Unprecedented Conformational Variability in Main Group Inorganic Chemistry: the Tetraazidoarsenite and Antimonite Salts A+[M(N3)4]- (A = NMe4, PPh4, (Ph3P)2N; M = As, Sb), Five Similar Salts, Five Different Anion Structures

Journal article, 2013

A unique example for conformational variability in inorganic main group chemistry has been discovered. The arrangement of the azido ligands in the pseudotrigonal bipyramidal [As(N3)4]− and [Sb(N3)4]− anions theoretically can give rise to seven different conformers which have identical MN4 skeletons but different azido ligand arrangements and very similar energies. We have now synthesized and structurally characterized five of these conformers by subtle variations in the nature of the counterion. Whereas conformational variability is common in organic chemistry, it is rare in inorganic main group chemistry and is usually limited to two. To our best knowledge, the experimental observation of five distinct single conformers for the same type of anion is unprecedented. Theoretical calculations at the M06-2X/cc-pwCVTZ-PP level for all seven possible basic conformers show that (1) the energy differences between the five experimentally observed conformers are about 1 kcal/mol or less, and (2) the free monomeric anions are the energetically favored species in the gas phase and also for [As(N3)4]− in the solid state, whereas for [Sb(N3)4]− associated anions are energetically favored in the solid state and possibly in solutions. Raman spectroscopy shows that in the azide antisymmetric stretching region, the solid-state spectra are distinct for the different conformers, and permits their identification. The spectra of solutions are solvent dependent and differ from those of the solids indicating the presence of rapidly exchanging equilibria of different conformers. The only compound for which a solid with a single well-ordered conformer could not be isolated was [N(CH3)4][As(N3)4] which formed a viscous, room-temperature ionic liquid. Its Raman spectrum was identical to that of its CH3CN solution indicating the presence of an equilibrium of multiple conformers.