Y. Yang et al. / Journal of Molecular Structure 920 (2009) 401–408
407
agreement with values reported in the literature [20]. A compila-
tion of the most relevant hydrogen bond interactions in 2 is pro-
vided in Table 3.
very similar with respect to bond distances and angles but differ
at first sight with respect to conformation: Fig. 5 (left) shows that
the dication in 2 adopts an anti conformation with respect to the
S1–C2 bond, with a torsion angle C1–S1–C2–C3 of À165.25(15)°,
whereas the corresponding torsion in 2Á4 thiourea (right) amounts
to 61.86(15)°. This difference in the soft rotational degree of
freedom is reflected in a less compact overall shape of the dication
in 2, with longer N1ÁÁÁN3 and N2ÁÁÁN3 distances (6.495(3) and
5.383(3) Å) than in the thiourea solvate (5.724(2) and
4.477(2) Å). 2-Aminoethylisothiuronium cations of [C3H11N3S]Br2
[28] and [C3H11N3S]2[CuBr3]Br2 [29] are anti conformation too,
which the corresponding torsions are À89.0° and À74.6°, respec-
tively, and the corresponding distances are 6.136, 4.508 and
5.854, 4.348 Å, respectively.
3.4. Crystal structure of [(CH3)3NCH2CH2SC(NH2)2]Br2Á4(NH2)2CS (2Á4
thiourea)
Previous reports concerning the substitution reaction of thio-
urea and alkylbromides [23–25] agree that prolonged refluxing in
ethanol is required in order to achieve complete conversion. In
contrast to this expectation, our reaction product 2Á4 thiourea
was obtained from thiourea and (2-bromoethyl)trimethylammoni-
um bromide by stirring for one hour at room temperature and
hence under much milder conditions.
Compound 2Á4 thiourea represents a tetrathiourea solvate of
compound 2 discussed in the preceding section; we have been able
to elucidate the crystal structure of this more complex solid, too. It
The NBO [30,31] analyses of the 2-(trimethylamino)ethylisothi-
uronium dications in 2 and in 2Á4 thiourea indicate that the
charges on S1 atom are 0.399 and 0.401 au, respectively, and the
charges on N1 are À0.597 and À0.655 au, respectively, and on N2
are À0.612 and À0.670 au. For C1 atom, its NBO charges are
0.370 and 0.266 au, and for C2 are À0.472 and À0.487 au. Thus
the positive charge is mainly distributed on S1 and C1 atoms from
theoretical view. The calculation results show a higher bond order
for C1–S1 than for C2–S1.
According to the Cambridge Structural Database [32], prior
X-ray data on isothiuronium derivatives are limited to 37 observa-
tions in 26 relevant structures; their histograms for the shorter and
longer C–S bond distances are shown in Fig. 6.
Mean values of 1.745 and 1.813 Å are in excellent agreement
with our experimental results for 2 (1.742(2), 1.811(2) Å) and 2Á4
thiourea (1.751(2), 1.812(2) Å). For chemically related structures
values ranging from 1.727 to 1.752 for the shorter and from
1.785 to 1.858 Å for the longer carbon–sulfur bond have been
reported [33–37].
Fig. 7 allows to compare the modes of linkage between the thio-
urea molecules in our compound 2Á4 thiourea (Fig. 7f) to those
encountered in related inclusion compounds. In the compound
(n-C3H7)4N+IÀÁ(NH2)2CS (Fig. 7a) [38], a single chain is generated
by a pair of chelating N–HÁÁÁI hydrogen bonds between thiourea
and iodide anions. In the solids (n-C4H9)4N+BrÀÁ2(NH2)2CS
(Fig. 7b) [39] and (n-C4H9)4N+ClÀÁ2(NH2)2CS (Fig. 7c) [40] with a
2:1 molar ratio between thiourea and halide anions, and in
(n-C4H9)4N+FÁ3(NH2)2CS (Fig. 7d) [38] with its 3:1 stoichiometry,
six thiourea molecules interact via hydrogen bonds and form large
rings together with the anions. Even 10 thiourea molecules
aggregate with bromide anions in the 2:1 compound(n-C4H9)4
N+BrÀÁ2(NH2)2CS (Fig. 7e) [41]. This compilation underlines the
broad variability in possible host lattices as a function of stoichi-
ometry and chemical composition.
ꢀ
crystallizes in the triclinic space group P1, with one dication, two
bromide anions and four thiourea molecules in the asymmetric
unit. A perspective view of the crystal structure is shown in
Fig. 3. It illustrates that the overall arrangement can be regarded
as an extended three-dimensional framework with channels along
the [221] direction. The thiourea molecules and the bromide
anions interact via hydrogen bonds and form the framework, and
the 2-(trimethylamino)ethylisothiuronium dications are accom-
modated within the channels of rectangular cross-section. The
hydrogen bonds in this structure merit a closer inspection: The
interaction between the thiourea molecules in 2Á4 thiourea is
shown in Fig. 4, and a compilation of all relevant hydrogen bonds
can be found in Table 4. The thiourea molecules associated with
S2 and S3 on the one hand and those with S4 and S5 on the other
hand aggregate to two pairs via N–HÁÁÁS hydrogen bonds. Each of
these pairs contains two roughly coplanar thiourea molecules in
a shoulder-to-shoulder manner. The planes subtended by the S2/
S3 and the S4/S5 thiourea molecules are mutually perpendicular,
subtending an angle of 88.16(5)°.
Additional H bonds between thiourea molecules and bromide
anions contribute to the framework of the host structure. Their
range of donorÁÁÁacceptor distances (3.344(2)–3.772(2) Å) is in
good agreement with values obtained for the crystal structure of
[C6H5(CH2)2NH3]2Cd0.75Hg0.25Br4 [26].
The remaining hydrogen bonds link the guest cations in the
channels to the framework: The four H atoms in the amido groups
of the guest interact with two thiourea sulfur acceptors and two
anions. The NÁÁÁS donorÁÁÁacceptor distances of 3.324(2) and
3.336(2) Å agree well with those encountered for comparable
N–HÁÁÁS hydrogen bonds (3.40 0.20 Å) [27], and the NÁÁÁBr interac-
tions of 3.269(2) and 3.526(2) Å also match earlier reports (3.487–
3.857 Å) [26]. In agreement with expectation, the sulfur atom in
the guest cation does not act as a hydrogen bond acceptor, cf. Table
4.
4. Supplementary data
Crystallographic data (excluding structure factors) for the struc-
tures in this paper have been deposited with the Cambridge Crys-
tallographic Data Center as Supplementary Publication Nos.: CCDC
680006 (1), 680007 (2) and 680008 (2Á4 thiourea). These data can
tre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223
336033).
3.5. Structural features and relationships
In this section, we will discuss the relationship between the
structures. Electrostatic forces are surely important in all three
ionic solids. An obvious difference between the structure of 1
and those of 2 and its solvate 2Á4 thiourea stems from the nature
of the most relevant interactions. Coulomb interactions apart,
intermolecular contacts in 1 are limited to rather weak non-classi-
cal H bonds, and the overall space filling amounts to 0.685. In 2 and
its tetrathiourea solvate, classical hydrogen bonds contribute to
the overall lattice energy, and packing is slightly less efficient, with
values of 0.668 for 2 and 0.658 for 2Á4 thiourea.
Acknowledgements
Support from the NSFC (Grant No. 20571012) and the NSFBJ
(Grant No. 2042013) are gratefully acknowledged. The authors
thank Yutian Wang for help with the data collection.
Compound 2 and 2Á4 thiourea are derivatives of the same
2-(trimethylamino)ethylisothiuronium dication. Both residues are
Yang, Yuan
