Lanthanide(III) Sandwich and Half‐Sandwich Complexes with Bulky Cyclooctatetraenyl Ligands: Synthesis, Structures, and Magnetic Properties

Enhancement of Single-Molecule Magnet Properties by Manipulating Intramolecular Dipolar Interactions

A new single-molecule magnet (SMM) complex [K(18-crown-6)][(COT)Er(µ-Cl)3Er(COT)] (Er2Cl3, COT = cyclooctatetraenide dianion) is obtained by the reaction of [(COT)Er(µ-Cl)(THF)]2 (Er2Cl2, THF = tetrahydrofuran) with an equivalent of KCl in the presence of 18-crown-6. The two COT-Er units in the newly formed complex are triply bridged by µ-Cl ligands, leading to the "head-to-tail" alignment of the magnetic easy axes distinctly different from the "staggered" arrangement in the precursor complex. This structural transformation has led to significantly enhanced intramolecular dipolar interactions and a reduced transverse component of the crystal fields, increasing the energy barrier from 150(8) K for Er2Cl2 to 264(4) K for Er2Cl3 and extending its magnetic relaxation time at 2 K by 2500 times with respect to Er2Cl2. More importantly, the blocking temperature increased from lower than 2 K for Er2Cl2 to 8 K for Er2Cl3, and the magnetic hysteresis loops at 2 K changed from butterfly-shaped for Er2Cl2 to open hysteresis loop with coercive force of 7 kOe for Er2Cl3. These results suggest that the properties of SMMs can be effectively tuned and improved by rationally arranging magnetic spins via molecular engineering.

Dipolar Coupling as a Mechanism for Fine Control of Magnetic States in ErCOT-Alkyl Molecular Magnets

The design of molecular magnets has progressed greatly by taking advantage of the ability to impart successive perturbations and control vibronic transitions in 4fn systems through the careful manipulation of the crystal field. Herein, we control the orientation and rigidity of two dinuclear ErCOT-based molecular magnets: the inversion-symmetric bridged [ErCOT(μ-Me)(THF)]2 (2) and the nearly linear Li[(ErCOT)2(μ-Me)3] (3). The conserved anisotropy of the ErCOT synthetic unit facilitates the direction of the arrangement of its magnetic anisotropy for the purposes of generating controlled internal magnetic fields, improving control of the energetics and transition probabilities of the electronic angular momentum states with exchange biasing via dipolar coupling. This control is evidenced through the introduction of a second thermal barrier to relaxation operant at low temperatures that is twice as large in 3 as in 2. This barrier acts to suppress through-barrier relaxation by protecting the ground state from interacting with stray local fields while operating at an energy scale an order of magnitude smaller than the crystal field term. These properties are highlighted when contrasted against the mononuclear structure ErCOT(Bn)(THF)2 (1), in which quantum tunneling of the magnetization processes dominate, as demonstrated by magnetometry and ab initio computational methods. Furthermore, far-infrared magnetospectroscopy measurements reveal that the increased rigidity imparted by successive removal of solvent ligands when adding bridging methyl groups, along with the increased excited state purity, severely limits local spin-vibrational interactions that facilitate magnetic relaxation, manifesting as longer relaxation times in 3 relative to those in 2 as temperature is increased.

Tetraanionic arachno-Carboranyl Ligand Imparts Strong Axiality to Terbium(III) Single-Molecule Magnets

A family of fully sandwiched arachno-lanthanacarborane complexes formulated as {η⁶ -[μ-1,2-[o-C₆ H₄ (CH₂ )₂ ]-1,2-C₂ B₁₀ H₁₀ ]₂ Ln}{Li₅ (THF)₁₀ } (Ln=Tb, Dy, Ho, Er, Y) is successfully synthesized, where the "carbons-adjacent" carboranyl ligand (arachno-R₂ -C₂ B₁₀ H₁₀ ^4- ) bears four negative charges and coordinates to the central lanthanide ions using the hexagonal η⁶ C₂ B₄ face. Thus, the central lanthanide cations are pseudo-twelve-coordinate and have an approximate pseudo-D_6h symmetry or hexagonal-prismatic geometry. As the crystal field effect imparted by this geometry is still unknown, we thoroughly investigated the magnetic properties of this series of complexes and found that the crystal field imposed by this ligand causes a relation of Tb>Dy>Ho>Er for the energy gaps between the ground and the first excited states, which is of striking resemblance to the ferrocenophane and phthalocyanine ligands although the latter two ligands give disparate local coordination geometries. Moreover, the effective energy barrier to magnetization reversal of 445(10) K, the observable hysteresis loop up to 4 K and the relaxation time of the yttrium-diluted sample reaching 193(17) seconds at 2 K under an optimized field for the Tb analogue of this family of arachno-lanthanacarborane complexes, render a new benchmark for Tb³⁺ -based single-molecule magnets.

Lanthanide-mediated tuning of electronic and magnetic properties in heterotrimetallic cyclooctatetraenyl multidecker self-assemblies

The synthesis of a novel family of homoleptic COT-based heterotrimetallic self-assemblies bearing the formula [LnKCa(COT)3(THF)3] (Ln(iii) = Gd, Tb, Dy, Ho, Er, Tm, and Yb) is reported followed by their X-ray crystallographic and magnetic characterization. All crystals conform to the monoclinic P21/c space group with a slight compression of the unit cell from 3396.4(2) Å3 to 3373.2(4) Å3 along the series. All complexes exhibit a triple-decker structure having the Ln(iii) and K(i) ions sandwiched by three COT2- ligands with an end-bound {Ca2+(THF)3} moiety to form a non-linear (153.5°) arrangement of three different metals. The COT2- ligands act in a η8-mode with respect to all metal centers. A detailed structural comparison of this unique set of heterotrimetallic complexes has revealed consistent trends along the series. From Gd to Yb, the Ln to ring-centroid distance decreases from 1.961(3) Å to 1.827(2) Å. In contrast, the separation of K(i) and Ca(ii) ions from the COT-centroid (2.443(3) and 1.914(3) Å, respectively) is not affected by the change of Ln(iii) ions. The magnetic property investigation of the [LnKCa(COT)3(THF)3] series (Ln(iii) = Gd, Tb, Dy, Ho, Er, and Tm) reveals that the Dy, Er, and Tm complexes display slow relaxation of their magnetization, in other words, single-molecule magnet (SMM) properties. This behaviour is dominated by thermally activated (Orbach-like) and quantum tunneling processes for [DyKCa(COT)3(THF)3] in contrast to [ErKCa(COT)3(THF)3], in which the thermally activated and Raman processes appear to be relevant. Details of the electronic structures and magnetic properties of these complexes are further clarified with the help of DFT and ab initio theoretical calculations.

Heteroleptic Rare-Earth Tris(metallocenes) Containing a Dibenzocyclooctatetraene Dianion

Isolable heteroleptic tris(metallocenes) containing five-membered and larger rings remain extremely scarce. The utilization of tripositive rare-earth-metal ions with ionic radii >1 Å allowed access to unprecedented and sterically congested dibenzocyclooctatetraenyl (dbCOT) metallocenes, [K(crypt-222)][Cp^tet₂RE(η²-dbCOT)] (RE = Y (1), Dy (2); Cp^tet = tetramethylcyclopentadienyl), through a salt metathesis reaction involving Cp^tet₂RE(BPh₄) and the potassium salt of the dbCOT dianion. The solid-state structures were investigated by single-crystal X-ray diffraction, magnetometry, and IR spectroscopy and provided evidence for the first crystallographically characterized (dbCOT)^2- anion in a complex containing d- or f-block metals. Remarkably, the (Cp^tet)^- ligands force a distortion from planarity within the (dbCOT)^2- moiety, engendering a rare η²-bonding motif, as opposed to the classical η⁸ conformation observed in complexes bearing a (COT)^2- ion. The η² coordination mode was proven crystallographically between 100 and 298 K and computationally (DFT and NBO). Furthermore, nucleus independent chemical shift (NICS) calculations uncovered significant ring current within the dbCOT ligand. The solution-state properties of 1 and 2 were analyzed via cyclic voltammetry, NMR, and UV-vis spectroscopy. Cyclic voltammograms of 1 and 2 exhibit a quasi-reversible feature indicating the accessibility of complexes with dbCOT in two oxidation states (dbCOT^2-/3-•). Importantly, the dysprosium congener, 2, is a zero-field single-molecule magnet (SMM).

The chemical and physical properties of tetravalent lanthanides: Pr, Nd, Tb, and Dy

The thermochemistry, descriptive chemistry, spectroscopy, and physical properties of the tetravalent lanthanides (Pr, Nd, Tb and Dy) in extended phases, gas phase, solution, and as isolable molecular complexes are presented.

Selective reduction of 1,5-diazacyclooctatetraenes: synthesis and structures of aromatic diazacyclooctatetraenyl dianions and a 2,6-bipyrrolinyl dianionic Co(ii) complex

The first structurally well-defined aromatic 1,5-diazacyclooctatetraenyl dianion and a 2,6-bipyrrolinyl dianionic Co(ii) complex were obtained selectively via reduction of 1,5-diazacyclooctatetraenes.