Structural diversity in chemistry part VI1: Analysis of structure correlation in organoammonium bromothallates(III)

Article abstract of DOI:10.1002/hlca.200800316

New bromothallate(III) complexes have been synthesized using eleven organoammonium cations related to others employed previously. The crystal structure determinations of eight of these have been performed. The diethylenetriammonium cation yields the first

Full text of DOI:10.1002/hlca.200800316

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Structural Diversity in Thallium Chemistry
Part VI1)
Analysis of Structure Correlation in Organoammonium Bromothallates(III)
by Anthony Linden*a), Alexander Petridisb), and Bruce D. James*b)
a) Institute of Organic Chemistry, University of Zꢀrich, Winterthurerstrasse 190, CH-8057 Zꢀrich
b) Department of Chemistry, La Trobe University, Vic. 3086, Australia
New bromothallate(III) complexes have been synthesized using eleven organoammonium cations
related to others employed previously. The crystal structure determinations of eight of these have been
performed. The diethylenetriammonium cation yields the first compound containing [TlBr6]3ꢀ as the
only anionic species, while the N,N,N’,N’’,N’’-pentamethyldiethylenetriammonium cation gave a mixed
salt containing discrete [TlBr4]ꢀ and Brꢀ ions, in which the [TlBr4]ꢀ unit does not participate in hydrogen
bonding. In the pyrazolinium mixed salt, both the [TlBr4]ꢀ and Brꢀ ions are hydrogen-bonded to cations
and disposed in an arrangement similar to a [Tl2Br9]3ꢀ unit. The triethylenediammonium dication gives a
compound in which a highly distorted trigonal bipyramidal [TlBr5]2ꢀ system is present, while the 1-
benzylpiperazinium compound also shows a very weak [TlBr4]ꢀ and Brꢀ interaction. Six other
monovalent cations provided [TlBr4]ꢀ derivatives that were characterized either crystallographically or
via their Raman spectra. In a comprehensive structural summary, the various interactions between
cations and bromothallate(III) anionic groupings, giving rise to the structural diversity described in this
and previous reports, are discussed in terms of the cation charge, quaternization and steric influence,
NꢀH ··· Br hydrogen bonding, and secondary bonds present in the compounds.
1. Introduction. – Until a few years ago, little was known about the structural
diversity displayed by halothallate(III) anions, and, in particular, the bromothalla-
te(III) species. In an extensive study, the use of organic cations has led to the
characterization of a range of new bromothallate(III) derivatives. A variety of large
univalent cations, encompassing a variety of alkyl diammonium cations [2] and
heterocyclic cations [1], have yielded bromothallate(III) complexes of differing
stoichiometry. These complexes included, in addition to the well-known [TlBr4]ꢀ
species [3], examples of bromothallate(III) compounds containing unusual mixed
salts. For example, the N,N-diethylpropane-1,3-diammonium salt contains each of a
[TlBr4]ꢀ and an axially compressed octahedral [TlBr6]3ꢀ anion, while the hexane-1,5-
diammonium complex contains a tetrahedral [TlBr4]ꢀ anion, a slightly distorted
octahedral [TlBr6]3ꢀ and a Brꢀ anion.
Moreover, further examples of the somewhat lesser known [TlBr5]2ꢀ moieties have
also been synthesized. These anions display a range of geometries, such as regular and
highly distorted axially-stretched trigonal bipyramidal forms and also cis-bromo-
1
)
For Part V, see [1].
ꢁ 2009 Verlag Helvetica Chimica Acta AG, Zꢀrich

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bridged chains [4 – 6]. In addition, a bromothallate(III) complex appeared as a
distorted octahedron with one long contact from an adjacent anion, thus completing the
hexacoordination about an otherwise distorted square pyramid [2], further expanding
what was known pertaining to these systems.
In the next phase of this study, reported here, instead of employing an entirely new
series of cations, it was decided to focus on the use of a number of other
organoammonium cations that are chemically, electronically, or sterically related to
those employed in the earlier investigations. This selection of cations allows further
comparisons with known bromothallate(III) complexes, and could potentially lead to a
clearer picture regarding any controlling relationships between the bromothallate(III)
species obtained in the solid state, and the nature of the cation or the synthetic
parameters. The eleven cations chosen for this study are depicted in Table 1. The
introduction of two trivalent cations is to some degree new territory, but these are
related structurally to some of the open-chain cations already employed. The crystal
structures of eight bromothallate(III) compounds obtained with these cations are
described, while the anionic species present in the remaining compounds were deduced
from spectroscopic data.
In the second part of this work, as a culmination of our extensive study, the now
more substantial library of known structures of organoammonium bromothallate(III)
salts is analyzed with the aim of determining if any structure-correlation can be
established in terms of the influence of the cation shape, size, charge, quaternization,
acidity, or NꢀH ··· Br H-bond-donor capabilities on the nature of the anionic species
that then appears in the solid state.
2. Experimental. – 2.1. Syntheses. All reagents were obtained from the Aldrich Chemical Co.,
Milwaukee, WI, and used as received. Solns. of thallium(III) bromide in the chosen solvent were
generated from thallium(I) nitrate or sulfate, as described by Brauer [7]. N,N,N’,N’’,N’’-Pentamethyl-
diethylenetriammonium bromide was synthesized by reacting diethylenetriamine with MeBr, according
to the general quaternization procedure of Oae et al. [8]. Compounds 1 – 112) (Table 2) were prepared
conveniently by mixing the thallium(III) bromide solutions with those containing equimolar quantities of
the org. base bromide salts in the same solvent and then warming each mixture to dissolve any resulting
precipitate. For crystal growth, various evaporation techniques were employed to minimize the rate of
vapor diffusion and solvent loss so that well-formed crystals could be obtained. Generally, several crops
of crystals were obtained, and these were isolated by filtration. None of the compounds appeared to be
moisture-sensitive, but they were stored routinely in a desiccator over P2O5 and KOH.
2.2. Raman Spectroscopy. Solid-state Raman spectroscopy was employed wherever possible to locate
TlꢀBr vibrational modes, which are particularly diagnostic for the presence of [TlBr4]ꢀ species [9].
Spectra were recorded either on a Dilor XY confocal Raman microprobe instrument with the 514.3-nm
line from a Spectra Physics 2016 Arþ laser, or using a Renishaw Ramanscope model 2000 with 633- or 780-
nm laser excitation. Even though crystalline samples were crushed, spectra were recorded on crystals of
varying orientation and using different polarizations to ensure that any Raman bands were not missed
due to single-crystal selection rules. Reference spectra of the org. base cations as their bromide salts were
also recorded.
2
)
The cations employed in compounds 1 – 11 have been labelled I – XI (Tables 1 and 2) for
convenience. Other compounds studied previously in this project are mentioned in the references,
and the cations that were employed have been numbered from XII onwards (see Tables 12 – 14).

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Table 1. Cations Used to Generate New Bromothallate(III) Salts in This Work
Diethylenetriammonium (I)
N,N,N’,N’’,N’’-Pentamethyldiethylenetriammonium (II)
Triethylenediammonium (III)
1-Benzylpiperazinium (IV)
Pyrazolinium (V)
1,5-Diazabicyclo[4.3.0]non-5-enium (VI)
Trimethyl(phenyl)ammonium (VII)
(Pyridin-2-yl)-(2-pyridinium)amine (VIII)
1,8-Diazabicyclo[5.4.0]undec-7-enium (IX)
Piperidinium (X)
Quinolinium (XI)
2.3. X-Ray Crystallography. The crystallographic data for compounds 1 – 8 are given in Table 33). All
measurements were conducted at low-temp. on a Nonius KappaCCD area-detector diffractometer [10]
3
)
CCDC-697489 – 697496 contain the supplementary crystallographic data for complexes 1 – 8. These
data can be obtained free of charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.

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using graphite-monochromated MoKa radiation (l 0.71073 ꢂ) and an Oxford Cryosystems Cryostream
700 cooler. Data reduction was performed with HKL Denzo and Scalepack [11]. The intensities were
corrected for Lorentz and polarization effects. For 1, an empirical absorption correction was applied
using SORTAV, which is based on an analysis of symmetry-equivalent reflections in the highly redundant
data set [12]. A numerical absorption correction [13] was applied to each of the remaining data sets.
Each structure was solved by direct methods using SIR92 [14], which revealed, at a minimum, the
positions of the Tl- and Br-atoms. All remaining non-H-atoms were located in subsequent difference
Fourier maps.
In 1, the Tl-atom lies on a center of inversion, while the central N-atom of the cation lies on a C2 axis.
In 6, the anion sits across a mirror plane with the Tl- and two Br-atoms on the mirror. The cation lies over
a crystallographic center of inversion and is, therefore, disordered about this symmetry center. The
overlapping disordered orientations of the ions caused instability in the refinement of the atomic
positions. Therefore, bond-length restraints were applied to most of the bonds within the cation in order
to maintain reasonable geometry. Neighboring atoms within and between each disordered orientation of
the cation were also restrained to have similar atomic displacement parameters. Compound 8 contains
two symmetry-independent cations, each of which is disordered across a center of inversion. The disorder
superimposes inverted copies of the cations upon themselves. This arrangement necessitated defining the
amino N-atom in each cation with a site occupation factor of 0.5, while the site for each C-atom adjacent
to the amino N-atom, as well as the sites of the pyridinyl and pyridinium N-atoms, are occupied by 0.5C þ
0.5N, where the atoms used to define a composite atom site were constrained to have identical fractional
coordinates and anisotropic displacement parameters.
The non-H-atoms in each structure were refined anisotropically. For 1, the H-atom of the central
ammonium group of the cation was placed in the position indicated by a difference Fourier map, and its
position was allowed to refine. The remaining H-atoms, as well as those in each of the other structures,
were placed in geometrically calculated positions and constrained to ride on their parent atoms. Each H-
atom was assigned an isotropic displacement parameter with a value equal to 1.2 Ueq of its parent atom
(1.5Ueq for Me and ꢀNHþ3 groups). A circular difference Fourier map was calculated and used to
estimate the orientation of the H-atoms of the symmetry-unique ꢀNHþ3 group in 1 and of the Me groups
in 2 and 7. Although the H-atoms could not be located definitively for 6, in adding the H-atoms to the
model, it was assumed that the cation is protonated at the imine N-atom, N(5). This assumption was
supported by the fact that a logical NꢀH ··· Br H-bond is evident only if this N-atom is protonated. For 8,
the cations could be protonated at the amino N-atom or at a pyridinyl N-atom, or possibly even
disordered between the two pyridinyl N-atoms, but the disorder of the cations made it difficult to locate
the positions of these H-atoms definitively. Protonation at any of the possible sites leads to a reasonable
arrangement of H-bonds. While refinement of the various alternative models left the R-factor virtually
unchanged, a test refinement of the isotropic atomic displacement parameters of the amino and
pyridinium H-atoms led to reasonable values for these parameters only when the cations were defined as
having one amino and one pyridinium H-atom, with the latter not being disordered between the two
pyridinyl N-atoms. Also, the amino NꢀC bond lengths of ca. 1.35 ꢂ are shorter than would be expected if
the amino group was protonated.
The refinement of each structure was carried out on F2 using full-matrix least-squares procedures,
which minimized the function Sw(Fo2 ꢀ Fc2 )2. Complex 3 crystallizes in a polar space group, and the
correct absolute structure has been confirmed by refinement of the absolute structure parameter
[15][16], which converged to a value of ꢀ 0.016(6). Corrections for secondary extinction were applied in
each case. For 5, four reflections, whose intensities were considered to be outliers, were omitted from the
final cycles of refinement. For each structure, the largest peaks of residual electron density were always
within the vicinity of the Tl- or Br-atoms. The scattering factors for non-H-atoms were taken from [17],
and the scattering factors for H-atoms were taken from [18]. For 3, anomalous dispersion effects were
included in Fc [19]; the values for f’ and f’’ were those from [20]. The values of the mass attenuation
coefficients were those from [21]. All calculations were performed using SHELXL97 [22]. The figures
were drawn using ORTEPII [23] and the PLUTON routine in the program PLATON [24].

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3. Results and Discussion. – 3.1. Salts with Trivalent Cations. The diethylenetriam-
monium (I) and N,N,N’,N’’,N’’-pentamethyldiethylenetriammonium (II) cations
provide an extension of the open-chain alkyl diammonium series [2] and are
interesting, because they are the first trivalent cations employed in these investigations.
In addition, it was anticipated that such large cations would stabilize a similarly large
counter-anion with equal but opposite charge, in accordance with Basoloꢃs general-
ization [25].
Indeed, for the complex 1 with [C4H16N3] · [TlBr6] stoichiometry, the X-ray
crystallography revealed the presence of discrete [C4H16N3]3þ cations and [TlBr6]3ꢀ
anions (Fig. 1). The cation has crystallographic C2 symmetry about the central N-atom
and the anion lies on a center of inversion. The anion deviates somewhat from perfect
octahedral geometry in that it is squashed along one axis, with these TlꢀBr bonds being
ca. 0.19 ꢂ shorter than the remaining TlꢀBr bonds (Table 4). A similar TlꢀBr bond
shortening of ca. 0.18 ꢂ was observed along one axis of the Ci-symmetric octahedral
[TlBr6]3ꢀ anion in the N,N-diethylpropane-1,3-diammonium cation (XII) mixed salt
[2]. Mixed salts containing [TlBr6]3ꢀ anions have also been found in the complexes
involving the 1,1,3,3-tetramethylimidazolidinium and hexane-1,5-diammonium cations
(XIII and XIV, resp.) [2][4], but the distortions of the octahedra were minimal, with
the anions possessing S4 and S6 symmetry, respectively. Compound 1 is unique in that it
is the first example of a bromothallate(III) salt with an organic base cation in which a
[TlBr6]3ꢀ anion is present as the sole anionic species.
Fig. 1. The components in the structure of [C4H16N3] · [TlBr6] (1) showing the atom-labeling scheme and
50% probability for the displacement ellipsoids. The labels with ’ and ’’ indicate atoms generated from the
unique atoms by the inversion and C2 symmetry, respectively.
The ions in 1 are linked by a complex three-dimensional network of NꢀH ··· Br H-
bonds with each cation affording eight potential H-bond donors (Table 5 and Fig. 2).
Each of the H-atoms of the central amine group interacts with a different [TlBr6]3ꢀ
anion, and each H-atom forms bifurcated H-bonds with two different Br-atoms from
the same anion. For the symmetry-unique terminal amine group, one of the H-atoms
forms trifurcated H-bonds, although two of them are quite weak. The corresponding
acceptor Br-atoms all come from the same anion, but this anion is not one of the anions
that interact with the central N-atom. The other two H-atoms of the termnal amine

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Table 4. Selected Interatomic Distances [ꢂ] and Angles [8] for [C4H16N3] · [TlBr6] (1) and [C9H26N3]2 ·
[TlBr4]3 · 3 Br (2), with Standard Uncertainties in Parenthesesa)
Complex 1
TlꢀBr(1)
2.8064(4)
2.6616(4)
TlꢀBr(3)
2.7872(4)
TlꢀBr(2)
Br(1)ꢀTlꢀBr(1)i
Br(2)ꢀTlꢀBr(2)i
Br(3)ꢀTlꢀBr(3)i
180
180
180
Br(1)ꢀTlꢀBr(2)
Br(1)ꢀTlꢀBr(3)
Br(2)ꢀTlꢀBr(3)
89.32(1)
89.36(1)
89.72(1)
Complex 2
Tl(1)ꢀBr(4)
Tl(1)ꢀBr(5)
Tl(1)ꢀBr(6)
Tl(1)ꢀBr(7)
Tl(2)ꢀBr(8)
Tl(2)ꢀBr(9)
2.5337(7)
2.5821(6)
2.5544(7)
2.5365(7)
2.5549(7)
2.5497(7)
Tl(2)ꢀBr(10)
Tl(2)ꢀBr(11)
Tl(3)ꢀBr(12)
Tl(3)ꢀBr(13)
Tl(3)ꢀBr(14)
Tl(3)ꢀBr(15)
2.5727(7)
2.5406(7)
2.5570(7)
2.5402(7)
2.5701(7)
2.5300(7)
Br(4)ꢀTl(1)ꢀBr(5)
Br(4)ꢀTl(1)ꢀBr(6)
Br(4)ꢀTl(1)ꢀBr(7)
Br(5)ꢀTl(1)ꢀBr(6)
Br(5)ꢀTl(1)ꢀBr(7)
Br(6)ꢀTl(1)ꢀBr(7)
Br(8)ꢀTl(2)ꢀBr(9)
Br(8)ꢀTl(2)ꢀBr(10)
Br(8)ꢀTl(2)ꢀBr(11)
107.73(2)
115.68(3)
110.06(3)
103.89(2)
108.86(2)
110.24(3)
108.03(2)
108.53(2)
112.34(2)
Br(9)ꢀTl(2)ꢀBr(10)
Br(9)ꢀTl(2)ꢀBr(11)
Br(10)ꢀTl(2)ꢀBr(11)
Br(12)ꢀTl(3)ꢀBr(13)
Br(12)ꢀTl(3)ꢀBr(14)
Br(12)ꢀTl(3)ꢀBr(15)
Br(13)ꢀTl(3)ꢀBr(14)
Br(13)ꢀTl(3)ꢀBr(15)
Br(14)ꢀTl(3)ꢀBr(15)
105.72(2)
115.08(2)
106.77(2)
114.14(2)
107.31(2)
103.15(2)
105.15(2)
116.95(3)
109.85(3)
a
i
) Symmetry operator: 1 ꢀ x, ꢀ y, 1 ꢀ z.
group each interact singly with additional [TlBr6]3ꢀ anions, one of which is one of the
anions involved with one of the central amine H-atoms. The other terminal amine
group, being related to the first by the C2 axis of the cation, has an identical set of
interactions with symmetry-related anions. The interactions result in each ion being H-
bonded to six different counter-ions. Two of the three symmetry-independent Br-atoms
of the [TlBr6]3ꢀ anion each accept three H-bonds. The Br-atom of the shorter TlꢀBr
bond only accepts one H-bond, which is presumably the reason that this Br-atom forms
the shortest TlꢀBr bond. This geometry is consistent with the previously discussed
observation [2] that the shortest TlꢀBr bonds in the [TlBr6]3ꢀ anions of the mixed salts
from cations XII and XIValways involve the Br-atoms that accept the fewest, if any, H-
bonds of any of the Br-atoms in these anions. By contrast, the quaternized cation XIII
precludes any H-bonding, and the TlꢀBr bond lengths in the [TlBr6]3ꢀ anion of its
bromothallate(III) mixed salt are all found to be equal by symmetry [4]. This
observation further supports the hypothesis mentioned earlier [2] that the NꢀH ··· Br
H-bonding interactions are extracting the acceptor Br-atom from the Tl-atom by
decreasing the available electron density on that Br-atom, thus weakening the
coordination strength of the Br-atom and producing a slightly longer TlꢀBr bond.
The Raman spectrum of 1 shows three peaks at 161, 153, and 95 cmꢀ1, which have
been assigned to the n1(A1g), n2(Eg), and n5(F2g) modes, respectively, expected for an

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Helvetica Chimica Acta – Vol. 92 (2009)
Fig. 2. A projection of the crystal packing of 1 viewed down [0 1 0] showing the H-bonding as dashed lines.
The H-atoms bonded to C-atoms have been omitted for clarity.
octahedral [TlBr6]3ꢀ unit. These bands are consistent with those tentatively assigned to
the [TlBr6]3ꢀ anion in the other mixed salts (cations XII, XIII, and XIV), although it
has to be borne in mind that those salts also incorporated [TlBr4]ꢀ ions [2][4].
Unfortunately, a comparison spectrum from a well-characterized [TlBr6]3ꢀ complex is
not available in the literature. While these assignments are similar to those made for
Rb3TlBr6 · 8/7 H2O, the spectrum of the Rb salt and that of K3TlCl6 · H2O are more
complex than expected [26]. In the latter case, the crystal structure is reported to
contain one [TlCl5 · H2O]2ꢀ and two [TlCl6]3ꢀ units, which give rise to two Raman lines
[27]. Only preliminary results for the structure of hydrated rubidium hexabromothal-
late(III) have ever appeared [28], and it is possible that similar halide/H2O ligand
exchange also occurs in that salt and affects the spectrum.
In contrast to 1, the quite similar N,N,N’,N’’,N’’-pentamethyldiethylenetriammo-
nium cation (II) generates a bromothallate(III) 2, with [C9H26N3]2 · [TlBr5]3 stoichi-
ometry, whereas the crystal structure reveals a mixed salt containing discrete [TlBr4]ꢀ

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Table 5. H-Bonding Parameters for [C4H16N3] · [TlBr6] (1) and [C9H26N3]2 · [TlBr4]3 · 3 Br (2), with
Standard Uncertainties in Parentheses
DꢀH ··· Aa)
DꢀH [ꢂ]
H ··· A [ꢂ]
D ··· A [ꢂ]
DꢀH ··· A [8]
Complex 1
N(1)ꢀH(11) ··· Br(1)
N(1)ꢀH(11) ··· Br(2)
N(1)ꢀH(11) ··· Br(3)
N(1)ꢀH(12) ··· Br(1)i
N(1)ꢀH(13) ··· Br(3)ii
N(4)ꢀH(4) ··· Br(1)iii
N(4)ꢀH(4) ··· Br(3)iv
0.91
0.91
0.91
0.91
0.91
0.88(4)
0.88(4)
2.82
2.88
2.94
2.44
2.36
2.79(4)
2.80(5)
3.514(4)
3.510(4)
3.560(4)
3.339(4)
3.242(4)
3.4027(5)
3.483(4)
134
128
126
171
163
128(4)
136(4)
Complex 2
N(1)ꢀH(1) ··· Br(2)v
N(4)ꢀH(4) ··· Br(1)
N(7)ꢀH(7) ··· Br(1)vi
N(13)ꢀH(13) ··· Br(3)
N(16)ꢀH(16) ··· Br(3)
N(19)ꢀH(19) ··· Br(2)
0.93
0.93
0.93
0.93
0.93
0.93
2.31
2.37
2.34
2.47
2.26
2.32
3.227(5)
3.280(5)
3.250(5)
3.332(5)
3.184(5)
3.222(5)
167
167
166
154
175
163
a
1
) Symmetry operators: i 1 ꢀ x, 1/2 þ y, 3/2 ꢀ z; ii 1 ꢀ x, 1 ꢀ y, 1 ꢀ z; iii 1/2 ꢀ x, 1/2 ꢀ y, z; iv x ꢀ /2, 1/2 þ y, 1 ꢀ z;
1 ꢀ x, 2 ꢀ y, 1 ꢀ z; vi ꢀ x, 2 ꢀ y, 1 ꢀ z.
v
and Brꢀ anions (Fig. 3). The asymmetric unit in the structure contains two cations,
three [TlBr4]ꢀ anions, and three Brꢀ ions, with none of these entities occupying any
special symmetry sites. There are no Tl ··· bromide-ion distances shorter than 5.7 ꢂ,
which precludes any weak interactions between the [TlBr4]ꢀ and Brꢀ ions. The shortest
interionic Tl ··· Br distance of 4.2104(7) ꢂ is between two [TlBr4]ꢀ anions and is
marginally inside the accepted value of ca. 4.3 ꢂ for the sum of the Van der Waals radii
of the Tl- and Br-atoms [29]. This interaction involves Tl(2)ꢀBr(9) ··· Tl(3). Indeed, as
shown in Table 4, the distortions of the [TlBr4]ꢀ anions from ideal tetrahedral geometry
are minimal, but greatest for the [TlBr4]ꢀ anion involving the atom Tl(3), which is
consistent with a small influence of the atom Br(9) on the geometry of this anion. If this
interaction is taken as significant, the two anions involved could be described as a
[Tl2Br8]2ꢀ unit, and the overall structure would then have the formulation [C9H26N3]2 ·
[Tl2Br8] · [TlBr4] · 3 Br. However, given that the Tl(2)ꢀBr(9) ··· Tl(3) angle is quite
sharp at 1248, and the proximity of the Tl(3) ··· Br(9) distance to the Van der Waals
limit, it is doubtful that this arrangement of the [TlBr4]ꢀ anions represents the presence
of true interactions in the form of Tl ··· Br secondary bonding [30][31].
All of the potential donor H-atoms of the cations in 2 are involved in NꢀH ··· Br H-
bonding interactions, and the acceptors are always the isolated Brꢀ anions (Table 5).
The cations and Brꢀ anions intertwine themselves into columns which run parallel to
the [1 0 1] direction. All of the NꢀH groups point towards the center of the column, and
the Brꢀ anions occupy the core of the column (Fig. 4). Within the columns, the H-bonds
do not form continuous chains, but any one set of interactions is limited to a finite
segment of the column. Each segment is composed of four cations and six Brꢀ anions
involving two sets of all symmetry-independent constituent moieties arranged about a
center of inversion. The segment starts with cation B (N(13), N(16), and N(19)), which

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Helvetica Chimica Acta – Vol. 92 (2009)
Fig. 3. The asymmetric unit in the structure of [C9H26N3]2 · [TlBr4]3 · 3 Br (2) showing the atom-labeling
scheme and 50% probability for the displacement ellipsoids
forms two H-bonding interactions with the same Brꢀ ion, Br(3). The third interaction
from cation B is with a different Brꢀ ion, Br(2), which also accepts a H-bond from one
end of cation A (N(1), N(4), and N(7)). Thus, Br(2) is bridging between cations A and
B. The remaining two donors in cation A each form a H-bond with different symmetry-
related Br(1) ions. These anions are related by a center of inversion and, therefore, each
accepts two H-bonds from symmetry-related A cations. Thus, two Br(1) anions bridge
between two cations of type A. Because of the center of inversion, this second cation A
is further connected to another cation B, whereupon the segment ends exactly as it
began. The H-bonded segment can thus be depicted schematically as shown in Fig. 5
and represented in the packing diagram displayed in Fig. 6.
Mixed salts containing essentially discrete [TlBr4]ꢀ and Brꢀ ions in various ratios
have also been found in the salts from 1,10-phenanthrolinium (XV) [5], the
pyridinium-based species, 4,4’-dimethyl-2,2’-bipyridinium, pyridinium, and 2-bromo-

Helvetica Chimica Acta – Vol. 92 (2009)
41
Fig. 4. A projection of the crystal packing of 2 viewed down [1 0 1] showing the columns of cations
wrapped around the Brꢀ ions at their core. These columns are then interspersed by columns of [TlBr4]2ꢀ
anions. The H-atoms bonded to C-atoms have been omitted for clarity.
pyridinium (XVI, XVII, and XVIII, resp.) [1][5], and the alkyl cations, N,N,N’,N’-
tetramethylpropane-1,3-diammonium and N,N,N’,N’-tetraethylethylene-1,2-diammo-
nium (XIX and XX, resp.) [2]. In all these, as in 2, the [TlBr4]ꢀ anions never act as
acceptors of H-bonds, these interactions being restricted to the Brꢀ ions. An exception
is the pyrazolinium salt (5; vide infra), which also contains discrete [TlBr4]ꢀ and Brꢀ
ions, but both species act as acceptors of H-bonds.
Having obtained a [TlBr6]3ꢀ anion with the trivalent diethylenetriammonium cation
in 1, as predicted by Basoloꢃs generalization [25], it might reasonably be expected that
the closely related trivalent cation II should also yield a complex in which [TlBr6]3ꢀ
anions are present. That the mixed salt, 2, without any [TlBr6]3ꢀ anions, was obtained
instead, might be attributed to the larger size of the latter cation caused by the five
additional Me substituents. Whereas the relative sizes of the cation and anion in 1
apparently allow the ions to pack comfortably in a 1:1 ratio to give a cation/Tl/Br ratio
of 1:1:6, the larger cation in 2 must be less compatible with the size of the [TlBr6]3ꢀ

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Fig. 5. Schematic of the discrete H-bonded segment in 2
Fig. 6. A projection of the crystal packing of 2 viewed down [0 0 1] showing the H-bonding as dashed
lines. Three horizontal columns of cations and Brꢀ ions are shown, and each contains two discrete H-
bonded segments. The H-atoms bonded to C-atoms have been omitted for clarity.

Helvetica Chimica Acta – Vol. 92 (2009)
43
anion, so the anionic arrangement expands into one that requires more volume, but still
maintains the required charge balance. This requirement is achieved with the
[C9H26N3]2 · [TlBr4]3 · 3 Br stoichiometry in 2, which has a cation/Tl/Br ratio of 2 :3 :15.
3.2. Salts with Divalent Cations. The triethylenediammonium (DABCO) cation
(III) stabilizes a bromothallate(III) salt 3, with the [C6H14N2] · [TlBr5] stoichiometry. In
the crystal structure, the asymmetric unit contains one cation and a highly distorted
trigonal bipyramidal [TlBr5]2ꢀ anion with one very long axial Tl ··· Br distance of
3.6252(5) ꢂ (Fig. 7). This geometry can effectively be described as being derived from
the distortion of a tetrahedral [TlBr4]ꢀ unit by the close approach or secondary bonding
[30][31] of the fifth Br-atom. The TlꢀBr bonds involving atoms Br(3), Br(4), and
Br(5), which would be equatorial in a symmetrical trigonal-bipyramidal ion, are bent at
the metal atom away from atom Br(2) towards the more distant Br(1)-atom (Table 6).
The distances of the Tl-, Br(1)- and Br(2)-atoms from the least-squares equatorial
plane defined by Br(3), Br(4), and Br(5) are 0.5872(3), ꢀ 3.0324(6), and 3.1965(6) ꢂ,
respectively. Thus, the two axial Br-atoms are almost equidistant from the equatorial
plane, and all five Br-atoms clearly belong to the one trigonal-bipyramidal anionic unit
in which the Tl-atom is displaced significantly towards one of the axial Br-atoms. The
TlꢀBr(2) bond, which is trans to the long interaction (Br(1) ··· TlꢀBr(2) 175.09(2)8), is
only 0.06 ꢂ longer than the equatorial TlꢀBr bonds. Secondary bonding between Brꢀ
ions and TlIII to give very similar axially distorted trigonal bipyramidal [TlBr5]2ꢀ anions
with long Tl ··· Br distances of ca. 3.77 ꢂ has been found in the structures of the
piperazinium (XXI) and 2,2’-bipyridinium (XXII) salts [5], and a slightly shorter Tl ···
Br distance of 3.40 ꢂ is present in the [TlBr5]2ꢀ anion of the 2-(ammoniomethyl)pyr-
idinium (XXIII) salt [1]. These contrast with the more regular trigonal bipyramidal
[TlBr5]2ꢀ species (longest TlꢀBr 2.73 – 2.92 ꢂ) found in the structures of the N,N’-
diethyl-N,N,N’,N’-tetramethylethylene-1,2-diammonium (XXIV) [2] and N,N’-dieth-
yltriethylenediammonium (diethyl-DABCO; XXV) salts [4] and in both polymorphs
of the 1,1,4,4-tetramethylpiperazinium (XXVI) compound [4][6].
Fig. 7. The asymmetric unit in the structure of [C6H14N2] · [TlBr5] (3) showing the atom-labeling scheme
and highlighting the long secondary Tl ··· Br(1) bond of 3.6252(5) ꢀ. The displacement ellipsoids are
shown at the 50% probability level.

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Table 6. Selected Interatomic Distances [ꢂ] and Angles [8] for [C6H14N2] · [TlBr5] (3) and [C11H18N2] ·
[TlBr4] · Br (4), with Standard Uncertainties in Parentheses
3
4
TlꢀBr(1)
TlꢀBr(2)
TlꢀBr(3)
TlꢀBr(4)
TlꢀBr(5)
3.6252(5)
2.6123(6)
2.5429(5)
2.5532(4)
2.5537(5)
4.0224(5)
2.5980(5)
2.5173(5)
2.5496(5)
2.5713(5)
Br(1)ꢀTlꢀBr(2)
Br(1)ꢀTlꢀBr(3)
Br(1)ꢀTlꢀBr(4)
Br(1)ꢀTlꢀBr(5)
Br(2)ꢀTlꢀBr(3)
Br(2)ꢀTlꢀBr(4)
Br(2)ꢀTlꢀBr(5)
Br(3)ꢀTlꢀBr(4)
Br(3)ꢀTlꢀBr(5)
Br(4)ꢀTlꢀBr(5)
175.09(2)
75.89(1)
74.46(1)
172.41(1)
77.85(1)
68.92(1)
79.78(1)
78.07(1)
105.58(2)
100.78(2)
103.54(2)
115.61(2)
114.84(2)
114.14(2)
108.76(2)
104.33(2)
102.06(2)
115.00(2)
113.27(2)
112.11(2)
Each cation in 3 forms strong NꢀH ··· Br H-bonds with only the weakly bound axial
Br-atom of two different [TlBr5]2ꢀ anions, and this Br-atom, in turn, accepts H-bonds
from two different cations (Table 7). The H-bonds link the cations and anions into
continuous chains which run parallel to the [010] direction in the sequence: ···
(cation) ! Brꢀ (cation) ! Brꢀ ··· (Fig. 8). This H-bonding pattern can be described
by the graph set motif [32] of C12(7). Interestingly, although no chiral moieties are
present, 3 crystallizes in a polar space group, as did the complex from cation XXV [4],
and the absolute structure has been determined by refinement of the absolute structure
parameter [15][16], which converged to a value of ꢀ 0.016(6).
The 1-benzylpiperazinium cation (IV) stabilizes a bromothallate(III) salt 4, with
the [C11H18N2] · [TlBr5] stoichiometry. The asymmetric unit in the crystal structure
Table 7. H-Bonding Parameters for [C6H14N2] · [TlBr5] (3) and [C11H18N2] · [TlBr4] · Br (4), with
Standard Uncertainties in Parentheses
DꢀH ··· Aa)
DꢀH [ꢂ]
H ··· A [ꢂ]
D ··· A [ꢂ]
DꢀH ··· A [8]
Complex 3
N(1)ꢀH(1) ··· Br(1)
0.93
0.93
2.32
2.39
3.226(4)
3.238(4)
164
152
N(4)ꢀH(4) ··· Br(1)i
Complex 4
N(1)ꢀH(1) ··· Br(1)ii
N(4)ꢀH(41) ··· Br(1)
N(4)ꢀH(42) ··· Br(1)iii
N(4)ꢀH(41) ··· Br(5)iii
0.93
0.92
0.92
0.92
2.40
2.63
2.38
2.87
3.296(3)
3.344(4)
3.248(4)
3.393(4)
161
135
157
117
a
1
iii
) Symmetry operators: i 3/2 ꢀ x, 1 ꢀ y, z ꢀ /2; ii x ꢀ 1, y, z; 1 ꢀ x, 1 ꢀ y, 1 ꢀ z.

Helvetica Chimica Acta – Vol. 92 (2009)
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Fig. 8. A projection of the crystal packing of 3 viewed down [100] showing the H-bonding as dashed lines.
The H-atoms bonded to C-atoms have been omitted for clarity.
contains one divalent cation, and one each of almost discrete [TlBr4]ꢀ and Brꢀ anions,
with none of these entities occupying any special symmetry sites. The anionic species
are associated very weakly and could be considered to have an arrangement
somewhere between the completely discrete [TlBr4]ꢀ and Brꢀ entities found in 2,
and the closer [TlBr4] ··· Br association via secondary bonding to give the distorted
[TlBr5]2ꢀ anion found in 3. The Brꢀ ion in 4, Br(1), lies 4.0224(4) ꢂ from the Tl-atom
and is, therefore, just inside the sum of the Van der Waals radii for these two atoms
(Fig. 9). The very slight distortion of the tetrahedral geometry of the [TlBr4]ꢀ anion is
consistent with the influence caused by a very weak interaction between the [TlBr4]ꢀ
anion and the Brꢀ ion (Table 6). The Brꢀ ion is correctly positioned to be considered as
forming the distant apex of a distorted [TlBr5]2ꢀ trigonal bipyramid, as it is nearly trans
to Br(2) with a Br(1) ··· TlꢀBr(2) angle of 172.41(1)8, and the TlꢀBr(2) bond is
correspondingly marginally longer than the other three TlꢀBr bonds. Although the
[TlBr4]ꢀ anion is distorted only minimally from ideal tetrahedral geometry, the
tetrahedral angles are distorted such that all angles involving the Br(2)-atom have
become smaller than the ideal tetrahedral value, while all angles not involving Br(2) are

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slightly larger. The distances of the Tl-, Br(1)-, and Br(2)-atoms from the least-squares
plane defined by the pseudo-equatorial atoms, Br(3), Br(4), and Br(5), are 0.6633(3),
ꢀ 3.3370(5), and 3.2558(5) ꢂ, respectively. Thus, the two pseudo-axial Br-atoms are
almost equidistant from the equatorial plane, as found in compound 3, which further
suggests that all five Br-atoms belong to the one anionic unit. All of these features are
consistent with the notion that the Brꢀ ion is interacting very slightly with the Tl-atom
and distorting the [TlBr4]ꢀ tetrahedron towards a [TlBr5]2ꢀ trigonal bipyramid. A
similar arrangement of anions to that in compound 4 was found in the 2-
bromopyridinium (XVIII) salt, where the Brꢀ ion is 4.1546(6) ꢂ from the Tl-atom,
and the [TlBr4]ꢀ tetrahedron is also distorted very slightly [1]. In this latter case, the
distances of the Tl- and the two pseudo-axial Br-atoms from the least-squares
equatorial plane defined by the pseudo-equatorial Br-atoms are 0.6777(3),
ꢀ 3.3728(6), and 3.2551(5) ꢂ, respectively, which are very similar to the corresponding
distances in compound 4. Thus, 4 and the 2-bromopyridinium (XVIII) salt may be
considered to be extreme examples of the weak [TlBr4] ··· Br secondary bonding found
in 3, and in the compounds from cations XXI, XXII, and XXIII that were described
earlier [1][5].
Fig. 9. The asymmetric unit in the structure of [C11H18N2] · [TlBr4] · Br (4) showing the atom-labeling
scheme and 50% probability for the displacement ellipsoids. The Tl ··· Br(1) distance is 4.0224(4) ꢂ.
All three of the potential donor H-atoms of the cation in 4 are involved in NꢀH ···
Br H-bonding interactions with neighboring Brꢀ ions. One H-atom forms bifurcated H-
bonds, with the second, weaker interaction being with a Br-atom from the [TlBr4]ꢀ
anion (Table 7). This latter interaction has a long H ··· Br distance and a quite acute
NꢀH ··· Br angle, so is possibly an insignificant interaction. Thus, each cation interacts
with three different Brꢀ ions and one [TlBr4]ꢀ anion, while each Brꢀ ion interacts with
three different cations. The cation ··· Brꢀ-ion interactions link these ions into

Helvetica Chimica Acta – Vol. 92 (2009)
47
continuous columns, which run parallel to the [100] direction. The construction of the
columns can be thought of as a ladder, whose junctions between the rungs and vertical
supports are, alternately, a cation and a Brꢀ ion, and each rung has a cation at one end
and a Brꢀ ion at the other end (Fig. 10). The repeating unit within the columns thus
consists of two rungs, A and B, of the ladder. The rung A to rung B step consists of two
cations and two Brꢀ ions distributed about a center of inversion and involves both of the
H-atoms on the unsubstituted N-atom of the cation. The two Brꢀ ions bridge the two
cations, and vice versa, thus forming a four-membered H-bonded ··· cation ··· Brꢀ ···
cation ··· Brꢀ ··· loop, which has a binary graph set motif [32] of R24(8). The next rung B
to rung A connection is also a centrosymmetric ··· cation ··· Brꢀ ··· cation ··· Brꢀ ···
loop, which involves one of the interactions in the first loop plus the third H-bond to
each Brꢀ ion, which involves the single H-atom on the benzyl-substituted N-atom of the
cation. This latter loop has a binary graph set motif of R24(14).
Fig. 10. A projection of the crystal packing of 4 viewed down [010] showing the H-bonding as dashed
lines. The H-atoms bonded to C-atoms have been omitted for clarity.
Compounds 3 and 4 are further examples of the [TlBr5]2ꢀ anion being highly
distorted when it is involved in H-bonding interactions with the cation, particularly
when these interactions are predominantly with a single Br-atom, which then interacts
much less strongly with the Tl-atom. These very weak Tl ··· Br interactions, or even
structures with essentially discrete [TlBr4]ꢀ and Brꢀ ions, have been found to occur
consistently in the presence of cations with NꢀH H-bond donors, whereas the much
more regular [TlBr5]2ꢀ anions are obtained only in the absence of NꢀH H-bond
donors, such as with quaternary organoammonium cations. This hypothesis, first
introduced in [1][2], will be discussed later.
3.3. Salts with Monovalent Cations. The pyrazolinium cation (V) yields a
bromothallate(III) salt 5 with the [Tl2Br9]3ꢀ stoichiometry, but the crystal structure

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shows that a formal dinuclear [Tl2Br9]3ꢀ species is not present. Instead, the asymmetric
unit of 5 contains three symmetry-independent pyrazolinium cations, two [TlBr4]ꢀ
anions, and a Brꢀ ion (Fig. 11). The structure is very similar to that found when the
pyridinium cation (XVII) was employed [1], with the same relative arrangement of
cations and anions, a pseudo-[Tl2Br9]3ꢀ grouping, similar unit cell dimensions, and the
same space group. The two structures could be described as pseudo-isostructural, with
the different cations precluding true isostructurality. In 5, the Brꢀ ion, Br(9), is
positioned between the two [TlBr4]ꢀ anions with a Tl(1) ··· Br(9) ··· Tl(2) angle of
111.17(2)8, and is virtually trans to Br(1) in one [TlBr4]ꢀ anion and Br(8) in the other
(Table 8). Although this resembles a [Tl2Br9]3ꢀ grouping, the Brꢀ ion is 4.7923(8) and
4.4712(8) ꢂ from atoms Tl(1) and Tl(2), respectively, which is similar to the
corresponding distances in the pyridinium salt. These distances, being longer than
the sum of the Van der Waals radii of the Tl- and Br-atoms [29], are too long for any
significant interaction between the Brꢀ and [TlBr4]ꢀ anions. This conclusion is
supported by the minimal distortions of the [TlBr4]ꢀ anions from ideal tetrahedral
geometry.
Fig. 11. The asymmetric unit in the structure of [C3H5N2]3 · [TlBr4]2 · Br (5) showing the atom-labeling
scheme and the pseudo-[Tl2B9]3ꢀ grouping. Dashed lines represent H-bonds. The displacement ellipsoids
are shown at the 50% probability level.
The Brꢀ ion in 5 accepts four NꢀH ··· Br H-bonds; one from each of two of the
symmetry-independent cations and two from the third cation (Table 9). The former
two cations also form H-bonds with two symmetry-related [TlBr4]ꢀ anions (those

Helvetica Chimica Acta – Vol. 92 (2009)
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Table 8. Selected Interatomic Distances [ꢂ] and Angles [8] for [C3H5N2]3 · [TlBr4]2 · Br (5), with Standard
Uncertainties in Parentheses
Tl(1)ꢀBr(1)
Tl(1)ꢀBr(2)
Tl(1)ꢀBr(3)
Tl(1)ꢀBr(4)
Tl(1) ··· Br(9)
Tl(1) ··· Tl(2)
2.5506(7)
2.5442(7)
2.5716(8)
2.5306(7)
4.7923(8)
7.6442(3)
Tl(2) ꢀBr(5)
Tl(2)ꢀBr(6)
Tl(2)ꢀBr(7)
Tl(2)ꢀBr(8)
Tl(2) ··· Br(9)
2.5708(7)
2.5336(7)
2.5510(9)
2.5459(7)
4.4712(8)
Br(1)ꢀTl(1)ꢀBr(2)
Br(1)ꢀTl(1)ꢀBr(3)
Br(1)ꢀTl(1)ꢀBr(4)
Br(2)ꢀTl(1)ꢀBr(3)
Br(2)ꢀTl(1)ꢀBr(4)
Br(3)ꢀTl(1)ꢀBr(4)
Br(1)ꢀTl(1) ··· Br(9)
Br(2)ꢀTl(1) ··· Br(9)
Br(3)ꢀTl(1) ··· Br(9)
Br(4)ꢀTl(1) ··· Br(9)
Tl(1) ··· Br(9) ··· Tl(2)
109.52(2)
105.37(3)
111.33(2)
107.07(3)
113.08(2)
110.11(3)
173.55(2)
71.00(2)
Br(5)ꢀTl(2)ꢀBr(6)
Br(5)ꢀTl(2)ꢀBr(7)
Br(5)ꢀTl(2)ꢀBr(8)
Br(6)ꢀTl(2)ꢀBr(7)
Br(6)ꢀTl(2)ꢀBr(8)
Br(7)ꢀTl(2)ꢀBr(8)
Br(5)ꢀTl(2) ··· Br(9)
Br(6)ꢀTl(2) ··· Br(9)
Br(7)ꢀTl(2) ··· Br(9)
Br(8)ꢀTl(2) ··· Br(9)
113.08(3)
108.46(3)
105.65(2)
107.83(2)
112.96(3)
108.72(3)
69.65(2)
73.90(2)
68.98(2)
173.05(2)
68.64(2)
73.71(2)
111.17(2)
Table 9. H-Bonding Parameters for [C3H5N2]3 · [TlBr4]2 · Br (5), with Standard Uncertainties in
Parentheses
DꢀH ··· Aa)
DꢀH [ꢂ]
H ··· A [ꢂ]
D ··· A [ꢂ]
DꢀH ··· A [8]
N(1)ꢀH(1) ··· Br(9)
N(2)ꢀH(2) ··· Br(6)
N(6)ꢀH(6) ··· Br(9)
N(7)ꢀH(7) ··· Br(5)i
N(11)ꢀH(11) ··· Br(9)
N(12)ꢀH(12) ··· Br(9)
0.88
0.88
0.88
0.88
0.88
0.88
2.66
2.82
2.41
2.52
2.93
2.79
3.285(7)
3.455(7)
3.281(7)
3.365(7)
3.429(9)
3.368(7)
129
130
171
162
118
124
a
3
) Symmetry operator: i 1/2 þ x, y, /2 ꢀ z.
involving atom Tl(2)). These interactions serve to link the three cations, the Brꢀ ion,
and one of the two symmetry-independent [TlBr4]ꢀ anions into continuous chains
which run parallel to the [1 0 0] direction (Fig. 12). The sequence of H-bonded moieties
through one repeat of this chain is ··· Tl(2) cation B ! Brꢀ cation A ! Tl(2’) ··· ,
while cation C is a side group which forms two H-bonds to the same Brꢀ ion. The other
symmetry-independent [TlBr4]ꢀ anions (those involving atom Tl(1)) are not involved
in any H-bonding interactions.
The atomic displacement parameters for the atoms of the three symmetry-
independent cations in 5 are elongated in directions that are parallel to the plane of
each five-membered ring. This observation suggests each of these rings may be
disordered over two orientations related to each other by a pivot of the ring about an
axis perpendicular to the ring plane. A similar disorder is often observed with
cyclopentadienyl rings. Attempts to resolve this disorder into two orientations for each
ring did not produce a satisfactory or stable model, even when a complex series of

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Fig. 12. A projection of the crystal packing of 5 viewed down [010] showing the H-bonding as dashed
lines. The H-atoms bonded to C-atoms have been omitted for clarity.
similarity restraints was employed. As a consequence, the ordered model was employed
for the final refinement, but the cation geometry should be taken as being only
approximate due to the untreated disorder.
All of the other monovalent cations, VI – XI (Table 1), employed in this part of the
study produced simple binary salts which contain tetrabromothallate(III) anions and
have the [BaseH] · [TlBr4] stoichiometry (compounds 6 – 11; Table 2). These facts were
confirmed by X-ray crystal-structure analyses for compounds 6 – 9, as detailed below.
For the piperidinium (X) and quinolinium (XI) salts 10 and 11, crystal-structure
determinations were not undertaken, but the Raman spectra showed TlꢀBr stretching
frequencies that are very similar to those observed for other compounds known from
X-ray crystallography to possess tetrahedral [TlBr4]ꢀ species [1][26], while the overall
[TlBr4]ꢀ stoichiometry was confirmed by the elemental analyses.
The anions in the crystal structure of 1,5-diazabicyclo[4.3.0]non-5-enium tetrabro-
mothallate(III) (6) sit across mirror planes, while the cations lie over centers of
inversion and are, therefore, disordered about the latter. The anion geometry is clearly
defined and is a very regular tetrahedron (Table 10). The overlapping disordered
orientations of the cations caused instability in the refinement of the atomic positions.
Therefore, bond-length restraints were applied to most of the bonds within the cation in
order to maintain reasonable geometry. Suitable values for the restraints were derived
from the well-defined structure of (S)-9-(hydroxymethyl)-1,5-diazabicyclo[4.3.0]non-
5-ene [33]. The positions of the H-atoms could not be located because of the disorder,
and it is assumed that N(5) is protonated. This choice is based on the fact that the

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Table 10. Selected Interatomic Distances [ꢂ] and Angles [8] for [C7H13N2] · [TlBr4] (6), [C9H14N] ·
[TlBr4] (7), and [C10H10N3] · [TlBr4] (8), with Standard Uncertainties in Parenthesesa)
Complex 6
TlꢀBr(1)
2.5518(8)
2.5559(8)
TlꢀBr(3)
2.5528(5)
TlꢀBr(2)
Br(1)ꢀTlꢀBr(2)
Br(1)ꢀTlꢀBr(3)
110.31(3)
108.15(2)
Br(2)ꢀTlꢀBr(3)
Br(3)ꢀTlꢀBr(3)i
109.42(2)
111.38(3)
Complex 7
TlꢀBr(1)
TlꢀBr(2)
2.5562(5)
2.5326(7)
TlꢀBr(3)
TlꢀBr(4)
2.5697(6)
2.5418(7)
Br(1)ꢀTlꢀBr(2)
Br(1)ꢀTlꢀBr(3)
Br(1)ꢀTlꢀBr(4)
113.60(2)
106.75(2)
106.36(2)
Br(2)ꢀTlꢀBr(3)
Br(2)ꢀTlꢀBr(4)
Br(3)ꢀTlꢀBr(4)
110.11(2)
109.40(4)
110.53(3)
Complex 8
TlꢀBr(1)
TlꢀBr(2)
2.5675(4)
2.5459(4)
TlꢀBr(3)
TlꢀBr(4)
2.5693(4)
2.5524(4)
Br(1)ꢀTlꢀBr(2)
Br(1)ꢀTlꢀBr(3)
Br(1)ꢀTlꢀBr(4)
109.65(2)
109.98(1)
104.48(2)
Br(2)ꢀTlꢀBr(3)
Br(2)ꢀTlꢀBr(4)
Br(3)ꢀTlꢀBr(4)
107.66(2)
115.33(2)
109.69(2)
a
i
3
) Symmetry operator: x, /2 ꢀ y, z.
bridging bond, N(1)ꢀC(6), is relatively short, i.e., 1.32(1) ꢂ, which suggests some
delocalization of the electron density from the N(5)¼C(6) bond (1.292(8) ꢂ) to the
N(1)ꢀC(6) bond with the result that N(1) probably carries some of the positive charge.
Furthermore, the sum of the angles around N(1) is 3608, so there is no evidence for any
pyramidalization at this atom, which would occur if N(1) was protonated. Also, N(5) is
closer to any Br-atom than N(1) and is, therefore, the most likely H-bond donor. The
shortest N ··· Br distance is 3.79(2) ꢂ for N(5) ··· Br(3), and the corresponding H ··· Br
distance is 3.00 ꢂ (Table 11). These distances are longer than the NꢀH ··· Br H-
bonding distances normally found in the other bromothallate(III) complexes that have
been studied and are probably at the margin of what might be considered an actual H-
Table 11. H-Bonding Parameters for [C7H13N2] · [TlBr4] (6) and [C10H10N3] · [TlBr4] (8), with Standard
Uncertainties in Parentheses
DꢀH ··· Aa)
Complex 6
N(5)ꢀH(5) ··· Br(3)
DꢀH [ꢂ]
H ··· A [ꢂ]
D ··· A [ꢂ]
3.79(2)
DꢀH ··· A [8]
0.88
3.00
150
Complex 8
N(3)ꢀH(13) ··· Br(2)i
N(3)ꢀH(13) ··· Br(3)
N(4)ꢀH(14) ··· Br(1)ii
N(4)ꢀH(14) ··· Br(2)iii
0.88
0.88
0.88
0.88
3.03
2.67
2.84
2.63
3.418(6)
3.526(6)
3.345(6)
3.401(7)
109
163
118
148
a
1
1
iii
) Symmetry operators: i 1 ꢀ x, y ꢀ /2, 3/2 ꢀ z; ii 1 ꢀ x, /2 þ y, 3/2 ꢀ z; 1 ꢀ x, 1 ꢀ y, 1 ꢀ z.

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Helvetica Chimica Acta – Vol. 92 (2009)
bonding interaction between the cation and the anion. The shortest N(1) ··· Br distance
is 4.10(3) ꢂ. As a result of the mirror symmetry of the anion, the N(5)ꢀH(5) ··· Br(3)
H-bond links two mirror-related cations and one anion into ion triplets (Fig. 13).
Fig. 13. The structure of the H-bonded ion-triplet of [C7H13N2] · [TlBr4] (6) showing the atom-labeling
scheme and 50% probability for the displacement ellipsoids. The labels with superscripts indicate atoms
generated from the unique atoms by mirror symmetry; the other inverted, superimposed orientation of
the disordered cation has been omitted for clarity.
The determination of the crystal structure of the tetrabromothallate(III) salt 9 of
the related 1,8-diazabicyclo[5.4.0]undec-7-enium cation (IX) proved troublesome
because of severe irresolvable disorder within the two symmetry-independent cations
which reside in the asymmetric unit alongside two [TlBr4]ꢀ anions. The inadequacy of
the model makes it inadvisable to provide a full discussion of the structure, other than
to comment that the anions in the structure have been found unequivocally to be simple
[TlBr4]ꢀ species, which is in agreement with the Raman spectrum and the stoichiometry
derived from the elemental analysis.
The trimethyl(phenyl)ammonium tetrabromothallate(III) salt (7) is the only
example in this entire study involving a monovalent quaternized cation. In the solid
state, the ions do not possess any crystallographic symmetry. The anion is a pure
[TlBr4]ꢀ species with no short contacts to other Br-atoms, and the tetrahedron is
virtually undistorted (Table 10 and Fig. 14). Although quaternized cations have been
found to be a requirement for the formation of true [TlBr5]2ꢀ anions [1][2][4][6], an
additional requirement is that such cations must either be divalent or, if monovalent, in
sufficient excess to provide the required charge balance. In the case of compound 7, the
use of a 1:1 cation/anion ratio has resulted in the [TlBr4]ꢀ salt, which indicates that, in
the absence of excess cation, an equilibrium in favor of the formation of bis[trime-
thyl(phenyl)ammonium] pentabromothallate(III) does not exist.

Helvetica Chimica Acta – Vol. 92 (2009)
53
Fig. 14. A projection of the crystal packing of [C9H14N] · [TlBr4] (7) viewed down [100] showing the
atom-labeling scheme and 50% probability for the displacement ellipsoids
The anion in the crystal structure of (pyridin-2-yl)-(2-pyridinium)amine tetrabro-
mothallate(III) (8) is a pure [TlBr4]ꢀ species with no short contacts to other Br-atoms,
and the tetrahedron is virtually undistorted (Table 10). There are two half-cations in
the asymmetric unit, each of which is close to an inversion center. Expanding the
cations about the inversion centers generates two full independent disordered cations,
where the disorder superimposes inverted copies of the cations upon themselves
(Fig. 15). This arrangement necessitated defining the amino N-atom in each cation
with a site occupation factor of 0.5, while the site for each C-atom adjacent to the
amino N-atom, as well as the sites of the pyridinyl and pyridinium N-atoms, are
occupied by 0.5C þ 0.5N. The arrangement of the cations in this structure is very similar
to those in the [FeCl4]ꢀ analogue [34] and in the (pyridin-2-yl)-(2-pyridinium)amine
maleic acid (1/2) salt [35].
The positions of the H-atoms in 8 could not be located definitively because of the
disorder. The cations could be protonated at the amino N-atom or at a pyridinyl N-

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Helvetica Chimica Acta – Vol. 92 (2009)
Fig. 15. The components in the structure of [C10H10N3] · [TlBr4] (8) showing the atom-labeling scheme
and 50% probability for the displacement ellipsoids. The labels with ’ and ’’ indicate atoms generated from
the unique atoms by inversion centers. In the cations, N(3) and N(4) have occupancy 0.5, the sites
denoted X(1), X(2), X(7), and X(8) are each occupied by 0.5C þ 0.5N, and the H-atoms associated with
N(3), N(4), X(1), and X(8) all have occupancy 0.5.
atom, or possibly even disordered between the two pyridinyl N-atoms. Protonation at
any of these sites leads to a reasonable arrangement of H-bonds, so the presence or
absence of H-bonds could not be used as a distinguishing tool in this case. However,
while refinement of the various alternative models left the R-factor virtually
unchanged, a test refinement of the isotropic atomic displacement parameters of the
amino and pyridinium H-atoms led to reasonable values for these parameters only
when the cations were defined as having one amino and one pyridinium H-atom, with
the latter not being disordered between the two pyridinyl N-atoms. Also, the amino
NꢀC bond lengths of ca. 1.35 ꢂ are shorter than would be expected if the amino group
was protonated.
The cations in 8 form bifurcated NꢀH ··· Br H-bonds to the Br-atoms of adjacent
[TlBr4]ꢀ anions (Table 11). Ignoring the cation disorder, each of the symmetry-
independent cations interacts with two [TlBr4]ꢀ anions, while each anion interacts with
three cations, two of which are symmetry-related, and the third is the other symmetry-
independent cation. The interaction between the anion and its two neighboring
symmetry-related cations serves to link these ions into continuous chains, which run
parallel to the crystallographic [010] direction (Fig. 16) and have a graph set motif [32]
of C21(4). The other symmetry-independent cation connects the chains to a second
anion, but the interactions are not propagated further, so that this cation – anion pair
merely acts as a side group to the main chain. Propagation of the interionic H-bonds by

Helvetica Chimica Acta – Vol. 92 (2009)
55
Fig. 16. A projection of the crystal packing of 8 viewed down [1 0 0] showing the H-bonding as dashed
lines and 50% probability for the displacement ellipsoids. Only one orientation of each disordered cation
is displayed. The H-atoms bonded to C-atoms have been omitted for clarity.
inversion of the cations leads to cooperative two-dimensional planar networks which lie
perpendicular to the crystallographic [1 0 0] direction. The pyridinium N-atom is not
close enough to any Br-atoms to indicate the presence of H-bonds between these atoms,
but the usual intramolecular NꢀH ··· N interaction between the pyridinium and the
pyridinyl N-atoms is present.
4. An Overview of the Structural Diversity and Predictability of Bromothallate(III)
Anions. – In our extensive study of the structural diversity exhibited by bromothallate
(III) anions, a total of 34 new organoammonium bromothallate(III) compounds were
synthesized, and the crystal structures of 25 of these have been determined ([1][2][4 –
6] and this work). In the remaining cases, the nature of the anionic species present has
been elucidated unambiguously on the basis of their Raman spectra and elemental
analyses. The pertinent questions to which answers were sought in this project are:
ꢂ Do the anions in bromothallate(III) compounds show a rich structural diversity, or
are the preferred anionic species limited to a few principal forms?
ꢂ Is it possible to generate new species with [TlBr5]2ꢀ stoichiometry?
ꢂ Do discrete [TlBr5]2ꢀ anions exist?

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Helvetica Chimica Acta – Vol. 92 (2009)
ꢂ If so, is their geometry square-pyramidal, or is a trigonal-bipyramidal coordination
geometry preferred? One example of each geometry is known for discrete [TlCl5]2ꢀ
anions [36][37].
ꢂ Is there a preference for octahedral coordination of TlIII by bromide, even in
compounds without [TlBr6]3ꢀ stoichiometry, as found with some chlorothallate(III)
structures [38 – 44], which will lead to Br-bridged binuclear or polynuclear anions?
ꢂ What influence, if any, does NꢀH ··· Br H-bonding have on the anionic structures?
ꢂ What factors influence the metal ion complex groupings that are present?
ꢂ Can the structural groupings be rationalized, so that it is possible to predict them and/
or design them?
The accumulated data now allow a detailed examination of trends and the factors
influencing the nature of the anions that can be generated. This analysis forms the
subject of the following discussion.
4.1. The Structural Diversity of Bromothallate(III) Anions. One of the principal
aims of the project was to ascertain if the anions in bromothallate(III) salts exhibited a
broad structural diversity, similar to the chlorothallate(III) salts, or if there was a
preference for a more limited range of species, such as the already known [TlBr4]ꢀ and
[TlBr6]3ꢀ anions. Since TlIII, is a large d10 system, it could have the potential to exhibit a
rich structural diversity similar to the huge diversity found for HgII complexes. Indeed,
a diverse range of bromothallate(III) structures was found with eleven different types
of anion, anion grouping, or mixed salt systems being observed. These eleven types are:
ꢂ [BaseH] · [TlBr4]
ꢂ [Base2þ] · [TlBr5]
ꢂ [BaseH3] · [TlBr6]
ꢂ [BaseH]2 · [TlBr4] · Br or [BaseH2] · [TlBr4] · Br with discrete anions
ꢂ [BaseH2] · [TlBr5] with one long axial Tl ··· Br secondary bond
ꢂ [BaseH2] · [TlBr5] with polymeric cis-bromo-bridged [TlBr5]2ꢀ anions
ꢂ [BaseH]3 · [TlBr4]2 · Br arranged as a pseudo-[Tl2Br9]3ꢀ grouping
ꢂ [BaseH2]4 · [TlBr4] · [TlBr6] · 4 Br
ꢂ [Base2þ]3 · [TlBr4]3 · [TlBr6] with potential cis-bromo-bridged [TlBr4]33ꢀ trimers
ꢂ [BaseH2]2 · [TlBr4] · [TlBr6]
ꢂ [BaseH3]2 · [TlBr4]3 · 3 Br
Despite this variety, the structural diversity is generally limited to various
combinations of just four principal moieties, namely the [TlBr4]ꢀ, [TlBr5]2ꢀ, [TlBr6]3ꢀ,
and Brꢀ anions. Even the pseudo-[Tl2Br9]3ꢀ groupings in the salts obtained from the
pyrazolinium and pyridinium cations (V and XVII, resp.) are nothing more than
conveniently arranged, but quite discrete combinations of [TlBr4]ꢀ and Brꢀ ions. A
degree of novel variation is observed among those salts containing [TlBr5]2ꢀ anions
with one long axial Tl ··· Br secondary bond where the Tl ··· Br distance covers a wide
range of values (vide infra in Sect. 4.5, and also Sect. 3.2). One polynuclear structure
was found, that being the cis-bromo-bridged [TlBr5]2ꢀ anions of the N-methylpropane-
1,3-diammonium (XXVII) salt, in which secondary Tl ··· Br bonds of 3.632(4) ꢂ link
the [TlBr5]2ꢀ units [2]. A second structure might possibly be interpreted as having
trinuclear anions. In the 1,1,3,3-tetramethylimidazolidinium (XIII) mixed salt, which

Helvetica Chimica Acta – Vol. 92 (2009)
57
has the formulation [Base2þ]3 · [TlBr4]3 · [TlBr6], the tetrahedral [TlBr4]ꢀ anions are
slightly distorted, and two very long cis-positioned secondary Tl ··· Br bonds of
3.975(2) ꢂ from adjacent [TlBr4]ꢀ ions complete an octahedral arrangement [4]. These
interactions propagate about a crystallographic C3 axis to give a cis-double-Br-bridged
cyclic trinuclear [Tl3Br12]3ꢀ moiety. Despite the very long Tl ··· Br distances, the
interactions must be significant, because noticeable distortion of the [TlBr4]ꢀ
tetrahedron occurs.
Thus, from the paucity of multinuclear anions found in the data accumulated so far,
the bromothallate(III) anions do not appear to exhibit a structural diversity quite as
rich as that found among the chlorothallate(III) salts, where binuclear species like
[Tl2Cl10]4ꢀ [38][39], or extended chain structures, in which [TlCl5]2ꢀ units are
symmetrically or asymmetrically cis-Cl-bridged [39 – 43] or trans-Cl-bridged [44],
have been observed more frequently.
The formation of binuclear or polynuclear [TlCl5]2ꢀ species suggests that there
might be a preference for the Tl-atom in chlorothallate(III) compounds to have
octahedral coordination, even when the overall Tl/halogen stoichiometry is not 1:6. In
contrast, the fact that only two polynuclear bromothallate(III) anionic structures were
observed among the studied pentabromothallate(III) compounds, and then only as a
result of quite weak secondary Tl ··· Br bonds, suggests that there is less of a preference
for octahedral coordination at the Tl-atom in the case of bromothallate(III) salts,
although octahedrally coordinated TlIII was observed as discrete [TlBr6]3ꢀ anions in
four other compounds. Three of these involved the divalent cations XIII, XIV, and
XXVII (see Sect. 4.3), so given the constantly used cation/TlBr3 ratio of 1:1, the
presence of [TlBr6]3ꢀ as part of a mixed salt, instead of a [TlBr5]2ꢀ anion or a [TlBr4]ꢀ/
Brꢀ mixed salt, does suggest that octahedral Tl-coordination is favored under the right
conditions. The fourth compound involved the trivalent cation I (see Sect. 4.4), which
would be expected to favor the formation of a [TlBr6]3ꢀ anion. Another similar
trivalent cation, II, however, produced the [BaseH3]2 · [TlBr4]3 · 3 Br arrangement of
compound 2, which is completely free of octahedrally coordinated Tl-atoms. All of
these results suggest that octahedral coordination at the Tl-atom only occurs when all
other contributing factors like ion-size distribution and required charge balance are
appropriate, rather than the drive to attain octahedral coordination being a dominating
influence.
4.2. Salts Obtained with Monovalent Cations. Of the 34 organoammonium cations
employed, 17 were monovalent, and these are depicted in Table 12. Basoloꢃs general
principle [25] states: ꢁSolid salts separate from aqueous solution easiest for combinations
of either small cation – small anion or large cation – large anion, preferably with systems
having the same but opposite charges on the counterionsꢂ. As all syntheses were
restricted deliberately to the use of a cation/TlBr3 ratio of 1:1, one might, therefore,
expect the monovalent cations to yield bromothallate(III) salts with a corresponding
monovalent anion of similar size, namely [TlBr4]ꢀ. Although the syntheses employed
the bromide salt of the organic base, Basoloꢃs hypothesis suggests that the inherent size
mismatch between the bulky cations and the Brꢀ ion would not favor the reappearance
of the simple bromide salt upon crystallization. Indeed, 13 of the employed monovalent
cations yielded the straightforward [BaseH] · [TlBr4] salts, as do the 2,5-dimethylpyr-
azinium and (2-phenylethyl)ammonium cations [45][46].

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Helvetica Chimica Acta – Vol. 92 (2009)
Table 12. Monovalent Cations Used to Generate Bromothallate(III) Salts and Where Described
Cations that produced [TlBr4]ꢀ salts
(Pyridin-2-yl)-(2-pyridinium)amine (VIII)
This work
2-(Pyridin-2-yl)pyridinium (XXXIV)
Quinolinium (XI)
[5]
This work
1,5-Diazabicyclo[4.3.0]non-5-enium (VI)
1,8-Diazabicyclo[5.4.0]undec-7-enium (IX)
This work
This work
Trimethyl(phenyl)ammonium (VII)
Piperidinium (X)
This work
This work
2-Aminopyridinium (XXVIII)
[1]
[1]
[1]
2-Amino-3-methylpyridinium (XXIX)
2-Amino-4-methylpyridinium (XXX)
4-Methylpyridinium (XXXI)
[1]
[1]
4-(Dimethylamino)pyridinium (XXXII)