Arylhydrazones of barbituric acid: Synthesis, coordination ability and catalytic activity of their CoII, CoII/III and CuII complexes toward peroxidative oxidation of alkanes

Article abstract of DOI:10.1039/c5ra14078a

New ortho-substituted arylhydrazones of barbituric acid, 5-(2-(2-hydroxyphenyl)hydrazono) pyrimidine-2,4,6(1H,3H,5H)-trione (H₄L¹) and the sodium salt of 2-(2-(2,4,6-trioxotetra-hydropyrimidin-5(2H)-ylidene)hydrazinyl)benzenesulfonic

Full text of DOI:10.1039/c5ra14078a

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Adv., 2015, DOI: 10.1039/C5RA14078A.
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Arylhydrazones of Barbituric Acid: synthesis, coordination ability and
II
II/III
II
catalytic activity of their Co , Co
and Cu complexes toward
peroxidative oxidation of alkanes
a,b
a,c,
a,
Jessica Palmucci, Kamran T. Mahmudov, * M. Fátima C. Guedes da Silva, *
a,d,
b
e
Luísa M. D. R. S. Martins, * Fabio Marchetti, Claudio Pettinari,
a,
Armando J. L. Pombeiro *
a
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco
Pais, 1049–001 Lisbon, Portugal
b
School of Science and Technology, University of Camerino, Chemistry Section, via S. Agostino 1,
6
2032 Camerino, Italy
c
Department of Chemistry, Baku State University, Z. Xalilov Str. 23, Az 1148 Baku, Azerbaijan
Chemical Engineering Department, Instituto Superior de Engenharia de Lisboa, Instituto
d
Politécnico de Lisboa, R. Conselheiro Emídio Navarro, 1959ꢀ007 Lisboa, Portugal
e
School of Pharmacy, University of Camerino, Chemistry Section, via S. Agostino 1, 62032
Camerino, Italy
∗Corresponding authors
Eꢀmail addresses:
kamran_chem@mail.ru, kamran_chem@yahoo.com (Kamran T. Mahmudov)
fatima.guedes@tecnico.ulisboa.pt (M. Fátima C. Guedes da Silva)
lmartins@deq.isel.ipl.pt (Luísa M. D. R. S. Martins)
pombeiro@tecnico.ulisboa.pt (Armando J. L. Pombeiro)
1

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Graphical abstract
II
II/III
II
Arylhydrazones of barbituric acid are valuable ligands for Co , Co
and Cu leading to
mononuclear complexes which effectively catalyse the oxidation of cyclohexane.
Abstract
New orthoꢀsubstituted arylhydrazones of barbituric acid, 5ꢀ(2ꢀ(2ꢀhydroxyphenyl)hydrazono)
1
pyrimidineꢀ2,4,6(1H,3H,5H)ꢀtrione (H L ) and the sodium salt of 2ꢀ(2ꢀ(2,4,6ꢀtrioxotetraꢀ
4
2
2
hydropyrimidinꢀ5(2H)ꢀylidene)hydrazinyl)benzenesulfonic acid (H L ), [Na(H L )(ꢁꢀH O)(H O) ]
4
3
2
2
2 2
II
II
II/III
1
(
=
1), were used in the synthesis of Cu , Co and Co
complexes, [Cu(H
2
L )(H
2
O)(im)]ꢁ3H
2
O (im
O (5)
1
2
2
imidazole) (2), [Co(H
O)
2 6
][Co(H
2
2
L ) ]
2
ꢁ8H
2
O (3), [Co(H
2
L )(im)
3
] (4), [Cu(H
2
L )(im)
2
]ꢁH
2
2
and [Co(H
2
O)
6
][H
3
L ]
2
ꢁ8H
2
O (6). The complexes are water soluble and the monoꢀ or diꢀ
deprotonated ligands display different coordination modes, depending on the synthetic conditions.
The electrochemical behaviour of all compounds was investigated by cyclic voltammetry and
controlled potential electrolysis, revealing that also the ligands are redox active. All compounds
were evaluated as catalysts for the peroxidative (with H
temperature. The compounds 2 and 3 are the most active ones (yields up to 21 % and TON up to
13 are achieved, in the presence of 3).
2 2
O ) oxidation of cyclohexane at room
2
II
II
II/III
Keywords: Arylhydrazones of barbituric acid; Cu , Co or Co
complexes; oxidation of
cyclohexane; electrochemical study.
2

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Introduction
Barbiturates, besides their biological significance, can also be used as precursors in the
1
a
1b
1c
1d
synthesis of numerous compounds, such as polymers, pigments, dyes and Vitamin B2, and as
building blocks in the construction of supramolecular structures owing to their both Hꢀbond donor
2
and acceptor capabilities. The rich coordination and supramolecular chemistry of barbiturates has
2
been reviewed recently by some of us, but data on the complexes with arylhydrazones of barbituric
2
,3
acid (AHBAs, Scheme 1a) ligands are rather scarce and mainly involve copper and zinc
2,3g,h
complexes.
All the reported AHBA ligands are stabilized in the hydrazone form with formation
2
of intramolecular resonanceꢀassisted hydrogen bonding (RAHB) between the hydrazone =N–NH–
moiety and a carbonyl group giving a sixꢀmembered cycle (Scheme 1a). In order to increase the
coordination sites of AHBA ligands, we decided to focus on functionalized forms, in particular the
ortho–SO
3
H and –OH substituted versions (Scheme 1cꢀd), in view of their promising coordination
4
features. Metallacycles can be obtained and, as shown in related compounds, the ortho–SO H
3
group can give more stable two sixꢀmembered metallacycles than the matallacycles associated to
the ortho–OH and to the unsubstituted AHBA ligands (Scheme 1bꢀd). Moreover, functionalization
of a ligand with an hydrophilic polar group, such as –OH, –SO H, is a common way to increase the
3
4
,5
water solubility of its complexes, which is important for catalysis in aqueous medium.
From other perspective, due to protonꢀdonor or ꢀacceptor sites of imidazole (im), it has been
used as an auxiliary ligand or deprotonating agent in the synthesis and design of water soluble
4
coordination compounds. Thus, the use of im as an auxiliary ligand or a deprotonating agent for the
weakening/destroying RAHB system in the complexation of AHBA ligands with transition metals
II
II
such as Cu , Co , etc. is of practical importance for the development of new synthetic methods.
Scheme 1. RAHB in AHBA (a) and expected coordination modes (bꢀd).
No application in catalysis of any metal complex of AHBA had been reported so far. Thus,
II
II/III
II
the preparation and characterization of the first Co and Co
complexes, as well as of Cu , are of
significant interest due to, e.g., their possible catalytic applications in the peroxidative oxidation of
3

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alkanes, for instance cyclohexane. Thus, another main aim of the current work concerns the
extension of the application of AHBA complexes to this field, i.e. oxidative catalysis. The partial
oxidation of cyclohexane is the main industrial route to cyclohexanone and cyclohexanol, which are
important intermediates in the production of adipic acid and caprolactam, also used in the
6
manufacture of Nylonꢀ6 and Nylonꢀ66 polymers. However, the industrial aerobic oxidation of
cyclohexane to the alcohol/ketone mixture, catalysed by Co salts, requires the use of considerable
o
temperature (150−170 C) and proceeds with a low yield (~ 4 %) in order to achieve a selectivity of
6
h,k
ca. 85%. Hence, it is of a great practical interest to develop a more efficient, as well as easily
synthesizable catalyst for the selective cyclohexane oxidation process able to operate under milder
conditions. In addition, such catalyst should be active in a reaction medium that employs either
hydrogen peroxide or preferably dioxygen to become an attractive alternative to the currently used
industrial processes.
Thus, in this work we focused on the following aims: i) synthesis of new water soluble
AHBA ligands bearing ortho –OH and −SO
H groups, namely 5ꢀ(2ꢀ(2ꢀhydroxyphenyl)hydrazono)
3
1
pyrimidineꢀ2,4,6(1H,3H,5H)ꢀtrione (H L ) and sodium salt of 2ꢀ(2ꢀ(2,4,6ꢀtrioxotetrahydroꢀ
4
2
pyrimidinꢀ5(2H)ꢀylidene)hydrazinyl)benzenesulfonic acid (H
4
L ), and their application in the
II
II/III
II
preparation of new water soluble Co , Co
and Cu ꢀAHBA complexes; ii) study of the
physicochemical and electrochemical properties of the ligands and complexes; iii) application of the
complexes as catalyst precursors for the peroxidative oxidation of cyclohexane in aqueous
acetonitrile medium, under mild conditions.
Results and Discussion
1
I
II
II/III
II
Synthesis and characterization of H
4
L , Na salt and Co , Co
and Cu complexes
The new AHBAs, namely 5ꢀ(2ꢀ(2ꢀhydroxyphenyl)hydrazono)pyrimidineꢀ2,4,6(1H,3H,5H)ꢀ
1
trione (H
4
L ) and sodium salt of 2ꢀ(2ꢀ(2,4,6ꢀtrioxotetrahydropyrimidinꢀ5(2H)ꢀylidene)hydrazinyl)
2
2
7
4 3 2 2 2 2
benzenesulfonic acid (H L ), [Na(H L )(ꢁꢀH O)(H O) ] (1), were prepared by a modified known
aqueous diazotization of 2ꢀaminophenol and 2ꢀaminobenzenesulfonic acid and subsequent coupling
1
with barbituric acid in basic medium, respectively (Scheme 2). H
4
L and 1 are highly soluble in
DMSO, DMF, water, methanol, acetone and acetonitrile. Both compounds are stabilized in DMSOꢀ
2,7d
1
1
d6 solution in the hydrazone form, as reported analogues. In fact, HꢀNMR spectra of H L and 1
4
6
in DMSOꢀd solution at room temperature show only one signal at δ 14.44 and 14.80, respectively,
assigned to the proton of the protonated nitrogen atom adjacent to the aryl unit (=N−NH−
13
hydrazone form). In accord, two peaks of the carbonyl groups in the C NMR spectra (see
Experimental) indicate that one of these groups undergoes a shift due to hydrogen bonding of the
carbonyl moiety with the hydrazone NH group. This is in agreement with the low field chemical
shift (14.44 and 14.80 ppm) of the hydrazone NH proton and is also consistent with IR data: the
4

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1
stretching bands ν(C=O) and ν(C=OꢁꢁꢁH) are at 1697 and 1643 (for H L ), 1677 and 1604 (for 1)
4
−1
2,7d
cm , respectively, the latter being shifted on account of the Hꢀbond. Elemental analysis and the
+
+
1
2 4
ESIꢀMS peaks at 249.2 [Mr+H] and 702.1 [Mr−4H O+H] support the formulation of H L and 1,
respectively (Figures 1S and 2S). Moreover, Xꢀray diffraction analysis of 1 is demonstrate that it is
stabilized in the hydrazone form (see below).
1
II
II
Reaction of H
4
L with Cu or Co (nitrate salts) in the presence of imidazole (im) and
1
triethylamine (Et N) in acetoneꢀwater mixture (4:1, v/v) lead to [Cu(H L )(H O)(im)]ꢁ3H O (2) and
3
2
2
2
1
[
Co(H
2
6
O) ][Co(H
2
L )
2
]
2
ꢁ8H
2
O (3), respectively (Scheme 2, see Experimental for details). The
1
added base (im or Et
3
N) conceivably weakens the hydrogenꢀbonded system in H
4
L and its
reactivity increases towards N−H deprotonation and coordination to the metal ions. Both complexes
are characterized by elemental analysis, ESIꢀMS spectra, IR spectroscopy and single crystal Xꢀray
diffraction (see below). In the IR spectra of 2 and 3, the ν(C=O) and ν(C=N) signals appear at 1714
–1
and 1586, 1719 and 1594 cm , respectively, values that are significantly shifted in relation to the
1
corresponding signals of H
4
L (1731 and 1697 ν(C=O), 1597 ν(C=N)). Mass spectrometry of 2 and
+
3
dissolved in methanol shows parent peaks at m/z = 310.7 [C H CuN O +H] (for 2) and 167.0
10
6
4
4
2
+
ꢀ
[
Co(H
2
O)
6
] , 551.2 [C20
H
8 8
12CoN O ] (for 3) (Figures 3S and 4S). Elemental analyses are also in
agreement with the proposed formulations, which are also supported by Xꢀray crystallography.
HN
OH2
N
H O/acetone
2
O
Cu2+
Cu
O
C
O
3
H O
H
N
HN
C
C
C
N
2
O
N
HO
HN
.
..H
N
O
C
O
2
HN
C
C
N
O
C
N
O
-
N
C
HN
H4L1
C
N
NH
C
C
O
O
N
H
O
H O/acetone
CoIII
2
II
2+
H
N
8H O
2
[
Co (H O) ]
2
6
O
O
N
Et N, Co2+
3
O
C
C
HN
C
N
C
O
2
3
Scheme 2. Synthesis of 2 and 3.
Reactions of Co(NO
3
) ꢁ6H
2
2
3 2 2
O and Cu(NO ) ꢁ2.5H O with 1 in waterꢀacetone mixture (1/3,
II
II
2
v/v), in the presence of im led to the Co and Cu complexes [Co(H L )(im) ] (4) and
2
3
2
II
[
[
Cu(H
2
2
L )(im)
2
]ꢁH
2
O
(5), respectively (Scheme 3). The Co
salt of AHBA,
2
Co(H
O)
6
][H
3
L ]
2
ꢁ8H
2
O (6), was obtained by mixing (stirring for 10 min) of an acetoneꢀwater
mixture (4:1, v/v) solution of 1, Et N and Co(NO ) ꢁ6H O at 80 °C (Scheme 3).
3
3 2
2
5

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Scheme 3. Synthesis of 4−6.
All the isolated compounds, 4−6, were characterized by elemental analysis, ESIꢀMS, IR
spectroscopy and single crystal Xꢀray diffraction. In the IR spectra of 4, 5 and 6, the ν(C=O) signal
–1
appears at 1664, 1633 and 1675 cm , respectively, values that are significantly shifted in relation to
–1
3
the corresponding signal of 1 (1677 cm ). The ESI mass spectra of 4, 5 and 6 dissolved in CH OH
2
+
2
+
show the peaks at m/z = 574.0 [{Co(im)
3
H
2
L }+H] , 510.0 [{[Cu(im)
2 2 2 2
H L ]ꢁH O}−H O+H] , and
2
+
6
82.1 [Co(H L )
3 2
+H] (Figures 5S−7S). Elemental analyses are consistent with the proposed
formulations which are also supported by Xꢀray crystallography (see below). All the obtained
complexes are soluble in water, methanol, DMF and DMSO, but later one is the best solvent for
them.
The wavelength (λmax) and molar absorption values (ɛ) of all compounds in the UVꢀVis
region 200−700 nm have been recorded in water solution (Table 1S and Figure 8S). In the spectra
1
of H L and 1−6 the bands at 234−270 nm are due to π–π* electronic transition, while the bands at
4
7b
3
52−482 nm are mainly of n−π* type.
Xꢀray diffraction analyses
Crystals of 1−6 suitable for Xꢀray diffraction analysis were obtained upon crystallization from
waterꢀacetone mixtures (1:4, v/v) (the schematic representations of their formulae are shown in
Schemes 2 and 3). The structure of 1 (Figure 9S, Supplementary Information) will not be discussed
6

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since it is a preliminary one (78% completeness) and no further suitable crystals of the compound
were obtained. Representative plots of 2−6 are depicted in Figure 1. Crystallographic data and
refinement parameters are given in Tables 2S and 3S. Hꢀcontacts for 2−6 are outlined in Table 4S
and depicted in Figures 10S).
General aspects of the structures
n
In compound 7 the L species cocrystalized with hexaaquacobaltate cation, apart from water
molecules, and can be roughly planar as measured by the angles between the least square plane of
the heteronuclear ring (plane H) and the least square plane of the phenyl ring (plane P; Table 4S).
In the mononuclear structures 2–5 an Nꢀhydrazone atom, a phenoxyl (in 2 and 3) or a
n
sulfonyl (in 4 and 5) Oꢀatom and an additional Oꢀatom from the pyrimidine trione moiety of the L
n
ligands are involved in the chelation. The L ligands are almost planar in the structure of 2 as
measured by the angle of 2.01º (2) between planes H and P (see above; Table 4S). However, they
are considerably twisted in 4 and 5 with angles of 34.49 and 38.46º, respectively, involving those
planes; in 3 that parameter assumes intermediate values (8.61 and 11.42º).
In all the structures the bond distances of 1.202(8)–1.308(4) Å for the N−N hydrazone
groups suggest a double bond character.
Specific aspects of the structures
1
2−
In complex 2 one (H L ) anion, one water and one imidazole molecule fulfil a square
2
8a
II
pyramidal geometry (τ = 0.02) around the Cu cation, the hydrazone and the imidazole ligands
5
occupying the equatorial positions and the water the axial site. This complex shows a considerably
elongated axial Cu−Owater length [2.545(4) Å] relative to the other metalꢀoxygen distances [1.921(4)
8
a
and 1.925(4) Å] suggesting a tetragonal elongation, according to Addison’s criteria, of av. 32%
probably arising from different donor properties of the coordinated Nꢀ and Oꢀatoms or from steric
constrains from the ligands.
III
III
–
II
The structure of 3 comprises two Co (anionic part {[C20
H
12Co N
8
O
8
] }
2
) and one Co
2
+
(
(
[
cationic part [Co(H
2
O)
2
L ) anions and the other six coordinated water molecules (Scheme 2). For the anionic part of 3,
6
] ) complex units, one of them involving two ONO chelating hydrazone
1
2−
H
2
+
Co(H
2
6
O) ]
is counter ion rendering the compound neutral. Both metal centres show slightly
distorted octahedral environments as expressed by the octahedral quadratic elongation (OQE)
2
values of 1.002 and 1.001, and octahedral angle variations (OAV) of 6.03 and 1.47 º ,
8
b
respectively. The hydrazone ligands in 3 are oriented in such a way that the two pairs of M−O
bonds are mutually trans, as well as the pair of M−N bonds.
II
2 2–
The Co coordination sphere in 4 comprises one (H L ) anion and three imidazole ligands
2
accomplishing a distorted octahedral environment around the cobalt cation (OQE and OAV of
2
1
.008 and 25.55º , in this order). The Co−O bond lengths comprising the hydrazone derivative
assume values of 2.104(5) and 2.195(4) Å and concern the Oketone and Osulfonyl, respectively; the Coꢀ
7

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N distance, in turn, take a value of 2.118(5) Å. The Co−N distances concerning the imidazole
ligands in 4 range from 2.110(5) to 2.134(6) Å, the longer one conceivably resulting from a trans
effect of the coordinated sulfonyl O atom of the hydrazone ligand. The molecules of 4 are
associated into infinite 1D chains along the crystallographic b axis by means of intermolecular N–
H⋅⋅⋅O interactions involving one of the imidazole groups and one of the nonꢀcoordinated sulfonyl
Oꢀatoms (Figure 10S and Table 4S). The intermolecular metal⋅⋅⋅metal distance in 4 (8.608 Å) is
considerably longer than that found in 3 (5.653 Å).
II
8
The Cu cation in 5 presents a square pyramidal geometry (τ
5
= 0.02), the equatorial plane
2
2–
being occupied by the Oketoneꢀ and Nꢀatoms from (H
2
L ) , as well as two Nꢀatoms from two
imidazole ligands; the axial site is engaged with one of the Osulfonyl atoms from the hydrazone. As
found in 2, the coordination distance to the axial atom is longer than those to the equatorial atoms,
8
a
although the tetragonal elongation is shorter (avg. 16%). The intermolecular metal⋅⋅⋅metal distance
in 5 (6.603 Å) is slightly longer to that found in 2 (5.590 Å).
4
,7d
The structure of 6 also contains threeꢀcentred RAHB interactions
in the anionic
hydrazone units involving the Oketoneꢀ and Osulfonylꢀatoms, which relates to the synergistic mutual
reinforcement of intramolecular hydrogen bonding due to πꢀelectron delocalization. The typical
double bond lengths of Cpyrimidine−Nhydrazone in the structure of 6 [1.314(3) Å] and the formation of
the intramolecular N−HꢁꢁꢁO RAHB contacts indicate that hydrazone is the preferred tautomeric
form in these cases.
The structures of this work contain trapped water molecules (except 4) which interact
among themselves and with the metalꢀorganic (2, 3 and 5) hosts via intermolecular hydrogen bonds.
Such interactions, together with others involving the Oꢀ and Nꢀatoms of pyrimidine trione rings as
well as the coordinated water molecules (in 2, 3 and 6; Figure 10S), give rise to intricate 3D
supramolecular frameworks.
2
3
8

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4
5
6
Figure 1. Xꢀray molecular structures of 2−6 with atom numbering schemes. In 3 the
II
hexaaquacobalt cation and the crystallization water molecules are omitted for clarity. Symmetry
operations to generate equivalent atoms: i) 1ꢀx, ꢀy, ꢀz (6).
1
Electrochemical behaviour of H
4
L and 1−6
In view of the application of 2−6 as homogeneous catalysts in oxidation processes in
1
acetonitrile solution, the redox properties of H
4
L and 1−6 were studied by cyclic voltammetry
(CV) and controlled potential electrolysis (CPE) at a platinum working electrode at 25 ºC in a 0.2 M
n
1
I
[
Bu
4
N][BF
4
4
]/NCMe solution. H L and the Na complex 1 revealed to be redox active (Table 1,
I
ox
Figures 11S and 12S), exhibiting, by CV, an irreversible oxidation wave at E
p
1.2 V vs. SCE and
I
red
an irreversible reduction wave ( E
p
) at ca. ꢀ0.9 to ꢀ1.1 V vs. SCE. Whereas the oxidation process
appears not be affected by the nature or position of the substituents of the aromatic ring, the
reduction one is more sensitive (a difference of 0.16 V vs. SCE is observed, Table 1). The redox
II
processes at 1 are centred on the ligands. The copper complexes 2 and 5 exhibit, besides the redox
I
ox
I
red
processes centred on the ligands ( Ep and E , Table 1), a twoꢀelectrons (as measured by CPE)
p
II red
II
irreversible cathodic wave at E
p
−1.56 V or −1.67 vs. SCE, respectively, assigned to the Cu →
0
9a,b
Cu reduction (Figure 13S).
The CPE performed at these potentials lead to the deposition of
ox
metallic copper which is clearly detected by its sharp oxidation (with desorption) wave at E
p
=
−0.09 (2) (Figure 13S) or −0.11 (5) V vs. SCE. For the Co complexes 3, 4 and 6, the irreversible
I
ox
II ox
oxidation waves ( E
p
and E
p
, Table 1) are conceivably centred on the organic ligands, as well as
9

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the irreversible reduction waves in the range of −0.8 to −1.0 V vs. SCE. However, in 3, the extra
I
red
½
III
II
reversible reduction wave at E
= −0.21 V is assigned to the Co → Co reduction of the
III
1
−
[
(
Co (H
2
L )
2
] anionic complex part (see Figure 14S). Those potentials are much lower than that
I
ox
II
III
E
p
=1.63 V, Table 1) observed for the Co → Co oxidation in the salt Co(NO ) salt and also in
3 2
9c
9d
mononuclear Co complexes bearing Cꢀscorpionate or pyrazole
ligands (1.03 to 1.52 V vs.
9d
SCE).
a
1
Table 1. Cyclic voltammetric data for compounds H
4
L and 1−6.
I
ox
II ox
I
red
I
II red
Compound
Ep
Ep
ꢀ
Ep ( E )
Ep
½
1
H
4
L
1.22
1.21
0.78
0.92
0.82
1.04
1.11
1.63
ꢀ0.93
ꢀ1.09
ꢀ0.82
(ꢀ0.21)
ꢀ1.14
ꢀ1.16
ꢀ1.02
ꢀ
ꢀ
ꢀ
1b
2
ꢀ
ꢀ
ꢀ1.56
ꢀ0.83
ꢀ
ꢀ1.67
ꢀ
ꢀ
3
4c
5
1.08
1.13
ꢀ
6
1.19
ꢀ
d
3 2 2
Co(NO ) ꢁ6H O
a
Potential values in Volt ± 0.02 vs. SCE, in a 0.2 M [Bu
4
N][BF
4
]/NCMe solution, at a Pt disc
5
0/+
ox
working electrode determined by using the [Fe(η ꢀC
SCE ) as internal standard at a scan rate of 200 mVs . An anodic (with desorption) wave at E
=
desorption) wave at E
wave. For comparison purposes.
5
H
5
)
2
]
redox couple (E1/2 = 0.42 V vs.
1
0
ꢀ1
b
ox
p
c
ꢀ0.09 V is generated upon scan reversal following the second reduction wave. An anodic (with
ox
p
= ꢀ0.11 V is generated upon scan reversal following the second reduction
d
Catalytic Activity of Metal Complexes for Cyclohexane Oxidation
1
Compounds 1−6 and the proligand H
4
L were tested as catalysts for the peroxidative (with
aqueous hydrogen peroxide) oxidation, at room temperature, of cyclohexane (CyH) to the mixture
of cyclohexyl hydroperoxide (CyOOH, primary product) with cyclohexanol (CyOH) and
1
cyclohexanone (CyꢀH=O) (final products) (Scheme 4). As expected, the nonꢀmetallic species H
4
L ,
as well as the sodium compound 1, were not able to catalyse the above reaction in the tested
conditions. The compounds 2−6 act as catalyst precursors for such an oxidation, 2 and 3 being the
most active ones (Table 2). Thus, 2 and 3, derived from ortho–OH AHBA ligand, show high
3
activity in comparison to other complexes from its ortho–SO H substituted version. The catalytic
II
II
3
activity of Co complexes (4 and 6) is higher than of Cu one (5) derived from same ortho–SO H
2
substituted ligand, H L . Nevertheless, the catalytic performances of 4 and 6 are different (11 and
4
8
% correspondingly, entries 5 and 9). Such differences possibly reflect the different coordination
II
environment of Co and labilities of the coordinated imidazole and water ligands in those
II
complexes. The easy reduction of the Cu complex 2 detected by cyclic voltammetry (Table 1) can
also have a favourable effect on its cataltytic activity.
1
0

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Scheme 4. Oxidation of cyclohexane catalysed by 2−6.
In fact, after 6 h stirring the reaction mixture (CyH, H O 30% aq. and the copper or cobalt
2
2
catalyst 2 or 3, respectively, in MeCN) at room temperature, a 21% overall yield (relative to the
cyclohexane) of cyclohexanol and cyclohexanone was obtained with an overall turnover number
(
TON) of 213 (3) (71 per Co atom) or 10 (2) moles of products per mole of catalyst precursor
entries 3 and 19, respectively, Table 2).
(
The formation of CyOOH (under the conditions of Table 2) is proved by using the method
6
d,10
proposed by Shul'pin.
3
The addition of PPh prior to the GC analysis of the products results in a
marked increase of the amount of cyclohexanol (compare entries 1 and 11, Table 2) due to
reduction of CyOOH by PPh , with formation of phosphane oxide, and a corresponding decrease of
cyclohexanone, as observed in other catalytic systems.
3
1
1
The selectivity towards the cyclohexanol + cyclohexanone mixture is rather high, even for
the high yields (e.g. 96 % selectivity for 21 % yield, entry 3, Table 2). This is much better than what
6
a,h,12
obtained in the current industrial process (ca. 4 % yield for ca. 85 % selectivity),
in spite of the
II
mild reaction conditions used in our case. Our yields are similar to reported for dinuclear Cu
11a
II 11c
III 9c,11f,
complexes (22ꢀ29%) or scorpionate Cu
in Table 2, Cu(NO , Co(NO ), [Cu(im) ](NO
total yield (entries 23–26), respectively. The high catalytic activity of 3 is expected to be due to its
or Co
complexes (up to 23%). As can be seen
3
)
2
3
4
3
)
2
and [Co(im)
6
](NO provide only 4, 7, 8 and 9 %
3 2
)
III
–
2+
anionic part [C H Co N O ] since the cationic part [Co(H O) ] can also be derived from
2
0
12
8
8
2
6
Co(NO
3
)
2
in solution, which provides low yield (entry 24, Table 2).
The catalytic performance of 2 and 3 under the same conditions, but replacing the oxidant
by dioxygen, was assessed. It was found a negligible conversion of cyclohexane into the
oxygenated products, thus suggesting the need of a peroxide oxidant, such as hydrogen peroxide,
for this mild oxidative process.
1
1

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a
Table 2. Peroxidative oxidation of cyclohexane with H O (selected data) catalysed by 2−6.
2
2
b
c
Entry
Catalyst
n(Cat)/n(CyH) n(H
2
O
2
)/n(Cat.)
−3
Yield (%)
TON CyOH/ Select.
d
3
Cy
=O
ꢀ
(%)
×
10
× 10
CyOH CyꢀH=O
e
H
Total
ratio
1f
2
2
2
3
3
4
4
5
5
6
6
2
2
2
2
2
2
2
2
2
2
2
2
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0.2
3
10
20
1
1
1
1
1
2
2
2
2
2
2
2
2
2
15.7 0.5 16.2 162
31.4
1.0
22.7
4.0
1.0
17.0
1.1
6.3
2.1
ꢀ
0.6
0.9
1.9
1.6
1.6
1.5
1.8
1.4
7.2
1.3
1.6
2.4
2.4
1.8
1.3
1.3
98
96
96
89
91
94
89
94
98
93
64
89
91
89
41
93
88
84
83
39
56
99
39
61
73
84
0.1 0.1 0.2
20.4 0.9 21.3 213
2.8 0.7 3.5 35
5.2 5.3 10.5 105
2
3f
4
5
f
6
1.7 0.1 1.8
2.9 2.7 5.6
1.9 0.3 2.2
5.5 2.6 8.1
18
56
22
81
2
7f
8
9
f
1
0
2
2
0.2
0
0.2
g
1
1
5.3 9.4 14.7 147
12
0.1
0.2
1
10
2
2
2
2
2
2
2
2
2
2
2
1.1 1.2 2.3
5.4 2.9 8.3
23
83
1
1
1
1
1
1
1
3
4
5
6
7
8
9
8.6 5.3 13.9 139
4.2 2.7 6.9
3.1 2.1 5.2
12.2 6.7 18.9
11.7 8.3 20.0
18.1 2.5 20.6
0.8 0.6 1.4
1.9 1.2 3.1
69
260
63
20
10
h
2
0
14
31
i
j
2
2
1
2
11.2 4.6 15.8 158
2
2
2
2
3
Cu(NO
Co(NO3)2
3
)
2
2.9 1.2 4.1
4.7 2.6 7.3
4.3 3.3 7.6
5.2 3.9 9.1
41
73
76
81
4
5
6
[Cu(im) ](NO )
1
1
4
3 2
[Co(im) ](NO )
6
3 2
a
Reaction conditions (unless stated otherwise): MeCN (3 mL), cyclohexane (5.0 mmol), 0.5 ꢀ 20
mol of 2−6, H (10.0 mmol), 6 h, r.t.; yield and TON determined by GC analysis (upon
ꢀ
2 2
O
b
treatment with PPh
3
). Molar yield (%) based on substrate, i.e. moles of products (cyclohexanol
c
(CyOH) and cyclohexanone (CyꢀH=O)) per 100 mol of cyclohexane. Turnover number (moles of
d
product per mol of catalyst precursor). Selectivity (moles of cyclohexanol and cyclohexanone per
mole of converted substrate). Moles of cyclohexanol + cyclohexanone per 100 moles of
cyclohexane. Reaction in the presence of HNO [n(HNO )/n(catalyst) = 30]. Without addition of
PPh
e
f
g
3
3
h
i
3
. Reaction in the presence of CBrCl
3
(5.0 mmol). Reaction in the presence of Ph
2
NH (5.0
j
mmol). Reaction performed under nitrogen atmosphere.
11d,f,13
The previously recognized common promoting effect of an acidic medium
on the
peroxidative oxidation of alkanes is not observed for the present systems (Table 2). On the contrary,
the presence of nitric acid has a drastic inhibiting effect on the catalytic activity of all the
compounds tested (Figure 2). Due to formation, in acidic medium, of stable a RAHB system in
ligand molecules, complexes are not stable, i.e., protonation of ligands breaks down the active
species (complex molecules) and therefore catalytic activity decreases. A similar behaviour was
1
2

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III
II
14
found for Cu complexes bearing azathia macrocyles and also for Cꢀscorpionate Au
15
complexes.
Figure 2. Catalytic activity of 2−6 for the oxidation of cyclohexane. Effect of the presence of nitric
acid on the overall yield (cyclohexanol + cyclohexanone) using 2−6 as catalysts.
The effect of the peroxide to catalyst molar ratio was also investigated (entries 1, 12ꢀ15,
2 2
Table 2) and is depicted in Figure 3. The increase of the peroxide amount up to n(H O )/n(catalyst)
3
molar ratio of 2 × 10 leads to the maximum product yield and TON (Table 2, entry 1, Figure 4).
4
Further increase of the oxidant amount (up to 1 × 10 ) results in a strong yield drop (entry 15)
eventually due to overoxidation reactions. In fact, for this oxidant to catalyst molar ratio, 1,4ꢀ
cyclohexanedione was detected by CGꢀMS as the main overoxidation product (3.2% relative to
cyclohexane). 1,3ꢀcyclohexanediol, 1,4ꢀcyclohexanediol, 1,4ꢀhydroxycyclohexanone, and 1,2ꢀ
epoxycyclohexane were also detected, but in much lower amounts (1.4, 1.1, 0.3 and 0.2%,
respectively, relative to cyclohexane).
Figure 3. Effect of the molar H
2 2
O to catalyst ratio on the overall yield (cyclohexanol +
cyclohexanone) using 2 as catalyst precursor.
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3

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Figure 4. Effect of the molar catalyst to cyclohexane ratio on the overall yield (cyclohexanol +
cyclohexanone) using 2 as catalyst.
Figure 5. Dependence of the overall yield (mol%, based on substrate) of the products on the
reaction time. Reaction conditions: MeCN (3.0 mL), cyclohexane (5.0 mmol), 5 ꢂmol of 2 or 3
3
n(H
2
O
2
)/n(catalyst precursor) (2 × 10 ), r.t..
The activities of our catalytic systems are also dependent on the reaction time. Yields tend to
increase on the extension of reaction time from up to 6 h (Fig. 5) whereafter a further increase of
reaction time results in a yield drop due to subsequent reactions. In fact, overoxidation products
such
as
1,3ꢀcyclohexanediol,
1,4ꢀcyclohexanediol,
1,4ꢀhydroxycyclohexanone,
1,4ꢀ
cyclohexanedione or 1,2ꢀepoxycyclohexane (e.g., 1.1, 0.8, 0.5, 0.9 or 0.3%, respectively, relative to
cyclohexane, for 3) were detected by CGꢀMS for 9 h reaction time.
A noteworthy feature of the studied oxidations is the use of low catalyst loadings (typically
0
.1 mol% vs. substrate) with high yields of oxygenated products (Table 2) considering the inertness
of the substrate.
As observed for several metal catalytic systems,
NH) to the reaction mixture (Table 2, entries 20 and 21, respectively) results in a considerable
suppression of the catalytic activity. This behaviour, along with the formation of cyclohexyl
11,13,14
3
addition of a radical trap (CBrCl or
Ph
2
1
4

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hydroperoxide (CyOOH, typical intermediate product in radicalꢀtype reactions) suggests a freeꢀ
radical mechanism for cyclohexane oxidation, which will be further investigated.
Conclusions
In this study we have successfully used AHBA chelating ligands bearing barbituric acid
fragment, as well as –OH and –SO
3
H groups in orthoꢀposition of the aromatic part, and in several
cases, imidazole (as an auxiliary ligand or a deprotonating agent for the weakening/destroying
II
II/III
II
RAHB system in AHBA), to synthesis novel mononuclear Co , Co
and Cu complexes. AHBA
ligands play a crucial structural role in the organization of the Hꢀbonded assemblies, influencing the
overall 3D supramolecular arrangement (also depending on the metal ions) and imparting water
solubility to the complexes.
II
II/III
The Cu and Co
derivatives 2 and 3, respectively, display a high catalytic activity for the
peroxidative (with H
conditions.
2 2
O ) oxidation of cyclohexane in aqueous acetonitrile medium, under mild
Experimental
Materials and instrumentation
1
13
The H and C NMR spectra were recorded at room temperature on a Bruker Avance II +
TM
3
00 (UltraShield Magnet) spectrometer operating at 300.130 and 75.468 MHz for proton and
carbonꢀ13, respectively. The chemical shifts are reported in ppm using tetramethylsilane as the
−1
internal reference. The infrared spectra (4000–400 cm ) were recorded on a BIOꢀRAD FTS
000MX instrument in KBr pellets. Carbon, hydrogen, and nitrogen elemental analyses were
3
carried out by the Microanalytical Service of the Instituto Superior Técnico. Electrospray mass
spectra (ESIꢀMS) were run with an ionꢀtrap instrument (Varian 500ꢀMS LC Ion Trap Mass
Spectrometer) equipped with an electrospray ion source. For electrospray 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 solution. The compounds were
observed in the negative or positive mode (capillary voltage = 80–105 V). The UVꢀvis absorption
−1
spectra in the 200 – 700 nm region were recorded with a scan rate of 240 nmꢁmin by using a
Lambda 35 UVꢀvis spectrophotometer (PerkinꢀElmer) in 1.00 cm quartz cells at room temperature,
1
−5
4
with a concentration of H L and 1−6 of 2.00ꢁ10 M in water. Chromatographic analyses were
undertaken by using a Fisons Instruments GC 8000 series gas chromatograph with a DBꢀ624 (J&W)
capillary column (FID detector) and the JascoꢀBorwin v.1.50 software. The internal standard
method was used to quantify the organic products. The electrochemical experiments were
performed on an EG&G PAR 273A potentiostat/galvanostat connected to a personal computer
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5

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through a GPIB interface. Cyclic voltammetry (CV) studies were undertaken in 0.2 M
n
[
Bu
4
N][BF ]/MeCN, at a platinum disc working electrode (d = 0.5 mm) and at room temperature.
4
Controlledꢀpotential electrolyses (CPE) were carried out in electrolyte solutions with the aboveꢀ
mentioned composition, in a threeꢀelectrode Hꢀtype cell. The compartments were separated by a
sintered glass frit and equipped with platinum gauze working and counter electrodes. For both CV
and CPE experiments, a Luggin capillary connected to a silver wire pseudoꢀreference electrode was
used to control the working electrode potential. A Pt wire was employed as the counterꢀelectrode
for the CV cell. The CPE experiments were monitored regularly by CV thus assuring no significant
2
potential drift occurred along the electrolysis. The solutions were saturated with N by bubbling this
gas before each run, the redox potentials of the complexes were measured by CV in the presence of
ferrocene as the internal standard, and their values are quoted relative to the SCE by using the
5
0/+
ox
/2
9
[
Fe(η ꢀC
5
H
5
2
) ]
redox couple (E1 = 0.42 V vs. SCE).
1
Syntheses of H
4
L and 1
The arylhydrazones of barbituric acid, 5ꢀ(2ꢀ(2ꢀhydroxyphenyl)hydrazono)pyrimidineꢀ
1
2
,4,6(1H,3H,5H)ꢀtrione (H
4
L ) and sodium salt of 2ꢀ(2ꢀ(2,4,6ꢀtrioxotetrahydropyrimidinꢀ5(2H)ꢀ
2
2
4 3 2 2 2 2
ylidene)hydrazinyl) benzenesulfonic acid (H L ), [Na(H L )(ꢁꢀH O)(H O) ] (1), were synthesized
2,7
according to the JappꢀKlingemann reaction between the aromatic diazonium salt of substituted
aniline and barbituric acid.
Diazotization: 2ꢀsubstituted aniline (20 mmol) was dissolved in 50 mL water, and 0.40 g (10
mmol) of NaOH was added. The solution was cooled in an ice bath to 273 K and 1.38 g (20 mmol)
of NaNO were added; 4.00 mL HCl were then added in 0.5 mL portions for 1 h. The temperature
2
of the mixture should not exceed 278 K.
Azocoupling: NaOH (0.80 g, 20 mmol) was added to a mixture of 20 mmol (2.56 g) of barbituric
acid with 50.00 mL of water. The solution was cooled in an ice bath, and a suspension of 2ꢀ
substituted aniline diazonium (prepared according to the procedure described above) was added in
two equal portions under vigorous stirring for 1 h. The formed precipitate of the ligand was filtered
off, recrystallized from methanol and dried in air. The characterizations of the ligands were carried
1
13
out by IR, H and CꢀNMR spectroscopies and are given below.
1
H
4
L : Yield, 3.72 g, 75 % (based on barbituric acid), orange powder soluble in DMSO, DMF,
water, methanol, acetone and acetonitrile. Anal. Calcd for C10
H
8
N
4
O
4
(Mr = 248.05): C, 48.39; H,
+
3
.25; N, 22.57; found: C, 48.07; H, 3.11; N, 22.32 %. ESIꢀMS: m/z: 249.2 [Mr+H] . IR (KBr): 3453
−1
ν(OH), 3041, 2844 and 2736 ν(NH), 1731 and 1697 ν(C=O), 1643 ν(C=OꢁꢁꢁH), 1597 ν(C=N) cm .
1
H NMR (300.130 MHz) in DMSOꢀd
H, O−H), 11.23 (s, 1H, N−H), 11.44 (s, 1H, N−H), 14.44 (s, 1H, N−H). C{ H} NMR (75.468
MHz, DMSOꢀd
). δ: 115.10, 115.94, 117.79 and 120.09 (Ar−H), 126.82 (Ar−NHN=), 129.09
C=N), 146.63 (Ar−OH), 149.86 and 160.01 (C=O), 162.50 (C=OꢁꢁꢁH).
6
, internal TMS, δ (ppm): 6.94−7.60 (4H, Ar−H), 10.62 (s,
1
3
1
1
6
(
1
6

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1
: Yield, 4.42 g, 57 % (based on barbituric acid), light orange powder soluble in DMSO, DMF,
water, methanol, acetone and acetonitrile. Anal. Calcd for C20 Na (Mr = 776.06): C,
0.93; H, 3.37; N, 14.43; found: C, 30.76; H, 3.23; N, 14.24 %. ESIꢀMS: m/z: 702.1
H
26
N
8
2 18 2
O S
3
+
[
Mr−4H O+H] . IR (KBr): 3452 ν(OH), 3065 and 2964 ν(NH), 1739 and 1677 ν(C=O), 1604
2
−
1 1
ν(C=OꢁꢁꢁH), 1520 ν(C=N) cm . H NMR (300.130 MHz) in DMSOꢀd
.21−7.80 (4H, Ar−H), 11.24 (s, 1H, N−H), 11.35 (s, 1H, N−H), 14.80 (s, 1H, N−H). C{ H}
NMR (75.468 MHz, DMSOꢀd ). δ: 116.21, 118.25, 125.01 and 127.59 (Ar−H), 130.53
6
, internal TMS, δ (ppm):
1
3
1
7
6
(
Ar−NHN=), 135.86 (C=N), 138.17 (Ar−SO
Na), 150.11 and 160.32 (C=O), 160.74 (C=OꢁꢁꢁH). The
3
crystals of 1 suitable for Xꢀray structural analysis were obtained by slow evaporation of a methanol
solution of the light orange powder.
Syntheses of 2−6
1
Synthesis of 2. 248 mg (1 mmol) of H
4
L were dissolved in waterꢀacetone mixture (1:4, v/v), then
1
36 mg (2 mmol) of imidazole (im) were added, under stirring. The addition of 232 mg (1 mmol) of
Cu(NO ꢁ2.5H O together with 10 mL of distilled water resulted in a clear solution. The resulting
3
)
2
2
solution was allowed to stand at room temperature and brown crystals of 2 were obtained after 1
day.
1
2
: Yield, 229 mg, 51 % (based on H
and acetonitrile. Anal. Calcd for C13
Found: C, 34.45; H, 3.95; N, 18.44%. IR (KBr): 3424 (s br.) ν(H O), 3060, 2870 and 2740 ν(NH),
4
L ). It is soluble in water, DMSO, methanol, DMF, acetone
H
6 8
18CuN O (Mr = 449.86): C, 34.71; H, 4.03; N, 18.68 %.
2
−
1
+
1
6 4 4
714 ν(C=O), 1586 ν(C=N) cm . ESIꢀMS: m/z: 310.7 [C10H CuN O +H] .
1
Synthesis of 3. 248 mg (1 mmol) of H
4
L were dissolved in waterꢀacetone mixture (1:4, v/v), then
2
00 µL of triethylamine (Et N) and 291 mg (1 mmol) of Co(NO ) ꢁ6H O were added at room
3 3 2 2
temperature. The mixture was stirred for 30 min and left standing for slow solvent evaporation.
Red crystals of 3 suitable for Xꢀrays started to form in the reaction mixture after 1 day at room
temperature; after 2 d they were filtered off and dried in air.
3
: Yield, 282 mg, 60 % (based on Co). It is soluble in water, DMSO, methanol, DMF, acetonitrile
and acetone. Anal. Calcd for C40 30 (Mr = 1413.73): C, 33.98; H, 3.71; N, 15.85 %.
Found: C, 33.66 H, 3.80; N, 15.67%. IR (KBr): 3451 (s br.) ν(H O), 3073, 2875 and 2733 ν(NH),
3 16
H52Co N O
2
−
1
2+
ꢀ
1
719 ν(C=O), 1594 ν(C=N) cm . ESIꢀMS: m/z: 167.0 [Co(H
Syntheses of 4 and 5. 776 mg (1.0 mmol) of 1 was dissolved in 15 mL waterꢀacetone mixture (1:4,
v/v), then 291 mg (1.0 mmol) of Co(NO ꢁ6H O (232 mg of Cu(NO ꢁ2.5H O in the case of 5) and
04 mg (3.0 mmol) im (136 mg im in the case of 5) were added. The mixture was stirred for 5 min
2 6 8 8
O) ] and 551.2 [C20H12CoN O ] .
3
)
2
2
3
)
2
2
2
at 80 °C, then left for slow evaporation. Black (green in the case of 5) crystals of the product started
to form after ca. 2 d at room temperature; they were then filtered off and dried in air.
4
: Yield, 298 mg, 52 % (based on Co). It is soluble in water, DMSO, methanol and DMF. Anal.
Calcd for C19 S (Mr = 573.41): C, 39.80; H, 3.16; N, 24.43 %. Found: C, 39.74; H,
H18CoN10O
6
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7

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3
.05; N, 24.32%. IR (KBr): 3380, 3220 and 2801 ν(NH), 1738 and 1664 ν(C=O), 1589 ν(C=N)
−1
+
cm . ESIꢀMS: m/z: 574.0 [Mr+H] .
: Yield, 290 mg, 55 % (based on Cu). It is soluble in water, DMSO, methanol and DMF. Anal.
Calcd for C H CuN O S (Mr = 527.96): C, 36.40; H, 3.05; N, 21.22 %. Found: C, 36.14; H, 3.00;
5
16
16
8
7
N, 21.14%. IR (KBr): 3454 ν(OH), 3145, 3075, 2980, 2892 and 2797 ν(NH), 1720, 1633 and 1606
−
1
+
ν(C=O), 1521 ν(C=N) cm . ESIꢀMS: m/z: 510.0 [Mr−H
Synthesis of 6. 776 mg (1 mmol) of 1 was dissolved in waterꢀacetone mixture (1:4, v/v), then 200
µL of triethylamine (Et N) and 291 mg (1 mmol) of Co(NO ꢁ6H O were added and the system
2
O+H] .
3
3
)
2
2
heated to 80 °C for 10 min and left for slow evaporation. Red crystals of 7 started to form after ca. 2
d at room temperature; they were then filtered off and dried in air.
7
: Yield, 229 mg, 49 % (based on Co). It is soluble in water, DMSO, methanol, DMF, acetonitrile
and acetone. Anal. Calcd for C20 (Mr = 933.65): C, 25.73; H, 4.53; N, 12.00 %.
Found: C, 25.75; H, 4.42; N, 11.95 %. IR (KBr): 3455 ν(OH), 3064, 2872 and 2677 ν(NH), 1738,
8 26 2
H42CoN O S
−1
3
+
1
3
675 and 1580 ν(C=O), 1520 ν(C=N) cm . ESIꢀMS: m/z: 312.1 [H L +H] and 682.1
3
+
[
Co(H L ) +H] .
3 2
Xꢀray structure determinations
Xꢀray quality single crystals of the compounds were mounted in a nylon loop and measured at 150
K (1 and 3) or at room temperature (2 and 4−6). Intensity data were collected using a Bruker AXSꢀ
KAPPA APEX II diffractometer or a Bruker APEXꢀII PHOTON 100 diffractometer with graphite
monochromated MoꢀKα (λ 0.71069) radiation. Data were collected using phi and omega scans of
0
.5° per frame and a full sphere of data was obtained. Cell parameters were retrieved using Bruker
1
6a
16a
SMART software and refined using Bruker SAINT on all the observed reflections. Absorption
1
6a
corrections were applied using SADABS . Structures were solved by direct methods by using the
1
6b
16b
SHELXS package and refined with SHELXLꢀ2014. Calculations were performed using the
16c
WinGX System–Version 1.80.03. The hydrogen atoms of water, hydrazine or ammonium groups
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 oxygen or nitrogen atoms, frequently with their
distances restrained by using the DFIX and DANG commands. Coordinates of hydrogen atoms
bonded to carbon atoms were included in the refinement using the ridingꢀmodel approximation with
the Uiso(H) defined as 1.2Ueq of the parent aromatic or methylene atoms. There were disordered
molecules present in the structure of 4. Since no obvious major site occupations were found for
1
6d
those molecules, it was not possible to model them. PLATON/SQUEEZE was used to correct the
data. A total of 136 electrons were estimated but the assignment is dubious. Thus, they were simply
removed from the model. Least square refinements with anisotropic thermal motion parameters for
all the nonꢀhydrogen atoms and isotropic ones for the remaining atoms were employed. CCDC
1
055317, 1055319−1055322, 1055325 contain the supplementary crystallographic data for this
1
8

Page 19 of 21
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DOI: 10.1039/C5RA14078A
paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre
via w.ccdc.cam.ac.uk/data_request/cif.
Oxidation of cyclohexane
1
Typical reaction mixtures were prepared as follows: to 0.5−20.0 µmol of the compound H L
4
and 1−6 contained in the reaction flask were added 3 mL of MeCN, 5.00 mmol of C
6
H12 and 10
mmol of H solution (30 % in H O). In the experiments with radical traps, CBrCl (5.00 mmol)
2
O
2
2
3
or NHPh (5.00 mmol) was added to the reaction mixture. The reaction mixture was stirred for 6 h
2
at room temperature (ca. 298 K) and air atmospheric pressure, then 90 ꢂL of cycloheptanone (as
internal standard) and 5.0 mL of diethyl ether (to extract the substrate and the products from the
reaction mixture) were added. The resulting mixture was stirred for 15 min and then a sample taken
3
from the organic phase was analysed by GC. Subsequently, PPh was added to the final organic
phase (to reduce the cyclohexyl hydroperoxide), the mixture was analysed again to estimate the
10
amount of cyclohexyl hydroperoxide, according to Shul'pin's method. Blank experiments were
performed and confirmed that no cyclohexane oxidation products were obtained in the absence of
the metal catalyst precursor.
Acknowledgements
This work has been partially supported by the Foundation for Science and Technology (FCT),
Portugal (UID/QUI/00100/2013 program). The authors acknowledge the Portuguese NMR Network
(ISTꢀUTL Centre) for access to the NMR facility, and the IST Node of the Portuguese Network of
massꢀspectrometry (Dr. Conceição Oliveira) for the ESIꢀMS measurements. The authors also
acknowledge the financial support by the University of Camerino (Fondo di Ateneo per la Ricerca
2
011–2012) and Nuova Simonelli Company.
Supporting Information Available: Crystal data, experimental parameters and selected details of
1
4
the refinement calculations, cyclic voltammogram (for H L and 1−3) and electron absorption
spectra of compounds 1−6. This material is available free of charge via the Internet at
http://pubs.rsc.org/ or from the authors. CCDC 1055317, 1055319−1055322, 1055325 contain the
supplementary crystallographic data for complexes 1−6, respectively. 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, Cambridge CB2 1EZ, UK; fax: (+44) 1223ꢀ336ꢀ033;
or eꢀmail: deposit@ccdc.cam.ac.uk.
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