A Pressure Induced Structural Dichotomy in Isostructural Bis-1,2,3-thiaselenazolyl Radical Dimers

The pressure dependence of the crystal and molecular
structure
of the bis-1,2,3-thiaselenazolyl radical dimer [1b]2 has been investigated over the range 0–11 GPa by powder
diffraction methods using synchrotron radiation and diamond anvil
cell techniques. At ambient pressure, the dimer consists of a pair
of radicals linked by a hypervalent 4-center 6-electron S---Se–Se---S
σ-bond in an essentially coplanar arrangement. The dimers are
packed in cross-braced slipped π-stack arrays running along
the x-direction of the monoclinic (space group P21/c) unit cell. Pressurization
to 11 GPa causes the unit cell dimensions a and c to undergo a slow but uniform compression, while the b-axis is slightly elongated. There is virtually no change
in the molecular structure or in the slipped π-stack crystal
architecture. This behavior is in marked contrast to that of the isostructural
radical dimer [1a]2, where the basal fluorine
is replaced by hydrogen. Pressurization of this latter material induces
a phase change near 4–5 GPa, characterized by a sharp contraction
in a and c, and a correspondingly
large increase in b. At the molecular level, the
transition is associated with a buckling of the σ-bonded dimer
to a more conventional π-bonded arrangement. Geometry optimized
DFT band structure calculations on [1b]2 replicate
the observed structural changes and indicate that compression widens
both the valence and conduction bands but does not induce band gap
closure until >13 GPa. This result is consistent with the measured
thermal activation energy for conduction Eact, which indicates that a metallic state requires pressures > 10
GPa.