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)(η2η2-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 π-bound alkene with a supporting γ-agostic Rh?H₃C interaction at low temperature by single-crystal X-ray diffraction, variable temperature 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, H₂CCHCD₃, using [Rh(Cy₂PCH₂CH₂PCy₂)(η2η2-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?1, 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?1. [Rh(Cy₂PCH₂CH₂PCy₂)(η2η2-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)(η