Ln Complexes Research Articles

Syntheses of Pyrazolato‐ and Formamidinato‐lanthanoid(III) Diiodides from in Situ Generated lanthanoid(III) Pseudo‐Grignard Reagents

AbstractTrivalent lanthanoid pseudo‐Grignard reagents PhpLnIIII(3‐p) (P=1 or 2; Ln=La, Ce, Nd, Er and Lu) are readily prepared by the oxidation of lanthanoid metals with iodobenzene at 25 °C, following pre‐activation of the metal with 1 drop Hg in tetrahydrofuran (thf) or acetonitrile (CH3CN) and brief sonication. The protolysis of PhpLnIIII(3‐p) with 3,5‐diphenylpyrazole (Ph2pzH) yields pyrazolato complexes [Ln(Ph2pz)I2(thf)n] (n=4: Ln=La 1, Ce 2, Nd 3; n=3: Ln=Er 4, Lu 5). Interestingly, for Er, a competing complex [ErPh(Ph2pz)I(thf)3]⋅thf (4 a) was obtained along with the expected [Er(Ph2pz)I2(thf)3] (4). In contrast, in CH3CN, [Ln(Ph2pz)3(CH3CN)3]⋅2CH3CN (Ln=Nd 6, Gd 7) complexes were obtained through Schlenk equilibrium redistribution. Reactions with bulky N,N′‐bis(2,6‐di‐isopropylphenyl)formamidine (DippFormH) and N,N’ –bis(2,6‐dimethylphenyl)formamidine (XylFormH) produced the Ln(III) complexes, [Ln(DippForm)I2(thf)3]⋅nthf (n=1, Ln=Nd 8; n=0.5, Ln=Er 9) and [Ln(XylForm)I2(thf)3]⋅thf (Ln=Nd 10, Er 11). All the complexes isolated in this study are monomeric; 4–5 and 8–11 are seven coordinate, complexes 1–3 are eight coordinate with one η2‐N‐donor ligand and two trans iodine ligands, and complexes 6–7 are nine coordinate, featuring three η2‐Ph2pz ligands, and three CH3CN ligands.

Octadentate Bispidine Chelators for Tb(III) Complexation: Pyridine Carboxylate versus Pyridine Phosphonate Donors.

With their rigid and preorganized skeleton, bispidine (3,7-diazabicyclo[3.3.1]nonane) chelators are very appealing for the preparation of metal complexes with high kinetic inertness. With the aim to develop new Tb(III)-based medical imaging probes, this study describes the synthesis and physicochemical properties of two novel terbium(III) complexes with octadentate bispidine-based ligands substituted with either pyridine-phosphonate (H6L1) or picolinate (H4L2) subunits. Thermodynamic stability constants of the corresponding Tb(III) complexes have been determined by potentiometric, UV-visible absorption spectrophotometric and spectrofluorimetric methods. Despite their apparent similarity, these two octadentate ligands differ in their most stable conformation: chair-chair conformation for H4L2 and boat-chair conformation for H6L1, as confirmed by 1H NMR studies and suggested by physicochemical investigations. This conformational change induces different protonation schemes for the two ligands. The kinetic inertness of the Tb complexes has been studied in various media and assessed by transmetalation and transchelation experiments. In particular, Tb(L2) displayed a remarkable kinetic inertness with no measurable dissociation over two months in mouse serum at 10-5 M concentration. The complex was also very inert in the presence of a 50-fold excess of Zn(II) in H2O at pH = 7.4 (7% of dissociation over two months). The complexes with ligand L1 are significantly less inert, emphasizing the influence of the ligand conformation on the kinetic inertness of the Ln(III) complexes. Finally, the luminescence properties of the isolated complexes have also been investigated. A bright green luminescence was observed, especially for Tb(L2), which displays a high quantum yield value of 50% in H2O (60% in D2O; λexc = 263 nm). In addition, luminescence lifetimes of 1.9(2) and 1.7(2) ms have been measured for Tb(L1) and Tb(L2), respectively, hence confirming the formation of nona-coordinated complexes with one inner-sphere water molecule. These data on a bispidine scaffold pave the way for developing bright, inert luminescent probes for bioimaging and for radiolabeling applications with Tb(III) radioisotopes.

Efficient Organic Light-Emitting Diodes Obtained by Introducing Gadolinium (Gd) Complexes Based on Pyrazolone Derivative Ligands as Hole Trappers.

The utilization of lanthanide (Ln) complexes in the realm of organic light-emitting diodes (OLEDs) has garnered extensive interest, particularly in their role as luminescent materials or electron trappers. A series of gadolinium (Gd) complexes with energy levels of high HOMO/LUMO and different triplet state energies were designed and synthesized by introducing substituents with different electronic effects onto the pyrazolone derivative ligands. Subsequently, these complexes were precisely purified by vacuum sublimation and codoped into the light-emitting layer (EML) of the OLEDs. This process was facilitated through the well-matched HOMO/LUMO levels and triplet energies among various functional materials. Consequently, the maximum external quantum efficiencies of blue, red, and green OLEDs were simultaneously enhanced with the ratios of 119%, 28%, and 71%, respectively. This improvement can be credited to the introduction of Gd(III) complex molecules within EMLs, which helps to capture excess holes and improve carriers' balance.

Trigonal Planar Heteroleptic Lanthanide(III) Bis(silyl)amide Complexes Containing Aminoxyl Radicals and Anions.

Modulation of the crystal field (CF) in lanthanide (Ln) complexes can enhance optical and magnetic properties, and large CF splitting can be achieved with low coordination numbers in specific geometries. We previously reported that the homoleptic near-linear Sm2+ complex [SmII{N(SiiPr3)2}2] (1-Sm) is oxidized by the 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO•) radical to give the heteroleptic, approximately trigonal planar Sm3+ complex, [SmIII{N(SiiPr3)2}2(TEMPO-)] (2-Sm). Here, we report the synthesis of homologous [LnIII{N(SiiPr3)2}2(TEMPO-)] (2-Ln; Ln = Tm, Yb) complexes by the oxidation of the parent [Ln{N(SiiPr3)2}2] (1-Ln; Ln = Tm, Yb) with TEMPO•; complexes 2-Ln all contain TEMPO- anions. The homoleptic bent Ln3+ complexes [LnIII{N(SiiPr3)2}2][B(C6F5)4] (3-Ln; Ln = Sm, Tm, Yb) were also treated with TEMPO• to yield the heteroleptic, approximately trigonal planar Ln3+ complexes [LnIII{N(SiiPr3)2}2(TEMPO•)][B(C6F5)4] (4-Ln; Ln = Sm, Tm, Yb); the cations of 4-Ln all contain TEMPO• radicals. We have compared the electronic structures of the two geometrically similar families of Ln3+ complexes with the TEMPO- anion (2-Ln) or TEMPO• radical (4-Ln) using a combination of UV-vis-NIR and EPR spectroscopy, magnetic measurements, and ab initio calculations. These studies revealed no single-molecule magnet behavior for 2-Yb despite evidence for sizable CF splitting and a high degree of purity of the ground stabilized mJ = |±7/2⟩ state, while the radical TEMPO• in 4-Yb did not significantly improve performance.

Solution and solid state structures of lanthanides(III) and uranyl(II) complexes of tripodal tris(2-pyridyl)-containing ligand on Ph3P(O) platform

Reaction of tris[2-(2′-pyridylmethoxy)phenyl]phosphine oxide (L) with f-element nitrates resulted in 1:1 complexes. The isolated complexes of ligand L with La(III), Eu(III), Tb(III), and U(VI) nitrates, and with La(III) chloride were studied in the solid state and solution by IR, Raman, and NMR spectroscopy, X-ray analysis, and also DFT calculations. Composition and structure of the complexes vary with lanthanide cation radius. According to the data of elemental analysis, vibrational spectroscopy, and X-ray diffraction, the ligand is coordinated in tridentate mode in crystalline La(III) and solid Eu(III) complexes: [Ln(OPO,N,Oeth-L)(H2O)(O,O-NO3)3], and in monodentate mode in crystalline complex [Tb(OPO-L)(H2O)2(O,O-NO3)3]. Nitrogen atoms of pyridine fragments of the ligand not involved in coordination produce intra- and intermolecular H-bonds with coordinated water molecules in the second coordination sphere of Ln(III). According to IR, NMR spectrometry, and DFT calculations, the structure of coordination polyhedron of the main species of Ln(III) complexes in acetonitrile solutions is retained including coordination of one of pyridine fragments in the second coordination sphere: [Ln{OPO,N,(N*),Oeth-L}(H2O)(O,O-NO3)3] and [Tb{OPO,(N*)-L}(H2O)2(O,O-NO3)3], where Ln = La, Eu, and N* is the nitrogen atom of pyridine fragment producing intramolecular H-bond with coordinated water molecule. According IR and Raman spectroscopy, ligand L is coordinated in bidentate mode in solid complex [UO2(OPO,N-L)(O,O-NO3)], and uncoordinated nitrogen atoms remain free. Solution structure of uranyl complex is labile and depends on solvent nature. Equilibria of complex species with variable coordination of nitrate ions, ligand L, and with probable involvement of water take place in CD3CN and CDCl3 solutions. Photophysical properties of the prepared Eu(III) and Tb(III) complexes were studied. The preliminary assessment of extraction properties of compound L was made. Stability of studied compounds in acetonitrile solutions was examined, and the structure of one of protolysis product of Eu(III) complex, [Eu(LH)(H2O)(NO3)4], was established by micro-IR and X-ray analysis.

Synthesis of Bis(2,6-Diadamantyl Aryloxide) Ln(II) Complexes of Samarium, Europium, and Ytterbium and Their Ln(III) Precursors.

The syntheses of Ln(II) bis(aryloxide) complexes of Sm, Eu, and Yb have been examined with the bulky aryloxide ligand (OC6H2-2,6-Ad2-4-tBu)1- [(OAr*)1-; Ad = 1-adamantyl]. These LnII(OAr*)2(THF)2 complexes were pursued for comparison with the tris(aryloxide) Ln(II) complexes [LnII(OAr*)3]1- (Ln = La, Ce, Nd, Gd, Dy, and Lu), which have 4fn5d1 electron configurations, and with 4f14 [YbII(OAr*)3]1-. Although the Ln(II) chemistry of Sm, Eu, and Yb is often similar since they all adopt 4fn+1 electron configurations, their chemistry is surprisingly diverse with (OAr*)1-. Reactions of LnIII2(THF)2 with KOAr* in THF form the bis(aryloxide) Ln(II) complexes LnII(OAr*)2(THF)2, 1-Ln, that were characterized by X-ray diffraction for 1-Sm and 1-Yb, but crystals of 1-Eu have been elusive. Reduction of the tris(aryloxide) Sm(III) complex, SmIII(OAr*)3, 2-Sm, prepared from SmIII(NR2)3 (R = SiMe3) and HOAr* in refluxing toluene, generated the bis(aryloxide) complex, 1-Sm, and KOAr* rather than a Sm analogue of [YbII(OAr*)3]1-. Reduction of EuIII(OAr*)3, 2-Eu, prepared from EuCl3 with KOAr* in THF, caused an immediate dark blue-purple to bright red-orange color change, but crystals of the product were elusive. Neither reduction of TmIII(OAr*)3, 2-Tm, nor the reaction of TmIII2(DME)3 with KOAr* provided crystalline Tm(II) complexes.

Quantum–Chemical Study of the Photophysical Behavior of Mesogenic Europium(III) Complexes with β‐Diketones and Lewis Bases

AbstractMetal‐containing liquid crystals such as Ln(III) complexes possess unique structural, liquid‐crystalline (LC), optical, and magnetic properties and represent modern cutting‐edge multifunctional materials. Application of mesogenic Ln(III) complexes is hindered by their nontrivial structure–property relationships. These relationships, in turn, depend on various factors, including insufficiently studied physicochemical processes. Although a proper Ln(III) element and the respective ligand environment are selected prior to synthesis of such complexes, the structural features of the coordination polyhedra, especially upon photoexcitation, are not uniquely defined. Therefore, this work focuses on the development of theoretical approaches to creating multifunctional materials represented by highly luminescent mesogenic Eu(III) complexes with β‐diketones and Lewis bases. The relationships between their structure, the parameters of Voronoi–Dirichlet polyhedra, the luminescence efficiency, and the LC properties were considered. The calculated excited states and intramolecular energy transfer rates were used to determine intramolecular energy transfer channels. The LC behavior of the studied materials was shown to mainly depend on the ligand environment, whereas their optical properties were found to be mostly governed by the coordination polyhedra.

Effect of the Introduction of ZnII and CdII Ions on EuIII and TbIII Emission in M2Ln2 Heterometallic Molecules With 2‐Furoic Acid Anions

ABSTRACTHeterometallic d‐4f coordination complexes are of paramount interest in modern coordination chemistry because of their potential applications in organic light‐emitting devices and spintronic materials. Here we report the synthesis and thorough investigation of Ln and MLn (M = Zn, Cd; Ln = Sm, Eu, Gd, Tb) molecular complexes based on 2‐furancarboxylic acid anion (Hfur): [Ln2(NO3)2(fur)4(DME)2] (Ln = Eu, Gd, Tb; DME is dimethoxyethane) and [M2Ln2(NO3)2(fur)8(bpy)2] (M = Zn, Cd; Ln = Eu, Gd, Tb, Sm; bpy is 2,2′‐bipyridyl). The structure and isostructural nature of compounds were determined based on the single‐crystal and powder X‐ray diffraction data. The photophysical properties of the obtained compounds were studied in detail: The energies of the triplet levels of the furoate anion and d‐blocks {M(fur)2(bpy)} (M = Zn, Cd), the relaxation times of the excited states, and the quantum yields were determined. Critical step from Ln complexes to ZnLn and CdLn (Ln = Eu, Tb) is accompanied by an increase in quantum yields, which correlates with a change in the energy of the triplet level of the aromatic part of the complexes and with the results of quantum chemical calculations indicating different schemes for the origination of triplet levels in MLn compounds.

The nitrogen heterocyclic dithione rare earth complexes as a multi-response high-efficiency detection sensor for the detection of Cu2+, Co2+, Cr3+, Fe3+, Bi3+ cations and its magnetic study

In this paper, eight novel rare earth complexes Ln(H3L2)(DMA)3(H2O), (Ln = Sm (1), Tb(2), Dy(3), Gd(4), Ho(5), Tm(6), Er(H3L2)(DMA)2(H2O)2 (7), [(μ3-OH)(μ2OH2)3Eu3(H4/3L1)3]n (8)), have been successfully synthesized by solvothermal method. Herein, H4L1 is 1,4,1′,4′-Tetrahydro-[3,3′]bi[[1,2,4] triazolyl]-5,5′-dithione, H6L2 is 5-((5-thioxo-4,5-dihydro-1H,4′H-[3,3′-bi(1,2,4-triazol)]-5′-yl)thio)-1′,4′- dihydro-1H,5′H-[3,3′-bi(1,2,4-triazole)]-5′-thione which was synthesized through in situ condensation reaction of two H4L1. The complexes 1–7 are mononuclear zero-dimensional structures of SmIII, TbIII, DyIII, GdIII, HoIII, TmIII, and ErIII, respectively, and complex 8 is a trinuclear EuIII coordinational polymer. Complexes 1–8 were characterized by the single-crystal X-ray diffraction, powder XRD, infrared spectroscopy and elemental analysis. The luminescence sensing and basic magnetic properties will be studied further. It was found that complexes 2 and 3 could specifically recognize Cu2+, Fe3+, Co2+, Cr3+, and Bi3+ in DMF solution. In 2, the limit of detection (LOD) are 2.33 × 10–5 M, 1.10 × 10–5 M, 2.14 × 10–5 M, 2.15 × 10–5 M for Cu2+, Fe3+, Co2+, Cr3+, respectively, while in 3, the LOD is 7.98 × 10–6 M for Bi3+. Magnetic studies revealed that 3 and 5 exhibit dominant antiferromagnetic interactions.

Synthesis and Characterization of Diacylthiourea and Diacylthioureato Cu(I) Complexes

A series of six disubstituted diacylthioureas 1,3‐ and 1,4‐C6H4[C(O)NHC(S)NR1R2]2 (L1‐6) were synthesized with varied Rn substituents (R1 = R2 = Et for L1‐2; R1 = R2 = Bn for L3‐4; R1 = iPr and R2 = Ph for L5 (1) and L6 (2)). Treatment of Ln with Cu(I) halide precursors CuX(PPh3)3 (X = Cl, Br, I) produced the discrete binuclear adducts Ln[CuX(PPh3)2]2 (3‐11 and 14‐16; n = 1‐3 and 6) by binding to Cu(I) centres via the monodentate‐S mode. In contrast, L4 and L5 yielded only the chloride products of binuclear L4[CuCl(PPh3)2]2 (12) and mononuclear L5CuCl(PPh3)2 (13), respectively, with one S arm in the latter remaining dangling, while bromide or iodide analogues were not available for L4 and L5, possibly due to the steric hindrance imposed by larger halide anions or bulky isopropyl substituents. The reactions of Ln with nitrate [Cu(NO3)(PPh3)2]led to the double‐deprotonation of ligand protons to generate dianions Ln' (1,3‐/1,4‐C6H4[C(O)NC(S)NR1R2]22‐) and led to the consequent formation of binuclear diacylthioureato Cu(I) complexes Ln'[Cu(PPh3)2]2 (n=1 (17), 2 (18), 4 (19)) via the κ‐O,S‐bidentate mode. The obtained ligands and complexes were spectroscopically and structurally characterized. These Cu(I) products (3‐19) were experimentally used as catalysts for the oxidation of 1‐phenylethanol.

Lanthanide-based F-MOFs: Structure, hydrolytic stability, spectral and magnetic properties

A series of five novel lanthanide-based fluorinated metal-organic frameworks (Ln-F-MOFs) have been synthesized under solvothermal conditions by the reaction of 3,3′-bis(trifluoromethyl)-[1,1′-biphenyl]-4,4′-dicarboxylic acid (H2L) and lanthanide Ln(III) ions (Ln(III)La, Ce, Pr, Nd and Eu). Powder X-ray diffraction (PXRD) analysis revealed that all prepared complexes are isostructural. A single-crystal X-ray crystallographic study of one representative of the isostructural group, namely {[La2(L)3(DMF)2(H2O)2]}n showed, that compound crystallizes in a triclinic system with space group P-1. The lattice parameter values are a = 8.563(2) Å, b = 13.199(3) Å, c = 16.008(4) Å, α = 104.588(7) °, β = 92.904(7) ° and γ = 92.717(7) °, with two formula units in the unit cell. The overall structure is formed by 2D polymeric layers, which are arranged into a semi-3D supramolecular structure through hydrogen bonds and other intramolecular interactions. The study of the hydrophobic properties of the complexes showed that the complexes exhibit surface hydrophobicity with a “rose petal effect” and a contact angle of approximately 107°. However, the structures are not hydrolytically stable in the long term and the structure starts to delaminate after two days in water. This is a manifestation of the fact that the complexes do not form a 3D polymer network, it is made up of 2D layers connected only by weak non-bonding interactions. The photoluminescence properties of the Ln(III) complexes are determined by the characteristic 5d-4f or 4f-4f electron transitions for the individual lanthanide ions. The magnetic properties of Nd(III), Pr(III) and Eu(III) variants were studied. The magnetic properties of {[Pr2(L)3(DMF)2(H2O)2]}n are characterized by the presence of a low-lying quasi-doublet with 15.6 cm−1 energy splitting, whereas Eu(III) variant is nonmagnetic at low temperatures, but the magnetic 7F1 state is accessible by thermal excitation. For Nd(III) complex, the X-band EPR measurements were performed. Since 1D channels with dimensions of 4.90 × 7.23 Å2 are present within the structure, the adsorption of N2, CO2 and H2 gases was also studied.

6-(6-Methyl-1,2,4,5-Tetrazine-3-yl)-2,2'-Bipyridine: A N-Donor Ligand for the Separation of Lanthanides(III) and Actinides(III).

Here, we report the synthesis of the 6-(6-methyl-1,2,4,5-tetrazine-3-yl)-2,2'-bipyridine (MTB) ligand that has been developed for lanthanide/actinide separation. A multimethod study of the complexation of MTB with trivalent actinide and lanthanide ions is presented. Single-crystal X-ray diffraction measurements reveal the formation of [Ce(MTB)2(NO3)3], [Pr(MTB)(NO3)3H2O], and [Ln(MTB)(NO3)3MeCN] (Ln = Nd, Sm, Eu, Gd). In addition, the complexation of Cm(III) with MTB in solution was studied by time-resolved laser fluorescence spectroscopy. The results show the formation of [Cm(MTB)1-3]3+ complexes, which occur in two different isomers. Quantum chemical calculations reveal an energy difference between these isomers of 12 kJ mol-1, clarifying the initial observations made by time-resolved laser fluorescence spectroscopy (TRLFS). Furthermore, quantum theory of atoms in molecules (QTAIM) analysis of the Cm(III) and Ln(III) complexes was performed, indicating a stronger covalent contribution in the Cm-N interaction compared to the respective Ln-N interaction. These findings align well with extraction data showing a preferred extraction of Am and Cm over lanthanides (e.g., max. SFAm/Eu = 8.3) at nitric acid concentrations <0.1 mol L-1 HNO3.

Effect of Pyrrolidinedithiocarbamate Ligand on the Luminescence Properties of Heteroligand Samarium and Europium Complexes: Experimental and Theoretical Study.

The photophysical properties of two isostructural heteroligand lanthanide complexes of general formula Ln(pdtc)3(phen) (pdtc = pyrrolidinedithiocarbamate anion, phen = 1,10-phenanthroline), Ln = Sm3+ (1), Eu3+ (2)) were studied in solid state and dichloromethane (DCM) solution. The two lanthanide complexes were investigated by experimental techniques for structural (single-crystal X-ray diffraction analysis of 1, powder XRD, TG-DTA) and spectroscopic [electron paramagnetic resonance (EPR), infrared (IR), ultraviolet-visible (UV-vis), photoluminescence (PL)] characterization. DFT/TDDFT/ωB97xD and multireference SA-CASSCF/NEVPT2 calculations with perturbative spin-orbit coupling corrections were applied to construct the Jablonski energy diagrams and to discuss the excited state energy transfer mechanism with competing excited state processes and possible sensitized mechanism of metal-centered emission. The first excited state (S1) involved in the excited state energy transfer L(antenna)-to-Ln was predicted to have interligand (pdtc-to-phen) charge transfer character in contrast to the previously predicted ligand-to-metal charge transfer character. The theoretical consideration showed similar relaxation paths and luminescence quenching channels and appropriate Donor*(phen)-Acceptor*(Ln3+) energy gap for 1 and 2. The experimental measurements in the solid state, however, showed efficient luminescence and good ability to convert UV to visible light only for the Sm(pdtc)3(phen) complex. The minor emission of 2 was explained by partial reduction of Eu3+, confirmed by EPR and calculated electron density distribution data.

2,6-Bis(5-(tert-butyl)-1H-pyrazol-3-yl)pyridine: Effects of the Peripheral Aliphatic Side Chain on the Coordination of Actinides(III) and Lanthanides(III).

To improve our understanding of the interaction mechanism in trivalent lanthanide and actinide complexes, studies with structurally different hard and soft donor ligands are of great interest. For that reason, the coordination chemistry of An(III) and Ln(III) with 2,6-bis(5-(tert-butyl)-1H-pyrazol-3-yl)pyridine (C4-BPP) has been explored. Time-resolved laser fluorescence spectroscopy (TRLFS) studies have revealed the formation of [Cm(C4-BPP)n]3+ (n = 1-3) (log β1' = 7.2 ± 0.4, log β2' = 10.1 ± 0.5, and log β3' = 11.8 ± 0.6) and [Eu(C4-BPP)m]3+ (m = 1-2) (log β1' = 4.9 ± 0.2 and log β2' = 8.0 ± 0.4). The absence of the [Eu(C4-BPP)3]3+ complex shows a more favorable complexation of Cm(III) over that of Eu(III). Additionally, complementary NMR measurements have been conducted to examine the M(III)-N bond in Ln(III) and Am(III) C4-BPP complexes. 15N NMR data have revealed notable differences in the chemical shifts of the coordinating nitrogen atoms between the Am(III) and Ln(III) complexes. In the Am(III) complex, the coordinating nitrogen atoms have shown a shift by 260 ppm, indicating a higher fraction of covalent bonding in the Am(III)-N bond compared with the Ln(III)-N bond. This observation aligns excellently with the differences in the stability constants obtained from TRLFS studies.

Dynamics of Coordinated Phosphonate Group Directly Observed by 17O-NMR in Lanthanide(iii) Complexes of a Mono(ethyl phosphonate) DOTA Analogue.

Biological phosphates can coordinate metal ions and their complexes are common in living systems. Dynamics of mutual oxygen atom exchange in the tetrahedral group in complexes has not been investigated. Here, we present a direct experimental proof of exchange ("phosphonate rotation") in model Ln(III) complexes of monophosphonate H4dota analogue which alters phosphorus atom chirality of coordinated phosphonate monoester. Combination of macrocycle-based isomerism with P-based chirality leads to several diastereoisomers. (Non)-coordinated oxygen atoms were distinguished through 17O-labelled phosphonate group and their mutual exchange was followed by various NMR techniques and DFT calculations. The process is sterically demanding and occurs through bulky bidentate (κ2-PO2)- coordination and was observed only in twisted-square antiprism (TSA) diastereoisomer of large Ln(III) ions. Its energy demands increase for smaller Ln(III) ions (298ΔG≠(exp./DFT)=51.8/52.1 and 61.0/71.5 kJ mol-1 for La(III) and Eu(III), respectively). These results are helpful in design of such complexes as MRI CA and for protein paramagnetic NMR probes. It demonstrates usefulness of 17O NMR to study solution dynamics in complexes involving phosphorus acid derivatives and it may inspire use of this method to study dynamics of phosphoric acid derivatives (as e. g. phosphorus acid-based inhibitors of metalloenzymes) in different areas of chemistry.

Solid State and Solution Structures of Lanthanide Nitrate Complexes of Tris-(1-napthylphosphine oxide).

Coordination complexes of lanthanide metals with tris-1-naphthylphosphine oxide (Nap3PO, L) have not been previously reported in the literature. We describe here the formation of lanthanide(III) nitrate complexes Ln(NO3)3L4 (Ln = Eu to Lu) and the structures of [Ln(NO3)3L2]·2L (Ln = Eu, Dy, Ho, Er) and L. The core structure of the complexes is an eight-coordinate [Ln(NO3)3L2] with the third and fourth ligands H-bonded via their oxygen atoms to one of the naphthyl rings. The structures are compared with those of the analogous complexes of triphenylphosphine oxide and show that the Ln-O(P) bond in the Nap3PO complexes is slightly longer than expected on the basis of differences in coordination numbers. The reaction solutions, investigated by 31P and 13C NMR spectroscopy in CD3CN, show that coordination of L occurs across the lanthanide series, even though complexes can only be isolated from Eu onwards. Analysis of the 31P NMR paramagnetic shifts shows that there is a break in the solution structures with a difference between the lighter lanthanides (La-Eu) and heavier metals (Tb-Lu) which implies a minor difference in structures. The isolated complexes are very poorly soluble, but in CDCl3, NMR measurements show dissociation into [Ln(NO3)3L2] and 2L occurs.

Influence of Aqueous Phase Acidity on Ln(III) Coordination by N,N,N',N'-Tetraoctyldiglycolamide.

This study highlights the importance of combining distribution ratio measurements with multiple spectroscopic techniques to provide a more comprehensive understanding of organic phase Ln coordination chemistry. Solvent extraction investigations with N,N,N',N'-tetraoctyldiglycolamide (TODGA) in n-heptane reveal the sensitivity of Ln complexation to the HNO3 concentration. Distribution ratio measurements in tandem with UV-Vis demonstrated that increasing the concentration of HNO3 above 0.5 M with a constant NO3- of 1 M increases the number of coordinating TODGA molecules, from a 1:2 to a 1:3 Ln:TODGA complex. At each concentration of HNO3 considered herein (from 0.01 to 1 M), Eu lifetime analysis demonstrated no evidence of H2O coordination. Results from Fourier transform infrared investigations suggest the presence of inner-sphere NO3- under low concentrations of HNO3 when the 1:2 Ln:TODGA complex is present. Increasing the HNO3 concentration above 0.5 M increases the propensity for outer-sphere interactions by removing the coordinated NO3- and saturating the Ln coordination sphere with three TODGA molecules, resulting in the well-established cationic, trischelate homoleptic [Ln(TODGA)3]3+ complex. This work demonstrates the importance in considering the NO3- source for solvent extraction systems. In particular, for systems with an affinity for outer-sphere interactions with molar concentrations of HNO3, changing the NO3- source can change the inner-sphere coordination of the Ln complex, which, in turn, affects the separation efficacy.

Insights into coordination and ligand trends of lanthanide complexes from the Cambridge Structural Database

Understanding lanthanide coordination chemistry can help develop new ligands for more efficient separation of lanthanides for critical materials needs. The Cambridge Structural Database (CSD) contains tens of thousands of single crystal structures of lanthanide complexes that can serve as a training ground for both fundamental chemical insights and future machine learning and generative artificial intelligence models. This work aims to understand the currently available structures of lanthanide complexes in CSD by analyzing the coordination shell, donor types, and ligand types, from the perspective of rare-earth element (REE) separations. We obtain four sets of lanthanide complexes from CSD: Subset 1, all Ln-containing complexes (49472 structures); Subset 2, mononuclear Ln complexes (27858 structures); Subset 3, mononuclear Ln complexes without cyclopentadienyl ligands (Cp) (26156 structures); Subset 4, Ln complexes with at least one 1,10-phenanthroline (phen) or its derivative as a coordinating ligand (2226 structures). The subsequent analysis of lanthanide complexes in these subsets examines the trends in coordination numbers and first shell distances as well as identifies and characterizes the ligands and donor groups. In addition, examples of Ln-complexes with commercially available complexants and phen-based ligands are interrogated in detail. This systematic investigation lays the groundwork for future data-driven ligand designs for REE separations based on the structural insights into the lanthanide coordination chemistry.

Unexpected Supramolecular-Induced Redox Switching in Sandwich Gd Bisphthalocyaninate.

The redox state of the phthalocyanine in sandwich lanthanide complexes is crucial for their applications. In this work, we demonstrate that the cation-induced supramolecular assembly of crown-substituted phthalocyanine lanthanide complexes Ln[(15C5)4Pc]2 can be used to control the redox state of the ligand simultaneously with the coordination sphere of the central metal. We achieve unprecedented redox switching of phthalocyanine ligands in a double-decker Gd(III) complex, resulting from the intramolecular inclusion of potassium cations between the decks with simultaneous twisting of the ligands (the skew angle between them decreases from 44.61 to 0.21°). Such a structural change leads to an increase in the deck-to-deck distance and drastically facilitates ligand reduction. This process was shown to be anion-dependent: only potassium salts of weak acids (KOPiv and KOAc) induce intramolecular inclusion of cations with redox switching in contrast to salts of strong acids (KBr, KOPic, KSCN, and KPF6), where such a redox process does not occur. This breakthrough opens new avenues for controlling the electrochromic properties, of phthalocyanines, along with other properties, such as electrical conductivity, optics, etc.

Engineering Clock Transitions in Molecular Lanthanide Complexes.

Molecular lanthanide (Ln) complexes are promising candidates for the development of next-generation quantum technologies. High-symmetry structures incorporating integer spin Ln ions can give rise to well-isolated crystal field quasi-doublet ground states, i.e., quantum two-level systems that may serve as the basis for magnetic qubits. Recent work has shown that symmetry lowering of the coordination environment around the Ln ion can produce an avoided crossing or clock transition within the ground doublet, leading to significantly enhanced coherence. Here, we employ single-crystal high-frequency electron paramagnetic resonance spectroscopy and high-level ab initio calculations to carry out a detailed investigation of the nine-coordinate complexes, [HoIIIL1L2], where L1 = 1,4,7,10-tetrakis(2-pyridylmethyl)-1,4,7,10-tetraaza-cyclododecane and L2 = F- (1) or [MeCN]0 (2). The pseudo-4-fold symmetry imposed by the neutral organic ligand scaffold (L1) and the apical anionic fluoride ion generates a strong axial anisotropy with an mJ = ±8 ground-state quasi-doublet in 1, where mJ denotes the projection of the J = 8 spin-orbital moment onto the ∼C4 axis. Meanwhile, off-diagonal crystal field interactions give rise to a giant 116.4 ± 1.0 GHz clock transition within this doublet. We then demonstrate targeted crystal field engineering of the clock transition by replacing F- with neutral MeCN (2), resulting in an increase in the clock transition frequency by a factor of 2.2. The experimental results are in broad agreement with quantum chemical calculations. This tunability is highly desirable because decoherence caused by second-order sensitivity to magnetic noise scales inversely with the clock transition frequency.