Crystal and molecular structures of Mo2Cl4(PMePh2)4 and Re2Cl4(PMePh2)4

Abstract

The dropwise addition of SiMe₃Cl to a stirred suspension of Mo₂(O₂CMe)₄ and PMePh₂ in CH₂Cl₂ results in the formation of the quadruply bonded dimolybdenum complex Mo₂Cl₄(PMePh₂)₄1. The triply bonded complex Re₂Cl₄(PMePh₂)₄2 was synthesised by reaction of PMePh₂[NBu₄]₂[Re₂Cl₈], and NaBH₄ in a MeOH–EtOH slurry and by direct reduction of ReCl₅ with Na/Hg in the presence of PMePh₂. The crystal structures of Mo₂Cl₄(PMePh₂)₄·C₆H₅1a, Re₂Cl₄(PMePh₂)₄·C₆H₆2a and Re₂Cl₄(PMePh₂)₄·Me₂CO 2b have been determined. The complexes were also characterized by ¹H and ³¹P-{¹H} NMR spectroscopy. The M₂Cl₄(PMePh₂)₄ molecules have qualitatively the same structure in all three cases, with effective S₄() symmetry. In 1a the MoMo distance is 2.135(1)Å and for 2a and 2b the ReRe distances for the two structural determinations average to 2.257 Å. It is shown that in addition to the observed structures of 1 and 2 there are three enantiomeric pairs of isomers obtainable by different combinations of rotations of the PMePh₂ ligands about the M–P bonds, all of which are of higher energy. It is suggested that the unstable green isomer of 1 has one of these structures.