From field-induced to zero-field SMMs associated with open/closed structures of bis(ZnDy) tetranuclear complexes: a combined magnetic, theoretical and optical study†

Dalton Transactions PAPER Cite this: Dalton Trans., 2024, 53, 7971 Received 17th January 2024, Accepted 11th April 2024 DOI: 10.1039/d4dt00148f rsc.li/dalton From field-induced to zero-field SMMs associated with open/closed structures of bis(ZnDy) tetranuclear complexes: a combined magnetic, theoretical and optical study† Andoni Zabala-Lekuona, * a Xabier Lopez de Pariza, b Ismael F. Díaz-Ortega, c,d Javier Cepeda, a Hiroyuki Nojiri, d Nina P. Gritsan, e Alexey A. Dmitriev, e Alberto López-Ortega, f,g,h Antonio Rodríguez-Diéguez, i José M. Seco * a and Enrique Colacio * i We have prepared a bis(compartmental) Mannich base ligand H 4 L (1,4,8,11-tetraaza-1,4,8,11-tetrakis(2hydroxy-3-methoxy-5-methylbenzyl)cyclotetradecane) specifically designed to obtain bis(TM II Ln III ) tetranuclear complexes (TM = transition metal). In this regard, we have succeeded in obtaining three new complexes of the formula [Zn 2 (µ-L)(µ-OAc)Dy 2 (NO 3 ) 2 ]·[Zn 2 (µ-L)(µ-OAc)Dy 2 (NO 3 )(OAc)]·4CHCl 3 ·2MeOH (1) and [TM 2 (µ-H 2 L) 2 (µ-succinate)Ln 2 (NO 3 ) 2 ] (NO 3 ) 2 ·2H 2 O·6MeOH (TM II = Zn, Ln III =Dy(2); TM II =Co, Ln III =Dy(3)). Compound 1contains two different bis(ZnDy) tetranuclear molecules that cocrystallize in the structure, in which acetato bridging ligands connect the Zn II and Dy III ions within each ZnDy subunit. This compound does not exhibit slow magnetic relaxation at zero field, but it is activated in the presence of an applied dc magnetic field and/or by Dy/Y magnetic dilution, showing two relaxation processes corresponding to each of the two different bis(ZnDy) units found in the structure. As revealed by the theoretical calculations, magnetic relaxation in 1is single-ion in origin and takes place through the first excited state of each Dy III ion. When using the succinato dicarboxylate bridging ligand instead of acetate, compounds 2and 3were serendipitously formed, which have a closed structure with the succinate anion bridging two ZnDy subunits belonging to two different ligands. It should be noted that only compound 2 exhibits slow relaxation of magnetization in the absence of an external magnetic field. According to experimental and theoretical data, 2relaxes through the second excited Kramers doublet (U eff = 342 K). In contrast, 3displays field-induced SMM behaviour (U eff = 203 K). However, the Co/Zn diluted version of this compound 3 Zn shows slow relaxation at zero field (U eff = 347 K). Ab initio theoretical calculations clearly show that the weak ferromagnetic coupling between Co II and Dy III ions is at the origin of the lack of slow relaxation of this compound at zero field. Compound 2and its diluted analogues 2 Y and 3 Zn show hysteresis loops at very low temperature, thus confirming their SMM behaviour. Finally, compounds 1and 2show Dy III based emission even at room temperature that, in the case of 2, allows us to extract the splitting of the ground 6 H 15/2 term, which matches reasonably well with theoretical calculations. a Departamento de Química Aplicada, Facultad de Química, Universidad del País Vasco (UPV/EHU), 20018 Donostia-San Sebastián, Spain. E-mail: [email protected], [email protected] b POLYMAT and Department of Polymers and Advanced Materials: Physics, Chemistry and Technology, Faculty of Chemistry, University of the Basque Country UPV/EHU, Paseo Manuel de Lardizabal 3, Donostia-San Sebastián 20018, Spain c Departamento de Química y Física-CIESOL, Universidad de Almería, Ctra. Sacramento s/n, 04120 Almería, Spain d Institute for Materials Research, Tohoku University, Katahira, Sendai, 980-8577, Japan e Institute of Chemical Kinetics and Combustion, Siberian Branch, Russian Academy of Sciences, 630090 Novosibirsk, Russia f Departamento de Ciencias, Universidad Pública de Navarra, E-31006 Pamplona, Spain g Institute for Advanced Materials and Mathematics (INAMAT2), Universidad Pública de Navarra, E-31006 Pamplona, Spain h CIC nanoGUNE BRTA, Tolosa Hiribidea 76, Donostia-San Sebastian, 20018, Spain i Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain. E-mail: ecola[email protected] †Electronic supplementary information (ESI) available. CCDC 2226682–2226685. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4dt00148f This journal is © The Royal Society of Chemistry 2024 Dalton Trans.,2024,53,7971–7984 | 7971 Open Access Article. Published on 12 April 2024. Downloaded on 10/3/2024 12:04:10 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online View Journal | View Issue Introduction Single-molecule magnets (SMMs) are open-shell coordination compounds displaying slow relaxation of magnetization and, as a result, magnetic hysteresis and magnetic memory below the so-called blocking temperature (T B ). 1 These properties make SMMs potential candidates for magnetic data storage 2 and molecular spintronic 3 devices. The SMM behavior mainly depends on the magnetic anisotropy, which arises from the combined action of spin–orbit coupling (SOC) and ligand field effects. Therefore, it is not surprising that the best SMM properties have been observed for metal complexes containing very anisotropic lanthanide ions, particularly Dy III . In fact, dysprosocenium analogues have been shown to be very efficient as SMMs with a U eff (effective energy barrier for magnetization reversal) of up to 1540 cm −1 and T B values surpassing the temperature limit of liquid nitrogen. 4 Even though many other factors should be considered, a fundamental one to observe SMM behaviour is the easy-axis anisotropy of the Dy III ion. To achieve this requirement, the Dy III ion should have as strong an axial field and as weak an equatorial ligand field as possible. 5 It is worth noting that T B and U eff are not always strictly linked, since under-barrier relaxation pathways may co-exist shortcutting the U eff and diminishing the functional T B value. Indeed, a wide variety of systems have recently been reported with large U eff values, but with widely varying T B s 6–9 that are typically smaller than those expected based on U eff values. Inspired by these guidelines, we and others have prepared a series of aminophenol Mannich and Schiffbased compartmental ligands that can simultaneously host both 3d and 4f ions in separate coordination pockets. 10–12 In such compounds, the largest negative charge, and consequently the largest ligand field, is provided by phenoxido groups, while the remaining neutral heteroatoms provide weaker ligand fields. Under these conditions, the magnetic anisotropy of the Dy III ion mainly depends on the location of the phenoxido groups in the Dy III coordination sphere. For instance, in trinuclear ZnDyZn complexes with two pairs of phenoxido groups placed at opposite positions of the Dy III ion, the crystal field favours the easy-axis magnetic anisotropy, which usually leads to an improvement of the SMM properties. Moreover, the inclusion of diamagnetic Zn II can also be an adjuvant factor to increase the easy-axis magnetic anisotropy of the Dy III ion in this type of 3d–4f complex. 13 Additionally, it needs to be pointed out that when both (3d and 4f) paramagnetic ions are combined in a system, in some cases magnetic interactions suppress quantum tunnelling of magnetization (QTM). This fact could also favour the SMM behaviour at zero field, as observed in some strongly coupled CrDy or NiDy based compounds. 14,15 However, it should be noted that the strategy of combining two exchanged anisotropic metal ions into a compound does not ensure SMM behaviour even in the presence of an applied magnetic field. 16,17 Thus, in order to better understand the magnetic interactions between 3d and 4f ions and, most importantly, their effect on the SMM behaviour, it would be very useful to prepare relatively simple 3d–4f systems. In addition, lanthanides can emit light in both the visible and NIR regions. 18,19 Thus, multifunctional systems with magnetic and luminescence properties can be obtained. Besides, the emission bands are directly related to the electronic structure of the lanthanide ion, and therefore extremely valuable information could be obtained from the emission spectra. In view of the foregoing, it becomes clear that the use of compartmental ligands to prepare relatively simple 3d–4f systems is still of paramount interest. In this regard, Comba et al. 17 are pioneers in preparing potential compartmental ligands by incorporating aminophenol groups as substituents at the nitrogen atoms of triazaand tetraaza-macrocycles. These ligands allow the preparation of linear 3d–4f–3d trinuclear and 3d–4f dinuclear complexes, respectively. 17,20 In the latter case, no tetranuclear complexes were formed, since two of the aminophenol groups linked to the 1,4,7,10-cyclododecane macrocycle in alternating positions do not coordinate the metal ions. In this regard, and in order to obtain tetranuclear bis(ZnDy) compounds, we have designed and synthesized a novel Mannich ligand, 1,4,8,11-tetraaza-1,4,8,11-tetrakis(2hydroxy-3-methoxy-5-methylbenzyl) cyclotetradecane, H 4 L (Scheme 1). This ligand has four (two and two) individual pockets ideal for bis(3d–4f) systems, where all aminophenoxido groups would bridge 3d and 4f metal ions. These bis(3d– 4f) systems can be connected by different linkers to give very interesting systems not only from a structural, but also from a magnetic point of view. Herein, we report the X-ray single-crystal structures, dc and ac magnetic properties, theoretical ab initio calculations and photoluminescence properties of the following bis(3d–4f) complexes of the formula [Zn 2 (µ-L)(µ-OAc)Dy 2 (NO 3 ) 2 ]·[Zn 2 (µ-L)(µOAc)Dy 2 (NO 3 )(OAc)]·4CHCl 3 ·2MeOH (1) and [TM 2 (µ-H 2 L) 2 (µsuccinate)Ln 2 (NO 3 ) 2 ](NO 3 ) 2 ·2H 2 O·6MeOH (TM II = Zn, Ln III = Dy (2); TM II = Co, Ln III =Dy(3)). The aim of this work is fourfold: (i) to confirm the formation of bis(ZnDy) tetranuclear complexes and to analyse how the nature of the carboxylate ancillary ligand (monoor dicarboxylate) affects the final tetranuclear structure; (ii) to determine whether complexes 1and 2 exhibit SMM behaviour and, if so, to perform theoretical anaScheme 1 Structure of ligand H 4 L. Paper Dalton Transactions 7972 |Dalton Trans.,2024,53,7971–7984 This journal is © The Royal Society of Chemistry 2024 Open Access Article. Published on 12 April 2024. Downloaded on 10/3/2024 12:04:10 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online lysis of their electronic structure to support their magnetization relaxation mechanisms; (iii) to perform experimental and theoretical studies on compound 3, as well as on its dilute counterparts 2 Y ,3 Zn and 3 Y , which are isostructural versions of 2, to elucidate the origin of the influence of the Co–Dy magnetic coupling on the SMM properties of 2; and (iv) to study the luminescence properties of 1and 2and to correlate the structure of their experimental emission spectra with the theoretically calculated electronic structure. Results and discussion Synthetic procedures and details of the experimental and theoretical methodologies used to study the reported complexes are described in the ESI.† The compartmental ligand H 4 L (Scheme 1) possesses two double pockets. Each pocket consists of an inner N 2 O 2 site suitable for transition metal ions and another outer O 4 site more suitable for harder oxophilic metal ions, such as lanthanides. Therefore, each ligand can give rise to two dinuclear systems in one, i.e. a bis(ZnDy) system, in which the macrocycle backbone links the ZnDy subunits. In good agreement with this, the reaction of H 4 L with Zn(OAc) 2 ·2H 2 O, Dy (NO 3 ) 3 ·5H 2 O and Et 3 N in a CHCl 3 /MeOH solvent mixture produced the tetranuclear bis(ZnDy) complex (1). In 1, the Zn II and Dy III ions within each dinuclear subunit are bridged by an acetate group. Each ZnDy subunit is very similar in the coordination environment of metal ions to the acetate-bridged dinuclear compound we reported previously. 10 In the synthesis of compounds 2–3,H 4 L reacted with the corresponding TM (NO 3 ) 2 ·xH 2 O and Ln(NO 3 ) 3 ·xH 2 O in methanol in the first step, and with succinic acid and Et 3 N in the second step. The final complexes 2–3exhibit a closed bis(TMLn) tetranuclear structure in which two dinuclear TMLn units are bridged by two aminophenolate groups belonging to neighbouring ligands and a succinate linker. Crystal structures The compartmental ligand H 4 L crystallizes in the triclinic P1 ˉ space group. The asymmetric unit consists of half of the ligand, where the phenol groups are stabilized by intramolecular hydrogen bonds towards amines (crystallographic data are given in Table S3,†and the structure is shown in Fig. S1†). Compound 1crystallizes in the monoclinic P2/cspace group (Table S3†for crystallographic data). The asymmetric unit contains two different half molecules and three crystallization solvent molecules (two chloroform and one methanol molecules). As mentioned above, two slightly different structures co-crystallize within the crystal structure, which will be referred to as 1A and 1B hereinafter (Fig. 1). In both cases, the fully deprotonated L 4− ligand encapsulates two pairs of Zn II Dy III systems, which are triply bridged by two phenoxido groups of the main ligand and one syn–syn acetate group. In both 1A and 1B, the transition metal possesses the same ZnN 2 O 3 square pyramidal coordination environment according to SHAPE 21 calculations (Table S6†). The shortest bond lengths correspond to Zn–O (1.969(2)–2.064(2) Å), while the Zn–N distances are somewhat longer (2.140(3)–2.221(3) Å). The difference between 1A and 1B (besides bond distances and angles, Table S4†) lies in the coordination environment of the Dy III ions. They both exhibit DyO 9 coordination spheres, and seven of the nine oxygen atoms belong to the same groups, namely, two phenoxido and two methoxy belong to L 4− , two oxygen atoms come from the bidentate nitrate and one oxygen belongs to the bridging acetate. The last two oxygen atoms, however, belong to an additional bidentate nitrate in 1A and another additional chelating acetate in 1B.In terms of bond lengths, Dy–O bonds fall into four different categories: the shortest Dy–O phenoxido bonds (in the range of 2.262 (2)–2.285(2) Å), intermediate Dy–O µ-acetate (2.329(3) Å and 2.346 (3) Å), longer Dy–O nitrate (between 2.441(3) and 2.500(3) Å) and the longest Dy–O methoxy (in the range of 2.515(2)–2.572(2) Å). For the chelating acetate, the Dy–O acetate bond lengths are longer than those for bridging acetate, but shorter than those Fig. 1 Perspective view of the molecular structure of 1(1A top panel and 1B bottom panel). Hydrogen atoms are omitted for clarity. Dalton Transactions Paper This journal is © The Royal Society of Chemistry 2024 Dalton Trans.,2024,53,7971–7984 | 7973 Open Access Article. Published on 12 April 2024. Downloaded on 10/3/2024 12:04:10 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online of Dy–O nitrate : 2.407(3) Å and 2.441(3) Å. Although the differences between 1A and 1B are small, they seem to have a significant effect on the dynamic magnetic properties related to them (vide infra). SHAPE analysis shows that the coordination polyhedra in both cases are far from ideal symmetry, but in both cases, they best fit a capped square antiprism (Table S7†). Finally, the shortest intermolecular Dy III ⋯Dy III distance is 8.3079(7) Å, while intramolecular distances are 12.3011(6) Å and 12.1047(6) Å for 1A and 1B, respectively. Compounds 2and 3are isostructural and crystallize in the orthorhombic Pba2 space group (for crystallographic data, see Table S3†). In view of this, only compound 2will be described hereinafter as an example. The X-ray crystal structure of 2consists of the [Zn 2 (µ-H 2 L) 2 (µ-succinate)Dy 2 (NO 3 ) 2 ] 2+ cation, two NO 3 − anions, six methanol and two water crystallization molecules (Fig. 2). Only one of the two double pockets in each ligand acts as predicted, encapsulating a dinuclear Zn II Dy III entity. As for the other pocket, only one aminophenolate group coordinates to the Dy III ion (located in the pocket of the other H 2 L 2− ligand) in a bidentate form via the oxygen atoms of the phenoxido and methoxy groups, while the other aminophenol group remains uncoordinated with the phenol group in the protonated state. Hence, three of the four phenol groups are deprotonated, acting as Zn II Dy III bridging phenoxido groups (O1 and O3 from the pocket) or as monocoordinating non-bridging phenoxido group (O7). Nonetheless, the ligands exhibit the H 2 L 2− mode, since part of them have a zwitterionic form. More specifically, the protons from O7 migrate to the nearest amine N4, forming one asymmetric centre in each ligand, which is stabilized by a hydrogen bond between N4H⋯O7. Taking into account that the asymmetric unit contains only half of the molecule, both new centres in the molecule have the same configuration (R,R′or S,S′). However, bearing in mind that there is no chiral directing agent to force the structure to be one enantiomer or the other, they both co-crystallize in the crystal structure (Fig. S2†). The role of the succinate ligand is essential in this structure because it acts as a double bridging group. On the one hand, it forms a bridge between Zn II and Dy III ions in the same way as in 1, and, on the other, it connects two dinuclear entities located in the pocket of each ligand. In 2, the ZnN 2 O 3 coordination sphere is very similar to that in 1. However, although the DyO 9 coordination environment is formed by the same seven out of nine oxygen atoms belonging to the same groups as in 1(two phenoxido, two methoxy, two nitrate and one succinate oxygen atoms), the additional chelate nitrate (or acetate in 1B) is replaced by a chelate formed by the oxygen atoms of the phenoxido and methoxy groups of the neighbouring ligand, resulting in a distorted muffin geometry (Table S7†). Therefore, the ZnDy subunits are connected by two aminophenolate groups belonging to neighbouring ligands and by a bis(chelate) succinate connector, forming a closed structure. Dy–O phenoxido bond lengths are the shortest ones in the range of 2.227(4)–2.313(4) Å, the Dy– O carboxylate is slightly longer at 2.320(4) Å and the rest of Dy–O lengths are larger than 2.4 Å (Table S5†). Remarkably, among the wide variety of donor atoms in this system, the most negatively charged phenoxido groups somehow provide the system with an appropriate crystal field leading to an axial ground state, which favours the SMM behaviour. Indeed, open angles with values of 137.32(16)° and 143.73(15)°are found between O1–Dy1–O7 and O3–Dy1–O7, respectively. As will be shown in the following sections, such a ligand field is important in explaining the dynamic magnetic properties of this system. Within the molecule, the Dy III ⋯Dy III and Zn II ⋯Zn II distances are 8.6991(6) Å and 7.6269(9) Å, respectively, and the shortest intermolecular Dy III ⋯Dy III and Zn II ⋯Zn II distances are 10.8644(6) Å and 11.5391(9) Å, respectively. In compound 2, the solvent molecules could not be properly refined. However, Fig. S3 and S4†show the supramolecular structure of 3to clarify how these molecules are arranged in the 3D structure. It is worth mentioning that nitrate counterions as well as methanol and water molecules form a complex hydrogenbonding pattern along the baxis (Fig. S3†), while the cationic [Co 2 (µ-H 2 L) 2 (µ-succinate)Dy 2 (NO 3 ) 2 ] 2+ units are linked by hydrogen bonds between O5H⋯O3N. Thus, the cationic fragments grow to form a 2D framework in the ab plane (Fig. S4†). Static magnetic properties The temperature dependence of the χ M Tproduct (χ M being the molar susceptibility) of polycrystalline samples of complexes 1–3in the 2–300 K temperature range at an applied field of 0.1 T is given in Fig. 3. The room temperature χ M Tvalues for 1 and 2of 57.90 and 28.26 cm 3 mol −1 K, respectively, are in the expected ranges for four and two non-interacting Dy III ions (14.17 cm 3 mol −1 K; 6 H 15/2 , and g= 4/3). Compound 3, containing additional anisotropic Co II ions, has a value of 33.95 cm 3 mol −1 K at room temperature, which is greater than the Fig. 2 Perspective view of the molecular structure of compound 2. Hydrogen atoms (except the phenolic and zwitterionic ones) and the counterions are omitted for clarity. Paper Dalton Transactions 7974 |Dalton Trans.,2024,53,7971–7984 This journal is © The Royal Society of Chemistry 2024 Open Access Article. Published on 12 April 2024. Downloaded on 10/3/2024 12:04:10 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online expected value for two Co II (with S= 3/2 and g= 2; 1.875 cm 3 mol −1 K) and two Dy III ions of 32.09 cm 3 mol −1 K. This fact is mainly due to the unquenched orbital angular momentum of Co II ions. Complexes containing a sole type of anisotropic ion (1and 2) demonstrate the usual temperature dependence of the χ M Tproduct. Thus, χ M Tgradually decreases with decreasing temperature due to the depopulation of the M J sublevels of the Dy III ion arising from the splitting of multiplets by the ligand field. The final drastic drop for 2could be indicative of magnetic blocking. 22 In contrast, for compound 3, a gradual decrease in χ M Tis followed by a sharp increase below 25 K, reaching a value of 35.08 cm 3 mol −1 K at 2 K. This behaviour below 25 K clearly indicates the existence of a ferromagnetic coupling between Co II and Dy III ions via bis(phenoxido)succinate bridges. As can be seen in Fig. 3, χ M Tvs.Tcurves for 1–3 are very well reproduced by theoretical calculations (see below in the “Theoretical studies”section). For all compounds, there is both a lack of saturation (Fig. S5 and S6†), and some nonsuperimposable features in the reduced magnetization curves (Fig. S7 and S8†) in the 2–5 K temperature range, which suggests the occurrence of magnetic anisotropy. Dynamic magnetic properties At zero field, only compound 2exhibits well-defined maxima in the χ″ M (T) plot below 28 K. Compounds 1and 3display non-zero signals, but without maxima (Fig. S9 and S10†). This may be due to the presence of a fast QTM process that prevents SMM behaviour. In order to suppress it, well known techniques have been used, such as the application of external dc fields and magnetic dilution in isostructural diamagnetic matrices. 23–25 Frequency and temperature dependent ac susceptibility measurements were performed under an optimum dc field of 2.5 kOe for 1(see field optimization data in Fig. S11 and S12†). To estimate the relaxation times of the two observed processes (FR, fast relaxation and SR, slow relaxation) the χ″ M (v) curves were fitted in the temperature range of 2.0–2.6 K and 7.0–13.0 K, respectively (Fig. S15 and S16†). The significant difference in operating temperatures for these processes allowed us to make this distinction. It is worth mentioning that for the FR process, only data above 1000 Hz were fitted to the generalized Debye model, since data below this frequency are significantly affected by the SR process. The temperature dependence of the magnetization relaxation time in molecules exhibiting slow relaxation of the magnetization is commonly described by eqn (1): τ1¼AH4TþB1=ð1þB2H2ÞþCTn þτ1 0expðUeff=kBTÞð1Þ where the first two terms correspond to the field dependent direct and QTM relaxation processes, respectively, whereas the third and fourth terms represent the field independent Raman and Orbach relaxation processes, respectively. Despite the high αvalues obtained from the Cole–Cole plots for 1in both regimes (FR: 0.37 (2.0 K)–0.62 (2.6 K); SR: 0.52 (7.0 K)–0.49 (13.0 K)), which may be due to the mixing of the FR and SR processes, the temperature dependence of the relaxation times can be fitted to the Orbach process (Fig. 4). The best fit U eff and τ 0 values are given in Table 1. Additional data analysis was accomplished using the CCFIT software. 26 This methodology gave results comparable to those obtained by separating the two temperature regimes (Fig. S19†). Fig. 4 Arrhenius plot for relaxation times for 1and 1 Y under 2.5 kOe applied field. Fig. 3 Experimental temperature dependences of the χ M Tproduct under a field of 0.1 T for complexes 1–3and the SINGLE_ANISO (1and 2) and POLY_ANISO (3,J= 0.48 cm −1 ) simulated data using the results of ab initio calculations (solid red lines). Table 1 U eff ,τ 0 ,C,n, and τ QTM parameters generated from the fit of the relaxation time–temperature dependence for 1,1 Y ,2,2 Y ,3,3 Zn and 3 Y Comp. dc field Orbach Raman QTM kOe U eff (K) τ 0 (s) C(s −1 K −n )nτ QTM (s) 12.5 9.2 (FR) 6.4 × 10 −7 66.9 (SR) 1.2 × 10 −7 1 Y 2.5 46.8 (FR) 2.6 × 10 −10 285.4 2.04 94.0 (SR) 1.6 × 10 −8 20 261.0 2.1 × 10 −9 1 342.4 2.4 × 10 −10 4.9 × 10 −4 5.11 2 Y 1 326.7 1.5 × 10 −10 1.0 × 10 −4 5.64 32.5 203.0 1.4 × 10 −8 9.8 1.61 3 Zn 0 347.2 9.0 × 10 −11 0.052 3.86 1.2 × 10 −3 2.5 334.8 2.4 × 10 −10 1.0 × 10 −4 5.65 3 Y bis(CoY) 1.5 588.5 (FR) 2.59 4.9 × 10 −4 3 Y (CoDy)(CoY) 209.1 (SR) 6.0 × 10 −9 Dalton Transactions Paper This journal is © The Royal Society of Chemistry 2024 Dalton Trans.,2024,53,7971–7984 | 7975 Open Access Article. Published on 12 April 2024. Downloaded on 10/3/2024 12:04:10 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Magnetically diluted samples were also studied to avoid possible intraor intermolecular dipolar Dy⋯Dy interactions that may contribute to QTM. At zero applied dc field, the dynamic behaviour of 1 Y is very similar to that of 1under an external field of 1 kOe (Fig. S20†) in terms of SR, whereas FR appears to be hidden by a strong QTM. Therefore, magnetic dilution does not completely suppress QTM, but it is sufficient to trigger the SMM behaviour of the SR process. We repeated the measurements for 1 Y under the same optimal external field as for 1(2.5 kOe, Fig. S22†). Again, FR and SR processes were individually analysed by choosing the appropriate temperature range in χ″ M (ν) curves. The temperature dependence of the relaxation times for the FR process was fitted to a combination of Raman and Orbach processes, whereas a single Orbach mechanism was considered for the SR process (Fig. 4). The corresponding magnetic parameters are given in Table 1. The analysis with CCFIT is shown in Fig. S27,†where consistent results are displayed. The simultaneous appearance of two sets of maxima in the χ″ M (T) and χ″ M (ν) plots may be due to (i) the presence of intraor intermolecular interactions, 27,28 (ii) the emergence of novel relaxation mechanisms due to the presence of an external magnetic field, 11 or (iii) the coexistence of two crystallographically independent Dy III ions, 29 although it is known that even a unique type of Dy III ion can show two maxima. 30 We have ruled out the first hypothesis, since 1 Y still exhibits two maxima. Based on the strategic rules of ligand field design proposed by Rinehart and Long 5 for oblate ions such as Dy III , axial ligand fields are optimal for stabilizing higher M J states. Therefore, when donor atoms with higher electron density are located in opposite positions in the Dy III coordination sphere, the ligand field ensures the formation of an axial ground state. Thus, we conclude that the molecule that owns an acetate group as a chelating ligand provides better axiality of the ground state of the Dy III ion than the one that contains a second nitrate (acetates provide more electron density than nitrates, as will be discussed later in the computational part). In view of this, we associate FR to the molecule possessing two chelate nitrates (1A), whereas SR is attributed to the molecule with acetate as a chelate (1B). Compound 2behaves like a zero-field SMM, displaying temperatureand frequency-dependent maxima in the ac susceptibility plots (Fig. 5 and Fig. S28 and S29†) below 28 K. The long tails below the maxima at low temperatures indicate the presence of QTM. Relaxation times were extracted from the 17.2–27.6 K temperature range (Fig. 5, top). Although the curvature of the Arrhenius plot at intermediate/low temperatures suggests that other relaxation mechanisms besides Orbach take place, it was not possible to obtain a reasonable fit (namely, combinations of Orbach + Raman and/or QTM were attempted). Considering the foregoing, only the linear part of the high temperature regime was fitted giving the values of U eff and τ 0 given in Table 1. With the aim of qualitatively investigating the influence of field and dilution on QTM, both quenching techniques were applied. For this purpose, we analysed the dynamic magnetic properties of 2and 2 Y under an optimum external magnetic field of 1 kOe (Fig. S31 and S32†). The fact that the QTM is gradually quenched is evidenced by the decrease of the tails in the χ″ M (T) plots (Fig. S34 and S38†). Consequently, the relaxation times become longer in the studied temperature range. On this occasion, we were able to fit the temperature dependence of the relaxation times for 2and 2 Y with the simultaneous occurrence of Orbach and Raman processes (Fig. 5, bottom). The best fit parameters are summarized in Table 1. Despite the fact that the coordination environment of 2is very similar to that of 1, the replacement of the second nitrate (or chelating acetate) with phenoxido and methoxy groups remarkably improves its SMM behaviour. This is because the donor oxygen atoms belonging to the phenoxide groups have the shortest Dy–O distances and the highest electron density, and are located in opposite positions in the Dy III coordination sphere: on one side O7, on the opposite side O1 and O3, belonging to the phenoxido groups connecting Zn II and Dy III ions. This arrangement of the shortest Dy–O bonds (Fig. 5 bottom, inset) creates a rather strong ligand field contributing to the axiality of the ground state of Dy III ions 31,32 with the anisotropy axis passing near the Dy–O7 bond and between the Fig. 5 Variable-temperature frequency dependence of the χ’’ M signal under zero applied field for 2(top panel). Arrhenius plots for relaxation times for 2–2 Y under different experimental conditions (bottom panel). The solid lines represent the best fits. Inset: reduced fragment of 2. Paper Dalton Transactions 7976 |Dalton Trans.,2024,53,7971–7984 This journal is © The Royal Society of Chemistry 2024 Open Access Article. Published on 12 April 2024. Downloaded on 10/3/2024 12:04:10 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online Dy–O1 and Dy–O3 directions, which agrees well with ab initio calculations (vide infra). Furthermore, the strong axiality is accompanied by a weak equatorial ligand field, since nitrate, carboxylate, and methoxy groups exhibit larger Dy–O distances and lower electronic densities on oxygen atoms than phenoxido groups; hence, they generate weaker fields. Thus, the serendipitous formation of such a coordination environment in 2, caused by the coordination of the succinate connector, is suitable for enhancing the magnetic anisotropy of the trivalent dysprosium ion. To elucidate the effect of replacing in 2the Zn II ion with the paramagnetic and anisotropic Co II ion on the SMM properties, its related heterometallic tetranuclear bis(CoDy) system 3was prepared. This compound does not show any maxima in the χ′ M (T) and χ″ M (T) plots at zero dc field (Fig. S10†), which could be attributed to the weak exchange and/or dipolar Co–Dy interactions operating in this compound (see the section on theoretical studies). These interactions could favour the fast QTM relaxation process. Nevertheless, it is known from the literature that some 3d–4f systems, where both metal cations are paramagnetic, exhibit improved SMM properties compared to systems in which the 3d ion is diamagnetic. 14 In such systems, the anisotropy axes of both ions are collinear, and relatively large exchange interactions slow down the relaxation processes and reduce the QTM. However, in many other 3d–4f systems, the introduction of a paramagnetic 3d ion had the opposite effect, 33 particularly when the above features are not fulfilled. Measurements carried out at the optimal field of 2.5 kOe revealed two sets of well-defined maxima: the first in the range of 2.0–3.0 K and the second in the range of 12.0–27.0 K (Fig. S45†). However, the first set shows a low frequency dependence of the maxima with little or no temperature shift from 60 to 10 000 Hz. To better evaluate this effect, the Mydosh parameter ϕwas calculated using the formula ϕ=ΔT p / [T p Δ(log f)], where T p is the peak temperature and fis the frequency. The frequency shifts from 200 to 10 000 Hz (we discarded the 60 Hz curve due to the evident mixing of both maxima) gave a value of ϕ= 0.09, which corresponds to glasslike behaviour. 34 High temperature relaxation, on the contrary, demonstrates a noticeable frequency dependence, which is consistent with SMM behaviour. Frequency-dependent out-ofphase susceptibility curves were fitted over the temperature range of 14.4–26.4 K using the generalized Debye model giving the relaxation time for each temperature (Fig. 6). The curvature of the data in the Arrhenius plot, as well as the relatively large values of αextracted from the Cole–Cole plots, prompted us to fit the temperature dependence of the relaxation time using the combined Orbach and Raman relaxation paths (eqn (1)), which resulted in the set of parameters given in Table 1. Compared to the U eff obtained for 2, the barrier height is noticeably lower, but agrees well with the energy of the second excited exchange-coupled doublet (see the computational part). It is noteworthy that the low-temperature set of maxima can presumably be associated with relaxation through the first excited exchange doublet, which involves a very low energy barrier in agreement with the maxima at extremely low temperatures. With the aim of disclosing the effect of magnetic dilution on the dynamic magnetic properties of 3, we prepared two isostructural derivatives, namely 3 Zn and 3 Y , by individually diluting Co II ions with Zn II or Dy III ions with Y III , respectively. These compounds contain 1 : 10 Co : Zn and Dy : Y, respectively (see Table S2†for ICP-MS results). Estimates show that the crystal structure of 3 Zn is dominated (82.63%) by bis(ZnDy) (2), there are significantly fewer (16.53%) (ZnDy)(CoDy) complexes, as well as a small amount (0.83%) of bis(CoDy) (3). Therefore, it is not surprising that 3 Zn behaves like a zero-field SMM, as seen in Fig. 7 and Fig. S47 and S48.†Below 28.0 K, a thermally activated relaxation process comparable to that exhibited by 2 occurs, consistent with the predominant contribution of bis (ZnDy) to the structure. The long tails appearing at low temperatures below the maxima indicate a pronounced QTM contribution (Fig. S48†). The temperature dependence of the relaxation time (Fig. 7) was fitted assuming the simultaneous presence of the Orbach, Raman and QTM mechanisms (eqn (1)) with the set of best-fit parameters collected in Table 1. The values related to Orbach and Raman mechanisms are comparable to those obtained for 2or 2 Y under an external magnetic field. When the same measurements were repeated under an external magnetic field of 2.5 kOe (field selected according to the field dependent measurements carried out for 2and 3), the QTM was successfully quenched (Fig. S52†). The small tails observed in Fig. S51 and S52†at very low temperature Fig. 6 Variable-temperature frequency dependence of the χ’’ M signal under 2.5 kOe applied field for 3(top panel). Arrhenius plot for relaxation times considering the simultaneous presence of Orbach and Raman mechanisms (bottom panel). Dalton Transactions Paper This journal is © The Royal Society of Chemistry 2024 Dalton Trans.,2024,53,7971–7984 | 7977 Open Access Article. Published on 12 April 2024. Downloaded on 10/3/2024 12:04:10 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online may be due to the residual QTM contribution and/or a relatively small number of exchange-coupled (ZnDy)(CoDy) molecules within the material. The absence of clear maxima at very low temperatures in the χ″ M (T) plot even at the highest frequency is consistent with a small energy gap between the ground and first excited exchange-coupled states. The relaxation times in the high temperature region were fitted by taking into account the combination of Orbach and Raman processes in eqn (1), affording the values indicated in Table 1. Almost identical values were obtained for 2under an external magnetic field (Table 1). In the case of 3 Y , the situation is reversed with respect to 3 Zn , and the dominant species in the structure is bis(CoY), followed by a substantially smaller amount of (CoDy)(CoY) and an overwhelming minority of bis(CoDy) (3). It should be noted that in the bis(CoY) species, the Co II ions have a square-pyramidal coordination environment, for which slow relaxation at zero-field is rarely observed. 35,36 This fact, along with the lack of slow relaxation in 3, explains why 3 Y does not exhibit out-ofphase ac susceptibility peaks at zero-field (Fig. S55†). When an external magnetic field of 1 kOe is applied, the temperature dependence of the out-of-phase ac susceptibility exhibits two maxima: the first below 4.0 K and the second (much less intense) below 26.0 K (Fig. S55†). We attribute the former to the relaxation of Co II ions (for square-pyramidal Co II complexes, field-induced χ″ M peaks usually appear below 6 K 35,36 ), while the latter corresponds to the exchange coupled Co–Dy system. Field-dependent measurements of χ″ M (ν) were carried out for the two individual processes at 2.0 K and 22.6 K. The field dependence of the relaxation times, obtained from fitting the χ″ M (ν) data with a generalized Debye model, allowed us to conclude that an external magnetic field of 1.5 kOe was adequate for slowing down the relaxation times for both processes (Fig. S56–S59†). When measuring the ac dynamic magnetic properties of 3 Y under the optimal field, the set of maxima at low temperature becomes frequency-dependent, and the Mydosh parameter ϕ= 1.0 is consistent with the SMM behaviour (Fig. S61†). The temperature dependence of the relaxation time was fitted taking into account both Raman and QTM processes (eqn (1)). The best fit afforded the set of parameters shown in Table 1. Accounting for three mechanisms (including also the Orbach process) did not give any reasonable fit. The presence of multiple relaxation paths is in agreement with the large αvalues (0.18 (2.0 K)–0.36 (4.6 K)). In the case of the slower relaxation process, a small proportion of (CoDy)(CoY) species in the structure leads to a weaker and noisier signal. We attempted to fit the relaxation times considering the simultaneous presence of the Orbach and Raman processes because αvalues (0.46 and 0.12 at 13.6 and 25.6 K, respectively) indicate the participation of several mechanisms. However, the fit did not afford any reasonable result. Thus, only the Orbach process was used to fit data for the high temperature regime, yielding the parameters gathered in Table 1. Note that the barrier height is almost identical to that found for 3, which is consistent with relaxation through the second excited state of the exchangecoupled Co–Dy system (see details in the Theoretical studies section). Last but not least, Fig. S66†displays all the Arrhenius plots involving data for pure 2and 3and for their diluted counterparts. As can be seen from this figure, compounds containing the (CoDy)(CoY) (3 Y ) or bis(CoDy) (3) species within the structure exhibit lower energy barriers and faster relaxation times than those containing the bis(ZnDy) (2), (ZnDy)(ZnY) (2 Y )or (ZnDy)(CoDy) (3 Zn ) systems. This fact will be confirmed by ab initio calculations, which are extensively discussed in the “Theoretical studies”section. As it can be observed in Table 1, τ 0 values decrease as the U eff values increase. This trend is not unexpected because τ 0 is expected to be proportional to |D| −3 whereas U, the thermal energy barrier, is proportional to D. 37 Hysteretic behavior We performed the magnetization hysteresis loop measurements on powdered samples of 2,2 Y and 3 Zn at 2 K to confirm their SMM behaviour (Fig. S67†). For 2, we used an average sweep rate of 7.7 × 10 −3 Ts −1 over the 0.44 to −0.63 T range and 8.8 × 10 −3 Ts −1 over the −0.44 to 0.63 T range. For 2 Y ,an average sweep rate of 4.4 × 10 −3 Ts −1 was employed in the range of 0.50 to −0.50 T and −0.50 to 0.50 T. Finally, for 3 Zn , an average sweep rate of 7.0 × 10 −3 Ts −1 was used in the range of 0.55 to −0.55 T and −0.55 to 0.55 T (in all cases slower in low fields and faster in high fields). Even though these compounds exhibited butterfly-shaped hysteresis loops without any remnant magnetization at zero field, a larger opening was observed for 2 Y in agreement with partial quenching of QTM by diamagnetic dilution. This is consistent with the differences observed in the QTM regimes in the χ″ M (T) plots. To better understand the magnetization dynamics, the magnetization curves for the most promising compounds (2, 2 Y ,3and 3 Zn ) were measured in a full cycle pulsed magnetic field (Fig. 8). 38 These measurements were carried out at 0.4 K with a maximum field of 10.4 T. They allowed us to observe Fig. 7 Arrhenius plots for the relaxation times and their fitting when considering the simultaneous presence of several mechanisms (experimental conditions are summarized in the Figure). Paper Dalton Transactions 7978 |Dalton Trans.,2024,53,7971–7984 This journal is © The Royal Society of Chemistry 2024 Open Access Article. Published on 12 April 2024. Downloaded on 10/3/2024 12:04:10 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online much larger hysteresis loops, because the sweep rates of 4.2 × 10 3 Ts −1 are extremely fast compared to what we used in the continuous field measurements (Fig. S67†). It is important to note that the field strength is not symmetrical in the positive and negative directions during the pulsing (Fig. S68†). Let us first compare the hysteresis curves for 2and 3. The latter compound exhibits a butterfly-shaped hysteresis loop with a sharp magnetization reversal close to zero field and a very small (compared to 2,2 Y and 3 Zn ) coercive field of 0.14 T (Fig. 8). This behaviour is consistent with the fact that 3exhibits neither slow magnetization relaxation at zero field, as indicated by ac measurements, nor hysteresis loops in dc measurements at much slower sweep rates. In contrast, compound 2, even at zero field, shows a clear open loop with a relatively large coercive field of 0.75 T. This behaviour would be in line with both the slow relaxation at zero field observed in the ac measurements and the butterfly-shaped hysteresis loop obtained from continuous field measurements (Fig. S67†). Surprisingly, diamagnetically diluted 3 Zn (or, in other words, compound 2, doped with Co II ) displayed a larger hysteresis loop than 2with a notably smaller QTM contribution at zero field (Fig. 8). In fact, the coercive field for 3 Zn was estimated to be 2.0 T, which is more than twice that of 2. Likewise, the remnant magnetization is slightly higher for the diluted compound (67.6% of the saturation values for 3 Zn and 63.5% for 2, respectively). For an unknown reason, pulse magnetization measurements indicate that a low concentration of Co II replacing Zn II ions improves the SMM properties. This is unexpected given the previous ac measurements (faster relaxation times for 3 Zn than for 2, see Fig. S69†) and the hysteresis loops measured in a continuous field (both compounds exhibit almost identical hysteresis loops). This astonishing behaviour of pulse magnetization is what we will try to explain in a future work by investigating doped versions of 2with variable Co : Zn ratios. Finally, we also studied compound 2 Y , which was synthesized to reduce intraand intermolecular Dy⋯Dy interactions that could facilitate QTM (Fig. 8). Similar to what we observed in the ac measurements, the QTM is almost completely quenched, and the loop opening for the diluted 2 Y counterpart is even larger than that for 3 Zn . Theoretical studies To better understand the dynamics of magnetization relaxation in the studied compounds, SA-CASSCF (state-averaged complete active space self-consistent field) calculations, followed by SO-RASSI (restricted active space state interaction) calculations, were performed for all binuclear fragments of compounds 1–3 (named 1′A,1′B,2′and 3′) with one paramagnetic ion using the geometries from the crystal structure data. For all these calculations, the OpenMolcas program was used (see the ESI†). It is worth noting that in 3,diamagneticZn II or Y III ions replaced the paramagnetic Co II or Dy III ions, respectively. To calculate the magnetic properties of such complexes, we used the SINGLE_ANISO procedure implemented in Molcas. As described above, compound 1consists of molecules 1A and 1B where in each ZnDy unit, in addition to a nitrate chelating the Dy III ion, there is a nitrate or acetate chelating ligand coordinated to the same Dy III ion, respectively. Ab initio calculations demonstrate that completely different relaxation behaviour is expected for these counterparts. On the one hand, the ground Kramers doublet (KD) for 1′Bis almost pure Ising type (KD1: M J = ±15/2, 98.6%; see Table S8†), while for 1′Ait is noticeably mixed (KD1: M J = ±15/2, 83.8%; see Table S9†). In good agreement with this, the ground state KD for 1′Bhas a g zz value close to 20 (the ideal Ising ground state has g x =g y =0and g z = 20 when using pseudospin S eff = 1/2) and almost negligible g xx /g yy values (Table S10†). The strong axiality of the ground KD suppresses the QTM within this doublet state, since the matrix element of the transverse magnetic moment within the +1/−1 ground KD (0.004) is significantly smaller than the required threshold of 0.1 for an efficient relaxation mechanism (Fig. S70†). 39 This fact justifies why 1 Y exhibits slow magnetic relaxation at zero applied dc field when the intermolecular interactions that facilitate QTM are suppressed (Fig. S20†). On the other hand, the ground KD of 1′Apresents significant transverse anisotropy, with g xx /g yy values much higher than those for 1′B(Table S11†). This factor, together with the mixed nature of the ground state wave function, results in a matrix element of the transverse magnetic moment within the ground KD of 0.23 (Fig. S71†), which indicates that the QTM relaxation within this KD is plausible. This explains why there is no zero-field SMM behaviour for this counterpart (Fig. S20†). In addition, we assume that both molecules (1′A and 1′B) relax through the first excited KD when applying an H dc field, since the matrix elements for the vertical and diagonal transitions (Orbach process) are large enough to allow the magnetization relaxation through these pathways. The experimentally and theoretically evaluated energy barriers are of 46.8 K and 46.0 K for 1A and 94.0 K and 173 K for 1B, respectively. The improved properties of 1′Bcompared to 1′Acan be easily rationalized by analysing the ligand field around the respective Dy III ions. In the Dy III coordination sphere of both units 1′Aand 1′B, the oxygen atoms of the phenoxido groups Fig. 8 Pulsed-field magnetization curves at a maximum field of 10.4 T and temperature of 0.4 K for 2,2 Y ,3and 3 Zn . Inset: Loop expansion to compare coercive fields. Note that the values for 2 Y have been normalized to give comparable loops. Dalton Transactions Paper This journal is © The Royal Society of Chemistry 2024 Dalton Trans.,2024,53,7971–7984 | 7979 Open Access Article. Published on 12 April 2024. Downloaded on 10/3/2024 12:04:10 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online