Mixed valence trinuclear cobalt (II/III) complexes: Synthesis, structural characterization and phenoxazinone synthase activity

Article abstract of DOI:10.1016/j.poly.2017.06.007

Three new mixed valence trinuclear Co(II/III) compounds [Co₃L₂(μ₂-C₆H₅CO₂^?)₂(CH₃CN)₂](ClO₄^?)₂·(CH₃CN)₃ (1) [Co₃L^R₂(μ₂-C₆H₅CO₂^?)₂(C₆H₅CO₂^?)₂]·(CH₃CN)₂ (2) and [Co₃L^R₂(μ₂-C₆H₅CH[dbnd]CH–CO₂^?)₂(C₆H₅CH[dbnd]CH–CO₂^?)₂]·((CH₃)₂CO)₂ (3) have been synthesized by reacting the di-Schiff base ligand [H₂L] or its reduced analog [H₂L^R] (where H₂L?=?N,N′-bis(salicylidene)-1,3-propanediamine) and (H₂L^R?=?N,N′-bis(2-hydroxybenzyl)-1,3-propanediamine) with cobalt perchlorate hexahydrate and sodium benzoate or sodium cinnamate. The complexes have been characterized by IR, UV–Vis and single crystal X-ray diffraction analyses. All the complexes are linear trinuclear species [Co^III–Co^II–Co^III] in which two terminal octahedral Co^IIIN₂O₂ cores coordinate to the central octahedral Co^II ion through μ₂-phenoxido oxygen atom and the bridging carboxylato ions. In addition, the complexes (2 and 3) derived from reduced Schiff base ligand have, rather unusually, a terminally coordinated carboxylato ion to Co^III, which is acetonitrile molecule in case of complex (1) with unreduced ligand. Complex 2 has been found to be an excellent functional model for the phenoxazinone synthase activity, in the aerial oxidation of 2-aminophenol to the corresponding 2-aminophenoxazine-3-one chromophore in acetonitrile solvent medium. Detailed kinetic data analysis of this oxidation reaction reveals a fairly high phenoxazinone synthase activity of 2 with k_cat?=?153.60?h^?1.

Full text of DOI:10.1016/j.poly.2017.06.007

Accepted Manuscript
Mixed valence trinuclear cobalt (II/III) complexes: Synthesis, structural char-
acterization and phenoxazinone synthase activity
Alokesh Hazari, Avijit Das, Prithwish Mahapatra, Ashutosh Ghosh
PII:
S0277-5387(17)30424-2
DOI:
http://dx.doi.org/10.1016/j.poly.2017.06.007
POLY 12687
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
5 April 2017
7 June 2017
Please cite this article as: A. Hazari, A. Das, P. Mahapatra, A. Ghosh, Mixed valence trinuclear cobalt (II/III)
complexes: Synthesis, structural characterization and phenoxazinone synthase activity, Polyhedron (2017), doi:
http://dx.doi.org/10.1016/j.poly.2017.06.007
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Mixed valence trinuclear cobalt (II/III) complexes: Synthesis, structural
characterization and phenoxazinone synthase activity
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Alokesh Hazari,a,b Avijit Dasa, Prithwish Mahapatraa, Ashutosh Ghosha*
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aDepartment of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata-700 009,
India. E-mail: ghosh_59@yahoo.com
bDepartment of Chemistry, Government General Degree College Singur, Jalaghata, Hooghly-712 409
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ABSTRACT
-
Three new mixed valence trinuclear Co(II/III) compounds [Co3L2(2C6H5CO2)2(CH3CN)2](ClO4


)2·(CH3CN)3
(1)
[Co3LR (2C6H5CO2 )2(C6H5CO2 )2]·(CH3CN)2
(2)
and
2
[Co3LR (2C6H5CH=CHCO2)2(C6H5CH=CHCO2 )2]·(CH3)2CO (3) have been synthesized by

2
reacting the di-Schiff base ligand [H2L] or its reduced analogue [H2LR] (where H2L = N,N-
bis(salicylidene)-1,3-propanediamine) and (H2LR = N,N-bis(2-hydroxybenzyl)-1,3-propanediamine) with
cobalt perchlorate hexahydrate and sodium benzoate or sodium cinnamate. The complexes have been
characterized by IR, UV-Vis and single crystal X-ray diffraction analyses. All the complexes are linear
trinuclear species [CoIIICoIICoIII] in which two terminal octahedral CoIIIN2O2 cores coordinate to the
central octahedral CoII ion through μ2-phenoxido oxygen atom and the bridging carboxylato ions. In
addition, the complexes (2 and 3) derived from reduced Schiff base ligand have, rather unusually, a
terminally coordinated carboxylato ion to CoIII, which is acetonitrile molecule in case of complex (1) with
unreduced ligand. Complex 2 has been found to be an excellent functional model for the phenoxazinone
synthase activity, in the aerial oxidation of 2-aminophenol to the corresponding 2-aminophenoxazine-3-
one chromophore in acetonitrile solvent medium. Detailed kinetic data analysis of this oxidation reaction
reveals a fairly high phenoxazinone synthase activity of 2 with kcat = 153.60 h−1.
Keywords: Reduced Schiff base, Mixed valence cobalt (II/III), Phenoxazinone synthase.
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1. Introduction
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The synthesis of cobalt Schiff base complexes is of continual interest due to their
important structural variations [1], various catalytic activities [2], intriguing magnetic properties
[3] and a wide range of medicinal applications [4]. The oxidation state of cobalt in these
complexes in most cases is either +3 or +2 depending upon the nature of the Schiff bases and
methods of syntheses. Besides, a few mixed valence (+3/+2) dinuclear species [5], trinuclear [6]
and tetranuclear [7] cobalt Schiff base complexes are also reported in the literature. Among
them, the trinuclear complexes deserve special attention as they can be derived easily from the
salen type di-Schiff base ligands [8]. The common structural feature of these trinuclear
complexes is that a central CoII ion is connected with two terminal CoIII ions through double
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phenoxido bridges along with an additional anionic bridge (carboxylato [8,9]or chloride [10] or
sulphito [11] or azido [6,12]) between the terminal and central cobalt atoms. It is also intriguing
to note that the complexes derived from the di-Schiff base ligand of 1,2-ethanediammine are
invariably mixed valence [CoIII–CoII–CoIII] [8,9] but those of 1,3-propanediammine can be either
mixed valence [CoIII–CoII–CoIII] [13] or only divalent [CoII–CoII–CoII] [14]. Recently, we
observed interesting structural features in a mixed valence cobalt complex of a reduced Schiff
base ligand, where the azido ion formed a bridge (μ1,3-N3) between the two terminal Co(III)
centers and participated in pnicogen bonding interactions [6]. The increased flexibility of less
constrained –CH2–NH– moiety of the reduced Schiff base compared to unreduced one and the
presence of H-atoms onto the reduced nitrogens seemed to be responsible for such exceptional
phenomena. To have comprehensive knowledge in the structural variations of such complexes,
we are interested in synthesizing some more cobalt complexes using di-Schiff ligands and their
reduced analogues.
The phenoxazinone synthase is a multicopper oxidase enzyme [15,16]. It is found
naturally in the bacterium Streptomyces antibioticus and known to catalyze the oxidative
coupling of 2-aminophenol to the phenoxazinone chromophore at the final step for the
biosynthesis of actinomycin D [17]. This is medicinally used for the treatment of different type
of tumors [18]. Therefore, the synthesis and characterization of functional model complexes for
metalloenzymes that imitate this oxidase activity, is of great significance in the field of catalysis
for the oxidation reactions [19]. Literature survey shows that some Co(II) [20] or Co(III)
[20b,21] Schiff base complexes can mimic phenoxazinone synthase activity like complexes of
Cu(II) [22] and Mn(II/III/IV) [23]. However, all the cobalt-Schiff base complexes which are
known to exhibit this catalytic activity are mononuclear or dinuclear in which the cobalt ion is
either in +2 or in +3 oxidation state [20,21]; till date, no mixed valence trinuclear cobalt complex
is reported as functional model for this activity.
In this present report, we have synthesized and structurally characterized three trinucler

-
mixed valence cobalt complexes, [Co3L2(2C6H5CO2 )2(CH3CN)2](ClO4 )2·(CH3CN)3 (1)


[Co3LR (2C6H5CO2 )2(C6H5CO2 )2]·(CH3CN)2
(2)
and
2


[Co3LR (2C6H5CH=CHCO2 )2(C6H5CH=CHCO2 )2]·(CH3)2CO (3) using Schiff base
2
ligand, H2L (where H2L = N,N'-bis(salicylidene)-1,3-propanediamine) and its reduced form
[H2LR] (where H2LR = N,N'-bis(2-hydroxybenzyl)-1,3-propanediamine) and carboxylato
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coligands (benzoato for 1 and 2; cinnamato for 3). Among them, only complex 2 exhibit
phenoxazinone synthase activity. The detailed kinetic study reveals that the turnover number (kcat
= 153.60 h-1) for this oxidase activity is significantly higher than similar reported complexes.
Thus, complex 2 is the first example of mixed valence trinuclear cobalt complex showing rather
high phenoxazinone synthase activity.
2. Experimental
2.1. Starting Materials
Salicylaldehyde, 1,3-propanediamine and sodium borohydride were purchased from
Lancaster and were of reagent grade. They were used without further purification.
Caution! Perchlorate salts of metal complexes with organic ligands are potentially explosive.
Only a small amount of material should be prepared and it should be handled with care.
2.2. Synthesis of the Schiff Base Ligand, N,N′-bis(salicylidene)-1,3-propanediamine (H2L)
The Schiff base ligand was synthesized by a standard method [24]. 5 mmol of 1,3-
propanediamine (0.42 mL) was mixed with 10 mmol of the salicylaldehyde (1.04 mL) in
methanol (30 mL). The resulting solution was refluxed for ca. 2 h and allowed to cool.
2.3. Synthesis of the reduced Schiff base ligand, N,N'-bis(2-hydroxybenzyl)-1,3-propanediamine
(H2LR)
The di-Schiff base ligand (H2L) was synthesized as stated above. Then 30 mL (5 mmol)
of this prepared methanolic ligand solution (H2L) was cooled to 0°C, and solid sodium
borohydride (570 mg, 15 mmol) was added to this methanolic solution with stirring. After
completion of addition, the resulting solution was acidified with concentrated HCl (10 mL) and
then evaporated to dryness on a hot water bath [25]. The reduced Schiff base ligand (H2LR) was
extracted from the solid mass with methanol and this methanol solution (ca. 30 mL) was used for
complex formation.
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
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2.4. Synthesis of the complex [Co3L2(2C6H5CO2 )2(CH3CN)2](ClO4 )2·(CH3CN)3 (1)
The yellow coloured methanolic solution (12 mL) of the di-Schiff base (H2L) ligand (2
mmol) was taken in a beaker. To this methanolic solution, a water solution (2 mL) of
Co(ClO4)2·6H2O (1.098 g, 3 mmol) followed by an aqueous solution (2 mL) of sodium benzoate
(0.576 g, 4 mmol) was added. The mixture was stirred for 1 h and then filtered. The filtrate was
allowed to stand overnight till brown coloured microcrystalline product of 1 appeared at the
bottom of the beaker. The crystalline product was recrystallized from acetonitrile solution. The
crystals were washed with an acetonitrile–water mixture and dried in a desiccator containing
anhydrous CaCl2 and then characterized by elemental analysis, spectroscopic methods, and X-
ray diffraction.
Complex 1. Yield: 0.917 g, 67%. Anal. Calc. for C58H57Co3Cl2N9O16: C 50.34, H 4.15, N 9.11;
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found: C 50.52, H 4.35, N 9.31%; UV/vis: [λmax in nm (εmax in M−1 cm−1)] (MeCN) = 621(223),
362(2908). IR (KBr) in cm−1: ν(C=N) 1633; ν(C=O) 1597; ν(C=O) 1558.
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

2.5. Synthesis of the complexes [Co3LR (2C6H5CO2 )2(C6H5CO2 )2]·(CH3CN)2 (2) and
2


[Co3LR (2C6H5CH=CHCO2 )2(C6H5CH=CHCO2 )2]·(CH3)2CO (3)
2
Complexes 2 and 3 were prepared using the reduced Schiff base ligand with the same
molar ratios of reactants as used for 1, but here only in one case sodium cinnamate (4 mmol,
0.680 g) was used for the synthesis complex 3 instead of using sodium benzoate. The
microcrystalline compound of 3 was recrystallized from acetone solution instead of acetonitrile.
Complex 2. Yield: 0.734 g, 56%. Anal. Calc. for C66H66Co3N6O12: C 55.53, H 4.62, N 7.31;
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found: C 55.72, H 4.83, N 7.37%; UV/vis: [λmax in nm (εmax in M−1 cm−1)] (MeCN) = 625(611),
374(2990), and 421(2698). IR (KBr) in cm−1: ν(N-H) 3211; ν(C=O) 1698; ν(C=O) 1600.
Complex 3. Yield: 0.841 g, 58%. Anal. Calc. for C76H80Co3N4O14: C 62.94, H 5.56, N 3.86;
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found: C 62.82, H 5.42, N 3.76 %; UV/vis: [λmax in nm (εmax in M−1 cm−1)] (MeCN) = 620(637),
367(3715), and 419(3156). IR (KBr) in cm−1: IR (KBr) in cm−1: ν(N-H) 3245; ν(C=O) 1637;
ν(C=O) 1597.
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2.6. Physical Measurements
Elemental analyses (C, H and N) were performed using a Perkin-Elmer 2400 series II
CHN analyzer. IR spectra in KBr pellets (4000−400 cm−1) were recorded using a Perkin- Elmer
RXI FT-IR spectrophotometer. Electronic spectra in acetonitrile (800−200 nm) were recorded in
a Hitachi U-3501 spectrophotometer.
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2.7. ESI-HRMS
Electrospray ionization mass spectrometry (ESI-MS positive) ion mass spectra were
acquired using a Xevo G2-S QTof (Waters) mass spectrometer, equipped with a Z-spray
interface, over a mass range of 100–1200 Da, in a continuum mode. Aqueous sodium formate
was used for Q-oa-Tof calibration. L-Leucine was used as the external mass calibrant lock mass
[M+H]+ = 556.2771 Da. The solution of compound 2 was injected at the flow rate of 5µl/min.
The solution was made in mass-spectrometric grade acetonitrile. The concentrations of complex
2 as well as complexes + substrates in solutions were the same (10-5 M) during acquisitions of
mass spectra.
2.8. Crystallographic data collection and refinement
Well formed single crystals of each compound had been mounted on a Bruker-AXS SMART
APEX II diffractometer equipped with a graphite monochromator and Mo Kα (λ = 0.71073 Å)
radiation. The crystals were located at 60 mm from the CCD, and frames (360) were measured
with a counting time of 10 s. The structures were solved using the Patterson method through the
SHELXS 97 program. Non hydrogen atoms were refined with independent anisotropic
displacement parameters, while difference Fourier synthesis and least-squares refinement
showed the positions of any remaining non-hydrogen atoms. The hydrogen atoms bound to
carbon atoms were included in geometric positions and set thermal parameters equivalent to 1.2
times those of the atom to which they were attached. Hydrogen atoms that bonded to N or O
were placed in a difference Fourier map and refined with distance constraints. Successful
convergence was indicated by the maximum shift/error of 0.001 for the last cycle of the least-
squares refinement. All the acetonitrile solvent molecules outside the coordination sphere are
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refined isotropically using distance constraints with 50% occupancy for structures, 1 and 2. In
addition, for structure 1, the perchlorate oxygen atoms were disordered and refined isotropically
using distance constraints with 50% occupancy of each O-atom. Absorption corrections were
carried out using the SADABS program [26], while all calculations were made via SHELXS 97
[27], SHELXL 97 [28], PLATON 99 [29], ORTEP-32 [30], and WINGX system ver-1.64 [31]
and refined using SHELXL-2014 [31]. Data collection, structure refinement parameters, and
crystallographic data for all three complexes are given in Table S1.
2.9. Oxidation of 2-aminophenol
In order to study the phenoxazinone synthase activity of 1-3, a 10−4 (M) solution of each
complex in acetonitrile was treated with 100 equiv of 2-aminophenol (OAPH) dissolved in
acetonitrile under aerobic conditions at room temperature. Absorbance vs wavelength
(wavelength scan) of these solutions was recorded at a regular time interval of 5 min in the
wavelength range 300−500 nm for all complexes. Amongst, only complex 2 showed this
activity. To determine the dependence of rate on substrate concentration and various kinetic
parameters, a 10−4 M solution of complex 2 was treated with 10, 30, 50, 70, and 100 equiv of
substrate. The reactions were followed spectrophotometrically by monitoring the increase in the
maximum absorbance of the phenoxazinone band at 432 nm as a function of time (time scan).
3. Results and Discussion
3.1. Syntheses of the Complexes
The Schiff-base ligand (H2L) and its reduced analouge (H2LR) were synthesized using the
reported procedures. These ligands (H2L or H2LR) on reaction with cobalt(II) perchlorate
hexahydrate and sodium benzoate or sodium cinnamate in 2:3:4 molar ratios in MeOH-H2O
medium (10:1, v/v) resulted in three new trinuclear mixed valence Co(II/III) complexes,

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[Co3L2(2C6H5CO2 )2(CH3CN)2](ClO4 )2·(CH3CN)3
(1)


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[Co3LR (2C6H5CO2 )2(C6H5CO2 )2]·(CH3CN)2
(2)
and
2
[Co3LR (2C6H5CH=CHCO2 )2(C6H5CH=CHCO2 )2]·(CH3)2CO (3) (Scheme 1).


2
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Scheme 1. Synthesis of complexes 13.
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3.2. IR and UV−Vis Spectra
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Besides elemental analyses, all three complexes were initially characterized by the IR
spectra. A strong and sharp band due to the azomethine ν(C=N) group of the Schiff base appears
at 1633 cm−1 for the complex 1 (Fig. S1). On the other hand, a moderately strong peak at a range
of 3210-3245 cm−1 (due to a N–H stretching vibration) for complexes 2 and 3 (Figs. S2 and S3)
indicates that the imine group of the Schiff base is reduced [32]. In addition, the presence of
carboxylato ligands in all three complexes are confirmed by the appearance of strong and sharp
peaks at 1597, 1698, and 1637 cm−1 along with shoulders at 1558, 1600 and 1597 cm−1 in the
spectra of 1-3, respectively (Figs. S1S3). The splitting of the band is indicative of the presence
of two different types (symmetric and asymmetric) of stretching vibration of carboxylato
coligands in each case.
The electronic spectra of the compounds were recorded in acetonitrile solutions. The complexes
show a broad absorption band in the visible region at 621, 625 and 620 nm in acetonitrile
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solution for 1-3 respectively, attributed to d−d transitions of Co(II) ions. Besides these bands, an
absorption band in the range 362−421 nm was observed in acetonitrile solution assignable to
ligand-to-metal charge transfer transitions for all the complexes (Figs. S4S5).
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3.3. Description of Structures
3.3.1. Complex 1
The
linear
trinuclear
structure
of
1
having
molecular
formula

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[Co3L2(2C6H5CO2 )2(CH3CN)2](ClO4 )2·(CH3CN)3 is shown in Fig. 1 together with the atomic
numbering scheme. Dimensions of the metal coordination sphere of 1 are given in Table S1 in
the Supporting Information. The structure contains a crystallographic inversion center
(symmetry: 2-x, -y, 2-z), which sits on the central cobalt atom [Co(2)]. Thus, the asymmetric unit
consists of two metal centers [Co(III)-terminal and Co(II)-central], one deprotonated di-Schiff
base ligand [(L)2-], one benzoate co-anion and one perchlorate anion. The basal plane of the
terminal Co(III) center is formed by two 2-phenoxido O atoms and two imine N atoms of the
chelated dianionic tetradentate Schiff-base ligand [L2-] and the axial positions are occupied by
one oxygen atom of benzoate anion and one nitrogen from acetonitrile solvent molecule. The
basal Co–O bond distances [1.898(4)1.912(4) Å] are shorter than the basal Co–N bond
distances [1.922(6)1.960(5) Å] (Table S2). The four donor atoms in the basal plane show root
mean square (r.m.s) deviations of 0.009 Å for Co(1) with the metal atom 0.001(1) Å shifted from
the mean plane towards the axially coordinated nitrogen atom. The dihedral angle between two
NCoO planes [0.78(21)°] and the ranges of trans [172.8(2)–176.8(2)°] and cis [83.3(2)–
95.6(2)°] angles (Table S2) around Co(1) suggested a distort octahedral geometry of the terminal
cobalt center.
The central cobalt atom is bonded to four oxygen atoms from the two ligands [(L)2-], which form
the basal plane of the Co(II), while the trans axial positions are occupied by the two oxygen atom
from benzoate anions.
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Fig. 1. The structure of 1 with ellipsoids at 30% probability. H-atoms are removed for clarity. The solvent
acetonitrile molecules are also omitted for clarity. Symmetry element a = 2-x, -y, 2-z.
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All the central CoO bond distances [2.030(4)‒2.089(4) Å] are considerably greater than that of
the terminal cobalt centers [1.893(4)‒1.912(4) Å] as the central cobalt atom is in +2 oxidation
state whereas the terminal atoms are in +3 oxidation states. The range of cis angles
[75.0(2)85.8(2)°] around this central Co center is an indicative of a distortion from its ideal
octahedral geometry. The Co(1)Co(2) distance is 3.030(1) Å.
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3.3.2. Complexes 2 and 3
Partially labeled diagrams of trinuclear structures of 2 and 3 are depicted in Figs. 2 and 3
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respectively, and selected bond parameters are summarized in Table S2. Both structures (2 and
3) contain three cobalt atoms [CoIIICoIICoIII] in a perfect linear arrangement, owing to the
presence of a crystallographic inversion center (symmetry: 2-x, -y, 2-z and 1-x, 1-y, 1-z for 2 and
3, respectively) at the central metal atom Co(2). In each of the structures (2 and 3) every cobalt
atom has a six-coordinated octahedral geometry like 1. The basal plane of the two terminal
Co(III) centers of each structure is formed by two μ2- phenoxido O atoms and two imine N atoms
of the dianionic tetradentate reduced Schiff-base ligand [(L2−)R] but unlike 1, the axial positions
are occupied by two oxygen atoms from one bridging and another terminal carboxylato ions
[benzoato and cinnamato for 2 and 3, respectively]. The r.m.s deviations of the four basal atoms
for Co(1) are 0.033 and 0.018 Å and these are 0.001(1) and 0.021(1) Å shifted from the mean
plane for 2 and 3, respectively. The basal Co–O bond distances of the terminal cobalt are shorter
than the basal Co–N bond distances (Table S2) like 1. It is noticeable that all the terminal Co–
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O/N bond distances are longer for 2 and 3 compared to those of 1. The ranges of trans [173.3(3)–
176.6(3)° for 2 and 172.9(2)–176.0(2)° for 3 ] and cis [81.0(2)–95.8(3)° for 2 and 81.3(2)–
95.5(2)° for 3] angles (Table S2) indicate a slight deviation from the ideal octahedral geometry
of the terminal cobalt centers like 1. The dihedral angle between two NCoO planes for 2 and 3
(2.79(23)º and 2.16(19)º, respectively) are considerably greater compared to that of complex 1.
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Fig. 2. The structure of 2 with ellipsoids at 30% probability. H-atoms are removed for clarity. The solvent
acetonitrile molecules are also omitted for clarity. Symmetry element a = 2-x, -y, 2-z.
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The central Co(II) atom in each case (2 and 3) is bonded to four phenoxido oxygen atoms of
deprotonated tetradentate reduced Schiff base ligands and two oxygen atoms from the two
bridging carboxylate ions like 1. Here, all Co(II)–O bond distances are almost same (Table S2)
range like 1. Again, here also the central Co–O bond distances are significantly greater than that
of the terminal cobalt centers just like 1 (Table S2). The central cobalt atom [Co(2)] presents a
six-coordinated slightly distorted octahedral geometry and this is confirmed by the range of cis
angles [75.6(2)–84.5(2)° for 2 and 74.6(1)84.7(2)º for 3]. The Co(2)⋯Co(1) distances are
3.055(1) and 3.045(3) Å for 2 and 3, respectively.
10

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280
Fig. 3. The structure of 3 with ellipsoids at 30% probability. H-atoms and the solvent acetone molecules are also
omitted for clarity. Symmetry element a = 1-x, 1-y, 1-z.
281
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283
284
285
286
Both structures (2 and 3) contain an intramolcular H-bonding between the O atom [O43]
of the terminal carboxylao ligand and the hydrogen atoms (H18 and H22) of the reduced imine
moiety [N(18)–H(18)⋯O(43) and N(22)–H(22)⋯O(43) with dimensions H18⋯O43 2.927(77)
and 1.818(23) Å, H22⋯O43 2.014(31) and 3.111(45) Å, N18–H18⋯O43 116.2(36)° and
173.6(21)°, N22–H22⋯O43 143.8(27)° and 113.2(18)° for 2 and 3, respectively (Table S3)]
[Fig. 4].
287
288
289
290
Fig. 4. The hydrogen bonding interaction (O43…H22/H18N22/N18) in the structures of 2 and 3. Symmetry
element a = 2-x, -y, 2-z and a = 1-x, 1-y, 1-z for 2 and 3, respectively.
11

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302
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308
309
310
311
312
313
314
A CSD search for trinuclear CoIII–CoII–CoIII complexes containing a deprotonated
tetradentate Schiff base ligand reveals that twelve such structures have been reported so far [8-
13]. Among them, eight complexes are linear in which the terminal Co(III) and the central Co(II)
are linked through acetato (six structures) [8,9,13] or sulfito (one structure) [11] or chlorido (one
structure) [10] bridges in addition to double phenoxido bridges. The remaining four are azido
bridged angular trinuclear species [6,12]. In case of the structures with acetato bridges (six
examples), the remaining axial position of the octahedral terminal metal center is coordinated to
another coligand. Four of these structures are derived from 1,2-ethanediammine based Schiff
bases where terminal coligand is acetato in three [8,9a] and thiocyanato in the other one [9b].
The terminal coligand is pyridine [13b] or thiocyanato [13a] in the two structures derived from
1,3-propanediammine based Schiff base ligand. It has been argued that crystal field stabilization
energy of the carboxylate ion is not strong enough to stabilize the terminal Co ion in +3
oxidation state in case of Schiff base derived from 1,3-propanediammine due its longer bond
distances compared to those derived from 1,2-ethanediammine. Hence there is no structure in
which carboxylato ion is coordinated to the terminal Co(III) ion and the Schiff base is derived
from 1,3-propanediammine. However, stronger CFSE of pyridine or thiocyanate ligand is
assumed to be crucial in the stabilization of the two structures with 1,3-propanediammine. In the
present study, structures 2 and 3 are the first examples in which terminal coordination mode of
carboxylato ion is observed in Co(III) with 1,3-propanediammne based Schiff base ligand. The
strong hydrogen bonding interaction (Fig. 4) between non coordinated carboxylato O-atom and
H-atom onto the reduced nitrogen seems to be responsible for stabilization of these two
structures (2 and 3).
3.4. Phenoxazinone synthase activity studies with kinetics
315
316
317
318
319
320
321
The phenoxazinone synthase activity of the complexes for the oxidation of 2-
aminophenol (OAPH) to 2-aminophenoxazine-3-one in aerobic condition according to the
reaction shown in Scheme 2 was evaluated spectrophotometrically in acetonitrile solution at
room temperature. The oxidation product, phenoxazinone is highly stable and has characteristic
absorption band maxima at around 432 nm in a pure acetonitrile solvent. Before going into the
details of kinetics studies, we examined the catalytic activity of all three complexes for the
oxidation of 2-aminophenol in acetonitrile solvent.
12

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323
Scheme 2. Catalytic oxidation of OAPH to 2-aminophenoxazine-3-one in an acetonitrile solvent.
324
325
326
327
328
329
330
331
332
For this purpose, (1 x 10−4) M acetonitrile solution of the complexes (1-3) were reacted with 100
equivalents of substrate OAPH at room temperature under aerobic condition. After addition of
the substrate (OAPH) into the complexes, the progress of the reaction was followed by recording
the UV−vis spectrum of the said mixture at 5 min times interval. The gradual increase of an
absorption band around 432 nm was observed in UV−vis spectroscopy only for complex 2, but
not for complexes 1 and 3. This indicates that only complex 2 catalyzes the aerobic oxidation of
OAPH to 2-aminophenoxazine-3-one. Hence, the detailed kinetic studies have been performed
only on complex 2. The variation of the spectral behavior after the addition of OAPH in the
complex 2 for the period of 1.5 h is shown in Fig. 5.
333
334
335
Fig. 5. Increase of the phenoxazinone band at around 432 nm after the addition of 100 equiv. of OAPH to an
acetonitrile solution of complex 2. The spectra were recorded at 5 min intervals.
336
337
The kinetics of the oxidation of OAPH to 2-aminophenoxazine-3-one by the complex 2
was determined by the method of initial rates by monitoring the development of the
13

338
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340
341
342
343
344
345
346
347
348
349
350
351
352
phenoxazinone band at 432 nm as a function of time. To maintain pseudo-first-order kinetics, the
substrate (OAPH) concentration was always set at least 10 times higher than that of the complex
2. The rate constant for a particular complex–substrate mixture was determined from the log
[Aα/(Aα − At)] vs. time plot. The oxidation rates and various kinetic parameters of the substrate
concentration were determined by using (1× 10−4) M solutions of 2 with different concentrations
(From 1× 10−2 M to 0.1 × 10−2 M) of OAPH under aerobic conditions. In this kinetic study, at
lower concentration of OAPH the first-order kinetic was observed, whereas saturation kinetics
resulted in higher concentration. This observation pointing out that the oxidation of OAPH
passes through a relatively stable complex–substrate intermediate adduct, most probably formed
by the coordination of substrate to catalyst by the replacement of a terminal benzoate ion (Figs.
S6 and S7). The observed rates versus the concentration of substrate data were then analyzed on
the basis of the Michaelis–Menten approach of enzymatic kinetics to obtain the Lineweaver–
Burk (double reciprocal) plot and values of kinetic parameters e.g. Vmax, KM, and kcat. Both the
curves of the observed rate vs. [substrate] and the Lineweaver–Burk plot for complex 2 are
shown in Fig. 6.
353
354
355
356
Fig. 6. Plot of the rate vs substrate concentration for complex 2. Inset shows the corresponding Lineweaver–Burk
plot.
357
358
The kinetic parameter for the complex 2 is listed in Table 1. The kcat value can be calculated by
dividing the Vmax value by the concentration of the complex (Table 1). The kcat (in h−1) value is
14

359
360
153.60 for 2. This value lies in the higher range of the kcat values of the cobalt Schiff Base
complexes behaving as functional models of phenoxazinone synthase [see Table 2].
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
To get a better understanding of the complexsubstrate intermediate of phenoxazinone synthase
activity during the oxidation reaction, we have recorded ESI-MS spectra for complex 2, and a
1:10 mixture of the complex to and OAPH within 10 min of mixing in acetonitrile solvent (Figs.
S6 and S7, Supporting Information). Complex 2 shows the base peak at m/z = 431.9913 (calcd
432.0294) which can be assigned to the dinuclear species [Co2(LR-2H)(MeOH)]+. In addition,
peaks due to the other dinuclear [Co2(LR-2H)]+, [Co2(LR-H)(C6H5CO2)]+ and trinuclear [Co3(LR-
H)2(C6H5CO2)]+, [Co3(LR)2(C6H5CO2)3]+ species at m/z = 400.00 (calcd 400.0032), 522.04 (calcd
522.0400) and 864.1172 (calcd 864.1179), 1108.19 (calcd 1108.1914), respectively are also
observed for complex 2. After the addition of OAPH to the solutions of complex 2, drastic
changes are observed. In addition to the protonated 2-aminophenoxazine-3-one [C12H11N2O2]+
base peak at 215.08 (calcd 215.0821), a new peak appears at m/z = 509.05 (calcd 509.0560)
which corresponds to the coordination of OAPH to the dinuclear species [Co2(LR-H)(OAPH)]+.
These results reveal the formation of the catalystsubstrate as intermediates which take part in
substrate activation during the oxidation of OAPH to 2-aminophenoxazine-3-one.
Thus, this mixed valence Co(III)–Co(II)Co(III) compound can be considered as a functional
model of phenoxazinone synthase activity. It is important to mention that the kcat value obtained
for complex (2) is considerably higher than most of those reported for mononuclear or dinuclear
cobalt compounds in which the cobalt ion is either in +2 or in +3 oxidation state. Therefore,
complex 2 is not only the first example of mixed valence trinuclear cobalt Schiff base complex
exhibiting this phenoxazinone synthase activity but its kcat value is one of the highest among the
reported Co(II) or Co(III) compounds (Table 2).
Detailed investigations with copper or cobalt based model complexes of phenoxazinone synthase
activity have shown that various factors may affect their catalytic activity, such as the
metal−metal distance, ligand flexibility, exogenous bridging ligand, and coordination geometry
around the metal center [19-22]. Among the Co(III)/Co(II) mixed valence compounds reported
herein, phenoxazinone synthase activity is observed only for 2, whereas 1 and 3 are inactive
towards OAPH oxidation. In case of complex 3, perhaps the extra π electron conjugation in
exogenous cinnamato ligand makes it weaker leaving group in comparison to benzoato in 2 (pKa
15

389
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391
392
393
394
395
396
397
398
399
400
401
= 4.46 and 4.20 at 25ºC for cinnamic acid and benzoic acid, respectively) and consequently
binding of the substrate (OAPH) to the metal center is restricted for 3. However, in the same
light of explanation complex 1 should show this activity but it is inactive and this is may be due
to the relatively less conformational flexibility on the unreduced Schiff base moiety (-C=N-) and
absence of any terminal benzoate ion. In addition, although there is a terminal acetonitrile
solvent molecule in 1 but may be due to strong coordination of acetonitrile molcule to the
terminal Co(III) at a distance 1.899(2)Å prevents the substitution of acetonitrile molecule by the
substrate OAPH. Therefore, the approach and coordination of the substrate to the metal center
are more favorable in those cases where the flexible ligand is used for complexation and
existence of easily removable exogenous coligand and these criteria are full filled in only
complex 2 (supported from ESI-MS spectra, Figs. S6 and S7) and hence it is highly reactive to
this catalytic activity.
402
403
404
4. Conclusion
In this present work, we synthesized three mixed valence [CoIIICoIICoIII] trinuclear
complexes using unreduced and reduced forms of N,N′-bis(salicylidene)-1,3-propanediamine
and carboxylato ion. The unreduced ligand produces the usual linear trinuclear complex (1) in
which two benzoate ions bridge the central and terminal cobalt atoms as reported before.
However, in complexes (2 and 3) with the corresponding reduced Schiff base ligand two types of
coordination mode of carboxylato ion are observed: one is bridging and other one is terminal. In
these two structures the terminal coordination mode of carboxylato ion to the Co(III) centres
seems to be stabilized through H-bonding interaction between carboxylato O-atom and H-atom
onto the reduced nitrogen. Examination of phenoxazinone synthase activity for all three
complexes reveals that only complex 2 catalyzes the oxidation of OAPH to the corresponding
phenoxazinone derivative and it is the first example of mixed valence trinuclear cobalt complex
that shows this catalytic activity with turnover number, kcat = 153.60 h−1 which is significantly
high compared to the reported Co(II) or Co(III) complexes.
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
Acknowledgements
16

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430
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432
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435
Crystallography was performed at the DST-FIST, India, funded Single Crystal Diffractometer
Facility at the Department of Chemistry, University of Calcutta. The authors also thank the
Department of Science and Technology (DST), New Delhi, India, for financial support
(SR/S1/IC/0034/2012).
Appendix A. Supplementary data
CCDC 15291861529188 contains the supplementary crystallographic data for (1–3). These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data (IR spectra, UV–Vis
spectra and mass spectrum (Figs. S1–S7) as well as tables for bond parameters (Tables S1 and
S2) associated with this article can be found, in the online version, at http://dx.doi.org/10.
1016/j.poly.2016.07.035.
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519
520
521
Table 1
Kinetic Parameters for the Oxidation of OAPH Catalyzed by Complex 2.
Complex
Vmax (M min−1)
12.8 X 10-5
Std. error
KM (M)
Std. error
Kcat (h−1)
153.60
2.47 X 10-5
9.66 X 10-3
1.98 X 10-4
2
522
523
524
525
526
Table 2
kcat Values for the oxidation of OAPH to 2-aminophenoxazine-3-one catalyzed by complex 2 and other reported
cobalt complexesa.
Complexes
[CoIII (L1)3]
[CoIII2(L2)2(μ-L3)2Cl2] Cl2·2H2O
kcat (h−1) in CH3CN
Not done
kcat (h−1) in CH3OH
Not calculated kcat
13.75
References
21c
Not done
21b
[CoIII(L4)(N3)3]
Not done
20.37
21a
19

[CoIII(L5)(N3)3]
Not done
Not done
Not done
Not done
Not done
Not done
33.26
30.09
23.04
3.36
21a
20b
20b
20b
20b
20b
[CoIII2(L6)2(μ-O2)](ClO4)4·2CH3CN
[CoIII2(L7)2(μ-O2)](ClO4)4
[CoII(L8)(CH3CN)](ClO4)2
[CoII(L9)(H2O)](ClO4)2
[CoII(L10)(H2O)](ClO4)2
6.37
8.28
[CoII(L11)Cl(H2O)]Cl.H2O
Not done
Not done
Not done
247.20
257.40
Not done
77.52
13.68
7.38
4.10
Not done
Not done
499.20
Not done
Not done
Not done
20a
20a
20a
21d
21d
21e
21f
21f
[CoII(L11)(NCS)2]
[CoII(L12)Cl2]
[CoIII(L13)(bzan)(N3)]
[CoIII(L13)(bzan)(NCS)]
[CoIII(L14)(N3)(L15)]
[CoIII(L16)(acna)(N3)]
[CoIII(L17)(bzan)(N3)]
77.12
153.60
[Co3LR (2PhCO2 )2(PhCO2 )2]


Present study
2
527
528
529
530
531
532
533
534
535
536
537
538
539
540
a Where L1 = 4,4-dimethyl-2,2’-bipyridine; L2 = 2-aminomethylpyridine; L3 = 2-iminomethylpyridine anion; L4 =
Bis-(2-pyridylmethyl)amine; L5
yl)methyleneamino)propyl)-N3-((pyridin-2-yl)methylene)propane-1,3- diamine; L7
yl)ethylideneamino)propyl)-N3-(1-(pyridin-2-yl)ethylidene)propane-1,3-diamine; L8
=
(2-pyridylmethyl)(2-pyridylethyl) amine; L6
=
N1-(3-((pyridin-2-
N1-(3-(1-(pyridin-2-
N1-(3-(methoxy(6-
=
=
methylpyridin-2-yl)methylamino)propyl)-N3-((6-methylpyridin-2-yl)methylene)propane-1,3-diamine; L9 = N1-(3-
((6-methylpyridin-2-yl)methyleneamino)propyl)-N3-((6-methylpyridin-2-yl)methylene)propane-1,3-diamine; L10
N1-(3-((6-methylpyridin-2-yl)methyleneamino)propyl)-N1-methyl-N3-((6-methylpyridin-2-yl)methylene)propane-
=
1,3-diamine; L11 = N,N’-bis(pyridin-2-ylmethylene)-2,2- dimethylpropane-1,3-diamine; L12 = N,N-bis(6-
methylpyridin-
2-ylmethylene)-2,2-dimethylpropane-1,3-diamine;
Hbzan 1-benzoylacetone;
HL13
=
2-(3-
1-((2-
(dimethylamino)propyliminomethyl)-6-ethoxyphenol;
=
HL14
=
(diethylamino)ethylimino)methyl)naphthalene-2-ol; HL15 = acetylacetone; HL16 = 2((2(2-hydroxyethylamino)
ethylimino)methyl)-6-ethoxyphenol,
Hacna
=
2-acetyl-1-naphthol;
HL17
=
1
((2(diethylamino)ethylimino)methyl)naphthalen-2-ol.
541
542
Research highlights:
 Mixed valence Co(II/III) complexes derived from reduced salen-type Schiff base ligand.
 All three complexes with carboxylato coligand are linear structures [CoIIICoIICoIII].
543
544
545
 Complex 2 is the first example of mixed valence trinuclear cobalt complex with a high
phenoxazinone synthase activity (kcat = 153.60 h−1).
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547
548
549
20

550
551
Graphical Abstract (Pictogram)
552 Mixed valence trinuclear cobalt (II/III) complexes: Synthesis, structural
553 characterization and phenoxazinone synthase activity
554
555
Alokesh Hazari,a,b Avijit Dasa, Prithwish Mahapatraa, and Ashutosh Ghosh a*
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557
aDepartment of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata-700 009,
India. E-mail: ghosh_59@yahoo.com
bDepartment of Chemistry, Government General Degree College Singur, Jalaghata, Hooghly-712409
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559
560
561
562
563
21

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565
Graphical Abstract (Synopsis)
566 Mixed valence trinuclear cobalt (II/III) complexes: Synthesis, structural
567 characterization and phenoxazinone synthase activity
568
569
Alokesh Hazari,a,b Avijit Dasa, Prithwish Mahapatraa, and Ashutosh Ghosh a*
570
571
aDepartment of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata-700 009,
India. E-mail: ghosh_59@yahoo.com
bDepartment of Chemistry, Government General Degree College Singur, Jalaghata, Hooghly-712409
572
573
574
575
576
577
578
Two of the three complexes are the first examples of the terminal coordination of carboxylato
ion to Co(III) of a mixed valence CoIII-CoII-CoIII complex with 1,3-propanediammine based
Schiff base ligand and one shows phenoxazinone synthase activity with considerably high
turnover number.
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580
22