Inorganic Chemistry
Article
FTIR data were collected on a PerkinElmer Frontier FT-IR in the
400−650 cm−1 range. Electrochemical measurements were recorded
on a Metrohm Autolab PGSTAT302N potentiostat using a standard
electrode configuration: a glassy-carbon working electrode, a Pt-mesh
counter electrode, and a saturated Ag/AgCl reference electrode.
Tetrabutylammonium hexafluorophosphate was used as the support-
ing electrolyte. Relevant single-crystal X-ray crystallographic data and
Supporting Information.
conversion of the anthraquinone/anthrahydroquinone redox
pair. Attempts with stronger reductants such as ascorbic acid
and sodium dithionite were carried out but led to framework
degradation under the investigated conditions.
Hydrogen Peroxide Production via Autoxidation of
DPAHq. In analogy to the anthraquinone process, we
investigated if a similar reactivity would be observed during
the thermal oxidation of dipyridyl-substituted anthraquinone 1.
To qualitatively determine the formation of H2O2, low-
concentration peroxide strips were used. A small sample of 1
was oxidized at 100 °C and extracted with water and the
solution separated by filtration. Immersion of a strip led
immediately to a color change to greenish blue (see the
quantify the amount of hydrogen peroxide, we used a modified
FOX colorimetric assay.20 The production of H2O2 was
achieved with 37% yield, and the MOF material did not
degrade during the process within one cycle (see the
could neither be oxidized nor generate any peroxide under the
same conditions or even at higher temperatures, once again
supporting the role of hydrogen bonding and structural
features toward the reactivity. This facile and practical
switching process highlights the potential of redox-active
porous materials for hydrogen peroxide production.
2,6-Di(pyridin-4-yl)-9,10-anthraquinone (DPAq). 2,6-Dibro-
moanthraquinone (366 mg, 1.0 mmol), K3PO4 (638 mg, 3.0 mmol),
and 4-pyridinylboronic acid (271 mg, 2.2 mmol) were weighed out
into a Schlenk pressure tube. The flask was evacuated (residual
pressure ∼10−3 mbar) and backfilled with argon three times. A 20 mL
portion of 1,4-dioxane was then added, and the suspension was
saturated with argon for at least 45 min. Pd(OAc)2 (22.5 mg, 0.1
mmol) and dppf (110 mg, 0.2 mmol) were thereafter added in one
portion under positive argon pressure, and the mixture was heated to
reflux for 2 days. After completion of the reaction, the resulting brown
suspension was left to cool and was diluted with EtOAc (30 mL). The
precipitate was separated by vacuum filtration and washed with
EtOAc (50 mL) and excess water. The desired product was
recrystallized from DMF and obtained as pale beige needles. For
elemental analysis, a small sample was dried under high vacuum (225
mg, 62% yield). 1H NMR (400 MHz, CDCl3): δ 8.79 (d, 4H, 3JH−H
=
6 Hz) 8.63 (d, 2H, 4JH−H = 1.6 Hz) 8.49 (d, 2H, 3JH−H = 8 Hz) 8.11
3,4
3
(dd, 2H,
J
= 8, 1.6 Hz) 7.67 (d, 4H, JH−H = 6 Hz) ppm.
H−H
Selected IR bands: ν 1672, 1590, 1396, 1314, 1273, 1244, 1170, 957,
814 cm−1. UV−vis (CH2Cl2): λ 276, 341 nm. Anal. Calcd: C, 79.55;
H, 3.89; N, 7.73. Found: C, 79.73; H, 3.83; N, 7.70.
CONCLUSION
■
2,6-Di(pyridin-4-yl)-9,10-anthrahydroquinone (DPAHq).
DPAq (110 mg, 0.3 mmol) was suspended in 10 mL of a 4/1
DMF/H2O mixture. AlCl3 (0.4 mg, 0.003 mmol) was added, and the
resulting mixture was heated to 100 °C for 18 h. The precipitate was
collected by filtration, washed with DMF, water, and EtOH, and then
air-dried and isolated as a glittering dark wine red solid (95 mg, 87%
yield). For elemental analysis, a small sample was dried under high
vacuum. 1H NMR (400 MHz, DMSO-d6, F3CCOOH): δ 9.15 (d, 2H,
This work comprises an approach to investigate the reversible
redox chemistry of the anthraquinone/anthrahydroquinone
redox pair in the solid state. A bipyridine-like linker, DPAq,
was chosen for the construction of pillar-layered MOFs based
on Zn and Cd. The quinone moiety of DPAq was shown to be
reducible under solvothermal conditions to the corresponding
hydroquinone, DPAHq, yielding the first crystal structure of an
anthrahydroquinone with unsubstituted hydroxy groups. The
hydrogen-bonded crystalline structure of DPAHq stabilizes the
phenolic groups in the solid state. Following the successful
ligand synthesis, the selective incorporation of both DPAq and
DPAHq into crystalline frameworks was performed using
different conditions. The choice of metal source and solvent
mixture proved to be able to control the oxidation state of the
ligand and its arrangement in the solid state. Finally, one of the
isolated compounds was shown to be reversibly redox
switchable using a facile thermal treatment/soaking procedure.
This process was used to produce hydrogen peroxide via
autoxidation.
4JH−H = 1.6 Hz) 9.03 (d, 4H, JH−H = 6.8 Hz) 8.62 (m, 6H) 8.07 (dd,
3,4
2H,
J
= 9.2, 1.6 Hz) ppm. IR: ν 3000 (broad), 1617, 1597,
H−H
1418, 1385, 1308, 1207, 1147, 1060, 841, 806 cm−1. Anal. Calcd: C,
79.11; H, 4.43; N, 7.69. Found: C, 79.34; H, 4.35; N, 7.65.
[Zn2(BDC)2(DPAHq)2]n·4DMF (1). Zn(NO3)2·6H2O (36.0 mg,
0.12 mmol), 1,4-benzenedicarboxylic acid (9.6 mg, 0.06 mmol), and
DPAq (10.8 mg, 0.03 mmol) were suspended in 7.5 mL of a 4/1
DMF/H2O mixture and heated at 100 °C for 18 h. The precipitate
was separated by filtration while hot to prevent recrystallization of the
linker and was washed with DMF and EtOH. The desired phase was
separated from colorless impurities by flotation using a CHCl3/
CHBr3 mixture and washed again with EtOH. The phase was
obtained as dark wine red crystals and was stored under a saturated
DMF atmosphere. Anal. Calcd: C, 61.67; H, 4.63; N, 7.57. Found: C,
60.88; H, 4.86; N, 7.81.
EXPERIMENTAL SECTION
■
[Zn2(BDC)2(DPAq)]n·DMA (2). Zn(NO3)2·6H2O (12.0 mg, 0.04
mmol), 1,4-benzenedicarboxylic acid (3.2 mg, 0.02 mmol), and DPAq
(3.6 mg, 0.01 mmol) were suspended in 2.0 mL DMA and heated at
100 °C for 18 h. The precipitate was separated by filtration while hot
to prevent recrystallization of the linker and was washed with DMA
and EtOH. The phase was isolated as a glittering tan yellow powder.
Anal. Calcd: C, 58.15; H, 3.41; N, 4.62. Found: C, 53.94; H, 4.02; N,
5.07.
[Cd2(BDC)2(DPAq)2]n·DMA (3). Cd(NO3)2·4H2O (12.0 mg, 0.04
mmol), 1,4-benzenedicarboxylic acid (3.2 mg, 0.02 mmol), and DPAq
(3.6 mg, 0.01 mmol) were suspended in 2.5 mL of a 4/1 DMA/H2O
mixture and heated at 100 °C for 18 h. The precipitate was separated
by filtration while hot to prevent recrystallization of the linker and was
washed with DMA and EtOH. The phase was isolated as dark yellow
crystals. Anal. Calcd: C, 59.83; H, 3.30; N, 5.13, Found: C, 57.51; H,
3.67; N, 5.47.
All reagents were obtained from commercial sources and used as
received, unless otherwise stated. Deuterated solvents were purchased
from Sigma-Aldrich. 2,6-Dibromoanthraquinone was synthesized
according to a previously reported literature procedure.21
1
General Procedures. Solution-state H NMR spectra of ligands
and digested MOFs were recorded on a Bruker UltraShield 400 MHz
spectrometer at 298 K. Residual solvent peaks (CDCl3, δ 7.26 ppm;
DMSO-d6, δ 2.50 ppm) were used as internal references for chemical
shifts. Elemental analysis was performed at the Microanalytical
Laboratory of the Catalysis Research Center−Technical University of
Munich. PXRD data were collected on a PANanalytical Empyrean X-
ray diffractometer (Cu Kα, λ = 1.5406 Å) or on a Rigaku MiniFlex
benchtop X-ray diffractometer. TGA measurements were performed
on a Mettler Toledo TGA/DSC 3+ instrument at a 5 K min−1 heating
rate, under a constant stream of synthetic air. Solution-state UV−vis
spectra were recorded on an Agilent Cary 60 UV−vis spectrometer.
4680
Inorg. Chem. 2021, 60, 4676−4682