Thermal, fluorescence, structural and solution studies of a thallium(I) one-dimensional coordination polymer with 4-aminobenzoate (AB-), [Tl(μ4-AB)]n

Article abstract of DOI:10.1016/j.jorganchem.2007.07.043

A thallium(I) one-dimensional polymer, [Tl(μ₄-AB)]_n (1) [HAB = 4-aminobenzoic acid], has been synthesized and characterized. The single-crystal X-ray data of compound show the coordination number in the Tl^I ions is five, the thallium atoms have irregular coordination sphere containing stereo-chemically active lone pair and bi-hapto (η²) interactions, thus attaining a total hapticity of seven with environment C₂O₅Tl. The thermal stability of 1 was studied by thermal gravimetric (TG) and differential thermal analyses (DTA). The ligand HAB and compound 1 are luminescent in the solution state, with emission maxima at 395 nm. The results of studies of the stoichiometry and formation of complex of 1 in DMF solution were found to be in support of their solid state stoichiometry.

Full text of DOI:10.1016/j.jorganchem.2007.07.043

Journal of Organometallic Chemistry 692 (2007) 5141–5146
www.elsevier.com/locate/jorganchem
Thermal, fluorescence, structural and solution studies
of a thallium(I) one-dimensional coordination polymer
with 4-aminobenzoate (ABÀ), [Tl(l4-AB)]n
*
Kamran Akhbari, Ali Morsali
Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Islamic Republic of Iran
Received 8 May 2007; received in revised form 17 July 2007; accepted 24 July 2007
Available online 2 August 2007
Abstract
A thallium(I) one-dimensional polymer, [Tl(l4-AB)]n (1) [HAB = 4-aminobenzoic acid], has been synthesized and characterized. The
single-crystal X-ray data of compound show the coordination number in the TlI ions is five, the thallium atoms have irregular coordi-
nation sphere containing stereo-chemically active lone pair and bi-hapto (g2) interactions, thus attaining a total hapticity of seven with
environment C2O5Tl. The thermal stability of 1 was studied by thermal gravimetric (TG) and differential thermal analyses (DTA). The
ligand HAB and compound 1 are luminescent in the solution state, with emission maxima at 395 nm. The results of studies of the
stoichiometry and formation of complex of 1 in DMF solution were found to be in support of their solid state stoichiometry.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Thallium(I); 4-Aminobenzoic acid; Coordination polymer; Lone pair
1. Introduction
the coordinating sites in the organic spacers, and the
solvent of recrystallization, among others [2]. The TlI
compounds are interesting and frequently discussed in
considering the ‘‘stereo-chemical activity’’ of valence shell
electron lone pairs, potential ability to form metal–metal
bonds and also complexes with aromatic hydrocarbons
[3–8]. From recent structural study of Tl(I) complexes of
anthranilates and salicylates [5], it has been argued that
polyhapto-aromatic interactions also play an important
role in determining the solid state lattices of such com-
pounds. The ‘‘inert-pair effect’’ has, in a general way, been
explained by Pitzer [9] and by Pyykko¨ and Desclaux [10]
causing the outer s electrons to be more strongly attracted
to the nucleus than normally expected. A theoretical study
of relativistic effects and electron correlation effects in
group 13 and period 6 hydrides and halides by Peter
Schwerdtfeger et al. [11] shows that although relativistic
contributions play an important role in the chemistry of
the 6th period elements, there is no evidence that the 6s
electrons are more inert than the s electrons of lighter
elements. Thus, there is no special ‘‘inert-pair effect’’ for
Metal–organic coordination polymers are attracting a
great deal of attention because of their potential as func-
tional materials [1]. In metal–organic crystal engineering,
one takes advantage of the coordinating ability of metal
centers and organic ligands in order to build new coordina-
tion polymers. The main goal in metal–organic crystal
engineering is to predict the topology of supramolecular
architectures in order to synthesize extended solid-state
materials with desired properties. However, there still
remain many problems to overcome before the synthesis
of coordination polymers with a desired topology is possi-
ble. Many parameters are involved in the formation of the
metal–organic framework (MOF), such as the metal and its
possibilities of coordination, the nature of the counter-
anion, the metal-to-ligand ratio, the flexibility of the
organic building blocks, the number and orientation of
*
Corresponding author. Tel.: +98 2188011001; fax: +98 2188011101.
E-mail address: morsali_a@yahoo.com (A. Morsali).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.07.043

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K. Akhbari, A. Morsali / Journal of Organometallic Chemistry 692 (2007) 5141–5146
Table 1
the 6th period elements, and it is inappropriate to use this
term to designate the low valancies of heavier elements, in
addition it is true that, in many of the reported TlI com-
pounds [11] the s electrons play only a small part in bond-
ing and in that sense can be regarded as core like, even for
the lighter elements. Thus, r-bonding involves mainly the
valence p electrons, and the familiar concept of s–p hybrid-
ization has to be taken with care [11].
Crystal data and structure refinement for compound [Tl(l4-AB)]n (1)
Identification code
1
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
C7H6NO2Tl
340.50
298(2)
0.71073
Cubic
Iba2
In several organometallic compounds, the stereoactive
lone pair is manifested by a ‘‘half-naked’’ cation, with
ligands only coordinating one hemisphere, as observed in
the crystal structure of [Tl2(l-Htdp)2(l-H2O)]n (H2tdp
= 4,4-thiodiphenol) [12], where the lone pair occupies the
hemisphere of Tl(I) coordination sphere with an obvious
gap in the coordination sphere due to the lone pair. Our
search show that Zn(II), Co(II), Cd(II) [13] and Ag(I)
[14] coordination polymers of 4-aminobenzoic acid
(HAB) were recently reported and similar coordination
polymers of thallium(I) ion with benzoate-derived ligands
were reported in the works by Schmidbaur and Wiesbrock
[5,6] and by Kristianson [8]. Continuing with our previous
work on TlI coordination polymers [12,15–22], in this work
we wish to report a coordination polymer of Tl(I) with
HAB via hydrogen bonds and covalent bonds with ther-
mal, emission and solution studies of this compound.
Unit cell dimensions
˚
a (A)
14.652(3)
15.095(3)
6.9642(14)
90.00
90.00
90.00
1540.3(5)
8
2.937
20.12
1216
0.28 · 0.21 · 0.13
1.94–25.28
À17 6 h 6 17
À18 6 k 6 18
À8 6 l 6 6
1179
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
Volume (A )
Z
Dcalc (g cmÀ3
)
Absorption coefficient (mmÀ1
F(000)
)
Crystal size (mm3)
Theta range for data collection (ꢁ)
Index ranges
Reflections collected
Independent reflections
Absorption correction
Maximum and minimum transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
1083
Multi-scan
0.065 and 0.006
Full-matrix least-squares on F2
1179/1/89
1.055
R1 = 0.0320
wR2 = 0.0684
R1 = 0.0287
wR2 = 0.0671
O
H2N
Final R indices [I > 2r(I)]
OH
HAB
R Indices (all data)
À3
˚
Largest difference Peak, hole
1.552 and À0.980 e A
2. Experimental
direct methods and refined by full-matrix least-squares
techniques on F2 using SHELXTL [23]. The molecular struc-
ture plots were prepared using ORTEP and Mercury [24].
Crystallographic data and details of the data collection
and structure refinements are listed in Table 1. The
observed anisotropic thermal parameters, the calculated
structure factors, and full lists of bond distances, bond
angles and torsion angles are given in the Supplementary
material.
2.1. Materials and physical techniques
All chemicals were of reagent grade and were used as
commercially obtained without further purification. IR
spectra were recorded using Perkin–Elmer 597 and Nicolet
510P spectrophotometers. Microanalyses were carried out
using a Heraeus CHN–O-Rapid analyzer. Melting points
were measured on an Electrothermal 9100 apparatus and
are uncorrected. The thermal behavior was measured with
a PL-STA 1500 apparatus. The luminescent properties
were investigated with a Shimadzu RF-5000 spectrofluoro-
photometer. All UV–Visible spectra were recorded on a
computerized double-beam Shimadzu 2550 spectropho-
tometer, using two matched 10-mm quartz cells. Conducto-
metric measurements were carried out with a Metrohm 712
conductometer equipped with a Julabo F12-MB circulator.
Crystallographic measurements were made at 298(2) K
using a Bruker SMART APEX2 CCD area detector. The
intensity data were collected using graphite monochromat-
3. Synthesis of [Tl(l4-AB)]n (1)
4-Aminobenzoic acid (1 mmol, 0.137 g) was dissolved in
20 ml methanol and was mixed and stirred with solution of
1 mmol (0.057 g) KOH in 3 ml H2O, then a solution of
1 mmol (0.266 g) TlNO3 in 5 ml H2O was added to the mix-
ture and was refluxed for 3 h. After filtering it was allowed
to evaporate for several days and then suitable white crys-
tals were obtained. The crystals were washed with acetone
and air dried, s.p. = 222 ꢁC, Yield: 0.163 g (48%). IR
(selected bands; in cmÀ1) : 490 m, 610 s, 777 s, 846 m,
1074 m, 1123 m, 1167 s, 1260 s, 1285 s, 1359 vs, 1395 vs,
1482 vs, 1540 vs, 1588 vs, 3195 m and 3395 m. Anal. calc.
˚
ed Mo–Ka radiation (k = 0.71073 A). Accurate unit cell
parameters and the orientation matrix were obtained from
least–squares refinement. The structure has been solved by

K. Akhbari, A. Morsali / Journal of Organometallic Chemistry 692 (2007) 5141–5146
5143
for C7H6NO2Tl: C, 24.67; H, 1.76; N, 4.11. Found: C,
24.80; H, 1.57; N, 4.60%.
atoms and another oxygen atom bridges to two other thal-
lium atoms. The amine nitrogen atom does not link to any
thallium atom. The oxygen atom of carboxylate groups
and hydrogen atom of –NH2 groups of 4-aminobenzoate
(ABÀ) anions in compound 1 are involved in a hydrogen
bonding network (Fig. 3).
4. Results and discussion
4.1. Structure description
In compound 1, the lone pair of Tl(I) is ‘active’ in the
solid state [5]. However, the arrangement of O-atoms sug-
gest a gap or hole in coordination geometry around the
Tl(I) coordination sphere (Figs. 1 and 2), a gap possibly
occupied by a ‘stereo-active’ electron lone pair. Hence,
the geometry of the nearest coordination environment of
every Tl(I)-atoms is likely to be caused by the geometrical
constraints of coordinated O-atoms, and by the influence
of a stereo-chemically ‘active’ electron lone pair.
With low coordination number of 5 for large thallium(I)
ions in compound 1, one tends to look for secondary inter-
actions that could possibly relieve this coordinative unsat-
uration. To find secondary interactions and any other
The reaction between 4-aminobenzoic acid and
TlI(NO3) provided crystalline materials of the general for-
mula [Tl(l4-AB)]n (1). Determination of the structure of
the compound 1 by X-ray crystallography (Tables 1 and
2) showed the complex to be a novel one-dimensional poly-
mer. There are five (TlO5) coordinate Tl atoms (Fig. 1) and
each ABÀ anion has five bonding interactions and four
lone pairs of ABÀ connect to four TlI ions (Fig. 2). The car-
boxylate group of each ABÀ ligand acts as both bidentate
chelating, and bridging group where two oxygen atoms of
the carboxylate group coordinate to a thallium(I) ion, also
one of this oxygen atoms bridges to one other thallium
Fig. 1. (a) ORTEP diagram of compound 1. (b) Environment of Tl-atoms in compound 1 after extending the bonding limit, Tl Á Á Á C interactions. (c) Space-
filling representation, close approaches of Tl to the face of phenyl rings can be seen (Tl = purple, O = red, C = gray, H = white). (i) Àx, Ày, z. (For
interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

5144
K. Akhbari, A. Morsali / Journal of Organometallic Chemistry 692 (2007) 5141–5146
Fig. 2. A fragment of the one-dimensional polymer in compound 1. H atoms are omitted for clarity.
Fig. 3. Showing hydrogen bonding in the compound 1.
potential donor center the bonding limit to be extend and a
search was made for Tl Á Á Á C approaches. Our search show
that Tl in compound 1 may also be involved in an g2 inter-
actions with the phenyl groups of neighboring molecules
(Fig. 1b and c). Thus, the Tl(I) atoms are linked to two
carbon atoms of phenyl groups with distances Tl Á Á Á C of
factor which can make varying contributions to the stabil-
ity of complex.
5. Thermogravimetric analysis
To examine the thermal stability of compound 1, ther-
mal gravimetric (TG) and differential thermal analyses
(DTA) were carried out between 30 and 700 ꢁC in the static
atmosphere of air (Fig. 4). Compound 1 does not melt and
TG curve exhibits a very sharp loss of weight between 222
and 239 ꢁC with a mass loss of 92.3% that could be related
to sublimation of this compound. DTA curve displays only
a broad exothermic peak with a maximum intensity at
322 ꢁC.
˚
3.362(2) and 3.617(5) A. Hence, the Tl(I) coordination
sphere is completed and rather than a TlO5 coordination
sphere, the complex can be considered to contain an g2
(C2O5Tl) center with a seven coordination number. The
˚
reported Tl Á Á Á C separations range is 3.20–4.00 A in recent
reported species [5,25] and the sum of the vander Waals
2
˚
radii of carbon and Tl atoms is 3.66 A [26]. Thus, g
aromatic coordination of Tl(I) appears to be yet another
Table 2
˚
Selected bond lengths (A) and angles (ꢁ) for compound [Tl(l4-AB)]n (1)
6. Luminescent properties
Tl1–O2
Tl1–O2i
Tl1–O1
2.633(7)
2.772(8)
2.786(8)
O2–Tl1–O2
O2–Tl1–O1
O2–Tl1–O1
70.2(3)
48.4(2)
112.1(2)
The UV–Vis and emission spectra of complex 1 as well
as the pure ligand HAB were studied. The UV–Vis spectra
of both ligand and complex in DMF display intense
(i) Àx, Ày, z.

K. Akhbari, A. Morsali / Journal of Organometallic Chemistry 692 (2007) 5141–5146
5145
Fig. 4. Thermal behaviour of compound 1.
Fig. 6. (a) Electronic absorption spectra of ligand HAB in DMF
(5.0 · 10À5 M) in the presence of increasing concentration of thallium(I)
ion at room temperature and (b) corresponding mole ratio plot at 283 nm.
Fig. 5. Electronic absorption of ligand HAB (a), Electronic absorption of
compound 1 (b) Absorption: c = 5.0 · 10À5 mol lÀ1, DMF, d = 1 cm for
both ligand HAB and compound 1. Solution-state emission spectrum for
compound 1 (c). Solution-state emission spectrum for ligand HAB (d).
Room temperature, kexc = 350 nm.
Fig. 7. Conductivity vs. [L]/[Tl+] mole ratio plot DMF solution.
absorption bands with the maximum intensity at 283 nm
(Fig. 5a and b, respectively), indicating that electronic tran-
sitions are mostly of p–p* character, originating from the
aromatic groups of the ligand. In DMF emission bands
have been observed both for compound 1 and also the
ligand HAB at 395 nm upon excitation at 350 nm,
(Fig. 5c and d, respectively). The emission intensity of com-
pound 1 is fairly weaker than that of the uncoordinated
ligand, which can be attributed to the heavy atom effect
[27,28] due to the coordination of the ligand to the Tl(I)
center. Thus, compounds 1 may have potential applica-
tions as a luminescent material in organic light emitting
devices.
thallium(I) nitrate solution in DMF was monitored as a
function of [L]/[Tl+] mole ratio at 25.00 0.05 ꢁC and
the resulting plot is shown in Fig. 7. As it is seen, the initial
conductivity is relatively low, probably due to some degree
of ion pairing which is common in solvents of relatively low
dielectric constant like DMF. Addition of the ligand to the
metal salt solution then do not causes a variation in the
solution conductivity possessing a rather distinct inflection
point at a molar ratio of about one, indicating the forma-
tion of a 1:1 Tl+–L complex in solution.
For evaluation of the conditional formation constants,
the mole ratio data obtained by the two different physico-
chemical methods employed were fitted to the previously
reported equations [29,30] using a non-linear least-squares
curve fitting program KINFIT [31]. The conditional
formation constants of the complex were evaluated as
0.60 0.02 and 0.52 0.03 from the spectrophotometric
and conductometric methods, respectively.
7. Solution studies
The electronic absorption spectra of the ligand ABÀ in
the presence of increasing concentration of thallium(I)
ion in DMF at room temperature are shown in Fig. 7.
As is obvious, the strong absorption of ligand at 283 nm
increases with increasing concentration of the metal ion.
The resulting absorbance (at 283 nm) against [Tl+]/[L]
mole ratio plot, shown in the inset of Fig. 6, revealed a dis-
tinct inflection point a metal-to-ligand molar ratio of about
1, emphasizing the formation of a 1:1 complex in solution.
The formation and stoichiometry of the Tl+–L complex in
DMF solution was also investigated by a conductometric
method. The conductivity of a 5.0 · 10À5 M solution of
8. Conclusions
A TlI one-dimensional coordination polymer of 4-
aminobenzoic acid (HAB) ligand, [Tl(l4-AB)]n, was synthe-
sized and characterized. The aromatic carboxylate ligand in
this complex exhibits a less-common g2-coordination mode
of the phenyl rings, in addition to the normal carboxylate

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K. Akhbari, A. Morsali / Journal of Organometallic Chemistry 692 (2007) 5141–5146
(e) Rodney P. Feazell, C.E. Carson, K.K. Klausmeyer, Inorg. Chem.
45 (2006) 2635;
(f) R.P. Feazell, C.E. Carson, K.K. Klausmeyer, Inorg. Chem. 45
(2006) 2627;
(g) R.P. Feazell, C.E. Carson, K.K. Klausmeyer, Inorg. Chem. 45
(2006) 935.
coordination modes. The individual one-dimensional units
in the compound [Tl(l4-AB)]n are almost parallel to each
other and further linked by hydrogen bonding, resulting
in a two-dimensional framework. The results of studies of
the stoichiometry and formation of [Tl(l4-AB)]n in DMF
solution were found to be in support of their solid state
stoichiometry. This compound has shown luminescent
properties in DMF solution.
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Acknowledgement
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Support of this investigation by Iran National Science
Foundation, INSF (Project No. 84118) is gratefully
acknowledged.
Appendix A. Supplementary material
[12] K. Akhbari, A. Morsali, H.D. Hunter, M. Zeller, Inorg. Chem.
Commun. 10 (2007) 178.
CCDC 645563 contains the supplementary crystallo-
graphic data for 1. These data can be obtained free of
[13] H. Chen, X. Chen, Inorg. Chim. Acta 329 (2002) 13.
[14] R. Wang, M. Hong, J. Luo, F. Jiang, L. Han, Z. Lin, R. Cao, Inorg.
Chim. Acta 357 (2004) 103.
[15] A.R. Mahjoub, A. Morsali, H. Bagherzadeh, Polyhedron 21 (2002)
2555.
[16] A. Askarinejad, A. Morsali, Helv. Chim. Acta 89 (2006) 265.
[17] A. Askarinejad, A. Morsali, Inorg. Chem. Commun. 9 (2006) 143.
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charge
via
http://www.ccdc.cam.ac.uk/conts/retriev-
ing.html, or from the Cambridge Crystallographic Data
Centre, 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.jorganchem.
2007.07.043.
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