Investigation on catecholase activity of salen Co(III) octahedral complexes

Article abstract of DOI:10.1016/j.poly.2019.114129

We have synthesized three octahedral Co(III) compounds in order to study their catalytic efficiency for the oxidation of 3,5-di-tertiarybutylcatechol (3,5-DTBC) into the corresponding quinone. Among the tested compounds, only one compound, 3 shows the cat

Full text of DOI:10.1016/j.poly.2019.114129

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Investigation on catecholase activity of Salen Co(III) octahedral complexes
Nithya Mohan, S.S. Sreejith, M.R. Prathapachandra Kurup
PII:
S0277-5387(19)30559-5
DOI:
https://doi.org/10.1016/j.poly.2019.114129
POLY 114129
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Polyhedron
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Please cite this article as: N. Mohan, S.S. Sreejith, M.R. Prathapachandra Kurup, Investigation on catecholase
activity of Salen Co(III) octahedral complexes, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.2019.114129
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Investigation on catecholase activity of Salen Co(III) octahedral complexes
Nithya Mohan†,a S. S. Sreejith†,a M.R. Prathapachandra Kurupa,b*
aDepartment of Applied Chemistry, Cochin University of Science and Technology, Kochi 682 022, Kerala,
India
bDepartment of Chemistry, School of Physical Sciences, Central University of Kerala, Tejaswini Hills,
Periye, Kasaragod, 671 316, India
Abstract
We have synthesized three octahedral Co(III) compounds in order to study their catalytic
efficiency for the oxidation of 3,5-ditertiarybutylcatechol (3,5-DTBC) into the corresponding
quinone. Among the tested compounds, only one compound, 3 shows the catecholase activity.
The catalytic efficiency of this compound arises from its capability to stabilise two different
oxidation states of the metal centre. The initial oxidation state of cobalt ion (Co(III)) changes
into Co(II) in order to accommodate the substrate. The kinetic parameters were calculated using
the Michaelis-Menten equation. The catalytic loop was proposed and the byproduct formed in
the cycle was experimentally determined.
1. Introduction
Metal ions especially from the transition metals have significant contribution in the biological
field and also plays a pivotal role in many biological functions. The commonly observed fundamental
processes in the environment as well biology like oxygen transport, photosynthesis and nitrogen
fixation are metal-centric ones.1 Also, the smooth functioning of our body is difficult without the
presence of these large number metalloenzymes. So, it is of utmost importance to design simple as
well as novel coordination compounds which are capable of mimicking the bio-activity of
metalloenzymes.
*Corresponding author
E mail address: mrpcusat@gmail.com, mrp_k@yahoo.com (M.R. Prathapachandra Kurup)
Phone: +91-484-2862423
Fax: +91-484-2575804
† N.M. and S.S.S. have contributed equally to this work.

Keywords: Catecholase, Salen-type; DFT study; Co(III) octahedral
Catecholase oxidase is a copper-containing enzyme and they are found in plants and the
activity of this enzyme is similar to tyrosinase.2 The crystal structure of this enzyme including deoxy
and met forms was early reported by Krebs and co-workers and reveals the binuclear structure of the
enzyme.3, 4 In catecholase oxidase, two copper atoms are present in +2 oxidation state and each metal
is coordinated to four entities in order to satisfy its coordination number as four. Several attempts
were made by researchers from worldwide to improve the efficiency of the catalysis by changing
reaction conditions like the solvent, pH and temperature of the medium, redox potentials and the
metal centre etc.5-9
The literature reports reveal that most of the coordination compounds reported for the
catalytic oxidation of 3,5-ditertiarybutylcatechol (3,5-DTBC) into the corresponding quinone are
copper-based ones.10-15 Reports on transition metal complexes other than Cu(II) atom are limited in
number.1, 9, 16 Also, among the non-copper coordination compounds reported, mononuclear cobalt-
based catalysts are less in number.17-20
Schiff bases21 and their complexes22-26 have found a wide variety of applications and the
synthetic flexibility and tunability of these species still grab the interest of researchers worldwide. In
the past few decades, it has been reported that Schiff bases containing Co(III) or Co(II) with different
nuclearity have been increasingly used in the field of catalysis which includes industrially important
reactions and also different oxidation reaction of various organic functionalities.27-35 Also, they can
bio-mimic various metalloenzymes like cyanocobalamin.
Cobalt(III) complexes are exclusively selected for catecholase oxidation reaction owing to the
ability of these compounds to exist in two different oxidation states and further there are only limited
reports on mononuclear cobalt containing systems in this regard. In this work, we have used Co(III)
octahedral complex as a model compound and an alternative for Cu(II) compounds and have also
predicted the catalytic loop for the oxidation reaction.
2. Experimental
2.1. Materials and methods
The solvents used for the syntheses and catalysis are analytical and spectroscopic grade
respectively. The aldehydes, 5-bromo-2-hydroxy-3-methoxybenzaldehyde and 2-hydroxy-3-
methoxybenzaldehyde are purchased from Sigma Aldrich. The diamines except 2,2’-dimethylpropane

diamine and the metal salts were purchased from Sigma Aldrich and 2,2’-dimethylpropane diamine is
from Nice chemicals.
The purity of synthesized compounds can be determined by elemental analysis. Elemental
analyses of carbon, hydrogen and nitrogen present in all the compounds were done on a Vario EL III
CHNS elemental analyzer. The metal content in the complexes were investigated using Inductively
coupled plasma mass spectrometry (ICP-MS) in a Thermo Fischer Scientific iCAP RQ instrument. Mass
data and the IR spectral measurements were done by Trace 1300 GC and ISQ QD single quadrupole
GC-MS instrument and JASCO FT-IR-5300 spectrometer in the 4000-400 cm-1 range using KBr pellets
respectively. The electronic spectra were recorded on a Thermo Scientific Evolution 220 UV-Vis
spectrophotometer in the 200-900 nm range at room temperature.
2.2. Syntheses of Schiff bases
Ligands of the compounds 1 and 2 were prepared by adapting an earlier reported 1:2
condensation method.36, 37 The aldehyde (1 mmol) was mixed with 2 mmol of the diamine in a mixture
of acetonitrile and DMF (1:1, v/v) and refluxed for about 1 hour. A mixture of solvents was selected
because we were able to isolate single crystals only when we use this mixture of solvents. The
resultant mixture after the completion of the reaction was kept for evaporation. Trial and error
methods were adopted to crystalize compound 3 and was obtained from the one-pot method of
synthesis.
2.3. Syntheses of compounds 1 to 3
2.3.1 Compound 1 ([CoL1(DMF)(H2O)]ClO4)
The metal salt, cobalt(II) perchloride hexahydrate (1 mmol) was added to 1 mmol of the
corresponding ligand solution which was taken in DMF-acetonitrile mixture. The resultant mixture was
then heated under reflux condition for about two hours. The yellow color of the ligand solution
gradually changes into dark brown as the complex was formed. The reaction mixture was then kept
for evaporation and the product formed was recrystallized from DMSO. It was washed with ether and
dried over P2O10 in vacuo (Scheme 1). Yield: 76%. Elemental analysis (%) for compound 1
([C24H31CoBr2N3O6]ClO4) Calc.: C, 37.16; H, 4.03; N, 5.42; Co, 8.32%, Found: C, 36.95; H, 4.00; N, 5.71,
Co, 8.21%.
2.3.2 Compound 2 ([CoL2(DMF)(H2O)]ClO4)
The same procedure was adopted for the synthesis of compound 2, but we could not isolate
single crystals of compound 2. Several attempts were made by varying the solvents and metal salts in

order to obtain pure single crystals. The separated powder was then washed with ether and dried over
P2O10 in vacuo (Scheme 1). Yield: 78%. Elemental analysis (%) for compound 2 ([C21H25CoBr2N3O6]ClO4)
Calc.: C, 34.38; H, 3.43; N, 5.73, Co, 8.65%; Found: C, 34.12; H, 3.11; N, 5.57; Co, 8.59%. Mass data:
735.68 (M+2, 100%, Fig. S1).
3
⊂
2.3.3 Compound 3 ([CoL (MSA) (H2O)])
One-pot method was adopted for the syntheses of compound 3. 2-hydroxy-3-
methoxybenzaldehyde (1 mmol) (MSA) and 2 mmol of the diamine 2,2’-dimethyl-1,3-propane diamine
were mixed together in the mixture of acetonitrile and DMF (1:1, v/v) and heated under reflux
condition for one hour. Once the formation of ligand was confirmed, 1 mmol of cobalt(II) chloride
hexahydrate was added and the reaction was continued for 2 h. The resultant solution was green in
colour and kept aside for evaporation. The product was recrystallized from DMSO and was washed
with ether and dried over P2O10 in vacuo (Scheme 1). Yield: 71%. Elemental analysis (%) for compound
3 (C28H31N2CoO6) Calc.: C, 61.09; H, 5.68; N, 5.09; Co, 10.19%; Found: C, 60.94; H, 5.18; N, 5.33; Co,
10.08%.

Scheme 1 Syntheses of compounds 1 to 3.
2.3. X-ray structural determinations
Single-crystal X-ray diffraction studies of the crystalline samples were performed on a
Bruker SMART APEXII diffractometer, equipped with a graphite crystal incident-beam
monochromator, and a fine focus sealed tube with Mo Kα (λ = 0.71073 Å) as the X-ray source.
The unit cell dimensions were measured and the data collection was performed at 293(2) K.
Bruker SMART software was used for data acquisition and Bruker SAINT software for data
integration.38 Absorption corrections were carried out using SADABS based on Laue symmetry
using equivalent reflections.39 The structure was solved by direct methods and refined by full-
matrix least-squares calculations with the SHELXL-14 software40 in the WINGX software
package.41 The structures were plotted using ORTEP-3 for windows41 and DIAMOND42 version
3.2g. All non-hydrogen atoms were refined anisotropically and positions of hydrogen atoms on
carbon were derived from Fourier difference maps and were placed geometrically with C–H
bond distances of 0.93 Å and refined in the riding model approximation with Uiso(H) set to 1.2
Ueq(C). The hydrogen atoms attached to nitrogen atoms and water molecules are located from
difference Fourier maps. The water H-atoms were restrained using DFIX and DANG instructions.
2.5 Theoretical studies
DFT calculations were done employing the ORCA 3.0.3 software package43, 44 using Becke’s
three-parameter hybrid exchange functional including the Lee–Yang–Parr nonlocal correlation
functional (B3LYP)45, 46 and the triple zeta valence basis set with one set of polarization function
(TZVP)47. To accelerate the calculations, the resolution of identity (RI) approximation was utilized with
the decontracted auxiliary def2-TZV/J Coulomb fitting basis sets and the chain-of-spheres (RIJCOSX)
approximation to exact exchange as implemented in ORCA.48 The optimized geometries were then
confirmed to be global energy minima by performing frequency calculations, which showed no
imaginary frequency. The TZVP basis set was used since it reduces the basis set superposition error
(BSSE) to negligible in the calculation of systems with noncovalent interactions. The Avogadro
software was used for the visualization of orbitals.49

2.6. Catalytic oxidation of 3,5-di-tert-butylcatechol
A stock solution of the catalysts was prepared in 10-4 M concentration in DMF and different
equivalents (10, 30, 50, 70 etc) of the substrate (3,5-DTBC) were chosen for testing the catecholase
activity. According to different literature reports, the best solvent for conducting catecholase activity
is acetonitrile and methanol but in our case as the catalysts (compounds 1, 2 and 3) were not
completely soluble in these solvents; DMF solvent was selected.1, 50
Initially, 10-4 M concentration of the catalysts was mixed with 100 equivalence of the
substrate, 3,5-DTBC in an open beaker and stirred for about 2 hours at room temperature and in the
presence of atmospheric oxygen. Formation of 3,5-di-tert-butylquionone (3,5-DTBQ) was monitored
by withdrawing 2 µL of the sample solution from the beaker. The electronic spectra of this solution
were taken and the peak around 400 nm was monitored. This procedure was repeated every 10-
minute interval to monitor the formation of the product.
2.7. Iodometric detection of hydrogen peroxide
The formation of hydrogen peroxide in the reaction medium as a byproduct was identified
and confirmed from the iodometric experiment. Once the completion and the formation of quinone
were confirmed, the solution was acidified with con. H2SO4 to stop the reaction. It is then diluted with
3 ml of water and the reaction mixture was extracted three times with chloroform to remove the
formed 3,5-DTBQ during the reaction. In order to accelerate the formation I3- ion, 1 ml of 0.3 M
potassium iodide and a catalytic amount of 3% ammonium molybdate are then added to the aqueous
layer and the electronic spectra were recorded.
3. Results and discussion
3.1. IR spectral data
Schiff base shows its characteristic phenolic –OH stretching frequency at ~3500 cm-1 but
complexation leads to the disappearance of this. However, in all the three Co(III) compounds, a peak
can be seen in the same region which is due to the presence of water molecule. But, in the case of

compound 3, which is an inclusion compound with water as the guest molecule, as opposed to the
several reports no twin peaks are observed in this region.51 This is due to the presence of OH group in
free aldehydic group coordinated to the metal center which effectively masks the twin peaks normally
seen in this region (Fig. S2, Table S1). The stretching frequencies of these water molecules are found
to be shifted into the lower frequency region and the shifting of this band towards lower wavenumber
region and its strength is indicative of the presence of strong hydrogen bonding which is evident from
the crystal structure which would be discussed later.52-54
The C–H asymmetric and symmetric stretching vibrations are seen in the region 2980-2850
cm-1. Aromatic C–C stretches are seen in all the spectra around 1470 cm-1 while phenolic C–O stretches
which is seen at 1385 cm-1 in the ligand, shifts towards lower wavenumber region confirming the
involvement of phenolic C–O bond in complexation
A peak at 1099 and 621 cm-1 in the compound 1 and the peaks at 1089 and 619 cm-1 in
-
compound 2 indicate the ν3 and ν4 vibrations of perchlorate ion (ClO4 ) and this suggests a tetrahedral
geometry (Td) of the perchlorate anion present in these compounds. Also, a sharp peak around 920
cm-1 which corresponds to ν1 was found to be absent in the above compounds which suggests that the
perchlorate ion found in the compounds was located outside the coordination sphere.55, 56
3.2. Electronic spectral data
Electronic spectra of the synthesized Co(III) compounds (1 to 3) were recorded in DMF
solution of spectroscopic grade at room temperature. The intraligand transitions and the dd bands
were measured by preparing the stock solution of 10-3 M and 10-5 M respectively.
All the three compounds reported here have an octahedral or distorted octahedral geometry
with Co(III) atom at its center. These octahedral d6 cobalt complexes have 5D atomic ground state and
the electronic ground state observed is 5T2g. Most of these octahedral Co(III) compounds are reported
to be low spin having A1g ground state and a few high spin compounds from this class are also reported.
5
For an octahedral complex we could expect a single transition from T2g→ Eg but due to Jahn Teller
distortion the excited state may split into two and two such transitions are possible under these
situations. These transitions are from A1g → T1g and T2g.57
In the case of compound 3, a broad peak at around 376 nm was observed and this is the only
peak observed in the compound. The peak at 376 nm arises due to the charge transfer transition from
the ligand to metal (LMCT) which appears to be broad and may mask the other possible transitions in
these two compounds (Fig. S3). In addition to the LMCT transition observed at ca. 370 nm, a shoulder

could be seen in the 280-340 nm region in compound 2. This shoulder indicates the n→π* or π*→ π*
transition due to the ligand present in the compound.
The dd band could not be observed in the lower concentration range so we have prepared a
10-3 M solution for monitoring the dd bands present in the compounds. A peak at ca. 633 nm in
compound 1 and 625 nm in compound 2 can be assigned to the dd transition present in the
compounds (Fig. S4, Table S2). In compound 3 any peak beyond 500 nm could not be seen which may
be due to the masking of these peaks by the intense CT bands present in the nearby region.
3.3. Crystal structure description
3.3.1. Compound 1 ([CoL1(DMF)(H2O)]ClO4)
1
Compound 1 crystallizes into triclinic P space group (Fig. 1, Table S6) and has an octahedral
coordination environment around Co(III) centre.
Fig. 1 ORTEP diagram of the asymmetric unit of compound 1 with atom labelling (for clarity
perchlorate anion is not labelled and hydrogen atoms are not shown).

The asymmetric unit of the compound consists of the molecule and a perchlorate ion which is
present outside of the coordination sphere. The oxidation number of the central cobalt atom is at +3
which is maintained by the deprotonated phenolic moieties of the ligand as well as the perchlorate
anion present in the asymmetric unit of the crystal system. The coordination number is 6 and is
satisfied by the four donor atoms (N2O2) of the ligand and rest of the two are from the solvent
molecules. The compound falls in the category Ma4b2 and has a C2v symmetry.2 Water molecule, and
the DMF solvent molecule which is used for the synthesis is coordinated to the metal centre (Fig. 2).
The key crystallographic information is tabulated in Table S3.
Fig. 2 Coordination polyhedron around Co(III) in compound 1.
The bond length and the bond angles around Co(III) suggest a pseudo octahedral geometry to
the compound.58, 59 It also reveals that the solvent molecules ie. O1s of DMF and O1w of water
molecule occupy the axial positions of the distorted octahedra with bond length Co1–O1s (2.20 Å),
Co1–O1w (2.27 Å) respectively and the bond angle formed between O1s–Co1–O1w is 177.02°. Thus,
the compound possesses a cis Ma2b4 type conformation. The Co(III)–N bond distances are observed
to be quite high compared to the other reported systems whereas the Co(III)–O distances are
comparable with the similar systems.14, 58, 60
The central metal atom deviates 0.0147 Å from the coordinating atoms and from their square
plane comprising of atoms N(1)–N(2)–O(2)–O(3) which is evidenced from least-square plane
calculations. The two metallocyclic chelate rings (#1; O(2)–C(1)–C(6)–C(7)–N(1)–Co(1), #2; O(3)–
C(13)–C(14)–C(19)–N(2)–Co(1)) are found to be coplanar with each other with an angle of deviation
of 4.50°. The third metallocycle involving the spacer diimine moiety is twisted from the other two (#1
and #2) by an average angle of 22.07°. The compound forms bifurcated hydrogen bond with the
nearest neighbour and therefore comes under the category of pseudo-inclusion compounds (Fig. 3).61

The coordinated water molecule in one compound (O1w) makes bifurcated hydrogen bond with the
oxygen atom of phenolic oxygen atoms (O(2), O(3)) and the methoxy oxygen atoms (O(1), O(4)) of the
neighboring unit. Apart from these classic intermolecular hydrogen bonding interactions, a number of
non-classical intramolecular hydrogen bonding interactions are also present in the crystal structure
(Fig. S5). These intramolecular interactions connect the perchlorate ion to the rest of the molecule.
The oxygen atoms of the perchlorate unit interact via non-classical hydrogen bonding with the diimine
spacer as well as the DMF molecule. Further, the hydrogen atom of this DMF (H22) interacts with the
phenolic oxygen atom of the aldehydic group by the same non-classical hydrogen bonding interaction
(Table S4, Fig. S5).
Fig. 3 Pseudo encapsulation in compound 1.
In this compound, there are three metallochelate rings formed via the coordination of the
metal. Out of the three, the metallocycle #1 (Cg(1) ; O(2)–C(1)–C(6)–C(7)–N(1)–Co(1) and #2 (Cg(2);
O(3)–C(13)–C(14)–C(19)–N(2)–Co(1) adopt a half chair (H) conformation (Table S5).
In addition to the above-discussed interactions, a conventional π···π and C–H ···π interaction
altogether stabilizes the solid-state architecture. Along with the bifurcated hydrogen bonding
interactions, the two adjacent molecules are paired by a type of chelate–chelate interaction between
Cg(4) and Cg(5) accounting for metalloaromaticity (Fig. 4, Table S4). The dimer formed via pseudo-
encapsulation is connected to the nearest dimer through a C–H ···π interaction and results in a
supramolecular tetrameric form (Fig. 5).

Fig. 4 Conventional π···π interaction present in compound 1.
Fig. 5 Formation of a tetramer via C– H ···π interaction.
3
⊂
3.3.2. Compound 3 ([CoL (MSA) (H2O)])
The structure of compound 3 has been solved in monoclinic P21/c space group. The
asymmetric unit of the compound houses two crystallographically independent molecules and a guest
water molecule in its N2O4 cosmopolitan compartment. The ORTEP image of the compound is shown
in Fig. 6. We have included only one out of the two molecules in the asymmetric unit for clarity. The
refinement data and other crystallographic information are tabulated in Table S6.

Fig. 6 ORTEP diagram of the asymmetric unit of compound 3 (for clarity hydrogen atoms are not
shown).
Similar to compound 1, 3 also falls under Ma4b2 type, 4+2 coordination geometry and thus the
metal atom is octacoordinated. Four out of six in this octacoordination are fulfilled by the two
azomethine nitrogen atoms and two by phenoxo oxygen atoms of the salen-type ligand as seen in the
case of 1. In contrast to 1, apart from the salen ligand which is formed by the 2:1 condensation of
diamine with an aldehyde, a bare aldehydic group (3-methoxysalicylaldehyde (MSA)) is also ligated to
the central metal atom in 3 which acts as the coligand. This coligand chelates the remaining two sites
via the heteroatoms (O) present in this aldehydic moiety. The phenolic oxygen of this free aldehyde is
deprotonated and is covalently linked to the central metal atom. The simultaneous attachment of the
deprotonated and the carbonyl oxygens restricts the oxidation state of cobalt as +3 and into
octahedral coordination. In the ORTEP image, atoms O5 and O6 are the coordinating atoms of the
bare aldehydic moiety whereas O2, O3, N1 and N2 are from the salen Schiff base ligand.
Even though there are a number of different facets to this compound when compared to the
others, one significant difference which can be highlighted is the near to perfect octahedral geometry.
The calculated bond lengths and bond angles around the Co(III) atom in 3 supports its near-ideal
geometry (Table S7). Further, the mean plane deviation calculations show that the metal atom
deviates least from the ligating atoms (0.021 Å).

Apart from the three metallocycles formed as seen in 1, here this type of octacoordination
results into four metallocyclic chelate rings in which metallocycle #1 comprising of atoms O(2)–C(6)–
C(1)–C(7)–N(1)–Co(1) adopts a half chair type conformation (H-type). The puckering parameters for
this H conformation are Q = (0.434), ɵ = (65.5°) and ɸ = (11.05°). It is interesting to note that the four
metallocycles formed are almost mutually perpendicular to each other.
As discussed earlier, compound 1 is in the family of pseudo-inclusion compounds where the
coordinated water molecule exists in the guest cavity of its neighbor. In contrast to this, here in 3, the
guest water molecule is encapsulated in the O4 cavity of the salen Schiff base ligand. This encapsulated
water molecule is displaced by 1.623 Å from least-squares N2O2 plane which shows that the
encapsulation is of intermediate strength. The bifurcated hydrogen bonds formed by the guest water
molecule with the four oxygen atoms in the O(phenoxo)2O(methoxy)2 compartment exhibits strong
hydrogen bonds (1.98-2.56 Å). Two types of supramolecular synthons are formed due to this
푅2
푅2
encapsulation and can be represented as ring motifs 2(6) and 1(5) (Fig. 7, Table S8).
Fig. 7 Formation of different ring motifs via encapsulation of guest water molecule in 3.
Apart from these hydrogen bonding interactions, a number of non-classical hydrogen bonding
and a conventional CH···π interactions are present in the crystal system and these supramolecular
interactions altogether stabilize the solid-state structure of the compound (Table S8).
3.4. Theoretical studies
3.4.1 Optimised geometry of compound 2 ([CoL2(DMF)(H2O)][ClO4])

Several attempts were done to crystallize compound 2 but we could not separate any single
crystals of diffraction quality. We have studied the structural properties of compound 2 theoretically
using DFT calculations. From the data obtained using various spectroscopic results, a tentative
structure was optimized using gaussian at B3LYP/TZVP level of theory (Fig. 8). In the octacoordinated
Co(III) compound, the two axial positions are occupied by the solvent molecule used for the synthesis
(DMF) and a water molecule respectively. Rest of the coordination is satisfied by the hetero donor
atoms of Salen type Schiff base which also defines the square plane. The theoretically obtained bond
angle and bond length are tabulated in Table S9 and it suggests the occupancy of equatorial position
by N2O4 atoms and the axial occupancy by solvent molecules (DMF and water).
Fig. 8 DFT optimized structure of compound 2 at B3LYP/TZVP level of theory.
3.5. Catalytic oxidation of 3,5-di-tert-butylcatechol into the corresponding
quinone
The catecholase activity was tested with three of our synthesized compounds (1, 2 and 3).
Catecholase activity was initially checked with 100 equivalence of the substrate 3,5-DTBC in an open
beaker and stirred for about 2 hours at room temperature and in presence of atmospheric oxygen.
Formation of 3,5-di-tert-butylquionone (3,5-DTBQ) was monitored by withdrawing 2 µL of the sample
solution from the beaker. The electronic spectra of this solution were taken and the peak around 400

nm was monitored. This procedure was repeated every 10-minute interval to monitor the formation
of the product. The absorbance of the peak around 400 nm, which is the characteristic peak of 3,5-
DTBQ was analysed and was found to increase as the time proceeds. This hyperchromic nature of the
peak confirms the product formation. Once we confirm the product formation, the same procedure
was repeated with the different equivalence of the substrate and the electronic spectra were recorded
at room temperature and at equal intervals of time. Out of the three catalysts tested, only 3 shows
catecholase activity which is discussed in the following sections.
3.6. Kinetics of the reaction
The absorption experiments were carried out to investigate the catalytic activity of the
compounds 1 to 3. The absorption spectra obtained (Fig. 9) corroborate the capacity of compound 3
to oxidize the substrate 3,5-DTBC into 3,5-DTBQ. The kinetics of the reaction was studied using initial
rate method by following the increase of absorbance at 400 nm in each case. The rate constant of the
reaction is calculated from the log[Aα/(Aα−At)] vs time plot. The dependence of substrate
concentration on the oxidation process was monitored under the aerobic condition at 10-4
M
concentration of the compound with the different equivalence of the substrate. As the compounds
show saturation kinetics, Michaelis- Menten kinetics were applied to obtain the data and with the
help of Lineweaver-Burk plot of linearization, different kinetic parameters were determined (Fig.10).
1/V = (KM/Vmax) )(1/[S]) + 1/Vmax
Lineweaver-Burk plot was constructed by using the above equation and Michaelis-Menten
constant (KM = 15.0 × 10-3 M) and maximum initial rate (Vmax = 3.06×10-3 Mmin-1) were determined.
Fig. 9 Electronic spectral profile for the formation of 3,5-DTBQ by the addition of 100 equivalent of
the substrate in 3.

Fig. 10 Michaelis -Menton and Lineweaver-Burk plot of 3.
3.7. Reaction mechanism–Catalytic loop
The mechanism behind the catalytic oxidation of 3,5-DTBC into 3,5-DTBQ can be explained by
the proposed catalytic loop (Fig. 11). Once the substrate is added to the catalytic solution, one of the
coordinated labile ligands get detached momentarily from the metal centre and the 3,5-DTBC gets
attached to the metal centre. In between the substrate attack and the detachment of one of the labile
ligand, the oxidation state of the cobalt ions changes momentarily from Co(III) to Co(II). The substrate
is coordinated to the central metal atom via the deprotonation of one of its phenolic –OH. Subsequent
removal of the second proton and the reduction of atmospheric oxygen happens before the substrate
gets oxidized and removed from the catalytic centre. The byproduct H2O2 was also identified
iodometrically which further supports the above proposition.
The oxidation state of cobalt changes from Co(III) to Co(II) at some stages of catalytic reaction
and such an exchange is possible only in compound 3 where the free aldehydic moiety, 3-
methoxysalicylaldehyde (MSA) (by the deprotonation of phenolic –OH) is coordinated to the metal

centre in 3. In the remaining compounds (1 and 2) the oxidation state +3 of the cobalt metal centre is
fulfilled by the perchlorate ion present outside the coordination sphere.
Fig. 11 Catalytic loop for the formation of 3,5-DTBQ.
3.8. Iodometric detection of hydrogen peroxide
The above-proposed mechanism was confirmed by analyzing H2O2, which is the byproduct
formed through the catechol oxidation reaction. It was determined by iodometric method which is
already discussed in the experimental section. Electronic spectra of the aqueous layer formed after
the addition of 3% ammonium molybdate were recorded and a characteristic absorption band at 353
nm (ɛ ca. 24, 000 M-1 cm-1 in water) corresponding to I3- was observed in 3 (Fig. S6).
4. Conclusion
We have synthesized and characterized three different Co(III) compounds of octahedral
coordination geometry. The coordination environment and solid-state interactions were studied with
the aid of single-crystal X-ray diffraction analyses. Among the three compounds, compound 3 falls in
the category of inclusion compound with water guest molecule and the rest of them are of pseudo-
inclusion in nature. The octacoordination and the possibility to exist in multiple oxidation states
prompted us to conduct homogeneous catalytic studies with these compounds. The oxidation
reaction of 3,5-DTBC to 3,5-DTBQ was conducted with three synthesized compounds and only

compound 3 shows catalytic activity. The labile coordination nature and the possibility to exist as
Co(III) and Co(II) are the reason for the catalytic efficiency.
Appendix A. Supplementary data
CCDC 1943425 and 1943427 contain the supplementary crystallographic data for 1 and 3 respectively.
These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 IEZ, UK; fax: (+44) 1223-336-
033; or e-mail: deposit@ccdc.cam.ac.uk.
Supporting Information: IR, UV-VIS, mass spectra, Hydrogen bonding and ring interactions, Crystal
data tables.
Acknowledgements
Nithya Mohan is grateful to Cochin University of Science and Technology, Kochi, India for financial
assistance in the form of a Senior Research Fellowship. The authors are thankful to the Sophisticated
Analytical Instrumentation Facility (SAIF), Kochi, India for single crystal X-ray diffraction
measurements.
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Investigation on catecholase activity of Salen Co(III) octahedral complexes
Nithya Mohan†,a S. S. Sreejith†,a M.R. Prathapachandra Kurupa,b*
Graphical Abstract
Three octahedral Co(III) Salen Schiff base complexes are synthesized and has been
investigated for their catecholase activity. It has been found that the labile coordination nature
and the possibility to exist as Co(III) and Co(II) helps one of the prepared complex to catalyze the
oxidation reaction.
*Corresponding author
E mail address: mrpcusat@gmail.com, mrp_k@yahoo.com (M.R. Prathapachandra Kurup)
Phone: +91-484-2862423
Fax: +91-484-2575804
† N.M. and S.S.S. have contributed equally to this work.

Investigation on catecholase activity of Salen Co(III) octahedral complexes
Nithya Mohan†,a S. S. Sreejith†,a M.R. Prathapachandra Kurupa,b*
*Corresponding author
E mail address: mrpcusat@gmail.com, mrp_k@yahoo.com (M.R. Prathapachandra Kurup)
Phone: +91-484-2862423
Fax: +91-484-2575804
† N.M. and S.S.S. have contributed equally to this work.