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A Rhodium-Pentane Sigma-Alkane Complex: Characterization in the Solid State by Experimental and Computational Techniques.

ABSTRACT: The pentane ?-complex [Rh{Cy2 P(CH2 CH2 )PCy2 }(?(2) :?(2) -C5 H12 )][BAr(F) 4 ] is synthesized by a solid/gas single-crystal to single-crystal transformation by addition of H2 to a precursor 1,3-pentadiene complex. Characterization by low temperature single-crystal X-ray diffraction (150?K) and SSNMR spectroscopy (158?K) reveals coordination through two Rh???H-C interactions in the 2,4-positions of the linear alkane. Periodic DFT calculations and molecular dynamics on the structure in the solid state provide insight into the experimentally observed Rh???H-C interaction, the extended environment in the crystal lattice and a temperature-dependent pentane rearrangement implicated by the SSNMR data.

SUBMITTER: Chadwick FM

PROVIDER: S-EPMC4797708 | biostudies-other | 2016 Mar

REPOSITORIES: biostudies-other

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Publications

A Rhodium-Pentane Sigma-Alkane Complex: Characterization in the Solid State by Experimental and Computational Techniques.

Chadwick F Mark FM Rees Nicholas H NH Weller Andrew S AS Krämer Tobias T Iannuzzi Marcella M Macgregor Stuart A SA

Angewandte Chemie (International ed. in English) 20160216 11

The pentane σ-complex [Rh{Cy2 P(CH2 CH2 )PCy2 }(η(2) :η(2) -C5 H12 )][BAr(F) 4 ] is synthesized by a solid/gas single-crystal to single-crystal transformation by addition of H2 to a precursor 1,3-pentadiene complex. Characterization by low temperature single-crystal X-ray diffraction (150 K) and SSNMR spectroscopy (158 K) reveals coordination through two Rh⋅⋅⋅H-C interactions in the 2,4-positions of the linear alkane. Periodic DFT calculations and molecular dynamics on the structure in the solid ...[more]

PMID: 26880330

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Project description:The non-oxidative catalytic dehydrogenation of light alkanes via C-H activation is a highly endothermic process that generally requires high temperatures and/or a sacrificial hydrogen acceptor to overcome unfavorable thermodynamics. This is complicated by alkanes being such poor ligands, meaning that binding at metal centers prior to C-H activation is disfavored. We demonstrate that by biasing the pre-equilibrium of alkane binding, by using solid-state molecular organometallic chemistry (SMOM-chem), well-defined isobutane and cyclohexane σ-complexes, [Rh(Cy2PCH2CH2PCy2)(η:η-(H3C)CH(CH3)2][BArF4] and [Rh(Cy2PCH2CH2PCy2)(η:η-C6H12)][BArF4] can be prepared by simple hydrogenation in a solid/gas single-crystal to single-crystal transformation of precursor alkene complexes. Solid-gas H/D exchange with D2 occurs at all C-H bonds in both alkane complexes, pointing to a variety of low energy fluxional processes that occur for the bound alkane ligands in the solid-state. These are probed by variable temperature solid-state nuclear magnetic resonance experiments and periodic density functional theory (DFT) calculations. These alkane σ-complexes undergo spontaneous acceptorless dehydrogenation at 298 K to reform the corresponding isobutene and cyclohexadiene complexes, by simple application of vacuum or Ar-flow to remove H2. These processes can be followed temporally, and modeled using classical chemical, or Johnson-Mehl-Avrami-Kologoromov, kinetics. When per-deuteration is coupled with dehydrogenation of cyclohexane to cyclohexadiene, this allows for two successive KIEs to be determined [kH/kD = 3.6(5) and 10.8(6)], showing that the rate-determining steps involve C-H activation. Periodic DFT calculations predict overall barriers of 20.6 and 24.4 kcal/mol for the two dehydrogenation steps, in good agreement with the values determined experimentally. The calculations also identify significant C-H bond elongation in both rate-limiting transition states and suggest that the large kH/kD for the second dehydrogenation results from a pre-equilibrium involving C-H oxidative cleavage and a subsequent rate-limiting β-H transfer step.

Dataset Information

A Rhodium-Pentane Sigma-Alkane Complex: Characterization in the Solid State by Experimental and Computational Techniques.

Publications

A Rhodium-Pentane Sigma-Alkane Complex: Characterization in the Solid State by Experimental and Computational Techniques.

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