Thermal defect engineering of precious group metal–organic frameworks: impact on the catalytic cyclopropanation reaction†
We report on the engineering of defects in precious group metal (PGM)-based HKUST-1 (Hong Kong University of Science and Technology) analogues (RhII,II, RuII,II, RuII,III) and the ramification on the catalytic activity by using the cyclopropanation of styrene with ethyl diazoacetate (EDA) as an analytical probe to investigate complex metal–organic framework (MOF) structures. We have characterized the active sites within the extended frameworks by their activity, product distribution and stereoselectivity. The role of the metal, its oxidation state and the availability of open metal sites is elucidated. With a set of 17 samples including reference to Cu-HKUST-1, metal nanoparticles and existing literature, conclusions on the tuneability of paddlewheel complexes within self-supported porous and crystalline frameworks are presented. In particular, additional axial ligands (OAc−/Cl−) accounting for charge compensation at the mixed-valent RuII,III nodes seem responsible for side-product formation during catalysis. Thermal defect-engineering allows for controlled and preferential removal of those axial ligands accompanied by reduction of the average metal oxidation state. This enhances the number of open metal sites (OMS) and the catalytic activity as well as improving the chemoselectivity towards cyclopropanes. The preference towards formation of trans-cyclopropane is assigned to the steric crowding of the paddlewheel moiety. This diastereoselectivity gradually diminishes with rising defectiveness of the PGM-HKUST-1 analogues featuring modified paddlewheel nodes.