Selective coordination of multidentate ligands in manganese(II) complexes: Syntheses, structures and phenoxazinone synthase mimicking activity

Article abstract of DOI:10.1016/j.poly.2014.05.017

The present report describes the syntheses, structures and catalytic activity mimicking the function of phenoxazinone synthase of four new mononuclear manganese(II) complexes, [Mn(L¹)Cl₂] (1), [Mn(L²)Cl₂] (2), [Mn(L³)Cl₂] (3) and [Mn(L⁴)Cl₂]·H₂O (4), derived from the multidentate ligands L¹-L⁴, which are the 1:2 condensation products of triamines (or tetraamine) and 2-pyridinecarboxaldehyde (or 6-methyl-2-pyridinecarboxaldehyde). X-ray crystallography reveals the preferential coordination ability of all the ligands (cyclic isomers over the acyclic analogues) to a common metal center (Mn²⁺) and in one case (2) methanol addition across the -CN- bond. All the complexes show moderate phenoxazinone synthase activity and the experimental data suggest that a correlation can be drawn between E_1/2 and V_max of the complexes, namely the lower the E_1/2 value the higher the catalytic activity. The effect of methyl substitution on the pyridine ring (in 3) on the matrix parameters and on the phenoxazinone synthase activity is also noticeable. The ESI-MS positive spectrum of a mixture of a representative complex (1 or 4) and o-aminophenol shows a peak corresponding to [Mn(L)(OAP)]⁺ (OAPH = o-aminophenol), suggesting that the catalytic activity proceeds through the formation of a stable complex-substrate intermediate.

Full text of DOI:10.1016/j.poly.2014.05.017

Accepted Manuscript
Selective coordination of multidentate ligands in manganese(II) complexes:
Syntheses, structures and phenoxazinone synthase mimicking activity
Anangamohan Panja
PII:
S0277-5387(14)00314-3
DOI:
http://dx.doi.org/10.1016/j.poly.2014.05.017
POLY 10730
Reference:
Polyhedron
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Accepted Date:
1 March 2014
1 May 2014
Please cite this article as: A. Panja, Selective coordination of multidentate ligands in manganese(II) complexes:
Syntheses, structures and phenoxazinone synthase mimicking activity, Polyhedron (2014), doi: http://dx.doi.org/
10.1016/j.poly.2014.05.017
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Selective coordination of multidentate ligands in manganese(II)
complexes: Syntheses, structures and phenoxazinone synthase
mimicking activity
Anangamohan Panja*,a
a Postgraduate Department of Chemistry, Panskura Banamali College, Panskura RS, Purba
Medinipur, West Bengal 721 152, India
E-mail: ampanja@yahoo.co.in
Abstract
The present report describes the syntheses, structures and catalytic activity mimicking the
function of phenoxazinone synthase of four new mononuclear manganese(II) complexes,
[Mn(L1)Cl2] (1), [Mn(L2)Cl2] (2), [Mn(L3)Cl2] (3) and [Mn(L4)Cl2]·H2O (4), derived from the
multidentate ligands L1–L4, which are the 1:2 condensation products of triamines (or
tetraamine) and 2-pyridinecarboxaldehyde (or 6-methyl-2-pyridinecarboxaldehyde). X-ray
crystallography reveals the preferential coordination ability of all the ligands (cyclic isomers
over the acyclic analogues) to a common metal center (Mn2+) and in one case (2) methanol
− bond. All the complexes show moderate pheno
addition across the –C=N
xazinone synthase
activity and the experimental data suggest that a correlation can be drawn between E1/2 and Vmax
of the complexes, namely the lower the E1/2 value the higher the catalytic activity. The effect of
methyl substitution on the pyridine ring (in 3) on the matrix parameters and on the
phenoxazinone synthase activity is also noticeable. The ESI-MS positive spectrum of a mixture
of a representative complex (1 or 4) and o-aminophenol shows a peak corresponding to
1

[Mn(L)(OAP)]+ (OAPH = o-aminophenol), suggesting that the catalytic activity proceeds
through the formation of a stable complex-substrate intermediate.
Keywords: Manganese(II) complexes; Selective coordination of ligands; Helical polymer;
Cyclic voltammetry; Phenoxazinone synthase activity; Catalyst-substrate aggregate
1. Introduction
Transition-metal based metalloenzymes capable of processing molecular dioxygen under
ambient conditions have received a great deal of attention as they facilitate the spin-forbidden
interaction between dioxygen and organic matter in various biochemical reactions [1-3].
Understanding of such processes gives us plenty of information for the development of small
molecule catalysts for specific oxidation reactions of organic substances under mild conditions.
Model studies on metalloenzymes with oxidase (oxygenase) activity are therefore of particular
interest for the development of bioinspired catalysts for oxidation reactions in industrial and
synthetic processes. As far as biochemistry is concerned, the enzyme phenoxazinone synthase
[4,5], a multicopper oxidase, is found naturally in the bacterium Streptomyces antibioticus.
This metalloenzyme is known to catalyze the oxidative coupling of two molecules of a
substituted o-aminophenol to the phenoxazinone chromophore in the final step for the
biosynthesis of actinomycin D [6].The later is used clinically for the treatment of Wilm’s tumor,
gestational choriocarcinoma and other tumors, in which the phenoxazinone chromophore is
recognized to inhibit DNA-dependent RNA synthesis by intercalation to DNA [7,8]. In
addition to copper-based models, some functional models for other transition metals have also
been reported which show phenoxazinone synthase activity, though none of the reports
illustrate any structure-property relationship [9-16]. Therefore, the challenge of modeling
phenoxazinone synthase activity with copper and other transition metal ions is still open.
2

On the other hand, Schiff-base compounds that contain a carbon-nitrogen double bond
are an attractive area of research because they can easily undergo reversible reactions under
certain conditions, such as E−Z isomerization reactions, nucleophilic additions, transamination
reactions, tautomerism and aza Diels-Alder reactions. Among these, the condensation reaction
of aromatic aldehydes with α,ω-di-primarypolyamines, consisting of both primary and
secondary amines, is known to afford both Schiff dibases and the isomers with a saturated
heterocyclic ring (imidazolidines or hexahydropyrimidines) [17,18]. It is also well explored
that isomerization among these products through formation or opening of the heterocyclic ring
in the presence of metal ions is quite common [19,20]. Two factors, namely the Lewis acidity
and the size of the metal ions, are mainly responsible in this isomerization process. According
to Boča et al. [21,22], the relative acidity of the imine-C and the metal center ultimately decides
in which direction the isomerization of the ligand will shift. For instance, weak Lewis acids,
such as Fe2+, Co2+ and Cu2+, are incapable (or only partly capable) of opening the heterocyclic
ring, whereas stronger Lewis acids (e.g. Fe3+ and Zn2+) can readily do it in order to gain the
appropriate denticity of the ligands. On the other hand, Tuchagues et al. [23] suggested that the
size of the metal ion is the determining factor in shifting the ligand equilibrium towards the
tautomeric form most appropriate to each specific cation. Low-spin Fe2+ has a small enough
ionic radius (75 pm) to fit inside the pocket provided by the imine isomers during the
complexation process, while the ionic radius of Ni2+ (83 pm) is probably too large to allow for
coordination of the imine tautomer, and thus it stabilizes the cyclic analogue.
We have recently reported a study on the oxidation of o-aminophenol by molecular
dioxygen in the presence of some mono- and dinuclear cobalt complexes as catalysts [24-28].
Notably, in one case, a mononuclear cobalt(III) complex showed only stoichiometric oxidation
of o-aminophenol [24], while the remaining compounds were capable of catalyzing the
oxidative coupling of o-aminophenol under aerobic conditions [25-28]. The reason for the
3

catalytic inability of the mononuclear cobalt(III) complex is primarily due to the failure of
regeneration of the catalyst during the course of the reaction, while the presence of labile sites
allows binding of the substrate to the metal center in other complexes, leading to the efficient
catalytic activity. These results indicate that the electronic environment around the metal
centers, along with the labile position, plays a significant role for such activity. Our continuing
interest in the field of bio-inspired catalytic chemistry thus lead us to explore the reaction
between 2-pyridinecarboxaldehyde (or 6-methyl-2-pyridinecarboxaldehyde) and various
α,ω-di-primarypolyamines in a 2:1 molar ratio that are known to afford a mixture of both the
Schiff dibases and their cyclic analogues. These ligands having several donor sites, namely
amine, imine, pyridine and imidazolidine (hexahydropyrimidine), may provide different kinds
of nitrogen donor environments around the metal center, thereby leading to significant
modulation on the catalytic activity. In this context, the syntheses and characterization of four
new manganese(II) complexes, [Mn(L1)Cl2] (1), [Mn(L2)Cl2] (2), [Mn(L3)Cl2] (3) and
[Mn(L4)Cl2]·H2O (4) where L1–L4 are multidentate ligands as shown in Scheme 1, are reported
here. The preferential coordination ability of all the ligands to a common metal center (Mn2+)
and in one case (2) methanol addition across a –C=N− bond, are determined by X-ray
crystallography. The phenoxazinone synthase-like activity and its detailed kinetic studies to
evaluate various kinetic parameters, including turnover numbers for all the complexes, have
also been described. This study not only provides plenty of information regarding role of the
metal ions and other factors on the preferential complexation ability of the ligands, but also
shows the potential of such complexes to exhibit phenoxazinone synthase mimicking activity.
2. Experimental
2.1. Materials and physical measurements
Chemicals, such as 2-pyridinecarboxaldehyde, 6-methyl-2-pyridinecarboxaldehyde,
3-[(2-aminoethyl)amino]-propylamine,
N,N-bis(3-aminopropyl)ethylenediamine
and
4

diethylenetriamine (Aldrich), and manganese(II) chloride tetrahydrate and o-aminophenol
(Merck, India), were of reagent grade and used without further purification. Solvents, namely
methanol, diethyl ether and acetonitrile (Merck, India), were of reagent grade and used as
received.
Elemental analyses for C, H and N were carried out using a Perkin-Elmer 240 elemental
analyzer. The infrared spectra of the complexes in KBr pellets were recorded in the 400-4000
cm–1 range on a Perkin Elmer Infrared spectrometer. Absorption spectra were measured using a
UV-2450 spectrophotometer (Shimadzu) with a 1 cm pathlength quartz cell. Electrochemical
data were collected on an EG&G Princeton Applied Research potentiostat model 263A with a
Pt working electrode, Pt auxiliary electrode and Ag/AgCl reference electrode. Electrospray
ionization mass (ESI-MS positive) spectra were recorded on a MICROMASS Q-TOF mass
spectrometer.
2.2. Synthesis of [Mn(L1)Cl2] (1)
A
mixture
of
2-pyridinecarboxaldehyde
(0.428
g,
4.0
mmol)
and
3-[(2-aminoethyl)amino]-propylamine (0.234 g, 2.0 mmol) in 30 ml of methanol was heated to
reflux for 30 min. To the resulting yellow solution a methanol solution of MnCl2·4H2O (0.396 g,
2 mmol) was added and the mixture refluxing was continued for 30 min. The solution was
allowed to cool to room temperature and upon addition of ether a pale-yellow powder
separated out from the solution. The precipitate was collected by filtration, washed with
methanol/ether and air dried. Yield: 0.674 g (80%). Crops of pale-yellow crystals suitable for
X-ray analysis were obtained from slow diffusion of ether into a methanolic solution of the
complex. Anal. Calc. for C17H21Cl2MnN5: C, 48.47; H, 5.03; N, 16.63. Found: C, 48.37; H,
5.15; N, 16.38 %. IR (KBr, cm–1): 3233 br (νN–H ); 1639 s (νC=N, imine); 1597 s (νC=N, pyridine).
A major peak was detected at m/z = 385.12, corresponding to [Mn(L1)Cl]+.
5

2.3. Synthesis of [Mn(L2)Cl2] (2)
An identical synthesis was performed using 2-pyridinecarboxaldehyde,
diethylenetriamine and MnCl2·4H2O as starting materials, which led to 0.614 g (70%) of a light
yellow precipitate. Crystallization of complex 2 was performed by slow diffusion of ether into
a methanol solution of the complex, which afforded light-yellow crystals suitable for X-ray
crystallography. Anal. Calc. for C17H23Cl2MnN5O: C, 46.49; H, 5.28; N, 15.94. Found: C,
46.37; H, 5.29; N, 15.80%. IR (KBr, cm–1): 3257 br (νNH); 1604 m (νC=N, pyridine). A major
peak was detected at m/z =403.10, corresponding to [Mn(L2)Cl]+.
2.4. Synthesis of [Mn(L3)Cl2] (3)
A similar synthetic methodology was applied using 6-methyl-2-pyridinecarboxaldehyde,
diethylenetriamine and MnCl2·4H2O as starting materials, which afforded 0.680 g (78%) of a
light-yellow crystalline material. A single crystal of X-ray diffraction quality was obtained
from slow diffusion of ether into a methanolic solution of 3. Anal. Calc. for C18H23Cl2MnN5: C,
49.67; H, 5.32; N, 16.09. Found: C, 49.57; H, 5.20; N, 15.98%. IR (KBr, cm–1): 3226 br (νNH);
1636 m (νC=N); 1600 m (νC=N, pyridine). A major peak was detected at m/z = 399.10,
corresponding to [Mn(L3)Cl]+.
2.5. Synthesis of [Mn(L4)Cl2]·H2O (4)
An identical synthesis was carried out using 2-pyridinecarboxaldehyde,
N,N-bis(3-aminopropyl)ethylenediamine and MnCl2·4H2O as starting materials, yielding
0.745 g (85%) of a dark yellow precipitate. Dark-yellow crystals suitable for X-ray diffraction
were obtained from slow diffusion of ether into a methanol solution of 4. Yield: 0.794 g (80%).
Anal. Calc. for C20H30Cl2MnN6O: C, 48.40; H, 6.09; N, 16.93. Found: C, 48.37; H, 6.19; N,
16.74%. IR (KBr, cm–1): 3444 br (νNH, OH); 1600 m (νC=N, pyridine). A major peak was detected
at m/z = 442.18, corresponding to [Mn(L4)Cl]+.
6

2.6. X-ray crystallography
Single crystal X-ray diffraction data for complexes 1 and 4 were collected on a Bruker
SMART APEX-II CCD diffractometer using graphite monochromated Mo-Kα radiation (λ =
0.71073 Å), while diffraction data for 2 and 3 were collected on a Nonius Kappa CCD
diffractometer (λ = 0.71073 Å). Experiments were performed at 120(2) K for 2, at 150(2) K for
3 and at room temperature for 1 and 4. Cell refinement, indexing and scaling of the data sets
were carried out using Bruker Apex2 suite [29], SADABS [29], DENZO-SMN and
SCALEPACK programs [30]. All the structures were solved by direct methods and all
non-hydrogen atoms were refined by full-matrix least squares based on F2 using the
SHELXL-97 [31] program. Hydrogen atoms attached to carbon atoms were included in the
structure factor calculations in geometrically idealized positions, while those connected to
nitrogen and oxygen atoms were located in the difference Fourier maps and were isotropically
treated using a riding model with their isotropic displacement parameters fixed at 120% of their
respective parent atoms. The restraints on the N–H bond distances were applied for complexes
2 and 4 and on the O–H bond distances for complex 4 as without these restrictions the
refinement was not stable. Relevant crystallographic data together with refinement details for
1-4 are given in Table 1.
2.7. Kinetics of the phenoxazinone synthase activity
The kinetics of the aerobic oxidation of o-aminophenol catalyzed by 1-4 was investigated
monitoring the growth of the absorbance as a function of time at 435 nm (ε = 24 × 103 M–1
cm–1) [12], which is characteristic of 2-aminophenoxazin-3-one in methanol. To determine the
dependence of the rate of the reaction on the substrate concentration and to evaluate various
kinetic parameters, 1 × 10–4 M solutions of the complexes were treated with at least 10
equivalents of substrate so as to maintain the pseudo first order conditions. Similarly, varying
7

the amounts of the catalyst, (5-100) × 10–5 M, were mixed with a fixed concentration (1 × 10–2
M) of substrate in methanol (2.0 ml) saturated with dioxygen in order to examine the catalyst
dependency on the rate of the reaction. The initial rate method was applied to determine the rate
of a reaction, and the average initial rate over three independent measurements was determined
by linear regression from the slope of the absorbance versus time plot.
3. Results and discussion
3.1. Syntheses and general characterizations
It has been well explored that the condensation of a polyamine with
2-pyridinecarboxaldehyde in a 1:2 molar ratio yields a mixture of the Schiff dibase and its
heterocyclic analogue, the ratio of which can be examined by NMR spectroscopy [22,23,32]. It
is also expected that substitution, like with a methyl group, at the pyridine ring should not have
a significant effect on such a tautomerization reaction. Recent reports disclosed that
metal-assisted ring opening/closing processes are common for such ligand systems, and
several factors are responsible in this event, including the size of the metal ions, Lewis acidity
and stereochemistry of the metal centers [22,23,32]. In this context, four different multidentate
ligands prepared in situ by the condensation of two equivalents of 2-pyridinecarboxaldehyde
(or 6-methyl-2-pyridinecarboxaldehyde) and one equivalent of polyamine were treated with
the hydrated salt of manganese(II) chloride in methanol in order to examine their coordinating
ability and the stability of their different isomeric forms upon complexation of a common metal
ion. On the basis of the X-ray crystal structures (vide infra), they are unambiguously
characterized as [Mn(L1)Cl2] (1), [Mn(L2)Cl2] (2), [Mn(L3)Cl2] (3) and [Mn(L4)Cl2]·H2O (4),
in which all the ligands exclusively bind the metal center through the cyclic isomeric form. The
elemental analyses of the complexes are consistent with the proposed molecular formula.
In the IR spectra of complexes 1-3, a medium intense peak in the range 3223-3257 cm-1
indicates the N−H stretching vibration of the ligands. This N−H stretching band overlaps with
8

the O–H stretching vibration of the lattice water molecule, and a resultant broad band was
found at 3444 cm-1 in 4. In the IR spectra of complexes 1 and 3, a strong and sharp band due to
azomethine ν(C=N) appears at 1639 and 1636 cm-1, respectively. The absence of a band in this
region for 2 clearly suggests modification of the azomethine bond, which is consistent with the
structural characterization of 2 where addition of a methanol molecule across the –C=N– bond
was noticed. A similar peak was also absent in the spectrum of 4, indicating the presence of two
hexahydropyrimidine rings in 4. In addition, a sharp peak varying from 1597 to1604 cm-1 was
observed for all of the complexes, which might be assigned to the stretching vibration of the
pyridyl C=N bond.
3.2. Structural studies
The crystal structures of complexes 1-4 were determined by single crystal X-ray
diffraction analyses. The molecular structures of the four complexes, along with the atom
numbering scheme, are shown in Figs. 1–4, respectively, and selected bond lengths and angles
are given in Table 2 for 1-3 and in Table 3 for 4. Complexes 1 and 3 crystallize in the triclinic
centrosymmetric space group Pī, and in the monoclinic centrosymmetric space group C2/c,
respectively, leading to racemic compounds. Complex 2 crystallizes in the orthorhombic chiral
P212121 space group, suggesting that it resolves as an optical isomer and is not likely to
represent the bulk sample. In complexes 1 and 3, the geometry of the metal center is
pseudo-octahedral, being comprised of one imine, one amine (hexahydropyrimidine in 1 or
imidazolidine in 3) and two pyridyl nitrogen donors from the folded dissymmetric ligands, and
two exogenous chloride ions in a cis disposition. Although the geometry of the metal center in
2 is a distorted octahedronl, the essential difference between 2 and the other complexes
originates from nature of the tetradentate ligand L2, which is a dissymmetric ligand including
an imidazolidine ring, derived from the addition of methanol across the azomethine bond,
leading to the conversion of the rigid imine double bond into a more flexible C–N single bond.
9

The concerning C–N bond length of 1.452(5) Å (C6–N2) in 2 is significantly longer than that
of the imine bond found in 1 [C6–N2, 1.262(2) Å] and 3 [C7–N2, 1.265(3) Å], confirming it is
a single bond in nature. The structure of complex 2 also differs from 1 and 3 in the relative
positions of the terminal pyridine groups, which are trans to each other in 2, while they are cis
to each other in 1 and 3. This difference may originate from the increased flexibility in 2
because of the addition of the methanol molecule across the azomethine double bond. The
Mn–Cl bond lengths, varying in the range 2.428(1)-2.440(1) Å in 1, 2.460(1)-2.465(1) Å in 2
and 2.431(1)-2.439(1) Å in 3, are usual. The average Mn–Npy bond distances [2.358(2) Å for 1
and 2.268(3) for 2] are considerably shorter than the Mn–Namine bond distances [2.472(2) Å for
1 and 2.351(2) Å for 2], which is in agreement with their state of hybridization. However, the
above does not agree in 3, as the average pyridine bond distance of 2.426 Å is significantly
longer than that of the imine bond [2.230(2) Å] and it is even longer than the amine bond
[2.374(2) Å]. All the matrix parameters are comparable to closely related systems reported in
the literature [32-36]. The degree of distortion from an ideal octahedral geometry (90/180°) is
reflected in both the cisoid and transoid angles, as shown in Table 2. Such a distortion from an
ideal octahedral geometry is presumably due to the shorter bite angles of the dissymmetric
ligands, especially those forming five-membered chelate rings. The five-membered chelates
involving the imine-N are conjugated and have a nearly planar conformation, as expected,
while rest are in an envelope conformation, indicating saturated non-planar characteristics.
Moreover, the six-membered hexahydropyrimidine ring in 1 is in the more stable chair
conformation, as found in most systems [23]. The crystal packing is essentially stabilized by
moderately strong π–π stacking interactions in 1 and 3 (Figs. S1 and S2 for 1 and 3,
respectively), but hydrogen bonding interactions in 2 are forefront in stabilizing the molecular
packing in the solid state. In addition to an intramolecular hydrogen bond with the amine group
of the imidazolidine ring, one of the coordinated Cl atoms is involved in intermolecular
10

hydrogen bonding with the amine hydrogen atom that resulted from the addition of the
methanol molecule across the azomethine group in 2 (Table 4). This ultimately yields a
hydrogen bonded supramolecular chain propagating along the b axis (Fig. 5). Interestingly,
these hydrogen bonded supramolecular chains eventually construct helical polymers of 2 in the
direction of the b axis, as shown in Fig. 5, and they weakly interact with each other by means of
π–π stacking interactions (not shown).
Complex 4 crystallizes in the monoclinic centrosymmetric P21/c space group with a lattice
water molecule, suggesting the meso-form is present in the solid state in which two carbon
centers C6 and C15 have different chirality. X-ray crystallography reveals that the geometry of
the central Mn2+ ion is pseudo-octahedral, comprising of two pyridyl nitrogen atoms and two
amine nitrogen atoms from ligand L4 and two chloride ions. In the crystal structure, the folded
pyridyl nitrogen atoms from the ligand L4 are trans coordinated, while the two chloride ions are
cis coordinated. Both the hexahydropyrimidine rings in L4 are in the more stable chair
conformation. The Mn–Npy bond distances of 2.261(4) and 2.274(4) Å are considerably shorter
than the Mn–Namine distances [2.443(5) and 2.365(4) Å] as expected. Again, the Mn–Cl bond
distances of 2.458(2) and 2.467(2) Å are usual. In the structure, all three chelate rings are
five-membered, which is mainly responsible for the structural distortion from the ideal
octahedral geometry [Table 3]. Careful inspection of the packing diagram shows that the
solvent water molecule, together with the secondary amine group of the hexahydropyrimidine
ring and one coordinated Cl atom form a 2D supramolecular sheet in the ac plane through
moderately strong hydrogen bonding interactions [Fig. 6 and Table 4].
A number of inferences can be obtained from the present study, and some of them nicely
agree well with reported literature results. Here, two types of multidentate ligands are used;
ligands L1-L3 are similar in that they have been derived from the condensation of triamines
with 2-pyridinecarboxaldehyde (or 6-methyl-2-pyridinecarboxaldehyde), while L4 is obtained
11

from the condensation of a tetraamine with 2-pyridinecarboxaldehyde. In the crystal structures,
all these ligands exclusively bind the Mn2+ ion through the cyclic
(hexahydropyrimidine/imidazolidine) form of the ligands and the substitution of a methyl
group at the sixth position of the pyridine ring (in 3) does not have any influence on such
selectivity. Although ligands L1 and L2 are very similar, the only difference arising from the
heterocyclic rings (a hexahydropyrimidine ring in L1 and an imidazolidine ring in L2), the
diverse patterns in the crystal structures are quite interesting. As the five-membered
imidazolidine ring is not flexible enough, a mismatch of the tetradentate imine form of the
ligand L2 with the coordination requirements of the Mn2+ ion probably allows the nucleophilic
attack on the C=N imine bond by the solvent methanol molecule, which results in MeOH
addition across the azomethine bond. This gain in flexibility favors the complexation of the
ligand with the Mn2+ ion in 2. On the other hand, as the hexahydropyrimidine ring is more
flexible, L1 is able to form complex 1 with the Mn2+ ion comfortably. The substantial
difference between the crystal structures of 2 and 3 is, at a first glance, quite surprising as both
complexes are derived from diethylenetriamine, with 6-methyl-2-pyridinecarboxaldehyde
being used in 3 instead of 2-pyridinecarboxaldehyde in 2. X-ray crystallography shows that
only in 2 has the imine group been modified by the addition of a methanol molecule across the
–C=N– bond. This difference probably arises from the substitution of the methyl group at the
sixth position of pyridine, which increases some sort of steric congestion at the donor site. This
steric hindrance prevents a closer approach of the pyridyl groups towards the MnII center,
thereby leading to longer metal-pyridyl bond distances, which have been mentioned earlier
[28,37,38]. In such a situation, the ligand L3 faces a lesser constraint to bind the metal center in
3. The addition of a methanol molecule across the azomethine group has a diverse impact on
the supramolecular motif in 2 because that amine nitrogen constitutes the helical chain
12

structure along the b axis, whereas π–π stacking interactions are mainly responsible for the
stability of the crystal packing in the solid state in 3.
3.3. Cyclic voltammetry
Redox active metalloenzymes catalyze the redox processes in biochemical reactions, in
which the redox flexibility of the metal centers play a crucial role in the processes. Therefore,
the electrochemical behaviors of their models are of profound importance as they could give us
an insight into their catalytic ability. The electrochemical behavior of complexes 1-4 was
investigated in acetonitrile solution containing 0.1 M tetraethylammonium perchlorate (TEAP)
as a supporting electrolyte in a conventional three electrode configuration using a Pt disk
working electrode, Pt auxiliary electrode and Ag/AgCl reference electrode. As the cyclic
voltammetric features of all the complexes are very similar, a representative profile for 3 is
depicted in Fig. 7, while electrochemical data for all the complexes are given in Table 5. All the
complexes show nearly quasi-reversible electrode responses that can be attributed to the
Mn(II)/Mn(III) oxidation process, which is in line with closely related manganese(II)
complexes of similar ligand environments [36,39]. The E1/2 value of 1.05 V for complex 3 is
significantly higher than the other complexes, which might be due to the steric congestion
arising from the methyl substitution at the sixth position of the pyridine ring. As we have seen
longer metal-pyridyl bond distances in complex 3, the stability of the higher oxidation state is
not favorable and as a consequence it shows higher electrode responses. Similar findings were
reported upon switching from a regular pyridine to a 6-methylpyridine ligand in other
manganese, iron and cobalt complexes [28,40-43].
3.4. Phenoxazinone synthase-like activity
The phenoxazinone synthase mimicking activity of the complexes was examined by
monitoring the oxidation of o-aminophenol (OAPH) spectrophotometrically in
13

dioxygen-saturated methanol because of the good solubility of the complexes, the substrate,
and the final product in this solvent. In order to avoid the autoxidation of the substrate by air,
the catalytic activity was examined in the absence of an added base. Before going into a
detailed kinetic investigation, it is necessary to check the ability of the complexes to behave as
catalysts for the oxidation of 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. Immediately after the addition of
substrate to the solutions of the catalysts, the growth of the absorption peak at ca. 435 nm,
characteristic of the phenoxazinone chromophore [12], was observed, which indicates catalytic
oxidation of o-aminophenol to the corresponding 2-aminophenoxazin-3-one. The time
dependent spectral profiles over a period of 2 h after the addition of OAPH are shown in Figs.
S3, S4, S5 and 8, for complexes 1-4, respectively. A control experiment was carried out using a
manganese(II) salt instead of the complexes, and this did not show any significant increase in
the band intensity at 435 nm under identical reaction conditions. These observations may
rationalize the way in which all the complexes exhibit phenoxazinone synthase-like activity
under aerobic conditions.
In order to understand the degree of catalytic efficiency, kinetic studies of all the
complexes were performed at 25 °C. For this purpose, 1 × 10−4 M solutions of the complexes
were treated with at least a 10-fold concentrated substrate solution to maintain
pseudo-first-order conditions. For a particular complex-substrate mixture, a time scan at the
maximum band of 2-aminophenoxazine-3-one was carried out over a period of 30 min, and the
initial rate was determined by linear regression from the slope of the absorbance versus time
plot. The initial rate of the reaction verses concentration of the substrate plot shows rate
saturation kinetics (Fig. 9 for 2 and 4 and Fig. S6 for 1 and 3). This observation indicates that
2-aminophenoxazine-3-one formation proceeds through a relatively stable intermediate, a
complex-substrate adduct, followed by the redox decomposition of the intermediate at the rate
14

determining step. The initial rate versus substrate concentration data were then analyzed on the
basis of the Michaelis-Menten approach of enzymatic kinetics to get a Lineweaver-Burk plot,
as well as values of the parameters Vmax, KM and kcat. The observed and simulated initial rates
versus substrate concentration plots for 2 and 4 are depicted in Fig. 9, while similar plots for 1
and 3 are shown in Fig. S6, and the representative Lineweaver-Burk plots for 1 and 4 are
displayed in Fig. 10. Analysis of the experimental data show that the Michaelis binding
constants (KM) for all the complexes are very similar and vary in the narrow range from (2.34
0.46) × 10–2 to (5.62 0.92) × 10–2, whereas the Vmax values are significantly different from
each other and they vary in the wide range from (2.27 0.07) × 10–7 for 3 to (7.31 0.14) ×
10–7 for 4. The turnover number (kcat) value is obtained by dividing the Vmax value by the
concentration of the corresponding complexes, and they are listed in Table 6. Moreover, for a
particular substrate concentration, varying the complex concentration, a linear relationship for
the initial rates was obtained, which shows a first-order dependence on the complex
concentration (Fig. S7).
3.5. Electrospray ionization mass spectral study
The electrospray ionization mass spectra (ESI-MS positive) of methanol solutions of
compounds 1-4 were recorded and they are very similar. Each spectrum contains three
common peaks with the virtually the same line-to-line separations (to within one unit); m/z =
349.15, 385.12 (base peak) and 296.22 for 1, m/z = 367.05, 403.10 (base peak) and 314.14 for
2, m/z = 363.09, 399.10 (base peak) and 310.10 for 3 and m/z = 406.20, 442.18 (base peak) and
353.28 for 4. These peaks, together with the isotopic distribution patterns, are assignable to the
[MnII(L–H)]+, [MnII(L)Cl]+ and (L+H)+ ions, respectively.
To get an insight into the nature of the possible complex-substrate intermediate, ESI-MS
positive spectra of a 1:50 mixture of the two most reactive complexes, 1 and 4, with OAPH
were recorded after 5 minutes of mixing in methanol and the results are depicted in Figs. S8
15

and 11, respectively. Both spectra are comprised of all the peaks that were found for the
complexes alone. In addition, one minor peak at m/z = 458.12 for 1 and 515.21 for 4 is quite
interesting because the peak position in each case clearly indicates that this peak arises from the
1:1 complex-substrate aggregate [MnII(L)(OAP)]+, which is consistent with the rate saturation
kinetics as discussed earlier.
3.6. Comparative discussion and probable mechanism
At this juncture, it is clear that all the complexes are reactive towards the catalytic
oxidation of o-aminophenol, and the kinetic analysis as well as ESI-MS spectral study clearly
indicates that the reaction proceeds through stable complex-substrate intermediate formation.
In order to justify the involvement of molecular dioxygen in the catalytic cycle, UV-vis spectra
of a mixture of a 1 × 10−4 M solution of the most reactive complex, 4, with 100 fold excess of
OAPH were recorded in nitrogen atmosphere, and no spectral growth at 435 nm was observed
until one hour of mixing. This finding unambiguously proves that molecular dioxygen oxidizes
OAPH to the corresponding 2-aminophenoxazine-3-one in the catalytic cycle in the presence
of the manganese(II) complex as a catalyst. Based on the above results, it is tempting to
propose a mechanism involving stepwise pathways, keeping in mind that it is impossible to
prove any single mechanism. A tentative catalytic cycle for the formation of the
phenoxazinone chromophore is shown in Scheme 2. In the first step OAP forms an adduct with
the manganese(II) complex, which yields an OAP radical by the reaction with molecular
dioxygen in the rate determining step. The OAP radical may generate o-benzoquinone
monoamine (BQMI) in many ways, including the disproportionation of the OAP radical itself.
Finally, 2-aminophenoxazine-3-one is produced via several oxidative dehydrogenation
processes involving OAPH, O2 and BQMI, as shown in Scheme 2. The turnover numbers
suggest that the reactivity of the complexes follows the order 4 > 1 > 2 > 3. It has been well
explored that the catalytic activity of a complex depends on several factors, such as the
16

geometry of the complex, redox potential of the central metal ion and structural flexibility. The
present results can be nicely interpreted by considering the electrochemical potential in
combination with the structural flexibility of the complexes. The most flexible complex in the
present investigation is 4 as it contains two hexahydropyrimidine moieties in addition to two
pyridine groups, which not only favors the binding of the incoming substrate but also helps to
shuffle between the two different oxidation states that may occur during the catalytic cycles. In
addition, the lower electrochemical response of complex 4 facilitates the interaction of the
metal center with molecular dioxygen in the catalytic cycle. In comparison to 4, the remaining
three complexes exhibit significant lower reactivity; the reason behind the lower reactivity is
probably the presence of a shorter ethylenic linker which makes the systems stiffer to structural
reorganization during the course of the reactions. Although ligand L2 and L3 in 2 and 3,
respectively, are very similar, the gain in flexibility by the addition of methanol across the
azomethine bond increases the reactivity of 2 in comparison to 3. Furthermore, the presence of
methyl substitution at the sixth position of pyridine enforces the pyridyl bond length to be
longer than the others, and thereby the possibility of stabilization of the higher oxidation state
is significantly reduced in 3. Therefore, both electronic and steric factors significantly reduce
the efficiency of the phenoxazinonone synthase activity of 3.
4. Conclusions
Four novel mononuclear manganese(II) complexes have been synthesized and
structurally characterized by X-ray crystallography. The structural characterizations reveal that
all these ligands exclusively bind the Mn2+ ion through the cyclic
(hexahydropyrimidine/imidazolidine) form of the ligands and the substitution of a methyl
group at the sixth position of the pyridine ring (in 3) does not have any influence on such
selectivity. The crystal packing of the complexes is mainly stabilized by hydrogen bonding and
π–π stacking interactions, and in one case (2), a helical polymer results from the hydrogen
17

bonding interactions. All the complexes are moderately reactive towards the catalytic oxidation
of o-aminophenol, and their relative catalytic efficiency mainly arises from the structural
flexibility of the ligands as well as the electrochemical behaviors of the complexes. From the
experimental results a nice correlation can be established between the E1/2 values and the
catalytic activity, with easily oxidizable metal centers in the complexes showing higher
catalytic activity. The ESI-MS positive spectra of a mixture of representative complexes (1 or
4) and OAPH show a peak corresponding to [Mn(L)(OAP)]+, which supports complex
substrate adduct formation in the catalytic cycle.
Appendix A. Supplementary data
The electronic supplementary information file contains Figs. S1-S9. CCDC 962740-962743
contains the supplementary crystallographic data for complexes 1-4. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or email:deposit@ccdc.cam.ac.uk.
Acknowledgements
A. P. is grateful to the University Grants Commission, India for financial support of this
research (Sanction No. PSW-173/11-12). Crystallography was performed at the DST-FIST,
India, funded single crystal diffractometer facility in the Department of Chemistry, Jadavpur
University, India.The instrumental facility from Indian association for the Cultivation of
Science, India is also gratefully acknowledged.
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21

Table 1. Crystal data and structure refinement for complexes 1-4.
1
2
3
4
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C17H21Cl2MnN5 C17H23Cl2MnN5O C18H23Cl2MnN5 C20H30Cl2MnN6O
421.23
439.24
120(2)
435.25
150(2)
496.34
298(2)
298(2)
0.71073
Triclinic
Pī
0.71073
Orthorhombic
P212121
8.2268(1)
13.5183(3)
17.9246(4)
90
0.71073
Monoclinic
C2/c
0.71073
Monoclinic
P21/c
7.4731(3)
8.6373(3)
15.7397(5)
83.325(2)
76.944(2)
69.424(1)
925.80(6)
2
32.5419(10)
7.4072(4)
15.8346(5)
90
8.5271(3)
16.1648(6)
16.5322(6)
90
b (Å)
c (Å)
α (°)
90
90.460(3)
90
91.897(2)
90
β (°)
90
γ (°)
Volume (Å3)
1993.44(7)
4
3816.7(3)
8
2277.53(14)
4
Z
Dcalc (g cm–3)
µ (mm–1)
F(000)
1.511
1.464
1.515
1.448
1.012
0.946
0.984
0.839
434
908
1800
1036
2.52-27.12
1.89-27.46
8848
3.10-27.47
7080
1.76-27.33
35246
θ range (°)
Reflections collected 13715
Independent
4032 / 0.0190
4536 / 0.0291
4377 / 0.0281
5068 / 0.0736
reflection / Rint
Observed reflections 3405
3967
3384
3024
Goodness-of-fit on
F2
1.038
1.154
1.050
1.042
Final R indices
[I>2σ(I)]
R1 = 0.0303,
wR2 =0.0715
R1 = 0.0383,
wR2 =0.0758
R1 = 0.0329,
wR2 =0.0868
R1 = 0.0482,
wR2 =0.1397
0.716 / –0.746
R1 = 0.0380,
wR2 =0.0895
R1 = 0.0591,
wR2 =0.0991
0.419/ -0.518
R1 = 0.0753,
wR2 =0.1864
R1 = 0.1288,
wR2 =0.2108
1.248/ –0.831
R indices (all data)
Largest
diff. 0.566/ –0.240
peak/hole (e Å–3)
22

Table 2. Selected bond lengths (Å) and bond angles (º) for complexes 1-3.
1
2
3
Bond distance
Mn1–N1
Mn1–N2
Mn1–N3
Mn1–N5
Mn1–Cl1
Mn1–Cl2
2.290(1)
2.216(1)
2.472(1)
2.426(1)
2.440(1)
2.428(1)
2.282(3)
2.322(3)
2.379(3)
2.254(3)
2.465(1)
2.460(1)
2.365(2)
2.230(2)
2.374(2)
2.487(2)
2.431(1)
2.439(1)
Bond angle
N1–Mn1–N2
N1–Mn1–N3
N1–Mn1–N5
N2–Mn1–N3
N2–Mn1–N5
N3–Mn1–N5
N1–Mn1–Cl1
N1–Mn1–Cl2
N2–Mn1–Cl1
N2–Mn1–Cl2
N3–Mn1–Cl1
N3–Mn1–Cl2
N5–Mn1–Cl1
N5–Mn1–Cl2
Cl1–Mn1–Cl2
71.52(5)
140.39(5)
89.65(5)
73.39(5)
82.57(5)
68.02(5)
102.08(4)
99.35(4)
165.88(4)
91.36(4)
96.52(4)
110.96(4)
86.71(4)
164.42(4)
101.25(2)
72.14(11)
98.09(11)
170.42(12)
75.74(11)
102.09(12)
72.81(11)
95.15(9)
71.22(7)
145.66(7)
101.68(6)
74.57(7)
75.53(7)
72.14(6)
111.00(5)
91.38(5)
161.17(5)
95.16(5)
102.34(5)
88.71(5)
85.81(5)
160.27(4)
103.40(2)
92.73(9)
88.58(8)
160.15(8)
155.17(8)
94.34(8)
92.29(8)
91.00(10)
105.93(4)
23

Table 3. Selected bond lengths (Å) and bond angles (º) for complex 4.
Bond
Distance
2.261(4)
2.443(5)
2.365(4)
Bond
Distance
2.274(4)
2.458(2)
2.467(2)
Mn1–N1
Mn1–N3
Mn1–N4
Mn1–N6
Mn1–Cl1
Mn1–Cl2
Bond
Angle
Bond
Angle
N1–Mn1–N3
N1–Mn1–N4
N1–Mn1–N6
N3–Mn1–N4
N3–Mn1–N6
N4–Mn1–N6
N1–Mn1–Cl1
N1–Mn1–Cl2
71.78(15)
94.54(15)
160.99(17)
76.68(15)
91.55(15)
72.18(15)
95.71(12)
94.60(12)
N3–Mn1–Cl1
N3–Mn1–Cl2
N4–Mn1–Cl1
N4–Mn1–Cl2
N6–Mn1–Cl1
N6–Mn1–Cl2
Cl2–Mn1–Cl1
160.74(12)
96.03(12)
90.17(11)
165.87(11)
97.81(12)
96.24(12)
99.61(6)
Table 4. Geometry of important hydrogen bonds (Å, º) for 1-4.
D–H···A
D–H
0.85
0.89
0.93
0.83
0.86
0.97
0.90
1.00
H···A
2.64
2.81
2.50
2.82
2.08
2.63
2.11
2.40
D···A
<D–H···A
161
N4–H4···Cl2
N4–H4···Cl2
N2–H2···Cl2
N4–H4A···Cl1
O1–H1O···N2
N2–H2A···Cl2
O1–H2O···N5a
N5–H5···Cl2b
3.458(2)
3.455(4)
3.337(3)
3.474(2)
2.898(9)
3.526(7)
2.969(8)
3.359(5)
1
2
130
150
137
3
4
158
155
161
161
Symmetry code: a = –1+x, 3/2–y, –1/2+z, b = 1+x, y, z.
24

Table 5. Electrochemical data for 1-4 in acetonitrile a
c
a
Complex
Ep (V)
Ep (V)
E1/2(V)
0.93
∆Ep (V)
0.15
0.86
0.88
0.99
0.76
1.01
1.02
1.12
0.88
1
2
3
4
0.13
0.95
0.13
1.05
0.12
0.82
a
Pt working and Ag/AgCl reference electrodes; supporting electrolyte tetraethylammonium
a
c
perchlorate; Scan rate 100 mVs–1; Ep and Ep are anodic and cathodic peak potentials
a
c
a
c
respectively; E1/2 = 0.5(Ep – Ep ); ∆Ep = Ep – Ep .
.
Table 6. Kinetic parameters for phenoxazinone synthase activity for complexes 1-4
complex
Vmax (mol dm-3 s-1)
(3.30 0.09) × 10–7
(2.76 0.15) × 10–7
(2.27 0.07) × 10–7
(7.31 0.14) × 10–7
KM (mol dm-3)
kcat (h-1)
11.90
9.66
(3.25 0.53) × 10–2
(2.96 0.52) × 10–2
(2.34 0.46) × 10–2
(5.46 0.98) × 10–2
1
2
3
4
8.20
26.32
25

Scheme 1. Drawings of the ligands characterized in the present study
.
O
OH
Catalyst
O2
[Adduct]
NH2
NH2
OAP radical
OAPH
[O]
O
O
N
O
OAPH
NH2
NH
O2
BQMI
Phenoxazinone
Scheme 2. Plausible mechanistic pathway showing the formation of
2-aminophenoxazine-3-one.
26

Fig. 1. Molecular structure of Mn(L1)Cl2 (1) with the atom labelling scheme. Ellipsoids are
presented at the 50% probability level.
27

Fig. 2. Crystal structure of Mn(L2)Cl2 (2) with the atom labelling scheme. Ellipsoids are drawn
at the 50% probability level.
Fig. 3. Molecular structure of Mn(L3)Cl2 (3) showing atom labelling scheme. Ellipsoids are
presented at the 50% probability level.
28

Fig. 4. Crystal structure of Mn(L4)Cl2·H2O (4) together with the atom numbering scheme at the
metal coordination sphere. Ellipsoids are drawn at the 50% probability level and the solvent
molecule is omitted for clarity.
Fig. 5. 1D supramolecular hydrogen bonded chain of 2 propagating along the b axis (top).
Space-fill model of 2 showing the helical tunnel parallel to the b axis (bottom)
29