Isoreticular Linker Substitution in Conductive Metal–Organic Frameworks with Through-Space Transport Pathways

Article abstract of DOI:10.1002/ange.202004697

The extension of reticular chemistry concepts to electrically conductive three-dimensional metal–organic frameworks (MOFs) has been challenging, particularly for cases in which strong interactions between electroactive linkers create the charge transport

Full text of DOI:10.1002/ange.202004697

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Titel: Isoreticular Linker Substitution in Conductive Metal-Organic
Frameworks with Through-Space Transport Pathways
Autoren: Lilia Xie, Sarah S Park, Michał J Chmielewski, Hanyu Li, Ruby
A Kharod, Michael G Campbell, and Mircea Dincă
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Zitierweise: Angew. Chem. Int. Ed. 10.1002/anie.202004697
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10.1002/ange.202004697
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Isoreticular Linker Substitution in Conductive Metal–Organic
Frameworks with Through-Space Transport Pathways
Lilia S. Xie,[a,c] Sarah S. Park,[a,c] Michał J. Chmielewski,[a,b] Hanyu Liu,[a] Ruby A. Kharod,[a] Luming
Yang,[a] Michael G. Campbell,[a] and Mircea Dincă*[a]
[a]
L. S. Xie, Dr. S. S. Park, Dr. M. J. Chmielewski, H. Liu, R. A. Kharod, L. Yang, Dr. M. G. Campbell, Prof. M. Dincă
Department of Chemistry
Massachusetts Institute of Technology
77 Massachusetts Avenue, Cambridge, MA 02139 (USA)
Email: mdinca@mit.edu
[b]
[c]
Dr. M. J. Chmielewski
Faculty of Chemistry, Biological and Chemical Research Centre
University of Warsaw
Żwirki i Wigury 101, 02-089 Warszawa (Poland)
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: The extension of reticular chemistry concepts to electrically
conductive three-dimensional metal-organic frameworks (MOFs) has
been challenging, particularly for cases in which strong interactions
between electroactive linkers create the charge transport pathways.
Here, we report the successful replacement of tetrathiafulvalene
(TTF) with a nickel glyoximate core in a family of isostructural
conductive MOFs with Mn2+, Zn2+, and Cd2+. Different coordination
environments of the framework metals lead to variations in the linker
stacking geometries and optical properties. Single crystal conductivity
data are consistent with charge transport along the linker stacking
direction, with conductivity values only slightly lower than those
reported for the analogous TTF materials. These results serve as a
case study demonstrating how reticular chemistry design principles
can be extended to conductive frameworks with significant
intermolecular contacts.
structural flexibility.[23] These characteristics are responsible for
the interesting electronic properties, but also hinder translation of
design strategies from TTFTB-based MOFs to other linkers.
Extending the scope of reticular chemistry – i.e. the controlled
assembly of frameworks based on conserved secondary building
units (SBUs)[28]
– to include structures with pronounced
intermolecular interactions is hence an unmet need.[29]
Complexes of nickel(II) with glyoximate ligands have been
known for more than a century,[30] and have been employed for
analytical determination of nickel content.[31] Like TTF
derivatives,[32] they stack extensively in the solid state[33] and form
conductive charge transfer salts.[34] Therefore, we hypothesized
that a tetratopic linker with a nickel glyoximate core would behave
similarly to TTFTB. Here, we present a new metallolinker,
nickel(II) bis(dibenzoateglyoximate) (Ni(dbg)2), and show that it
forms frameworks with Mn2+, Zn2+, and Cd2+ exhibiting the same
topology as the corresponding TTFTB MOFs. These structures
contain helical linker stacks with Ni×××Ni distances as close as 3.70
Å, resulting in electrical conductivities up to 10–6 S cm–1. These
results demonstrate a novel instance of isoreticular substitution of
linkers in MOFs with through-space transport pathways.
Electrically conductive metal–organic frameworks (MOFs) are a
promising class of porous conductors.[1,2] They are appealing for
applications benefitting from facile charge transport and high
surface
area,
including
chemiresistive
sensing,[3–6]
electrochemical energy storage,[7–11] and electrocatalysis.[12–14]
Many conductive MOFs have incorporated building blocks from
molecular organic conductors into frameworks.[15–21] Although a
diverse library of ligands exists, techniques to regulate the spatial
arrangement of electroactive units are less developed.[22–24] Since
electronic coupling among linkers often facilitates charge
transport, synthetic control over stacking interactions would
enable conductive MOFs with tailored structures and transport
properties.
The H4Ni(dbg)2 linker was synthesized via metalation of the
glyoxime dibenzoic acid (dbg) ligand (Scheme 1), which was
obtained via benzoin condensation of methyl 4-formylbenzoate
and subsequent oxidation and treatment with hydroxylamine
Many studies of linker stacking interactions in conductive
MOFs have focused on the tetrathiafulvalene tetrabenzoate
(TTFTB) linker.[16,22,23,25–27] These materials exhibit a variety of
stacking arrangements involving the tetrathiafulvalene (TTF)
cores, with close and continuous stacking correlating with the
highest conductivities (up to 10–4 S cm–1).[22,23,27] Importantly, a
comparative analysis of MOFs with tetratopic carboxylate linkers
indicated that TTFTB frameworks tend to exhibit unique
topologies due to this linker’s propensity for π–π interactions and
Scheme 1. Synthesis of H4Ni(dbg)2.
1
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Figure 1. Portions of the crystal structure of Zn, viewed (a) along the c axis, showing rhombic pores, and (b) perpendicular to the c axis, showing helical stacking
of nickel glyoximate cores. Hydrogen atoms are omitted for clarity.
(Scheme S1 and Figures S1–S10; a cobaloxime MOF linker was
recently obtained via a similar route[35]). Combining H4Ni(dbg)2
with Mn(NO3)2×4H2O, Zn(NO3)2×6H2O, and Cd(NO3)2×4H2O under
solvothermal reaction conditions mimicking those used for the
M2(TTFTB) MOFs[16,22] led to the isolation of Mn2[Ni(dbg)2] (Mn),
Zn2[Ni(dbg)2] (Zn), and Cd2[Ni(dbg)2] (Cd).
486.7(5) m2 g–1, respectively (Figures S13–S19), similar to the
M2(TTFTB) materials.[16,22]
Searches of the Cambridge Structural Database[36] for entries
in space groups P61, P65, P6122, and P6522 showed that these
M2[Ni(dbg)2] phases (and the isostructural M2(TTFTB) MOFs) are
topologically unique among polymeric extended structures. The
lack of other MOFs with this topology is consistent with evidence
that the geometry and π–π stacking of TTFTB are uncommon
among tetratopic linkers.[23,26,29] Closely matched geometries and
stacking propensities appear to enable the isoreticular
substitution of Ni(dbg)2 for TTFTB.
Single-crystal X-ray diffraction (SCXRD) showed all three
phases are isostructural to one another and to the M2(TTFTB)
MOFs, with Mn and Cd crystallizing in space groups P65/P61, and
Zn in space groups P6522/P6122 (Tables S1–S3 and Figures
S11–S13). (The measured crystals of Mn and Cd exhibited
inversion twinning, suggesting that the products are not
enantiopure.) In each phase, the linkers form helical stacks along
the c axis (Figure 1). The framework metal atoms are coordinated
by linker carboxylates and solvent atoms to form one-dimensional
(1D) inorganic SBUs. Rhombic 1D pores also extend along the c
direction. Nitrogen adsorption isotherms of activated Mn, Zn, and
Cd revealed them to be permanently porous, with Brunauer–
Emmett–Teller (BET) surface areas of 543.2(3), 539.8(4), and
The pronounced stacking of nickel glyoximate complexes
often lead to anisotropic optical and electronic properties.[37,38]
Hence, we sought to further quantify the stacking geometries in
these MOFs (Figure 2). Each nickel glyoximate core is nearly
planar, with < 0.1 Å deviation of all atoms from the least-squares
plane (Tables S4–S6). The separation between adjacent least-
squares planes is slightly longer for Mn than Zn and Cd (Table 1).
Because the Ni(dbg)2 ligands are not exactly centered on the six-
fold screw axes, the Ni×××Ni distances are longer than the
interplanar spacings. These contact distances are similar to the
S×××S distances observed in the M2(TTFTB) analogs (3.65–3.77 Å),
and ~0.1–0.6 Å longer than Ni×××Ni distances in other nickel
glyoximate complexes.[37] The slip distances between adjacent
cores (defined as the magnitude of the projection of the Ni×××Ni
vector onto the least-squares plane) are shortest for Zn and
longest for Cd.
Figure 2. (a) Illustration of the interplanar and Ni×××Ni distances between
adjacent Ni(dbg)2 linkers, with the least-squares planes shown in transparent
yellow. (b) Illustration of the slip distance, defined as the projection of the
Ni×××Ni vector onto the least-squares plane.
2
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Table 1. Crystallographic linker stacking distances in M2[Ni(dbg)2] MOFs.
MOF
Mn
Zn
Interplanar (Å)
3.6663
Ni×××Ni (Å)
3.8131(9)
3.7027(6)
3.7704(11)
Slip (Å)
1.0480(2)
0.9582(2)
1.1566(3)
3.5767
Cd
3.5887
Distinctions in the linker stacking geometries can be explained
by the different coordination environments of the metal centers in
the inorganic secondary building units (SBUs). In each phase, two
crystallographically independent framework M2+ centers
coordinated by linker carboxylates and solvent oxygen atoms
form continuous 1D SBUs encircling the three-fold screw axis
(Figure 3). The five-coordinate Mn2+ centers in Mn form corner-
sharing polyhedra that pack relatively inefficiently, resulting in the
longest interplanar spacing. In Zn, the alternating tetrahedra and
pseudo-octahedra do not share any oxygen atoms. The small
ionic radius of Zn2+ coupled with a relatively straight SBU results
in the shortest interplanar, Ni×××Ni, and slip distances.[39] Finally,
the Cd2+ centers in Cd form edge-sharing polyhedra that
effectively compress the pitch of the linker stacking helix while
increasing its diameter compared to Mn and Zn. As a result, the
interplanar spacing of Cd is similar to Zn, while the slip distance
is the largest among the three phases.
Figure 3. Kubelka–Munk-transformed diffuse reflectance UV-visible spectra of
Mn, Zn, and Cd, and absorption spectrum of a 0.025 mM solution of H4Ni(dbg)2
in DMF.
spectrum of H4Ni(dbg)2, with the bands blue-shifted by 1,000–
3,000 cm–1 compared to the MOFs.
Bands between 16,000 and 22,000 cm–1 are present in all of
the solid-state MOF spectra, but absent from the solution-phase
linker spectrum. We tentatively assign these features to
transitions originating from filled Ni 3d orbitals to the empty Ni 4pz
orbitals. Closer Ni×××Ni distances result in red-shifted bands in this
region for nickel glyoximate complexes.[37,38,42] The presence of
these features in the MOF spectra and their absence in the
solution-phase spectrum of H4Ni(dbg)2 are consistent with
previous suggestions that the 3dz2 ® 4pz transition is intensified
by mixing with interatomic charge transfer interactions in the solid
state.[41] The more intense features in Zn and Cd likely correlate
with their closer Ni×××Ni distances and interplanar spacings
compared to Mn.
Because stacking often influences the electronic transitions
of d8 square planar complexes in the solid state,[40] we examined
the diffuse reflectance UV–visible (DRUV–vis) spectra of Mn, Zn,
and Cd (Figure 4). Bands in the region 30,000 to 40,000 cm–1 are
likely transitions with predominantly π ® π* character from the
dbg ligand (Figure S20).[38] Bands between 22,000 and 28,000
cm–1 are assigned to metal-to-ligand charge transfer (MLCT)
transitions from Ni 3d orbitals to states with π* character.[38,41] All
of these features are present in the solution-phase absorbance
Motivated by the structural analogy to the conductive
M2(TTFTB) MOFs and the precedent of stacking-dependent
transport properties in nickel glyoximate complexes,[43] we
investigated the electrical conductivities of Mn, Zn, and Cd. Two-
contact probe single-crystal devices of Mn and Cd (Figures S21
and S22) revealed champion conductivity values of 2.0 ´ 10–7
S
cm–1 for Mn and 2.7 ´ 10–6 S cm–1 for Cd along the stacking
direction (Tables S7 and S8). These values are about 2–3 orders
of magnitude lower than the TTFTB analogs,[22] but higher than
neutral nickel glyoximate complexes with similar interplanar
spacings (Table S9).[34] (Due to crystal size limitations, we were
unable to carry out single-crystal measurements on Zn.) Two-
contact probe pressed pellets yielded typical values of < 10–10
S
cm–1 for Mn, Zn, and Cd (Tables S10–S12 and Figure S23). The
lower pellet conductivities are consistent with the expected
anisotropic charge transport pathways. One important distinction
from the M2(TTFTB) analogs[16,22] is the lack of linker oxidation or
mixed-valency in the as-synthesized M2[Ni(dbg)2] MOFs. Neither
DRUV–vis nor electron paramagnetic resonance spectroscopies
exhibited signatures of radical species (Figure S24), suggesting
that the somewhat inferior charge transport properties of these
Figure 4. Different coordination environments in the inorganic secondary
building units of (a) Mn, (b) Zn, and (c) Cd, which comprise carboxylate-
bridged chains of metal centers.
3
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materials relative to their TTFTB congeners are likely due to their
lower carrier concentrations. However, attempts to post-
synthetically oxidatively or reductively dope these materials have
not resulted in higher conductivities thus far (Figure S25 and
Table S13), leading us to hypothesize that other oxidation states
of the linker may not be stable in these structures.
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Entry for the Table of Contents
Matching the geometry and stacking propensity of linkers allows for tetrathiafulvalene to be replaced with a nickel glyoximate core in
a family of metal–organic frameworks with charge transport properties that are in line with their charge density.
6
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