Molecular properties of linear amino acids in water.

Four linear amino acids of increased separation of the carboxyl and amino groups, namely glycine (aminoacetic acid), ß-alanine (3-aminopropanoic acid), GABA (4-aminobutanoic acid) and DAVA (5-aminopentanoic acid), have been studied by quantum chemical ab initio and DFT methods including the solvent effect in order to get electronic structure and molecular descriptors, such as ionisation energy, electron affinity, molecular electronegativity, chemical hardness, electrophilicity index, dipole moment, quadrupole moment and dipole polarizability. Thermodynamic functions (zero-point energy, inner energy, enthalpy, entropy, and the Gibbs energy) were evaluated after the complete vibrational analysis at the true energy minimum provided by the full geometry optimization. Reaction Gibbs energy allows evaluating the absolute redox potentials on reduction and/or oxidation. The non-local non-additive molecular descriptors were compared along the series showing which of them behave as extensive, varying in match with the molar mass and/or separation of the carboxyl and amino groups. Amino acidic forms and zwitterionic forms of the substances were studied in parallel in order to compare their relative stability and redox properties. In total, 24 species were investigated by B3LYP/def2-TZVPD method (M1) including neutral molecules, molecular cations and molecular anions. For comparison, MP2/def2-TZVPD method (M2) with full geometry optimization and vibrational analysis in water has been applied for 12 species; analogously, for 24 substances, DLPNO-CCSD(T)/aug-cc-pVTZ method (M3) has been applied in the geometry obtained by MP2 and/or B3LYP. It was found that the absolute oxidation potential correlates with the adiabatic ionisation energy; the absolute reduction potential correlates with the adiabatic electron affinity and the electrophilicity index. In order to validate the used methodology with experimental vertical ionisation energies and vibrational spectrum obtained in gas phase, calculations were done also in vacuo.

Large Magnetic Anisotropy in Mono- and Binuclear cobalt(II) Complexes: The Role of the Distortion of the Coordination Sphere in Validity of the Spin-Hamiltonian Formalism.

To get a better insight into understanding the factors affecting the enhancement of the magnetic anisotropy in single molecule (single ion) magnets, two cobalt(II) complexes based on a tridentate ligand 2,6-di(thiazol-2-yl)pyridine substituted at the 4-position with N-methyl-pyrrol-2-yl have been synthesized and studied by X-ray crystallography, AC and DC magnetic data, FIRMS and HFEPR spectra, and theoretical calculations. The change of the counteranion in starting Co(II) salts results in the formation of pentacoordinated mononuclear [Co(mpyr-dtpy)Cl2]·2MeCN (1) complex and binuclear [Co(mpyr-dtpy)2][Co(NCS)4] (2) compound. The observed marked distortion of trigonal bipyramid geometry in 1 and cationic octahedral and anionic tetrahedral units in 2 brings up a question about the validity of the spin-Hamiltonian formalism and the possibility of determining the value and sign of the zero-field splitting D parameter. Both complexes exhibit field-induced slow magnetic relaxation with two or three relaxation channels at BDC = 0.3 T. The high-frequency relaxation time in the reciprocal form τ(HF)-1 = CTn develops according to the Raman relaxation mechanism (for 2, n = 8.8) and the phonon-bottleneck-like mechanism (for 1, n = 2.3). The high-frequency relaxation time at T = 2.0 K and BDC = 0.30 T is τ(HF) = 96 and 47 µs for 1 and 2, respectively.

Limitations on the D-Parameter in Ni(II) Complexes.

A number of hexacoordinate, pentacoordinate, and tetracoordinate Ni(II) complexes have been investigated by applying ab initio CASSCF + NEVPT2 + SOC calculations and Generalized Crystal Field Theory. The geometry of the coordination polyhedron covers D4h, D3h, D2h, D2d, C4v, C3v, and C2v symmetry. The calculated spin-Hamiltonian parameters D and E were compared to the available experimental data. The limiting values of the D-parameter in the class of Ni(II) complexes are identified. Magnetic anisotropy in Ni(II) complexes, expressed by the axial zero-field splitting parameter D, seriously depends upon the ground and first excited electronic states. In hexacoordinate complexes, the ground electronic term is nondegenerate 3B1g for the D4h symmetry; D is slightly positive or negative. In tetracoordinate systems, D is only positive when the electronic ground state is nondegenerate 3A or 3B; this diverges on the τ4 path when oblate bisphenoid approaches the prolate geometry and a level crossing with 3E occurs. In pentacoordinate systems, D could be extremely negative when approaching a trigonal bipyramid (Addison index τ5 ∼ 1, ground state 3E″). In pentacoordinate Ni(II) complexes with the D3h and C3v symmetry of the coordination polyhedron, the ground electronic term is orbitally doubly degenerate which causes the D-parameter stays undefined. It is emphasized that one has to inspect compositions of the spin-orbit multiplets from the spin states |MS⟩ and check whether the weights confirm the expected spin-Hamiltonian picture: with D > 0, the ground state contains a dominant part of |0⟩ (close to 100%) whereas with D < 0 the spin-orbit doublet is formed of |±1⟩ with high weights (approaching 50 + 50%). The calculations show that the situations are not black and white, and the mixing of the states might be more complex especially when the rhombic zero-field splitting parameter E is in the play. In the case of the 3E ground term, six spin-orbit multiplets are formed by mixing six |MS⟩ states from the ground and quasi-degenerate excited states.

Possible Violation of the Spin-Hamiltonian Concept in Interpreting the Zero-Field Splitting.

The majority of experimental data in electron spin resonance and molecular magnetism are interpreted in terms of the spin-Hamiltonian (SH) formalism. However, this is an approximate theory that requires a proper testing. In the older variant, the multielectron terms are used as a basis in which the D-tensor components are evaluated by employing the second-order perturbation theory (PT) for nondegenerate states; here, the spin-orbit interaction expressed via the spin-orbit splitting parameter λ serves for the perturbation. The model space is restricted only to the fictitious spin functions |S, M⟩. In the case of the orbital (quasi) degeneracy of the ground term, the PT tends to diverge and the subtracted D, E, and g parameters are false. In the second variant working in the "complete active space" (CAS), the spin-orbit coupling operator is involved by the variation method resulting in the spin-orbit multiplets (energies and eigenvectors) The multiplets can be evaluated either by applying ab initio CASSCF + NEVPT2 + SOC calculations or by using semiempirical generalized crystal-field theory (with the one-electron SOC operator depending upon ξ). The resulting states can be projected onto the subspace of the spin-only kets in the way that the eigenvalues stay invariant. Such an effective Hamiltonian matrix can be reconstructed using six independent components of the symmetric D-tensor from which the D and E values are obtained by solving linear equations. The eigenvectors of the spin-orbit multiplets in the CAS allow determining the dominating composition of the spin projection─cumulative weights of |±M⟩. These are conceptually different from those generated by the SH alone. It is shown that in some cases, the SH theory works satisfactorily for a series of transition-metal complexes; however, sometimes it fails. The ab initio calculations on the SH parameters are compared with the approximate generalized crystal-field theory conducted at the experimental geometry of the chromophore. In total, 12 metal complexes have been analyzed. One of the criteria that assesses the validity of SH is the projection norm N for spin multiplets (this has not to be far from 1). Another criterion is the gap in the spectrum of the spin-orbit multiplets that separates the hypothetical (fictitious) spin-only manifold from the rest of the states.

Quantified Quasi-symmetry in Metal Complexes.

Shapeness of the coordination polyhedra is quantified by a procedure that moves arbitrary Cartesian coordinates of the complex to the origin, rotates them, reorders them, and compares with the predefined model complex of exact symmetry by calculating the square Euclidian distance and/or R-factor as agreement factors. The generalized crystal-field theory has been enriched by considering a non-perfect match of the characters of the irreducible representations borne by the eigenvectors representing the crystal-field terms with those assigned to a perfect symmetry. The agreement of quasi-symmetry with the perfect one is quantified by an array of square Euclidian distances and/or R-factors. This procedure allows assignment of electronic d-d transitions in the case of non-perfect (quasi) symmetry.

A Mixed Valence CoIICoIII2 Field-Supported Single Molecule Magnet: Solvent-Dependent Structural Variation.

One-pot reaction of the Schiff base N,N'-ethylene bis(salicylaldimine) (H2L), CoCl2.6H2O, and [Ph2SnCl2] in acetone produces the mixed valence CoIICoIII2 compound [CoIICoIII2(µ-L)2(Ph)2(µ-Cl)2]·(CH3)2CO·H2O (1). Our recent study already revealed that the same reaction mixtures in methanol or ethanol produced a heterometallic SnIVCoIII (2) or monometallic CoIII complex (3), respectively. Comparison of these organometallic systems shows that the 2,1-intermetallic Ph shift occurs in any of those solvents, but their relevant structural features (mononuclear, dinuclear-heterometallic, and trinuclear mixed valence) are solvent dependent. Geometrical structural rotation is also discussed among the related organometallic CoIICoIII2 systems. The AC magnetic susceptibility measurements indicate that 1 is a single molecule magnet (SMM), exhibiting a field-induced slow magnetic relaxation with two modes. The relaxation time for the low-frequency channel is as slow as τ~0.6 s at T = 2.0 K and BDC = 1.0 T.

Effect of the Distant Substituent to Slow Magnetic Relaxation of Pentacoordinate Fe(III) Complexes.

Pentacoordinate Fe(III) complexes [Fe(LMeO)2X] and [Fe(LEtO)2X], X = Cl and Br, show the slow magnetic relaxation that is enhanced by the applied static magnetic field. A substitution of the distant ethoxy group to the methoxy group residing at the phenyl ring of a Schiff base N,O-donor ligand (LMeO vs LEtO) considerably influences the relaxation characteristics. In the chlorido complex [Fe(LMeO)2Cl], the following three slow relaxation channels are recognized as possessing different relaxation times: τLF = 0.47 s, τIF = 13 ms, and τHF = 26 µs at the static field BDC = 0.2 T and T = 1.9 K. In the bromido complex [Fe(LMeO)2Br], only the following two relaxation channels are seen: τLF = 0.30 ms and τHF = 139 µs at BDC = 0.15 T and T = 1.9 K. Due to D > 0, the Orbach relaxation mechanism does not apply, and the temperature dependence of the high-frequency relaxation time can be described by two Raman-like terms.

Above Room Temperature Spin Transition in Thermally Stable Mononuclear Fe(III) Complexes.

Two solvent-free mononuclear Fe(III) complexes [Fe(L)2]NO3 (1) and [Fe(L)2]ClO4 (2) have been synthesized by employing a new π-conjugated azo-phenyl substituted ligand, 2-(( E)-((2-(ethylamino)ethyl)imino)methyl)-4-(2-phenyldiazenyl)phenol (HL). The noncoordinated azo-phenyl part of the ligand adopts two different conformations which can exert a varied local distortion around the metal center affecting the spin crossover behavior. The magnetic data (2-450 K) reveal that complex 1 displays spin crossover above room temperature where the ligand is in linear form, while complex 2 shows an incomplete spin transition where the ligand adopts a skew form in the solid state. These complexes represent rare examples of high-temperature spin transition for mononuclear Fe(III) complexes with T1/2 > 350 K with very high thermal stability. Presence of strong intermolecular interactions and solvent-free nature of the complexes leads to exceptional thermal stability up to 485 K (for 1) and 496 K (for 2) as revealed by thermogravimetric analysis. The magnetic data for complex 1 have been analyzed by employing an Ising-like model with vibrations yielding the enthalpy change Δ H and entropy change Δ S of the spin transition along with the critical temperature T1/2 and the solid-state cooperativeness Γ. Spin crossover behavior of complex 1 has also been characterized by differential scanning calorimetry and electron paramagnetic resonance measurements. Ab initio calculations have been performed to analyze the difference in energies of the ground state and excited states of the complexes.

Breaking the Magic Border of One Second for Slow Magnetic Relaxation of Cobalt-Based Single Ion Magnets.

Instead of assembling complex clusters and/or expensive lanthanide-based systems as single ion magnets, we are focusing on mononuclear cobalt(II) systems among which the complex under study, [Co( pydca)( dmpy)]2·H2O (1), shows a field supported slow magnetic relaxation on the order of seconds at low temperature ( pydca = pyridine-2,6-dicarboxylato, dmpy = 2,6-dimethanolpyridine). The low-frequency relaxation time is as slow as τ(LF) = 1.35(6) s at T = 1.9 K and BDC = 0.4 T. The properties of 1 are compared to the previously reported nickel and copper analogues which were the first examples of single ion magnets in the family of Ni(II) and Cu(II) complexes.

Octahedral-Tetrahedral Systems [Co( dppm O, O)3]2+[CoX4]2- Showing Slow Magnetic Relaxation with Two Relaxation Modes.

Three compounds with octahedral-tetrahedral Co(II) moieties of [Co( dppm O, O)3][CoX4] type, where X = SCN (1), Cl (2), or I (4) have been synthesized and characterized by the X-ray structure analysis (1 and 4), and spectroscopic methods. The dc magnetic measurements show high magnetic anisotropy for octahedral centers whereas tetrahedral sites possess moderate D values. These results are confirmed by the ab initio calculations. The ac susceptibility data reveals a slow magnetic relaxation for 2 and 4, similar to that of the X = Br analogue (3), whereas 1 displays no ac-absorption signal. There are two relaxation channels; the slower for 2 (4) possesses a relaxation time as long as τLF= 178 (588) ms at T = 1.9 K and Bdc = 0.7 T. Also, the half-Zn analogue, [Co( dppm O, O)3][ZnI4], shows slow magnetic relaxation with two relaxation channels conditioned by the cationic unit [Co( dppm O, O)3]2+.

Slow Magnetic Relaxation in Cobalt(II) Field-Induced Single-Ion Magnets with Positive Large Anisotropy.

Three pentacoordinate complexes of the type [Co( pypz)X2], where pypz is a tridentate ligand 2,6-bis(pyrazol-1-yl)pyridine and X = Cl- (1), NCS- (2), and NCO- (3), have been synthesized, and their structures have been determined by X-ray analysis. The DC magnetic data show a sizable magnetic anisotropy, which was confirmed by high-field high-frequency electron paramagnetic resonance (HF EPR) measurements. Well-resolved HF EPR spectra of high spin cobalt (II) were observed over the microwave frequency range 100-650 GHz. The experimental spectra of both complexes were simulated with axial g tensor components, a very large positive D value, and different E/ D ratios. To determine the exact D value for 2 (38.4 cm-1) and 3 (40.92 cm-1), the far-infrared magnetic spectroscopy method was used. Knowledge of the zero field splitting parameters and their signs is crucial in interpreting the single-molecule magnet or single chain magnet behavior. The AC susceptibility data confirm that these complexes exhibit a slow magnetic relaxation under small applied DC field with two (1 and 3) or three (2) relaxation modes.

Solvent-Controlled Phase Transition of a CoII -Organic Framework: From Achiral to Chiral and Two to Three Dimensions.

An unprecedented reversible dynamic transformation is reported in a metal-organic framework involving bond formation, which is accompanied by two important structural changes; achiral to chiral and two- to three-dimensions. Using two bent organic ligands (diimpym=4,6-di(1H-imidazol-1-yl)pyrimidine; H2 npta=5-nitroisophthalic acid) and CoII (NO3 )2 ⋅6 H2 O the coordination polymer Co(diimpym)(npta)⋅CH3 OH, (1⋅CH3 OH), was obtained solvothermally. Its structure consists of knitted pairs of square layers (44 -sql net) of five-coordinated Co and disordered methanol, and it crystallized in the achiral Pbca space group at room temperature. It undergoes a single crystal to single crystal (SC-SC) transformation to a 3D interpenetrated framework (α-polonium-type net, pcu) of six-coordinated Co and ordered methanol in the chiral P21 21 21 space group below 220 K. Most unusual is the dynamic temperature-dependent shortening of a Co⋅⋅⋅O connection from a non-bonded 2.640 Š(298 K) to a bonded 2.347 Šdistance (100 K) transforming the square pyramidal cobalt polyhedron to a distorted octahedron. The desolvated crystals (1) obtained at 480 K retain the full crystallinity and crystallize in the achiral Pbca space group between 100 and 298 K but the dynamic shortening of the Co⋅⋅⋅O distance connecting the layers into the 3D pcu framework structure is observed. Following post-synthetic insertion of ethanol (1⋅CH3 CH2 OH) it does not exhibit the transformation and retains the knitted 2D achiral Pbca structure for all temperatures (100-298 K) and the ethanol is always disordered. The structural analyses thus conclude that the ordering of the methanol induces the chirality while the available space controls the dynamic motion of the knitted 2D networks into the 3D interpenetrated framework. Consequently, 1 selectively adsorbs CO2 to N2 and exhibits Type-III isotherms indicating dynamic motion of the 2D networks to accommodate the CO2 at 273 and 298 K in contrast to the rigidity of the 3D framework at 77 K preventing N2 from penetrating the solid. The magnetic properties are also reported.

Field Supported Slow Magnetic Relaxation in a Mononuclear Cu(II) Complex.

A mononuclear hexacoordinate Cu(II) complex shows a field induced slow magnetic relaxation that is not facilitated by an energy barrier to spin reversal due to the zero-field splitting. Two relaxation channels were found: the magnetic field strongly supports the low-frequency relaxation path with a relaxation time as long as τ = 0.8 s at T = 1.9 K and B = 1.5 T. The mechanism of the relaxation at low temperature involves the dominant Raman process for this S = 1/2 spin system along with a temperature-independent term belonging to a quantum tunneling.

Field-Assisted Slow Magnetic Relaxation in a Six-Coordinate Co(II)-Co(III) Complex with Large Negative Anisotropy.

The reaction of Co(CH3COO)2·4H2O with the Schiff base ligand LH4 derived from o-vanillin and tris(hydroxymethyl)aminomethane produces the dinuclear mixed-valence complex [CoIICoIII(LH2)2(CH3COO)(H2O)](H2O)3 (1), which has been investigated using IR spectroscopy, X-ray crystallography, temperature-dependent magnetic susceptibility, magnetization, HFEPR spectroscopy, and ac susceptibility measurements at various frequencies, temperatures, and external magnetic fields. The structure of 1 consists of neutral molecules in which two cobalt ions with distorted octahedral geometries, CoIIO6 and CoIIIN2O4, are bridged by two deprotonated -CH2O- groups of the two LH22- ligands. 1 completes a series with Cl, Br, NO3, and NCS anions published before by different authors. Low-temperature HFEPR measurements reveal that the ground electronic state of the Co(II) center in 1 is a highly anisotropic Kramers doublet; the effective g values of 7.18, 2.97, and 1.96 are frequency-independent over the frequency ranges 200-630, 200-406, and 200-300 GHz for the highest, intermediate, and lowest geff values, respectively. The two lower values were not seen at higher frequencies because the magnetic field was not high enough. Temperature-dependent magnetic susceptibility and field-dependent magnetization data confirm high magnetic anisotropy of the easy axis type. Complex 1 behaves as a single-ion magnet under a small applied external field and demonstrates two relaxation modes that strongly depend on the applied static dc field. The observation of multiple relaxation pathways clearly distinguishes 1 from the Cl and Br analogues.

Physical and Structural Characterization of Imidazolium-Based Organic-Inorganic Hybrid: (C3N2H5)2[CoCl4].

(C3N2H5)2[CoCl4] (ICC) was characterized in a wide temperature range by the single-crystal X-ray diffraction method. Differential scanning calorimetry revealed two structural phase transitions: continuous at 245.5 K (from phase I to II) and a discontinuous one at 234/237 K (cooling/heating) (II → III). ICC adopts monoclinic space groups C2/c and P21/c in phase (I) and (III), respectively. The intermediate phase (II) appears to be incommensurately modulated. Dynamic properties of polycrystalline ICC were studied by means of dielectric spectroscopy and proton magnetic resonance ((1)H NMR). The presence of a low frequency dielectric relaxation process in phase III reflects libration motion of the imidazolium cations. The temperature dependence of the (1)H spin-lattice relaxation time indicated two motional processes with similar activation energies that are by about an order of magnitude smaller than the activation energy obtained from dielectric studies. There are no abrupt changes in the (1)H relaxation time at the phase transitions indicating that the dynamics of the imidazolium rings gradually varies with temperature; that is, it does not change suddenly at the phase transition. Negative values of the Weiss constant and the intermolecular exchange parameter were obtained, confirming the presence of a weak antiferromagnetic interaction between the nearest cobalt centers. Moreover, the magnitude of zero field splitting was determined. The AC susceptibility measurements show that a slow magnetic relaxation is induced by small external magnetic field.

Iron oxides in human spleen.

Iron is an essential element for fundamental cell functions and a catalyst for chemical reactions. Three samples extracted from the human spleen were investigated by scanning (SEM) and transmission electron microscopy (TEM), Mössbauer spectrometry (MS), and SQUID magnetometry. The sample with diagnosis of hemosiderosis (H) differs from that referring to hereditary spherocytosis and the reference sample. SEM reveals iron-rich micrometer-sized aggregate of various structures-tiny fibrils in hereditary spherocytosis sample and no fibrils in hemochromatosis. Hematite and magnetite particles from 2 to 6 µm in TEM with diffraction in all samples were shown. The SQUID magnetometry shows different amount of diamagnetic, paramagnetic and ferrimagnetic structures in the tissues. The MS results indicate contribution of ferromagnetically split sextets for all investigated samples. Their occurrence indicates that at least part of the sample is magnetically ordered below the critical temperature. The iron accumulation process is different in hereditary spherocytosis and hemosiderosis. This fact may be the reason of different iron crystallization.

Structural, magnetic and luminescent properties of lanthanide complexes with N-salicylideneglycine.

A series of anionic heavy lanthanide complexes, involving the N-salicylideneglycinato(2-) Schiff base ligand (salgly) and having the general formula K[Ln(salgly)2(H2O)2]∙H2O (1-6), where Ln stands for Gd, Tb, Dy, Ho, Er and Tm, was prepared using the one-pot template synthesis. The complexes were thoroughly characterized by elemental and Thermogravimetric/Differential Thermal Analyses (TG/DTA), Fourier Transform Infrared Spectroscopy (FT-IR), and photoluminescence spectroscopies, electrospray-ionization mass spectrometry, and their magnetic properties were studied by temperature-dependent dc magnetic measurements using the superconducting quantum interference device (SQUID). The X-ray structure of the terbium(III) complex (2), representing the unique structure between the lanthanide complexes of N-salicylideneamino acids, was determined. The results of spectral and structural studies revealed the isostructural nature of the prepared complexes, in which the lanthanide ion is octacoordinated by two O,N,O-donor salgly ligands and two aqua ligands. The analysis of magnetic data confirmed that the complexes behave as paramagnets obeying the Curie law. The results of photoluminescence spectral studies of the complexes showed the different origin in their luminescent properties between the solid state and solution. An antenna effect of the Schiff base ligand was observed in a powder form of the complex only, while it acts as a fluorophore in a solution.

Simple mononuclear cobalt(II) complex: a single-molecule magnet showing two slow relaxation processes.

Complex [Co(PPh3)2Br2] possesses intermediate magnetic anisotropy, D/hc = -13 cm(-1). It displays superparamagnetic behavior either in the absence of the magnetic field or in fields of Bdc = 0.05, 0.1, 0.15, and 0.2 T. At Bdc = 0.1 T, the barrier to spin reversal U/kB = 37 K and the extrapolated relaxation time τ0 = 9.4 × 10(-11) s confirm its classification as a single-molecule magnet. At Bdc = 0.2 T, two relaxation processes are evidenced.

Single-molecule magnetism in a pentacoordinate cobalt(II) complex supported by an antenna ligand.

Pentacoordinate complex [CoL(3)Cl2] with a tridentate antenna-like ligand L(3) forms a dimer held by short π-π stacking with head-to-head contacts at 3.4 Å. The direct-current (dc) magnetic susceptibility and magnetization data confirm weak ferromagnetic interaction and a large-magnetic anisotropy, D/hc = 150 cm(-1) and E/hc = 11.6 cm(-1). The system shows superparamagnetic behavior at low temperature that depends upon the applied magnetic field. At Bdc = 0.2 T, a low-frequency peak at the out-of-phase susceptibility is seen (ν ∼ 0.3 Hz), whereas the onset of the second peak appears at ν > 1500 Hz, indicating the existence of two slow relaxation processes.

Tetranuclear hetero-metal [Co(II)2Ln(III)2] (Ln = Gd, Tb, Dy, Ho, La) complexes involving carboxylato bridges in a rare µ4-η(2):η(2) mode: synthesis, crystal structures, and magnetic properties.

A new family of 3d-4f heterometal 2 × 2 complexes [Co(II)2(L)2(PhCOO)2Ln(III)2(hfac)4] (1-5) (Ln = Gd (compound 1), Tb (compound 2), Dy (compound 3), Ho (compound 4), and La (compound 5)) have been synthesized in moderate yields (48-63%) following a single-pot protocol using stoichiometric amounts (1:1 mol ratio) of [Co(II)(H2L)(PhCOO)2] (H2L = N,N'-dimethyl-N,N'-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine) as a metalloligand and [Ln(III)(hfac)3(H2O)2] (Hhfac = hexafluoroacetylacetone) as a lanthanide precursor compound. Also reported with this series is the Zn-Dy analog [Zn(II)2(L)2(PhCOO)2Dy(III)2(hfac)4] 6 to help us in understanding the magnetic properties of these compounds. The compounds 1-6 are isostructural. Both hexafluoroacetylacetonate and benzoate play crucial roles in these structures as coligands in generating a tetranuclear core of high thermodynamic stability through a self-assembly process. The metal centers are arranged alternately at the four corners of this rhombic core, and the carboxylato oxygen atoms of each benzoate moiety bind all of the four metal centers of this core in a rare µ4-η(2):η(2) bridging mode as confirmed by X-ray crystallography. The magnetic susceptibility and magnetization data confirm a paramagnetic behavior, and no remnant magnetization exists in any of these compounds at vanishing magnetic field. The metal centers are coupled in an antiferromagnetic manner in these compounds. The [Co(II)2Dy(III)2] compound exhibits a slow magnetic relaxation below 6 K, as proven by the AC susceptibility measurements; the activation energy reads U/kB = 8.8 K (τ0 = 2.0 × 10(-7) s) at BDC = 0, and U/kB = 7.8 K (τ0 = 3.9 × 10(-7) s) at BDC = 0.1 T. The [Zn(II)2Dy(III)2] compound also behaves as a single-molecule magnet with U/kB = 47.9 K and τ0 = 2.75 × 10(-7) s.