M–Sn–Q (M = Zn, Cd; Q = S, Se) Compounds Templated by (Alkyl)ammonium Species: Synthesis, Crystal Structure, and Sr²⁺ Adsorption Property

Abstract

Deep investigations on the synthetic and structural chemistry of heterometallic chalcogenidostannates bear fundamental significance for the establishment of the structure-property relationship that would offer guidance on the functional material innovation. Presented here are four ammonium- and/or alkylammonium-directed M-Sn-Q (M = Zn, Cd; Q = S, Se) compounds, namely, [NH₄]₇[H₃O]₃Zn₄Sn₄S₁₇ (<b>1</b>), [NH₄]₅[(CH₃)₂NH₂]Zn₄Sn₅S₁₇ (<b>2</b>), [CH₃CH₂NH₃]₂₂Zn₁₆Sn₁₂Se₅₁(H₂O)₄·16H₂O (<b>3</b>), and [NH₄]₂CdSnSe₄ (<b>4</b>). All four compounds were synthesized in deep eutectic solvents (DESs) or ethylamine aqueous solution, both of which function simultaneously as reaction media and structure-directing agents. Compound <b>1</b> consists of discrete P1-[Zn₄Sn₄S₁₇]^10- clusters templated by mixed [NH₄]⁺/[H₃O]⁺ cations. In compound <b>2</b>, such P1 clusters are bridged by Sn⁴⁺ ions in a 4,4-connection mode to form a [Zn₄Sn₅S₁₇]_<i>n</i>^6<i>n</i>- framework with three types of cavities (I-III) varying in size. The two smaller cavities (I and II) accommodate NH₄⁺ while the larger one(III) is occupied by [(CH₃)₂NH₂]⁺, reflecting the rational size-dependence of cations on cavities. Compound <b>3</b> features an [Zn₁₆Sn₁₂Se₅₁(H₂O)₄]_<i>n</i>^22<i>n</i>- open framework constructed from the 4,3-connection of P1-[Zn₄Sn₄Se₁₇]^10- clusters and {Zn(H₂O)}²⁺ bridges. This linkage mode contributes to a large cage-like subunit (inner dimension: 21.99 × 9.06 Ų) and therefore an ultrahigh porosity that are occupied by [CH₃CH₂NH₃]⁺ cations and water molecules (volume fraction: 57.7%). Compound <b>4</b> exists as a stacking of [CdSnSe₄]_<i>n</i>^2<i>n</i>- chains, which are composed of alternatively arranged {CdSe₄} and {SnSe₄} tetrahedra, in combination with [NH₄]⁺ cations as both charge-compensating and space-filling agents. Detailed synthetic, structural, and topological analyses were performed on these solid materials, coupled with extensive investigations on their optical and thermal properties. Compound <b>3</b> exhibits an efficient Sr²⁺ adsorption performance, featuring ultrafast kinetics (94.69% in 5 min), high removal rate (98.57% in 20 min) at equilibrium, and high capacity (104.17 ± 23.53 mg g^-1).