Synthesis, characterization, and reactivity of zinc carboxylate complexes of 2,3-pyridine dicarboxylic acid and (3-oxo-2,3-dihydro-benzo[1,4]oxazin-4-yl) acetic acid

Article abstract of DOI:10.1016/j.ica.2010.08.045

Ethylenediammonium tris-2,3-pyridine dicarboxylato zinc(II) trihydrate (enH₂)₂[ZnL₃]·2H₂O (1) (where H₂L = 2,3-pyridine dicarboxylic acid, en = ethylenediamine) and a mixed metal coordination polymer with composition [Na₂ZnL′ ₃(OAc)(H₂O)₃]_n (2) {where L′ = anion of (3-oxo-2,3-dihydro-benzo[1,4]oxazin-4-yl)acetic acid} are synthesized and characterized. The complex 1 is mono nuclear complex with three coordinating carboxylate anion along with nitrogen chelating zinc ion and there is three uncoordinated carboxylate group one each from three ligand molecules making a complex anion of zinc(II). The zinc(II) ion are in distorted octahedral coordination geometry. In this complex diprotonated ethylenediamines serve as cations. The complex 2 has a polymeric structure with one acetate and three carboxylate of L′ binding to zinc ion provides a tetrahedral environment and these ligands further hold dinuclear units of tri-aquated sodium ions; each dinuclear sodium units are bridged by one water molecule make the coordination polymer. The catalytic ability of these two complexes 1 and 2 towards carbon-carbon bond formation reaction between 3,4-dimethoxy benzaldehyde and acetone are studied. Both the complexes as well as sodium salt of L′ are found to be catalyst for such reactions.

Full text of DOI:10.1016/j.ica.2010.08.045

Inorganica Chimica Acta 372 (2011) 37–41
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, characterization, and reactivity of zinc carboxylate complexes
of 2,3-pyridine dicarboxylic acid and (3-oxo-2,3-dihydro-benzo[1,4]oxazin-4-yl)
acetic acid
⇑
W. Marjit Singh, Bigyan R. Jali, Babulal Das, Jubaraj B. Baruah
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 3 September 2010
Ethylenediammonium tris-2,3-pyridine dicarboxylato zinc(II) trihydrate (enH ) [ZnL ]ꢀ2H O (1) (where
2
2
3
2
2
H L = 2,3-pyridine dicarboxylic acid, en = ethylenediamine) and a mixed metal coordination polymer
0
0
with composition [Na ZnL (OAc)(H O) ] (2) {where L = anion of (3-oxo-2,3-dihydro-benzo[1,4]oxa-
2 3 2 3 n
This article is dedicated to Prof. S.S.
Krishnamurthy on the occasion of his 70th
birthday
zin-4-yl)acetic acid} are synthesized and characterized. The complex 1 is mono nuclear complex with
three coordinating carboxylate anion along with nitrogen chelating zinc ion and there is three uncoordi-
nated carboxylate group one each from three ligand molecules making a complex anion of zinc(II). The
zinc(II) ion are in distorted octahedral coordination geometry. In this complex diprotonated ethylenedi-
amines serve as cations. The complex 2 has a polymeric structure with one acetate and three carboxylate
Keywords:
2
,3-Pyridine dicarboxylic acid
0
(
3-Oxo-2,3-dihydro-benzo[1,4]oxazin-4-
of L binding to zinc ion provides a tetrahedral environment and these ligands further hold dinuclear units
yl)acetic acid
Carboxylate complex
Zinc
Sodium
Coordination polymer
of tri-aquated sodium ions; each dinuclear sodium units are bridged by one water molecule make the
coordination polymer. The catalytic ability of these two complexes 1 and 2 towards carbon–carbon bond
formation reaction between 3,4-dimethoxy benzaldehyde and acetone are studied. Both the complexes as
0
well as sodium salt of L are found to be catalyst for such reactions.
Ó 2010 Elsevier B.V. All rights reserved.
1
. Introduction
Zinc carboxylate complexes are useful catalyst for C–C bond for-
determining their crystal structures and their catalytic activity to-
wards condensation reaction between 3,4-dimethoxy benzalde-
hyde with acetone is described.
mation reactions [1]. Some of the zinc carboxylate complexes find
place in carbon-dioxide activation [2–6]. Binuclear zinc carboxyl-
ate complexes are useful as biological mimic [7,8]. There is need
for studying structural and reactivity of carboxylate complexes
that have additional functional groups or binding sites so that they
provide extra supramolecular interactions [9]. Moreover, when
such units are attached to chiral units, they should cause chiral
transformations [1]. Thus, there is a need to synthesize new car-
boxylate complexes with pendant functional groups having scope
for synthetic modifications. Further to this amino acid containing
heterocycle such as proline and its zinc complexes are active cata-
lyst for carbon–carbon bond formation reactions [15–18]. So, we
have chosen two carboxylic acid compound having heterocycle
attached to them and prepared zinc complexes for such studies.
2
. Experimental
Synthesis of ethylenediammonium tris-2,3-pyridine dicarboxy-
lato zinc(II) trihydrate (1): to a methanolic solution (20 ml) of H
0.516 g, 3 mmol), zinc(II) acetate dihydrate (0.219 g, 1 mmol) was
added and stirred for half an hour. A white precipitate was
formed. Ethylenediamine (140 l, 2 mmol) was added dropwise
2
L
(
l
at room temperature to dissolve the precipitate. The solution
was filtered, and kept for crystallization. Colourless blocks were
formed in 2–3 days after keeping them in a beaker suitable for
ꢁ1
X-ray crystallography. Isolated yield, 45%. IR (KBr, cm ): 3418
b), 2924 (s), 2852 (w), 1609 (s), 1578 (s), 1392 (m), 1362 (s),
(
1
4
1
268 (m), 1232 (w), 1103 (s), 828 (m), 709 (w). H NMR (D
00 MHz, ppm): 8.31 (3H, d, J = 4.0 Hz), 7.89 (3H, d, J = 7.60 Hz),
2
O,
The two acids chosen are 2,3-pyridine dicarboxylic acid (H
and (3-oxo-2,3-dihydro-benzo[1,4]oxazin-4-yl)acetic acid (HL )
2
L)
0
7.61 (3H, dd, J = 2.40 Hz, 5.20 Hz), 3.23 (8H, s). Molar conduc-
tance: 207.0 S cm mol in water.
The (3-oxo-2,3-dihydro-benzo[1,4]oxazin-4-yl)acetic acid was
prepared by reported procedure [10].
2
ꢁ1
(Fig. 1). The complex derived from them are characterized by
⇑
Corresponding author. Tel.: +91 361 2582082; fax: +91 361 2690762.
E-mail address: juba@iitg.ernet.in (J.B. Baruah).
Synthesis of the complex [Na
of sodium salt of the ligand (3-oxo-2,3-dihydro-benzo[1,4]
2 3 2 3 n
ZnL (OAc)(H O) ] : to a solution
0
020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.08.045

3
8
W. Marjit Singh et al. / Inorganica Chimica Acta 372 (2011) 37–41
O
O
O
OH
OH
O
N
OH
N
O
pyridine-2,3-dicarboxylic acid (H L) (3-oxo-2,3-dihydro-benzo[1,4]oxazin-4-yl)-acetic acid (HL')
2
Fig. 1. Structure of ligands.
0
oxazin-4-yl)-acetic acid (HL ) (0.229 g, 1 mmol) dissolved in
methanol/water (15 ml of 2:1 by volume), a solution of zinc(II)
acetate dihydrate (0.110 g, 0.5 mmol) in methanol (5 ml) was
added. A colorless solution was observed, the solution was kept
undisturbed for crystallization. After four days, colorless crystals
located in both the structures 1 and 2. The crystallographic parame-
ters are given in Table 1.
Typical catalytic reaction for preparation of (E)-4-(3,4-dime-
thoxyphenyl) but-3-en-2-one: to a solution of 3,4-dimethoxy benz-
aldehyde (0.33 g, 2 mmol), and sodium hydroxide (0.002 g, 5 mol%)
in acetone (15 ml), complex 1 (0.007 g, 1 mol%) was added. The
resulting solution was kept stirring for 4 h at room temperature.
After slow evaporation of the filtrate, a yellow color semi solid
was obtained, which was purified by column chromatography
ꢁ1
of the complex were obtained. Yield 78%, FT-IR (KBr, cm ):
450 (bs), 2960 (w), 1660 (s), 1614 (s), 1504 (m), 1471 (w),
323 (s), 1388 (s), 1316 (w), 1281 (s), 1243 (m), 1135 (m),
088 (w), 1056 (m), 902 (m), 755 (s). H NMR (DMSO-d
3
1
1
6
1
6
):
1
.93 (4H, m, aromatic protons), 4.60 (2H, s, cyclic aliphatic –
–), 4.30 (2H, s, –CH – group), 3.16 (s, from H O), 1.77 (3H,
): 176.2, 170.9, 163.8,
44.6, 129.4, 123.1, 122.6, 116.2, 115.8, 67.1, 44.7, 23.7.
and characterized by recording its H NMR, IR and comparing with
CH
s, –CH
2
2
2
authentic sample.
1
3
3
of acetate). C NMR (DMSO-d
6
Catalyst optimization: to the above reaction mixture naphtha-
lene (0.126 g) was added and the consumption of the 3,4-dime-
thoxybenzaldehyde with time during the reaction was monitored
by GC–MS. The GC–MS were recorded in a Perkin–Elmer Clarus
500 with oven, temperature 200 °C, injector temperature 250 °C,
inlet line a source temperature 150 °C with a capillary column of
5% phenyl 95% methyl-polysiloxane. The relative peak areas are
calibrated independently by using naphthalene as a standard in
each experiment. Similar reactions were performed with other cat-
1
The X-ray crystallographic data were collected at 296 K with Mo
radiation (k = 0.71073 Å) using a Bruker Nonius SMART CCD dif-
Ka
fractometer equipped with graphite monochromator. The SMART
software was used for data collection and also for indexing the
reflections and determining the unit cell parameters; the collected
data were integrated using SAINT software [11]. 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
0
alysts namely L Na, 2 and without catalyst were monitored.
2
in the anisotropic approximation against F of all reflections. The H-
atoms, except those attached to O were placed at their calculated
positions and refined in the isotropic approximation; those attached
to hetero-atoms (O) were located in the difference Fourier maps, and
refined with isotropic displacement coefficients. Some of the hydro-
gen atoms of oxygen atoms of water of crystallization could not be
3
. Results and discussion
The reaction of zinc(II) acetate with 2,3-pyridine dicarboxylic
acid followed by ethylenediamine gives a mononuclear complex
Fig. 2) with composition (enH [ZnL O (where H L = 2,3-
]ꢀ2H
pyridine dicarboxylic acid, en = ethylenediamine) (Eq. (1)).
(
2
)
2
3
2
2
ZnðOAcÞ ꢀ 2H
2
O þ H
2
L þ en ƒƒƒƒƒƒƒƒƒ! ðenH
2
Þ2½ZnL
3
ꢂ ꢀ 3H
2
O þ AcOH
ð1Þ
2
en¼ethylenediamine
Table 1
The crystallographic parameters for complex 1 and 2.
Compound
1
2
Empirical formula
C
25
H
33
7
N O
14Zn
32 27 3 2
C H N Na O19Zn
M
r
720.92
296
868.92
296
Temperature (K)
3
Crystal size (mm )
Crystal systems
Space group
a (Å)
b (Å)
c (Å)
0.40 ꢃ 0.28 ꢃ 0.16
0.44 ꢃ 0.26 ꢃ 0.20
orthorhombic
monoclinic
P2
1
1
2 2
1
P2
1
8.0064(4)
16.0942(10)
23.1032(13)
90.00
90.00
90.00
2977.0(3)
4
1.600
8.0001(4)
14.7338(7)
16.1445(8)
90.00
103.673(3)
90.00
1849.05(16)
2
1.561
a
(°)
b (°)
(°)
c
3
V (Å )
Z
D
calc (g cm 3
ꢁ
)
l
(Mo K
a
) (mmꢁ1)
0.908
0.120
F(0 0 0)
1480
888
h (°)
27.00
28.32
Reflections collected
Independent reflections (Rint
Data/restraints/parameters
GOF on F2
35390
6201
6201/0/428
1.012
19747
8422
8422/0/515
0.929
)
R
R
1
, wR
, wR
2
[I > 2
(all data)
r
(I)]
0.0599, 0.1452
0.0821, 0.1600
–
0.0816, 0.1596
0.1986, 0.2471
0.04(3)
1
2
Fig. 2. Structure of tris-(2,3-pyridine dicarboxylato) zinc anion (the cations of the
complex are removed for clarity).
Flack parameter

W. Marjit Singh et al. / Inorganica Chimica Acta 372 (2011) 37–41
39
Table 2
(a)
Selected bond distances (Å) and bond angles (°) of complex 1.
R1
H O
2
OH2
Bond parameter
For 1
Bond parameter
For 1
Zn(1)–N(1)
Zn(1)–N(2)
Zn(1)–N(3)
Zn (1)–O(5)
Zn(1)–O(1)
Zn(1)–O(3)
2.123(4)
2.149(4)
2.124(4)
2.144(3)
2.076(3)
2.136(4)
162.30(16)
98.86(16)
95.64(14)
88.50(15)
77.98(14)
<O(3)–Zn(1)–N(2)
<N(2)–Zn(1)–N(3)
<N(3)–Zn(1)–O(5)
<O(1)–Zn(1)–O(3)
<O(5)–Zn(1)–O(3)
<N(3)–Zn(1)–O(1)
<N(1)–Zn(1)–O(3)
<N(3)–Zn(1)–O(3)
<O(5)–Zn(1)–O(1)
<N(1)–Zn(1)–O(1)
175.36(15)
93.46(16)
77.64(14)
100.21(15)
86.60(14)
96.54(16)
77.11(15)
90.98(15)
171.20(14)
98.41(17)
O
O
OH2
Na
Zn
O
R = CH3
O
N
O
O
O
O
R1
=
O
R1
<
<
<
<
<
N(3)–Zn(1)–N(1)
N(2)–Zn(1)–N(1)
N(2)–Zn(1)–O(5)
N(1)–Zn(1)–O(5)
N(2)–Zn(1)-O(1)
O
R1
R
Na
(
b)
The complex has characteristic infra-red stretching at
ꢁ
1
3
418 cm due to N–H stretching with an strong overtone band
ꢁ1
at 1609 cm . The carboxylate carbonyl frequency appears at
ꢁ1
1
2
578 cm
.
The molar conductance is found to be
2
ꢁ1
07.0 S cm mol corresponds to the 1:2 electrolyte. The proton
NMR spectrum of the complex shows the corresponding peaks
for the ligands and the ethylene diammonium cations. The crystal
structure of the complex, reveals it to have an octahedral geome-
tries around the zinc ion. The complex 1 is obtained either from
the reaction of zinc(II) acetate to dicarboxylic acid (H
2
L) ratio
(
1:2) or in (1:3); later composition gives the best yield. This sug-
gests that the reaction to be thermodynamically controlled. The
zinc ions are in octahedral geometry and the some of the metal–li-
gand bond distances and bond angles of the zinc complex are listed
in Table 2.
There are examples of 2,3-pyridine dicarboxylato complexes in
which complexes in chelating mode derived from partially depro-
tonated acid are formed and such complexes have strong intramo-
lecular hydrogen bonds [12]. The 2,3-pyridine dicarboxylato
complexes with polymeric structures are well known [13,14]. In
fact depending on the reaction procedure and cation used poly-
meric zinc complex of 2,3-pyridine dicarboxylato anion can be pre-
pared [12,19–23].
Fig. 3. (a) Representation of
coordination polymer 2; (b) crystal structure of the repeated units of the
coordination polymer 2.
a portion of structural backbone of the zinc
0
The sodium salt of HL on reaction with zinc(II) acetate dihy-
0
drate gave a complex with a composition [Na
2
ZnL
3
(OAc)(H
2
O)
3
]
n
.
–
1
The H NMR of the complex shows the presence of two –CH
2
0
groups of L in different environment. There is also signal for –
formed by partial substitution of acetate ligand of zinc(II) acetate.
The complex shows its characteristic peaks for acetate methyl
CH
70.9, and 163.8 due to the presence of three different carbonyl
groups. The signals at 67.1, 44.7 corresponds to the –CH – groups
of L’ and the signal appearing at 23.7 corresponds of CH -group of
acetate. The complex has two strong carbonyl absorptions at 1660
3
group of acetate. In the 13C NMR, there are signals at 176.2,
1
group and the other CH
2
groups flanked by oxygen and nitrogen
0
2
of the ligand L . The repeated units of the polymeric structure is
shown in Fig. 3a, the structure of the coordination polymer is
shown in Fig. 3b. The coordination polymer is formed by zinc an-
ions coordinated to three carboxylato oxygen atoms of the ligand
L’ and one oxygen atom of acetate group The zinc ions are in dis-
torted tetrahedral environment. Each of the carboxylate group is
3
ꢁ1
and 1614 cm due to two types of carboxylate groups. The com-
plex has broad and sharp absorption at 3450 that arises from
OH-stretching of water attached to sodium ions.
O
O
N
MeOH/H2O
ONa + Zn(OAc) 2H O
[Na ZnL' (OAc)(H O) ] + NaOAc
ð2Þ
2
.
2
2
3
2
3 n
O
2
L'Na
Synthesis of coordination polymer
This reaction is interesting as it is observed that partial ligand
exchange took place but a mixed metal complex with mixed car-
boxylate ligand is formed. The complex can be considered to be
anchored to sodium ion. The sodium ions are further coordinated
to three water molecules. The two coordinated water molecules
are terminal ligands whereas one acts as bridging ligands. The

4
0
W. Marjit Singh et al. / Inorganica Chimica Acta 372 (2011) 37–41
Table 3
Selected bond distances (Å) and bond angles (°) of complex 2.
Table 4
Catalytic C–C bond formation reactions between 3,4-dimethoxybenzaldehyde with
acetone.
Bond parameter
For 2
Bond parameter
For 2
Sl. No.
Catalysta
Isolated yield (%)
Time (h)
Zn(1)–O(8)
Zn(1)–O(13)
Zn(1)–O(11)
Zn (1)–O(4)
Na(1)–O(3)
Na(1)–O(17)
Na(1)–O(16)
Na(2)–O(19)
Na(1)–O(15)
Na(2)–O(18)
Na(1)–O(14)
Na(1)–O(12)
Na(2)–O(7)
1.942(6)
1.951(8)
1.973(5)
1.980(6)
2.402(7)
2.352(9)
2.404(10)
2.485(13)
1.943(18)
2.461(17)
2.378(11)
2.403(7)
2.292(8)
123.1(3)
109.1(2)
95.9(3)
<O15–Na1–O3
<O17–Na1–O3
<O14–Na1–O3
<O15–Na1–O12
<O17 Na1–O12
<O14–Na1–O12
<O3–Na1–O12
<O15–Na1–O16
<O17–Na1–O16
<O14–Na1–O16
<O3–Na1–O16
<O12–Na1–O16
<O15–Na1–O8
<O17–Na1–O8
<O14–Na1–O8
<O3–Na1–O8
53.9(4)
175.4(4)
93.6(4)
52.6(4)
85.7(3)
170.9(5)
89.9(3)
126.4(4)
89.6(4)
94.6(4)
92.1(4)
93.7(3)
52.0(4)
96.5(3)
82.7(4)
82.0(2)
89.5(2)
173.3(4)
123.9(4)
0
2
3
4
6
NaL
1
2
98
92
94
96
3
5
3
4
zinc-oxide
a
In all cases 1 mol% of catalyst is taken with 5 mol% sodiumhydroxide in acetone
15 ml) at room temperature.
(
[ZnLn]
+
NaOH
Ln-1Zn(OH)
+
LNa
Na
Where L is ligand
+
LnZn + H2O
(i)
<
<
<
<
<
<
<
O8–Zn1–O13
O8–Zn1–O11
O13–Zn1–O11
O8–Zn1–O4
O13–Zn1–O4
O15–Na1–O17
O17–Na1–O14
112.1(3)
103.1(3)
121.8(5)
90.6(4)
<O12–Na1–O8
<O16–Na1–O8
<O15–Na1–O14
H C
H3C
H3C
2
(
ii)
O
Ln-1Zn(OH) + LNa +
O
H3C
H2 OH
C
H
H2O
bond angles and bond distances describing the coordination envi-
ronment are listed in Table 3. There are good numbers of literature
of aqua bridged sodium complexes either leading to dimeric or
polymeric structures [24,26]. Further to this the sodium ions can
adopt six to eight coordination numbers [26]. These aqua bridges
together with the carboxylate bridges make a polymeric structure,
in which the sodium ions are in seven coordination geometry.
Aqua bridged sodium complexes are common in literature [24–
3
H C
CH2
O
Na
O
H
O
+
H3C
LnZn
H
C
H3C
Ln-1Zn(OH)
+
O
H
product
active catalyst
2
6]. However, the importance about this carboxylato complex is
Scheme 1. Proposed mechanism for zinc catalysed reactions.
the observation of mixed metal as well as mixed carboxylato
ligands, which is uncommon in literature.
It is interesting to note that the acetate groups act as bridge
between two sodium ions along with a side-on interaction with
zinc(II) ion. Each sodium ion is coordinated to three carboxylate
ligands to make a network structure. In structure determination
by single crystal X-ray crystallography we could not locate
precisely the hydrogen atoms anchored to oxygen atoms of water
molecules. The compound has tetrahedral geometry around each
zinc ions and the sodium ions are hepta-coordinated. The mononu-
clear coordination complex of L’ with copper(II) is recently
reported [27], however the present complex is a coordination
polymer.
4-methoxybenzaldehyde. The reaction time and reaction condition
for different catalysts are listed in Table 4. Based on the results
obtained from control experiments, a mechanism is proposed in
Scheme 1. In the absence of these catalysts, the reaction does not
take place. Further to this sodium hydroxide is also required in
each case to cause the catalytic reactions, or in other word the
metal complexes alone do not catalyse the reactions. The reactivity
of the reactions are similar to zinc oxide in sodium hydroxide or
similar to sodium salt of the ligand. No homo-coupling reaction
between acetones is observed. This is due to the ease at which
the cross coupling takes place, that dominates. The homo-coupling
is also not observed in acetone earlier [19]. Thus, the role of sodium
hydroxide is to generate zinc hydroxide derivatives in situ to act as
catalyst. Since chiral induction was not possible and unsaturated
carbonyl compounds were formed as the final product in these
reactions, obvious interpretation is that the zinc complexes helps
in dehydration and leads to active catalyst in the final step, so that
the catalytic reactions continues. The zinc proline complex cata-
lysed reaction do not eliminate water [19] but we have obtained
exclusively the unsaturated carbonyl compounds which are
formed by elimination of water from their corresponding alcohols.
The coordination polymer 2 is made up of dinuclear sodium units
holding tetrahedral zinc centers, this complex can be easily decom-
pose by sodium hydroxide and the original polymeric complex can
not be obtained after dissolution in dilute sodium hydroxide. This
also suggests the role of zinc hydroxyl species as active catalyst in
these reactions.
MeO
H3C
O
H
O
Excess acetone
Catalyst
MeO
MeO
OMe
ð3Þ
Selective catalytic reactions by proline [15–18] and its zinc
complexes [19] are well studied. Since the zinc complexes that
catalyses such aldol type reactions have heteroatom on a cyclic
counterpart with a carboxylate functional group, we investigated
the possibility of catalytic aldol reactions by complex 1 and 2. It
0
is found that the sodium salt of HL as well as 1, 2 is suitable cat-
alyst for aldol reactions. The complexes 1 and 2 independently
catalyse the reaction between aromatic aldehydes and acetone in
mild basic medium. The reactions led to unsaturated carbonyl
derivatives as illustrated in Eq. (3). One such reaction is mentioned
here but the reaction is also applicable to condensation reaction
between acetone and other electron rich aromatic aldehydes like
In conclusion we have synthesized and characterized two zinc
carboxylate complexes that have the carboxylate ligands attached
to heterocyclic rings. The catalytic activities of these two com-
plexes are tested and it is found that in combination with sodium
hydroxide these complexes catalyses condensation cum elimina-
tion reactions of acetone with aromatic aldehydes.

W. Marjit Singh et al. / Inorganica Chimica Acta 372 (2011) 37–41
41
[
[
[
6] D.J. Darensbourg, M.S. Zimmer, Macromolecules 32 (1999) 2137.
7] B.-H. Ye, X.-Y. Li, Inorg. Chem. 41 (2002) 6426.
8] B. Singh, J.R. Long, J. Am. Chem. Soc. 119 (1997) 7030.
Acknowledgements
The authors thank Department Science and Technology, India,
for financial support and one of the author W.M.S thanks Council
of Scientific and Industrial Research, India, for a Senior Research
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