Unravelling the Mechanism of CO2 Activation: Insights Into Metal-Metal Cooperativity and Spin-Orbit Coupling with {3d-4f} Catalysts

Converting CO2 into useful chemicals using metal catalysts is a significant challenge in chemistry. Among the various catalysts reported, transition metal lanthanide hybrid {3d-4f} complexes stand out for their superior efficiency and site selectivity. However, unlike transition metal catalysts, understanding the origin of this efficiency in lanthanides poses a challenge due to their orbital degeneracy, rendering the application of DFT methods ineffective. In this study, we employed a combination of density functional theory (DFT) and ab initio CASSCF/RASSI-SO calculations to explore the mechanism of CO2 conversion to cyclic carbonate using a 3d-4f heterometallic catalyst for the first time. This work unveils the importance of 3d and 4f metal cooperativity and the role of individual spin-orbit states in dictating the overall efficiency of the catalyst.

Hourglass-Shaped Homo- and Heteronuclear Nonanuclear Lanthanide Clusters: Structures, Magnetism, Photoluminescence, and Theoretical Analysis

Synthesis of nonameric cationic clusters [Dy9(acac)16(μ3-OH)8(μ4-OH)2]OH·6H2O (1), [Dy8Tb (acac)16(μ3-OH)8(μ4-OH)2]OH·2H2O (2), and [Gd9(acac)16(μ3-OH)8(μ4-OH)2]OH·6H2O (3) (acac = acetylacetonate) is reported. The emission spectrum of 1 shows Dy(III) ion characteristic bands assignable to the 4F9/2 → 6HJ (J = 15/2 to 9/2) transitions. Emission due to both Dy(III) and Tb(III) ions is observed for 2 in the visible range, with Tb(III) specific bands appearing due to the 5D4 → 7FJ (J = 6, 4, and 3) transitions. Cluster 3 exhibits a significant magnetocaloric effect (MCE), with -ΔSm values increasing with decrease in temperature and increase in field, reaching -ΔSmmax = 20.98 J kg-1 K-1 at 2 K and 9 T. Isotropic magnetic coupling constants (Js) in 3 derived from density functional theory (DFT) calculations reveal that the exchange interactions are antiferromagnetic and weak. Compound 3 possesses S = 7/2 ground state arising from the central Gd(III) ion along with several nested excited states due to competing antiferromagnetic interactions that yield reasonably large MCE values. Utilizing computed exchange coupling interactions, we have performed ab initio CASSCF/RASSI-SO/POL_ANISO calculations on antiferromagnetic 1 and 2 to estimate the exchange interactions using the Lines model. For 2, Dy(III)···Tb(III) exchange interactions were extracted for the first time and were found to be weakly antiferromagnetically coupled.

Self-assembly of octanuclear Ln(III)-based clusters: their large magnetocaloric effects and highly efficient conversion of CO2

The design and construction of high-nuclear lanthanide clusters with fascinating topology and functional properties have been an active area of research, however, the development of an effective approach for obtaining high-nuclear lanthanide clusters with multifunctional properties is still extremely difficult. Up to now, a systematic approach for guiding the further expansion of Ln(III)-based clusters showing good functional properties is lacking. Herein, we design and synthesize a polydentate Schiff base ligand (HL), which reacts with β-diketonate salts Ln(acac)₃·2H₂O, and a series of Ln₈ clusters [Ln₈(acac)₆(L)₂(μ₃-O)₆(μ₂-C₂H₅O)₄(μ₂-Hacac)₂]·2CH₃CN (Ln(III) = Gd (1), Dy (2), and Ho (3); HL = pyridine-2-carboxylic acid (5-hydroxymethyl-furan-2-ylmethylene)-hydrazide, Hacac = acetylacetone) have been successfully synthesized. Single-crystal X-ray diffraction studies reveal that clusters 1-3 are isostructural and can be viewed as a Ln₈ core bridged by eighteen μ₂-O atoms, six μ₃-O atoms and two μ₄-O atoms. Magnetic studies show that cluster 1-Gd8 displays a large magnetocaloric effect with -ΔS_m = 46.14 J kg^-1 K^-1 (T = 2.0 K and ΔH = 7.0 T); cluster 2-Dy8 exhibits single-molecule magnet behavior under zero-field conditions. It is worth mentioning that the -ΔS_m of cluster 1-Gd8 is larger than that of most reported polynuclear Gd(III)-based clusters; the 2-Dy8 cluster is one of the rare polynuclear Ln_n SMMs (n ≥ 8) under zero dc field. Importantly, these Ln(III)-based clusters (1-3) can catalyze the cycloaddition of CO₂ with epoxides with high efficiency under mild conditions; and cluster 1-Gd8 as a catalyst could be reused at least three times without obvious loss of catalytic performance.

Molecular assemblies from linear-shaped Ln4 clusters to Ln8 clusters using different β-diketonates: disparate magnetocaloric effects and single-molecule magnet behaviours

A series of tetranuclear lanthanide-based clusters [Ln₄(dbm)₆(L)₂(CH₃OH)₄]·2CH₃OH (Ln(III) = Gd (1), Dy (2), and Ho (3); H₃L = 2-[(2-(hydroxyimino)propanehydrazide)methyl]-2,3-dihydroxybenzaldehyde, Hdbm = dibenzoylmethane) and octanuclear lanthanide-based clusters [Ln₈(HL)₁₀(CH₃O)₄(CH₃OH)₂]·6CH₃OH (Ln(III) = Gd (4), Dy (5)) were assembled using a polydentate Schiff-base ligand H₃L and two different β-diketone salts via a solvothermal method, and their structures and magnetic properties have been characterized. Interestingly, β-diketones play an important role in assembling and affecting the structures of Ln₄ to Ln₈ clusters. This is the first use of β-diketone to affect the structures of polynuclear Ln(III)-based clusters from linear-shaped Ln₄ clusters to Ln₈ clusters. Magnetic studies revealed that antiferromagnetic interactions exist in clusters 1-Gd4 and 4-Gd8. More importantly, clusters 1-Gd4 and 4-Gd8 display significant cryogenic magnetic refrigeration properties (-ΔS_m = 24.88 J kg^-1 K^-1 for 1-Gd4 and -ΔS_m = 32.52 J kg^-1 K^-1 for 4-Gd8); the results show that cluster 4-Gd8 exhibits a larger magnetocaloric effect than 1-Gd4. Cluster 2-Dy4 shows remarkable single-molecule magnet (SMM) behavior (ΔE/k_B = 67.5 K and τ₀ = 3.06 × 10^-7 s) under a zero dc field, and 5-Dy8 exhibits a field-induced SMM-like behavior (ΔE/k_B = 39.83 K and τ₀ = 2.12 × 10^-7 s) under a 5000 Oe dc field.

Ring-forming transformation associated with hydrazone changes of hexadecanuclear dysprosium phosphonates

{Dy16(μ6-C10H7PO3)2(μ5-C10H7PO3)8(spch)8(μ3-OH)2(μ2-OH)2(μ2-AcO)6(μ3-COO)2(DMF)2(H2O)6}·0.5CH3OH·4.5H2O (1) and {Dy16(μ5-C10H7PO3)4(μ3-C10H7PO3)12(μ2-C10H7PO3H)8(opch)4(DMF)8(MeOH)4}·2.5CH3OH·3H2O (2), where H2spch is ((E)-N'-(2-hydroxybenzylidene)pyrazine-2-carbohydrazide, C10H7PO3H2 is 1-naphthylphosphonic acid and H2opch is (E)-N'-(2-hydroxy-3-methoxybenzylidene)pyrazine-2-carbohydrazide, were successfully synthesized by varying the hydrazone ligands in the Dy-phosphonate system. It is important that the ellipsoidal core experiences a ring forming structural transformation to the supramolecular square motif upon the incorporation of an ortho-methoxy substituent into the hydrazone. Alternating-current (ac) magnetic susceptibility studies of 1 and 2 suggest that similar single molecule magnet behaviors occur for these two complexes. The result represents an effective molecular assembly tactic to develop highly complicated lanthanide coordination clusters through the multicomponent self-assembly of the coalescence of phosphonate- and hydrazone-based ligands and metal salts.

Regulating the solution structural integrity and slow magnetic relaxation behavior of two Dy6 clusters with a pyridine–triazole ligand

Two {Dy₆} skeletons consolidated by nitrate (1) and hydroxide (2) have been isolated. HRESI-MS reveals that 1 retains a higher structural integrity and fragment stability. While 2 has a typical slow magnetic relaxation of SMM behavior.

Metal-Organic Frameworks Based on Group 3 and 4 Metals

Metal-organic frameworks (MOFs) based on group 3 and 4 metals are considered as the most promising MOFs for varying practical applications including water adsorption, carbon conversion, and biomedical applications. The relatively strong coordination bonds and versatile coordination modes within these MOFs endow the framework with high chemical stability, diverse structures and topologies, and interesting properties and functions. Herein, the significant progress made on this series of MOFs since 2018 is summarized and an update on the current status and future trends on the structural design of robust MOFs with high connectivity is provided. Cluster chemistry involving Y, lanthanides (Ln, from La to Lu), actinides (An, from Ac to Lr), Ti, and Zr is initially introduced. This is followed by a review of recently developed MOFs based on group 3 and 4 metals with their structures discussed based on the types of inorganic or organic building blocks. The novel properties and arising applications of these MOFs in catalysis, adsorption and separation, delivery, and sensing are highlighted. Overall, this review is expected to provide a timely summary on MOFs based on group 3 and 4 metals, which shall guide the future discovery and development of stable and functional MOFs for practical applications.

Monometallic lanthanide salicylhydrazone complexes exhibiting strong near-infrared luminescence

We report the synthesis of monometallic lanthanide complexes supported by one salicylhydrazone ligand. Complexes of neodymium, erbium, and ytterbium have been synthesized, and exhibit near infrared luminescence with modest to high quantum yields. The luminescence of ytterbium is increased by a factor of five when comparing our complex to other salicylhydrazone-supported complexes.

Phosphonate-assisted tetranuclear lanthanide assemblies: observation of the toroidic ground state in the TbIII analogue

The reaction of LnCl3·6H2O with a multidentate flexible Schiff base ligand (LH4), H2O3PtBu and trifluoroacetic acid (tfaH) afforded a series of homometallic tetranuclear complexes, [Ln4(LH2)2(O3PtBu)2(μ2-η1η1tfa)2][2Cl] (Ln = DyIII (1), TbIII (2) and GdIII (3)). The tetranuclear lanthanide core contains two structurally different lanthanide centres, one being in a distorted trigonal dodecahedron geometry and the other in a distorted trigonal prism. Complexes 1-3 were investigated via direct and alternating current (DC and AC) magnetic susceptibility measurements. Only 1 revealed a weak single-molecule magnet (SMM) behaviour. Alternating current (ac) magnetic susceptibility measurements on 1 reveal a frequency-dependent out-of-phase signal. However, the absence of distinct maxima in the χ'' peak (within the temperature/frequency range of our experiments) prevented deduction of the experimental energy barrier for magnetization reversal (Ueff) and the relaxation time. We have carried out extensive ab initio (CASSCF + RASSI-SO + SINGLE_ANISO + POLY_ANISO) calculations on complexes 1-2 to gain deeper insights into the nature of magnetic anisotropy. Our calculations yielded only one exchange coupling parameter between the two LnIII centres bridged by the ligand (neglecting the exchange between the LnIII centres that are not proximal wrt each other). All the extracted J values indicate a weakly antiferromagnetic coupling between the metal centres (J = -0.025 cm-1 for 1 and J = -0.015 cm-1 for 2). Calculated exchange coupled Ucal values of ∼5 and ∼1 cm-1 in 1 and 2 respectively nicely corroborated the experimental observations regarding weak and no SMM characteristics. Our calculations indicated the presence of a net single-molecule toroidal (SMT) behaviour in complex 2. On the other hand, fitting the magnetic data (susceptibility and magnetization) in the isotropic cluster 3 revealed weak AFM exchange couplings of J1 = 0.025 cm-1 and J2 = -0.020 cm-1 which are consistent with those for GdIII ions.

Mononuclear and trinuclear DyIII SMMs with Schiff-base ligands modified by nitro-groups: first triangular complex with a N–N pathway

Three Dy^III SMMs were obtained with Schiff-base ligands containing nitro-groups. The triangular complex supported by the cis–trans form of the hnc³⁻ ligand displayed the first air-stable triangular complex with a N–N pathway.

Triethylamine-templated nanocalix Ln12 clusters of diacylhydrazone: crystal structures and magnetic properties

Three {Ln₁₂} (Ln = Gd (1), Tb (2), Dy (3)) nanocalix clusters with a novel ligand of N,N′-bis(o-vanillidene)-1H-imidazole-4,5-dicarbohydrazide (H₅ovih) were synthesized via the amine-templating strategy.