Crystal Growth & Design
Article
dabcoHI cations and NH+···N bonds are exactly aligned along
the chains (angles η and η′ are exactly 180°),4,6 and both H
sites are equally favored. Likewise, the protons are nearly 50:50
disordered in the 44′biPyHA·xH2O structures where angles η
and η′ are close to 180° (Table 2).
The similar structural elements of NH+···N-bonded chains
lead to isostructural features of the 44′biPyHA monosalts.
Monosalts can be classified as 3-, 2-, and at least 1-
dimensionally isostructural.28 Crystals α-44′biPyHI·H2O and
44′biPyHBr·H2O are 3-D isostructural and crystallize in the
same space group P212121. They are also 3-D isostructural with
the 44′biPyHCl·2H2O crystal: it has similar unit-cell parame-
ters; however, due to the conformational disorder the space
group symmetry is increased to Pnma.
Figure 4. Iodide−water OH···I− hydrogen-bonded aggregates
44′biPyHI·H2O in (a) polymorph α and (b) polymorph β.
The triclinic structure of dihydrate 44′biPyHClO4·2H2O
considerably differs from the monohydrates. In 44′biPyHClO4·
2H2O there are two independent 44′biPyH+ cations; four
independent water molecules and two independent anions
form H-bonded layers along the (001) planes (Figures 1d and
R24(8). The tetramers do not interact between themselves but
form the close contacts with pyridine rings (Table 3).
The NH+···N bonds (Table 4) linking the 4,4′-bipyridinium
cations into linear chains are on average 2.718 Å long and about
0.104 Å shorter than those in the dabcoHA6,32 and 0.115 Å
shorter than in pyrazine salts.7 In this respect, activation of
proton dynamics should be the easiest in 44′biPyHA hydrates.
The shortest NH+···N distance is in 44′biPyHBr·H2O, where
the proton is most ordered of all 44′biPyHA·xH2O structures
investigated at 296 K in this study. It suggests that the effects of
the crystal environment and displacements of the H-bonded
cations are more significant for the proton site than the N+···N
distance. The disorder of protons in the crystal can be in
principle due either to the disorder of chains or to the disorder
of protons within the NH+···N-bonded chains. It was confirmed
by dielectric spectroscopy2−8,18 and observation of ionic
disparity in pyrazole33 that the protons are disordered within
the chains.
−
−
2d). Despite the structural resemblance of ClO4 and BF4
anions, the crystal structure of 44′biPyHBF4·1/2H2O is very
different than 44′biPyHClO4·2H2O. Unlike in other 44′biPy-
HA·xH2O compounds, where the NH+···N-bonded chains are
all parallel to one of the crystal axes (Table 2), the chains in
44′biPyHBF ·1/2H O run along diagonals [110] and [110].
̅
4
2
Another difference between 44′biPyHClO4·2H2O and 44′bi-
−
−
PyHBF4·1/2H2O is that anion ClO4 is ordered and BF4 is
disordered. The disorder of the BF4− anion may be due to weak
intermolecular interactions of the fluorine atoms, which may be
also connected with the low contents of crystalline water in this
structure.
Polymorphs of 44′biPyHI·H2O. 44′biPyHI·H2O poly-
morphs α and β can be easily distinguished by their
orthorhombic and triclinic crystal habits and different colors,
yellow and orange, respectively (Figure 3). These colors
The mechanism of proton disorder is still being investigated:
the protons can dynamically hop between their two sites, and
there may be a static contribution of disproportionated 44′biPy
molecules, 44′biPyH+ cations, and 44′biPyH2 dications, as
2+
illustrated in Figure 5. It was established that the protons are
ordered in 44′biPyHBr·H2O at 90 K,13 which suggest that the
dynamic disorder of protons is dominant in this structure. It
was reported that protons are ordered in 44′biPyHClO4 at 296
K; however, refinements of our data consistently refined to the
disordered model (Table 2). Most recently we showed that the
defects generated by proton disordering in NH+···N bonds in
pyrazole can be segregated at the symmetry-independent sites
in the structure, and in this way it can contribute to the
polarization of the crystal.33 In the 44′biPyHA·xH2O structures,
it is also possible that the proton disorder is associated with the
changed polarity of the neighboring chains, or that both the
disorder types, within the chains and between the chains, exist
simultaneously.
Figure 3. Crystals of the polymorphic forms of 44′biPyHI·H2O: (a)
polymorph α and (b) polymorph β.
correspond to the conformations of cations. The larger dihedral
angle between pyridine rings in α-44′biPyHI·H2O, correspond-
ing to a weaker conjugation of aromatic electrons across the
C4−C4′ bond, which is consistent with the less intense color.
In α-44′biPyHI·H2O, water molecules and anions form zigzag
chains, running between the chains of 44′biPyH+ cations, along
the [100] direction (Figure 4). The graph descriptor of this
chain is C12(4).29−31 The water oxygen atom forms short CH···
O contacts to the hydrogens at pyridinium carbons C5′ and C6
(Table 3); there are also weak interactions between atoms H2′
and I1. In polymorph β the iodide ions and water molecules
aggregate into planar cyclic tetramers located around the
inversion centers. The graph descriptor of the cyclomer is
4. CONCLUSIONS
It can be concluded that hydrogen-bonded 44′biPyHA
monosalts exhibit a series of characteristic features which can
be attributed to the structure and properties of 44′biPyH+
cations (Table 5). Most importantly, they aggregate into NH+···
N-bonded chains, analogous to those associated with ferro-
electric and relaxor properties in dabcoHA analogues (Table 5).
At normal conditions the protons are disordered at different
ratios in the NH+···N bonds of 44′biPyHA chains.
Several other structural features of 44′biPyHA salts are
puzzling, particularly when referred to analogous dabcoHA
4382
dx.doi.org/10.1021/cg400752y | Cryst. Growth Des. 2013, 13, 4378−4384
Ratajczak-Sitarz, Malgorzata
