Synthesis and Decarbonylation Reactions of Diiron Cyclopentadienyl Complexes with Bent-Phosphinidene Bridges

The phosphinidene complexes [Fe2Cp2(μ-PR)(μ-CO)(CO)2] (Cp = η5-C5H5; R=Cy, Ph, Mes, Mes*; Mes = 2,4,6-C6H2Me3, Mes* = 2,4,6-C6H2tBu3) were readily prepared in high yields through a two-step procedure starting from the corresponding phosphine complexes [Fe2Cp2(μ-CO)2(CO)(PH2R)]. When R = Cy, Ph, Mes, the first step required the oxidation of the phosphine complex with 1 equiv of [FeCp2]BF4, this being followed by spontaneous dehydrogenation to yield the cationic phosphide complexes [Fe2Cp2(μ-PHR)(μ-CO)(CO)2]BF4, while the formation of the related PMes*H-bridged complex required a double oxidation of the corresponding neutral precursor in the presence of a deprotonating agent. In all cases the final step involved the deprotonation of the above cations with strong bases such as M(OH) (M = Na, K). Attempts to decarbonylate the above phosphinidene complexes by either thermolysis or photolysis in solution gave results strongly dependent on the nature of R. Thus, the photolysis of the cyclohexylphosphinidene complex gave a mixture of the clusters [Fe4Cp4(μ3-PCy)2(μ3-CO)(μ-CO)(CO)] (Fe−Fe distances 2.6115(7) and 2.5332(8) Å) and [Fe3Cp3(μ3-PCy)(μ-PCyH)(μ-CO)2], while the thermal decarbonylation of the mesitylphosphinidene complex in refluxing toluene gave the trinuclear derivative [Fe3Cp3(μ-PMes)(μ-PMesH)(μ-CO)2(CO)], having a trigonal phosphinidene ligand bridging two iron atoms (Fe−P = 2.161(8), 2.165(8) Å). The formation of all these compounds presumably involves the participation of the corresponding intermediate complex [Fe2Cp2(μ-PR)(μ-CO)2], a species likely to contain a trigonal (four-electron-donor) phosphinidene ligand, not detected. In contrast, the supermesitylphosphinidene complex undergoes a C(tBu)−H oxidative addition to the P atom under mild thermal conditions to give the phosphine derivative [Fe2Cp2(μ-CO)2(CO){PH(CH2CMe2)C6H2tBu2}], while its photolysis gave a mixture of the above phosphine complex and the phosphide−hydride derivatives cis- and trans-[Fe2Cp2(μ-H){μ-P(CH2CMe2)C6H2tBu2}(CO)2], the latter resulting from the oxidative addition of a P−H bond to the dimetal center.