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

The complex [Fe2Cp2(mu-PMes*)(mu-CO)(CO)(2)] (Mes* = 2,4,6-(Ce6H2Bu3)-Bu-t), 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 [Fe2Cp2(mu-CO)(2)(CO)(PH(2)Mes*)] 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(mu-CO)(2)(CO){PH(CH2CMe2)(C6H2Bu2)-Bu-t}], the dimer [Fe2Cp2(CO)(4)], and free phosphine PH(2)Mes*. 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(mu-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(2)Mes*)]-(BF4) with Na(OH), the latter in turn being prepared via oxidation of [Fe2Cp2(CO)(4)] with [FeCp2](BF4) in the presence of PH(2)Mes*. 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 [Fe2Cp2(mu-PCy)(mu-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 [Fe2Cp2(mu-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) angstrom, Fe-P = 2.149(1) angstrom).