Solvent-mediated crystal-to-crystal transformations from a cationic homometallic metal–organic framework to heterometallic frameworks

Abstract

Two unprecedented heterometallic metal–organic frameworks (MOFs), Ag₂(btr)₂Cr₂O₇·0.5H₂O (1) and Ag₉(btr)₆(Cr₂O₇)₄·PF₆·6H₂O (2) [btr = 4,4′-bis(1,2,4-triazole)], were synthesized through crystal-to-crystal transformation when the monometallic MOF Ag₂(btr)₂·2ClO₄·3H₂O was immersed in the aqueous solutions of KPF₆–K₂Cr₂O₇ and NaBF₄–K₂Cr₂O₇, respectively. The transformation follows a solvent-mediated anion-induced mechanism through the dissolution–reaction–crystallization process. Single-crystal X-ray diffraction analyses reveal that both 1 and 2 are three-dimensional structures based on Ag⁺, Cr₂O₇²⁻ and btr. In 1, Cr₂O₇²⁻ adopts a bidentate bridging mode, and Ag⁺ ions are linked by Cr₂O₇²⁻ and btr into a neutral framework. However, Cr₂O₇²⁻ in 2 exhibits two types of unprecedented bridging modes through bridging four and five Ag⁺ ions. Ag⁺ ions in 2 are bridged by Cr₂O₇²⁻ and btr to form a cationic framework. The non-coordinated BF₄⁻/PF₆⁻ anions show a structure-directing effect during the crystal-to-crystal transformations and can be considered as structure-directing agents. The second-harmonic-generation (SHG) measurement shows that 1 is a non-linear optical complex.