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

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 (1), [NH4]5[(CH3)2NH2]Zn4Sn5S17 (2), [CH3CH2NH3]22Zn16Sn12Se51(H2O)4·16H2O (3), and [NH4]2CdSnSe4 (4). 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 1 consists of discrete P1-[Zn4Sn4S17]10– clusters templated by mixed [NH4]+/[H3O]+ cations. In compound 2, such P1 clusters are bridged by Sn4+ ions in a 4,4-connection mode to form a [Zn4Sn5S17]n6n– 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 3 features an [Zn16Sn12Se51(H2O)4]n22n– 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 Å2) and therefore an ultrahigh porosity that are occupied by [CH3CH2NH3]+ cations and water molecules (volume fraction: 57.7%). Compound 4 exists as a stacking of [CdSnSe4]n2n– 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 3 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).