Copper(II) Coordination Polymers Self-Assembled from Aminoalcohols and Pyromellitic Acid: Highly Active Precatalysts for the Mild Water-Promoted Oxidation of Alkanes

Article abstract of DOI:10.1021/acs.inorgchem.5b01983

Three novel water-soluble 2D copper(II) coordination polymersˉ[{Cu₂(μ₂-dmea)₂(H₂O)}₂(μ₄-pma)]_n·4nH₂O (1), [{Cu₂(μ₂-Hedea)₂}₂(μ₄-pma)]_n·4nH₂O (2), and [{Cu(bea)(Hbea)}₄(μ₄-pma)]_n·2nH₂O (3)ˉwere generated by an aqueous medium self-assembly method from copper(II) nitrate, pyromellitic acid (H₄pma), and different aminoalcohols [N,N-dimethylethanolamine (Hdmea), N-ethyldiethanolamine (H₂edea), and N-benzylethanolamine (Hbea)]. Compounds 2 and 3 represent the first coordination polymers derived from H₂edea and Hbea. All the products were characterized by infrared (IR), electron paramagnetic resonance (EPR), and ultraviolet-visible light (UV-vis) spectroscopy, electrospray ionization-mass spectroscopy (ESI-MS(±)), thermogravimetric and elemental analysis, and single-crystal X-ray diffraction (XRD), which revealed that their two-dimensional (2D) metal-organic networks are composed of distinct dicopper(II) or monocopper(II) aminoalcoholate units and μ₄-pyromellitate spacers. From the topological viewpoint, the underlying 2D nets of 1-3 can be classified as uninodal 4-connected layers with the sql topology. The structures of 1 and 2 are further extended by multiple intermolecular hydrogen bonds, resulting in three-dimensional (3D) hydrogen-bonded networks with rare or unique topologies. The obtained compounds also act as highly efficient precatalysts for the mild homogeneous oxidation, by aqueous H₂O₂ in acidic MeCN/H₂O medium, of various cycloalkanes to the corresponding alcohols and ketones. Overall product yields up to 45% (based on cycloalkane) were attained and the effects of various reaction parameters were investigated, including the type of precatalyst and acid promoter, influence of water, and substrate scope. Although water usually strongly inhibits the alkane oxidations, a very pronounced promoting behavior of H₂O was detected when using the precatalyst 1, resulting in a 15-fold growth of an initial reaction rate in the cyclohexane oxidation on increasing the amount of H₂O from ～4 M to 17 M in the reaction mixture, followed by a 2-fold product yield growth.

Full text of DOI:10.1021/acs.inorgchem.5b01983

Article
pubs.acs.org/IC
Copper(II) Coordination Polymers Self-Assembled from
Aminoalcohols and Pyromellitic Acid: Highly Active Precatalysts for
the Mild Water-Promoted Oxidation of Alkanes
Tiago A. Fernandes,† Carla I. M. Santos,† Vania Andre,† Julia Kłak,‡ Marina V. Kirillova,*,†
̂
́
and Alexander M. Kirillov*,†
†Centro de Química Estrutural, Complexo I, Instituto Superior Tec
́
nico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001, Lisbon,
Portugal
‡Faculty of Chemistry, University of Wrocław, ul. F. Joliot-Curie 14, 50-383 Wrocław, Poland
S
* Supporting Information
ABSTRACT: Three novel water-soluble 2D copper(II) coordination
polymers[{Cu2(μ2-dmea)2(H2O)}2(μ4-pma)]n·4nH2O (1), [{Cu2(μ2-
Hedea)2}2(μ4-pma)]n·4nH2O (2), and [{Cu(bea)(Hbea)}4(μ4-pma)]n·
2nH2O (3)were generated by an aqueous medium self-assembly method
from copper(II) nitrate, pyromellitic acid (H4pma), and different amino-
alcohols [N,N-dimethylethanolamine (Hdmea), N-ethyldiethanolamine
(H2edea), and N-benzylethanolamine (Hbea)]. Compounds 2 and 3 represent
the first coordination polymers derived from H2edea and Hbea. All the
products were characterized by infrared (IR), electron paramagnetic resonance
(EPR), and ultraviolet−visible light (UV-vis) spectroscopy, electrospray
ionization−mass spectroscopy (ESI-MS( )), thermogravimetric and elemen-
tal analysis, and single-crystal X-ray diffraction (XRD), which revealed that
their two-dimensional (2D) metal−organic networks are composed of distinct
dicopper(II) or monocopper(II) aminoalcoholate units and μ4-pyromellitate spacers. From the topological viewpoint, the
underlying 2D nets of 1−3 can be classified as uninodal 4-connected layers with the sql topology. The structures of 1 and 2 are
further extended by multiple intermolecular hydrogen bonds, resulting in three-dimensional (3D) hydrogen-bonded networks
with rare or unique topologies. The obtained compounds also act as highly efficient precatalysts for the mild homogeneous
oxidation, by aqueous H2O2 in acidic MeCN/H2O medium, of various cycloalkanes to the corresponding alcohols and ketones.
Overall product yields up to 45% (based on cycloalkane) were attained and the effects of various reaction parameters were
investigated, including the type of precatalyst and acid promoter, influence of water, and substrate scope. Although water usually
strongly inhibits the alkane oxidations, a very pronounced promoting behavior of H2O was detected when using the precatalyst 1,
resulting in a 15-fold growth of an initial reaction rate in the cyclohexane oxidation on increasing the amount of H2O from ∼4 M
to 17 M in the reaction mixture, followed by a 2-fold product yield growth.
of metal−organic networks,6 there are some simple and
INTRODUCTION
■
commercially available aminoalcohol building blocks that have
Within the sweeping progress of research on the design,
never been applied toward the design of CPs. For example, a
search of the Cambridge Structural Database (CSD) reveals no
synthesis, and properties of metal−organic networks,1 water-
soluble coordination polymers (CPs) represent a significantly
examples of coordination polymers derived from N-ethyl-
less explored class of compounds which, however, possess a
diethanolamine (H2edea) and N-benzylethanolamine (Hbea),6
diversity of notable applications in modern chemistry. In
particular, bioactive and/or biomimetic CPs2 have found
while their coordination chemistry is limited to a few discrete
metal complexes.7 Considering the above-mentioned points
significance in the fields of drug delivery and antimicrobial
and following our general interest in exploring various
treatment, selective sorption, and homogeneous bioinspired
catalysis.3,4 The solubility of CPs in aqueous medium can be
aminoalcohol building blocks for the generation of
multicopper(II) cores and coordination polymers with
achieved by introducing into the structure at least one organic
building block that is soluble in water. Aminoalcohols are
particularly attractive examples of such building blocks, because
significance in aqueous-medium catalysis, molecular magnetism,
and supramolecular chemistry,8 the principal objectives of the
current study were as follows:
of their coordination flexibility and versatility, low toxicity and
bioactivity, as well as high stability, solubility, and low cost.5
Although a few common aminoalcohols (e.g., diethanolamine
Received: August 27, 2015
and triethanolamine) have been well-explored for the synthesis
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b01983
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Article
decomposition of dmea), 260−370 °C (decomposition of pma,
formation of CuO; total Δm: 66.3% expt., 67.0% calcd.).
(1) To probe diverse aminoalcohol ligands for the
preparation of novel mixed-ligand copper(II) CPs that
would potentially show solubility in water.
(2) To verify whether a slight variation of the type of
aminoalcohol block would alter the structural features
and properties of coordination polymers derived from a
common organic linker: pyromellitic acid (H4pma).
(3) To test the obtained products as water-soluble
precatalysts in the mild homogeneous oxidation of
alkanes.
[{Cu2(μ2-Hedea)2}2(μ4-pma)]n·4nH2O (2). This product was ob-
tained following the procedure described for 1, but using a mixture of
N-ethyldiethanolamine (H2edea; 2.0 mmol, 525 μL) and KOH (3.00
mmol, 3 mL, 1 M aqueous solution) instead of Hdmea. Dark-blue X-
ray-quality crystals of 2 were obtained in ∼50% yield (based on
copper(II) nitrate). Compound 2 is barely soluble in H2O (S25 °C ≈ 1
mg mL−1). Anal. Calcd (%) for 2+H2O, C34H68Cu4N4O21: C 36.36, H
6.10, N 4.99; found C 36.49, H 6.18, N 4.82. FT-IR (KBr): 3418 (s br)
ν(H2O), 2978 (w), 2859 (w) and 2845 (m) ν(CH), 1611 (s br)
νas(COO), 1492 (w), 1467 (w), 1406 (s) and 1362 (s) and 1333 (sh)
νs(COO), 1247 (w), 1139 (s), 1072 (s), 1042 (m), 1028 (w), 920
(w), 897 (w), 815 (w), 752 (w), 618 (w), 571 (w), and 512 (w) cm−1.
ESI−MS( ) (H2O), selected fragments: MS(+), m/z: 1032
[Cu4(Hedea)4(Hpma)]+, 839 [Cu3(Hedea)3(H2pma)]+, 643
[Cu2(Hedea)2(H3pma)]+, 134 [H2edea + H]+; MS(−), m/z: 1019
[Cu4(Hedea)2(pma) (Hpma)]−, 641 [Cu2(Hedea)2(Hpma)]−, 507
[Cu2(Hedea) (pma)]−, 253 [H3pma]−. TGA (N2, 5 °C/min): 60−170
°C (−4H2O; Δm: 5.8% expt., 6.5% calcd.), 170−350 °C
(decomposition of Hedea), 350−500 °C (decomposition of pma,
formation of CuO; total Δm: 68.1% expt., 71.2% calcd.).
Thus, in the present study, we report a simple aqueous
medium self-assembly synthesis, full characterization, structural
and topological features, and catalytic properties of three novel,
water-soluble and mixed-ligand two-dimensional (2D) copper-
(II) coordination polymers[{Cu2(μ2-dmea)2(H2O)}2(μ4-
pma)]n·4nH2O (1), [{Cu2(μ2-Hedea)2}2(μ4-pma)]n·4nH2O
(2), and [{Cu(bea)(Hbea)}4(μ4-pma)]n·2nH2O (3)which
were assembled from rigid pyromellitic acid linkers and
different flexible aminoalcohol building blocks (Scheme 1).
[{Cu(bea)(Hbea)}4(μ4-pma)]n·2nH2O (3). This product was ob-
tained by following the procedure described for 1, but using a mixture
of N-benzylethanolamine (Hbea; 5.0 mmol, 710 μL) and KOH (3.00
mmol, 3 mL, 1 M aqueous solution) instead of Hdmea. Light-blue X-
ray-quality crystals of 3 were obtained in ∼60% yield (based on
copper(II) nitrate). Compound 3 is soluble in H2O (S25 °C ≈ 4 mg
mL−1). Anal. Calcd (%) for 3+NO3, C82H106Cu4N9O21: C 54.47, H
5.91, N 6.97; found C 54.37, H 5.72, N 6.56. FT-IR (KBr): 3231 (s)
and 3184 (s) ν(H2O) + ν(NH), 2940 (w), 2885 (w), and 2829 (w)
ν(CH), 1454 (s) νas(COO), 1393 (sh) and 1365 (vs br) and 1319
(sh) νs(COO), 1250 (w), 1076 (m), 1049 (m), 1039 (m), 1015 (w),
970 (w), 943 (w), 902 (w), 827 (w), 754 (s), 702 (s), 641 (w), 596
(w), 534 (w), 458 (w), and 431 (w) cm−1. ESI−MS( ) (H2O),
selected fragments: MS(+), m/z: 152 [Hbea + H]+; MS(−), m/z: 679
[Cu(Hbea)2(pma)]−, 363 [Cu(bea)2 − H]−, 253 [H3pma]−. TGA
(N2, 5 °C/min): 50−110 °C (−2H2O; Δm: 2.4% expt., 2.1% calcd.),
200−415 °C (decomposition of bea/Hbea), 415−520 °C (decom-
position of pma, formation of CuO; total Δm: 83.0% expt., 81.8%
calcd.).
X-ray Crystallography. Crystals of 1−3 suitable for X-ray
diffraction (XRD) study were mounted with Fomblin in a cryoloop.
Data were collected on a Bruker AXS-KAPPA APEX II diffractometer
with graphite-monochromated radiation (Mo Kα, λ = 0.17073 Å) at
298 K. The X-ray generator was operated at 50 kV and 30 mA and the
X-ray data collection was monitored by the APEX29 program. All data
were corrected for Lorentzian, polarization, and absorption effects,
using SAINT9 and SADABS9 programs. SIR-9710 and SHELXS-9711
were used for structure solution and SHELXL-9711 was applied for full
matrix least-squares refinement on F2. These three programs are
included in the package of programs WINGX-Version 1.80.05.12 Non-
hydrogen atoms were refined anisotropically. A full-matrix least-
squares refinement was used for the non-hydrogen atoms with
anisotropic thermal parameters. All the H atoms were inserted in
idealized positions and allowed to refine in the parent C or O atom.
TOPOS 4.013 and PLATON14 were applied for topological analysis
and hydrogen bond interactions, respectively. Crystal data and details
of the data collection for 1−3 are reported in Table 1.
Scheme 1. Structural Formulae of Organic Building Blocks
The latter vary in sterical hindrance, number of ethanolamine
arms, and presence of different N-aliphatic (methyl, ethyl) or
N-aromatic (benzyl) groups. Apart from representing the first
coordination polymers derived from H2edea (compound 2)
and Hbea (compound 3), all of the obtained products (1−3)
also act as highly efficient precatalysts for the mild oxidation of
different alkanes to alcohols and ketones, by H2O2 and in
aqueous acidic MeCN medium, revealing a very unusual
promoting role of water.
EXPERIMENTAL SECTION
■
Self-Assembly Synthesis and Characterization. [{Cu2(μ2-
dmea)2(H2O)}2(μ4-pma)]n·4nH2O (1). An excess of N,N′-dimethyle-
thanolamine (Hdmea; 5.0 mmol, 0.50 mL) and pyromellitic acid (0.50
mmol, 127 mg) were added in this order, to an aqueous solution (10
mL) containing Cu(NO3)2·2.5H2O (1.00 mmol, 233 mg) and with
continuous stirring at room temperature. The resulting reaction
mixture was stirred during 2 h and then filtered off. The filtrate was left
to evaporate in a beaker at ambient temperature. Blue X-ray-quality
crystals were formed within 2−3 weeks, then collected and dried in air
to furnish compound 1 in ∼60% yield (based on copper(II) nitrate).
Compound 1 is very soluble in H2O (S25 °C > 15 mg mL−1). Anal.
Calcd (%) for 1−H2O, C26H52Cu4N4O17: C 32.98, H 5.54, N 5.92;
found C 33.09, H 5.48, N 6.30. FT-IR (KBr): 3453 (s br), 3408 (s),
and 3276 (sh) ν(H2O), 2970 (w), 2878 (w), and 2851 (w) ν(CH),
1669 (s), 1612 (s) and 1601 (s, one broad band with two maxima)
νas(COO), 1489 (m), 1468 (m), 1405 (s) and 1385 (s) and 1355 (s)
νs(COO), 1281 (m), 1131 (s), 1090 (m), 1066 (m), 1018 (w), 950
(w), 868 (w), 826 (w), 785 (w), 656 (m), 562 (w), 530 (w), and 473
(w) cm−1. ESI−MS( ) (H2O), selected fragments: MS(+), m/z: 977
[Cu5(Hdmea)4(pma)(H2O)3]+, 797 [Cu3(Hdmea)4(pma)]+; MS(−),
m/z: 1109 [Cu4(dmea)4(pma)(H3pma)]−, 929 [Cu4(dmea)4(pma)-
(H2O)4]−, 554 [Cu2(Hdmea)2(pma)]−, 377 [Cu2(pma)]−, 315
[Cu(Hpma)]−, 253 [H3pma]−. TGA (N2, 5 °C/min): 50−180 °C
(−4H2O; Δm: 7.3% expt., 7.5% calcd.), 180−260 °C (−2H2O,
Catalytic Studies. The alkane oxidations were typically carried out
in an air atmosphere in thermostated glass reactors equipped with a
condenser under vigorous stirring and using MeCN as the solvent (up
to a total volume of 2.5 mL) at 50 °C. In a typical experiment,
precatalyst 1−3 (5 × 10−3 mmol) and gas chromatography (GC)
internal standard (MeNO2, 25 μL) were introduced into a MeCN
solution, followed by the addition of an acid promoter (0.05 mmol,
optional) used as a stock solution in MeCN. The alkane substrate (1
mmol) was then introduced, and the reaction started upon addition of
hydrogen peroxide (50% in H2O, 5 mmol) in one portion. The
reactions were monitored by withdrawing small aliquots after different
B
DOI: 10.1021/acs.inorgchem.5b01983
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Inorganic Chemistry
Article
UV-vis spectroscopy, ESI-MS( ), thermogravimetric and
elemental analysis, and single-crystal X-ray diffraction.
Table 1. Crystal Data and Structure Refinement Details for
1−3
The selection of pyromellitic acid was governed by its
recognized application as a linker in crystal engineering of
copper(II)-organic networks.16,17 Despite similar network
dimensionality, compounds 1−3 are built from the structurally
distinct dicopper(II) or monocopper(II) aminoalcoholate
secondary building units (SBUs). Their self-assembly appeared
to be primarily guided by the type of aminoalcohol building
block, which also influences the solubility of the resulting
product in H2O. In fact, water solubility values vary from ∼1
mg mL−1 for a barely soluble 2 to ∼4 mg mL−1 for 3, and >15
mg mL−1 for a well-soluble product 1. Compounds 1−3 can
partially dissociate upon dissolution in water, as confirmed by
the ESI-MS( ) data, revealing the presence of heaviest
fragments derived from molecular units, namely,
[Cu5(Hdmea)4(pma)(H2O)3]+ (m/z = 977) and
[Cu4(dmea)4(pma)(H2O)4]− (m/z = 929) for 1,
1
2
3
formula
fw
C13H27Cu2N2O9
482.44
C17H29Cu2N2O10
548.50
C23H28CuN2O7
508.01
crystal form,
color
block, blue
block, blue
plate, blue
crystal size,
mm
0.05 × 0.03 ×
0.04 × 0.03 × 0.02 0.08 × 0.05 ×
0.03
0.04
cryst. syst.
space group
a, Å
triclinic
monoclinic
P21/c
triclinic
P1
̅
P1
̅
8.5488(7)
11.1202(9)
12.2436(10)
111.443(3)
107.668(3)
97.411(3)
2
13.0515(12)
14.5150(14)
12.6791(12)
90.00
10.9067(11)
11.0800(12)
11.1272(12)
83.799(4)
85.737(4)
65.227(2)
2
b, Å
c, Å
α, deg
β, deg
γ, deg
Z
100.520(3)
90.00
4
[ C u 4 ( H e d e a ) 4 ( H p m a ) ] + ( m / z
= 1 0 3 2 ) a n d
V, Å3
993.77(14)
293(2)
2361.6(4)
293(2)
1213.2(2)
293(2)
[Cu4(Hedea)2(pma) (Hpma)]− (m/z = 1019) for 2, and
[Cu(Hbea)2(pma)]− (m/z = 679) for 3. A relatively low
intensity of these peaks in both positive and negative modes
can be explained by the difficulty in fragmentation of polymeric
networks, probably suggesting their stability in aqueous
medium. In fact, the compounds can be recrystallized or self-
assembled again after dissolution. However, the relative
intensity of the ESI-MS( ) patterns of 1−3 increases upon
an addition of a small amount of HNO3, suggesting that an
acidic medium promotes a partial disaggregation of these
metal−organic networks.
T, K
Dc, g cm−3
1.612
1.543
1.391
μ(Mo Kα),
2.185
1.853
0.944
mm−1
θ range (deg)
2.597−25.242
2.492−25.242
2.624−31.102
no. of refl.
collected
39065
27502
66251
no. of
independent
refl.
6778
4243
7776
Rint
a
0.1148
0.0882
0.0607
b
R1 , wR2 [I ≥ 0.0593, 0.1008
2σ(I)]
0.0933, 0.2240
0.0439, 0.0852
The EPR spectra of powdered samples of 1 and 2 recorded
in the X-band at 293 and 77 K are essentially similar and show a
few poorly resolved lines (Figure S1 in the Supporting
Information). These spectral shapes can be compatible with
the presence of magnetically nonequivalent copper(II) centers
in the ligand fields of the distorted square-pyramidal geo-
metries.18−20 Moreover, in a low field part of the spectrum (at
1500 G), there is one ΔMS = 2 transition that confirms an
exchange interaction between metal atoms in dicopper(II)
blocks. The EPR data of 3 are typical for copper(II) centers in a
ligand field of the distorted octahedral {CuN2O4} symme-
try.18−20 Only a broad isotropic signal with the g value of ∼2.14
was observed. The UV−vis spectra of solid samples 1−3 display
absorptions both in the UV and visible regions (Figure S2 in
the Supporting Information). The strong absorption bands near
330 and 360 nm are due to π → π* and n → π* transitions of
ligands. The broad bands in the visible region are due to d−d
transitions. Five-coordinated copper(II) compounds with O-
donor ligands usually have18,21 two absorption bands lying near
1220 and 980 nm. The spectra of 1 and 2 are very similar, being
consistent with a small difference in ligand field. They both
display a very broad d−d band at 850 nm. Six-coordinated
compounds typically give rise to one absorption band in the
visible region18,21 and the peak near 700 nm is observed in 3.
The thermogravimetric analyses (TGA) of 1−3 show a series
of typical thermal effects corresponding to a removal of
crystallization water molecules and a stepwise decomposition of
metal−organic networks (see Figures S3−S5 in the Supporting
Information). For example, four crystallization H2O molecules
of 1 are released in the 50−180 °C range, whereas a next more-
complex thermal effect in the 180−260 °C temperature interval
corresponds to the removal of two water ligands along with the
decomposition of dmea moieties. Further heating up to 370 °C
GOF on F2
1.001
1.041
1.056
a
b
2
2
R1 = ∑∥F0| − |Fc∥/∑|F0|. wR2 = [∑[w(F0 − Fc )2]/
2
∑[w(F0 )2]]1/2
.
periods of time, which were treated with PPh3 (following a method
developed by Shul’pin)15 for reduction of remaining H2O2 and alkyl
hydroperoxides that are typically formed as major primary products in
alkane oxidations. The samples were analyzed by gas chromatography
(GC), using nitromethane as an internal standard. Attribution of peaks
was made by comparison with chromatograms of authentic samples.
Chromatographic analyses were run on an Agilent Technologies
7820A series gas chromatograph (with helium as the carrier gas)
equipped with the FID detector and BP20/SGE (30 m × 0.22 mm ×
0.25 μm) capillary column. Blank tests confirmed that alkane
oxidations do not proceed in the absence of copper precatalyst.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The 2D coordination
polymers [{Cu2(μ2-dmea)2(H2O)}2(μ4-pma)]n·4nH2O (1),
[{Cu2(μ2-Hedea)2}2(μ4-pma)]n·4nH2O (2), and [{Cu(bea)
(Hbea)}4(μ4-pma)]n·2nH2O (3) were generated as micro-
crystalline solids by an aqueous medium self-assembly method.
It is based on a simple combination, in water at ∼25 °C and in
air, of Cu(NO3)2·2.5H2O with an aminoalcohol [N,N′-
dimethylethanolamine (Hdmea), N-ethyldiethanolamine
(H2edea), or N-benzylethanolamine (Hbea) for 1−3, respec-
tively] as a main building block and pyromellitic acid (H4pma)
as a spacer or linker. An excess of aminoalcohol (for 1−3) and
additionally an aqueous solution of potassium hydroxide (for 2
and 3) were required to maintain the basicity (pH 10−12) of
the reaction mixtures. Compounds 1−3 were isolated as air-
stable microcrystalline solids and characterized by IR, EPR and
C
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Figure 1. Structural fragments of 1 showing (a) interlinkage of two distinct dicopper(II) [Cu2(μ2-dmea)2]2+ and [Cu2(μ2-dmea)2(H2O)2]2+ units,
(b) 2D metal−organic layer, and (c) its topological representation displaying a uninodal 4-connected underlying net with the sql topology. Further
details: (panels a and b) H atoms and crystallization H2O molecules are omitted for clarity, Cu (green), O (red), N (blue), and C (gray); (c)
terminal H2O ligands were omitted for clarity; Cu centers (green), centroids of μ2-dmea (blue) and μ4-pma (gray) blocks. Symmetry codes: (a): (i)
−x + 1, −y + 1, −z + 2; (ii) −x + 1, −y + 2, −z + 1. Selected distances (Å): Cu1−N2, 2.018(4); Cu1−O1, 1.919(4); Cu1−O2, 2.713(3); Cu1−O5,
1.920(3); Cu1i−O5, 1.886(4); Cu1···Cu1i, 2.9763(8); Cu2−N1, 2.009(4); Cu2−O1w, 2.337(6); Cu2−O3, 1.940(2); Cu2−O6, 1.918(4); Cu2ii−
O6, 1.906(4); Cu2···Cu2i, 2.9561(9).
leads to the decomposition of pma linkers. In contrast, the
TGA pattern of 2 is shifted to higher temperatures, with the
major thermal effects due to the loss of four crystallization H2O
molecules (60−170 °C), decomposition of Hedea (170−350
°C) and pma (350−500 °C) ligands. Similarly, the TGA
pattern of 3 shows the removal of two crystallization H2O
molecules (50−110 °C) and the decomposition of bea/Hbea
(200−415 °C) and pma (415−520 °C) moieties. The observed
total weight loss in all samples 1−3 (Δm of 66.3%, 68.1%, and
83.0%, respectively) is very close to the calculated values
assuming the formation of CuO residue.
Structural and Topological Description. The crystal
structure of the 2D coordination polymer [{Cu2(μ2-
dmea)2(H2O)}2(μ4-pma)]n·4nH2O (1) is composed of two
distinct types of dicopper(II) [Cu2(μ2-dmea)2]2+ and [Cu2(μ2-
dmea)2(H2O)2]2+ blocks (based on Cu1 and Cu2 centers,
respectively), a μ4-pma(4−) spacer, and four crystallization
water molecules (Figure 1). The Cu1 atoms reveal a distorted
{CuNO4} square-pyramidal environment (τ5 = 0.21; this value
is zero for an ideal square-pyramidal geometry),22 filled by the
N2, O5, and O5i atoms of bidentate μ2-dmea ligands [Cu1−N2
2.018(4), Cu1−O5 1.919(4), Cu1−O5i 1.886(4) Å] and the
O1 atom of μ4-pma [Cu1−O1 1.920(3) Å] in equatorial sites,
whereas the axial position is occupied by a weakly bound O2
carboxylate atom [Cu1−O2 2.713(3) Å] (Figure 1a). The Cu2
atoms also adopt a distorted {CuNO4} square-pyramidal
environment (τ5 = 0.29), formed by the aminoalcoholate N2,
O6, and O6ii and carboxylate O3 atoms [Cu2−N1, 2.009(4);
Cu2−O6, 1.918(4); Cu2−O6ii, 1.906(4); Cu2−O3, 1.940(2)
Å] in equatorial positions, while an axial site is taken by the
terminal O1w water ligand [Cu2−O1w 2.337(6) Å]. In both
types of the dicopper(II) blocks, the {Cu2(μ-O)2} cores are
planar and possess rather short Cu1 ···Cu1i [2.9763(9) Å] and
Cu2···Cu2i [2.9561(9) Å] separations, with the corresponding
Cu1−O5−Cu1i and Cu2−O6−Cu2ii angles of 102.92(14) and
101.23(12)°, respectively. The μ4-pma(4−) moieties bear two
pairs of distinct η1:η1- and η1:η0−COO groups that interlink the
adjacent dicopper(II) aminoalcoholate blocks into a 2D metal−
organic layer (see Figures 1b and 1c).
Similar to 1, the structure of [{Cu2(μ2-Hedea)2}2(μ4-pma)]n·
4nH2O (2) also features a resembling 2D metal−organic
network assembled from the [Cu2(μ2-Hedea)2]2+ and μ4-
pma(4−) blocks (see Figure S6 in the Supporting Informa-
tion), and thus is not discussed in detail. The structural
difference between 1 and 2 mainly concerns the absence in 2 of
H2O ligands and the presence of only one η1:η0−COO mode of
carboxylate groups in μ4-pma. The bonding parameters in 1 and
2 are comparable to those of other copper(II) compounds
bearing dmea,6,8c Hedea,7a,b or pyromellitate16,17 moieties.
Besides, compound 2 represents the first coordination polymer
derived from N-ethyldiethanolamine.6
Although the compound [{Cu(bea)(Hbea)}4(μ4-pma)]n·
2nH2O (3) also discloses a 2D metal−organic network, it is
however assembled from the monocopper(II) [Cu(bae)2] and
[Cu(Hbea)2]2+ blocks (based on Cu1 and Cu2 centers,
respectively) and μ4-pma(4−) spacers (Figure 2). In contrast
to five-coordinate Cu atoms and {CuNO4} environments in 1
and 2, coordination polymer 3 possesses the six-coordinated
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Figure 2. Structural fragments of 3 showing (a) interlinkage of two distinct monocopper(II) [Cu(bea)2] and [Cu(Hbea)2]2+ units, (b) 2D metal−
organic layer, and (c) its topological representation displaying a uninodal 4-connected underlying net with the sql topology. Further details: (a, b) H
atoms and crystallization H2O molecules are omitted for clarity, Cu (green), O (red), N (blue), and C (gray); (c) Cu centers (green), centroids of
bea/Hbea (blue) and μ4-pma (gray) blocks. Selected distances (Å): Cu1−N1, 2.039(2); Cu1−O1, 2.583(2); Cu1−O2, 1.9546(13); Cu2−N2,
2.040(2); Cu2−O4, 1.9583(13); Cu2−O6, 2.4068(16).
Cu centers with a {CuN2O4} octahedral geometry. Hence, the
Cu1 atoms are surrounded by symmetry equivalent pairs of the
N1 and O1 (from bea) and O2 (from μ4-pma) atoms in a
slightly distorted octahedral fashion [Cu1−N1, 2.039(2) Å;
Cu1−O1, 2.583(2) Å; Cu1−O2, 1.9546(13) Å], wherein two
aminoalcoholate O1 atoms occupy the axial sites. Similarly, the
Cu2 centers bind the symmetry equivalent pairs of N2 and O6
atoms from Hbea and the O4 atoms from μ4-pma [Cu2−N2,
2.040(2) Å; Cu2−O4, 1.9583(13) Å; Cu2−O6, 2.4068(16) Å].
As in 2, the μ4-pma(4−) spacers in 3 interlink the adjacent
monocopper(II) units via four η1:η0−COO groups, resulting in
a 2D metal−organic layer (Figures 2b and 2c) with the shortest
Cu1···Cu1, Cu1···Cu2, and Cu2···Cu2 separations through μ4-
pma of 11.127(2), 7.415(2) (8.265(2)), and 11.080(2) Å,
respectively. To our knowledge, compound 3 represents the
first Cu coordination compound and the first coordination
polymer (of any metal) derived from N-benzylethanolamine.6
Topological analysis of the 2D metal−organic networks in
1−3 was performed following the concept of the simplified
underlying net;13 such nets were generated by contracting all
ligands to their centroids maintaining their connectivity (see
Figures 1b and 2b). Despite being constructed from different
dicopper(II) or monocopper(II) blocks, all the 2D underlying
nets of 1−3 can be topologically classified as uninodal 4-
connected layers with the sql [Shubnikov tetragonal plane net]
topology and the point symbol of (44.62).
intermolecular hydrogen bonds and involving crystallization
water molecules, to give very complex 3D supramolecular
frameworks (see Figure S7 in the Supporting Information). To
better understand the structures of these supramolecular nets,
we carried out their topological analysis.13,23 Hence, the
hydrogen-bonded net of 1 was simplified by contracting
dicopper(II) aminoalcoholate blocks, crystallization H2O
molecules, and pyromellitate spacers to their centroids,
maintaining their connectivity via both coordination and
hydrogen bonds; only conventional hydrogen bonds were
considered [H···A < 2.50 Å, D···A < 3.50 Å, and ∠(D−H···A) >
120°; D and A stand for donor and acceptor atoms].13 The
resulting underlying net (Figure 3a) can be classified as a
trinodal 3,4,8-connected framework with a rare 3,4,8T9
topology.13 It is defined by the point symbol of
(3.4.5)2(32.42.52.614.74.83.9)(32.63.7), wherein the notations
(3.4.5), (32.42.52.614.74.83.9), and (32.63.7) correspond to the
3-connected H2O, 8-connected pma, and 4-connected [Cu2(μ2-
dmea)2(H2O)2]2+ nodes, respectively; the [Cu2(μ2-dmea)2]2+
blocks act as the 2-connected linkers. Similarly, a three-
dimensional (3D) hydrogen-bonded network of 2 was
simplified and the obtained underlying framework (Figure
3b) was topologically classified as a trinodal 4,6,8-connected
net. As confirmed by a search of various databases,6,13,24 this net
features the unique topology defined by the point symbol of
(32.42.52)2(34.44.54.615.8)(34.44.54.63), wherein the (32.42.52),
(34.44.54.615.8), and (34.44.54.63) indices are those of the 4-
connected (H2O)2 cluster nodes, 8-connected pma, and 6-
Furthermore, the 2D metal−organic layers in 1 and 2 are
further extended [2D → 3D], by means of multiple
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(TFA) promoter (see Figure 4 for the accumulation of
products with time) proceeds with the formation of cyclo-
Figure 3. Topological representations of the underlying three-
dimensional (3D) hydrogen-bonded frameworks in (a) 1 and (b) 2.
Panel (a) shows a trinodal 3,4,8-connected net with the 3,4,8T9
topology and point symbol of (3.4.5)2(32.42.52.614.74.83.9)(32.63.7),
and panel (b) shows a trinodal 4,6,8-connected net in 2 with the
unique topology and point symbol of (32.42.52)2(34.44.54.615.8)-
(34.44.54.63). Further details: (a) centroids of 2-connected [Cu2(μ2-
dmea)2]2+ blocks and 4-connected [Cu2(μ2-dmea)2(H2O)2]2+ nodes
(green), centroids of 3-connected H2O nodes (red); (b) centroids of
2- and 6-connected [Cu2(μ2-Hedea)2] blocks (green), and centroids of
4-connected (H2O)2 nodes (red); (a, b) centroids of 8-connected pma
nodes (gray).
Figure 4. Accumulation of products (cyclohexanol and cyclohexanone,
total yield, %) with time in the cyclohexane oxidation by H2O2
catalyzed by compounds 1−3 in the presence of TFA promoter.
Reaction conditions: precatalyst 1−3 (0.005 mmol), TFA (0.05
mmol), C6H12 (1.0 mmol), H2O2 (5.0 mmol), CH3CN (up to 2.5 mL
of the total volume), 50 °C.
connected [Cu2(μ2-Hedea)2] (based on Cu1 atoms) nodes,
respectively; the [Cu2(μ2-Hedea)2] blocks based on the Cu2
atoms act the 2-connected linkers.
Mild Catalytic Oxidation of Alkanes. Although metal−
organic networks are usually applied in heterogeneous catalysis,
the coordination polymers that are soluble in aqueous and/or
organic medium can act as precursors of catalytically active
species in homogeneous catalytic systems.25,26 Inspired by a
solubility of compounds 1−3 in aqueous medium, we tested
their catalytic activity toward the mild oxidation of cycloalkanes
by aqueous H2O2 to give the corresponding alcohols and
ketones. Cyclohexane was used as a model substrate27 and
typical oxidation reactions were carried out in aqueous
acetonitrile medium, at 50 °C in air, and in the presence
(optional) of an acid promoter.25 It should be mentioned that
the compounds 1−3 are not intact during the catalytic
experiments and in the presence of an acid promoter and
hydrogen peroxide lead to the formation of oligomeric
catalytically active species, eventually via additional protonation
and partial decoordination of aminoalcoholate or pyromellitate
ligands.
hexanol (main product) and cyclohexanone with their total
yields of 24%, 30%, and 33%, respectively; for the detailed
kinetic curves of product accumulation, see Figure S8 in the
Supporting Information. In the absence of an acid promoter, 1
is inactive (Figure 5a), 2 shows modest activity (12% total
yield, Figure S9 in the Supporting Information), while 3 reveals
an appreciable total yield of 21% (Figure 5b) which is, however,
inferior to that achieved in the presence of an acid promoter.
The effect of different acid promoters was studied (see Figure
5, as well as Figure S9), revealing that the highest activity is
observed in the systems containing TFA and HCl, followed by
HNO3, while H2SO4 is a less-active promoter. Following the
literature background,25,28−30 we believe that the acid promoter
facilitates some proton transfer steps and activates the
precatalysts by partial protonation of ligands and unsaturation
of copper centers; it also improves the oxidation power of H2O2
and prevents its decomposition (e.g., catalase activity). In fact, a
partial decomposition of H2O2 can be observed when using
compound 1 in the absence of an acid promoter, thus
explaining the very low yields that are observed in the
oxidation of cyclohexane.
All compounds showed a good activity under common
reaction conditions with the total product yields of up to 36%
based on cycloalkane (Table 2). The C6H12 oxidation catalyzed
by 1−3 in the presence of a small amount of trifluoroacetic acid
In the presence of TFA, the reaction rate of the cyclohexane
oxidation catalyzed by 1 is increased when the TFA
concentration is increased up to 20 equiv, relative to the
precatalyst amount (see Figure 6). After that, the initial reaction
rate (W0) is stabilized and is no longer dependent on the TFA
concentration. The dependence of W0 on the TFA amount has
an S-shape, thus indicating second-order reaction kinetics, with
respect to TFA. Hence, two TFA-containing species (or
protons) are taking place in the rate-limiting step of the
catalytic generation of hydroxyl radicals. Similar dependences of
the total yield and initial reaction rate on the TFA amount were
observed in the presence of compound 2 (Figure S10 in the
Supporting Information). In contrast, an initial reaction rate of
cyclohexane oxidation catalyzed by 3 has a linear dependence
on the HNO3 amount (Figure 7). The total yield is also rising,
from 21% (in the absence of any acid promoter) to 36%, by
increasing the amount of HNO3 up to 20 equiv, relative to the
Table 2. Oxidation of Cycloalkanes by H2O2 in the Presence
of Precatalysts 1−3
a
b
Total Product Yield (Alcohol + Ketone) (%)
cycloalkane
1
2
3
C5H10
C6H12
C7H14
C8H16
16.5
23.9
21.1
25.7
35.5
29.8
35.8
35.5
26.2
32.5
33.9
31.6
a
Reaction conditions: precatalyst 1−3 (0.005 mmol), acid promoter
(TFA, 0.05 mmol), cycloalkane (1 mmol), H2O2 (50% aq. 5 mmol),
CH3CN (up to 2.5 mL of the total volume), 50 °C, 5 h. Yields are
b
based on alkanes [(mol of products per mol of cycloalkane) × 100%].
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Figure 5. Effect of different acid promoters on the oxidation of cyclohexane catalyzed by (a) 1 and (b) 3. Reaction conditions: precatalyst 1 or 3
(0.005 mmol), acid promoter (0.05 mmol), C6H12 (1.0 mmol), H2O2 (5.0 mmol), CH3CN (up to 2.5 mL of the total volume), 50 °C.
Figure 6. Effect of TFA amount on (a) the total yield of the products and (b) the maximum initial reaction rate (W0) of the cyclohexane oxidation
catalyzed by 1 in the presence of H2O2. Reaction conditions: precatalyst 1 (0.005 mmol), TFA (0.025−0.15 mmol), C6H12 (1.0 mmol), H2O2 (5.0
mmol), CH3CN (up to 2.5 mL of the total volume), 50 °C.
Figure 7. Effect of HNO3 amount on (a) the total yield of the products and (b) the maximum initial reaction rate (W0) of the cyclohexane oxidation
catalyzed by 3 in the presence of H2O2. Reaction conditions: precatalyst 3 (0.005 mmol), HNO3 (0.025−0.1 mmol), C6H12 (1.0 mmol), H2O2 (5.0
mmol), CH3CN (up to 2.5 mL of the total volume), 50 °C.
precatalyst. Besides, the increase of the acid promoter and
water concentrations may also lead to higher extent of
dissociation of the coordination polymers, generating catalyti-
cally active species of lower nuclearity, as confirmed by the ESI-
MS tests.
The unusual promoting effect of water affecting both the
total yield and reaction rate of cyclohexane oxidation was
observed in the 1/TFA/H2O2 system (Figure 8). The total
yield of the products increases from 24% to 45% (Figure 8a)
when increasing the water concentration in the system from 4
M (corresponding the amount of H2O contained in 50%
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Figure 8. Effect of the amount of H2O on (a) the total yield of the products and (b) the maximum initial reaction rate (W0) of the cyclohexane
oxidation catalyzed by 1/TFA/H2O2 system. Reaction conditions: precatalyst 1 (0.005 mmol), TFA (0.05 mmol), C6H12 (1.0 mmol), H2O2 (5.0
mmol), CH3CN (up to 2.5 mL of the total volume), added H2O (up to 0.6 mL), 50 °C.
Figure 9. Effect of H2O2 amount on (a) the total yield of the products and (b) the maximum initial reaction rate (W0) of the cyclohexane oxidation
catalyzed by 3/TFA/H2O2 system. Reaction conditions: precatalyst 3 (0.005 mmol), TFA (0.05 mmol), C6H12 (1.0 mmol), H2O2 (3.0−7.5 mmol),
CH3CN (up to 2.5 mL of the total volume), 50 °C.
aqueous H2O2) to 17.2 M (after introduction of an additional
amount of water to the reaction mixture). Besides, it is clearly
seen from the dependence of the initial reaction rate (Figure
8b) that the addition of water dramatically accelerates the
oxidation. Thus, we can conclude that there is a direct
involvement of water in the rate-limiting step of catalytic
generation of hydroxyl radicals. A similar behavior was earlier
observed by us in the oxidation of alkanes catalyzed by a
tetracopper(II) triethanolaminate derivative,28b as well as in the
hydrocarboxylation reactions of alkanes that undergo only in a
mixed H2O/MeCN solvent.26,31 Probably, in the present case,
an additional amount of water helps to extend the catalytic life
of the Cu-containing active species that most likely also bear
some water ligands, as confirmed by the ESI-MS( )
investigation of the 1/HNO3/H2O2 system. Furthermore, a
water-assisted mechanism for the generation of hydroxyl
radicals in vanadium(V)- and rhenium(VII)-catalyzed alkane
oxidation was earlier proposed based on the DFT calcu-
lations.32
(Figure S11 in the Supporting Information). The W0 exhibits a
linear dependence on the H2O concentration.
The H2O2 amount also has an influence on both the product
yield and reaction rate of the cyclohexane oxidation. By rising
the amount of hydrogen peroxide in the 3/TFA/H2O2 system
from 3.0 to 7.5 mmol, the total product yield is gradually
increases from ∼25% to 45% (Figure 9a), followed by a
noticeable accelerating effect. Although the reaction rate
linearly increases with hydrogen peroxide concentration
(Figure 9b), there can also be an additional contribution due
to the effect of water. The need for an excess of H2O2 relatively
to the substrate to achieve higher yields of the oxidation
products indicates that hydrogen peroxide also undergoes a
partial catalytic decomposition, which is a common side
reaction in Cu-catalyzed alkane oxidations.4 Only traces of
products were observed when t-BuOOH (70% in H2O) was
used instead of H2O2 under identical reaction conditions in the
presence of all three precatalysts 1−3, either with or without
TFA promoter.
We have also studied the effect of catalyst amount on the
reaction rate and total product yield of cyclohexane oxidation.
For both compounds 1 and 2, the linear dependence of W0 on
the precatalyst concentration in the presence of TFA promoter
was found (see Figures S12 and S13 in the Supporting
The promoting role of water on the reaction rate of
cyclohexane oxidation was also observed for the 2/TFA/H2O2
system. However, in this case, the water accelerating effect is
not so pronounced and does not alter the total product yield
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Information), thus revealing the first-order reaction kinetics,
which supports an involvement of one type of Cu-containing
species in the rate-limiting catalytic step. The total yield of the
products is also rising on increasing the precatalyst amount in
the system, achieving the values of 26% and 36% when using
0.0075 mmol of compounds 1 and 2, respectively.
In order to understand the nature of oxidizing species
involved, we studied the oxidation of linear and branched
alkanes and measured the corresponding bond selectivity,
regioselectivity, and stereoselectivity parameters (Table 3) in
Table 3. Selectivity Parameters in the Oxidation of Linear
a
It should be mentioned that the maximum overall product
yields (up to 45% based on cyclohexane) obtained in the
present study are superior to those achieved when using a
variety of monocopper or dicopper complexes as homogeneous
catalysts or precatalysts,4,25,33a including some related discrete
dicopper(II) aminoalcoholate derivatives [Cu2(H2tea)2(ba)2]·
2H2O (15% yield) {H3tea, triethanolamine; Hba, benzoic
acid}33a and [Cu2(Hbdea)2(mba)2]·2H2O (18% yield)
{H2bdea, N-butyldiethanolamine; Hmba, 4-methylbenzoic
acid}.33b Better activity of 1−3 can thus be regarded to the
presence of an increased number of copper centers, their
cumulative action, as well as higher stability of the catalytically
active multicopper species derived from the parent precatalysts.
To get some information about such possible catalytically
active species, we investigated by ESI-MS( ) technique
(including the MS/MS studies of the parent fragments) the
most active precatalyst 1 before and after its treatment with a
nitric acid promoter and hydrogen peroxide oxidant, using the
conditions typical to those of catalytic tests. Hence, in addition
to the [Cu5(Hdmea)4(pma)(H2O)3]+ (m/z = 977) and
[Cu3(Hdmea)4(pma)]+ (m/z = 797) fragments that were
present in the ESI-MS(+) spectrum of the parent compound 1,
the ESI-MS(+) plot of the 1/HNO3/H2O2 system reveals many
new peaks, namely [Cu5(Hdmea)4(pma)2]+ (m/z = 1173),
[Cu4(Hdmea)4(pma) (Hpma)]+ (m/z = 1111), [Cu3-
(Hdmea)4(pma)(H3pma)]+ (m/z = 1050), [Cu6(dmea)4-
(H3pma)]+ (m/z = 987), and [Cu5(dmea)3(H2pma)(H2O)2]+
(m/z = 869). Similarly, a few new rather intense signals also
appeared in the ESI-MS(−) plot of 1 after the treatment with
HNO3 and H2O2, corresponding to the [Cu3(Hdmea)4(pma)-
(Hpma)(H2O)]− (m/z = 1065), [Cu5(dmea)4(pma)(H2O)]−
(m/z = 937), and [Cu3(dmea)4(Hpma)(H2O)]− (m/z = 811)
fragments with expected isotopic distribution patterns. Various
ions were selected and subjected to the MS/MS study, which
indicated that the heaviest [Cu5(Hdmea)4(pma)2]+ (m/z =
1173) and [Cu4(dmea)4(pma)(H3pma)]− (m/z = 1109)
fragments can be considered as the most relevant parent ions
in positive and negative ESI modes, respectively. For these and
other peaks, the fragmentation pathways including the MS2
and, in some cases, MS3 and MS4 experiments have been
established (see the Supporting Information). These data
suggest that upon dissolution and in the course of catalytic
reactions the coordination network of 1 partially disaggregates
to generate a series of different high-molecular-weight
tetracopper and/or pentacopper blocks bearing both amino-
alcohol and pyromellitate ligands. Although, because of the
complexity of mass spectrometry patterns, it is not evident
which of these fragments can potentially constitute the
catalytically active species in the present alkane oxidation
reactions, we would like to speculate that such species are most
likely [Cux(dmea)4(pma)y(H2O)z] {x = 4 or 5; y = 1 or 2, z = 0
or 1}, given some similarities in fragmentation patterns and
typical peaks observed in both positive and negative ESI modes
and also by considering the relative peak intensities and data of
MS/MS studies. The Cu-containing peroxo species have not
been detected, although a possibility of their formation at initial
stages of catalytic reactions cannot be excluded.4,28
and Branched Alkanes
precatalyst 1 precatalyst 2 precatalyst 3
Regioselectivity
b
C(1):C(2):C(3):C(4)
1:4:4:4
1:5:5:5
1:4:4:4
(n-C7H16)
Bond Selectivity
c
1°:2°:3° (methylcyclohexane)
1:5:16
1:5:13
3.4
1:5:23
3.3
d
3°/2° (adamantane)
3.4
Stereoselectivity
0.6
trans/cis (cis-
0.7
0.8
0.8
0.8
e
dimethylcyclohexane)
trans/cis (trans-
0.9
e
dimethylcyclohexane)
a
Reaction conditions: precatalyst 1−3 (0.005 mmol), TFA (0.05
mmol), alkane (1.0 mmol), H2O2 (5.0 mmol), MeCN up to 2.5 mL
total volume, 3 h, 50 °C. All parameters were calculated based on the
ratios of isomeric alcohols. The calculated parameters were
normalized, i.e., recalculated taking into account the number of H
b
atoms at each C atom. Parameters C(1):C(2):C(3):C(4) are the
relative reactivities of H atoms at carbons 1, 2, 3, and 4 of the n-
c
heptane chain. Parameters 1°:2°:3° are the relative normalized
reactivities of the hydrogen atoms at primary, secondary, and tertiary C
d
atoms of methylcyclohexane. Parameters 3°/2° are the relative
normalized reactivities of the hydrogen atoms at tertiary and secondary
C atoms of adamantane, determined as the ratio of the formed tertiary
e
and secondary alcohol isomers. Parameter trans/cis is determined as
the ratio of the formed tertiary alcohol isomers with mutual trans and
cis orientation of the methyl groups.
the presence of all three precatalysts. Oxidation of n-heptane
proceeds without a specific preference to any secondary carbon
atom of the hydrocarbon chain. The oxidation of methyl-
cyclohexane and adamantane results in a moderate bond
selectivity, suggesting that the tertiary C atom is oxidized with
some preference over the secondary C atoms. The oxidations of
both cis- or trans-1,2-dimethylcyclohexane proceed in a
nonstereoselective manner, as confirmed by the trans/cis ratios
of 0.6−0.9 between the generated isomeric tertiary alcohols
with the mutual trans and cis orientation of the methyl groups.
A partial inversion of the configuration was also detected, with
the cis isomers being predominant products in both cis- and
trans-1,2-dimethylcyclohexane oxidations.
The obtained bond selectivity, regioselectivity, and stereo-
selectivity parameters (Table 3) are indicative of a powerful and
rather indiscriminate oxidizing species.34 Moreover, these
selectivity parameters are similar to those reported earlier for
other copper-containing catalytic systems operating with HO•
radicals.4,8a,b,25,28 Therefore, based on the selectivity parameters
in the oxidation of linear and branched alkanes, as well as on
some kinetic data in the oxidation of cyclohexane, we can
propose a general free-radical mechanism undergoing with the
participation of hydroxyl radicals as principal oxidizing species.
These are formed from H2O2 via an interaction with a Cu
precatalyst. Then, the hydroxyl radicals abstract H atoms from
an alkane forming the alkyl radicals R•, which further react with
O2 (e.g., from air or via partial H2O2 decomposition) resulting
in the ROO• radicals that are then transformed to alkyl
hydroperoxides ROOH as primary intermediate products.
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However, alkyl hydroperoxides decompose (conceivably by Cu-
catalyzed processes and additionally after the treatment by
PPh3) to furnish the corresponding alcohols and ketones as
final oxidation products.8a,b,25
and can also be of potential practical significance, namely,
permitting the use of diluted, in situ-generated aqueous
solutions of H2O2 as oxidant. Further research to widen both
synthetic and catalytic directions toward the design of new
copper(II) aminoalcoholate metal−organic networks and their
exploration in oxidation catalysis will be pursued.
CONCLUSIONS
■
In the present study, we have further explored the application
of a versatile aqueous medium self-assembly synthetic protocol
toward the generation of a novel series of mixed-ligand 2D
coordination polymers 1−3, derived from three different
aminoalcohols as main building blocks and pyromellitic acid
as a linker. The work thus opens up the application of simple
and commercially available aminoalcohols, namely, N-ethyl-
diethanolamine and N-benzylethanolamine, as versatile N,O-
ligands for the construction of coordination polymers. The
structural and topological features, water solubility and catalytic
behavior of 1−3 appear to be dependent on the type of
aminoalcohol building block used.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b01983.
Materials and methods; additional details on ESI-MS
studies; EPR (Figure S1) and UV-vis (Figure S2)
spectra; TGA plots (Figures S3−S5); additional
structural representations (Figures S6 and S7); and
catalytic results (Figures S8−S13) (PDF)
Crystallographic file for C13H27Cu2N2O9 (1) (CCDC
File No. 1419236) (CIF)
Crystallographic file for C17H29Cu2N2O10 (2) (CCDC
In fact, despite possessing resembling metal−organic 2D
layers with the sql topology, the coordination networks of 1−3
are built from distinct dinuclear or mononuclear copper(II)
aminoalcoholate SBUs and μ4-pyromellitate spacers. Besides,
the 2D → 3D extension of metal−organic networks by means
of intermolecular hydrogen bonds has been observed in 1 and
2, furnishing 3D supramolecular nets with a rare 3,4,8T9 or
unprecedented topology, respectively, thus contributing to the
identification of novel topological motifs.
An interesting feature of 1−3 consists in their water
solubility, thus opening up the application of these CPs as
precatalysts in homogeneous catalytic reactions that proceed in
an aqueous medium. Indeed, compounds 1−3 act as highly
efficient precatalysts for the mild oxidation, by aqueous H2O2 in
acidic MeCN/H2O medium, of various cycloalkanes to the
corresponding alcohols and ketones, resulting in up to 45%
overall product yields in the case of cyclohexane oxidation. In
contrast to the oxidation of more reactive substrates than
alkanes (e.g., olefins, alcohols), the obtained herein product
yields can be rated as very high in the field of alkane oxidation,
especially considering the high inertness of these hydrocarbons
and the very mild reaction conditions that were ap-
plied.4,15,27−35 A diversity of reaction parameters has been
investigated, including the type of precatalyst, the type of acid
promoter, substrate scope, relative amounts of reagents and
different selectivity parameters. The obtained compounds can
also be considered as bioinspired oxidation precatalysts with
some potential relevance to the catalytic function of particulate
methane monooxygenase. This enzyme comprises a multi-
copper active site with a N,O-environment, which is capable of
hydroxylating alkanes.36,37
Moreover, a remarkable feature of the present catalytic
systems concerns an unusual promoting role of water. Although
water typically plays a strongly inhibiting role in mild alkane
oxidations (e.g., due to the reduction of the H2O2
concentration and lowering of the alkane solubility), we have
found a very pronounced promoting behavior of H2O when
using precatalyst 1 and, to a lesser extent, precatalyst 2. For
example, an initial reaction rate (W0) in the cyclohexane
oxidation by the 1/TFA/H2O2 system can be increased from
∼6 × 10−8 to ∼93 × 10−8 M s−1 (∼15-fold growth) upon
changing the amount of H2O in the reaction mixture from ∼4
M to 17.2 M (Figure 8), respectively, followed by a total yield
growth from 24% to 45%. We believe that such a highly
promoting behavior of water should deserve further exploration
File No. 1419237) (CIF)
Crystallographic file for C23H28CuN2O7 (3) (CCDC File
No. 1419238) (CIF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: kirillova@ist.utl.pt (M. V. Kirillova).
*E-mail: kirillov@ist.utl.pt (A. M. Kirillov).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Foundation for Science and
Technology (FCT) (Project Nos. IF/01395/2013/CP1163/
CT005, PTDC/QUI-QUI/121526/2010, RECI/QEQ-QIN/
0189/2012, UID/QUI/00100/2013, SFRH/BPD/78854/
2011, and REM 2013), Portugal. We thank Dr. M. C. Oliveira
and Ms. A. Dias for ESI-MS( ) measurements.
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