Ethoxysilane appended M(II) complexes and their SiO2/MCM-41 supported forms as catalysts for efficient oxidation of secondary alcohols

Article abstract of DOI:10.1016/j.ica.2017.09.024

Divalent transition metal complexes ML₂ (M = Mn 1; Co 2; Cu 3; Zn 4), possessing an ethoxysilane group as a part of the bidentate Schiff base ((E)-1-((3-(triethoxysilyl)propylimino)methyl)naphthalen-2-ol (L)), have been synthesized. While the copper complex 3 has been isolated in an analytically pure form and characterized by spectroscopic and single crystal XRD studies, the formation of complexes 1, 2, and 4 in solution has been verified by ESI mass spectroscopy and subsequently used for further catalyst preparation without their isolation. Treatment of the in situ formed 1–4 with pre-activated silica in boiling toluene produces the catalysts 5–8, respectively. The copper complex 3 was also treated with MCM-41 in boiling toluene to obtain CuL₂@MCM-41 (9). Elemental analysis (CHN), ESI MS, IR, UV–vis., ¹³C & ²⁹Si NMR, EPR, P-XRD, TGA, BET, SEM and TEM have been used to characterize the compounds. Compounds 3 (homogeneous) and 5–9 (heterogeneous) have been utilized as catalysts in the oxidation of secondary alcohols to corresponding carbonyls in the presence of H₂O₂, t-BuOOH, and C₆H₅C(CH₃)₂OOH. 3 and 9 have shown better catalytic activity than the rest of the catalysts investigated. Combination of 9 with H₂O₂ is the best catalytic system due to its efficiency and reusability besides being environment friendly.

Full text of DOI:10.1016/j.ica.2017.09.024

Accepted Manuscript
Research paper
Ethoxysilane appended M(II) complexes and their SiO2/MCM-41 supported
forms as catalysts for efficient oxidation of secondary alcohols
Rajendran Antony, Rajendiran Marimuthu, Pratap Vishnoi, Ramaswamy
Murugavel
PII:
S0020-1693(17)30891-5
http://dx.doi.org/10.1016/j.ica.2017.09.024
ICA 17880
DOI:
Reference:
Inorganica Chimica Acta
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Received Date:
Revised Date:
Accepted Date:
7 June 2017
12 August 2017
8 September 2017
Please cite this article as: R. Antony, R. Marimuthu, P. Vishnoi, R. Murugavel, Ethoxysilane appended M(II)
complexes and their SiO2/MCM-41 supported forms as catalysts for efficient oxidation of secondary alcohols,
Inorganica Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.09.024
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Ethoxysilane appended M(II) complexes and their SiO2/MCM-41 supported forms
as catalysts for efficient oxidation of secondary alcohols
Rajendran Antony, Rajendiran Marimuthu, Pratap Vishnoi and Ramaswamy Murugavel*
Organometallics and Materials Chemistry Lab, Department of Chemistry,
Indian Institute of Technology Bombay, Mumbai-400076, India
E-mail: rmv@chem.iitb.ac.in and muruks@iitb.ac.in
ABSTRACT
Divalent transition metal complexes ML2 (M = Mn 1; Co 2; Cu 3; Zn 4), possessing an ethoxysilane
group as a part of the bidentate Schiff base ((E)-1-((3-(triethoxysilyl)propylimino)methyl)naphthalen-2-
ol (L)), have been synthesized. While the copper complex 3 has been isolated in an analytically pure
form and characterized by spectroscopic and single crystal XRD studies, the formation of complexes 1,
2, and 4 in solution has been verified by ESI mass spectroscopy and subsequently used for further
catalyst preparation without their isolation. Treatment of the in situ formed 1-4 with pre-activated silica
in boiling toluene produces the catalysts 5-8, respectively. The copper complex 3 was also treated with
MCM-41 in boiling toluene to obtain CuL2@MCM-41 (9). Elemental analysis (CHN), ESI MS, IR,
UV-vis., 13C & 29Si NMR, EPR, P-XRD, TGA, BET, SEM and TEM have been used to characterize the
compounds. Compounds 3 (homogeneous) and 5-9 (heterogeneous) have been utilized as catalysts in
the oxidation of secondary alcohols to corresponding carbonyls in the presence of H2O2, t-BuOOH, and
C6H5C(CH3)2OOH. 3 and 9 have shown better catalytic activity than the rest of the catalysts
investigated. Combination of 9 with H2O2 is the best catalytic system due to its efficiency and
reusability besides being environment friendly.
KEYWORDS: Schiff base, coordination, spectroscopy, heterogeneous catalysis, oxidation, green
chemistry
1

1. Introduction
Owing to the applications of carbonyl compounds as building blocks for drugs, dyes,
plastic additives and fine chemicals,1 their preparation is pivotal in synthetic chemistry. The
oxidation of alcohols to carbonyl compounds has been carried out initially by the use of
stoichiometric amount of inorganic oxidants such as HNO3, producing redundant wastes.2
Replacement of these oxidants by metal free peroxo oxidants, including hydrogen peroxide
(HP), tert-butyl hydrogen peroxide (TBHP) and cumene hydroperoxide (CHP) has been well
acknowledged in the recent literature.2 These attractive oxidants are however ineffective in the
absence of catalysts. A variety of catalysts have been reported for alcohol oxidation such as
metal oxides,3 layered double hydroxides,4 polyoxometalates,2g photocatalyst,5 metal complexes
of carbenes6 and Schiff base complexes.7 Schiff base complexes of first row late transition
metals are attractive due to their concerted advantages of catalytic activity, low cost and
reusability, and well cooperation with the metal free peroxo oxidants.2e,7-8
In the perspective of green chemistry, heterogeneous catalysts are preferred over the
corresponding homogenous analogues. Schiff base complexes can be heterogenized by
anchoring them on solid supports such as alumina,9 graphene,10 carbon nanotubes,11 synthetic
and bio-polymers,12 zeolite,7,13 and montmorillonite clay14. Among these, great emphasis has
been placed on the use of SiO2 due to its remarkable physical (high surface area, easy
availability and, good mechanical and thermal stability) and chemical properties (flexible
surface silanol groups) and tunable porosity.15 Moreover, SiO2 supported complexes combine
the advantages of homogeneous and heterogeneous catalysts as organic-inorganic hybrids.15a,16
Schiff base complexes can be chemically installed on SiO2 through a suitably designed
organic linker. Ethoxysilane compounds (–Si(OR)3), due to their ability to condense effectively
with surface –OH groups of SiO2, are being often employed as linkers.17 3-
(Aminopropyl)triethoxysilane (APTES) is of particular interest due to its versatile –NH2 group
which could form attractive imine bond (showing excellent electron affinity to transition metals)
with aldehydes. This imine bond can fix the metal ions firmly onto SiO2 through coordination
(M-N).18 As it is a chemical adsorption, it could effectively minimize the leaching of metal ions
from silica surface and extend the catalyst’s reusability further.
Though many imine functionalized SiO2 supported catalysts have been synthesized using
APTES in the literature,15a,16a,18b,19 no model Schiff base complex compound possessing APTES
tail has been isolated in analytically pure form, except the Pd(II) complex reported by Bedford
et al.17a If such APTES attached Schiff base complexes are isolated and characterized by single
crystal XRD, they can be considered as the molecular models for their corresponding SiO2
supported heterogeneous catalysts. On the other hand, isolating APTES appended Schiff base
2

complexes is indeed difficult due to hydrolysable ethoxysilane groups,20 as evidenced by the
presence of only two reports on isolation of such Schiff base compounds.17a, 20b
Keeping these facts in mind, we have attempted and successfully isolated a new Cu(II)
Schiff base complex
complexes (Mn(II)
complex have been synthesized from the same APTES ligand system. Using these synthesized
3
containing APTES in the present study. Analogous SiO2 supported
5
, Co(II) , Cu(II) and Zn(II) ) and also MCM-41 anchored Cu(II)
6
7
8
9
complexes, we have developed a sufficiently environmental benign catalytic system for the
oxidation of secondary alcohols with different metal free peroxo oxidants (HP, TBHP and CHP)
in acetonitrile. The details are described in this contribution.
2. Experimental Section
2.1. Materials and methods
APTES, cetyltrimethylammonium bromide (CTAB) and tetraethylorthosilicate (TEOS)
were purchased from Sigma Aldrich. 2-Hydroxy-1-napthaldehyde (Fluka) and
diphenylmethanol (Loba Chemie) were used as procured. Metal acetates (Mn(OAc)2.4H2O,
Co(OAc)2.4H2O, Cu(OAc)2.H2O and Zn(OAc)2.4H2O) were purchased from Merck.
Mesoporous SiO2 (MCM-41) was synthesized by following an earlier reported procedure.21
Melting point of compound
3 was determined in capillary tube and reported uncorrected.
CHN data for was obtained on a Thermo Finnigan (FLASH EA 1112) microanalyzer. IR
3
spectral studies of the KBr diluted compounds were performed on a Perkin Elmer Spectrum One
Infrared Spectrometer. Varian Cary Bio 100 and Shimadzu 3600 UV-vis spectrophotometers
were used to record solution and solid-state electronic spectra of the compounds, respectively.
1
Bruker 400 and 500 MHz NMR spectrometers were used to obtain H and CP MAS (13C and
29Si) NMR spectra, respectively. Mass spectral data of
L and 3 were collected using water Q-
TOF micro mass spectrometer. Thermal studies of the prepared compounds were carried out in
the range of 30-800 °C using Perkin Elmer Pyris Diamond TGA instrument under dynamic N2
atmosphere. P-XRD analysis was performed on a Philips X’pert Pro (PANAnalytical)
diffractometer using Cu-Kα radiation (λ= 1.54190 Å). X-band EPR spectra were analyzed using
JEOL FA 200 EPR spectrometer at 100 K. N2 adsorption-desorption studies of the mesoporous
materials were carried out on a Quantachrome Autosorb-1C analyzer.
2.2. Synthesis of
L
Ethanolic solution of 2-hydroxy-1-naphthaldehyde was added to APTES (1:1 ratio) and
stirred at room temperature (RT) for 8 h. In this reaction, activated type 4A molecular sieve was
used to scavenge the water from the reaction because this molecular sieve is sodium
3

aluminosilicate possessing no surface hydroxyl groups, which can interact with APTES.22 After
removing the molecular sieves by filtration, the solvent was removed to obtain a brown viscous /
sticky liquid, which was characterized by both NMR and mass spectroscopy. 1H NMR (400
MHz, CDCl3): 0.7 (m, 2H, CH2CH2CH2Si); 1.2 (m, 9H, OCH2CH3); 1.9 (m, 2H,
CH2CH2CH2Si); 3.6 (m, 2H, CH2CH2CH2Si); 3.9 (m, 6H, OCH2CH3); 6.9-7.9 (m, 6H, ArH);
8.7 (s, 1H, C
[M+Na]+(found).
H=N); 14.3 (s, 1H, OH
). ESI-MS: 375.19 (calculated); 376.19 (M+H]+ and 398.19
2.3. Synthesis of
3
Equimolar (1 mmol) mixture of 2-hydroxy-1-naphthaldehyde and APTES was taken in
ethanol in the presence of activated molecular sieves and allowed to stir under RT for 8 h. To
the resultant brown solution, 0.5 mmol of Cu(OAc)2.H2O was added and stirring was continued
o
for 8 h. After filtration, the obtained mixture was stored at 5 C for about a week to obtain
brown crystals. Yield: 0.698 g (86%). M.p: ≈ 230 °C. Anal. Calcd. for C40H56CuN2O8Si2: C,
58.71; H, 7.29; N, 3.67. Found: C, 59.12; H, 6.95; N, 3.45. FT-IR (KBr disc, cm-1): 2962 (s),
2873 (s), 1617 (s), 1463 (s), 1381 (s), 975 (s), 953 (s), 798 (s). ESI-MS: 811.29 (calculated);
812.34 [M+H]+ (found).
2.4. Synthesis of 5-9
Equimolar mixture of 2-hydroxy-1-naphthaldehyde and APTES (1 mmol) was taken in
ethanol in presence of activated molecular sieves and allowed to stir at RT for 8 h. To the brown
solution obtained, 0.5 mmol of M(OAc)2.xH2O was added and stirred for 8 h. After removing
ethanol under vacuum, toluene was added to the in situ formed complexes
1-4 (M: Mn(II) 1; M:
Co(II) 2; M: Cu(II) and M: Zn(II) ). These toluene solutions were treated under reflux with
3
4
1.5 g of pre-activated silica for 48 h. The obtained solid was filtered, washed repeatedly with
o
ethanol and dried at 100 C to yield
5
-
8
, respectively.
, in situ formed compound
1.5 g MCM-41. The anchoring and reaction work was carried out as described above for the
preparation of
For the synthesis of MCM-41 based catalyst
9
3 was treated with
5-8.
2.5. Synthesis of ditolylmethanol
Ditolylmethanol was prepared by following a modified version of the previously reported
procedure.23 Magnesium turnings (1.87 g, 77 mmol) and a pinch of iodine were mixed in THF
under N2 atmosphere followed by the addition of 4-bromotoluene (10.48 g, 59 mmol) at RT and
o
heated (40 C) for 1 h. After cooling this mixture to RT, ethyl formate (1.95 g, 26.5 mmol) and
4

o
THF were added. The obtained solution was kept under heating (40 C) for 30 mins and
overnight stirring at RT. To this, saturated NH4Cl solution was added and the organic part was
extracted with CH2Cl2 to get an oily product. Upon trituration with petroleum ether, white
crystals of ditolylmethanol was collected. M.p≈81 oC. IR (KBr disc, cm-1): 3313 (br); 3100 (w);
1
3020 (w); 2910 (w); 1495 (s). H NMR (400 MHz, CDCl3): 7.3 (d, 4H, ArH), 5.8 (d, 1H,
JHH=32 Hz, CH(OH)) 7.1 (d, 4H, ArH) 2.3 (s, 6H).
2.6. Procedure for oxidation reactions
A two-necked RB flask equipped with a magnetic bar was charged with 10 ml acetonitrile.
To this, alcohol (1 mmol), oxidant (15 mmol) and catalyst (16 mg for
3
and 50 mg for 5-9) were
o
added sequentially. This catalytic mixture was kept under reflux at 60 C until the completion of
reaction. The progress of reaction has been monitored by analysing the aliquots of reaction at
regular intervals using GC-MS analysis. A Perkin-Elmer gas chromatograph annexed with mass
spectrometer (GC-MS CLAURUS 500) was used to analyse the reaction aliquots under the flow
of helium carrier gas. The retention time was assigned to the products by comparing with
standard values. They were further supported by mass spectrometry.
internal standard.
n-Decane was used as an
2.7. SEM and TEM analyses
Changes in morphology of SiO2 after chemical modification were studied using a
LEO1530 Gemini Scanning Electron Microscope. FEG-SEM images of platinum coated MCM-
41 and
9 were obtained on a JEOL JSM-7600 microscope. TEM images of MCM-41 and 9 were
taken using Philips CM 200 microscope at 100 kV. Prior to TEM imaging, samples were
dispersed in toluene and placed on holey carbon grids.
2.8. Single crystal X-ray diffraction studies
Single crystal X-ray diffraction data of compound 3 were collected on a Rigaku Saturn 724+ CCD
diffractometer with a Mo-Kα radiation source (λ = 0.71075 Å) at 120 K under continuous flow of
cooled N2 gas. Rigaku CrystalClear software was used in data integration and indexing. Multi-scan
method was employed to correct absorption. WinGX module24 was used for performing all the
calculations. The structure was solved by direct methods using SIR-92.25 The final refinement of the
structure was carried out using full least-square methods on F2 using SHELXL-2014.26 All non-
hydrogen atoms were refined anisotropically. The hydrogen atoms were refined isotropically as rigid
atoms in their idealized locations.
5

3. Results and Discussion
3.1. Synthesis of compounds
L and 1-9
Scheme 1 Synthesis of complexes
Schiff base ligand ( ) was prepared by the condensation of APTES with 2-hydroxy-1-
naphthaldehyde in ethanol and isolated as a viscous brown liquid. To prepare
1-4
L
1-4,
M(OAc)2.xH2O was directly added to the reaction mixture of APTES and 2-hydroxy-1-
naphthaldehyde in the presence of activated molecular sieves at RT (Scheme 1). After filtration,
solvent was slowly evaporated. In the case of Cu(II) complex 3, the complex could be isolated
as brown single crystals. The as synthesized complexes 1-4 were dried under vacuum for
extended periods to remove any occluded ethanol. In order to prepare the heterogeneous
catalysts
5
-
9
(Scheme 2 and 3), the dried samples of 1-4 were treated with required amount of
) in toluene and subsequently refluxed for 48 h to ensure the
SiO2 (for 5-8) or MCM-41 (for
9
maximum condensation of the silica surface hydroxyl groups with the Si-OEt groups of the
complexes.

Scheme 2 Anchoring of complexes 1-4 on SiO2 to produce supported catalysts 5-8.
Scheme 3 Anchoring of copper complex
3 inside MCM-41 to produce catalyst 9
3.2. Single crystal X-ray crystallography
Although several APTES based materials have been reported, the examples of discrete
compounds of APTES isolated in pure form are very few due to the easily hydrolysable ethoxy
silane groups.16b,17a,17b,27 In the present work, we were able to obtain single crystals of
3 through
o
slow evaporation of an ethanol solution of the compound at 5 C. The ORTEP diagram of the
centrosymmetric molecule of
one independent Cu(II) ion with 50% occupancy (present at the inversion center) and one
chelating ligand L-. Selected crystallographic data for
are summarised in Table 1. The four-
3 is shown in Fig. 1. The asymmetric part of the unit cell contains
3
coordinate Cu(II) ion adopts square-planar coordination geometry, and is surrounded by trans
distribution of phenolate oxygen and aldimine nitrogen from two N,O-chelates of deprotonated
ligand L-. The average Cu-O and Cu-N distances and O-Cu-N bond angles are similar to those
found in analogous systems.28 Further, the naphthalene rings are in the same plane (dihedral
angle between both the naphthaldiminato planes is < 1⁰).
7

Fig. 1 ORTEP diagram of
3 (ellipsoids are drawn at 50% probability level; hydrogen atoms are
omitted for clarity)
3.3. IR Spectroscopy
IR spectrum of
3 shows the metal-coordinated azomethine (M-N=CH-) stretching at 1617
cm-1 and vibrations of Si-OC2H5 at 969, 1072 and 1101 cm-1, respectively. Aromatic and
aliphatic –CH vibrations are found in the region of 2900-3050 cm-1 while the aromatic C-C
vibrations appear at 1470 and 1545 cm-1. Similarly, the IR spectra of
recorded prior to their treatment with SiO2. The spectral characteristics of these complexes are
similar to those of (see ESI). SiO2 and MCM-41 exhibit their characteristic peaks at ≈1090 and
1, 2 and 4 have also been
3
800 cm-1 (anti-symmetric and symmetric stretches of Si–O–Si bond, respectively), 460 cm-1
(bending vibrations of O–Si–O units), 980 cm-1 (Si–OH stretching) and 3450 cm-1 (SiO-H
stretch). SiO2 and MCM-41 supported complexes 5-9 also reveal these peaks along with the
characteristic peaks of –CH=N, -CH and -C=C at the respective wavenumbers. This confirms
the presence of aromatic functionalities on silica surface as a result of chemical modification.
3.4. UV-visible spectroscopy
Fig. 2 illustrates UV-visible spectrum of
3 in ethanol, showing four different electronic
transitions. The π-π* transition of aromatic ring and the -CH=N unit appears at 230 nm while the
n-π*transition of non-bonded electrons is observed at 305 nm. Ligand to metal charge transfer
(LMCT) transition (L-Cu) is found at 380 nm.29 The characteristic d-d transition appears at ≈650
nm, indicating possible square planar geometry around copper in
3
(d9 Cu(II) system), with an
unpaired electron of dx -y ground state.30 These electronic transitions such as π-π*, n-π* and
charge transfer are invariably present in SiO2 and MCM-41 supported complexes 9, albeit with
2
2
5
-

lower intensities (see ESI). The d-d transitions are found at ≈510 nm for 531 and at around 650
nm for
and 9 18d,32
6,
7
.
Fig. 2 UV-visible spectrum of 10-5 M solution of
3 in ethanol (inset: d-d transition of 3
at 10-3 M
concentration)
Fig. 3 a) Solid state CP MAS 13C NMR of
8 8
and b) Solid state CP MAS 29Si NMR of
3.5. NMR spectroscopy
For the NMR spectral studies, the diamagnetic Zn(II) complexes
4
and
8 have been
examined as model compounds. Unlike in the case of
L, no –OH resonance has been found in
1
the H NMR spectrum of
4 (see ESI). Besides, noticeable shift is observed for the –CH=N
13
29
resonance due to complexation. Solid state CP MAS C and Si NMR spectra of
8 are shown
in Fig. 3. In the 13C NMR spectrum, distinctive resonances for the three carbons of the propyl
chain are observed at 43.01 (Cl), 23.67 (Cm) and 10.35 ppm (Cn), respectively. The resonances
appearing in the region 105-163 ppm are due to the aromatic carbons (Cb ). The phenolic (Ca)
-
j
and imine carbons (Ck) resonate at 177.13 and 170.03 ppm, respectively. Additional peaks at 60
29
ppm are due to residual ethoxy groups.33 The Si NMR spectrum of
8 shows a major peak at -
112.61 ppm (a distinctive peak of Q4 sites (SiO4Si)). Appearance of low intensity peaks
corresponding to Q3 (-101.71 ppm) (SiO3Si(OR)) and Q2 (-97.20 ppm) (SiO2Si(OR)2) sites
9

indicates a high degree of condensation between ethoxy groups of
of SiO2 during the preparation of
. Presence of T2 (-58.82 ppm) (CSi(OSi)3) and T3(-67.01
ppm) (CSi(OSi)2(OR)) resonances also lend evidence for the high degree of condensation.
4 and surface silanol groups
8
Fig. 4 TGA profiles of SiO2, 5-7, MCM-41 and 9 (inset: TGA profile of 3)
3.6. Thermal Analysis
Thermal analysis of complex
3
and the supported catalysts
5
-
9
were carried out in the
o
range 30-800 °C at a heating rate of 10 C/min (Fig. 4). Complex
3 is thermally stable up to 270
°C and then it undergoes decomposition in three different steps: 0-100 °C (3%), 270-290°C
(36%) and 290-600°C (39%). First step corresponds to the removal of physically adsorbed water
molecules (3%) while second step is assigned to the decomposition of ethoxy silane groups. The
final weight loss occurs due to the removal of aromatic moieties. The remaining 22% matches
with the expected amount of CuO and SiO2 residue. To support the identity of the residual
o
compounds,
3
has been bulk pyrolyzed at 800 C under N2 and subsequently subjected to P-
XRD analysis (see ESI).34 The SiO2 supported complexes
decomposition steps as compared to 3. However the weight-loss during thermal analysis of
5
-
7
do not exhibit any sharp
may
5-7
include the elimination of physically adsorbed water molecules and, organic moieties and
dehydroxylation of surface silanols. The remnant weight % could be due to the metal oxides and
dehyroxylated SiO2. MCM-41 and catalyst
SiO2 and . The percentage of physisorbed water is higher in case of MCM-41 than SiO2 as
this is also evident from the IR studies. The difference between residual amounts (%) of
modified and bare forms of SiO2/MCM-41 supports firm anchoring of on SiO2/MCM-41. The
suggests the effective loading of in MCM-41
9 show different TG patterns as compared to those of
5-7
3
lowest residue (%) that obtained in case of
than SiO2.
9
3
10

3.7. EPR Study
To understand the electronic and geometrical properties of the compounds in detail, their
X-band (9.427 Hz) EPR spectra were recorded at 100 K under a flow of liquid N2. EPR
spectrum of (Fig. 5a) shows the typical hyperfine structure corresponding to four coordinated
(N2O2) Cu(II) mononuclear complexes. The anisotropic pattern of reveals four well resolved
peaks in the low field region and one intense peak in the high field region (characteristic of
3
3
Cu(II) system with one unpaired electron). The order of
g
tensor values (gll>g⊥>2.0023)
2
2
2
indicates the presence of unpaired electron in dx -y orbital with B1g ground state and suggests
the square planar geometry. The calculated gll/All value (130 cm) further supports this without
any appreciable tetrahedral distortion.35 Kivelson and Neiman have reported that gll value is less
than 2.3 for Cu(II) complexes with covalent type of Cu-ligand interactions and accordingly Cu-
L interaction is found to be covalent in 3
.36 The exchange coupling factor (G) is calculated from
the equation, G=(gll-2)/(g⊥-2) to be 3.63. This proves that complex
3 is axial with minimal
exchange interaction.35b,37
Fig. 5. EPR spectrum of a)
The solid state EPR spectra of Cu(II) complexes supported SiO2 and MCM-41 samples
are shown in Fig. 5b and 5c, respectively, while those for and are given in
supplementary data (see ESI). EPR spectrum of compound shows a 11 lines pattern at 100 K
due to the hyperfine interaction, matching with the spectra of other monomeric Mn(II)
complexes (I=5/2).38 EPR spectrum of
at RT exhibits a sextet line pattern with g≈2 and a
3 in toluene; Solid state EPR spectra of b) 7 and c) 9 at 100 K.
7
and
9
5
6
5
5
hyperfine coupling constant of 93.7 G. These parameters further support +2 oxidation state of
Mn and presence of Mn(II) species in SiO2’s surface. The broadness of these signals is due to
the distribution of Mn(II) species on SiO2’s surface which further evidences the heterogeneity
around the grafted species as found in the similar hybrid materials.39 The EPR spectrum
obtained for
6 is very broad due to the differences in relaxation parameters and spin-spin
interaction between Co(II) ions.40
As in the case of EPR spectrum of
3
, EPR spectrum (Fig. 5b) of SiO2 supported Cu(II)
2
2
catalyst
7 shows axial nature and indicates dx -y ground state (gll>g⊥> 2.0023 and G=4.65). The
larger G value is due to the absence of exchange coupling as a result of well dispersion of Cu(II)

centres from each other with appreciable distance. The structural index gII/AII of
7
is 107.4 cm,
distinctive of a square-derived geometry. MCM-41 supported Cu(II) catalyst
9 also exhibits
EPR pattern (Fig. 5c) similar to
7
with the parameters (gll
>
g⊥> 2.0023, G=4.46 and
2
2
gII/AII=104.22 cm), suggesting square-derived geometry with dx -dy ground state. The larger G
value supports well dispersion of Cu(II) centers on MCM-41. The larger value of All for both
7
and
9
may be due to the electrostatic interaction of silica skeleton.7 These observations suggest
that the geometry of Cu(II) is same in free state (3) as well as supported forms (7 and 9).
3.8. Surface area measurements
MCM-41 exhibits Type IV isotherm, high BET surface area and large pore diameter
analogous to other mesoporous materials.41 Anchoring of
on MCM-41 greatly alters the
3
isotherm pattern of MCM-41 (Fig. 6). BET surface area and pore volume of MCM-41 are found
to change after anchoring (Table 2; see ESI). While pure MCM-41 displays pore diameters in
the range 1.2-34 nm,
decrease in pore size could be due to the filling of Cu(II) complex inside the pores of MCM-41.
Decrease in surface area of MCM-41 is the evidence for the successful installation of on
MCM-41. Partial amorphization of layered phase of MCM-41 during the course of anchoring
9 exhibits maximum number of pores typically of 1.6 nm diameter. This
3
3
may also contribute to such decrease.42 N2 sorption studies of SiO2 and
7 were also carried out
(see ESI).
Fig. 6. N2 adsorption-desorption isotherms of MCM-41 and
9 at 77 K
3.9. Powder XRD (P-XRD) studies
Wide angle P-XRD patterns of SiO2, MCM-41 and
5-9 are given in supplementary data
(see ESI). They look very similar except for minor variations in the intensity of the peaks.
12

⁰
Compounds 5-8 show peaks with less intensity compared to parent SiO2 (particularly at 2θ≈20 ,
characteristic peak of the topological structure of SiO2), due to the filling of pores or a reduction
in X-ray scattering contrast between the channel walls of silicate framework and the organic
part. Low angle P-XRD pattern of MCM-41 (Fig. 7) shows the characteristic peaks of its
hexagonal ordered mesoporous lattice at 2θ=2.1° (100), 3.9o (110) and 4.4o (200).18c,43 As
compared to MCM-41, the intensity and the positions of these characteristic peaks are
significantly changed in case of 9. This could once again be due to the lowering of local order
like variations in wall thickness or reduction of scattering contrast between the channel wall of
18b,44
silicate framework and Schiff base ligand in 9.
Fig. 7 Low angle P-XRD patterns of MCM-41 and
9
3.10. SEM and TEM analysis
The changes in surface properties of SiO2 and MCM-41 after the anchoring of complexes
have been investigated by SEM and TEM analyses (Fig. 8 and 9, respectively). Parent SiO2
surface is found to exhibit relatively smooth surface as compared to
images of the catalysts confirm the incorporation of (see ESI). FEG-SEM and TEM images
reveal spherical shaped MCM-41 particles (average size 250 nm) with properly arranged
mesoporous channels. These mesoporous channels cannot be seen in case of (Fig. 9d) as a
5-7. The elemental mapping
3
9
result of metal complex anchoring and these anchored particles are visible in the surface of
MCM-41.

Fig. 8 SEM images of a) SiO2, b) 5, c) 6 and d) 7
Fig. 9 a) SEM images of a) MCM-41 (inset: FEG-SEM) and b)
5 (inset: FEG-SEM); TEM
images of c) MCM-41 and d)
9
3.11. Catalytic studies
Owing to the industrial and commercial applications of carbonyl compounds, the prepared
catalysts and were utilized in the conversion of secondary alcohols (diphenylmethanol,
ditolylmethanol and isopropanol) to corresponding carbonyl compounds in the presence of three
different oxidants (H2O2, -BuOOH and C6H5C(CH3)2COOH). According to Corma et al.
3
,
5
-
7
9
t
,
solvent plays crucial role in catalysis45 and we have used acetonitrile as it functioned well both
in homogeneous and heterogeneous catalysis.15a,19a,46 The reaction temperature was optimized to
o
60 C. Both TLC and GC methods were utilized to monitor the progress of the reaction. The
results obtained for the catalytic oxidation are summarized in Scheme 4 and Table 3.
14

o
Scheme 4. Catalytic oxidation of secondary alcohols with peroxides in CH3CN at 60 C.
To compare the catalytic activity of homogeneous and similar heterogeneous catalysts,
catalytic runs were carried out with
behaviour, the marginally better catalytic activity of
homogeneity with the substrates in the catalytic mixture. Among the heterogeneous catalysts,
has shown better catalytic activity than due to the mesoporosity and high surface area of
MCM-41. Catalyst exhibited better catalytic activity than other SiO2 supported catalysts (
and ). Irrespective of the catalysts and oxidants, maximum conversion (99%) is quickly
3
,
7
and 9. Although
3
and
9 exhibit similar catalytic
3
is consistent with its complete
9
7
6
5
7
reached for isopropanol oxidation, while selectivity has however been found to be similar for all
the three substrates. The catalytic activity of present catalysts is comparable to (often better
than) the previously reported catalytic systems (see ESI).2d,2e,3,5,7,47
3.11.1. Plausible mechanism
Scheme 5. Schematic showing the formation of pentacoordinated Cu(II)-peroxo complex.
In general, oxidation of organic compounds like alkanes and alcohols with peroxides
catalyzed by transition metal complexes follows peroxidative mechanism.48 Occurrence of this
mechanism can be easily verified by the formation of corresponding peroxo complex. By
following the procedure reported earlier,48b one equivalent of
3 was mixed with ten equivalents
of H2O2 in presence of two equivalent triethylamine using CH3OH:CH3CN (1:1) solvent
mixture. The formed pentacoordinated peroxo complex (Scheme 5) was characterized by
analysing its geometry using UV-vis. spectroscopy (Fig. 10). Below 450 nm, the spectrum looks
similar to the parent complex
3 while above 450 nm it looks quite different. The expected
LMCT band of HOO-Cu (≈370 nm) may be superimposed with that of L-Cu. Instead of
showing only one broad d-d transition band at 650 nm, [CuH(L)2OOH] displays two d-d
transition bands at 621 and 740. Bands at 621 and 740 nm are collectively assigned to square
pyramidal geometry and an additional shoulder at 700 nm proposes that the metal geometry has
undergone distortion.49
This implies that the peroxidative mechanism of oxidation of alcohols is operative in the
present case. In the first step hydrogen peroxide gets coordinated to the complex, followed by
15

.
the abstraction of OH radical from the coordinated hydrogen peroxide. This would lead to the
formation of the unstable hypervalent [(L)2HCu(III)O] metal centre. With the addition of
another hydrogen peroxide to this Cu(III) centre, peroxo radical (.OOH) is forced to leave the
coordination sphere which leads to abstraction of hydrogen radical from alcohol. This leads to
regeneration of peroxide and catalyst, and also the formation of ketones (Scheme S1, see ESI).
Fig. 10 UV-vis spectrum showing d-d transitions of [Cu(L)2OOH] in CH3OH:CH3CN (1:1)
3.11.2. Reusability of
7
and
9
The reusability of
7
and 9 was examined over five consecutive catalytic runs under
identical reaction conditions. Prior to the next catalytic run, the used catalyst was filtered from
o
the reaction mixture, washed twice with diethyl ether and dried at 100 C. The immediate loss of
conversion percentage (8 and 3% for
7 and 9, respectively) was observed during the second run
due to the leaching of physically adsorbed catalytic sites from SiO2 and MCM-41. However
after second run, no such loss was noted until at least the fifth catalytic run. In fifth run, catalyst
7
and 9 showed 82% and 93% conversion, respectively (Fig. 11). Moreover, it is found that the
structural integrity of catalyst has not been changed even after its reuse as evidenced by the IR
spectral study (see ESI).

Fig. 11. Reusability of 7 and 9 in diphenylmethanol with H2O2 in acetonitrile at 60 oC for 7 and 2 h,
respectively
4. Conclusions
We have successfully isolated an ethoxysilane appended Cu(II) complex 3 and anchored it on both
SiO2 and MCM-41. Analogous SiO2 supported Mn(II), Co(II) and Zn(II) systems were also synthesized.
All the new heterogeneous systems have been characterized quite exhaustively using multiple
techniques and further employed as catalysts for the oxidation of isopropanol, diphenylmethanol and
ditolylmethanol. MCM-41 supported catalyst 9 has shown better catalytic activity than the other
catalytic systems investigated. Based on the UV-vis spectroscopic control, a peroxidative mechanism
has been proposed. From the catalytic reusability observed, it can be concluded that the catalysts
reported here can be effectively used in the repeated catalytic cycles using H2O2 as the oxidant.
ACKNOWLEDGEMENTS
RM is grateful to the SERB, New Delhi for a JC Bose Fellowship. RA thanks IIT Bombay and
DST-SERB (PDF/2016/000037) for providing Postdoctoral Fellowship. SAIF, IIT Bombay is greatly
acknowledged for analytical and instrumental facilities.
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Table 1
Crystal data for
3
Compound
Formula
Formula weight
[Cu(L)2] (3)
C40H56CuN2O8Si2
812.58
Temperature/K
Wavelength/Å
120(2)
0.71075
Crystal system
Space group
Monoclinic
P21/c
a/Å
b/Å
18.252(4)
7.3054(15)
17.044(4)
90
117.001(4)
90
2024.9(8)
2
1.333
c/Å
α/o
β/o
γ/o
Volume/Å3
Z
Density (calculated)/Mgm-3
Absorption coefficient/mm-1
0.651
F
(000)
962
Reflections collected
Rint
14114
0.0375
Data/restraints/parameters
3546/0/242
1.070
Goodness-of-fit on F2
R1
R1
,
,
wR2
wR2 [all data]
[
I
> 2σ(
I
)]
0.0371, 0.0989
0.0375, 0.0992
Table 2
Results of N2 sorption studies of SiO2,
7
, MCM-41 and
9
Sample
BET Surface area Pore volume
Pore size
(nm)
1.4
1.5
1.2-3.4
1.6
(m2/g)
155
(cc/g)
0.52
0.33
0.6
SiO2
7
86
MCM-41 1087
81
0.09
9
20

Table 3
Results of alcohol oxidation catalyzed by current catalysts with three different oxidants in acetonitrile at
60 oC
Yield &
Time [h]
Conversiond
selectivityd (%)
Cat.
Substrate
a
1
6
5
7
2
b
1
6
c
1
3
a
b
c
>99 >99 >99 100 100 100
>99 >99 >99 100 100 100
4.5 2.5 >99 >99 >99 100 100 100
>99 >99 >99 100 100 100
1.5 1.5 >99 >99 >99 100 100 100
3
5
6
7
9
3
5
6
7
9
3
5
6
7
9
7
4
4
8
2.5
7
2
>99 >99 >99 100 100 100
5.5 >99 >99 >99 100 100 100
6.5 5.5
10 8.5
5
7
3
>99 >99 >99 100 100 100
>99 >99 >99 100 100 100
>99 >99 >99 100 100 100
5
3.5
0.5 0.5 0.5 >99 >99 >99 100 100 100
3.5
3
4
3
1.5
3
2
1
>99 >99 >99 100 100 100
>99 >99 >99 100 100 100
2.5 >99 >99 >99 100 100 100
1
0.5 0.5 >99 >99 >99 100 100 100
Reaction conditions: catalyst (116 mg and 2-650 mg), substrate (1mmol) oxidant (15 mmol), solvent (CH3CN, 10 ml)
aH2O2; b
t
-BuOOH; cC6H5C(CH3)2OOH; d Analysed by GC
Note: No notable conversion was obtained either in the absence of catalyst and oxidant

Graphical Abstract
22

Highlights
• A new Cu(II) square planar complex possessing ethoxysilane tail has been isolated.
• SiO2 supported Mn(II), Cu(II), Co(II) and Zn(II) catalysts have been synthesized and characterized.
• The synthesized catalysts are found to be useful in catalyzing secondary alcohol oxidation.
23