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.

Recent Developments of Nontraditional Single-Molecule Toroics.

Single-molecule toroics (SMTs), defined as a type of molecules with toroidal arrangement of magnetic moment associated with bi-stable non-magnetic ground states, are promising candidates for high-density information storage and the development of molecule based multiferroic materials with linear magneto-electric coupling and multiferroic behavior. The design and synthesis of SMTs by arranging the magnetic anisotropy axis in a circular pattern at the molecular level have been of great interest to scientists for last two decades since the first detection of the SMT behavior in the seminal Dy3 molecules. DyIII ion has long been the ideal candidate for constructing SMTs due to its Kramer ion nature as well as high anisotropy. Nevertheless, other LnIII ions such as TbIII and HoIII ions, as well as some paramagnetic transition metal ions, have also been used to construct many nontraditional SMTs. Therefore, we review the progress in the studies of SMTs based on the nontraditional perspective, ranging from the 3D topological to 1D&2D&3D polymeric SMTs, and 3d-4f to non Dy-based SMTs. We hope the understanding we provide about nontraditional SMTs will be helpful in designing novel SMTs.

Gaining Insights into the Interplay between Optical and Magnetic Properties in Photoexcited Coordination Compounds.

We describe early and recent advances in the fascinating field of combined magnetic and optical properties of inorganic coordination compounds and in particular of 3d-4f single molecule magnets. We cover various applied techniques which allow for the correlation of results obtained in the frequency and time domain in order to highlight the specific properties of these compounds and the future challenges towards multidimensional spectroscopic tools. An important point is to understand the details of the interplay of magnetic and optical properties through performing time-resolved studies in the presence of external fields especially magnetic ones. This will enable further exploration of this fundamental interactions i. e. the two components of electromagnetic radiation influencing optical properties.

Elevating the Performance of Heterometallic 3d/4f SMMs: The Role of Diamagnetic CoIII and ZnII Ions in Magnetization Dynamics.

Recent advances in the synthesis of 3d/4f Single-Molecule Magnets (SMMs) have revealed the effective role of incorporating diamagnetic CoIII or ZnII ions to enhance the magnetic properties of LnIII ions. This concept highlights notable examples of CoIII/LnIII and ZnII/LnIII SMMs documented in the recent literature, illustrating how the selection of various peripheral and/or bridging ligands can modulate the magnetic anisotropy of 4f metal ions, thereby increasing their energy barriers.

Enhancing the Magnetization Blocking Energy of Biradical-Metal System by Merging Discrete Complexes into One-Dimensional Chains.

The reaction of nitronyl nitroxide biradical NITPhMeImbis [5-(2-methylimidazole)-1,3-bis(1-oxyl-3'-oxido-4',4',5',5'-tetramethyl-4,5-hydro-1H-imidazol-2-yl)-benzene] with Ln(hfac)3 ⋅ 2H2 O and Cu(hfac)2 (hfac=hexafluoroacetylacetonate), led to two series of 2p-3d-4f complexes, namely, nona-spin clusters, [Ln2 Cu3 (hfac)12 (NITPhMeImbis)2 ] (Ln=Gd 1, Dy 2), or one-dimensional chains [LnCu2 (hfac)7 (NITPhMeImbis)] (Ln=Y 3, Dy 4, Tb 5) depending on the temperature of the reaction. All five complexes contain a biradical-Ln unit in which the biradical chelates the LnIII ion by the means of one aminoxyl (i. e. NO) group of each NIT unit. For the discrete complexes, a Cu(hfac)2 links two biradical-Ln units via one of the remaining NO groups, while for the chain compounds, the two remaining NO groups of the biradical-Ln moiety are each coordinated to a Cu(hfac)2 unit to form a 1D coordination polymer. Moreover, a terminal Cu(hfac)2 unit is coordinated to the imidazole-N atom of the NITPhMeImbis ligand. Spin dynamics investigations evidenced the onset of slow relaxation of the magnetization for 2, whereas 4 and 5 exhibit a typical single-chain magnet behavior. This highlights the vital role of the 1D spin correlation in the blocking of the magnetization. These results illustrate that from the same basic building blocks, magnetic relaxation can be carefully modulated by structural adjustments.

Russian Doll-like 3d-4f Cluster Wheels with Slow Relaxation of Magnetization.

The solvothermal reactions of LnCl3·6H2O and MCl2·6H2O (M = Co, Ni) with 2,2'-diphenol (H2L1) and 5,7-dichloro-8-hydroxyquinoline (HL2) gave three 3d-4f heterometallic wheel-like nano-clusters [Ln7M6(L1)6(L2)6(µ3-OH)6(OCH3)6Cl(CH3CN)6]Cl2·xH2O (Ln = Dy, M = Co, x = 3 for 1; Ln = Dy, M = Ni, x = 0 for 2; Ln = Tb, M = Ni, x = 0 for 3) with similar cluster structure. The innermost Ln(III) ion is encapsulated in a planar Ln6 ring which is further embedded in a chair-conformation M6 ring, constructing a Russian doll-like 3d-4f cluster wheel Ln(III)⸦Ln6⸦M6. 2 and 3 show obvious slow magnetic relaxation behavior with negligible opening of the magnetic hysteresis loop. Such a Russian doll-like 3d-4f cluster wheel with the lanthanide disc isolated by transition metallo-ring is rarely reported.

SMM Behaviour of the Butterfly {CrIII 2 DyIII 2 } Pivalate Complex and Magneto-structurally Correlated Relaxation Thermal Barrier.

We are reporting the synthesis, single-crystal X-ray structure characterization, and magnetic property investigations of the pivalate butterfly {CrIII 2 LnIII 2 } complexes with Ln= Gd and Dy and the isostructural Y(III) sample. We found an anti-ferromagnetic Cr(III)-Ln(III) exchange interaction, which, as previously observed in related Cr(III)/Ln(III) systems, plays a key role in suppressing quantum tunnelling of magnetization and enhances the SMM performance in the Dy(III) complex. In fact, a pure Orbach relaxation mechanism, with absence of QT regime, is observed with a thermal barrier of 50 cm-1 , leading to magnetization hysteresis opening, measured with a commercial magnetometer, up to 3.6 K with a coercive field of 2.9 T. Analysis of SMM behaviour in literature-known butterfly {CrIII 2 DyIII 2 } complexes, reveals the existence of a magneto-structural correlation between Ueff , the thermal barrier size, and the mean Cr-Dy bond distances. Moreover, a clear correlation is found for the thermal barrier magnitude and the maximum temperature hysteresis opening and coercive field.

A Self-Assembled ZnII-NdIII Heterohexanuclear Dimer Based on a Hexadentate N2O4-Type Ligand and Terephthalic Acid: Synthesis, Structure, and Fluorescence Properties.

A self-assembled ZnII-NdIII heterohexanuclear coordination compound [Zn4Nd2(L)4(bdc)2]·2NO3 based on a hexadentate Salamo-like chelating ligand (H2L = 1,2-bis(3-methoxysalicylideneaminooxy)ethane]) and H2bdc (H2bdc = terephthalic acid) has been synthesized and characterized by elemental analyses, IR and UV/Vis spectra, and X-ray crystallography. Two crystallographically equivalent [Zn2Nd(L)2] moieties lie in the inversion center linked by two (bdc)2- ligands leading to a heterohexanuclear dimer in which the carboxylato group bridges the ZnII and NdIII atoms. The heteropolynuclear 3d-4f coordination compound includes four ZnII atoms, two NdIII atoms, four completely deprotonated (L)2- units, two fully deprotonated (bdc)2- units, and two crystalling nitrate ions. All of the ZnII atoms in the ZnII-NdIII coordination compound possess trigonal bipyramidal geometries and the NdIII atoms possess distorted bicapped square antiprism coordination arrangements. In addition, the fluorescence properties of the ligand and the ZnII-NdIII coordination compound were investigated.

Heterometallic ZnII⁻LnIII⁻ZnII Schiff Base Complexes with Linear or Bent Conformation-Synthesis, Crystal Structures, Luminescent and Magnetic Characterization.

A series of racemic, heteronuclear complexes [Zn2Nd(ac)2(HL)2]NO3·3H2O (1), [Zn2Sm(ac)2(HL)2]NO3·3CH3OH·0.3H2O (2), [Zn2Ln(ac)2(HL)2]NO3·5.33H2O (3⁻5) (where HL is the dideprotonated form of N,N'-bis(5-bromo-3-methoxysalicylidene)-1,3-diamino-2-propanol, ac = acetate ion, and Ln = Eu (3), Tb (4), Dy (5), respectively) with an achiral multisite coordination Schiff base ligand (H3L) were synthesized and characterized. The X-ray crystallography revealed that the chirality in complexes is centered at lanthanide(III) ions due to two vicinally located µ-acetato-bridging ligands. The presented crystals have isoskeletal coordination units but they crystallize in monoclinic (1, 2) or trigonal crystal systems (3⁻5) with slightly different conformation. In 1 and 2 the ZnII⁻LnIII⁻ZnII coordination core is linear, whereas in isostructural crystals 3⁻5 the chiral coordination cores are bent and lie on a two-fold axis. The complexes 1, 3⁻5 show a blue emission attributed to the emission of the ligand. For ZnII2SmIII complex (2) the characteristic emission bands of f-f* transitions were observed. The magnetic properties for compounds 1, 4 and 5 are characteristic for the paramagnetism of the corresponding lanthanide(III) ions.

Heterometallic Zn3 Ln3 Ensembles Containing (µ6 -CO3 ) Ligand and Triangular Disposition of Ln3+ ions: Analysis of Single-Molecule Toroic (SMT) and Single-Molecule Magnet (SMM) Behavior.

Two new heterometallic Zn3 Ln3 (Ln3+ =Dy, Tb) complexes, with a double triangular topology of the metal ions, have been assembled from the polytopic Mannich base ligand 6,6'-{[2-(dimethylamino)ethylazanediyl]bis(methylene)}bis(2-methoxy-4-methylphenol) (H2 L) with the aid of an in situ generated carbonate ligand from atmospheric CO2 fixation. Theoretical calculations indicate axial ground states for the Ln3+ ions in these complexes, with their local magnetic moments being almost coplanar and tangential to the Ln3+ atoms that define the equilateral triangle. Therefore, they can be considered as single-molecule toroics (SMTs) with almost zero total magnetic moment. Micro-SQUID measurements on the Dy3+ counterpart show hysteresis loops below 3 K that have an S-shape, with large coercive fields opening upon cooling. This behavior is typical of a single molecule magnet (SMM) with very slow zero-field relaxation. At around ±0.35 T, the loops have a broad step, which is due to a direct relaxation process and corresponds to an acceleration of the relaxation of the magnetization, also observed at this magnetic field from ac susceptibility measurements. Simulations suggest that the broad step corresponds to two level avoidance of crossing points where the spin chiral Kramers doublet meets excited states of the coupled manifold, whose position is defined by exchange and dipole interactions. The Tb3+ counterpart does not exhibit SMM behavior, which is due to the fact that the degeneracy of the ground state of the exchange coupled system is lifted at zero field, thus favoring quantum tunneling of magnetization (QTM).

Role of Magnetic Exchange Interactions in the Magnetization Relaxation of {3d-4f} Single-Molecule Magnets: A Theoretical Perspective.

Combined density functional and ab initio calculations are performed on two isomorphous tetranuclear {Ni3 (III) Ln(III) } star-type complexes [Ln=Gd (1), Dy (2)] to shed light on the mechanism of magnetic exchange in 1 and the origin of the slow magnetization relaxation in complex 2. DFT calculations correctly reproduce the sign and magnitude of the J values compared to the experiments for complex 1. Acute ∢Ni-O-Gd bond angles present in 1 instigate a significant interaction between the 4fxyz orbital of the Gd(III) ion and 3d${{_{x{^{2}}- y{^{2}}}}}$ orbital of the Ni(II) ions, leading to rare and strong antiferromagnetic Ni⋅⋅⋅Gd interactions. Calculations reveal the presence of a strong next-nearest-neighbour Ni⋅⋅⋅Ni antiferromagnetic interaction in complex 1 leading to spin frustration behavior. CASSCF+RASSI-SO calculations performed on complex 2 suggest that the octahedral environment around the Dy(III) ion is neither strong enough to stabilize the mJ |±15/2〉 as the ground state nor able to achieve a large ground-state-first-excited-state gap. The ground-state Kramers doublet for the Dy(III) ion is found to be the mJ |±13/2〉 state with a significant transverse anisotropy, leading to very strong quantum tunneling of magnetization (QTM). Using the POLY_ANISO program, we have extracted the JNiDy interaction as -1.45 cm(-1) . The strong Ni⋅⋅⋅Dy and next-nearest-neighbour Ni⋅⋅⋅Ni interactions are found to quench the QTM to a certain extent, resulting in zero-field SMM behavior for complex 2. The absence of any ac signals at zero field for the structurally similar [Dy(AlMe4 )3 ] highlights the importance of both the Ni⋅⋅⋅Dy and the Ni⋅⋅⋅Ni interactions in the magnetization relaxation of complex 2. To the best of our knowledge, this is the first time that the roles of both the Ni⋅⋅⋅Dy and Ni⋅⋅⋅Ni interactions in magnetization relaxation of a {3d-4f} molecular magnet have been established.

A single-ion magnet based on a heterometallic Co(III) 2 Dy(III) complex.

We report a Co(III) 2 Dy(III) complex, which shows single-ion-magnet behaviour. AC susceptibility data of this compound reveals the presence of slow relaxation of the magnetization in zero-field below 15 K. The relaxation barrier is 88 K.

The coordination chemistry and magnetism of some 3d-4f and 4f amino-polyalcohol compounds.

Triethanolamine, teaH3, and diethanolamine, RdeaH2, 3d-4f and 4f compounds demonstrate an enormous variety in their structure and bonding. This review examines the synthetic strategies to these molecules and their magnetic properties, whilst trying to assess these ligands' suitability towards new SMMs and magnetic refrigerants.

Cu(II)-Gd(III) cryogenic magnetic refrigerants and Cu8Dy9 single-molecule magnet generated by in situ reactions of picolinaldehyde and acetylpyridine: experimental and theoretical study.

A series of heterometallic [Ln(III)(x)Cu(II)(y)] complexes, [Gd2Cu2]n (1), [Gd4Cu8] (2), [Ln9Cu8] (Ln=Gd, 3·Gd; Ln=Dy, 3·Dy), were successfully synthesized by a one-pot route at room temperature with three kinds of in situ carbonyl-related reactions: Cannizzaro reaction, aldol reaction, and oxidation. This strategy led to dysprosium analogues that behaved as single-molecule magnets (SMMs) and gadolinium analogues that showed significant magnetocaloric effect (MCE). In this study a numerical DFT approach is proposed by using pseudopotentials to calculate the exchange coupling constants in three polynuclear [Gd(x)Cu(y)] complexes; with these values exact diagonalization or quantum Monte Carlo simulations have been performed to calculate the variation of the magnetic entropy involved in the MCE. For the [Dy9Cu8] complexes, local magnetic properties of the Dy(III) centers have been determined by using the CASSCF+RASSI method.

A Giant 3d-4f Polyoxometalate Super-Tetrahedron with High Proton Conductivity.

The assembly of gigantic heterometallic metal clusters remains a great challenge for synthetic chemistry. Herein, based on the slow release strategy of lanthanide ions and in situ formation of lacunary polyoxometalates, two giant 3d-4f polyoxometalate inorganic clusters [LaNi12 W35 Sb3 P3 O139 (OH)6 ]23- (LaNi12 ) and [La10 Ni48 W140 Sb16 P12 O568 (OH)24 (H2 O)20 ]86- (La10 Ni48 ) are obtained. The nanoscopic inorganic cluster La10 Ni48 possesses a super tetrahedron structure, which can be viewed as assembly from four LaNi12 molecules encapsulating a central [La6 (SbO3 )4 (H2 O)20 ]6+ octahedron core. This giant aesthetic La10 Ni48 tetrahedron containing 214 metal ions is the largest 3d-4f cluster reported thus far in polyoxometalate system. More interestingly, the LaNi12 and La10 Ni48 display high stability in solution and La10 Ni48 displays excellent proton conductivity.

Fabrication of Hollow Cube Dual-Semiconductor Ln2O3/MnO/C Nanocomposites with Excellent Microwave Absorption Performance.

Metal-organic frameworks (MOFs) have been verified as ideal precursors for preparing highly effective microwave absorbers. However, it is still challenging to fabricate a thin, lightweight, and well-organized nanostructure with strong microwave absorption (MA) capability and wide absorption bandwidth. In this study, hollow cube dual-semiconductor Ln2O3/MnO/C (Ln = Nd, Gd, Er) nanocomposites, which are effective microwave absorbers, have been fabricated via one-step high-temperature carbonization of Ln-Mn-MOFs. The effect of band gap on the MA performance of various nanocomposites synthesized at the same carbonization temperature is investigated. Gd2O3/MnO/C-800 shows superior MA capacity with maximum reflection loss (RLmax) of -64.4 dB at 12.8 GHz and 1.86 mm-thickness. When the thickness is 1.44 mm, the RL value is obtained as -52.7 dB at 16.8 GHz, and at a low frequency of 4.36 GHz and thickness of 4.59 mm, the RL value reaches -56.4 dB. Further, the effect of temperature on the MA properties of Gd2O3/MnO/C is examined. The results reveal that Gd2O3/MnO/C-700 has an ultrahigh MA bandwidth of 6.6 GHz, covering the entire Ku bands at 2.09 mm-thickness. Overall, this work demonstrates a facile strategy to construct hollow, homogeneous ternary composites with outstanding MA performance.

Synthesis and characterization of two isostructural 3d-4f coordination compounds based on pyridine-2,6-dicarboxylic acid and 4,4'-bipyridine.

The design and synthesis of 3d-4f heterometallic coordination polymers have attracted much interest due to the intriguing diversity of their architectures and topologies. Pyridine-2,6-dicarboxylic acid (H2pydc) has a versatile coordination mode and has been used to construct multinuclear and heterometallic compounds. Two isostructural centrosymmetric 3d-4f coordination compounds constructed from pyridine-2,6-dicarboxylic acid and 4,4'-bipyridine (bpy), namely 4,4'-bipyridine-1,1'-diium diaquabis(µ2-pyridine-2,6-dicarboxylato)tetrakis(pyridine-2,6-dicarboxylato)bis[4-(pyridin-4-yl)pyridinium]cobalt(II)dieuropium(III) octahydrate, (C10H10N2)[CoEu2(C10H9N2)2(C7H3NO4)6(H2O)2]·8H2O, (I), and 4,4'-bipyridine-1,1'-diium diaquabis(µ2-pyridine-2,6-dicarboxylato)tetrakis(pyridine-2,6-dicarboxylato)bis[4-(pyridin-4-yl)pyridinium]cobalt(II)diterbium(III) octahydrate, (C10H10N2)[CoTb2(C10H9N2)2(C7H3NO4)6(H2O)2]·8H2O, (II), were synthesized under hydrothermal conditions and characterized by IR and fluorescence spectroscopy, thermogravimetric analysis and powder X-ray diffraction. Both compounds crystallize in the triclinic space group P-1. The EuIII and TbIII cations adopt nine-coordinated distorted tricapped trigonal-prismatic geometries bridged by three pydc2- ligands. The CoII cation has a six-coordination environment formed by two pydc2- ligands, two bpy ligands and two coordinated water molecules. Adjacent molecules are connected by π-π stacking interactions to form a one-dimensional chain, which is further extended into a three-dimensional supramolecular network by multipoint hydrogen bonds.

Heterometallic 3d-4f Complexes as Single-Molecule Magnets.

Heterometallic 3d-4f complexes are being investigated, for some time, as being useful in molecular magnetism, particularly as single-molecule magnets (SMMs). This interest is primarily because of the possibility of an increased ligand-mediated super-exchange phenomenon between the 3d and 4f metal ions. Such an interaction, apart from bestowing a favorable ground-state spin to the complex, also assists in reducing quantum tunneling of magnetization that is widely prevalent in SMMs making them to lose magnetization. However, assembling both 3d as well as 4f ions using same ligand system is challenging and involves the design of multi-site coordination ligands with specific coordination compartments for the 3d and the 4f metal ions while at the same time allowing these disparate metal ions to be linked to each other through a bridging ligating atom. This review presents a summary of the 3d-4f complexes primarily derived from the author's work while alluding to important examples from the literature. We also provide an outlook for the future design of such complexes.

Single Crystal Investigations Unravel the Magnetic Anisotropy of the "Square-In Square" Cr4Dy4 SMM Coordination Cluster.

In the search for new single molecule magnets (SMM), i.e., molecular systems that can retain their magnetization without the need to apply an external magnetic field, a successful strategy is to associate 3d and 4f ions to form molecular coordination clusters. In order to efficiently design such systems, it is necessary to chemically project both the magnetic building blocks and the resultant interaction before the synthesis. Lanthanide ions can provide the required easy axis magnetic anisotropy that hampers magnetization reversal. In the rare examples of 3d/4f SMMs containing CrIII ions, the latter turn out to act as quasi-isotropic anchors which can also interact via 3d-4f coupling to neighbouring Ln centres. This has been demonstrated in cases where the intramolecular exchange interactions mediated by CrIII ions effectively reduce the efficiency of tunnelling without applied magnetic field. However, describing such high nuclearity systems remains challenging, from both experimental and theoretical perspectives, because the overall behaviour of the molecular cluster is heavily affected by the orientation of the individual anisotropy axes. These are in general non-collinear to each other. In this article, we combine single crystal SQUID and torque magnetometry studies of the octanuclear [Cr4Dy4(µ3-OH)4(µ-N3)4(mdea)4(piv)8]·3CH2Cl2 single molecule magnet (piv=pivalate and mdea=N-methyldiethanol amine). These experiments allowed us to probe the magnetic anisotropy of this complex which displays slow magnetization dynamics due to the peculiar arrangement of the easy-axis anisotropy on the Dy sites. New ab initio calculations considering the entire cluster are in agreement with our experimental results.

Magnetic Nature of the CrIII-LnIII Interactions in [CrIII2LnIII3] Clusters with Slow Magnetic Relaxation.

Two 3d-4f hetero-metal pentanuclear complexes with the formula {[CrIII2LnIII3L10(OH)6(H2O)2]Et3NH} [Ln=Tb (1), Dy (2); HL=pivalic acid, Et3N=triethylamine] have been produced. The metal core of each cluster is made up of a trigonal bipyramid with three LnIII ions (plane) and two CrIII ions (above and below) held together by six µ3-OH bridges. Also reported with this series is the diamagnetic CrIII-YIII analogue (3). Fortunately, we successfully prepared AlIII-LnIII analogues with the formula {[AlIII2LnIII3L10(OH)6(H2O)2]Et3NH⋅H2O} [Ln=Tb (4), Dy (5)], containing diamagnetic AlIII ions, which can be used to evaluate the CrIII-LnIII magnetic nature through a diamagnetic substitution method. Subsequently, static (dc) magnetic susceptibility studies reveal dominant ferromagnetic interactions between CrIII and LnIII ions. Dynamic (ac) magnetic susceptibility studies show frequency-dependent out-of-phase (χ'') signals for [CrIII2TbIII3] (1), [CrIII2DyIII3] (2), and [AlIII2DyIII3] (5), which are derived from the single-ion behavior of LnIII ions and/or the CrIII-LnIII ferromagnetic interactions.