Processes involved in the reduction of a cyclometalated palladium(II) complex

Reaction of [Pd(TFA)₂] (1; TFA = trifluoroacetate) with 2 equiv of benzyldiisopropylphosphine resulted in formation of the metalated complex [Pd {C₆H₄(CH₂PⁱPr₂)}{(C ₆H₅CH₂)PⁱPr₂ }(TFA)] (2). The dinuclear trifluoroacetate complex [Pd(C₆H₄(CH ₂PⁱPr₂)}[TFA)]₂ (3) was formed when the reaction was performed with an equimolar amount of the phosphine. Both complexes were structurally characterized. Reduction of the cyclometalated palladium complex 2 with sodium metal in THF gave a mixture of cis and trans isomers of the dimetalated bis(o-benzyldiisopropylphosphine)palladium(II) (6a,b) and bis(benzyldiisopropylphosphine)palladium(0) (7). A mechanism for the reduction process is presented. Treatment of the reaction mixture of 6a,b and 7 with an equimolar amount of hydrochloric acid led to a mixture of 7, [Pd(C ₆H₄(CH₂PⁱPr₂)Ji(C ₆H₅CH₂)PⁱPr₂}(Cl)] (9), [Pd{(C₆H₅CH₂)PⁱPr₂J ₂(H)(Cl)] (8), [Pd((C₆H₅CH₂)-P ⁱPr₂}₂(Cl)₂] (4), and both isomeric forms of [Pd(C₆H₄(CH₂PⁱPr ₂)}₂] (6a,b). All complexes were independently prepared and fully characterized. The addition of another 1 equiv of hydrochloric acid to this reaction mixture resulted in the exclusive formation of 4. Reduction of 4 with sodium metal in THF cleanly yielded the Pd⁰ complex 7 in high yields, offering a new, facile, and high-yield route toward the synthesis of dicoordinated bis(phosphine) Pd⁰ complexes. Reaction of 7 with an equimolar amount of HCl led to the clean formation of [PdJ(C₆H ₅CH₂)PⁱPr₂}₂(H)(Cl)] (8). The addition of another 1 equiv of HCl led to the quantitative formation of 2 and H₂. Reactions of 9 and an equimolar amount of NaBHEt3 cleanly yielded complex 7, which was also exclusively formed by treatment of 4 with 2 equiv of NaBHEt3. Mixtures of the cis and trans isomers 6a,b were formed by the addition of 2 equiv of (lithiobenzyl)diisopropylphosphine to benzene solutions of bis(diethyl sulfide)palladium dichloride (5) at room temperature.