Structural characterization of a new manganese(III)-salen complex [H2salen = N,N′-bis(salicylidene)ethane-1,2-diamine] and study of its electron transfer kinetics with hydroquinone and catechol

Article abstract of DOI:10.1016/S0277-5387(03)00068-8

The octahedral manganese(III) complex, [Mn(salen)Cl(H₂O)]·H₂O (H₂salen = N,N′-bis(salicylidene)ethane-1,2-diamine), has been synthesized and structurally characterized by single-crystal X-ray diffraction study. The X-ray structural analysis shows that the metal center is six-coordinate with distorted octahedral geometry. In this compound, the Mn-Cl and Mn-OH₂ bond lengths are 2.621(6) and 2.333(2) A?, respectively, slightly longer than the usual values. The kinetics of electron transfer reaction of the complex with catechol and hydroquinone has been followed spectrophotometrically over the pH range 5.75-8.15 at 30 °C. The nature of the kinetic trace is indicative of a biphasic reaction with each reductant. Detailed analysis of kinetic data supports the formation of a relatively stable intermediate complex between manganese(III) and the dihydroxybenzenes (H₂A) at the initial stage. The decay of the intermediate is the bimolecular electron transfer process involving another molecule of reductants leading to the manganese(II) complex and benzoquinones. Mn^III(salen)(H₂O)(OH) generated in situ from the aquation and proton equilibrium of the parent complex has been assigned to be the reactive species during the formation of the intermediate. The corresponding species for the redox step is Mn^III(L)(H₂O)(HA), sufficient experimental evidence for which has been gathered from pH-rate profiles.

Full text of DOI:10.1016/S0277-5387(03)00068-8

Polyhedron 22 (2003) 1191ꢀ1198
/
www.elsevier.com/locate/poly
Structural characterization of a new manganese(III)ꢀ
/
salen complex
[H2salenꢁ
/
N,N?-bis(salicylidene)ethane-1,2-diamine] and study of its
electron transfer kinetics with hydroquinone and catechol
Anangamohan Panja a, Nizamuddin Shaikh a, Mahammad Ali b, Pavel Vojt´ısˇek c,
Pradyot Banerjee a,*
a Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata 700032, India
b Department of Chemistry, Jadavpur University, Kolkata 700032, India
c Department of Inorganic Chemistry, Universita Karlova, Albertov 2030, 12840 Prague 2, Czech Republic
Received 1 May 2002; accepted 10 January 2003
Abstract
The octahedral manganese(III) complex, [Mn(salen)Cl(H2O)]×
been synthesized and structurally characterized by single-crystal X-ray diffraction study. The X-ray structural analysis shows that
the metal center is six-coordinate with distorted octahedral geometry. In this compound, the MnꢀCl and MnꢀOH2 bond lengths are
2.621(6) and 2.333(2) A, respectively, slightly longer than the usual values. The kinetics of electron transfer reaction of the complex
with catechol and hydroquinone has been followed spectrophotometrically over the pH range 5.75ꢀ8.15 at 30 8C. The nature of the
/
H2O (H2salenꢁN,N?-bis(salicylidene)ethane-1,2-diamine), has
/
/
/
˚
/
kinetic trace is indicative of a biphasic reaction with each reductant. Detailed analysis of kinetic data supports the formation of a
relatively stable intermediate complex between manganese(III) and the dihydroxybenzenes (H2A) at the initial stage. The decay of
the intermediate is the bimolecular electron transfer process involving another molecule of reductants leading to the manganese(II)
complex and benzoquinones. MnIII(salen)(H2O)(OH) generated in situ from the aquation and proton equilibrium of the parent
complex has been assigned to be the reactive species during the formation of the intermediate. The corresponding species for the
redox step is MnIII(L)(H2O)(HA), sufficient experimental evidence for which has been gathered from pH-rate profiles.
# 2003 Elsevier Science Ltd. All rights reserved.
Keywords: MnIII ꢀ
Schiff base complex; Crystal structures; Redox activity; Hydroquinone; Catechol
/
1. Introduction
only magnetic moment of d4 high-spin systems. How-
ever, no prominent band in the 900ꢀ
700 cmꢂ1 region of
IR spectrum was located, and this was attributed to the
absence of MnꢀOꢀ
Mn bridge of the dimer. The MnIIIꢀ
salen complex reported by Ashmawy et al. [3] was
considered to have a dimeric structure. In the absence of
a single-crystal X-ray structure, the authors have
established it from IR data and antiferromagnetic
interactions in the solid state. Supporting evidence has
been put forward from fast-atom bombardment mass
spectroscopy. On the other hand, conductivity measure-
ments at 295 K lends support for the existence of an
equilibrium between monomeric and dimeric formula-
tions as given by
/
The Schiff base complexes of manganese(III) are
considered to be the simplest models for the reactivity
of OEC active site of manganese catalases. Three main
structural types for the Schiff base ligand N,N?-bis(sa-
licylidene)ethane-1,2-diamine (H2salen) have been re-
/
/
/
ported [1]: (i) mononuclear M(salen)X, where Xꢁ
halide; (ii) binuclear [M(salen)]2; and (iii) bridged bi-
nuclear [M(salen)O]2.
The manganese(III)ꢀsalen complex reported by Bou-
cher [2] in powder form shows a magnetic moment value
of 4.84 BM (in solid), little below the theoretical spin-
/
/
2[Mn(salen)(H2O)]ClO4
* Corresponding author. Fax: ꢃ91-33-2473-2805.
/
X[Mn(salen)(H2O)]2(ClO4)2
(1)
E-mail address: icpb@mahendra.iacs.res.in (P. Banerjee).
0277-5387/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0277-5387(03)00068-8

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A. Panja et al. / Polyhedron 22 (2003) 1191ꢀ1198
/
In the following year, Pecoraro and Butler [4] reported a
mononuclear species, [Mn(salen)Cl]CH3CN, which dis-
plays a square-pyramidal geometry similar to that of
[FeIII(salen)Cl] [5].
cmꢂ1): 213 (sh), 234 (37,600), 279 (17,100), 300 (sh), 386
(sh). meff 4.94 BM.
2.3. Physical measurements
In the attempts [3,6ꢀ8] of finding artificial catalysis
/
for the decomposition of water, investigation on the new
complexes of manganese(III) with salen [9] and its
derivatives [10] is a continuing process. We present
here the crystallographic characterization of a new
Microanalysis (CHN) was performed by a Perkinꢀ
/
Elmer 240C elemental analyzer. pH measurements
were made with a Systronics digital pH meter (model
335, India). Magnetic susceptibility measurement was
carried out on a PAR 155 vibrating sample magnet-
ometer, and the EPR spectra were obtained on a Varian
model 109 E-line X-band spectrometer equipped with a
low-temperature quartz Dewar for measurements at 77
K. IR spectra were obtained on a Nicolet, MAGNA-IR
750 spectrometer with samples prepared as KBr pellets.
Conductivity measurements were made with Systronics
(India) direct reading conductivity meter (model 304).
Cyclic voltammetry was performed at a planar EG&G
PARC G0229 glassy carbon millielectrode using an
EG&G PARC electrochemical analysis system (model
250/5/0) in acetonitrile under dry nitrogen atmosphere in
conventional three-electrode configurations with a scan
mononuclear manganese(III)ꢀsalen complex in a nearly
/
octahedral environment prepared in aqueous methano-
lic medium. The electron transfer reactivity of this
manganese(III) complex with catechol and hydroqui-
none has also been studied over the pH range 5.75ꢀ8.15.
/
Although dihydroxybenzene is not involved in PS II, the
intervention of the oxidized (semiquinone) form of this
structural element in PS I is reported [11]. The precise
mechanism of the reactions is thereby worth pursuing.
2. Experimental
rate of 0.05 V sꢂ1
Spectral and kinetic measurements were performed in
a UVꢀVis Spectrophotometer (UV-2100, Shimadzu,
.
2.1. Materials
/
Ethylenediamine, salicylaldehyde, hydroquinone, and
catechol were of analytical reagent grade (Aldrich) and
used without further purification. All other chemicals,
e.g. manganous acetate tetrahydrate, manganous chlo-
ride, acetonitrile, methanol, sodium hydroxide, and
perchloric acid were of reagent grade. Ionic strength
and pH were adjusted by NaClO4 (Fluka AR) and
NaH2PO4 (Fluka AR), respectively. Both were recrys-
tallized before use. Stock solutions were prepared with
doubly distilled water.
Japan). The reactions were initiated by injecting a
constant proportion of the complex into the solution
containing the substrate, buffer, and NaClO4 (for
maintaining ionic strength at 0.2 mol dmꢂ3) according
to the conditions required. Dilute solutions of NaOH
and HClO4 were used to adjust the pH to a desired
value. Although the measured pH is defined by the
activity of the hydrogen ion, the concentration of
hydrogen ion in terms of ꢂ
log[Hꢃ] for each solution
/
was obtained by calibrating the pH-electrode with
analytically prepared solutions to the desired ionic
strength. The reactions were studied under pseudo-
first-order conditions at 400 nm taking the complex as
a minor component. The corresponding rate constants
were evaluated by means of suitable nonlinear acquisi-
tion analysis system with data taken for at least three
half-lives of the reactions.
2.2. Synthesis of the complex
The trans-[Mn(salen)Cl(H2O)]×
pared through a two-step process, the first step of which
involves the preparation of [MnIII(salen)OAc]×
H2O
following the standard literature procedure [12]. In the
second step, the [MnIII(salen)OAc]×
H2O (0.8 g, 2 mmol)
dissolved in methanol (20 cm3) was mixed well with a
solution of MnCl2×4H2O (0.4 g, 2 mmol) in water (15
/
H2O complex was pre-
/
/
2.4. Crystal structure determination and structural
H2O
/
refinement of trans-[Mn(salen)Cl(H2O)]×
/
cm3) which caused an immediate color change from
greenish brown to brownish black. The resulting mix-
ture was kept at room temperature for several days.
During this period, a dark brownish black microcrystal-
line compound separated out, which was collected by
vacuum filtration and washed with distilled water,
followed by diethyl ether to remove excess water. Yield:
0.67 g (85%) (Found: C, 49.2; H, 4.6; N, 7.2. Calc. for
C16H18N2O4ClMn: C, 48.91; H, 4.58; N, 7.13%). FTIR
Single crystals of trans-[Mn(salen)Cl(H2O)]×
were obtained by recrystallization of the complex from
methanolꢀwater mixture. Data were collected by v-
scan method in the 2u range 3.28ꢀ27.58 on an Enraf-
Nonious CAD4 diffractometer with graphite-mono-
/
H2O
/
/
˚
0.71070 A) at 293 K.
chromated Mo Ka radiation (lꢁ
/
Information concerning crystallographic data collection
and refinement of the structure is compiled in Table 1.
Unit-cell parameters and orientation matrix were deter-
mined by a least-squares fit. Intensity data were
(KBr, cmꢂ1): nCÄN 1598, nCꢀO 1286, nOH 3404ꢀ
/
3448.
Vis spectrum (H2O): lmax (nm) (omax, molꢂ1 dm3
UVꢀ
/

A. Panja et al. / Polyhedron 22 (2003) 1191ꢀ
/
1198
1193
Table 1
Data collection and structure refinement of trans-[Mn(salen)Cl(H2O)]×
/
H2O
Empirical formula
Formula weight
Crystal system
C16H18N2O4ClMn
392.71
monoclinic
P21/n
Space group
Unit cell dimensions
˚
a (A)
6.6470(2)
7.3330(2)
33.8260(10)
95.1650(17)
1642.07(8)
4
˚
b (A)
˚
c (A)
b (8)
3
˚
V (A )
Z
m(Mo Ka) (mmꢂ1
T (K)
)
0.990
293
Total data
Unique data, Rint
6921
3320(0.023)
3080
Fig. 1. Molecular structure of the complex trans-[Mn(salen)Cl(H2O)]×
H2O showing the atom labeling scheme. Hydrogen atoms and solvent
water molecule have been omitted for clarity.
/
Data [I ꢀ
R [I ꢀ2s(I)]
wR [I ꢀ2s(I)]
/2s(I)]
/
0.0332
/
0.0811
The MnIII
[MnIII(salen)Cl] [4] is 2.461 A with manganese(III) ion
ꢀ
/
Cl bond length in square-pyramidal
˚
˚
being displaced by 0.19 A above the equatorial plane,
whereas in this octahedral complex the MnIII
Cl and
MnꢀOH2 bond lengths are 2.621(6) and 2.333(17) A,
corrected for Lorentz polarization but not for absorp-
tion. All calculations for structure solution and refine-
ment were done by standard procedures (SHELXS-97 [13]
and SHELXL-97 [14]). All non-hydrogen atoms were
refined anisotropically by full-matrix least-squares tech-
niques on F2. All the hydrogen atoms were located in
Fourier difference maps and refined isotropically. The
final crystal structure was drawn using ORTEP program
[15].
ꢀ
/
˚
/
respectively, which are little longer than usual values [10]
probably due to the hydrogen bonds (Fig. 2). The
coordinated Cl ion takes part in two H bonds with
Table 2
˚
Selected bond distances (A) and bond angles (8) for complex trans-
[Mn(salen)Cl(H2O)]×H2O
/
Bond distances
3. Results and discussion
Mnꢀ
Mnꢀ
Mnꢀ
Mnꢀ
Mnꢀ
Mnꢀ
/
Cl
2.6209(6)
/
O(1)
O(2)
O(3W)
N(1)
N(2)
1.8641(14)
1.9010(15)
2.3329(17)
1.9846(17)
1.9818(17)
/
3.1. Description of the structure of
/
[Mn(salen)Cl(H2O)]×
/H2O
/
/
An ORTEP view of the complex with atom numbering
scheme is shown in Fig. 1 and the important bond
distances and angles are summarized in Table 2. In this
monomeric complex, the manganese atom displays a
slightly distorted octahedral coordination in which
manganese(III) ion is being displaced from the O(1),
Bond angles
Clꢀ
Clꢀ
Clꢀ
Clꢀ
Clꢀ
O(1)ꢀ
O(1)ꢀ
O(1)ꢀ
O(1)ꢀ
O(2)ꢀ
O(2)ꢀ
O(2)ꢀ
O(3W)ꢀ
O(3W)ꢀ
N(1)ꢀMnꢀ
/
Mnꢀ
Mnꢀ
Mnꢀ
Mnꢀ
Mnꢀ
/
O(1)
94.71(5)
95.50(5)
174.08(5)
87.44(5)
89.60(5)
93.73(6)
88.96(7)
91.42(7)
172.35(6)
88.89(7)
173.83(7)
92.14(7)
87.82(7)
86.26(6)
82.44(7)
/
/O(2)
/
/
O(3W)
N(1)
N(2)
/
/
/
/
/
Mnꢀ
Mnꢀ
Mnꢀ
Mnꢀ
Mnꢀ
Mnꢀ
Mnꢀ
/O(2)
˚
N(1), N(2), O(2) planes by 0.066 A. The nearly planar
tetradentate salen ligand is tightly bound to mangane-
/
/O(3W)
/
/N(1)
/
/N(2)
se(III) ion via Nimine and Ophenol atoms (Mnꢀ
/
Nimine
/
/O(3W)
˚
bond lengths of 1.985(17) and 1.982(17) A, and the Mnꢀ
/
/
/N(1)
˚
Ophenol bond lengths of 1.864(14) and 1.901(15) A). The
/
/N(2)
/
Mnꢀ
/
N(1)
distortion of the coordination octahedron is due to the
˚
O(1) and
/
Mnꢀ
/N(2)
slight difference (0.037 A) between the Mnꢀ
/
/
/
N(2)
Mnꢀ
N(1) and O(2)ꢀ
deviation of N(2)ꢀ
(82.45(7)8) to equatorial plane is also noteworthy.
However, the NꢀMnꢀN and OꢀMnꢀN bond angles
are typical of metal complexes with salen as a ligand.
/
O(2) bond lengths as well as between O(1)ꢀ
MnꢀN(2) bond angles (0.728). Slight
MnꢀN(1) axis from orthogonality
/
Mnꢀ
/
Bond distances
O(3W)ꢀ ꢀ ꢀO(4W)
O(3W)ꢀ ꢀ ꢀCl
/
/
2.808(3)
3.261(17)
2.936(3)
3.251(2)
/
/
O(4W)ꢀ ꢀ ꢀO(2)
O(4W)ꢀ ꢀ ꢀCl
/
/
/
/

1194
A. Panja et al. / Polyhedron 22 (2003) 1191ꢀ1198
/
3.2. Cyclic voltammetric studies
The cyclic voltammetric study on [MnIII(sa-
len)Cl(H2O)] was carried out in acetonitrile medium
using a three-electrode assembly consisting of a glassy
carbon working electrode, a platinum counterelectrode,
and a saturated calomel as reference electrode. The
cyclic voltammogram is depicted in Fig. 3. In this case,
the metal center reduction takes place quasi-reversibly
as evidenced from the fact that the peak-to-peak
separation, DEp, is 759
determined to be ꢂ0.237 V vs. SCE, which is higher
than that observed in the complex [MnIII(salen)Cl]
(E1/2
0.264 V at a scan rate of 50 mV sꢂ1).
/5 mV. E1/2 of the complex was
/
ꢁ
/
ꢂ
/
3.3. Spectrophotometric determination of pK of
[MnIII(salen)(H2O)2]ꢃ
Spectral change observed during the reaction of the
MnIII complex with H2A (where H2A represents the
generalized form of both hydroquinone (H2Q) and
catechol (H2cat)) at pH 7.00 showed an initial increase
in absorbance followed by a decay (Fig. 4). The initial
increase in absorbance may be due to the formation of
an intermediate, which then undergoes redox transfor-
mation. This observation prompted us to determine
spectrophotometrically the pKm of the complex
[MnIII(salen)(H2O)2]ꢃ which is actually generated in
situ from [MnIII(salen)(Cl)(H2O)] in aqueous solution.
For this purpose, a fixed concentration of the complex
10ꢂ5 mol dmꢂ3) was allowed to react with 0.003
8.15. The
Fig. 2. Molecular packing diagram showing intermolecular hydrogen
(5.0ꢄ
/
bonding in [Mn(salen)Cl(H2O)]×H2O.
/
mol dmꢂ3 of H2Q over a wide pH range 5.0ꢀ
/
maximum absorbance at 400 nm was noted and plotted
against [Hꢃ] (Fig. 5). It is apparent that the concentra-
tion of the intermediate complex decreases with the
˚
O3W and O4W (O3Wꢀ ꢀ ꢀCl: 3.261 A and O4Wꢀ ꢀ ꢀCl:
˚
3.251 A) from the next unit-cell. On the side of Mnꢀ
salen plane, the MnꢀOH2 bond is affected by two
hydrogen bonds (O3Wꢀ ꢀ ꢀO4W: 2.808 A). The elonga-
/
/
increase in pH until it reaches Â6.00, below which no
/
˚
detectable amount of the intermediate is formed. It also
appears that [MnIII(salen)(H2O)2]ꢃ does not form any
intermediate, and [MnIII(salen)(H2O)(OH)], owing to
the enhanced lability of the aqua group having a OHꢂ
˚
tion from 2.461 to 2.621 A definitely points to a very
weak bonding of MnIII
Cl in the octahedral environ-
ꢀ
/
ment, and supports the observation that when dissolved
in water (or in polar solvent such as CH3CN), the
dissociation of chloride occurs instantaneously produ-
cing a 1:1 electrolyte (molar conductance, 110 Vꢂ1 cm2
molꢂ1
)
presumably due to the formation of
[Mn(salen)(H2O)2]ꢃ complex. The packing diagram
reveals a three-dimensional framework due to four
intermolecular hydrogen bonds involving [Mn(sa-
len)Cl(H2O)] molecules. Non-coordinated water mole-
cules also participate in this framework. The ligands
(salen) are nearly planar, parallel, and form endless
‘‘piles’’ parallel with the b-axis. The distances between
˚
them are about 6.76 A with the Mnꢀ
/
Mn distances of
˚
7.333(2) A. Thus, no appreciable magnetic interaction is
expected in the solid state and the observed magnetic
data (4.94 BM) is consistent with the purely monomeric
structure of the complex.
Fig. 3. Cyclic voltammogram of trans-[Mn(salen)Cl(H2O)]×
1.0ꢄ
10ꢂ3 mol dmꢂ3, in acetonitrile with 0.1 mol dmꢂ3 TEAP, and
scan rate of 50 mV sꢂ1 measured at a glassy carbon electrode.
/
H2O,
/

A. Panja et al. / Polyhedron 22 (2003) 1191ꢀ
/
1198
1195
A
[Hꢃ] ꢃ KmAOH
Km ꢃ [Hꢃ]
ꢂ
H2O
Aꢁ
(5)
where AH O is the absorbance of the complex when it is
ꢂ
in 100% aqua form. AOH is the absorbance of the
2
complex when it is in 100% hydroxo form that leads to
the formation of the intermediate in 100% yield. A is the
absorbance of the complex in between the two extreme
ꢂ
conditions where both AH O and AOH exist. A non-
linear fit of the experimental data to Eq. (5) by means of
2
a computer-fit program gives the value of Kmꢁ
/
(1.239
/
0.16)ꢄ 6.91). The same results could be
/
10ꢂ7 (pKmꢁ
extracted from reactions involving manganese(III) com-
plex and H2cat.
/
3.4. Stoichiometry and reaction products
Fig. 4. Absorbance vs. time graph (400 nm) for the reduction of
[MnIII(salen)(H2O)2]ꢃ (5.0ꢄ10ꢂ5 mol dmꢂ3) with H2Q (4ꢄ10ꢂ3
mol dmꢂ3) at 30 8C, pH 7.00, Iꢁ0.2 mol dmꢂ3 (NaClO4), and 0.05
mol dmꢂ3 phosphate buffer. (*
) Best fit curve and (k) experimental
points.
/
/
/
The stoichiometry of the reaction was determined by
reacting fixed and excess concentration of complex,
varying the concentration of the H2A and then estimat-
ing the unreacted concentration of the complex spectro-
photometrically at 400 nm and pH 7.00. These
measurements confirm the following stoichiometric
reactions:
/
ð6Þ
ð7Þ
Fig. 5. Plot of absorbance vs. [Hꢃ] for the spectrophotometric
determination of pKm of [MnIII(salen)(H2O)2]ꢃ (5.0ꢄ10ꢂ5 mol
dmꢂ3) at 30 8C, [H2Q]ꢁ 10ꢂ3 mol dmꢂ3, Iꢁ0.2 mol dmꢂ3
3ꢄ
(NaClO4), and 0.05 mol dmꢂ3 phosphate buffer.
The formation of ortho- and para-benzoquinones was
confirmed by IR and 1H NMR spectroscopy of the
isolated products. The reduction product of the complex
is the corresponding MnII species and was confirmed by
EPR measurement showing 6-line EPR signals.
/
/
/
/
in the trans-position, forms the intermediates with H2A.
This can be expressed as
3.5. Description of the reactivity of the system
Km
[MnIII(L)(H2O)2 ]ꢃ X[MnIII(L)(H2O)(OH)]ꢃHꢃ
[MnIII(L)(H2O)(OH)]
(2)
(3)
(4)
Under the pseudo-first-order reaction condition, the
absorbance (At) vs. t curve shows an exponential
increase followed by an exponential decay. These
observations indicate that the oxidation of H2A occurs
through the formation of a relatively stable intermediate
followed by its redox decomposition and can be
represented by the following sequence of reactions:
HAꢂ
X [MnIII(L)(HA)(OH)]ꢂ ꢃH2O
[MnIII(L)(H2O)(OH)]
H2A
X [MnIII(L)(H2A)(OH)]ꢂ ꢃH2O
k0(1)
A X B k00(2) C
(8)
where L denotes salen. This change in absorbance with
pH allows to determine the pKm of the complex
[MnIII(L)(H2O)2ꢃ] by using the equation
k0(ꢂ1)
k0(1), k0(ꢂ1), and k0(2) were evaluated by fitting the
/

1196
A. Panja et al. / Polyhedron 22 (2003) 1191ꢀ1198
/
Table 3
Pseudo-first-order rate constants k0(1) a for the formation of intermediate complexes and k0(2) a for the redox decomposition of intermediate
complexes by dihydroxybenzenes at 30 8C, pH 7.00, Iꢁ
/
0.2 mol dmꢂ3 (NaClO4), and 0.05 mol dmꢂ3 phosphate buffer a
k0(1) (ꢄ k0(2) (ꢄ
103 sꢂ1 103 sꢂ1
H2Q H2Q
[H2A]T (ꢄ
/
103 mol dmꢂ3
)
/
)
/
)
H2cat
H2cat
0.70
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
10
5.219
7.239
14.59
23.59
31.69
39.89
46.29
54.29
59.39
72.69
/
0.02
ꢀ
/
6.209
9.049
18.19
26.59
28.99
32.49
33.09
33.79
34.99
35.99
/
0.08
ꢀ
/
/
0.03
0.11
0.05
0.04
0.08
0.12
0.22
0.10
0.26
1.289
ꢀ/
2.129
3.099
ꢀ/
/
0.06
/
0.06
0.04
0.08
0.06
0.12
0.02
0.11
0.13
0.09
4.839
ꢀ/
9.629
13.79
ꢀ/
/
0.04
/
/
/
/0.03
/
/0.04
/
/0.02
/
/0.09
/
/
/
3.629
ꢀ/
/0.05
/
16.39
ꢀ/
/0.06
/
/
/
4.419
5.959
10.39
18.89
31.39
/
0.08
0.04
0.09
0.12
0.08
/
18.19
18.89
23.89
28.79
31.79
/
0.12
0.10
0.04
0.08
0.09
/
/
/
/
20
40
ꢀ/
ꢀ/
ꢀ/
/
ꢀ/
ꢀ/
ꢀ/
/
/
/
60
/
/
a
k0(ꢂ
/
1) in each trace as computed from KFSIM40 program is B
10ꢂ5 sꢂ1 and not shown.
/
Km
X[MnIII(L)(H2O)(HA)]
[MnIII(L)(H2O)(OH)]ꢃHAꢂ
experimental trace to the above sequence of reactions
using KFSIM40 program [16]. The inputs for the program
are the At vs. t data along with molar absorption
coefficients (o) for A and B together with that of C, and
Fig. 4 demonstrates such a typical fit.
(12)
k2
X[MnIII(L)(OH)(HA)]ꢂ ꢃH2O (13)
kꢂ2
The corresponding rate law is
3.6. Kinetics of the formation of intermediate complexes
ꢀ
ꢁ
k1[Hꢃ] ꢃ k2Ka
Ka ꢃ [Hꢃ]
Km
Km ꢃ [Hꢃ]
k0(1)ꢁ
[H2A]T
Dependence of the pseudo-first-order rate constants
k0(1) on [H2A] was investigated at 30 8C, pH 7.0 (0.05
kꢂ1[Hꢃ] ꢃ Km? kꢂ2
Km? ꢃ [Hꢃ]
mol dmꢂ3 phosphate buffer), and Iꢁ0.2 mol dmꢂ3
/
ꢃ
(14)
(NaClO4). Plot of k0(1) vs. [H2A] is a straight line (Table
3) with slope (7.399
0.16 dm3 molꢂ1 sꢂ1 for H2Q and
0.499
0.01 dm3 molꢂ1 sꢂ1 for H2cat) and a small
intercept (8.7898.72ꢄ
10ꢂ4 sꢂ1 for H2Q and 6.599
3.39ꢄ
10ꢂ4 sꢂ1 for H2cat), although not significant or
/
[H2A]T is the total analytical concentration of H2A.
/
/
/
/
/
precise, can be attributed to contributions from the
reverse reaction and/or self decomposition of the parent
complex in the medium. To check the effect of pH on
the rate of the reaction, kinetic experiments were
conducted in the pH range 5.75ꢀ
mol dmꢂ3, [H2cat]ꢁ0.004 mol dmꢂ3, and other con-
ditions were same. k0(1) vs. ꢂ
log [Hꢃ] plot (Fig. 6)
/
7.60 at [H2Q]ꢁ
/
0.003
/
/
shows that the rate of the formation of the intermediate
increases slowly at lower pH region and then rises
steeply. The above observation can be explained by Eqs.
(10)ꢀ(13):
/
Ka
H2AXHAꢂ ꢃHꢃ
(9)
Km
[MnIII(L)(H2O)2]ꢃ X[MnIII(L)(H2O)(OH)]ꢃHꢃ
(10)
[MnIII(L)(H2O)(OH)]ꢃH2A
Fig. 6. Plot of second-order rate constants vs. ꢂ
/
log [Hꢃ] for the
0.02 mol dmꢂ3
k1
formation of intermediate complex at 30 8C, Iꢁ
/
X [MnIII(L)(OH)(H2A)]ꢃH2O
(11)
(NaClO4), and 0.05 mol dmꢂ3 phosphate buffer. (k) For H2Q
kꢂ1
k
(0.003 mol dmꢂ3) and (j) for H2cat (0.004 mol dmꢂ3); best fit curve
[MnIII(L)(OH)(HA)]ꢂ ꢃHꢃ
is shown by (*).
/

A. Panja et al. / Polyhedron 22 (2003) 1191ꢀ
/
1198
1197
k0(ꢂ
/
1), the observed rate constant for the backward
reaction extracted through KFSIM40 program, is B
/
10ꢂ5
sꢂ1 and almost same in magnitude in each case. This is
associated with the second term of the above equation
and can be neglected leading to
ꢀ
ꢁ
k1[Hꢃ] ꢃ k2Km
Ka ꢃ [Hꢃ]
Km
Km ꢃ [Hꢃ]
k0(1)ꢁ
[H2A]T
(15)
The above equation was solved by means of a nonlinear
least-squares program and the computed parameters for
H2Q are k1ꢁ
0.21)ꢄ
103 dm3 molꢂ1 sꢂ1, Kaꢁ
and Kmꢁ(1.6290.29)ꢄ
10ꢂ8. The corresponding va-
lues for H2cat are k1ꢁ1.949
0.41 dm3 molꢂ1 sꢂ1, k2ꢁ
(2.2190.29)ꢄ (3.1590.43)ꢄ
102 dm3 molꢂ1 sꢂ1, Kaꢁ
10ꢂ10, and Kmꢁ 10ꢂ8
(4.6290.29)ꢄ
/
21.799
/
0.49 dm3 molꢂ1 sꢂ1, k2ꢁ
/
(1.379
/
/
/
(1.059
/
0.17)ꢄ
/
10ꢂ9
,
/
/
/
/
/
/
/
/
/
/
/
Fig. 7. Plot of second-order rate constants vs. ꢂ
/
log[Hꢃ] for the redox
/
/
/
.
decomposition of intermediate complex by dihydroxybenzenes at
30 8C, Iꢁ
0.02 mol dmꢂ3 (NaClO4), and 0.05 mol dmꢂ3 phosphate
buffer. (k) For H2Q (0.003 mol dmꢂ3) and (j) for H2cat (0.004 mol
dmꢂ3); best fit curve is shown by (*
).
/
3.7. Kinetics of the redox decomposition of the
intermediate
/
[MnIII(L)(H2O)(HA)]ꢃH2A
An analysis of the kinetic data from the second stage
of the reaction demonstrates that the plot of k0(2) vs.
[H2A] shows a first-order dependence on [H2A] (Table
3) with no intercept on the rate axis. A rate saturation is,
however, noted at higher concentrations of H2A. The
intramolecular electron transfer thereby cannot be a
reasonable explanation and the pre-equilibrium forma-
tion of a second adduct between the intermediate and
the H2A/HAꢂ species followed by electron transfer is
implicated.
Q1
k1
XIP1 0 MnII ꢃHAo ꢃHꢃ
(18)
Q2
k2
[MnIII(L)(H2O)(HA)]ꢃHAꢂ XIP2 0 MnII ꢃHAo (19)
The radicals thus generated react in a fast step with
another molecule of complex to give the final oxidation
products as benzoquinones.
The rate law derived from the above scheme is
ꢀ
ꢁ
k1Q2[Hꢃ] ꢃ k2Q2Ka
Ka ꢃ [Hꢃ]
Hꢃ
k(2)ꢁ
(20)
The theoretical rate law is
Km? ꢃ [Hꢃ]
2k2Q[H2A]T
k0(2)ꢁ
(16)
Here, k(2)ꢁk0(2)/2[H2A]T, 2 being the stoichiometric
/
1 ꢃ Q[H2A]T
factor of the reactions.
H2A is always less reactive than its deprotonated form
due to the change in the oxidation potential value [18],
and its contribution towards the redox step can be
assumed to be negligible. It is also evident from the fact
that fitting of the experimental data to Eq. (20) gives a
negative value for k1. Thus, Eq. (20) can be reduced to
Eq. (21) neglecting the k1 path as
where k2 is the second-order rate constant for the decay
step of the reactions and Q the association constant of
the second adduct.
The values of k2 and Q were evaluated by fitting the
experimental data to Eq. (16) and the evaluated para-
meters are k2ꢁ
Qꢁ(2.9390.12)ꢄ
(3.4090.07)ꢄ
10ꢂ3 dm3 molꢂ1 sꢂ1 and Qꢁ
0.20)ꢄ
102 dm3 molꢂ1 for H2cat at a fixed pH (7.00).
/
(5.009
102 dm3 molꢂ1 for H2Q, and k2ꢁ
(1.429
/
0.02)ꢄ
/
10ꢂ3 dm3 molꢂ1 sꢂ1 and
/
/
/
/
ꢀ
ꢁ
/
/
/
/
k2Q2Ka
Ka ꢃ [Hꢃ] Km? ꢃ [Hꢃ]
[Hꢃ]
/
k(2)ꢁ
(21)
The pH variation study for the electron transfer step
shows a bell-shaped curve (Fig. 7) which indicates that
MnIII(L)(H2O)(HA) is the reactive intermediate. Analo-
gous observations were encountered in the oxidation of
The evaluated parameters are k2Q2ꢁ
molꢂ1 sꢂ1, Kaꢁ 10ꢂ8, and Km?
(2.0190.25)ꢄ
0.36)ꢄ 0.389
10ꢂ7 for H2Q; and k2Q2ꢁ
molꢂ1 sꢂ1 10ꢂ8
Kaꢁ(1.9490.25)ꢄ and Km?
(1.2890.18)ꢄ
10ꢂ7 for H2cat.
/
0.779
/
0.05 dm3
/
/
/
/
ꢁ
/
(2.399
/
/
/
/
0.03 dm3
several
inorganic
and
organic
species
by
,
,
/
/
/
,
/
ꢁ
/
[MnIII(cdta)(H2O)]ꢂ, where [MnIII(cdta)(H2O)(OH)]2ꢂ
/
/
, the hydroxo complex, was found to be redox-inactive
[17]. A reasonable reaction scheme thereby can be
framed as
It is worth noting that the proton dissociation
constants (Ka) for H2A are found to be slightly higher
in contrast to the reported value [19] and could be due to
the fact that on association the phenolic proton becomes
slightly labile. The pKm value for the complex
[MnIII(L)(H2O)2]ꢃ obtained by spectrophotometric
[MnIII(L)(H2O)(HA)]
Km?
X[MnIII(L)(OH)(HA)]ꢂ ꢃHꢃ (17)

1198
A. Panja et al. / Polyhedron 22 (2003) 1191ꢀ1198
/
method agrees well to the kinetically evaluated value in
the formation step.
CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK
(fax: ꢃ44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk
or www: http://www.ccdc.cam.ac.uk).
/
The interaction of H2A with the complex
[MnIII(L)(Cl)(H2O)] occurs through the initial forma-
tion of relatively stable intermediates which then react
with another molecule of H2A leading to the formation
of the final products, manganese(II) and 1,4- or 1,2-
benzoquinones. This proposition of the sequence of
reactions is supported by the kinetic traces. Similar
behavior was also observed in the oxidation of ascorbic
acid [20] and sulfur(IV) [21] by the complex
[MnIII(L)(H2O)2]ꢃ. The formation reactions have been
shown to be much faster than the decay steps in earlier
studies. The reactions reported here are much more
Acknowledgements
This work has been supported through a research
project from the Council of Scientific and Industrial
Research, New Delhi, to P.B. and a CSIRꢀNET
/
fellowship to A.P. We would like to thank Prof. R.
Banerjee, Jadavpur University, Kolkata, for some help
in operating the KFSIM40 program.
complicated, and the formation steps are only about 2ꢀ
/
4 times higher than the decay ones.
Several interesting aspects in this study deserve special
mention.
References
1) The complex [MnIII(salen)(Cl)(H2O)] is octahedral
in the solid state as evidenced from its single-crystal
X-ray analysis, with the coordinated water molecule
being moved far away from the metal center. This is
different from the studies reported earlier where it
was assumed to have a square-pyramidal structure.
2) The cyclic voltammetric studies showed that the
prepared complex has a significantly higher E1/2
value compared to the previously reported values on
related complexes. This implies that in the later
complexes the metal center was coordinatively
unsaturated and had natural tendency to dimerise
to fulfil the coordination number 6.
[1] M.D. Hobday, T.D. Smith, Coord. Chem. Rev. 9 (1972) 311.
[2] L.J. Boucher, J. Inorg. Nucl. Chem. 36 (1974) 531.
[3] F.M. Ashmawy, C.A. McAuliffe, R.V. Parish, J. Tames, J. Chem.
Soc., Dalton Trans. (1985) 1391.
[4] V.L. Pecoraro, W.M. Butler, Acta Crystallogr., Sect. C 42 (1986)
1151.
[5] M. Gerloch, F.E. Mabbs, J. Chem. Soc. A (1967) 1598.
[6] R. Ramraraj, A. Kira, M. Kaneko, Angew. Chem., Int. Ed. Engl.
25 (1986) 825.
[7] J. Limburg, J.S. Vrettos, L.M. Liable-Sands, A.L. Rheingold,
R.H. Crabtree, G.W. Brudvig, Science 283 (1999) 1524.
[8] N. Auranzeb, C.E. Helme, C.A. McAuliffe, R.G. Pritchard, M.
Watkinson, R. Bermejo, A. Sousa, J. Chem. Soc., Chem.
Commun. (1994) 1153.
[9] K. Oyaizu, T. Nakagawa, E. Tsuchida, Inorg. Chim. Acta 305
(2000) 184.
3) The
oxidation
of
L
-ascorbic
acid
by
[10] M. Maneiro, M.R. Bermejo, A. Sousa, M. Fondo, A.M.
Gonzalez, A. Sousa-Pedrares, C.A. McAuliffe, Polyhedron 19
(2000) 47.
[MnIII(salen)(H2O)2]ꢃ was found to be inner-sphere
in nature leading to the formation of intermediate
ascorbate complex which was believed to inhibit the
electron transfer act. The observations in the pre-
sent study are also in the same line but the pH-rate
profiles for the redox step clearly point out that it is
the protic equilibrium (17) responsible for such
observations rendering [MnIII(L)(OH)(HA)]ꢂ to
be a silent partner towards redox act.
[11] G.W. Brudvig, W.F. Beck, J.C. de Paula, Annu. Rev. Biophys.
Chem. 18 (1989) 25.
[12] A. van den Bergen, K.S. Murray, M.J. O’Connor, B.O. West,
Aust. J. Chem. 22 (1969) 39.
[13] G.M. Sheldrick, Acta Crystallogr., Sect. A 46 (1990) 467.
[14] G.M. Sheldrick, SHELXL-97, Program for Crystal Structure
Refinement, University of Gottingen, 1997.
¨
[15] C.K. Johnson, M.N. Burnett, ORNL-6895, Oak Ridge National
Laboratory, Oak Ridge, TN, 2000.
4) Comparison of reactivity showed that H2Q is more
reactive than H2cat for both the formation and
decay steps which is at par with the earlier findings
[22].
[16] (a) A. Barshop, R.F. Wren, C. Frieden, Anal. Biochem. 130
(1983) 134;
(b) B. Chaudhuri, R. Banerjee, J. Chem. Soc., Dalton Trans.
(1998) 3451.
[17] S. Gangopadhyay, M. Ali, P. Banerjee, Coord. Chem. Rev. 135/
136 (1994) 399.
[18] S. Bhattacharya, A. Chakravorty, Comments Inorg. Chem. 4
(1985) 1.
4. Supplementary data
[19] B. Saha, A. Dutta, P. Banerjee, Coord. Chem. Rev. 170 (1998) 47.
[20] I.A. Salem, A.H. Gemeay, Transition Met. Chem. 21 (1996) 130.
[21] A.C. Dash, A. Das, Int. J. Chem. Kinet. 31 (1999) 627.
[22] (a) J.C. Brodovitch, A. McAuley, T. Oswald, Inorg. Chem. 21
(1982) 3442;
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data
[MnIII(salen)(Cl)(H2O)]×
tion may be obtained free of charge from The Director,
Centre,
CCDC
No.
168827
for
/H2O. Copies of this informa-
(b) E. Pelizzetti, E. Mentasi, C. Baiocchi, J. Chem. Soc., Dalton
Trans. (1977) 132.