Highly Active Gas Phase Organometallic Catalysis Supported Within Metal–Organic Framework Pores

Abstract

Metal-organic frameworks (MOFs) can act as a platform for the heterogenization of molecular catalysts, providing improved stability, allowing easy catalyst recovery and a route toward structural elucidation of the active catalyst. We have developed a MOF, <b>1</b>, possessing vacant <i>N</i>,<i>N</i>-chelating sites which are accessible via the porous channels that penetrate the structure. In the present work, cationic rhodium(I) norbornadiene (NBD) and bis(ethylene) (ETH) complexes paired with both noncoordinating and coordinating anions have been incorporated into the <i>N</i>,<i>N</i>-chelation sites of <b>1</b> via postsynthetic metalation and facile anion exchange. Exploiting the crystallinity of the host framework, the immobilized Rh(I) complexes were structurally characterized using X-ray crystallography. Ethylene hydrogenation catalysis by <b>1</b>·[Rh(NBD)]X and <b>1</b>·[Rh(ETH)₂]X (X = Cl and BF₄) was studied in the gas phase (2 bar, 46 °C) to reveal that <b>1</b>·[Rh(ETH)₂](BF₄) was the most active catalyst (TOF = 64 h^-1); the NBD materials and the chloride salt were notably less active. On the basis of these observations, the activity of the Rh(I) bis(ethylene) complexes, <b>1</b>·[Rh(ETH)₂]BF₄ and <b>1</b>·[Rh(ETH)₂]Cl, in butene isomerization was also studied using gas-phase NMR spectroscopy. Under one bar of butene at 46 °C, <b>1</b>·[Rh(ETH)₂]BF₄ rapidly catalyzes the conversion of 1-butene to 2-butene with a TOF averaging 2000 h^-1 over five cycles. Notably, the chloride derivative, <b>1</b> [Rh(ETH)₂]Cl displays negligible activity in comparison. XPS analysis of the postcatalysis sample, supported by DFT calculations, suggest that the catalytic activity is inhibited by the strong interactions between a Rh(III) allyl hydride intermediate and the chloride anion.