New copper(II) tetramer with arylhydrazone of barbituric acid and its catalytic activity in the oxidation of cyclic C5–C8alkanes

Article abstract of DOI:10.1016/j.poly.2016.07.002

2-(2-(2,4,6-Trioxotetrahydropyrimidin-5(2H)-ylidene)hydrazinyl)terephthalic acid (H₅L) was synthesized by azocoupling of diazonium salts of 2-aminoterephthalic acid with barbituric acid and characterized by IR,¹H and¹³C NMR spectroscopies and ESI-MS analysis. In solution and solid state, H₅L exists in the hydrazone form. The reaction of H₅L with copper(II) nitrate hydrate in methanol affords the tetramer species [Cu₂(μ₂-1κNOO′:2κO′O″-H₃L)₂]₂(1) where carboxylate groups bridge two Cu(II) centers in a bridging chelate μ₂-(O,O′,O′) fashion, giving rise to a cyclic Cu₄O₄core. A rare and intense intermolecular non-covalent interaction of the type O_carboxylic(lone pair)?π(pyrimidine), with O?centroid distance of 2.705??, was found in 1. In addition, medium intense [Formula presented] and [Formula presented] contacts expand the structure to a 3D network. Compound 1 behaves as efficient catalyst in the mild oxidation, by aqueous H₂O₂in acidic MeCN/H₂O media, of different cyclic C₅–C₈alkanes to produce the corresponding alcohols and ketones. Overall product yields up to ca. 25% (based on alkane) were achieved.

Full text of DOI:10.1016/j.poly.2016.07.002

Polyhedron 117 (2016) 666–671
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
New copper(II) tetramer with arylhydrazone of barbituric acid
and its catalytic activity in the oxidation of cyclic C –C alkanes
5
8
Manas Sutradhar a, , Elisabete C.B.A. Alegria , Tannistha Roy Barman , M. Fátima C. Guedes da Silva
⇑
,
⇑
a,b
a
a,
a,c
d,e
a,
⇑
Kamran T. Mahmudov , Firuddin I. Guseynov , Armando J.L. Pombeiro
a
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisbon, Portugal
Chemical Engineering Departament, ISEL-Instituto Superior de Engenharia de Lisboa, Instituto Politécnico de Lisboa, Rua Conselheiro Emídio Navarro, 1959-007 Lisboa, Portugal
Department of Chemistry, Baku State University, Z. Xalilov Str. 23, Az 1148 Baku, Azerbaijan
Department of Chemistry, M. V. Lomonosov Moscow State University, Leninskie Gory, 119992 Moscow, Russian Federation
Institute for Chemistry and Biology Immanuel Kant Baltic Federal University, Universitetskaya str., 236000 Kaliningrad, Russian Federation
b
c
d
e
a r t i c l e i n f o
a b s t r a c t
Article history:
2-(2-(2,4,6-Trioxotetrahydropyrimidin-5(2H)-ylidene)hydrazinyl)terephthalic acid (H L) was synthe-
sized by azocoupling of diazonium salts of 2-aminoterephthalic acid with barbituric acid and character-
5
Received 6 June 2016
Accepted 2 July 2016
Available online 12 July 2016
1
13
ized by IR, H and C NMR spectroscopies and ESI-MS analysis. In solution and solid state, H
5
L exists in
L with copper(II) nitrate hydrate in methanol affords the tetramer
(1) where carboxylate groups bridge two Cu(II) centers in a
core. A rare and intense intermolecular
(pyrimidine), with Oꢀ ꢀ ꢀcentroid distance of
.705 Å, was found in 1. In addition, medium intense NAHꢀ ꢀ ꢀO and OAHꢀ ꢀ ꢀO contacts expand the struc-
ture to a 3D network. Compound 1 behaves as efficient catalyst in the mild oxidation, by aqueous H in
acidic MeCN/H O media, of different cyclic C –C alkanes to produce the corresponding alcohols and
ketones. Overall product yields up to ca. 25% (based on alkane) were achieved.
the hydrazone form. The reaction of H
5
0
0
00
species [Cu
2
(
l
2
-1
jNOO :2
j
O O -H
3
L)
2
]
2
Keywords:
0
0
bridging chelate
l
2
-(O,O ,O ) fashion, giving rise to a cyclic Cu
4
O
4
Arylhydrazone of barbituric acid
Tetrameric copper(II) complex
Peroxidative oxidation of alkanes
X-ray structure
non-covalent interaction of the type Ocarboxylic(lone pair)ꢀ ꢀ ꢀ
p
2
2 2
O
Non-covalent interaction
2
5
8
Ó 2016 Elsevier Ltd. All rights reserved.
1
. Introduction
On the other hand, multinuclear copper complexes have been
well investigated in catalyzing the peroxidative oxidation of alka-
nes to the corresponding alcohol and ketones in the past decade
[8,9]. In this catalytic system, an acid (e.g., nitric acid, trifluo-
roacetic acid, pyrazine-2-carboxylic acid) can act as a promoter
[10,11]. Our group has functionalized arylhydrazone ligands by
Due to the presence of several potential donors, such as two
imine nitrogens and three carbonyl oxygens, barbiturates are very
interesting polyfunctional ligands in coordination chemistry [1].
Moreover, the imine and carbonyl moieties of barbiturates can
bind other substrates (as sensors or analyst) through H-bonds with
some degree of complementarity, giving rise to an analytical signal
3
the introduction of AOH, ASO H or ACOOH groups, in order to
increase the water solubility of the catalyst [5,12]. Thus, it would
be attractive to prepare a new hydrazone ligand bearing two
ACOOH groups (more acidic source) and the barbituric acid moiety
(Scheme 1), to synthesize a water-soluble copper(II) complex and
apply it as a catalyst in the peroxidative oxidation of alkanes. In
comparison to other transition metal complexes, copper catalysts
are commercially inexpensive, less polluting, easy to be prepared
and handled [8,9].
(
absorbance or emission intensity, etc.), and making barbiturates
very interesting building blocks in the context of molecular recog-
nition [1,2]. Arylhydrazones of barbituric acid (AHBA, Scheme 1 for
the particular example of this study – see below) contain the
C@NNHAr substituent at the 5-position on the barbiturate ring.
This substituent can provide an extra coordination site compared
with barbituric acid itself [1,3–7], thus affecting the type of the
resulting structures.
Herein, we report the synthesis and characterization of a water
soluble copper(II) complex of an AHBA, i.e., 2-(2-(2,4,6-trioxote-
trahydropyrimidin-5(2H)-ylidene)hydrazinyl) terephthalic acid
⇑
Corresponding authors.
E-mail addresses: manaschem@yahoo.co.in (M. Sutradhar), fatima.guedes@
(
H
5
L), and its catalytic activity (as homogeneous catalyst) towards
peroxidative oxidation of different cyclic C –C alkanes in acidic
MeCN/H O media.
5
8
tecnico.ulisboa.pt (M.F.C. Guedes da Silva), pombeiro@tecnico.ulisboa.pt
A.J.L. Pombeiro).
(
2
http://dx.doi.org/10.1016/j.poly.2016.07.002
277-5387/Ó 2016 Elsevier Ltd. All rights reserved.
0

M. Sutradhar et al. / Polyhedron 117 (2016) 666–671
667
H
N
(M = 320.22): C, 45.01; H, 2.52; N, 17.50. Found: C, 44.87; H,
O
O
..
O
OH
+
ꢁ1
.
.
.
2.43; N, 17.22%. ESI-MS: m/z: 321.2 [M+H] . IR, cm : 3428
OH), 3368, 2992 and 2878 (NH), 1716 and 1632 (C@O), 1614
(C@N). H NMR (300.130 MHz) in DMSO-d
internal TMS, d (ppm): 5.66 (2H, COOH), 7.06–8.45 (3H, ArAH),
1.37 (s, 1H, NAH), 11.53 (s, 1H, NAH), 15.25 (s, 1H, NNAH). 13
m
.
C
C
C
H
N
C
(
m
m
m
1
HN
(C@Oꢀ ꢀ ꢀH), 1598
m
6
,
C
O
N
1
C
1
{
H} NMR (75.468 MHz, DMSO-d
(ArACOOH), 120.37 and 124.77 (ArAH), 132.12 (ArACOOH),
36.00 (C@N), 143.37 (ArANHN@), 150.03 and 160.05 (C@O),
161.08 (C@Oꢀ ꢀ ꢀH), 166.37 and 167.34 (ArACOOH).
6
). d: 116.65 (ArAH), 119.90
H5L
COOH
1
Scheme
terephthalic acid (H
1. 2-(2-(2,4,6-Trioxotetrahydropyrimidin-5(2H)-ylidene)hydrazinyl)
L).
5
0
0
00
2
2 2 3 2 2
.3. Synthesis of [Cu (l -1jNOO :2jO O -H L) ] (1)
2
. Experimental
3
20 mg (1 mmol) of H
5
L were dissolved in 30 mL of methanol
ꢀ2.5H O were added, with stir-
and 255 mg (1.1 mmol) of Cu(NO
3
)
2
2
2.1. Materials and instrumentation
ring. The mixture was refluxed for 2 h in an oil bath, in open air.
The resultant green solution was filtered and the filtrate was kept
in air for slow evaporation. After ca. 3–4 d, X-ray quality single
crystals were isolated.
Yield, 67% (based on Cu). Anal. Calc. for C48H N O28Cu
24 16 4
M = 1526.81): C, 37.76; H, 1.58; N, 14.68. Found: C, 37.39; H,
The 1H and 13C NMR spectra were recorded at room tempera-
TM
ture on a Bruker Avance II + 300 (UltraShield Magnet) spectrom-
eter operating at 300.130 and 75.468 MHz for proton and carbon-
1
3, respectively. The chemical shifts are reported in ppm using
(
1
m
tetramethylsilane as the internal reference. The infrared spectra
ꢁ1
.69; N, 14.32%. IR, cm : 3324, 2986
(C@O), 1598
m
(NH), 1706
m
(C@O), 1609
ꢁ1
(
4000–400 cm ) were recorded on a Bruker Vertex 70 instrument
+
m
(C@N). ESI-MS: m/z: 1528 [M+H]
in KBr pellets. Carbon, hydrogen, and nitrogen elemental analyses
were carried out by the Microanalytical Service of the Instituto
Superior Técnico. Electrospray mass spectra (ESI-MS) were run
with an ion-trap instrument (Varian 500-MS LC Ion Trap Mass
Spectrometer) equipped with an electrospray ion source. For elec-
trospray ionization, the drying gas and flow rate were optimized
according to the particular sample with 35 p.s.i. nebulizer pressure.
Scanning was performed from m/z 100 to 1200 in methanol solu-
tion. The compounds were observed in the negative or positive
mode (capillary voltage = 80–105 V). Chromatographic analyses
were undertaken by using a Fisons Instruments GC 8000 series
gas chromatograph with a DB-624 (J&W) capillary column (FID
detector) and the Jasco-Borwin v.1.50 software. The internal stan-
dard method was used to quantify the organic products.
2
.4. X-ray structure determinations
A single crystal of 1 was immersed in cryo-oil mounted in Nylon
loops and measured at 296 K. Intensity data were collected using a
Bruker APEX-II PHOTON100 with graphite monochromated Mo K
k 0.71073) radiation. Data were collected using phi and omega
a
(
scans of 0.5° per frame and a full sphere of data was obtained. Cell
parameters were retrieved using Bruker SMART [14a] software and
refined using Bruker SAINT [14a] on all the observed reflections.
Absorption corrections were applied using SADABS [14b]. Structure
was solved by direct methods by using the SIR97 [14c] and
refined with SHELXL-2014 [14d]. Calculations were performed
using the WinGX System-Version 1.80.03 [14e]. The hydrogen
atoms attached to nitrogen were found in the difference Fourier
map and the isotropic thermal parameters were set at 1.5 times
the average thermal parameters of the belonging N-atoms, with
their distances restrained by using the DFIX commands. The
carboxylic H-atom was inserted in a calculated position and its
position had to be fixed during refinement. Coordinates of
hydrogen atoms bonded to phenyl and pyrimidine groups were
included in the refinement using the riding-model approximation
with the Uiso(H) defined as 1.2Ueq of the parent C- or N-atoms.
The O8 water hydrogen atoms could not be located or included
in calculated positions; in addition, the unit-occupancy of O8 had
abnormally large Ueq and Uii values and by refining it with 0.5
occupancy the values of such parameters lowered considerably
which thus means that, statistically, 50% of the unit cells have a
water molecule in that site. Crystal data and refinement
parameters are presented in Table 1 and selected features, bond
distances and angles are given in Table 2.
2.2. Synthesis of H
5
L
5
H L was synthesized according to the Japp–Klingemann reac-
tion [13] between the diazonium salt of 2-aminoterephthalic acid
with barbituric acid in alkali medium.
2
.2.1. Diazotization
0 mmol of 2-aminoterephthalic acid were dissolved in
0.00 mL of water upon addition of 1.00 g of crystalline NaOH.
1
5
The solution was cooled in an ice bath to 0 °C, and 10 mmol of
NaNO were added with subsequent addition of 2.00 mL HCl in
2
portions of 0.20 mL for 1 h under vigorous stirring. During the reac-
tion the temperature of the mixture must not exceed +5 °C.
2.2.2. Azocoupling
1
.00 g of NaOH was added to a mixture of 10 mmol of barbituric
acid with 50.00 mL of water. The solution was cooled in an ice bath,
and a suspension of 2,5-dicarboxybenzenediazonium chloride
2.5. Peroxidative oxidation of alkanes
(
prepared according to the procedure of Section 2.2.1) was added
in two equal portions under vigorous stirring for 1 h. The formed
yellow precipitate of 2-(2-(2,4,6-trioxotetrahydropyrimidin-5
Typical reaction mixtures were prepared as follows: to 5
of the Cu(II) complex 1 contained in the reaction flask, 5 mmol of
cyclic alkane (C –C ), 10 mmol of H solution (30% in H O), an
acid cocatalyst (0.025–0.125 mmol) such as nitric acid (HNO ),
chloridric acid (HCl), sulfuric acid (H SO ) or trifluoroacetic acid
(TFA, in the form of a stock solution in acetonitrile) and acetonitrile
(MeCN) as the solvent (up to 5.0 mL total volume), were added. The
reaction mixture was stirred for 6 h at room temperature and air
lmol
(
2H)-ylidene)hydrazinyl)terephthalic acid (H
washed with water, recrystallized from ethanol and dried in air.
The characterization of H L was undertaken by elemental and
5
L) was filtered off,
5
8
2
O
2
2
3
5
2
4
1
13
ESI-MS analyses, IR, H and C NMR spectroscopies.
Yield, 85% (based on barbituric acid), yellow powder, soluble in
methanol, ethanol, acetone, dichloromethane, chloroform, tetrahy-
drofuran and insoluble in water. Anal. Calc. for
C
12
8
H N
4
O
7
atmospheric pressure, whereafter 90 lL of cycloheptanone (as

6
68
M. Sutradhar et al. / Polyhedron 117 (2016) 666–671
Table 1
Crystal data for 1.
the proton of the protonated nitrogen atom adjacent to the aryl
unit (@NANHA hydrazo form). This is also supported by IR data:
Compound
1
3428
614
m
(OH), 3368, 2992 and 2878
m(NH), 1716 and 1632 m(C@O),
1
m(C@Oꢀ ꢀ ꢀH), 1598 (C@N). Elemental analysis and ESI-MS in
m
Empirical formula
Formulla weight
T (K)
k (Å)
Crystal system
Space group
C
48 4 16 30
H28Cu N O
+
1563.02
296(2)
0.71073
Tetragonal
I4 /a
methanol (peak at m/z 321.2 [M+H] ) confirm the formulation of
H
5
L.
The tetranuclear copper(II) complex [Cu
L) (1) (Scheme 2), was obtained with 67% yield by refluxing
L and copper(II) nitrate hydrate for 2 h.
0
0
00
2 2
(l -1jNOO :2jO O -
H
3
2 2
]
1
a methanol solution of H
5
Unit cell dimensions
Green crystals suitable for single-crystal X-ray diffraction analysis
were obtained after 3 d. The IR spectrum bands of the complex,
1706
a (Å)
b (Å)
c (Å)
15.2632 (7)
15.2632 (7)
24.8484 (9)
90
90
90
5788.8 (6)
4
1.793
ꢁ1
m(C@O), 1609 m(C@O), 1598 m(C@N) cm , are significantly
a
(°)
b (°)
(°)
shifted in relation to those of the free ligand.
c
3
V (Å )
3.2. X-ray structure description
Z
q
calc (mg/m3)
F(000)
(Mo K
No. reflections
Unique/Observed reflections
3136
1.560
Single crystals of 1 suitable for X-ray analysis were obtained
l
a
) (mm 1
ꢁ
)
upon slow evaporation, at room temperature, of a methanolic solu-
tion of the compound. The crystallographic data and processing
parameters are summarized in Table 1 and selected dimensions
are presented in Table 2. The asymmetric unit of 1 (Fig. 1a) con-
tains one copper cation, one coordinated ligand, and a crystalliza-
tion water molecule; the tetrametallic motif of 1, with a central
59098
2662/1993
0.1027
0.0492
0.1279
1.085
R
R
wR
int
a
1
(I P 2r)
b
2
(I P 2r)
2
Goodness-of-fit (GOF) on (F )
a
Cu
O
core (Fig. 1b), is generated via a 4-fold rotational axis. Each
coordination environ-
R
wR
1
=
R
= [
||F
R
o
| ꢁ |F
c o
||/R|F |.
4
4
b
2
2
2
2
)2]]1/2.
2
[w(F
o
ꢁ F
c
) ]/
R
[w(F
o
Cu atom presents square pyramidal CuNO
4
ment where the basal plane is defined by the keto O3-, the imine
N1- and the carboxylate O1-atom of one ligand, and the carboxy-
late O2-atom from a symmetry generated one; the carboxylate
O1-atom from this symmetry generated ligand occupies the axial
position and is, thus, only 2.111(3) Å above the basal least-square
plane. The CuAO bond distances (Table 2) range from 1.878(3) to
Table 2
Selected distances (Å) and angles (°) of 1.
N1ACu1
O1ACu1
O3ACu1
Cu1AO2
1.954(4)
1.878 (3)
1.897 (3)
2.024 (3)
2.475(3)
171.64(15)
93.60(15)
93.73(14)
86.62(14)
86.45(14)
174.79(14)
2
.025(3) Å while that of the elongated (axial) Cu1AO1 bond is
i
2.475(3) Å; the CuAN bond distance assumes the value of 1.954
Cu1AO1
ꢁ
2
O1ACu1AO3
O1ACu1AN1
O3ACu1AN1
O1ACu1AO2
O3ACu1AO2
(4) Å. Resulting from the chelation mode of H
cations are involved in 6- and 4-membered monometallic rings
CuC O, CuC NO and CuCO ), but 8- to 16-membered tetram-
3
L , the metal
(
2
N
2
3
2
i
i
etallic rings can also be envisaged, the shortest corresponding to
the central core.
i
N1ACu1AO2
The intra- and intermolecular interactions that can be found in
the structure of 1 are worth to be discussed. A special mention con-
Symmetry codes: (i) 1/4 ꢁ y, 1/4 + x, 1.25 ꢁ z.
cerns the short lone-pairꢀ ꢀ ꢀ
p contacts (2.706 Å) involving every O5
atoms from the carboxylic groups (as donors) in the cluster and the
pyrimidine systems of adjacent molecules (Fig. S1). Such interac-
tions are important from the chemical and biological viewpoints
internal standard) and 10 mL of diethyl ether (to extract the sub-
strate and the products from the reaction mixture) were added.
The resulting mixture was stirred for 10 min and then a sample
from the organic phase was taken and usually treated with PPh
to reduce the alkyl hydroperoxides, typically the major primary
3
(
COOH
products formed in alkane oxidation) according to Shul’pin’s
method [15] and finally analyzed by gas chromatography (GC).
Blank experiments were performed and confirmed that no alkane
oxidation products were obtained in the absence of the metal cat-
alyst precursor.
H
N
O
C
O
C
NH
C
O
C
N
N
N
C
O
C
C
O
NH
N
Cu
Cu
O
C
O
O
C
HOOC
O
N
H
3
. Results and discussion
O
C
O
H
O
C
3
.1. Synthesis and characterization of H
5
L and
N
C
O
0
0
00
O
[Cu
2
2
(l -1jNOO :2
3 2
jO O -H L) ]
2
(1)
C
COOH
HN
O
Cu
Cu
N
C
C
O
O
C
2
-(2-(2,4,6-Trioxotetrahydropyrimidin-5(2H)-ylidene)hy-
L) was prepared by a modified
known [13] aqueous diazotization of 2-aminoterephthalic acid
and subsequent coupling with barbituric acid. H L is stabilized in
DMSO-d solution in the hydrazone form, as reported analogs
4–7]. In fact, the H NMR spectrum of H
at room temperature shows only one signal (d 15.25) assigned to
N
N
N
O
C
drazinyl)terephthalic acid (H
5
C
HN
C
O
C
5
N
H
O
6
HOOC
1
6
[
5
L in DMSO-d solution
Scheme 2. Schematic representation of 1.

M. Sutradhar et al. / Polyhedron 117 (2016) 666–671
669
Fig. 1. (a) The asymmetric unit of 1 with atom numbering scheme and (b) the tetrametallic cluster of 1 viewed down the crystallographic b axis. Dashed light blue lines
represent the H-bond interactions. Symmetry codes: (i) 1/4 + y, 1/4 ꢁ x, 1/4 + z; (ii) 1/4 ꢁ y, ꢁ1/4 + x, ꢁ1/4 + z; (iii) 1 ꢁ x, ꢁy, 1 ꢁ z. H-bond interactions [d Dꢀ ꢀ ꢀA (Å), \ D–HꢀꢀꢀA
(°)]: N3AH3Nꢀ ꢀ ꢀO6 2.731(5), 173(5); N4AH4Nꢀ ꢀ ꢀO7 2.830(5), 170(5); O4AH4Oꢀ ꢀ ꢀO8 2.735(11), 170. (Colour online.)
[
3
16] and their length is usually considerably larger (higher than
.0 Å) than that found in our system.[17] These interactions are
The type of acid additive and its amount influence the alkane
oxidation catalyzed by 1, as shown for cyclohexane (Table 3,
entries 5–12). The addition of TFA in an acid/catalyst 1 M ratio of
5 leads already to a yield growth to 11% (Table 3, entry 5), whereas
the maximum yield of 23% is achieved for the molar ratio of 25
(Table 3, entry 6) beyond which the yield drops, e.g., to 18% for
the molar ratio of 50 (Table 3, entry 8).
Compared with TFA, and for the same acid/catalyst molar ratio
of 25, nitric and hydrochloric acids have a slightly weaker promot-
ing effect, resulting in the maximum total yields of 21% and 19%
(Table 3, entries 9 and 11, respectively). Sulfuric acid displays the
weakest promoting effect among the tested strong acids with
reinforced by means of medium intense H-bond contacts involving
the two pyrimidine N- and the carboxylic O-atoms (as donors), and
both the ketone O-atoms as well as the non-coordinated water
molecule (as acceptors) (Fig. 1a). Intramolecular
pꢀ ꢀ ꢀp stacking
interactions (Fig. S2) also contribute for the stabilization of the
structure of 1, and include the mutual contacts not only between
the C2 > C7 phenyl groups (3.715 Å) but also between the 6-mem-
bered CuC3NO metallacycles (3.979 Å). All these features play a
crucial role in the extension of the structure to a 3D network
(
Fig. S3).
1
5% of total yield of oxygenated products (Table 3, entry 12).
The promoting effect of an acid was already observed for other
3.3. Catalytic activity for peroxidative cycloalkane oxidation
Cu-based systems in the oxidative transformation of alkanes
17,19]. The role of an acid additive may be associated to the acti-
[
Following our interest in the oxidative functionalization of alka-
vation of the catalyst by protonation of ligands with unsaturation
of the metal centers, or to the promotion of the oxidizing proper-
ties of H O , as well as to the prevention of its decomposition to
2 2
water and oxygen (i.e., suppressing eventual catalase activity in
acidic medium) [12,20].
nes under mild conditions catalyzed by multicopper(II) complexes
18] we tested compound 1 as a catalyst for the oxidation with
of various cyclic alkanes (C –C ) to the corresponding alco-
hols and ketones (Scheme 3 and Table 3) in an MeCN/H O medium
[
H
2
O
2
5
8
2
at room temperature in the absence and in the presence of an acid
promoter.
Compound 1 catalyzes the peroxidative oxidation of cyclic
5 8
C –C alkanes with modest yields (maximum of ca. 6%) in the
Among the cyclic alkanes tested, the oxidation of cycloheptane
appears to be the most efficient, resulting in 25% total yields of
cycloheptanol and cycloheptanone in the presence of TFA [n
(acid)/n(catalyst) = 25]. For the same reaction conditions, cyclooc-
tane and cyclohexane are somewhat less reactive substrates (max-
imum of 20% and 23% total yields) and cyclopentane is the least
reactive cycloalkane under study (maximum yield of 11%).
The formation of cyclohexyl hydroperoxide (CyOOH) as a pri-
mary product was confirmed by following Shul’pin’s method
[10d,15]. The duplicate GC analyses of the reaction mixture,
absence of any acid additive.
The presence of a strong acid, such as trifluoroacetic acid (TFA),
improves significantly the catalytic performance of 1. For example,
a marked growth in total yield of the oxygenated products from 3%
to 6% range (Table 3, entries 1, 3, 13 and 15) in the absence of any
additive to the 11–25% range in the presence of TFA [n(acid)/n(cat-
alyst) = 25] is observed (Table 3, entries 2, 6, 14 and 16).
3
before and after addition of PPh , has shown that the amount of
OOH
OH
O
Cu(II) catalyst
(n = 1-4)
aq. H O /MeCN
n
2
2
n
n
n
Scheme 3. Oxidation of cycloalkanes by 1/H
2
O
2
system.

6
70
M. Sutradhar et al. / Polyhedron 117 (2016) 666–671
Table 3
elemental analysis, IR spectroscopy, ESI-MS and X-ray crystallogra-
(pyrim-
idine) contacts are found in the cluster which are reinforced by
means of medium intense H-bond contacts. Compound 1 acts as
catalyst for the efficient peroxidative oxidation of different cyclic
Total yield of oxygenated products in the oxidation of C
5
–C
8 2 2
cycloalkanes by H O
phy. Short intermolecular lone-pair (O-carboxylic)ꢀ ꢀ ꢀ
p
(
1
30% aqueous solution) at room temperature in an MeCN/H O medium catalyzed by
.
2
a
Entry
Substrate
Acid co-catalystb
Total yield of
oxygenated products
c
C
5
–C
and at room temperature to produce the corresponding alcohols
and ketones. The presence of an acid (TFA, HNO , HCl or H SO
8
alkanes with aqueous hydrogen peroxide in acetonitrile
CyOH
CyO
Total
1
2
–
2.8
9.8
0.2
1.2
3
11
4
)
3
2
TFA (25)
has a promoting effect in these reactions, TFA being the best
promoter.
Acknowledgements
3
4
–
–
4.8
1.6
9.7
21.7
8.5
16.7
19.2
10.8
16.5
13.5
1.2
2.4
1.3
1.3
9.4
1.3
1.8
6.2
2.5
1.5
6
4
d
Authors are grateful to the Fundação para a Ciência e a Tecnolo-
gia: FCT (projects UID/QUI/00100/2013 and PTDC/QEQ-
QIN/3967/2014), Portugal, for financial support. M.S. acknowledges
5
6
7
TFA (5)
TFA (25)
TFA (25)
TFA (50)
HNO
HNO
11
23
18
18
21
17
19
15
d
8
9
the FCT, Portugal for
a
postdoctoral fellowship (SFRH/
3
(25)
(25)
0d
BPD/86067/2012). Authors are thankful to the Portuguese NMR
Network (IST-UL Centre) for access to the NMR facility and the
IST Node of the Portuguese Network of mass-spectrometry for
the ESI-MS measurements.
1
1
1
3
1
2
HCl (25)
H
2 4
SO (25)
1
1
3
4
–
3.8
23.4
2.2
1.6
6
25
TFA (25)
Appendix A. Supplementary data
CCDC 1478509 for 1 contains the supplementary crystallo-
graphic data for . These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/10.
1
1
5
6
–
3.2
18.1
0.8
1.9
4
20
TFA (25)
a
Reaction conditions: cycloalkane (5 mmol), catalyst precursor
.1 mol% vs. substrate), H (10 mmol, 30% aq. solution), MeCN (up to 5 mL total
volume), room temperature.
n(acid)/n(catalyst 1).
Moles of oxygenated products [cycloalkanol + cycloalkanone]/100 mol of
3
cycloalkane, determined by GC after treatment with PPh .
1 (5 lmol,
0
2 2
O
1
016/j.poly.2016.07.002.
b
c
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