Monometallic Analogues Research Articles

Ligand‐Directed Assembly of Multimetallic Aluminum Complexes: Synthesis, Structure and ROP Catalysis

AbstractMultinuclear aluminum complexes are useful catalysts for the ring opening polymerization (ROP) of ϵ‐caprolactone (ϵ‐CL), often outperforming monometallic analogues in the production of biodegradable polycaprolactone (PCL). These complexes require control over metal‐metal distance, ligand structure, and substituents. Herein, we report on the preparation of mono‐, bi‐, and octametallic aluminum complexes through direct chelation of multidentate bis(phosphino)‐2,2‐dimethyl‐1,3‐diamino ligands Me2C[CH2NHR2P(O)]2; R=Ph(1), iPr(2), tBu(3). Ligands 1–3 yielded bimetallic complexes with two types of coordination pockets, as well as bimetallic systems with identical coordination environments. In addition to the diversity of structural patterns within the present series, the catalytic performances of [AlR2{κ2‐N,N’,–(Me2C{CH2NPh2P(O)}2}‐(κ2‐O,O’‐AlR2)] (R=Me(4), Et(5), iBu(6)) outperformed that of the monometallic [AlEt2{κ2‐O,O’‐(Me2C(CH2NtBu2P(O))2H} (10), showing controlled and immortal ROP of ϵ‐CL in the presence of BnOH as co‐initiator. DFT calculations suggest that the primary active site for 4–6 is the N−Al–N center. Gaining insight into the underlying principles that lead to high activity and controlled production of PCL in bimetallic systems can help design tailor‐made complexes with enhanced performance.

Hydrogenation Reactions with Heterobimetallic Complexes

AbstractHydrogenations are fundamentally and industrially important reactions that are atom economical paths to synthesize value‐added products from feedstock chemicals. The cooperative effects of two or more metal centers in multimetallic active sites is a successful strategy to activate small molecules and facilitate catalytic reactions, and this strategy has been recently applied to catalytic hydrogenation reactions. Furthermore, heterobimetallic complexes have been well‐documented to provide novel reaction pathways and improved selectivity, compared to their homo‐bimetallic and monometallic analogues. This minireview provides a historical perspective on the development of heterobimetallic catalysts for the hydrogenation of unsaturated substrates and describes recent developments in this burgeoning research area.

Hydrogenation Reactions with Heterobimetallic Complexes.

Hydrogenations are fundamentally and industrially important reactions that are atom economical paths to synthesize value-added products from feedstock chemicals. The cooperative effects of two or more metal centers in multimetallic active sites is a successful strategy to activate small molecules and facilitate catalytic reactions, and this strategy has been recently applied to catalytic hydrogenation reactions. Furthermore, heterobimetallic complexes have been well-documented to provide novel reaction pathways and improved selectivity, compared to their homo-bimetallic and monometallic analogues. This minireview provides a historical perspective on the development of heterobimetallic catalysts for the hydrogenation of unsaturated substrates and describes recent developments in this burgeoning research area.

Unsymmetric Co-Facial "Salixpyrrole" Hydrogen Evolution Catalysts: Two Metals are Better than One.

Designing ligand architectures that can mimic enzyme active sites is a promising approach for developing efficient small molecule activation catalysts for sustainable energy applications. Some key design features include chemically distinct binding pockets for multiple metal centers and a three-dimensional structure that controls the positioning of catalytic sites. With these principles in mind, mono- and bimetallic unsymmetric cofacial palladium complexes, 2 and 3, respectively, bearing ligands with calixpyrrole and salen coordination sites, or "salixpyrrole" ligands, are reported. These species were accessed in a straightforward Schiff-base reaction with appreciable yields. In addition, both 2 and 3 were found to be active hydrogen evolution electrocatalysts using para-toluenesulfonic acid monohydrate as the proton source. The two salixpyrrole species displayed different mechanisms of action, with 2 showing a second-order dependence on acid concentration, whereas 3 exhibited a first-order dependence. Moreover, the bimetallic catalyst was significantly more efficient, with higher turnover frequencies, 4640 s-1 vs 1680 s-1 for 2, and lower overpotentials, 0.39 V vs 0.69 V for 2. The results reported herein provide proof-of-concept that bimetallic catalysts with chemically distinct binding sites demonstrate enhanced catalytic properties in comparison to monometallic or symmetric analogues.

Synthesis, Characterization, and Catalytic Performance of a New Heterobimetallic Y/Tb Metal-Organic Framework with High Catalytic Activity.

A three-dimensional heterobimetallic porous structure with the formula {[Y3.5Tb1.5L6(OH)3(H2O)1.5 (DMF)1.5] n ·1.5H2O·DMF} n (L = 3-amino-4-hydroxybenzoate) (Y/Tb-MOF) has been synthesized and characterized by single crystal and powder X-ray diffraction, scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), inductively coupled plasma mass spectrometry (ICP-MS), electrophoretic mobility, and Fourier transform infrared (FTIR) spectroscopy. The structure presents two metal environments: a bioaugmented isosceles wedge (mm2) MO8 and a tricapped trigonal prism (-6m2) MN3O6. These configurations facilitate the creation of channels with a diameter of 10.7 Å, enabling its utilization as an active catalyst where the heterobimetallic nature of the assembly will be explored. This mixed-metal metal-organic framework has been tested in the cycloaddition of epoxides with carbon dioxide as well as in the cyanosilylation and hydroboration reactions of carbonylic substrates. Additionally, a monometallic Tb-MOF analogue has been synthesized for comparative evaluation of their catalytic performances. Both the mixed metal and monometallic variants exhibit outstanding activity in the cyanosilylation and hydroboration of carbonyls and in the synthesis of carbonates under CO2 pressure. However, only the latter exhibits high recyclability.

To Be or Not To Be… Cooperative? Trimetallic Catalysts for the Ring‐Opening Polymerisation of Cyclic Esters

AbstractMultimetallic catalysts have emerged as a promising route to enhance catalyst performance in cyclic ester ring‐opening polymerisation (ROP), as these complexes can outperform the monometallic analogues in terms of reactivity and/or polymerisation control. Such enhancements are often attributed to “multimetallic cooperativity”, yet the origins of this cooperativity often remain unclear. Here, we report the synthesis and characterisation of two trimetallic Al‐salen complexes (L1(AlMe)3 and L1(AlEt)3), containing three Al‐salen subunits in close proximity, and their monometallic analogues (L2(AlMe) and L2(AlEt)). These complexes have been applied as catalysts for cyclic ester ROP under a range of conditions, to probe their potential for cooperative behaviour. Trimetallic L1(AlMe)3 and L1(AlEt)3 both display high activities in rac‐lactide ROP, giving kobs values of up to 11×10−3 min−1. Yet careful benchmarking against the monometallic complexes reveals that whether these systems display multimetallic cooperativity or not depends upon the reaction conditions. Detailed kinetic and spectroscopic studies into the origins of cooperativity revealed that the neighbouring metals play a key role in the initiation step, rather than propagation. Overall, these studies highlight the importance of understanding the reaction conditions in order to accurately define whether a catalyst displays multimetallic cooperativity.

Exploiting Multimetallic Cooperativity in the Ring-Opening Polymerization of Cyclic Esters and Ethers.

The use of multimetallic complexes is a rapidly advancing route to enhance catalyst performance in the ring-opening polymerization of cyclic esters and ethers. Multimetallic catalysts often outperform their monometallic analogues in terms of reactivity and/or polymerization control, and these improvements are typically attributed to "multimetallic cooperativity". Yet the origins of multimetallic cooperativity often remain unclear. This review explores the key factors underpinning multimetallic cooperativity, including metal-metal distances, the flexibility, electronics and conformation of the ligand framework, and the coordination environment of the metal centers. Emerging trends are discussed to provide insights into why cooperativity occurs and how to harness cooperativity for the development of highly efficient multimetallic catalysts.

Tuning Pd-to-Ag Ratio to Enhance the Synergistic Activity of Fly Ash-Supported PdxAgy Bimetallic Nanoparticles.

Fly ash (FA)-supported bimetallic nanoparticles (PdxAgy/FA) with varying Pd:Ag ratios were prepared by coprecipitation of Pd and Ag involving in situ reduction of Pd(II) and Ag(I) salts in aqueous medium. All the supported nanoparticles were thoroughly characterized with the aid of powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), electron microscopy (field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM)), and elemental analyses, which include inductively coupled plasma-optical emission spectroscopy (ICP-OES) and energy-dispersive X-ray spectroscopy (EDS). A gradual broadening and shifting of PXRD peaks, ascribable to Ag, to higher angles with an increase in the Pd:Ag ratio affirms the alloying of interface between Pd and Ag nanoparticles. The coexistence of Pd and Ag was further confirmed by EDS elemental mapping as well as by the presence of bimetallic lattices on the FA surface, as evident from the high-resolution TEM analysis. The dependency of crystallite size and average size of bimetallic nanoparticles on Ag loading (mol %) was elucidated with the help of a combination of PXRD and TEM studies. Based on XPS analysis, the charge transfer phenomenon between contacting Pd-Ag sites could be evident from the shifting of 3d core electron binding energy for both Pd and Ag compared with monometallic Pd and Ag nanoparticles. Following a pseudo-first-order reaction kinetics, all the nanocatalysts were able to efficiently reduce 4-nitrophenol into 4-aminophenol in aqueous NaBH4. The superior catalytic performance of the bimetallic nanocatalysts (PdxAgy/FA) over their monometallic (Pd100/FA and Ag100/FA) analogues has been demonstrated. Moreover, the tunable synergistic effect of the bimetallic systems has been explored in detail by varying the Pd:Ag mol ratio in a systematic manner which in turn allowed us to achieve an optimum reaction rate (k = 1.050 min-1) for the nitrophenol reduction using a Pd25Ag75/FA system. Most importantly, all the bimetallic nanocatalysts explored here exhibited excellent normalized rate constants (K ≈ 6000-15,000 min-1 mmol-1) compared with other supported bimetallic Pd-Ag nanocatalysts reported in the literature.

Copper Doping Boosts Electrocatalytic CO2 Reduction of Atomically Precise Gold Nanoclusters.

Unraveling the atomistic synergistic effects of nanoalloys on the electrocatalytic CO2 reduction reaction (eCO2RR), especially in the presence of copper, is of paramount importance. However, this endeavor encounters significant challenges due to the lack of the crystallographically determined atomic-level structure of appropriate monometallic and bimetallic analogues. Herein, we report a one-pot synthesis and structure characterization of a AuCu nanoalloy cluster catalyst, [Au15Cu4(DPPM)6Cl4(C≡CR)1]2+ (denoted as Au15Cu4). Single-crystal X-ray diffraction analysis reveals that Au15Cu4 comprises two interpenetrating incomplete, centered icosahedra (Au9Cu2 and Au8Cu3) and is protected by six DPPM, four halide, and one alkynyl ligand. The Au15Cu4 cluster and its closest monometal structural analogue, [Au18(DPPM)6Br4]2+ (denoted as Au18), as model systems, enable the elucidation of the atomistic synergistic effects of Au and Cu on eCO2RR. The results reveal that Au15Cu4 is an excellent eCO2RR catalyst in a gas diffusion electrode-based membrane electrode assembly (MEA) cell, exhibiting a high CO Faradaic efficiency (FECO) of >90%, and this efficiency is substantially higher than that of the undoped Au18 (FECO: 60% at -3.75 V). Au15Cu4 exhibits an industrial-level CO partial current density of up to -413 mA/cm2 at -3.75 V with the gas CO2-fed MEA, which is 2-fold higher than that of Au18. The density functional theory (DFT) calculations demonstrate that the synergistic effects are induced by Cu doping, where the exposed pair of AuCu dual sites was suggested for launching the eCO2RR process. Besides, DFT simulations reveal that these special dual sites synergistically coordinate a moderate shift in the d-state, thus enhancing its overall catalytic performance.

Bimetallic Single-Atom Catalysts for Electrocatalytic and Photocatalytic Hydrogen Production

Electrocatalytic and photocatalytic hydrogen evolution reactions (HERs) provide a promising approach to clean energy generation. Bimetallic single-atom catalysts have been developed and explored to be advanced catalysts for HER. It is urgent to review and summarize the recent advances in developing bimetallic single-atom HER catalysts. Firstly, the fundamentals of bimetallic single-atom catalysts are presented, highlighting their unique configuration of two isolated metal atoms on their supports and resultant synergistic effects. Secondly, recent advances in bimetallic single-atom catalysts for electrocatalytic HER under acidic/alkaline conditions are then reviewed, including W-Mo, Ru-Bi, Ni-Fe, Co-Ag, and other dual-atom systems on graphene and transition metal dichalcogenides (TMDs) with enhanced HER activity versus monometallic analogs due to geometric and electronic synergies. Then, photocatalytic bimetallic single-atom catalysts on semiconducting carbon nitrides for solar H2 production are also discussed. Finally, an outlook is provided on opportunities and challenges in precisely controlling bimetallic single-atom catalyst synthesis and gaining in-depth mechanistic insights into bimetallic interactions. Further mechanistic and synthetic studies on bimetallic single-atom catalysts will be imperative for developing optimal systems for efficient and sustainable hydrogen production.

Investigating Periodic Table Interpolation for the Rational Design of Nanoalloy Catalysts for Green Hydrogen Production from Ammonia Decomposition

Developing highly active catalysts for the decomposition of ammonia to produce hydrogen is an important goal in the context of renewable energy. Allied with this is a need for identification strategies to efficiently design novel catalysts integral to ensuring rapid progress in this research field. We investigated the efficacy of N–binding energy and periodic table interpolation to predict active bimetallic nanoparticle catalysts. Supported iron-platinum and iron-palladium were identified and experimentally shown to be more active than their monometallic analogues. Atomic resolution electron microscopy indicated that the most active catalyst (5 wt% Fe80Pt20/γ-Al2O3) was principally formed of alloyed nanoparticles. It restructured during testing, yet no activity loss was noted at 20 h time-on-line. While these findings show that periodic table interpolation may be a viable tool for identifying active combinations of metals, the activity of the catalysts in the current work were not able to outperform the Ru/Al2O3 benchmark. Further catalyst optimization or refinement of reaction descriptors may facilitate the development of catalysts with higher intrinsic activity than the current state-of-the-art catalysts.Graphical

Removal of gaseous formaldehyde by portable photocatalytic air purifier equipped with bimetallic Pt@Cu-TiO2 filter

In this research, a mini-scale air purifier (AP) system was built with a series of honeycomb (HC) ceramic filters coated with 1 wt% Pt@Cu/TiO2 (AP (PCT-x), where x = 1, 2, and 3 for Pt:Cu weight ratios). The designed AP (PCT-x) photocatalytic systems were investigated for the removal of formaldehyde (FA) vapor under varying operational conditions (e.g., FA concentration (0.5–5 ppm), AP-flow rate (100–160 L/min), and UV light intensity (0.004–0.982 W)). The enhanced photocatalytic activities of AP (PCT-x) system (e.g., compared to the monometallic analogues) are suspected to reflect the synergistic role of Pt metal sites (in terms of high work function (5.93 eV) for the 111 crystal plane) in the Pt@Cu bimetallic structures and the high electronic transitions between Pt and Cu sites on the TiO2 surface. Nonetheless, their photocatalytic activities tend to decrease at higher Pt content (e.g., PCT-3 relative to PCT-2) due possibly to the formation of Pt clusters and associated shielding effect. Overall, AP (PCT-2) exhibited the highest efficacy (over five reuse cycles) to attain 100% removal of FA (0.5 ppm) within 5 min at an AP-flow rate of 160 L/min and UV light intensity of 0.982 W (quantum yield (QY) of 1.94E-04 molecules/photon and figure of merit (FOM) of 1.21E-06 L∙mol/ (mg∙ J∙h)). According to the in-situ DRIFTS analysis, Pt2@Cu metal sites were favorable to promote the transfer of photo-induced charge carriers by TiO2 so as to accelerate the oxidation reaction of FA vapor (e.g., through the generation of OH• and O2•- radicals). This study offers practical insights into the potential of controlled tuning of redox dynamics (i.e., between Pt and Cu metal sites on TiO2) in the enhancement of the photocatalytic performance for air purification system.

Selective Oxidation of Methane to Methanol via In Situ H2O2 Synthesis.

The selective oxidation of methane to methanol, using H2O2 generated in situ from the elements, has been investigated using a series of ZSM-5-supported AuPd catalysts of varying elemental composition, prepared via a deposition precipitation protocol. The alloying of Pd with Au was found to offer significantly improved efficacy, compared to that observed over monometallic analogues. Complementary studies into catalytic performance toward the direct synthesis and subsequent degradation of H2O2, under idealized conditions, indicate that methane oxidation efficacy is not directly related to H2O2 production rates, and it is considered that the known ability of Au to promote the release of reactive oxygen species is the underlying cause for the improved performance of the bimetallic catalysts.

Colorimetric and fluorescence detection of circulating tumor cells based on a bimetallic-organic framework

An ultrasensitive colorimetric and fluorescence dual-readout method was developed for detecting circulating tumor cells (CTCs) using a CuFe-based bimetallic-organic framework (CuFe-NH2-MIL-101, bi-MOF). Incorporation of Cu into an Fe-based metal–organic framework (MOF) led to improved fluorescence compared with that of monometallic analogues. The synthesized bi-MOF exhibited improved fluorescence. The synthesized bi-MOF also exhibited enzyme-mimetic activity. The CTCs were bound by biotinylated antibodies and then selected and enriched by streptavidin-coated magnetic beads. With peroxidase-mimicking activity and enhanced fluorescence, the CuFe bi-MOF functioned as a signal-amplifying nanoprobe for colorimetric and fluorescence detection. The absorbance increased with increasing concentration of tumor cells, whereas the fluorescence intensity gradually decreased. Under optimized conditions, the sensor exhibited a detection range of 102–107 cells mL−1. The detection limit of this cytosensor could reach 7 cells mL−1. Moreover, this sensor showed reliable applicability in real serum samples. Overall, this dual-readout strategy holds potential for the detection of CTCs in clinical diagnostics.

Selective Ammoximation of Ketones via In Situ H2O2 Synthesis

The ammoximation of ketones to the corresponding oxime via the in situ production of H2O2 offers a viable alternative to the current means of industrial-scale production, in particular for the synthesis of cyclohexanone oxime, a key precursor to Nylon-6. Herein, we demonstrate that using a bifunctional catalyst, consisting of Pd-based bimetallic nanoparticles immobilized onto a TS-1 carrier, it is possible to bridge the considerable condition gap that exists between the two key distinct reaction pathways that constitute an in-situ approach (i.e., the direct synthesis of H2O2 and ketone ammoximation). The formation of PdAu nanoalloys is found to be crucial in achieving high reactivity and in promoting catalytic stability, with the optimal formulation significantly outperforming both alternative Pd-based materials and the monometallic Pd analogue.

Synthesis of bimetallic rhodium phosphinine complexes with enhanced catalytic activity towards alkyne hydrosilylation

Reaction of alkynes with PCcarbeneP complex 1 (featuring an alkylidene anchored pincer ligand) results in the formation of η3-benzo-λ5-phosphinine rhodium complexes 2. Reacting two equivalents of 1 with diynes allows easy access to bimetallic η3-benzo-λ5-phosphinine rhodium complexes 3. Using this approach, compounds 2a-c,e and 3a-c are synthesized and characterized. Testing these compounds as catalysts in the hydrosilylation of alkynes reveals that bimetallic complexes 3a-c show enhanced activity to monometallic and monocationic analogues (i.e. 2d) with improved yields of up to 13 times and reaction rates of over 4 times for the same rhodium concentration. We show that the dicationic monometallic η3-benzo-λ5-phosphinine rhodium complex 2e also displays enhanced activity and that the higher activity in bimetallic complexes 3a-c likely arises from their dicationic nature.

Bimetallic polymerization of lactide with binaphthol-derived bis-heteroscorpionate dizinc and dimagnesium complexes.

Discrete bimetallic catalysts often provide enhanced reactivity and selectivity in lactone polymerization, making metal-metal cooperativity an important design principle for new catalyst development. However, the poor modularity of binucleating ligands limits structure-reactivity analysis and optimization. This report describes a modular, binucleating bis(pyrazolyl)alkane ligand series (1-R) bridged by a chiral binaphthol unit, prepared by nucleophile-catalyzed condensation between a dialdehyde and a bis(pyrazolyl)methanone. A bis(ethylzinc) complex was characterized by single-crystal X-ray diffraction, but in situ complexation with Zn(HMDS)2 and Mg(HMDS)2 provided more active catalysts for lactide polymerization (HMDS- = hexamethyldisilazide). Structure-reactivity studies identified complexes of 1-Me2 as the most active, and these catalysts show significant enhancements in rate compared to their monometallic analogues. Kinetic analysis resulted in first-order dependence on both mono- and bimetallic catalysts, suggesting metal-metal cooperativity as the basis for this rate enhancement. End-group analysis and low dispersity implicate a coordination-insertion mechanism through an alkoxide. Despite rapid transesterification observed by MALDI, we still demonstrated controlled polymerization in the block copolymerization of ε-caprolactone and L-lactide. Although we observed rate differences in the polymerization of L-lactide by opposite enantiomer catalysts, we did not observe catalyst-directed stereoselectivity in the polymerization of rac- or meso-lactide.

Heterodimetallic iridium-rhenium system: Synthesis, computational and photocatalytic aspects

Herein, we present an example of heterodimetallic iridium-rhenium system [(ppy)2IrIII(µ-phpy)ReI(CO)3Cl]+ ([3]+) using a semi-flexible phenanthroline-pyrazine-based ligand exhibiting excellent photocatalytic activity towards photodecomposition of congo red dye. The formation of heterodimetallic [3]+ was confirmed by HRMS, 1H–1H COSY, and 13C NMR spectroscopy. The complexes are luminescent in CH3CN solvent. The lowest energy absorption band for complex [3]+ is 1MRe/IrLLCT in nature (where both Ir and Re contribute), whereas, for mononuclear [1]+ and [2], it is 1MLLCT corresponding to the individual metal centers. The spin density analysis suggests that the emission bands originate from 3MLLCT states. The heterodimetallic [3]+ display excellent photocatalytic activity even under diffused sunlight, whereas, the monometallic analogue [(ppy)2Ir(phpy)]+ ([1]+) and [(phpy)Re(CO)3Cl] ([2]) are showing poorer activity under visible light photoirradiation. The mechanistic investigation suggests that the reactive oxygen species (•OH/O2• −), generated during the photoexcitation of [3]+, is responsible for photodecomposition of dye molecules.

Enhancing the Chemo-Enzymatic One-Pot Oxidation of Cyclohexane via In Situ H2O2 Production over Supported Pd-Based Catalysts

The combination of an evolved unspecific peroxygenase (UPO), from Agrocybe aegerita (PaDa-I variant) and bimetallic Pd-based catalysts, is demonstrated to be highly effective for the one-pot oxidative valorization of cyclohexane to cyclohexanol and cyclohexanone (collectively KA oil), via the in situ formation of H2O2 from the elements. The alloying of Pd with Zn in particular is found to significantly enhance catalytic performance compared to bimetallic PdAu or monometallic Pd analogues. The improved activity of the PdZn/TiO2/PaDa-I system is attributed to the facile formation of PdZn alloys and the resulting electronic modification of Pd, which results in an inhibition of competitive chemo-catalyzed H2O2 degradation reactions and the total suppression of the overoxidation of cyclohexanol. By comparison, the large population of Pd-only clusters present in both the PdAu and monometallic Pd catalysts is considered to be responsible for the promotion of further oxidation products and the unselective conversion of H2O2 to H2O, which hampers overall process efficiency. Notably, given the susceptibility of the enzyme to deactivation at moderate H2O2 concentrations, the continual supply of low levels of the oxidant via in situ production represents a highly attractive alternative to the continuous addition of preformed H2O2 or co-enzyme-based systems.

Metal‐Mediated Interactions in Homo‐ and Heterobimetallic Edifices with Lanthanides: A Study in Solution

AbstractWe report here on the coordination properties of a series of lanthanide cations (Ln=La, Nd, Eu, Tb, Er) with a tripodal homoditopic ligand L, which is bearing three diamido‐dihydroxamate arms anchored on a tertiary amine. The complexes of L with lanthanides were studied in a MeOH/H2O (80/20 by weight) solvent and a marked size‐discriminating effect depending on the intramolecular interactions was observed in the tripodal dinuclear lanthanide edifice. Positive cooperative effects favour the formation of the bimetallic LLn2 complexes compared with the monometallic analogues for the heavier lanthanide cations (Eu, Tb, Er), while the lighter ones (La, Nd) show a decrease of cooperative interactions in the corresponding bimetallic species. Moreover, increased levels of heterobimetallic complexes are detected for Ln(III) pairs containing a lighter Ln(III) ion and a heavier one. In contrast with the adjunctive ligand exchange reported for LFe2 and CDTA, the corresponding substitution reaction for LLn2 occurs according to a disjunctive mechanism. Our work thus highlights the importance of the balance between the rigidity of a tripodal structure and the flexibility of the side arms to accommodate two trivalent cations in a supramolecular ensemble.