Dinuclear mixed valence cobalt(II/III) and hetero-tetranuclear cobalt(III)/Na complexes with a compartmental ligand: Synthesis, characterization and use as catalysts for oxidative dimerisation of 2-aminophenol

Article abstract of DOI:10.1016/j.ica.2020.120044

A N₂O₄ donor compartmental reduced Schiff base ligand, H₂L [(2,2-dimethyl-1,3-propanediyl)bis(iminomethylene)bis(6-methoxyphenol)], obtained on 1:2 condensation of 2,2-dimethyl-1,3-propanediamine with ortho-vanillin follow

Full text of DOI:10.1016/j.ica.2020.120044

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Dinuclear mixed valence cobalt(II/III) and hetero-tetranuclear cobalt(III)/Na
complexes with a compartmental ligand: Synthesis, characterization and use
as catalysts for oxidative dimerisation of 2-aminophenol
Abhisek Banerjee, Shouvik Chattopadhyay
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https://doi.org/10.1016/j.ica.2020.120044
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Please cite this article as: A. Banerjee, S. Chattopadhyay, Dinuclear mixed valence cobalt(II/III) and hetero-
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Dinuclear mixed valence cobalt(II/III) and hetero-
tetranuclear cobalt(III)/Na complexes with a compartmental
ligand: Synthesis, characterization and use as catalysts for
oxidative dimerisation of 2-aminophenol
Abhisek Banerjee, Shouvik Chattopadhyay*
Department of Chemistry, Inorganic Section, Jadavpur University, Kolkata - 700032, India.
Tel: +9133-2457-2941; E-mail: shouvik.chem@gmail.com
Abstract
A N2O4 donor compartmental reduced Schiff base ligand, H2L [(2,2-dimethyl-1,3-
propanediyl)bis(iminomethylene)bis(6-methoxyphenol)], obtained on 1:2 condensation of 2,2-
dimethyl-1,3-propanediamine with ortho-vanillin followed by reduction with NaBH4 in methanol
solution, has been used to prepare two cobalt complexes, [(N3)CoIIIL(µ-OAc)CoII(N3)] (1) and
[(μ-N3)2{(AcO)CoIIILNa(CH3OH)}2]·2CH3OH (2). Complex 1 is a dinuclear mixed valence
cobalt(III)/cobalt(II) complex with CoIIIO2CoII core. Complex 2, on the other hand, is a
tetranuclear cobalt(III)/sodium complex with CoO2Na(N3)2NaO2Co core. Formation of complex
1 or 2 is mainly governed by the amount of cobalt(II) precursors present in the reaction mixture.
Each complex has been characterized by elemental and spectral analysis. X-ray diffraction
analysis has confirmed their structures. Complex 1 crystallized in a chiral space group Pna21
where both the cobalt(III) and cobalt(II) centers adopt six-coordinate distorted octahedral
geometry with cobalt(III) and cobalt(II) centers residing respectivelyat inner N2O2 and outer O4
1

cavities of the reduced Schiff base. Complex 2 crystallized in triclinic system with P space
1
group, where both cobalt(III) and sodium centers adopt distorted octahedral geometry. Oxidation
states of cobalt centers have been confirmed by bond length consideration, BVS calculations as
well as from room temperature magnetic moment measurement. Both complexes 1 and 2 show
phenoxazinone synthase mimicking activity with kcat values 250.21 and 493.73 h−1 respectively.
1. Introduction
3d metal mixed valence complexes have sparked large interests owing to their potential
application in molecular electronics and molecular computing [1-4]. The most renowned mixed
valence complex of cobalt is Co3O4, a normal spinel, where the cobalt(III) centers are octahedral
and low spin, whereas cobalt(II) centers are tetrahedral and high spin [5]. The mixed valence
cobalt complexes are less common in literature compared to manganese and iron, and therefore
their importance in catalysis, magnetism and electro-chromism is less explored [6-8]. Recently,
synthetic inorganic chemists are trying to synthesize mixed valence cobalt(II/III) complexes via
partial oxidation of cobalt(II) precursors and some di and trinuclear mixed valence cobalt(II/III)
complexes are reported in literature [9-18].
The synthetic inorganic chemists are also attracted in the field of synthesis,
characterization of heteronuclear complexes containing alkali metal ions for their wide range of
applications since long [19-23]. Recent example of sodium mediated electron transfer reactions
between the two metal salicylaldimine species in [Na{Ni(salen)}2]+ type complexes has
increased this attraction [24]. The easiest way to synthesize such heteronuclear complexes is to
use compartmental ligands [25-29] of which ‘salen’ based N2O4 donor compartmental Schiff
base ligands are the most common, as their inner N2O2 core is suitable for accommodation of
2

different transition metal ions and outer O4 core can accommodate alkali metal ions easily [30-
32].
Reduction of Schiff bases with borohydride may produce ‘reduced Schiff bases’, which
are better complexing agent considering their more flexibility due to the absence of imine
(>C=N) functionality [33-35]. Many compartmental ‘reduced Schiff bases’ have been used by
several groups to prepare mixed valence cobalt(II/III) complexes and also heteronuclear
cobalt(III)/Na complexes [17-18, 31,34]. In this work, we have used a reduced Schiff base to
prepare a mixed valence cobalt(II/III) complex and also heteronuclear cobalt(III)/Na complex.
The formation of two different complexes is primarily linked with the reaction condition used for
the synthesis of these complexes. Both complexes were characterized by elemental and spectral
analysis. Their structures were confirmed by single crystal X-ray diffraction study. The ability of
both complexes to catalyze the oxidative dimerization of 2-aminophenol (i.e. phenoxazinone
synthase mimicking activity) is also explored and correlated with their structures.
Herein, we would like to report the synthesis, structure and catalytic ability of two cobalt
complexes with a compartmental N2O4 donor ‘reduced Schiff base’.
Keywords: Cobalt, mixed valence, compartmental ligand, phenoxazinone synthase.
2. Experimental Section
Materials
All chemicals used were purchased from Sigma-Aldrich and were of reagent grade. They
were used without further purification.
2.1. Synthesis
3

2.1.1. Synthesis of
a
reduced Schiff base ligand, H2L [(2,2-dimethyl-1,3-
propanediyl)bis(iminomethylene)bis(6-methoxyphenol)]
2 mmol of ortho-vanillin (304 mg) was mixed with 1 mmol of 2,2-dimethyl-1,3-
propanediamine(0.12 mL) in 10 ml methanol and the resulting solution was allowed to reflux for
ca. 2 hr. to synthesize the hexadentate Schiff base ligand, H2La {[N,N'-bis(3-
ethoxysalicylidene)2,2-dimethyl-1,3-propanediamine]}. Then this prepared Schiff base solution
was cooled to 0°C, followed by the addition of 4 mmol solid sodium borohydride (150 mg) to it
with constant stirring. After completion of sodium borohydride addition the resulting solution
was acidified with 10 mL glacial acetic acid and placed under reduced pressure in a rotary
evaporator maintaining the temperature to ~60°C. The residue was dissolved finally in minimum
volume of water and extracted with 15 mL of dichloromethane using a separating funnel. The
ligand solution in dichloromethane was dried using anhydrous sodium acetate and then
dichloromethane was evaporated under reduced pressure and the ligand, H2L, was extracted in
methanol. The ligand was not isolated and the methanol solution was used directly for the
synthesis of complexes 1 and 2.
2.2. Synthesis of the complexes
2.2.1. Synthesis of [(N3)CoIIIL(µ-OAc)CoII(N3)] (1)
10 mL methanol solution of 2 mmol cobalt(II) acetate tetrahydrate (500 mg) was added to
the methanol solution (10 mL) of the reduced Schiff base ligand, H2L, with constant stirring for
ca. 1 hr. 10 mL methanol/water solution (4:1 ratio) of 4 mmol sodium azide (260 mg) was then
added to it and stirring was continued for about 2 h more. Few drops of triethylamine were added
into the final solution and stirring was continued for about 1 hr. Dark brown crystals were
4

appeared after 4-6 days on slow evaporation of the filtrate in an open atmosphere. X-ray quality
single crystals were collected from the product.
Yield: 392 mg (~62%, based on cobalt). Anal. Calc. for C23H31Co2N8O6 (FW = 633.42): C,
43.57; H, 4.89; N, 17.68 %. Found: C, 43.5; H, 4.8; N, 17.8 %. FT-IR (KBr, cm-1): 3195 (νN-H),
2935-2848 (νC-H), 2056(νN3), 2035 (νN3), 1562-1480 (νCOO). UV-Vis, max (nm), [εmax (L mol-1
cm-1)] (CH3CN), 620 (2.01x102), 362 (2.03x103), 245 (9.5x103), Magnetic moment, μ = 5.17
B.M.
2.2.2. Synthesis of [(μ-N3)2{(AcO)CoIIILNa(CH3OH)}2]·2CH3OH (2)
10 mL methanol solution of 1 mmol cobalt(II) acetate tetrahydrate (250 mg) was added to
the methanol solution (10 mL) of the reduced Schiff base ligand, H2L, with constant stirring for
ca. 1 hr. 10 mL methanol/water solution (4:1 ratio) of 4 mmol sodium azide (260 mg) was then
added to it. Dark brown crystals were appeared after 4-6 days on slow evaporation of the filtrate
in an open atmosphere. X-ray quality single crystals were collected from the product.
Yield: 340 mg (~55%, based on cobalt). Anal. Calc. for C50H76Co2N10Na2O16 (FW =1237.05):
C, 48.50; H, 6.14; N, 11.31 %. Found: C, 48.4; H, 6.1; N, 11.4 %. FT-IR (KBr, cm-1): 3210 (νN-
H), 2960-2885 (νC-H), 2051 (νN3), 1577-1485 (νCOO). UV-Vis, max (nm), [εmax (dm3 mol-1 cm-1)]
(CH3CN), 706 (1.05x102), 352 (3.80x103), 255 (8.1x103), Diamagnetic.
2.3. Physical measurements
Elemental analyses (carbon, hydrogen and nitrogen) were performed using a Perkin
Elmer 240C elemental analyzer. IR spectra in KBr (4500-500 cm-1) were recorded with a Perkin
Elmer Spectrum Two spectrophotometer. Electronic spectra in acetonitrile (800-200 nm) were
5

recorded on a Perkin Elmer Lambda 35 UV-visible spectrophotometer. The magnetic
susceptibility measurement was done with an EG & PAR vibrating sample magnetometer
(Model 155) at 25 °C and diamagnetic corrections were performed using Pascal's constants.
Molar conductivity measurements were performed on a Jencons 4010 Conductivity Meter in
nitrobenzene at 25 0C.
2.4. X-ray crystallography
Suitable single crystals of both complexes were used for data collection using a ‘Bruker
D8 QUEST area detector’ diffractometer equipped with graphite-monochromated Mo Kα
radiation ( = 0.71073 Å). The molecular structures were solved by direct method and refined by
full-matrix least squares on F2 using the SHELXL-14 package [36]. Non-hydrogen atoms were
refined with anisotropic thermal parameters. Hydrogen atoms attached to nitrogen and oxygen
atoms were located by difference Fourier maps and were kept at fixed positions. All other
hydrogen atoms were placed in their geometrically idealized positions and constrained to ride on
their parent atoms. Multi-scan empirical absorption corrections were applied to the data using the
program SADABS [37]. In complex 1, three carbon atoms (C8, C37 and C38) have been refined
as isotropic only. The poor quality of the crystals of 1 gave rise to misshapen large spots in the
diffraction pattern which in turn led to high R values. A summary of the crystallographic data
has been given in Table S1 (Supplementary Information). Selected bond lengths have been listed
in Tables 2 (Unit-I of complex 1 and complex 2) and S2 (Unit-II of complex 1). Bond angles of
both complexes have been given in Table S3.
6

2.5. Kinetics of the phenoxazinone synthase activity
The phenoxazinone synthase (PHS) mimicking activity of both complexes has been
checked by monitoring the oxidation of 2-aminophenol (2-AP) spectrophotometrically in
acetonitrile medium at 25 °C. Every catalytic reaction has been studied spectrophotometrically
for ca. 2 h in the wavelength range 300-600 nm. Before entering into to the kinetic study in
detail, it is mandatory to check the ability of these complexes to behave as a catalyst for carrying
out the oxidation of 2-aminophenol. Thus 1.0 × 10− 4 M solutions of the complexes have been
treated with a 0.01 M solution of 2-aminophenol, and the spectra have been recorded in
acetonitrile medium at 25°C. The steady increase in the absorbance as a function of time at ca.
433 nm, is the characteristic of its corresponding chromophore, 2-aminophenoxazin-3-one.
Furthermore, the catalytic rate dependency on concentration of catalyst has been monitored by
performing similar sets of experiments at a fixed concentration of substrate with variable catalyst
concentrations. Rate of the reactions have been calculated from the initial rate method, and the
average initial rate over three independent measurements was also recorded. One thing must be
emphasized here that a blank experiment without catalyst (only complex) has been carried out in
an identical reaction condition and does not show significant growth of the band at ~ 433 nm,
characteristic of phenoxazinone chromophore.
3. Results and discussion
3.1. Synthesis
2,2-dimethyl-1,3-propanediamine and ortho-vanillin have been used to synthesize the
potential hexadentate compartmental reduced Schiff base ligand, H2L, following the literature
method [31] (Scheme 1).
7

Scheme 1: Synthetic route to the reduced Schiff base ligand, H2L.
The methanol solution of H2L, on reaction with cobalt(II) acetate tetrahydrate followed
by the addition of sodium azide in slightly alkaline medium produced a dinuclear mixed valence
cobalt(III)/cobalt(II) complex, 1. In addition, the treatment of methanol solution of H2Lwith the
cobalt(II) acetate tetrahydrate in presence of excess sodium azide produced a centrosymmetric
tetrameric cobalt(III)/sodium complex, 2. The synthetic routes to the complexes have been
shown in Scheme 2. Cobalt(II) is definitely oxidized by areal oxidation. The inner cobalt centers
in both complexes reside in the stronger field (N3O3) coordination environment compared to the
outer cobalt center (which resides in O5N) in complex 1. This may be the driving force to carry
out this oxidation of inner cobalt center in both complexes.
It is to be noted here that the starting materials are more or less same for the synthesis of
both complexes. However, the ratio of the reactants is different. 1 mmol cobalt(II) acetate and
excess sodium azide were used for the synthesis of complex 2, whereas 2 mmol cobalt(II) acetate
and excess sodium azide was used for the synthesis of complex 1. Thus the variation in
structures is primarily governed by the different amounts of cobalt salt used as precursors. H2L is
a compartmental ligand and its inner N2O2 cavity traps the cobalt(III) produced by the partial
oxidation of cobalt(II) and outer O4 compartment binds with the remaining cobalt(II) to form
complex 1. However, when only 1 mmol cobalt salt is added in the reaction mixture, cobalt(III)
8

is trapped in inner N2O2 compartment and outer O4 compartment traps sodium to form complex
2.
Scheme 2: Synthetic routes to complexes 1 and 2. Solvent methanol molecules in complex 2
have been omitted for clarity.
Table: 2. Selected bond lengths (Å) of complexes 1 (Unit-I) and 2.
1
2
1
2
Co(1)-N(1)
Co(1)-N(2)
1.929(10)
2.048(11)
1.993(4)
1.981(4)
M(2)-O(2)
M(2)-O(3)
2.121(10)
2.394(11)
2.357(3)
2.482(2)
Co(1)-O(1)
Co(1)-O(2)
Co(1)-N(3)
Co(1)-O(5)
M(2)-O(1)
2.005(11)
1.850(8)
1.943(12)
1.932(9)
1.945(8)
1.908(15)
1.914(1)
1.933(4)
1.954(2)
2.317(3)
M(2)-O(4)
M(2)-O(6)
M(2)-O(7)
M(2)-N(4)
M(2)-N(5)a
2.244(11)
2.508(2)
2.093(10)
-
-
2.389(3)
-
1.962(11)
-
2.418(3)
(M= Co in 1 and Na in 2), Symmetry transformationa=1-x,1-y,2-z.
9

The reduced Schiff base H2L and its precursor Schiff base H2La have been used
previously to prepare several complexes [38-50]. In most of the cases, H2La and H2L forms
multimetallic complexes, although mononuclear complexes were also formed. In one such
mononuclear complex, water molecule is placed in the outer O4 environment to form water
tetramer [38]. Complexes with H2La and H2L including nuclearity of the complexes and denticity
of the ligand have been listed in Table 3.
Table 3: A list of some reported complexes of H2La and H2L: Nuclearity of the complexes and
denticity of the ligand
Complexes with H2La
Complex
[CuLa]·2H2O
Nuclearity of the Denticity of the ligand
complex
Reference
Mononuclear
Dinuclear
Dinuclear
Dinuclear
Dinuclear
Dinuclear
Dinuclear
Trinuclear
Trinuclear
Tetranuclear
Tetradentate
Tetradentate
Hexadentate
Tetradentate
Hexadentate
Hexadentate
Hexadentate
Hexadentate
Hexadentate
Hexadentate
38
39
40
41
[(DMSO)2Ni(La)Zn(NCS)2]
[CuLaHgCl2]
[(La)Fe(H2O)(µ-Htrim)(H2O)Fe(La)]·H2O
[(MeOH)Co(La)Gd(NO3)3]
[Co(La)(µ-AcO)2Gd(NO3)2]
[Zn(La)(µ-AcO)La(thd)2]
[Ni(La)Na(La)Ni(NCS)]·H2O
[Ni(La)Na(La)Ni(N3)]
42
43
44
[(SCN)Co(La)(µ-AcO)Na]2
45
Complexes with H2L
10

Complex
Nuclearity of the Denticity of the ligand
complex
Reference
[(SCN)Cu(L)Pb(SCN)]
Dinuclear
Hexadentate
Tetradentate
Hexadentate
Hexadentate
Hexadentate
Hexadentate
Hexadentate
46
47
[Ni2(HL)2(DMSO)2(H2O)2](ClO4)2∙H2O
[(SCN)Ni(L)(μ-OAc)Pb]
Dinuclear
Dinuclear
48
[{(DMSO)(H2O)Ni(L)}2Pb](ClO4)2
(μ-N3)2[(H2O)Cu(L)Cd(N3)]2∙2CH3OH
(μ-NCS)2[Cu(L)Cd(SCN)]2∙2CH3OH
[(μ-dca)2{(dca)Zn(L)Zn}2]
Trinuclear
Tetranuclear
Tetranuclear
Tetranuclear
49
50
Htrim = dianion of trimesic acid; thd=tetramethylheptanedionato; dca= dicyanamide
3.2. Description of structures
3.2.1. [(N3)CoIIIL(µ-OAc)CoII(N3)] (1)
Single crystal X-ray diffraction analysis reveals that the complex 1 crystallizes in the
orthorhombic space group, Pna21. There are two independent dinuclear units (I and II) with
equivalent geometry in complex 1. A perspective view of the Unit I of complex 1 is given in
Figure 1 (Unit II has very similar molecular structure: Figure S1, Supplementary Information).
This complex falls under the example of mixed valence category containing both cobalt(II) and
cobalt(III) centers in the neutral dinuclear core which can be confirmed by total charge balance
considerations, inter-atomic cobalt-nitrogen and cobalt-oxygen distances and BVS (bond valence
sum) calculations [51-56].
11

Figure 1: ORTEP view of complex 1 with 30% thermal ellipsoid probablity. Hydrogen atoms
have been omitted for clarity.
Cobalt(III) center, Co(1), and cobalt(II) center, Co(2) occupy the inner N2O2 and outer
O4 cavities respectively of the reduced Schiff base unit, H2L. Co(1) is equatorially coordinated
by two amine nitrogen atoms and two phenoxo oxygen atoms, {N(1), N(2), O(1) and O(2)} and
Co(2) is equatorially coordinated by two phenoxo oxygen atoms and two methoxy oxygen
atoms, {O(1), O(2), O(3) and O(4)}. The fifth coordination sites of Co(1) and Co(2) are
coordinated by nitrogen atoms N(3) and N(4) respectively from two terminal azides. The oxygen
atoms, O(5) and O(6), of a bridging acetate are bonded to the sixth coordination sites of Co(1)
and Co(2) respectively to complete their distorted octahedral geometries. The intramolecular
separation between Co(1) and Co(2) is close to 2.95 Å which is not satisfactorily short to infer
any metal–metal bonding [57-58]. The saturated six membered chelate ring, Co(1)-N(1)-C(9)-
C(10)-C(13)-N(2) represents chair conformation (Figure 2) with puckering parameters [59], q(2)
= 0.551(14)Å, and φ = 186(6).
12

Figure 2: Chair conformation of a saturated six-membered chelatering in unit-I of complex 1.
Only selected atoms have been shown.
3.2.2. [(μ-N3)2{(AcO)CoIIILNa(CH3OH)}2]·2CH3OH (2)
Complex 2 is centrosymmetric and it crystallizes in the triclinic space group, P . An
1
ORTEP view of the asymmetric unit is shown in Figure 3. In this complex, the cobalt(III) center,
Co(1), occupies the inner N2O2 cavity and the sodium center, Na(1) occupies the outer O4 cavity
of the compartmental reduced Schiff base. Co(1) is coordinated by two amine nitrogen atoms
and two phenoxo oxygen atoms, {N(1), N(2), O(1) and O(2)}, and Na(1) is coordinated by two
phenoxo oxygen atoms, O(1) and O(2) and two methoxy oxygen atoms, O(3) and O(4) of the
deprotonated reduced Schiff base ligands in the equatorial plane. An oxygen atom, O(5) of a
terminal acetate coordinates axially to the cobalt(III) center at its fifth coordination site. The fifth
coordination site of Na(1), on the other hand, is occupied by an oxygen atom, O(7) of a methanol
molecule. An end to end end bridging azide [(N(3)-N(4)-N(5)] connects Co(1) with a symmetry
related (symmetry transformation,a = 1-x,1-y, 2-z) Na(1)a to form a tetranuclear complex with
CoO2Na(N3)2NaO2Co core. A tentative view of the complete tetranuclear unit of complex 2 has
been depicted in Figure 4.
13

Figure 3: ORTEP view of the asymmetric unit of complex 2 with 30% thermal ellipsoid
probablity. Hydrogen atoms have been omitted for clarity.
Figure 4: Perspective view of tetranuclear unit of complex 2. Only the metal centers have been
labeled here for clarity. Symmetry transformation:a= 1-x,1-y,2-z.
3.3. BVS calculation
Taking the help of bond valence sum (BVS) calculations we are able to perform the
calculation of oxidation state of cobalt centers in the above prepared complexes. The results are
14

in good agreement for confirming the existence of mixed valence states (+2 and +3) of cobalt in
complex 1 while the cobalt center in complex 2 is approximately in +3 state. Thus this bond
valence sum calculative strategy is considered as one of the promising tools which strongly
supports the assignment of mixed valence states of cobalt in complex 1. The BVS values
calculated for all the cobalt centers in both complexes have been listed in Table 4.
Table 4: BVS (Bond Valence Sum) values of the cobalt centers in both complexes.
Complex
Cobalt centers BVS of cobalt
Co(1)
Co(2)
Co(3)
Co(4)
Co(1)
3.05
1.96
3.08
2.05
3.03
1
2
3.4. Supramolecular interactions
In complex 1, the hydrogen atoms, H(1) [attached to amine nitrogen atom, N(1)] and
H(2) [attached to amine nitrogen atom, N(2)] are involved in intermolecular hydrogen bonding
with symmetry related nitrogen atoms, N(7)b and N(5)b, respectively of the terminal azide groups
{symmetry transformation,b =-1/2+x,1/2-y,1-z} of a neighboring molecule leading to the
formation of a chain like structure (Figure 5). Similar kind of interaction has been observed in
Unit-II. In complex 2, on the other hand, two hydrogen atoms, H(1) and H(2), attached to amine
nitrogen atoms, N(1) and N(2), are involved in intramolecular hydrogen bonding with an oxygen
atom, O(6), of the acetate group (Figure 6). The details of hydrogen bonding interactions in both
complexes have been provided in Table 5.
Table 5: Hydrogen bond distances (Å) and angles (◦) for complexes 1 and 2.
15

Complex
1
D–H···A
D-H
H···A
2.470(6)
2.120(4)
2.150(4)
2.570(4)
2.25(3)
D···A
D-H···A
164.00(1)
174.00(9)
169.00(9)
166.00(9)
149.00(3)
150.00(3)
N(1)-H(1)···N(8)b
N(2)-H(2)···N(7)b
N(9)-H(9)···N(15)c
N(9)-H(9)···N(16)c
N(1)-H(1)···O(6)
N(2)-H(2)···O(6)
0.980(6)
0.980(6)
0.980(6)
0.980(6)
0.80(3)
0.85(3)
3.430(2)
3.093(4)
3.115(3)
3.530(2)
2.962(3)
3.039(3)
2
2.28(3)
c
D = donor; H = hydrogen; A = acceptor, Symmetry transformation:b= -1/2+x,1/2-y,1-z, =
1/2+x,3/2-y,z.
Figure 5: Perspective view of a chain structure through intermolecular hydrogen bonding
interactions in Unit-I of complex 1. Only relevant atoms have been shown here for clarity.
Symmetry transformation:b=1/2+x,1/2-y,1-z.
16

Figure 6: Perspective view of intramolecular hydrogen bonding interactions in complex 2. Only
a dinuclear fragment of the complex and only relevant atoms have been shown here for clarity.
3.5. Hirshfeld surface analyses
Hirshfeld surface analysis has been accomplished in order to examine thoroughly the
strength and role of hydrogen bonds and other intermolecular contacts, and to estimate their
importance for the crystal lattice stability.The Hirshfeld surfaces of both complexes have been
mapped over dnorm (range -0.1 Å to 1.5 Å); shape index and curvedness (Figure 7). Graphical
plots of the molecular Hirshfeld surfaces mapped with dnorm used a red-white-blue colour
scheme, where red highlights shorter contacts, white is used for contacts around the Van der
Walls separation, and blue is for longer contacts. The intense circular depressions (deep red) seen
on the surfaces is indicative of hydrogen bonding contacts whereas small extent of area and light
colour on the surface indicates weaker and longer contact other than hydrogen bonds.
17

Figure 7: Hirshfeld surfaces mapped with dnorm(left), shape index(middle) and curvedness(right).
Two-dimensional fingerprint plots (Figure 8) obtained from Hirshfeld surface analysis
may also be useful to shed light on the supramolecular interactions, which appear as distinct
spikes in this plot. Complementary regions are observable in the two-dimensional fingerprint
plots where one molecule act as donor (de > di) and the other as an acceptor (de < di). In the
fingerprint plot of complex 1, the most dominant interactions are found for N∙∙∙H/H∙∙∙N contacts,
which contribute to 26.0% of the total Hirshfeld surface. However in complex 2, N∙∙∙H/H∙∙∙N
interactions contribute only 5.9%. The proportions of C∙∙∙H/H∙∙∙C interactions comprise 11.7 and
11.8% of the Hirshfeld surfaces for each molecule of complexes 1 and 2 respectively. On the
other hand very less contribution of interactions is coming from O∙∙∙H contacts i.e. 5.5% and 9%
in complexes 1 and 2 respectively.
18

Figure 8: Fingerprint plot: Full (a), resolved into H∙∙∙C/C∙∙∙H (b), H∙∙∙N/N∙∙∙H (c) and
H∙∙∙O/O∙∙∙H (d) contacts contributed to the total Hirshfeld Surface.
At this stage it is relevant to construct a histogram of relative areas as discussed above.
Thus with the help of the histogram one may minutely differentiate the magnitude of the
supramolecular interactions between two complexes. Hence the percentage contributions of
significant atom-type/atom-type contacts to the Hirshfeld surfaces have been elucidated in Figure
9 in the form of histograms, which clearly indicate that in the solid state structure,
supramolecular interactions involving N∙∙∙H hydrogen bonding are highly significant in complex
1.
19

26
30
25
20
15
10
5
C-H
N-H
O-H
11.7
11.8
5.9
9
5.5
0
Complex 1
Complex 2
Figure 9: Percentage contributions to the Hirshfeld surface area for significant close
intermolecular contacts in complexes
3.6. Spectral and magnetic details
The IR spectra of both complexes exhibit moderately strong sharp peaks in the range of
3195-3210 cm-1 which may be assigned as N–H stretching vibrations [60]. In the IR spectrum of
complex 1, a bifurcated sharp band in the range of 2035-2056 cm-1 corresponds to the presence
of azide groups. On the other hand, a sharp peak at 2051 cm-1 in the IR spectrum of complex 2
also supports the presence of azide groups [2]. The presence of carboxylato ligand has been
confirmed by the appearance of a strong and sharp peak (at 1562 cm-1 in 1 and 1577 cm-1 in 2)
along with a shoulder (at 1480 cm-1 in 1 and 1485 cm-1 in 2) in the IR spectrum of each complex
[61-63]. The bands corresponding to alkyl C-H stretching vibrations occur in the range of 2960-
2848 cm-1. IR spectra of complexes 1 and 2 have been given in Figures S2 and S3 (SI)
respectively.
The electronic spectra (800-200 nm) of complexes 1 and 2 have been depicted in Figures
S4 and S5 (SI) respectively. Most often low spin cobalt(III) complexes show two broad
1
1
1
absorption bands at low energy region due to A1g 1T1g and A1g T2g transitions which are
→
20

spin allowed [64]. The broad absorption band (at 620 nm for 1 and 706 nm for 2) in the
electronic spectra of complexes 1 and 2 may be assigned as one of the two expected (mentioned
above) transitions. The other spin allowed transition gets obscured due to the large intensity of
LMCT bands [65]. The intense absorption bands of the complexes, at around 355 nm have been
assigned as charge transfer transitions from the coordinated ligands to the cobalt(III) centers
(LMCT) [66]. High energy absorption bands at 245 nm in complex 1 and 255 nm in complex 2
may be assigned to intra ligand π – π* transition [67].
The effective magnetic moment of complex 1 has been measured to about 5.17 B.M. at
room temperature, and this is slightly higher from that expected for three unpaired electrons
6
present in the high spin (HS) cobalt(II) system. Cobalt(III) is essentially low spin with t2g
electronic configuration and therefore do not contribute any spin magnetic moment. From the
magnetic point of view, therefore, this dinuclear mixed valence cobalt(III/II) complex may be
4
considered as a cobalt(II) monomer with T1g ground state term. Therefore, it shows high
magnitude of orbital contribution to the total magnetic moment in its ground state, which is the
reason for deviation from spin only magnetic moment value. Recently a similar kind of dinuclear
mixed valence cobalt(III/II) complex has been synthesized by our group [68] and its magnetic
nature is also explored. It suggests that such kind of complex belongs to localized class-I mixed
3
valence system with isolated S = /2 ion and there may be no significant spin delocalisation
between cobalt(II) and cobalt(III) center. On the other hand, complex 2 is found to be
diamagnetic, as expected for low spin cobalt(III) complexes [69-76].
3.7. Molar conductivity
The molar conductivity values of complexes 1 and 2 in nitrobenzene were found as 0.03
and 0.002 Scm2mol-1 respectively, which were in good agreement with values expected for a
21

non-electrolyte. This suggested that the coordinated azide and acetate did not ionize in
nitrobenzene.
3.8. Kinetics study and explanation of the turnover numbers
The saturation rate dependency on the concentration of the substrate can be calculated
using Michaelis-Menten model. According to the Michaelis-Menten equation
푉푚푎푥 [푆]
V = 퐾
∙∙∙ (1)
푀 + [푆]
Where, V = initial rate; [S] = concentration of the substrates; KM = Michaelis-Menten
constant for complex and Vmax = maximum initial rate achieved for a particular concentration of
the complex in the presence of a large amount of the substrate.
On the other hand, rearrangement of Michaelis-Menten equation produces three new
equations i.e. Lineweaver-Burk equation (eq. 2), Hanes equation (eq. 3) and Eadie-Hofstee
equation (eq. 4).
퐾푀
1
1
1
+
∙∙∙(2)
∙∙∙ (3)
∙∙∙ (4)
=
푉
푉푚푎푥[푆] 푉푚푎푥
퐾푀
[푆]
[푆]
=
+
푉
푉푚푎푥 푉푚푎푥
푉
−
푉 = 푉푚푎푥 퐾푀
[푆]
Variety of kinetic parameters, including turnover number (kcat) and specificity constant
(kcat/KM) for phenoxazinone synthase mimicking activity of synthesized complexes can be
analysed using all these equations. Kinetic parameters like Vmax and KM have been calculated by
the analysis of entire experimental data. Figure 10 depicts the time dependent spectral change for
the period of 2 h after the addition of 2-aminophenol to the acetonitrile medium of complex 1.
The turnover number value (kcat) has been calculated by dividing the Vmax by the concentration
22

of the catalyst (complex) used. Figure 11 represents the Michaelis-Menten plot, Lineweaver-
Burk plot, Hanes plot and Eadie-Hofstee plot of complex 2 for catalytic oxidation of 2-AP,
respectively. Such kind of plots for complex 1 has been provided in Figures S6-S7 (SI). Both the
complexes show a reactivity pattern very similar to each other. In order to calculate kinetic
parameters more accurately, several enzyme kinetic plots have been used. Similar kind of kinetic
parameters have been obtained in all cases which indicate the accuracy of the experimental
results. The kcat values (Table 6) suggest that both the complexes act as effective catalyst for the
conversation of 2-aminophenol to 2-aminophenoxazine-3-one. Kinetic parameters of some of the
reported cobalt complexes for phenoxazinone synthase like activity have been depicted in Table
6. Control experiments were performed using 10-4 M Co(OAc)2∙4H2O and 10-2 M of 2-AP,
where no drastic change has been observed in absorption intensity even after several hours.
Figure 10: Time resolved UV-Vis spectral profile indicating the increment of 2-
aminophenoxazine-3-one peak at ~ 433 nm upon addition of 10-2 M 2-aminophenol to the 10-4 M
of complex 2 in acetonitrile medium at room temperature (298 K).
23

Figure 11: Michaelis–Menten plot (a), Lineweaver–Burk plot (b), Hanes plot (c) and Eadie-
Hofstee plot (d) for catalytic oxidation of 2-AP in acetonitrile medium at room temperature (298
K) using complex 1 as catalyst.
Table 6: Kinetic parameters for the 2-AP mimicking activity of complexes 1and 2 in acetonitrile.
Vmax
KM
KCat
(h-1)
Complex (mol L-1 min-1)
(mol L-1)
14.02x10-3
2.32x10-2
1
2
4.17 x10-4
8.23 x10-4
250.21
493.73
3.8. Mechanism of catalysis
The experimental results are in nice agreement with the moderate catalytic activity of
both complexes to mimic phenoxazinone synthase. At the beginning, both complexes liberate the
metal centers from the outer O4 compartments (Cobalt(II) in 1 and Na in 2) to produce the
intermediate species (say A), as concluded from the ESI mass spectra of the complexes and the
mass spectra of the complexes with 2-aminophenol in 1:50 ratio (Figures 12, 13, S8 and S9, see
24

below). 2-aminophenol then forms adduct with this intermediate species (A) liberating
monodentate azide and acetate, and this adduct, in turn, produces 2-aminophenol radical by the
reaction with molecular dioxygen and thereby regenerates the intermediate (A). The 2-
aminophenol radical may generate ortho-benzoquinone monoamine, which can then easily be
converted to 2-aminophenoxazine-3-one by the reaction with dioxygen and 2-aminophenol [77].
Scheme 3 highlights the probable mechanism for catalytic conversation of 2-aminophenol to 2-
aminophenoxazine-3-one by the individual complexes 1 and 2.
25

Scheme 3: Probable mechanistic pathway showing the formation of 2-aminophenoxazine-3-one
by the prepared cobalt complexes, 1 and 2.
From the detailed kinetic analysis, it is evident that complex 2 acts as more efficient
catalyst for the aerobic oxidation of 2-aminophenol, while complex 1 is relatively less reactive.
This may be correlated with the lability of the metal ion present in the outer O4 compartment.
26

Since cobalt(II) is more crystal field stabilized than sodium ion (CFSE = 0 for Na ion), sodium
ion eliminates out from the outer compartment at much faster rate compared to cobalt(II),
leading to higher kcat value for complex 2. Synonymous plausible catalytic cycles of this
phenoxazinone synthase mimicking catalytic activity have been also projected by other groups
over the past years (see below).
Table 7: Turn over numbers for the oxidation of 2-AP to 2-aminophenoxazine-3-one catalyzed
by various reported cobalt complexes.
Complexes
TON ( h-1)
4.10
Solvent
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
Reference
[CoII(L1)Cl2]
[CoII(L2)(NCS)2]
7.38
78
[CoII(L2)Cl(H2O)]Cl·H2O
[CoII(L3)(CH3CN)](ClO4)2
[CoII(L4)(H2O)](ClO4)2
[CoII(L5)(H2O)](ClO4)2
13.68
3.36
6.37
79
80
8.28
[CoIII (L6)2(μ-O2)](ClO4)4
23.04
30.09
8.32
2
[CoIII (L7)2(μ-O2)](ClO4)4·2CH3CN
2
[CoIII(L8)(L9)(N3)]
[CoIII(L10)(L11)2]ClO4·MeOH·H2O
[CoIII(L10)(L12)2Na(ClO4)2]·0.5H2O
11.48
27.90
81
82
[CoIII (L13)2(μ-L14)2Cl2]Cl2·2H2O
13.75
8.37
Methanol
Methanol
2
[CoIII(HL15)2][CoII(NCS)4]NCS
27

[CoIII(L15)2]2[CoII(NCO)4]
[CoIII(L15)(NCS)2]·0.5H2O
[CoIII(L15)(N3)2]·0.5CH3CN
[CoIII(L16)(N3)3]
26.1
48.6
54
Methanol
Methanol
Methanol
Methanol
Methanol
Methanol
83
20.37
33.26
30.60
84
85
86
18
87
88
[CoIII(L17)(N3)3]
[CoIII(L18)2(L19)2]Cl·8H2O
[CoIII(L20)( L21)(N3)]
72.12
77.52
Acetonitrile
Acetonitrile
Acetonitrile
[CoIII(L22)( L23)(N3)]
-
-
[CoIII CoII(L24)2(μ2-C6H5CO2 )2(C6H5CO2
153.60
2
)2]·(CH3CN)2
[CoIII(L20)(L25)(N3)]
[CoIII(L20)(L25)(NCS)]
[CoIII(L26)(L27)(N3)]
[CoIII(L27)(L28)(N3)]
247.2
257.4
Acetonitrile
Acetonitrile
351.68
355.28
Methanol +
Acetonitrile
Methanol +
Acetonitrile
Acetonitrile
Acetonitrile
[CoIII(L29)(L30)(N3)]
166.64
626.40
89
[CoIII(L31)(L32)(NCS)]
[CoIII(L33)2](ClO4)3
5120
Acetonitrile
Methanol
69
34
[(N3)CoIIIL34(µ-
153.90
C6H5COO)CoII(N3)]·CH3OH
[(N3)CoIIIL(µ-OAc)CoII(N3)]
250.21
Acetonitrile
28

(μ-N3)2[(N3)CoIII(L)Na(OMe)]2·CH3OH
493.73
Acetonitrile
This Work
L1 = N,N′-bis(6-methylpyridin-2-ylmethylene)-2,2-dimethylpropane-1,3-diamine; L2 = N,N′-
bis(pyridin-2-ylmethylene)-2,2-dimethylpropane-1,3-diamine; L3 = 1 : 2 condensation product of
diethylenetriamine and 2-pyridinecarboxaldehyde; L4 = 1 : 2 condensation product of 3,3′-
bisaminopropylamine and 6-methyl-2-pyridinecarboxaldehyde; L5= 1 : 2 condensation product
of 3,3′-diamino-N-methyldipropylamine and 6-methyl-2-pyridinecarboxaldehyde; L6 = 1 : 2
condensation product of 3,3′-bisaminopropylamine and 2-acetylpyridine; L7 = 1 : 2
condensation product of 3,3′-bisaminopropylamine and 2-pyridinecarboxaldehyde; HL8 = 1-((2-
(diethylamino)ethylimino)methyl)naphthalene-2-ol; HL9 = acetylacetone, H2L10 = N,N′-bis(3-
methoxysalicylidehydene)cyclohexane-1,2-diamine); L11 = 4-aminopyridine; L12 = 1-
methylimidazole; L13 = 2-aminomethylpyridine; L14 = 2-iminomethylpyridine anion; HL15
=
condensation product of N,N′-dimethyldipropylenetriamine and o-vanillin; L16 = bis(2-
pyridylmethyl)amine; L17 = (2-pyridylmethyl)(2-pyridylethyl)amine; H2L18 = 3,5,6-tribromo-4-
pyridiniumcatechol;
L19
=
pyridine;
HL20
=
1-benzoylacetone;
HL21
=
=
1((2(diethylamino)ethylimino)methyl)naphthalen-2-ol; HL22 = 2-acetyl-1-naphthol; HL23
2((2(2-hydroxyethylamino)ethylimino)methyl)-6-ethoxyphenol; H2L24 = N-(2-hydroxyethyl)-3-
methoxysalicylaldimine; HL25 = 2-(3-(dimethylamino)propyliminomethyl)-6-ethoxyphenol;
H2L26=
pyridyl)tetrazole; H2L28 = 2((3-(methylamino)propylimino)methyl)-6-methoxyphenol; HL29
2((2(piperidin-1-yl)ethylimino) methyl)-6-ethoxyphenol; HL30 = 1-acetyl-2-naphthol; HL31 = 2-
2((3-(dimethylamino)propylimino)methyl)-6-ethoxyphenol;
HL27
=
5-(2-
=
((3(dimethylamino)propylimino)methyl)-6-methoxyphenol; HL32 = 2,4-pentanedione; L33
=
29