A novel thermally stable hydroperoxo-copper(ii) complex in a Cu(N 2O2) chromophore of a potential N4O2 donor Schiff base ligand: Synthesis, structure and catalytic studies

Article abstract of DOI:10.1039/c3dt51359a

The generation and study of metal-hydroperoxo/metal-peroxo (LCu ^II-OOH or LCu^II-OO) complexes is a fascinating area of research of many chemical and biochemical researchers, because of their involvement as active intermediates in man

Full text of DOI:10.1039/c3dt51359a

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A novel thermally stable hydroperoxo–copper(II)
complex in a Cu(N2O2) chromophore of a potential
N4O2 donor Schiff base ligand: synthesis, structure
and catalytic studies†
Cite this: Dalton Trans., 2013, 42, 13210
Surajit Biswas, Arpan Dutta, Mainak Debnath, Malay Dolai, Kalyan K. Das and
Mahammad Ali*
The generation and study of metal–hydroperoxo/metal–peroxo (LCuII–OOH or LCuII–OO˙) complexes is a
fascinating area of research of many chemical and biochemical researchers, because of their involvement
as active intermediates in many biological and industrial catalytic oxidation processes. For this purpose
we have designed a bulky hexa-coordinating ligand with potential N4O2 donor atoms which could
provide an opportunity to synthesize a mononuclear Cu(II) complex with an aim to utilize it in the cata-
lytic oxidation of aromatic hydrocarbons by an environmentally benign oxidant, H2O2. The Cu(II) complex (1)
was structurally characterized and found to have square-planar geometry with the two pyrazolyl groups
remaining in dangling mode. A novel mononuclear complex [Et3NH][LCuII–OOH] (2) was found to form
in the reaction between 1 and H2O2 in the presence of Et3N. The presence of this dangling groups
favours the stability of hydroperoxo species, [LCu–OOH]− (2) through H-bonding with the coordinated
phenoxo oxygen atom, which was confirmed by ESI-MS+ and MS− (m/z) mass analysis and DFT calcu-
lations. This complex was found to be thermally stable at room temperature [kd = (5.67 0.03) × 10−5 s−1
at 25 °C] and may be due to the formation of O–O–H⋯O(phenoxo) H-bonding as delineated by the DFT
calculations. Complex 1 was found to be an efficient catalyst for the oxidation of aromatic hydrocarbons
to the corresponding aldehyde and alcohol in 2 : 1 mole ratio with TON ∼300.
Received 28th May 2013,
Accepted 18th June 2013
DOI: 10.1039/c3dt51359a
www.rsc.org/dalton
synthesized and investigated as model complexes.9,10 The for-
Introduction
mation of LCuII–OOH species through the abstraction of an
The generation and study of metal–hydroperoxo (LCuII–OOH) H-atom from a substrate by LCuII–OO˙ species has been
and metal–peroxo (LCuII–OO˙) complexes is a fascinating area supported by DFT calculations.8
of interest for many chemical and biochemical researchers,
Mononuclear CuII–hydroperoxo complexes9,11–21 are also
because of their involvement as active intermediates in many believed to be formed by the addition of H2O2/base to a pre-
biologically and industrially important catalytic oxidation cursor copper(II) complex. Though there are numerous reports
processes.1–3 The characterization of these thermally un- on LCu–OOH species, most of these are limited to CuI/II-com-
stable hydroperoxo intermediates is very difficult due to their plexes of tripodal N4 donor ligands, which were characterized
very short lifetimes. The mononuclear hydroperoxo4 and only in solution phase at very low temperature (∼liquid nitro-
superoxo5–8–CuII species, formed by the reaction between LCuI gen temperature) by spectroscopic means. Masuda and co-
and O2, have been proposed as reactive intermediates in the workers9 were the only ones who have been able to obtain an
aliphatic hydroxylation of substrates by many enzymes such X-ray crystal structure of a LCu–OOH complex employing a
as peptidylglycine α-amidating monooxygenase (PAM) and tripodal tetradentate ligand with additional (CO–NH) groups
dopamine β-monooxygenase (DβM) and have vigorously been as H-donor to the coordinated oxygen atom of –OOH to
stabilize the species in solid state. Thus, the chemistry of
L–CuII–(O2˙−)/L–Cu–OOH species has mostly been limited to
tripodal tetradentate N4 donor ligands such as tris(2-pyridyl-
Department of Chemistry, Jadavpur University, Kolkata 700 032, India.
E-mail: mali@chemistry.jdvu.ac.in
methyl)amine (TMPA) and analogues or derivatives of tris-
†Electronic supplementary information (ESI) available. CCDC 905948. For
(2-aminoethyl)amine, tren22–28 and extensively studied only in
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt51359a
solution.
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described the reactivity of copper–peroxo species with hydro-
carbons to produce the corresponding alcohols and acids.45–48
All these lead us to design and synthesize a molecule which
could have potential ability to catalyze hydrocarbon oxidation.
For this purpose we have designed a bulky hexa-coordinating
flexidentate ligand with potential N4O2 donor atoms which
could provide an opportunity to synthesize mono- as well as
multinuclear complexes. In the case of mononuclear copper(II)
complexes with dangling pyrazole groups there is ample possi-
bility to obtain stable hydroperoxo-species due to H-bonding
between the –OOH group and pyrazole N-atoms as delineated
by Masuda.9 Though there are numerous reports on structure–
reactivity relationship of LCu–OOH intermediates that are tran-
siently generated in the medium and mostly characterized by
spectroscopic methods, these are based on tetradentate N4
donor ligands. To the best of our knowledge this constitutes
the first report on a stable LCu–OOH− species of a tetradentate
N2O2 Schiff base ligand that can be stabilized thermally. We
report here the synthesis and crystal structure of a mono-
Scheme 1
Scheme 2
The only report by Masuda et al. gives us an idea to design nuclear copper(II) complex (1) and catalytic oxidation of aro-
a ligand in bulky environment with phenoxo oxygen atoms matic hydrocarbons by H2O2. We have selected hydrogen
which simultaneously coordinates to the metal center and pro- peroxide as the oxidant of choice due to its ability to function
vides an opportunity to form an H-bond with the H-atom of a under ambient mild conditions as well as its low cost.
coordinated –OOH group thereby stabilizing the otherwise
unstable LCu–OOH species (Scheme 1). In this research endea-
vour, we have been successful in designing such a ligand (H2L)
Experimental section
Materials and reagents
with mixed donor atoms (N4O2) and found to stabilize LCuII–
OOH through H-bonding (Scheme 2).
2-(Hydroxymethyl)-6-carbaldehyde-4-methylphenol and 2-
Soil and sediment bacteria like Pseudomonas putida is the
(chloromethyl)-6-carbaldehyde-4-methylphenol were prepared
by reported methods.49 All reagents and solvents are commer-
cially available. THF was dried using sodium metal and benzo-
phenone and other solvents were used without further
purification.
most extensively characterized catabolic plasmid that encodes
enzymes for the mineralization of toluene, m- and p-xylenes,
m-ethyltoluene and 1,3,4-trimethylbenzene29,30 in which the
methyl group at carbon 1 in the aromatic ring is oxidized to
yield the corresponding carboxylic acid, which is subsequently
oxidized in a number of steps to give pyruvate and aldehydes
as the final products.31 Chemical catalytic oxidation of hydro-
carbons to the corresponding alcohol, phenol, carbonyl or car-
boxylic acid is an extremely fascinating area of research, owing
to their extensive applications as precursors in a wide variety
of organic synthesis in the laboratory as well as in
industry.32–34 Recently it has been observed that many tran-
sition-metal complexes, in combination with various oxidizing
agents, can catalyze the oxidation of a variety of hydrocarbons
including lower alkanes.35
Physical measurements
Elemental analyses were carried out using a Perkin-Elmer 240
elemental analyzer. 1H NMR were recorded in CDCl3 on a
Bruker 300 MHz NMR Spectrophotometer using tetramethylsi-
lane (δ = 0) as an internal standard. Electronic spectra were
recorded on Agilent-8453 diode array UV-Vis spectrophoto-
meter. ESR spectra were recorded on JOEL JES-FA 200 ESR
spectrometer. ESI-MS+ and MS− (m/z) were recorded on Waters
Qtof Micro YA263 mass spectrometer.
Monomeric copper–Schiff base complexes are important in
modelling metallobioenzymes36–39 as well as from the indus-
trial perspective40 and a great amount of research attention
Synthesis of proligand 2-formyl-4-methyl-6-
[3,5-dimethylpyrazole]phenol
has been paid to understand the reactivity of metalloradicals 2-Chloromethyl-6-carbaldehyde-4-methylphenol
(1.52
g,
involved in free-radical catalysis such as GOase41–43 along with 8.2 mmol) was dissolved in 15 ml dry THF in a round bottom
some industrially important catalytic reactions such as oxi- flask. 0.7883 g (8.2 mmol) 3,5-dimethylpyrazole and 1.659 g
dative coupling polymerization of phenol derivatives,44 oxi- (16.4 mmol) triethylamine (Et3N) were dissolved in 5–10 ml
dation of hydrocarbons45–48 etc., which provide an dry THF separately. Then the latter solution was added to the
environmentally benign method. Though, copper–Schiff base previous one and this mixture was further added dropwise to
compounds have been widely used as catalysts in oxidation of the 2-chloromethyl-6-carbaldehyde-4-methylphenol solution.
hydrocarbons, yields and TON (turn over number) obtained An instant precipitation of Et3NHCl was observed and the
are not very remarkable so far.45–48 Recent reports have colour of the solution turned bright yellow. After 24 h of
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stirring the precipitate was filtered off. The filtrate was then DFT calculations
concentrated by removing THF under reduced pressure. After a
The calculations were started with molecular structures of
[Cu(L)] (1) obtained from the single-crystal X-ray diffraction
and also on its hydroperoxo derivative [LCu–OOH]− (2) with
peroxide ligated axially to the metal center. Their ground-state
geometries were fully optimized by DFT calculations using the
Gaussian 03 program51 with the Becke three-parameter hybrid
functional (B3LYP).52,53 The LanL2DZ54 valence and effective
core potential functions were used for all atoms including
copper. These basis functions were augmented with additional
few days under vacuum a light yellow solid was obtained. Yield
∼70% 1H NMR (in CDCl3, 300 MHz, ppm): δ 2.23–2.28 (9 H,
m, –ArCH3), 5.27 (2H, s, –CH2), 7.09 (1H, s, –ArH), 7.27 (1H, s,
–ArH), 7.28 (1H, s, –ArH), 9.88 (1H, s, –CHO), 11.23 (1H, br s,
–ArOH).
Syntheses of [CuII(L)] (1)
A mixture of 2-formyl-4-methyl-6-(3,5-dimethylpyrazole)phenol
(0.244 g, 1 mmol) and ethylenediamine (0.03 g, 0.5 mmol) in
30 ml methanol was refluxed for 45 min. After cooling to room
temperature copper perchlorate hexahydrate (0.185 g,
0.5 mmol) was added and refluxing was continued for a
further 90 min. The yellow coloured solution turned brown. It
was filtered and the filtrate was kept in a rack. Slow evapo-
ration of methanol gave rod-shaped brown crystals suitable for
X-ray studies. Elemental analysis: Calc. for C30H34N6O2Cu (M =
574.18): C, 62.70; H, 5.92; N, 14.63; Found: C, 62.34; H, 6.09;
N, 14.46%.
polarization functions of the exponents: Cu (ζf = 0.8), C (ζd
=
0.8), N (ζd = 0.8), O (ζd = 0.8). The nature of all stationary
points was confirmed by performing a normal-mode analysis.
Experimental set up for catalytic oxidation
A mixture of complex 1 (the catalyst) (32 mg, 0.0555 mmol)
and the substrate (hydrocarbon) (20 mmol) was dissolved in
10 ml dry MeCN solvent and the resulting solution was taken
into a 50 ml capacity two-neck round bottom flask of which
one neck was closed with a rubber septum and the other was
fitted with a condenser. To the above solution was then added
2 ml of 30% H2O2 and the resulting solution immediately
turned brownish green. The solution was then heated on an
oil-bath to reflux for 24 h. An aliquot (0.10 ml) of the reaction
solution was withdrawn with the help of long needle syringe at
regular time intervals (Table 3) and was subjected to multiple
ether extraction and 1 μL of concentrated ether extract was
then injected into the GC port with the help of a 10 μL syringe.
The retention times of the peaks were compared with those of
commercial standards.
Single-crystal X-ray diffraction studies
Intensity data for complex 1 was collected at 293(2) K on a
Bruker SMART APEX-II CCD diffractometer using graphite-
monochromated Mo-Kα radiation (λ = 0.71073 Å and the ω–2θ
scan mode in the range 2.3 < 2θ < 27.5°). No decomposition of
the crystal occurred during the data collection. The intensities
were corrected for Lorentz and polarization effects and for
absorption using the ψ-scan method. The cell parameters were
refined from all strong reflections. The data reductions were
carried out using the CrysAlis RED (Oxford Diffraction, UK)
program, and analytical absorption corrections were applied.
The structure was determined by direct methods using
SHELXS-9750 and refined anisotropically on F2 using the full-
matrix least-squares procedure of SHELXL-97.50 The crystallo-
graphic data for 1 are given in Table 1.
Structural descriptions
Single-crystal X-ray structural analysis reveals a square-planar
Cu(II)–Schiff-base complex 1 (Fig. 1) where Cu1 is found to be
tetra-coordinated. The two ethylenediamine N atoms (N1 and
N2) and two phenoxo O atoms (O2 and O3) satisfy four
Table 1 Details of crystallographic data collection and refinements
Formula
Mr
C30H34N6O2Cu
574.18
Crystal system
Space group
a/Å
b/Å
Monoclinic
C2/c (no. 15)
21.53600
9.92000
c/Å
13.03300
108.9800
2632.954
4
β/°
V/Å3
Z
Dc/g cm−3
1.448
μ(Mo-Kα)/mm
F(000)
0.870
1204
T/K
λ/Å
293
0.71073
θmin,max/°
2.3, 27.5
Dataset, hkl
Total, unique data, Rint
Observed data [I > 2.0σ(I)]
Nref, Npar
−27, 27; −12, 12; −16, 16
5843, 3028, 0.013
2734
3028, 245
0.0310, 0.1185, 1.09
Fig. 1 Molecular view with atom numbering scheme of 1. All H-atoms are
omitted for clarity. Symmetry code: (A) x, y, z; (B) −x, y, 1/2 − z; (C) 1/2 + x, 1/2
+ y, z; (D) 1/2 − x, 1/2 + y, 1/2 − z; (E) −x, −y, −z; (F) x, −y, 1/2 + z; (G) 1/2 − x,
1/2 − y, −z; (H) 1/2 + x, 1/2 − y, 1/2 + z.
R, wR2, S
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Table 2 Selected bond lengths (Å) and bond angles (°) for complex 1; values
in parentheses are for the DFT optimized structure of 1
Bond lengths/Å
Bond angles/°
Cu1–O1
Cu1–N1
Cu1–O1a
Cu1–N1a
1.8985(12) [1.930]
O1–Cu1–N1
O1–Cu1O1a
O1–Cu1–N1a
O1a–Cu1–N1
N1–Cu1–N1a
O1a–Cu1–N1a
93.38(5) [92.0]
88.96(5) [93.3]
175.22(5) [92.0]
175.22(5) [171.1]
84.61(6) [83.6]
93.38(6) [92.0]
1.9406(14) [1.987]
1.8985(12) [1.935]
1.9406(14) [1.987]
Fig. 3 The π-stacking (dotted lines in magenta color) and CH⋯π interaction
(dotted lines in yellow color) leads to a supramolecular sheet in the bc plane for
the Cu complex.
Fig. 2 Chelate ring–π(phenyl), chelate ring–chelate ring and Cu–π(phenyl)
interactions in complex 1.
coordination sites. Cu1–N (ethylenediamine) and Cu1–O
(phenoxo) bond distances are typical of square-planar Cu com-
plexes55 (Table 2).
Interesting π-stacking interactions govern the supramole-
cular assembly of the mononuclear units in 1 (Fig. 2). In complex
1 there are chelate ring–π(phenyl) and chelate ring–chelate ring
π-stacking interactions and also the Cu–π(phenyl) interactions
that stabilize the dimer of two mononuclear units. Also an
additional CH–π interaction comes into play between the
methyl group attached to the phenyl ring and the six-
membered chelate ring involving Cu in 1.
The π-stacking interactions are self-complementary and suc-
cessively join symmetry related mononuclear units along the
crystallographic c-axis (Fig. 3). Simultaneously each mono-
nuclear unit is also involved in CH–π interactions (Fig. S1†) in
which the ethylene C–H group acts as a donor to the pyrazole
ring of the symmetry related unit. This interaction propagates
the supramolecular assembly along the crystallographic b-axis.
Thus a 2D supramolecular sheet in the bc-plane is generated
(Fig. 3) which is further stacked along the crystallographic
a-axis (Fig. 4).
Fig. 4 The packing of supramolecular sheets along the crystallographic a-axis
for the Cu complex.
triethylamine (TEA) in a solvent mixture of CH3CN and
CH3OH (1 : 1) at 25 °C; the color of the solution changed from
brown to brownish-green (Scheme 2). The UV-Vis spectrum of
2 shows an intense band at 374 nm (ε = 2589 M−1 cm−1) and a
weak band at 572 (ε = 234 M−1 cm−1) (Fig. 5), which are similar
to those of previously reported CuII–OOR complexes.14,55–57
The former absorption peak has been assigned to a HOO−
→
CuII ligand-to-metal charge-transfer transition (LMCT) and the
latter band as d–d transition of the CuII ion in a square-pyra-
midal geometry. The corresponding bands of complex 1
appear at 360 nm (ε = 2420 M−1 cm−1) and 550 nm (ε =
155 M−1 cm−1). On addition of H2O2 in presence of a base there
are red shifts by ∼14 and ∼22 nm for the absorption peaks at
360 and 550 nm, respectively of the parent complex 1 along
with the increase in intensities. Though the complex [TEAH]
[Cu(L)(OOH)] was found to be stable in solution we could not
isolate it in the solid state. The ESI-MS+ of 1 in presence of
Copper(II)–hydroperoxo complex and its characterization
The copper(II)–hydroperoxo complex, [Cu(L)(OOH)]− (2), was
prepared by adding 10 equivalents of H2O2 to a reaction solu-
tion containing
1 in the presence of 2 equivalents of
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Fig. 5 Spectral change at room temperature for (a) 1 (2.0 mM); (b) 1 (2.0 mM)
+ H2O2 (50.0 mM) + NEt3 (50.0 mM); (c) 1 (0.1 mM); (d) 1 (0.1 mM) + H2O2
(1.0 mM) + NEt3 (1.0 mM).
Fig. 7 ESI-MS− spectra for 2-16O2 (bottom left hand side) and 2-18O2 (bottom
right hand side) and their corresponding simulated spectra (top).
Fig. 6 (a) ESI-MS+ of
1 in the presence of H2O2 and added HNO3 and
(b) ESI-MS− (m/z) spectra for 1 in the presence of H2O2 and TEA.
H2O2 and HNO3 (1 : 10) exhibits a prominent ion peak at 574
(Fig. 6a), whose mass corresponds to [Cu(L)]•+ (calculated m/z,
574.18), characteristic of the parent complex. There are three
other ion peaks at 608 ([Cu(L)(HOOH)]•+); 631 ([Cu(LH)(OOH)] +
Fig. 8 IR spectra of complex 1 in MeCN (black) and also in presence of H216O2
Na+) and 647 ([Cu(LH)(OOH)] + K+) (Fig. 6a). The ESI-MS− (2-16O2) (magenta) and H218O2 (2-18O2) (blue).
spectrum of 1 in MeCN in the presence of H2O2 and TEA
appears at 607 (Fig. 6b) whose mass and isotope distribution
pattern corresponds to [CuII(L)(OOH)]− (2-16O).
is very close to the DFT optimized value of 896 cm−1 (vide
infra). When we treated complex 1 with H218O2 in presence of
When the reaction was carried out with isotopically labeled
H218O2, a mass peak corresponding to [CuII(L)(18O18OH)]− TEA the same band appears at 869 cm−1 indicating firmly the
(2-18O) appeared at m/z of 611 (Fig. 7). The shift in four atomic coordination of HOO– to the Cu(II) center of the complex.
mass units on substitution of 16O with 18O proves that 2 con-
The stability of [LCu–OOH] (2) in MeCN was determined
tains an O2 unit. The ESI-MS− (m/z) peaks in the ESI-MS spectrophotometrically by following the decomposition of
spectrum indicate the absence of the formation of in situ formed species by reacting 1 (3.0 × 10−4 M) with H2O2
[LCu(μ-OH)2CuL]2− species in solution. So it could be concluded
(3.0 × 10−3 M) in the presence of NEt3 (3.90 × 10−3 M). The rate
that [LCuII–OOH]− is definitely formed in the solution. This is of decomposition was determined by initial rate method and
further proved by IR studies (Fig. 8) where a new band appears found to be kd = (5.67 0.03) × 10−5 s−1. This clearly points out
at 880 cm−1 corresponding to O–O stretching vibration which
that [LCu–OOH]− is fairly stable in MeCN.
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Fig. 10 DFT optimized geometry of 2-OOH.
Table 3 Selected FT optimized bond lengths (Å) and bond angles (°) for
complex 2
Fig. 9 X-Band EPR at 70 K: (a) 1 (0.5 mM in MeCN); (b) 1 (0.5 mM in MeCN) +
5.0 mM H2O2 + 5.0 mM NEt3.
Bond lengths/Å
Bond angles/°
The ESR spectra of 1 and 2 are shown in Fig. 9. The spec-
trum of 1 (g∥ = 2.264, g⊥ = 2.12, A∥ = 200 G) is clearly different
from the spectrum of 2 (g∥ = 2.566, g⊥ = 2.38, A∥ = 170 G). Thus
shifts in g and A values clearly indicates a change in geometry
on going from 1 to 2 i.e. –OOH definitely coordinates to the
metal center axially and in solution both the complexes are
roughly in square-pyramidal geometry.
Cu1–O2
Cu1–N3
2.04
2.28
2.06
2.04
1.97
1.44
2.26
O2–Cu1–N3
85.2
88.0
77.3
O38–Cu1–ON39
N3–Cu1–N39
O2–Cu1–O38
O2–Cu1–N39
O38–Cu1–N3
Cu1–O7–O75
Cu1–O38
Cu1–N39
Cu1–O74
O74–O75
O2⋯H76
98.5
161.9
126.6
113.0
DFT optimized structure
Table 2 displays some important geometrical parameters opti-
mized for the ground state. These gas-phase data are com-
pared with those obtained from the X-ray diffraction studies.
In the basal plane the Cu–O and Cu–N bond distances of the
parent complex 1 and DFT optimized geometry of 1 differ only
by 0.047 and 0.031 Å, respectively. The general agreement
between these two sets of data is very good. We have also opti-
mized the [Cu(L)(OOH)]− species 2, where the –OOH group is
attached to the copper centre axially, which is expected to be
the reactive intermediate of the catalytic cycle in the oxidation
of aliphatic and also aromatic hydrocarbons. Fig. 10 displays
the optimized geometry of 2 and corresponding bond lengths
and bond angles are listed in Table 3. MO diagrams for com-
plexes 1 and 2 are depicted in Fig. 11 and 12, respectively. The
HOMO–LUMO energy gaps appear to be 3.52 and 3.65 eV for
complexes 1 and 2, respectively. The differences in Cu–O and
Cu–N bond lengths between 1 and 2 are 0.161 and 0.34 Å,
respectively. The larger differences in bond lengths between 1
and 2 are quite logical, and are comparable to previous obser-
vations.9 The Cu–peroxide geometry obtained is, as expected,
of the end-on η1-type with the coordinating atoms of the
Fig. 11 HOMO and LUMO diagrams of [Cu(L)].
ligand in a square-pyramidal fashion with τ = 0.85, in agree- the value reported by Masuda (114.5°). It is interesting to note
ment with the EPR spectroscopic results described above. The that there is a strong O⋯H–O–O H-bonding interaction
Cu–O74 (hydroperoxo) bond length is 1.97 Å and close to the (O2⋯H76 = 2.27 Å) between phenoxo–O and peroxo H atoms
value of 1.888 Å reported by Masuda and co-workers.9 The which is shorter than the NH⋯O(OOH) bond distance
Cu–O74–O75 angle of 113° is also in good agreement with (2.78 Å), as found by Masuda and co-workers, and believed to
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Table 4 Oxidation of toluene by complex 1 (0.055 mmol)
Yield (%)
Total Sel.
conv. benzyl
n(H2O2)/
n(catalyst 1) t/h Benzyl alcohol Benzaldehyde (%)
alcohol TON
150
300
500
3
6
9
13
27
41
4
7
10
11
13
17
34
51
62
70
76.4
79.4
80.3
82.2
12 51
24 57
81.4
255
273
298
3
6
9
15
29
44
5
8
9
12
14
20
37
53
66
75
75.0
78.3
83.0
81.8
81.3
12 54
24 61
3
6
9
18
32
49
8
9
11
14
16
26
41
60
69
82
69.2
78.0
81.6
79.7
80.4
12 55
24 66
Table 5 Oxidation of o-xylene by complex 1 (0.055 mmol)
Fig. 12 HOMO and LUMO diagrams of [Cu(L)–OOH]−.
Yield (%)
Total
conv. Selectivity
n(H2O2)/
n(catalyst 1) t/h Alcohol Aldehyde + acid (%)
alcohol
TON
236
251
280
be responsible for the stability of hydroperoxo species. The IR
spectrum showed a prominent peak at ∼879 cm−1 and
matches reasonably well with the DFT optimized value
(896 cm−1) in the gas phase.
150
300
500
3
6
9
11
24
40
2
5
8
10
12
13
29
48
59
65
84.6
82.7
83.3
83.0
81.5
12 49
24 53
3
6
9
13
25
40
3
5
9
11
13
16
30
49
63
69
81.2
83.3
81.6
82.5
81.1
Probable mechanism of catalytic hydrocarbons oxidation
Complex 1 has been tested for the peroxidative oxidation of
cyclohexane and toluene by H2O2 as oxidant, in a slightly
acidic (HNO3) medium under ambient conditions. A mixture
of complex 1 (the catalyst) (0.0555 mmol), and the hydro-
carbon substrate (20 mmol) was refluxed in 10 ml dry MeCN
solvent in a 50 ml two-necked round bottom flask and the cat-
alysis were initiated by adding 2 ml of 30% H2O2. Toluene and
xylenes were oxidized to the corresponding alcohol and alde-
hyde. The yield was optimized by varying the relative pro-
12 52
24 56
3
6
9
14
29
47
3
7
11
13
15
17
36
58
66
77
82.3
80.5
81.0
80.3
80.5
12 53
24 62
portion of hydrogen peroxide with respect to the catalyst, and Tables 4–6. It can be clearly seen from Tables 4–6 that complex
also varying the reaction time. It has been evident from pre- 1 is a very efficient catalyst for the oxidation of toluene with
vious studies by other groups that the presence of nitric acid 82% conversion (TON = 298) when the n(H2O2)/n(catalyst) ratio
has a positive role in such catalytic reactions.58–61 The added is 500, with a reaction time of 24 h. It has been observed for
nitric acid has numerous roles in that (i) it increases the oxi- all the catalytic conversions that the yield increases with time.
dation properties of the catalyst by increasing the unsaturation Catalytic conversions are also dependant on the amount of
at the metal center (by protonation of the ligand); (ii) prevents oxidant used. These results are comparable with the previously
decomposition of peroxide present in the reaction medium as reported studies.62 Fig. S2† shows the plot of the percentage
oxidant; and (iii) enhances the stability of the peroxo inter- yield of alcohol with reaction time. Simple copper salts, like
mediate formed in the reaction. The ratio n(HNO3)/n(catalyst) Cu(NO3)2, under the same reaction conditions exhibit less
has been optimized as 10 and all experiments were performed than 4% conversion using the above mentioned substrates at
under this condition.
[n(H2O2)/n(catalyst) = 500]. So it is evident that the presence of
ESI MS+ (m/z) mass spectra of complex 1 in the presence of N and O donor ligands is quite relevant. The mechanism of
nitric acid as well as in TEA and H2O2 showed the presence of the catalytic conversion is schematically given in Scheme 3.
copper–hydroperoxo species in the medium. The results of the The LCuII complex reacts with H2O2 to form the LHCuII–OOH
oxidation of toluene, o-xylene and p-xylene are shown in intermediate where one proton from HOOH is attached to a
13216 | Dalton Trans., 2013, 42, 13210–13219
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and one HOO˙ radicals out of two H2O2 has recently been
demonstrated by Shul’pin et al. through DFT studies.63 The
metal-assisted homolytic cleavage of alkyl hydroperoxide gen-
erates alkoxyl (RO˙, upon O–O bond rupture) and alkylperoxyl
(ROO˙, upon O–H bond fission) radicals which can form
an alcohol (ROH) upon H-abstraction from the alkane (RH)
by RO˙ or both ROH and the >CvO upon decomposition
of ROO˙.
Table 6 Oxidation of p-xylene by complex 1 (0.055 mmol)
Yield (%)
Total
conv. Selectivity
n(H2O2)/
n(catalyst 1) t/h Alcohol Aldehyde + acid (%)
alcohol
TON
255
273
287
150
300
500
3
6
9
10
22
38
4
5
8
12
15
14
27
46
60
70
71.4
81.4
82.6
80.0
78.6
12 48
24 55
3
6
9
13
27
42
3
6
11
13
15
16
33
53
67
75
81.2
81.8
79.2
80.6
80.0
Conclusions
12 54
24 60
Complex 1 has been optimized structurally and the gas-phase
data are found to be in good agreement with the X-ray data. It
reacts with hydrogen peroxide in presence of a TEA to generate
[TEAH][Cu(L)(OOH)] (2) which is reasonably stable in solution.
DFT calculations reveal that the Cu–O74 (hydroperoxo) bond
length is 1.97 Å and is close to the 1.888 Å reported by Masuda
and co-workers.9 The Cu–O74–O75 angle 113° is also in good
agreement with the value reported by Masuda (114.5°). It is
interesting to note that there is a strong O⋯H–O–O H-bonding
interaction (O2⋯H76 = 2.27 Å) between phenoxo–O and
peroxo H atoms which is shorter than the NH⋯O(OOH) bond
distance (2.78 Å) as found by Masuda and co-workers and
believed to be responsible for the stability of hydroperoxo
species. These results clearly indicate that a non-covalent inter-
action is essential for the coordination and stability of small
molecules such as HOOH, as predicted by Masuda. The IR
spectrum showed a prominent peak at ∼880 cm−1 and
matches reasonably well with the DFT optimized value
(896 cm−1) in the gas phase. Thus these hydroperoxo com-
plexes could serve as models for the active center in biological
systems such as peptidylglycine α-amidating monooxygenase
(PAM) and dopamine β-monooxygenase (DβM) and is evi-
denced by its catalytic oxidation of toluene with high TON.
This reports opens up a new dimension for the stabilization
of small molecules such as –OOH coordinated to the
metal center through H-bonding in the Schiff-base ligand
environment.
3
6
9
14
33
47
5
8
12
13
19
19
41
59
70
79
73.6
80.5
79.6
81.4
74.7
12 57
24 59
Scheme 3
phenoxo oxygen atom. This proposition is supported from the
optimized geometry of [LCu–OOH]− where the peroxy H atom
is strongly H-bonded to the phenoxo oxygen atom of the co-
ordinated ligand. The intermediate LHCuII–OOH can release
one HO˙ radical with the formation of LHCuIIIvO which on
further steps reacts with another molecule of HOOH leading
to the formation of LHCuII–OH with the concerted release of
HOO˙ radical. LHCuII–OH releases one water molecule to
regenerate the catalyst. Thus in one catalytic cycle two H2O2
molecules release one HO˙ and one HOO˙ radicals. Thus
these radicals could be formed from the metal-assisted
decomposition of hydrogen peroxide i.e., through Fenton-like
reactions.
Acknowledgements
Financial supports from CSIR (Ref No. 01(2490)/11/EMR-II)
and UGC [39-735/2010(SR)], New Delhi, India are gratefully
acknowledged.
Notes and references
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The HO˙/HOO˙ radicals abstract H atoms from the sub-
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