Bio-relevant manganese(II) compartmental ligand complexes: Syntheses, crystal structures and studies of catalytic activities

Article abstract of DOI:10.1016/j.molcata.2011.01.025

Three new mono-manganese(II) complexes of a compartmental ligand, namely [Mn(HL)(H₂O)₃](NO₃)₂·H ₂O (1), [Mn(HL)(SCN)₂(H₂O)]·H ₂O (2), and [Mn(HL){N(CN)₂}(H<

Full text of DOI:10.1016/j.molcata.2011.01.025

Journal of Molecular Catalysis A: Chemical 338 (2011) 51–57
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Bio-relevant manganese(II) compartmental ligand complexes: Syntheses, crystal
structures and studies of catalytic activities
Averi Guhaa, Kazi Sabnam Banua, Arpita Banerjeea, Totan Ghosha, Santanu Bhattacharyab,
Ennio Zangrandoc,∗, Debasis Dasa,∗
a Department of Chemistry, University of Calcutta, 92, A.P.C. Road, Kolkata 700009, India
b Department of Chemistry, Maharaja Manindra Chandra College, Kolkata 700003, India
c Dipartimento di Scienze Chimiche e Farmaceutiche, University of Trieste, Via L. Giorgieri 1, 34127 Trieste, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Three
new
mono-manganese(II)
complexes
of
a
compartmental
ligand,
namely
Received 22 September 2010
Received in revised form 23 January 2011
Accepted 24 January 2011
[Mn(HL)(H2O)3](NO3)2·H2O (1), [Mn(HL)(SCN)2(H2O)]·H2O (2), and [Mn(HL){N(CN)2}(H2O)2](NO3)·H2O
(3), where L = 2,6-bis{2-(N-ethyl)pyridineiminomethyl}-4-methylphenolato, have been synthesized and
characterised by routine physicochemical techniques and complexes 1, 2 also by X-ray single crystal
structure analysis. All the mono nuclear complexes contain MnII high spin species at octahedral core as
evidenced by magnetic moment (measured at 300 K) and EPR study at 77 K. As revealed by crystal struc-
ture analyses, the protonation of one imine nitrogen atom of the potential dinucleating ligand L hampers
to form the expected dinuclear MnII complex. However, complexes 1–3 show excellent catecholase-like
activity with both 3,5-di-tert-butylcatechol and tetrachlorocatechol as substrates. In addition complexes
1 and 2 also exhibit phosphatase activity, while 3 forms an adduct with p-nitrophenyl phosphate as
substrate. To the best of our knowledge this is the first report of MnII complexes being able to catalyze
the oxidation of TCC to TCQ. Catecholase and phosphatase activities have been monitored by UV–vis
spectrophotometer and Michaelis–Menten equation has been applied to rationalize all the kinetic
parameters where complex 1 shows maximum kcat value followed by 2 and 3 (where for phosphatase
activity 3 only forms an adduct).
Available online 24 February 2011
Keywords:
Manganese(II)
Schiff-base complexes
Catecholase activity
Phosphatase activity
Crystal structure
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
and arginase [19] and as the tetra nuclear cluster found in the oxy-
gen evolving complex (OEC) of photo system II (PS II) [20–24]) the
Phenol based compartmental ligands are of current inter-
est to the bio-inorganic chemists as the most desirable target
to synthesize dinuclear complexes as corroborative models of
some metallobiosites namely catechol oxidase, urease, catalase etc.
[1–12]. Recently our group have got excellent results while working
withphenolbased compartmentalligandcomplexesofNiII, CuII and
ZnII [13]. Manganese is one of the most important trace transition
element in biological systems, being present in several metalloen-
zymes of varied nuclearity. Literature survey reveals that although
manganese is one of the most important trace transition element in
biological systems (being present in several metalloenzymes with
varying nuclearity, e.g. mononuclear manganese species exist in
superoxide dismutase [14] and manganese dioxygenase [15], as
dinuclear species in catalase [16,17], ribonucleotide reductase [18],
reports on bio-relevant catalytic activities of manganese(II) com-
partmental ligand complexes are surprisingly scanty.
We started this work with the goal to generate manganese
mimics of catecholase [dicopper(II) containing type-III copper
protein] and of phosphatase [dizinc(II) containing hydrolase
enzyme] since such reports are rare in literature. In order
to stabilize manganese in its +2 oxidation state, we have
accordingly designed
2,6-bis{2-(N-ethyl)pyridineiminomethyl}-4-methylphenolato
(L) as depicted in Scheme 1, system having vacant ␲*
a phenol based compartmental ligand,
a
orbitals, necessary condition to stabilize metals in low oxidation
states. Interestingly, instead of getting dinuclear manganese(II)
complexes we have obtained mono-manganese(II) species,
[Mn(HL)(H2O)3](NO3)2·(H2O) (1), [Mn(HL)(SCN)2(H2O)]·0.5H2O
(2) and [Mn(HL){N(CN)2}(H2O)2](NO3)·H2O (3) most likely due to
the protonation of one of the imine nitrogen atoms of the dinu-
cleating ligand, L. As a result of such peculiarities of the structures
these three complexes show excellent catalytic activities. Here
∗
Corresponding authors. Tel.: +91 33 24837031; fax: +91 33 23519755.
E-mail address: dasdebasis2001@yahoo.com (D. Das).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.01.025

52
A. Guha et al. / Journal of Molecular Catalysis A: Chemical 338 (2011) 51–57
Fig. 1. ORTEP drawing (30% probability ellipsoid) of the complex cation of 1. Of the disordered ethylene bridge only the C17/C18 atoms at higher occupancy are shown.
Fig. 2. Detail of the H-bonding pattern in the crystal packing of 1. Ow4 and nitrate N7 are close to a centre of symmetry and their positions can be interchanged.
Fig. 3. ORTEP drawing (40% probability ellipsoid) of complex 2.

A. Guha et al. / Journal of Molecular Catalysis A: Chemical 338 (2011) 51–57
53
Table 1
Selected bond lengths (Å) and angles (◦) for 1 and 2 with esds in parentheses.
1
2
Mn–O(1)
2.102(2)
2.195(3)
2.264(3)
2.190(3)
2.238(4)
2.180(3)
Mn–O(1)
Mn–N(2)
Mn–N(3)
Mn–O(1w)
Mn–N(5)
Mn–N(6)
2.142(4)
2.219(5)
2.319(6)
2.278(6)
2.231(7)
2.139(7)
Mn–N(2)
Mn–N(3)
Mn–O(1w)
Mn–O(2w)
Mn–O(3w)
_
O
N
N
N
N
O(1)–Mn–N(2)
O(1)–Mn–N(3)
O(1)–Mn–O(1w)
N(2)–Mn–N(3)
83.45(9)
172.55(10)
91.03(9)
O(1)–Mn–N(2)
O(1)–Mn–N(3)
O(1)–Mn–O(1w)
N(2)–Mn–N(3)
N(2)–Mn–O(1w)
O(1w)–Mn–N(3)
N(2)–Mn–N(5)
N(2)–Mn–N(6)
N(3)–Mn–N(5)
N(3)–Mn–N(6)
O(1)–Mn–N(5)
O(1)–Mn–N(6)
O(1w)–Mn–N(5)
O(1w)–Mn–N(6)
N(5)–Mn–N(6)
82.60(18)
170.5(2)
94.75(19)
88.9(2)
84.27(19)
80.2(2)
97.8(2)
173.5(3)
97.9(2)
90.6(3)
87.5(2)
97.4(3)
177.1(2)
89.3(3)
88.7(3)
89.15(11)
92.34(14)
90.06(10)
97.56(17)
173.02(12)
89.96(12)
97.77(12)
90.23(12)
89.62(10)
170.10(14)
88.58(12)
81.61(15)
Scheme 1. Chemical structure of compartmental ligand L.
N(2)–Mn–O(1w)
N(3)–Mn–O(1w)
N(2)–Mn–O(2w)
N(2)–Mn–O(3w)
N(3)–Mn–O(2w)
N(3)–Mn–O(3w)
O(1)–Mn–O(2w)
O(1)–Mn–O(3w)
O(1w)–Mn–O(2w)
O(1w)–Mn–O(3w)
O(2w)–Mn–O(3w)
we are reporting their synthetic technique, characterisation, and
catalytic activities.
2. Results and discussion
2.1. Syntheses and characterisation
Complex
1 is prepared by applying template synthesis
technique by treating manganese(II) nitrate hydrate with the
Schiff-base formed in situ via condensation of 2,6-diformyl-
4-methylphenol with 2-(2-aminoethyl)pyridine in methanol.
Complexes 2 and 3 are obtained by adding an excess of sodium
thiocyanate and sodium dicyanamide (at least twice with respect
to the manganese (II) nitrate salt) to the solution of 1, respectively.
Complexes 1–3 show bands in IR spectra in the range
1642–1645 cm−1 and 1532–1540 cm−1 due to C N stretching
and skeletal vibration, respectively. The bands in the range
1320–1383 cm−1 observed in the IR spectra of complexes 1 and
arrangement (Fig. 2). However, being the nitrate and the water
molecules disordered as described in Section 3, this aspect will not
be described in details. A ␲–␲ interaction is realized between the
uncoordinated py and the phenolato ring of an adjacent complex
◦
˚
(distance between ring centroids 3.675 A, angle 6.7 ).
The molecular structure of complex 2 is similar to that of 1
by replacing two aqua ligands in the axial and equatorial posi-
tions by SCN− anions coordinated via N. The coordination bond
lengths appear systematically slightly longer than the correspond-
ing ones of complex 1. A picture of the complex is depicted in
Fig. 3, and a selection of bond lengths and angles is reported in
−
3 are due to NO3 group [25]. Complex 2 shows a band centred
at 2165 cm−1 corresponds to the SCN− moiety. The very strong
absorptions in the region of 2140–2160 cm−1 observed in the IR
spectrum of complex 3 is due to ꢀ(C N) of N(CN)2− moiety present
in it. The effective magnetic moment for complexes 1–3 in the range
of 5.85–5.90 B.M. at 300 K is very much consistence with the spin
only value (5.92 B.M.) for high spin MnII. The oxidation state of the
metal complexes has further been examined by EPR study at liq-
uid nitrogen temperature (77 K) in methanol. All the complexes
1–3 exhibit characteristics six line EPR spectra (in SI file) as it is
expected for octahedral MnII species.
˚
Table 1. The equatorial Mn–N(6) CS distance (2.139(7) A) is shorter
˚
than the axial Mn–N(5) CS of 2.231(7) A, and the relative C–N–Mn
angles are 146.3(7) and 166.5(6)◦. The larger value for the latter
can be explained with the requirement of sulphur S1 to establish a
S1· · ·H–O1w (at −1 + x, y, z) interaction with an adjacent molecule
˚
(S· · ·O distance 3.219 A). The intramolecular N1–H· · ·O1 interaction
˚
has a distance of 2.564 A. The molecules are arranged in pairs about
a centre of symmetry connected by a strong O1w–H· · ·N4(pyridine)
˚
hydrogen bond (N· · ·O distance = 2.720 A), as shown in Fig. 4.
2.2. Description of structure of complexes 1 and 2
Complex 1 shows the manganese ion in a distorted octahedral
geometry being chelated in the equatorial plane by the compart-
mental ligand through the phenolato oxygen, the imino and the
pyridine nitrogen donors and completing the coordination sphere
with three water molecules. A picture of the complex is depicted
in Fig. 1, and a selection of bond lengths and angles is reported in
Table 1.
The geometrical values are as expected, with Mn–N(py)
and Mn–O2w that manifest the longest values of 2.264(3) and
˚
2.238(4) A, respectively. The other coordination bond lengths fall
˚
in range 2.102(2)–2.195(3) A. The distortions in the coordination
bond angles are evident from Table 1, the larger from ideal value
being O(1w)–Mn–O(2w) of 170.10(14)◦. The protonated imino
nitrogen N1 forms an intramolecular H-bond with the pheno-
˚
lato oxygen O1 (N· · ·O distance 2.580 A), and the molecules are
arranged in pairs about a centre of symmetry connected by a strong
˚
O1w–H· · ·N4(pyridine) hydrogen bond (N4· · ·O1w = 2.580 A), an
arrangement similar to that observed in 2 (see below). Moreover
the coordinated and lattice water molecules are engaged in a H-
bonding scheme with the nitrate anions leading to a 3D polymeric
Fig. 4. H-bonded centrosymmetric arrangement of complex 2.

54
A. Guha et al. / Journal of Molecular Catalysis A: Chemical 338 (2011) 51–57
Enzyme Kinetics Data for 1 with 3,5
DTBC in MeOH
0.0006
0.0004
0.0002
0
4000
2000
0
0
200 400 600 8001000
1 / [Substrate]
0
0.02
0.04
0.06
[Substrate] (M)
Parameter
Value
Std. Error
Vmax
Km
0.0006
0.0018
3.31185e-006
4.39600e-005
Fig. 5. UV–vis spectra of (i) complex 1 (1 × 10−4 M) in methanol; (ii) 3,5-DTBC
(1 × 10−2 M) in methanol; (iii) changes in UV–vis spectra of 1 upon addition of
100-fold 3,5-DTBC observed at intervals of 10 min.
Fig. 6. Plot of rate vs concentration of 1. Plot of inset shows reciprocal
Lineweaver–Burk plot for the same complex.
Any attempt to obtain suitable single crystals for X-ray anal-
ysis of complex 3 failed. However preliminary results from a
disordered crystal allowed to confirm the formation of a mononu-
clear manganese complex, namely [Mn(HL)(H2O)3](N(CN)2)(NO3),
a composition confirmed by routine physicochemical techniques
MeOH
2
−
(vide supra). Since the coordination ability of N(CN)2 is well doc-
umented, we cannot rule out that in solution this anion might be
in the first coordination sphere of the metal, and thus committed
in the catalytic activity of this complex (see below).
complex1
2.3. Catalytic activity and kinetic study
2.3.1. Catecholase activity
The enzymes catechol oxidases [26,27], found in bacteria, fungi
and plants, belong to the class of type-III copper enzyme [28]
like hemocyanin and tyrosinase and are also known as o-diphenol
oxidases or polyphenol oxidases. Catechol oxidases catalyze exclu-
sively the two-electron oxidation of o-diphenols (i.e. catechols)
to the corresponding o-quinones (called catecholase activity) cou-
pled with the reduction of dioxygen to water. The resulting highly
reactive quinones auto polymerize to form brown polyphenolic
catechol melanins. Some higher valent manganese complexes are
observed to exhibit this property [29,30]. We have examined the
catalytic activity of complexes 1–3 evaluating the oxidation of 3,5-
di-tert-butylcatechol (3,5-DTBC) and tetrachlorocatechol (TCC) by
O2 to o-quinone (3,5-DTBQ and TCQ, respectively) in air-saturated
methanol solution at 300 K. All the three mononuclear MnII com-
plexes show significant catalytic oxidation of both the substrates as
monitored by means of UV–vis spectroscopy. The kinetics for the
oxidation of the substrates 3,5-DTBC and TCC were determined by
monitoring the increase of the products, 3,5-DTBQ and TCQ, respec-
tively, following the procedure reported in Section 3. All the species
exhibit saturation kinetics and complex 1, with three labile water
molecules bound to the metal centre, shows the highest kcat value
and 3 the lowest among the three complexes. The UV–vis spectral
scan and enzyme kinetics data of complex 1 with 3,5-DTBC and
TCC are depicted in Figs. 5–8, the corresponding spectral scans and
enzyme kinetics data for complexes 2 and 3 are reported as SI file.
TCC
0
300
400
500
600
Wavelength/nm
Fig. 7. UV–vis spectra of (i) complex 1 (1 × 10−4 M) in methanol; (ii) TCC (1 × 10−2 M)
in methanol; (iii) changes in UV–vis spectra of 1 upon addition of 100-fold TCC
observed at intervals of 10 min.
Enzyme Kinetics Data for 1 with
TCC in MeOH
0.00001
0.00001
4e5
0
2e5
0
0
0
200 400 600 8001000
1 / [Substrate]
0
0
0.02
0.04
0.06
[Substrate] (M)
Value
Parameter
Std. Error
Vmax
Km
7.22828e-006
0.0025
2.70873e-008
3.79520e-005
2.3.2. Phosphatase activity
Phosphatase or phosphohydrolase enzymes exclusively cat-
alyze the hydrolytic cleavage of P–O linkage of phosphate esters
(phosphatase activity) [31–33]. It is well-known that phosphate
Fig. 8. Plot of rate vs concentration of 1. Plot of inset shows reciprocal
Lineweaver–Burk plot for the same complex.

A. Guha et al. / Journal of Molecular Catalysis A: Chemical 338 (2011) 51–57
55
2
MeOH-Water
2
p-Nitrophenol
complex1
1
3,5-DTBC + Mn+2 solution
(no change of spectral patterns)
3,5-DTBC
p-Nitrophenyl Phosphate
Mn+2solution
0
400
500
600
Wavelength/nm
0
300
350
400
450
500
550
600
Fig. 9. UV–vis spectra of (i) complex 1 (1 × 10−4 M) in methanol–water; (ii) PNPP
(1 × 10−2 M) in methanol–water; (iii) changes in UV–vis spectra of 1 upon addition
of 100-fold PNPP observed at intervals of 10 min.
Wavelength/nm
Fig. 11. UV–vis spectrum of (i) Mn2+ salt solution (1 × 10−4 M) in methanol; (ii)
3,5-DTBC (1 × 10−2 M) in methanol; (iii) UV–vis spectra of Mn2+ salt solution upon
addition of 100-fold 3,5-DTBC observed at intervals of 10 min.
ester backbone is usually highly resistant towards hydrolytic cleav-
age and therefore it is still a big challenge to synthesize catalysts
reactive enough to hydrolyze the target bonds rapidly under phys-
iological conditions. Complexes 1 and 2 are potential catalysts to
cleave the P–O linkage due to their structural property. The activity
was monitored by UV–vis spectroscopy using PNPP as substrate in
MeOH–water medium and kinetic parameters were evaluated on
the basis of initial rate method at 300 K using the Michaelis–Menten
equation where a saturation kinetics were attained after a short
period of time. The kinetic study was performed following the pro-
cedure reported by Chattopadhyay et al. [13a]. In this case also
complex 1 shows a greater kcat value than that of complex 2 and
complex 3 forms an adduct with PNPP. The UV–vis spectral scan
and enzyme kinetics data of complex 1 with PNPP are shown in
Figs. 9 and 10, while the corresponding spectral scans and enzyme
kinetics data of 2 and 3 are given in SI file.
reported as SI) suggesting the effectiveness of our complexes as
catalyst.
2.3.3. Rationalization of the observed kcat values
The catalytic activities exhibited by these low valent Mn-
complexes depend mainly on the ease of interaction between the
substrate and the catalyst. More facile interaction would lead to
greater kcat value. Again the feasibility of the interaction depends
on the structure of the catalyst where unsaturation of the coordina-
tion site or presence of labile groups attached to the metal centre in
the coordination sphere promotes the approach of the substrate to
a greater extent. Complex 1 has three labile water molecules in its
coordination sphere and thus the approach of substrates towards
the metal centre should be easier in comparison to the other two
complexes, where one and two water molecules are present in 2
and in 3, respectively, leading complex 1 to display the highest kcat
value (Table 2). The bulkier dicyanamide present in 3 (with regard
to thiocyanate in 2) could justify a more facile interaction of the
substrate to the metal centre in 2, thus validating the observed
higher kcat value of 2 than of 3 in case of catecholase activity, while
only an adduct is formed between 3 and PNPP in the phosphatase
activity study. However, electronic effects of dicyanamide moiety
and thiocyanate anions cannot be excluded to influence these reac-
tions. Although the formal oxidation state of manganese in our
complexes is +2, the catalytic activity exhibited by them is largely
due to the presence of positive charge (on imine N) very near to the
metal centre. This extra positive charge can furnish a path to facil-
itate the approach of the negatively charged substrate to bind the
metal centre, a well accepted argument nowadays in biology/bio-
inorganic chemistry to explain the activity of metallobiosites such
as CuZn–SOD [34–37].
A blank experiment, carried out with only Mn2+ salt and the
substrates in the absence of the ligand, showed no band around
∼400 nm in the UV–vis spectra, as illustrated in Fig. 11 with 3,5-
DTBC as substrate (results for TCC and PNPP as substrates are
Enzyme Kinetics Data for 1 with
pnpp in MeOH-Water
0.00006
0.00004
1.2e5
1e5
80000
60000
40000
20000
0
0.00002
0
200 400 600 8001000
1 / [Substrate]
0
0
0.02
0.04
0.06
[Substrate] (M)
Value
Table 2
kcat values (h−1) of complexes 1, 2 and 3 catalyzed reactions with 3,5-DTBC, TCC and
Parameter
Std. Error
PNPP as substrates.
Vmax
Km
5.65636e-005
0.0060
5.51194e-007
0.0002
Complex
3,5-DTBC
TCC
PNPP
1
2
3
2.16 × 104
1.44 × 104
7.2 × 103
2.60 × 102
2.46 × 102
2.16 × 102
2.03 × 103
1.23 × 103
Adduct
Fig. 10. Plot of rate vs concentration of 1. Plot of inset shows reciprocal
Lineweaver–Burk plot for the same complex.

56
A. Guha et al. / Journal of Molecular Catalysis A: Chemical 338 (2011) 51–57
Table 3
3.3.2. Complex 2: [Mn(HL)(SCN)2(H2O)]·H2O
Crystal data and details of structural refinements for 1 and 2.
The complex was prepared following the same procedure as for
1 with the addition of solid NaSCN (0.3242 g, 5 mmol) on the result-
ing cooled mixture and stirred for 3 h. The clear orange solution
was kept in a CaCl2 desiccator in dark, from which square-shaped
orange crystals, suitable for X-ray data collection, were obtained
after a few days (yield 70%). Anal. Calcd. for C25H27N6O2.5S2Mn (2):
C, 52.63; H, 4.77; N, 14.73. Found: C, 52.59; H, 4.66; N, 14.62%.
1
2
Empirical formula
fw
Cryst syst
Space group
a (Å)
C23H32MnN6O11
623.49
Monoclinic
C 2/c
17.571(4)
17.069(3)
20.509(4)
109.87(3)
5785(2)
8
1.432
0.523
2600
3.56–26.02
38868
C25H26MnN6O2S2
561.58
Monoclinic
P 21/c
9.2223(10)
19.601(2)
14.6243(16)
94.664(2)
2634.8(5)
4
1.416
0.694
1164
1.74–23.24
13890
b (Å)
c (Å)
ˇ (◦)
3.3.3. Complex 3: [Mn(HL){N(CN)2}(H2O)2](NO3)·H2O
V (Å3)
The complex was synthesized by adopting a similar procedure as
for 2 by using solid NaN(CN)2 (0.3561 g, 5 mmol) instead of NaSCN
(yield 71%). Anal. Calcd. for C25H30N8O7Mn (3): C, 49.26; H, 4.96;
N, 18.38. Found: C, 49.06; H, 4.88; N, 18.31%.
Z
Dcalcd (mg/m3)
M (Mo K␣) (mm−1
F(0 0 0)
)
ꢁ range (◦)
Collected reflections
Indep reflections
Rint
3.4. Catalytic activities and kinetic studies
5629
0.0380
3153
0.0516
The kinetics of the 3,5-DTBC and TCC oxidation was determined
by the initial rates method by monitoring the increase of the prod-
uct 3,5-DTBQ (at 423, 422 and 423 nm for 1, 2, and 3, respectively)
and of the product TCQ (at 428, 430 and 425 nm for 1, 2 and 3,
respectively). The concentration of the substrate 3,5-DTBC and TCC
was always kept at least 10 times larger than that of the MnII
complex so as to maintain the pseudo-first-order condition. All
of the kinetic experiments were conducted at constant temper-
ature of 20 ◦C, monitored with a thermostat. Initially, a series of
solutions of substrate 3,5-DTBC and TCC having different concen-
trations (8 × 10−4 to 5 × 10−2 mol dm−3), were prepared from a
substrate concentrated stock solution using methanol as solvent.
Then, 2 mL of such substrate solution was poured in a 1 cm quartz
cell and kept in the spectrophotometer to equilibrate the temper-
ature at 20 ◦C. A 0.04 mL (2 drops) of 5 × 10−3 mol dm−3 of MnII
complex in methanol was quickly added and mixed thoroughly to
get an ultimate MnII complex concentration of 1 × 10−4 mol dm−3
and the absorbencies were measured accordingly. Initial rates were
determined from slope of the tangent to the absorbance vs time
curve at t = 0. By applying GraFit32 program for enzymatic kinetics,
Km and Vmax are calculated from the graphs, and the inset shows
the Lineweaver–Burk plot. kcat values were calculated by divid-
ing the Vmax values with the concentration of the corresponding
complexes. Furthermore, for a particular substrate concentration,
varying the MnII complex concentration, a linear relationship for
the initial rates was obtained, which shows a first order dependence
on the MnII complex concentration. On the other hand, varying
the concentration of substrates 3,5-DTBC and TCC, a first-order
dependence was observed at low 3,5-DTBC and TCC concentration.
However, all MnII complexes showed a saturation kinetic at higher
3,5-DTBC and TCC concentrations. Thus a treatment on the basis of
Michaelis–Menten model was seemed to be appropriate.
Obs reflections [I > 2ꢂ(I)]
Parameters
R1 [I > 2ꢂ(I)]
3426
434
0.0576
0.1712
1.036
0.380, −0.318
2307
333
0.0623
0.1672
1.081
0.876, −0.598
wR2 [I > 2ꢂ(I)]
GOF on F2
Residuals/e A
−3
˚
3. Experimental
3.1. Starting materials
All chemicals were obtained from commercial sources and used
as received. Solvents were dried according to standard procedure
and distilled prior to use. 2,6-Diformyl-4-methylphenol was pre-
pared according to the literature method [38]. 2-(2-Aminoethyl)
pyridine (Lancaster Chemical Company Inc.), manganese(II) nitrate
hydrate (Aldrich), sodium thiocyanate (SRL Pvt. Ltd.), sodium
dicyanamide (Fluka) were purchased from commercial sources and
used as received. All other chemicals used were of AR grade.
3.2. Physical measurements
Elemental analyses (carbon, hydrogen, and nitrogen) were per-
formed using a PerkinElmer 240C elemental analyzer. Infrared
spectra (4000–400 cm−1) were recorded at 300 K using a Shimadzu
FTIR-8400S with KBr as medium. Electronic spectra (800–200 nm)
were obtained at 300 K using a Shimadzu UV-3101PC with dry
methanol as medium as well as reference. EPR experiment was
done at 298 K and 77 K in pure methanol using Bruker EMX-X
band spectrometer. The magnetic susceptibilities were measured
at 300 K using a Magway MSB Mk 1 magnetic susceptibility balance
made by Sherwood Scientific Ltd.
Similar procedure as described above was adopted to study
phosphatase activity by using PNPP as substrate. The reactions were
spectroscopically monitored by the increase of the product PNP at
436 nm for 1 and at 435 nm for 2, while 3 forms an adduct with
PNPP and therefore no kinetic study was performed for the latter.
3.3. Syntheses of the complexes
3.3.1. Complex 1: [Mn(HL)(H2O)3](NO3)2·H2O
A
methanolic solution (5 mL) of 2-(2-aminoethyl)pyridine
3.5. X-ray data collection and structure determination
(0.244 g, 2 mmol) was added dropwise to a heated methanolic solu-
tion (10 mL) of 2,6-diformyl-4-methylphenol (0.164 g, 1 mmol),
and the resulting orange colour solution was boiled for half an hour.
Then, a methanolic solution (15 mL) of Mn(NO3)2·3H2O (0.7176 g,
2.5 mmol) was added and reflux was further continued for 2 h. After
cooling, the clear orange solution was kept in a CaCl2 desiccator in
dark. Square-shaped orange crystals, suitable for X-ray data collec-
tion were obtained after a few days from that solution (yield 68%).
Anal. Calcd. for C23H32N6O11Mn (1): C, 44.31; H, 5.17; N, 13.48.
Found: C, 44.12; H, 5.26; N, 13.38%.
Diffraction data for the structures reported were collected
at room temperature on a Nonius DIP-1030H system and on a
BRUKER SMART APEX diffractometer (in both cases Mo K␣ radi-
˚
ation, ꢃ = 0.71073 A) equipped with CCD. Cell refinement, indexing
and scaling of the data sets were carried out using Bruker SMART
APEX and SAINT package [39], Denzo, and Scalepack programs [40].
The crystal of 2 revealed to be weakly diffracting (ꢁmax angle 23.4◦),
but the results obtained are of good quality. The structures were
solved by direct methods and subsequent Fourier analyses [41]

A. Guha et al. / Journal of Molecular Catalysis A: Chemical 338 (2011) 51–57
57
and refined by the full-matrix leastsquares method based on F2
References
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Acknowledgements
The authors wish to thank the University Grants Commission,
New Delhi [UGC Major Research Project, F. No. 34-308\2008 (SR)
Dated: 31.12.2008 (DD) for financial support. A.G. is thankful to
Shailabala Biswas Research Foundation, University of Calcutta for
financial support.
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Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2011.01.025.