Activation of H–H and H–O Bonds at Phosphorus with Diiron Complexes Bearing Pyramidal Phosphinidene Ligands

The complex [Fe2Cp2(μ-PMes*)­(μ-CO)­(CO)2] (Mes* = 2,4,6-C6H2tBu3), which in the solid state displays a pyramidal phosphinidene bridge, reacted at room temperature with H2 (ca. 4 atm) to give the known phosphine complex [Fe2Cp2(μ-CO)2(CO)­(PH2Mes*)] as the major product, along with small amounts of other byproducts arising from the thermal degradation of the starting material, such as the phosphindole complex [Fe2Cp2(μ-CO)2(CO)­{PH­(CH2CMe2)­C6H2tBu2}], the dimer [Fe2Cp2(CO)4], and free phosphine PH2Mes*. During the course of the reaction, trace amounts of the mononuclear phosphide complex [FeCp­(CO)2(PHMes*)] were also detected, a compound later found to be the major product in the carbonylation of the parent phosphinidene complex, with this reaction also yielding the dimer [Fe2Cp2(CO)4] and the known diphosphene Mes*PPMes*. The outcome of the carbonylation reactions of the title complex could be rationalized by assuming the formation of an unstable tetracarbonyl intermediate [Fe2Cp2(μ-PMes*)­(CO)4] (undetected) that would undergo a fast homolytic cleavage of a Fe–P bond, this being followed by subsequent evolution of the radical species so generated through either dimerization or reaction with trace amounts of water present in the reaction media. A more rational synthetic procedure for the phosphide complex was accomplished through deprotonation of the phosphine compound [FeCp­(CO)2(PH2Mes*)]­(BF4) with Na­(OH), the latter in turn being prepared via oxidation of [Fe2Cp2(CO)4] with [FeCp2]­(BF4) in the presence of PH2Mes*. To account for the hydrogenation of the parent phosphinidene complex it was assumed that, in solution, small amounts of an isomer displaying a terminal phosphinidene ligand would coexist with the more stable bridged form, a proposal supported by density functional theory (DFT) calculations of both isomers, with the latter also revealing that the frontier orbitals of the terminal isomer (only 5.7 kJ mol–1 above of the bridged isomer, in toluene solution) have the right shapes to interact with the H2 molecule. In contrast to the above behavior, the cyclohexylphosphinidene complex [Fe2Cp2(μ-PCy)­(μ-CO)­(CO)2] failed to react with H2 under conditions comparable to those of its PMes* analogue. Instead, it slowly reacted with HOR (R = H, Et) to give the corresponding phosphinous acid (or ethyl phosphinite) complexes [Fe2Cp2(μ-CO)2(CO)­{PH­(OR)­Mes*}], a behavior not observed for the PMes* complex. The presence of BEt3 increased significantly the rate of the above reaction, thus pointing to a pathway initiated with deprotonation of an O–H bond of the reagent by the basic P center of the phosphinidene complex, this being followed by the nucleophilic attack of the OR– anion at the P site of the transient cationic phosphide thus formed. The solid-state structure of the cis isomer of the ethanol derivative was determined through a single crystal X-ray diffraction study (Fe–Fe = 2.5112(8) Å, Fe–P = 2.149(1) Å).