Experimental and Theoretical Investigations of Magnetic Exchange Pathways in Structurally Diverse Iron(III) Schiff-Base Complexes

The synthesis, and the structural and magnetic properties, of the following new iron­(III) Schiff base complexes with the {O′,N,O″}-chelating ligand H₂L (2-hydroxyphenylsalicylaldimine) are reported: K­[FeL₂]·H₂O (1), (Pr₃NH)­[FeL₂]·2CH₃OH (2), [FeL­(bpyO₂) (CH₃OH)]­[FeL₂]·CH₃OH (3), [Fe₂L₃(CH₃OH)]·2CH₃OH·H₂O (4), and [{Fe₂L₂}­(μ–OH)₂{FeL­(bpyO₂)}₂]­[BPh₄]₂·2H₂O (5), where Pr₃NH⁺ represents the tripropylammonium cation and bpyO₂ stands for 2,2′-bipyridine-N-dioxide. A thorough density functional theory (DFT) study of magnetic interactions (the isotropic exchange) at the B3LYP/def-TZVP level of theory was employed, and calculations have revealed superexchange pathways through intramolecular/intermolecular noncovalent contacts (π–π stacking, C–H···O and O–H···O hydrogen bonds, diamagnetic metal cations) and/or covalent bonds ((μ-O_Ph, μ-OH) or bis­(μ-O_Ph) bridging modes), which helped us to postulate trustworthy spin Hamiltonians for magnetic analysis of experimental data. Within the reported family of compounds 1–5, the mediators of the antiferromagnetic exchange can be sorted by their increasing strength as follows: π–π stacking (J^DFT = −0.022 cm^–1/J^mag = −0.025(4) cm^–1 in 2) < C–H···O contacts and π–π stacking (J^DFT = −0.19 cm^–1/J^mag = −0.347(9)­cm^–1 in 1) < O–H···O hydrogen bonds (J^DFT = −0.53 cm^–1/J^mag = −0.41(1) cm^–1 in 3) < bis­(μ-O_Ph) bridge (J^DFT = −13.8 cm^–1/J^mag = −12.3(9) cm^–1 in 4) < (μ-O_Ph, μ-OH) bridge (J^DFT = −18.0 cm^–1/J^mag = −17.1(2) cm^–1 in 5), where J^DFT and J^mag are the isotropic exchange parameters derived from DFT calculations, and analysis of the experimental magnetic data, respectively. The good agreement between theoretically calculated and experimentally derived isotropic exchange parameters suggests that this procedure is applicable also for other chemical and structural systems to interpret magnetic data properly.