A hundredfold enhancement of relaxation times among Er(<sc>iii</sc>) single-molecule magnets with comparable energy barriers

Magneto-structural correlations are of great importance in developing single-molecule magnets (SMMs) for storage and quantum processing of information. In this work, three cyclooctatetraenylido (COT) ligands, namely COTTMS1, COTTMS2, and COTTMS3 equipped with one, two, and three trimethylsilyl (TMS) substituent(s), respectively, were used to prepare the corresponding Er(III) complexes, namely [K(DME)(2)][Er(COTTMS1)(2)] (1: DME, 1,2-dimethoxyethane), [K(DME)(2)][Er(COTTMS2)(2)] (2), and [K(18-crown-6)(1.5)][Er(COTTMS3)(2)] (3). The complexes, each featuring an Er(III) ion sandwiched by two cyclooctatetraenyl rings, possess uni-axial magnetic anisotropy and display single-molecule magnet behaviors with energy barriers of 208(1) K for 1, 193(2) K for 2, and 217(18) K for 3, and blocking temperatures of 11, 10, and 9 K, respectively. The relaxation times are found to be closely related to the number of TMS substituents, with that of 1 being about 10 times longer than that of 2, and more significantly, 100 times longer than that of 3 at the same temperatures. Studies using infrared spectroscopy indicate that the molecular vibrations of the complexes intensify upon increasing the number of TMS substituents in the ligands, resulting in faster magnetic relaxation. Further analyses reveal that the molecular vibrations in these complexes are related to the pre-exponential factor tau(0) for the Orbach process or C value for the Raman process.