Mn(III) and Cu(II) complexes of 1-((3-(dimethylamino)propylimino)methyl) naphthalen-2-ol): Synthesis, characterization, catecholase and phenoxazinone synthase activity and DFT-TDDFT study

Article abstract of DOI:10.1080/00958972.2018.1453065

Two new complexes, [MnL₂](ClO₄) (1) and [CuL₂] (2) (where LH?=?(E)-1-((3-(dimethylamino)propylimino)methyl)naphthalen-2-ol), have been synthesized and characterized by spectroscopic techniques and their molecular structures are established by single-crystal X-ray diffraction study. Complex 1 adopts an octahedral geometry around the central manganese atom which is in?+?3 oxidation state, whereas in complex 2, the Cu⁺² ion preferred a square pyramidal environment around it through the ligand donor atoms. Both complexes were tested for catecholase and phenoxazinone synthase activity. Complex 1 catalyzes the oxidation of 3,5-ditertiary-butyl catechol with a k_cat value of 6.8424?×?10²?h^?1 in acetonitrile whereas the same for complex 2 is 3.7485?×?10²?h^?1 in methanol. Phenoxazinone synthase activity was shown only by complex 2 having k_cat?=?74.225?h^?1. Structures of both the title complexes have been optimized by means of DFT calculations. Experimental electronic spectra of the complexes have been corroborated by TDDFT analysis. Electrochemical investigations by means of cyclic voltammetry have been carried out to study the electron transfer processes in the complexes.

Full text of DOI:10.1080/00958972.2018.1453065

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Mn(III) and Cu(II) complexes of 1-((3-
(dimethylamino)propylimino)methyl)
naphthalen-2-ol): Synthesis, characterization,
catecholase and phenoxazinone synthase activity
and DFT-TDDFT study
Swaraj Sengupta, Binitendra Naath Mongal, Suman Das, Tarun K. Panda,
Tarun K. Mandal, Michel Fleck, Shyamal K. Chattopadhyay & Subhendu
Naskar
To cite this article: Swaraj Sengupta, Binitendra Naath Mongal, Suman Das, Tarun K. Panda,
Tarun K. Mandal, Michel Fleck, Shyamal K. Chattopadhyay & Subhendu Naskar (2018): Mn(III)
and Cu(II) complexes of 1-((3-(dimethylamino)propylimino)methyl) naphthalen-2-ol): Synthesis,
characterization, catecholase and phenoxazinone synthase activity and DFT-TDDFT study, Journal
of Coordination Chemistry, DOI: 10.1080/00958972.2018.1453065
To link to this article: https://doi.org/10.1080/00958972.2018.1453065
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Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://doi.org/10.1080/00958972.2018.1453065
Mn(III) and Cu(II) complexes of 1-((3-(dimethylamino)propylimino)methyl)
naphthalen-2-ol): Synthesis, characterization, catecholase and phenoxazinone
synthase activity and DFT-TDDFT study
a,b
a
c
c
SWARAJ SENGUPTA , BINITENDRA NAATH MONGAL , SUMAN DAS , TARUN K. PANDA ,
d
e
b
a
TARUN K. MANDAL , MICHEL FLECK , SHYAMAL K. CHATTOPADHYAY and SUBHENDU NASKAR*
a
Department of Chemistry, Birla Institute of Technology, Mesra, Ranchi-835215, Jharkhand, India
b
Department of Chemistry, Indian Institute of Engineering Science and Technology-Shibpur, Howrah-711103, India
Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, Sangareddy 502 285, Telangana, India
c
d
e
Department of Biotechnology, Haldia Institute of Technology, Hatiberia, Haldia 721 657, West Bengal, India
Institute for Mineralogy and Crystallography, University of Vienna, Althanstr. 14, 1090 Vienna, Austria
Two new complexes, [MnL ](ClO ) (1) and [CuL ] (2) (where LH = (E)-1-((3-
2
4
2
(dimethylamino)propylimino)methyl)naphthalen-2-ol), have been synthesized and characterized
by spectroscopic techniques and their molecular structures are established by single-crystal
X-ray diffraction study. Complex 1 adopts an octahedral geometry around the central manganese
+
2
atom which is in +3 oxidation state, whereas in complex 2, the Cu ion preferred a square
pyramidal environment around it through the ligand donor atoms. Both complexes were tested
for catecholase and phenoxazinone synthase activity. Complex 1 catalyzes the oxidation of 3,5-
2
-1
ditertiary-butyl catechol with a k value of 6.8424×10 h in acetonitrile whereas the same for
cat
2
-1
complex 2 is 3.7485×10 h in methanol. Phenoxazinone synthase activity was shown only by
-1
complex 2 having k = 74.225 h . Structures of both the title complexes have been optimized
cat
by means of DFT calculations. Experimental electronic spectra of the complexes have been
corroborated by TDDFT analysis. Electrochemical investigations by means of cyclic
voltammetry have been carried out to study the electron transfer processes in the complexes.
Keywords: Metal complex; Catecholase; Phenoxazinone synthase; DFT
1
. Introduction
*
Corresponding author. Email: subnaskar@yahoo.co.in
1

Oxidation is a fundamental reaction in organic transformation. Molecular oxygen has been
employed as a popular oxidizing agent for its availability and environmentally benign nature. But
activation of molecular oxygen is a major challenge due to its kinetic inertness. This kinetic
inertness arises due to the mismatch of the spin state of the reactants in ground state: O has spin
2
triplet with two unpaired electrons whereas typical organic substrates are singlet with no
unpaired electrons. However nature has evolved an excellent solution to overcome this obstacle
by incorporating metal ions in the protein chain in the form of metalloenzymes which provide
suitable alternative pathway and reduces the energy barrier. Metalloenzymes use various types of
active sites [1], e.g. heme and non-heme iron site, heteronuclear iron-copper site or mono- and
binuclear copper site for dioxygen activation. Oxidation of alcohol to carbonyl functionality is an
important class of oxidative transformation [2].
Catechol oxidase (CO) is a type-III copper protein which catalyzes the oxidation of
o-diphenol to corresponding quinone (scheme 1). Highly reactive quinone then undergoes auto-
polymerization forming melanin. In this way, CO protects damaged tissues of plants against
pathogens or insects.
Klabunde et al. [3] determined the X-ray crystal structure of CO from sweet potato
II
II
I
I
(
Ipomoea batatas) both in oxidized Cu -OH-Cu and reduced Cu -OH-Cu form. Both contain a
binuclear hydroxo-bridged Cu site, each copper being coordinated by three histidine nitrogen
2
atoms. Quite naturally large number of binuclear copper complexes [4] with similar ligand
environment have been synthesized and screened for the catecholase activity for the elucidation
of the catalytic mechanism. Besides, other metals, e.g. Mn- and Co-based models, have also been
reported with comparable catecholase activity to Cu-based mimics.
Details of catecholase activity have been described in earlier reviews [5]. Recently,
Mukherjee et al. published an excellent review article [6] describing different mechanisms for
catecholase activity proposed to date where the authors deal the active site with a number of
metals, e.g. Cu, Co and Mn. For binuclear Cu mimics, a catecholato-bridged intermediate is
II
II
formed prior to the formation of quinine [7]. For mononuclear Cu complex, chelated Cu -
I
DTBC (3,5 DTBCH = 3,5-ditertiary-butyl catechol) is converted to a Cu -semiquinone
2
intermediate which reacts with molecular oxygen to form superoxo-complex. The latter then
II
releases DTBQ (3,5-ditertiary-butyl quinone) and H O [8]. Among other metals, Mn mimics
2
2
have a comparable catecholase activity to that of dicopper system. This may be due to the multi-
2

electron transfer capability of manganese which can easily change the oxidation state between
II
IV
Mn and Mn . Recently progress in development of nickel- and zinc-based mimics for CO has
been started. Das et al. [9] and Ghosh et al. [10] reported binuclear nickel complexes with quite
high k value.
cat
Phenoxazinone synthase (PHS) is another kind of multicopper oxidase [11] which
catalyzes the oxidative condensation of two molecules of 3-hydroxy-4-methylanthranilic acid
pentapeptide lactone, the last step in the biosynthesis of the antibiotic actinomycin D (scheme 1).
This reaction produces the phenoxazinone chromophore. The reaction proceeds via three
consecutive two-electron oxidation processes which cumulatively lead to six-electron oxidation
process. The enzyme PHS was first isolated in 1962 by Katz and Weissbach from Streptomyces
antibiotus [12]. However, the structure was solved only in 2006 by Allen and Francisco [13].
PHS exists in two oligomeric forms with varied catalytic activity; low-active dimer and hexamer
of high activity. The hexamer contains five copper atoms: one type 1, two type 2 and one
binuclear type 3 centers. Biomimetic model systems of PHS with copper were reported by
different groups: Speier and co-workers [14], Halligudi et al. [15] and Chaudhuri et al. [16].
Although not observed in nature, manganese- [17] and cobalt-based [18] mimics have been
successfully employed as potential candidates to show PHS activity.
OH
OH
O
O
Catecholase
+
O2
O
N
NH2
O
Phenoxazinone Synthase
+
O2
NH2
OH
Scheme 1. Catecholase and phenoxazinone synthase activity.
Researchers have tried to get the mimics not only to understand the underlying mechanism of the
oxidation process but also to enhance the catalytic activity of the synthetic mimics for the
industrial application.
In this work, we demonstrate the synthesis and characterization of a Mn(III) and a Cu(II)
3

complex of a Schiff base ligand derived from the condensation of 2-hydroxyl-naphthaldehyde
and N,N-dimethylpropanediamine (scheme 2). Details of spectroscopic and single-crystal X-ray
crystallographic characterization, electrochemical and magnetic measurements along with
catalytic study for the catecholase and phenoxazinone synthase-like activity using these
complexes have also been reported. Although the copper [19] and nickel [20] complexes with the
ligand have been reported earlier by different groups, nobody has studied the biological
oxidation catalysis with the reported molecules.
H C
+
3
N
NH2
OH
MeOH
OH
CH3
Reflux, 5 Hr
O
N
MnL ClO
2
4
M(ClO ) . 6 H O + 2 HL
4
2
2
CH3
HL
N
M = Mn and Cu
MeOH
Reflux, 6 Hr
CuL2
CH3
+
N
O
O
N
N
N
CuII
MnIII
O
N
N
N
N
O
Scheme 2. Synthesis of the ligand and complexes.
2
2
. Experimental
.1. Materials
N,N-Dimethyltrimethylenediamine, 2-hydroxy-1-naphthaldehyde and 3,5-di-tert-butyl catechol
were purchased from Acros Organics and used as received. For spectrochemical and
electrochemical analysis HPLC grade solvents from Rankem were used. Manganese and copper
perchlorate salts were synthesized from respective metal carbonates in laboratory according to
the procedure reported in literature [21]. Tetraethylammonium perchlorate (TEAP) used for the
4

electrochemical studies was prepared according to the literature procedure [22].
Caution: Perchlorate salts of the metals are potentially explosive and must be handled
with care. In present work no problem was faced.
2
.2. Physical measurements
Infrared (IR) spectra of the ligand and the complexes were recorded on model IR prestige 21,
-1
-1
SHIMADZU Corporation (Japan) between 400 cm to 4000 cm in KBr pellet. UV-vis spectra
were recorded in a PERKIN-ELMER (LAMBDA-750) UV/VIS/NIR spectrophotometer in the
1
range 250-1800 nm. H-NMR spectra of the ligand were recorded in a JEOL JNM-ECZ400S/L1
spectrometer. Cyclic voltammograms and differential pulse voltammograms were recorded in
CHI6003E Potentiostat in a typical three-electrode system: Pt or glassy carbon working
electrode, Ag/AgNO reference electrode and Pt counter electrode. TEAP was used as supporting
3
0
electrolyte. Ferrocene/ferrocenium couple was observed at E (∆E ) = 0.4 V (100 mV) under
p
these experimental conditions.
2
.3. Procedure for the oxidation of 3,5-di-tertiary-butyl catechol (3,5-DTBC) and
determination of the rate of formation of 3,5-di-tertiary-butyl quinone (3,5-DTBQ)
-5
In the kinetic investigation air saturated 10 mL methanolic solution of 1×10 M of complex was
-3
prepared and then the solution was treated with 1×10 M of substrate 3,5-DTBC. 2 mL solution
of catalyst was taken in a quartz cell and externally 100 eq of substrate was added to the cell
solution. The investigation was then followed by UV-vis spectral scan at the interval of every
1
0 min up to 3 h. Catalytic study of complexes 1 and 2 was studied in methanol and acetonitrile
solvent. In both acetonitrile and methanol, the absorbance was monitored at 400 nm (ε =
-1
-1
1
2
900 M cm ) as reported by Krebs et al. [23]. The kinetic measurements were performed at
5 C. Time-dependent spectral profiles for the catalytic study by complex 1 are shown in
figure 1 (in acetonitrile) and figure S11 (in methanol), while for complex 2 the same profiles are
shown in figure S13 (in methanol) and figure S15 (in acetonitrile), respectively. Same
experiments performed without catalyst under identical conditions did not show any significant
growth of the band at respective positions.
For the detection of hydrogen peroxide during catalysis, a fresh set of reaction mixture
-5
-5
was prepared by mixing 10 mL (50×10 M) of catechol and 10 mL (2.5×10 M) of the
5

complexes. The reaction mixture was stirred at 25 °C. After 1 h, pH of the solution was adjusted
to 2 by adding H SO and an equal volume of water was added to stop further oxidation. The
2
4
quinone formed was extracted with dichloromethane followed by the addition of 1 mL of 10%
potassium iodide solution and 4 drops of 3% ammonium molybdate solution to the aqueous
-
layer. Formation of I was monitored spectrophotometrically by measuring the intensity of the
3
-1
-1
characteristic band at 353 nm (ε = 26,000 M cm ).
2
.4. Procedure for the oxidation of 2-amino phenol and determination of the rate of formation
of phenoxazinone chromophore
In this kinetic investigation, same procedure is applied. Here 10 mL methanol solution of
-5
-2
1
×10 M of complex 2 was prepared and then the solution was treated with 1×10 M of
substrate: o-amino phenol (OAPH). The UV-vis spectral total scan time was 3 h with intervals of
-1
-1
1
0 min. In MeOH, the absorbance was monitored at 430 nm (ε = 9095 M cm ) following earlier
report [24]. The kinetic measurements were performed at 25 C. Time-dependent spectral
profiles for the catalytic study by complex are shown in figure 2. Similar experiment performed
without catalyst under identical conditions did not show any significant growth of the absorption
band at respective positions.
2
.5. Synthesis of the ligand (E)-1-((2-(dimethylamino)ethylimino)methyl)naphthalene-2-ol
LH)
In a round bottomed flask, 0.688 g (4 mmol) of 2-hydroxy-napthaldehyde was dissolved in
(
2
2
5 mL of methanol. Another solution of 0.408 g (4 mmol) of N,N-dimethylpropanediamine in
5 mL methanol was added dropwise to the former. The solution mixture turned from pink to
light yellow. Then the reaction mixture was refluxed for 5 h with constant stirring. The resulting
solution was subjected to evaporation under vacuum to give a deep brown viscous liquid. Yield-
crude: 73.5%. The liquid mass was purified in silica using 1:1 hexane-CH Cl mixture as eluent.
2
2
1
Yield-pure: 50.5%. NMR ( H, 400 MHz, CDCl ), δ (ppm): 8.1-7.0 (6H, ArH), 6.1 (1H, ArCH),
3
1
3
5
1
1
.0 (s, 1H, O-H), 2.0-2.1 (6H, CH ), 1.7 (m, 6H, CH ). NMR ( C, 100 MHz, CDCl ) δ (ppm):
2 3 3
35.0 (C ), 132.7 (C ), 132.2 (C ), 131.6 (C ), 130.4 (C ), 130.0 (C ), 129.58 (C ), 129.4 (C ),
a
k
j
g
i
d
f
e
29.1 (C ), 124.7 (C ), 56.8 (C ), 37.1 (C ), 32.7 (C ), 30.9 (C ), 29.7 (C ). Notation for NMR is
c
b
l
n
o
p
m
-1
given in figure S1. FTIR (KBr) (vmax/cm ): 3414(s, br OH), 2943 (s board, C-H of -N(CH ),
3
6

-1
-1
1
3
635 (m, C=N), 1546 (w, C –O), 1041 (s, C-N). MS: m/z 256 {LH}. λ /nm (ε/M cm ):
ar
max
05(5680), 399(4312), 418(4443).
2
.6. Synthesis of the complexes
Complex 1, [MnL ](ClO ): In a round bottomed flask, 0.502 g ligand LH (2 mmol) was
2
4
dissolved in 25 mL of methanol with the subsequent addition of 0.202 g (2 mmol) of Et N. Then
3
a methanolic solution of 0.363 g (1 mmol) Mn(ClO ) was added dropwise into the ligand
4
2
solution. The mixture was refluxed for another 6 h with constant stirring. A deep-brown solid
appeared which was filtered and washed with methanol (5 mL) and diethyl ether (5 mL). Yield:
3
48 mg (52.4%). X-ray quality single crystals were obtained by evaporation of acetronitrile
+
-1`
solution of complex 1. ESI-MS(+ve) (m/z): 565.4 ([MnL ] ); FTIR (KBr, v/cm ): 2993(w, br),
2
1
7
597(s) (νC=N), 1539(m) (νAr-C=C), 1454(m), 1336(m), 1255(w), 1067(s), 983(w), 833(w),
59(m); Anal. Calc. for C H N O ClMn: C, 57.83; H, 5.72; N, 8.43. Found: C, 57.56; H, 5.44;
3
2
38
4
6
-1
-1
N, 8.76. λmax/nm (ε/M cm ): 314 (9263), 336 (9166), 454 (1913), 531 (392), 575 (329).
Complex 2, [CuL ]: The complex was prepared in similar method described as that of
2
complex 1 using Cu(ClO ) ·6H O as the metal precursor. In this case, deep-green crystalline
4
2
2
solid was obtained. Yield: 317 mg (55.23%). Single-crystals were obtained from the evaporation
+
of a methanol solution of the complex. ESI-MS(+ve) (m/z): 672.70 ([Cu(LH) ClO ] ); FTIR
2
4
-1`
(
KBr, v/cm ): 2926(w, br), 1614(s) (νC=N), 1532(w), 1462(m), 1369(m), 1255(w), 1184(w),
1
9
060(w), 968(w), 827(w), 750(m), 489(w); Anal. Calc. for C H N O Cu: C, 66.89; H, 6.62; N,
32 38 4 2
-1
-1
.75. Found: C, 67.12; H, 6.47; N, 9.44. λmax/nm (ε/M cm ): 323 (11423), 384 (4912), 402
(4287), 611 (155).
2
.7. X ray crystallography
Single-crystals of complexes 1 and 2 were obtained from the evaporation of acetonitrile and
methanol solutions, respectively. In each case, a crystal of suitable dimensions was mounted on a
CryoLoop (Hampton Research Corp.) with a layer of light mineral oil and placed at 200 (for 1)
and 293 (for 2) K. The measurements were made either on an Enraf Nonius Kappa CCD
diffractometer with Mo-Kα (0.71073 Å for 1) or an Agilent Supernova X-calibur Eos CCD
detector with graphite-monochromatic Cu-Kα (1.54184 Å, for 2) radiation. Crystal data and
structure refinement parameters are summarized in table 1. The structures were solved by direct
7

2
methods (SIR2004) [25] and refined on F using the full-matrix least-squares method, using
SHELXL-97 [26]. Non-hydrogen atoms were anisotropically refined. Hydrogens were included
in the refinement on calculated positions riding on their carrier atoms. The function minimized
2
2 2
2
2
2
2
2
was [Σw(Fo -Fc ) ] (w = 1 / [σ (Fo ) + (aP) + bP]), where P = (Max(Fo ,0) + 2Fc ) / 3 with
2
2
σ (Fo ) from counting statistics. The function R1 and wR2 were (Σ||Fo|-|Fc||) / Σ|Fo| and
2
2 2
4
[
Σw(Fo -Fc ) / Σ(wFo )]1/2, respectively. Crystallographic data (excluding structure factors) for
the structures reported in this article have been deposited with the Cambridge Crystallographic
Data Centre as supplementary publication nos. CCDC 1561130 (1) and 1561131 (2). Copies of
the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK (Fax: + (44)1223-336-033; E-mail: deposit@ccdc.cam.ac.uk).
2
.8. Computational details
The calculations were performed by density functional theory (DFT) implemented in
GAUSSIAN09 [27]. The ligand and the complexes were optimized using B3LYP hybrid
functional and 6-31g(d) basis set for hydrogen, carbon, oxygen and nitrogen and LANL2DZ
basis set for metals. The electronic spectra were simulated by Time-Dependent Density
Functional Theory (TD-DFT). Computational studies were carried out in DMSO solvent using
Polarizable Continuum Model (PCM) implemented in Gaussian 09.
3
3
. Results and discussion
.1. Synthesis
The ligand was synthesized by simple Schiff base condensation reaction. The ligand was liquid
oil at room temperature and the crude product contained starting materials, particularly the
amine. The pure compound was obtained from a 1:1 hexane:dichloromethane mixture in silica
column. Both complexes 1 and 2 were obtained in good yield from the reaction between
respective metal perchlorates and the protic ligand in the presence of triethyl amine under
refluxing condition in methanol.
3
.2. Description of the X-ray crystal structures
Representative molecular structures along with the atom numbering scheme for complexes 1 and
are depicted in figure 3.
2
8

[
MnL ](ClO ): Complex 1 crystallizes in P2 /n space group. Although the crystal
2 4 1
structure gives a formula of [(L )Mn] (ClO )(MeCN), which implies a mixed-valent Mn(II/III)
2
2
4
formulation analysis of the Mn-O and Mn-N bond lengths suggests the metal to be present in +3
oxidation state. This is further supported by the fact that the structures of all four cations are
essentially identical. Despite several attempts, we could not grow good-quality single-crystals
which led us to report the present crystal structure for this molecule. The ligand coordinates to
the metal in a tridentate facial manner through phenolic-O, imine-N and amine-N atoms. Thus,
two ligands fulfill the coordination sphere leading to an octahedral geometry around the metal
center. In the unit cell, there are four different Mn-residues, besides there are disordered
perchlorate ions and MeCN molecule. For any given Mn-residue, half of the molecule is
symmetric with the other half. In the unit cell, the MnL moiety has occupancy 0.5, whereas
2
perchlorate anions have full occupancy. As the ligand is mono-deprotonated, the charge of
III
III
manganese can be assigned to +3. The Mn -O and Mn -N bond lengths are comparable to a
previous report [28]. The equatorial plane is formed by the imine nitrogen atoms (N1A and
NiAa) and phenolic oxygen atoms (O1A and O1Aa), whereas the axial sites are occupied by the
amine nitrogen atoms (N2A and N2Aa). Equatorial Mn-O and Mn-N distances are 1.858 Å and
2
.02 Å, respectively. Axial Mn-N distances (2.45 Å) are much longer than the equatorial
distances, indicating the presence of strong Jahn-Teller elongation along the Mn-Nimine bonds.
Except two cis angles (80.91°) N1A-Mn1A-N2A and N1A_a-Mn1A-N2A_a, all the cis and
-3
trans angles are very close to the ideal values. High electron density of 1.79 e A in complex 1 is
due to perchlorate ion which is loosely bound to the crystal structure. The percholate ion in
complex 1 is only loosely bound in the crystal structure. Thus, the oxygen atoms of this ion show
strong displacement with high residual electron density in their close environment.
[
CuL ]: Green-colored complex 2 crystallizes in monoclinic P2 /c (No. 14) space group
2
1
having four molecules in the unit cell. Complex 2 possesses an interesting geometrical feature.
Considering the usual Cu-N and Cu-O covalent bond distances, the molecule adopts distorted
square planar geometry. All cis angles are very close to 90 (89.02° - 91.16°). One trans angle
O1-Cu1-O2 (162.74°) deviates from the standard value whereas the other (N1-Cu1-N3 =
1
78.90°) is in close proximity to 180°. Value of four-coordinate geometry index τ (τ = [360 -
4
4
(α+β)]/141, α and β are two largest angles) for complex 2 is 0.13. This suggests the structure is
close to perfect square planar (For perfect geometry: τ = 0 for square planar and 1 for
4
9

tetrahedral). However this description is not absolutely correct if we consider the terminal -NMe2
groups of the ligands. C-C bonds connecting the imine and amine functionality of the ligands
rotate in such a way that one -NMe group comes very close to the Cu center while the other
2
moves apart a large distance from Cu. Therefore, the former -NMe group can interact with
2
metal. The Cu1-N4 bond distance (2.511 Å) is longer than the covalent Cu-N distance but
shorter than the sum of their van der Waals radii. The N atom is also placed in such a position
that it can provide a square pyramidal ligand environment around Cu. In this second type of
geometry, all the bonds (viz., Cu-N and Cu-O) are similar to the literature except Cu-N4, the
axial bond which is longer to some extent. All the cis angles containing the axial Cu1-N4 bond
deviate from regular value of 90°. The values are in the range 82.73° to 105.22°.
3
.3. Electronic spectra
Electronic spectra of both complexes were measured in acetonitrile medium. Complex 1 displays
-1
-1
two high-energy ligand-based transition bands at 314 nm (ε = 9263 M cm ) and 336 nm (ε =
-
1
-1
9
1
166 M cm ) (figure 4a). One band of moderate intensity is observed at 454 nm (ε =
-1
-1
913 M cm ). This latter band is attributable to LMCT transition presumably occurring as
III
phenolic-O → Mn ion. In the low-energy region, low-intensity bands are observed at 531 nm
-1
-1
-1
-1
(
ε = 392 M cm ) and 575 nm (ε = 329 M cm ). Absorptivity at these band positions and
comparison with the existing report [29] clearly suggests ligand-field transitions. Comparatively
high value of extinction coefficient may be due to intensity borrowing from the nearby allowed
LMCT transitions. Presence of two such d-d bands clearly indicates the existence of fairly strong
tetragonal distortion in the octahedral ligand field.
In case of complex 2, the ligand-centered transition band is observed at 323 nm (ε =
-1
-1
-1
-1
1
1423 M cm ). Absorptions of medium intensity are observed at 384 nm (ε = 4912 M cm )
-1
-1
and 402 nm (ε = 4287 M cm ) (figure 4b). These two bands are assigned to be phenolic-O to
II
-1
-1
Cu transitions. The low energy d-d transition band is observed at 611 nm (ε = 155 M cm ).
All these transitions observed are in accordance to the earlier reports [30].
3
.4. FTIR spectra
FTIR spectra of the ligand and the complexes are given in SI (figures S7-S9). FTIR spectrum of
-1
the ligand shows a strong broad band at 3414 cm attributed to the stretching of the O-H bond.
1
0

-1
Another strong band at 2943 cm may be due to stretching of the C-H bond of -N(CH ) .
3
2
-1
Stretching of the C=N bond is indicated by a moderately strong band at 1635 cm . In both
-1
complexes, the broad band at 3414 cm in the ligand is completely vanished, indicating
deprotonation of the -OH functionality during coordination to the metals. Moreover, the νC=N
-1
-1
band in the ligand at 1635 cm shifts to lower energy in both complexes at 1597 cm and
-1
1
614 cm for 1 and 2, respectively. This also supports the coordination through the imine-N
atom in the ligand.
3
.5. Magnetic properties
Experimentally observed magnetic moment of complex 1 is 4.878 BM. It suggests the presence
4
of four unpaired electrons usually obtained in the case of high spin d electronic configuration of
Mn(III) ion in complex 1. Magnetic moment for complex 2 was 1.8386 BM. This also confirms a
9
Cu(II) ion with d electronic configuration in complex 2.
3
.6. DFT Study
Quantum chemical calculation using DFT with the B3LYP hybrid functional was performed to
get the optimized geometry of the ligand and complexes 1 and 2 in acetonitrile. Table 2
summarizes the energy and composition of the frontier molecular orbitals of the complexes while
the metrical parameters of the optimized geometry and crystal geometry for both 1 and 2 have
been compared in tables 3 and 4, respectively. Similarity among the parameters for crystal and
optimized structure suggests that the optimization process was suitable for this type of
molecules. Composition of frontier molecular orbitals for complex 1 shows that HOMO-3,
LUMO and LUMO+3 possess major contribution from Mn, HOMO-2 is equally contributed by
Mn and the ligand while HOMO-1, HOMO, LUMO+1 and LUMO+2 are majorly occupied by
ligand electrons. In complex 2, only LUMO has significant contribution of metal copper, while
the rest of the MOs in the range HOMO-3 to LUMO+3 are exclusively contributed by the
ligands. Optimized structures of the complexes are displayed in figure S18. According to single-
crystal X-ray study, complex 2 can possess either the square planar or the square pyramidal
geometry. Therefore both structures were optimized which shows that the square pyramid is
slightly more stable than the other geometry.
1
1

3
.7. TDDFT Analysis
UV-visible spectra of the complexes are well reproduced with TDDFT analysis. Vertical
transitions are shown in figure 5 (complex 1) and figure 6 (complex 2), while the band
assignments are displayed in table 5.
In complex 1, high-energy intraligand transition band at 313 nm in the experimental
spectrum is observed at 304 nm in the calculated spectrum. The transition band at 338 nm
corresponds to the transition at 348 nm in the theoretical spectrum. Analyzing the composition of
the participating orbitals, this transition may be interpreted as mixed intra-ligand and MLCT
nature. Experimental band at 454 nm is observed at 460 nm in the simulated spectrum and
corresponds to HOMO-4 → LUMO transition. This transition band is mostly of LMCT nature.
There are two transition bands in the experimental spectrum at 531 nm and 575 nm, absorptivity
of which shows to be d-d transitions. A careful inspection in the calculated spectrum enabled us
to find those d-d transitions at 524 nm (HOMO-3→LUMO) and 623 nm
(
HOMO-3→LUMO+3). Although there is some ligand-character, these last two transitions are
mostly of d-d nature.
In complex 2 the intraligand transition band at 310 nm is observed at 288 nm in the
calculated spectrum and corresponds to the HOMO(B)→LUMO+3(B) excitation. Two LMCT
transition bands at 384 nm and 400 nm appear at 377 nm (HOMO-6(B)→LUMO(B)) and
3
6
94 nm (HOMO-3(B) →LUMO(B)) in the theoretical spectrum. The d-d transition band at
10 nm in the experimental spectrum is also supported by the simulated spectrum at 538 nm
(HOMO-14(B)→LUMO(B)), where significant d-d nature is observed along with the LMCT
character.
3
.8. Catecholase activity by complexes 1 and 2
For the kinetic study, initial rate method was followed. To determine the dependence of rate on
-5
the substrate concentration, solution of fixed concentration (1×10 M) of catalyst (complex 1 or
complex 2) was treated with increasing concentration of substrate 3,5-DTBCH (20 to 200 eq) in
2
a pseudo-first order environment under aerobic condition at 25 °C. Time-dependent spectral
profiles are displayed in figure 1 (1 in MeCN), figure S11 (1 in MeOH), figure S13 (2 in MeOH)
and figure S15 (2 in MeCN). The rate constant of catalyst-substrate mixture was determined
from the difference in absorbance with increasing time. A linear relationship was obtained
1
2

between the differences in absorbance versus time plot. The slope of initial rate of reaction
versus concentration of substrate plot shows a rate-saturation kinetic (figure 7a for 1 in MeCN,
figure S12a for 1 in MeOH and figure S14a for 2 in MeOH). The dependence of the saturation
rate on the concentration of substrate can be treated with the Michaelis-Menten approach. The
treatment on the basis of the Michaelis-Menten approach was applied by means of Lineweaver-
Burk plot (figure 7b for 1 in MeCN, figure S12b for 1 in MeOH and figure S14b for 2 in MeOH)
to calculate all the kinetic parameters such as K , V and K . Kinetic data are given in table 6.
M
max
cat
The turn-over number (TON) of the complex was calculated by dividing the value of Vmax by the
complex concentration. From the experimental data, Michaelis binding constant (K ) were
m
-5
-5
-2
2
.13×10 M and 2.53×10 M for 1 in acetonitrile and methanol, respectively, and 9.87×10 M
for 2 in methanol. However, 1 shows better catalytic activity than 2. Complex 1 has k value of
cat
-1
-1
6
84.24 h in acetonitrile and 547.45 h in MeOH, whereas 2 has slightly lower k value
cat
-1
(
374.85 h ) in MeOH.
There are two different mechanisms for the catecholase activity where H O and H O are
2
2
2
formed as by-products. To get an idea about the possible pathway, the catalytic reaction was
performed in the presence of potassium iodide. The procedure has been explained in the
-
Experimental section. Production of hydrogen peroxide was confirmed by the oxidation of I to
-
I ; the latter was detected by the appearance of peak at 353 nm in the UV-vis spectrum.
3
Formation of hydrogen peroxide was detected for both complexes 1 (figure 8) and 2 (figure S16).
Therefore, the results obtained may be summarized as: complex 1 has less activity than
complex 2 in acetonitrile and methanol. Further, complex 2 is almost inactive in acetonitrile. A
possible reason for the lower catalytic activity of complex 2 may be due to its structure. As a
mimic of natural catecholase enzyme, binuclear copper complexes usually exhibit high turnover
number following the mechanism (3,7) which includes the formation of catecholase-bridged
binuclear copper core. Complex 2 is mononuclear in nature with an almost square planar
geometry that cannot stabilize the catecholase-bridged intermadiates. Still complex 2 has higher
activity than the other mononuclear copper complexes reported earlier [31, 32]. Inactivity of
I
complex 2 in acetonitrile may be due to the stabilization of the reduced Cu species by
acetonitrile which restricts the vacant axial sides to make available to the substrate. This type of
I
stabilization of Cu species has also been reported by other groups [33]. Although manganese is
not the part of the native catecholase enzyme, manganese complexes in different oxidation states
1
3

of the metal show excellent catalytic catecholase activity due to its large redox window. In the
III
present case, the higher activity of Mn species (complex 1) may also be due to its
electrochemical properties. MnIII/II couple (complex 1) is observed at higher potential than the
II/I
Cu (complex 2) couple, i.e. formation of the reduced species is more feasible in complex 1.
3
.9. Phenoxazinone synthase like activity by complex 2
For this kinetic study, initial rate method was also followed using o-amino phenol (OAPH) as
substrate. Both complexes 1 and 2 were employed as catalysts. Only complex 2 exhibited
significant catalytic activity whereas complex 1 was inactive. Time dependent spectral profile
(figure 2), rate saturation kinetics (figure 9a) and Lineweaver-Burk plot (figure 9b) for the
oxidation of o-aminophenol are shown below. The TON of the complex was calculated by
-4
-1
dividing the value of Vmax (Vmax = 8.84×10 M h ) by the complex concentration. From the
-4
experimental data, Michaelis binding constant (K ) was 8.02×10 M and k value was
m
cat
-1
7
4.23 h .
In order to get an idea about the substrate-catalyst interaction we study the possible
complex-substrate intermediates by monitoring the ESI-MS spectra of both complexes mixed
with the respective substrates, i.e. 3,5-DTBCH and 2-aminophenol in 1:100 ratio. The spectrum
2
was recorded within 15 min of mixing in methanol. After addition of 3,5-DTBCH to the
2
solutions of complex 1 (figure 10), some new peaks were generated. The new peaks can be
III
II
+
III
+
assigned as m/z 530.25 [Mn Mn L(CH O)(ClO )(Cl)] , m/z 583.13 [Mn L H O] , m/z 682.29
3
4
2
2
III
II
+
III
III
III
III
[
Mn Mn L (CH O) ] , m/z 716.30 [Mn Mn L (CH O) (Cl)], m/z 773.02 [Mn Mn L(3,5-
2 3 2 2 3 2
+
III
III
+ +
DTBC-H)(ClO )(Cl) (H O)] , m/z 919.75 [Mn Mn L(3,5-DTBC) (HO)(OOH)+2Na ] , m/z
4
2
2
2
III
II
+
III
III
9
58.80 [Mn Mn L (3,5-DTBC-H)(ClO )(H O)] and m/z 1083.09 [Mn Mn L(3,5-
2 4 2
+
DTBC-H) +Na ].
2
For complex 2 and 3,5-DTBCH mixture, the peaks in the mass spectrum (figure S17)
2
II
I
+ +
II
can be correlated as m/z 538.08 [Cu Cu L(ClO )(Cl)+Na ] , m/z 751.83 [Cu L(3,5-DTBC-
4
2
+
I
+ +
H)(CH OH)(H O)(ClO )] , m/z 781.00 [Cu L(3,5-DTBCH )(ClO )(OOH)+2Na ] , m/z 816.97
3
2
4
2
2
4
II
+ +
II
[
Cu L(3,5-DTBC-H)(ClO )(Cl) +2Na ] , m/z 849.11 [Cu L(3,5-DTBC-
2
4
2
2
+
+
II
+
H)(Cl)(CH OH)(ClO )+2Na ] , m/z 856.82 [Cu L (3,5-DTBC-H)] , m/z 1035.96
3
4
2
2
II
I
+ +
II
I
+ +
[
Cu Cu L (3,5-DTBC-H)(Cl)(ClO )+2Na ] , m/z 1101.01 [Cu Cu L (3,5-DTBC-H) +Na ] and
2
4
2
2
II
I
+ +
m/z 1134.01 [Cu Cu L (3,5-DTBC-H)(3,5-DTBC-H )(CH OH)+ Na ] .
2
2
3
1
4

Analysis of the above mentioned peaks suggests, although the catalyst is a mononuclear
copper/manganese complex, in presence of substrate, the former undergoes the dissociation of
+
one ligand to form the species ML which further combines with other
ligands/substrates/solvents etc. in a number of ways, mostly forming a binuclear species. Here
we have been successful in interpreting almost all the peaks produced during the catalytic
reaction. In this ESI-MS study, the most significant peak at m/z 781.00 suggests the formation of
an intermediate having a hydroperoxo species that indicates the production of hydrogen peroxide
during the reaction. Production of hydrogen peroxide has also been observed experimentally.
However the low rate in comparison to the binuclear species is also expected as the distance
between the copper centers is not fixed like binuclear complexes.
The same procedure is applied for the study of phenoxazinone synthase-like activity by
complex 2. ESIMS(+ve) spectrum of the methanol solution of the mixture of 2-amino phenol and
complex 2 (figure S17) was recorded after 15 min of mixing. Three major peaks obtained in this
+
case indicate formation of the intermediates as mentioned here: m/z 744.12 [Cu L (OAP)] , m/z
2
2
II
+
II
II
+ +
8
75.11 [Cu L (OAP) +Na ] and m/z 1024.03 [Cu Cu L (OAP) (H O)+2Na ] . Just like
2 2 2 2 3 2
catecholase activity, here we also found peaks consisting of binuclear species.
3
.10. Electrochemistry
Cyclic voltammograms (figure 11) were measured in acetonitrile medium for ligand and
complexes at ambient temperature using TEAP as supporting electrolyte. The measured
+
potentials were referenced versus ferrocinium/ferrocene (Fc /Fc) couple. Both complexes as well
as the ligand show one quasi-reversible peak on cathodic scan in the range of -1.00 to-1.22 V.
Complex 1 shows one more irreversible peak at -0.44 V, which may be due to reduction of
Mn(III) to Mn(II). One irreversible cathodic wave at -0.68 V displayed by complex 2 may be due
to Cu(II)/Cu(I) reduction. Scanning on the anodic side shows a peak at 0.72 V for ligand and at
0
.72 V and 0.66 V for complex 1 and complex 2, respectively, which may be due to the
oxidation of phenolic -OH to the phenoxyl radical. In the anodic scan, the reversible anodic wave
at 1.525 V depicts MnIII/IV couple. Complex 2 on the anodic side shows another peak at 1.02 V,
which may be the phenolic -OH to the phenoxyl radical oxidation of the second ligand.
4
. Conclusion
1
5

Two mononuclear Schiff base complexes of manganese and copper have been synthesized and
well characterized by spectroscopy and single-crystal X-ray diffraction techniques. Both
complexes are active in oxidizing catechol with a moderate value of k . Possible intermediates
cat
in the catalytic reaction have been interpreted on the basis of the ESI-MS techniques. Formation
of the hydroperoxo species has been manifested from the analysis of the mass spectroscopy data.
Production of hydrogen peroxide has also been monitored during the catalysis by iodometric
titration. Copper complex is active for phenoxazinone synthase activity. In this case, we were
also able to explain the observed peaks in the mass spectrum. All the processes show the
involvement of several binuclear species in the catalytic cycle.
Supplementary data accompanies the on-line manuscript.
Acknowledgements
S. N. thanks CSIR, New Delhi, Govt. of India for funding this work through the CSIR Project
(
No. 01(2577)/12/EMR-II, 03/04/2013). S.N. is grateful to AICTE, New Delhi, Govt. of India
for sanctioning a UV–Vis-NIR spectrophotometer through AICTE-RPS Project
No. 8023/RID/RPS-10/Pvt (II Policy)/2011–12). S. N. also acknowledges the NMR spectral
facility established in the Chemistry Dept. BIT-Mesra through the DST FIST Program
SR/FST/CSI-242/2012). Different analytical services obtained from CIF, BIT-Mesra are greatly
(
(
acknowledged.
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1
9

Table 1. Crystal data and structure refinement parameters for complexes 1 and 2.
Complex 1
Complex 2
Empirical formula
2(C32
H
38MnN
4
O
2
) ClO
4
, CH
3
CN
32 4 2
C H38CuN O
Formula weight
1271.71
200
574.20
293
Temperature/K
Crystal system
Monoclinic
Monoclinic
P2 /c (No. 14)
Space group
P 2
1
/n
1
a/Å
18.1890(9)
20.3144(7)
19.2340(7)
90
17.731(2)
15.664(2)
10.5388(7)
90
b/Å
c/Å
α/°
β/°
100.008(2)
90
92.609(8)
90
γ/°
Volume/Å3
6998.8(5)
4
2924.0(5)
4
Z
ρ
calcg/cm3
1.2069(1)
0.455
1.304
μ/mm-1
1.323
F(000)
2676.0
1212
Crystal size/mm3
0.04×0.06×0.08
0.21×0.26×0.27
CuKα (λ = 1.54184)
3.8 to 70.8
Radiation
MoKα (λ = 0.71073)
2
Θ range for data collection/°
1.7 to 31.6
Dataset
Tot., Uniq. Data, R(int)
-26: 26; -29: 29; -28: 27
162430, 23318, 0.154
-16: 21; -18: 14; -12: 8
11014, 5382, 0.062
Reflections collected
Independent reflections
Nref, Npar
Goodness-of-fit on F2
Observed Data [I > 2.0 sigma(I)]
R, wR2, S
162430
11014
23318
5382
23318, 854
2.26=s
5382,357
1.03
7300
2889
0.1486, 0.2961, 2.26
0.0726, 0.2332, 1.03
2
2
2
2
2
2
2
w = 1/[σ (Fo )+(0.0999P) ]
Largest diff. peak/hole /e Å-3
Max. and Av. Shift/Err
where P=(Fo +2Fc )/3
1.79/-0.57
where P=(Fo +2Fc )/3
0.43/-0.67
0.00, 0.00
0.02, 0.00
2
0

Table 2. Compositions and energies of the frontier molecular orbitals of complexes 1 and 2.
Complex 1
Energy (eV)
Atom/ligand %
Mn
-6.57
-6.26
-6.1
-5.89
-3.29
LUMO
73
-2.06
LUMO+1
0
-1.97
-1.34
HOMO-3 HOMO-2 HOMO-1 HOMO
LUMO+2 LUMO+3
62
38
50
50
23
77
0
7
71
29
Ligand
100
27
100
93
Complex 2
Energy (eV)
Atom/ligand %
Cu
-6.15
-5.81
-5.35
-5.27
2.48
LUMO
60
-1.5
-1.46
-0.36
HOMO-3 HOMO-2 HOMO-1 HOMO
LUMO+1
LUMO+2 LUMO+3
5
0
2
2
1
1
2
Ligand
95
100
98
98
40
99
99
98
Table 3. Bond length and bond distances of the crystal geometry and optimized
geometry for complex 1.
Crystal geometry
Bond/
Optimized geometry
Distance (Å)
Bond/
Distance (Å)
Mn1A-O1A
Mn1A-N1A
Mn1A-N2A
Mn1A-O1A_a
Mn1A-N1A_a
Mn1A-N2A_a
1.858(4)
2.022(4)
2.449(5)
1.858(4)
2.022(4)
2.449(5)
(1-2)
1.86981
1.9905
2.2767
1.8698
1.9905
2.27666
(1-21)
(1-31)
(1-40)
(1-59)
(1-69)
Angle
Angle (°)
Angle
Angle (°)
O1A-Mn1A-N1A
O1A-Mn1A-N2A
88.62(18)
90.82(17)
180.00
91.38(18)
89.19(17)
80.91(17)
91.38(18)
180.00
99.09(17)
89.19(17)
99.09(17)
180.00
88.62(18)
90.82(17)
80.91(17)
(2-1-21)
(2-1-31)
(2-1-40)
(2-1-59)
88.711
89.014
179.98
91.29
O1A-Mn1A-O1A_a
O1A-Mn1A-N1A_a
O1A-Mn1A-N2A_a
N1A-Mn1A-N2A
N1A-Mn1A-O1A_a
N1A-Mn1A-N1A_a
N1A-Mn1A-N2A_a
N2A-Mn1A-O1A_a
N2A-Mn1A-N1A_a
N2A-Mn1A-N2A_a
O1A_a-Mn1A-N1A_a
O1A_a-Mn1A-N2A_a
N1A_a-Mn1A-N2A_a
(2-1-69)
90.98
(21-1-31)
(21-1-40)
(21-1-59)
(21-1-69)
(31-1-40)
(31-1-59)
(31-1-69)
(40-1-59)
(40-1-69)
(59-1-69)
84.745
91.289
179.998
95.243
90.965
95.257
179.988
88.71
89.037
84.755
2
1

Table 4. Bond length and bond distances of the crystal geometry and
optimized geometry for complex 2.
Crystal geometry
Bond/
Optimized geometry
Distance (Å)
Bond/
Distance (Å)
Cu(1)-O(1)
Cu(1)-O(2)
Cu(1)-N(1)
Cu(1)-N(3)
Cu(1)-N(4)
1.908(3)
1.931(4)
1.991(4)
1.979(4)
2.511(5)
(1-3)
(1-2)
(1-5)
(1-4)
(1-6)
1.99782
1.97384
2.04129
2.02812
2.49889
Angle
Angle (°)
Angle
Angle (°)
O(1)-Cu(1)-O(2)
O(1)-Cu(1)-N(1)
O(1)-Cu(1)-N(3)
O(1)-Cu(1)-N(4)
O(2)-Cu(1)-N(1)
O(2)-Cu(1)-N(3)
O(2)-Cu(1)-N(4)
N(1)-Cu(1)-N(3)
N(1)-Cu(1)-N(4)
N(3)-Cu(1)-N(4)
162.74(19)
89.02(17)
89.92(15)
105.22(17)
89.77(17)
91.16(16)
92.00(16)
178.90(16)
97.83(17)
82.73(17)
(3-1-2)
(3-1-5)
(3-1-4)
(3-1-6)
(2-1-5)
(2-1-4)
(2-1-6)
(5-1-4)
(5-1-6)
(4-1-6)
164.332
91.444
87.637
101.264
88.669
89.803
93.941
170.977
103.376
85.600
Table 5. Vertical excitations with band position, oscillator strength and character assignment for complexes 1 and 2.
Experimental Wavelength
Oscillator
strength
(f)
Percentage
%)
Transition
assignment
Complex
wavelength
nm)
obtained from
TD-DFT (nm)
Contributing orbital (s)
(
(
1
260
257.37(0.1025)
303.99(0.1547)
348.39(0.57)
459.81(0.0209)
524.68(0.000)
623.78(0.000)
0.102
0.155
0.57
0.021
0.00
HOMO->LUMO +6
HOMO-4->LUMO+2
HOMO-2->LUMO+1
HOMO-4-> LUMO
HOMO-3->LUMO
HOMO-3->LUMO+3
(37%)
(66%)
(60%)
(97%)
Intra ligand
Intra ligand
Intra ligand +MLCT
LMCT
d-d
d-d
3
3
4
5
5
13
38
54
31
75
0.00
2
310
288.44(0.2338) 0.233
HOMO(B)-> LUMO+3(B)
HOMO-6(B)->LUMO(B)
HOMO-3(B)->LUMO(B)
HOMO-14(B)->LUMO(B)
(19%)
19%)
(43%)
(16%)
Intra ligand
LMCT
LMCT
3
4
6
84
00
10
377.39(0.0795)
394.86(0.0628)
0.079
0.063
0.010
5
38.098(0.0105)
d-d + LMCT
2
2

Table 6. Kinetic parameters for the oxidation of 3,5-DTBC catalyzed by complexes 1 and 2.
-1
cat (h-1)
-1 -1
Complex
Solvent
V
max (M h )
K
m
(M)
k
m
kcat/K (M h )
1
MeOH
MeCN
MeOH
6.78×10-3
8.47×10-3
4.46×10-3
2.53×10-5
2.13×10-5
9.87×10-2
547.45
2.16×107
3.21×107
3.79×103
6.8424×102
3.75×102
2
Figure captions
Figure 1. Changes in spectral pattern for complex 1 after addition of 3,5-DTBCH up to 3 h in
2
acetonitrile.
Figure 2. Changes in spectral pattern in complex 2 after addition of OAPH up to 3 h in MeOH.
+
+
Figure 3. ORTEP diagram (at 30% probability level) of [MnL ] (1 ) and [CuL ] (2).
2
2
Figure 4. Electronic spectra (acetonitrile medium) of (a) complex 1 and (b) complex 2.
Figure 5. Electronic transitions in complex 1 in DMSO solution.
Figure 6. Electronic transitions in complex 2 in DMSO solution.
Figure 7. (a) Rate versus substrate concentration plot for the oxidation of 3,5-DTBCH in
2
acetonitrile catalyzed by complex 1 at 25 C. (b) Lineweaver-Burk plot for the oxidation of
3
,5-DTBCH2.
-
Figure 8. Electronic spectrum of I (catecholase activity by complex 1).
3
Figure 9. (a) Rate versus substrate concentration plot for the oxidation of o-aminophenol in
methanol catalyzed by complex 2 at 25 C. (b) Lineweaver-Burk plot for the oxidation of
o-aminophenol.
Figure 10. ESI mass spectrum (+ve) of 1:100 mixture of complex 1 and 3,5-DTBCH in
2
methanol recorded after 15 min of mixing.
Figure 11. Cyclic voltammograms of ligand, complex 1 and complex 2 recorded in acetonitrile
solution with TEAP supporting electrolyte and glassy carbon working electrode at a scan rate
-
1
1
00 mV/s .
2
3

2
4

2
5

2
6

2
7

2
8