New copper(II) coordination compounds assembled from multifunctional pyridine-carboxylate blocks: Synthesis, structures, and catalytic activity in cycloalkane oxidation

Article abstract of DOI:10.3390/molecules24010006

Two new copper(II) coordination compounds, namely a 1D coordination polymer [Cu(μ-cpna)(phen)(H₂O)]n (1) and a discrete tetracopper(II) derivative [Cu(phen)₂(H₂O)]2[Cu2(μ-Hdppa)₂(Hdppa)₂] (2), were hydrothermally synthesized from copper(II) chloride as a metal source, 5-(4-carboxyphenoxy)nicotinic acid (H₂cpna) or 5-(3,4-dicarboxylphenyl)picolinic acid (H3dppa) as a principal building block, and 1,10-phenanthroline (phen) as a crystallization mediator. Compounds 1 and 2 were isolated as air-stable microcrystalline solids and fully characterized by elemental and thermogravimetric analyses, IR spectroscopy, powder and single-crystal X-ray diffraction. In the solid state, the structure of 1 discloses the linear interdigitated 1D coordination polymer chains with the 2C1 topology. The crystal structure of an ionic derivative 2 shows that the mono-and dicopper(II) units are extended into the intricate 1D hydrogen-bonded chains with the SP 1-periodic net (4,4)(0,2) topology. Thermal stability and catalytic properties of 1 and 2 were also investigated. In fact, both Cu derivatives act as efficient homogeneous catalysts (catalyst precursors) for the mild oxidation of cycloalkanes by hydrogen peroxide to give the corresponding alcohols and ketones; the substrate scope and the effects of type and amount of acid promoter as well as bond-, regio-, and stereo-selectivity features were investigated.

Full text of DOI:10.3390/molecules24010006

molecules
Article
New Copper(II) Coordination Compounds
Assembled from Multifunctional
Pyridine-Carboxylate Blocks: Synthesis, Structures,
and Catalytic Activity in Cycloalkane Oxidation
Na Zhao 1, Yu Li 1,*, Jinzhong Gu 2, Tiago A. Fernandes 3, Marina V. Kirillova 3 and
Alexander M. Kirillov 3,4,
*
1
Foshan Research Center for Special Functional Building Materials and Their Green Preparation Technology,
Guangdong Industry Polytechnic, Guangzhou 510300, China; 2012009035@gdip.edu.cn
College of chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China;
gujzh@lzu.edu.cn
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais,
1049-001 Lisbon, Portugal; tiago.a.fernandes@tecnico.ulisboa.pt (T.A.F.);
kirillova@tecnico.ulisboa.pt (M.V.K.)
2
3
4
Peoples’ Friendship University of Russia (RUDN University), Research Institute of Chemistry,
6 Miklukho-Maklaya st., Moscow 117198, Russia
*
Correspondence: liyuletter@163.com (Y.L.); kirillov@tecnico.ulisboa.pt (A.M.K.);
Tel.: +86-20-6123-0629 (Y.L.); +351-218-417-178 (A.M.K.)
Academic Editors: Luís Cunha Silva and Filipe Alexandre Almeida Paz
Received: 25 November 2018; Accepted: 18 December 2018; Published: 20 December 2018
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Abstract:
polymer [Cu(
[Cu(phen)2(H2O)]2[Cu2(
copper(II) chloride as
or 5-(3,4-dicarboxylphenyl)picolinic acid (H3dppa) as a principal building block, and
1,10-phenanthroline (phen) as a crystallization mediator. Compounds and were isolated
Two new copper(II) coordination compounds, namely
-cpna)(phen)(H2O)]n and
-Hdppa)2(Hdppa)2] ), were hydrothermally synthesized from
metal source, 5-(4-carboxyphenoxy)nicotinic acid (H2cpna)
a
1D coordination
µ
(
1
)
a
discrete tetracopper(II) derivative
µ
(2
a
1
2
as air-stable microcrystalline solids and fully characterized by elemental and thermogravimetric
analyses, IR spectroscopy, powder and single-crystal X-ray diffraction. In the solid state, the structure
of
The crystal structure of an ionic derivative
into the intricate 1D hydrogen-bonded chains with the SP 1-periodic net (4,4)(0,2) topology. Thermal
stability and catalytic properties of and were also investigated. In fact, both Cu derivatives act
1
discloses the linear interdigitated 1D coordination polymer chains with the 2C1 topology.
2
shows that the mono- and dicopper(II) units are extended
1
2
as efficient homogeneous catalysts (catalyst precursors) for the mild oxidation of cycloalkanes by
hydrogen peroxide to give the corresponding alcohols and ketones; the substrate scope and the
effects of type and amount of acid promoter as well as bond-, regio-, and stereo-selectivity features
were investigated.
Keywords: hydrothermal synthesis; crystal structure; coordination polymers; self-assembly; catalysis;
alkanes; cyclohexane; oxidation reactions; hydrogen peroxide
1. Introduction
The design of new coordination polymers and derived materials has become a hot research topic
in the fields of inorganic, coordination and materials chemistry, namely because of an almost unlimited
structural diversity [1–3], unusual properties and promising applications of such compounds as
Molecules 2019, 24, 6; doi:10.3390/molecules24010006
www.mdpi.com/journal/molecules

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functional materials. Examples of such applications range from the areas of luminescence [
4–6
] and
molecular magnetism [7–9] to gas storage [10–12], sensing [13–16], and catalysis [17–24].
The structures and functional properties of target coordination polymers are influenced by a
variety of factors that, among the reaction conditions, include the kind of metal nodes and organic
building blocks [1–3,25–30]; the latter play a noteworthy structurally-driven role. A very common
example of such building blocks concerns aromatic carboxylic acids, which attracted a considerable
attention for assembling diverse coordination polymers [27,28,30–33].
Following our general research interest toward the exploration of yet poorly investigated
multicarboxylic acids for the synthesis of novel coordination polymers or metal-organic
frameworks [31–33], in the present work we focused our attention on copper(II) ions as a metal
source and 5-(4-carboxyphenoxy)nicotinic acid (H2cpna) or 5-(3,4-dicarboxylphenyl)picolinic acid
(H3dppa) as main dicarboxylate or tricarboxylate building blocks, respectively (Scheme 1). The use
of Cu(II) as metal nodes can be explained by their versatile coordination behavior and significance
in bioinorganic chemistry [34,35] and oxidation catalysis [36–39]. On the other hand, the use of still
little explored H2cpna and H3dppa can be justified by their multifunctionality (presence of N-pyridyl
functionality and several COOH groups), good thermal stability and suitability for hydrothermal
synthesis, as well as flexibility wherein pyridyl and phenyl rings can rotate around the C-O-C or C-C
single bonds [40–42].
Scheme 1. Structural formulae of H2cpna, H3dppa, and phen.
Hence, in the present study we probed the hydrothermal generation of coordination compounds
from a multicomponent reaction mixture comprising of water as a solvent, copper(II) chloride as a
metal source, H2cpna or H3dppa as a main building block, sodium hydroxide as a base (deprotonating
agent), and phen (1,10-phenanthroline) as a crystallization mediator. As a result, two new products
were isolated, i.e., [Cu(µ-cpna)(phen)(H2O)]n (1) and [Cu(phen)2(H2O)]2[Cu2(µ-Hdppa)2(Hdppa)2]
(2). Their full characterization, thermal behavior, structural and topological features, as well as catalytic
properties toward the mild oxidation of cycloalkanes by H2O2 are reported herein.
2. Results and Discussion
2.1. Hydrothermal Synthesis
Hydrothermal treatment of the aqueous mixtures composed of a copper(II) chloride,
5-(4-carboxyphenoxy)nicotinic acid (H2cpna) or 5-(3,4-dicarboxylphenyl)picolinic acid (H3dppa) as
a principal building block, sodium hydroxide as a deprotonating agent, and 1,10-phenanthroline
as a crystallization mediator gave rise to the generation of two novel coordination compounds,
namely a 1D coordination polymer [Cu(
derivative [Cu(phen)2(H2O)]2[Cu2( -Hdppa)2(Hdppa)2] (
microcrystalline solids and analyzed by standard solid-state characterization methods, including by
single-crystal X-ray diffraction. The crystal structures of and are driven by pyridine-carboxylate
µ-cpna)(phen)(H2O)]n (
1) and a discrete 0D 2Cu1 + Cu2
µ
2). Both products were isolated as stable
1
2
blocks that adopt different coordination modes (Scheme 2).
2.2. Crystal Structure of [Cu(µ-cpna)(phen)(H2O)]n (1)
In the solid state, compound
µ
1
features a 1D coordination polymer structure that is driven by
-cpna2−
the
-cpna2− blocks (Figure 1). An asymmetric unit is composed of a Cu(II) center, a
µ

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block, a terminal phen ligand, and a coordinated H2O molecule. The Cu1 atom is six-coordinate and
has a distorted octahedral {CuN2O4} environment, which comprises three carboxylate O donors
from two
µ
-cpna2− moieties, one H2O ligand, and two phen N donors (Figure 1a). The Cu-O
[1.940(2)–2.932(3) Å] and Cu-N [2.031(3)–2.035(3) Å] distances are comparable to those in relate−d
Cu(II) derivatives [31,40–42]. In 1 µ-linker (mode I, Scheme 2) with its COO
, the cpna2− moiety acts a
groups adopting a monodentate or bidentate modes; the nicotinate N atom remains uncoordinated.
In
-cpna2−, the dihedral angle between the two aromatic rings attains 78.26◦, while the C-Oether-C
angle reaches 117.96◦. The -cpna2− linkers multiply interconnect the adjacent Cu(II) centers to
generate a 1D linear chain structure (Figure 1b); the adjacent chains are interdigitated. Topologically,
such chains are composed of the 2-connected Cu1 nodes and the
-cpna2− linkers (Figure 1c), and are
classified as a uninodal 2-connected underlying net with the 2C1 topology.
µ
µ
µ
Scheme 2. Coordination modes of
in 1 and 2.
µ
-cpna2− (mode I) and
µ
-Hdppa2−/Hdppa2− (modes II, III) blocks
(a)
(b)
(c)
Figure 1. Structural fragments of
1
. (
a
) Coordination environment around the Cu(II) atom; H atoms
1. ( ) Linear 1D coordination polymer chain (view
) Topological representation of two 1D chains showing a uninodal 2-connected
are omitted for clarity. Symmetry code: i = x, y, z
along the a axis). (
−
b
c
underlying net with the 2C1 topology; color codes: 2-connected Cu nodes (green balls), centroids of
2-connected µ-cpna2− linkers (gray).
2.3. Crystal Structure of [Cu(phen)2(H2O)]2[Cu2(µ-Hdppa)2(Hdppa)2] (2)
The crystal structure of
monocopper(II) blocks, 2Cu1) and a [Cu2(
2
(Figure 2a) is composed of two [Cu(phen)2(H2O)]2+ cations (two
-Hdppa)2(Hdppa)2]4− anion (a dicopper(II) block, Cu2).
µ
In the cation, the Cu2 atom adopts a distorted trigonal bipyramidal {CuN4O} geometry filled by four N
atoms from two phen moieties and one H2O ligand. In the anion, the Cu1 center is also five-coordinate
and reveals a distorted trigonal bipyramidal {CuN2O3} environment, which is taken by two carboxylate
O and one N atom from two different µ
-Hdppa2− blocks, as well as one carboxylate O and one N
atom from the terminal Hdppa2− ligand. The Cu-O and Cu-N bonds span in the 1.944(3)–2.406(4)
and 1.963(4)–2.060(4) Å range, respectively; these are within the normal values for related Cu(II)
derivatives [31
Scheme 2) or as a
,
40
–
µ
42]. In
-linker (mode II), having the corresponding dihedral angles of 9.64◦ or 19.24◦
2
, the Hdppa2− blocks act as a terminal monodentate ligand (mode III,

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between pyridyl and phenyl rings. In the anion, two
µ
-Hdppa2− moieties link two adjacent Cu1
atoms to form a Cu2 unit with a Cu· · · Cu separation of 9.375(2) Å (Figure 2a). The dicopper(II)
[Cu2( -Hdppa)2(
µ
µ
-Hdppa)2]4− anions and monocopper [Cu(H2O)(phen)2]2+ cations are arranged
into the intricate 1D H-bonded double chains (Figure 2b). After simplification, such 1D chains can be
topologically defined as a uninodal 3-connected underlying net (Figure 2c) with the SP 1-periodic net
(4,4)(0,2) topology and the point symbol of (42.6).
(a)
(b)
(c)
Figure 2. Structural fragments of
are omitted for clarity except those in COOH groups. Symmetry code: i =
chain (view along the c axis); H bonds are shown as dotted lines. ( ) Topological representation of a
2. (
a) Coordination environment around the Cu(II) atoms; H atoms
−
x, y, z. ( ) 1D H-bonded
−
−
b
c
decorated 1D chain showing a uninodal 3-connected underlying net with the SP 1-periodic net (4,4)(0,2)
topology (view along the a axis); color codes: 3-connected Cu1 nodes (dark green balls), centroids
of 3-connected
[Cu(H2O)(phen)2]2+ linkers (green balls).
µ
-Hdppa2− nodes or 2-connected Hdppa2− linkers (gray), centroids of 2-connected

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2.4. Thermogravimetric and Powder X-ray Diffraction Analysis
Thermal behavior and stability of compounds
1
and
2
were studied by thermogravimetric analysis
(TGA) in the 20–800 ◦C temperature range under N2 atmosphere (Figure S1). TGA curve of
1
shows
◦
a release of one coordinated water molecule between 111 and 162 C (exptl, 3.3%; calcd, 3.5%);
a dehydrated solid remains stable on further heating up to 225 C. In 2, a weight loss in the 112–156 C
◦
◦
range corresponds to a removal of two H2O ligands (exptl, 1.9%; calcd, 1.7%) and the dehydrated
material keeps its integrity on heating up to 210 ◦C.
Microcrystalline samples of 1 and 2 were also subjected to PXRD (powder X-ray diffraction) study.
PXRD patterns of the bulk products are given in Figures S2 and S3. The experimental results well
match the diffractograms simulated from the single-crystal X-ray diffraction data, thus confirming a
phase purity of the bulk samples of 1 and 2.
2.5. Mild Catalytic Oxidation of Cycloalkanes
Compounds
1 and 2 were tested as homogeneous catalysts (catalyst precursors) in the mild
oxidation of C6–C8 cycloalkanes to the corresponding alcohols and ketones (Scheme 3). Reactions
◦
were typically run in acetonitrile medium at 50 C in air, using 50% aqueous hydrogen peroxide
as an oxidant, and in the presence (optional) of an acid promoter. Trifluoroacetic acid (TFA),
HNO3, and HCl were tested as typical promoters [36
–39]. It should be mentioned that both the
coordination polymer and an ionic complex dissolve in the catalytic reaction medium and produce
1
2
homogeneous catalytically active species. Cyclohexane was used as a model substrate for detailed
catalytic studies due to the industrial significance of its oxidation products, cyclohexanone and
cyclohexanol, that are intermediates in the production of nylon [43,44]. In fact, an industrial process for
the oxidation of cyclohexane (DuPont) also operates with a homogeneous metal carboxylate catalyst
(cobalt naphthenate), requires harsher reaction conditions, and shows a maximum C6H12 conversion
of only ~5–10% [43,44]. Herein, a higher catalytic activity was achieved in the presence of 2, resulting
in up to 25% of the total product yield. Both catalysts (catalyst precursors) show a similar trend toward
substrate reactivity: C6H12 < C8H16 < C7H14.
Scheme 3. Cu-catalyzed oxidation of cycloalkanes to cyclic alcohols and ketones.
Compounds 1 and 2 catalyze the oxidation of cyclohexane and show a comparable level of activity,
resulting in ~10–12% of the total yield (Figure 3) with cyclohexanol being formed in higher amount in
comparison with cyclohexanone (~1.5–2:1 molar ratio). However, there is a difference in kinetic curves.
In the case of
1
, there is a smooth accumulation of the products up to 120 min, whereupon no yield
, there is a lag period
increase is observed. In contrast, in the cyclohexane oxidation catalyzed by
2
of ~45 min when the reaction is accelerating, showing then a maximum reaction rate up to 120 min.
The lag period can be associated with a lower solubility of 2 in the reaction medium.
Oxidations of cycloheptane and cyclooctane proceed more efficiently than that of cyclohexane for
both
cycloheptanol and cycloheptanone) of ~25 and 23% for
leads to 17–20% total product yields. The corresponding kinetic curves of products accumulation in
C7H14 and C8H16 oxidations are different for compounds and . In the case of , reactions proceed up
to 120 min and then the yields practically do not change (no overoxidation was observed). In contrast,
in the oxidations catalyzed by and after an achievement of the maximum value (at 120 min), the yield
1
and
2
(Figure 4). Cycloheptane is the most reactive substrate resulting in the yields (total of
1
and , respectively. Oxidation of cyclooctane
2
1
2
1
2

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drop was detected due to an overoxidation. Besides, the lag period, detected for C6H12 oxidation in
the presence of 2, is less pronounced for cyclooctane and cycloheptane.
(a)
(b)
Figure 3. Kinetic curves of product accumulation in the cyclohexane oxidation by H2O2 catalyzed by
) and ) in the presence of TFA promoter. Reaction conditions: Cu catalyst or catalyst precursor
(5.0 ( ) or 2.5 (
1
(
a
2 (b
1
2
)
µ
mol), TFA (0.05 mmol), C6H12 (1.0 mmol), H2O2 (5.0 mmol), CH3CN (up to 2.5 mL
◦
of the total volume), 50 C.
(a)
(b)
Figure 4. Mild oxidation of cycloalkanes (C6–C8) to the corresponding alcohols and ketones (total
yield) by H2O2 catalyzed by 1 (a) and 2 (b). Reaction conditions are those of Figure 3.
In contrast to other Cu-based catalytic systems, the compounds
promoter to catalyze the oxidation of cycloalkanes (Figure 5). However, in the case of
1
and
2
do not require any acid
, the presence
1
of an acid promoter (HCl, TFA or HNO3) in a low amount (acid-to-catalyst molar ratio of 10:1) leads
to the acceleration of the reaction and removes a minor lag period (Figure 5a). Overall efficiency of
the system is higher in the presence of HCl and TFA, wherein the reactions proceed faster and result
in slightly superior total yields. Although HNO3 is capable of removing a lag period, the oxidation
of cyclohexane catalyzed by the
reaction rate was observed in the presence of HCl (Figure 5b).
A different behavior is noticed for catalyst in the presence of acid promoter (Figure 5c,d).
Interestingly, the highest activity (total yield and W0max) is attained in the absence of any promoter.
The presence of HCl results in the full suppression of catalytic activity of , whereas the addition of
1/HNO3 system is slower and less efficient. The maximum initial
2
2
TFA leads to a slight deceleration of the reaction and extends an existing lag period; the same product
yield was observed as in the absence of acid. An addition of HNO3 results in lowering the product
yield and the reaction rate.

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(a)
(b)
(c) (d)
Figure 5. Effect of the acid promoter (a,c) on the total yield of cyclohexanol and cyclohexanone and
(b,d) maximum initial reaction rate (W0) in the cyclohexane oxidation with H2O2 catalyzed by 1 (a,b)
and 2 (c,d). Reaction conditions are those of Figure 3.
Since the highest activity was observed for the cycloheptane oxidation catalyzed by 1, we studied
the effect of the amount of
and the maximum initial reaction rate (Figure 6). Both the total yield and W0
increasing the catalyst precursor amount from 0.5 to 2.0 mM, revealing the W0
1 on the total yield of the products (cycloheptanol and cycloheptanone)
max
are growing by
max
dependence on the
concentration of 1 with an order > 1. It possibly indicates that more than one Cu containing moiety
participates in the rate limiting step of the reaction.
It should be noted that the coordination polymer
1
is not intact in the course of catalytic tests
and undergoes a partial disaggregation upon dissolution and in the presence of oxidant and/or acid
promoter to give a homogeneous catalytically active species. To get more information on the type
of species present in solution, we investigated the model aqueous solutions of
1 and 1/H2O2 by
ESI-MS(+), using the conditions typical to those of catalytic tests. The following main fragments can
be observed in the MS(+) spectrum of 1: [Cu(µ µ-cpna)(phen) +
-cpna)(phen)2 + H]+ (m/z 681), [Cu(
H]+ (m/z 501), and [Cu(phen)2]+ (m/z 423). Besides, an intense peak of the molecular ion (m/z 501) is
also detected after addition of H2O2. Such data suggest a fragmentation (can also be ESI-induced) of
coordination polymer
1
upon dissolution to generate a series of monocopper species. These, along
, most likely
with the related cationic [Cu(phen)2(H2O)]2+ and anionic [Cu2(Hdppa)4]4− blocks of
2
represent the homogeneous catalytically active species in the present oxidation reactions.
The observed efficiency of the present catalytic systems is comparable to other copper-based
systems applied in the mild oxidation of cycloalkanes [20
catalytic systems also requires the use of an acid promoter in contrast with the catalytic behavior of 1
and . These are capable of catalyzing the oxidation of cycloalkane in the absence of added acid, what
can be associated with the presence of pyridine-carboxylate ligands in the structures of 1 and 2 [45].
,36–39]. However, the majority of these
2

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(a)
(b)
Figure 6. Effect of the amount of
1
on (
a) the total yield of cycloheptanol and cycloheptanone and (b)
the maximum initial reaction rate (W0) in the cycloheptane oxidation with the
1/H2O2 system. Reaction
conditions:
50 C, CH3CN (up to 2.5 mL total volume).
1 (0.5–2.0 mM), acid promoter (TFA, 0.05 mmol), C7H14 (1.0 mmol), H2O2 (5.0 mmol),
◦
2.6. Regio- and Bond Selectivity Investigation and Proposed Mechanism
In order to obtain additional information on the nature of the oxidizing species, we tested the
oxidation of branched alkanes in the presence of both catalytic systems (Table 1). In the oxidation
of methylcyclohexane, the normalized bond selectivity parameters 1◦:2◦:3◦ of 1:4:11 (
1) and 1:5:17
(2) suggest that the tertiary C atom is oxidized with some preference over the secondary C atoms.
Similar behavior is observed in the adamantane oxidation that shows the 2◦:3◦ parameter of 1:3.2
(determined as the ratio of the formed tertiary and secondary alcohol isomers). Oxidation of
n-heptane proceeds without specific preference to any secondary C atom of the hydrocarbon chain,
revealing the C(1):C(2):C(3):C(4) value of 1:6:6:7 and 1:5:5:7 in the presence of
1 and 2, respectively.
Trans-dimethylcyclohexane is oxidized with low stereoselectivity (trans/cis ratio of the isomer products
is 0.9). The above selectivity parameters are indicative of a powerful and rather indiscriminate
oxidizing species such as hydroxyl radicals [20,36–39,46].
Table 1. Different selectivity parameters in the mild oxidation of methylcyclohexane, adamantine,
a
n-heptane, and trans-dimethylcyclohexane catalyzed by 1 and 2.
Selectivity Parameter
1
2
Bond selectivity
◦
◦
◦
1 :2 :3 (methylcyclohexane) b
1:4:11
1:3.3
1:5:17
1:3.2
◦
◦
2 :3 (adamantane) b
Regioselectivity
C(1):C(2):C(3):C(4) (n-heptane) c
Stereoselectivity
1:6:6:7
0.9
1:5:5:7
0.9
trans/cis (trans-dimethylcyclohexane) d
a
Reaction conditions: alkane (1.0 mmol), catalyst or catalyst precursor (5 (1) or 2.5 (2) µmol), TFA (0.05 mmol),
H2O2 (50% aq., 5.0 mmol), MeCN (up to 2.5 mL total volume), 50 ◦C, 2 h. All parameters were calculated based
on the ratios of isomeric alcohols. The parameters were normalized, i.e., recalculated taking into account the
number of H atoms at each carbon atom; b Parameters 1◦:2◦:3◦ and 2◦:3◦ are relative normalized reactivities of the
H atoms at primary (only for methylcyclohexane), secondary, and tertiary carbon atoms of methylcyclohexane and
adamantane, respectively; these are determined as the ratio of the formed primary, secondary, and tertiary alcohol
isomers; c Parameters C(1):C(2):C(3):C(4) are the relative reactivities of hydrogen atoms at carbons 1, 2, 3 and 4 of
the n-heptane chain; d Parameter trans/cis is determined as the ratio of the formed tertiary alcohol isomers with
mutual trans and cis orientation of the methyl groups.
Summarizing these results, we can propose that the general reaction mechanism involves the
following steps [20,44,45,47,48]. H2O2 interacts with a copper catalyst (catalyst precursor) causing

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the formation of oxo/peroxo-copper intermediates via the coordination of H2O2 followed by the
elimination of HO• radicals. Then, the hydroxyl radicals abstract H atoms from a cycloalkane
producing the alkyl radicals R•, which further react with O2 (from air) and result in the ROO•
radicals. These can be transformed into cycloalkyl hydroperoxides ROOH as primary intermediate
products. Then, cycloalkyl hydroperoxides decompose (conceivably by Cu-catalyzed processes in the
course of the reaction) to furnish the corresponding alcohols and ketones as final products.
3. Experimental
3.1. Materials and Physical Measurements
All chemicals were of analytical reagent grade and used as received. H2cpna and H3dppa were
obtained from Jinan Henghua Sci. & Tec. Co., Ltd (Jinan, China). IR spectra were recorded on a Bruker
EQUINOX 55 spectrometer (Bruker Corporation, Billerica, MA, USA) using KBr pellets. Elemental
(C, H, N) analyses were run on an Elementar Vario EL elemental analyzer (Elementar, Langenselbold,
Germany). Thermogravimetric analyses (TGA) were performed under N2 atmosphere on a LINSEIS
STA PT1600 thermal analyzer (Linseis Messgeräte GmbH, Selb, Germary) with a heating rate of
10 ◦C/min. Powder X-ray diffraction patterns (PXRD) were measured on microcrystalline samples
using a Rigaku-D/MAX 2400 diffractometer (Cu-Kα radiation; λ = 1.54060 Å) (Rigaku Corporation,
Tokyo, Japan). ESI-MS(+) measurements were performed on a LCQ Fleet mass spectrometer with an
ESI source (Varian, Inc., Palo Alto, CA, USA).
3.2. Synthesis of [Cu(µ-cpna)(phen)(H2O)]n (1)
A mixture of CuCl2 2H2O (34.1 mg, 0.2 mmol), H2cpna (51.8 mg, 0.2 mmol), phen (39.6 mg,
·
0.2 mmol), NaOH (16.0 mg, 0.4 mmol), and H2O (10 mL) was stirred at room temperature for 15 min.
◦
Then, it was sealed in a 25 mL Teflon-lined stainless steel vessel and heated at 120 C for 3 days,
◦
followed by cooling to room temperature at a rate of 10 C/h. Green block-shaped crystals were
isolated manually, washed with distilled H2O, and dried in air to give compound 1. Yield: 65% (based
on H2cpna). Calcd for C25H17CuN3O6: C 57.86, H 3.30, N 8.10%. Found: C 58.01, H 3.32, N 8.06%. IR
(KBr, cm−1): 3383 w, 3067 w, 2927 w, 1610 s, 1558 m, 1517 w, 1496 m, 1428 m, 1367 s, 1340 m, 1288 w,
1242 m, 1206 w, 1164 m, 1097 w, 1024 w, 1009 w, 962 w, 905 w, 848 m, 780 m, 724 m, 693 w, 651 w, 614 w.
3.3. Synthesis of [Cu(phen)2(H2O)]2[Cu2(µ-Hdppa)2(Hdppa)2] (2)
Compound 2 was prepared following the procedure described for 1 but using H3dppa (57.4 mg,
0.2 mmol) instead of H2cpna. Blue block-shaped crystals were isolated manually, washed with distilled
H2O, and dried in air to give 2. Yield: 55% (based on H3dppa). Calcd for C104H64Cu4N12O26: C 58.05,
H 3.00, N 7.81%. Found: C 57.84, H 3.02, N 7.75%. IR (KBr, cm−1): 3378 w, 3062 w, 1685 w, 1622 s,
1591 m, 1516 m, 1428 m, 1368 s, 1253 w, 1166 w, 1100 w, 1039 w, 964 w, 913 w, 848 m, 819 w, 772 m,
725 m, 698 w, 644 w, 535 w.
3.4. X-ray Crystallography and Topological Analysis
Single-crystal X-ray data for
a graphite-monochromated Mo Kα radiation (
1
and
2
were collected on a Bruker APEX-II CCD diffractometer, using
= 0.71073 Å). Semiempirical absorption corrections
λ
were applied using the SADABS program. Crystal structures were determined using direct methods
and refined by full-matrix least-squares on F2 with the SHELXS-2015 and SHELXL-2015 programs [49].
All the non-H atoms were refined anisotropically by full-matrix least-squares methods on F2. All the H
atoms (except those of H2O/COOH) were placed in calculated positions with fixed isotropic thermal
parameters, and included in structure factor calculations at the final stage of full-matrix least-squares
refinement. Hydrogen atoms of H2O/COOH moieties were located by difference maps and constrained
to ride on their parent oxygen atoms. Crystal data for
1 and 2 are given in Table 2. Selected bond

Molecules 2019, 24, 6
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lengths and hydrogen bonding details are given in Tables S1 and S2, respectively (Supplementary
Material). CCDC-1877525 and 1877526 for 1 and 2 contain the supplementary crystallographic data.
Table 2. Crystal data for compounds 1 and 2.
Compound.
1
2
Chemical formula
Molecular weight
Crystal system
Space group
a/Å
C25H17CuN3O6
518.95
Monoclinic
P21/c
C104H64Cu4N12O26
2151.87
Monoclinic
P21/c
17.9277(7)
14.6238(8)
16.4405(7)
90
98.168(4)
90
4266.5(3)
8.7501(4)
23.0910(7)
11.0499(4)
90
110.506(4)
90
b/Å
c/Å
◦
α/( )
◦
β/( )
◦
γ/( )
V/Å3
Z
2091.15(14)
4
4
F(000)
1060
2192
Crystal size/mm
θ range for data collection
0.25 × 0.23 × 0.22
3.298–28.624
−8 ≤ h ≤ 11, −15 ≤ k ≤ 29,
−14 ≤ l ≤ 13
9097/4733 (0.0400)
1.648
0.25 × 0.23 × 0.21
3.322–28.586
−24 ≤ h ≤ 18, −16 ≤ k ≤ 19,
−20 ≤ l ≤ 22
19416/9809 (0.0568)
1.675
Limiting indices
Reflections collected/unique (Rint
)
−3
Dc/(Mg·cm
)
−1
1.096
1.080
µ/mm
Data/restraints/parameters
4733/0/316
1.051
9809/0/660
1.035
Goodness-of-fit on F2
Final R indices[(I
≥
2
σ
(I))] R1, wR2
0.0503, 0.0943
0.0828, 0.1113
0.0627, 0.1080
0.1345, 0.1446
R indices (all data) R1, wR2
Largest diff. peak and
0.402 and −0.414
0.443 and −0.556
−3
hole/(e·Å
)
Topological analysis of coordination (
concept of the simplified underlying net [50]. Underlying nets were generated by: (i) eliminating
terminal ligands and contracting bridging ligands to their centroids in or (ii) contracting
-Hdppa2−/Hdppa2− ligands and [Cu(H2O)(phen)2]2+ blocks to their centroids in
. Connectivity of
) or both coordination and hydrogen bonds
1) or H-bonded (2) networks was performed following the
1
µ
2
nodes and linkers was maintained via coordination (in
1
(in
2
). Only strong D–H· · · A hydrogen bonds were considered, wherein the H· · · A < 2.50 Å, D· · · A <
3.50 Å, and ∠(D−H· · · A) > 120◦; D and A stand for donor and acceptor atoms, respectively [51].
3.5. Mild Oxidation of Cycloalkanes
Cycloalkane oxidation reactions were typically performed in air atmosphere in thermostated glass
reactors equipped with a condenser under vigorous stirring at 50 ◦C under atmospheric pressure and
using MeCN as solvent (up to 2.5 mL total volume). These conditions of temperature and pressure are
considered as rather mild in the field of alkane oxidation [20
catalyst or catalyst precursor (5.0 mol for or 2.5 mol for
and gas chromatography (GC) internal standard (MeNO2, 25
,
39
), acid promoter (optional, 0.05 mmol)
L) were introduced into MeCN solution,
,40]. In a typical experiment, copper(II)
µ
1
µ
2
µ
followed by an addition of alkane substrate (1 mmol). Reaction started by adding hydrogen peroxide
(50% in H2O, 5 mmol) in one portion. The oxidation reactions were monitored by withdrawing small
aliquots of the reaction mixture after different periods of time, which were treated with PPh3 for
the reduction of remaining H2O2 and alkyl hydroperoxides that are typically formed as primary
products in alkane oxidations [52,53]. The samples were then analyzed by GC using nitromethane
as an internal standard. The formation of alkyl hydroperoxides as primary intermediate products
was also confirmed by GC analyses of the reaction mixtures before and after the treatment with PPh3

Molecules 2019, 24, 6
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(Shul’pin’s method) [52
,
53]. Attribution of peaks was made by comparison with chromatograms of
authentic samples. Blank tests confirmed that alkane oxidations do not proceed in the absence of
copper catalyst.
4. Conclusions
In this work, we studied the hydrothermal generation of copper(II) coordination compounds from
a three-component system comprising Cu(II) chloride—pyridine carboxylic acid—1,10-phenanthroline.
Two new products 1 and 2 were generated depending on the type of the main multicarboxylate building
block. Their solid-state structures revealed different types of metal-organic or H-bonded 1D chains,
which also represent rare examples of coordination compounds derived from H2cpna and H3dppa as
principal building blocks. The present study also widens the application of hydrothermal synthetic
protocols and use of water as a benign solvent for the generation of novel coordination compounds.
Besides, copper(II) compounds
1 and 2 act as homogeneous catalysts (catalyst precursors) for the
mild oxidation of C6–C8 cycloalkanes (cyclohexane, cycloheptane, and cyclooctane) by H2O2 to give
a mixture of the respective cyclic alcohols and ketones, resulting in up to 25% total product yields
based on cycloheptane. Such yields are considered rather high in the area of alkane functionalization,
especially taking into account a high inertness of these saturated hydrocarbons and the mild reaction
◦
conditions applied (e.g., 50 C temperature, atmospheric pressure, aqueous H2O2 oxidant). Additional
studies on the heterogenization of the obtained compounds on a solid support will be pursued aiming
at the development of recoverable heterogeneous catalysts.
Further research on exploring the present types of multifunctional pyridine-carboxylic acids for
the hydrothermal synthesis of coordination polymers or metal-organic frameworks and search for
their applications in oxidation catalysis are currently in progress in our laboratories.
Supplementary Materials: The following data are available online. Figure S1: TGA curves, Figures S2 and S3:
PXRD patterns, Tables S1 and S2: selected bonding and H-bonding parameters for compounds 1 and 2, Schemes
S1 and S2: additional catalysis schemes (bond selectivity study).
Author Contributions: Conceptualization, Y.L. and A.M.K.; Data curation, N.Z. and J.G.; Funding acquisition,
Y.L. and A.M.K.; Investigation, N.Z., Y.L., J.G., T.A.F. and M.V.K.; Methodology, M.V.K.; Supervision, Y.L.;
Visualization, T.A.F. and M.V.K.; Writing—original draft, N.Z., J.G., T.A.F., M.V.K. and A.M.K.; Writing—review &
editing, A.M.K.
Funding: This research was funded by Guangdong Province Higher Vocational Colleges & Schools Pearl
River Scholar Funded Scheme (2015, 2018), Innovation Team Project on University of Guangdong Province
of P. R. China (2017GKCXTD001, 2017GWCXTD002), the Foundation for Science and Technology (FCT)
and Portugal 2020 (projects IF/01395/2013/CP1163/CT005, CEECIND/03708/2017, UID/QUI/00100/2013,
LISBOA-01-0145-FEDER-029697) and RUDN University (the publication was prepared with the support of the
RUDN University Program 5-100).
Acknowledgments: A.M.K. acknowledges the EU COST Action CA15106 (CHAOS). T.A.F. acknowledges the FCT
for BPD grant SFRH/BPD/119980/2016. We thank M. C. Oliveira and MSc A. Dias for ESI-MS measurements.
Conflicts of Interest: The authors declare no conflict of interest.
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2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).