Synthesis and reactions of low-valent palladium complexes.

Abstract

The reaction of Pd(NO₂)₂L₂(L = P(4-Cl-C₆H₄)₃, PEtPh₂,PMePh₂) with CO was investigated. A technique was developed to analyze the gaseous products of the reaction, CO₂ and N₂O, and to quantify the molar amounts of CO₂ and N₂O evolved using infrared spectroscopy. The reaction solution was analyzed by variable temperature ultraviolet-visible spectroscopy to give evidence for at least five species in the reaction solution. The reaction with CO leads to three moles of CO₂ and one mole of N₂O per mole of Pd(NO₂)₂L₂. Less than stoichiometric amounts were observed when significant amounts of nitrosyl complexes remained in solution, as, for example, when L = PMePh₂ at low concentrations. The initial rate of CO₂ evolution is dependent upon the concentration of the starting material and precedes N₂O evolution. The rate of N₂O evolution depends upon the phosphine ligand and the palladium concentration with no N₂O being evolved at low Pd(NO₂)₂L₂ concentrations. The observed initial rate constant for the disappearance of Pd(NO₂)₂ (PMePh₂)₂ is 0.04642 min⁻¹ (25°C, 1.1 x 10⁻⁴ M), and the observed initial rate constant for the evolution of CO₂ is 0.001963 min⁻¹ (25°C, 0.95 x 10⁻⁴ M), indicating the initial formation of the five-coordinate intermediate, PdCO(NO₂)₂L₂, prior to CO₂ evolution. This intermediate subsequently evolves CO₂ and forms the nitro-nitrosyl complex, Pd(NO₂)(NO)L₂. A third intermediate was identified as the five-coordinate PdCO(NO₂)(NO)L₂. This third species also decomposed, liberating a second mole of CO₂ and producing the dinitrosyl complex, Pd(NO)₂L₂. It is proposed that the dinitrosyl complex subsequently undergoes bimolecular reactions leading to the evolution of the third mole of CO₂ and to the formation of N₂O.