The first report of a tetra-azide bound mononuclear cobalt(iii) complex and its comparative biomimetic catalytic activity with tri-azide bound cobalt(iii) compounds

Article abstract of DOI:10.1039/d0nj02339f

Three new azide-bound cobalt(iii) complexes, [Co(HL1)(N3)4] (1), [Co(L2)(N3)3] (2) and [Co(L3)(N3)3] (3), where L1, L2 and L3 are N,N-dimethyldipropylenetriamine, N1-isopropyldiethylenetriamine and N,N-diethyldiethylenetriamine, respectively, were synthesized and structurally characterised. X-ray crystallographic studies reveal that the structures of both 2 and 3 are very similar in which three terminal azide ions together with a triamine coordinate the metal centre. Complex 1 on the other hand is significantly different from the other two as the metal centre in 1 is bonded with four terminal azide ions and two donor sites of triamine L1, leaving the tertiary amine group protonated. All the complexes are stabilized by rich hydrogen bonding interactions, leading to hydrogen bonded supramolecular chain structures. It is worth noting that complex 1 is the first example in cobalt(iii) coordination chemistry in which all four azide ions coordinate the metal centre terminally. All these compounds exhibited efficient catalytic activity towards the oxidative coupling of o-aminophenols to phenoxazinone chromophores under aerobic conditions and the role of the structural factors in the catalytic activity has been explored. A mass spectrometry study was carried out to identify the products and important reactive intermediates and to support the mechanistic proposal. This journal is

Full text of DOI:10.1039/d0nj02339f

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The first report of a tetra-azide bound mononuclear cobalt(III)
DOI: 10.1039/D0NJ02339F
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complex and its comparative biomimetic catalytic activity with tri-
azide bound cobalt(III) compounds
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Narayan Ch. Jana a, Paula Brandão b and Anangamohan Panja *a,c
a
Postgraduate Department of Chemistry, Panskura Banamali College, Panskura RS, WB
721152, India
b
Department of Chemistry, CICECO-Aveiro Institute of Materials, University of Aveiro,
3810-193 Aveiro, Portugal
c
Department of Chemistry, Gokhale Memorial Girls' College, 1/1 Harish Mukherjee
Road, Kolkata-700020, India.
Three new azide-bound cobalt(III) complexes, [Co(HL1)(N3)4] (1), [Co(L2)(N3)3] (2)
and [Co(L3)(N3)3] (3), where L1, L2 and L3 are N,N-dimethyldipropylenetriamine, N1-
isopropyldiethylenetriamine and N,N-diethyldiethylenetriamine, respectively, were
synthesized and structurally characterised. X-ray crystallographic studies reveal that the
structures of both 2 and 3 are very similar in which three terminal azide ions together with a
triamine coordinate the metal centre. Complex 1 on the other hand significantly different from
other two as the metal centre in 1 is bonded with four terminal azide ions and two donor sites
of triamine L1, leaving tertiary amine group protonated. All the complexes are stabilized by
rich hydrogen bonding interactions, leading to hydrogen bonded supramolecular chain
structures. It is worth noting that complex 1 is the first example in cobalt(III) coordination
chemistry in which all four azide ions coordinate the metal centre terminally. All these
compounds exhibited efficient catalytic activity towards the oxidative coupling of o-
aminophenols to phenoxazinone chromophores in aerobic condition and role of the structural

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factors on the catalytic activity has been explored. The mass spectrometry studyDwOIa: 1s0.c10a3r9r/iDe0dNJ02339F
out to identify products and important reactive intermediates and to support the mechanistic
proposal.
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Introduction
Dioxygen, a “green oxidant”, is considered as an essential oxidant for the oxidation
reaction of organic compounds, particularly to manufacture agrochemicals, pharmaceuticals,
and other fine chemicals.1–2 Utilization of such “green oxidant” is in great demand because it
can oxidise organic molecules at mild condition and the oxidation process is economically
favourable. But this environmentally benign oxidant is kinetically inert due to its triplet
ground state, and thus it is essential to activate triplet aerial oxygen to its active state to
execute oxidation process. However, metalloproteins can easily promote spin prohibited
communication among dioxygen and organic matter for biochemical oxidation reaction by
dioxygen activation.3–5 Thus, the bioinorganic chemists have paid considerable attention to
explore the mechanistic pathway of such enzymes by applying simple model study to get
insight into the enzymatic reaction.6–8 Moreover, the development of new model catalysts
that mimic the function of these enzymes may be utilized to achieve specific industrial
transformation of organic substances9,10 and therefore could be an alternative of traditional
hazardous inorganic catalysts.11–13
We are presently working on 1st row transition metal complexes which can mimic the
function of phenoxazinone synthase14–16 and catechol oxidase.17–19 The former is a multi-
copper metalloenzyme,20 that catalyses the oxidative coupling of substituted o-aminophenols
to produce the phenoxazinone chromophore in the final stage of biosynthesis of actinomycin-
D.21 This potent antineoplastic agent is known to use clinically for the treatment of various
22,23
cancers
through intercalation of phenoxazinone chromophore into DNA-base pair,
thereby inhibiting the synthesis of RNA.24 Apart from 2-aminophenoxazin-3-one (Phx-1)

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derivative, an oxygen transport enzyme, bovine hemoglobin, is capable to prDoOdIu: 1c0e.103o9t/hD0eNrJ02339F
phenoxazinone chromophore like 2-amino-4,4a-dihydro-4a-7-dimethyl-3H-phenoxazin-3-one
(Phx-2), and 3-amino-1,4a-dihydro-4a-8-dimethyl-2H-phenoxazin-2-one (Phx-3) (Scheme 1),
which can act as antibacterial agents.25 Moreover, the phenoxazine chromophore itself is
considered as a promising radical-trapping antioxidant.26 Furthermore, the oxidative coupling
of two o-aminophenol moieties could be considered as a novel click reaction for chemical and
biochemical studies.27 Thus, much work has to be done to develop the chemistry of such a
phenoxazinone chromophore.
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NH2
N
Phx-1
O
O
NH2
N
Phx-2
Phx-3
O
O
H3C
H3C
CH3
O
N
O
NH2
CH3
Scheme 1: Three different phenoxazinone chromophores.
Though both catechol oxidase and phenoxazinone synthase are multicopper oxidase
enzymes, several reports with mono and multinuclear metal complexes are available in which
other transition metals were also taken into an account for such oxidase activity.14–19, 25, 28–35
Moreover, these reports tell us that the catalytic efficiency of the complexes is governed by
the availability of labile position(s) as well as the electronic environment around the metal
centre. Therefore, the ligands with fewer donor atoms are obvious choice for the development
of efficient synthetic analogues. With this aim, we reacted cobalt(II) salts with three different
triamines in presence of azide ions, and successfully isolated three mononuclear octahedral
cobalt(III) complexes having labile sites. Herein, we report the syntheses and crystal

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DOI: 10.1039/D0NJ02339F
structures of [Co(HL1)(N3)4] (1), [Co(L2)(N3)3] (2) and [Co(L3)(N3)3] (3), where L1, L2 and L3
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are
N,N-dimethyldipropylenetriamine,
N1-isopropyldiethylenetriamine
and
N,N-
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diethyldiethylenetriamine, respectively. The phenoxazinone synthase mimicking activity of
these complexes has been examined, and the results are interpreted and discussed. To the best
of our knowledge the tetra-azide bound cobalt(III) complex (1) is the first example in which
all four azide ions coordinate the cobalt(III) centre terminally.
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Experimental section
Materials and measurements
Cobalt(II)
nitrate
hexahydrate,
N,N-dimethyldipropylenetriamine,
N1-
isopropyldiethylenetriamine and N,N-diethyldiethylenetriamine, 2-amino-5-methylphenol and
2-amino-4-methylphenol were purchased from Sigma Aldrich or Alfa Aesar Chemicals.
Sodium azide, o-aminophenol (OAPH) and solvents used in the present work were of reagent
grade and used as received.
Warning! Azide salts of metal complexes especially in presence of organic ligands are
potentially explosive. Appropriate protective measures should always be taken when handling
these compounds and small quantities should be used.
Synthesis of [Co(HL1)(N3)4] (1)
A methanol solution of cobalt(II) nitrate hexahydrate (0.291 gm, 1.0 mmol) was added
dropwise to a methanolic (20 ml) solution of N,N-dimethyldipropylenetriamine (L1) (0.159
gm, 1.0 mmol) at room temperature. The resulting solution was stirred in air for 10 min and
then an aqueous solution of sodium azide (excess amount) was added to the solution. The
stirring was continued for about 20 min. The resulting solution was filtered to remove a small
amount of precipitate appeared during this process and the filtrate was kept in a beaker for
slow evaporation. The reddish-brown crystals suitable for single crystal X-ray diffraction
study appeared after 2–3 days, which were filtered out and dried in air. Yield: 0.329 gm (85

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DOI: 10.1039/D0NJ02339F
%). Anal. Calcd. for C8H22CoN15: C 24.81 %; H 5.73 %; N 54.25 %. Found: C 24.89%, H
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5.72%, N 54.06 %. FTIR (KBr, cm–1): (N–H) stretching 3288 (m), 3224 (s), 3149 (m);
휈
휈
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–
(C–H) stretching 2946 (m); (N ) 2065 (s), 2017(s).
휈
3
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Synthesis of complex [Co(L2)(N3)3] (2) and complex [Co(L3)(N3)3] (3)
Complexes 2 and 3 were synthesized following a very similar procedure as that applied for
N1-isopropyldiethylenetriamine
N,N-
complex
1,
except
that
(L2),
and
diethyldiethylenetriamine (L3), were used instead of N,N-dimethyldipropylenetriamine,
respectively.
Complex 2. Yield: 0.258 gm (78 %). Anal. Calcd. for C7H19CoN12: C 25.46 %; H 5.80 %; N
50.90 %. Found: C 25.62%, H 5.91%, N 50.65 %. FTIR (KBr, cm–1): (N–H) stretching
휈
–
3262 (s), 3219 (s), 3144 (s); (C–H) stretching 2984 (m); (N ) 2067 (s), 2024 (s).
휈
휈
3
Complex 3. Yield: 0.279 gm (81 %). Anal. Calcd. for C8H21CoN12: C 27.91 %; H 6.15 %; N
48.82 %. Found: C 27.99%, H 6.22%, N 48.60 %. FTIR (KBr, cm–1): (N–H) stretching
휈
–
3238 (m), 3174 (s), 3120 (m); (C–H) stretching 2960 (m); (N ) 2038 (s).
휈
휈
3
Physical measurements
Elemental analysis for C, H, and N were carried out in a Perkin-Elmer 240C elemental
analyser. UV-vis spectrophotometric studies were performed in an Agilent Carry-60 diode
array UV–vis spectrophotometer with 1-cm path-length quartz cell at room temperature in
methanol solvent. IR spectra of the samples, prepared as KBr pellets, were recorded in a
PerkinElmer Spectrum Two FTIR spectrophotometer in the range 400 to 4000 cm–1. The
cyclic voltammetric studies were performed using an electrochemical workstation model
CHI630E, CH Instruments, within the potential range of 0 to −1.60 V vs. Ag/AgCl at a scan
rate of 100 mVs−1. Data were recorded using 1.0 × 10−3 M complexes in presence of a
supporting electrolyte, tetrabutylammonium perchlorate (TBAP, 0.1 M), in methanol using a
conventional three-electrode assembly made up of a platinum working electrode, a platinum

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wire auxiliary electrode and a Ag/AgCl reference electrode. Electrospray ioniDzOaIt:i1o0n.103m9/aD0sNsJ02339F
spectrometry (ESI-MS positive) were carried out in a Micromass Q-tof-Micro Quadruple
mass spectrometer.
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Phenoxazinone synthase activity
The phenoxazinone synthase activity of all the complexes were examined by the reaction of
the complexes with o-aminophenol (OAP) as a substrate in methanol under aerobic condition
at room temperature. Similar oxidase activity was also carried out with two other o-
aminophenol derivatives, namely 2-amino-5-methylphenol and 2-amino-4-methylphenol,
under identical rection conditions. The progress of the reactions was followed by UV-vis
spectrophotometrically by monitoring increase in the absorbance maxima of the respective
phenoxazinone chromophores at 435, 405, and 388 nm for OAPH, 2-amino-5-methylphenol,
and 2-amino-4-methylphenol as a function of time. The initial rates were calculated by linear
regression from the slope of absorbance against time plot, obtained maintaining the pseudo-
first-order reaction condition, using literature reported molar extinction coefficient values.36
Different kinetic parameters, namely KM, Vmax and Kcat were evaluated applying Michaelis
Menten equation and the turnover number (Kcat value) is obtained by dividing the Vmax by the
initial concentration of the complex used.
X-ray crystallographic data collection and refinement
X-ray crystallographic data were collected using monochromated Mo-Kα radiation (λ =
0.71073 Å) on a Bruker Smart Apex-II diffractometer, equipped with a CCD area detector at
293 K for complex 1 and at 150 K for complexes 2 and 3. Data analysis was carried out with
the Bruker Saint program. The semi-empirical absorption correction to reflection data was
made using SADABS program.37 The initial structures were solved by the direct method and
all non-hydrogen atoms were refined with anisotropic thermal parameters using the full-
matrix least squares method on F2.38 The hydrogen atoms bonded to carbon were included in

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geometrically idealized positions with given thermal parameters equivalent to 1.2DoOrI:110.5.10t3i9m/De0NsJ02339F
that of the other atoms to which they are attached. Hydrogen atoms attached to nitrogen atoms
were located on the difference Fourier map and isotropically treated using a riding model. The
crystal structures of these complexes were generated using MERCURY-4.1.2. software. The
crystallographic information of these structures are provided in Table 1.
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Table 1. Crystal details and structure refinement parameters of complexes 1−3.
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2
3
Empirical formula
Temperature (K)
Formula weight (g mol-1)
Crystal system
Space group
C8H22CoN15
293(2)
387.33
Monoclinic
P21/n
C7H19CoN12
150(2)
330.27
Monoclinic
P21/n
C8H21CoN12
150(2)
344.30
Triclinic
Pī
a (Å)
b (Å)
c (Å)
9.159(4)
7.816(3)
24.420(11)
90.00
8.5905(4)
15.1046(7)
10.9917(5)
90.00
8.1748(18)
9.1211(19)
11.034(2)
89.394(9)
78.199(9)
66.964(8)
738.9(3)
2
 (deg)
 (deg)
 (deg)
volume (Å3)
99.059(7)
90.00
99.511(2)
90.00
1726.3(13)
4
1406.63(11)
4
Z
Dcalc (g cm‒3)
µ (mm‒1)
1.490
1.022
1.560
1.233
1.547
1.177
F(000)
808
688
360
1.689−23.454
11785
2421(0.1086)
1647
2.312−29.220
22163
3803(0.0363)
3101
1.891−25.854
12414
2800(0.0253)
2638
 Range (deg)
Reflections collected
Independent reflections (Rint)
Observed reflections [I>2(I)]
Restraints/parameters
Goodness-of-fit on F2
Final R indices [I>2(I)]
0/219
1.143
0/203
1.018
0/222
1.123
R1 = 0.0879,
wR2 =0.2218
R1 = 0.1198,
wR2 =0.2626
0.993/−1.049
R1 = 0.0285,
wR2 =0.0645
R1 = 0.0410,
wR2 =0.0701
0.710/−0.241
R1 = 0.0262,
wR2 = 0.0619
R1 = 0.0282,
wR2 = 0.0627
0.405/−0.319
R indices (all data)
Largest diff. peak/hole (e Å‒3)
Results and Discussion
Synthesis and general characterizations

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There cobalt complexes were obtained in good yields by the one-pot reaction of cobalt(II)
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nitrate hexahydrate with three different triamines in presence of an excess sodium azide in
methanol at ambient temperature. Slow evaporation of methanolic solutions afforded three
complexes [Co(HL1)(N3)4] (1), [Co(L2)(N3)3] (2), and [Co(L3)(N3)3] (3) as dark brown block-
shaped crystals. All the complexes are mononuclear cobalt(III) complexes in which starting
cobalt(II) metal centre is air-oxidized to cobalt(III), bonded to triamine and azide ions.
Interestingly, complex 1 has different composition than other two complexes. In complex 1,
two nitrogen atoms of the triamine and four nitrogens of the four terminal azide ions are
bonded to the metal centre, while in complexes 2 and 3, all nitrogen atoms of the triamines are
coordinated to the metal centre along with three nitrogens of the three terminal azide ions.
Furthermore, the tertiary amine nitrogen atom is protonated in 1 to balance the charge of the
system. It is worth mentioning that the mononuclear octahedral cobalt(III) complex bound to
all four terminal azide ions is the first example in cobalt(III) coordination chemistry. All
complexes are soluble in common solvents like methanol, acetonitrile and DMF.
IR spectra of complexes 1–3 show sharp bands in the range 2018–2065 cm–1, which
can be assigned to the stretching vibration of azide ions.15,39 Bifurcation of this band in 1
indicates the presence of both axial and equatorial azide ions in the octahedral geometry of the
cobalt(III) centre. Similarly, broadening of the peaks in 2 and 3 indicates that azide ions are
not electronically equivalent. Additionally, several bands of medium intensity appeared in the
range 3100–3300 cm–1 can be assigned to the N–H stretching vibrations of the triamines in 1–
3.40
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Description of crystal structures
Single crystal X-ray diffraction study was carried out to establish the solid-state
structures of the complexes. The crystal structures of complexes 1–3 along with atom
numbering scheme in each complex are shown in Figs. 1–3, respectively. Crystallographic

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information of all the complexes are assembled in Table 1. Important bond distances for all
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the complexes are given in Table 2 while bond angles are given in Table S1. X-ray
crystallographic studies reveal that 1‒3 are mononuclear cobalt(III) complexes with
compositions [Co(HL1)(N3)4], [Co(L2)(N3)3] and [Co(L3)(N3)3], respectively. Complexes 1
and 2 crystallise in the monoclinic P21/n space group, while complex 3 crystallises in the
triclinic Pī space group, and the asymmetric unit in all the crystal systems is comprised with a
complete set of neutral complex molecule. The geometry of the metal centre in all the
complexes is best described as a pseudo-octahedral structure with CoN6 coordination
environment as suggested by slight deviation of the cisoid (83.43(6) – 97.49(6) °) and
transiod (170.76(6) –177.5(8)°) bond angles around the metal centre from the ideal values.42–
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43
In 2 and 3, the Co–N bond lengths of the primary amine and secondary amines vary in
narrow range 1.9368(13)–1.9552(17) Å, whereas the Co–N bond length (2.088(16) Å) of
tertiary amine nitrogen is significantly longer in 3. This is consistent with weaker basicity of
the tertiary amine nitrogen atom, which is because of increased steric crowding around the
donor nitrogen atom, resulting in a longer Co–N bond of the tertiary amine over analogues
primary and secondary amines.15,39,44-46 The Co–N bond length of primary amine of 1.947(6)
Å in 1 falls within the range that found in 2 and 3, but in 1 the Co–N bond length of
secondary amine (2.033(6) Å) is slightly longer than the primary amine bond. Although the
tertiary amine bond length in 3 is significantly longer, the structures of 2 and 3 are eventually
similar in that the metal centres are connected with three nitrogen atoms of the triamine
ligands and the remaining three positions are occupied by three terminal azide ions. The
structure of complex 1 is significantly different from other two complexes as the metal centre
in 1 is bonded with four azide ions terminally and the remaining two positions are occupied
by two nitrogen atoms of the triamine ligand, leaving the tertiary amine group protonated to
balance the charge of the system. It is interesting to observe that the tertiary amine nitrogen

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atom of triamine L3 is connected to the metal centre in 3, while the tertiary amDinOeI: 1n0.i1t0r3o9/gDe0nNJ02339F
atom of ligand L1 in 1 is protonated, which is the origin of the structural differences in these
systems. We have already observed weaker binding tendency of the tertiary amine in 3 and
this is consistent with the literature reports on similar systems.15,39 The relatively higher
stability of five membered chelate ring over six-membered one may further play crucial role
in the diverse coordination ability of the triamines in these complexes. The amine groups in
L3 is separated by the ethylenic spacer, while these are separated through the propylenic
spacer in L1. Although the tertiary amine is weakly coordinated in 3 as reflected from the
longer Co–N bond length, the relatively more stable five-membered chelate ring brings
adequate stability in the system to favour the coordination of tertiary amine in 3. Both the
weaker basicity of the tertiary amine group and the relatively lower stability of six-membered
chelate ring allow to bind the stronger azide ion with the metal centre, leaving the tertiary
amine nitrogen protonated to ensure the electroneutrality in the system. The bond parameters
associated with azide ions are typical for terminally coordinated azide ions.15,39,41
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Crystal packing of all the complexes are mainly stabilized by the extensive hydrogen
bonding interactions in which amine groups are exclusively hydrogen bond donors and azide
ion is the sole acceptor. The details of the hydrogen bonding parameters are given in Table 3.
As can be seen from Fig. 4, each complex molecule in 1 is engaged with neighbouring
molecules through eight moderate-to-strong intermolecular hydrogen bonds, leading to a
hydrogen-bonded supramolecular chain structure along the crystallographic b axis. Similarly,
the hydrogen bonding interactions between the adjacent molecules constitute the
supramolecular chains as shown in Figs. S1 and S2 for 2 and 3, respectively. In 3, the primary
amine forms intermolecular hydrogen bonds with two azide ions one each from two
neighbouring molecules, constituting a zig-zag supramolecular chain along the direction of
the c axis as depicted in Fig. S2. In 2, the primary amine group forms hydrogen bonds with

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two azide ions from a neighbouring molecule that results a hydrogen bonded supramolecular
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dimer. These supramolecular dimeric units further connect each other through the hydrogen
bonds involving the terminal secondary amine group and azide ions from adjacent dimers,
leading to the formation of a supramolecular chain running along the a axis (Fig. S1).
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Fig. 1. Ellipsoid view of [Co(HL1)(N3)4] (1). Hydrogen atoms are wireframed (except on N-
atoms of the triamine) for clarity and ellipsoids are drawn at the 30% probability level.
Fig. 2. Molecular diagram of [Co(L2)(N3)3] (2). Hydrogen atoms are wireframed (except on
N-atoms of the triamine) for clarity and ellipsoids are drawn at the 30% probability level.

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Fig. 3. Molecular diagram of [Co(L3)(N3)3] (3). Hydrogen atoms are wireframed (except on
N-atoms of the triamine) for clarity and ellipsoids are drawn at the 30% probability level.
Fig. 4. Hydrogen bonded supramolecular chain structure of 1 propagating along the b axis.
Table 2. Selected bond distances (Å) for complexes 1–3.
1
2
3
1.947(6)
2.033(6)
1.9518(14)
1.9368(13)
1.9875(14)
1.9552(17)
1.9487(17)
2.0866(16)
CoN (primary)
CoN (secondary)
CoN (tertiary)
Co–N (azide)
1.975(6)
1.956(7)
1.956(6)
1.942(7)
1.9575(13)
1.9592(14)
1.9719(13)
1.9634(17)
1.9365(17)
1.9556(16)

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Table 3. Distances (Å) and Angles (°) of hydrogen bonds for compounds 1–3#
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D–H···A
d(H ···A)
2.352
2.409
2.271
1.928
2.350
2.400
2.444
2.412
2.214
2.342
2.368
2.194
2.257
2.222
2.248
d(D ···A)
3.176
3.237
3.087
2.850
2.652
2.702
3.117
2.899
3.061
3.102
2.679
2.625
3.059
2.996
2.641
<D–H···A
N1–H1A···N4a
N1–H1B···N10b
N2–H2···N15c
N3–H3···N9c
N1–H1A···N10
N1–H1B···N4
N1–H1C···N6d
N1–H1C···N11
N1–H1D···N7d
N2–H2C···N4e
N2–H2C···N4
N3–H3C···N10
N1–H1C···N6f
154.09
154.81
139.99
156.88
100.00
100.00
136.55
116.94
165.47
152.69
103.04
110.69
167.04
150.85
107.20
9
Complex 1
Complex 2
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Complex 3 N1–H1D···N12g
N2–H2C···N4
# Symmetry codes: a = 3/2‒x,1/2+y,3/2‒z; b = 3/2‒x,‒1/2+y,3/2‒z; c = x,‒1+y,z,; d =
1‒x,1‒y,‒z; e = 2‒x,1‒y,1‒z; f = 1‒x,2‒,1‒z and g = 1‒x,2‒y,2‒z; D and A stand for donor
and accepter, respectively
Electrochemistry
The redox potentials of the transition metals in the active site of metalloenzymes play vital
role for their efficient catalytic activity in the redox transformation of the biologically
important compounds in the living system. Thus, electrochemical studies of the synthetic
analogues may provide adequate information to predict their catalytic efficiency in model
study. The electrochemical study of complexes 1–3 was carried out in methanol solution at a
scan rate of 100 mVs–1 with respect to an Ag/AgCl reference electrode using
tetrabutylammonium perchlorate as a supporting electrolyte. A cathodic scan (0 to ‒1.0 V) of
the complexes shows an irreversible wave varying in the range ‒0.59 to ‒0.68 V and these
values are comparable to reported results on similar systems those exhibited efficient
oxidative catalytic activity (Fig. 5).41 The irreversible reductive response is expected for the

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reduction of cobalt(III) to cobalt(II) as found in the similar complexes with CoN6 coordination
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environment.41 The irreversible nature of this response is due to increased lability of the
coordinated azide ions upon reduction of cobalt(III) centre.
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Fig. 5. Cyclic voltammograms of 1−3 in methanol using a platinum working electrode in
presence of tetrabutylammonium perchlorate as a supporting electrolyte at room temperature
with scan rate 100 mV s‒1.
Phenoxazinone synthase like activity
Phenoxazinone synthase activity of the complexes was carried out using o-aminophenol as a
substrate. Deploying identical reaction conditions, 1.0 ×10–5 M concentration of complexes 1–
3 was treated with 1.0 ×10–2 M concentration of o-aminophenol under aerobic condition in
methanol at room temperature, and the progress of the reaction was monitored UV-vis
spectrophotometrically at 5 min time interval. A gradual increase in the absorption spectra at
435 nm, characteristic for phenoxazinone chromophore, was observed in the UV-vis Spectra
for all the complexes. Fig. 6 represents the wavelength vs. absorption plot for the catalytic
oxidation of o-aminophenol for complexes 1 and 2, whereas the same spectral profile for
complex 3 is depicted in Fig. S3. A comparative catalytic efficiency was investigated in terms
of their initial rates, which were determined from the log[A/(A – At)] vs. time plots for the
reactions catalysed by complexes 1–3. The plots of initial rates vs. concentrations of the
substrate show the first-order reaction kinetics at lower concentrations and the saturation

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kinetics at higher substrate concentrations of the substrate (Fig. 7). These kinetic data suggest
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that the course of the reaction proceeds through the formation of a stable catalyst-substrate
intermediate before the redox degradation of the intermediate in the rate determining step.
Michaelis Menten model has been applied to analyse rate saturation kinetics and to extract
different kinetic parameters KM, Vmax and Kcat. The turnover number (Kcat value) is obtained
by dividing the Vmax by the initial concentration of the complex used, and the summarized
data for all the complexes were given in Table 4.
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Fig. 6. UV-vis spectral scans for the catalytic oxidation of o-aminophenol in methanol
showing the increase in phenoxazinone product at 435 nm for 1 and 2.
Fig 7. Initial rate versus substrate concentration plot for the oxidation of OAPH in dioxygen-
saturated methanol catalysed by complexes 1–3 at room temperature. Symbols and solid lines
represent the experimental and simulated profiles, respectively.

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Again, on the basis of our current interest to get insight into a clear mechanistic
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pathway we recently carried out model oxidations of o-aminophenol together with some
substituted o-aminophenols.39 Like o-aminophenol, we carried out the catalytic oxidation of
two substituted o-aminophenols (2-amino-5-methylphenol and 2-amino-4-methylphenol). It
was observed that the absorption maxima for 2-amino-4,4a-dihydro-4a-7-dimethyl-3H-
phenoxazine-3-one (Phx-2) gradually increased when the reaction was carried out in an
identical reaction condition for 1 and 2 as depicted in Figs. 8 and in Fig. S4 for complex 3.
Similar spectral features were also observed for the formation 3-amino-1,4a-dihydro-4a,8-
dimethyl-2H-phenoxazin-2-one (Phx-3) when 2-amino-4-methylphenol was used and a
representative spectral scan is displayed in Fig. S5. Various parameter like KM, Vmax and Kcat
were extracted by analysing the rate saturation kinetics (Fig S6) and listed in Table 4.
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Table 4: Kinetic parameters of phenoxazinone synthase like activity for complexes 1–3.
Substrate
Catalyst
Vmax (Ms−1)
KM (M)
kcat (h−1)
61.92
1
2
3
1
2
(1.72 ± 0.03) × 10−7 (7.46 ± 0.44) × 10−4
(1.12 ± 0.03) × 10−7 (2.68 ± 0.22) × 10−3
OAPH
40.34
(1.18 ± 0.03) × 10−7
(1.17 ± 09) × 10−3
42.63
(3.12 ± 0.03) × 10−7 (2.60 ± 0.08) × 10−3
(2.48 ± 0.04) × 10−7 (2.34 ± 0.11) × 10−3
112.12
89.35
2-amino-5-
methylphenol
3
1
(2.44 ± 0.05) × 10−7 (3.33 ± 0.19) × 10−3
(2.32 ± 0.04) × 10−7 (2.47 ± 0.14) × 10−3
87.81
83.59
2-amino-4-
methylphenol
2
3
(1.82 ± 0.02) × 10−7 (2.17 ± 0.09) × 10−3
(1.67 ± 0.05) × 10−7 (3.06 ± 0.25) × 10−3
65.61
60.12

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Fig 8. Increase of the phenoxazinone chromophore band at 402 nm after the addition of 2-
amino-5-methylphenol to a methanol solution with complex 1 and 2. The spectra were
recorded at 5 min intervals.
Comparative catalytic activity and mechanism
Structural studies on metalloenzymes and several efficient functional models reveal that the
available vacant sites or labile positions around the metal centre is accountable for their
remarkably high catalytic reactivity. A similar observation is also noticed in the present report
where the presence of substitutionally labile azide ions at the metal coordination sphere in all
the complexes is responsible for their strong catalytic activity towards the oxidative coupling
of o-aminophenols in aerobic condition. From the crystallographic data, it is clear that
complex 1 comprises of four labile azide ions while complexes 2 and 3 hold three azide ions
around the cobalt(III) centre. Therefore, the probability of formation of the complex-substrate
intermediate is expected to be higher in 1 compared to 2 and 3, and consequently compound 1
is found to be a better catalyst in this series. It is also to be noted that a comparable activity is
observed for complexes 2 and 3 towards the catalytic oxidation of o-aminophenols, which
could be due to the consequence of their structural similarity as discussed earlier. Again, it is
found that the methyl substitution on the benzene ring does not inhibit the formation of the
complex-substrate adduct as suggested by the rate saturation kinetics as well as mass

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spectrometry study (vide infra).39 Moreover, the methyl substituation facilitates the oxidation
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of aminophenol moiety, leading to their greater turnover rates over parent o-aminophenol.
The mass spectrometry study of a mixture of complex 2 and o-aminophenols in 1:20
molar ratio was carried out for possible identification of products and reactive intermediates.
The mass spectra of compound 2 in presence of o-aminophenol or 2-amino-5-methylphenol as
depicted in Figs. S7 and S8 respectively are quite clean in that they consist of only few peaks
and all of them are well assignable. When the mass spectrometry of complex 2 is performed
in presence of o-aminophenol, we observed the base peak at m/z = 110.0560 which is
matched with the protonated species of o-aminophenol itself (calc. m/z = 110.06). The next
most abundant peak at m/z = 215.0814 is quite interesting as it represents the important
reactive intermediate II-B (Scheme 2), produced during the oxidative coupling of two o-
aminophenols at the penultimate step of the formation of 2-aminophenoxazin-3-one, in the
form of its protonated species (calc. m/z = 215.08). Another major peak observed at m/z =
311.1297 is the most important one as it agrees well with the complex-substrate intermediate
of composition [CoIII(L2)(OAPH) ‒2H]+ (calc. m/z = 311.13). The mass spectra of 2 in
presence of 2-amino-5-methylphenol provides us also significant information in which the
base peak at m/z = 325.1441 is matched with the intermediate species of composition
[CoIII(L2)(2-amino-5-methylphenol) ‒2H]+ (calc. m/z = 325.14). The next highest peak is a
product related peak as it matched nicely with the protonated species of the product (calc. m/z
= 243.11). Therefore, from the mass spectrometry study as well as from the rate saturation
kinetics, it is quite clear that the catalytic cycle proceeds with the formation of a stable
complex-substrate intermediate in which the substrate binds the metal centre by the
replacement of available terminal azide ions. This intermediate species experiences
intramolecular redox transformation in the rate determine step in which the cobalt(III) centre
is reduced to the cobalt(II) state with concomitant oxidation of coordinated o-aminophenol to
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its mono-radical species, which thereafter oxidises to o-benzoquinone monoamDinOeI: 1(0B.10Q39M/D0IN)J02339F
intermediate. In order to further probe the mechanistic pathway, we measured the EPR spectra
of complex 2 both in presence and absence of o-aminophenol, but both were EPR silent. The
trapping of o-aminophenol monoradical through EPR spectroscopy is rarely achieved.47
Furthermore, high spin cobalt(II) centre is EPR silent at the liquid nitrogen temperature.
However, it is well accepted that mononuclear manganese(III) and cobalt(III) comlexes
oxidise o-aminophenol and themselves reduced to the +II oxidation state, and for the
manganese systems characteristic manganese(II) species had been detected through EPR
spectroscopy.14,18,48 The influence of radical trapping agent namely TEMPO is also found
insensitive in the present study, which is quite justified as TEMPO mediated catalytic
oxidation of o-aminophenol is reported in the literature.49 The kinetic study at variable
dioxygen concentration further reveals that the dioxygen is not involved in the rate
determining step (Fig. S9). Facile oxidation of methyl substituted o-aminophenols over parent
o-aminophenol also justifies that the redox degradation of the complex-substrate intermediate
is the rate determining step, resulting in overall higher turnover rates for methyl substituted
analogues. The intermediate (BQMI) thereafter reacts with another o-aminophenol moiety and
finally yields the phenoxazinone chromophore through couple of oxidative dehydrogenation
steps as shown in Scheme 2. However, due to the presence of methyl substitution, final
oxidative dehydrogenation step is inhibited for methyl substituted o-aminophenols, thereby
ending with the formation of dihydro-phenoxazinone chromophores instead of phenoxazinone
one.
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OH
R
H+ + 2 N3
NH2
N
N
O
N
CoIII
N3
R
N3
N
N
N
H2
CoIII
H
H
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N3
N
N3
H
H
1/2H2O2
N
.
.
O
N
O
R
CoII
N
R
NH2
N
H+ + 2 N3 + 1/2O2
H2
N3
-e
-H+
H
H
OH
H
N
R
H
N
H
NH
O
NH2
OH
NH2
NH2
R
R
R
OH
OH
O
R
R
BQMI
-2e
-2H+
I-A
I-B
H
-2e
NH
NH2
NH2
OH
N
N
O
N
-2H+
2
R = H
R
OH
II-A
O
(OAPH)
O
O
R
2-aminophenoxazin-3-one
II-B
R = CH3
R = CH3
(2-amino-4-methylphenol)
(2-amino-5-methylphenol)
10
N
10
9
1
9
1
NH2
10a
9a
5a
H3C
10a
O
9a
5a
N
8
7
8
7
2
2
4a
4a
3
O
O
3
O
NH2
H3C
6
4
6
4
5
5
CH3
CH3
3-amino-1,4a-dihydro-4a-8-
dimethyl-2H-phenoxazin-2-one
2-amino-4,4a-dihydro-4a-7-
dimethyl-3H-phenoxazin-3-one
Scheme 2. Plausible mechanistic pathway for the aerobic oxidation of o-aminophenol
catalysed by 2.
Conclusions
As a whole, one-pot reaction of cobalt(II) nitrate hexahydrate with three different triamines in
presence of an excess sodium azide afforded [Co(HL1)(N3)4] (1), [Co(L2)(N3)3] (2), and
[Co(L3)(N3)3] (3) as dark brown block-shaped crystals in high yield. X-ray structural analysis
reveals that the geometry of the cobalt(III) centre in all these complexes is pseudo-octahedral

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with CoN6 coordination sphere. Moreover, the structures of 2 and 3 are very similar in that the
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metal centre is connected with all three nitrogen atoms of a triamine together with three
terminal azide ions. However, they are significantly different from complex 1 in which the
metal centre is bonded with four terminal azide ions together with primary and secondary
amines of triamine L1, leaving tertiary amine group protonated to balance the charge of the
system. The origin of such structural diversity has been analysed and discussed, and this
structural difference is reflected on their catalytic efficiency towards the aerobic oxidation of
o-aminophenols. As the more numbers of substitutionally labile azide ions are available in 1,
the reactivity of it is also found highest in this series. The solid-state structures of these
complexes are stabilized by extensive hydrogen bonding interactions, resulting in the
supramolecular hydrogen-bonded chain structures. Moreover, the reactivity of methyl-
substituted o-aminophenols is found higher than o-aminophenol itself which is because of the
effect of electronic factor upon methyl substitution, However, the effect of this substitution on
the complex-substrate adduct formation is not noticed, but the final dehydrogenation step is
inhibited which ended up with the production of dihydro-phenoxazinone chromophores
instead of phenoxazinone one. The mass spectrometry study provides us the information of
final products as well as the reactive intermediates based on which a deep insight into the
reactivity of the catalytic oxidation of o-aminophenols is possible. Finally, Finally, complex 1
provides the first example in cobalt(III) coordination chemistry of a complex with four
terminal azide ions bonded to the metal centre terminally .
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Acknowledgments
A. P. gratefully acknowledges the financial support of this work by the Government of West
Bengal through the Department of Science & Technology and Biotechnology, Kolkata, India
(Sanction No. 331/ST/P/S&T/15G-8/2018, dated-19.06.2019).

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Appendix A. Supplementary data
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Electronic supplementary information (ESI) available. The ESI file contains Fig. S1–S9 and
Table S1. CCDC 2001493-2001495 contain the supplementary crystallographic data for 1‒3.
For ESI and crystallographic data in CIF or other electronic format see DOI:
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Graphical Abstract
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The first report of a tetra-azide bound mononuclear cobalt(III) complex and its
comparative biomimetic catalytic activity with tri-azide bound cobalt(III) compounds
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Narayan Ch. Jana, Paula Brandão, and Anangamohan Panja
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Three new azide-bound cobalt(III) complexes derived from three different triamines with
extensive hydrogen bonded supramolecular chain structures and role of their structural
factors on oxidative coupling of o-aminophenols have been reported.