Anhydrous, Homoleptic Lanthanide Frameworks with the

Article pubs.acs.org/IC

Anhydrous, Homoleptic Lanthanide Frameworks with the Pentafluoroethyltricyanoborate Anion Tatjana Ribbeck,† Sven H. Zottnick,† Christoph Kerpen,† Johannes Landmann,† Nikolai V. Ignat’ev,‡ Klaus Müller-Buschbaum,*,† and Maik Finze*,†,§ †

Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany Institut für nachhaltige Chemie & Katalyse mit Bor (ICB), Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany ‡ Dr. Ignatiev-Chemistry Consultancy, 47058 Duisburg, Germany §

S Supporting Information *

ABSTRACT: Pentafluoroethyltricyanoborate frameworks of rare-earth metal ions of the general formula [Ln{C2F5B(CN)3}3(OH2)n] (Ln = La, Eu, Ho; n = 0, 3; [Ln13(OH2)n]) were synthesized using the oxonium salt (H3O)[C2F5B(CN)3] ((H3O)1) and lanthanide chlorides LnCl3·nH2O as starting 3 compounds. Single-crystals of ∞ [La{C 2 F 5 B(CN) 3 } 3 ] 3 (∞[La13]) are obtained from the room temperature ionic liquid (RTIL) [EMIm]1 using either a ionothermal approach or by recrystallization of anhydrous microcrystalline [La13] that is obtained from reactions in aqueous media after drying in a vacuum. Removal of water from [Ln13(OH2)3] (Ln = Eu, Ho) to give microcrystalline 3∞[Ln13] is achieved in a vacuum at elevated temperatures. All compounds were characterized by vibrational and NMR spectroscopy, thermogravimetry, and elemental analysis. The structures of the three-dimensional coordination polymers 3∞[Eu13(OH2)3] and 3∞[La13] were elucidated by single-crystal X-ray-diffraction. According to powder diffraction studies on anhydrous 3∞[Ln13] (Ln = La, Eu, Ho), the three compounds are isotypic. A study of the photoluminescence properties reveals that both Eu3+ compounds, [Eu13] and [Eu13(OH2)3], are strongly luminescent, the emission of the anhydrous framework being significantly more intense than the one of the hydrate. The Eu-compounds benefit from a sensitizer effect of the anion. In contrast, the Ho-containing framework 3 3+ ions. ∞[Ho13] exhibits separate chromophores and a strong reabsorption of the fluorescence by the Ho

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(Ln = Sm, Gd, Tb, Y).19 Besides the aforementioned lanthanide complexes of the [B(CN)4]− anion, lanthanide complexes of cyanoborate anions have been reported with the [B2(CN)6]2− dianion, only.20 Perfluoroalkylcyanoborate anions are new interesting anions, for example, as building blocks for ionic liquids.21 The properties of metal complexes and materials derived from these anions can be influenced, in principle, by (i) the number of perfluoroalkyl and cyano groups and (ii) the length of the perfluoroalkyl chain(s). However, in contrast to the homoleptic cyanoborate anion [B(CN)4]−, only little is known about this class of compounds. Tris(trifluoromethyl)cyanoborates were the first examples that have been obtained as pure compounds.22−24 The [(CF3)3B(CN)]− anion was found to act as a ligand in coordination chemistry in [(Ph3P)3Rh{(CF3)3B(CN)}],25 [Cp2ZrCH3{(CF3)3B(CN)}],6 and [Au(NCCH3)2][Au{(CF3)3B(CN)}].26 Preparation of salts of monoperfluoroalkyltricyanoborate anions of the general for3 ∞[LnCl3{1,4-C6H4(CN)2}]

INTRODUCTION Cyanoborate anions are valuable building blocks, especially for low-viscosity hydrophobic room temperature ionic liquids (RTILs)1 and as counteranions or ligands in transition metal chemistry. Most coordination compounds of cyanoborate anions have been reported with the tetracyanoborate anion [B(CN)4]−,2−4 including complexes of main group metals (for example alkali metal cations2,5), transition metals (e.g., [Cp2ZrCH3{B(CN)4}],6 [Fe{B(CN)4}2(OH2)2]7), and rareearth metals (e.g., [Ln{B(CN)4}(OH2)7][B(CN)4]2 (Ln = Y, Tb, Dy, Ho, Er, Tm, Yb, Lu)8). The aqua complexes can be dehydrated to yield the corresponding homoleptic cyanoborate complexes. No structural details have been reported, and therefore the coordination of the cyanoborate anions to the rare-earth metal centers in these compounds is unknown. However, compounds of the type [Ln{B(CN)4}3] are related to homoleptic acetonitrile complexes [Ln(NCCH3)x]3+ (x = 3, 8, 9) that are stabilized by weakly coordinating anions9−16 and to Ln3+-based metal−organic frameworks (MOFs) with 1,3- and 1,4-benzodinitrile with interesting luminescence properties, e.g., 17 3 Ho 18 ) and ∞ [LnCl 3 {1,3-C 6 H 4 (CN) 2 }] (Ln = Eu, © 2017 American Chemical Society

Received: December 8, 2016 Published: February 2, 2017 2278

DOI: 10.1021/acs.inorgchem.6b02984 Inorg. Chem. 2017, 56, 2278−2286

Article

Inorganic Chemistry mula [RFB(CN)3]− including ionic liquids (ILs) has been reported,21,27 but their coordination chemistry is unexplored. ILs with anions of the type [RFB(CN)3]− show properties similar to ILs with the parent [B(CN)4]− anion.21 Especially their hydrophobic nature is remarkable and makes them promising building blocks to gain hydrophobic properties for coordination compounds. As outlined, until now, only very few coordination compounds of lanthanides with cyanoborate anions are known. The few structurally characterized complexes contain a lanthanide cation with a substantial number of aqua ligands or solely with water molecules bonded to the metal center.8,20 In coordination compounds and especially in coordination polymers and metal−organic frameworks of lanthanides, significant luminescence has been observed.28−30 For this luminescence, a combination of ligand and metal based transitions is essential, as the parity forbidden nature of 4f transitions of Ln3+ ions limits their light uptake strongly.31−33 Ligands that function as effective sensitizers are valuable for an intense lanthanide based emission.34−36 In this contribution, we report on first lanthanide frameworks with the perfluoroalkyltricyanoborate anion [C2F5B(CN)3]− (1). They have been obtained either using an ionothermal37,38 approach or via an aqueous synthetic route. The compounds were studied by vibrational spectroscopy, elemental analysis, and thermogravimetric measurements. [Eu{C 2 F 5 B(CN)3}3(OH2)3] ([Eu13(OH2)3]) and [La{C2F5B(CN)3}3] ([La13]) were characterized by single-crystal X-ray diffraction and the bulk materials [Ln13] (Ln = La, Eu, Ho) by X-ray powder diffraction. Several coordination polymers [Ln13] (Ln = La, Eu, Ho) as well as [Eu13(OH2)3] were investigated by photoluminescence (PL) spectroscopy indicating a range of PL properties from 4f emission to sensitizer effects and reabsorption by inner filter effects.

LaNO3 ·6H 2O + 3[EMIm][C2F5B(CN)3 ] ([EMIm]1) [EMIm]1

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ La[C2F5B(CN)3 ]3 + 3[EMIm]NO3 + 6H 2O 120 ° C → rt

(2)

Single crystals of [La13] were also obtained starting from the microcrystalline material that was synthesized according to eq 1 and suspended in [EMIm]1 at 120 °C. However, the solubility of [La13] in [EMIm]1 is very low and visually no dissolution was recognized. The solubility of anhydrous [Ln13] (Ln = Eu, Ho) is even lower because recrystallization of the microcrystalline solids derived from the aqua complexes by drying in a vacuum at elevated temperatures remained unsuccessful, so far. In agreement with this observation the photoluminescence spectrum of [Eu13] in [EMIm]1 is very weak in intensity (vide inf ra). Crystal Structure of 3∞[La{C2F5B(CN)3}3] (3∞[La13]). The three-dimensional coordination polymer 3∞[La13] crystallizes in the hexagonal crystal system in the space group P63/m. Selected interatomic distances are given in Table 1. In the homoleptic Table 1. Selected Interatomic Distances [Å] and Angles [deg] of Anion 1 in the Structures of 3∞[La13] and a 3 ∞[Eu13(OH2)3]

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RESULTS AND DISCUSSION Synthesis of Solvent-Free Lanthanide Frameworks [Ln{C2F5B(CN)3}3] (Ln = La, Eu, Ho; [Ln13]). Coordination polymers of the formula [Ln{C2F5B(CN)3}3(OH2)n] (Ln = La, Eu, Ho; n = 0, 3; [Ln13(OH2)n]) were obtained by metathesis reactions of the metal chlorides LnCl3·mH2O (Ln = La, Eu, Ho; m = 0, 6) with the Brønsted acid (H3O)[C2F5B(CN)3] ((H3O) 1)21,39 under formation of HCl. After removal of all volatiles in a vacuum, anhydrous [La{C2F5B(CN)3}3] ([La13]) and [Ln{C2F5B(CN)3}3(OH2)3] (Ln = Eu, Ho; [Ln13(OH2)3]) were isolated (eq 1). The anhydrous europium and holmium complexes [Eu13] and [Ho13] were obtained as microcrystalline compounds after drying of [Ln13(OH2)3] (Ln = Eu, Ho) for 5 days at 110 °C in a vacuum.

a

((H3O)1)

⎯⎯⎯→ [Ln{C2F5B(CN)3 }3(OH 2)n ] + 3HCl rt

parameter

3 ∞[Eu13(OH2)3]

3 ∞[La13]

B−CN CN B−CF2 CF2−CF3 C−F (CF2) C−F (CF3) NC−B−CN NC−B−C2F5 B−CF2−CF3

1.593(10) 1.138(9) 1.643(10) 1.517(11) 1.368(8) 1.330(9) 109.1(5) 109.8(6) 117.7(6)

1.595(6) 1.137(5) 1.634(6) 1.529(7) 1.362(3) 1.334(3) 108.45(9) 110.5(3) 116.9(4)

Mean values where applicable.

complex, the La3+ ion exhibits a tricapped trigonal prismatic coordination polyhedron that is constituted by nine N atoms belonging to the pentafluoroethyltricyanoborate anions 1 (Figure 1). The two independent La···N distances of 2.632(2) and 2.641(2) Å are in good agreement with La···N distances found for related lanthanum complexes with cyanoborate and cyanometallate anions, e.g., 2 ∞ [{La 2 {B 2 (CN) 6 } 3 (OH 2 ) 8 }·2.7H 2 O] (La···N: 2.646(2)− 2.649(2) Å)20 and 2∞{K[La{Pt(CN)4}1.5(OH2)6[Pt(CN)4]0.5· 2.75H2O} (La···N: 2.623(7)−2.699(7) Å).40 For related homoleptic acetonitrile complexes of La3+, a coordination number of nine and a tricapped trigonal prismatic coordination with similar La···N distances have been reported,11,12,14 e.g., [La(NCCH3)9][AlCl4]3·CH3CN (La···N: 2.634(5)−2.657(5) Å).14 In the crystal of 3∞[La13], each anion interconnects three different lanthanum ions via La···N coordination, thus leading to an uncharged, three-dimensional coordination polymer. 3 Crystal Structure of ∞ [Eu{C 2 F 5 B(CN) 3 } 3 (OH 2 ) 3 ] 3 (∞[Eu13(OH2)3]). Slow evaporation of water in the presence of sulfuric acid resulted in single crystals of 3∞[Eu13(OH2)3] from concentrated aqueous solution. 3∞[Eu13(OH2)3] crystallizes in the triclinic crystal system in the space group P1̅ and forms a three-dimensional coordination polymer. Like the La3+ ion in the structure of 3∞[La13], the coordination polyhedron of the Eu3+ ion is a distorted tricapped trigonal prism (Figure 2).

LnCl3·m(H 2O) + 3(H3O)[C2F5B(CN)3 ] H 2O

([La13 ])

([Ln13 (OH 2)n ])

(Ln = La, Eu, Ho; m = 0, 6; n = 0(La), 3(Eu, Ho)) (1) 3 3 ∞[La{C2F5B(CN)3}3] (∞[La13]) 37,38

Single crystals of were obtained using an ionothermal approach starting from La(NO3)3·6H2O and the room temperature ionic liquid [EMIm][C2F5B(CN)3] ([EMIm]1)21 (eq 2). 2279


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Figure 1. Coordination sphere of La3+ in 3∞[La13] with thermal ellipsoids at the 50% probability level (left) and view along the a-axis of the structure of 3∞[La13] (right).

Figure 2. Coordination sphere of Eu3+ in 3∞[Eu13(OH2)3] with thermal ellipsoids at the 50% probability level (left) and view along the b-axis of the crystal packing of the coordination polymer 3∞[Eu13(OH2)3] (right).

and water.43,44 The hydrogen bonds result in connected layers in 3∞[Eu13(OH2)3] and thus in an overall three-dimensional supramolecular network (Figure 2). Powder X-ray Diffraction. The powder X-ray diffraction patterns of 3∞[Ln{C2F5B(CN)3}3] (Ln = La, Eu, Ho; 3∞[Ln13]) pictured in Figure 3 are in good accordance with the diffraction pattern simulated from single-crystal data of 3∞[La13], thus proving the phase purity of the bulk materials of 3∞[La13], 3 3 ∞[Eu13], and ∞[Ho13]. As the experimental temperatures of the powder investigation (room temperature) and the single crystal structure determination (100 K) are different, a slight shift of the reflections to larger 2θ angles and thus shorter cell axes can be expected upon cooling. For 3∞[La13], this is the case especially for Miller indices attributed to the b and c axes. Most prominent is the slight shift of the main reflection (hkl = 021) at 2θ = 18.63° in the powder diffraction pattern to 18.98° calculated from single-crystal data of 3∞[La13]. Accordingly, the three compounds 3∞[Ln13] (Ln = La, Eu, Ho) are isotypic to the structure shown in Figure 1, thereby pointing out the absence of water in the final products. In Table 2, the cell parameters of 3∞[Ln13] (Ln = La, Eu, Ho) are listed. As expected, the cell volume also decreases with decreasing ionic

Six pentafluoroethyltricyanoborate anions are bonded to a single Eu3+ ion via one cyano group, and three additional aqua ligands complete the coordination sphere. All six Eu···NC distances are slightly different and range from 2.521(5) to 2.573(5) Å, and the three Eu···OH2 distances are 2.390(5), 2.417(5), and 2.439(4) Å. The Eu···NC and Eu···OH2 distances compare well to distances reported for related Eu3+ complexes, for example, in 3∞[EuCl3{1,3-C6H4(CN)2}] (Eu···N: 2.610(2) Å)17 and 1∞[Na{Eu(2,6-PDA2}(OH2)2}·3H2O] (2,6PDA2− = 2,6-pyridinedicarboxylate; Eu···O: 2.384(3) and 2.423(3) Å).41 Each cyanoborate anion 1 is coordinated to two Eu3+ ions via a Eu···N bond resulting in a layered structure. The third cyano group is not bonded to one of the europium centers but acts as a hydrogen-bond acceptor to an aqua ligand. Although the H atoms of the aqua ligands were located in the difference map, the quality of the data did not allow a free refinement. So, calculated positions were applied for the H atoms and the discussion of the hydrogen-bonding is limited to the N···O distances of 2.736(8), 2.772(7), and 2.796(7) Å. These values are similar to d(N···O) found in (H5O2)[B(CN)4] that range from 2.654(3) to 2.791(3) Å42 and significantly shorter than typical d(N···O) between nitriles 2280


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Figure 3. Powder X-ray diffractograms of 3∞[Ln13] (Ln = La, Eu, Ho) as well as the simulated diffractogram of 3∞[La13] based on singlecrystal X-ray data.

Figure 4. IR and Raman spectra of [Eu13(OH2)3] (top) and [Eu13] (bottom).

Table 2. ν̃(CN) (Raman Spectroscopy) and Cell Parameters Derived from Powder Diffraction Studies on 3∞[Ln13] (Ln = La, Eu, Ho) at Room Temperature [Ln13]

units

Ln = La

Ln = Eu

Ln = Ho

ν̃(CN) a=b c V

cm−1 Å Å Å3

2259, 2262 (sh) 13.1500(8) 8.4134(10) 1259.9(2)

2263, 2264 (sh) 12.9573(11) 8.2230(5) 1193.8(2)

2263, 2264 (sh) 12.9318(9) 8.1782(12) 1184.4(2)

the range typical for OH moieties involved in hydrogen bonds,48,49 which is in agreement to the structure of [Eu13(OH2)3] (Figure 2). Thermogravimetric Investigations. The thermal properties of [Ln13] (Ln = La, Eu, Ho), [Eu13(OH2)3], and [Ho13(OH2)3] were examined by simultaneous differential thermoanalysis (DTA) and thermogravimetry (TG) with a scan rate of 10 °C min−1 up to 875 °C in a constant flow of inert gas (Ar). The thermal properties of the anhydrous compounds are complex, and defined decomposition products cannot be unambiguously assigned. Surprisingly, the processes observed are exothermic, with the origin not being known, yet. Figure 5

radii in the order La3+ (1.356 Å), Eu3+ (1.260 Å), and Ho3+ (1.212 Å).45 Since the lattice energy increases with decreasing molecular volume,46,47 the lattice energy of 3∞[La13] should be the smallest in the series of pentafluoroethyltricyanoborate complexes, which is in perfect agreement with the observation that only the lanthanum complex can be recrystallized from [EMIm]1. Vibrational Spectroscopy. In the Raman spectrum of [Ln13], two very strong overlapping bands are found for ν̃(CN) that are slightly shifted to higher wavenumbers in the order La, Eu, and Ho (Table 2, Figure 4, and Figure S1). In the IR spectrum, only a single band is resolved at approximately the same position. For [EMIm]1 the CN band is observed at 2208 cm−1, which is much lower than ν̃(CN) of [Ln13] and reflects the strong interaction between the lanthanide ions and the cyanoborate anion 1. The small shift to higher ν̃(CN) in the series La, Eu, and Ho is indicative of a slight increase in the coordination strength due to the decreasing ionic radius of Ln3+. A similar increase in ν̃(CN) with increasing atomic number was reported for acetonitrile complexes of Ln3+ (La, Sm, Gd, Dy, Ho, Tm, Yb) that are present in solution of Ln(ClO4)3 in anhydrous acetonitrile.16 This nicely parallels the decreasing cell volumes derived from powder diffraction data (Table 2). In Figure 4, the vibrational spectra of [Eu13] and [Eu13(OH2)3] are displayed. ν̃(CN) of [Eu13(OH2)3] with 2262 cm−1 (Raman) and 2261 cm−1 (IR) are close to the wavenumber found for [Eu13] (Table 2). The presence of water in [Eu13(OH2)3] is proven by characteristic broad OH stretching bands at 3332 and 3220 cm−1 and δ(OHO) at 1631 and 1617 cm−1. The wavenumbers of the OH stretches are in

Figure 5. Thermal properties of [Eu13] by simultaneous DTA/TG with a heating rate of 10 °C min−1 from room temperature to 850 °C.

shows the mass signal and heat flow of 3∞[Eu13], which is stable up to more than 200 °C. A small exothermic event at 240 °C is followed by two strong overlapping exothermic signals starting at approximately 340 °C that can be addressed to decomposition of the product. The thermal behavior of [La13] (Figure S4) is analogous to that of [Eu13] and similar to that of [Ho13] (Figure S5). For [Eu13(OH2)3] an additional endothermic effect with an onset temperature of 120 °C and a 2281


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for 3∞[Eu13] in [EMIm]1 (Figure 6). Hence, 3∞[Eu13] shows a limited solubility in [EMIm]1, which is in agreement with the discussion on the lattice energy and the problems in recrystallization attempts of 3∞[Eu13] in [EMIm]1, as discussed before. PL spectra were also recorded for the lanthanum-containing framework 3∞[La13] to determine the potential sensitizer character of the pentafluoroethyltricyanoborate anion 1. Similar excitation and emission corresponding to the broad excitation and emission bands of 3∞[Eu13] were observed. As La3+ does not participate in 4f transitions, the isotypic nature of the structure gives the best possible comparison for the identification of the fluorescence of 1 (see Figure S10). Because Ho3+ is a potential emitter in the NIR region, PL studies were also carried out on 3∞[Ho13]. However, no such emission could be detected. Instead, the compound shows intense reabsorption phenomena. Thereby, Ho3+ absorbs energy from the anion fluorescence from UV to the vis range (see Figure 7). Upon absorption of some part of the

mass loss of ca. 3% occurs, which is associated with the loss of the water equivalents (Figure S6). Photoluminescence. The photoluminescence properties of the new Ln-pentafluoroethyltricyanoborates, both water-free and hydrated compounds, were investigated with PL spectroscopy. Both, anhydrous framework 3∞[Eu13] and the corresponding hydrate 3∞[Eu13(OH2)3] show photoluminescence if excited by UV light. Excitation is constituted by two parts, mainly by *S1 ← S0 transitions, as well as direct 4f−4f excitation of Eu3+ with a maximum at 394 nm. Emission exclusively occurs from characteristic f−f transitions of Eu3+ Table 3. Excitation and Emission Wavelengths of the 4f−4f Transitions of 3∞[Eu13] and the Reabsorption Transitions of 3 ∞[Ho13] compound 3

∞[Eu13]

3

∞[Ho13]

transitions

corresponding wavelengths (nm)

Observed 4f−4f Transitions F0 → 5D4, 5L8, 5G3, 5L7, 5L6 361, 374, 379, 385, 394 5 D0 → 7F0, 7F1, 7F2, 7F3, 7F4 582, 591, 615, 649, 694 Observed Reabsorption 5 I8 → 3K6, 3L9, 3H6, (5G, 3G)5, 331, 344, 359, 417, 451, 5 486, 538, 645 G6, 5F8, 5F4, 5F5 7

(Table 3) and shows a maximum at 615 nm for the transition 5 D0 → 7F2.50 The luminescence intensity of 3∞[Eu13] is more intense by a factor of ∼2.5 compared to 3∞[Eu13(OH2)3], which is attributed to the high vibrational states of OH in water that are typically known to efficiently quench luminescence. In both cases, anion 1 exhibits a sensitizer effect and transfers energy to the Eu3+ ions (see Figure 6 and Figure S8). We also determined the luminescence lifetime by an investigation of the overall process decay time. Long decay times, typical for Eu3+,51,52 of τ1 = 3.032(11) ms for 3∞[Eu13(OH2)3] and τ1 = 3.741(16) ms for 3 ∞ [Eu13] corroborated hydrate and water-free framework (Table S1). In order to prove and evaluate the solubility of the product 3 framework, microcrystalline ∞ [Eu13] was suspended in [EMIm]1 and the ionic liquid phase was filtrated and studied by luminescence spectroscopy, subsequently. Excitation and emission are dominated by transitions of the IL (Figure S9), showing broad fluorescence bands. However, additional weak emission bands of 4f−4f transitions of Eu3+ are also observed

Figure 7. Excitation (black trace) and emission (red trace) spectra of 3 ∞[Ho13].

fluorescence, distinct 4f transitions of Ho3+ are observed, the strongest being 5G6 ← 5I8 (see Table 3). Such an inner filter effect originates from the anions and cations acting as separate chromophores, indicating that there is no sensitizer effect, and

Figure 6. Left: Excitation (black traces) and emission (red traces) spectra of 3∞[Eu13(OH2)3] (solid lines) and the respective spectra of 3∞[Eu13] (dotted lines). Right: Excitation (black trace) and emission (red trace) spectra of 3∞[Eu13] in [EMIm]1 after filtration. 2282


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Fluorolog 3 spectrometer equipped with a 450 W Xe lamp, doublegrated excitation and emission monochromators, and a photomultiplier tube (R928P) using the FluorEssence software. Furthermore, both excitation and emission spectra were corrected for the spectral response of the monochromators and detector using correction spectra provided by the manufacturer. All samples were examined in spectroscopically pure quartz cuvettes in front face mode at room temperature. Additionally, an edge filter (GG400, Reichmann Optics) was used for emission spectra. NIR. NIR investigations were performed on a Photon Technology International QuantaMaster model QM-2000-4 spectrometer with an InGaAs-NIR detector and a 75 W short arc lamp (UXL-75XE, Ushio). Additionally, a filter for excitation (300 nm bandpass, Δ = 20 nm, OD = 5, Edmund Optics) and an edge filter for emission (RG780, Edmund Optics) were used. Fluorescence Lifetime. The fluorescence lifetimes were obtained as process decay times with an Edinburgh Instruments (FLS920) spectrometer. The samples were prepared in quartz glass cuvettes under inert-gas atmosphere. The decay times were recorded by timecorrelated single-photon counting (TCSPC) with a microsecond flash lamp with an excitation wavelength of 395 nm. The fluorescence emission was collected at right angles to the excitation source, and the emission wavelength was selected with a monochromator and detected by a single-photon avalanche diode (SPAD). The resulting intensity decays were calculated through tail fit analysis (Edinburgh F900 analysis software). The quality of the fits was evidenced by good χ2 values (χ2 < 1.7). Powder X-ray Diffraction. Samples for powder diffraction were ground in a mortar and placed into Lindemann glass capillaries (⌀ 0.5 mm). Diffraction data was collected on a powder X-ray diffractometer BRUKER AXS D8 Discover equipped with a Lynx-Eye detector in transmission geometry. X-ray radiation (Cu Kα1; λ = 154.06 pm) was focused with a Goebel mirror, whereas Cu Kα2 radiation was eliminated by use of a Ni absorber. Diffraction patterns were recorded and analyzed using the BRUKER AXS Diffrac-Suite. Thermal and Elemental Analysis. Thermal properties of the bulk substances were investigated by simultaneous DTA/TG (NETZSCH STA-409). Therefore, 25−35 mg of the sample was kept in an argon (Linde 5.0)/nitrogen (Linde 5.0) mixture with a gas flow of 40 mL min−1 and heated with a constant rate of 10 K min−1 to a maximum temperature of 850 °C. Carbon, nitrogen, and hydrogen elemental analysis was conducted on a vario MICRO cube (Elementar Analysensysteme GmbH). Single-Crystal X-ray Diffraction. Single crystals of 3 3 3 ∞[La{C2F5B(CN)3}3] (∞[La13]) and ∞[Eu{C2F5B(CN)3}3(OH2)3] (3∞[Eu13(OH2)3]) were obtained by recrystallization or an ionothermal approach from [EMIm]1 and by slow evaporation of an aqueous solution, respectively. The crystals were examined on a Bruker X8Apex II diffractometer using Mo Kα radiation (λ = 0.71073 Å) at 100 K. Crystallographic data including CCDC numbers are given in Table 4. The structures were solved by intrinsic phasing methods (SHELXT).57,58 Refinement is based on full-matrix least-squares calculations on F2,57,59 and the calculations were carried out using either the OLEX2 software60 or the ShelXle graphical interface.61 All non-hydrogen atoms were refined anisotropically. For aqua ligands idealized O−H bond lengths and angles were used. Molecular diagrams and packing diagrams were drawn using the Diamond 4.2.2 program.62 Supplementary crystallographic data for this publication are deposited in the Supporting Information or can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Synthesis of [Eu{C2F5B(CN)3}3(OH2)3] ([Eu13(OH2)3]). A 100 mL round-bottom flask equipped with a magnetic stirring bar was charged with (H3O)1 (1 g, 4.4 mmol), which was dissolved in H2O (50 mL). Solid EuCl3·6H2O (538 mg, 1.5 mmol) was added, and the solution was stirred for 1 h at room temperature. Subsequently, all volatiles were removed under reduced pressure, and the obtained light yellow solid was dried in a vacuum. Yield: 995 mg (1.2 mmol, 80%, purity according to 11B NMR spectroscopy 99.9%). 13C NMR ((CD3)2SO, δ ppm): 123.2 (q, 3C, 1J(13C,11B) = 69.4 Hz). 11B NMR ((CD3)2SO, δ

has, e.g., been shown for Ln-N-coordination in 3D frameworks of Ln-1,2,3-triazolates, or Ln-dipyridyl-ethylene coordination polymers.53,54 This effect can also be shown in the excitation process, if the source is not saturating the overall absorption, by competition of both chromophores. Besides this strong effect, excitation and emission are also derived from the anion 1. It is remarkable that these contradicting effects occur for Eu3+ and Ho3+ for isotypic structures.

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CONCLUSION A versatile entry toward lanthanide frameworks of the pentafluoroethyltricyanoborate anion 1 has been developed using the acid (H3O)1 as starting compound. The straightforward preparation of anhydrous [La13] and the remarkably unhindered removal of water from the coordination polymers with three aqua ligands [Ln13(OH2)3] (Ln = Eu, Ho) emphasizes the hydrophobic nature of anion 1. The coordination polymers [Ln13] (Ln = La, Eu, Ho) reveal an unprecedented high thermal stability for Ln-cyanoperfluoroalkylborate compounds considering the intrinsic high Lewis acidic character of Ln3+ ions and show the inherent stability of 1 against loss of F− and CN−. Both Eu3+ compounds, [Eu13] and [Eu13(OH2)3], are strongly luminescent due to 4f emission of Eu3+, the emission of the anhydrous framework being significantly more intense. The compounds show a sensitizer effect of the anion as well as direct Eu excitation. In contrast, the Ho-containing framework [Ho13] exhibits separate chromophores and a remarkably strong reabsorption of the anion emission by the Ho3+ ions. Thus, the photoluminescence properties of the first lanthanide frameworks of the pentafluoroethyltricyanoborate anion 1 are quite distinct. Due to the availability of closely related luminescent nonaqueous and hydrate compounds, this class of compounds has a potential to explore and quantify the influence of high vibrational states of quenching coligands, such as water, on the luminescence intensity almost independent from other influences. Thereby it may help to gain further insights for the sensing of such molecules based on luminescent coordination polymers.

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EXPERIMENTAL SECTION

Chemicals. All chemicals were purchased from commercial sources. La(NO3)3·6H2O was purchased from Merck KGaA with a purity of >99.9%. The rare-earth metal chlorides were purchased from Alfa Products (LaCl3), Auer-Remy (HoCl3), and ABCR (EuCl3· 6H2O) with purities of >99.9%. [EMIm][C2F5B(CN)3] ([EMIm]1)21 and (H3O)[C2F5B(CN)3] ((H3O)1)39 were synthesized as described elsewhere. All solvents were used without further purification. NMR Spectroscopy. 1H, 11B, 13C, and 19F NMR spectra were recorded on a Bruker Avance 500, 400, or 200 MHz NMR spectrometer. The NMR signals were referenced against tetramethylsilane (TMS, 1H and 13C) and BF3·OEt2 in CDCl3 with Ξ[11B] = 32.083974 MHz and CFCl3 with Ξ[19F] = 94.094011 MHz as external standards.55 1H and 13C chemical shifts were calibrated against the residual solvent signal and the solvent signal respectively [δ(1H), (CD2H)(CD3)SO 2.50 ppm; δ(13C), (CD3)2SO 39.52 ppm].56 Vibrational Spectroscopy. IR spectra were recorded on a Bruker Alpha Platinum-ATR spectrometer equipped with either a diamond or a Ge crystal with a resolution of 1 cm−1 in the range of 400 to 4000 cm−1. Raman spectra were recorded at room temperature on a Bruker IFS-120 spectrometer with an apodized resolution of 2 cm−1 using the 1064 nm excitation line of a Nd/YAG laser on crystalline samples contained in melting point capillaries in the region of 4000−100 cm−1. Photoluminescence Spectroscopy. UV/Vis. Excitation and emission spectra were measured with a HORIBA Jobin Yvon Spex 2283


Article

Inorganic Chemistry

NMR ((CD3)2SO, δ ppm): − 81.2 (s, 3F), − 123.0 (q, 2F, 2J(19F,11B) = 25.0 Hz). Anal. Calcd for [Ho13(OH2)3]: C, 21.38; H, 0.72; N, 14.96. Found: C, 22.50; H, 0.85; N, 15.94. Drying of [Ho13(OH2)n] for 5 days at 110 °C in a vacuum yielded 3 ∞[Ho13]. Anal. Calcd for Ho13: C, 22.85; N, 15.99. Found: C, 24.14; N, 17.02. Similar results for the elemental analysis of [Ho13] were obtained for three different samples.

Table 4. Crystallographic Data of 3∞[La13] and 3 ∞[Eu13(OH2)3] empirical formula formula wt/g mol−1 cryst syst space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z Dcalcd/Mg cm−3 μ/mm−1 F(000) T/K no. of collected reflns no. of unique reflns R(int) no. of params R1 (I > 2σ(I))a wR2 (all)b GOF on F2 largest diff peak/hole/e Å−3 CCDC no. a R1 = ∑[|Fo| − |Fc|]/∑|Fo|. Fo|2)2]1/2.57

3 ∞[La13]

3 ∞[Eu13(OH2)3]

C15B3N9F15La 762.58 hexagonal P63/m 13.162(4) 13.162(4) 8.359(2)

C15H6B3N9F15EuO3 829.68 triclinic P1̅ 10.6951(12) 10.7990(13) 12.9833(15) 79.437(3) 84.777(3) 80.451(3) 1450.8(3) 2 1.899 2.294 792 100(2) 10094 5695 0.0389 418 0.0431 0.1159 1.032 2.034/−1.777 1519370

1254.2(4) 4 2.019 1.840 720 100(2) 18408 1120 0.0455 75 0.0199 0.0542 0.995 0.49/−0.42 1518443 b

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02984. X-ray crystallographic file for 3∞[Eu13(OH2)3] (CIF) X-ray crystallographic file for 3∞[La13] (CIF) Vibrational spectra, DTA/TG curves, excitation and emission spectra, and photoluminescence properties of 3 3 ∞[Eu13] and ∞[Eu13(OH2)3] (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: maik.fi[email protected]. *E-mail: [email protected]. ORCID

Maik Finze: 0000-0002-6098-7148 Funding

The authors gratefully acknowledge financial support by Deutsche Forschungsgemeinschaft (DFG) within the priority program SPP 1708, both, within the projects FI 1628/4-1 and MU-1562/8-1. S.H.Z. also thanks the Evangelisches Studienwerk Villigst e.V. for a PhD scholarship.

R2 = [∑w(|Fo|2 − |Fc|2)2/∑w(|

ppm): − 32.0 (t, 1B, 2J(11B,19F) = 25.0 Hz). 19F NMR ((CD3)2SO, δ ppm): − 80.5 (s, 3F,), − 122.4 (q, 2F, 2J(19F,11B) = 25.0 Hz). Anal. Calcd for [Eu13(OH2)3]: C, 22.20; H, 0.50; N, 15.53. Found: C, 21.72; H, 0.73; N, 15.19. Recrystallization from H2O over H2SO4 in a desiccator afforded colorless crystals of 3∞[Eu13(OH2)3], which were studied by singlecrystal X-ray diffraction. Drying of crystalline 3∞[Eu13(OH2)3] for 5 days at 110 °C in a vacuum yielded 3∞[Eu13]. Anal. Calcd for Eu13: C, 23.23; N, 16.53. Found: C, 23.58; N, 16.43. Synthesis of [La{C2F5B(CN)3}3] ([La13]). Method A. [La13] was synthesized starting from (H3O)1 (680 mg, 3 mmol) dissolved in H2O (50 mL) and LaCl3 (245 mg, 1 mmol) as described for the preparation of [Eu13(OH2)2]. Yield: 626 mg (0.82 mmol, 82%, purity according to 11 B NMR spectroscopy 99.9%). 13C NMR ((CD3)2SO, δ ppm): 122.8 (q, 3C, 1J(13C,11B) = 69.5 Hz). 11B NMR ((CD3)2SO, δ ppm): − 32.1 (t, 1B, 2J(19F,11B) = 25.2 Hz). 19F NMR ((CD3)2SO, δ ppm): 81.1 (d, 3F, 3J(19F,19F) = 6.5 Hz), − 122.9 (q, 2F, 2J(19F,11B) = 25.0 Hz). Anal. Calcd for La13: C, 23.63; N, 16.53. Found: C, 23.59; N, 16.39. Single crystals of 3∞[La13] were obtained by recrystallization of the amorphous material from [EMIm]1. Synthesis of [La{C2F5B(CN)3}3] ([La13]). Method B. La(NO3)3· 6H2O (0.20 mmol, 86.6 mg) and [EMIm]1 (0.8 mmol, 255.2 mg) were mixed and heated at 100 °C for 5 h. The reaction mixture was cooled to room temperature overnight whereupon the formation of needle-shaped crystals was observed. In order to complete crystallization, the mixture was kept in a refrigerator for 7 days. Single crystal were isolated manually. Yield: 6.1 mg (0.01 mmol, 4%). Synthesis of [Ho{C2F5B(CN)3}3(OH2)n] ([Ho13(OH2)n]; n = 2−3). The synthesis of [Ho13(OH2)n] was conducted as described for [Eu13(OH2)2] using (H3O)1 (680 mg, 3 mmol) and HoCl3 (271 mg, 1 mmol) as starting materials in aqueous solution. Yield: 630 mg (0.8 mmol, 80%, purity according to 11B NMR spectroscopy 99.9%). 13C NMR ((CD3)2SO, δ ppm): 123.1 (q, 3C, 1J(13C,11B) = 64.4 H. 11B NMR ((CD3)2SO, δ ppm): − 32.1 (t, 1B, 2J(19F,11B) = 24.0 Hz). 19F

Notes

The authors declare no competing financial interest.

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