Mild oxidative functionalization of cycloalkanes catalyzed by novel dicopper(II) cores

Article abstract of DOI:10.1016/j.mcat.2021.111401

The search for new transition metal based catalytic systems that are active in the oxidative functionalization of such inert substrates as saturated hydrocarbons continues to be an important research direction in molecular catalysis. In the present study, two new copper(II) coordination compounds, namely a discrete dimer [Cu₂(μ-H₂tea)₂(nfa)₂]·2H₂O (1) and a 1D coordination polymer [Cu₂(μ-H₂tea)₂(μ-Htma)]_n·4nH₂O (2) were synthesized and applied as homogeneous catalysts for the mild oxidative functionalization of cycloalkanes (cyclopentane, cyclohexane, cycloheptane, and cyclooctane). Both products 1 and 2 were self-assembled in aqueous medium from copper(II) nitrate, triethanolamine (H₃tea), sodium hydroxide, and 2-naphthoic (Hnfa) or trimesic (H₃tma) acids, isolated as stable crystalline solids, and fully characterized by standard methods including single-crystal X-ray diffraction. Their structures feature a similar type of dicopper(II) triethanolaminate cores that are decorated by terminal or bridging aromatic carboxylate ligands. Two model catalytic reactions were investigated, namely the oxidation of cycloalkanes by H₂O₂ to produce cycloalkyl hydroperoxides as intermediate products and then a mixture of cyclic alcohols and ketones as final products, and the carboxylation of cycloalkanes with CO/S₂O₈^2?/H₂O to form cycloalkanecarboxylic acids as main products. These model reactions undergo under very mild conditions (50?60 °C) and show good efficiency. Substrate scope, selectivity features, and the effects of reaction parameters were investigated and discussed in detail. This study widens the family of multicopper(II) cores capable of catalyzing the oxidative functionalization of saturated hydrocarbons under mild conditions.

Full text of DOI:10.1016/j.mcat.2021.111401

Molecular Catalysis 503 (2021) 111401
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Mild oxidative functionalization of cycloalkanes catalyzed by novel
dicopper(II) cores
a
,
b
a,
a
b
Kiryl I. Trusau , Marina V. Kirillova *, V aˆ nia Andr ´e , Andrew I. Usevich , Alexander
M. Kirillov
a,c,
*
a
Centro de Química Estrutural and Departamento de Engenharia Química, Instituto Superior T ´e cnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001, Lisboa,
Portugal
b
Department of Oil and Gas Processing and Petroleum Chemistry, Belarusian State Technological University, 13a Sverdlova St., 220006, Minsk, Belarus
c
Research Institute of Chemistry, Peoples’ Friendship University of Russia (RUDN University), 6 Miklukho-Maklaya st., Moscow, 117198, Russian Federation
A R T I C L E I N F O
A B S T R A C T
Keywords:
The search for new transition metal based catalytic systems that are active in the oxidative functionalization of
such inert substrates as saturated hydrocarbons continues to be an important research direction in molecular
Cꢀ H functionalization
Cycloalkanes
Cyclohexane
catalysis. In the present study, two new copper(II) coordination compounds, namely a discrete dimer [Cu
2
( -
μ
H
2
tea) (nfa) O (1) and a 1D coordination polymer [Cu -H tea) -Htma)] O (2) were synthesized
2
2
]⋅2H
2
2
(
μ
2
2
(
μ
n
⋅4nH
2
Oxidation
and applied as homogeneous catalysts for the mild oxidative functionalization of cycloalkanes (cyclopentane,
Carboxylation
Homogeneous catalysis
Copper complexes
Alcohols
cyclohexane, cycloheptane, and cyclooctane). Both products 1 and 2 were self-assembled in aqueous medium
3 3
from copper(II) nitrate, triethanolamine (H tea), sodium hydroxide, and 2-naphthoic (Hnfa) or trimesic (H tma)
acids, isolated as stable crystalline solids, and fully characterized by standard methods including single-crystal X-
ray diffraction. Their structures feature a similar type of dicopper(II) triethanolaminate cores that are decorated
by terminal or bridging aromatic carboxylate ligands. Two model catalytic reactions were investigated, namely
Ketones
Carboxylic acids
the oxidation of cycloalkanes by H
mixture of cyclic alcohols and ketones as final products, and the carboxylation of cycloalkanes with CO/S
O to form cycloalkanecarboxylic acids as main products. These model reactions undergo under very mild
2 2
O to produce cycloalkyl hydroperoxides as intermediate products and then a
2
8
ꢀ
2
O
/
H
2
conditions (50ꢀ 60 ◦C) and show good efficiency. Substrate scope, selectivity features, and the effects of reaction
parameters were investigated and discussed in detail. This study widens the family of multicopper(II) cores
capable of catalyzing the oxidative functionalization of saturated hydrocarbons under mild conditions.
1
. Introduction
The search for efficient and versatile catalytic systems capable of
industrial scales, the catalytic systems incorporating copper are partic-
ularly attractive in view of different notable characteristics of this metal.
These include the versatile redox behavior and catalytic applications,
rich coordination chemistry, natural abundance and low cost [11–14],
as well as the presence in many enzymatic active sites, including mul-
ticopper oxidases and pMMO (particulate methane monooxygenase)
[15–18]. Hence, there is an intense research in the areas of bioinorganic
copper chemistry and bioinspired and/or biomimetic catalytic trans-
formations, aiming at the development of new Cu-based catalytic sys-
tems for the oxidative functionalization of different substrates including
alkanes [15–20]. A recognized inertness of saturated hydrocarbons
precludes their wide application as an attractive feedstock for the mild
and single-step generation of added value oxidation products with
activating such inert substrates as saturated hydrocarbons continues to
be an inspiring research direction in molecular catalysis, especially
given an abundance of these hydrocarbons as main components of the
petroleum and natural gas feedstocks [1–3]. Although many attractive
transition metal based catalytic systems have been proposed for the
activation and oxidative functionalization of alkanes, there is still a need
for catalytic solutions based on abundant and low-cost metals and
relatively simple ligands [4–10].
Among a variety of transition metal coordination compounds
commonly applied in alkane functionalization both at laboratory and
*
Corresponding authors at: Centro de Química Estrutural and Departamento de Engenharia Química, Instituto Superior T ´e cnico, Universidade de Lisboa, Av.
Rovisco Pais, 1049-001, Lisboa, Portugal.
E-mail addresses: kirillova@tecnico.ulisboa.pt (M.V. Kirillova), kirillov@tecnico.ulisboa.pt (A.M. Kirillov).
https://doi.org/10.1016/j.mcat.2021.111401
Received 29 July 2020; Received in revised form 18 November 2020; Accepted 5 January 2021
Available online 6 February 2021
2
468-8231/© 2021 Elsevier B.V. All rights reserved.

K.I. Trusau et al.
Molecular Catalysis 503 (2021) 111401
alcohol, ketone, aldehyde or carboxylic acid functional groups [1–3].
However, a combination of appropriate ligand types around the redox
active copper centers along with the selection of suitable oxidants might
result in new molecular catalytic systems for the oxidative functionali-
zation of alkanes, which would proceed efficiently at low temperature
and pressure conditions.
w–weak, br–broad, sh ꢀ shoulder). EA (elemental analyses) were carried
out on a Perkin Elmer PE 2400 Series II analyzer (Laboratory of Ana-
lyses, IST). ESI-MS(+) studies were performed on a LCQ Fleet mass
spectrometer with an ESI source (Thermo Scientific). In catalytic ex-
periments, GC (Gas chromatography) analyses were performed on an
Agilent Technologies 7820A series gas chromatograph (carrier gas: he-
lium; detector: flame ionization; capillary column: BP20/SGE, 30m ×
Following our continuing interest in the design and catalytic explo-
ration of copper(II) coordination compounds, including discrete com-
plexes [21,22] and metal-organic networks [23,24], the principal goal of
this study has been to synthesize novel dicopper(II) cores stabilized by
aminoalcoholate and carboxylate ligands, as well as to probe their cat-
alytic potential in the mild oxidative functionalization of cycloalkanes as
0.22mm × 0.25
μ
m).
2.2. Synthesis and characterization of 1 and 2
[Cu2(
μ
-H tea) (nfa) ]⋅2H O (1). In a round-bottom 50 mL flask, an
2
2
2
2
model substrates. Herein, triethanolamine (H
3
tea) has been selected as a
aqueous 1 M solution of triethanolamine (H tea; 1.0 mmol, 1 mL) was
3
principal N,O-ligand given its aqueous solubility, chelating properties,
added dropwise to an aqueous solution of Cu(NO ) ∙2.5H O (10 mL, 1.0
3
2
2
low cost, and capability to stabilize multicopper cores [9]. Simple aro-
mmol, 233 mg) with continuous stirring (magnetic stirring bar) at room
temperature, producing a blue solution. In a separate small vial, 2-naph-
thoic acid (Hnfa, 1 mmol, 172 mg) was dissolved in an aqueous 1 M
solution of NaOH (3.0 mmol, 3 mL). The obtained colorless solution was
added dropwise to the main reaction mixture under constant stirring,
producing a deep blue solution. This was stirred further for 12 h and
then filtered off to remove any traces of precipitate using a filter paper.
The deep blue filtrate was left in an open beaker (25 mL) to slowly
evaporate in air at room temperature, producing greenish-blue crystals
(including those of X-ray quality) in ~2 weeks. The mother liquor was
decanted and the crystals of the product were washed with a minimum
amount of distilled water and then manually transferred onto a filter
paper, followed by their drying in air or in desiccator at room temper-
ature. As a result, crystalline sample of compound 1 was produced in
~65 % yield, based on copper(II) nitrate. Additional crops of 1 can be
obtained by further evaporation of the above-mentioned mother liquor.
Small portions of crystals of 1 were isolated manually without being
washed and dried, and subjected to single-crystal X-ray diffraction. Anal.
Calculated for C34H46Cu N O (MW 801.8): C 50.93, H 5.78, N 3.49;
3
matic carboxylic acids such as 2-naphthoic (Hnfa) and trimesic (H tma)
acids were chosen as supporting ligands that can also promote the
oxidation of alkanes [10]. Besides, carboxylate complexes of
redox-active metal centers (e.g., Co, Fe, Cu, Mn) that are soluble in
nonpolar systems are widely applied as homogeneous oxidation cata-
lysts in petroleum industry [25]. For example, cobalt(II) naphthenate
and related derivatives are classical industrial catalysts for the oxidation
of cyclohexane to KA oil (cyclohexanone and cyclohexanol mixture)
which, despite proceeding at elevated temperatures and pressures (T =
◦
1
50ꢀ 160 C, Pair = 10ꢀ 15 atm), shows low KA oil yields not exceeding
ꢀ 10 % [25–27]. The remaining substrate is then recycled and returned
5
to an oxidation reactor.
Hence, this work reports on the self-assembly generation, structural
features and catalytic application of two new heteroleptic copper(II)
derivatives, namely [Cu
tea) -Htma)] O (2) (Scheme 1). These coordination com-
⋅4nH
pounds were explored as homogeneous catalysts in two model reactions:
a) the oxidation of cycloalkanes by H to produce a mixture of cyclic
alcohols and ketones, and (b) the carboxylation of cycloalkanes with
2
(
μ-H
2
tea)
2
(nfa)
2
2
]⋅2H O
(1) and [Cu
2
(
μ-
H
2
2
(
μ
n
2
(
2 2
O
2
2 12
ꢀ
1
found: C 50.90, H 5.75, N 3.41. FT-IR (KBr, cm ): 3363 (s) and 3150
(m) (OH/H O), 3051 (w) as(CH), 2870 (w) s(CH), 1626 (m sh), 1599
(s sh), 1584 (s sh), 1548 (s) (COO), 1464 (m), 1432 (m), 1391 (s sh),
2
ꢀ
CO/S
2
O
8
/H
2
O to form carboxylic acids as main products (Scheme 2).
ν
ν
ν
2
◦
These model reactions proceed under very mild conditions (50ꢀ 60 C)
and exhibit good efficiency. Substrate scope, selectivity features, and the
effects of reaction parameters were investigated.
ν
as
ν
s
1377 (s) (COO), 1228 (m), 1097 (m), 1027 (w), 1015 (w), 967 (w), 953
(m), 926 (m), 837 (m), 789 (s), 767 (s), 730 (m), 673 (m), 638 (m), 602
+
(
m), 491 (m), 477 (m), 419 (m). ESI-MS(+) (H
2
O/CH
3
CN/H ), m/z: 593
+
+
2
. Experimental
[Cu (H tea) (nfa)] , 421 [Cu (H tea) (Htea)] .
2
2
2
2
2
2
[
Cu
2
(
μ-H
2
2
tea) (
μ-Htma)]
n
2
⋅4nH O (2). This compound was prepared
2
.1. Reagents and methods
following a procedure described for 1 but using trimesic acid (Htma, 0.5
mmol, 105 mg) instead of 2-naphthoic acid. Blue crystals of the com-
pound 2 were obtained in ~55 % yield, based on copper(II) nitrate.
Small portions of crystals of 2 were isolated manually without being
washed and dried, and subjected to single-crystal X-ray diffraction. Anal.
All reagents and solvents were obtained from commercial sources.
Infrared spectra were measured on a JASCO FT/IR-4100 instrument
using KBr discs (abbreviations: vs ꢀ very strong, s ꢀ strong, m–medium,
Calculated for C21
found: C 36.06, H 5.52, N 4.14. FT-IR (KBr, cm ): 3361 (vs)
O), 2963 (w) as(CH), 2874 (w) (CH), 1613 (s) as(COO), 1438 (s),
383 (s) and 1361 (s sh)
2 2
H40Cu N O16 (2, MW 703.6): C 35.85, H 5.73, N 3.98;
ꢀ
1
ν(OH/
H
2
ν
ν
s
ν
1
4
ν
s
(COO), 1050 (m), 929 (w), 760 (m), 724 (s),
+
78 (m), 427 (m). ESI-MS(+) (H
2
O/CH
3
CN/H ), m/z: 631
+
+
[
Cu
(H
2 2
2
tea) (H
2
tma)] , 421 [Cu
2
(H
2
tea)(Htea)] .
2
.3. X-ray crystal structure determination
The single crystals of 1 and 2 were mounted using Fomblin© and a
cryoloop. The X-ray data was collected on a BRUKER D8 QUEST
diffractometer (graphite-monochromated radiation, Mo K
α, λ = 0.7107
Å). APEX3 program [28] was used for monitoring the collection of data,
which was also corrected for Lorentzian, polarization, and absorption
effects with SAINT [29] and SADABS [30]. SHELXT [31] and SHELXL-97
[
32] software was used for structure solution and full matrix
2
least-squares refinement on F , respectively. These programs are a part
of the WINGX-Version 2014.1 package [33,34]. Full-matrix least-s-
quares refinement was applied for the non-H atoms with anisotropic
thermal parameters. All hydrogen atoms of CH moieties were placed
Scheme 1. Structural formulae of compounds 1 and 2.
2

K.I. Trusau et al.
Molecular Catalysis 503 (2021) 111401
Scheme 2. Mild Cu-catalyzed oxidation (a) and carboxylation (b) of cyclohexane as a model substrate.
into the idealized positions and refined at a parent C atom. The OH/H
2
O
an ice bath, degassed and opened. The reaction mixture was transferred
to a Schlenk flask. Diethyl ether (9.0 mL) and cycloheptanone (45 L, GC
internal standard) were added. In the carboxylation of cycloheptane,
cyclohexanone (45 L) was used instead as a GC standard. The obtained
hydrogen atoms were located from the electron density map and fixed.
In 2, a squeeze procedure was applied to remove disordered water sol-
vent molecules. CCDC 2,018,678 ꢀ 2,018,679.
μ
μ
Crystal data for 1: C34
H
46Cu
2
N
2
O
12, M = 801.81, λ = 0.71073 Å (Mo-
mixture was vigorously stirred for 10 min, and the organic layer was
analyzed by GC (internal standard method). Cycloalkane carboxylic
acids were formed as principal carboxylation products along with the
corresponding cyclic alcohols and ketones (oxidation products). Attri-
bution of GC peaks was made by comparison with chromatograms of
authentic samples.
K
α), T = 296(2) K, triclinic, space group P-1, a = 7.5163(4), b = 8.0649
(
4), c = 15.5293(8) Å,
α
= 95.232(2), β = 94.502(2), γ = 109.528(2)º, V
3
3
-1
=
877.54(8) Å , Z = 1, D
c
=1.517 g/cm ,
μ
= 1.277 mm , 45,979 re-
flections collected, 5417 unique, I > 2
σ
(I) (Rint = 0.0333), R = 0.0280,
1
2
wR
solvent), M = 631.59, λ = 0.71073 Å (Mo-K
rhombic, space group Pbcn, a = 14.9193(9), b = 18.1974(10), c =
2
= 0.0661, GOF on F =1.038. Crystal data for 2: C21
2 2 12
H32Cu N O ∙
(
α), T = 296(2) K, ortho-
3
. Results and discussion
◦
3
1
2.4831(6) Å,
α
= β = γ = 90 , V = 3389.1(3) Å , Z = 4, D
c
=1.238 g/
3
-1
cm ,
μ
= 1.304 mm , 28,125 reflections collected, 3481 unique, I > 2σ
3
.1. Synthesis of copper (II) catalysts 1 and 2
2
(
I) (Rint = 0.0582), R
1
= 0.0401, wR = 0.0970, GOF on F = 1.044.
2
◦
The self-assembly reactions, at ~25 C in water, between copper(II)
nitrate (metal source), triethanolmaine (principal ligand), 2-naphthoic
or trimesic acid (supporting ligand), and sodium hydroxide (regulator
2
.4. Catalytic oxidation reactions
Cycloalkane oxidation reactions were run in air atmosphere in
of pH) led to the generation of new copper(II) derivatives [Cu
2
( -
μ
H
2
tea) (nfa) (1) and [Cu -H tea) -Htma)] O (2).
2
2
]⋅2H
2
O
2
(
μ
2
2
(
μ
n
⋅4nH
2
thermostated glass vessels equipped with a reflux condenser. The re-
◦
These compounds were obtained as stable crystalline solids in good
yields and were characterized by FTIR spectroscopy, elemental analysis
and single crystal X-ray diffraction. Both compounds feature a similar
actions were performed at 50 C under vigorous stirring in CH
3
CN as
solvent (up to 5 mL of the total reaction mixture volume). In a typical
catalytic test, catalyst 1 or 2 (10 mol), acid promoter (0.1 mmol as a
stock solution in CH CN), GC (gas chromatography) internal standard
CH NO , 50 L), and cycloalkane substrate (2 mmol) were added into a
reaction vessel containing CH CN solution. Then, the reaction was
initiated by adding H (50 % in H O, 10 mmol) in one portion. The
μ
2
+
type of dicopper(II) triethanolaminate [Cu
2
(μ
-H
2
2
tea) ]
cores that are
3
ꢀ
2ꢀ
stabilized by the terminal nfa ligands in 1 or extended by the
μ-Htma
(
3
2
μ
linkers into a 1D metal-organic chain in 2. These copper(II) derivatives
3
also dissolve in the reaction mixtures typically used for the oxidation
2
O
2
2
+
2ꢀ
(
H
2
O/CH
3
CN/H
2
O
2
/H ) or carboxylation (H
2
3
O/CH CN/S
2
O
8
)
of
reaction progress was monitored by withdrawing small aliquots in reg-
ular time periods. Prior to GC analysis, the aliquots were treated with
cycloalkanes, thus enabling their application as homogeneous catalysts
in the cycloalkane functionalization reactions.
2 2
triphenylphosphine for reducing the remaining H O and cycloalkyl
hydroperoxides that are commonly formed as initial products in the
oxidation of cycloalkanes. In several cases, the samples were analyzed
3
.2. Crystal structures
by GC two times, before and after the addition of PPh
firming the formation of cycloalkyl hydroperoxides (Shul’pin’s method
35–37]). GC peak allocation was performed by comparing the chro-
3
, aiming at con-
The structure of [Cu
2
(
μ
-H
2
tea)
2
(nfa)
2
]⋅2H
2
O (1) contains a discrete
[
dicopper(II) molecular unit, which is composed of two symmetry
matograms with those of authentic samples. The quantification of
products was performed using nitromethane as an internal standard.
Blank tests confirmed that cycloalkane oxidations do not proceed in the
absence of copper catalyst (only traces of oxidation products can be
detected).
ꢀ
ꢀ
equivalent Cu1 centers, two
μ
-H
2
tea moieties, and two terminal nfa
ligands (Fig. 1a). The Cu1 atom is 6-coordinate and adopts a distorted
{
CuNO
5
}
octahedral environment. This is constructed from one
ꢀ
carboxylate O atom from nfa [Cuꢀ O 1.9533(1) Å] as well as one N
[
Cuꢀ N 2.0482(1) Å] and two
μ-O [Cuꢀ O 1.9445(1) and 1.9382(1) Å]
ꢀ
donors from the
μ
-H
2
tea moiety in equatorial sites, while the apical
ꢀ
2
.5. Catalytic carboxylation reactions
positions are taken by the remaining OH groups of the
μ-H
2
tea ligands
[
Cuꢀ O 2.4584(1) and 2.5513(1) Å]. These bonding parameters as well
The cycloalkane carboxylation reactions were run in stainless steel
as a short Cu… Cu separation of 2.9130(6) Å are comparable with those
found in related dicopper(II) cores [38,39].
autoclaves (20.0 mL volume), equipped with a Teflon-coated magnetic
stirring bar. In a typical catalytic test, catalyst 1 or 2 (10 mol) was
added to an autoclave, followed by the introduction of K (1.50
mmol), H O (2.0 mL) and CH CN (4.0 mL), and cycloalkane (1.00
μ
The crystal structure of [Cu
2
(
μ-H
2
tea)
2
(
μ
-Htma)]
n
2
⋅4nH O (2) fea-
2
S
2
O
8
tures a dicopper(II) triethanolaminate core (Fig. 1b) that is essentially
2
3
similar to that of 1. In 2, the symmetry equivalent Cu1 centers are also 6-
mmol). Then the autoclave was sealed, flushed three times with CO, and
finally pressurized CO (20 atm). Caution: Due to toxicity of CO, all op-
erations must be carried out in a well-ventilated hood! The reaction was
5
coordinate and show a distorted {CuNO } octahedral geometry. The
main difference lies in the type of aromatic carboxylate ligand which is a
ꢀ
μ
-bridging in 2 (
μ
-Htma2 ), thus generating a 1D metal-organic chain
◦
2ꢀ
run at 60 C for 4 h using a magnetic stirrer and an oil bath with tem-
structure (Fig. 1c). The
μ-Htma ligand acts as a linker between the
perature control. After this time, the autoclave was cooled down using
adjacent dicopper(II) units with a Cu… Cu separation of 10.7085(9) Å,
3

K.I. Trusau et al.
Molecular Catalysis 503 (2021) 111401
Fig. 1. Ball-and-stick representation of the crystal structures of 1 (a) and 2 (b, c). (a) Molecular [Cu
2
(
μ
-H
2
tea)
2
(nfa)
2
] unit. (b) [Cu
2
(
μ
-H
2
tea)
2
( -Htma)] structural
μ
fragment. (c) 1D metal-organic chain with coordination polyhedra around Cu(II) centers. H atoms and crystallization water molecules are omitted; Cu (green), O
red), N (blue), C (gray) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).
(
while the Cu… Cu distance within the Cu
2.8545(7) Å].
2
core is also rather short
treatment with a reducing agent (PPh
3
, Shul’pin’s method [35–37])
[
testify the formation of cyclohexyl hydroperoxide as a primary (inter-
mediate) product in the present cyclohexane oxidation.
As the catalytic systems based on 1 and 2 require the use of an acid
promoter, the influence of various acids on the cyclohexane oxidation
was investigated (Fig. 3). Prior studies demonstrated [7–10] that the
presence of a catalytic amount of strong acid can demonstrate a number
of advantages in the reaction system, namely (1) an activation of Cu
catalyst via protonation of aminoalcoholate and carboxylate ligands, (2)
3
.3. Cu-catalyzed oxidation of cyclohexane
Compounds 1 and 2 were screened as homogeneous catalysts for the
mild oxidation of C cycloalkanes by hydrogen peroxide to give the
ꢀ C
corresponding cyclic alcohols and ketones as final products (Scheme
a). Cyclohexane was used as a model substrate due to an industrial
5
8
2
+
a facilitation of H -transfer steps, and (3) an improvement of the
relevance of its oxidation products (KA oil) [25–27]. The oxidation re-
actions undergo in aqueous acetonitrile medium under mild conditions
oxidation properties of hydrogen peroxide in acidic medium and pre-
venting its decomposition via catalase activity. In the oxidation of
cyclohexane, the product accumulation curves display distinct trends in
the presence of sulfuric, nitric, and trifluoroacetic (TFA) acids as pro-
moters (Fig. 3), with the maximum yields observed for both catalysts 1
◦
(
atmospheric pressure, aqueous H
2
O
2
as oxidant, 50 C).
Given the presence of similar dicopper(II) triethanolaminate cores,
the compounds 1 and 2 exhibit the comparable product accumulation
patterns (Fig. 2) with the total product yields of 21ꢀ 22 % after 90 min of
the reaction; hereinafter, all the product yields are based on cycloalkane
substrate: (moles of products per moles of cycloalkane)×100 %. For both
catalytic systems, the cyclohexane oxidation undergoes with a linear
increase of product yields in the first 90 min of reaction, resulting in TOF
and 2 when using H
seen in the first 90 (H
sponding TOF values of 29, 25, and 35 h , respectively. For 2, a linear
trend is present in the first 90 (H SO ) or 120 (HNO , TFA) min with
2 4
SO . For 1, a linear trend of product accumulation is
2
SO , HNO ) or 60 (TFA) min with the corre-
4
3
ꢀ 1
2
4
3
ꢀ
1
ꢀ 1
TOFs of 28, 18, and 14 h , respectively. After approaching the steady
state, the oxidation of cyclohexane stops eventually due to the deacti-
vation of catalyst and/or consumption of oxidant.
values of 29 and 28 h for 1 and 2, respectively (turnover frequencies,
TOFs, were calculated as moles of products formed per mole of catalyst
per hour). These oxidation reactions require the presence of a catalytic
amount of acid promoter such as sulfuric acid. Cyclohexanol is gener-
ated predominantly to cyclohexanone at the beginning of reaction. This
observation as well as the fact that the alcohol yields differ if the GC
analyses of the reaction mixtures performed before and after the
Given a good performance of sulfuric acid in terms of both product
yields and reaction rates, its loading was also optimized for both cata-
lysts (Fig. 4), resulting in an optimum acid-to-catalyst molar ratio of
1
0:1 with the total product yields of 21ꢀ 22 %. The best cyclohexane
Fig. 2. Product accumulation curves in the oxidation of cyclohexane to cyclohexanol and cyclohexanone catalyzed by 1 (a) and 2 (b). Conditions: catalyst (10
μ
mol),
◦
H
2
SO
4
(0.1 mmol), C
6
H
12 (2.0 mmol), H
2
O
2
3
(aq. 50 %, 10.0 mmol), CH CN (total reaction volume up to 5 mL), 50 C.
4

K.I. Trusau et al.
Molecular Catalysis 503 (2021) 111401
Fig. 3. Effect of the acid promoter in the oxidation of cyclohexane to cyclohexanol and cyclohexanone (total yield vs. time) catalyzed by 1 (a) and 2 (b). Conditions:
◦
catalyst (10
μ
mol), acid promoter (0.1 mmol), C
6
H
12 (2.0 mmol), H
2
O
2
(aq. 50 %, 10.0 mmol), CH
3
CN (total reaction volume up to 5 mL), 50 C.
Fig. 4. Effect of the amount of H
2
SO
4
on the oxidation of cyclohexane to cyclohexanol and cyclohexanone (total yield vs. time) catalyzed by 1 (a) and 2 (b).
◦
Conditions: catalyst (10 mol), H SO
μ
2
4
promoter (0ꢀ 0.15 mmol), C
6
H
12 (2.0 mmol), H
O
2 2
(aq. 50 %, 10.0 mmol), CH
3
CN (total reaction volume up to 5 mL), 50 C.
oxidation performance obtained when using this acid-to-catalyst ratio
capability of chloride anions to behave as additional terminal or
bridging ligands as well as to stabilize copper(I) species or eventual Cu-
peroxo intermediates [39]. It should also be mentioned that the cyclo-
hexane oxidation reactions catalyzed by CuO (0.2 %), CuO/HCl (6.6 %),
+
suggests the need for an increased concentration of H ions to allow the
activation of catalyst and the prevention of an eventual catalase activity
2 2
(H O decomposition) in acidic medium. However, higher amount of
acid (acid-to-catalyst molar ratio of 15:1, Fig. 4) is not beneficial as this
may provoke full protonation of ligands and their decoordination from
copper centers, thus resulting in inferior catalytic activity. In fact, under
similar reaction conditions (conditions of Fig. 3, 2 h), various control
CuCl
2 2
(6.2 %) and CuCl /HCl (7.3 %) are less efficient as those in the
presence of 1 and 2 (15 ꢀ 17 % total yields).
3
.4. Cu-catalyzed oxidation of C
5
ꢀ C
8
cycloalkanes
3 2
tests on the cyclohexane oxidation catalyzed by the Cu(NO ) (3.6 %),
Cu(NO
3
)
2
/H
3
tea/Hnfa (5.7 %), Cu(NO
(9.8 %), Cu(NO /Hnfa/H
SO
3
)
2
/H
SO
2
SO
(11.1 %), and Cu(NO
4
(5.3 %), Cu(NO
3
)
)
2
/
/
After the above optimization tests, further reactions were run to
H
H
3
tea/H
2
SO
4
3
)
2
2
4
3
2
explore a substrate scope in the Cu-catalyzed cycloalkane oxidations
Fig. 5). The catalysts 1 and 2 behave in a different way concerning the
product accumulation patterns and the maximum total yields in the
oxidation of C cycloalkanes. For the catalyst 1, the highest activity
ꢀ C
29 % overall product yield) was detected in the oxidation of cyclo-
3
tea/Hnfa/H
2
4
(11.4 %) systems resulted in inferior total product
(
yields (% values in brackets) than those obtained in the presence of
compounds 1 (21.8 %) and 2 (20.8 %) as catalysts. Despite appreciable
product yields attained when using the in-situ generated catalytic sys-
tems containing full combination of ligands and acid promoter, the
performance of isolated Cu(II) compounds 1 and 2 is significantly su-
perior. This behavior is also in good agreement with the results reported
for other copper catalytic systems [9].
6
8
(
heptane, followed by cyclooctane (27 %) and cyclohexane (22 %). In the
presence of catalyst 2, the trend is different with a superior activity
observed in the oxidation of cyclohexane (21 % overall product yield),
cyclooctane (16 %), and then cycloheptane (13 %). A decreased activity
of 2 vs. 1 can be associated with its structural features (presence of a
Interestingly, a different promoting behavior is detected in the
presence of hydrochloric acid with a drastically increased reaction rate
of cyclohexane oxidation (Fig. 3). As a result, the reaction is very fast
and completes in 3 or 7 min with the total product yields of 17 or 15 %
μ
-carboxylate linker and 1D coordination polymer structure). Although
some overoxidation processes can occur at prolonged reaction times
Fig. 5), good overall selectivities toward main products, i.e. cyclic al-
(
ꢀ
1
for catalysts 1 and 2, respectively (TOFs = 680ꢀ 250 h ). Although an
unusual reaction accelerating role of hydrochloric acid is not clearly
implicit if compared to other strong acids, it may be associated to a
cohols and ketones (commonly above 90 % within the first reaction
hours), were observed in the present systems catalyzed by 1 and 2. Both
catalysts 1 and 2 reveal a decreased efficacy in the oxidation of
5

K.I. Trusau et al.
Molecular Catalysis 503 (2021) 111401
Fig. 5. Oxidation of C
5
–C
8
cycloalkanes to the corresponding alcohols and ketones (total yield vs. time) catalyzed by 1 (a) and 2 (b). Conditions: catalyst (10
μ
mol),
◦
H
2
SO
4
(0.1 mmol), cycloalkane (2.0 mmol), H
O
2 2
(aq. 50 %, 10.0 mmol), CH
3
CN (total reaction volume up to 5 mL), 50 C.
cyclopentane with the overall product yields of 10 and 7%, respectively
of 1:5:8 (1) and 1:4:7 (2). The Cu-catalyzed oxidation reactions of cis-
and trans-1,2-dimethylcyclohexane undergo in a non-stereoselective
way with the trans/cis parameters of 0.8ꢀ 1.0. All these selectivity fea-
tures, the formation of cycloalkyl hydroperoxides as intermediate
products, and the previous data on Cu-catalyzed alkane oxidation [7–10,
21–24] testify the presence of indiscriminate and potent oxidizing spe-
cies like hydroxyl radicals.
(
Fig. 5). With an exception of cyclopentane, the total 22ꢀ 29 % yields of
cyclic alcohols and ketones obtained in the oxidation of other cyclo-
alkanes (cyclohexane, cycloheptane, and cyclooctane) are significantly
higher than those commonly observed in, for example, industrial
6
oxidation of C H12 to KA oil with a homogeneous Co(II) naphthenate
catalyst [25,26]. The attained herein yields of products when using 1
and 2 are also comparable or better than those shown by various cata-
lysts in related cycloalkane oxidation, including the reactions catalyzed
by other Cu coordination compounds composed of resembling types of
ligands [9,21,22].
Hence, having this experimental evidence and considering a prior
background for resembling Cu-based homogeneous catalysts [9,10], the
following radical type mechanistic steps can occur in the present
cycloalkane oxidation reactions: (i) reaction of Cu catalyst with
•
hydrogen peroxide to furnish hydroxyl radicals (HO ); (ii) cycloalkane
•
•
(
CyH) reaction with HO to form cycloalkyl radicals (Cy ); (iii) coupling
3
.5. Selectivity and mechanistic features in Cu-catalyzed oxidation of
•
•
of Cy with O
2
to give cycloalkyl peroxo radicals (CyOO ); (iv) conver-
alkanes
•
ꢀ
sion of CyOO to cycloalkylperoxide anions (CyOO ), (v) generation of
cycloalkyl hydroperoxides (CyOOH) as primary products, (vi) decom-
position of CyOOH to furnish cyclic alcohols (CyOH) and ketones
To get some further details on the nature of oxidizing species in the
present Cu-catalyzed reactions, we investigated the oxidation of specific
alkane substrates and determined different selectivity parameters
(
Cy’=O) as final products.
.6. Cu-catalyzed carboxylation of C
Apart from the oxidation of C
(
Table 1). Thus, the oxidation of a linear alkane, n-heptane, undergoes
3
5
ꢀ C
8
cycloalkanes
without specific preference to a particular secondary C atom, as attested
by the regioselectivity C(1):C(2):C(3):C(4) parameters of 1:4:5:6 and
5
ꢀ C cycloalkanes, the compounds 1
8
1
:4:4:5 for the catalysts 1 and 2, respectively. The oxidation of meth-
◦
◦
◦
and 2 were also tested as catalysts for the single-step carboxylation of
these substrates to give the corresponding cycloalkanecarboxylic acids
as main carboxylation products along with cyclic alcohols and ketones
as oxidation products (Scheme 2b). In these Cu-catalyzed carboxylation
reactions, cycloalkane is treated with carbon monoxide (source of
ylcyclohexane shows the moderate bond selectivity 1 :2 :3 parameters
Table 1
Regio-, bond- and stereoselectivity parameters in the Cu-catalyzed oxidation of
a
alkanes.
carbonyl functionality), water (source of hydroxyl group), and potas-
Selectivity (alkane substrate)
1
2
◦
sium peroxodisulfate (oxidizing agent) at 60 C in H
2
O/CH
3
CN medium
Regioselectivity
[
9,40–42]. It should be mentioned that there is no need for an acid
b
C(1):C(2):C(3):C(4) (n-heptane)
1:4:5:6
1:4:4:5
promoter in the carboxylation reactions. The results are collected in
Table 2.
Bond selectivity
◦
◦
◦
c
Both catalysts 1 and 2 show a comparable level of activity in the
carboxylation of cycloalkanes with the overall product yields attaining
1
:2 :3 (methylcyclohexane)
1:5:8
1:4:7
ꢀ
1
Stereoselectivity
trans/cis (cis-1,2-dimethylcyclohexane)
28 and 33 % based on cycloalkane (TOFs 6.9 and 8.2 h ). For the
cyclopentane and cyclohexane substrates, a slightly superior activity
was observed when using 1, resulting in the yields of cyclo-
pentanecarboxylic and cyclohexanecarboxylic acids in the 29ꢀ 30 %
d
1.0
0.9
0.8
0.8
d
trans/cis (trans-1,2-dimethylcyclohexane)
a
Conditions: catalyst (5
μ
mol), H
2 4 2 2
SO (0.05 mmol), alkane (1.0 mmol), H O
◦
ꢀ 1
(
aq. 50 %, 5.0 mmol), CH
3
CN (total reaction volume up to 2.5 mL), 2 h, 50 C.
range and TOFs of 8.0ꢀ 8.2 h (Table 2, entries 1, 3). These cyclo-
Selectivities were calculated on the basis of the molar ratios of isomeric alcohol
products; normalization was made considering the number of H atoms at each C
atom.
alkanes are also more prone to undergo the carboxylation in comparison
with the oxidation reactions, as attested by rather low yields of cyclic
alcohols and ketones (3ꢀ 4% total yields). However, the carboxylation
reactions of cycloheptane and cyclooctane lead to the modest yields of
carboxylic acids (14ꢀ 16 % of cycloheptanecarboxylic acid; 7% of
cyclooctanecarboxylic acid) and more pronounced yields of oxidation
products (9ꢀ 16 % total yields). This reactivity of cycloalkanes with
regard to carboxylation reactions can be explained by a stability of
b
C(1):C(2):C(3):C(4) refers to reactivity of H atoms at C(1), C(2), C(3), and C
(
4) atoms of n-heptane.
c
◦
◦ ◦
1
:2 :3 stands for reactivity of H atoms at primary, secondary and tertiary C
atoms of methylcyclohexane.
d
Parameter trans/cis refers to the ratio of the tertiary alcohol products with
3
mutual trans and cis orientation of CH groups.
6

K.I. Trusau et al.
Molecular Catalysis 503 (2021) 111401
Table 2
a
Single-step carboxylation of cycloalkanes catalyzed by 1 and 2.
b
Product yield, %
Acid (CyCOOH)
Entry
Cycloalkane (CyH)
Catalyst
ꢀ
1 c
Ketone (Cy’=O)
Alcohol (CyOH)
Total
TOF, h
1
2
3
1
2
1
29.1
24.6
29.8
2.4
2.2
1.8
1.2
0.8
0.5
32.7
27.6
32.1
8.2
6.9
8.0
4
5
2
1
24.0
14.4
2.2
6.3
0.5
2.8
26.7
23.5
6.7
5.9
6
7
2
1
16.0
6.8
8.8
2.5
6.2
27.3
23.8
6.8
6.0
10.8
8
a
2
7.0
11.7
6.0
24.7
6.2
◦
2 2 8 2
Conditions: cycloalkane (1.00 mmol), Cu catalyst (10 mol), CO (20 atm), K S O (1.50 mmol), H O (2.0 mL)/MeCN (4.0 mL), 60 C, 4 h in an autoclave (20.0 mL
μ
capacity).
b
(
Moles of product per moles cycloalkane)×100 %; CyCOOH are generally the main carboxylation products; Cy’=O and CyOH are also generated as oxidation
products.
c
Moles of products per mol of catalyst per hour.
cycloalkyl radicals, following a reverse trend (Cy
43]. This means that the carboxylation is not a favorable process for
cycloalkyl radicals having higher stability (Cy and Cy ), which are
6
≤ Cy
5
< Cy
7
< Cy
8
)
% product yields based on the cycloalkane substrate. Given a recognized
inertness of saturated hydrocarbons and very mild reaction conditions
[
◦
7
8
(50ꢀ 60 C, aqueous acetonitrile medium), the achieved herein product
more susceptible to undergo oxidation reactions thus explaining
significantly higher yields of the oxidation products in the reactions of
yields can be considered as rather high in the field of oxidative func-
tionalization of alkanes [1–5], despite being modest from an organic
synthesis perspective. In fact, an industrial oxidation of cyclohexane to a
mixture of cyclohexanone and cyclohexanone (KA oil, intermediate in
nylon synthesis) utilizes a homogeneous Co(II) naphthenate catalyst and
C
7
H
14 and C
8
H
16 (Table 2, entries 5ꢀ 8). Despite not being very high
from a synthetic viewpoint, the overall product yields (up to 33 %) in the
present carboxylation reactions are appreciable given the inertness of
cycloalkanes and rather mild reaction conditions applied. Attempts to
further increase the yields of carboxylic acid products by prolonging the
reaction time, augmenting the temperature or using increased amounts
of carboxylating agent and oxidant were not successful.
◦
proceeds under harsher reaction conditions (150ꢀ 160 C), resulting in
significantly lower overall product yields (5ꢀ 10 %).
Since the compounds 1 and 2 are not only active in the oxidative
functionalization of saturated hydrocarbons but also display an N,O-
donor environment at Cu centers, these can be classified as bioinspired
catalysts with a relevance to pMMO (particulate methane mono-
oxygenase) [18], a unique multicopper enzyme capable of oxidizing the
CH bonds of inert alkanes. In addition, the reported herein compounds
widen the family of multicopper core catalysts applied in various
oxidation reactions [9,44–46]. Further research on the synthesis of new
types of multicopper catalysts assembled from simple and readily
available building blocks, as well as the exploration of such Cu coordi-
nation compounds in the mild oxidative functionalization of alkanes and
other substrates is underway.
The obtained results on the carboxylation of cycloalkanes as well as
prior background for related Cu-catalyzed transformations indicate that
the reactions proceed via a free-radical mechanism involving sulfate
•
ꢀ
SO
4
radical anions as main species [9,40–42]. Hence, the following
reaction steps can be present in these carboxylation reactions: (i) gen-
•
–
–
2ꢀ
eration of sulfate radical anions (SO
4
) via homolysis of S
2
O
8
; (ii) re-
•
•
action of cycloalkane (CyH) with SO
4
to give cycloalkyl radicals (Cy );
•
•
(
iii) carbonylation of Cy with CO to form cycloacyl radicals (CyCO ); (iv)
•
+
oxidation of CyCO to furnish cycloacyl cations (CyCO ); (v) hydrolysis
+
of CyCO to give cycloalkanecarboxylic acid product (CyCOOH). The
•
competitive oxidation processes of Cy can also occur explaining the
formation of cyclic alcohols and ketones as oxidation products.
Declaration of Competing Interest
4
. Conclusions
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
In this work, we showed that new copper(II) coordination com-
pounds with notable catalytic potential in mild oxidative functionali-
zation of cycloalkanes can be easily generated by self-assembly in water
and using simple and commercially available chemicals. The obtained
CRediT authorship contribution statement
[
Cu
2
(
μ
-H
2
tea)
2
(nfa)
2
]⋅4H
2
O (1) and [Cu
2
(
μ
-H
2
tea)
2
(
μ
-Htma)]
n
⋅4nH
2
O
Kiryl I. Trusau: Investigation, Data curation, Validation. Marina V.
Kirillova: Investigation, Conceptualization, Methodology, Visualiza-
tion, Writing - original draft, Writing - review & editing. V aˆ nia Andr e´ :
Investigation, Data curation, Validation. Andrew I. Usevich: Data
curation, Supervision, Validation. Alexander M. Kirillov: Conceptual-
ization, Visualization, Funding acquisition, Writing - original draft,
Writing - review & editing.
(
2) products were structurally characterized, revealing a similar type of
dicopper(II) triethanolaminate cores but distinct stabilizing or linking
carboxylate ligands.
Apart from extending the family of heteroleptic copper(II) de-
rivatives bearing aminoalcoholate and carboxylate ligands, these
dicopper(II) cores were also applied as efficient homogeneous catalysts
for the oxidation of cycloalkanes leading to the formation of cyclic
alcohol and ketone products. Besides, both catalysts are also active for
the mild carboxylation of cycloalkanes to give the corresponding
cycloalkanecarboxylic acids along with alcohols and ketones in up to 33
Acknowledgements
This work has been supported by the Foundation for Science and
7

K.I. Trusau et al.
Molecular Catalysis 503 (2021) 111401
Technology (FCT) and Portugal 2020 (projects CEECIND/03708/2017,
UIDB/00100/2020, and LISBOA-01-0145-FEDER-029697). This paper
has also been supported by the RUDN University Strategic Academic
Leadership Program.
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2
939–2945;
K.A.D.F. Castro, J.M.M. Rodrigues, R.F. Mendes, P.M.S. Maria da Graça, M.M.Q.
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[
Appendix A. Supplementary data
(
2
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[
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