Photoluminescent and Slow Magnetic Relaxation Studies on

Article pubs.acs.org/IC

Photoluminescent and Slow Magnetic Relaxation Studies on Lanthanide(III)-2,5-pyrazinedicarboxylate Frameworks Maria Vanda Marinho,*,† Daniella O. Reis,‡ Willian X. C. Oliveira,‡ Lippy F. Marques,§ Humberto O. Stumpf,*,‡ Mariadel Déniz,∥ Jorge Pasán,∥,⊥ Catalina Ruiz-Pérez,∥ Joan Cano,⊥,# Francesc Lloret,⊥ and Miguel Julve⊥ †

Instituto de Química, Universidade Federal de Alfenas, Campus Sede, Alfenas, MG 37130-000, Brazil Departamento de Química, Universidade Federal de Minas Gerais, Av Antônio Carlos, 6627, Belo Horizonte, MG 31270-901, Brazil § Grupo de Materiais Inorgânicos Multifuncionais, Instituto de Química, Universidade do Estado do Rio de Janeiro, Rio de Janeiro, RJ 20550-900, Brazil ∥ Laboratorio de Rayos X y Materiales Moleculares, Departamento de Física, Facultad de Ciencias (Sección Física), Universidad de La Laguna, Edifício de Física y Matemáticas, Apdo. 456, E-38200 La Laguna, Tenerife, Spain ⊥ Instituto de Ciencia Molecular−Departament de Química Inorgànica, Universitat de València, C/Catedrático José Beltrán 2, 46980 Paterna, València, Spain # Fundació General de la Universitat de València, Universitat de València, 46010 València, Spain ‡

S Supporting Information *

ABSTRACT: In the series described in this work, the hydrothermal synthesis led to oxidation of the 5-methyl-pyrazinecarboxylate anion to the 2,5pyrazinedicarboxylate dianion (2,5-pzdc) allowing the preparation of threedimensional (3D) lanthanide(III) organic frameworks of formula {[Ln2(2,5pzdc)3(H2O)4]·6H2O}n [Ln = Ce (1), Pr (2), Nd (3), and Eu (4)] and {[Er2(2,5-pzdc)3(H2O)4]·5H2O}n (5). Single-crystal X-ray diffraction on 1−5 reveals that they crystallize in the triclinic system, P1̅ space group with the series 1−4 being isostructural. The crystal structure of the five compounds are 3D with the lanthanide(III) ions linked through 2,5-pzdc2− dianions acting as two- and fourfold connectors, building a binodal 4,4-connected (4·648)(426282)-mog network. The photophysical properties of the Nd(III) (3) and Eu(III) (4) complexes exhibit sensitized photoluminescence in the nearinfrared and visible regions, respectively. The photoluminescence intensity and lifetime of 4 were very sensitive due to the luminescence quenching of the 5D0 level by O−H oscillators of four water molecules in the first coordination sphere leading to a quantum efficiency of 11%. Variable-temperature magnetic susceptibility measurements for 1−5 reveal behaviors as expected for the ground terms of the magnetically isolated rare-earth ions [2F5/2, 2H4, 4I9/2, 7F0, and 4I15/2 for Ce(III), Pr(III), Nd(III), Eu(III), and Er(III), respectively] with MJ = 0 (2 and 4) and ±1/2 (1, 3, and 5). Q-band electron paramagnetic resonance measurements at low temperature corroborate these facts. Frequency-dependent alternating-current magnetic susceptibility signals under external direct-current fields in the range of 100−2500 G were observed for the Kramers ions of 1, 3, and 5, indicating slow magnetic relaxation (single-ion magnet) behavior. In these compounds, τ−1 decreases with decreasing temperature at any magnetic field, but no Arrhenius law can simulate such a dependence in all the temperature range. This dependence can be reproduced by the contributions of direct and Raman processes, the Raman exponent (n) reaching the expected value (n = 9) for a Kramers system.

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INTRODUCTION

can be achieved by the high coordination number of the rareearth ions.7−17 In this context, the spectroscopy of these ions differs considerably from that of the d-shell transition metal ions. The shielding of the 4f orbitals by filled 5d66s2 subshells in the lanthanide elements results in special optical features with characteristic narrow lines, such as optical pure color emissions that are nearly unaffected by the ligand field.18,19 These

The crystal engineering on inorganic compounds has become a research field of intense activity as a consequence of the growth in the synthesis of novel functional metal−organic assemblies (MOAs). The dimensionality of MOAs varies from discrete metal−organic polyhedral (MOPs) to three-dimensional (3D) metal−organic frameworks (MOFs).1−6 Restricting ourselves to the lanthanide-organic frameworks (LnOFs), an increasing attraction in crystal engineering has been observed, owing to their potential applications and topological architectures, which © 2017 American Chemical Society

Received: November 18, 2016 Published: February 3, 2017 2108

DOI: 10.1021/acs.inorgchem.6b02774 Inorg. Chem. 2017, 56, 2108−2123

Article

Inorganic Chemistry

being well-known that unexpected results can occur when hydro(solvo)thermal syntheses are employed (see some selected examples listed in Table S1).8,40−49 For instance, the 2,5-pyrazinedicarboxylate (dianion of 2,5-H2pzdc) was obtained in situ under hydrothermal conditions through the oxidation of the methyl group of the initial Hmpca reagent for the series of complexes [Ln2(2,5-pzdc)3(H2O)4]·6H2O}n [Ln = Ce (1), Pr (2), Nd (3), and Eu (4)] and {[Er2(2,5-pzdc)3(H2O)4]· 5H2O}n (5), which are the subject of this work. Such an oxidation is very attractive, because two carboxylate groups as substituents in the heterocyclic pyrazine ring allow multiple coordination modes50 as shown in Chart 1b. This variety of the coordination modes of the 2,5-pzdc2− dianion makes it a suitable molecule for the straightforward design and construction of hybrid organic−inorganic materials with different structures and physical properties.1,15,44,51−54 It deserves to be noted that the oxidation to carboxylate of the methyl group of the Hmpca acid leading to the 2,5-H2pzdc acid was previously observed involving Eu2O315 in the presence of HNO3 as oxidant, and also in the hydrothermal reaction of Tm(NO3)3· 6H2O and Hmpca in water.55 Herein we report the hydrothermal synthesis, crystallographic analysis, thermal investigation, and variable-temperature magnetic study of five new 2,5-pdz-containing lanthanide compounds 1−5 together with the photoluminescent studies of two of them, namely, 3 and 4. It is important to highlight that the study of the magnetic properties of the lanthanide(III)-2,5pzdc series in this work provides new findings that contribute to establish the magneto−structural possibilities with the two coordination modes adopted by the 2,5-pzdc2− ligand. Although a work reporting the existence of antiferromagnetic interactions between the cerium(III) ions bridged by 2,5-pzdc2− ligands in the 3D compound of formula [Ce 2 (2,5pdz)3(H2O)2]n has been published,56 our magneto−structural data on 1−5 allow to substantiate the single-ion magnet (SIM) behavior observed in 1, 3, and 5. Additionally, it is important to emphasize that previous works on lanthanide organic-frameworks (LnOFs) [especially containing the Eu(III) ion] with this ligand55,57 were reported, where some discuss briefly the photophysical investigations.15,44,50 Hence, a more detailed study of the photoluminescent properties [radiative (Arad) and nonradiative (A nrad) decay rates, quantum efficiency (η), and CIE coordinates] of the Eu(III) compound with 2,5-pzdc2− ligand is included herein.

compounds are also regarded as promising molecular magnetic materials, due to the high magnetic anisotropy and strong spin−orbit coupling of the lanthanide ions, many of their complexes behaving as single-molecule magnets (SMMs).16,17 Until now, most contributions have focused primarily on the use of “nodes” (such as transition-metal ions, clusters, or preformed complexes whose coordination sphere is unsaturated) to prepare tailor-made new multifunctional materials.1,20−31 A suitable strategy is the choice of a potential ligand, preferably with conformational flexibility, diversity of binding modes, and ability to form hydrogen bonds, since MOFs can be prepared from the simple combination and self-assembly of nodes with organic ligands. Having in mind this strategy, we selected the 5-methyl-2pyrazinecarboxylate (mpca−) as the primary ligand for the coordination to the lanthanide ions due to its known chemical versatility and complex binding capabilities (see Chart 1a).32−36 Chart 1. Different Coordination Modesa of the (a) mpca− (A−C) and (b) 2,5-pzdc2− (A′−F′) Ligands

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EXPERIMENTAL SECTION

Materials and Methods. All the chemicals were reagent grade and were used as commercially obtained. Elemental analyses (C, H, N) were performed on an EA 1108 CHNS-O microanalytical analyzer of the SEGAI service of the University of La Laguna. Syntheses of the Complexes. {[Ln2(2,5-pzdc)3(H2O)4]·6H2O}n (Ln = Ce(III) (1), Pr(III) (2), Eu(III) (4), and {[Er2(2,5pzdc)3(H2O)4]·5H2O}n (5)). A mixture of Hmpca (0.069 g, 0.50 mmol) and Ce(NO3)3·6H2O (0.130 g, 0.30 mmol) in 30 mL of water was stirred for 1 h at room temperature. Then it was placed in a 45 mL stainless steel reactor with a Teflon linear autoclave under autogenous pressure at 170 °C for 72 h and slowly cooled to room temperature for 48 h. 2, 4, and 5 were obtained by following the same procedure of 1 but replacing Ce(NO3)3·6H2O by Pr(NO3)3·6H2O (0.131 g, 0.30 mmol), Eu(NO3)3·6H2O (0.134 g, 0.30 mmol), and Er(NO3)3·6H2O (0.138 g, 0.30 mmol), respectively. X-ray quality yellow needles of 1, 2, 4, and 5 were collected and used for the data collection. Yield: 0.035 (27%), 0.030 (30%), 0.040 (22%), and 0.035 g (25%) for 1, 2, 4, and

κ N,O (A), μ-κ2O:κN (B) and μ-κ2O:κO′ (C); μ-κ2N,O:κ2N′,O″ (A′), μ4-κ2N,O:κO′:κ2N′,O″:κO‴ (B′), μ-κ2N,O:κ2O″,O‴ (C′), μκO:κO″ (D′), μ-κO′:κ2O″,O‴ (E′) and μ-κ2O,O′:κ2O″,O‴ (F′).

a 2

Since the lanthanide(III) ions present high affinity for ligands with oxygen donors and the transition-metal ions have a strong tendency to coordinate to nitrogen donors, a ligand with nitrogen and oxygen as donor atoms like mpca− is an excellent candidate to get stable molecule-based architectures37−39 with both types of metal ions. The prediction of the crystal structures made via hydro(solvo)thermal in situ ligand reactions is still a difficult task, it 2109


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Inorganic Chemistry Table 1. Crystal Data and Details of Structure Determination for Compounds 1−5 compound

1

2

3

4

5

formula formula weight crystal system space group crystal size a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z T, K ρcalc (g cm−3) λ (Cu Kα Å) F(000) reflections collected unique reflections [I ≥ 2σ(I)] no. parameters/restraints goodness-of-fit R1, [I > 2σ(I)]/all Rw, [I > 2σ(I)]/all

C9H13N3CeO11 479.26 triclinic P1̅ 0.37 × 0.05 × 0.03 6.099(5) 10.501(5) 13.688(5) 72.607(5) 88.789(5) 88.980(5) 836.3(8) 2 293(2) 1.903 1.541 84 448 5004 3200 3118 226/0 1.073 0.0298/0.0305 0.0819/0.0829

C9H13N3PrO11 480.05 triclinic P1̅ 0.55 × 0.04 × 0.04 6.0820(5) 10.5091(8) 13.6280(10) 72.398(7) 88.980(6) 89.043(6) 830.08(11) 2 293(2) 1.921 1.541 84 450 5500 3227 3124 227/0 1.096 0.0372/0.0382 0.1013/0.1029

C9H13N3NdO11 483.38 triclinic P1̅ 0.71 × 0.06 × 0.04 6.0736(2) 10.4912(3) 13.5790(4) 72.133(3) 89.023(3) 89.009(3) 823.33(4) 2 293(2) 1.950 1.541 84 452 12 878 3260 3224 226/0 1.109 0.0283/0.0286 0.0790/0.0792

C9H13N3EuO11 491.10 triclinic P1̅ 0.25 × 0.08 × 0.05 6.0520(2) 10.4994(3) 13.4547(4) 71.630(3) 89.254(3) 89.101(3) 811.25(4) 2 293(2) 2.010 1.541 84 458 12 344 3204 3141 226/0 1.096 0.0274/0.0286 0.0727/0.0731

C18H24N6Er2O21 994.80 triclinic P1̅ 0.18 × 0.13 × 0.08 6.0327(2) 12.7752(4) 13.1775(5) 118.990(4) 90.575(3) 91.670(2) 887.61(5) 1 293(2) 1.861 1.541 84 460 13 431 3512 3422 235/0 1.112 0.0283/0.0302 0.0826/0.0834

Table 2. Selected Bonds Lengths (Å) for 1−5 Ln(1)−O(1) Ln(1)−O(3) Ln(1)−O(5) Ln(1)−O(6a) Ln(1)−O(2a) Ln(1)−O(1W) Ln(1)−O(2W) Ln(1)−N(1) Ln(1)−N(2) Ln(1)−N(3)

1

2

3

4

5

2.490(3) 2.448(3) 2.504(3) 2.485(3)

2.473(4) 2.432(4) 2.486(3) 2.464(3)

2.462(3) 2.420(3) 2.466(3) 2.452(3)

2.423(3) 2.387(3) 2.429(3) 2.414(3)

2.363(3) 2.285(3) 2.302(3)

2.487(3) 2.533(3) 2.746(4) 2.757(4) 2.710(4)

2.516(4) 2.468(4) 2.728(4) 2.731(4) 2.702(4)

2.498(3) 2.451(4) 2.709(4) 2.716(4) 2.674(4)

2.457(3) 2.401(3) 2.668(4) 2.683(4) 2.634(3)

2.342(3) 2.369(3) 2.363(3) 2.622(3) 2.740(3) 2.600(3)

alumina crucibles containing samples of ∼3 mg under a synthetic air atmosphere (3) and dinitrogen (1, 2, 4, and 5) flow (100 mL min−1) at a heating rate of 10 °C min−1. The TG/DTA equipment was calibrated using an indium standard for temperature and an alumina calibration weight for mass. IR spectra were recorded on a Nicolet Impact 400 spectrometer using KBr pellets in the wavenumber range of 4000−400 cm−1 with an average of 128 scans and 4 cm−1 of spectral resolution. XRPD data were obtained on a powder X-ray diffractometer (model Ultima IV, Rigaku, Japan) using a Cu Kα tube (λ = 1.5418 Å) at a voltage of 40 kV and a current of 30 mA in the 2θ range of 5−55°. Q-band electron paramagnetic resonance (EPR) spectra of polycrystalline samples of 1−5 were recorded at different temperatures with a Bruker ER 200 spectrometer equipped with a helium continuous-flow cryostat. Direct-current (dc) and alternating-current (ac) magnetic susceptibility measurements were performed on crushed polycrystalline samples of 1−5 with a Quantum Design SQUID magnetometer in the temperature range of 1.9−300 K under applied dc fields of 1 T (T ≥ 50 K) and 100 G (T < 50 K). The experimental magnetic susceptibility data were corrected for diamagnetic contributions of the constituent atoms and the sample holder (a plastic bag). The luminescence excitation and emission spectra were recorded using a Jobin-Yvon model Fluorolog FL3−22 spectro-photometer equipped with an R928 Hamamatsu photomultiplier and 450 W xenon lamp as excitation source, and the

5, respectively. Anal. Calcd for C18H26Ce2N6O22 (1): C, 22.55; H, 2.73; N, 8.77. Found: C, 22.49; H, 2.85; N, 8.45%. Anal. Calcd for C18H26N6Pr2O22 (2): C, 22.51; H, 2.71; N, 8.75. Found: C, 22.38; H, 2.83; N, 8.52%. Anal. Calcd for C18H26Eu2N6O22 (4): C, 22.01; H, 2.67; N, 8.55. Found: C, 20.95; H, 2.41; N, 8.25%. Anal. Calcd for C18H24Er2N6O21 (5): C, 21.73; H, 2.43; N, 8.45. Found: C, 20.81; H, 2.34; N, 8.15%. {[Nd2(C6H2N2O4)3(H2O)4]·6H2O}n (3). A mixture of Hmpca (0.069 g, 0.50 mmol) and Nd(NO3)3·6H2O (0.132 g, 0.30 mmol) in 30 mL of water was stirred for 1 h at room temperature. Then it was poured into a 45 mL stainless steel reactor with a Teflon linear autoclave under autogenous pressure at 130 °C for 72 h and slowly cooled to room temperature for 48 h. A few single crystals of 3 as dark yellow needles were formed in the reactor. They were manually selected and dried on filter paper in the open air. Yield: 0.058 g (28%). Anal. Calcd for C18H26Nd2N6O22 (3): C, 22.36; H, 2.71; N, 8.69. Found: C, 21.56; H, 2.81; N, 8.48%. Although we repeated this synthesis using the 2,5-H2pzdc acid and adjusting to pH 6−6.5 by addition of NaOH or alternatively, the sodium salt of the Hmpca, similar results were obtained. Physical Measurements. Thermogravimetry (TG) and differential thermal analysis (DTA) were performed simultaneously with the same modulus employing a thermobalance (model SDT Q600, TA Instruments, USA) in the temperature range of 25−1000 °C, using 2110


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Inorganic Chemistry Table 3. Selected Bond Angles (deg) for 1−5a O(1)−Ln(1)−O(3) O(1)−Ln(1)−O(5) O(1)−Ln(1)−O(6a) O(1)−Ln(1)−O(1W) O(1)−Ln(1)−O(2W) O(1)−Ln(1)−N(1) O(1)−Ln(1)−N(2) O(1)−Ln(1)−N(3) O(3)−Ln(1)−O(5) O(3)−Ln(1)−O(6a) O(3)−Ln(1)−O(1W) O(3)−Ln(1)−O(2W) O(3)−Ln(1)−N(1) O(3)−Ln(1)−N(2) O(3)−Ln(1)−N(3) O(5)−Ln(1)−O(6a) O(5)−Ln(1)−O(1W) O(5)−Ln(1)−O(2W) O(5)−Ln(1)−N(1) O(5)−Ln(1)−N(2) O(5)−Ln(1)−N(3) O(6a)−Ln(1)−O(1W) O(6a)−Ln(1)−O(2W) O(6a)−Ln(1)−N(1) O(6a)−Ln(1)−N(2) O(6a)−Ln(1)−N(3) O(1W)−Ln(1)−O(2W) O(1W)−Ln(1)−N(1) O(1W)−Ln(1)−N(2) O(1W)−Ln(1)−N(3) O(2W)−Ln(1)−N(1) O(2W)−Ln(1)−N(2) O(2W)−Ln(1)−N(3) N(1)−Ln(1)−N(2) N(1)−Ln(1)−N(3) N(2)−Ln(1)−N(3) a

1

2

3

4

5b

144.50(12) 117.19(10) 67.18(10) 76.82(12) 128.03(11) 60.62(11) 125.92(11) 71.59(11) 68.70(10) 137.72(10) 72.63(12) 87.46(12) 93.67(11) 61.25(11) 129.39(11) 131.77(10) 124.33(12) 71.63(11) 69.68(10) 116.89(10) 61.30(10) 103.76(12) 71.19(11) 126.75(10) 77.09(10) 77.95(10) 144.01(11) 74.61(12) 73.83(11) 144.85(11) 137.84(11) 70.30(11) 70.48(11) 144.36(11) 76.69(11) 138.48(11)

144.01(14) 117.58(12) 67.19(12) 128.27(13) 76.49(14) 60.91(12) 125.75(13) 71.81(13) 68.27(13) 137.90(12) 87.70(14) 72.72(15) 92.91(13) 61.49(13) 129.47(13) 132.37(12) 71.83(13) 124.55(14) 69.66(13) 116.66(13) 61.78(12) 71.34(13) 102.97(14) 127.15(12) 76.95(12) 78.09(13) 143.61(14) 138.12(14) 70.21(13) 70.54(13) 74.74(14) 73.48(14) 145.03(14) 143.90(13) 77.20(13) 138.43(13)

143.64(12) 118.01(10) 66.94(10) 128.12(11) 76.36(12) 61.32(11) 125.37(12) 72.04(11) 68.11(12) 138.14(11) 88.23(12) 72.62(13) 92.29(11) 61.83(11) 129.47(11) 132.71(10) 72.05(11) 124.93(12) 69.71(11) 116.62(11) 61.87(10) 71.29(11) 102.28(12) 127.35(11) 76.77(11) 78.36(10) 143.18(12) 138.41(12) 70.17(11) 70.58(11) 75.01(12) 73.07(12) 145.26(11) 143.64(11) 77.41(11) 138.47(11)

142.62(11) 118.52(11) 66.74(10) 128.43(11) 75.77(11) 61.78(10) 125.20(10) 72.14(10) 67.48(10) 138.65(10) 88.95(11) 72.89(12) 90.58(11) 62.69(10) 129.70(10) 133.99(9) 72.09(10) 125.83(11) 69.96(10) 116.28(10) 62.61(9) 72.04(10) 100.13(11) 127.88(10) 76.22(10) 78.89(10) 142.52(10) 138.99(10) 69.95(10) 70.66(10) 75.17(11) 72.59(10) 145.39(10) 142.86(11) 78.49(11) 138.18(10)

72.23(10) 95.97(11) 136.86(9) 139.70(10) 76.42(11) 62.77(9) 123.63(10) 71.04(10) 136.58(10) 140.27(10) 84.85(10) 84.22(10) 130.84(10) 62.49(10) 71.43(10) 75.95(10) 78.18(11) 134.72(10) 69.47(11) 140.05(11) 65.27(10) 80.95(10) 80.06(10) 74.95(9) 77.78(10) 135.28(10) 134.75(11) 143.33(10) 68.34(10) 70.40(10) 67.47(10) 67.67(10) 143.89(11) 130.49(10) 109.13(10) 119.33(10)

Symmetry code: (a) = x + 1, y, z. bO(6a) corresponds to O(2a) for compound 5.

Scheme 1. Synthetic Route to Obtain the Lanthanide Complexes 1−5 under Hydrothermal Conditions

with an Agilent Supernova X-ray diffractometer equipped with μ-focus Cu radiation (λ = 1.5418 Å) at 293 K. These data were indexed, integrated, and scaled through the Agilent CrysalisPro software.58 The crystal structures of 1−5 were solved by direct methods and refined with full-matrix least-squares techniques on F2 using SHELXS-97 and SHELXL-97 programs included in the WINGX software package.59,60

spectra were corrected with respect to the Xe lamp intensity and spectrometer response. Measurements of emission decay were performed with the same equipment by using a pulsed Xe (3 μs bandwidth) source. Crystallographic Data Collection and Structure Determination. X-ray diffraction data on single crystals of 1−5 were collected 2111


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Inorganic Chemistry All non-hydrogen atoms were refined anisotropically. The hydrogen atoms of the organic ligands were set on geometrical positions and refined with a riding model, whereas those of the water molecules were neither found nor set. The water molecules present some disorder, and two positions were encountered for some of them. The site occupation factors were refined in a first step, and when they converge, the values were fixed in the final refinements. The final geometrical calculations and the graphical manipulations were performed with the DIAMOND program.61 Crystal data and details of the data collection and refinement for 1−5 are listed in Table 1, whereas selected bond distances and angles are given in Tables 2 and 3, respectively. The experimental and calculated powder X-ray diffraction patterns (see Figure S1, in the Supporting Information) regarding all five compounds exhibit a great coincidence of the positions of all peaks expected, each pattern confirming that the obtained structure from the single crystal is equal to one of the bulk. Additional crystallographic information is available in the Supporting Information.

compound was also prepared by the hydrothermal method, but a different temperature ramp without pH control was used here, and similar results were found. Crystal Structures of 1−4. Complexes 1−4 crystallize in the triclinic group space P1̅. Their structure is 3D with the lanthanide(III) ions linked through 2,5-pzdc2− dianions acting as two- and fourfold connectors, building a bimodal 4,4connected (4·648)(426282)-mog network. There are few examples reported of these quartz-like net in coordination polymers,63−66 and to our knowledge, this is the first example with lanthanide ions. Since 1−4 are isostructural compounds, we will only discuss the structure of 3 as the representative example of the series making reference to 1, 2, and 4 for comparative purposes. Each neodymium ion is nine-coordinated by four oxygen atoms [O(1), O(3), O(5), and O(6a); symmetry code (a) = x + 1, y, z] and three nitrogen [N(1), N(2), and N(3)] atoms of three crystallographically independent 2,5-pzdc2− ligands plus two oxygen atoms [(O1w) and O(2w)] of water molecules building a polyhedron that can be described as a somewhat distorted tricapped trigonal prism (Figure 1). The O(3)O(5)-

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RESULTS AND DISCUSSION Synthetic Details, Infrared Spectroscopy, and Thermal Study. The preparation of all the complexes was performed by reaction between Hmpca and the corresponding lanthanide(III) nitrate hydrothermally at different temperatures (see Scheme 1). The oxidation of the methyl group of the Hmpca acid under hydrothermal conditions afforded the 2,5pzdc2− anion, which is present as a ligand in 1−5. To our knowledge, the mechanism involving the oxidation of the methyl group of the carboxylate is not still clear. However, some studies described in the literature15,55 show that in hydrothermal conditions, with and without presence of strong oxidants, the oxidation of alkyl groups on the pyridine ring to the corresponding aromatic carboxylic acids have been described.48 We believe that this oxidation of the methyl group for these derivated pyridyl ligands can be initiated or catalyzed (see also Table S1) in the presence of lanthanide ions15,55 or metal ions48 under hydrothermal conditions. Thus, the general procedure in their syntheses is as follows: the mixture of the lanthanide(III) nitrate and Hmpca in water is stirred for 1 h at room temperature and then heated at 130 °C (Ln = Nd) and 170 °C (Ln = Ce, Pr, Eu, and Er) during 72 h; the final step consisted of slowly cooling to room temperature for 48 h, as shown in Scheme 1. The IR spectra of the complexes 1−5 resemble each other and show characteristic bands of the carboxylate groups, pyrazine ring, and water molecules (see Table S2).50 The IR spectra of 1−5 are shown in Figure S2, and the main bands in the fingerprint region are listed in Table S2 (in Supporting Information). All these spectroscopic features on 1−5 were confirmed by the crystallographic study (see below). The TG curves for 1−5 show similar behavior. For all, the residue of the decomposition process corresponds to 1/2 mol of Ln2O3 as observed during the thermal decomposition of other lanthanide compounds.62 The TG (black line) and DTA (blue line) curves for 3 are shown in Figure S3a (Supporting Information). The TG and DTA curves for the complexes 1, 2, 4, and 5 are given in Figure S3b and S4, respectively, in Supporting Information). Description of the Structures. The coordination sphere of the Ln(III) ions in 1−5 [Ln = Ce (1), Pr (2), Nd (3), Eu (4), and Er (5)] contains the same number of 2,5-pzdc2− groups and water molecules as ligands. However, despite the erbium(III) derivative (5) is not isomorphous with 1−4, it is isostructural with them and also isomorphous with the TmMOF complex {[Tm2(2,5-pzdc)3(H2O)4]·6H2O}n.55 This last

Figure 1. (top) A view of the coordination environment around the neodymium(III) ion in 3 [symmetry code: (a) = x + 1, y, z; (b) = −x, −y + 2, −z + 1; (c) = −x + 1, −y + 1, −z + 2; (d) = −x, −y + 1, −z + 1; (e) = −x + 1, −y + 1, −z + 1]. (bottom) Perspective view of the coordination sphere of the neodymium(III) ion in 3 with the atom numbering [the same drawing is valid for 1, 2, and 4 by replacing the neodymium ion by cerium (1), praseodymium (2), and europium (4); Figures S5−S7]. Thermal ellipsoids are drawn at the 50% probability level. 2112


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Inorganic Chemistry O(1W) and O(1)O(2W)O(6a) set of atoms constitute the triangular bases and the rectangular faces are capped by the N(1), N(2), and N(3) atoms, resulting in an axial symmetry around each lanthanide center of 1−4 (roughly D3h point group). The three crystallographically independent 2,5-pzdc2− ligands are centrosymmetric, and they exhibit two types of bridging modes: μ−κ 2 N,O:κ 2 N′,O″ (A′) and μ 4 κ2N,O:κO′:κ2N′,O″:κO‴ (B′; Chart 1), the sets of donor atoms involved being N(1),O(1):N(1b),O(1b) and N(2),O(3):N(2c),O(3c) in A′ and N(3),O(5):O(6):N(3d),O(5d):O(6d) in B′ [(b) = −x, −y + 2, −z + 1; (c) = −x + 1, −y + 1, −z + 2; (d) = −x, −y + 1, −z + 1; see Figure 1]. The mean values of the neodynium(III) to pyrazine-nitrogen and carboxylate-oxygen bond lengths for the two 2,5-pzdc2− ligands exhibiting the bis-bidentate bridging mode (A′) are 2.712(4) and 2.441(3) Å, respectively, while in B′ the Nd(1)−N(3) distance is 2.673(4) Å; those at the carboxylate group [O(5)C(7)O(6)], which exhibits the anti-syn coordination mode, are 2.466(3) [Nd(1)−O(5)] and 2.453(3) Å [Nd(1)− O(6a)]. The values of Nd(III) ion-to-water bond distances are 2.498(3) [Nd(1)−O(1W)] and 2.451(5) Å [Nd(1)−O(2W)]. The corresponding values for the isostructural 1, 2, and 4 compounds agree well with those found for 3 (see Table 3). The coexistence of the A′ and B′ coordination modes of the 2,5-pzdc2− ligand leads to the formation of hexagon-like rings characteristic of a quartz-like net where the lanthanide ions act as fourfold nodes connected by 2,5-pzdc2− ligands acting either as simple spacers (A′ coordination mode) or as fourfold connectors (B′; Figures 2 and 3). Each hexagon contains six neodymium ions shared by three different rings at the vertexes with water molecules of crystallization located inside the hexagonal pores, the estimated void volume per unit cell being 261.6 Å3 (ca. 32% of the unit cell). Each Nd(III) ion is connected to other seven adjacent ones through two A′ and two B′ 2,5-pzdc2− dianions. The Nd···Nd separations through the A′ bridges are 8.1995(4) [Nd(1)···Nd(1b)] and 8.2040(5) Å [Nd(1)···Nd(1c)], whereas the same bis-bidentate bridge in the B′ coordination mode produces a separation of 8.0765(5) Å [Nd(1)···Nd(1d)]. The other bridges within the B′ 2,5pzdc2− dianions give separations of 6.0736(3) Å through the anti-syn carboxylate bridge [Nd(1)···Nd(1a)] and 7.9507(5) Å for the N(3),O(5):O(6d) bridge [Nd(1)···Nd(1e); (e) = −x + 1, −y + 1, −z + 1]. The values of the separations between the lanthanide(III) ions in the compounds 1−4 follow the trend of the values of the ionic radii of the lanthanide cations involved.50 Therefore, the Ln···Ln separations through the A′ bridges decrease following the series Ce(III), Pr(III), Nd(III), and Eu(III) [8.276(3) (1), 8.2438(8) (2), 8.2040(5) (3), and 8.1326(5) Å (4)]. Also, the unit cell parameters and volume decrease when going from 1 to 4, and the estimated void volume per unit cell follows the sequence 269.78 (1), 269.48 (2), 261.64 (3), and 243.98 Å3 (4). Description of the Structure of 5. This compound crystallizes also in the space group P1,̅ but the cell parameters are different from those of 1−4. The 3D structure is formed by erbium(III) ions linked by 2,5-pzdc2− anions to form a 4,4connected mog-net as in 1−4. Each Er(III) ion is ninecoordinate with two oxygen atoms from water molecules [O(1w) and O(2w)], three nitrogen [N(1), N(2), and N(3)], and four oxygen atoms [O(1), O(2a), O(3), and O(5); (a) = x + 1, y, z] from four 2,5-pzdc2− ligands building a tricapped

Figure 2. (a) View of a fragment of the quartz-like arrangement of 3 down the crystallographic a axis (the same applies for 1, 2, and 4). The hydrogen atoms and the crystallization water molecules are omitted for clarity. (b) View of the moganite network of 1−5. Neodymium(III) ions are depicted in purple, while 2,5-pzdc2− dianions with A′ and B′ coordination modes are depicted in yellow and green, respectively (see text and Chart 1b).

trigonal prism that is less distorted than those in 1−4 (Figure 4). The O(1)O(3)O(2W) and O(5)O(1W)O(2a) set of atoms built the bases of the trigonal prism, while N(1), N(2), and N(3) capped the rectangular faces of the polyhedron. The Er− O and Er−N bond lengths vary in the ranges of 2.285(3)− 2.368(3) and 2.600(3)−2.739(4) Å, respectively, and they are comparable to those reported for these bonds in other 2,5pzdc-containing erbium(III) complexes.56 Three crystallographically independent centrosymmetric 2,5pzdc2− ligands occur in 5, and they exhibit the same coordination modes (A′ and B′; see Chart 1) reported for 1−4. The sets of donor atoms involved are N(2),O(3):N(2b),O(3b) and N(3),O(5):N(3c),O(5c) in A′ and N(1),O(1):O(2):N(1d),O(1d):O(2d) in B′ [(b) = −x, −y, −z + 1; (c) = −x + 1, −y +1, −z +1; (d) = −x + 1, −y, −z; see Figure 4]. They build the same hexagonal pattern with the channels arranged along the crystallographic a axis, where the 2113


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water molecules are located (Figures 5 and S8). The calculated void volume per unit cell once the crystallization water molecules are removed is 342.6 Å3, which accounts for 38.6% of the unit cell. This value is larger than those observed for 1− 4. The small change in the coordination of the 2,5-pzdc2− ligands around the Er(III) ion produces larger hexagons in 5. The Nd···Nd separations through the bis-bidentate bridge in the 2,5-pzdc2− anions with A′ coordination mode are 8.2454(5) [for Er(1)···Er(1b)] and 7.9749(5) Å [for Er(1)···Er(1c)]. These values are similar to Er(1)···Er(1d) separation by the bisbidentate bridge on B′ conformation [7.9675(5) Å]. The other bridges within the B′ 2,5-pzdc2− anions give separations of 6.0237(3) Å for the anti-syn carboxylate bridge [Er(1)···Er(1a)] and 8.0528(5) Å for the N(1),O(1):O(2d) bridge [Er(1)··· Er(1e); (e) = −x, −y, −z]. These values are similar to those observed for 1−4. Photoluminescence Studies. The trivalent lanthanide ions are characterized by their partially filled 4f shells, except for lanthanum La(III) and lutetium Lu(III), which have a completely filled 4f shell. Some of these lanthanide ions and their organic complexes are known to exhibit photoluminescence in solution and in the solid state, exhibiting linelike emission bands, relatively long luminescence lifetimes, and a

Figure 3. View of a fragment of the structure of 3 along the crystallographic b (left) and c (right) axes. Crystallization water molecules are omitted for clarity.

Figure 4. (top) Coordination environment of the erbium(III) ion in 5. The thermal ellipsoids are drawn at the 50% probability level [symmetry code: (a) = x + 1, y, z; (b) = −x, −y, −z + 1; (c) = −x + 1, −y + 1, −z + 1; (d) = −x + 1, −y, −z; (e) = −x, −y, −z]. (bottom) Perspective view of the coordination sphere of the erbium(III) ion in 5 with the atom numbering. Thermal ellipsoids are drawn at the 50% probability level. 2114


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Figure 6. Photoluminiscence (a) excitation and (b) emission spectra of 3 at 303 K. Excitation spectra were obtained at emission maximum, while the emission spectra were obtained upon excitation at 315 nm.

nm) transitions. These features indicate that the 2,5-pzdc2− ligand is suitable for the sensitization of the NIR luminescence for the Nd(III) ion. The excitation spectrum of 4 recorded at 303 K in the 250− 500 range by monitoring the Eu(III) emission at 615 nm is shown in Figure 7a. The broad band between 250 and 350 nm

Figure 5. View of the structure of 5 along the crystallographic a axis (crystallization water molecules were removed for clarity).

strong sensitivity toward quenching by high-frequency vibrations such as the O−H vibration.67 An important feature of the luminescence of the lanthanide ions is that it can be easily photosensitized by the appropriate antenna chromophores (sensitizers). In this process, which can be very efficient, the sensitizer transfers its excitation energy to the lanthanide ion, which results in the population of the lanthanide luminescence state and subsequent emission. Most of the investigations in the field of luminescent lanthanide complexes have been devoted to Eu(III) and Tb(III) compounds, where they emit in the visible spectral region, and they are used as sensors and as luminescent labels in fluoroimmuno assays. Currently much work is being devoted to Nd(III) lanthanide compounds emitting in the near-infrared (NIR) region (800− 1600 nm) for both fundamental reasons and possible applications in some optical fields, specially, telecommunication networks and novel laser materials.68 Owing to the excellent luminescent properties of Nd(III) and Eu(III) ions, the photoluminescence of 3 and 4 was investigated. The normalized excitation spectrum of 3, which was recorded at 303 K and monitored around the intense 4 F3/2→4I11/2 transition of the Nd(III) ion, is shown in Figure 6a. The excitation spectrum exhibits a broad band between 280 and 460 nm, which is attributable to the S0→S1 (π,π*) ligandcentered transition of the aromatic pyrazine moiety. Additionally, a series of bands arising from 4f−4f transitions from the ground-state 4I9/2 level to 4G9/2 (523 nm), 4G5/2 + 2G7/2 (578 nm), 2H11/2 (632 nm), 4I9/2→4F7/2 + 4S3/2 (747 nm), and 4 I9/2→2H9/2 (800 nm) excited states occurs. Upon excitation through 2,5-pzdc2− ligand singlet state at 315 nm, the emission spectrum of 3 at room temperature (Figure 6b) shows the main characteristic emission lines corresponding to the expected f−f NIR transitions and those corresponding to the 4F3/2→4I9/2 (948 nm), 4F3/2→4I11/2 (1063 nm), and 4F3/2→4I13/2 (1341

Figure 7. Photoluminiscence (a) excitation and (b) emission spectra of 4 at 303 K. The excitation spectrum was obtained for the emission maxima, while the emission spectrum was monitored on the ligand band at 325 nm.

in this spectrum is attributable to the S0→S1 (π,π*) ligandcentered transition of the pyrazine moiety or to a possible ligand-to-metal charge transfer (LMCT). The absorption bands are originated from the 7F0 ground state to the excited levels 5 LJ: 5G6 (361 nm), 5H4 (374 nm), 5L7 (384 nm), 5L6 (394 nm), 5 D3 (415 nm), and 5D2 (464 nm) excited states. The 7F0→5L6 transition exhibits the highest absorption intensity among the 4f6 intra-configurational transitions of the Eu(III) ion, indicating that this transition is more efficient for the direct excitation in this metal center. However, these transitions are less intense than those attributable to the ligand levels, proving that the luminescence sensitization is more efficient than the direct excitation of the absorption levels of the Eu(III) ion. 2115


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Figure 8. (a) Emission spectrum of 4 in the solid state at 77 K upon excitation at 325 nm. (b) 5D0→7F1 and (c) 5D0→7F2 transitions showing the local-ligand field splitting.

luminescence decay curves obtained at room temperature results in a slight decrease in the emission lifetime of the Eu(III) complex. This is caused by increase of vibrational contributions that decrease both the population of the level transmitter and the emission intensities. To get further information on the chemical environment of the Eu(III) ion in 4, radiative rates (A0λ, where λ = 2 and 4, represents the spontaneous emission coefficients of 5D0→7F2 and 5D0→7F4 transitions) and emission quantum efficiency (η) values were determined as described in the literature.72,73 The following expression is used to calculate the values of A0λ [eq 1]

The emission spectrum of 4 (Figure 7b) provides a lot of information about the point-group symmetry of the EuIII ion.69 This spectrum exhibits several characteristic 5D0→7FJ (J = 0−4) emission bands upon excitation in the ligand absorption band at 325 nm. The 5D0→7F0 transition (581 nm) consists of only one peak, which can be a strong indication that the Eu(III) ions experience the same crystal-field strength and occupy sites of the same symmetry. To confirm these results, an emission spectrum in 5D0→7F0 region was obtained at 77 K (inset of Figure 7b), and no splitting of this band was observed, being thus consistent with the crystal structure analyses. In addition, because the Eu(III) 7F0 level is nondegenerated and the ligand field cannot split it, the single peak at 581 nm indicates that there is only one emitter EuIII center in the compound.70 Another feature in the photoluminescence data is the absence of the phosphorescence broad band from the 2,5-pzdc2− ligands, suggesting that the intramolecular energy transfer is efficient. It is noteworthy that the spectra of the EuIII compound recorded at 303 and 77 K showed similar profiles. However, the emission spectra at room temperature (Figure 7b) are less resolved, compared to the spectrum obtained at the liquid nitrogen (Figure 8a). Three Stark levels for the 5D0→7F1 transition (Figure 8b) and five levels in the hypersensitive 5 D0→7F2 transition (Figure 8c) can be observed in this spectrum. These results indicate the presence of at least one site without inversion center for the Eu(III) ion (i.e., C1, C2, or Cs, with triclinic crystal system).71 Another feature of this spectrum is that the transitions from the 5D1 excited state to the 7 F0, 7F1, and 7F2 states can be also observed (inset of Figure 8a). Such transitions exhibit extremely low intensity, because the 5D0 and 5D1 levels have small energy gap, and so, the 5 D1→5D0 transfer of energy is preferred in relation to the 5 D1→7FJ (J = 0−6) electronic transitions. The values of the Eu (5D0) lifetime (τ) in the solid state were determined at 303 (τ = 0.34 ms) and 77 K (τ = 0.39 ms), under excitation at 325 nm with emission monitored at the 5D0→7F2 transition. Each of the decay curves (Figure S9 in Supporting Information) follows a monoexponential decay law, and the equation Intensity = A1·exp(−x/t1) + y0 was used to fit the fluorescence decay curves. These data are also consistent with only one symmetry site for 4, in agreement with the Eu(III) complex emission spectrum and X-ray diffraction analysis. The

A 0λ =

ν01 S0λ · . (A 01) ν0λ S01

(1)

In this equation, S01 and S0λ are the areas under the curves of the 5D0→7F1 and 5D0→7Fλ transitions, ν01 and ν0λ being their respective energy barycenters (in cm−1). As is known, the magnetic dipole-allowed 5D0→7F1 transition was taken as reference,74 since the emission coefficient A01 = 0.31 × 10−11(n)3(ν01)3 is almost insensitive to chemical environment changes around the Eu(III) ion with A0→1 ≅ 50 s−1. The emission quantum efficiency (η) of europium(III) ion in 4 was determined on the basis of the emission spectrum and lifetime of the 5D0 emitting level. Initially, the emission coefficients A02 and A 04 corresponding to the 5 D 0 → 7 F 2 and 5 D 0 → 7 F 4 transitions, respectively, were calculated according to eq 1. Considering the ratio between the emitting-state lifetime and the total decay rate, (Atotal = 1/τ = Arad + Anrad), the value of η can be calculated through eq 2:72 η=

A rad A rad +A nrad

(2)

The low value of η of ca. 11% reflects the large value of the nonradiative rate (Anrad = 2539 s−1) due to the luminescence quenching of the 5D0 level by the O−H oscillators of four water molecules in the first coordination sphere. Values of the emission quantum efficiency (η) and radiative rates for compounds of Eu(III) with 2,5-pzdc2− ligand are extremely rare in the literature making impossible a comparison with other similar compounds.15,44,50 However, Soares-Santos et al. reported the photophysical study of the compound [Eu2(2,3pzdc)3(ox)2(H2O)2]n [2,3-pzdc2− = 2,3-pyrazinedicarboxylate 2116


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Inorganic Chemistry and ox2− = oxalate)8 with η ≈ 13%, a similar value to that obtained herein for 4 (η ≈ 11%). The Comission Internationale L’Eclairage (CIE) coordinates were also calculated for the Eu(III) compound as x = 0.668 and y = 0.329, values that are located in the red region (Figure S10 in the Supporting Information). Despite the low-intensity emission for this compound, the CIE coordinates show the characteristic emission of the monochromatic nature for the Eu(III) compound herein. Magnetic Properties of 1−5. The magnetic properties for 1−5, in the form of the χMT product as a function of the temperature [χM is the magnetic susceptibility per mole of Ln(III) ion] are given in Figures 9−13. When cooled, the

Figure 11. Temperature dependence of the χMT product (o) for 3 together with the calculated curves through the Hamiltonian of eq 3 as a function of Δ with λ = 260 cm−1.

Figure 9. Temperature dependence of the χMT product (○) for 1 together with the calculated curves through the Hamiltonian of eq 3 as a function of Δ with λ = 600 cm−1. Figure 12. Temperature dependence of the χMT product (○) for 4 together with the calculated curves through the Hamiltonian of eq 3 as a function of Δ with λ = 300 cm−1.

Figure 10. Temperature dependence of the χMT product (○) for 2 together with the calculated curves through the Hamiltonian of eq 3 as a function of Δ with λ = 360 cm−1. Figure 13. Temperature dependence of the χMT product (○) for 5 together with the calculated curved through the Hamiltonian of eq 3 as a function of Δ with λ = −680 cm−1.

values of χMT for all these compounds decrease with the temperature, and they vanish at low temperature for 2 and 4, whereas they tend to finite values of 0.44, 0.80, and 5.80 cm3 mol−1 K at 2.0 K for 1, 3, and 5, respectively. These behaviors are as expected for the ground term of these metal ions, 2F5/2 [Ce(III)], 2H4 [Pr(III)], 4I9/2 [Nd(III)], 7F0 [Eu(III)], and 4 I15/2 [Er(III)]. The fact that the χMT value for 2 and 4 tends to zero is indicative of the occurrence of a MJ = 0 as ground state in both compounds. The coordination polyhedron of the Ln(III) ions for 1−5 is shown in Scheme 2 (left), and its ideal D3h symmetry (trigonal axial symmetry) is depicted in Scheme 2 (right). In this

situation, we can simplify and describe their magnetic properties by using the Hamiltonian of eq 3 ⎤ ⎡ 2 1 Ĥ = λLŜ ̂ + Δ⎢Lẑ − L(L + 1)⎥ + βH( −κL̂ + 2S)̂ ⎦ ⎣ 3 (3)

where the first term describes the spin−orbit coupling, the second one accounts for an axial ligand-field component (x = y 2117


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Inorganic Chemistry Scheme 2. Coordination Polyhedron in 1−5

≠ z), and the last one is the Zeeman effect. λ is the spin−orbit coupling parameter, Δ describes the energy gap between the ML components, and κ is an orbital reduction parameter. The best-fit parameters obtained through this Hamiltonian and using the VPMAG pack program75 are listed in Table 4 (we Table 4. Best-Fit Parameters for 1−5 through eq 3a compound

λ, cm−1

Δ, cm−1

E(MJ±1) − E(MJ)

Ce(III) 1 Pr(III) 2 Nd(III) 3 Eu(III) 4 Er(III) 5

600(30) 360(20) 260(10) 300(12) −680(25)

78(2) 50(1) 80(1) 11(2) 56(7)

196 68 217 284 68

Figure 14. Q-Band EPR spectra of powdered samples of 1, 3, and 5 recorded at 4.0 K.

the large metal−metal separation through possible exchange pathways (anti-syn carboxylato and pyrazine bridge type) that are present in these compounds. Although the magnetic memory has been often associated with a strong uniaxial magnetic anisotropy,76,77 1, 3, and 5 present a slow magnetic relaxation under an external applied magnetic field, in spite of the fact that their ground state is the lowest MJ value (MJ = ±1/2). No out-of-phase signal (χM″) of the ac magnetic susceptibility was observed for 1, 3, and 5 in absence of an external magnetic field suggesting a fast quantum tunneling of the magnetization (QTM) in these compounds. However, a nonzero value of this signal is observed for them under the application of dc magnetic fields that suppress the QTM (see Figures 15, 16a, and 17a). Figures S11−S13 show the in-phase and out-of-phase ac magnetic susceptibility for other applied dc magnetic fields. The magnetic relaxation rate τ−1 was determined from the maxima of the χM″ curves (Figures 15a, 16a, 17a, and S11− S13) and by performing Cole−Cole fits (Figures S14−S16 and Tables S3−S5)78 of the frequency-dependent susceptibility data. The results are depicted in Figures 15(inset), 16b, and 17b. In all cases, τ−1 decreases with decreasing temperature at any magnetic field, but no exponential law (Arrhenius law) can simulate such a dependence in all the temperature range. This dependence can be reproduced very well through eq 479,80

a

The last column concerns the values of the energy gap between the ground and first excited states (in cm−1).

kept constant κ = 1 in the fitting process of the magnetic data of 1−5). The comparison of the experimental data of 1−5 with several theoretical curves calculated from different Δ values can be seen in Figures 9−13. A quite good match between the experimental and calculated curves through the paramenters from Table 4 is achieved in all the cases. The more important point in this analysis of the thermal dependence of the χMT product is that it is clearly indicative of the occurrence of a positive sign of Δ with a few tens of inverse centimeters in 1−5. A negative value would imply a very different shape of the χMT curve and a very different magnetic moment for the ground state. Δ > 0 means that the smallest value of ML (ML = 0) corresponds to that of the lowest energy, and so, the lowest spin−orbit MJ value will be the ground state (that is, MJ = 0 for 2 and 4 and MJ = ±1/2 for 1, 3, and 5). Qband EPR spectra at low temperature corroborate these facts. 2 and 4 are EPR-silent (MJ = 0 as ground state), whereas the EPR spectra for 1, 3, and 5 unambiguously support an MJ = ±1/2 as ground state (see Figure 14). Therefore, a splitting of the energy levels as indicated in Scheme S1 is expected and where the energy values were calculated from the best-fit parameters listed in Table 4. Although a lower symmetry can mix different MJ values in the wave functions, Scheme S1 shows the MJ component of the largest contribution for an ideal D3h symmetry, and in Table 4 we give the energy gap between the ground and the first excited states. The fact that the values of χMT at low temperature are very close to those expected through the Hamiltonian of eq 3 is indicative of the absence of any important magnetic interaction between the Ln(III) ions, which strongly influence the spin dynamics (see below). These very weak, if any, magnetic interactions are in agreement with

τ −1 ≈ Adirect T + Braman T 9

(4)

where the first term represents the contributions of onephonon direct processes, which are dominant below ca. 3.5, 5.0, and 3.0 K for 1, 3, and 5, respectively. The second term represents two-phonon Raman processes, which give rise to faster spin−lattice relaxation at higher temperatures. The bestfit parameters are depicted in Table S6. However, for these high-temperature regions we can fit the experimental data to an Arrhenius law, τ = τ0 exp(Ea/kT), as expected for a thermally activated Orbach process [see the insets of Figures 15b, 16b, and 17b]. The best-fit parameters are indicated in Table S7. 2118


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Figure 15. (a) Frequency dependence of the out-of-phase ac susceptibility for 1 under an applied static field of Hdc = 2000 G with a ±5.0 G oscillating field at frequencies in the range of 0.3−10 kHz. (b) Temperature dependence of the spin−lattice relaxation rates for 1 obtained from Cole−Cole fits under static dc fields of 1000 (blue) and 2000 G (red). (inset) The Arrhenius plot for 1 as ln(τ) against 1/T.

Figure 16. (a) Frequency dependence of the out-of-phase ac susceptibility for 3 under an applied static field of Hdc = 2000 G with a ±5.0 G oscillating field at frequencies in the range of 0.2−10 kHz. (b) Temperature dependence of the spin−lattice relaxation rates for 3 obtained from Cole−Cole fits under static dc fields of 1000 (blue) and 2000 G (red). (inset) The Arrhenius plot for 3 as ln(τ) vs 1/T.

Figure 17. (a) Frequency dependence of the out-of-phase ac susceptibility for 5 under an applied static field of Hdc = 1000 G with a ±5.0 G oscillating field at frequencies in the range of 0.05−10 kHz. (b) Temperature dependence of the spin−lattice relaxation rates for 5 obtained from Cole−Cole fits under static dc fields of 500 (green), 1000 (blue), and 2500 G (red). (inset) The Arrhenius plot for 5 as ln(τ) vs 1/T.

explain the energy barriers expected for an Orbach process.79 Therefore, a two-phonon Raman process must be the most plausible mechanism at high temperature.81 In fact, when we used a general power law, Tn, the best-fit parameters give values for n that are very close to 9. The electronic spin interacts with

However, the values of the energy barriers obtained from these fits (10−12, 35−42, and 19−23 cm−1 for 1, 3, and 5, respectively) are much lower even than the energy gap between the ground doublet state and the first excited doublet (see Table 4), and so, no level exists at lower energies that could 2119


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Inorganic Chemistry the external magnetic fields and also with the corresponding nuclear spins. The hyperfine interactions with the nuclear spins I = 5/2 (3) or 7/2 (5) turn the electronic doublet into a manifold of 12 or 16 electronuclear spin states, respectively, and direct transitions between some of these levels could be allowed, providing a relaxation pathway.82 However, 1 exhibits a very close behavior to that of 3 and 5, but no nuclear spin is present in that compound. In general, magnetic slow relaxation has often been associated with the combination of a ground state with a high spin value and a strong uniaxial magnetic anisotropy.76,77 Recently, slow magnetic relaxation has been also observed in some Kramers ions with dominant easy-plane magnetic anisotropy but only under an external magnetic field.81−88 In fact, the observed magnetic behavior for 1, 3, and 5 resembles closely that found for some high-spin Co(II) mononuclear compounds with dominant easy-plane anisotropy.81−88 Recently, Gómez-Coca et al.81 studied this phenomenon from both experimental and theoretical points of view.

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*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Humberto O. Stumpf: 0000-0001-7756-0987 Notes

The authors declare no competing financial interest. CCDC 1510395 (1), 1510396 (2), 1510398 (3), 1510399 (4), and 1510397 (5) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44(0)1223−336033; e-mail: [email protected]].

CONCLUSIONS The use of hydrothermal conditions in the Ln(III)/2,5-pzdc2− system provided some advantages, since the crystals were obtained directly from the Teflon linear autoclave in the temperatures employed, and also the oxidation of the methyl group of the initial Hmpca acid into to the 2,5-pzdc2− dianion provides an efficient and relatively cheap synthetic route to prepare all the 2,5-pzdc-Ln(III) systems described herein. In these complexes, 2,5-pzdc−2 is coordinated to the lanthanide(III) ions in two coordination modes (bis-bidentate and bisbidentate/outer bis-monodentate) leading to interesting hexagon-like cycles containing six rare-earth atoms with four water molecules inside of the porous. The hexagon-like channel pore of 5 presented calculated void volume larger than other four lanthanides. The photophysical properties of the Eu(III) and Nd(III) complexes were studied: the complexes exhibited sensitized photoluminescence in the visible and NIR regions, respectively. According to the photoluminescence study, the phosphorescence broad bands from ligands are not present in any spectra, suggesting the intramolecular ligand-to-metal energy transfer. Although the possibility of intramolecular energy transfer exists, the presence of water molecules decreases the luminescence intensity of Eu(III) complex, through nonradiative decays. Finally, compounds 1, 3, and 5, where the lanthanide(III) centers are Kramers ions, behave as SIMs exhibiting slow magnetic relaxation at low temperatures under external applied dc fields. The relaxation times of 1, 3, and 5 in the low-temperature range investigated follow the combination of one-phonon direct and two-phonon Raman processes.

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ACKNOWLEDGMENTS The authors thank Prof. S. J. Lima Ribeiro (UNESPAraraquara) to allow the access to the photoluminescent equipment. The Conselho Nacional de Desenvolvimento Cientifı ́co e Tecnoloǵico (CNPq), the Fundacão de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG), the Coordenaçaõ de Aperfeico̧amento de Pessoal de Nı ́vel Superior (CAPES), FINEP (Ref.134/08), the Ministerio Español de Economı á y Competitividad MINECO (Project Nos. CTQ2013-44844P, MAT2014-57465-R, and Unidad de Excelencia MDM-2015-0538), and the Generalitat Valenciana (PROMETEOII/2014/070) are gratefully acknowledged for financial support. C.R.-P. thanks a visiting Professorship of the Universitat de València (2015-2016).

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02774. crystallographic crystallographic crystallographic crystallographic crystallographic

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PXRD data for 1−5; IR spectra for 1−5; thermal behavior for 1−5 and differential thermal analysis for 1− 5; coordination sphere for 1, 2, and 4; structure of 5; luminescence decay profiles and chromaticity diagram for 4; ac susceptibilities measurements and Cole−Cole plots for 1, 3, and 5; hydrothermal in situ ligand reaction; main stretching vibrations for 1−5; ac magnetic data for 1, 3, and 5; best-fit parameters and energy levels for 1−5 (PDF)

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