Iron({II}) Complexes of Two Amine/Imine N$_{\textrm{5}}$ Chelate Ligands Containing a 1,4-Diazepane Core ??? To Crossover or Not To Crossover

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 N5 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 ({nBu}4N){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 ??-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.