Synthesis and structural characterization of nonanuclear lanthanide complexes

Hourglass-Shaped Homo- and Heteronuclear Nonanuclear Lanthanide Clusters: Structures, Magnetism, Photoluminescence, and Theoretical Analysis

Synthesis of nonameric cationic clusters [Dy9(acac)16(μ3-OH)8(μ4-OH)2]OH·6H2O (1), [Dy8Tb (acac)16(μ3-OH)8(μ4-OH)2]OH·2H2O (2), and [Gd9(acac)16(μ3-OH)8(μ4-OH)2]OH·6H2O (3) (acac = acetylacetonate) is reported. The emission spectrum of 1 shows Dy(III) ion characteristic bands assignable to the 4F9/2 → 6HJ (J = 15/2 to 9/2) transitions. Emission due to both Dy(III) and Tb(III) ions is observed for 2 in the visible range, with Tb(III) specific bands appearing due to the 5D4 → 7FJ (J = 6, 4, and 3) transitions. Cluster 3 exhibits a significant magnetocaloric effect (MCE), with -ΔSm values increasing with decrease in temperature and increase in field, reaching -ΔSmmax = 20.98 J kg-1 K-1 at 2 K and 9 T. Isotropic magnetic coupling constants (Js) in 3 derived from density functional theory (DFT) calculations reveal that the exchange interactions are antiferromagnetic and weak. Compound 3 possesses S = 7/2 ground state arising from the central Gd(III) ion along with several nested excited states due to competing antiferromagnetic interactions that yield reasonably large MCE values. Utilizing computed exchange coupling interactions, we have performed ab initio CASSCF/RASSI-SO/POL_ANISO calculations on antiferromagnetic 1 and 2 to estimate the exchange interactions using the Lines model. For 2, Dy(III)···Tb(III) exchange interactions were extracted for the first time and were found to be weakly antiferromagnetically coupled.

Yb-to-Eu Cooperative Sensitization Upconversion in a Multifunctional Molecular Nonanuclear Lanthanide Cluster in Solution

Lanthanide metal clusters excel in combining molecular and material chemistry properties. Here, we report an efficient cooperative sensitization UC phenomenon of a Eu3+/Yb3+ nonanuclear lanthanide cluster in CD3OD. The synthesis and characterization of the heteronuclear cluster in the solid state and solution are described together with the UC phenomenon showing Eu3+ luminescence in the visible region upon 980 nm NIR excitation of Yb3+ at concentrations as low as 100 nM. Alongside being the Eu/Yb cluster to display UC (with a quantum yield value of 4.88 × 10-8 upon 1.13 W cm-2 excitation at 980 nm), the cluster exhibits downshifted light emission of Yb3+ in the NIR region upon 578 nm visible excitation of Eu3+, which is ascribed to sensitization pathways for Yb through the 5D0 energy levels of Eu3+. Additionally, a faint emission is also observed at ca. 500 nm upon 980 nm excitation, originating from the cooperative luminescence of Yb3+. The [Eu8Yb(BA)16(OH)10]Cl cluster (BA = benzoylacetonate) is also a field-induced single-molecular magnet (SMM) under 4K with a modest Ueff/kB of 8.48 K, thereby joining the coveted list of Yb-SMMs and emerging as a prototype system for next-generation devices, combining luminescence with single-molecular magnetism in a molecular cluster.

Lanthanide molecular cluster-aggregates as the next generation of optical materials

In this perspective, we provide an overview of the recent achievements in luminescent lanthanide-based molecular cluster-aggregates (MCAs) and illustrate why MCAs can be seen as the next generation of highly efficient optical materials. MCAs are high nuclearity compounds composed of rigid multinuclear metal cores encapsulated by organic ligands. The combination of high nuclearity and molecular structure makes MCAs an ideal class of compounds that can unify the properties of traditional nanoparticles and small molecules. By bridging the gap between both domains, MCAs intrinsically retain unique features with tremendous impacts on their optical properties. Although homometallic luminescent MCAs have been extensively studied since the late 1990s, it was only recently that heterometallic luminescent MCAs were pioneered as tunable luminescent materials. These heterometallic systems have shown tremendous impacts in areas such as anti-counterfeiting materials, luminescent thermometry, and molecular upconversion, thus representing a new generation of lanthanide-based optical materials.

Thiacalix[4]arene-Sandwiched Sandglass-like Ln9 Clusters (Ln = Tb and Eu): Insights into the Selective Luminescence Quenching Properties by p-Nitrobenzene Derivatives

Nonanuclear lanthanide clusters Ln₉ (Ln = Tb and Eu) based on p-tert-butylthiacalix[4]arene (H₄TC4A) have been synthesized by the solvothermal reaction and were structurally determined by single-crystal X-ray diffraction. The framework of Ln₉ can be termed as a sandglass-like structure whose two Ln₄-TC4A polynuclear secondary building units are bridged by one octa-coordinate {Ln(μ₃-O)₈} unit. Efficient TC4A-to-Ln energy transfer was observed for Tb₉ but not for Eu₉. The luminescence quantum yield (QY) of Tb₉ in the solid state at room temperature was determined to be 17.6%, while its highest QY in a methanolic solution (2 × 10^-5 mol/L) is 59.2% upon excitation at 318 nm. The luminescence of Tb₉ was quenched selectively by derivatives of p-nitrobenzene, as demonstrated by the results of photoluminescence and UV-vis titration experiments and supported by density functional theory calculations. We believe that the interactions between the analyte molecules and the pocket of Tb₉ are primarily responsible for the observed quenching. As such, this work represents one of the few examples of utilizing structurally interesting lanthanide cluster complexes as a sensory platform for the recognition of meaningful analytes and portends the further development of lanthanide-calixarene-complex-based functional materials.

Oxidation of europium with ammonium perfluorocarboxylates in liquid ammonia: pathways to europium(II) carboxylates and hexanuclear europium(III) fluoridocarboxylate complexes

The novel coordination polymer [Eu(O₂CCF₃)₂(dmf)₂]_∞ (1) (dmf = N,N-dimethylformamide) containing europium(II) and the two new compounds (NH₄)₂[Eu₆F₈(O₂CCF₃)₁₂(CF₃COOH)₆] (2) and (NH₄)₂[Eu₆F₈(O₂CC₂F₅)₁₂(C₂F₅COOH)₆]·8C₂F₅COOH (3), both based on hexanuclear europiate(III) complexes, were synthesized from precursors with a Eu²⁺ : Eu³⁺ ratio >1, obtained by reaction of europium metal with ammonium perfluorocarboxylates in liquid ammonia. In the crystal structure of 1 the europium(II) ions are bridged by carboxylate groups and N,N-dimethylformamide to form polymeric chains with Eu²⁺⋯Eu²⁺ distances of 408.39(13)-410.49(13) pm. The compound crystallizes in the triclinic space group P1̄ (Z = 2). To the best of our knowledge, this is the first example of a (solvated) perfluorocarboxylate containing a lanthanoid in a subvalent oxidation state. In the crystal structures of 2 and 3 the europium(III) ions are bridged by fluoride ions and carboxylate groups to form hexanuclear complex anions with an octahedral arrangement of the cations. The Eu³⁺⋯Eu³⁺ distances are in the range of 398.27(15)-400.93(15) pm in 2 and 395.37(4)-399.78(5) pm in 3, respectively. Both compounds crystallize in the monoclinic space group type P2₁/n (Z = 4) and are the first examples of octahedro-hexanuclear europium carboxylates for which fluoride is reported as a bridging ligand. In all compounds the oxidation state of europium was monitored via¹⁵¹Eu Mössbauer and photoluminescene spectroscopy.

Two types of Ln2Cu2 hydroxo-trimethylacetate complexes with 0D and 1D motifs: synthetic features, structural differences, and slow magnetic relaxation

Two series of heterometallic Ln^III-Cu^II compounds containing a butterfly-like tetranuclear metal core were synthesized and characterized by X-ray diffraction and magnetometry. The structures of the new compounds were shown to depend on the nature of the hydroxide used for the synthesis. The reactions of copper(II) and lanthanide(III) salts with Hpiv (Hpiv is trimethylacetic acid) and LiOH in a MeCN-EtOH mixture afford the molecular complexes [Ln₂Cu₂(μ₃-OH)₂(piv)₈(H₂O)₄]·4EtOH (1Ln, Ln = Gd, Tb, Dy, Ho, Yb), whereas the similar reactions using NaOH instead of LiOH give the 1D coordination polymers [Na₂Ln₂Cu₂(μ₃-OH)₂(piv)₁₀(EtOH)₂]·EtOH (2Ln, Ln = Gd, Tb, Dy, Ho, Yb). According to ac susceptibility measurements, the Dy^III-Cu^II compounds (1Dy and 2Dy) exhibit slow relaxation of magnetization indicative of single-molecule magnet (SMM) behavior. In the series of Yb^III-Cu^II compounds, only complex 2Yb shows frequency-dependent out-of-phase ac susceptibility signals. This is the first reported example of carboxylate-based Yb^III-Cu^II compound displaying slow magnetic relaxation.

Upconversion in molecular hetero-nonanuclear lanthanide complexes in solution

Here we show that nonanuclear lanthanide complexes respresent a new class of solution state upconversion (UC) molecules. For a composition of one Tb per eight Yb the nonanuclear complexes display a very efficient UC phenomenon with Tb luminescence in the visible region upon 980 nm NIR excitation of Yb. An unprecedented value of 1.0 × 10-7 was obtained for the UC efficiency at only 2.86 W cm-2, demonstrating these new molecular complexes to be up to 26 times more efficient than the best current molecular systems, the UC being observed down to a concentration of 10 nM.

Twists to the Spin Structure of the Ln9-diabolo Motif Exemplified in Two {Zn2Ln2}[Ln9]{Zn2} Coordination Clusters

Two pentadecanuclear Zn₄Ln₁₁ [with Ln = Gd(1) or Dy(2)] coordination clusters, best formulated as {Zn₂Ln₂}[Ln₉]{Zn₂}, are presented. The central {Ln₉} diabolo core has a {Zn₂Ln₂} handle motif pulling at two outer Ln ions of the central core via two {ZnLn} units, which also invest the system with C₂ point symmetry. The resulting cluster motif is supported on two Zn "feet", corresponding to the {Zn₂} unit in the formula. A thorough investigation of the magnetic properties in the light of the properties of previously reported {Ln₉} diabolo compounds was undertaken. Up to now, the spin structure of such diabolo motifs usually proves ambiguous. Our magnetic studies show that the orientation of the central spin in the {Gd₉} diabolo plays a decisive role. In stabilizing the core by attachment of the {Zn}²⁺ "feet" and using the C₂ symmetry related {ZnGd}⁵⁺ handles to influence the spin direction of the central Gd of the {Gd₉} diabolo, we can understand why the "naked" {Gd₉} diabolo shows ambiguous spin structure. This then allowed us to elucidate the single-molecule magnetic (SMM) properties of the Dy-based compound 2 through disentangling the magnetic properties of the isostructural Gd-based compound 1.

Diverse Lanthanide Coordination Polymers with 3,3'-Dimethylcyclopropane-1,2-dicarboxylate Ligand: Synthesis, Crystal Structure, and Properties

In an attempt to investigate the influence of many variables on the synthesis of lanthanide coordination polymers (Ln-CPs) assembled from the ligand 3,3-dimethylcyclopropane-1,2-dicarboxylic acid, three different Ln-CPs with formulae [La9(μ4-dcd)12(μ3-O)2(H)] n (1), [Gd4(μ4-dcd)6(H2O)] n (2), and [Gd2(μ3-OH)2(μ3-dcd)(μ2-ac)2(H2O)] n (3) (dcd = 3,3-dimethylcyclopropane-1,2-dicarboxylate, ac = acetate) have been hydrothermally synthesized and structurally characterized by elemental analysis, IR spectrum, thermal analysis, powder X-ray diffraction, and single X-ray diffraction techniques. 1 represents the first report of the three-dimensional (3D) Ln-CPs based on nonanuclear lanthanide clusters, although it shows extremely low gas uptakes. 2 exhibits one of the previously reported 3D lanthanide wheel cluster-like frameworks. 3 characterizes a novel one-dimensional ladder-like chain [Gd4(OH)4] n decorated with mixed ligand ribbons. Variable-temperature magnetic susceptibility measurement reveals that the shortest Gd···Gd distance in 3 induces the antiferromagnetic interactions between adjacent Gd3+ cations within the hydroxyl-bridged binuclear unit. Remarkably, magnetic investigation for 2 indicates a unique metamagnetic transition from the antiferromagnet to ferromagnet. Furthermore, magnetic studies for 2 also exhibit the presence of significant magnetocaloric effect with a large magnetic entropy change.

Probing the magnetic relaxation and magnetic moment arrangement in a series of Dy4 squares

Modulations of the first and secondary coordination geometries by dimerizing and reducing the Schiff base ligands result in the enhancement of the magnetic relaxation and rearrangement of the magnetic moments in μ₄-O²⁻ bridged Dy₄ squares.

A series of six-membered lanthanide rings based on 2,2-bis(hydroxymethyl)-2,2′,2″-nitrilotriethanol: synthesis, crystal structures and magnetic properties

A series of six-membered ring cyclic lanthanide(iii) clusters have been constructed by utilizing the pentol ligand.

Synthesis, structure, and physical properties of new rare earth ferrocenoylacetonates

New ferrocenoylacetonate complexes of several rare earth elements, [Ln(fca)3(bpy)]·MeC6H5 (Ln = Pr (), Eu (), Gd (), Tb (), Dy (), Ho (), Y (); bpy - 2,2'-bipyridine; Hfca - FcCOCH2COMe) as well as scandium ferrocenoylacetonate [Sc(fca)3]·0.5MeC6H5 (), were synthesized and characterized by single crystal X-ray diffraction analysis. In the crystal lattice of the isostructural complexes , two [Ln(fca)3(bpy)] molecules form a pair due to stacking interactions between the bpy ligands. The Ln(3+) ions are coordinated in a square antiprism geometry with a coordination number of 8. The Sc(3+) ions in complex are coordinated in an octahedral geometry. Thermolysis of complexes was studied under air and argon atmospheres; in the first case, it affords perovskites LnFeO3 as one of the products. Complexes display single-molecule magnet properties, and the effective relaxation barrier for the Dy complex , was found to be Δeff/kB = 241 K, which is one of the highest values obtained for a mononuclear β-diketonate lanthanide complex.

Pentanuclear [2.2] spirocyclic lanthanide(iii) complexes: slow magnetic relaxation of the DyIII analogue

A series of [2.2] spirocyclic complexes [Ln₅(LH)₄(η¹-Piv)(η²-Piv)₃(μ₂–η² η¹Piv)₂(H₂O)]·Cl (Ln = Dy^III, Tb^III and Ho^III) were synthesized. Magnetic analysis reveals that the Dy^III analogue showed slow relaxation of magnetization.

Slight synthetic changes eliciting different topologies: synthesis, structure and magnetic properties of novel dinuclear and nonanuclear dysprosium complexes

Using the Schiff-base ligand, H₂hmc, dinuclear and nonanuclear compounds are generated through the addition of different bases. The unique Dy9 core structure of complex 2 was obtained and the magnetic properties of both compounds are fully studied.

New NIR-emissive tetranuclear Er(iii) complexes with 4-hydroxo-2,1,3-benzothiadiazolate and dibenzoylmethanide ligands: synthesis and characterization

Two new Er complexes were prepared and studied.

Characterization of Nonanuclear Europium and Gadolinium Complexes by Gas-Phase Luminescence Spectroscopy

Gas-phase measurements using mass-spectrometric techniques allow determination of the luminescence properties of selected molecular systems with knowledge of their exact composition. Furthermore, isolated luminophores are unaffected by matrix effects like solvent interactions or crystal packing. As a result, the system complexity is reduced relative to the condensed phase and a direct comparison with theory is facilitated. Herein, we report the intrinsic luminescence properties of nonanuclear europium(III) and gadolinium(III) 9-hydroxyphenalen-1-one (HPLN)-hydroxo complexes. Luminescence spectra of [Eu9(PLN)16(OH)10](+) ions reveal an europium-centered emission dominated by a 4-fold split Eu(III) hypersensitive transition. The corresponding Gd(III) complex, [Gd9(PLN)16(OH)10](+), shows a broad emission from a ligand based triplet state with an onset of about 1000 wavenumbers in excess of the europium emission. As supported by photoluminescence lifetime measurements for both complexes, we deduce an efficient europium sensitization via PLN-based triplet states. The luminescence spectra of the complexes are discussed in terms of a square antiprismatic europium/gadolinium core structure as suggested by density functional computations.

Two isomorphous lanthanide crotonate complexes: di-μ-but-2-enoato-bis[diaquabis(but-2-enoato)dysprosium(III)] adenine monosolvate heptahydrate and the samarium(II) analogue

The asymmetric unit of the title dimeric compounds, [Ln(2)(C(4)H(5)O(2))(6)(H(2)O)(4)]·C(5)H(5)N(5)·7H(2)O, with Ln = Dy, (I), and Sm, (II), consists of an Ln(III) cation, three crotonate (but-2-enoate) anions and two coordinated water molecules forming the neutral complex, cocrystallized with half of an external adenine molecule and 3.5 water molecules. The metal complex has crystallographic inversion symmetry. The LnO(9) coordination polyhedra are connected through the sharing of a single edge to form isolated dimeric units, with Ln...Ln separations of 4.1766 (12) Å for (I) and 4.2340 (12) Å for (II). The unbound adenine molecule and one of the solvent water molecules are disordered around an inversion centre into two overlapping, equally populated, units. The structure is sustained by a complex hydrogen-bonding scheme involving all possible O-H and N-H groups as donors, and crotonate and water O and adenine N atoms as acceptors. The system is compared with recently published related compounds.

Poly[[diaquabis(μ2-crotonato-κ3O:O,O')(crotonato-κ2O,O')lanthanum(III)] adenine monosolvate monohydrate]

The asymmetric unit of the title compound, {[La(C(4)H(5)O(2))(3)(H(2)O)(2)]·C(5)H(5)N(5)·H(2)O}(n), consists of an La(III) cation, three crotonate (but-2-enoate) anions and two coordinated water molecules forming the neutral complex, completed by an external adenine molecule and one hydration water molecule. The LaO(10) coordination polyhedra, connected through the sharing of a single edge, form isolated chains running along the [100] direction. These one-dimensional structures are characterized by two different centrosymmetric La(2)O(2) loops, with La...La distances of 4.5394 (6) and 4.5036 (6) Å. The unbound adenine and water solvent molecules form a highly planar hydrogen-bonded array parallel to (110) (r.m.s. deviation from the mean plane < 0.10 Å) which intersects the isolated La-crotonate chains in a slanted fashion to form an extremely connected hydrogen-bonded three-dimensional structure.