A ferromagnetic Ni(ii)–Cr(iii) single-chain magnet based on pentagonal bipyramidal building units

Abstract

A Ni(II) complex harnessed in a pentagonal bipyramidal environment by a rigid N₅-macrocyclic ligand (i.e. 1,3,10,12-tetramethyl-1,2,11,12-tetraaza[3](2,6)-pyridino[3](2,9)-1,10-phenanthrolinophane-2,10-diene) is shown to exhibit axial anisotropy with a ZFS D parameter in the order of −10 to −15 cm⁻¹, and this anisotropy is not significantly changed by apical ligand substitution. Using the latter complex as a building unit, a ferromagnetic 1-D CN-bridged Ni–Cr coordination polymer with SCM behavior characterized by a barrier of magnetization reversal U_eff/k_B = 54 K (37 cm⁻¹) with τ₀ = 2.28 × 10⁻¹¹ s was obtained. The Cr(III) complex involved in the design was chosen to favor parallel organization of the anisotropy axes of the Ni units along the 1-D array, and thus maximize the magnetic anisotropy of the supramolecular system. A related chain compound prepared with diamagnetic [Ni(CN)₄]²⁻ units, and a trinuclear CN-bridged [NiCr₂] complex enabled assessing respectively the magnetic anisotropy of Ni(II) in this coordination sphere and its exchange interaction with Cr(III) found to be ferromagnetic and of the order of 10 cm⁻¹. The crystal structures and magnetic behaviors of all compounds are reported.