A New Approach to Non-Coordinating Anions: Lewis Acid Enhancement of Porphyrin Metal Centers in a Zwitterionic Metal-Organic Framework

Article abstract of DOI:10.1021/jacs.6b05626

We describe a new strategy to generate non-coordinating anions using zwitterionic metal-organic frameworks (MOFs). By assembly of anionic inorganic secondary building blocks (SBUs) ([In(CO₂)₄]^-) with cationic metalloporphyrin-based organic linkers, we prepared zwitterionic MOFs in which the complete internal charge separation effectively prevents the potential binding of the counteranion to the cationic metal center. We demonstrate the enhanced Lewis acidity of Mn^III- and Fe^III-porphyrins in the zwitterionic MOFs in three representative electrocyclization reactions: [2 + 1] cycloisomerization of enynes, [3 + 2] cycloaddition of aziridines and alkenes, and [4 + 2] hetero-Diels-Alder cycloaddition of aldehydes with dienes. This work paves a new way to design functional MOFs for tunable chemical catalysis.

Full text of DOI:10.1021/jacs.6b05626

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Article
A New Approach to Non-coordinating Anions: Lewis Acid Enhancement
of Porphyrin Metal Centers in a Zwitterionic Metal-Organic Framework
Jacob A. Johnson, Brenna M Petersen, Attila Kormos, Elena Echeverría, Yu-Sheng Chen, and Jian Zhang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b05626 • Publication Date (Web): 20 Jul 2016
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A New Approach to Non-coordinating Anions: Lewis Acid Enhance-
ment of Porphyrin Metal Centers in a Zwitterionic Metal-Organic
Framework
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Jacob A. Johnson,† Brenna M. Petersen,† Attila Kormos,‡ Elena Echeverría,# Yu-Sheng Chen,§
and Jian Zhang*,†
†Department of Chemistry, University of Nebraska−Lincoln, Lincoln, Nebraska 68588, United States
‡Chemical Biology Research Group, Institute of Organic Chemistry, Research Centre for Natural Sciences, Hungari-
an Academy of Sciences, Magyar tudósok krt. 2, H-1117, Budapest, Hungary
#Department of Physics and Astronomy, Theodore Jorgensen Hall, 855 N 16th St., University of Nebraska, Lincoln,
Nebraska 68588-0299, United States
§ChemMatCARS, Center for Advanced Radiation Sources, The University of Chicago, 9700 South Cass Avenue, Ar-
gonne, Illinois 60439, United States
ABSTRACT: We describe a new strategy to generate non-coordinating anions using zwitterionic metal-organic frame-
works (MOFs). By assembly of anionic inorganic secondary building blocks (SBUs) ([In(CO2)4]–) with cationic metal-
loporphyrin-based organic linkers, we prepared zwitterionic MOFs in which the complete internal charge separation ef-
fectively prevents the potential binding of the counter anion to the cationic metal center. We demonstrate the enhanced
Lewis acidity of MnIII- and FeIII-porphyrins in the zwitterionic MOFs in three representative electrocyclization reactions:
[2+1] cycloisomerization of enynes, [3+2] cycloaddition of aziridines and alkenes, and [4+2] hetero-Diels-Alder cycloaddi-
tion of aldehydes with dienes. This work paves a new way to design functional MOFs for tunable chemical catalysis.
INTRODUCTION
portantly, in such zwitterionic MOFs the anionic SBU is
essentially “non-coordinating”, which results in an en-
hanced Lewis acidity of the cationic metal center in the
organic linker (Scheme 1c). Zwitterionic MOFs should
also compare favorably to MOFs that are composed of
cationic metalloligands with extraframework WCAs in
which only the partial charge separation is present
(Scheme 1b). Herein, we describe a proof of concept study
by synthesizing porphyrinic MOFs12 that are comprised of
anionic [In(CO2)4]– SBUs and cationic metalloporphyrin
Cationic transition metal complexes are frequently used
to promote organic reactions, during which the ion pair-
ing interaction between the metal center and the counter
anion significantly affects the activation of substrates and
catalytic activity in general.1 Oftentimes, weakly coordi-
nating anions (WCAs),2,3 realized by either increasing the
volume or introducing electron-withdrawing substitu-
ents,4 are employed to increase the net positive charge of
the cationic metal center and enhance the Lewis acidity
(i.e. electrophilicity).5 Another attractive approach in-
volves the use of zwitterionic metal complexes, in which a
cationic metal fragment and a negatively charged ancil-
lary ligand are covalently bonded together to achieve a
formal charge separation.6 The internal charge neutraliza-
tion and tunable electrophilicity of zwitterionic metal
complexes has been particularly appealing for a variety of
catalytic reactivity studies.7
Scheme 1. Design Strategy to Enhance Lewis Acidity
of Metal Center Using Zwitterionic MOFs
MOFs are a class of highly porous crystalline solid ma-
terials8 that have found vast application potential in
chemical catalysis.9 Since both components of MOFs, the
inorganic SBU (secondary building unit) and organic
linker, can be constructed to exhibit net ionic charge,10 it
is therefore possible to prepare zwitterionic MOFs,11 for
example, by the assembly of anionic inorganic SBUs with
cationic metal complex-based linkers (Scheme 1). Im-
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linkers. We demonstrate that, with increased net positive Single crystal X-ray diffraction structural analysis re-
vealed that UNLPF-13-MnIIICl, UNLPF-14-MnIII, UNLPF-
15-FeIIICl, and UNLPF-16-FeIII crystallize in the same or-
thorhombic Pnnm space group and are isostructures of
UNLPF-10 (Supporting Information, Table S1-4). A chlo-
ride anion was crystallographically located on the porphy-
rin metal centers in the case of UNLPF-13-MnIIICl and -15-
FeIIICl, with MIII…Cl distances of 2.16Å and 2.20Å, respec-
tively (Figure S2). As expected, anion coordination was
not present in UNLPF-14-MnIII and -16-FeIII. Instead, the
coordination of water was observed with MIII…O distances
of 2.25Å and 2.29Å, respectively, in good agreement with
reported values.15 Thorough solvent treatment of UNLPF-
14-MnIII and -16-FeIII ensured the complete removal of the
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charge in the cationic metal center, such zwitterionic
MOFs exhibit enhanced catalytic activities for three rep-
resentative organic transformations, that is, the [2+1] cy-
cloisomerization of enynes, the [3+2] cycloaddition of
aziridines and alkenes, and the [4+2] hetero-Diels-Alder
cycloaddition of aldehydes with dienes. Due to the in-
creased Lewis acidity of the porphyrin metal center, the
zwitterionic MOFs indeed afford higher reaction yields as
well as an improved chemo-, and/or regioselectivity in all
three reactions.
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RESULTS AND DISCUSSION
Ligand and MOFs Synthesis. Using the extended oc-
tatopic ligand TBCPPP (tetrakis-3,5-bis((4-carboxy)-
phenyl)phenyl-porphyrin) developed in our group,13 we
first synthesized TBCPPP-MnIIICl and -FeIIICl linkers (Fig-
ure 1a) (See Supporting Information for detailed synthetic
procedure). Anion metathesis with AgBF4 was then em-
ployed to replace the strongly (S) coordinated Cl– with
–
labile extra-framework BF4 , evidenced by the absence of
the signal of 19F (δ ~ –149 ppm) in the 19F NMR spectra of
acid-digested MOF samples (Figure S4). The cationic dye
uptake analysis13c for UNLPF-14-MnIII and -16-FeIII reveals
an overall framework charge of “–1” per cage, consistent
with the average charge occupation of the two [In(CO2)4]–
SBUs and one [MIII-porphyrin]+ center per cage.
–
weakly (W) coordinated BF4 . The subsequent solvother-
mal reaction of octatopic TBCPPP-MnIII[BF4] and -
FeIII[BF4] ligands with In(NO3)3 resulted in non-
coordinating (N) UNLPF-14-MnIII and UNLPF-16-FeIII,
respectively. We also prepared the following MOFs as
controls for the catalytic studies (vide infra): (1) UNLPF-
13-MnIIICl and UNLPF-15-FeIIICl using TBCPPP-MIIICl
linkers (M = Mn and Fe) (Figure 1a); (2) neutral metal-
loporphyrinic MOFs PCN-223-MnIIICl and PCN-223-FeIIICl
using tetratopic TCPP-MIIICl linkers (TCPP = tetrakis(4-
carboxyphenyl)-porphyrin, M = Mn and Fe); and (3) PCN-
223-MnIII[SbF6] and PCN-223-FeIII[BF4] via post-synthetic
anion metathesis with AgSbF6 and AgBF4, respectively
(reported by Zhou et al14) (Figure 1b).
Figure 2. UV-vis spectra of (a) UNLPF-13,14 and (b) UNLPF-
15,16 and XPS spectra showing the binding energy of N1s for
(c) UNLPF-13,14, PCN-223-MnIIICl, and PCN-223-MnIII[SbF6]
and (d) UNLPF-15,16, PCN-223-FeIIICl, and PCN-223-
FeIII[BF4].
The change of coordination environment in the por-
phyrin metal center resulted from anion metathesis was
further confirmed using UV-vis and X-ray photoemission
spectroscopy (XPS). Specifically, in the absence of a
strongly binding of the chloride ligand, a small blue shift
of 5 nm and 8 nm of the Q1 band of the metalloporphyrin
macrocycle in UNLPF-14-MnIII and in -16-FeIII (Figure 2a-
b), respectively, consistent with the literature values16 as
well as the corresponding linkers (Supporting Infor-
mation, Figure S1). Moreover, the disappearance of the Cl
(2p) signal at 201.2 and 200.6 eV, respectively (Figure S5),
as well as the increase of the binding energy of N1s in the
XPS spectra (Figure 2c-d) of UNLPF-14-MnIII (∆BE = 1.30
eV) and -16-FeIII (∆BE = 1.45 eV) also provide experimental
proofs of successful preparation of zwitterionic MOF
structures that exhibit the complete charge separation of
Figure 1. Structures of porphyrinic MOFs used in this work:
(a) UNLPF-13-MnIIICl, UNLPF-14-MnIII, UNLPF-15-FeIIICl, and
UNLPF-16-FeIII; (b) PCN-223-MnIIICl, PCN-223-MnIII[SbF6],
PCN-223-FeIIICl, and PCN-223-FeIII[BF4].
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(TPP = tetraphenylporphyrin) (entry 5). This result high-
the anionic SBUs and metalloporphyrin’s cationic center.
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Notably, the observed increase in the binding energy for
the anion-exchanged PCN-223-MnIII[SbF6] and PCN-223-
FeIII[BF4] was smaller (0.30 eV and 0.73 eV, respectively),
likely due to the presence of the labile extra-framework
counter anions (i.e. SbF6 and BF4 ). In addition, thermal-
gravimetric analysis (TGA) reveals a substantial initial
weight loss (61.5-68.2 wt%) internally trapped solvent
molecules, demonstrating the highly porous nature of
UNLPF-13-MnIIICl, -14-MnIII, -15-FeIIICl, and -16-FeIII (Fig-
ure S7).
lights the essential role of the complete charge separation
that is only attainable in our zwitterionic MOFs. Fur-
thermore, UNLPF-14-MnIII exhibits an excellent recycla-
bility: a 92% yield was obtained after three times of cata-
lyst reuse (entry 8), while still maintaining good crystal-
linity (Figure S8). Additionally, ICP-MS performed on the
supernatant of the reaction mixture indicated a low cata-
lyst leaching of 0.20 mol% (Scheme S3).
–
–
9
To demonstrate the scope of the cycloisomerization of
1,6-enynes using UNLPF-14-MnIII as the catalyst we car-
ried out the reaction using different enynes 4 (Table 2)
under the optimized reaction condition (Table 1, entry 1).
The reaction shows good tolerance toward terminal, al-
kyl- and dialkyl-substituted, as well as phenyl-substituted
alkenes (4a-4d). Interestingly, we did not observe the
decomposition of substrate 4d in our catalytic system, as
was the case in the presence of MnTPP[TFPB].18 Our
MOF-based catalyst also exhibits excellent to good reac-
tivity toward 1,6-enynes that contain internal alkynes
such as 4e (88% yield of 5e) and 4f (67% yield of 5f), for
which the cationic MnTPP[SbF6] catalyst exhibited negli-
gible catalytic activity (Table 2). According to the pro-
posed mechanism18 (Scheme S4), the coordination of the
Lewis acid to the β-C of the alkyne with respect to the
nitrogen tether is the first and likely the rate limiting step
leading to the endo-product.18 However, in the presence
of a weakly coordinating anion (e.g. OTf–) the catalysts’
active site is likely sterically hindered, in which case the γ-
C of the alkyne may coordinate instead further leading to
the exo-product (Table 1, entry 6). Thus, the high selectiv-
ity of UNLPF-14-MnIII towards the endo-product is likely
due to the “non-coordinating” nature of the SBU-based
counter anion. In effect this renders the Lewis acidic met-
al center more “naked”, allowing for substrates with steri-
cally demanding internal alkynes (4e and 4f) to coordi-
nate and further undergo the [2+1] cycloaddition. To the
best of our knowledge, transition-metal (Rh, Ir, Pt, and
Au) complexes are usually required to prepare endo cyclic
products 5e or 5f with however poor to moderate yields.19
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[2+1] Cycloisomerization of Enynes. In order to
demonstrate the enhanced Lewis acid catalytic activity of
the synthesized zwitterionic MOFs, we first examined
their reactivity toward the cycloisomerization of enynes.17
Cationic MnIII-porphyrin is known to catalyze the cycloi-
somerization of 1,6-enynes where the coordination capa-
bility of the counter anion is essential toward the
chemoselectity.18 In general, weakly coordinated anions
seem to promote the rare cyclopropane-annulated endo-
cyclized six-membered product via an intramolecular
stepwise [2+1] cycloaddition, with the highest yield of 75%
(with TFPB, tetrakis[3,5-bis(trifluoromethyl)-phenyl]-
borate).18 To our delight, employing only 1 mol% of
UNLPF-14-MnIII as the catalyst, an excellent yield and
selectivity toward the endo-product 5a (95% isolated
yield) was obtained within only 8h (Table 1, entry 1). In
contrast, PCN-223-MnIII[SbF6] exhibits essentially the
same catalytic activity and selectivity for the endo-
product 5a (63%/31% yield of endo/exo product) as the
homogenous catalyst MnTPP[SbF6] (entries 4 and 7), like-
–
ly due to the presence of the extra-framework SbF6 . As
expected, the presence of the strongly coordinating chlo-
ride anion in UNLPF-13-MnIIICl completely inhibits the
catalytic activity (entry 2), consistent with PCN-223-
MnIIICl (entry 3) and the homogeneous catalyst MnTPPCl
Table 1. [2+1] Cycloisomerization of enynesa
Table 2. Scope of [2+1] Cycloisomerization of Enynesa
yield 5a
(%)
yield 6a
(%)
entry
catalyst
t (h)
1
2
UNLPF-14-MnIII
UNLPF-13-MnIIICl
PCN-223-MnIIICl
PCN-223-MnIII[SbF6]
MnTPPCl
8
95
<1
<1
63
<1
<1
61
92
<1
<1
<1
31
24
24
24
24
24
24
8
H
TsN
TsN
TsN
3
H
4
5a, 8h, 95%
5b, 8h, 92%
5c, 8h, 96%
5b,c
6b,c
7b,c
8d
<1
95
30
<1
MnTPP[OTf]
MnTPP[SbF6]
UNLPF-14-MnIII
aReaction conditions: MnIII-porphyrin catalysts (1
mol%), and enyne (0.2 mmol) in 1.6 mL of anhydrous xy-
aReaction conditions: catalyst (1 mol %) and enyne (0.2
mmol) in 1.6 mL of anhydrous xylenes at 160°C.
bMnTPP[SbF6] as the catalyst.
b
c
lenes at 160°C. From Ref 18. Catalyst loading: 10 mol%.
dAfter 3rd catalyst recycle.
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[3+2] Cycloaddition of Aziridines and Alkenes. En- pyrrolidine 9a in 95% yield as a diastereomeric mixture
(Table 3 and Table S5). UNLPF-14-MnIII also exhibited
excellent recyclability. After three times of catalyst reuse,
a 92% yield was obtained (Table S5). ICP-MS performed
on the supernatant of the reaction mixture revealed a low
catalyst leaching of 0.05 mol% (Scheme S6). The catalytic
activity of zwitterionic UNLPF-14-MnIII is significantly
enhanced compared to that of MnTPP[SbF6] (5 mol%,
63% yield in 12h)21, TBCPPP-MnIII[BF4] (5 mol%, 54% yield
in 12h), and PCN-223-MnIII[SbF6] (1 mol%, 60% yield in
24h) (see Supporting Information Table S5). Strong coor-
dination of chloride completely quenched the reaction in
UNLPF-13-MnIIICl and PCN-223-MnIIICl (Table S5), similar
to MnTPPCl.21
We next examined the UNLPF-14-MnIII catalyzed [3+2]
cycloaddition of aziridine 7 with different alkenes 8. Sty-
renes with electron-withdrawing substituents such as
chloride (8a) and bromide (8c) as well as electron-
donating substituents such as methoxy (8d) and tert-
butyl (8e) all gave rise to the pyrrolidine product 9 in ex-
cellent yields (>88%, Table 3). Remarkably, the internal
alkene, anethole (8h), is also a suitable substrate for
aziridine 7a to give pyrrolidine 9h (87% yield, 12h). In
contrast, MnTPP[SbF6] only afforded a low yield (19%,
24h), in which the major by-product was determined to
be the Friedel-Craft arylation product.22 We attribute this
result to capability of UNLPF-14-MnIII to better stabilize
the 1,3-dipole intermediate following the C-N cleavage of
the aziridine, further allowing for more challenging sty-
rene substrates to attack (Scheme S5). To the best of our
knowledge, this is the first reported example of a pyrroli-
dine formed from an aromatic internal alkene and an
aziridine. Our catalyst protocol may be used to prepare
the derivatives of pharmaceutically relevant compounds
such as Atrasentan (Table 3, inset). Moreover, aziridines
with electron-withdrawing substituting groups such as
fluoride (7b), chloride (7g), and trifluromethyl (7f) were
also suitable substrates, and the functionalized pyrroli-
dines were obtained in excellent yields (>88%).
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couraged by this result, we next tested the catalytic activi-
ty of UNLPF-14-MnIII towards a Lewis acid catalyzed bi-
molecular [3+2] cycloaddition of aziridines and alkenes. In
general, Lewis acids are able to activate aziridines, gener-
ating a zwitterionic 1,3-dipolar intermediate via C-N bond
cleavage, which may undergo stereo- and regioselective
[3+2] cycloaddition to form pyrrolidines.20 It was recently
found that cationic MnIII-porphyrin allowed for styrene
derivatives to react with aziridines in the presence of a
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WCA such as SbF6 . With the essential non-coordinating
SBU-based anions in UNLPF-14-MnIII, an enhanced cata-
lytic performance of the MnIII-porphyrin centers was an-
ticipated. Indeed, the cycloaddition of 2-phenyl-1-
tosylaziridine (7a) with 4-chlorostyrene (8a) in the pres-
ence of UNLPF-14-MnIII (1 mol%) in 1,2-dichloroethane
(DCE) at 100°C for 8h afforded the corresponding
Table 3. [3+2] Cycloaddition of Aziridines and Al-
kenesa
To further test the scope of this reaction we employed
an aziridine functionalized with an even stronger elec-
tron-withdrawing group, NO2 (7g). Remarkably, when
styrene is used as the alkene the corresponding pyrroli-
dine (9g) can be obtained in 93% yield albeit a longer
reaction time (24h) is required. This may be attributed to
the decreased nucleophilicily of the nitrogen which, ac-
cording to the proposed mechanism (Scheme S5), coordi-
nates to the cationic MnIII-porphyrin center. When the
sterically demanding substrate 4-benzhydrylstyrene (8i)
was combined with 7a in the presence of UNLPF-14-MnIII,
a poor yield (10% in 24h) was achieved for the corre-
sponding pyrrolidine 9i. Further, increasing the catalyst
loading to 5 mol% had minimal effect on the catalysts’
performance (24% in 24h). However, the homogenous
catalyst MnTPP[SbF6], was capable of achieving a good
yield of 67% in 24h. We tentatively attribute the inferior
performance of UNLPF-14-MnIII towards this reaction to
the inefficient product diffusion within the MOF pores
due to the geometric dimensions of 9i being too large to
aReaction conditions: catalyst (1 mol %), aziridine (0.2
mmol), and alkene (0.6 mmol) in 0.8 mL of anhydrous DCE
b
c
at 80°C. From Ref 21. 1 mol% of PCN-223-MnIII[SbF6] as the
catalyst. dThermal ellipsoids are set at the 50% probability
f
level. eCatalyst loading increased to 5 mol%. 5 mol% of
MnTPP[SbF6] as the catalyst.
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decreased nucleophility of the aldehyde oxygen, which
fit through cage windows of the framework (Figures S9-
must coordinate to the cationic FeIII-porphyrin center to
activate the substrate according to the proposed mecha-
nism (Scheme S7). Nevertheless, comparing to
FeTPP[BF4] and PCN-223-FeIII[BF4], UNLPF-16-FeIII clearly
exhibited a faster reaction rate and higher reaction yield
(92% and 83% yield for 12d and 12e, respectively). The
enhanced reactivity is likely due to the better polarization
and further activation of the C=O bond upon coordina-
tion of the benzaldehyde substrate to zwitterionic
UNLPF-16-FeIII.
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5
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7
8
11). This result also strongly suggests that the catalytic
reaction indeed mostly occurs inside the MOF’s channels.
Interestingly, pyrrolidines 9a and 9f crystallize in the chi-
ral monoclinic space group P21/c as a disordered mixture
of diastereomers (Figure S13 and Tables S6-7).
[4+2] Hetero-Diels-Alder Cycloaddition of Alde-
hydes with Dienes. To extend our zwitterionic MOF
catalysis protocol to other cationic metalloporphyrin sys-
tems, we tested the catalytic activity of UNLPF-16-FeIII
towards the [4+2] hetero-Diels-Alder cycloaddition of
aldehydes with dienes.23 This reaction can be used to pre-
pare pyrans, a class of synthetic targets for biologically
relevant compounds.24 It was recently reported that cati-
onic FeIII-porphyrin can promote the chemoselective cy-
cloaddition of unactivated, electron deficient aldehydes
with simple, poorly reactive dienes.25 Similarly, the coor-
dination strength of the counter anion dictates the cata-
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CONCLUSIONS
In summary, we have provided a proof of concept study
in which the zwitterionic MOFs indeed mimic the zwit-
terionic metal complexes and cationic metal complexes
consisting of WCAs. The enhanced Lewis acid catalytic
activities of the porphyrin metal centers in the anionic
indium metal-organic frameworks are due to the com-
plete charge separation and the resulting change of the
electronic properties including the orbital energies and
the distribution of electronic density. This change of in
terms of electronic structure considerably affects the acti-
vation of substrates and lowers the activation barrier of
subsequent transformation steps in three representative
electrocyclization reactions. This study paves a new way
for designing functional MOFs for tunable chemical ca-
talysis, which is currently underway in our laboratory.
–
lytic activity in that the WCA, SbF6– or BF4 , was necessary
for a high catalytic activity.25 Indeed, the reaction of ben-
zaldehyde (10a) with 2,3-dimethyl-1,3-butadiene (11) in
the presence of zwitterionic UNLPF-16-FeIII in toluene at
80°C for 8h afforded pyran 12a in 96% yield (Table 4 and
Table S8), faster than FeTPP[BF4] (5 mol% loading, 92%
in 12h).25 Again a low catalyst leaching of 0.06 mol% was
demonstrated by ICP-MS for the supernatant of the reac-
tion mixture (Scheme S8). As expected, in the presence of
UNLPF-15-FeIIICl or PCN-223-FeIIICl the reaction became
retarded (Table S8) due to the presence of the strongly
coordinating Cl–, similar to the result from FeTPPCl.25
ASSOCIATED CONTENT
Supporting Information. Materials, general experimental
procedures, synthesis of UNLPF-13, -14, -15, and -16 and cata-
lytic studies, characterizations of compounds, and crystallo-
graphic data. This material is available free of charge via the
Internet at http://pubs.acs.org.
UNLPF-16-FeIII also has a good substrate scope: benzal-
dehydes with both electron-withdrawing (bromide 10b)
and electron-donating (methyl 10c) substituents are suit-
able to furnish the pyran products in excellent yields (Ta-
ble 4). Nitrile and nitro substituents are also tolerated
albeit a longer reaction time is required, likely due to the
AUTHOR INFORMATION
Corresponding Author
Table 4. [4+2] Hetero-Diels-Alder Cycloaddition of
Aldehydes with Dienesa
jzhang3@unl.edu
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
The authors acknowledge the support from the University of
Nebraska−Lincoln and NSF through Nebraska MRSEC
(DMR-0820521). ChemMatCARS Sector 15 is principally sup-
ported by the Divisions of Chemistry (CHE) and Materials
Research (DMR), National Science Foundation, under grant
number NSF/CHE-1346572. Use of the Advanced Photon
Source, an Office of Science User Facility operated for the
U.S. Department of Energy (DOE) Office of Science by Ar-
gonne National Laboratory, was supported by the U.S. DOE
under Contract No. DE-AC02-06CH11357. A. K. thanks the
Rosztoczy Foundation for a fellowship.
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