Abstract
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, thus 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 reveals 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 multihapto 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.
Highlights
Metallocenes [M(CpR)2] (CpR = substituted cyclopentadienyl, C5R5) are one of the most celebrated families of compounds in organometallic chemistry.[1]
Lanthanide (Ln) metallocenium cations [Ln(CpR)2]+ were proposed by Birmingham and Wilkinson in 1956,3 and many solvated derivatives have been reported, but complexes with no equatorial interactions were elusive until the dysprosocenium complex [Dy(Cpttt)2][B(C6F5)4] (Cpttt = C5H2tBu3-1,2,4, 1Dy) was reported in 2017.4
For sandwich-type Ln single-molecule magnet (SMM), initial studies of homo- and heteroleptic bis-COTR (COTR = substituted cyclooctatetraenyl) complexes showed that COTR ligands present an equatorial crystal field (CF) potential, yielding more favorable electronic structures and SMM properties for ErIII over DyIII,[6−10] and more recently, pointlike charges on a single axis have been shown to generate a strong axial CF yielding excellent SMM characteristics for DyIII.[11−15] It is intuitive that shrinking the size of the organic sandwich ligand from C8 COTR down to C5 CpR and approaching a more point-like axial charge distribution would result in large magnetic anisotropy for 1-Dy
Summary
34 GHz) of 2-Er and 2-Yb at 10 K (Figures S121 and S123) show only one broad absorption-like feature at low field, characteristic of a single large g-value, with some hyperfine structure visible for 2-Yb Simulations of these spectra show that the g-values are 17.5 and 7.7 for 2-Er and 2-Yb, respectively, in excellent agreement with the CASSCF-SO predictions of 17.59 and 7.98 (Tables S37 and S45) and directly indicative of maximal mJ = ±15/2 and ±7/2 ground states, respectively, as predicted by electrostatic arguments. Simulation of the spectrum gives g2 = 2.65 and g3 = 5.71, in good agreement with CASSCF-SO (Table S17, g1 = 1.09, g2 = 3.50, and g3 = 5.49; g1 not observed experimentally), where the hyperfine coupling is well reproduced using the empirical expressions based on the effective g-values described by Denning et al.[54] These g-values are indicative of a rhombic Kramers doublet ground state, dominated by an mJ = ±1/2 component (mJ = ±1/2 has g1 = 1.14, g2 = g3 = 4.57), as expected from electrostatic considerations. It appears that the unique spin-phonon coupling in 1-Ln is crucial for observing magnetic hysteresis at high temperatures in 1-Dy, compared to complexes containing ligands with localized donor atoms, such as [Dy(tBuO)2(pyridine)5][B(Ph)4].14 the exclusive employment of multihapto ligands, such as arenes and other πdonors, may be useful as a design strategy to realize better performance in the generation of high-temperature SMMs
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