Toward the catalytic synthesis of phosphiranes. A computational study

Reaction pathways for the formation of zirconocene phosphinidene complex Cp₂Zr(PR₃)PR from Cp₂ZrCl₂ and LiH and LiPRH and its reactivity to 1,2-dichloroethane are explored with density functional theory using model structures that are devoid of substituents. After the initial Cp₂Zr(Cl)PH₂ is generated with LiPH ₂ reaction with LiH is likely to eliminate HCl in a single step to give directly the 16-electron complex Cp₂ZrPH, which is stabilized by the PH₃ phosphine ligand. The intermediate formation of a phosphine hydride complex, Cp₂Zr(H)PH₂ resulting from hydride substitution, is unlikely both on the basis of unfavorable reaction energies and calculated ³¹P NMR chemical shifts that indicate that such a species cannot have been observed experimentally. It is likely that a diphosphine complex, Cp₂Zr(PH₂)₂, results on using an excess of the lithium phosphide, which on H-transfer gives directly the phosphine-stabilized phosphinidene complex. The reactivity of this species is dominated by the release of its stabilizing phosphine ligand to give a highly reactive 16-electron phosphinidene complex, Cp₂ZrPH, which reacts with 1,2-dichloroethane after coordination to one of the chlorine atoms in two asynchronous metathesis steps to the three-membered phosphirane ring. In this process, ZrCl₂ is reformed enabling its recycling to regenerate the phosphinidene complex. This study highlights the special reactivity of the 16-electron Cp₂ZrPH and suggests that related complexes may be generated similarly, thereby expanding the synthetic potential of these nucleophilic reagents.