Activation of a coordinated alkyne by electron transfer: crystal structures of [Pd{PPh2CHC(But)NNC(But)CH2PPh2}{C(CO2Me)CH(CO2Me)}] and [Pd{(Z,Z)PPh2CH2C(But)NNC(But)CH2PPh2}{C(CO2Me)C(CO2Me)}]

A new zerovalent complex [Ph2}{C(CO2Me)C(CO2Me)}] (1) was prepared and its crystal structure was determined by X-ray diffraction analysis, which shows it crystallises in the C2/C monoclinic space group with a=10.940(1) Å, b=22.086(1) Å, c=19.042(2) Å and β=92.692(9)°. The azine diphosphine ligand chelates the metal in a (Z,Z) configuration with a PPdP bite angle of 114.63(4)°. The alkyne carbon atoms lie essentially in the PPdP plane. The electrochemical reduction of 1 promoted the conversion of the alkyne ligand to a vinyl species, and of the azine diphosphine to ene-hydrazone diphosphine, with formation of [Ph2}{C(CO2Me)CH(CO2Me)}] (2). X-ray diffraction analysis of complex 2 shows it crystallises in the triclinic space group P1̄ with a=11.302(1) Å, b=12.528(1) Å, c=16.028(2) Å, α=107.64(2)°, β=92.27(1)° and γ=111.79(2)°. It displays a square-planar geometry with the two phosphorus atoms in trans position. Extended Hückel MO calculations were performed in order to elucidate the redox process.