Evasive Group 14 polyanions observed in solution for the first time

The Zintl ions based on the group 14 elements, Si₄^4– and Sn₄^4– have been observed in solution by NMR spectroscopy for the first time, according to scientists from the University of Regensburg in Germany. In the first case, Sn₄^4– was prepared in ammonia solution by the direct reduction of elemental Sn with elemental Rb. The coupling constants and the ¹¹⁷Sn signals of the product were consistent with Sn₄^4– but the possibility of slightly smaller or larger clusters could not be excluded. However, a different method of preparation involving dissolution of Rb₄Sn₄ in ammonia in the presence of a [2.2.2]cryptand definitely produced Sn₄^4–, as described in Angewandte Chemie International Edition.

The equivalent Si polyanion was prepared by dissolving K₆Rb₆Si₁₇ in liquid ammonia in the presence of the cryptand, using 20% ²⁹Si labelling to assist NMR detection. The first silicide to be reported in solution, Si₄^4–, was confirmed from the NMR spectra. The notion that protons derived from the solvent can oxidise Zintl ions was also confirmed for the first time with the observation of NH₂^– and SiH₃^– as the degradation product.

Both Si₄^4– and Sn₄^4– displayed "amazingly high" signal intensities and stabilities, due largely to the presence of the cryptand.

Zintl ions are of interest to synthetic chemists as starting blocks for hybrid materials containing transition metals and main group elements.