P–C and C–C Coupling Processes in the Reactions of the Phosphinidene-Bridged Complex [Fe2(η5‑C5H5)2(μ-PCy)(μ-CO)(CO)2] with Alkynes

The title complex reacts with excess HCC(p-tol) at room temperature to give selectively the complex [Fe2Cp{μ–η5:κ1-C5H4PCyCHCH(p-tol)}(CO)3] (Fe–Fe = 2.6930(7) Å), which displays a bridging (cyclopentadienylidene)(alkenyl)phosphine ligand resulting from an unprecedented coupling of the phosphinidene ligand to both the alkyne and one of the cyclopentadienyl (Cp) groups. At 348 K, however, a mixture is obtained of the above compound and two isomers of the phosphide-acyl complex [Fe2Cp2{μ–κ1:κ1,η1-CyPCHC(p-tol)C(O)}(μ-CO)(CO)] differing in the relative orientation of the Cp rings with respect to the Fe2P plane (Fe–Fe = 2.6069(8) Å in the cis isomer). The latter complex is decarbonylated photochemically to give the dicarbonyl derivative [Fe2Cp2{μ–η3:η1-PCyCHC(p-tol)}(μ-CO)(CO)], which displays a rare four-electron donor phosphaallyl ligand bridging the dimetal center. The stoichiometric reaction with dimethyl acetylenedicarboxylate proceeded analogously to yield the complexes [Fe2Cp{μ–η5:κ1-C5H4PCyC(CO2Me)CH(CO2Me)}(CO)3] and [Fe2Cp2{μ–κ1:κ1,η1-CyP(CO2Me)C(CO2Me)C(O)}(μ-CO)(CO)]. However, the reaction with 2 equiv of alkyne at 273 K gave selectively the dicarbonyl complex [Fe2Cp2{μ–κ1:η2-PCy(CCO2Me)4}(μ-CO)2] with a four-electron-donor phosphole ligand bridging the metal atoms derived from the coupling of the phosphinidene ligand with two alkyne molecules. The reactions with methyl propiolate turned out to be particularly complex since several compounds were formed in significant amounts depending on the reaction conditions. The (cyclopentadienylidene)(alkenyl)phosphine complex [Fe2Cp{μ–η5:κ1-C5H4PCyCHCH(CO2Me)}(CO)3] was selectively formed in the reaction with 2 equiv of alkyne in toluene at 348 K, whereas the slow addition of a dichloromethane solution of the title complex to a solution of 2 equiv of alkyne in the same solvent at 273 K led to the exclusive formation of the (alkenyl)(alkynyl)phosphine complex [Fe2Cp2(μ-CO)2(CO){κ1-PCy(C2H2CO2Me)(C2CO2Me)}] (Fe–Fe = 2.5246(8) Å in the cis isomer). All these products seem to follow from of a common zwitterionic intermediate resulting from the nucleophilic attack of the pyramidal phosphinidene ligand of the title complex to the less protected α carbon of the alkyne, which then evolves differently depending on the alkyne and experimental conditions.