Isostructural lanthanide oxalatophosphonates Ln(5pm8hqH₃)(C₂O₄)_1.5(H₂O)·2H₂O [Ln(III) = Eu, Gd, Tb, Dy] (5pm8hqH₃ = 5-phosphonomethyl-8-hydroxyquinoline): structures, magnetic and fluorescent properties

Abstract

Isostructural lanthanide oxalatophosphonates Ln(5pm8hqH₃)(C₂O₄)_1.5(H₂O)·2H₂O [Ln(III) = Eu(1), Gd(2), Tb(3), Dy(4)] have been obtained through hydrothermal reactions of 5-phosphonomethyl-8-hydroxyquinoline (5pm8hqH₃), oxalate, and Ln(NO₃)₃·6H₂O. Four compounds reveal layer structures, in which {LnO₈} polyhedra are linked through oxalate units into a netlike {Ln(C₂O₄)_1.5}_n layer containing 24-member ring composed of six Ln(III) ions and six oxalate anions. The noncoordinated 8-hydroxyquinoline groups protrude from two sides of the layer. Neighboring layers are further connected to each other through hydrogen bonds and aromatic stacking of the 8-hydroxyquinoline rings, forming a supramolecular 3D structure. In magnetic measurements, compound 2 shows weak antiferromagnetic interactions. The magnetic behaviors of 1, 3 and 4 are ascribed to the depopulation of the Stark levels and weak antiferromagnetic interactions. In the solid-state fluorescent measurements, strong ligand-centered emissions and weak Ln(III)-centered emissions can be observed for 1 and 4, while only ligand-centered emissions for 2 and 3.

---

Introduction

In recent years, considerable effort has been focused on lanthanide phosphonates due to their structural diversity and interesting luminescent¹ and magnetic properties.² In the preparation of lanthanide phosphonates, the introduction of oxalate is one of the effective approaches because of three main advantages of oxalate: (i) improving the crystallization of the products^1b,3 (ii) mediating electronic effects between paramagnetic metal ions, and (iii) linking metal centers into various structures with dimensionalities ranging from zero to three.⁴ On the other hand, lanthanide phosphonates with structural diversity and related physical and chemical properties could be obtained by modifying phosphonates with different functional organic groups, such as crown ether,⁵ carboxylate,^2c,6 amino,^2c,7 and pyridyl,^7b,8etc. 8-Hydroxyquinoline and its derivatives are versatile coordination ligands towards a wide range of metal ions, including Ln(III) ions, resulting in some materials with promising photophysical properties.^9,10 The incorporation of the 8-hydroxyquinoline group into a phosphonate ligand could not only provide additional coordination sites for the metal ions but also influence the packing of the structures through weak interactions such as aromatic stacking and hydrogen-bond interactions. So far, only one case of phosphonate ligand containing 8-hydroxyquinoline group has been reported, i.e. 8-quinolinyloxymethylphosphonic acid, synthesized by phosphonomethylation of 8-hydroxyquinoline at the hydroxyl position.¹¹ Based on the corresponding diethyl-8-quinolinyloxymethylphosphonate, zinc compound with chain structure was obtained. In order to obtained more lanthanide phosphonates with interesting structures and properties, we synthesized 5-phosphonomethyl-8-hydroxyquinolin (5pm8hqH₃) (Scheme 1a). Based on 5pm8hqH₃ and the second ligand of oxalate, isostructural lanthanide oxalatophosphonates Ln(5pm8hqH₃)(C₂O₄)_1.5(H₂O)·2H₂O [Ln(III) = Eu(1), Gd(2), Tb(3), Dy(4)] have been prepared. Their structures, magnetic and fluorescent properties were studied.

Scheme 1

Experimental

Materials and physical measurements

The 5-chloromethyl-8-hydroxyquinol hydrochloride was prepared according the literature.¹² All the other starting materials were of reagent grade and were obtained from commercial sources without further purification. Elemental analyses were performed on a Perkin Elmer 240C elemental analyzer. IR spectra were obtained as KBr disks on a VECTOR 22 spectrometer. Thermal analyses were performed under N₂ gas flow using a Mettler-Toloedo TGA/DSC instrument. The heating rate was 10 °C min⁻¹ from room temperature to 800 °C. The powder XRD patterns were recorded on a Shimadzu XD-3A X-ray diffractometer. Magnetic susceptibility data were obtained on microcrystalline samples (6.01 mg for 1, 3.88 mg for 2, 3.96 mg for 3, and 4.59 mg for 4), using a Quantum Design MPMS-XL7 SQUID magnetometer. Diamagnetic corrections were made for both the sample holder and the compound estimated from Pascal's constants.¹³ The luminescent spectra were measured at room temperature on a Perkin Elmer LS55 fluorescence spectrometer. The luminescent lifetimes were measured at room temperature on an Edinburgh FL-FS920 fluorescence spectrometer.

Synthesis of 5-phosphonomethyl-8-hydroxyquinoline hydrochloride (5pm8hqH₃·HCl·H₂O)

5-Chloromethyl-8-hydroxyquinol hydrochloride (10 g, 40.8 mmol) was dissolved in 200 mL of deionized water, and saturated Na₂CO₃ aqueous solution was added under stirring until the pH was near to 7.0. The ashen solid was collected, washed with water, and dried in air. The mixture of ashen solid and P(OEt)₃ (68 g, 408 mmol) was heated at 160 °C for 32 h. After removing unreacted P(OEt)₃ by rotary evaporation, the resultant brown oil was mixed with 6M HCl (100 mL). The mixture was refluxed for 12 h, and then evaporated until yellow solid just appears. After cooling, yellow needlelike crystals were collected, washed with ethanol, and dried in air. Yield: 8.7 g (72.6%). Anal. found (calcd) for C₁₀H₁₃O₅NPCl: C, 41.52 (40.90); H, 4.55 (4.46); N, 4.51 (4.77). IR (KBr, cm⁻¹): 3410(s), 2950(s, br), 2665(m), 2098(w), 1628(m), 1597(s), 1553(s), 1508(w), 1472(w), 1421(w), 1390(s), 1306(s), 1250(m), 1203(w), 1158(w), 1088(s), 990(s), 923(s), 849(w), 829(m), 818(s), 779(w), 733(w), 703(w), 667(w), 637(w), 547(w), 515(w), 478(s), 415(m).

Syntheses of Ln(C₂O₄)_1.5(5pm8hqH₃)(H₂O)·2H₂O [Ln(III) = Eu(1), Gd(2), Tb(3), Dy(4)]

Compounds 1–4 were synthesized according to the same procedure. In the synthesis of 1, a mixture of Eu(NO₃)₃·6H₂O (0.10 mmol, 0.0446 g), 5pm8hqH₃·HCl·H₂O (0.14 mmol, 0.0414 g), and Na₂C₂O₄ (0.15 mmol, 0.0201 g) in 5 mL of water was kept in a Teflon-lined autoclave at 140 °C for one day. After cooling to around 90 °C, yellow blocky crystals were collected as a monophasic material based on the powder XRD patterns (Fig. S1†). Compounds 2–4 were synthesized using Gd(NO₃)₃·6H₂O, Tb(NO₃)₃·6H₂O and Dy(NO₃)₃·6H₂O, respectively, instead of Eu(NO₃)₃·6H₂O. The yields, elemental analyses, and IR data for compounds 1–4 were shown in the ESI.† Thermal analyses of compounds 1–4 revealed a weight loss of 9.6% in 46–235 °C for 1, 9.4% in 41–248 °C for 2, 9.1% in 37–260 °C for 3, and 9.3% in 47–280 °C for 4, respectively, close to the theoretical values of one coordination and two lattice water molecules (9.4% for 1, 9.3% for 2, 9.2% for 3, and 9.2% for 4 (Fig. S2†). In addition, TG curves of 1–4 indicate that the removal of coordination water molecules resulted in the structural decomposition of compounds, which was confirmed by XRD patterns.

X-ray crystallographic studies

The diffraction data of 1–4 were collected on a Bruker SMART APEX CCD diffractometer using graphite-monochromated Mo-Kα radiation (λ = 0.710 73 Å) at room temperature. A hemisphere of data was collected using a narrow-frame method with scan widths of 0.30° in ω and an exposure time of 10 s per frame. The data were integrated using the Siemens SAINT program,¹⁴ with the intensities corrected for the Lorentz factor, polarization, air absorption, and absorption due to variation in the path length through the detector faceplate. Multi-scan absorption corrections were applied. The structures were solved by direct methods and refined on F² by full matrix least squares using SHELXTL.¹⁵ All the non-hydrogen atoms were located from the Fourier maps, and were refined anisotropically. All H atoms were put in calculated positions using riding model, and were refined isotropically, with the isotropic vibration parameters related to the non-H atom to which they are bonded. The crystallographic data for 1–4 are listed in Table 1. Selected bond lengths are given in Table 2. CCDC 810422–810425 contain the supplementary crystallographic data for 1–4 for this paper.†

Table 1 Crystallographic data and refinement for 1–4

Compound	1	2	3	4
a R 1 = Σ||Fo| − |Fc||/Σ|Fo|, wR2 = [Σw(Fo^2 − Fc^2)^2/Σw(Fo^2)^2]^1/2.
Empirical formula	C13H16O13NPEu	C13H16O13NPGd	C13H16O13NPTb	C13H16O13NPDy
Fw	577.20	582.49	584.16	587.74
Crystal system	Triclinic	Triclinic	Triclinic	Triclinic
Space group	P1̄	P1̄	P1̄	P1̄
a/Å	7.864(3)	7.8439(12)	7.8354(13)	7.8235(2)
b/Å	9.964(3)	9.9333(15)	9.9100(17)	9.9093(2)
c/Å	12.842(4)	12.839(2)	12.852(2)	12.840(3)
α (°)	67.521(5)	67.578(3)	67.780(3)	67.621(1)
β (°)	89.856(6)	89.872(3)	89.721(3)	89.760(1)
γ (°)	80.711(6)	80.597(2)	80.236(3)	80.470(1)
V Å^3	915.6(5)	910.3(2)	908.5(3)	905.88(4)
Z	2	2	2	2
ρ c/g cm^−3	2.094	2.125	2.135	2.155
F(000)	566	568	570	572
Crystal size/mm	0.09 × 0.07 × 0.06	0.10 × 0.08 × 0.07	0.10 × 0.08 × 0.06	0.11 × 0.09 × 0.08
Theta range (°)	1.72–25.00°	2.25–24.99°	2.25–24.99°	2.25–25.25°
Reflections collected/unique	4581, 3154	4587, 3144	4551, 3133	4887, 3240
	(Rint = 0.0181)	(Rint = 0.0251)	(Rint = 0.0277)	(Rint = 0.0617)
GOF on F^2	1.074	1.002	1.057	1.031
R 1, wR2^a [I > 2σ(I)]	0.0317, 0.0781	0.0288, 0.0450	0.0287, 0.0506	0.0269, 0.0580
R 1, wR2^a (all data)	0.0357, 0.0793	0.0340, 0.0454	0.0336, 0.0511	0.0338, 0.0597
(Δρ)max, (Δρ)min/e Å^−3	1.335, −0.627	0.788, −0.686	1.112, −0.684	0.855, −1.623

---

Table 2 Selected bond lengths (Å) for 1–4^a

	1	2	3	4
a Symmetry codes: A: −x + 1, −y + 1, −z + 1; B: −x + 1, −y + 2, −z + 1; C: −x + 2, −y + 1, −z + 1.
Ln1–O1	2.250(4)	2.235(3)	2.223(3)	2.209(3)
Ln1–O1W	2.420(4)	2.389(3)	2.391(3)	2.372(3)
Ln1–O9	2.417(3)	2.411(3)	2.397(3)	2.385(3)
Ln1–O6A	2.420(4)	2.409(3)	2.403(3)	2.389(3)
Ln1–O10B	2.423(3)	2.403(3)	2.387(3)	2.402(3)
Ln1–O7	2.455(3)	2.429(3)	2.413(3)	2.408(3)
Ln1–O8C	2.429(3)	2.415(3)	2.404(3)	2.404(3)
Ln1–O5	2.435(4)	2.419(3)	2.416(3)	2.408(3)
P1–O1	1.501(4)	1.500(3)	1.506(3)	1.500(3)
P1–O2	1.559(4)	1.562(3)	1.570(4)	1.564(4)
P1–O3	1.504(4)	1.506(3)	1.504(4)	1.505(3)
O4–C8	1.349(6)	1.347(5)	1.350(5)	1.342(6)
O5–C11	1.242(6)	1.243(5)	1.244(6)	1.240(5)
O6–C11	1.247(6)	1.255(5)	1.259(6)	1.265(5)
O7–C12	1.249(6)	1.261(5)	1.261(6)	1.267(5)
O8–C12	1.240(6)	1.235(5)	1.236(6)	1.250(5)
O9–C13	1.246(6)	1.240(5)	1.232(6)	1.248(5)
O10–C13	1.254(6)	1.261(5)	1.271(5)	1.258(5)

---

Results and discussion

Syntheses

Compounds 1–4 were prepared through hydrothermal reactions of 5pm8hqH₃·HCl·H₂O, Na₂C₂O₄ and corresponding lanthanide nitrates at 140 °C for one day. It was found that the Ln/oxalate/ligand molar ratio could greatly affect the formation of 1–4. The molar ratio of 1/1.5/1 results in a mixture of product and some white powders, while the molar ratio of 1/1.5/1.4 leads to a mixture of product and a small amount of yellow needlelike crystals of phosphonate ligand. Finally, compounds 1–4, with the best crystal quality and relative high yield, were prepared by using the Ln/oxalate/ligand molar ratio of 1/1.5/1.4, and by filtrating product when the temperature of reaction mixture is about 90 °C to avoid the crystallization of excess phosphonate ligand in solution.

Crystal structures of 1–4

Compounds 1–4 are isostructural and crystallize in the triclinic space group P1̄. As shown in Fig. 1, the asymmetric unit of 1 contains one Eu(III) ion, one 5pm8hqH₃ ligand, 1.5 oxalate anions, one coordination and two lattice water molecules. The Eu(III) is eight-coordinated and is surrounded by six oxalate oxygen atoms (O5, O7, O9, O6A, O8C, O10B), one phosphonate oxygen (O1), and one water molecule (O1W). The Eu–O bond distances are in the range of 2.250(4)–2.455(3) Å, comparable to those in compounds [Eu₂(Hpmp)(C₂O₄)_2.5(H₂O)₃]·H₂O (H₂pmp = N-piperidinomethane-1-phosphonic acid) [2.258(4)–2.509(5) Å]¹⁶ and [Eu((HO₃PCH₂)₂NHCH₂C₆H₅)(C₂O₄)]·2H₂O [2.324(3)–2.478(3) Å].¹⁷ Each oxalate anion adopts the most commonly observed bis-bidentate coordination mode (Scheme 1b), chelating to two Eu(III) ions. The connections of oxalate units and Eu(III) ions result in a lanthanide oxalate layer of {Ln(C₂O₄)_1.5}_n in approximate ab plane (Fig. 2a). This layer contains parallelogram-like 24-member ring made up of six Ln(III) ions and six oxalate units, showing the Ln⋯Ln separation of 6.255(1)–6.272(6) Å over the oxalate unit (Fig. 2b). The similar lanthanide oxalate layer was observed in compounds [Ln₂(H₄L)(C₂O₄)₃(H₂O)]₃·2H₂O [Ln = Gd, Tb, Dy, Y, Er, Lu; H₄L = 3-pyridyl-CH₂N(CH₂PO₃H₂)₂], where the {Ln₂(C₂O₄)₃}_n layers are further interlinked by phosphonate groups through sharing Ln(III) ions to form three dimensional framework.¹⁸ In the 5pm8hqH₃ ligand, only phosphonate oxygen O1 is involved in the coordination to one Eu(III) ion (Scheme 1c). The phosphonate oxygen atom O2, hydroxyl oxygen atom O4 and pyridine nitrogen atom N1 are protonated [P1–O2 = 1.559(4) Å, C8–O4 = 1.349(6) Å], while the remaining phosphonate oxygen O3 is pendant [P1–O3 = 1.504(4) Å]. Thus, 5pm8hqH₃ molecules are hung on both sides of the {Ln(C₂O₄)_1.5}_n layer, forming a layer structure. As shown in Fig. 3a, the neighboring layers are further linked in the sequence of ⋯AAA⋯ through extensive hydrogen bonds including O2⋯O3^b [2.592(5) Å], O4⋯O7^c [2.678(5) Å], N1⋯O3^d [2.770(6) Å] (symmetry codes: b = −x + 1, −y + 1, −z + 2; c = x, y − 1, z + 1; d = −x + 1, −y, −z + 2), thus forming a three dimensional supramolecular structure (Fig. 3a). The 8-hydroxyquinoline rings from neighboring layers are parallel displaced with respect to each other, and reveal aromatic stacking interactions with a displacement angle of 27.8(1)° and a plane⋯plane distance of 3.220(1) Å (Fig. 3b, Table S1†).¹⁹

Fig. 1 The asymmetric unit of structure 1 with the atomic labeling scheme (50% probability). Lattice water O2W, O3W and all H atoms except attached to N1, O2 and O4 atoms are omitted for clarity.

Fig. 2 The lanthanide oxalate layer of {Ln(C₂O₄)_1.5}_n (a) and the parallelogram-like 24-membered ring (b) in compound 1.

Fig. 3 The packing structures with H–bond interactions (a) and aromatic stacking interactions (b) in 1.

The structures of 2–4 are analogous to 1 except that the Eu(III) in 1 is replaced by Gd(III) in 2, Tb(III) in 3, and Dy(III) in 4, respectively. The crystal cell volume decreases in the sequence 1 > 2 > 3 > 4, in agreement with the decreasing sequence of ionic radii of the corresponding Ln(III) ions. The Ln(III) ions are bridged by oxalate units to form {Ln(C₂O₄)_1.5}_n layers, where the Ln⋯Ln distances across the oxalate unit are in the range of 6.227(1)–6.248(1) Å for 2, 6.188(1)–6.222(1) Å for 3, and 6.193(1)–6.224(1) Å for 4, respectively. The 8-hydroxyquinoline groups are arranged on two sides of the {Ln(C₂O₄)_1.5}_n layers due to the coordination of phosphonate oxygen (O1) to Ln(III) with Ln1–O1 distances of 2.235(3) Å in 2, 2.223(3) in 3, and 2.209(3) in 4, respectively. These layers are connected by hydrogen-bond interactions among phosphonate oxygen atoms O2 and O3, oxalate oxygen O7, hydroxyl oxygen atom O4, and pyridine nitrogen atom N1 (Table 3), and forming similar three dimensional supramolecular structure to that of 1. The aromatic stacking interactions between parallel 8-hydroxyquinoline rings exhibit a displacement angle of 28.0(1)° and a plane⋯plane distance of 3.213(1) Å in 2, 28.2(1)° and 3.202(1) Å in 3, and 27.8(1)° and 3.218(1) Å in 4, respectively (Table S1).

Table 3 Hydrogen bonds in 1–4 (Å and °)

a Symmetry codes = x, y + 1, z. b = −x + 1, −y + 1, −z + 2. c = x, y − 1, z + 1. d = −x + 1, −y, −z + 2. e = −x + 2, −y + 1, −z + 1.
	1		2
D–H⋯A	d(D⋯A)	< (DHA)	d(D⋯A)	< (DHA)
O1W–H1WA⋯O2W^a	2.746(6)	167.2(1)	2.738(4)	166.4(1)
O2–H2A⋯O3^b	2.592(5)	165.2(1)	2.590(5)	164.7(1)
O4–H4A⋯O7^c	2.678(5)	170.9(1)	2.687(4)	171.2(1)
N1–H1A⋯O3^d	2.770(6)	152.6(1)	2.771(5)	152.5(1)
O2W–H2WD⋯O1W^e	2.981(6)	120.7(1)	2.986(5)	121.9(1)
	3		4
D–H⋯A	d(D⋯A)	< (DHA)	d(D⋯A)	< (DHA)
O1W–H1WA⋯O2W^a	2.732(5)	166.6(1)	2.730(5)	166.1(1)
O2–H2A⋯O3^b	2.583(5)	164.3(1)	2.598(5)	163.8(1)
O4–H4A⋯O7^c	2.683(4)	170.5(1)	2.680(4)	170.4(1)
N1–H1A⋯O3^d	2.791(5)	152.9(1)	2.787(5)	153.8(1)
O2W–H2WD⋯O1W^e	2.977(5)	122.6(1)	2.985(5)	122.9(1)

---

So far, most of the reported lanthanide oxalatophosphonates reveal three dimensional structures,^3,18,20 and only few cases show layer structures including [Ln₄(C₂O₄)₅(2-pmpH)₂(H₂O)₇]·5H₂O [Ln = Gd, Tb, Dy; 2-pmpH₂ = 2-pyridylmethylphosphonicacid],²¹ [Ln₃(Hpmp)₃(C₂O₄)₂(H₂O)₅](ClO₄)₂·9H₂O [Ln(III) = Gd, Eu, Tb; H₂pmp = N-piperidinomethane-1-phosphonic acid],¹⁶ and [Ln(H₃L)(C₂O₄)]·2H₂O [Ln(III) = La–Dy, Er and Y, H₄L = C₆H₅CH₂N(CH₂PO₃H₂)₂].¹⁷ These compounds reveal clearly different layer structures from title compounds. In [Ln₄(C₂O₄)₅(2-pmpH)₂(H₂O)₇]·5H₂O, the double layer structures were observed, where net-like {Ln₄(ox)₅}_n layers are connected by phosphonate oxygens. [Ln₃(Hpmp)₃(C₂O₄)₂(H₂O)₅](ClO₄)₂·9H₂O have a microporous layer, where each [Ln₆(Hpmp)₆] unit is linked to six neighboring equivalents by eight C₂O₄²⁻ ligands. In [Ln(H₃L)(C₂O₄)]·2H₂O, two {LnO₈} polyhedra and four {PO₃C} tetrahedra are interconnected into a subunit through corner-sharing, and these subunits are further bridged by the oxalate anions into a layer. Consequently, compounds 1–4 provide a kind of new layer structure in lanthanide oxalatophosphonate materials.

Magnetic and fluorescent properties

The temperature-dependent magnetic susceptibilities of 1–4 were studied in the temperature range of 1.8–300 K at the magnetic field of 1000 Oe. As shown in Fig. 4, the χ_MT value of 1 at 300 K (1.36 cm³ K mol⁻¹) is much higher than the theoretical value of 0 cm³ K mol⁻¹ for one isolated Eu(III) ion (S = 3, L = 3, ⁷F₀), because not only the ground state (⁷F₀) but also the first (⁷F₁) and even higher exited states can be populated at room temperature.¹³ Upon lowering the temperature, the χ_MT value gradually decreases to 0.013 cm³ K mol⁻¹ at 1.8 K, corresponding to a nonmagnetic ground state of ⁷F₀ for Eu(III) ions. The similar magnetic behaviors were also observed in other Eu(III) compounds.²² For compound 2, the χ_MT value at 300 K is 8.04 cm³ K mol⁻¹, close to the value of 7.88 cm³ K mol⁻¹ for one isolated Gd(III) ion (S = 7/2). Upon cooling, the χ_MT value slowly decreases to 7.86 cm³ K mol⁻¹ at 14 K, and then rapidly reaches 7.23 cm³ K mol⁻¹ at 1.8 K. This feature indicates the presence of very weak antiferromagnetic interactions between Gd(III) ions, which is further confirmed by a small Weiss constant θ = −0.36 K (Fig. S3†). For 3 and 4, the observed χ_MT values at 300 K are 12.09, and 14.60 cm³ K mol⁻¹, respectively, which are comparable to 11.82 cm³ K mol⁻¹ for one isolated Tb(III) ion (J = 6, g = 3/2), and 14.17 cm³ K mol⁻¹ for one uncoupled Dy(III) ion (J = 15/2, g = 4/3), respectively.²³ Upon cooling, the χ_MT values of both 3 and 4 remain nearly constants before 60 K, and then dramatically decrease to 4.45 and 8.74 cm³ K mol⁻¹ at 1.8 K, respectively. The decrease in χ_MT values with lowering the temperature can be attributed to the depopulation of the Stark levels together with weak antiferromagnetic interactions between the lanthanide ions.^23,24 In the whole temperature range, the magnetic data were fitted according to the Curie–Weiss law, giving Weiss constant −3.40 K for 3 and −1.41 K for 4, respectively (Fig. S4 and S5†).

Fig. 4 The χ_MT vs. T plots for 1–4.

The solid-state fluorescent properties of 1–4 and 5pm8hqH₃·HCl·H₂O were investigated at room temperature under the excitation at 409 nm for 1, 360 nm for 2 and 5pm8hqH₃·HCl·H₂O, 390 nm for 3, and 387 nm for 4. As shown in Fig. 5, compounds 1–4 show the ligand-centered (LC) fluorescence with emission band at 529 nm for 1, 524 nm for 2, 523 nm for 3, and 523 nm for 4, confirmed by the broad emission band of 5pm8hqH₃·HCl·H₂O (λ_max = 523 nm).²⁵ In contrast to 5pm8hqH₃·HCl·H₂O, 1–4 reveal enhanced fluorescence, which might be due to the aromatic stacking interactions between 8-hydroxyquinoline rings.²⁶ Exception the ligand-centered (LC) emission, the weak Ln(III)-centred emissions were found in 1 and 4, showing the characteristic band of 614 nm attributed to ⁵D₀ → ⁷F₂ for Eu(III), and 580 nm attributed to ⁴D_9/2 → ⁶H_13/2 for Dy(III), respectively.^2c,17 The fluorescent lifetimes of 1, 3 and 4 were measured. However, only the lifetimes from the ligand-centered (LC) fluorescence were obtained, 1.5 ns for 1, 1.8 ns for 3, and 1.6 ns for 4.

Fig. 5 Solid-state emission spectra for compounds 1–4 and 5pm8hqH₃·HCl·H₂O at room temperature.

Conclusions

5-Phosphonomethyl-8-hydroxyquinoline (5pm8hqH₃) and its lanthanide oxalatophosphonates Ln(5pm8hqH₃)(C₂O₄)_1.5(H₂O)·2H₂O [Ln(III) = Eu(1), Gd(2), Tb(3), Dy(4)] have been synthesized. Compounds 1–4 show isostructural layer structures, where 5pm8hqH₃ ligands are hung on both sides of the {Ln(C₂O₄)_1.5}_n layer through the coordination of one phosphonate oxygen to the Ln(III) ion. Neighboring layers are linked through hydrogen bonds and aromatic stacking of the 8-hydroxyquinoline rings into a supramolecular 3D structure. Magnetic studies indicate the presence of weak antiferromagnetic interactions in 2, and the coexistence of depopulation of the Stark levels and weak antiferromagnetic interactions in 1, 3 and 4. In solid-state fluorescent measurements, 1–4 show stronger ligand-centered emissions than that of 5pm8hqH₃·HCl·H₂O. In addition, 1 and 4 reveal the Ln(III)-centered emissions although their intensities are weak.

Acknowledgements

We thank financial support from the NSF of China (No. 90922006), the NSF of Jiangsu Province (No. BK2009009), and the NSF of China (No. 21171090).

References

1. (a) Q. Yue, J. Yang, G.-H. Li, G.-D. Li and J.-S. Chen, Inorg. Chem., 2006, 45, 4431; (b) J.-G. Mao, Coord. Chem. Rev., 2007, 251, 1493; (c) S.-S. Bao, L.-F. Ma, Y. Wang, L. Fang, C.-J. Zhu, Y.-Z. Li and L.-M. Zheng, Chem.–Eur. J., 2007, 13, 2333; (d) L. Cunha-Silva, S. Lima, D. Ananias, P. Silva, L. Mafra, L. D. Carlos, M. Pillinger, A. A. Valente, F. A. Almeida Paz and J. Rocha, J. Mater. Chem., 2009, 19, 2618; (e) Z. Amghouz, S. Garcia-Granda, J. R. Garcia, A. Clearfield and R. Valiente, Cryst. Growth Des., 2011, 11, 5289.
2. (a) Z.-Y. Du, H.-B. Xu and J.-G. Mao, Inorg. Chem., 2006, 45, 9780; (b) X. Li, Q. Liu, J. Lin, Y. Li and R. Cao, Inorg. Chem. Commun., 2009, 12, 502; (c) T.-H. Zhou, F.-Y. Yi, P.-X. Li and J.-G. Mao, Inorg. Chem., 2010, 49, 905.
3. S.-M. Ying and J.-G. Mao, Cryst. Growth Des., 2006, 964.
4. (a) C. N. R. Rao, S. Natarajan and R. Vaidhyanathan, Angew. Chem., Int. Ed., 2004, 43, 1466 , and some references therein; (b) L. Z. Zhang, W. Gu, B. Li, X. Liu and D. Z. Liao, Inorg. Chem., 2007, 46, 622.
5. H. L. Ngo and W.-B. Lin, J. Am. Chem. Soc., 2002, 124, 14298.
6. (a) D.-K. Cao, S.-Z. Hou, Y.-Z. Li and L.-M. Zheng, Cryst. Growth Des., 2009, 9, 4445; (b) J.-L. Song, F.-Y. Yi and J.-G. Mao, Cryst. Growth Des., 2009, 9, 3273.
7. (a) X.-G. Liu, K. Zhou, J. Dong, C.-J. Zhu, S.-S. Bao and L.-M. Zheng, Inorg. Chem., 2009, 48, 1901; (b) S.-F. Tang, J.-L. Song, X.-L. Li and J.-G. Mao, Cryst. Growth Des., 2007, 7, 360.
8. (a) D.-K. Cao, Y.-Z. Li, Y. Song and L.-M. Zheng, Inorg. Chem., 2005, 44, 3599; (b) J. Galezowska, R. Janicki, H. Kozlowski, A. Mondry, P. Mlynarz and L. Szyrwiel, Eur. J. Inorg. Chem., 2010, 1696.
9. (a) M. Albrecht, R. Frohlich, J.-C. G. Bunzli, A. Aebischer, F. Gumy and J. Hamacek, J. Am. Chem. Soc., 2007, 129, 14178; (b) M. Albrecht, M. Fiege and O. Osetska, Coord. Chem. Rev., 2008, 252, 812; (c) M. Albrecht, O. Osetska, J.-C. G. Bunzli, F. Gumy and R. Frohlich, Chem.–Eur. J., 2009, 15, 8791; (d) G. Bozoklu, C. Marchal, J. Pecaut, D. Imbert and M. Mazzanti, Dalton Trans., 2010, 39, 9112.
10. (a) H.-B. Xu, J. Li, L.-X. Shia and Z.-N. Chen, Dalton Trans., 2011, 40, 5549; (b) H.-B. Xu, H.-M. Wen, Z.-H. Chen, J. Li, L.-X. Shi and Z.-N. Chen, Dalton Trans., 2010, 39, 1948; (c) H.-B. Xu, X.-M. Chen, Q.-S. Zhang, L.-Y. Zhang and Zh.-N. Chen, Chem. Commun., 2009, 7318.
11. S. P. Man, D. M. Benoit, E. Buchaca, F. Esan, M. Motevalli, J. Wilson and A. Sullivan, Inorg. Chem. Commun., 2006, 45, 5328.
12. J. H. Burckhalter and R. I. Leib, J. Org. Chem., 1961, 26, 4078.
13. O. Kahn, Molecular Magnetism, VCH, New York, 1993.
14. SAINT, Program for Data Extraction and Reduction, Siemens Analytical X-ray Instruments, Madison, WI, 1994–1996.
15. (a) SHELXTL, Reference Manual, version 5.0, Siemens Industrial Automation, Analytical Instruments, Madison, WI, 1997; (b) G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008, 64, 112.
16. X. Li, T. Liu, Q. Lin and R. Cao, Cryst. Growth Des., 2010, 10, 608.
17. Y.-Y. Zhu, Z.-G. Sun, F. Tong, Z.-M. Liu, C.-Y. Huang, W.-N. Wang, C.-Q. Jiao, C.-L. Wang, C. Li and K. Chen, Dalton Trans., 2011, 40, 5584.
18. L. Liu, Z.-G. Sun, N. Zhang, Y.-Y. Zhu, Y. Zhao, X. Lu, F. Tong, W.-N. Wang and C.-Y. Huang, Cryst. Growth Des., 2010, 10, 406.
19. C. Janiak, J. Chem. Soc., Dalton Trans., 2000, 3885.
20. (a) J.-L. Song and J.-G. Mao, Chem.–Eur. J., 2005, 11, 1417; (b) Y.-L. Huang, M.-Y. Huang, T.-H. Chan, B.-C. Chang and K.-H. Lii, Chem. Mater., 2007, 19, 3232; (c) Y. Zhao, J. Li, Z.-G. Sun, J. Zhang, Y.-Y. Zhu, X. Lu, L. Liu and N. Zhang, Inorg. Chem. Commun., 2008, 11, 1057; (d) Y.-Y. Zhu, Z.-G. Sun, Y. Zhao, J. Zhang, X. Lu, N. Zhang, L. Liu and F. Tong, New J. Chem., 2009, 33, 119; (e) Y.-Y. Zhu, Z.-G. Sun, H. Chen, J. Zhang, Y. Zhao, N. Zhang, L. Liu, X. Lu, W.-N. Wang, F. Tong and L.-C. Zhang, Cryst. Growth Des., 2009, 9, 3228; (f) L. Zhang, S. Xu, Y. Zhou, X. Zheng, C. Yu, Z. Shi, S. Hassan and C. Chen, CrystEngComm, 2011, 13, 6511.
21. T.-H. Yang, D.-K. Cao, Y.-Z. Li and L.-M. Zheng, J. Solid State Chem., 2010, 183, 1159.
22. (a) Q.-Y. Liu, W.-F. Wang, Y.-L. Wang, Z.-M. Shan, M.-S. Wang and J. Tang, Inorg. Chem., 2012, 51, 2381; (b) X.-Y. Qian, J.-H. Zhang, T.-H. Zhoua and J.-G. Mao, Dalton Trans., 2012, 41, 1229; (c) M.-F. Wu, M.-S. Wang, S.-P. Guo, F.-K. Zheng, H.-F. Chen, X.-M. Jiang, G.-N. Liu, G.-C. Guo and J.-S. Huang, Cryst. Growth Des., 2011, 11, 372.
23. C. Benelli and D. Gatteschi, Chem. Rev., 2002, 102, 2369.
24. (a) J.-C. G. Bunzli and G. R. Chopin, Lanthanide Probes in Life, Chemical and Earth Sciences, Elsevier, Amsterdam, 1989; (b) J.-P. Costes and F. Nicodeme, Chem.–Eur. J., 2002, 8, 3442.
25. M. D. Allendorf, C. A. Bauer, R. K. Bhakta and R. J. T. Houk, Chem. Soc. Rev., 2009, 38, 1330.
26. S.-J. Yoon, J. H. Kim, J. W. Chung and S. Y. Park, J. Mater. Chem., 2011, 21, 18971.

---

Footnote

† Electronic supplementary information (ESI) available: The yield, elemental analyses, and IR data for 1–4, the distances and angles for aromatic stacking interactions in 1–4, X-ray crystallographic files in CIF format, XRD patterns, TG curves, and the χ_M⁻¹vs. T curves for 2–4. CCDC reference numbers 810422–810425. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2ra20111a

---