Metal-Organic Architectures Assembled from Multifunctional Polycarboxylates: Hydrothermal Self-Assembly, Structures, and Catalytic Activity in Alkane Oxidation

Article abstract of DOI:10.1021/acs.inorgchem.8b02926

A three-component aqueous reaction system comprising copper(II) acetate (metal node), poly(carboxylic acid) with a phenylpyridine or biphenyl core (main building block), and 1,10-phenanthroline (crystallization mediator) was investigated under hydrothermal conditions. As a result, four new coordination compounds were self-assembled, namely, {[Cu(μ₃-cpna)(phen)]·H₂O}_n (1), {[Cu(μ-Hbtc)(phen)]·H₂O}_n (2), {[Cu(μ₃-Hcpic)(phen)]·2H₂O}_n (3), and [Cu₆(μ-Hcptc)₆(phen)₆]·6H₂O (4), where H₂cpna = 5-(2′-carboxylphenyl)nicotinic acid, H₃btc = biphenyl-2,4,4′-tricarboxylic acid, H₃cpic = 4-(5-carboxypyridin-2-yl)isophthalic acid, H₃cptc = 2-(4-carboxypyridin-3-yl)terephthalic acid, and phen = 1,10-phenanthroline. Crystal structures of compounds 1-3 reveal that they are 1D coordination polymers with a ladder, linear, or double-chain structure, while product 4 is a 0D hexanuclear complex. All of the structures are extended further [1D a?' 2D (1 and 2), 1D a?' 3D (3), and 0D a?' 3D (4)] into hydrogen-bonded networks. The type of a multicarboxylate building block has a considerable effect on the final structures of 1-4. The magnetic behavior and thermal stability of 1-4 were also investigated. Besides, these copper(II) derivatives efficiently catalyze the oxidation of cycloalkanes with hydrogen peroxide under mild conditions. The obtained products are the unique examples of copper derivatives that were assembled from H₂cpna, H₃btc, H₃cpic, and H₃cptc, thus opening up their use as multicarboxylate ligands toward the design of copper-organic architectures.

Full text of DOI:10.1021/acs.inorgchem.8b02926

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Metal−Organic Architectures Assembled from Multifunctional
Polycarboxylates: Hydrothermal Self-Assembly, Structures, and
Catalytic Activity in Alkane Oxidation
Jinzhong Gu,*,† Min Wen,† Yan Cai,† Zifa Shi,† Aliaksandr S. Arol,‡ 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, Avenida Rovisco Pais, 1049-001 Lisbon,
Portugal
§Research Institute of Chemistry, Peoples’ Friendship University of Russia (RUDN University), 6 Miklukho-Maklaya Street,
Moscow 117198, Russian Federation
S
* Supporting Information
ABSTRACT: A three-component aqueous reaction system comprising
copper(II) acetate (metal node), poly(carboxylic acid) with a phenylpyridine
or biphenyl core (main building block), and 1,10-phenanthroline (crystal-
lization mediator) was investigated under hydrothermal conditions. As a
result, four new coordination compounds were self-assembled, namely,
{[Cu(μ3-cpna)(phen)]·H2O}n (1), {[Cu(μ-Hbtc)(phen)]·H2O}n (2), {[Cu-
(μ3-Hcpic)(phen)]·2H2O}n (3), and [Cu6(μ-Hcptc)6(phen)6]·6H2O (4),
where H2cpna = 5-(2′-carboxylphenyl)nicotinic acid, H3btc = biphenyl-2,4,4′-
tricarboxylic acid, H3cpic = 4-(5-carboxypyridin-2-yl)isophthalic acid, H3cptc
= 2-(4-carboxypyridin-3-yl)terephthalic acid, and phen = 1,10-phenanthroline.
Crystal structures of compounds 1−3 reveal that they are 1D coordination
polymers with a ladder, linear, or double-chain structure, while product 4 is a
0D hexanuclear complex. All of the structures are extended further [1D → 2D
(1 and 2), 1D → 3D (3), and 0D → 3D (4)] into hydrogen-bonded
networks. The type of a multicarboxylate building block has a considerable effect on the final structures of 1−4. The magnetic
behavior and thermal stability of 1−4 were also investigated. Besides, these copper(II) derivatives efficiently catalyze the
oxidation of cycloalkanes with hydrogen peroxide under mild conditions. The obtained products are the unique examples of
copper derivatives that were assembled from H2cpna, H3btc, H3cpic, and H3cptc, thus opening up their use as multicarboxylate
ligands toward the design of copper−organic architectures.
owing to a possible rotation of two aromatic cycles and variable
levels of deprotonation of carboxylic acid groups. These
characteristics of multifunctional carboxylic acid ligands can
lead to the generation of new CPs with distinct structures.25−27
In this work, an aqueous-medium system comprising
copper(II) acetate, a multicarboxylic acid building block, and
a crystallization mediator has been explored toward the
hydrothermal generation of a new series of copper-based
metal−organic architectures. The selection of copper as a
metal node was governed by its low cost, rich coordination and
bioinorganic chemistry, and a plethora of potential applications
of copper coordination compounds, including molecular
magnetism and oxidation catalysis.4,5,28,29
INTRODUCTION
■
Coordination polymers (CPs) and related metal−organic
architectures are of great interest in modern inorganic and
materials chemistry because of their structural features,
functional properties, and variety of applications in catalysis,
magnetism, sensing, luminescence, and selective sorption.1−10
The generation of CPs can be affected by numerous
parameters such as the nature of the metal nodes, organic
building blocks, and supporting ligands, stoichiometry, type of
solvent, and reaction temperature.11−16 In particular, diverse
aromatic multicarboxylic acids are commonly applied for the
design of CPs because such building blocks are thermally
stable and flexible and can easily satisfy the charge balance of
metal nodes, thus leading to a variety of coordination
modes.1−3,17−22 Carboxylic acids with phenylpyridine or
biphenyl cores and a different arrangement of the COOH
and pyridine N sites have been of particular interest,23−27
Received: October 15, 2018
© XXXX American Chemical Society
A
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Hence, following our research line on the synthesis of novel
copper-based CPs and their application in catalysis,4,30,31
herein we report the assembly (hydrothermal synthesis),
crystal structures, thermal stability and magnetic behavior, and
catalytic activity of four compounds, which are driven by
different and poorly explored multicarboxylate building blocks
(Scheme 1). The obtained products are 1D CPs {[Cu(μ3-
Furthermore, a Cambridge Structural Database (CSD) search
revealed that 1−4 are the first copper coordination compounds
assembled from H2cpna, H3btc, H3cpic, and H3cptc.
EXPERIMENTAL SECTION
■
Reagents and Methods. All reagents were from commercial
sources (analytical reagent grade) and were used as received.
Elemental analyses (C/H/N) were obtained using an Elementar
Vario EL elemental analyzer. Fourier transform infrared (FTIR)
analyses were run using KBr disks and a Bruker EQUINOX 55
spectrometer. Thermogravimetric analyses (TGA) were carried out
on a LINSEIS STA PT1600 thermal analyzer (N2 atmosphere; 2.5
°C/min heating rate). Powder X-ray diffraction (PXRD) data were
obtained on a Rigaku Dmax 2400 diffractometer (Cu Kα radiation; λ
= 1.54060 Å). Measurements of the magnetic susceptibility were
performed on a Quantum Design MPMS XL-7 SQUID magneto-
meter (2−300 K; 0.1 T magnetic field strength). Prior to analysis of
the data, a correction for the diamagnetic contribution was performed.
The χMT and 1/χM versus T plots and a description of the magnetic
behavior of 1−4 are given in the Supporting Information (SI).
Analysis of the reaction solutions in catalytic tests was performed by
gas chromatography (GC) using an Agilent Technologies 7820A
series gas chromatograph (carrier gas, He; detector, FID detector;
capillary column, BP20/SGE, 30 m × 0.22 mm × 0.25 μm).
Synthesis and Analytical Data for 1−4. {[Cu(μ3-cpna)(phen)]·
H2O}n (1). The mixture of Cu(CH3COO)2·H2O (0.1 mmol, 20.0 mg),
H2cpna (0.1 mmol, 24.3 mg), and phen (0.1 mmol, 20.0 mg) in water
(H2O; 10 mL) was vigorously stirred for 15 min and then transferred
to a Teflon-lined stainless steel reactor (25 mL volume). It was heated
for 3 days at 120 °C and then slowly cooled to ambient temperature
(cooling rate: 10 °C/h). This reaction produced blue crystals, which
were separated from the reaction mixture (either manually or by
filtration), washed with H2O, and dried in air to furnish product 1.
Yield: 65% (based on H2cpna). Calcd for C25H17CuN3O5: C, 59.70;
H, 3.41; N, 8.35. Found: C, 59.88; H, 3.39; N, 8.41. FTIR (KBr,
cm−1): 3492w, 3062w, 1642s, 1517w, 1492w, 1427m, 1381m, 1362s,
1284w, 1147w, 1108w, 1030w, 945w, 912w, 854 w, 769w, 723m,
671w, 645w, 534w.
Scheme 1. Multifunctional Carboxylic Acids
cpna)(phen)]·H2O}n (1), {[Cu(μ-Hbtc)(phen)]·H2O}n (2),
and {[Cu(μ3-Hcpic)(phen)]·2H2O}n (3), as well as a discrete
0D hexacopper(II) complex, [Cu6(μ-Hcptc)6(phen)6]·6H2O
(4), where H2cpna = 5-(2′-carboxylphenyl)nicotinic acid,
H3btc = biphenyl-2,4,4′-tricarboxylic acid, H3cpic = 4-(5-
carboxypyridin-2-yl)isophthalic acid, H3cptc = 2-(4-carboxy-
pyridin-3-yl)terephthalic acid, and phen = 1,10-phenanthro-
line. Their structural diversity suggests that the type of
multicarboxylate block affects the structural characteristics of
1−4. Besides, these copper(II) derivatives were explored as
effective homogeneous catalysts for cycloalkane oxidation by
hydrogen peroxide (H2O2) under mild conditions. These
copper(II) catalysts are highly active without the need for an
additive (strong acid), which constitutes a remarkable feature.
Such a behavior can be explained by the incorporation of
multicarboxylic acid building blocks into the structures of 1−4.
{[Cu(μ-Hbtc)(phen)]·H2O}n (2). CP 2 was prepared following a
method described for 1 but using H3btc instead of H2cpna. Blue
crystals were separated from the reaction mixture (either manually or
Table 1. Crystal and Structure Refinement Data for 1−4
1
2
3
4
chemical formula
fw
C25H17CuN3O5
502.95
C27H18CuN2O7
545.98
C26H19CuN3O8
564.98
C78H51Cu3N9O21
1640.90
cryst syst
space group
a/Å
b/Å
c/Å
α/deg
β/deg
γ/deg
V/Å3
triclinic
triclinic
monoclinic
P21/n
14.4510(11)
9.9874(6)
15.933(3)
90
94.362(11)
90
2292.9(5)
293(2)
4
triclinic
P1
̅
P1
̅
P1
̅
9.611(3)
10.217(2)
12.399(2)
98.798(17)
106.65(2)
110.31(2)
1049.9(5)
293(2)
2
9.1647(9)
10.4481(15)
12.1649(14)
94.385(11)
93.136(9)
91.169(10)
1159.3(2)
293(2)
9.6278(6)
19.0451(11)
19.4115(13)
84.497(5)
77.500(5)
78.582(5)
3401.0(4)
293(2)
T/K
Z
2
2
Dc/g cm−3
μ/mm−1
F(000)
1.591
1.086
514
1.561
0.995
556
1.637
1.014
1156
1.602
1.019
1674
reflns measd
unique reflns (Rint)
GOF on F2
R1 [I > 2σ(I)]
wR2 [I > 2σ(I)]
6292
3722 (0.0538)
0.994
0.0614
0.1185
7552
4107 (0.1233)
0.992
0.0896
0.1099
7753
4053 (0.1107)
0.998
0.0803
0.0988
20685
12022 (0.0639)
0.993
0.0734
0.1405
B
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by filtration), washed with H2O, and dried in air to furnish product 2.
Yield: 60% (based on H3btc). Calcd for C27H18CuN2O7: C, 59.40; H,
3.32; N, 5.13. Found: C, 59.57; H, 3.33; N, 5.10. FTIR (KBr, cm−1):
3347w, 3063w, 1691w, 1586s, 1541w, 1517w, 1430w, 1390s, 1360w,
1256w, 1222w, 1147w, 1106w, 1002w, 909w, 874w, 857w, 793w,
776w, 723w, 690w, 660w, 551w.
{[Cu(μ3-Hcpic)(phen)]·2H2O}n (3). CP 3 was prepared following a
method described for 1 but using H3cpic instead of H2cpna. Blue
crystals were separated from the reaction mixture (either manually or
by filtration), washed with H2O, and dried in air to furnish product 3.
Yield: 50% (based on H3cpic). Calcd for C26H19CuN3O8: C, 55.27;
H, 3.39; N, 7.44. Found: C, 55.13; H, 3.36; N, 7.39. FTIR (KBr,
cm−1): 3433m, 2925w, 1694w, 1602s, 1518w, 1433w, 1362s, 1270w,
1244w, 1128w, 1050w, 1023w, 945w, 841w, 801 w, 776w, 723w,
691w, 651w, 541w.
copper catalyst (blank tests) confirmed that there was no oxidation of
the alkane substrates.
RESULTS AND DISCUSSION
Structural Description. Compound 1. CP 1 features a 1D
ladder chain (Figure 1). In the asymmetric unit of 1, there is a
■
[Cu6(μ-Hcptc)6(phen)6]·6H2O (4). Compound 4 was prepared
following a method described for 1 but using H3cptc instead of
H2cpna. Blue crystals were separated from the reaction mixture
(either manually or by filtration), washed with H2O, and dried in air
to furnish product 4. Yield: 55% (based on H3cptc). Calcd for
C78H51Cu3N9O21: C, 57.09; H, 3.13; N, 7.68. Found: C, 56.94; H,
3.15; N, 7.63. FTIR (KBr, cm−1): 3318w, 3046w, 1674s, 1598s,
1517w, 1430w, 1413w, 1337s, 1286w, 1262w, 1176w, 1106w, 1048w,
950w, 858w, 776w, 724w, 695w, 648w, 567w.
X-ray Diffraction Study. For 1−4, the X-ray diffraction data were
obtained using a Bruker Smart CCD diffractometer (λ = 0.71073 Å;
graphite-monochromated Mo Kα radiation). The SADABS program
was used for an absorption correction (semiempirical). The SHELXS-
97 and SHELXL-97 programs were used for solving the structures
(direct methods), followed by their refinement (full-matrix least
squares on F2 procedure).32 The full-matrix least squares on F2
procedure was applied for an anisotropic refinement of all non-
hydrogen atoms. Hydrogen atoms (with the exception of those in the
H2O/OH moieties) were added in the respective calculated positions
(with fixed isotropic thermal parameters) and considered in the
structure factor calculations during the last stage of refinement with a
full-matrix least-squares method. Hydrogen atoms of the H2O/OH
moieties were placed using difference maps with a constrainment to
the respective oxygen atoms. The crystal data of 1−4 are given in
Table 1, while the relevant bonding and hydrogen-bonding
parameters are collected in Tables S1 and S2, respectively.
Figure 1. Fragments of the crystal structure of 1. (a) Connectivity and
coordination environment of the Cu1 center (hydrogen atoms are not
shown). (b) 1D ladder CP chain (view down the c axis).
Cu1 center, a μ3-cpna2− block, a phen moiety, and a H2O
molecule of crystallization. The five-coordinate copper(II)
atom is surrounded by two carboxylate oxygen donors coming
from two different μ3-cpna2− moieties, an nitrogen donor from
another μ3-cpna2− ligand, and two phen nitrogen donors, thus
forming a distorted square-pyramidal {CuN3O2} environment
with the τ parameter of 0.0792 (τ = 0 or 1 for a regular square-
pyramidal or trigonal-bipyramidal geometry, respectively).35
The Cu−N and Cu−O bonds are in the 2.034(4)−2.382(4)
and 1.934(3)−1.965(3) Å ranges, respectively (Figure 1a);
these bond lengths are within the typical values for related
copper(II) compounds22,36,37 and CPs of other metals derived
from H2cpna.25 In 1, cpna2− acts as a μ3-N,O2 spacer, with the
carboxylate groups being monodentate (mode I, Scheme 2). A
Besides, to better understand the structures of 1−4, their
topological analysis was carried out by applying an underlying net
concept.33 A simplified net was constructed by omitting all terminal
ligands and transforming all bridging ligands into the corresponding
centroids; connectivity of the bridging ligands with the copper(II)
nodes via coordination bonds was maintained.
Scheme 2. Coordination Modes of μ3-cpna2− (I), μ-Hbtc2−
(II), μ3-Hcpic2− (III), and μ-Hcptc2− (IV and V) in
Compounds 1−4, Respectively
Catalytic Studies. The oxidation of alkanes under mild conditions
(ambient pressure and air atmosphere) was investigated in glass
reactors that were thermostated at 50 °C and equipped with the reflux
condenser. The reactions were carried out under vigorous stirring in
acetonitrile (MeCN) as a solvent (total volume of the reaction
mixture was up to 2.5 mL). Typically, a copper(II) catalyst (5.0
μmol), trifluoroacetic acid (TFA, CF3COOH, optional; 50 μmol as a
stock solution in MeCN), an alkane substrate (1 mmol), and a GC
internal standard (MeNO2, 25 μL) were added to an MeCN solution.
Then, the oxidation reaction was initiated by adding H2O2 (5 mmol;
aqueous 50% solution). In the course of the oxidation reactions,
aliquots were taken from the reaction mixtures and then treated with a
minimum amount of solid triphenylphosphine (PPh3).34 Such a
treatment allows reduction of the alkyl hydroperoxide (ROOH,
common initial product in the oxidation of alkanes with H2O2) and
the remaining oxidant. The obtained samples of the reaction mixture
were then subjected to GC analysis. The presence of ROOH primary
products was further confirmed by analyzing the selected samples
twice by GC, with and without treatment with PPh3, following a
method developed by Shul’pin.34 Assignment of the peaks on the gas
chromatograms was performed by recording the GC plots of
commercial samples; the GC internal standard was used for
quantification of the products. Experiments in the absence of a
dihedral angle of 88.74° is observed between the rings of μ3-
cpna2−. The μ3-cpna2− blocks multiply interconnect the
adjacent copper(II) centers to form a 1D ladder chain (Figure
1b). Adjacent chains then further assemble to a 2D hydrogen-
bonded layer through O−H···O hydrogen bonds (Figure S1
and Table S2). From a topological viewpoint (Figure S2), the
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1D ladders are assembled from 3-connected copper and μ3-
cpna2− nodes, resulting in a 3-connected underlying chain with
the SP 1-periodic net (4,4)(0,2) topology. It is defined by a
(42.6) point symbol.
Compound 2. Product 2 is also a 1D CP (Figure 2). An
asymmetric unit of 2 has a Cu1 center, a μ-Hbtc2− linker, a
Figure 3. Fragments of the crystal structure of 3. (a) Connectivity and
coordination environment of the Cu1 center (CH hydrogen atoms are
not shown). (b) 1D double chain rotated along the c axis (phen rings
are not shown).
S5). There is also a further extension of the structure by
hydrogen bonds, generating a complex 3D hydrogen-bonded
net (Table S2 and Figure S6).
Figure 2. Fragments of the crystal structure of 2. (a) Connectivity and
coordination environment of the Cu1 center (CH hydrogen atoms are
not shown). (b) 1D linear CP chain (view down the c axis).
Compound 4. The structure of 4 has a discrete hexacopper-
(II) molecular unit with a cyclic tetracopper core (Figure 4).
Its asymmetric unit contains three crystallographically
independent copper(II) atoms, three μ-Hcptc2− ligands,
three phen moieties, and three crystallization H2O molecules.
All copper(II) centers are five-coordinate and possess the
distorted square-pyramidal {CuN3O2} (Cu1), {CuN2O3}
(Cu2), and {CuN4O} (Cu3) geometries with τ parameters
of 0.256, 0.154, and 0.128, respectively. The Cu1 center is
surrounded by two carboxylate oxygen atoms and one nitrogen
donor from two μ-Hcptc2− ligands, in addition to two phen
nitrogen donors. The Cu2 center is coordinated by three
oxygen atoms and two nitrogen donors from three μ-Hcptc2−
blocks. A “lateral” Cu3 center is connected to the carboxylate
oxygen atom from a μ-Hcptc2− moiety and two pairs of phen
nitrogen donors. The Cu−N and Cu−O distances are within
the 1.962(5)−2.241(6) and 1.941(5)−2.298(4) Å ranges,
correspondingly. All of the Hcptc2− blocks act as μ-N,O2
spacers, and their COO−/COOH groups are monodentate and
bidentate or remain uncoordinated (Scheme 2, modes IV and
V). Dihedral angles in μ-Hcptc2− vary from 89.20° to 44.50°
and 44.49°. The oxygen and nitrogen donors of six μ-Hcptc2−
ligands interconnect the copper atoms into a discrete
hexacopper(II) unit (Figure 4b), wherein the central Cu4
core is assembled from two pairs of Cu1/Cu2 nodes and μ-
Hcptc2− linkers (Figure S7). Besides, the intermolecular O−
H···O hydrogen bonds provide an extension of the discrete
Cu6 units into a 3D hydrogen-bonded net (Table S2 and
Figure S8).
phen moiety, and a crystallization H2O molecule. The four-
coordinate Cu1 atom shows a distorted {CuN2O2} square-
planar geometry, which is taken by two oxygen donors
(coming from two μ-Hbtc2− blocks) and a couple of phen
nitrogen donors (Figure 2a). The Cu−O [1.9429(3)−
1.9435(3) Å] and Cu−N [1.986(8)−1.994(6) Å] distances
are comparable to those in related copper(II) deriva-
tives.26,37,38 The Hbtc2− block functions as a μ-linker (mode
II, Scheme 2), and its two deprotonated COO− functionalities
are monodentate. The dihedral angle of 53.58° is observed
between the aromatic rings in μ-Hbtc2−. The μ-Hbtc2− blocks
interlink the neighboring Cu1 atoms into linear metal−organic
chains having a Cu···Cu distance of 9.165 Å (Figure 2b). Such
chains are classified within the 2C1 topological type (Figure
S3). Besides, the 1D metal−organic chains extend into a 2D
hydrogen-bonded network via the O−H···O hydrogen bonds
(Table S2 and Figure S4).
Compound 3. This compound features a double 1D metal−
organic chain (Figure 3). There is a Cu1 center, a μ3-Hcpic2−
block, a phen moiety, and a couple of crystallization H2O
molecules. The five-coordinate Cu1 atom forms a well-
distorted square-pyramidal {CuN3O2} environment (τ =
0.256) that is occupied by two oxygen and one nitrogen
atoms from three μ3-Hcpic2− ligands and two phen nitrogen
atoms (Figure 3a). The Cu−O [1.911(4)−1.975(4) Å] and
Cu−N [1.999(6)−2.559(5) Å] distances are well comparable
to those of related copper derivatives.36−38 In 3, the Hcpic2−
spacers show a μ3-mode (Scheme 2, mode III), with the
COO− groups being monodentate; the dihedral angle that
separates the aromatic functionalities attains 58.86°. The
neighboring Cu1 atoms are interconnected via the oxygen and
nitrogen donors from the μ3-Hcpic2− spacers, thus forming a
CP with a 1D double-chain structure (Figure 3b). These
double chains are topologically similar to those in 1 (Figure
Synthetic Aspects and Structural Comparison. All of
the products 1−4 were synthesized using a similar hydro-
thermal protocol (using H2O as a green solvent) by exploring a
three-component reaction system: copper(II) acetate (metal
node)−multicarboxylic acid with a phenylpyridine or biphenyl
core (main building block)−1,10-phenanthroline (crystalliza-
tion mediator). The effect of the type of multicarboxylate
ligand containing a phenylpyridine or biphenyl core was
explored, resulting in the successful crystallization of four new
D
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Figure 4. Fragments of the crystal structure of 4. (a) Connectivity and coordination environment of copper centers (CH hydrogen atoms are not
shown). (b) Discrete hexacopper(II) molecular unit with a cyclic Cu4 core (phen rings are not shown).
Table 2. Selected Structural Data and Catalytic Properties for Compounds 1−4
Cu coordination number
{environment}
presence of uncoordinated
dimensionality (→ hydrogen-
maximum yield in cycloalkane
a
compound
−COOH groups
bonded net)
oxidation (%)
{[Cu(μ3-cpna)(phen)]·
H2O}n (1)
{[Cu(μ-Hbtc)(phen)]·
H2O}n (2)
{[Cu(μ3-Hcpic)(phen)]·
2H2O}n (3)
[Cu6(μ-Hcptc)6(phen)6]·
6H2O (4)
5 {CuN3O2}
4 {CuN2O2}
5 {CuN3O2}
0
1
1
1
1D → 2D
1D → 2D
1D → 3D
0D → 3D
20−30
18−22
15−18
13−19
5 {CuN2O3}, {CuN3O2},
{CuN4O}
a
For details, see Table 3.
copper(II) coordination compounds. The oxidation state of
copper in all compounds is 2+, as confirmed by detailed
magnetic studies (see the SI). The multicarboxylate blocks act
as μ3-spacers (in 1 and 3) or μ-linkers (in 2 and 4), while their
COO− groups take a monodentate or bidentate mode. In 2−4,
one COOH group remains protonated and uncoordinated but
participates in the intermolecular hydrogen-bonding inter-
actions. The pyridyl nitrogen atoms in the cpna2−, Hcpic2−,
and Hcptc2− blocks behave as nitrogen donors for copper(II)
centers in products 1, 3, and 4.
To achieve the required environment of the copper(II)
coordination sphere during hydrothermal synthesis, the C−C
bond between the two aromatic rings in the cpna2−, Hbtc2−,
Hcpic2−, or Hcptc2− blocks should have a certain degree of
flexibility and rotation, as attested by the corresponding
dihedral angles that are in the 44.49−89.20° range.
Compounds 1−3 possess different types of 1D metal−organic
chains (ladder, zigzag, or double chain, respectively), while
compound 4 features a discrete 0D hexacopper(II) structure.
An observed variation of the structures suggests that their
hydrothermal generation depends on the type of multi-
carboxylate block. The copper(II) centers in 1−4 adopt
square-pyramidal (in 1, 3, and 4) or square-planar (in 2)
geometries. Table 2 provides a brief summary of the main
structural data and catalytic properties of the obtained
compounds.
temperature interval; the remaining solid shows stability upon
heating until 239 °C. The TGA curve of 2 indicates a thermal
effect (102−163 °C), which is associated with the loss of a
H2O molecule of crystallization (exptl, 3.5%; calcd, 3.3%);
heating the sample further above 227 °C provokes its
decomposition. In CP 3, mass loss due to the release of two
crystallization H2O molecules can be seen in the 105−144 °C
range (exptl, 6.3%; calcd, 6.4%), followed by decomposition of
the sample above 240 °C. In complex 4, a principal thermal
effect (48−108 °C) is associated with a loss of six lattice H2O
molecules (exptl, 3.4%; calcd, 3.3%). A sample obtained after
dehydration maintains its stability up to 285 °C. Cu2O is
expected as a final decomposition product of 1−4 at 1000 °C.
A summary of the main TGA data is provided in Table S3).
The PXRD patterns were obtained at ambient temperature
using microcrystalline samples of 1−4. Experimental PXRD
plots well match the pattern calculated from the single-crystal
X-ray diffraction data (Figure S10), indicating that the as-
synthesized bulk materials are pure products.
Cycloalkane Oxidation Catalyzed by 1−4. The catalytic
activity of all products 1−4 was studied in the mild
homogeneous oxidation of C6−C8 cycloalkanes to give a
mixture of respective alcohols and ketones (Scheme 3).
Scheme 3. Mild Catalytic Oxidation of Cycloalkanes
Discussion of the TGA and PXRD Data. For 1−4, the
TGA (N2 atmosphere; 25−1000 °C) are shown in Figure S9
and briefly discussed below. CP 1 exhibits a loss of the lattice
H2O molecule (exptl, 3.4%; calcd, 3.6%) in the 104−162 °C
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Figure 5. Cyclohexane oxidation (total yield of products, cyclohexanol, and cyclohexanone vs time) by H2O2 catalyzed by 1−4 (a−d) in the
presence and in the absence of CF3COOH (TFA). Conditions of the reactions: compounds 1−4, 5 μmol; cyclohexane, 1 mmol; TFA (optional),
50 μmol; H2O2, 5 mmol; MeCN, until 2.5 mL of total reaction volume; temperature, 50 °C.
As model substrates, cycloalkanes were chosen because of
the similarity of all of their carbon atoms. Besides, such
oxidation reactions are relevant in different fields39−41 that
span from the caprolactam production (precursor of nylon-6)
to the biological oxidation of alkanes with an enzyme
(particulate methane monooxygenase) that contains a multi-
copper active center.40 Herein, the mild oxidation of
cycloalkanes was investigated in air at 50 °C and in MeCN/
H2O solution, applying H2O2 as the oxidant (50% in H2O).
Catalytic data are shown in Figures 5 and 6 as well as in Table
3; hereinafter, the yields of products refer to the yields based
on the substrate, i.e., (moles of product per mole of
cycloalkane) × 100%.
Given the recognized effect of strong acids on promoting the
mild oxidation of alkanes by H2O2,4,38,39 we studied the
catalytic behavior of 1−4 in cyclohexane oxidation, with and
without the use of an acid additive. As a typical acid promoter,
TFA was used, and the obtained results are shown in Figure 5.
For catalyst 1, better activity (20% total product yield) is
seen when the acid additive is not used (Figure 5a), whereas
the total product yield is lower (15%) in the presence of TFA.
The effect of TFA is negligible when using 2 as a catalyst,
resulting in 18% total product yield in both systems with and
without acid promoter (Figure 5b). When using the 3/TFA
system, the cyclohexane oxidation is quicker, although the total
yields achieved in both systems with and without TFA are
similar (16−17%) after 6 h of the reaction (Figure 5c).
Compound 4 shows a resembling trend but is less active than 3
(Figure 5d). In the case of catalysts 2−4, a lag period is
observed (up to 120 min for 4). Such a lag period is not
detected when using the TFA promoter, which facilitates the
generation of catalytically active species. The observed
variations in the catalytic behavior of 1−4 are associated
with their structural differences. Besides, in contrast to many
other catalytic systems that require an acid promoter,4,30,39,41,42
all of the tested herein catalysts show similar or superior
activity in the absence of acid additives. This can be explained
by the presence of multicarboxylate blocks in the structure of
the catalyst. Hence, further tests were performed in the
absence of TFA additive.
To explore the substrate scope for these catalytic systems,
we investigated the oxidation of cyloheptane and cyclooctane
(Figure 6 and Table 3). Among all catalysts, the highest
activity is shown by 1 for the oxidation of cycloheptane,
leading to 30% total yield of the corresponding alcohol and
ketone (Figure 6a). The same catalytic system is also active in
the oxidation of cyclooctane with the total product yield of
22%. Interestingly, for all other catalysts (2−4), cyclooctane is
the most reactive substrate (18−22% total yields), followed by
cycloheptane (15−17% total yields) and cyclohexane (13−
18% total yields).
In the research on the oxidative C−H functionalization of
saturated hydrocarbons, the obtained yields of oxidation
products (up to 30%) can be considered as very significant.39,41
For instance, an industrial method (DuPont) for cyclohexane
oxidation uses a homogeneous catalyst (cobalt naphthenate)
and proceeds with a maximum substrate conversion of ∼5−
10%, also requiring higher temperatures and pressures.41d The
yields of cyclohalkane oxidation products achieved herein with
catalysts 1−4 are also superior or similar to the ones reported
for other catalytic systems.4,39,42 However, because of the
presence of multicarboxylic acid ligands, compounds 1−4 can
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Figure 6. C6−C8 cycloalkane oxidation: cyclohexane, cycloheptane, and cyclooctane (alcohol and ketone total yield vs time) with H2O2 in the
presence of catalysts 1−4 (a−d). Reaction conditions: compounds 1−4, 5 μmol; cycloalkane, 1 mmol; H2O2, 5 mmol; MeCN, until 2.5 mL of total
reaction volume; temperature, 50 °C.
To get additional information on the selectivity parameters
and the type of oxidizing species in reactions catalyzed by 1−4,
the mild oxidation of a linear alkane (n-heptane), a substituted
cycloalkane (methylcyclohexane), and stereoisomeric cyclo-
alkanes (cis- and trans-dimethylcyclohexane) was investigated
(Table 4). For all of the tested copper(II) catalysts, n-C7H16
oxidation occurs with no preference toward a specific
secondary carbon atom at the n-heptane skeleton, showing a
regioselectivity parameter C1:C2:C3:C4 equal to 1:4:5:5 (for
1 and 2) or 1:4:4:4 (for 3 and 4). When using
methylcyclohexane as a model substrate, the bond selectivity
values (1°:2°:3°) of 1:5:16 (1), 1:14:17 (2), 1:5:14 (3), and
1:5:13 (4) are quite resembling as well, thus indicating
preferable oxidation of the tertiary carbon atom in comparison
with the secondary carbon atoms. Besides, the application of
cis or trans isomers of 1,2-dimethylcyclohexane as substrates
indicates that their oxidation proceeds in a nonstereoselective
way, as evidenced by the molar ratios (trans/cis = 0.8−0.9)
among the produced tertiary alcohol isomers. Inversion of the
configuration was also observed to some extent, wherein the cis
isomers are formed in higher amounts in the oxidation of both
substrates (cis and trans isomers of 1,2-dimethylcyclohexane).
Analysis of the obtained catalysis data and various selectivity
parameters shows that these well compare with other copper-
based systems for cycloalkane oxidation, also supporting a
mechanism proceeding with HO• radicals as indiscriminate
and powerful oxidizing agents.30,42,43 Hence, a proposed
reaction mechanism (Scheme 4) involves the formation of
hydroxyl radicals from H2O2. Then, these radicals abstract
hydrogen atoms of the substrate (cycloalkane, CyH) to
produce Cy• (cycloalkyl radicals). These radicals further
Table 3. Mild Oxidation of Cycloalkanes (C6−C8) in the
a
Presence of Catalysts 1−4
b
yield (%)
c
hydrocarbon
cyclic alcohol
Catalyst 1
cyclic ketone
total
cyclohexane
cycloheptane
cyclooctane
15.1
14.7
5.6
4.9
14.8
16.4
20.0
29.5
22.0
Catalyst 2
cyclohexane
cycloheptane
cyclooctane
14.3
8.7
5.4
4.0
11.7
16.5
18.3
20.4
21.9
Catalyst 3
cyclohexane
cycloheptane
cyclooctane
12.5
6.6
9.6
4.0
8.0
8.6
16.5
14.6
18.2
Catalyst 4
cyclohexane
cycloheptane
cyclooctane
9.6
6.8
11.0
3.3
10.2
8.0
12.9
17.0
19.0
a
Conditions of the reactions: cycloalkane, 1 mmol; catalyst 1−4, 5
μmol; H2O2, 5 mmol; MeCN, until 2.5 mL of total reaction volume;
b
temperature, 50 °C; time, 5 h. Yields were calculated as (moles of
c
product per mole of cycloalkane) × 100%. Sum of the alcohol and
ketone yields.
catalyze the oxidation of cycloalkanes without the need for acid
additive (promoter), which is crucial for many catalytic
systems for alkane oxidation by H2O2 under mild con-
ditions.4,30,38,42,43
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Besides, compounds 1−4 efficiently catalyze the oxidation of
C6−C8 cycloalkanes by H2O2 to generate the respective
alcohols and ketones with total yields attaining 30%. Such
yields are significant in the field of alkane C−H chem-
istry,39,41,43 particularly considering that these saturated
hydrocarbons are very inert and the reactions studied herein
can occur at a low temperature (50 °C) and in a MeCN/H2O
medium.
Further research on probing various types of aromatic
multicarboxylic acids as versatile spacers to assemble new
copper-based CPs or metal−organic frameworks with certain
functional properties will be pursued in our laboratories. A
special focus will be made on developing recoverable,
heterogeneous catalysts based on the present type of
copper(II) coordination compounds.
Table 4. Different Selectivity Parameters in the Oxidation of
Alkanes
a
alkane and selectivity
parameter
1
2
3
4
b
Regioselectivity
1:4:5:5
n-heptane, C1:C2:C3:C4
1:4:5:5
1:4:4:4
1:5:14
1:4:4:4
1:5:13
Bond Selectivity
c
methylcyclohexane, 1°:2°:3°
1:5:16
Stereoselectivity
0.8
1:4:17
d
cis-dimethylcyclohexane,
trans/cis
trans-dimethylcyclohexane,
trans/cis
0.9
0.8
0.9
0.8
0.8
0.9
0.8
a
Reaction conditions: catalyst, 5 μmol; alkane, 1 mmol; H2O2, 5
mmol; MeCN, until 2.5 mL of total reaction volume; temperature, 50
°C; time, 2 h. The values of the selectivity parameters were calculated
on the basis of the molar ratios of alcohol product isomers; all
selectivity parameters underwent normalization, wherein a number of
hydrogen atoms at every carbon center was taken into consideration.
ASSOCIATED CONTENT
* Supporting Information
■
S
b
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02926.
Parameters C1:C2:C3:C4 refer to a reactivity of hydrogen atoms at
c
C1, C2, C3, and C4 atoms of the n-C7H16 skeleton. Parameters
1°:2°:3° refer to the reactivity of hydrogen atoms at different types
(primary, secondary, or tertiary) of carbon atoms in methylcyclohex-
Additional structural data (Tables S1 and S2), summary
of the thermal analysis data (Table S3), additional
structural representations (Figures S1−S8), TGA plots
(Figure S9), PXRD patterns (Figure S10), and magnetic
behavior (Figure S11) and its detailed discussion (PDF)
d
ane. Parameters trans/cis refer to a molar ratio of the obtained
isomeric tertiary alcohols with trans- or cis-oriented CH3 groups.
Scheme 4. Proposed Mechanistic Pathway for the Copper-
Catalyzed Oxidation of Cycloalkane Substrates, Cn (n = 1−
3)
Accession Codes
CCDC 1821255−1821258 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. Fax: +86-
931-8915196.
■
react with dioxygen (present in air or generated from H2O2) to
form CyOO• (cycloalkylperoxy radicals). CyOO• radicals are
converted into CyOOH products (cycloalkyl hydroperoxides),
which represent the primary formed products; their presence
in the reaction system can be evidenced from GC analyses of
the reaction mixtures (Shul’pin’s method).34 CyOOH
products decompose in the course of the reaction to give the
final oxidation products (cyclic alcohols and ketones).
*E-mail: kirillov@tecnico.ulisboa.pt. Tel.: +351 218417178.
ORCID
Jinzhong Gu: 0000-0001-6704-7370
Zifa Shi: 0000-0001-9231-5963
Alexander M. Kirillov: 0000-0002-2052-5280
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
CONCLUSIONS
■
■
This work was supported by the National Natural Science
Foundation of China (Project 21572086), the 111 Project of
MOE (111-2-17), and the Foundation for Science and
Technology and Portugal 2020 (Projects IF/01395/2013/
CP1163/CT005, CEECIND/03708/2017, UID/QUI/00100/
2013, and LISBOA-01-0145-FEDER-029697). The paper was
also prepared with support of the RUDN University Program
5-100. A.S.A. acknowledges the BL-CQE/2017-019 fellowship
(UID/QUI/00100/2013). A.M.K. acknowledges the COST
Action CA15106 (CHAOS).
Herein we studied hydrothermal generation of copper metal−
organic architectures using a three-component system: copper-
(II) acetate−multicarboxylic acid with a phenylpyridine or
biphenyl core −1,10-phenanthroline. Four new products were
generated by varying the main multicarboxylate ligand. A
search of the CSD44 confirmed that all of the obtained
products constitute the first copper compounds assembled
from H2cpna, H3btc, H3cpic, and H3cptc. The obtained
products exhibit different metal−organic architectures, which
include ladder, linear, or double 1D chains, in addition to a
discrete 0D hexacopper aggregate with a cyclic Cu4 core. The
structural, topological, and hydrogen-bonding features of the
obtained products were highlighted. The magnetic properties
of compounds 1−4 were also studied.
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Inorganic Chemistry
Article
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DOI: 10.1021/acs.inorgchem.8b02926
Inorg. Chem. XXXX, XXX, XXX−XXX