A Dinuclear Palladium(I) Ethynyl Complex:  Synthesis, Structure, and Dynamics

The reaction of Pd(η³-C₃H₅)₂ with PⁱPr₃ at −30 °C affords yellow crystals of the Pd^II complex (ⁱPr₃P)Pd(η³-C₃H₅)(η¹-C₃H₅) (1). At 20 °C 1 transforms into the dinuclear Pd^I complex {(ⁱPr₃P)Pd}₂(μ-C₃H₅)₂ (2) due to oxidative C−C coupling of two allyl groups with elimination of 1,5-hexadiene. Heating 1 or 2 in 1,6-heptadiene to 80 °C produces the Pd⁰ complex (ⁱPr₃P)Pd(η²,η²-C₇H₁₂) (3). {(η³-C₃H₅)PdCl}₂ reacts with ⁱPr₃P to give (ⁱPr₃P)Pd(η³-C₃H₅)Cl (4b), from which further derivatives (ⁱPr₃P)Pd(η³-C₃H₅)X (X = OSO₂CF₃ (4a), C⋮CH (5a), CH₃ (5b)) are obtained by replacement reactions. The mononuclear Pd^II-acetylide 5a and complex 3 combine to give the dinuclear Pd^I derivative {(ⁱPr₃P)Pd}₂(μ-C₃H₅)(μ₂-η²-C₂H) (6). The Pd−Pd bond in 6 is unsymmetrically bridged by a π-allyl and a σ−π-ethynyl group, as determined by X-ray structure analysis. The structures of 1, 4a,b, and 6 are dynamic in solution, with 1 undergoing an exchange of the binding modes of the π- and σ-coordinated allyl groups and 4a,b displaying a π/σ-allyl group rearrangement, and in 6 the C⋮CH substituent oscillates in its π-coordination from one Pd^I atom to the other.