Redox-Active 1D Coordination Polymers of Iron-Sulfur Clusters

Article abstract of DOI:10.1021/jacs.8b12339

Here we describe the combination of an archetypal redox-active metal sulfide cluster, Fe₄S₄, with an organic linker, 1,4-benzenedithiolate, to prepare coordination polymers containing infinite chains of Fe₄S₄ cl

Full text of DOI:10.1021/jacs.8b12339

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Article
Redox-Active 1D Coordination Polymers of Iron-Sulfur Clusters
Noah E. Horwitz, Jiaze Xie, Alexander S. Filatov, Robert J. Papoular, William
E. Shepard, David Z. Zee, Mia P. Grahn, Chloe Gilder, and John S. Anderson
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b12339 • Publication Date (Web): 04 Feb 2019
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Redox‐Active 1D Coordination Polymers of Iron‐Sulfur Clusters
Noah E. Horwitz,†‡ Jiaze Xie,†‡ Alexander S. Filatov,† Robert J. Papoular,§ William E. Shepard,⊥ Da-
vid Z. Zee,∥ Mia P. Grahn,† Chloe Gilder,† and John S. Anderson*†
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†Department of Chemistry, University of Chicago, Chicago, Illinois 60637, United States
§Saclay Institute for Matter and Radiation [ IRAMIS ], Leon Brillouin Laboratory, CEA-Saclay, 91191 Gif-sur-Yvette,
France.
⊥Synchrotron SOLEIL, L'Orme des Merisiers Saint-Aubin, BP 48, 91192 Gif-sur-Yvette CEDEX, France
∥Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
ABSTRACT: Here we describe the combination of an archetypal redox-active metal sulfide cluster, Fe4S4, with an organic
linker, 1,4-benzenedithiolate, to prepare coordination polymers containing infinite chains of Fe4S4 clusters. The crystal struc-
tures of two solid materials have been solved from synchrotron X-ray powder diffraction data using simulated annealing and
refined by a least-squares Rietveld refinement procedure. The electronic properties of these chains have also been character-
ized by UV-visible and Mössbauer spectroscopies. Additional experiments demonstrated that these chains can be solubilized
by variation of the countercation and that the chain structure is maintained in solution. The redox-activity of the Fe4S4 clusters
can be accessed with chemical reagents. Introduction of charge carriers by reduction of the Fe4S4 clusters is found to increase
the electrical conductivity of the materials by up to four orders of magnitude. These results highlight the utility of Fe4S4 clus-
ters as redox-active building blocks in preparing new classes of coordination polymers.
INTRODUCTION
majority of these materials, however, are based on monometal-
lic nodes. Comparatively, the use of metal-chalcogenide clus-
ters as building blocks has been underexplored. The use of
metal-chalcogenide clusters to rationally impart desired prop-
erties to materials is well demonstrated by recent work in
which hydrogen evolution catalysts can be prepared from Mo-
S clusters linked by organosulfur ligands. 7 Among possible
metal chalcogenide clusters, the cubane-type Fe4S4 cluster
stands out as an attractive building block due to its redox-ac-
tivity, stability, electronic delocalization, and complex mag-
netic features – properties also exploited by nature in ubiqui-
tous iron-sulfur proteins.8 Of particular relevance are proteins
containing chains of Fe4S4 clusters that facilitate electron
transport over long distances.9 Several synthetic examples fur-
ther demonstrate the utility of Fe4S4 clusters towards building
materials with unusual physical and chemical properties. For
instance, Kanatzidis and coworkers have prepared a crystalline
framework10 and a family of amorphous chalcogels with Fe4S4
units connected by inorganic chalcogenide linkers. The Fe4S4
clusters retain their redox-activity in the chalcogels, enabling
redox catalysis by these materials.11 Additionally, Pickett and
coworkers have studied the synthesis and conductivity of a se-
ries of materials in which Fe4S4 clusters are bound to substi-
tuted polypyrroles through electrostatic interactions or
Hybrid inorganic-organic materials are an attractive area of
research as they can exhibit a wide array of physical and chem-
ical properties that can be tuned through substitution of their
components. Recent interest in these types of materials is ex-
emplified by coordination polymers such as metal-organic
frameworks (MOFs)1 and hybrid perovskites.2 In particular,
there has been a surge of interest in coordination polymers ex-
hibiting intrinsic magnetic and conductive properties as these
materials show potential for applications in energy storage and
electronics.3 It is crucial to carefully match the properties, par-
ticularly the electronic properties, of the metal and linker in
these materials to optimize their performance. Detailed studies
have shown that moving from O-based linkers to less electro-
negative and consequently more donating N-based linkers im-
proves coupling and delocalization.3 By the same rationale, fur-
ther work has shown that using heavier chalcogenide-based
linkers (S-based in particular) in place of more common O- or
N-based ligands can enhance electronic coupling leading to im-
provement in conductivity.4
These studies have generated several examples of materials
which highlight that the combination of organosulfur ligands
with transition metal ions leads to coordination polymers with
desirable properties such as high electrical conductivity.5,6 The
Scheme 1. Synthesis of [Fe4S4(BDT)2][NR4]2 coordination polymers.
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reaction mixture. We speculate that this effect may be at-
tributed to ionic screening as discussed below.
1
We have performed several experiments to validate the pro-
posed compositions of 1 and 2. Digestion of 1 and 2 in 12 M
hydrochloric acid followed by extraction with C6D6 or CD2Cl2
and 1H NMR analysis supports the presence of BDT in the struc-
tures (Figures S4 and S6). No thiophenol from the
[Fe4S4(SPh)4]2- starting material is observed, which is also con-
sistent with complete ligand substitution in the precipitated
material. The infrared spectra of 1 and 2 do not show any fea-
tures attributable to S-H stretching modes near 2500 cm-1, fur-
ther demonstrating that BDT is fully deprotonated in 1 and 2
(Figures S39 and S40). Additionally, digestion in D2SO4 fol-
lowed by dilution with (CD3)2SO and 1H NMR analysis confirms
the presence of TBA in 1 and TMA in 2 (Figures S5 and S7). The
presence of intact Fe4S4 clusters is more difficult to confirm
with the same method, as these clusters are not stable to acid
digestion. Mössbauer spectroscopy, however, provides strong
evidence for the presence of intact Fe4S4 clusters in 1 and 2. The
80 K 57Fe Mössbauer spectrum (Figure 1a) of 1 consists of a
quadrupole doublet with isomer shift (δ) of 0.4345(7) mm/s
and quadrupole splitting (ΔEQ) of 0.714(1) mm/s. These values
are similar to those reported for other [Fe4S4]2+ cluster materi-
als and to [Fe4S4(SPh)4]2-,8,10,11b suggesting that the cluster is
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present in
1
in the same oxidation state as the
[Fe4S4(SPh)4][TBA]2 precursor. The spectrum (Figure 1b) of 2
can be fitted by two quadrupole doublets with δ of 0.4339(9)
and 0.447(1) mm/s and ΔEQ of 1.330(4) and 0.610(4) mm/s,
suggesting that two distinct [Fe4S4] sites are present in the
structure of 2. All spectra are fit with Voigt line profiles,
potentially reflecting a distribution of Mössbauer parameters
due to structural disorder.13 Spectra of 1 and 2 at 25 K and 4.2
K are similar to those at 80 K, showing increases in quadrupole
splittings upon cooling (Figures S34, S35).
Additional experiments were performed to verify the empir-
ical formulas of 1 and 2. X-ray photoelectron spectroscopy
(XPS) gives an Fe:S atomic ratio of 1:1.96 and 1:1.95, as ex-
pected for 1 and 2 respectively. In addition, inductively-cou-
pled plasma mass spectrometry (ICP-MS) measurements on ni-
tric acid digests of 1 and 2 indicate that little Li+ (1.4% and 7%
relative to TBA or TMA respectively) is incorporated into the
materials (Table S1). Finally, the results from combustion anal-
ysis are also consistent for these materials. All of this data sup-
ports the assigned formulas of [(Fe4S4)(BDT)2][TBA]2 and
[(Fe4S4)(BDT)2][TMA]2 for 1 and 2 respectively.
Figure 1. 57Fe Mössbauer spectrum of (a) 1 and (b) 2 recorded
at 80 K.
pendant thiolates.12 These examples illustrate the potential of
materials synthesized with Fe4S4 clusters.
In this work we report the preparation of coordination poly-
mers containing 1D chains of Fe4S4 clusters linked by an orga-
nochalcogenide ligand, 1,4-benzenedithiolate (BDT, Scheme 1).
This represents a highly unusual example where Fe4S4 clusters
can be combined with organic linkers to form crystalline coor-
dination polymers. The choice of counterion and solvent con-
trols the packing and solubility of the anionic chains, allowing
isolation and characterization of two distinct crystalline solids.
The redox-activity of the Fe4S4 clusters is maintained in the pol-
ymer, manifesting as redox-dependent electrical conductivity
in the solid materials. These results show the promise of using
redox-active metal-chalcogenide clusters as building blocks for
coordination polymers with tunable physical properties.
Structural Determination
The crystal structures of 1 and 2 were solved ab initio using
simulated annealing and then further refined with the Rietveld
method from synchrotron XRPD data with input from the ex-
perimentally determined formulas (Figure 2 and Figure 3). The
structure of 1 consists of chains of Fe4S4 clusters connected by
pairs of BDT groups surrounded by TBA cations. Each Fe4S4
cluster is equidistant from four TBA cations 5.97 Å from the
Fe4S4 centroid. The axis-to-axis separation between adjacent
chains is 11.93 Å, and the separation between Fe4S4 clusters
within the chain is 9.92 Å (centroid to centroid). The rings of
the BDT linkers in each pair and the Fe atoms to which they are
coordinated are coplanar. In the structure of 2, similar Fe4S4-
BDT chains are present. In the absence of the bulky TBA cati-
ons, the separation between chains decreases to 8.59 Å with
the TMA cations confined to channels between the chains. Two
Fe4S4 sites alternate along each chain, with distances of 5.77
and 6.77 Å respectively to each of the nearest four TMA cations.
In contrast to 1, the ring formed by the pair of BDT groups and
RESULTS AND DISCUSSION
Synthesis and Composition
Heating [Fe4S4(SPh)4][TBA]2 (TBA = tetra-n-butylammo-
nium) with 2 equivalents of 1,4-benzenedithiol (BDTH2) in
N,N-dimethylformamide (DMF) leads to the precipitation of
[(Fe4S4)(BDT)2][TBA]2 (1) as a dark purple to black solid. Sim-
ilarly, [(Fe4S4)(BDT)2][TMA]2 (2) is obtained by heating in
MeCN when TBA is replaced by tetramethylammonium (TMA)
as the counterion. X-ray powder diffraction (XRPD) analysis
demonstrates that both of these materials are crystalline. Fur-
thermore, we have observed that the crystallinity of both 1 and
2 can be increased by addition of excess [Li][CF3SO3] to the
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Figure 2. X-ray powder diffraction pattern (a) and structure of
Figure 3. X-ray powder diffraction pattern (a) and structure of
1 solved from synchrotron X-ray powder diffraction data
viewed (b) parallel to the Fe4S4-BDT chain and (c) perpendicu-
lar to the chain. Atoms shown as balls and sticks with Fe = or-
ange, S = yellow, C = gray, N = blue; H atoms omitted for clarity;
tetrabutylammonium ions are rendered as sticks to highlight
the Fe4S4-BDT chain.
2 solved from synchrotron X-ray powder diffraction data
viewed (b) parallel to the Fe4S4-BDT chain and (c) perpendicu-
lar to the chain. Atoms shown as balls and sticks with Fe = or-
ange, S = yellow, C = gray, N = blue; H atoms omitted for clarity;
tetramethylammonium ions are rendered as sticks to highlight
the Fe4S4-BDT chain.
the Fe4S4 clusters in 2 is buckled, with a Fe-S-C6H4-S-Fe dihe-
dral angle of 43.5°. The closest separation between Fe4S4 clus-
ters is 10.24 Å within each chain and 10.00 Å between chains.
The presence of two distinct sites for the Fe4S4 clusters is con-
sistent with the Mössbauer spectrum of 2.
We had initially hypothesized that the combination of Fe4S4
clusters and BDT could produce an extended 3D framework,
but the observed 1D structures suggest that other confor-
mations of these materials are accessible. We hypothesize that
the choice of cation may be an important point of modulation
to control morphology. In 1, the Fe4S4-BDT chains are sepa-
rated by TBA cations, but in 2, the smaller TMA cations allow
closer contact between the chains. Additionally, the size of the
cation is found to affect the conformation of the chains, which
highlights that the coordination of BDT to Fe is flexible. The dif-
ferent structures imbued by these cations, as well as the differ-
ent synthetic conditions required to isolate 2, prompted us to
investigate the solution phase behavior of these materials.
Solution Behavior of Fe4S4‐BDT Chains
The observation that addition of Li+ during synthesis of 1 and
2 slows the rate of precipitation and improves crystallinity led
us to hypothesize that the Fe4S4-BDT chains might form in so-
lution prior to packing with TBA or TMA and subsequent pre-
cipitation. We note that initial attempts to synthesize 2 by heat-
ing of BDTH2 with [Fe4S4(SPh)4][TMA]2 in DMF resulted in neg-
ligible formation of a precipitate (in some cases, a small amount
of an amorphous solid degradation product formed and was
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Figure 5. Normalized UV-visible diffuse reflectance spectra of
1 and 2 in a [Mg][SO4] matrix plotted as a Kubelka-Munk func-
tion and absorbance spectrum of 2 in DMF solution.
Figure 4. Small angle X-ray scattering (SAXS) from a DMF solu-
tion of 2.
filtered out). Furthermore, addition of
5 equivalents of
of 2 in DMF show similar features, providing further evidence
that the Fe4S4-BDT chains persist in solution. The 560 nm fea-
ture is red-shifted from the 450 nm absorption observed for
monomeric [Fe4S4(SPh)4]2- in solution (Figure S26). This band
has been attributed to a ligand-to-metal charge transfer transi-
tion involving the thiolate ligand.19 The more electron rich na-
ture of the BDT ligand in 1 versus the thiophenolate ligand in
the monomer may explain this shift.
The electron transfer series of biologically relevant cubane-
type Fe4S4 clusters has been studied extensively. Of these, the
[Fe4S4]2+ and [Fe4S4]+ cores are the most stable and thoroughly
characterized oxidation states.20 The redox behavior of these
Fe4S4-BDT chains was investigated by cyclic voltammetry (CV)
on DMF solutions of 2 (Figure 6). These measurements are
complicated by the dependence of solubility on counterion as
noted above. When 0.1 M [TMA][PF6] is used as the electrolyte,
the Fe4S4-BDT chains partially precipitate as an amorphous
solid, forming a fine suspension. We suspect that this precipi-
tation is due to a much higher concentration of TMA as com-
pared with the conditions for the synthesis of 2. When 0.1 M
[Li][CF3SO3] is used instead, no precipitation is observed. With
[TMA][PF6] as the electrolyte, the CV response of this
[TBA][PF6] to this solution after cooling to room temperature
resulted in rapid precipitation of poorly-crystalline 1. This sug-
gests that chains of 2 may be forming in solution but are suffi-
ciently soluble in DMF to avoid precipitation. Addition of
[TBA][PF6] then putatively triggers rapid packing of the chains
which forces the observed precipitation.
To test this hypothesis the solubility of isolated 2 was also
investigated. Isolated crystalline 2 is soluble in DMF, and addi-
tion of [TBA][PF6] to this solution also results in the immediate
precipitation of poorly-crystalline 1. This precipitation can be
slowed by the addition of excess [Li][CF3SO3], suggesting that
ionic strength likely plays a role in ion packing and hence in
controlling the rate of crystallization (Figure S47). In this way,
the [Li][CF3SO3] may act similarly to the competing ligands of-
ten used as modulators in syntheses of MOFs.14 The use of non-
coordinating salts as modulators is less explored, but ionic
strength has been invoked as a factor affecting the dynamics of
other soluble coordination polymers.15 To further examine the
solution structure of these chains, small angle X-ray scattering
(SAXS) experiments were carried out on a near-saturated DMF
solution of 2 (Figure 4). The SAXS data are best fit to a power-
law behavior at higher q, with a slope on a log-log plot near -5/3
that is indicative of a swollen polymer coil structure.16 The full
range of data can also be fit well by the unified exponen-
tial/power-law model described by Beaucage,16 yielding a sim-
ilar result (Figure S23). This type of scattering has been re-
ported for other charged polymers such as DNA.17 Taken to-
gether, these results indicate that the Fe4S4-BDT chain persists
in solution, and it seems that the identity of the counterion
plays an important role in solubility and crystallization.
The Fe4S4-BDT chains found in both 1 and 2 contain Fe4S4
clusters with a similar separation to those in biological electron
transport chains.9,18 This raises the possibility that electrical
conductivity could occur along these chains. Furthermore, we
hypothesized that the shorter chain-chain contacts in 2 might
also facilitate enhanced electronic coupling. As such, we next
investigated the electronic structure and conductivity of these
materials.
Electronic Properties of Fe4S4‐BDT Chains
The diffuse reflectance UV-visible spectra of 1 and 2 (Figure
5) show a broad band near 560 nm, along with a shoulder near
820 nm and additional absorption in the UV region. Solutions
Figure 6. Cyclic voltammograms of 2 and monomeric
[Fe4S4(SPh)4][TMA]2. Arrow denotes scan direction. Condi-
tions: DMF, 0.1 M [Li][CF3SO3], 0.1 V/s.
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suspension is similar to that of the [Fe4S4(SPh)4][TMA]2 mono-
enhancement (7(5) × 10-12 S/cm and 7(1) × 10-10 S/cm for 1
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mer in solution (Figure S43), showing quasi-reversible reduc-
tions at -1.43 and -2.13 V vs. FeCp2+/FeCp2 (Figure S44). With
[Li][CF3SO3] as the electrolyte, reductive features are similarly
observed at -1.6 and -2.2 V vs. FeCp2+/FeCp2 (Figure 6). By
comparison to the monomer, these features correspond to the
[Fe4S4]2+/[Fe4S4]+ and [Fe4S4]+/[Fe4S4]0 redox couples. Upon
scanning past the first reductive feature in [Li][CF3SO3] electro-
lyte, a deposit is observed on the working electrode which re-
dissolves at more oxidizing potentials. This suggests that the
Fe4S4-BDT chain containing [Fe4S4]+ clusters is less soluble, and
may explain the non-ideal shape of the CV features and changes
upon repeated scans (Figure S46). Despite this complexity, the
electrochemistry results demonstrate that the redox-activity of
the Fe4S4 clusters is preserved in the Fe4S4-BDT chains present
in 2 and likely in 1 as well.
and 2 respectively). The crystallinity of all doped materials was
maintained as verified by their XRPD patterns (Figures S15-
S18).
Reduction of the [Fe4S4]2+ cluster requires incorporation of
an additional cation for charge balance, either Li+ or CoCp* +
.
2
ICP-MS was therefore used to assess the degree of reduction in
1 and 2 (Table 1). For analysis of the ICP-MS data, it is assumed
that exchange of TBA or TMA with Li+ or CoCp* + does not oc-
2
cur. This assertion is supported by the observation that the Li+
content in 1 and 2 does not increase upon soaking in a THF so-
lution of [Li][CF3SO3] without reductant. The increase in Li and
Co content upon treatment indicates reduction by 0.07(2) and
0.24(6) electrons per formula unit for 1 and 2 respectively. The
lower degree of reduction in 1 is consistent with the smaller
increase in conductivity upon treatment and may be due to dif-
ficulty incorporating the additional cations into the densely
packed structure. Mössbauer spectroscopy was used to further
characterize the reduced materials (Figures S33-S37). For both
1 and 2, the spectra of the reduced materials show no addi-
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In addition to these reductive features, an irreversible oxida-
tion was observed at approximately -0.3 V vs. FeCp2+/FeCp2
with both electrolytes. Because bulky ligands have been shown
to improve the stability of the [Fe4S4]3+ oxidation state,21 we
had hypothesized that this state might be stabilized in a coor-
dination polymer. However, the irreversible oxidation near the
potential observed in the monomer suggests that this is not the
case for the Fe4S4-BDT chains in solution or suspension.
As-synthesized 1 and 2 are both poor electrical conductors,
with room temperature pressed pellet conductivities of σ =
3(3) × 10-11 S/cm and 5(3) × 10-10 S/cm respectively. For com-
parison, the conductivities of the monomeric starting materials
([Fe4S4(SPh)4][TBA]2 and [Fe4S4(SPh)4][TMA]2) are both below
10-12 S/cm. The similarity in values for 1 and 2 suggests that
insulation of the Fe4S4-BDT chains by the large TBA cation is
not the only factor leading to low conductivity. Instead, we pro-
pose that these insulating behaviors may be ascribed to two
factors. Firstly, there is likely a low concentration of charge car-
tional signals attributable to free Fe2+ or Fe3+ 23
, indicating that
the Fe4S4 clusters remain intact. Mössbauer spectra of mono-
meric Fe4S4 clusters show small increases in isomer shift and
quadrupole splitting upon reduction from [Fe4S4]2+ to
[Fe4S4]+.23, 24 With 1, no significant changes are seen in the
Mössbauer spectra, consistent with the small degree of reduc-
tion estimated from ICP-MS. In contrast, the spectrum of 2
shows small changes in shape (Figure S37). This suggests an
additional, unresolved contribution from a species with higher
isomer shift than the as-synthesized 2. Subtraction of the spec-
tra of the as-synthesized and reduced materials reveals a broad
additional signal in the same region as literature spectra of
[Fe4S4]+ compounds (Figure S38).23 This contribution can be
approximated in fits to the spectrum of reduced 2 by addition
of a third site with similar δ and ΔEQ to literature [Fe4S4]+ spec-
tra (full treatment of the [Fe4S4]+ species with two Fe sites was
not possible given the low intensity of this feature and overlap
with signals from [Fe4S4]2+ sites). These changes are reversible
by brief exposure to dry air, further supporting their attribu-
tion to reduced [Fe4S4]+ clusters. While interpretation of the
Mössbauer spectra is complicated by the presence of overlap-
ping signals, these data support that the reversible redox-activ-
ity of the Fe4S4 clusters is preserved.
Finally, we have also investigated chemical reduction of 2 in
the solution phase. Upon treatment with approximately 0.5
equivalents of sodium acenaphthylene ([Na][C12H8]), DMF so-
lutions of 2 show changes in their UV-visible absorption spec-
tra consistent with partial reduction of the Fe4S4 clusters from
[Fe4S4]2+ to [Fe4S4]+ (Figure S27).25 A small amount of precipi-
tate was observed, consistent with the lower solubility of the
reduced polymer observed during cyclic voltammetry experi-
ments. SAXS and IR measurements support the persistence of
the polymer structure in the reduced solution (Figures S24,
S25, S41, S42). Following precipitation of the partially reduced
solution by addition of Et2O, an amorphous solid was obtained
with conductivity of 1.5(5) × 10-5 S/cm. In contrast, material
precipitated from a DMF solution of 2 without added reductant
showed a conductivity of 3(4) × 10-9 S/cm. ICP-MS measure-
ments of the Na/Fe ratio were used to assess the degree of re-
duction between these two materials. These data indicate that
the material was reduced by 0.6(4) electrons per cluster. This
result further supports that the introduction of charge carriers
by accessing the [Fe4S4]+ oxidation state is advantageous for
preparing materials with enhanced electrical conductivity.
riers along the chain due to the S = 0 ground state of [Fe4S4]2+
.
Secondly, we note the observed bulk conductivity of pressed
pellet samples is a weighted average of the conductivity in each
crystallographic direction plus grain boundary resistance. Both
of these factors will lower the measured conductivity.22
Doping experiments were carried out to introduce additional
charge carriers and improve the conductivities of these mate-
rials. Solid 1 and 2 were soaked in a THF solution containing
0.5 equivalents of bis(pentamethylcyclopentadienyl)cobalt(II)
(CoCp* ) per Fe4S4 unit and excess [Li][CF3SO3]. The reduc-
2
tively-doped analogues of 1 and 2 exhibited enhanced conduc-
tivities of 6(2) × 10-9 S/cm and 5(2) × 10-6 S/cm respectively.
In contrast, similar doping experiments with an oxidizing
agent, ferrocenium tetrafluoroborate ([Fc][BF4]), in the pres-
ence of excess [TMA][Br] resulted in no measurable
Table 1. Li and Co content of 1 and 2 before and after reduc-
tion in the solid state
Compound
Reduced 1
Compound
Reduced 2
Molar ratio
Li/Fe4S4
Before reduction
After reduction
Increase amount
Co/Fe4S4
3(1)%
9(1)%
6(2)%
14(5)%
28(4)%
15(6)%
Before reduction
After reduction
Increase amount
Reduction degree
0.14(4)%
1.2(3)%
1.1(3)%
7(2)%
0.08(2)%
9.0(6)%
9.0(6)%
24(6)%
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CONCLUSION
(82%). Ref. b.p. 168-169 °C. 1H NMR (400 MHz, CDCl3): δ 7.31
(4 H, s), 3.36 (2 H, spt, J = 3.4 Hz), 1.28 (12 H, d, J = 1.3 Hz) ppm.
1
2
3
4
5
6
7
8
We have synthesized and characterized two new materials
incorporating redox-active Fe4S4 clusters with an organochal-
cogenide ligand, BDT. Synchrotron X-ray powder diffraction
data analysis revealed that the materials feature 1D chains of
Fe4S4 clusters connected by pairs of BDT ligands. Small angle X-
ray scattering measurements and precipitation experiments
proved the persistence of these chains in solution. The choice
of counterion is important in determining the packing and sol-
ubility of these anionic chains. Mössbauer spectroscopy and cy-
clic voltammetry demonstrate that the redox-activity of the
Fe4S4 cluster is preserved in these materials, and chemical re-
duction is shown to impart enhanced electrical conductivity.
These results lay the groundwork for a new class of materials
based on metal-sulfur clusters with organosulfur ligands.
1,4-Benzenedithiol
Safety note: In our experience any excess Na was consumed by
side reactions with the solvent and was not present after heating.
However, care should be taken to ensure that no Na metal is pre‐
sent before adding the HCl solution. Addition of a few drops of the
acid solution first is recommended to check for bubbling that
would indicate the presence of Na metal.
Sodium (1.8 g, 80 mmol), dry DMA (40 mL) from the glove-
box, and 1,4-Bis(isopropylthio)benzene (4.5 g, 20 mmol) were
added in sequence to a 125 mL three-neck flask under N2 at-
mosphere and the mixture was heated to 100 °C for 8 h. During
this time the reaction mixture became thick with precipitate.
The reaction solution was quenched with diluted HCl solution
(concentrated HCl solution (34-37% w/w, 10 mL) + H2O (50
mL)) in an ice bath and stirred for another 0.5 h under an inert
atmosphere. The mixture was extracted with Et2O (2×75 mL).
The organic layer was washed with H2O (5×30 mL), dried with
[Mg][SO4], and evaporated. A white powder was obtained by
washing with hexanes (20 mL). More product can be recovered
by storing the hexane washes in a freezer (-35 °C) overnight,
resulting in a pale yellow to white powder. Overall yield: 1.9 g
(67%). 1H NMR (400 MHz, CDCl3): δ 7.16 (4 H, s) and 3.41 (2 H,
s) ppm.
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EXPERIMENTAL
General Methods
All manipulations were performed under an inert atmos-
phere of dry N2 using a Schlenk line or MBraun UNIlab glovebox
unless otherwise noted. 1H NMR measurements were per-
formed on Bruker DRX 400 or 500 spectrometers. Elemental
analyses (C, H, N) were performed by Midwest Microlabs. In-
ductively coupled plasma mass spectrometry (ICP-MS) data
was obtained with an Agilent 7700x ICP-MS and analyzed using
ICP-MS Mass Hunter version B01.03. The samples were diluted
in 2% HNO3 matrix and analyzed with a 159Tb internal standard
against a 12-point standard curve over the range from 0.1 ppb
to 500 ppb. The correlation was > 0.9997 for all analyses of in-
terest. Data collection was performed in Spectrum Mode with
five replicates per sample and 100 sweeps per replicate. Dime-
thylformamide (DMF), tetrahydrofuran (THF), and acetonitrile
(MeCN) used in preparing the coordination polymers were ini-
tially dried and purged with Ar on a solvent purification system
from Pure Process Technology. DMF was then passed through
activated alumina before use. THF was stirred with liquid NaK
alloy and then filtered through activated alumina and stored
over 4Å molecular sieves. MeCN was stored over 4Å molecular
sieves. Dimethylacetamide (DMA) was sparged with N2, trans-
ferred into the glovebox, passed through activated alumina and
stored over 4Å molecular sieves. [TMA][PF6] was recrystallized
from H2O and dried at 160 °C before use.26 All other chemicals
were purchased from commercial sources and used as re-
ceived. [Fe4S4(SPh)4][TBA]2 was prepared as previously de-
scribed.27
[Fe4S4(SPh)4][TMA]2
The synthesis of this compound has been reported previously,29
but details of the synthetic method and purification had not been
published.
MeOH (80 mL) was added to a 250 mL Schlenk flask with a
stir bar and deoxygenated by four pump-purge cycles with N2.
The flask was then cooled in an ice/water bath. Sodium (1.8 g,
80 mmol) was added with stirring and continued cooling and
allowed to dissolve fully before returning the flask to room
temperature. PhSH (8.2 mL, 80 mmol) was then injected into
the reaction mixture. In a separate 250 mL Schlenk flask, anhy-
drous FeCl3 (3.2 g, 20 mmol) and MeOH (50 mL) were deoxy-
genated by four pump-purge cycles with N2 and transferred via
cannula to the flask containing the [Na][PhS] solution. Sulfur
(0.64 g, 20 mmol) was added and the reaction mixture stirred
overnight at room temperature. This mixture was then filtered
into a solution of [TMA][Br] (2.3 g, 15 mmol) in methanol (45
mL), resulting in precipitation of the crude product as a black
solid. This suspension was filtered and the resulting solid dried
under vacuum. The crude solid was dissolved in ~70 °C deoxy-
genated MeCN (40 mL) and passed through a glass frit before
deoxygenated MeOH (130 mL) was added via slow cannula
transfer while heating to ~60 °C. Slow cooling of this solution
to −35 °C resulted in the formation of large black crystals that
were collected by filtration, washed with MeOH, and dried un-
der vacuum (4.0 g, 85%). 1H NMR (400 MHz, CD3CN): δ 8.19 (8
H, d, J = 5.7 Hz), 5.91 (8 H, br. s), 5.30 (4 H, t, J = 5.6 Hz), 3.07
(24 H, s) ppm.
Synthetic Procedures
1,4-Bis(isopropylthio)benzene
1,4‐Bis(isopropylthio)benzene was synthesized following a re‐
ported procedure,28 but the synthetic method and purification
have been modified as follows:
A dispersion of 60% NaH in mineral oil (19.2 g, 480 mmol)
was added to a 500 mL three-neck flask and then washed with
hexane twice and DMA once under a N2 atmosphere. After add-
ing DMA (100 mL), 2-propanethiol (52 mL, 480 mmol) was in-
jected slowly and in portions to avoid excessive foaming (if nec-
essary, use of ice bath and drop funnel is helpful). Subse-
quently, DMA (100 mL) and a solution of 1,4-dibromobenzene
(28.3 g. 120 mmol) in DMA (150 mL) were injected, and the
mixture was heated for 17 h to 100 °C. The mixture was cooled,
poured into 100 mL saturated NaCl solution and extracted with
Et2O (3×100 mL). The organic layer was washed with H2O
(5×50 mL), dried with [Mg][SO4], and evaporated. The yellow-
ish oil was further purified by vacuum distillation. Yield: 22 g
[Fe4S4(BDT)2][TBA]2 (1)
[Fe4S4(SPh)4][TBA]2 (192 mg, 0.15 mmol) and [Li][CF3SO3]
(117 mg, 0.75 mmol) were added to a 24 mL vial and dissolved
in DMF (12 mL). A solution of 1,4-benzenedithiol (43 mg, 0.30
mmol) in DMF (3 mL) was added and the vial was sealed and
placed in a heating block on a 140 °C hot plate. The reaction
mixture was heated for 2 days, after which 1 was separated by
centrifugation, washed with DMF (4x2 mL) and THF (3x2 mL),
and dried under vacuum. Compound 1 was obtained as a black
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powder (102 mg, 61%). Anal. calc. for Fe4S8C44H80N2: C 47.31%,
XRPD (Figures S13 and S14). Further ICP-MS analysis sug-
1
2
3
4
5
6
7
8
H 7.22%, N 2.51%; found: C 44.61%, H 6.62%, N 2.18%. The
combustion analysis for this material is improved upon soaking
with excess TBA (see below).
gested that the Li content decreases (Table S1). The C, H, N
analysis results of as-synthesized 1 with [Li][CF3SO3] deviate
from the theoretical values slightly but by washing with
[TBA][PF6], the C, H, N analysis matches with the assigned for-
mula. Anal. calc. for 1, Fe4S8C44H80N2: C 47.31%, H 7.22%, N
2.51%; found: C 47.40%, H 7.33%, N 2.36%.
[Fe4S4(BDT)2][TMA]2 (2)
[Fe4S4(SPh)4][TMA]2 (140 mg, 0.15 mmol) and [Li][CF3SO3]
(936 mg, 6.0 mmol) were added to a 24 mL vial and dissolved
in MeCN (9 mL). A solution of 1,4-benzenedithiol (43 mg, 0.30
mmol) in MeCN (6 mL) was added and the vial was sealed and
placed in a heating block on a 100 °C hot plate. The reaction
mixture was heated for 2 days, after which 2 was separated by
centrifugation, washed with MeCN (4x2 mL), and dried under
vacuum. 2 was obtained as a black powder (68 mg, 58%). Anal.
calc. for Fe4S8C20H32N2: C 30.78%, H 4.13%, N 3.59%; found: C
30.46%, H 4.11%, N 3.62%.
Digestion Experiments
1 and 2 (~5 mg) were digested in 12 M hydrochloric acid (~1
mL) under air to check for BDT and PhSH content. The acid so-
lution was extracted with C6D6 or CD2Cl2 (~0.5-1 mL; in the
presence of TBA, Fe-containing digestion products were found
to be soluble in CD2Cl2) and analyzed by 1H NMR spectroscopy.
To test for the presence of quaternary ammonium ions, 1 and
2 (~5 mg) were digested in D2SO4 (~0.1-0.2 mL),30 resulting in
oxidation of BDT to an insoluble polymer. This suspension was
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1
Reductive doping experiments of solid state 1 and 2
diluted with (CD3)2SO and analyzed by H NMR spectroscopy.
1 (0.05 mmol, 56 mg) or 2 (0.05 mmol, 39 mg) was soaked in a
solution of CoCp*2 (0.5 equivalents, 0.025 mmol, 8 mg) and ex-
cess [Li][CF3SO3] (5 equivalents, 0.25 mmol, 39 mg) in THF (5
mL) and then stirred at room temperature overnight. The re-
duced product was isolated by centrifugation, washed with
THF (typically 4x2 mL) until the wash solvent was colorless,
and dried under vacuum. Crystallinity was maintained as veri-
fied by XRPD (Figures S15 and S17). The degree of reduction
was analyzed by ICP-MS (Table S2).
Addition of [TBA][PF6] or [TMA][Cl] standards was used to
confirm the identity of the species in solution.
Sample Preparation for ICP‐MS
Solutions for ICP-MS were prepared by digesting 2 mg of ma-
terial in 2 mL HNO3 (trace metal grade) solution in a fume hood
overnight at room temperature. Samples analyzed for Na con-
tent were digested in polypropylene to avoid contamination of
Na from glass containers. The solution was diluted with ultra-
filtered deionized water for ICP-MS analysis. Reported errors
are the standard deviation of measurements on three batches
of each material, except for the Co content before reduction
(measured in duplicate).
Oxidative doping experiments of solid state 1 and 2
1 (0.05 mmol, 56 mg) or 2 (0.05 mmol, 39 mg) was soaked in a
solution of [Fc][BF4] (0.5 equivalents, 0.025 mmol, 7 mg) and
excess [TMA][Br] (5 equivalents, 0.25 mmol, 38 mg) in MeCN
(4 mL) and MeOH (1 mL) and then stirred at room temperature
overnight. The oxidized product was isolated by centrifugation,
washed with MeCN (typically 4x2 mL) until the wash solvent
was colorless, and dried under vacuum. Crystallinity was main-
tained as verified by XRPD (Figures S16 and S18).
X‐ray Powder Diffraction
Reaction screening:
X-ray powder diffraction measurements for screening reac-
tion conditions were performed on a SAXSLAB Ganesha
equipped with a Xenocs GeniX3D Cu Kα source. Samples were
loaded into 0.8-1.1 mm ID, 0.25 mm wall borosilicate capillar-
ies and sealed with wax under N2. Data reduction/integration
was performed using Saxsgui software and a background cor-
rection for the capillary was applied.
Chemical reduction of 2 in solution
The concentration of 2 in DMF was determined by induc-
tively-coupled plasma optical emission spectroscopy (ICP-
OES). Three aliquots of each solution batch were dried, di-
gested in HNO3, and the Fe content was measured versus a Cu
internal standard.
A solution of [Na][C12H8] was prepared in THF (2.5 mL) by
stirring sodium (10 mg, 0.43 mmol) and excess acenaphthylene
(80 mg, 0.53 mmol) at room temperature overnight. This solu-
tion was passed through a glass microfiber filter and diluted
10-fold with DMF prior to use.
The [Na][C12H8] solution (0.5 equivalents, 1.24 mL, 0.022
mmol) was added dropwise to a stirred solution of 2 in DMF
(27 mL, 34 mg, 0.043 mmol) and then stirred overnight. The
reduced solid was precipitated by addition of an equal volume
Et2O and then separated by centrifugation, washed with THF
(4x2 mL), and dried under vacuum. The material was amor-
phous as checked by XRPD.
11-BM APS synchrotron beamline (bending magnet source) /
Argonne National Laboratory (Lemont, Il, USA):
The samples were loaded into 1.0 mm OD, 0.01 mm wall bo-
rosilicate capillaries in a N2-filled glovebox (using an ionizer to
reduce static and repeatedly dropping the capillary into a glass
vial) and then capped with a septum with a small amount of
vacuum grease. Capillaries were then evacuated via an inserted
needle through a septum, and sealed using an electric arc
lighter. The powders were rotated during the measurement at
~50 cycles per second. The powder patterns were measured at
295 K at beamline 11-BM of the Advanced Photon Source at Ar-
gonne National Laboratory using a wavelength of λ = 0.41275
Å, from 0.5 to 50° 2θ with a step size of 0.001° and a counting
time of 0.1 sec/step.
Treatment with [R4N][PF6]
PROXIMA 2A / SOLEIL synchrotron beamline (Saint-Aubin,
France):
A mixture of 1 or 2 (0.05 mmol, 55 mg or 39 mg) and a solu-
tion of [TBA][PF6] or [TMA][PF6] respectively (0.25 mmol, 67
mg or 55 mg) in DMF/MeCN (5 mL) was stirred overnight at
100 °C in the glovebox. In the next morning, the solid was iso-
lated by centrifugation and washed with fresh DMF/MeCN
(3×2 mL) and THF (3×2 mL). After drying for hours under vac-
uum, the crystallinity was still maintained, as examined by
To confirm the stability of 1 under the intense X-ray beam of
the synchrotron (~1 hour under the beam at APS at ambient
conditions), additional powder X-ray diffraction data for 1
were recorded at the PROXIMA 2A beamline (PX2A) of the
SOLEIL synchrotron (E = 17 keV , λ = 0.7293 Å) using an EIGER
X 9M 2D hybrid photon counting detector and measured at 295
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Page 8 of 13
K.31 The optical layout included a cryogenically cooled channel-
ambient temperature. Data reduction/integration was per-
1
2
3
4
5
6
7
8
cut Si [111] monochromator, a convex horizontal prefocusing
mirror and a pair of focusing bimorph mirrors in Kirpatrick-
Baez configuration, resulting in a 100 m wide X-ray beam at
the sample position. The 2D detector was set at a distance of
300 mm from the sample. 1 was packed in a flame-sealed 1.0
mm diameter glass capillary, which was not only rotated dur-
ing the measurement, but was also translated along its spinning
axis, resulting in a helical path of the beam along the exterior of
the capillary, all to minimize sample damage from the intense
X-ray beam. The capillary was translated along a 0.8 mm path
(i.e. ca. 10 times the horizontal beamsize). The measurement
time was 72 s, much smaller than for the 11-BM/APS related
measurement, making radiation damage to the sample very un-
likely. The 2D nature of the measurement, a 2D image display-
ing full Debye-Scherrer rings, provided a direct check for the
maintenance of the crystallinity and homogeneity of the sam-
ple, as well as for the absence of significant preferred orienta-
tion in the latter (Figure S8).
formed using Saxsgui software, and background subtraction
and fitting were performed in the Irena software suite.16c Lin-
ear fits were performed in OriginPro software. Data were also
fit using the Unified Fit module in Irena, based on the work of
Beaucage.16
Optical Spectroscopy
UV-visible absorption spectra were recorded on a Thermo
Scientific Evolution 300 spectrometer with the VISIONpro soft-
ware suite. All spectra were recorded at room temperature. Dif-
fuse reflectance spectra were obtained with the use of a Harrick
Praying Mantis accessory. Solid 1 or 2 was ground with
[Mg][SO4] in air to produce the diffuse reflectance sample and
pure [Mg][SO4] was used as a white reference material. All
spectra were normalized and had a background subtraction ap-
plied.
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X‐ray Photoelectron Spectroscopy
X-ray photoelectron spectra (XPS) were collected with the
AXIS Nova spectrometer (Kratos Analytical) equipped with a
monochromatic Al Kα X-ray source. The Al anode was powered
at 10 mA and 15 kV. The instrument work function was cali-
brated to give an Au 4f7/2 metallic gold binding energy of 83.95
eV. Instrument base pressure was ca. 1×10−10 Torr. The analy-
sis area size was 0.3 x 0.7 mm2. For calibration purposes, the
binding energies were referenced to C 1s peak at 284.8 eV. Sur-
vey spectra were collected with a step size of 1 eV and 160 eV
pass energy. The high-resolution spectra were collected with a
pass energy of 40 eV and 0.1 eV step size. Powder samples were
affixed to conductive carbon tape in air before loading into the
spectrometer.
For 1, Fe, S, N, C, and O (likely from atmospheric adsorbates)
are present. Quantification results provide the molar ratio of
Fe:S:N = 1:1.96:0.52. For 2, Fe, S, N, C, and O are again detected.
Only Fe and S could be quantified, giving a molar ratio Fe:S =
1:1.95. For both 1 and 2, XPS seems consistent with mixed va-
lent Fe in the Fe2+/3+ state; C 1s shows C-C/C-H and C-N bonds
(indicated by broadening at about 286 eV); The S 2p3/2 shift of
161.5 eV corresponds to S2- as in a sulfide ion or organic thiols
– these species have close shifts and closely overlap. The N 1s
spectrum appears as a sharp single peak located at 401.8 eV
that fits well with reported binding energies of alkyl ammo-
nium salts.41 Li 1s overlaps with Fe 3p so it is difficult to assess
Li content by XPS.
The free ALBULA / DECTRIS software was used to convert
the PX2A synchrotron data from 2D I(x,y) binary frames (CBF
& HDF5 formats) into conventional 1D ASCII I(2θ) XRPD histo-
grams, with an average 2θ step size of 0.015°.32 For both the
11-BM/APS and the PX2A/SOLEIL data, the two ubiquitous in-
dexing programs DICVOL1433 and N-TREOR1434 were used, to-
gether with their respective graphical interfaces PreDICT35 and
EXPO2014,36 yielding nearly identical unit cells for a given 1D
histogram (Tables S2, S3). For structure solution, the 11-BM
data for 1 and 2 were used and simulated annealing applied as
implemented in EXPO201436 and FOX,37 yielding similar start-
ing models albeit with different success rates. The Rietveld
least-squares refinement of the structure solutions against the
synchrotron data was carried out using the GSAS-I software,38
based on starting models obtained with FOX. Restraints for
bond distances and bond angles were introduced in the refine-
ments based on published Fe4S4 cubane structures,39 as well as
from geometry information gathered from the CCDC/MOGUL
module of the Cambridge Structural Database (CSD).40 Besides
appraisal of the usual figures of merit for powder structures,
the crystallographic arrangements were also analyzed graph-
ically using the CCDC Mercury software41 to ensure that the ob-
tained refined crystallographic structures were chemically
plausible. It is worth noting that even the PX2A data allowed
for a successful structure solution, and not merely for success-
ful indexing and Rietveld refinement. Eventually, the two GSAS-
I Rietveld-refined structures for both 1 and 2 were obtained as
CIF (Crystallographic Information; CCDC 1869639-1869640)
files which were themselves validated by the standard IUCr /
CheckCIF procedure.42 Details of the structure solution and re-
finement are listed in Tables S3 and S4. Atomic coordinates are
collected in Tables S5 and S6. Final Rietveld refinement fits are
shown in Figures S9-S12.
Mössbauer Spectroscopy
57Fe Mössbauer spectra were measured at 80 K, 25 K, or 4.2
K with zero applied magnetic field, using a constant accelera-
tion spectrometer with a 57Co on Rh source. Samples were en-
cased in Paratone-N oil and placed in a polyethylene sample
cup inside a N2-filled glovebox. Samples were frozen in liquid
N2 immediately upon removal from the glovebox and kept cold
while loading into the spectrometer. Spectra were analyzed us-
ing WMOSS software. The sample of reduced, air-exposed 2
was prepared by exposing the reduced 2 Mössbauer sample to
dry air (passed over P2O5) for 15 minutes.
Small Angle X‐ray Scattering
Small angle X-ray scattering (SAXS) experiments were per-
formed on a SAXSLAB Ganesha equipped with a Xenocs Ge-
niX3D Cu Kα source. SAXS samples were loaded into 1.0 mm OD
x 0.01 mm wall borosilicate glass tubes and sealed under N2
with wax. A near-saturated solution of 2 in dry DMF (approx. 2
mg/mL) was prepared and passed through a 0.2 µm PET sy-
ringe filter prior to loading into the tube. A tube with neat DMF
was used as the blank. SAXS data were collected in six 1-hour
increments and compared to check for radiation damage,
which was not found. All experiments were performed at
Infrared Spectroscopy
Infrared spectra were recorded on a Bruker Tensor II FTIR
spectrometer with MCT detector operated at 77 K. Data were
processed and background corrected with OPUS software (ver-
sion 7.5). An additional manual correction for scattering was
also applied. Samples were prepared under N2 by grinding
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Journal of the American Chemical Society
solid 1 or 2 with Nujol, placed between two KBr crystal plates,
and measured in air under ambient conditions.
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Electrochemical Measurements
Electrical conductivity measurements were performed in a
two-contact geometry at room temperature under N2. Samples
were prepared as pressed pellets clamped between two brass
electrodes (4.8 mm diam.) in a plastic sleeve, allowing meas-
urement of the sample thickness with calipers. Linear sweep
voltammetry was conducted using a BASi Epsilon potenti-
ostat/galvanostat, with the reference and counter electrode
terminals connected to one electrode and the working elec-
trode terminal to the other. The resulting data were fit to a
straight line to obtain the sample resistance. All values have
been measured in triplicate on separate batches.
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Electrochemical experiments were performed using cyclic
voltammetry (CV) or differential pulse voltammetry (DPV) on
a BASi Epsilon potentiostat/galvanostat. A glassy carbon work-
ing electrode, platinum wire counter electrode, and silver wire
pseudoreference electrode were used for all measurements.
Due to overlap of the irreversible [Fe4S4]2+ oxidation with the
FeCp2+/FeCp2 couple, CoCp2 was used as an internal standard,
and the CoCp2+/CoCp2 and CoCp2/CoCp2 couples were refer-
-
enced to FeCp2+/FeCp2 in a separate electrolyte solution under
the same conditions. Unless otherwise noted, all scans were
performed with a reductive initial scan direction.
ASSOCIATED CONTENT
Analytical data and additional spectra and electrochemistry
data. This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*jsanderson@uchicago.edu
Conductivity
in
Ni3(2,3,6,7,10,11-Hexaiminotriphenylene)2,
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‡These authors contributed equally. The manuscript was writ-
ten through contributions of all authors. All authors have
given approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank Dr. Jordan DeGayner and Prof. T. David Harris for as-
sistance with Mössbauer spectroscopy. We thank Wenbo Han
and Prof. Wenbin Lin for assistance with ICP-MS measure-
ments. We thank Vlad Kamysbayev and Prof. Dmitri Talapin for
assistance with ICP-OES measurements. This work was sup-
ported by the U. S. Department of Energy, Office of Science, Of-
fice of Basic Energy Sciences, under Award No. DE-SC0019215.
This work was supported by the Chicago MRSEC, which is
funded by NSF through Grant DMR-1420709. The University of
Chicago is thanked for startup funds. Use of the Advanced Pho-
ton Source at Argonne National Laboratory was supported by
the U. S. Department of Energy, Office of Science, Office of Basic
Energy Sciences, under Contract No. DE-AC02-06CH11357, and
we thank the 11-BM beamline staff. SOLEIL Synchrotron beam-
time at the PROXIMA 2A beamline is gratefully acknowledged.
Mia Grahn was supported by a partnership between the Insti-
tute for Molecular Engineering and After School Matters. Chloe
Gilder was supported through the NSF by an NSF CAREER
award Grant No. 1654144 to John S. Anderson.
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