Structure and thermal reactivity of a novel Pd(0) metalloenediyne

We report the X-ray diffraction structure and thermal reactivity of the met alloenediyne compound bis(1,2-bis(diphenylphosphinoethynyl)benzenepalladium (0) (Pd(dppeb)(2), 1). The structure of 1 features a tetrahedral Pd(0) cent er with four phosphorus atoms from two chelaling ligands. The P-Pd-P bond a ngles nearly match the idealized 109.5 degrees geometry expected for a d(10 ) metal center in a tetrahedral ligand field. The tetrahedral geometry of t he metal center forces the alkyne termini separation of the enediyne ligand to a distance of 3.47 Angstrom, which results in a thermally stable compou nd at room temperature, However, at 115 degrees C 1 exhibits solvent-depend ent reactivity. In sigma-fluorotoluene, 1 decomposes via ligand dissociatio n, while in sigma-dichlorobenzene, carbon-halide bond activation of solvent occurs leading to the oxidative addition product trans-Pd((2-chlorophenyl) diphenylphosphine)(2)Cl-2 and free (2-chlorophenyl)diphenylphosphine. The t hermal reactivity of 1 is markedly more endothermic (44 kcal/mol) than that of the known Pd(dppeb)Cl-2 analogue (12.3 kcal/mol), The diminished reacti vity can be attributed to two factors: the increased alkyne termini separat ion in 1 (3.47 vs 3.3 Angstrom) due to the metal-mandated tetrahedral geome try of the Pd(0) center, and the resistance of the Pd(0) to adopting a plan ar transition state geometry to promote Bergman cyclization. Overall this s tudy demonstrates that metal binding can impose structural consequences upo n the enediyne ligand governed by the oxidation state and corresponding lig and field geometry of the metal center.