Article pubs.acs.org/crystal
First Eu(II)/Ln(III) Mixed Complex with High Oxidative Stability Yongqin Wei, Gaoji Wang, and Kechen Wu* State Key Laboratory of structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fujian 350002, China S Supporting Information *
ABSTRACT: Due to the instability of divalent europium ions, the heterometallic Eu(II)/Ln(III) complex has not yet been reported. By utilizing coordination chemistry principles, a macrocyclic ligand, N,N′,N″,N‴-tetra(2-hydroxy-3-methoxy-5-methylbenzyl)-1,4,7,10-tetraazacyclododecae (H4L), has been rationally designed to encapsulate Eu2+ and to enable direct formation of the first mixed Eu(II)/Ln(III) complexes, namely, EuII2LnIII4(OH)4(NIC)4L2 (Ln = Sm, Eu, Tb; HNIC = nicotinic acid). Two divalent europium ions are trapped within the macrocyclic cavities of designed ligands L and are further isolated from the environment by outside phenyl rings and the tetrahedral 4Ln(III) cluster, resulting in the enhanced stability of Eu2+. Cyclic voltammetry experiments showed that the oxidation potential of Eu2+ in the heterovalent 2Eu(II)/4Ln(III) cluster is larger than that for the ferrocene/ferrocenium redox couple, which has never been reported previously for Eu2+-containing complexes. Further development of Eu(II) complexes has been limited because Eu2+ could be easily oxidized to Eu3+. The dramatic oxidative stability of as-synthesized complexes not only verifies the synthetic feasibility but also highlights the prospective applications of mixed Eu(II)/Ln(III) coordination complexes.
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the extreme tendency of Eu2+ to be oxidized into Eu3+ severely hinders the use of Eu(II) complexes.14,15 Although, in some instances, Eu2+ has been stabilized by organic ligands such as crown ethers and cryptands,16−20 these Eu(II) complexes in solution were often oxidized when exposed to air. Thus, our major challenge is to design an appropriate organic ligand for stabilizing Eu2+ and further linking it with Ln3+. Compared with trivalent lanthanides, Eu2+ is considered to be a large, soft, electron-rich ion. In principle, divalent europium and trivalent lanthanide ions could be incorporated into a single coordination complex by utilizing coordination chemistry principles such as hard−soft and acid−base theory. We hypothesized that the ideal ligand for linking Eu(II)/ Ln(III) centers should satisfy the following requirements: (1) soft or moderately hard donor sites coordinating preferentially to Eu2+; (2) adequate coordinated atoms to encapsulate Eu2+ in order to protect the ion from interacting with the outside environment, thereby stabilizing Eu2+ and preventing its oxidation; and (3) additional hard donors to connect to Ln3+. Macrocyclic complexes often exhibit enhanced kinetic and thermodynamic stabilities as well as unusual redox properties including stabilization of less common oxidation states.21−25 On the basis of the above considerations, we designed a tetrasubstituted cyclen derivative ligand, N,N′,N″,N‴-tetra(2hydroxy-3-methoxy-5-methylbenzyl)-1,4,7,10-tetraazacyclodo-
INTRODUCTION In coordination complexes, lanthanide elements exist primarily as trivalent ions in both liquid solutions and solid state. Because of their reducing properties and sensitivity to air and moisture, divalent lanthanide complexes are generally handled in a strictly anaerobic and anhydrous atmosphere.1−8 Due to the instability of divalent lanthanide ions, there still have been no reports of any heterometallic lanthanide coordination complexes including both divalent and trivalent species and thus no detailed reports on their structural properties. In contrast, studies of well-known mixed d/f complexes involving transition metal ions and trivalent lanthanide ions have seen tremendous progress due to special interactions between the different metal centers and their fascinating effects on magnetic, optical, or catalytic properties.9−11 Thus, incorporation of mixed lanthanide ions into a single complex by using coordination chemistry principles remains a significant challenge in inorganic chemistry. In this article, we describe efforts to obtain the first mixed Eu(II)/Ln(III) coordination complex using primitive divalent europium and trivalent lanthanide salts. The main reason for choosing Eu2+ as a trial ion is that this ion is the most accessible of the divalent lanthanides (the standard reduction potential for trivalent europium to its divalent species is −0.35 V).12 Due to the higher oxidation potential of Eu2+ compared to that of other divalent lanthanide ions and its special magnetic, optical, and catalytic properties, a large number of Eu2+-containing complexes have been reported over the past few decades, paving the way to these complexes becoming commonplace in coordination chemistry.13 However, © XXXX American Chemical Society
Received: June 10, 2015 Revised: September 5, 2015
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DOI: 10.1021/acs.cgd.5b00804 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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that it is feasible to design mixed Eu(II)/Ln(III) complexes in a rational design process.
decae (H4L, Scheme 1). The four phenyl rings were introduced to decrease the electron-donating ability of the adjacent
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Scheme 1. Cyclen Derivative Ligand H4L with Four Pendant Arms
EXPERIMENTAL SECTION
General. All chemicals were obtained from commercial sources without further purification. Elemental analyses for C, H, N, and O were carried out using a Vario MICRO CHNOS elemental analyzer. Inductively coupled plasma spectroscopy (ICP) was performed using an Ultima2 inductively coupled plasma OES spectrometer. Sm/Eu and Tb/Eu ratios were determined by ICP analysis, and ICP samples were prepared by digesting the dry samples into concentrated HCl (30%). X-band electron paramagnetic resonance (EPR) spectra were recorded using a Bruker-BioSpin E500 spectrometer. Synthesis of N,N′,N″,N‴-Tetra(2-hydroxy-3-methoxy-5methylbenzyl)-1,4,7,10-tetraazacyclododecae (H4L). Paraformaldehyde (2.40 g, 80 mmol) was added to a solution of cyclen (1.72 g, 10 mmol) in methanol (100 mL). After refluxing for 2 h, 2methoxy-4-methyl-phenol (8.28 g, 60 mmol) was added, and reflux was continued for 24 h. Water (300 mL) was then added to the solution, resulting in a white precipitate that was separated by filtration and washed with water. The pure product was obtained by recrystallizing the precipitate in acetone (4.72 g, yield: 61%, based on cyclen). Anal. Calcd for C44H60N4O8 (FW 772.97): C, 68.37; H, 7.82; N, 7.25; O, 16.56. Found: C, 68.32; H, 7.87; N, 7.19; O, 16.62. 1 H NMR (CD3OD; Figure S1, Supporting Information): 2.23 (m, 3H), 2.81 (s, 2H), 3.33 (dt, 2H), 3.56 (s, 2H), 3.81 (t, 3H), 4.94 (d, 1H), 6.56 (m, 1H), 6.69 (dd, 1H). Syntheses of EuII2LnIII4(OH)4(NIC)4L2 (M-EuIILnIII-1, Ln = Sm, Eu, Tb). The title complexes were synthesized by reaction of EuBr2 (0.2 mmol), Ln(NO3)3·6H2O (0.4 mmol), HNIC (0.4 mmol), and H4L (0.2 mmol) in a sealed methanol solution (6 mL) at 70 °C for 12 h. Crystals were obtained by natural volatilization of the clear solutions in air at room temperature. X-ray Crystallography. Suitable single crystals of M-EuIILnIII-1 (Ln = Sm, Eu, Tb) were carefully selected and glued to thin glass fibers with epoxy resin. Because of the ease with which single crystals collapse due to the loss of solvent molecules, intensity data were collected at low temperature (173 K) using a Rigaku Mercury CCD diffractometer with graphite-monochromatized Mo Kα radiation (λ =
phenolate donors by a resonance-withdrawing effect,26,27 and the four moderately hard amine donors of cyclen would preferentially accommodate Eu2+. Thus, the eight-coordinate environment, completed by the four oxygen and four nitrogen atoms, was expected to encapsulate and stabilize Eu2+. Due to their oxophilicity, Ln3+ ions were expected to selectively bind to O-donor sites adjacent to the phenyl rings. Fortunately, a series of isostructural clusters, EuII2LnIII4(OH)4(NIC)4L2 (Ln = Sm, Eu, Tb, defined as M-EuIILnIII-1, mixed Eu(II)/Ln(III) coordination complexes-1), was successfully synthesized by introducing nicotinic acid (HNIC) as the second ligand. Moreover, the clusters were not oxidized when exposed to air in either solution or solid state. Cyclic voltammetry experiments showed that they are the most oxidatively stable EuIIcontaining complexes known so far. This work demonstrates
Table 1. Crystallographic Data for the Three Mixed Eu(II)/Ln(III) Complexes
a
complex
M-EuIISmIII-1
M-EuIIEuIII-1
M-EuIITbIII-1
formula formula weight crystal color dimensions/mm3 crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z F(000) μ/mm−1 θ for data collection/deg reflections collected unique reflections/R(int) parameters GOF R1a, wR2b (I > 2σ(I))
Eu2Sm4C112H132N12O28 2999.62 yellow 0.406 × 0.323 × 0.206 triclinic P1 15.183(7) 15.207(8) 15.218(8) 89.263(19) 78.254(15) 66.414(10) 3144(3) 1 1486 2.892 2.08 to 27.57 21 019 18 932/0.0741 1399 1.021 0.0575, 0.1480
Eu6C112H132N12O28 3006.06 yellow 0.368 × 0.235 × 0.196 triclinic P1 15.246(4) 15.252(4) 15.280(4) 66.002(5) 89.090(7) 78.068(7) 3166.5(14) 1 1490 2.998 2.08 to 27.52 21 067 18 743/0.0744 1387 1.038 0.0672, 0.1672
Eu2Tb4C112H132N12O28 3033.90 yellow 0.336 × 0.298 × 0.243 triclinic P1 15.1243(5) 15.1725(6) 15.1882(7) 78.901(4) 66.852(4) 89.556(3) 3135.8(2) 1 1498 3.282 2.08 to 27.52 21 665 12 013/0.0860 1375 1.023 0.0900, 0.2248
R1 = ∑(||Fo| − |Fc||)/∑|Fo|. bwR2 = {∑w[(Fo2 − Fc2)]/∑w[(Fo2)2]}0.5. B
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Figure 1. Molecular structure of M-EuIISmIII-1: (a) heterovalent and heterometallic 2Eu(II)/4Sm(III) cluster (black dashed lines represent four intramolecular hydrogen bonds between μ3-OH and the uncoordinated methoxyl oxygen atom); (b) eight-coordinate environment of Eu2+ and coordination mode of ligand L; (c) tetrahedral 4Sm(III) cluster connected by μ3-OH and μ2-bridging NIC.
can be explained only by the presence of two Eu2+ and four Sm3+ ions. The existence of Eu2+ was further confirmed by ICP and EPR measurements. The assignment of Eu2+ and Sm3+ ions to the respective sites was based on hard−soft and acid−base theory. Eu2+ as a soft acid is preferentially accommodated by the four moderately hard amine donors of cyclen, and the coordination environment of Sm3+ is completely occupied by oxygen atoms due to the oxophilicity of trivalent lanthanide ion. Figure 1b shows the coordination mode of ligand L. The four phenolate donors encapsulate Eu2+ and also connect two Sm3+ ions together with the two cooperative methoxyl oxygen atoms. The remaining two methoxyl oxygen atoms are uncoordinated. As shown in Figure 1a, two ligands L with the same coordination mode are located at the two sides, with each of them linking two Sm3+ ions, leaving a tetrahedral 4Sm(III) cluster in the middle of the molecule. The tetrahedral 4Sm(III) cluster is further stabilized by four hydroxyls and four ligands NIC (Figure 1c). Each of the hydroxyls that adopts the μ3bridging mode is located at the apex of a triangle surface, and the four edges of the tetrahedral 4Sm(III) cluster are linked by μ2-bridging carboxylate groups of the NIC ligands. Therefore, a novel 2Eu(II)/4Sm(III) cluster with a multiplexed coordination bonding is obtained. In addition, there are four strong intramolecular hydrogen bonds between μ3-OH and the uncoordinated oxygen atom of the methoxyl group (dashed lines, Figure 1a). Oxidative Stability of EuII. The two divalent europium ions in the cluster are trapped within the macrocyclic cavities of designed ligands L and are further isolated from the environment by outside phenyl rings and the tetrahedral 4Ln(III) cluster. Such a tight structure is expected to increase the oxidative stability of divalent europium ions. Although crystals of M-EuIILnIII-1 are easily collapsed in air due to the loss of solvent molecules at room temperature, X-band EPR spectra of the solid samples exhibit signals over a wide range of magnetic fields (namely, 0.17−0.56 T; Figure 2), confirming the existence of Eu2+. To verify the oxidative stability of MEuIILnIII-1, the solid samples were exposed to air for 2 months and then dissolved and recrystallized in methanol. X-ray analysis of the obtained crystals showed that the primitive 2Eu(II)/4Ln(III) cluster remains unchanged. Cyclic voltammetry experiments were performed to probe redox responses from the as-synthesized complexes, with the ferrocene/ferrocenium (Fc/Fc+) redox couple as an internal
0.71073 Å). Empirical absorption corrections were performed using the CrystalClear program.28 Structures were solved and refined with respect to F2 by the full-matrix least-squares technique using the SHELX-97 program package.29,30 Anisotropic thermal parameters were applied to all non-hydrogen atoms. Hydrogen atoms were generated geometrically. There were extensive areas of residual electron density that could not be reasonably modeled as solvent molecules.31,32 Therefore, they were removed by using the SQUEEZE function in PLATON. Crystallographic data for the three complexes are listed in Table 1. Electrochemical Measurements. Cyclic voltammetry experiments were performed to probe redox responses from ligand H4L and M-EuIILnIII-1 (Ln = Sm, Eu, Tb). Due to their poor solubility in strongly polar solutions such as H2O and DMF, electrochemistry experiments were performed in ultrapure methanol solution using 0.1 mol dm−3 [NnBu4]ClO4 as a supporting electrolyte. A standard threeelectrode cell was used with a glassy-carbon working electrode, a Ptwire auxiliary electrode, and a Ag/AgCl reference electrode (1.0 M KCl). Following each measurement, ferrocene was added to the sample, and the measurement was repeated to enable referencing to the ferrocene/ferrocenium (Fc/Fc+) redox couple as an internal standard.33
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RESULTS AND DISCUSSION Synthesis and Characterization. The title complexes MEuIILnIII-1 (Ln = Sm, Eu, Tb) were obtained by solvothermal reaction of EuBr2, Ln(NO3)3, HNIC, and H4L in a sealed methanol solution. X-ray analyses (Figure 1) revealed the isostructure of a huge cluster with a composition of EuII2LnIII4(OH)4(NIC)4L2. The formation of these neutral molecules with balanced ionic charges can be explained only by the presence of two Eu2+ and four Ln3+ ions. In addition, ICP spectroscopy analyses for M-EuIISmIII-1 and M-EuIITbIII-1 showed that the measured Sm/Eu and Tb/Eu ratios were 1.975 and 1.986, respectively, approaching the ideal value of 2; this confirms the heterovalent and heterometallic character of the two complexes. Structural Descriptions. In light of the isostructural character of the three complexes, the molecular structure of M-EuIISmIII-1 is depicted as an example. As expected, a divalent europium ion is trapped within a macrocyclic cavity of designed ligand L. The bond lengths of EuII−N and EuII−O are in the ranges of 2.626−2.689 and 2.331−2.407 Å, respectively. We note that the interatomic distances of EuII−O are similar to that of SmIII−O, which appears to be inconsistent with the larger ionic radius of Eu2+. As mentioned above, the neutral molecule C
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has been no reported oxidation potential of a Eu(II) complex that is larger than the reference Fc/Fc+ redox couple. The previously reported 4,7,13,16,21,24-hexaoxa-1,10diazabicyclo[8.8.8]hexacosane europium(II) is a well-known oxidatively stable Eu(II) complex.39 By improving the cryptand derivative ligand, Gamage et al. generated the most stable aqueous Eu(II) complex at that time (−0.035 V vs Fc/Fc+).36
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CONCLUSIONS In summary, mixed Eu(II)/Ln(III) coordination complexes were synthesized for the first time from primitive metal salts, although some homometallic Eu(II)/Eu(III) complexes have been accidentally obtained by spontaneous reduction of Eu(III) complex or oxidation of Eu metal.40−42 The study herein illustrates how a rationally designed macrocyclic ligand can be employed to encapsulate Eu2+ and enable direct formation of a novel heterovalent 2Eu(II)/4Ln(III) cluster. The dramatic oxidative stability of M-EuIILnIII-1 not only verifies the synthetic feasibility but also highlights the prospective applications of mixed Eu(II)/Ln(III) coordination complexes. It will be highly interesting to investigate the interactions between heterovalent Eu(II)/Ln(III) centers and their effects on the magnetic, spectral, or catalytic properties. We are currently pursuing this research and will further focus on synthesizing other heterovalent lanthanide complexes such as mixed Sm(II)/Ln(III) and Yb(II)/Ln(III) complexes.
Figure 2. X-band EPR spectra of M-EuIILnIII-1 (Ln = Sm, Eu, Tb) recorded at room temperature.
standard.33 Notably, only one anodic current peak appears in the measured range of −0.6 to 1.0 V vs Ag/AgCl due to the oxidation of Eu2+, observed at 0.337, 0.340, and 0.344 V vs Fc/ Fc+ for M-EuIILnIII-1 (Ln = Sm, Eu, and Tb), respectively (Figure 3). The higher anodic current peak at approximately 0.76 V vs Fc/Fc+ is attributed to the oxidation of ligand L (Figure S2, Supporting Information). In all cases, the electrode reactions are found to be irreversible by the cyclic voltammetry. The obtained oxidative stabilities of EuII in solution are unprecedented.16,19,34−38 To the best of our knowledge, there
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00804. 1 H NMR of H4L in CD3OD solution and cyclic staircase voltammograms of H4L and M-EuIISmIII-1 (PDF) Crystallographic data in CIF format for M-EuIILnIII-1 (Ln = Sm, Eu, Tb, CCDC reference numbers 1054733− 1054735) (CIF, CIF, CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Natural Science Foundation of China (nos. 21171165, 21201165, and 91122015).
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Figure 3. Cyclic staircase voltammograms of M-EuIILnIII-1 (Ln = Sm, Eu, Tb). D
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