Mn2(2,5-disulfhydrylbenzene-1,4-dicarboxylate): A microporous metal-organic framework with infinite (-Mn-S-)∞ chains and high intrinsic charge mobility

Article abstract of DOI:10.1021/ja4037516

The reaction of MnCl₂ with 2,5-disulfhydrylbenzene-1,4- dicarboxylic acid (H₄DSBDC), in which the phenol groups in 2,5-dihydroxybenzene-1,4-dicarboxylic acid (H₄DOBDC) have been replaced by thiophenol units, led to the isolation of Mn₂(DSBDC), a thiolated analogue of the M₂(DOBDC) series of metal-organic frameworks (MOFs). The sulfur atoms participate in infinite one-dimensional Mn-S chains, and Mn₂(DSBDC) shows a high surface area and high charge mobility similar to that found in some of the most common organic semiconductors. The synthetic approach to Mn₂(DSBDC) and its excellent electronic properties provide a blueprint for a potentially rich area of exploration in microporous conductive MOFs with low-dimensional charge transport pathways.

Full text of DOI:10.1021/ja4037516

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Communication
2
Mn(2,5-disulfhydrylbenzene-1,4-dicarboxylate): a Microporous MOF
#
with Infinite (—Mn—S—) Chains and High Intrinsic Charge Mobility
Lei Sun, Tomoyo Miyakai, Shu Seki, and Mircea Dinca
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja4037516 • Publication Date (Web): 14 May 2013
Downloaded from http://pubs.acs.org on May 15, 2013
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Mn2(2,5-disulfhydrylbenzene-1,4-dicarboxylate): a Mi-
croporous MOF with Infinite (−M−S−)∞ Chains and High Intrin-
sic Charge Mobility
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Lei Sun,† Tomoyo Miyakai,‡ Shu Seki,‡ and Mircea Dincă†*
†Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA,
02139, USA
‡Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
Supporting Information Placeholder
ABSTRACT: Replacement of phenol groups in 2,5-
dihydroxy-1,4-benzenedicarboxylate by thiophenol units and
reaction of 2,5-disulfhydrylbenzene-1,4-dicarboxylic acid
(H4DSBDC) with MnCl2 led to the isolation of Mn2(DSBDC),
a thiolated analogue of the M2(DOBDC) series of metal-
organic frameworks. The sulfur atoms participate in infinite
one-dimensional Mn-S chains, and Mn2(DSBDC) shows high
surface area and high charge mobility similar to some of the
most common organic semiconductors. The synthetic ap-
proach to Mn2(DSBDC) and its excellent electronic proper-
ties provide a blueprint for a potentially rich area of explora-
tion in microporous conductive MOFs with low-dimensional
charge transport pathways.
Myriad applications have been found or proposed for mi-
croporous metal-organic frameworks (MOFs),1 yet those that
require electron transport or conductivity in combination
with permanent porosity still lag behind because the vast
Figure 1. Conceptual design of MOFs consisting of
majority of microporous MOFs are electrical insulators.2
(−M−S−)∞ chains based on replacing phenol groups in
However, recent inroads in this area suggest that judicious
M2(DOBDC) with thiophenol groups. The purple bonds
choice of the metal and organic components can lead to crys-
indicate the infinite (−M−O−)∞ and expected (−M−S−)∞
chains.
talline porous materials with promising conductivity or
charge mobility properties.3 In fact, taking a cue from the
fur can alleviate the energy mismatch,8 and has led to semi-
more mature area of molecular conductors and conductive
conducting non-porous coordination polymers9 and one of
coordination polymers, one can envision that two main ap-
the only examples of a porous and conductive material, a Ni-
proaches can be employed to construct new MOFs with good
dithiolene MOF.3a We sought to pursue a related approach,
charge transport properties.4,5 One is “through-space” and
where oxygen atoms in metal-oxide chains of existing MOFs
relies on π stacking interactions between electroactive moie-
are replaced isomorphically by sulfur atoms, thereby giving
ties. This approach has led to remarkable results with cova-
lent-organic frameworks6 and has also recently been used by
rise to infinite metal-sulfur chains of potential interest to-
wards charge transport. In essence, this approach would cre-
us to make a tetrathiafulvalene (TTF)-based MOF with high
charge mobility.3c The second approach relies on a “through-
ate dimensionally-reduced metal-organic chalcogenides,
wherein organic spacers would create porosity while main-
bond” formalism, where both symmetry and energy overlap
must exist between the covalently-bonded components to
taining (−M-S−)∞ units along one direction. Although theo-
retical and experimental studies indicated that dimensional
promote good charge transport. This approach is synthetical-
reduction in bulk metal chalcogenides, which are usually
ly more tractable because it allows control of the direction-
intrinsic semiconductors or metals,10 increases the band gap
ality of the metal-ligand bond. It has nevertheless seen lim-
of these materials, the lower-dimensionality does not elimi-
ited success in MOFs, which are typically constructed from
nate charge transport.11
hard metal ions and oxygen- or nitrogen-based redox-
inactive ligands7 that rarely provide a good pathway for
charge delocalization. Replacement of oxygen atoms by sul-
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There are numerous MOFs that contain infinite one-
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dimensional secondary building units (SBUs) where single
oxygen atoms bridge pairs of two metal ions. One of the
most iconic examples is the series of materials with the for-
mula M2(DOBDC) (M = Mg, Mn, Fe, Co, Ni, Zn, H4DOBDC
= 2,5-dihydroxybenzene-1,4-dicarboxylic acid).12 The SBUs in
these MOFs are (−M−O−)∞ chains where each pair of metal
atoms is bridged by one phenolate group. We surmised that
replacing the phenol groups by thiophenol functionalities
would give rise to isomorphous materials with linear
(−M−S−)∞ units and high charge mobility (Figure 1). Herein
we show that the thiolated analogue of H4DOBDC, 2,5-
disulfhydrylbenzene-1,4-dicarboxylic acid (H4DSBDC) gener-
ates Mn2(DSBDC), a new material with permanent porosity
and high charge mobility. To our knowledge, the new MOF
has the highest surface area among MOFs that display elec-
tron delocalization, improving the previous record by almost
50% and reinforcing the idea that high surface area and
charge delocalization can indeed coexist in crystalline hybrid
materials.
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The ligand H4DSBDC was synthesized starting from hy-
droquinone in five steps and 19% overall yield, according to
Scheme S1.13 Heating a solution of H4DSBDC and anhydrous
MnCl2 in
a
dry and degassed mixture of N,N-
dimethylformamide (DMF) and methanol (10:1 by volume) at
120 °C under an N2 atmosphere for 1 day generated yellow
crystals of [Mn2(DSBDC)(DMF)2]·0.2DMF (as-synthesized 1)
and an unidentified orange powder. Single crystal X-ray dif-
fraction analysis of 1 revealed a structure consisting of infi-
nite Mn2+ chains bridged by both carboxylate groups and
thiophenoxide groups pertaining to anionic DSBDC4- ligands
(Figure 2). Although related to the structure of M2(DOBDC),
the structure of 1 differs from the latter because two crystal-
lographically independent Mn2+ ions are found in the asym-
metric unit, whereas only one is present in M2(DOBDC). In 1,
one Mn2+ ion is coordinated by four carboxylate oxygen at-
oms and two thiophenoxide groups, while another is coordi-
nated by two carboxylate oxygen atoms, two thiophenoxide
groups and two cis-oriented DMF molecules. Importantly, 1
contains infinite (−Mn−S−)∞ chains defined by Mn2+-
thiophenoxide linkages wherein the sulfur atoms at both
crystallographic Mn sites are oriented trans with respect to
each other, with Mn−S distances of 2.493(1) and 2.632(1) Å,
respectively. This indicates that the S atoms interact with the
same d-orbital on Mn, an important symmetry requirement
for charge delocalization along the (−Mn−S−)∞ chain. Bridg-
ing in each pair of neighboring Mn atoms is completed by
Figure 2. Portions of the X-ray crystal structure of
Mn2(DSBDC). Top: A view of a (−Mn−S−)∞ chain SBU. Mn4
and Mn6 represent 4-coordinated and 6-coordinated Mn
sites in activated Mn2(DSBDC). Bottom: A view of one-
dimensional infinite pores along the c-axis. H atoms and
DMF molecules were omitted for clarity.
content visibly decreased the amount of undesired orange
product, it also decreased the crystal size of 1. Pure 1 was
obtained as a microcrystalline yellow material when the DMF
to methanol ratio reached 2:1. Its identity and purity was
verified by powder X-ray diffraction, which revealed a pat-
tern corresponding to that simulated from the single crystal
X-ray structure (Figure 3).
Thermogravimetric analysis (TGA) of as-synthesized 1 ex-
hibited a gradual weight loss of 31.8% between 50 ˚C and 365
˚C (Figure S1), which matched well with the expected loss of
2.2 DMF molecules, including both guest and bound DMF
molecules. A second weight loss was observed above 400 ˚C
and likely corresponds to ligand and framework decomposi-
tion. A better defined TGA trace is obtained if the guest and
bound DMF molecules are exchanged with lower-boiling
methanol. To achieve this, as-synthesized 1 was subjected to
a Soxhlet extraction with freshly dried and deaerated metha-
nol for two days, during which the material acquired a green
hue (methanol-exchanged 1). Replacement of DMF by meth-
anol was tested by infrared (IR) spectroscopy, which con-
firmed the absence of the strong DMF C=O stretch at 1647
one oxygen atom from carboxylate groups and ꢀ-carboxylate
linkages. Neighboring chains are connected by DSBDC4- lig-
ands, which together define a three-dimensional framework
containing hexagonal one-dimensional pores with a van der
Waals diameter of ~16 Å (for fully desolvated 1). As expected
based on the longer Mn−S and C−S distances relative to M−O
and C−O, respectively, this is approximately 2.4 Å larger than
that found in M2(DOBDC) analogues12c and is much larger
than those found in previous conductive MOFs,3 suggesting
potential applications in donor-acceptor studies.
Attempts to eliminate the unidentified secondary orange
product led us to increase the methanol content of the sol-
vent mixture. Although gradual increase of the methanol
cm-1 previously prominent in as-synthesized 1, and the ap-
,
pearance of a methanol C−O stretch at 1005 cm-1 (Figure S3).
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TGA of methanol-exchanged Mn2(DSBDC) showed a rapid
weight loss below ~ 100 °C and a stable plateau between 150
methanol-exchanged and activated 1 are virtually identical,
displaying maximum values of
Σμ = 2 × 10-5 cm2V-1s-1. The
1
2
φ
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Figure 3. PXRD patterns of as-synthesized 1, methanol-
exchanged 1, activated 1, and simulated 1. The inset shows
an N2 adsorption isotherm in activated 1 at 77 K.
°C and 400 °C (Figure S2). Accordingly, activation of
Mn2(DSBDC) was achieved by heating methanol-exchanged 1
at 150 °C and high vacuum (3 mTorr) for two days, to obtain
activated 1. The structural integrity of activated 1 was verified
by PXRD (Figure 3). The absence of guest and bound metha-
nol was confirmed by IR spectroscopy, and the purity of the
sample was confirmed by C, H, and S microanalysis. Upon
activation, Mn2(DSBDC) adsorbs approximately 240 cm3g-1 of
N2 at 77 K and exhibits a Type I isotherm, shown in Figure 3,
inset. The corresponding BET surface area of 978 m2g-1 (329
m2mmol-1) is comparable with those reported for
M2(DOBDC) (287 ~ 416 m2mmol-1).12c,d
Figure 4. (a) Conductivity transients observed by FP-TRMC
upon excitation at 355 nm with 1.4 × 1016 cm-2 photons per
pulse for methanol-exchanged 1 and activated 1. (b) Photo-
current transients observed by TOF upon excitation at 355
nm with 1.9 × 1014 cm-2 photons per pulse for methanol-
exchanged 1 and activated 1. The transients were observed
with a terminate resistance of 10 kΩ under applied bias at 3
× 104 Vcm-1.
The charge carrier mobilities of methanol-exchanged 1 and
activated 1 were evaluated by flash-photolysis time-resolved
microwave conductivity (FP-TRMC). FP-TRMC is a contact-
less technique which utilizes high frequency (GHz) micro-
waves to probe the laser-induced transient conductivity in-
crease, giving information on charge transport on the multi-
nanometer length scale. It is particularly informative when
measuring anisotropic crystalline materials because it elimi-
nates perturbations related to grain boundaries, defects, im-
purities, contact resistance, and high electric fields that affect
the results obtained by the more common direct current
time-of-flight and field-effect-transistor measurements,
thereby revealing the intrinsic charge carrier mobility of a
material.14 FP-TRMC experiments were performed on films
made from PMMA and the above two Mn2(DSBDC) materi-
als, which were mixed in 1:1 weight ratios. The films were
inserted in a resonant microwave cavity, and were irradiated
with 355 nm laser light. We obtained time-dependent values
values of
φ were estimated by a time-of-flight (TOF) transi-
ent current integration method, shown in Figure 4b. The
TOF-derived
φ values for methanol-exchanged 1 and activat-
ed 1 were 9.7 × 10-4 and 1.4 × 10-3, respectively.
The combination of FP-TRMC and TOF results revealed
excellent charge carrier mobilities of 0.02 and 0.01 cm2V-1s-1
for methanol-exchanged and activated Mn2(DSBDC), respec-
tively. The similarity between the two values suggests that
the charge transport pathway, presumably the Mn-S chain, is
not affected greatly by the presence of solvent. Based on the
observed charge carrier mobility values and the frequency of
the microwave field (9.1 GHz), the displacement lengths of
charge carriers (defined as the distance that charge carriers
can move in one period of microwave) in these two
Mn2(DSBDC) materials are found to be approximately 2 ~ 3
nm, corresponding to charge delocalization over 8 ~ 12 Mn-S
units.15 This short displacement length of charge carriers
confirms that the results indeed reveal the intrinsic charge
of the transient conductivity, φΣμ, where φ is the quantum
efficiency for the generation of charge carriers upon one-
photon absorption, and Σμ is the sum of electron and hole
mobilities. As shown in Figure 4a, the FP-TRMC profiles of
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carrier mobility of the materials. As shown in figure 4b, the
World-Leading Researchers (NEXT Programs) of the Japan
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photo-current transients are principally dispersive ones;
some inflection points are detectable to give mean flight time
of charge carriers (figure S5). The estimated values of mobili-
ty are 0.001 cm2V-1s-1 (E = 0 Vcm-1) according to TOF results,
which are one order of magnitude lower than those in
TRMC, and reflect long-range translational motion of charge
carriers. An alternative charge transport pathway may in-
volve transfer through the benzene ring, through the 1,4-
benzenedithiol moiety.16 Experiments that probe this possi-
bility are currently underway.
Society for the Promotion of Science (JSPS).
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Importantly, the charge mobility values observed for
Mn2(DSBDC) are comparable with those found in organic
conductors such as polythiophenes (Σμ = 0.003 ~ 0.1 cm2V-1s-
1
)
14a,17 and rubrene (Σμ = 0.05 cm2V-1s-1),18 as measured by the
same technique, and highlight the potential utility of MOFs
for the construction of various electronic devices that com-
bine high surface area and high charge mobility. In fact, at
978 m2g-1, Mn2(DSBDC) has the highest surface area by al-
most 50% among MOFs that have demonstrated intrinsic
charge delocalization thus far, such as Cu[Ni(pyrazine-2,3-
dithiolate)2] (385 m2g-1),3a Fe(1,2,3-triazolate) (450 m2g-1),3b
Cu- and Ni-catecholates (~425-490 m2g-1),3d and Zn2(TTF-
tetrabenzoate) (662 m2g-1).3c Notably, because the pore size
of the M2(DOBDC) structure type can be extended into the
mesoporous regime19 and assuming that a similar isoreticular
approach is applicable to Mn2(DSBDC), these results rein-
force the idea that high surface area, porosity, and high
charge mobility are not mutually exclusive.
In summary, a redox-matching strategy8 aimed at isomor-
phous substitution of O atoms by S atoms to yield infinite
one-dimensional metal-sulfur chains has led to the synthesis
of a new MOF with high charge mobility. Because the iso-
morphous replacement strategy could be amenable to many
other MOFs containing metal-oxygen chains, the study pro-
vides a potentially general mechanism for the formation of
other porous crystalline materials with high charge mobility,
not least of which are members of the M2(DEBDC) structure
types with other transition metals.
ASSOCIATED CONTENT
Supporting Information
Experimental details, table of X-ray refinement details, TGA
traces, IR spectra, BET linear fit, and TOF data. This material
is available free of charge via the Internet at
http://pubs.acs.org.
13. Vial, L.; Ludlow, R. F.; Leclaire, J.; Pérez-Fernández, R.; Otto, S. J.
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AUTHOR INFORMATION
Corresponding Author
mdinca@mit.edu
ACKNOWLEDGMENT
This work was supported by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences under
Award Number DE-SC0006937. The NSF provided support to
the DCIF at MIT (CHE-9808061, DBI-9729592). S.S. is sup-
ported by the Funding Program for the Next-Generation
18. Saeki, A.; Seki, S.; Takenobu, T.; Iwasa, Y.; Tagawa, S. Adv. Mater.
2008, 20, 920.
19. Deng, H. et al. Science 2012, 336, 1018.
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ACS Paragon Plus Environment

Journal of the American Chemical Society
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81x80mm (300 x 300 DPI)
ACS Paragon Plus Environment

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Journal of the American Chemical Society
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81x131mm (300 x 300 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
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75x75mm (300 x 300 DPI)
ACS Paragon Plus Environment

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Journal of the American Chemical Society
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76x123mm (300 x 300 DPI)
ACS Paragon Plus Environment