Influence of the alkali cation size on the Cu${}^{2+}$ coordination environments in (AX)[Cu(HSeO${}_3$)${}_2$] (A=Na, K, NH${}_4$, 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(HSeO${}_3$)${}_2$] with A = Na, K, Rb, Cs, and NH${}_4$, 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(HSeO${}_3$)${}_2$] and (KX)[Cu(HSeO${}_3$)${}_2$] (X = Cl, Br) are monoclinic, whereas (AX)[Cu(HSeO${}_3$)${}_2$] (A = NH${}_4$, Rb, Cs; X = Cl, Br) are orthorhombic. Upon the enlargement of the A${}^{+}$ ionic radii inserted in the interlayer between the neighboring [Cu(HSeO${}_3$)${}_2$] slabs, the effective distance is increasing and results in essential elongation of the apical Cu-X (X = Cl, Br) distances. Three different types of CuO${}_4$X${}_n$ (n = 0–2) polyhedra are formed. The observed trend is an interesting example of the chemical tuning of the Cu${}^{2+}$ coordination environments.