Cycloaddition and C-S Bond Cleavage Processes in Reactions of Heterometallic Phosphinidene-Bridged MoRe and MoMn Complexes with Alkynes and Phenyl Isothiocyanate.

Reactions of [MoReCp(μ-PMes*)(CO)] with internal alkynes RC≡CR yielded the phosphapropenylidene-bridged complexes [MoReCp(μ-κ:η-PMesCRCR)(CO)] (Mes = 2,4,6-CHBu; R = COMe, Ph). Terminal alkynes HC≡CR gave mixtures of isomers [MoReCp(μ-κ:η-PMesCHCR)(CO)] and [MoReCp(μ-κ:η-PMesCRCH)(CO)], with the first isomer being major (R = COMe) or unique (R = Bu), indicating the relevance of steric repulsions during the [2 + 2] cycloaddition step between Mo=P and C≡C bonds in these reactions. Similar reactions were observed for [MoMnCp(μ-PMes*)(CO)]. Addition of ligands to these complexes promoted rearrangement of the phosphapropenylidene ligand into the allyl-like μ-η:κ mode, as shown by the reaction of [MoReCp(μ-κ:η-PMesCHC(COMe)}(CO)] with CN(-CHOMe) to give [MoReCp{μ-η:κ-PMesCHC(COMe)}(CO){CN(-CHOMe)}]. The MoRe phosphinidene complex reacted with S=C=NPh to give as major products the phosphametallacyclic complex [MoReCp{μ-κ:κ-PMesC(NPh)S}(CO)] and its thiophosphinidene-bridged isomer [MoReCp(μ-η:κ-SPMes)(CO)(CNPh)]. The first product follows from a [2 + 2] cycloaddition between Mo=P and C=S bonds, with specific formation of P-C bonds, whereas the second one would arise from the alternative cycloaddition involving the formation of P-S bonds, more favored on steric grounds. The prevalence of the μ-η:κ coordination mode of the SPMes* ligand over the μ-η:κ mode was investigated theoretically to conclude that steric congestion favors the first mode, while the kinetic barrier for interconversion between isomers is low in any case.