Proton-Coupled Electron Transport in Anthraquinone-Based Zirconium Metal-Organic Frameworks

Article abstract of DOI:10.1021/acs.inorgchem.7b01656

The ditopic ligands 2,6-dicarboxy-9,10-anthraquinone and 1,4-dicarboxy-9,10-anthraquinone were used to synthesize two new UiO-type metal-organic frameworks (MOFs; namely, 2,6-Zr-AQ-MOF and 1,4-Zr-AQ-MOF, respectively). The Pourbaix diagrams (E vs pH) of the MOFs and their ligands were constructed using cyclic voltammetry in aqueous buffered media. The MOFs exhibit chemical stability and undergo diverse electrochemical processes, where the number of electrons and protons transferred was tailored in a Nernstian manner by the pH of the media. Both the 2,6-Zr-AQ-MOF and its ligand reveal a similar electrochemical pK_a value (7.56 and 7.35, respectively) for the transition between a two-electron, two-proton transfer (at pH < pK_a) and a two-electron, one-proton transfer (at pH > pK_a). In contrast, the position of the quinone moiety with respect to the zirconium node, the effect of hydrogen bonding, and the amount of defects in 1,4-Zr-AQ-MOF lead to the transition from a two-electron, three-proton transfer to a two-electron, one-proton transfer. The pK_a of this framework (5.18) is analogous to one of the three electrochemical pK_a values displayed by its ligand (3.91, 5.46, and 8.80), which also showed intramolecular hydrogen bonding. The ability of the MOFs to tailor discrete numbers of protons and electrons suggests their application as charge carriers in electronic devices.

Full text of DOI:10.1021/acs.inorgchem.7b01656

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
pubs.acs.org/IC
Proton-Coupled Electron Transport in Anthraquinone-Based
Zirconium Metal−Organic Frameworks
Paula J. Celis-Salazar, Charity C. Epley, Spencer R. Ahrenholtz, William A. Maza, Pavel M. Usov,
and Amanda J. Morris*
Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, United States
*
S Supporting Information
ABSTRACT: The ditopic ligands 2,6-dicarboxy-9,10-anthraquinone and 1,4-
dicarboxy-9,10-anthraquinone were used to synthesize two new UiO-type metal−
organic frameworks (MOFs; namely, 2,6-Zr-AQ-MOF and 1,4-Zr-AQ-MOF,
respectively). The Pourbaix diagrams (E vs pH) of the MOFs and their ligands
were constructed using cyclic voltammetry in aqueous buffered media. The MOFs
exhibit chemical stability and undergo diverse electrochemical processes, where the
number of electrons and protons transferred was tailored in a Nernstian manner by
the pH of the media. Both the 2,6-Zr-AQ-MOF and its ligand reveal a similar
electrochemical pK value (7.56 and 7.35, respectively) for the transition between a
a
two-electron, two-proton transfer (at pH < pK ) and a two-electron, one-proton
a
transfer (at pH > pK ). In contrast, the position of the quinone moiety with respect
a
to the zirconium node, the effect of hydrogen bonding, and the amount of defects
in 1,4-Zr-AQ-MOF lead to the transition from a two-electron, three-proton transfer
to a two-electron, one-proton transfer. The pK of this framework (5.18) is analogous to one of the three electrochemical pKa
a
values displayed by its ligand (3.91, 5.46, and 8.80), which also showed intramolecular hydrogen bonding. The ability of the
MOFs to tailor discrete numbers of protons and electrons suggests their application as charge carriers in electronic devices.
1
. INTRODUCTION
proton process that generates the respective hydroquinone
9
(
QH ). At alkaline pH, the concentration of protons is too low
2
The application of metal−organic frameworks (MOFs) as
active materials in electronic devices has been proposed as a
suitable alternative to achieve efficient charge storage and
to support PCET; therefore, the redox process is a two-electron
2
−
9
transfer to form the dianion (Q ). At neutral pH, the quinone
can undergo either a two-electron, one-proton process resulting
1
energy conversion. The characteristic synthetic tunability, high
−
in the protonated dianion species (QH ), or alternatively it
permanent porosity, and structural uniformity of MOFs
position these materials as prospective candidates for
assembling high surface electrodes containing well-organized
redox moieties that are capable to store electricity. In
particular, the ability of MOFs to undergo electron transport
could lead to the formation of the dianion (Supporting
9
Information, Scheme S2). The electrochemical pK
values of
a
2
the different redox and protonation states of quinone species
can be represented using a potential (E) versus pH plot
(Pourbaix diagram).
has been achieved through three different synthetic approaches:
3
(
1) charge transfer through metal−ligand orbital overlap, (2)
Quinone-based ligands have served as an inspiration to the
10,11
π-stacking interactions between electroactive ligands within the
synthesis of covalent-organic frameworks (COFs)
and
4
5
framework, and (3) redox hopping. Nevertheless, the
synthetic design of MOFs that exhibit proton-coupled electron
transport (PCET) remains in its early stages. The two main
necessities are the ability to facilitate high electron and proton
conduction and a high water stability over a range of pH.
One way to address these requirements is to integrate ligands
capable of undergoing PCET into isoreticular arrays of highly
stable MOFs, such as the UiO series. These zirconium-based
MOFs have demonstrated an exceptional stability in water and
other MOFs that show potential applications in batteries and
12−14
supercapacitors.
However, understanding the effect of
local pH in the pore environment of these frameworks remains
vague. A hitherto study of the reduction of MOF UMCM-1
containing a nanoconfined anthraquinone molecule (Alizarin
red S) in its pores showed that a typical two-electron, two-
proton process at acidic pH was experienced only in
15,16
anthraquinone molecules located at the MOF surface.
In
6
,7
contrast, the deficiency of protons inside the MOF pores
created a local alkaline environment during the two-electron
reduction that led to the formation of the negatively charged
in acidic pH. Similarly, quinone ligands are recognized as
excellent PCET agents, since they play an important role in
biochemical processes, including photosynthesis and oxidative
8
phosphorylation. The redox chemistry of quinones is strongly
influenced by the pH of the media. Generally, at acidic pH, the
Received: July 10, 2017
quinone (Q) reduction is a single-step, two-electron, two-
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01656
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Inorganic Chemistry
Article
dianion, which was ejected from the framework to stabilize
cooled to room temperature, the yellow precipitate was filtered under
1
5,16
vacuum and washed with water. The solid was dried overnight,
internal charge.
Furthermore, the protonation state of the zirconium nodes of
1
9
1
affording pure 2 with 69% yield. H NMR (400 MHz, DMSO): δ
17
13.37 (s, 2H), 8.19 (dd, J = 5.6 Hz, 3.2 Hz, 2H), 7.99 (dd, J = 5.6 Hz,
UiO-MOFs is also directly influenced by the local pH. As a
result, assembling these frameworks with ligands that exhibit
quinone moieties in the proximity of the zirconium cluster may
result in the occurrence of other complex processes, such as
hydrogen bonding or linker-to-node proton transfer. Addition-
ally, the pore size and the amount of defects (missing ligands)
in the framework may play an important role in the transport of
protons within the MOF pores. To elucidate these factors, we
sought to study the effect of applied potential and pH on the
equilibrium between different quinone species in these
materials, where the specific number of protons and electrons
transferred can be accounted. Herein, we report the first
example of Pourbaix diagrams for two different anthraquinone-
based zirconium MOFs that exhibit diverse electrochemical
response in aqueous electrolytes due to the positioning of the
quinone moiety with respect to the zirconium node, the
amount of defects, and the pore size. To the best of our
knowledge, these types of studies have not been performed on
redox-active frameworks to date. These results demonstrate the
potential of MOFs to be employed as proton−electron carriers
in energy transport systems.
+
3
.2 Hz, 2H), 7.89 (s, 2H). HRMS: (M+H) : 297.0376.
Synthesis of 2,6-Zr-AQ MOF [Zr O (OH) (C O H ) -
6
4
4
16
6
6 6
(
C H NO) (H O) ]. The synthesis was performed using a similar
3
7
17
2
22
21
procedure previously reported in the literature. ZrCl (23.0 mg, 0.1
4
mmol) was disolved in 2.0 mL of dimethylformamide (DMF) and
sonicated for 5 min. Then, the ligand 2,6-dicarboxy-9,10-anthraqui-
none 1 (29.6 mg, 0.1 mmol) was added to the solution, as well as 47
equiv (0.3 mL) of acetic acid as a modulator. The mixture was
sonicated for 15 min before being set at 120 °C for 24 h. The 2,6-Zr-
AQ-MOF was obtained as a pink powder and isolated by
centrifugation. The pink solid was washed and centrifuged three
times with fresh DMF and dried under vacuum at room temperature
(yield 31.3 mg, 75%).
Synthesis of 1,4-Zr-AQ-MOF [Zr O (OH) (C O H ) -
2 2 3 2.76 2 1.24 3 7 11 2 40 4
6
4
4
16
6
6 4
(
C O H ) (CO H) (C H NO) (H O) ]. A mixture of ZrCl
(23.0 mg, 0.1 mmol), the ligand 1,4-dicarboxy-9,10-anthraquinone 2
(29.6 mg, 0.1 mmol), acetic acid (1.2 mL, 200 equiv), and DMF (3.0
mL) was sonicated for 15 min. The mixture was heated at 120 °C for
48 h. After it cooled at a rate of 1 °C/min, the 1,4-Zr-AQ-MOF was
obtained as a pink powder, which was isolated by centrifugation. The
pink solid was washed and centrifuged three times with fresh DMF
and dried under vacuum at room temperature (yield 30.0 mg, 72%).
2
. EXPERIMENTAL SECTION
3. RESULTS AND DISCUSSION
Synthesis of 2,6-Dicarboxy-9,10-anthhraquinone (1). The
The ligands 2,6-dicarboxy-9,10-anthraquinone (1) and 1,4-
dicarboxy-9,10-anthraquinone (2) were synthesized in two
synthesis of 1 was achieved in two steps by adapting previous
18,19
procedures described in the literature (Scheme S1).
Benzoquinone
steps by means of a Diels−Alder reaction followed by
(
(
3.25 g, 30 mmol), isoprene (10.0 mL), and traces of hydroquinone
two spatula tips), were suspended in 20.0 mL of absolute ethanol.
18−20
oxidation, according to Scheme S1.
The powdered
samples of Zr O (OH) (C O H ) (C H NO) (H O)
The mixture was placed in a 40.0 mL autoclave and heated at 130 °C
for 6 h. Once the autoclave reached room temperature, the mixture
was dissolved in a potassium hydroxide solution in ethanol (8.5 g of
KOH in 200 mL of EtOH) in a round-bottom flask. The solution was
stirred and heated for 8 h at 40 °C under air. Stirring was continued
for 12 h at room temperature under air. The mixture was heated again
at 50 °C for 2 h, allowed to reach room temperature, and filtered in
vacuum to give a white cream solid. The solid was placed in a vial
containing absolute ethanol and left in the fridge overnight.
Subsequently, the solid was filtered again, washed with cold ethanol
6
4
4
16
6
6
6
3
7
17
2
22
[
2 , 6 - Z r - A Q - M O F ] a n d Z r O ( O H ) -
6
4
4
(C16
O
H
)
(C
O
H
)
2.76(CO
H)1.24(C
H
NO)11(H
O)40
6
6
4
2
2
3
2
3
7
2
[1,4-Zr-AQ-MOF] were obtained by the solvothermal synthesis
of ZrCl and either 1 or 2 in DMF, using acetic acid as a
4
modulator (Figure 1). Specifically, the addition of 1 and 47
equivalents of acetic acid to a solution of ZrCl in DMF,
4
followed by heating at 120 °C for 1 d, afforded 2,6-Zr-AQ-
21
MOF as a pink powder. Similarly, the treatment of 2 and
ZrCl in a mixture of acetic acid and DMF (1:2.5 by volume) at
120 °C for 2 d, gave 1,4-Zr-AQ-MOF as a pink powder.
1
and water, and dried under air. H NMR confirmed the identity of the
4
18
product as 2,6-dimethyl-9,10-anthraquinone. A solution of 2,6-
dimethyl-9,10-anthraquinone (1.0 g) in 12.0 mL of 25% nitric acid was
placed in a 40.0 mL autoclave and heated at 220 °C for 3 h. After it
cooled to room temperature, the yellow precipitate was filtered under
vacuum and washed with water. The solid was dried overnight,
19
1
affording pure 1 with 69% yield. H NMR (400 MHz, dimethyl
sulfoxide (DMSO)): δ 13.77 (s, 2H), 8.69(dd, J = 1.8 Hz, 0.4 Hz, 2H),
8
.44(dd, J = 8 Hz, 1.8 Hz, 2H), 8.36(d,d J = 8 Hz, 0.4 Hz, 2H). High-
+
resolution mass spectrometry (HRMS): (M+H) : 297.0375.
Synthesis of 1,4-Dicarboxy-9,10-anthhraquinone (2). The
synthesis of 2 was achieved in two steps by adapting previous
19,20
procedures described in the literature (Scheme S1).
First, a
solution of 1,4-naphthoquinone (9.35 g, 0.059 mol) and 2,4-hexadiene
(
5.0 g, 0.605 mol) in toluene (35 mL) was heated at 65 °C for 4 d.
Subsequently, the solvent was evaporated, and the remaining oily solid
was used in the next step without further purification. The oily solid
(
15.0 g) was disolved in 250 mL of absolute ethanol and added to a
solution of KOH (30 g) in ethanol (1.25 L), while cooled at 10 °C.
Then a current of oxygen was bubbled through the solution for 1 h,
while the temperature was maintained at 10 °C. After the solvent was
evaporated, the semisolid residue was treated with water (600 mL) and
extracted with benzene. Removal of the aromatic solvent afforded 1,4-
20
dimethyl-9,10-anthraquinone with 70% yield. A solution of 1,4-
dimethyl-9,10-anthraquinone (1.0 g) in 12.0 mL of 25% nitric acid was
placed in a 40.0 mL autoclave and heated at 220 °C for 3 h. After it
Figure 1. Synthesis of 2,6-Zr-AQ-MOF and 1,4-Zr-AQ-MOF.
B
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Powder X-ray diffraction (PXRD) patterns of 2,6-Zr-AQ-
MOF and 1,4-Zr-AQ-MOF reveal that both MOFs are
isostructural with the UiO series (Figure 2). Le Bail refinement
−1
Figure 3. Cyclic voltammograms (10 mV s ) of (A) ligand 1, 0.1 M
phosphate buffer−DMF (9:1 volume) as supporting electrolyte, and
Figure 2. PXRD patterns of (A) 2,6-Zr-AQ-MOF and (B) 1,4-Zr-
AQ-MOF. (insets) The corresponding N adsorption isotherms.
2
(
B) ligand 2, 0.1 M phosphate buffer as supporting electrolyte.
Pourbaix diagrams of (C) ligand 1 and (D) ligand 2.
of the powder patterns using crystallographic data for UiO-67
and UiO-66 as starting models, respectively, revealed a lattice
parameter of a = 27.0472 Å for 2,6-Zr-AQ-MOF and a =
This linear dependence of the reduction potential with
respect to pH is expected to follow the modified Nernst
equation:
20.6825 Å for 1,4-Zr-AQ-MOF (Figures S9 and S10), which is
E = E° − 0
.059 16
n
log
1
consistent with the length of each ligand. On the one hand,
scanning electron microscopy (SEM) images of 2,6-Zr-AQ-
MOF show that it consists of octahedral-shaped crystallites,
characteristic of UiO-type frameworks. The 1,4-Zr-AQ-MOF,
on the other hand, exhibited a more inconsistent morphology
+
m
[H ]
(1)
where m is the number of protons, and n is the number of
electrons involved in the process. Therefore, the derivative of
this relationship can be expressed as
(Figures S11 and S12), presumably due to incorporated defects
∂
E
pH
m
n
(
vide infra). The N adsorption isotherms of both MOFs are
= 0.059 16
2
∂
(2)
shown in Figure 2. A Brunauer−Emmett−Teller (BET) surface
2
−1
area of 175.6 m g was found for 2,6-Zr-AQ-MOF, while the
and represents the slope of the linear regression between E and
pH in the different segments of the Pourbaix diagrams (Figure
2
−1
1
,4-Zr-AQ-MOF exhibited a surface area of 554.16 m g .
The redox behavior of the ligands in aqueous phosphate
3
C,D). As a result, this expression allows the elucidation of the
buffer was studied by cyclic voltammetry (CV), using a glassy
carbon (GC) working electrode. Reduction potentials were
recorded at different pH, to construct the Pourbaix diagrams.
Both ligands show a single-step, reversible CV wave, which
shifted to more negative reduction potentials with increasing
pH (Figure 3A,B).
number of protons and electrons transferred during the
reduction. For instance, two different electrochemical processes
are apparent in the Pourbaix diagram of ligand 1 (Figure 3C).
An experimentally determined relationship of dE/d(pH) of
−53 ± 6 between pH 1.33 and 7.35 suggests a two-electron,
two-proton transfer to the quinone moiety of the ligand to form
C
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the respective hydroquinone. In addition, from pH 7.35 to
explanation for the transfer of additional protons during the
reduction of 2 at pH < 3.91 with the third/fourth proton
originating from the protonation of the carboxylate in the
hydroquinone form of 2 and stabilized through intramolecular
hydrogen bonding. This stabilization is less likely in 1, since
carboxylic acid and hydroxyl functionalities are further apart.
CV of the anthraquinone frameworks was performed on the
dried slurry of the MOF powder (10 mg), graphite (10 mg),
poly(methyl methacrylate) (3.34 mg), and tetrahydrofuran
(THF; 1 mL) deposited on a GC electrode. Subsequently, this
working electrode was used in a three-electrode assembly, with
a 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid
(HEPES)-buffered aqueous solution as supporting electrolyte
(see Supporting Information for detailed procedure). UV−vis
spectroscopy was used to monitor the stability of the
frameworks, evidenced as the absence of ligand absorption in
solution before and after the CV measurements over the range
of pH (Figures S20−S22). The 2,6-Zr-AQ-MOF showed
stability from pH 1.74 to 9.67, while the 1,4-Zr-AQ-MOF
was stable from pH 1.66 to 6.75. Both MOFs showed a
reversible redox process that shifted to more negative potential
with increasing pH (Figure 4A,B).
11.38, a slope (dE/d(pH)) of −26 ± 6 evidenced a two-
electron, one-proton transfer from the quinone to the
correspondent protonated dianion. The appearance of these
two processes in ligand 1 is typical of many anthraquinone
systems in aqueous buffer and has been extensively discussed in
8
,9,22,23
the literature.
The boundary between the two types of electrochemical
processes defines the electrochemical pK of 1 (7.35). In
a
addition, the two chemical pK values for the carboxylic acids of
a
1
were determined via the titration with NaOH (Figure S7).
These pK values (pK = 4.2 and pKa2 = 6.1) are also
a
a1
represented in the Pourbaix Diagram of 1 as vertical lines
Figure 3C). Similarly, the titration of 2 (Figure S8) allowed
(
the determination of pKa1 = 3.7 and pKa2 = 5.6 for the two
carboxylic acid in 2 (Figure 3D).
Interestingly, ligand 2 was found to undergo four different
PCET processes (Figure 3D). Similar to ligand 1, a two-
electron, one-proton transfer from the quinone to the
corresponding protonated dianion was observed from pH
5
.46 to 8.80, as evidenced by a slope of −27 ± 2. In addition,
for pH between 3.91 and 5.46, the slope was found to be −66
16, suggesting a two-electron and two-proton transfer
±
The behavior of 2,6-Zr-AQ-MOF was very similar to that
observed for its free ligand in solution. The Pourbaix diagram of
this MOF showed two electrochemical processes, defined by an
process of the quinone moiety of 2 to form the hydroquinone.
However, two other processes were evidenced at the highest
and lowest pH ranges tested in this study: at high pH (from pH
electrochemical pK of 7.56 (Figure 4C). For pH < 7.56, a slope
a
8
.80 to 12.22) a four-electron one-proton transfer, represented
of −63 ± 4 was observed indicating the two-electron, two-
proton transfer to the quinone resulting in the hydroquinone
state of the ligand inside 2,6-Zr-AQ-MOF. Furthermore, for pH
> 7.56 either a two-electron, one-proton transfer or a two-
electron, two-proton transfer took place, as evidenced by a
slope of −39 ± 3. For clarity, only the two-electron, one-proton
transfer is shown in the corresponding segment of the Pourbaix
Diagram in Figure 4C. The similarity in the electrochemical pKa
of 2,6-Zr-AQ-MOF (7.56) and its ligand in solution (7.35)
reveals that the pore dimensions of this framework allow a
sufficient transport of protons inside the MOF pores.
Consequently, the pH in the pores is the same as in the bulk
solution, and only the pH tailors the number of protons
transferred to the quinone moiety during the reduction. This
fact corroborates the benefits of introducing anthraquinone
moieties as ligands instead of encapsulating them in the MOF
by a slope of −13 ± 3, involved the reaction of two ligand
molecules to produce both the dianion and protonated
dianion.
of 2 at alkaline pH, which was not observed in 1, may be a
result of the difficulty to protonate this moiety due to the
proximity of the carboxylic acid.
Moreover, at pH below 3.91, a slope of −105 ± 7 involving a
transfer of either two electrons and three protons or two
electrons and four protons characterized the last process. Since
the carboxylic acid groups of 2 are fully protonated at pH < 3.7,
a maximum of two protons can be transferred to the quinone to
form the hydroquinone. The apparition of a third or fourth
proton during the reduction can be explained based on the
proximity of the carboxylic acids to the quinone moiety in 2.
First, it is expected that the pK of the carboxylic acids of 2 in
the hydroquinone state is much lower (∼2) compared to the
quinone state.
electrons and three/four protons could be attributed to a
simultaneous protonation of the carboxylic acid of the reduced
hydroquinone moiety that is stabilized through hydrogen
bonding. Figure 3D shows the proposed seven-membered ring
structure formed during the two-electron, three-proton transfer.
Predominantly, hydroquinones have been found to form both
2
4−26
The formation of equal amounts of these species
a
15,16
pores as nanoconfined molecules.
2
7,28
As a result, the observed transfer of two
Unlike its ligand, 1,4-Zr-AQ-MOF revealed only two
electrochemical processes (Figure 4D). For pH > 5.18, the
formation of the protonated dianion through a two-electron,
one-proton transfer was characterized by a slope of −29 ± 8.
This behavior is comparable to the one of its ligand in solution
at pH between 5.46 and 8.80 (Figure 3D) and to the 2,6-Zr-
AQ-MOF (Figure 4C). This observation suggests that 1,4-Zr-
AQ-MOF also allows the efficient penetration of protons into
the MOF structure and demonstrates the consistency between
the pore environment with respect to the bulk.
The Pourbaix diagram of 1,4-Zr-AQ-MOF also showed a
slope of −96 ± 5 for pH < 5.18, representing a two-electron,
three-proton transfer, similar to the one observed for 2 at pH <
3.91 (Figure 3D). Since the ligand of 1,4-Zr-AQ-MOF can only
accept maximum of two protons without breaking the metal-
carboxylate bonds, this behavior could be explained based on
the level of defects in the MOF. Particularly, the UiO
frameworks exhibit two different types of defects. On one
hand, “missing linker defects” refer to the absence of ligands
coordinated to the Zr-oxide cluster, which is commonly
2
9,30
intermolecular
and intramolecular hydrogen bonding with
31
other −OH functionalities. In fact, the existence of hydrogen
bonding between hydroquinone moieties and carboxylic acids is
well-documented.
3
1,32
Consequently, the proximity of the
carboxylic acids to the hydroxyl group in the hydroquinone
species could also lead to intramolecular stabilization due to
hydrogen bonding. To test this claim, the Pourbaix diagram of a
similar derivative, 1,4-dimethyl-9,10-anthraquinone, was ob-
tained under identical conditions (Figure S18). In this
analogue, the carboxylic acid groups are replaced with methyl
groups. A slope of −52 ± 2 was determined for pH between
1
.69 and 6.15 indicating that this analogue undergoes a two-
electron, two-proton transfer. This finding supports the earlier
D
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Article
the observed two-electron, three-proton transfer shown in
Figure 4D, when considering two different possibilities.
The existence of ligands connected only to one node could
lead to intramolecular hydrogen bonding between the free
carboxylic acid and the reduced hydroquinone moiety, in a
similar fashion to its ligand in solution (Figure 3D).
Alternatively, the proximity of the quinone moiety to the
zirconium node (2.6−2.8 Å) inside the framework could also
lead to hydrogen bonding between the hydroquinone and the
protons present on the inorganic cluster, as represented in
Figure 4D for pH < 5.18. Two of the three types of protons
present in the zirconium node are recognized as −OH and
1
7
−
OH groups that occupy the place of missing linkers. The
2
acidity of these protons in UiO-66 has been recently studied,
finding pK values of 8.30 and 6.79, respectively. This fact
17
a
suggests that at pH < 5.18 the highly defected node of 1,4-Zr-
AQ-MOF is most likely protonated, facilitating hydrogen
bonding between the hydroquinone and the protons in the
node (Figure 4D). A related cooperative behavior between
linker and node was postulated for the electrocatalytic water
oxidation in PCN-224, where the node acts as a proton
37
acceptor. These processes are not observed in the Pourbaix
diagram of 2,6-Zr-AQ-MOF, because the quinone moiety is
relatively distant (5.0−5.2 Å) from the zirconium node (Figure
1
).
4
. CONCLUSION
In summary, we have shown that PCET can be modulated in
anthraquinone-based Zr MOFs as a function of the pH. The
positioning of the quinone moiety with respect to the
zirconium node, the amount of defects, and the pore size of
these materials are directly influencing the amount of protons
transported to the frameworks. These results demonstrate that
MOFs have potential as proton−electron carriers for electronic
applications and encourage further research in this area.
−1
Figure 4. Cyclic voltammograms (100 mV s , 0.1 M HEPES buffer as
supporting electrolyte) of (A) 2,6-Zr-AQ-MOF and (B) 1,4-Zr-AQ-
MOF. Pourbaix diagrams of (C) 2,6-Zr-AQ-MOF and (D) 1,4-Zr-AQ-
MOF.
ASSOCIATED CONTENT
■
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01656.
Detailed experimental procedures, NMR, HR-MS
spectra, IR spectra, titration curves of ligands, SEM
images, PXRD refinement, elemental analysis, TGA
traces, and CV and UV−vis spectra for determination
of MOF stability (PDF)
3
3
compensated by modulator molecules connected to the node.
On the other hand, the recently discovered “missing cluster
defects” correspond to the absence of Zr (μ -O) (μ -OH)
4
node, as well as the 12 ligands connected to it, in correlated
6
3
4
3
34,35
nanoregions of the reo topology.
the defects in 1,4-Zr-AQ-MOF using quantitative NMR analysis
Figure S23), finding a total modulator-to-linker molar ratio of
.96, suggesting an equivalent amount of linker and modulator
We sought to calculate
(
0
AUTHOR INFORMATION
■
*
Corresponding Author
E-mail: ajmorris@vt.edu.
in the framework, presumably due to the high concentration of
acetic acid utilized during the synthesis (Figure 1). Elemental
analysis on the framework (Table S1) also supported these
findings. This framework is an analogue of the UiO-66 MOF,
which has been typically recognized to have a high amount of
ORCID
William A. Maza: 0000-0002-9126-2959
Amanda J. Morris: 0000-0002-3512-0366
3
3,35,36
defects,
and the shape of the anthraquinone ligand
Notes
suggests steric hindrance during its formation (Figure 1). More
importantly, the PXRD pattern of 1,4-Zr-AQ-MOF (Figure 2B)
displays a broad peak at 2θ range of ca. 3−6°, which has been
recently assigned as a fingerprint of missing node defects in
UiO frameworks, where one-quarter of the MOF clusters are
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This material is based upon work supported by the National
Science Foundation under Grant No. 1551964.
3
4
missing. The high defectivity of 1,4-Zr-AQ-MOF can explain
E
DOI: 10.1021/acs.inorgchem.7b01656
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(
18) Cabellero, A. G.; Croft, A. K.; Nalli, S. M. Remote aromatic
REFERENCES
■
stabilization in radical reactions. Tetrahedron Lett. 2008, 49, 3613−
615.
19) Arient, J.; Podstata, J. Nitration and oxidation of anthraquinone
dimethyl derivatives. Collect. Czech. Chem. Commun. 1974, 39, 3117−
123.
20) Garay, R. O.; Naarmann, H.; Muellen, K. Synthesis and
characterization of poly(1,4-anthrylenevinylene). Macromolecules 1994,
7, 1922−1927.
21) Bon, V.; Senkovska, I.; Weiss, M. S.; Kaskel, S. Tailoring of
(
1) Allendorf, M. D.; Schwartzberg, A.; Stavila, V.; Talin, A. A. A
roadmap to implementing metal-organic frameworks in electronic
devices: challenges and critical directions. Chem. - Eur. J. 2011, 17,
1372−88.
2) DeBlase, C. R.; Hernan
Rodríguez-Calero, G. G.; Bisbey, R. P.; Abrun
3
(
1
(
3
(
́
dez-Burgos, K.; Silberstein, K. E.;
a, H. D.; Dichtel, W.
̃
R. Rapid and Efficient Redox Processes within 2D Covalent Organic
2
(
Framework Thin Films. ACS Nano 2015, 9, 3178−3183.
(
̆
3) Sun, L.; Miyakai, T.; Seki, S.; Dinca, M. Mn2(2,5-
network dimensionality and porosity adjustment in Zr- and Hf-based
MOFs. CrystEngComm 2013, 15, 9572−9577.
(
Anthraquinones1. J. Am. Chem. Soc. 1948, 70, 3055−3061.
(
of 9,10-anthraquinone with DNA 1. Hanging mercury drop electrode.
Microchem. J. 1998, 59, 294−306.
(
the aqueous electrochemistry of some anthraquinone derivatives on
carbon paste electrode. Iranian Journal of Chemistry and Chemical
Engineering (IJCCE) 2001, 20, 75−81.
disulfhydrylbenzene-1,4-dicarboxylate): A Microporous Metal−Organ-
ic Framework with Infinite (−Mn−S−)∞ Chains and High Intrinsic
Charge Mobility. J. Am. Chem. Soc. 2013, 135, 8185−8188.
22) Furman, N. H.; Stone, K. G. A Polarographic Study of Certain
(
̆
4) Narayan, T. C.; Miyakai, T.; Seki, S.; Dinca, M. High Charge
23) Zhu, Z. W.; Li, N. Q. Electrochemical studies of the interaction
Mobility in a Tetrathiafulvalene-Based Microporous Metal−Organic
Framework. J. Am. Chem. Soc. 2012, 134, 12932−12935.
(
5) Ahrenholtz, S. R.; Epley, C. C.; Morris, A. J. Solvothermal
24) Ojani, R.; Raoof, J.; Ebranimi, M. A cyclic voltammetric study of
Preparation of an Electrocatalytic Metalloporphyrin MOF Thin Film
and its Redox Hopping Charge-Transfer Mechanism. J. Am. Chem. Soc.
014, 136, 2464−2472.
6) Cavka, J. H.; Jakobsen, S.; Olsbye, U.; Guillou, N.; Lamberti, C.;
2
(
Bordiga, S.; Lillerud, K. P. A New Zirconium Inorganic Building Brick
Forming Metal Organic Frameworks with Exceptional Stability. J. Am.
Chem. Soc. 2008, 130, 13850−13851.
(25) Petrova, S. A.; Kolodyazhny, M. V.; Ksenzhek, O. S.
Electrochemical properties of some naturally occurring quinones. J.
Electroanal. Chem. Interfacial Electrochem. 1990, 277, 189−196.
(26) Raoof, D. B.; Golabi, S. M. Electrochemical Properties of
Carbon-Paste Electrodes Spiked with Some 1,4-Naphthoquinone
Derivatives. Bull. Chem. Soc. Jpn. 1995, 68, 2253−2261.
(27) Driebergen, R. J.; Den Hartigh, J.; Holthuis, J. J. M.; Hulshoff,
A.; van Oort, W. J.; Postma Kelder, S. J.; Verboom, W.; Reinhoudt, D.
N.; Bos, M.; van der Linden, W. E. Electrochemistry of potentially
bioreductive alkylating quinones. Anal. Chim. Acta 1990, 233, 251−
268.
(
7) DeCoste, J. B.; Peterson, G. W.; Jasuja, H.; Glover, T. G.; Huang,
Y.-g.; Walton, K. S. Stability and degradation mechanisms of metal-
organic frameworks containing the Zr6O4(OH)4 secondary building
unit. J. Mater. Chem. A 2013, 1, 5642−5650.
(
8) Guin, P. S.; Das, S.; Mandal, P. C. Electrochemical Reduction of
Quinones in Different Media: A Review. Int. J. Electrochem. 2011,
011, 1.
9) Guin, P. S.; Das, S.; Mandal, P. Electrochemical reduction of
2
(
sodium 1,4-dihydroxy-9,10-anthraquinone-2-sulphonate in aqueous
and aqueous dimethyl formamide mixed solvent: a cyclic voltammetric
study. Int. J. Electrochem. Sci. 2008, 3, 1016−1028.
(28) Ornskov, E.; Linusson, A.; Folestad, S. Determination of
dissociation constants of labile drug compounds by capillary
̈
electrophoresis. J. Pharm. Biomed. Anal. 2003, 33, 379−391.
(
̃
10) DeBlase, C. R.; Silberstein, K. E.; Truong, T.-T.; Abruna, H. D.;
(29) Nemkevich, A.; Spackman, M. A.; Corry, B. Mechanism of
Dichtel, W. R. β-Ketoenamine-Linked Covalent Organic Frameworks
Concerted Hydrogen Bond Reorientation in Clathrates of Dianin’s
Compound and Hydroquinone. J. Am. Chem. Soc. 2011, 133, 18880−
Capable of Pseudocapacitive Energy Storage. J. Am. Chem. Soc. 2013,
1
(
35, 16821−16824.
11) Chandra, S.; Roy Chowdhury, D.; Addicoat, M.; Heine, T.; Paul,
1
8888.
(30) Ilczyszyn, M.; Selent, M.; Ilczyszyn, M. M. Participation of
A.; Banerjee, R. Molecular Level Control of the Capacitance of Two-
Dimensional Covalent Organic Frameworks: Role of Hydrogen
Bonding in Energy Storage Materials. Chem. Mater. 2017, 29, 2074−
Xenon Guest in Hydrogen Bond Network of β-Hydroquinone Crystal.
J. Phys. Chem. A 2012, 116, 3206−3214.
(31) Salazar, R.; Vidal, J.; Martinez-Cifuentes, M.; Araya-Maturana,
2
080.
R.; Ramirez-Rodriguez, O. Electrochemical characterization of hydro-
quinone derivatives with different substituents in acetonitrile. New J.
Chem. 2015, 39, 1237−1246.
(
12) Gui, B.; Meng, X.; Chen, Y.; Tian, J.; Liu, G.; Shen, C.; Zeller,
M.; Yuan, D.; Wang, C. Reversible Tuning Hydroquinone/Quinone
Reaction in Metal−Organic Framework: Immobilized Molecular
Switches in Solid State. Chem. Mater. 2015, 27, 6426−6431.
(
32) Astudillo, P. D.; Valencia, D. P.; Gonzal
́
ez-Fuentes, M. A.; Díaz-
Sanchez, B. R.; Frontana, C.; Gonzalez, F. J. Electrochemical and
chemical formation of a low-barrier proton transfer complex between
́
́
(
13) Zhang, Z.; Yoshikawa, H.; Awaga, K. Monitoring the Solid-State
Electrochemistry of Cu(2,7-AQDC) (AQDC = Anthraquinone
Dicarboxylate) in a Lithium Battery: Coexistence of Metal and Ligand
Redox Activities in a Metal−Organic Framework. J. Am. Chem. Soc.
the quinone dianion and hydroquinone. Electrochim. Acta 2012, 81,
1
(
97−204.
33) Wu, H.; Chua, Y. S.; Krungleviciute, V.; Tyagi, M.; Chen, P.;
2
(
014, 136, 16112−16115.
Yildirim, T.; Zhou, W. Unusual and Highly Tunable Missing-Linker
Defects in Zirconium Metal−Organic Framework UiO-66 and Their
Important Effects on Gas Adsorption. J. Am. Chem. Soc. 2013, 135,
14) Zhang, Z.; Yoshikawa, H.; Awaga, K. Discovery of a “Bipolar
Charging” Mechanism in the Solid-State Electrochemical Process of a
Flexible Metal−Organic Framework. Chem. Mater. 2016, 28, 1298−
1
(
0525−10532.
1
303.
34) Shearer, G. C.; Chavan, S.; Bordiga, S.; Svelle, S.; Olsbye, U.;
(
15) Halls, J. E.; Ahn, S. D.; Jiang, D.; Keenan, L. L.; Burrows, A. D.;
Lillerud, K. P. Defect Engineering: Tuning the Porosity and
Composition of the Metal−Organic Framework UiO-66 via
Modulated Synthesis. Chem. Mater. 2016, 28, 3749−3761.
Marken, F. Proton uptake vs. redox driven release from metal−
organic-frameworks: Alizarin red S reactivity in UMCM-1. J.
Electroanal. Chem. 2013, 689, 168−175.
(35) Cliffe, M. J.; Wan, W.; Zou, X.; Chater, P. A.; Kleppe, A. K.;
(
16) Halls, J. E.; Jiang, D.; Burrows, A. D.; Kulandainathan, M. A.;
Tucker, M. G.; Wilhelm, H.; Funnell, N. P.; Coudert, F. X.; Goodwin,
A. L. Correlated defect nanoregions in a metal-organic framework.
Nat. Commun. 2014, 5, 4176.
(36) Øien, S.; Wragg, D.; Reinsch, H.; Svelle, S.; Bordiga, S.;
Lamberti, C.; Lillerud, K. P. Detailed Structure Analysis of Atomic
Positions and Defects in Zirconium Metal−Organic Frameworks.
Cryst. Growth Des. 2014, 14, 5370−5372.
Marken, F. Electrochemistry within metal-organic frameworks. In
Electrochemistry; The Royal Society of Chemistry, 2014; Vol. 12, pp
1
(
87−210.
17) Klet, R. C.; Liu, Y.; Wang, T. C.; Hupp, J. T.; Farha, O. K.
Evaluation of Bronsted acidity and proton topology in Zr- and Hf-
based metal-organic frameworks using potentiometric acid-base
titration. J. Mater. Chem. A 2016, 4, 1479−1485.
F
DOI: 10.1021/acs.inorgchem.7b01656
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(
37) Usov, P. M.; Ahrenholtz, S. R.; Maza, W. A.; Stratakes, B.; Epley,
C. C.; Kessinger, M. C.; Zhu, J.; Morris, A. J. Cooperative
electrochemical water oxidation by Zr nodes and Ni-porphyrin linkers
of a PCN-224 MOF thin film. J. Mater. Chem. A 2016, 4, 16818−
1
6823.
G
DOI: 10.1021/acs.inorgchem.7b01656
Inorg. Chem. XXXX, XXX, XXX−XXX