Influence of the alkali cation size on the Cu2+ coordination environments in (AX)[Cu(HSeO3)2] (A=Na, K, NH4, Rb, Cs; X=Cl, Br) layered copper hydrogen selenite halides

Using solution evaporation techniques, we succeeded in preparation of new members essentially extending the layered copper hydrogen selenite family, (AX)[Cu(HSeO3)2] with A = Na, K, Rb, Cs, and NH4, and X = Cl and Br. Bromides and chlorides are isostructural in the family of described new compounds crystallizing in three different structure types. (NaX)[Cu(HSeO3)2] and (KX)[Cu(HSeO3)2] (X = Cl, Br) are monoclinic, whereas (AX)[Cu(HSeO3)2] (A = NH4, Rb, Cs; X = Cl, Br) are orthorhombic. Upon the enlargement of the A + ionic radii inserted in the interlayer between the neighboring [Cu(HSeO3)2] slabs, the effective distance is increasing and results in essential elongation of the apical Cu-X (X = Cl, Br) distances. Three different types of CuO4 X n (n = 0–2) polyhedra are formed. The observed trend is an interesting example of the chemical tuning of the Cu2+ coordination environments.