Interplay between H-bonding and interpenetration in an aqueous copper(ii)-aminoalcohol-pyromellitic acid system: self-assembly synthesis, structural features and catalysis

Article abstract of DOI:10.1039/C8DT02983K

Two new copper(ii) coordination compounds, [Cu(H_1.5mdea)₂]₂(H₂pma) (1a) and [{Cu₂(μ-Hmdea)₂}₂(μ₄-pma)]_n·2nH₂O (1b), were self-assembled at different temperatures from the same multicomponent reaction system, comprising copper(ii) nitrate, N-methyldiethanolamine (H₂mdea), pyromellitic acid (H₄pma), and potassium hydroxide. Products 1a and 1b were isolated as microcrystalline solids and fully characterized and their structures were established by single-crystal X-ray diffraction. Compound 1a features the bis-aminoalcohol(ate) monocopper(ii) units and H₂pma^2? anions that are multiply interconnected by strong H-bonds into a firm 2D H-bonded layer. Compound 1b reveals the bis-aminoalcoholate dicopper(ii) motifs that are interlinked by the μ₄-pma^4? spacers into a 3D + 3D interpenetrated metal-organic framework. From a topological perspective, both networks of 1a and 1b are uninodal and driven by similar 4-connected H₂pma^2? or pma^4? nodes, but result in distinct sql and dia topologies, respectively. Compound 1a was applied as an efficient catalyst for two model cycloalkane functionalization reactions: (1) oxidation by H₂O₂ to form cyclic alcohols and ketones and (2) hydrocarboxylation by CO/H₂O and S₂O₈^2? to form cycloalkanecarboxylic acids. The substrate scope, effects of various reaction parameters, selectivity and mechanistic features were also investigated.

Full text of DOI:10.1039/C8DT02983K

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Interplay between H-bonding and interpenetration
in an aqueous copper(II)–aminoalcohol–
pyromellitic acid system: self-assembly synthesis,
structural features and catalysis†
Cite this: Dalton Trans., 2018, 47,
16674
Tiago A. Fernandes,a Marina V. Kirillova,*a Vânia Andréa and
a,b
Alexander M. Kirillov
*
Two new copper(II) coordination compounds, [Cu(H1.5mdea)2]2(H2pma) (1a) and [{Cu2(µ-Hmdea)2}2(µ4-
pma)]n·2nH2O (1b), were self-assembled at different temperatures from the same multicomponent reac-
tion system, comprising copper(II) nitrate, N-methyldiethanolamine (H2mdea), pyromellitic acid (H4pma),
and potassium hydroxide. Products 1a and 1b were isolated as microcrystalline solids and fully character-
ized and their structures were established by single-crystal X-ray diffraction. Compound 1a features the
bis-aminoalcohol(ate) monocopper(II) units and H2pma2− anions that are multiply interconnected by
strong H-bonds into a firm 2D H-bonded layer. Compound 1b reveals the bis-aminoalcoholate dicopper
(II) motifs that are interlinked by the µ4-pma4− spacers into a 3D + 3D interpenetrated metal–organic
framework. From a topological perspective, both networks of 1a and 1b are uninodal and driven by similar
4-connected H2pma2− or pma4− nodes, but result in distinct sql and dia topologies, respectively.
Compound 1a was applied as an efficient catalyst for two model cycloalkane functionalization reactions:
(1) oxidation by H2O2 to form cyclic alcohols and ketones and (2) hydrocarboxylation by CO/H2O and
Received 21st July 2018,
Accepted 29th October 2018
2−
DOI: 10.1039/c8dt02983k
S2O8 to form cycloalkanecarboxylic acids. The substrate scope, effects of various reaction parameters,
rsc.li/dalton
selectivity and mechanistic features were also investigated.
stability and solubility, as well as their commercial availability
and low cost and due to the presence of different N,O-donor
Introduction
The synthesis of new metal–organic architectures assembled sites.7 However, it is often difficult to assemble and crystallize
from different types of copper(II) cores has been of increased copper(II)–aminoalcohol derivatives in an aqueous medium
interest in recent years owing to a versatile coordination behav- due to their high solubility; therefore, such synthetic
ior of copper ions along with their promising redox, catalytic, approaches require the introduction of an additional, prefer-
and biological properties.1–4 Synthetic approaches for copper ably anionic ligand that acts as a crystallization mediator or
(II) coordination compounds often rely on the self-assembly supporting block.5–7 Carboxylic acids are very promising
protocols, wherein various types of organic molecules are used examples of such anionic ligands with a high affinity to copper
as principal building blocks.3–6 Among the great diversity of (II) ions and rich coordination chemistry.5,6
organic ligands, aminoalcohols and, in particular, ethanol-
Following the above discussion and our general interest in
amine derivatives represent an interesting class of compounds the aqueous medium self-assembly synthesis of diverse
in view of their coordination versatility, flexibility, aqueous copper(II) cores and coordination polymers,8 a primary objec-
tive of the present study has been to synthesize new metal–
organic architectures from an aqueous three-component
system, copper(II)–aminoalcohol–carboxylic acid. Herein, we
have selected simple diethanolamine derivative,
aCentro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa,
Av. Rovisco Pais, 1049-001 Lisboa, Portugal. E-mail: kirillova@tecnico.ulisboa.pt,
kirillov@tecnico.ulisboa.pt; Tel: +351 218417178
bPeoples’ Friendship University of Russia (RUDN University), Research Institute of
Chemistry, 6 Miklukho-Maklaya st., Moscow, 117198, Russian Federation
†Electronic supplementary information (ESI) available: Additional structural
(Tables S1 and S2, Fig. S1–S3) and catalytic (Fig. S4–S7) data, as well as crystallo-
graphic CIF files for 1a and 1b. CCDC 1857110 and 1857111. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/c8dt02983k
a
N-methyldiethanolamine (H2mdea), and an aromatic tetracar-
boxylic acid, pyromellitic acid (H4pma), as principal building
blocks (Scheme 1). Despite their recognized use in the design
of diverse coordination compounds,9–11 the combination of
H2mdea and H4pma within one reaction system has not been
explored and no Cu compounds containing both these blocks
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tion of N-methyldiethanolamine (H2mdea; 1 mL, 1 mmol) was
added dropwise under constant stirring, followed by an
addition of pyromellitic acid (H4pma; 127 mg, 0.50 mmol) pre-
viously dissolved in an aqueous solution of KOH (3 mL,
3 mmol). The obtained mixture was stirred overnight at
ambient temperature (∼25 °C) and then filtered. The filtrate
was left to slowly evaporate in a beaker at ambient tempera-
ture, resulting in the formation of blue crystals (including
those suitable for single crystal X-ray diffraction) within
2 weeks. These were collected and dried in air to give com-
pound 1a in ∼60% yield, based on copper(II) nitrate.
Alternatively, compound 1a can be generated by following the
above procedure and using an excess of H2mdea (0.573 mL,
5 mmol) instead of KOH. Compound 1a is ionic and well
Scheme 1 Structural formulae of N-methyldiethanolamine and pyro-
mellitic acid used as main building blocks.
have been documented, as confirmed by a search of the CSD
(Cambridge Structural Database).9
Hence, in the present study we report an aqueous medium
self-assembly synthesis, characterization, and structural
and topological features of two new copper(II) coordination
soluble in H2O (S25
> 20 mg mL−1). Anal. calcd (%) for
°C
compounds,
namely
a
discrete
0D
derivative
1a·6H2O: C30H66Cu2N4O22 (MW 962): C 37.46, H 6.92, N 5.82;
found C 37.42, H 6.77, N 6.03. FT-IR (KBr): 3519 (s br), 3456
(s br) and 3349 (s br) ν(OH/H2O), 2991 (w), 2955 (w), 2914 (w),
2884 (w) ν(CH), 1902 (s br), 1624 (m sh), 1556 (s) νas(COO),
1481 (s), 1449 (w), 1402 (s sh), 1369 (vs br) νs(COO), 1321 (m),
1243 (w), 1133 (m), 1059 (s), 1025 (w), 997 (m), 904 (m), 890
(m), 844 (m), 795 (m), 763 (m), 665 (w), 618 (w), 582 (w), and
538 (m) cm−1. ESI-MS( ) (H2O), selected fragments: MS(+), m/z:
887 (5%) [{Cu(H1.5mdea)2}2(H2pma) + 2H2O + H]+, 734 (27%)
[Cu(H1.5mdea)2]2(H2pma) (1a) and a 3D interpenetrated MOF
[{Cu2(µ-Hmdea)2}2(µ4-pma)]n·2nH2O (1b), which have been
obtained at different temperatures from the same multicompo-
nent reaction system. Besides, compound 1a has been applied
as an efficient homogeneous catalyst for the oxidative
functionalization of cycloalkanes to give cyclic alcohols and
ketones (oxidation reaction) or cycloalkanecarboxylic acids
(hydrocarboxylation reaction). The selection of alkanes as sub-
strates in catalytic tests has been governed by (1) the impor-
tance of these saturated hydrocarbons as abundant feed-
stocks,12 (2) the industrial relevance of alkane oxidation pro-
ducts (e.g., cyclohexanol and cyclohexanone are intermediates
in nylon industry),13 and (3) the presence of copper in the
active sites of some enzymes, including the particulate
methane monooxygenase that is capable of hydroxylating
alkanes.1,14
[Cu2(Hmdea)2(mdea)(H4pma)
+
H]+, 555 (6%) [Cu
(Hmdea)2(H4pma) + H]+, 480 (98%) [Cu2(Hmdea)3 + H]+, 300
(100%) [Cu(Hmdea)(H2mdea)]+; MS(−), m/z: 553 (5%)
[Cu(Hmdea)(H2mdea)(H2pma)]−, 253 (100%) [(H3pma)]−.
[{Cu2(µ-Hmdea)2}2(µ4-pma)]n·2nH2O (1b). This compound
was obtained by following the main procedure described for
1a but performing an overnight reaction at higher temperature
(∼60 °C) and without stirring. This resulted in a deposition of
blue microcrystals that were filtered to give 1b as a blue solid
in ∼15% yield, based on copper(II) nitrate. The filtrate can be
left to slowly evaporate in a beaker at r.t., resulting in the for-
mation of a further crop of 1b (∼5% yield) along with 1a.
Compound 1b is insoluble in water and organic solvents. Anal.
calcd (%) for 1b: C30H54Cu4N4O18 (MW 1013): C 35.57, H 5.37,
N 5.53; found C 35.29, H 5.48, N 5.41. FT-IR (KBr): 3302 (s br)
ν(OH/H2O), 2980 (w) and 2891 (w) ν(CH), 1636 (m), 1613 (m)
and 1564 (vs) νas(COO), 1421 (m), 1384 (s) and 1333 (m)
νs(COO), 1193 (w), 1140 (w), 1064 (s), 1017 (m), 901 (w),
Experimental
Materials and methods
All chemicals were obtained from commercial sources.
Synthesis of 1a and 1b was carried out under an air atmo-
sphere. Elemental (C, H, N) analyses were carried out by the
Laboratory
of
Analyses
of
IST.
Infrared
spectra
(4000–400 cm−1) were recorded on a JASCO FT/IR-4100 equip-
ment using KBr pellets (abbreviations: vs – very strong, s –
strong, m – medium, w – weak, br – broad, sh – shoulder).
ESI-MS( ) measurements were run on a LCQ Fleet mass
spectrometer with an ESI source (Thermo Scientific), using
∼10−3 M solutions of 1a or its model mixtures with HNO3 and
H2O2 in the aqueous medium. Gas chromatography (GC) ana-
lyses of the catalytic reaction mixtures were run on an Agilent
Technologies 7820A series gas chromatograph (He as carrier
gas) equipped with the FID detector and the BP20/SGE (30 m ×
0.22 mm × 0.25 μm) capillary column.
896 (w), 754 (m), 667 (w) and 542 (w) cm−1
.
X-ray crystallography
Single crystals of 1a and 1b were mounted with Fomblin© in a
cryoloop and the crystal data were collected on Bruker
AXS-KAPPA APEX II and D8QUEST diffractometers with graph-
ite-monochromated radiation (Mo Kα, λ = 0.71073 Å) and the
X-ray generator operating at 50 kV and 30 mA. Data collection
was monitored by the APEX2 program.15 A correction for
Lorentzian, polarization, and absorption effects was made
using the SAINT and SADABS software.15 SIR97 and/or
SHELXS-97 programs16,17 were applied for structure solution
Synthesis and characterization
[Cu(H1.5mdea)2]2(H2pma) (1a). To an aqueous solution and SHELXL-97 was used for full matrix least-squares refine-
(10 mL) of Cu(NO3)2·3H2O (242 mg, 1 mmol), an aqueous solu- ment on F2;17 these programs are in the WINGX-Version
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1.80.05 package.18 Non-H atoms were refined anisotropically. stock solution in MeCN) and an alkane substrate (2 mmol).
A full-matrix least-squares refinement was used for the non-H The reaction was then initiated upon addition of H2O2 (50% in
atoms with anisotropic thermal parameters. All the H atoms H2O, 10 mmol) in one portion. The reaction progress was
were inserted in idealized positions and allowed to refine in monitored by withdrawing small aliquots of the reaction mix-
their parent carbon atom, except for the OH/H2O moieties; tures after different periods of time. The aliquots were treated
these were located from the electron density map. The disorder with PPh3 for the reduction of remaining H2O2 and alkyl
model was applied to compound 1a. PLATON software19 was hydroperoxides that are typically formed as primary products
applied for the analysis of H-bond interactions. Crystal data in alkane oxidation.22 The samples were analyzed by GC using
and details of data collection for 1a and 1b are summarized in nitromethane as an internal standard. Formation of alkyl
Table 1.
hydroperoxides was confirmed by performing the GC analyses
of selected samples two times, before and after the treatment
with PPh3 (Shul’pin’s method).22 GC peak assignment was
made by comparison with chromatograms of authentic
samples. Blank tests for all cycloalkane substrates tested con-
firmed that their oxidation does not proceed in the absence of
the copper catalyst. Moreover, free ligands (H2mdea, H4pma)
do not catalyze the oxidation of alkanes. Copper(II) nitrate as
well as a mixture of Cu(NO3)2 with H2mdea and H4pma (in situ
system resembling 1a) results in significantly lower product
yields (Fig. S4, ESI†) if compared with 1a, thus confirming the
need for a prior synthesis of the Cu(II) catalyst.7a,8b
Topological analysis
Topological analysis of H-bonded (in 1a) or metal–organic (in
1b) networks was performed following the concept of the sim-
plified underlying net.20,21 Such underlying nets were gener-
ated by contracting discrete cations and anions (in 1a) or brid-
ging ligands (in 1b) to the corresponding centroids, maintain-
ing their connectivity via hydrogen or coordination bonds,
respectively. In 1a, 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.20 The obtained underlying nets were then classified
from the topological viewpoint.
Catalytic hydrocarboxylation of cycloalkanes
In a typical hydrocarboxylation experiment, the reaction
mixture was prepared as follows. The catalyst (10 µmol) was
placed into a 20.0 mL stainless steel autoclave equipped with a
Teflon-coated magnetic stirring bar, followed by the addition
of K2S2O8 (1.5 mmol), solvent (2.0 mL H2O and 4.0 mL of
MeCN; total solvent volume 6.0 mL), and cycloalkane substrate
(1 mmol). 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 reaction mixture
was stirred for 4 h at 60 °C using a magnetic stirrer and an oil
bath. Then, the autoclave was cooled in an ice bath, degassed,
and opened, and the reaction mixture was transferred to a
glass flask. Diethyl ether (9.0 mL) and 45 μL of cycloheptanone
(typical GC internal standard) were added. For the cyclohep-
tane hydrocarboxylation, cyclohexanone (45 μL) was used as a
GC standard instead of cycloheptanone. The obtained mixture
was vigorously stirred for 10 min, and the organic layer was
analyzed by gas chromatography (internal standard method),
revealing the formation of the corresponding monocarboxylic
acids as major hydrocarboxylation products, along with cyclic
alcohols and ketones as minor oxidation products. Attribution
of peaks was made by comparison with chromatograms of
authentic samples.
Catalytic oxidation of alkanes
The catalytic reactions were performed in thermostated glass
reactors equipped with a reflux condenser under vigorous stir-
ring at 50 °C, under an air atmosphere, and using MeCN as
the solvent (up to 5 mL total volume). In a typical test, the cata-
lyst (10 µmol) and the GC internal standard (MeNO2, 50 µL)
were introduced into MeCN solution, followed by the addition
of an acid promoter (typically 0.1 mmol, optional; used as a
Table 1 Crystal data and structure refinement details for 1a and 1ba
1a
1b
Formula
Fw
C30H54Cu2N4O16
853.87
C15H25Cu2N2O8·H2O
506.49
Block, blue
0.08 × 0.05 × 0.03
Tetragonal
I41/acd
13.8211(8)
13.8211(8)
41.437(2)
90
Crystal form, color
Crystal size (mm)
Cryst. syst.
Space group
Block, blue
0.18 × 0.08 × 0.06
Triclinic
ˉ
P1
a, Å
b, Å
c, Å
α, °
β, °
γ, °
Z
8.744(2)
10.490(2)
11.542(3)
102.033(7)
91.299(12)
100.042(9)
1
90
90
16
T, K
298(2)
1.404
1.123
2.370–30.738
57 135
6150
0.0376
0.0370, 0.1059
1.062
298(2)
1.700
2.199
2.304–27.131
26 048
2203
0.0593
0.0325, 0.0702
1.048
Dc, g cm−3
μ(Mo Kα), mm−1
θ range (°)
Refl. Collected
Independent refl.
Rint
Results and discussion
Synthesis of 1a and 1b
A self-assembly reaction, at ∼25 °C and in the aqueous
R1 a, wR2 b [I ≥ 2σ(I)]
GOF on F2
medium, between copper(II) nitrate as
a metal source,
N-methyldiethanolamine (H2mdea) as a principal building
block, pyromellitic acid (H4pma) as a crystallization mediator,
a R1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = [∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]]1/2
.
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and potassium hydroxide as a base, resulted in the generation O/OH functionalities of H2mdea/Hmdea− ligands [O5–H⋯O1,
of a novel coordination compound [Cu(H1.5mdea)2]2(H2pma) 2.525(3) Å; O6A/B–H⋯O2, 2.608(10)/2.484(10) Å; O7–H⋯O7,
(1a). This product is composed of two monocopper(II) bis- 2.652(4) Å; O8–H⋯O4 2.483(3) Å]. As a result of these multiple
aminoalcohol(ate) [Cu(H2mdea)2]2+ and [Cu(Hmdea)2] units interactions, a firm 2D H-bonded layer is generated (Fig. 1b).
and the H2pma2− anion, which are H-bonded into a 2D supra- Topological classification of this layer was performed by ana-
molecular network. However, a similar synthetic procedure lyzing its simplified underlying network (Fig. 1c), which is
performed at higher temperature (∼60 °C) gave rise to a built from the 4-connected H2pma2− nodes and centroids of
different product [{Cu2(µ-Hmdea)2}2(µ4-pma)]n·2nH2O (1b), the 2-connected [Cu(H2mdea)2]2+ and [Cu(Hmdea)2] linkers.
wherein the dicopper(II) bis-aminoalcoholate [Cu2(µ- This net can be classified as a uninodal 4-connected layer with
Hmdea)2]2+ units are interlinked by the µ4-pma4− spacers into the sql [Shubnikov tetragonal plane net] topology and a point
a
3D
+
3D interpenetrated metal–organic framework. symbol of (44·62).20
[{Cu2(µ-Hmdea)2}2(µ4-pma)]n·2nH2O (1b). Despite being self-
Compound 1b typically crystallizes along with 1a and the
attempts to convert 1a to 1b in the aqueous medium at assembled from the same reaction mixture as 1a, the structure
50–60 °C were not successful. Phase purity of 1a was con- of 1b is very different and shows a 3D + 3D interpenetrated
firmed by powder X-ray diffraction (Fig. S1†). Both products metal–organic framework (Fig. 2). The asymmetric unit of 1b
were isolated as blue, air-stable microcrystalline solids and comprises one Cu1 center, one µ-Hmdea− block, a quarter of
were characterized by standard methods, including elemental µ4-pma4− spacer, and half of a H2O molecule of crystallization.
analysis, IR spectroscopy, ESI-MS, single crystal X-ray diffrac- The Cu1 atoms and µ-Hmdea− blocks are arranged into the
tion and topological analysis.
dicopper(II) bis-aminoalcoholate [Cu2(µ-Hmdea)2]2+ subunits
with a rather short Cu1⋯Cu1 separation of 2.8559(6) Å
(Fig. 2a). Within these Cu2 subunits, the five-coordinate Cu1
centers display a distorted square-pyramid {CuNO4} environ-
ment. Herein, the basal sites are taken by the two symmetry
generated O1 atoms and the N3 atom of µ-Hmdea− along with
the O3 atom of µ4-pma4− [Cu1–O1 1.929(2), Cu1–N3 2.059(3),
Cu1–O3 1.932(2) Å], whereas an apical site is occupied by the
O2 atom (OH group) of the µ-Hmdea− moiety [Cu1–O2
2.393(2) Å]. The µ4-pma4− spacer acts as a 4-connected node
and interlinks four [Cu2(µ-Hmdea)2]2+ subunits into an intri-
cate 3D metal–organic framework (Fig. 2b).
From a topological perspective, a 3D MOF structure of 1b is
composed of the 4-connected µ4-pma4− nodes, 2-connected
Cu1 centers, and 2-connected µ-Hmdea− linkers (Fig. 2c).
Topological analysis of such a net reveals a uninodal 4-con-
nected framework with the dia (Diamond) topology and a
point symbol of (66).20 A notable feature of this structure also
concerns the presence of two independent 3D nets (Fig. 2d).
Structural and topological description
[Cu(H1.5mdea)2]2(H2pma) (1a). The structure of 1a features a
[Cu(H2mdea)2]2+ cation, a neutral [Cu(Hmdea)2] unit, and an
H2pma2− anion per formula unit (Fig. 1a). In both mono-
copper(II) blocks, the six-coordinate Cu1 and Cu2 atoms are
located at an inversion center and display an octahedral
{CuN2O4} geometry. It is filled by a pair of H2mdea (Cu1) or
Hmdea− (Cu2) moieties that act as tridentate N,O2-chelating
ligands. The Cu–N [2.027(2)–2.038(3) Å] and Cu–O [2.031(3)–
2.326(3) Å] distances are within typical values for such
copper(II) derivatives.10
Although the H2pma2− moiety remained uncoordinated,
there are very strong hydrogen bonding interactions between
the H2pma2− and [Cu(H2mdea)2]2+ and [Cu(Hmdea)2] units
(Fig. 1a and Table S2†). In fact, the crystal packing pattern of
1a reveals that each H2pma2− anion is bound by four pairs of
H-bonds involving all pyromellitate COO/COOH groups and all
Fig. 1 Structural fragments of 1a. (a) Molecular [Cu(H1.5mdea)2]2(H2pma) unit with atom labelling and hydrogen bonds (red-gray dotted lines); CH
hydrogen atoms were omitted for clarity. (b) 2D H-bonded layer; view along the a axis; Cu (green balls), N (blue), O (red), C (pale green), H (gray); H
bonds are shown as red-gray dotted lines. (c) Topological representation of a 2D H-bonded net showing a uninodal 4-connected layer with the sql
topology; centroids of 2-connected [Cu(Hmdea)2] and [Cu(H2mdea)2]2+ units (green balls); centroids of 4-connected H2pma2− nodes (pale green),
view along the a axis.
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Fig. 2 Structural fragments of 1b. (a) Interlinkage of two [Cu2(µ-Hmdea)2]2+ blocks and µ4-pma4− spacer; CH hydrogen atoms were omitted for
clarity. (b) 3D metal–organic framework; view along the c axis; Cu (green balls), N (blue), O (red), C (pale green). (c) Topological representation of a
3D underlying net showing a uninodal 4-connected framework with the dia topology; 2-connected Cu centers (green balls), centroids of 4-con-
nected µ4-pma4− nodes (pale green), centroids of 2-connected µ-Hmdea− linkers (blue); view along the c axis. (d) Topological representation of two
3D + 3D interpenetrated nets represented by different colors (green and gray); Cu centers are shown as balls.
Analysis of their interpenetration discloses an extended 3D +
3D interpenetrated network (class IIa, Z = 2).
Mild catalytic oxidation of cycloalkanes
Despite being abundant hydrocarbon feedstocks, alkanes are
very inert substrates and their oxidative functionalization
Scheme 2 Mild oxidation (a) and hydrocarboxylation (b) of C5–C8
cycloalkanes (CyH) to give the corresponding cyclic alcohols (CyOH)
and ketones (Cy’vO), or cycloalkanecarboxylic acids (CyCOOH).
under mild conditions continues to be a challenging research
topic.12 In industrially relevant processes, the oxidation of
alkanes usually requires high temperatures and pressures, in
addition to showing rather low product yields and selectivity.
For example, an industrial oxidation of cyclohexane to cyclo-
hexanol and cyclohexanone (nylon precursors) uses a homo- with comparable reaction rates, although leading to different
geneous cobalt naphthenate catalyst, proceeds at ∼150 °C, and total product yields (Fig. 3 and S5†); hereinafter, all the yields
requires multiple substrate recycling due to low overall are based on the cycloalkane substrate. Cyclooctane and cyclo-
product yields that do not exceed 5–10% (based on heptane appear to be more reactive substrates leading to total
cyclohexane).12,13
Given the solubility of 1a in the aqueous medium and fol- and cyclopentane (∼10%).
product yields of ∼22–23%, followed by cyclohexane (∼18%)
lowing our interest in the search for new catalytic systems for
Oxidation of cycloalkanes in the presence of 1a is promoted
mild alkane functionalization, we applied herein 1a as a by a catalytic amount of acid. We studied the effect of different
homogeneous catalyst for the mild oxidation of C5–C8 cycloalk- acid promoters on the oxidation of C6H12 (Fig. 4). In the pres-
anes by aqueous H2O2. The reaction proceeds at 50 °C and ence of HCl, the oxidation proceeds very quickly and results in
gives a mixture of the corresponding alcohols and ketones ∼17% total yield after 10 min; further increase of reaction time
(Scheme 2a). The oxidation of all tested cycloalkanes proceeds leads to product consumption due to overoxidation. Although
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Fig. 3 Oxidation of C5–C8 cycloalkanes (total yield of alcohol and
ketone vs. time) with H2O2 catalyzed by 1a in the presence of TFA pro-
moter. Reaction conditions: Catalyst 1a (0.01 mmol), TFA (0.1 mmol),
cycloalkane (2 mmol), H2O2 (50% in H2O, 10 mmol), CH3CN (up to
5.0 mL total volume), and 50 °C.
Fig. 5 Effect of the H2O amount on the total product yield in the
cyclooctane oxidation with H2O2 catalyzed by 1a/TFA. Reaction con-
ditions: Catalyst 1a (0.01 mmol), TFA (0.1 mmol), C8H16 (2 mmol), H2O2
(50% in H2O, 10 mmol), added H2O (up to 0.8 mL), CH3CN (up to 5.0 mL
total volume), and 50 °C.
We also studied the effect of the catalyst amount and
observed a linear dependence of the initial reaction rate W0 on
the catalyst concentration (Fig. S7, ESI†) in the cyclooctane oxi-
dation catalyzed by the 1a/TFA system. This behavior indicates
the first-order reaction kinetics, which supports an involve-
ment of one type of Cu-containing species in the rate-limiting
catalytic step. The increase of the catalyst concentration from
0.5 to 2.0 mM also results in a linear growth of the maximum
product yield (up to 22%).
An interesting accelerating effect of water on the initial reac-
tion rate of cyclooctane oxidation was observed in the 1a/TFA
system (Fig. 5). The W0 increases on increasing the water con-
centration in the system from 4.1 M (corresponds to the
amount of H2O in 50% aqueous H2O2) to 12.9 M (after intro-
duction of an additional amount of water to the reaction
mixture). However, a higher water amount in the system leads
to a drop of the total product yield from 22 to 16%, respect-
ively. This can be related to a lower solubility of cyclooctane in
Fig. 4 Effect of the type of acid promoter on the total product yield in
the cyclohexane oxidation with H2O2 catalyzed by 1a. Reaction con-
ditions: Catalyst 1a (0.01 mmol), acid promoter (0.1 mmol), C6H12
(2 mmol), H2O2 (50% in H2O, 10 mmol), CH3CN (up to 5.0 mL total
volume), and 50 °C.
such an accelerating effect of HCl is not yet clearly understood, the reaction mixture at a higher content of water. Thus, we can
it can be associated with an ability of chloride anions to stabil- propose an involvement of water in the rate limiting step of
ize a Cu(I) oxidation state (or Cu–peroxo intermediates) via the catalytic generation of hydroxyl radicals.8
formation of copper aminoalcohol(ate) moieties bearing term-
inal or bridging Cl− ligands.7a
To evaluate the stability of the catalytic system and eluci-
date some mechanistic aspects, we also performed a stepwise
A slightly better yield of 18% is achieved after 120 min in oxidation of cyclooctane (Fig. 6). Since the C8H16 oxidation in
the presence of trifluoroacetic acid (TFA) as a promoter. Nitric the presence of 1/TFA requires 120 min to achieve the
acid and sulfuric acid also promote the oxidation of cyclo- maximum product yield (Fig. 3), after this period of time we
hexane, although in these cases the reactions are slower and introduced a new portion of cyclooctane and H2O2 into the
lead to ∼15% yields after 240 min. TFA was thus selected as a reaction medium without an addition of a new loading of cata-
promoter for studying the effects of other reaction parameters. lyst. The oxidation restarted even with a higher reaction rate.
The influence of the TFA amount on the oxidation of One more addition of both the oxidant and substrate after 4 h
cyclooctane catalyzed by 1a was also investigated (Fig. S6, again allows continuing the cyclooctane oxidation. This shows
ESI†). The increase of the TFA amount from 0 to 5 equivalents that the catalytically active Cu species is stable in the reaction
relatively to the catalyst leads to a gradual growth of both the solution and alive for a long time. The oxidation reaction ends
total product yield and an initial reaction rate, W0. The latter due to the consumption of H2O2, since the introduction of
reveals an s-shape dependence on the TFA concentration only the substrate (without an addition of H2O2) does not
(Fig. S6b†). Higher TFA amounts (10 or 20 equivalents vs. cata- allow to restart the reaction.
lyst) almost do not affect the initial reaction rate, but result in
superior product yields (e.g., 22% at 10 equivalents of TFA).
To gain additional insight into the nature of catalytically
active species, we studied the aqueous solutions of 1a (1a/H+)
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Table 2 Different selectivity parameters in the oxidation of linear and
branched alkanes catalyzed by 1aa
Selectivity (substrate)
Selectivity value
Regioselectivity
C(1) : C(2) : C(3)b (n-C5H12
C(1) : C(2) : C(3)b (n-C6H14
)
)
1 : 13 : 14
1 : 8 : 13
1 : 9 : 12 : 16
C(1) : C(2) : C(3) : C(4)b (n-C7H16
)
Bond selectivity
1° : 2° : 3° (methylcyclohexane)c
1 : 9 : 36
3.8
3°/2° (adamantane)d
Stereoselectivity
trans/cis (cis-Dimethylcyclohexane)e
trans/cis (trans-Dimethylcyclohexane)e
0.7
0.7
Fig. 6 Stepwise oxidation of cyclooctane with H2O2 catalyzed by 1a/
TFA. On each step (at 120 and 240 min) a new portion of C8H16 (1 mmol)
and H2O2 (5 mmol) was added. Reaction conditions: Catalyst 1a
(0.01 mmol), TFA (0.1 mmol), C8H16 (1 mmol), H2O2 (50% in H2O,
5 mmol), CH3CN (up to 5.0 mL total volume), and 50 °C.
a Reaction conditions: Catalyst 1a (0.005 mmol), TFA (0.05 mmol),
alkane (1 mmol), H2O2 (50% in H2O, 5 mmol), MeCN up to 2.5 mL
total volume, 2 h, and 50 °C. All selectivity parameters were calculated
based on the ratios of isomeric alcohols. Normalization of the values
was performed by taking into account the number of hydrogen atoms
at each carbon atom. b Parameters C(1) : C(2) : C(3) : (C(4)) are the rela-
tive reactivities of H atoms at C(1), C(2), C(3), and C(4) (optional)
carbon atoms of the n-alkane chain. c Parameters 1° : 2° : 3° are the
relative normalized reactivities of the hydrogen atoms at primary, sec-
ondary and tertiary carbon atoms of methylcyclohexane. d Parameters
3°/2° are the relative normalized reactivities of the hydrogen atoms at
tertiary and secondary carbon atoms of adamantane, determined as
the ratio of the formed tertiary and secondary alcohol isomers.
e Parameter trans/cis is determined as the ratio of the formed tertiary
alcohol isomers with mutual trans and cis orientation of the methyl
groups.
and 1a/H+/H2O2 by ESI-MS( ), using the conditions typical
to those of catalytic tests. The following main fragments
can be observed in the MS(+) spectra of 1a (1a/H+):
[{Cu(H1.5mdea)2}2(H2pma)
[Cu2(Hmdea)2(mdea)(H4pma)
+
2H2O
+
+
H]+ (m/z 887),
(m/z 734)
H]+
[Cu(Hmdea)2(H4pma) + H]+ (m/z 555), [Cu2(Hmdea)3 + H]+ (m/z
480), and [Cu(Hmdea)(H2mdea)]+ (m/z 300). Apart from
expected monocopper bis-aminoalcohol(ate) species, there are
also heavier fragments due to their aggregation and interaction
with a pyromellitate anion. A characteristic molecular ion
peak, [Cu(Hmdea)(H2mdea)(H2pma)]− (m/z 553), was also
observed with the expected isotopic distribution pattern in the
MS(−) mode, along with the [(H3pma)]− (m/z 253) anion. The
ESI-MS plots of a model 1a/H+/H2O2 solution also show some
of the above-mentioned fragments (e.g., [Cu(Hmdea)(H2mdea)
(H2pma)]− (m/z 553), in addition to new intense peaks that are
particularly pronounced in a negative mode. These are the
considerable preference over to the secondary C atoms. In the
oxidation of adamantane, the 3°/2° parameter of 3.8 is less
pronounced if compared to the respective ratio (1° : 2° : 3° of
1 : 9 : 36) in the oxidation of methylcyclohexane. The oxidations
of both cis- or trans-1,2-dimethylcyclohexanes proceed non-
stereoselectively, as attested by the trans/cis ratios of 0.7
between the generated isomeric tertiary alcohols with the
mutual trans and cis orientation of the CH3 groups. Partial
inversion of the configuration was detected and the cis
isomers are the predominant products in the oxidation of both
cis- and trans-1,2-dimethylcyclohexane.
Analysis of all these selectivity patterns and prior literature
background8,22,23 suggest that the present reactions involve
powerful and indiscriminate oxidizing species, namely the
HO• radicals that are formed as a result of the interaction of
H2O2 with the copper catalyst. The proposed reaction mecha-
nism thus relies on hydroxyl radicals that abstract the H atom
from an alkane substrate (RH) to give alkyl radicals (R•). These
couple with O2 (e.g., formed from H2O2 or from air) to furnish
the alkyl peroxo (ROO•) radicals, which then give rise to ROO−
(and a regeneration of the CuII catalyst) and then alkyl hydro-
peroxides (ROOH) as primary intermediate products. The
latter were detected by GC following a method of Shul’pin;22
ROOH then decompose to furnish the corresponding alcohols
and ketones as final oxidation products.
[Cu(Hmdea)2(H3pma)2
−
2(COOH)]− (m/z 719) and
[Cu(Hmdea)2(H2pma) − COOH]− (m/z 507) fragments, gener-
ated as a result of ESI- and H2O2-induced decarboxylation of
pyromellitate anions. This observation, along with a catalyst
stability study (Fig. 6), suggests that the monocopper bis-
aminoalcohol(ate) units and respective adducts with pyromelli-
tate anions may represent the catalytically active species in the
present system.
Selectivity and mechanistic features in mild alkane oxidation
To obtain further mechanistic evidence, we performed the oxi-
dation of linear and branched alkanes in order to determine
various types of selectivity parameters (Table 2). The oxidation
of linear alkanes (n-pentane, n-hexane, n-heptane, n-octane)
results in regioselectivity C(1) : C(2) : C(3) : (C4) parameters of
1 : 13 : 14, 1 : 8 : 13, 1 : 9 : 12 : 16, and 1 : 17 : 24 : 23, respectively;
these also reveal some preference to the oxidation at the
internal C(3) or C(4) secondary carbon atom of the hydro-
carbon chain. For the oxidation of methylcyclohexane, a nor-
Catalytic hydrocarboxylation of cycloalkanes
malized bond selectivity 1° : 2° : 3° parameter of 1 : 9 : 36 was Apart from the mild alkane oxidation reactions, we also inves-
obtained, suggesting that the tertiary C atom is oxidized with a tigated the catalytic behavior of 1a in the hydrocarboxylation
16680 | Dalton Trans., 2018, 47, 16674–16683
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of cycloalkanes. This transformation consists of the reac-
tion of a Cn cycloalkane with carbon monoxide (carbonyl
Conclusions
source) and water (hydroxyl source) to form directly Cn+1 In the present work, we investigated the self-assembly synthesis
cycloalkanecarboxylic acid. The reaction requires the pres- of copper(II) coordination compounds from a multicomponent
ence of a potent peroxodisulfate oxidant (e.g., K2S2O8) and aqueous
proceeds at 60 °C in the water–acetonitrile medium N-methyldiethanolamine, pyromellitic acid and a base (KOH).
(Scheme 2b). Interestingly, two distinct products can be generated depending
system
composed
of
Cu(II)
nitrate,
Compound 1a acts as an efficient catalyst toward the on the reaction temperature. At 25 °C, a discrete 0D compound
hydrocarboxylation of cyclopentane, cyclohexane, cyclohep- [Cu(H1.5mdea)2]2(H2pma) (1a) is formed as a sole product in a
tane, and cyclooctane with overall product yields up to 45% good yield. However, at a higher reaction temperature of 60 °C,
based on cycloalkane (Table 3). The highest activity was a 3D interpenetrated MOF [{Cu2(µ-Hmdea)2}2(µ4-pma)]n·2nH2O
observed for the cyclohexane substrate, leading to 43% yield (1b) is generated as a minor product along with 1a. The analysis
of C6H11COOH, followed by cyclopentane (30% of of crystal structures, packing patterns, and topological features
C5H9COOH) and cycloheptane (23% of C7H13COOH). The of 1a and 1b suggests that there is an interplay between the
yield of carboxylic acid (C8H15COOH) drops to 9% when H-bonding interactions and interpenetration during the crystal-
using cyclooctane as the substrate. The corresponding cyclic lization of these products. In fact, both structures are driven by
ketones and alcohols are also formed as by-products of a the H2pma2−/pma4− blocks that act as similar 4-connected
competing oxidation reaction. The total yield of oxygenates nodes and lead to the extension of 1a into a 2D H-bonded layer
(ketone is formed predominantly to alcohol) increases with or the generation of an intricate interpenetrated 3D + 3D metal–
the hydrocarbon size, namely from 0.5% for C5H10 to 12.8% organic network in 1b. The 2D and 3D underlying nets in 1a
for C8H16
.
and 1b were topologically classified revealing the sql and dia
As in the case of cycloalkane oxidation, there is strong evi- topologies, respectively. Hence, the present study also contrib-
dence that the present type of hydrocarboxylation proceeds via utes to intensive research on the assembly of various interpene-
a free-radical mechanism.7a,24 In fact, sulfate radical anions trated metal–organic architectures.25,26
2−
(SO4•−) are the key species formed from the S2O8 oxidant
Given good solubility of 1a in the aqueous medium, this
through the Cu-assisted and/or temperature-assisted oxidant compound was also tested as a homogeneous catalyst for the
activation processes. Then, SO4•− react with a cycloalkane (RH) mild oxidative functionalization of C5–C8 cycloalkanes (C5H10,
to abstract an H atom and generate cycloalkyl radicals (R•), fol- C6H12, C7H14, and C8H16) and two model oxidation and hydro-
lowed by their reaction with CO to give cycloacyl radicals carboxylation reactions were explored. Hence, compound 1a
(RCO•). The latter undergo oxidation by CuII to form cycloacyl catalyzes the oxidation of cycloalkanes by H2O2 to form a
cations (RCO+) which were then hydrolyzed by water to give mixture of the respective cyclic alcohols and ketones.
cycloalkanecarboxylic acids (RCOOH) as main hydrocarboxyl- Interestingly, the product yields grow on increasing the carbon
ation products.24
ring size in the homologous series of cycloalkane substrates,
which can be associated with an increased stability of
cycloalkyl radicals having more carbon atoms within the ring.
Besides, the catalytic efficiency of 1a in cycloalkane oxi-
dation depends on the type and amount of an acid promoter
and the quantity of water in the system. Furthermore, 1a also
catalyzes the mild hydrocarboxylation of cycloalkanes, by CO/
H2O and S2O82−, to give the corresponding cycloalkanecar-
boxylic acids. In both oxidation and hydrocarboxylation pro-
cesses, the obtained total product yields (up to 45%, based on
cycloalkane) are considered high in the area of alkane
functionalization, especially if taking into account an intrinsic
inertness of such hydrocarbons and rather mild reaction con-
Table 3 Single-pot hydrocarboxylation of Cn (n = 5–8) cycloalkanes
into the corresponding Cn+1 cycloalkanecarboxylic acids catalyzed by
1aa
Product yieldb, %
Cycloalkane-
Entry Cycloalkane carboxylic acid
Ketone Alcohol Totalc
1
2
29.7
42.6
0.5
1.8
Traces
0.5
30.2
44.9
ditions
(e.g.,
50–60
°C
temperature,
aqueous
medium).12,13,22–24,27,28 Furthermore, the total product yields
achieved herein are better or comparable to those commonly
observed in the oxidative functionalization of cycloalkanes
catalyzed by other Cu-containing systems containing amino-
alcohol(ate) and carboxylate blocks.7a,8,24b
3
4
22.8
8.8
7.0
2.2
6.1
32.0
27.7
12.8
a Cyclic ketones and alcohols are also formed as by-products of
cycloalkane oxidation. Reaction conditions: Catalyst 1a (0.01 mmol),
cycloalkane (1 mmol), p(CO) (20 atm), K2S2O8 (1.5 mmol), H2O
(2.0 mL)/MeCN (4.0 mL), 60 °C, 4 h in an autoclave (20 mL capacity).
The research on exploring such a type of multicomponent
copper(II)–aminoalcohol–carboxylic acid system for the self-
assembly generation of diverse copper cores and metal–
organic frameworks with notable structural, topological, and
functional characteristics is underway in our laboratory.
b (Moles of product/mole of cycloalkane)
products.
×
100%. c Yield of all
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248, 101; (c) J. G. Verkade, Acc. Chem. Res., 1993, 26,
483.
Conflicts of interest
There are no conflicts to declare.
8 (a) T. A. Fernandes, C. I. M. Santos, V. André, J. Kłak,
M. V. Kirillova and A. M. Kirillov, Inorg. Chem., 2016, 55,
125; (b) S. S. P. Dias, M. V. Kirillova, V. André, J. Kłak and
A. M. Kirillov, Inorg. Chem. Front., 2015, 2, 525;
(c) S. S. P. Dias, M. V. Kirillova, V. André, J. Kłak and
A. M. Kirillov, Inorg. Chem., 2015, 54, 5204.
9 See the Cambridge Structural Database: C. R. Groom,
I. J. Bruno, M. P. Lightfoot and S. C. Ward, Acta Crystallogr.,
Sect. B: Struct. Sci., Cryst. Eng. Mater., 2016, 72,
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Acknowledgements
This work was supported by the Foundation for Science and
Technology (FCT) and Portugal 2020 (projects IF/01395/2013/
CP1163/CT005, CEECIND/03708/2017, UID/QUI/00100/2013,
LISBOA-01-0145-FEDER-029697, and REM2013). The publi-
cation was also prepared with the support of the RUDN
University Program 5-100. We thank Ms S. Dias for experi- 10 For selected examples of Cu/H2mdea derivatives, see:
mental assistance, Dr M. C. Oliveira and Ms A. Dias for ESI-MS
( ) measurements. AMK also acknowledges the EU COST
Action CA15106 (CHAOS).
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