Steric effects in controlling co-ordination environment in zinc 2-nitrobenzoate complexes

Article abstract of DOI:10.1016/j.ica.2008.01.044

A dinuclear zinc complex having exclusive bis(unidentate) carboxylato co-ordination mode is synthesized through two-step reactions, involving solid phase reaction followed by solution reaction. The formation of such dinuclear species is governed by the ligand under consideration. For example, 2-nitrobenzoate zinc complex having 1,10-phenanthroline is mononuclear whereas a similar complex with 8-aminoquinoline is dinuclear having bis(unidentate) carboxylato co-ordination mode. The reasons for the formation such complex is explained on the basis of the distance of separation of uncoordinated oxygen atoms of the carboxylate oxygen of mononuclear and dinuclear complexes. Zinc 2-nitrobenzoate complexes having 2,2′-bipyridine is mononuclear but with 2-aminopyrimidine has a dinuclear paddle wheel structure. Solution phase reaction of 2-nitrobenzoic acid with zinc(II)acetate in the presence of 8-aminoquinoline gives bis-(8-aminoquinoline)zinc(II) 2-nitrobenzoate.

Full text of DOI:10.1016/j.ica.2008.01.044

Available online at www.sciencedirect.com
Inorganica Chimica Acta 361 (2008) 2777–2784
www.elsevier.com/locate/ica
Steric effects in controlling co-ordination environment in zinc
2-nitrobenzoate complexes
Abhilasha M. Baruah, Anirban Karmakar, Jubaraj B. Baruah *
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781 039, Assam, India
Received 23 November 2007; received in revised form 30 January 2008; accepted 31 January 2008
Available online 8 February 2008
Abstract
A dinuclear zinc complex having exclusive bis(unidentate) carboxylato co-ordination mode is synthesized through two-step reactions,
involving solid phase reaction followed by solution reaction. The formation of such dinuclear species is governed by the ligand under
consideration. For example, 2-nitrobenzoate zinc complex having 1,10-phenanthroline is mononuclear whereas a similar complex with
8-aminoquinoline is dinuclear having bis(unidentate) carboxylato co-ordination mode. The reasons for the formation such complex is
explained on the basis of the distance of separation of uncoordinated oxygen atoms of the carboxylate oxygen of mononuclear and dinu-
clear complexes. Zinc 2-nitrobenzoate complexes having 2,20-bipyridine is mononuclear but with 2-aminopyrimidine has a dinuclear pad-
dle wheel structure. Solution phase reaction of 2-nitrobenzoic acid with zinc(II)acetate in the presence of 8-aminoquinoline gives bis-(8-
aminoquinoline)zinc(II) 2-nitrobenzoate.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Paddle wheel structure; Bis-unidentate-carboxylate bridge; Mononuclear zinc complexes; Binuclear zinc complexes; Solid phase synthesis;
Crystal structure
1. Introduction
dentate) bridged complexes is known as the secondary part
in polynuclear carboxylate complexes [3d,4]. Mode II of
Fig. 1 is not independently observed. In fact the literature
examples on such bridges are part of other bonding mode,
arising out of steric requirements [4]. Thus, it is essential to
understand the condition to achieve such bis(unidentate)
carboxylato complex (Mode II of Fig. 1) in simple metal
carboxylate and also as an independent co-ordination
mode. We have recently shown that depending on the
method of synthesis mono-nuclear and dinuclear metal car-
boxylate complexes can be prepared [5]. In the case of 2-
nitrobenzoate complexes polymorphs as well as pseudo-
polymorphs are also established [5a–i]. In these cases it
was illustrated that the nitro groups can be oriented in dif-
ferent directions and that it is sensitive to environment.
Recently competitive reactivity of pyrazole with carboxyl-
ate ligands is used to synthesize interesting zinc complexes
[1i]. In this study we report a novel example of bis(uniden-
tate) carboxylato bridged dinuclear zinc carboxylate com-
plex, which is much different from the conventional
Carboxylate chemistry is indispensable for the construc-
tion of inorganic motifs having interesting material proper-
ties [1]. The success in designing the target metal
carboxylate synthon/s depends on the modulation of vari-
ous co-ordination modes of carboxylates for synthetic [2]
and biologically occurring molecules [2f,2g]. Among the
different co-ordination modes chelating, monodentate,
and bridging are the three modes which generally decide
the structural features of inorganic carboxylate complexes
[3], and they are also studied from the theoretical point
of view also [3f]. Some possible bridging modes for metal
carboxylate complexes are shown in Fig. 1. Bridging
through two oxygen atoms of carboxylato group (biden-
tate) generally leads to paddle wheel structure or polymeric
structures [2a,4]. But the carboxylato group having bis(uni-
*
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 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.01.044

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A.M. Baruah et al. / Inorganica Chimica Acta 361 (2008) 2777–2784
1H), 7.44 (dd, J = 4 Hz, 1H), 7.57–7.67 (m, 4H), 7.75–
M
O
O
7.80 (m, 4H), 8.15 (dm, J = 8 Hz, 1H), 8.71 (dm,
J = 4 Hz, 1H). IR (KBr, cmꢀ1) 3448(bw), 3027(w),
1659(s), 1576(s), 1526(s), 1512(s), 1409(s), 1360(s),
1379(s), 1318(m), 1145(w), 808(m), 783(s), 742(s), 698(s).
Elemental Anal. Calc. for C46H36N8O18Zn2: C, 49.30;
H,3.22. Found: C, 49.46; H, 3.28%.
O
M
O
M
O
O
M
M
M
M
I
II
III
Fig. 1. Different bridging modes of carboxylate group.
2.2.2. [Zn(oNBz)2(PHEN)(H2O)] (2)
1H NMR (DMSO-d6) 7.44 (t, J = 8 Hz, 2H), 7.53 (t,
J = 8 Hz, 2H), 7.55–7.63 (m, 4H), 8.08 (br s, 1H), 8.87
paddle wheel structure by using 8-aminoquinoline (AQ) as
ancillary ligand, and compare its structure with different
mono-nuclear and paddle wheel zinc(II) complexes.
(d, J = 8 Hz, 1H), 9.20 (br s, 1H). IR (KBr, cmꢀ1
)
3186(br s), 3085(s), 1599(br s), 1481(s), 1391(s), 1351(s),
1307(s), 1266(s), 1150(s), 1090(w), 1076(s), 866(m), 785(s),
726(s), 702(s), 649(s). Elemental Anal. Calc. for
C26H18N4O9Zn: C, 52.42; H, 3.02. Found: C, 52.38; H,
3.05%.
2. Experimental
The 8-aminoquinoline (AQ), 2-nitrobenzoic acid
(oNBz), PHEN = 1,10-phenanthroline, 2-aminopyrimidine
(AMP), 2,20-bipyridine were obtained from Aldrich Chem-
ical Co. USA and used as received.
2.2.3. [Zn (oNBz)2(AQ)2] (3)
1H NMR (DMSO-d6) 5.89 (br s, 4H), 6.85 (dm,
J = 7 Hz, 2H), 7.04 (dt, J = 8 Hz, 2H), 7.29 (t, J = 8 Hz,
2H), 7.44 (dd, J = 4 Hz, 2H), 7.57–7.67 (m, 4H), 7.75–
7.80 (m, 4H), 8.15 (dm, J = 8 Hz, 2H), 8.71 (dm,
J = 4Hz, 2H). IR (KBr, cmꢀ1) 3309(s), 2971(br s),
1634(s), 1501(w), 1393(wb), 1219(m), 1040(m), 825(s),
789(s), 692(s). Elemental Anal. Calc. for C32H24N6O8Zn:
C, 55.98; H,3.50. Found: C, 56.01; H, 3.54%.
2.1. Synthesis of complexes
2.1.1. Synthesis of complex [Zn2(oNBz)4(AQ)2(H2O)2] (1)
In a typical procedure a mixture of zinc(II) acetate dihy-
drate (0.219 g, 1 mmol) and 2-nitrobenzoic acid (0.334 g,
2 mmol) was ground in a mortar and pestle and heated at
80 °C for 20 min. The mixture was then transferred to a
round-bottom flask and methanol (10 ml) was added and
stirred for half an hour. To this reaction mixture a solution
of 8-aminoquinoline (0.288 g, 2 mmol) dissolved in a mix-
ture of toluene (5 ml) and methanol (5 ml) was added drop-
wise. A clear solution was observed. The clear solution was
kept for crystallization and after 3 days white needle-like
crystals appeared and were filtered. Yield: 82%. For the
synthesis of other complexes 2, 4 and 5 identical reaction
conditions except the nitrogen donor ligands namely 1,10-
phenanthroline, 2,20-bipyridine and 2-aminopyrimidine,
respectively, are used. Isolated yield of pure crystalline
compounds of 2, 4 and 5 are 86%, 90%, 94%, respectively.
2.2.4. [Zn (oNBz)2(bpy)] (4)
1H NMR (DMSO-d6) 7.50 (t, J = 8 Hz, 2H) 7.58 (t,
J = 8 Hz, 2H), 7.67 (dd, J = 8 Hz, 4H), 8.18 (s, 2H),
8.57(s, 2H), 8.75 (s, 2H). IR (KBr, cmꢀ1) 3533(s),
3445(s), 1579(s), 1571(s), 1525(s), 1492(m), 1442(s),
1399(s), 1371(s), 1316(s), 1020(s), 774(s), 736(s), 694(s). Ele-
mental Anal. Calc. for C24H16N4O8Zn: C, 52.01; H, 2.89.
Found: C, 52.07; H, 2.90%.
2.2.5. [Zn2(l-oNBz)4(AMP)2] (5)
1H NMR (DMSO-d6) 6.53 (m, 1H), 6.60 (br s, 2H), 7.58
(t, J = 8 Hz, 2H), 7.62 (t, J = 8 Hz, 2H), 7.74–7.79 (m, 4H),
8.19 (br s, 1H). IR (KBr, cmꢀ1) 3443(s), 3323(s), 3181(s),
3096(s), 1568(s), 1595(s), 1569(s), 1530(s), 1485(s), 1408(s),
1363(s), 1344(s), 1228(s), 1263(s), 1195(s), 1154(s),
1073(w), 877(m), 863(s), 839(s), 801(s), 787(s), 754(s),
740(s), 698(s) 658(s). Elemental Anal. Calc. for C36H26
N10O16Zn2: C, 43.84; H, 2.64. Found: C, 43.81; H, 2.63%.
2.1.2. Synthesis of complex [Zn (oNBz)2(AQ)2] (3)
To a solution of zinc(II)acetate dihydrate (0.219 g,
1 mmol) and 2-nitrobenzoic acid (0.334 g, 2 mmol) in
methanol (25 ml) a solution of 8-aminoquinoline (0.288 g,
2 mmol) in methanol (10 ml) was added dropwise. A
homogeneous solution was obtained. The solution was stir-
red at room temperature for 2 h. The solution was filtered
through an ordinary filter paper to remove any undissolved
residue. The filtrate was kept standing for two days and
then a white crystalline product of 3 was obtained. Yield
was 92% with respect to 8-aminoquinoline.
2.3. X-ray crystallography
The X-ray diffraction data were collected at 296 K with
˚
MoKa radiation (k = 0.71073 A) using a Bruker Nonius
SMART CCD diffractometer equipped with graphite mono-
chromator. 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. The structures were solved by direct meth-
ods and refined by full-matrix least-squares calculations
2.2. Spectroscopic data for complexes 1–4
2.2.1. [Zn2(oNBz)4(AQ)2(H2O)2] (1)
1H NMR (DMSO-d6) 5.89 (br s, 2H), 6.85 (dm,
J = 7 Hz, 1H), 7.04 (dt, J = 8 Hz, 1H), 7.29 (t, J = 8 Hz,

A.M. Baruah et al. / Inorganica Chimica Acta 361 (2008) 2777–2784
2779
Table 1
Crystallographic parameters for compounds 1, 3–5
Compound no.
1
3
4
5
Formulae
Mw
C46H36 N8O18Zn2
1119.57
monoclinic
P2(1)/c
296
0.71073
7.6674(5)
21.4398(15)
14.2719(10)
90.00
C32H24N6O8Zn
685.94
monoclinic
P2(1)/c
296
0.71073
7.6959(5)
20.2231(14)
9.5743(6)
90.00
C24H16N4O8Zn
553.78
monoclinic
P2(1)/c
296
0.71073
7.6111(8)
17.332(2)
18.156(2)
90.00
C36H26N10O16Zn2
985.41
monoclinic
P2(1)/c
Crystal system
Space group
Temperature (K)
296
˚
Wavelength (A)
0.71073
12.1172(2)
17.3050(4)
10.2873(2)
90.00
110.6550(10)
90.00
2018.47(7)
2
˚
a (A)
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
96.484(4)
90.00
2331.1(3)
2
99.590(4)
90.00
1469.27(17)
2
98.333(7)
90.00
2369.8(5)
4
3
˚
V (A )
Z
Dcalc (mg mꢀ3
)
1.595
1.550
1.552
1.621
Absortion coefficient
(mmꢀ1
1.116
0.901
1.095
1.275
)
Absortion correction
F(000)
none
1144
none
704
none
1128
none
1000
Total no. of reflections
Reflections, I > 2r(I)
Max. 2h (°)
24840
5782
28.39
19765
3597
28.34
28473
5804
28.34
27123
4924
28.24
ꢀ16 6 h 6 15
ꢀ23 6 k 6 23
ꢀ13 6 l 6 13
98.7
Ranges (h,k,l)
ꢀ10 6 h 6 10
ꢀ10 6 h 6 8
ꢀ9 6 h 6 9
ꢀ28 6 k 6 28
ꢀ26 6 k 6 26
ꢀ20 6 k 6 23
ꢀ18 6 l 6 19
ꢀ12 6 l 6 12
ꢀ24 6 l 6 22
Completeness to 2h (%)
98.8
98.2
98.2
Refinement method
full-matrix least-squares on
F2
full-matrix least-squares on
F2
full-matrix least-squares on
F2
full-matrix least-squares on
F2
Data/restraints/parameters 5782/0/350
3597/0/214
1.008
0.0539
0.0935
5804/0/334
1.026
0.0370
0.0698
4924/0/297
1.021
0.0305
0.0507
Goodness-of-fit on F2
R indices [I > 2r(I)]
R indices (all data)
1.148
0.0717
0.0992
[Zn(oNBz)2(PHEN)(H2O)]
2
PHEN (soln)
oNBzH
solid phase
oNBzH
solid phase
AQ
AQ + oNBzH
MeOH
[Zn(oNBz)2(AQ)2]
[Zn2(oNBz)4(AQ)2(H2O)2]
1
Zn(OAc)2 2H2O
3
MeOH/toluene
oNBzH
solid phase
oNBzH
solid phase
AMP (soln)
bpy (soln)
[Zn2(oNBz)4(AMP)2]
5
[Zn(oNBz)2(bpy)]
4
O
OH
NO2
NH2
N
N
N
N
N
PHEN =
N
bpy =
AMP =
AQ
=
oNBzH =
N
NH2
Scheme 1.

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using SHELXTL software. All the non-H atoms were refined in
the anisotropic approximation. The crystallographic
parameters of the compounds studied are given in Table 1.
nitrobenzoate complexes having different nitrogen donor
ligands could be obtained as illustrated in Scheme 1.
The reaction of 2-nitrobenzoic acid with zinc(II)acetate
dihydrate followed by treatment with 8-aminoquinoline
3. Results and discussion
gave a dinuclear zinc complex having composition
[Zn2(oNBz)4(AQ)2(H2O)2] (1) where AQ is 8-aminoquino-
line and oNBz is 2-nitrobenzoato anion. The structure of
the complex determined by crystallography is shown in
Fig. 2. The complex has monodentate carboxylate, chelat-
ing 8-aminoquinoline, water, and bridging carboxylate, one
each per zinc center. The co-ordination mode of carboxyl-
ate group in the bridge is mediated by one oxygen of the
carboxylate group to have a bis(unidentate) carboxylate
bridge. Such binding modes are reported in polycarboxylic
acids, in which it occurs due to steric reasons [2e]. How-
ever, in isolated molecules such interactions are not pre-
ferred [2e,3d,4b] and to the best of our knowledge they
are not isolated as exclusive bridging mode in dinuclear
systems. Even when such bis(unidentate) co-ordination of
carboxylate group is present they generally occur comple-
mentary to a commonly occurring bidentate bridging mode
[2e]. The distance between two zinc atoms in the dinuclear
It is observed that depending on the reaction condition
different types of co-ordination environments in zinc 2-
˚
˚
motif is 3.31 A, which is higher than 3.03 A for complexes
of zinc having paddle wheel structure [5d,5i].
The complex is highly symmetric and may be considered
as a self-assembly of two mono-nuclear halves with a com-
position [Zn(oNBz)2(AQ)(H2O)] and having a structure as
shown in Fig. 3a. This part has structural similarity with a
mononuclear [Zn(oNBz)2(PHEN)(H2O)] (2) complex [5i]
(where PHEN = 1,10-phenanthroline). The structure of
[Zn(oNBz)2(PHEN)(H2O)] (2) is shown in Fig. 3b. This
compound 2 is mono-nuclear and we have not been able
to get a dinuclear counterpart of this composition when
synthesis is carried out in solid phase followed by solution
Fig. 2. The ORTEP diagram of complex 1 (20% thermal ellipsoid, hydrogen
˚
atoms are omitted for clarity) The bond distances (A) and bond angles (°)
for [Zn2(oNBz)4(AQ)2(H2O)2] (1) are Zn2–N1, 2.12; Zn2–N2, 2.13; Zn2–
O9, 2.11, Zn2–O1, 2.61 Zn2–O6, 2.25; <O9–Zn2–O6, 91.35; <N1–Zn2–
O9, 100.96; <O1–Zn2–O9, 88.80; <N2–Zn2–O6, 92.72; <N1–Zn2–O6,
92.58; <N1–Zn2–N2, 79.09.
˚
Fig. 3. The structure of (a) symmetric half of dinuclear zinc complex 1; (b) mononuclear complex 2 [5i]. The bond distances (A) and bond angles (°) for 2
are Zn1–N2, 2.08; Zn1–N1, 2.17; Zn1–O9, 2.04; Zn1–O2, 2.08; Zn1–O5, 1.99; <O9–Zn1–O2, 87.71; <N1–Zn1–O9, 93.62; <O2–Zn1–O9, 87.71; <N2–Zn1–
O5, 117.59; <N1–Zn1–O5, 99.53; <N1–Zn1–N2, 78.02.

A.M. Baruah et al. / Inorganica Chimica Acta 361 (2008) 2777–2784
2781
chemistry or completely in solution. On comparison of
these two structures, it is observed that the bite angle
between the two nitrogen donor atoms and the zinc atom
is comparable. Same is the case with the other bond dis-
tances and the bond angles around the zinc in the two zinc
complexes. But there is a significant difference in the dis-
tance of separation between the oxygen atoms involved
in bridging between the zinc ions and the un-coordinated
oxygen atoms of the monodentate 2-nitrobenzoate ligand
of complex 1, with the distance between the oxygen atoms
of the two un-coordinated oxygen atoms of the monoden-
tate benzoate group of 2. These distances of separations are
˚
˚
found to be 5.64 A and 3.70 A, respectively (Fig. 3a and b).
This difference in distances arises from the twist conferred
to the benzoate group due to the orientations of the nitro
groups. Such comparatively large separation between the
two oxygens in compound 1 is facilitated by the inter-
molecular hydrogen bonding between the N1–H ꢁ ꢁ ꢁ O5
˚
(dD–A,inter 3.06A; <D–H ꢁ ꢁ ꢁ A, 142.95°) and also by intra-
˚
molecular hydrogen bonding O9–H ꢁ ꢁ ꢁ O2 (dD–A 2.69 A;
<D–H ꢁ ꢁ ꢁ A 150.1°) and inter-molecular hydrogen bonding
˚
between O9–H ꢁ ꢁ ꢁ O5 (dD–A 2.65 A, <D–H ꢁ ꢁ ꢁ A 154.29°).
Thus, in the case of 1 the orientation of the oxygen atom
is such that it becomes easy to form dinuclear complex,
whereas in the case of 2 it is not the case. The dinuclear
complex formation in 1 is also facilitated by the separation
between the free oxygens of carboxylate group; it allows
another molecule to come close to form a dinuclear struc-
ture through bis(unidentate) bridging co-ordination. In
Fig. 4. The structure of bis (8-aminoquinoline) zinc(II) 2-nitrobenzoate
(3) (drawn with 20% thermal ellipsoid, hydrogen atoms are omitted for
˚
clarity). The bond distances (A) and bond angles (°) are Zn1–N2, 2.13,
˚
complex 1 Zn2–O6 distance is 2.25 A which is well in the
Zn1–N1, 2.15, Zn1–O2, 2.14, <N1–Zn1–N2, 79.71, <N1–Zn–O2, 89.59,
permissible limit to have a dative bond. In a paddle wheel
and <N2–Zn–O2, 92.43.
Fig. 5. 1H NMR spectra of (i) 8-aminoquinoline (ii) complex 3 in DMSO-d6 and (iii)–(v) complex 1 in different solvents in the range of 6–9 ppm.

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1
structure such distances are large, for example the bis-pyr-
sociate in dimethyl sulfoxide. In these two cases H NMR
idine tetra (l-benzoato)zinc(II) complex has a paddle wheel
structure [5i] and has such distance to be 3.33 A (see
Fig. 4).
signals arising from 8-aminoquinoline ligand of the com-
plexes resemble the parent ligand [Fig. 5i, ii and iv]. How-
ever, complex 1 shows entirely different spectra in
methanol-d4 and benzene-d6. The solution of complex 1
in benzene-d6 has 10 aromatic proton signals (six from 8-
aminoquinoline and two each from bridging and terminal
2-nitrocraboxylate) in contrast to eight aromatic proton
signals (six from 8-aminoquinoline and two from 2-nitro-
benzoate) that are observed from the solution of complex
1 in DMSO-d6. This suggests that the protons in the aro-
matic ring of the bridging and the terminal carboxylate
are not magnetically equivalent. In the case of methanol-
d4 solution of complex 1 there are eight aromatic proton
˚
The reaction between zinc(II) acetate 8-aminoquinoline
and 2-nitrobenzoic acid in 1:1:2 molar ratio in methanol
solution gives bis (8-aminoquioline) zinc(II) 2-nitrobenzo-
ate (3). The side product in the reaction is found to be zin-
c(II) 2-nitrobenzoate. The formation of side product can be
decreased by using 1:2:2 stoichiometry; in which case only
3 is formed. The solvents have a profound effect in the
1
chemical shifts of the protons in the H NMR spectra of
complex 1 (Fig. 5). The 1H NMR spectra recorded in
DMSO-d6 solution shows that both complexes 1 and 3 dis-
Fig. 6. The structure of complex (a) [Zn(oNBz)2(bpy)] (4) and (b) [Zn2(l-oNBz)4(AMP)2] (5); (c) the self-assembling of 5 through hydrogen bonding
interactions (the figures are drawn with 20% thermal ellipsoids).

A.M. Baruah et al. / Inorganica Chimica Acta 361 (2008) 2777–2784
2783
Table 2
signals corresponding to the ligands but the aromatic sig-
˚
Selected bond distances (in A) and angles (in °) in 4 and 5
nals appear at much different chemical shift positions from
the parent 8-aminoquinoline. This result supports the fact
that the compound does not remain in dinuclear form
and most likely it occurs as mono-nuclear species. Recrys-
tallisation of compound 1 from methanol gave a mixture of
compounds 1 and 3. Formation of mixture of a 1 and 3 on
re-dissolution in methanol is confirmed by recording the
powder X-ray diffraction of the residue from methanolic
solution (Supplementary material) and also by picking up
crystals from the mixture and determining the X-ray struc-
ture. It may be noted that the dinuclear complex 1 can be
crystallized only when the aromatic solvent such as toluene
or benzene is used along with methanol.
It is obvious that the X-ray powder diffraction patterns
of products 1 and 3 are different (Supplementary material),
and each of them could be prepared in pure state in good
yield. Analysis of powder X-ray shows that the product
obtained from the solid phase reaction followed by solu-
tion chemistry does not possess super-imposable peaks of
the product obtained from only solution route. This sug-
gests that product 3 is not formed in the solid phase reac-
tion followed by solution reaction. The 1H NMR
integration of complex 3 confirms the composition of the
complex in solution. The structure determined by X-ray
crystallography shows that complex 3 has a distorted
octahedral geometry around zinc(II)ion and that the
2-nitrobenzoate groups are located trans to each other.
The 8-aminoquinoline ligand occupies the rest of the four
co-ordination positions. The nitrogen atoms of the
pyridine ring of 8-aminoquinoline are trans to each other.
The structural study on related compounds shows the
role of the bite angles of ligands or the co-ordination of
amino group in the formation of bis(unidentate) carboxyl-
ate bridged dinuclear complex. Two other zinc(II) 2-nitro-
benzoate complexes, one having 2,20-bipyridine and the
other having 2-aminopyrimidine ligand were prepared. In
the former case a mononuclear complex, namely 2,20-bipyr-
idine zinc(II) 2-nitrobenzoate (4) was formed, whereas in
the case of 2-aminopyrimidine (AMP) a binuclear complex
having composition [Zn2(l-oNBz)4(AMP)2] (5) was
obtained. The structures of the complexes were determined
and the structures of the two complexes are shown in Figs.
6a and b. Some important bond lengths and bond angles
are listed in Table 2.
For 4
For 5
Zn2–O6
Zn2–N1
Zn2–N2
Zn2–O1
Zn2–O2
Zn2–O5
2.06
2.08
2.10
2.11
2.25
2.26
109.1
101.9
78.7
148.2
98.7
98.2
93.6
157.3
96.0
Zn1–O1
Zn1–O8
Zn1–N3
Zn1–O7
2.03
2.03
2.04
2.05
2.06
157.9
106.4
95.6
90.1
87.8
102.4
86.8
87.2
98.9
158.5
Zn1–O4
O1–Zn1–O8
O1–Zn1–N3
O8–Zn1–N3
O1–Zn1–O7
O8–Zn1–O7
N3–Zn1–O7
O1–Zn1–O4
O8–Zn1–O4
N3–Zn1–O4
O7–Zn1–O4
O6–Zn2–N1
O6–Zn2–N2
N1–Zn2–N2
O6–Zn2–O1
N1–Zn2–O1
N2–Zn2–O1
O6–Zn2–O2
N1–Zn2–O2
N2–Zn2–O2
O1–Zn2–O2
O6–Zn2–O5
N1–Zn2–O5
N2–Zn2–O5
O1–Zn2–O5
O2–Zn2–O5
59.8
60.6
100.1
161.4
100.4
91.8
aminopyrimidine is coordinating the other nitrogen gets
involved in the formation of H-bonded assembly though
R22 (8) type of H-boned geometry (Fig. 6c) via N4–
˚
H ꢁ ꢁ ꢁ N5 interactions (dD–A
,
3.05 A; <D–H ꢁ ꢁ ꢁ A
176.19°). No intra-molecular hydrogen bonding is
observed in this case. The oxygen–oxygen separation in 4
and 5 is also much shorter in comparison to 1; thus, this
suggests that the orientation of the carboxylate group by
the steric requirement of nitro groups decides the stability
of different structures. In complex 4 the distance between
the zinc and the two oxygen atoms of carboxylate groups
˚
˚
namely Zn–O1 and Zn–O2 is 3.181 A and 2.078 A, respec-
tively (Fig. 6a), whereas in the case of complex 2 similar
Zn–O1 and Zn–O2 distances (Fig. 3b) are 2.252 A and
2.108 A respectively. These clearly suggest that in the for-
mer case the 2-nitrobenzoate group is monodentate,
whereas in the later it is a bidentate chelate.
In conclusion different types of structures of the 2-nitro-
benzoate complexes of zinc can be constructed depending
on the reaction conditions and ancillary ligand used. The
formation of dinuclear zinc complex having exclusive
bis(unidentate) carboxylate bridging co-ordination mode
is observed in a binuclear zinc complex and the rationality
of its formation is described by the comparison of its struc-
ture with several analogous zinc complexes prepared with
different ancillary ligands.
˚
˚
Complex 4 has six-coordination around the zinc atom.
The zinc ion is in a six-coordinated environment consisting
of two carboxylato groups and a bidentate 2,2-bipyridine
ligand. Complex 5 has a conventional paddle wheel struc-
ture with one of the ring nitrogen atoms of 2-aminopyrim-
idine coordinating to the zinc atoms at the two ends. The
Appendix A. Supplementary material
˚
Zn1–Zn1 distance in the complex is 3.00 A. A square pyra-
CCDC 647910, 647911, 647912, 656163 and 639912 con-
tain 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.
ac.uk/data_request/cif. The X-ray powder pattern of com-
plexes 1 and 3. Supplementary data associated with this
midal geometry is formed around each zinc atom (Table 2).
The distance between the two zincs in the paddle wheel
˚
structure is 3.01 A, which is in very close similarity to the
common binuclear zinc carboxylates having paddle wheel
structures [5d]. Since only one of the nitrogen atoms of 2-

2784
A.M. Baruah et al. / Inorganica Chimica Acta 361 (2008) 2777–2784
[3] (a) M. Eddaoudi, J. Kim, J.B. Wachter, H.K. Chae, M. O’Keeffe, O.M.
article can be found, in the online version, at doi:10.1016/
j.ica.2008.01.044.
Yaghi, J. Am. Chem. Soc. 123 (2001) 4368;
(b) M. Casarin, C. Corvaja, C.D. Nicola, D. Falcomer, L. Franco, M.
Monari, L. Pandolfo, C. Pettinari, F. Piccinelli, Inorg. Chem. 44 (2005)
6265;
References
(c) M. Eddaoudi, H. Li, O.M. Yaghi, J. Am. Chem. Soc. 127 (2005)
[1] (a) C. Mellot-Draznieks, G. Ferey, Prog. Solid State Chem. 33 (2005)
187;
7110;
(d) H. Kumagai, C.J. Kepert, M. Kurmoo, Inorg. Chem. 41 (2002)
(b) O.M. Yaghi, M.O. Keeffe, N.W. Ockwig, H.K. Chae, M.
Eddaoudi, J. Kim, Nature 423 (2003) 705;
3410;
(e) C. Costentin, M. Robert, J.M. Saveant, J. Am. Chem. Soc. 128
(c) X. Zhao, B. Xiao, A.J. Fletcher, K.M. Thomas, D. Bradshaw, M.J.
Rosseinsky, Science 306 (2004) 1012;
(2006) 8726;
(f) V. Robert, G. Lamercier, J. Am. Chem. Soc. 128 (2006) 1183.
(d) B. Moulton, M.J. Zaworotko, Chem. Rev. 101 (2001) 1629;
(e) S. Noro, S. Kitagawa, M. Kondo, K. Seki, Angew. Chem., Int. Ed.
Engl. 39 (2000) 2081;
[4] (a) F.S. Delgado, M. Hernandez-Molina, J. Sanchiz, C. Riuz-Perz, Y.
Rodriguez-Martin, T. Lopez, F. Lloret, M. Julve, CrystEngCommun.
6 (2004) 106;
(f) P.J. Hagrman, D. Hagrman, J. Zubieta, Angew. Chem., Int. Ed.
Engl. 38 (1999) 2638;
(b) S.O.H. Gutchke, D.J. Price, A.K. Powell, P.T. Wood, Eur. J. Inorg.
Chem. (2001) 2739.
(g) J.S. Seo, D. Whang, H. Lee, S.I. Jun, J. Oh, Y.J. Jeon, K. Kim,
Nature 404 (2000) 982;
[5] (a) A. Karmakar, R.J. Sarma, J.B. Baruah, Eur. J. Inorg. Chem. (2007)
643;
(h) S.M. Humphrey, J.-S. Chang, S.H. Jhung, J.W. Yoon, P.T. Wood,
Angew. Chem., Int. Ed. Engl. 46 (2007) 272;
(b) A. Karmakar, R.J. Sarma, J.B. Baruah, Eur. J. Inorg. Chem. (2006)
4673;
(i) A. Cingolani, S. Galli, N. Masciocchi, L. Pandolfo, C. Pettinari, A.
Sironi, Dalton Trans. (2006) 2479.
(c) N. Barooah, A. Karmakar, R.J. Sarma, J.B. Baruah, Inorg. Chem.
Commun. 9 (2006) 1251;
[2] (a) F.A. Cotton, C. Lin, C.A. Murillo, Acc. Chem. Res. 34 (2001) 759;
(b) M. Eddaoudi, D.B. Moler, H. Li, B. Chen, T.M. Reineke, M.
Keeffe, O.M. Yaghi, Acc. Chem. Res. 34 (2001) 319;
(c) S.L. James, Chem. Soc. Rev. 32 (2003) 276;
(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;
(d) R. Murugavel, M.G. Walawalkar, M. Dan, H.W. Roesky, C.N.R.
Rao, Acc. Chem. Res. 37 (2004) 763;
(f) N. Barooah, R.J. Sarma, J.B. Baruah, Eur. J. Inorg. Chem. (2006)
2942;
(e) Y. Rodriguez-Martin, M. Hernandez-Molina, F.S. Delgado, J.
Pasan, C. Ruiz-Perez, J. Sanchiz, F. Lloret, M. Julve, CrystEngCom-
mun. 4 (2002) 522;
(g) A. Karmakar, K. Deka, R.J. Sarma, J.B. Baruah, Inorg. Chem.
Commun. 9 (2006) 836;
(h) K. Deka, N. Barooah, R.J. Sarma, J.B. Baruah, J. Mol. Struct. 827
(2007) 44;
(f) D.E. Wilkox, Chem. Rev. 96 (1996) 2435;
(g) G. Lemercier, E. Mulliez, C. Brouca-Cabarrecq, F. Dahan, J.P.
Tuchagues, Inorg. Chem. 43 (2004) 2105.
(i) A. Karmakar, K. Bania, A. Baruah, J.B. Baruah, Inorg. Chem.
Commun. 10 (2007) 959.