Ralf Haiges - 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.

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      Publication Details (including relevant citation   information): Haiges, Ralf, Rahm, Martin, Christe, Karl   O., Inorg. Chem., 2013, 52  (1), pp 402-414

      Abstract: A unique example for conformational   variability in inorg. main group chem. was discovered. The   arrangement of the azido ligands in the pseudotrigonal   bipyramidal [As(N3)4]- and [Sb(N3)4]- anions theor. can give rise   to seven different conformers which have identical MN4 skeletons   but different azido ligand arrangements and very similar   energies. The authors 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 org. chem., it is rare in inorg. main group chem.   and is usually limited to two. To the authors' best knowledge,   the exptl. observation of five distinct single conformers for the   same type of anion is unprecedented. Theor. calcns. at the   M06-2X/cc-pwCVTZ-PP level for all seven possible basic conformers   show that (1) the energy differences between the five exptl.   obsd. conformers are ∼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]- assocd. anions are energetically favored in the solid   state and possibly in solns. Raman spectroscopy shows that in the   azide antisym. stretching region, the solid-state spectra are   distinct for the different conformers, and permits their   identification. The spectra of solns. are solvent dependent and   differ from those of the solids indicating the presence of   rapidly exchanging equil. of different conformers. The only   compd. for which a solid with a single well-ordered conformer   could not be isolated was [NMe4][As(N3)4] which formed a viscous,   room-temp. ionic liq. Its Raman spectrum was identical to that of   its MeCN soln. indicating the presence of an equil. of multiple   conformers. Safety caution: though no explosion occurred in the   course of the study, the compds. may be extremely   shock-sensitive. [on SciFinder(R)]

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