Synthesis of [Ni(η2-CH2C6H4R-4){PPh(CH2CH2PPh2)2}]+ (R = H, Me or MeO) and protonation reactions with HCl

The complexes [Ni(η2-CH2C6H4R-4)(triphos)]BPh4 {R = H, Me or MeO; triphos = PhP(CH2CH2PPh2)2} have been prepared and characterised by spectroscopy and X-ray crystallography. In all cases the coordination geometry of the nickel is best described as square-planar with an η2-benzyl ligand occupying one of the positions. The orientation of the η2-benzyl ligand is dictated by the steric restrictions imposed by the phenyl groups on the triphos ligand, so that the phenyl group on the unique secondary phosphorus and the aromatic group of the benzyl ligand (which are trans to one another) are oriented in the same direction. [Ni(η2-CH2C6H4R-4)(triphos)]+ react with an excess of anhydrous HCl in MeCN to form [NiCl(triphos)]+ (characterised as the [BPh4]− salt by X-ray crystallography) and the corresponding substituted toluene. The kinetics of the reaction of all [Ni(η2-CH2C6H4R-4)(triphos)]+ and HCl in the presence of Cl− have been determined using stopped-flow spectrophotometry. All reactions exhibit a first-order dependence on the concentration of complex and a first-order dependence on the ratio [HCl]/[Cl−]. Varying the 4-R-substituent on the benzyl ligand shows that electron-withdrawing substituents facilitate the rate of the reaction. It is proposed that the mechanism involves initial rapid protonation at the nickel to form [NiH(η2-CH2C6H4R-4)(triphos)]2+, followed by intramolecular proton migration from nickel to carbon to yield the products.