Four new dual-functional electro-catalysts formed from small molybdenum clusters and Cu-pyridyl complexes

Article abstract of DOI:10.1039/c9dt03560e

By changing N-heterocyclic ligands in the same Mo₇/Cu/N-ligand reaction systems, four new organic-inorganic hybrids based on isopolymolybdates, [Cu₂(tpy)₂(β-Mo₈O₂₆)_0.5(γ-Mo₈O₂₆)_0.5]·0.25H₂O (1), [Cu₂(tpy)₂(H₂O)₂(β-Mo₈O₂₆)] (2), [Cu(bpy)(Mo₃O₁₀)]·H₂O (3), and [Cu(bpy)(H₂O)(β-Mo₈O₂₆)_0.5]_0.5 (4) (tpy = 2,2′:6′,2′′-Terpyridine and bpy = 2,6-bis(pyrazol-1-yl) pyridine), were prepared using hydrothermal methods at different pH values. X-ray structural analysis shows that compound 1 has a 1D {-β-[Mo₈O₂₆]-Cu₂-γ-[Mo₈O₂₆]}_n straight chain structure with mixed β-[Mo₈O₂₆] and γ-[Mo₈O₂₆] polyoxoanions; compound 2 possesses a 3D supramolecular structure based on [Cu(tpy)]²⁺ motifs and β-[Mo₈O₂₆] clusters; and compound 3 has a 1D chain structure built from [Cu(bpy)]²⁺ and [Mo₃O₁₀]^2- units. In compound 4, [β-Mo₈O₂₆] clusters are linked by [Cu(bpy)]²⁺ motifs to give rise to a 2D sheet structure including {(β-Mo₈O₂₆)₄Cu₄} 8-membered rings. Cyclic voltammograms of compounds 1-4 display discrepant dual-functional electro-catalytic activities toward the reduction of nitrite and the oxidation of ascorbic acid in acidic solution. Electrocatalytic tests indicate that the [Mo₃O₁₀]^2--based organic-inorganic hybrid exhibits better electro-catalytic performances than [Mo₈O₂₆]^4--Type hybrids towards oxidation and reduction.

Full text of DOI:10.1039/c9dt03560e

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Four new dual-functional electro-catalysts formed
from small molybdenum clusters and Cu-pyridyl
complexes†
Cite this: Dalton Trans., 2019, 48,
16350
Wanli Zhou, *a Ping Liu,a Yanping Zheng,a Xuekun Liu,a Yong Zhang,a Gang Yuana
and Jun Peng*b
By changing N-heterocyclic ligands in the same Mo7/Cu/N-ligand reaction systems, four new organic–
inorganic hybrids based on isopolymolybdates, [Cu2(tpy)2(β-Mo8O26 0.5
)
(γ-Mo8O26 0.5
)
]·0.25H2O (1),
[Cu2(tpy)2(H2O)2(β-Mo8O26)] (2), [Cu(bpy)(Mo3O10)]·H2O (3), and [Cu(bpy)(H2O)(β-Mo8O26 0.5 0.5
)
]
(4) (tpy =
2,2’:6’,2’’-terpyridine and bpy = 2,6-bis(pyrazol-1-yl) pyridine), were prepared using hydrothermal
methods at different pH values. X-ray structural analysis shows that compound 1 has a 1D {-β-[Mo8O26]-
Cu2-γ-[Mo8O26]}n straight chain structure with mixed β-[Mo8O26] and γ-[Mo8O26] polyoxoanions; com-
pound 2 possesses a 3D supramolecular structure based on [Cu(tpy)]2+ motifs and β-[Mo8O26] clusters;
2−
and compound 3 has a 1D chain structure built from [Cu(bpy)]2+ and [Mo3O10
]
units. In compound 4,
[β-Mo8O26] clusters are linked by [Cu(bpy)]2+ motifs to give rise to a 2D sheet structure including
{(β-Mo8O26)4Cu4} 8-membered rings. Cyclic voltammograms of compounds 1–4 display discrepant
dual-functional electro-catalytic activities toward the reduction of nitrite and the oxidation of ascorbic
Received 3rd September 2019,
Accepted 6th October 2019
acid in acidic solution. Electrocatalytic tests indicate that the [Mo3O10
]
2−-based organic–inorganic
]
4−-type hybrids towards oxidation and
DOI: 10.1039/c9dt03560e
hybrid exhibits better electro-catalytic performances than [Mo8O26
reduction.
rsc.li/dalton
obtain a carboxylate-coordinated {Mo142} Mo-blue nanoring in
Introduction
the presence of carboxylic acids,3 and so on. Small Mo clusters
4−
Isopolymolybdates are important members of the polyoxome- include octamolybdates (α, β, γ, δ, ε, ζ, η and θ-type [Mo8O26
]
talate (POM) family, and they have been the focus of materials clusters), Mo6 clusters,4 and the like. Cu is also an important
scientists in recent decades to build multifunctional com- d10 transition metal owing to its widespread application in cat-
plexes because of their tailored structures and broad range of alysis for organic synthesis. For instance, S. L. Buchwald et al.
applications in medicine, chiral materials, lithium-ion bat- prepared a Cu-based catalyst for the X-arylation (X = O, N) of
teries, electro-catalysts, photo-catalysts, supercapacitors, and aminophenols;5 Shi et al. reported the cuprous catalyzed dia-
so on.1 Isopolymolybdates can be divided into two types of mination of conjugated dienes;6 A. Leyva-Pérez et al. reported
building units in reported POM materials: huge Mo and small C–X (X = N, C, O, S, P) bond-forming reactions through Cu
Mo clusters, for example, {Mo132} type Keplerate clusters are clusters within a polymeric film;7 Zhang et al. reported asym-
first reported by Müller et al., using medium ratios of [MoV]/ metric nitrone synthesis by using ligand-enabled copper-cata-
[MoVI] atoms present in ammonium molybdate;2 T. Yamase lyzed hydroamination of cyclopropene,8 and so on. A feasible
6−
carried out the photoreductive self-assembly of [Mo7O24
]
to strategy to prepare organic–inorganic hybrids is to utilize
small Mo clusters/mixed Mo clusters and transition metals
linked by flexible N-donor ligands as building units, through
aFaculty of Chemistry, Tonghua Normal University, Tonghua, Jilin 134002,
PR China. E-mail: zhouwl222@nenu.edu.cn
a
one-pot method,9 as these kinds of hybrids show
bKey Laboratory of Polyoxometalate Science of Ministry of Education,
Faculty of Chemistry, Northeast Normal University, Changchun, Jilin 130024,
PR China. E-mail: jpeng@nenu.edu.cn
concerted catalysis including fragments of isopolymolybdates
and Cu–organic complexes. In this work, we select
(NH4)6Mo7O24·4H2O, hydrated copper nitrate, and tpy/bpy
as starting materials to generate small Mo cluster-
based organic–inorganic hybrid compounds. Four new
compounds, [Cu2(tpy)2(β-Mo8O26)0.5(γ-Mo8O26)0.5]·0.25H2O (1),
[Cu2(tpy)2(H2O)2(β-Mo8O26)] (2), [Cu(bpy)(Mo3O10)]·H2O (3),
†Electronic supplementary information (ESI) available. Crystal data and struc-
ture refinement for compounds 1–4. Selected bond lengths and angles, IR
spectra, TG curves, PXRD patterns, CVs and CAT% data obtained for compounds
1–4. CCDC 1950999–1951002. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/C9DT03560E
16350 | Dalton Trans., 2019, 48, 16350–16357
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and [Cu(bpy)(H2O)(β-Mo8O26 0.5 0.5
)
]
(4), were obtained by the HCl (2 M), and the resulting suspension was sealed in a 25 ml-
hydrothermal method. The electrochemistry and electro-cata- reaction still and heated at 433 K for 5 days. Blue crystals were
lytic properties of compounds 1–4 have also been studied by obtained (0.0230 g, yield 4.65% based on Mo). Elemental ana-
cyclic voltammetry.
lysis for C30H22.50N6O26.25Cu2Mo8: found (calcd) (%): Mo 42.80
(43.08), Cu 7.00 (7.13), C 20.00 (20.21), H 1.35 (1.26), N 4.50
(4.72). IR (cm−1): 3450 (s), 3123 (w) 1586 (s), 1509 (s), 1315 (s),
1252 (s), 1134 (m), 1078 (s), 940 (s), 890 (s), 850 (s), 703 (s)
640 (s) 703 (w).
Experimental
All the reagents purchased were of reagent grade and used
without further purification. Elemental analyses of C, H and N
were performed on a Perkin-Elmer 2400 CHN elemental ana-
lyzer. Elemental analyses of Cu and Mo were performed on
a Leeman inductively coupled plasma (ICP) spectrometer.
IR spectra were recorded on a Nicolet 6700 FTIR spectro-
[Cu2(tpy)2(H2O)2(β-Mo8O26)] (2)
Compound 2 was prepared by a method similar to that of com-
pound 1, except that pH was adjusted to 4.10. Blue crystals
were harvested (0.0200 g, yield 7.95% based on Mo). Elemental
analysis for C30H26N6O28Cu2Mo8: found (calcd) (%): Mo 41.70
(42.33), Cu 6.50 (7.00), C 20.00 (19.85), H 1.30 (1.43), N 4.50
(4.63). IR (cm−1): 3560 (s), 1605 (s), 1475 (s), 1328 (s), 1088 (m),
943 (s), 882 (s), 720 (s), 550 (m).
meter with KBr pellets in the range of 4000–400 cm−1
.
Thermogravimetric (TG) analyses were carried out in a N2
environment on a Diamond TG analyzer from 50 °C to 1000 °C
at a rate of 10 °C min−1. Powder X-Ray Diffraction (PXRD) was
performed on a Bruker D2 Phaser diffractometer equipped [Cu(bpy)(Mo3O10)]·H2O (3)
with a monochromatic Cu-Kα (λ = 1.5418 Å) radiation source in
A mixture of (NH4)6Mo7O24·4H2O (0.1480 g), Cu(NO3)2·6H2O
the range of 5 ≤ 2θ ≤ 50, at a scanning rate of 4° s−1. Field-
emission scanning electron microscopy (FE-SEM) was per-
formed to investigate the crystal surface by using a Hitachi
SU-8010 FE-SEM. Cyclic voltammograms were obtained with a
CHI-660E electro-chemical workstation at ambient tempera-
ture, with a platinum wire as the counter electrode, an Ag/AgCl
electrode as the reference electrode, and the carbon paste elec-
trode (CPE) modified with compounds 1–4 as working electro-
des. The electrochemical and electro-catalytic properties of
1–4-CPEs were studied in a 1 M H2SO4 + Na2SO4 supporting
electrolyte (pH = 1) through CV measurements in the potential
(0.1300 g) and bpy (0.0450 g) was dissolved in 7 mL of distilled
water and stirred for 1 h. The pH of the solution was adjusted
to 4.50 with HCl (2 M), and the resulting suspension was
sealed in a 25 ml-reaction still and heated at 433 K for 5 days.
Green crystals were harvested (0.0020 g, yield 5.22% based on
Mo). Elemental analysis for C11H11N5O11CuMo3: found (calcd)
(%): Mo 37.60 (38.86), Cu 9.00 (8.57), C 18.00 (17.82), H 1.40
(1.49), N 9.50 (9.45). IR (cm−1): 3490 (s), 3122 (m), 1610 (s),
1571 (s), 1490 (s), 1405 (s), 1349 (s), 1216 (m), 1167 (m),
1050 (s), 978 (s), 917 (s), 882 (s), 790 (s), 650 (s), 527 (s).
range from +300 to −1000 mV at a scan rate of 0.1 V s−1
.
[Cu(bpy)(H2O)(β-Mo8O26)0.5 0.5 (4)
]
The n-CPEs (n = 1–4) were processed as follows: 0.01 g of
compounds 1–4 and 0.10 g of carbon dust were mixed and
ground together to form a homogeneous mixture, into which
two drops of paraffin oil was added with stirring. The such-
obtained paste was packed into a glass tube with 2.0 mm inner
diameter, and the surface was wiped with smooth paper.
Electrical contact was established with a thin copper rod
through the back of the electrode.
The same synthetic method as for 3 was used except for pH =
4.70. Blue crystals were obtained (0.0220 g, yield 9.02% based
on Mo). Elemental analysis for C11H9N5O14CuMo4: found
(calcd) (%): Mo 44.00 (43.48), Cu 7.00 (7.20), C 15.00 (14.96),
H 0.95 (1.02), N 8.00 (7.93). IR (cm−1): 3428 (m), 1620 (s),
1460 (m), 1397 (s), 1342 (s), 1167 (s), 1050 (s), 507 (s), 458 (m).
Selected good single crystals were fixed onto thin glass
fibers by epoxy glue for data collection. Diffraction data were
collected on a Bruker D8 Quest Eco CCD with Mo-Kα radiation
(λ = 0.71073 Å). The structures of the crystals were solved
through the direct method and refined by full-matrix least-
squares on F2. All calculations were performed using the
SHELX-2014/7 program package.10 All the Mo, Cu, O, C and N
atoms were anisotropically refined except for H atoms. Further
details of the X-ray structural analysis of compounds 1–4 are
given in Table S1† and the selected bond lengths and angles
are listed in Tables S2–S5.†
Results and discussion
Fig. 1 shows the SEM images of compounds 1–4, in which
block-like crystals represent the title compounds. Single-crystal
X-ray diffraction analyses indicate that compound 1 consists of
mixed Mo8 clusters (γ-Mo8O26 and β-Mo8O26) and two
[Cu(tpy)]2+ motifs with a 1D straight chain and crystallizes in
ˉ
the triclinic space group P1. As shown in Fig. 2, the asym-
metric unit of compound 1 contains two copper ions, two tpy
ligands, γ-Mo8O26, β-Mo8O26 clusters and one dissociative
water molecule. The γ-[Mo8O26]4− cluster consists of six dis-
torted {MoO6} octahedra and two distorted {MoO5} pentahedra
including four kinds of O atoms: two μ4-O, four μ3-O, six μ2-O
[Cu2(tpy)2(β-Mo8O26)0.5(γ-Mo8O26)0.5]·0.25H2O (1)
A mixture of (NH4)6Mo7O24·4H2O (0.1500 g), Cu(NO3)2·6H2O and fourteen Ot atoms.11 The β-[Mo8O26 4− anion included two
]
(0.1200 g) and tpy (0.0430 g) was dissolved in 7 mL of distilled centro-symmetric [Mo4O13]2− interleaving units linked via a
2−
water and stirred for 1 h. The pH was adjusted to 4.30 with bridging oxygen atom. Each [Mo4O13
]
unit was made up of
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Fig. 3 The1D straight chain of {-γ-Cu2Mo8O26- Cu-β-Mo8O26-}n in
compound 1 along the c-axis.
The 1D straight chain of compound 1 is featured by two
moieties: {γ-Cu2Mo8O26} clusters and the β-octamolybdate
4−
anion [Mo8O26
]
(Fig. 3). Firstly, [Cu1(tpy)]2+ and its crystallo-
graphic partners are linked by γ-Mo8O26 through Cu1–O1–Mo5
and Cu1–O2–Mo6 bridges to form {γ-Cu2Mo8O26} clusters.
Secondly, {γ-Cu2Mo8O26} clusters and β- Mo8O26 are linked by
[Cu2(tpy)]2+ alternately by using Cu2–O3–Mo1 and Cu2–O4–
Mo4 bonds leading to a 1D straight chain of {-γ-Cu2Mo8O26-
Cu-β-Mo8O26-}n.
Fig. 1 SEM images of compounds 1–4.
As shown in Fig. 4, the asymmetric unit of compound 2
consists of one Cu ion, a [β-Mo8O26]4− anion, a tpy ligand and
one crystallization water molecule. Cu1 is five-coordinated in a
square pyramidal geometry by three N atoms (N1, N2, and N3)
from one tpy ligand and two oxygen atoms (O1 and O1 W)
from β-Mo8O26 clusters and the water molecule. The distances/Å
for Cu1 are: Cu1–N1 = 2.021(8), Cu1–N2 = 1.932(8), Cu1–N3 =
2.015(8), Cu1–O1 W = 1.969(7), and Cu1–O1 = 2.227(6) and
the angles/° for Cu1 are: N1–Cu1–N2 = 80.3(3), N2–Cu1–N3 =
80.2(4), and N3–Cu1–O1 W = 97.1(3).
The 3D supramolecular structure of compound 2 stems
from two building units: [Cu1(tpy)]2+ and β-[Mo8O26]4− anions
(Fig. 5). Firstly, [Cu1(tpy)]2+ and its crystallographic partners
are linked by β-Mo8O26 through Cu1–O1–Mo2 bonds to form a
{β-Mo8O26Cu2} unit. Secondly, the adjacent {β-Mo8O26Cu2}
units are linked via O–H⋯O interactions to form a 3D supra-
molecular structure.
As shown in Fig. 6, the asymmetric unit of compound 3
consists of one Cu ion, a [Mo3O10]2− unit, a bpy ligand and
one water molecule. Cu1 is five-coordinated in a square pyra-
midal geometry by three N atoms (N1, N2, and N3) from one
Fig. 2 Ball/stick view of the asymmetric unit of compound 1. The
hydrogen atoms and crystallization water molecules are omitted for
clarity. Symmetry operation: #1 −x, 1−y, 1−z; #2 −1−x, −y, 2−z.
four {MoO6} edge-sharing octahedra. The oxygen atoms of the
β-[Mo8O26]4− anion could be divided into four groups accord-
ing to their bonding features, namely, Ot atoms with Mo–O
distances of 1.685–1.973 Å, μ2-O atoms with Mo–O distances of
1.751–2.290 Å, μ3-O atoms with Mo–O distances of
2.006–2.275 Å, and a μ5-O atom (O12) with a Mo–O distance of
2.290 Å. The average Mo–O distance of these four groups was
1.722, 2.057, 2.010 and 2.290 Å, respectively. The result showed
that these {MoO6} polyhedra were distorted octahedra. In the
asymmetric unit, there are two crystallographically indepen-
dent copper atoms (Cu1 and Cu2), which exhibit the same
coordination geometries. The two Cu atoms are five-co-
ordinated in a square pyramidal geometry by three N atoms
from a tpy ligand and two oxygen atoms from γ-Mo8O26 and
β-Mo8O26 clusters, completed by N1, N2, N3, O1, and O2
atoms for the Cu1 atom and N3, N4, N5, O3, and O4 atoms for
the Cu2 atom. The bond distances and angles around Cu1 are
1.85(3)–1.91(3)
Å (Cu1–N), 2.561 Å (Cu1–O1), 174.21(4)°
(N–Cu1–N), and 90.36–95.33° (N–Cu1–O1) and the bond distances
and angles around Cu2 are 1.82(3)–1.89(3) Å (Cu2–N), 2.785 Å
(Cu2–O2), 178.63° (N–Cu2–N), and 86.61–92.43° (N–Cu2–O1).
Fig. 4 Ball/stick view of the asymmetric unit of compound 2. The
hydrogen atoms and crystallization water molecules are omitted for
clarity. Symmetry operation: #1 2−x, −y, 1−z.
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Fig. 7 The infinite molybdenum oxide chain in compound 3.
Fig. 5 View of the 3D supramolecular structure of compound 2.
Fig. 8 Ball/stick view of the asymmetric unit of compound 4. The
hydrogen atoms and crystallization water molecules are omitted for
clarity. Symmetry operation: #1 x, 1.5 − y, 0.5 + z; #2 1 − x, 1 − y, 2 − z;
#3 1 − x, −0.5 + y, 1.5 − z.
Fig. 6 Ball/stick view of the asymmetric unit of compound 3. The
hydrogen atoms and crystallization water molecules are omitted for
clarity. Symmetry operation: #1 −1−x, 1−y, 1−z.
tances/Å for Cu1 are: Cu1–N1 = 1.951(16), Cu1–N2 = 2.019(19),
Cu1–N3 = 1.99(2), Cu1–O1 W = 1.949(15), Cu1–O1 = 2.341(16),
and Cu1–O2 = 2.532 and the angles/° for Cu1 are: O1 W–Cu1–
N1 = 169.0(7), O1 W–Cu1–N3 = 104.8(7), N1–Cu1–N3 = 78.9(8),
and N1–Cu1–N2 = 79.3(7). Bond valence sum calculations12 for
compounds 1–4 indicate that all the Mo atoms are in the +VI
oxidation state and copper atoms in the +II oxidation state.
The 2D sheet structure of compound 4 is featured by
two subunits: [Cu1(bpy)]2+ units and β-Mo8O26 clusters.
One bpy ligand is connected by Cu1 atoms coordinated to
three N atoms (N1, N2, and N3) to generate a [Cu1(bpy)]2+
group. The β-Mo8O26 anion acts as a four-connected inorganic
ligand to coordinate with Cu1 to form Mo–O–Cu1 bridges,
leading to a 2D sheet structure with {(β-Mo8O26)4Cu4} 8-mem-
bered rings (Fig. 9).
Generally, hydrothermal reaction systems are regarded as a
riddle, in which many conditions can affect reaction mecha-
nisms and conformations of final products. As shown in
Fig. 10, in this work, compounds 1–4 were obtained under the
same reaction conditions, except for organic ligands and pH:
tpy for compounds 1 and 2 and bpy for compounds 3 and 4.
Parallel experiments showed that pH and organic ligands are
crucial for the formation of the title compounds 1–4.
Compounds 1 and 2 with two different structures were
bpy ligand and two oxygen atoms (O1 and O2) from Mo1O6
and Mo2O6 clusters. The distances/Å for Cu1 are: Cu1–N1 =
2.013(3), Cu1–N2 = 1.987(3), Cu1–N3 = 1.948(3), Cu1–O1 =
1.893(3), and Cu1–O2 = 2.347(3) and the angles/° for Cu1 are:
O1–Cu1–N2 = 97.20(13), N2–Cu1–N3 = 79.06(14), and N1–Cu1–
O1 = 101.99(13). The [Mo3O10]2− cluster consists of three dis-
torted {MoO6} octahedra including two kinds of O atoms: four
μ3-O (O3, O6, O7 and O8) and six Ot (O1, O2, O4, O5, O9, and
O10). The bond lengths of Mo–O (1.701–2.280 Å) are in a
reasonable range.
As shown in Fig. S1,† two Mo atoms (Mo1 and Mo2) and
their crystallographic partners are linked by Mo3 through four
μ3-O (O3, O6, O7, and O8) to build a Mo5 cluster; the adjacent
Mo5 clusters are linked by Mo3’s partners to form an infinite
molybdenum oxide chain along the a-axis, and Cu1(bpy) and
its crystallographic partners are added to reinforce it through
Mo2–O1–Cu1 and Mo1–O2–Cu1 bonds (Fig. 7).
As shown in Fig. 8, the asymmetric unit of compound 4
contains one Cu ion, a [β-Mo8O26]
4− anion, the bpy ligand and
water molecules. The Cu1 ion is six-coordinated in a distorted
octahedron geometry defined by three N atoms (N1, N2, and
N3) from one bpy ligand, three O atoms (O1 and O2) from
β-Mo8O26 and one from the water molecule (O1 W). The dis-
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The IR spectra of compounds 1–4 are shown in Fig. S2.† In
the spectra, the characteristic bands at 645, 842, and 945 cm−1
for 1, 640, 855, and 922 cm−1 for 2, 655, 877, and 921 cm−1 for
3 and 665, 789, and 950 cm−1 for 4 are ascribed to ν(MovOt)
and ν(Mo–O–Mo) of the [Mo8O26]
4−/[Mo3O10]2− anion, respect-
ively. The bands in the regions of 1076–1623 cm−1 for 1,
1025–1602 cm−1 for 2, 1060–1614 cm−1 for 3 and 1177–1629
cm−1 for 4 can be assigned to the N-heterocyclic ligand. The
bands at around 3400 cm−1 are due to the water molecules.
The TG curves of 1–4 show roughly two steps of weight loss.
A total weight loss of ca. 28.0% in the range of 50–800 °C
corresponds to the loss of the water molecules and organic
ligands, in detail, 27.8% (calculated 27.0%) for compound 1,
24.6% (calculated 26.5%) for compound 2, 30.5% (calculated
30.9%) for compound 3, and 26.0% (calculated 24.8%) for
compound 4 (Fig. S3).†
Fig. 9 The 2D sheet structure of compound 4.
To indicate the phase purities of compounds 1–4, PXRD
experiments were carried out. As shown in Fig. S4,† the experi-
mental and simulated patterns of XRD for compounds 1–4 are
consistent, which shows that the phase purities of compounds
1–4 are good.
The energy dispersive spectra of compounds 1–4 are shown
in Fig. 11. EDX data were collected for the four compounds,
which indicated the presence of Mo, Cu, C, N and O at the
levels expected.
Fig. 10 Illustration of the structural features of compounds 1–4 gener-
ated at different pH values and using different organic ligands.
obtained in the same Cu/POM/tpy reaction process except for
different pH values. Compound 1 was obtained at a higher pH
value (4.30). When the pH value was decreased, a mixture of
compounds 1 and 2 was obtained. By further decreasing the
pH to 4.10, only compound 1 was isolated. The ligand tpy and
copper ions exhibit the same coordination modes in com-
pounds 1 and 2: two types of mixed Mo8 clusters (γ-Mo8O26
and β-Mo8O26) in 1 and only the β-Mo8O26 anion in 2; tpy acts
as a tridentate ligand coordinated to Cu2+ ions that are co-
ordinated to three N atoms in 1 and 2, and although the
copper ions in compounds 1 and 2 are all five-coordinated, the
coordinated atoms are different. Similar to 1 and 2, compound
4 was obtained at a higher pH value (4.70). When the pH value
was decreased to 4.50, compound 3 was obtained. Cu1 exhibits
different coordination modes, five-coordinated for 3 and six-
coordinated for 4. The connection mode of molybdenum
atoms in these two compounds is different, an infinite molyb-
denum oxide chain in 3 and β-Mo8O26 clusters in 4. It can be
concluded that different pH values not only influence the for-
mation of compounds 1–4, but also affect the types of small
molybdenum clusters as well as their final structures. Among
the organic N-donor ligands, tpy and bpy as rigid ligands
usually exhibit μ3 coordination modes to link transition metals
to give transition metal–organic complexes through three
neighbouring N-donors. The difference is that the alkalinity of
bpy is higher than tpy owing to diverse molecular groups,
three pyridine rings in tpy and one pyridine ring and two imid-
azole rings in bpy. We find that higher pH allows for obtaining
a higher dimensional structure, a 0D discrete structure for
compound 2, 1D chain structures for compounds 1 and 3; and
a 2D sheet structure for compound 4.
The cheap 1–4-CPEs were assembled to investigate their
electrochemical properties owing to the insolubility of the four
compounds in water and most organic solvents. The cyclic vol-
tammograms are shown in Fig. S5.† The midpoint potentials
(Em) of the redox couples I–I′, II–II′, III–III′ and IV–IV′, esti-
mated by using the two tangent method, are 0.13, −0.38, and
−0.62 V for 1-CPE; 0.12, −0.33, and −0.55 V for 2-CPE; 0.12,
0.01, −0.32, and −0.53 V for 3-CPE; and 0.10, −0.45, and−0.65
V for 4-CPE. The couple I–I′ can be attributed to the redox pro-
cesses of Cu(II)/Cu, and the couples II–II′, III–III′ and IV–IV′
can be derived from the redox processes of MoVI/MoV centers.
Electrocatalysis is an important application of POMs
because they store multiple protons/electrons during redox
processes. Nitrite (NO2−) and ascorbic acid (AA) are often
selected as probes of electrocatalytic redox reactions in the
electrocatalytic field.13 The electrocatalytic properties of com-
pounds 1–4 are measured by using CV measurements with
n-CPEs (n = 1–4) in the presence of nitrite and AA in a 1 M
Fig. 11 Energy dispersive spectrum from compounds 1–4.
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−
Fig. 12 Cyclic voltammograms showing the electrocatalytic reduction of NO2 by (a) 1-CPE, (b) 2-CPE, (c) 3-CPE and (d) 4-CPE in 1 M H2SO4
Na2SO4 solutions (pH = 1) with increase of concentration of NO2−, from top to bottom: 0, 2, 4, 6, 8 mM. Scan rate: 0.1 V s−1
+
.
H2SO4 + Na2SO4 supporting electrolyte (pH = 1), respectively of AA concentration is observed. The oxidation reactions of AA
(Fig. 12 and 13). are electro-catalyzed clearly by means of the redox couples
It can be easily found from Fig. 12 that a distinct change in CuII/Cu and MoVI/MoV. In Fig. 13c, it is interesting to note that
the reduction peak currents (II′, III′ and IV′) following the an unexpected change in the oxidation peak currents (II for 3-
addition of nitrite is observed and the corresponding oxidation CPE) is observed along with the increase of AA concentration
peak currents remained unchanged basically, showing that II′, at a potential of 100 mV, indicating electro-catalytic activity
III′ and IV′ cathodic peaks of n-CPEs (n = 1–4) indicate good towards the oxidation of AA ascribed to the MoVI-centre.
electrocatalytic activity for the reaction of nitrite reduction. It Electrocatalytic oxidation of AA ascribed to both MoVI and
−
−
is well known that NO2 disproportionates into NO and NO3
CuII-centres is firstly found.
in acidic solutions.14 We infer that the reduction mechanism
involves the reduction of NO2 and NO by the heteropoly blue We deduce the oxidation mechanism as follows because
The proposed oxidation mechanism is provided in eqn (3).
−
species (eqn (1) and (2) in the ESI†).
ascorbic acid undergoes a two-proton dehydrogenation process:
Proposed mechanisms for the electrocatalytic reduction of
nitrite by compounds 1–4 are as follows:
CuII=MoVI þ C6H8O6 ! Cu=MoV þ C6H6O6
ð3Þ
VI
Mo MoVn O26ð4þnÞꢀ þ NO2ꢀ þ xHþ
According to the method proposed by Keita, the electro-
catalytic efficiencies (CAT) of n-CPEs (n = 1–4) are calculated by
the following formula:15
8ꢀn
ð1Þ
! MoV8IO264ꢀ þ products ðcontaining NÞ
Mo MoVn O26ð4þnÞꢀ þ NO þ yHþ
VI
8ꢀn
ð2Þ
CAT ¼ 100% ꢁ ½IpðC; POM þ substrateÞ ꢀ IpðPOMÞꢂ=IpðPOMÞ
! MoV8IO264ꢀ þ products ðcontaining NÞ
In Fig. 13, an obvious increase of the oxidation peak cur- where Ip (C, POM + substrate) is the current intensity of a cata-
−
rents of CuII/MoVI in compounds 1–4 along with the addition lytic wave for an electro-catalyst in the presence of NO2 or AA
Fig. 13 Cyclic voltammograms showing the electrocatalytic oxidation of AA by (a) 1-CPE, (b) 2-CPE, (c) 3-CPE and (d) 4-CPE in 1M H2SO4
Na2SO4 solutions (pH = 1) with increase of concentration of AA, from bottom to top: 0, 2, 4, 6, 8 mM. Scan rate: 0.1 V s−1
+
.
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to the CuII/MoVI-centre. We have done three parallel experi-
ments to check the stability of the electrocatalysts for the
−
electrocatalytic reduction of NO2 (2 mM) by 2-CPE under
identical conditions. Fig. S6† demonstrates that the electro-
−
catalytic activity for reduction of NO2 is unchanged basically
during the reduplicate catalytic experiments.
Conclusions
−
Fig. 14 Charts of CAT% vs. concentration of NO2 (a) and concen-
tration of AA (b) for 1–4-CPEs.
We have prepared four new organic–inorganic hybrid com-
pounds based on small Mo clusters and Cu-pyridyl segments
by adjusting pH. Compound 1 shows a 1D {-β-[Mo8O26]-
Cu2-γ-[Mo8O26]}n straight chain structure including mixed
β-[Mo8O26] and γ-[Mo8O26] clusters; compound 2 possesses a
3D supramolecular structure stemming from β-[Mo8O26] units
and [Cu(tpy)]2+ motifs; and compound 3 has a 1D chain struc-
ture built from [Mo3O10]2− anions and [Cu(bpy)]2+ units. The
[β-Mo8O26] clusters are bridged by [Cu(bpy)]2+ motifs leading
to a 2D sheet structure in compound 4. Furthermore, the
electrochemical and electro-catalytic properties of compounds
1–4 were investigated in 1 M H2SO4 + Na2SO4 electrolyte solu-
tion through CV measurements. The results show that n-CPEs
(n = 1–4) have different dual electro-catalytic activities for not
and Ip (POM) is the current intensity of the catalyst without
NO2 and AA. In this work, the CAT% is defined according to
−
the current intensities of the cathodic wave of IV′ in the pres-
−
ence of NO2 and the current intensities of the anodic wave of
CuII/MoVI (I for CuII and II for MoVI) in the presence of AA.
Histograms of CAT% for 1–4-CPEs along with concentrations
−
of NO2 and AA are shown in Fig. 14 for an intuitional com-
parison of their electro-catalytic properties.
As shown in Tables S6 and S7,† the CAT% of 2-CPE towards
−
2.0, 4.0, 6.0, and 8.0 mM NO2 was calculated to be 359%,
400%, 616% and 1721%, and that of 3-CPE was calculated
to be 392%, 547%, 1012% and 1093%; and the CAT% of
3-CPE-MoII towards 2.0, 4.0, 6.0, and 8.0 mM AA was calculated
to be 29%, 58%, 90% and 123%, and that of 3-CPE-CuII was
calculated to be 26%, 61%, 87% and 96%, respectively. The
electrocatalytic efficiencies of n-CPEs are in the order of 3-CPE
−
only the reduction of NO2 but also the oxidation reaction of
AA. We will extend our research to isopolyvanadates to prepare
POV/M/L hybrid compounds with novel structures and multi-
functional properties.
−
> 2-CPE > 4-CPE > 1-CPE for low concentration of lean NO2
Conflicts of interest
(2 mM) and 2-CPE > 4-CPE >3-CPE > 1-CPE for high concen-
tration of NO2 (8 mM). The electrocatalytic efficiencies of
−
There are no conflicts to declare.
n-CPE are in the order of 1-CPE >2-CPE > 3-CPE-MoII >3-
CPE-CuII > 4-CPE for low concentration of lean AA (2 mM) and
3-CPE-MoII >3-CPE-CuII > 2-CPE > 1-CPE > 4-CPE for high con-
centration of AA oxidation. The results indicate that the
Acknowledgements
[Mo3O10]
2−-type organic–inorganic hybrid exhibits better per-
We thank the National Natural Science Foundation of China
(Grant 21373044); the Department of Science and Technology
of Jilin Province, China (20180311016YY, 20191102016YY); the
Education Department of Jilin Province (JJKH20180867KJ,
JJKH20170434KJ); and the Youth Foundation of Department of
Science and Technology of Jilin Province (20160520172JH) for
financial support.
formance than [Mo8O26]4−-type (γ-Mo8O26 and β-Mo8O26 for 1
and β-Mo8O26 for 2 and 4) hybrids towards the electrocatalytic
−
reduction of NO2 (2 mM) and the β-Mo8O26−-type hybrids
show better properties than the [Mo3O10]2− unit and mixed
Mo8O26-type hybrids during the process of electrocatalytic
−
reduction of NO2 (8 mM); in a similar way, the [Mo3O10]2−
-
type hybrid exhibits better activities towards the electro-
catalytic oxidation of AA than [Mo8O26]
4−-type hybrids. It’s
worth mentioning that 3-CPE-MoVI is superior to 3-CPE-MoVI
for the oxidation of AA. In a word, the [Mo3O10]2−-based
organic–inorganic hybrid exhibits better performances than
Notes and references
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