Open framework structures based on Se_x^2- fragments: Synthesis of (Ph₄P)[M(Se₆)₂] (M = Ga, In, Tl) in Molten (Ph₄P)₂Se_x

Polychalcogenide compounds with open polymeric frameworks are rare, and they represent a class of compounds in which microporosity might be achieved. Microporous frameworks that are not oxide-based are now attracting interest because of the combination of catalytic and electronic properties they may simultaneously possess. Three new compounds that may be forerunners to such materials have been discovered and are reported here. The reaction of gallium (Ga), indium (In), and thallium (Tl) metal with (Ph₄P)₂Se₅(Ph, phenyl) and an excess of elemental selenium (Se) in a sealed, evacuated Pyrex tube at 200°C yielded small red crystals of (Ph₄P)[Ga(Se₆)₂] (I), (Ph₄P)[In(Se₆)₂] (II), and (Ph₄P)[Tl(Se₆)₂] (III), respectively. The [M(Se₆)₂]^- (M = Ga, In, Tl) ions form a two-dimensional, open framework filled with Ph₄P⁺ ions. The [M(Se₆)₂]_n^n- structure consists of tetrahedral M³⁺ centers and bridging Se₆^2- ligands, leading to an extended structure in two dimensions. These layers stack perfectly one on top of the other giving rise to one-dimensional channels running down the c axis that are filled with Ph₄P⁺ cations. These cations are situated in the layers, as opposed to between the layers, and the whole structure can be viewed as a template. Compound II shows remarkable thermal stability and melts congruently at 242°C. Upon cooling to room temperature it gives a glassy phase that recrystallizes upon subsequent heating to 160°C.