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).