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

Single-crystal to single-crystal solid/gas reactivity and catalysis starting from the precursor sigma-alkane complex [Rh(Cy(2)PCH(2)CH(2)PCy(2))(η(2)η(2)-NBA)][BAr(F)(4)] (NBA = norbornane; Ar(F) = 3,5-(CF(3))(2)C(6)H(3)) is reported. By adding ethene, propene and 1-butene to this precursor in solid/gas reactions the resulting alkene complexes [Rh(Cy(2)PCH(2)CH(2)PCy(2))(alkene)(x)][BAr(F)(4)] are formed. The ethene (x = 2) complex, [Rh(Cy(2)PCH(2)CH(2)PCy(2))(ethene)(2)][BAr(F)(4)]-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(Cy(2)PCH(2)CH(2)PCy(2))(ethene)(2)][BAr(F)(4)]-Hex, that has a hexagonal microporous structure (P6(3)22). The propene complex (x = 1) [Rh(Cy(2)PCH(2)CH(2)PCy(2))(propene)][BAr(F)(4)] is characterized as having a π-bound alkene with a supporting γ-agostic Rh···H(3)C 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 d(3)-propene, H(2)C[double bond, length as m-dash]CHCD(3), using [Rh(Cy(2)PCH(2)CH(2)PCy(2))(η(2)η(2)-NBA)][BAr(F)(4)] 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(Cy(2)PCH(2)CH(2)PCy(2))(propene)(2)][BAr(F)(4)], is formed. [Rh(Cy(2)PCH(2)CH(2)PCy(2))(butene)][BAr(F)(4)] (x = 1) is characterized as having 2-butene bound as the cis-isomer and a single Rh···H(3)C 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(Cy(2)PCH(2)CH(2)PCy(2))(η(2)η(2)-NBA)][BAr(F)(4)] and the polymorphs of [Rh(Cy(2)PCH(2)CH(2)PCy(2))(ethene)(2)][BAr(F)(4)] 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(Cy(2)PCH(2)CH(2)PCy(2))(η(2)η(2)-NBA)][BAr(F)(4)] is also shown to catalyze the transfer dehydrogenation of butane to 2-butene at 298 K using ethene as the sacrificial acceptor.