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-κ2P,C:η3-PMes*CRCR)(CO)5] (Mes* = 2,4,6-C6H2tBu3; R = CO2Me,
Ph). Terminal alkynes HCCR1 gave mixtures of isomers
[MoReCp(μ-κ2P,C:η3-PMes*CHCR1)(CO)5] and [MoReCp(μ-κ2P,C:η3-PMes*CR1CH)(CO)5], with the first isomer being major (R1 = CO2Me) or unique (R1 = tBu), 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:κ1C mode, as shown by the reaction
of [MoReCp(μ-κ2P,C:η3-PMes*CHC(CO2Me)}(CO)5] with CN(p-C6H4OMe) to give [MoReCp{μ-η3:κ1C-PMes*CHC(CO2Me)}(CO)5{CN(p-CH4OMe)}2]. The
MoRe phosphinidene complex reacted with SCNPh to give
as major products the phosphametallacyclic complex [MoReCp{μ-κ2P,S:κ2P,S-PMes*C(NPh)S}(CO)5] and its thiophosphinidene-bridged isomer [MoReCp(μ-η2:κ1S-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:κ1S coordination mode of the SPMes* ligand over the
μ-η2:κ1p 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.