Phase transition investigations of closo-hydroborates

Abstract

Decahydrodecaborates M2Ba10H10 (M  Men4, Et4N, Et3NH, n-Bu4N, Na, K) and dodecahydrododecaborates M2B12H12 (M  Et4N, Li) have been studied in the solid state between 200 and 400 K using DSC, XRD and 11B MAS NMR techniques. Reversible phase transitions were observed with (Et4N)2B10H10 at 263–277 K (ΔH= 10.03 kJ mol−1), (Et3NH)2B10H10 at 324 K (ΔH=15.32 kJ mol−1), Na2B10H10 at 382 K (ΔH=9.81 kJ mol−1) and (Et4N)2B12H12 between 200 and 220 K (ΔH= 3.99 kJ mol−1). The transitions were first order except for (Et4N)2B12H12. The well resolved signals in the MAS NMR spectra recorded at high temperature became wide quadrupolar signals at temperatures lower than the transition temperature. These results were interpreted as to be caused by changes in the motions of the hydroborate cages in the cation framework. The sharp lined MAS NMR spectra are caused by large motions of the cages which average the observed quadrupolar magnetic moments of the boron atoms. When these large motions are stopped or hindered due to a crystallographic or an order-disorder transition, the MAS NMR spectra are broadened. Attempts to determine the structure of tetraethylammonium salts probably failed because the positions of the anions in the solid are disordered even at temperatures lower than the transition temperature.