Solid-Phase “Self-Hydrolysis” of [Zn(NH₃)₄MoO₄@2H₂O] Involving Enclathrated Water—An Easy Route to a Layered Basic Ammonium Zinc Molybdate Coordination Polymer

• Main Authors: Kende Attila Béres, István E. Sajó, György Lendvay, László Trif, Vladimir M. Petruševski, Berta Barta-Holló, László Korecz, Fernanda Paiva Franguelli, Krisztina László, Imre Miklós Szilágyi, László Kótai
• Format: Article
• Language: English
• Published: MDPI AG 2021-06-01
• Series: Molecules
• Subjects: ammine; hydrolysis; vibrational spectroscopy; thermal analysis; zinc molybdate; photocatalysis
• Online Access: https://www.mdpi.com/1420-3049/26/13/4022

An aerial humidity-induced solid-phase hydrolytic transformation of the [Zn(NH₃)₄]MoO₄<b>@</b>2H₂O (compound <b>1</b><b>@</b>2H₂O) with the formation of [(NH₄)<i>_x</i>H_{(1−<i>x</i>)}Zn(OH)(MoO₄)]_n (x = 0.92–0.94) coordination polymer (formally NH₄Zn(OH)MoO₄, compound <b>2</b>) is described. Based on the isostructural relationship, the powder XRD indicates that the crystal lattice of compound <b>1@</b>2H₂O contains a hydrogen-bonded network of tetraamminezinc (2+) and molybdate (2−) ions, and there are cavities (O₄N₄(μ-H₁₂) cube) occupied by the two water molecules, which stabilize the crystal structure. Several observations indicate that the water molecules have no fixed positions in the lattice voids; instead, the cavity provides a neighborhood similar to those in clathrates. The <b>@</b> symbol in the notation is intended to emphasize that the H₂O in this compound is enclathrated rather than being water of crystallization. Yet, signs of temperature-dependent dynamic interactions with the wall of the cages can be detected, and <b>1@</b>2H₂O easily releases its water content even on standing and yields compound <b>2</b>. Surprisingly, hydrolysis products of <b>1</b> were observed even in the absence of aerial humidity, which suggests a unique solid-phase quasi-intramolecular hydrolysis. A mechanism involving successive substitution of the ammonia ligands by water molecules and ammonia release is proposed. An ESR study of the Cu-doped compound <b>2</b> (<b>2</b>#dotCu) showed that this complex consists of two different Cu²⁺(Zn²⁺) environments in the polymeric structure. Thermal decomposition of compounds <b>1</b> and <b>2</b> results in ZnMoO₄ with similar specific surface area and morphology. The ZnMoO₄ samples prepared from compounds <b>1</b> and <b>2</b> and compound <b>2</b> in itself are active photocatalysts in the degradation of Congo Red dye. IR, Raman, and UV studies on compounds <b>1@</b>2H₂O and <b>2</b> are discussed in detail.