Divergent Reactivity of a Phosphinidene-Bridged Dimolybdenum Complex Toward 1-Alkynes: P–C, P–H, C–C, and C–H Couplings

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.

Reactions of Heterometallic Phosphinidene-Bridged MoMn and MoRe Complexes with Sulfur and Selenium: From Chalcogenophosphinidene- to Trithiophosphonate-Bridged Derivatives

Reactions of [MoReCp(μ-PR*)(CO)₆] with S₈ were strongly dependent on experimental conditions (R* = 2,4,6-C₆H₂^tBu₃). When using 1 equiv of sulfur, complex [MoReCp(μ-η²:κ¹_S-SPR*)(CO)₆] was slowly formed at 313 K, with a thiophosphinidene ligand unexpectedly bridging the dimetal center in the novel μ-κ¹_S:η² coordination mode, as opposed to the μ-κ¹_P:η² mode usually found in related complexes. The latter underwent fast decarbonylation at 363 K to give [MoReCp(μ-η²:η²-SPR*)(CO)₅], with a six-electron donor thiophosphinidene ligand rearranged into the rare μ-η²:η² coordination mode. Depending on reaction conditions, reactions with excess sulfur involved the addition of two or three S atoms to the phosphinidene ligand to give new complexes identified as the dithiophosphinidene-bridged complex [MoReCp(μ-η²:κ²_S,S'-S₂PR*)(CO)₅], its dithiophosphonite-bridged isomer [MoReCp(μ-κ²_S,S':κ²_S,S'-S₂PR*)(CO)₅], or the trithiophosphonate-bridged derivative [MoReCp(μ-κ²_S,S':κ²_S,S'-S₃PR*)(CO)₅], all of them displaying novel coordination modes of their PRS₂ and PRS₃ ligands, as determined by X-ray diffraction studies. In contrast, the related MoMn complex yielded [MoMnCp(μ-η²:η²-SPR*)(CO)₅] under most conditions. A similar output was obtained in reactions with gray selenium for either MoRe or MoMn phosphinidene complexes, which under different conditions only gave the pentacarbonyl complexes [MoMCp(μ-η²:η²-SePR*)(CO)₅] (M = Re, Mn), these providing a new coordination mode for selenophosphinidene ligands.

Efficient Synthesis and Multisite Reactivity of a Phosphinidene-Bridged Mo-Re Complex. A Platform Combining Nucleophilic and Electrophilic Features

The heterometallic complex [MoReCp(μ-PR*)(CO)₆] (3) was prepared in 60% overall yield from syn-[MoCp(PHR*)(CO)₂] via a three-step procedure involving complexes syn-[MoCp(PClR*)(CO)₂] and [MoReCp(μ-PR*)(CO)₇] as intermediate species (R* = 2,4,6-C₆H₂^tBu₃). The PR* ligand in 3 displays a novel asymmetric interaction with the dimetal center, involving a double bond with one atom (Mo) and a dative single bond with the other one (Re). Compound 3 underwent thermal isomerization involving a C-H bond cleavage to yield the hydride [MoReCp(μ-H){μ-P(CH₂CMe₂)C₆H₂^tBu₂}(CO)₆] and reacted with I₂ to give [MoReCpI₂(μ-PR*)(CO)₆], which displays a symmetrical phosphinidene bridge. Its reaction with methyl propiolate at 293 K proceeded with [2 + 2] cycloaddition of the alkyne and decarbonylation to yield the phosphapropenylidene-bridged complex [MoReCp{μ-κ²_P,C:η³-PR*CHC(CO₂Me)}(CO)₅] as the major product, whereas its reaction with excess CN(4-C₆H₄OMe) at 273 K proceeded with formal [2 + 1] cycloaddition of the isocyanide and further isocyanide addition at the Re site to yield the complex [MoReCp{μ-η²_P,C:κ¹_P-PR*CN(4-C₆H₄OMe)}(CO)₆{CN(4-C₆H₄OMe)}], which displays an azaphosphaallene ligand in a novel bridging coordination mode.