Manganese complexes as models for manganese-containing pseudocatalase enzymes: Synthesis, structural and catalytic activity studies

Article abstract of DOI:10.1016/j.poly.2007.03.049

Manganese complexes of the type [TpMn(X)] and [TpMn(μ-N₃)(μ-X)MnTp] (X = acetylacetonate, acac; picolinate, pic and Tp = Tp^Ph,Me for acac, Tp = Tp^ipr2 for pic complexes) having Tp^Ph,Me (hydrotris(3-phenyl,5-methyl-pyrazol-1-yl)borate)/Tp^ipr2 (hydrotris(3,5-diisopropyl-pyrazol-1-yl)borate) as a supporting ligand have been synthesized and structurally characterized. IR and X-ray structures suggest that complexes 7 and 9 are binuclear with azido and bidentate ligands (acac/pic) bridging, whereas complexes 6 and 8 are mononuclear with a 5-coordinated metal center. In complex 9 the picolinate is coordinated as tridentate in a η₃-fashion, but in complex 7 acac behaves as bidentate, whereas azide is coordinated in a bridging bidentate μ-1,3-manner in both 7 and 9. Since the coordination geometry of the manganese ions in complex 9 is very similar to the active site structure of manganese-containing pseudocatalase, we have tested the catalytic activity of the same towards the disproportionation of hydrogen peroxide. The catalytic results indicated that complex 9 has reasonably good catalase activity and may be suitable, structurally as well as functionally, as a model for the pseudocatalase enzyme.

Full text of DOI:10.1016/j.poly.2007.03.049

Polyhedron 26 (2007) 3625–3632
www.elsevier.com/locate/poly
Manganese complexes as models for manganese-containing
pseudocatalase enzymes: Synthesis, structural and catalytic
activity studies
a,
a
b
Udai P. Singh *, Pooja Tyagi , Shailesh Upreti
a
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India
Department of Chemistry, Indian Institute of Technology Delhi, New Delhi 110 016, India
b
Received 18 January 2007; accepted 29 March 2007
Available online 10 April 2007
Abstract
Manganese complexes of the type [TpMn(X)] and [TpMn(l-N3)(l-X)MnTp] (X = acetylacetonate, acac; picolinate, pic and
Tp = TpPh,Me for acac, Tp = Tpipr2 for pic complexes) having TpPh,Me (hydrotris(3-phenyl,5-methyl-pyrazol-1-yl)borate)/Tpipr2 (hydro-
tris(3,5-diisopropyl-pyrazol-1-yl)borate) as a supporting ligand have been synthesized and structurally characterized. IR and X-ray struc-
tures suggest that complexes 7 and 9 are binuclear with azido and bidentate ligands (acac/pic) bridging, whereas complexes 6 and 8 are
mononuclear with a 5-coordinated metal center. In complex 9 the picolinate is coordinated as tridentate in a g3-fashion, but in complex 7
acac behaves as bidentate, whereas azide is coordinated in a bridging bidentate l-1,3-manner in both 7 and 9. Since the coordination
geometry of the manganese ions in complex 9 is very similar to the active site structure of manganese-containing pseudocatalase, we have
tested the catalytic activity of the same towards the disproportionation of hydrogen peroxide. The catalytic results indicated that complex
9 has reasonably good catalase activity and may be suitable, structurally as well as functionally, as a model for the pseudocatalase
enzyme.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Keywords: Pyrazolylborate; Carboxylate; Azide; Manganese; Pseudocatalase; Crystal structure
1. Introduction
On the basis of various model complexes of manganese
reported in the literature, it has been concluded that the
active site structure of both catalase as well as pseudocata-
lase is basically the same, however their working mecha-
nism may be different.
The recently reported crystal structure of Mn-containing
catalase from L. plantarum [4] demonstrated that it has
hexameric units. Each subunit contains a dimanganese
active site. The manganese ions are linked by a l-1,3-bridg-
ing glutamate carboxylate and two single atom solvent
bridges (oxo, hydroxo or aquo) that electronically couple
the centre. In addition, each manganese ion is coordinated
by one histidine nitrogen and one carboxylate. For the
Mn1 subsite, the carboxylate (Glu35) is monodentate, with
the non-coordinated oxygen forming a six atom hydrogen-
bonded loop including the solvent bridge, indicating that
the bridge is protonated in the resting state. The sixth
Catalase enzymes are present in most aerobic forms of
life and are responsible for the decomposition of hydrogen
peroxide to molecular oxygen and water [1]. Although,
most catalases contain the iron-protoporphyrin IX pros-
thetic group, some bacteria utilize a non-heme, Mn-con-
taining catalase. Many Mn catalases have been identified
[2,3] but those from Thermus thermophilus and Lactobacil-
lus plantarum have been best studied. Some microorgan-
isms, which are unable to synthesized heme can
nevertheless produce a catalase. This catalase, which in
contrast to heme-containing catalase, is not inhibited by
CNÀ or N3À, and is called pseudocatalase/azide insensitive.
*
Corresponding author. Tel.: +91 1332 285329; fax: +91 1332 273560.
E-mail address: udaipfcy@yahoo.co.in (U.P. Singh).
0277-5387/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.03.049

3626
U.P. Singh et al. / Polyhedron 26 (2007) 3625–3632
(apical) position of this subsite is occupied by a third non-
bridging solvent molecule coordinated to the dimanganese
cluster. Terminally bound water molecules are easily dis-
placed from the metal complexes and this apical site likely
serves as the initial substrate binding site during catalysis.
The other subsite (Mn2) has a bidentate (chelating) coordi-
nation mode for glutamate, which may prevent exogenous
ligands from binding and makes each of the manganese
ions six-coordinate in the cluster, with approximately octa-
hedral geometry at each subsite. The Mn–Mn separation in
2.2. Physical methods
IR spectra were obtained on a Thermo Nikolet Nexus
FT-IR spectrometer in KBr discs. Room temperature mag-
netic susceptibility measurements were done on a Princeton
applied research vibrating sample magnetometer Model
155 whereas the magnetic susceptibility measurements at
variable temperature (from liquid helium to room temper-
ature) were performed on a Quantum Design Squid
Magnetometer Model MPMS. The X-ray data collection
and processing for 6 and 9 were performed on a Kappa
Apex Bruker CCD detector with Mo Ka radiation (k =
˚
the resting state of the enzyme is 3.03 A and both manga-
nese centres are in the three oxidation state. The EXAFS
analysis of the same protein predicted a Mn–Mn separa-
˚
0.71070 A). In the reduction of data, Lorentz and polariza-
˚
tion of 3.53 A in the fully reduced state, i.e. 2,2 oxidation
tion corrections, and empirical absorption corrections were
made. All non-hydrogen atoms were refined aniostropi-
cally. The structures were solved by Patterson methods
and refined anisotropically with the SHELX program suite
[16]. Hydrogen atoms were constrained by a rigid model.
state [5]. It is well known that the azide ion acts as an inhib-
itor of L. plantarum catalase forming an active site com-
plex. The crystal structure of the azide coordinated
protein is virtually indistinguishable from the native struc-
ture except in the vicinity of the active site, where signifi-
cant differences occur. In the azide complex of the
oxidized form of the enzyme, the anion occupies a terminal
position on the Mn1 subsite, replacing the solvent present
in the native structure. Since azide is an analog of hydrogen
peroxide, this mode of bonding is likely to mimic that of
the coordinated substrate. The X-ray structure of azide
bonded catalase in the oxidized state revealed the terminal
bonding mode for the coordinated azide, however the coor-
dination mode of azide in the reduced state of the enzyme is
not definitely known.
Carboxylato bridged dinuclear Mn complexes have
attracted much attention in recent years because of their
potential biological relevance [6] in different manganese
containing enzymes [7]. While several dinuclear Mn com-
plexes having (l-oxo)bis(l-carboxylato) [8], (l-hydroxo)-
bis(l-carboxylato) [9], (l-hydroxo)(l-acetato) [10],
(l-hydroxo)bis(l-acetato) [11] bridges are known, no
example exist of dinuclear manganese complexes with a
(l-azido)(l-carboxylato) bridging unit. As a part of our
synthetic effort towards modelling the active site of
Mn-containing protein by using the hindered tris(pyraz-
olyl)borate as a supporting ligand, we now report on
dinuclear complexes bridged with a single carboxylato
and a single azido unit along with other carboxylate com-
plexes of manganese (II), and their catalytic activity.
2.3. Synthesis of the complexes
2.3.1. [TpPh,MeMnCl] (5)
MnCl2 Æ 4H2O (0.198 g, 1.0 mmol) was stirred with 3
(0.521 g, 1.0 mmol) in 10 mL CH3OH and 20 mL CH2Cl2
for 1 h. The solution was filtered over celite and the solvent
was evaporated to dryness under vacuum. Crystallization
of complex 5 from acetonitrile yielded a colorless solid in
89% (0.51 g, 0.89 mmol.) yield. Anal. Calc. for C30H28-
N6BClMn: C, 62.79; H, 4.91; N, 14.64; Cl, 6.17. Found:
C, 63.00; H, 4.82; N, 14.36; Cl, 6.21%. IR (KBr, cmÀ1):
m(BH) 2541.
2.3.2. [TpPh,MeMn(acac)] (6)
To an acetonitrile (30 mL) solution of complex 5, one
equivalent of sodium acetylacetonate (0.109 g, 0.89 mmol)
was added and the resulting solution was stirred for 8 h.
The reaction mixture was filtered over celite and the sol-
vent was evaporated to dryness under vacuum. Crystals
suitable for X-ray data collection were obtained by slow
cooling of the compound in a mixture of CH2Cl2 and
CH3CN in 75% (0.43 g, 0.67 mmol.) yield. Anal. Calc.
for C35H36N6O2BMn: C, 65.84; H, 5.68; N, 13.16. Found:
C, 66.00; H, 5.82; N 13.06%. IR (KBr, cmÀ1): m(BH)
2539, m(CO) 1594. Magnetic moment leff (295 K) = 5.78
BM.
2. Experimental
2.3.3. [TpPh,MeMn(l-N3) (l-acac)Mn TpPh,Me] (7)
The reaction of complex 6 (0.427 g, 0.67 mmol) in
dichloromethane (20 mL) and sodium azide (0.043 g,
0.67 mmol) in methanol (10 mL) for 3 h followed by fil-
tration over celite and evaporation to dryness under vac-
uum resulted in the formation of compound 7 in 77%
(0.35 g, 0.52 mmol) yield. Anal. Calc. for C65H64-
N15O2B2Mn2: C, 64.05; H, 5.29; N, 17.23. Found: C,
63.90; H, 5.22; N, 17.16%. IR (KBr, cmÀ1): m(BH) 2514,
m(CO) 1596, m(N3) 2075. Magnetic moment leff
2.1. Materials
All solvents used were purified by the literature methods
[12]. Reagents of the highest grade commercially available
were used without further purification. All manipulations
were carried out under a nitrogen atmosphere using stan-
dard Schlenk tube techniques unless otherwise stated. 3,5-
iPr2pzH [PziPr2H] (1), KHB(3,5-iPr2pz)3 [TpiPr2] (2) [13],
KHB(3-Ph,5-Mepz)3 [TpPh,Me] (3) [14] and TpiPr2MnCl
(4) [15] were synthesized by the literature methods.
(295 K) = 5.57 BM/Mn2+
.

U.P. Singh et al. / Polyhedron 26 (2007) 3625–3632
3627
2.3.4. [Tpipr2Mn(pic)] (8)
R1
R2
To a dichloromethane (20 mL) solution of 4 (0.556 g,
1.0 mmol) was added a 10 mL methanolic solution of
potassium picolinate (0.161 g, 1.0 mmol), and the resulting
solution was stirred for 3 h. After this time, the reaction
mixture was filtered over celite. The evaporation of solvent
under vacuum resulted in a white crystalline compound
in 64% (0.41 g, 0.64 mmol) yield. Anal. Calc. for
C33H50N7O2BMn: C, 61.68; H, 7.84; N, 15.23. Found: C,
61.59; H, 7.94; N, 15.43%. IR (KBr, cmÀ1), m(BH) 2538,
mas(COO) 1592, ms(COO) 1470. Magnetic moment leff
(295 K) = 5.83 BM.
N
N
R1, R2 = (CH3)2CH-
1
H
R2
N
K+
B
R2
R2
N
N
N
N
N
N
=
N
N
R1
R1
R1
2.3.5. [Tpipr2Mn(l-N3) (l-pic)Mn(Pzipr2)Tpipr2] (9)
The reaction of complex 8 (0.411 g, 0.64 mmol), Pzipr2
H
2
3
R1, R2 = (CH3)2CH-
(0.097 g, 0.64 mmol) and sodium azide (0.041 g,
0.64 mmol) in 20 mL dichloromethane and 10 mL metha-
nol for 3 h resulted in the formation of complex 9. The
resulting solution was filtered over celite and evaporated
to dryness under vacuum. The solid was dissolved in a mix-
ture of dichloromethane and acetonitrile (1:1 ratio) and a
crystalline compound was obtained by slow cooling of
the solution at 4 ꢁC in 36% (0.310 g, 0.23 mmol) yield.
Anal. Calc. C69H112N18O2B2Mn2: C, 61.06; H, 8.32; N,
18.57. Found: C, 61.39; H, 8.56; N 18.69%. IR (KBr,
cmÀ1), m(BH) 2545, mas(COO) 1593, ms(COO) 1469, m(N3)
R1 = CH3 R2 = C6H5-
Fig. 1. Chemdraw structures of 1, 2 and 3.
N
N
N
Mn
Cl
4
5
R1, R2 = (CH3)2CH-
R1 = CH3 R2 = C6H5-
2077. Magnetic moment leff (295 K) = 5.68 BM/Mn2+
.
Fig. 2. Chemdraw structures of 4 and 5.
3. Results and discussion
CH3
N
N
N
C
As reported in our previous work [17], the reaction of a
binuclear Mn(II) di(l-hydroxo) complex with one equiva-
lent of benzoic acid results in the replacement of one
hydroxide ligand to give the corresponding bimetallic
(l-hydroxo)(l-benzoato) complex. Further replacement
of the bridging hydroxide with azide gave a binuclear
Mn(II) complex [18]. Due to the high solubility of this
compound in the majority of organic solvents, we could
not get a suitable single crystal for X-ray structure determi-
nation. However on the basis of IR and other spectroscopic
techniques, a (l-azido)(l-benzoato) bridged manganese
(II) dimer complex was proposed.
O
O
O
N
Mn
8
C
Mn
6
H
C
N
N
O
N
C
CH3
Fig. 3. Chemdraw structures of 6 and 8.
and fall in the range of reported Mn–N bond distances
of manganese complexes having other Tp ligands [19].
The Mn–O bond distances of the coordinated acetylaceto-
˚
nate, Mn(1)–O(1), Mn(1)–O(2), are 2.078(2) A. These Mn–
O bond distances are well within the range of other Mn–O
bond distances [20]. The IR band at 1592, (mas(COO)) and
1470, (ms(COO)) cmÀ1 in the picolinate complex suggest the
bidentate coordination mode of the ligand. Although, a
crystal structure is not available for complex 8, other data
suggest the same structure with a 5-coordinated manganese
center as is seen for 6. Both complexes 6 and 8 were used
for the synthesis of the corresponding azido complexes.
When complexes 6 and 8 in dichloromethane solvent were
treated with a methanolic solution of sodium azide for 3 h,
7 and 9 (Fig. 5) were formed, respectively. Although both
these reaction were carried out in the presence of an excess
amount of azide, no detectable amount of [TpMn(N3)] was
formed. Thus, the formation of 7 and 9 is a thermodynam-
ically favorable reaction. Detailed characterization of these
In our effort to prepare a model compound for Mn-con-
taining pseudocatalase, first we prepared the starting com-
plexes TpiPr2MnCl (4) and TpPh,MeMnCl (5). The starting
materials were prepared by reacting manganese (II) chlo-
ride with both 3-phenyl,5-methyl and 3,5-diisopropyl-
substituted pyrazolylborate ligands (Fig. 1). The starting
materials viz., 4 and 5 (Fig. 2) when treated with sodium
acetylacetonate/sodium picolinate gave 5-coordinated
manganese (II) complexes TpPh,MeMn(acac) (6) and
TpiPr2Mn(pic) (8) (Fig. 3). The IR data suggested that both
acac and pic are coordinated in a bidentate manner. The
single crystal X-ray study of 6 suggested that the manga-
nese ion is coordinated by three nitrogen atoms of TpPh,Me
and two oxygen atoms of acetylacetonate (Fig. 4). The
˚
Mn–N bond distances range from 2.178(2) to 2.230(2) A

3628
U.P. Singh et al. / Polyhedron 26 (2007) 3625–3632
complexes revealed that both complexes have azide coordi-
nation where each metal center is 6-coordinated in 9 and 5-
coordinated in complex 7. IR data demonstrated that in
both 7 and 9, the azide ligands are coordinated in a l-
1,3-fashion. Complex 7 could not be crystallized as single
crystals, but slow recrystallization of 9 from dichlorometh-
ane and acetonitrile at 4 ꢁC gave single crystals suitable for
X-ray studies. The X-ray data collection for 9 was per-
formed at 100 K (Table 1) and a thermal ellipsoid view is
given in Fig. 6. The molecular structure of 9 (Fig. 6) repre-
sents a novel (l-1,3-azido)(l-picolinato) bridging core.
This is the first example of such a structure, while a
(l-hydroxo)(l-1,3-azido) binuclear copper complex with
TpiPr2 was reported previously [21].
As shown in Fig. 6, each manganese is in a 6-coordina-
tion sphere as is present in Mn-containing catalase/pseudo-
catalase. The most interesting feature of this structure is
that the picolinate ligand is behaving as a tridentate ligand
in this complex, where one nitrogen and one oxygen is
coordinated with one manganese center and only one oxy-
gen is coordinated with the other manganese center in an
asymmetric manner. The tridentate coordination nature
Fig. 4. Thermal ellipsoid view of complex 6 drawn at the 30% probability
level. All hydrogen atoms have been omitted for clarity.
N
N
H
N
N
N=N=N
N
N
Mn
O
Mn
O
N
N
N
N
N=N=N
N
N
N
Mn
Mn
N
O
O
C
C
N
C
C
H
CH3
CH3
9
7
Fig. 5. Chemdraw structures of 7 and 9.
Fig. 6. Thermal ellipsoid view of complex 9 drawn at the 30% probability level. All hydrogen atoms have been omitted for clarity.

U.P. Singh et al. / Polyhedron 26 (2007) 3625–3632
3629
Table 1
of picolinate is not uncommon and a few examples have
been reported earlier [22], but to the best of our knowledge
this is the first report of a (l-azido)(l-picolinato) binuclear
manganese (II) complex where picolinate is showing triden-
tate behavior. Mn1 is coordinated with three nitrogen
atoms of TpiPr2 and one nitrogen of free pyrazole whereas
Mn2 is coordinated with only three nitrogen atoms of
TpiPr2 ligand. The azide molecule is coordinated with both
manganese centres, bridging in a g-1,3-fashion. The strik-
ing feature of the tridentate behaviour of picolinate is that
in this complex the coordination sphere of both manganese
ions are six, as in native catalase/pseudocatalase enzyme,
with a slight difference of the coordinating ligands in the
vicinity of the metal ions. In the native enzyme it has been
reported that the azide ions act as inhibitors and the crystal
structure of the azide bonded catalase demonstrated that
the binding of azide is terminal where as in complex 9
the azide is bonded in a bridging mode. Binuclear transi-
tion metal complexes with (l-azido)(l-carboxylato) are
very rare in the literature [23]. The coordination mode of
the azide ligand in the oxidized state of catalase is well
known, but still there is speculation about the binding
mode of this ligand in the reduced state. Also it has been
reported in the literature that the metal–metal separation
in the reduced state of the enzyme is more than that of
the oxidized form and there is strong belief that the binding
mode of the azide ligand may also change to bridging in the
reduced form, which may be responsible for the large sep-
aration of the manganese ions. In fact the Mn–Mn separa-
tion in binuclear complexes is very much dependent on the
presence of bridging ligands as evident from the Mn–Mn
separation in different complexes with the TpiPr2 ligand
(Table 3). The Mn–Mn separation in di(l-hydroxo)
Crystallographic data for [TpPh,MeMn(acac)] (6) and [Tpipr2Mn(l-N3)-
(l-pic)Mn(Pzipr2)Tpipr2] (9)
6
9
Formula weight
Crystal system
Space group
723.37
triclinic
1496.44
monoclinic
P21=n
ꢀ
P1
Lattice parameters
˚
a (A)
11.7457(17)
11.8283(17)
15.670(2)
101.839(2)
94.360(2)
119.440(2)
1815.2(4)
2
13.290(2)
31.649(5)
21.206(3)
90.00
106.881(7)
90.00
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
8535(2)
4
Z
Dcalc (g/cm3)
1.324
1.165
Data collection
l (Mo Ka) (cmÀ1
hmax (ꢁ)
Number of parameters refined
R
Rw
)
0.551
28.79
433
0.352
25.55
978
0.0758
0.1966
0.0542
0.0854
Table 2
Selected bond lengths (A) and angles (ꢁ) for [TpPh,MeMn(acac)] (6) and
˚
[Tpipr2Mn(l-N3)(l-pic)Mn(Pzipr2) Tpipr2] (9)
TpPh.MeMn(acac) (6)
˚
Bond lengths (A)
Mn(1)–O(1)
Mn(1)–N(2)
Mn(1)–N(6)
2.078(2) Mn(1)–O(2)
2.202(2) Mn(1)–N(4)
2.230(2)
2.078(2)
2.178(2)
Bond angles (ꢁ)
O(2)–Mn(1)–O(1)
O(1)–Mn(1)–N(4)
O(1)–Mn(1)–N(2)
O(2)–Mn(1)–N(6)
N(4)–Mn(1)–N(6)
85.16(8)
108.30(8)
93.25(8)
92.12(8)
93.29(8)
O(2)–Mn(1)–N(4)
111.93(9)
156.95(9)
90.44(8)
157.69(9)
80.67(8)
O(2)–Mn(1)–N(2)
N(4)–Mn(1)–N(2)
O(1)–Mn(1)–N(6)
N(2)–Mn(1)–N(6)
˚
˚
dimanganese (II) (3.33 A), changes to 3.603(2) A in
˚
(l-hydroxo)(l-pyrazolato) dimanganese(II) [24], 2.706 A
in di(l-oxo)(l-acetato) dimanganese (III,IV) [10] and
Tpipr2Mn(l-pic)(l-N3)Mn (Pzipr2)Tpipr2 (9)
˚
Bond lengths (A)
2.70 A in di(l-oxo) dimanganese (III) [15]. This distance
˚
further increases when azide is present in the bridging mode.
Mn(1)–N(2)
Mn(1)–N(6)
Mn(1)–N(16)
Mn(2)–N(9)
Mn(2)–N(13)
Mn(2)–N(18)
2.281(5) Mn(1)–N(4)
2.231(5)
2.313(5)
2.231(4)
2.208(5)
2.207(5)
2.251(4)
˚
In complex 9, the Mn–Mn separation is 5.355 A. The Mn–
2.253(5) Mn(1)–N(7)
2.183(5) Mn(1)–O(1)
2.216(5) Mn(2)–N(11)
2.262(5) Mn(2)–N(17)
2.297(5) Mn(2)–O(2)
˚
Mn separation of 5.355 A in complex 9 gives us strong
support to propose that the terminal mode of azide in the
oxidized form of pseudocatalase changes to a bridging
mode in the reduced form. The Mn–N bond distances of
both coordinated TpiPr2 ligands are in the range 2.208–
Bond angles (ꢁ)
N(16)–Mn(1)–O(1)
O(1)–Mn(1)–N(4)
O(1)–Mn(1)–N(6)
N(16)–Mn(1)–N(2)
N(4)–Mn(1)–N(2)
N(16)–Mn(1)–N(7)
N(4)–Mn(1)–N(7)
N(2)–Mn(1)–N(7)
N(11)–Mn(2)–N(9)
N(11)–Mn(2)–O(2)
N(9)–Mn(2)–O(2)
85.29(17) N(16)–Mn(1)–N(4)
174.35(17) N(16)–Mn(1)–N(6)
93.97(16) N(4)–Mn(1)–N(6)
92.54(19) O(1)–Mn(1)–N(2)
81.64(18) N(6)–Mn(1)–N(2)
97.42(19) O(1)–Mn(1)–N(7)
94.08(18) N(6)–Mn(1)–N(7)
169.18(17) N(11)–Mn(2)–N(17)
87.13(18) N(17)–Mn(2)–N(9)
108.76(16) N(17)–Mn(2)–O(2)
163.19(17) N(11)–Mn(2)–N(13)
90.14(19)
173.7(2)
90.20(17)
95.22(16)
81.25(18)
89.81(16)
88.86(17)
95.59(19)
91.31(18)
81.93(16)
80.72(19)
86.84(18)
Table 3
The distances between Mn–Mn ions in different binuclear complexes of the
hydrotris(3,5-diisopropyl-pyrazol-1-yl)borate ligand
Complex
Mn1–Mn2 distance
(A)
Reference
˚
TpMnII(l-OH)2MnIITp
TpMnIII(l-O)2MnIIITp
TpMnII(l-OH)(l-3,5-Pri2pz)
MnIITp
TpMnII(l-OBz)3MnII (3,5-Pr2i pz)2
TpMnIII(l-O)2(l-OAc)MnIVTp
TpMnII(l-OBz)3MnII (TpiPr2)
3.314(1)
2.696(2)
3.603(2)
[15]
[15]
[24]
N(17)–Mn(2)–N(13) 175.9(2)
N(9)–Mn(2)–N(13)
100.80(16) N(11)–Mn(2)–N(18) 171.39(19)
93.01(19) N(9)–Mn(2)–N(18)
72.10(16) N(13)–Mn(2)–N(18)
O(2)–Mn(2)–N(13)
N(17)–Mn(2)–N(18)
O(2)– Mn(2)–N(18)
3.753(2)
2.709(1)
4.006(1)
[19]
[10]
[17]
93.03(18)
90.69(19)

3630
U.P. Singh et al. / Polyhedron 26 (2007) 3625–3632
˚
2.281 A, as reported for other complexes of this ligand. The
bond distance of coordinated 3,5-diisopropylpyrazole
In complex 9, both manganese centres (Mn1 and Mn2)
are also linked to each other by a l-1,3-OCO bridge,
whereas the nitrogen of picolinate is coordinated to Mn2
only, forming a five coordinate ring as shown in Fig. 6.
Fig. 7 shows the interaction between two independent mol-
ecules and Fig. 8 shows the packing diagram of the com-
pound. As shown in Fig. 7, a CH3–p interaction appears
˚
(Mn1–N7, 2.313 A) is larger than the Mn–N bond distance
of TpiPr2 (Table 2). The Mn–N bond distances of the coor-
˚
dinated azide (Mn1–N16, 2.183; Mn1–N17, 2.207 A) are
very similar to the Cu–N bond distance in the azide bridged
dinuclear copper complex (Cu–N4, 2.06 A) [21].
˚
Fig. 7. Molecular structure showing CH3–p interaction in complex 9.
Fig. 8. Packing view of complex 9.

U.P. Singh et al. / Polyhedron 26 (2007) 3625–3632
3631
to exist between the two independent molecules with a sep-
aration of 3.177(3) A.
3.2.2. Kinetics
˚
Since the coordination geometry of complex 9 is very
similar to the active site structure of pseudocatalase, the
kinetic experiment of complex 9 with H2O2 was performed
in detail. An excess concentration of H2O2 was used in the
reaction to measure the initial rate of oxygen evolution and
formation of new manganese species in methanol at room
temperature. The order of the reaction with respect to the
catalyst was measured by varying the catalyst concentra-
tion at a constant concentration of H2O2. The initial rate
versus catalyst concentration showed a good linear correla-
tion (Fig. 10). The linear dependence of the rate with
respect to the catalyst revealed that the reaction is first
order. The slope of the line gives a first order rate constant
3.1. Magnetic studies
The room temperature magnetic moment for all the
reported complexes are well within the range of magnetic
moment reported for manganese (II) complexes [25,26].
Since the coordination structure of complex 9 is very close
to the active site structure of manganese containing cata-
lase/pseudocatalase, its magnetic properties has been
reported at variable temperatures ranging from helium to
room temperature. Fig. 9 shows the temperature depen-
dence plot of leff (ꢀ) and vM (n). The value of leff at
300 K amounts to 5.69 lB, close to the spin-only value of
5.92 lB for S = 1/2. The effective magnetic moment
smoothly decreases with temperature. The vM versus T
curve shows maxima at approximately 44 K and then
declines rapidly to zero at 6 K, indicating the presence of
weak antiferromagnetic coupling between the manganese
ions [26].
(k) = 9.725 · 10À4 sÀ1 for
a H2O2 concentration of
24.47 mM. The rate constant for complex 9 is slower as
compared to other model compounds reported in the liter-
ature [28,29]. At a constant concentration, complex 9
exhibits saturation kinetics with H2O2 and follows the
Michaelis–Menten equation. The catalytic turnover num-
ber (kcat) = 6.676 · 10À3 sÀ1 and the Michaelis constant
(KM) is 114 mM (Fig. 11). As reported in the literature,
some catalases whose activity was not inhibited by CNÀ
or azide (N3À) are called pseudocatalase. The interesting
aspect of 9 is that in spite of azide binding, it exhibits cat-
alytic activity and catalyses the disproportionation of
hydrogen peroxide.
3.2. Catalytic activity studies
3.2.1. Electronic spectra
Complexes 7 and 9 exhibit no absorption band in the
visible region, suggesting that both manganese ions in
these complexes are in the two oxidation state. In order
to get an insight into the disproportionation of H2O2 cat-
alyzed by complexes 7 and 9, the reaction was monitored
by absorption spectroscopy. The UV–vis absorption spec-
tra of these complexes (showing no bands) are very simi-
lar to the optical spectrum (lack of visible absorption) of
manganese catalase from T. thermophilus (TTC) in the
reduced (II, II) state [27]. During the reaction of 9 with
H2O2 in methanol, a new band at 379 nm wavelength
appeared and the intensity of this band increases with
increasing concentration of hydrogen peroxide until the
reaction is completed.
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Catalyst (mM)
Fig. 10. Effect of the [catalyst] on the initial rate of H2O2 disproportion-
ation at 25 ꢁC and [H2O2] = 24.47 mM in methanol.
0.06
0.05
0.04
0.03
0.02
0.01
0
6
5
4
3
2
1
0
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
50
100
150
[H2O2] (mM)
0
45
95
145
195
245
295
Temp (K)
Fig. 11. Effect of the [H2O2] on the initial rate of H2O2 disproportionation
Fig. 9. Plot of leff (ꢀ) and v (n) vs. temperature for complex 9.
at 25 ꢁC and [catalyst] = 368 lM in methanol.

3632
U.P. Singh et al. / Polyhedron 26 (2007) 3625–3632
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Sov. Phys. Dokl. 31 (1986) 877;
In conclusion, we have reported the synthesis, structural
as well as catalytic studies of some manganese compounds
as suitable models for manganese-containing pseudocata-
lase. To the best of our knowledge complex 9 is the first
manganese complex that is a l-1,3-azido bridged dimer
having picolinate as a bridging ligand. The most striking
feature of this complex is that the picolinate is behaving
as tridentate ligand, which is not very common. Due to
the tridentate nature of picolinate, each manganese center
is 6-coordinated which is very important for the catalytic
activity of pseudocatalase. The coordination number of
each manganese ion in 7 is five where the acetylacetonate
behaves as a bidentate ligand. The interesting observation
is that complex 9 has the capability for disproportionation
of hydrogen peroxide but complex 7 does not because the
coordination geometry of this complex is not very similar
to the active site structure of the native enzyme. Thus, com-
plex 9 may be a true model for manganese-containing
pseudocatalase both structurally as well as functionally.
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Acknowledgements
This research was supported by DST, New Delhi. The
authors are grateful to Dr. W. Thomas, Max-Planck Insti-
tute for Bioinorganic Chemistry, Muelheim, Germany for
fruitful discussion on the X-ray structure. We are also
thankful to the Head, Institute Instrumentation Centre, In-
dian Institute of Technology Roorkee, for the single crystal
X-ray facility.
[16] (a) G.M. Sheldrick, Acta Cryst. A. 46 (1990) 467;
(b) G.M. Sheldrick, SHELXTL – NT 2000 version 6.12, reference
manual, University of Go¨ttingen, Go¨ttingen, Germany.
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Chim. Acta 310 (2000) 273.
Appendix A. Supplementary material
CCDC 633945 and 633946 contain the supplementary
crystallographic data for this paper. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with
this article can be found, in the online version, at
doi:10.1016/j.poly.2007.03.049.
[18] R. Singh, Ph.D. dissertation 2001, Indian Institute of Technology
Roorkee, Roorkee 247 667, India.
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Chem. Soc., Chem. Commun. (1993) 943.
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