Solid–state molecular organometallic chemistry. Single–crystal to single–crystal reactivity and catalysis with light hydrocarbon substrates

摘要

Single-crystal to single-crystal solid/gas reactivity and catalysis starting from the precursor sigma–alkane complex [Rh(Cy2PCH2CH2PCy2)(–NBA)][BArF4] (NBA = norbornane; ArF = 3,5–(CF3)2C6H3) is reported. By adding ethene, propene and 1–butene to this precursor in solid/gas reactions the resulting alkene complexes [Rh(Cy2PCH2CH2PCy2)(alkene)x][BArF4] are formed. The ethene (x = 2) complex, [Rh(Cy2PCH2CH2PCy2)(ethene)2][BArF4]–Oct, has been characterized in the solid–state (single-crystal x-ray diffraction) and by solution and solid–state NMR spectroscopy. Rapid, low temperature recrystallization using solution methods results in a different crystalline modification, [Rh(Cy2PCH2CH2PCy2)(ethene)2][BArF4]–Hex, that has a hexagonal microporous structure (P6322). The propene complex (x = 1) [Rh(Cy2PCH2CH2PCy2)(propene)][BArF4] is characterized as having a –bound alkene with a supporting –agostic Rh···H3C interaction at low temperature by single–crystal x-ray diffraction, variable tempertaure solution and solid–state NMR spectroscopy, as well as periodic density functional theory (DFT) calculations. A fluxional process occurs in both the solid–state and solution that is proposed to proceed via a tautomeric allyl-hydride. Gas/solid catalytic isomerization of d3–propene, H2C=CHCD3, using [Rh(Cy2PCH2CH2PCy2)(–NBA)][BArF4] scrambles the D-label into all possible positions of the propene, as shown by isotopic perturbation of equilibrium measurements for the agostic interaction. Periodic DFT calculations show a low barrier to H/D exchange (10.9 kcal/mol, PBE-D3 level), and GIPAW chemical shift calculations guide the assignment of the experimental data. When synthesized using solution routes a bis-propene complex, [Rh(Cy2PCH2CH2PCy2)(propene)2][BArF4], is formed. [Rh(Cy2PCH2CH2PCy2)(butene)][BArF4] (x = 1) is characterized as having 2–butene bound as the cis-isomer and a single Rh··H3C agostic interaction. In the solid–state two low–energy fluxional processes are proposed. The first is a simple libration of the 2–butene that exchanges the agostic interaction, and the second is a butene isomerization process that proceeds via an allyl–hydride intermediate with a low computed barrier of 14.5 kcal/mol. [Rh(Cy2PCH2CH2PCy2)(–NBA)][BArF4] and the polymorphs of [Rh(Cy2PCH2CH2PCy2)(ethene)2][BArF4] are shown to be effective in Solid-state Molecular OrganoMetallic catalysis (SMOM–cat) for the isomerization of 1–butene to a mixture of cis– and trans–2–butene at 298 K and 1 atm, and studies suggest that catalysis is likely dominated by surface–active species. [Rh(Cy2PCH2CH2PCy2)(–NBA)][BArF4] is also shown to catalyze the transfer dehydrogenation of butane to 2–butene at 298 K using ethene as the sacrificial acceptor.