Ligand Substituents Research Articles

From isostructural to hetero-structural one-dimensional coordination polymers adjusted by changing the ligand substituent group from pyrazine to furan

A new ligand, 4-(4-carboxyphenyl)-6-(pyrazin-2-yl)picolinic acid (H2L1), was used to construct four isostructural one-dimensional (1 D) zig-zag chain coordination polymers ([M(L1)(H2O)2]·3H2O, M = Mn (1), Co (2), Cd (3) and Zn (4)). Moreover, when changing the ligand substituent group from pyrazine ring to furan ring, another new ligand, 4-(4-carboxyphenyl)-6-(furan-2-yl)picolinic acid (H2L2), was synthesized. Based on H2L2, different structural 1 D coordination polymers, ([Mn(L2)(H2O)2] (5) and [Ni(L2)(H2O)2]·H2O (6)), have been obtained under the same solvothermal conditions as for the syntheses of isostructural 1–4. Interestingly, only through changing the ligand substituent group, the 1 D isostructural zig-zag chain (1–4) was adjusted to a 1 D molecular box chain (5) and 1 D linear chain (6), which should be ascribed to ligand substituent group effects. The various 1 D chain structures originate from the types of ligand substituent group: the substituent group with a pyrazine ring only yields isostructural coordination polymers 1–4, whereas the substituent group with a furan ring can construct different structural coordination polymers, such as a 1 D molecular box chain (5) or a 1 D linear chain (6). The photoluminescent properties of all coordination polymers were also examined.

Solid‐state Fluorescence from Carbon Dots Widely Tunable from Blue to Deep Red through Surface Ligand Modulation

AbstractCarbon dots (CDs) find widespread attention due to their remarkable fluorescent and electronic properties. However, aggregation‐caused quenching currently limits the application of CDs in colored displays. The construction of CDs with color‐tunable solid‐state fluorescence (SSF) is rarely reported, since the preparation of SSF CDs is technically challenging. Herein, through surface ligand modulation, SSF CDs with an emission‐color span of almost 300 nm (from blue to deep red) were obtained. In‐depth structure‐property studies reveal that intra‐ and inter‐molecular hydrogen‐bonding inside SSF CDs provokes the emission properties in the aggregated state. Photodynamic characterizations demonstrate emission wavelengths can be switched smoothly by deliberately altering conjugation ability between substituent ligands and CDs core. Three‐dimensional printing patterning is used to create a range of emissive objects, demonstrating the commercial potential for use in optical lamps.

Solid-state Fluorescence from Carbon Dots Widely Tunable from Blue to Deep Red through Surface Ligand Modulation.

Carbon dots (CDs) find widespread attention due to their remarkable fluorescent and electronic properties. However, aggregation-caused quenching currently limits the application of CDs in colored displays. The construction of CDs with color-tunable solid-state fluorescence (SSF) is rarely reported, since the preparation of SSF CDs is technically challenging. Herein, through surface ligand modulation, SSF CDs with an emission-color span of almost 300 nm (from blue to deep red) were obtained. In-depth structure-property studies reveal that intra- and inter-molecular hydrogen-bonding inside SSF CDs provokes the emission properties in the aggregated state. Photodynamic characterizations demonstrate emission wavelengths can be switched smoothly by deliberately altering conjugation ability between substituent ligands and CDs core. Three-dimensional printing patterning is used to create a range of emissive objects, demonstrating the commercial potential for use in optical lamps.

High-Temperature Spin Crossover in FeIII N4O2 Complexes Incorporating an [R-sal2323] Backbone

Of the multitude of [FeIII(R-sal2323)]X complexes reported in the literature, only four have demonstrated spin crossover (SCO). Herein, we report four additional examples of thermal spin crossover in [FeIII(R-sal2323)]X complexes (where R = Br, NEt2, and X = ClO4–, BF4–). Magnetic susceptibility measurements reveal gradual, high-temperature spin transitions in all four compounds with onsets near room temperature. To investigate the emergence of SCO behaviors being observed in these compounds, a range of intramolecular and intermolecular structural parameters were examined. The effect that ligand substituents may have on the electronic environment, as well as the effect of counterions and various intermolecular interactions on the crystal packing, were investigated and compared to the literature of [FeIII(R-sal2323)]X compounds for which magnetic measurements are reported. This comparison found that neither intramolecular subtleties nor intermolecular interactions have a large impact on whether or not these compounds are SCO active. Instead, it is shown and proposed that many compounds in the [FeIII(R-sal2323)]X family may demonstrate SCO activity if measured to higher temperatures (above 300 K). This would provide a wide range of FeIII compounds that are SCO active near or above room temperature to be explored in future work.

Dioxaphosphabicyclooctanes: small caged phosphines from tris(hydroxymethyl)phosphine.

Dioxaphosphabicyclo[2.2.2]octanes (L1-L4) have been prepared in a one-pot reaction from tris(hydroxymethyl)phosphine and various α,β-unsaturated ketones. The non-volatile phosphines oxidise very slowly in air. They possess highly upfield 31P chemical shifts (-59 to -70 ppm), small cone angles (121-140°) and a similar electronic parameter to PPh3. Reaction of L1 with [Rh(acac)(CO)2] gave the complex [Rh(acac)(CO)(L1)] with a ν(CO) of 1981.5 cm-1, whereas reaction L1 with [Rh(CO)2Cl]2 gave [Rh(CO)(L1)2Cl] with a ν(CO) of 1979.9 cm-1, remarkably similar to the CO stretching frequencies reported for analogous PPh3 complexes. The cage phosphines were explored as ligands in rhodium catalysed hydroformylation of 1-octene. All of the ligands gave a linear selectivity to n-nonanal of 68%, regardless of the substituents. However the ligand substituents had a significant effect on the catalyst activity, with increased steric bulk around the coordination environment giving a three-fold increase in aldehyde yield. The phosphines undergo ligand subsitution with [Pd(MeCN)2Cl2] forming square planar trans-[Pd(L)2Cl2] complexes. Subsequent reduction with hydrazine furnishes homoleptic tetravalent [Pd(L1)4] which was applied as a catalyst in Suzuki-Miyaura couplings, furnishing the C-C coupled products in moderate yields.

Spin-state switching: chemical modulation and the impact of intermolecular interactions in manganese(III) complexes.

A series of mononuclear manganese(III) complexes [Mn(X-sal2-323)](ReO4) (X = 5 Cl, 1; X = 5 Br, 2; X = 3,5 Cl, 3; X = 3,5 Br, 4; and X = 5 NO2, 5), containing hexadentate ligands prepared using the condensation of N,N'-bis(3-aminopropyl)ethylenediamine and 5- or 3,5-substituted salicylaldehyde, has been synthesized. Variable temperature single-crystal X-ray diffraction, magnetic, spectroscopic, electrochemical, and spectroelectrochemical analyses, and theoretical calculations have been used to explore the role of various ligand substituents in the spin-state switching behavior of the prepared manganese(III) complexes. All five complexes consist of an analogous distorted octahedral monocationic MnN4O2 surrounding offered by the flexible hexadentate ligand and ReO4- as the counter anion. However, a disordered water molecule was detected in complex 4. Complexes 1 (X = 5 Cl) and 5 (X = 5 NO2) show gradual and complete spin-state switching between the high-spin (HS) (S = 2) and the low-spin (LS) (S = 1) state with T1/2 values of 146 and 115 K respectively, while an abrupt and complete transition at 95 K was observed for complex 2 (X = 5 Br). Alternatively, complex 3 (X = 3, 5 Cl) exhibits an incomplete and sharp transition between the HS and LS states at 104 K, while complex 4 (X = 3, 5 Br) (desolvated) remains almost LS up to 300 K and then displays gradual and incomplete SCO at a higher temperature. The nature of the spin-state switch and transition temperature suggest that the structural effect (cooperativity) plays a more significant role in comparison with the electronic effect coming from various substituents (Cl, Br, and NO2), which is further supported by the detailed structural, electrochemical, and theoretical studies.

Fine-tuning of the spin-crossover properties of Fe(III) complexes via ligand design.

Exploring the chemical space of a given ligand aiming to modulate its ligand field strength is a versatile strategy for the fine-tuning of physical properties such as the transition temperature (T1/2) of spin-crossover (SCO) complexes. The computational study presented herein aims at systematically exploring the extent to which the ligand substituent effects can modulate T1/2 in two families of Fe(III) SCO systems with a N4O2 coordination environment and at identifying the best descriptors for fast and accurate prediction of changes in T1/2 upon ligand functionalization. B3LYP* calculations show that the attachment of substituents to β-ketoiminato fragments (L1) leads to drastic changes in T1/2, while functionalization of phenolato moieties (L2) allows for a finer degree of control over T1/2. Natural Bond Orbital (NBO) charges of the donor atoms, Hammett parameters for both para and meta-functionalization of L2, and Swain-Lupton parameters for L1 and para-functionalization of L2 have been found to be the suitable descriptors for predicting the changes in T1/2. Further analysis of the ligand-field splitting in such systems rationalizes the observed trends and shows that ligand substituents modify both the σ and π bonds between the Fe(III) center and the ligands. Thus, we provide simple yet reliable guidelines for the rational design of new SCO systems with specific values of T1/2 based on their ligand design.

Spin crossover in Hofmann-like coordination polymers. Effect of the axial ligand substituent and its position

Thermally induced spin crossover in the Fe(3-hydroxypyridine)2[M(CN)4] series with M = Ni, Pd, and Pt is reported. The role of the distortion of the iron atom coordination environment in the possibility of observing the spin crossover is explored.

Surface bonding of MN4 macrocyclic metal complexes with pyridine-functionalized multi-walled carbon nanotubes for non-aqueous Li-O2 batteries

It is essential to develop bifunctional catalysts with high activity and stability for reversible oxygen reduction reactions (ORRs) and oxygen evolution reactions (OERs) in lithium-oxygen (Li-O2) batteries. In this work, pyridine (Py) functionalized multi-walled carbon nanotubes (MWCNTs) were prepared to immobilize various solid MN4 macrocyclic metal complexes (MN4-MC) as cathode electrocatalysts for Li-O2 batteries. Three types of MN4-MC molecules, including iron phthalocyanine (FePc), cobalt phthalocyanine (CoPc) and iron protoporphyrin IX (Heme) were examined to evaluate the influence of central metal atoms and ligand substituents found in MN4-MC molecules on the electrocatalytic performance of the study samples. The order of the ORR/OER catalytic activity of the bifunctional catalysts is FePc > Heme > CoPc. The central metal atom in FePc molecule has the highest occupied molecular orbital (HOMO) energy than the corresponding metal atoms in CoPc and Heme molecules. This made the molecule to have better dioxygen-binding ability and higher catalytic activity in the ORR process; it also made it to easily lose electrons that were oxidized in the OER process. This study proposed a simplified scheme of the electrode surface route to assist in understanding the diverse ORR/OER performances of MN4-MC. It is discovered that the positive core of the MN5 coordination sphere in MN4-MC/Py/MWCNTs composite is the primary active site that can influence the formation of MN5···O2* and MN5-LOOLi cluster in the ORR process. The interfacial electron could be easily delivered between MWCNTs and MN5 active site through the Py bridge. This facilitated the formation and decomposition of MN5-LOOLi species during the ORRs/OERs, leading to the enhancement of its catalytic performance. This work provides a new insight into the effects of the molecular structure and organization of MN4-MC on the catalytic activity of O2 electrodes in Li-O2 batteries.

K([2.2.2]‐crypt)]K[Pt3(μ2‐CO)3(PPh3)3] ⋅ 3NH3 – A New Chini‐Type Platinum Carbonyl Complex

AbstractThe titular compound [K([2.2.2]‐crypt)]K[Pt3(μ2‐CO)3(PPh3)3] ⋅ 3NH3 (1) ([2.2.2]‐crypt=4,7,13,16,21,24‐Hexaoxa‐1,10‐diazabicyclo[8.8.8]hexacosane) contains a triangular platinum complex, the approximate D3 symmetry of which is only broken by the phenyl substituents of the PPh3 ligands.. The dianionic complex represents only the second triangular carbonyl platinate characterized via single crystal X‐ray diffraction in monomeric form. The compound crystallizes in the space group P with the unit cell parameters a=13.8826 Å, b=14.2899 Å and c=21.3254 Å. For the formation of the platinate, liquid ammonia seems to play a crucial role as a solvent for the polygermanide used for the reduction as well as for the stabilization of the dianion.

The choice of μ-vinyliminium ligand substituents is key to optimize the antiproliferative activity of related diiron complexes.

Diiron vinyliminium complexes constitute a large family of organometallics displaying a promising anticancer potential. The complexes [Fe2Cp2(CO)(μ-CO){μ-η1:η3-C(R3)C(R4)CN(R1)(R2)}]CF3SO3 (2a-c, 4a-d) were synthesized, assessed for their behavior in aqueous solutions (D2O solubility, Log Pow, stability in D2O/Me2SO-d6 mixture at 37°C over 48 h) and investigated for their antiproliferative activity against A2780 and A2780cisR ovarian cancer cell lines and the nontumoral one Balb/3T3 clone A31. Cytotoxicity data collected for 50 vinyliminium complexes were correlated with the structural properties (i.e. the different R1-R4 substituents) using the partial least squares methodology. A clear positive correlation emerged between the octanol-water partition coefficient and the relative antiproliferative activity on ovarian cancer cell lines, both of which appear as uncorrelated to the cancer cell selectivity. However, the different effects played by the R1-R4 substituents allow tracing guidelines for the development of novel, more effective compounds. Based on these results, three additional complexes (4p-r) were designed, synthesized and biologically investigated, revealing their ability to hamper thioredoxin reductase enzyme and to induce cancer cell production of reactive oxygen species.

Transmembrane Chloride Transport by Diphosphine‐Pd(II) Complexes: Effect of the Ligand Geometry

AbstractTransmembrane chloride transport is a fundamental biological process, and its impairment is at the origin of severe genetic diseases such as cystic fibrosis. Synthetic anion carriers able to transport chloride and other anions across biological membranes are considered as putative candidates for treating these syndromes. We have proposed diphosphine–Pd(II) complexes as a new class of anion carriers and in this contribution we have investigated the ionophoric activity of a family of Pd(II) complexes with diphosphine chelating ligands differing for the ligand bite angle that is varied from 71° (dppm) to 98° (dppb). Moreover, in the case of the dppe ligand we also investigated the effect of the introduction in the ligand structure of alkyl substituents of increasing length and hydrophobicity and of electron donor and withdrawing substituents. All the complexes investigated are able to transport chloride across the phospholipid membrane of liposomes and the most important parameter influencing their relative efficacy is the lipophilicity of the complex with the highest activity observed for [Pd(pEt‐dppe)Cl2]. On the contrary the bite angle of the ligand appears to be not relevant while the activity is diminished by the insertion of both EWG and EDG groups on the phenyl substituents of the phosphine ligands. Finally, also Ni(II) and Cu(I) complexes display ionophoric activity demonstrating that the transport ability of metal complexes is not limited to Pd(II) metal ion.

Perchlorate reduction catalyzed by dioxidomolybdenum(VI) complexes: Effect of ligand substituents

Mimicking the molybdoenzyme (per)chlorate reductase allows for the development of homogeneous catalysts capable of reducing perchlorate. Here, we report several novel dioxidomolybdenum(VI) complexes, [MoO2(SN)2] (SN = bidentate ligand) employing 5-nitro-6-methylpyridine-2-thiolate, 4-(trifluoromethyl)pyrimidine-2-thiolate, 3-chloropyridine-2-thiolate, 3-methylpyridine-2-thiolate as well as the literature-known [MoO2(PyS)2], (PyS = pyridine-2-thiolate) as homogeneous catalysts for perchlorate reduction. These complexes were designed and chosen to evaluate the electronic effect of the ligands on the catalytic activity. Using PPh3 as a sacrificial oxygen acceptor, we followed the reduction of ClO4− indirectly by the determination of the conversion of PPh3 to OPPh3 by GC–MS, under different catalytic conditions. Electron-withdrawing substituents were found to reduce the kinetic barrier for the initial oxygen atom transfer reaction from [Mo(VI)O2L2] to PPh3 supported by kinetic investigations using UV–Vis spectroscopy. However, their presence also decreases the stability of the complexes under the explored conditions.

Cover Feature: Ligand Substituent Effects on Light‐Driven Hydrogen Evolution by Cobalt(II) Tripodal Iminopyridine Catalysts under Precious‐Metal‐Free Conditions (Eur. J. Inorg. Chem. 31/2022)

The Cover Feature shows light-driven hydrogen evolution mediated by cobalt(II) tripodal iminopyridine complexes, using tri(n-butyl)triazatriagulenium (TATA+) as the organic sensitizer and triethylamine (TEA) as the sacrificial donor in aqueous acetonitrile. The activity of the complexes depends highly on the electronic properties of the substituents on the pyridine moieties. Under acidified conditions, the activity for H2 production was enhanced for the substituted catalysts. The 5-Cl derivative showed higher activity and lower catalytic potential (Ecat), which can be attributed to the stabilized CoI, favorable for the formation of CoIII(H). The improved H2 productivity and lower Ecat of the 5-CH3O, 6-CH3O, and 5-CH3 derivatives are suggested to be associated with the increased electron density on the CoIII(H) intermediate, which is more susceptible to protonation. More information can be found in the Research Article by J. Xiang, C.-F. Leung, C.-C. Ko and co-workers.

Development of biomimetic triazine-based affinity ligands for efficient immunoglobulin G purification from human and rabbit plasma

Immunoglobulin purification from different biological fluids is considered one of the most critical steps in antibody production for diagnostic, therapeutic, and research purposes. The current study aimed to elucidate the role of the different aryl substituents in triazine-based affinity ligands on the performance of an affinity chromatography purification media to separate immunoglobulin G (IgG). The biomimetic triazine-based affinity ligand was chosen as a varied containing fix spacer and support. The sepharose beads were activated by epichlorohydrin, and five types of aryl substituents were replaced in the triazine ring and covalently immobilized to the resin surface by 1, 4-diaminobutane spacer. All affinity resins with various ligands were characterized and validated using FTIR, SEM, EDX, and microscopic images. The findings revealed that using R1=3-aminophenol and R2=3-aminophenol substituents in the triazine ring, as affinity ligands attached to the sepharose surface with a 10-atom linker CAES-6B-Cl@R1= MAF, R2= MAF (No. 4), leads to better purification of IgG from human and rabbit plasma with 22.8 mg/mL resin binding capacity in 73±5% yield and 95% of purity. All results confirmed that the designed triazine-based affinity ligands could effectively purify IgG compatible with a fast and low-cost approach.

Switchable ethylene tri-/tetramerization with high catalytic performance: Subtle effect presented by P-alkyl substituent of PCCP ligands

Electron-rich Ph2PC(R″) = CHPRR′ ligands with a P-alkyl group were prepared, and, in conjunction with chromium, showed very high activities in ethylene tri-/tetramerization, leading to, in most cases, a very low PE selectivity of ≤0.1 wt%. The electronic property of ligands has a remarkable influence on the catalytic activity: the stronger the σ-donor ability, the higher the activity. Dialkyl phosphine ligands show a stronger σ-donor ability, and thus exhibited higher activities than their mixed alkyl/Ph phosphine counterparts. Reducing the ligand steric bulk could convert the selectivity from predominant trimerization into mixed tri-/tetramerization with 1-octene:1-hexene ratio of >1 without compromising high combined 1-C6/1-C8 selectivity (>90 wt%). Furthermore, the ligands with a dialkyl phosphine showed high low-temperature activity at 25 °C with a relatively high 1-C8 selectivity. Notably, under a lower pressure of 20–30 bar, using PPhiPr-PCCP 7 showed almost no formation of PE with high activity and selectivity.

Triggering of Valence Tautomeric Transitions in Dioxolene-Based Cobalt Complexes Influenced by Ligand Substituents, Co-ligands, and Anions

We report the multistep synthesis of the 1,1′-(piperazine-1,4-diyl)bis(N,N-bis(pyridin-2-ylmethyl)methanamine)(Ltpbap) octadentate ligand, which, in combination with the known 3,5-di-tert-butylcatechol (3,5-dbcat), allowed the preparation of two di-nuclear cobalt complexes [Co2(Ltpbap)(3,5-dbcat)2](SO4)·5.5MeOH·2H2O (3a) and [Co2(Ltpbap)(3,5-dbcat)2 ](ClO4)2·1.5H2O (3b). We also report the synthesis of two mono-nuclear cobalt complexes [Co(3,5-dbsq)(3,5-dbcat)(4-Mepip)2] (1) with 4-Mepip being 4-methylpiperidine and (Hpip)[Co(tbcat)2(pip)2]·CH3CN (2) where Hpip denotes a piperidinium cation and tbcat is the tetra-bromocatechol ligand. The obtained complexes were characterized by single-crystal X-ray crystallography, SQUID magnetometry, and IR spectroscopy. The structure of the crystalline material in all the cases was determined at 173 K. The magnetic properties of all complexes were measured between 2 and 380 K. The magnetic data clearly show that mono-nuclear complex 1 and di-nuclear complex 3a exhibit valence tautomerism with onset around 300 K and 370 K, respectively, whereas the other two complexes 2 and 3b remain unchanged over the measured temperature range.

RNA-binding of Ru(II) complexes [Ru(phen)2(7-OCH3-dppz)]2+ and [Ru(phen)2(7-NO2-dppz)]2+: The former serves as a molecular “light switch” for poly(A)•poly(U)

To further determine the factors that affect the binding properties of ruthenium(II) polypyridine complexes with RNA duplex and to find excellent RNA-binding agents, the binding properties of ruthenium(II) complexes [Ru(phen)2(7-OCH3-dppz)]2+ (Ru1, phen = 1,10-phenan- throline, 7-OCH3-dppz = 7-methoxy-dipyrido-[3,2-a,2′,3′-c]-phenazine) and [Ru(phen)2(7-NO2- dppz)]2+ (Ru2, 7-NO2-dppz = 7-nitro-dipyrido-[3,2-a,2′,3′-c]-phenazine) with RNA poly(A)•poly(U) duplex have been investigated by spectroscopic methods and viscosity measurements in this work. The results show that complexes Ru1 and Ru2 bind to poly(A)•poly(U) through intercalation and the binding affinity between Ru2 and poly(A)•poly(U) is greater than that of Ru1. Thermal denaturation experiments suggest that both ruthenium(II) complexes exhibit a significant stabilizing effect on poly(A)•poly(U) duplex. Moreover, fluorescence emission spectra exhibit that, deviating from Ru2, Ru1 exhibits a “light switch” effect for poly(A)•poly(U). This effect can be observed by the naked eye under UV light and adjusted by pH, meaning that Ru1 may act as a reversible pH controlled molecular “light switch”. The results obtained in this work will contribute to our understanding of the significant influence of the intercalative ligand substituent effect in the binding process of ruthenium(II) complexes with RNA duplex.

Ligand Substituent Effects on Light‐Driven Hydrogen Evolution by Cobalt(II) Tripodal Iminopyridine Catalysts under Precious‐Metal‐Free Conditions

AbstractCobalt tripodal iminopyridine complexes [Co(L)]2+ (1–4), where L=cis,cis,–1,3,5‐tris(pyridine‐2‐carboxaldimino)‐cyclohexane (1) and its derivatives, bearing substituents of varying electronic properties at the pyridine moieties, act as water reduction catalysts (WRCs) under visible light (λ>420 nm) in aqueous acetonitrile (2.5 %, v/v), using tri(n‐butyl)triazatriagulenium (TATA+) as the organic photosensitizer and triethylamine (TEA) as the sacrificial donor. In the presence of added acetic acid, enhanced turnover numbers (TONs) of 1430–2810 were recorded with the substituted catalysts (2–4), which are attributed to the varied catalytic potential (Ecat) upon the introduction of the substituents.

Coordination Chemistry-Driven Approaches to Rare Earth Element Separations.

Current projections for global mining indicate that unsustainable practices will cause supply problems for many elements, called critical raw materials, in the next 20 years. These include elements necessary for renewable technologies as well as artisanal sources. Energy critical elements (ECEs) comprise a group used for clean, renewable energy applications that are in low abundance in the Earth's crust or require an economic premium to extract from ores. Sustainable practices of acquiring ECEs is an important problem to address through fundamental research to provide alternative energy technologies such as wind turbines and electric vehicles at cheaper costs for our global energy generation and usage. Some of these green technologies incorporate rare-earth (RE) metals (Sc, Y and the lanthanides), which are challenging to separate from mineral sources because of their similar sizes (i.e., ionic radii) and chemical properties. The current process used to provide REs at requisite purities for these applications is counter-current solvent-solvent extraction, which is scalable and works efficiently for any ore composition. However, this method produces large amounts of caustic waste that is environmentally damaging, especially to areas in China that house major separation facilities. Advancement of the selectivity of this process is challenging since exact molecular speciation that affords separations is still relatively unknown. In this context, we developed a program to investigate new RE separations systems that were aimed at minimizing solvent use, controlled by molecular speciation, and could be targeted at problems in recycling these critical metals.The first ligand system that was developed to impart solubility differences between light and heavy rare-earth ions was [{(2-tBuNO)C6H4CH2}3N]3- (TriNOx3-) (graphic below). A differential solubility allowed for a separation of Nd and Dy of SFNd:Dy = ∼300 in a single step. In other words, a 50:50 Nd/Dy sample was enriched to give 95% pure Nd and Dy through a simple filtration, which is potentially impactful to recycling magnetic materials found in wind turbines. This separations system compares favorably to other state-of-the-art molecular extractants that are based on energetic differences of the thermodynamic parameter to affect separations for neighboring elements. This straightforward, thermodynamically driven method to separate REs primed our future research for new coordination chemistry approaches to separations.Another separations system was accomplished through the variable rate of a redox event from one arm of the TriNOx3- ligand. It was determined that the rate of this one electron oxidation, which operated through an electrochemical-chemical-electrochemical mechanism, was dependent on the identity of the RE ion. This kinetically driven separation afforded a separation factor (SF) of SFEu:Y = 75. We have also described other transformations such as ligand exchange, substituent dependent, and redox-driven chelation processes with well-defined speciation to afford purified RE materials. Recently, we determined that magnetic properties can be used to enhance both thermodynamic and kinetic RE separations processes to give an approximately 100% boost for pairs of paramagnetic/diamagnetic REs. These results have shown that both thermodynamic and kinetic RE separations were efficient for different selected RE binary pairs through coordination chemistry. The focus of this Account will detail the differences that are observed for RE separations when promoted by thermodynamic or kinetic factors. Overall, the development of rationally adjusted speciation of REs provides a basis for future industrial separations processes for technologies applied to ECEs derived from wind turbines, batteries for electric vehicles, and LEDs.