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Mechanism of thermal decomposition of trans-diethylbis(tertiary phosphine)palladium(II). Steric effects of tertiary phosphine ligands on the stability of diethylpalladium complexes

Journal Article · · J. Am. Chem. Soc.; (United States)
DOI:https://doi.org/10.1021/ja00541a013· OSTI ID:6674807
; ;
trans-(PdR'/sub 2/(PR/sub 3/)/sub 2/) with a series of alkyl groups and tertiary phosphine ligands of various steric bulkiness (R' = Et, PR/sub 3/ = PMe/sub 2/Ph, PEt/sub 3/, PEt/sub 2/Ph, PMePh/sub 2/, PEtPh/sub 2/; R' = Pr, Bu, PR/sub 3/ = PMe/sub 2/Ph) are thermolyzed in solution by ..beta..-elimination processes liberating alkane and alkene in a 1:1 ratio. Kinetic studies on thermolysis of trans-(PdEt/sub 2/(PR/sub 3/)/sub 2/) revealed that they decompose obeying the first-order rate law with respect to the concentration of the palladium complexes. Thermolysis predominantly proceeds from the four-coordinated complex without dissociation of the tertiary phosphine ligand. The minor parallel thermolysis pathway involving the dissociation of the phosphine is completely blocked by addition of the phosphine. Activation enthalpies for thermolysis of the ethylpalladium complexes having various steric bulkiness were virtually constant in the range of 26.0 +- 1.0 kcal/mol whereas activation entropies showed considerable variation with increasing bulkiness of the phosphine. Thermolysis of trans-(Pd(CH/sub 2/CD/sub 3/)/sub 2/(PMePh/sub 2/)/sub 2/) cleanly liberated CH/sub 2/==CD/sub 2/ and CH/sub 2/DCD/sub 3/ with a small isotope effect (k/sub H//k/sub D/ = 1.4 +- 0.1). The kinetic results together with examination of the molecular model suggest that the interaction between the bulky phosphine ligands and the ethyl groups causes the destabilization of the Pd-Et bonds. A thermolysis mechanism consistent with the kinetic results assuming an activation state distorted from the square-planar ground state is proposed. 6 figures, 5 tables.
Research Organization:
Tokyo Inst. of Tech., Japan
OSTI ID:
6674807
Journal Information:
J. Am. Chem. Soc.; (United States), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 102:21; ISSN JACSA
Country of Publication:
United States
Language:
English

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Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400302* -- Organic Chemistry-- Isotope Effects-- (-1987)
ACTIVATION ENERGY
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
COMPLEXES
DECOMPOSITION
DEUTERIUM
ENERGY
ENTHALPY
ENTROPY
HYDROGEN ISOTOPES
ISOTOPE EFFECTS
ISOTOPES
KINETICS
LIGHT NUCLEI
NUCLEI
ODD-ODD NUCLEI
PALLADIUM COMPLEXES
PHOSPHINES
PHOSPHORUS COMPOUNDS
PHYSICAL PROPERTIES
PYROLYSIS
REACTION KINETICS
STABLE ISOTOPES
THERMOCHEMICAL PROCESSES
THERMODYNAMIC PROPERTIES
TRANSITION ELEMENT COMPLEXES