Synthesis, Structural, and Magnetic Characterization of a Mixed 3d/4f 12-Metallacrown-4 Family of Complexes

A new family of complexes (tBu4N){[LnIII(O2CBut)4][12-MC-Mn(III)N(shi)-4]}·5CH2Cl2 (Ln = Gd (1) and Tb (2)), (tBu4N)2{[YIII(O2CBut)4][12-MC-Mn(III)N(shi)-4]}(ClO4) (3); where shiH3 = salicylhydroxamic acid; ButCO2− = pivalate ions; tBu4N = tetrabutylammonium and ClO4− = perchlorate ions, has been isolated. The reaction of salicylhydroxamic acid with Mn(O2CBut)2·2H2O, Ln(NO3)3·xH2O, tBu4NClO4 in the presence of morpholine (C4H9NO) led to the isolation of compounds 1–3. The complexes belong to the 12-MC-4 family of Metallacrowns (MCs) possessing a central {Mn4IIILnIII(µ-NO)4}11+ core with the four MnIII atoms occupying the periphery positions, while the formed [Mn–N–O] repeating unit, assists in the accommodation of the LnIII atom in the center of the ring. Peripheral ligation is provided by four η1:η1:μ pivalate ions. Direct current magnetic susceptibility (dc) measurements revealed the presence of predominant antiferromagnetic exchange interactions within the metal centers. A 1-J fitting model was used in order to quantify the MnIII–MnIII interactions and fitting of the data, for the diamagnetic YIII analogue, gave J = −3.74 cm−1 and gMn(III) = 2.07. Fitting of the {Mn4Gd} compound using a 2-J model, counting additionally for the MnIII–GdIII interactions, revealed values of J1 = −3.52 cm−1, J2 = −0.45 cm−1, and gMn(III) = 1.99.


Introduction
Heterometallic 3d/4f complexes continue to attract the interest of the scientific community as they have been proven to be good candidates for possible applications in various fields such as optics [1,2], catalysis [3], and molecular magnetism [4]. In the field of molecular magnetism, the use of paramagnetic 3d metal ions in combination with highly anisotropic lanthanides such as Dy III or Tb III with large and unquenched orbital angular momenta [5] can lead to single-molecule magnetism (SMM) behavior with large anisotropy barriers (or energy barriers) for the magnetization reversal. In a common understanding, an SMM is able to retain its magnetization only as long as it is kept below a characteristic blocking temperature, T B , in the absence of an applied magnetic field [6]. The magnitude of the energy barrier to spin reversal (U eff ) in 3d SMMs is equal to S 2 |D| for integer and (S 2 − 1/4)|D| for half-integer spin systems, where D is the zero-field splitting parameter. Thus, the total spin of the molecule, S, and the Ising-type magnetic anisotropy, are the two factors that block the magnetization reversal. In transition metal complexes, the ground state bistability arises from the total spin S with the ensuing [2S + 1] m s microstates, while in lanthanides, the spin-orbit-coupled ground term 2S+1 L J splits into [2J + 1] m J microstates that are responsible for the magnetic bistability of those complexes [5]. Experimentally, we can detect the slow magnetic relaxation of SMMs by performing alternating-current (ac) susceptibility measurements and, most importantly, by the observation of hysteresis loops, which is the ultimate diagnostic property of bulk classical magnets [7]. Recently, quantum tunneling of magnetization (QTM) [8,9] and quantum interference promote the discussion of SMMs as being ideal candidates for even more advanced applications such as spintronics and quantum computing [9][10][11].
Metallacrowns (MCs) is a class of compounds that since their discovery has attracted the immense attention of the scientific community [12][13][14]. Most of these complexes have repeatedly demonstrated their ability to encapsulate a central metal ion in their MC cavity, similar to crown ethers, and till now a wide range of MC sizes has been reported [15]. The first example of a 12-MC-4 was reported in 1989 by Pecoraro and Lah, and it was a Mn(OAc) 2 [12-MC-MnIII(N)shi -4] complex where OAc − is acetate ions and shi 3− is salicylhydroximate ions [13]. Pecoraro and coworkers have exceedingly demonstrated that salicylhydroxamic acid (shaH 2 , Scheme 1) can possibly be subjected to a metal-assisted 2-amide-iminol tautomerism, which leads to salicylhydroxime (shiH 3 , Scheme 1); the latter being an excellent chelating-bridging ligand which has also been shown to possess the appropriate geometry to afford the 12-MC-4 motif [1,15]. The formed repeating unit of [Mn III -N-O], along with the triply deprotonated salicylhydroximate, the central Mn II ion and the two acetate bridges were responsible for the aforementioned configuration. Usually, in the central MC cavity sits a transition metal ion, even though there are also reports where alkali and alkaline earth metals occupy the cavity [15,16]. Lately, the focus has been turned into the incorporation of lanthanide ions in the center, since these compounds have been proposed as excellent candidates for molecular recognition [17,18], molecular magnetism [19] and luminescent [1,[19][20][21][22] technologies. Single-crystal diffraction studies revealed that compounds 1 and 2 are isostructural and crystallize in the P4/n tetragonal space group, while complex 3 crystallizes in the P4cc tetragonal space group (Table S1). Although complex 3 is not isostructural with complexes 1 and 2, still it does possess the same core with them and thus, only complex 2 will be thoroughly described for simplicity reasons. The structure of 2 consists of a [Tb III (O2CBu t )4][12-MC-Mn(III)N(shi)-4]} − anion (Figure 1), one t Bu4N + cation and, five non-coordinated CH2Cl2 molecules. Its asymmetric unit features one-quarter of the [Tb III (O2CBu t )4][12-MC-Mn(III)N(shi)-4]} − anion, with the C4 axis passing through the central Tb III ion. There are two isomers in the structure and only the main part will be discussed. Selected interatomic distances and angles for all complexes are listed in Tables S2 and S3. The core of 2 comprises four Mn III and one Tb III atoms arranged in a square pyramidal-like topology with the Tb atom occupying the apical position of the pyramid and the Mn atoms completing the base ( Figure  S1). Scheme 1. Illustrative representation and abbreviation of organic molecules discussed in the text.
Although there have been numerous 12-MC-4 complexes reported to date, only a few comprise the 3d/4f motif with salicylhydroxamic acid (shiH 3 ) [23][24][25]. If we further restrict the above requirements, by exclusively using Mn III as the periphery ring metal ion, only a very few papers have been published featuring the above qualifications. Pecoraro and Zaleski reported a complex with the general formula being Ln III M I (OAc) 4 [12-MC-MnIII(N)shi -4](H 2 O) 4 ·6DMF where M I = Na I and K I , and Ln = various lanthanides [26][27][28]. This family of compounds has been extensively structurally studied but the authors did focus their investigations mainly towards the effect that the Na I or K I ions had on magnetic measurements and not on the pure {Mn 4 III Ln III (µ-NO)4} 11+ magnetic core, without emphasizing on the evaluation of exchange interactions by fitting of the data. Our group has a great interest in the synthesis and magnetic characterization of MCs and so far has dealt with the isolation of homometallic and heterometallic ones based on transition metal ions [29][30][31][32] 4 ]·(ClO 4 )) for 3 in high yields (>59%). The chemical and structural identities of the compounds were confirmed by single-crystal X-ray crystallography, elemental analyses (C, H, N) and IR spectral data (Supplementary Materials).
Single-crystal diffraction studies revealed that compounds 1 and 2 are isostructural and crystallize in the P4/n tetragonal space group, while complex 3 crystallizes in the P4cc tetragonal space group (Table S1). Although complex 3 is not isostructural with complexes 1 and 2, still it does possess the same core with them and thus, only complex 2 will be thoroughly described for simplicity reasons. There are two isomers in the structure and only the main part will be discussed. Selected interatomic distances and angles for all complexes are listed in Tables S2 and S3. The core of 2 comprises four Mn III and one Tb III atoms arranged in a square pyramidal-like topology with the Tb atom occupying the apical position of the pyramid and the Mn atoms completing the base ( Figure S1). Single-crystal diffraction studies revealed that compounds 1 and 2 are isostructural and crystallize in the P4/n tetragonal space group, while complex 3 crystallizes in the P4cc tetragonal space group (Table S1). Although complex 3 is not isostructural with complexes 1 and 2, still it does possess the same core with them and thus, only complex 2 will be thoroughly described for simplicity reasons.  The basal Mn III atoms are bridged by diatomic oximate bridges provided by the shi 3− ligand, giving a Mn•••Mn separation of 4.642(9) Å. Note that the coordinated anion of shi 3− was generated in situ from the metal ion-assisted transformation of shaH2 under basic conditions. The large Mn-N-O-Mn torsion angle (173.3(4)°), which is very close to the ideal linearity of 180°, is responsible for the approximately ideal planarity of the Mn4 assembly, whilst the Tb III atom lies 1.789(8) Å out of the Mn4 plane. The connection between the basal Mn III and the Tb III atoms is provided by the oximate O atoms of the shi 3− ligand resulting in a Mn…Tb separation of 3.739(2) Å. Further ligation is provided by four η 1 :η 1 :μ bridging pivalate ions ( Figure 1). The coordination sphere around the Mn atoms is completed by the alkoxido and phenoxido O atoms provided by the organic moiety which possesses a η 1 :η 1 :η 1 :η 2 :μ3 coordination mode leading to an overall inorganic core of {Mn4 III Ln III (μ-NO)4} 11+ ( Figure S1).
All Mn III ions are five-coordinate with almost perfect square pyramidal geometry. This has been confirmed by the analysis of the shape-determining bonds and angles using the Reedjik and Addison et al. method [33], which gives us a trigonality index, τ, of 0.11 for the four Mn III ions. The τ = 0.1 value is consistent with a square pyramidal geometry, as for a perfect square pyramidal geometry a τ value of 0 is expected, while a τ value of 1 is consistent for a trigonal bipyramidal geometry. The oxidation The basal Mn III atoms are bridged by diatomic oximate bridges provided by the shi 3− ligand, giving a Mn···Mn separation of 4.642(9) Å. Note that the coordinated anion of shi 3− was generated in situ from the metal ion-assisted transformation of shaH 2 under basic conditions. The large Mn-N-O-Mn torsion angle (173.3(4) • ), which is very close to the ideal linearity of 180 • , is responsible for the approximately ideal planarity of the Mn4 assembly, whilst the Tb III atom lies 1.789(8) Å out of the Mn4 plane. The connection between the basal Mn III and the Tb III atoms is provided by the oximate O atoms of the shi 3− ligand resulting in a Mn . . . Tb separation of 3.739(2) Å. Further ligation is provided by four η 1 :η 1 :µ bridging pivalate ions ( Figure 1). The coordination sphere around the Mn atoms is completed by the alkoxido and phenoxido O atoms provided by the organic moiety which possesses a η 1 :η 1 :η 1 :η 2 :µ 3 coordination mode leading to an overall inorganic core of {Mn 4 III Ln III (µ-NO) 4 } 11+ ( Figure S1). All Mn III ions are five-coordinate with almost perfect square pyramidal geometry. This has been confirmed by the analysis of the shape-determining bonds and angles using the Reedjik and Addison et al. method [33], which gives us a trigonality index, τ, of 0.11 for the four Mn III ions.
The τ = 0.1 value is consistent with a square pyramidal geometry, as for a perfect square pyramidal geometry a τ value of 0 is expected, while a τ value of 1 is consistent for a trigonal bipyramidal geometry. The oxidation states of Mn atoms were established by charge balance considerations, metric parameters and bond-valence sum (BVS) calculations [34], with the last providing us with a value of 3.03 for Mn1. Note that the oxidation of Mn II to Mn III occurs undoubtedly by the atmospheric O 2 under the prevailing basic conditions [35,36]. The central lanthanide ion is eight-coordinate possessing a slightly distorted square antiprismatic geometry with a continuous shape measurement factor of CShM = 0.71 ( Figure S2) [37]. The closest this number is to zero, the closest is the geometry of the lanthanide to the ideal one. Finally, the crystal packing of complex 2 ( Figure S2) has revealed that the ionic compounds do not communicate which each other (well separated) in the crystal by any means such as hydrogen bonding or π-π stacking interactions but are solely surrounded by the tetrabutylammonium cations and the solvate molecules in the lattice.
Several pivotal geometrical parameters were obtained for complex 2 in order to gain a better understanding of the inner coordination sphere around the lanthanide ion ( Figure 2). To be more descriptive, the angle between the four-fold axis and the Ln-O bond direction, θ, corresponds to compression or elongation along the tetragonal axis, depending on its value. The magic value for perfect square-antiprismatic (SAP) geometry is θ = 54.74 • , while smaller angles correspond to elongation and wider ones lead to compression [38][39][40]. In complex 2, the average value of θ was found to be 56.15 • , indicating axial compression. The distance between the upper and lower O 4 -planes, interplanar distance (dpp), was found to be 2.639(0) Å, while the distances d 1 and d 2 were found to be 1.103(4) Å and 1.535(6) Å, counting for the plane spanned by the carboxylate oxygen atom (O 5 ) and the one by the oximate oxygen (O 3 ), respectively. The symmetry of lanthanide's coordination geometry can be further described by another important parameter which is called skew or twist angle. This is the ϕ angle which basically defines the angle between the diagonals of the two O 4 -planes. This is a vital parameter for the determination of point group symmetry at the lanthanide site, which consequently leads to assisting on the description of the crystal field substrate composition of lanthanide complexes. When ϕ = 0 an ideal square prismatic geometry is expected, while when ϕ = 45 • an ideal square antiprismatic geometry is observed. In complex 2 an average ϕ value of 43.28(4) • was calculated, further supporting the square antiprismatic geometry of the Tb III ion.
Inorganics 2018, 6, x FOR PEER REVIEW 4 of 10 states of Mn atoms were established by charge balance considerations, metric parameters and bondvalence sum (BVS) calculations [34], with the last providing us with a value of 3.03 for Mn1. Note that the oxidation of Mn II to Mn III occurs undoubtedly by the atmospheric O2 under the prevailing basic conditions [35,36]. The central lanthanide ion is eight-coordinate possessing a slightly distorted square antiprismatic geometry with a continuous shape measurement factor of CShM = 0.71 ( Figure  S2) [37]. The closest this number is to zero, the closest is the geometry of the lanthanide to the ideal one. Finally, the crystal packing of complex 2 ( Figure S2) has revealed that the ionic compounds do not communicate which each other (well separated) in the crystal by any means such as hydrogen bonding or π-π stacking interactions but are solely surrounded by the tetrabutylammonium cations and the solvate molecules in the lattice. Several pivotal geometrical parameters were obtained for complex 2 in order to gain a better understanding of the inner coordination sphere around the lanthanide ion ( Figure 2). To be more descriptive, the angle between the four-fold axis and the Ln-O bond direction, θ, corresponds to compression or elongation along the tetragonal axis, depending on its value. The magic value for perfect square-antiprismatic (SAP) geometry is θ = 54.74°, while smaller angles correspond to elongation and wider ones lead to compression [38][39][40]. In complex 2, the average value of θ was found to be 56.15°, indicating axial compression. The distance between the upper and lower O4planes, interplanar distance (dpp), was found to be 2.639(0) A˚, while the distances d1 and d2 were found to be 1.103(4) A˚ and 1.535(6) A˚, counting for the plane spanned by the carboxylate oxygen atom (O5) and the one by the oximate oxygen (O3), respectively. The symmetry of lanthanide's coordination geometry can be further described by another important parameter which is called skew or twist angle. This is the φ angle which basically defines the angle between the diagonals of the two O4-planes. This is a vital parameter for the determination of point group symmetry at the lanthanide site, which consequently leads to assisting on the description of the crystal field substrate composition of lanthanide complexes. When φ = 0 an ideal square prismatic geometry is expected, while when φ = 45° an ideal square antiprismatic geometry is observed. In complex 2 an average φ value of 43.28(4)° was calculated, further supporting the square antiprismatic geometry of the Tb III ion.

Magnetic Studies of Complexes 1-3
Solid state, direct-current (dc) magnetic susceptibility (χ M ) measurements were collected in the temperature range of 2.0-300 K for freshly prepared crystalline samples of 1, 2, and 3, under an applied field of 0.1 T. The obtained data are presented as a χ M T vs. T plot in Figure 3. The experimental values at 300 K for all complexes (18.1 cm 3 ·mol −1 ·K for 1, 21.1 cm 3 ·mol −1 ·K for 2 and 11.4 cm 3 ·mol −1 ·K for 3) are lower than the theoretical ones (19.88 cm 3 ·mol −1 ·K for 1, 23.82 cm 3 ·mol −1 ·K for 2 and 12.0 cm 3 ·mol −1 ·K for 3) expected for four non-interacting Mn III ions (S = 2, g = 2) and one Gd III ( 8 S 7/2 , S = 7/2, L = 0, g = 2), one Tb III ( 7 F 6 , S = 3, L = 3, g = 3/2), or one diamagnetic Y III ion [41]. All complexes possess a similar magnetic behavior, with the χ M T steadily decreasing with decreasing temperature from 300 K till 2 K, where it reaches values of 6.95 cm 3 ·K·mol −1 for 1, 3.30 cm 3 ·K·mol −1 for 2 and 0.2 cm 3 ·K·mol −1 for 3, respectively. The shape of the χ M T vs. T plots for all complexes indicates the presence of predominant antiferromagnetic exchange interactions within the metal centers. This is further supported by the fact that the χ M T values at 300 K for all complexes are lower than the expected theoretical ones. In order to gain better insight in the strength of the intramolecular Mn III -Mn III magnetic exchange interactions, the magnetic susceptibility data of complex 3 were fit using PHI [42] program. The magnetic susceptibility data of complex 3, which comprises the diamagnetic Y III ion in the central cavity, were fit using a 1-J model according to the spin Hamiltonian: An excellent simulation of the data (solid green line, Figure 3) was achieved with a J = −3.74 cm −1 and g = 2.07. The antiferromagnetic exchange interactions are anticipated for a system that is exclusively coupled via oximate bridges that give very large Mn-N-O-Mn torsion angles, which are known to promote antiferromagnetic exchange interactions [43].
In order to also quantify the nature of the Mn III -Ln III exchange interactions, complex 1 was also fit, using a 2-J model, which counts for the outer Mn III -Mn III (J1) interactions as well as for inner the Mn III -Gd III (J2) interaction, shown in Figure 3b. The data were fit according to the spin Hamiltonian shown below: An excellent fit of the data (solid green line, Figure 3) could be obtained with J1 = −3.35 cm −1 , J2 = −0.45 cm −1 and gMn(III) = 1.99. The values of both models are in an excellent agreement. Note that antiferromagnetic exchange interactions, in Mn III -Gd III and other 3d-Gd III species, possessing the 3dx2−y2 orbital unoccupied, are quite often observed [44][45][46][47]. Here is the first time that fitting of the magnetic susceptibility data for 3d/Gd III interactions has been reported, within the MC family of complexes.
Field-dependent magnetization measurements were also performed for complexes 1 and 2 at temperatures between 2 K and 10 K over the range of 0-7 T (Figures 4 and S8). The magnetization of 1 and 2 shows a rapid increase below 1 T followed by a slow, nearly linear increase without reaching saturation. The lack of saturation in magnetization of 1 and 2 indicates the presence of magnetic anisotropy and/or population of the Ln III low-lying excited states, as well as the effect from some weak antiferromagnetic components between the metal centers. In order to gain better insight in the strength of the intramolecular Mn III -Mn III magnetic exchange interactions, the magnetic susceptibility data of complex 3 were fit using PHI [42] program. The magnetic susceptibility data of complex 3, which comprises the diamagnetic Y III ion in the central cavity, were fit using a 1-J model according to the spin Hamiltonian: An excellent simulation of the data (solid green line, Figure 3) was achieved with a J = −3.74 cm −1 and g = 2.07. The antiferromagnetic exchange interactions are anticipated for a system that is exclusively coupled via oximate bridges that give very large Mn-N-O-Mn torsion angles, which are known to promote antiferromagnetic exchange interactions [43].
In order to also quantify the nature of the Mn III -Ln III exchange interactions, complex 1 was also fit, using a 2-J model, which counts for the outer Mn III -Mn III (J 1 ) interactions as well as for inner the Mn III -Gd III (J 2 ) interaction, shown in Figure 3b. The data were fit according to the spin Hamiltonian shown below: An excellent fit of the data (solid green line, Figure 3) could be obtained with J 1 = −3.35 cm −1 , J 2 = −0.45 cm −1 and g Mn(III) = 1.99. The values of both models are in an excellent agreement. Note that antiferromagnetic exchange interactions, in Mn III -Gd III and other 3d-Gd III species, possessing the 3d x2−y2 orbital unoccupied, are quite often observed [44][45][46][47]. Here is the first time that fitting of the magnetic susceptibility data for 3d/Gd III interactions has been reported, within the MC family of complexes.
Field-dependent magnetization measurements were also performed for complexes 1 and 2 at temperatures between 2 K and 10 K over the range of 0-7 T (Figure 4 and Figure S8). The magnetization of 1 and 2 shows a rapid increase below 1 T followed by a slow, nearly linear increase without reaching saturation. The lack of saturation in magnetization of 1 and 2 indicates the presence of magnetic anisotropy and/or population of the Ln III low-lying excited states, as well as the effect from some weak antiferromagnetic components between the metal centers.

General Information
All chemicals and solvents used for synthesis were of reagent grade and used as purchased without further purification. The starting material Mn(O2CBu t )2•2H2O was synthesized using literature procedures [48]. C, H and N elemental analyses were carried out on a Foss Heraeus Vario EL (Elementar Analysensysteme GmbH, Langenselbold, Germany) at the Institute of Organic Chemistry at the Johannes Gutenberg University Mainz. Infrared absorption spectra were recorded at room temperature in a range of 3000-400 cm −1 on a Thermo Fischer NICOLET Nexus FT/IR-5700 spectrometer (Thermo Fischer Scientific, Waltham, MA, USA) equipped with Smart Orbit ATR Diamond cell (Thermo Fischer Scientific, Waltham, MA, USA). UV-Vis absorption measurements were performed between for complexes 1, 2 and 3 in MeCN between 200 mm and 1000 nm on a JASCO V-570 UV/Vis/NIR spectrophotometer (JASCO Inc, Easton, MD, USA) ( Figure S7 in Supplementary Materials).
A similar procedure has been used to isolate compounds 1-3.

General Information
All chemicals and solvents used for synthesis were of reagent grade and used as purchased without further purification. The starting material Mn(O 2 CBu t ) 2 ·2H 2 O was synthesized using literature procedures [48]. C, H and N elemental analyses were carried out on a Foss Heraeus Vario EL (Elementar Analysensysteme GmbH, Langenselbold, Germany) at the Institute of Organic Chemistry at the Johannes Gutenberg University Mainz. Infrared absorption spectra were recorded at room temperature in a range of 3000-400 cm −1 on a Thermo Fischer NICOLET Nexus FT/IR-5700 spectrometer (Thermo Fischer Scientific, Waltham, MA, USA) equipped with Smart Orbit ATR Diamond cell (Thermo Fischer Scientific, Waltham, MA, USA). UV-Vis absorption measurements were performed between for complexes 1, 2 and 3 in MeCN between 200 mm and 1000 nm on a JASCO V-570 UV/Vis/NIR spectrophotometer (JASCO Inc., Easton, MD, USA) ( Figure S7 in Supplementary Materials).
A similar procedure has been used to isolate compounds 1-3.

Magnetic Measurements
Variable-temperature direct current (dc) magnetic susceptibility measurements were performed on polycrystalline samples with the use of Quantum Design SQUID magnetometer MPMS-7 equipped with a 7 T magnet. The samples were embedded in eicosane to avoid orientation of the crystallites under the applied field. Experimental susceptibility data were corrected for the underlying diamagnetism using Pascal's constants [55]. The temperature dependent magnetic contribution of the holder and of the embedding matrix eicosane were experimentally determined and subtracted from the measured susceptibility data. Variable temperature susceptibility data were collected in a temperature range of 2-300 K under an applied field of 0.1 Tesla, while magnetization data were collected between 2 K and 10 K and magnetic fields up to 7 Tesla. Alternating-current (ac) measurements were performed with an oscillating magnetic field of 3 Oe at frequencies ranging from 1 Hz to 1400 Hz.

Conclusions
In summary, we reported a new family of Mn III /Ln III 12-MC-4 complexes, derived from the reaction of Mn(O 2 CBu t ) 2 ·2H 2 O with various nitrate salts of lanthanides in the presence of salicylhydroxamic acid. Direct-current (dc) magnetic susceptibility studies revealed the presence of antiferromagnetic exchange interactions between the metal centers, while fitting of the data using the {Mn 4 Y} complex allowed us to quantify the strength of the interactions within the outer Mn III ions, which was found to be J = −3.74 cm −1 with g = 2.07. Moreover, fitting of the {Mn 4 Gd} (1) data gave an extra insight into the strength of the magnetic exchange interactions, especially for the Mn III -Gd III intramolecular interaction, revealing values of J Mn-Mn = −3.35 cm −1 , J Mn-Gd = −0.45 cm −1 and g Mn(III) = 1.99. Note that this is the first example within the family of metallacrowns (MCs), where simulation of the magnetic data has been reported. In-phase and out-of-phase (ac) magnetic susceptibility measurements as a function of temperature did not reveal any slow relaxation in fields of H dc = 0-3000 Oe. In order to further improve the magnetic properties of such compounds, the chemistry will be broadened to the use of other magnetic or diamagnetic 3d and 4f metal ions, by means of modifying the structural and/or physical properties of the resulting molecular compounds.
Author Contributions: A.A.A. designed and performed the experiments. She analyzed most of the data and wrote the paper. L.M.C. refined the structures and helped a lot with his knowledgeable input throughout the paper-writing process. E.R. was involved during the writing of the paper to every step of the process giving valuable feedback.
Funding: This research received no external funding.