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

Abstract

Single-crystal to single-crystal solid/gas reactivity and catalysis starting from the precursor sigma-alkane complex <b>[Rh(Cy₂PCH₂CH₂PCy₂)(η²η²-NBA)][BAr^F ₄]</b> (NBA = norbornane; Ar^F = 3,5-(CF₃)₂C₆H₃) is reported. By adding ethene, propene and 1-butene to this precursor in solid/gas reactions the resulting alkene complexes <b>[Rh(Cy₂PCH₂CH₂PCy₂)(alkene) <i>_x</i> ][BAr^F ₄]</b> are formed. The ethene (<i>x</i> = 2) complex, <b>[Rh(Cy₂PCH₂CH₂PCy₂)(ethene)₂][BAr^F ₄]-Oct</b>, 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, <b>[Rh(Cy₂PCH₂CH₂PCy₂)(ethene)₂][BAr^F ₄]-Hex</b>, that has a hexagonal microporous structure (<i>P</i>6₃22). The propene complex (<i>x</i> = 1) <b>[Rh(Cy₂PCH₂CH₂PCy₂)(propene)][BAr^F ₄]</b> 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 <i>via</i> a tautomeric allyl-hydride. Gas/solid catalytic isomerization of d₃-propene, H₂C[double bond, length as m-dash]CHCD₃, using <b>[Rh(Cy₂PCH₂CH₂PCy₂)(η²η²-NBA)][BAr^F ₄]</b> 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, <b>[Rh(Cy₂PCH₂CH₂PCy₂)(propene)₂][BAr^F ₄]</b>, is formed. <b>[Rh(Cy₂PCH₂CH₂PCy₂)(butene)][BAr^F ₄]</b> (<i>x</i> = 1) is characterized as having 2-butene bound as the <i>cis</i>-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 <i>via</i> an allyl-hydride intermediate with a low computed barrier of 14.5 kcal mol^-1. <b>[Rh(Cy₂PCH₂CH₂PCy₂)(η²η²-NBA)][BAr^F ₄]</b> and the polymorphs of <b>[Rh(Cy₂PCH₂CH₂PCy₂)(ethene)₂][BAr^F ₄]</b> are shown to be effective in solid-state molecular organometallic catalysis (SMOM-Cat) for the isomerization of 1-butene to a mixture of <i>cis</i>- and <i>trans</i>-2-butene at 298 K and 1 atm, and studies suggest that catalysis is likely dominated by surface-active species. <b>[Rh(Cy₂PCH₂CH₂PCy₂)(η²η²-NBA)][BAr^F ₄]</b> is also shown to catalyze the transfer dehydrogenation of butane to 2-butene at 298 K using ethene as the sacrificial acceptor.