Skip to main content
NCBI home page
ACS AuthorChoice logoLink to ACS AuthorChoice
. 2024 Dec 26;64(1):723–730. doi: 10.1021/acs.inorgchem.4c03055

Crown-Ether Coordination Compounds of Europium and 24-Crown-8

Maxime A Bonnin 1, Sina Leicht 1, Claus Feldmann 1,*
PMCID: PMC11734689  PMID: 39725383

Abstract

graphic file with name ic4c03055_0007.jpg

Crown-ether coordination compounds of europium(II/III) and the crown ether (C2H4O)8 (24-crown-8, 24c8) are prepared, aiming at novel compounds, structures, and coordination modes as well as potential luminescence properties. By reacting EuCl2, EuI2, or EuCl3 with 24c8 or its derivatives in ionic liquids, the novel compounds [Bu3MeN]2[Eu(II)(NTf2)4] (1), [BMIm]6[Eu3I12] (2), [EuCl2(dibenzo-18c6)] (3), [EuI2(dibenzo-24c8)] (4), [(Eu(III)Cl3)2(C14H30O8)](24c8) (5), and [Eu(III)Cl(24c8)]I2 (6) are obtained (BMIm: 1-butyl-3-methylimidazolium; EMIm: 1-ethyl-3-methylimidazolium). Based on different reaction conditions, different coordinative modes including the absence of the crown ether in the product (1, 2), splitting of the crown ether (5), and coordination of 24c8 via six of eight oxygen atoms (4) and, finally, via all oxygen atoms (6) are observed. Crystallization of the title compounds is generally difficult, which can be attributed to the flexibility of the crown-ether molecule that can be rotated around all 24 tetrahedral (C) and pseudo-tetrahedral (O) centers. Besides structural characterization via single-crystal structure analysis and X-ray powder diffraction with Rietveld refinement, compounds 1–6 are examined by infrared spectroscopy and thermal analysis. The title compounds show blue to red emission, and the influence of structure and coordinative mode on the luminescent properties is analyzed.

Short abstract

Synthesis, coordinative motives, structures, and luminescence of crown-ether coordination compounds of Eu(II/III) and the crown ether 24c8 are studied, resulting in the novel compounds [Bu3MeN]2[Eu(II)(NTf2)4] (1), [BMIm]6[Eu3I12] (2), [EuCl2(dibenzo-18c6)] (3), [EuI2(dibenzo-24c8)] (4), [(Eu(III)Cl3)2(C14H30O8)](24c8) (5), and [Eu(III)Cl(24c8)]I2 (6).

Introduction

Crown ethers are known as versatile ligands and have led to a rich coordination chemistry.1 Coordination compounds with crown ethers as ligands were reported for almost all metal cations.1,2 Even alkali metals show strong and selective coordination to crown ethers, such as in [Na(15c5)]+ or [K(18c6)]+ (15c5: 15-crown-5/(C2H4O)5; 18c6: 18-crown-6/(C2H4O)6).3 In contrast to this well-established coordination chemistry, the luminescence properties of crown-ether coordination compounds have rarely been addressed. To this end, luminescent crown-ether coordination compounds need to be separated into compounds with additional ligands/functional groups as the origin of luminescence and compounds with the coordinated cation being the origin of light emission.4 Crown-ether compounds with distinct fluorescent dyes (e.g., anthracenes, pyrenes, etc.) as additional ligands/functional groups of the crown-ether scaffold, in fact, are well-known and widely used for sensing or in analytical chemistry to detect the presence of certain metal cations.5,6 In contrast, luminescence of the coordinated cation is rare. Crown-ether coordination compounds with luminescent cations were described, for instance, with Mn2+, Sm2+, Eu2+/3+, or Tm3+ but usually show only week emission or emission at low temperatures (≤77 K).7−13 In many cases, moreover, water, OH, alcohols, or amines are coordinated to the metal center, so the emission is expected to be quenched due to relaxation via high-energy vibronic transitions.

Serendipitously, we recently observed intense emission and unexpected high quantum yields (>80%) for some crown-ether coordination compounds of 18c6 with divalent cations such as Mn2+ and Eu2+.14 Specific examples are [Mn3I6(18c6)2], [Mn2I4(18c6)], and [EuI2(18c6)]. The occurrence of these surprising luminescence properties could be related to the specific structure and bonding situation. In regard of intense emission and high quantum yields, the following conditions turned out to be favorable: (i) the formation of finite absorber–emitter pairs with short distance (<550 pm) and a significantly larger distance to all other luminescent centers (>850 pm) to avoid concentration quenching; (ii) the absence of ligands with high-energy vibronic transitions (i.e., O–H, N–H, C=O, or similar); (iii) a rigid coordination of the luminescent centers; and (iv) a coordination by ligands with high atomic mass (e.g., iodine preferred over chlorine) to reduce vibrational relaxation processes.15 In the case of crown ethers as ligands, moreover, the mismatch of cation radius (e.g., Mn2+ with 83 pm, CN = 6)16 and the ring opening of the crown ether (e.g., 18c6 with 300 pm)17 can result in a strong bending of the crown-ether molecule, which promotes a rigid coordination of the luminescent center.15 Taking these considerations into account, the extraordinary properties of [Mn2I4(18c6)], showing a quantum yield of 100% as well as nonlinear optical properties, can be rationalized.14,15

Based on the interesting coordination, structures, and the resulting promising luminescence properties of coordination compounds with Mn2+ and 18c6, here we aim at coordination compounds of Eu2+/3+ with the larger crown ether 24c8 (24-crown-8/(C2H4O)8). The knowledge on such coordination compounds is generally rare. For Eu2+/Eu3+ and 24c8, the ratio of cation radius (r) and ring-opening diameter (o) with 2r:o = 2.0 to 2.5 is almost similar to (Eu2+) or even larger (Eu3+) compared to that of Mn2+ and 18c6 (2r:o = 1.8). Thus, coordination compounds of Eu2+/Eu3+ with 24c8 could also show certain bending of the crown-ether molecule, potentially resulting in a rigid coordination of Eu2+/Eu3+ with promising luminescent features.

Results and Discussion

Ionic Liquid-Based Synthesis

Because of the sensitivity of europium halides to moisture and the necessity to exclude low-weight ligands (H2O, THF, acetonitrile, etc.) in regard to optimal luminescent properties, all syntheses were performed in ionic liquids (i.e., [Bu3MeN][NTf2], [EMIm][NTf2], [BMIm]I; BMIm: 1-butyl-3-methylimidazolium; EMIm: 1-ethyl-3-methylimidazolium). Besides the absence of moisture, ionic liquids have good thermal stability, weakly coordinating properties, and good solubility of europium halides EuX2/EuX3.18 Thus, the weakly coordinating properties promote the coordination of Eu2+/3+ with 24c8 and avoid the formation of solvato complexes, which may occur, for instance, with conventional solvents like THF or acetonitrile. However, certain disadvantage of ionic liquids relates to the difficult separation of the product and ionic liquid subsequent to synthesis because the solubility of product and ionic liquid are often very similar. To this end, we used small portions of cold CH2Cl2 (0 °C) to wash the crystals of the title compounds and to remove most of the ionic liquid. Adsorption of the remaining ionic liquid on the crystal surfaces, however, can hardly be avoided.

In detail, EuI2, EuCl3, and 24c8 or dibenzo-24c8 were reacted at 60–180 °C for 1–2 weeks in [EMIm][NTf2], [Bu3MeN][NTf2], or [BMIm]I (Figure 1). In regard to the moisture sensitivity of the starting materials and the title compounds, all handling and manipulation were performed with inert conditions (argon, nitrogen) using a glovebox or Schlenk techniques. All reactions were performed in glass ampules sealed under argon. After the synthesis, the title compounds [Bu3MeN]2[Eu(II)(NTf2)4] (1), [BMIm]6[Eu3I12] (2), [EuCl2(dibenzo-18c6)] (3), [EuI2(dibenzo-24c8)] (4), [(Eu(III)Cl3)2(C14H30O8)](24c8) (5), and [Eu(III)Cl(24c8)]I2 (6) were obtained as single crystals (Figure 1).

Figure 1.

Figure 1

Open in a new tab

Exemplary illustration of the ionic liquid-based synthesis of the title compounds by reaction of EuX2 or EuX3 with 24c8.

Chemical Composition and Structural Properties

[Bu3MeN]2[Eu(NTf2)4] (1)

As a first approach, Eu(II)I2 was reacted with 24c8 in [Bu3MeN][NTf2] as the ionic liquid. Already at room temperature (25 °C), a yellow solid formed, which, however, turned out to be noncrystalline according to powder X-ray diffraction (PXRD). After heating to 60 °C for 2 weeks, colorless, platelet-shaped crystals were obtained in addition to the yellow solid. These platelet-shaped crystals were identified as [Bu3MeN]2[Eu(NTf2)4] (1) by single-crystal structure analysis. As 24c8 is not present in 1, the synthesis was repeated without the crown ether, which again resulted in the formation of 1 and the formation of the yellow solid. Based on these findings, the reaction can be rationalized based on the following equation:

graphic file with name ic4c03055_m001.jpg

The yellow solid was indeed identified as [Bu3MeN][EuI3] with energy dispersive X-ray spectroscopy indicating an Eu:I ratio of 1.0:2.8 (calculated: 1:3). Although the specific composition [Bu3MeN][EuI3] is new, comparable compounds (e.g., [Ph4P][EuI3] × 0.5 H2O)19 are well-known, so [Bu3MeN][EuI3] was not characterized in detail. In addition to the single-crystal structure analysis (Table S1, Figure S1), the composition and structure of 1 were confirmed by PXRD with Rietveld refinement (Figure S11). In regard to 24c8, it must be noted that the crystalline product 1, against our intention, does not contain the crown ether. Interestingly, only [NTf2]−, known as a weakly coordinating anion, is present instead of the chelating crown-ether ligand.

Despite the absence of 24c8 in 1, the compound nevertheless has interesting structural features. 1 crystallizes in the triclinic space group P1̅ and consists of [Bu3MeN]+ cations and [Eu(NTf2)4]2– anions (Table S1, Figures S1 and S2). Due to its size and the delocalization of the anion charge, the [NTf2]− anion is usually considered as weakly coordinating and chemically inert.18,20 In this regard, it is surprising that a coordination complex of Eu2+ with four [NTf2]− anions as ligands is formed, although 24c8 as a chelating agent is available. Metal complexes with [NTf2]− as a ligand are of course known and made, for instance, by the dissolution of metal halides in [NTf2]−-containing ionic liquids.21,22 However, the metal cation usually coordinates via the more nucleophilic nitrogen atom instead of the oxygen atoms of the SO3 groups. In 1, all four [NTf2]− anions serve as chelating ligands with a coordination of two oxygen atoms of each −SO3 group. In sum, Eu2+ is coordinated by a total of eight oxygen atoms, forming a distorted squared prism (Figure 2a). It should be noted that the [NTf2]− anions show positional disorder, which was tackled by split-atom positions with an occupancy of 50% for each (Figure S2). Such κ2 coordination of the [NTf2]− anions via O atoms was already reported for several trivalent lanthanides (e.g., [Sm(H2O)5(NTf2)3], [BMPyr]2[Pr(NTf2)5]),23−26 usually resulting in a 9-fold coordination of Ln3+.24 For europium, only [NTf2]− complexes of Eu3+ are known (e.g., [BMPyr]2[Eu(NTf2)5]).27 The structure and coordination of 1 are most comparable to those of [MPPyr]2[Yb(II)(NTf2)4],28 which, however, exhibits a distorted squared antiprismatic coordination instead of the prismatic coordination of Eu2+ in 1.

Figure 2.

Figure 2

Open in a new tab

Anionic building units in 1 and 2: (a) [Eu(NTf2)5]2– anion in 1 and (b) [Eu3I12]6– anion in 2 (H atoms are not shown for clarity; only one position is shown for positional disorder in 1, while both positions are shown in Figure S2).

The Eu–O distances (258.4(6)–269.3(4) pm) in 1 are longer than in EuO (257.3 pm) but comparable to those of coordination compounds such as [EuCl2(18c6)] (265–275 pm).14 The distortion of the squared prism is confirmed by significantly diverging O–Eu–O angles (58.3(2)–121.7(2)°), whereas the O–Eu–O angles of opposite O atoms are 180° due to the lattice symmetry.

[BMIm]6[Eu3I12] (2)

To exclude coordination with [NTf2]−, we next reacted EuI2 in [BMIm]I as the ionic liquid for 7 days at 80 °C. The obtained green crystals were identified by single-crystal structure analysis and PXRD with Rietveld refinement as [BMIm]6[Eu3I12] (Figure S11). 2 crystallizes in the trigonal space group R3̅ and consists of [BMIm]+ cations and [Eu3I12]6– anions (Table S1, Figure S3). Herein, three face-sharing EuI6 octahedra form a [Eu3I12]6– anion, which is orientated along the crystallographic c-axis (Figure 2b, Figure S3). This anion is first observed and compares to [Bi3I12]3–29 or infinite 1∞[EuI6/2] chains of face-sharing EuI6 octahedra in [Ph4P][EuI3].19 The Eu–I distances (320.1(1)–336.6(1) pm) in 2 show wide variation with a central EuI6 octahedron (Eu–I: 323.4(1) pm) and two distorted terminal EuI6 octahedra with shorter distances of terminal (Eu–Iterm: 320.1(1) pm) than bridging (Eu–Ibridge: 336.6(1) pm) iodine atoms.

Similar to 1, the crystalline product 2 again does not contain the chelating crown ether 24c8 as a ligand. Therefore, we also reacted the more ionic EuCl2 with 24c8 in [Bu3MeN][NTf2]. However, no crystalline product with 24c8 could be obtained here. The only observation was a recrystallization of EuCl2 with the formation of colorless, millimeter-sized crystals.

[EuCl2(dibenzo-18c6)] (3)

In order to verify whether the absence of crystalline products after heating EuCl2/EuI2 and 24c8 relates to an insufficient reaction and/or an insufficient crystallization, a similar reaction was performed with the dibenzylated derivative of the crown ether. Due to the benzyl groups, on the one hand, the crown-ether molecule becomes less flexible as the rotation around the tetrahedral centers is limited. On the other hand, the intermolecular π-interaction of the benzyl groups could support crystallization. To verify this attempt, we first reacted the smaller, even less flexible dibenzo-18c6 with EuCl2 and EuI2. As dibenzo-functionalized crown ethers show poor solubility in ionic liquids with purely aliphatic cations (i.e., [Bu3MeN][NTf2]), here, [EMIm][NTf2] and [BMIm]I were used.

The reaction of EuCl2 with dibenzo-18c6 in [EMIm][NTf2] at 180 °C for 7 days resulted in colorless crystals of [EuCl2(dibenzo-18c6)] (3). 3 crystallizes in the monoclinic space group C2/c and consists of noncharged molecular units (Table S1, Figures S4 and S11). As intended, these molecular units show intermolecular π-stacking with distances of <360 pm between benzene rings (Figure S5). Eu2+ is equatorially coordinated by six oxygen atoms of dibenzo-18c6 with two chlorine atoms in the axial position, resulting in a hexagonal bipyramidal coordination (Figure 3a). The Eu–Cl distances (282.7(1), 289.2(1) pm) are elongated in comparison to those in [EuCl2(18c6)] (273, 283 pm) but shorter than those in EuCl2 (292–344 pm).14,30 The Cl–Eu–Cl angle (180.0(1)°) is perfectly linear. The Eu–O distances (265.9(1)–267.7(1) pm) are comparable to those of similar compounds ([EuCl2(18c6)]: 3 × 265, 3 × 275 pm).14

Figure 3.

Figure 3

Open in a new tab

Molecular units of (a) [EuCl2(dibenzo-18c6)] (3) and (b) [EuI2(dibenzo-24c8)] (4) (H atoms are not shown for clarity).

While dibenzo-18c6-coordinated compounds of trivalent lanthanides are well-known (e.g., [GdCl2(MeCN)(dibenzo-18c6)][SbCl6]·2MeCN, [Dy2Cl4(dibenzo-18c6)2][Dy2(MeCN)2Cl8], [Nd(BH4)2(dibenzo-18c6)][BPh4]·2THF), [Yb(MeCN)3(dibenzo-18c6)][BPh4]2·3MeCN is yet the only crystal structure with a divalent lanthanide cation.31−33 Most often, further ligands (e.g., solvents such as THF) are present in addition to the crown ether and halide. Compounds such as 3 without further ligands are rare and include [EuCl2(18c6)], [SmI3(dibenzo-18c6)], and [HgCl2(dibenzo-18c6)].14,34,35 Here, the absence of further ligands can be ascribed to the use of weakly coordinating ionic liquids as solvents.

[EuI2(dibenzo-24c8)] (4)

After the successful reaction with dibenzo-18c6, EuI2 was reacted with the larger dibenzo-24c8 in [BMIm]I for 7 days at 150 °C. According the single-crystal structure analysis and PXRD, the resulting colorless crystals crystallize in the triclinic space group P1̅ (Table S2, Figures S6 and S11). Similar to 3, 4 consists of noncharged molecular units with significant intermolecular π-stacking between benzene rings (distances of <355 pm; Figure S7). Eu2+ is axially coordinated by two iodine atoms and equatorially by the crown ether, but only with six of eight oxygen atoms of dibenzo-24c8. In summary, this results in a hexagonal bipyramidal coordination with two oxygen atoms of dibenzo-24c8 left uncoordinated (Figure 3b). The Eu–I distances (318.1(1), 326.8(1) pm) are comparable to those of [EuI2(18c6)] (323 pm).14 The I–Eu–I angle (174.3(1)°) points to slight tilting of the iodine atoms toward the noncoordinating oxygen atoms of the crown ether (Figure 3b). The Eu–O distances (265.4(2)–279.2(3) pm) are in agreement with those of comparable compounds ([EuI2(18c6)]: 4 × 272, 2 × 273 pm).14 The O–Eu–O angles (55.8(1)–68.6(1)°) indicate a 6-fold planar coordination. Comparable coordination compounds of dibenzo-24c8 are known with examples like [Ba(ClO4)2(dibenzo-24c8)] and [Hg(CN)2(dibenzo-24c8)]·H2O.36,37 Coordination compounds of dibenzo-24c8 and the lanthanides, however, were not described.

[(EuCl3)2(C14H30O8)](24c8) (5)

Since reactions of Eu(II)Cl2 and Eu(II)I2 with 24c8 did not result in crystalline crown-ether coordination compounds, similar syntheses with the more Lewis-acidic and more strongly coordinating Eu(III)Cl3 were examined. Indeed, the reaction of EuCl3 with 24c8 in [Bu3MeN][NTf2] as the ionic liquid after 14 days at 60 °C resulted in colorless, rhombic crystals, which were identified by single-crystal structure analysis and PXRD as [(EuCl3)2(C14H30O8)](24c8) (5) (Table S2, Figures S8 and S11). 5 crystallizes in the monoclinic space group P21/n and consists of noncharged, dinuclear [(EuCl3)2(C14H30O8)] molecules with additional noncoordinating 24c8 in between.

Along with the higher Lewis acidity of Eu(III)Cl3 in comparison with Eu(II)Cl2, Eu3+ is now coordinated by 24c8, but the crown-ether ring was split to an open heptaethylene-glycol chain (Figure 4a). Such splitting of 24c8 is unexpected. Since 5 is reproducibly prepared in larger quantities and the splitting was only observed for 5, the presence of moisture can be excluded. On the other hand, ionic liquids are known to be C–H acidic,38 so the splitting of 24c8 most likely results from the following combined effects: (i) the interaction of EuCl3 as a Lewis acid with oxygen atoms of 24c8; (ii) the C–H acidity of the ionic liquid to protonate oxygen atoms; and (iii) the coordination of Eu3+ by the heptaethylene-glycol ligand, which is obviously preferred over coordination with 24c8. Such splitting of crown ethers is rare but was nevertheless observed before.39,40 As a result, 5 is a dinuclear complex with two EuCl3 units and each Eu3+ being coordinated by four oxygen atoms of a heptaethylene-glycol ligand. In summary, this results in a distorted pentagonal bipyramidal coordination of Eu3+ with three chlorine and four oxygen atoms. The terminal hydroxyl groups of the two Eu3+-ion-bridging heptaethylene-glycol ligands show hydrogen bonding to a noncoordinating 24c8 molecule (Figure 4b). This hydrogen bonding also confirms the presence of OH groups terminating the heptaethylene-glycol ligand and leads to a chain-like structure of 5. Comparable ethylene glycol-type ligands are known for several Ln(III) cations. These compounds, however, are mononuclear complexes throughout and exhibit smaller tri-, tetra-, or pentaethylene-glycol units.41−43 Only [MCl3(C6H14O4)](18c6) (M = Dy, Y) exhibits a chain-type structure via hydrogen bonding between triethylene glycol and 18c6.

Figure 4.

Figure 4

Open in a new tab

Building units in [(EuCl3)2(C14H30O8)](24c8) (5): (a) single (24c8)[(EuCl3)2(C14H30O8)] unit and (b) chain-like [(EuCl3)2(C14H30O8)](24c8) (anisotropic displacement parameters with 50% probability of finding; for clarity, only H atoms of the O–H hydrogen bridge are shown).

Both the Eu–Cl (261.2(1)–264.7(1) pm) and Eu–O distances (236.3(3)–247.5(2) pm) in 5 are comparable to those in [EuCl3(C8H18O5)] (Eu–Cl: 265.2–278.0 pm; Eu–O: 244.8–253.0 pm).42 The O–Eu–O (64.1(1)–66.0(1)°) and Cl–Eu–O (85.0(1)–92.2(1)°) angles are also comparable with literature data ([DyCl3(C6H14O4)](18c6): O–Eu–O: 66.0–66.9°; Cl–Eu–O: 80.6–91.5°).41 Together with the equatorial chlorine atom, four oxygen atoms around Eu3+ form an almost planar pentagon with the axial chlorine atoms located almost perpendicular to the pentagon (Clax–Eu–O/Cleq: 94.2(1)–94.6(1)°; Clax–Eu–Clax: 170.9(1)°). Finally, it should be noted that the noncoordinating 24c8 molecule is disordered in regard of the two oxygen atoms being involved in the hydrogen bonds. This was tackled by split-atom positions with 50% occupancy for each of these atoms so that there is a shorter and a longer O···H–O distance (200(3)/202(2) and 242(3)/265(3) pm).

[EuCl(24c8)]I2 (6)

As reactions of Eu2+ and 24c8 did not result in crystalline products and since EuCl3 seems to be too acidic, finally, a reaction of 24c8 with EuI3 was tested. EuI3, however, is unknown as a pure compound, as it decomposes with the formation of EuI2 and I2.44 In order to at least allow an exchange of the halide, we performed a synthesis with EuCl3 in [BMIm]I as the ionic liquid. By reacting EuCl3 and 24c8 in [BMIm]I over 14 days at 60 °C, indeed, colorless crystals were obtained, which, according to single-crystal structure analysis, were identified as [EuCl(24c8)]I2 (6). 6 crystallizes in the noncentrosymmetric orthorhombic space group Cmc21 (Table S2, Figure S9) and contains a [EuCl(24c8)]2+ cation and two isolated I– anions (Figure 5). In contrast to 1–5, 6 was obtained with only a few single crystals accompanied by major amounts of nonreacted starting materials. Attempts to accelerate the reaction by increasing the temperature (60 → 80 °C), however, only led to the decomposition of 24c8. In fact, slow and poor crystallization of 6 is not a surprise and coincides with the high flexibility of the 24c8 molecule scaffold, which can rotate around all 24 tetrahedral (C) and pseudo-tetrahedral (O) centers.

Figure 5.

Figure 5

Open in a new tab

[EuCl(24c8)]2+ cation in 6 (H atoms are not shown for clarity).

Despite the low quantities, 6 exhibits the intended coordination of Eu3+ with all eight oxygen atoms of 24c8 as well as with significant bending of the crown-ether molecule. Taking the additional chlorine atom into account, a 9-fold coordination results for Eu3+. It should be noted that 24c8 shows positional disorder over two positions with 50% occupancy for each (Figure S10). The Eu–Cl distance (265.4(3) pm) in 6 is comparable to literature data ([EuCl3(C8H18O5)]: Eu–Cl: 265.2–278.0 pm).42 In contrast, the Eu–O distances (241.2(11)–256.1(13) pm) are elongated compared to those of 5 (236.3(3)–247.5(2) pm), which can be attributed to the 8-fold coordination of 24c8 and the limited flexibility of the ethylene groups. The angles in 6 are 60.7(4)–67.4(4)° for O–Eu–O and 72.4(3)–85.2(3)° for Cl–Eu–O. Finally, two isolated iodide anions are observed, which stem from the ionic liquid and which show long Eu–I distances (599.4(1), 705.0(1) pm). In sum, such coordination of trivalent lanthanides with 24c8 is rare. So far, only spectrophotometric studies of trivalent lanthanides with dibenzo-24c8 are known.45,46

Material Properties

Besides X-ray diffraction based on single crystals and powders, the compounds 1–5 were additionally characterized by Fourier transform infrared (FT-IR) spectroscopy, differential thermal analysis (DTA), and thermogravimetry (TG) (Figures S12–S14). As discussed before, only a few single crystals of 6 were obtained together with large amounts of nonreacted starting materials, so no further analysis could be performed here.

The FT-IR spectra of 1–5 are dominated by the respective organic constituents, as indicated by comparison with the relevant starting materials (Figure S12). Thus, 1 shows the characteristic vibrations of [Bu3MeN]+ and [NTf2]−. Similarly, the FT-IR spectrum of 2 is dominated by vibrations of the [BMIm]+ cation. For 3 and 4, vibrations of the crown ethers dibenzo-18c6 and dibenzo-24c8 are most prominent. For 5, the dominating ν(C–H) (3000–2800 cm–1) and ν(C–O) vibrations (1150–1000 cm–1) relate to the heptaethylene-glycol unit and 24c8 (Figure S12). Moreover, weaker ν(S=O) (1400–1200 cm–1) and ν(C–F) vibrations (750–500 cm–1) point to the presence of surface-adsorbed ionic liquid. Due to the low concentration and the hydrogen bonding, the ν(O–H) vibration of the terminal OH group of the heptaethylene-glycol unit can be expected to become broad and have low intensity, and it is therefore not visible in the spectra.47

Regarding the thermal properties, first, DTA shows endothermal peaks at 80 to 220 °C for 1–5, with 1 exhibiting the lowest (80 °C) and 3 the highest melting point (220 °C) (Figure S13). Moreover, TG shows a one-step decomposition for 2–4 and a two-step decomposition for 1 and 5 at 250–450 °C (Figure S14). The thermal decomposition can be rationalized based on the decomposition of the organic constituents (e.g., [Bu3MeN]+ and [NTf2]− for 1) or the evaporation of the crown ether (e.g., dibenzo-24c8 for 4). The fact that the observed mass losses are in accordance with the calculated values also points to the composition of the title compounds (Table S3).

Finally, the luminescence properties of 1–6 were examined. As both Eu2+ and Eu3+ are well-known luminescent centers,48 in principle, visible emission can be expected for all title compounds. Specifically, d → f transitions may occur on Eu2+, and f → f transitions may occur on Eu3+.48 The compounds 2–5 indeed show visible emission ranging from blue (3, 4) and green (2) to red light (5) (Figure 6). In the case of 1, the colorless crystals did not show any emission at room temperature. Only the yellow side product [Bu3MeN][EuI3] exhibits intense green emission (Figure 6a). Despite the intended coordination of Eu3+ by 24c8, 6 neither spectroscopically nor visually shows any emission at room temperature as well (Figure 6f). Excitation and emission spectra of 5 show characteristic f → f transitions of Eu3+, whereas 2–4 exhibit the typical features of d → f transitions of Eu2+.48 Due to strong coupling of the d orbitals with the ligands, the wavelength of excitation and emission of 2–4 strongly depends on the specific ligands. Here, a red-shifted emission of 2 (λem(max) = 510 nm) with iodine coordination of Eu2+ is expected in comparison to the mixed coordination with oxygen and halide of 3 (λem(max) = 405 nm) and 4 (λem(max) = 414 nm). The emission of [EuI2(dibenzo-24c8)] (4) is nevertheless surprising due to its narrow bandwidth and the low wavelength, although iodine as a polarizable ligand is present (Figure 6d). In principle, such narrowband blue-emitting features can be interesting for organic light-emitting diodes (OLEDs) to replace the current Ru- or Ir-containing dyes.49

Figure 6.

Figure 6

Open in a new tab

Optical properties: (a) 1 with photos in daylight and with excitation (luminescence only of side product [Bu3MeN][EuI3]). Excitation and emission spectra and photos with excitation of (b) [BMIm]6[Eu3I12] (2), (c) [EuCl2(dibenzo-18c6)] (3), (d) [EuI2(dibenzo-24c8)] (4), and (e) [(EuCl3)2(C14H30O8)](24c8) (5). (f) Photos of 6 in daylight and with excitation. Excitation for all was at λex = 366 nm.

Although the compounds 2–5 show good emission intensity, their quantum yields with up to 13% (4) and 36% (5) are much below those of recently discovered crown-ether coordination compounds of Eu2+ and Mn2+ with quantum yields of 90–100% (Table 1).14 A comparison of 1–6 with comparable structural features and with Eu2+/3+ as the only luminescent center can nevertheless help to understand the influence of the coordination modes on the luminescent processes and the efficiency of crown-ether coordination compounds. Besides the crown-ether ligand, first, heavy ligands such as iodine are preferred over chlorine to decrease vibrational loss processes (Table 1). In addition, rigid binding of the luminescent Eu2+/3+ center by the ligands, as indicated by, for example, short distances and low coordination numbers, is supportive. The isotropic thermal displacement parameter (Ueq) can be indicative as well to evaluate the vibration of the cation itself (Table 1). For this purpose, data collection for single-crystal structure analysis at the same temperature with the same type of diffractometer and a comparable crystal quality is a prerequisite (Tables S1 and S2). Ueq was evaluated instead of the anisotropic thermal displacement parameters in order to be independent from the different site symmetry of compounds with different space-group symmetries (Table 1, Tables S1 and S2).

Table 1. Distances, Isotropic Displacement Parameter, and Quantum Yield of 1–6 as well as [EuCl2(18c6)] (Structural Data from Single-Crystal Data Collected at 210 K for 1–6 and 213 K for [EuCl2(18c6)]).

compound Eu–O/pm Eu–X/pm C.N. Eu Ueq of Eu/Å2 × 103 quantum yield/% T/K
[Bu3MeN]2[EuII(NTf2)4] (1) 258.4–269.3 / 8 57.4(1) <5 210
[EuIICl2(18c6)]14 265.0–275.4 273.3–283.4 8 37.1(1) 36(4) 213
[EuIICl2(dibenzo-18c6)] (3) 265.9–267.7 282.7, 289.2 8 25.3(1) 6(3) 210
[(EuIIICl3)2(C14H30O8)](24c8) (5) 236.3–247.5 261.2–264.7 7 32.0(1) 36(3) 210
[EuIIICl(24c8)]I2 (6) 241.2–256.1 265.4 9 31.7(1) <5 210
[EuIII2(18c6)]14 271.5–272.5 323.0 8 23.1(3) 82(3) 210
[BMIm]6[EuII3I12] (2) / 320.1, 323.4, 336.6 6 47.7(2), 52.3(1) 30(5) 210
[EuIII2(dibenzo-24c8)] (4) 265.4–279.2 318.1, 326.8 8 33.0(1) 13(5) 210
Open in a new tab

It must also be taken into account that no single parameter is decisive for efficient luminescence, but rather a combination of several is. Based on these considerations, 1 and 6 can be expected to show poor emission (i.e., Eu2+/3+ coordinated only by light elements, high coordination number, high Ueq). For the iodine-coordinated Eu2+ compounds, 2 and 4 can be clearly expected to be less efficient than [EuIII2(18c6)] due to higher distances and higher Ueq, indicating a higher probability of vibrational losses (Table 1). For the chlorine-coordinated compounds, it must be noted that the f → f transitions of Eu3+ are generally less sensitive to the lattice compared to the d → f transitions on Eu2+.48 Due to the high coordination number at comparable distances and Ueq value, 6 can be expected to be less efficient. Compound 3, however, could have shown a quantum yield similar to or even higher than that of [EuIICl2(18c6)] (Table 1). Possibly the intermolecular π-stacking of benzene rings results in an additional loss process for [EuCl2(dibenzo-18c6)] (3).

Conclusions

The synthesis, coordination modes, structures, and luminescence of crown-ether coordination compounds of Eu2+/3+ and crown ether 24c8 and derivatives were examined. As a result, the novel compounds [Bu3MeN]2[Eu(II)(NTf2)4] (1), [BMIm]6[Eu3I12] (2), [EuCl2(dibenzo-18c6)] (3), [EuI2(dibenzo-24c8)] (4), [(Eu(III)Cl3)2(C14H30O8)](24c8) (5), and [Eu(III)Cl(24c8)]I2 (6) (BMIm: 1-butyl-3-methylimidazolium; EMIm: 1-ethyl-3-methylimidazolium) were obtained by ionic liquid-based synthesis. Ionic liquids were used because of their weekly coordinating properties to avoid ligands other than the crown ether and halogen. The reaction and crystallization of 24c8-coordinated Eu2+/3+ compounds generally turned out to be difficult, which can be ascribed to the flexibility of the crown-ether scaffold with possible rotation around all 24 tetrahedral/pseudo-tetrahedral centers. Reactions with EuCl2 and EuI2 resulted in [Bu3MeN]2[Eu(II)(NTf2)4] (1) and [BMIm]6[Eu3I12] (2) without any coordination of 24c8. Only when using the more rigid crown ethers dibenzo-18c6 and dibenzo-24c8, [Eu(II)Cl2(dibenzo-18c6)] (3) and [Eu(II)I2(dibenzo-24c8)] (4) were realized. The more Lewis-acidic EuCl3 causes splitting of 24c8 with the formation of the dinuclear [(Eu(III)Cl3)2(C14H30O8)](24c8) (5) with a heptaethylene-glycol unit. Since EuI3 is sensitive to decomposition to EuI2/I2, finally, EuCl3 was reacted with 24c8 in [BMIm]I. Indeed, this resulted in the formation of [Eu(III)Cl(24c8)]I2 (6) as the first example of a lanthanide cation μ8-coordinated by 24c8.

The compounds 2–5 show visible emission ranging from blue (3, 4) to green (2) and red (5) due to d → f transitions of Eu2+ (3, 4) or f → f transitions of Eu3+ (2). Based on the comparable structural features and with Eu2+/3+ as the only luminescent center, 1–6 can be compared in order to rationalize the influence of the coordination modes on the luminescent processes and the efficiency of crown-ether coordination compounds. In this regard, heavy ligands such as iodine in addition to the crown-ether ligand and a rigid coordination of the luminescent Eu2+/3+ center by the ligands with short distances, a low coordination number, and low vibration of the luminescent center itself are supportive for efficient emission. Besides the synthesis of new compounds with different types of coordination, the emission colors ranging from blue and green to red are interesting. Specifically, the narrowband blue emission of 4 is relevant for the use of OLEDs. Finally, the comparison and understanding of crown-ether coordination compounds can initiate the realization of further examples with interesting luminescence properties.

Acknowledgments

M.A.B. and C.F. acknowledge the Deutsche Forschungsgemeinschaft (DFG) for funding within the project “Crown-Ether-Coordination-Compounds with Unusual Structural and Optical Properties/Crown I (FE 911/14-1)”.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.4c03055.

  • Details regarding analytical techniques, synthesis of the title compounds, crystallographic details, and spectroscopic and thermal analyses (PDF)

The authors declare no competing financial interest.

Supplementary Material

ic4c03055_si_001.pdf (1.4MB, pdf)

References

  1. Liu Z.; Nalluri S. K. M.; Stoddart J. F. Surveying macrocyclic chemistry: from flexible crown ethers to rigid cyclophanes. Chem. Soc. Rev. 2017, 46, 2459–2478. 10.1039/C7CS00185A. [DOI] [PubMed] [Google Scholar]
  2. Bradshaw J. S.; Izatt R. M. A.; Bordunov V.; Zhu C. Y.; Hathaway J. K.. Crown ethers. In Comprehensive Supramolecular Chemistry; Gokel G. W., Ed.; 1996; Vol. 1, p 35. [Google Scholar]
  3. Steed J. W. First- and second-sphere coordination chemistry of alkali metal crown ether complexes. Coord. Chem. Rev. 2001, 215, 171–221. 10.1016/S0010-8545(01)00317-4. [DOI] [Google Scholar]
  4. Zhou W.-L.; Chen Y.; Lin W.; Liu Y. Luminescent lanthanide-macrocycle supramolecular assembly. Chem. Commun. 2021, 57, 11443–11456. 10.1039/D1CC04672A. [DOI] [PubMed] [Google Scholar]
  5. Li J.; Yim D.; Jang W.-D.; Yoon J. Recent progress in the design and applications of fluorescence probes containing crown ethers. Chem. Soc. Rev. 2017, 46, 2437–2458. 10.1039/C6CS00619A. [DOI] [PubMed] [Google Scholar]
  6. Gokel G. W.; Leevy W. M.; Weber M. E. Crown Ethers: Sensors for Ions and Molecular Scaffolds for Materials and Biological Models. Chem. Rev. 2004, 104, 2723–2750. 10.1021/cr020080k. [DOI] [PubMed] [Google Scholar]
  7. Xemard M.; Cordier M.; Molton F.; Duboc C.; Le Guennic B.; Maury O.; Cador O.; Nocton G. Divalent Thulium Crown Ether Complexes with Field-Induced Slow Magnetic Relaxation. Inorg. Chem. 2019, 58, 2872–2880. 10.1021/acs.inorgchem.8b03551. [DOI] [PubMed] [Google Scholar]
  8. White F. D.; Celis-Barros C.; Rankin J.; Solís-Céspedes E.; Dan D.; Gaiser A. N.; Zhou Y.; Colangelo J.; Páez-Hernández D.; Arratia-Perez R.; Albrecht-Schmitt T.-E. Molecular and Electronic Structure, and Hydrolytic Reactivity of a Samarium(II) Crown Ether Complex. Inorg. Chem. 2019, 58, 3457–347. 10.1021/acs.inorgchem.8b03566. [DOI] [PubMed] [Google Scholar]
  9. Voss D. A.; Farquhar E. R.; Horrocks W. D.; Morrow J. R. Lanthanide(III) complexes of amide derivatives of DOTA exhibit an unusual variation in stability across the lanthanide series. Inorg. Chim. Acta 2004, 357, 859–863. 10.1016/S0020-1693(03)00337-2. [DOI] [Google Scholar]
  10. Starynowicz P. Two complexes of Sm(II) with crown ethers-electrochemical synthesis, structure and spectroscopy. Dalton Trans. 2004, 825–832. 10.1039/B316253B. [DOI] [PubMed] [Google Scholar]
  11. Bunzli J. C. G.; Klein B.; Chapuis G.; Schenk K. J. Diffusion of cadmium acetate and self-diffusion of cadmium ions in cadmium acetate in agar gel medium. Inorg. Chem. 1982, 21, 808–810. 10.1021/ic00132a066. [DOI] [Google Scholar]
  12. Fiolka C.; Pantenburg I.; Meyer G. Transition-Metal(II)-Crown-Ether-Polyiodides. Cryst. Growth Des. 2011, 11, 5159–5165. 10.1021/cg201198t. [DOI] [Google Scholar]
  13. Braga D.; Gandolfi M.; Lusi M.; Polito M.; Rubini K.; Grepioni F. Solution and Solid-State Preparation of 18-Crown-6 and 15-Crown-5 Adducts of Hydrogen Sulfate Salts and an Investigation of the Reversible Dehydration Processes. Crystal Growth Des. 2007, 7, 919–924. 10.1021/cg0607598. [DOI] [Google Scholar]
  14. Merzlyakova E.; Wolf S.; Lebedkin S.; Bayarjargal L.; Neumeier B. L.; Bartenbach D.; Holzer C.; Klopper W.; Winkler B.; Kappes M.; Feldmann C. 18-Crown-6 Coordinated Metal Halides with Bright Luminescence and Nonlinear Optical Effects. J. Am. Chem. Soc. 2021, 143, 798–804. 10.1021/jacs.0c09454. [DOI] [PubMed] [Google Scholar]
  15. Bonnin M. A.; Feldmann C. Insights of the Structure and Luminescence of Mn2+/Sn2+-Containing Crown-Ether Coordination Compounds. Inorg. Chem. 2021, 60, 14645–14654. 10.1021/acs.inorgchem.1c01662. [DOI] [PubMed] [Google Scholar]
  16. Shannon R. D. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr. A 1976, 32, 751–767. 10.1107/S0567739476001551. [DOI] [Google Scholar]
  17. De Mello J. C.; Wittmann H. F.; Friend R. H. An improved experimental determination of external photoluminescence quantum efficiency. Adv. Mater. 1997, 9, 230–232. 10.1002/adma.19970090308. [DOI] [Google Scholar]
  18. Ionic Liquids in Synthesis; Wasserscheid P., Welton T., Eds.; Wiley-VCH, Weinheim, Germany, 2008. [Google Scholar]
  19. Siai A.; Ströbele M.; Enseling D.; Jüstel T.; Meyer H.-J. Solid-State Synthesis of (Ph4P)MI3 (M = Eu2+, Sr2+, Sn2+) and Investigation of Photoluminescence Properties of Green Emitting Phosphor. Eur. J. Inorg. Chem. 2021, 2021, 1846–1851. 10.1002/ejic.202100059. [DOI] [Google Scholar]
  20. Freudenmann D.; Wolf S.; Wolff M.; Feldmann C. Ionic Liquids: New Perspectives for Inorganic Synthesis?. Angew. Chem., Int. Ed. 2011, 50, 11050–11060. 10.1002/anie.201100904. [DOI] [PubMed] [Google Scholar]
  21. Bowring M. A.; Bergman R. G.; Tilley T. D. Pt-Catalyzed C–C Activation Induced by C–H Activation. J. Am. Chem. Soc. 2013, 135, 13121–13128. 10.1021/ja406260j. [DOI] [PubMed] [Google Scholar]
  22. Xiao J.; Cui Y.; Li C.; Xu H.; Zhai Y.; Zhang X.; Ma S. Room Temperature Allenation of Terminal Alkynes with Aldehydes. Angew. Chem., Int. Ed. 2021, 133, 25912–25917. 10.1002/ange.202109879. [DOI] [PubMed] [Google Scholar]
  23. Vander Hoogerstraete T.; Brooks N. R.; Onghena B.; Van Meervelt L.; Binnemans K. Crystal structures of hydrated rare-earth bis(trifluoromethylsulfonyl)imide salts. CrystEngComm 2015, 17, 7142–7149. 10.1039/C5CE01270H. [DOI] [Google Scholar]
  24. Babai A.; Mudring A.-V. Anhydrous Praseodymium Salts in the Ionic Liquid [bmpyr][Tf2N]: Structural and Optical Properties of [bmpyr]4[PrI6][Tf2N] and [bmyr]2[Pr(Tf2N)5]. Chem. Mater. 2005, 17, 6230–6238. 10.1021/cm051137x. [DOI] [Google Scholar]
  25. Babai A.; Mudring A.-V. The first homoleptic bis(trifluoromethanesulfonyl)amide complex compounds of trivalent f-elements. Dalton Trans. 2006, 1828–1830. 10.1039/b517694h. [DOI] [PubMed] [Google Scholar]
  26. Babai A.; Pitula S.; Mudring A.-V. Structural and Electrochemical Properties of YbIII in Various Ionic Liquids. Eur. J. Inorg. Chem. 2010, 2010, 4933–4937. 10.1002/ejic.201000323. [DOI] [Google Scholar]
  27. Tang S.; Babai A.; Mudring A.-V. Europium-Based Ionic Liquids as Luminescent Soft Materials. Angew. Chem., Int. Ed. 2008, 47, 7631–7634. 10.1002/anie.200801159. [DOI] [PubMed] [Google Scholar]
  28. Mudring A.-V.; Babai A.; Arenz S.; Giernoth R. The “Noncoordinating” Anion Tf2N– Coordinates to Yb2+: A Structurally Characterized Tf2N– Complex from the Ionic Liquid [mppyr][Tf2N]. Angew. Chem., Int. Ed. 2005, 44, 5485–5488. 10.1002/anie.200501297. [DOI] [PubMed] [Google Scholar]
  29. Carmalt C. J.; Farrugia L. J.; Norman N. C. Structural studies on some iodoantimonate and iodobismuthate anions. Z. Anorg. Allg. Chem. 1995, 621, 47–56. 10.1002/zaac.19956210110. [DOI] [Google Scholar]
  30. Sanchez J. P.; Friedt J. M.; Baernighausen H.; Van Duyneveldt A. J. Structural, magnetic, and electronic properties of europium dihalides EuX2 (X = Cl, Br, I). Inorg. Chem. 1985, 24, 408–415. 10.1021/ic00197a031. [DOI] [Google Scholar]
  31. Willey G. R.; Meehan P. R.; Rudd M. D.; Clase H. J.; Alcock N. W. Synthesis crystal structure of [GdCl2dibenzo-18-crown-6-MeCN][SbCl6]·2MeCN dibenzo-18-crown-6–2,3,11,12-dibenzo-1,4,7,10,13,16-hexaoxacyclooctadeca-2,11-diene. Inorg. Chim. Acta 1994, 215, 209–213. 10.1016/0020-1693(93)03687-6. [DOI] [Google Scholar]
  32. Crisci G.; Meyer G. Kationische und anionische kantenverknüpfte Dimere in [Dy2(Dibenzo-18-Krone-6)2Cl4][Dy2(CH3CN)2Cl8] Synthese und Kristallstruktur. Z. anorg. allg. Chem. 1994, 620, 1023–1027. 10.1002/zaac.19946200612. [DOI] [Google Scholar]
  33. Evans D. J.; Guo Z.; Deacon G. B.; Junk P. C. Synthesis of cationic lanthanoid tetraphenylborate crown ether complexes. J. Organomet. Chem. 2024, 1013, 123186 10.1016/j.jorganchem.2024.123186. [DOI] [Google Scholar]
  34. Runschke C.; Meyer G. Synthese und Kristallstruktur des monomeren, ,in-cavity″-Komplexes [SmI3(Dibenzo-18-Krone-6)]. Z. anorg. allg. Chem. 1997, 623, 981–984. 10.1002/zaac.199762301153. [DOI] [Google Scholar]
  35. Kawasaki Y.; Matsuura Y. Preparation of Dibenzo-18-crown-6 Complexes of Cadmium(II) and Mercury(II), [MX2(dbc)], and Molecular Structure of [HgCl2(dbc)]. Chem. Lett. 1984, 13, 155–158. 10.1246/cl.1984.155. [DOI] [Google Scholar]
  36. Hughes D. L.; Mortimer C. L.; Truter M. R. The Crystal Structure of a Complex between Barium Perchlorate and Dibenzo24-crown-8: Bisperchlorato(6,7,9,10,12,13,20,21,23,24,26,27-dodecahydrodibenz[b,n]-[1,4,7,10,13,16,19,22]octaoxacyclotetracosin)barium. Acta Crystallogr. 1978, B34, 800–807. 10.1107/S0567740878004094. [DOI] [Google Scholar]
  37. Luis S. V.; Frías J. C.; Salvador R. V.; Bolte M. Complexes with Hg(II) and macrocyclic ligands. J. Chem. Crystallogr. 1999, 29, 403–408. 10.1023/A:1009506809212. [DOI] [Google Scholar]
  38. Amarasekara A. S. Acidic Ionic Liquids. Chem. Rev. 2016, 116, 6133–6183. 10.1021/acs.chemrev.5b00763. [DOI] [PubMed] [Google Scholar]
  39. Grobelny Z.; Stolarzewicz A.; Maercker A. Self-decomposition of K–, K+(15-crown-5)2 tetrahydrofuran solution via organometallic intermediates. J. Organomet. Chem. 2001, 628, 65–67. 10.1016/S0022-328X(01)00760-4. [DOI] [Google Scholar]
  40. Notter S.; Donsbach C.; Feldmann C. On Iodido Bismuthates, Bismuth Complexes and Polyiodides with Bismuth in the System BiI3/18-crown-6/I2. Z. Naturforsch. B 2021, 76, 765–774. 10.1515/znb-2021-0127. [DOI] [Google Scholar]
  41. Rogers R. D.; Voss E. J.; Etzenhouser R. D. f-Element/crown ether complexes. 17. Synthetic and structural survey of lanthanide chloride triethylene glycol complexes. Inorg. Chem. 1988, 27, 533–542. 10.1021/ic00276a020. [DOI] [Google Scholar]
  42. Rogers R. D.; Etzenhouser R. D.; Murdoch J. S.; Reyes E. Macrocycle complexation chemistry. 35. Survey of the complexation of the open chain 15-crown-5 analog tetraethylene glycol with the lanthanide chlorides. Inorg. Chem. 1991, 30, 1445–1455. 10.1021/ic00007a006. [DOI] [Google Scholar]
  43. Rogers R. D.; Rollins A. N.; Etzenhouser R. D.; Voss E. J.; Bauer C. B. Structural investigation into the steric control of polyether complexation in the lanthanide series: macrocyclic 18-crown-6 versus acyclic pentaethylene glycol. Inorg. Chem. 1993, 32, 3451–3462. 10.1021/ic00068a013. [DOI] [Google Scholar]
  44. Asprey L. B.; Keenan T. K.; Kruse F. H. Preparation and Crystal Data for Lanthanide and Actinide Triiodides. Inorg. Chem. 1964, 3, 1137–1141. 10.1021/ic50018a015. [DOI] [Google Scholar]
  45. Elbanowski M.; Staninski K.; Schroeder G. Spectrophotometric Study of Lanthanide (III) Complexes with Macrocyclic Polyethers: 18c6 and DB24c8. Polym. J. Chem. 1993, 67, 267–271. [Google Scholar]
  46. Tongbu L.; Xinmin G.; Ning T.; Minyu T. Studies on rare earth complexes with crown ethers-XIX. Synthesis and characterization of the complexes of lanthanide thiocyanates with dibenzo-24-crown-8. Polyhedr. 1990, 9, 2371–2374. 10.1016/S0277-5387(00)86815-1. [DOI] [Google Scholar]
  47. Socrates G. In Infrared and Raman characteristic group frequencies: Tables and charts; Wiley, Chichester, England, 2001. [Google Scholar]
  48. Blasse G.; Grabmeier B. C. In Luminescent Materials; Springer, Berlin, Germany, 1994. [Google Scholar]
  49. Lee J.-H.; Chen C.-H.; Lee P.-H.; Lin H.-Y.; Leung M.; Chiu T.-L.; Lin C.-F. Blue organic light-emitting diodes: current status, challenges, and future outlook. J. Mater. Chem. C 2019, 7, 5874–5888. 10.1039/C9TC00204A. [DOI] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

ic4c03055_si_001.pdf (1.4MB, pdf)

Articles from Inorganic Chemistry are provided here courtesy of American Chemical Society

RESOURCES