Mechanistic Insights into Molecular Crystalline Organometallic Heterogeneous Catalysis through Parahydrogen-Based Nuclear Magnetic Resonance Studies.

The heterogeneous solid-gas reactions of crystals of [Rh(L 2 )(propene)][BAr F 4 ] ( 1 , L 2 = t Bu 2 PCH 2 CH 2 P t Bu 2 ) with H 2 and propene, 1-butene, propyne, or 1-butyne are explored by gas-phase nuclear magnetic resonance (NMR) spectroscopy under batch conditions at 25 °C. The temporal evolution of the resulting parahydrogen-induced polarization (PHIP) effects measures catalytic flux and thus interrogates the efficiency of catalytic pairwise para -H 2 transfer, speciation changes in the crystalline catalyst at the molecular level, and allows for high-quality single-scan 1 H, 13 C NMR gas-phase spectra for the products to be obtained, as well as 2D-measurements. Complex 1 reacts with H 2 to form dimeric [Rh(L 2 )(H)(μ-H)] 2 [BAr F 4 ] 2 ( 4 ), as probed using EXAFS; meanwhile, a single-crystal of 1 equilibrates NMR silent para -H 2 with its NMR active ortho isomer, contemporaneously converting into 4 , and 1 and 4 each convert para -H 2 into ortho -H 2 at different rates. Hydrogenation of propene using 1 and para -H 2 results in very high initial polarization levels in propane (>85%). Strong PHIP was also detected in the hydrogenation products of 1-butene, propyne, and 1-butyne. With propyne, a competing cyclotrimerization deactivation process occurs to afford [Rh( t Bu 2 PCH 2 CH 2 P t Bu 2 )(1,3,4-Me 3 C 6 H 3 )][BAr F 4 ], while with 1-butyne, rapid isomerization of 1-butyne occurs to give a butadiene complex, which then reacts with H 2 more slowly to form catalytically active 4 . Surprisingly, the high PHIP hydrogenation efficiencies allow hyperpolarization effects to be seen when H 2 is taken directly from a regular cylinder at 25 °C. Finally, changing the chelating phosphine to Cy 2 PCH 2 CH 2 PCy 2 results in initial high polarization efficiencies for propene hydrogenation, but rapid quenching of the catalyst competes to form the zwitterion [Rh(Cy 2 PCH 2 CH 2 PCy 2 ){η 6 -(CF 3 ) 2 (C 6 H 3 )}BAr F 3 ].