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, 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.

Selectivity of Rh⋅⋅⋅H−C Binding in a σ‐Alkane Complex Controlled by the Secondary Microenvironment in the Solid State

“…Diagnostic signals in the high‐field region of the 1 H NMR spectrum are attributed to agostic Rh⋅⋅⋅H−C interactions, and a Rh−H group [ δ =−29.04 ppm, J (RhH)=32 Hz] in the allyl hydride, 3 . Whereas similar butene complexes to 1 and 2 are formed from solid/gas reactions with Rh I σ‐alkane complexes having R 2 P(CH 2 ) 2 PR 2 ligands (R=Cy, [36] t Bu [20] ), tautomeric Rh III allyl hydrides that come from C−H oxidative cleavage have only been identified indirectly by mechanistic and DFT studies as higher‐energy intermediates in alkene double‐bond isomerization processes for these [36] and related MOF systems [37] . Here, we suggest the larger P−Rh−P ligand bite‐angle makes the Rh III oxidation state more accessible, [38] and this key intermediate can now be observed.…”

Coordination modulated on-off switching of flexibility in a metal–organic framework

“…To overcome this issue, we employed anion metathesis of 1 ·[CuCl] with NaBr or NaI in dry methanol for 24 and 48 hours, respectively. 29,30 In both cases, EDX analysis of the crystals indicated that quantitative halide exchange had occurred (see Mn : Cu : X 3 : 1 : 1 ratios; X = Br, I; Table S3 and Fig. S20 † ).…”

“…33 SCXRD analysis revealed a linear AuCl complex coordinated to a single N-donor site in 1 (Figure 1, Figures S15, S16), and EDX analysis of the crystals (Table S2, Figure S17) confirmed the expected Mn:Au:Cl ratio of 3:1: with NaBr or NaI in dry methanol for 24 and 48 hours, respectively. 29,30 In both cases, EDX analysis of the crystals indicated that quantitative halide exchange had occurred (see Mn:Cu:X 3:1:1 ratios; X= Br, I; Table S3, Figure S20). Single crystal quality for the bromide and iodide exchanged MOFs was excellent, permitting their examination by SCXRD.…”

“…Please do not adjust margins Please do not adjust margins DFT simulations. A representative molecular model of the framework, as used in a previous study, 29 where the framework was cleaved at the pyrazole C4 position and capped with methyl functionality was employed to investigate the coordination of bis-pyrazole system. Geometry optimizations of this molecule coordinated to a Cu centre with a series of halogens (Cl, Br, I) in either a two or three-coordinate conformation used the PBE0 density functional 55 employed by the ORCA software package.…”

“…27 Work in our laboratory has shown that a manganese(II)based MOF, 1 ([Mn3(L)2L'], where L=L'= bis(4-carboxyphenyl-3,5-dimethyl-pyrazol-1-yl)methane, and L' possesses a free N,N' chelating site) can retain crystallinity following post-synthetic metalation and consecutive chemical reactions at the extraneous metal site. [28][29][30][31][32] We have shown that this is primarily due to the intrinsic flexibility of the organic linker which alleviates strain on the framework caused by changes in geometry of the chelated metal ion during reactions. 33 A salient example is cobalt(II) metalated 1 which undergoes a reversible, temperature-dependent, conversion between octahedral 1•[Co(H2O)4]Cl2 and tetrahedral 1•[CoCl2] geometries at the post-synthetic metalation site (Figure 1).…”