8
2
A.R. Silva et al. / Catalysis Today 203 (2013) 81–86
The mechanism of the reaction was attributed to be the competing
radical pathways involving hydroxyl radicals and more selective
non-radical high valent iron species pathways, as the major prob-
lem with Fenton chemistry is the lack of selectivity in alkane
oxidation [5]. This conclusion is also supported by experiments
using other systems where a radical inhibitor was added to the
reaction after which oxygenated products were still obtained [12].
One of the most active catalysts for the mild oxidation cyclo-
hexane is a copper(II) triethanolaminate complex, whose activity
is drastically accelerated by the presence of nitric acid additive (co-
catalyst) leading to 39% yield of oxygenates in a single batch based
on the substrate [4,13].
H salhd is (a) with Y H, H (Brsalhd) is (a) with Y Br, H (Mesalhd)
2 2 2
is (b) with Y CH3 and pyhd is (b).
ꢀ
H salhd,
N,N -bis(salicylaldehyde)-cyclohexanodiimine:
2
1
C20H22N O . H NMR (CDCl , 300 MHz, 297 K) ı/ppm: 13.5, 13.3 (s,
2
2
3
2H, HO), 8.4, 8.3 (s, 2H, N CH), 7.2–7.1 (m, 4H, aromatic), 6.9–6.7
(m, 4H, aromatic), 3.6, 3.3 (m, 2H, CH ), 2.0–1.5 (m, 8H, CH ). EA,
2
2
%: calculated C 74.51, N 6.88, H 8.69, experimental C 73.96, N 7.09,
−
1
H 8.68. FTIR, ꢀ/cm : 2933 m, 1629 vs, 1579 m, 1280 s. UV–vis,
ꢁmax/nm: 262, 357, 410.
ꢀ
H Brsalhd,
N,N -bis(5-bromosalicylaldehyde)-
2
1
cyclohexanodiimine: C20H20Br N O . H NMR (CDCl , 300 MHz,
2
2
2
3
297 K) ı/ppm: 13.4, 13.2 (s, 2H, HO), 8.3, 8.2 (s, 2H, N CH), 7.4–7.3
Several heterogenized metallosalen systems have been reported
as heterogeneous catalysts for the alkane oxidation [8,14–16]. To
the best of our knowledge, however, a comprehensive study is
not available of alkane oxidation at room temperature by these
transition-metal complexes in homogeneous phase and the effect
of the addition of additives to the reaction. In this context, here we
wish to present a systematic study on the catalytic activity of sus-
tainable first-row transition metal complexes (Fe, V, Cu, Co, Mn)
(m, 4H, aromatic), 6.8 (m, 2H, aromatic), 3.6, 3.3 (m, 2H, CH ),
2
−
1
2.0–1.5 (m, 8H, CH ). FTIR, ꢀ/cm : 2925 m, 1631 vs, 1567 m, 1475;
2
s, 1500 m, 1280 s, 827 m.
ꢀ
H Mesalhd,
N,N -bis(5-methylsalicylaldehyde)-
2
1
cyclohexanodiimine: C22H26N O .
H NMR (CDCl3, 300 MHz,
2
2
297 K) ı/ppm: 13.2, 13.1 (s, 2H, HO), 8.3, 8.2 (s, 2H, N CH), 7.1–6.9
(m, 4H, aromatic), 6.8 (m, 2H, aromatic), 3.6, 3.3 (m, 2H, CH ), 2.3,
2
−
1
2.2 (m, 6H, CH ), 1.9–1.6 (m, 8H, CH ). FTIR, ꢀ/cm : 2927 m, 1631
2
2
with biomimetic salen-type polydentate ligands with N O2 or N4
vs, 1589 m, 1494 s, 1280 s.
2
ꢀ
coordination sphere, in homogeneous phase, in the oxidation of
cyclohexane at room temperature, using environmentally-benign
reactants: hydrogen peroxide as the oxygen source and acetoni-
trile as the solvent. The use of catalysts based on more abundant
transition metals is also desirable as it would decrease costs and
increase sustainability. Therefore the best metal catalyst was iden-
tified and the effect of the different co-catalysts and temperature
in the reaction studied. Other oxidants, such as H O ·urea and
pyhd,
N,N -bis(2-pyridinealdehyde)-cyclohexanodiimine:
1
C18H20N . H NMR (CDCl , 300 MHz, 297 K) ı/ppm: 8.6, 8.5 (s,
4
3
2H, N CH), 8.3, 8.2 (m, 2H, aromatic), 7.9–7.8 (m, 4H, aromatic),
7.4 (m, 2H, aromatic), 3.7, 3.5 (m, 2H, CH ), 1.8–1.5 (m, 8H, CH ).
2
2
+
ESI-HRMS, m/z: calculated (C20H21N4 ) 293.17607, experimental
−
1
293.17618. FTIR, ꢀ/cm : 2927 s, 2854 s, 1644 vs, 1585 s, 1564 s,
1467 s, 1436 s, 788 m, 771 m.
2
2
iodosylbenzene, were also tested.
2.3. Synthesis of the metal complexes
The transition-metal complexes of vanadyl(IV), manganese(III),
iron(III), cobalt(III) and copper(II) were synthesized by procedures
described in the literature [10,17]; equimolar solutions of ligand
and metal salt (17 mmol) were refluxed for 1–2 h under magnetic
stirring. After precipitation of the solids they were collected by vac-
uum filtration and dried under reduced pressure during several
days. The yields were between 49 and 62% and their structure is
represented in Scheme 1c and d. The complexes were character-
ized by FTIR, UV–visible, C, N and H elemental analysis and high
2
. Materials and methods
2.1. Materials and solvents
The reagents used in the synthesis of the ligands were used as
received. In the synthesis of the ligands and corresponding com-
plexes 1,2-cyclohexanodiamine (99%), salicylaldehyde (98%), 5-
bromosalicyladehyde (98%), 2-hydroxymethylbenzaldehyde (98%),
2
-pyridinecarboxaldehyde (99%), iron(III) chloride hexahydrate
resolution mass spectrometry.
(
p.a.), manganese(II) chloride dehydrate (p.a.), vanadyl(IV) sul-
ꢀ
[
VO(salhd)], [N,N -bis(salicylaldehyde)-cyclohexanodiminate]
phate pentahydrate (97%), vanadyl(IV) acetylacetonate (95%) and
deuterated solvents were purchased from Sigma–Aldrich. Cobalt(II)
chloride hexahydrate (p.a.), copper(II) acetate monohydrate (p.a.)
and ethanol were from Panreac.
In the catalytic tests cyclohexane (≥99.5%), chloroben-
zene (≥99.5%), hydrogen peroxide 30% wt in water, hydrogen
peroxide–urea adduct (p.a.), nitric acid (p.a.) and triphenylphos-
phine (99%) were purchased from Sigma–Aldrich; acetonitrile was
HPLC grade and from Romil. n-Hexane (99.8%) was from Fisher Sci-
entific. For the FTIR potassium bromide was used spectroscopic
grade.
vanadyl(IV):
VC20H20N O .
ESI-HRMS,
m/z:
calculated
2
3
(
VC20H20N O3+) 387.09081, experimental 387.09053. EA, %:
2
calculated C 62.02, N 7.23, H 5.20, experimental C 61.63, N 7.30, H
−
1
5
.38. FTIR, ꢀ/cm : 2933 m, 1614 vs, 1311 s. UV–vis, ꢁmax/nm: 262,
3
97, 458 (i), 597, ∼650 (i).
ꢀ
[
Mn(salhd)Cl],
chloro-[N,N -bis(salicylaldehyde)-
cyclohexanodiminate]
ESI-HRMS, m/z: calculated (MnC20H20N O ) 375.08998, experi-
mental 375.08948. FTIR, ꢀ/cm : 2931 m, 1621 vs, 1598 vs, 1311
manganese(III):
MnC20H20N O Cl.
2 2
+
2
2
−
1
s, 1278 s. UV–vis, ꢁmax/nm: 264, 412, 455 (i), ∼686 (i).
ꢀ
[Fe(salhd)Cl],
chloro-[N,N -bis(salicylaldehyde)-
cyclohexanodiminate] iron(III): FeC20H20N O Cl. ESI-HRMS, m/z:
2
2
+
2.2. Synthesis of the ligands
calculated (FeC20H20N O ) 376.0869, experimental 376.0864. EA,
2 2
%
: calculated C 58.35, N 6.80, H 4.90, experimental C 58.34, N 6.76,
−
1
The synthesis of the ligands was performed according to the
H 4.99. FTIR, ꢀ/cm : 2937 m, 1625 vs, 1608 vs, 1311 s. UV–vis,
process described by Holm et al. [17]; ethanolic solutions of salic-
ylaldehyde, or its derivatives or 2-pyridinecarboxaldehyde, were
refluxed for 1–2 h with cyclohexanediamine in the proportion
of 2:1 with vigorous magnetic stirring. Tipically 0.015 mmol of
diamine were refluxed with 0.03 mmol of salicylaldehyde in 20 ml
of ethanol for 1 h. The solution was kept in the freezer overnight
and the precipitated ligand was collected after filtration under vac-
uum. All synthesized ligands were yellow and yields were between
ꢁmax/nm: 256, 475.
ꢀ
[Fe(Brsalhd)Cl],
chloro-[N,N -bis(5-bromosalicylaldehyde)-
cyclohexanodiminate] iron(III): FeC20H18N O Br Cl. ESI-HRMS,
m/z: calculated (FeC2 H18N O Br
531.90549. FTIR, ꢀ/cm : 2937 m, 2859 m, 1644 vs, 1625 vs, 1625
vs, 1608 vs, 1460 s, 1384 s, 1314 s. UV–vis, ꢁmax/nm: 266, 394, 534.
[Fe(Mesalhd)Cl], chloro-[N,N -bis(5-methylsalicylaldehyde)-
cyclohexanodiminate] iron(III): FeC22H24N O Cl. ESI-HRMS, m/z:
calculated (FeC22H24N O ) 404.11817, experimental 404.11686.
2
2
2
+
) 531.90796, experimental
0
2
2
2
−
1
ꢀ
2
2
+
5
5 and 95%. The ligand structure is represented in Scheme 1 where
2 2