We report the first near-linear bis(amide) 4f-block compound and show that this novel structure, if implemented with dysprosium(III), would have unprecedented single molecule magnet (SMM) properties with an energy barrier, U_eff, for reorientation of magnetization of 1800 cm⁻¹.

The recent discovery of single-ion magnets with magnetic hysteresis above liquid-nitrogen temperatures placed these compounds among the best candidates to realize high-density storage devices. Starting from a prototypical dysprosocenium molecule, showing hysteresis up to 60 K, we derive here a general recipe to design high-blocking-temperature rare-earth single-ion magnets. The complex magnetic relaxation is unraveled by combining magnetization and nuclear magnetic resonance measurements with inelastic neutron scattering experiments and ab initio calculations, thus disentangling the different mechanisms and identifying the key ingredients behind slow relaxation.

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.

The proposal that paramagnetic transition metal complexes could be used as qubits for quantum information processing (QIP) requires that the molecules retain the spin information for a sufficient length of time to allow computation and error correction. Therefore, understanding how the electron spin-lattice relaxation time (T₁) and phase memory time (T_m) relate to structure is important. Previous studies have focused on the ligand shell surrounding the paramagnetic centre, seeking to increase rigidity or remove elements with nuclear spins or both. Here we have studied a family of early 3d or 4f metals in the +2 oxidation states where the ground state is effectively a²S state. This leads to a highly isotropic spin and hence makes the putative qubit insensitive to its environment. We have studied how this influencesT₁andT_mand show unusually long relaxation times given that the ligand shell is rich in nuclear spins and non-rigid.

Following our report of the first near-linear lanthanide (Ln) complex, [Sm(N^††)₂] (1), herein we present the synthesis of [Ln(N^††)₂] [N^†† = {N(SiⁱPr₃)₂}; Ln = Eu (2), Tm (3), Yb (4)], thus achieving approximate uniaxial geometries for a series of “traditional” Ln^II ions. Experimental evidence, together with calculations performed on a model of 4, indicates that dispersion forces are important for stabilization of the near-linear geometries of 1–4. The isolation of 3 under a dinitrogen atmosphere is noteworthy, given that “[Tm(N″)(μ-N″)]₂” (N″ = {N(SiMe₃)₂}) has not previously been structurally authenticated and reacts rapidly with N₂(g) to give [{Tm(N″)₂}₂(μ-η²:η²-N₂)]. Complexes 1–4 have been characterized as appropriate by single-crystal X-ray diffraction, magnetic measurements, electrochemistry, multinuclear NMR, electron paramagnetic resonance (EPR), and electronic spectroscopy, along with computational methods for 3 and 4. The remarkable geometries of monomeric 1–4 lead to interesting physical properties, which complement and contrast with comparatively well understood dimeric [Ln(N″)(μ-N″)]₂ complexes. EPR spectroscopy of 3 shows that the near-linear geometry stabilizes m_J states with oblate spheroid electron density distributions, validating our previous suggestions. Cyclic voltammetry experiments carried out on 1–4 did not yield Ln^II reduction potentials, so a reactivity study of 1 was performed with selected substrates in order to benchmark the Sm^III → Sm^II couple. The separate reactions of 1 with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), azobenzene, and benzophenone gave crystals of [Sm(N^††)₂(TEMPO)] (5), [Sm(N^††)₂(N₂Ph₂)] (6), and [Sm(N^††){μ-OPhC(C₆H₅)CPh₂O-κO,O′}]₂ (7), respectively. The isolation of 5–7 shows that the Sm^II center in 1 is still accessible despite having two bulky N^†† moieties and that the N-donor atoms are able to deviate further from linearity or ligand scrambling occurs in order to accommodate another ligand in the Sm^III coordination spheres of the products.