Activation of H-H and H-O bonds at phosphorus with diiron complexes bearing pyramidal phosphinidene ligands

摘要

The complex [Fe _2Cp _2(μ-PMes*)(μ-CO)(CO) _2] (Mes* = 2,4,6-C _6H _2 ~tBu _3), which in the solid state displays a pyramidal phosphinidene bridge, reacted at room temperature with H _2 (ca. 4 atm) to give the known phosphine complex [Fe _2Cp _2(μ-CO) _2(CO)(PH 2Mes*)] 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 [Fe _2Cp _2(μ-CO) _2(CO){PH(CH _2CMe _2)C _6H _2 ~tBu _2}], the dimer [Fe _2Cp _2(CO) _4], and free phosphine PH _2Mes*. 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 [Fe _2Cp _2(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 [Fe _2Cp _2(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(PH _2Mes*)](BF _4) with Na(OH), the latter in turn being prepared via oxidation of [Fe _2Cp _2(CO) _4] with [FeCp _2](BF _4) in the presence of PH _2Mes*. 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 H _2 molecule. In contrast to the above behavior, the cyclohexylphosphinidene complex [Fe _2Cp _2(μ-PCy)(μ-CO)(CO) _2] failed to react with H _2 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 [Fe _2Cp _2(μ-CO) _2(CO){PH(OR)Mes*}], a behavior not observed for the PMes* complex. The presence of BEt _3 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) ?, Fe-P = 2.149(1) ?).