Liquid-Phase Synthesis of Multidecker Organoeuropium Sandwich

Article pubs.acs.org/JPCC

Liquid-Phase Synthesis of Multidecker Organoeuropium Sandwich Complexes and Their Physical Properties Takashi Tsuji,† Natsuki Hosoya,† Suguru Fukazawa,† Rion Sugiyama,† Takeshi Iwasa,†,‡ Hironori Tsunoyama,†,‡ Hirofumi Hamaki,§ Norihiro Tokitoh,§ and Atsushi Nakajima*,†,‡ †

Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan ‡ JST, ERATO, Nakajima Designer Nanocluster Assembly Project, 3-2-1 Sakado, Takatsu-ku, Kawasaki 213-0012, Japan § Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan S Supporting Information *

ABSTRACT: Multidecker organoeuropium sandwich complexes were synthesized with the organic ligands 1,3,5,7-cyclooctatetraene (COT) and 1,4- and 1,6bis(trimethylsilyl)cyclooctatetraenes (COT″) in a liquid ammonia solution of europium (Eu) metal. Infrared (IR) absorption and Raman spectroscopies and magnetic measurements were used to identify local structures and charge distributions. It was found that the organoeuropium complexes have Eu2+−COT2− (Eu2+−COT″2−) charge distributions, including a local structure of ligand−Eu− ligand sandwiches. Together with elemental analysis by inductively coupled plasma atomic emission spectroscopy, it is shown that the organoeuropium complexes form a multidecker sandwich structure that is insouble in many solvents. In addition to their paramagnetic behaviors at 150−300 K, the organoeuropium complexes of Eu−COT and Eu−COT″ showed red and orange−red emissions, respectively, under UV irradiation. The introdution of bulky trimethylsilyl groups onto COT prominently enhanced the emission intensity by more than a factor of 10 compared to that of Eu−COT, and similar emission enhancement was observed when ethylenediamine vapor was contacted with Eu−COT solid. The intensity enhancement and the shifts in the emission wavelength are discussed from the viewpoint of orbital interactions between Eu2+ and COT2− (COT″2−). unique ligand field around the spin carrier of lanthanide (Ln) ions. Metal−ligand interactions, including 4f orbital splitting in the field of the organic molecule, have been revealed by spectroscopic studies of the formation and properties of Ln− COT complexes. Previously, we reported the gas-phase chemistry for the formation of multidecker sandwich Ln−COT complexes of the form [Lnn(COT)n+1] [Ln = cerium (Ce), neodymium (Nd), Eu, holmium (Ho), and Yb] and their characterization by mass, photoionization, and anion photoelectron spectroscopies.32,33 The formation mechanism and bonding nature of the organolanthanide complexes were further analyzed by density functional theory (DFT) calculations.34,35 In addition, the magnetic moments of [Lnn(COT)m] [Ln = Eu, terbium (Tb), Ho, and thulium (Tm)] were measured using Stern−Gerlach magnetic deflection experiments.36 In particular, our gas-phase studies revealed that COT complexes, together with divalent lanthanide elements such as Eu and Yb, are favorably generated by a sequentially mediated charge-transfer process, the so-called “harpoon mechanism”,

1. INTRODUCTION The magnetic and optical properties of organolanthanide complexes (based on 4f electrons) have been the focus of considerable interest. In particular, multinuclear complexes are expected to show novel properties originating from intermetallic communication,1−7 with applicability to molecular magnets,8−11 molecular catalysis,12,13 and luminescent devices.14−16 Furthermore, the stability of the half-filled f7 and completely filled f14 electronic configurations renders the 2+ oxidation state accessible to europium (Eu) and ytterbium (Yb), and divalent compounds of these elements have been extensively employed in organic synthesis.17,18 Since the 1970s, the 1,3,5,7-cyclooctatetraene (COT, C8H8) dianion and its derivatives have played an important role in organo−f-element chemistry.19 In particular, bulky silylsubstituted COT ligands, such as 1,4-bis(trimethylsilyl)cyclooctatetraenide, have demonstrated utility in the synthesis of novel sandwich complexes of f elements, yielding multidecker structures.20 COT molecules possess nonplanar tublike structures and are nonaromatic (i.e., they do not satisfy Hückel’s rule).21 However, because COT molecules can be converted into planar molecules by electron attachment22−27 or metal coordination,14,19,20,28−31 the planar dianion ligands of COT2− permit a new coordination environment that provides a © 2014 American Chemical Society

Received: November 2, 2013 Revised: January 10, 2014 Published: March 5, 2014 5896

dx.doi.org/10.1021/jp4108014 | J. Phys. Chem. C 2014, 118, 5896−5907

The Journal of Physical Chemistry C

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Scheme 1. Reaction Scheme and Structures of 1,4-COT″ and 1,6-COT″

Scheme 2. Reaction Scheme and Structures of Eu−COT and Eu−COT″

and can form one-dimensional multidecker sandwich structures EunCOTn+1 with lengths of up to 8 nm (18 layers).34 In the case of Eu2+, which has the 8S7/2 electronic structure, magnetic and optical properties based on 4f electrons are expected. For example, photoluminescence properties of Eu2+ compounds originating from 4f65d1 → 4f7 transitions are well-known.16 Moreover, the total magnetic moment of Eu n COT n+1 complexes linearly increases with the number of Eu atoms (the average magnetic moment per Eu atom is approximately 7 μB),36 and thus, their spintronic properties have also received considerable attention.37 In fact, the liquid-phase synthesis and isolation of the dinuclear triple-decker and trinuclear quadruple-decker sandwich complexes were successfully accomplished by Evans et al.38 and Edelmann et al.,39 respectively, and their physical properties have attracted considerable interest.20d In 1969, furthermore, a Eu2+ compound with COT ligands was successfully synthesized using liquid ammonia as a solvent by Hayes and Thomas.40 Neutral Eu and neutral COT were mixed with liquid ammonia, and upon solvent removal, they formed a divalent Eu compound with a chemical composition of Eu/ COT ≈ 1:1, although there was no statement regarding the lengths of the complexes. Because this reaction is based on charge transfer from a Eu atom to a COT ligand, from the viewpoint of the reaction mechanism, this reaction is similar to the gas-phase formation of EunCOTn+1 complexes. This similarity suggests that the Eu compound produced in liquid ammonia solvent has a multidecker sandwich structure. Because of poor solubility and high sensitivity to air, Eu compounds are difficult to handle; therefore, characterization of their structures and investigation of their physical properties have been limited to model compounds of minimum units of Eu−COT complexes terminated by potassium ions or a cyclopentadienyl (Cp) ligand.38,39

In this study, we synthesized Eu−COT complexes using the method developed by Hayes and Thomas40 and characterized their structures and magnetic and optical properties. In addition, because 1,4-(trimethylsilyl)cyclooctatetraene (COT″) is commonly used as a ligand of lanthanide complexes for easy handling,20,41 Eu complexes with COT″ ligands were also synthesized by the same reaction. The chemical and physical properties of Eu−COT and Eu−COT″ were compared.

2. EXPERIMENTAL SECTION All synthesis procedures were performed under argon using a Schlenk line or glovebox. COT was purchased from Aldrich and degassed using a freeze-and-pump method before use. [(DME)Li]2COT″ (DME = dimethoxyethane) was prepared as detailed in the literature.42 2.1. Preparation of COT″. As shown in Scheme 1, first, anhydrous cobalt(II) chloride (CoCl2; 1.33 g, 10.3 mmol) was added to a stirred toluene solution (90 mL) of [(DME)Li]2COT″ (4.5 g, 10.1 mmol) at room temperature. After 24 h, the solvent was removed by an evaporator, and hexane was added to the dark red slurry. After 48 h, the mixture was filtered to remove Co particles, and then the hexane solvent was removed using a rotary evaporator. The residue was distilled under a vacuum (about 10 Pa, 70 °C) to give a mixture of 1,4and 1,6-COT″ as a pale yellow liquid (1.38 g, 55%) (see the structures of 1,4- and 1,6-COT″ in Scheme 1). Anal. Calcd for C14H24Si2: C = 67.7, H = 9.74. Found: C = 67.2, H = 9.59. 1H NMR (CDCl3, 400 MHz): 1,4-COT″ δ 6.11 (2H, s, ring-CH), 5.93−5.86 (2H, m, ring-CH), 5.76−5.72 (2H, m, ring-CH), 0.07 [18H, s, Si(CH3)3]; 1,6-COT″ δ 5.97− 5.96 (2H, s, ring-CH), 5.90−5.88 (4H, m, ring-CH), 0.08 [18H, s, Si(CH3)3]. 5897



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2.2. Preparation of Eu−COT. Eu−COT was synthesized as described in the literature,40 as shown in Scheme 2. Briefly, Eu metal (1.3 g, 8.53 mmol) was dissolved in 40 mL of liquid ammonia. Cyclooctatetraene (1.0 mL, 8.89 mmol) was added to the deep blue solution at −34 °C, and the mixture was stirred for 3 h, yielding a light-green solution with a light-green precipitate. The remaining ammonia was removed at room temperature, and a light-green powder was obtained. Upon heating to 200 °C under 10−1 Pa, the color of the powder changed to orange. The obtained powder was washed with toluene and dried under a vacuum. 2.3. Preparation of Eu−COT″. Eu−COT″ was synthesized by the same procedure as used for the synthesis of Eu−COT, as shown in Scheme 2. Briefly, Eu metal (500 mg, 3.29 mmol) was dissolved in 40 mL of liquid ammonia. COT″ (955 mg, 3.85 mmol) was added to the deep blue solution at −34 °C, and the mixture was stirred for 3 h, yielding a yellow precipitate. The remaining ammonia was removed at room temperature, and a yellow powder was obtained. After being heated at 200 °C under 10−1 Pa for 1 h, the powder was washed with toluene to remove unreacted COT″, and an orange−yellow product was obtained. 2.4. Infrared and Raman Spectroscopic Measurements. For Eu−COT and Eu−COT″, IR absorption spectra were recorded by the KBr method using an FT/IR-460Plus spectrometer (JASCO). Sample pellets were sealed in a homemade stainless steel cell with two NaCl windows under an argon atmosphere. Raman spectra were recorded using an STR300-3L Raman spectrometer (Seki Technotron) with excitation of a He−Ne laser (632.8 nm), where the sample was sealed in a quartz capillary under an argon atmosphere. 2.5. Magnetic and Photoluminescence Measurements. The magnetic susceptibilities of Eu−COT and Eu− COT″ were determined by superconducting quantum interference device (SQUID) magnetometry (MPMS-XL, Quantum Design) at 2−300 K in an applied field of 500 Oe. Samples were sealed in NMR tubes to avoid exposure to air. Photoluminescence measurements were performed using a spectrofluorometer (Fluorolog3; FL3-21, Horiba Jobin Yvon). Powder samples were sandwiched between two glass slides and sealed using vacuum grease to avoid exposure to air. 2.6. Elemental Analysis for Eu, C, and H in Eu−COT and Eu−COT″. The Eu concentrations in Eu−COT and Eu− COT″ complexes were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES; Seiko Instruments, SPS1700HVR). Eu−COT and Eu−COT″ samples (15−17 mg) were weighed directly into volumetric flasks in the glovebox. After oxidation through slow exposure to air, the samples were dissolved completely in a mixed solution of sulfuric and nitric acids (4 and 6 mL, respectively). The dissolved samples were diluted to 100 mL with ultrapure water. The Eu concentrations were quantified by the external standard method at an emission wavelength of 393.048 nm. In combustion analysis for C and H, all elemental analyses were performed in the Microanalytical Laboratory of the Institute for Chemical Research, Kyoto University. 2.7. X-ray Diffraction Measurements. Powder X-ray diffraction patterns were recorded on an X-ray diffractometer (Bruker AXS, D8 Discover) operating at 50 kV and 100 mA with Cu Kα radiation. Samples were sealed in a Lindemann glass (lithium borate) capillary under argon atmosphere.

3. COMPUTATIONS Theoretical calculations were employed for vibrational spectroscopic analyses of COT, COT″, and their Eu complexes. Geometry optimizations and harmonic frequency analyses for COT, 1,4-COT″, and 1,6-COT″ were performed using density functional theory (DFT) with the Gaussian 03 quantum chemistry program.43 The B3LYP fuctional44 with the 6311G** basis set45 was used for the calculations, and a scaling factor of 0.96 was applied to the obtained harmonic vibrational frequencies. The calculations for Eu−COT and Eu−COT″ sandwiches terminated by Li, Li2Eu(COT)2, and Li2Eu(COT″)2 were performed with TURBOMOLE 6.2−6.446 at the B3LYP level44 using the def-SV(P) basis set47 along with the 28-electron relativistic effective core potential for Eu.48 Raman spectra were computed using the static polarizability gradient obtained with the EGRAD module49 implemented with the TURBOMOLE. Line spectra were convoluted by a Lorentzian function with a width of 4 cm−1, and a scaling factor of 0.97 was used for the latter caluclations. 4. RESULTS AND DISCUSSION 4.1. Synthesis of COT″. The synthesis of 1,4- and 1,6COT″ from [(THF)xLi]2COT″ (THF = tetrahydrofuran) was reported by Burton et al.,20b who used HgCl2 as an oxidant. Because HgCl2 is highly toxic, CoCl2 was used as the oxidant in this study, as reported by Edelmann et al.,20e who successfully produced triple-decker sandwich complexes Ln2(COT″)3 (Ln = Nd, Ho) from Li[Ln(COT″)2]. From 1H NMR analyses, it was found that the neutral COT″ species obtained by oxidation of [(DME)xLi]2COT″ using CoCl2 comprised an equilibrium isomeric mixture of 1,4-COT″ and 1,6-COT″ in a ratio of 2:3 at 298 K (see Supporting Information 1 and Figure S1). COT″ was further purified by filtration to remove Co compounds and then used as a reagent. Note that it is suggested that the Co compound ligated by COT″ would exist as an intermediate. During the reaction, the Li2COT″ toluene solution underwent a color change from transparent pale green to red, accompanied by the precipitation of a black Co nanoparticle powder. This could be caused by the removal of COT″ from cobalt compounds, followed by the aggregation of Co compounds. 4.2. Synthesis of Eu−COT and Eu−COT″. As described in the Experimental Section, Eu−COT and Eu−COT″ were synthesized by adding COT and COT″ to dissolved Eu in liquid ammonia and were obtained as orange and yellow powders, respectively, upon removal of the ammonia. Previously, our gas-phase study revealed that compounds composed of divalent lanthanide ions and COT2− ligands can efficiently form multidecker sandwich structures through a harpoon mechanism,35 and no length limit is expected. Thus, the composition of the multidecker sandwich Ln−COT complexes is expected to be Ln/COT = 1:1. In fact, Hayes and Thomas showed that the chemical composition of Yb− COT complex synthesized by the reaction between Yb and COT in liquid ammonia is Yb/COT = 1:1,40 in which Yb is characterized as divalent. Quantitative elemental analyses of Eu−COT and Eu−COT″ were conducted by ICP-AES for Eu and by combustion analysis for C and H. As reported in Table 1a, the concentrations of Eu, C, and H in Eu−COT were 64.2, 31.22, and 2.75 wt %, respectively, which differed from the estimated values of 59.3, 37.51, and 3.15 wt %, respectively, assuming that a Eu(COT) 5898



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Table 1. Elemental Analysis of Eu, C, and H in (a) Eu−COT and (b) Eu−COT″ and Calculation without Europium Oxide Contamination, (c) Contamination Ratio of EuO and Eu2O3 Estimated by XRD Measurements and M−H Plots, and the Recalculated Concentrations of Eu, C, and H in (d) Eu−COT and (e) Eu−COT″ Including EuO and Eu2O3 (a) Eu, C, and H in Eu−COT without Europium Oxide Estimation Eua found 64.2 calculated 59.3 Δ 4.9 (b) Eu, C, and H in Eu−COT″ without Eua found calculated Δ

Cb

Hb

31.22 2.75 37.51 3.15 −6.29 −0.40 Europium Oxide Estimation Cb

Hb

40.4 40.68 38.0 41.98 2.4 −1.30 (c) Europium Oxide Estimation Eu−COT or Eu−COT″ (wt %)

EuO (wt %)

5.99 6.04 −0.05 Eu2O3 (wt %)

Figure 1. Powder X-ray diffraction patterns of multiple-decker sandwich organoeuropium solids: (a) Eu−COT and (b) Eu−COT″ in the range of 3−80° at a scan speed of 1.2°/min. Simulated XRD patterns of EuO, Eu2O3, Eu, and Eu(OH)3, which are possible impurities, are also shown for identification of the impurities. In panels a and b, diffraction peaks of Eu and Eu(OH)3 were not observed, whereas those of EuO and Eu2O3 were observed, as indicated by the plus signs and triangles, respectively. The EuO/Eu2O3 concentration ratios in the products were roughly estimated to be (a) 1:2 and (b) 3:1 by comparing the maximum peak intensity divided by I/Icor. The remaining peaks in panels a and b are attributed to Eu(COT) and Eu(COT″), as indicated by solid circles and solid squares, respectively.

Eu−COT 85 5 10 Eu−COT″ 97.1 2.2 0.7 (d) Eu, C, and H in Eu−COT Including Europium Oxide Estimation Eua

Cb

Hb

found 64.2 31.22 2.75 calculated 63.6 31.88 2.67 Δ 0.6 −0.66 0.07 (e) Eu, C, and H in Eu−COT″ Including Europium Oxide Estimation found calculated Δ a

Eua

Cb

Hb

40.4 39.4 1.0

40.68 40.76 −0.08

5.99 5.87 0.12

observed at 30−35° only for Eu−COT. Although the symmetries of the crystals have not yet been determined, the results suggest that Eu−COT and Eu−COT″ have similar crystal structures. The most intense peak at 14.1° (for Eu− COT) was shifted to a lower angle of 7.6° in Eu−COT″ (the d values of the peaks were 6.33 and 11.5 Å for Eu−COT and Eu−COT″, respectively). Because the d values of the peaks were more than twice the Eu−C8H8(center) distance in [(THF)3K(COT)]2Eu (dEu−c‑C8H8 = 2.15 Å),38a these two peaks probably correspond to diffraction from intercomplex Eu atoms. Indeed, the shift of d values in Eu−COT″ (6.33 Å) from Eu−COT (11.5 Å) is twice that of the trimethylsilyl (TMS) groups (2.7 Å). In contrast, the peaks at 20−30° and 40−50° were not shifted between the two complexes. This suggests that these peaks presumably correspond to diffraction from intracomplex Eu atoms. Indeed, the d values of the peaks at 22° (4.0−4.1 Å) are close to twice the dEu−c‑C8H8 distance,38a which suggests that (1) the intracomplex Eu−Eu distances are almost the same in the two complexes and (2) the intercomplex distances become longer in Eu−COT″ than Eu−COT, as expected from the molecular structures. Figure 2 shows M−H plots for Eu−COT and Eu−COT″ at 50 K, both of which can be adequately fitted with a linear relationship at low and high magnetic fields. Because the magnetic response of ferromagnetic EuO (Tc < 70 K) is rapidly saturated at low magnetic field compared to that of paramagnetic compounds,50 the gradient of the curve at high magnetic field can be ascribed to the target complexes Eu− COT and Eu−COT″, whereas the first gradient can be ascribed mainly to the magnetic response of EuO. The saturated magnetization of EuO corresponds to the intercept of the lines

b

Determined by ICP-AES. Determined by combustion analysis.

compound, namely, [Eu(COT)]n, would be synthesized without any contamination. Similar discrepancies in the elemental analyses were found for Eu−COT″, as shown in Table 1b: The concentrations of Eu, C, and H in Eu−COT″ were 40.4, 40.68, and 5.99 wt %, respectively, which differed from the corresponding estimated values of 38.0, 41.98, and 6.04 wt %, assuming that a Eu(COT″) compound, namely, [Eu(COT″)]n, would be synthesized without any contamination. These discrepancies correspond to the concentration of europium oxides such as EuO and Eu2O3, which could be byproducts of the reaction with trace oxygen. Indeed, contamination by EuO and Eu2O3 can be identified in the powder XRD patterns of Eu−COT and Eu−COT″, as shown in Figure 1. In the XRD patterns of Eu−COT and Eu−COT″, the diffraction peaks of Eu and Eu(OH)3 could not be observed. Thus, the rest of the diffraction peaks not corresponding to EuO and Eu2O3 can be assigned to Eu− COT and Eu−COT″. The concentrations of EuO and Eu2O3 in Eu−COT and Eu−COT″ were quantified during the magnetic measurements: The amount of EuO was estimated from M−H plots at 50 K, and then the concentration ratio between EuO and Eu2O3 was determined using the XRD pattern. As shown in Figure 1, the peaks assignable to Eu−COT (solid circles) and Eu−COT″ (solid rectangles) show similar intensity profiles as follows: Pairs of intense peaks were observed at 10−20°, followed by weak multiple peaks at approximately 25° and 40°. Relatively intense peaks were 5899



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Figure 2. M−H plots at 50 K for (a) Eu−COT and (b) Eu−COT″. Curving of the plots is due to the EuO impurity, which behaves as a ferromagnetic compound below 70 K.48 To estimate the amounts of EuO impurity, linear fits of the plots from 10000 to 50000 Oe are indicated by the red lines. From the intercept of these approximate curves, the amounts of EuO in the samples were estimated.

fitting the M−H plots above 15000 Oe, which are shown by red lines in Figure 2. In general, saturated magnetization can be expressed as

Ms = NgmBJ

(1)

where N, g, mB, and J represent the number of magnetic ions, the Lande g factor, the Bohr magneton, and the total angular momentum, respectively. Using eq 1, the concentrations of EuO in Eu−COT and Eu−COT″ were calculated to be 5 and 2.2 wt %, respectively. Furthermore, from the XRD patterns shown previously, the concentration ratios between EuO and Eu2O3 were estimated as 1:2 for Eu−COT and 3:1 for Eu− COT″, and therefore, the concentrations of Eu2O3 in Eu−COT and Eu−COT″ were calculated as 10 and 0.7 wt %, respectively. The revised compositions of Eu−COT and Eu− COT″ including europium oxides are reported in Table 1d,e. As shown in Table 1c, the concentrations of Eu(COT) and Eu(COT″) complexes in Eu−COT and Eu−COT″ samples were evaluated to be 85 and 97.1 wt %, respectively, in which discrepancies of 0.6 and 1.0 wt % were within the experimental uncertainties. As reported previously, EuO was produced through the reaction of dissolved Eu with oxygen under liquid ammonia.51 4.3. Infrared Absorption Spectra of Eu−COT and Eu− COT″. It is well-known that, whereas neutral COT is an antiaromatic compound with a tub-shaped structure, the dianion, COT2−, is an aromatic compound with a planar structure. This is because the number of π electrons satisfies Hückel’s 4n + 2 rule for COT2− (10e) but not for COT (8e). The structural differences between COT and COT2− reflect a change in the vibrational IR and Raman spectra, and the charge states of COT and COT″ in Eu−COT and Eu−COT″ can be deduced from the IR spectra. Figure 3 shows the IR spectra of COT and Eu−COT together with calculated spectra for neutral COT and Li2Eu(COT)2. The observed vibrational frequencies of the IR spectra and their assignments are listed in Table 2, together with those calculated for Li2EuCOT2. Whereas the IR spectrum of COT exhibits more than 20 peaks in the range of 500−3250 cm−1 (Figure 3b), the IR spectrum of Eu−COT has only 7 peaks (Figure 3c). This difference implies that the higher symmetry from tub-shaped COT to planar-shaped COT2− would result in vibrational mode degeneracy of out-of-plane CH bending and in-plane CH bending observed at approximately 673 and 888 cm−1, respectively. The calculated IR spectrum for Li2EuCOT2 (Figure 3d), in which an Eu2+ ion is sandwiched between two planar COT2− ligands, reproduces the experimental curve well, showing that COT ligands exist as

Figure 3. Experimental IR spectra (solid lines) of (b) COT (purple) and (c) Eu−COT (blue) in the range of 500−3250 cm−1. Calculated spectra (dotted lines) of (a) COT and (d) Li2Eu(COT)2 were obtained for comparison at the B3LYP/6-311G** (scaling factor = 0.96) and B3LYP/def-SV(P) (scaling factor = 0.97) levels, respectively. The peaks of Eu−COT labeled by numbers are listed in Table 2 to show the vibrational assignments.

planar COT2− ions in Eu−COT. Because no IR peaks attributable to neutral COT were detected in the IR spectrum of Eu−COT, it is concluded that all of the COT ligands exist as dianions. Figure 4 shows the IR spectra of COT″ and Eu−COT″, together with those calculated for neutral 1,4−COT″/1,6− COT″ and Li2Eu(COT″)2. The observed vibrational frequencies of the IR spectra and their assignments are listed in Table 2, together with those calculated for Li2EuCOT″2. The IR spectrum in Figure 4c shows strong peaks at 841 and 1247 cm−1, which can be assigned to the CH bending and CH3 umbrella modes, respectively, of TMS groups. The difference between COT″ and Eu−COT″ can be clearly seen in the range of 950−1250 cm−1, corresponding to the vibrations derived from in-plane CH bending and TMS−COT stretching. Magnified spectra of COT″ and Eu−COT″ in this range are shown in Figure 5. As is apparent from a detailed comparison of the calculated and experimental spectra, the vibrational frequencies of CC stretching and Si−C−C stretching are shifted to higher and lower frequencies, respectively, with the complexing of Eu. The frequency shift shows that COT″ molecules exist as dianions in Eu−COT″. With the structural change from the tub-shaped neutral COT″ to the planar 5900



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Table 2. Experimental and Calculated IR Absorption Frequencies (cm−1) and Vibrational Assignments for (a) Eu−COT and (b) Eu−COT″, Together with Those Calculated for Li2EuCOT2 and Li2EuCOT″2 (a) Eu−COT Eu−COT (exp)

Li2EuCOT2 (calca)

673

688

777

747

888 1448 2852 2929 3007

884 1408

3044

Eu−COT″ (exp)

Li2EuCOT″2 (calca)

635 682 730

607 656 728

749

746

779

778

833 929 974 1049 1201 1246 2893 2951 3034

841 922 963 1037 1197 1247 2907 2990 3071

a

vibrational assignment

number in Figure 1c

out-of-plane CH bend out-of-plane CH bend in-plane CH bend in-plane CH bend CH stretch CH stretch CH stretch (b) Eu−COT″

1 2 3 4 5 6 7

vibrational assignmentb TMS Si−C stretch TMS Si−C stretch out-of-plane C−H bend +C−H bend (TMS) out-of-plane C−H bend +C−H bend (TMS) out-of-plane C−H bend +C−H bend (TMS) C−H bend (TMS) CCH bend (COT) Si−C−C stretch asym Si−C stretch C−C stretch CH3 umbrella C−H stretch (TMS) C−H stretch (TMS) C−H stretch (COT)

Calculated at the B3LYP/def-SV(P) level. trimethylsilyl.

b

number in Figure 2d

Figure 4. Experimental IR spectra (solid lines) of (c) COT″ (green) and (d) Eu−COT″ (red) in the range of 500−3250 cm−1. Calculated spectra (dotted lines) of (a) 1,4−COT″, (b) 1,6−COT″, and (e) Li2Eu(COT″)2 were obtained for comparison at the B3LYP/6311G** (scaling factor = 0.96) and B3LYP/def-SV(P) (scaling factor = 0.97) levels, respectively. Peaks marked by * and ** were derived from the absorbances of CO2 and H2O contaminants, respectively. The peaks of Eu−COT″ labeled by numbers are listed in Table 2 to show the vibrational assignments.

1 2 3 3 3 4 5 6 7 8 9 10 11 12

TMS represents

structure of COT″2−, the bonding order of CC stretching is strengthened from 1 to 1.5 corresponding to a stiffer carbon ring, resulting in the blue shift of the vibrational frequency of the CC stretching. On the other hand, the bonding order of CC in Si−C−C stretching is weakened from 2 to 1.5, resulting in a red shift of the vibrational frequency. 4.4. Raman Spectra of Eu−COT and Eu−COT″. Figure 6 shows Raman spectra of Eu−COT and Eu−COT″ in the range of 120−1800 cm−1 with an excitation wavelength of 632.8 nm; the calculated Raman spectra for Li2Eu(COT)2 and Li2Eu(COT″)2 are also shown. The observed vibrational frequencies of the Raman spectra and their assignments are reported in Table 3, together with those calculated for Li2EuCOT2 and Li2EuCOT″2. In the Raman experiments, an excitation wavelength of 785 nm gave Raman spectra similar to those obtained at 633 nm, but excitation at 532 nm was rarely used owing to fluorescence disturbances. Consistent with the IR spectra, the Raman spectra reveal that the oxidation states of COT and COT″ in the complexes were 2−. Three representative peaks attributable to COT2− and COT″2− were observed in the range of 350−1600 cm−1: (1) CCC in-plane bending, (2) ring breathing, and (3) CC stretching. These three correspond to peaks 3−5 for Eu−COT and to peaks 7, 11, and 12 for Eu−COT″ in Figure 6. Note

Figure 5. Magnified spectra of Figure 4 in the range of 950−1300 cm−1 to show the shift of the corresponding vibrational modes for (c) COT″ and (d) Eu−COT″, together with calculated spectra for (a) 1,4-COT″, (b) 1,6-COT″, and (e) Li2Eu(COT″)2.

that, comparing COT2− and COT″2−, the CCC in-plane bending mode exhibits a blue shift from 345 to 449 cm−1 with the introduction of TMS groups, which is attributed to coupling with the COT−TMS stretching mode. Moreover, additional peaks assignable to the Si−CH3 stretching mode, ν(Si−C), at 630 and 685 cm−1; TMS torsion; and Si−CH3 bending below 400 cm−1 were observed for the Eu−COT″ complex. All of these additional peaks were also well reproduced in the calculated spectrum. 5901



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the peaks at 191 and 211 cm−1 were assignable to the metal− ring tilting mode based on the theoretical calculations. For the smallest Li2Eu(COT)2 complex, the tilting and stretching mode is quasidegenerate in frequency, as shown in Table 3. As the number of layers increased, the ring−M−ring stretching mode was red-shifted, whereas the corresponding tilting mode remained the same or slightly blue-shifted, as shown in Figure S2 (see Supporting Information 2), in which Ba2+ was used for simplicity instead of Eu2+. The results indicate that the observed peaks 1 and 2 in Eu−COT are assignable to the tilting modes, although the corresponding stretching modes could not be observed, as they were below the detection limit. Although the frequency of the tilting mode in Ba3(COT)4 is slightly lower than the corresponding peaks (1 and 2) in Eu−COT, this difference probably originates from the existence of different numbers of layers. In addition to the theoretical results, the Eu−COT−Eu stretching mode was also estimated to be approximately 100 cm−1 based on the peak observed in K2(COT)54 at 169 cm−1 after correcting the mass of the metal atoms to Eu. This also suggests a larger red shift of the stretching mode in Eu−COT. Discrepancies between our assignment of peaks in U(COT)2, Th(COT)2, and K[La(COT)2] and those previously reported probably originate from differences in the numbers of layers and charge states (4+ for U and Th and 3+ for La), both of which cause red shifts of ring−M−ring stretching. For Eu−COT″, however, ring−M−ring tilting and stretching were observed at the same frequency of 132 cm−1, which is similar to the value for the calculated spectrum of the smallest compound, Li2Eu(COT)2. Because Eu−COT″ was confirmed to have multiple sandwich structures based on elemental analysis and powder XRD, the observed quasidegeneracy of tilting and stretching is due to (1) the length of Eu−COT″ (i.e., the number of sandwich layers might be less than the length of Eu−COT) and (2) bulky TMS groups, which might prevent the ring−Eu−ring stretching mode of Eu−COT″ from being lowered because of intercomplex steric interference in the layered structures. Regarding the latter factor, indeed, it has

Figure 6. Experimental Raman spectra (solid lines) of (b) Eu−COT (blue) and (c) Eu−COT″ (red) in the range of 120−1800 cm−1 with excitation at 632.8 nm. Backgrounds derived from fluorescence were eliminated from these spectra. Calculated spectra (dotted lines) of (a) Li2Eu(COT)2 (blue) and (d) Li2Eu(COT″)2 (red) were obtained for comparison at the B3LYP/def-SV(P) (scaling factor = 0.97) level. The peaks of Eu−COT and Eu−COT″ labeled by numbers are listed in Table 3 to show the vibrational assignments.

In addition to the aforementioned peaks, intermolecular Eu− COT−Eu (or Eu−COT″−Eu) vibrational modes were also observed below 500 cm−1; metal−ring tilting modes were observed at 191 and 211 cm−1 for Eu−COT, but they were quasidegenerate with the ring−metal−ring stretching mode in Eu−COT″ observed at 132 cm−1 (Table 3). For other metal− COT sandwich complexes, ring−M−ring stretching and tilting modes were observed at 212 and 236 cm−1, respectively, for U(COT)2;52 222 and 242 cm−1, respectively, for Th(COT)2;52 and 202 and 216 cm−1, respectively, for K[La(COT)2].53 Although the frequency of the observed doublet peaks in Eu− COT was similar to those of other metal−COT complexes,52,53

Table 3. Observed Vibrational Frequencies (cm−1) of Raman Spectra for Eu−COT and Eu−COT″ and Their Assignments, Together with Those Calculated for Li2EuCOT2 and Li2EuCOT″2, as Well as Data Reported for Related Sandwich Compounds Eu−COT (exp)

1 2

a

191 211

Li2Eu(COT)2 (calca)

Eu−COT″ (exp)

Li2Eu(COT″)2 (calca)

K2(COT)b Th(COT)2c U(COT)2c K[La(COT)2]d

130

1

132

109

169

222

212

202

133

1

132

169

242

236

216

2 3

181 208

109 (127) 142 166 184

4 5 6 7 8 9 10 11 12

241 299 377 449 511 630 685 749 1474

203 228 300 386 450 508 616 677 766 1519

3

353

345

4 5

740 1486

760 1524

344

364

379

370

736 1488

750 1498

752 1450

750 −

modee ring−metal−ring stretch metal−ring tilt TMS torsion TMS torsion out-of-plane CCC bend Si−CH3 bend Si−CH3 bend in-plane CCC bend Li−COT stretch in-plane CCC bend in-plane CCC bend Si−C stretch Si−C stretch ring breath CC stretch

Calculated at the B3LYP/def-SV(P) level. bReference 54. cReference 52. dReference 53. eTMS represents trimethylsilyl. 5902



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4.6. Photoluminescence Properties of Eu−COT and Eu−COT″. The photoluminescence properties of the synthesized Eu−COT and Eu−COT″ materials were measured because one attractive application of organolanthanide complexes is luminescent devices.15,16,57 Figure 8 shows the

been reported for molecular wires of Rh(CO)2Cl(amine) that the van der Waals interaction between the alkyl amine groups induces the elongation of the metal−metal bonds.55 It is worth noting that the Raman spectra of Eu−COT and Eu−COT″ exhibit peaks assignable to metal−ring vibrations, as well as modes of COT2− and COT″2− ligands. The latter is consistent with the spectral features in their IR spectra, as discussed in section 4.3. These spectroscopic results reasonably show that Eu−COT and Eu−COT″ are ionic bonding complexes in which there is a sandwich structure between Eu2+ and COT2− and COT″2−. 4.5. Magnetic Properties of Eu−COT and Eu−COT″. Eu ions in an organometallic complex can exist in either divalent or trivalent states. It is reasonable to expect that Eu ions in Eu− COT and Eu−COT″ exist in divalent states because Eu ions function as counterions to COT and COT″ dianions. The divalent state of Eu2+ can be experimentally examined through magnetic properties. The effective magnetic moment (μeff) of Eu ions differs significantly between Eu2+ and Eu3+ in complexes. Because the electronic ground state of Eu2+ is 8 S7/2, μeff of Eu2+ is 7.94 μB.56 On the other hand, in the case of Eu3+, the electronic excited states of 7Fn (n = 1−6) are located slightly above the electronic ground state of 7F0, and μeff of Eu3+ is approximately 3.5 μB at room temperature.56 Therefore, the oxidation states of Eu ions were determined from the effective magnetic moments estimated from the temperature dependence of the magnetic susceptibilities of Eu−COT and Eu− COT″. Figure 7 shows the molar susceptibilities of Eu ions in Eu− COT and Eu−COT″ in the range of 2−300 K. As mentioned

Figure 8. Normalized photoluminescence excitation spectra (solid lines) and emission spectra (dotted lines) for solid samples of Eu− COT (blue) and Eu−COT″ (red). Emission spectra were recorded at 485-nm excitation, whereas excitation spectra of Eu−COT and Eu− COT″ were obtained with 620- and 610-nm emission, respectively.

emission and excitation spectra of Eu−COT, and the emission spectrum excited at 485 nm shows a broad band located at λmax = 623 nm. This broad band can be assigned to the 4f65d1 → 4f7 transition of Eu2+ sandwiched between two COT ligands, because the Eu ions in the Eu−COT complex take divalent states as discussed above. Furthermore, as shown in Figure 8, the emission spectrum of Eu−COT″ exhibits a similar band located at λmax = 608 nm. Comparing the emission wavelengths of Eu−COT and Eu− COT″, the emission bands are blue-shifted by the introduction of TMS groups with COT ligands. Although the TMS group has both geometric and electronic effects, the latter seem to be dominant because the emission spectra are shifted under almost the same excitation spectra for Eu−COT and Eu−COT″. If geometrical changes were involved in the optical responses, both the emission and excitation spectra would be shifted similarly because of geometrical changes in the ground and excited states, which cause a shift in the opposite direction between the excitation and emission spectra. As for the electronic factor, the electron-donation character of TMS groups in COT″, rather than the orbital energy, seems to be the major reason for the blue shift of the emission bands assignable to 5d → 4f. When the energy of the π orbital increases, the ligand π ring becomes closer to the empty 5d orbitals of Eu2+, leading to stronger interactions between them. As shown in Supporting Information 3 and Figure S3, however, our DFT calculations predict that the π orbital states for COT″2− are deeper than those for COT2−, suggesting that the energy difference between the 4f and 5d orbitals in Eu−COT″ would be smaller than that in Eu−COT. This result shows that the blue shift of the emission bands cannot be explained by the orbital energy difference in the π orbital states. Instead, a strong ionic interaction between Eu2+ and the ligand molecules might cause a blue shift of the emission bands. In fact, TMS groups exhibit an electron-donation characteristic. Because more negatively charged ligands of COT″2− can strongly interact

Figure 7. Inverse susceptibility plots of χm−1 at 500 Oe against T from 2 to 300 K: (a) Eu−COT and (b) Eu−COT″. The effective magnetic moments of Eu−COT and Eu−COT″ were evaluated as 8.86 and 7.43 μB, respectively, from the gradient of a linear fit for the data at 150(100)−300 K.

above, the samples of Eu−COT and Eu−COT″ contain EuO, which shows ferromagnetic behavior below approximately 70 K.50 Indeed, the plots of magnetic susceptibility deviate from linearity, especially at temperatures below 100 K. This deviation is more significant for Eu−COT than for Eu−COT″ because the EuO concentration in Eu−COT is more than that in Eu− COT″. From the linear fitting above 150 and 100 K of the plots for Eu−COT and Eu−COT″, respectively, the effective magnetic moments of the Eu ions were determined to be 8.86 μB and 7.43 μB, respectively. Although oxide contamination introduced experimental uncertainties, it is concluded that Eu ions in Eu−COT and Eu−COT″ exist as divalent states. It is noted that magnetic exchange interaction between Eu ions could not be observed for both of the complexes due to the contamination of ferromagnetic EuO. 5903



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with Eu2+, the ionic interaction should result in a larger energy gap between the 4f and 5d orbitals in Eu−COT″ compared to Eu−COT. Interestingly, it has been revealed that COT ligands bearing a TMS group can prominently enhance the photoluminescence intensity of organoeuropium complexes. Figure 9a shows the

Table 4. Luminescence Properties of Organoeuropium Sandwich Complexes with and without Ethylenediamine (EDA)a Eu−COT Eu−COT″ Eu−COT + EDA Eu−COT″ + EDA

maximum of emission (nm)

relative intensity

623 608 560 549

1 11 55 38

a

Emission spectra without and with EDA recorded at 485- and 350nm excitation, respectively.

exhibited green photoluminescence upon 350-nm excitation, as shown in the photographs in Figure 9. Note that the emission enhancement of Eu−COT with ethylenediamine is dramatic compared with that of Eu−COT″. As shown in Figure 9a,b, the emission intensity of Eu−COT and Eu−COT″ became stronger in contact with ethylenediamine, with the enhancement rates of Eu−COT and Eu−COT″ with ethylenediamine approximately 55 and 3 times larger, respectively. The different color changes depending on the type of amine compound can be attributed to the difference in steric hindrance in the coordination of amine compounds to Eu ions. Because triethylamine and propylamine are geometrically larger compounds than ethylendiamine and ammonia, it seems that only smaller amine compounds can coordinate to Eu2+ ions in Eu−COT and Eu−COT″. In fact, ethylenediamine and ammonia effectively enhance the emission, irrespective of their smaller basicity for coordination compared to triethylamine and propylamine. For Eu−COT, in particular, amine coordinations result in not only 5d orbital tuning but also symmetry lowering and elongation of the interatomic distance between Eu species, and then the enhancement might become larger than that for Eu−COT″. Interestingly, the emissions of Eu−COT and Eu−COT″ become closer in intensity when ethylenediamine vapor comes into contact with Eu−COT, as shown in Figure 9b. The introduction of bulky TMS groups into COT lowers the symmetry of the compounds, and the symmetry lowering by COT″ is one of the reasons that the emission intensity is enhanced by more than a factor of 10. On the other hand, the intensity difference can be overcome by the coordination of amine compounds to Eu−COT, which also lowers the symmetry. Therefore, the similar emission intensities of Eu− COT and Eu−COT″ in contact with ethylenediamine show that the low emission intensity in Eu−COT without amine compounds is attributed to symmetry restrictions for electronic transitions between 4f and 5d orbitals. Finally, it is noted that the solubility effects when TMS groups are introduced into COT were not prominent in the Eu−COT complexes. Eu−COT is insoluble in hydrocarbon and ether solvents and is soluble in dimethylformamide (DMF), resulting in an orange solution. Eu−COT shows no solubility in hydrocarbon and ether solvents, such as THF, and no solubility improvements were observed for Eu−COT″. Although Eu−COT and Eu−COT″ seemed to dissolve in DMF, the solvent could not be removed without changes to the optical properties. Indeed, once they were dissolved in DMF, neither sample exhibited photoluminescence, even after the removal of DMF in a vacuum. This is probably due to strong chemical bonds between Eu and DMF or conversion to reaction products.

Figure 9. (Left) Photoluminescence emission spectra of Eu−COT (blue) and Eu−COT″ (red): (a) without EDA and (b) with EDA. Spectra of Eu−COT and Eu−COT″ magnified by a factor of 5 are also shown as dotted lines in panel a. Emission spectra of Eu−COT and Eu−COT″ without EDA were recorded at 485-nm excitation, whereas those of Eu−COT and Eu−COT″ with EDA were recorded at 350nm excitation. (Right) Photographs of Eu−COT and Eu−COT″ (top) without EDA and (bottom) with EDA under 366-nm excitation.

emission spectra of Eu−COT and Eu−COT″ excited at 485 nm. The integrated intensity of the emission bands for Eu− COT″ is approximately 11 times higher than that for Eu−COT. This is probably because (1) symmetry lowering from the TMS group could contribute to a higher electronic transition probability and (2) a reduction of nonradiative quenching could result from an elongated intercluster distance in Eu− COT″. Under lower symmetries, restrictions for electronic transitions are reduced between 4f and 5d, such as 4fσ/5dσ. For the latter, energy transfer from the photoluminescent excited state to the nonradiative state in neighboring sandwich complexes is possibly diminished with the longer distance between them, although similar energy transfer is also possible inside the complexes. On the basis of the XRD results, it is suggested that Eu−COT″ has a relatively longer distance between the sandwich complexes. Similar enhancement of the photoluminescence was observed for the molecular coordinations to Eu−COT and Eu−COT″, as reported in Table 4. Figure 9b shows the emission spectra of Eu−COT and Eu−COT″ in contact with ethylenediamine vapor recorded under 350-nm excitation. Peak maxima of the emission bands of Eu−COT and Eu−COT″ shifted to λmax = 560 and 549 nm, respectively, although no solubility of Eu−COT and Eu−COT″ was observed in ethylenediamine. No color changes were observed for Eu− COT and Eu−COT″ in contact with triethylamine and propylamine (instead of ethylenediamine), as was similarly observed for Yb(COT) complexes.58 In contact with ethylenediamine, solid Eu−COT and Eu−COT″ turned yellow and 5904



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(6) Parker, D. Excitement in f Block: Structure, Dynamics and Function of Nine-Coordinate Chiral Lanthanide Complexes in Aqueous Media. Chem. Soc. Rev. 2004, 33, 156−165. (7) Edelmann, F. T. Lanthanides and Actinides: Annual Survey of Their Organometallic Chemistry Covering the Years 2003 and 2004. Coord. Chem. Rev. 2006, 250, 2511−2564. (8) Ishikawa, N.; Sugita, M.; Ishikawa, T.; Koshihara, S.; Kaiz, Y. Lanthanide Double-Decker Complexes Functioning as Magnets at the Single-Molecular Level. J. Am. Chem. Soc. 2003, 125, 8694−8695. (9) Le Roy, J. J.; Jeletic, M.; Gorelsky, S. I.; Korobkov, I.; Ungur, L.; Chibotaru, L. F.; Murugesu, M. An Organometallic Building Block Approach to Produce a Multidecker 4f Single-Molecule Magnet. J. Am. Chem. Soc. 2013, 135, 3502−3510. (10) Magnani, N.; Apostolidis, C.; Morgenstern, A.; Colineau, E.; Griveau, J. C.; Bolvin, H.; Walter, O.; Caciuffo, R. Magnetic Memory Effect in a Transuranic Mononuclear Complex. Angew. Chem., Int. Ed. 2011, 50, 1696−1698. (11) Jiang, S.-D.; Wang, B.-W.; Sun, H.-L.; Wang, Z.-M.; Gao, S. An Organometallic Single-Ion Magnet. J. Am. Chem. Soc. 2011, 133, 4730−4733. (12) Molander, G. A.; Romero, J. A. C. Lanthanocene Catalysts in Selective Organic Synthesis. Chem. Rev. 2002, 102, 2161−2185. (13) Kobayashi, S.; Sugiura, M.; Kitagawa, H.; Lam, W. W. L. RareEarth Metal Triflates in Organic Synthesis. Chem. Rev. 2002, 102, 2227−2302. (14) Avdeef, A.; Raymond, K. N.; Hodgson, K. O.; Zalkin, A. Two Isostructural Actinide π Complexes: The Crystal and Molecular Structure of Bis(cyclooctatetraenyl)uranium(IV), U(C8H8)2, and Bis(cyc1ooctatetraenyl)thorium(IV), Th(C8H8)2. Inorg. Chem. 1972, 11, 1083−1088. (15) Nonat, A. M.; Quinn, S. J.; Gunnlaugsson, T. Mixed f-d Coordination Complexes as Dual Visible- and Near-Infrared-Emitting Probes for Targeting DNA. Inorg. Chem. 2009, 48, 4646−4648. (16) Marks, S.; Heck, J. G.; Habicht, M. H.; Oña-Burgos, P.; Feldmann, C.; Roesky, P. W. [Ln(BH4)2(THF)2] (Ln = Eu, Yb)A Highly Luminescent Material. Synthesis, Properties, Reactivity, and NMR Studies. J. Am. Chem. Soc. 2012, 134, 16983−16986. (17) Kagan, H. B. Divalent Samarium Compounds: Perspectives for Organic Chemistry. New J. Chem. 1990, 14, 453−460. (18) Evans, W. J. The Organometallic Chemistry of the Lanthanide Elements in Low Oxidation States. Polyhedron 1987, 6, 803−835. (19) (a) Edelmann, F. T.; Freckmann, D. M. M.; Schumann, H. Synthesis and Structural Chemistry of Non-Cyclopentadienyl Organolanthanide Complexes. Chem. Rev. 2002, 102, 1851−1896. (b) Edelmann, F. T. Multiple-Decker Sandwich Complexes of f-Elements. New J. Chem. 2011, 35, 517. (20) (a) Burton, N. C.; Cloke, F. G. N.; Hitchcock, P. B.; de Lemos, H. C.; Sameh, A. A. Scandium, Yttrium, Uranium, and Thorium Derivatives of the 1,4-Bis(trimethylsilyl)cyclo-octatetraene Dianion; the X-ray Crystal Structure of [Sc2(η-C8H6{1,4-(SiMe3)2})2(μ-Cl)2(μthf)] (thf = tetrahydrofuran). J. Chem. Soc., Chem. Commun. 1989, 1462−1464. (b) Burton, N. C.; Cloke, F. G. N.; Joseph, S. C. P.; Karamallakis, H.; Sameh, A. A. Trimethylsilyl Derivatives of Cyclooctatetraene. J. Organomet. Chem. 1993, 462, 39−43. (c) Lorenz, V.; Edelmann, A.; Blaurock, S.; Freise, F.; Edelmann, F. T. A Unique Organolanthanide Cluster Containing Bulky Cyclooctatetraenyl Ligands. Organometallics 2007, 26, 4708−4710. (d) Summerscales, O. T.; Jones, S. C.; Cloke, F. G. N.; Hitchcock, P. B. Anti-Bimetallic Complexes of Divalent Lanthanides with Silylated Pentalene and Cyclooctatetraenyl Bridging Ligands as Molecular Models for Lanthanide-Based Polymers. Organometallics 2009, 28, 5896−5908. (e) Lorenz, V.; Blaurock, S.; Hrib, C. G.; Edelmann, F. T. The First Linear, Homoleptic Triple-Decker Sandwich Complex of an fElement: A Molecular Model for Organolanthanide Nanowires. Organometallics 2010, 29, 4787−4789. (21) Fray, G. I.; Saxton, R. G. The Chemistry of Cyclooctatetraene and Its Derivatives; Cambridge University Press: New York, 1978.

5. CONCLUSIONS We synthesized europium cyclooctatetraene complex Eu−COT and its TMS devrivatives Eu−COT″ using liquid ammonia as a solvent. Characterizations by ICP-AES, XRD, combustion analysis, and magnetic measurements showed that Eu−COT and Eu−COT″ mainly consist of Eu(COT) or Eu(COT″) with some impurites of EuO and Eu2O3. IR absorption spectra and inverse magnetic susceptibility plots as a function of temperature showed that Eu(COT) and Eu(COT″) have Eu2+− COT2− and Eu2+−COT″2− charge distributions. The chemical composition of the complex (Eu/COT = Eu/COT″ = 1:1) and the observation of the ring−metal−ring vibrational mode in Raman spectra clearly indicate that Eu(COT) and Eu(COT″) form multidecker sandwich structures. In the case of Eu(COT), in particular, vibrational coupling was observed. Eu−COT and Eu−COT″ exhibited red and orange−red emissions under UV irradiation, and the emission intensity of Eu−COT″ was approximately 11 times higher than that of Eu−COT, which clearly shows that the introdution of fuctional groups onto COT rings induced the change in photoluminescence properties. In addition, a blue shift in the emission wavelength and enhancement of the emission intensity of Eu−COT and Eu− COT″ were also observed when the complexes were in contact with ethylenediamine. The photoluminescence poperties of Eu complexes ligated by cyclooctatetaene molecules can easily be changed by their chemical environment.

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ASSOCIATED CONTENT

* Supporting Information S

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +81-45-566-1697. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Professor F. Kakiuchi and Dr. T. Kochi (Keio University) for valuable discussion on the synthetic procedures, to Professor K. Iwata and Ms. Y. Nojima (Gakushuin University) for measurements of Raman spectra, and to Dr. Y. Mizuhata and Ms. T. Hirano (Kyoto University) for combustion analysis. This work was partially supported by MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2009−2013.

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REFERENCES

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