Gaining Insights into the Interplay between Optical and Magnetic Properties in Photoexcited Coordination Compounds.

We describe early and recent advances in the fascinating field of combined magnetic and optical properties of inorganic coordination compounds and in particular of 3d-4f single molecule magnets. We cover various applied techniques which allow for the correlation of results obtained in the frequency and time domain in order to highlight the specific properties of these compounds and the future challenges towards multidimensional spectroscopic tools. An important point is to understand the details of the interplay of magnetic and optical properties through performing time-resolved studies in the presence of external fields especially magnetic ones. This will enable further exploration of this fundamental interactions i. e. the two components of electromagnetic radiation influencing optical properties.

Fe-Gd Ferromagnetic Cyclic Coordination Cluster [FeIII4GdIII4(teaH)8(N3)8(H2O)] with Magnetic Anisotropy─Theory and Experiment.

The synthesis, structural, and magnetic characterization of [FeIII4LnIII4(teaH)8(N3)8(H2O)] (Ln = Gd and Y) and the previously reported isostructural Dy analogue are discussed. The commonly held belief that both FeIII and GdIII can be regarded as isotropic ions is shown to be an oversimplification. This conclusion is derived from the magnetic data for the YIII analogue in terms of the zero-field splitting seen for FeIII and from the fact that the magnetic data for the new GdIII analogue can only be fit employing an additional anisotropy term for the GdIII ions. Furthermore, the Fe4Gd4 ring shows slow relaxation of magnetization. Our analysis of the experimental magnetic data employs both density functional theory as well as the finite-temperature Lanczos method which finally enables us to provide an almost perfect fit of magnetocaloric properties.

An Effectively Uncoupled Gd8 Cluster Formed through Fixation of Atmospheric CO2 Showing Excellent Magnetocaloric Properties.

The [Gd8(opch)8(CO3)4(H2O)8]·4H2O·10MeCN coordination cluster (1) crystallises in P1¯. The Gd8 core is held together by four bridging carbonates derived from atmospheric CO2 as well as the carboxyhydrazonyl oxygens of the 2-hydroxy-3-methoxybenzylidenepyrazine-2-carbohydrazide (H2opch) Schiff base ligands. The magnetic measurements show that the GdIII ions are effectively uncoupled as seen from the low Weiss constant of 0.05 K needed to fit the inverse susceptibility to the Curie-Weiss law. Furthermore, the magnetisation data are consistent with the Brillouin function for eight independent GdIII ions. These features lead to a magnetocaloric effect with a high efficiency which is 89% of the theoretical maximum value.

Mn12 -Acetate Complexes Studied as Single Molecules.

The phenomenon of single molecule magnet (SMM) behavior of mixed valent Mn12 coordination clusters of general formula [MnIII 8 MnIV 4 O12 (RCOO)16 (H2 O)4 ] had been exemplified by bulk samples of the archetypal [MnIII 8 MnIV 4 O12 (CH3 COO)16 (H2 O)4 ] (4) molecule, and the molecular origin of the observed magnetic behavior has found support from extensive studies on the Mn12 system within crystalline material or on molecules attached to a variety of surfaces. Here we report the magnetic signature of the isolated cationic species [Mn12 O12 (CH3 COO)15 (CH3 CN)]+ (1) by gas phase X-ray Magnetic Circular Dichroism (XMCD) spectroscopy, and we find it closely resembling that of the corresponding bulk samples. Furthermore, we report broken symmetry DFT calculations of spin densities and single ion tensors of the isolated, optimized complexes [Mn12 O12 (CH3 COO)15 (CH3 CN)]+ (1), [Mn12 O12 (CH3 COO)16 ] (2), [Mn12 O12 (CH3 COO)16 (H2 O)4 ] (3), and the complex in bulk geometry [MnIII 8 MnIV 4 O12 (CH3 COO)16 (H2 O)4 ] (5). The found magnetic fingerprints - experiment and theory alike - are of a remarkable robustness: The MnIV 4 core bears almost no magnetic anisotropy while the surrounding MnIII 8 ring is highly anisotropic. These signatures are truly intrinsic properties of the Mn12 core scaffold within all of these complexes and largely void of the environment. This likely holds irrespective of bulk packing effects.

NMR Relaxivities of Paramagnetic, Ultra-High Spin Heterometallic Clusters within Polyoxometalate Matrix as a Function of Solvent and Metal Ion.

Selectivity and image contrast are always challenging in magnetic resonance imaging (MRI), which are - inter alia - addressed by contrast agents. These compounds still need to be improved, and their relaxation properties, i. e., their paramagnetic relaxation enhancement (PRE), needs to be understood. The main goal is to improve specificity and relaxivities, especially at the high magnetic fields currently exploited not only in material science but also in the medical environment. Longitudinal and transverse relaxivities, r1 and r2 , which correspond to the longitudinal and transverse relaxation rates R1 and R2, normalized to the concentration of the paramagnetic moieties, need to be considered because both contribute to the image contrast. 1 H-relaxivities r1 and r2 of high-spin heterometallic clusters were studied containing lanthanide and transition-metal ions within a polyoxometalate matrix. A wide range of magnetic fields from 0.5 T/20 MHz to 33 T/1.4 GHz was applied. The questions addressed here concern the rotational and diffusion correlation times which determine the relaxivities and are affected by the solvent's viscosity. Moreover, the variation of the lanthanide and transition-metal ions of the clusters provided insights into the sensitivity of PRE with respect to the electron spin properties of the paramagnetic centers as well as cooperative effects between lanthanides and transition metal ions.

From the {FeIII 2 Ln2 } Butterfly's Perspective: the Magnetic Benefits and Challenges of Cooperativity within 3 d-4 f Based Coordination Clusters.

In this Review we discuss the tuning handles which can be used to steer the magnetic properties of FeIII -4 f "butterfly" compounds. The majority of presented compounds were produced in the context of project A3 "Di- to tetranuclear compounds incorporating highly anisotropic paramagnetic metal ions" within the SFB/TRR88 "3MET". These contain {FeIII 2 Ln2 } cores encapsulated in ligand shells which are easy to tune in a "test-bed" system. We identify the following advantages and variables in such systems: (i) the complexes are structurally simple usually with one crystallographically independent FeIII and LnIII , respectively. This simplifies theory and anaylsis; (ii) choosing Fe allows 57 Fe Mössbauer spectroscopy to be used as an additional technique which can give information about oxidation levels and spin states, local moments at the iron nuclei and spin-relaxation and, more importantly, about the anisotropy not only of the studied isotope, but also of elements interacting with this isotope; (iii) isostructural analogues with all the available (i. e. not Pm) 4 f ions can be synthesised, enabling a systematic survey of the influence of the 4 f ion on the electronic structure; (iv) this cluster type is obtained by reacting [FeIII 3 O(O2 CR)6 (L)3 ](X) (X=anion, L=solvent such as H2 O, py) with an ethanolamine-based ligand L' and lanthanide salts. This allows to study analogues of [FeIII 2 Ln2 (µ3 -OH)2 (L')2 (O2 CR)6 ] using the appropriate iron trinuclear starting materials. (v) the organic main ligand can be readily functionalised, facilitating a systematic investigation of the effect of organic substituents on the ligands on the magnetic properties of the complexes. We describe and discuss 34 {MIII 2 Ln2 } (M=Fe or in one case Al) butterfly compounds which have been reported up to 2020. The analysis of these gives perspectives for designing new SMM systems with specific electronic and magnetic signatures.

Terminal Ligand and Packing Effects on Slow Relaxation in an Isostructural Set of [Dy(H2 dapp)X2 ]+ Single Molecule Magnets*.

We report three structurally related single ion Dy compounds using the pentadentate ligand 2,6-bis((E)-1-(2-(pyridin-2-yl)-hydrazineylidene)ethyl)pyridine (H2 dapp) [Dy(H2 dapp)(NO3 )2 ]NO3 (1), [Dy(H2 dapp)(OAc)2 ]Cl (2) and [Dy(H2 dapp)(NO3 )2 ]Cl0.92 (NO3 )0.08 (3). The (H2 dapp) occupies a helical twisted pentagonal equatorial arrangement with two anionic ligands in the axial positions. Further influence on the electronic and magnetic structure is provided by a closely associated counterion interacting with the central N-H group of the (H2 dapp). The slow relaxation of the magnetisation shows that the anionic acetates give the greatest slowing down of the magnetisation reversal. Further influence on the relaxation properties of compounds1 and 2 is the presence of short nitrate-nitrate intermolecular ligand contact opening further lattice relaxation pathways.

Breaking Symmetry Relaxes Structural and Magnetic Restraints, Suppressing QTM in Enantiopure Butterfly Fe2 Dy2 SMMs*.

The {Fe2 Dy2 } butterfly systems can show single molecule magnet (SMM) behaviour, the nature of which depends on details of the electronic structure, as previously demonstrated for the [Fe2 Dy2 (µ3 -OH)2 (Me-teaH)2 (O2 CPh)6 ] compound, where the [N,N-bis-(2-hydroxyethyl)-amino]-2-propanol (Me-teaH3 ) ligand is usually used in its racemic form. Here, we describe the consequences for the SMM properties by using enantiopure versions of this ligand and present the first homochiral 3d/4 f SMM, which could only be obtained for the S enantiomer of the ligand for [Fe2 Dy2 (µ3 -OH)2 (Me-teaH)2 (O2 CPh)6 ] since the R enantiomer underwent significant racemisation. To investigate this further, we prepared the [Fe2 Dy2 (µ3 -OH)2 (Me-teaH)2 (O2 CPh)4 (NO3 )2 ] version, which could be obtained as the RS-, R- and S-compounds. Remarkably, the enantiopure versions show enhanced slow relaxation of magnetisation. The use of the enantiomerically pure ligand suppresses QTM, leading to the conclusion that use of enantiopure ligands is a "gamechanger" by breaking the cluster symmetry and altering the intimate details of the coordination cluster's molecular structure.

Assisted Self-Assembly to Target Heterometallic Mn-Nd and Mn-Sm SMMs: Synthesis and Magnetic Characterisation of [Mn7 Ln3 (O)4 (OH)4 (mdea)3 (piv)9 (NO3 )3 ] (Ln=Nd, Sm, Eu, Gd)*.

In an assisted self-assembly approach starting from the [Mn6 O2 (piv)10 (4-Me-py)2 (pivH)2 ] cluster a family of Mn-Ln compounds (Ln=Pr-Yb) was synthesised. The reaction of [Mn6 O2 (piv)10 (4-Me-py)2 (pivH)2 ] (1) with N-methyldiethanolamine (mdeaH2 ) and Ln(NO3 )3 ⋅ 6H2 O in MeCN generally yields two main structure types: for Ln=Tb-Yb a previously reported Mn5 Ln4 motif is obtained, whereas for Ln=Pr-Eu a series of Mn7 Ln3 clusters is obtained. Within this series the GdIII analogue represents a special case because it shows both structural types as well as a third Mn2 Ln2 inverse butterfly motif. Variation in reaction conditions allows access to different structure types across the whole series. This prompts further studies into the reaction mechanism of this cluster assisted self-assembly approach. For the Mn7 Ln3 analogues reported here variable-temperature magnetic susceptibility measurements suggest that antiferromagnetic interactions between the spin carriers are dominant. Compounds incorporating Ln=NdIII (2), SmIII (3) and GdIII (5) display SMM behaviour. The slow relaxation of the magnetisation for these compounds was confirmed by ac measurements above 1.8 K.

NiII 36 -Containing 54-Tungsto-6-Silicate: Synthesis, Structure, Magnetic and Electrochemical Studies.

The 36-NiII -containing 54-tungsto-6-silicate, [Ni36 (OH)18 (H2 O)36 (SiW9 O34 )6 ]6- (Ni36 ) was synthesized by a simple one-pot reaction of the Ni2 -pivalate complex [Ni2 (µ-OH2 )(O2 CCMe3 )4 (HO2 CCMe3 )4 ] with the trilacunary [SiW9 O34 ]10- polyanion precursor in water and structurally characterized by a multitude of physicochemical techniques including single-crystal XRD, FTIR, TGA, elemental analysis, magnetic and electrochemical studies. Polyanion Ni36 comprises six equivalent {NiII 6 SiW9 } units which are linked by Ni-O-W bridges forming a macrocyclic assembly. Magnetic studies demonstrate that the {Ni6 } building blocks in Ni36 remain magnetically intact while forming a hexagonal ring with antiferromagnetic exchange interactions between adjacent {Ni6 } units. Electrochemical studies indicate that the first reduction is reversible and associated with the WVI/V couple, whereas the second reduction is irreversible attributed to the NiII/0 couple.

NMR Relaxivities of Paramagnetic Lanthanide-Containing Polyoxometalates.

The current trend for ultra-high-field magnetic resonance imaging (MRI) technologies opens up new routes in clinical diagnostic imaging as well as in material imaging applications. MRI selectivity is further improved by using contrast agents (CAs), which enhance the image contrast and improve specificity by the paramagnetic relaxation enhancement (PRE) mechanism. Generally, the efficacy of a CA at a given magnetic field is measured by its longitudinal and transverse relaxivities r1 and r2, i.e., the longitudinal and transverse relaxation rates T1-1 and T2-1 normalized to CA concentration. However, even though basic NMR sensitivity and resolution become better in stronger fields, r1 of classic CA generally decreases, which often causes a reduction of the image contrast. In this regard, there is a growing interest in the development of new contrast agents that would be suitable to work at higher magnetic fields. One of the strategies to increase imaging contrast at high magnetic field is to inspect other paramagnetic ions than the commonly used Gd(III)-based CAs. For lanthanides, the magnetic moment can be higher than that of the isotropic Gd(III) ion. In addition, the symmetry of electronic ground state influences the PRE properties of a compound apart from diverse correlation times. In this work, PRE of water 1H has been investigated over a wide range of magnetic fields for aqueous solutions of the lanthanide containing polyoxometalates [DyIII(H2O)4GeW11O39]5- (Dy-W11), [ErIII(H2O)3GeW11O39]5- (Er-W11) and [{ErIII(H2O)(CH3COO)(P2W17O61)}2]16- (Er2-W34) over a wide range of frequencies from 20 MHz to 1.4 GHz. Their relaxivities r1 and r2 increase with increasing applied fields. These results indicate that the three chosen POM systems are potential candidates for contrast agents, especially at high magnetic fields.

Inorganic Approach to Stabilizing Nanoscale Toroidicity in a Tetraicosanuclear Fe18Dy6 Single Molecule Magnet.

Cyclic coordination clusters (CCCs) are proving to provide an extra dimension in terms of exotic magnetic behavior as a result of their finite but cyclized chain structures. The Fe18Dy6 CCC is a Single Molecule Magnet with the highest nuclearity among Ln containing clusters. The three isostructural compounds [Fe18Ln6(µ-OH)6(ampd)12(Hampd)12(PhCO2)24](NO3)6·38MeCN for Ln = DyIII (1), LuIII (2), or YIII (3), where H2ampd = 2-amino-2-methyl-1,3-propanediol, are reported. These can be described in terms of the cyclization of six {Fe3Ln(µOH)(ampd)2(Hampd)2(PhCO2)4}+ units with six nitrate counterions to give the neutral cluster. The overall structure consists of two giant Dy3 triangles sandwiching a strongly antiferromagnetically coupled Fe18 ring, leading to a toroidal arrangement of the anisotropy axis of the Dy ions, making this the biggest toroidal arrangement on a molecular level known so far.

The First Use of a ReX5 Synthon to Modulate FeIII Spin Crossover via Supramolecular Halogen⋠⋠⋠Halogen Interactions.

We have added the {ReIV X5 }- (X=Br, Cl) synthon to a pocket-based ligand to provide supramolecular design using halogen⋅⋅⋅halogen interactions within an FeIII system that has the potential to undergo spin crossover (SCO). By removing the solvent from the crystal lattice, we "switch on" halogen⋅⋅⋅halogen interactions between neighboring molecules, providing a supramolecular cooperative pathway for SCO. Furthermore, changes to the halogen-based interaction allow us to modify the temperature and nature of the SCO event.

Exploring the Vibrational Side of Spin-Phonon Coupling in Single-Molecule Magnets via 161 Dy Nuclear Resonance Vibrational Spectroscopy.

Synchrotron-based nuclear resonance vibrational spectroscopy (NRVS) using the Mössbauer isotope 161 Dy has been employed for the first time to study the vibrational properties of a single-molecule magnet (SMM) incorporating DyIII , namely [Dy(Cy3 PO)2 (H2 O)5 ]Br3 ⋅2 (Cy3 PO)⋅2 H2 O ⋅2 EtOH. The experimental partial phonon density of states (pDOS), which includes all vibrational modes involving a displacement of the DyIII ion, was reproduced by means of simulations using density functional theory (DFT), enabling the assignment of all intramolecular vibrational modes. This study proves that 161 Dy NRVS is a powerful experimental tool with significant potential to help to clarify the role of phonons in SMMs.

Chasing BODIPY: Enhancement of Luminescence in Homoleptic Bis(dipyrrinato) ZnII Complexes Utilizing Symmetric and Unsymmetrical Dipyrrins.

Dipyrromethene metal complexes are fascinating molecules that have applications as light-harvesting systems, luminophores, and laser dyes. Recently, it has been shown that structurally rigid bis(dipyrrinato) zinc(II) complexes exhibit high fluorescence with comparable quantum yields to those of boron dipyrromethenes or BODIPYs. Herein, eight new bis(dipyrrinato) ZnII complexes, obtained from symmetric and unsymmetrical functionalization of the dipyrromethene structure through a Knoevenagel reaction, are reported. It was possible not only to vary the maximum visible absorption from 490 to 630 nm, but also to enhance the emission quantum yield up to 66 %, which is extraordinarily high for homoleptic bis(dipyrrinato) zinc complexes. These results pave the way for designing highly luminescent bis(dipyrrinato) zinc complexes.

Trinuclear and Hexanuclear Lanthanide(III) Complexes of the Chiral 3+3 Macrocycle: X-ray Crystal Structures and Magnetic Properties.

A new triphenolic hexaaza chiral macrocyclic amine L forms trinuclear complexes 1-3 with rare earth metal lanthanide(III) ions (Ln = Dy, Eu, and Y) with the general formula [Ln3L(µ3-OH)2(NO3)4(H2O)2]· xCH3OH. The crystal structures of the nitrate derivatives of this type reveal the presence of a {Ln3(µ3-OH)2} core within the macrocycle. For the chloride derivative of dysprosium(III) 4, a duplex of the trinuclear compound is formed to give the hexanuclear [Dy6L2(µ3-OH)3(µ3-O)(µ2-Cl)3Cl4(H2O)2] compound, in which two trinuclear macrocyclic units are linked by bridging chloride anions, supplemented by a hydrogen bond connecting the central oxo and hydroxo bridges as well as by weak interactions at the periphery of the macrocycle. The nuclear magnetic resonance spectra of these complexes reveal a dynamic behavior in solution related to exchange of axial ligands and hindered rotation of phenyl substituents. Magnetic studies of the nitrate (1-3) and chloride (4) dysprosium(III) complexes suggest the presence of weak ferromagnetic interactions between neighboring metal centers. The interaction is strongest for compound 1, and for the related duplex compound 4, it appears to be somewhat weaker. The ac susceptibility measurements for complexes 1 and 4 confirm their field-induced single-molecule magnet behavior with the following characteristics: Ueff = 10.6 cm-1 (15.2 K), τ0 = 2.05 × 10-4 s under 2500 Oe dc fields for 1; Ueff = 7.9 cm-1 (11.4 K), τ0 = 1.68 × 10-4 s under a 3000 Oe dc field for 4.

Multimodeling Approach to Ferromagnetic Spin-Wave Excitations in the High-Spin Cluster Mn18Sr Observed by Inelastic Neutron Scattering.

The magnetism of the mixed-valence high-spin cluster [Mn18SrO8(N3)7Cl(MedhmpH)12(MeCN)6]Cl2 (1) exhibiting intramolecular ferromagnetic interactions was studied using inelastic neutron scattering (INS), and reliable values for the exchange coupling constants were determined based on the quality of simultaneous fits to the INS and magnetic data. The challenge of the huge size of the Hilbert space (3 375 000) and many exchange coupling constants (7 assuming a C3 symmetry) generally encountered in large spin clusters was resolved as follows: (a) The results of the restricted Hilbert space ferromagnetic cluster spin wave theory were compared to the experimental spectroscopic data. The observed INS transitions were thus assigned to spin wave excitations in a bounded ferromagnetic spin cluster and moreover could be visualized in a straightforward way based on this theory. (b) Simultaneously, Quantum Monte Carlo (QMC) calculations of the temperature-dependent magnetic susceptibility with the same parameter set were compared to the experimental data. Application of state-of-the-art QMC algorithms, as available in the open source ALPS package, in ferromagnetic clusters avoids the full Hamiltonian diagonalization without sacrificing calculation accuracy of the magnetic susceptibility down to the lowest temperatures, which was crucial for the successful analysis. The combined fits revealed two exchange-coupling models with equally good overall agreement to the data. Our preferred model was inspired by magnetostructural correlations and is consistent with them. The model involves three different exchange interactions, one describing the interaction between the core MnIII spins Ja = 14.3(1.0) K and two interactions linking the core and the peripheral MnII spins: Jb = 8.3(4) K and J6 = 3.6(4) K. The use of open-source QMC software and our systematic approach to fitting multiple sets of data obtained by different experimental techniques are described in detail and are generally applicable for understanding large ferromagnetically coupled clusters.

Twists to the Spin Structure of the Ln9-diabolo Motif Exemplified in Two {Zn2Ln2}[Ln9]{Zn2} Coordination Clusters.

Two pentadecanuclear Zn4Ln11 [with Ln = Gd(1) or Dy(2)] coordination clusters, best formulated as {Zn2Ln2}[Ln9]{Zn2}, are presented. The central {Ln9} diabolo core has a {Zn2Ln2} handle motif pulling at two outer Ln ions of the central core via two {ZnLn} units, which also invest the system with C2 point symmetry. The resulting cluster motif is supported on two Zn "feet", corresponding to the {Zn2} unit in the formula. A thorough investigation of the magnetic properties in the light of the properties of previously reported {Ln9} diabolo compounds was undertaken. Up to now, the spin structure of such diabolo motifs usually proves ambiguous. Our magnetic studies show that the orientation of the central spin in the {Gd9} diabolo plays a decisive role. In stabilizing the core by attachment of the {Zn}2+ "feet" and using the C2 symmetry related {ZnGd}5+ handles to influence the spin direction of the central Gd of the {Gd9} diabolo, we can understand why the "naked" {Gd9} diabolo shows ambiguous spin structure. This then allowed us to elucidate the single-molecule magnetic (SMM) properties of the Dy-based compound 2 through disentangling the magnetic properties of the isostructural Gd-based compound 1.

161 Dy Time-Domain Synchrotron Mössbauer Spectroscopy for Investigating Single-Molecule Magnets Incorporating Dy Ions.

Time-domain synchrotron Mössbauer spectroscopy (SMS) based on the Mössbauer effect of 161 Dy has been used to investigate the magnetic properties of a DyIII -based single-molecule magnet (SMM). The magnetic hyperfine field of [Dy(Cy3 PO)2 (H2 O)5 ]Br3 ⋅2 (Cy3 PO)⋅2 H2 O⋅2 EtOH is with B0 =582.3(5) T significantly larger than that of the free-ion DyIII with a 6 H15/2 ground state. This difference is attributed to the influence of the coordinating ligands on the Fermi contact interaction between the s and 4f electrons of the DyIII ion. This study demonstrates that 161 Dy SMS is an effective local probe of the influence of the coordinating ligands on the magnetic structure of Dy-containing compounds.

Magnetization Blocking in Fe2 III Dy2 III Molecular Magnets: Ab Initio Calculations and EPR Spectroscopy.

The magnetism and magnetization blocking of a series of [Fe2 Dy2 (OH)2 (teaH)2 (RC6 H4 COO)6 ] complexes was investigated, in which teaH3 =triethanolamine and R=meta-CN (1), para-CN (2), meta-CH3  (3), para-NO2  (4) and para-CH3  (5), by combining ab initio calculations and EPR measurements. The results of broken-symmetry DFT calculations show that in all compounds the Fe-Fe exchange interaction is antiferromagnetic and stronger by far than the Fe-Dy and Dy-Dy interactions. As a result, the lowest two exchange doublets probed by EPR spectroscopy mostly originate from the Ising interaction of the dysprosium ions in all compounds. A correct quantitative description of the splitting of these two doublets requires, however, an explicit account of the Fe-Dy and Fe-Fe interactions. Due to the inversion symmetry of the complexes, the doublets under consideration are described by a collinear Ising exchange interaction. This picture is also supported by the EPR spectra, which could be simulated with parameters close to those extracted from the calculations. The magneto-structural analysis shows an increase of the antiferromagnetic Fe-Fe exchange interaction with increasing Fe-O-Fe angle and Fe-Fe distance. For the Dy-Fe interaction, the opposite tendency is observed, that is, a decrease of antiferromagnetic exchange coupling with increasing Dy-O-Fe angle and Dy-Fe distance. The transversal g factors extracted from the ab initio calculations have values in the range of 0.01-0.2, testifying to the lack of high axiality of the ground state of the dysprosium ions. This explains the lack/poor single-molecule magnetic behavior of this series of compounds at the investigated temperatures of a few Kelvin. Due to a very small gap (fractions of a wavenumber) between the ground and first-excited exchange doublet, relaxation takes place by magnetic moment reversal at individual dysprosium sites in the considered temperature domain.