The relationship between the strength of hydrogen bonding and spin crossover behaviour in a series of iron(III) Schiff base complexes.

X-ray crystal structures and magnetic properties of an isostructural series of iron(III) Schiff base complexes with the general formula [Fe(L(5))(NCX)]·Solv (where H2L(5) = N,N'-bis(2-hydroxy-naphthylidene)-1,6-diamino-4-azahexane, X = S, Solv = tetrahydrofuran, 1a; X = S, Solv = methanol and 0.5 pyrazine, 1b; X = S, Solv = butanone, 1c; Solv = N,N'-dimethylformamide, X = S (1d) or X = Se (1d'); X = S, Solv = dimethyl sulfoxide, 1e) are reported. In the crystals, the individual [Fe(L(5))(NCX)] molecules are connected through weak C-H···O, C-H···π or C-H···S non-covalent contacts into 2D supramolecular networks, while the guest-solvent (Solv) molecules are trapped in the cavities between two adjacent layers, which are furthermore stabilized by N-H···O hydrogen bonds connecting the Solv oxygen atom with the amine group of the [Fe(L(5))(NCX)] molecule, with the N···O distances varying from 2.921(6) Å (in 1d') to 3.295(2) Å (in 1a). The magnetic properties of the complexes were tuned by the different Solv molecules and as a result of this, four new spin crossover (SCO) compounds with cooperative spin transitions are reported, which are accompanied by thermal hysteresis in two cases (1d and 1e): , T1/2 = 84 K; 1d, T1/2↓ = 232 K, T1/2↑ = 235 K and 1e, T1/2↓ = 127 K, T1/2↑ = 138 K. The role of the N-H···O hydrogen bonding in the occurrence and tuning of SCO was also computationally studied using a topological analysis, and also by evaluation of non-covalent interaction (NCI) indexes. Both theoretical approaches showed a clear relationship between the strength of the N-H···O hydrogen bonds and T1/2, as already inferred from X-ray structural and magnetic data.