Cation-exchanged conductive Mn2DSBDC metal–organic frameworks: Synthesis, structure, and THz conductivity

Article abstract of DOI:10.1016/j.poly.2021.115182

Conductive metal–organic frameworks (MOFs) are an emerging class of materials that rely upon crystallographically-defined charge-transport pathways that can be synthetically designed. Such conductive MOFs, including Mn-based MOFs, have not yet found applications in electrocatalysis at least partly due to restrictions in their tunability, as compositional or structural changes may interrupt the purposefully-designed charge-transport pathways. In this work, we provide an original strategy to exchange a portion of the Mn²⁺ cations in the conductive MOF Mn₂DSBDC (where DSBDC = 2,5-dimercaptoterephthalate) for either Ni²⁺, Cu²⁺, or Co²⁺. The bulk and local structures were characterized using powder X-ray diffraction and element-specific X-ray absorption spectroscopy, respectively, to understand the structural effects of cation exchange, supporting that cation exchange does not alter the overall structure of the MOF. Importantly, using time-domain THz spectroscopy, it was discovered that the cation exchange does not alter the conductivity of the MOF. This finding opens the door to functionalization and tunability with respect to the cation composition in Mn₂DSBDC, strongly suggesting applications in electrocatalysis.

Full text of DOI:10.1016/j.poly.2021.115182

Polyhedron 203 (2021) 115182
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Cation-exchanged conductive Mn2DSBDC metal–organic frameworks:
Synthesis, structure, and THz conductivity
Brian Pattengale a, Jens Neu b, Ayano Tada a,1, Gongfang Hu a, Christopher J. Karpovich a,2, Gary W. Brudvig a,
⇑
a Department of Chemistry and Yale Energy Sciences Institute, Yale University, New Haven, CT 06520-8107, United States
b Department of Molecular Biophysics and Biochemistry and Yale Microbial Sciences Institute, Yale University, New Haven, CT 06520-8107, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Conductive metal–organic frameworks (MOFs) are an emerging class of materials that rely upon crystal-
lographically-defined charge-transport pathways that can be synthetically designed. Such conductive
MOFs, including Mn-based MOFs, have not yet found applications in electrocatalysis at least partly due
to restrictions in their tunability, as compositional or structural changes may interrupt the purpose-
fully-designed charge-transport pathways. In this work, we provide an original strategy to exchange a
portion of the Mn2+ cations in the conductive MOF Mn2DSBDC (where DSBDC = 2,5-dimercaptoterephtha-
late) for either Ni2+, Cu2+, or Co2+. The bulk and local structures were characterized using powder X-ray
diffraction and element-specific X-ray absorption spectroscopy, respectively, to understand the structural
effects of cation exchange, supporting that cation exchange does not alter the overall structure of the
MOF. Importantly, using time-domain THz spectroscopy, it was discovered that the cation exchange does
not alter the conductivity of the MOF. This finding opens the door to functionalization and tunability with
respect to the cation composition in Mn2DSBDC, strongly suggesting applications in electrocatalysis.
Ó 2021 Elsevier Ltd. All rights reserved.
Received 30 January 2021
Accepted 25 March 2021
Available online 26 March 2021
Keywords:
Metal–organic frameworks
Conductive polymers
X-ray absorption
Time-domain terahertz spectroscopy
Electrocatalysis
1. Introduction
Furthermore, in a more nascent area of MOFs, several Mn-based
electrically conductive MOFs have been reported [27–34].
The field of metal–organic frameworks (MOFs) relates to micro-
porous crystalline polymeric structures composed of a wide variety
of organic linkers and inorganic nodes [1–3]. The linkers and nodes,
which are alternately repeated throughout the structure, are
referred to as secondary building units (SBUs) that allow the syn-
thetic chemist to design and direct the three-dimensional structure
by modifying the structure of the SBUs in a process termed reticu-
lar chemistry [4,5]. One metal ion that has found an important role
in metal–organic frameworks is manganese, owing to its structural
diversity and stability in MOF structures [6–9]. MOFs containing
manganese nodes have been utilized in several important applica-
tions including catalysis [8,10–13], gas storage [6,14–16], batteries
and supercapacitors [17–19], and as MRI contrast agents [20–22].
Linker moieties comprising an inorganic Mn complex have also
been incorporated into MOFs for catalytic applications [23–26].
A widely-appreciated method to alter and tune the properties of
MOFs is through a cation exchange procedure, wherein the metal
ions that form the inorganic node of the MOF are substituted with
another cation [35–38]. One advantage of this synthetic route is
the ability to prepare cation-exchanged frameworks with the orig-
inal framework structure of the parent, enabling the inclusion of
transition metal nodes in which direct synthesis of said framework
structure is not feasible [39,40]. This approach has proven to be
advantageous in Mn-based MOFs for various applications. Dincǎ
and Long showed that the hydrogen adsorption enthalpy of the
MOF Mn3[(Mn4Cl)3(BTT)8(CH3OH)10 2
etristetrazolate) could be significantly altered and, in some cases
improved, by exchange of Mn2+ ions for Li+, Cu+, Fe2+, Co2+, Ni2+
] (where BTT = 1,3,5-benzen-
,
Cu2+, or Zn2+ [15]. In some cases, full exchange was obtained
whereas in other cases, only partial exchange of Mn2+ occurred.
Zhang et al. reported a porphyrin-based MOF constructed from
biphenyl-3,40,5-tricarboxylate and Cd2+ that could be fully
exchanged with Mn2+ to yield an isostructural MOF with both
cation-exchanged nodes and a cation-exchanged porphyrin linker
[41]. The resulting Mn-based MOF was shown to have significantly
enhanced catalytic activity for an epoxidation reaction compared
to the isostructural parent Cd-based MOF. Further examples of
⇑
Corresponding author.
E-mail address: gary.brudvig@yale.edu (G.W. Brudvig).
Current address: Department of Materials Science, Graduate School of Engineer-
1
ing, Osaka Prefecture University, 1-1 Gakuen-cho, Nakuku, Sakai, Osaka 599-8531,
Japan.
2
Current address: Department of Materials Science and Engineering, Mas-
sachusetts Institute of Technology, Cambridge, MA 02139, United States.
https://doi.org/10.1016/j.poly.2021.115182
0277-5387/Ó 2021 Elsevier Ltd. All rights reserved.

B. Pattengale, J. Neu, A. Tada et al.
Polyhedron 203 (2021) 115182
MOFs involving Mn2+ exchange to alter their properties have been
reported [42–47].
the completion of the reaction. 20 mL of boiling ethanol was then
added. The resulting mixture was allowed to cool to room temper-
ature and centrifuged (3000ꢀg, 15 min). The supernatant was dis-
carded. The precipitate was dried under high vacuum to afford a
pale brown solid (745 mg, 74%): 1H NMR (CDCl3, 400 MHz) d
With an eye toward functional applications in electrocatalysis,
we hypothesized that it may be feasible to extend this strategy
to electrically conductive MOFs in order to install catalytically
active metal cations directly within a crystallographically-defined
charge-transport pathway. In this study, we report on the cation
exchange of Mn2DSBDC (where DSBDC = 2,5-dimercaptotereph-
thalate). A cation exchange procedure was developed to exchange
up to approximately 10 at% of Mn2+ for Co2+, Cu2+, or Ni2+ while
maintaining the crystal structure that harbors a well-defined con-
ductivity pathway in the MOF. The bulk structural aspects of the
exchanged MOFs were investigated via powder X-ray diffraction
(pXRD) and the local structure of each ion was studied via ele-
ment-specific X-ray absorption spectroscopy. UV–visible spec-
troscopy demonstrated that the Co2+, Cu2+, or Ni2+ exchanged
MOFs have a smaller bandgap than pristine Mn2DSBDC owing to
modulation of the electronic properties of the MOF upon exchange.
Time-domain THz spectroscopy (THz-TDS), a non-contact spectro-
scopic AC conductivity technique, was used to determine the con-
ductivities of each MOF, thereby demonstrating that cation
exchange does not significantly disrupt conductivity. As a whole,
the results suggest a promising route to obtain cation-functional-
ized conductive MOFs potentially applicable in electrocatalytic
applications.
3
8.11 (s, 2H), 4.34 (quartet, JHH = 7.1 Hz, 4H), 3.10 (s, 6H), 3.04 (s,
3
6H), 1.37 (t, JHH = 7.2 Hz, 6H).
2.4. Synthesis of 2,5-dimercaptoterephthalic acid (H2DSBDC)
A mixture of S3 (745 mg, 1.74 mmol, 1.0 equiv) in a deaerated
solution of sodium hydroxide (1.3 M) in ethanol/water (1:1, v/v,
16 mL) was refluxed under N2 for 3 h, and allowed to cool to room
temperature. The resulting solution was poured into 2 M HCl (aq.
50 mL), and yellow precipitates formed. The mixture was then fil-
tered under reduced pressure. The solid residue was washed three
times with water (70 mL) and dried under high vacuum to afford a
yellow solid (340 mg, 85%): 1H NMR (DMSO d6, 400 MHz) d 8.05 (s,
2H).
2.5. Synthesis of Mn2DSBDC MOF
Mn2DSBDC was prepared under air-free conditions utilizing a
modified synthesis procedure based upon the published procedure
[27]. MnCl2ꢁ4H2O (377.2 mg, 1.91 mmol), H2DSBDC (83.1 mg,
0.36 mmol), dry DMF (9.9 mL), and methanol (4.8 mL) were added
to a pressure tube (Ace Glass, pressure tube with plunger valve)
along with a PTFE-coated stir bar. Three freeze–pump–thaw steps
were used to evacuate and refill the tube with nitrogen. The result-
ing clear yellow solution was heated at 120 °C in an oil bath and
stirred at 350 rpm for about 21 h to afford a yellow precipitate.
(Fig. S2, photographs) The air-stable solid was collected by vacuum
filtration and washed by 10 mL of methanol and 10 mL of DMF. In a
particular synthesis, the total mass of Mn2DSBDC obtained was
about 160.2 mg. The procedure was repeated four times to
yield > 600 mg of undoped Mn2DSBDC. Each of the four products
were individually characterized using pXRD to confirm phase pur-
ity before combining into a single vial to be used for the following
transition metal exchange procedures or further study on the pris-
tine material.
2. Materials and methods
2.1. General notes
All commercially available chemicals were used as received
without further purification. Deionized water was used for all
aqueous solutions. 1H NMR spectra were collected at room temper-
ature on a 400 MHz Bruker NMR spectrometer and referenced to
the residual solvent signal of CDCl3 (d = 7.26 ppm) or DMSO d6
(d = 2.50 ppm). H2DSBDC was synthesized following the reported
procedure [48], see Fig. S1.
2.2. Synthesis of diethyl 2,5-bis((dimethylcarbamothioyl)oxy)
terephthalate (S2)
2.6. Cation exchange procedure
A round-bottom flask was charged with a mixture of diethyl
2,5-dihydroxyterephthalate (S1, 500 mg, 1.97 mmol, 1.0 equiv)
and 1,4-diazabicyclo[2.2.2]octane (DABCO, 884 mg, 7.88 mmol,
4.0 equiv), purged with N2 and treated with anhydrous N,N-
dimethylacetamide (DMA, 10 mL). The solution was cooled to
0 °C with an ice-water bath. Dimethylthiocarbamoyl chloride
(974 mg, 7.88 mmol, 4.0 equiv) was added to another flask, which
was then purged with N2 and treated with anhydrous DMA
(2.5 mL). The solution of the acid chloride was added to the solu-
tion of the terephthalate dropwise at 0 °C. The resulting mixture
was stirred for 17 h at room temperature, diluted with water
(50 mL), and filtered. The solid filter cake was washed with water
(200 mL) and dried under high vacuum to afford a white solid
(828 mg, 98%): 1H NMR (CDCl3, 400 MHz), d 7.72 (s, 2H), 4.30
The exchange of Mn2+ in Mn2DSBDC for Co2+, Cu2+, or Ni2+
cations was accomplished via a newly developed post synthetic
exchange procedure under air free conditions. Co@Mn2DSBDC
was prepared by first dispersing 140 mg of Mn2DSBDC in 30 mL
of dry DMF via sonication bath for 20 min. Cobalt acetylacetonate
(Co(acac)2; 47.2 mg; 0.0183 mmol) was added and dissolved. The
suspension containing the MOF and the acetylacetonate metal pre-
cursor was then transferred to a pressure tube along with a PTFE-
coated stir bar. The tube was evacuated and filled with nitrogen
three times, and then was heated at 100 °C for 21 h to afford a yel-
low green precipitate. The color change was observed after a short
heating time but the exchange was performed for a long period of
time to ensure that the exchange went to completion and that the
MOF structure was stable under the exchange conditions. The solid
was rinsed three times with DMF and once with methanol via cen-
trifugation and dried under ambient conditions. Ni@Mn2DSBC was
prepared under the same procedure as Co@Mn2DSBDC using Ni
(acac)2 as the precursor (55.2 mg, 0.215 mmol) and 164 mg of Mn2-
DSBDC. Cu@Mn2DSBDC was prepared using the same procedure;
however, approximately 5ꢀ less Cu(acac)2 was required due to a
higher post-synthetic exchange affinity than the other two
exchanged metals (Cu(acac)2, 10.0 mg, 0.038 mmol, and 150 mg
Mn2DSBDC).
3
3
(quartet, JHH = 7.2 Hz, 4H), 3.45 (s, 6H), 3.39 (s, 4H), 1.33 (t,
-
JHH = 7.1 Hz, 6H).
2.3. Synthesis of diethyl 2,5-bis((dimethylcarbamoyl)thio)
terephthalate (S3)
A flask was charged with S2 (1.00 g, 2.33 mmol) and purged
with N2. The solid was then heated to 215 °C and stirred for 1 h.
A balloon was attached to balance the pressure in the flask. The
while solid melted and became a pale brown liquid, indicating
2

B. Pattengale, J. Neu, A. Tada et al.
Polyhedron 203 (2021) 115182
2.7. Standard characterization of MOFs
Mn2. Both Mn1 and Mn2 are coordinated by four in-plane O atoms
and two axial S ligands to give an octahedral structure. The inter-
atomic distances determined from the crystal structure are shown
in Table 1 [27]. Mn1 is coordinated by four O atoms of the DSBDC
linker, two of which bridge Mn1 to Mn2 and two of which are
non-bridging. The shorter of the two Mn1-O distances (2.15 Å)
are the cis non-bridging atoms whereas the longer Mn1-O distances
(2.28 Å) are the cis bridging atoms. In addition to the bridging O
atoms, Mn1 and Mn2 are bridged by the S atom (2.49 Å Mn-S dis-
tance), which provides the continuous Mn-S-Mn-S chains through-
out the structure that provide through-bond conductivity. It was
previously confirmed that the Mn-S-Mn-S chains provide a con-
ductivity pathway because the isostructural MOF constructed from
DOBDC linkers, which replace -S with -O, has reduced conductivity
[34]. Like Mn1, Mn2 has two cis bridging O atoms (2.24 Å); how-
ever, Mn2 is not coordinatively saturated by the MOF linkers. As
shown in Fig. 1, two solvent atoms coordinate Mn2 (DMF, Mn2-O
distance 2.13 Å) in the as-prepared material. In the activated form
of the MOF, the solvent atoms can be removed by CH2Cl2 exchange
and drying resulting in a 4-coordinate Mn2+ center at the Mn2 posi-
tion [34]. The as-prepared MOF with DMF coordination is investi-
gated in this work.
Powder X-ray diffraction (pXRD) was performed using a Rigaku
SmartLab X-ray diffractometer. Mercury (CCDC) was used to calcu-
late the reference pXRD pattern using the published single crystal
structure [27]. The powdered samples were packed into a glass
sample holder with a cavity using a flat edge to ensure uniform
sample height. UV–visible spectroscopy was performed using a
Shimadzu UV-2600 spectrometer in diffuse reflectance mode with
an integrating sphere attachment. X-ray fluorescence (XRF) was
performed with a Rigaku ZSX Primus II XRF spectrometer.
2.8. X-ray absorption spectroscopy
X-ray absorption spectroscopy was performed at Brookhaven
National Laboratory in the National Synchrotron Light Source II
(Beamline 6-BM). The powder samples were evenly dispersed on
Kapton tape and measured in transmission mode. Several reference
compounds were measured in addition to metal foil at each edge
for energy calibration. Hydrated complexes were used as refer-
ences. The EXAFS data were analyzed using the Demeter suite of
programs (version 0.9.25) which implements FEFF6 [49]. The pub-
lished crystal structure [27] was used to prepare input file models
for each as described in the Results and Discussion section.
Mn2DSBDC was prepared according to a modified procedure
based upon the published procedure [27] as described in the Mate-
rials and Methods section. Briefly, the H2DSBDC linker was dis-
solved in DMF and the MnCl2ꢁ4H2O precursor in MeOH after
which the solutions were slowly mixed and added to a pressure
tube. Air was removed from the pressure tube and synthesis pro-
ceeded at 120 °C under stirring for approximately 21 h. The pro-
gression of MOF precipitation during synthesis is shown
photographically in Fig. S2. The MOF was characterized via pXRD
as shown in Fig. 2a, giving excellent agreement to the pXRD pat-
tern calculated from the published single crystal structure [27].
In this work, a new post-synthetic exchange procedure was
developed to install transition metal cations other than Mn2+
within the inorganic nodes of the MOF in order to explore function-
alization and structural diversity of the conductive MOF. Initial
attempts to synthesize the MOF entirely from first-row transition
2.9. Time-domain Terahertz spectroscopy
Terahertz spectra were collected with a self-built Terahertz
Time Domain spectrometer (THz-TDS) within a liquid nitrogen
temperature controlled cryostat described in detail previously
[50]. The system is based on a femtosecond oscillator, repetition
rate 90 GHz, center wavelength 800 nm, average power 300 mW,
sub 50 fs pulses. These pulses are split in THz generation and
detection, while the detection beam is routed via a mechanical
delay line. This line is used to adjust the timing between the gen-
erated THz pulse and the gate pulse for detection. The detection in
time domain provides sign resolved electrical fields with femtosec-
ond time resolution. These time traces can be transformed into the
frequency domain, providing amplitude and also phase informa-
tion of the THz beam. Referencing on air allows determination of
the complex refractive index of the sample, and then also determi-
nation of the high frequency permittivity and conductivity [50,51].
The complex permittivity and conductivity data were modeled
using the Drude-Smith equation as shown in Equation (1):
metal chloride, nitrate, or acetate salts (i.e. Co2+, Cu2+, and Ni2+
)
failed to produce materials with the correct pXRD pattern. Such
materials were not further investigated, as the goal of this study
was to maintain the structure and thereby preserve the infinite
metal-sulfur chains that endow Mn2DSBDC with conductivity.
From these results, it was hypothesized that some amount of
Mn2+ is required to maintain the highly ordered crystalline MOF
structure. Attempts to synthesize the MOF from a mixture of MnCl2
and chloride, nitrate, or acetate salts of Co2+, Cu2+, and Ni2+ also
resulted in amorphous or incorrect structures. Moving then to a
post-synthetic exchange procedure, the chloride, nitrate, or acetate
salts of Co2+, Cu2+, and Ni2+ again resulted in loss of crystallinity in
the originally crystalline Mn2DSBDC. Finally, it was discovered that
a post-synthetic exchange procedure employing acetylacetonate
salts could be successfully applied to replace up to approximately
10 at% of the native Mn2+ ions with Co2+, Cu2+, or Ni2+ while still
maintaining the crystalline structure of the host Mn2DSBDC MOF.
The procedure is described in detail in the Materials and Methods
section and is shown photographically in Fig. 2d.
!
!
xp2
ꢀ
ꢁ
s
c
ꢀð
x
Þ ¼ ꢀlattice
ꢂ
ꢃ
1 þ
ð1Þ
ð
s
x2 þ i
xÞ
1 ꢂ i
xs
sffiffiffiffiffiffiffiffiffiffiffi
ne2
ꢀ
xp
¼
ð2Þ
0mꢃ
The variable
time constant,
e is the elementary charge, n is the carrier density, and m* is the
x
is the angular frequency 2pm
,
s
is the scattering
ꢀ
lattice is the lattice permittivity, c is the c parameter,
carrier effective mass. Each of the parameters xp, s, ꢀlattice, and c
were fit.
The pXRD pattern of Mn2DSBDC is compared to its calculated
pattern and the patterns for Co@Mn2DSBDC, Cu@Mn2DSBDC, and
Ni@Mn2DSBDC (i.e., the cation-exchanged analogues) in Fig. 2a.
The samples measured in this study are each exchanged at approx-
imately 8–10 at% with respect to Mn; the at% values obtained via
X-ray fluorescence are shown in Table S1. From the pXRD patterns
in Fig. 2a, it can be observed that the cation exchange procedure
does not alter the crystal structure of Mn2DSBDC. The reduction
in pXRD intensity in the Cu@Mn2DSBDC and Co@Mn2DSBDC ana-
3. Results and discussion
3.1. Structure, synthesis, and characterization
In the reported structure of Mn2DSBDC, the hexagonal pores of
the structure feature continuous chains of the Mn2+ ions at the ver-
tices of the pores (Fig. 1). The continuous chains feature crystallo-
graphically-distinct alternating Mn2+ centers denoted Mn1 and
3

B. Pattengale, J. Neu, A. Tada et al.
Polyhedron 203 (2021) 115182
Fig. 1. Crystal structure of Mn2DSBDC at left and continuous chains of alternating Mn2+ ions depicted at right (from ref. 27). Non-coordinating solvent atoms removed for
clarity. (Mn = Violet, S = Yellow, O = Red, C = Brown).
Table 1
Mn K-edge EXAFS fit parameters.
Description
Vector
Mn2DSBDC
Ni@Mn2DSBDC
Cu@Mn2DSBDC
Co@Mn2DSBDC
Ref.
S02
D
E0
r2
R
S02
D
E0
r2
R
S02
D
E0
r2
R
S02
D
E0
r2
R
Mn1-O
Mn1-O
Mn1-S
Mn1-Mn2
Mn2-O
Mn2-O
Mn2-S
2.15
2.28
2.49
3.34
2.13
2.24
2.63
3.34
0.50
0.50
0.50
0.24
0.50
0.50
0.50
0.24
0.48
0.48
0.48
0.48
0.48
0.48
0.48
0.48
9.0
9.0
9.0
9.0
9.0
9.0
9.0
9.0
2.14
2.27
2.48
3.35
2.06
2.17
2.76
3.35
0.50
0.50
0.50
0.20
0.50
0.50
0.50
0.20
ꢂ0.75
ꢂ0.75
ꢂ0.75
ꢂ3.92
ꢂ0.75
ꢂ0.75
ꢂ0.75
ꢂ3.92
6.0
6.0
6.0
6.0
6.0
6.0
6.0
6.0
2.11
2.24
2.49
3.27
2.06
2.17
2.75
3.27
0.50
0.50
0.50
0.18
0.50
0.50
0.50
0.18
ꢂ0.88
ꢂ0.88
ꢂ0.88
ꢂ4.17
ꢂ0.88
ꢂ0.88
ꢂ0.88
ꢂ4.17
7.0
7.0
7.0
7.0
7.0
7.0
7.0
7.0
2.12
2.26
2.47
3.25
2.05
2.16
2.75
3.25
0.50
0.50
0.50
0.18
0.50
0.50
0.50
0.18
ꢂ0.75
ꢂ0.75
ꢂ0.75
ꢂ4.21
ꢂ0.75
ꢂ0.75
ꢂ0.75
ꢂ4.21
6.0
6.0
6.0
6.0
6.0
6.0
6.0
6.0
2.10
2.23
2.49
3.25
2.07
2.18
2.76
3.25
Mn2-Mn1
*Units defined as follows:
D
E0 = eV; r2 = Å2 ꢀ 10ꢂ3; R = Å; Uncertainty in R is approximately 0.02 Å.
**Reference (Ref.) vector distances determined from crystal structure; Coordination number (N) fixed to 2 for each vector.
Fig. 2. (a) pXRD patterns comparing Mn2DSBDC to the calculated pattern and the cation-exchanged analogues Co@Mn2DSBDC, Cu@Mn2DSBDC, and Ni@Mn2DSBDC. (b) and
(c) show enlarged regions of two pXRD peaks with dashed guides to the eye located at the Mn2DSBDC peaks. (d) shows example photographs of Cu exchanged Mn2DSBDC
(Cu@Mn2DSBDC) during the exchange process.
4

B. Pattengale, J. Neu, A. Tada et al.
Polyhedron 203 (2021) 115182
logues is indicative of partial loss of crystallinity as the cation
exchange percentage approaches the maximum tolerable value of
about 10 at% before the crystallinity is severely diminished. By
comparing the peak positions (Fig. 2b and c), it can be observed
that the doping procedure shifts the pXRD peaks at ~6.5 and
~11.3° to slightly higher values of 2h, which is consistent with unit
cell contraction. This finding suggests that cation exchange results
in a bulk structure effect of shortening the interatomic distances in
the material.
The qualitative conclusions from the XANES results were cor-
roborated by quantitative EXAFS analysis. The spectra and fitting
results are shown in k-space (Fig. 3c) and R-space (Fig. 3d). In R-
space, it can be observed that first coordination shell appears to
be slightly shortened in the cation-exchanged variants compared
to Mn2DSBDC. As shown in Fig. 1, two nonequivalent Mn centers
(i.e., Mn1 and Mn2) are present in Mn2DSBDC each at 50% contribu-
tion. Therefore, the model utilized to fit the data accounted for
each Mn by amplitude-weighting the first shell Mn-O and Mn-S
contributions at 50% using the S20 amplitude parameter (Table 1).
In the second shell, a Mn-Mn path was included and its S20 ampli-
tude was allowed to vary to account for the small amplitude in
the second shell scattering observed, likely due to the saturated
coordination of the Mn2+ ions in the structure. For Mn2DSBDC,
the obtained coordination distances, R in Table 1, generally agree
well with the reference (Ref.) values obtained from the published
single crystal structure [27]. Mn1 fits within uncertainty of the
Ref. values, while Mn2 shows a contracted Mn2-O coordination
with elongated axial Mn-S distances. The Mn centers in Mn2DSBDC
fit to the expected Mn-Mn distance.
In the cation-exchanged analogues, the same EXAFS model as in
pristine Mn2DSBDC was used for the Mn K edge data. In agreement
with the qualitative observation of a shortened first shell coordina-
tion environment in the R-space spectra (Fig. 3d), the Mn1-O dis-
tances are quantified to be shortened in the three M@Mn2DSBDC
samples while the coordination distance to the axial S ligands
remains unchanged. The Mn2-O distances are also identical to pris-
tine Mn2DSBDC. A clear trend is observed in the Mn1-Mn2 and
Mn2-Mn1 distances, which are each shortened by approximately
0.1 Å in the cation-exchanged samples compared to pristine Mn2-
DSBDC. As a whole, the Mn K edge results suggest that, in agree-
ment with the pXRD results in Fig. 2a, the cation exchange
results in, to some degree, shortening of the first and second coor-
dination spheres of Mn.
Example photographs of the cation exchange procedure for Cu2+
are shown in Fig. 2d. Due to cation exchange, the color of the MOF
undergoes a visible darkening. The photograph is shown at 2 h of
heating and the color change persists for the full 21-hour
exchange, after washing with DMF and EtOH, and in the dried
product. This qualitative observation was investigated using dif-
fuse reflectance UV–visible spectroscopy (Fig. S3). The UV–visible
results show that cation exchange appears to alter the bandgap
and also results in the observation of a new, relatively weak
absorption feature in the 500–700 nm region of the spectrum.
Due to the relatively weak intensity, this feature is likely due to
the forbidden d-manifold transitions of the exchanged cation. Tauc
analysis was used to extract the bandgaps of the pristine Mn2-
DSBDC and cation-exchanged variants (Fig. S4 and Table S2). In
each cation-exchanged sample, the bandgap (exclusive of the
new absorption feature in the 500–700 nm range) is shortened
from 510 nm (Mn2DSBDC) to as small as 605 nm (Ni@Mn2DSBDC).
This result indicates that, in addition to the bulk structural effects
observed in pXRD, the exchanged cations have an electronic effect
on the material. An important aspect of the MOFs that is not under-
stood from these techniques is the local structure of each metal ion
in the MOFs, which was analyzed using X-ray absorption spec-
troscopy as described in the following section.
3.2. X-ray absorption spectroscopy
Each cation-exchanged sample (Ni, Cu, and Co) was probed at
the K edge of each substituted metal to provide insight into the
changes in the local structure. The Cu K-edge XANES spectra
(Fig. 4a) were measured for Cu@Mn2DSBDC, and several reference
samples. Based upon the edge position of Cu@Mn2DSBDC relative
to the references, the oxidation state of Cu can be assigned as
Cu2+. The Cu K edge within the Cu2+ region shows variation
between the shape of the 1s-4p transition with different coordina-
tion environments. Cu@Mn2DSBDC appears to have a feature at
8.9825 keV, a lower energy than the first feature in any of the ref-
erences measured. The first derivative plots of the Cu2+ spectra
(Fig. 4b) reveal a trend in the energy position of the first peak with
Cu@Mn2DSBDC < CuO < CuCl2 < CuNO3 ffi Cu(acac)2. This trend is in
excellent agreement with literature results which show that a
trend in increasing ligand covalency leads to a reduction in the
energy of the first feature on the edge Cu K edge [52,53]. Intu-
itively, the increased covalency would place more electron density
on the metal and lead to the trend observed. With respect to
Cu@Mn2DSBDC, the covalency is likely attributed to the S ligands
that are implicated in the conductive charge-transfer pathway of
the MOF. This result indicates that Cu2+ occupies a S-coordinated
site in Cu@Mn2DSBDC, and furthermore that Cu2+ experiences
covalency with its axial S ligands, potentially suggesting electronic
communication and conductivity through S-Cu-S moieties in the
structure.
The local structures of as-prepared Mn2DSBDC and the cation-
exchanged samples were investigated using element-specific X-
ray absorption spectroscopy. Both qualitative X-ray absorption
near edge structure (XANES) and quantitative extended X-ray
absorption fine structure (EXAFS) provide insight into the local
structure of Mn2+ in Mn2DSBDC. In the cation-exchanged variants,
both the Mn2+ and the exchanged cations were probed individually
to provide element-specific information on the local structure.
Mn2+ metal centers were probed at the Mn K-edge as shown in
the XANES data in Fig. 3a, which captures the Mn 1s-4p transition.
Reference compounds with Mn oxidation states of I-IV were mea-
sured, including MnCl2ꢁ4H2O, the Mn precursor used in the synthe-
sis of Mn2DSBDC. A clear trend in the edge energy is observed with
oxidation state, demonstrating that the Mn in Mn2DSBDC retains
its 2+ oxidation state. The XANES spectrum of Mn2DSBDC generally
agrees well with that of the octahedral MnSO4ꢁH2O complex,
including the observation of a relatively weak dipole-forbidden
1s-3d pre-edge feature as expected for a centrosymmetric com-
plex. The XANES spectra for the cation-exchanged variants are
shown in Fig. 3b. In each, the shape of the XANES spectrum is
nearly identical with only slight variation in the intensity of the
main 1s-4p transition feature above the edge which could indicate
slightly different electronic environments. In each, the relatively
weak 1s-3d forbidden pre-edge transition suggests that Mn retains
a similar coordination structure in the cation-exchanged variants.
The peak in the first derivative of the XANES spectra (inset of
Fig. 3b) shows that edge position does not vary significantly. These
XANES results, therefore, indicate that the cation exchange process
does not significantly impact the local structure of the Mn2+ cen-
ters in M@Mn2DSBDC.
While rich electronic information was observed at the Cu K
edge in Cu@Mn2DSBDC, the insights from XANES are more limited
in Co@Mn2DSBDC (Fig. 4c) and Ni@Mn2DSBDC (Fig. 4d), which did
not display significantly varying features on the transition edges.
Each does display a relatively weak 1s-3d pre-edge feature at
7.701 keV and 8.334 keV for Co@Mn2DSBDC and Ni@Mn2DSBDC
at their respective K edges. A weak 1s-3d pre-edge feature is con-
5

B. Pattengale, J. Neu, A. Tada et al.
Polyhedron 203 (2021) 115182
Fig. 3. (a) Mn K-edge XANES spectra of Mn2DSBDC (solid black) and reference samples (dashed). (b) Mn K-edge XANES spectra of Mn2DSBDC compared to M@Mn2DSBDC
samples with the first derivative shown in the inset. (c) Mn K-edge K-space and (d) R-space spectra with the data shown as open points and the best fits as solid lines.
Fig. 5. Comparison of (a) K-space and (b) R-space EXAFS data shown as open points
and best fits shown as solid lines for Co@Mn2DSBDC, Cu@Mn2DSBDC, and Ni@Mn2-
DSBDC at their respective Co, Cu, or Ni K-edges.
tion distances needed to be adjusted significantly in order provide
a reasonable fit. From the quantified distances in Table 2, it can be
clearly seen that, while each metal ion agrees well with the same
coordination environment (i.e., four oxygen ligands and two larger
Fig. 4. (a) Cu K-edge XANES spectra and (b) first derivative spectra with arrows
sulfur atoms displaced axially), each of the M-O and M-S (where
indicating the first peak of the K-edge. (c) Co K-edge XANES spectra and (d) Ni K-
M = Co, Cu, or Ni) distances are at least ~0.1 Å shorter than the cor-
edge XANES spectra.
responding Mn-O and Mn-S distances shown in Table 1.
One curious aspect of the EXAFS data is the lack of a second
shell feature in Co@Mn2DSBDC and Cu@Mn2DSBDC, while a clear
Ni-X (where X = Mn and/or Ni, the two metal ions present in
Ni@Mn2DSBDC) feature is observed. The second shell scattering
at the Ni K edge is a prominent feature that fits to approximately
0.2 Å shorter than Mn-Mn distances obtained at the Mn K-edge.
Two possibilities seem most likely to explain this observation.
The first is that, while the coordination number of Ni in the first
shell is equivalent to the other exchanged metal ions, the structure
may be significantly distorted. A second possibility could be that,
while Co and Cu disperse homogeneously within the MOF, Ni ions
sistent with a centrosymmetric octahedral geometry and the
agreement of each to their respective octahedral nitrate salts sug-
gests that each has a 2+ oxidation state and octahedral geometry.
Quantitative EXAFS analysis was utilized to further characterize
each of the Co-, Cu-, and Ni-exchanged MOFs. The EXAFS spectra
are compared in k-space (Fig. 5a) and R-space (Fig. 5b) and it is
observed that each sample has a prominent first shell feature
and only Ni@Mn2DSBDC appears to have a significant second shell
scattering feature. The EXAFS data were fit using a model based
upon the corresponding Mn K-edge model; however, the coordina-
6

B. Pattengale, J. Neu, A. Tada et al.
Polyhedron 203 (2021) 115182
Table 2
Co, Cu and Ni K-edge EXAFS fit parameters.
Shared
Vector
Co@Mn2DSBDC Co K Edge
Cu@Mn2DSBDC Cu K Edge
Ni@Mn2DSBDC Ni K Edge
N
S02
D
E0
r2
R
S02
D
E0
r2
R
S02
D
E0
r2
R
M1-O
M1-O
M1-S
M1-X
M2-O
M2-O
M2-S
M2-X
2
2
2
2
2
2
2
2
0.50
0.50
0.50
0
0.50
0.50
0.50
0
ꢂ5.48
ꢂ5.48
ꢂ5.48
–
ꢂ1.94
ꢂ1.94
ꢂ1.94
–
5.0
2.03
2.03
2.23
–
1.99
2.00
2.48
–
0.50
0.50
0.50
0
0.50
0.50
0.50
0
6.12
6.12
6.12
–
ꢂ5.60
ꢂ5.60
ꢂ5.60
–
7.0
7.0
7.0
–
5.0
5.0
5.0
–
2.01
2.00
2.24
–
1.99
2.02
2.45
–
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0.50
ꢂ11.2
ꢂ11.2
ꢂ11.2
ꢂ4.37
ꢂ0.88
ꢂ0.88
ꢂ0.88
ꢂ4.37
4.0
4.0
4.0
6.0
4.0
4.0
4.0
6.0
1.97
1.98
2.31
3.12
2.01
2.09
2.64
3.03
5.0
5.0
–
10.0
10.0
10.0
–
*Units defined as follows:
D
E0 = eV; r2 = Å2 ꢀ 10ꢂ3; R = Å; Uncertainty in R is approximately 0.02 Å.
may prefer to occupy adjacent sites within the metal ion chains. If
this is the case, the M-M distance may be shorter, resulting in the
observation of the second shell feature.
As a whole, the XANES and EXAFS results complement the pXRD
analysis and suggest that exchange of Mn2+ in Mn2DSBDC for Co2+
,
Cu2+, and Ni2+ results in both a more contracted local structure at
Mn2+ ions and a more contracted bulk structure. This constriction
is due to the incorporation of the Co2+, Cu2+, and Ni2+ ions, each
of which has a significantly shorter coordination sphere relative
to the equivalent Mn2+ ion in the structure.
3.3. Time domain THz spectroscopy
Time domain THz spectroscopy (THz-TDS) was used as a non-
contact probe of the complex ground state conductivity of each
of the conductive MOF samples [50]. A known mass of each sample
was mixed with a known mass of a THz transparent host material
(PTFE) to allow for reproducible pressed pellet sample fabrication.
Each of the samples was prepared with between 20 and 30 mg of
Mn2DSBDC mixed with about 1 g of Teflon. The resulting powder
mix was then pressed with 11 kbar to achieve a solid pellet. The
sample pellet was mounted on a translation stage in a liquid nitro-
gen cooled cryostat. The THz transmission through air (reference),
bare PTFE (control) and the pelletized sample were each collected
within minutes of each other at each temperature. These three
measurements were then used to calculate the complex refractive
index of the pellet. The conductivities of the inclusion materials
(Mn2DSBDC or cation-exchanged analogue) were determined
using effective medium theory [54].
The high frequency (THz) conductivity of Mn2DSBDC - averaged
over three individual samples measured on three independent
points each - is shown in Fig. 6a at each temperature (300 K,
200 K, and 100 K). There is no observable temperature dependence
in Mn2DSBDC or any of its cation exchanged analogues (Fig. S5).
Interestingly, a representative comparison at 200 K (Fig. 6b) indi-
cates that there is no significant difference between the conductiv-
ity of Mn2DSBDC and its ca. 10 at% Ni2+, Cu2+, or Co2+ cation-
exchanged analogues. The same lack of temperature dependence
is observed at 300 K and 100 K as well (Fig. S6). Because the con-
ductivity pathway is described as infinite Mn-S-Mn-S chains in
the MOF structure, this conductivity result suggests that, even
though the continuous Mn-S-Mn-S chains are interrupted with
the exchanged cations, the conductivity is not adversely affected.
This finding is suggestive that Mn2DSBDC offers a large degree of
tunability in terms of cation composition without a loss of the
desired conductivity property, which is exciting for potential elec-
trocatalytic and/or photocatalytic applications.
Fig. 6. (a) Comparison of the real part of the conductivity of Mn2DSBDC across
varying temperatures. (b) Representative comparison of the real part of the
conductivity for Mn2DSBDC and the cation-exchanged analogues at 200 K. (c) The
complex permittivity and (d) complex conductivity with data shown as open points
and best fits to the Drude-Smith model shown as lines.
is on the order of some of the best reported conductive MOFs,
although the comparison should be taken with caution as the AC
THz conductivity should not be equated with the probe-based DC
conductivity results [28,55]. For most materials, the conductivity
depends on the temperature. There are several possible thermal
mechanisms influencing the conductivity. In doped materials for
example, thermal energy provides the necessary ionization energy
to the dopants, meaning at lower temperature fewer mobile
charges are present and, hence, the conductivity is reduced. Simi-
larly, for a thermal activated hopping mechanism, lower tempera-
tures correspond to lower mobility, i.e., reduced conductivity. For
metallic conductors, which exhibit an abundance of mobile
charges, the main thermal contribution is related to phonon scat-
tering of mobile electrons. With lower temperature fewer phonons
are present causing a reduction in scattering and, therefore, an
increase in conductivity. However, the strength of this effect
depends on crystallinity and perfection of the material. In the
absence of phonons, the conductivity is still limited by defect scat-
tering effects or grain boundaries. Therefore, this result suggests
that Mn2DSBDC has a metal-like conductivity limited mainly by
defect or grain boundary scattering.
To gain additional insight into the underlying physics we
employed the Drude-Smith model to describe the conductivity
(see Materials and Methods for details). The fit was carried out
for the complex permittivity instead of the more common
The overall conductivity is orders of magnitude higher than pre-
viously reported THz photoconductivity results, the best point of
reference currently available for spectroscopic conductivity results
[55]. As compared to probe-based measurements, the conductivity
7

B. Pattengale, J. Neu, A. Tada et al.
Polyhedron 203 (2021) 115182
approach of fitting the conductivity. The advantage of fitting the
permittivity is that the unknown static background permittivity
is an additional fit variable in the model. This allows us to simulta-
neously fit the contribution from bound electrons and mobile elec-
trons. This is necessary here as we are measuring ground state
conductivity compared to the more common approach of measur-
ing photoexcited conductivity. A representative Drude-Smith fit-
ting result for the complex permittivity of Mn2DSBDC at 200 K is
shown in Fig. 6c and the corresponding complex conductivity is
shown in Fig. 6d. The full comparison of fitting results is shown
in Figs. S7 and S8 for the permittivity and conductivity, respec-
tively, for each sample at each temperature. The fit for Mn2DSBDC
with the Drude-Smith model shows a weak dependence of carrier
density and scattering on the temperature; furthermore, the c-
parameter is also temperature insensitive (ꢂ0.99 0.03 at 300 K,
comparison of this AC value to DC results is not straightforward
and further THz AC conductivity studies of conductive MOFs will
be needed. The result that the exchange of cations did not cause
a loss of conductivity implies a great degree of tunability in the
cation composition of Mn2DSBDC without a loss of the functionally
relevant conductivity property for potential electrocatalytic appli-
cations. With respect to Mn2DSBDC specifically, these findings also
demonstrate a need for further studies, including input from the-
ory, to better understand the charge-transport pathways and
whether exchanged cations play an active role in charge transport.
CRediT authorship contribution statement
Brian Pattengale: Conceptualization, Methodology, Investiga-
tion, Writing - original draft. Jens Neu: Investigation, Writing -
review & editing. Ayano Tada: Investigation. Gongfang Hu: Inves-
tigation, Writing - review & editing. Christopher J. Karpovich:
Investigation. Gary W. Brudvig: Project administration, Supervi-
sion, Writing - review & editing.
ꢂ1.00
0.01 at 200 K and 100 K) (Table S3). The c-parameter
describes the interaction of a mobile charge with a boundary or
interface. A c-parameter of ꢂ1 can be interpreted as meaning that
a carrier cannot propagate past a defect or grain boundary. These
same findings extend to each of the cation-exchanged variants
and are in agreement with the lack of temperature dependence
observed in the THz conductivity traces.
Declaration of Competing Interest
The fact that the dopants do not alter the conductivity leaves us
with two potential interpretations to draw from this work. The first
is that even though the exchanged cations interrupt the continuous
Mn-S-Mn-S chains in the structure, the carrier is still free to prop-
agate through those exchanged cations. This conclusion appears to
be difficult to understand because exchanged cations would be
expected to interrupt energetically matched Mn-S bands. However,
the compacted bulk structure in cation-exchanged variants and the
shorter bond lengths in the local structure for each exchanged
cation may be compensating factors for such electronic mis-
match/interruption. A second interpretation is that the conductiv-
ity in this MOF actually proceeds in a different crystallographic
orientation through the DSBDC linkers in a Mn-S-BDC-S-Mn-S-
BDC-S pathway. This potential pathway was raised as a possibility
in earlier work [27] and then largely abandoned in favor of the
through-metal pathway when replacement of Mn2+ with Fe2+
resulted in a million-fold conductivity enhancement [34]. In light
of the results here, this pathway may need to be considered further
in Mn2DSBDC. Irrespective of the charge-transport pathway, this
work provides evidence that cation-exchanged Mn2DSBDC variants
are particularly intriguing for development as electrocatalytic or
photocatalytic materials due to the tunability of the cation compo-
sition without hindering charge transport.
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
This work was supported by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, under award
DE-FG02-07ER15909, as well as from a generous donation from
the TomKat Foundation. This research used 6-BM of the National
Synchrotron Light Source II, a U.S. Department of Energy (DOE)
Office of Science User Facility operated for the DOE Office of
Science by Brookhaven National Laboratory under Contract DE-
SC0012704. A.T. acknowledges funding from the Nakatani Founda-
tion for performing research at Yale University.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.poly.2021.115182.
References
[1] H.-C. Zhou, J.R. Long, O.M. Yaghi, Introduction to metal–organic frameworks,
Chem. Rev. 112 (2012) 673–674, https://doi.org/10.1021/cr300014x.
[2] Q.i. Wang, D. Astruc, State of the art and prospects in metal–organic
framework (MOF)-based and MOF-derived nanocatalysis, Chem. Rev. 120
(2020) 1438–1511, https://doi.org/10.1021/acs.chemrev.9b00223.
[3] S. Yuan, L. Feng, K. Wang, J. Pang, M. Bosch, C. Lollar, Y. Sun, J. Qin, X. Yang, P.
Zhang, Q.i. Wang, L. Zou, Y. Zhang, L. Zhang, Y.u. Fang, J. Li, H.-C. Zhou, Stable
metal–organic frameworks: design, synthesis, and applications, Adv. Mater. 30
(2018) 1704303, https://doi.org/10.1002/adma.201704303.
[4] O.M. Yaghi, M. O’Keeffe, N.W. Ockwig, H.K. Chae, M. Eddaoudi, J. Kim, Reticular
synthesis and the design of new materials, Nature 423 (2003) 705–714,
https://doi.org/10.1038/nature01650.
[5] M. Li, D. Li, M. O’Keeffe, O.M. Yaghi, Topological analysis of metal–organic
frameworks with polytopic linkers and/or multiple building units and the
minimal transitivity principle, Chem. Rev. 114 (2014) 1343–1370, https://doi.
org/10.1021/cr400392k.
4. Conclusions
This work investigated the structure and conductivity of cation-
exchanged conductive Mn2DSBDC metal–organic frameworks
(MOFs) (where DSBDC = 2,5-dimercaptoterepthalate). The cation-
exchanged structure is compressed compared to the native MOF,
verified with pXRD. The local structure of each metal ion was
investigated using element-specific X-ray absorption spectroscopy,
providing rationale for the bulk structure effects and electronic/
structural information on the exchanged cations. UV–visible spec-
troscopy indicated that the cation exchange altered the optical
properties and resulted in a smaller bandgap. Time-domain THz
spectroscopy yielded spectroscopic AC conductivity information
on pristine Mn2DSBDC and the cation-exchanged variants, showing
that the conductivity in each is highly similar and mainly limited
by defect or grain boundary scattering. A conductivity value of
approximately 25 S/m was observed in each of the native and
cation-exchanged MOFs, which is among the highest reported val-
ues compared to the existing DC conductivity results; however, the
[6] M. Dincaꢀ, A. Dailly, Y. Liu, C.M. Brown, D.A. Neumann, J.R. Long, Hydrogen
storage in
a
microporous metalꢂorganic framework with exposed Mn2+
coordination sites, J. Am. Chem. Soc. 128 (2006) 16876–16883, https://doi.
org/10.1021/ja0656853.
[7] E.V. Khramenkova, M.V. Polynski, A.V. Vinogradov, E.A. Pidko, Degradation
paths of manganese-based MOF materials in a model oxidative environment: a
computational study, Phys. Chem. Chem. Phys. 20 (2018) 20785–20795,
https://doi.org/10.1039/C8CP00397A.
[8] T. Ladrak, S. Smulders, O. Roubeau, S.J. Teat, P. Gamez, J. Reedijk, Manganese-
based metal–organic frameworks as heterogeneous catalysts for the
8

B. Pattengale, J. Neu, A. Tada et al.
Polyhedron 203 (2021) 115182
cyanosilylation of acetaldehyde, Eur. J. Inorg. Chem. 2010 (2010) 3804–3812,
https://doi.org/10.1002/ejic.201000378.
[9] Y.-K. Lv, C.-H. Zhan, Y.-L. Feng, A chiral manganese–potassium heterometallic
MOF with an unusual (3,7)-connected network, CrystEngComm 12 (2010)
3052–3056, https://doi.org/10.1039/B925546J.
[10] Q. Sun, M. Liu, K. Li, Y. Han, Y.i. Zuo, F. Chai, C. Song, G. Zhang, X. Guo, Synthesis
of Fe/M (M = Mn Co, Ni) bimetallic metal organic frameworks and their
catalytic activity for phenol degradation under mild conditions, Inorg. Chem.
Front. 4 (2017) 144–153, https://doi.org/10.1039/C6QI00441E.
[31] S.S. Park, E.R. Hontz, L. Sun, C.H. Hendon, A. Walsh, T. Van Voorhis, M. Dinca˘,
Cation-dependent intrinsic electrical conductivity in isostructural
tetrathiafulvalene-based microporous metal–organic frameworks, J. Am.
Chem. Soc. 137 (2015) 1774–1777, https://doi.org/10.1021/ja512437u.
[32] Q. Yu, J. Su, J.-P. Ma, C.F. Leong, D.M. D’Alessandro, H.-Y. Wang, M. Kurmoo, J.-L.
Zuo, Progressive structure designing and property tuning of manganese(II)
coordination polymers with the tetra(4-pyridyl)-tetrathiafulvalene ligand,
Cryst. Growth Des. 19 (2019) 3012–3018, https://doi.org/10.1021/acs.
cgd.9b00239.
[11] K. Singh, J.d.J. Guillen Campos, F. Dinic, Z. Hao, T. Yuan, O. Voznyy, Manganese
MOF enables efficient oxygen evolution in acid, ACS Mater. Lett. 2 (2020) 798–
800, https://doi.org/10.1021/acsmaterialslett.0c00123.
[12] S. Horike, M. Dincaꢀ, K. Tamaki, J.R. Long, Size-selective Lewis acid catalysis in a
microporous metal-organic framework with exposed Mn2+ coordination sites,
J. Am. Chem. Soc. 130 (2008) 5854–5855, https://doi.org/10.1021/ja800669j.
[13] J. Chen, M. Chen, B. Zhang, R. Nie, A.o. Huang, T.W. Goh, A. Volkov, Z. Zhang, Q.
Ren, W. Huang, Allylic oxidation of olefins with a manganese-based metal–
organic framework, Green Chem. 21 (2019) 3629–3636, https://doi.org/
10.1039/C9GC01337G.
[33] S.S. Shinde, C.H. Lee, J.-Y. Jung, N.K. Wagh, S.-H. Kim, D.-H. Kim, C. Lin, S.U. Lee,
J.-H. Lee, Unveiling dual-linkage 3D hexaiminobenzene metal–organic
frameworks towards long-lasting advanced reversible Zn–air batteries,
Energy Environ. Sci. 12 (2019) 727–738, https://doi.org/10.1039/C8EE02679C.
[34] L. Sun, C.H. Hendon, M.A. Minier, A. Walsh, M. Dinca˘, Million-fold electrical
conductivity enhancement in Fe2(DEBDC) versus Mn2(DEBDC) (E = S, O), J. Am.
Chem. Soc. 137 (2015) 6164–6167, https://doi.org/10.1021/jacs.5b02897.
[35] C.K. Brozek, M. Dinca˘, Cation exchange at the secondary building units of
metal–organic frameworks, Chem. Soc. Rev. 43 (2014) 5456–5467, https://doi.
org/10.1039/C4CS00002A.
[14] Z.-B. Han, R.-Y. Lu, Y.-F. Liang, Y.-L. Zhou, Q. Chen, M.-H. Zeng, Mn(II)-based
porous metal–organic framework showing metamagnetic properties and high
hydrogen adsorption at low pressure, Inorg. Chem. 51 (2012) 674–679,
https://doi.org/10.1021/ic2021929.
[36] J. An, N.L. Rosi, Tuning MOF CO2 adsorption properties via cation exchange, J.
Am. Chem. Soc. 132 (2010) 5578–5579, https://doi.org/10.1021/ja1012992.
[37] Y. Noori, K. Akhbari, Post-synthetic ion-exchange process in nanoporous
metal–organic frameworks; an effective way for modulating their structures
[15] M. Dincaꢀ, J.R. Long, High-enthalpy hydrogen adsorption in cation-exchanged
and properties, RSC Adv.
C6RA24958B.
[38] S. Mandal, S. Natarajan, P. Mani, A. Pankajakshan, Post-synthetic modification
of metal–organic frameworks toward applications, Adv. Funct. Mater. 31
(2021) 2006291, https://doi.org/10.1002/adfm.202006291.
7 (2017) 1782–1808, https://doi.org/10.1039/
variants
of
the
microporous
metalꢂorganic
framework
Mn3[(Mn4Cl)3(BTT)8(CH3OH)10]2, J. Am. Chem. Soc. 129 (2007) 11172–11176,
https://doi.org/10.1021/ja072871f10.1021/ja072871f.s001.
[16] A. Asghar, N. Iqbal, L. Aftab, T. Noor, B.M. Kariuki, L. Kidwell, T.L. Easun,
Ethylenediamine loading into a manganese-based metal-organic framework
enhances water stability and carbon dioxide uptake of the framework, R. Soc.
Open Sci. 7 (2020) 191934, https://doi.org/10.1098/rsos.191934.
[17] S. Li, Y.u. Dong, J. Zhou, Y. Liu, J. Wang, X. Gao, Y. Han, P. Qi, B.o. Wang, Carbon
dioxide in the cage: manganese metal–organic frameworks for high
performance CO2 electrodes in Li–CO2 batteries, Energy Environ. Sci. 11
(2018) 1318–1325, https://doi.org/10.1039/C8EE00415C.
[39] Y. Kim, S. Das, S. Bhattacharya, S. Hong, M.G. Kim, M. Yoon, S. Natarajan, K.
Kim, Metal-ion metathesis in metal–organic frameworks: a synthetic route to
new metal–organic frameworks, Chem. Eur. J. 18 (2012) 16642–16648,
https://doi.org/10.1002/chem.v18.5210.1002/chem.201202899.
[40] C.K. Brozek, M. Dinca˘, Ti3+-, V2+/3+-, Cr2+/3+-, Mn2+-, and Fe2+-substituted MOF-5
and redox reactivity in Cr- and Fe-MOF-5, J. Am. Chem. Soc. 135 (2013) 12886–
12891, https://doi.org/10.1021/ja4064475.
[18] S.H. Kim, Y.J. Lee, D.H. Kim, Y.J. Lee, Bimetallic metal–organic frameworks as
efficient cathode catalysts for Li–O2 Batteries, ACS Appl. Mater. Interfaces 10
(2018) 660–667, https://doi.org/10.1021/acsami.7b15499.
[41] Z. Zhang, L. Zhang, L. Wojtas, P. Nugent, M. Eddaoudi, M.J. Zaworotko,
Templated synthesis, postsynthetic metal exchange, and properties of
a
porphyrin-encapsulating metal–organic material, J. Am. Chem. Soc. 134
(2012) 924–927, https://doi.org/10.1021/ja209643b.
[19] X. Wang, X. Liu, H. Rong, Y. Song, H. Wen, Q.i. Liu, Layered manganese-based
metal–organic framework as
supercapacitors, RSC Adv. 7 (2017) 29611–29617, https://doi.org/10.1039/
C7RA04374K.
a
high capacity electrode material for
[42] H. Fei, J.F. Cahill, K.A. Prather, S.M. Cohen, Tandem postsynthetic metal ion and
ligand exchange in zeolitic imidazolate frameworks, Inorg. Chem. 52 (2013)
4011–4016, https://doi.org/10.1021/ic400048g.
[20] K.M.L. Taylor, W.J. Rieter, W. Lin, Manganese-based nanoscale metalꢂorganic
frameworks for magnetic resonance imaging, J. Am. Chem. Soc. 130 (2008)
14358–14359, https://doi.org/10.1021/ja803777x.
[21] Y.u. Yang, J. Liu, C. Liang, L. Feng, T. Fu, Z. Dong, Y.u. Chao, Y. Li, G. Lu, M. Chen,
Z. Liu, Nanoscale metal–organic particles with rapid clearance for magnetic
resonance imaging-guided photothermal therapy, ACS Nano 10 (2016) 2774–
2781, https://doi.org/10.1021/acsnano.5b07882.
[22] M.F. Bennewitz, T.L. Lobo, M.K. Nkansah, G. Ulas, G.W. Brudvig, E.M. Shapiro,
Biocompatible and pH-sensitive PLGA encapsulated MnO nanocrystals for
molecular and cellular MRI, ACS Nano 5 (2011) 3438–3446, https://doi.org/
10.1021/nn1019779.
[43] H. Zhang, Y. Lu, Z. Zhang, H. Fu, Y. Li, D. Volkmer, D. Denysenko, E. Wang, A 12-
connected metal–organic framework constructed from an unprecedented
cyclic dodecanuclear copper cluster, Chem. Comm. 48 (2012) 7295–7297,
https://doi.org/10.1039/C2CC32120C.
[44] M.T. Huxley, C.J. Coghlan, W.M. Bloch, A. Burgun, C.J. Doonan, C.J. Sumby, X-ray
crystallographic insights into post-synthetic metalation products in a metal-
organic framework, Philos. Trans. A Math Phys. Eng. Sci. 375 (2017) 20160028,
https://doi.org/10.1098/rsta.2016.0028.
[45] A.W. Stubbs, M. Dincǎ, Selective oxidation of C-H bonds through a manganese
(III) hydroperoxo in MnII-exchanged CFA-1, Inorg. Chem. 58 (2019) 13221–
13228, https://doi.org/10.1021/acs.inorgchem.9b02068.
[23] H. Fei, M.D. Sampson, Y. Lee, C.P. Kubiak, S.M. Cohen, Photocatalytic CO2
reduction to formate using a Mn(I) molecular catalyst in a robust metal–
organic framework, Inorg. Chem. 54 (2015) 6821–6828, https://doi.org/
10.1021/acs.inorgchem.5b00752.
[46] A.W. Stubbs, L. Braglia, E. Borfecchia, R.J. Meyer, Y. Román- Leshkov, C.
Lamberti, M. Dinca˘, Selective catalytic olefin epoxidation with MnII-exchanged
MOF-5,
ACS
Catal.
8
(2018)
596–601,
https://doi.org/10.1021/
acscatal.7b02946.
[24] S.-H. Cho, B. Ma, S.T. Nguyen, J.T. Hupp, T.E. Albrecht-Schmitt, A metal–organic
framework material that functions as an enantioselective catalyst for olefin
epoxidation, Chem. Comm. (2006) 2563–2565, https://doi.org/10.1039/
B600408C.
[47] C.K. Brozek, A.F. Cozzolino, S.J. Teat, Y.-S. Chen, M. Dinca˘, Quantification of site-
specific cation exchange in metal–organic frameworks using multi-
wavelength anomalous X-ray dispersion, Chem. Mater. 25 (2013) 2998–
3002, https://doi.org/10.1021/cm400858d.
[25] J.W. Brown, Q.T. Nguyen, T. Otto, N.N. Jarenwattananon, S. Glöggler, L.-S.
[48] L. Vial, R.F. Ludlow, J. Leclaire, R. Pérez-Fernández, S. Otto, Controlling the
biological effects of spermine using a synthetic receptor, J. Am. Chem. Soc. 128
(2006) 10253–10257, https://doi.org/10.1021/ja062536b.
[49] B. Ravel, M. Newville, ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray
absorption spectroscopy using IFEFFIT, J. Synchtrotron Radiat. 12 (2005) 537–
541, https://doi.org/10.1107/S0909049505012719.
[50] J. Neu, C.A. Schmuttenmaer, Tutorial: An introduction to terahertz time
domain spectroscopy (THz-TDS), J. Appl. Phys. 124 (2018) 231101, https://doi.
org/10.1063/1.5047659.
Bouchard, Epoxidation of alkenes with molecular oxygen catalyzed by
a
manganese porphyrin-based metal–organic framework, Catal. Comm. 59
(2015) 50–54, https://doi.org/10.1016/j.catcom.2014.09.040.
[26] F. Song, C. Wang, J.M. Falkowski, L. Ma, W. Lin, Isoreticular chiral
metalꢂorganic frameworks for asymmetric alkene epoxidation: tuning
catalytic activity by controlling framework catenation and varying open
channel sizes, J. Am. Chem. Soc. 132 (2010) 15390–15398, https://doi.org/
10.1021/ja1069773.
[27] L. Sun, T. Miyakai, S. Seki, M. Dinca˘, Mn2(2,5-disulfhydrylbenzene-1,4-
[51] J. Neu, H. Nikonow, C.A. Schmuttenmaer, Terahertz spectroscopy and density
functional theory calculations of dl-Norleucine and dl-Methionine, J. Phys.
Chem. A 122 (2018) 5978–5982, https://doi.org/10.1021/acs.jpca.8b04978.
[52] L.S. Kau, D.J. Spira-Solomon, J.E. Penner-Hahn, K.O. Hodgson, E.I. Solomon, X-
ray absorption edge determination of the oxidation state and coordination
number of copper. Application to the type 3 site in Rhus vernicifera laccase
and its reaction with oxygen, J. Am. Chem. Soc. 109 (1987) 6433–6442, https://
doi.org/10.1021/ja00255a032.
[53] J. Garcia, M. Benfatto, C.R. Natoli, A. Bianconi, A. Fontaine, H. Tolentino, The
quantitative Jahn-teller distortion of the Cu2+ site in aqueous solution by xanes
spectroscopy, Chem. Phys. 132 (1989) 295–302, https://doi.org/10.1016/0301-
0104(89)80095-3.
dicarboxylate):
a microporous metal–organic framework with infinite
(ꢂMn–Sꢂ)1 chains and high intrinsic charge mobility, J. Am. Chem. Soc.
135 (2013) 8185–8188, https://doi.org/10.1021/ja4037516.
[28] L.S. Xie, G. Skorupskii, M. Dinca˘, Electrically conductive metal–organic
frameworks, Chem. Rev. 120 (2020) 8536–8580, https://doi.org/10.1021/acs.
chemrev.9b00766.
[29] P.I. Scheurle, A. Mähringer, A.C. Jakowetz, P. Hosseini, A.F. Richter, G.
Wittstock, D.D. Medina, T. Bein, A highly crystalline anthracene-based MOF-
74 series featuring electrical conductivity and luminescence, Nanoscale 11
(2019) 20949–20955, https://doi.org/10.1039/C9NR05431F.
[30] L. Liu, J.A. DeGayner, L. Sun, D.Z. Zee, T.D. Harris, Reversible redox switching of
magnetic order and electrical conductivity in a 2D manganese benzoquinoid
framework, Chem. Sci. 10 (2019) 4652–4661, https://doi.org/10.1039/
C9SC00606K.
[54] V.A. Markel, Introduction to the Maxwell Garnett approximation: tutorial, J.
Optical Soc. Amer.
JOSAA.33.001244.
A 33 (2016) 1244–1256, https://doi.org/10.1364/
9

B. Pattengale, J. Neu, A. Tada et al.
Polyhedron 203 (2021) 115182
[55] B. Pattengale, J. Neu, S. Ostresh, G. Hu, J.A. Spies, R. Okabe, G.W. Brudvig, C.A.
Schmuttenmaer, Metal–organic framework photoconductivity via time-
resolved terahertz spectroscopy, J. Am. Chem. Soc. 141 (2019) 9793–9797,
https://doi.org/10.1021/jacs.9b04338.
10