Synthesis and characterization of pyridine N-oxide complexes of manganese, copper and zinc

Article abstract of DOI:10.1016/j.ica.2007.10.029

A series of metal carboxylates containing pyridine N-oxide are prepared via one pot synthesis and solid phase synthesis. The structural variations from metal to metal are observed. In the case of reactions of manganese(II) acetate with pyridine N-oxide in the presence of aromatic carboxylic acids, polymeric complexes with bridging aromatic carboxylate as well as bridging pyridine N-oxide are observed. Whereas, the reaction of copper(II) acetate with pyridine N-oxide in the presence of an aromatic carboxylic acid led to mononuclear or binuclear paddle wheel carboxylate complexes with monodentate pyridine N-oxide. Co-crystal of two neutral complexes having composition [Cu₂(OBz)₄(MeOH)₂][Cu₂(OBz)₄(pyO)₂] (where OBz = benzoate, pyO = pyridine N-oxide) each neutral parts have paddle wheel structure. Solid phase reaction of zinc chloride with sodium benzoate prepared in situ and pyridine N-oxide leads to a tetra-nuclear zinc complex.

Full text of DOI:10.1016/j.ica.2007.10.029

Available online at www.sciencedirect.com
Inorganica Chimica Acta 361 (2008) 2081–2086
www.elsevier.com/locate/ica
Synthesis and characterization of pyridine N-oxide complexes
of manganese, copper and zinc
*
Rupam Sarma, Anirban Karmakar, Jubaraj B. Baruah
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039 Assam, India
Received 22 June 2007; accepted 21 October 2007
Abstract
A series of metal carboxylates containing pyridine N-oxide are prepared via one pot synthesis and solid phase synthesis. The struc-
tural variations from metal to metal are observed. In the case of reactions of manganese(II) acetate with pyridine N-oxide in the presence
of aromatic carboxylic acids, polymeric complexes with bridging aromatic carboxylate as well as bridging pyridine N-oxide are observed.
Whereas, the reaction of copper(II) acetate with pyridine N-oxide in the presence of an aromatic carboxylic acid led to mononuclear or
binuclear paddle wheel carboxylate complexes with monodentate pyridine N-oxide. Co-crystal of two neutral complexes having compo-
sition [Cu2(OBz)4(MeOH)2][Cu2(OBz)4(pyO)2] (where OBz = benzoate, pyO = pyridine N-oxide) each neutral parts have paddle wheel
structure. Solid phase reaction of zinc chloride with sodium benzoate prepared in situ and pyridine N-oxide leads to a tetra-nuclear zinc
complex.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Metal-carboxylates; Pyridine N-oxide; Co-ordination polymers; Co-crystals
1. Introduction
complicacy as the carboxylate group it self have multiple
ways to bind to a metal [4h,5]. Thus, a general approach
to synthesize pyridine N-oxide containing metal carboxyl-
ate complexes is essential. We have made a series of study
to prepare aromatic carboxylate complexes through solu-
tion phase and solid phase reactions [8]. We used nitrogen
containing ligand in all these studies [8]; here we present
preliminary results on the general methodology for synthe-
sis of metal carboxylate complexes containing pyridine N-
oxide.
Pyridine N-oxide is an important reagent for oxo-trans-
fer reactions [1]. Such reactions are facilitated by transition
metal complexes [2]. Many N-oxide complexes have been
prepared and studied [3]. The N-oxide containing metal
complexes are also of special interest as magnetic material
[4]. The nitroxide complexes also form polymeric structures
and their magnetic properties [5] and electrochemical prop-
erties [6] are studied. Nitroxides can co-ordinate as mono-
dentate and bridging ligand, which complicates structural
studies [7]. For example a complex having composition
[Co(pyO)Cl2(H2O)] (where pyO = pyridine N-oxide) has
halogen and also N-oxide bridge in its polymeric structure
[3h]. The carboxylate containing nitroxide complexes are
known in the literature but they have further structural
2. Experimental
The complexes [Mn(OBz)2(pyO)]n 1a, [Mn(pNO2-
OBz)2(pyO)]n
1b,
[Cu2(OBz)4 Æ (MeOH)2][Cu2(OBz)4
(pyO)2] 2a, Cu(pNO2OBz)2(pyO)2] 2b were prepared
through solution chemistry. Whereas complex [Zn4(OBz)8
(pyO)3(H2O)2]H2O 3 was prepared by solid phase synthe-
sis. (where OBz = C6H5CO2, pNO2OBz = 4-NO2C6H4
CO2, and pyO = C5H5NO)
*
Corresponding author. Tel.: +91 361 2582082; fax: +91 361 2690762.
E-mail address: juba@iitg.ernet.in (J.B. Baruah).
0020-1693/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2007.10.029

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R. Sarma et al. / Inorganica Chimica Acta 361 (2008) 2081–2086
3. Typical procedure for synthesis of 1a
3453 (br m), 3067 (m), 1627 (s), 1572 (s), 1470 (w), 1405
(s), 1224 (m), 1020 (m), 1176 (s), 719 (m). Molar conduc-
tance: 12.80 S cm2 molꢁ1 in methanol. Magnetic moment,
leff: 1.93 B.M.(RT). kmax (methanol): 713 nm;
e = 350 Mꢁ1 cmꢁ1. Complex 2b: IR (KBr, cmꢁ1): 1630
(m), 1583 (s), 1514 (s), 1469 (m), 1392 (w), 1340 (s), 1199
(m), 829(m), 723 (m); Magnetic moment, leff: 1.91
To a solution of benzoic acid (2 mmol, 0.242 g) in meth-
anol (20 ml), Mn(OAc)2 Æ 4H2O (1 mmol, 0.245 g) was
added and stirred for 10 min to obtain a homogeneous
solution. To this reaction mixture pyridine N-oxide
(3 mmol, 0.285 g) was added with constant stirring at room
temperature. The colour of the solution changes to yellow.
A small amount (ꢀ2 ml) of water was added to dissolve the
precipitate that appeared after addition of pyridine N-
oxide. Yellow coloured crystals were collected after 3 days
and dried in air. Yield of the pure crystalline complex was
found to be 27% (crude yield >70%). IR (KBr, cmꢁ1): 1593
(m), 1557 (s), 1477 (w), 1397 (s), 1219 (m), 731 (m). Molar
conductance: 129.3 S cm2 molꢁ1 in water and 55.0 S cm2
molꢁ1 in methanol. Magnetic moment leff: 5.38 B.M.(RT).
Complexes 1b, 2a and 2b were prepared with similar
procedure and crystalline pure products with 21%, 24%
and 29% yield, respectively, were obtained. Complex 1b:
IR (KBr, cmꢁ1): 1580 (s), 1518 (m), 1471 (w), 1398 (s),
1343 (s), 1208 (m), 1102 (w), 1020 (m), 820 (m), 727 (m).
Molar conductance: 124 S cm2 molꢁ1 in water. Magnetic
moment leff: 5.62 B.M. Complex 2a: IR (KBr, cmꢁ1):
B.M.(RT). kmax (methanol): 705 nm; e = 349 Mꢁ1 cmꢁ1
.
Synthesis of 3: A mixture of benzoic acid (0.242 g,
2 mmol) and potassium hydroxide (0.113 g, 2 mmol) was
finely ground in a pastel mortar and heated at 90 °C for
15 min. The mixture was cooled to room temperature.
Anhydrous ZnCl2 (0.136 g, 1 mmol) was added. The mix-
ture was finely ground and heated again at 90 °C for
another 15 min. The mixture was cooled to room tempera-
ture, transferred to a round bottom flask, and dissolved in
a mixture of methanol/toluene (15 ml/5 ml). To this reac-
tion mixture pyridine N-oxide (0.285 g, 3 mmol) was added
with continuous stirring. The solution was then allowed to
stand. Colourless crystals were collected after 5 days and
dried in air. Yield of the crystalline complex was found
1
to be 26%. HNMR (CDCl3, ppm): 8.41 (s, 6H), 8.08 (s,
16H), 7.48 (br m, 9H), 7.35 (br s, 24H), 4.2 (br s, exchange-
Table 1
The crystal parameters of the complexes 1a–b, 2a–b and 3
Complexes
1a
1b
2a
2b
3
Formulae
C
19H15MnNO5
C19H13MnN3O9
482.26
monoclinic
C2/c
C68 H58 Cu4 N2O20
1477.32
triclinic
C24 H18 Cu N4 O10
585.96
monoclinic
P2(1)/n
296
0.71073
6.6443(4)
7.0988(4)
25.1865(18)
90.00
C71 H63 N3O23 Zn4
1587.72
monoclinic
C2/c
Molecular weight
Crystal system
Space group
392.26
orthorhombic
pbcn
ꢀ
P1
Temperature (K)
296
296
296
296
˚
Wavelength (A)
0.71073
16.5652(14)
13.3047(10)
7.5755(6)
90.00
90.00
90.00
1669.6(2)
4
1.561
0.71073
25.6908(8)
9.7787(3)
7.6613(3)
90.00
101.421(2)
90.00
1886.58(11)
4
0.71073
10.6502(8)
10.6939(8)
15.4901(11)
101.970(4)
105.362(4)
96.011(4)
1640.2(2)
1
0.71073
25.3108(10)
9.6114(3)
31.4127(14)
90.00
112.617(4)
90.00
7054.1(5)
4
˚
a (A)
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
90.402(4)
90.00
1187.93(13)
2
3
˚
V (A )
Z
Dcalc. (Mg mꢁ3
)
1.698
1.496
1.638
1.495
Absolute coefficient
(mmꢁ1
0.822
0.763
1.355
0.988
1.423
)
Absolute correction
F(000)
none
804
none
980
none
756
none
598
none
3256
Total number of
reflections
17272
11049
17502
11021
26988
Reflections, I > 2r(I) 2033
2263
5558
2833
6570
Maximum 2h (°)
Ranges (h,k,l)
28.37
28.29
25.00
28.36
25.50
ꢁ20 6 h 6 20
ꢁ14 6 k 6 17
ꢁ10 6 l 6 10
97.1
ꢁ34 6 h 6 32
ꢁ12 6 k 6 13
ꢁ10 6 l 6 9
96.4
ꢁ12 6 h 6 11
ꢁ12 6 k 6 12
ꢁ18 6 l 6 18
96.2
ꢁ8 6 h 6 8
ꢁ9 6 k 6 9
ꢁ29 6 l 6 32
95.8
ꢁ30 6 h 6 30
ꢁ11 6 k 6 11
ꢁ37 6 l 6 38
99.8
Complete to 2h (%)
Refinement method
full-matrix least-
squares on F2
2033/0/121
full-matrix least-
squares on F2
2263/0/148
full-matrix least-
squares on F2
5558/0/429
full-matrix least-
squares on F2
2833/0/178
full-matrix least-
squares on F2
6750/0/473
Data/restraints/
parameters
Goodness-of-fit (F2)
R indices [I > 2r(I)]
R indices (all data)
1.053
0.0263
0.0358
1.058
0.0276
0.0300
1.049
0.0299
0.0406
1.201
0.0604
0.0801
1.042
0.0367
0.0638

R. Sarma et al. / Inorganica Chimica Acta 361 (2008) 2081–2086
2083
able from water), 2.64 (br s, exchangeable from water). IR
(KBr, cmꢁ1): 3378 (br s), 1595 (m), 1559 (s), 1475 (m) 1392
(s), 1218 (m), 1023 (m), 829 (m), 731 (m).
3.1. Crystallography
The X-ray diffraction data were collected at 296 K with
˚
Mo Ka radiation (k = 0.71073 A) using a Bruker Nonius
SMART CCD diffractometer equipped with graphite
monochromator. The SMART software was used for data
collection and also for indexing the reflections and deter-
mining the unit cell parameters; the collected data were
integrated using SAINT software. The structures were solved
by direct methods and refined by full-matrix least-squares
calculations using SHELXTL software. All the non-H atoms
were refined in the anisotropic approximation. The crystal-
lographic parameters of the complexes are given in Table 1.
4. Results and discussion
The reaction of pyridine N-oxide along with aromatic
carboxylic acid with metal acetates such as manganese(II)
acetate and copper(II) acetate leads to pyridine N-oxide
complexes. However, the formation of the type of pyridine
N-oxide complexes varies with metal and carboxylic acid
under consideration. The reaction of pyridine N-oxide
and aromatic carboxylic acid with manganese(II) acetate
tetrahydrate under ambient condition is illustrated in
Scheme 1. Similar reaction also proceeds smoothly with
other acids such as 4-chlorobenzoic acid and 4-nitroben-
zoic acid. In each case co-ordination polymers are formed
by bridging carboxylate and N-oxide. The crystal struc-
tures of the polymeric compounds are determined and
two structures are shown in Figs. 1b and c. The complexes
have six co-ordination geometry around the manganese
ions. The geometry around the manganese (II) in complex
1a is shown in Fig. 1a. The repeated units in the polymeric
manganese complexes can be compared with dinuclear
aqua bridged metal carboxylate complexes [8a,8b,8j]. In
aqua and carboxylato bridged manganese(II) complexes
the Mnꢂ ꢂ ꢂMn separation [8j] are in the range of 3.65–
Fig. 1. (a) The co-ordination environment around manganese in complex
1a, (b) co-ordination polymer 1a, (c) co-ordination polymer 1b.
higher Mnꢂ ꢂ ꢂMn separation than the aqua bridged dinucle-
ar manganese carboxylate complexes. At room tempera-
ture the two complexes 1a and 1b, show relatively low
magnetic moments, e.g. 1a has 5.38 B.M. and 1b has 5.62
B.M. The calculated spin only magnetic moment for a d5
system is 5.92 B.M. Thus, there is magnetic interaction/s
between the manganese ions in the lattice which reduces
the magnetic moment from the expected value. The com-
pound 1a is non-ionic in methanol and has molar conduc-
tance 55 S cm2 molꢁ1 whereas in water it shows ionization
and has molar conductance 124 S cm2 molꢁ1. The com-
pound 1b is not soluble in methanol and its molar conduc-
tance was recorded in water and found to be
125 S cm2 molꢁ1, which suggests that in water it ionizes.
In contrast to these results the copper(II) pyridine N-
oxide complex has paddle wheel type structure which was
˚
3.74 A. The Mnꢂ ꢂ ꢂÆMn separation in the one-dimensional
˚
˚
polymers 1a and 1b are 3.79 A, 3.83 A, respectively. This
shows that pyridine N-oxide bridged complex has slightly
Ar
N
O
O
*
*
MeOH
O
*
Mn
Mn
*
+ ArCO2H
Mn(OAc)2
+
N
O
*
*
-
O
O
n
Ar
When Ar = C6H5-(1a) and 4-NO2-C6H4-(1b)
Scheme 1.

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R. Sarma et al. / Inorganica Chimica Acta 361 (2008) 2081–2086
˚
obtained when benzoic acid along with pyridine N-oxide
was reacted with copper(II) acetate. This reaction led to
the formation of very interesting co-crystals of two paddle
wheel molecules. Recently, we have reported co-crystals
that can arise from self-assembling of two or more neutral
inorganic complexes [8c,8d,8e]. These are generally held in
the lattices by weak interactions, such as C–Hꢂ ꢂ ꢂO, C–
Hꢂ ꢂ ꢂp interactions. The co-crystal 2a is also not an excep-
tion to this. The two co-crystals are held together by
O3–Hꢂ ꢂ ꢂO70 (dH–A 3.17 A and \D–Hꢂ ꢂ ꢂA 166.91°) interac-
tions. The crystal structure of the compound is shown in
Fig. 2a. The co-crystals are non-ionic in methanol and have
molar conductance value 13 S cm2 molꢁ1. The co-crystal 2a
shows magnetic moment 1.93 B.M. at room temperature.
This shows that there is no significant interaction among
the copper(II) ions in the co-crystal. The compound has a
broad absorption in visible spectra at 713 nm which is
attributed to E–T2 transition. The two Cu ꢂ ꢂ ꢂCu separation
C15–Hꢂ ꢂ ꢂO80 (dH–A 3.22 A and \D–Hꢂ ꢂ ꢂA 106.86°) and
in the two neutral paddle wheel are 2.616 A and 2.620 A,
respectively, which is slightly longer than the distance
˚
˚
˚
˚
observed in copper(II) acetate ( 2.614 A) but comparable
to copper carboxylate complexes with aromatic carboxy-
lato groups [8h].
The reaction of 4-nitro-benzoic acid and pyridine N-
oxide with copper(II) acetate led to the formation of
mono-nuclear complex 2b having two molecules of pyri-
dine N-oxide co-ordinated to copper(II) 4-nitrobenzoate
(Scheme 2). From the crystal structure it is clear that the
complex has six co-ordination geometry. The compound
is highly symmetric and has Cu1–O1, Cu1–O2 and Cu1–
˚
˚
˚
O5 bond distances as 1.95 A, 2.66 A and 1.98 A, respec-
tively. Thus, the two oxygen atoms occupying axial posi-
tions have longer Cu-O bond distances than the other
two and suggest a strong Jahn–Teller distortion in the com-
plex. The compound has very broad and distorted absor-
bance at 705 nm due to E ! T2 transition which supports
a distorted six co-ordination environment in solution.
The zinc(II)acetate on reaction with benzoic acid and
pyridine N-oxide gave a tetra-nuclear complex having com-
position [Zn4(OBz)8(pyO)3(H2O)2]H2O (3). The reaction is
shown in Scheme 3. The complex has six co-ordination
around all the four zinc ions and the repeated unit in a unit
cell of the complex is illustrated in Fig. 3a. Each unit has
three bridging pyridine N-oxide and six bridging carboxyl-
ates. There are two monodentate carboxylates attached one
Fig. 2. Structure of (a) 2a and (b) 2b.
Ar
Ar
Ar
Ar
O
O
O
Cu
O
O
O
N
MeOH
H
O
O
O
H
O
Cu
O
Cu
+ ArCO2H
O
Cu
O
Me
Cu(OAc)2
+
N
O
O
Me
O
O
O
N
-
O
O
O
Ar
Ar
Ar
Ar
2a
When Ar = C6H5-
MeOH
Ar
Ar
O
O
O
Cu
O
O
O
N
N
2b
When Ar = 4-NO2-C6H4-
Scheme 2.

R. Sarma et al. / Inorganica Chimica Acta 361 (2008) 2081–2086
2085
NaOH
ZnCl2
+
+ ArCO2H
[Zn4(ArCOO)8(pyO)3(H2O)2]H2O
pyO
Methanol/toluene
When Ar = C6H5-(3)
pyO =
N
O
-
Scheme 3.
ing with the co-ordinated water molecule and also with the
water of crystallization. The complex has some similarity
to the polynuclear complexes of carboxylate complexes of
cobalt and few other metals [9] in which the role of pyridine
N-oxide is played by a carboxylate group which has chelat-
ing as well as bridging co-ordination mode.
In these complexes the N–O stretching absorptions are
prominent in IR spectroscopy. The bridging N–O absorp-
tion in the complex 1a, 1b and 3 occurs at 1219 cmꢁ1
,
1208 cmꢁ1 and 1218 cmꢁ1 whereas in the mono-dentate
N–O absorption in 2a and 2b occurs at 1224 cmꢁ1 and
1199 cmꢁ1
.
In conclusion, metal carboxylate complexes containing
pyridine N-oxide can easily be prepared by ligand exchange
reactions of metal acetates in the presence of an aromatic
carboxylic acid. The structural diversity of the complexes
and formation of co-ordination polymer with uniform
chain under ambient condition provides wide scope for
synthesis of varieties of new class of mixed ligand
complexes.
Acknowledgement
The authors thank Department of Science and Technol-
ogy, Government of India, New-Delhi, for financial
assistance.
Appendix A. Supplementary data
CCDC 650977, 650978, 650979, 650980, 650982 contain
the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.a-
c.uk/data_request/cif. Supplementary data associated with
this article can be found, in the online version, at
doi:10.1016/j.ica.2007.10.029.
Fig. 3. Structure of the (a) repeated unit in the tetra-nuclear zinc complex
3, (b) the complex 3.
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tate carboxylate group. The complex shows absorbance at
1595 cmꢁ1 and at 1559 cmꢁ1 due to terminal and bridging
carboxylate groups. The terminal monodentate carboxyl-
ate absorption is at lower frequency due to hydrogen bond-
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