Two samarium(III) complexes with tunable fluorescence from in situ reactions of 2-ethoxy-6-((pyridin-2-ylmethylimino)methyl)phenol with Sm³⁺ ion

Abstract

Two samarium(III) complexes showing different luminescent intensity, namely Sm(HL¹)₂·NO₃·2H₂O (1) and [SmL²(C₂H₅OH)₂·NO₃]₂ (2), were constructed via the solvothermal in situ lanthanide metal–ligand reactions of 2-ethoxy-6-((pyridin-2-ylmethylimino)-methyl)phenol (HL) from pyridin-2-ylmethanamine and 3-ethoxy-2-hydroxybenzaldehyde with Sm³⁺ ion at different temperatures, in which H₂L¹ resulted from C–C coupling dimerization of HL, while H₂L² from C–C coupling of HL and pyridin-2-ylmethanamine.

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The recently developed solvothermal in situ metal–ligand reaction might be a very effective means for preparation of complicated metal complexes,¹ and the existence of –CH=N–CH₂– moiety in some Schiff base ligands can result in in situ effective formation of the novel Schiff base ligands or aza-heterocycles, especially for the multi-arm podand-type ligands and complicated aza-heterocycles difficultly obtained from common organic synthetic reactions, such as inmidazo[1,5-a]pyridine, pyrazine, piperazine, imidazolidine and imidazole, as well as their derivatives from C–C/C–N coupling cycloaddition reactions of simple Schiff base ligands mediated by Cd²⁺, Zn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Ti⁴⁺ etc.² Compared to these widely studied in situ transition metal–ligand reactions, the corresponding in situ reactions induced by lanthanide metal ions have not yet been reported so far.

On the other hand, the assembly of lanthanide(III) complexes has attracted much attention because of their interesting luminescent and magnetic properties.³ In order to obtain strongly luminescent lanthanide complexes, the selection and design of the organic ligand is very important, and it should be able to (i) efficiently absorb and transfer energy to the central metal ion and (ii) encapsulate and protect the lanthanide ion from the coordination of the solvent molecules.⁴ For these purposes, macrocyclic, macrobicyclic (cryptand) and podand type ligands containing aromatic rings are often good choices. In particular, podand ligands with many suitably designed arms could provide hosts for the lanthanide ions and well encapsulate them, by deliberate incorporation of appropriate multiple absorption groups (such as phenyl, benzimidazolyl, pyridyl groups, etc.) suitable for energy transfer. Therefore solvothermal in situ lanthanide metal–ligand reaction can provide a very good assembly strategy to prepare strongly luminescent lanthanide complexes.⁵

Our recent studies have focused on the use of a variety of salicylal/pyridyl-base Schiff base ligands for the synthesis of mono/poly-nuclear metal complexes.⁶ The research results show these simple Schiff base ligands can in situ convert into podand ligands with three/four suitably salicylal/pyridyl arms via transition metal-mediated C–C/C–N bond-forming strategy under solvothermal conditions. As part of our continuing studies focused on the solvothermal in situ metal–ligand reactions of simple Schiff base ligand, we report here two samarium(III) complexes Sm(HL¹)₂·NO₃·2H₂O (1) and [SmL²(C₂H₅OH)₂·NO₃]₂ (2), which are obtained from the solvothermal in situ lanthanide metal–ligand reactions of simple Schiff base ligand of 2-ethoxy-6-((pyridin-2-yl-methylimino)methyl)phenol (HL, Scheme 1) with Sm³⁺ ion at different temperatures. Interestingly, in situ generated complex 1 has much better luminescent properties comparing with complex 2.

Scheme 1 Molecular structure of in situ formed Schiff base ligands H₂L^1–2 in complexes 1–2.

To the best of our knowledge, it is the first example of complexes formed from the solvothermal lanthanide metal–ligand reactions in Schiff base system with –CH=N–CH₂– moiety, in which two in situ generated novel multidentate Schiff base ligands of 6,6′-(5-phenyl-3-(pyridin-2-ylmethyl)-imidazolidine-2,4-diyl)bis(2-ethoxy-phenol)(H₂L¹), 2-((2-amino-1,2-di(pyridin-2-yl)-ethylimino)-methyl)-6-ethoxy-phenol(H₂L²) (Scheme 1) were involved.

As outlined in Scheme 1, the simple Schiff base ligand of 2-ethoxy-6-((pyridin-2-ylmethylimino)-methyl)phenol (HL) was synthesized in good yield by the 1 : 1 condensation of pyridin-2-ylmethanamine with 3-ethoxy-2-hydroxybenzaldehyde.⁶ Heating Sm(NO₃)₃·6H₂O and HL (a 1 : 2 at molar ratio) in the mixed solvents of ethanol and water (a 3 : 1 ratio at volume) at 80 °C for 8 h afforded yellow block-like crystals Sm(HL¹)₂·NO₃·2H₂O (1) in good yield.‡ The products are all stable in air, and insoluble in water or common organic solvents. The formula of 1 was determined by single-crystal X-ray diffraction studies, elemental analysis (EA), infrared spectroscopy (IR) and thermogravimetric analysis (TGA).

A single-crystal X-ray diffraction study performed on 1 reveals a neutral 0-D mononuclear compound.§ 1 crystallizes in the triclinic space group P1̄ and each molecular unit contains one Sm³⁺ ion, two in situ formed (HL¹)⁻ ligands, one nitrate ion and two lattice water molecules. Each Sm(III) center in 1 has a ten-coordinated environment and adopts a distorted bicapped quadrangular prismatic coordination geometry with the N₈O₂ donor set of two η⁵-mode (HL¹)⁻ ligands as shown in Fig. 1. The Sm–N and Sm–O bond lengths range from 2.625(3) to 2.771(3) Å and from 2.372(3) to 2.398(2) Å (Table S1†), respectively. Intermolecular hydrogen bonding C(O)–H⋯O(N) (Table S2†) and π⋯π packing (the distances between centroids of two parallel pyridyl rings 4.077 Å) interactions result in the assembly of a 3-D supramolecular network in the ab plane (Fig. S1†).

Fig. 1 In 1, (a) the coordination environment of the Sm³⁺ ion and coordination mode of ligand (HL¹)⁻ in cationic species [Sm(HL¹)₂]⁺; (b) a distort bicapped quadrangular prismatic geometry around central Sm³⁺ ion.

A solvothermal reaction of Sm(NO₃)₃·6H₂O and HL (a 1 : 2 molar ratio) in a mixed solvent of ethanol and water (a 1 : 1 ratio at volume) at 120 °C for 8 h afforded yellow strip-like crystals [SmL²(C₂H₅OH)₂·NO₃]₂ (2).‡ The products are all stable in air, and insoluble in water or common organic solvents. The formula of 2 was determined by single-crystal X-ray diffraction studies,§ elemental analysis (EA), infrared spectroscopy (IR) and thermogravimetric analysis (TGA).

The crystal analysis result reveals that complex 2 crystallized in triclinic, P1̄ space group, and shows a discrete binuclear structure with a centrosymmetry, which comprised of two Sm³⁺ ions, two in situ formed (L²)²⁻ ligands, four coordinated ethanol molecules and two free nitrate ions. In the asymmetric unit, Sm1 atom was octo-coordinated to two μ₂-bridging amide nitrogen atoms (N3, N3a), two pyridine nitrogen atoms (N2a, N4), one imine nitrogen atom (N1a), one hydroxyl oxygen atom (O2a) and two ethanol oxygen atoms (O3, O4), showing a bicapped trigonal prismatic coordination geometry (Fig. 2). The Sm–N and Sm–O bond lengths range from 2.315(4) to 2.680(6) Å and from 2.226(4) to 2.471(5) Å (Table S1†), respectively. Intermolecular hydrogen bonding C(O)–H⋯O (Table S2†) interactions result in the assembly of a 2-D supramolecular layer in the ab plane (Fig. S2†).

Fig. 2 In 2, (a) the coordination environment of the Sm³⁺ ion and coordination mode of ligand (L²)²⁻ in cationic species [Sm(L²)]₂²⁺; (b) a distort bicapped trigonal prismatic coordination geometry of the central Sm³⁺ ion. The symmetric code: (a) −x, −y, 1 − z.

From above analyses and Scheme 1, the multidentate ligand H₂L¹ in compound 1 resulted from C–C coupling dimerization of HL. Different from H₂L¹, ligand H₂L² in 2 originated from C–C coupling of simple Schiff base ligand 2-ethoxy-6-((pyridin-2-ylmethylimino)-methyl)phenol (HL) and pyridin-2-ylmethanamine from partial hydrolysis of HL in solution state at high temperature (120 °C). Obviously, simple Schiff base HL in the assembly process of two compounds 1–2 underwent in situ C–C coupling reactions to form two Sm(III)-complexes 1 and 2 containing two new different ligands H₂L^1–2.

On the basis of the related literatures⁷ and our previous works,⁶ two possible mechanisms should be addressed in Schemes 2 and 3, respectively. At the stage of low temperature (80 °C), upon coordination to Sm(III) center via the imino nitrogen atom (the transition state (a) in Scheme 2) may result in a significantly increase of the activity of a C–H bond in the α-position to an imino group –CH=N–CH–,⁸ pyridine, as the basic acceptor, has the ability to deprotonate the imino carbon-bound hydrogen atom to form carbanion.⁹ Such the methylene proton (α-H, H3′) was activated to produce a strong nucleophilic carbanion formed at C3′ (the transition state (b) in Scheme 2), and another molecule of H₂L¹ was drawn close by coordination bond between Sm(III) and pyridine nitrogen atom, then by nucleophilic attack of C3′ to C1, a C–C coupling reaction occurred to form the final product 1 containing ligand H₂L¹ ((c) in Scheme 2). In order to meet high coordination number of samarium ion (CN: 10), the resulting twisted molecular conformation of in situ formed H₂L¹ molecule prevented further occurrence of intramolecular C–C or C–N coupling reaction.

Scheme 2 Proposed mechanism for the in situ formation of H₂L¹ shown in the asymmetric unit of complex 1.

Scheme 3 Proposed mechanism for the in situ formation of H₂L² shown in molecule of complex 2.

With the increase of temperature (120 °C), Schiff base ligand HL is easy to hydrolyze and the resulting product pyridin-2-ylmethanamine together with HL coordinated to Sm³⁺ ion to give intermediate (a) (Scheme 3). Similar to compound 1, coordination of the imino nitrogen atom to a metal Sm(III) center resulted in markedly increase of the acidity of a C–H bond in the α-position to an imino group –CH=N–CH–, and then deprotonated the methylene proton (α-H1) in the presence of pyridine groups to produce a strong nucleophilic carbanion formed at C1 as intermediate (b) in Scheme 3. Subsequently, carbanion attacks the electrophilic carbon C3′ from a pyridin-2-ylmethanamine ligand to form C–C bond. After removal of amine hydrogen atoms in the existence of pyridine, the final complex 2 was formed ((c) in Scheme 3).

The phase purity of both 1 and 2 were supported by the powder X-ray diffraction (XRD) pattern of the bulk sample, which are consistent with the calculated patterns (Fig. S3†). TGA show that 1 loses 2.77% of its total weight upon heating to 153 °C, which corresponds to the loss of two lattice water molecules per formula unit (calcd 2.82%) and the molecular structure is stable up to 330 °C (Fig. S4†). While 2 loses 13.82% of its total weight upon heating to 155 °C, corresponding to four ethanol molecules per formula (calcd 13.84%). The TGA result shows that 2 is stable up to 385 °C.

Solid-state luminescent properties of two complexes 1–2 are measured at room temperature. As can be seen from Fig. 3, when excited at 345 nm, complex 1 presents samarium-centered luminescence with its emission peak at 562 nm (⁴G_5/2 → ⁶H_5/2), 597 nm (⁴G_5/2 → ⁶H_7/2) and 643 nm (⁴G_5/2 → ⁶H_9/2), which is in good agreement with previous reported Sm³⁺ complexes.¹⁰ The most intense emission peak at 597 nm was monitored, and its lifetime was measured as 34.5 μs. According to the following equation, η = τ/τ₀, where τ is the fluorescence lifetime (observed lifetime) of ⁴G_5/2 excited state, and τ₀ is the calculated natural lifetime (radiative lifetime) of ⁴G_5/2 excited state,¹¹ its quantum yield was calculated as 1.10%. While the ligand-centered emission at 468 nm is much weaker relative to that of Sm³⁺, indicating that the energy transfer from ligand to Sm³⁺ center is very efficient. However, samarium-centered luminescence is barely detected in complex 2, which could be probably ascribed to the existence of high energy O–H oscillators from four coordinated ethanol molecules, causing significant non-radiative decays of energy and resulting in luminescent quenching of 2 (details in Table S3 and Fig. S6†).¹²

Fig. 3 Emission spectra of complexes 1–2 in the solid state at room temperature (λ_ex = 345 nm).

In summary, through solvothermal in situ Sm–ligand (Schiff base HL) reaction, two samarium(III) complexes 1–2 are obtained at different temperature. The central Sm³⁺ ion in 1 was tightly encapsulated by two in situ generated ligands (HL¹)⁻, well prevented from coordination of solvent molecules, therefore compound 1 shows very good Sm(III)-centered luminescent property. While in complex 2, the ligand (L²)²⁻ failed to protect Sm³⁺ from coordination to ethanol molecules, because of which its Sm-based luminescence is quenched. Moreover, the possible formation mechanisms of two in situ generated ligands H₂L^1–2 are also carefully proposed. To the best of our knowledge, it is the first example of the solvothermal in situ C–C coupling reaction promoted by lanthanide metal ion in Schiff base system with –CH=N–CH₂– moiety. Obviously, this synthetic strategy provides a very effective approach for obtaining the multi-arm podand-type ligands and strongly lanthanide luminescent materials.

Acknowledgements

This work was supported by the national natural science foundation of P. R. China (Grant No. 21471061, 91122008 and 21671071), research fund for the doctoral program of higher education of China (Grant No. 20124407110007), Guangdong province higher school science and technology innovation key projects (Grant No. cxzd1113), Science and Technology Planning Project of Guangdong Province (Grant No. 2013B010403024 and 2015B010135009).

Notes and references

1. S. Hu, J.-C. Chen, M.-L. Tong, B. Wang, Y.-X. Yan and S. R. Batten, Angew. Chem., Int. Ed., 2005, 44, 5471; S. M.-F. Lo, S. S.-Y. Chui, L.-Y. Shek, Z. Lin, X. X. Zhang, G.-H. Wen and I. D. Williams, J. Am. Chem. Soc., 2000, 122, 6293; Y.-J. Ou, Z.-P. Zheng, X.-J. Hong, L.-T. Wan, L.-M. Wei, X.-M. Lin and Y.-P. Cai, Cryst. Growth Des., 2014, 14, 5339.
2. S. Hu, J.-C. Chen, M.-L. Tong, B. Wang, Y.-X. Yan and S. R. Batten, Angew. Chem., Int. Ed., 2005, 117, 5607; L. Han, W.-N. Zhao, Y. Zhou, X. Li and J.-G. Pan, Cryst. Growth Des., 2008, 8, 3504; J. Carranza, C. Brennan, J. Sletten, J. M. Clemente-Juan, F. Lioret and M. Julve, Inorg. Chem., 2003, 42, 8716; B. A. Frazier, V. A. Villiams, P. T. Wolczanski, S. C. Bart, K. Meyer, T. R. Cundary and E. B. Lobkovsky, Inorg. Chem., 2013, 52, 3295; B. A. Frazier, P. T. Wolczanski, I. Keresztes, S. DeBeer, E. B. Lobkovsky, A. W. Pierpont and T. R. Cundari, Inorg. Chem., 2012, 51, 8177; B. A. Frazier, P. T. Wolczanski, E. B. Lobkovsky and T. R. Cundari, J. Am. Chem. Soc., 2009, 131, 3428.
3. E. Pershagen, J. Nordholm and K. E. Borbas, J. Am. Chem. Soc., 2012, 134, 9832; S. Comby, E. M. Surender, O. Kotova, L. K. Truman, J. K. Molloy and T. Gunnlaugsson, Inorg. Chem., 2014, 53, 1867; Z. Zhang, W.-X. Feng, P.-Y. Su, X.-Q. Lv, J.-R. Song, D.-D. Fan, W.-K. Wong, R. A. Jones and C.-Y. Su, Inorg. Chem., 2014, 53, 5950.
4. N. Sabbatini, M. Guardigli and J. M. Lehn, Coord. Chem. Rev., 1993, 123, 201; X.-P. Yang, C.-Y. Su, B.-S. Kang, X.-L. Feng, W.-L. Xiao and H.-Q. Liu, J. Chem. Soc., Dalton Trans., 2000, 3253.
5. S. J. Bradberry, A. J. Savyasachi, M. Martinez-Calvo and T. Gunnlaugsson, Coord. Chem. Rev., 2014, 273–274, 226; J.-M. Lehn and J.-B. Regnouf de Vains, Helv. Chim. Acta, 1992, 75, 1221; V. M. Mukkala and J. J. Kankare, Helv. Chim. Acta, 1992, 75, 1578; B. Alpha, J.-M. Lehn and G. Mathis, Angew. Chem., Int. Ed. Engl., 1987, 26, 266; V. Balzani, E. Berghmans, J.-M. Lehn, N. Sabbatini, A. Mecai, R. Therorde and R. Ziessel, Helv. Chim. Acta, 1990, 73, 2083.
6. Z.-P. Zheng, Y.-J. Ou, X.-J. Hong, L.-M. Wei, L.-T. Wan, W.-H. Zhou, Q.-G. Zhan and Y.-P. Cai, Inorg. Chem., 2014, 53, 9625; Z.-P. Zheng, Q. Wei, W.-X. Yin, L.-T. Wan, X. Huang, Y. Yu and Y.-P. Cai, RSC Adv., 2015, 5, 27682; Y.-J. Ou, Y.-J. Ding, Q. Wei, X.-J. Hong, Z.-P. Zheng, Y.-H. Long, Y.-P. Cai and X.-D. Yao, RSC Adv., 2015, 5, 27743.
7. M. Roy, B. V. S. K. Chakravarthi, C. Jayabaskaran, A. A. Karande and A. R. Chakravarty, Dalton Trans., 2011, 40, 4855; M. E. Bluhm, M. Ciesielski, H. Görls, O. Walter and M. Döring, Inorg. Chem., 2003, 42, 8878; D. H. Ess and K. N. Houk, J. Am. Chem. Soc., 2008, 130, 10187.
8. V. M. Aryuzina and M. N. Shchukina, Khim. Geterotsikl. Soedin., 1968, 3, 506; M. P. Kochergin and V. A. Lifanov, Khim. Geterotsikl. Soedin., 1994, 4, 490; M. E. Bluhm, M. Görls, H. Ciesielski, O. Walter and M. Döring, Inorg. Chem., 2003, 42, 8878.
9. R. Huisgen, in 1,3-Dipolar Cycloaddition Chemistry, ed. A. Padwa, John Wiley & Sons, New York, 1984, vol. 1.
10. M. D. Regulacio, M. H. Pablico, J. A. Vasquez, P. N. Myers, S. Gentry, M. Prushan, S. Wah, T. Chang and S. L. Stoll, Inorg. Chem., 2008, 47, 1512; H. S. Wang, B. Zhao, B. Zhai, W. Shi, P. Cheng, D. Z. Liao and S. P. Yan, Cryst. Growth Des., 2007, 7, 1851; S. Quici, M. Cavazzini, G. Marzanni, G. Accorsi, N. Armaroli, B. Ventura and F. Barigelletti, Inorg. Chem., 2005, 44, 529.
11. Z.-F. Li, J.-B. Yu, L. Zhou, H.-J. Zhang and R.-P. Deng, Inorg. Chem. Commun., 2008, 11, 1284; X.-Y. Chen, M. P. Jensen and G.-K. Liu, J. Phys. Chem. B, 2005, 109, 13991.
12. J.-C. G. Bünzli, Chem. Rev., 2010, 110, 2729; S. V. Eliseeva and J.-C. G. Bünzli, Chem. Soc. Rev., 2010, 39, 189; Y.-W. Yip, H.-L. Wen, W.-T. Wong, P. A. Tanner and K.-L. Wong, Inorg. Chem., 2012, 51, 7013; D. Parker, Coord. Chem. Rev., 2000, 205, 109; X. J. Hong, M. F. Wang, H. G. Jin, Q. G. Zhan, Y. T. Liu, H. Y. Jia, X. Liu and Y. P. Cai, CrystEngComm, 2013, 15, 5606; M.-F. Wang, J.-Q. Su, H.-M. Peng, H.-Y. Jia, Q.-G. Zhan, H.-G. Jin, J.-H. Chen and Y.-P. Cai, Inorg. Chem. Commun., 2012, 21, 8.
13. G. M. Sheldrick, SADABS, Version 2.05, University of Göttingen, Göttingen, Germany.
14. G. M. Sheldrick, SHELXS-97, Program for X-ray Crystal Structure Determination, University of Göttingen, Göttingen, Germany, 1997.
15. G. M. Sheldrick, Acta Crystallogr., Sect. C: Struct. Chem., 2015, 71, 3.

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Footnotes

† Electronic supplementary information (ESI) available: Details of the synthetic procedures, spectra data and crystallographic data. CCDC 1061273 for 1 and 1061274 for 2. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra20957b

‡ Preparation of 1A mixture of Sm(NO₃)₃·6H₂O (17.78 mg, 0.04 mmol) and HL (40.96 mg, 0.16 mmol) was placed in a small vial containing ethanol (15 mL) and H₂O (5 mL). The vial was sealed and heated at 80 °C for three day; yellow block-like crystals were collected, washed with ethanol and dried in air. Yield: 37.13 mg (73% based on Sm). Elemental analysis (%) calcd for C₆₀H₆₆N₉O₁₃Sm (Mr = 1271.56): C 56.62, H 5.19, N 9.91. Found: C 56.57, H 5.24, N 9.95. IR (KBr, cm⁻¹): 3415(br, vs), 2981(w), 2893(w), 1631(s), 1603(m), 1556(m), 1488(m), 1460(m), 1426(w), 1389(m), 1275(m), 1246(w), 1216(s), 1110(w), 1046(s), 992(w), 904(w), 842(w), 785(m), 739(m), 663(m), 557(w), 469(m).Preparation of 2A mixture of Sm(NO₃)₃·6H₂O (35.56 mg, 0.08 mmol) and HL (40.96 mg, 0.16 mmol) was placed in a Teflon-lined stainless steel autoclave containing ethanol (10 mL) and H₂O (10 mL), and heated to 120 °C for 72 h. After the mixture was cooled to room temperature at the rate of 10 °C h⁻¹, pale yellow crystals of 2 were collected, washed with water and dried at room temperature, yield: 34.58 mg (65% based on Sm). Elemental analysis (%) calcd for C₅₀H₆₄N₁₀O₁₄Sm₂ (Mr = 1329.81): C 45.12, H 4.81, N 10.53. Found: C 45.17, H 4.83, N 10.52. IR (KBr, cm⁻¹): 3479(br, vs), 3305(w), 2971(w), 1636(vs), 1559(vs), 1421(m), 1414(s), 1305(w), 1281(w), 1235(w), 1210(s), 1150(w), 1079(m), 1040(m), 907(w), 769(m), 744(m), 645(m), 521(w), 442(w).

§ Crystallographic data for 1M = 1271.56, triclinic, space group P1̄, a = 12.4150(18) Å, b = 15.719(2) Å, c = 16.661(2) Å, α = 97.375(2)°, β = 103.260(2)°, γ = 105.382(2)°, V = 2988.8(7) ų, Z = 2, μ = 1.053 mm⁻¹, D_c = 1.413 Mg m⁻³, F(000) = 1310, 6278 unique (R_int = 0.0495), R₁ = 0.0709, wR₂ = 0.1617[I > 2σ(I)], GOF = 1.031. The intensity data were collected on a Bruker Smart Apex II CCD diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 298 K. All absorption corrections were performed by using the SADABS program.¹³ Structural solutions and full-matrix least-squares refinements based on F² were performed with the SHELXS-97 (ref. 14) and SHELXL-2014/7 (ref. 15) program packages, respectively. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms on organic ligand was generated by the riding mode (C–H = 0.93 or 0.96 Å), and the water hydrogen atoms were located from difference maps and refined with isotropic temperature factors. Two oxygen (O10, O11) atoms of nitrate in 1 are disordered into two positions, and each position has a site occupancy factor of 0.5.Crystallographic data for 2M = 1329.81, triclinic, space group P1̄, a = 10.718(4) Å, b = 11.181(4) Å, c = 13.217(5) Å, α = 79.809(5)°, β = 81.813(5)°, γ = 65.165(4)°, V = 1408.3(9) ų, Z = 1, μ = 2.135 mm⁻¹, D_c = 1.568 Mg m⁻³, F(000) = 670, 8348 unique (R_int = 0.0247), R₁ = 0.0439, wR₂ = 0.0880[I > 2σ(I)], GOF = 1.037. The intensity data were collected on a Bruker Smart Apex II CCD diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 298 K. All absorption corrections were performed by using the SADABS program.¹³ Structural solutions and full-matrix least-squares refinements based on F² were performed with the SHELXS-97 (ref. 14) and SHELXL-2014/7 (ref. 15) program packages, respectively. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms on organic ligand was generated by the riding mode (C–H = 0.93 or 0.96 Å).

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