Highly polyhapto-aromatic interactions in thallium(I) coordination

Article abstract of DOI:10.1016/j.jorganchem.2007.08.019

In the solid state, the Tl(I) complex of 4-hydroxybenzoate (HB^-), [Tl(HB)]_n (1), can be regarded as containing polymeric chains linked through 2 × η⁶ interactions of the Tl atoms with phenyl groups from adjacent units. The thallium atoms contain close Tl^I ? π (aromatic) contacts, thus attaining a total hapticity of 16 with environments TlO₄C₁₂. The unusually high coordination number in the this compound may reflect the capacity of Tl(I) to act as both a Lewis acid and a Lewis base.

Full text of DOI:10.1016/j.jorganchem.2007.08.019

Journal of Organometallic Chemistry 692 (2007) 5109–5112
www.elsevier.com/locate/jorganchem
Communication
Highly polyhapto–aromatic interactions in thallium(I) coordination
*
Kamran Akhbari, Ali Morsali
Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Islamic Republic of Iran
Received 3 June 2007; received in revised form 30 July 2007; accepted 10 August 2007
Available online 23 August 2007
Abstract
In the solid state, the Tl(I) complex of 4-hydroxybenzoate (HBꢀ), [Tl(HB)]n (1), can be regarded as containing polymeric chains linked
through 2 · g6 interactions of the Tl atoms with phenyl groups from adjacent units. The thallium atoms contain close TlI ꢁ ꢁ ꢁ p (aromatic)
contacts, thus attaining a total hapticity of 16 with environments TlO4C12. The unusually high coordination number in the this com-
pound may reflect the capacity of Tl(I) to act as both a Lewis acid and a Lewis base.
ꢀ 2007 Published by Elsevier B.V.
Keywords: Thallium(I); 4-Hydroxybenzoate; Polyhapto interaction; Lone pair
The TlI compounds are interesting and frequently dis-
cussed 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 [1]. From recent structural study of Tl(I)
complexes of anthranilates, salicylates [2], and modified
poly(pyrazolyl)borate ligands [3], it has been argued that
polyhapto–aromatic interactions also play an important
role in determining the solid state lattices of such com-
pounds. Continuing with our previous work on TlI coordi-
nation polymers [4–11], we have now determined the
structure of thallium(I) complex with the ligand 4-hydroxy-
benzoate (HBꢀ), that provides a two-dimensional polymer
construction of polyhapto–aromatic interactions and
hydrogen bonds, interestingly, show the strong ability of
TlI for forming of polyhapto interactions. Although discus-
sions of compound 1 and similar compounds that describe
primarily vibrational studies of thallium carboxylates and
focus on the Tl–O2CR interaction could be found in the
literature [12–15], but no structural studies were done for
compound 1. We also should mention that the structural
study of 2-hydroxybenzoate(salicylate) [9,16] and 3-
hydroxybenzoate [17] were done and structure of com-
pound 1 is as similar as thallium(I) salicylate, addition of
methyl groups to hyroxybenzoate were studied by Wies-
brock and Schmidbaur [2] and Olof Kristiansson [16].
The reaction between 4-hydroxybenzoic acid and
TlI(NO3) provided crystalline materials of the general
formula [Tl(HB)]n (1) [18]. Determination of the structure
of the compound 1 by X-ray crystallography [19] showed
the complex to be a novel two-dimensional polymer.
There are four (TlO4) coordinate Tl atoms with distances
of Tl1–O1 = 2.556(4), Tl1–O1iii (iii: ꢀx, ꢀy,ꢀz) = 2.792(4),
ꢀ
˚
Tl–O2 = 2.810(3), Tl–O3 = 3.005(3) A (Fig. 1). Each HB
anion has four bonding interactions and four lone pairs of
HBꢀ connect to three TlI ions. The carboxylate groups of
the HBꢀ ligand act 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
bridge to two other thallium atoms. The phenolic oxygen
atom only link to one thallium atom. The oxygen atoms of
carboxylate group and hydrogen atom of –OH groups of
4-hydroxybenzoate (HBꢀ) anions in compound 1 are
involved in a hydrogen bonding network (Fig. S2).
In compound 1, the lone pair of Tl(I) is ‘active’ in the
solid state. However, the arrangement of O-atoms suggest
a gap or hole in coordination geometry around the Tl(I)
coordination sphere (Fig. 1), 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
*
Corresponding author. Tel.: +98 2188011001; fax: +98 2188011101.
E-mail address: morsali_a@yahoo.com (A. Morsali).
0022-328X/$ - see front matter ꢀ 2007 Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2007.08.019

5110
K. Akhbari, A. Morsali / Journal of Organometallic Chemistry 692 (2007) 5109–5112
Fig. 2. A fragment of the two-dimensional layer in compound 1, showing
the p ꢁ ꢁ ꢁ Tl ꢁ ꢁ ꢁ p and Tl ꢁ ꢁ ꢁ p ꢁ ꢁ ꢁ Tl interactions. H atoms are omitted for
clarity.
Table 1
Distances of thallium(I) with carbon atoms of phenyl groups in compound
1
Tl ꢁ ꢁ ꢁ C2ii
Tl ꢁ ꢁ ꢁ C3ii
Tl ꢁ ꢁ ꢁ C4ii
Tl ꢁ ꢁ ꢁ C5ii
Tl ꢁ ꢁ ꢁ C6ii
Tl ꢁ ꢁ ꢁ C7ii
3.554(2)
3.487(2)
3.487(2)
3.575(3)
3.688(5)
3.654(2)
Tl ꢁ ꢁ ꢁ C2iii
Tl ꢁ ꢁ ꢁ C3iii
Tl ꢁ ꢁ ꢁ C4iii
Tl ꢁ ꢁ ꢁ C5iii
Tl ꢁ ꢁ ꢁ C6iii
Tl ꢁ ꢁ ꢁ C7iii
3.695(5)
3.579(2)
3.492(2)
3.500(2)
3.552(3)
3.658(5)
((ii) x + 1/2, ꢀy + 1/2, ꢀz and (iii) ꢀx, ꢀy, ꢀz ).
˚
[Tl(PhNNNPh)]2, 3.33 A in [Tl(TolNNNNNTol)]2 [17]
˚
and 3.29 A in catena-[bis(l3-salicylato)dithallium(I)] [16].
Hence, the Tl(I) coordination sphere is completed and
rather than a TlO4 coordination sphere, the complex can
be considered to contain a 2 · g6 (C12O4Tl) center with a
16 coordination number, this coordination number and
this structural behavior were exactly observed in thal-
lium(I) salicylate [9,16]. The reported Tl ꢁ ꢁ ꢁ C separations
Fig. 1. (a) Space-filling representation, close approaches of Tl to the face
of two phenyl rings can be seen. (b) Environment of Tl-atoms in
compound 1 after extending the bonding limit, Tl ꢁ ꢁ ꢁ C interactions. (i)
ꢀx, y + 1/2, ꢀz + 1/2, (ii) x + 1/2, ꢀy + 1/2, ꢀz, (iii) ꢀx, ꢀy, ꢀz, (iv) x,
ꢀy + 1/2, z + 1/2.
˚
range is 3.20–4.00 A in recent reported species [2,20] and
the sum of the vander Walls radii of carbon and Tl atoms
6
˚
is 3.66 A [21]. Thus, 2 · g aromatic coordination of Tl(I)
appears to be yet another factor which can make varying
contributions to the stability of complexes of this metal
ion.
of coordinated O-atoms, and by the influence of a stereo-
chemically ‘active’ electron lone pair.
With low coordination number of 4 for large thallium(I)
ions in 1, one tends to look for secondary interactions that
could possibly relieve this coordinative unsaturation. To
find any other potential donor center, it is necessary to
extend the bonding limit. A search was made generally
for Tl ꢁ ꢁ ꢁ C approaches and it appears that Tl in 1 may also
be involved in an 2 · g6 interactions with the phenyl
groups of neighboring molecules (Figs. 1b, 2 and S1). Thus,
the Tl(I) atoms are linked to 12 carbon atoms of phenyl
groups (Table 1). The Tl ꢁ ꢁ ꢁ C6(plane) and the interplanar
angle of the two phenyl groups surrounding the Tl-atom
However, the TlI is one of the post-transition metal
elements that exhibit ‘‘inert-pair effect’’, referring to the
resistance of the pair of outer electrons on Tl(I) to removal
or to participation in covalent bond formation or hydrogen
bonding. The obvious question then is whether the lone
pair in the TlI of the compound 1 may or may not be
involved in donor bonding. Though a qualitative explana-
tion of the nature of these particular interactions may be
offered in terms of precise geometrical considerations,
identification of the exact nature of the interaction is not
always obvious and however, the distinction here of
acceptor vs. donor is perhaps artificial. Actually, whether
the thallium atoms in 1 act only as a Lewis acid or whether
˚
are 3.287(3) A and 151.60(3)ꢁ, respectively, this
˚
Tl ꢁ ꢁ ꢁ C6(plane) value is similar to that of 3.27 A in

K. Akhbari, A. Morsali / Journal of Organometallic Chemistry 692 (2007) 5109–5112
5111
as both a Lewis acid and a Lewis base is not clear and
answer to this question definitively for compound 1 would
require a computational study of the electron density or a
high resolution charge density study. Since somewhat sur-
prisingly, the recent report of polyhapto–aromatic interac-
tions in lead(II) coordination suggested that the active lone
pair in the Pb(II) may be involved in donor bonding [22].
Comparison with this reported Pb(II) compound [22] and
because of the following reasons it may strongly be
reflected that Tl(I) may be had higher capacity than Pb(II)
to act as a Lewis base.
the Tl+–HBꢀ complex in methanol solution was also inves-
tigated by a conductometric method. Twenty milliliters of
thallium(I) nitrate solution (5.0 · 10ꢀ5 M) in methanol
was placed in a two-wall thermostated glass cell, the tem-
perature was adjusted to 25.00 0.05 ꢁC, and the conduc-
tance of solution was measured. Then a known amount of
the concentrated solution of ligand (HBꢀ) in methanol
(5.0 · 10ꢀ3 M) was added in a stepwise manner using a
5-ll Hamilton syringe. The conductance of the solution
was measured after each addition. The ligand solution
was continually added until the desired ligand to metal
ion mole ratio was achieved. The conductivity of a
5.0 · 10ꢀ5 M solution of thallium nitrate solution in meth-
anol was monitored as a function of [HBꢀ]/[Tl+] mole ratio
at 25.00 0.05 ꢁC and the resulting plot is shown in
Fig. S5. As it is seen, addition of the ligand to the metal salt
solution then causes a increase in the solution conductivity
possessing a inflection point at a molar ratio of about one,
indicating the formation of a 1:1 Tl+–HBꢀ complex in
solution.
1. The higher polyhapto interactions for Tl atoms in the
reported compound here than the Pb compound [22],
coordination sphere of Tl(TlO4C12) in compound 1
compared with Pb(PbO5C8) reported recently [22].
2. The shorter M ꢁ ꢁ ꢁ p for Tl atoms in the reported com-
pound here than the Pb compound [22], Tl ꢁ ꢁ ꢁ p =
˚
˚
3.287(3) A compare to Pb ꢁ ꢁ ꢁ p = 3.408(3) A from the
centroid of the phenyl groups.
3. The lower oxidation number +1 for Tl atoms compare
to +2 for Pb atoms in the reported Pb compound [22].
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 [23,24] using a non-linear least-squares
curve fitting program KINFIT [25]. The conditional
formation constants of the complex were evaluated as
0.36 0.01 and 0.34 0.01 from the spectrophotometric
and conductometric methods, respectively, suggesting that
these values of Kf are very low. In the 13C NMR, there is no
direct measure of changes in aromatic carbon electron den-
sity between compound 1 and the ligand HBꢀ which would
enable a distinction between donor and acceptor functions
of the aromatic ring to be made in the solution state. This
point and also very low formation constant of the complex
may say that interaction between ligand HBꢀand TlI ions
in the solution state is very weak and the polymeric com-
plex in compound 1 cannot retain in solution state and
probably as it anticipate extensive solvation occur in both
methanol and DMSO.
However structural determination of compound 1 show
two factors, hydrogen bonding and polyhapto interactions
that may control the packing of this compound.
In order to examine the thermal stability of the com-
pound 1, thermal gravimetric (TG) and differential thermal
analyses (DTA) were carried out between 30 and 700 ꢁC.
Compound 1 does not melt and is stable up to 185 ꢁC at
which temperature it begins to melt and the decomposition
of 4-hydroxybenzoate (HBꢀ) anions take place at 200–
420 ꢁC with one endothermic and two exothermic effects
at 245, 320 and 395 ꢁC, respectively. The solid residue
formed at around 420 ꢁC is thus suggested to be Tl2O,
(Fig. S3).
In spectrophotometric procedure, 2.0 ml of ligand solu-
tion (2.5 · 10ꢀ5 M), after deprotonate the carboxylic acid
group of H2B with KOH, in methanol was placed in the
spectrophotometer cell and the absorbance of solution
was measured. Then a known amount of the concentrated
solution of thallium(I) nitrate in methanol (1.3 · 10ꢀ3 M)
was added in a stepwise manner using a 5-ll Hamilton
syringe. The absorbance spectrum of the solution was
recorded after each addition. The thallium(I) ion solution
was continually added until the desired metal to ligand
mole ratio was achieved. The electronic absorption spectra
of the ligand 4-hydroxybenzoate (HBꢀ) in the presence of
increasing concentration of thallium(I) ion in methanol at
room temperature are shown in Fig. S4. As is obvious,
the strong absorption of ligand at 267.2 nm, which is
related to p ! p* transition, increases with increasing con-
centration of the metal ion. The resulting absorbance
against [Tl+]/[HBꢀ] mole ratio plot, shown in the inset of
Fig. S4, revealed a 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
In summary, the structure of compound 1 is novel and
interesting because it represents a new, rarely observed
two-dimensional framework self-assembled from polyha-
pto–aromatic interactions in thallium(I) coordination,
and indicates the active lone pair on TlI in this compound
may be covered by involvement in donor bonding.
Acknowledgement
Support of this work by the Iran National Science
Foundation, INSF (Project No. 84118) is gratefully
acknowledged.
Appendix A. Supplementary material
CCDC 628812 contains the supplementary crystallo-
graphic data for 1. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.

5112
K. Akhbari, A. Morsali / Journal of Organometallic Chemistry 692 (2007) 5109–5112
H2O was added to the mixture and was heated and stirred for an
hour. After filtering it was allowed to evaporate for several days and
then suitable colorless crystals were obtained. The crystals were
washed with acetone and air dried, d.p. = 185 ꢁC, Yield: 0.150 g
(44%, based on H2B). C7H503Tl: C, 24.59; H, 1.45; Tl, 59.82. Found:
C, 24.80; H, 1.60; Tl, 59.10. IR (selected bands; in cmꢀ1): 614 m, 777
m, 851 m, 1096 w, 1160 m, 1248 s, 1382 vs, 1522 vs, 1585 s. 1H NMR
(DMSO): 6.62–6.81 (d, 2H), 7.61–7.83 (d, 2H), 9.34–10.22 (broad,
1H) ppm. 13C–{1H} NMR (DMSO): 114.42, 130.92, 132.15, 159.88
and 172.21 ppm.
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.
2007.08.019.
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˚
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