Iron(II) Complexes of Two Amine/Imine N5 Chelate Ligands Containing a 1,4-Diazepane Core – To Crossover or Not To Crossover

Abstract Two new chelate ligands, 6‐methyl‐6‐(pyridin‐2‐yl)‐1,4‐bis[(pyridin‐2‐yl)methyl]‐1,4‐diazepane ( 4a ) and 6‐methyl‐6‐(pyridin‐2‐yl)‐1,4‐bis[2‐(pyridin‐2‐yl)ethyl]‐1,4‐diazepane ( 4b ), were synthesized from pyridine‐derived precursors in three‐step procedures. Both ligands have N 5 donor sets consisting of two tertiary amine and three pyridyl N atoms. Complexation with FeCl 2 or FeBr 2 in MeOH followed by anion exchange with ( n Bu 4 N)PF 6 gave the complexes [Fe( 4a )X]PF 6 and [Fe( 4b )X]PF 6 (X = Cl, Br) in moderate‐to‐good yields. The coordination geometry around the iron(II) centre, as determined by single‐crystal X‐ray diffraction, is strongly distorted octahedral for ligand 4a and more regular for ligand 4b . Magnetic susceptibility measurements show the complexes to contain high‐spin iron(II) over the whole range of temperatures (2 textless T textless 300 K). DFT calculations for the complexes of ligands 4a and 4b reproduce the high‐spin ground state and suggest that exchange of the halide ligand for ligands that exhibit some degree of π‐type interaction could induce SCO behaviour. Also, calculations of the zero‐field splitting (ZFS) parameters of these complexes rationalize the observed sign change on the basis of different degrees of structural distortion imparted by ligands 4a and 4b .