Polytype Selection by Intercalated Anions: Design and Synthesis of the 3R2 Polytype of the Layered Double Hydroxide of Zn and Al

The layered double hydroxides are obtained by the stacking of metal hydroxide layers one above another. The stacking sequence is determined by the molecular symmetry of the intercalated anion. Anions select for those stacking sequences which provide interlayer sites having a local symmetry compatible with their own molecular symmetry. Oxoanions, XO3n- (X = S, I), are unique in that their molecular symmetry, C-3v, is compatible with two different polytypes of rhombohedral symmetry. The SO32- ion selects for the more ubiquitous 3R(1) polytype, whereas the IO3- ion selects for the much rarer 3R(2) polytype. Structure refinement shows that neither anion departs significantly from the structure of the free species on intercalation. The higher charge-to-size ratio of the SO32- ion compared to the IO3- ion and its consequent stronger basicity is responsible for the nucleation of the 3R(1) polytype, wherein the trigonal prismatic interlayer sites facilitate the superior hydrogen bonding capacity of the SO32- ion. The poorer basicity of the IO3- ion and its consequent poorer proclivity to form hydrogen bonds select for the 3R(2) polytype wherein the IO3- ion is lodged in interlayer sites of octahedral symmetry.