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 [MAMBMB] and [MAMBMC] with FeIIICuIINiII1, FeIIINiIINiII2, CoIIICuIINiII3, FeIIICuIICuII4, FeIIINiIICuII5 and FeIIIMnIICuII6 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 FeIII and six-co-ordinated NiII 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 dx2 − y2∥sp2∥dx2 − y2∥sp2∥dx2 − y2. The coupling constant JMA-MB ranges from −5 to −20 cm−1, while JMB-MC ranges from −66 to −395 cm−1 (except for JNi-Ni = −21 cm−1). The coupling constant JMA-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.