Direct Conversion of Benzothiadiazole to Benzimidazole: New Benzimidazole-Derived Metal-Organic Frameworks with Adjustable Honeycomb-Like Cavities

Abstract

Up to now, the direct conversion of the thiadiazole ring to other heterocyclic rings has been a very challenging task. Herein, a Cd^II -mediated alcohol-substitution strategy for direct conversion from benzothiadiazole to benzimidazole is reported. Experimental and molecular modeling studies on the role of the chelated metal ion in this in situ alcohol-substitution reaction revealed that it serves as an all-rounder that is involved in the insertion of alcohol, activation of the thiadiazole ring by coordinative interaction, and the sulfur-extrusion process. Interestingly, the insertion of alcohol occurs much earlier than the sulfur-extrusion process, supported by a water-mediated proton-transfer process. This strategy also is suitable for constructing new benzimidazole-derived MOFs [Cd₂ (HMBIDC^2- )₂ ]⋅4 H₂ O (Cd-BID-MOF-1, HMBIDC^2- =2-methyl-1H-benzimidazole-4,7-dicarboxylate) and [Cd₂ (HPBIDC^2- )₂ ]⋅1/3 H₂ O (Cd-BID-MOF-2, HPBIDC^2- =2-(3-hydroxypropyl)-2H-benzimidazole-4,7-dicarboxylate). Because the terminal hydroxyl group on the imidazole ring protrudes into the circular channel in rhombohedral Cd-BID-MOF-2, the cavity is closer to hydrophilic than the honeycomb-like cavity in Cd-BID-MOF-1 with similar 3D structure. This rare observation will provide a new strategy to develop in situ ligand-reaction synthesis of functional MOFs and useful chelation-assisted catalytic reactions in heteroaromatic chemistry.

Keywords: cadmium; heterocycles; metal-organic frameworks; molecular modeling; solvothermal synthesis.