Syntheses, crystal structures and magnetic properties of a series of luminescent lanthanide complexes containing neutral tetradentate phenanthroline-amide ligands

Qian-Qian Sua, Kun Fanb, Xin-Xin Jina, Xin-Da Huangb, Shun-Cheung Chengc, Li-Juan Luoa, Yao-Jie Lia, Jing Xiang*a, Chi-Chiu Koc, Li-Min Zheng*b and Tai-Chu Lau*c
aCollege of Chemistry and Environmental Engineering, Yangtze University, Jingzhou 434020, Hubei, P. R. China. E-mail:
bState Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China. E-mail:
cDepartment of Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong. E-mail:

Received 12th March 2019 , Accepted 11th April 2019

First published on 12th April 2019

A series of lanthanide (Ln) compounds, [EuIII(L)2(MeOH)(H2O)](ClO4)3 (1), [TbIII(L)2(H2O)](Cl3) (2) and [MIII(L)2(NO3)](ClO4)2 (M = Eu (3); Sm (4); Tb (5); Dy (6)), have been prepared by the treatment of a neutral tetra-dentate ligand (N2,N9-dibutyl-1,10-phenanthroline-2,9-dicarboxamide, L) with various Ln(III) salts in MeOH. All of the compounds have been structurally characterized by X-ray crystallography. It is noted that the anions of the Ln(III) salts play crucial roles in the solid-state structures and the properties of the resulting complexes. In compounds 1 and 2, the metal centres are 10- and 9-coordinated, respectively, which are surrounded by two L ligands and solvent molecules. When the nitrate salts are used instead, four isostructural 10-coordinate Ln(III) compounds are obtained. The photophysical properties show that most of the compounds exhibit the characteristic emission peaks of Ln(III). Ligand L acts as a very good ‘antenna’ to sensitize the Eu(III) emission, but not for other Ln(III) emissions. The quantum yield of 3 is 75.4%, which is much higher than that of 1 (17.6%) due to the –OH oscillators of the coordinated H2O and MeOH molecules in 1. The magnetic properties of the Tb(III) compounds 2 and 5 and the Dy(III) compound 6 were investigated in detail as a natural extension of our previous study of the effect of high-coordination number on the single-ion magnet (SIM) behaviour of 3d compounds. The results show that subtle changes in the coordination environment have significant effects on the magnetic behaviour. The 9-coordinate compound 2 is not a single ion magnet (SIM), but both 10-coordinate compounds 5 and 6 exhibit field-induced SIM behaviour. It is also interesting to note that, upon the application of a small dc field, 5 exhibits two distinct relaxation processes, which is uncommon in Ln-based SIMs.


Multi-functional molecule-based materials that contain two or more useful physical properties in one molecule have attracted wide attention in recent years due to their potential applications in various fields.1–3 Lanthanide(III) ions are ideal candidates for constructing such molecular materials because their high coordination number (CN) and flexible coordination geometry can provide structural diversity. Ln(III) complexes also possess unique optical and magnetic properties that are very different from those of transition metal complexes.4

Luminescent Ln(III) compounds can display emissions from visible to near-infrared regions, with various special features including large Stokes shifts, narrow emission bands and very long-lived excited-state lifetimes, which enable them to be promising in widespread applications including display devices, luminescent probes in biology and tunable lasers.5,6 Although the emission quantum yield and lifetime of Ln(III) compounds are usually influenced by their coordination environment, their emissions are very characteristic and could be readily distinguished with the naked eye. For example, Tb(III) compounds usually exhibit green luminescence, while Eu(III) compounds usually exhibit red luminescence. The luminescence of Ln(III) compounds usually originate from metal-centred f–f transitions, which are both spin- and parity-forbidden. As a consequence, these transitions result in low-intensity absorption and emission.7 Therefore, direct excitation of Ln(III) ions is not efficient; however, the use of organic ligands with suitable π-systems is a feasible method for the construction of highly luminescent Ln(III) materials, because these organic chromophores are able to act as ‘antennas’ to harvest incident light and then transfer the energy to Ln(III), ultimately leading to the emissive 4f* excited states.8

Ln(III) complexes are also excellent candidates for the design of single-molecule magnets (SMMs) or single ion magnets (SIMs), because their strong spin–orbit coupling effect leads to significant single ion magnetic anisotropy.9 The intrinsic magnetic anisotropy D values together with the large spin S of Ln(III) ions make them suitable for the design of SMMs. However, the magnetic anisotropy of the Ln(III) compounds is strongly influenced by the ligand field and local coordination geometry.10 Generally speaking, compounds with a highly local symmetry can maximize the axial anisotropy and thus suppress the quantum tunnelling of magnetization (QTM), leading to high energy barriers and/or blocking temperatures. In previously reported Ln-based SIMs, complexes with a lower coordination number and higher local symmetry are considered as ideal candidates to display SIMs. In contrast, a complex with a high coordination number easily leads to a spherical ligand field that readily minimizes the magnetic anisotropy. Thus, most of the reported Ln-based SIMs have coordination numbers less than 9.11,12

Recently, we have designed a series of homoleptic mononuclear FeII and CoII compounds with coordination number 8 by employing the neutral tetradentate ligand (2,9-dialkylcarboxylate-1,10-phenanthroline or 6,6′-dialkylcarboxylate-2,2-bipyridine).13 Such a high CN is uncommon for late transition metal complexes; however, these 8-coordinate FeII and CoII complexes exhibit interesting field-induced SIM behaviour. Inspired by these results, we intend to synthesize similar high coordinate Ln(III) complexes as a natural extension of our previous work to be suitably modified by using amide carbonyl instead of the ester functional groups in order to have a stronger affinity for Ln(III).20b A new ligand, N2,N9-dibutyl-1,10-phenanthroline-2,9-dicarboxamide (L), has been prepared (Fig. 1). The presence of the n-butyl side chain gives better solubility and suppresses possible inter-molecular dipole–dipole interactions. A series of Ln(III) complexes with a CN 9 or 10 and an L/Ln(III) ratio of 2 have been obtained. The optical and magnetic properties of these complexes are fine-tuned using additional solvent molecules and/or counterions in the coordination sphere. Relatively large metal–ligand distances are found in these complexes, which may result in more localized metal electron density and hence a higher anisotropy in the Ln(III) complexes.14 Besides the intrinsic magnetic properties, the solid-state photophysical properties of these compounds were also investigated in detail in order to determine whether L is able to sensitize the emission of various lanthanide ions.

image file: c9qi00238c-f1.tif
Fig. 1 Structure of L.

Results and discussion

Synthesis and characterization

Ligand L was synthesized in high yield by direct condensation of 2,9-dicarboxyl-1,10-phenanthroline (PDA) with n-butylamine. The reaction of L with hydrated EuCl3 in the presence of excess NaClO4 in MeOH afforded the mononuclear compound [Eu(L)2(MeOH)(H2O)](ClO4)3 (1). Under similar conditions, its reaction with TbCl3 in the absence of NaClO4 afforded the compound [Tb(L)2(H2O)](Cl3) (2). The coordination numbers are 10 and 9, respectively, for 1 and 2, possibly due to their different ionic radii. Addition of NaClO4 does not change the coordination environment around the metal centres but only leads to different anions in 1 and 2. In order to reduce the variables for comparison of their properties, the nitrate salts are used in preparing compounds 3–6. The reaction of L with hydrated MIII(NO3)3 in MeOH afforded the isostructural compounds [MIII(L)2(NO3)](ClO4)2 (M = Eu (3), Sm (4), Tb (5) and Dy (6)). As shown by their X-ray crystal structures, all the metal centres have the same coordination number 10 in 3–6 (Fig. 2). Actually, our initial aim is to synthesize Ln(III) compounds with higher CN (CN = 12) by employing more than 3 equiv. L such that the metal centre is coordinated by three L ligands without any anions or H2O molecules. However, this approach is unsuccessful, possibly due to the steric effects of L which preclude the formation of a 3[thin space (1/6-em)]:[thin space (1/6-em)]1 (L to M) complex. The IR spectra of these compounds show the v(N–H) stretches in the range of 3215 cm−1 to 3299 cm−1 and the v(C[double bond, length as m-dash]O) stretches in a narrow range of 1634 cm−1 to 1640 cm−1. As compared with the free ligand L (v(N–H) at 3122 cm−1 and v(C[double bond, length as m-dash]O) at 1719 cm−1), these v(N–H) stretches are shifted to higher wavenumbers, while the v(C[double bond, length as m-dash]O) stretches are shifted to lower wavenumbers. These results indicate that L in these compounds is not deprotonated and remains as a neutral tetra-dentate ligand that coordinates with the metal via two carbonyl O atoms and two N atoms. Except for 2, all the compounds show a very strong broad stretching band at around 1100 cm−1, which is assigned to the ClO4 anion. The peak at around 1310 cm−1 in compounds 3–6 is assigned to the v(N–O) stretch of the coordinated NO3.
image file: c9qi00238c-f2.tif
Fig. 2 Syntheses of Ln(III) complexes 1–6.

The electrospray ionization mass spectrometry (ESI/MS) of these Ln(III) complexes was also performed. For the Eu(III) complex 1, the ESI/MS in MeOH (a +ve mode) shows two predominant peaks at m/z 303.4 and 504.3, which are assigned to the cations [Eu(L)2]3+ and {[Eu(L)2] + (ClO4)}2+, respectively (Fig. S1). There is also a minor peak at m/z 454.4 ([Eu(L)2–H]2+), which most likely comes from its deprotonation in MS. The ESI/MS of 3 is similar to that of 1 (Fig. S2), but an additional peak at m/z 485.9 is observed due to {[Eu(L)2] + (NO3)}2+. ESI/MS of the Tb(III) complex 2 in MeOH shows two predominant peaks at m/z 305.4 and 457.4, which are assigned to [Tb(L)2]3+ and [Tb(L)2–H]2+, respectively (Fig. S3). Compounds 4, 5 and 6 are isostructural to 3 and the ESI/MS of these compounds in MeOH also shows similar fragmental peaks to those of 3.

X-ray crystal structures

Single crystals suitable for X-ray crystallography were obtained by slow evaporation of their MeOH solutions. Their structures have been determined and the perspective viewings are shown in Fig. 3. Selected bond parameters (Å, °) are summarized in Table 1 and the related structural refinement details are listed in Tables S1 and S2.
image file: c9qi00238c-f3.tif
Fig. 3 Perspective viewings for the cationic structures of 1–6.
Table 1 Selected bond parameters (Å) for compounds 1–6
  1 2 3 4 5 6
M–O1 2.459(4) 2.442(4) 2.476(4) 2.489(4) 2.443(5) 2.406(4)
M–O2 2.446(4) 2.376(4) 2.479(4) 2.479(4) 2.447(5) 2.439(4)
M–O3 2.424(4) 2.431(4) 2.454(4) 2.449(4) 2.415(5) 2.424(5)
M–O4 2.469(4) 2.452(4) 2.436(4) 2.449(4) 2.421(5) 2.416(4)
M–O5 2.456(4) 2.373(4) 2.547(4) 2.567(4) 2.541(5) 2.544(5)
M–O6 2.421(4) 2.555(4) 2.562(4) 2.548(5) 2.569(4)
M–N2 2.630(5) 2.524(4) 2.564(4) 2.624(5) 2.531(6) 2.576(5)
M–N3 2.658(5) 2.497(4) 2.617(4) 2.576(5) 2.593(6) 2.518(5)
M–N6 2.696(5) 2.501(4) 2.631(4) 2.543(5) 2.513(6) 2.517(5)
M–N7 2.681(5) 2.553(4) 2.537(4) 2.631(5) 2.599(7) 2.583(5)
O2–M–O3 68.61(14) 80.21(12) 135.42(12) 81.82(14) 81.61(18) 78.89(14)
O2–M–O4 94.26(13) 73.58(12) 72.20(12) 72.05(13) 72.62(17) 74.03(15)

An asymmetric unit of 1 contains one cationic Eu(III) moiety, three anionic ClO4 molecules and three solvate H2O molecules (Fig. 3). It crystallizes in the triclinic P[1 with combining macron] space group. The crystal structure shows that Eu(III) in 1 is 10-coordinated (Fig. 3(1) and 4), with eight coordination sites occupied by two L ligands and the remainder by a H2O molecule and a MeOH molecule. To identify the polyhedron of 1, continuous shape measure (CShM) analysis was carried out by employing SHAPE 2.1 software.15 The result indicates that the coordination geometry around Eu(III) can be described as a distorted sphenocorona (C2v) (Table S3). The bond lengths of Eu–O (C[double bond, length as m-dash]O) are comparable with that of Eu–O (H2O or MeOH) with a narrow range of 2.421(4) to 2.459(4) Å, which are much shorter than the Eu–N bond lengths which are in the range of 2.630(5) to 2.696(5) Å. As shown in Fig. S4, H-bonding interactions are found among the anions, coordinated H2O and solvates.

Compound 2 crystallizes in the monoclinic P21/n space group and its asymmetric unit contains one cationic Tb(III) moiety, three anionic Cl molecules and six solvate H2O molecules. The Tb(III) ion is 9-coordinated by two L ligands involving four N atoms and four O atoms, and an aqua ligand. The analysis based on CShM software (Table S4) indicates that the coordination geometry around Tb(III) can be described as a muffin (Cs symmetry) (Fig. 3 and 4). The Tb–O bond lengths vary from 2.376(4) to 2.452(4) Å and the Tb–N bond lengths vary from 2.497(4) to 2.553(4) Å (Table 1). In the packing diagram, the closest Tb⋯Tb distance is about 15.85 Å. Similar to 1, extensive H-bonding interactions are found among the Cl molecules, solvate H2O, and the aqua ligand (Fig. S5).

image file: c9qi00238c-f4.tif
Fig. 4 Representations of the coordination geometries around Eu(III) and Tb(III) ions in 1, 2 and 3, respectively.

Compounds 3–6 are isostructural and crystallize in the triclinic P[1 with combining macron] space group. All the metal centres are 10-coordinated by four N atoms and four O atoms from two L ligands, and two O atoms from an η2-nitrate. Similar to 1, the coordination geometries of the compounds are close to the sphenocorona with an approximate C2v crystal-field symmetry (Tables S5 and S6).15,16 Since these four compounds are isostructural, only compound 3 will be discussed in detail. An asymmetric unit of 3 contains one cationic Eu(III) moiety, two disordered anionic ClO4 and two solvate H2O molecules (Fig. 3). One feature of 3 is that the coordinated H2O and MeOH molecules in 1 are replaced by a bidentate NO3 molecule. The Eu–N(L) and Eu–O(L) bond lengths in 1 and 3 are comparable, while the Eu–O(NO3) bond lengths (2.547(4) and 2.555(4) Å) in 3 are obviously longer than those of Eu–O(solvates) in 1.

These four isostructural compounds 3–6 show the expected decrease in bond lengths ongoing from Sm(III), Eu(III), Tb(III) to Dy(III). In all cases, the M–O (C[double bond, length as m-dash]O) bond lengths are significantly shorter than the M–O (NO3) bond lengths. It is noteworthy that the CN 10 in these compounds is the same along the series since the CN of Sm(III) with a larger radius is usually greater than that of Dy(III). The packing diagrams of complexes 3–6 show that the Ln(III) ions are well-separated by the anions ClO4 and the bulk butyl groups, with the closest M⋯M separation in the range of 10.78 to 11.07 Å. The presence of H2O molecules, anions ClO4, coordinated NO3 and amide N–H groups in the structure generates a set of hydrogen bonds, which are shown in Fig. S6–S8, respectively.

Optical spectroscopy of L and 1–6

The absorption spectra of the free ligand L and the lanthanide complexes 1–6 were studied in MeOH at room temperature, and the data are summarized in Table 2. As shown in Fig. 5, the UV-visible absorption spectrum of L features three main bands with λmax at around 240, 280 and 315 nm, which are attributed to π–π* transitions of L. The UV-visible absorption spectra of the Ln(III) complexes are similar to that of the free ligand L. Compared with L, the latter two absorption bands are slightly shifted to lower energy, which suggests increased rigidity and π-conjugation upon coordination of L with the Ln(III) complex. Thus, these absorptions are also attributed to ligand-centred π–π* transitions of L. Since f–f transitions are Laporte forbidden, the observation of these peaks with appreciable molar absorptivity coefficients indicates that ligand L acts as an ‘antenna’ to sensitize this lanthanide luminescence.
image file: c9qi00238c-f5.tif
Fig. 5 The UV/Vis spectra of L and complexes 1–6 in MeOH.
Table 2 Photophysical data for compounds 1–6
  Emissions ϕem/102 τ0/μs Absorptions λ/nm (ε/M−1 cm−1)
1 590; 612; 649; 684 17.6 524 240 (89[thin space (1/6-em)]540), 268 (34[thin space (1/6-em)]430), 295 (77[thin space (1/6-em)]970), 315sh (25[thin space (1/6-em)]070), 331sh (18[thin space (1/6-em)]230), 346sh (3310)
2 490; 543; 590; 615 0.93 13.5; 332 240 (126[thin space (1/6-em)]990), 260 (48[thin space (1/6-em)]970), 294 (98[thin space (1/6-em)]670), 319sh (31[thin space (1/6-em)]590), 330sh (22[thin space (1/6-em)]700), 347 (4290)
3 588; 612; 648; 684 75.4 814 240 (127[thin space (1/6-em)]710), 268 (49[thin space (1/6-em)]780), 295 (108[thin space (1/6-em)]400), 316sh (35[thin space (1/6-em)]100), 327sh (27[thin space (1/6-em)]660), 348sh (4990)
4 563; 596; 606; 643; 652 3.48 52.5 240 (126[thin space (1/6-em)]320), 268 (50[thin space (1/6-em)]790), 289sh (94[thin space (1/6-em)]730), 294 (106[thin space (1/6-em)]530), 317sh (35[thin space (1/6-em)]500), 329sh (27[thin space (1/6-em)]850), 347 (5340), 349 (5910)
5 543; 590; 613; 649 0.46 713 240 (114[thin space (1/6-em)]880), 268 (45[thin space (1/6-em)]410), 294 (85[thin space (1/6-em)]370), 320sh (26[thin space (1/6-em)]280), 327sh (21[thin space (1/6-em)]330), 341sh (6580)
6 239 (120[thin space (1/6-em)]290), 268 (48[thin space (1/6-em)]760), 294 (88[thin space (1/6-em)]300), 318sh (31[thin space (1/6-em)]240), 330sh (20[thin space (1/6-em)]500), 345sh (5390)

The solid-state luminescence spectra of these compounds were recorded at room temperature (Fig. 6). Except for the Dy(III) compound 6 which is non-emissive, all other compounds exhibit luminescence. In the solid state, upon excitation by λex = 355 nm, compounds 1 and 3 exhibit bright red emission. The emission bands occurring at 590, 617, 650, and 694 nm in 1 and 3 can be assigned to 5D07FJ (J = 1, 2, 3, and 4) transitions, while the 5D07F0 emission transition induced by the crystal field J mixing is not observed in both complexes. It is well known that the 5D07F2 and 5D07F1 transitions of the Eu(III) ion are electric-dipole (ED) and magnetic-dipole (MD), respectively. The former is extremely sensitive to site symmetry, while the latter emission intensity is mainly dependent on the crystal field around the Eu(III) ion. The intensity of the 5D07F2 transitions in 1 and 3 is much stronger than that of the 5D07F1 transition, indicating that the local symmetry environment around the Eu(III) ion in these two complexes is asymmetric,17 in agreement with the single-crystal X-ray analysis. In contrast to 1, both 5D07F1 and 5D07F2 transitions in 3 are split into two bands.18 The splitting of the magnetic dipole 5D07F1 transition typically indicates the presence of strong ligand fields, while that of 5D07F2 is possibly due to the absence of time-averaged rotational symmetry.19 The absence of ligand-based emission in 1 and 3 indicates that the efficient energy transfer from the ligand to the Eu(III) centre (the ‘antenna effect’) greatly enhances the optical performance.

image file: c9qi00238c-f6.tif
Fig. 6 Solid state emission spectra (λex = 355 nm) of 1 (a), 2 (b), 3 (c) and 5 (d).

The Eu(III) (5D0) lifetimes (λex = 355 nm) are similar with τ = 0.52 and 0.81 ms for 1 and 3, respectively. The decay profiles of 1 and 3 fit a single-exponential law, confirming that the Eu(III) ions in both complexes lie in the same environment in the solid samples. However, the measured luminescence quantum yield of 3 (Φ = 75.4%) is much higher than that of 1 (Φ = 17.6%) in the solid state at room temperature. The shorter lifetime and lower quantum yield of 1 compared with those of 3 indicate the quenching effect of the bound OH oscillators invoked by the coordinated H2O and MeOH molecules in 1. Another possible quenching mechanism via the population of ligand-to-metal charge transfer excited states from the 5D0 Eu(III) emitting state, followed by the efficient non-radiative decay to the ground state,20 should be ruled out because these two Eu(III) complexes have nearly the same coordination environments.

Upon excitation by λex = 355 nm, the emission spectrum of complex 2 shows characteristic transitions of Tb(III) ions at 490, 543, 590 and 615 nm, which can be assigned to 5D47F6, 5D47F5, 5D47F4 and 5D47F3 transitions, respectively. However, the Tb(III)-centred emission strongly mixes with ligand-centred emission at λem = 434 nm. The result indicates that energy transfer from L to Tb(III) is not as efficient as that in the Eu(III) complexes 1 and 3. Obviously, the emission intensity observed at I543 is the strongest in the Tb(III)-based emission bands in 2. Under similar conditions, complex 5 also shows characteristic transitions of Tb(III) ions, from the 5D4 level to the 7FJ levels and the ligand-centred emission of L is not observed in this solid-state sample. Although the emission positions of Tb(III)-based emissions in 5 are comparable with those in 2, the emission at 613 nm (I613) is the most intense transition, which is much stronger than that of I543. The change in the relative emission intensity of Ln(III)-centred transitions is mainly due to the variation of local symmetry of the Tb(III) ion from Cs to C2v upon substitution of an aqua ligand by a nitrate, which is also found in some related Ln(III)-based complexes.20b,21 The absence of ligand-based emission in 5 indicates that efficient energy transfer from the ligand to the Tb(III) centre (the ‘antenna effect’) greatly enhances the optical performance of the metal ion by simply replacing the aqua ligand with a nitrate. Luminescence decay of 2 exhibits a biexponential curve, yielding lifetimes of 13.4 μs and 0.33 ms, which are tentatively assigned to ligand-centred 3π–π* and metal-centred Tb(III) (5D4), respectively. In contrast, luminescence decay of 5 exhibits only monoexponential kinetics with a lifetime of τ = 0.71 ms. The measured luminescence quantum yields for 2 (Φ = 0.9%) and 5 (Φ = 0.5%) in the solid state at room temperature are much lower than those of the Eu(III) complexes 1 and 3. Moreover, another interesting feature of the emission of 2 is that its emission spectrum is strongly dependent on the excitation wavelength. As shown in Fig. S9, when excited at 355–390 nm, 2 presents an emission variation through a dual-emitting pathway; the emission intensity of ligand-centred emission gradually increases, while the relative intensity for the characteristic transitions of Tb(III) ions gradually decreases.22

The Dy(III) compound 6 is non-emissive. These results indicate that the excited-state energy (3LC) of L matches well only with the lower excited-state energy of 5D0 (Eu) (17[thin space (1/6-em)]200 cm−1), but not with the higher excited-state energy of 5D4 (Tb) (20[thin space (1/6-em)]400 cm−1) and 4F9/2 (Dy) (21[thin space (1/6-em)]000 cm−1) and thus the energy transfer from ligand L to the latter two lanthanide ions is not efficient.

For the Sm(III) compound 4, the visible emission bands centred at 561, 595 and 642 nm (associated with the 4G5/2-6HJ (J = 5/2, 7/2 and 9/2) transitions) were evident but were weak due to poor sensitization of the metal (Fig. S10). The Sm(III) (4G5/2) lifetime (τ = 52.5 μs) of 4 at λex = 355 nm is similar to the related luminescent Sm(III) complexes, which is much shorter than the Eu(III) and Tb(III) analogues.23 This complex exhibits moderate emission intensity (Φ = 3.48%) at room temperature in the solid state. The observation of ligand-centred emissions also indicates that ligand L is not a good sensitizer for Sm(III).23

In MeOH, except for the Eu(III) complexes 1 and 3 which exhibit characteristic metal-centred sharp emissions, all the other complexes exhibit highly structured blue emissions from ligand-centred π–π* transitions (Fig. S11). The peaks from metal-centred emissions are very weak and barely detectable, which appear at similar wavelengths to those in the solid state. The weaker emissions are possibly due to the decrease of energy gaps between 3π–π* and the excited state of Ln(III) ongoing from the solid state to solution, allowing a subsequent back-transfer from the excited state of the metal to the ligand.24 In addition, the H-bonding interaction of the amide NH group with solvents and/or the dissociation of nitrate occurring in MeOH may also result in quenching of emission, since ESI/MS of these compounds in MeOH suggest that only the [M(L)2]3+ moiety remains intact.

Magnetic properties

Static magnetic properties of compounds 2, 5 and 6 were investigated between 2 and 300 K under a dc field of 1 kOe. Plots of χMT versus T (χM is the molar magnetic susceptibility per Ln(III) molecule and T is the absolute temperature) for these compounds are shown in Fig. 7. At 300 K, the χMT values of 2, 5 and 6 are 10.79, 11.10 and 15.08 cm3 K mol−1, respectively, which are comparable with the expected value of 11.82 cm3 K mol−1 for a free single Tb(III) cation (S = 3, L = 3, 7F6, gJ = 3/2) and 14.17 cm3 K mol−1 for a free Dy(III) (S = 5/2, L = 5, 6H15/2, gJ = 4/3) ion. Upon cooling, the χMT values decrease continuously to reach 6.79 cm3 K mol−1 for 2, 8.45 cm3 K mol−1 for 5 and 10.09 cm3 K mol−1 for 6. The decrease of χMT on lowering the temperature is likely due to the depopulation of the MJ sublevels of the anisotropic Ln(III) ions that arises from the splitting of the ground term by the ligand field. It is also reasonable to neglect the dipolar coupling interaction between the mononuclear units because of the relatively long distance between the metal centres (see the Structural determination for 1–6 section).
image file: c9qi00238c-f7.tif
Fig. 7 Temperature dependence of χMT for compounds 2, 5 and 6. Data are collected at 1 kOe.

At 2 K, the magnetizations for compounds 5 and 6 show a relatively rapid increase at a low field and then a very slow linear increase with increasing H. The values reach 4.93 and 6.05B at 70 kOe, which are much smaller than the calculated saturation magnetization (Ms) values of 9 and 10 B for non-interacting Tb(III) and Dy(III) ions, respectively. This is most likely due to the strong crystal-field effect for the Ln(III) ion that eliminates the degeneracy of the ground state. The presence of low-lying excited states is another possible reason for the differences. As shown in Fig. 8, the non-superimposition of the M vs. H curves below 10 K suggests the presence of a significant magnetic anisotropy and/or low-lying excited states for the Tb(III) and Dy(III) ions in these compounds.

image file: c9qi00238c-f8.tif
Fig. 8 Low-temperature magnetization data for 5 and 6 under applied dc fields from 0 to 7 T at different temperatures.

Dynamic magnetic properties

In order to assess whether the Tb(III) and Dy(III) complexes exhibit SMM behaviour, dynamic alternating current (ac) magnetic susceptibility measurements as a function of temperature at different frequencies were carried out with or without an external applied static dc field. Unfortunately, under a zero external field, no maxima in in-phase (χ′) and out-of-phase (χ′′) susceptibilities emerge above 2 K for compounds 2, 5 and 6, indicating the presence of significant quantum tunnelling of the magnetization (QTM) in the ground state, which is commonly observed in the related Ln(III) compounds.11,12 In order to suppress the QTM, the relaxation of magnetization were collected at different external fields. Even the applied field was increased up to 1 kOe, and there is no ac signal for 2, indicating the lack of SMM behaviour. In contrast, compounds 5 and 6 exhibit ac susceptibility signals under an applied dc field (Fig. S12). The isostructural compounds 5 and 6 showed obvious frequency dependence with a maximum in their χ′′ curves (Fig. 9). This could be explained by the difference in their solid state structures. The Tb(III) site in 2 is in a highly distorted muffin (Cs symmetry) geometry. Upon substitution of the aqua ligand of 2 by an η2-NO3 ligand, the coordination geometries for both metal centres in 5 and 6 are close to a distorted dicapped square antiprism (D4d) or a sphenocorona (C2v) geometry with a symmetry higher than that of 2. It is well known that a higher symmetry, especially for a D4d symmetry, is in favour of suppressing the quantum tunnelling process more effectively and removing the transverse crystal field.25 Thus, it is not surprising to observe SMM behaviour due to the improvement of the local symmetry in 5 and 6. More importantly, it may also represent a simple and feasible strategy to modulate the SMM behaviour of the Ln(III) complexes by changing auxiliary ligands or anions.
image file: c9qi00238c-f9.tif
Fig. 9 Frequency dependence of in-phase (χ′) and out-of-phase (χ′′) ac magnetic susceptibilities for 5 (left) and 6 (right) under a 1.5 kOe (for 5) and 1.0 kOe (for 6) dc field.

The temperature-dependent magnetic dynamics of 5 was investigated under 1.5 kOe. Well-shaped peaks with two maxima are observed below 3 K, while the peaks with only one maximum from 3 to 5.5 K were observed and the maxima gradually shift to higher frequencies upon heating. These results indicate the presence of two distinct relaxation processes including a slow relaxation process (a) and a fast relaxation process (b), in the χ′′ vs. v curves for 5. The fast relaxation process (b) probably undergoes a thermally activated relaxation mechanism. The dual relaxation process in 5 could be further confirmed by the Cole–Cole plots with two obvious semicircles (Fig. 10 and S13). However, in the range of 1.8 K to 2.4 K, two partially merged semicircles are clearly observed, while the higher part in the temperature range of 2.6 K to 5.5 K could be well fitted by using the generalized Debye model,26 yielding a values in the range of 0.15 to 0.32, indicating a narrow distribution of relaxation time.

image file: c9qi00238c-f10.tif
Fig. 10 Cole–Cole plots for 5 (left) and 6 (right) under a 1 kOe dc field at indicated temperatures.

The relaxation process (a) below 3 K is much more complicated. On the basis of the previously reported related Ln-based SMMs, this relaxation mechanism for Tb(III) compounds may involve not only the Orbach mechanism but also the Raman direct and/or QTM processes (Fig. 11). The temperature-dependent relaxation time profiles can be fitted as a sum of the contributions of the Raman, Orbach, and quantum tunnelling relaxation mechanisms, as shown in eqn (1)

τ−1 = CTn + τ0−1[thin space (1/6-em)]exp(−Ueff/kT) + τQTM−1. (1)

image file: c9qi00238c-f11.tif
Fig. 11 ln(τ) vs. T−1 plots for 5 (left) and 6 (right). Red lines are the best fits for 5 and 6.

These three terms represent the Raman, the Orbach relaxation and the zero-field QTM, respectively. The energy barrier (Ueff) and the pre-exponential factor (τ0) were calculated to be 76.0 K and 1.35 × 10−11 s, respectively, which are typically found related to Ln(III)-based SIMs.9,10 In addition, C, n and τQTM were determined to be 3.27 K−4.72 s−1, 4.72 and 3.93 × 10−4 s, respectively.

Interestingly, dual magnetization relaxation processes were observed in 5, which is not so common in Ln-based SIMs, because in most cases, these processes result from the presence of two different metal sites at low temperature or from intermolecular interactions including H-bonding and π⋯π stacking. Nevertheless, it is noteworthy that the presence of dual relaxation processes was also observed in some monometallic complexes with unique crystallographically equivalent metal sites.27 The recently reported Ln-compounds, [Er(tpm)3(bipy)] and [Zn(μ-L)(μ-OAc)Ln(NO3)2]·CH3CN (Ln(III) = Er and Yb), with unique metal sites also underwent two thermally activated relaxation processes when a dc field was applied, which may be associated with a direct process favoured by the lifting of the Kramers degeneracy by the applied dc field.28

When 1.0 kOe dc fields were applied to the Dy(III) compound 6, both in-phase (χ′) and out-of-phase (χ′′) susceptibilities of 6 show significant frequency-dependent maxima typical of a field-induced SIM (Fig. 9). The maximum χ′′ values gradually shift to higher frequencies with increasing temperature until they move beyond the high-frequency limit of the instrument. However, it is interesting to note that only one relaxation process was observed, although complex 6 is isostructural to 5. At present, we have no reasonable explanation for the observed diversity of these two isostructural complexes. The single relaxation process in 6 could also be confirmed by the Cole–Cole plots with only one semicircle in the temperature range of 1.8 to 7 K (Fig. 10 and S14). The fitting by using the generalized Debye model gives a values in the range of 0.09 to 0.37. The temperature-dependent relaxation time profiles for 6 can be fitted using eqn (1). The energy barrier (Ueff) and the pre-exponential factor (τ0) for 6 are determined to be 44.3 K and 5.17 × 10−7 s, respectively. C and n values are determined to be 0.475 K−4.29 s−1 and 4.29, respectively (the QTM effect is not considered in 6).


In conclusion, we have reported a family of mononuclear Ln(III) complexes 1–6 based on the neutral tetra-dentate ligand L. As shown by their X-ray crystal structures, these compounds have the same L to M ratio (1[thin space (1/6-em)]:[thin space (1/6-em)]2), leaving only limited vacant sites for access of solvents or anions. By using amide carbonyl functions instead of ester groups, L is able to stabilize these Ln(III) compounds due to stronger binding affinity. In both the solid state and solution, L is able to sensitize effectively the Eu(III) emission via the so-called ‘antenna’ effect, which is evidenced by their high quantum yields in the solid state. The quantum yields are significantly affected by the small ancillary ligands. On substituting a H2O molecule and a MeOH molecule by a NO3 molecule, the quantum yield increases from 17.6% in 1 to 75.4% in 3, possibly due to the reducing quenching effects of –OH oscillators from coordinated solvents. In contrast, L is not a good sensitizer towards Tb(III)/Dy(III), due to their high-lying excited states. The magnetic properties of 2, 5 and 6 have been investigated in detail due to the presence of stronger anisotropy for the Tb(III)/Dy(III) ions. This work could be considered as an extension of our previous study on the effect of high coordination number 3d compounds towards SIM behaviour. Although compounds of CN 12 with L to M ratio = 3 were not obtained due to the significant steric effects of L, the CN 10 observed in these compounds is still higher than the normal Ln-based SIMs, which usually have a CN from 2 to 9. Moreover, the ancillary ligands influence not only the photophysical properties of these compounds but also their magnetic behaviour. Compounds 5 and 6 show field-induced SIM behaviour, but a similar phenomenon is not observed in 2. In addition, compound 5 undergoes dual magnetization relaxation processes, which is not so common in Tb(III)-based SIMs. The tuning of photophysical and magnetic properties by the small molecules deserves more attention, as it may be a promising means to design new multi-functional materials.

Experimental section

Materials and physical measurements

IR spectra were obtained as KBr discs using a Nicolet 360 FT-IR spectrophotometer. Electronic spectra were recorded using a PerkinElmer Lambda 19 spectrophotometer in 1 cm quartz cuvettes. ESI mass spectra were recorded using a PE-SCIEX API 150 EX single-quadruple mass spectrometer. Elemental analysis was performed using an ElementarVario MICRO Cube elemental analyzer. Solid-state emission spectra and lifetimes at room temperature were measured by using an Edinburgh Instruments LP920 laser flash photolysis system with a flashlamp pumped Q-switched Nd:YAG laser using 355 nm excitation. Absolute solid-state luminescent quantum yields were measured using an Edinburgh Instruments FLS980 spectrofluorometer by the integrating sphere method. Elemental analysis was carried out by using an Elementar Vario EL analyzer. Powder samples for magnetic analysis were made from their crystalline samples. The magnetic properties, including the variable-temperature magnetic susceptibility, field dependence of magnetization, and ac magnetic susceptibility, were investigated using a Quantum Design MPMS XL-7 or VSM SQUID system. Background corrections were done experimentally on the corresponding sample holder. The diamagnetism of the constituent atoms (Pascal's tables) was used to correct the experimental susceptibilities.
Structural determination for 1–6. Crystals of compounds 1–6 that are suitable for X-ray diffraction analysis were obtained by slow evaporation of their methanolic solutions. X-ray diffraction data for 1–6 were collected at low temperature (100 K) on an Oxford CCD diffractometer (Mo Kα, λ = 0.71073 Å). Their structures were resolved by the heavy-atom Patterson method and refined by full-matrix least-squares using SHELX-97 and expanded using Fourier techniques.29,30 CCDC 1898271–1898276 contain supplementary crystallographic data for compounds 1–6.

Synthesis of compounds

N2,N9-Dibutyl-1,10-phenanthroline-2,9-dicarboxamide (L). A solution of 2,9-dicarboxyl-1,10-phenanthroline (PDA) (900 mg, 3.36 mmol) and n-butyl amine was refluxed for 48 h to give a light yellow solution. The MeOH was removed under reduced pressure and the residue was poured into ice water to afford pale yellow needle crystals. After maintaining the mixture at 0 °C for 2 h, the pale yellow crystals were collected by filtration. Yield (800 mg, 63%). Selected IR (KBr, cm−1): v(N–H) 3122; v(C[double bond, length as m-dash]O) 1719; UV/vis (MeOH): λmax [nm] (ε [mol−1 dm3 cm−1]) 237 (47[thin space (1/6-em)]470), 246sh (41[thin space (1/6-em)]990), 260sh (19[thin space (1/6-em)]430), 281 (25[thin space (1/6-em)]210), 316 (9630), 330sh (5230), 346 (1890); elemental analysis of C22H26N4O2: calcd C 69.82, H 6.92, N 14.80; found C 69.88, H 6.80, N 14.87.
[EuIII(L)2(MeOH)(H2O)](ClO4)3 (1). A mixture of L (50 mg, 0.13 mmol) and EuCl3·6H2O (33 mg, 0.09 mmol) in MeOH (25 mL) was heated at reflux for 0.5 h. The resulting white precipitate was removed by filtration and then NaClO4·H2O (126 mg, 0.9 mmol) was added into the filtrate. Slow evaporation of the MeOH solution for 2 d gave white block crystals suitable for X-ray crystallography. Yield (63 mg, 53.4%). Selected IR (KBr, cm−1): v(N–H) 3217; v(C[double bond, length as m-dash]O) 1637; v(Cl–O) 1100. Elemental analysis of C45H58Cl3EuN8O18·2H2O: calcd C 41.79, H 4.83, N 8.66; found C 41.82, H 4.91, N 8.57. UV/vis (MeOH): λmax [nm] (ε [mol−1 dm3 cm−1]): 240 (89[thin space (1/6-em)]540), 268 (34[thin space (1/6-em)]430), 295 (77[thin space (1/6-em)]970), 315sh (25[thin space (1/6-em)]070), 331sh (18[thin space (1/6-em)]230), 346sh (3310).
[TbIII(L)2(H2O)]Cl3 (2). A mixture of L (50 mg, 0.13 mmol) and TbCl3·6H2O (34 mg, 0.09 mmol) in MeOH (25 mL) was refluxed for 0.5 h. The white precipitate was removed by filtration and slow evaporation of the MeOH solution for 3 d gave white block crystals suitable for X-ray crystallography. Yield (42 mg, 40.9%). Selected IR (KBr, cm−1): v(N–H) 3220; v(C[double bond, length as m-dash]O) 1638. Elemental analysis of C44H54Cl3N8O5Tb·3H2O: calcd C 48.29, H 5.53, N 10.24; found C 48.31, H 5.60, N 10.17. UV/vis (MeOH): λmax [nm] (ε [mol−1 dm3 cm−1]): 240 (126[thin space (1/6-em)]990), 260 (48[thin space (1/6-em)]970), 294 (98[thin space (1/6-em)]670), 319sh (31[thin space (1/6-em)]590), 330sh (22[thin space (1/6-em)]700), 347 (4290).
[EuIII(L)2(NO3)](ClO4)2 (3). White block crystals of 3 were obtained by a procedure similar to that for 1 except that Eu(NO3)3·6H2O was used instead of EuCl3·6H2O. Yield (49 mg, 45.2%). Selected IR (KBr, cm−1): v(N–H) 3222; v(C[double bond, length as m-dash]O) 1637, v(Cl–O) 1120. Elemental analysis of C44H52Cl2N9O15Eu·2H2O: calcd C 43.83, H 4.68, N 10.45; found C 43.77, H 4,72, N 10.35. UV/vis (MeOH): λmax [nm] (ε [mol−1 dm3 cm−1]): 240 (127[thin space (1/6-em)]710), 268 (49[thin space (1/6-em)]780), 295 (108[thin space (1/6-em)]400), 316sh (35[thin space (1/6-em)]100), 327sh (27[thin space (1/6-em)]660), 348sh (4990).
[SmIII(L)2(NO3)](ClO4)2 (4). The synthetic procedure for 4 is similar to that of 3 except that Sm(NO3)3·6H2O (40 mg, 0.09 mmol) was used instead of Eu(NO3)3·6H2O. Yield (39 mg, 37%). Selected IR (KBr, cm−1): v(N–H) 3215; v(C[double bond, length as m-dash]O) 1635, v(Cl–O) 1108. Elemental analysis of C44H52Cl2N9O15Sm·2H2O: calcd C 43.88, H 4.69, N 10.47; found C 43.70, H 4.73, N 10.43. UV/vis (MeOH): λmax [nm] (ε [mol−1 dm3 cm−1]): 240 (126[thin space (1/6-em)]320), 268 (50[thin space (1/6-em)]790), 289sh (94[thin space (1/6-em)]730), 294 (106[thin space (1/6-em)]530), 317sh (35[thin space (1/6-em)]500), 329sh (27[thin space (1/6-em)]850), 347 (5340), 349 (5910).
[TbIII(L)2(NO3)](ClO4)2 (5). The synthetic procedure for 5 is similar to that of 3 except that Tb (NO3)3·6H2O (41 mg, 0.09 mmol) was used instead of Eu(NO3)3·6H2O. Yield (57 mg, 50.9%). Selected IR (KBr, cm−1): v(N–H) 3299; v(C[double bond, length as m-dash]O) 1638, v(Cl–O) 1100. Elemental analysis of C44H52Cl2N9O15Tb·2H2O: calcd C 43.57, H 4.65, N 10.39; found C 43.60, H 4.60, N 10.30. UV/vis (MeOH): λmax [nm] (ε [mol−1 dm3 cm−1]): 240 (114[thin space (1/6-em)]880), 268 (45[thin space (1/6-em)]410), 294 (85[thin space (1/6-em)]370), 320sh (26[thin space (1/6-em)]280), 327sh (21[thin space (1/6-em)]330), 341sh (6580).
[DyIII(L)2(NO3)](ClO4)2 (6). The synthetic procedure for 6 is similar to that of 3 except that Dy(NO3)3·6H2O (41 mg, 0.09 mmol) was used instead of Eu(NO3)3·6H2O. Yield (43 mg, 38.9%). Selected IR (KBr, cm−1): v(N–H) 3243; v(C[double bond, length as m-dash]O) 1638, v(Cl–O) 1121. Elemental analysis of C44H52Cl2N9O15Dy·2H2O: calcd C 43.45, H 4.64, N 10.36; found C 43.41, H 4.65, N 10.27. UV/vis (MeOH): λmax [nm] (ε [mol−1 dm3 cm−1]): 239 (120[thin space (1/6-em)]290), 268 (48[thin space (1/6-em)]760), 294 (88[thin space (1/6-em)]300), 318sh (31[thin space (1/6-em)]240), 330sh (20[thin space (1/6-em)]500), 345sh (5390).

Author contributions

J.X., L.M.Z., and T.C.L. designed the experiments. Q.Q.S., K.F., and X.X.J. synthesized the compounds and solved the X-ray structures. S.C.C. and C.C.K investigated photophysical properties. X.D.H. L.J.L. and Y.J.L. performed magnetic studies. J.X and T.C.L. designed the study. J. X., L.M.Z. and T.C.L. analyzed the data and wrote the paper. All authors discussed the results and commented on the manuscript.

Conflicts of interest

There are no conflicts to declare.


The authors gratefully acknowledge the financial support of the Natural Science Foundation of China (21771026) and the Hubei Provincial Natural Science Foundation of China (2018CFA047).

Notes and references

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Electronic supplementary information (ESI) available. CCDC 1898271–1898276. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9qi00238c
These authors contributed equally to this work.

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