Synthesis, characterization and catalytic properties of dinuclear complexes of copper(II) and nickel(II): Oxidation of cyclohexane, toluene and cyclopentane

Article abstract of DOI:10.1016/j.ica.2019.03.005

Oxidation of saturated hydrocarbon by transition metal complexes is of great importance because this would give industrially valuable organic fine chemicals. Here, dinuclear copper(II) and dinuclear nickel(II) complexes with same N,O donor ligand have been used as catalyst for the oxidation of cyclohexane, toluene and cyclopentane using hydrogen peroxide as the oxidant to examine their catalytic activity and effect of nature of metal center on catalysis. [Cu₂(L¹)₂(μ₂-Cl)Cl]·2.5H₂O (1) and [Ni₂(L¹)₂(μ₂-N₃)₂ (CH₃OH)₂]·CH₃OH (2) have been synthesized under mild conditions where HL¹ is 1-((2-hydroxyethylimino)methyl)naphthalen-2-ol. These complexes have been characterized by elemental analysis, FT-IR, UV-Vis, mass spectroscopy, TGA-DTA and cyclic voltametric studies. Their structures have been confirmed by single crystal X-ray diffraction analysis. Some theoretical calculations have been performed to investigate spectral transitions. Copper(II) complex shows effective catalytic ability towards oxidation of the saturated hydrocarbons such as cyclohexane, toluene and cyclopentane in the presence of hydrogen peroxide when corresponding alcohols and ketones have been obtained as major products. However, complex 2 has been found to be inactive as catalyst for the oxidation. It is possible that Cu-hydroperoxo or Cu-peroxo species is formed during catalysis (as revealed from UV-vis spectral analysis) which could be the active species for the oxidation reaction.

Full text of DOI:10.1016/j.ica.2019.03.005

Accepted Manuscript
Research paper
Synthesis, characterization and catalytic properties of dinuclear complexes of
copper(II) and nickel(II): oxidation of cyclohexane, toluene and cyclopentane
Aradhita Bhattacharjee, Sudipto Dey, Partha Roy
PII:
S0020-1693(19)30019-2
https://doi.org/10.1016/j.ica.2019.03.005
ICA 18810
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
9 January 2019
2 March 2019
4 March 2019
Please cite this article as: A. Bhattacharjee, S. Dey, P. Roy, Synthesis, characterization and catalytic properties of
dinuclear complexes of copper(II) and nickel(II): oxidation of cyclohexane, toluene and cyclopentane, Inorganica
Chimica Acta (2019), doi: https://doi.org/10.1016/j.ica.2019.03.005
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Synthesis, characterization and catalytic properties of dinuclear complexes of
copper(II) and nickel(II): oxidation of cyclohexane, toluene and cyclopentane
‡
‡
Aradhita Bhattacharjee , Sudipto Dey , Partha Roy*
Department of Chemistry, Jadavpur University, Jadavpur, Kolkata-700 032, India
E-mail: partha.roy@jadavpuruniversity.in; proy@chemistry.jdvu.ac.in
‡These authors contributed equally to this work.
1

Abstract
Oxidation of saturated hydrocarbon by transition metal complexes is of great importance because
this would give industrially valuable organic fine chemicals. Here, dinuclear copper(II) and
dinuclear nickel(II) complexes with same N,O donor ligand have been used as catalyst for the
oxidation of cyclohexane, toluene and cyclopentane using hydrogen peroxide as the oxidant to
examine their catalyticactivityandeffect of nature of metal center on catalysis. [Cu
2
(L1)
2
(μ -
2
Cl)Cl]·2.5H O(1) and [Ni (μ -N (CH OH) ]·CH OH(2) have been synthesized under
2
2
(L1)
2
2
3
)
2
3
2
3
mild conditions where HL1 is 1-((2-hydroxyethylimino)methyl)naphthalen-2-ol. These
complexes have been characterized by elemental analysis, FT-IR, UV-Vis, mass spectroscopy,
TGA-DTA and cyclic voltametric studies. Their structures have been confirmed by single crystal
X-ray diffraction analysis. Some theoretical calculations have been performed to investigate
spectral transitions. Copper(II) complex shows effective catalytic ability towards oxidation of the
saturated hydrocarbons such as cyclohexane, toluene and cyclopentane in the presence of
hydrogen peroxide when corresponding alcohols and ketones have been obtained as major
products. However, complex 2 has been found to be inactive as catalyst for the oxidation. It is
possible that Cu–hydroperoxo or Cu–peroxo species is formed during catalysis (as revealed from
UV-vis spectral analysis) which could be the active species for the oxidation reaction.
Keywords: Homogenous catalysis; Saturated hydrocarbon; Oxidation; Copper; X-ray Structure.
2

1
. Introduction
Activation of Csp3-H bond is one of the most difficult tasks in chemistry. Natural gas and
fuel are mainly saturated hydrocarbons. This bond in methane is inert and of high energy (104
kcal/mol) at ambient temperature, thus, the oxidation requires high temperature and pressure
and/or acidic reaction media in addition to the presence of metal as catalyst [1]. Transformation
of these compounds may give different valuable organic fine chemicals which are of great
industrial importance. For example, methane can be converted to methanol with one functional
group which could be converted to other compounds easily [2]. Modeling of metalloenzymes
with particular catalytic activity is of great interest for the design and development of bio-
inspired catalysts. In connection with activation of Csp3-H bond, we could recall an enzyme,
methane monooxogenase [3]. It exists in two forms; (a) particulate methane monooxogenase
(pMMO) and (b) soluble methane monooxogenase (sMMO). Both of these forms are able to
convert methane to methanol.sMMO contains two iron(III) centers in the active site. Active site
of pMMO is not identified unambiguously by structural characterization of the enzyme.
However, the existence of two Cu(II) centers in the active site of pMMO is of most acceptable
belief [1a, 3]. Therefore, there are several copper(II) complexes including dinuclear copper(II)
compounds which have used as catalyst for the activation of Csp3-H bond [4]. Different
hydrocarbons have utilized as the substrates such as methane, cyclohexane, cyclopentane.
Oxidation of cyclohexane is useful because its oxidized products, mainly cyclohexanol and
cyclohexanone, are of great industrial importance [5]. Cyclohexanol is, mainly, used as the raw
material for the manufacture of adipic acid. Adipic acid is used for the preparation of nylon-6,6′,
soaps and detergents, pesticides, etc. Cyclohexanone is, mainly, utilized as a solvent in industries
and activator in oxidation reactions. Pombeiro et al. have been working for long time on the
development of different copper(II) complexes as active catalyst for such oxidation reactions [6].
Few other groups have also reported C-H activation reactions by Cu(II) complexes [7]. But,
conversion of substrate to product is of comparatively low percentage because of high C-H bond
energy. Although there is significant importance of copper(II) complexes as catalysts in various
catalytic transformations [7i-7n], the field of alkane oxidation by copper complexes is less
explored. It is to be noted that in addition to the original metal present in the metalloenzyme,
some functional models with other transition metals have been designed and reported. For
example, phenaxozinone synthase mimicking activity has been reported by complexes with
3

Mn(II), Co(III), etc.[8]. So other transition metal complexes can be designed for MMO activity
to enable Csp3-H bond activation. Thus, search for active catalyst for high conversion and better
selectivity of a particular product continues. Effort should be given to design the catalyst which
can mimic activity of naturally abundant enzymes.
In this respect, we report synthesis, characterization and catalytic properties of dinuclear
Cu(II) and Ni(II) complexes, [Cu
2
(L1)
2
(μ
2
-Cl)Cl]·2.5H
2
O(1) and [Ni
2
(L1)
2
(μ
2
3 2
-N )
1
(
CH
3
OH)
2
]·CH
3
OH(2) where HL is 1-((2-hydroxyethylimino)methyl)naphthalen-2-ol (Scheme
1
). Some theoretical calculations have been performed to investigate spectral transitions of these
complexes. Cu(II) complex has been prepared by the reaction between copper(II) chloride and
1
HL under mild conditions. It has been used as catalyst for oxidation of cyclohexane, toluene and
cyclopentane using hydrogen peroxide as the oxidant. We wanted to check if replacement of
copper by other metal, say nickel, in similar environment has any effect on catalytic activity. In
other words, we wanted to use Ni for pMMO mimicking activity. Although Ni(II) complexes
have been used as catalyst for different reactions including olefin epoxidation, C-C bond
formation, etc.[9], attempt to use of nickel compounds in alkane oxidation is rare. Dinuclear
Ni(II) complex has been synthesized with same ligand to provide similar chemical environment
around the metal center.
2
. Experimental
2
.1. Materials and characterization
2
-Hydroxy1-naphthaldehyde,
2-aminoethanol,
copper(II)
chloride
dihydrate,
nickel(II)nitrate hexahydrate, sodium azide and tetrabutylammonium perchlorate (TBAP) were
purchased from Sigma Aldrich and used without further purification. Other reagents and solvents
were obtained from commercial sources and used as received. NMR spectra of the compounds
were recorded on a Bruker 500 MHz spectrometer. Elemental analyses (carbon, hydrogen and
nitrogen) were performed using a Perkin–Elmer 2400C elemental analyzer. FT-IR spectra were
recorded on a Perkin Elmer spectrometer (Spectrum Two) with the samples by the use of
attenuated total reflectance (ATR) technique. The UV-visible spectral measurements were
recorded in Agilent 8453 diode array spectrophotometer. Analysis of reaction mixture of
catalytic reactions was performed with a Shimadzu next generation high speed gas
chromatography system (model: GC-2025 AF) equipped with a fused silica capillary column and
4

FID detector. All electrochemical measurements were performed using a personal computer
PC)-controlled PAR model 273A electrochemistry system. A GC working electrode, a platinum
(
wire auxiliary electrode and Ag/AgCl saturated KCl reference electrode were employed in a
standard three-electrode configuration. TBAP was used as supporting electrolyte in MeCN and
was done under nitrogen atmosphere. TG-DTA analysis was carried out on Perkin-Elmer Pyris
Diamond TG/DTA unit. All experiments were carried out at room temperature in air unless
reported otherwise.
CAUTION: Organic perchlorates are potentially explosive. Only small amount of the
perchlorate salt should be handled with care.
2
2
.2. Synthesis
.2.1. Synthesis of 1-((2-hydroxyethylimino)methyl)naphthalen-2-ol (HL1)
1
1
-((2-Hydroxyethylimino)methyl)naphthalen-2-ol (HL ) has been synthesized following
a published procedure with slight modification [10]. Typically, 2-aminoethanol (0.5 mmol, 0.030
mL) was added to a methanolic solution (10.0 mL) of 2-hydroxy1-napthaldehyde (0.5 mmol,
0
.086 g) under stirring condition. The mixture was stirred for 30 min. Then the resulting solution
was refluxed for 2 h. The color of the mixture turned yellow. It was then cooled to room
temperature to obtain yellow color solid product. The product was collected by filtration and
dried in air. Yield: 85% (0.091 g); C, H, N analysis: anal. calc. for C13
H13NO
2
: C, 72.54; H, 6.09;
1
N, 6.51; found: C, 72.44; H, 5.98; N, 6.56%. H NMR (500 MHz, CD
3
CN) δ (ppm): 14.23 (s,
1
H), 8.80 (s, 1H), 7.92 (d, J = 8.5Hz, 1H), 7.66 (d, J = 9.5 Hz, 1H), 7.58 (d, J = 7.5 Hz, 1H), 7.39
(1H, dd), 7.17 (dd , J = 6.0 Hz, 1H), 6.74 (d, J = 9.0 Hz, 1H), 4.11(t, J = 9.0 Hz, 2H), 3.71 (t, J =
1
3
4
1
2
.0 Hz, 2H), 3.18 (s, 1H); C NMR (500 MHz, CD CN) δ (ppm): 55.63, 62.08, 107.35, 119.29,
3
‒
1
23.48, 126.10, 127.01, 129.02, 130.04, 135.45, 137.90, 160.69 and 177.62; FT-IR cm : 3152,
918, 1635, 1539, 1358, 1072, 838, 744, 500; ESI-MS (m/z): [M + H]+ = 215.13.
1
2
.2.2. Synthesis of [Cu
2
(L )
2
(μ
2
-Cl)Cl]·2.5H O (1)
2
A methanolic solution (10.0 mL) of copper(II) chloride dihydrate (0.5 mmol, 0.067 g)
1
was added to 5.0 mL methanolic solution of HL (0.5 mmol, 0.108 g). The mixture was stirred
till it turned greenish. It was then refluxed for 1 h. Color of the solution became dark green. The
mixture was finally cooled to room temperature and filtered to remove any precipitate or
5

suspended materials. The filtrate was kept at ambient temperature. Green crystals suitable for X-
ray diffraction study were produced within few days. Yield 65%, 0.218 g; C, H, N analysis: anal.
calc. for C26
H29Cl
2
Cu
2
2
N O6.5: C, 46.50; H, 4.35; N, 4.17; found: C, 46.44; H, 4.23; N, 4.32%.
2
.2.2. Synthesis of [Ni
2
(L1)
2
(N
3
)
2
(CH
3
OH)
2
]·CH OH (2)
3
A methanolic solution (10.0 mL) of nickel(II) nitrate hexahydrate (0.5mmol, 0.091 g)
1
was added to 5.0 mL methanolic solution of HL (0.5 mmol, 0.108 g)while the mixture was
constantly stirred. It was stirred for 30 min. Then, sodium azide (0.5mmol, 0.034 g) in 2.0 mL of
methanol/water was added dropwise to the resulting green solution. After addition of azide, the
solution turned dark green. The solution was stirred for another 30 min before it was refluxed for
1
h. The mixture was finally cooled to room temperature and filtered to remove any precipitate
or suspended materials. The filtrate was kept at ambient temperature. Green crystals suitable for
X-ray diffraction analysis were produced within few days. Yield 70%, 0.253 g; C, H, N analysis:
anal. calc. for C29
H
36
N
8
Ni
2
7
O : C, 47.97; H, 5.00; N, 15.43; found: C, 47.84; H, 4.92; N, 15.36%.
2
.3. X-ray data collection and structure determination
Details of the data collection and refinement parameters for complexes 1 and 2 are
summarized in Table 1. The diffraction experiments were carried out on a Bruker APEX-II CCD
diffractometer using graphite monochromated Mo Kα radiation at 298 K for both the complexes
1
and 2. Data were processed using the Bruker APEX2 and SAINT packages [11]. Absorption
corrections based on multi-scans using the SADABS software [11] were applied to the intensity
data. The structures were solved by direct methods using SHELXT [12] and refined with full-
2
matrix least-squares on F on all unique reflections using SHELXL-2014/7[13]. All the non-
hydrogen atoms of the complexes were refined anisotropically. A few hydrogen atoms, which
are highly acidic, are disordered due to thermal disturbances. These have been assigned to their
best possible positions by chemical speculation.
2
.4. Computational details
1
The singlet ground state (S
0
)
geometry of ligand, HL and its copper and nickel
complexes i.e. complexes 1 and 2 have been fully optimized by DFT method with B3LYP
6

(Becke’s three-parameter hybrid functional and Lee–Yang–Parr’s gradient corrected correlation
functional) exchange correlation functional approach using the Gaussian 09 program [14]. The
B3LYP functional has been adopted along with the 6-31G basis set for H, C, N, O atoms
whereas the 6-311G (d,p) basis set was utilized for Cl atoms and LANL2DZ was adopted as the
basis set for Cu and Ni atoms [15]. The nature of all the stationary points was confirmed by
carrying out a normal mode analysis, where all vibrational frequencies were found to be positive.
On the basis of the optimized ground state (S ) geometrical structures, the UV–vis absorption
0
transition properties of the corresponding ligand and complexes 1 and 2 in methanol were
computed by a time-dependent density functional theory (TDDFT) [16] approach associated with
the conductor-like polarizable continuum model (CPCM) [17] using the same B3LYP level and
the previous basis sets. Frontier Molecular Orbital (FMO) analysis has been done with gauss
view software [18].
2
.5. Catalytic studies
Oxidation of cyclohexane, toluene and cyclopentane has been performed in a similar
procedure in the presence of both the metal complexes as catalysts. Typically, Metal complex
0.03 mmol) was taken in 5.0 mL of acetonitrile in a two-neck round bottom flask which was
fitted with a condenser. To this, 1.5 mmol of the substrate and varying amount of HNO were
added. Catalytic reaction started as soon as 3-10 mmol of hydrogen peroxide (30% in H O) was
(
3
2
added to it. The mixture was continuously stirred for 48 h at desired temperature under
atmospheric pressure. Aliquots were collected at regular time intervals. The substrate and
products from the reaction mixture were extracted with 2.0 mL diethyl ether. The product
mixture was analyzed in gas chromatograph or GCMS before and after the treatment with
triphenylphosphine (PPh
standards or GCMS.
3
). The identification was done by either the comparison with known
7

3
. Results and Discussion
3
.1. Synthesis and ligand characterization
Synthesis of complexes 1 and 2 has been performed following route given in Scheme 1.
1
First, HL has been synthesized by one step Schiff-base condensation between 2-hydroxy-1-
napthaldehyde and 2-aminoethanol in 1:1 molar ratio in methanol. The ligand has been
characterized by elemental analysis, H and 13C NMR spectra as well as by FT-IR and mass
1
1
spectral analyses (Fig. s1-s4). H NMR spectrum (Fig. s1) of the ligand shows a peak at 14.23
ppm indicating the presence of phenolic OH group. Presence of azomethine proton has been
confirmed by the singlet peak at 8.80 ppm. Bands in the range of 4.19 to 3.66 ppm exhibit the
presence of methylene protons while the peak at 3.18 ppm has been assigned to the presence of
alcoholic proton. Signals for aromatic protons appear in the range of 7.92 to 6.72 ppm. 13C
spectrum of the ligand corroborates well with the formation of the Schiff-base compound (Fig.
s2). IR spectrum of the ligand shows most significant bands at 3152, 2918 and 1635 cm‒1
indicating the presence of phenolic OH, methylene and C=N moieties, respectively (Fig. s3). ESI
mass spectrum of the compound shows m/z band at 215.13 confirming the formation of the
1
Schiff-base ligand (Fig. s4) (calculated value 215.09). Reaction between HL and copper(II)
chloride dihydrate gives 1. Complex 2 has been synthesized by the reaction between the ligand,
nickel(II) nitrate hexahydrate and sodium azide where azide acts as a bridging ligand. HL1
undergoes deprotonation in the reaction medium without any external deprotonating base.
3
.2. Crystal structures of 1 and 2
Complex 1 crystallizes from methanol in orthorhombic system with P212121 space group. A
perspective view of the complex is given in Fig. 1. Selected bond lengths and bond angles are
given in Table 2. Cu1 is tetracoordinated whereas Cu2 is in pentacoordination geometry. The
two chloride ions from the copper(II) salt used for the complex formation, bind the Cu atoms
differently as confirmed by the crystal structure. One chlorido is bridging while the other is
terminal. Cu1 is in a distorted square planar environment bonded to O1, O2, N1 and Cl1. Cl1 is
the bridging chlorine atom which links Cu1 and Cu2. There is deviation of donor-metal-donor
bond angle from 90°. The O2-Cu1-N1 bond angle is 83°, O1-Cu1-N1 angle is 92°, Cl1-Cu1-O2
angle is 93° and Cl1-Cu1-O1 angle is 94°.The Cu1-Cl1 bond is slightly out of plane. This
confirms the approximate square planar geometry around the Cu1 center. Cu2, on the other hand,
8

is bonded to O3, O4, N2, Cl1, Cl2. It is in distorted square pyramidal geometry as indicated by
the value of the trigonal index, τ. The trigonal index is calculated as the difference between the
two largest donor‒metal‒donor angles divided by 60[19]. Its value is 1 for the ideal trigonal
bipyramid and 0 for the square pyramid. Here the τ value is calculated to be 0.25 which indicates
its significantly distorted square pyramidal environment. O3, O4, N2 and Cl2 atoms form the
basal plane of the square pyramid and C11 occupies the apical position. However, Cu center is
out of the mean plane by 0.150 Å towards Cl1. There are 2.5 water molecules in the crystal
structure. The solvent water molecules are disordered due to molecular vibrations. The H atoms
in the alcoholic moiety and the water molecule have been assigned their best possible positions
in spite of being disordered. The crystal has been refined as a merohedral twin. In packing
structure, there are π-π and CH-π interactions (Fig. s5). The metal-metal distance is 3.485 Å.
Metal-chlorine bond distances are long compared to other metal-donor distances as expected.
Although Cu-Cl bond distances are long here (Table 2), similar long copper-chlorine distances
are also reported [20].
Complex 2 crystallizes from methanol into a monoclinic system with P 21/n space group.
A perspective view of the complex is given in Fig. 2. Selected bond lengths and bond angles are
given in Table 2.The molecule consists of two nickel atoms, two deprotonated ligands, 1-((2-
1
hydroxyethylimino)methyl)naphthalen-2-ol (HL ), two azido ligands and two coordinated
methanol molecules. One methanol is present as solvent of crystallization. Each Ni atom is in
similar coordination geometry i.e. in octahedral geometry. Ni1 is bonded with O4, N8 and O6
from the ligand, N1 and N4 from two different azido moieties and O5 from a methanol molecule.
On the other hand, Ni2 is coordinated to O2, N7 and O3 atoms from the ligand, Ni and N4 from
two different azido species and O1 from a methanol molecule. Two intramolecular hydrogen
bonds are present in this molecule involving non-coordinated methanol (Fig. s6). Coordinated
O3 atom is present in H-bonding with H-O moiety of methanol and second hydrogen bond is
found involving O atom of methanol and H-O6 moiety of alcoholic group. All of the donor-Ni
bond distances are in agreement with the reported values [21].
3
.3. IR spectral studies
FT-IR spectra of all of the complexes have been obtained with powder samples by ATR
‒
1
technique. Peaks at 1622 and 1617 cm in the IR spectra of Cu and Ni complex respectively,
9

confirm the presence of azomethine bond in both the complexes (Fig. s7). A sharp peak at 2054
‒
1
cm indicates the presence of azido moiety in complex 2. Both the complexes show broad band
‒
1
in the range of 3500-3400 cm . Broad peak in complex 1 is due to the presence of water
molecule which is present as solvent of crystallization in complex 1 and that for complex 2 is
probably due to the presence of methanol molecules in the Ni complex.
3
.4. UV-vis spectral studies
The electronic spectra of complexes 1 and 2 have been recorded in methanol at room
temperature (Fig. s8,s9). For complex 1, high intensity bands appear at 383 and 316 nm (molar
-1
-1
extinction coefficient of 12100 and 13500 Lmol cm , respectively) which may occur due to
−
ligand to metal charge transfer (PhO → Cu(II) and N(imino) → Cu(II)) and intra ligand charge
transfer respectively. The Cu complex shows broad bands at 646, 673 and 720 nm with low
intensity. These spectral characteristics are in consistence with copper(II) complexes of distorted
square pyramidal (SP) geometry. The observed bands originate from dxz,dyz → d
x
2‒y2 transitions
[
22]. They are accompanied by a low energy shoulder at 844 nm due to dxy, d
x
2
‒y2 → d 2
z
transitions. As the structure moves more towards TBP structure the low energy band along with
high energy spin forbidden band becomes more pronounced. The characteristic bands observed
here are results for distorted square pyramidal geometry.
UV spectrum of complex 2 shows high intensity bands at 312 and 395 nm with molar
-1
-1
extinction coefficient of 9335 and 8261 Lmol cm , respectively. These bands are probably due
−
to the intraligand charge transfer, and LMCT (PhO → Ni(II) and N(imino) → Ni(II)),
respectively.
Complex 2 also shows d-d transitions. The broad bands are obtained at 750 and 860 nm which
probably indicate dxz,dyz → d
z
2 and dxy, d
x
2
‒y2 → d 2 transitions [22].
z
3
.5. Geometry optimization and computational studies
1
Molecular structures of HL and its Cu, Ni complex have been optimized at their electronic
ground state (S ) by means of DFT at B3LYP/(6-31G) level. For better understanding of the
0
electronic transitions involved in absorption process, TD-DFT calculations have been carried out
by the B3LYP/CPCM method in methanol by utilizing the ground state optimized geometries.
We have computed the lowest 40 singlet–singlet transition and results of the TD calculations are
1
0

qualitatively very similar with the experimental results. The computed absorption energies along
with their oscillator strengths, the main configurations, are summarized in Table 3. Fig. 3
displays the energy levels of different Frontier Molecular Orbital’s of the ligand and complexes 1
and 2 associated with their HOMO-LUMO energy gap. Due to the presence of electronic
correlation in the TD-DFT (B3LYP) method, it can yield more accurate electronic excitation
energies. The UV spectra computed by TD-DFT calculations in methanol show the important
peaks in the range 200–500 nm.
1
HL shows lowest lying distinguishable singlet → singlet absorption band at 418.14 nm
(experimentally 415 nm, experimental absorption band in methanol are shown in Fig. s10) which
is due to the contribution of HOMO-1→LUMO+1, HOMO→LUMO, HOMO→LUMO+1
transitions, also a moderately intense peak at 400.12 nm (experimentally 400 nm) that
corresponds to HOMO-1→LUMO, HOMO→LUMO, HOMO→LUMO + 1 transitions along
with a shoulder at 311.24 nm (experimentally 306 nm) corresponding to HOMO-3→LUMO+1,
HOMO→LUMO+1, HOMO-1→LUMO+2 and HOMO-1→LUMO transitions.
Complex 1 shows an intense absorption peak at 384.21 nm (experimentally 383 nm)
corresponding to HOMO-1→LUMO+1, HOMO-3→LUMO, HOMO→LUMO transitions along
with moderately intense absorption band at 317.88 nm (experimentally 316 nm) corresponding to
HOMO-2→LUMO+4, HOMO→LUMO+6 transitions (FIFURE4A). Besides the nickel dimer
shows absorption peaks at 396.12 nm (experimentally 395 nm) due to the contribution of
HOMO-3→LUMO, HOMO-2→LUMO, HOMO-1→LUMO+1 transitions and peak at 314.40
nm (experimentally 312 nm) corresponding to HOMO-1→LUMO+4, and HOMO→LUMO+1
transitions (Fig. 4B).
3
.6. Thermal analysis
Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) have been
performed on powdered samples of complexes 1 and 2 from 35 to 800 °C (for complex 1) and 35
to 750 °C (for complex 2) to examine thermal stability of the complexes and fate of solvent of
crystallization molecules.
In the TGA DTA plot of complex 1, an initial loss of 2.6% is seen at temperature 50 °C
which corresponds to an endothermic DTA peak. It depicts the loss of surface water probably
(Fig. s11). The next visible loss at 190 °C is a weight loss of 6% which probably indicates the
1
1

loss of all the water molecules (calculated loss 6.7%) also depicted by an endothermic peak. A
loss of almost 32% occurs at 280 °C which probably shows loss of one ligand moiety (calculated
loss 32.2%). The next decomposition at 370 °C probably indicates loss of two Cl (observed
~10% vs calculated 10.6%). The final loss of 33% could be attributed to the loss of another
ligand moiety after which a stable residue of CuO is obtained. All of the last three losses
accompany a corresponding exothermic peak in the DTA curve.
In the TGA DTA plot of complex 2, the initial loss of 13% at 86 °C can be attributed to
the loss of 3 methanol molecules (bound and unbound (calculated loss 13.2%)) as confirmed by
endothermic DT peak (Fig. s12). There are next two consecutive losses, each of 4%, which can
be assigned to the loss of N
C of temperature. The next loss of another 4% is the final decomposition of the nitrogens of the
azide bridge and its evolution as another molecule of N at and the process completes at 270 °C.
2
twice from the azide bridges. This loss occurs between 132 and 180
°
2
The next loss of 40% probably confirms the loss of one part of the dinuclear complex which
continues upto 400 °C. The residue as stable NiO is left behind. All show exothermic DT peaks
other than the loss of solvent molecules at the starting temperatures.
3
.7. Cyclic voltametric studies
Electrochemical studies of complexes 1 and 2, and the ligand have been carried out in
acetonitrile using TBAP as supporting electrolyte. A typical cyclic voltammogram (CV) has
been obtained by using a glassy carbon electrode and an Ag/AgCl saturated KCl reference
electrode. CV plots of the complexes are shown in Fig. 5.
The CV plot of the dinuclear complex, 1 has been obtained by scanning potential -2.0 to
+1.5, wherein the electrochemical behavior of the complex is elucidated properly. The scan rate
-1
has been maintained at 50 mV s . The forward scan gives an anodic peak at 0.62 V and Ipa 8 ×
-6
-5
1
0 A and the cathodic counterpart in the reverse scan at Epc 0.48 V and Ipc of 2.2 × 10 A. These
peaks have been attributed to the redox couple Cu(II)-Cu(III) as the peaks clearly show a
quasireversible one electron transfer process [23a-23d]. The similarity with literature values
confirms the peaks. Of the copper centers, Cu2 is more competent to undergo oxidation to +3
state as it satisfies the 18-electron rule after the oxidation process. Comparison of the results of
voltammetric peak current with those of the ferrocene–ferrocenium couple under the same
1
2

experimental conditions establishes that these responses involve one electron in each step. The
electrochemical responses of the complex may be assigned as:
II
II
III
II
Cu ; Cu ‒ e ⇌ Cu ; Cu ;
III
II
III
III
Cu ; Cu ‒ e ⇌ Cu ; Cu
The anodic peak at 1.2 V indicates an irreversible ligand oxidation which is characteristic of the
oxidation of the Schiff base groups [24a-24c].
-6
In the scan of range 0.0 to -2.0 V, the cathodic peak at Epc -0.78 V with Ipc 6 × 10 A is
seen as a broad peak which indicates two simultaneous reductions in this region. An irreversible
Cu(II) to Cu(I) reduction occurs in this potential region. This is well supported by literature data
[
23b,25a-25b]. The ligand also undergoes an irreversible reduction at this potential as confirmed
by the cyclic voltammogram of the isolated ligand as well. The peak in the isolated ligand is at -
.05 V, which has shifted to the new potential (shifted gradually to slightly higher potential
1
values) due to coordination of the ligand to the Cu(II) center [25b-25c], may be assigned to the
-
reduction of imine group [(-C=N-)/(-C=N-) ] [25a]. The cathodic peak at -1.5 V indicates a
further reduction of Cu(I) to Cu(0). This is a one electron process as ascertained from literature
values as well [23b]. The peak in the reverse scan is the anodic dissolution wave of deposited
copper, for Cu(0)→Cu(I) which is indicated by along peak at -0.19 V [23b].
-7
For the nickel complex, the quasireversible redox couple at Epc 0.22 V and Ipc 7 × 10 A
-6
and Epa 0.38 V and Ipa 2 × 10 A can be assigned to the Ni(II)/Ni(I) couple [25d-25f]. The scan
-1
rate has been kept at 100 mV s . The cathodic peak at -0.64 V indicates a ligand reduction which
has also been observed in the free ligand at -1.05 V but shifted to higher potentials due to the
binding to Ni(II) ion. The anodic peak at 1.05 V corresponds to a ligand oxidation as almost
similarly observed in the Cu(II) complex.
The CV plot of the free ligand has been given in the supplementary information (Fig
s16). It shows two cathodic peaks at -0.56 V and -1.05 V corresponding to the reductions on the
ligand and an anodic peak at -0.57 V which corresponds to an oxidation. Scan rate has been
-1
maintained at 100 mV s .
3
.8. Catalysis studies
We have checked catalytic activity of both the complexes for oxidation of cyclohexane,
toluene and cyclopentane. The oxidation of the substrates has been performed by H as the
2
O
2
1
3

oxidant in a slightly acidic medium under ambient conditions. To set optimum conditions, we
have varied amount of hydrogen peroxide (30% in H O)as well as the acid (HNO )keeping fixed
amount of the metal complex (0.03 mmol) in 5.0 mL of acetonitrile. The ratio
n(HNO )/n(catalyst) has been varied in the range of 5 to 25. The n(H )/n(catalyst) has been
varied from 100–500 with the best yield obtained at 500.
2
3
3
2
O
2
Results of oxidation are summarized in Table 4. The primary products of the oxidation of
cyclohexane are cyclohexanol and cyclohexanone; toluene is oxidized mainly to benzyl alcohol
and benzaldehyde, whereas oxidation of cyclopentane gives cyclopentanol and cyclopentanone
as the major products. Cyclohexane oxidation gives a small amount of adipic acid (~9%) in
addition to the major products when the ratio n(H
n(H )/n(catalyst), say 100, no adipic acid has been detected and at still higher ratio of 500
which is the most optimized condition) the amount of adipic acid formed remains the same i.e.
%. Benzoic acid has been obtained as a minor product from oxidation of toluene. Benzoic acid
2
O
2
)/n(catalyst) is 250-300. At lesser ratio
2
O
2
(
9
is generally obtained by aerial oxidation of benzaldehyde, hence this benzoic acid could have
come from benzaldehyde which has been formed during the oxidation of toluene. Oxidation of
cyclopentane gives some high boiling combustion products along with the major cyclopentanol
and cyclopentanone.
The yield has been optimized by varying the relative proportions of nitric acid and
hydrogen peroxide with respect to the catalysts, temperature and varying the reaction time. It has
been obvious from previous studies [6,7,26] that the presence of nitric acid has positive role in
these catalytic reactions. Nitric acid has, mainly, two important roles: (i) it could increase the
vacant coordination site at the metal center by the protonation of the ligand of these complexes
and hence enhances oxidative properties of the catalyst; and (ii) decomposition of peroxide is
retarded in the presence of nitric acid, the stability of peroxo intermediate is increased. However,
it is difficult to determine the exact structure of the complex in solution. Copper center in
dinuclear complex is in both tetra and penta-coordination. Thus, these metal centers would need
nitric acid to increase the unsaturation at the metal center. The highest conversion has been
ascertained at n(HNO )/n(catalyst) ratio 20, while the yield decreases on further increasing the
3
ratio. Hence, this ratio is maintained throughout for all the reaction sets.
The reaction mixture has been stirred for 48 h at room temperature (35°C) under
atmospheric pressure. Different temperatures conditions have been applied to optimize the
1
4

highest yield. Reaction has been carried out at 40, 50, 60 and70°C. The best yield was obtained
at 40°C. Upto 50°C, the reaction proceeds showing slight decrease in yield at 50°C which
gradually declines upto 60°C. However, after 60°C, there is a drastic decrease in the yield of
products as it has been confirmed by gas chromatographic analysis.
The catalysis has also been done at high pressure conditions which fail to yield the
products. This indicates that normal pressure conditions are most desired condition for the
reaction. The reaction has also been performed with TBHP as the oxidant under the similar
experimental conditions but it shows lower yield, 5% in total. This can, probably, be due to its
bulky structure which destabilizes the intermediate. Then, we have performed the same catalytic
reaction under inert (argon) atmosphere to examine effect of the presence of oxygen. However, it
has been found that this reaction shows almost equal yield when same catalytic reaction is
carried out under normal experimental conditions. There is only a small decrease in amount of
products formed under inert atmosphere which indicates only little participation of atmospheric
oxygen in the oxidation process.
Instead of nitric acid, organic acid such as acetic acid has been employed in the reaction.
The yield of product remains the same showing that organic acid is as effective in the role played
by nitric acid. In order to find out the probable mechanisms, we have carried out the catalytic
reaction of cyclohexane with the complex maintaining n(HNO
3
)/n(catalyst) ratio of 20 and
n(H )/n(catalyst) ratio of 500 in the presence of TEMPO (2,2,6,6-tetramethylpiperidin-1-
2
O
2
oxyl). The yield of the reaction is greatly suppressed in its presence. TEMPO is well known as a
radical trap. This indirectly indicates that oxidation reactions occur mainly by mechanisms
involving the formation of radicals.
Copper salts like Cu(NO
3
2
) , under the same reaction conditions exhibit at much lower
activity towards oxidation of all substrates under same experimental condition. It shows only
about 6% conversion. So it is evident that the presence of N and O donor ligands is quite
relevant.
To check the possibility of oxidation of all the substrates under acidic conditions, we
have performed blank reactions for each of the substrates without any metal center. The amounts
of nitric acid and hydrogen peroxide have been chosen at which concentration of these reagents
maximum conversions of the substrates were achieved with the catalyst. Blank reactions have
been carried out with substrate (1.5 mmol), hydrogen peroxide (15 mmol) and nitric acid (0.6
1
5

mmol) at 40 °C for 48 h. Conversion rate is very low (~3.7% for cyclohexane and toluene, and
.5% for cyclopentane). This indicates that the conversion of the substrate to product is difficult
4
reaction and accelerated by the presence of our copper complex.
The reaction is believed to proceed through the formation of alkyl hydroperoxide
(ROOH) (e.g. cyclohexyl peroxideor benzyl peroxide) along with the alcohol (e.g. cyclohexanol,
benzyl alcohol and cyclopentanol) and carbonyl compounds(such as cyclohexanone,
benzaldehyde and cyclopentanone).The formation and quantification of these compounds have
been carried out by following a method introduced by G. B. Shul’pin [27]. According to this
procedure, the reaction mixture has been analyzed by GC before and after treating with excess
PPh
alcohol with subsequent formation of phosphane oxide (OPPh
reduction with PPh , the alcohol peak in the GC analyses rises markedly for each of the substrate
3
. Treatment of the reaction mixture (alkyl hydroperoxide) by PPh
3
leads to the formation
3
). We have observed that after the
3
while the intensity of the>C=O compound’s peak reduces. Alkyl hydroperoxides are, in many
cases, the main primary products of alkane oxidation with hydrogen peroxide. The peroxides
formed in the oxidation with H
2
2
O can, sometimes, decompose in the injector of GC during the
analysis with the formation of the corresponding alcohol and ketone. If we compare the yields of
reaction before and after the treatment with triphenylphosphine, we can see that yield of alcohol
increases tremendously after the treatment with PPh (representative case, Table 4). Before the
3
treatment with PPh3, product of oxidation of cyclohexnae is almost solely cyclohexanone. This
indicates that the saturated hydrocarbon oxidation may follow the Shul’pin pathway.
•
The hydroxyl radical HO could be formed as a result of metal-assisted decomposition of
.
hydrogen peroxide. The hydroxyl radical abstracts H from the substrate (RH) to form R [26a].
The formation of ROOH may be occurred by the reaction between a metal-peroxo intermediate,
.
e.g. bearing a Cu(II)–OOH type moiety and the organoradical R , to form ROOH [28]. The
formation of Cu–peroxo species has been determined by UV-vis spectra. The metal-assisted
.
homolytic cleavage of alkyl hydroperoxide generates alkoxyl (RO , upon O–O bond rupture) and
.
alkylperoxyl (ROO , upon O–H bond breakage) radicals which can form an alcohol (ROH) upon
.
H-abstraction from the alkane (RH) by RO or both ROH and the>C=O upon decomposition of
.
ROO [29].
ESI-mass spectrum of complex 1 in methanol shows the presence of both mononuclear
and dinuclear species (Fig. s13). Peak at 641.09 indicates the presence of dinuclear species and
1
6

may be attributed to the presence of [Cu
2
(L1)
2
(μ
2
-Cl)(H
2
O)(CH OH)]. It shows additional m/z
3
1
+
peak at 277.05 which is assigned to [Cu(L )] . This indicates that dinuclear species also exists in
solution. UV-vis spectra for the complexes have been recorded in methanol at room temperature
as discussed earlier. To further observe the effect of hydrogen peroxide, we have recorded the
UV-vis spectra of the complexes in the presence of hydrogen peroxide (Fig. s14 and s15). It has
been observed that an intense peak at 400 nm, with a shoulder in the range of 410–420 nm,
appears for complex 1. This may be attributed to the existence of Cu–hydroperoxo or Cu–peroxo
species [30] However, there is no observable change in the UV-vis spectrum of complex 2 when
it is recorded in the presence of hydrogen peroxide (Fig. s15).
Complex 2 has also been checked for its catalytic property in oxidizing the substrates.
But the Ni catalyst shows very poor conversion i.e. about 7% total yield for cyclohexane, 5% for
toluene and 4% for cyclopentane. This could be attributed to the coordinatively saturated nature
of the each Ni center in complex 2 with coordination number 6. The UV vis data of the complex
in the presence of H
2
2
O couldnot confirm the presence of Ni-hydroperoxo or peroxo species like
Cu– hydroperoxo or Cu–peroxo in case of complex 1. Also electrochemical studies of the
complex indicate no formation of higher oxidation state species in the said voltage range whereas
Cu(III) species has been detected in the said voltage range indicating more vulnerability
character of complex 1. Thus replacement of Cu by Ni in the complex actually stops the catalytic
ability of the material.
Conversion of saturated hydrocarbons is one of the most difficult reactions in organic
chemistry. Results from past decade [6,7] and recent past [31] indicate that our catalyst is very
good in this respect.Recent report on catalysis with copper(II) complexes of vinylphosphonic
acid and 1,10-phenanthroline shows that only ~32% products are formed where main product is
corresponding carboxylic acid [31a]. Formation of cyclohexanol and cyclohexanone is extremely
low. Copper(II) complexes of functionalized 2,2’:6’,2’’-terpyridines and 2,6-di(thiazol-2-
yl)pyridine could be used as catalyst for cyclohexane oxidation in the presence of hydrogen
peroxide and highest conversion achieved is ~23% [31b]. Three copper(II) complexes of
pyromellitic acid and different aminoalcohols can catalyze cyclohexane oxidation and highest
conversion is ~33% [31c]. Conversion for cyclopentane is also difficult as revealed from the
small yield with Cu(II) complexes [31d].
1
7

4
. Conclusions
We have been able to synthesize and characterize two dinuclear transition metal
complexes with a Schiff-base ligand. One of them is copper containing complex, complex 1 and
another with nickel, complex 2. Complex 1 has been found to be active catalyst for the oxidation
of cyclohexane, toluene and cyclopentane in the presence of hydrogen peroxide as the terminal
oxidant. Corresponding alcohol and aldehyde have been produced as the major products.
Conversion of the substrates is quite high in comparison to the results published by other groups
in recent time. However, complex 2 is not able to convert these substrates under similar
conditions. UV-vis spectral analysis shows that complex 1 could generate Cu-peroxo or Cu-
hydroperoxo species in the presence of hydrogen peroxide whereas formation of such type of
species with nickel complex is not evident from the UV-vis spectral studies.CV of complex 1
shows that two Cu(II) centers could be converted to Cu(III) center with transfer of one electron
in each of two steps. Formation of higher oxidation state species of Ni is not indicated from its
electrochemical studies.These analyses support that formation of Cu-peroxo as active species for
the catalytic conversions. Possible mechanism indicates the involvement of Cu(III) species.
Probably other metals also need higher oxidation states for this catalysis. As generation of
Cu(III) center is feasible, not the higher oxidation state for nickel center evident from
electrochemical studies under normal condition, the dinuclear copper complex can catalyze the
oxidation of saturated hydrocarbons mimicking the activity of particulate methane
monooxygenase while dinuclear nickel complex cannot.
Acknowledgements
AB wishes to thank CSIR, New Delhi for providing her senior research fellowship.
Appendix A. Supplementary data
CCDC 1848336-1848337 contains the supplementary crystallographic data for this paper. 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) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data to this article can
be found online at xxxxxx.
1
8

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2
3

Table 1 Crystal data, data collection and structure refinement of complexes
Complex
Formula
1
2
C
26
H
26Cl
2
Cu
2
N
2
O
6
C
29
H
36
N
8
Ni
2
O
7
Formula weight
T (K)
660.47
298 K
726.08
298 K
Green
Crystal color
Crystal system
Dark green
orthorhombic
P212121
Monoclinic
P 21/n
Space group
a (Å)
13.5722(7)
11.8931(6)
17.3627(8)
90.00
16.7002(5)
7.4796(2)
25.7434(7)
90.00
b (Å)
c (Å)
α (°)
β (°)
90.000(3)
90.00
100.802(2)
90.00
γ (°)
V (Å3)
2802.6(2)
4
3158.65(15)
4
Z
Crystal dimensions (mm)
0.5 × 0.2 × 0.15
1344
0.4 × 0.2 × 0.1
1512
F(0 0 0)
−
3
D
c
(g cm )
1.565
1.527
λ (Mo Kα) (Å)
θ Range (°)
0.71073
0.71073
1.90- 24.99
33619,4936 ,4098
1.601 – 27.220
51406, 7017, 5070
Reflection collected/
unique/observed
Absorption correction
multi-scan
0.0735
0.0479
0.1391
1.150
multi-scan
0.0342
0.0504
0.1450
1.033
R
int
Final R
Final wR
Goodness-of-fit
1
index [I > 2σ(I)]
2
index (all reflections)
2
4

Table 2 Selected bond lengths (Å) and bond angles (°) of complexes 1 and 2
Complex 1
Cu1‒O2
Cu1‒ N1
Cu1‒O1
Cu1‒Cl1
1.997(6)
1.911(8)
1.906(6)
2.250(3)
Cu2‒O3
Cu2‒N2
Cu2‒O4
Cu2‒Cl2
Cu2‒Cl1
1.885(6)
1.918(7)
2.022(6)
2.280(3)
2.724(3)
O2‒Cu1‒N1
O2‒Cu1‒O1
N1‒Cu1‒O1
O2‒Cu1‒Cl1
N1‒Cu1‒Cl1
O1‒Cu1‒Cl1
O3‒Cu2‒N2
O3‒Cu2‒O4
83.0(3)
N2‒Cu2‒O4
O3‒Cu2‒Cl2
N2‒Cu2‒Cl2
O4‒Cu2‒Cl2
O3‒Cu2‒Cl1
N2‒Cu2‒Cl1
O4‒Cu2‒Cl1
Cl2‒Cu2‒Cl1
82.5(3)
170.1(3)
91.7(3)
96.7(2)
156.8(2)
89.9(2)
93.0(2)
163.0(2)
94.45(19)
92.7(3)
89.9(2)
107.1(2)
85.88(19)
94.03(9)
172.4(3)
Complex 2
Ni1- N8
Ni1- O6
Ni1- N1
Ni1- N4
Ni1- O4
Ni1- O5
1.969(3)
1.977(2)
2.035(3)
2.133(3)
2.155(2)
2.170(3)
Ni2- N7
Ni2- O2
Ni2- N4
Ni2- N1
Ni2- O3
Ni2- O1
1.968(3)
1.990(2)
2.038(3)
2.144(3)
2.162(3)
2.164(3)
N8- Ni1- O6
N8- Ni1- N1
O6- Ni1- N1
N8- Ni1-N4
O6- Ni1- N4
N1- Ni1-N4
91.24(10)
169.95(12)
97.73(11)
95.65(12)
92.44(12)
79.55(16)
N7- Ni2- O2
N7- Ni2- N4
O2- Ni2- N4
N7- Ni2- N1
O2- Ni2- N1
N4- Ni2-N1
89.62(11)
171.16(13)
94.28(11)
92.80(12)
91.65(12)
79.18(12)
2
5

N1- Ni1- O4
O6- Ni1- O4
N1- Ni1- O4
N4- Ni1- O4
N8- Ni1- O5
O6- Ni1- O5
N1- Ni1- O5
N4- Ni1- O5
O4- Ni1- O5
82.19(12)
173.12(9)
89.08(11)
87.84(11)
91.09(11)
92.49(11)
93.03(12)
171.56(11)
88.08(11)
N7- Ni2- O3
O2- Ni2- O3
N4- Ni2- O3
N1- Ni2- O3
N7- Ni2- O1
O2- Ni2- O1
N4- Ni2- O1
N1- Ni2- O1
O3- Ni2- O1
81.57(12)
171.06(10)
94.66(12)
90.28(12)
96.10(13)
93.62(12)
91.58(13)
169.69(12)
85.88(13)
2
6

1
Table 3 Main calculated UV-Vis transition for HL , complexes 1 and 2 with vertical excitation
energies (Ecal), oscillator strengths (fcal) of the lowest few excited singlets obtained from
TDDFT/B3LYP/CPCM method in methanol.
Compound Excitation Electronic excitation
Osc.
strength
(f)
CI
Key transitions
(eV)
transition
State
(nm)
2
.904
S
S
S
0-
S
S
S
1
418.14
0.0423
0.1965 (08%)HOMO-1 →LUMO+1
0
0
0
0
0
.6204 (77%)HOMO→LUMO
.1274 (03%)HOMO →LUMO+1
.5570 (62%)HOMO-1→LUMO
.1192 (02%)HOMO-1→LUMO+1
.2872 (16%)HOMO→LUMO+1
HL1
0- 2
2.991
400.12
311.24
0.0252
0.6564
3
.983
0-
5
0.1188 (03%)HOMO-3→LUMO+1
0
0
0
.4700 (44%)HOMO-1→LUMO
.1195 (43%)HOMO-1→LUMO+2
.4220 (35%)HOMO→LUMO+1
3
3
.070
.227
S
0-
S
1
390.05
384.21
0.0642
0.1501
0.6714 (89%)HOMO→LUMO
0
0
.2031 (08%)HOMO-2→LUMO
.1180 (02%)HOMO→LUMO+1
S
0-
S
2
0.6418 (82%)HOMO-1→LUMO+1
0
.1114 (02%)HOMO-3→LUMO
2
7

0
.1834 (06%)HOMO→LUMO
3
.254
S S
0- 4
380.93
0.0402
0.2215 (10%)HOMO-2→LUMO
Complex 1
0
0
.1046 (02%)HOMO-9→LUMO
.5468 (60%)HOMO→LUMO+1
3
3
.900
.956
S
0-
S
8
9
317.88
313.40
0.2581
0.0304
0.1483 (04%)HOMO-2→LUMO+3
0
.6705 (90%)HOMO→LUMO+1
S
0-
S
0.6101 (74%)HOMO-4→LUMO+3
0
0
.2635 (14%)HOMO-4→LUMO+5
.1190 (03%)HOMO-4→LUMO+8
3
3
.083
.120
S
0-
S
1
400.01
396.12
0.0104
0.1033
0.2156 (10%)HOMO-2→LUMO
0
0
.1169 (02%)HOMO-2→LUMO+1
.6153 (74%)HOMO→LUMO
S
0-
S
2
0.1788 (06%)HOMO-2→LUMO
0
0
.3422 (23%)HOMO-3→LUMO
.4418 (39%)HOMO-1→LUMO+1
Complex 2
3
3
.961
.986
S
0-
S
5
6
314.40
311.21
0.0309
0.0571
0.2967 (17%)HOMO-1→LUMO+4
0
.5512 (61%)HOMO→LUMO+1
S
0-
S
0.2806 (15%)HOMO-3→LUMO+2
0
.1165 (02%)HOMO-3→LUMO+1
2
8

a
Table 4 Oxidation of cyclohexane, toluene and cyclopentane
Cyclohexane
b
Totalc
d
TONe
n(H
2
2
O )/n(catalyst)
Time
Yield (%)
Selectivity of
(
h)
Cyclohexanone Cyclohexanol
cyclohexanol (%)
1
2
5
00
50
00
8
3.2
6.4
5.8
9.1
9
64.4
58.7
65
4.5
4
8
8
15.5
21.7
30.7
30.4
48.2
47.5
7.75
10.8
15.3
15.2
24.1
23.5
8
7.6
14.1
21.4
19.3
29.8
0.5
4
9.3
69.7
63.4
62
8
11.1
18.4
47
4
8
4
8f
0.01
Toluene
n(H )/n(catalyst)
b
Totalc
Selectivity of TONe
benzyl alcohol
(%)
d
2
O
2
Time (h)
Yield (%)
Benzyl
alcohol
7.4
Benzaldehyde
1
2
5
00
50
00
8
4.8
7.2
12.2
18.3
21.1
33.8
32.9
39.7
60.6
60.6
56
6
4
8
11.1
11.8
19.4
19.1
23.8
9
8
9.3
10.55
16.9
16.45
19.8
4
4
8
14.4
13.8
15.9
57.4
58
8
8
60
Cyclopentane
n(H )/n(catalyst)
b
Totalc
d
TONe
2
O
2
Time (h)
Yield (%)
Cyclopentanone Cyclopentanol
Selectivity of
Cyclopentanol
(
%)
1
2
5
00
50
00
8
5.2
9.2
5.8
11
52.7
55.3
56.1
52.2
54.2
53.3
5.5
10.3
9.8
4
8
11.4
10.9
14.9
17.2
19.1
20.6
19.4
28.5
31.7
35.8
8
8.5
4
8
13.6
14.5
16.7
14.25
15.85
17.9
8
4
8
2
9