Iron Hydride Complexes Bearing Phosphinite-Based Pincer Ligands: Synthesis, Reactivity, and Catalytic Application in Hydrosilylation Reactions

Treatment of resorcinol-derived bis(phosphinite) ligands 1,3-(R2PO)2C6H4 (R = iPr and Ph) with Fe(PMe3)4 furnishes iron POCOP-pincer hydride complexes [2,6-(R2PO)2C6H3]Fe(H)(PMe3)2 (R = iPr, 1a; R = Ph, 1b) with two PMe3 cis to each other. The isopropyl complex 1a undergoes ligand substitution upon mixing with CO to give [2,6-(iPr2PO)2C6H3]Fe(H)(PMe3)(CO). The kinetic product (2a) of this process contains a CO ligand trans to the hydride, whereas the thermodynamic product (2a′) has a CO ligand cis to the hydride. The displacement of PMe3 in 2a by CO takes place at an elevated temperature, resulting in the formation of [2,6-(iPr2PO)2C6H3]Fe(H)(CO)2 (3a). These new iron POCOP-pincer hydride complexes catalyze the hydrosilylation of aldehydes and ketones with different functional groups, and 1a is the most efficient catalyst for this process. Isotopic labeling experiments rule out the hydride ligand being directly involved in the reduction. The hydrosilylation reactions are more likely to proceed via the activation of silanes or carbonyl substrates after ligand (PMe3, or CO in the case of 3a) dissociation from the iron center.