Structure and Thermal Reactivity of a Novel Pd(0) Metalloenediyne

We report the X-ray diffraction structure and thermal reactivity of the metalloenediyne compound
bis(1,2-bis(diphenylphosphinoethynyl)benzene)palladium(0) (Pd(dppeb)2, 1). The structure of 1 features a
tetrahedral Pd(0) center with four phosphorus atoms from two chelating ligands. The PPdP bond angles
nearly match the idealized 109.5° geometry expected for a d10 metal center in a tetrahedral ligand field. The
tetrahedral geometry of the metal center forces the alkyne termini separation of the enediyne ligand to a distance
of 3.47 Å, which results in a thermally stable compound at room temperature. However, at 115 °C 1 exhibits
solvent-dependent reactivity. In o-fluorotoluene, 1 decomposes via ligand dissociation, while in o-dichlorobenzene, carbon−halide bond activation of solvent occurs leading to the oxidative addition product
trans-Pd((2-chlorophenyl)diphenylphosphine)2Cl2 and free (2-chlorophenyl)diphenylphosphine. The thermal
reactivity of 1 is markedly more endothermic (44 kcal/mol) than that of the known Pd(dppeb)Cl2 analogue
(12.3 kcal/mol). The diminished reactivity can be attributed to two factors:  the increased alkyne termini
separation in 1 (3.47 vs 3.3 Å) due to the metal-mandated tetrahedral geometry of the Pd(0) center, and the
resistance of the Pd(0) to adopting a planar transition state geometry to promote Bergman cyclization. Overall
this study demonstrates that metal binding can impose structural consequences upon the enediyne ligand governed
by the oxidation state and corresponding ligand field geometry of the metal center.