Coordination of tetradentate X2N2 (X = P, S, O) ligands to iron(II) metal center and catalytic application in the transfer hydrogenation of ketones

A series of mixed tetradentate ligands associating two iminophosphorane moieties with two phosphino, thiophosphino, or phosphine oxide groups (labelled 2, 3, and 4 respectively) have been prepared from the corresponding aminophosphonium derivatives. Their coordination to the iron dichloride metal fragment was achieved using the [FeCl2(THF)(1.5)] precursor leading to [(P2N2)FeCl2] (5), [(S2N2)FeCl2] (6), and [(O2N2)FeCl2] (7). These complexes were shown to be paramagnetic. Moreover, those ligands can act as bi, tri or tetradentate, as evidenced by X-ray structure analysis of the complexes, depending on the nature of the pendant PY coordinating ligand (Y = lone pair, S, O). Indeed, while only one phosphorus atom is coordinated to iron in 5 (PNN coordination), no thiophosphine moiety is connected to Fe in 6 (NN coordination), whereas both phosphine oxide arms are linked to the metal in 7 (ONNO) coordination. For ligand 2, coordination reactions were also performed with a non-chlorinated iron precursor (either [Fe(CH3CN)(6)](BF4)(2) or [Fe(H2O)(6)](BF4)(2)) leading to complexes 8 and 9 depending on the reaction conditions. The different iron(II) complexes 5-9 were tested in catalytic transfer hydrogenation of acetophenone, and were found to be efficient for reactions carried out at 82 degrees C.