Heterometallic Rare-Earth Complexes: A Review on Magnetism, Catalysis and Transformation to Nanomaterials

1. Introduction

Rare-earth elements (REs), named for their scarcity in Earth’s crust, are the group of seventeen metals containing scandium, yttrium, and fifteen lanthanides. Natural RE deposits are infrequent, and separating compounds containing these elements is challenging due to their chemical similarities. Despite the nomenclature suggesting rareness, many of them are more abundant in the Earth’s upper crust than tin, molybdenum, silver, mercury, gold, or platinum. Remarkably, cerium, the most abundant rare-earth element, surpasses lead and copper in commonality. A notable observation is the greater abundance of rare-earth elements with even atomic numbers compared to those with off values, a phenomenon recognized as the Oddo-Harkins rule [1,2,3].

It is essential to highlight that promethium does not naturally occur due to the short half-lives of its isotopes. The elements from cerium to lutetium are collectively known as the 4f elements, reflecting the occupation of the 4f subshell. Yttrium and lanthanum are categorized under group 3 in the periodic table. Scandium stands apart from other rare-earth elements and is often regarded as a ferromagnesian trace element.

On the contrary, despite its atomic number and mass differences, yttrium exhibits numerous physical and chemical resemblances to the lanthanides, particularly those commonly categorized as heavy rare-earth elements (Gd through Lu). One noteworthy similarity lies in the size of their RE^III cation radii, especially notable for Y^III and Ho^III. That and similar oxidation are stated to provide rare-earth metals with the capacity to substitute for one another within crystal lattices. Additionally, differences in cation radii lead to natural deposits usually being enriched in either heavy REs or light REs (La through Eu).

Rare-earth elements and their various derivatives exhibit numerous similarities [1,4], including high electrical conductivity, high coordination numbers, preferably binding with strongly electronegative elements, tarnishing of their surface when exposed to air (especially in humid conditions), and large stability of their oxides.

In the chemistry of rare-earths elements, a series of characteristics distinguish them from transition metals. Unlike the d-block elements, they do not form stable carbonyl complexes and exhibit almost no activity for the oxidation state of 0. The reactivity of these elements is more akin to alkaline earth metals. They commonly adopt the +3 oxidation state, especially in aqueous solutions. Some elements also exhibit the +2 oxidation state (e.g. Nd, Sm, Eu) or +4 oxidation state (e.g. Ce, Pr, Tb) to achieve a more stable electron configuration. Due to the presence of the 4f subshell, RE ions exhibit coordination numbers in the range of 7-12. Due to the predominantly electrostatic nature of 4f–ligand interactions, the coordination geometry of Ln^III complexes is determined by repulsive interactions and steric effects. The 4f orbitals in the Ln^III ion do not directly participate in bonding as the 5s² and 5p⁶ orbitals will shield them. The general notion is that the lanthanides form predominantly ionic bonds, at least in their +3 oxidation state. The interconnection between the orbital overlap and energy-degeneracy-driven covalency and the subsequent impact on bond stability for lanthanide compounds is poorly understood [5]. Rare-earth ions are hard Lewis acids and readily form complexes with hard bases (strongly electron donating ligands) such as F^-, Cl^-, O^2-, RO^-, RCOO^-, H₂O. Due to their high hydration energy, RE^III ions easily form aqua complexes [RE(H₂O)₉]³⁺ pictured in Figure 1 [6].

Though employed in relatively modest amounts, REs boast an extensive array of both current and potential applications. Their versatility is particularly pronounced in catalysis [7] and digital technologies [8], contributing to advancements in magnets [9], rechargeable batteries [10], lasers [11], coatings [12], phosphors [13], optical sensors [14], and even nuclear reactors [15]. Notably, certain RE ions possess the unique ability to emit visible luminescence.

One of the most cost-effective and straightforward methods for enhancing alloy properties involves microalloying and modification with rare-earth elements. Even a small addition of lanthanides or yttrium can significantly enhance the physicochemical characteristics of alloys by altering their microstructure. For instance, REs alloys with magnesium stand out as the most promising high-performance materials [16,17]. Unlimited solubility of REs in molten iron leads to an increase in molten iron viscosity and a reduction of surface tension [18]. Other examples include mischmetals or samarium-cobalt and neodymium-iron magnets, with the last ones being the strongest type of permanent magnet available commercially.

The majority of rare-earth oxides in nature exist in the form of sesquioxides, following the formula RE₂O₃. However, there are exceptions with Ce, Pr, and Tb. Cerium naturally forms CeO₂, praseodymium oxide is Pr₆O₁₁, and terbium forms Tb₄O₇. Despite being non-sesquioxides, these compounds can, under specific conditions, adopt structures isostructural to other REs. Within catalysis, they stand out as highly active and selective catalysts, facilitating processes such as the oxidation coupling of CH₄ to higher hydrocarbons [19], isomerization of 1-butene [20], acetone aldol addition [21], hydrogenation of 1,3-butadiene [22], and the dehydration of alkanediols into unsaturated alcohols [23]. The distinctive magnetic and optical properties of glasses derived from rare-earth metal oxides position them as ideal candidates for applications in optics and electronics. They have also been incorporated into electrochemical sensors for detecting humidity, toxins, drugs, and gases.

Rare-earth complexes are used in many practical applications as molecular probes, contrast agents, catalysts, molecular magnetic or luminescence materials, dopants, and modifiers for optical materials.

For this work, in the following sections, we discussed various methods for the synthesis of heterometallic rare earth compounds, then we described their magnetic properties, and we made a concise description of their catalytic applications in organic synthesis, polymerization of cyclic monomers or photocatalytic energy conversion processes. The last paragraph describes the use of heterometallic complexes as molecular precursors for synthesizing functional nanomaterials. Because luminescent properties were described thoroughly before [24,25,26], we have not discussed them in this review.

2. Heteromatallic rare-earth complexes, synthesis and applications

2.1. Synthesis routes

Usually, the synthesis of heterometallic transition metal-rare-earth complexes is performed by dissolution of RE(NO)₃ and transition metal reagents (chlorides, nitrates, acetates) in the water-alcohol mixture as solvent, using amines to deprotonate the phenolic ligands. These reactions usually lead to thermodynamically stable crystalline products with molecular structure mostly depending on the reaction conditions, used reagents, or crystallization procedure. The more controlled synthesis method was used to prepare group 4 – rare earth oxo alkoxides by substituting the halogen atoms in RECl₃ by K[Zr₂(OⁱPr)₉] in THF solution, leading to the formation of [REZr₂(OⁱPr)₉Cl₂]_x (RE^III = Y^III, Nd^III, Ho^III, Ce^III for x = 2; Er^III, Yb^III for x = 1) [27]. Alternative method claims the synthesis of titanium-lanthanide oxo clusters pictured in Figure 2, i.e., [RETi₄(O)₃(OⁱPr)₂(OMc)₁₁] (RE = La, Ce; OMc = methacrylate), [RE₂Ti₆O₆(OMc)₁₈(HOⁱPr)] (RE = La, Ce, Nd, Sm) and [RE₂Ti₄O₄(OMc)₁₄(HOMc)₂] (RE = Sm, Eu, Gd, Ho) by the reaction of titanium isopropoxide, lanthanide acetate and methacrylic acid [28].

The direct route for the rare-earth complexes from the pure metals has not been considered as attractive because it requires activation by Hg, HgCl_2, or I₂ and prolonged heating [29].

Recently Sobota and coworkers reported the synthesis of heterometallic oxo-alkoxides by the reaction of metallic lanthanides with divalent transition metal chlorides MCl₂ (M^II = Mn^II, Ni^II or Co^II) or group 4 metallocene dichlorides Cp₂MCl₂ (where M^IV = Ti^IV, Zr^IV, Hf^IV), using 2-methoxy ethanol or ethanol as solvents and reagents [30,31]. The reaction of RECl₃ with the molar excess of alkaline or alkaline earth ligand precursors is used as the main method for synthesizing heterometallic rare earth – group 1/group 2 complexes [32,33].

Employing precursors with well-defined structures in synthesizing heterometallic complexes minimizes the risk of side reactions or the formation of multiple reaction products, ensuring a more controlled and selective outcome that enhances the synthesis efficiency and selectivity. The structurally authenticated precursors act as templates, guiding the coordination of specific ligands to metal sites to yield a predetermined complex. The advantages of this methodology are particularly evident in the enhanced reproducibility of heterometallic complexes and the ability to scale up the synthesis for practical applications. Moreover, the structurally authenticated precursors enable a deeper understanding of the reaction mechanism, facilitating the design of more sophisticated and tailored heterometallic rare-earth complexes with desired properties for diverse catalytic and material applications. For example, trinuclear ionic zinc compound based on [Zn₃(OAc)₂(L)₂(H₂O)₂]²⁺ cation (where LH = 2-ethoxy-6-((pyridin-2-ylmethylimino)methyl)phenol)) [34] could be used as attractive molecular platform for the synthesis of a wide range heterometallic Zn^IIYb^III clusters by the reaction with various ytterbium salts as shown in Figure 3.

The use of Yb(ClO₄)₃ led to the [Zn₂Yb(L)₂(OAc)₄]ClO₄, and Yb(NO₃)₃ led to [ZnYb(L)₂(NO₃)₃]. The treatment of trinuclear zinc cation with Yb(OAc)₃ allows the formation of [ZnYb(OAc)₄(L)], but the reaction with YbCl₃ led to the [Yb₂(L)₂(OAc)(H₂O)Cl]₂Cl₄. The reversed sequence was applied in the reaction of [RE(L)(THF)_n] (where RE^III = Y^III, Sm^III, Nd^III, La^III; n == 1, 2) with Co(OAc)₂ that as the results of ligand redistribution lead to the formation of [Co₂RE₂(OAc)₄(L)₂(THF)_n] containing acetato bridged RE^III dimers capped by Co^II trisphenolato units, as seen in Figure 4 below [35].

2.2. Magnetism

Coordination compounds that behave as single-molecule magnets (SMMs), which exhibit slow magnetization reversal and distinct hysteresis in the absence of long-range magnetic order, are of particular interest because of their potential applications in high-density information storage [36]. Lanthanide complexes show great potential for SMMs because of their strong magnetic anisotropy and large-spin ground states, of which the best candidates are Tb^III, Dy^III, and Ho^III. However, their combination with 3d metal ions is essential to suppress quantum tunneling effectively and establish a magnetic exchange between metal ions (3d–4f) [37,38,39]. The combination of different paramagnetic metal ions within the same molecular entity leads to a wide variety of magnetic properties of the heterometallic compounds. 3d–4f complexes have been intensively studied in order to reveal the factors governing the nature and magnitude of the exchange interaction between a 3d metal ion and various 4f metal ions. The comparison of magnetic properties of the series of [M₃Ln₂(opba)₃] (for M^II = Cu^II, Ni^II; opba^4- = ortho-phenylenebis(oxamato)) based on a ladder-type structure with isostructural zinc compounds allow to determine the nature of interactions in Ln^III−M^II pairs. This led to the conclusion that the interaction is ferromagnetic for Gd^III, Tb^III, and Dy^III and seems to be antiferromagnetic for Ce^III, Pr^III, Nd^III, and Er^III [40,41]. For the series of trinuclear complexes of general formula [Cu₂Ln(L)₂(NO₃)₂]⁺ for (LH₂ = 2,6-di(acetoacetyl)pyridine), the presence of an antiferromagnetic interaction for Ln^III = Ce^III, Pr^III, Nd^III and Sm^III, and ferromagnetic for Ln^III = Gd^III, Tb^III, Dy^III, Ho^III, and Er^III was observed [42]. The structures of those complexes can be found in Figure 5.

The nature of the Ni^II−Ln^III exchange interaction within binuclear [NiLn(valpn)(CH₃CN)₂(NO₃)₃] (for Ln^III = Pr^III, Nd^III, Sm^III, Eu^III, Gd^III, Er^III) found in Figure 6, [NiLn(valpn)(CH₃CN)(H₂O)₄(NO₃)](NO₃)₂ (for Ln^III = Tb^III, Dy^III) and [NiCe(valpn)(CH₃CN)₂(H₂O)(NO₃)₃]·[NiCe(valpn)(CH₃CN)(H₂O)₂(NO₃)₂](NO₃) (for valpn^2- = 1,3-propanediylbis(2-iminomethylene-6-methoxy-phenolato)) has been emphasized by comparison with the cryomagnetic behavior of the related [Zn^IILn^III] derivatives. This route allowed for establishing that the interaction within these compounds is antiferromagnetic with the 4f ions of the beginning of the Ln series (Ce^III – Eu^III) and turns ferromagnetic from Gd^III to Er^III. The [Ni^IIDy^III] complex shows slow relaxation processes of the magnetization close to 2 K [43].

For the heterodinuclear [CuLn(L)(NO₃)₃] (Figure 7) complexes (Ln^III = Ce^III−Yb^III; LH₂ = N,N‘-ethylenebis(3-ethoxysalicylaldiimine)), ferromagnetic interactions seem to be exhibited for the Gd^III, Tb^III, Dy^III, Ho^III, Tm^III, and Yb^III, while for Ce^III, Nd^III, and Sm^III the interaction was antiferromagnetic. Pr^III and Eu^III analogs behaved as spin-uncorrelated systems, and no definite conclusions were reached for the Er^III complex [44]. Within the series of [CuLn(L)(Me₂CO)(NO₃)₃] complexes pictured in Figure 8, the nature of the coupling between the Cu^II–Ln^III ions was antiferromagnetic for Ce^III, Nd^III, Sm^III, Tm^III, and Yb^III, and ferromagnetic for Gd^III, Tb^III, Dy^III, Ho^III, and Er^III, with Cu^II–Pr/Eu^III pairs devoid of any significant interaction [45].

The first example of a 3d–4f SMM was [CuTb(hfac)₂(L)]₂ (LH₃ = 1-(2-hydroxybenzamido)-2-(2-hydroxy-3-methoxy-benzylideneamino)-ethane; hfacH = hexafluoroacetylacetone), which has an effective energy barrier of 21 K, a relaxation time of 2.7 ×10^–8 s, and an estimated blocking temperature of 1.2 K [46]. Since this discovery, combinations of Tb^III , Dy^III, Er^III, Sm^III, Yb^III, Gd^III, or Ho^III with transition metal ions such as Co^II, Mn^II/III, Fe^II/III, Ni^II, and Cu^II have been extensively studied. SMM behavior has also been observed for Tb^III and Dy^III complexes within the series of trimetallic [CuLn(L)(C₃H₆O)(NO₃)₃] (where Ln = Gd^III, Tb^III, Dy^III, Ho^III, Er^III; L^2- = 2,2'-((2,2-dimethylpropane-1,3-diyl)bis((nitrilo)methylylidene))bis(6-methoxyphenolato))) with activation energies of magnetization reversal equal to 42.3(4) and 11.5(10) K, respectively. The magnetic exchange couplings in Cu^II−Ln^III display a monotonic decrease of ferromagnetic J_Ln–Cu in the order of the atomic number, from ⁶⁴Gd to ⁶⁸Er (J_Ln–Cuk_B⁻¹/K = 6.9 (Gd), ≥ 3.3 (Tb), 1.63(1) (Dy), 1.09(2) (Ho), 0.24(1) (Er)) [47].

The family of hetero-tri-metallic complexes [{CuTb(L)(H₂O)₄}{M(CN)₆}]_n (for M^III = Co^III, Cr^III, Fe^III; L^2- = 2,2'-(propane-1,3-diylbis(nitrilomethylylidene))bis(6-methoxyphenolato), 2,2'-(ethane-1,2-diylbis(nitrilomethylylidene))bis(6-methoxyphenolato); n = 1, 2, ∞) are interesting examples that illustrate how SMM behavior of the [CuTb(L)]³⁺ moiety can be modulated via the control of intermolecular interactions with [M(CN)₆]³⁻ species [48].

When [Cr/Fe(CN)₆]³⁻ are used, weak antiferromagnetic interactions are responsible for the decrease of the SMM efficiency. The combination of the diamagnetic [Co(CN)₆]³⁻ with a [CuTb(L)]³⁺ moiety resulted in an improvement of the SMM properties compared to the reference [CuTb] complex, with a significantly longer relaxation time. The increase of the effective anisotropic barriers (U_eff) from 5–7 cm⁻¹ present for the [{CuTb}Cr/Fe] compounds to 15–18 cm⁻¹ for [{CuTb}Co] was also observed [49].

The heterometallic [MnLn₂(QCl)₈] (Ln = Dy^III, Tb^III; QCl^- = 5-chloro-8-quinolinolate) display clearly resolved out-of-phase susceptibility maxima below 10 K originating from SMM behavior. Both of them are interesting examples demonstrating that the co-complexation of [Bu₄N][Ln(QCl)₄] with Mn(NO₃)₂ led to a structural transformation from mononuclear to trinuclear compounds enhancing single molecule magnetic (SMM) behavior. Molecular structure of [MnLn₂(QCl)₈] is presented in Figure 9 [50].

Heterometallic Ni^IIDy^III complexes generally exhibit field-induced single-molecule magnet behavior with an effective energy barrier of 6.6 - 16.9 K for reversal of the magnetization, i.e., [NiDy(valpn)(hfac)₂(N₃)(H₂O)₂] [51], [NiDy(L)(ArCOO)(NO₃)₂] (LH_{2 =} N,N′,N″-trimethyl-N,N″-bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylenetriamine); Ar = benzyl, 9-anthracenyl) [52], [Ni₂Dy₂(CH₃COO)₃(HL)₄(H₂O)₂](NO₃)₃ (LH^-=2-methoxy-6-[(E)-2′-hydroxymethyl-phenyliminomethyl]-phenolate) [53], [Ni₂Dy₂(μ₄-CO₃)₂(3-MeOsaltn)₂(H₂O)₂(NO₃)₂] [54] (3-MeOsaltn^- = N,N′-bis(3-methoxy-2-oxybenzylidene)-1,3-propanediaminato), [Ni₂Dy₂(L)₄(NO₃)₂(MeOH)₂] [55] (L^2- = 2-(((2-hydroxyphenyl)imino)methyl)-6-methoxyphenolato), [R₃NH]₂[Ni₂Dy₂(μ₃-OH)₂(^tBuCOO)₁₀] [56] and many others [57,58,59]. Chosen Ni^IIDy^III complexes are presented in Figure 10.

The compounds [Zn₂Ln(L)₂(NO₃)(SCN)₂] (where Ln^III = Ce^III, Nd^III; L^2- = 2,2'-(ethane-1,2-diylbis(nitrilomethylylidene))bis(6-methoxyphenolato)) based on a linear Zn^II–Ln^III–Zn^II motif with an axially stressed ligand field demonstrated the appearance of field-induced SMM behavior, which was correlated with the even-numbered J_z sublevels of Ce(III) and Nd^III ions known as the Kramers system [60]. Field-induced SMM with an estimated U_eff barrier of 59K was [Zn₂Dy(L)(NO₃)₃(OH)], despite the fact that mononuclear [Dy(H₃L)(H₂O)(NO₃)](NO₃)₂ do not show slow relaxation of the magnetization [61]. An uncommon example of heterometallic clusters is [Zn₂Dy₄(HL)₄(o-vanillin)₂(OH)₄(CH₃OH)₂](NO₃)₂] that shows typical ferromagnetic single molecule magnetic behavior with a slow zero-field relaxation [62].

In the first bifunctional SMM [ZnDy(NO₃)₃(L)(H₂O)] (where LH₂ = N,N’-bis(3-methoxysalicylidene)-1,2-diaminoethane), pictured in the Figure 11, diamagnetic zinc cation provides the compound with two significant profits: it increases the negative charge of the phenolates and it elevates rare-earth cations’ crystal field splitting without the suppression of the Dy^III emission in the visible spectral window [63,64].

2.3. Catalysis, ring-opening polymerization and copolymerization of cyclic esters

Rare earth complexes offer the promise of catalytic performance of many organic reactions due to their high coordination numbers as well as their ability to change the coordination environment quickly. The catalytic applications of heterometallic rare earth complexes are mainly related to Shibasaki catalysts of the formula [M₃RE(binol)₃] (where M^I is alkali metal ion, binol^2- - binaphtholato as shown in Figure 12). Since the beginning of the 1990s, Shibasaki catalysts have been mainly used in asymmetric synthesis, and in particular in the Michael addition, [65] aldol [66]/nitro-aldol [67] condensation, cyclopropanation of enons, [68] aldol–Tishchenko reaction, [69] cyanation of aldehydes, [70] reduction of 1,4-benzoquinones, [71] Mannich reaction, [72] protonation, [73] Diels-Alder reaction, [74] hydrophosphonation of imines and aldehydes [75].

In asymmetric synthesis, the increase in enantioselectivity was mainly dependent on the type of M^I, when the influence of the RE^III was much lower. The effectiveness of these catalysts is due to their ability to behave as both a Lewis acid via RE^III ions to activate electrophiles and Brönsted base via Mbinol to activate pronucleophiles, which is analogous to metalloenzyme reactions. The synergic action of the two metal centers allows for transformations that were not possible with conventional catalysts.

Schelter published the ethylzinc Shibasaki catalyst analogs [(EtZn)₃RE(binol)₃ (THF)₃][76] (where RE = La^III, Pr^III, and Eu^III) presented in Figure 13, which were used as catalysts for the enantioselective addition of ethyl groups to benzaldehyde in toluene at room temperature (yields 95% (La^III), 89% (Pr^III), 99% (Eu^III). The greatest control over the process (ee = 70%) was achieved using an equimolar mixture of Zn^II₃Eu^III complex, ZnEt_2, and triphenylphosphine oxide.

Dual activity of [NiRE(L)₃] [77] (where RE = Lu^III, Y^III, La^III, L^- = (ⁱPr₂PCH₂NPh)^-) was discovered in the hydrogenation of diphenylacetylene (DPA) to (E)-stilbene (4.6 atm H₂ at 70 °C, 2.5 mol % of cat.). In the presence of DPA, [NiRE(L)₃] compounds catalyze the production of (Z)-stilbene (99-98% conversion after 24 h). In the absence of DPA they are responsible for the cis to trans isomerization of stilbene, with trans:cis ratio: >99:1 (for Lu^III and Y^III) or 20:80 (for La^III)), as presented in Figure 14. In performed reactions, electron-rich Ni^II centers were engaged in alkyne binding, while RE^III ions provide an open, intramolecular coordination site to favor the hemilability of the phosphine ligand.

Another example was bimetallic rare-earth coordination polymers of formula {[Fe₇RE(Hpmida)₆]·2H₂O}_n [78] (where RE^III = Eu^III, Dy^III, Ho^III, Y^III; H₄pmida = N-(phosphonomethyl)iminodiacetic acid), which were useful as highly selective (95-98%) heterogenous catalysts for Knöevenagel condensation, an important C-C coupling reaction widely used for the synthesis of fine chemicals and pharmaceuticals. Depending on the reactants chosen, the conversion reached 20% to 57% (27 h, toluene, 60 °C), with the most active europium and dysprosium. In the experiments performed, it turned out that the presence of RE^III improves the course of the reaction and significantly increases the conversion (from non-catalyzed 7-8%).

Heterometallic rare-earth complexes may also be used as initiators in the ring-opening polymerization (ROP) of cyclic esters or their copolymerization (ROCOP) with cyclic epoxides and anhydrides. ROP of cyclic esters initiated by metal catalysts permits the preparation of well-defined polyesters with strictly controlled molecular weight, dispersity, chain microstructure, and tacticity [79]. ROP and ROCOP initiators featuring f-block metals have been developed to take advantage of their Lewis acidity and large coordination spheres. The impact of RE^III in monomer activation usually was observed for the alkali metal-rare earth initiators.

In the ε-caprolactone (ε-CL) polymerization studies initiated by [Na₈Sm₂(OCH₂CH₂NMe₂)₁₂(OH)₂], [K₁₂Na₄Sm₂(OCH₂CH₂NMe₂)₁₈(OH)₄] and [K₂₀RE₄(OCH₂CH₂NMe₂)₂₆(OH)₆] (where RE^III = Sm^III, Nd^III, Pr^III, Yb^III) it has been shown that the interaction of heterobimetallic centers (Figure 15) allows for achieving within 1 min from 76 to 100% conversion of monomer at stoichiometry of ε-CL/I = 6.000 - 15.000, while homometallic alkoxides were inactive in the study process [80].

Another example is [LiY(C₅Me₄SiMe₂NCH₂CH₂X)₂] [81] (where X = OMe, NMe₂), which in the ROP of ε-CL in toluene at room temperature reached monomer conversion of 75-94% in just 90 minutes. Poly(ε-caprolactone)s obtained were of high molecular weight (M_n > 30 kDa) with the highest values over 170 kDa (for ratio [ε-CL]/[initiator] = 188/1) and dispersity (Đ) < 2.0. Reported complexes provided higher conversion and lower Đ than [Y(N(SiMe₃)₂)₃] for identical reaction conditions. Other examples of heterometallic initiators in ROP of ε-CL are mixed-metal allyl complexes of formula [RE(η³-C₃H₅)₃(μ-C₄H₈O₂)·Mg(η¹-C₃H₅)₂(μ-C₄H₈O₂)_1.5]_n[82] (where RE^III = La^III, Y^III). Both compounds are highly effective under mild conditions and reach monomer conversion of 99% (t = 0.3 min) and 61% (t = 1.3 min) with M_n values of 42.1 and 29.2 kDa with Đ ~1.4, respectively. The catalytic activity of both initiators was better than monometallic [Mg(η¹-C₃H₅)₂(BDI)(THF)] (BDIH = (2-(2,6-diisopropylphenyl)amino-4-(2,6-diisopropylphenyl)imino-2-pentene; [ε-CL]/[initiator] = 200/1, conversion 92% in 6 min, M_n = 13.6 kDa, Đ = 1.4). The interesting group of heterometallic complexes was obtained using Ferrocene-based tetradentate Schiff base ligands.[83] The key finding of this study was the use of [Ce(phosphen)(THF)₂] (where phosphen = 1,10-di(2-tert-butyl-6-diphenylphosphiniminophenoxy)ferrocene) as redox control initiator for ROP of l-lactide (l-LA), which course was dependent on the oxidation state of the Ce^III/^IV ion [84]. The Ce^III complex presented in Figure 16 was active in l-LA polymerization at 0 °C with a reagents stoichiometry of l-LA/Ce = 100/1, and it allowed to achieve 96% monomer conversion over 0.5 h, whereas under these conditions Ce^IV compound was inactive. In turn, the isostructural Y^III compound was used as a model system to determine the influence of the oxidation state of Fe atoms in the ferrocene backbone on the Y^III–OR activity in the l-LA polymerization. An initiator containing Fe^II atoms allowed to achieve 24% conversion of monomer in polymerization performed at a ratio of l-LA/Y = 100 through 1h at 25 ° C, while the complex containing Fe^III centers was completely inactive in the studied reaction [85].

Series of heterometallic Ni^II-RE^III complexes (where RE^III = Ce^III, Nd^III, Sm^III, Eu^III, Tb^III, Ho^III, Tm^III) with the acyclic Salen-type ligand (LH₂ = N,N’-bis(3-methoxysalicylidene)ethylene-1,2-diamine), with the structure shown in Figure 17, were used as initiators for ROP of l-LA in bulk at 160 °C for 12 h (for ratio [l-LA][catalyst] = 1000/1). The presence of rare-earth ions was an influential factor because it effectively passivated the catalytic behaviors on the ROP, leading to increased molecular weights of obtained polymers (M_n = 28 961 – 31 555), and improved the polymerization control (Đ = 1.12 – 1.19). For the mentioned complexes, it was discovered that the catalytic activity is relative to the intramolecular Ni^II-RE^III separations without the lanthanide contraction sequence [86].

Two-dimensional coordination polymer (Figure 18) [CuEr(pdc)₂(Hpdc)(H₂O)₄]_n [87] (where pdcH₃ = 3,5-pyrazole dicarboxylic acid) was used as a heterogeneous catalyst in the cyclopropanation of styrene with ethyldiazoacetate at room temperature in dichloromethane, leading to achieving high diastereoselectivity (84%) at the 12% conversion rate after 24 hours.

A series of heterometallic {[Cu₃Ln₂(L)₆(H₂O)₆]·10H₂O}_n [88] (where Ln^III = La^III, Gd^III, Yb^III, Lu^III) coordination polymers were used for catalytic oxidation of olefins and aromatic benzylic substrates using ^tBuOOH or O₂ as oxidants. The series of used compounds showed that their activity increases with the increase in the atomic number within the lanthanide group. The most active in the cyclohexene oxidation reaction (1,2-dichloroethane, ^tBuOOH (70%), 75 °C, 0.001 mol% [Cu]) was the Cu^II-Lu^III complex (Figure 19), which after 24 h led to the achievement of 60% substrate conversion, while the remaining compounds allowed to achieve conversions of 48% (Cu^II-La^III), 52% (Cu^II-Gd^III), and 57% (Cu^II-Yb^III). An identical trend in the reactivity of compounds was also maintained in the styrene oxidation reaction, in which the substrate conversion values were 75% for Cu^II-Lu^III, 64% for Cu^II-La^III, 69% for Cu^II-Gd^III, and 73% for Cu^II-Yb^III for the same reaction conditions.

2.4. CO₂ conversion

In recent decades, rapid increases in atmospheric carbon dioxide concentrations, primarily driven by population growth, economic development, and energy consumption, have become a global concern. The elevated CO₂ levels, mainly from fossil fuel use, contribute to ecological imbalances, including temperature growth, melting snow cover, permafrost thaw, and rising sea levels [89]. Consequently, mitigating CO₂ emissions and reducing atmospheric levels have become crucial global objectives to combat climate change. Efforts have been directed toward capturing, utilizing, and storing CO₂ to achieve decarbonization and emissions reduction goals. Carbon dioxide offers a broad spectrum of potential applications, ranging from direct uses in oil recovery, food processing, water treatment, fire retardants, coolants, and cleaning agents to chemical conversions into value-added products. CO₂ offers an accessible, non-toxic, low-cost, renewable carbon feedstock for producing chemicals, fuels, plastics, and raw materials. However, due to its low reactivity, carbon dioxide conversion needs suitable reagents, catalysts, or high-energy sources.

Most published studies use highly reactive substrates and harsh reaction conditions to overcome the high thermodynamic stability and chemical inertness of CO₂. Therefore, molecular catalysis has recently focused on developing more efficient systems that promote CO₂ transformation, especially under mild conditions, in order to reduce production costs and energy consumption. Numerous metal-based catalysts, including main group elements, transition elements, and rare-earth elements, have been used for the chemical fixation of CO₂ into value-added products. Both homogenous and heterogenous metal catalysts play a crucial role in CO₂ conversion reactions, examples of which are shown in Figure 20. The synthesis of cyclic carbonates by cycloaddition of CO₂ and epoxides is one of the most studied and significant reactions in green and sustainable chemistry. In this reaction, the Lewis acidic RE^III site activates the epoxide molecule towards the nucleophilic attack of the Lewis base (X^- = Cl^-, Br^-, I^-), leading to the epoxide’s ring opening.

Then CO₂ insertion occurs, forming a carbonate intermediate that undergoes intramolecular ring closure to release the cyclic carbonate. Lanthanum complex [La(L)(THF)] stabilized by tris(phenolato) ligand (L^3- = 2,2’-[({2-[{[3,5-di-t-butyl-2-(hydroxy)phenyl]methyl}(methyl)amino]ethyl}azanediyl)bis(methylene)]bis(4,6-di-t-butylphenolato)}) is a rare example of catalyst that enables the cycloaddition of terminal epoxides with CO₂ under mild conditions (i.e., 25 °C, 1 bar CO₂, 0.3 mol% cat., TBAI 0.6 mol%), leading to cyclic carbonates with a yield of 49–99%, as seen in Figure 21 [90]. Several other studies have shown that synergistic interactions between different metal centers can improve the catalytic activity of the catalyst and the selectivity of this reaction [91]. The commonly accepted mechanism assumes simultaneous activation of epoxide and CO₂ on adjacent metal centers.

For example, [ZnLa₂(OBn)₂(L)₂] for L^3- = 2,2’-[{[2-(2-oxidoethoxy)ethyl]azanediyl}bis(methylene)]bis(4,6-di-t-butylphenolato), a catalyst presented in Figure 22, showed 2 to 8 times better catalytic activity in the reaction of CO₂ and 1,2-epoxyhexane (0.5 mol% cat., 1 mol% TBAB, 25 °C, 24 h, 1 atm CO₂) than [La₂(L)₂(THF)₂] or zinc aryloxide [Zn(OBn)₂] [92].

Observed enhancement of the catalytic activity of heterometallic [EtZnY(L)(THF)] (L^4- = N,N,N’,N'-tetrakis(3,5-di-t-butyl-2-oxybenzyl)ethane-1,2-diamine)), occurs as a result of the presence of Zn^II centers, which allows for better delocalization of the electron density in the complex and drastically changes the energy barrier of the ring opening step by decreasing the electrostatic repulsion between yttrium center and bromine anion from the cocatalyst. Therefore, the epoxide conversion (0.2 mol% cat., 0.8 mol% TBAB, 40 °C, 18 h, 1 atm CO₂) of 81% for heterometallic catalyst was much better than for [Y(HL)(THF)] amounting to 41% or Zn(OAc)₂ 16% [93]. Another example of catalysts with a synergistic interaction between RE^III and Zn^II Lewis acidic sites were [Zn₄RE₂(μ₃-OH)₂L₄(OAc)₂(NO₃)₂(DMF)₂] (for RE^III = Dy^III, Nd^III, Tb^III; L^2- = N-[(3-methoxy-2-oxidophenyl)methylidene]pyridine-3-carbohydrazonato), which showed higher catalytic activity than equivalent amounts of a Zn^II salt, RE^III salts, and a ligand mixture of each. The structure of the mentioned complexes can be found in Figure 23 [94].

Excellent catalytic activity during cycloaddition between CO₂ and styrene oxide (0.01 mol% cat., 0.8 mol% TBAB, 80 °C, 1 bar CO₂) was also reported for a series of heterometallic clusters [Zn₂RE₂(μ₃-OH)₂L₄(NO₃)₄] (for RE^III = Eu^III, Tb^III, Er^III, Yb^III, Nd^III; L^- = 2-methoxy-6-(methoxycarbonyl)phenolato), which convert from 88 to 93% of epoxide within 14h.[95] There are also several examples of heterometallic lanthanide−zinc clusters that are less efficient than their homometallic counterparts due to the steric effect of the ligands and crowded coordination environments [96].

A reaction of great interest in the synthesis of cyclic carbonates is the oxidative carboxylation of olefins. However, so far, only a few lanthanide-based MOFs have been investigated as heterogeneous catalysts for this purpose. The application of [Nd₂(BIPA-TC)_1.5]_n, [Eu(H₂BIPA-TC)(BIPA-TC)_0.5]_n or [Tb(H₂BIPA-TC)(BIPA-TC)_0.5]_n as catalysts in the reaction of styrene, tert-butyl hydroperoxide, and CO₂, gave 80 – 87% cyclic carbonate after 10 h (0.18 mol % of MOF cat., 1 bar of CO₂, 80 °C). The proposed mechanism for this reaction can be found in Figure 24 below [97].

Much attention has been paid to the construction of C–N bonds through CO₂ fixation, but these reactions require high temperatures and pressures and the use of equivalent amounts of base. The bis(amidate) lanthanide complex (Figure 25) [Ln₂L₂(N(SiMe₃)₂)₂(THF)₂] (Ln^III = Eu^III, Yb^III; L^- = N-(2,6-diisopropylphenyl)benzenecarboximidato) showed good catalytic performance in the synthesis of 2,4-quinazolidinones from CO₂ and 2-aminobenzonitriles (5 mol % cat., 5mol% DBU, 1 bar of CO₂, 100 °C, 24h) leading to final product with a yield of 61 or 91% [98].

Other rare earth amides [RE₂L₂(N(SiMe₃)₂)₂(THF)₂] (RE^III = La^III, Nd^III, Y^III; LH₂ = N, N’-(cyclohexane-1,2-diyl)bis(4-tert-butylbenzamide) turned out to be effective catalysts for the direct carboxylation of terminal alkynes at ambient pressure, leading to the formation of the C–C bond and the synthesis of acetylenic carboxylic acids with a yield of 80-89% (phenylacetylene (1.0 mmol), Cs₂CO₃ (3.0 mmol), cat. (0.04 mmol), 1 atm of CO₂, 60 °C, DMSO (10 mL)) [99].

The transformation of CO₂ into macromolecular compounds can be carried out by direct copolymerization of CO₂ with epoxides/aziridines, polycondensation with amines, alcohols, and amino alcohols, or by the synthesis of CO₂-based monomers that will be used in polymerization reactions dependent on the nature and functionality of the appropriate monomer. These synthetic routes enable the production of a wide range of polycarbonates (PC), polyurethanes (PU), polyureas (PUA), and polyesters using versatile polymerization techniques, including ring-opening polymerization (ROP), ring-opening copolymerization (ROCOP), polycondensation and terpolymerization. However, for synthesizing CO₂-based polymers, the low reactivity of CO₂ or its derivatives requires elevated temperatures, removal of by-products, and activation of the monomers by multiple metal centers to complete the reaction.

Direct polymerization routes to CO₂-based polymers are mainly limited to the ROCOP of CO₂ with epoxides or aziridines. CO₂ is also used as a comonomer in the synthesis of multiblock copolymers by sequential monomer addition and tandem approach [100].

In the indirect approach, CO₂ is first converted into linear or cyclic building blocks, which are then used to synthesize polymeric materials. Cyclic monomers (carbonates, carbamates) can be ring-opening polymerized to form the corresponding homopolymers (PCs, PUs). Cyclic 5-membered carbonates can also copolymerize with diols, diamines, diamines, and diols or cyclic ureas to form PCs, PUAs, or PUs [101]. Copolymerization of cyclic carbonates with lactones is also widely studied to obtain copolyesters.

Copolymerization of CO₂ and epoxides is the most investigated method of synthesis of PCs [102]. High molecular weight polycarbonates exhibit properties suitable for replacing petrochemical polymers in sectors including packaging, coatings, rigid plastics, and medical materials. Recently, developing and understanding the catalytic performance of heterometallic catalysts that exhibit metal synergy are among the most frequently studied aspects of the synthesis of biodegradable polymers. It was shown that some heterometallic complexes having two different metal centers showed much higher activity than their homometallic counterparts. It is assumed that different metals play different roles in copolymerization; the stronger Lewis acid activates the epoxide, and the softer Lewis acid forms a labile bond with the carbonate of the propagating chain, making the carbonate more nucleophilic. Several heterometallic zinc-lanthanide complexes have been shown to initiate the copolymerization of CO₂ and cyclohexene oxide (CHO).

For example, the heterometallic cluster pictured in Figure 26, [Zn₂Nd₂(μ-OBn)₂(L)₂] (for L^2- = (((2-(bis(3,5-di-t-butyl-2-oxidobenzyl)amino)phenyl)amino)methyl)-4,6-di-t-butylphenolato)) gave polycarbonates with high molecular weight (Mn up to 295.8 kDa) with narrow dispersity (Đ = 1.65) and high selectivity (99%) at [CHO]:[cat.] = 2000:1 ratio, 25 °C, 12h and 7 bar CO₂, and its reactivity was nine times greater than that of the isostructural yttrium catalyst [103]. Using [ZnLn(L)(L^’)(NSiHMe₂)₂] (for L^- = (cyclohexane-1,2-diylbis(azanylylidenemethylylidene))bis(6-methylphenolato); L’^- = 2,2’-{[(2-methoxyethyl)azanediyl]bis(methylene)}bis(4,6-di-t-butylphenolato) as catalysts for CO₂/CHO copolymerization, it was shown that the radius of the RE^III ion significantly influences their activity. The most effective catalysts turned out to be Zn^IIDy^III and Zn^IISm^III complexes containing lanthanide ions with a moderate ionic radius, leading to polycarbonates with M_n = 148 kDa, Đ = 1.52–1.62, and selectivities of 99% (carbonate bonds). Another example was heterometallic Zn₃RE clusters (RE^III = La^III, Ce^III, Pr^III, Nd^III, Sm^III, Eu^III, Gd^III, and Dy^III) based on macrocyclic tri(salen) ligands, which showed a unique and rapid exchange of intra- and intermolecular acetate ligands. Lanthanide complexes with larger ionic radii (La^III, Ce^III, Pr^III, Nd^III) showed higher catalytic activity than those based on smaller lanthanides (Sm^III, Eu^III, Gd^III, Dy^III). The most active of them was Zn₃Ce, which enables the synthesis of polycarbonates with M_n = 14 kDa, Đ = 1.3 at 100 °C within 3 h [104].

All the above-mentioned heterometallic catalysts for CO₂ copolymerization operate via the chain-shuttling mechanism in which the Lewis acidic RE^III enhances CHO coordination, and the unstable Zn-carbonate bond enhances nucleophilic attack to open the epoxide ring. The resulting RE-alkoxide and CO₂ coordinated to Zn^II form a carbonate complex, leading to the growth of the polycarbonate chain [105]. Introducing Co^II instead of Zn^II into heterometallic complexes led to the discovery of one of the most efficient multimetallic catalysts, Co₃Nd, which had a turnover number (TON) of 13,000. This gave a polymer with >99% selectivity of carbonate bonds, M_n = 114 kDa and Đ = 1.05 at 2 MPa CO₂, 130 °C after 8 h. Pathway of that reaction may be found in Figure 27 [106].

2.5. Catalysts for energy conversion processes

Recently, particular attention was placed on using 3d-4f clusters as promising catalysts for energy conversion processes like hydrogen evolution reaction (HER), oxygen evolution reaction (OER), overall water splitting, and CO₂ reduction. The synergic cooperation between both RE^III/M^II centers led to their enhanced activity. For example, [Ni₃₆Gd₁₀₂(μ₃-OH)₁₃₂(L)₁₈(L’)₁₈(H₂L’)₂₄(OAc)₈₄(SO₄)₁₈(NO₃)₁₈(H₂O)₃₀]Br₆,(NO₃)₆ (Ni₃₆Gd₁₀₂, LH = 2-mercapto-5-methyl-1,3,4-thiadiazide; L’H₃ = 2,2-dimethylol propionic acid) found in Figure 28 shows remarkable activity in photocatalytic CO₂ reduction, providing a TON of 29700 and a turnover frequency (TOF) of 1.2 s⁻¹ over 10 h with a selectivity of 90.2% for CO formation. This performance is much better than those of most homogeneous CO₂-reduction catalysts because the Lewis-acidic Gd^III modulates the electronic structure of the catalytic Ni^II centers, enhancing photocatalytic activity [107].

[Co₁₂Eu₃₆(μ₄-O)₆(μ₃-OH)₈₄(OAc)₁₈(Cl)₂(NO₃)]³³⁺ shows effective water oxidation activity under acidic conditions (TOF of 1.5 s-1 at 1.8 V) owing to the synergistic effect of Eu^III and Co^II ions on O–O bond formation [108]. Heterometallic cooperativity in water oxidation has also been reported for [Mn₂RE₂(O₂CMe)₆(pdmH)₂₍L)](NO₃) (RE^III = Dy^III and Gd^III; pdmH₂ = 2,6-pyridine dimethanol; LH₂ = (6-hydroxymethylpyridin-2-yl)-(6-hydroxymethylpyridin-2-ylmethoxy-methanol)) which structure can be found in Figure 29 [109].

The series [Co₃RE(hmp)₄(OAc)₅(H₂O)], where RE = Ho^III, Er^III, Tm^III, or Yb^III (hmpH = 2-(hydroxymethyl)pyridine)), [110] and [NdCo₃(btp)₂(OAc)₂(NO₃)₂](NO₃) (btp = 2,6-bis(1,2,3-triazol-4-yl)pyridine) anchored in phospho-doped graphitic carbon nitride, [111] are further examples of water-oxidation catalysts, being mimetics of the {CaMn₄O₅} oxygen-evolution complex in photosystem II. [RE₅₂Ni₅₆(IDA)₄₈(OH)₁₅₄(H₂O)₃₈]¹⁸⁺ clusters (RE = Pr^III, Eu^III, Gd^III; IDA = iminodiacetate) supported on CdS forms lanthanide–transition metal catalysts RE₅₂Ni_56-xCd_x/CdS that show activity for photocatalytic hydrogen evolution. The high photocatalytic efficiency of 25353 μmol h⁻¹ g⁻¹ was achieved usingNi_56-xCd_xEu₅₂/CdS, which enhances H₂ production under visible-light irradiation (≥ 420 nm) owing to the formation of catalytic Eu^II centers by the transfer of photoexcited electrons from CdS to the LUMO of Eu^III [112]. Furthermore, the iodide-templated 3D-coordination polymer {[Cu₅Eu₂(OH)₂(pydc)₆(H2O)₈]·I₈}_n (pydc = pyridine-2,6-dicarboxylate) has been demonstrated to be an efficient photocatalyst for H₂ production under UV irradiation, providing H₂ evolution at a rate of 2262 μmol h⁻¹ g⁻¹ [113].

2.6. Molecular precursors of functional inorganic materials.

The use of heterometallic 3d−4f compounds as single-source molecular precursors for the synthesis of functional inorganic materials is also very limited. One of the few examples is the series of isostructural compounds [Fe₂Ln₂((OCH₂)₃CR)₂(O₂C^tBu)₆(H₂O)₄] (Ln = La, Gd and R = Me, Et), which were used to prepare lanthanide orthoferrite perovskites LnFeO₃ [114]. Another example is the use of an equimolar mixture of (NH₄)[Ln(EDTA)] (Ln^III = Pr^III, Sm^III, Eu^III, Gd^III, Dy^III, Er^III) and (NH₄)₃[V(O)₂(EDTA)] at 800 °C for the preparation of lanthanide vanadates (LnVO₄) [115]. Furthermore, the solid-phase thermal decomposition of [Ln(Mn(CO)₃Cp^COOH)₂(OAc)(MeOH)]_n (Ln^III = Nd^III, Gd^III, Dy^III), [Ln₂(Mn(CO)₃Cp^COOH)₄(OAc)₂(H₂O)₄] (Ln^III = Ho^III, Er^III, Tm^III), and [Ln₂(Mn(CO)₃Cp^COOH)₄(NO₃)₂(DME)₂] (Ln^III = Eu^III, Tb^III) at 670–900 °C has been investigated for the synthesis of metamagnetic LnMn₂O₅ [116,117].

Various synthesis methods have been explored in the literature to synthesize La₂CuO₄. One of them involves the thermal decomposition of an amorphous precursor [La₂Cu(DTPA) _1.6]·6H₂O[118] (H₅DTPA = diethylenetriaminepentaacetic acid) initially at 450 °C to eliminate volatile organic constituents, and then by heat treatment at 650 °C yielded La₂CuO₄ with an average crystallite size of around 29 nm. An alternate approach to synthesize La₂CuO₄ included hydrolyzing a mixture of [La₄(CO₃)(O₂CNBu₂)₁₀] and [Cu(O₂CNBu₂)(py)₂] (py = 4-dimethylaminopyridine) in a toluene solution, resulting in the isolation of the compound [La₂Cu(CO₃)₄]·5H₂O [119]. The subsequent procedure involved heating the isolated precursor at 600°C, yielding a tetragonal polymorph with an average crystallite size of 15 nm. Above 850°C, the tetragonal form transformed into a rhombohedral phase with crystallites of sizes of 50 nm.

Recently, we have developed a simple and efficient synthetic strategy for the preparation of industrially important heterometallic perovskite-type materials of LaMnO₃, GdMnO₃, NdMnO₃, Pr_0.9MnO₃ (Figure 30)_, and PrCoO₃ by thermal decomposition of heterometallic 3d–4f alkoxides [Mn₂Ln₄(µ₆-O)(µ₃-OR)₈(HOR)₄Cl₆] (Ln = La^III, Nd^III, Gd^III); [Co₂Pr₄(µ₆-O)(µ₃-OR)₈(HOR)₂Cl₆]; and [Mn₂Pr₄(µ₃-OH)₂(µ₃-OR)₄(µ-OR)₄(µ-Cl)₂(HOR)₄Cl₆], which molecular structures can be found in Figure 31, and external [MnCl₂(HOR)]_n or [Co₄(µ₃-OR)₄(HOR)₄Cl₄] at 1100 °C [30]. When we performed the thermolysis of [Ni₂Pr₄(µ₆-O)(µ₃-OR)₈(HOR)₄Cl₆] and [NiCl₂(HOR)₂] the formation of a mixture of the homo- and heterometallic oxides PrOCl, PrO₂, NiO, PrNiO₃, and Pr₂NiO₄ was observed. The representative PXRD pattern of Pr_0.9MnO₃ is presented in Figure 32. The synthesis of 3d–4f precursors was performed using an uncommon synthetic method involving the reaction of metallic lanthanides (Ln^III = La^III, Pr^III, Nd^III, Gd^III) with divalent transition metal chlorides (MCl₂, where M = Mn^II, Ni^II, or Co^II) using 2-methoxyethanol (ROH) as the solvent and ligand precursor.

Another work reported that group 4–lanthanide ethoxides are attractive starting materials for the production of pyrochlore type phases Ln₂M₂O₇ of considerable interest because of their use as materials for thermal coatings of turbine components for protection against hot and corrosive gas streams [120], high-temperature electrolytes in solid-oxide fuel cells [121], or radiation-resistant materials [122]. Thermal decomposition of the isostructural compounds [Ln₂Ti₄(µ₄-O)₂(µ₃-OEt)₂(µ-OEt)₈(OEt)₆(HOEt)₂Cl₂] (Ln^III = La^III, Nd^III) at 950 °C gave La_0.66TiO₃ or a mixture of Nd₄Ti₉O₂₄ and Nd_0.66TiO₃. Calcination of [M₂La₂(µ₃-O)(µ-OEt)₅(µ-Cl)(OEt)₂(HOEt)₄Cl₄]_n and [M₄Nd₄(µ₃-O)₂(µ-OEt)₁₀(µ-Cl)₄(OEt)₈(HOEt)₁₀Cl₂] (M^IV = Zr^IV, Hf^IV) at 950–1500 °C led to the selective formation of heterometallic La₂Zr₂O₇, La₂Hf₂O₇, Nd₂Zr₂O₇, and Nd₂Hf₂O₇ phases, respectively [31].

3. Conclusions

Discussed here, heterometallic rare-earth-transition metal, alkaline earth, or alkali metal complexes are attractive molecular materials with a number of applications in energy storage and conversion process, molecular magnetism, organic synthesis, material and polymer engineering. This review focused on the correlation of molecular structures of heterometallic compounds with displayed magnetic, catalytic, or photocatalytic properties. The general routes for synthesizing heterometallic rare-earth complexes with various solid-state structures, efficiency, and reproducibility were also presented. We discussed catalytic applications in asymmetric synthesis, CO2 transformations, polymerization/copolymerization of heterocyclic monomers, water splitting, and hydrogen or oxygen evolution reactions in the context of multimetallic synergy and cooperativity effects. Furthermore, the transformative potential of these complexes into advanced nanomaterials with diverse applications underscores their significance in the field of materials science. The complexes contribute to understanding fundamental chemical principles and open new routes for applied research and technological advancements. The intricate synergy between metal ions in these complexes unravels a wealth of possibilities, inspiring many researchers to further studies. Heterometallic rare-earth complexes hold promise as key players in developing innovative materials and technologies.