Synthesis and structural elucidation of (pyridyl)imine Fe(II) complexes and their applications as catalysts in transfer hydrogenation of ketones

Reactions of (pyridyl)imine ligands: 2,6-diisopropyl-N-[(pyridine-2-yl)methylene]aniline (L1), 2,6-diisopropyl-N-[(pyridine-2-yl)ethylidene]aniline (L2), 2,6-dimethyl-N-[(pyridine-2-yl)methylene]aniline (L3), 2,6-dimethyl-N-[(pyridine-2-yl)ethylidenelaniline (IA) and N-[(pyridine-2-yl)methylenelaniline (L5) with FeCl 2 salt afforded the corresponding paramagnetic Fe(II) complexes [Fe(L1)(2)CI][FeCl4] (Fe1), [Fe (L2)(2)Cl][FeCl4 ] (Fe2), [Fe(L3)(2)Cl][FeCl4] (Fe3), [Fe(L4)(2)Cl][FeCl4], (Fe4), [Fe(L5)(2)Cl-2] (Fe5) in good yields. On the other hand, reactions of L1 with FeCl2 in the presence of NaPF6 afforded complex [Fe(L1)(2)Cl] [PF6] (Fe6) in moderate yields. Molecular structures of complexes Fe1 and Fe2 reveal the formation of cationic species containing two N boolean AND N bidentate ligands and one chlorido co-ligand to give five-coordinate geometry with [FeCl4] as counter-anion. On the other hand, complex Fe5, is an octahedral neutral species containing two bidentate L5 and two chlorido ligands. All the complexes (Fe1-Fe6) formed active catalysts in the transfer hydrogenation of ketones affording average yields of about 85%. The ligand architecture, reaction conditions and nature of substrate influenced the catalytic activities of the complexes. Mercury and subs-stoichiometric poisoning tests pointed to the existence of both Fe(0) nanoparticles and homogeneous Fe(II) species as the active intermediates. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

The reactions of (pyridyl)imine ligands with FeCl2 salt produced various Fe(II) complexes with different coordination modes, some of which showed excellent catalytic activities in the transfer hydrogenation of ketones. The existence of both Fe(0) nanoparticles and homogeneous Fe(II) species as the active intermediates was confirmed through mercury and sub-stoichiometric poisoning tests.