The reaction of triorganophosphorus diiodides, R3PI2, with zinc metal powder; Dependency of product on R; The X-ray crystal structures of dimeric {ZnI2[P(NMe2)3]}2 and monomeric ZnI2(PPh2Me)2

Article abstract of DOI:10.1016/S0020-1693(99)00197-8

Seventeen zinc(II) tertiary phosphine complexes have been synthesised directly from elemental zinc by reaction with the reagents R₃PI₂. The complexes have been characterised by elemental analysis and ³¹P{H} NMR spectroscopy. The present work represents the first comprehensive study of a wide variety of zinc(II) tertiary phosphine complexes containing different parent tertiary phosphines and the majority of the complexes are reported for the first time. In most cases, reaction of R₃PI₂ with zinc metal powder in diethyl ether in a 1:1 stoichiometric ratio, produces the dimeric complexes [ZnI₂(PR₃)]₂, analogous to the previously reported [ZnI₂(PEt₃)]₂. In contrast, reaction of R₃PI₂ (R=Ph₃, Ph₂Et, Ph₂Me) with zinc metal powder produces the monomeric bis complexes ZnI₂(PR₃)₂ and an equimolar quantity of zinc(II) iodide, the latter product being identified by X-ray powder diffraction. The X-ray crystal structures of dimeric {ZnI₂[P(NMe₂)₃]}₂ and monomeric ZnI₂(PPh₂Me)₂ are also described. The formation of the bis complexes ZnI₂(PR₃)₂ (R₃=Ph₃, Ph₂Me, Ph₂Et) is surprising and cannot be due to steric factors since complexes containing less bulky tertiary phosphines are shown to be dimeric and the adoption of a monomeric zinc(II) centre increases steric crowding at the metal atom. The existence of the bis complexes is therefore reasoned to be due to favourable Π-Π interactions on the ligands and crystal packing forces.