Lanthanide-based 0D and 2D molecular assemblies with the pyridazine-3,6-dicarboxylate linker

Abstract

Two distinct series of lanthanides-based coordination polymers involving the pyridazine-3,6-dicarboxylate (pzdc) ligand have been structurally characterized by means of single-crystal XRD analysis. The structure of the compounds Ln₂(H₂O)₄(pzdc)₃·(3/3.5)H₂O with Ln = Pr (1), Nd (2), Sm (3), Eu (4) is built up from dinuclear bricks [Ln₂O₈N₅(H₂O)₄] containing two μ₃-oxo bridged nine-fold coordinated lanthanide cations, connected via the pzdc ligand, which acts as tetratendate, tridentate or monodentate linkers. It results in the generation of layers of mixed pzdc-Ln₂O₈N₅(H₂O)₄ networks, intercalated by free water molecules. The second series with chemical formula Ln₂(H₂O)₆(pzdc)₃·3H₂O (Ln = Gd (5), Tb (6), Dy (7), Er (8), Lu (9)) is a molecular assembly of discrete dinuclear bricks similar to those of the previous compounds. They only differ by the number of terminal water molecules within the dimer [Ln₂O₆N₅(H₂O)₆]. Interactions viahydrogen bond occur between terminal water species attached the lanthanide and free intercalated water molecules. Europium- and terbium-based compounds 4 and 6 exhibit fluorescence emission in the visible region at room temperature.

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Introduction

Since the last decade, a large number of studies has been devoted to the synthesis of crystalline hybrid materials exhibiting extended open structures with porosity properties. In this context, the so-called metal–organic frameworks (MOF) or coordination polymers^1–6 associate inorganic building bricks connected to each other through organic spacers in order to generate multidimensional 1–3D architectures delimiting cavities or/and tunnels. Depending on the nature of the inorganic center and organic ligand, many potential industrial applications could be developed in the fields of gas adsorbers, molecular separation, catalysis, drug delivery, etc.^4,7–10 Among the different metal candidates, the hybrid lanthanide-organic compounds have paid a special attention since specific properties have been observed in the areas of magnetism,¹¹ molecular recognition,¹² luminescence,^13–15 sensors,¹⁶ catalysis^17–20etc. Moreover, due to the wide variety of coordination environments of f-block elements, numerous infinite networks have been described with diverse structural topologies.^15,21,22 In the other hand, investigations have been also focused on the functionalities and geometries of the organic ligands. For instance, the family of rigid poly-carboxylates containing benzene ring has been intensively used for the construction of such structural frameworks. Particularly, the 1,4-benzenedicarboxylate (terephthalate) linker has attracted many interests for the generation of unusual flexible networks incorporating trivalent metals (Cr,²³Fe,^24,25Al,^26,27 Ga,^27–29 In³⁰) in the MIL-53 series, for instance. This bifunctional ligand was also successfully incorporated in many coordination polymers structures with the trivalent lanthanides,^31–39 some for them exhibiting both porous and luminescence properties. At the same time, mixed O- and N-donor molecules gave the opportunity to discover new polymeric structures with higher complexity due to the fact of possible bonding via both nitrogen or carboxyl oxygen species. They include the pyridine-2,5-dicarboxylate^40–50 (one N-donor center) or pyrazine-2,5-dicarboxylate ligands,^51,52 which are derived from the terephthalate motif. The latter has two N-donor centers in the 1,4 position within the benzene-like ring. Another closely related organic derivative called pyridazine-3,6-dicarboxylate is characterized by the occurrence of two bonded nitrogen atoms, forming an azine function within the aromatic cycle. The reactivity of this azo derivative (hereafter noted pzdc) towards metals was mainly investigated with divalent cations, giving rise to the isolation of discrete mononuclear (Mn²⁺,⁵³Cu²⁺,⁵⁴Pb2 +⁵⁵) or dinuclear (Mn²⁺,^56,57Co²⁺,^57,58Ni²⁺,⁵⁸Zn2 +^57,59) complexes in molecular hydrogen-bonded assemblies.

In this contribution, we have explored the possibility of the reaction of the pyridazine-3,6-dicarboxylate molecules as a linker between trivalent lanthanide cations in order to build atomic arrangements with higher dimensionalities. Lanthanides cations are known displaying various coordination surroundings from VI to IX, establishing bonding with oxygen as well as nitrogen from the mixed functionalized organic ligands. The occurrence of high coordination state for lanthanide centers could favour the formation of multiple bonds for the generation of infinite extended polymeric networks. Here two series of phases have been synthesized in water solvent. The compounds Ln₂(H₂O)₄(pzdc)₃·(3/3.5)H₂O with Ln = Pr (1), Nd (2), Sm (3), Eu (4) exhibit a two-dimensional structure whereas the compounds Ln₂(H₂O)₆(pzdc)₃·3H₂O with Ln = Gd (5), Tb (6), Dy (7), Er (8), Lu (9) are characterized by the molecular arrangement of discrete dinuclear bricks. The paper deals with the synthesis, structure description of the different phases as well as the luminescence spectrum of the europium- and terbium-based solids.

Experimental

Synthesis

All the lanthanide nitrates or chlorides were purchased from Acros (Pr, Nd, Sm, Eu, Gd, Er) or Aldrich (Tb, Dy, Lu) and used as received without any further purification. Pyridazine-3,6-dicarboxylic acid (H₂pzdc) was obtained from the synthesis procedure described by Sueur et al.⁶⁰

Pr₂(H₂O)₄(pzdc)₃·3.5H₂O (1). 0.25 g of praseodymium nitrate pentahydrate (99.9%, 0.6 mmol) was dissolved in 25 ml deionized hot water with 0.1 g pyridazine-3,6-dicarboxylic acid (0.6 mmol) and then left at room temperature. After one week, well formed pale yellow single crystals of 1 appeared in the mother liquid and dried in air.

Nd₂(H₂O)₄(pzdc)₃·3.5H₂O (2). Pale purple single-crystals of 2 were prepared by using the method similar to 1, with neodymium nitrate hexahydrate (99.99%, 0.26 g, 0.6 mmol) instead of praseodymium nitrate pentahydrate.

Sm₂(H₂O)₄(pzdc)₃·3.5H₂O (3). Pale yellow single-crystals of 3 were prepared by using the method similar to 1, with samarium nitrate hexahydrate (99.9%, 0.27 g, 0.6 mmol) instead of praseodymium nitrate pentahydrate.

Eu₂(H₂O)₄(pzdc)₃·3H₂O (4). Pale brown single-crystals of 4 were prepared by using the method similar to 1, with europium nitrate hexahydrate (99.9%, 0.27 g, 0.6 mmol) instead of praseodymium nitrate hydrate.

Gd₂(H₂O)₆(pzdc)₃·3H₂O (5). Pale yellow single-crystals of 5 were prepared by using the method similar to 1, with gadolinium nitrate hexahydrate (99.9%, 0.27 g, 0.6 mmol) instead of praseodymium nitrate pentahydrate.

Tb₂(H₂O)₆(pzdc)₃·3H₂O (6). Pale brown single-crystals of 6 were prepared by using the method similar to 1, with terbium chloride hexahydrate (99.9%, 0.26 g, 0.7 mmol) instead of praseodymium nitrate pentahydrate.

Dy₂(H₂O)₆(pzdc)₃·3H₂O (7). Pale yellow single-crystals of 7 were prepared by using the method similar to 1, with dysprosium nitrate hydrate (99.9%, 0.27 g, 0.7 mmol) instead of praseodymium nitrate pentahydrate.

Er₂(H₂O)₆(pzdc)₃·3H₂O (8). Pink single-crystals of 8 were prepared by using the method similar to 1, with erbium nitrate pentahydrate (99.9%, 0.27 g, 0.6 mmol) instead of praseodymium nitrate pentahydrate.

Lu₂(H₂O)₆(pzdc)₃·3H₂O (9). Pale yellow single-crystals of 9 were prepared by using the method similar to 1, with lutetium nitrate hydrate (99.9%, 0.22 g, 0.6 mmol) instead of praseodymium nitrate pentahydrate.

Single-crystal X-ray diffraction

Crystals were selected under polarizing optical microscope and glued on a glass fiber for a single-crystal X-ray diffraction experiments. X-Ray intensity data were collected on a Bruker X8-APEX2 CCD area-detector diffractometer using Mo-Kα radiation (λ = 0.71073 Å) with an optical fiber as collimator. Several sets of narrow data frames (20 s per frame) were collected at different values of θ for two initial values of ϕ and ω, respectively, using 0.3° increments of ϕ or ω. Data reduction was accomplished using SAINT V7.53a.⁶¹ The substantial redundancy in data allowed a semi-empirical absorption correction (SADABS V2.10⁶²) to be applied, on the basis of multiple measurements of equivalent reflections. The structure was solved by direct methods, developed by successive difference Fourier syntheses, and refined by full-matrix least-squares on all F² data using SHELX⁶³ program suite. Hydrogen atoms of the benzene ring were included in calculated positions and allowed to ride on their parent atoms. The final refinements include anisotropic thermal parameters of all non-hydrogen atoms, except the oxygen atoms of the water molecules. The crystal data are given in Table 1.

Table 1 Crystal data and structure refinement for lanthanides pyridazine-3,6-dicarboxylates

	1	2	3	4	5	6	7	8	9
Formula	C18H21N6O19.5Pr2	C18H21N6Nd2O19.5	C18H21N6O19.5Sm2	C18H20Eu2N6O19	C18H24Gd2N6O21	C18H24N6O21Tb2	C18H24Dy2N6O21	C18H24Er2N6O21	C18H24Lu2N6O21
Formula weight	915.2	921.86	934.08	928.32	974.91	978.13	985.41	994.93	1010.35
T/K	292 (2)	296(2)	296(2)	302(2)	296(2)	293(2)	296(2)	296(2)	296(2)
Crystal type	yellow platelet	purple platelet	yellow platelet	Light brown platelet	yellow platelet	Light brown platelet	yellow platelet	pink platelet	yellow platelet
Crystal size/mm	0.45 × 0.20 × 0.16	0.25 × 0.12 × 0.03	0.27 × 0.14 × 0.03	0.08 × 0.05 × 0.01	0.16 × 0.14 × 0.04	0.38 × 0.23 × 0.09	0.48 × 0.19 × 0.10	0.37 × 0.10 × 0.07	0.37 × 0.10 × 0.07
Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic
Space group	P21/c	P21/c	P21/c	P21/c	P21/n	P21/n	P21/n	P21/n	P21/n
a/Å	10.5283(8)	10.4836(5)	10.4555(19)	10.4063(10)	11.8519(2)	11.8076(6)	11.8365(3)	11.8162(8)	11.7980(7)
b/Å	13.4817(12)	13.4043(8)	13.333(3)	13.1997(13)	19.6425(3)	19.6020(10)	19.5914(5)	19.4943(13)	19.4168(12)
c/Å	20.5006(18)	20.4238(13)	20.372(3)	20.244(2)	14.9977(3)	14.9743(7)	15.0554(4)	15.0763(10)	14.8932(10)
β/°	102.016(4)	102.163(3)	102.245(10)	102.695(5)	102.5080(10)	102.750(2)	102.6320(10)	102.854(3)	103.041(3)
V/Å^3	2846.1(4)	2805.6(3)	2775.2(9)	2712.8(5)	3408.61(10)	3380.4(3)	3406.74(15)	3385.8(4)	3323.7(4)
Z, ρcalcd/g cm^−3	4, 2.101	4, 2.147	4, 2.199	4, 2.273	4, 1.865	4, 1.887	4, 1.889	4, 1.951	4, 2.018
μ/mm^−1	3.480	3.758	4.290	4.682	3.944	4.237	4.960	5.010	5.996
θ range/°	1.82–33.73	1.83–26.14	1.84–31.00	1.86–25.13	1.73–30.54	1.74–30.51	1.73–36.28	1.99–33.32	1.75 to 30.51
Limiting indices	−16 ≤ h ≤ 16	−12 ≤ h ≤ 12	−14 ≤ h ≤ 14	−12 ≤ h ≤ 12	−16 ≤ h ≤ 16	−16 ≤ h ≤ 16	−17 ≤ h ≤ 19	−18 ≤ h ≤ 18	−16 ≤ h ≤ 16
	−21 ≤ k ≤ 20	−16 ≤ k ≤ 16	−19 ≤ k ≤ 19	−15 ≤ k ≤ 15	−28 ≤ k ≤ 28	−28 ≤ k ≤ 28	−32 ≤ k ≤ 32	−29 ≤ k ≤ 30	−26 ≤ k ≤ 27
	−32 ≤ l ≤ 31	−25 ≤ l ≤ 25	−29 ≤ l ≤ 29	−24 ≤ l ≤ 2 4	−21 ≤ l ≤ 21	−21 ≤ l ≤ 20	−25 ≤ l ≤ 25	−23 ≤ l ≤ 23	−20 ≤ l ≤ 20
Collected reflections	95 040	82 298	194 149	53 377	103 337	115 552	138 135	62 837	122 645
Unique reflections	11 292 [Rint = 0.0473]	5570 [Rint = 0.0752]	8585 [Rint = 0.1317]	4822 [Rint = 0.0480]	10 419 [Rint = 0.0628]	10 305 [Rint = 0.0447]	16 270 [Rint = 0.0632]	12 985 [Rint = 0.0530]	10 139 [Rint = 0.0410]
Parameters	415	415	415	406	433	433	433	433	434
Goodness-of-fit on F^2	1.025	1.084	1.067	0.977	1.024	1.085	1.019	1.041	1.157
Final R indices [I > 2σ(I)]	R 1 = 0.0238	R 1 = 0.0318	R 1 = 0.0322	R 1 = 0.0224	R 1 = 0.0355	R 1 = 0.0353	R 1 = 0.0352	R 1 = 0.0433	R 1 = 0.0336
	wR2 = 0.0680	wR2 = 0.0802	wR2 = 0.0778	wR2 = 0.0546	wR2 = 0.1147	wR2 = 0.1156	wR2 = 0.1176	wR2 = 0.1301	wR2 = 0.1041
R indices (all data)	R 1 = 0.0316	R 1 = 0.0493	R 1 = 0.0637	R 1 = 0.0295	R 1 = 0.0520	R 1 = 0.0486	R 1 = 0.0564	R 1 = 0.0656	R 1 = 0.0428
	wR2 = 0.0801	wR2 = 0.0965	wR2 = 0.1025	wR2 = 0.0582	wR2 = 0.1362	wR2 = 0.1359	wR2 = 0.1377	wR2 = 0.1451	wR2 = 0.1229
Largest diffraction peak and hole/e Å^−3	1.865 and −1.571	1.134 and −0.954	1.281 and −1.507	0.651 and −0.493	3.714 and −1.059	3.338 and −1.329	3.784 and −1.396	2.990 and −2.130	3.413 and −1.258

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Thermogravimetric analysis

The thermogravimetric experiments have been carried out on a thermoanalyzer TGA 92 SETARAM under dry air atmosphere with a heating rate of 1 °C min⁻¹ from room temperature up to 800 °C.

Luminescence spectroscopy

Luminescence (emission) spectra were measured on a SAFAS FLX-Xenius fluorescence spectrometer between 400 and 800 nm, equipped with a xenon lamp. Slid widths were 1 nm and scan rates of 125 nm s⁻¹ were used.

Results

Crystal structures of 1–4

The compounds Ln₂(H₂O)₄(pzdc)₃·(3/3.5)H₂O (Ln = Pr (1), Nd (2), Sm (3), Eu (4)) exhibit identical layered networks for the lightest lanthanides and are described in the monoclinic symmetry with the P2₁/c space group with a ≈ 10.46, b ≈ 13.34, c ≈ 20.36 Å, β ≈ 102.3°, V ≈ 2775 ų (Table 1). The structure contains two crystallographically independent sites for lanthanides with the same nine-fold coordination surroundings, described by distorted capped antiprism polyhedra (Fig. 1). Ln1 cation is bonded to six carboxyl oxygen atoms, two nitrogen atoms (N1B and N1C) from two distinct pyridazinedicarboxylate groups and one terminal water molecule (O1H). The Ln–O distances are ranging from 2.408(2) up to 2.741(2) Å and Ln–N distances from 2.723(2) up to 2.738(2) Å for Pr (1). Due to the lanthanide radius contraction, these bond distances decrease when the atomic number of lanthanide increases, with Ln–O distances in the range 2.346(3)–2.687(3) Å and Ln–N distances in the range 2.635(4)–2.674(4) Å in the Eu (4) compound. The second lanthanide cation Ln2 is coordinated to three carboxyl oxygen atoms, three nitrogen atoms from the three distinct pzdc linkers and three terminal aquo species. Typical distances Ln–O of 2.398(2)–2.564(2) Å and Ln–N of 2.699(2)–2.748(2) Å are observed for the Pr (1) compound and they decrease, as expected, for the heaviest lanthanides with Ln–O = 2.347(3)–2.491(3) Å and Ln–N = 2.618(4)–2.684(4) Å for the Eu (4) phase. For both cations, the existence of terminal aquo groups agrees well with the bond valence calculations⁶⁴ with values in the range 0.44–0.31 (expected value for H₂O: 0.4). The two lanthanide polyhedra Ln1O₆N₂(H₂O) and Ln2O₃N₃(H₂O)₃ are linked through an oxo group O4A, which also belongs to one of the carboxylate arm of the pzdc ligand. It results in the occurrence of discrete lanthanide-centered dinuclear motifs [Ln₂O₈N₅(H₂O)₄], bridged by μ₃-oxo groups, with Ln⋯Ln distances ranging from 4.475(5) for Pr (1) down to 4.3897(4) Å for Eu (4) (Fig. 1). The inorganic dimers are connected to each other via the three crystallographically independent pyridazinedicarboxylate ligands [O₂C–C₄H₂N₂-CO₂]²⁻ (noted A, B and C), which adopt different connection modes (Fig. 1). Two organic molecules (B and C) act as bis(bidentate) linker towards the two lanthanide cations; the two nitrogen atoms are bound to two distinct lanthanide centers and one of the carboxyl oxygen is bound to one lanthanide center. Three of the remaining carboxyl oxygen atoms of the two pzdc species are non-bonded with relatively short C–O distance (1.20–1.22 Å) resulting in a monodentate bridging mode for the carboxylate groups (configuration μ₂-η₁:η₁ for B). The fourth one is connected to another lanthanide cation (Ln1) from a distinct dimeric unit (syn–anti bidentate bridging mode for the carboxylate group: configuration μ₃-η₁:η₁:η₁ for C). This particular tetradentate coordination fashion was previously reported in other isolated molecular dinuclear complexes involving divalent transition metals such as Zn,^57,59Mn,^56,57Co,^57,58Ni.^57,58 In these compounds, the dimeric species is tetra-coordinated by three pzdc ligands, describing a trigonal prismatic molecular brick around a C₃ axis (inducing an angle of 120° between the pzdc species). In our lanthanide-based series, the two pzdc molecules have an angle of nearly 90° and three other organic ligands are attached to the dimer. One of them (noted A) is tridendate and links one lanthanide cation via one of the nitrogen atom (Ln2) and the two lanthanide centers via the carboxyl μ₃-oxo group O4A. The carboxylate arm acts as chelating bridge with Ln1 and monodentate bridge with Ln1 and Ln2. The two last other carboxylates are in a monodentate bridging mode with a lanthanide cation from an adjacent dimeric unit. The carboxylate groups of the pzdc molecule A thus adopt the following configuration μ₃-η₁:η₂:η₁. The two last pyridazine-based species (A and C) are linked to the inorganic dimer in a monodentate fashion. The monodentate bridging mode of the species A and C allows for the connection of the dimers and this generates the formation of neutral mixed organic-inorganic sheets, developing in the (a,b) plane (Fig. 2 and Fig. 3). The corrugated layers are stacked along the c axis and intercalated by free water molecules. The latter interact viahydrogen bond with the terminal water molecules attached to lanthanide cations. Other hydrogen bonds also exist between the bonded water molecules and remaining free carboxyl oxygen atoms of the pzdc ligand, ensuring the three-dimensional cohesion of the structure. One observes that the europium-based compound (4) incorporates less water with only three crystallographic sites instead of four sites (one of them has a 50% occupancy) in the other members of the 1–4 series.

Fig. 1 Views of the nine-fold coordination environments of lanthanide cations in the series 1–4 (top). Polyhedral representation of the dinuclear unit LnO₈N₅(H₂O)₃ (bottom). Both lanthanide centers have a distorted capped square antiprism surrounding and are connected to each other via a carboxyl μ₃-oxo group. The oxygen species OnH, (n = 1–4) are water molecules in terminal position. A, B and C labels indicate the three crystallographically independent pzdc linkers. Purple = lanthanide; red = oxygen; blue = nitrogen; grey = carbon. Hydrogen atoms have been omitted for clarity.

Fig. 2 View of layer in the compounds Ln₂(H₂O)₄(pzdc)₃·(3/3.5)H₂O, 1–4, along the c axis.

Fig. 3 View showing the stacking of two corrugated layers [Ln₂(H₂O)₄(pzdc)₃] intercalated by water molecules in 1–4, along the c axis.

Crystal structures of 5–9

The compounds Ln₂(H₂O)₆(pzdc)₃·3H₂O (Ln = Gd (5), Tb (6), Dy (7), Er (8), Lu (9)) are closely related to the previous 1–4 series although they crystallize in a monoclinic cell (P2₁/n) with distinct parameters a ≈ 11.82, b ≈ 19.52, c ≈ 14.95 Å, β ≈ 102.8°, V ≈ 3365 ų (Table 1). The lanthanide cations lie on two crystallographically independent sites, coordinated to nine ligands, describing distorted capped antiprism polyhedra (Fig. 4). Ln1 is surrounded by three carboxyl atoms, three nitrogen atoms from the three distinct pzdc molecules and three water molecules in terminal position. For the Gd-based compound (5), the Gd–O distances are ranging from 2.322(3) up to 2.456(4) Å and Gd–N distances from 2.666(4) up to 2.707(4) Å in a GdO₃N₃(H₂O)₃ polyhedron. As expected with the heaviest lanthanides, the Ln–O and Ln–N distances decrease, with Lu–O = 2.240(3)–2.362(4) Å and Lu–N = 2.585(4)–2.668(4) Å in (9). The nine-fold coordinated Ln2 cation involves four carboxyl oxygen atoms, two nitrogen atoms and three terminal water molecules. The Gd–O distances are in the range 2.354(4)–2.595(4) Å and Gd–N equal to 2.662(4) Å for a GdO₄N₂(H₂O)₃ polyhedron in (5). With the radius contraction of lanthanides, shorter bond distances are observed: Lu–O = 2.265(4)–2.538(4) Å and Lu–N = 2.566(4)–2.565(4) Å in (9). The presence of terminal aquo species is well confirmed from bond valence⁶⁴ considerations. The connection of the lanthanide cations by a μ₃-oxo group (O1C) is identical to that found in compounds 1–4. That generates a dinuclear unit [Ln₂O₆N₅(H₂O)₆], which is linked to three crystallographically independent pyridazinedicarboxylate ligands (Fig. 4). The same connection mode of the organic occurs in the compounds 5–9, except the absence of two additional monodentate pzdc ligands. Two of them (noted A and B) are tetra-coordinated to the lanthanides through the nitrogen and carboxyl atoms. The remaining non-bonded carboxyl oxygen atoms are free and short terminal C–O distances (1.20–1.25 Å) reflect their non-protonated character. The third pzdc molecule (noted C) is linked to Ln2 in chelating bridging mode and also connects Ln1via the carboxyl μ₃-oxo species and one nitrogen atom as in compounds 1–4. The second carboxylate arm is not bonded and exists in its negatively charged form R–CO₂⁻. The relatively short C–O bonding (1.22–1.25 Å) agree well with this configuration. A larger C–O distance (≈ 1.29 Å) would be expected in the case of protonated form of the carboxylate.⁶⁵ The presence of this negative non-bonded carboxylate balances the charge of the dimer and ensures the electroneutrality of the dimeric brick. The structure is built up from the molecular assembly of isolated mixed organic-inorganic dinuclear units (Fig. 5) intercalated by free water molecules. The building blocks interact to each other through a hydrogen bond network involving terminal aquo species and free water (O7H⋯O1H, O8H⋯O2H, O9H⋯O6H, O9H⋯O2H, O10H⋯O5H), terminal carboxyl oxygen atoms (O8H⋯O4A, O9H⋯O3C, O9H⋯O2B, O10H⋯O1B) and free water molecules themselves (O7H⋯O10H, O8H⋯O10H). Beside the presence of free water, PLATON⁶⁶ calculations also indicate the positions (around 0 ½ 0 and ½ 0 ½) of small unoccupied cavities (72 Å).

Fig. 4 View of the nine-fold coordination environments of lanthanide cations in the series 5–9 (top). Polyhedral representation of the discrete dinuclear unit LnO₆N₅(H₂O)₆ (bottom). Both lanthanide centers have a distorted capped square antiprism surrounding and are connected to each other via a carboxyl μ₃-oxo group. The oxygen species OnH, (n = 1–6) are water molecules in terminal position. A, B and C labels indicate the three crystallographically independent pzdc linkers. Purple = lanthanide; red = oxygen; blue = nitrogen; grey = carbon. Hydrogen atoms have been omitted for clarity.

Fig. 5 View of the molecular assembly of discrete dinuclear units [Ln₂O₆N₅(H₂O)₆] along the b axis, in the compounds series 5–9, Ln₂(H₂O)₆(pzdc)₃·3H₂O.

Thermal analysis

Only the two compounds 3 and 8 belonging to distinct series 1–4 or 5–9 have been selected for the thermogravimetric and differential thermogravimetric analyses. Data of the europium-based 4 compound are given in the ESI.†

The TG curves (Fig. 6) show two main events of weight losses. For the samarium-based compound (3), the first step is assigned to the departure of free water together with the terminal bonded water, with a total weight loss of 15.4% at 180 °C. It is also correlated to endothermic peaks at 80 °C (free water) and 110 & 165 °C (bonded water) respectively. This corresponds to a total water content of 8 H₂O (calcd: 15.2%) per Sm₂ unit, or 4 H₂O (coordinated to Sm) and 4 H₂O (free). The amount the intercalated water species is slightly higher than the value observed from the single-crystal analysis (3.5 H₂O) and the difference may come from additional water adsorbed on the surface. It is also observed on the TG curve (ESI)† of the europium-based compound a smaller water weight loss with a value of 13.8%, corresponding to a total amount of water of 7.2 H₂O, or 4 H₂O (coordinated to Eu) and 3.2 H₂O (free). This is in agreement with the X-ray diffraction analysis which indicates a lower water content. The second event occurs from 250 up to 600 °C and is attributed to the thermal decomposition the organic linker, with a series of large exothermic peaks at 335, 355, 405 and 495 °C (obsd: 48.3%; calcd: 48.1%). The final yellowish residue is samarium oxide (confirmed by powder X-ray diffraction) with a final weight of 36.3% (calcd: 36.7%).

Fig. 6 TG and DTG curves of Sm₂(H₂O)₄(pzdc)₃·3.5H₂O (3) (top) and Er₂(H₂O)₆(pzdc)₃·3H₂O (8) (bottom) under air (heating rate: 1 °C min⁻¹).

The TG curve of the erbium-based (8) phase is shown in Fig. 6. A first weight loss (16.1% or 8.9 H₂O for one Er₃ unit; expected 9 H₂O) up to 150 °C is assigned to the departure of total content of water molecules (free and bonded) associated with an endothermic peaks at 85 °C. The second event is attributed to the thermal decomposition of the organic ligand with weight loss of 45.1% (calcd: 45.6%). Two exothermic peaks are visible at 390 and 420 °C. At 700 °C, the final residue is erbium oxide (Er₂O₃). It corresponds to a remaining weight of 38.8% (calcd: 38.2%).

Luminescence properties

The solid-state luminescent properties were investigated at room temperature for the europium- and terbium-based compounds (4) and (6).

Under excitation at 394 nm, the europium-based phase (4) exhibits a red luminescence with five groups of signals, which are typical for the Eu³⁺ cation (Fig. 7). They are located at 579, 592, 617, 649, 694 nm and are assigned to the five ⁵D₀ → ⁷F_J transitions (J = 0, 1, 2, 3, 4, respectively).⁶⁷ The ⁵D₀ → ⁷F₂ (as well as ⁵D₀ → ⁷F₄) transition is predominantly due to the electric dipole character, depending on the nature of bonded ligands around the Eu³⁺ cation whereas the ⁵D₀ → ⁷F₁ one is mainly sensitive to the magnetic dipole effect, reflecting crystal field environment. The strongest emissions are visible for the ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ transitions, with an intensity ratio of 2. This indicates that europium cations lie on low symmetry sites, involving the absence of inversion center. Moreover, the observation of the weak ⁵D₀ → ⁷F₀ transition, which is induced by crystal field Jmixing, is allowed on non-inversion sites. Two different signatures are consistent with the X-ray diffraction crystal structure of the compounds 1–4, which shows that the two crystallographically independent lanthanide cations occupy the general position 4e with no symmetry operation in the space groupP2₁/c. For this preliminary experience, it was also observed some splittings for different transitions (⁵D₀ → ⁷F_1,), which could be related to the distinct Stark components due to the crystal sites multiplicity observed for Eu³⁺ in the structure.

Fig. 7 solid-state emission spectra of Eu₂(H₂O)₄(pzdc)₃·3H₂O (4) (excitation at 394 nm) Tb₂(H₂O)₆(pzdc)₃·3H₂O (6) (excitation at 292 nm) at room temperature.

Under excitation at 292 nm, the terbium-based compound (6) emits a green luminescence with peaks located at 488, 545, 584 and 620 nm (Fig. 7). These signals typically correspond to the transitions ⁵D₄ → ⁷F₆, ⁵D₄ → ⁷F₅, ⁵D₄ → ⁷F₄ and ⁵D₄ → ⁷F₃, respectively.⁶⁷ As expected, the intensity of the ⁵D₄ → ⁷F₄ transition is the strongest one⁶⁸ and is sensitive to the nature of the atoms surrounding the rare-earth cation.

Conclusion

To conclude, two types of coordination polymers based on pyridazine-3,6-dicarboxylate linkers have been successfully synthesized for the first time with trivalent metals such as lanthanides. Depending on the size of lanthanide cations, two series of compounds based on the similar dinuclear building brick of two μ₃-oxo-bridged capped antiprisms have been isolated. The first ones (1–4), with the lightest lanthanides (Pr, Nd, Sm, Eu), consists of the connection of the dimeric units to each other through the pzdc ligands, generating a two-dimensional network. Free water molecules are intercalated between the hybrid organic-inorganic sheets. The second series 5–9 with the heaviest lanthanides (Gd, Tb, Dy, Er, Lu) contains the closely related dinuclear unit, coordinated by three pzdc species, which are isolated to each other by water molecules. The structural differences of the two molecular assemblies lie on the distinct number of water molecules in the coordination sphere of the metallic cations, correlated to the number of the pzdc species surrounding them. In series 1–4, five organic pzdc moieties are connected through a tetradentate, tridentate and monodentate connection scheme. In the series 5–9, two of the pzdc moieties are replaced by terminal water molecules, which prevent any further connection of the dimers to each other. It results in a 0D molecular arrangement of the isolated dinuclear units. From these observations, the lanthanides size seems to have a significant influence for the connection mode of the building blocks since for an identical nine-fold coordination, the number of water molecules attached to the cations varies drastically from the lightest to the heaviest lanthanides. It is also interesting to notice that with the multi-functionalized pzdc ligand, dimensionalities of frameworks higher than 0D can be generated in case of trivalent lanthanides. This structural feature has not been previously reported with the divalent metals.^53–59

Acknowledgements

The authors would like to thank the GNR MATINEX of PACEN interdisciplinary program and the French ANR project no. ANR-08-BLAN-0216-01 for financial support. We also thank the technical help of Dr Pascal Roussel and Dr Frédéric Capet for the single-crystal X-ray diffraction intensities collection of 4 by means of a Bruker X8 apparatus equipped with an X-ray microsource.

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Footnote

† Electronic supplementary information (ESI) available: TGA data. CCDC reference numbers 770963–770971. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0ce00013b

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