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

Journal Article · Wed Apr 02 04:00:00 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), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 108:7; ISSN JACSA
• Country of Publication: United States
• Language: English

Related Subjects

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