Syntheses, structures and catecholase activity of two cobalt(III) complexes derived from N,N′-ethylenebis(3-ethoxysalicylaldiimine): A special host-guest system from a special ligand

Article abstract of DOI:10.1016/j.ica.2015.06.007

Abstract The work in this report deals with syntheses, crystal structures, catechol oxidase activity and ESI-MS (positive) studies of two mononuclear cobalt(III) compounds [Co^IIIL^OEt-en(N₃)₂?(H₃O

Full text of DOI:10.1016/j.ica.2015.06.007

Inorganica Chimica Acta 435 (2015) 38–45
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Syntheses, structures and catecholase activity of two cobalt(III)
complexes derived from N,N0-ethylenebis(3-ethoxysalicylaldiimine):
A special host–guest system from a special ligand
⇑
Priyanka Chakraborty, Sasankasekhar Mohanta
Department of Chemistry, University of Calcutta, 92 A.P.C. Road, Kolkata 700 009, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The work in this report deals with syntheses, crystal structures, catechol oxidase activity and ESI-MS
(positive) studies of two mononuclear cobalt(III) compounds [CoIIILOEt–en(N3)2ꢀ(H3O+)]ꢁ2MeOH (1) and
[CoIIILOEt–en(NCS)(H2O)]ꢁDMFꢁH2O (2), where H2LOEt–en is N,N0-ethylenebis(3-ethoxysalicylaldiimine).
The metal center in both the compounds occupies N(imine)2O(phenoxo)2 compartment. Two apical coor-
dination positions are occupied by one anionic ligand (thiocyanate) and one neutral ligand (water) in 2
but by two anionic ligands (azide) in 1. The extra negative charge in [CoIIILOEt–en(N3)2]ꢂ is balanced by an
oxonium ion (H3O+), which is trapped in the O(phenoxo)2O(ethoxy)2 compartment to result in the forma-
tion of host–guest species [CoIIILOEt–en(N3)2ꢀ(H3O+)] in 1. Both the compounds show catechol oxidase
activity (1 in MeCN, 2 in DMF) as monitored by the UV–Vis spectroscopy of the aerial oxidation of 3,5-
DTBCH2 to 3,5-DTBQ. The kinetic parameters have been determined using Michaelis–Menten approach.
The Kcat values for 1 in MeCN and 2 in DMF are 10.0 hꢂ1 and 11.2 hꢂ1, respectively. ESI-MS (positive) of 1
and the mixture of 1 and 3,5-DTBCH2 have been recorded and the positive ions have been well assigned.
Interestingly, a 1:2 adduct of composition [CoIILOEt–en(3,5-DTBQ)2+H+]+ has been identified from the spec-
trum of the mixture of 1 and 3,5-DTBCH2. Unique features of 1 and 2 in terms of showing catechol oxidase
activity and, more importantly, unique structural features of compound 1 have been discussed.
Ó 2015 Elsevier B.V. All rights reserved.
Received 22 April 2015
Received in revised form 8 June 2015
Accepted 11 June 2015
Available online 18 June 2015
Keywords:
Oxonium ion
Catecholase activity
Cobalt(III)
Hydrogen bonds
ESI-MS
1. Introduction
is worth mentioning that a number of the metal compounds
derived from H2LOEt ligands are weak interaction directed self-
Many mono/di/oligonuclear metal complexes have been
reported from the following types of acyclic Schiff base ligands:
(i) Single-compartment ligands (H2LSingle), obtained on [2+1] con-
densation of salicylaldehyde/2-hydroxyacetophenone/2-hydrox-
assemblies [1,19–35] and thus lie in the domain of general per-
spectives of crystal engineering [36–39]. In spite of our extensive
works on H2LOEt ligands [1,19–35] and in spite of our observation
of a number of interesting self-assemblies over a decade, we have
been hopeful that some more new types of weak interaction direc-
ted systems can be stabilized with this family of special ligands
and therefore we are yet in the process of continuous development.
Although catechol oxidase, which catalyzes aerial oxidation of
catechol to o-quinone, contains a dicopper(II) active site [40,41],
a number of functional model compounds of other metal ions are
known to show this activity [24,42–53]. Those model compounds
include manganese(II) [42], manganese(III) [24,43], manganese(IV)
ypropiophenone and
a diamine [1–11]; Double-compartment
ligands H2LOMe [1,12–18] and H2LOEt [1,19–35] ligands, obtained
on [2+1] condensation of 3-methoxysalicylaldehyde and
3-ethoxysalicylaldehyde, respectively, and a diamine. It has been
established that H2LOEt ligands are something special because its
O(phenoxo)2O(ethoxy)2 compartment is strongly potential to act
as a hydrogen bond acceptor with hydrogen bond donors such as
water [1,19–31], aquated proton [32,33], ammonium ion [22]
and diprotonated diamine [34,35]. The metal complexes derived
from H2LOEt ligands include mononuclear host–guest systems, din-
uclear systems, double/triple/quadruple-decker compounds,
supramolecular dimers and finite/infinite cocrystals [1,19–35]. It
[44],
manganese(III)manganese(II)
[45],
manganese(III)
manganese(III) [46], iron(II) [47], iron(III) [48], cobalt(II) [49,50],
cobalt(III)cobalt(II) [51–53] and cobalt(III)cobalt(III) systems [53].
Recently, we have reported
a
series of mononuclear
manganese(III) compounds derived from H2LOEt–en (N,N0-ethylene-
bis(3-ethoxysalicylaldiimine); Scheme 1) as functional models of
catechol oxidase activity [24]. Role of auxiliary anionic ligands
⇑
Corresponding author.
E-mail address: sm_cu_chem@yahoo.co.in (S. Mohanta).
http://dx.doi.org/10.1016/j.ica.2015.06.007
0020-1693/Ó 2015 Elsevier B.V. All rights reserved.

P. Chakraborty, S. Mohanta / Inorganica Chimica Acta 435 (2015) 38–45
39
2.2.2. [CoIIILOEt–en(NCS)(H2O)]ꢁDMFꢁH2O (2)
To a stirred suspension of H2LOEt–en (0.100 g, 0.28 mmol) in
MeOH (15 mL), was dropwise added
a MeOH solution of
O
cobalt(II) perchlorate hexahydrate (0.205 g, 0.56 mmol). To the
resulting brown solution, an aqueous solution (5 mL) of NH4SCN
(0.085 g, 1.12 mmol) was added after 30 min. A dark brown precip-
itate started to deposit which was collected by filtration and
washed with MeOH. Recrystallization was done on diffusing
diethyl ether to DMF solution of the brown solid to yield crystalline
compound containing diffraction quality single crystals. Yield:
0.100 g (61%). Anal. Calc. for C24H33N4O7SCo (FW: 580.55): C,
49.65; H, 5.73; N, 9.65. Found: C, 49.81; H, 5.67; N, 9.54%. FT-IR
OH
OH
N
N
O
(cmꢂ1, KBr):
1642s. Diamagnetic.
m(H2O) 3446m and 3267m; m(SCN) 2119vs; m(C@N)
Scheme 1. Chemical structure of the ligand.
2.3. Crystal structure determination of 1 and 2
(azide, thiocyanate, etc.) on the catechol oxidase activity has also
been reported in that investigation. We therefore have been inter-
ested to explore whether mononuclear cobalt(III) compounds
derived from the same ligand show such activity.
With the above mentioned twofold aims, namely, exploration of
self-assembly aspects and catechol oxidase activity of mononu-
clear cobalt(III) complexes derived from H2LOEt ligand(s), we have
isolated two CoIII compounds of composition [CoIIILOEt–en(N3)2ꢀ
(H3O+)]ꢁ2MeOH (1) and [CoIIILOEt–en(NCS)(H2O)]ꢁDMFꢁH2O (2).
Herein, we report syntheses, crystal structures and catechol
oxidase activity of 1 and 2, ESI-MS (positive) studies of 1 and
uniqueness in the structure of 1.
The crystallographic data for the two compounds, 1 and 2 are
summarized in Table 1. Diffraction data of these two compounds
were collected on a Bruker-APEX II CCD diffractometer at 296 K
using graphite-monochromated Mo K
a radiation (k = 0.71073 Å).
For data processing and absorption correction the packages SAINT
[54] and SADABS [54] were used. The structures were solved by direct
and Fourier methods and refined by full-matrix least-squares
based on F2 using SHELXTL [55] and SHELXL-97 packages [56].
The three hydrogen atoms of the oxonium ion in 1 and the two
hydrogen atoms of the coordinated water molecule in 2 were
located from Fourier difference maps. The following hydrogen
atoms could not be located: One hydroxyl hydrogen atom (linked
with O5) of one of the two methanol molecules in 1; Two hydrogen
atoms linked with the solvated water oxygen atom (O7) in 2. All
other hydrogen atoms in the two compounds were inserted on
geometrical calculated positions with fixed thermal parameters
and were refined isotropically, while all the nonhydrogen atoms
were refined anisotropically. The final least-squares refinements
2. Experimental
2.1. Materials and physical measurements
All the reagents and solvents were purchased from the commer-
cial sources and used as received. The Schiff base ligand H2LOEt–en
was prepared by 2:1 condensation of 3-ethoxysalicylaldehyde
and ethylenediamine in MeOH [20]. Elemental (C, H and N) analy-
ses were performed on a Perkin–Elmer 2400 II analyzer. IR spectra
were recorded in the region 400–4000 cmꢂ1 on a Bruker-Optics
Alpha-T spectrophotometer with samples as KBr disks. Studies on
catecholase activity study were performed with a Shimadzu
UV-3600 spectrophotometer. The electrospray ionization mass
(R1) based on I > 2r(I) converged to 0.0563 and 0.0576 for 1 and
2, respectively.
Table 1
Crystallographic data for 1 and 2.
Compound
1
2
Empirical formula
Fw
C
22H32N8O7Co
C24H31N4O7SCo
578.52
triclinic
spectra were recorded on
a Micromass Qtof YA 263 mass
579.49
spectrometer. Molar conductivity was measured at 25 °C with a
Systronics conductivity bridge. Magnetic susceptibility measure-
ments of at 300 K were carried out with a Sherwood Scientific
Co., U.K. magnetic susceptibility balance.
Crystal system
Space group
a (Å)
b (Å)
c (Å)
orthorhombic
Pbca
ꢀ
P1
16.823(2)
11.1895(16)
28.186(4)
90.00
10.889(4)
11.834(4)
12.344(4)
116.917(4)
92.120(4)
105.574(4)
1342.8(8)
2
1.431
0.766
296(2)
604
3.76–50.00
ꢂ12 6 h 6 12
ꢂ14 6 k 6 14
ꢂ13 6 l 6 14
9194
4605(0.0388)
346
1.061
a
(°)
b (°)
90.00
2.2. Syntheses
c
(°)
90.00
5305.7(13)
8
1.451
0.704
V (Å3)
2.2.1. [CoIIILOEt–en(N3)2ꢀ(H3O+)]ꢁ2MeOH (1)
Z
To a stirred suspension of H2LOEt–en (0.100 g, 0.28 mmol) in
Dcalc (g cmꢂ3
)
l
(mmꢂ1
)
MeOH (15 mL), was dropwise added
a MeOH solution of
T (K)
296(2)
cobalt(II) perchlorate hexahydrate (0.205 g, 0.56 mmol). To the
resulting brown solution, an aqueous solution (5 mL) of NaN3
(0.074 g, 1.12 mmol) was added after 30 min. A dark brown precip-
itate started to deposit which was dissolved by heating. The solu-
tion was filtered to remove any suspended particles and the filtrate
was kept at ambient temperature for slow evaporation. After
2–3 days, brown crystalline compound containing diffraction
quality single crystals were deposited, which was collected by
filtration and was washed with MeOH. Yield: 0.100 g (61%). Anal.
Calc. for C22H33N8O7Co (FW: 580.48): C, 45.52; H, 5.73; N, 19.30.
F (000)
2h (°)
Index ranges
2424
2.88–49.94
ꢂ19 6 h 6 19
ꢂ12 6 k 6 13
ꢂ33 6 l 6 33
34702
Reflections collected
Independent reflections (Rint
)
4641(0.0379)
363
1.054
0.0563, 0.2008
0.0714, 0.2261
Parameters refined
Goodness-of-fit (GOF) on F2
b
R1a, wR2 (I > 2
r
(I))
0.0576, 0.1619
0.0773, 0.1834
b
R1a, wR2 (for all data)
Found: C, 45.38; H, 5.80; N, 19.45%. FT-IR (cmꢂ1, KBr):
m
(H3O+)
P
P
a
R1 = [ ||Fo| ꢂ |Fc||/ |Fo|].
P
P
b
wR2 = [ w(F2o ꢂ Fc2)2/ wFo4]1/2
.
3472m and 3417m; (N3) 2018vs; (C@N) 1635s. Diamagnetic.
m
m

40
P. Chakraborty, S. Mohanta / Inorganica Chimica Acta 435 (2015) 38–45
3. Results and discussion
3.1. Syntheses and characterization
The two mononuclear cobalt(III) complexes 1 and 2 were
prepared by the reaction of the ligand (H2LOEt–en
with
)
Co(ClO4)2ꢁ6H2O and NaN3 (for 1)/and NH4SCN (for 2) in 1:2:4 ratio.
Clearly, aerial oxidation of cobalt(II) to cobalt(III) and metal
assisted deprotonation of the phenolic moieties take place during
the formation of 1 and 2. It is worth mentioning that two CoIII com-
pounds derived from H2LOEt ligands have been reported previously
[57,58]. Those were produced on reacting the corresponding ligand
with CoCl2ꢁ6H2O [58] or Co(ClO4)2ꢁ6H2O [57]. In both those cases a
fraction of H2LOEt ligand decomposes to give rise to either the
aldehyde or the half-condensed ligand (aldehyde:diamine = 1:1).
In contrast, no such ligand decomposition takes place in case of 1
or 2, probably because azide and thiocyanate are used as auxiliary
ligand in the syntheses of these two compounds.
The characteristic C@N stretching frequency is observed as a
strong band in the range 1635–1642 cmꢂ1 for the complexes 1
and 2. The presence of azide in 1 and thiocyanate in 2 is evidenced
by the appearance of a band of very strong intensity at 2018 and
2119 cmꢂ1, respectively. OAH stretching of water and oxonium
ion in 2 and 1, respectively, is observed as a broad band at 3446
and 3472 cmꢂ1 with a shoulder at 3267 and 3417 cmꢂ1
.
Fig. 1. Crystal structure of [CoIIILOEt–en(N3)2ꢀH3O+]ꢁ2MeOH (1). All the hydrogen
atoms, except those participate in hydrogen bonding, are omitted for clarity.
Symmetry: D, ꢂ1 + x, y, z.
Compound 2 is practically insoluble in common organic sol-
vents except DMF, while compound 1 is soluble in DMF and
MeCN. Therefore, all the solution studies of 1 were carried out in
DMF and MeCN, while for 2, the solution studies were performed
in DMF only. The molar conductance values of 1 were measured
in DMF and MeCN, while for 2, conductance was measured in
DMF only. The nonelectrolytic nature of the compound 1 and 2
in the solid state is retained in solution as evidenced by the low
molar conductivity values 9.3 and 18.9
X
ꢂ1 Mꢂ1 cmꢂ1 L respec-
tively for 1 and 2 in DMF [59]. Compound 1 retains the nonelec-
trolytic nature in MeCN also as evidenced by its very low value
of conductance (6.3
X
ꢂ1 Mꢂ1 cmꢂ1 L) [59].
The UV–Vis spectra of 0.125 ꢃ 10ꢂ4 M and 2 ꢃ 10ꢂ3 M solutions
of complexes 1 (in DMF and MeCN), and 2 (in DMF) were mea-
sured. The band position and extinction coefficient are listed in
Table S1 in ESI. For complex 1, there are three bands in the range
248–355 nm (
bands in the range 264–352 nm (
DMF. For complex 2 in DMF, three bands are observed in the range
264–404 nm (
= 8000–46000 Mꢂ1 cmꢂ1). All these bands arise
e
= 12800–54160 Mꢂ1 cmꢂ1) in MeCN but only two
= 12800–38400 Mꢂ1 cmꢂ1) in
e
e
due to internal ligand transition or ligand to metal charge transfer.
The two compounds exhibit one low intensity band at 621 nm
which is clearly due to the d-d band of CoIII center.
Room temperature magnetic moment was measured for the
two complexes; The values of leff implies that both are diamag-
netic in nature.
3.2. Description of crystal structures of 1 and 2
Crystal structures of [CoIIILOEt–en(N3)2ꢀ(H3O+)]ꢁ2MeOH (1) and
[CoIIILOEt–en(NCS)(H2O)]ꢁDMFꢁH2O (2) are shown in Figs. 1 and 2,
respectively. The structures reveal that both 1 and 2 are mononu-
Fig. 2. Crystal structure of [CoIIILOEt–en(NCS)(H2O)]ꢁDMFꢁH2O (2). All the hydrogen
atoms except those of coordinated water molecule, and the solvated DMF and water
molecules are omitted for clarity.
clear cobalt(III) compounds containing [LOEt–en 2ꢂ
]
. The metal cen-
ter in both the compounds occupies the N(imine)2O(phenoxo)2
compartment of [LOEt–en 2ꢂ
. In both the compounds, cobalt(III)
]
center is hexacoordinated; the additional two coordination sites
are satisfied by two nitrogen atoms of two terminal azide ligands
in 1, while those are satisfied by the nitrogen atom of a terminal
thiocyanate ligand and the oxygen atom of a water molecule in
2. While [CoIIILOEt–en(NCS)(H2O)] (2; excluding solvent molecules)
is a neutral system, charge of [CoIIILOEt–en(N3)2] moiety (a part of 1)
is ꢂ1 and this negative charge is balanced by an oxonium ion
(H3O+) in compound 1. The oxonium ion in this compound is
trapped in the O(phenoxo)2O(ethoxy)2 compartment by forming
hydrogen bonds; each of the two of the three hydrogen atoms in

P. Chakraborty, S. Mohanta / Inorganica Chimica Acta 435 (2015) 38–45
41
H3O+ forms bifurcated hydrogen bonds with a phenoxo and an
ethoxy oxygen atoms. Compound 1 contains two methanol mole-
cules as solvent of crystallization, one of which is interlinked with
the trapped oxonium ion by hydrogen bonding interactions; the
third hydrogen atom of the oxonium ion forms a hydrogen bond
with the oxygen atom of one methanol molecule. In 2, coordinated
water molecule interacts with the O(phenoxo)2O(ethoxy)2
compartment of a neighboring symmetry related complex by
forming hydrogen bonds (Fig. 3); each of the two water hydrogen
atoms forms bifurcated hydrogen bonds with one phenoxo and
Table 2
Comparative structural parameters (distances in Å and angles in °) in the coordination
environment of the CoIII center in 1 and 2.
Parameters
Compounds
1
2
Co–O(phenoxo)
Co–N(imine)
Co–First axial
1.897(3), 1.883(3)
1.875(3), 1.882(3)
Co–N(N3) = 1.983(4)
Co–N(N3) = 1.962(4)
178.32(15)–179.41(16)
84.36(11)–95.02(13)
0.008
1.895(3), 1.902(3)
1.885(3), 1.885(3)
Co–N(NCS) = 1.887(4)
Co–O(water) = 1.934(3)
176.85(12)–178.42(12)
85.08(14)–94.15(13)
0.028
Co–Second axial
All trans angles’ range
All cis angles’ range
a
one ethoxy oxygen atoms. There are also two
pꢁ ꢁ ꢁp stacking
dCo
a
interactions in 2 between the two neighboring aromatic rings,
where the center-to-center distance and dihedral angle between
the least-squares planes of the adjacent aromatic rings are
3.630 Å and 5.01°, respectively. This way, two symmetry related
mononuclear cobalt(III) complex units are interlinked to form
dimer-of-mononuclear type self-assembly in 2.
dN/O
dO
0.018
0.035
0.008
0.105
a
a
dCo, dN/O and dO are the displacement of the metal center, the average deviation
of the constituent atoms from the corresponding least-squares basal
N(imine)2O(phenoxo)2 plane and the average deviation of the constituent atoms
from the corresponding least-squares O4 plane respectively.
Selected bond lengths and bond angles in the coordination envi-
ronment of the metal center in 1 and 2 are listed in Table 2 in a
comparative way, while the individual bond lengths and bond
angles of the metal centers are listed in Tables S2 and S3 for com-
pound 1 and 2, respectively. The coordination geometry of the
metal center in both the compounds is distorted octahedral in
which the N(imine)2O(phenoxo)2 compartment defines the basal
plane. Two CoIII–phenoxo and two CoIII–imine bond distances in
compound 1 as well as in compound 2 are very close. Again, the
CoIII–phenoxo/imine bond distances in both the compounds
are also very close. The overall range of these bond distances in
the two compounds is 1.875–1.902 Å. The CoIII–N(NCS) bond
distance, 1.887 Å, in 2 is almost same as those in the basal plane,
while the second apical bond distance (CoIII–O(water) = 1.934 Å)
in this compound is slightly longer. In the case of 1, both the apical
CoIII–N(azide) bond distances (1.962 and 1.983 Å) are clearly
longer than those in the basal plane. For both the compounds, both
the cisoid (84.36–95.02° in 1; 85.08–94.15° in 2) and transoid
angles (178.32–179.41° in 1; 176.85–178.42° in 2) don’t deviate
much from the ideal values. The average deviation (dN/O = 0.018
Å in 1; 0.008 Å in 2) of the imine nitrogen and phenoxo oxygen
atoms and displacement (dCo = 0.008 Å in 1; 0.028 Å in 2) of the
metal center from the least-squares N(imine)2O(phenoxo)2 basal
plane is very small. All in all, it is evident that the coordination
geometry of the cobalt(III) center in both 1 and 2 is deviated only
little from the ideal octahedral geometry.
Two phenoxo and two ethoxy oxygen atoms in both the
compounds form almost a perfect plane, as evidenced by the small
values (0.035 Å for 1 and 0.105 Å for 2) of the average deviation
of the constituting oxygen atoms from the least-squares
O(phenoxo)2O(ethoxy)2 plane. The oxygen atom of the oxonium
ion in
1 is displaced by 0.277 Å from the least-squares
O(phenoxo)2O(ethoxy)2 plane.
The parameter values of the hydrogen bonds in 1 and 2 are
listed in Table 3. Although the Dꢁ ꢁ ꢁA contacts for the oxonium
ionꢁ ꢁ ꢁO(phenoxo/ethoxy) hydrogen bonds in 1 lie in the range
2.320–2.607 Å, the D–Hꢁ ꢁ ꢁA angles are smaller, 83.86–132.50°,
indicating that these hydrogen bonds are moderately strong.
The oxonium ionꢁ ꢁ ꢁmethanol hydrogen bond (Dꢁ ꢁ ꢁA = 2.419 Å,
D–Hꢁ ꢁ ꢁA = 120.04°) are also moderately strong. In the case of 2,
the waterꢁ ꢁ ꢁO4 hydrogen bonds are also moderately strong; the
ranges of Dꢁ ꢁ ꢁA contacts and D–Hꢁ ꢁ ꢁA angles are 2.720–3.041 Å
and 130.49–151.54°.
Bond valence sum (BVS) calculations of the CoIII center of 1 and
2 have been performed. The values are 3.93 and 4.4 for 1 and 2,
respectively. The values are much larger than 3 but such large
BVS values of CoIII centers are not unusual [51,53,60].
As already mentioned, a number of mononuclear MIII com-
pounds are known from H2LOEt ligands [1,19,24–26,57,58,61–70].
Table 3
Geometries (distances in Å and angles in °) of hydrogen bonds in 1 and 2. Symmetry:
D, ꢂ1 + x, y, z; E, 1 ꢂ x, ꢂy, 1ꢂ z.
Compound
D–Hꢁ ꢁ ꢁA
Dꢁ ꢁ ꢁA
Hꢁ ꢁ ꢁA
D–Hꢁ ꢁ ꢁA
1
O6D–H6BDꢁ ꢁ ꢁO3
O6D–H6BDꢁ ꢁ ꢁO1
O6D–H6CDꢁ ꢁ ꢁO2
O6D–H6CDꢁ ꢁ ꢁO4
O6D–H6CDꢁ ꢁ ꢁO5D
O5–H5WAꢁ ꢁ ꢁO4E
O5–H5WAꢁ ꢁ ꢁO2E
O5–H5WBꢁ ꢁ ꢁO3E
O5–H5WBꢁ ꢁ ꢁO1E
2.607
2.320
2.332
2.600
2.419
3.026
2.821
3.041
2.720
2.538
1.892
1.591
2.016
1.816
2.302
2.184
2.468
2.009
83.86
105.76
132.50
118.50
120.04
151.54
137.20
130.49
149.52
2
Fig. 3. Perspective view of the dimeric self-assembly of 2. Ethoxy carbon atoms, the
solvated DMF and water molecules and the hydrogen atoms, except those
participating in hydrogen bonding, are omitted for clarity. Symmetry: E, 1 ꢂ x,
ꢂy, 1 ꢂ z.

42
P. Chakraborty, S. Mohanta / Inorganica Chimica Acta 435 (2015) 38–45
The trivalent metal ion, which resides in N(imine)2O(phenoxo)2
compartment, in all of those compounds is either FeIII or MnIII
[24–26,61–70] and it is CoIII in only two cases [57,58]. In those
two examples of the cobalt(III) compounds, ligand decomposition
takes place and the fifth and sixth coordination positions are occu-
pied by phenolate oxygen atom and aldehyde oxygen atom or
imine nitrogen atom of the decomposed ligand. In the case of
one of the several manganese(III) compounds, the fifth and sixth
coordination positions are occupied by a chelating anionic ligand,
acetate [66]. In all other cases, the general composition of the
MIII (clearly, MnIII/FeIII) compounds are either [MIIILOEt(H2O)(S)]Y
or [MIIILOEt(H2O)(X)] where S is water [26,64,65,69], or methanol
[25,26], Y is an anion such as nitrate [26,64,65] or perchlorate
(0.25 ꢃ 10ꢂ4 M) of a complex in MeCN/DMF was treated with a
100-fold concentrated solution (in the same solvent) of 3,5-
DTBCH2 and the course of the reaction was followed by recording
UV–Vis spectra of the mixture under aerobic condition. The spectra
of the mixture of a complex and 3,5-DTBCH2 were recorded after
every 5 min up to a total time of 90 min.
As already mentioned, complex 1 has an intense band at
355 nm (in MeCN) which is gradually red shifted to 400 nm with
steady increase in intensity after mixing with 3,5-DTBCH2, indicat-
ing more and more formation of 3,5-DTBQ (Fig. 4). It has also been
mentioned that complex 2 (in DMF) shows two intense signals at
404 and 345 nm (Table S1). After mixing with 3,5-DTBCH2, both
these two peaks become blue/red shifted initially and then merged
at 400 nm (Fig. 5). However, steady increase in intensity of the two
bands or the single merged band takes place after mixing with
3,5-DTBCH2. The observation here also in line with more and more
formation of 3,5-DTBQ because, as already mentioned, it is well
known that 3,5-DTBQ shows an intense band at around 400 nm
[25,26,69] and
X
is an anionic ligand such as acetate
[24,61,64,70], azide [24,25,68], chloride [24,25,63,67], thiocyanate
[24,62] and selenocyanate [24]. A common feature in these latter
two types of compounds is that at least one water molecule is coor-
dinated with the MIII center (which occupies N2O2 compartment)
and that water molecule interacts with the O4 compartment of
the neighboring symmetry related molecule to form dimer-of-
mononuclear type structure [24–26,61–65,67–70].
0.8
It is well known now that O(phenoxo)2O(ethoxy)2 compart-
ment in copper(II)/nickel(II) systems derived from H2LOEt ligands
can accommodate a noncoordinated guest species such as water,
aquated proton, ammonium ion and diprotonated diamine
[1,19–35]. In contrast, it is quite unlikely that a noncoordinated
guest species (water, oxonium ion, etc.) be incorporated in the O4
compartment of mononuclear MIII compounds of H2LOEt ligands.
If otherwise, that would be definitely an unexpected and interest-
ing observation. Of the two CoIII compounds in this investigation,
the composition of [CoIIILOEt–en(NCS)(H2O)]ꢁDMFꢁH2O (2) belongs
to the expected general composition [MIIILOEt(H2O)(X)] and its
coordinated water molecule interacts as usual with the O4 com-
partment of neighboring symmetry related molecule (Fig. 3).
However, [CoIIILOEt–en(N3)2ꢀ(H3O+)]ꢁ2MeOH (1) represents a new
type of composition. Unlike in all the previous MIII compounds,
both the two axial ligands in 1 are anionic (azide). Therefore, an
oxonium ion is present in this molecule to balance charges.
Existence of oxonium ion as charge balancing cation is very usual
in various compounds [1]. But, as the O(phenoxo)2O(ethoxy)2 com-
partment is strongly potential to form hydrogen bonds, the oxo-
nium ion in 1 is trapped in the O4 compartment. As a matter of
fact, compound 1 is the first example of a MIII compound of
H2LOEt ligands where a noncoordinated guest species (oxonium
ion here) is trapped in the O(phenoxo)2O(ethoxy)2 compartment.
Another unique aspect is the presence of two apical anionic
ligands. Several mononuclear MIII compounds (M = MnIII, FeIII,
CoIII) having the metal ion in the N(imine)2O(phenoxo)2
compartment are known derived from double-compartment
acyclic Schiff base ligands (both 3-ethoxysalicylaldehyde-diamine
and 3-methoxysalicylaldehyde-diamine) [1,24–26,57,58,61–70].
In none of those compounds, the MIII center is coordinated with
two apical anionic ligands as in 1. Clearly, 1 represents a new type
from that aspect also.
3,5-DTBQ
0.6
0.4
Complex 1
3,5-DTBCH2
0.2
0.0
300
400
500
600
Wavelength (nm)
Fig. 4. Spectral profile showing the increase of quinone band at 400 nm after the
addition of 100 fold of 3,5-DTBCH2 to
a solution containing complex 1
(0.25 ꢃ 10ꢂ4 M) in MeCN. Spectra were recorded after each 5 min.
0.3
3,5-DTBQ
0.2
Complex 2
3,5-DTBCH2
0.1
3.3. Catecholase activity
In most of the catecholase activity studies, 3,5-di-tert-butylcat-
echol (3,5-DTBCH2) has been employed as the model substrate
because (i) It is easily oxidized to 3,5-DTBQ and (ii) the generated
product, 3,5-di-tert-butylquinone (3,5-DTBQ), has a characteristic
absorption band at around 400 nm [24,40–53]. Therefore, activity
and kinetic parameters can be determined by monitoring the
absorption maximum of the quinone.
0.0
300 350 400 450 500 550 600
Wavelength (nm)
Fig. 5. Spectral profile showing the increase of quinone band at 400 nm after the
Catecholase activity studies were performed in both DMF and
MeCN for compound 1 and only in DMF for compound 2. A solution
addition of 100 fold of 3,5-DTBCH2 to
a solution containing complex 2
(0.25 ꢃ 10ꢂ4 M) in DMF. Spectra were recorded after each 5 min.

P. Chakraborty, S. Mohanta / Inorganica Chimica Acta 435 (2015) 38–45
43
was monitored. This difference in optical densities was plotted
against time and the rate constant for a catalyst complex was
determined from that plot by initial rate method. The rate constant
versus substrate concentration data were then analyzed on the
basis of the Michaelis–Menten approach of enzymatic kinetics to
get the Lineweaver–Burk plot as well as the values of the parame-
ters Vmax, KM, and Kcat. Observed and simulated initial rate versus
[substrate] plots and the Lineweaver–Burk plots for the complexes
1 (in MeCN) and 2 (in DMF) are shown in Figs. 6 and 7, respectively.
As listed in Table 4, turnover numbers (Kcat) are 10.0 hꢂ1 for
complex 1 in MeCN and 11.2 hꢂ1 for complex 2 in DMF. Clearly,
complex 1 in MeCN and complex 2 in DMF are functional models
of catechol oxidase.
Fig. 6. Initial rates vs. substrate concentration for the 3,5-DTBCH2 ? 3,5-DTBQ
oxidation reaction catalyzed by complex 1 in MeCN. Inset shows Lineweaver–Burk
plot. Symbols and solid lines represent the observed and simulated profiles,
respectively.
3.4. Electrospray ionization mass spectral study
The electrospray ionization mass spectrum in positive mode
(ESI-MS positive) of MeCN solution of 1 was recorded. ESI-MS of
complex 2 could not be recorded as it is not soluble in low boiling
solvents. The observed and simulated spectra of complex 1 are
shown in Fig. 8 and the assigned species are listed in Table S4.
Compound 1 exhibits five peaks at m/z = 413 (100%, line to line
separation 1.0), 436 (70%, line to line separation 1.0), 826 (30%, line
to line separation 1.0), 843 (15%, line to line separation 1.0) and
868 (35%, line to line separation 1.0). These five peaks can be
assigned as follows: (i) m/z = 413, a mononuclear CoIII species
[CoIIILOEt–en + (I; C20H22N2O4Co); (ii) m/z = 436, a CoIINaI species
]
{[CoIILOEt–en] + Na+} (II; C20H22N2O4CoNa); (iii) m/z = 826, a compo-
sition containing both a CoIII and CoII moieties {[CoIIILOEt–en] +
[CoIILOEt–en]}+ (III; C40H44N4O8Co2)
– probably an aggregate
between the two; (iv) m/z = 843, a hydroxo bridged dinuclear
CoIII species [(CoIIILOEt–en)2(OH)]+ (IV; C40H45N4O9Co2); (v) m/z =
868, an azide-bridged dinuclear CoIII species [(CoIIILOEt–en)2(N3)]+
(V; C40H44N7O8Co2).
Fig. 7. Initial rates vs. substrate concentration for the 3,5-DTBCH2 ? 3,5-DTBQ
oxidation reaction catalyzed by complex 2 in DMF. Inset shows Lineweaver–Burk
plot. Symbols and solid lines represent the observed and simulated profiles,
respectively.
Coordination of catechol moiety to the metal center during the
catalytic cycle by any possible way is an essential criterion to show
catecholase activity. It is worth mentioning that the intermediate
species identified from ESI-MS (or in solid state) may not be a real
intermediate during the catalytic cycle in solution. However,
ESI-MS can give an idea about the possible intermediates in cat-
alytic cycle. Therefore, we have recorded the ESI-MS spectrum of
a mixture of 1 and 3,5-DTBCH2 in MeCN after 15 min of mixing
to check if any complex–substrate aggregate could be identified.
The observed and simulated spectra of the mixture of compound
1 and a 100-fold 3,5-DTBCH2 are shown in Fig. 9 and the assigned
species are listed in Table S5. The mixture exhibits five peaks, at
m/z = 243 (100%, line to line separation 1.0), 413 (20%, line to line
separation 1.0), 436 (28%, line to line separation 1.0), 463 (75%, line
to line separation 1.0) and 853 (5%, line to line separation 1.0).
Among these five peaks, the peaks at m/z = 413 and m/z = 436 are
also observed in the spectrum of the original complex. Three other
peaks can be assigned as follows: (i) m/z = 243, a 1:1 quinone–
sodium aggregate [(3,5-DTBQ)Na]+ (VI; C14H20O2Na); (ii)
[24,40–53]. Clearly, complex 1 in MeCN and complex 2 in DMF
show catechol oxidase activity. At this point, it is relevant to men-
tion that blank experiment without complex showed no formation
of the quinone up to 12 h in MeCN and 3 h in DMF. After mixing
complex 1 with 3,5-DTBCH2 in DMF, no band around 400 nm is
generated, indicating that complex 1 in DMF does not show cate-
chol oxidase activity.
Kinetic studies of the catecholase activity of the catalysts,
complexes 1 (in MeCN) and 2 (in DMF), have been performed to
understand the extent of their efficiency. For this purpose,
0.25 ꢃ 10ꢂ4 M solution of a complex in respective solvent was
treated with 3,5-DTBCH2 solution of concentration ranging from
0.25 ꢃ 10ꢂ3 M to 0.25 ꢃ 10ꢂ2 M. For a particular complex–sub-
strate mixture, time scan at the maximum of the quinone band
was carried out for a period of 90 min at a constant temperature,
25 °C. The optical density of 3,5-DTBQ is associated with that of
the original band of the complex species, so optical density
obtained in the time scan was subtracted by the optical density
of the original complex at the wavelength at which time scan
m/z = 463,
a
2:1 quinone–sodium aggregate [(3,5-DTBQ)2Na]+
(VII; C28H40O4Na); (iii) m/z = 853, a mononuclear CoII species
[CoIILOEt–en(3,5-DTBQ)2+H+]+ (VIII; C48H63N2O8Co). It is worth
mentioning that the identification of the two quinone species (VI
and VII) as well as a CoII–quinone aggregate (VIII) in the ESI-MS
Table 4
Kinetic parameters for the complexes 1 and 2.
Complex
Solvent
Vmax (M minꢂ1
)
Std. Error
KM (M)
Std. Error
Kcat (hꢂ1
)
1
2
MeCN
DMF
4.18 ꢃ 10ꢂ6
5.15 ꢃ 10ꢂ7
1.26 ꢃ 10ꢂ3
3.28 ꢃ 10ꢂ4
10.0
11.2
4.68 ꢃ 10ꢂ6
3.55 ꢃ 10ꢂ7
1.24 ꢃ 10ꢂ3
2.08 ꢃ 10ꢂ4

44
P. Chakraborty, S. Mohanta / Inorganica Chimica Acta 435 (2015) 38–45
413
100
90
868
80
70
413
826
843
436 436
Simulated
60
% 50
40
30
20
868
868
413
436
826
843
826
Observed
843
10
m/Z
200
300
700
800
100
500
600
400
Fig. 8. Electrospray ionization mass spectrum (ESI-MS positive) of [CoIIILOEt–en(N3)2ꢀH3O+]ꢁ2MeOH (1) in MeCN showing observed and simulated isotopic distribution
patterns.
243
413
436 463
243
100
90
80
70
853
Simulated
463
60
50
%
243
413
436
853
700
463
40
30
20
10
Observed
853
436
413
m/Z
400
500
600
800
100
200
300
Fig. 9. Electrospray ionization mass spectrum (ESI-MS positive) of a 1:100 mixture of [CoIIILOEt–en(N3)2ꢀH3O+]ꢁ2MeOH (1) and 3,5-DTBCH2 in MeCN recorded after 15 min of
mixing, showing observed and simulated isotopic distribution patterns.
spectra of complex 1 is in line with the catecholase activity of the
complex.
of CoIII compounds from these ligands where no decomposition of a
part of the Schiff base ligand takes place.
Compound 1 is a unique example where a guest species (H3O+
here) is trapped in O(phenoxo)2O(ethoxy)2 compartment of a sys-
4. Conclusions
tem in which N(imine)2O(phenoxo)2 compartment of [LOEt 2ꢂ
is
]
To the best of our knowledge, The two cobalt(III) compounds
[CoIIILOEt–en(N3)2ꢀ(H3O+)]ꢁ2MeOH (1) and [CoIIILOEt–en(NCS)(H2O)]ꢁ
DMFꢁH2O (2) are the first examples mononuclear cobalt(III) com-
pounds from any type of ligands to show catecholase activity.
The title CoIII compounds 1 and 2 are only the third examples of
CoIII compounds derived from H2LOEt ligands and the first examples
occupied by a trivalent metal ion. Coordination of a MIII ion by
two anionic, apical ligands is also a new feature in 3-ethoxysalicy-
laldehyde-diamine ligand systems. It is also worth mentioning
that, the special structural features in the CoIII compound, 1 have
not been observed in any MIII (M = Fe, Mn, Co) compound derived
from not only 3-ethoxysalicylaldehyde-diamine ligands but also

P. Chakraborty, S. Mohanta / Inorganica Chimica Acta 435 (2015) 38–45
45
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Acknowledgements
Financial support from the Government of India through
Department of Science and Technology (Project number SR/S1/IC-
42/2011) and University Grant Commission (Fellowship to P.
Chakraborty) is gratefully acknowledged. Crystal data of 1 and 2
were collected at the DST (Government of India)–FIST funded
Single Crystal X-ray Diffractometer Facility at the Department of
Chemistry, University of Calcutta.
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