Substituent Positioning Effects on the Magnetic Properties of Sandwich-Type Erbium(III) Complexes with Bis(trimethylsilyl)-Substituted Cyclooctatetraenyl Ligands.

Lanthanide complexes with judiciously designed ligands have been extensively studied for their potential applications as single-molecule magnets. With the influence of ligands on their magnetic properties generally established, recent research has unearthed certain effects inherent to site differentiation due to the different types and varying numbers of substituents on the same ligand platform. Using two new sandwich-type Er(III) complexes with cyclooctatetraenyl (COT) ligands featuring two differently positioned trimethylsilyl (TMS) substituents, namely, [Li(DME)Er(COT)] () and [Na(DME)][Er(COT)] () [COT and COT donate 1,3- and 1,5-bis(trimethylsilyl)-substituted cyclooctatetraenyl ligands, respectively; DME = 1,2-dimethoxyethane], and with reference to previously reported [Li(DME)][Er(COT)] () and [K(DME)][Er(COT)] (), any possible substituent position effects have been explored for the first time. The rearrangement of the TMS substituents from the starting COT to COT and COT, by way of formal migration of the TMS group, was thermally induced in the case of , while for the formation of , the use of Na in the placement of its Li and K congeners is essential. Both and display single-molecule magnetic behaviors with energy barriers of 170(3) and 172(6) K, respectively. Magnetic hysteresis loops, butterfly-shaped for and wide open for , were observed up to 12 K for and 13 K for . Studies of magnetic dynamics reveal the different pathways for relaxation of magnetization below 10 K, mainly by the Raman process for and by quantum tunneling of magnetization for , leading to the order of magnitude difference in magnetic relaxation times and sharply different magnetic hysteresis loops.