Ring opening reactions of pyromellitic dianhydride for the synthesis of first row transition metal dicarboxylate complexes

Article abstract of DOI:10.1016/j.poly.2007.05.054

The ring opening reaction of pyromellitic dianhydride by methanol is an effective method to prepare first row transition metal dicarboxylate complexes. The reactions of different first row transition metal salts with pyromellitic dianhydride in the presence of nitrogen donating bidentate ligands such as 1,10-phenanthroline and 2,2′-bipyridine gives different compositions depending on the ligand and the metal salts used. For example, the reaction of nickel(II) acetate with pyromellitic dianhydride in the presence of 1,10-phenanthroline results in the formation of a carboxylato bridged nickel(II) metallacycle through the ring opening reaction of pyromellitic dianhydride (PAH) at the 1 and 3-positions, whereas a mononuclear tetra-aqua 2,2′-bipyridine nickel(II) complex is formed in a similar reaction of nickel(II) acetate through ring opening at the 1,4-position of PAH. Mononuclear cobalt(II) dicarboxylate complexes are formed from the ring opening reaction of pyromellitic dianhydride in methanol in the presence of the nitrogen donor ligands 1,10-phenanthroline or 2,2′-bipyridine. Copper(II) chloride on reaction with PAH and 2,2′-bipyridine gives a mononuclear complex via ring opening at the 1 and 4-positions; having chlorides inside and outside the coordination sphere. Whereas, the reaction of copper(II)acetate gives dinuclear copper complexes having a monodentate carboxylato bridge arising from the carboxylato groups at the 1 and 4-positions on the aromatic ring. The crystal structures of all the complexes have been determined.

Full text of DOI:10.1016/j.poly.2007.05.054

Polyhedron 26 (2007) 4479–4488
www.elsevier.com/locate/poly
Ring opening reactions of pyromellitic dianhydride for the synthesis
of first row transition metal dicarboxylate complexes
*
Abhilasha M. Baruah, Anirban Karmakar, Jubaraj B. Baruah
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, Assam, India
Received 3 May 2007; accepted 31 May 2007
Available online 15 June 2007
Abstract
The ring opening reaction of pyromellitic dianhydride by methanol is an effective method to prepare first row transition metal dicarb-
oxylate complexes. The reactions of different first row transition metal salts with pyromellitic dianhydride in the presence of nitrogen
donating bidentate ligands such as 1,10-phenanthroline and 2,20-bipyridine gives different compositions depending on the ligand and
the metal salts used. For example, the reaction of nickel(II) acetate with pyromellitic dianhydride in the presence of 1,10-phenanthroline
results in the formation of a carboxylato bridged nickel(II) metallacycle through the ring opening reaction of pyromellitic dianhydride
(PAH) at the 1 and 3-positions, whereas a mononuclear tetra-aqua 2,20-bipyridine nickel(II) complex is formed in a similar reaction of
nickel(II) acetate through ring opening at the 1,4-position of PAH. Mononuclear cobalt(II) dicarboxylate complexes are formed from the
ring opening reaction of pyromellitic dianhydride in methanol in the presence of the nitrogen donor ligands 1,10-phenanthroline or 2,20-
bipyridine. Copper(II) chloride on reaction with PAH and 2,20-bipyridine gives a mononuclear complex via ring opening at the 1 and 4-
positions; having chlorides inside and outside the coordination sphere. Whereas, the reaction of copper(II)acetate gives dinuclear copper
complexes having a monodentate carboxylato bridge arising from the carboxylato groups at the 1 and 4-positions on the aromatic ring.
The crystal structures of all the complexes have been determined.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Ring-opening reactions; Anhydride; Metal carboxylates; Metallacycles; Coordination polymers
1. Introduction
able nickel metallacycles contain metal–carbon bonds
along with the metal carboxylate bond [4]. Many of the
first row transition metal complexes can have six coordina-
tion, thus to design a coordination polymer or cyclic com-
plexes some of these sites need to be occupied by labile
ligands and some by non-labile ligands. One of the ways
to generate poly carboxylato anions in situ is by ring open-
ing reactions of dianhydrides. Ring opening reactions of
anhydrides have been well studied for the synthesis of
ordinary esters [5a] as well as chiral esters [5b,5c]. The ring
opening reaction of pyromellitic dianhydride by an alcohol
can lead to 2,6-dicarbomethoxy benzene 1,4-dicarboxylic
acid or 2,4-dicarbomethoxy benzene 1,5-dicarboxylic acid
[5a]. In the present study, we have chosen pyromellitic
dianhydride as a substrate for the ring opening reactions
in order to establish the selectivity of the metal in the ring
Bonding of carboxylato groups in metallo-organic
frameworks has been of great interest to material chemists
as well as to synthetic chemists [1]. The complexity
involved in such systems is being reviewed from time to
time; yet there are big challenges in designed syntheses
[2]. An understanding of the binding ability of carboxylate
group/s to transition metal ions [3] is useful to delineate the
type/s of bonding as well as to have control on directed
syntheses. It is also a well known fact that metallacycles
containing a carboxylate ligand with a metal like nickel
are useful in organic synthesis; however, commonly avail-
*
Corresponding author. Tel.: +91 361 2582301; fax: +91 361 2690762.
E-mail address: juba@iitg.ernet.in (J.B. Baruah).
0277-5387/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.05.054

4480
A.M. Baruah et al. / Polyhedron 26 (2007) 4479–4488
O
O
+
M(OAc)2
+
L
L
O
O
O
O
L'
L'
ROH
ROH
ROH
2-
RO
OR
O
O
O
O
O
O
L
L
2+
OR
O
O
MLn
L' M
L'
RO
O
O
O
L
L
L'
M
L'
OO
L'
O
O
RO
O
O
L
L
*
O
OR
O
O
O
M
O
L'
O
O
Or other isomer
O
O
OR
RO
OO
OO
n
*
RO
OR
L' is a monodentate ligand
Scheme 1. Some possibilities for metal carboxylate complex formation by ring opening reactions of pyromellitic dianhydride by alcohols.
opening, as illustrated in Scheme 1 and also to use the ring
opening reactions for formation of metal complexes of first
row transition metals.
ture was cooled and transferred to a solution of 2,20-bipyri-
dine (234 mg, 3 mmol) in 10 ml of methanol. The mixture
was stirred for half an hour, after which time a green col-
ored solution was obtained and filtered. The filtrate was
kept undisturbed for crystallization to give crystals of A
(isolated yield 87%). Elemental Anal. Calc. for
C24H18N2NiO4: C, 63.01; H, 3.94. Found C, 63.03; H,
3.96%. IR (KBr, cmꢀ1): 3434(b), 1593(m), 1523(s),
1426(s), 1073(w), 1026(w), 857(m), 726(m), 734(m),
685(m). Visible kmax, 645 nm (eethanol = 9.11 Mꢀ1 cmꢀ1).
2. Experimental
2.1. General
The reagents used here were purchased from commer-
cial sources (Sigma–Aldrich or Fluka) and were used as
received unless otherwise stated. The X-ray diffraction
data were collected at 296 K with Mo Ka radiation
2.3. The ring opening reactions of pyromellitic dianhydride
˚
(k = 0.71073 A) using a Bruker Nonius SMART CCD
diffractometer equipped with a graphite monochromator.
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. The structures were solved by direct
methods and refined by full-matrix least-squares calcula-
tions using SHELXTL software. All the non-H atoms were
refined in the anisotropic approximation. The crystallo-
graphic parameters of the compounds studied are given
in Table 1.
The complexes with different compositions are prepared
by reactions of in situ generated 2,5-dicarbomethoxy ben-
zene 1,4-dicarboxylic acid or its isomer 2,4-dicarbometh-
oxy benzene 1,5-dicarboxylic acid (abbreviated as LH2)
with different metal salts and nitrogen containing heterocy-
cles such 1,10-phenanthroline (phen) or 2,20-bipyridine
(bpy) as follows:
2.3.1. Synthesis of dicarboxylate complexes
[Ni2L2(phen)2(CH3OH)2(H2O)2] Æ 2CH3OH [B]: To a
solution of pyromellitic dianhydride (218 mg, 1 mmol) in
methanol (15 ml) a solution of nickel(II) acetate tetrahy-
drate (248 mg, 1 mmol in 5 ml methanol) was added, giving
a green cloudy solution. To this solution a solution of 1,10-
phenanthroline (200 mg, 1 mmol in 5 ml methanol) was
added, followed by stirring at room temperature for 1 h.
The solution was refluxed for 8 h and filtered, the filtrate
on standing gave complex B (isolated yield of pure crystals
14%). Elemental Anal. Calc. for C54H60N4Ni2O24: C, 51.21;
2.2. Synthetic procedures for the complexes
2.2.1. Synthesis of (2,20-bipyridyl)dibenzoato nickel(II)
[A]
Benzoic acid (733 mg, 6 mmol) and potassium hydrox-
ide (337 mg, 6 mmol) were mixed together with nickel(II)
chloride hexahydrate (710 mg, 3 mmol) in a mortar. The
whole content was heated at 100 °C for 30 min. The mix-

A.M. Baruah et al. / Polyhedron 26 (2007) 4479–4488
4481
Table 1
Crystallographic parameters of compounds A–H
A
B
C
D
E
F
G
H
Formulae
Molecular
weight
C24H18N2NiO4 C54H60N4Ni2O24 C22H24N2Ni1O12 C24H18N2O11Zn C32H28CoN4O12 C25H26CoN2O12 C26H25Cl2CuN4O6 C84H58Cu2N8O24
457.11
1266.48
567.12
575.79
719.51
605.41
623.94
1690.46
Crystal system monoclinic
monoclinic
P21/c
296(2)
monoclinic
P2/c
296(2)
triclinic
P1
296(2)
monoclinic
C2/c
296(2)
triclinic
P1
296(2)
triclinic
P1
296(2)
triclinic
P1
296(2)
ꢀ
ꢀ
ꢀ
ꢀ
Space group
Temperature
(K)
P21/n
296(2)
˚
Wavelength (A) 0.71073
0.71073
12.9793(17)
17.354(2)
14.1181(17)
90.00
116.589(3)
90.00
2843.7(6)
2
0.71073
9.5081(2)
10.3025(2)
14.6321(2)
90.00
120.6260(10)
90.00
1233.39(4)
2
0.71073
0.71073
25.5622(17)
15.8261(17)
19.5923(16)
90
110.010(6)
90
7447.6(11)
8
0.71073
0.71073
0.71073
˚
a (A)
7.4322(2)
16.7224(4)
17.3528(4)
90.00
101.3810(10)
90.00
10.7940(11)
11.2328(11)
11.5938(11)
77.154(6)
76.933(6)
72.510(6)
1287.9(2)
2
9.3313(3)
12.6779(6)
13.2919(4)
105.996(2)
110.445(10)
100.325(2)
1348.47(9)
2
8.0962(2)
12.9513(3)
13.9321(4)
70.8500(10)
84.086(2)
81.5520(10)
1362.70(6)
2
11.9018(11)
13.2077(11)
15.9836(15)
68.741(5)
78.601(6)
69.457(5)
2185.5(3)
1
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
3
˚
V (A )
2114.27(9)
4
Z
Density
1.436
1.479
1.527
1.485
1.283
1.491
1.521
1.284
(Mg mꢀ3
)
Absorption
coefficient
(mmꢀ1
Absorption
correction
F(000)
0.951
0.750
none
0.854
none
1.016
none
0.522
none
0.703
none
1.046
none
0.563
none
)
none
944
1320
588
588
2968
626
640
868
Total number of 13,307
reflections
31,303
13,284
9359
50,847
6549
16,125
25,024
Reflections,
5195
7073
2971
4150
9061
5485
6033
10,090
I > 2r(I)
Maximum 2h (°) 29.09
Ranges (h, k, l) ꢀ9 6 h 6 9,
ꢀ22 6 k 6 19,
28.39
28.28
28.28
28.30
28.33
28.29
28.44
ꢀ17 6 h< = 17, ꢀ12 6 h< = 12, ꢀ14 6 h< = 11, ꢀ34 6 h< = 33, ꢀ34 6 h< = 33, ꢀ12 6 h< = 12,
ꢀ15 6 h< = 15,
ꢀ16 6 k 6 17,
ꢀ17 6 l 6 21
91.7
ꢀ21 6 k 6 23, ꢀ13 6 k 6 13,
ꢀ14 6 k 6 12,
ꢀ15 6 l 6 15
69.6
ꢀ21 6 k 6 18,
ꢀ25 6 l 6 25
98.0
ꢀ21 6 k 6 18,
ꢀ25 6 l 6 25
97.3
ꢀ16 6 k 6 16,
ꢀ17 6 l 6 17
89.1
ꢀ23 6 l 6 21
ꢀ18 6 l 6 18
ꢀ19 6 l 6 19
Complete to 2h 91.8
(%)
99.3
96.5
Refinement
method
full- matrix least- full-matrix least- full- matrix least- full- matrix least- full- matrix least- full- matrix least- full- matrix least- full-matrix least-
squares on F2
squares on F2
squares on F2
squares on F2
squares on F2
squares on F2
squares on F2
squares on F2
Data/restraints/ 5195/0/280
parameters
7073/0/397
2971/0/169
4150/0/353
9061/0/449
6549/0/364
6033/0/373
10090/0/535
Goodness-of-fit 1.031
1.018
1.010
1.056
1.065
1.045
1.020
1.070
(F2)
R indices
[I > 2r(I)]
R indices (all
0.0298
0.0432
0.0640
0.1284
0.0374
0.0424
0.0666
0.1106
0.0920
0.1606
0.0497
0.1229
0.0425
0.0866
0.1022
0.1589
data)
H, 4.74. Found C, 51.28; H, 4.87%. IR (KBr, cmꢀ1):
3421(bs), 30669(s), 2951(s), 1726(s), 1624(s), 1587(s),
1496(s), 1430(s), 1392(s), 1342(s), 1305(s), 1258(s),
1106(s), 1052(w), 963(w), 920(w), 867(m), 854(s), 797(s),
779(s), 728(s), 671(s), 643(w), 582(s), 540(s).
[Ni(bpy)(H2O)4]L [C] was prepared in an analogous
manner except the reaction was carried out with 2,20
-bipyridine (reaction time 12 h at room temperature) and
crystals were obtained after 15 days standing (isolated yield
of pure crystals 43%). Elemental Anal. Calc. for
C22H24N2NiO12: C, 46.55; H, 4.23. Found C, 46.48; H,
4.31%. IR (KBr, cmꢀ1): 3525(s), 3105(bs), 1716(s),
1567(s), 1493(bs), 1440(s), 1405(s), 1335(s), 1305(s),
1295(s), 1173(w), 1194(w), 1111(s), 1057(w), 1027(m),
956(m), 903(m), 804(s), 763(s), 736(s).
[{Zn(phen)(H2O)L} Æ 3H2O]n [D] was prepared in a
similar manner to B but zinc(II)acetate tetrahydrate was
used as the metal salt (stirred at room temperature for
1 h followed by heating at 60 °C for 8 h and allowed to
crystallise, crystals obtained after about 7 days, yield of
pure crystals 25%). Elemental Anal. Calc. for
C24H18N2O11Zn: C, 50.02; H, 3.13. Found C, 50.11; H,
3.26%. IR (KBr, cmꢀ1): 3566(s), 3421(bs), 3066(bs),
1726(s), 1624(s), 1587(bs), 1430(s), 1392(s), 1342(s),
1258(s), 1106(s), 963(w), 854(s), 797(s), 728(s).
[Co(bpy)2(H2O)L] Æ 4H2O [E]: Cobalt(II) acetate
tetrahydrate (249 mg, 1 mmol) was added to a solution of
pyromellitic dianhydride (218 mg, 1 mmol) in methanol
(15 ml) with constant stirring. A turbid pink coloured solu-
tion appeared, and to this mixture a methanolic solution of
2,20-bipyridine (312 mg, 2 mmol in 5 ml) was added drop-
wise with constant stirring. The solution turned dark yel-
low and stirring was continued for 2 h. On crystallization
pure crystals of complex E could be obtained after 6 days;
isolated yield of pure crystals 34%. Elemental Anal. Calc.
for C32H28CoN4O12: C, 53.36; H, 3.89. Found C, 53.24;
H, 3.96%. IR (KBr, cmꢀ1): 3402(s), 1738(s), 1717(s),
1575(s), 1493(bs), 1474(s), 1443(s), 1418(s), 1376(s),
1295(s), 1250(s), 1111(s), 1022(s), 850(s), 797(s) 772(s). Vis-
ible kmax, 482 nm (eMeOH = 5.94 molꢀ1 dm3 cmꢀ1). Mag-
netic moment 6.51 BM.
[Co(phen)(H2O)3L]CH3OH (F): Cobalt(II) acetate
tetrahydrate (249 mg, 1 mmol) was added to a solution of
pyromellitic dianhydride (218 mg, 1 mmol) in methanol

4482
A.M. Baruah et al. / Polyhedron 26 (2007) 4479–4488
(15 ml) with constant stirring. A turbid pink coloured solu-
tion appeared, and to this mixture a methanolic solution of
1,10-phenanthroline (200 mg, 1 mmol in 5 ml) was added
dropwise with constant stirring. The solution turned dark
yellow. The reaction mixture was stirred at room tempera-
ture for further 2 h. A turbidity appeared and the reaction
mixture was heated at 60 °C for 6 h. On crystallization pure
crystals of complex E could be obtained after about 12 days;
isolated yield of pure crystals 30%. Elemental Anal. Calc.
for C25H26CoN2O12: C, 49.55; H, 4.29. Found C, 49.58;
H, 4.30%. IR (KBr, cmꢀ1): 3421(bs), 1726(s), 1598(s),
1518(w), 1494(bs), 1427(s), 1347(w), 1296(w), 1255(s),
1196(w), 1134(w), 1108(s), 1013(s), 868(w), 848(s). Visible
complex (2,20-bipyridyl)dibenzoato nickel(II) [A] could be
easily prepared in this reaction. Alternatively the same
complex also could be prepared by reaction of nickel(II)
chloride with potassium benzoate followed by treatment
with 2,20-bipyridine. This complex has an IR absorption
at 1593 cmꢀ1 (mCOO ) corresponding to stretching of a
ꢀ
coordinated carboxylate group; this absorption is lower
in frequency than in free benzoic acid (mCOO 1680 cmꢀ1).
ꢀ
The synthetic procedure for complex A reported here is
much simpler than the earlier reported procedure [6].
The complex was further characterised by determining
its crystal structure. It has a distorted octahedral geometry
around the nickel with benzoate groups acting as chelating
ligands and one bipyridine ligand occupying the remainder
of the coordination sites, as shown in Fig. 1. The carbox-
ylate chelates are unsymmetrical, for example, the Ni1–O1
and Ni1–O2 bond distances are not equal; same is true for
the Ni1–O3 and Ni1–O4 pair. Although the structure of
the complex A is very simple, it serves as a starting point
to design the synthesis for low dimensional coordination
polymers or cyclic complexes.
The reaction of pyromellitic dianhydride with nickel(II)
acetate tetrahydrate in methanol followed by treatment with
1,10-phenanthroline gave the metallacycle [B] (Scheme 2).
By directly using 1,3-benzene dicarboxylic acid as a precur-
sor, we could not prepare such cyclic complexes. However,
adopting the ring opening methodology of pyromellitic
dianhydride, as illustrated in Scheme 2, we could prepare
novel metallacycles of nickel(II). The product has a charac-
teristic ester frequency at 1726 cmꢀ1, showing that the
pyromellitic dianhydride ring has opened to form the cor-
responding ester. The identity of the cyclic product has
been established by crystallography (Fig. 2). In this com-
plex each nickel(II) ion is attached to two mono-dentate
k
max, 487 nm (eMeOH = 38.75 molꢀ1 dm3 cmꢀ1).
[Cu(bpy)2Cl]Cl Æ LH Æ 2H2O [G]: To a methanolic solu-
tion of pyromellitic dianhydride (218 mg, 1 mmol, 15 ml)
cupric chloride dihydrate (171 mg, 1 mmol) was added
and the mixture was stirred for 15 min at room tempera-
ture. The solution became light green. A solution of
sodium acetate (82 mg, 1 mmol) in methanol (2 ml) was
added. To this mixture a methanolic solution of 2,20-bipyri-
dine (0.312 g, 2 mmol, 10 ml) was added dropwise with
constant stirring. A turbid solution of intense blue colour
appeared. The mixture was refluxed for 8 h and filtered.
The residue was discarded and solution was kept for crys-
tallization. Blue crystals of G appeared after about 15 days;
isolated yield of pure crystals, 28%. Elemental Anal. Calc.
for C26H25Cl2CuN4O6: C, 50.00; H, 4.01. Found C,
50.12; H, 4.03%. IR (KBr, cmꢀ1): 3348(s), 1732(s),
1599(s), 1493(bs), 1472(s), 1442(s), 1400(s), 1311(s),
1261(s), 1105(w), 1030(w), 1013(w), 775(s), 769(s). Visible
k
max, 737 nm (eMeOH = 0.85 molꢀ1 dm3 cmꢀ1). Molar con-
ductance in methanol 67.5 ohmꢀ1 cm2 molꢀ1
.
[Cu2(phen)4L2]L Æ LH [H]: To a solution of pyromel-
litic dianhydride (218 mg, 1 mmol) in methanol (15 ml)
cupric acetate monohydrate (199 mg, 1 mmol) was added.
A blue precipitate was formed. The mixture was stirred
for 30 min. Then a methanolic solution of 1,10-phenan-
throline (200 mg, 1 mmol, 10 ml) was added dropwise.
The reaction mixture was further stirred overnight and fil-
tered. The filtrate, having a light blue colour, was kept for
crystallization. After five days pure crystals of H were
obtained; isolated yield of pure crystals 19%. Elemental
Anal. Calc. for C84H58Cu2N8O24: C, 59.63; H, 3.43. Found
C, 59.70; H, 3.51%. IR (KBr, cmꢀ1): 3421(s), 1718(s),
1591(s), 1429(s), 1396(s), 1383(s), 1248(s), 1109(s), 852(s),
812(w), 724(s).
3. Results and discussion
In order to prepare different first row transition metal
carboxylates, as depicted in Scheme 1, we have tested
the feasibility of a basic one-pot reaction of a nitrogen
donor ligand and carboxylic acid with a first row transi-
tion metal acetate. For this purpose the reaction of
benzoic acid with nickel(II) acetate in the presence of 2,20-
bipyridine was carried out in an one pot reaction. The
Fig. 1. The structure of (2,20-bipyridyl)dibenzoato nickel(II) [A] (Drawn
with 20% thermal ellipsoids; hydrogen atoms are omitted for clarity).
˚
Some bond distances (A) and angles (°) are Ni1–N2, 2.03; Ni1–N1, 2.05;
Ni1–O4, 2.05; Ni1–O2, 2.07; Ni1–O1, 2.15; Ni1–O3, 2.15; \N2–Ni1–N1,
79.48; \N2–Ni1–O4, 100.69; \N1–Ni–O4, 99.99; \N2–Ni1–O2, 96.40;
\N1–Ni1–O2, 98.95; \N2–Ni1–O1, 96.26; \O4–Ni1–O1, 99.35; \N1–
Ni1–O3, 97.57; \O1–Ni1–O3 91.91.

A.M. Baruah et al. / Polyhedron 26 (2007) 4479–4488
4483
MeO
OMe
O
O
O
O
O
O
O
HOMe
O
O
N
N
MeOH
Reflux
N
N
OH2
N
N
O
+
+ Ni(OAc)2.4H2O
Ni
Ni
OH2
O
O
O
O
HOMe
O
O
O
O
pyromellitic dianhydride
2.MeOH
MeO
OMe
B
Scheme 2.
Fig. 3. The crystal structure of C (20% thermal ellipsoids; all hydrogen
˚
atoms are omitted for clarity). Some important bond distances (A), Ni1–
N1, 2.06; Ni1–N10, 2.06; Ni1–O1, 2.07; Ni1–O2, 2.07; Ni1–O10, 2.07; and
bond angles (°) \N1–Ni1–N10, 78.85; \N1–Ni1–O1, 90.86; \N1–Ni–O2,
94.41.
Fig. 2. The crystal structure of the cyclic nickel complex B (20% thermal
˚
ellipsoids; for clarity hydrogen atoms are omitted). The bond distances (A)
Ni1–N1, 2.05; Ni1–N2, 2.09; Ni1–O10, 2.09; Ni1–O1, 2.04; Ni1–O7, 2.08;
Bond angles (°) \N1–Ni1–N2, 79.27; \N1–Ni1–O10, 93.77; \N1–Ni–O7,
93.68; \O1–Ni1–O9, 86.80; \O1–Ni1–O10, 92.04; \O7–Ni1–O10, 91.69;
\O1–Ni1–O7, 89.71.
temperature which prefers to form thermodynamically sta-
ble products. In general such ring opening reactions of
pyromellitic dianhydride by alcohol (without any other
reagent being added) leads to equimolar amounts of the
1,3- and 1,4-dicarboxylic acids [5a]. Formation of C pro-
vides a clue that under adequate reaction conditions the
1,4-dicarboxylato anion should act as ligand by replacing
the labile aqua ligand, leading to polymeric materials.
The TGA of the compound C shows a weight loss at
129 °C due to the loss of four water molecules (experimen-
tal wt. loss 13.4%; theoretical 12.7%). When the IR spec-
trum of compound C is compared with the spectrum
after heating C at 130 °C for 15 min, the absorption at
3525 cmꢀ1 disappears and also the carboxylate frequency
is shifted from 1567 cmꢀ1 to 1578 cmꢀ1, showing coordina-
tion of carboxylate group/s (Fig. 4a). On heating at 130 °C
compound C becomes very insoluble in all common sol-
vents. The process is attributed to the dehydration of the
aqua ligands. The dehydration may lead to dinuclear spe-
cies as observed. However, the poor solubility of the prod-
uct prevented us from further study in this direction. On
prolonged heating of compound C at 200 °C, a profound
effect on the carbonyl frequency of the ester group could
carboxylate groups from the dicarboxylic acids; these
groups occupy cis-positions around the nickel ion. Addi-
tional to these, the six coordinations positions around the
nickel(II) ion are completed by 1,10-phenanthroline, water
and methanol molecules. The carboxylate groups attached
at the 1,3-position of the aromatic rings facilitate the for-
mation of the cyclic metallacarboxylato complex. The
coordinated water molecules on two metals are trans to
each other and the same is the case with the methanol mol-
ecules. Fig. 3.
In this particular case the other complex from the 1,4-
dicarboxylate isomer could not be obtained and insolubil-
ity of the products has prevented further characterization
of open chain compounds, if any, formed in the reaction.
However, from the room temperature reaction of nickel(II)
acetate tetrahydrate with pyromelletic dianhydride and
2,20-bipyridine in methanol we could obtain the mononu-
clear nickel complex C (Scheme 3). Formation of the
1,4-dicarboxylato complex occurs due to the synthetic
procedure, as we have carried out the reaction at room

4484
A.M. Baruah et al. / Polyhedron 26 (2007) 4479–4488
2+
O O
2-
O
OMe
O
O
O
O
OH2
N
N
MeOH
RT
N
N
OH2
OH2
O
+
+ Ni(OAc)2.4H2O
Ni
MeO
OH2
O
O
O
O
C
Scheme 3.
be seen. A new absorption at 1778 cmꢀ1 appeared, in addi-
tion to an absorption at 1717 cmꢀ1 (Fig. 4a), and the
remaining peaks became slightly broadened without much
shift being observed. The OH group absorption peak at
3105 cmꢀ1 of the parent compound was shifted to
3368 cmꢀ1 on heating at 200 °C. This suggests that the
complex formed on heating at 200 °C absorbs moisture.
We have tried making the anhydrous derivative, but the
attempts were not successful.
Although we could not succeed in characterising the
polymeric product/s from the reactions with nickel, from
a similar reaction of pyromellitic dianhydride with zinc(II)
acetate and 1,10-phenanthroline in methanol, a crystalline
compound D having a polymeric structure (Scheme 4)
could be characterised. This dicarboxylate complex D has
five coordination around the zinc ions (Fig. 4b). In the
polymer, each zinc ion has a distorted square-pyramidal
geometry. The five coordination positions are completed
by a water molecule, 1,10-phenanthroline and two monod-
entate carboxylate groups. The crystals qualities of the
polymer were not very good for crystallography, but the
available data are good enough to describe the polymeric
chain. Since all these reactions gave a low yield of poly-
meric product in the pure form, we looked into the other
possible products in these reactions.
Fig. 4a. FT-IR (KBr) spectra of C (a) at room temperature, (b) after C
was heated at 130 °C for 15 min, (c) after C was heated at 200 °C for
30 min.
Due to limited solubility of the products we looked care-
fully for a reaction from which we could obtain different
types of crystals. From the reaction of zinc(II) chloride
with pyromellitic dianhydride in methanol with 2,20-bipyri-
dine (1:1:2 molar ratio); we got two crystalline compounds
and a white solid. The crystal structures of the crystalline
compounds were determined and they were found to be
[Zn(bpy)3Cl2] Æ 5H2O and Zn(bpy)Cl2. The compound
[Zn(bpy)3Cl2] Æ 5H2O has an analogous structure to that
already reported [8]. The white solid compound has charac-
teristic features of 2,5-dicarbomethoxy benzene1,4-dicar-
boxylic acid as well as bipyridine, suggesting it to be a
zinc complex, but lack of crystal structure and limited sol-
ubility limits our further discussion on this issue. However,
it is clear that in these reactions metal bipyridine complexes
are formed as side products, lowering the yield of the poly-
meric product, which may be attributed to such side reac-
tions which compete with metal–oxygen bond formation
by metal–nitrogen bond formation. It may also be noted
Fig. 4b. (a) The asymmetric unit of the zinc complex D showing the
coordination environment. Some important bond distances (A) Zn1–N1,
˚
2.17; Zn1–N2, 2.10; Zn1–O1, 2.05; Zn1–O3, 2.00; Zn1–O6, 2.22; and bond
angles (°) \N1–Zn1–N2, 78.13; \O1–Zn1–O3, 90.13; \O3–Zn–O6,
105.91. (b) The polymeric structure of D (all hydrogen atoms and water
of crystallisation are omitted for clarity; drawn with 20% thermal
ellipsoids).

A.M. Baruah et al. / Polyhedron 26 (2007) 4479–4488
4485
N
O
N
xH2O
O
O
O
H2O Zn
O
MeOH
Reflux
N
N
O
O
O
+
+
Zn(OAc)2.2H2O
O
O
MeO
O
OMe
O
O
OMe
MeO
O O
O
O O
n
D
Scheme 4.
that the use of a 1:3 stoichiometric ratio of zinc to biden-
tate ligand, namely 2,2’-bipyridine, led to the formation
of [Zn(bpy)3]Cl2 Æ 5H2O along with formation of 2,5-dicar-
bomethoxy benzene 1,4-dicarboxylic acid (the free acid is
of the carboxylato groups remains free and participates in
hydrogen bonding in the self-assembled structure. The
structure of the cobalt complex E obtained from the reac-
tion of cobalt(II) acetate with pyromellitic dianhydride in
the presence of 2,20-bipyridine is illustrated in Fig. 5. The
complex has a distorted octahedral structure with a monod-
entate carboxylate group and the other carboxaylate group
at the 4-position remains as the carboxylato anion. Besides
the two 2,20-bipyridine ligands, the complex has one water
molecule in the coordination sphere. In addition to the one
aqua ligand the complex has four water of crystallisation
molecules in the lattice and has the composition [Co(bpy)2-
(H2O)L] Æ 4H2O (where LH2 = 2,5-dicarbomethoxy ben-
zene 1,4-dicarboxylic acid). From the crystallographic
study we could not locate the positions of the six hydrogen
atoms around the three lattice water molecules. However,
thermogravimetric analysis confirmed the loss of water
molecules corresponding to the four molecules of water of
crystallisation. The presence of ester group absorptions at
1738 cmꢀ1 and 1717 cmꢀ1 suggests that the two ester
groups have slightly different environments and supports
the results obtained from the crystallographic study. The
complex has a visible absorption at 482 nm in methanol
due to a d–d transition of the d7-electronic configuration.
It has magnetic moment of 6.51 BM corresponding to a
1
characterised by recording its IR and H NMR spectra
and comparing them with those of an authentic sample).
This suggests that the anhydride ring opening and the com-
plex formation both occur in a competitive manner and the
reaction can be represented by Scheme 5. This shows that
the competitive reaction between the nitrogen donating
ligands and carboxylic acid groups to occupy the maximum
coordination sites makes the yield of the carboxylate com-
plexes less. Nevertheless it provides a better method to
obtain characterisable metal dicarboxylate complexes hav-
ing optimum solubility.
Recently it has been shown that zinc complexes of phe-
nylene diacrylic acid form clusters leading to hydrogen stor-
age material [7]. However, complex D presented here has a
much simpler structure with a five coordination geometry
around the zinc ions. From the available data, use of a
2:1 stoichiometry of the chelating ligand with respect to
the metal acetate, along with one equivalent of pyromellitic
dianhydride, has not been fruitful for polymeric product
formation. For example, we have observed a self-assembled
structure from such a reaction with cobalt(II), in which one
O
O
N
N
Zn
N
Cl2.5H2O
MeO
O
ZnCl2.4H2O +
+
+
O
N
N
N
N
N
N
Zn
O
N
O
Cl
Cl
O
O
OH
+
HO
O
O
OMe
Scheme 5. Products from the reaction of zinc(II) chloride with pyromellitic dianhydride in methanol in the presence of 1,10-phenanthroline.

4486
A.M. Baruah et al. / Polyhedron 26 (2007) 4479–4488
We have also carried out independent reactions of
pyromellitic dianhydride with methanol and copper(II)
chloride, as well as with copper(II) acetate in the presence
of bidentate aromatic heterocyclic ligands. Two different
products obtained from such reactions are illustrated in
Scheme 6. The reaction of copper(II) chloride with
pyromellitic dianhydride in methanol with 2,20-bipyridine
led to the formation of a dinuclear complex. The
complex has two 2,20-bipyridine ligands attached to each
copper and is bridged by two monodentate carboxylato
groups at the 1,4-positions of 2,5-dicarbomethoxy
benzene 1,4-dicarboxylic acid. The complex has the
composition [CuCl(bpy)2]Cl Æ 2H2O Æ LH2; where LH2 is
2,5-dicarbomethoxy benzene 1,4-dicarboxylic acid and
bpy = 2,20-bipyridine. The ionic chloride is estimated by
treatment of G with dilute silver nitrate and weighing
the chloride as the silver chloride. An interesting feature
of the complex is that in the lattice the chloride ion is
hydrogen bonded to form a layered structure. The water
molecules occur in pairs and each pair are hydrogen
bonded to each other and are further hydrogen bonded
to the chloride anion as depicted in Fig. 7c. The hydro-
gen bonding forms alternate four and six member rings
and these rings are repeated along the crystallographic
a-axis. The chloride ions are in T-shape three coordina-
tion environments in the hydrogen bonding scheme. Such
a T-shaped hydrogen bonding environment in anion
binding is important in anion recognition by amides
and related compounds [9].
Fig. 5. The ORTEP (20% thermal ellipsoids) diagram of the cobalt(II)
complex obtained from the reaction of pyromellitic dianhydride, cobalt(II)
acetate and 2, 20-bipyridine in a 1:1:2 molar ratio in methanol (hydrogen
˚
atoms are omitted for clarity). Some important bond distances (A) Co1–
O9, 2.09; Co1–N3, 2.10; Co1–N2, 2.14; Co1–N1, 2.11; Co1–O2, 2.13 and
bond angles (°) \O1–Co1–O2, 91.75; \N1–Co1–N2, 76.35; \N2–Co2–
N3, 92.34; \N1–Co1–O2, 86.87.
d7-octahedral cobalt(II) complex. The reaction of pyromel-
litic dianhydride, cobalt(II) acetate and 1,10-phenanthroline
in a 1:1:1 molar ratio in methanol gave a complex having
the composition [Co(phen)(L)(H2O)3] Æ CH3OH, [F] and
its crystal structure is shown in Fig. 6. This compound has
a distorted octahedral geometry with one monodentate
carboxylate.
The compound has a weak absorption at 487 nm corre-
sponding to the d–d transition of an octahedral d7 cobal-
t(II) system. The TGA shows that three coordinated
water molecules are released on heating up to 250 °C,
and they are lost in three steps.
On the other hand, at room temperature the reaction of
copper(II) acetate with pyromellitic dinahydride at room
temperature followed by treatment with 1,10-phenanthro-
line (Scheme 6) led to a dinuclear complex having the
composition [Cu2(phen)4l-(L)]L Æ LH2; (phen = 1,10-phe-
nanthroline). In the complex two copper(II) ions are
bridged by the 2,5-dicarbomethoxy benzene 1,4-dicarboxy-
lato dianion. The charges on the metal ions are further neu-
tralised by a dianion of an uncoordinated dicarboxylic
acid. The structures of complexes G and H are shown in
Fig. 7a and b. The complex has also a neutral dicarboxylic
acid included as a co-crystal. In addition to the character-
istic IR absorptions of 1,10-phenanthroline, the complex
has the characteristic IR frequency for the ester groups at
1718 cmꢀ1
.
The bond distances in the two copper(II) complexes are
shown in Table 2. It clearly depicts that the compounds
have five coordinated square-pyramidal geometries around
the copper(II) centre. Complex G has a visible absorption
at 737 nm due to a d–d transition. The complex shows a
molar conductance of 67.5 ohmꢀ1 cm2 molꢀ1 in methanol
which suggests it is a 1:1 electrolyte. Complex H, being
highly insoluble in common solvents, prevented us from
comparing their visible spectra in solution.
In conclusion, we have demonstrated a simple yet very
useful synthetic methodology for the synthesis of first
row transition metal carboxylate complexes through ester-
ification of pyromellitic dianhydride. The ring opening
Fig. 6. The ORTEP (20% thermal ellipsoids) diagram of the cobalt(II)
complex obtained from the reaction of pyromellitic dianhydride, cobalt(II)
acetate and 1,10-phenathroline in a 1:1:1 molar ratio in methanol. Some
˚
important bond distances (A) Co2–O1, 2.16; Co2–N1, 2.10; Co2–N2, 2.14;
Co2–O2, 2.10; Co2–O3, 2.06; Co2–O5, 2.11 and bond angles (°) \O1–
Co2–O2, 88.29; \N1–Co2–N2, 77.80; \N2–Co2–O1, 84.20; \N1–Co1–
O2, 92.18.

A.M. Baruah et al. / Polyhedron 26 (2007) 4479–4488
4487
O
O
O
O
O
O
CuCl2.4H2O
MeOH
N
N
N
N
Cu(OAc)2.H2O
MeOH
2+
+
O O
O O
HO
OMe
2H2O
-O
N
N
OMe
Cl
N
O
N
O
OMe
O
Cu
N
Cl
O
Cu
N
N
N
MeO
OH
O-
MeO
HO
N
O
O
O
O
Cu
N
MeO
O
O
N
N
O O
OMe
G
MeO
OH
O
O
H
Scheme 6.
reactions of pyromellitic dianhydride with methanol can
lead to either 1,4-dicarboxylic acid or 1,5-dicarboxylic acid
depending on the metal salts and ancillary ligands used.
Several such metal complexes are characterised by crystal-
lography. In order to establish the selectivity in product
formation, a greater understanding of the ring opening
reactions by first row transition metal ions is needed. The
limited solubility of the products and formation of multiple
products made us take up selective cases where crystals
suitable for X-ray diffraction were obtained. Present results
suggest that the synergic competition between the nitrogen
donor and carboxylato ligand leads to the formation of dif-
ferent compositions of dicarboxylate complexes. The reac-
tion of chloride and acetate salts of copper in ring opening
reactions of pyromellitic dianhydride suggested that the
chloride occupies one of the coordination sites, yielding a
mononuclear carboxylate complex, whereas using cop-
per(II) acetate the product prefers coordination of the
dicarboxylic acid as a bridging ligand through the two
distal ends to give a dinuclear copper complex. From the
Fig. 7. The structure of (a) G and (b) H (drawn with 20% thermal ellipsoids) (c) the hydrogen bonding between the water of crystallisation and the
chloride anion in the lattice of G (chloride, hydrogen and oxygen are represented by green, white and red balls, respectively). (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of this article.)

4488
A.M. Baruah et al. / Polyhedron 26 (2007) 4479–4488
(i) O.R. Evans, H.L. Ngo, W. Lin, J. Am. Chem. Soc. 123 (2001)
Table 2
The selected bond parameters in G and H
10395;
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Angew. Chem., Int. Ed. 44 (2005) 3845;
(k) R. Xiong, X. You, B.F. Abrahams, Z. Xue, C. Che, Angew. Chem.,
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127 (2005) 5495;
˚
Bonds
Bond distances (A)
Angles
Bond angles (°)
For F
Cu1–N1
Cu1–Cl1
Cu1–N3
Cu1–N2
Cu1–N4
1.98
2.29
1.99
2.10
2.09
\N1–Cu1–Cl1
\N3–Cu1–Cl1
\N3–Cu1–N4
\N1–Cu1–N2
\N4–Cu1–Cl1
94.11
91.78
79.94
79.64
109.15
(m) S.G. Baca, M.T. Reetz, R. Goddard, I.G. Fillippova, Y.A.
Simonov, M. Gdaiec, N. Gerbeleu, Polyhedron 25 (2006) 1215.
[2] (a) F.A. Cotton, C. Lin, C.A. Murillo, Acc. Chem. Res. 34 (2001)
759;
For G
Cu1–N1
Cu1–N2
Cu1–N3
Cu1–N4
Cu1–O1
2.02
2.18
2.00
2.06
1.99
\N1–Cu1–O1
\N3–Cu1–O1
\N2–Cu1–N3
\N1–Cu1–N2
\N4–Cu1–O1
92.57
92.10
101.86
79.32
(b) M. Eddaoudi, D.B. Moler, H. Li, B. Chen, T.M. Reineke, M.
Keeffe, O.M. Yaghi, Acc. Chem. Res. 34 (2001) 319;
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Ouahab, V.V. Pavlishchuk, J. Am. Chem. Soc. 127 (2005) 12247;
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164.35
present results it is observed that the copper complexes
show 1,4-selectivity while the cobalt complexes show 1,5-
selectivity on the position of the carboxylic acid groups
in the ring opening reaction of pyromellitic dianhydride.
(g) M. Eddaoudi, H. Li, O.M. Yaghi, J. Am. Chem. Soc. 127
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Acknowledgement
[3] (a) A. Karmakar, R.J. Sarma, J.B. Baruah, Eur. J. Inorg. Chem. (2007)
643;
The authors thank the Department of Science and Tech-
nology, New-Delhi for financial support.
(b) A. Karmakar, R.J. Sarma, J.B. Baruah, Eur. J. Inorg. Chem. (2006)
4673;
(c) N. Barooah, A. Karmakar, R.J. Sarma, J.B. Baruah, Inorg. Chem.
Commun. 9 (2006) 1251;
Appendix A. Supplementary material
(d) A. Karmakar, R.J. Sarma, J.B. Baruah, Inorg. Chem. Commun. 9
(2006) 1169;
(e) K. Deka, R.J. Sarma, J.B. Baruah, Inorg. Chem. Commun. 9 (2006)
931;
(f) N. Barooah, R.J. Sarma, R.J. Baruah, Eur. J. Inorg. Chem. (2006)
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(g) A. Karmakar, K. Deka, R.J. Sarma, J.B. Baruah, Inorg. Chem.
Commun. 9 (2006) 836;
CCDC 638667–638669, 638978, 641189, 641651, 641190
and 644270 contain the supplementary crystallographic
data for 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 Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.poly.2007.
05.054.
(h) K. Deka, N. Barooah, R.J. Sarma, J.B. Baruah, J. Mol. Struct. 827
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