Tuning of spin crossover behaviour in iron(iii) complexes involving pentadentate Schiff bases and pseudohalides

Abstract

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.