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Processes Involved in the Reduction of a Cyclometalated Palladium(II) Complex

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posted on 10.03.2008, 00:00 by Christian M. Frech, Gregory Leitus, David Milstein
Reaction of [Pd(TFA)2] (1; TFA = trifluoroacetate) with 2 equiv of benzyldiisopropylphosphine resulted in formation of the metalated complex [Pd{C6H4(CH2PiPr2)}{(C6H5CH2)PiPr2}(TFA)] (2). The dinuclear trifluoroacetate complex [Pd{C6H4(CH2PiPr2)}(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{C6H4(CH2PiPr2)}{(C6H5CH2)PiPr2}(Cl)] (9), [Pd{(C6H5CH2)PiPr2}2(H)(Cl)] (8), [Pd{(C6H5CH2)PiPr2}2(Cl)2] (4), and both isomeric forms of [Pd{C6H4(CH2PiPr2)}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 [Pd{(C6H5CH2)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 (o-lithiobenzyl)diisopropylphosphine to benzene solutions of bis(diethyl sulfide)palladium dichloride (5) at room temperature.