Cobalt(II) Coordination Polymers Assembled from Unexplored Pyridine-Carboxylic Acids: Structural Diversity and Catalytic Oxidation of Alcohols

Article abstract of DOI:10.1021/acs.inorgchem.9b00242

New coordination polymers of cobalt(II), namely, [Co(μ₄-cpna)(H₂O)₂]_n (1), [Co(μ₃-cpna)(phen)(H₂O)]_n·nH₂O (2), [Co₃(μ₄-dppa)₂(H₂O)₆]_n·2nH₂O (3), and [Co₃(μ₅-dppa)₂(μ-4,4-bipy)(H₂O)₂]_n·4nH₂O (4), have been generated under hydrothermal conditions from CoCl₂·6H₂O, two different multifunctional pyridine-carboxylic acids {H₂cpna: 5-(4-carboxyphenoxy)nicotinic acid; H₃dppa: 5-(3,4-dicarboxylphenyl)picolinic acid}, and optional N,N-supporting ligands {phen: 1,10-phenanthroline; 4,4-bipy: 4,4-bipyridine} acting as mediators of crystallization. These Co(II) coordination polymers (CPs) have been obtained as stable crystalline materials and characterized by conventional solid-state techniques, including X-ray crystallography. The obtained products are 3D metal-organic frameworks (MOFs 1 and 4) or 2D coordination polymers (CPs 2 and 3). Analysis of the topologies of simplified nets has revealed the sra (1), fes (2), and 3,4L13 (3) networks, in addition to a very complex topologically unique framework in 4. An observed diversity of structures is driven by types of carboxylate building blocks and crystallization mediators. Thermal stability and magnetic and catalytic properties of 1-4 have also been studied. In fact, the Co(II) compounds act as heterogeneous catalysts for the oxidation of alcohols with tBuOOH (tert-butylhydroperoxide) under mild conditions. Compound 2 features a good catalytic activity (up to 45% yield) in the oxidation of 1-indanol to 1-indanone. Finally, products 1-4 broaden a still very small number of CPs or MOFs driven by the present type of multifunctional pyridine-carboxylic acids (H₂cpna, H₂dppa).

Full text of DOI:10.1021/acs.inorgchem.9b00242

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Cobalt(II) Coordination Polymers Assembled from Unexplored
Pyridine-Carboxylic Acids: Structural Diversity and Catalytic
Oxidation of Alcohols
Jinzhong Gu,*,† Min Wen,† Yan Cai,† Zifa Shi,† Dmytro S. Nesterov,‡ Marina V. Kirillova,‡
and Alexander M. Kirillov*,‡,§
†State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of
Gansu Province, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People’s Republic of
China
‡
́
Centro de Química Estrutural, Instituto Superior Tecnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001, Lisbon, Portugal
§Peoples’ Friendship University of Russia (RUDN University), 6 Miklukho-Maklaya st., Moscow, 117198, Russian Federation
S
* Supporting Information
ABSTRACT: New coordination polymers of cobalt(II), namely, [Co(μ4-cpna)-
(H2O)2]n (1), [Co(μ3-cpna)(phen)(H2O)]n·nH2O (2), [Co3(μ4-dppa)2(H2O)6]n·
2nH2O (3), and [Co3(μ5-dppa)2(μ-4,4′-bipy)(H2O)2]n·4nH2O (4), have been
generated under hydrothermal conditions from CoCl2·6H2O, two different
multifunctional pyridine-carboxylic acids {H2cpna: 5-(4-carboxyphenoxy)nicotinic
acid; H3dppa: 5-(3,4-dicarboxylphenyl)picolinic acid}, and optional N,N-supporting
ligands {phen: 1,10-phenanthroline; 4,4′-bipy: 4,4′-bipyridine} acting as mediators of
crystallization. These Co(II) coordination polymers (CPs) have been obtained as
stable crystalline materials and characterized by conventional solid-state techniques,
including X-ray crystallography. The obtained products are 3D metal−organic
frameworks (MOFs 1 and 4) or 2D coordination polymers (CPs 2 and 3). Analysis
of the topologies of simplified nets has revealed the sra (1), fes (2), and 3,4L13 (3)
networks, in addition to a very complex topologically unique framework in 4. An
observed diversity of structures is driven by types of carboxylate building blocks and crystallization mediators. Thermal stability
and magnetic and catalytic properties of 1−4 have also been studied. In fact, the Co(II) compounds act as heterogeneous
catalysts for the oxidation of alcohols with tBuOOH (tert-butylhydroperoxide) under mild conditions. Compound 2 features a
good catalytic activity (up to 45% yield) in the oxidation of 1-indanol to 1-indanone. Finally, products 1−4 broaden a still very
small number of CPs or MOFs driven by the present type of multifunctional pyridine-carboxylic acids (H2cpna, H2dppa).
such as cobalt are more abundant and less expensive, and thus
can be of significant practical importance. In particular, some
cobalt compounds with the ligands containing pyridyl and
carboxylate functional groups catalyze the oxidation of alcohols
and amines with peroxides or dioxygen.24−26
By combining our recent research directions in the areas of
(i) hydrothermal synthesis of functional coordination poly-
mers27−30 and (ii) oxidation catalysis,31−33 herein we focused
our attention on two poorly explored pyridine-(di/tri)-
carboxylic acids (Scheme 1), namely, H2cpna (5-(4-carboxy-
phenoxy)nicotinic acid) and H3dppa (5-(3,4-dicarboxyl-
phenyl)picolinic acid).27,34−39 Along with optional N,N-type
mediators of crystallization (co-ligands), these carboxylic acids
were probed as main organic blocks (Scheme 1) for
hydrothermal synthesis of new cobalt(II) CPs. Besides,
considering that these ligands (H2cpna and H3dppa) comprise
two types of principal functional groups (pyridine and
INTRODUCTION
■
Design and synthesis of new coordination polymers and
metal−organic frameworks (abbr. CPs and MOFs, respec-
tively) has recently attracted an enormous attention due to an
unlimited diversity of their structural types, architectures,
topologies, as well as important properties and applications of
these functional solids.1−10 In particular, the use of CPs in
catalysis became a recognized research area,11−15 which is
primarily associated with the high porosity, crystallinity,
inorganic−organic hybrid structures, thermal stability, and
tunable physicochemical properties of such compounds.11−15
Among the vast variety of metal-complex and coordination
polymer catalysts, coordination compounds that incorporate
cobalt nodes and carboxylate blocks as spacers or linkers have
deserved a special attention in catalysis.16−19 In fact, cobalt
coordination compounds can catalyze a wide range of organic
reactions, which span from C−H activation and cross-coupling
to CO2 reduction and water oxidation.20−23 Although many of
these processes can be efficiently promoted by the platinum
group complexes as catalysts, the first-row transition metals
Received: January 24, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b00242
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XL-7 model, field strength of 0.1 T) at 2−300 K; diamagnetic
contribution was corrected before analyzing the data. Catalytic
oxidation of alcohols was monitored by GC (gas chromatography)
analyses that were carried out on an Agilent Technologies 7820 A
chromatograph using a flame ionization detector, capillary column
(BP20/SGE), and helium as a carrier gas.
Scheme 1. Main Pyridine-Carboxylic Acid Ligands and N,N-
Donor Crystallization Mediators
Synthesis and Characterization of 1−4. [Co(μ4-cpna)(H2O)2]n
(1). CoCl2·6H2O (0.3 mmol, 71.0 mg), H2cpna (0.3 mmol, 77.7 mg),
NaOH (0.6 mmol, 24.0 mg), and H2O (10 mL) were combined in a
stainless steel (Teflon-lined) reactor of the 25 mL capacity. After
vigorous stirring for 15 min at ambient temperature, a reactor was
closed and kept at 160 °C for 3 days in the oven. Then, the reactor
was gradually cooled down (10 °C/h) to ambient temperature and
opened. Yellow crystals (needles) were removed from the reaction
mixture manually or by filtration, washed with H2O, and then air-
dried to furnish 1. Yield: 65%, based on H2cpna. Analysis calculated
for C13H11CoNO7: C 44.34, H 3.15, N 3.98%. Found: C 44.67, H
3.13, N 4.01%. IR (cm−1): 3596w, 3068w, 1628s, 1596s, 1544m,
1499w, 1453w, 1401s, 1335s, 1297m, 1244w, 1206w, 1160w, 1095w,
1049w, 1003w, 958w, 918w, 860w, 821w, 770m, 699w, 652w, 554w.
[Co(μ3-cpna)(phen)(H2O)]n·nH2O (2). Compound 2 was synthe-
sized by adapting a procedure for 1 and using a mixture of H2cpna
(0.3 mmol, 77.7 mg) with phen (0.30 mmol 60.0 mg). Orange
crystals (blocks) were removed from the reaction mixture manually or
by filtration, washed with H2O, and then air-dried to furnish 2. Yield:
60%, based on H2cpna. Analysis calculated for C25H19CoN3O7: C,
56.40; H, 3.60; N, 7.89. Found: C, 56.58; H, 3.62; N, 7.93%. IR
(cm−1): 3414w, 3062w, 1609s, 1564s, 1517w, 1492 w, 1453w, 1427w,
1395s, 1297w, 1244w, 1199w, 1160w, 1101w, 1043w, 1010w, 958w,
899w, 854m, 782m, 730w, 691w, 638w, 541w.
carboxylate), it was also interesting to investigate the catalytic
behavior of respective cobalt(II) compounds toward the
oxidation of alcohols.
Thus, the present study describes the hydrothermal
preparation, crystal structures, characterization, thermal
behavior, magnetic properties, as well as catalytic activity of
a new series of cobalt(II) CPs and MOFs, namely, [Co(μ4-
cpna)(H2O)2]n (1), [Co(μ3-cpna)(phen)(H2O)]n·nH2O (2),
[Co3(μ4-dppa)2(H2O)6]n·2nH2O (3), and [Co3(μ5-dppa)2(μ-
4,4′-bipy)(H2O)2]n·4nH2O (4). The obtained compounds
represent the very rare (1, 2) or unique (3, 4) examples of
cobalt(II) CPs derived from H2cpna or H3dppa, as confirmed
by searching the Cambridge Structural Database (CSD).
[Co3(μ4-dppa)2(H2O)6]n·2nH2O (3). Compound 3 was synthesized
by adapting a procedure for 1 and using H3dppa (0.20 mmol, 57.4
mg) instead of H2cpna. Pink crystals (blocks) were removed from the
reaction mixture manually or by filtration, washed with H2O, and then
air-dried to furnish 3. Yield: 60%, based on H3dppa. Analysis
calculated for C28H28Co3N2O20: C, 37.82; H, 3.17; N, 3.15. Found: C,
37.95; H, 3.18; N, 3.12%. IR (cm−1): 3387m, 1622s, 1596s, 1570w,
1486w, 1413s, 1368s, 1290w, 1251w, 1205w, 1160w, 1095w, 1036w,
899w, 841w, 808w, 782w, 704w, 658w, 638w, 541w.
EXPERIMENTAL SECTION
■
Reagents and Equipment. All chemicals and solvents (A.R.
grade) were furnished by commercial suppliers. Elemental content (C,
H, N) in 1−4 was studied on an Elementar Vario EL elemental
analyzer. TGA (thermogravimetric analysis) data were collected on a
LINSEIS STA PT1600 thermal analyzer (flow of N2, 10 °C/min
heating rate). A Bruker EQUINOX 55 spectrometer was used to
record the IR spectra (KBr discs). PXRD (powder X-ray diffraction)
patterns were measured on a Rigaku-Dmax 2400 diffractometer (λ =
1.54060 Å Cu Kα radiation). Magnetic measurements were
performed with a Quantum Design SQUID Magnetometer (MPMS
[Co3(μ5-dppa)2(μ-4,4′-bipy)(H2O)2]n·4nH2O (4). Compound 4 was
synthesized by adapting a procedure for 1 and using a mixture of
H3dppa (0.20 mmol, 57.4 mg) with 4,4′-bipy (0.3 mmol, 46.8 mg)
Table 1. Crystallographic Data for Compounds 1−4
1
2
3
4
chemical formula
formula weight
crystal system
space group
a, Å
b, Å
c, Å
α, deg
β, deg
γ, deg
V, Å3
T, K
Z
Dc, g cm−3
μ, mm−1
C13H11CoNO7
352.16
monoclinic
P21/n
6.1155(2)
8.1678(3)
26.0705(10)
90
92.335(4)
90
1301.14(8)
293(2)
4
1.798
1.357
C25H19CoN3O7
532.36
monoclinic
P21/n
12.1205(4)
10.2942(4)
18.4120(6)
90
91.373(3)
90
2296.61(13)
293(2)
4
1.540
0.800
C28H28Co3N2O20
889.31
triclinic
C38H32Co3N4O18
1009.46
monoclinic
I2/c
25.592(2)
7.2957(3)
23.7096(18)
90
120.372(11)
90
3819.3(6)
293(2)
P1
̅
7.4924(5)
9.5818(5)
12.0040(9)
88.207(5)
75.573(6)
76.919(5)
812.66(10)
293(2)
1
4
1.756
1.607
1.756
1.377
F(000)
716
1092
451
2052
reflns measured
unique reflns (Rint)
GOF on F2
R1 [I > 2σ(I)]
wR2 [I > 2σ(I)]
4587
2306 (0.0355)
0.998
0.0373
0.0689
8123
4071 (0.0388)
1.040
0.0442
0.0827
5117
2853 (0.0203)
1.018
0.0362
0.0932
6800
3396 (0.0450)
1.051
0.0466
0.0921
B
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Figure 1. Crystal structure of MOF 1. (a) Co1 coordination environment. (b) Dicobalt(II) carboxylate motif. (c) 3D metal−organic framework.
(d) Simplified topological representation of 3D MOF displaying a 4-connected uninodal network of the sra topological type; centroids of 4-
connected μ4-cpna2− nodes (gray), 4-connected Co1 nodes (magenta balls). Further details: (a−c) H atoms are not shown; (c) view along the bc
plane; (d) view along the a axis.
instead of H2cpna. Pink crystals (blocks) were removed from the
reaction mixture manually or by filtration, washed with H2O, and then
air-dried to furnish 4. Yield: 62%, based on H3dppa. Analysis
calculated for C38H32Co3N4O18: C, 45.21; H, 3.20; N, 5.55. Found: C,
45.07; H, 3.18; N, 5.52%. IR (cm−1): 3590 w, 3498w, 1596s, 1564s,
1492w, 1401s, 1257w, 1212w, 1160w, 1095w, 1062w, 1036w, 1010w,
919w, 847w, 808w, 704m, 658w, 625w, 560w.
a small amount of solid triphenylphosphine (Shul′pin’s method41,42
)
to reduce the remaining peroxide. GC peaks were attributed based on
chromatograms of commercial samples. Blank tests indicated that
oxidation reactions are significantly slower in the absence of Co
catalyst (Figure S7). A slightly elevated temperature of 50 °C is
essential for the activation of tBuOOH oxidant; in fact, the reaction
rates at 50 and 20 °C differ by more than 20-fold (for catalyst 2,
Figure S7). Acetonitrile was selected as a standard solvent for this
type of reactions due to its stability toward oxidation, sufficient boiling
point, and solvation properties.31
Single-Crystal X-ray Diffraction. Single-crystal X-ray data of 1−
4 were collected on a Bruker Smart CCD diffractometer (λ = 0.71073
Å, graphite-monochromated Mo Kα radiation). The SADABS
program was applied for semiempirical absorption corrections.
Structures were solved by direct methods and refined by full-matrix
least-squares on F2 using SHELXS-97/SHELXL-97 programs.40 All
non-hydrogen atoms were anisotropically refined (full-matrix least-
squares on F2). CH hydrogen atoms were positioned in calculated
sites with fixed isotropic thermal parameters; these were included in
structure factor calculations when performing a final stage refinement.
Water H atoms were located by difference maps and constrained to
ride on the respective O atoms. Crystallographic data for compounds
1−4 are listed in Table 1, whereas Tables S1 and S2 (Supporting
Information, SI) provide a summary of selected structural parameters.
Catalytic Study. Alcohol oxidation reactions were investigated
under vigorous stirring in an air atmosphere in glass reactors
(thermostated at 50 °C) that were equipped with a reflux condenser.
In a typical test, a heterogeneous Co(II) catalyst selected from 1−4 (5
μmol; calculated per one cobalt atom) and CH3CN solvent (up to 5
mL) were added to a glass reactor and then alcohol (1 mmol),
nitromethane (GC internal standard, 0.2 M final concentration), and
oxidant (tBuOOH, 70% in H2O, 7 mmol) were introduced. Progress
of catalytic reactions was observed by periodically taking samples from
reaction mixture. Prior to GC analysis, these samples were mixed with
RESULTS AND DISCUSSION
■
Structural Description. [Co(μ4-cpna)(H2O)2]n (1). Cobalt-
(II) product 1 is a three-dimensional MOF (Figure 1) that is
composed of a Co(II) center, a μ4-cpna2− block, and two H2O
molecules of crystallization (per asymmetric unit). The six-
coordinate Co1 atom displays a slightly distorted {CoNO5}
octahedral environment (Figure 1a), which is occupied by
three oxygen and one nitrogen donor atoms coming from four
different μ4-cpna2− moieties, in addition to a couple of terminal
H2O ligands. The Co−N [2.157(3) Å] and Co−O [2.053(2)−
2.199(2) Å] distances are typical for the present type of Co(II)
compounds.7a,18,27 The cpna2− block behaves as a μ4-N,O2-
ligand, wherein the COO− groups are monodentate or
bidentate (Scheme 2, mode I); the dihedral angle between
pyridyl and benzene functionalities attains 85.13°, while the
C−Oether−C angle is 118.17°.
Two μ-carboxylate O atoms of two μ4-cpna2− spacers
interlink the adjacent Co(II) centers into the dicobalt(II)
C
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76.35°. The μ3-cpna2− spacers interlink the Co1 centers into a
two-dimensional layer (Figures 2b and S3), wherein both the
Co1 and μ3-cpna2− nodes are 3-connected and equivalent from
a topological point of view (Figure 2c). A simplified layer
(Figure 2c) is topologically described as the 3-connected
uninodal net with the fes topology and the point symbol of
(4.82).
Scheme 2. Modes of Coordination of Main Ligands (cpna2−:
Modes I and II; dppa3−: Modes III and IV) in Compounds
1−4
[Co3(μ4-dppa)2(H2O)6]n·2nH2O (3). Product 3 also displays
a two-dimensional coordination polymer structure (Figure 3).
However, 3 is more complex than 2 given the presence of two
different Co(II) centers (Co2 with a full occupancy; Co1 with
a half occupancy), one μ4-dppa3− spacer, three water ligands,
and a lattice H2O molecule per asymmetric unit (Figure 3a).
Co1 atom is six-coordinate and is located on a 2-fold rotation
axis. It adopts an ideal octahedral {CoO6} geometry, which is
taken by four oxygen donors (from four μ4-dppa3− spacers)
and two terminal water ligands. The Co2 atom also features a
slightly distorted {CoNO5} octahedral environment that is
filled by three carboxylate oxygen and one nitrogen atoms from
two μ4-dppa3− spacers, in addition to two terminal water
ligands. The Co−N distance is 2.122(3) Å, whereas the Co−O
bonds range from 2.032(2) to 2.142(2) Å; these distances are
in accord with related cobalt derivatives.18,27,29 The dppa3−
block functions as the μ4-ligand (Scheme 2, mode III), in
which carboxylate groups are bridging bidentate or mono-
dentate. In μ4-dppa3−, a dihedral angle among the picolinate
and isophthalate functionalities attains 31.29°. Both Co1/Co2
centers are interlinked by neighboring carboxylate groups
(phthalate functionality of μ4-dppa3−) to generate a one-
dimensional double chain pattern (Figure 3b). These patterns
are extended further, through a picolinate functionality of μ4-
dppa3−, into a complex 2D layer (Figure 3c). From a
topological perspective, a simplified 2D net of this CP is
built from the 3-connected μ4-dppa3− nodes, the 4-connected
Co1 nodes, and the 2-connected Co2 atoms acting as linkers
(Figure 3d). Topological classification reveals a 3,4-connected
motifs (Figure 1b) with a Co···Co separation of 3.350(2) Å.
Such motifs are further assembled, via the remaining COO−
groups and N atoms of μ4-cpna2−, to a three-dimensional MOF
(Figures 1c, S1, and S2). After simplification, topological
classification of this 3D MOF (Figure 1d) reveals a 4-
connected uninodal net of sra topological type; the point
symbol is (42.63.8).
[Co(μ3-cpna)(phen)(H2O)]n·nH2O (2). Compound 2 fea-
tures a two-dimensional coordination polymer network (Figure
2). The asymmetric unit is composed of a Co(II) center, a μ3-
cpna2− block, a phen moiety, a terminal water ligand, and a
lattice H2O molecule (Figure 2a). The six-coordinate Co1
center shows a distorted {CoN3O3} octahedral geometry. It is
occupied by two oxygen and one nitrogen atoms from three
μ3-cpna2− spacers, a pair of phen N atoms, and an H2O ligand,
with the standard Co−N [2.138(3)−2.170(3) Å] and Co−O
[2.030(2)−2.145(3) Å] distances.7b,27,28 In 2, cpna2− behaves
as the μ3-N,O2-ligand (mode II, Scheme 2) in which both
carboxylate groups are monodentate; a C−Oether−C angle is
115.77°, while a dihedral angle between the rings of cpna2− is
Figure 2. Crystal structure of 2. (a) Co1 coordination environment. (b) 2D metal−organic layer. (c) Simplified topological representation of 2D
layer displaying a 3-connected uninodal net of the fes topological type; 3-connected Co1 nodes (magenta balls), centroids of 3-connected μ3-
cpna2− nodes (gray). Further details: (a, b) H atoms and phen ligands in (b) are not shown; (b) view along the bc plane; (c) view along the a axis.
D
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Figure 3. Crystal structure of 3. (a) Co1 and Co2 coordination environment. (b) 1D double chain motif. (c) 2D metal−organic layer. (d)
Simplified topological view of two-dimensional CP showing a 3,4-connected binodal layer of the 3,4L13 topological type; centroids of 3-connected
μ4-dppa3− nodes (gray), 2-connected Co2 linkers, and 4-connected Co1 nodes (magenta balls). Further details: (a−c) H atoms are not shown; (b)
view along the bc plane; (c, d) view along the b axis.
binodal net of the 3,4L13 topological type. This net is defined
by the (4.62)2(42.62.82) point symbol in which the (4.62) and
(42.62.82) notations refer to the μ4-dppa3− and Co1 nodes,
correspondingly.
5-connected μ5-dppa3− nodes, and 2-connected μ-4,4′-bipy
linkers (Figure 4d). Hence, MOF 4 discloses a 4,4,5-connected
trinodal underlying net of a unique topological type. This is
defined by the (4.52.6.7.8)2(42.52.62.73.8)2(42.52.62) point
symbol, wherein the (4.52.6.7.8), (42.52.62.73.8), and
(42.52.62) indices belong to Co1, μ5-dppa3−, and Co2 nodes,
correspondingly. The unique type of this topology can be
attested by screening of various databases.43−46 Further
simplification of this framework (μ-4,4′-bipy linkers omitted)
results in a trinodal net with a rare pkb2 topology.43
[Co3(μ5-dppa)2(μ-4,4′-bipy)(H2O)2]n·4nH2O (4). If com-
pared to 3, the structure of 4 possesses a more intricate 3D
metal−organic framework due to the presence of an additional
μ-4,4′-bipy linker (Figure 4). The asymmetric unit of 4 bears
two Co(II) atoms (Co1 with a complete and Co2 with a half
occupancy), a μ5-dppa3− block, a half of μ-4,4′-bipy moiety, a
H2O ligand, and two lattice H2O molecules. The Co2/Co1
atoms are six-coordinate and possess the octahedral {CoO6}
and {CoN2O4} environments, correspondingly (Figure 4a).
The Co1 atom is bound by four oxygen and one nitrogen
donors coming from three μ5-dppa3− ligands, in addition to
one 4,4′-bipy N donor. The Co2 atom is surrounded by four
oxygen donors of μ5-dppa3− blocks and a couple of terminal
H2O ligands. The Co−N and Co−O bonds are in the
2.156(4)−2.176(4) and 2.066(3)−2.139(3) Å range, respec-
tively. The dppa3− block acts as a μ5-N,O6-spacer with all three
COO− groups adopting a bidentate bridging fashion (mode
IV, Scheme 2). The μ5-dppa3− block features the dihedral angle
of 48.19° (angle involving the aromatic rings). Carboxylate
groups of μ5-dppa3− provide an interconnection of cobalt(II)
centers into a 2D sheet pattern (Figure 4b). Such patterns are
extended further, via an additional functionality of μ5-dppa3−
and the μ-4,4′-bipy linkers, into a 3D MOF structure (Figure
4c).
Synthesis and Structural Comparison of 1−4.
Compounds 1 and 2 were obtained by performing similar
syntheses but using phen as an additional mediator of
crystallization when preparing product 2. In this case, phen
chelator can block two coordination sites of cobalt(II) ions,
resulting in a structure of lower dimensionality (2D CP 2 vs
3D MOF 1). In contrast, the use of 4,4′-bipy as an additional
linker and crystallization mediator when synthesizing 4 gives
rise to a more complex structure (3D MOF 4 vs 2D CP 3).
These results show that the use of an N,N-type crystallization
mediator affects the final structure and dimensionality of the
metal−organic network during the hydrothermal synthesis.
Compounds 1−4 are nonporous as attested by the calculation
of an effective free volume of the crystal volume by PLATON,
showing that the unit cell contains no residual solvent
accessible voids.
In compounds 1−4, the cpna2− and dppa3− blocks are fully
deprotonated and show diverse coordination modes (Scheme
2), ranging from μ4-N,O2-cpna2− and μ3-N,O2-cpna2− (in 1 and
2) to μ4-N,O5-dppa3− and μ5-N,O6-dppa3− (in 3 and 4). To
Topologically, a simplified net of 4 is built from the Co1 and
Co2 nodes (both are 4-connected but topologically distinct),
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Figure 4. Crystal structure of 4. (a) Co1 and Co2 coordination environment. (b) 2D layer pattern. (c) 3D MOF. (d) Simplified representation of
3D MOF displaying a 4,4,5-connected trinodal underlying framework of the new topological type; centroids of 5-connected μ5-dppa3− nodes
(gray), 4-connected Co1/Co2 nodes (magenta balls), centroids of 2-connected μ-4,4′-bipy linkers (blue). Further details: (a−c) H atoms are not
shown; (b) view along the ab plane; (c) view along the ac plane; (d) view along the b axis.
□
○
Figure 5. Magnetic susceptibility data for compounds 1−4 (χMT ( ) and 1/χM ( ) vs T).
achieve a required coordination geometry at cobalt(II) centers,
both pyridine-carboxylate ligands undergo some rotation of
aromatic rings owing to the presence of the C−Oether−C and
C−C functionality, with respective angles of 115.77−118.17°
(C−Oether−C angle in cpna2−) and 31.29−48.19° (dihedral
angle in dppa3−).
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Apart from distinct dimensionalities, the obtained com-
pounds also feature different types of topological networks,
namely, (a) 4-connected uninodal 3D framework of an sra
(SrAl2) topological type in compound 1, (b) 3-connected
uninodal layer with an fes [Shubnikov plane net (4.82)]
topology in compound 2, (c) 3,4-connected binodal layer of a
3,4L13 topological type in compound 3, and (d) 4,4,5-
connected trinodal framework of the novel topological type in
compound 4.
TGA and PXRD Results. Thermogravimetric analysis
(TGA) was used to investigate the stability of compounds
1−4 upon heating under a flow of N2 (Figure S4, SI). A loss of
two H2O ligands in 1 (calcd, 10.2%; exptl, 10.4%;) is observed
at a rather high temperature interval (151−297 °C); the
dehydrated sample then decomposes starting at 351 °C. TGA
data of 2 show two minor thermal effects in the 61−195 °C
range, which are associated with a release of one solvent H2O
molecule and one water ligand (calcd, 6.8%; exptl, 6.5%). After
dehydration, the sample of 2 is stable until 297 °C. For
compound 3, two thermal effects in the 92−230 °C
temperature range refer to the loss of two crystallization
water molecules and six H2O ligands (calcd, 16.2%; exptl,
16.3%); the sample then starts to decompose at 331 °C. Two
minor thermal effects are also observed in the 116−258 °C
interval in the TGA plot of 4, which correspond to the removal
of four solvent molecules and a couple of H2O ligands (calcd,
10.7%; exptl, 10.3%). After dehydration, the sample maintains
stability up to 380 °C.
to a value (5.61 cm3·mol−1·K) expected for three isolated high-
spin Co(II) ions (S = 3/2). This can be explained by the
significant orbital contribution.7a,27,29 The value of χMT
declines to 6.13 cm3·mol−1·K (minimum value) on decreasing
the temperature to 10 K, followed by an abrupt increase to
8.03 cm3·mol−1·K (maximum value) at 2 K, thus showing a
complex behavior. According to the structure of 4 (Figure 4b),
adjacent Co(II) centers possess a single type of the magnetic
exchange path, namely, through one syn-anti carboxylate
bridge, which explains a ferromagnetic exchange observed in
this compound.
Catalytic Study. Catalytic performance of 1−4 was
evaluated for oxidation of alcohols with peroxides under mild
conditions (Scheme 3). The compounds 1−4 were hydro-
Scheme 3. Mild Oxidation of Alcohols to Ketones Catalyzed
by 1−4
Powder X-ray diffraction (PXRD) experiments were under-
taken using microcrystalline samples of 1−4. The obtained
powder diffractograms (Figure S5, SI) well match the
calculated patterns (on the basis of single-crystal X-ray
diffraction data). This proves a phase homogeneity of the as-
synthesized compounds.
Magnetic Properties. Magnetic susceptibility study at
different temperatures (2−300 K) was performed using the
crystalline samples of cobalt(II) compounds 1−4 (Figure 5).
For 1, the χMT value of 3.30 cm3·mol−1·K at 300 K is superior
if compared to the value (1.83 cm3·mol−1·K) calculated for one
isolated (magnetically) high-spin cobalt(II) ion (g = 2.0, S = 3/
2). Such an observation represents a phenomenon commonly
seen for cobalt(II) ions owing to powerful spin−orbital
coupling.18,27,29 The χMT values slowly decline to 2.89 cm3·
mol−1·K (minimum value) on decreasing the temperature up
to 24 K. Further decrease of the temperature to 4 K leads to an
abrupt increase of χMT (3.29 cm3·mol−1·K), which then
quickly decreases on further cooling.
thermally generated and are stable in water (Figure S5, SI).
They are insoluble in the catalytic reaction medium composed
of an alcohol substrate, aqueous tBuOOH oxidant, and
acetonitrile solvent, thus acting as heterogeneous catalysts.
Oxidation of 1-indanol (0.2 M) with tert-butylhydroperoxide
(tBuOOH; 1.4 M) catalyzed by 1−4 (0.5 mol %) in
acetonitrile afforded 1-indanone as a main reaction product.
The highest product yield (45%) was observed for the catalyst
2, followed by 1, 4, and 3 (26−39% yields, Table 2).
Table 2. Mild Oxidation of Alcohols with tBuOOH
a
Catalyzed by 1−4
b
yield of ketone (%)
For 2, the χMT value at 300 K (3.04 cm3·mol−1·K) is higher
than the parameter (χMT = 1.83 cm3·mol−1·K) expected for an
isolated high-spin cobalt(II) ion (g = 2.0, S = 3/2). Upon
cooling from 300 to 2 K, χMT decreases from 3.04 to 1.77 cm3·
mol−1·K, respectively (Figure 5). Such a decrease of χMT can
be explained by the general antiferromagnetic interaction
between cobalt(II) centers.
substrate/catalyst
1
2
3
4
1-indanol
cyclohexanol
fenchyl alcohol
39.4
9.6
6.1
44.5
20.9
7.3
26.3
9.0
5.0
37.0
7.1
5.6
a
Reaction conditions: Co catalyst (5 μmol; 0.5 mol %), alcohol (1
mmol), tBuOOH (aq. 70%, 7 mmol), acetonitrile (until 5 mL of total
reaction volume), 24 h, 50 °C. Molar yield based on substrate:
(moles of ketone product per mol of alcohol substrate) × 100%.
b
For compound 3, room temperature χMT (8.36 cm3·mol−1·
K) is significantly higher than the value of 5.61 cm3·mol−1·K
expected for three isolated Co(II) ions (S = 3/2), indicating
the presence of a significant spin−orbital coupling. On
decreasing the temperature, χMT values steadily go down,
suggesting the dominant antiferromagnetic interactions within
the spin network.
The accumulation plots of 1-indanone (Figure 6) exhibit a
linear dependence for all the catalysts, with almost equal initial
reaction rates for 1, 3, and 4 of 8.4 × 10−7 M s−1 and a
considerably higher rate of 1.5 × 10−6 M s−1 for compound 2
(Figure 6).
Metal−organic framework 4 shows a room temperature χMT
of 9.44 cm3·mol−1·K, which is significantly higher if compared
G
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due to a low catalyst loading of 0.5 mol %, as a main scope of
the present study was to investigate kinetic effects of the
catalysts rather than to optimize the reaction to achieve high
yields. In comparison, a series of Co(II)-cluster-based MOFs
in a very high loading (20 mol %) can produce up to 90% of 1-
indanone yield in the oxidation of indane with tBuOOH.49
Despite showing modest activity, different cobalt salts can also
be applied as simple homogeneous catalysts for the oxidation
of alcohols with tBuOOH.49,50
We also investigated an influence of the oxidant type on
product yields using a catalytic system that revealed the highest
activity. In fact, the oxidation of 1-indanol with H2O2,
catalyzed by 2, showed a low yield of 1-indanone (4%) after
24 h. A slightly higher yield (7%) but with a very low
selectivity of 6% was observed when using peracetic acid
(CH3CO3H) as a terminal oxidant (conversion of 1-indanol
was 97% with formation of a wide range of oxidation
products). The use of 3-chloroperbenzoic acid improved the
yield of 1-indanone up to 47%, which is comparable with the
system operating with the tBuOOH oxidant. Finally, we
attempted to perform the aerobic oxidation, as some
coordination compounds of cobalt are known to catalyze the
oxidation of alcohols with dioxygen.25 However, no 1-
indanone or any other product was detected. These data
suggest that tBuOOH is a preferable oxidant in the present Co-
catalyzed oxidation of alcohols. Better reactivity of tBuOOH in
comparison with H2O2 can be related to an enhanced
miscibility of the former with organic solvents. Besides,
tBuOOH is more stable and not prone to O2 release at high
concentrations (this side reaction is very typical for H2O2).
Despite showing high reactivity, peracids as oxidants typically
lead to overoxidation products due to Baeyer−Villiger
oxidation of ketones and other processes, thus resulting in a
low overall selectivity.
Figure 6. Accumulation of 1-indanone vs time in the 1-indanol
oxidation by tBuOOH in the presence of catalysts 1−4 (conditions of
the reactions are those of Table 2).
Cyclohexanol can also be used as a substrate, leading to a
maximum cyclohexanone yield of 21% for 2 and 7−10% for 1,
3, and 4 (Table 2). Similarly to 1-indanol, compound 2
exhibits the highest activity among all the catalysts studied. In
contrast to 1-indanone (Figure 6), the accumulation curves for
cyclohexanone demonstrate a complex behavior with a quasi-
linear dependence in the 30−150 min range (Figure S6, SI).
Replacement of cyclohexanol by a structurally related fenchyl
alcohol (Figure S6, SI) caused only a slight change in the
product yield when using 1, 3, and 4 after 24 h (Table 2),
while a yield drop was observed in the system catalyzed by 2.
Notably, all the catalytic systems exhibit almost 100%
selectivity in the case of cyclohexanol and fenchyl alcohol
oxidation, as no byproducts were detected. In the case of 1-
indanol oxidation, the selectivity to 1-indanone is at the 85%
level.
The highest yield of 1-indanone as comparing to other
substrates can be explained by an enhanced activity of the
proton located at the benzylic position; activation of this
proton is a required step. The same reason can explain a lower
selectivity in the oxidation of 1-indanol, where activation of the
second benzylic C−H bond can produce various products up
to 1,3-indandione. Lower activity of the fenchyl alcohol relative
to cyclohexanol can be due to a pronounced sterical hindering
of the methyl and methylene groups close the alcohol one.
Although all the compounds 1−4 are insoluble in the
reaction medium and act as heterogeneous catalysts; some
leaching of cobalt(II) ions can be detected at a prolonged
reaction time, as confirmed by chelation experiments with 2,2′-
bipyridine (bpy).33a In contrast to a linear dependence of the
ketone accumulation demonstrated by 2 (Figure S7), the
2+bpy system shows acceleration with the initial reaction rate
W0 of 2.3 × 10−6 M s−1. Such a reaction rate is very close to
that observed (Figure S7) for the system containing only 2
(1.5 × 10−6 M s−1). This behavior can be explained by a small
leaching of the cobalt ions during the reaction. At a low
concentration, homogeneous cobalt species do not show
notable activity and the accumulation curves for 1−4 remain
linear. On the other side, introduction of bpy as a strong
chelator may additionally contribute to a disaggregation of the
heterogeneous catalyst, leading to generation of homogeneous
Co-bpy species.
CONCLUSIONS
■
We showed that new types of cobalt(II) CPs and MOFs can be
easily generated, by hydrothermal synthesis in water, from
Co(II) chloride and two flexible and still very poorly explored
pyridine-carboxylic acid ligands (H2cpna and H3dppa). The
structures of the obtained products vary from 2D metal−
organic layers (2 and 3) to 3D frameworks (1 and 4) and
feature different topologies and architectures, which also
include unique types. An observed diversity of the structures
of 1−4 suggests that both a main pyridine-carboxylate ligand
and an optional N,N-donor crystallization mediator affect the
construction and resulting structures of these CPs or MOFs.
Thermal and aqueous stability, and catalytic and magnetic
properties of compounds 1−4 were studied as well. In
particular, catalytic studies disclosed that 1−4 are active
heterogeneous catalysts in the oxidation of alcohols with
tBuOOH. Compound 2, bearing μ3-cpna2− and phen ligands,
revealed a considerably higher activity than the other products,
suggesting a particular catalytic efficiency of such a
combination of ligands.
Furthermore, the obtained products 1 and 2 represent a rare
type of coordination polymers assembled from H2cpna, while
compounds 3 and 4 are the first Co(II) CPs assembled from
H3dppa, according to the Cambridge Structural Database
(CSD). Hence, the present study also widened an application
of such flexible pyridine-carboxylate blocks toward designing
new metal−organic architectures. Further research with a focus
on assembling related cobalt(II) CPs and MOFs and on the
Although yields of products exhibited by the catalysts 1−4
are moderate if compared to other heterogeneous catalysts,47,48
it should be noted that, in the case of 1−4, the lower yields are
H
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00242.
Selected bond lengths (Table S1) and H-bond
parameters (Table S2), additional figures with repre-
sentation of crystal structures (Figures S1−S3), TGA
(Figure S4) and PXRD data (Figure S5), kinetic curves
(Figures S6 and S7) (PDF)
Accession Codes
CCDC 1874595−1874598 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: gujzh@lzu.edu.cn. Tel.: +86-931-8915196 (J.G.).
*E-mail: kirillov@tecnico.ulisboa.pt. Tel.: +351-218417178
(A.M.K.).
(6) (a) Lin, R. B.; Li, F.; Liu, S. Y.; Qi, X. L.; Zhang, J. P.; Chen, X.
M. A noble-metal-free porous coordination framework with excep-
tional sensing efficiency for oxygen. Angew. Chem., Int. Ed. 2013, 52,
13429−13433. (b) Lustig, W. P.; Mukherjee, S.; Rudd, N. D.; Desai,
A. V.; Li, J.; Ghosh, S. K. Metal-organic frameworks: functional
luminescent and photonic materials for sensing applications. Chem.
Soc. Rev. 2017, 46, 3242−3285. (c) DeCoste, J. B.; Peterson, G. W.
Metal-organic frameworks for air purification of toxic chemicals.
Chem. Rev. 2014, 114, 5695−5727.
ORCID
Jinzhong Gu: 0000-0001-6704-7370
Zifa Shi: 0000-0001-9231-5963
Dmytro S. Nesterov: 0000-0002-1095-6888
Alexander M. Kirillov: 0000-0002-2052-5280
Notes
(7) (a) Chen, Q.; Lin, J. B.; Xue, W.; Zeng, M. H.; Chen, X. M. A
porous coordination polymer assembled from 8-connected
The authors declare no competing financial interest.
{CoII (OH)} clusters and isonicotinate: multiple active metal sites,
3
ACKNOWLEDGMENTS
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■
The present work has been financially supported by the
National Natural Science Foundation of China (Project
21201091), the Foundation for Science and Technology
(FCT), and Portugal 2020 (projects IF/01395/2013/
CP1163/CT005, CEECIND/03708/2017, UID/QUI/
00100/2013, IST-ID/086/2018, and LISBOA-01-0145-
FEDER-029697). The publication was also prepared with the
support of the RUDN University Program 5-100. A.M.K.
acknowledges the COST Action CA15106 (CHAOS).
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