ic3021996_si_001.pdf (3.76 MB)
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 contribution
posted on 2016-02-20, 02:30 authored by Ralf Haiges, Martin Rahm, Karl O. ChristeA 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.