Single-Ion Magnet and Photoluminescence Properties of Lanthanide(III) Coordination Polymers Based on Pyrimidine-4,6-Dicarboxylate

magnetochemistry Article Single-Ion Magnet and Photoluminescence Properties of Lanthanide(III) Coordination Polymers Based on Pyrimidine-4,6-Dicarboxylate Oier Pajuelo-Corral 1, Jose Angel García2, Oscar Castillo 3,4, Antonio Luque 3,4, Antonio Rodríguez-Diéguez 5 and Javier Cepeda 1,*   Citation: Pajuelo-Corral, O.; García, J.A.; Castillo, O.; Luque, A.; Rodríguez-Diéguez, A.; Cepeda, J. Single-Ion Magnet and Photoluminescence Properties of Lanthanide(III) Coordination Polymers Based on Pyrimidine-4,6-Dicarboxylate. Magnetochemistry 2021,7, 8. https://doi.org/10.3390/ magnetochemistry7010008 Received: 4 December 2020 Accepted: 29 December 2020 Published: 4 January 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1Departamento de Química Aplicada, Facultad de Química, Universidad del País Vasco/Euskal Herriko Unibertsitatea (UPV/EHU), 20018 Donostia, Spain; oier[email protected] 2Departamento de Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco/Euskal Herriko Unibertsitatea (UPV/EHU), 48940 Leioa, Spain; [email protected] 3 Departamento de Química Inorgánica, Facultad de Ciencia y Tecnología, Universidad del País Vasco/Euskal Herriko Unibertsitatea (UPV/EHU), 48940 Leioa, Spain; oscar[email protected] (O.C.); [email protected] (A.L.) 4BCMaterials, Basque Center for Materials, Applications and Nanostructures, UPV/EHU Science Park, 48940 Leioa, Spain 5 Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain; [email protected] *Correspondence: javier[email protected]; Tel.: +34-943015409 Abstract: Herein, we report the magnetic and photoluminescence characterization of coordination polymers (CP) built from the combination of lanthanide(III) ions, pyrimidine-4,6-dicarboxylate (pmdc) ligand and a co-ligand with formula {[Dy( µ -pmdc)( µ -ox) 0.5 (H 2 O) 3 ] · 2H 2 O} n ( 1-Dy ), {[Dy( µ3 - pmdc)( µ -ox) 0.5 (H 2 O) 2 ] ~2.33H 2 O} n ( 2-Dy ), {[Dy 2 ( µ3 -pmdc)( µ4 -pmdc)( µ -ox)(H 2 O) 3 ] · 5H 2 O} n ( 3-Dy ), {[Ln( µ3 -pmdc)( µ -ox) 0.5 (H 2 O) 2 ] · H 2 O} n (where Ln(III) = Nd ( 4-Nd ), Sm ( 4-Sm ), Eu ( 4-Eu ) and Dy ( 4-Dy )) and {[Dy( µ4 -pmdc)(NO 3 )(H 2 O)] · H 2 O} n ( 5-Dy ). It must be noted the presence of oxalate anion acting as ditopic co-ligand in compounds 1-Dy , 2-Dy , 3-Dy and 4-Ln , whereas in 5-Dy the nitrate anion plays the role of terminal co-ligand. Direct current measurements carried out for the dysprosium-based CPs reveal almost negligible interactions between Dy 3+ ions within the crystal structure, which is confirmed by computed values of the exchange parameters J. In addition, alternating current measurements show field-induced single-molecule magnet (SMM) behavior in compounds 1-Dy , 2-Dy , 4-Dy and 5-Dy , whereas slight-frequency dependence is also observed in 3-Dy . Solid state emission spectra performed at room temperature for those compounds emitting in visible region confirm the occurrence of significant ligand-to-lanthanide charge transfer in view of the strong characteristic emissions for all lanthanide ions. Emission decay curves were also recorded to estimate the emission lifetimes for the reported compounds, in addition to the absolute quantum yields. Among them, the high quantum yield of 25.0% measured for 4-Eu is to be highlighted as a representative example of the good emissive properties of the materials. Keywords: single-molecule magnets; lanthanide(III) ions; coordination polymers; slow relaxation of the magnetization; photoluminescence 1. Introduction The pre-designed synthesis and characterization of coordination polymers (CPs) has clearly become a primary research strategy in order to yield a particular multifunctional material which exhibits good performance in a particular research field by combining multiple physical properties [ 1 , 2 ]. A main reason is derived from the fact that these materials may be oriented to a particular spatial arrangement and topology which better fits the desired property, usually achieved by selecting the appropriate metal ions and organic ligands among Magnetochemistry 2021,7, 8. https://doi.org/10.3390/magnetochemistry7010008 https://www.mdpi.com/journal/magnetochemistry Magnetochemistry 2021,7, 8 2 of 20 an almost infinite bench of combinations [ 3 , 4 ]. In fact, the actual research on these materials is mainly focused on their more classical properties, including adsorption/purification of pollutant or fuel gases [ 5 – 9 ], liquids [ 10 ], or drugs, biomolecules delivery in a controlled way [ 11 , 12 ], catalysis [ 9 , 13 ], magnetism [ 14 – 16 ] and luminescence [ 17 , 18 ], although very recently it has been also expanding to other yet unexplored novel areas such as molecular sensing [ 19 , 20 ], enantiospecific resolution [ 21 ] and spintronics [ 22 ]. Among all these interesting properties, magnetism and luminescence have gained an increasing interest due to the potential of CPs to behave as single-molecule magnets (SMMs) [ 23 – 26 ] and solids-state lighting materials (organic light-emitting diodes, OLEDs) [ 27 ], by taking advantage of previous properties, respectively. Inside this sorts of materials, those built up from the coordination of organic ligands to lanthanide(III) ions have shown enormous potential during the last years owing to their particular electronic structure, which being hardly disturbed by the ligand field in the crystal structure, affords the aforementioned properties to the resulting CP [28–31]. To start with their magnetic properties, once it was corroborated in 2014 by MínguezEspallargas et al. with a Ln(III)-based MOF (metal-organic framework) presented SMM behavior [ 32 ], the research about the way the magnetic anisotropy is manifested in the MOF’s structure was studied in depth in the following years [ 23 ]. As is known, SMMs exhibit slow relaxation of the magnetization (SRM) below certain temperature (called blocking temperature, TB) as a consequence of the concurrence of large spin state in the molecule and magnetic anisotropy (usually axial, D; though other distributions such as plane anisotropy are also known, E). Paramagnetic lanthanide(III) ions usually present a large magnetic moment and sizeable single-ion anisotropy derived from spin-orbit coupling (soc) effect [ 29 , 33 ] that varies according to nature and symmetry of their local environment, in such a way that they may lead to astonishing energy barriers which impede the reversal of the magnetization in the compound [ 34 ]. On another level, the use of these metal centers is often compatible with the occurrence of luminescence on the CPs, given f-f based intraionic emissions can occur in lanthanide-organic frameworks (Ln-OFs) provided that inner levels of lanthanides(III) keep undistorted by the molecular ligand-based electronic structure [ 35 , 36 ]. Moreover, although the light-absorbing capacity of these metal ions is quite low due to their narrow absorption bands (despite the high intensity of many bands in the ultraviolet (UV) region), their coordination to organic ligands may largely improve this problem by means of the antennae effect, by which the energy absorbed by the ligand is transferred from its excited triplet state to an excited Ln(III)-based inner state from which the emissive state is eventually populated [37,38]. Bearing these ideas in mind, together with the fact that Ln-OFs require the use of ligands that are able to not only isolate lanthanide(III) centers from each other in the network to bring SRM but also provide both good light absorption and effective transfers to the lanthanide(III), we have turned to pyrimidine-4,6-dicarboxylate (pmdc) [ 39 ]. As previously proved by our group [ 40 , 41 ], this ligand fulfills both requisites by presenting low capacity to set magnetic exchange interactions among lanthanide(III) centers while it brings good absorption capacity. Therefore, in this study we undertake a detailed analysis of the single-ion magnet and photoluminescent properties of Ln-pmdc based compounds to evaluate their potential in the field of magneto-luminescent functional materials. 2. Results and Discussion 2.1. Brief Structural Overview There are five different crystalline families of coordination polymers described in this work, which consist of lanthanide(III) ions, pmdc ligand and a co-ligand that changes for a particular family. In particular, the co-ligand is oxalate for compounds 1-Dy , 2-Dy , 3-Dy and 4-Ln because it is in situ generated in the reaction media (as part of a well-known ringopening concomitant reaction that happens in aqueous mixtures containing lanthanide(III) ions in acidic pH and open atmosphere, observed not only for pmdc ligand but also for many other similar aromatic molecules) [ 42 ] and thus incorporated to the final products. Magnetochemistry 2021,7, 8 3 of 20 Despite the fact that oxalate ligand shares a unique coordination pattern in all compounds (the most habitual bis-bidentate mode as expected for its large affinity to lanthanide(III) ions according to the Hard-Soft Acid-Base (HSAB) principle) [ 43 ], pmdc ligand shows a variable coordination flexibility that depends on the entropy driven desolvation pathway [ 44 , 45 ]. In this sense, lanthanide(III) ions coordinate to less water molecules as the temperature of hydrothermal reaction rises, in such a way that the vacant positions in the coordination shell are more easily occupied by carboxylate oxygen atoms belonging to the pmdc ligand. Hence, pmdc ligand increases its denticity moving from the basal bis-chelating mode ( µ - κ2 N,O: κ2 N’,O’) in 1-Dy to other highly coordinating modes ( µ3 - κ2 N,O: κ2 N’,O’: κ O” in 2-Dy , 3-Dy and 4-Ln and µ4 - κ2 N,O: κ2 N’,O’: κ O”: κ O”’ in 5-Dy ) as the temperature rises. The aforementioned effect is also noticed by the presence of less water molecules (including coordination and crystallization molecules) per lanthanide(III) atom enclosed in the crystal packings of the CPs, which eventually generates more condensed frameworks ( 1-Dy < 2-Dy <3-Dy <4-Ln) with the increasing synthesis temperature. It is worth highlighting that all these coordination modes share theoccurrence of two N/O-chelating rings, a fact that forces the ligand to acquire a planar arrangement. In fact, this may be taken as a characteristic fact of pmdc ligand because such preponderant coordination mode is only prevented when a bulky substituent is present in the ligand such as the phenyl ring in ppmdc (2-phenylpyrimidine-4,6-dicarboxylic acid), in which the coordination through the nitrogen atoms is hindered in the CP of {[Eu(ppmdc)(phen)(C2O4)0.5]·0.5H2O}nformula [46]. On the other hand, compound 5-Dy possesses nitrate anions as co-ligand due to the solvent-free procedure employed for its synthesis. In this case, the reagents (dysprosium(III) nitrate hydrate and H 2 pmdc) are directly mixed and heated at 190 ◦ C in a closed vessel, such that the melt formed is very rich in nitrate ions and yield single crystals in which those anions are able to compete with water molecules for occupying coordination sites. Apart from the structures included in this work, it must be mentioned that pmdc ligand is able to render some additional crystalline compounds with most of lanthanide ions involving no additional linkers, namely {[Ln( µ -pmdc) 1.5 (H 2 O) 3 ] · xH 2 O} n (where x = 5.5–6.5 depending on the metal ion) and {[Ln 2 ( µ4 -pmdc) 2 ( µ -pmdc)(H 2 O) 2 ] · H 2 O} n CPs [ 47 ], in addition to the MOF of {[Eu( µ4 -pmdc) 2 ( µ3 -OH) 2 ] · 3H 2 O} n formula [ 48 ] in which hydroxide anion acts a co-ligand. In both cases, the compounds are synthesized starting from aqueous solutions with basic pH under solvothermal conditions, a medium that avoids the aforementioned spontaneous generation of oxalate [ 42 ] in such a way that these alternative frameworks are crystallized. 2.2. Structural Description A detailed structural description of most of the compounds, except for compounds 4-Ln (whose crystallographic data are summarized in Table S1), has been already done in previously reported works [ 40 , 41 ], so their structural description will be briefly discussed. Compound 1-Dy crystallizes in the monoclinic P2 1 /n space group with {[Dy( µ -pmdc) ( µ -ox) 0.5 (H 2 O) 3 ] · 2H 2 O} n formula as part of a family of other isostructural homologues with other lanthanide(III) ions. The Dy atom is linked to two pmdc ligands, one oxalate anion and three water molecules, rendering a nine-coordinated N 2 O 4 Ow 3 donor set which can be described as a spherical capped square antiprism (CSAPR) polyhedron (S CSAPR = 0.806, see Supporting information, Table S2) as depicted by the continuous shape measurements (CShMs) using the SHAPE program [ 49 ]. The coordination bonds are in the range of 2.656–2.674 Å for Dy-N, 2.283–2.398 Å for Dy-O carboxylate and 2.373–2.493 Å for Dy-O water . Detailed distances are shown in Table S3 on Electronic Supplementary Information (ESI). The metal centers are linked by planar bis-bidentate pmdc ligands into a corrugated chain along the crystallographic baxis, in which an intermetallic distance of ca. 7.24 Å is imposed. Moreover, the adjacent chains are joined to one another by the bis-chelating oxalate anion, which separates the dysprosium atoms at ca. 6.14 Å and gives rise to 2D sheets of hcb topology that spread along the (101) plane. The successive planes are connected into the 3D packing by means of hydrogen bonds established between the Magnetochemistry 2021,7, 8 4 of 20 coordinated water molecules and uncoordinated oxygen atoms of pmdc ligands belonging to adjacent sheets (Figure 1). Moreover, lattice water molecules placed between sheets take part into additional hydrogen bonding interactions with the 2D metal-organic sheets, which reinforces the overall crystal structure. For comparative purposes, this framework resembles the isoreticular architecture shown by the oxalate-free compound of {[Dy( µ - pmdc) 1.5 (H 2 O) 3 ] · 5.5H 2 O} n formula in which the oxalate bridge is replaced by an additional bis-chelating pmdc linker yields a similar 2D-layered structure [ 47 ]. In fact, the most significant change of 1-Dy with respect to {[Dy( µ -pmdc) 1.5 (H 2 O) 3 ] · 5.5H 2 O} n is the slight change occurring in the network as result of replacement a µ -ox by a µ -pmdc, with the subsequent enlargement in the Dy···Dy distance of ca. 1 Å (from ca. 6.1 to 7.3 Å). Magnetochemistry 2021, 7, x FOR PEER REVIEW 4 of 21 of hcb topology that spread along the (101) plane. The successive planes are connected into the 3D packing by means of hydrogen bonds established between the coordinated water molecules and uncoordinated oxygen atoms of pmdc ligands belonging to adjacent sheets (Figure 1). Moreover, lattice water molecules placed between sheets take part into additional hydrogen bonding interactions with the 2D metal-organic sheets, which reinforces the overall crystal structure. For comparative purposes, this framework resembles the isoreticular architecture shown by the oxalate-free compound of {[Dy(μpmdc)1.5(H2O)3]·5.5H2O}n formula in which the oxalate bridge is replaced by an additional bis-chelating pmdc linker yields a similar 2D-layered structure [47]. In fact, the most significant change of 1-Dy with respect to {[Dy(μ-pmdc)1.5(H2O)3]·5.5H2O}n is the slight change occurring in the network as result of replacement a μ-ox by a μ-pmdc, with the subsequent enlargement in the Dy···Dy distance of ca. 1 Å (from ca. 6.1 to 7.3 Å). Figure 1. Crystal packing of 1-Dy showing the 2D sheets. Inset represents a fragment of the compound with the coordination polyhedron. Color coding: green, dysprosium; red, oxygen; blue, nitrogen; grey, carbon; pink, hydrogen. Being crystallized in the P21/n space group, the titled compound 2-Dy presents the {[Dy(μ3-pmdc)(μ-ox)0.5(H2O)2] ~2.33H2O}n formula and shows a similar structure to that of 1-Dy, but with one of the coordinated water molecules being replaced by a non-chelating oxygen atom of the pmdc ligand (which was uncoordinated in 1-Dy), a fact that increases the dimensionality of the structure to give rise to a 3D framework. Hence, in the core of the building crystal, the dysprosium atom shows a N2O5Ow2 donor set coming from two symmetrically related pmdc ligands, a centrosymmetric oxalate anion and two coordinated water molecules (Figure 2). Distances between dysprosium and donor atoms in the coordination center are listed in Table S3. This replacement in the coordination sphere increases the distortion of the coordination polyhedron with regard to the previously analyzed ideal geometry, in such a way that the environment of Dy1 atom in 2-Dy corresponds to an intermediate between CSAPR and spherical tricapped trigonal prism (TCTPR) as shown by the CShMs (SCSAPR = 0.828 and STCTPR = 0.738, Table S2). Despite the similar connection shown for 1-Dy, the rearrangement of the coordination sphere promotes some distance changes between metal centers (Dy···Dy distance is 7.03 Å between for pmdc bridging centers and 6.23 Å for oxalate bridging centers, respectively). Moreover, the μ3-κN,O:κN’,O’:κO’’ coordination mode displayed by the pmdc ligand in 2-Dy yields an open 3D framework of ins topology (it is a bimodal network because not only lanthanide but also the pmdc ligand act as nodes). Solvent water molecules are placed in the channels of the architecture by bringing irregular patterns owing to the crystallographic modulation (the structure is more precisely described as a 1 × 1 × 3 supercell) existing in the structure which, in any case, does not affect the already described overall crystal architecture (Figure 2). Figure 1. Crystal packing of 1-Dy showing the 2D sheets. Inset represents a fragment of the compound with the coordination polyhedron. Color coding: green, dysprosium; red, oxygen; blue, nitrogen; grey, carbon; pink, hydrogen. Being crystallized in the P2 1 /n space group, the titled compound 2-Dy presents the {[Dy( µ3 -pmdc)( µ -ox) 0.5 (H 2 O) 2 ] ~2.33H 2 O} n formula and shows a similar structure to that of 1-Dy , but with one of the coordinated water molecules being replaced by a non-chelating oxygen atom of the pmdc ligand (which was uncoordinated in 1-Dy ), a fact that increases the dimensionality of the structure to give rise to a 3D framework. Hence, in the core of the building crystal, the dysprosium atom shows a N 2 O 5 Ow 2 donor set coming from two symmetrically related pmdc ligands, a centrosymmetric oxalate anion and two coordinated water molecules (Figure 2). Distances between dysprosium and donor atoms in the coordination center are listed in Table S3. This replacement in the coordination sphere increases the distortion of the coordination polyhedron with regard to the previously analyzed ideal geometry, in such a way that the environment of Dy1 atom in 2-Dy corresponds to an intermediate between CSAPR and spherical tricapped trigonal prism (TCTPR) as shown by the CShMs (S CSAPR = 0.828 and S TCTPR = 0.738, Table S2). Despite the similar connection shown for 1-Dy , the rearrangement of the coordination sphere promotes some distance changes between metal centers (Dy ··· Dy distance is 7.03 Å between for pmdc bridging centers and 6.23 Å for oxalate bridging centers, respectively). Moreover, the µ3 - κ N,O: κ N’,O’: κ O” coordination mode displayed by the pmdc ligand in 2-Dy yields an open 3D framework of ins topology (it is a bimodal network because not only lanthanide but also the pmdc ligand act as nodes). Solvent water molecules are placed in the channels of the architecture by bringing irregular patterns owing to the crystallographic modulation (the structure is more precisely described as a 1 × 1 × 3 supercell) existing in the structure which, in any case, does not affect the already described overall crystal architecture (Figure 2). Compound 3-Dy, obtained as a polycrystalline powder, corresponds to a family of isostructural CPs of {[Ln 2 ( µ3 -pmdc)( µ4 -pmdc)( µ -ox)(H 2 O) 3 ] · 5H 2 O} n formula (Figure S1). These compounds crystallize in the non-centrosymmetric Pna2 1 space group and show a crystal structure related to that of 2-Dy because it also exhibits a 3D open framework. However, in 3-Dy there are two crystallographic independent metal centers, one of which Magnetochemistry 2021,7, 8 5 of 20 (Ln1) shows the previously specified LnN 2 O 5 Ow 2 coordination environment, whereas in the second center (Ln2) one of the coordinated water molecules is replaced by an oxygen of pmdc ligand, thus displaying a LnN 2 O 6 Ow coordination environment (Figure 3). Despite the fact that no further details on the coordination environment can be given for the dysprosium counterpart, based on the Gd analogue, it may be concluded that the larger connectivity afforded by pmdc brings an increased distortion in the metal center (S TCTPR = 1.074 and S CSAPR = 0.587 for Ln 1 and Ln 2, respectively). The linkage of the metal centers by the highly connected pmdc ligands gives rise to a three-dimensional architecture that possesses the jcr1 topology and exhibits tubular channels along the crystallographic aaxis, which are filled by lattice water molecules. Putting aside the specific topology of shown by this framework, it shares strict structural relationship with the previously reported oxalate-free CP of {[Dy 2 ( µ4 -pmdc) 2 ( µ -pmdc)(H 2 O) 2 ] · H 2 O} n formula [ 47 ], since the latter may be considered as the next step to bring higher metal-organic connectivity in the 3D framework. In the same way as was explained for the 1-Dy ↔2-Dy pair, the aforementioned oxalate-free CP may be considered the result of a subsequent replacement of a coordination water molecule by a non-chelating carboxylate oxygen atom of pmdc ligand in the architecture of 3-Dy , a fact that is only achieved in the absence of oxalate ligand, in such a way that pmdc linker can accommodate the required arrangement in the framework to increase its coordination to dysprosium(III) ions. Magnetochemistry 2021, 7, x FOR PEER REVIEW 5 of 21 Figure 2. Crystal packing of 2-Dy showing the environment around the metal center. Color coding: green, dysprosium; red, oxygen; blue, nitrogen; grey, carbon; pink, hydrogen. Compound 3-Dy, obtained as a polycrystalline powder, corresponds to a family of isostructural CPs of {[Ln2(μ3-pmdc)(μ4-pmdc)(μ-ox)(H2O)3]·5H2O}n formula (Figure S1). These compounds crystallize in the non-centrosymmetric Pna21 space group and show a crystal structure related to that of 2-Dy because it also exhibits a 3D open framework. However, in 3-Dy there are two crystallographic independent metal centers, one of which (Ln1) shows the previously specified LnN2O5Ow2 coordination environment, whereas in the second center (Ln2) one of the coordinated water molecules is replaced by an oxygen of pmdc ligand, thus displaying a LnN2O6Ow coordination environment (Figure 3). Despite the fact that no further details on the coordination environment can be given for the dysprosium counterpart, based on the Gd analogue, it may be concluded that the larger connectivity afforded by pmdc brings an increased distortion in the metal center (STCTPR = 1.074 and SCSAPR = 0.587 for Ln 1 and Ln 2, respectively). The linkage of the metal centers by the highly connected pmdc ligands gives rise to a three-dimensional architecture that possesses the jcr1 topology and exhibits tubular channels along the crystallographic a axis, which are filled by lattice water molecules. Putting aside the specific topology of shown by this framework, it shares strict structural relationship with the previously reported oxalate-free CP of {[Dy2(μ4-pmdc)2(μ-pmdc)(H2O)2]·H2O}n formula [47], since the latter may be considered as the next step to bring higher metal-organic connectivity in the 3D framework. In the same way as was explained for the 1-Dy ↔ 2-Dy pair, the aforementioned oxalate-free CP may be considered the result of a subsequent replacement of a coordination water molecule by a non-chelating carboxylate oxygen atom of pmdc ligand in the architecture of 3-Dy, a fact that is only achieved in the absence of oxalate ligand, in such a way that pmdc linker can accommodate the required arrangement in the framework to increase its coordination to dysprosium(III) ions. Figure 2. Crystal packing of 2-Dy showing the environment around the metal center. Color coding: green, dysprosium; red, oxygen; blue, nitrogen; grey, carbon; pink, hydrogen. Magnetochemistry 2021, 7, x FOR PEER REVIEW 6 of 21 Figure 3. (a) Crystal building of the previously reported Gd-based coordination polymer (CP) isostructural to 3-Dy. (b) Fragment of compound 3-Gd showing the coordination polyhedron and the environment around the metal centers. Color coding: green, metal; red, oxygen; blue, nitrogen; grey, carbon; pink, hydrogen. Compounds 4-Ln are all isostructural as confirmed by the single-crystal X-ray diffraction measurements for compounds 4-Nd, 4-Sm and 4-Eu and by the PXRD data analysis of a polycrystalline sample of 4-Dy (Figure S2). They crystallize in the P21/n crystallographic space group and belong to the previously reported family with {[Ln(μ3pmdc)(μ-ox)0.5(H2O)2]·H2O}n (Ln = La and Ce) formula [40]. The crystal structure is very similar to 2-Dy, since it shows an almost identical 3D framework of the same topological class but which differs in the number of lattice molecules, being dropped to solely one. Accordingly, there are no significant changes in the coordination sphere, which keeps the LnN2O5Ow2 chromophore that resembles CSAPR (Table S2). Further analysis of these structures reveal that their coordination shell is less distorted as the ion size of the lanthanide gets smaller, as confirmed by CShMs (STCTPR = 0.955, 0.889 and 0.845 respectively for Nd3+, Sm3+, and Eu3+), although the distortion is still higher than that found for compound 1-Dy. The distances between metal centers and donor atoms are listed in Table S3. To end up with the description of these structures, it must be highlighted that the loss of the water molecules results in a contraction of the framework, which presents no continuity of accessible voids for solvent molecules (that is, channels) and, therefore, a more compacted crystal building (Figure 4). Figure 4. Packing of compound 4-Nd with a detailed illustration of the coordination sphere. Color coding: green, dysprosium; red, oxygen; blue, nitrogen; grey, carbon; pink, hydrogen. Figure 3. ( a ) Crystal building of the previously reported Gd-based coordination polymer (CP) isostructural to 3-Dy . ( b ) Fragment of compound 3-Gd showing the coordination polyhedron and the environment around the metal centers. Color coding: green, metal; red, oxygen; blue, nitrogen; grey, carbon; pink, hydrogen. Compounds 4-Ln are all isostructural as confirmed by the single-crystal X-ray diffraction measurements for compounds 4-Nd , 4-Sm and 4-Eu and by the PXRD data analysis of a polycrystalline sample of 4-Dy (Figure S2). They crystallize in the P2 1 /n crystallographic space group and belong to the previously reported family with {[Ln( µ3 -pmdc)( µ - Magnetochemistry 2021,7, 8 6 of 20 ox) 0.5 (H 2 O) 2 ] · H 2 O} n (Ln = La and Ce) formula [ 40 ]. The crystal structure is very similar to 2-Dy , since it shows an almost identical 3D framework of the same topological class but which differs in the number of lattice molecules, being dropped to solely one. Accordingly, there are no significant changes in the coordination sphere, which keeps the LnN 2 O 5 Ow 2 chromophore that resembles CSAPR (Table S2). Further analysis of these structures reveal that their coordination shell is less distorted as the ion size of the lanthanide gets smaller, as confirmed by CShMs (S TCTPR = 0.955, 0.889 and 0.845 respectively for Nd 3+ , Sm 3+ , and Eu 3+ ), although the distortion is still higher than that found for compound 1-Dy . The distances between metal centers and donor atoms are listed in Table S3. To end up with the description of these structures, it must be highlighted that the loss of the water molecules results in a contraction of the framework, which presents no continuity of accessible voids for solvent molecules (that is, channels) and, therefore, a more compacted crystal building (Figure 4). Magnetochemistry 2021, 7, x FOR PEER REVIEW 6 of 21 Figure 3. (a) Crystal building of the previously reported Gd-based coordination polymer (CP) isostructural to 3-Dy. (b) Fragment of compound 3-Gd showing the coordination polyhedron and the environment around the metal centers. Color coding: green, metal; red, oxygen; blue, nitrogen; grey, carbon; pink, hydrogen. Compounds 4-Ln are all isostructural as confirmed by the single-crystal X-ray diffraction measurements for compounds 4-Nd, 4-Sm and 4-Eu and by the PXRD data analysis of a polycrystalline sample of 4-Dy (Figure S2). They crystallize in the P21/n crystallographic space group and belong to the previously reported family with {[Ln(μ3pmdc)(μ-ox)0.5(H2O)2]·H2O}n (Ln = La and Ce) formula [40]. The crystal structure is very similar to 2-Dy, since it shows an almost identical 3D framework of the same topological class but which differs in the number of lattice molecules, being dropped to solely one. Accordingly, there are no significant changes in the coordination sphere, which keeps the LnN2O5Ow2 chromophore that resembles CSAPR (Table S2). Further analysis of these structures reveal that their coordination shell is less distorted as the ion size of the lanthanide gets smaller, as confirmed by CShMs (STCTPR = 0.955, 0.889 and 0.845 respectively for Nd3+, Sm3+, and Eu3+), although the distortion is still higher than that found for compound 1-Dy. The distances between metal centers and donor atoms are listed in Table S3. To end up with the description of these structures, it must be highlighted that the loss of the water molecules results in a contraction of the framework, which presents no continuity of accessible voids for solvent molecules (that is, channels) and, therefore, a more compacted crystal building (Figure 4). Figure 4. Packing of compound 4-Nd with a detailed illustration of the coordination sphere. Color coding: green, dysprosium; red, oxygen; blue, nitrogen; grey, carbon; pink, hydrogen. Figure 4. Packing of compound 4-Nd with a detailed illustration of the coordination sphere. Color coding: green, dysprosium; red, oxygen; blue, nitrogen; grey, carbon; pink, hydrogen. At last, compound 5-Dy crystallizes in the hexagonal P6 3 space group and possesses the {[Dy( µ4 -pmdc)(NO 3 )(H 2 O)] · H 2 O} n formula. In this case, the coordination around the central lanthanide(III) atom is completed by the N 2 O 6 Ow donor set, which is formed by two N/O-chelating pmdc ligands, two O-terminal pmdc ligands, a chelating nitrate anion and a water molecule. The resulting polyhedron resembles a muffin (S MFF = 0.410, see Table S2), which can be considered as a less symmetric shape. Note that the calculated geometry was obtained for the Gd-analogue [ 41 ], whose geometry is not expected to be significantly different from compound 5-Dy . The presence of the nitrate anion in the coordination shell, compared to the oxalate, drops the connectivity of the co-ligand, though this fact is taken as an advantage by pmdc ligand because it links to four metal centers by acquiring the µ4 - κ N,O: κ N’,O’: κ O”: κ O”’ coordination mode previously seen in compound 3-Dy . Hence, pmdc ligands bridge dysprosium ions one another by building a kind of triangle-shaped building units which are further linked into the resulting 3D framework that belongs to the ssa topology. The architecture possesses tubular channels occupied by lattice water molecules (Figure 5). Magnetochemistry 2021, 7, x FOR PEER REVIEW 7 of 21 At last, compound 5-Dy crystallizes in the hexagonal P63 space group and possesses the {[Dy(μ4-pmdc)(NO3)(H2O)]·H2O}n formula. In this case, the coordination around the central lanthanide(III) atom is completed by the N2O6Ow donor set, which is formed by two N/O-chelating pmdc ligands, two O-terminal pmdc ligands, a chelating nitrate anion and a water molecule. The resulting polyhedron resembles a muffin (SMFF = 0.410, see Table S2), which can be considered as a less symmetric shape. Note that the calculated geometry was obtained for the Gd-analogue [41], whose geometry is not expected to be significantly different from compound 5-Dy. The presence of the nitrate anion in the coordination shell, compared to the oxalate, drops the connectivity of the co-ligand, though this fact is taken as an advantage by pmdc ligand because it links to four metal centers by acquiring the μ4-κN,O:κN’,O’:κO’’:κO’’’ coordination mode previously seen in compound 3-Dy. Hence, pmdc ligands bridge dysprosium ions one another by building a kind of triangle-shaped building units which are further linked into the resulting 3D framework that belongs to the ssa topology. The architecture possesses tubular channels occupied by lattice water molecules (Figure 5). Figure 5. (a) Packing of reported isostructural 5-Gd showing the tubular channels along c axes. (b) Fragment of 5-Gd showing the coordination environment. Color coding: green, metal; red, oxygen; blue, nitrogen; grey, carbon; pink, hydrogen 2.3. Static Magnetic Properties The analysis of the variable-temperature evolution of the molar magnetic susceptibility (χM) of the dysprosium-based counterparts, previously measured and reproduced here (Figure 6) [40,41], indicates that these compounds present a usual magnetic behavior dominated by the spin-orbit coupling (soc) of the paramagnetic ions. According to this, the χMT product at room temperature per one dysprosium(III) atom is ca. 14 cm3 K mol–1, close to the expected one for an isolated Dy(III) with a gJ of 1.25. All curves describe a similar shape by which the χMT product shows a slight and progressive drop cooling down from room temperature down to 50 K, below which it describes a subtle drop to reach a value of ca. 10.3 cm3 K mol–1. Although a priori this kind of shape could be also attributed to the presence of antiferromagnetic exchange interactions, all compounds can be considered practically paramagnetic in view of the rectilinear drop of the χM–1 vs T plot, which gives values for the Weiss constant of nearly zero (θ about −3.0 and −4.0 for all compounds). Figure 5. ( a ) Packing of reported isostructural 5-Gd showing the tubular channels along c axes. ( b ) Fragment of 5-Gd showing the coordination environment. Color coding: green, metal; red, oxygen; blue, nitrogen; grey, carbon; pink, hydrogen Magnetochemistry 2021,7, 8 7 of 20 2.3. Static Magnetic Properties The analysis of the variable-temperature evolution of the molar magnetic susceptibility ( χM ) of the dysprosium-based counterparts, previously measured and reproduced here (Figure 6) [ 40 , 41 ], indicates that these compounds present a usual magnetic behavior dominated by the spin-orbit coupling (soc) of the paramagnetic ions. According to this, the χM Tproduct at room temperature per one dysprosium(III) atom is ca. 14 cm 3 K mol −1 , close to the expected one for an isolated Dy(III) with a g J of 1.25. All curves describe a similar shape by which the χM Tproduct shows a slight and progressive drop cooling down from room temperature down to 50 K, below which it describes a subtle drop to reach a value of ca. 10.3 cm 3 K mol −1 . Although a priori this kind of shape could be also attributed to the presence of antiferromagnetic exchange interactions, all compounds can be considered practically paramagnetic in view of the rectilinear drop of the χM−1 vs T plot, which gives values for the Weiss constant of nearly zero ( θ about − 3.0 and − 4.0 for all compounds). 1 Figure 6. χM Tvs Tand χM−1 vs Tplots for all dysprosium-based compounds. The plot sited at the bottom on the right-hand side shows the isothermal (2 K) variable-field magnetization of the compounds. In any case, to gain deeper insights, the Density-Functional Theory-(DFT) computed values of the coupling constant, performed on an isostructural Gd(III)-based model in which all the possible superexchange pathways present in these compounds have been also analyzed [(i) through O,O’/O”,O”’-bis-chelating ox ligand, (ii) through N,O/N’,O’- bis-chelating pmdc ligand, (iii) through N,O/O’-mode of pmdc ligand (the short bridge Magnetochemistry 2021,7, 8 8 of 20 across the same carboxylate moiety) and (iv) through N,O/O’-mode of pmdc ligand (the long bridge across the other carboxylate moiety, only present in compound 3-Dy )]. Given that these calculations give very low Jvalues (Figure 7), either of positive or negative signs, we can conclude that the contribution of exchange interactions, if occurring, is practically negligible. Magnetochemistry 2021, 7, x FOR PEER REVIEW 9 of 21 Figure 7. Spin distributions represented on the three main (i), (ii), and (iii) superexchange pathways. Note that pathway (iv) has not been shown because it involves J = 0 cm–1. Perhaps, in the general comparison the curve of compound 5-Dy is the most remarkable one because it significantly differs from the rest in the following aspects: (i) it shows a slightly lower value of χMT at room temperature, (ii) the value of χMT remains more constant in the high temperature range (100–300 K) compared to the rest of compounds and more importantly, (iii) the drop at low temperature is not as abrupt as for previous compounds. Taking into account that Dy(III) is a Kramers ion with a 6H15/2 ground term, the coordination shell can induce a different distribution of the Stark sublevels of the ion, thus influencing the magnetization resulting from the thermal depopulation occurring when lowering the temperature [50]. Therefore, such a conclusion should involve a multiconfigurational calculation on all the environments of the compounds, and the slightly higher remnant value of the magnetic susceptibility for 5-Dy could be attributed to a larger axial magnetic anisotropy in this compound compared to the oxalate-based ones. The M(H) isothermal measurements carried out at 2 K for the compounds point indeed in the same direction, since the curve for 5-Dy exhibits a more abrupt increase and slightly larger value of magnetization compared to the rest of the compounds. These values are of ca. 6 NμB, which is a bit lower that the expected saturation value. 2.4. Dynamic Magnetic Properties In order to study the SMM behavior, alternative current (ac) measurements were performed with the temperature and frequency range of 2–20 K and 60–10,000 Hz respectively, over polycrystalline samples of compounds 1-Dy, 2-Dy, 3-Dy, 4-Dy and 5-Dy in the absence and the presence of an external applied magnetic field. With no external field, none of the compounds present signal in the χ’’ vs. T plot, probably due to the fast relaxation through quantum tunneling of the magnetization (QTM) process. When the measurements were repeated with and external direct current (dc) field of 1000 Oe in order to suppress the QTM, compounds 1-Dy, 2-Dy, 4-Dy and 5-Dy present temperature and frequency dependent maxima in the out-of-phase curves, indicating a slow relaxation of the magnetization (Figure 8). For compound 3-Dy however, there is no a significant frequency dependency of the out-of-phase signal above 2 K (Figure S3), which could be attributed to either the fact that the external field does not suppress the QTM or to a very small thermal energy barrier in which the maxima appear under the detection limit of the apparatus, thus impeding any further study. Figure 7. Spin distributions represented on the three main ( i ), ( ii ), and ( iii ) superexchange pathways. Note that pathway (iv) has not been shown because it involves J= 0 cm−1. Perhaps, in the general comparison the curve of compound 5-Dy is the most remarkable one because it significantly differs from the rest in the following aspects: (i) it shows a slightly lower value of χM Tat room temperature, (ii) the value of χM Tremains more constant in the high temperature range (100–300 K) compared to the rest of compounds and more importantly, (iii) the drop at low temperature is not as abrupt as for previous compounds. Taking into account that Dy(III) is a Kramers ion with a 6H15/2 ground term, the coordination shell can induce a different distribution of the Stark sublevels of the ion, thus influencing the magnetization resulting from the thermal depopulation occurring when lowering the temperature [ 50 ]. Therefore, such a conclusion should involve a multiconfigurational calculation on all the environments of the compounds, and the slightly higher remnant value of the magnetic susceptibility for 5-Dy could be attributed to a larger axial magnetic anisotropy in this compound compared to the oxalate-based ones. The M(H) isothermal measurements carried out at 2 K for the compounds point indeed in the same direction, since the curve for 5-Dy exhibits a more abrupt increase and slightly larger value of magnetization compared to the rest of the compounds. These values are of ca. 6 N µB , which is a bit lower that the expected saturation value. 2.4. Dynamic Magnetic Properties In order to study the SMM behavior, alternative current (ac) measurements were performed with the temperature and frequency range of 2–20 K and 60–10,000 Hz respectively, over polycrystalline samples of compounds 1-Dy , 2-Dy , 3-Dy , 4-Dy and 5-Dy in the absence and the presence of an external applied magnetic field. With no external field, none of the compounds present signal in the χ ”vs. Tplot, probably due to the fast relaxation through quantum tunneling of the magnetization (QTM) process. When the measurements were repeated with and external direct current (dc) field of 1000 Oe in order to suppress the QTM, compounds 1-Dy , 2-Dy , 4-Dy and 5-Dy present temperature and frequency dependent maxima in the out-of-phase curves, indicating a slow relaxation of the magnetization (Figure 8). For compound 3-Dy however, there is no a significant frequency dependency of the out-of-phase signal above 2 K (Figure S3), which could be attributed to either the fact that the external field does not suppress the QTM or to a very small thermal energy barrier in which the maxima appear under the detection limit of the apparatus, thus impeding any further study. Magnetochemistry 2021,7, 8 9 of 20 1 Figure 8. Temperature dependence of out-of-phase components of the ac susceptibility in a direct current (dc) applied magnetic field of 1000 Oe for ( a ) 1-Dy , ( b ) 2-Dy , ( c ) 4-Dy and ( d ) 5-Dy . The insets represent the temperature dependence for the relaxation times and the best fits of the data to different relaxation processes. When a dc field of 1000 Oe is applied, compounds 1-Dy, 4-Dy and 5-Dy show frequency dependent maxima with the appearance of small shoulders at low temperatures, suggesting the QTM is still operating. The Cole–Cole plots generated for the corresponding compounds show semi-circular shapes that fit well the generalized Debye model (Figures S8 and S9), affording α values in the range of 0.5–0.1. Note that α parameter determine the width of the different relaxation processes, meaning that α = 1 stands for infinitely wide relaxation processes whereas α = 0 indicates a narrow relaxation processes. The obtained values indicates the existence of more than one relaxation mechanism, especially in the low temperature regime. In fact, the Arrhenius plots in the form of ln( τ ) versus 1/T (selecting the temperature range according to the Cole–Cole plots) feature a curvilinear shape that only retains linear behavior in the high temperature region, which was fitted according to the Arrhenius law (Orbach process, in which the obtained U eff and τ0 values are represented in Table 1) [Equation (1)]. Besides the thermally activated Orbach relaxation mode, compounds 1-Dy and 4-Dy require the inclusion of the QTM process to correctly reproduce the low temperature range, whereas a combination of Orbach and Raman relaxations were employed to fit the temperature-dependent representation of the relaxation times for 5-Dy , yet the presence of QTM cannot be discarded in the low temperature region in this case. Best fits parameters (which were obtained with low errors and R 2 lower than 9.0 × 10 –3 ) are listed in Table 1. τ−1=τ0−1exp (−Ueff/kbT) + τQTM−1+ BRamanTn(1) Table 1. Ueff and τ0values for compounds 1-Dy,2-Dy,4-Dy and 5-Dy. Compound Orbach Orbach + QTM (Quantum Tunneling of the Magnetization) Orbach + Raman Ueff (K) τ0(s) Ueff (K) τ0(s) τQTM (s) Ueff (K) τ0(s) BRaman (s−1K−1) n 1-Dy 35(2) 1(2) ×10−841(2) 4(2) ×10−96(4) ×10−4- - - - 2-Dy 18(3) (FR) 4(3) ×10−9(FR) - - - - - - - -23(3) (SR) 2(3) ×10−8(SR) - - - - - - - 4-Dy 22(2) 8(3) ×10−929(3) 1(3) ×10−91.1(3) ×10−4- - - - 5-Dy 44(2) 7(2) ×10−8- - - 70(2) 5(1) ×10−923(5) 3.3(2) Magnetochemistry 2021,7, 8 16 of 20 achieved for the most ideal polyhedra. Nonetheless, the replacement of oxalate ligand by nitrate anion in the equatorial plane leads to a higher stabilization of the ligand field and thus, a higher energy barrier in spite of the fact that 5-Dy presents a more distorted polyhedron. On the other hand, the photoluminescence emission spectra recorded for the Dy-based compounds, as well as for 4-Sm and 4-Eu compounds, show the characteristic emission peaks for each lanthanide(III) ions in the visible region as a consequence of the f-f intraionic transitions, in addition to the weaker ligand based emission. The relative intensity of the ligand-based emission and f-f intraionic transitions vary for Dy-based compounds, which is explained by different effects. On the one hand, a variable change in the effectiveness of the ligand-to-lanthanide energy transfer due to the changes occurring in the coordination shell of the structures, which is mainly observed when comparing the stronger emissions for 3D systems of 2-Dy , 3-Dy and 4-Dy to the 2D system of 1-Dy . On the other hand, the bright Dy-based emission observed for 5-Dy seems to proceed through a direct Dy-centered intraionic excitation, which bypasses the ligand excitation and associated antennae effect and helps decreasing the non-radiative pathways within the CP, in such a way the 5-Dy yields the most effective emission for all analyzed Dy-based compounds. On another level, the analysis on the rest of the counterparts ( 4-Sm and 4-Eu ) shows that the antennae effect is presumably maximized for 4-Eu in view of the similar energy of triplet state of pmdc ligand and the excited states of Eu(III) ion, in such a way that 4-Eu affords a QY of 25% at room temperature. Supplementary Materials: The following are available online at https://www.mdpi.com/2312-7 481/7/1/8/s1. The ESI file includes Fourier Transform Infrared (FTIR) spectra, Powder X-Ray Diffraction (PXRD) analysis, crystallographic data, structural details, ac magnetic susceptibility measurements, photoluminescence measurements and computational calculation of the triplet state. Figure S1: Pattern-matching analyses of previously reported Dy-based compounds Figure S2: Patternmatching analyses for 4-Nd , 4-Sm , 4-Eu and 4-Dy , Figure S3: χ M’(T) and χ M”(T) signals plots for compound 3-Dy under H dc = 1000 Oe, Figures S4 and S5: Temperature dependence of in-phase components of the ac susceptibility in a dc applied field of 1000 Oe for 1-Dy , 2-Dy , 4-Dy and 5-Dy Figures S6 and S7: Cole-Cole plots in dc applied field of 1000 Oe for 1-Dy , 2-Dy , 4-Dy and 5-Dy , Figure S8 and S9: Variable-temperature frequency dependence of the χM´´ signal under 1000 Oe applied field of 1-Dy , 2-Dy , 4-Dy and 5-Dy , Figure S10: Theoretical orientation of the magnetic moments for Dy(III) ions in compounds 1-Dy and 5-Dy , Figure S11: Excitation spectra monitored at 573 nm for compounds 1-Dy, 2-Dy, 3-Dy, 4-Dy and 5-Dy , Figure S12: Excitation spectra monitored at 596 nm for 4-Sm and at 618 nm for 4-Eu , Figure S13: Emission decay curves for compounds 1-Dy, 2-Dy, 3-Dy, 4-Dy and 5-Dy under 325 nm excitation laser showing the best fitting, Figure S14: Emission decay curves for compounds 4-Sm and 4-Eu under 308 nm excitation laser showing the best fitting, Figure S15: Illustration of the triplet and ground single state of pmdc ligand with the energy gap between them, Figure S16: IR spectra of 4-Nd, 4-Sm, 4-Eu and 4-Dy , Figure S17: FTIR spectra of all Dy-based compounds, Table S1: Crystallographic data for compounds 4-Nd , 4-Sm and 4-Eu , Table S2: CShMs for the coordination environment of compounds 1-Dy, 2-Dy, 3-Gd, 4-Nd, 4-Sm, 4-Eu and 5-Gd , Table S3: Selected bond lengths (Å) for compounds 1-Dy, 2-Dy, 4-Nd , 4-Sm and 4-Eu , Table S4: Relaxation Fitting Parameters from Least-Squares Fitting of χ ( φ ) data for compound 2-Dy . Author Contributions: Conceptualization, J.C. and A.R.-D.; synthesis of the samples, O.P.-C.; structural analysis, A.R.-D. and J.C.; validation, J.C., O.C. and J.A.G.; formal analysis, resources, O.C. and J.A.G.; data curation of PL, J.A.G.; data curation of magnetism, O.P.-C.; writing—original draft preparation, O.P.-C and J.C.; writing—review and editing, O.C. and A.L.; supervision, O.C. and A.L.; project administration, J.C.; funding acquisition, O.C. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Spanish Ministry of Science, Innovation and Universities (MCIU/AEI/FEDER, UE) (PGC2018-102052-A-C22, PGC2018-102052-B-C21, PID2019-108028GBC21), Junta de Andalucía (FQM-394), Gobierno Vasco/Eusko Jaurlaritza (IT1005-16, IT1291-19) and University of the Basque Country (UPV/EHU) (GIU 17/13, GIU17/50). Data Availability Statement: The data presented in this study are available in this article or Supplementary Material. 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