X-ray crystal structures and magnetic properties of an isostructural series of iron(III) Schiff base complexes with the general formula [Fe(L⁵)(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(L⁵)(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(L⁵)(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): 1c, T_1/2 = 84 K; 1d, T_1/2↓ = 232 K, T_1/2↑ = 235 K and 1e, T_1/2↓ = 127 K, T_1/2↑ = 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 T_1/2, as already inferred from X-ray structural and magnetic data.

The tetranuclear Ni ^II complexes with the Ni ₄ O ₄ cubane-like core ( 1 ) and the Ni ₄ O ₆ defective dicubane core ( 2 ) were studied, and the latter identified as a first single-molecule magnet with such topology.

A series of the trinuclear [{Fe(3MeO-L)₂}₂{μ⁶-Ln(η²-NO₃)(H₂O)}]·nH₂O, (Ln = Gd (2a), Tb (2b), Dy (2c), Ho (2d), Er (2e), Y (2f), H₂-3MeO-L = 2-hydroxy-3-methoxy-phenylsalicylaldimine) complexes were prepared and thoroughly characterized. The crystal structure of 2b was determined and it revealed that the heterotrinuclear complex consists of two anionic [Fe(3MeO-L)₂]⁻ subunits coordinated to the [Tb(H₂O)(η²-NO₃)]²⁺ bridging moiety through the phenolato and methoxy oxygen atoms. The angular distortion within the coordination polyhedron of the [Fe(3MeO-L)₂]⁻ subunits grows significantly upon coordination to the Ln atom of the bridging moiety, which consequently induces an increase in the parameter of the axial magnetic anisotropy. This conclusion is obvious from the comparison and analysis of the structural (XRD) and magnetic data of the yttrium trimer 2f and the precursor complex (Pr₃NH)[Fe(3MeO-L)₂] (1, Pr₃NH = the tripropylammonium cation), where D_Fe(1) = +0.80 cm⁻¹ and D_Fe(2f) = +1.64 cm⁻¹. Furthermore, a weak antiferromagnetic interaction between the Fe(III) centres was found in 2f (J_FeFe = −0.26 cm⁻¹). The magnetic parameters of 2f were used in the fitting of the magnetic properties of 2a as constraints. The ferromagnetic nature of the Fe–Gd interaction in 2a was confirmed, with J_GdFe = +1.40 cm⁻¹, D_Gd = −0.26 cm⁻¹. Moreover, in the case of the Tb (2b) and Dy (2c) compounds, a slow relaxation of the magnetization at low temperature (below 1.9 K) was observed upon the dehydration of the parent compounds.

Investigations on a series of eight novel mononuclear iron(III) Schiff base complexes with the general formula [Fe(L⁵)(L¹)]·S (where H₂L⁵ = pentadentate Schiff-base ligand, L¹ = a pseudohalido ligand, and S is a solvent molecule) are reported. Several different aromatic 2-hydroxyaldehyde derivatives were used in combination with a non-symmetrical triamine 1,6-diamino-4-azahexane to synthesize the H₂L⁵ Schiff base ligands. The consecutive reaction with iron(III) chloride resulted in the preparation of the [Fe(L⁵)Cl] precursor complexes which were left to react with a wide range of the L¹ pseudohalido ligands. The low-spin compounds were prepared using the cyanido ligand: [Fe(3m-salpet)(CN)]·CH₃OH (1a), [Fe(3e-salpet)(CN)]·H₂O (1b), while the high-spin compounds were obtained by the reaction of the pseudohalido (other than cyanido) ligands with the [Fe(L⁵)Cl] complex arising from salicylaldehyde derivatives: [Fe(3Bu5Me-salpet)(NCS)] (2a), [Fe(3m-salpet)(NCO)]·CH₃OH (2b) and [Fe(3m-salpet)(N₃)] (2c). The compounds exhibiting spin-crossover phenomena were prepared only when L⁵ arose from 2-hydroxy-1-naphthaldehyde (H₂L⁵ = H₂napet): [Fe(napet)(NCS)]·CH₃CN (3a, T_1/2 = 151 K), [Fe(napet)(NCSe)]·CH₃CN (3b, T_1/2 = 170 K), [Fe(napet)(NCO)] (3c, T_1/2 = 155 K) and [Fe(napet)(N₃)], which, moreover, exhibits thermal hysteresis (3d, T_1/2↑ = 122 K, T_1/2↓ = 117 K). These compounds are the first examples of octahedral iron(III) spin-crossover compounds with the coordinated pseudohalides. We report the structure and magnetic properties of these complexes. The magnetic data of all the compounds were analysed using the spin Hamiltonian formalism including the ZFS term and in the case of spin-crossover, the Ising-like model was also applied.

A thorough study of structural and magnetic properties was performed on a series of trinuclear and dinuclear Mn(III)/Fe(III) complexes consisting of [M(L4)(Solv)]⁺ and [Fe(CN)₅(NO)]²⁻ moieties (M = Fe(III) or Mn(III), Solv = H₂O or CH₃OH, L4 = tetradentate salen-type ligands), in which dominant magnetic exchange is mediated by O_S–H⋯O_Ph hydrogen bonds in [M(L4)(Solv)]⁺⋯[M(L4)(Solv)]⁺ supramolecular dimers. As deduced from magnetic analysis involving the determination of zero-field splitting (ZFS) parameters for Mn(III) and Fe(III) ions, as well as from comprehensive DFT calculations and modelling, it may be concluded that the strength of the magnetic exchange is correlated with the number of hydrogen bonds and with the O_Ph⋯O_S distance between the phenolic oxygen atom of the salen-type ligand (O_Ph) and the oxygen atom of the solvent molecule coordinated to the adjacent metal atom (O_S).