Reversible First-Order Single Crystal to Single Crystal Thermal Phase Transition in [(CH 3 ) 3 CNH 3 ] 4 [V 4 O 12 ].

The well-known compound tetrakis(tert-butylammonium)-cyclo-tetrametavanadate (V), [(CH 3 ) 3 CNH 3 ] 4 [V 4 O 12 ] (1h_RT), which crystallizes in the tetragonal I4/m space group, undergoes an irreversible solid state transformation upon heating, constituting one of the few examples in which the initial and the final stages are structurally characterized by sc-XRD. Now, we observed the ability of the same compound to undergo an additional single-crystal-to-single-crystal (SCSC) transformation upon thermal stimuli, but this time at low temperatures (153 K). Compound 1h_RT contains a discrete unprotonated [V 4 O 12 ] 4- tetrahedral anion in which V and O bridging atoms are coplanar. In both phases, these tetrameric anions are linked through tert-butylammonium cations in an extensive network of hydrogen bonds, but at low temperatures, this phase loses its characteristic O-V-O coplanarity, with the resulting rearrangement of the crystal packing and hydrogen-bond network which provide its reversibility at low temperatures. Again, the initial and final stages have been characterized structurally by sc-XRD.