Preliminary chemical reduction for synthesizing a stable porous molecular conductor with neutral metal nodes

Article abstract of DOI:10.1039/d0cc03541f

Preliminary chemical reduction of naphthalenediimide (NDI)-based organic ligands was applied to the synthesis of a porous molecular conductor (PMC) with neutral metal nodes (cobalt(ii) acetylacetonate). The obtained semiconductive PMC (PMC-2) was stable due to the neutral metal nodes, providing an advantage over electrochemical reduction. This journal is

Full text of DOI:10.1039/d0cc03541f

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Preliminary chemical reduction for synthesizing
a stable porous molecular conductor with
Cite this:DOI: 10.1039/d0cc03541f
neutral metal nodes†
Received 18th May 2020,
Accepted 23rd September 2020
Shohei Koyama,a Tappei Tanabe,a Shinya Takaishi,
Masahiro Yamashitaab and
a
a
Hiroaki Iguchi
*
DOI: 10.1039/d0cc03541f
rsc.li/chemcomm
Preliminary chemical reduction of naphthalenediimide (NDI)-based of Cd(NO3)2 in an N,N-dimethylacetamide (DMA) solution.1
organic ligands was applied to the synthesis of a porous molecular PMC-1 has a large carrier density of 0.7 ꢀ 0.1 eꢂ per NDI core.
conductor (PMC) with neutral metal nodes (cobalt(II) acetylacetonate). However, the electrochemical method cannot be used with com-
The obtained semiconductive PMC (PMC-2) was stable due to the binations of metal sources and organic linkers that react and
neutral metal nodes, providing an advantage over electrochemical precipitate immediately, because the generation of radical species
reduction.
is limited at the surface of the electrode and is slower than
precipitation is. For instance, it is difficult to introduce neutral
Porous molecular conductors (PMCs) are new electrically con- metal complexes as the metal nodes in PMCs because neutral
ductive materials, which share features of both metal–organic non-conductive coordination polymers typically precipitate prior
frameworks (MOFs) and molecular conductors.1 Although to electrochemical reaction. In addition, electrocrystallization is
electron-conductive MOFs2–10 have been extensively studied not suitable for large-scale synthesis. In order to overcome these
over the last decade, owing to their potential electronics difficulties, a new methodology to synthesize PMCs is necessary.
applications,11–13 they are generally prepared using
a
Herein, we present a new method for synthesizing PMCs
‘‘through-bond approach’’2–11 with the aim of delocalizing involving chemical reduction of the ligands prior to the
electrons along the frameworks. On the other hand, the design formation of the coordination network. To the best of our
of PMCs involves a ‘‘through-space approach’’, in which charge knowledge, a similar method was used to the synthesis of
transport pathways (e.g. p-stacking) among organic ligands are radical MOFs only once, whereas their conducting properties
incorporated. This approach is advantageous due to the variety were not reported.26 To prove the validity of the method,
of useable ligands and the availability of single crystals.14–19 [Co(acac)2] (acac = acetylacetonate) was chosen to act as the
However, only a few MOFs possess sufficient amounts of conduc- neutral metal node because it can form stable linear coordina-
tion carriers in the through-space conduction pathways.20–22
tion polymers with pyridyl-based bridging ligands.27,28 As
Incorporating numerous conduction carriers was possible expected, upon mixing DMA solutions of [Co(acac)2(MeOH)2]
by applying an electrocrystallization method, which is typically and NDI–py, the linear coordination polymer [Co(acac)2(NDI–
used for the preparation of molecular conductors.23–25 The py)]ꢁ2DMA (20) rapidly precipitated (Fig. S1 and Table S2, ESI†).
first PMC [Cd(OH2)4(NDI–py)](NO3)1.3ꢀ0.1ꢁnDMA (PMC-1) was There are no conduction carriers or p–p interactions between
synthesized by electrochemically reducing N,N0-di(4-pyridyl)- the NDI cores in 20. Thus, through-space conduction does
1,4,5,8-naphthalenetetracarboxydiimide (NDI–py) in the presence not occur. To employ the [Co(acac)2] complex in PMCs, we
chemically reduced NDI–py using cobaltocene29,30 in advance,
and then the DMA solution was layered onto a solution of
[Co(acac)2(MeOH)2] with RbBPh4 for slow liquid–liquid diffu-
a Department of Chemistry, Graduate School of Science, Tohoku University,
6-3 Aramaki-Aza-Aoba, Aoba-ku, Sendai, Miyagi 980-8578, Japan.
sion. A new semiconducting coordination polymer with a neutral
metal node, Rb0.5[Co(acac)2(NDI–py)]ꢁ2DMA (PMC-2), was obtained.
From single-crystal X-ray diffraction (SXRD) analysis, PMC-2
crystallized as conglomerates in the space groups P6422 and P6222
like PMC-1 did.1 PMC-2 has a threefold infinite p-stacked colum-
nar structure along the c axis constructed of linear coordination
polymers of [Co(acac)2] and NDI–py (Fig. 1 and Fig. S2, S3 and
E-mail: h-iguchi@tohoku.ac.jp
b School of Materials Science and Engineering, Nankai University, Tianjin 300350,
China
† Electronic supplementary information (ESI) available: Experimental details,
crystallographic details, thermogravimetric analysis, PXRD patterns and mag-
netic susceptibility. CCDC 2004231 (20) and 2004232 (PMC-2). For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
d0cc03541f
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with the coordination environment for a high-spin Co2+
ion.27,28,41 Moreover, the electron configuration is supported
by the magnetic properties (Fig. S6 and S7, ESI†), and thus, the
charge of the [Co(acac)2] moiety is considered to be neutral.
The Rb+ ion is disordered in three different positions
between two adjacent [Co(acac)2] moieties along the c axis
(Fig. 1c). The total occupancy of the Rb+ ion was fixed to 50%
to agree with the results of inductively coupled plasma (ICP)
analysis (48.3%). Since the Rb+ ion compensates for the charge
of the reduced NDI–py, we concluded that the mean charge of
NDI–py was nearly ꢂ0.5. Thus, the charge of the NDI core is
quantifiable via the number of countercations. This is one of
the advantages of PMCs with neutral metal nodes. In the case of
PMC-1, the cationic [Cd(OH2)4]2+ nodes require counteranions
(NO3ꢂ) in the pores to compensate for the charge. However, the
ꢂ
NO3 ions were difficult to quantify, and thus, there is a large
uncertainty in the charge of NDI–py (ꢂ0.7 ꢀ 0.1).
Four lattice DMA molecules coordinate to a Rb+ ion via the
oxygen atoms (Fig. S2, ESI†), and their occupancy could be
reasonably fixed to 50%, which is consistent with the total
occupancy of Rb+ ion. As a result, there are two DMA molecules
in the pore per [Co(acac)2(NDI–py)] moiety. From thermogravi-
metric analysis (TGA) of PMC-2, the 20% weight loss up to
200 1C (Fig. S8, ESI†) indicates the liberation of two DMA
molecules (19.5% in theory). The lattice DMA molecules were
fixed only when the crystal was slowly cooled (ꢂ2 K minꢂ1) from
room temperature. The residual electron density in the pores
was too dispersed to establish the structure of DMA when the
crystal was rapidly cooled to 120 K, suggesting that the DMA
molecules fluctuate at room temperature.
From the viewpoint of crystal engineering, the Co2+ ions
help to form linear coordination polymers as metal nodes and
provide a platform for modifying the pore sizes and charge of
counterions by replacing coordinating ligands. The moderate
substitution inertness of Co2+ ions enables the formation of the
coordination polymers in PMC-2 without exchanging the acac
ligands for solvent molecules. No precipitate formed when
Fig. 1 Crystal structure of PMC-2. (a) One-dimensional coordination
polymer in PMC-2. (b) p-Stacked NDI column along the c axis. (c) Thermal
ellipsoid plot for the [Co(acac)2] moieties and Rb+ ions, which are dis-
ordered in three different positions with a total occupancy of 50%. (d)
Perspective view of PMC-2 along the c axis. Hydrogen atoms are omitted
for clarity in (b and c). Green, Rb; purple, Co; red, O; blue, N; gray, C; white, H.
Table S1, ESI†). The distance between p-conjugated planes of more labile [Zn(acac)2] moieties were used likely due to the
NDI–py was determined to be 3.186 Å, which is shorter than that elimination of the acac ligands from the Zn2+ ion.
of neutral NDI molecules (43.3 Å),31 suggesting that there are
Solid-state absorption spectra of PMC-2 acquired in both air
attractive interactions between NDI cores enhanced by the and inert gas (Ar) are shown in Fig. 2 along with the spectra of
presence of radicals. In fact, the results of calculations on the PMC-1 and NDI–py. According to previous research,1,42–46
LUMO of NDI–py monomer and dimer extracted from the crystal broad bands around 2.0 and 2.7 eV were assigned to the
structure (Fig. S4, ESI†) indicate that there is significant orbital intramolecular electron transition of NDIꢃꢂ. Weak broad bands
overlap between the NDI–py molecules. Such an infinite p-stacked in the near infrared region at 0.3 aꢂnd 1.1 eV were assigned to
ꢂ
array of NDI cores in coordination polymers is still rare,1,17,32–38 charge transfer (CT) between NDIꢃ and NDI0 cores (NDIꢃ
+
even though vast numbers of coordination polymers with NDI- NDI0 - NDI0 + NDIꢃ ) and the CT between two NDIꢃ cores
ꢂ
ꢂ
based ligands have been reported.39,40 The LUMO level of NDI–py (NDIꢃ + NDIꢃꢂ - NDI0 + NDI2ꢂ), respectively. The spectrum of
ꢂ
in PMC-2 was calculated to be ꢂ4.17 eV from the first reduction PMC-2 is similar to that of PMC-1 under inert atmospheres.
potential in the solid-state cyclic voltammogram (Fig. S5, ESI†). From DFT calculations on a NDI–py monomer (Fig. S4, ESI†),
This value is comparable to the LUMO level of NDI dimer in the HOMOꢂ2 and LUMO are mainly made of the p-orbitals of the
DFT calculation.
NDI core. The energy of the (HOMOꢂ2)–LUMO gap (3.44 eV) is
Focusing on Co ion, the four coordination Co–O(acac) bonds consistent with the intense band around 3.3 eV observed in the
are crystallographically equivalent. The coordination bond absorption spectrum of the NDI–py ligand. Thus, the band is
lengths for Co–O(acac) and Co–N(NDI–py) were determined to thought to be an intramolecular p–p* transition in a neutral
be 2.031(4) Å and 2.191(3) Å, respectively, and are consistent NDI core. It is noteworthy that the p–p* transition band is
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Fig. 3 Temperature dependence of electrical conductivity of single
crystalline PMC-2 (black) and PMC-1 (red) measured along the c axis
(the direction of p-stacked columnar structure).
Fig. 2 Solid-state absorption spectra of PMC-2 (black line), PMC-1 (red
line), and NDI–py ligand (orange line) acquired as KBr pellets.
almost negligible in the spectra of PMC-2, whereas it appears in
the spectrum of PMC-1 measured in air, suggesting that PMC-2
is more resistant to air oxidation than PMC-1 is.
previous work on PMC-1 with an average charge of NDI–py of
ꢂ0.7 ꢀ 0.1, we have observed a similar phenomenon probably
ꢂ
because the randomly accommodated counterions (NO3
)
The color of PMC-2 did not change even after it was exposed
to ambient air for 120 hours in the dark, whereas the color of
PMC-1 gradually faded. The powder XRD (PXRD) patterns of
PMC-2 were also unchanged over 120 hours in ambient air
(Fig. S10, ESI†). During TGA of PMC-2, no significant weight
loss was observed in a nitrogen gas flow (100 mL minꢂ1) over
10 hours at room temperature (Fig. S9, ESI†). Weight loss only
occurred when the temperature was over 70 1C, whereas for
PMC-1, it started at room temperature (Fig. S8, ESI†). These
results indicate that the solvent molecules in the pores block
oxygen molecules from entering and, thus, improve the oxida-
tive resistance of PMC-2. In addition, the counterions in the
pores may also improve the resistance to oxygen via the SOMO
energy of the NDIꢃꢂ radical. The radical is stabilized by the Rb+
induce a non-periodic charge localization. In the case of
PMC-2, Rb+ ions locating near the conductive pathways are
disordered with a 50% occupancy, and this would cause a
random charge localization and decrease the electrical con-
ductivity. The calculated transfer integral between adjacent
LUMO orbitals (255.8 meV) is larger than those of NDI-based
organic semiconductors47,48 and comparable to those of highly
conducting molecular conductors.49 Thus, the p-stacked array
of NDI cores has the potential to exhibit a higher intrinsic
conductivity if a random charge localization does not occur.
Moreover, despite the difference in the counterions, PMC-2 has
almost the same p-stacked arrays of NDI cores as PMC-1 does.
This feature is promising for studying intrinsic conduction
properties of molecular conductors without a structural pertur-
bation of the p-stacked arrays typically caused by surrounding
counterions.
In conclusion, the chemical reduction method could be used
to synthesize PMC-2, which contains neutral metal nodes. NDI–py
was reduced by cobaltocene prior to mixing with the neutral
[Co(acac)2] via liquid–liquid slow diffusion. This method is more
versatile than the previously used electrochemical method
because it can be used with reactive combinations of metal
sources and organic ligands. In addition, the introduction of
neutral metal nodes improved PMC-2 as follows: (1) durability
was increased. DMA molecules in the pore remained up to 70 1C,
increasing the resistance to oxidation. (2) The charge of NDI core
could be determined more precisely by counting the counter-
cations (Rb+). Since various countercations and metal complexes
can be used to prepare PMCs using our method, cation size and
ligand bulkiness can be varied to modulate the electronic proper-
ties, thermal stability and porosity of PMCs. Finally, additional
physical properties can be introduced by using neutral metal
nodes, advancing the research on conductive soft crystals50 and
functional framework materials.
ꢂ
ions in the case of PMC-2 but destabilized by the NO3 anions
in the case of PMC-1. The increased durability toward the
elimination of solvent molecules is probably due to the follow-
ing three reasons. (1) Pore size and volume are smaller due to
the introduction of bulky [Co(acac)2] moieties as metal nodes in
comparison with those of PMC-1 with compact [Cd(OH2)4]2+
nodes (Fig. S3, ESI†). (2) There are attractive interactions
between the DMA molecules and the Rb+ ions. (3) [Co(acac)2]
nodes are more stable than the [Cd(OH2)4]2+ nodes in PMC-1,
which can release H2O molecules.
Direct current (DC) electrical conductivities (s) of single
crystals were measured along the c axis by using a two probe
method. The temperature dependence of s (Fig. 3) indicated
that PMC-2 exhibited semiconducting behaviour with a low
activation energy (Ea) of 82 meV, which was calculated by fitting
the data with the Arrhenius equation in the range of 150–222 K
(blue dotted line in Fig. S11, ESI†). The s value of PMC-2 at
300 K (s300 K) was (1.2–1.6) ꢄ 10ꢂ6 S cmꢂ1, which is much lower
than that of PMC-1 (s300 K = (1.0–3.3) ꢄ 10ꢂ3 S cmꢂ1). Although
the charge of NDI–py is a non-integer value, the relatively low
conductivity and the semiconducting (not metallic) behaviour
of PMC-2 suggest that the carriers are localized. In fact, in our
We acknowledge Prof. Akutagawa and Dr. Hoshino at Tohoku
University for acquiring the ESR spectrum. We also acknowledge
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