Hydration-Controlled Layer Stacking in (NH3)2Cu5(SeO3)2(OH)6·(H2O)2+ <i>x</i> ( <i>x</i> = 0, 1, 3)

Hydration and dehydration are powerful yet underexplored variables for controlling the architecture of layered inorganic materials because intercalated water can modify interlayer separation, hydrogen-bonding networks, and layer stacking. Here, we report the reflux synthesis of a new family of hydrated layered copper selenites, (NH3)2Cu5(SeO3)2(OH)6·(H2O)2+x (x = 0, 1, 3). From the crystal structures determined using electron diffraction and single-crystal X-ray diffraction, we deduce that all three compounds share an identical layer built from Cu(OH)4 squares and Cu-centered square pyramids forming a distorted kagomé-like Cu2+ network. While the intralayer atomic arrangement is preserved across the series, the degree of hydration governs both the interlayer separation and the stacking sequence. These compounds therefore provide a rare platform relevant to the design of hydration-responsive materials for sensing, ion transport, separations, actuation, and energy-related applications. The preservation of distorted kagomé-like Cu2+ layers across hydration states further suggests potential interest for examining how interlayer water and stacking sequence affect low-dimensional magnetic coupling. Under reflux conditions, these phases are also shown to act as reactive intermediates in the formation of Cu2OSeO3, establishing them as tunable precursors for copper oxoselenite synthesis.