On the rational synthesis and properties of exchange-coupled heterotrinuclear systems containing [MAMBMB] and [MAMBMC] cores

Abstract

Six new modular linear heterotrinuclear complexes of the types [M_AM_BM_B] and [M_AM_BM_C] with Fe^IIICu^IINi^II1, Fe^IIINi^IINi^II2, Co^IIICu^IINi^II3, Fe^IIICu^IICu^II4, Fe^IIINi^IICu^II5 and Fe^IIIMn^IICu^II6 cores were synthesized using selective template reactions involving the ligands 1,4,7-trimethyl-1,4,7-triazacyclononane, tmtacn, and N,N′-bis(2-hydroxy-3-hydroxyiminomethyl-5-methylphenylmethylene)-1,3-propanediamine, H₄Lox. The complexes were characterised by elemental analysis (C, H, N, and the metals), IR, UV-vis, Mössbauer and EPR spectroscopies and variable temperature and variable field magnetic susceptibility measurements. The crystal structure of 2 was determined by X-ray diffraction. It is comparable to those of the previously published compounds 1 and 3 and consists of high-spin Fe^III and six-co-ordinated Ni^II embedded in the imine–oxime ligand, with an intramolecular Fe⋯Ni distance of 3.79 Å and Ni⋯Ni distance of 3.14 Å. The magnetic behaviour of the compounds is complex and exhibits a predominant antiferromagnetic exchange coupling acting along the pathway d_{x² − y²}∥sp²∥d_{x² − y²}∥sp²∥d_{x² − y²}. The coupling constant J_MA-MB ranges from −5 to −20 cm⁻¹, while J_MB-MC ranges from −66 to −395 cm⁻¹ (except for J_Ni-Ni = −21 cm⁻¹). The coupling constant J_MA-MC was kept fixed to zero during the fitting procedure. Compounds 1 and 5 with isoelectronic structures exhibit ground and first excited states inverted to one another due to their spin topology.