Three-Component Copper-Phosphonate-Auxiliary Ligand Systems: Proton Conductors and Efficient Catalysts in Mild Oxidative Functionalization of Cycloalkanes

Article abstract of DOI:10.1021/acs.inorgchem.8b01315

The synthesis, structural characterization, topological analysis, proton conductivity, and catalytic properties are reported of two Cu(II)-based compounds, namely a dinuclear Cu(II) complex [Cu₂(μ-VPA)₂(phen)₂(H₂O)₂]·8H₂O (1) (H₂VPA = vinylphosphonic acid, phen = 1,10-phenanthroline) and a 1D coordination polymer [Cu(μ-SO₄)(phen)(H₂O)₂]_∞ (2). Their structural features and H-bonding interactions were investigated in detail, showing that the metal-organic structures of 1 and 2 are extended by multiple hydrogen bonds to more complex 2D or 1D H-bonded architectures with the kgd [Shubnikov plane net (3.6.3.6)/dual] and SP 1-periodic net (4,4)(0,2) topology, respectively. These nets are primarily driven by the H-bonding interactions involving water ligands and H₂O molecules of crystallization; besides, the (H₂O)₄/(H₂O)₅ clusters were identified in 1. Both 1 and 2 are moderate proton conductors, with proton conductivity values, σ = 3.65 × 10^-6 and 3.94 × 10^-6 S·cm^-1, respectively (measured at 80 °C and 95% relative humidity). Compounds 1 and 2 are also efficient homogeneous catalysts for the mild oxidative functionalization of C₅-C₈ cycloalkanes (cyclopentane, cyclohexane, cycloheptane, and cyclooctane), namely for the oxidation by H₂O₂ to give cyclic alcohols and ketones and the hydrocarboxylation by CO/H₂O and S₂O₈^2- to the corresponding cycloalkanecarboxylic acids as major products. The catalytic reactions proceed under mild conditions (50-60 °C) in aqueous acetonitrile medium, resulting in up to 34% product yields based on cycloalkane substrate.

Full text of DOI:10.1021/acs.inorgchem.8b01315

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Three-Component Copper-Phosphonate-Auxiliary Ligand Systems:
Proton Conductors and Efficient Catalysts in Mild Oxidative
Functionalization of Cycloalkanes
†
§
‡
‡
́
Eirini Armakola, Rosario M. P. Colodrero, Montse Bazaga-García, Ines R. Salcedo,
Duane Choquesillo-Lazarte,∥ Aurelio Cabeza,‡ Marina V. Kirillova,⊥ Alexander M. Kirillov,*,⊥,#
and Konstantinos D. Demadis*,†
†Crystal Engineering, Growth and Design Laboratory, Department of Chemistry, University of Crete, Voutes Campus, Crete,
GR-71003, Greece
‡
́
́
́
Departamento de Química Inorganica, Universidad de Malaga, Campus Teatinos s/n, Malaga-29071, Spain
§Faculty of Science and Engineering, University of Wolverhampton, Wulfruna Street, Wolverhampton, WV1 1LY, United Kingdom
∥
́
Laboratorio de Estudios Cristalograficos, IACT, CSIC-Universidad de Granada, Granada-18100, Spain
⊥
́
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-Maklay st., Moscow, 117198, Russian Federation
S
* Supporting Information
ABSTRACT: The synthesis, structural characterization,
topological analysis, proton conductivity, and catalytic proper-
ties are reported of two Cu(II)-based compounds, namely a
dinuclear Cu(II) complex [Cu2(μ-VPA)2(phen)2(H2O)2]·
8H2O (1) (H2VPA = vinylphosphonic acid, phen = 1,10-
phenanthroline) and a 1D coordination polymer [Cu(μ-
SO4)(phen)(H2O)2]∞ (2). Their structural features and H-
bonding interactions were investigated in detail, showing that
the metal−organic structures of 1 and 2 are extended by
multiple hydrogen bonds to more complex 2D or 1D H-
bonded architectures with the kgd [Shubnikov plane net
(3.6.3.6)/dual] and SP 1-periodic net (4,4)(0,2) topology,
respectively. These nets are primarily driven by the H-bonding interactions involving water ligands and H2O molecules of
crystallization; besides, the (H2O)4/(H2O)5 clusters were identified in 1. Both 1 and 2 are moderate proton conductors, with
proton conductivity values, σ = 3.65 × 10−6 and 3.94 × 10−6 S·cm−1, respectively (measured at 80 °C and 95% relative
humidity). Compounds 1 and 2 are also efficient homogeneous catalysts for the mild oxidative functionalization of C5−C8
cycloalkanes (cyclopentane, cyclohexane, cycloheptane, and cyclooctane), namely for the oxidation by H2O2 to give cyclic
alcohols and ketones and the hydrocarboxylation by CO/H2O and S2O82− to the corresponding cycloalkanecarboxylic acids as
major products. The catalytic reactions proceed under mild conditions (50−60 °C) in aqueous acetonitrile medium, resulting in
up to 34% product yields based on cycloalkane substrate.
stable and robust.12 Their stability is one of the reasons for
their increasing use in catalysis.
Currently, synthetic efforts in metal phosphonate chemistry
INTRODUCTION
■
Metal phosphonates have attracted the interest of coordination
chemists during the last decades. There is a plethora of
reasons. (a) Phosphonate ligands display different coordina-
have been devoted to including a secondary auxiliary ligand
(SAL).13 Such ligands are usually neutral polypyridines,14
tion modes and can bridge several metal binding sites, thus
yielding a phalanx of architectures.1 (b) Phosphonic acids (and
although other anionic ligands (such as carboxylates) have also
been utilized.15 Incorporation of a SAL can induce profound
their deprotonated forms) can coordinate virtually to any
metal ion from the Periodic Table.2−6 (c) Their sensitivity to
changes in the final product, such as a change in
dimensionality, creation of hydrophobic domains, changes in
the coordination mode of the metal centers, and, in certain
cases, rupture of the “coordination polymer” architecture. In
pH offers the opportunity to perform syntheses in a wide
spectrum of pH values, with different results.7,8 (d) The
“metal/phosphonate” system can accommodate other auxiliary
ligands (such as N-heterocycles), thus altering the dimension-
ality and architecture of the resulting product.9−11 (e) Metal
phosphonate coordination polymers or complexes are usually
Received: May 14, 2018
© XXXX American Chemical Society
A
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the case of copper-based materials several examples have been
reported. Representative examples include the following. A 3D
bimetallic copper vanadium phosphonate framework can be
isolated from the Cu/V/terpyridine/methane tetra-p-phenyl-
phosphonic acid system.16 Low-dimensional Cu(II)-based
organophosphonates can be prepared from the Cu/2,2′-bpy/
anthracenephosphonate system.17 Various structural motifs
have been observed in compounds produced from the
combination of Cu(II) with 1,3,5-benzenetriphosphonic acid
and N-donor ligands such as pyrazine, 1,2-bis(4-pyridyl)-
ethylene, and 1,3-bis(4-pyridyl)propane.18 Reaction of Cu(II)
with 1-hydroxyethylidenediphosphonate and 2,2′-bipyridine
afforded a 1D coordination polymer.19 A zigzag 1D
coordination polymer has been structurally characterized that
contains Cu(II)-phenanthroline nodes and 1,2-ethylenediphos-
phate.20 Further useful information can be found in a recent
review.21
also been evaluated. Both 1 and 2 act as efficient homogeneous
catalysts for the mild oxidative functionalization of C5−C8
alkanes to the corresponding cyclic alcohols and ketones
(oxidation) or cyclalkanecarboxylic acids (hydrocarboxyla-
tion).
EXPERIMENTAL SECTION
■
Reagents and Equipment. All chemicals (copper salts, ligands,
cycloalkanes, and solvents) were purchased commercially and were
used as received without further purification. Ion exchange column-
deionized (DI) water was used for the syntheses. Stock aqueous
solutions of NaOH and HCl (1.0 and 0.1 M) were used for pH
adjustment during the synthesis. In catalytic oxidation and hydro-
carboxylation of cycloalkanes, gas chromatography (GC) analyses of
the reaction mixtures were run on an Agilent Technologies 7820A
series gas chromatograph (He as carrier gas) equipped with the FID
detector and BP20/SGE (30 m × 0.22 mm × 0.25 μm) capillary
column.
Metal phosphonate compounds have recently entered the
arena of catalytic transformations.22 Some representative
catalytic transformations using metal phosphonates as catalysts
include CO2 coupling with epoxides to give cyclic
carbonates,23 selective liquid phase oxidation of cyclohexanone
to adipic acid,24 hydrogen evolution,25 heterogeneous water
oxidation,26 alcoholysis of styrene oxide, acetalization of
benzaldehyde and cyclohexanaldehyde, and ketalization of
cyclohexanone,27 oxidation of thioethers,28 conversion of ethyl
levulinate to γ-valerolactone,29 heterogeneous asymmetric
allylboration, propargylation, Friedel−Crafts alkylation and
sulfoxidation reactions,30 photocatalytic water-splitting H2
evolution,31 and cyanosilylation of aldehydes and ring opening
of meso-carboxylic anhydrides.32
Among various types of organic substrates for catalytic
transformations, alkanes are particularly important given their
high inertness and natural abundance as hydrocarbon feed-
stocks.33,34 Hence, the search for new catalytic systems and
reaction protocols capable of transforming alkanes into value-
added products represents an important research direction in
modern chemistry.33,34 Particular attention is focused on the
design of low-cost transition metal catalysts for the oxidative
functionalization of alkanes under mild conditions. Given the
presence of copper in the active sites of various oxidation
enzymes (e.g., particulate methane monooxygenase, multi-
copper oxidases) along with its rich coordination chemistry
and recognized application in oxidation catalysis,35,36 the
synthesis and exploration of novel catalysts based on copper
complexes or coordination polymers are particularly attractive.
An interesting idea also concerns the design of catalytically
active copper cores that would combine versatile and relatively
labile phosphonate ligands with strong chelating N,N-donors,
thus resulting in a three component (Cu-phosphonate-auxiliary
ligand) system.
Syntheses. [Cu2(μ-VPA)2(phen)2(H2O)2]·8H2O (1). A quantity of
H2VPA (9 μL of a 90% aqueous solution, 0.10 mmol) was dissolved
in 10 mL of DI water. Equimolar quantities of phen (0.020 g, 0.10
mmol) and CuSO4·5H2O (0.025 g, 0.10 mmol) were added to the
above solution under rigorous stirring. The pH was adjusted to 6.8
with HCl and NaOH stock solutions. The resulting blue solution was
left undisturbed for ∼7 days at room temperature. Blue crystals were
formed due to solvent evaporation (final pH = 6.71) and were
isolated by filtration, washed with small amounts of DI water, and air-
dried. The yield was 74% based on the metal salt. Elemental analysis
for the formula {[Cu(C12H8N2)(C2H3O3P)(H2O)]·4H2O}2 (MW =
879.67g/mol): Calculated C 38.23%, H 4.81%, N 6.37%. Found C
37.93%, H 4.96%, N 6.46%.
[Cu(μ-SO4)(phen)(H2O)2]∞ (2). Identical procedures and quantities
as for compound 1 were followed, with the exception that the
synthesis pH was 3.6 (final pH = 3.44). The yield was 68% based on
the metal salt. Elemental analysis for the formula [Cu-
(H2O)2(C12H8N2)][(SO4)] (MW = 375.84 g/mol): Calculated C
38.40%, H 3.23%, N 7.47%. Found: C 37.90%, H 3.19%, N 7.49%.
Instrumentation. Thermogravimetric Analysis (TGA). Thermog-
ravimetric analysis (TGA) data were recorded on an SDT-Q600
analyzer from TA Instruments. The temperature varied from RT to
900 °C at a heating rate of 10 °C·min−1. Measurements were carried
out on samples in open platinum crucibles under a flow of air.
Single Crystal X-ray Determination. Crystals were handled under
inert conditions and immersed in perfluoropolyether as protecting oil
for manipulation. Suitable crystals of 1 were mounted on MiTeGen
Micromounts, and these samples were used for data collection. Data
were collected on a Bruker D8 Venture diffractometer. The data were
processed with an APEX2 program41 and corrected for absorption
using SADABS.42 The structures were solved by direct methods,
which revealed the position of all non-hydrogen atoms.43 These atoms
were refined on F2 by a full-matrix least-squares procedure using
anisotropic displacement parameters. All hydrogen atoms were
located in difference Fourier maps and included as fixed contributions
riding on attached atoms with isotropic thermal displacement
parameters 1.2 (−C-H) or 1.5 (−O-H) times those of the respective
atom. Crystallographic data for 1 are reported in Table 1.
Aiming at exploring a catalytic potential of such multi-
component copper cores and following our general interest in
the mild oxidative functionalization of alkanes,37−39 the present
paper reports the synthesis, full characterization, proton
conductivity, and structural and topological features, as well
as oxidation catalytic activity of a novel dinuclear Cu(II)
compound [Cu2(μ-VPA)2(phen)2(H2O)2]·8H2O (1) (H2VPA
= vinylphosphonic acid, phen = 1,10-phenanthroline). For
comparative purposes, the properties and catalytic behavior of
a structurally related 1D coordination polymer [Cu(μ-
SO4)(phen)(H2O)2]∞ (2, byproduct in the synthesis of 1)40
wherein a μ-VPA block is substituted by a μ-sulfate ligand have
Topological Analysis. The H-bonded architectures in 1 and 2 were
analyzed from the topological viewpoint by following the concept of
the simplified underlying net.44−47 Such nets were generated by
contracting (a) the [Cu2(μ-VPA)2(phen)2(H2O)2] molecular units
and (H2O)4 clusters in 1 or (b) the [Cu(phen)2(H2O)2]2+ and μ-
SO42− moieties in 2 to the corresponding centroids, maintaining their
connectivity via hydrogen bonds. 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.44,45 Topological classification of the obtained underlying nets
was performed.
Powder X-ray Diffraction. Laboratory X-ray powder diffraction
(XRPD) patterns were collected on a PANanalytical EMPYREAN
B
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The samples were then analyzed by GC using nitromethane as an
internal standard. Attribution of peaks was made by comparison with
chromatograms of authentic samples.
Table 1. Selected Crystallographic Data for Cu2(phen)2(μ-
VPA)2(H2O)2·8H2O (1)
Cu2(phen)2(μ-VPA)2(H2O)2·8H2O
Hydrocarboxylation of Cycloalkanes. In a typical experiment, the
reaction mixtures were prepared as follows: a stainless steel autoclave
(20.0 mL) equipped with a Teflon-coated magnetic stirring bar was
filled with copper catalyst (0.01 mmol), K2S2O8 (1.5 mmol), H2O
(2.0 mL) and MeCN (4.0 mL; total solvent volume was 6.0 mL), and
cycloalkane (1.0 mmol). Then, the autoclave was closed and flushed
with CO three times to remove the air, and finally pressurized with 20
atm of CO. CAUTION: Due to the toxicity of CO, all operations should
be carried out in a well-ventilated hood! The obtained reaction mixture
was stirred for 3 h at 60 °C using a magnetic stirrer and an oil bath,
whereupon it was cooled in an ice bath, degassed, opened, and
transferred to a flask with tap. Diethyl ether (9.0 mL) and
cycloheptanone (45 μL, typical GC internal standard) were added.
Cyclohexanone was used as a GC internal standard in cycloheptane
hydrocarboxylation. The obtained mixture was vigorously stirred for
10 min using a magnetic stirrer. Then, the organic layer was analyzed
by gas chromatography (internal standard method), revealing the
formation of the corresponding cycloalkanecarboxylic acids as the
dominant products. The formation of oxidation products (cyclic
alcohols and ketones) was also detected. Attribution of peaks was
made by comparison with chromatograms of authentic samples.
Compound
Empirical formula
(1)
C28H42Cu2N4O16P2
879.67
100 K
F.W. (g mol−1
Temperature
Space group
λ (Å)
)
C2/c
0.71073
27.720(8)
8.091(2)
20.036(7)
90.0
127.944(9)
90.0
3544(2)
0.12 × 0.08 × 0.06
8
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Crystal size (mm)
Z
ρcalc (g·cm−3
)
1.649
2θ range (deg)
Data/Restraints/Parameters
No. reflections
2.578−27.618
4087/0/244
15998
Independent reflections [I > 2σ(I)]
GoF, F2
R Factor [I > 2σ(I)]
R Factor (all data)
CCDC Reference Code
4087
RESULTS AND DISCUSSION
■
1.014
a
a
Synthesis and Characterization of the Catalysts.
Syntheses of the compounds [Cu2(μ-VPA)2(phen)2(H2O)2]·
8H2O (1) and [Cu(μ-SO4)(phen)(H2O)2]∞ (2) were carried
out in aqueous solutions under identical conditions, except the
solution pH value (6.8 for 1 and 3.6 for 2). Products were
isolated as crystalline solids that precipitated out of solution
without addition of any secondary solvent.
R1 = 0.0393, wR2 = 0.0876
a
a
R1 = 0.0585, wR2 = 0.0956
1837061
a
R1(F) = ∑||Fo| − |Fc||/∑|Fo|; wR2(F2) = [∑w(Fo2 − Fc2)2/∑F4]1/2
.
diffractometer, in the Bragg−Brentano reflection configuration by
using Cu Kα1,2 radiation and a PIXcel detector. X-ray patterns were
recorded between 4 and 70° (2θ), with a 0.026° step size and an
equivalent counting time of ∼24 s/step. Thermodiffractometric
studies were carried using an Anton Paar TTK450 Camera under
static air. Data were collected at different temperatures up to 200 °C
with a delay time of 15 min to ensure thermal stabilization. XRPD
patterns were recorded between 4 and 50° in 2θ, with a step size of
0.013° and an equivalent counting time of ∼ 150 s/step.
Proton Conductivity. Impedance measurements were carried out
on cylindrical pellets (diameter ∼5 mm and thickness ∼1 mm)
obtained by pressing 40 mg of sample at 250 MPa, for 1 min. The
pellets were pressed between porous C electrodes (Sigracet, GDL 10
BB, no Pt). The sample cell was placed inside a temperature- and
humidity-controlled chamber (Espec SH-222). AC impedance data
were collected using an AUTOLAB PGSTAT302N analyzer over the
frequency range from 20 Hz to 105 Hz with an applied voltage of 0.35
V. To equilibrate water content, pellets were first preheated (0.2 °C/
min) from 25 to 80 °C and 95% RH. Impedance spectra were
recorded on cooling using a stabilization time of 3 h for each
temperature (80, 60, 40, 25 °C). Water condensation on sample was
avoided by reducing first the relative humidity before decreasing
temperature. All measurements were electronically controlled by the
NOVA 2.1 package of programs.
Mild Oxidation of Cycloalkanes. The oxidation reactions of
cycloalkanes were performed in air atmosphere in a thermostated
glass reactor system equipped with a condenser, under vigorous
stirring at 50 °C and using MeCN as solvent (up to 5 mL total
volume). In a typical experiment, copper catalyst (0.01 mmol) and gas
chromatography (GC) internal standard (MeNO2, 50 μL) were
introduced into MeCN solution, followed by the addition of a
cycloalkane substrate (2 mmol). The reaction started upon addition
of hydrogen peroxide (50% in H2O, 10 mmol) in one portion. The
oxidation reactions were monitored by withdrawing small aliquots
after different periods of time; these were treated with PPh3 for the
reduction of remaining H2O2 and cycloalkyl hydroperoxides48,49 that
are typically formed as primary products in cycloalkane oxidations.
The pKa values of the H2VPA ligand are 2.6 and 7.3.50,51
The synthesis of 1 was carried out at the pH value of 6.8;
therefore, VPA should be at least monodeprotonated
(-PO3H−) with a “−1” charge and partially bis-deprotonated
(based on the proximity of the pKa2 value and the synthesis
pH). The phosphonic group of VPA in the structure of 1 (vide
infra) is completely deprotonated (-PO32−) carrying a “−2”
charge. Thus, the binuclear complex 1 is neutral. Such
deprotonation behavior is not surprising in metal phosphonate
chemistry.52 Hence, the following equation can be envisioned
for the synthesis of the copper(II) dimer 1:
2CuSO ·5H2O + 2H2VPA + 2phen
4
water
⎯⎯⎯⎯→ [Cu2(μ − VPA)2(phen)2(H2O)2]·8H2O (1)
+ 2H2O + H2SO
4
Decrease in the pH value to 3.6 during synthesis affords a
related product 2, which does not contain μ-VPA, but instead a
bridging sulfate (originating from the copper(II) sulfate). In
contrast to 1, compound 2 is a 1D coordination polymer
containing Cu2+ centers in an octahedral arrangement,
coordinated by the two N atoms of the chelating phen ligand,
two water molecules in a cis configuration, and two oxygen
atoms (from the bridging sulfate) in a trans configuration. The
synthesis of 2 can be rationalized based on the fact that the
sulfate dianion is a stronger ligand for Cu2+ at pH 3.6,
effectively competing with the monoanionic HVPA− ligand.
Furthermore, formation of 2 is driven by the hydrogen
bonding interactions developed between the 1D chains (see
structural description below). The following equation can be
envisioned for the synthesis of 2:
C
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Figure 1. Left: Structure of the binuclear unit in 1, showing the coordination environment of the Cu2+ centers, with numbering of the essential
atoms. Right: π−π interactions between the phen pyridyl rings in 1. VPA and water ligands are omitted for clarity.
9.65%) giving a semicrystalline anhydrous phase that remains
stable up to 340 °C. Above this temperature, decomposition of
the solid takes place. Rehydration of the anhydrous compound
does not take place upon cooling although it is fully reversible
after exposing the sample at high humidity for 3 days (Figure
S3 in the Supporting Information).
CuSO ·5H2O + H2VPA + phen
4
water
⎯⎯⎯⎯→ [Cu(μ‐SO )(phen)(H2O)2]∞ (2) + 5H2O
4
+ H2VPA (unreacted)
It should be noted that 2 does not form in the absence of
H2VPA under the same conditions. It is likely that H2VPA has
a templating role in the reaction.
The purity of compounds 1 and 2 was evaluated by XRPD,
revealing a good agreement between the simulated X-ray
diffraction patterns from the single-crystal data and those
obtained for the polycrystalline samples (Figure S1 in the
Supporting Information).
Structural Description of 1 and 2. Compound 1 is
composed of two symmetry equivalent [Cu(μ-VPA)(phen)-
(H2O)] units that are interconnected by the μ-VPA linkers
into the dicopper(II) complex with a Cu···Cu separation of
4.188 Å, Figure 1 (left). Each structurally equivalent Cu2+
center is found in a square pyramidal {CuN2O3} coordination
environment. The pyramid base is formed by two N atoms
from the chelating phen and two O atoms from two different
VPA molecules. A water ligand occupies the pyramid apex.
Cu−N bond distances [2.018(2) Å and 2.024(2) Å] are
within the reported values.53 Cu−O(P) bonds [1.9059(19) Å
and 1.9260(18) Å] fall within the reported range of similar
Cu−O(phosphonate) distances.54 The Cu−O(H2O) bond is
elongated [2.3829(19) Å] due to the Jahn−Teller effect.55
There are two “short” P−O distances [1.5213(19) and
1.516(2) Å] corresponding to the P−O(Cu) bonds and one
“long” uncoordinated PO bond [1.544(2) Å].
The TGA curves for both compounds, 1 and 2, are given in
Figure S2 in the Supporting Information. Compound 1
undergoes several consecutive weight loss steps from room
temperature up to 160 °C corresponding to the removal of the
eight lattice and two coordinated water molecules. According
to the TG curve and the thermodiffractometric study (Figures
S2 and S3 in the Supporting Information) of solid 1, the first
weight loss, between 50 and 80 °C, corresponds to the loss of
six lattice water molecules (calculated weight loss 12.27%;
observed, 12.43%), while the loss of the two additional lattice
water molecules takes place between 80 and 120 °C
(calculated weight loss up to 120 °C, 16.36%; observed,
15.83%). Finally, the coordinated water molecules are lost in
the range 120−160 °C. The total weight loss, 20.49%, is in
good agreement with the calculated one, 20.46%. Each
dehydration stage has associated structural modifications
which lead to an anhydrous semicrystalline phase. No
additional weight loss is observed up to 370 °C temperature,
at which the decomposition of the compound begins. The
rehydration of solid 1 takes place after exposing for several
days the anhydrous phase at high humidity atmosphere (98%
RH) obtained by a saturated K2SO4 solution in a closed vessel.
The weight loss found by thermal analysis (Figure S2 in the
Supporting Information), 18.6%, agrees well with the uptake of
nine water molecules (calculated, 18.4%). The rehydration
process leads to a new semicrystalline compound, for which
the XRPD pattern could not be indexed, with diffraction peaks
at higher d-space than the original solid (Figure S3 in the
Supporting Information). This fact suggests a different
arrangement of the dinuclear Cu(II) unit, as well as of the
incorporated lattice water molecules after rehydration.
The dimers interact via two kinds of weak interactions:
hydrogen bonds via the lattice and coordinated water
molecules, and π−π interactions of the phen aromatic rings.
Due to the opposite arrangement of the neighboring phen
rings, two π−π interactions (centroid-to-centroid distance
3.614 Å) are formed between the pyridyl edge rings, Figure 1,
right.
There are a total of ten water molecules (eight in the lattice
and two ligands) per dimeric unit of 1. Hence, there is a
plethora of hydrogen bonds with the O···O distances ranging
from 2.736 to 2.865 Å (Figure S4 in the Supporting
Information), which lead to a generation of water clusters
and an extension of the structure (see below).
The structure of 2 has been reported before.40 However,
herein we will present some salient structural features of 2 that
will be useful in rationalizing the proton conductivity processes
that will be discussed later. Compound 2 is a 1D linear chain
coordination polymer (Figure 2).
The Cu2+ centers are found in an octahedral {CuN2O4}
environment. The octahedron base is formed by the two N
atoms of the chelating phen ligand [Cu−N distance
2.0099(29) Å] and the two water molecules [Cu−O distance
1.9714(26) Å] in a cis configuration. The axial ligands are O
atoms from the bridging sulfate dianion [Cu···Cu distance
7.006 Å, Cu−O(sulfate) distance 2.4592(28) Å, similar to the
The TG curve of compound 2 and its thermodiffractometric
study (Figures S2 and S3 in the Supporting Information)
confirm that the coordinated water molecules are lost between
120 and 160 °C (calculated weight loss, 9.57%; observed,
D
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(43)2(4·66·683), wherein the (43) and (4·66·683) notations are
those of the dicopper(II) and (H2O)4 nodes, respectively.
In 2, two adjacent 1D metal−organic chains feature
intermolecular hydrogen bonds between water ligands and
sulfate linkers, thus resulting in the formation of a more
complex double chain (Figure 4a). From the topological
perspective, an underlying double chain is built from the 3-
connected [Cu(phen)(H2O)2]2+ and μ-SO4 nodes that are
2−
topologically equivalent (Figure 4b). As a result, such chain
can be classified as a uninodal 3-connected network with the
SP 1-periodic net (4,4)(0,2) topology and point symbol of
(4·26).
Figure 2. Portion of the 1D metal−organic chain in 2, showing the
chelating phen ligands, the bridging sulfates, and the coordinated
water molecules. Only selected atoms are labeled.
Proton Conductivity Studies. Impedance measurements
were carried out between 80 and 25 °C at 95% relative
humidity (RH). The overall pellet conductivities, determined
from the semicircles in the Nyquist plots (Figure S6 in the
Supporting Information), are presented in Figure 5 in a
traditional Arrhenius plot.
Cu−O(sulfate) distances in CuSO4·5H2O.56 The 1D chains in
2 interact via two interchain “double” hydrogen bonds which
develop between the Cu-coordinated water ligands and the
bridging sulfates (O···O distance 2.672 Å) (Figure S5 in the
Supporting Information, left). There is also an intrachain
hydrogen bond between one of the water molecules and the
sulfate (O···O distance 2.608 Å). Interlocking π−π interactions
also develop between the benzene rings (centroid-to-centroid
distance 3.520 Å) (Figure S5 in the Supporting Information,
right).
H-Bonded Nets and Topological Analysis. The
structure of 1 features a discrete dicopper(II) [Cu2(μ-
VPA)2(phen)2(H2O)2] molecular unit and eight crystallization
water molecules. The latter are assembled by hydrogen bonds
into a pair of acyclic tetrameric (H2O)4 clusters (Figure 3a).
These clusters can be extended to more complex (H2O)5
aggregates if considering an additional interaction with H2O
ligand. According to the classification of discrete water
clusters,57 the identified (H2O)4 and (H2O)5 associates can
be classified within the D3 and R4 type, respectively.58,59 The
adjacent dicopper(II) blocks and (H2O)4 clusters are multiply
interlinked by strong H-bonds, resulting in the generation of
intricate 2D H-bonded layers (Figure 3).
Both compounds 1 and 2 display similar proton conductivity
values, 3.65 × 10−6 and 3.94 × 10−6 S·cm−1, respectively,
measured at 80 °C and 95% RH. This level of conductivity
may be due to the absence of readily ionizable acid groups in
their H-bonded or metal−organic networks as observed for
other coordination polymers.60,61 However, the proton transfer
mechanism exhibited by both compounds is quite different.
Likely, the more acidic coordinated water molecules [Cu−
O(H2O) ∼ 1.97 Å] in 2 together with their stronger H-bond
interactions with the oxygen atoms of sulfate groups (O···O
distance 2.608 Å) (Figure 5) favor the hopping of protons
between them, which is consistent with a Grotthuss-like
mechanism and low activation energy (0.26 eV). In contrast,
the less acidic coordinated water in compound 1 [Cu−
O(H2O) ∼ 2.38 Å] together with weaker H-bond interactions
(D···A > 2.7 Å) between the bonded/lattice water molecules
and the O atoms of the phosphonate group (Figures 1 and 3)
suggest that a vehicle-type mechanism prevails in 1 with a
higher activation energy (0.59 eV).61−63
After impedance measurements, pelletized samples were
analyzed by XRPD (Figure S7 in the Supporting Information)
and TGA (Figure S8 in the Supporting Information) to notice
possible structural and/or water content changes. The XRPD
patterns did not reveal significant changes, as compared to the
initial sample, and no significant weight gain was observed. In
Topological analysis of the underlying 2D layers in 1 (Figure
3b) reveals that they are composed of the 6-connected [Cu2(μ-
VPA)2(phen)2(H2O)2] molecular nodes and the 3-connected
(H2O)4 nodes. This net can be classified as a binodal 3,6-
connected layer with the kgd [Shubnikov plane net (3.6.3.6)/
dual] topology. It is described by the point symbol of
Figure 3. Structural fragments of 1. (a) H-bonding interlinkage of two dicopper(II) molecular units [Cu2(μ-VPA)2(phen)2(H2O)2] with (H2O)4
clusters; water O atoms within (H2O)4 clusters are shown in yellow; Cu (green balls), O (red), N (blue), P (orange), C (gray), H (pale gray). (b)
Topological representation of the underlying 2D H-bonded network showing a binodal 3,6-connected layer with the kgd [Shubnikov plane net
(3.6.3.6)/dual] topology; view along the a axis; centroids of 6-connected dicopper(II) molecular nodes (green balls), centroids of 3-connected
(H2O)4 nodes (yellow).
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Figure 4. Structural fragments of 2. (a) H-bonding interlinkage of two linear 1D metal−organic chains into a double chain; Cu (green balls), O
(red), N (blue), S (yellow), C (gray), H (pale gray). (b) Topological representation of the underlying 1D H-bonded network showing a uninodal
3-connected double chain with the SP 1-periodic net (4,4)(0,2) topology; view along the a axis; centroids of 3-connected [Cu(phen)(H2O)2]2+
2−
nodes (green balls), centroids of 3-connected μ-SO4 nodes (yellow).
total yields (Figure 6). Hence, the oxidations of cyclohexane
and cycloheptane are slightly more efficient (maximum total
product yields of ∼23% and ∼20%, respectively) than those of
cyclopentane and cyclooctane (total yields of ∼17−18%).
Hereinafter all the yields are relative to cycloalkane substrate.
In the oxidation of cyclopentane (Figure 6a), cyclo-
pentanone is formed predominantly to cyclopentanol along
all the reaction time. In the case of cyclohexane oxidation
(Figure 6b), cyclohexanol is formed predominantly (∼2:1
molar ratio) to cyclohexanone in the beginning of the reaction;
however, their yields become comparable with time. The yields
of both oxidation products are similar in the oxidation of
cycloheptane (Figure 6c). Interestingly, cyclooctanone is a
predominant product in the oxidation of cyclooctane (Figure
6d).
For comparative purposes, the oxidation of cyclohexane in
the presence of catalyst 2 was also investigated (Figure 7).
Given the presence of only one Cu atom per formula unit, an
excess of 2 (20 μmol) was used in comparison with 1 (10
μmol). A lag period is observed for 2 within the initial 90 min
of the oxidation reaction. After this period, the reaction rate
Figure 5. Arrhenius plots for 1 (red) and 2 (black) at 95% of relative
humidity (RH).
increases significantly but the activity of 2 does not reach the
addition, the water content of compound 1 was also
level showed by 1. Such a lag period can potentially be related
determined by thermal analysis after keeping the sample for
4 h at 80 °C and 95% RH (Figure S8 in the Supporting
Information). The observed weight loss confirms that the
compound maintains the same hydration degree as the as-
synthesized sample. The high relative humidity inside the
with a 1D polymeric nature of 2, wherein additional time is
required for a partial disaggregation of the network and
generation of a catalytically active species.
The obtained yields of oxygenates can be considered as
rather good in the field of mild oxidative functionalization of
alkanes.39,64−66 For example, an industrial DuPont process of
chamber used for the proton conductivity measurements
prevents dehydration of the sample and shifts it to higher
the cyclohexane oxidation operates with a homogeneous cobalt
temperatures than those shown by the thermodiffractometric
naphthenate catalyst and proceeds only with ∼5% cyclohexane
conversion under harsher reaction conditions.67 Moreover,
study carried out under dry conditions.
Mild Catalytic Oxidation of Cycloalkanes. Compound
catalyst 1 does not require the use of an acid-type promoter or
1 was also tested in the mild homogeneous oxidation of C5−C8
cocatalyst, which is usually required by various catalytic
systems for the mild oxidation of alkanes.39,68,69
cycloalkanes, by aqueous H2O2 in MeCN/H2O medium at 50
°C, to give a mixture of the corresponding cyclic alcohols and
ketones (Scheme 1). The oxidation of cycloalkanes proceeds
with comparable reaction rates but results in slightly different
Mild Catalytic Hydrocarboxylation of Cycloalkanes.
Compound 1 was also applied as an efficient catalyst for the
hydrocarboxylation of C5−C8 cycloalkanes (Scheme 2). This
transformation consists of treating a cycloalkane substrate with
CO (carbonyl source), potassium peroxodisulfate (oxidant),
and H2O (hydroxyl source) to form a cycloalkanecarboxylic
acid in a single-step.70 The hydrocarboxylation occurs in H2O/
MeCN medium at 60 °C and requires the use of a
homogeneous metal containing catalyst. The obtained results
on the activity of 1 are summarized in Table 2.
Scheme 1. Oxidation of C5−C8 Cycloalkanes
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Figure 6. Product accumulation curves in the oxidation of (a) cyclopentane, (b) cyclohexane, (c) cycloheptane, and (d) cyclooctane with H2O2
catalyzed by 1. Reaction conditions: 1 (10 μmol), cycloalkane (2.0 mmol), H2O2 (10.0 mmol; 50% in H2O), CH3CN (up to 5.0 mL total volume),
50 °C.
Scheme 2. Hydrocarboxylation of C5−C8 Cycloalkanes
acids were generated in 30% or 29% yields, respectively;
hereinafter all the yields are relative to cycloalkane substrate.
Lower product yields were obtained in the reactions using
cycloheptane and cyclooctane as substrates (19% and 7%,
respectively). No dicarboxylic acid formation was observed
during the reactions. Nevertheless, the corresponding cyclic
ketones and alcohols were also formed as a result of partial
cycloalkane oxidation. The total yield of oxygenates (ketone is
Figure 7. Product accumulation curves in the oxidation of
formed predominantly to alcohol) increases with the hydro-
cyclohexane with H2O2 catalyzed by 1 and 2. Reaction conditions:
carbon size, namely from 0.6% for C5H10 and 4.0% for C6H12
1 (10 μmol) or 2 (20 μmol), cyclohexane (2.0 mmol), H2O2 (10.0
to 10.3% for C7H14 and 14.6% for C8H16. Interestingly, an
mmol, 50% in H2O), CH3CN (up to 5.0 mL total volume), 50 °C.
opposite trend is observed when considering the yields of
cycloalkanecarboxylic acids, which gradually decline on
The hydrocarboxylation of C5−C8 cycloalkanes results in the
formation of one monocarboxylic acid product due to the
presence of a single type of carbon atom in substrate
molecules. Cyclopentanecarboxylic or cyclohexanecarboxylic
increasing the hydrocarbon ring size. For comparative
purposes, the hydrocarboxylation of cyclohexane in the
presence of catalyst 2 was also attempted, resulting in up to
34% total product yield. This is almost similar to that (33%)
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Table 2. Single-Pot Hydrocarboxylation of Cn (n = 5−8) Cycloalkanes into the Corresponding Cn+1 Cycloalkanecarboxylic
Acids.a
a
Cyclic ketones and alcohols are also formed as products of cycloalkane oxidation. Reaction conditions (unless stated otherwise): cycloalkane (1.00
mmol), catalyst 1 (10 μmol), p(CO) = 20 atm, K2S2O8 (1.50 mmol), H2O (2.0 mL)/MeCN (4.0 mL), 60 °C, 3 h in an autoclave (20.0 mL
b
c
d
capacity). Moles of product/100 mol of alkane determined by GC analysis. Yield of all products. Compound 2 was tested as catalyst (20 μmol)
for comparative purposes.
Scheme 3. Proposed Mechanism for Cu-Catalyzed Oxidation (a) and Hydrocarboxylation (b) of C5−C8 Cycloalkanes
attained in the reaction catalyzed by 1, thus indicating a
comparable activity of both catalysts.
last step, the cycloacyl cations are hydrolyzed by H2O to
furnish cycloalkanecarboxylic acids (CyCOOH) as final
products.
Proposed Mechanism for Mild Oxidative Functional-
ization of Cycloalkanes. Considering our previous data and
literature background on the mild Cu-catalyzed oxidation37,38
and hydrocarboxylation70 of alkanes, the free radical
mechanisms can be proposed for both types of model
reactions studied in the present work (Scheme 3). Hence, in
the oxidation of cycloalkanes (Scheme 3a), the copper catalyst
facilitates the generation of hydroxyl radicals (HO•) that are
the principal oxidizing species. The HO• radicals abstract H
atoms from cycloalkane (CyH) to form cycloalkyl radicals
(Cy•), which then react with O2 (e.g., from air or formed from
H2O2) to give rise to CyOO• radicals. The latter are
transformed to CyOO− (followed by a regeneration of CuII
catalyst) and then to cycloalkyl hydroperoxides (CyOOH) as
primary intermediate products. CyOOH then rapidly decom-
pose to form the corresponding cyclic alcohols and ketones as
final products.
For the hydrocarboxylation of cycloalkanes (Scheme 3b),
the free-radical mechanism begins from the thermal and/or
Cu-assisted generation of sulfate radical anions (SO4•−) from
peroxodisulfate. These radical anions are a key species to
abstract H atoms from cycloalkanes (CyH), leading to the
formation of cycloalkyl radicals (Cy•). These then react with
CO to produce cycloacyl radicals (CyCO•), which are oxidized
by CuII to cycloacyl cations CyCO+ (oxidation of CuI by
K2S2O8 contributes to the regeneration of CuII species). In the
CONCLUSIONS
■
In this paper we reported the synthesis, structural character-
ization, topological analysis, proton conductivity, and catalytic
properties of two di- or multicopper(II) compounds,
containing the resembling [Cu(phen)(H2O)n]2+ (n = 1, 2)
blocks and anionic linkers (μ-VPA2− or μ-SO42−). Both
products 1 and 2 were generated under similar conditions
except the reaction pH, which is a key parameter that guides
the structural differences between a 0D copper(II) dimer 1 and
a linear chain 1D coordination polymer 2.
Structural features and H-bonding interactions in the
obtained compounds were investigated in detail, allowing
identification and classification of tetrameric and pentameric
water clusters in 1. Besides, the metal−organic structures of 1
and 2 are extended by multiple hydrogen bonds to more
complex 2D or 1D H-bonded architectures, respectively. They
were also classified from a topological perspective, disclosing
the kgd [Shubnikov plane net (3.6.3.6)/dual] net in 1 and the
SP 1-periodic net (4,4)(0,2) in 2. Both compounds 1 and 2 are
also moderate proton conductors, with proton conductivity
values, σ = 3.65 × 10−6 and 3.94 × 10−6 S·cm−1, respectively
(measured at 80 °C and 95% relative humidity).
H
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Furthermore, we showed that compounds 1 and 2 act as
efficient homogeneous catalysts for the mild oxidative
functionalization of C5−C8 cycloalkanes, namely cyclopentane,
cyclohexane, cycloheptane, and cyclooctane. Specifically, 1
catalyzes the mild oxidation of cycloalkanes, by aqueous H2O2
in MeCN/H2O medium at 50 °C, to give a mixture of the
corresponding cyclic alcohols and ketones with up to 23% total
yields. Both copper(II) compounds are also active in the mild
Science and Technology (FCT) and Portugal 2020 (projects
IF/01395/2013/CP1163/CT005, UID/QUI/00100/2013,
and LISBOA-01-0145-FEDER-029697). The publication has
been prepared with the support of the RUDN University
Program 5-100. AMK acknowledges the COST Action
CA15106 (CHAOS).
Notes
The authors declare no competing financial interest.
2−
hydrocarboxylation of cycloalkanes, by CO/H2O and S2O8
at 60 °C in H2O/MeCN medium, allowing generation of the
corresponding cycloalkanecarboxylic acids in up to 30% yields.
These yields are rather high in the field of oxidative
functionalization of alkanes, especially considering the high
inertness of alkanes and mild reaction conditions (e.g., aqueous
medium, 50−60 °C temperatures, absence of acid cocatalysts
or promoters). Due to the homogeneous nature of the present
catalytic systems, catalyst recovery still remains a challenging
task. Hence, future studies can be devoted to the fabrication of
recoverable heterogeneous catalytic systems that incorporate
such copper(II) coordination compounds. Besides, further
research on widening the family of related copper(II) catalysts
and extending the substrate scope of catalytic transformations
will be pursued.
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CCDC 1837061 contains the supplementary crystallographic
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: kirillov@tecnico.ulisboa.pt (AMK).
*E-mail: demadis@uoc.gr (KDD).
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