Equatorial coordination optimization for enhanced axiality of mononuclear Dy(iii) single-molecule magnets

We present a controlled synthetic route to optimize the equatorial coordination environment of three Dy(iii) borohydride complexes: Dy(BH₄)₃(THF)₃ (1), [Dy(BH₄)₂(THF)₅][BPh₄] (2) and [Dy(BH₄)₂(18-C-6)][Na(THF)₂(18-C-6)][BPh₄]₂ (3) (THF = tetrahydrofuran, BPh₄^- = tetraphenyl borate, 18-C-6 = 18-crown-6-ether), which have the same axial coordination environment, while different equatorial sites. Alteration of the coordination environment on the equatorial sites leads to a significant change in their magnetic properties. In the absence of the dc field, complex 1 with three THF molecules and one BH₄^- ligand in the equatorial plane shows no single-molecule magnet (SMM) behaviour, complex 2 having five THF molecules at equatorial sites displays small tails of out-of-phase (χ′′) signals, and complex 3 containing one 18-C-6 with six O atoms in the equatorial plane exhibits χ′′ signals at higher temperatures. These results show the optimization of the equatorial coordination environment of Dy(iii) mononuclear single-molecule magnets.