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-κ 2 P,C :η 3 -PMes*CRCR)(CO) 5 ] (Mes* = 2,4,6-C 6 H 2 t Bu 3 ; R = CO 2 Me, Ph). Terminal alkynes HC≡CR 1 gave mixtures of isomers [MoReCp(μ-κ 2 P,C :η 3 -PMes*CHCR 1 )(CO) 5 ] and [MoReCp(μ-κ 2 P,C :η 3 -PMes*CR 1 CH)(CO) 5 ], with the first isomer being major (R 1 = CO 2 Me) or unique (R 1 = t 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-η 3 :κ 1 C mode, as shown by the reaction of [MoReCp(μ-κ 2 P,C :η 3 -PMes*CHC(CO 2 Me)}(CO) 5 ] with CN( p -C 6 H 4 OMe) to give [MoReCp{μ-η 3 :κ 1 C -PMes*CHC(CO 2 Me)}(CO) 5 {CN( p -CH 4 OMe)} 2 ]. The MoRe phosphinidene complex reacted with S=C=NPh to give as major products the phosphametallacyclic complex [MoReCp{μ-κ 2 P,S :κ 2 P,S -PMes*C(NPh)S}(CO) 5 ] and its thiophosphinidene-bridged isomer [MoReCp(μ-η 2 :κ 1 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-η 2 :κ 1 S coordination mode of the SPMes* ligand over the μ-η 2 :κ 1 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.