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)₆] with internal alkynes RCCR yielded the phosphapropenylidene-bridged complexes [MoReCp(μ-κ²_P,C:η³-PMes*CRCR)(CO)₅] (Mes* = 2,4,6-C₆H₂^<i>t</i>Bu₃; R = CO₂Me, Ph). Terminal alkynes HCCR¹ gave mixtures of isomers [MoReCp(μ-κ²_P,C:η³-PMes*CHCR¹)(CO)₅] and [MoReCp(μ-κ²_P,C:η³-PMes*CR¹CH)(CO)₅], with the first isomer being major (R¹ = CO₂Me) or unique (R¹ = ^<i>t</i>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)₆]. Addition of ligands to these complexes promoted rearrangement of the phosphapropenylidene ligand into the allyl-like μ-η³:κ¹_C mode, as shown by the reaction of [MoReCp(μ-κ²_P,C:η³-PMes*CHC(CO₂Me)}(CO)₅] with CN(<i>p</i>-C₆H₄OMe) to give [MoReCp{μ-η³:κ¹_C-PMes*CHC(CO₂Me)}(CO)₅{CN(<i>p</i>-CH₄OMe)}₂]. The MoRe phosphinidene complex reacted with SCNPh to give as major products the phosphametallacyclic complex [MoReCp{μ-κ²_P,S:κ²_P,S-PMes*C(NPh)S}(CO)₅] and its thiophosphinidene-bridged isomer [MoReCp(μ-η²:κ¹_S-SPMes*)(CO)₅(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-η²:κ¹_S coordination mode of the SPMes* ligand over the μ-η²:κ¹_p 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.