Ductility, which is a common phenomenon in most
metals and metal-based alloys, is hard to achieve in molecular crystals. Organic
crystals have been recently shown to deform plastically, but only on one or two
faces, and fracture when stressed in any other arbitrary direction. Here, we
report an exceptional metal-like ductility in crystals of two globular
molecules, BH<sub>3</sub>NMe<sub>3</sub> and BF<sub>3</sub>NMe<sub>3</sub>,
with characteristic stretching, necking and thinning with deformations as large
as ~ 500%. Surprisingly, the mechanically deformed samples not only retained good
long range order, but also allowed structure determination by single crystal
X-ray diffraction. Molecules in these high symmetry crystals interact predominantly
via electrostatic forces (B<sup>–</sup>–N<sup>+</sup>) and form columnar
structures, thus forming multiple slip planes with weak dispersive forces among
columns. While the former interactions hold molecules together, the latter
facilitate exceptional malleability. On the other hand, the limited number of
facile slip planes and strong dihydrogen
bonding in BH<sub>3</sub>NHMe<sub>2</sub> negates ductility. We show the possibility
to simultaneously achieve both exceptional ductility and crystallinity in solids
of certain globular molecules, which may enable designing highly modular,
easy-to-cast crystalline functional organics, for applications in
barocalorimetry, ferroelectrics and soft-robotics.
The role of flexibility and molecular shape in the crystallization of proteins by surface mutagenesis