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

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, 1, possessing
vacant N,N-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 N,N-chelation sites of 1 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 1·[Rh­(NBD)]­X and 1·[Rh­(ETH)2]­X (X = Cl and BF4) was studied in the gas phase (2 bar,
46 °C) to reveal that 1·[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, 1·[Rh­(ETH)2]­BF4 and 1·[Rh­(ETH)2]­Cl, in butene isomerization
was also studied using gas-phase NMR spectroscopy. Under one bar of
butene at 46 °C, 1·[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, 1 [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.