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)(propene)][BAr] (, L = BuPCHCHPBu) with H 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 -H transfer, speciation changes in the crystalline catalyst at the molecular level, and allows for high-quality single-scan H, C NMR gas-phase spectra for the products to be obtained, as well as 2D-measurements. Complex reacts with H to form dimeric [Rh(L)(H)(μ-H)][BAr] (), as probed using EXAFS; meanwhile, a single-crystal of equilibrates NMR silent -H with its NMR active ortho isomer, contemporaneously converting into , and and each convert -H into -H at different rates. Hydrogenation of propene using and -H 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(BuPCHCHPBu)(1,3,4-MeCH)][BAr], while with 1-butyne, rapid isomerization of 1-butyne occurs to give a butadiene complex, which then reacts with H more slowly to form catalytically active . Surprisingly, the high PHIP hydrogenation efficiencies allow hyperpolarization effects to be seen when H is taken directly from a regular cylinder at 25 °C. Finally, changing the chelating phosphine to CyPCHCHPCy results in initial high polarization efficiencies for propene hydrogenation, but rapid quenching of the catalyst competes to form the zwitterion [Rh(CyPCHCHPCy){η-(CF)(CH)}BAr].