Solid-state molecular organometallic chemistry. Single-crystal to single-crystal reactivity and catalysis with light hydrocarbon substrates. Issue 9 (19th July 2017)

• Record Type: Journal Article
• Authors: Chadwick, F. Mark; McKay, Alasdair I.; Martinez-Martinez, Antonio J.; Rees, Nicholas H.; Krämer, Tobias; Macgregor, Stuart A.; Weller, Andrew S.
• Is Part Of: Chemical science. Volume 8:Issue 9(2017)
• Page Start: 6014
• Page End: 6029
• Publication Date: 2017-07-19
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/c7sc01491k
• Languages: English
• ISSNs: 2041-6520

Abstract

Abstract : Solid-state molecular organometallic catalysis (SMOM-cat): synthetic routes, unique structural motifs, mobility in the solid-state and very active gas/solid isomerization catalysts. Abstract : Single-crystal to single-crystal solid/gas reactivity and catalysis starting from the precursor sigma-alkane complex[Rh(Cy2 PCH2 CH2 PCy2 )(η 2 η 2 -NBA)][BAr F 4 ] (NBA = norbornane; Ar F = 3, 5-(CF3 )2 C6 H3 ) is reported. By adding ethene, propene and 1-butene to this precursor in solid/gas reactions the resulting alkene complexes[Rh(Cy2 PCH2 CH2 PCy2 )(alkene) x ][BAr F 4 ] are formed. The ethene ( x = 2) complex, [Rh(Cy2 PCH2 CH2 PCy2 )(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(Cy2 PCH2 CH2 PCy2 )(ethene)2 ][BAr F 4 ]-Hex, that has a hexagonal microporous structure ( P 63 22). The propene complex ( x = 1)[Rh(Cy2 PCH2 CH2 PCy2 )(propene)][BAr F 4 ] is characterized as having a π-bound alkene with a supporting γ-agostic Rh⋯H3 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 d3 -propene, H2 CCHCD3, using[Rh(Cy2 PCH2 CH2 PCy2 )(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(Cy2 PCH2 CH2 PCy2 )(propene)2 ][BAr F 4 ], is formed.[Rh(Cy2 PCH2 CH2 PCy2 )(butene)][BAr F 4 ] ( x = 1) is characterized as having 2-butene bound as the cis -isomer and a single Rh⋯H3 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(Cy2 PCH2 CH2 PCy2 )(η 2 η 2 -NBA)][BAr F 4 ] and the polymorphs of[Rh(Cy2 PCH2 CH2 PCy2 )(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(Cy2 PCH2 CH2 PCy2 )(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.