Linker Deficiency, Aromatic Ring Fusion, and Electrocatalysis in a Porous Ni8-Pyrazolate Network

Article abstract of DOI:10.1021/acs.inorgchem.0c02662

The cruciform linker molecule here features two designer functions: the pyrazole donors for framework construction, and the vicinal alkynyl units for benzannulation to form nanographene units into the Ni8-pyrazolate scaffold. Unlike the full 12 connections of the Ni8(OH)4(H2O)2 clusters in other Ni8-pyrazolate networks, significant linker deficiency was observed here, leaving about half of the Ni(II) sites capped by acetate ligands, which can be potentially removed to open the metal sites for reactivity. The crystalline Ni8-pyrazolate scaffold also retains the crystalline order even after thermal treatments (up to 300 °C) that served to partially graphitize the neighboring alkyne units. The resultant nanographene components enhance the electroactive properties of the porous hosts, achieving hydrogen evolution reaction (HER) activity that rivals that of topical nickel/palladium-enabled materials.

Full text of DOI:10.1021/acs.inorgchem.0c02662

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Article
Linker Deficiency, Aromatic Ring Fusion, and Electrocatalysis in a
Porous Ni ‑Pyrazolate Network
8
Jieying Hu, Xiangling Deng, Hu Zhang, Yingxue Diao, Shengxian Cheng, Sai-Li Zheng, Wei-Ming Liao,
Jun He,* and Zhengtao Xu*
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sı Supporting Information
ABSTRACT: The cruciform linker molecule here features two
designer functions: the pyrazole donors for framework con-
struction, and the vicinal alkynyl units for benzannulation to form
nanographene units into the Ni -pyrazolate scaffold. Unlike the full
8
1
2 connections of the Ni (OH) (H O) clusters in other Ni -
8 4 2 2 8
pyrazolate networks, significant linker deficiency was observed
here, leaving about half of the Ni(II) sites capped by acetate
ligands, which can be potentially removed to open the metal sites
for reactivity. The crystalline Ni -pyrazolate scaffold also retains
8
the crystalline order even after thermal treatments (up to 300 °C)
that served to partially graphitize the neighboring alkyne units. The
resultant nanographene components enhance the electroactive properties of the porous hosts, achieving hydrogen evolution reaction
(
HER) activity that rivals that of topical nickel/palladium-enabled materials.
24−28
ecently, we have studied cascade cyclization of alkyne
In this connection, the more robust Ni -pyrazolate nets
8
R
units built into the linker molecules of metal−organic
are of interest. These consist of the secondary building unit of
4
+
−
frameworks (e.g., the relatively stable Zr -carboxyl sys-
a Ni cube with its six faces occupied by OH or H O species,
8
2
1
−3
tems).
The thermally triggered cyclization (e.g., above
and its 12 edges paralleled by the N-N units of the pyrazolate
ligands, each of which straddles two Ni centers. The 12-
connected Ni O node can be considered topologically
3
00 °C) leads to cross-linked, partially graphitized products
that feature enhanced polarizability (e.g., black in color) and
chemical stability. Topically, this line of work bears on the
endeavors to assemble polarizable, large-π molecules into a
MOF scaffold to enhance photophysical and electronic
8
6
1
4
2+
equivalent to the Zr O (OH)
unit (e.g., as in the UiO
6
4
series), which features a Zr octahedron and an O or OH−
2−
6
species on its 8 faces. One difference, however, is noted: while
Ni -pyrazolate nets reported so far uniformly feature full linker
8
4
−16
properties.
The two-step (i.e., crystallization-cyclization)
approach, however, serves two important purposes. First, it
utilizes open-arm alkyne precursors to avoid the solubility
problems caused by the tight-packing large-π aromatics.
Second, the extensive cross-linking and merging of the linker
molecules transforms the coordination precursor into a
covalent framework. Conceptually, this work can also be
considered a leap into the infinite network from the
intramolecular ring closures reported by Hupp, Stoddart, and
occupancy (i.e., one Ni O node per six ditopic linkers to give
8
6
the 12 connections), the Zr O (OH) unit was often found to
6
4
4
21,29−31
fall short of the full 12 connections,
e.g., with the
missing linkers filled in by monotopic formate/acetate or
−
OH /H O ligands. Another difference lies in the softer Ni(II)-
2
4+
pyrazolate pair (relative to Zr -carboxylate), which imparts
more covalency and stability to the bonding, making the
resultant framework less prone to deformation and degrada-
tion. In the context of the thermocyclized carbon framework,
the more active Ni centers can be useful for catalysis, and the
pyrazolate−Ni bonds also feature closer orbital overlap and
1
7,18
7
Farha,
and more recently by Zhou.
The Zr(IV)-carboxylate coordination links are, however,
highly ionic and often exhibit structural lability leading to
contraction or partial collapse of the host framework (e.g.,
1
,19−23
when the solvent molecules are evacuated).
Such
Received: September 5, 2020
structural lability hampers the thermocyclization studies,
because it can degrade the structural order of the coordination
scaffold and that of the thermocyclized product, thereby
complicating characterization. The framework contraction/
collapse can also significantly reduce the accessible surface area
in the solid state, and limit the use as porous materials.
©
XXXX American Chemical Society
https://dx.doi.org/10.1021/acs.inorgchem.0c02662
A
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Article
electronic coupling to promote, for example, electrocatalytic
performance.
To implement cascade cyclization in the Ni -pyrazolate
8
system, we design the cruciform molecule L1 (Figure 1), which
Figure 2. PXRD patterns (Cu Kα, λ = 1.5418 Å) for NiL1: (a)
calculated from the crystal structure model based on the fcu topology;
(
b) measured from an as-made sample (inset: a photograph of an as-
made sample of NiL1); (c) from an activated sample; (d) from an as-
made sample heated at 300 °C for 2 h, i.e., NiL1-300 (inset: a
photograph of NiL1-300).
interference in the Sonogashira coupling reaction. The
solubilizing BOC group also makes for easier purification,
and its in situ demasking in the solvothermal synthesis
promotes the product’s crystallinity. Powder X-ray diffraction
(
PXRD) reveals a cubic lattice (a = 32.366 Å; Figure 2b;
details of refinement are included in the Supporting
Information (SI)). A crystal structure can be modeled based
on the secondary building block of [Ni (OH) (H O) ]
Figure 1. (A) The cruciform molecule Boc L1 (featuring four alkyne
8
4
2
2
2
arms). (B) A Ni (OH) (H O) (abbreviated as Ni O ) cluster with
(Figure 1B), and the resultant network is isoreticular to a
8
4
2
2
8
6
24 24
1
2 associated pyrazolate units. (C) View of an octahedral unit in an
Ni(II)-MOF reported by the groups of Bordiga, Galli, and
35
idealized model (i.e., assuming full linker occupancy) of the fcu net of
NiL1 along the a axis. H atoms were omitted for clarity. Red spheres,
O; gray, C; blue, N; green; Ni-based polyhedral.
Li which features the fcu topology (with an fcc array of the
Ni clusters).
8
Elemental analysis found C (47.73%), H (3.70%), N
(
6.17%), and S (6.83%) for an activated sample of NiL1,
features two pyrazolate units in a linear array, and two pairs of
contiguous alkynyl units on the central benzenoid ring for
cyclization in the prospective MOF scaffold. As a handle for
functionalization, we also attached the thioether donors to the
two side arms. We will describe the porous cubic Ni O -L1
with a fitting formula determined to be Ni (OH) (H O) -
(L1) (CH COO) (H O) , which gives a calculated profile of
C (48.14%), H (4.11%), N (5.91%), and S (6.76%). The
presence of the acetate (CH COO ) component is also
8
4
2
2
3
3
6
2
15
−
3
verified by dissolving the solid sample in DCl/D O for solution
8
6
2
framework product (denoted as NiL1), which maintains the
crystalline order even when heated to 300 °C for effecting the
cyclization of the vicinal alkyne units. The thermally induced
partial graphitization expands the polycyclic conjugated system
throughout the solid state and enhances electroactivity as is
reflected in the hydrogen evolution reaction studies. Also
noted is the discovery of substantial linker deficiency in Ni O -
NMR measurement (Figure S2). Compared with the formula
Ni (OH) (H O) (L1) dictated by the idealized fcu net, in
8
4
2
2
6
which the Ni cluster is connected to 12 pyrazolate units (as
8
proffered by the 6 ditopic L1 linkers), the determined formula
indicates that about half of the L1 sites are vacant (e.g.,
−
replaced by CH COO ) in the current sample. Comparably,
3
motifs of Ni-O clusters featuring mixed carboxyl/pyrazolate
8
6
34,36
L1, leaving a significant fraction of the Ni(II) sites as
ligands are known.
Thermogravimetric analysis (TGA,
32,33
potentially open metal sites
Lewis acids).
for functional uses (e.g., as
conducted in air; Figure S3) reveals a steep weight loss of
about 60% between 310 and 410 °C, which can be ascribed to
linker decomposition. PXRD on the residue features distinctly
the NiO phase (Figure S4). The residual weight found (25.4
wt %) is slightly higher than the NiO fraction (23.2%) in
Ni (OH) (H O) (L1) (CH COO) : the difference can be
The yellow, crystalline powder of NiL1 (see Figure 2 for a
photograph of the as-made sample of NiL1) was solvother-
mally prepared from a Boc L1 molecule (Figure 1A) and
2
Ni(OAc) ·4H O (e.g., with L1/Ni ratio = 1:2), using N,N-
2
2
8
4
2
2
3
3
6
dimethylacetamide and H O as solvent (water also helps
accounted for by some amorphous constituent, e.g., as revealed
by the diffuse hump at 2θ = 25° in the PXRD pattern of Figure
S4. The linker deficiency in NiL1 is likely due to the steric
2
deprotect the t-butyl dicarbonate). The t-butoxycarbonyl
group (BOC) masks the pyrazole donors and reduces their
B
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interference from the long and rigid alkyne side arms, which
PXRD pattern. The enhanced base stability of NiL1-300 can
be ascribed to the inter-linker covalent links (e.g., the aryl−S−
aryl bonds) formed in the vigorous thermal treatment process.
NiL1 and NiL1-300 are also stable to HCl solutions of pH = 3,
but at higher acidity (e.g., 0.5 M H SO or pH = 1 HCl), some
degradation can occur, as evidenced by the PXRD peak
broadening after soaking for several hours (Figure S11).
make it harder to fit the bulky linkers around the Ni O cluster.
8
6
The as-made NiL1 solid was heated under an argon flow
e.g., 109 mg; for 2 h) at various temperatures. PXRD indicates
(
that its crystalline order can withstand up to 300 °C (PXRD
pattern d in Figure 2; with the product, ca. 89 mg, being
denoted as NiL1-300), whereas higher temperatures (e.g., 350
2
4
°
C; see Figure S5 for the PXRD patterns) result in amorphous
Similar acid sensitivity was reported of other Ni -pyrazolate
8
25,35,37
products. Unlike the yellow as-made NiL1, NiL1-300 is dark
brown (see insets of Figure 2 for the photographs; see Figure
S6 for the diffuse reflectance spectra). The IR spectra indicate
that the distinct alkyne stretching at 2205 cm− of NiL1
completely disappeared in the thermally treated sample of
frameworks.
So exposure to acids should be minimized
(e.g., < a few minutes, as in the electrocatalytic tests below) in
order to retain the MOF solids in the pristine states (Figure
S11).
The activated NiL1 and NiL1-300 solids are also stable in
air and vacuum and can be conveniently used for gas sorption
1
−
1
NiL1-300 (Figure 3). In addition, the C−H peak at 2923 cm
studies. The N adsorption−desorption isotherms collected at
2
7
7 K (Figures S12−S15) exhibit typical type-I characteristics,
with the corresponding BET surface areas of NiL1 and NiL1-
3
2
00 being 465 and 340 m /g, respectively. Perhaps, the larger
surface area of NiL1 here arises from the extended side arms
that offer additional contact with the sorbate molecules
(
whereas in NiL1-300, the side branches are fused with the
backbone). The measured pore volume of NiL1-300 is 0.829
3
3
cm /g, greater than the 0.611 cm /g of NiL1, which can also
be rationalized by the fact that merging of the side arms
empties out more space in the framework matrix. In a
preliminary two-probe measurement, the conductivity of NiL1-
−
5
3
00 exhibits a higher conductivity (1.08 × 10 S/m) than
−
7
NiL1 (7.94 × 10 S/m), which is consistent with the
enhanced π-conjugation and electron delocalization in the
thermally cyclized NiL1-300 framework.
Nickel, coupled with graphene, amorphous carbon or other
carbon substrates, is often used for the electrocatalysis of the
38−42
hydrogen evolution reaction (HER).
The electrocatalytic
performance of NiL1 and NiL1-300 as HER catalysts was
carried out by the method of rotating disk electrode in a three-
electrode system (details can be seen in SI). In a 0.5 M H SO
Figure 3. FT-IR spectra for samples of (a) activated NiL1 and (b)
NiL1-300.
2
4
solution, the NiL1 electrode gave an η10 of 891 mV, and a
Tafel slope of 258 mV/dec (Figure 4). Compared to NiL1, the
thermocyclized sample of NiL1-300 exhibits a smaller
overpotential of 698 mV at the current density of 10 mA/
associated with the CH S groups of NiL1 was also greatly
3
weakened in NiL1-300, indicating the cleavage of the CH −S
3
bond, e.g., to form aryl−S−aryl bonds, and to emit CH SCH
3
3
2
and other small molecules (see Figure S7 for a proposed
reaction scheme).
cm and a Tafel slope of 158 mV/dec, which is consistent with
the enhanced charge transport in NiL1-300. As shown in
Figure S19, the η10 overpotential increases along the time
points of 0, 1, 3, 6, and 12 h, which parallels the progressive
sample change observed in the PXRD patterns (Figure S11).
Notice that the electrocatalytic run lasts only a few minutes,
and the initial (time = 0 in Figure S19) data ought to be closely
reflective of the original samples of NiL1 and NiL1-300,
therefore allowing for a comparison of the electrocatalytic
performance between NiL1 (before thermocyclization) and
To further characterize the emitted molecules, a sample of
NiL1 (activated by Soxhlet extraction at 90 °C with methanol
for 24 h) was heated in a sealed tube at 300 °C for 2 h, after
which CDCl was then added into the tube prefrozen by liquid
3
nitrogen to collect the soluble products for NMR analysis. The
1
H NMR spectrum (Figure S8) indicates the formation of
CH SCH , and (methylthio)benzene which can be rationalized
3
3
as being split from the thermocyclizing linker molecules.
1
Curiously though, the H NMR spectrum also features a strong
NiL1-300 (after thermocyclization at 300 °C). For practical H
2
peak for benzene (δ = 7.36 ppm), whose origin remains to be
further elucidated. As shown in Figure S9, analysis of exhaust
by thermogravimetric, gas chromatography, and mass spec-
trometry (TG-GC-MS) coupling also confirms the eluents of
CH SCH , and (methylthio)benzene, as well as benzene.
production research, however, improvement of the sample
stability to acids is needed.
For better catalytic performance, Pd(II) was loaded to form
NiL1-300-Pd by soaking the NiL1-300 solid in a CH CN
3
solution of PdCl . PXRD (Figure S10) of the resultant NiL1-
3
3
2
NiL1 and NiL1-300 exhibit comparable stability in boiling
water, and in acid and base solutions, as shown by the PXRD
patterns in Figures S10 and S11. Notably, the crystallinity of
both remains intact after immersion in 10 M NaOH for 24 h.
Treatment by 15 M NaOH (for 24 h) does not affect the
crystallinity of the thermocyclized sample of NiL1-300, but
degrades that of NiL1, as seen in the weaker peaks in the
300-Pd indicates retention of the crystalline order. Elemental
analysis by ICP-OES quantifies the Ni/Pd atomic ratio to be
17.3:1, indicating a small loading of Pd; nevertheless,
significantly enhanced HER performance was achieved, with
a much smaller overpotential of 383 mV at the current density
of 10 mA/cm and a Tafel slope of 137 mV/dec. SEM
elemental mapping indicates palladium to be uniformly
2
C
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Article
AUTHOR INFORMATION
■
Corresponding Authors
Jun He − School of Chemical Engineering and Light Industry,
Guangdong University of Technology, Guangzhou 510006,
Guangdong, People’s Republic of China; orcid.org/0000-
0
001-7062-4001; Phone: 86 138 2211 8686;
Email: junhe@gdut.edu.cn
Zhengtao Xu − Department of Chemistry, City University of
Hong Kong, Kowloon, Hong Kong, People’s Republic of
China; orcid.org/0000-0002-7408-4951; Phone: 852
34424679; Email: zhengtao@ cityu.edu.hk
Authors
Jieying Hu − School of Chemical Engineering and Light
Industry, Guangdong University of Technology, Guangzhou
10006, Guangdong, People’s Republic of China
5
Xiangling Deng − School of Chemical Engineering and Light
Industry, Guangdong University of Technology, Guangzhou
510006, Guangdong, People’s Republic of China
Hu Zhang − School of Chemical Engineering and Light
Industry, Guangdong University of Technology, Guangzhou
510006, Guangdong, People’s Republic of China
Yingxue Diao − Department of Chemistry, City University of
Hong Kong, Kowloon, Hong Kong, People’s Republic of
China
Shengxian Cheng − Department of Chemistry, City University
of Hong Kong, Kowloon, Hong Kong, People’s Republic of
China
Sai-Li Zheng − School of Chemical Engineering and Light
Industry, Guangdong University of Technology, Guangzhou
510006, Guangdong, People’s Republic of China
Wei-Ming Liao − School of Chemical Engineering and Light
Figure 4. (a) Linear sweep voltammetry (LSV) curves of HER for
NiL1, NiL1-300, NiL1-300-Pd, and NiL1-300-Pd/CB samples
dispersed on GCE in N -saturated 0.5 M H SO electrolyte. (b)
Industry, Guangdong University of Technology, Guangzhou
510006, Guangdong, People’s Republic of China
2
2
4
Tafel plots derived from HER polarization curves in (a).
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02662
Notes
dispersed in NiL1-300-Pd (Figure S20), while XPS spectra of
The authors declare no competing financial interest.
the Pd 3d region reveals only Pd2 (i.e., no Pd ) in NiL1-300-
Pd before and after the electrochemical tests (Figure S21).
Finally, carbon black was ground with NiL1-300-Pd to
enhance the conductivity in order to further improve the HER
performance. In 0.5 M H SO electrolyte, the NiL1-300-Pd/
+
0
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (21871061, 21901046), Local Innovative
and Research Teams Project of Guangdong Pearl River Talents
Program (2017BT01Z032), Science and Technology Planning
Project of Guangdong Province (2017A050506051), and
Science and Technology Program of Guangzhou (20180701-
0026). Z.X. acknowledges a GRF grant from the Research
Grants Council of HKSAR (CityU 11306018) and an SRG
grant of the City University of Hong Kong (Project 7004820).
2
4
carbon blend successfully reduced the overpotential down to
2
2
93 mV at the current density of 10 mA/cm and a Tafel slope
of 97 mV/dec, thus achieving HER activity comparable with
that of some of the topical nickel/palladium-enabled materials
2
(
e.g., the overpotential at 10 mA/cm and Tafel slope of
THTA-Ni single layer: 315 mV, 76 mV/dec; Ni@C N: 450
2
43−47
mV, 165 mV/dec; Pd@C2N: 289 mV, 188 mV/dec).
The thermal treatment of NiL1 as well as the metal loading
steps can likely be further modulated for potentially better
electrocatalytic properties.
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sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02662.
■
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