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 H2L5 = 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 center dot center dot center dot O, C-H center dot center dot center dot pi or C-H center dot center dot center dot 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 center dot center dot center dot O hydrogen bonds connecting the Solv oxygen atom with the amine group of the [Fe(L-5)(NCX)] molecule, with the N center dot center dot center dot O distances varying from 2.921(6) angstrom (in 1d') to 3.295(2) angstrom (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): 1c, T-1/2 = 84 K; 1d, T-1/2 down arrow = 232 K, T-1/2 down arrow = 235 K and 1e, T-1/2 down arrow = 127 K, T-1/2 down arrow = 138 K. The role of the N-H center dot center dot center dot 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 center dot center dot center dot O hydrogen bonds and T-1/2, as already inferred from X-ray structural and magnetic data.