Structural, kinetic, and thermodynamic study of the reversible thermal C-H activation/reductive elimination of alkanes at iridium

Journal Article · Wed Apr 02 00:00:00 EST 1986 · J. Am. Chem. Soc.; (United States)

DOI:https://doi.org/10.1021/ja00267a025· OSTI ID:5282221

Buchanan, J M; Stryker, J M; Bergman, R G

The hydrido alkyl iridium complex Cp*(PMe/sub 3/)Ir(Cy)(H) (1, Cp* = eta/sup 5/-C/sub 5/; Cy = cyclohexyl) has been isolated by air-free chromatography at -80/sup 0/C, and its molecular structure has been determined by X-ray diffraction. Thermolysis of 1 in benzene cleanly produces cyclohexane and Cp*(PMe/sub 3/)Ir(Ph)(H) (2). The rate of reaction is first-order in 1, zero-order in benzene, and inhibited by cyclohexane; its activation parameters are ..delta..H/sup + +/ = 35.6 +/- 0.5 kcal/mol and ..delta..S/sup + +/ = +10 +/- 2 eu. An inverse isotope effect, kappa/sub h/kappa/sub d/ = 0.7 +/- 0.1, is calculated from rates of cyclohexane and cyclohexane-d/sub 12/ reductive elimination at 130/sup 0/C, and deuterium scrambling between the hydride and ..cap alpha..-cyclohexyl positions is observed to occur competitively with reductive elimination. A mechanism is proposed in which cyclohexane loss from 1 is reversible and produces (Cp*(PMe/sub 3/)Ir), which oxidatively adds to a C-H bond in a benzene solvent molecule to form 2. Evidence is also presented for the possible intermediacy of a cyclohexane/(Cp*(PMe/sub 3/)Ir) sigma-complex, which is formed before free (Cp*(PMe/sub 3/)Ir) is released. Equilibrium constants for the equilibration of several pairs of alkanes and their corresponding iridium(III) hydrido alkyl complexes have been determined and imply the following trend in solution phase iridium-carbon bond dissociation enthalpies: phenyl >> n-pentyl > 2,3-dimethylbutyl > cyclopentyl approx. cyclohexyl > neopentyl.

• Research Organization: Univ. of California, Berkeley
• DOE Contract Number: AC03-76SF00098
• OSTI ID: 5282221
• Journal Information: J. Am. Chem. Soc.; (United States), Vol. 108:7
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
DEUTERIUM COMPOUNDS
CHEMICAL REACTION KINETICS
MOLECULAR STRUCTURE
THERMODYNAMICS
IRIDIUM COMPLEXES
ORGANOMETALLIC COMPOUNDS
DECOMPOSITION
EXPERIMENTAL DATA
X-RAY DIFFRACTION
CHEMICAL REACTIONS
COHERENT SCATTERING
COMPLEXES
DATA
DIFFRACTION
HYDROGEN COMPOUNDS
INFORMATION
KINETICS
NUMERICAL DATA
ORGANIC COMPOUNDS
REACTION KINETICS
SCATTERING
TRANSITION ELEMENT COMPLEXES
400301* - Organic Chemistry- Chemical & Physicochemical Properties- (-1987)