Two Successive Single Crystal Phase Transitions Involving the Coordination Sphere of Antimony in PhSb(dmit), the First Organo-Antimony(III) Dithiolene Complex

PhSb(dmit) (dmit2-, 4,5-dithiolato-1,3-dithiole-2-thione), the first neutral organo-antimony dithiolene complex,
has been synthesized by addition of PhSbCl2 on a suspension of Na2(dmit). The complex was characterized by
spectroscopic (1H and 13C NMR and IR) methods and elemental analysis. Its crystal structure was determined by
X-ray diffraction at room temperature in the monoclinic P21/c space group, with a = 12.580(3), b = 8.9756(18),
c = 15.905(3) Å, β = 109.06(3)°, V = 1697.5(6) Å3, Z = 4. A coordinating THF molecule was found in the
structure and the coordination geometry around the antimony atom is of distorted pseudopentagonal bipyramid
type, if taking into account the Sb···O and secondary Sb···S interactions, as well as the stereochemically active
5s2 lone pair. The intermolecular Sb···S and S···S contacts, shorter than the sum of van der Waals radii of
corresponding atoms, lead to the formation of a three-dimensional polymeric network in the solid state. A second
X-ray diffraction experiment, performed at 85 K, revealed a very similar monoclinic unit cell with the
noncentrosymmetrical space group P21 with a = 12.613(3), b = 8.9876(18), c = 15.109(3) Å, β = 107.01(3)°,
V = 1637.8(6), Z = 4. The structural differences with the first one are basically due to the rotation of the THF
ligand in the coordination sphere of the antimony center, leading to the loss of every inversion center found at
room temperature. A temperature variable X-ray diffraction study on a PhSb(dmit) single-crystal allowed the
detection, with a remarkable accuracy, of two successive first-order phase transitions, the first occurring at T =
162.5 K, while the second was observed at T = 182.5 K. Subsequently, a third set of X-ray data was collected
at 180 K and the resulting structure (monoclinic, P21/c, a = 16.736(3), b = 8.9653(18), c = 33.132(7) Å, β =
91.98(3)°, V = 4968.2(17), Z = 12) derives from the two others by a common b axis, a 3-fold cell volume
increase, and the presence of only one-third of the inversion centers present at room temperature. A DSC analysis,
showing two endothermic peaks at the expected temperatures, confirms the occurrence of the two structural phase
transitions, also in agreement with preliminary Raman data.