Since the discovery of AlQ3 (Q = 8-quinolinolato) quinolinato complexes, they have been extensively scrutinized as emitter materials for organic lighting. Herein, we report on the first representatives of a series of tetrabutylphosphonium tetrakis(8-quinolinolato)lanthanidate complexes [P4444][Ln(Q)4]·2X (Ln = Dy-Lu and Y; X = H2O for Ln = Dy-Tm, Lu and Y and (CH3)2CO for Ln = Yb), which are synthesised by a simple metathesis reaction of the respective potassium tetrakis(8-quinolinolato)lanthanidate salts with tetrabutylphosphonium bromide in acetone at room temperature. Single-crystal X-ray diffraction reveals that Ln(iii) is coordinated by four bidentate 8-quinolinato ligands in the form of a distorted square antiprism. The distinct [Ln(Q)4]− anions interact with the [P4444]+ cations through secondary bonding interactions, such as CH-π and van der Waals interactions, in addition to electrostatic coulombic interactions. Although these compounds contain crystal water/solvent molecules (and their synthesis does not require an inert atmosphere), they do not enter the metal coordination sphere but form pairwise intramolecular hydrogen bonds with the two 8-quinolinato ligands of the complex lanthanide anions. Combined differential scanning calorimetry-thermogravimetric analysis indicates that crystal water is lost at around 100 °C and [P4444][Ln(Q)4] is formed, which is stable up to 300 °C, where further degradation occurs. All compounds feature strong emission in the green region, originating from the π* → π transitions within the 8-quinolinato ligand, with lifetimes in the nanosecond range. The luminescence colour changes from blue-green to yellow-green depending on Ln3+, which opens up additional directions in the colour tuning of emitters for organic lighting applications.

A set of IL-forming ion combinations has been studied to gain a deeper understanding of how, aside from obvious electrostatic interactions and ion size effects, secondary bonding such as hydrogen as well as halogen bonding and van der Waals interactions along with conformational and structural flexibility influence the crystallization behavior of potentially IL forming salts. The scrutinized ions have been specifically chosen to allow for unraveling preferential interactions of functional groups that may favor or disfavor crystallization with respect to secondary bonding interactions, i.e., primary and quaternary ammonium cations of variable alkyl chain lengths, which were also endowed with hydroxy groups, combined with formate and bis(trifluoromethanesulfonyl)amide anions. The background is to provide a deeper fundamental understanding of how to intentionally pair cations and anions that will not support the formation of a crystalline solid but rather IL formation, an approach described as “anti-crystal engineering”. This concept is based on the idea to avoid combining ions that are strong supramolecular synthons for crystallization. To this avail, the crystallization behavior of salts constituted of combinations of selected ions bearing different structural, supramolecular crystallization motifs has been studied in detail by low-temperature differential scanning calorimetry (DSC). Single crystal X-ray structure analysis has been used to elucidate ion packing and preferential interactions whenever crystalline solid formation is observed. The study reveals that the lowest melting points are supported by cation-anion combinations that have the least hydrogen bonding. However, if there are multiple possibilities of H-bonding for an ion with its counteranion, this bonding frustration leads as well to low melting points-albeit they are still higher compared to ion combinations with no H-bonding capacity. Through a careful balance of primary and secondary, directional and nondirectional interactions, it was possible to rationally identify a record class of ionic liquids, which combine exceptionally high decomposition points (440-450 °C) with an enormously high liquid range around of more than 500 °C and no tendency for solidification down to well below ambient temperature (−90 °C). These ILs are formed by bis(trifluoromethane)sulfonylamides with quaternary ammonium ions that bear an −OH group in the side chain.

Commonly accepted design concepts for ionic liquids (ILs) state that the constituting ions must be large and carry low, well-dispersed charges. A series of ILs based of pentadeca charged ILs with pentanuclear linear {Ln₅} units ([Ln₅(C₂H₅-C₃H₃N₂-CH₂COO)₁₆(H₂O)₈](Tf₂N)₁₅ (C₃H₃N₂ = imidazolium moiety, Tf₂N = bis(trifluoromethanesulfonyl)amide) with Ln = Er, Ho, Tm) demonstrates that these criteria are not absolute. Highly charged ions can also support IL formation, provided they are sufficiently large. Expanding the series of these unconventional, record pentadeca charged with new lanthanide representatives, led to the discovery of additional unprecedented properties for ILs: The Gd compound exhibits a strong magnetocaloric effect (MCE) in the liquid state with a maximum magnetic entropy change of −ΔS_M = −11 J⋅kg⁻¹⋅K⁻¹ at 2 K for Δμ₀H = 7 T. Albeit the Dy representative shows slow magnetic relaxation, the relaxation times are not favorable for practical application as a molecular magnet. Lastly, for both the Gd and the Y compound, phosphorescence in the seconds time scale is observed, which is, to the best of our knowledge, the longest ever reported for an IL.

Our recent observations of an unexpected Ce(III) hydrolysis product from the reaction of 4-amino-1,2,4-triazole (4-NH₂-1,2,4-Triaz) with CeCl₃·7H₂O, [Ce₆(μ₃-O)₄(μ₃-OH)₂(μ₃-Cl)₂(Cl)₆(μ₂-4-NH₂-1,2,4-Triaz)₁₂]·7H₂O, the first high-nuclearity lanthanide complex where all Ln atoms are connected pairwise through 12 N-donor ligands or 12 neutral bridging ligands of any type, prompted us to explore the utility of this ligand in trapping additional f-element examples. Reactions of LnCl₃·6H₂O (Ln = Nd, Eu, Ho) with a large excess of 4-NH₂-1,2,4-Triaz (20 equiv) and with the addition of small amounts of water to help solubilize the metal salts led to the isolation of the unique hydrolysis products [Nd₆(μ₃-OH)₈Cl₆(μ₂-4-NH₂-1,2,4-Triaz)₁₂][Cl₄]·2H₂O, [Eu₆(μ₆-Cl)_0.23(μ₃-O_0.77)4(μ₃-O)_2.6(μ₃-Cl)_0.4Cl₆(μ₂-4-NH₂-1,2,4-Triaz)₁₂], and [Ho₆(μ₆-Cl)_0.21(μ₃-O_0.79)₄(μ₃-OH)₂Cl₆(μ₂-4-NH₂-1,2,4-Triaz)₁₂][Cl]_3.4. We also report a Ce(III) analogue prepared in glassware contaminated with Pb(OAc)₂, namely, [Ce₆(μ₃-OH)₈(BrPbBr₅)(μ₂-4-NH₂-1,2,4-Triaz)_11.5(OH₂)₆][Pb_0.84Br_4.2][Br]_3.8·2(4-NH₂-1,2,4-Triaz)·3.6H₂O. The Nd(III) complex is the structurally most ordered with a clear [Nd₆(μ₃-OH)₈] cluster core, while the Eu(III) and Ho(III) compounds contain partial occupancy of a μ6 position and thus result in an incomplete Ln₆O₉ cluster core formation. The crystallographic results suggest that the 4-NH₂-1,2,4-Triaz ligand brings Ln(III) ions together, followed by the formation of an Ln₆O₈ or Ln₆O₉ core with whatever remaining anions or ligands can be incorporated. Given the complexity of the hydrolysis products of nuclear waste, we expect to continue to find a myriad of closely related complex structures of these types for the f-elements. 

Two series of isostructural intermetallics have been discovered in our search for new compounds with fused honeycomb motifs, both stable at elevated temperatures (1073 K). They crystallize with orthorhombic unit cells - La4Co4M (M = Sn, Sb, Te, Pb, Bi, SG Pbam, a = 8.247-8.315(2), b = 21.913-22.137(7), c = 4.750-4.664(2) angstrom, V = 850.5-869.5(4) angstrom 3, Z = 4) and La3Ni3M (M = Al, Ga, SG Cmcm, a = 4.1790-4.2395(1), b = 10.4921-10.6426 (6), c = 13.6399-13.7616(8) angstrom, V = 606.72-612.05(7), Z = 3). The crystal structures represent interesting variations of semiregular tilings of corrugated anionic layers and predominantly cationic zigzag motifs. The La4Co4M compounds reveal a complex type of ordering with a high degree of frustration as could be expected for the Kagome ' -related lattices, while magnetic ordering in the La3Ni3M series is less evident. Electronic structure calculations have been performed for multiple compounds within both series revealing metallic character and visible local minima around the Fermi level. The bonding picture is characterized by nearly equal contributions from the anionic and the cationic components.

The La-poor part of the ternary La–Ni–Si system has been explored leading to the discovery and structural characterization of four new polar intermetallic compounds. LaNi₅Si₂ [BaAu₅Ga₂ type, oP64, space group Pnma, a = 7.8223(7) Å, b = 6.3894(6) Å, c = 17.843(2) Å, V = 891.8(2) ų, Z = 8] features a diamond (lonsdaleite)-like homoatomic Ni framework and is the first Ni representative of a larger family of compounds typically formed by aurides. La₂Ni₈Si₃ [Eu₂Ni₈Si₃ type, tP52, P4₂/nmc, a = 10.0278(3) Å, c = 7.5047(4) Å, V = 754.65(6) ų, Z = 4] is characterized by homoatomic Ni₄ tetrahedra and rectangles. LaNi₅Si₃ [SrNi₅P₃ type, oS36, Cmcm, a = 3.722(2) Å, b = 11.759(5) Å, c = 11.622(3) Å, V = 508.7(3) ų, Z = 4] is governed by extensive heteroatomic bonding and characterized by homopolyhedral packing. La₃Ni₄Si₂ [Ce₃Ni₄Si₂ type, mC36, C2/c, a = 15.819(1) Å, b = 6.0068(5) Å, c = 7.4918(6) Å, β = 103.163(5)°, V = 693.17(10) ų, Z = 4] is a new member of homologous series that includes La₃Ni₃Si₂ and La₃Ni_3.5Si₂. We conclude that the similar radii and electronegativities of Ni and Si are the reason for the incredible diversity of compositions and structures in this system.

Emerging organic light-emitting devices, such as light-emitting electrochemical cells (LECs), offer a multitude of advantages but currently suffer from that most efficient phosphorescent emitters are based on expensive and rare metals. Herein, it is demonstrated that a rare metal-free salt, bis(benzyltriphenylphosphonium)tetrabromidomanganate(II) ([Ph₃PBn]₂[MnBr₄]), can function as the phosphorescent emitter in an LEC, and that a careful device design results in the fact that such a rare metal-free phosphorescent LEC delivers broadband white emission with a high color rendering index (CRI) of 89. It is further shown that broadband emission is effectuated by an electric-field-driven structural transformation of the original green-light emitter structure into a red-emitting structure. 

Light-emitting materials based on earth-abundant metals, such as manganese hold great promise as emitters for organic lighting devices. In order to apply such emitter materials and, in particular, to overcome the problem of self-quenching due to cross-relaxation, we investigated a series of tetrabromidomanganate ([MnBr₄]²⁻) salts with bulky tetraalkylphosphonium counter cations [P_nnn]⁺, namely [P_nnnn]₂[MnBr₄] (n = 4 (1), 6 (2) and 8 (3)), which can be obtained by a straightforward reaction of the respective phosphonium bromide and MnBr₂. Variation of the cation size allows control of the properties of the resulting ionic materials. 1 and 3 qualify as ionic liquids (ILs), where 1 features a melting point of 68 °C, and 3 is liquid at room temperature and even at very low temperatures. Furthermore, 1 and 2 show the formation of higher-ordered thermotropic mesophases. For 1 a transition to a thermodynamically metastable smectic liquid crystalline phase can be observed at room temperature upon reheating from the metastable glassy state; 2 appears to form a plastic crystalline phase at ∼63 °C, which persists up to the melting point of 235 °C. The photoemission is greatly affected by phase behaviour and ion dynamics. A photoluminescence quantum yield of 61% could be achieved, by balancing the increase in Mn²⁺-Mn²⁺ separation and reducing self-quenching through increasingly large organic cations which leads to adverse increased vibrational quenching. Compared to analogous ammonium compounds, which have been promoted as @#x0308;inorganic hybrid perovskite, the phosphonium salts show superior performance, with respect to photoluminescent quantum yield and thermal and air/humidity stability. As the presented compounds are not sensitive to the atmosphere, in particular moisture, and show strong visible electroluminescence in the green region of light, they are important emitter materials for use in organic light-emitting devices.

In search of ionic liquids (ILs) with multiple types of soft donor atoms capable of preferentially complexing a range of soft metal ions over harder ions, we investigated structural clues to the role of hydrogen bonding in IL behavior through a series of salts with anions containing both N- and S-donor atoms based on azole thiolates. Reaction of equimolar amounts of triethylamine (Et₃N) or diisobutylamine (DBA) with 1-phenyl-1H-tetrazole-5-thiol (PhTzSH), 1-methyl-1H-tetrazole-5-thiol (MeTzSH), or 5-methyl-1,3,4-dithiazole-2-thiol (MeDiTSH) yielded [Et₃NH][MeTzS] (1), a yellow liquid, and the low melting yellow solids [DBAH][MeTzS] (2), [Et₃NH][PhTzS] (3), [DBAH][PhTzS] (4), [Et₃NH][MeDiTS] (5), and [DBAH][MeDiTS] (6). Thermal analysis revealed that all of them qualify as ILs with melting points below 100 °C. Single crystal X-ray structure analysis of 2–6 revealed the presence of an extensive H-bonding network that includes the rare N–H⋯S hydrogen bonds in 3, 4, and 6. These weaker interactions appear to significantly influence thermal behavior, where strong bonding leads to higher melting temperatures and lower decomposition points.

Our attempts to use an ionic liquid strategy to force f-element coordination of soft donor adenine and 4,5-dicyanoimidazole using 1-pot reactions in water using hydrated lanthanide salts failed, instead resulting in the isolation of anhydrous lanthanide nitrate salts. Reactions of previously ground mixtures of 40 % aq. [P₄₄₄₄][OH] and adenine with Nd(NO₃)₃·6H₂O or Sm(NO₃)₃·6H₂O followed by hand grinding and heating to 90 °C for 10 h led to hot crystallization of anhydrous [P₄₄₄₄]₂[Nd(NO₃)₅] and [P₄₄₄₄]₂[Sm(NO₃)₅]. The same reaction with 4,5-dicyanoimidazole and Nd(NO₃)₃·6H₂O but without heating (10 h, room temperature) to prevent ligand decomposition, led to crystallization of anhydrous [P₄₄₄₄]₃[Nd(NO₃)₆]. [P₄₄₄₄]₃[Nd(NO₃)₆] could also be isolated by conducting the first reaction without using adenine. The single crystal X-ray diffraction structures of the three new anhydrous salts were examined to provide clues to the observed behavior. The [P₄₄₄₄]⁺ cations allow charge balance while forming box-type encapsulations around the anionic counterpart which has been previously reported in similar structures. The fact that these anhydrous salts were prepared from water with hydrated starting materials and no precautions taken to keep the crystal growing process anhydrous, suggests synthetic strategies that can potentially be beneficial for more advanced separations procedures.