Modulation of the Slow Relaxation of the Magnetization Dynamics through Second Coordination Sphere in Macrocyclic Dysprosium(III) Complexes

Single-molecule magnets (SMMs) exhibit unique magnetic properties related to the molecular nature of these "isolated" coordination complexes. In this study, we investigate the influence of counterions on two macrocyclic dysprosium-based SMMs, namely, [DyLN6(OAc)2][OAc] (1) and [DyLN6(OAc)2][B(C6H5)4] (2) (LN6 = 2,7,13,18-tetramethyl-3,6,14,17,23,24-hexaazatricyclo[17.3.1.18,12]tetracosa 1(23),2,6,8,10,12(24),13,17,19,21-decaene, OAc = acetate), differing by the nature of the counterion. We demonstrate that the nature of the counterion indirectly modulates the degree of coordination geometry distortion, particularly in macrocycle folding, through subtle intermolecular interactions. This subsequently affects the slow relaxation dynamics, as supported by both magnetic and theoretical calculations. A noticeable decrease in the transverse magnetic anisotropy is observed for 2, attributed to a reduced degree of macrocycle distortion in the equatorial plane. Theoretical calculations further suggest a greater crystal-field splitting in 2 with the first excited state estimated to be twice as large in energy compared to 1. Both complexes exhibit magnetization relaxation dynamics primarily driven by a Raman process.