A rare flattened tetrahedral Mn(II) salen type complex: Synthesis, crystal structure, biomimetic catalysis and DFT study

Article abstract of DOI:10.1016/j.ica.2019.119176

A new flattened tetrahedral high spin Mn(II) complex (1) has been synthesized using N₂O₄ donor Schiff base ligand. Complex 1 was characterized by X-ray diffraction and various spectroscopic techniques. For further understanding of el

Full text of DOI:10.1016/j.ica.2019.119176

InorganicaChimicaActa499(2020)119176
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Inorganica Chimica Acta
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Research paper
A rare flattened tetrahedral Mn(II) salen type complex: Synthesis, crystal
structure, biomimetic catalysis and DFT study
Saikat Banerjeea, Pravat Ghoraia, Papiya Sarkarb, Anangamohan Panjac,d, Amrita Sahaa,⁎
a Department of Chemistry, Jadavpur University, Kolkata 700032, India
b Chemistry and Biomimetics Group, CSIR-Central Mechanical Engineering Research Institute, M. G. Avenue, Durgapur 713209, India
c Department of Chemistry, Panskura Banamali College, Panskura RS, WB 721152, India
d Department of Chemistry, Gokhale Memorial Girls' College, 1/1 Harish Mukherjee Road, Kolkata 700020, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Mn(II) salen type complex
Crystal structure
Spectroscopic studies
Phenoxazinone synthase like activity
Theoretical calculations
A new flattened tetrahedral high spin Mn(II) complex (1) has been synthesized using N2O4 donor Schiff base
ligand. Complex 1 was characterized by X-ray diffraction and various spectroscopic techniques. For further
understanding of electronic structure of the complex, DFT calculations and electrochemical studies have been
performed. This is a rare example of a flattened tetrahedral Mn(II) salen type Schiff base complex. High-spin d5
configuration of the metal center provides no crystal-field stabilization energy to the system and that is the main
reason behind the significant deviation of this salen-type ligand from planarity. Notably, the propylenic linker in
the ligand provides adequate flexibility so that such an uncommon binding mode of the salen type Schiff base
ligand becomes possible. Complex 1 exhibits excellent catalytic property towards oxidation of o-aminophenols in
aerobic condition. Detailed kinetic investigations together with the mass spectrometry studies reveal several
important information relating to biomimetic catalytic activity of the present complex.
1. Introduction
used as a primary oxidant, [13–17] direct oxidation of small organic
molecules by molecular oxygen is still difficult because of its spin re-
Manganese is the 12th most naturally occurring trace metal found in
the living systems. Coordination chemistry of manganese is driven by a
part of its occurrence in the active sites of several enzymes in the bio-
logical systems [1–4]. For example, in photosystem-II (PS-II), manga-
nese centers constitute oxygen evolving complex (OEC) which photo-
lytically oxidizes water to oxygen. In the active site structures of Mn
containing catalase [5–7] and peroxidase, the manganese centers are
found to coordinate with N or O donor ligands [8,9]. It is clear that
nature has chosen Mn in the active site of different metalloenzymes due
to its rich redox properties and possibilities of presence of Mn ions in
different geometries and stable oxidation states. These enzymatic ac-
tivities of Mn inspired us to use its model complexes for selective oxi-
dation of organic molecules. It is important to mention that synthesis of
biologically-compatible, environment-friendly and energetically-effi-
cient metal complexes is a challenging task for the development of new
chemicals for industrial processes and subsequently facilitating the
advancement of science in different fields. Oxidation process plays a
crucial role in organic reaction for the synthesis of several valuable
organic compounds in the fields of pharmaceuticals, agrochemicals, etc.
[10–12]. Although in chemical industries mainly molecular oxygen is
striction that reduces its reactivity severely with ending up of poor yield
[18–21]. In this connection phenoxazinone synthase (PHS) needs spe-
cial mention for its biological importance, which is a penta copper
oxidase that efficiently activates molecular dioxygen at ambient con-
dition to catalyze the oxidative coupling of two molecules of a sub-
stituted o-aminophenol to the phenoxazinone chromophore in the final
step for the biosynthesis of actinomycin D [22,23]. Actinomycin D is an
aromatic heterocyclic natural product which is clinically used for
treatment of choriocarcinoma, wilms tumors, rhabdomyosarcoma, and
Kaposi's sarcoma [24]. So, it is important to develop metal complexes
which can efficiently mimic PHS by oxidizing o-aminophenol to 2-
aminophenoxazin-3-one chromophore [25].
On the other hand, Schiff base ligands are classical chelating ligands
which are vigorously used to understand molecular processes occurring
in biochemistry, material science, catalysis, encapsulation, activation,
transport and separation phenomena, hydrometallurgy, etc. [26,27].
Their ease of synthesis and reactivity with almost all metal ions present
in the periodic table make them suitable synthons for the development
of coordination chemistry. Literature has witnessed rich coordination
chemistry involving H2L (Scheme 1) ligand with reports of numerous
⁎ Corresponding author.
E-mail address: amritasahachemju@gmail.com (A. Saha).
https://doi.org/10.1016/j.ica.2019.119176
Received 27 July 2019; Received in revised form 26 September 2019; Accepted 26 September 2019
Availableonline27September2019
0020-1693/©2019ElsevierB.V.Allrightsreserved.

S. Banerjee, et al.
InorganicaChimicaActa499(2020)119176
Table 1
Crystal parameters and selected refinement details for complex 1.
Complex
1
Empirical formula
Formula weight/g mol−1
Temperature/K
Crystal system
Space group
C19H20MnN2O4
395.31
293(2)
Orthorhombic
Pbcn
a/Å
24.7944(10)
b/Å
8.0362(3)
c/Å
8.6956(4)
α/°
90
90
90
β/°
γ/°
Volume/Å3
1732.62(12)
Z
4
ρcalc g/cm3
1.515
0.790
820.0
0.2 × 0.1 × 0.06
3.286–54.32
−31 ≤ h ≤ 31, −10 ≤ k ≤ 10, −11 ≤ l ≤ 11
μ/mm−1
F(0 0 0)
Crystal size/mm3
2θ range for data collection/°
Index ranges
Scheme 1. The route to the syntheses of complex 1.
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indexes [I > 2σ (I)]
Final R indexes [all data]
Largest diff. peak/hole/e Å−3
25,178
1926 [Rint = 0.0362, Rsigma = 0.0178]
1926/0/120
1.061
mono-, di-, tri-, tetra- and polynuclear complexes having 3d and 4d
metals [28,29]. A close inspection of the manganese chemistry clearly
suggests that in all cases Mn(II) salt upon reaction with different types
of salen ligands produce Mn(III) complexes where Mn(II) centers un-
dergo aerial oxidation [29]. The enormous applicability of such Mn(III)
complexes in diverse oxidation reactions is well known, and the redox
flexibility between the Mn(III) and Mn(II) plays the crucial role in those
redox processes. Interestingly, the crystallographic information of Mn
(II) salen type Schiff base complex is rarely available in the literature
[30] that motivated us to prepare Mn(II) complex using salen type li-
gand H2L as that could provide significant information of role of the
manganese center in the oxidation reactions. With this aim, we are first
time reporting here the crystallographic information of flatten tetra-
hedral Mn(II) salen type Schiff base complex [Mn(L)] (1). It is also
important to mention that although various Mn(III) octahedral com-
plexes are known in literature, a few examples are known to have other
coordination geometries [31]. Complex 1 has been characterized by
various spectroscopic techniques and structure has been elucidated by
X-ray crystallography. UV–Vis spectrum and X-ray data are further
verified using DFT and TDDFT calculations. Furthermore, the oxidation
of o-aminophenols catalyzed by the synthesized complex has been ex-
plored.
R
R
1 = 0.0477, wR2 = 0.1495
1 = 0.0585, wR2 = 0.1598
0.60/−0.43
2.2. X-ray crystallography
Single crystal X-ray data of complex 1 was collected on a Bruker
SMART APEX-II CCD diffractometer using graphite monochromated Mo
Kα radiation (λ = 0.71073 Å) at room temperature. Data processing,
structure solution, and refinement were performed using Bruker Apex-II
suite program. All available reflections in 2θmax range were harvested
and corrected for Lorentz and polarization factors with Bruker SAINT
plus [32]. Reflections were then corrected for absorption, inter-frame
scaling, and other systematic errors with SADABS [33]. The structure
was solved by the direct methods and refined by means of full matrix
least-square technique based on F2 with SHELX-2017/1 software
package [34]. All the non-hydrogen atoms were refined with aniso-
tropic thermal parameters. C–H hydrogen atoms were inserted at geo-
metrical positions with Uiso = 1.2Ueq to those they are attached. Crystal
data and details of data collection and refinement for 1 are summarized
in Table 1.
2. Experimental
2.3. Synthesis of Schiff base ligand (H2L = N,N′-bis(3-
methoxysalicylidene)propylene-1,3-diamine)
2.1. Materials and physical measurements
The tetradentate Schiff base ligand (H2L) was prepared by the well-
known method described in literature [28,29,35]. Briefly, a mixture of
o-vanillin (4.0 mmol, 0.608 g) and 1,3-diaminopropane (2.0 mmol,
0.148 g) in 25 mL methanol was heated to reflux for 2 h. The resulting
light yellow colored Schiff base ligand (H2L) was directly used for the
following reaction.
All reagent or analytical grade chemicals and solvents were pur-
chased from commercial sources and used without further purification.
Elemental analysis for C, H and N was carried out using a
Perkin–Elmer
240C
elemental
analyzer.
Infrared
spectra
(400–4000 cm−1) were recorded from KBr pellets on a Nicolet Magna
IR 750 series-II FTIR spectrophotometer. Absorption spectra were
measured using a Cary 60 spectrophotometer (Agilent) with a 1-cm-
path-length quartz cell. The electrochemical data of complex 1 have
been recorded in methanol containing 0.1 M tetraethylammonium
perchlorate as a supporting electrolyte in a conventional three electrode
configuration using a Pt disk working electrode, Pt auxiliary electrode
and Ag/AgCl reference electrode using a PC-controlled PAR model
273A electrochemical system. Electron spray ionization mass (ESI-MS
positive) spectra were recorded on a MICROMASS Q-TOF mass spec-
trometer. Electron paramagnetic resonance (EPR) spectra were re-
corded in standard quartz EPR tubes using a Varian E-109C spectro-
meter.
2.4. Preparation of [Mn(L)] (1)
A methanolic solution (2 mL) of manganese chloride tetrahydrate
(1.0 mmol, 0.198 g) was added drop wise to 20 mL methanolic solution
of H2L (1.0 mmol) followed by addition of triethylamine (2.0 mmol,
∼0.4 mL) and the resultant reaction mixture was heated to reflux for
4 h under nitrogen atmosphere. The solution was then cooled and fil-
tered. Light green colored needle shaped crystals resulted from the slow
evaporation of methanolic solution of the complex at room tempera-
ture. Yield: 0.2963 g (75%). Anal. Calc. for C19H20MnN2O4: C 57.73%;
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S. Banerjee, et al.
InorganicaChimicaActa499(2020)119176
H 5.10%; N 7.09%. Found: C 57.25%; H 5.01%; N 6.98%. IR (cm−1
,
KBr): ν(C]N) 1627m; ν(C–N) 1220s; ν(C–H) 739 s. UV–Vis, λmax (nm),
(ε (dm3 mol−1 cm−1)) in methanol: 280 (55262) and 370 (14120).
2.5. Computational method
All computations were performed using the GAUSSIAN09 (G09)
[36] software. Full geometry optimizations were carried out using the
density functional theory method at the B3LYP level [37,38] for the
complex 1. The coordinates obtained from single crystal X-ray diffrac-
tion data was used for optimization using the lanL2DZ effective po-
tential (ECP) set of Hay and Wadt [39–41] for manganese atom and the
standard 6-31+G(d) basis set for C, H, N and O atoms, respectively
[42,43]. To get local minima and only positive eigen values of opti-
mized geometries, the vibrational frequency calculation were carried
out. Time-dependent density functional theory (TDDFT) was computed
[44,45] in methanol using conductor-like polarizable continuum model
(CPCM) [46–49] with the same B3LYP level and basis sets to get ver-
tical electronic excitations. GAUSSSUM 3.0 [50] was used to calculate
the percentage contributions of ligand, coligand and metal ion to each
molecular orbital.
Fig. 1. Ortep view of complex 1. Atoms are shown as 30% thermal ellipsoids. H
atoms are omitted for clarity. [Symmetry Code: A = −x, y, 1/2 − z].
in the present endeavor, MnCl2·4H2O upon reaction with H2L in pre-
sence of Et3N in 1:1:2 M ratio in methanol afforded the mononuclear
complex [Mn(L)] (1) strictly under inert atmosphere in which oxidation
state of manganese is +II (Scheme 1). It is important to note that only
one report of crystallographically characterized Mn(II) complex with
salen type ligands is available in the literature, and thereby present
report could be further helpful in the better understating of manganese
salen chemistry and would motivate the researchers working in this
field to isolate and structurally characterized similar complexes of high
importance.
2.6. Hirshfeld surface analysis
Hirshfeld surface analysis have been done using Crystal Explorer
version 3.1 [51]. The normalized contact distance (dnorm) based on di
and de has been determined by the given equation where rvdW is the van
der Waals (vdW) radius of the appropriate atom internal or external to
the surface.
(di − rvdW
)
(de − revdW
)
i
dnorm =
+
rvdW
revdW
i
3.2. Crystal structure description of [Mn(L)] (1)
dnorm becomes negative for shorter contacts than vdW separations and
becomes positive for contacts greater than vdW separations, and is
displayed using a red–white–blue color scheme, where red highlights
shorter contacts, white is used for contacts around the vdW separation,
and blue is for longer contacts [52].
The molecular structure of 1 is shown in Fig. 1. The selected bond
distances and angles are given in Table 2.
The X-ray data reveal that complex 1 crystallizes in the orthor-
hombic Pbcn space group. The Mn(II) complex is a mononuclear com-
pound and the metal center is tetra coordinated, where the coordina-
tion numbers are satisfied by a pair of imine nitrogen atoms (N1 and
N1A) and phenoxido oxygen atoms (O2 and O2A) provided by the
deprotonated tetradentate Schiff base ligand in order to adopt distorted
tetrahedral geometry. The dipositive charge of the manganese ion is
satisfied by two phenoxido oxygen atoms. The structure of the complex
possesses 2-fold rotation axis where the Mn(II) ion sits on an inversion
centre. The angle between two planes i.e. N1–Mn1–O2 and
N1A–Mn1–O2A is 39.53° implying flatten tetrahedral geometry. The
Mn–N (1.938(2)Å) and Mn–O (1.907(2) Å) bond distances are relatively
shorter with those found in the similar tetrahedral structures reported
earlier [54]. However, the EPR spectrum of the complex at liquid ni-
trogen temperature shows characteristic six-line spectrum at g = 2.04,
ensuring the +II oxidation state of manganese (Fig. 2). The bite angles
around manganese (N(1)–Mn(1)–N(1A) = 92.41(15)°, N(1A)/N(1)–Mn
(1)–O(2)/O(2A) = 93.98(7)° and O(2)–Mn(1)–O(2A) = 93.68(14)°
2.7. Biomimetic catalysis (catalytic oxidation of o-aminophenols)
Oxidation of o-aminophenols catalysed by the complex was ex-
amined by the reacting 1.0 × 10−5 M dioxygen-saturated methanolic
solutions of the complex with 10−2 M solution of o-aminophenols at
25 °C. The progress of the reactions was monitored by the successive
increase in absorbance band of phenoxazinone chromophore using lit-
erature reported extinction coefficients values [53]. To evaluate the
rate dependency of the reaction on o-aminophenols and to evaluate
various kinetic parameters like Vmax, KM, kcat, 2.0 × 10–5 (M) solution
of the complex was mixed with various concentration of the substrates
maintaining minimum 10 folds excess to that of catalyst to retain the
pseudo-first-order condition. Rate of a reaction was evaluated from the
initial rate method, and the average initial rate over three independent
measurements was recorded.
Table 2
3. Results and discussion
Selected bond lengths (Å) and bond angles (°) for complex 1.
X-ray
Calculated
3.1. Preparation of Schiff base ligand (H2L) and complex 1
Mn1–N1
Mn1–N1A
Mn1–O2
Mn1–O2A
C9–C10–C9A
N1–Mn1–O2
N1–Mn1–N1A
1.938
1.938
1.907
1.907
115.6
94.0
1.9552
1.9569
1.8365
1.8365
111.32
91.71
The ligand H2L was synthesized by the condensation reaction of 1,3-
diaminopropane and o-vanillin in a 1:2 M ratio in methanol following
the standard procedure [28,29,35]. The ligand possesses six potential
donor sites; two azomethine nitrogen atoms, two phenolic oxygen
atoms and two oxygen atoms of methoxy groups. Salen type ligands by
reacting with Mn(II) salts in aerobic conditions exclusively produced
Mn(III) complexes in which Mn(II) center is air oxidized. Interestingly,
92.4
88.97
[Symmetry Code: A = −x, y, 1/2 − z].
3

S. Banerjee, et al.
InorganicaChimicaActa499(2020)119176
(III) complexes with H2L ligand are hexa-coordinated with octahedral
geometry in which the ligand coordinates the metal center at equatorial
positions as expected [55]. However, in the present case it is sig-
nificantly deviated from planarity resulting in an outcome of flattened
tetrahedral geometry, which is extremely rare in the coordination
chemistry with salen type ligands. Close inspection of the crystal
structure reveals that the propylenic linker in the ligand provides
adequate flexibility so that such an uncommon binding mode of the
salen type Schiff base ligand become a reality.
3.3. Spectroscopic study
Besides elemental analysis, ligand (H2L) and complex 1 were in-
itially characterized by IR spectroscopy. A strong and sharp band ap-
peared at 1629 and 1627 cm−1, respectively for the ligand (H2L) and
complex 1 due to azomethine ν(C]N) (Figs. S1 and S2). Complex 1
exhibits two absorption bands around 280 nm and 370 nm (Fig. S3).
These absorption bands can be assigned to π → π* and n → π* transi-
tions of the Schiff base ligand. The spectroscopic values agree with the
literature values for distorted tetrahedral Mn(II) compounds [56]. The
magnetic moment µeff. is around 5.78 BM at 300 K as expected for an
isolated S = 5/2 Mn(II) monomer.
Fig. 2. X-band EPR spectrum of complex 1 at 77 K.
respectively) deviate from the ideal tetrahedral values. The two six-
membered MnC3NO chelate rings are not planar. The dihedral angle
between two chelate rings is 42.72°. The propylenic part of the Schiff
base, N1–C9–C10–C9A–N1A, is to some extent puckered due to the sp3
hybridization of the saturated portion of the chelating ligand. The bond
angle (C9–C10–C9A, 115.68(6)°) deviates appreciably from its ideal
value.
In complex 1, one dimensional self-assembly along the c axis is
observed, supported by significantly strong hydrogen bonds between
oxygen atom of methoxy group and H-atom of azomethine group. This
H-bonding distance is around 2.717 Å. Also, an intermolecular H-
bonding interaction between same azomethine H atom and the phenolic
oxygen from the neighboring molecule with donor-acceptor distance of
2.692 Å has been observed (Fig. 3).
3.4. DFT study
The coordinates obtained from single crystal X-ray diffraction data
of complex 1 was used for optimization using DFT/B3LYP method.
Bond lengths and angles obtained from the DFT optimized structure of
the complex 1 show well agreement with that of X-ray structures
(Table 2).
Mulliken charge distribution shows that positive charge on metal
atom i.e. 1.496537. Mulliken charge distribution of complex 1 is de-
picted in Table S1. Energy (eV) of some selected molecular orbitals of
complex 1, key electronic transitions and composition of M.O.s of some
selected ones are presented in Tables S2 and S3, respectively. Contour
plots of some selected M.O.s are given in Fig. S4 and S5. The energies of
HOMO and LUMO are −2.5 eV (α-spin), −5.1 eV (β-spin) and
−1.97 eV (α-spin), −2.12 eV (β-spin) respectively.
It is important to note that salen type ligands generally coordinate
the metal centers in equatorial positions with minimal deviation from
planarity because of its inherent righty. Notably, all the reported Mn
From DFT study it has been found that molecular orbitals (M.O.s)
Fig. 3. 1D supramolecular architecture of complex 1 showing intermolecular hydrogen bonding interactions along the b axis. Hydrogen atoms of least interest are
omitted for clarity.
4

S. Banerjee, et al.
InorganicaChimicaActa499(2020)119176
HOMO to HOMO−5 (both α-spin and β-spin) have major contribution
from ligand i.e. these orbitals are rich in π(L) character. Whereas LUMO
+4 (α-spin), LUMO to LUMO+3 (β-spin) molecular orbitals have
major contribution from dπ of Mn centre. LUMO (α-spin), LUMO+2 (α-
spin) and LUMO+5(α-spin) have significant contributions from ligand
π*(L) site. LUMO+1 (α-spin), LUMO+1 (β-spin) and LUMO+5 (β-
spin) have contributions from both metal and ligand in comparable
extend.
been modified chemically. Huge structural difference of the Mn centre
in different oxidation states is presumably responsible for such elec-
trochemical events. Interestingly, when the cyclic voltammogram was
recorded in aerobic condition, only an irreversible reduction response
at −1.25 V was observed which is comparable to the cathodic response
when the electrothermal study was conducted in nitrogen atmosphere
(see Fig. S9b). Similar electrochemical response was also observed in
recently reported Mn(III)-salen type complex, [29h] but only difference
is that in the present case the reduction is less favourable, which can be
realised from the fact that the high reorganisation energy required for
the structural change at the metal centre in both oxidation and reduc-
tion processes brings lower redox flexibility in the present system.
Furthermore, absence of any oxidative response in aerobic condition
indicates that this Mn(II)-salen type complex is highly susceptible to
aerial oxidation leading the fast conversion to Mn(III) species, even
faster than the electrochemical time scale.
3.5. TDDFT study
For better understanding of electronic transition TDDFT calcula-
tions were performed using B3LYP/CPCM method using same basis sets
in methanol solvent. The calculated electronic transitions along with
the calculated Oscillator Strength (f) are given in Table S3. Complex 1
shows intense absorption band for ligand based π → π* and n → π*
transitions around 280 nm and 370 nm, respectively. The bands at
280 nm and 370 nm are theoretically assigned as the following excita-
tions at 4.24 eV (λ = 292.72 nm, f = 0.0188), 4.22 eV (λ = 293.75 nm,
f = 0.0473) and 3.41 eV (λ = 363.05 nm, f = 0.0688), 3.30 eV
(λ = 375.25 nm, f = 0.0083), respectively and these are due to the
contribution of HOMO(β) → LUMO+2(β) (73%), HOMO−1(β) →
LUMO+2(β) (54%) and HOMO−1(α) → LUMO+5(α) (51%),
HOMO−3(α) → LUMO(α) (49%) transitions.
3.8. Functional model for phenoxazinone synthase like activity
3.8.1. Spectrophotometric study
The phenoxazinone synthase like activity of the complex was ex-
amined by monitoring the oxidation of o-aminophenol (OAPH) spec-
trophotometrically in dioxygen-saturated methanol as both complex
and substrate are well soluble in methanol. Catalytic oxidation of o-
aminophenol (OAPH) was studied in absence of added base to avoid
autoxidation of the substrate by air. To investigate the catalytic effi-
ciency of the complex, spectral scan of the resultant mixture of
1 × 10–5 M methanolic solution of complex 1 and 0.01 M OAPH was
performed in 5 min time interval at 25 °C under aerobic condition. As
can be seen from Fig. 4, the characteristic absorbance band of phe-
noxazinone chromophore ca. at 433 nm gradually increases upon suc-
cessive catalytic oxidation of o-aminophenol (OAPH) to 2-aminophe-
noxazin-3-one.
3.6. Hirshfeld surface analysis
Supramolecular interactions are further investigated using Hirshfeld
Surface analysis. Complex 1 is mapped over dnorm (range of −0.1 to
1.5 Å), shape index (range of −1.0 to 1.0 Å) and curvedness (range of
−4.0 to 0.4) respectively and presented in Figs. S6 and S7, respectively.
During mapping surfaces are kept transparent for visualization of
different supramolecular interactions. For Complex 1, H-bonding in-
teractions between the O atom of methoxy group and H atoms of the
imine group has been predominantly found as bright red area in the
Hirshfeld surfaces. Other longer and weaker interactions appeared as
light colour in the surfaces. Fingerprint plots consist of all type of in-
termolecular interactions. So, fingerprint plots need to be decomposed
to have idea of individual contacts. In the decomposed fingerprint plot,
complementary regions are obtained where one molecule acts as a
donor (de > di) (bottom left of fingerprint plot) and the other as an
acceptor (de < di) (bottom right of fingerprint plot). For complex 1,
O⋯H/H⋯O and H⋯H interactions comprise 20.60% and 46.80% of the
total Hirshfeld surface. O⋯H interactions comprise around 11.3% of
the total Hirshfeld surface and represented by a spike (di = 1.025 Å,
de = 1.35 Å) in the bottom left (donor) area. Whereas H⋯O interactions
comprise around 9.3% of the total Hirshfeld surface and represented by
a spike (di = 1.355 Å, de = 1.025 Å) in the bottom right (acceptor) re-
gion (Fig. S8).
A blank experiment has been performed in absence of the complex 1
under identical condition, which does not result any significant en-
hancement in the band intensity ca. at 433 nm. The result of the ex-
periment implies that complex 1 is catalytically efficient to oxidize
OAPH to 2-aminophenoxazin-3-one under aerobic condition. In order to
understand the degree of catalytic efficiency of complex 1, detail ki-
netic studies were performed at 25 °C. For this purpose, 1 × 10–5
methanolic solution of complex was reacted with various
M
1
3.7. Electrochemical study
The electrochemical behaviour of the complex has been checked in
methanol in the presence of 0.1 M tetraethylammonium perchlorate as
a supporting electrolyte at ambient temperature. In order to get more
insight into the real electronic states of the complex and its sensitivity
towards the molecular dioxygen, the electrochemical studies have been
performed both in nitrogen atmosphere and in aerobic condition, and
the cyclic voltammograms of complex 1 are displayed in Fig. S9, in
which the potentials are referenced to the standard Ag/AgCl electrode.
When the cyclic voltammogram was recorded in nitrogen atmosphere,
an irreversible oxidation at 1.28 V was observed, which can be assigned
to the oxidation of Mn(II) to Mn(III). Moreover, a reductive response at
−1.38 V was observed, which could be due to the reduction of Mn(III)
to Mn(II) (Fig. S9a). These potential values suggest that both after
oxidation and reduction the electrochemically generated species had
Fig. 4. UV–Vis spectral scans showing the increase in phenoxazinone chromo-
phore band at 433 nm after the addition of o-aminophenol (10−2 M) to a so-
lution of complex 1 (1 × 10−5 M) in methanol at 25 °C. The spectra were re-
corded in 5 min time interval.
5

S. Banerjee, et al.
InorganicaChimicaActa499(2020)119176
concentration of the substrate maintaining at least 10 folds excess with
respect to the catalyst used to maintain pseudo first order reaction
condition. For each kinetic measurement time scan at the maximum
band (433 nm) of 2-aminophenoxazin-3-one was carried out for a
period of 10 min using particular substrate ratio to that of the complex.
The initial rate was determined by linear regression from the slope of
the absorbance versus time, and each experiment was performed thrice
and average values were noted. The initial rate of the reactions verses
concentrations of the substrate plot shows rate saturation kinetics as
depicted in Fig. S10. These kinetic data can be fitted to Michae-
lis–Menten model to get different kinetic parameters like Vmax, KM, and
kcat. These can also be determined by linearization of Michaelis–Menten
equation which gives double reciprocal Lineweaver-Burk plot (Fig.
S11). Michaelis binding constant (KM) and Vmax were calculated to be
substrate, the base peak was found at m/z = 365.10, which can be as-
signed to the sodium salt of ligand H2L (calculated m/z = 365.15). The
peak at m/z = 343.12 can be assigned to the protonated Schiff base H2L
(calculated m/z = 343.16). The second most abundant peak at m/
z = 244.05 is a molecular ion peak of product 2-aminophenoxazin-3-
one along with
a solvated methanol (calculated m/z = 244.08).
Another product related peak at m/z = 267.10 is nicely matched with
sodium adduct of 2-aminophenoxazin-3-one together with a solvent
methanol molecule. The peak at m/z = 110.02 is nothing but a proto-
nated substrate of OAPH. One of the moderately abundant peak at m/
z = 215.04 is quite interesting as the isotopic distribution patterns
match well with the protonated intermediate II-B (calculated m/
z = 215.08) as shown in scheme 2. Furthermore, a peak at m/
z = 213.03 is also a product related peak as that matched with the
protonated species of 2-aminophenoxazin-3-one (calculated m/
z = 213.06). Integrity of the original complex was ensured by the
presence of two minor peaks at m/z = 395.03 and 418.02 as these are
well matched with the monocationic species of compositions [Mn(L)]+
(calculated m/z = 395.08) and [NaMn(L)]+ (calculated m/
z = 418.07), respectively (see Fig. S14).
Further inspection of the mass spectrum (Fig. S14) discloses several
minor peaks which are related to the intermediates in the course of
production of 2-aminophenoxazin-3-one. The peaks at m/z = 485.24
and 507.22 are nicely matched with dioxygen bound manganese spe-
cies of compositions [KMn(L)(HO2)(H2O)]+ (calculated m/z = 485.05)
and [NaKMn(L)(O2)(H2O)]+ (calculated m/z = 507.03), respectively,
clearly indicating the interaction of dioxygen with the metal centre at
the catalytic cycle. The peak at m/z = 523.19 is a substrate bound
species of formula [HMn(L)(OAPH)(H2O)]+ that suggests binding of
the substrate to the metal centre during the catalytic oxidation of the
substrate, which is consistent with the rate saturation kinetics.
(1.09
turnover number (kcat) is calculated by dividing the Vmax by the con-
centration of the complex used, and is found to be 22.10 h−1
0.35) × 10–2 and 6.14 × 10–8 M s−1
M , respectively. The
.
A reasonable numbers of model complexes are known in the lit-
erature mimicking the function of phenoxazinone synthase to better
understand the possible mechanistic pathway and their catalytic effi-
ciency. In order to increase the scope for wider perspective, we also
examined the detailed catalytic activity using 2-amino-5-methylphenol
(5-MeOAPH) as a substrate to get further insights into the reactive in-
termediates and the mechanistic details of the catalytic reactions and
finally to examine the effect of substitution on the catalytic reactivity.
The spectral profile shown in Fig. 5 discloses the catalytic oxidation of
2-amino-5-methylphenol in aerobic condition leading to cumulative
increase of the product 2-amino-4,4a-dihydro-4a-7-dimethyl-3H-phe-
noxazine-3-one. Detailed kinetic analysis further gives us the values
K
M = (1.50
0.30) × 10−2 M Vmax = (8.58
1.21) × 10–7 M s−1
and kcat = 30.9 h−1
.
Similarly, the mass spectrum (Fig. S15) of the complex in presence
of substrate 2-amino-5-methylphenol is also quite informative. The base
peak at m/z = 124.07 is matched with the protonated species of sub-
strate 2-amino-5-methylphenol (calculated m/z = 124.08). A product
related peak was found at m/z = 243.09 (Fig. S15) as it matched with
the protonated species of the product 2-amino-4,4a-dihydro-4a-7-di-
methyl-3H-phenoxazine-3-one (calculated m/z = 243.11). Another
minor peak at m/z = 567.14 (Fig. S16) is also interesting as it agrees
well with the monocationic species of the complex-substrate adduct of
composition [Mn(L)(5-Me-OAP)(MeOH)(OH2)]+ (calculated m/
z = 567.18), which clearly justifies that effect of methyl substitution
does not inhibit the formation of a stable complex substrate adduct.
Isotopic distribution of key m/z values and their simulated pattern are
3.8.2. ESI mass spectral study
Mass spectrometry is considered one of the most useful technique as
that could provide significant information regarding the important in-
termediates of a chemical reaction from which one can frame the most
possible mechanistic pathway of a catalytic reaction. Accordingly, the
ESI (positive) mass spectra of compound 1 alone and in the presence of
excess OAPH after 15 min of mixing were recorded, and the spectra are
depicted in Figs. S12 and S13 (expanded view in Fig. S14), respectively.
In the mass spectrum of complex 1, the base peak at m/z = 395.04 is
nicely matched with [MnIII(L)]+ cationic species (calculated m/
z = 395.08).
When the mass spectrum was carried out in presence of excess
Fig. 5. UV–Vis spectral profile (left) and non-linear plot (right) of catalytic oxidation of 2-amino-5-methylphenol in dioxygen-saturated methanol at 25 °C catalysed
by complex 1.
6

S. Banerjee, et al.
InorganicaChimicaActa499(2020)119176
Scheme 2. Probable mechanistic pathway.
presented in supplementary section (Fig. S17(a)–S17(n)).
mass spectrometry study does not support the formation of any adduct
in which both the substrate and dioxygen simultaneously bonded to the
mental centre, therefore indicating that such an intermediate may not
be a probable one in the present system, although it has been observed
in many reported model complexes with different transition metals
[29h,58]. Now we are at the right position to propose the most possible
mechanism through which the present complex showing phenox-
azinone synthase activity. At the first step of which the metal centre
activates the dioxygen which thereafter oxidises Mn(II) to Mn(III) and
itself reduces to H2O. At the next step OAPH forms a complex-substrate
aggregate which thereafter through redox transformation generates o-
amninophenolate monoradical with regeneration of Mn(II)L. This OAP
radical might be converted into o-benzoquinone monoamine (BQMI) in
several ways including self-disproportionation reaction. Finally, 2-
aminophenoxazin-3-one is produced through several oxidative
3.8.3. Probable mechanism and catalytic efficiency
Both rate saturation kinetics and the mass spectrometry studies
suggest that the catalytic reactivity proceeds through the formation of a
stable complex-substrate aggregate. As the present manganese complex
is only four coordinated and thus substrate binding is quite probable. It
is well known that the oxidation reactions catalysed by the manganese
complexes proceed through the redox shuttling between the different
oxidation states of manganese especially in between Mn(III) and Mn(II)
[29h,57]. It is also obvious that the interactions of both the substrate
and molecular dioxygen with the metal centre are important steps in
the catalytic cycle exhibiting oxidase activity. From the mass spectro-
metry study, it is clear that both the dioxygen and substrate in-
dividually form stable adducts with the mental centres. However, the
7

S. Banerjee, et al.
InorganicaChimicaActa499(2020)119176
dehydrogenation processes involving OAPH, O2, and BQMI as shown in
Scheme 2.
step was blocked by the methyl substitution.
Remarkably, intermediate II-B, shown in Scheme 2, was eventually
detected in the mass spectrum of the complex in presence of excess
substrate, thereby validating the said scheme. Two steps in the catalytic
cycles i.e., dioxygen activation and the redox transformation of the
complex-substrate aggregate leading to the generation of the OAP ra-
dicals are deserved to be special mention as either of them could be the
rate limiting step of the catalytic cycle. The rate saturation kinetics with
the varying substrate concentrations clearly justifies the involvement of
the substrate in the rate determining step i.e., the later one. We should
keep in mind that the different aggregates identified in the ESI-MS
spectra may not necessarily mirror the real intermediate in the catalytic
cycle because such aggregates may be formed in situ because of the
relative stability when compared to the other cationic species in the
time scale of ESI-MS spectrum. Therefore, although we have not ob-
served any peak related to both substrate and dioxygen bound metal
complex in the ESI-MS spectrum, the existence of such intermediate
cannot be ruled out in the solution phase. In such a situation both
substrate and dioxygen bound metal complex could be involved in the
rate determining step. In order to get better insight into the mechanistic
pathway we have measured the reaction kinetics in an identical con-
dition but in different dioxygen concentrations. Remarkably, the initial
rate of the reactions of all the cases found comparable, which clearly
indicates that dioxygen does not involved in the rate determining step.
Now it become clear that dioxygen rapidly oxidizes Mn(II) to Mn(III) in
the catalytic cycle and itself reduces to water, and this process is so fast
that the formation of both the substrate and dioxygen bound metal
complex become least possible. A brief literature survey has been made
and presented in Table S4. Table S4 exemplifies the kcat values for the
oxidation of OAPH by earlier reported catalyst transition metal com-
plexes [29,59]. It clearly suggests that complex 1 exhibits moderate
catalytic activity towards OAPH oxidation, but the reactivity of the
present complex is somewhat lower than the recently published similar
Mn(III)-salen type complex [29h]. The lower redox flexibility of the
present complex as suggested by the cyclic voltammetric study, asso-
ciated with the requirement of high reorganization energy for the
structural change of the complex, is responsible for slightly lower ac-
tivity. Like o-aminophenol, the catalytic oxidation of 2-amino-5-me-
thylphenol proceeds through the identical pathway in which methyl
substitution does not inhibit the formation of a stable complex-substrate
aggregate as supported by the mass spectrometry and rate saturation
kinetics. Interestingly, the presence of methyl substation in the benzene
ring favours the substrate oxidation as indicated by the turn over
numbers and these results also support the intramolecular electron
transfer between the substrate and metal centre in the rate determining
step. Moreover, the methyl substitution does not inhibit the coupling of
two aminophenols, but the final dehydrogenation step was blocked by
the methyl substitution, leading to the formation of dihydro-phenox-
azinone chromophore as a final product (see Scheme 2).
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper
Acknowledgments
A. S. gratefully acknowledges the financial support of this work by
the DST, India (Sanction No. SB/FT/CS-102/2014, dated- 18.07.2015.
A. P. also gratefully acknowledges the financial support of this work by
the University Grant Commission, India (Sanction no. F.PSW-229/15-
16(ERO)).
Appendix A. Supplementary data
CCDC 1403382 contains the supplementary crystallographic data
for complex 1. This 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 email: deposit@ccdc.cam.ac.uk.
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.ica.2019.119176.
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9