphy (Figure 2a,b), and the phase purity of the bulk
materials was confirmed by powder X-ray diffraction
(PXRD)[17] (Figure S22).
As anticipated, the reaction between six equivalents of
1 and eight equivalents of 2 in chloroform in the presence of
a catalytic amount of trifluoroacetic acid (TFA) resulted in
quantitative formation of the porphyrin box, PB-1, without
1
forming any side product (Scheme 1b). The H NMR spec-
trum of PB-1 is relatively simple, reflecting their highly
symmetric structure (Supporting Information, Figures S2–6).
The mass spectrum of PB-1 shows exclusively a single peak at
m/z = 6981.4 [M + H+] that corresponds to the [6+8] cage
composed of six units of 1 and eight units of 2 with a molecular
formula C456H444N48O24 (Figure S8). Taken together, these
results suggest that PB-1 is the thermodynamically most
stable and sole product formed during the reversible covalent
self-assembly.
To date, the majority of organic cages have been
constructed by reversible imine bond formation; however,
only a few of them are reported to be stable in aqueous
media.[10b,15] To check the chemical stability of PB-1, a series
of tests were conducted and results were confirmed by
1H NMR spectroscopy (Figures S9,S10). Surprisingly, PB-
1 shows exceptional chemical stability in the pH range
between 4.8 and 13. This unexpected chemical stability of PB-
1 in aqueous media may be partially attributed to the
hydrophobic nature of the butoxy groups located near the
imine bonds, which may significantly slow down the rate of
hydrolysis (Figure S11).
PB-2 crystallizes in the tetragonal space group P4/n with
two molecules in the unit cell. Unlike PB-1, it has a distorted
rhombicuboctahedron structure that is close to a cube shape
(Figure 2a,b). It has a cavity size of 1.13 nm in diameter and
the windows with a dimension of 1 ” 3 Š. A different packing
mode of PB-2 compared to that of PB-1 leads to a porous 3D
structure with extrinsic voids featuring 1D channels with
a diameter of 4.1 Š along the c axis (Figures S23–S25). The
solvent-accessible volume of PB-2 is estimated to be
42.1%.[16]
The structure of PB-1 was determined by single-crystal X-
ray crystallography. PB-1 crystallized in the trigonal space
Having established that the porphyrin boxes, consisting of
rigid building units, have 3D structures with intrinsic and/or
¯
group R3c with six molecules in the unit cell. Crystal structure
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gas-sorption.[19] PB-1 crystals exhibit a moderate CO2 uptake
(7.32 wt%) and selectivity for CO2 over N2 (12.43) and CH4
(3.96) at 273 K at 1 bar (Figures S29,S30). In contrast, PB-2
crystals show a much higher CO2 uptake (14.58 wt% at 273 K
at 1 bar; Figure 2d) even though their BET surface area is
much smaller than that of PB-1. The selectivity for CO2 over
N2 and CH4 is 201.06 and 47.91 (273 K), respectively (Fig-
ure S30). Such highly selective sorption of CO2 may be
attributed to the additional basic amine sites and narrow
channels in the packing structure of PB-2, which discriminate
CO2 with a small kinetic diameter (3.3 Š) from other gases
with large kinetic diameters such as N2, CO and CH4 (3.64,
3.76, and 3.84 Š, respectively).
In summary, we reported the porphyrin boxes, porous
organic cages which are rationally designed by a combination
of well-defined and rigid 3-connected triangular and 4-
connected square shaped building blocks. The size of cavities
and windows of the organic cages can be controlled using
different-sized building units. The porous organic cage
crystals exhibit not only a high surface area, but also unusual
gas sorption behavior. With exceptional chemical stability,
they may find useful applications especially in selective gas
sorption. We believe that the general approach presented
here is applicable to the synthesis of ultra-large, shape-
persistent organic cages leading to highly porous molecular
solids. Besides the porosity, the shape-persistent cages built
with porphyrin units may exhibit interesting chemical, photo-
physical, electrochemical, and biological properties.[20] Fur-
thermore, the large organic cages may encapsulate intriguing
guests such as fullerenes in the hydrophobic cavity through
host–guest interactions.[21] Finally, once metalated, the por-
phyrin boxes can be utilized as a building block to generate
metal-organic frameworks (MOFs) with many exciting appli-
cation perspectives. Work along these lines is in progress.
Figure 2. Crystal structure of PB-2: a) wireframe and b) space filling
model (C gray, N blue). Hydrogen atoms are omitted for clarity. Gas
sorption isotherms for PB-2 c) N2 at 77 K and d) CO2 (red triangles),
N2 (black circles), CO (blue squares), and CH4 (magenta stars) at
273 K (filled symbols: adsorption; open symbols: desorption).
extrinsic channels in the packing structure, their porosity was
investigated by N2 sorption measurements. The N2 sorption
behavior of PB-1 crystals follows a type I isotherm with small
hysteresis (Figure 1c). The Brunauer–Emmett–Teller (BET)
surface area was found to be 1370 m2gÀ1, making it one of the
largest among the known organic molecular porous materials.
The pore size distribution profile exhibits a narrow size
distribution at around 1.74 nm that is close to the cavity size of
PB-1 in the crystal structure (Figure 1d). PXRD analysis also
showed that the crystalline nature of desolvated-PB-1 crystals
was maintained (Figure S26). These crystals were further
analyzed by TGA which shows no weight loss at temperatures
below 3508C (Figure S27). Taking all these results together,
we concluded that the crystalline porous structure of PB-
1 crystals are intact even after desolvation. As demonstrated
by others,[8d] the porosity of PB-1 crystals may be increased by
introducing different substituents to the molecule and/or
using different crystallization conditions. Work along this line
is in progress.
Acknowledgements
This work was supported by Institute for Basic Science (IBS)
[IBS-R007-D1]. We thank Dr. H. W. Kim for theoretical
calculations. The X-ray crystallography experiment with
synchrotron radiation was performed at the Pohang Accel-
erator Laboratory (PLS-II BL2D SMC beamline).
Interestingly, unusual N2 sorption isotherms with large
hysteresis for PB-2 crystals were observed (Figure 2c). A
rapid uptake of N2 gas at low pressure followed by a plateau
region and further gradual uptake of N2 gas was observed.
Although we do not know the exact mechanism of this
unusual phenomenon at the moment, it may be due to the
gate-opening behavior or the narrow sized channels in the
crystal structure of PB-2. Similar phenomena have been
observed in MOFs and organic cages.[18] Further work to
elucidate the mechanism of this behavior is in progress. In
addition, CO2 sorption isotherm at 195 K revealed a micro-
porosity of PB-2 crystals with BET surface area of 935 m2gÀ1
(Figure S28). Since both PB crystals have permanent porosity
with different sized cavities (or channels) and chemical
environment, further sorption experiments were conducted
to better understand the structure–function relationship in
Keywords: chemical stability · organic cages ·
permanent porosity · porphyrins · selective gas sorption
How to cite: Angew. Chem. Int. Ed. 2015, 54, 13241–13244
Angew. Chem. 2015, 127, 13439–13442
[1] a) K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D.
Bloch, Z. R. Herm, T.-H. Bae, J. R. Long, Chem. Rev. 2012, 112,
724 – 781; b) J.-R. Li, J. Sculley, H.-C. Zhou, Chem. Rev. 2012,
112, 869 – 932; c) Y, He, W. Zhou, G. Qian, B. Chen, Chem. Soc.
Rev. 2014, 43, 5657 – 5678.
[2] a) X. Feng, X. Ding, D. Jiang, Chem. Soc. Rev. 2012, 41, 6010 –
6022; b) S.-Y. Ding, W. Wang, Chem. Soc. Rev. 2013, 42, 548 –
568.
[3] a) J. R. Holst, A. Trewin, A. I. Cooper, Nat. Chem. 2010, 2, 915 –
920; b) T. Hasell, H. Zhang, A. I. Cooper, Adv. Mater. 2012, 24,
Angew. Chem. Int. Ed. 2015, 54, 13241 –13244
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 13243

.
Angewandte
Communications
5732 – 5737; c) M. Brutschy, M. W. Schneider, M. Mastalerz,
S. R. Waldvogel, Adv. Mater. 2012, 24, 6049 – 6052; d) A. F.
Bushell, P. M. Budd, M. P. Attfield, J. T. A. Jones, T. Hasell, A. I.
Cooper, P. Bernardo, F. Bazzarelli, G. Clarizia, J. C. Jansen,
Angew. Chem. Int. Ed. 2013, 52, 1253 – 1256; Angew. Chem.
2013, 125, 1291 – 1294; e) M. W. Schneider, I. M. Oppel, A.
Griffin, M. Mastalerz, Angew. Chem. Int. Ed. 2013, 52, 3611 –
3615; Angew. Chem. 2013, 125, 3699 – 3703; f) M. Brutschy,
M. W. Schneider, M. Mastalerz, S. R. Waldvogel, Chem.
Commun. 2013, 49, 8398 – 8400; g) A. Kewley, A. Stephenson,
L. Chen, M. E. Briggs, T. Hasell, A. I. Cooper, Chem. Mater.
2015, 27, 3207 – 3210.
Schneider, I. M. Oppel, H. Ott, L. G. Lechner, H.-J. S. Hauswald,
R. Stoll, M. Mastalerz, Chem. Eur. J. 2012, 18, 836 – 847.
[9] a) A. Granzhan, T. Riis-Johannessen, R. Scopelliti, K. Severin,
Angew. Chem. Int. Ed. 2010, 49, 5515 – 5518; Angew. Chem.
2010, 122, 5647 – 5650; b) K. E. Jelfs, X. Wu, M. Schmidtmann,
J. T. A. Jones, J. E. Warren, D. J. Adams, A. I. Cooper, Angew.
Chem. Int. Ed. 2011, 50, 10653 – 10656; Angew. Chem. 2011, 123,
10841 – 10844.
[10] a) M. W. Schneider, I. M. Oppel, M. Mastalerz, Chem. Eur. J.
2012, 18, 4156 – 4160; b) M. Mastalerz, Chem. Eur. J. 2012, 18,
10082 – 10091.
[11] a) G. Zhang, O. Presly, F. White, I. M. Oppel, M. Mastalerz,
Angew. Chem. Int. Ed. 2014, 53, 1516 – 1520; Angew. Chem.
2014, 126, 1542 – 1546; b) G. Zhang, O. Presly, F. White, I. M.
Oppel, M. Mastalerz, Angew. Chem. Int. Ed. 2014, 53, 5126 –
5130; Angew. Chem. 2014, 126, 5226 – 5230.
[12] There are some exceptions: a) S. M. Elbert, F. Rominger, M.
Mastalerz, Chem. Eur. J. 2014, 20, 16707 – 16720; b) A. Avella-
neda, P. Valente, A. Burgun, J. D. Evans, A. W. Markwell-Heys,
D. Rankine, D. J. Nielsen, M. R. Hill, C. J. Sumby, C. J. Doonan,
Angew. Chem. Int. Ed. 2013, 52, 3746 – 3749; Angew. Chem.
2013, 125, 3834 – 3837.
[13] Although organic cages were synthesized using a similar strategy
by Warmuth and co-workers, the resulting organic cages were
not investigated in terms of porosity and chemical stability.
Furthermore, no X-ray crystal structures were reported: Y. Liu,
X. Liu, R. Warmuth, Chem. Eur. J. 2007, 13, 8953 – 8959.
[14] A preliminary computational study suggested that the formation
of a rhombicuboctahedron structure is most favorable among
possible Archimedean structures (see the Supporting Informa-
tion).
[4] J. Tian, P. K. Thallapally, B. P. McGrail, CrystEngComm 2012,
14, 1909 – 1919.
[5] Extrinsic porous materials: a) P. Sozzani, A. Comotti, R.
Simonutti, T. Meersmann, J. W. Logan, A. Pines, Angew.
Chem. Int. Ed. 2000, 39, 2695 – 2699; Angew. Chem. 2000, 112,
2807 – 2810; b) D. V. Soldatov, I. L. Moudrakovski, J. A. Rip-
meester, Angew. Chem. Int. Ed. 2004, 43, 6308 – 6311; Angew.
Chem. 2004, 116, 6468 – 6471; c) P. Sozzani, S. Bracco, A.
Comotti, L. Ferretti, R. Simonutti, Angew. Chem. Int. Ed.
2005, 44, 1816 – 1820; Angew. Chem. 2005, 117, 1850 – 1854;
d) M. Mastalerz, I. M. Oppel, Angew. Chem. Int. Ed. 2012, 51,
5252 – 5255; Angew. Chem. 2012, 124, 5345 – 5348; Intrinsic
´
porous materials: e) P. K. Thallapally, L. Dobrzanska, T. R.
Gingrich, T. B. Wirsig, L. J. Barbour, J. L. Atwood, Angew.
Chem. Int. Ed. 2006, 45, 6506 – 6509; Angew. Chem. 2006, 118,
6656 – 6659; f) S. J. Dalgarno, P. K. Thallapally, L. J. Barbour,
J. L. Atwood, Chem. Soc. Rev. 2007, 36, 236 – 245; g) S. Lim, H.
Kim, N. Selvapalam, K.-J. Kim, S. J. Cho, G. Seo, K. Kim, Angew.
Chem. Int. Ed. 2008, 47, 3352 – 3355; Angew. Chem. 2008, 120,
3400 – 3403; h) J. Tian, P. K. Thallapally, S. J. Dalgarno, P. B.
McGrail, J. L. Atwood, Angew. Chem. Int. Ed. 2009, 48, 5492 –
5495; Angew. Chem. 2009, 121, 5600 – 5603; i) H. Kim, Y. Kim,
M. Yoon, S. Lim, S. M. Park, G. Seo, K. Kim, J. Am. Chem. Soc.
2010, 132, 12200 – 12202.
[15] T. Hasell, M. Schmidtmann, C. A. Stone, M. W. Smith, A. I.
Cooper, Chem. Commun. 2012, 48, 4689 – 4691.
[16] A. L. Spek, PLATON, a multipurpose crystallographic tool,
Utrecht University, Utrecht, The Netherlands, 2001.
[17] It seems to undergo phase change upon reversible gas sorption.
But, at this stage, we do not know the exact crystal structure of
new phase.
[18] a) A. Schneemann, V. Bon, I. Schwedler, I. Senkovska, S. Kaskel,
R. A. Fischer, Chem. Soc. Rev. 2014, 43, 6062 – 6096; b) T. Mitra,
X. Wu, R. Clowes, J. T. A. Jones, K. E. Jelfs, D. J. Adams, A.
Trewin, J. Bacsa, A. Steiner, A. I. Cooper, Chem. Eur. J. 2011, 17,
10235 – 10240.
[19] The adsorption selectivity of PB-1 and PB-2 for CO2/N2 (15/85
molar ratio) and CO2/CH4 (equimolar ratio) mixtures at 273 K,
1 bar was calculated using the ideal adsorbed solution theory
(IAST): A. L. Myers, J. M. Prausnitz, AIChE J. 1965, 11, 121 –
130.
[20] S. Durot, J. Taesch, V. Heitz, Chem. Rev. 2014, 114, 8542 – 8578.
[21] Preliminary results indicate that C60 is encapsulated by PB-1.
Further investigation along this line is in progress.
[6] A significant progress has been made recently: J. T. A. Jones, T.
Hasell, X. Wu, J. Bacsa, K. E. Jelfs, M. Schmidtmann, S. Y.
Chong, D. J. Adams, A. Trewin, F. Schiffman, F. Cora, B. Slater,
A. Steiner, G. M. Day, A. I. Cooper, Nature 2011, 474, 367 – 371.
[7] T. Tozawa, J. T. A. Jones, S. I. Swamy, S. Jiang, D. J. Adams, S.
Shakespeare, R. Clowes, D. Bradshaw, T. Hasell, S. Y. Chong, C.
Tang, S. Thompson, J. Parker, A. Trewin, J. Bacsa, A. M. Z.
Slawin, A. Steiner, A. I. Cooper, Nat. Mater. 2009, 8, 973 – 978.
[8] a) Y. Jin, B. A. Voss, R. D. Noble, W. Zhang, Angew. Chem. Int.
Ed. 2010, 49, 6348 – 6351; Angew. Chem. 2010, 122, 6492 – 6495;
b) M. J. Bojdys, M. E. Briggs, J. T. A. Jones, D. J. Adams, S. Y.
Chong, M. Schmidtmann, A. I. Cooper, J. Am. Chem. Soc. 2011,
133, 16566 – 16571; c) H. Ding, Y. Yang, B. Li, F. Pan, G. Zhu, M.
Zeller, D. Yuan, C. Wang, Chem. Commun. 2015, 51, 1976 –
1979; d) G. Zhang, M. Mastalerz, Chem. Soc. Rev. 2014, 43,
1934 – 1947; e) A. G. Slater, A. I. Cooper, Science 2015, 348,
DOI: 10.1126/science.aaa8075; f) M. Mastalerz, M. W.
Schneider, I. M. Oppel, O. Presly, Angew. Chem. Int. Ed. 2011,
50, 1046 – 1051; Angew. Chem. 2011, 123, 1078 – 1083; g) M. W.
Received: June 18, 2015
Published online: August 25, 2015
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