Alexandros S.
Armenis
a,
Dimitris I.
Alexandropoulos
b,
Anne
Worrell
c,
Luís
Cunha-Silva
d,
Kim R.
Dunbar
b and
Theocharis C.
Stamatatos
*ae
aDepartment of Chemistry, University of Patras, 26504 Patras, Greece. E-mail: thstama@upatras.gr; alexarmenis1996@gmail.com; Tel: +30 2610 996730
bDepartment of Chemistry, Texas A&M University, College Station, Texas 77843, USA. E-mail: dalexandrop4@gmail.com; dunbar@chem.tamu.edu
cDepartment of Chemistry, 1812 Sir Isaac Brock Way, Brock University, L2S 3A1 St Catharines, Ontario, Canada. E-mail: aw12vt@brocku.ca
dLAQV/REQUIMTE & Department of Chemistry and Biochemistry, Faculty of Sciences, University of Porto, 4169-007 Porto, Portugal. E-mail: l.cunha.silva@fc.up.pt
eInstitute of Chemical Engineering Sciences, Foundation for Research and Technology – Hellas (FORTH/ICE – HT), Platani, P.O. Box 1414, 26504, Patras, Greece
First published on 12th September 2023
The first use of the organic chelate N-hydroxy-1,8-naphthalimide (hynadH) in DyIII chemistry has unveiled access to a synthetic ‘playground’ composed of four new dinuclear complexes, all of which possess the same planar {Dy2(μ-OR)2}4+ diamond-shaped core, resulting from the bridging and chelating capacity of the hynad− groups. The structural stability of the central {Dy2} core has allowed for the modulation of the peripheral coordination sites of the metal ions, and specifically the NO3−/hynad− ratio of capping groups, thus affording the compounds [Dy2(hynad)2(NO3)4(DMF)2] (1), (Me4N)2[Dy2(hynad)2(NO3)6] (2), [Dy2(hynad)4(NO3)2(H2O)2] (3), and [Dy2(hynad)6(H2O)2] (4). Because of the chemical and structural modifications in the series 1–4, the DyIII coordination polyhedra are also dissimilar, comprising the muffin (1 and 3), tetradecahedral (2), and spherical tricapped trigonal prismatic (4) geometries. Complexes 1, 2, and 4 exhibit a ferromagnetic response at low temperatures, while 3 is antiferromagnetically coupled. All compounds exhibit out-of-phase (χ′′M) ac signals as a function of ac frequency and temperature, thus behaving as single-molecule magnets (SMMs), in the absence or presence of applied dc fields. Interestingly, the hynad−-rich and nitrato-free complex 4, demonstrates the largest energy barrier (Ueff = 69.62(1) K) for the magnetization reversal which is attributed to the presence of the two axial triangular faces of the spherical tricapped trigonal prism by the negatively charged O-atoms of the hynad− ligands.
A key ‘player’ in the field of 4f metal-based SMMs is the DyIII ion, due to the large magnetic anisotropy which arises from strong spin–orbit coupling, as well as the bistable ground state resulting from the odd number of electrons (4f9) according to Kramers theorem.9 Consequently, the DyIII ion has played a pivotal role in the pursuit of efficient SMMs with large energy barriers (Ueff) for the magnetization reversal and high blocking temperatures (TB).10 The enhancement of the magnetic anisotropy for an oblate-shaped ion, such as DyIII, can be achieved by either chemical or structural means, such as placing strong axial ligand fields with weak ligand fields in the equatorial plane,11 or preparing highly symmetric complexes with ideal lanthanide coordination geometries (i.e., D4d, D5h, and D6h).12 Maximizing the magnetic anisotropy results in large energy splitting of the mJ microstates of the ground state (6H15/2) and subsequently large Ueff values (exceeding 2000 K), high blocking temperatures (up to 80 K), and dominant thermally activated relaxation mechanisms (Orbach process) observed in a number of mononuclear organometallic DyIII complexes.13 However, there are some under-barrier demagnetization pathways, such as the Raman process, which are favored due to the perturbation of the relaxing spins from the lattice thermal energy (phonons) which are detrimental to the retention of the magnetization.14 Another major undesired mechanism is Quantum Tunneling of Magnetization (QTM) in which the spin tunnels through the barrier without following the thermally activated, stepwise Orbach process and is typically operative at low temperatures. The tunneling process is observed as a step in the hysteresis loops of magnetization (M) vs. field (H) plots due to the loss of magnetization near zero field and is a common feature in most non-organometallic, high-performance DyIII SMMs.14,15
One of the main synthetic strategies to circumvent under barrier relaxation processes is the preparation of radical-bridged dinuclear lanthanide (Ln) complexes, such as the family of [Ln3+–N23−–Ln3+] complexes,16a in which radical anionic ligands penetrate the inner 4f orbitals which leads to strong magnetic coupling between the 4f-metal ions.16 Furthermore, metal–metal bonding between Ln ions has shown considerable promise, for example the mixed-valence dinuclear complex [(CpiPr5)2Ln2I3], where CpiPr5 is the pentaisopropylcyclopentadienyl anion and Ln = Gd, Tb, or Dy.17 The metal ions in this family of metal–metal bonded systems exhibit a Ln–Ln bond by sharing a d electron in the 5dz2 orbital, rendering this molecular system an ultrahard magnet with open hysteresis loops up to 80 K and coercivities comparable with those of commercial magnets (i.e., SmCo5 and Nd2Fe14B).
Among the polynuclear lanthanide complexes that have been investigated in the area of molecular magnetism, dinuclear complexes have been widely explored because they constitute the most simple platform to investigate magnetic exchange coupling between two spin-carriers.2,18 The choice of chelating/bridging organic ligands is of paramount importance in such molecular systems, as they should mainly consist of O-donor atoms to satisfy the oxophilicity of Ln ions. In this regard, we recently targeted the ligand N-hydroxy-1,8-naphthalimide (hynadH; Scheme 1), which, upon deprotonation, can act as a pocket-like, C2 symmetric ligand for bridging and chelating two LnIII atoms.
![]() | ||
Scheme 1 Structural formula and abbreviation of the ligand N-hydroxy-1,8-naphthalimide (hynadH) used in this study. |
Herein we report the syntheses, structures, and magnetic characterization of a new family of {Dy2} complexes with a {Dy2(hynad)2}4+ planar core comprising the four compounds [Dy2(hynad)2(NO3)4(DMF)2] (1), (Me4N)2[Dy2(hynad)2(NO3)6] (2), [Dy2(hynad)4(NO3)2(H2O)2] (3), and [Dy2(hynad)6(H2O)2] (4). The ferromagnetic vs. antiferromagnetic coupling for compounds 1, 2, and 4versus3, respectively, has been attributed to their dissimilar metrical parameters, including the intramolecular Dy⋯Dy separations, the Dy–O core distances, and the ligand's configuration with respect to the {Dy2O2} core. Interestingly, all complexes exhibit slow relaxation of their magnetization and SMM properties in the presence or absence of an external magnetic field. The different magnetic dynamics of 1–4 have been rationalized by means of analyzing the DyIII coordination polyhedra and the orientation of the axial magnetic anisotropy along the stronger Dy–O bonds.
Substantial electron density was found in the data of compounds 3 and 4 due to disordered solvent molecules occupying the interstices. Our efforts to properly locate, model and refine these residues were unsuccessful, and the investigation for the total potential solvent area using the software package PLATON clearly confirmed the existence of cavities with solvent accessible void volume. Consequently, the original data sets were treated with the program SQUEEZE,30 a part of the PLATON package of crystallographic software, which calculates the contribution of the disordered electron density in the void spaces and adds this to the calculated structure factors from the structural model when refining against the .hkl file.
Figures of the structures were created using Diamond 3 and Mercury software packages.31,32 Unit cell parameters, structure solution and refinement details for 1–4 are summarized in Table S1.† Further crystallographic details can be found in the corresponding CIF files provided in the ESI.†
The first reaction that was performed was the 1:
1
:
1 reaction between Dy(NO3)3·5H2O, hynadH and NEt3 in a solvent mixture of MeCN/DMF (1
:
1, v/v) to increase the solubility of the reactants and the final product. The yellow crystalline compound was revealed to be the dinuclear complex [Dy2(hynad)2(NO3)4(DMF)2] (1) bearing a double alkoxido-bridged {Dy2(μ-OR)2} core supported by peripheral nitrato groups and DMF solvate molecules (vide infra). A subsequent systematic study was conducted,35 firstly by replacing NEt3 with the stronger base Me4NOH without altering other synthetic parameters. After a week, yellow crystals of the anionic dinuclear complex (Me4N)2[Dy2(hynad)2(NO3)6]·2MeCN (2·2MeCN) were formed, revealing the first peripheral site modulation of the {Dy2(μ-OR)2} core from the set of hynad2−vs. NO3− ligands (1
:
2 ratio in 1vs. 1
:
3 ratio in 2). Additional changes to the nature and strength of the external base did not yield any crystalline material but only oily products or amorphous precipitates.
The next synthetic step was to exert a chelate stress on the initial reaction which led to compound 1 by increasing the quantity of the hynadH ligand with respect to that of available nitrates, thus further manipulating the ratio of the two coordinating groups without affecting the dinuclear core structure. Indeed, the 1:
2
:
2 reaction between Dy(NO)3·5H2O, hynadH and NEt3, in the same solvent mixture of MeCN/DMF (1
:
1, v/v), yielded orange-yellow crystals of the [Dy2(hynad)4(NO3)2(H2O)2]·2DMF (3·2DMF) compound featuring the same {Dy2(μ-OR)2} core albeit in a new hynad2−vs. NO3− ligand ratio of 2
:
1. A further increase in the amount of chelate ligand by using a 1
:
3
:
3 reaction of Dy(NO)3·5H2O, hynadH and NEt3 in MeCN/DMF (1
:
1, v/v) yielded a red colored solution and single-crystals of [Dy2(hynad)6(H2O)2]·2DMF (4·2DMF), the fourth member of this family of diamond-shaped {Dy2} complexes and the first containing exclusively hynad2− bound ligands. Although it would not have been possible to predict the formation of complex 4, given the steric bulk of the chelate it is evident that the planarity of the central {Dy2(hynad)2}4+ subunit provides the available space for its formation.
For all four compounds 1–4 (Fig. 1–4), the two DyIII atoms are doubly bridged by the deprotonated alkoxido-type O atom of two nearly planar, η1:η2:η1:μ hynad− ligands, yielding a planar {Dy2(μ-OR)2}4+ diamond-shaped core (highlighted bonds in Fig. 1–4). The intra-dimer Dy1⋯Dy1′ distances are 4.018(4), 4.130(3), 4.094(3), and 4.133(5) Å, for 1–4, respectively. In all cases, the central diamond-shaped core is completed by the two five-membered chelating rings from each hynad− ligand. In addition, for 1, peripheral ligation is provided by two bidentate chelating NO3− groups and one terminally bound DMF molecules on each DyIII atom. The NO3− groups are perpendicular to the nearly planar {Dy2(μ-hynad)2}4+ unit, while the coordinated DMF molecules are close to the axis that passes through the two DyIII centers (Fig. S1a†). The displacement of the ligand's donor atoms O9 and O10 (and their symmetry-equivalent partners) out of the {Dy2(μ-O)2} best-mean-plane is 0.354 and 0.090 Å, respectively. The Dy1–O8–Dy1′ intra-dimer angle is 119.0(1)°. Each DyIII atom in 1 is nine-coordinate, possessing a distorted “muffin-type” geometry (Fig. 1b), as confirmed by the continuous shape measures (CShM) approach of the SHAPE program36 which allows one to numerically evaluate how much a particular polyhedron deviates from the ideal shape. The best fit was obtained for the muffin geometry (CShM value = 2.73 and Table S6†). Values of CShM larger than 3 correspond to a significant distortion from the ideal geometry.
In the case of (Me4N)2[Dy2(hynad)2(NO3)6] (2), the coordinated DMF molecules have been replaced by two bidentate chelating NO3− groups (Fig. 2a), thus increasing the coordination number of each DyIII atom from nine (in 1) to ten. As a result, the coordination geometry of DyIII atoms in 2 can be best described as tetradecahedral (Fig. 2b) with a CShM value of 2.24 (Table S7†). The methyl groups of the Me4N+ cations are weakly interacting with the dangling O-atoms of the nitrato ligands, holding together the dianionic coordination compound. In 2, the displacement of the ligand's donor atoms O1 and O3 (and their symmetry equivalents) out of the {Dy2(μ-O)2} best-mean-plane is 0.748 and 0.216 Å, respectively, thus imposing a significant twist on the ligand's backbone (Fig. S1b†), which is a noticeable difference in the structures of 1 and 2. The Dy1–O2–Dy1′ intra-dimer angle is 121.2(1)°, very close to the value found in 1. A comparison of the stereochemical features of 1 and 2 reveals a noteworthy feature which deserves discussion; the ligand hynad− (as defined by the best-mean-plane of all its atoms) in 2 is significantly tilted with respect to the planar {Dy2O2} core by an angle of 13.0°, while the same angle in 1 is only 5.4°. These features emphasize the flexible nature of hynad− upon coordination with the DyIII centers.
In terms of intermolecular interactions, both 1 and 2 exhibit π–π stacking interactions with their neighboring counterparts through the naphthalene units of hynad− ligands (Fig. S2 and S3†). The centroid-to-centroid separations are 3.612 and 3.654 Å, while the shortest intermolecular Dy⋯Dy distance is 7.209 and 8.739 Å for 1 and 2, respectively.
Complex [Dy2(hynad)4(NO3)2(H2O)2] (3) is the first of its kind within the reported family of dinuclear compounds in that two hynad− groups occupy peripheral sites of the {Dy2(hynad)2}4+ core, acting as η1:η1-bidentate chelating ligands (Fig. 3a) and each having an uncoordinated carbonyl O atom. Additional ligation is provided by two bidentate chelating NO3− groups and two terminal H2O molecules, which complete the nine-coordinate geometry about each DyIII atom. According to the SHAPE program, the best geometry that describes the DyIII centers is that of a distorted muffin (CShM = 3.29; Fig. 3b and Table S6†). The Dy1–O2–Dy1′ intra-dimer angle is 117.9(2)°. The displacement of the core ligand's donor atoms O1 and O3 (and their symmetry-equivalent partners) out of the {Dy2(μ-O)2} best-mean-plane is 0.813 and 0.195 Å, respectively, while the ligand hynad− in whole forms an angle of 14.7° with respect to the {Dy2O2} core. This leads to a more pronounced distortion of 3 as compared to the nitrato-rich 1 and 2 (Fig. S1c†).
Finally, the coordinated H2O molecules (O1W and O1′W) form intramolecular H-bonds with the lattice DMF solvate molecules (O10 and O10′) and the deprotonated O-atoms of the bidentate chelating hynad− ligands (O5 and O5′); their dimensions are: O1W⋯O10 = 2.754(1) Å and O1W⋯ O5′ = 2.762(6) Å (Fig. S4†). Furthermore, the {Dy2} complexes in the crystal of 3·2DMF interact intermolecularly with each other through π–π stacking interactions along the crystallographic a- and c-axes, thus creating an overall 2-D porous framework (Fig. S5 and S6†). The shortest intermolecular Dy⋯Dy distance in 3 is 10.885 Å.
The nitrato-free complex [Dy2(hynad)6(H2O)2] (4) consists of two nine-coordinate DyIII atoms (Fig. 4a), but, in this case, the metal centers adopt a spherical tricapped trigonal prismatic geometry as established by the SHAPE program (CShM = 2.05, Fig. 4b and Table S6†). Peripheral ligation about the {Dy2(hynad)2}4+ core is provided by four additional η1:η1-bidentate chelating hynad− ligands (as in 3) and two terminally-bound H2O molecules. The Dy1–O1–Dy1′ intra-dimer angle is 117.6(2)°, essentially the same as that of 3. The displacement of the core ligand's donor atoms O2 and O3 (and their symmetry-equivalent partners) out of the {Dy2(μ-O)2} best-mean-plane is 0.089 and 1.211 Å, respectively, which means that the O2/O2′ atoms are nearly co-parallel with the core subunit whereas the O3/O3′ atoms are distal from the core (Fig. S1d†). Interestingly, the bridging hynad− core ligands in 4 form an angle of 28.6° with respect to the {Dy2O2} core, which is nearly twice the corresponding value found in 3, thus imposing a significant twist on the structure; this is most likely due to the presence of the additional chelating hynad− groups at the peripheral sites of the compound.
Akin to 3, the coordinated H2O molecules (O11 and O11′) form intramolecular H-bonds with the interstitial DMF solvate molecules (O10 and O10′) and the deprotonated O-atoms of two bidentate chelating hynad− ligands (O7 and O7′); their dimensions are: O11⋯O10 = 2.780(1) Å and O11⋯ O7′ = 2.737(6) Å. Moreover, the {Dy2} complexes in the crystal of 4·2DMF are strongly interacting with each other through an extensive array of π–π stacking intermolecular interactions with centroid-to-centroid distances of 3.597 and 3.511 Å along the crystallographic a- and c-axes, respectively (Fig. S7 and S8†). The shortest intermolecular Dy⋯Dy distance in 4 is 11.013 Å, slightly larger than that of 3.
![]() | ||
Fig. 5 Temperature dependence of the χMT product for complexes 1–4 recorded at a 0.1 T static dc field. |
The field (H) dependence of the magnetization (M) for all complexes 1–4 at 2, 5, and 7 K are shown in Fig. S9–S12.† All of the compounds exhibit a relatively rapid increase at low fields without reaching saturation at the maximum applied field of 7 T, indicating the presence of magnetic anisotropy and/or low-lying excited states. The magnetization values at 7 T are 15.01 (1), 14.40 (2), 8.36 (3), and 16.34NμB (4), much lower than the expected saturation value (MS) for two DyIII ions (MS/NμB = 20NμB); this is mainly attributed to the crystal field effects that induce strong magnetic anisotropy.
To further examine the distribution of relaxation times (α), the Cole–Cole plots of both 1 and 2 were fit using a generalized Debye model (Fig. S13 and S14†).37 The shapes of the plots deviate from the typical semicircular ones, yielding α values in the range of 0.16–0.03 (Tables S8 and S9†), indicative of a wide distribution of relaxation times which is consistent with the presence of multiple relaxation processes most likely due to a combination of thermally assisted and under-barrier relaxation mechanisms. Hence, to extract the temperature dependence of relaxation times (τ), and construct an Arrhenius-like plot, we fitted the data including Orbach, Raman, and QTM relaxation mechanisms to the overall magnetization behavior of complexes 1 and 2, by using the following eqn (1):
τ−1 = τ0−1![]() | (1) |
![]() | ||
Fig. 7 Temperature dependence of the relaxation times (τ) according to the Arrhenius plot for 1 (top) and 2 (bottom) under zero applied dc field. The red circles correspond to experimental data and the black line is the best-fit of the data to eqn (1); see the insets for the fit parameters. |
As shown in Fig. 7, the non-linear shape of the Arrhenius plots further corroborates the significant contribution of Raman and (possibly) QTM processes to the magnetization dynamics as the temperature is lowered. Specifically, at the intermediate and low-T regime, the relaxation time appears to be dominated by the Raman process as illustrated by the curvature of the lnτ vs. T−1 plots (Fig. 7) denoted by a power-law dependence (second term in eqn (1)).39 In the high-T regime, the thermally assisted Orbach process dominates and the relaxation time has an exponential dependence on temperature (linear region), giving similar Ueff values of 7.24(2) K (1) and 6.30(2) K (2) as well as τ0 values of 1.49(1) × 10−4 s (1) and 1.61(1) × 10−4 s (2). The best-fit parameters, C and n, of the Raman process (inset of Fig. 7) are within the expected range for DyIII SMMs.9,10,38 In summary, the different coordination environments of the DyIII centers in complexes 1 and 2 (muffin vs. tetradecahedral, respectively), do not appear to significantly affect the relaxation dynamics of the resultant complexes.
Complex 3 does not show any out-of-phase (χ′′M′) signals at zero applied dc field; however, from the field dependence of the χ′′Mvs. frequency (v) plots at 2 K (Fig. S15†), an optimum dc field of 2000 Oe was extracted, and this was used to carry out detailed ac studies. At this field, complex 3 shows frequency- and temperature-dependent χ′′M signals in the temperature range of 1.8–4.6 K (Fig. 8), indicative of the slow magnetization relaxation of an SMM. The shapes of the Cole–Cole plots deviate significantly from the ideal semicircles (Fig. S16†), indicating the coexistence of thermally assisted and through barrier relaxation processes. This is further supported by the derived α values, which span the range 0.35–0.16 (Table S10†), reflecting a wide distribution of relaxation times and, a fortiori, the presence of multiple relaxation processes. Although application of an external magnetic field is known to suppress or even eliminate the QTM mechanism9,12,39 we fitted the experimental lnτ vs. T−1 data (Fig. 9) over the entire temperature range using eqn (1), which includes the contribution from the tunneling process. A very good fit was obtained, and this gave us Ueff and τ0 values of 13.64(1) K and 9.34(1) × 10−6 s, respectively (inset of Fig. 9). As in the cases of 1 and 2, the curved shape of the ln
τ vs. T−1 plot for 3 also suggests the presence of Raman and QTM processes, which turned out to be the case given the derived fitting parameters: C = 1.21(2) × 10−2 s−1 K−n, n = 8.53(4), and τQTM = 1.71(2) × 10−3 s.
![]() | ||
Fig. 9 Temperature dependence of the relaxation times (τ) according to the Arrhenius plot for 3 under a 2000 Oe applied dc field. The blue circles correspond to experimental data and the black line is the best-fit of the data to eqn (1); see the inset for the fit parameters. |
Complex 4, the final member of this family of {Dy2} complexes and the only one that is nitrato-free, contains (as in 1 and 3) nine-coordinate DyIII atoms albeit in a spherical tricapped trigonal prismatic geometry. At zero external dc field, complex 4 exhibits tails of out-of-phase (χ′′M) signals at temperatures below 10 K (Fig. S17†), indicative of a fast relaxation process, which is predominantly driven by the QTM process. To slow down the relaxation process and force the magnetization to thermally relax via the excited state(s), we applied an optimum dc field of 800 Oe determined from the fit of the χ′′Mvs. frequency data at various fields (Fig. S18†). Indeed, under a small dc field of 800 Oe, fully visible out-of-phase signals, as a function of the ac frequency in the temperature range of 2.0–8.5 K, were observed for 4 (Fig. 10), which is characteristic of an SMM with an appreciable energy barrier for the magnetization reversal. A good fit of the temperature-dependent relaxation times, according to eqn (1), yielded a Ueff barrier of 69.62(1) K and a τ0 of 3.91(3) × 10−5 s (Fig. 11), together with the corresponding parameters from the operating Raman and QTM processes (inset of Fig. 11). The α parameters resulting from the fit of the Cole–Cole plots (Fig. S19†), over the temperature range of 2.0–8.5 K, span the range 0.21–0.08, in agreement with a wide distribution of relaxation times.
![]() | ||
Fig. 11 Temperature dependence of the relaxation times (τ) according to the Arrhenius plot for 4 under an 800 Oe applied dc field. The purple circles correspond to experimental data and the black line is the best-fit of the data to eqn (1); see the inset for the fit parameters. |
The derived Ueff value of 4 is almost five times larger than that of 3 and an order of magnitude larger than those of 1 and 2. This result is attributed to the DyIII coordination environment in 4, which likely induces a larger crystal field splitting of the ground 6H15/2 state, as well as the exclusive presence of hynad− ligands, which foster a larger separation of the {Dy2} complexes which serves to minimize the transverse fields resulting from dipolar interactions between neighboring molecules, thereby reducing the efficiency of the tunneling relaxation. Retrospectively, the rare spherical tricapped trigonal prismatic polyhedron tends to have the capping vertices at the same distance from the center of the polyhedron.40 This is the case for 4 (Fig. 4b); the distance of Dy1 to the three equatorial capping O donor atoms (O2′, O3 and O11), which belong to the neutral charged carbonyl O-atoms of hynad− and H2O ligands, are 2.509(6), 2.522(7) and 2.429(7) Å, respectively. The two axial triangular faces of the prism contain the atoms O4/O5/O8 and O1/O1′/O7, most of which belong to the deprotonated O-donor atoms of the hynad− ligands. The bond distances between these atoms and the central Dy1 atom are much shorter (2.309(7)–2.447(6) Å) than those of the equatorial capping atoms. Thus, the anionic O-donors of the hynad− ligands are much closer to the DyIII ions than the three equatorial ligands, and they will thus dominate the electronic structure. In turn, this will induce a relatively strong and axial crystal field above and below the DyIII metal ions, which would enhance the oblate nature of the electron density of DyIII in its electronic ground state, which explains the experimentally observed anisotropy barrier.41
As a final comparison, all the pertinent features of compounds 1–4 with respect to their structural, and static and dynamic magnetic properties are compared in Table 1. In addition to the aforementioned impact of the DyIII coordination geometry on the magnetic dynamics of 4, only some tentative conclusions can be further derived by examining the information in Table 1, and these are restricted to a comparison between 1 and 3, both of which contain 9-coordinate DyIII atoms with muffin-like geometries. Following the conclusions extracted by the work of Tang and coworkers on complexes bearing the {Dy2(μ-OR)2}4+ core with the same coordination geometries, albeit with distinctly different Dy–O–Dy angles,42 we mainly attribute the ferromagnetic response of 1 (versus the antiferromagnetic behavior of 3) to the closer intramolecular Dy⋯Dy separations, the shorter Dy–O core distances, and the planarity of the bridging hynad− ligands with respect to the {Dy2O2} core, provided that the Dy–O–Dy angles in 1 and 3 are essentially the same. To further reach a level of understanding on the differences between the obtained magnetic dynamics of 1 and 3, we analyzed the muffin-like geometries of the corresponding DyIII atoms (Fig. 1b and 3b).43 To this end, in 1, the equatorial pentagonal plane, which encompasses the oblate DyIII center, is made of O1, O2, O4, O5, and O8 atoms, while the basal trigonal plane and the single atom vertex of the muffin, which are located below and above the DyO5 plane, are formed by O6, O8′, O10′, and O9 atoms, respectively.44 Interestingly, the stronger Dy–O bonds within the muffin topology are distributed among the atoms occupying both the equatorial pentagon [Dy1–O1 = 2.286(2) and Dy1–O8 = 2.331(2)] and the axial triangular [Dy1–O8′ = 2.332(3)] subunits, likely causing a disorientation of the magnetic anisotropy from the pure axiality which explains the small Ueff value of 1. In contrast to 1, in complex 3, the stronger Dy–O bond belongs to the single atom vertex [Dy1–O5 = 2.332(4)] of the muffin-like polyhedron, which could direct the projection of the magnetic anisotropy towards the axiality and away from the transverse crystal field, thus yielding a larger Ueff value (vide supra).
Complex | 1 | 2 | 3 | 4 |
---|---|---|---|---|
Intramolecular Dy⋯Dy (Å) distance | 4.018(4) | 4.130(3) | 4.094(3) | 4.133(5) |
Intermolecular Dy⋯Dy (Å) distance | 7.209(4) | 8.739(3) | 10.885(3) | 11.013(5) |
DyIII coord. number/geometry | 9/muffin | 10/tetradecahedron | 9/muffin | 9/spherical tricapped trigonal prism |
Dy–O core distances (Å) | 2.331(2) | 2.377(2) | 2.361(4) | 2.384(6) |
2.332(3) | 2.364(2) | 2.417(4) | 2.447(6) | |
Dy–O–Dy angle (°) | 119.0(1) | 121.2(1) | 117.9(2) | 117.6(2) |
Distortion of hynad− over the {Dy2O2} core (°) | 5.4 | 13.0 | 14.7 | 28.6 |
Predominant magnetic exchange interactions | Ferromagnetic | Ferromagnetic | Antiferromagnetic | Ferromagnetic |
U eff (K) | 7.24(2) (0 dc) | 6.30(2) (0 dc) | 13.64(1) (dc = 2000 Oe) | 69.62(1) (dc = 800 Oe) |
Current efforts are directed at seeking new synthetic methods for retaining the {Dy2(hynad)2}4+ core while introducing strongly bound alkoxides, phenoxides, or siloxides at the apical positions of the DyIII coordination sites as a means of increasing the crystal field strength at the axial positions and therefore enhancing the easy-axis magnetic anisotropy and the energy barriers for the magnetization reversal. Given the stereochemical conformation of the ligand hynad− and its coordinating flexibility about the DyIII atoms observed in 1–4, it is reasonable to expect that upon additional chemical variations and synthetic modification, this chelate ligand will yield 4f-compounds of different nuclearities and topologies. The results of these studies will be reported in due course.
Footnote |
† Electronic supplementary information (ESI) available: Crystal data and refinement parameters, structural data (tables and figures), and additional magnetism figures and tables for complexes 1–4. CCDC 2278244 (1), 2278245 (2), 2278246 (3) and 2278247 (4). For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3dt02596a |
This journal is © The Royal Society of Chemistry 2023 |