organic compounds
narrow ranges of 1.377 (2)–1.400 (2) and 1.537 (2)–
˚
1.541 (2) A, respectively, and are practically equal to the
corresponding values in (II) [1.357 (5)–1.398 (3) and
˚
1.539 (3)–1.541 (3) A, respectively].
The crystal packings of the molecules in (I) and (II) are
topologically similar. They both consist of stacks along the a
axis and these stacks form layers parallel to the ab plane
(Figs. 3a and 3b). However, the arrangements of the molecules
relative to each other in neighbouring stacks, and conse-
quently within the layers, differ considerably. In (I), molecules
in neighbouring layers are oriented with the amino groups
facing each other, which favours the formation of the afore-
mentioned N—Hꢀ ꢀ ꢀN hydrogen bonds, while in (II), the
amino groups of neighbouring stacks both within and between
the layers are oriented away from each other (Figs. 3c and 3d).
Since the orthorhombic polymorph was obtained by
recrystallization from a solution in the polar solvent
dichloromethane, while the triclinic polymorph was isolated
from a nonpolar hexane solution, we decided to elucidate the
influence of solvent polarity on the formation of the different
polymophic modifications of triphenylmethylamine. For this
purpose, we recrystallized commercially available triphenyl-
methylamine from solutions in the polar solvents ethanol,
diethyl ether and dichloromethane, and the nonpolar solvents
hexane, heptane and benzene. It was found that only the
orthorhombic modification of triphenylmethylamine is formed
from all these solutions at room temperature. Thus, the
polarity of solvent does not affect the capability of
triphenylmethylamine to crystallize in the different poly-
morphic modifications. Moreover, the orthorhombic poly-
morph, (II), is more thermodynamically stable under normal
conditions than the triclinic polymorph, (I). It is interesting to
note that even the presence of hydrogen bonding in poly-
morph (I) does not result in its greater stability under ambient
conditions compared with polymorph (II).
The possibility of a phase transition from the orthorhombic
to the triclinic modification upon cooling was studied by X-ray
diffraction analysis in the temperature interval 120–293 K.
Our experimental data show that a phase transition does not
occur. The densities of the orthorhombic (1.231 Mg mꢁ3) and
triclinic (1.235 Mg mꢁ3) modifications at 120 K are practically
equal. This result implies that factors other than thermo-
dynamics might be responsible for their formation (Burger &
Ramberger, 1979). In the present case, it would seem that
either the kinetic factors or the effects of the other products of
the reaction facilitating the hydrogen-bonded dimerization of
triphenylmethylamine molecules were critical for the isolation
of the triclinic polymorph of triphenylmethylamine, (I).
Figure 2
A comparison of the conformations of the molecules of the two
polymorphs. The molecules of (I) and (II) are drawn with solid and open
lines, respectively.
As a rule, the formation of hydrogen bonds of different
types results in a decrease in the total energy of a system and
serves as its stabilizing factor. Taking this into consideration, it
seemed surprising that triphenylmethylamine, possessing two
active H atoms and a hydrogen-bond acceptor, forms only one
polymorphic modification without hydrogen bonds (Glidewell
& Ferguson, 1994; Clegg & Elsegood, 2005). Therefore, one
might expect the existence of another polymorphic modifica-
tion of this compound, which should contain N—Hꢀ ꢀ ꢀN
hydrogen bonds. A new triclinic polymorph, (I), of tri-
phenylmethylamine was serendipitously obtained by crystal-
lization of a hexane reaction mixture after treatment of
LiGe(OCH2CH2NMe2)3 with Ph3CN3 and we report its
structure here.
Polymorph (I) crystallizes in the triclinic space group P1,
rather than in the previously known orthorhombic modifica-
tion of this compound (space group P212121), (II). The main
difference between the two polymorphs is the formation of
dimers via N—Hꢀ ꢀ ꢀN hydrogen bonds in (I) (Fig. 1 and
Table 1), whereas (II) consists of isolated molecules. Despite
the fact that the dimers lie across crystallographic inversion
centres, the molecules are not really connected by them. The
centrosymmetric structure is due to the statistical disordering
of the amino H atoms participating in the N—Hꢀ ꢀ ꢀN hydrogen
bonds, and thus the inversion centre is superpositional.
The conformation of the molecules in (I) is such that there
is an almost perfect staggering of the N—H and C—Ph bonds.
A similar conformation is also characteristic of the molecules
in polymorph (II) (Fig. 2). Nevertheless, the mutual disposi-
tion of the phenyl rings in the molecules of the two poly-
morphs is slightly different. In the orthorhombic structure,
(II), the phenyl rings have a propeller-like arrangement, with
N—C—C—C torsion angles of ꢁ12.0 (1), ꢁ47.2 (2) and
ꢁ60.3 (2)ꢂ, while in the triclinic structure, (I), the same N—
C—C—C torsion angles are ꢁ35.2 (2), ꢁ39.2 (1) and
ꢁ53.2 (1)ꢂ (Fig. 2).
Experimental
An excess (3 ml) of a tetrahydrofuran (THF) solution of Ph3CN3
(0.636 g, 2.23 mmol) was added to a THF solution of LiGe-
(OCH2CH2NMe2)3 (0.5548 g, 1.61 mmol) at 225 K. The liberation of
an amount of gas was observed. The reaction mixture was then
heated to room temperature for 30 min and allowed to stand over-
night. Removal of the solvent by filtration and recrystallization from
The aromatic C—C bond lengths in the phenyl rings and the
C—Ph bond lengths of the central C atom of (I) fall in the
ꢃ
o32 Khrustalev et al.
C19H17
N
Acta Cryst. (2009). C65, o31–o34
Khrustalev, Victor N.
