Tetranuclear NIR luminescent Schiff-base Zn–Nd complexes

Abstract

With the simple dinuclear Zn–Nd complex [ZnNdL(H₂O)(NO₃)₃] (1) (H₂L = N,N′-bis(3-methoxysalicylidene)ethylene-1,2-diamine) as the precursor, tetranuclear [Zn₂Nd₂L₂(4,4′-bpy)(NO₃)₆]·Et₂O (2) and [Zn₂Nd₂L₂(4,4′-bpe)]·2H₂O (3) (4,4′-bpy = 4,4′-bipyridine, 4,4′-bpe = trans-bis(4-pyridyl)ethylene) complexes are formed: suitable choice of bidentate linkers could construct multinuclear heterometallic 3d–4f Schiff-base complexes with improved luminescent properties, and the controlling of linker character shows a potentially effective way to the fine-tuning properties of NIR luminescence from Nd ions.

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Introduction

Much recent effort has been devoted to luminescent lanthanide (Ln) complexes with long-lived and characteristic line like emission bands (near UV to visible and NIR regions),¹ because of their potential applications in bioassays,² medicine³ and materials science.⁴ The limit of low absorption coefficients due to the parity forbidden f–f transitions for these lanthanide ions,⁵ has led to considerable exploration of suitable chromophores,⁶ with intense and tunable absorption maxima in the visible region, to act as sensitizers of luminescence from lanthanides in complex systems. Several recent investigations have focused on organic dyes⁷ and d-block metal complexes (i.e.Cr(III),⁸Ru(II),⁹Pt(II),¹⁰Re(I) or Os(II)¹¹), which display absorption bands in the near-UV or visible ranges, and have been demonstrated to effectively transfer energy to the lanthanide ions indirectly.

Our past research¹² has focused on the use of compartmental salen-type Schiff-base ligands, to bind both 3d Zn(II) and 4f Nd(III), Yb(III) or Er(III) ions. These heterometallic systems, as suitable chromophores, can act as antennae or sensitizers for NIR lanthanide luminescence, and the π-conjugation of those salen-type Schiff-base ligands helps to enhance luminescent properties. In the view of fine-tuning of their NIR properties, three effective routes are adopted: (1) change of spacers or (2) electronic properties of the substituents on the flanking phenyl rings of the Schiff-base ligands to extend the conjugation of the complexes,^12g and (3) of the functional bridge from OH to other anionic groups, to obtain multinuclear 3d–4f complexes.^12c It is noticeable that in the above 3d–4f heterometallic complexes, the axial position of the 3d ions was occupied by anions (OAc^–, ^12a,f Cl^–12c,e or NO₃^–12e,g) or H₂O^12b,e,g solvates from different reaction conditions, which may lead to partial luminescent quenching with OH solvates such as EtOH or H₂O around the 4f ions. If replaced by axial groups with rigid multidentate linkers of stronger coordination ability, polynuclear 3d–4f complexes should be obtained, which could effectively extend the electronic conjugation of the whole molecule, and endow multi-chromophores to increase the rate of the energy-transfer process, leading to better luminescent properties than that of the dinuclear complexes. Herein, with the simple dinuclear Zn–Nd complex [ZnNdL(H₂O)(NO₃)₃] (1) as the precursor, we report the syntheses, structures and photophysical properties of two Zn–Nd tetranuclear complexes [Zn₂Nd₂L₂(4,4′-bpy)(NO₃)₆]·Et₂O (2) and [Zn₂Nd₂L₂(4,4′-bpe)]·2H₂O (3), which are formed from different bidentate (4,4′-bpy or 4,4′-bpe) linkers to connect with the two Zn–Nd salen-type Schiff-base architecture units on the axial positions of Zn ions; and their NIR luminescent characteristics can be effectively fine-tuned as a function of the linkage.

Results and discussion

NMR

As shown in Scheme 1, reaction of an equimolar amount of H₂L, Zn(OAc)₂·2H₂O and Nd(NO₃)₃·6H₂O in absolute ethanol afforded dinuclear precursor 1 in a good yield of ca. 80%. Further reaction of 1 with the second 4,4′-bipyridyl ligand 4,4′-bpy or 4,4′-bpe in 2 : 1 ratio in DMF–EtOH resulted in the formation of tetranuclear 2 or 3, respectively.

Scheme 1 Controlled design of tetranuclear heterometallic 3d–4f complexes.

Unchanged NMR spectroscopic results (as shown in ESI,† Fig. 1s) reveals that complexes 1––3 are stable in CD₃OD solution at room temperature for up to one month. The ¹H NMR spectra of 2 or 3 show large shifts (δ from 14.22 to –6.70 ppm for 2, 14.31 to –7.87 ppm for 3) of the resonances to high fields in the neodymium compounds due to the lanthanide-induced shift. Two down-field signals (δ 14.22 and 13.25 ppm for 2; δ 14.31 and 13.27 ppm for 3) are assigned to the pyridyl protons of complexed linkers, shifted down-field relatively compared with free 4,4′-bpy (δ 7.82 and 8.69 ppm) or 4,4′-bpe (δ 7.48, 7.73 and 8.74 ppm). The signals of the coordinated ligand (δ from 11.77 to –5.36 ppm for 2 and δ from 11.77 to –5.17 ppm for 3) are significantly spread relative to those of the free H₂LSchiff base (δ from 8.42 to 3.82 ppm) and in diamagnetic 1 (δ from 11.72 to –5.07 ppm).

Crystallography

The X-ray crystal structure analysis unambiguously revealed the tetranuclear 3d–4f structures for complexes 2 and 3. Fig. 1 and 2 show the molecular structure of 2 or 3 in the asymmetric unit: two independent dimers lie about the inversion center, and one 4,4′-bipyridyl linker (4,4′-bpy for 2, 4,4′-bpe for 3) bridges the two Zn ions. Each Zn(II) ion lies in a five-coordinate environment and adopts a distorted square pyramidal geometry, where the inner N₂O₂ core from the Schiff-base H₂L ligand comprises the basal plane, and one N atom from the 4,4′-bipyridyl ligand occupies the apical position. Each Nd(III) ion is ten-coordinated, composed of the outer O₂O₂ cavity of the Schiff-base H₂L ligand and six O atoms from three bidentate nitrate anions. The average length of Zn–O (phenolic), Nd–O (phenolic, methoxy or nitrate) or Zn–N bonds (as shown in ESI,† Table 1s) is in the normal range and comparable to those of other dinuclear Zn–Nd Schiff-base complexes.^12a,g The Zn–Nd distances of each dimer in 2 and 3 are 3.544(2) and 3.581(2) Å, respectively, slightly longer than those of previous Zn–Nd Schiff-base complexes,¹² in which the apical position of Zn ions was occupied by O atoms from solvates (H₂O) or anions (OAc^– or NO₃^–), instead of N atoms from the 4,4′-bipyridyl ligands in complexes 2 and 3.

Fig. 1 Perspective drawing of compound 2; H atoms and solvates are omitted for clarity.

Fig. 2 Perspective drawing of compound 3; H atoms and solvates are omitted for clarity.

The change of 4,4′-bipyridyl linker length from 4,4′-bpy to 4,4′-bpe leads to a significant change of the conjugation extension in the two tetranuclear complexes. In 2, the Zn⋯Zn separation from bridged 4,4′-bpy is 11.221(2) Å, and as shown in ESI,† Fig. 2s, no appreciable aromatic π–π stacking interactions are present, though various complementary intermolecular C–H⋯O (C11–H11⋯O13 and C20–H20⋯O8) hydrogen bonds were generated to lead to the formation of a multi-dimensional hydrogen bonded framework. For 3, The existence of C=C of 4,4′-bpe endows extended electronic conjugation of the tetranuclear molecule system, and increases the distance of the Zn⋯Zn separation (13.295(2) Å). A detailed examination of the crystal packing of 3 revealed weak aromatic π–π stacking (3.751(5) Å), which together with complementary intermolecular C–H⋯O (C11–H11⋯O7) hydrogen bonds (as shown in ESI,† Fig. 3s), cause the close stacking of the multi-dimensional network. The solvate molecules of 2 and 3 are not bound to the framework and they exhibit no observable interactions with the host structure.

Photophysical properties

The UV-Vis absorption spectra of complexes 1––3 in MeOH solution at room temperature are shown in Fig. 3. Similar ligand-centered solution absorption spectra (204, 228, 271 and 348 nm) are observed, red-shifted upon coordination to the metal ions compared with that (226, 248 and 337 nm) of the free Schiff-base ligand H₂L. The smaller molar absorption coefficients of complex 2 in the same condition (solvent and concentration), should be attributed to the larger π-system of complex 3. At room temperature in MeOH solution, the emission spectra of complexes 1––3 are shown in Fig. 4: the typical NIR emission bands of Nd³⁺ (4F_3/2 → 4I_J/2, J = 9, 11, 13), show no shifts but obvious differences of relative intensity. The emissions at 863 nm must be assigned to the 4F_3/2 → 4I_9/2, 1060 nm to 4F_3/2 → 4I_11/2 and 1328 nm to 4F_3/2 → 4I_13/2 transition of Nd³⁺. Due to the limitation of our equipment, the excitation spectra and the NIR luminescent lifetime could not be measured. The free H₂L¹ ligand, 4,4′-bipyridyl linkers and Nd(NO₃)₃ do not exhibit NIR luminescence under the same conditions, and the ligand-centered fluorescence of complexes 1––3 in the visible region is almost quenched in solution at room temperature, which suggests that the energy transfer from the antenna (3d Zn complexes) to lanthanide ions takes place efficiently.¹³

Fig. 3 UV-Vis spectra of 1 (solid line), 2 (dotted line) and 3 (dashed line) in MeOH solution at 2 × 10^–5 M at room temperature.

Fig. 4 NIR emission spectra of 1 (solid line), 2 (dotted line) and 3 (dash-dot-dot line) in MeOH solution at 2 × 10^–5 M at room temperature.

It is noteworthy that the relative intensities of the three compound transitions are structured, both in dilute MeOH solution and in the solid state. In solution, almost identical NIR emission spectra were obtained, and the relative emission intensities of complexes 2 or 3 at 1060 nm were ca. 1.6 or 2.6 times to that of the dinuclear Zn–Nd precursor 1, and ca. 1.3 or 1.7 times to that of the pyridine adduct of 1, when compared the relative intensity using solutions with their concentrations adjusted to give the same absorbance values at 337 nm, which indicates that the emission should result from the Zn–Nd bimetallic species, and higher nuclearity species (Zn₂Nd₂) with two chromophores could effectively increase the luminescent properties of their complexes. Moreover, the route to multinuclear formation, completely avoids luminescent quenching from OH-containing bridges or solvents.^12c Especially for the superior luminescent complexes 2 and 3, the function of the linkers at the axial position of Zn ions endows the adjustment of whole conjugation extension to some extent, which should be due to the larger size of spectral overlap¹⁴ between the same broad ligand-centered emission spectra (donor) and the Nd-centered narrow line absorption spectrum (acceptor), and may be a new way to make better use of fine-tuning sensitizers to lanthanide ions. With respect to their luminescent properties in the solid state, similar NIR emission spectra are observed, and well resemble their characteristics in dilute solution. Differences of relative intensities of complexes 1––3 in the solid state from those in solution, further demonstrates that the directional close stacking of crystal molecules has a significant effect on their luminescence besides high nuclearity.

Conclusion

In conclusion, suitable choice of 4,4′-bipyridyl linkers replacing axial bound H₂O solvates of 3d Zn²⁺ ions from the dinuclear precursor could construct tetranuclear heterometallic Zn–Nd complexes, with extended molecular conjugation, and more chromophores, leading to better luminescent properties than those of dinuclear complexes. The controlling of function from the linkage shows a potentially effective way to the fine-tuning of luminescent properties in 3d–4f Schiff-base coordination complexes in the future.

Experimental

All chemicals were commercial products of reagent grade and were used without further purification. Elemental analyses were performed on a Perkin-Elmer 240C elemental analyzer. Infrared spectra were recorded on a Nicolet Nagna-IR 550 spectrophotometer in the region 4000–400 cm^–1 using KBr pellets. ¹H NMR spectra were recorded on a JEOL EX270 spectrometer in CD₃OD at room temperature. Electronic absorption spectra in the UV/Vis region were recorded with a Hewlett Packard 8453 UV/Vis spectrophotometer, and steady-state visible fluorescence on a pico-N2 laser system (PLT Time Master) with λ_ex = 337 nm. NIR emission was detected by a liquid-nitrogen cooled INSb IR detector (EG & G) with a preamplifier and recorded by a lock-in amplifier system. The third harmonic, 355 nm line of a Nd-YAG laser (Quantel Brilliant B), was used as the excitation light source.

Syntheses

H₂L: N,N′-bis(3-methoxysalicylidene)ethylene-1,2-diamine. Schiff base H₂L was obtained from o-vanilin (6.3 g, 40 mmol) and 1,2-diaminoethane (1.4 ml, 20 mmol) according to a well-established procedure from the literature.¹⁵ Yield: 5.0 g, 76%. Calc. for C₁₈H₂₀O₄N₂: C, 65.84; H, 6.14; N, 8.53%. Found: C, 65.58; H, 6.06; N, 8.63%. ¹H NMR (270 MHz, CD₃OD): δ (ppm) 8.42 (2H), 6.91 (4H), 6.71 (2H), 3.96 (4H), 3.82 (6H).

[ZnNdL(H₂O)(NO₃)₃] (1). To a stirred suspension of H₂L (0.033 g, 0.10 mmol) in absolute ethanol (5 ml), Zn(OAc)₂·2H₂O (0.022 g, 0.10 mmol) was added and heated under reflux for 5 h. A solution of Nd(NO₃)₃·6H₂O (0.044 g, 0.10 mmol) in absolute ethanol (2 ml) was then added and the mixture was refluxed for another 3 h. The clear yellow solution was then cooled to room temperature and volatile materials were removed under reduced pressure to give the product in ca. 77% yield. Calc. for C₁₈H₂₀O₁₄N₅ZnNd: C, 29.22; H, 2.72; N, 9.46%. Found: C, 28.38; H, 2.66; N, 9.33%. ¹H NMR (270 MHz, CD₃OD): δ (ppm) 11.72 (1H), 9.41 (1H), 8.35 (1H), 8.02 (1H), 7.91 (1H), 7.70 (1H), 3.85 (1H), 3.64 (1H), 2.99 (1H), 2.85 (1H), 1.15 (3H), 0.29 (1H), –3.45 (1H), –5.17 (3H). IR (KBr, cm^–1): 3422 (br), 2933 (w), 1635 (s), 1607 (m), 1457 (vs), 1386 (m), 1344 (s), 1281 (vs), 1220 (m), 1074 (m), 1030 (m), 966 (m), 855 (w), 785 (w), 737 (m), 639 (w).

[Zn₂L₂Nd₂(4,4′-bpy)(NO₃)₆]·Et₂O (2). To a solution of 1 (74 mg, 0.10 mmol) in absolute ethanol (4 ml) was added 4,4′-bpy (7.8 mg, 0.05 mmol) and the mixture was refluxed for 2 h. Then 2 ml absolute DMF was added to give a clear yellow solution. Diethyl ether was allowed to diffuse slowly into this solution at room temperature and pale yellow single crystals were obtained in three weeks. Yield: 46 mg (55%). Calc. for C₅₀H₄₉N₁₂O₂₇Zn₂Nd₂: C, 35.98; H, 2.96; N, 10.07%. Found: C, 38.13; H, 3.05; N, 9.96%. ¹H NMR (270 MHz, CD₃OD): δ (ppm) 14.22 (1H), 13.25 (1H), 11.77 (1H), 10.60 (1H), 8.91 (1H), 8.23 (1H), 8.01 (1H), 7.96 (1H), 7.90 (1H), 4.64 (1H), 3.61 (1H), 3.35 (1H) 2.99 (1H), 2.85 (1H), 2.71 (1H), 1.19 (3H), 0.20 (1H), –0.42 (1H), –1.40 (1H), –3.56 (1H), –5.36 (3H), –6.70 (1H). IR (KBr, cm^–1): 3435 (w), 2939 (w), 1638 (s), 1610 (m), 1461 (vs), 1392 (m), 1318 (s), 1277 (vs), 1218 (s), 1074 (m), 1030 (m), 958 (w), 849 (w), 820 (w), 737 (m), 639 (w).

[Zn₂L₂Nd₂(4,4′-bpe)(NO₃)₆]·2H₂O (3). To a solution of 1 (74 mg, 0.10 mmol) in absolute ethanol (4 ml) was added 4,4′-bpe (9.2 mg, 0.05 mmol) and the mixture was refluxed for 2 h. Then 2 ml absolute DMF was added to give a clear yellow solution. Diethyl ether was allowed to diffuse slowly into this solution at room temperature and pale yellow single crystals were obtained in two weeks. Yield: 51 mg (61%). Calc. for C₄₈H₅₀N₁₂O₂₈Zn₂Nd₂: C, 34.68; H, 3.03; N, 10.11%. Found: C, 34.76; H, 3.07; N, 10.24%. ¹H NMR (270 MHz, CD₃OD): δ (ppm) 14.31 (1H), 13.27 (1H), 11.77 (1H), 8.65 (1H), 8.26 (1H), 8.01 (1H), 7.94 (1H), 7.68 (1H), 7.53 (1H), 5.20 (1H), 4.78 (1H), 3.65 (1H), 3.54 (1H), 3.11 (1H), 3.01 (1H), 2.79 (1H), 1.28 (3H), 0.31 (1H), –0.50 (1H), –2.69 (1H), –3.46 (1H), –5.17 (3H), –6.71 (1H), –7.87 (1H). IR (KBr, cm^–1): 3463 (br), 2936 (w), 1666 (s), 1616 (vs), 1582 (m), 1550 (m), 1458 (vs), 1389 (s), 1303 (vs), 1232 (s), 1194 (s), 1096 (w), 1073 (w), 1082 (w), 965 (m), 846 (w), 734 (m), 510 (w).

Crystal structure determination. Single crystals of 2 and 3 of suitable dimensions were mounted onto thin glass fibers. All the intensity data were collected at 298 K on a Bruker SMART CCD diffractometer (Mo-Kα radiation, λ = 0.71073 Å) in Φ and ω scan modes. Structures were solved by Patterson methods followed by difference Fourier syntheses, and then refined by full-matrix least-squares techniques against F² using SHELXTL.^16a All other non-hydrogen atoms were refined with anisotropic thermal parameters. Absorption corrections were applied using SADABS.^16b All hydrogen atoms were placed in calculated positions and refined isotropically using a riding model. Crystallographic data and refinement parameters for the complexes are presented in Table 1. Relevant atomic distances and bond angles are collected in ESI,† Table 1s.

Table 1 Crystal data and refinement for complexes 2 and 3

Compound	2	3
Empirical formula	C50H49N12Nd2O27Zn2	C48H50N12Nd2O28Zn2
M	1669.23	1662.22
Crystal system	Triclinic	Triclinic
Space group	P1̄ (no. 2)	P1̄ (no. 2)
a/Å	9.5651(5)	9.1671(14)
b/Å	10.5104(6)	10.4152(16)
c/Å	16.2462(9)	16.667(3)
α/°	77.6550(10)	92.882(3)
β/°	80.6800(10)	100.841(3)
γ/°	74.2530(10)	104.315(3)
V/Å^3	1527.92(15)	1506.5(4)
Z	1	1
D c/g cm^–3	1.814	1.832
Crystal size/mm	0.31 × 0.25 × 0.21	0.35 × 0.31 × 0.23
µ(Mo-Kα)/mm^–1	2.542	2.579
T/K	293(2)	293(2)
Data/restraints/parameters	6658/0/422	6121/0/415
Quality-of-fit indicator	1.028	0.948
Final R indices [I > 2σ(I)]	R 1 = 0.0256	R 1 = 0.0595
	wR 2 = 0.0680	wR 2 = 0.1322
R indices (all data)	R 1 = 0.0281	R 1 = 0.1145
	wR 2 = 0.0690	wR 2 = 0.1600

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CCDC reference number 652822 (for 2) and 652823 (for 3).

For crystallographic data in CIF or other electronic format see DOI: 10.1039/b710204f

Acknowledgements

We thank Hong Kong Grants Council and Natural Science Foundation (2007B03) of Shaanxi Province for financial support.

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Footnote

† Electronic supplementary information (ESI) available: Table 1s: Interatomic distances (Å) and bond angles (°) with esds for complexes 2 and 3. Fig. 1s: ¹H NMR spectra of complexes 1–3 in CD₃OD at room temperature. Fig. 2s: View of the stacking of 2. Fig. 3s: View of the stacking of 3. See DOI: 10.1039/b710204f

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