Synthesis and Electronic Structures of Heavy Lanthanide Metallocenium Cations

Since the origin of 60 K magnetic hysteresis in the dysprosocenium complex [Dy(Cpttt)2][B(C6F5)4] (Cpttt = C5H2tBu3-1,2,4, 1-Dy) remains mysterious, we envisaged that analysis of a series of [Ln(Cpttt)2]+ (Ln = lanthanide) cations could shed light on these properties. Herein we report the synthesis and physical characterization of a family of isolated [Ln(Cpttt)2]+ cations (1-Ln; Ln = Gd, Ho, Er, Tm, Yb, Lu), synthesized by halide abstraction of [Ln(Cpttt)2(Cl)], (2-Ln; Ln = Gd, Ho, Er, Tm, Yb, Lu). Complexes within the two families 1-Ln and 2-Ln are isostructural, and display pseudo-linear and pseudo-trigonal crystal fields, respectively. This results in archetypal electronic structures, determined with CASSCF-SO calculations and confirmed with SQUID magnetometry and EPR spectroscopy, showing easy-axis or easy-plane magnetic anisotropy depending on the choice of Ln ion. Study of their magnetic relaxation dynamics reveal that 1-Ho also exhibits an anomalously low Raman exponent similar to 1-Dy, both being distinct from the larger and more regular Raman exponents for 2-Dy, 2-Er and 2-Yb. This suggests that low Raman exponents arise from the unique spin-phonon coupling of isolated [Ln(Cpttt)2]+ cations. Crucially, this highlights a direct connection between ligand coordination modes and spin-phonon coupling, and therefore we propose that the exclusive presence of multi-hapto ligands in 1-Dy is the origin of its remarkable magnetic properties. Controlling the spin-phonon coupling through ligand design thus appears vital for realizing the next-generation of high-temperature single-molecule magnets.