Reaction of [Ni(Ph2PCH2CH2PPh2)2] with DCl: controlling the formation of HD and D2 *

Abstract

The reaction between anhydrous DCl and [Ni(Ph₂PCH₂CH₂PPh₂)₂] in dichloromethane produced mixtures of HD and D₂. The relative amounts of the dihydrogen isotopomers produced depends on the concentration of acid. Mechanistic investigations showed that the reaction involves initial formation of [NiD(Ph₂PCH₂CH₂PPh₂)₂]⁺. This deuteriation labilises the nickel to phosphine dissociation. At low concentrations of acid, phosphine chelate ring opening produces [NiCl₂(Ph₂PCH₂CH₂PPh₂)] (confirmed by X-ray crystallography) and free Ph₂PCH₂CH₂PPh₂ together with HD (65 ± 5) and D₂ (35 ± 5%). The hydrogen atom of HD originates from a phosphine ligand. At high concentrations of acid the rate of attack of DCl at [NiD(Ph₂PCH₂CH₂PPh₂)₂]⁺ becomes faster than phosphine chelate ring opening and results in the formation of [Ni(Ph₂PCH₂CH₂PPh₂)₂]Cl₂ and predominantly D₂. Experimentally, the highest concentration of DCl that can be used without appreciable decomposition of the complex is 0.2 mol dm^–3. At this concentration of acid the dihydrogen isotopomer distribution is HD (35 ± 5) and D₂ (65 ± 5%); however, analysis of the product distribution indicates that at much higher acid concentrations D₂ could be the exclusive isotopomer.