Published on 05 December 2014. Downloaded by UNIVERSITY OF ALABAMA AT BIRMINGHAM on 13/02/2015 13:26:41.
Dalton Transactions View Article Online
PAPER
Cite this: Dalton Trans., 2015, 44, 2325
View Journal | View Issue
Synthesis, structure and near-infrared photoluminescence of hexanitratoneodymate ionic liquids† Ling He, Shun-Ping Ji, Ning Tang, Ying Zhao and Guo-Hong Tao* Five hexanitratoneodymate-based rare earth complexes (1–5) were synthesized using a straightforward method. Purple plate crystals of 1 were isolated and the crystal structure was determined by single-crystal X-ray diffraction with respect to the coordination mode of the nitrate anion to the central Nd(III) ion. (1: monoclinic system P21/c, a = 15.9460(3) Å, b = 10.2457(6) Å, c = 33.323(3) Å, β = 91.8108(17)°, V = 3109.11(11) Å3, Z = 4). The central Nd(III) ion is surrounded by six bidentate nitrate ligands, with a major trend towards high symmetry of the [Nd(NO3)6]3− anion as an icosahedron. Thermal properties were determined from differential scanning calorimetry (DSC) combined with thermogravimetric analysis (TGA)
Received 25th October 2014, Accepted 5th December 2014
tests. Complexes 3–5 are found to be room temperature liquids, and their excitation and emission spectra
DOI: 10.1039/c4dt03294b
were recorded. These complexes exhibit intense near-infrared (NIR) luminescence emission, which originates from interconfigurational f–f transitions 4F3/2→4IJ multiplet ( J = 9/2–13/2). These liquid Nd(III) com-
www.rsc.org/dalton
plexes are of interest as potential NIR luminescent soft materials with high thermal stability.
Introduction Ionic liquids are low temperature/room temperature molten salts, composed completely of ions.1 The increased interest in ionic liquids in recent years arises from the realization of novel soft materials including solvents,2 catalysts,3 electrolytes,4 lubricants,5 extractants,6 surfactants,7 propellants,8 magnetic fluids,9 and optical fluids,10 etc. The combination of different cations and anions could lead to the construction of functionalized ionic liquids with various properties. Metalcontaining ionic liquids are multifunctional soft materials, with the combined advantages of ionic liquids and metal features.11 Rare earth metals and their compounds have important applications in many technological devices, such as superconductors, permanent magnets, magnesium alloys, phosphors, electronic polishers, refining catalysts, opticalfiber communication systems, night vision goggles, rangefinders and hybrid car components ( primarily batteries and magnets). Therefore, the interest in lanthanide ionic liquids strongly originates from the attractive properties for applications as soft materials in optics, magnetics and catalysis.12
College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, China. E-mail: [email protected] † Electronic supplementary information (ESI) available: X-ray crystallography data of 1, 1H NMR and 13C NMR of 1–5, excitation and emission spectra of 3–5. CCDC 1012407. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt03294b
This journal is © The Royal Society of Chemistry 2015
Research on the development of new lanthanide ionic liquid compositions focuses largely on the creation of coordination compounds that constitute lanthanide-containing anions including [Ln(TiW11O39)2]13−·xH2O,13 [LnBr6]3−,14 [LnI6]3−,15 [Ln(NCS)x(H2O)y](x−3)−,16 [Ln(Tf2N)x+3]x−,17 [Ln(NO3)6]3−,18 and [Ln(dcnm)6]3− (dcnm = dicyanonitrosomethanide).19 In most of them, water is either an indispensable part of the ionic liquid, or a potential displacing ligand, which always results in the hydrolysis of ionic liquids.13–17 Furthermore, water efficiently absorbs the energy of the excited state through the excitation of O–H vibrations, leading to high radiationless decay rates, which can cause the complete quenching of any emission in the near-infrared (NIR) region of f-element luminescence in optical materials.17 The Nd(III) ion is fascinating and presents intricate properties in view of singular spectroscopic and magnetic properties, which originate from the special features of its electronic [Xe]4f 46s2 configuration. The investigation of Nd(III)-based materials is of relevance for various fields in NIR luminescence for applications as optical-fibers, lasers, light-emitting diodes, safety inks, luminescent labels, and so forth.20 Until now, most of the studied lanthanide-containing ionic liquids are focused on Eu or Tb, whereas Nd-containing compounds which emit in the NIR region have been less explored. To avoid interference from water, water-free Nd(III)-based ionic liquids are expected to be explored as NIR luminescent soft materials. Lanthanides, when coordinated with organic ligands, almost exclusively exist in the Ln3+ oxidation state and behave
Dalton Trans., 2015, 44, 2325–2332 | 2325
View Article Online
Published on 05 December 2014. Downloaded by UNIVERSITY OF ALABAMA AT BIRMINGHAM on 13/02/2015 13:26:41.
Paper
Scheme 1 Structures ([Cnmim]3[Nd(NO3)6]).
Dalton Transactions
of
hexanitratoneodymate
complexes
1–5
as “hard” acids, with a binding preference of O > N > S. The nitrate anion (NO3−) is a typical O-donor ligand, and is also a “hard” base, offering the possibility to construct lanthanide complexes of high coordination for high symmetry. Based on HSAB (Hard and Soft Acids and Bases) theory, the choice of NO3− incorporated with the Nd(III) ion should give a stronger affinity interaction, which helps to minimize the coordination of neutral water. Such a feature may possess a high degree of stability, and also avoid elements such as halides, sulfur, or phosphorus. In our previous work, water-free lanthanide ionic liquids built up from the [La(NO3)6]3− anion have been prepared with good chemical stability.18 Thus, water-free neodymium-containing ionic liquids, especially their coordination structures and optical properties in the NIR region, are worth looking at next. Herein, we report for the first time five novel neodymium-containing ionic liquids, with the general composition of [Cnmim]3[Nd(NO3)6] (n = 1, 2, 4, 6 and 8) (Scheme 1). The structure of these Nd(III) complexes was confirmed by the single crystal structure of complex 1. Their luminescence performances were also studied by NIR luminescence measurements.
Experimental General methods All chemicals of analytical grade were obtained commercially. Solvents were dried by standard procedures. 1,3-Dimethylimidazolium iodide ([C1mim]I), 1-alkyl-3-methylimidazolium bromides ([Cnmim]Br, n = 2, 4, 6 and 8), and 1-alkyl-3methylimidazolium nitrates ([Cnmim]NO3, n = 1, 2, 4, 6 and 8) were synthesized according to the literature procedures.2,21 Infrared spectra (IR) were recorded on a NEXUS 670 FT-IR spectrometer using KBr pellets at 25 °C. 1H and 13C NMR spectra were recorded on a Bruker 400 MHz nuclear magnetic resonance spectrometer operating at 400 (1H) and 100 (13C) MHz, respectively, with d6-DMSO as the locking solvent unless otherwise stated. 1H and 13C chemical shifts are reported in ppm relative to TMS. Differential scanning calorimetry (DSC) measurements were performed on a NETZSCH DSC 200 PC calorimeter equipped with a cooling accessory and calibrated
2326 | Dalton Trans., 2015, 44, 2325–2332
using standard pure indium, which was gently flooded with N2 at a flow rate of 20 mL min−1. Measurements were carried out at a heating rate of 10 °C min−1 from −100 °C to 100 °C. The reference sample was an Al container with nitrogen. Thermogravimetric analysis (TGA) measurements were accomplished on a NETZSCH TG 209F1 thermogravimetric analyzer at a heating rate of 10 °C min−1 from 25 to 500 °C in a dynamic nitrogen atmosphere at a flow rate of 70 mL min−1. Elemental analyses (H, C, N) were performed on an Elementar Vario MICRO CUBE elemental analyzer. UV-vis absorption spectra were recorded on a Rayleigh UV-1601 UV-Vis spectrophotometer. Luminescence measurements were recorded on an Edinburgh FLS920 fluorescence spectrophotometer with a xenon lamp as the excitation source and a photomultiplier tube for detection. Excitation and emission spectra were collected at 2.5 nm band pass at 293 K and corrected for detector response and lamp spectrum. Tri(1,3-dimethylimidazolium) hexanitratoneodymate, [C1mim]3[Nd(NO3 )6], (1). 1,3-Dimethylimidazolium nitrate (478 mg, 3 mmol) was dissolved in acetonitrile (30 mL), and then neodymium(III) nitrate hexahydrate (Nd(NO3)3·6H2O, 438 mg, 1 mmol) was added. The resulting mixture was stirred at 40 °C for 24 h. Then, the mixture was dried to yield 1 as a lilac solid (791 mg, 98%). Clear purple plate crystals of 1 suitable for X-ray structure determination were obtained after recrystallization from acetonitrile/ethyl acetate. 1H NMR: δ = 9.03 (s, 3H), 7.67 (d, 6H), 3.83 (s, 18H) ppm. 13C NMR: δ = 136.96, 123.36, 35.56 ppm. IR (KBr, 25 °C): ν = 3162, 3100, 2961, 2865, 1578, 1430, 1320, 1170, 1039, 852, 821, 736, 621 cm−1. Anal. Calcd for C15H27NdN12O18 (807.68): C, 22.31; H, 3.37; N, 20.81. Found: C, 22.29; H, 3.42; N, 20.61. Tri(1-ethyl-3-methylimidazolium) hexanitratoneodymate, [C2mim]3[Nd(NO3)6], (2). A similar procedure was followed as that described above for the preparation of 1. 1-Ethyl-3-methylimidazolium nitrate (520 g, 3 mmol) and neodymium(III) nitrate hexahydrate (438 mg, 1 mmol) were reacted in acetonitrile to obtain a purple solid 2 (824 mg, 97%). 1H NMR: δ = 9.14 (s, 3 H), 7.77 (s, 3 H), 7.68 (s, 3 H), 4.17 (q, 6 H, J = 7.2 Hz), 3.83 (s, 9 H), 1.39 (t, 9 H, J = 7.2 Hz) ppm. 13C NMR: δ = 136.22, 123.49, 121.90, 44.05, 35.59, 15.02 ppm. IR (KBr, 25 °C): ν = 3155, 3118, 2987, 2890, 1569, 1440, 1320, 1162, 1039, 849, 814, 735, 621 cm−1. Anal. Calcd for C18H33NdN12O18 (849.76): C, 25.44; H, 3.91; N, 19.78. Found: C, 25.36; H, 3.87; N, 19.69. Tri(1-butyl-3-methylimidazolium) hexanitratoneodymate, [C4mim]3[Nd(NO3)6], (3). A similar procedure was followed as that described above for the preparation of 1. 1-Butyl-3-methylimidazolium nitrate (604 mg, 3 mmol) and neodymium(III) nitrate hexahydrate (438 mg, 1 mmol) were reacted in acetonitrile to obtain a purple liquid 3 (924 g, 99%). 1H NMR: δ = 9.15 (s, 3 H), 7.76 (s, 3 H), 7.68 (s, 3 H), 4.13 (t, 6 H, J = 7.2 Hz), 3.82 (s, 9 H), 1.73 (m, 6 H), 1.23 (m, 6 H), 0.86 (t, 9 H, J = 7.2 Hz) ppm. 13C NMR: δ = 136.57, 123.57, 122.24, 48.44, 35.63, 31.31, 18.72, 13.21 ppm. IR (KBr, 25 °C): ν = 3153, 3118, 2970, 2874, 1569, 1448, 1326, 1162, 1039, 855, 820, 735, 652, 623 cm−1. Anal. Calcd for C24H45NdN12O18 (933.92): C, 30.87; H, 4.86; N, 18.00. Found: C: 30.76; H: 4.91; N: 17.92.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 05 December 2014. Downloaded by UNIVERSITY OF ALABAMA AT BIRMINGHAM on 13/02/2015 13:26:41.
Dalton Transactions
Tri(1-hexyl-3-methylimidazolium) hexanitratoneodymate, [C6mim]3[Nd(NO3)6], (4). A similar procedure was followed as that described above for the preparation of 1. 1-Hexyl-3-methylimidazolium nitrate (688 mg, 3 mmol) and neodymium(III) nitrate hexahydrate (438 mg, 1 mmol) were reacted in acetonitrile to obtain a purple liquid 4 (1.008 g, 99%). 1H NMR: δ = 9.12 (s, 3 H), 7.77 (s, 3 H), 7.70 (s, 3 H), 4.14 (t, 6 H, J = 7.2 Hz), 3.84 (s, 9 H), 1.77 (m, 6 H), 1.26 (m, 18 H), 0.86 (t, 9 H, J = 7.2 Hz) ppm. 13C NMR: δ = 136.42, 123.52, 122.17, 48.66, 35.64, 30.43, 29.22, 25.04, 21.76, 13.73 ppm. IR (KBr, 25 °C): ν = 3150, 3110, 2935, 2865, 1569, 1448, 1320, 1162, 1039, 859, 815, 731, 652, 623 cm−1. Anal. Calcd for C30H57NdN12O18 (1018.08): C, 35.39; H, 5.64; N, 16.51. Found: C, 35.27; H, 5.72; N, 16.58. Tri(1-methyl-3-octylimidazolium) hexanitratoneodymate, [C8mim]3[Nd(NO3)6], (5). A similar procedure was followed as that described above for the preparation of 1. 1-Methyl-3-octylimidazolium nitrate (924 mg, 3 mmol) and neodymium(III) nitrate hexahydrate (438 mg, 1 mmol) were added. The resulting mixture was reacted in acetonitrile to obtain a purple liquid 5 (1.091 g, 99%). 1H NMR: δ = 9.16 (s, 3 H), 7.75 (s, 3 H), 7.67 (s, 3 H), 4.10 (t, 6 H, J = 7.2 Hz), 3.80 (s, 9 H), 1.72 (m, 6 H), 1.18 (m, 30 H), 0.77 (t, 9 H, J = 6.8 Hz) ppm. 13C NMR: δ = 136.65, 123.58, 122.27, 48.75, 35.62, 31.15, 29.40, 28.47, 28.33, 25.49, 22.04, 13.90 ppm. IR (KBr, 25 °C): ν = 3153, 3110, 2930, 2865, 1569, 1457, 1326, 1162, 1039, 858, 823, 742, 656, 623 cm−1. Anal. Calcd for C36H69NdN12O18 (1102.24): C, 39.23; H, 6.31; N, 15.25. Found: C, 39.33; H, 6.29; N, 15.21. X-ray crystallography Single crystals of 1 were removed from the test tube; a suitable crystal was selected, attached to a glass fiber and the data were collected at 293 K using an Oxford Xcalibur diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The structure was solved by direct methods and refined by the full-matrix least-squares method on F2 with the SHELXTL program package.26 The structure was solved in the space group P21/c by analysis of systematic absences. All nonhydrogen atoms were refined anisotropically, and hydrogen atoms were located and refined. No decomposition was observed during data collection. CCDC 1012407 contains the supplementary crystallographic data for this paper.†
Results and discussion Syntheses The hexanitratoneodymate ionic liquids were formed by the addition of neodymium nitrate hexahydrate to the corresponding 1,3-dialkylimidazolium nitrates [Cnmim][NO3] (1 : 3 molar ratio) in acetonitrile (Scheme 2). Then, water (as the exclusive by-product) was removed by distillation to give the pure complexes. Complex 1 was obtained as a lilac solid. Slow recrystallization from an acetonitrile/ethyl acetate solution afforded purple plates of 1 suitable for X-ray diffraction. Analogous routes were employed for complexes 2–5. Complex
This journal is © The Royal Society of Chemistry 2015
Paper
Scheme 2
Synthesis route to complexes 1–5 [Cnmim]3[Nd(NO3)6].
Fig. 1 Hexanitratoneodymate complexes 1–5, from left to right: 1, [C1mim]3[Nd(NO3)6]; 2, [C2mim]3[Nd(NO3)6]; 3, [C4mim]3[Nd(NO3)6]; 4, [C6mim]3[Nd(NO3)6]; 5, [C8mim]3[Nd(NO3)6].
2 is a purple solid, while complexes 3–5 are purple liquids (Fig. 1). 1,3-Dialkylimidazolium cations were introduced to produce these complexes with a major trend towards ionic liquids with low melting points, derived from the delocalization of positive charge on the imidazolium ring. The size and the tenacity of the alkyls attached on the imidazolium ring have a further influence on the melting points of the complexes. The higher conformational flexibility of butyl, hexyl or octyl groups combined with a well-shielded or delocalized charge on the imidazolium ring, leads to stronger disorder in the molecules for complexes 3–5 in the liquid state, rather than their derivatives 1 and 2. The structures of [Cnmim]3[Nd(NO3)6] 1–5 were fully characterized by IR, NMR and elemental analysis. NMR spectra The existence of 1-alkyl-3-methylimidazolium cations was confirmed in all complexes by the data from the 1H and 13C NMR spectra (Fig. S3–S12, see ESI†). For example, the 1H NMR spectrum of 1 shows singlet resonances at 9.03 and 7.67 ppm attributed to hydrogens in the imidazolium ring, which are also confirmed in other imidazolium complexes.21 The resonance for the remaining hydrogens of the methyl group is shifted upfield and appears at 3.83 ppm. In addition, no hydrated or solvated water molecules were observed based on the data of the 1H NMR spectrum. In the 13C NMR spectrum of 1, the signals assigned to the carbons in the imidazolium ring are also observed, with values of 136.95 and 123.36 ppm, respectively. The chemical shift of the methyl group is centered upfield (35.56 ppm). This signal is well separated from those of the imidazolium ring. Such data were found to be similar to those of imidazolium complexes obtained in previous literature.21
Dalton Trans., 2015, 44, 2325–2332 | 2327
View Article Online
Published on 05 December 2014. Downloaded by UNIVERSITY OF ALABAMA AT BIRMINGHAM on 13/02/2015 13:26:41.
Paper
Dalton Transactions
Analogous data were also obtained for complexes 2–5. Different from complex 1, more signals are recorded in the 1H NMR spectra. Two resonances of the 4- and 5-position hydrogens in the imidazolium ring are found at 7.77 and 7.68 ppm, as a result of the different alkyl groups attached on the imidazolium ring. The presence of the alkyl group produces more complicated spectra, with three (4.18, 3.83 and 1.39 ppm, 2) or five signals (4.13, 3.82, 1.73, 1.23 and 0.86 ppm, 3–5) recorded. In the 13C NMR spectra of 2–5, differences are also found. Three chemical shifts assigned to the imidazolium ring appear at 136.22, 123.49 and 121.90 ppm, respectively. The signals attributed to the alkyl group are also observed to be more complicated than those found in 1. IR spectra Complexation of Nd(III) by the nitrate ligand was illustrated using infrared spectroscopy (Fig. 2). Characteristic absorption bands of complexes 1–5 arising from the lattice and Nd–O vibrations, the nitrato groups, and the counterions were observed, similar to those of the reported hexanitratolanthanate ([La(NO3)6]3−) and hexanitratocerate ([Ce(NO3)6]3−) complexes.18 The asymmetric N–O stretching vibrations assigned to the oxygen atoms of the NO3− anion coordinated with a Nd(III) ion were found at δ = 1454 and 1320 cm−1 in each [Cnmim]3[Nd(NO3)6] complex. In the IR spectra, the absorbance peak at around 1385 cm−1 is assigned to the symmetric free nitrate radical associated with ν3(E′) of the D3h point group. This signal is sensitive to the complex formation, and indeed for 1, this band disappeared and further split into two peaks (ν4 and ν1 modes), recorded at 1454 and 1325 cm−1, respectively. Such changes in the absorbance peaks show that all nitrate ions are coordinated with the central Nd(III) ion, and no free nitrate ion exists in these complexes. The N–O symmetrical stretching vibration ν2, along with the out-of-plane bending vibration ν6 are observed at about 1028–1039 cm−1
Fig. 2
IR spectra of complexes 1–5.
2328 | Dalton Trans., 2015, 44, 2325–2332
and 817–822 cm−1. No other N–O stretching mode was found. Such features are also found in the other four [Cnmim]3[Nd(NO3)6] complexes 2–5. The existence of the 12-coordinated hexanitratoneodymate, [Nd(NO3)6]3−, is confirmed. As a further investigation, the samples of hydrous [C4mim]3[Nd(NO3)6] with 1 wt% water, and neodymium nitrate hexahydrate Nd(NO3)6·6H2O were detected by IR measurement (Fig. S13, see ESI†). The water molecule has three IR active vibrations due to its C2v symmetry, the bending mode ν5 absorbed in the region 1595–1650 cm−1, the antisymmetric and symmetric stretching modes lie in the region 3000–3800 cm−1. Two absorbance peaks assignable to the absorbed water around 3400 cm−1 and 1625 cm−1 were found in these two samples. All of the absorbance of the ν1–ν6 mode peaks was observed in the spectrum of Nd(NO3)6·6H2O, showing the existence of the coordinated nitrate, coordinated water and trace free water. For [C4mim]3[Nd(NO3)6] with 1 wt% water, absorbance peaks around 1385 cm−1 assigned to the non-coordinated nitrate anion, 1625 cm−1 assigned to water, along with the peaks 1450 and 1325 cm−1 belonging to the coordinated nitrate anions, were observed. When the complex was dried again, the absorbance peak around 1385 cm−1 disappeared. A reversible process between the non-coordinated nitrate anion in the aqueous phase and the coordinated nitrate anion in the dried complex is realized. Thus, moisture stable, water-free Nd(III) complex liquid materials may be obtained and handled. Thermal behavior The melting points of complexes 1–5 were determined by differential scanning calorimetry (DSC). No loss of water for the hydrated species was observed, indicating that no neutral water molecule coordinated to the Nd(III) ion. [C1mim]3[Nd(NO3)6] (1) and [C2mim]3[Nd(NO3)6] (2) afford distinct melting points at 108 °C and 61 °C, respectively. Such a value for 2 is analogous to the data for the [C2mim]3[Nd(dcnm)6] complex (61 °C).19 Complexes 3–5 are found to be liquids or supercooled liquids at room temperature, with only glass transition temperatures recorded to be −42 to −38 °C. The size, charge delocalization and symmetry of the cations are of importance to determine the melting points of these Nd(III)-based complexes. The alkyl chain can produce changes in the tendency of the complex to form glasses rather than crystalline solids on cooling, by changing the efficiency of ion packing and hydrogen bonding. Compared with 2–5, the 1,3-dimethylimidazolium cation of 1 has a smaller size and higher charge density along with a higher point symmetry to form stronger electrostatic interactions. It therefore tends towards a more ordered structure resulting in a higher melting point. Such a trend is consistent with their hexanitratolanthanate ([Cnmim]3[La(NO3)6]) analogues. However, the major changes in the radius of Ln(III) from La(III) to Nd(III) yields lower melting points/glass transition temperatures, due to changes in the interaction of the metal(III)–nitrate anion (Nd–O, 2.603(3)–2.629(3) Å & La–O, 2.6200(18)–2.6831(18) Å).18 Furthermore, the small size of the nitrate ligand coordinated to Nd(III) and the smaller charges of
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 05 December 2014. Downloaded by UNIVERSITY OF ALABAMA AT BIRMINGHAM on 13/02/2015 13:26:41.
Dalton Transactions
the anions can produce complex 3 with a lower melting point compared to the Nd(III) complex with an isothiocyanate anion and coordinated water molecules ([C4mim]4[Nd(NCS)7(H2O)], 28 °C).16 The thermal stabilities of complexes 1–5 were assessed by thermogravimetric analysis (TGA) tests. The curves are similar, with one step decomposition at 300–320 °C. Such considerable thermal stabilities of complexes 1–5 of over 300 °C are substantially comparable to the performance of the La(III)-based complexes [Cnmim]3[La(NO3)6].18 Their thermal stabilities are also predicted to be better than the other Nd(III)-based ionic liquid, [C4mim]4[Nd(NCS)7(H2O)] (294 °C).16 Therefore, these Nd(III)-based ionic liquids are sufficiently thermally stable materials, with a large liquidus range of over 300 °C illustrated. X-ray analyses The crystal structure of 1 was determined by single-crystal Xray diffraction. The purple plate crystals of 1 were obtained by slow recrystallization from acetonitrile/ethyl acetate at room temperature. The molecular structure of 1 is shown in Fig. 3 and the crystallographic data are summarized in Table 1. Selected bond lengths and angles are given in Table 2. All nonhydrogen atoms were refined anisotropically. 1 crystallizes in the monoclinic space group P21/c with four molecular moieties in each unit cell, consistent with the data reported for [C1mim]3[La(NO3)6].18 The calculated density of 1 is 1.726 g cm−3 with a unit cell volume of 3109.11(11) Å3 (a = 15.9460(3) Å, b = 14.2772(3) Å, c = 13.6634(3) Å). In 1, each molecule consists of one hexanitratoneodymate anion ([Nd(NO3)6]3−) and three 1,3-dimethylimidazolium cations to satisfy the charge equilibrium of the total coordination polymer. No disordered solvent or water is observed in the crystal lattice. The central Nd(III) ion is surrounded by six bidentate nitrate ligands, with twelve oxygen donors bidentate-chelated, to form one hexanitratoneodymate anion. The data indicate that the ionic radius of the Nd(III) ion and imidazolium allows the accommodation of six nitrate ligands surrounding the Nd(III) ion. It is immediately noticeable that this twelve-coordination
Paper Table 1
Formula FW (g mol−1) Temperature (K) Size (mm3) Crystal system Space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z ρ (g cm−3) μ (mm−1) F(000) λ(Mo Kα) (Å) Reflections Rint Parameters S on F2 R1 (I > 2σ(I))a wR2 (I > 2σ(I))b R1 (all data)a wR2 (all data)b Δρmin and max (e Å−3) a
Molecular structure of 1.
This journal is © The Royal Society of Chemistry 2015
C15H27N12NdO18 807.73 293(2) 0.40 × 0.40 × 0.35 Monoclinic P21/c 15.9460(3) 14.2772(3) 13.6634(3) 90 91.8108(17) 90 3109.11(11) 4 1.726 1.764 1620 0.71073 12893 0.0236 424 1.074 0.0360 0.0887 0.0744 0.1098 0.55 and −0.86
R1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2.
Table 2 1
Selected bond lengths (Å) and bond angles (°) in the crystal of
Bond lengths (Å) Nd–O1a Nd–O1 Nd–O2 Nd–O4 Nd–O6 Nd–O8 Nd–O9
2.611(3) 2.611(3) 2.619(3) 2.607(3) 2.603(3) 2.629(3) 2.604(3)
O1–N1 O2–N1 O3–N1 O5–N2 O6–N2 O7–N3 O8–N3
1.259(5) 1.250(5) 1.237(5) 1.220(5) 1.265(5) 1.213(5) 1.256(4)
180 48.55(12) 131.45(12) 68.78(11) 111.22(11) 48.31(10) 67.47(11) 112.53(11)
N1–O1–Nd N2–O4–Nd N2–O6–Nd O2–N1–O1 O3–N1–O1 O4–N2–O6 O5–N2–O6
96.7(3) 97.7(3) 97.0(3) 117.9(4) 120.6(4) 117.2(4) 121.8(4)
Bond angles (°) O1–Nd–O1a O1–Nd–O2 O1–Nd–O2a O1–Nd–O4a O1–Nd–O4 O4–Nd–O6 O4–Nd–O8 O8–Nd–O6 a
Fig. 3
Crystal data and structure refinement for 1
−1 − X, 1 − Y, −Z.
initially represents a high coordination of oxygen atoms to the Nd(III) ion with the major trend towards a high symmetry of the [Nd(NO3)6]3− anion. For the Nd(III) ion, the coordination environment can be considered as an ordered icosahedron, which consists of the Nd(III) ion located in the center and oxygen occupied at the vertices. The nitrates opposite to one another lie in a coplane, while the adjacent nitrates are almost vertical. This feature of 1 differs from the previously reported [Ln(dcnm)6]3− complex, in which six dcnm ligands formed an
Dalton Trans., 2015, 44, 2325–2332 | 2329
View Article Online
Published on 05 December 2014. Downloaded by UNIVERSITY OF ALABAMA AT BIRMINGHAM on 13/02/2015 13:26:41.
Paper
Dalton Transactions
octahedral-like environment around the central lanthanum(III) ion.22 This 12-coordinated polyhedron of the Nd(III) ion is similar to that of the hexanitratolanthanate anion ([La(NO3)6]3−),18 which gives the highest coordination numbers of rare earth metal complexes. The Nd–O bond lengths are found to be between 2.603(3)– 2.619(3) Å, which are slightly shorter than the reported values of the La–O bonds (2.6200(18)–2.6831(19)) in the [La(NO3)6]3− anion.18 The stronger interaction between the oxygen atom and Nd(III) ion, arises from the smaller ionic radius of Nd(III), with a tendency towards a higher electronic density in the [Nd(NO3)6]3− anion. The presence of some multiple bond character is confirmed in the crystal structure of 1. The nitrate anions are delocalized. The interatomic distances of the N–O bonds are 1.213(5)–1.265(5) Å. The N–O bonds coordinated to the Nd(III) are about 0.013–0.052 Å longer than the other N–O bonds. Such values of the N–O bonds in 1 are in contrast to those found in [C1mim]3[La(NO3)6]. The O–N–O angles range from 76.6(3) to 77.6(3)°, and are also comparable to the values found for [C1mim]3[La(NO3)6].18 Furthermore, the torsion angles show that the nitrate anion in 1 is coplanar. Delocalization of the aromatic system for the 1,3-dimethylimidazolium cation is also confirmed and in agreement with the structural motif found in other imidazolium-based complexes.21 An analogous trend is observed for the N–N bonds. In addition, a decrease of the N–C bond lengths is also measured. The infinite three-dimensional system of 1 is also derived from the simple ion pairs (Fig. 4). The imidazolium cations orderly fill the crystal lattices. However, no π-stacking interactions commonly seen in aromatic derivatives were found among the imidazolium cations. The reason may be the large volume of hexanitratoneodymate anions hindering the π-stacking of the cations. This type of packing structure is homologous to the arrangement observed in the crystal structure of the [C1mim]3[La(NO3)6] complex. However, this structure is predicted to be different from the normal imidazolium-based
Fig. 4
Packing diagram of 1 viewed down the a-axis.
2330 | Dalton Trans., 2015, 44, 2325–2332
systems combined with for example [LnI6]3−, [Ln(NCS)7(H2O)]4− or [Ln(dcnm)6]3− anions, in which the hydrogen atom in the 2-position normally participated in hydrogen bonding for its weakly acidic nature. In this instance, neither C–H⋯O intramolecular H-bond interactions nor C–H⋯N intermolecular H-bond interactions exist between the imidazolium cation and the nitrate anions. Indeed, complexes of the type [C1mim]3[Nd(NO3)6] do not possess any N–H⋯O hydrogen bonding capability. Compared with the hydrogen bonded lanthanide complexes, the packing structure of [C1mim]3[Nd(NO3)6] without hydrogen bonding could produce major changes in the melting points as well as their liquid characteristics. Near-infrared luminescence Trivalent lanthanide ions Ln(III) present singular optical properties, enabling easy spectral and time discrimination of their emission bands which span both the visible and nearinfrared ranges. Due to the intricate optical properties originating from the special features of its electronic [Xe]4f 3 configuration, Nd(III) exhibits a strong luminescence in NIR spectra. Herein, [Cnmim]3[Nd(NO3)6] ionic liquids were characterized by the excitation and emission spectra. Complexes 3–5 give similar excitation and emission spectra. No significant differences were observed for the ratios of the integrated intensities of the f–f transitions. A decline in the relative intensities could be observed for the excitation spectra recorded for complexes 3–5, due to an increase in the substituent length of the cation. The spectra of complex 3 is similar to the Nd(III) complexes found in aqueous 1.0 M perchloric acid,23 and the 1-butyl-1methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid (BMPTf2N).24 Such data indicate that the solvation environment has little influence on most of the 4f←4f electronic transitions observed for Nd(III) ions. Several low-intensity absorption bands at 510 nm, 525 nm, 735 nm, 796 nm, and 869 nm are recorded in the spectra of Nd(III) (Fig. 5). These sharp but weak bands arise from various Laporte forbidden intraconfigurational f←f electronic transitions.25 A maximum peak lies at 570–590 nm, associated with the UV-
Fig. 5 Excitation spectrum (blue) and emission spectrum (red) (λex = 581 nm) of complex 3 ([C4mim]3Nd(NO3)6) at room temperature.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 05 December 2014. Downloaded by UNIVERSITY OF ALABAMA AT BIRMINGHAM on 13/02/2015 13:26:41.
Dalton Transactions
vis-NIR spectra. The principal absorption peak of Nd(III) complexes in the visible region is the hypersensitive transition 4 G5/2, 2G7/2←4I9/2 at 570–590 nm, reflecting small changes in the ligand coordination sphere.25 Sharp emission bands originating from the metal ion are detected after rapid internal conversion to the emitting level. Electronic transitions were assigned according to the energy level diagrams of trivalent rare earth ions.20 The peak around 581 nm, attributed to overlap of the hypersensitive 4G5/2←4I9/2 and non-hypersensitive 2G7/2←4I9/2 transitions, can be observed as the most prominent line. Its intensity generally increases and the shape of the band often changes with an increase in complexation,25 while the 2 H9/2←4I9/2 transition at 796 nm is known to be largely unaffected by solvent interactions. Thus, the intensity of these two bands can be predicted to assess the coordination environment around the Nd(III) ion. For complex 3, the intensity ratio I581/I796 is predicted to be about 5.94. This value is significantly larger than that found in the NTf2− coordinated systems of the Nd(III) ion. A relatively higher coordination environment of the Nd(III) ion for this nitrate-coordinated system is illustrated. The ratios of the intensity of this transition to the other transitions are very similar in complexes 3–5. The emission spectra of complexes 3–5 were excited at 581 nm. The typical characteristic transitions for the Nd(III) ion are recorded between 800–1500 nm, excited by the band of the 4I9/2→4G5/2 transition. The hexanitratoneodymate complexes transmit a strong luminescence in the NIR part of the spectra, consisting of different lines of the 4F3/2→4IJ multiplet ( J = 9/2–13/2), which are attributed to f–f transitions, in accordance with the reported data.14 The emission spectrum of 3 is shown in Fig. 5. The strongest intensity assigned to the 4 F3/2→4I11/2 transition is found at 1057 nm. Its lineshape is extremely sharp in response to the luminescence colour. The 4 F3/2→4I9/2 transition is also intense. Splitting of these emission transitions was observed, with two overlapping peaks around 864 and 896 nm. The 4F3/2→4I13/2 transition occurs with a weak intensity at 1332 nm. However, in the water-added sample, all of the 4F3/2→4IJ transitions become even less intense, indicating a change in the coordination environment. The most obvious difference between the emission spectra measured is observed for the 4F3/2→4I9/2 transition. The peaks split into three peaks. The point symmetry of [Nd(NO3)6]3− is high with twelve-coordination, confirmed by X-ray diffraction. Compared with the luminescence spectra of the other Nd(III) complexes,20 the coordination sphere of the Nd(III) in complexes 3–5 is more symmetrical.
Conclusions Five high-coordinated NIR luminescent ionic liquids 1–5, of composition [Cnmim]3[Nd(NO3)6] (n = 1, 2, 4, 6 and 8), were synthesized for the first time without any neutral water coordinated. The complexes were obtained in high yields and fully characterized by NMR, IR and elemental analysis. The
This journal is © The Royal Society of Chemistry 2015
Paper
coordination of nitrate to the Nd(III) ion in the complexes was confirmed by IR spectroscopy, and no water was detected in the dried sample. A highly symmetrical structure of the [Nd(NO3)6]3− anion with twelve oxygen donors bidentatecoordinated to the central Nd(III) was strongly illustrated by the crystal structure of 1. This polyhedron can be described as an undistorted icosahedron. No coordinated water and hydrogenbonding networks were observed, which may result in their low melting points. These complexes are ionic liquids with low melting points, and exhibit high thermal stabilities and a desirable large liquidus range. The excitation and emission spectra of these 12coordinated hexanitratoneodymate complexes were recorded. These complexes exhibit excellent NIR luminescence of narrow band emission, which originates from interconfigurational f–f transitions 4F3/2→4IJ multiplet ( J = 9/2–13/2). Such data indicate that nitrate as an O-donor ligand may offer the possibility to form high-coordinated and highly symmetric hexanitratoneodymate complexes without any neutral coordinated water. In particular, suitable cations may lead these hexanitratoneodymate complexes to become ionic liquids as promising NIRluminescent soft materials with high monochromatic purity. The water-free feature may exhibit attractive optical applications of Nd(III)-based ionic liquids, to overcome the complete quenching of any emission in the NIR region.
Acknowledgements The financial support of NSFC (no. 21103116, 21303108, J1210004), SRF for ROCS, SEM (no. 2012170774), FRFCU (2013SCU04A12), and SSTIC (no. ZDSY20130331145131323) is gratefully acknowledged. We thank the Comprehensive training platform of specialized laboratory, College of chemistry, Sichuan University for instrumental measurements.
Notes and references 1 (a) T. Welton, Chem. Rev., 1999, 99, 2071–2083; (b) P. Wasserscheid and W. Keim, Angew. Chem., Int. Ed., 2000, 39, 3772–3789; (c) R. D. Rogers and K. R. Seddon, Science, 2003, 302, 792–793. 2 (a) S. Z. E. Abedin and F. Endres, Acc. Chem. Res., 2007, 40, 1106–1113; (b) G. H. Tao, M. Zou, X. H. Wang, Z. Y. Chen, D. G. Evans and Y. Kou, Aust. J. Chem., 2005, 58, 327–331; (c) G. H. Tao, L. He, W. S. Liu, L. Xu, W. Xiong, T. Wang and Y. Kou, Green Chem., 2006, 8, 639–646; (d) M. J. Earle, C. M. Gordon, N. V. Plechkova, K. R. Seddon and T. Welton, Anal. Chem., 2007, 79, 758–764; (e) J. P. Hallett and T. Welton, Chem. Rev., 2011, 111, 3508–3576. 3 (a) J. S. Wilkes, J. Mol. Catal. A: Chem., 2004, 214, 11–17; (b) G. H. Tao, Z. Y. Chen, L. He and Y. Kou, Chin. J. Catal., 2005, 26, 253–260; (c) K. Yuan and H. Ling, Prog. Chem., 2008, 20, 6–10; (d) T. Jiang and B. X. Han, Curr. Org. Chem., 2009, 13, 1278–1299; (e) J. Dupont and J. D. Scholtena,
Dalton Trans., 2015, 44, 2325–2332 | 2331
View Article Online
Published on 05 December 2014. Downloaded by UNIVERSITY OF ALABAMA AT BIRMINGHAM on 13/02/2015 13:26:41.
Paper
4
5 6 7 8
9
10
11
12 13
Chem. Soc. Rev., 2010, 39, 1780–1804; (f) L. L. Dong, L. He, G. H. Tao and C. W. Hu, RSC Adv., 2013, 3, 4806–4813. (a) M. Galiński, A. Lewandowski and I. Stępniak, Electrochim. Acta, 2006, 51, 5567–5580; (b) M. Armand, F. Endres, D. R. MacFarlane, H. Ohno and B. Scrosati, Nat. Mater., 2009, 8, 621–629; (c) X. Yang, L. He, S. Qin, G. H. Tao, M. Huang and Y. Lv, PLoS One, 2014, 9, e95832. C. F. Ye, W. M. Liu, Y. X. Chen and L. G. Yu, Chem. Commun., 2001, 2244–2245. J. S. Wang, C. N. Sheaff, B. Yoon, S. Addleman and C. M. Wai, Chem. – Eur. J., 2009, 15, 4458–4463. T. Torimoto, T. Tsuda, K. Okazaki and S. Kuwabata, Adv. Mater., 2010, 22, 1196–1221. (a) G. H. Tao, Y. Guo, Y. H. Joo, B. Twamley and J. M. Shreeve, J. Mater. Chem., 2008, 18, 5524–5530; (b) L. He, G. H. Tao, D. A. Parrish and J. M. Shreeve, Chem. – Eur. J., 2010, 16, 5736–5743; (c) S. Schneider, T. Hawkins, Y. Ahmed, M. Rosander, L. Hudgens and J. Mills, Angew. Chem., Int. Ed., 2011, 50, 5886–5888; (d) L. He, G. H. Tao, D. A. Parrish and J. M. Shreeve, Inorg. Chem., 2011, 50, 679– 685. (a) M. Okuno, H. Hamaguchi and S. Hayashi, Appl. Phys. Lett., 2006, 89, 132506; (b) R. E. Del Sesto, T. M. McCleskey, A. K. Burrell, G. A. Baker, J. D. Thompson, B. L. Scott, J. S. Wilkes and P. Williams, Chem. Commun., 2008, 447– 449. (a) S. Samikkanu, K. Mellem, M. Berry and P. S. May, Inorg. Chem., 2007, 46, 7121–7128; (b) B. Mallick, B. Balke, C. Felser and A. V. Mudring, Angew. Chem., Int. Ed., 2008, 47, 7635–7638. (a) J. Dupont, R. F. de Souza and P. A. Z. Suarez, Chem. Rev., 2002, 102, 3667–3692; (b) Z. F. Fei, T. J. Geldbach, D. B. Zhao and P. J. Dyson, Chem. – Eur. J., 2006, 12, 2122– 2130; (c) L. He, G. H. Tao, W. S. Liu, W. Xiong, T. Wang and Y. Kou, Chin. Chem. Lett., 2006, 17, 321–324; (d) N. V. Plechkova and K. R. Seddon, Chem. Soc. Rev., 2008, 37, 123–150; (e) R. Giernoth, Angew. Chem., Int. Ed., 2010, 49, 2834–2839. K. Binnemans, Chem. Rev., 2007, 107, 2592–2614. L. Dai, S. Yu, Y. Shan and M. He, Eur. J. Inorg. Chem., 2004, 237–241.
2332 | Dalton Trans., 2015, 44, 2325–2332
Dalton Transactions
14 A. Getsis and A. V. Mudring, Eur. J. Inorg. Chem., 2011, 3207–3213. 15 (a) A. Babai and A. V. Mudring, Chem. Mater., 2005, 17, 6230–6238; (b) A. Babai and A. V. Mudring, Inorg. Chem., 2005, 44, 8168–8169; (c) A. Babai and A. V. Mudring, Inorg. Chem., 2006, 45, 4874–4876. 16 P. Nockemann, B. Thijs, N. Postelmans, K. V. Hecke, L. V. Meervelt and K. Binnemans, J. Am. Chem. Soc., 2006, 128, 13658–13659. 17 S. Tang, A. Babai and A. V. Mudring, Angew. Chem., Int. Ed., 2008, 47, 7631–7634. 18 (a) G. H. Tao, Y. Huang, J. A. Boatz and J. M. Shreeve, Chem. – Eur. J., 2008, 14, 11167–11173; (b) S. P. Ji, M. Tang, L. He and G. H. Tao, Chem. – Eur. J., 2013, 19, 4452–4461. 19 A. S. R. Chesman, M. Yang, N. D. Spiccia, G. B. Deacon, S. R. Batten and A. V. Mudring, Chem. – Eur. J., 2012, 18, 9580–9589. 20 (a) M. P. Jensen, J. Neuefeind, J. V. Beitz, S. Skanthakumar and L. Soderholm, J. Am. Chem. Soc., 2003, 125, 15466– 15473; (b) K. Binnemans, Chem. Rev., 2009, 109, 4283– 4374; (c) S. V. Eliseeva and J. C. G. Bünzli, Chem. Soc. Rev., 2010, 39, 189–227. 21 (a) G. H. Tao, M. Tang, L. He, S. P. Ji, F. D. Nie and M. Huang, Eur. J. Inorg. Chem., 2012, 3070–3078; (b) L. L. Dong, L. He, H. Y. Liu, G. H. Tao, F. D. Nie, M. Huang and C. W. Hu, Eur. J. Inorg. Chem., 2013, 5009–5019. 22 A. S. R. Chesman, D. R. Turner, G. B. Deacon and S. R. Batten, Eur. J. Inorg. Chem., 2010, 2798–2812. 23 (a) K. Binnemans and C. Görller-Walrand, Chem. Phys. Lett., 1995, 235, 163–174; (b) C. V. Banks and D. W. Klingman, Anal. Chim. Acta, 1956, 15, 356–363. 24 L. H. Chou and C. L. Hussey, Inorg. Chem., 2014, 53, 5750– 5758. 25 C. Görller-Walrand and K. Binnemans, Spectral Intensities of f–f Transitions, in Handbook on the Physics and Chemistry of Rare Earths, ed. K. A. Gschneidner and L. Eyring, Elsevier, Amsterdam, 1998, vol. 25, ch. 167, pp. 101–264. 26 (a) O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard and H. Puschmann, J. Appl. Crystallogr., 2009, 42, 339–341; (b) G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam. Crystallogr., 2008, 64, 112–122.
This journal is © The Royal Society of Chemistry 2015
Dalton Transactions View Article Online
PAPER
Cite this: Dalton Trans., 2015, 44, 2325
View Journal | View Issue
Synthesis, structure and near-infrared photoluminescence of hexanitratoneodymate ionic liquids† Ling He, Shun-Ping Ji, Ning Tang, Ying Zhao and Guo-Hong Tao* Five hexanitratoneodymate-based rare earth complexes (1–5) were synthesized using a straightforward method. Purple plate crystals of 1 were isolated and the crystal structure was determined by single-crystal X-ray diffraction with respect to the coordination mode of the nitrate anion to the central Nd(III) ion. (1: monoclinic system P21/c, a = 15.9460(3) Å, b = 10.2457(6) Å, c = 33.323(3) Å, β = 91.8108(17)°, V = 3109.11(11) Å3, Z = 4). The central Nd(III) ion is surrounded by six bidentate nitrate ligands, with a major trend towards high symmetry of the [Nd(NO3)6]3− anion as an icosahedron. Thermal properties were determined from differential scanning calorimetry (DSC) combined with thermogravimetric analysis (TGA)
Received 25th October 2014, Accepted 5th December 2014
tests. Complexes 3–5 are found to be room temperature liquids, and their excitation and emission spectra
DOI: 10.1039/c4dt03294b
were recorded. These complexes exhibit intense near-infrared (NIR) luminescence emission, which originates from interconfigurational f–f transitions 4F3/2→4IJ multiplet ( J = 9/2–13/2). These liquid Nd(III) com-
www.rsc.org/dalton
plexes are of interest as potential NIR luminescent soft materials with high thermal stability.
Introduction Ionic liquids are low temperature/room temperature molten salts, composed completely of ions.1 The increased interest in ionic liquids in recent years arises from the realization of novel soft materials including solvents,2 catalysts,3 electrolytes,4 lubricants,5 extractants,6 surfactants,7 propellants,8 magnetic fluids,9 and optical fluids,10 etc. The combination of different cations and anions could lead to the construction of functionalized ionic liquids with various properties. Metalcontaining ionic liquids are multifunctional soft materials, with the combined advantages of ionic liquids and metal features.11 Rare earth metals and their compounds have important applications in many technological devices, such as superconductors, permanent magnets, magnesium alloys, phosphors, electronic polishers, refining catalysts, opticalfiber communication systems, night vision goggles, rangefinders and hybrid car components ( primarily batteries and magnets). Therefore, the interest in lanthanide ionic liquids strongly originates from the attractive properties for applications as soft materials in optics, magnetics and catalysis.12
College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, China. E-mail: [email protected] † Electronic supplementary information (ESI) available: X-ray crystallography data of 1, 1H NMR and 13C NMR of 1–5, excitation and emission spectra of 3–5. CCDC 1012407. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt03294b
This journal is © The Royal Society of Chemistry 2015
Research on the development of new lanthanide ionic liquid compositions focuses largely on the creation of coordination compounds that constitute lanthanide-containing anions including [Ln(TiW11O39)2]13−·xH2O,13 [LnBr6]3−,14 [LnI6]3−,15 [Ln(NCS)x(H2O)y](x−3)−,16 [Ln(Tf2N)x+3]x−,17 [Ln(NO3)6]3−,18 and [Ln(dcnm)6]3− (dcnm = dicyanonitrosomethanide).19 In most of them, water is either an indispensable part of the ionic liquid, or a potential displacing ligand, which always results in the hydrolysis of ionic liquids.13–17 Furthermore, water efficiently absorbs the energy of the excited state through the excitation of O–H vibrations, leading to high radiationless decay rates, which can cause the complete quenching of any emission in the near-infrared (NIR) region of f-element luminescence in optical materials.17 The Nd(III) ion is fascinating and presents intricate properties in view of singular spectroscopic and magnetic properties, which originate from the special features of its electronic [Xe]4f 46s2 configuration. The investigation of Nd(III)-based materials is of relevance for various fields in NIR luminescence for applications as optical-fibers, lasers, light-emitting diodes, safety inks, luminescent labels, and so forth.20 Until now, most of the studied lanthanide-containing ionic liquids are focused on Eu or Tb, whereas Nd-containing compounds which emit in the NIR region have been less explored. To avoid interference from water, water-free Nd(III)-based ionic liquids are expected to be explored as NIR luminescent soft materials. Lanthanides, when coordinated with organic ligands, almost exclusively exist in the Ln3+ oxidation state and behave
Dalton Trans., 2015, 44, 2325–2332 | 2325
View Article Online
Published on 05 December 2014. Downloaded by UNIVERSITY OF ALABAMA AT BIRMINGHAM on 13/02/2015 13:26:41.
Paper
Scheme 1 Structures ([Cnmim]3[Nd(NO3)6]).
Dalton Transactions
of
hexanitratoneodymate
complexes
1–5
as “hard” acids, with a binding preference of O > N > S. The nitrate anion (NO3−) is a typical O-donor ligand, and is also a “hard” base, offering the possibility to construct lanthanide complexes of high coordination for high symmetry. Based on HSAB (Hard and Soft Acids and Bases) theory, the choice of NO3− incorporated with the Nd(III) ion should give a stronger affinity interaction, which helps to minimize the coordination of neutral water. Such a feature may possess a high degree of stability, and also avoid elements such as halides, sulfur, or phosphorus. In our previous work, water-free lanthanide ionic liquids built up from the [La(NO3)6]3− anion have been prepared with good chemical stability.18 Thus, water-free neodymium-containing ionic liquids, especially their coordination structures and optical properties in the NIR region, are worth looking at next. Herein, we report for the first time five novel neodymium-containing ionic liquids, with the general composition of [Cnmim]3[Nd(NO3)6] (n = 1, 2, 4, 6 and 8) (Scheme 1). The structure of these Nd(III) complexes was confirmed by the single crystal structure of complex 1. Their luminescence performances were also studied by NIR luminescence measurements.
Experimental General methods All chemicals of analytical grade were obtained commercially. Solvents were dried by standard procedures. 1,3-Dimethylimidazolium iodide ([C1mim]I), 1-alkyl-3-methylimidazolium bromides ([Cnmim]Br, n = 2, 4, 6 and 8), and 1-alkyl-3methylimidazolium nitrates ([Cnmim]NO3, n = 1, 2, 4, 6 and 8) were synthesized according to the literature procedures.2,21 Infrared spectra (IR) were recorded on a NEXUS 670 FT-IR spectrometer using KBr pellets at 25 °C. 1H and 13C NMR spectra were recorded on a Bruker 400 MHz nuclear magnetic resonance spectrometer operating at 400 (1H) and 100 (13C) MHz, respectively, with d6-DMSO as the locking solvent unless otherwise stated. 1H and 13C chemical shifts are reported in ppm relative to TMS. Differential scanning calorimetry (DSC) measurements were performed on a NETZSCH DSC 200 PC calorimeter equipped with a cooling accessory and calibrated
2326 | Dalton Trans., 2015, 44, 2325–2332
using standard pure indium, which was gently flooded with N2 at a flow rate of 20 mL min−1. Measurements were carried out at a heating rate of 10 °C min−1 from −100 °C to 100 °C. The reference sample was an Al container with nitrogen. Thermogravimetric analysis (TGA) measurements were accomplished on a NETZSCH TG 209F1 thermogravimetric analyzer at a heating rate of 10 °C min−1 from 25 to 500 °C in a dynamic nitrogen atmosphere at a flow rate of 70 mL min−1. Elemental analyses (H, C, N) were performed on an Elementar Vario MICRO CUBE elemental analyzer. UV-vis absorption spectra were recorded on a Rayleigh UV-1601 UV-Vis spectrophotometer. Luminescence measurements were recorded on an Edinburgh FLS920 fluorescence spectrophotometer with a xenon lamp as the excitation source and a photomultiplier tube for detection. Excitation and emission spectra were collected at 2.5 nm band pass at 293 K and corrected for detector response and lamp spectrum. Tri(1,3-dimethylimidazolium) hexanitratoneodymate, [C1mim]3[Nd(NO3 )6], (1). 1,3-Dimethylimidazolium nitrate (478 mg, 3 mmol) was dissolved in acetonitrile (30 mL), and then neodymium(III) nitrate hexahydrate (Nd(NO3)3·6H2O, 438 mg, 1 mmol) was added. The resulting mixture was stirred at 40 °C for 24 h. Then, the mixture was dried to yield 1 as a lilac solid (791 mg, 98%). Clear purple plate crystals of 1 suitable for X-ray structure determination were obtained after recrystallization from acetonitrile/ethyl acetate. 1H NMR: δ = 9.03 (s, 3H), 7.67 (d, 6H), 3.83 (s, 18H) ppm. 13C NMR: δ = 136.96, 123.36, 35.56 ppm. IR (KBr, 25 °C): ν = 3162, 3100, 2961, 2865, 1578, 1430, 1320, 1170, 1039, 852, 821, 736, 621 cm−1. Anal. Calcd for C15H27NdN12O18 (807.68): C, 22.31; H, 3.37; N, 20.81. Found: C, 22.29; H, 3.42; N, 20.61. Tri(1-ethyl-3-methylimidazolium) hexanitratoneodymate, [C2mim]3[Nd(NO3)6], (2). A similar procedure was followed as that described above for the preparation of 1. 1-Ethyl-3-methylimidazolium nitrate (520 g, 3 mmol) and neodymium(III) nitrate hexahydrate (438 mg, 1 mmol) were reacted in acetonitrile to obtain a purple solid 2 (824 mg, 97%). 1H NMR: δ = 9.14 (s, 3 H), 7.77 (s, 3 H), 7.68 (s, 3 H), 4.17 (q, 6 H, J = 7.2 Hz), 3.83 (s, 9 H), 1.39 (t, 9 H, J = 7.2 Hz) ppm. 13C NMR: δ = 136.22, 123.49, 121.90, 44.05, 35.59, 15.02 ppm. IR (KBr, 25 °C): ν = 3155, 3118, 2987, 2890, 1569, 1440, 1320, 1162, 1039, 849, 814, 735, 621 cm−1. Anal. Calcd for C18H33NdN12O18 (849.76): C, 25.44; H, 3.91; N, 19.78. Found: C, 25.36; H, 3.87; N, 19.69. Tri(1-butyl-3-methylimidazolium) hexanitratoneodymate, [C4mim]3[Nd(NO3)6], (3). A similar procedure was followed as that described above for the preparation of 1. 1-Butyl-3-methylimidazolium nitrate (604 mg, 3 mmol) and neodymium(III) nitrate hexahydrate (438 mg, 1 mmol) were reacted in acetonitrile to obtain a purple liquid 3 (924 g, 99%). 1H NMR: δ = 9.15 (s, 3 H), 7.76 (s, 3 H), 7.68 (s, 3 H), 4.13 (t, 6 H, J = 7.2 Hz), 3.82 (s, 9 H), 1.73 (m, 6 H), 1.23 (m, 6 H), 0.86 (t, 9 H, J = 7.2 Hz) ppm. 13C NMR: δ = 136.57, 123.57, 122.24, 48.44, 35.63, 31.31, 18.72, 13.21 ppm. IR (KBr, 25 °C): ν = 3153, 3118, 2970, 2874, 1569, 1448, 1326, 1162, 1039, 855, 820, 735, 652, 623 cm−1. Anal. Calcd for C24H45NdN12O18 (933.92): C, 30.87; H, 4.86; N, 18.00. Found: C: 30.76; H: 4.91; N: 17.92.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 05 December 2014. Downloaded by UNIVERSITY OF ALABAMA AT BIRMINGHAM on 13/02/2015 13:26:41.
Dalton Transactions
Tri(1-hexyl-3-methylimidazolium) hexanitratoneodymate, [C6mim]3[Nd(NO3)6], (4). A similar procedure was followed as that described above for the preparation of 1. 1-Hexyl-3-methylimidazolium nitrate (688 mg, 3 mmol) and neodymium(III) nitrate hexahydrate (438 mg, 1 mmol) were reacted in acetonitrile to obtain a purple liquid 4 (1.008 g, 99%). 1H NMR: δ = 9.12 (s, 3 H), 7.77 (s, 3 H), 7.70 (s, 3 H), 4.14 (t, 6 H, J = 7.2 Hz), 3.84 (s, 9 H), 1.77 (m, 6 H), 1.26 (m, 18 H), 0.86 (t, 9 H, J = 7.2 Hz) ppm. 13C NMR: δ = 136.42, 123.52, 122.17, 48.66, 35.64, 30.43, 29.22, 25.04, 21.76, 13.73 ppm. IR (KBr, 25 °C): ν = 3150, 3110, 2935, 2865, 1569, 1448, 1320, 1162, 1039, 859, 815, 731, 652, 623 cm−1. Anal. Calcd for C30H57NdN12O18 (1018.08): C, 35.39; H, 5.64; N, 16.51. Found: C, 35.27; H, 5.72; N, 16.58. Tri(1-methyl-3-octylimidazolium) hexanitratoneodymate, [C8mim]3[Nd(NO3)6], (5). A similar procedure was followed as that described above for the preparation of 1. 1-Methyl-3-octylimidazolium nitrate (924 mg, 3 mmol) and neodymium(III) nitrate hexahydrate (438 mg, 1 mmol) were added. The resulting mixture was reacted in acetonitrile to obtain a purple liquid 5 (1.091 g, 99%). 1H NMR: δ = 9.16 (s, 3 H), 7.75 (s, 3 H), 7.67 (s, 3 H), 4.10 (t, 6 H, J = 7.2 Hz), 3.80 (s, 9 H), 1.72 (m, 6 H), 1.18 (m, 30 H), 0.77 (t, 9 H, J = 6.8 Hz) ppm. 13C NMR: δ = 136.65, 123.58, 122.27, 48.75, 35.62, 31.15, 29.40, 28.47, 28.33, 25.49, 22.04, 13.90 ppm. IR (KBr, 25 °C): ν = 3153, 3110, 2930, 2865, 1569, 1457, 1326, 1162, 1039, 858, 823, 742, 656, 623 cm−1. Anal. Calcd for C36H69NdN12O18 (1102.24): C, 39.23; H, 6.31; N, 15.25. Found: C, 39.33; H, 6.29; N, 15.21. X-ray crystallography Single crystals of 1 were removed from the test tube; a suitable crystal was selected, attached to a glass fiber and the data were collected at 293 K using an Oxford Xcalibur diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The structure was solved by direct methods and refined by the full-matrix least-squares method on F2 with the SHELXTL program package.26 The structure was solved in the space group P21/c by analysis of systematic absences. All nonhydrogen atoms were refined anisotropically, and hydrogen atoms were located and refined. No decomposition was observed during data collection. CCDC 1012407 contains the supplementary crystallographic data for this paper.†
Results and discussion Syntheses The hexanitratoneodymate ionic liquids were formed by the addition of neodymium nitrate hexahydrate to the corresponding 1,3-dialkylimidazolium nitrates [Cnmim][NO3] (1 : 3 molar ratio) in acetonitrile (Scheme 2). Then, water (as the exclusive by-product) was removed by distillation to give the pure complexes. Complex 1 was obtained as a lilac solid. Slow recrystallization from an acetonitrile/ethyl acetate solution afforded purple plates of 1 suitable for X-ray diffraction. Analogous routes were employed for complexes 2–5. Complex
This journal is © The Royal Society of Chemistry 2015
Paper
Scheme 2
Synthesis route to complexes 1–5 [Cnmim]3[Nd(NO3)6].
Fig. 1 Hexanitratoneodymate complexes 1–5, from left to right: 1, [C1mim]3[Nd(NO3)6]; 2, [C2mim]3[Nd(NO3)6]; 3, [C4mim]3[Nd(NO3)6]; 4, [C6mim]3[Nd(NO3)6]; 5, [C8mim]3[Nd(NO3)6].
2 is a purple solid, while complexes 3–5 are purple liquids (Fig. 1). 1,3-Dialkylimidazolium cations were introduced to produce these complexes with a major trend towards ionic liquids with low melting points, derived from the delocalization of positive charge on the imidazolium ring. The size and the tenacity of the alkyls attached on the imidazolium ring have a further influence on the melting points of the complexes. The higher conformational flexibility of butyl, hexyl or octyl groups combined with a well-shielded or delocalized charge on the imidazolium ring, leads to stronger disorder in the molecules for complexes 3–5 in the liquid state, rather than their derivatives 1 and 2. The structures of [Cnmim]3[Nd(NO3)6] 1–5 were fully characterized by IR, NMR and elemental analysis. NMR spectra The existence of 1-alkyl-3-methylimidazolium cations was confirmed in all complexes by the data from the 1H and 13C NMR spectra (Fig. S3–S12, see ESI†). For example, the 1H NMR spectrum of 1 shows singlet resonances at 9.03 and 7.67 ppm attributed to hydrogens in the imidazolium ring, which are also confirmed in other imidazolium complexes.21 The resonance for the remaining hydrogens of the methyl group is shifted upfield and appears at 3.83 ppm. In addition, no hydrated or solvated water molecules were observed based on the data of the 1H NMR spectrum. In the 13C NMR spectrum of 1, the signals assigned to the carbons in the imidazolium ring are also observed, with values of 136.95 and 123.36 ppm, respectively. The chemical shift of the methyl group is centered upfield (35.56 ppm). This signal is well separated from those of the imidazolium ring. Such data were found to be similar to those of imidazolium complexes obtained in previous literature.21
Dalton Trans., 2015, 44, 2325–2332 | 2327
View Article Online
Published on 05 December 2014. Downloaded by UNIVERSITY OF ALABAMA AT BIRMINGHAM on 13/02/2015 13:26:41.
Paper
Dalton Transactions
Analogous data were also obtained for complexes 2–5. Different from complex 1, more signals are recorded in the 1H NMR spectra. Two resonances of the 4- and 5-position hydrogens in the imidazolium ring are found at 7.77 and 7.68 ppm, as a result of the different alkyl groups attached on the imidazolium ring. The presence of the alkyl group produces more complicated spectra, with three (4.18, 3.83 and 1.39 ppm, 2) or five signals (4.13, 3.82, 1.73, 1.23 and 0.86 ppm, 3–5) recorded. In the 13C NMR spectra of 2–5, differences are also found. Three chemical shifts assigned to the imidazolium ring appear at 136.22, 123.49 and 121.90 ppm, respectively. The signals attributed to the alkyl group are also observed to be more complicated than those found in 1. IR spectra Complexation of Nd(III) by the nitrate ligand was illustrated using infrared spectroscopy (Fig. 2). Characteristic absorption bands of complexes 1–5 arising from the lattice and Nd–O vibrations, the nitrato groups, and the counterions were observed, similar to those of the reported hexanitratolanthanate ([La(NO3)6]3−) and hexanitratocerate ([Ce(NO3)6]3−) complexes.18 The asymmetric N–O stretching vibrations assigned to the oxygen atoms of the NO3− anion coordinated with a Nd(III) ion were found at δ = 1454 and 1320 cm−1 in each [Cnmim]3[Nd(NO3)6] complex. In the IR spectra, the absorbance peak at around 1385 cm−1 is assigned to the symmetric free nitrate radical associated with ν3(E′) of the D3h point group. This signal is sensitive to the complex formation, and indeed for 1, this band disappeared and further split into two peaks (ν4 and ν1 modes), recorded at 1454 and 1325 cm−1, respectively. Such changes in the absorbance peaks show that all nitrate ions are coordinated with the central Nd(III) ion, and no free nitrate ion exists in these complexes. The N–O symmetrical stretching vibration ν2, along with the out-of-plane bending vibration ν6 are observed at about 1028–1039 cm−1
Fig. 2
IR spectra of complexes 1–5.
2328 | Dalton Trans., 2015, 44, 2325–2332
and 817–822 cm−1. No other N–O stretching mode was found. Such features are also found in the other four [Cnmim]3[Nd(NO3)6] complexes 2–5. The existence of the 12-coordinated hexanitratoneodymate, [Nd(NO3)6]3−, is confirmed. As a further investigation, the samples of hydrous [C4mim]3[Nd(NO3)6] with 1 wt% water, and neodymium nitrate hexahydrate Nd(NO3)6·6H2O were detected by IR measurement (Fig. S13, see ESI†). The water molecule has three IR active vibrations due to its C2v symmetry, the bending mode ν5 absorbed in the region 1595–1650 cm−1, the antisymmetric and symmetric stretching modes lie in the region 3000–3800 cm−1. Two absorbance peaks assignable to the absorbed water around 3400 cm−1 and 1625 cm−1 were found in these two samples. All of the absorbance of the ν1–ν6 mode peaks was observed in the spectrum of Nd(NO3)6·6H2O, showing the existence of the coordinated nitrate, coordinated water and trace free water. For [C4mim]3[Nd(NO3)6] with 1 wt% water, absorbance peaks around 1385 cm−1 assigned to the non-coordinated nitrate anion, 1625 cm−1 assigned to water, along with the peaks 1450 and 1325 cm−1 belonging to the coordinated nitrate anions, were observed. When the complex was dried again, the absorbance peak around 1385 cm−1 disappeared. A reversible process between the non-coordinated nitrate anion in the aqueous phase and the coordinated nitrate anion in the dried complex is realized. Thus, moisture stable, water-free Nd(III) complex liquid materials may be obtained and handled. Thermal behavior The melting points of complexes 1–5 were determined by differential scanning calorimetry (DSC). No loss of water for the hydrated species was observed, indicating that no neutral water molecule coordinated to the Nd(III) ion. [C1mim]3[Nd(NO3)6] (1) and [C2mim]3[Nd(NO3)6] (2) afford distinct melting points at 108 °C and 61 °C, respectively. Such a value for 2 is analogous to the data for the [C2mim]3[Nd(dcnm)6] complex (61 °C).19 Complexes 3–5 are found to be liquids or supercooled liquids at room temperature, with only glass transition temperatures recorded to be −42 to −38 °C. The size, charge delocalization and symmetry of the cations are of importance to determine the melting points of these Nd(III)-based complexes. The alkyl chain can produce changes in the tendency of the complex to form glasses rather than crystalline solids on cooling, by changing the efficiency of ion packing and hydrogen bonding. Compared with 2–5, the 1,3-dimethylimidazolium cation of 1 has a smaller size and higher charge density along with a higher point symmetry to form stronger electrostatic interactions. It therefore tends towards a more ordered structure resulting in a higher melting point. Such a trend is consistent with their hexanitratolanthanate ([Cnmim]3[La(NO3)6]) analogues. However, the major changes in the radius of Ln(III) from La(III) to Nd(III) yields lower melting points/glass transition temperatures, due to changes in the interaction of the metal(III)–nitrate anion (Nd–O, 2.603(3)–2.629(3) Å & La–O, 2.6200(18)–2.6831(18) Å).18 Furthermore, the small size of the nitrate ligand coordinated to Nd(III) and the smaller charges of
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 05 December 2014. Downloaded by UNIVERSITY OF ALABAMA AT BIRMINGHAM on 13/02/2015 13:26:41.
Dalton Transactions
the anions can produce complex 3 with a lower melting point compared to the Nd(III) complex with an isothiocyanate anion and coordinated water molecules ([C4mim]4[Nd(NCS)7(H2O)], 28 °C).16 The thermal stabilities of complexes 1–5 were assessed by thermogravimetric analysis (TGA) tests. The curves are similar, with one step decomposition at 300–320 °C. Such considerable thermal stabilities of complexes 1–5 of over 300 °C are substantially comparable to the performance of the La(III)-based complexes [Cnmim]3[La(NO3)6].18 Their thermal stabilities are also predicted to be better than the other Nd(III)-based ionic liquid, [C4mim]4[Nd(NCS)7(H2O)] (294 °C).16 Therefore, these Nd(III)-based ionic liquids are sufficiently thermally stable materials, with a large liquidus range of over 300 °C illustrated. X-ray analyses The crystal structure of 1 was determined by single-crystal Xray diffraction. The purple plate crystals of 1 were obtained by slow recrystallization from acetonitrile/ethyl acetate at room temperature. The molecular structure of 1 is shown in Fig. 3 and the crystallographic data are summarized in Table 1. Selected bond lengths and angles are given in Table 2. All nonhydrogen atoms were refined anisotropically. 1 crystallizes in the monoclinic space group P21/c with four molecular moieties in each unit cell, consistent with the data reported for [C1mim]3[La(NO3)6].18 The calculated density of 1 is 1.726 g cm−3 with a unit cell volume of 3109.11(11) Å3 (a = 15.9460(3) Å, b = 14.2772(3) Å, c = 13.6634(3) Å). In 1, each molecule consists of one hexanitratoneodymate anion ([Nd(NO3)6]3−) and three 1,3-dimethylimidazolium cations to satisfy the charge equilibrium of the total coordination polymer. No disordered solvent or water is observed in the crystal lattice. The central Nd(III) ion is surrounded by six bidentate nitrate ligands, with twelve oxygen donors bidentate-chelated, to form one hexanitratoneodymate anion. The data indicate that the ionic radius of the Nd(III) ion and imidazolium allows the accommodation of six nitrate ligands surrounding the Nd(III) ion. It is immediately noticeable that this twelve-coordination
Paper Table 1
Formula FW (g mol−1) Temperature (K) Size (mm3) Crystal system Space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z ρ (g cm−3) μ (mm−1) F(000) λ(Mo Kα) (Å) Reflections Rint Parameters S on F2 R1 (I > 2σ(I))a wR2 (I > 2σ(I))b R1 (all data)a wR2 (all data)b Δρmin and max (e Å−3) a
Molecular structure of 1.
This journal is © The Royal Society of Chemistry 2015
C15H27N12NdO18 807.73 293(2) 0.40 × 0.40 × 0.35 Monoclinic P21/c 15.9460(3) 14.2772(3) 13.6634(3) 90 91.8108(17) 90 3109.11(11) 4 1.726 1.764 1620 0.71073 12893 0.0236 424 1.074 0.0360 0.0887 0.0744 0.1098 0.55 and −0.86
R1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2.
Table 2 1
Selected bond lengths (Å) and bond angles (°) in the crystal of
Bond lengths (Å) Nd–O1a Nd–O1 Nd–O2 Nd–O4 Nd–O6 Nd–O8 Nd–O9
2.611(3) 2.611(3) 2.619(3) 2.607(3) 2.603(3) 2.629(3) 2.604(3)
O1–N1 O2–N1 O3–N1 O5–N2 O6–N2 O7–N3 O8–N3
1.259(5) 1.250(5) 1.237(5) 1.220(5) 1.265(5) 1.213(5) 1.256(4)
180 48.55(12) 131.45(12) 68.78(11) 111.22(11) 48.31(10) 67.47(11) 112.53(11)
N1–O1–Nd N2–O4–Nd N2–O6–Nd O2–N1–O1 O3–N1–O1 O4–N2–O6 O5–N2–O6
96.7(3) 97.7(3) 97.0(3) 117.9(4) 120.6(4) 117.2(4) 121.8(4)
Bond angles (°) O1–Nd–O1a O1–Nd–O2 O1–Nd–O2a O1–Nd–O4a O1–Nd–O4 O4–Nd–O6 O4–Nd–O8 O8–Nd–O6 a
Fig. 3
Crystal data and structure refinement for 1
−1 − X, 1 − Y, −Z.
initially represents a high coordination of oxygen atoms to the Nd(III) ion with the major trend towards a high symmetry of the [Nd(NO3)6]3− anion. For the Nd(III) ion, the coordination environment can be considered as an ordered icosahedron, which consists of the Nd(III) ion located in the center and oxygen occupied at the vertices. The nitrates opposite to one another lie in a coplane, while the adjacent nitrates are almost vertical. This feature of 1 differs from the previously reported [Ln(dcnm)6]3− complex, in which six dcnm ligands formed an
Dalton Trans., 2015, 44, 2325–2332 | 2329
View Article Online
Published on 05 December 2014. Downloaded by UNIVERSITY OF ALABAMA AT BIRMINGHAM on 13/02/2015 13:26:41.
Paper
Dalton Transactions
octahedral-like environment around the central lanthanum(III) ion.22 This 12-coordinated polyhedron of the Nd(III) ion is similar to that of the hexanitratolanthanate anion ([La(NO3)6]3−),18 which gives the highest coordination numbers of rare earth metal complexes. The Nd–O bond lengths are found to be between 2.603(3)– 2.619(3) Å, which are slightly shorter than the reported values of the La–O bonds (2.6200(18)–2.6831(19)) in the [La(NO3)6]3− anion.18 The stronger interaction between the oxygen atom and Nd(III) ion, arises from the smaller ionic radius of Nd(III), with a tendency towards a higher electronic density in the [Nd(NO3)6]3− anion. The presence of some multiple bond character is confirmed in the crystal structure of 1. The nitrate anions are delocalized. The interatomic distances of the N–O bonds are 1.213(5)–1.265(5) Å. The N–O bonds coordinated to the Nd(III) are about 0.013–0.052 Å longer than the other N–O bonds. Such values of the N–O bonds in 1 are in contrast to those found in [C1mim]3[La(NO3)6]. The O–N–O angles range from 76.6(3) to 77.6(3)°, and are also comparable to the values found for [C1mim]3[La(NO3)6].18 Furthermore, the torsion angles show that the nitrate anion in 1 is coplanar. Delocalization of the aromatic system for the 1,3-dimethylimidazolium cation is also confirmed and in agreement with the structural motif found in other imidazolium-based complexes.21 An analogous trend is observed for the N–N bonds. In addition, a decrease of the N–C bond lengths is also measured. The infinite three-dimensional system of 1 is also derived from the simple ion pairs (Fig. 4). The imidazolium cations orderly fill the crystal lattices. However, no π-stacking interactions commonly seen in aromatic derivatives were found among the imidazolium cations. The reason may be the large volume of hexanitratoneodymate anions hindering the π-stacking of the cations. This type of packing structure is homologous to the arrangement observed in the crystal structure of the [C1mim]3[La(NO3)6] complex. However, this structure is predicted to be different from the normal imidazolium-based
Fig. 4
Packing diagram of 1 viewed down the a-axis.
2330 | Dalton Trans., 2015, 44, 2325–2332
systems combined with for example [LnI6]3−, [Ln(NCS)7(H2O)]4− or [Ln(dcnm)6]3− anions, in which the hydrogen atom in the 2-position normally participated in hydrogen bonding for its weakly acidic nature. In this instance, neither C–H⋯O intramolecular H-bond interactions nor C–H⋯N intermolecular H-bond interactions exist between the imidazolium cation and the nitrate anions. Indeed, complexes of the type [C1mim]3[Nd(NO3)6] do not possess any N–H⋯O hydrogen bonding capability. Compared with the hydrogen bonded lanthanide complexes, the packing structure of [C1mim]3[Nd(NO3)6] without hydrogen bonding could produce major changes in the melting points as well as their liquid characteristics. Near-infrared luminescence Trivalent lanthanide ions Ln(III) present singular optical properties, enabling easy spectral and time discrimination of their emission bands which span both the visible and nearinfrared ranges. Due to the intricate optical properties originating from the special features of its electronic [Xe]4f 3 configuration, Nd(III) exhibits a strong luminescence in NIR spectra. Herein, [Cnmim]3[Nd(NO3)6] ionic liquids were characterized by the excitation and emission spectra. Complexes 3–5 give similar excitation and emission spectra. No significant differences were observed for the ratios of the integrated intensities of the f–f transitions. A decline in the relative intensities could be observed for the excitation spectra recorded for complexes 3–5, due to an increase in the substituent length of the cation. The spectra of complex 3 is similar to the Nd(III) complexes found in aqueous 1.0 M perchloric acid,23 and the 1-butyl-1methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid (BMPTf2N).24 Such data indicate that the solvation environment has little influence on most of the 4f←4f electronic transitions observed for Nd(III) ions. Several low-intensity absorption bands at 510 nm, 525 nm, 735 nm, 796 nm, and 869 nm are recorded in the spectra of Nd(III) (Fig. 5). These sharp but weak bands arise from various Laporte forbidden intraconfigurational f←f electronic transitions.25 A maximum peak lies at 570–590 nm, associated with the UV-
Fig. 5 Excitation spectrum (blue) and emission spectrum (red) (λex = 581 nm) of complex 3 ([C4mim]3Nd(NO3)6) at room temperature.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 05 December 2014. Downloaded by UNIVERSITY OF ALABAMA AT BIRMINGHAM on 13/02/2015 13:26:41.
Dalton Transactions
vis-NIR spectra. The principal absorption peak of Nd(III) complexes in the visible region is the hypersensitive transition 4 G5/2, 2G7/2←4I9/2 at 570–590 nm, reflecting small changes in the ligand coordination sphere.25 Sharp emission bands originating from the metal ion are detected after rapid internal conversion to the emitting level. Electronic transitions were assigned according to the energy level diagrams of trivalent rare earth ions.20 The peak around 581 nm, attributed to overlap of the hypersensitive 4G5/2←4I9/2 and non-hypersensitive 2G7/2←4I9/2 transitions, can be observed as the most prominent line. Its intensity generally increases and the shape of the band often changes with an increase in complexation,25 while the 2 H9/2←4I9/2 transition at 796 nm is known to be largely unaffected by solvent interactions. Thus, the intensity of these two bands can be predicted to assess the coordination environment around the Nd(III) ion. For complex 3, the intensity ratio I581/I796 is predicted to be about 5.94. This value is significantly larger than that found in the NTf2− coordinated systems of the Nd(III) ion. A relatively higher coordination environment of the Nd(III) ion for this nitrate-coordinated system is illustrated. The ratios of the intensity of this transition to the other transitions are very similar in complexes 3–5. The emission spectra of complexes 3–5 were excited at 581 nm. The typical characteristic transitions for the Nd(III) ion are recorded between 800–1500 nm, excited by the band of the 4I9/2→4G5/2 transition. The hexanitratoneodymate complexes transmit a strong luminescence in the NIR part of the spectra, consisting of different lines of the 4F3/2→4IJ multiplet ( J = 9/2–13/2), which are attributed to f–f transitions, in accordance with the reported data.14 The emission spectrum of 3 is shown in Fig. 5. The strongest intensity assigned to the 4 F3/2→4I11/2 transition is found at 1057 nm. Its lineshape is extremely sharp in response to the luminescence colour. The 4 F3/2→4I9/2 transition is also intense. Splitting of these emission transitions was observed, with two overlapping peaks around 864 and 896 nm. The 4F3/2→4I13/2 transition occurs with a weak intensity at 1332 nm. However, in the water-added sample, all of the 4F3/2→4IJ transitions become even less intense, indicating a change in the coordination environment. The most obvious difference between the emission spectra measured is observed for the 4F3/2→4I9/2 transition. The peaks split into three peaks. The point symmetry of [Nd(NO3)6]3− is high with twelve-coordination, confirmed by X-ray diffraction. Compared with the luminescence spectra of the other Nd(III) complexes,20 the coordination sphere of the Nd(III) in complexes 3–5 is more symmetrical.
Conclusions Five high-coordinated NIR luminescent ionic liquids 1–5, of composition [Cnmim]3[Nd(NO3)6] (n = 1, 2, 4, 6 and 8), were synthesized for the first time without any neutral water coordinated. The complexes were obtained in high yields and fully characterized by NMR, IR and elemental analysis. The
This journal is © The Royal Society of Chemistry 2015
Paper
coordination of nitrate to the Nd(III) ion in the complexes was confirmed by IR spectroscopy, and no water was detected in the dried sample. A highly symmetrical structure of the [Nd(NO3)6]3− anion with twelve oxygen donors bidentatecoordinated to the central Nd(III) was strongly illustrated by the crystal structure of 1. This polyhedron can be described as an undistorted icosahedron. No coordinated water and hydrogenbonding networks were observed, which may result in their low melting points. These complexes are ionic liquids with low melting points, and exhibit high thermal stabilities and a desirable large liquidus range. The excitation and emission spectra of these 12coordinated hexanitratoneodymate complexes were recorded. These complexes exhibit excellent NIR luminescence of narrow band emission, which originates from interconfigurational f–f transitions 4F3/2→4IJ multiplet ( J = 9/2–13/2). Such data indicate that nitrate as an O-donor ligand may offer the possibility to form high-coordinated and highly symmetric hexanitratoneodymate complexes without any neutral coordinated water. In particular, suitable cations may lead these hexanitratoneodymate complexes to become ionic liquids as promising NIRluminescent soft materials with high monochromatic purity. The water-free feature may exhibit attractive optical applications of Nd(III)-based ionic liquids, to overcome the complete quenching of any emission in the NIR region.
Acknowledgements The financial support of NSFC (no. 21103116, 21303108, J1210004), SRF for ROCS, SEM (no. 2012170774), FRFCU (2013SCU04A12), and SSTIC (no. ZDSY20130331145131323) is gratefully acknowledged. We thank the Comprehensive training platform of specialized laboratory, College of chemistry, Sichuan University for instrumental measurements.
Notes and references 1 (a) T. Welton, Chem. Rev., 1999, 99, 2071–2083; (b) P. Wasserscheid and W. Keim, Angew. Chem., Int. Ed., 2000, 39, 3772–3789; (c) R. D. Rogers and K. R. Seddon, Science, 2003, 302, 792–793. 2 (a) S. Z. E. Abedin and F. Endres, Acc. Chem. Res., 2007, 40, 1106–1113; (b) G. H. Tao, M. Zou, X. H. Wang, Z. Y. Chen, D. G. Evans and Y. Kou, Aust. J. Chem., 2005, 58, 327–331; (c) G. H. Tao, L. He, W. S. Liu, L. Xu, W. Xiong, T. Wang and Y. Kou, Green Chem., 2006, 8, 639–646; (d) M. J. Earle, C. M. Gordon, N. V. Plechkova, K. R. Seddon and T. Welton, Anal. Chem., 2007, 79, 758–764; (e) J. P. Hallett and T. Welton, Chem. Rev., 2011, 111, 3508–3576. 3 (a) J. S. Wilkes, J. Mol. Catal. A: Chem., 2004, 214, 11–17; (b) G. H. Tao, Z. Y. Chen, L. He and Y. Kou, Chin. J. Catal., 2005, 26, 253–260; (c) K. Yuan and H. Ling, Prog. Chem., 2008, 20, 6–10; (d) T. Jiang and B. X. Han, Curr. Org. Chem., 2009, 13, 1278–1299; (e) J. Dupont and J. D. Scholtena,
Dalton Trans., 2015, 44, 2325–2332 | 2331
View Article Online
Published on 05 December 2014. Downloaded by UNIVERSITY OF ALABAMA AT BIRMINGHAM on 13/02/2015 13:26:41.
Paper
4
5 6 7 8
9
10
11
12 13
Chem. Soc. Rev., 2010, 39, 1780–1804; (f) L. L. Dong, L. He, G. H. Tao and C. W. Hu, RSC Adv., 2013, 3, 4806–4813. (a) M. Galiński, A. Lewandowski and I. Stępniak, Electrochim. Acta, 2006, 51, 5567–5580; (b) M. Armand, F. Endres, D. R. MacFarlane, H. Ohno and B. Scrosati, Nat. Mater., 2009, 8, 621–629; (c) X. Yang, L. He, S. Qin, G. H. Tao, M. Huang and Y. Lv, PLoS One, 2014, 9, e95832. C. F. Ye, W. M. Liu, Y. X. Chen and L. G. Yu, Chem. Commun., 2001, 2244–2245. J. S. Wang, C. N. Sheaff, B. Yoon, S. Addleman and C. M. Wai, Chem. – Eur. J., 2009, 15, 4458–4463. T. Torimoto, T. Tsuda, K. Okazaki and S. Kuwabata, Adv. Mater., 2010, 22, 1196–1221. (a) G. H. Tao, Y. Guo, Y. H. Joo, B. Twamley and J. M. Shreeve, J. Mater. Chem., 2008, 18, 5524–5530; (b) L. He, G. H. Tao, D. A. Parrish and J. M. Shreeve, Chem. – Eur. J., 2010, 16, 5736–5743; (c) S. Schneider, T. Hawkins, Y. Ahmed, M. Rosander, L. Hudgens and J. Mills, Angew. Chem., Int. Ed., 2011, 50, 5886–5888; (d) L. He, G. H. Tao, D. A. Parrish and J. M. Shreeve, Inorg. Chem., 2011, 50, 679– 685. (a) M. Okuno, H. Hamaguchi and S. Hayashi, Appl. Phys. Lett., 2006, 89, 132506; (b) R. E. Del Sesto, T. M. McCleskey, A. K. Burrell, G. A. Baker, J. D. Thompson, B. L. Scott, J. S. Wilkes and P. Williams, Chem. Commun., 2008, 447– 449. (a) S. Samikkanu, K. Mellem, M. Berry and P. S. May, Inorg. Chem., 2007, 46, 7121–7128; (b) B. Mallick, B. Balke, C. Felser and A. V. Mudring, Angew. Chem., Int. Ed., 2008, 47, 7635–7638. (a) J. Dupont, R. F. de Souza and P. A. Z. Suarez, Chem. Rev., 2002, 102, 3667–3692; (b) Z. F. Fei, T. J. Geldbach, D. B. Zhao and P. J. Dyson, Chem. – Eur. J., 2006, 12, 2122– 2130; (c) L. He, G. H. Tao, W. S. Liu, W. Xiong, T. Wang and Y. Kou, Chin. Chem. Lett., 2006, 17, 321–324; (d) N. V. Plechkova and K. R. Seddon, Chem. Soc. Rev., 2008, 37, 123–150; (e) R. Giernoth, Angew. Chem., Int. Ed., 2010, 49, 2834–2839. K. Binnemans, Chem. Rev., 2007, 107, 2592–2614. L. Dai, S. Yu, Y. Shan and M. He, Eur. J. Inorg. Chem., 2004, 237–241.
2332 | Dalton Trans., 2015, 44, 2325–2332
Dalton Transactions
14 A. Getsis and A. V. Mudring, Eur. J. Inorg. Chem., 2011, 3207–3213. 15 (a) A. Babai and A. V. Mudring, Chem. Mater., 2005, 17, 6230–6238; (b) A. Babai and A. V. Mudring, Inorg. Chem., 2005, 44, 8168–8169; (c) A. Babai and A. V. Mudring, Inorg. Chem., 2006, 45, 4874–4876. 16 P. Nockemann, B. Thijs, N. Postelmans, K. V. Hecke, L. V. Meervelt and K. Binnemans, J. Am. Chem. Soc., 2006, 128, 13658–13659. 17 S. Tang, A. Babai and A. V. Mudring, Angew. Chem., Int. Ed., 2008, 47, 7631–7634. 18 (a) G. H. Tao, Y. Huang, J. A. Boatz and J. M. Shreeve, Chem. – Eur. J., 2008, 14, 11167–11173; (b) S. P. Ji, M. Tang, L. He and G. H. Tao, Chem. – Eur. J., 2013, 19, 4452–4461. 19 A. S. R. Chesman, M. Yang, N. D. Spiccia, G. B. Deacon, S. R. Batten and A. V. Mudring, Chem. – Eur. J., 2012, 18, 9580–9589. 20 (a) M. P. Jensen, J. Neuefeind, J. V. Beitz, S. Skanthakumar and L. Soderholm, J. Am. Chem. Soc., 2003, 125, 15466– 15473; (b) K. Binnemans, Chem. Rev., 2009, 109, 4283– 4374; (c) S. V. Eliseeva and J. C. G. Bünzli, Chem. Soc. Rev., 2010, 39, 189–227. 21 (a) G. H. Tao, M. Tang, L. He, S. P. Ji, F. D. Nie and M. Huang, Eur. J. Inorg. Chem., 2012, 3070–3078; (b) L. L. Dong, L. He, H. Y. Liu, G. H. Tao, F. D. Nie, M. Huang and C. W. Hu, Eur. J. Inorg. Chem., 2013, 5009–5019. 22 A. S. R. Chesman, D. R. Turner, G. B. Deacon and S. R. Batten, Eur. J. Inorg. Chem., 2010, 2798–2812. 23 (a) K. Binnemans and C. Görller-Walrand, Chem. Phys. Lett., 1995, 235, 163–174; (b) C. V. Banks and D. W. Klingman, Anal. Chim. Acta, 1956, 15, 356–363. 24 L. H. Chou and C. L. Hussey, Inorg. Chem., 2014, 53, 5750– 5758. 25 C. Görller-Walrand and K. Binnemans, Spectral Intensities of f–f Transitions, in Handbook on the Physics and Chemistry of Rare Earths, ed. K. A. Gschneidner and L. Eyring, Elsevier, Amsterdam, 1998, vol. 25, ch. 167, pp. 101–264. 26 (a) O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard and H. Puschmann, J. Appl. Crystallogr., 2009, 42, 339–341; (b) G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam. Crystallogr., 2008, 64, 112–122.
This journal is © The Royal Society of Chemistry 2015
