65087-62-9Relevant academic research and scientific papers
Room Temperature Acceptorless Alkane Dehydrogenation from Molecular σ-Alkane Complexes
McKay, Alasdair I.,Bukvic, Alexander J.,Tegner, Bengt E.,Burnage, Arron L.,Mart?nez-Mart?nez, Antonio J.,Rees, Nicholas H.,Macgregor, Stuart A.,Weller, Andrew S.
, p. 11700 - 11712 (2019)
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.
Activation of benzene carbon-hydrogen bonds via photolysis or thermolysis of (η5-C5Me5)2Zr(alkyl)H. Isolation of (η5-C5Me5)2Zr(C 6H5)H and its conversion to a complex containing a tetramethylfulvene ligand
Miller, Frederick D.,Sanner, Robert D.
, p. 818 - 825 (2008/10/08)
A new high-yield synthesis of Cp*2ZrH2 (Cp* = η5-C5Me5) is described, and olefin insertion into its Zr-H bond is used to prepare several new Cp*2Zr(alkyl)H complexes. Photolysis or thermolysis of Cp*2Zr(alkyl)H in benzene yields the respective alkane by intramolecular reductive elimination of the cis alkyl and hydride ligands, as well as the benzene C-H bond activation product Cp*2Zr(C6H5)H. Photochemically induced reductive elimination is also observed for Cp*2Zr(C6H5)H and Cp*2ZrH2. Deuterium-labeling experiments show that hydrogen exchange between the hydride and Cp* methyl groups occurs in both Cp*2Zr(H)CH2CH(CH3)2 and Cp*2Zr(C6H5)H. An additional exchange process in Cp*2Zr(C6H5)H involves the hydride ligand and an ortho phenyl hydrogen atom. Thermolysis of Cp*2Zr(C6H5)H in benzene causes quantitative evolution of dihydrogen and reversibly forms the tetramethylfulvene complex Cp*(η6-C5Me4CH2)Zr(C 6H5). Reaction of this compound with iodine produces the Cp* ring substituted phenyl iodide Cp*(η5-C5Me4CH2I)Zr(C 6H5)I. Several of the transformations involving Cp*2Zr(C6H5)H are believed to proceed via β-hydrogen elimination from the phenyl group to yield a benzyne dihydride intermediate.
