Synthesis and characterization of some Mn(II) and Mn(III) complexes of N,N′-o-phenylenebis(salicylideneimine)(LH2) and N,N′-o-phenylenebis(5-bromosalicylideneimine)(L′H2). Crystal structures of [Mn(L)(H2O)(ClO4)], [Mn(L)(NCS)] and an infinite linear chain of [Mn(L)(OAc)]

Article abstract of DOI:10.1016/j.ica.2005.01.026

A series of manganese(II) [Mn(L)] and manganese(III) [Mn(L)(X)] (X = ClO₄, OAc, NCS, N₃, Cl, Br and I) complexes have been synthesized from Schiff base ligands N,N′-o- phenylenebis(salicylideneimine)(LH₂) and N,N′-o-phenylenebis(5- bromosalicylideneimine)(L′H₂) obtained by condensation of salicylaldehyde or 5-Br salicylaldehyde with o-phenylene-diamine. The complexes have been characterized by the combination of IR, UV-Vis spectroscopy, magnetic measurements and electrochemical studies. Three manganese(III) complexes 3 [Mn(L)(ClO₄)(H₂O)], 5 [Mn(L)(OAc)] and 13 [Mn(L)(NCS)] have been characterized by X-ray crystallography. The X-ray structures show that the manganese(III) is hexa-coordinated in 3, it is penta-coordinated in 13, while in 5 there is an infinite chain where the MnL moieties are connected by acetate ions acting as bridging bidentate ligand. The cyclic voltammograms of all the manganese(III) complexes exhibit two reversible/quasi-reversible/ irreversible responses assignable to Mn(III)/Mn(II) and Mn(IV)/Mn(III) couples. It was observed that the ligand L′H₂ containing the 5-bromosal moiety always stabilizes the lower oxidation states compared to the corresponding unsubstituted LH₂. Cyclic voltammograms of the manganese(II) complexes (1 and 2) exhibit a quasi-reversible Mn(III)/Mn(II) couple at E_1/2 -0.08 V for 1 and 0.054 V for 2.

Full text of DOI:10.1016/j.ica.2005.01.026

Inorganica Chimica Acta 358 (2005) 2473–2481
www.elsevier.com/locate/ica
Synthesis and characterization of some Mn(II) and Mn(III)
complexes of N,N0-o-phenylenebis(salicylideneimine)(LH2)
and N,N0-o-phenylenebis(5-bromosalicylideneimine)(L0H2).
Crystal structures of [Mn(L)(H2O)(ClO4)], [Mn(L)(NCS)]
and an infinite linear chain of [Mn(L)(OAc)]
a
Subhendu Biswas , Kamala Mitra , C.H. Schwalbe , C. Robert Lucas
a
c
b,
*
,
a,
a,
*
Shyamal K. Chattopadhyay *, Bibhutosh Adhikary
a
Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711 103, India
b
Department of Chemistry, Memorial University of Newfoundland, St. Johnꢀs, Newfoundland, Canada A1B 3X7
Department of Pharmaceutical Sciences, Aston University, Aston Triangle, Birmingham B4 7ET, UK
c
Received 20 July 2004; accepted 14 January 2005
Available online 14 March 2005
Abstract
A series of manganese(II) [Mn(L)] and manganese(III) [Mn(L)(X)] (X = ClO4, OAc, NCS, N3, Cl, Br and I) complexes have been
synthesized from Schiff base ligands N,N0-o-phenylenebis(salicylideneimine)(LH2) and N,N0-o-phenylenebis(5-bromosalicylidenei-
mine)(L0H2) obtained by condensation of salicylaldehyde or 5-Br salicylaldehyde with o-phenylene-diamine. The complexes have
been characterized by the combination of IR, UV–Vis spectroscopy, magnetic measurements and electrochemical studies. Three
manganese(III) complexes 3 [Mn(L)(ClO4)(H2O)], 5 [Mn(L)(OAc)] and 13 [Mn(L)(NCS)] have been characterized by X-ray crystal-
lography. The X-ray structures show that the manganese(III) is hexa-coordinated in 3, it is penta-coordinated in 13, while in 5 there
is an infinite chain where the MnL moieties are connected by acetate ions acting as bridging bidentate ligand. The cyclic voltam-
mograms of all the manganese(III) complexes exhibit two reversible/quasi-reversible/irreversible responses assignable to Mn(III)/
Mn(II) and Mn(IV)/Mn(III) couples. It was observed that the ligand L0H2 containing the 5-bromosal moiety always stabilizes
the lower oxidation states compared to the corresponding unsubstituted LH2. Cyclic voltammograms of the manganese(II) com-
plexes (1 and 2) exhibit a quasi-reversible Mn(III)/Mn(II) couple at E1/2 ꢀ0.08 V for 1 and 0.054 V for 2.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Manganese(II) and manganese(III) complexes; Phenolate Schiff bases; N2O2 donors; Acetate bridged manganese(III) chain; Electronic
spectra; X-ray structure; Electrochemistry
1. Introduction
of the role of this metal in biological systems [1–4]. Stud-
ies of spectroscopic and physicochemical properties
along with X-ray structure determination of model com-
plexes are especially important to gain insights into the
nature and mode of action of the manganese containing
prosthetic groups. It has been established that the bio-
logical sites containing manganese are of variable nucle-
arity and the oxidation states of manganese vary
between +2 and +5 [5–11]. Of particular interest is the
There has been considerable interest in the coordina-
tion chemistry of manganese involving nitrogen and
oxygen donor ligands due to the increasing recognition
*
Corresponding authors. Fax: + 91 33 2668 2916.
E-mail addresses: shyamal@chem.becs.ac.in (S.K. Chattopadhyay),
adhikarybibhu@yahoo.com (B. Adhikary).
0020-1693/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.01.026

2474
S. Biswas et al. / Inorganica Chimica Acta 358 (2005) 2473–2481
Y
Mn(ClO4)2 Æ 6H2O/Mn(OAc)2 Æ 4H2O/MnCl2 Æ 6H2O
(1 mmol) and Et3N (2mmol) in 5 ml MeCN in stirring
condition. After 1 h of stirring, a saturated aqueous
solution (5 ml) of KBr/KI/NH4NCS/NaN3 was added
which results in immediate separation of brown/black
colored corresponding compounds. They were collected
by filtration and washed with water, ethanol and finally
with diethyl ether. All the compounds were recrystal-
lized from MeCN–ether mixture, except 9 and 10, which
cannot be purified further due to its solubility only in
DMF and DMSO. Yield ranges from 80% to 94%.
Ligand
LH2
Y
O
N
N
H
Mn
X
O
L'H2
Br
Y
Scheme 1.
study of electron transfer behavior of manganese(II),
manganese(III) and manganese(IV) and its modulations
by change of substituents on ligand because of their po-
tential uses as oxidizing agents, catalysts [11–13] and
electro catalysts [14–16] for the oxidation of compounds
such as alcohols, esters and water [17–19]. In this paper,
we report synthesis, electronic spectra and redox activi-
ties of several manganese(II) and manganese(III) com-
plexes and structural characterization of three
manganese(III) complexes involving N2O2 Schiff-base li-
gand (see Scheme 1).
2.2.3. Complexes (1 and 2)
The complexes were prepared by adding solid
Mn(ClO4)2 Æ 6H2O (1 mmol) to a degassed MeCN solu-
tion (25 ml) of ligand (1 mmol, LH2/L0H2) and Et3N
(2 mmol) in 5 ml MeCN at room temperature. Yellow
compounds were separated within 20 min. The com-
pounds were collected by filtration under N2 and washed
in acetonitrile and finally with diethyl ether. Yield
ranges from 70% to 80%.
Caution! Perchlorate salts are potentially explosive,
and though we have not encountered any problem, nev-
ertheless they should be handled with care.
2. Experimental
2.1. Materials
2.3. Physical measurements
All the chemicals and solvents used for the syntheses
were of commercially available reagent grade and were
used without further purification. TEAP was prepared
following the literature procedure [20]. All the Schiff
base ligands were prepared by known methods [21].
Microanalyses (CHN) were performed on a Perkin–
Elmer 2400II elemental analyzer. Electronic spectra
were obtained using a JASCO 7850 spectrophotometer.
IR spectra were recorded on JASCO FT/IR-460 spec-
trometer with samples prepared as KBr pellets. Electro-
chemical experiments were carried out using a EG&G
PARC VersaStat II potentiostat with three-electrode
configuration consisting of a PARC glassy carbon
working electrode, a Pt wire as auxiliary electrode and
Ag/AgCl as the reference electrode and 0.1 mol dmꢀ3
NEt4ClO4 as supporting electrolyte in MeCN or DMF
solution under dry nitrogen atmosphere with a scan rate
0.1 V sꢀ1. Data were collected using ꢁECHEMꢀ software
supplied by PARC. For electrochemical and spectral
measurements HPLC grade solvents were used. All the
electrochemical potentials were calibrated against a fer-
rocene–ferrocenium couple. Room temperature mag-
netic susceptibility measurements were done by PAR
155 model vibrating sample magnetometer, which was
calibrated against Hg[Co(NCS)4].
2.2. Synthesis of the complexes
2.2.1. Complexes (3, 4, 5, 6, 7 and 8)
All of these were prepared by a similar method given
below.
The complexes were prepared by stirring a MeCN
solution (25 ml) of ligand (1 mmol, LH2/L0H2)
and adding successively a MeCN solution (5 ml)
of Mn(ClO4)2 Æ 6H2O/Mn(OAc)2 Æ 4H2O/MnCl2 Æ 6H2O
(1 mmol) and Et3N (2 mmol) in 5 ml MeCN in stirring
condition. The initial yellow color of the solution rap-
idly changed to brown. After 3 h of stirring at room
temperature slow evaporation of solvent lead to depo-
sition brown compounds. The products were collected
by filtration and washed with ethanol–diethyl ether
mixture (1:1). The compounds were recrystallized from
MeCN–diethyl ether mixture. Yield ranges from 80%
to 91%.
2.4. Crystal structure determination and structural
refinement of 3, 5 and 13
The single crystals of 3, 5 and 13 suitable for X-ray
diffraction were grown by slow diffusion of anhydrous
diethyl ether into a MeCN solution of the appropriate
compound at room temperature. Crystal data are sum-
marized in Table 3. Intensity data were collected on a
2.2.2. Complexes (9, 10, 11, 12, 13, 14 and 15)
The complexes were prepared by stirring a MeCN
solution (25 ml) of ligand (1 mmol, LH2/L0H2) and
adding successively
a MeCN solution (5 ml) of

S. Biswas et al. / Inorganica Chimica Acta 358 (2005) 2473–2481
2475
Table 1
List of the complexes reported in this paper
Nonius kappa-geometry based CAD4/PC V2.0 X-ray
diffractometer for 3 and in a Bruker P4/CCD system
for 5 and 13, using graphite monochromated Mo Ka
radiation in all cases. For 3, absorption corrections were
derived from psi-scan measurements [22] and Siemens
area detected absorption routine SADABS for 5 and 13.
The structures were solved by direct methods [23] refined
by least squares using the program SHELXL-97 [24]. The
non-hydrogen atoms were refined independently with
anisotropic displacement parameters. Most of the
hydrogen atoms appeared in difference Fourier maps.
For 3, water hydrogen atoms were refined indepen-
dently, subject to a distance restraint. All other hydro-
gen atoms for 3 and all hydrogens for 5 and 13 were
placed at calculated positions and refined using a riding
model with isotropic displacement parameters. The
refinements converged with the residuals summarized
in Table 3. Selected bond distances are given in Table 4.
Complex
X
Y
1
2
Mn(L)(H2O)
Mn(L0)(H2O)
H
Br
H
3
[Mn(L)(ClO4)(H2O)]
[Mn(L0)(ClO4)(H2O)] Æ 3H2O
[Mn(L)(OAc)]
ClO4
ClO4
OAc
OAc
Cl
4
Br
H
5
6
[Mn(L0)(OAc)] Æ 2H2O
[Mn(L)(Cl)(H2O)]
[Mn(L0)(Cl)(H2O)]
[Mn(L)(Br)(H2O)]
[Mn(L0)(Br)(H2O)]
[Mn(L)(I)(H2O)]
[Mn(L0)(I)(H2O)]
[Mn(L)(NCS)]
Br
H
7
8
Cl
Br
Br
H
9
10
11
12
13
14
15
Br
I
Br
H
I
Br
H
NCS
N3
[Mn(L)(N3)]
[Mn(L0)(N3)]
H
Br
N3
Table 2
Analytical data and magnetic moments for the complexes
3. Results and discussion
Complex
Analyses: Calc. (found)
leff (BM)
3.1. Synthesis
C (%)
H (%)
N (%)
1
2
62.00(62.10)
44.03(44.02)
49.33(49.32)
34.36(34.35)
61.68(61.67)
42.44(42.45)
56.80(56.81)
41.34(41.35)
51.39(51.40)
38.40(38.41)
46.69(46.70)
35.71(35.72)
59.01(59.00)
58.39(58.40)
42.18(42.17)
4.13(4.11)
2.57(2.56)
3.29(3.28)
2.86(2.87)
3.97(3.86)
3.05(3.04)
3.79(3.78)
2.41(2.42)
3.43(3.42)
2.24(2.25)
3.11(3.10)
2.08(2.07)
3.27(3.26)
3.41(3.40)
2.11(2.10)
7.24(7.23)
5.14(5.13)
5.76(5.77)
4.01(4.02)
6.54(6.55)
4.50(4.51)
6.63(6.62)
4.82(4.81)
5.99(5.98)
4.48(4.47)
5.45(5.46)
4.17(4.16)
9.83(9.84)
17.03(17.02)
12.30(12.22)
5.92
6.01
4.62
4.75
3.84
3.82
4.97
4.88
4.94
4.87
4.69
4.77
4.98
4.88
4.74
The manganese(II) complexes 1 and 2 have been pre-
pared in fairly good yields (70–80%) by reacting solid
Mn(ClO4)2 Æ 6H2O with the appropriate ligand in nitro-
gen atmosphere. The solutions of these two complexes
are not stable in air and turn deep chocolate immedi-
ately due to aerial oxidation. All the manganese(III)
complexes have been prepared by the direct reaction
of corresponding ligands and the manganese(II) salts
in presence of Et3N in aerobic condition, when the ini-
tially formed manganese(II) complexes rapidly oxidizes
to the corresponding manganese(III) species, probably
following similar mechanism as described by Boucher
and Coe [25].
3
4
5
6
7
8
9
10
11
12
13
14
15
We have also synthesized the same manganese(III)
complexes using the saturated ligand obtained by the
reduction of the Schiff-base ligand (LH2) by NaBH4.
The complexes obtained from the saturated ligand are
oxidized to original Schiff-base ligand during prepara-
tion of the complexes, which was confirmed by infrared
spectroscopy (absence of N–H band at ca. 3200 cmꢀ1) as
well as by X-ray structure determination of one of the
representative compound (the acetate coordinated deriv-
ative). Earlier workers also reported similar observa-
tions [26] (see Table 2).
3.2. Description of the structures
3.2.1. Complex 3 [MnL(ClO4)(H2O)]
The structural analysis of complex 3 (Fig. 1) has re-
vealed that the central manganese(III) ion adopts a dis-
torted octahedral structure. The N2O2 donor set coming
from ligand itself forms an equatorial plane, and the
Fig. 1. ORTEP diagram for the complex 3 [Mn(L)(ClO4)(H2O)]
showing the atom labeling scheme and 50% probability ellipsoid for
the non-hydrogen atom.

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S. Biswas et al. / Inorganica Chimica Acta 358 (2005) 2473–2481
Table 3
Data collection and structure refinements for 3, 5 and 13
Complex
3
5
13
Empirical formula
Formula weight
MnC20H16N2O7Cl
486.74
red, plate
C22H17N2O4Mn
428.33
C21H14N3O2MnS
427.36
Crystal color, habit
Crystal dimensions (mm)
Crystal system
black, prism
0.78 · 0.30 · 0.14
monoclinic
11.4055(6)
14.1242(7)
12.4671(6)
90
black, plate
0.50 · 0.24 · 0.04
monoclinic
12.8465(4)
10.0590(3)
14.1869(4)
90
0.35 · 0.25 · 0.08
triclinic
˚
a (A)
8.0481(11)
11.2063(19)
11.844(2)
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
98.555(13)
109 329(14)
93.359(13)
990.1(3)
115.2240(9)
90
1816.9(1)
100.0050(5)
90
3
˚
V (A )
1805.39(8)
P21/n (#14)
4
ꢀ
Space group
Z value
l (Mo Ka) (mmꢀ1
Tmax,min
P1
P21/n (#14)
4
0.760
2
0.849
)
0.870
0.6702, 0.9660
872
0.934, 0.737
496
0.5886, ꢀ0.194
880
F(0 0 0)
Index ranges
ꢀ9 6 h 6 1,
ꢀ13 6 k 6 13,
ꢀ14 6 l 6 14
3443/2/289
0.0384; 0.1104
0.66 and ꢀ0.39
ꢀ14 6 h 6 14,
ꢀ17 6 k 6 17,
ꢀ15 6 l 6 15
3714/0/262
0.029; 0.082
0.337 and ꢀ0.194
ꢀ16 6 h 6 16,
ꢀ12 6 k 6 12,
ꢀ17 6 l 6 17
3687/0/253
0.029; 0.078
0.354 and ꢀ0.170
Data/restraints/parameters
R1; Rw
Maximum and minimum
˚
peak in difference map (e A
Goodness-of-fit on F2
ꢀ3
)
1.012
1.049
1.053
˚
remaining axial positions are occupied by the O donor
set of H2O and perchlorate. The bond distances of
˚
Mn(1)–O(1) and Mn(1)–O(7) (2.3684 and 2.2370 A,
1.8458, 1.9729, 1.9755 A, respectively) which fall within
the range observed for structurally characterized manga-
nese(III) compounds [27].
respectively) are longer than those of Mn(1)–O(5),
Mn(1)–O(6), Mn(1)–N(8) and Mn(1)–N(15) (1.8770,
The elongated axial bonds may have arisen from the
Jahn–Teller effect. The basal bond angles are all close to
Table 4
˚
Selected bond distances (A) and bond angles (ꢁ) for complexes 3, 5 and 13
3
5
13
Bond distances
Mn(1)–O(5)
Mn(1)–O(6)
Mn(1)–O(1)
Mn(1)–O(7)
Mn(1)–N(8)
Mn(1)–N(15)
1.877(2)
1.846(2)
2.368(3)
2.238(3)
1.973(3)
1.976(3)
Mn(1)–O(1)
Mn(1)–O(2)
Mn(1)–O(3)
Mn(1)–O(4a)
Mn(1)–N(1)
Mn(1)–N(2)
1.8777(12)
1.8762(12)
2.2166(12)
2.2432(12)
1.9969(15)
1.9972(15)
Mn(1)–O(1)
Mn(1)–O(2)
Mn(1)–N(1)
Mn(1)–N(2)
Mn(1)–N(3)
1.8562(11)
1.8886(11)
1.9949(13)
1.9840(13)
2.1649(15)
Bond angles
N(8)–Mn(1)–N(15)
N(8)–Mn(1)–O(1)
N(8)–Mn(1)–O(7)
N(15)–Mn(1)–O(1)
N(15)–Mn(1)–O(7)
O(5)–Mn(1)–O(6)
O(5)–Mn(1)–N(8)
O(5)–Mn(1)–N(15)
O(5)–Mn(1)–O(1)
O(5)–Mn(1)–O(7)
O(6)–Mn(1)–N(8)
O(6)–Mn(1)–N(15)
O(6)–Mn(1)–O(1)
O(6)–Mn(1)–O(7)
O(1)–Mn(1)–O(7)
82.83(11)
83.25(10)
86.12(10)
89.31(10)
91.24 (10)
90.67(10)
93.45(10)
174.41(11)
86.10(10)
92.67(10)
175.56(10)
92.94(11)
95.39 (11)
95.35(11)
169.20(10)
N(1)–Mn(1)–N(2)
N(1)–Mn(1)–O(1)
N(1)–Mn(1)–O(2)
N(1)–Mn(1)–O(3)
N(1)–Mn(1)–O(4)
O(1)–Mn(1)–O(2)
O(1)–Mn(1)–N(2)
O(1)–Mn(1)–O(3)
O(1)–Mn(1)–O(4)
O(2)–Mn(1)–N(2)
O(2)–Mn(1)–O(3)
O(2)–Mn(1)–O(4)
O(3)–Mn(1)–O(4)
N(2)–Mn(1)–O(3)
N(2)–Mn(1)–O(4)
82.05(6)
93.24(5)
173.95(5)
83.45(5)
89.62(5)
92.12(5)
173.24(5)
96.17(5)
94.45(5)
92.85(5)
93.17(5)
92.78(5)
167.62(5)
88.13(5)
80.74(5)
N(1)–Mn(1)–N(2)
N(1)–Mn(1)–O(1)
N(1)–Mn(1)–O(2)
N(1)–Mn(1)–N(3)
N(2)–Mn(1)–N(3)
O(1)–Mn(1)–O(2)
O(1)–Mn(1)–N(3)
O(1)–Mn(1)–N(2)
O(2)–Mn(1)–N(2)
O(2)–Mn(1)–N(3)
82.29(5)
91.58(5)
170.47(5)
89.37(5)
96.41(6)
90.90(5)
99.61(5)
162.79(5)
92.72(5)
99.28(5)

S. Biswas et al. / Inorganica Chimica Acta 358 (2005) 2473–2481
2477
90ꢁ (O(5)–Mn–O(6), N(8)–Mn–O(5), N(15)–Mn–O(6)
are 90.67ꢁ, 93.67ꢁ, 92.96ꢁ, respectively) except N(15)–
Mn–N(8) that is 82.82ꢁ. The manganese atom is
3.2.2. Complex 5 [MnL(OAc)]
An ORTEP diagram along with atom numbering
scheme for the asymmetric unit of 5 is shown in Fig.
2. In the asymmetric unit the ligand L occupies the equa-
torial plane with the Mn–O and Mn–N distances very
similar to that of 3. The basal bond angles of the two
molecules are also quite similar. The atoms Mn(1),
O(2), N(1) and N(2) form a good least square plane with
the Mn(1) atom showing the maximum deviation of
˚
0.048(1) A out of the plane of these four atoms. Com-
pared to the [Mn(L)(ImzH)2]ClO4 complex recently
investigated in our laboratories [27], the angles sub-
tended at Mn by equatorial ligand atoms differ by at
most 1.2ꢁ. The diverse axial ligands in the present case
show greater variation and a less ideal geometry, e.g.,
O(1)–Mn(1)–O(7) is 169.3(1)ꢁ while the imidazole N–
Mn–N angle was 173.8(1)ꢁ. Participation of the perchlo-
rate ion as a ligand is evidently sensitive to conditions.
When the identical [MnIII(L)]+ complex, was crystallized
from methanol containing a little water, the axial posi-
tions were occupied by water and methanol at Mn–O
˚
0.036(4) A from this plane. The other donor atom in
the basal plane O(1) is appreciably deviated from this
˚
plane (0.221(1) A). The acetate ion is coordinated by
one of its oxygen O(3) to the axial position and the
Mn(1)–O(3) bond length is, as expected, appreciably
longer than the equatorial Mn(1)–O(1) bond. However,
the more interesting aspect of this structure is that the
other oxygen of the acetate O(4) is also participating
in coordination by binding to a second MnL moiety.
Thus in the solid state, the Mn(1) atom is in a 4 + 2
coordination geometry and the acetate ion joins the
individual distorted octahedrons to form a one dimen-
sional infinite chain (see Fig. 3).
˚
distances of 2.272(2) and 2.292(2) A [28]. Likewise, the
related
diaqua-(N,N0)-3,5-dichlorobis-(salicylidene)-
(1,2-diaminoethane)Mn(III) perchlorate has solvent
(water) molecules occupying both axial positions with
˚
Mn–O distances 2.220(5) and 2.272(5) A [29]. With
weakly coordinating axial ligands very long Mn–O
˚
distances can occur: 2.489(4) A to one triflate O atom
in [(3,30-17-crown-6-o-phsal)Mn(III)] (CH3COO) Ba-
(SO3CF3)2 Æ 2H2O [30]. Although in this series of four
N2O4 donor complexes there are differences in detail
among the tetradentate Schiff base ligands, the average
of the four equatorial Mn–O and Mn–N distances
˚
hardly changes: 1.918, 1.923, 1.924, 1.923 A, respec-
tively. Even the drastic changes in the axial ligands ap-
pear to self-compensate with one O atom drawing
nearer as the other becomes more distant; the average
axial Mn–O distances in the four complexes are 2.303,
˚
2.282, 2.246 and 2.298 A. In the present structure, the
single water molecule donates hydrogen bonds to per-
chlorate O(4) and (more strongly) to phenolate O(5)
˚
with Hꢁ ꢁ ꢁO distances 2.18(3) and 1.95(3) A, Oꢁ ꢁ ꢁO dis-
˚
tances 2.874(4) and 2.744(4) A, O–Hꢁ ꢁ ꢁO angles 148(5)
and 165(5)ꢁ, respectively. Even though the comparison
structure with water and methanol as ligands has one
more polar H atom available for hydrogen bonding,
the phenolato O atoms are not used as acceptors [28].
However, in the second comparison structure there are
hydrogen bonds from water ligands to phenolate O
atoms.
Fig. 2. ORTEP diagram and atom numbering scheme for the complex
5 [Mn(L)(OAc)] showing 50% probability ellipsoid for the non-
hydrogen atom.
Fig. 3. A view of the linear chain of complex 5.

2478
S. Biswas et al. / Inorganica Chimica Acta 358 (2005) 2473–2481
13 may be attributable to the N-bonded thiocyanate
[32]. For 14 and 15, a strong peak at ca. 2040 cmꢀ1 have
been shown to be diagnostic of anti symmetric stretch
m
asym(N3) vibrations. A broad absorption of most of
the complexes at ca. 3430 cmꢀ1 is due to m(H2O)
vibrations.
The analytical data of the complexes along with their
room temperature magnetic moment data are given in
Table 1. The value of magnetic moments for the com-
plexes 1 and 2 are 5.92 and 6.01 BM, which are consistent
with high-spin square pyramidal stereochemistry. Simi-
larly, the magnetic susceptibility data for all manga-
nese(III) complexes except 5 and 6 are in the range of
4.98–4.62 BM indicating high-spin d4 configuration.
The much lower magnetic moment of 5 and 6 indicates
that there is appreciable superexchange between the man-
ganese(III) centers through the bridging acetate ions.
Fig. 4. ORTEP diagram and atom numbering scheme for the complex
13 [Mn(L)(NCS)] with 50% thermal probability ellipsoid for the non-
hydrogen atom.
3.2.3. Complex 13 [MnL(NCS)]
The ORTEP representation of 13 (Fig. 4) shows that
the Mn center is in a square pyramidal geometry. The
3.4. Electronic spectra
The electronic spectra (mull) of manganese(II) com-
plexes exhibit (Table 5) lack of any characteristic
absorption peak in the visible region, presumably be-
cause the weak d–d bands are submerged in the tail of
the charge transfer bands. The spectra of all the manga-
nese(III) complexes are very similar to those previously
reported for closely related Mn(salen)(X) [21], Mn(a-
cen)(X) [33] and other manganese(III) complexes [34].
The earlier workers have assigned the visible region of
the spectra to d–d transitions of Jahn–Teller distorted
d4 system, which under ideal symmetric ligand field will
˚
Mn–O (phenolate) distances (average 1.8724 A) and
˚
Mn–N (imine) distances (average 1.9894 A) in 13 are
similar to those observed in 3 and 5. The manganese
atom is additionally bound to a N-bonded thiocyanate
anion in the axial position where the Mn–N bond dis-
˚
tance is longer (2.1643 A) than the usual due to Jahn–
Teller distortion. The N–C–S angle is 178.08ꢁ indicates
that the NCS is linear. It is interesting to note that
though thiocyanate is a well-known bridging ligand,
and we have solved the X-ray crystallographic structure
of the molecule, with a hope to obtain a chain like struc-
ture, similar to that of the corresponding acetate com-
plex 5, the actual structure shows that no such chain
exists in the solid state. The ꢁhardꢀ coordination atmo-
sphere of manganese(III) probably precludes the coordi-
nation of ꢁsoftꢀ sulfur of thiocyanate moiety and thus the
chain formation cannot proceed.
5
5
be E ! T transition. However, the complexes of the
type being discussed here are of much lower symmetry
(C1). Boucher and Coe [25] as well as Chaudhury and
co-workers [34c] assumed a pseudo-C4v symmetry for
Table 5
Electronic spectral data for the complexes
3.3. Infrared spectra and magnetic moment
Complex
UV–Vis [kmax (nm)/e (Mꢀ1 cmꢀ1)]
1
2
3
470, 400 (Nujol mull)
470, 410 (Nujol mull)
The IR spectra of all the complexes have several
prominent bands. A strong band at ca. 1605 cmꢀ1 is as-
signed to m(C@N) stretching vibration. Additional
strong bands at ca. 1585, 1532 and 1450 cmꢀ1 have been
previously assigned to m(C@C, phenyl ring),
m(C'N + C'C + C'O) stretching vibrations, respec-
tively [31]. The perchlorate complexes 3 and 4 show a
number of strong and broad bands with peaks at ca.
1106 and 1128 cmꢀ1 for 3 and at 1136 and 1055 cmꢀ1
for 4 together with bands at ca. 617, 638 cmꢀ1 for 3
and at 625, 636 cmꢀ1 for 4, that are indicative of the
presence of coordinated perchlorate. The ma(COOꢀ)
and ms(COOꢀ) bands of acetate complexes 5 and 6 ap-
pear at ca. 1575 and 1430 cmꢀ1, respectively. The strong
vibrations at ca. 2100 cmꢀ1 of the thiocyanate complex
600(592), 550(1015), 445(8230), 340(23 870)
600, 560, 500 (Nujol mull)
4
5
600(647), 550(886), 438(5179) 338(14 242)
595(515), 540(925), 440(4925), 345(14 328)
600, 550, 500 (Nujol mull)
6
7
600(278), 550(618), 410(10 069), 320(11 850)
595(423), 545(1083), 440(6456), 340(17 977)
600(448), 550(1048), 425(8267), 340(17 507)
595(344), 545(964), 440(6010), 341(16 932)
600(405), 550(628), 440(2921), 340(8082)
600(408), 545(756), 445(8692), 345(18 002)
600(514), 550(838), 440(4852), 340(12 822)
595(420), 545(755), 425(11 000), 340(17 480)
590(429), 545(715), 430(6645), 340(18 750)
600(134), 550(1130), 405(8686), 325(17 064)
8
9a
10a
11
12
13
14
15
a
In DMF and rests are in MeOH.

S. Biswas et al. / Inorganica Chimica Acta 358 (2005) 2473–2481
2479
such systems, and they predicted three spin-allowed d–d
5
5
5
transitions 5B1 ! A1, 5B1 ! E and 5B1 ! B2 are pos-
sible, of which the first one will involve an electron from
the d2z orbital and the spectra should be sensitive to the
nature of the axial ligands. However, as the data tabu-
lated in Table 5 show, the spectra of all the complexes
are nearly the same and insensitive to the identity of
the axial ligands. This may be due to the displacement
of the axial ligands by coordinating solvent, viz.,
MeOH, but since the mull spectra of the complexes
are nearly the same as solution spectra, and the com-
plexes are non-conducting in nature in solution so the
5
5
possibility of B1 ! A1 transition is ruled out. So, the
5
5
first band is due to the B1 ! E transition and thus it
is independent of energy of the d2z orbital. The second
5
band is then due to the 5B1 ! B2 transition. Beside
the d–d transitions, the complexes exhibit two charge
transfer bands in the near UV-region. The band at ca.
440 nm
may
be
attributable
to
phenolate
ꢂ
OðppÞ ! Mnðdp Þ ligand-to-metal charge transfer by
analogy with related manganese(III) complexes. The
remaining band at ca. 350 nm is most likely due to
p ! p* (azomethine) ligand transition. Though the elec-
tronic spectral assignments in the previous paragraph
were done following the works on similar systems re-
ported in the literature, in order to have a deeper under-
standing of the problem we have carried out UHF
ZINDO/S calculations [35,27] on 5 and 13.
Fig. 5. HOMO(71a, alpha) for 13. SHOMO(70a, alpha) for 13.
Our calculations show that the HOMO to HOMO-3
orbitals containing the four unpaired electrons for these
two molecules, contain at the most 2–3% Mn character
as measured by the sum of square of the coefficients of
the manganese orbitals in a given M.O. This is also ni-
cely illustrated in Fig. 5, which clearly shows that the
HOMO and SHOMO for 13 is mainly based on N atom
of thiocyanate. We have recently observed similar re-
sults using more accurate ab initio GAMESS calculation
[27]. Thus, the visible spectral bands are best assigned to
HOMO ! LUMO and SHOMO ! LUMO transitions.
Table 6
Electrochemical data for the complexes
*
Complex
E1=2ðMn
Þ; ½DEꢃ ðVÞ
E1=2ðMn
Þ; ½DEꢃ ðVÞ
III=MnII
IV=MnIII
1
2
ꢀ0.08[0.122]
ꢀ0.054[0.076]
ꢀ0.021[0.074]
0.160[0.172]
0.06b
3
0.426[0.108]
0.490[0.088]
0.444[0.070]
0.491[0.060]
0.886[0.120]
0.940[0.150]
0.487[0.094]
0.518[0.060]
4
5
6
0.068[0.140]
ꢀ0.092[0.086]
0.050[0.124]
ꢀ0.089(0.074)
ꢀ0.071[0.066]
7
8
9a
10a
11
12
13
14
15
a
3.5. Electrochemical studies
ꢀ0.045[0.105]
0.430b
The electrochemical properties of all the complexes
have been investigated by cyclic voltammetry at a glassy
carbon electrode. The experiments were carried out at
room temperature in MeCN and DMF solution using
0.1 mol dmꢀ3 NEt4 ClO4 as supporting electrolyte. The
ligands LH2 and L0H2 are electro-inactive up to
ꢀ1.5 V. The cyclic voltammogram of Mn(II) complexes,
1 and 2, have grossly identical features (Table 6), both
the complexes showing one oxidative response nearly
at 0.0 V assignable to Mn(II) to Mn(III) oxidation.
The Mn(III)/Mn(II) couple for complex 2 is situated at
more positive value compared to that of 1, indicating
that due to the presence of electronegative Br substitu-
ents in the ligand in 2, the oxidation of Mn(II) becomes
ꢀ0.011[0.094]
0.485[0.122]
0.510[0.144]
0.045[0.070]
DMF solvent and rest are in MeCN.
Irreversible peak potentials, rest are quasi and reversible peak
b
potentials, respectively.
*
E1/2 = (Epa + Epc)/2 V, DEp = jEpa ꢀ Epcj V, where Epa and Epc are
anodic and cathodic peak potentials, respectively.
more difficult than in 1, thus demonstrating an elec-
tronic effect on redox potential. The electron transfer
properties of all Mn(III) complexes have grossly similar
characteristics (Table 6). Most of the complexes display

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S. Biswas et al. / Inorganica Chimica Acta 358 (2005) 2473–2481
one reductive and another oxidative response, tenta-
tively assigned to Mn(III) ! Mn(II) reduction and
Mn(III) ! Mn(IV) oxidation, respectively. This type
of observation where manganese(III) shows both oxida-
tion and reduction has also been found earlier [34c]
although there are some exception where manga-
nese(III) show only Mn(III) ! Mn(II) reduction [36].
In all the cases when the redox potential of the com-
plexes of L0 containing the bromo-substituent salicylid-
ene moiety is compared with the analogous complexes
of L, the former complex always have higher E1/2 value
than the later, indicating the stabilization of the lower
oxidation state by the L0 ligand. Comparison of the
E1/2 values for the Mn(IV)/Mn(III) couple, between 7
and 9 and between 8 and 10, indicate that more polariz-
able bromide ligand makes the Mn(III) ! Mn(IV) oxi-
dation easier (less positive E1/2 value) [37] compared to
much less polarizable chloride, because bromide transfer
more electron density to the metal center than chloride.
However, the E1/2 values for the Mn(III)/Mn(II) couple
for these complexes are too close to each other to make
any such comparison, considering the fact that for 7 and
8 the data was obtained in MeCN where as for 9 and 10
it was in DMF. As the oxidation involves removal of an
electron from d2z orbital, while during reduction the elec-
tron is added to dxy orbital, so effect of axial ligand is
expected to be more pronounced on oxidation than on
reduction. Due to lack of solubility of bromo complexes
(9 and 10) in MeCN, we have run the voltammetric
experiments in DMF and we have the voltammograms,
which have identical features like other complexes.
Mn(II) reduction process. The redox behavior of iodo
complex involving the L0 ligand 12 shows an irreversible
oxidation at +0.43 V and a quasi-reversible reduction at
ꢀ0.04 V. A comparison of the electrochemical data
(Table 6), reveals that the complexes with electron with-
drawing substituent directly attach to the manga-
nese(III) center get more difficult to oxidize than the
complexes where the electron withdrawing group at-
tached to the ligand moiety.
Though the redox properties discussed above have
been described as metal-based redox reactions following
similar assignments of previous workers, the results of
ZINDO/S calculations suggest that they should better
be discussed in terms of electron addition to HOMO-3
for reduction and electron removal from HOMO for
the oxidation process.
4. Computational method
Single point ZINDO/S calculations were carried out
on the manganese complexes using UHF method, with
the help of HYPERCHEM 7.01 program[38], with the de-
fault parameters of the program (r–r overlapping fac-
tor 1.267, p–p overlapping factor 0.585, convergence
limit 0.01). The geometries of the complexes 3 and 5
for the single point ZINDO/S calculations were taken
directly from the crystal structure.
Acknowledgments
E1/2(red)
-e
E1/2(Ox)
-e
Financial assistance from UGC, CSIR and AICTE is
gratefully acknowledged. S.B. thanks CSIR for award
for a SRF. The authors thank the AICTE-MODROBS
program for the purchase of the electrochemical unit.
Authors acknowledge the help of D.O. Miller, Depart-
ment of Chemistry, Memorial University of Newfound-
land, for X-ray crystallography and Dr. Bob
McDonald, Department of Chemistry, University of
Alberta for data collection. We also thank DST–FIST
program for providing FTIR and UV–Vis
Spectrophotometer.
[MnIIL]0
[MnIVL]2+
[MnIII L]+
+e
+e
Cyclic voltammograms of azide complexes 14 and 15
exhibit a quasi-reversible Mn(III) ! Mn(IV) metal
centered oxidation process in the positive potential re-
gion (Fig. 6) and a quasi-reversible/reversible Mn(III) !
Appendix A. Supplementary data
Crystallographic data for the structure analysis have
been deposited with the Cambridge Crystallographic
Data, CCDC Nos. are 240638 (3), 238882 (5) and
238883 (13). Copy of the data can be obtained, free of
charge, on application to the CCDC, 12 Union Road,
Cambridge CB2 1 EZ, UK (fax: +44 1223 336033 or
e-mail: deposit@ccdc.cam.ac.uk). Supplementary data
associated with this article can be found, in the online
version, at doi:10.1016/j.ica.2005.01.026.
Fig. 6. Cyclic of 8 [Mn(L0)(Cl)(H2O)] in MeCN with 0.1 mol dmꢀ1
NEt4ClO4 as supporting electrolyte (working electrode, Glassy-Car-
bon; reference electrode, Ag/AgCl; Scan rate 100 mV sꢀ1).

S. Biswas et al. / Inorganica Chimica Acta 358 (2005) 2473–2481
2481
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