Synthesis, structures and magnetic properties of [(η9-C9H9)Ln(η8-C8H8)] super sandwich complexes

Organolanthanide Single-Molecule Magnets with Heterocyclic Ligands

Lanthanide (Ln) based mononuclear single-molecule magnets (SMMs) provide probably the finest ligand regulation model for magnetic property. Recently, the development of such SMMs has witnessed a fast transition from coordination to organometallic complexes because the latter provides a fertile, yet not fully excavated soil for the development of SMMs. Especially those SMMs with heterocyclic ligands have shown the potential to reach higher blocking temperature. In this minireview, we give an overview of the design principle of SMMs and highlight those "shining stars" of heterocyclic organolanthanide SMMs based on the ring sizes of ligands, analysing how the electronic structures of those ligands and the stiffness of subsequently formed molecules affect the dynamic magnetism of SMMs. Finally, we envisaged the future development of heterocyclic Ln-SMMs.

Single-Molecule Magnet Rods: Remarkably Elongated Lanthanide Phosphonate Cores with Quasilinear Hydrazones

Large metal-phosphonate clusters typically exhibit regular polyhedral, wheel-shaped, spherical, or capsule-shaped morphologies more effectively than high-aspect ratio topologies. A system of elongated lanthanide core topologies has now been synthesized by the reaction of lanthanide 1-naphthylmethylphosphonates and four differently terminated pyrazinyl hydrazones. Four new rod-shaped dysprosium phosphonate clusters, [Dy6(O3PC11H9)4(L1)4(μ4-O)(DMF)4]·2DMF·3MeCN·3H2O (1), [Dy8(O3PC11H9)4(L2)4(μ3-O)4(CO2)4(H2O)4]·6DMF·4MeCN·3H2O (2), [Dy12Na(O3PC11H9)6(L3)6(μ3-O)2(pyr)6]·DMF·2MeCN·H2O (3), and [Dy14(O3PC11H9)12(L4)8(μ3-O)2(DMF)4(MeOH)2(H2O)4]·5DMF·2MeCN·H2O (4), were obtained. Four single-pyrazinyl hydrazones function as pentadentate bis-chelate terminal co-ligands, coordinating the periphery of dysprosium phosphonate rods. A sodium ion serves as a cation template for constructing heterobimetallic 3 by occupying the void, demonstrating the ability to reliably control cluster length by modifying the hydrazone co-ligand structure and cation template. Additionally, it was observed that the elongation of the rods has a significant directional impact on the magnetic relaxation behavior, transitioning from a one-step process in 1 to a three-step process in 2, a two-step process in 3, and finally a two-step process in 4.

Construction of intermolecular σ-hole interactions in rare earth metallocene complexes using a 2,3,4,5-tetraiodopyrrolyl anion

The generation of noncovalent intermolecular interactions represents a powerful method to control molecular vibrations and rotations. Combining these with the axial ligand field enforced by the metallocene ligand scaffold provides a dual-pronged approach in controlling the magnetic-relaxation pathways for dysprosium-based single-molecule magnets (SMMs). Here, we present the first implementation of 2,3,4,5-tetraiodopyrrole (TIPH) in its anionic form [TIP]- as a ligand in three isostructural rare-earth metal complexes Cp*2RE(TIP) (1-RE, RE = Y, Gd, and Dy; Cp* = pentamethylcylopentadienyl), where the TIP ligand binds through the nitrogen and one iodine atom κ2(N,I) to the metal centre. The shallow potential energy surface of the intermolecular σ-hole interaction yields distortions of the interatomic distances at elevated temperatures which were investigated by variable-temperature SCXRD. 1-RE constitute the first crystallographically characterized molecules containing TIP as a ligand for any metal ion, and 1-Dy is the first SMM that employs the TIP ligand. The structural dependence on temperature allowed the mechanism of magnetic relaxation to be explored through ab initio calculations at different temperatures. The electronic influence of the coordinated iodine substituent was probed via magnetometry and cw-EPR spectroscopy on 1-Gd. To further scrutinize the impact of the iodine substituents on the physical properties, a second set of new complexes Cp*2RE(DMP) (2-RE, RE = Y, and Dy) where DMP = 2,5-dimethylpyrrolyl were synthesized. Here, the DMP ligand binds similarly to the TIP ligand and represents an all-hydrocarbon analogue to 1-RE. 2-Dy constitutes the first SMM bearing a DMP ligand.

Coercive Fields Exceeding 30 T in the Mixed-Valence Single-Molecule Magnet (CpiPr5)2Ho2I3

Mixed-valence dilanthanide complexes of the type (CpiPr5)2Ln2I3 (CpiPr5 = pentaisopropylcyclopentadienyl; Ln = Gd, Tb, Dy) featuring a direct Ln-Ln σ-bonding interaction have been shown to exhibit well-isolated high-spin ground states and, in the case of the Tb and Dy variants, a strong axial magnetic anisotropy that gives rise to a large magnetic coercivity. Here, we report the synthesis and characterization of two new mixed-valence dilanthanide compounds in this series, (CpiPr5)2Ln2I3 (1-Ln; Ln = Ho, Er). Both compounds feature a Ln-Ln bonding interaction, the first such interaction in any molecular compounds of Ho or Er. Like the Tb and Dy congeners, both complexes exhibit high-spin ground states arising from strong spin-spin coupling between the lanthanide 4f electrons and a single σ-type lanthanide-lanthanide bonding electron. Beyond these similarities, however, the magnetic properties of the two compounds diverge. In particular, 1-Er does not exhibit observable magnetic blocking or slow magnetic relaxation, while 1-Ho exhibits magnetic blocking below 28 K, which is the highest temperature among Ho-based single-molecule magnets, and a spin reversal barrier of 556(4) cm-1. Additionally, variable-field magnetization data collected for 1-Ho reveal a coercive field of greater than 32 T below 8 K, more than 6-fold higher than observed for the bulk magnets SmCo5 and Nd2Fe14B, and the highest coercive field reported to date for any single-molecule magnet or molecule-based magnetic material. Multiconfigurational calculations, supported by far-infrared magnetospectroscopy data, reveal that the stark differences in magnetic properties of 1-Ho and 1-Er arise from differences in the local magnetic anisotropy of the lanthanide centers.

Linear, Electron-Rich Erbium Single-Molecule Magnet with Dibenzocyclooctatetraene Ligands

Judicious design of ligand scaffolds to highly anisotropic lanthanide ions led to substantial advances in molecular spintronics and single-molecule magnetism. Erbium-based single-molecule magnets (SMMs) are rare, which is attributed to the prolate-shaped ErIII ion requiring an equatorial ligand field for enhancing its single-ion magnetic anisotropy. Here, we present an electron-rich mononuclear Er SMM, [K(crypt-222)][Er(dbCOT)2], 1 (where dbCOT = dibenzocyclooctatetraene), that was obtained from a salt metathesis reaction of ErCl3 and K2dbCOT. The dipotassium salt, K2dbCOT, was generated through a two-electron reduction of the bare dbCOT0 ligand employing potassium graphite and was crystallized from DME to give the new solvated complex, [K(DME)]2[dbCOT]n, 2. 1 was analyzed through crystallography, electrochemistry, spectroscopy, magnetometry, and CASSCF calculations. The structure of 1 consists of an anionic metallocene complex featuring a linear (180.0°) geometry with an ErIII ion sandwiched between dianionic dbCOT ligands and an outer-sphere K+ ion encapsulated in 2.2.2-cryptand. Two pronounced redox events at negative potentials allude to the formation of a trianionic erbocene complex, [Er(dbCOT)2]3-, on the electrochemical time scale. 1 shows slow magnetic relaxation with an effective spin-reversal barrier of Ueff = 114(2) cm-1, which is close in magnitude to the calculated energies of the first and second excited states of 96.9 and 109.13 cm-1, respectively. 1 exhibits waist-constricted hysteresis loops below 4 K and constitutes the first example of an erbocene-SMM bearing fused aromatic rings to the central COT ligand. Notably, 1 comprises the largest COT scaffold implemented in erbocene SMMs, yielding the most electron-rich homoleptic erbium metallocene SMM.

Substituent Positioning Effects on the Magnetic Properties of Sandwich-Type Erbium(III) Complexes with Bis(trimethylsilyl)-Substituted Cyclooctatetraenyl Ligands

Lanthanide complexes with judiciously designed ligands have been extensively studied for their potential applications as single-molecule magnets. With the influence of ligands on their magnetic properties generally established, recent research has unearthed certain effects inherent to site differentiation due to the different types and varying numbers of substituents on the same ligand platform. Using two new sandwich-type Er(III) complexes with cyclooctatetraenyl (COT) ligands featuring two differently positioned trimethylsilyl (TMS) substituents, namely, [Li(DME)Er(COT1,5-TMS2)2]n (Er1) and [Na(DME)3][Er(COT1,3-TMS2)2] (Er2) [COT1,3-TMS2 and COT1,5-TMS2 donate 1,3- and 1,5-bis(trimethylsilyl)-substituted cyclooctatetraenyl ligands, respectively; DME = 1,2-dimethoxyethane], and with reference to previously reported [Li(DME)3][Er(COT1,4-TMS2)2] (A) and [K(DME)2][Er(COT1,4-TMS2)2] (B), any possible substituent position effects have been explored for the first time. The rearrangement of the TMS substituents from the starting COT1,4-TMS2 to COT1,3-TMS2 and COT1,5-TMS2, by way of formal migration of the TMS group, was thermally induced in the case of Er1, while for the formation of Er2, the use of Na+ in the placement of its Li+ and K+ congeners is essential. Both Er1 and Er2 display single-molecule magnetic behaviors with energy barriers of 170(3) and 172(6) K, respectively. Magnetic hysteresis loops, butterfly-shaped for Er1 and wide open for Er2, were observed up to 12 K for Er1 and 13 K for Er2. Studies of magnetic dynamics reveal the different pathways for relaxation of magnetization below 10 K, mainly by the Raman process for Er1 and by quantum tunneling of magnetization for Er2, leading to the order of magnitude difference in magnetic relaxation times and sharply different magnetic hysteresis loops.

Synthesis, Structural Characterization, and Magnetic Properties of Lanthanide Arsolyl Sandwich Complexes

A series of trivalent lanthanide sandwich complexes [(η5-C4R4As)Ln(η8-C8H8)] using three different arsolyl ligands are reported. The complexes were obtained via salt elimination reactions between potassium arsolyl salts and lanthanide precursors [LnI(COT)(THF)2] (Ln = Sm, Dy, Er; COT = η8-C8H8). The resulting compounds exhibit classical sandwich complex structures with one notable exception. Characterization was conducted in both the solid state using single-crystal X-ray diffraction and in solution for the Sm compounds using NMR spectroscopy. Furthermore, the magnetic properties of an Er complex were investigated, revealing distinctive single-molecule-magnet behavior characterized by an energy barrier of Ueff = 323.3 K. Theoretical calculations were employed to support and interpret the experimental findings, with a comparative analysis performed against previously reported complexes.

Cycloheptatrienyl-Bridged Triple-Decker Complexes

The first structurally characterized organometallic multidecker sandwich complexes featuring a cycloheptatrienyl ring (Cht, C7H73-) in the coordination sphere are presented. The synthesis of inverse sandwich complexes of the rare earth elements YIII and ErIII with a bridging cycloheptatrienyl ligand of the type [(thf)(BH4)2LnIII(μ-η7:η7-Cht)LnIII(BH4)(thf)2] is described first. The subsequent introduction of the CotTIPS ligand (CotTIPS = 1,4-(iPr3Si)2C8H62-) into the coordination sphere of the rare earth cations resulted in the isolation of unprecedented triple-decker compounds with the formula [(thf)3K{(η8-CotTIPS)LnIII}2(μ-η7:η7-Cht)], bearing a seven-membered aromatic carbon ring as a middle deck. These compounds are also the first examples of rare earth triple-decker complexes not bridged by a Cot derivative, based on purely carbon-based ligands. The magnetic properties of the respective ErIII congeners were investigated in detail, leading to the observation of antiferromagnetic coupling of the ErIII cations and a blocking temperature of 13.5 K. The conversion of the YIII compound [(thf)3K{(η8-CotTIPS)YIII}2(μ-η7:η7-Cht)] with [YIII(Cot)I(thf)2] resulted in ligand rearrangement and the selective formation of the first triple-decker complex ([(η8-CotTIPSYIII)2(μ-η8:η8-Cot)]) featuring two Cot ligands with different substituents in its coordination sphere.

Unique Double and Triple Decker Arrangements of Rare-Earth 9,10-Diborataanthracene Complexes Featuring Single-Molecule Magnet Characteristics

Herein, we present the first report on the synthesis of rare-earth complexes featuring a 9,10-diborataanthracene ligand. This 14-π-electron ligand is highly reductive and was previously used in small-molecule activation. Salt elimination reactions between dipotassium 9,10-diethyl-9,10-diborataanthracene [K2(DEDBA)] and [LnIII(η8-CotTIPS)(BH4)(thf)x] (CotTIPS=1,4-(iPr3Si)2C8H6) in a 1 : 1 ratio yielded heteroleptic sandwich complexes [K(η8-CotTIPS)LnIII(η6-DEDBA)] (Ln=Y, Dy, Er). These compounds form Lewis-base-free one-dimensional coordination polymers when crystallised from toluene. In contrast, reaction of [K2(DEDBA)] and [LnIII(η8-CotTIPS)(BH4)(thf)x] in a 1 : 2 ratio led to the formation of heteroleptic triple-decker complexes [(η8-CotTIPS)LnIII(μ-η6:η6-DEDBA)LnIII(η8-CotTIPS)] (Ln=Y, Dy, Er). Notably, these are not only the first lanthanide triple-decker compounds featuring a six-membered ring as a deck but also the first trivalent lanthanide triple-decker featuring a heterocycle in the coordination sphere. Magnetic investigations reveal that [K(η8-CotTIPS)LnIII(η6-DEDBA)] (Ln=Dy, Er) and [(η8-CotTIPS)ErIII(μ-η6:η6-DEDBA)ErIII(η8-CotTIPS)] exhibit Single-Molecule Magnet (SMM) behaviour. In the case of [(η8-CotTIPS)LnIII(μ-η6:η6-DEDBA)LnIII(η8-CotTIPS)] (Ln=Dy, Er), the introduction of a second near lanthanide ion results in strong antiferromagnetic interactions, allowing the enhancement of the magnetic characteristic of the system, compared to the quasi isolated counterpart. This research renews the overlooked coordination chemistry of the DBA ligand and expands it to encompass rare-earth elements.

Covalency versus magnetic axiality in Nd molecular magnets: Nd-photoluminescence, strong ligand-field, and unprecedented nephelauxetic effect in fullerenes NdM2N@C80 (M = Sc, Lu, Y)

Nd-based nitride clusterfullerenes NdM2N@C80 with rare-earth metals of different sizes (M = Sc, Y, Lu) were synthesized to elucidate the influence of the cluster composition, shape and internal strain on the structural and magnetic properties. Single crystal X-ray diffraction revealed a very short Nd-N bond length in NdSc2N@C80. For Lu and Y analogs, the further shortening of the Nd-N bond and pyramidalization of the NdM2N cluster are predicted by DFT calculations as a result of the increased cluster size and a strain caused by the limited size of the fullerene cage. The short distance between Nd and nitride ions leads to a very large ligand-field splitting of Nd3+ of 1100-1200 cm-1, while the variation of the NdM2N cluster composition and concomitant internal strain results in the noticeable modulation of the splitting, which could be directly assessed from the well-resolved fine structure in the Nd-based photoluminescence spectra of NdM2N@C80 clusterfullerenes. Photoluminescence measurements also revealed an unprecedentedly strong nephelauxetic effect, pointing to a high degree of covalency. The latter appears detrimental to the magnetic axiality despite the strong ligand field. As a result, the ground magnetic state has considerable transversal components of the pseudospin g-tensor, and the slow magnetic relaxation of NdSc2N@C80 could be observed by AC magnetometry only in the presence of a magnetic field. A combination of the well-resolved magneto-optical states and slow relaxation of magnetization suggests that Nd clusterfullerenes can be useful building blocks for magneto-photonic quantum technologies.

It's not just the size that matters: crystal engineering of lanthanide-based coordination polymers

Synthesis and characterization of Lewis base free coordination polymers of selected lanthanides are presented. For this purpose, the substituted CotTIPS ligand (CotTIPS = 1,4-bis-triisopropylsilyl-cyclo-octatetraendiide) was used to synthesize homoleptic, anionic multidecker compounds of the type [K{LnIII(ɳ8-CotTIPS)2}]n. Depending on the solvent used for crystallization and the ionic radii of the lanthanide cations, three different categories of one-dimensional heterobimetallic coordination polymers were obtained in the solid state. For the early lanthanides La and Ce a unique helical conformation was obtained by crystallization from toluene, while the ionic radius of Pr seems to be a turning point towards the crystallization of zigzag polymers. For Er a third structural motif, a trapezoidal wave polymer was observed. Additionally, the zigzag polymer for all compounds could be obtained by changing the solvent from toluene to Et2O, reavealing a correlation between solid-state structure and ionic radii as well as solvent. While photoluminescence (PL) properties of Cot-lanthanide compounds are scarce, the La complexes show ligand centered green luminescence, whereas the Ce complexes reveal deep red emission origin from d-f transitions. The Er-compounds are single-molecule magnets, in which the magnetic relaxation of each Er ion occurs isolated from its neighbors at temperatures above 10 K, while below 9 K a strong antiferromagnetic coupling between the Er ions was seen.

Understanding electrostatics and covalency effects in highly anisotropic organometallic sandwich dysprosium complexes [Dy(CmRm)2] (where R = H, SiH3, CH3 and m = 4 to 9): a computational perspective

In this article, we have thoroughly studied the electronic structure and 4f-ligand covalency of six mononuclear dysprosium organometallic sandwich complexes [Dy(CmRm)2]n+/- (where R = H, SiH3, CH3; m = 4 to 9; n = 1, 3) using both the scalar relativistic density functional and complete active space self-consistent field (CASSCF) and N-electron valence perturbation theory (NEVPT2) method to shed light on the ligand field effects in fine-tuning the magnetic anisotropy of these complexes. Energy decomposition analysis (EDA) and ab initio-based ligand field theory AILFT calculations predict the sizable 4f-ligand covalency in all these complexes. The analysis of CASSCF/NEVPT2 computed spin-Hamiltonian (SH) parameters indicates the stabilization of mJ |±15/2〉 for [Dy(C4(SiH3)4)2]- (1), [Dy(C5(CH3)5)2]+ (2) and [Dy(C6H6)2]3+ (3) complexes with the Ucal value of 1867.5, 1621.5 and 1070.8 cm-1, respectively. On the other hand, we observed mJ |±9/2〉 as the ground state for [Dy(C7H7)2]3- (4) and [Dy(C8H8)2]- (5) complexes with significantly smaller Ucal values of 237.1 and 38.6 cm-1 respectively. For the nine-membered ring [Dy(C9H9)2]+ (6) complex, we observed the stabilization of the mJ |±1/2〉 ground state, with the first excited state being located ∼29 cm-1 higher in energy. AILFT-NEVPT2 ligand field splitting analysis indicates that the presence of π-type 4f-ligand interactions in complexes 1-3 help generate the axial-ligand field, while the δ-type interactions in complexes 4-5 generate the equatorial ligand field despite the ligands approaching from the axial direction. As the ring size increases, φ-type interactions dominate, generating a pure equatorial ligand field stabilising mJ |±1/2〉 as the ground state for 6. Calculations suggest that the nature of the ligand field mainly governs the Ucal values in the following order: 4f-Lσ > 4f-Lπ > 4f-Lδ > 4f-Lφ. Calculations were performed by replacing ligands with CHELPG charges to access the crystal field (CF) effects which suggests the stabilization of pure mJ |±15/2〉 in all the charge-embedded models (1Q-6Q). Our findings point out that the crystal field and ligand field effects complement each other and generate a giant barrier for magnetic relaxation in the small ring complexes 1-3, while a relatively weak crystal field and adverse 4f-Lδ/4f-Lφ interactions diminish the SMM behaviour in the large ring complexes 4-6.

Theoretical Magnetic Relaxation and Spin-Phonon Coupling Study in a Series of Molecular Engineering Designed Bridged Dysprosocenium Analogues

A detailed computational study of hypothetical sandwich dysprosium double-decker complexes, bridged by various numbers of aliphatic linkers, was performed to evaluate the effect of the structural modifications on their ground-state magnetic sublevels and assess their potential as candidates for single-molecule magnets (SMMs). The molecular structures of seven complexes were optimized using the TPSSh functional, and the electronic structure and magnetic properties were investigated using the complete active space self-consistent field method (CASSCF). Estimates of the magnetic moment blocking barrier (Ueff) and blocking temperatures (TB) are reported. In addition, a new method based on computed derivatives of effective demagnetization barriers Ueff with respect to vibrational normal modes was introduced and applied to evaluate the impact of spin-phonon coupling on the SMM properties. On the basis of the computed parameters, we have identified promising candidates with properties superior to those of the existing single-molecule magnets.

Dynamic Magnetic Properties of Germole-ligated Lanthanide Sandwich Complexes

The first germole-ligated single-molecule magnets are reported, with contrasting properties found for the near-linear sandwich complexes [(η8 -COT)Ln(η5 -CpGe ]- , where Ln=Dy (1Dy ) or Er (1Er ), COT is cyclo-octatetraenyl and CpGe is [GeC4 -2,5-(SiMe3 )2 -3,4-Me2 ]2- . Whereas 1Er has an energy barrier of 120(1) cm-1 in zero applied field and open hysteresis loops up to 10 K, the relaxation in 1Dy is characterized by quantum tunneling within the ground state.

Structural and Magnetization Dynamics of Borohydride-Bridged Rare-Earth Metallocenium Cations

The structure and magnetic properties of the bimetallic borohydride-bridged dysprosocenium compound [{(η⁵-Cp^ttt)(η⁵-Cp^Me4t)Dy}₂(μ:κ²:κ²-BH₄)]⁺[B(C₆F₅)₄]^- ([3_Dy][B(C₆F₅)₄]) are reported along with the solution-phase dynamics of the isostructural yttrium and lutetium analogues (Cp^ttt is 1,2,4-tri(tert-butyl)cyclopentadienyl, Cp^Me4t is tetramethyl(tert-butyl)cyclopentadienyl). The synthesis of [3_M][B(C₆F₅)₄] was accomplished in the 2:1 stoichiometric reactions of [(η⁵-Cp^ttt)(η⁵-Cp^Me4t)Dy(BH₄)] (2_M) with [CPh₃][B(C₆F₅)₄], with the metallocenes 2_M obtained from reactions of the half-sandwich complexes [(η⁵-Cp^ttt)M(BH₄)₂(THF)] (1_M) (M = Y, Dy, Lu) with NaCp^Me4t. Crystallographic studies show significant lengthening of the M···B distance on moving through the series 1_M, 2_M, and 3_M, with essentially linear {M···B···M} bridges in 3_M. Multinuclear NMR spectroscopy indicates restricted rotation of the Cp^ttt ligands in 3_Y and 3_Lu in solution. The single-molecule magnet (SMM) properties of [3_M][B(C₆F₅)₄] are characterized by Raman and Orbach processes, with an effective barrier of 533(18) cm^-1 and relaxation via the second-excited Kramers doublet. Although quantum tunneling of the magnetization (QTM) was not observed for [3_M][B(C₆F₅)₄], it was, surprisingly, found in its magnetically dilute version, which has a very similar barrier of U_eff = 499(21) cm^-1. Consistent with this observation, slightly wider openings of the magnetic hysteresis loop at 2 K are found for [3_M][B(C₆F₅)₄] but not for the diluted analogue. The dynamic magnetic properties of the dysprosium SMMs and the role of exchange interactions in 3_Dy are interpreted with the aid of multireference ab initio calculations.

Structure and bonding patterns in heterometallic organometallics with linear Ln-Pd-Ln motifs

Complexes with short intermetallic distances between transition metal fragments and lanthanide (Ln) fragments are fascinating objects of study, owing to the ambiguity of the nature of the interaction. The addition of the divalent lanthanide fragments Cp*₂Ln(OEt₂) (Ln = Sm or Yb) to a Pd(ii) complex bearing the deprotonated form of the redox-active, non-symmetrical ligand, 2-pyrimidin-2-yl-1H-benzimidazole (Hbimpm), leads to two isostructural complexes, of the general formula (Cp*₂Ln)₂[μ-Pd(pyridyl)₂] (Ln = Sm (4) and Yb (5)). These adducts have interesting features, such as unique linear Ln-Pd-Ln arrangements and short Ln-Pd distances, which deviate from the expected lanthanide contraction. A mixed computational and spectroscopic study into the formation of these adducts gathers important clues as to their formation. At the same time, thorough characterization of these complexes establishes the +3 oxidation state of all the involved Ln centers. Detailed theoretical computations demonstrate that the apparent deviation from lanthanide contraction is not due to any difference in the intermetallic interaction between the Pd and the Ln, but that the fragments are joined together by electrostatic interactions and dispersive forces. This conclusion contrasts with the findings about a third complex, Cp*₂Yb(μ-Me)₂PdCp* (6), formed during the reaction, which also possesses a short Yb-Pd distance. Studies at the CASSCF level of theory on this complex show several orbitals containing significant interactions between the 4f and 4d manifolds of the metals. This demonstrates the need for methodical and careful analyses in gauging the intermetallic interaction and the inadequacy of empirical metrics in describing such phenomena.

Back to the future of organolanthanide chemistry

At the dawn of the development of structural organometallic chemistry, soon after the discovery of ferrocene, the description of the LnCp₃ complexes, featuring large and mostly trivalent lanthanide ions, was rather original and sparked curiosity. Yet, the interest in these new architectures rapidly dwindled due to the electrostatic nature of the bonding between π-aromatic ligands and 4f-elements. Almost 70 years later, it is interesting to focus on how the discipline has evolved in various directions with the reports of multiple catalytic reactivities, remarkable potential in small molecule activation, and the development of rich redox chemistry. Aside from chemical reactivity, a better understanding of their singular electronic nature - not precisely as simplistic as anticipated - has been crucial for developing tailored compounds with adapted magnetic anisotropy or high fluorescence properties that have witnessed significant popularity in recent years. Future developments shall greatly benefit from the detailed reactivity, structural and physical chemistry studies, particularly in photochemistry, electro- or photoelectrocatalysis of inert small molecules, and manipulating the spins' coherence in quantum technology.

Molecular Lanthanide Switches for Magnetism and Photoluminescence

Solvation of [(CNT)Ln(η⁸ -COT)] (Ln=La, Ce, Nd, Tb, Er; CNT=cyclononatetraenyl, i.e., C₉ H₉ ^- ; COT=cyclooctatetraendiid, i.e., C₈ H₈ ^2- ) complexes with tetrahydrofuran (THF) gives rise to neutral [(η⁴ -CNT)Ln(thf)₂ (η⁸ -COT)] (Ln=La, Ce) and ionic [Ln(thf)_x (η⁸ -COT)][CNT] (x=4 (Ce, Nd, Tb), 3 (Er)) species in a solid-to-solid transformation. Due to the severe distortion of the ligand sphere upon solvation, these species act as switchable luminophores and single-molecule magnets. The desolvation of the coordinated solvents can be triggered by applying a dynamic vacuum, as well as a temperature gradient stimulus. Raman spectroscopic investigations revealed fast and fully reversible solvation and desolvation processes. Moreover, we also show that a Nd:YAG laser can induce the necessary temperature gradient for a self-sufficient switching process of the Ce(III) analogue in a spatially resolved manner.

Thorium- and Uranium-Mediated C-H Activation of a Silyl-Substituted Cyclobutadienyl Ligand

Cyclobutadienyl complexes of the f-elements are a relatively new yet poorly understood class of sandwich and half-sandwich organometallic compounds. We now describe cyclobutadienyl transfer reactions of the magnesium reagent [(η⁴-Cb'''')Mg(THF)₃] (1), where Cb'''' is tetrakis(trimethylsilyl)cyclobutadienyl, toward thorium(IV) and uranium(IV) tetrachlorides. The 1:1 stoichiometric reactions between 1 and AnCl₄ proceed with intact transfer of Cb'''' to give the half-sandwich complexes [(η⁴-Cb'''')AnCl(μ-Cl)₃Mg(THF)₃] (An = Th, 2; An = U, 3). Using a 2:1 reaction stoichiometry produces [Mg₂Cl₃(THF)₆][(η⁴-Cb'''')An(η³-C₄H(SiMe₃)₃-κ-(CH₂SiMe₂)(Cl)] (An = Th, [Mg₂Cl₃(THF)₆][4]; An = U [Mg₂Cl₃(THF)₆][5]), in which one Cb'''' ligand has undergone cyclometalation of a trimethylsilyl group, resulting in the formation of an An-C σ-bond, protonation of the four-membered ring, and an η³-allylic interaction with the actinide. Complex solution-phase dynamics are observed with multinuclear nuclear magnetic resonance spectroscopy for both sandwich complexes. A computational analysis of the reaction mechanism leading to the formation of 4 and 5 indicates that the cyclobutadienyl ligands undergo C-H activation across the actinide center.

Triangulo-{ErIII 3} complex showing field supported slow magnetic relaxation

The triangulo-{Er3} complex [Er3Cl(o-van)3(OH)2(H2O)5]Cl3·nH2O (n = 9.4; H(o-van) = o-vanillin) (1) was generated by an in situ method. The isolated Er(iii) complex 1 was characterized by elemental analysis and molecular spectroscopy. The results of single crystal X-ray diffraction studies have shown that 1 is built up of trinuclear [Er3Cl(o-van)3(OH)2(H2O)5]3+ complex cations, chloride anions and water solvate molecules. Within the complex cation the three Er(iii) central atoms are placed at the apexes of a triangle which are bridged by three (o-van)- ligands with additional chelating functions and two μ3-OH- ligands. Additionally five aqua and one chlorido ligands complete the octa-coordination of the three Er(iii) atoms. AC susceptibility measurements reveal that the compound exhibits slow magnetic relaxation with two relaxation modes.

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.

Silole and germole complexes of lanthanum and cerium

Using dianionic metallole ligands (silole or germole) and the cyclooctatetraendiide dianion, heteroleptic lanthanide multi-decker complexes have been prepared. Due to the heteroatom of the metallole ligands intermolecular bridging between the sandwich complexes takes place. Our work highlights that different combinations of the lanthanide and heterocycle lead to different intermolecular interactions including a dimeric La-silole sandwich complex, a La-germole ladder-type polymeric species and a Ce-germole coordination polymer.

Development of Nd (III)-Based Terahertz Absorbers Revealing Temperature Dependent Near-Infrared Luminescence

Molecular vibrations in the solid-state, detectable in the terahertz (THz) region, are the subject of research to further develop THz technologies. To observe such vibrations in terahertz time-domain spectroscopy (THz-TDS) and low-frequency (LF) Raman spectroscopy, two supramolecular assemblies with the formula [Nd^III (phen)₃ (NCX)₃] 0.3EtOH (X = S, 1-S; Se, 1-Se) were designed and prepared. Both compounds show several THz-TDS and LF-Raman peaks in the sub-THz range, with the lowest frequencies of 0.65 and 0.59 THz for 1-S and 1-Se, and 0.75 and 0.61 THz for 1-S and 1-Se, respectively. The peak redshift was observed due to the substitution of SCN⁻ by SeCN⁻. Additionally, temperature-dependent TDS-THz studies showed a thermal blueshift phenomenon, as the peak position shifted to 0.68 THz for 1-S and 0.62 THz for 1-Se at 10 K. Based on ab initio calculations, sub-THz vibrations were ascribed to the swaying of the three thiocyanate/selenocyanate. Moreover, both samples exhibited near-infrared (NIR) emission from Nd (III), and very good thermometric properties in the 300–150 K range, comparable to neodymium (III) oxide-based thermometers and higher than previously reported complexes. Moreover, the temperature dependence of fluorescence and THz spectroscopy analysis showed that the reduction in anharmonic thermal vibrations leads to a significant increase in the intensity and a reduction in the width of the emission and LF absorption peaks. These studies provide the basis for developing new routes to adjust the LF vibrational absorption.

An intermetallic molecular nanomagnet with the lanthanide coordinated only by transition metals

Magnetic molecules known as molecular nanomagnets (MNMs) may be the key to ultra-high density data storage. Thus, novel strategies on how to design MNMs are desirable. Here, inspired by the hexagonal structure of the hardest intermetallic magnet SmCo₅, we have synthesized a nanomagnetic molecule where the central lanthanide (Ln) Er^III is coordinated solely by three transition metal ions (TM) in a perfectly trigonal planar fashion. This intermetallic molecule [Er^III(Re^ICp₂)₃] (ErRe₃) starts a family of molecular nanomagnets (MNM) with unsupported Ln-TM bonds and paves the way towards molecular intermetallics with strong direct magnetic exchange interactions-a promising route towards high-performance 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.

Introduction of plumbole to f-element chemistry

Herein, we present the synthesis and characterization of heteroleptic lanthanide complexes bearing a dianionic η5-plumbole ligand in their coordination sphere. The reaction proceeds via a salt elimination reaction between the dilithioplumbole ([Li(thf)]2[1,4-bis-tert-butyl-dimethylsilyl-2,3-bis-phenyl-plumbolyl] = [Li2(thf)2(η5-LPb)]) and specifically designed [Ln(η8-COTTIPS)BH4] precursors (Ln = lanthanide, La, Ce, Sm, Er; COTTIPS = 1,4-bis-triisopropylsilyl-cyclooctatetraenyl), that are capable of stabilizing a planar plumbole moiety in the coordination sphere of different trivalent lanthanide ions. In-depth ab initio calculations show that the aromaticity of the dianionic plumbole is retained upon coordination. Electron delocalization occurs from the plumbole HOMO to an orbital of mainly d-character at the lanthanide ion. The magnetic properties of the erbium congener were investigated in detail, leading to the observation of magnetic hysteresis up to 5 K (200 Oe s-1), an unequivocal proof for single molecule magnet behavior in this system. The magnetic behavior of the erbium species can be modulated by manipulating the position of the lithium cation in the complex, which directly influences the bonding metrics in the central [(η5-LPb)Er(η8-COTTIPS)]- fragment. This allowed us to assess a fundamental magneto-structural correlation in an otherwise identical inner coordination sphere.

A Nd6 molecular butterfly: a unique all-in-one material for SMM, MCE and maiden photosensitized opto-electronic device fabrication

Besides iron, ironically neodymium (Nd) is the most ubiquitously used metal for magnetic purposes, even among the lanthanides, when it comes to the field of molecular magnetism, yet it ranks among the least studied metals. However, strong apathy towards this magnetic lanthanide means that vital information will be missed, which is required for the advancement of the subject. Herein, we have successfully demonstrated the usefulness of a hexanuclear neodymium complex as a magnetic material, and also in electronic device fabrication. A {NdIII6} cage with an aesthetically pleasing butterfly topology was synthesized using a rather non-conventional N-rich pyridyl-pyrazolyl based ligand. The cage shows single molecule magnet (SMM) properties, with an effective energy barrier, U_eff, value of 3.4 K and relaxation time, τ₀, of 3.1 × 10^-4 s, originating from an unusual occurrence of metal centres with different coordination environments. Furthermore, magnetic studies reveal significant cyrogenic magnetic cooling, with a magnetic entropy change of 8.28 J kg^-1 K^-1 at 5 T and 3 K. To the best of our knowledge, the titular compound is the only example of a Nd-complex that exhibits concomitant magnetocaloric effect (MCE) and SMM properties. Complete active space self-consistent field (CASSCF) calculations were carried out to shed light on the origin of the magnetic anisotropy and magnetic relaxation of the compound. The same uniqueness is also true for the first electronic investigation carried out on the Nd complex. The maiden electronic device fabricated using the Nd complex shows an interesting intertwining of electronic and optical features, which contribute towards its improved photosensitized optoelectronic data.

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).

Cyclooctatetraenide-based single-ion magnets featuring bulky cyclopentadienyl ligand

We report a family of organometallic rare-earth complexes with the general formula (COT)M(Cp^ttt) (where (COT)²⁻ = cyclooctatetraenide, (Cp^ttt)⁻ = 1,2,4-tri(tert-butyl)cyclopentadienide, M = Y(iii), Nd(iii), Dy(iii) and Er(iii)). Similarly to the prototypical Er(iii) analog featuring pentamethylcyclopentadienyl ligand (Cp*)⁻, (COT)Er(Cp^ttt) behaves as a single-ion magnet. However, the introduction of the sterically demanding (Cp^ttt)⁻ imposes geometric constraints that lead to a simplified magnetic relaxation behavior compared to the (Cp*)⁻ containing complexes. Consequently, (COT)Er(Cp^ttt) can be viewed as a model representative of this organometallic single-ion magnet architecture. In addition, we demonstrate that the increased steric profile associated with the (Cp^ttt)⁻ ligand permits preparation, structural characterization and interrogation of magnetic properties of the early-lanthanide complex, (COT)Nd(Cp^ttt). Such a mononuclear derivative could not be obtained when a (Cp*)⁻ ligand was employed, a testament to larger ionic radius of this early lanthanide ion.

Application of steric control principles allows for simplification of the magnetic behavior of an iconic single-ion magnet architecture as well as the preparation of its previously inaccessible representative.

Metal–metal bond in lanthanide single-molecule magnets

This review surveys recent critical advances in lanthanide SMMs, highlighting the influences of metal–metal bonds on the magnetization dynamics.

Understanding the Magnetic Anisotropy for Linear Sandwich [Er(COT)]+-based Compounds: A Theoretical Investigation

A series of linear sandwich single-ion magnets containing [Er(COT)]+ fragment were selected to probe the magneto-structural correlations using ab initio methods. For prolate shaped ErIII ion, an equatorially coordinating geometry...

The Archetypal Homoleptic Lanthanide Quadruple-Decker-Synthesis, Mechanistic Studies, and Quantum Chemical Investigations

Reduction of [SmIII (COT1,4-SiiPr3 )(BH4 )(thf)] (COT1,4-SiiPr3 =1,4-(i Pr3 Si)3 C8 H6 ) with KC8 resulted in [SmIII/II/III (COT1,4-SiiPr3 )4 ], the first example of a homoleptic lanthanide quadruple-decker. As indicated by an analysis of the bond metrics in the solid-state, the inner Sm ion is present in the divalent oxidation state, while the outer ones are trivalent. This observation could be confirmed by quantum chemical calculations. Mechanistic studies revealed not only insight into possible formation pathways of [SmIII/II/III (COT1,4-SiiPr3 )4 ] but also resulted in the transformation to other mixed metal sandwich complexes with unique structural properties. These are the 1D-polymeric chain structured [KSmIII (COT1,4-SiiPr3 )]n and the hexametallic species [(tol)K(COT1,4-SiiPr3 )SmII (COT1,4-SiiPr3 )K]2 which were initially envisioned as possible building blocks as part of different retrosynthetically guided pathways that we developed.

A simple one-pot route to stable formamidinatoiodidolanthanoid(III) complexes from lanthanoid metals

A number of formamidinatoiodidolanthanoid(III) complexes, [Ln(DFForm)₂I(thf)₂] and [Ln(DFForm)I₂(thf)₃] (DFFormH = N,N'-bis(2,6-difluorophenyl)formamidine) and [Ln(DippForm)I₂(thf)₃] (DippFormH = N,N'-bis(2,6-diisopropylphenyl)formamidine) have been synthesized in good yields by one-pot direct reactions of the corresponding free metals with iodine and DFFormH or DippFormH in suitable ratios and are stable to rearrangement.

Size-Controlled Hapticity Switching in [Ln(C9 H9 )(C8 H8 )] Sandwiches

Sandwich complexes of lanthanides have recently attracted a considerable amount of interest due to their applications as Single Molecule Magnet (SMM). Herein, a comprehensive series of heteroleptic lanthanide sandwich complexes ligated by the cyclononatetraenyl (Cnt) and the cyclooctatetraenyl (Cot) ligand [Ln(Cot)(Cnt)] (Ln=Tb, Dy, Er, Ho, Yb, and Lu) is reported. The coordination behavior of the Cnt ligand has been investigated along the series and shows different coordination patterns in the solid-state depending on the size of the corresponding lanthanide ion without altering its overall anisotropy. Besides the characterization in the solid state by single-crystal X-ray diffraction and in solution by 1 H NMR, static magnetic studies and ab initio computational studies were performed.

Slow magnetic relaxation and luminescence properties in neodymium(iii)-4,4,4-trifluoro-1-(2-naphthyl)butane-1,3-dionato complexes incorporating bipyridyl ligands

A new series of eight-coordinated Nd(iii) complexes derived from 4,4,4-trifluoro-1-(naphthalen-2-yl)butane-1,3-dionate and bi-pyridyl ligands revealed multifunctional molecular materials as photoluminescent single-molecule magnets (SMMs).

Pseudo-tetrahedral vs. pseudo-octahedral ErIII single molecule magnets and the disruptive role of coordinated TEMPO radical

Tetrahedral Er^III complexes are potential candidates for high-performance single molecule magnets (SMMs).