Synthetic stratagem and structures of two heteroleptic cobalt(III) complexes acting as biomimetic catalysts: Role of co-ligands in catalytic activities

Article abstract of DOI:10.1016/j.poly.2019.05.062

Two heteroleptic cobalt(III) complexes, [CoL¹(acac)(N₃)] (1) and [CoL¹(acac)(NCS)] (2) {where HL¹ = 1-((2-(dimethylamino)ethylimino)methyl)naphthalen-2-ol and Hacac = acetylacetone} have been synthesized and str

Full text of DOI:10.1016/j.poly.2019.05.062

Accepted Manuscript
Synthetic stratagem and structures of two heteroleptic cobalt(III) complexes
acting as biomimetic catalysts: Role of co-ligands in catalytic activities
Kousik Ghosh, Shouvik Chattopadhyay
PII:
S0277-5387(19)30400-0
https://doi.org/10.1016/j.poly.2019.05.062
POLY 13987
DOI:
Reference:
Polyhedron
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28 May 2019
30 May 2019
Please cite this article as: K. Ghosh, S. Chattopadhyay, Synthetic stratagem and structures of two heteroleptic
cobalt(III) complexes acting as biomimetic catalysts: Role of co-ligands in catalytic activities, Polyhedron (2019),
doi: https://doi.org/10.1016/j.poly.2019.05.062
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Synthetic stratagem and structures of two heteroleptic cobalt(III)
complexes acting as biomimetic catalysts: Role of co-ligands in
catalytic activities
Kousik Ghosh, Shouvik Chattopadhyay*
Department of Chemistry, Inorganic Section, Jadavpur University, Kolkata 700 032, India
e-mail: shouvik.chem@gmail.com Tel. +91-33- 2414-2941
Abstract
Two heteroleptic cobalt(III) complexes, [CoL1(acac)(N3)] (1) and [CoL1(acac)(NCS)] (2)
{where HL1=1-((2-(dimethylamino)ethylimino)methyl)naphthalen-2-ol, Hacac = acetylacetone,}
have been synthesized and structurally characterized by several analytical techniques including
single crystal X-ray diffraction analysis. Extended supra-molecular assemblies were generated in
them through weak noncovalent interactions. Synthesized complexes have also been found to
mimic the role of quite a lot of metalloenzymes like catechol oxidase, phenoxazinone synthase
and phosphatase efficiently by catalytic areal oxidation of respective substrates. Detailed kinetic
studies of catalytic reactions are performed spectrophotometrically, which confirm that catalytic
reactions follow Michaelis-Menten enzymatic reaction kinetics. Role of co-ligands in catalytic
activity has also been assessed.
Keywords: Cobalt(III); Schiff base; Crystal structure; Catechol oxidase, phenoxazinone synthase
and phosphatase mimicking activity.
1. Introduction

The efficiency and selectivity of various metalloenzymes in processing molecular
dioxygen have lead synthetic inorganic chemists to prepare metal complexes and investigate
their ability to mimic these enzymes [1-3]. A lot of information regarding the role of metal ions
in oxidative and hydrolytic metalloenzymes has been gained through several relative studies on
metalloenzymes and synthetic model metal complexes [4-6]. A number of investigations with
model complexes have been performed to mimic the structural and functional properties of these
metalloenzymes, which confirms that the presence of at least one labile site in the metal complex
is essential for binding of the substrate to initiate the catalytic process [7-9]. In one of our
previous work, it was shown that mononuclear cobalt(III) complexes of the type
[Co(ABC)(DE)X] {ABC is a tridentate ligand, DE is a bidentate ligand and X is a pseudo-
halide} are showing good catalytic activity due to the presence of easily replaceable pseudo-
halide, thereby producing catalytically active intermediate species, [Co(ABC)(DE)(substrate)]
[10]. It was also reported that octahedral bis-ligand cobalt(III) complexes of tridentate N2O
donor Schiff bases were not showing any catalytic activity, as expected [9, 11].
It is to be mentioned here that although mimicking activities of various cobalt(III)
complexes have been checked in our previous works [9-11], the change in mimicking activity on
changing only the monodentate co-ligand has not been checked. Keeping this fact in mind, we
have synthesized two similar complexes differing only in the nature of monodentate pseudo-
halides, e.g. azide and thiocyanate. Catalytic activities have been checked and correlated with the
ability of monodentate co-ligands to dissociate, thereby initiating catalysis.
In the present work, acetylacetone (Hacac) has been used as a blocking bidentate ligand
to form octahedral cobalt(III) complexes of the type, [CoL1(acac)X] with a tridentate N2O donor
Schiff base (1-((2-(dimethylamino)ethylimino)methyl)naphthalen-2-ol) and each is having one

easily replaceable pseudo-halide (X = azide for 1 and thiocyanate for 2). Catechol oxidase,
phenoxazinone synthase and phosphatase like activities of synthesized complexes have been
inspected using 3,5-di-tert-butylcatechol (3,5-DTBC) o-aminophenol (OAPH) and 4-
nitrophenylphosphate (4-NPP), as model substrates, respectively, to evaluate their catalytic
efficiency and role of the monodentate co-ligands in these catalytic mimicking activities. The
higher activity of complex 2 towards these bio-relevant catalyses may be correlated with the ease
of releasing co-ligand, thiocyanate (compared to azide) forming stable catalyst-substare adduct.
2. Material and methods
2.1. Starting materials and solvents
Starting materials and solvents used in this work were purchased from Sigma-Aldrich,
India (now Merck, India) and were of reagent grade. They were used as received, without any
further purification. The entire syntheses and manipulations were carried out under aerobic
conditions.
Caution!!! Even though no troubles were experienced in this work, metal complexes
containing azide salts and organic ligands are potentially explosive. Only a small amount of
material should be prepared, and it should be handled with care.
2.2.
Preparation
of
[CoL1(acac)(N3)]
(1)
[Where,
HL1
=
1-((2-
(dimethylamino)ethylimino)methyl)naphthalen-2-ol]
A methanol solution (20 ml) of N,N-dimethyl-1,2-diaminoethane (0.11 mL, ~1 mmol)
and 2-hydroxy-1-naphthaldehyde (172 mg, ~1 mmol) was refluxed for ca. 1 h to prepare
tridentate Schiff base ligand, HL1. A methanol solution (10 ml) of cobalt(II) acetate tetrahydrate

(250 mg, ~1 mmol) was added into the methanol solution of the Schiff base ligand, HL1, under
starring condition to get a dark brown solution. A methanol solution of acetylacetone (0.10 mL,
~1 mmol) was added to the resulting dark brown coloured solution, followed by the addition of a
methanol–water (10:1) solution of sodium azide (65 mg, ~1 mmol) with constant stirring. After
leaving the resulting solution in aerial condition to evaporate about half of the solvent,
diffraction quality dark brown coloured blocked shaped single crystals of the complex were
formed. The crystals were isolated, washed with mother liquor and dried in a vacuum desiccator
using anhydrous calcium chloride.
Yield: 330 mg (74.6%); based on cobalt(III). Anal. Calc. for C20H25CoN5O3 (FW =
442.38): C, 54.30; H, 5.70; N, 15.83%. Found: C, 54.2; H, 5.6; N, 15.9%. FT-IR (KBr, cm-1):
3028-2933 (υC-H), 2016 (υN3), 1617 (υC=N), 1520 (υC=O), 1440 (υC=C), 1278 (υC-O). UV-Vis, max
(nm), [εmax (dm3 mol-1 cm-1)] (CH3CN), 490 (9.22 × 102), 415 (5.31 × 103), 316 (2.73 × 104), 270
(7.24 × 104).
2.3. Preparation of [CoL1(acac)(NCS)] (2)
Complex 2 was prepared in a similar method as that followed for the preparation of
complex 1, except that sodium thiocyanate (81 mg, ~1 mmol) was used instead of sodium azide.
X-ray quality dark brown single crystals started to grow from methanol solution after several
days.
Yield: 340 mg (74.2%); based on cobalt(III). Anal. Calc. for C21H25CoN3O3S (FW =
458.43): C, 55.02; H, 5.50; N, 9.17%. Found: C, 54.8; H, 5.3; N, 9.3%. FT-IR (KBr, cm-1): 2995-
2874 (υC-H), 2124 (υN3), 1627 (υC=N), 1514 (υC=O), 1436 (υC=C), 1254 (υC-O). UV-Vis, max (nm),
[εmax (dm3 mol-1 cm-1)] 497 (2.29 × 102), 415 (1.21 × 104), 396 (1.19 × 104), 275 (6.89 × 104).

2.4. Details of instrumentation
Single crystal x-ray diffraction was performed using a ‘Bruker SMART APEX II’
diffractometer equipped with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). More
instrumentation details have been given in “Supplementary Information” part.
2.5. Hirshfeld surface analysis
Crystal Explorer [12] was used to calculate Hirshfeld surfaces [13,14] and associated 2D
fingerprint plots [15-17] of both complexes. Further details could be found in supplementary
information part.
2.6. Catalytic activity and kinetic study
Catechol oxidase, phenoxazinone synthase and phosphatase mimicking activity of both
complexes are monitored spectrophotometrically. The additional experimental details of catalytic
activity and kinetic study are given in supplementary information part.
3. Results and discussion
3.1 Synthesis of the complexes
The tridentate Schiff base ligand, HL1, was prepared by facile condensation of N,N-
dimethyl-1,2-diaminoethane with 2-hydroxy-1-naphthaldehyde in methanol following the
literature method [18-20]. The ligand was not isolated and the yellow coloured methanol solution
of the ligand was then reacted with a methanol solution of cobalt(II) acetate tetrahydrate.
Addition of acetylacetone into the resulting solution followed by the addition of aqueous
methanol solution of sodium azide produced a mononuclear Schiff base complex,
[CoL1(acac)(N3)] (1). On the contrary, on adding sodium thiocyanate in lieu of sodium azide

produces [CoL1(acac)(NCS)] (2). Cobalt(II) was converted into cobalt(III) by aerial oxidation in
presence of (strong field) Schiff base ligand, as observed in many previous cases [9-11]. Use of
anaerobic condition prevents the formation of this complex. The formation of the complex is
shown in Scheme 1.
Scheme 1: Synthetic route to complexes 1 and 2.
3.2 Description of solid state structures
3.2.1. Crystal structure of [CoL1(acac)(N3)] (1) and [CoL1(acac)(NCS)] (2)
Single crystal x-ray structure determination reveals that both complexes consist of
–
discrete mononuclear units having general formula, [CoL1(acac)(X)], where X = N3 (for 1) and
NCS– (for 2). Complex 1 crystallizes in monoclinic space group P21/c, whereas complex 2
crystallizes in monoclinic space group Cc. Perspective views of both complexes with selective
atom-numbering scheme are illustrated in Figure 1. Details of the crystallographic data and
refinement details of both complexes are given in Table 1. Selected bond lengths and bond
angles of both complexes are gathered in Tables 2 and 3 respectively. In each complex,

cobalt(III) centre, Co(1), is coordinated by one imine nitrogen atom, N(1), one amine nitrogen
atom, N(2), and one phenoxo oxygen atom, O(1), from one deprotonated Schiff base ligand,
(L1)2-, and two oxygen atoms, O(2) and O(3), of a different ligand, acac-. Nitrogen atom, N(3), of
a terminal azide (in 1) or thiocyanate (in 2) occupied the sixth coordination site of cobalt(III)
centre to complete its distorted octahedral geometry. Bond angles deviate from the ideal values
of 90° (for the cis angles) and 180° (for the trans angles), clearly indicate the distortion from
perfect octahedral geometry. The saturated five membered chelate ring, [Co(1)-N(1)-C(12)-
C(13)-N(2)], presents in half chair conformation, with puckering parameters [21,22] Q =
0.414(5) Å, ϕ = 287.7(5)° (for 1) and Q = 0.447(3) Å, ϕ = 90.0(3)° (for 2). The Co(III)-Nimine
distances are shorter than the Co(III)-Namine ones, due to different hybridization of nitrogen
atoms. This is a common phenomenon observed in many other cobalt(III) Schiff base complexes
[9-11]. The terminal azide in 1 and thiocyanate in 2 are quasi-linear with the N-N-N and N-C-S
angles being 176.2(4)° and 178.4(3)° respectively, as observed in similar complexes [9-11].
Figure 1: Molecular structure of both complexes with selective atom numbering scheme.
Hydrogen atoms are omitted for clarity.
3.2.2. Significant supramolecular interactions

Significant supramolecular interactions lead to the formation of extended supramolecular
architectures in solid state of both complexes. In complex 1, nitrogen atom, N(5) of terminal
azide, is involved in intra-molecular N···π interaction with six membered chelate ring, Cg2
[Co(1)-N(1)-C(11)-C(5)-C(6)-O(1)] (Figure 2).
Figure 2: Intra-molecular N···π interaction present in complex 1. Hydrogen atoms are omitted
for clarity.
Similarly in complex 2, hydrogen atom, H(14B), attached with methyl carbon atom,
C(14), is involved in intra-molecular C-H··· π interaction with six membered chelate ring, Cg3
[Co(1)-O(2)-C(17)-C(18)-C(19)-O(3)]. On the other hand, hydrogen atom, H(11), attached to
carbon atom, C(11), is involved in inter-molecular C-H··· π interaction with the symmetry
related (x,1/2-y,-1/2+z) chelate ring, Cg3 [Co(1)-O(2)-C(17)-C(18)-C(19)-O(3)] to form a 1D
array. Both the intra-molecular and inter-molecular C-H··· π interactions are depicted in Figure
3. Geometric features of all of these noncovalent interactions are given in Table 4.

Figure 3: 1D array generated through C-H··· π in solid state of complex 2. Only relevant
hydrogen atoms are shown for clarity.
3.3. Hirshfeld surface analysis
Solid state crystal structure of any complex can be determined by an amalgamation of a
number of important intermolecular and intramolecular interactions, and hence all these
interactions should be taken into account. Hirshfeld surface analysis helps us to visualize and
investigate these important supramolecular interactions. Visualization and investigation of these
major interactions using Hirshfeld surface based technique symbolize a vital progress in enabling
supramolecular chemists and crystal engineers to gain insight into crystal packing. Hirshfeld
surfaces of both complexes mapped over dnorm, shape index and curvedness (Figure 4). The
surfaces are shown as transparent so that molecular moieties around which Hirshfeld surfaces are
calculated could be easily picturize. The dominant interactions of both complexes are H∙∙∙H,
C∙∙∙H/H∙∙∙C and N∙∙∙H/H∙∙∙N (for 1) or O∙∙∙H/ H∙∙∙O (for 2). Red spots on the dnorm surface
(Figure 4) indicate that these interactions are dominant. Furthermore, 2D fingerprint plots
(Figure 5) exemplify various inter-molecular interaction patterns associated with both complexes
and their relative contributions are given in percentage scale. In 2D fingerprint plots inter-

molecular interactions become visible as distinct spikes. Complementary regions are visible in
the two dimensional fingerprint plots where one molecule act as donor (de>di) and the other as an
acceptor (de<di). The fingerprint plots can also be decomposed to highlight preferred atoms pair
close contacts. This decomposition enables separation of contributions from different interaction
types, which overlap in the full fingerprint [23].
Figure 4: Hirshfeld surfaces mapped over dnorm (left-side), shape index (middle) and curvedness
(right-side).

Figure 5: Fingerprint plots of 1 (left-side) and 2 (right-side): full and resolved into H∙∙∙H,
C∙∙∙H/H∙∙∙C, N∙∙∙H/H∙∙∙N or O∙∙∙H/H∙∙∙O contacts showing the percentages of contacts contributed
to the total Hirshfeld surface area of both complexes. Surfaces in the right hand columns
highlight the relevant surface patches associated with the specific contacts in the total Hirshfeld
surface area of both complexes.
3.4. IR and electronic spectroscopy

The IR and electronic spectra of both complexes are in sound agreement with the findings
obtained from x-ray crystallography. In IR spectra of both complexes, characteristic absorption
bands corresponding to azomethine (>C=N-) groups are observed at 1617 (for 1) and 1627 cm-1
(for 2) [9-11]. These bands are shifted to somewhat lower frequencies due to the coordination of
the azomethine group with the metal centre. Both complexes show strong bands around 1520 cm-
1
due to the presence of keto group (C=O) and moderately strong band around 1270 cm-1 due to
the presence of C-O (phenoxy) stretching [10]. Sharp absorption bands at 2016 cm-1 (in 1) or
2124 cm-1 (in 2) indicate the presence of terminal azide (for 1) and N-bonded thiocyanate (for 2)
respectively, which are also evident from crystal structure determination [9]. In addition, the IR
spectrum of both complexes also exhibits several weak bands for the aromatic and aliphatic C–H
stretching vibration frequencies in the region 3080-2874 cm-1 [9-11], and strong bands for the
aromatic C=C stretching bonds around 1440 cm-1 [24].
Electronic absorption spectra of both complexes in acetonitrile medium shows d-d
transition bands around 490 and 415 nm, as expected for a low spin cobalt(III) complex in
octahedral geometry [24,25]. Broad absorption bands at 316 nm (for 1) and 396 nm (for 2) are
also observed which is consistent with LMCT band [24,25]. Additionally, high energy
absorption bands around 270 nm may be attributed to intra-ligand transitions i.e. * and
n* transitions [24,25].
3.5. Catechol oxidase and phenoxazinone synthase mimicking activity
The catechol oxidase mimicking activity of synthesized complexes has been evaluated
using 3,5-DTBC as a model substrate due to its easy oxidation to 3,5-di-tert-butylbenzoquinone
(3,5-DTBQ), whereas OAPH has been used as model substrate for evaluating phenoxazinone

synthase mimicking activity of both complexes. Mimicking activity studies were performed in
acetonitrile medium because complexes, substrates and their products are highly soluble in
acetonitrile. Entire mimicking activity studies were carried out in absence of any supplementary
base to minimize the possibility of oxidation of substrates by molecular dioxygen. Progresses of
the reactions were followed spectrophotometrically. Spectrophotometric scans revealed a gradual
increase in intensity of the absorption band at ∼400 nm (characteristic for 3,5-DTBQ) or ∼433
nm (characteristic for phenoxazinone chromophore) for catechol oxidase or phenoxazinone
synthase like activities respectively. The resulting solutions were also monitored
spectrophotometrically after 48 h, which show the formation of 3,5-DTBQ or 2-
aminophenoxazine-3-one respectively as the sole product. These results undoubtedly indicate
that synthesized complexes are active towards both catechol oxidase and phenoxazinone
synthase mimicking activities. Blank experiments for both of these mimicking activities were
also performed under identical conditions and in each case, no significant growth of the
absorption bands around was observed at desired wavelengths.
3.6. Phosphatase mimicking activity
The complexes were also utilized as catalyst for hydrolysis of 4-nitrophenylphosphate (4-
NPP) to 4-nitrophenolate at room temperature. The catalytic activity was performed in aqueous
DMF (98% DMF, v/v) medium because of very good solubility of both 4-NPP and 4-
nitrophenolate. Efficiency of P-O bond cleavage of both complexes were evaluated by
monitoring the spectral change in the wavelength scan (300-500 nm) of a mixture solution for 2
h where complex to substrate ratio is maintained roughly 1:100 stoichiometrically. Band maxima
at ~425 nm confirm formation of 4-nitrophenolate ion. The resulting solutions were also
monitored spectrophotometrically after 48 h which show the formation of 4-nitrophenolate as the

sole product with no further increase of band maxima. These consequences clearly indicate that
the complexes are active towards phosphatase mimicking activity. Blank experiments without
any catalyst were also carried out under identical conditions and in this case, no significant
growth of the absorption bands around 425 nm was observed.
3.7. Kinetic investigations through a number of enzyme kinetic plots
The Michaelis-Menten model is the best-known approach for treating biochemical
reactions involving a single substrate. Initial rate of the reaction versus substrate concentration
was plotted for all of these mimicking activities. These plots suggest rate saturation kinetics. The
initial rate of the reaction was calculated by multiplying the substrate concentration with rate
constant. Rate constant of the reaction was obtained from the slope of the product concentration
vs. time plot. Linearization of Michaelis-Menten equation produces a double reciprocal
Lineweaver-Burk plot, which can be used to analyze kinetic parameters like Vmax and KM
directly. Additionally, rearrangement of Michaelis-Menten equation gives Hanes equation. In
this case, the ratio of the substrate concentration to the reaction rate is plotted against substrate
concentration. Eadie-Hofstee equation is sometimes used in biochemistry for a graphical
representation of enzyme kinetics. In this plot reaction rate is plotted as a function of the ratio
between reaction rate and substrate concentration. All these plots are used to evaluate various
kinetic parameters including kcat and specificity constant (kcat/KM) for catechol oxidase,
phenoxazinone synthase and phosphatase mimicking activity of both complexes. The kcat value is
obtained by dividing Vmax by the concentration of the catalyst (complex) used [9-11]. Figures 6
and 7 represent the Lineweaver–Burk plot, Hanes plot and Eadie-Hofstee plot of complex 2 for
catechol oxidase and phenoxazinone synthase mimicking activity, respectively. Similarly, the
Lineweaver-Burk plot, Hanes plot and Eadie-Hofstee plot for phosphatase mimicking activity of

complex 2 are illustrated in Figure 8. All these plots of complex 1 are given Figure S2-S4
(Supplementary information). A number of kinetic parameters for catechol oxidase,
phenoxazinone synthase and phosphatase mimicking activity of both complexes are gathered in
Tables 5, 6 and 7, respectively.
Figure 6: Michaelis-Menten plot (a), Lineweaver-Burk plot (b), Hanes-Woolf plot (c) and
Eadie-Hofstee plot (d) of complex 2 for catechol oxidase mimicking activity.

Figure 7: Michaelis-Menten plot (a), Lineweaver-Burk plot (b), Hanes-Woolf plot (c) and
Eadie-Hofstee plot (d) of complex 2 for phenoxazinone synthase mimicking activity.

Figure 8: Michaelis-Menten plot (a), Lineweaver-Burk plot (b), Hanes-Woolf plot (c) and
Eadie-Hofstee plot (d) of complex 2 for phosphatase mimicking activity.
3.8. Mechanistic pathway of catalytic activity
From the experimental results, it is clear that both complexes are able to mimic catechol
oxidase, phenoxazinone synthase and phosphatase like enzymes effectively. It is very obvious
that quite a few factors may influence the catalytic activity, such as the variable oxidation state
of the metal, coordination geometry around the metal centre, metal-metal distance, ligand
flexibility, exogenous bridging ligand etc [26,27]. Based on the several previously published

reports [26-30], tentative catalytic conversations for catechol oxidase, phenoxazinone synthase
and phosphatase mimicking activities are given in Schemes 2, 3 and 4, respectively.
Scheme 2: Possible mechanistic pathway for catechol oxidase mimicking activity of synthesized
complexes.

Scheme 3: Tentative mechanistic pathway for phenoxazinone synthase mimicking activity of
synthesized complexes.

Scheme 4: Plausible mechanistic pathway for phosphatase mimicking activity of synthesized
complexes.
It can easily be predicted that initially a catalyst-substrate adduct is formed by replacing
the monodentate azide or thiocyanate coligand, which subsequently leads to respective products,
regenerating catalyst molecules. Catalytic activities depend upon the interaction between
substrate and catalyst. Higher the interaction between substrate and catalyst, higher is the kcat
values. Interaction between the Catalyst and OAPH molecule is more than that of catalyst and
3,5-DTBC or 4-NPP, as less steric hindrance occur in case of OAPH. That’s why the turnover
number (kcat) values for phenoxazinone synthase mimicking activity of both complexes are
greater than catechol oxidase and phosphatase mimicking activity. It is very clear that, complex 2
acts as more active catalyst than complex 1. Most probably, this is due to the fact that
thiocyanate coligands are much more labile compared to azide. Therefore, substrates can form

catalyst-substare adduct replacing thiocyanate from complex 2 with greater ease, initiating
catalysis. This was found in some of our previous works too [9,10].
To elucidate active species present in catalytic pathways during mimicking activities, the
electron spray ionization mass spectrometry of the 1:50 mixtures of complexes and respective
substrates has been performed in the positive mode. Complex 2 was taken as a model complex as
it exhibits highest kcat value. ESI-MS positive spectra of 1 : 50 mixtures of the complex 2 with
3,5-DTBC, o-aminophenol and 4-NPP were recorded separately after 5 min from mixing and the
results are depicted in Figures 9, 10 and 11, respectively. In presence of 3,5-DTBC, the mass
spectrum exhibits a peak at m/z = 243.0888, corresponding to quinone sodium aggregate [(3,5-
DTBQ)(Na)]+, which confirms the termination of the catalytic cycle. Similarly, the mass
spectrum of complex 2 and o-aminophenol shows a peak at m/z = 242.2386, corresponding to
the formation of Na and Li adduct of the deprotonated form of 2-aminophenoxazin-3-one,
confirming the phenoxazinone synthase mimicking activity of the complex. On the other hand,
mass spectral analysis of complex 2 and 4-NPP confirms the presence of [(4-
nitrophenolate)(Li)(K)]+, which is indicated by the appearance of a peak at m/z = 184.14. From
this mass spectral analysis, phosphatase mimicking activity of the complex has been confirmed.
In each case, a peak is observed at m/z ≈ 399, which may be assigned as [Co(L1)(acac)]+,
generated via the loss of monodentate azide/thiocyanate coligand in the initial step of the
catalysis. So it is clear that axially coordinated azide/monodentate coligand can easily be
replaced. Mass spectral data of complex 1 is exactly similar with the above mention data,
indicating similar path of reaction in each catalytic reaction. The mechanistic pathways shown in
Schemes 2, 3 and 4 are therefore justified by the mass spectral analysis.

Figure 9: ESI-MS positive spectrum of 1 : 50 mixture of complex 2 and 3,5-DTBC in
acetonitrile medium at room temperature.

Figure 10: ESI-MS positive spectrum of 1 : 50 mixture of complex 2 and OAPH in acetonitrile
medium at room temperature.

Figure 11: ESI-MS positive spectrum of 1 : 50 mixture of complex 2 and 4-NPP in DMF-water
mixture at room temperature.
4. Concluding Remarks
In conclusion, we have discussed the synthetic stratagem and structural characterization
of two heteroleptic cobalt(III) complexes. Structures of both complexes have been confirmed by
single crystal X-ray diffraction technique. Significant solid state supramolecular interactions in
them are also investigated. Synthesized complexes are active towards catalytic conversation of
3,5-DTBC to 3,5-DTBQ (catechol oxidase mimicking activity) and OAPH to 2-
aminophenoxazine-3-one (phenoxazinone synthase mimicking activity) in acetonitrile medium.
Both complexes have also been found to mimic the role of phosphatase enzyme efficiently by

transforming 4-nitrophenylphosphate to 4-nitrophenolate at room temperature in aqueous DMF
(98% DMF, v/v) medium. Mimicking activities have been assessed by following conventional
Michaelis-Menten enzymatic kinetics. Kinetic studies of the catalytic cycles have been
performed in detail using a variety of enzyme kinetics plots to calculate a number of kinetic
parameters, including kcat. The catalytic activity of complex 2 (towards all these bio-relevant
catalyses) is higher compared to complex 1. This may be correlated with the presence of easily
replaceable thiocyanate in 2 (compared to azide in 1) initiating the catalytic cycle by forming
stable catalyst-substare adduct easily.
Acknowledgements
K.G. expresses his gratitude to UGC, India, for awarding a Senior Research Fellowship.
Crystallographic data were collected at the DST-FIST, India funded Single Crystal
Diffractometer Facility at the Department of Chemistry, Jadavpur University.
Appendix A. Supplementary data
CCDC 1913920-1913921 contain the supplementary crystallographic data of the
synthesized complexes. 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.
References:
[1] F. Nastri, M. Chino, O. Maglio, A. Bhagi-Damodaran, Y. Lu, A. Lombardi, Chem. Soc. Rev.
45 (2016) 5020-5054.

[2] M. Fontecave, J. -L. Pierre, Coord. Chem. Rev. 170 (1998) 125-140.
[3] D. J. Xuereb, R. Raja, Catal. Sci. Technol. 1 (2011) 517-534.
[4] M. Zhao, H. -B. Wang, L. -N. Ji, Z. -W. Mao, Chem. Soc. Rev. 42 (2013) 8360-8375.
[5] A. S. Borovik, Acc. Chem. Res. 38 (2005) 54-61.
[6] E. Y. Tshuva, S. J. Lippard, Chem. Rev. 104 (2004) 987-1012.
[7] N. Sarkar, K. Harms, A. Frontera, S. Chattopadhyay, New J. Chem. 41 (2017) 8053-8065.
[8] S. Purkait, P. Chakraborty, A. Frontera, A. Bauzá, E. Zangrando, D. Das, New J. Chem. 42
(2018) 12998-13009.
[9] K. Ghosh, S Roy, A. Ghosh, Abhisek Banerjee, A. Bauzá, A. Frontera, S. Chattopadhyay,
Polyhedron 112 (2016) 6-17.
[10] K. Ghosh, K. Harms, S. Chattopadhyay, ChemistrySelect 2 (2017) 8207-8220.
[11] K. Ghosh, K. Harms, S. Chattopadhyay, Polyhedron 123 (2017) 162-175.
[12] S. K. Wolff, D. J. Grimwood, J. J. McKinnon, D Jayatilaka, M. A. Spackman, Crystal
Explorer
2.0;
University
of
Western
Australia:
Perth,
Australia,
(2007).
http://hirshfeldsurfacenet.blogspot.com/.
[13] M. A. Spackman, D. Jayatilaka, CrystEngComm 11 (2009) 19-32.
[14] H. F. Clausen, M. S. Chevallier, M. A. Spackman, B. B. Iversen, New J. Chem. 34 (2010)
193-199.

[15] A. L. Rohl, M. Moret, W. Kaminsky, K. Claborn, J. J. McKinnon, B. Kahr, Cryst. Growth
Des. 8 (2008) 4517-4525.
[16] A. Parkin, G. Barr, W. Dong, C. J. Gilmore, D. Jayatilaka, J. J. McKinnon, M. A.
Spackman, C. C. Wilson, CrystEngComm 9 (2007) 648-652.
[17] M. A. Spackman, J. J. McKinnon, CrystEngComm 4 (2002) 378-392.
[18] P. Pandey, A. Verma, K. Bretosh, J. -P. Sutter, S. S. Sunkari, Polyhedron 164 (2019) 80-89.
[19] Y. Zhang, M. Avdeev, J. R. Price, I. Karatchevtseva, D. J. Fanna, I. Chironi, K. Lu,
Polyhedron 165 (2019) 125-131.
[20] B. -B. Tang, X. -P. Sun, G. -L. Liu, H. Li, J. Mol. Struct. 984 (2010) 111-116.
[21] D. Cremer, J. A. Pople, J. Am. Chem. Soc. 97 (1975) 1354-1358.
[22] A. D. Hill, P. J. Reilly, J. Chem. Inf. Model. 47 (2007) 1031-1035.
[23] M. A. Spackman, P. G. Byrom, Chem. Phys. Lett. 267 (1997) 215-220.
[24] K. Ghosh, K. Harms, A. Bauzá, A. Frontera, S. Chattopadhyay, Dalton Trans. 47 (2018)
331-347.
[25] K. Ghosh, K. Harms, A. Bauzá, A. Frontera, S. Chattopadhyay, CrystEngComm 20 (2018)
7281-7292.
[26] A. Hazari, A. Das, P. Mahapatra, A. Ghosh, Polyhedron 134 (2017) 99-106.
[27] K. Ghosh, K. Harms, A. Franconetti, A. Frontera, S. Chattopadhyay, J. Organomet. Chem.
883 (2019) 52-64.
[28] J. Adhikary, I. Majumdar, P. Kundu, H. Kornweitz, H. Kara, D. Das, ChemistrySelect 3
(2018) 1445-1454.

[29] M. Mitra, P. Raghavaiah, R. Ghosh, New J. Chem. 39 (2015) 200-205.
[30] K. Ghosh, A. Banerjee, A. Bauzá, A. Frontera, S. Chattopadhyay, RSC Adv. 8 (2018)
28216-28237.

Table 1: Crystal data and refinement details of complexes 1 and 2.
Complex 1
Complex 2
Formula
Formula Weight
Temperature (K)
Crystal System
Space group
C20H25CoN5O3
442.38
C21H25CoN3O3S
458.43
296(2)
296(2)
Monoclinic
Cc
Monoclinic
P21/c
a(Å)
7.7642(3)
18.2919(8)
14.3215(5)
98.123(2)
1.459
9.5965(5)
19.6256(10)
11.8519(6)
103.215(3)
1.401
b(Å)
c(Å)
β(°)
D(calc) [g/cm3]
µ [mm-1]
0.884
0.912
F(000)
924
956
Total Reflections
Unique Reflections
Observed data [I > 2 σ(I)]
No of parameters
R(int)
14053
31576
3811
3979
3684
3441
262
262
0.024
0.048
R1, wR2 (all data)
R1, wR2 ([I > 2 σ(I)]
Residual Electron Density (eÅ-3)
0.0309, 0.0802
0.0299, 0.0796
0.443, -0.301
0.0522, 0.1201
0.0443, 0.1138
0.709, -0.553