Valence tautomerism induced nucleophilic ipso substitution in a coordinated tetrabromocatecholate ligand and diverse catalytic activity mimicking the function of phenoxazinone synthase

Article abstract of DOI:10.1016/j.molcata.2015.11.008

Two new manganese(III) complexes, [pyH][Mn(Br₄Cat)₂(py)] (1) and [Mn(Br₄Cat)(Br₃pyCat)(py)₂] (2), where py is pyridine, Br₄CatH₂ is tetrabromocatechol and Br₃pyCatH₂ is 3,5,6-tribromo-4-pyridiniumcatechol, derived from redox 'noninnocent' bromo-substituted catecholate ligands are reported. Both complexes were characterized by various spectroscopic techniques in addition to the single crystal X-ray crystallography, and the electrochemical behavior of these complexes was investigated by cyclic voltammetry. Variable temperature UV-vis spectral studies for complex 1 reveal an unprecedented observation in which a dramatic increase of the ligand-to-metal charge transfer band at 592 nm associated with concomitant change in color of the solution from olive-green to dark-green is noticed with increase in temperature. This unprecedented spectral feature is consistent with the formation of a new species 2 in which valence tautomerism induced aromatic nucleophilic substitution of a tetrabromocatecholate ligand by pyridine is observed. DFT calculations have been used to rationalize this unexpected result. To the best of our knowledge, the nucleophilic aromatic substitution of tetrabromosemiquinone by pyridine to generate this pyridinium-containing catecholate ligand is the first example of valence tautomerism induced nucleophilic ipso substitution by a nitrogen containing ligand. Although the electrochemical behavior of both complexes is similar, the probable role of a positive charge on the ligand backbone has been discussed in order to justify the significantly higher catalytic activity of complex 2 over complex 1.

Full text of DOI:10.1016/j.molcata.2015.11.008

Journal of Molecular Catalysis A: Chemical 412 (2016) 56–66
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Valence tautomerism induced nucleophilic ipso substitution in a
coordinated tetrabromocatecholate ligand and diverse catalytic
activity mimicking the function of phenoxazinone synthase
Anangamohan Panjaa,∗, Narayan Ch. Janaa, Moumita Patraa, Paula Brandãob,
Curtis E. Moorec, David M. Eichhornd, Antonio Fronterae
a Postgraduate Department of Chemistry, Panskura Banamali College, Panskura RS, WB 721152, India
b Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
c Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093, USA
d Department of Chemistry, Wichita State University, Wichita, KS 67260, USA
e Departament de Química, Universitat de les Illes Balears, Crta. de Valldemossa Km 7.5, 07122 Palma de Mallorca (Baleares), Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new manganese(III) complexes, [pyH][Mn(Br4Cat)2(py)] (1) and [Mn(Br4Cat)(Br3pyCat)(py)2]
(2), where py is pyridine, Br4CatH2 is tetrabromocatechol and Br3pyCatH2 is 3,5,6-tribromo-4-
pyridiniumcatechol, derived from redox ‘noninnocent’ bromo-substituted catecholate ligands are
reported. Both complexes were characterized by various spectroscopic techniques in addition to the
single crystal X-ray crystallography, and the electrochemical behavior of these complexes was inves-
tigated by cyclic voltammetry. Variable temperature UV–vis spectral studies for complex 1 reveal an
unprecedented observation in which a dramatic increase of the ligand-to-metal charge transfer band at
592 nm associated with concomitant change in color of the solution from olive-green to dark-green is
noticed with increase in temperature. This unprecedented spectral feature is consistent with the for-
mation of a new species 2 in which valence tautomerism induced aromatic nucleophilic substitution of
a tetrabromocatecholate ligand by pyridine is observed. DFT calculations have been used to rationalize
this unexpected result. To the best of our knowledge, the nucleophilic aromatic substitution of tetra-
bromosemiquinone by pyridine to generate this pyridinium-containing catecholate ligand is the first
example of valence tautomerism induced nucleophilic ipso substitution by a nitrogen containing ligand.
Although the electrochemical behavior of both complexes is similar, the probable role of a positive charge
on the ligand backbone has been discussed in order to justify the significantly higher catalytic activity of
complex 2 over complex 1.
Received 15 September 2015
Received in revised form 4 November 2015
Accepted 10 November 2015
Available online 2 December 2015
Keywords:
Manganese-catecholate complex
Valence tautomerism
Ipso substitution
Crystal structure
Phenoxazinone synthase activity
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
mode of such enzymes and hence to their structures and spec-
troscopic characterizations. Modeling of such enzymes through
Metalloenzymes play vital roles in dioxygen binding and oxida-
tion of various biologically important small molecules, in which
they facilitate the spin-forbidden interaction between dioxygen
and organic compounds [1–3]. These enzymes may act as oxy-
genases, the functions of which are the incorporation of one or
both oxygen atoms from molecular oxygen into organic substrates,
or oxidases, which impart substrate oxidation with concomitant
reduction of molecular oxygen to water or hydrogen peroxide
[1–3]. Bioinorganic chemists have paid attention to the functioning
the characterization of low-molecular-weight metal complexes is
a way to gain structural and functional insights into the active
sites of enzymes. As a consequence, the development of new
redox catalysts that mimic the functions of these enzymes for the
selective oxidation reactions of organic substances in industrial
and synthetic processes has been an ongoing study [4–8]. Tradi-
tionally, inorganic and organometallic redox catalysts are derived
from second- and third-row transition-metal ions with redox-inert
ancillary ligands [4–8]. In these systems, the multielectron redox
activity is entirely metal-driven. In contrast, redox-active “non-
innocent” ligands may impart a multielectron redox capacity to
mononuclear first-row metal complexes, which typically prefer
only single-electron redox changes [9,10]. Aside from metallo-
∗
Corresponding author.
E-mail address: ampanja@yahoo.co.in (A. Panja).
http://dx.doi.org/10.1016/j.molcata.2015.11.008
1381-1169/© 2015 Elsevier B.V. All rights reserved.

A. Panja et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 56–66
57
porphyrin complexes, such ligand-derived multielectron reaction
2.2. Synthesis of [pyH][Mn(Br4Cat)2(py)] (1)
chemistry has been largely unexplored for redox transformations
of small-molecule substrates, but has received increasing attention
[9–13].
MnCl2·4H2O (99.0 mg, 0.5 mmol) and Br4CatH2 (425 mg,
1.0 mmol) were dissolved in 20 mL of methanol with stirring. A
solution of pyridine (316 mg, 4.0 mmol) in 10 mL of methanol was
added dropwise to the mixture with stirring and the resulting
olive-green solution was filtered and kept in the refrigerator for
crystallization. Crops of olive-green block-shaped crystals suitable
for structural studies separated out from the solution overnight.
The remaining crystals were isolated via suction filtration, washed
with methanol, and air dried at room temperature. Yield: 0.396 g
(75%). Anal Calcd. for C22H11Br8MnN2O4: C, 25.06; H, 1.05; N,
2.66. Found: C, 25.18; H, 1.09; N, 2.49. FTIR (KBr, cm−1): 1637 w,
1598 w, 1420 s, 1385 m, 1260 m, 1226 m, 925 m, 753 m, 675 w,
591 m. UV–vis, nm (␧, dm3 mol−1 cm−1): 295 (10726), 335 (sh,
6842), 592 (832).
Research on valence tautomeric (VT) complexes of transition
metal containing redox noninnocent dioxolene ligands is an impor-
tant topic because of their potential applications in future bistable
molecular switching materials and devices [14–16]. In addition
to the materials perspective, naturally occurring bromopyrocat-
echols are found in marine life and frequently isolated from
red algae of the family Rhodomelaceae [17]. Most of these com-
pounds have important biological activities. For example, some
bromopyrocatechols exhibit enzyme inhibition, cytotoxicity, feed-
ing deterrent and microbial activities [18–21]. Protein tyrosine
phosphatase inhibitory activity of bromopyrocatechols has also
been reported [22]. Therefore, synthesis of bromo-derivatives of
pyrocatechols is of immense importance for their biological appli-
cation. One of the ways to derive halogen-substituted catechols
is the metal catalyzed ipso substitution of aromatic halides, and
in those systems metal-assisted electron deficiency in aromatic
ring favors substitution of halides [23]. Furthermore, electron defi-
cient catechols such as tetrachlorocatechol bound manganese(III)
complexes were proved to be efficient catalysts for the production
of hydrogen peroxide from O2 in the presence of hydroxylamine
or hydrazine as a sacrificial reductant [24,25]. Thus, we specu-
lated that such dioxolene-based manganese(III) complexes may
provide a robust platform to exhibit oxidase activity toward the
oxidation of small molecule organic substances. As a part of our
ongoing study of transition-metal dioxolene chemistry [26–29] and
functional mimics of phenoxazinone synthase [30–33], we report
herein the syntheses, structures and spectroscopic investigations of
mononuclear manganese(III) complexes, [pyH][Mn(Br4Cat)2(py)]
(1) and [Mn(Br4Cat)(Br3pyCat)(py)2] (2), where py is pyridine,
Br4CatH2 is tetrabromocatechol and Br3pyCatH2 is 3,5,6-tribromo-
4-pyridiniumcatechol. The present report demonstrates, for the
first time, the valance tautomerism induced nucleophilic substitu-
tion reaction of an aromatic Br-ion by a nitrogen containing ligand,
supported by DFT calculations. The effect of the positive charge at
the ligand backbone on the diverse catalytic activity modeling the
function of phenoxazinone synthase has also been investigated.
2.3. Synthesis of [Mn(Br4Cat)(Br3pyCat)(py)2] (2)
MnCl2·4H2O (79.2 mg, 0.4 mmol) and Br4CatH2 (340 mg,
0.8 mmol) were mixed in 20 mL of methanol with stirring. A solu-
tion of pyridine (790 mg, 10.0 mmol) in 10 mL of methanol was
added dropwise to the mixture with continuous stirring and the
resulting olive-green solution was heated at 50 ◦C for 30 min dur-
ing which time the color of the solution changed to dark green.
Upon standing at ambient temperature, crops of dark-green block-
crystals suitable for structural studies separated out from the
solution within a couple of days. The remaining crystals were
isolated via suction filtration, washed with methanol, and air
dried at room temperature. Yield: 0.278 g (66%). Anal Calcd. for
C27H15Br7MnN3O4: C, 30.78; H, 1.43; N, 3.99. Found: C, 30.68; H,
1.49; N, 3.79. FTIR (KBr, cm−1): 1622 w, 1592 m, 1427 s, 1385 m,
1262 m, 1230 m, 928 m, 749 m, 696 m, 567 m. UV–vis, nm (␧,
dm3 mol−1 cm−1): 283 (12055), 329 (sh, 6313), 584 (1182).
2.4. X-ray crystallography
Single crystal X-ray diffraction data for 1 and 2 were col-
lected with monochromated Mo-K␣ radiation (ꢀ = 0.71073 Å) on
a Bruker Kappa Apex-II and a Bruker Smart Apex-II diffractome-
ters, equipped with a CCD area detector at 180(2) K and 100(2)
K, respectively. Several scans in ꢁ and ω directions were made to
increase the number of redundant reflections and were averaged
during refinement cycles. Data processing was performed using the
Bruker SAINT software package [34]. Multi-scan absorption correc-
tion was applied to all intensity data using the SADABS program
[35]. The structures were solved by direct methods and refined by
a full-matrix least squares-based method on F2 using the SHELXL
program [36]. The intensity data were corrected for Lorentz and
polarization effects, and all non-hydrogen atoms were refined using
anisotropic thermal parameters. Hydrogen atoms were placed at
calculated positions with individual isotropic thermal factors equal
to 1.2 times of those to which they are attached, but were not
refined. The hydrogen atom bound to nitrogen was located on
the difference Fourier map and refined isotropically with thermal
parameters equivalent to 1.2 times that of the parent atom. Rel-
evant crystallographic data together with refinement details are
given in Table 1.
2. Experimental
2.1. Materials and physical measurements
Tetrabromocatechol monohydrate, manganese(II) chloride
tetrahydrate and pyridine (py) were of commercially available
reagent or analytical grade chemicals and were used as received.
Other chemicals and solvents were of reagent or analytical grade
and used without further purification.
Elemental analyses for C, H, and N were performed in a
PerkinElmer 240C elemental analyzer. The infrared spectra of
the samples were recorded in the range of 400–4000 cm−1 on a
PerkinElmer Spectrum Two Infrared spectrometer using KBr pel-
lets. Cyclic voltammetric experiments were performed at room
temperature under nitrogen in DMF using tetraethylammonium
perchlorate as a supporting electrolyte on a BAS Epsilon electro-
chemical workstation. The conventional three-electrode assembly
is comprised of a BAS glassy carbon (GC) working electrode, a plat-
inum wire auxiliary electrode and a Ag/AgCl reference electrode.
Electronic absorption spectra were recorded using a PerkinElmer
Lamda-35 spectrophotometer with a 1-cm path-length quartz cell.
The X-band EPR spectra were recorded in DMF using a JEOL JES-
FA 200 instrument both at room temperature and liquid nitrogen
temperature (77 K).
2.5. Kinetics of the phenoxazinone synthase activity
Phenoxazinone synthase-like activity was studied by mix-
ing 100 equivalents of o-aminophenol (OAPH) with 1.0 × 10−4 M
solutions of 1 and 2 in DMF under aerobic conditions at room
temperature. The reaction was followed spectrophotometrically by
monitoring the increase of the absorbance as a function of time

58
A. Panja et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 56–66
Table 1
Crystal data and structure refinement for complexes 1 and 2.
1
2
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
C44H22Br16Mn2N4O8
2123.10
180(2)
Triclinic
P¯ı
11.4908(3)
15.0085(4)
16.5369(5)
92.1240(10)
95.5120(10)
103.0410(10)
2760.47(13)
2
C27H15Br7MnN3O4
1059.73
100(2)
Triclinic
P¯ı
8.3697(10)
9.9133(11)
10.4913(19)
107.230(11)
94.162(11)
113.089(8)
746.96(19)
1
b (Å)
c (Å)
˛ (◦)
ˇ (◦)
ꢂ (◦)
Volume (Å3)
Z
Dcalc (mg m−3
)
2.554
12.092
1976
1.24–27.94
72668
2.356
9.844
500
3.586–25.426
8935
ꢃ (mm−1
F (000)
)
 Range (◦)
Reflections collected
Independent reflections (Rint
)
13192 (0.0549)
8615
8615/0/675
1.019
R1 = 0.0333, wR2 = 0.0616
R1 = 0.0706, wR2 = 0.0721
0.843/–0.791
2717 (0.0305)
2107
2107/83/230
1.192
R1 = 0.0694, wR2 = 0.1259
R1 = 0.0910, wR2 = 0.1317
0.699/–0.833
Observed reflections [I > 2ı(I)]
Data/restraints/parameters
Goodness-of-fit on F 2
Final R indices [I > 2ı(I)]
R indices (all data)
Largest diff. peak/hole (e Å−3
)
at 434 nm (ε = 5.5 × 103 M−1 cm−1) [37], which is characteristic of
2-aminophenoxazin-3-one in DMF. To determine the dependence
of rate on substrate concentration and various kinetic parame-
ters, 1.0 × 10−4 M or 2.0 × 10−4 M solutions of the complexes were
treated with at least 10 equivalents of substrate, maintaining
pseudo first-order conditions. Similarly, a fixed concentration of
substrate (1 × 10−2 M) was mixed with varying concentrations of
the catalysts (2.0–50 × 10−5 M) in air-saturated DMF (2.0 mL) in
order to examine the effect of catalyst concentration on the rate of
the reaction. The initial rate method was applied to determine the
rate of a reaction by linear regression from the slope of absorbance
versus time plot.
but completely soluble in dimethylformamide (DMF), and thus all
the solution studies were performed in DMF.
IR spectroscopy is a good technique to distinguish the oxidation
state of coordinated dioxolene ligands [40]. IR spectra of quinone
C
O usually exhibit transitions at 1630–1700 cm−1. On the other
hand, semiquinone C O stretching frequencies are observed in
the range of 1430–1480 cm−1. IR spectra of fully reduced cate-
cholate ions exhibit transitions below 1430 cm−1. IR spectra of 1
and 2 were recorded using KBr pellets, and the spectra of both
complexes have similar features (Fig. S1†). These spectra con-
sist of strong bands at 1421 cm−1 for 1 and 1427 cm−1 for 2
corresponding to the catecholate C O stretching mode of metal-
catecholate complexes. Additionally, the infrared spectra display
medium bands in the region 1226–1262 cm−1 which are also typi-
cal for metal-catecholate compounds. The bifurcated bands at 1598
and 1637 cm−1 for 1 and 1592 and 1622 cm−1 for 2 are character-
istic of pyridyl C N stretching. The bifurcation of the band is the
signature of the presence of two types of pyridine ligands in both
complexes, consistent with the structural characterizations.
2.6. Theoretical methods
We have studied the valence tautomerism (compounds 1 and
1a, see Scheme 1) using DFT calculations (B3LYP/def2-SVP level of
theory). The calculations have been performed using Gaussian-09
program [38]. We have used Mulliken method [39] to derive atomic
charges in both compounds.
3.2. Structural descriptions of the complexes
3. Results and discussion
X-ray crystallography reveals that complex 1 crystalizes in
the triclinic P¯ı space group and the asymmetric unit consists of
two crystallographically independent molecules. The structure of
1 can be viewed as composed of a complex anion of formula
[Mn(Br4Cat)2(py)]− and a pyridinium counter ion. The perspec-
tive view of 1 along with the selected atom labeling scheme
is depicted in Fig. 1, while important bond distances and bond
angles are listed in Table 2. The geometry of each of the crystal-
lographically independent manganese centers is distorted square
pyramidal as reflected from Addison parameter (␶) of about 0.02
in which four oxygen atoms from two dioxolene ligands con-
struct the equatorial plane and the apical position is occupied
by a pyridine ligand. Mn O(dioxolene) bond distances fall in the
range 1.887(2) to 1.916(2)—these are similar to the Mn O lengths
observed for catecholato-manganese(III) complexes with similar
ligand systems [26–29,40,41]. The C O bond lengths of the coordi-
nated o-dioxolene ligand often provide interesting clues as to the
oxidation state of this redox-noninnocent ligand [26,40]. These lig-
3.1. Syntheses and general characterizations
Reaction of manganese(II) chloride tetrahydrate and tetrabro-
mocatechol in a 1: 2 molar ratio in methanol in the presence of
pyridine under aerobic conditions resulted in an olive-green solu-
tion, which upon standing at 4 ◦C afforded olive-green crystals of 1
overnight in high yield in which manganese(II) is converted to man-
ganese(III) by aerial oxidation. A similar reaction in the presence of
excess pyridine at elevated temperature ultimately results in com-
plex 2 within a couple of days, in which nucleophilic substitution of
a bromide (of tetrabromocatecholate ion) by pyridine is observed
in addition to the aerial oxidation of the manganese(II) ion. The
elemental analyses of both complexes are consistent with the pro-
posed molecular formulas. The role of the pyridine is noteworthy in
these systems as it serves as a base, a monodentate ligand, and also
a nucleophile for the ipso substitution reaction (vide infra). Both
complexes are poorly soluble in acetonitrile and common alcohols

A. Panja et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 56–66
59
Scheme 1. Schematically depicted plausible mechanism showing valence tautomerism induced ipso substitution reaction.
Table 2
Table 3
Selected bond distances (Å) and bond angles (◦) for complex 1.
Important bond distances (Å) and bond angles (◦) for complex 2.
Bond distances
Bond distances
Mn1A O3A
Mn1A O2A
Mn1A O4A
Mn1A O1A
Mn1A N1A
C1A O1A
C2A O2A
C7A O3A
C8A O4A
C1A C2A
1.887(2)
1.889(2)
1.890(2)
1.916(2)
2.249(3)
1.362(4)
1.333(4)
1.343(4)
1.341(4)
1.414(5)
1.423(5)
Mn1B O1B
Mn1B O2B
Mn1B O3B
Mn1B O4B
Mn1B N1B
C1B O1B
C2B O2B
C7B O3B
C8B O4B
C1B C2B
1.886(2)
1.888(2)
1.902(2)
1.903(2)
2.258(3)
1.336(4)
1.343(4)
1.340(4)
1.348(4)
1.413(5)
1.412(5)
Mn(1) O(1) a
Mn(1) O(1)
Mn(1) O(2)
Mn(1) O(2) a
Mn(1) N(1)
1.932(6)
1.932(6)
1.895(7)
1.895(7)
2.394(8)
Mn(1) N(1) a
O(1) C(1)
O(2) C(2)
C(1) C(2)
2.394(8)
1.301(12)
1.332(11)
1.411(14)
Bond angles
O(2) Mn(1) O(1)
O(2) a Mn(1) O(1) a
84.7(3)
84.7(3)
O(1) Mn(1) O(1) a
O(2) Mn(1) O(2) a
N(1) a Mn(1) N(1)
180.0
180.0
180.0
C7A C8A
C7B C8B
Bond angles
which is consistent with the high-spin d4 electronic configura-
tion of the central metal ion. The crystallographic inversion center
generates a second molecule for each molecule present in the
asymmetric unit such that the planar catecholate ligands of the
adjacent unit lie in a dimeric face-to-face orientation as shown in
Fig. 1. In these dimeric units, the manganese(III) center is weakly
bonded to one catecholate oxygen atom of the adjacent man-
ganese(III) unit. The dimeric units are further stabilized by ␲–␲
stacking interactions between catecholate rings of two adjacent
molecules with short contacts of 3.360 and 3.373 Å in combination
O1A Mn1A O2A
O3A Mn1A O4A
O1A Mn1A O4A
O2A Mn1A O3A
85.45(10)
86.61(11)
168.40(12)
169.85(12)
O1B Mn1B O2B
O3B Mn1B O4B
O1B Mn1B O4B
O2B Mn1B O3B
86.84(11)
85.34(11)
170.80(12)
169.51(12)
ands in the semiquinone state generally have shorter C O distances
(ca. 1.28 Å) compared to the fully reduced catecholate ligand (ca.
1.34 Å). The ring (O)C C(O) distances are also informative—with
distances in the range 1.36–1.44 Å for catecholates and in the
range 1.45–1.48 Å for semiquinones. The average C O bond dis-
tance of 1.343(4) Å of the chelated dioxolene ligands is fully
consistent with a manganese(III)-catecholate charge distribution.
The C C bonds bearing catecholate O− are similar for both the
crystallographically independent molecules (1.412(5) and 1.423(5)
Å), and also support the manganese(III)-catecholate formulation.
Mn N(pyridyl) bond distances of 2.249(3) and 2.253(3) Å for two
crystallographically independent molecules are axially elongated
with hydrogen bonding interactions involving the N H and C
H
protons of pyridinium ions and the coordinating oxygen atom of
the chelated catecholate ligands (N H· · ·O = 2.789(1)–2.790(1) Å;
C
H· · ·O = 3.003(1)–3.309(1) Å) (Fig. S2).
The monomeric structure of [Mn(Br4Cat)(Br3pyCat)(py)2] (2)
is shown in Fig. 2 together with the atom numbering scheme,
and the metrical parameters for the metal coordination sphere
are listed in Table 3. Unsymmetrical complex 2 crystallizes in

60
A. Panja et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 56–66
Fig. 1. Molecular view showing dimeric face-to-face orientation of the planar catecholate ligands in 1 . Ellipsoids are drawn at the 50% probability level.
Fig. 2. Molecular structure of 2 showing the atom numbering scheme. Ellipsoids are drawn at the 30% probability level.
the triclinic P¯ı space group in which the central metal ion sits
on an inversion center; therefore it has been refined using a
disorder model. The structure of 2 consists of a manganese(III)
ion residing in a distorted octahedral environment. Four oxygen
atoms from two distinct catecholate ligands comprise the equato-
rial plane in which one of the tetrabromocatecholate ligands has
been modified by nucleophilic aromatic substitution of a bromide
ion by pyridine. The coordination sphere of the manganese(III)
center is completed by two pyridine molecules in the axial posi-
tions. The symmetry imposed on the molecule results in there
being only one unique Mn N(pyridine) distance of 2.394(8) Å and
two unique Mn O(dioxolene) distances of 1.895(7) and 1.932(6)Å,
which are comparable to that of 1 and other structurally charac-
terized manganese(III)-catecholate complexes [26–29,24,41]. The
longer metal-pyridyl bond distances clearly suggest the tetrago-
nally elongated high spin manganese(III), having a d4 electronic

A. Panja et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 56–66
61
configuration. Similar to 1, the average C O distance of 1.316(11)
Å and the (O)C C(O) distance of 1.411(14) Å are indicative of the
fully reduced catecholate state of the ligand. As dictated by sym-
metry, the trans angles are found to be 180◦ but the distortion
from the ideal octahedral geometry is due to the smaller bite angle
(84.67◦) of the coordinated catecholate ligands—this value is again
typical for catechol complexes [26,28]. The dihedral angle between
pyridine and benzene ring (76.57◦) in Br3pyCat unit is nearly per-
pendicular, as expected.
3.3. Valence tautomerism induced ipso substitution
In our previous studies, we have observed that manganese(III)
complexes with electron withdrawing substituted catechols
(tetrachloro- and tetrabromo-catechols) are susceptible to undergo
valence tautomerism (VT) at about room temperature [26–29].
Magnetic susceptibility measurement is not a good technique to
observe the VT phenomenon in the manganese series because all
the redox isomers namely MnIV(Cat)2, MnIII(SQ)(Cat) and MnII(SQ)2
have S = 3/2 ground states, where Cat is catecholate and SQ
is semiquinonate. Similarly, both MnIII(Cat)2 and MnII(Cat)(SQ)
isomers have ground states of S = 2 [26,28]. But the variable temper-
ature electronic absorption spectral studies clearly indicated that
the manganese-dioxolene systems exhibit reversible intramolecu-
lar metal-to-ligand electron transfer at around room temperature
[26–29]. The electronic absorption spectrum of complex 1 was
recorded in DMF and consists of two distinct bands and a shoul-
der in the range 200–1100 nm. The low energy band spans from
500 to 800 nm with ꢀmax at 592 nm (ε = 8.32 × 102 cm−1 M−1),
and a more intense sharp band and shoulder appear at 295 nm
(ε = 1.07 × 104 cm−1 M−1) and 335 nm (ε = 6.84 × 103 cm−1 M−1),
respectively. The intense higher energy band at 295 nm is usually
assigned to the ␲–␲* transition, while the poorly resolved shoul-
der at 335 nm is assigned to an n → ␲* transition. The relatively
broad band observed in the region of 500–800 nm is attributed to
a catecholate-to-manganese(III) charge-transfer transition [28,42].
Upon increasing the temperature from 10 to 35 ◦C, a decrease of the
band intensity at 592 nm was observed which is consistent with
the thermodynamically favorable transformation of MnIII(Br4Cat)2
to MnII(Br4Cat)(Br4SQ)—the thermodynamic favorability of such a
conversion is largely due to the greater vibrational and electronic
entropy of the +2 oxidation state over the +3 oxidation state (Fig. 3).
Until now the present observation is in good agreement with those
observed for other valence tautomeric interconversion equilib-
ria involving dioxolene-based cobalt and manganese complexes
[26–29]. But upon increasing the temperature beyond 35 ◦C, a dra-
matic increase of the band intensity was observed up to heating at
65 ◦C. This novel observation in the field of metal-dioxolene chem-
istry initially puzzled us, as it does not agree with any reports in
similar systems. Previous studies on 3d metal complexes formed by
tetrahalo-dioxolene in both catecholato or semiquinonato oxida-
tion states have shown that some of these compounds are unstable
in solution. Indeed, with the exception of complexes kinetically
inert towards dissociation, the formation of orange hexahalo-2,3-
oxanthrenequinone, a product of the nucleophilic reaction between
two dioxolene ligands, is often observed [41,43,44]. But the increase
of the band intensity at 592 nm does not support the formation
of this kind of product. In the present case the change of color
from olive-green to dark-green suggests that the oxidation state
of the metal is at least preserved at +3. In our recent report, we
have also shown that the oxidation state of a dioxolene moiety
is affected by the acidity of the medium [28]. During the course
of that study, it has also been noted that, although the complexes
were stable enough in the acid medium, they were prone to decom-
pose in the presence of excess pyridine. All these observations
suggest the formation of a new species, and addition of pyridine
Fig. 3. Temperature-dependent changes in the electronic absorption spectrum of
1 showing unprecedented increase and decrease of band intensity with increase in
temperature.
base even accelerates the formation of that species. Indeed, the
spectrum of the final solution is identical with the spectrum of
[Mn(Br4Cat)(Br3pyCat)(py)2] (2) in DMF solution, originating from
the nucleophilic substitution of one of the trans Br− (with respect
to catecholate-O) by pyridine. As a whole, electronic absorption
spectral studies suggest that, although complex 1 is stable at lower
temperature, it is prone to undergo nucleophilic substitution reac-
tion of an aromatic bromide by pyridine at elevated temperatures,
resulting in the formation of a new complex 2. In order to establish
the importance of metal center in ipso substitution reaction, a con-
trol experiment was performed in which tetrabromocatechol was
allowed to react with excess pyridine. But in an identical reaction
condition in absence of metal complex it did not produce Br3pyCat
moiety which ensures the crucial role of metal center in aforesaid
ipso substitution process.
Mechanistically, the synthesis of 2 appears quite interesting and
probably involves the formation of 1a as an important intermedi-
ate which is subsequently converted to 2. A plausible mechanistic
pathway for the formation of 2 is shown in Scheme 1. As with
our previous report [26–29], complex 1 is initially formed and
then undergoes an intramolecular ligand-to-metal electron trans-
fer at around room temperature, leading to the transformation of
manganese(III)-catecholate (1) to its isomeric form, manganese(II)-
semiquinonate (1a). This intramolecular ligand-to-metal electron
transfer favors the aromatic nucleophilic ipso substitution of only
the bromide at para position with respect to semiquinonato oxy-
gen atom by pyridine at elevated temperature. The pyridine for Br−
substitution in the tetrabromosemiquinonate ring in 1a, followed
by the reversal of electron transfer (metal-to-ligand) generates
an unsymmetrical compound 2 . The nucleophilic aromatic sub-
stitution of tetrabromosemiquinone by pyridine to generate this
pyridinium-containing catecholate ligand is, to the best of our
knowledge, the first example of a valence tautomerism-induced
nucleophilic ipso substitution by a nitrogen containing ligand. The
only report in this regard, by Chaudhury, et al., is one in which
vanadium ion induced nucleophilic aromatic substitution of Cl−
by OH− results from formation of the chloranilate ion [23]. The
present study is also quite interesting in that both a weaker base
and a weaker nucleophile like pyridine may also be capable of
carrying out such a nucleophilic aromatic substitution reaction.
This could be a new and potentially useful synthetic route to pyri-

62
A. Panja et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 56–66
Fig. 4. Spin density and LUMO plots of 1 (A) and 1a (B) at the B3LYP/def2-SVP level of theory.
dinium substituted aromatic compounds. These compounds may
also be useful in conferring water solubility properties to aro-
matic compounds, the formation of ionic liquids, the tuning of
electronic properties in coordination compounds, etc. Interestingly,
a similar type of manganese(III) complex with a pyridine sub-
stituted tetracholorocatecholate ligand has been reported in the
literature, and in that case the complex was prepared by adopting
a different methodology [24]. At the first step, 1,2-dihydroxy-
3,5,6-trichlorobenzene-4-pyridinium chloride was derived from a
nucleophilic aromatic substitution of tetrachloro-o-benzoquinone
by pyridine followed by the reduction of o-quinone to the catechol
by hydroxylamine, and finally treatment of this ligand with a man-
ganese(II) salt under aerobic conditions resulted in the reported
complex.
In order to further support that the intramolecular valence tau-
tomerism favors the aromatic nucleophilic ipso substitution we
have performed a DFT study. We have computed the Mulliken
charge distribution in 1 and 1a, and the carbon atom that suf-
fers the ipso substitution increases its positive charge from 0.016 e
to 0.020 e upon the valence tautomerism thereby becoming more
electrophilic. We have also computed the spin density plots and the
LUMOs of compounds 1 and 1a (see Fig. 4). Remarkably, in com-
pound 1 the spin density is located at the Mn metal center with
some delocalization to the atoms directly bonded to the Mn(III).
In compound 1a, apart from the spin density located at the Mn(II)
metal center, there is also spin density equally distributed in both
tetrabromocatechol ligands. More interestingly, the LUMO is totally
different in both tautomers. In 1 the LUMO is located in the pyridine
ligand, therefore the aromatic substitution in the tetrabromocate-
chol ring is not favored. In contrast the LUMO in 1a is located in both
tetrabromocatechol and the largest atomic coefficients that partic-
ipate in this molecular orbital are located at the carbon atoms that
suffer the nucleophilic attack of pyridine. These calculations fur-
ther support the higher ability of 1a to participate in nucleophilic
aromatic substitution reactions.
Fig. 5. Cyclic voltammograms of 1 and 2 in acetonitrile solution using a glassy car-
bon working electrode in the presence of tetraethylammonium perchlorate as a
supporting electrolyte at ambient temperature with scan rate 100 mV s−1. Electrode
potential values are reported versus ferrocene/ferrocenium couple.
dation response at 0.20 V (E1/2 = 0.01 V,
Ep = 390 mV) versus
ferrocene/ferrocenium, which is attributable to the oxidation of the
coordinated Br4Cat2– ligands to the corresponding semiquinonate
forms. Unlike for complex 1, the cyclic voltammogram of 2 consists
of one quasi-reversible and one irreversible oxidative responses at
0.23 V (E1/2 = 0.02 V, Ep = 420 mV) and 0.42 V. It is usually found
in a family of metal-polyoxolene complexes that even if differ-
ent metal ions have the same oxidation states, the ligand-centered
redox processes are characterized by similar free energy values
[45]. The quasi-reversible oxidative response at 0.23 V is compara-
ble to the oxidation at 0.20 V for complex 1, and thus is responsible
for the oxidation of the Br4Cat2− ligand. The higher oxidation poten-
tial at 0.42 V is likely to correspond to the oxidation of Br3pyCat2– to
its semiquinonate form. The positive charge on the pyridinium ion
results in a more positive oxidation potential. In addition to the
ligand-centered redox activity, an electrochemically irreversible
reduction wave is observed at −0.87 and −0.85 V for 1 and 2, respec-
tively, and these can be assigned to the reduction of manganese(III)
to manganese(II) [26,28].
3.4. Electrochemical studies
Electrochemical studies on dioxolene-chelated transition-metal
complexes containing both redox-active metal centers and electro-
active ligands are important to get an insight into the electronic
nature of the complexes. The cyclic voltammogram of 1 in
DMF solution depicted in Fig. 5 displays a quasi-reversible oxi-

A. Panja et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 56–66
63
Fig. 7. Initial rate versus substrate concentration plot for the oxidation of o-
aminophenol in dioxygen-saturated DMF catalyzed by the complexes at 25 ◦C.
Symbols and solid lines represent the experimental and simulated profiles, respec-
tively.
Fig. 6. The spectral profile showing the growth of 2-aminophenoxazine-3-one
at 434 nm upon addition of 0.01 M o-aminophenol to a solution containing 2
(1 × 10−4 M) in DMF. The spectra were recorded for the period of 1 h in aerobic
condition at 25 ◦C.
3.5. Phenoxazinone synthase like activity
The aerial oxidation of o-aminophenol (OAPH) catalyzed by
the complexes was investigated by monitoring the increase of the
absorption at 434 nm, characteristic of the phenoxazinone chro-
mophore [31–33,46,47]. The catalytic studies were conducted in
the absence of added base to minimize the possibility of autoxida-
tion of the substrate. Before proceeding with the detailed kinetic
investigation, it was necessary to get an estimate of the ability
of the complexes to oxidize o-aminophenol, and for this purpose
1.0 × 10−4 M solutions of the complexes were treated with a 0.01 M
solution of OAPH, and the spectra were recorded at 5 min time
interval. The time dependent spectral profiles for a period of 1 h
after the addition of OAPH are shown in Fig. S3† and Fig. 6 for 1
and 2, respectively. The spectral scans reveal the rapid increase of
peak intensity at ca. 434 nm for 2, suggesting the excellent catalytic
conversion of OAPH to 2-aminophenoxazine-3-one under aero-
bic conditions. Unlike for 2, a significantly slower increase of the
phenoxazinone chromophore band is noticed for 1 which at least
tentatively indicates the lower reactivity of 1 in comparison to 2 .
We also speculate that in DMF solution pyridine may be released
from the coordination sphere and thus it is necessary to check if
pyridine is involved in the autoxidation of the substrate. But the
spectral scan in the presence of 2.0 × 10−4 M pyridine solution does
not show any significant growth during 1 h of reaction in an identi-
cal condition. These spectral behaviors suggest that both complexes
show phenoxazinone synthase-like activity in aerobic condition.
Kinetic studies were performed to understand the extent of
the catalytic efficiency. For this purpose, a 1.0 × 10−4 M solu-
tion of 2 or 2.0 × 10−4 M solution of 1 was combined with
OAPH maintaining pseudo-first-order conditions. For a particular
complex-substrate mixture, time scan at the absorbance maxi-
mum of 2-aminophenoxazine-3-one was carried out for a period
of 20 min, and the initial rate was determined by linear regression
from the slope of the absorbance versus time plot. As depicted in
Fig. 7, the initial rate of the reactions versus concentrations of the
substrate plot shows rate saturation kinetics. This observation indi-
cates that an intermediate complex-substrate adduct is formed in
Fig. 8. Lineweaver–Burk plots for the oxidation of o-aminophenol catalyzed by 1
and 2 in DMF. Symbols and solid lines represent the experimental and simulated
profiles, respectively.
a pre-equilibrium stage and that the irreversible substrate oxida-
tion is the rate determining step of the catalytic cycle. This type of
rate saturation kinetics on the concentration of the substrate can
be treated with a Michaelis–Menten model, which on lineariza-
tion gives a double-reciprocal Lineweaver–Burk plot to analyze
values of the parameters Vmax, KM, and kcat. The observed and
simulated initial rate versus substrate concentration plot and the
Lineweaver–Burk plot are displayed in Figs. 7 and 8, respectively.
Analysis of the experimental data yields Michaelis binding con-
stant (KM) values of (6.76 2.07) × 10−2 and (2.17 0.47) × 10−2
and Vmax values of (4.83 0.97) × 10−7 and (6.22 0.62) × 10−7 M
s
−1 for 1 and 2, respectively. The turnover number (kcat) values are
obtained by dividing the Vmax by the concentration of the com-
plexes used, and are found to be 8.69 and 22.39 h−1 for 1 and 2,
respectively. Kinetic measurements of the reaction with respect to
catalyst indicate first-order dependency in both cases (Fig. S4).

64
A. Panja et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 56–66
Fig. 10. UV–vis spectra of complex 1 alone and after addition of 50 equivalents of
o-aminophenol in DMF.
Fig. 9. X-band EPR spectra of 1 and after addition of 50 equivalents of OAPH in DMF
at 77 K.
step processes involving OAPH, BQMI and molecular dioxygen
(Scheme 2).
3.6. Plausible mechanism for o-aminophenol oxidation with
spectroscopic supports
3.7. Comparative phenoxazinone synthase activity
The EPR spectrum of frozen DMF solution of complex 1 at 77 K
shows a well-resolved six-line hyperfine-split band at g ≈2.00. The
observed six-line spectrum could be due to solvated MnII ions,
which might result from the decomposition of the complex in
solution. Interestingly, EPR spectra of 1 in DMF solution at room
temperature do not exhibit such characteristic feature. In the case
of decomposition of the complexes in solution, one would expect
free MnII even at room temperature, which usually exhibits a
six-line spectrum. The absence of characteristic six line EPR sig-
nal precludes the possibility of decomposition of the complex in
solution, and thus we suggest that the observed transitions arise
from the mixed valence semiquinonecatecholate manganese(II)
form, MnII(Cat)(SQ), as the MnIII(Cat)2 isomer is expected to be
X-band EPR silent (Fig. 9). This spectral feature suggests that,
even at lower temperature, some amount of MnII(Cat)(SQ)− iso-
mer is present. Similar spectral features have also been observed
in our previous systems [26,28]. Interestingly, upon addition of
50 equivalents of substrate the intensity of the six-line hyperfine
splitting band is abruptly increased which suggests that the con-
centration of manganese(II) species is increased many-fold after
addition of substrate. A similar observation has been made in
the UV–vis spectroscopic method, in which the catecholate-to-
manganese(III) charge-transfer band of the complexes disappears
upon addition of 50 equivalents of substrate (Fig. 10). These spec-
tral studies together suggest that although a minute quantity
of manganese(II)-SQ isomer may be present in the solution, the
dominant manganese(III)-Cat isomer oxidizes o-aminophenol to 2-
aminophenoxazin-3-one and itself reduces to the manganese(II)
state. At this stage, we can suggest a plausible mechanism in
which, at the first step, substrate binds to the manganese(III) cen-
ter through one of the apical positions, and this complex-substrate
intermediate decomposes in the rate determining step to gener-
ate OAP radical, which may generate o-benzoquinonemonoamine
(BQMI) in many ways, including disproportionation of OAP radical
itself. Finally 2-aminophenoxazin-3-one is formed in a multi-
The structural features associated with 1 essentially point to a
mononuclear manganese(III) complex, and the dinuclear nature is
due to weak bonding of manganese(III) with a catecholate oxygen
atom from the adjacent manganese(III) unit together with other
weak interactions like hydrogen bonding and ␲–␲ stacking inter-
actions as discussed earlier. Therefore, it is reasonable to consider
that the structure of both complexes are similar in DMF solution, as
they are expected to exist as a mononuclear manganese(III) entity.
In our previous studies, we have noted that reduction potential of
the metal centers play an important role in the biomimetic cat-
alytic activity, and it has been found that easily reducible metal
centers in the complexes exhibited stronger oxidase activity [31].
Electrochemical reduction potentials of the manganese(III) centers
in both the complexes are similar, and therefore, the significant dif-
ferences in their catalytic ability raises the question as to whether
the positively charged center in the ligand backbone in 2 may have
a role in the significantly increased catalytic activity over 1 . It is
well known that in biological systems non-covalent interactions
such as hydrogen bonding, electrostatic attraction and hydropho-
bic interaction play crucial roles in molecular recognition in various
biochemical processes and it is also reported that such weak inter-
actions may also stabilize the intermediates and transition states
in several enzymatic processes [48,49]. For example, a scheme has
been proposed to explain the activity of CuZn-superoxide dismu-
tase (SOD), where the copper(II) lies at the bottom of a narrow
channel, and the positively charged arginine and lysine residues
are believed to play a role in attracting the anions and guiding
them into the channel [50,51]. Therefore, the significantly higher
catalytic activity of 2 over complex 1 is likely due to the presence of
a positive charge on the ligand backbone in which the pyridinium
moiety may attract the substrate OAPH towards the formation of
the complex-substrate adduct. Being overall electronically neutral,
complex-substrate intermediate formation in 2 is also quite favor-
able in comparison to 1 .

A. Panja et al. / Journal of Molecular Catalysis A: Chemical 412 (2016) 56–66
65
Scheme 2. Plausible mechanistic pathway for the metal catalyzed oxidation of o-aminophenol.
4. Conclusions
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molcata.2015.11.
008.

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