Selectively Regulating Lewis Acid-Base Sites in Metal-Organic Frameworks for Achieving Turn-On/Off of the Catalytic Activity in Different CO 2 Reactions.

Regulating Lewis acid-base sites in catalysts to investigate their influence in the chemical fixation of CO 2 is significant but challenging. A metal-organic framework (MOF) with open metal Co sites, {(NH 2 Me 2 )[Co 3 (μ 3 -OH)(BTB) 2 (H 2 O)]⋅9 H 2 O⋅5 DMF} n (1), was obtained and the results of the catalytic investigation show that 1 can catalyze cycloaddition of CO 2 and aziridines to give 99 % yield. The efficiency of the cyclization of CO 2 with propargyl amines is only 32 %. To improve the catalytic ability of 1, ligand XN with Lewis base sites was introduced into 1 and coordinated with the open Co sites, resulting in a decrease of the Lewis acid sites and an increase in the Lewis base sites in a related MOF 2 ({(NH 2 Me 2 )[Co 3 (μ 3 -OH)(NHMe 2 )(BTB) 2 (XN)]⋅8 H 2 O⋅4 DMF} n ). Selective regulation of the type of active centers causes the yield of oxazolidinones to be enhanced by about 2.4 times, suggesting that this strategy can turn on/off the catalytic activity for different reactions. The catalytic results from 2 treated with acid solution support this conclusion. This work illuminates a MOF-construction strategy that produces efficient catalysts for CO 2 conversion.