Mononuclear manganese carboxylate complexes: Synthesis and structural studies

Article abstract of DOI:10.1016/j.poly.2006.07.029

Several manganese carboxylates complexes having Pz^iPr2H (3,5-diisopropylpyrazole), Tp^Ph,Me (hydrotris(3-phenyl,5-methyl-pyrazol-1-yl)borate), Tp^ipr2 (hydrotris(3,5-diisopropyl-pyrazol-1-yl)borate) as supporting ligands have been synthesised and structurally characterized. Single-crystal X-ray diffraction studies suggest that the manganese center in complexes (Pz^iPr2H)₄Mn(NO₂-OBz)₂ (5) and (Pz^iPr2H)₄Mn(F-OBz)₂ (6) have same coordination environment and geometry whereas the complex [Tp^Ph,MeMn(OAc)Pz^Ph,MeH] (7) has a five coordinate manganese center. In all these complexes, the carboxylate groups are coordinated as monodentate and the uncoordinated oxygen atom of the carboxylate groups form intramolecular hydrogen bonds with the NH group of the corresponding coordinated pyrazole (Pz^iPr2H/Pz^Ph,MeH). The complexes 5-8 and 10 were tested for their superoxide dismutase activity and it was found that only complex 7 has SOD activity as its structure is very similar to the active site structure of the native Mn-SOD enzyme. The SOD activity studies on these carboxylate complexes suggest that any model compound with analogous active site structure and intramolecular hydrogen bonding may be a suitable mimic for the Mn-SOD enzyme.

Full text of DOI:10.1016/j.poly.2006.07.029

Polyhedron 25 (2006) 3628–3638
www.elsevier.com/locate/poly
Mononuclear manganese carboxylate complexes:
Synthesis and structural studies
a,
a
a
b
a
Udai P. Singh *, Asish K. Sharma , Pooja Tyagi , Shailesh Upreti , Raj K. Singh
a
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee, Uttranchal 247 667, India
b
Department of Chemistry, Indian Institute of Technology Delhi, New Delhi 110 016, India
Received 24 April 2006; accepted 17 July 2006
Available online 7 August 2006
Abstract
Several manganese carboxylates complexes having PziPr2H (3,5-diisopropylpyrazole), TpPh,Me (hydrotris(3-phenyl,5-methyl-pyrazol-
1-yl)borate), Tpipr2 (hydrotris(3,5-diisopropyl-pyrazol-1-yl)borate) as supporting ligands have been synthesised and structurally charac-
terized. Single-crystal X-ray diffraction studies suggest that the manganese center in complexes (PziPr2H)4Mn(NO2–OBz)2 (5) and
(PziPr2H)4Mn(F–OBz)2 (6) have same coordination environment and geometry whereas the complex [TpPh,MeMn(OAc)PzPh,MeH] (7)
has a five coordinate manganese center. In all these complexes, the carboxylate groups are coordinated as monodentate and the
uncoordinated oxygen atom of the carboxylate groups form intramolecular hydrogen bonds with the NH group of the corresponding
coordinated pyrazole (PziPr2H/PzPh,MeH). The complexes 5–8 and 10 were tested for their superoxide dismutase activity and it was found
that only complex 7 has SOD activity as its structure is very similar to the active site structure of the native Mn–SOD enzyme. The SOD
activity studies on these carboxylate complexes suggest that any model compound with analogous active site structure and intramolec-
ular hydrogen bonding may be a suitable mimic for the Mn–SOD enzyme.
Ó 2006 Elsevier Ltd. All rights reserved.
Keywords: Pyrazole ligand; Carboxylate ligand; Manganese; SOD; Crystal structure
1. Introduction
are also known. Among these, the most extensively studied
enzyme is MnSOD. It catalyzes the dismutation of the
superoxide ion and protects living cells against dioxygen
dependent toxicities of various organic compounds [10].
The crystal structure from Thermus Thermophilus [1] shows
that it is tetramer and Mn adopts a trigonal-bipyramidal
coordination geometry with a N3O2 ligand donor set, one
histidyl nitrogen occupies the apical position while water
is suggested to sit at the opposite site. The carboxylate oxy-
gen from aspartate is bound to the manganese uniden-
tately. The non-ligating oxygen from the aspartate forms
a hydrogen bond with an amino acid residue or the peptide
backbone, which is important for the activity of the enzyme
(Fig. 1a).
Manganese is an important transition metal, required
for the growth and survival of many living organisms.
There are many enzymes that not only need manganese
specifically but utilize its redox capabilities also. These
are manganese superoxide dismutase (MnSOD) [1], manga-
nese peroxidase (MnP) [2], manganese thiosulfate oxidase
[3], manganese catalase [4], ribonucleotide reductase [5],
acid phosphatase [6] and oxygen evolving complexes in
photosystem II [7].
A few other manganese-containing enzymes i.e., manga-
nese dioxygenase, manganese lipoxygenase (MnLO) [8] and
oxalate oxidase [9] with one manganese as an active center
Germin is a manganese-containing enzyme with oxalate
oxidase and superoxide dismutase activities [9]. As shown
in Fig. 1b, the manganese(II) ion is in an octahedral geom-
etry comprising three histidine, a glutamate and two water
*
Corresponding author. Tel.: +91 1332 285329; Fax: +91 1332 273560.
E-mail address: udaipfcy@iitr.ernet.in (U.P. Singh).
0277-5387/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.07.029

U.P. Singh et al. / Polyhedron 25 (2006) 3628–3638
3629
Fig. 1. Active site structure for Mn–SOD (a) and Mn(II)oxlate-oxidase (b).
molecules. The active site structures for other mononuclear
2. Experimental
manganese enzymes are not known with certainty, but
based on their analogous iron enzymes, a square-pyramidal
arrangement of two histidines, a glutamate and two water
molecules has been proposed for Mn(II)dioxygenase [11]
and an active site structure similar to the iron-containing
soybean lipoxygenase has been proposed for the manga-
nese lipoxygenase enzyme [12]. According to Minor et al.
[13], the iron in iron-lipoxygenase, is coordinated to three
nitrogens of histidine residues, one oxygen of a monoden-
tate isoleucine and one oxygen of a weakly coordinated
asparagine. A water molecule forming a hydrogen bond
with the non-coordinated carboxylate oxygen of isoleucine,
completes the distorted octahedral coordination sphere. A
common structural feature found in most of the above
proteins and enzymes is the presence of one or more car-
boxylate derived from aspartate/glutamate/isoleucine or
asparagine side chains of the protein. These carboxylates
act as a bridge between metal ions present in the binuclear
manganese containing enzyme and also are suggested to
play an important role for structural holding and proton
transfer via hydrogen-bonding interactions in proteins
[14]. The role of hydrogen bonding in the stabilization of
structures as well as in the functions of various metallo-
proteins is well known in biochemistry [15].
There are several examples of mononuclear and binu-
clear manganese carboxylate complexes where the hydro-
gen atoms of the ligand alcohol groups, the bridging
aqua or the lattice water molecules form inter/intramolec-
ular hydrogen bonds with coordinated carboxylate groups
available in the literature [16], but manganese carboxylate
complexes with hydrogen bonds involving pyrazole groups
are limited in number. In light of the importance of hydro-
gen bonded carboxylate and the limited examples of man-
ganese carboxylate complexes with hydrogen bonded
pyrazole, we have undertaken the synthesis, structural
and SOD activity studies of some manganese carboxylate
complexes in the present paper.
2.1. Materials
All manipulations were carried out under nitrogen
atmosphere using standard Schlenk tube techniques unless
otherwise stated. The required organic solvents were care-
fully purified and distilled under nitrogen prior to use, by
the literature method [17]. MnCl2 Æ 4H2O (reagent grade)
was purchased from E. Merck. Other reagents were of
the highest grade commercially available and were used
without further purification. 3,5-i-Pr2pzH [PziPr2H] (1),
KHB(3,5-i-Pr2pz)3 [TpiPr2] (2), [18] KHB(3-Ph,5-Mepz)3
[TpPh,Me] (3) [19] and TpiPr2MnCl (4) [20] were synthesized
by the literature methods.
2.2. Physical methods
IR spectra were obtained on a Thermo Nikolet Nexus
FT-IR spectrometer in KBr and FD-MS spectra were
obtained on a Hitachi M-80 mass spectrometer. Room
temperature magnetic susceptibility measurements were
done on a Cahn Faraday magnetic susceptibility balance.
The X-ray data collection and processing for 5 and 6 were
performed on a Rigaku RAXIS-IV imaging plate area
˚
detector with Mo Ka radiation (k = 0.71070 A). In the
reduction of data, Lorentz and polarization corrections,
and empirical absorptions were made [21]. The structure
analysis was performed on an IRIS O2 (Silicon Graphics)
using the teXsan structure solving program system
obtained from the Rigaku Corp., Tokyo, Japan [22]. Neu-
tral scattering factors were obtained from the standard
source [23]. The structures were solved by a combination
of direct methods (SHELXS-86) [24] and Fourier synthesis
(DIRDIF 94) [25]. Least-squares refinements were carried
out using SHELXS-97 [24] linked to teXsan. All non-
hydrogen atoms were refined anisotropically. Diffraction
data for complex 7 were recorded at ꢀ80 °C with a Bruker

3630
U.P. Singh et al. / Polyhedron 25 (2006) 3628–3638
Smart CCD diffractometer. Emperical absorption correc-
tions were applied. The structure was solved by Patterson
methods and refined anisotropically with the SHELX pro-
gram suite. Hydrogen atoms involved in hydrogen bonding
were obtained by refinement, not by fixing the position.
Single crystal diffraction studies for 8 and 10 were carried
out on a Bruker Smart CCD diffractometer with Mo Ka
romethane and acetonitrile (1:1 ratio) and crystals were
obtained by cooling the solution at ꢀ20 °C (Yield 65%).
Anal. Calc. (%) for C42H41BMnN8O2: C, 66.76; H, 5.47;
N, 14.83. Found: C, 66.56; H, 5.39; N, 14.90%. IR (KBr,
cmꢀ1), m(BH) 2539, mas(COO) 1572, ms(COO) 1435,
leff = 5.68 BM at 295 K.
2.3.4. [TpiPr2Mn(Cl)PziPr2H] (8)
˚
radiation (k = 0.71073 A) in a sealed tube at 25 °C. Crystal
structures were solved by direct methods and in aniso-
tropic approximation refined using the SHELX TL package
[26,27]. Hydrogen atoms were constrained by the rigid
model.
MnCl2 Æ 4H2O (0.197 g, 1.00 mmol), KTpiPr2 (0.504 g,
1.00 mmol) and PziPr2H (0.761 g, 5.00 mmol) were stirred
in 25 mL CH2Cl2 and 5 mL CH3OH for 1 h. The mixture
was filtered over celite and the solvent was evaporated to
dryness under vacuum. The compound was dissolved in
5.0 mL pentane and 3.0 mL ether. Colorless crystals were
obtained from the filtrate at ꢀ20 °C in 85% yield. Anal.
Calc. for C36H62N8BClMn: C, 61.05; H, 8.82; N, 15.82;
Cl, 5.00. Found: C, 61.12; H, 8.83; N, 15.97; Cl, 4.80%.
IR (KBr, cmꢀ1) m(BH) 2548, m(NH of pyrazole) 3292,
m(CN of pyrazole) 1560, leff = 5.70 BM at 295 K.
2.3. Synthesis of complexes
2.3.1. [(PziPr2H)4Mn(NO2–OBz)2] (5)
(0.197 g, 1.00 mmol) MnCl2 Æ 4H2O and (0.604 g,
4.00 mmol) PziPr2H were stirred in 25 mL CH2Cl2 and
5 mL CH3OH for 1 h. To this solution was added a 5 mL
acetonitrile solution of sodium p-nitrobenzoate (0.378 g,
2.00 mmol) and the reaction mixture was stirred for 12 h
at room temperature. The mixture was filtered over celite
and the solvent was evaporated to dryness. The compound
was dissolved in 5 mL CH3CN and colorless crystals were
obtained at ꢀ20 °C (yield 84%). Anal. Calc. (%) for
C50H72N10O8Mn: C, 60.26; H, 7.28; N, 14.06. Found: C,
60.09; H, 7.31; N, 13.72%. IR (KBr, cmꢀ1), m(NH of pyra-
zole) 3211, mas(COO) 1576, ms(COO) 1473, leff = 5.75 BM
at 295 K.
2.3.5. [TpiPr2Mn(F–OBz)] (9)
To a 30 mL toluene solution of 4 (0.489 g, 0.882 mmol),
was added a 10 mL acetonitrile solution of sodium p-fluoro-
benzoate (0.143 g, 0.882 mmol) and the reaction mixture
was stirred for 16 h. The mixture was filtered over celite
and the solvent was evaporated under vacuum. A colorless
microcrystalline solid was obtained by slow cooling of an
acetonitrile solution at ꢀ20 °C in 45% yield. Anal. Calc.
(%) for C34H51N6BFO2Mn: C, 61.82; H, 7.78; N, 12.27.
Found: C, 61.65; H, 7.90; N, 12.07%. IR (KBr, cmꢀ1),
m(BH) 2541, mas(COO) 1600, ms(COO) 1469, leff = 5.83 BM
at 295 K, FD–MS (m/z) = 660.
2.3.2. [(PziPr2H)4Mn(F–OBz)2] (6)
(0.197 g, 1.00 mmol) MnCl2 Æ 4H2O and (0.604 g,
4.00 mmol) PziPr2H were stirred in 30 mL CH2Cl2 and
5 mL CH3OH for 1 h. A 5 mL acetonitrile solution of
sodium p-fluorobenzoate (0.324 g, 2.00 mmol) was added
to this solution and stirred for 12 h. The mixture was filtered
over celite and the solvent was evaporated to dryness. The
compound was dissolved in 5 mL acetonitrile and colorless
crystals were obtained by slow cooling at ꢀ20 °C (Yield
80%). Anal. Calc. (%) for C50H72N8O4F2Mn: C, 63.72; H,
7.70; N, 11.89. Found: C, 63.49; H, 7.67; N, 11.44%. IR
(KBr, cmꢀ1) m (NH of pyrazole) 3297, mas(COO) 1570,
ms(COO) 1442, leff = 5.78 BM at 295 K.
2.3.6. [TpiPr2Mn2(l-FOBz)3(PziPr2H)2] (10)
MnCl2 Æ 4H2O (0.168 g, 0.855 mmol), complex
4
(0.474 g, 0.855 mmol), sodium p-fluorobenzoate (0.419 g,
2.57 mmol) and PziPr2H (0.260 g, 1.71 mmol) in a mixture
of toluene and acetonitrile (1:1) were stirred for 12 h at
room temperature. The mixture was filtered over celite
and the solvent was evaporated to dryness. Colorless crys-
tals were obtained by slow cooling of acetonitrile solution
at ꢀ20 °C in 65% yield. Anal. Calc. (%) C66H90N10-
BF3O6Mn2; C, 61.11; H, 6.99; N, 10.79. Found: C, 60.85;
H, 6.95; N, 10.85%. IR(KBr, cmꢀ1), m(BH) 2538, mas(COO)
1594, ms(COO) 1469, leff = 5.90/Mn2+ BM at 295 K.
2.3.3. [TpPh,MeMn(OAc)PzPh,MeH] (7)
KTpPh,Me (0.521 g, 1.00 mmol) was stirred with
Mn(OAc)2 Æ 4H2O (0.245 g, 1.00 mmol) and 1.0 equivalent
of PzPh,MeH (0.158 g, 1.00 mmol) in 25 mL dichloromethane
and 5 mL methanol for 2 h. The mixture was filtered
through celite and the filtrate was dried under vacuum.
The colorless powder was dissolved in a mixture of dichlo-
3. Results and discussion
The reaction of MnCl2 Æ 4H2O with four equivalents of
pyrazole 1 and two equivalent of sodium p-nitrobenzoate/
sodium p-fluorobenzoate gave six coordinated mononuclear
(PziPr2H)4Mn(X-OBz)2
+
4 eq. PziPr2
H + 2 eq. NaOBzX
MnCl2.4H2O
(X= NO2, 5)
(X= F, 6)
Scheme 1.

U.P. Singh et al. / Polyhedron 25 (2006) 3628–3638
3631
Table 1
manganese high spin complexes 5 {C50H72N10O8Mn} and
6 {C50H72N8O4F2Mn} (Scheme 1). Selected bond distances
and bond angles are listed in Table 1 and crystallographic
data in Table 2. The Thermal ellipsoid view of the crystal
structures for complexes 5 and 6 are given in Figs. 2 and
3, whereas the structure showing intramolecular hydrogen
bonding and CH3–p interactions for these complexes are
given in Figs. 4 and 5, respectively. The coordination envi-
ronment and geometry in complexes 5 and 6 are very simi-
lar. As shown in the X-ray structures, both complexes are
six coordinated with a distorted octahedral geometry. The
manganese–nitrogen and manganese–oxygen bond dis-
tances (Table 1) for 5 and 6 are nearly the same. The Mn–
N bond distances are slightly longer than the Mn–N bond
distances in Mn–TpiPr2 complexes [28]. Also the Mn–O
bond distances of coordinated p-nitrobenzoate/p-fluoro-
benzoate are slightly longer than the reported manganese
carboxylate complexes having the Tpipr2 ligand [28]. In both
these complexes, the pyrazole as well as the benzoate groups
are behaving as monodentate ligands; the p-nitrobenzoate/
p-fluorobenzoate are coordinated to manganese by only
one oxygen atom. The uncoordinated oxygen atom of each
benzoate group forms intramolecular hydrogen bonds with
two N–H hydrogen atoms attached to the pyrazole rings of
PziPr2H. The different values of the N–O bond distances
Selected bond lengths (A) and angles (°) for [(PziPr2H)4Mn(NO2–OBz)2]
(5), [(PziPr2H)4Mn(F–OBz)2] (6), [TpPh,MeMn(OAc)PzPh,MeH] (7),
[TpiPr2Mn(Cl)PziPr2H] (8) and [TpiPr2Mn2(l-FOBz)3(PziPr2H)2] Æ CH3CN
(10 Æ CH3CN)
˚
[(PziPr2H)4Mn(NO2–OBz)2]
˚
Bond lengths (A)
Mn1–N11
Mn1–O1
O2–C1
2.327(1)
2.174(2)
1.249(3)
Mn1–N21
O1–C1
2.312(2)
1.268(3)
Bond angles (°)
O1–Mn1–O1
O1–Mn1–N11
O1–Mn1–N21
N21–Mn1–N21
180.00(1)
92.16(6)
90.75(7)
180.00(0)
O1–Mn1–N11
O1–Mn1–N21
N11–Mn–N11
87.84(6)
89.25(7)
180.00(0)
[(PziPr2H)4Mn(F–OBz)2]
Bond lengths (A)
Mn1–N11
Mn1–O1
O2–C1
˚
2.329(2)
2.175(1)
1.263(3)
Mn1–N21
O1–C1
2.327(2)
1.251(2)
Bond angles (°)
O1–Mn1–O1
O1–Mn1–N11
O1–Mn1–N21
N21–Mn1–N21
180.00(0)
89.08(6)
89.32(6)
180.00(0)
O1–Mn1–N11
O1–Mn1–N21
N11–Mn–N11
90.92(6)
90.68(6)
180.00(0)
[TpPh,MeMn(OAc)PzPh,MeH]
Bond lengths (A)
Mn1–O1
Mn1–N6
Mn1–N2
˚
˚
(O2–N12, 2.833 (3); O2–N22 is 2.873(3) A in 5 and O2–
2.024 (16)
2.199(18)
2.282(17)
Mn1–N4
Mn1–N7
2.149(18)
2.233(18)
˚
N12, 2.803 (2); O2–N22 is 2.831(3) A in 6), clearly indicate
that the hydrogen bonding is stronger in complex 6 than
in complex 5. Besides the existence of intramolecular hydro-
gen bonding, both of these complexes exhibit a CH3–p inter-
action between the CH3 group of PziPr2H and the benzene
ring of p-nitrobenzoate/p-fluorobenzoate as can be judged
from the distances between the hydrogen and the centroid
Bond angles (°)
O1–Mn1–N4
N4–Mn1–N6
N4–Mn1–N7
O1–Mn1–N2
N6–Mn1–N2
118.91(7)
O1–Mn1–N6
O1–Mn1–N7
N6–Mn1–N7
N4–Mn1–N2
N7–Mn1–N2
146.99(7)
95.65(7)
88.18(6)
84.81(6)
170.32(6)
93.36(7)
94.20(7)
93.25(7)
82.27(6)
˚
˚
of the ring, 3.022 A in 5 and 3.217 A in 6 (Figs. 4 and 5).
The presence of CH3–p as well as the p–p stacking interac-
tion is not uncommon in metal complexes [16h]. Several
manganese benzoate complexes having Tp ligands have
been reported [28], but mononuclear six coordinated
manganese carboxylate complexes with intramolecular
hydrogen bonding are not very common, except for some
octahedral manganese (II) carboxylate complexes [29].
The presence of intramolecular hydrogen bonding as well
as a CH3–p interaction may be responsible for the stability
of these complexes, as both are stable under air in the solid
as well as in solution state, and they do not react with O2
or CO2.
When Mn(OAc)2 Æ 4H2O was allowed to react with
TpPh,Me in the presence of pyrazole (PzPh,MeH) (Scheme
2), a five coordinated intramolecular hydrogen bonded
manganese (II) complex 7 {C42H41BMnN8O2} was isolated.
As shown in Fig. 6 (Thermal ellipsoid view), the manganese
is coordinated with three nitrogen of the TpPh,Me ligand.
The Mn–N bond distances in complex 7 (Table 1) are very
similar to the Mn–N bond distances present in the mono-
nuclear benzoate complex of manganese with TpiPr2
[TpiPr2Mn(Cl)PziPr2H]
Bond lengths (A)
Mn1–N3
Mn1–N1
Mn1–Cl1
˚
2.151 (19)
2.243(19)
2.419(8)
Mn1–N5
Mn1–N7
2.164(2)
2.306(2)
Bond angles (°)
N3–Mn1–N5
N5–Mn1–N1
N5–Mn1–N7
N5–Mn1–Cl1
N7–Mn1–Cl1
N2–N1–Mn1
95.61(7)
80.64(7)
89.16(7)
139.15(5)
87.89(5)
114.89(13)
N3–Mn1–N1
N3–Mn1–N7
N3–Mn1–Cl1
N1–Mn1–Cl1
N5–Mn1–Cl1
82.08(7)
90.56(7)
125.15(5)
105.37(5)
89.16(7)
[TpiPr2Mn2(l-FOBz)3(PziPr2H)2] Æ CH3CN
˚
Bond lengths (A)
Mn1–O2
Mn1–N3
Mn1–O6
Mn2–O3
Mn2–N7
Mn2–N9
2.133(3)
2.238(3)
2.255(3)
2.096(3)
2.216(3)
2.272(3)
Mn1–O4
Mn1–N5
Mn1–N1
Mn2–O1
Mn2–O5
2.193(3)
2.240(3)
2.293(3)
2.107(3)
2.271(3)
Bond angles (°)
O2–Mn1–O4
O4–Mn1–O6
N5–Mn1–O6
88.80(7)
93.10(7)
87.31(8)
O2–Mn1–O6
N3–Mn1–O6
O2–Mn1–N1
91.69(7)
169.71(8)
94.40(8)
˚
˚
(Mn1–N1, 2.195(4) A; Mn1–N2, 2.277(4) A; Mn1–N3,
˚
(continued on next page)
2.161(3) A) [28b] but longer than the Fe–N bond distances

3632
U.P. Singh et al. / Polyhedron 25 (2006) 3628–3638
Table 1 (continued)
[TpiPr2Mn2(l-FOBz)3(PziPr2H)2] Æ CH3CN
Bond angles (°)
O4–Mn1–N1
O1–Mn2–N7
O3–Mn2–N9
O1–Mn2–N9
O5–Mn2–N9
174.02(8)
88.63(9)
96.62(8)
171.62(8)
81.00(7)
O3–Mn2–N7
91.82(9)
113.94(8)
81.00(7)
88.06(9)
N7–Mn2–O5
O5–Mn2–N9
N7–Mn2–N9
˚
˚
(Fe1–N1, 2.2626(15) A; Fe1–N2, 2.0745(15) A; Fe1–N3,
˚
2.0855(16) A) in a mononuclear iron acetate complex with
TpPh2 [30]. The Mn–N bond distance of the coordinated
PzPh,MeH (Mn1–N7, 2.234 (18) A) is similar to the Fe–N
˚
bond distance with 3,5-diphenylpyrazole (Fe1–N4, 2.2347
˚
(17) A), but shorter than the Mn–N bond distance with
PziPr2H coordination (Mn1–N4, 2.304 (4) A). The acetate
˚
behaves as a monodendate ligand in 7 where one oxygen
atom is bonded with the manganese center and the other
oxygen atom forms a hydrogen bond with the proton lying
on the nitrogen of PzPhMeH coordinated at the apical posi-
tion to the manganese ion. The carboxylate oxygen from
asparate in Mn–SOD is also bound to the manganese
unidentately as is the acetate anion in complex 7. The
existence of a hydrogen bond in complex 7 is clearly estab-
lished on the basis of the location of the hydrogen bond
˚
with mean bond distances of Ha–O2, 1.909 A and
˚
Ha–N8, 0.859 A. The presence of a hydrogen bond also
affects the appearance of the m(NH) band in the IR spec-
trum of the complex as the free PzPhMeH gives the band
at 3180 cmꢀ1. The short distance of the carbonyl oxygen
(O2) of the acetate anion from the nitrogen of neutral pyr-
Fig. 2. Thermal ellipsoid view of complex 5 drawn at the 30% probability
level. All hydrogen atoms have been omitted for clarity.
Table 2
Crystallographic data for [(PziPr2H)4Mn(NO2–OBz)2] (5), [(PziPr2H)4Mn(F–OBz)2] (6), [TpPh,MeMn(OAc) PzPh,MeH] (7), [TpiPr2Mn(Cl)PziPr2H] (8) and
[TpiPr2Mn2(l-FOBz)3(PziPr2H)2] Æ CH3CN (10 Æ CH3CN)
5
6
7
8
10 Æ CH3CN
Empirical formula
Formula weight
Crystal system
Space group
C
996.12
triclinic
50H72N10O8Mn
C50H72N8O4F2Mn
942.10
monoclinic
P21/n
C42H41BMnN8O2
755.58
triclinic
C36H62BClMnN8
708.14
monoclinic
P21/c
C68H90BF3Mn2N11O6
1335.20
triclinic
ꢀ
ꢀ
ꢀ
P1
P1
P1 (no. 2)
Lattice parameters
˚
a (A)
10.715(6)
11.344(4)
12.747(8)
100.33(3)
107.16(2)
105.97(3)
1364(1)
1
10.948(2)
12.663(3)
18.695(3)
90.00(0)
96.035(9)
90.00(0)
2577.5(9)
2
11.7003(19)
11.7630(19)
15.425(3)
86.927(3)
87.581(3)
64.696(2)
1916.1(5)
2
13.312(2)
16.941(3)
17.528(3)
90.00(0)
92.428(3)
90.00(0)
3949.5(12)
4
12.3069(16)
13.4695(18)
24.205(3)
81.3(0)
80.22(0)
64.15(0)
3544.87(80)
2
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
3
˚
V (A )
Z
Dcalc (g/cm3)
Data collection
1.212
1.214
1.310
1.191
1.251
l(Mo Ka) (cmꢀ1
hmax (°)
)
0.300
27.4
5582
4752
329
0.313
27.5
5613
4352
311
0.392
28.79
17275
6410
0.437
28.28
7344
6922
442
0.420
28.02
26591
10845
841
Number of measured reflections
Number of observed reflections
Number of parameters refined
491
R
Rw
0.0625
0.1756
0.0588
0.1664
0.0442
0.1313
0.0514
0.1335
0.0579
0.1591

U.P. Singh et al. / Polyhedron 25 (2006) 3628–3638
3633
Fig. 3. Thermal ellipsoid view of complex 6 drawn at the 30% probability level. All hydrogen atoms have been omitted for clarity.
˚
˚
azole (N8) (O2–N8 bond, 2.76 A), also suggest the presence
2.043(4) A) [28b] but larger than the Fe–O1 bond distance
in a mononuclear hydrogen bonded iron acetato complex
of hydrogen bonding in 7.
˚
˚
The Mn–O bond distance (Mn1–O1, 2.0240 (16) A) is
(Fe–O1, 1.9634 (14) A) [30]. The geometry of this complex
very similar to the Mn–O bond distance in a mononuclear
hydrogen bonded manganese benzoate complex (Mn1–O1,
is trigonal bipyramidal with one nitrogen (N2) from
TpPhMe and N7 of the neutral pyrazole in apical positions.
Fig. 4. Molecular structure showing intramolecular hydrogen bonding and CH3–p interactions in complex 5.

3634
U.P. Singh et al. / Polyhedron 25 (2006) 3628–3638
In another set of experiments, we have prepared
TpiPr2MnClPziPr2H, {C36H62N8BClMn} (8) as a starting
material for the preparation of other various manganese
complexes. Complex 8 was prepared by the reaction of
MnCl2 Æ 4H2O with KTpiPr2 in the presence of 1.5 equiva-
lents of PziPr2H, as shown in Scheme 3. Complex 8 is a mono-
nuclear high spin manganese (II) species. As shown in the
crystal structure (Fig. 7), it is five coordinate having a
N4Cl ligand donor set with a distorted geometry. The
˚
Mn1–Cl1 bond distance is 2.4193 A, which is longer than
ꢀ2
the Mn–Cl bond distance in Mn2Cl4ðOAC6H4Ap-CH3Þ2
˚
˚
(2.32 A) [31] and [MnTPPCl](K222)] (2.364 A) [31c], but
similar to the Mn–Cl terminal bond distance in LMn2Cl2Br
˚
(2.411 A) [31b]. The distances between manganese and nitro-
gens of the coordinated TpiPr2 slightly differ from each other
˚
˚
(Mn1–N1, 2.2425 (19) A; Mn1–N3, 2.1507 A; Mn1–N5,
˚
2.164 (2) A) and are considerably longer than the distances
reported for [Mn(HB(3,5-Mepz)3)]+2 (1.96–1.98 A) [32]
˚
˚
and 2.05–2.23 A in [Mn2O(O2(CH3)2)(HB(pz)3)2] Æ CH3CN
[33].
Using the literature method [20] we have prepared the
complex TpiPr2MnCl, {C34H51N6BFO2Mn} (4) by reacting
MnCl2 Æ 4H2O and KTpiPr2 in a 1:1 ratio, to act as a
starting material for the synthesis of complexes
9
{C34H51N6BFO2Mn} and 10 {C66H90N10BF3O6Mn2}.
Complex 9 was prepared by the reaction of 4 with one
equivalent of sodium p-fluorobenzoate (Scheme 4). The
spectroscopic characterization revealed that the benzoate
group coordinates in a bidentate fashion. Although the
crystal structure of 9 could not be determined, based on
other spectroscopic data and the X-ray structure of a five
coordinate cobalt fluorobenzoate complex with TpiPr2
[34], we infer that 9 is also a monomeric five coordinate
Mn(II) complex.
We have also succeeded in synthesizing an unsymmetri-
cal fluorobenzoate bridged binuclear manganese (II) com-
plex. The reaction of 4 with 1, MnCl2 Æ 4H2O and sodium
Fig. 5. Molecular structure showing intramolecular hydrogen bonding
and CH3–p interactions in complex 6.
KTpPh,Me
PzPh,Me
H
TpPh,MeMn(OAc)PzPh,Me
H
Mn(OAc)2.4H2O
+
+
7
Scheme 2.
Fig. 6. Thermal ellipsoid view of complex 7. All hydrogen atoms have been omitted for clarity.

U.P. Singh et al. / Polyhedron 25 (2006) 3628–3638
3635
KTpiPr2
PziPr2
H
TpiPr2Mn(Cl)PziPr2
H
MnCl2.4H2O
+
+
8
Scheme 3.
fluorobenzoate groups in an unsymmetrical manner with
˚
a Mn–Mn separation of 3.732(4) A. Like the previously
reported benzoate bridged binuclear manganese complex
by Osawa et al. [35], in complex 10 both manganese ions
are also in an octahedral environment and two of the l-flu-
orobenzoate groups are symmetrically bridging, whereas
the third one adopts a unique coordination mode i.e., only
one oxygen from the carboxylate serves as a bridging
ligand, while the other one binds to Mn2 terminally. A
structurally similar (l-carboxylato)3 unit has been reported
in a linear trinuclear manganese(II) carboxylato complex
[36]. The Mn–Mn separation found in complex 10 is
˚
˚
˚
3.732(4) A, which is close to 3.739(2) A [37] and
3.612(2) A [38] in (l-aqua)(bis(l-carboxylato)dimanga-
˚
nese(II)) complexes but shorter than 4.034(2) A in a sym-
metrical tris(l-carboxylato)3 complex [39]. The short
metal–metal distance in 10 reflects the unusual nature of
the tris(l-fluorobenzoato)3 unit with one oxygen atom
from a fluorobenzoate group acting as a bridging ligand.
The magnetic susceptibility of the powdered sample is
8.34 BM per mol. Efforts were also made for the successful
removal of one bridging fluorobenzoate without disturbing
the manganese bimetallic core, but the reaction of 10 with
one equivalent of NaOH in toluene resulted in the forma-
tion of a mononuclear manganese (II) fluorobenzoate com-
plex similar to 9, in contrast to the pyrazole adduct as
reported by Osawa et al. [35]. The resultant complex is five
coordinated with a N3O2 ligand donor set and the fluoro-
benzoate group is coordinated to the manganese
bidentately.
Fig. 7. Thermal ellipsoid view of complex 8 drawn at the 30% probability
level. All hydrogen atoms have been omitted for clarity.
COONa
1 eq.
O
N
N
II
Mn
F
N
Mn
N
Cl
C
F
N
N
O
9
Scheme 4.
3.1. Superoxide dismutase activity studies
p-fluorobenzoate in mixed solvent (toluene:acetonitrile in a
1:1 ratio) gave complex 10 (Scheme 5). The presence of an
IR band at 3297 cmꢀ1 (due to the mN–H stretching band) in
the IR spectrum of this complex suggested the existence of
pyrazole in the protonated form. The molecular structure
of 10 was determined by X-ray crystallography and its
Thermal ellipsoid view is given in Fig. 8. As shown in
Fig. 8, both manganese (II) ions are in the same environ-
ment. The two manganese (II) ions are bridged with three
The complexes 5–8 and 10 were tested for their SOD
activity using xanthine–xanthine oxidase–nitro blue tetra-
zolium (NBT) methods [40], as the coordination number
around manganese in these complexes is close to the
coordination number around the manganese in manga-
nese-containing SOD enzyme [1] and also they have intra-
molecular hydrogen bonding, except in complexes 8 and
10. The test was performed separately in duplicate for each
compound in 3 ml of 50 mM potassium phosphate buffer
(pH 7.4) at 25 °C in the absence of EDTA. The reaction
mixture was prepared as follows:
Fifty micromoles of NBT, 50 lM xanthine, 1000 U/ml
catalase and 0.04 U/ml xanthine oxidase were used to pro-
duce superoxide ions in solution. The formation of diforma-
zan was monitored at 560 nm wavelength. The IC50 value in
Fig. 9 means the concentration of the complex which exerts
SOD activity equivalent to one unit of native SOD. Out of
these tested complexes only complex 7 exhibited IC50 values
COONa
N
II
N
PziPr2H
+
3 eq.
Mn Cl
+
MnCl2.4H2O
+ 2 eq.
N
F
TpiPr2Mn2(μ-FOBz)3(PziPr2H)2
10
Scheme 5.

3636
U.P. Singh et al. / Polyhedron 25 (2006) 3628–3638
Fig. 8. Thermal ellipsoid view of complex 10 drawn at the 50% probability level. All hydrogen atoms have been omitted for clarity.
at 3.5 lM concentration (Fig. 9), clearly indicating that this
may be a suitable model for a SOD mimic. The SOD activ-
ity value for 7 is much better than the previously reported
SOD model complexes screened by the NBT method under
the same conditions [41], but lower than the SOD model
complex reported by Kitajima et al.ꢀ[28b]. Also the ability
of complex 7 to disproportionate O2 is comparatively bet-
ter than the most active systems based on salen or porphy-
rin ligands reported by several workers [42]. The better SOD
activities of complex 7 may be attributable to the very close
structural similarities of this complex with the active site of
native Mn–SOD.
4. Conclusion
We have reported several manganese carboxylate com-
plexes, both with hydrogen as well as non-hydrogen bind-
ing, having five and six coordination numbers. The results
presented in this paper demonstrated that different coordi-
nation modes of the carboxylate groups are possible simply
by changing the supporting nitrogen donor ligands. It is
possible to get the hydrogen bonded carboxylate complexes
simply by adding the corresponding pyrazole to the reac-
tion mixture as demonstrated in the formation of complex
7, or by using pyrazole as a supporting ligand in the reac-
tion as shown by the preparation of complexes 5 and 6. The
reaction of PziPr2H with MnCl2 Æ 4H2O and sodium p-nitro-
benzoate/sodium p-fluorbenzoate resulted in the formation
of octahedral complexes with intramolecular hydrogen
bonding and CH3–p interactions. The SOD activity studies
on complexes 5–8 and 10 suggested that complex 7 seems
to be a good model mimic of the native Mn–SOD enzyme
as complex 7 has a suitable active site structure as well as
intermolecular hydrogen bonding.
5. Supporting data
The crystallographic data have been deposited with the
CCDC. Supplementary data are available from the CCDC,
12 Union Road, Cambridge CB2 1EZ, UK on requesting,
Fig. 9. SOD activity of complex 7 in xanthene oxidase-NBT assay.

U.P. Singh et al. / Polyhedron 25 (2006) 3628–3638
3637
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