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

Abstract

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 1,5-TMS2 ) 2 ] n (Er1) and [Na(DME) 3 ][Er(COT 1,3-TMS2 ) 2 ] (Er2) [COT 1,3-TMS2 and COT 1,5-TMS2 donate 1,3-and 1,5-bis(trimethylsilyl)-substituted cyclooctatetraenyl ligands, respectively; DME = 1,2-dimethoxyethane], and with reference to previously reported [Li(DME) 3 ][Er(COT 1,4-TMS2 ) 2 ] (A) and [K(DME) 2 ]-[Er(COT 1,4-TMS2) 2 ] (B), any possible substituent position effects have been explored for the first time. The rearrangement of the TMS substituents from the starting COT 1,4-TMS2 to COT 1,3-TMS2 and COT 1,5-TMS2 , by way of formal migration of the TMS group, was thermally induced in the case of Er1, while for the formation of Er2, the use of Na + in the placement of its Li + and K + congeners is essential. Both Er1 and Er2 display single-molecule magnetic behaviors with energy barriers of 170(3) and 172(6) K, respectively. Magnetic hysteresis loops, butterfly-shaped for Er1 and wide open for Er2, were observed up to 12 K for Er1 and 13 K for Er2. Studies of magnetic dynamics reveal the different pathways for relaxation of magnetization below 10 K, mainly by the Raman process for Er1 and by quantum tunneling of magnetization for Er2, leading to the order of magnitude difference in magnetic relaxation times and sharply different magnetic hysteresis loops.

Ligand–Metal Complementarity in Rare-Earth and Actinide Chemistry

“…While regioisomers of silylated COT rings have been reported previously, methods for their purposeful isolation have remained elusive; therefore, the influence of multiple substitution patterns on the SMM properties of Er III has not been systematically explored. In their work, Ding, Zhu, and Zheng’s team presented a unique twist on the area by exploring the differences between 1,3-, 1,4-, and also 1,5-substituted analogues of the {C 8 H 6 -(SiMe 3 ) 2 } ligand on the SMM properties of Er III complexes . Rogachev, Shatruk, Petrukhina, and their team continued the COT sandwich complex theme but instead have focused on a benzannulated analogue, dibenzo­[ a , e ]­cyclooctatetraene (DBCOT), which forms charge-separated ion pairs, [RE­(DBCOT)­(THF) 4 ]­[RE­(DBCOT) 2 ] (RE = Y, La, Gd, Tb, Dy, Er) .…”