Metal Switcheroo Tunes Selectivity

Changing the ratio of metal ions in multi­metal catalytic framework compounds can be an effective strategy for steering multicomponent reactions toward a desired product, according to researchers in Spain (J. Am. Chem. Soc. 2015, DOI: 10.1021/jacs.5b02313). To demonstrate the principle, which can be used to reduce waste and bypass purification steps, M. Ángeles Monge and coworkers at the Materials Science Institute of Madrid prepared three isostructural metal-organic framework (MOF) compounds, each containing one type of metal—aluminum, gallium, or indium. They used the MOFs individually to catalyze a one-pot Strecker reaction between benzaldehyde, trimethylsilyl cyanide, and aniline. The aluminum MOF generated an α-aminonitrile, the gallium MOF formed a cyanosilyl derivative, and the indium MOF yielded an imine. The team also prepared mixed indium-gallium MOFs in which the metals, located at equivalent lattice positions, were present in various ratios. They found that tuning the indium-to-gallium ratio altered the reaction rates and product selectivities. For example, a gallium-rich MOF functioned similarly to the aluminum compound and catalyzed formation of the α-aminonitrile in 96% yield in 30 minutes. In contrast, an indium-rich MOF led to just 64% yield after four days.