Influence of the phonon-bottleneck effect and low-energy vibrational modes on the slow spin-phonon relaxation in Kramers-ions-based Cu(II) and Co(II) complexes with 4-amino-3,5-bis-(pyridin-2-yl)-1,2,4-triazole and dicyanamide

Two new complexes, bis-[4-amino-3,5-bis-(pyridin-2-yl)-1,2,4-triazole-κ2N2,N6]bis-(dicyanamide-κN8)copper(II), [Cu(abpt)2(dca)2] (1) and bis-[4-amino-3,5-bis-(pyridin-2-yl)-1,2,4-triazole-κ2N2,N6]bis-(dicyanamide-κN8)cobalt(II), [Co(abpt)2(dca)2] (2), have been prepared and magneto-structurally characterised. Single crystal studies of both complexes have shown that their crystal structures are molecular, in which the central atoms are six-coordinated in the form of a distorted octahedron by two bidentate abpt and two monodentate dca ligands. Even if both complexes have the same composition and crystallize in the same P1̄ space group, they are not isostructural. Both structures contain strong intermolecular N-H⋯N hydrogen bonds and π-π stacking interactions. IR spectra are consistent with the solved structures of both complexes and confirmed the terminal character of the dca ligands and the bidentate coordination of the abpt ligands. The analysis of the magnetic properties showed that both complexes exhibit field-induced slow spin-phonon relaxation. In both complexes, the slow spin-phonon relaxation is influenced by a severe phonon-bottleneck effect that affects the direct process, a dominant relaxation mechanism at low temperatures in both complexes. The phonon-bottleneck effect in 1 was suppressed by simply reducing the crystallite size, and further analysis of the field dependence of the relaxation time yielded the characteristic energy of vibrational modes of 11 cm-1 participating in the Raman process at low magnetic fields. The analysis of magnetic properties and ab initio calculations confirmed that 2 represents a system with a moderate uniaxial anisotropy yielding an average energy barrier of 82 cm-1 (from all four nonequivalent Co(II) sites in the structure of 2).

Field-Induced Slow Magnetic Relaxation in CoII Cyclopropane-1,1-dicarboxylates

New Co^II substituted malonate field-induced molecular magnets {[Rb₆Co₃(cpdc)₆(H₂O)₁₂]∙6H₂O}_n (1) and [Cs₂Co(cpdc)₂(H₂O)₆]_n (2) (where cpdc²⁻ stands for cyclopropane-1,1-dicarboxylic acid dianions) were synthesized. Both compounds contain mononuclear bischelate fragments {Co^II(cpdc)₂(H₂O)₂}²⁻ where the quasi-octahedral cobalt environment (CoO₆) is complemented by water molecules in apical positions. The alkali metal atoms play the role of connectors between the bischelate fragments to form 3D and 2D polymeric structures for 1 and 2, respectively. Analysis of dc magnetic data using the parametric Griffith Hamiltonian for high-spin Co^II supported by ab initio calculations revealed that both compounds have an easy axis of magnetic anisotropy. Compounds 1 and 2 exhibit slow magnetic relaxation under an external magnetic field (H_DC = 1000 and 1500 Oe, respectively).

Effect of Ligand Chain Length for Tuning of Molecular Dimensionality and Magnetic Relaxation in Redox Active Cobalt(II) EDOT Complexes (EDOT = 3,4-Ethylenedioxythiophene)

Four cobalt(II) complexes, [Co(L1)2(NCX)2(MeOH)2] (X = S (1), Se (2)) and {[Co(L2)2(NCX)2]}n (X = S (3), Se (4)) (L1 = 2,5dipyridyl-3,4,-ethylenedioxylthiophene and L2 = 2,5diethynylpyridinyl-3,4-ethylenedioxythiophene), were synthesized by incorporating ethylenedioxythiophene based redox-active luminescence ligands. All these complexes have been well characterized using single-crystal X-ray diffraction analyses, spectroscopic and magnetic investigations. Magneto-structural studies showed that 1 and 2 adopt a mononuclear structure with CoN4O2 octahedral coordination geometry while 3 and 4 have a 2D [4 x 4] rhombic grid coordination networks (CNs) where each cobalt(II) center is in a CoN6 octahedral coordination environment. Static magnetic measurements reveal that all four complexes displayed a high spin (HS) (S = 3/2) state between 2 and 280 K which was further confirmed by X-band and Q-band EPR studies. Remarkably, along with the molecular dimensionality (0D and 2D) the modification in the axial coligands lead to a significant difference in the dynamic magnetic properties of the monomers and CNs at low temperatures. All complexes display slow magnetic relaxation behavior under an external dc magnetic field. For the complexes with NCS- as coligand observed higher energy barrier for spin reversal in comparison to the complexes with NCSe- as coligand, while mononuclear complex 1 exhibited a higher energy barrier than that of CN 3. Theoretical calculations at the DFT and CASSCF level of theory have been performed to get more insight into the electronic structure and magnetic properties of all four complexes.

Field-induced mononuclear cobalt(II) single-molecule magnet (SMM) based on a benzothiadiazole-ortho-vanillin ligand

A unique π-conjugated benzothiadiazole-ortho-vanillin ligand (HL), characterized by single crystal X-ray diffraction and DFT calculations, has been prepared by condensation between 4-amino-benzothiadiazole (BTD) and ortho-vanillin. Its reaction with cobalt(II) acetate...

Field-induced single-ion magnet based on a quasi-octahedral Co(II) complex with mixed sulfur-oxygen coordination environment

Synthesis and characterization of structure and magnetic properties of the quasi-octahedral complex (pipH₂)[Co(TDA)₂] 2H₂O (I), (pipH₂²⁺ = piperazine dication, TDA^2- = thiodiacetic anion) are described. X-ray diffraction studies reveal the first coordination sphere of the Co(II) ion, consisting of two chelating tridentate TDA ligands with a mixed sulfur-oxygen strongly elongated octahedral coordination environment. SQUID magnetometry, frequency-domain Fourier-transform (FD-FT) THz-EPR spectroscopy, and high-level ab initio SA-CASSCF/NEVPT2 quantum chemical calculations reveal a strong "easy-plane" type magnetic anisotropy (D ≈ +54 cm^-1) of complex I. The complex shows field-induced slow relaxation of magnetization at an applied DC field of 1000 Oe.

Building-up host-guest helicate motifs and chains: a magneto-structural study of new field-induced cobalt-based single-ion magnets

In this work, we present the synthetic pathway, a refined structural description, complete solid-state characterization and the magnetic properties of four new cobalt(ii) compounds of formulas [Co(H₂O)₆][Co₂(H₂mpba)₃]·2H₂O·0.5dmso (1), [Co(H₂O)₆][Co₂(H₂mpba)₃]·3H₂O·0.5dpss (2), [Co₂(H₂mpba)₂(H₂O)₄]_n·4nH₂O (3), and [Co₂(H₂mpba)₂(CH₃OH)₂(H₂O)₂]_n·0.5nH₂O·2ndpss (4) [dpss = 2,2'-dipyridyldisulfide and H₄mpba = 1,3-phenylenebis(oxamic) acid], where 2 and 4 were obtained from [Co(dpss)Cl₂] (Pre-I) as the source of cobalt(ii). All four compounds are air-stable and were prepared under ambient conditions. 1 and 2 were obtained from a slow diffusion method [cobalt(ii) : H₂mpba^2- molar ratio used 1 : 1] and their structures are made up of [Co₂(H₂mpba)₃]^2- anionic helicate units and [Co(H₂O)₆]²⁺ cations, exhibiting supramolecular three-dimensional structures. Interestingly, a supramolecular honeycomb network between the helicate units interacting with each other through R₂²(10) type hydrogen bonds occurs in 2 hosting one co-crystallized dpss molecule. On the other hand, for the first time, linear (3) and zigzag (4) cobalt(ii) chains were isolated by slow evaporation of stirred solutions of mixed solvents with cobalt(ii) : H₂mpba^2- in 1 : 2 molar ratio at room temperature. Magnetic measurements of Pre-I revealed a quasi magnetically isolated S = 3/2 spin state with a significant second-order spin-orbit contribution as expected for tetrahedrally coordinated cobalt(ii) ions. The analysis of the variable temperature static (dc) magnetic susceptibility data through first- (1 and 3) and second-order spin-orbit coupling models (2 and 4) reveals the presence of magnetically non-interacting high-spin cobalt(ii) ions with easy-axis (1 and 4)/easy-plane magnetic anisotropies (2 and 4) with low rhombic distortions. Dynamic (ac) magnetic measurements for Pre-I and 1-4 below 8.0 K show that they are examples of field-induced Single-Ion Magnets (SIMs).

Insertion of single-ion magnets based on mononuclear Co(II) complexes into ferromagnetic oxalate-based networks

The 1 : 2 and 1 : 1 Co(ii) complexes of the L ligand (L = 6-(3,5-diamino-2,4,6-triazinyl)2,2'-bipyridine) with formulas [CoII(L)2](ClO4)2·0.5MeCN·Et2O (1) and [CoII(L)(CH3CN)2(H2O)](ClO4)2·MeCN (2) have been prepared. The structural and magnetic characterization of the two compounds shows that they contain octahedral high-spin Co(ii) and present a field-induced slow relaxation of the magnetization. 1 has been inserted into a bimetallic oxalate-based network leading to a novel achiral 3D compound of formula [CoII(L)2][MnIICrIII(ox)3]2·(solvate) (3) exhibiting ferromagnetic ordering below 4.6 K. EPR measurements suggest a weak magnetic coupling between the two sublattices.

Magnetism in a helicate complexes arising with the tetradentate ligand

The synthesis of [M(dimphen)(NCS)₂] (1; M = Fe^II), (2; M = Co^II), (3; M = Mn^II) and [Fe(dimphen)(NCSe)₂] (4), where dimphen = [1,2-bis(9-methyl-1,10-phenanthrolin-2-yl)ethane], are reported. The crystal packing structures of 1-3, show intermolecular π-π stacking and NCSSCN interactions. The complex 1 shows ferromagnetic interaction, and the complex 2 displays single-molecular magnet behaviour.

Syntheses, structures and magnetic properties of two isostructural dicyanamide-bridged 2D polymers

Complexes [Co(biq)(μ_1,5-dca)₂]_n (1) and [Ni(biq)(μ_1,5-dca)₂]_n (2) (biq is 2,2′-biquinoline, dca is dicyanamide anion, N(CN)₂⁻) have been characterized by crystal structure analysis, and spectral and magnetic measurements.

Non-traditional thermal behavior of Co(ii) coordination networks showing slow magnetic relaxation

Three new Co(ii) coordination polymers show the DC magnetic data consistent with the S = 3/2 spin system with large zero-field splitting D > 0, which was confirmed by HF EPR and FIRMS measurements.

X-ray Structure and Magnetic Properties of Heterobimetallic Chains Based on the Use of an Octacyanidodicobalt(III) Complex as Metalloligand

The assembly of [Co2III(μ-2,5-dpp)(CN)8]2− anions and [MII(CH3OH)2(DMSO)2]2+ cations resulted into the formation of two heterobimetallic 1D coordination polymers of formula [MII(CH3OH)2(DMSO)2(μ-NC)2Co2III(μ-2,5-dpp)(CN)6]n·4nCH3OH [M = CoII (1)/FeII (2) and 2,5-dpp = 2,5-bis(2-pyridyl)pyrazine. The [Co2III(μ-2,5-dpp)(CN)8]2− metalloligand coordinates the paramagnetic [MII(CH3OH)2(DMSO)2]2+ complex cations, in a bis-monodentate fashion, to give rise to neutral heterobimetallic chains. Cryomagnetic dc (1.9–300 K) and ac (2.0–13 K) magnetic measurements for 1 and 2 show the presence of Co(II)HS (1) and Fe(II)HS (2) ions (HS – high-spin), respectively, with D values of +53.7(5) (1) and −5.1(3) cm−1 (2) and slow magnetic relaxation for 1, this compound being a new example of SIM with transversal magnetic anisotropy. Low-temperature Q-band EPR study of 1 confirms that D value is positive, which reveals the occurrence of a strong asymmetry in the g-tensors and allows a rough estimation of the E/D ratio, whereas 2 is EPR silent. Theoretical calculations by CASSCF/NEVPT2 on 1 and 2 support the results from magnetometry and EPR. The analysis of the ac magnetic measurements of 1 shows that the relaxation of M takes place in the ground state under external magnetic dc fields through dominant Raman and direct spin-phonon processes.

Co(II)-Based single-ion magnets with 1,1'-ferrocenediyl-bis(diphenylphosphine) metalloligands

Herein, we report on investigations of magnetic and spectroscopic properties of three heterobimetallic Fe(ii)-Co(ii) coordination compounds based on the tetracoordinate {CoP2X2} core encapsulated by dppf metalloligand, where X = Cl (1), Br (2), I (3), dppf = 1,1'-ferrocenediyl -bis(diphenylphosphine). The analysis of static magnetic data has revealed the presence of axial magnetic anisotropy in compounds (1) and (2) and this was further confirmed by high-frequency electron spin resonance (HF-ESR) spectroscopy. Dynamic magnetic data confirmed that (1) and (2) behave as field-induced Single-Ion Magnets (SIMs). Together with bulk studies, we have also tested the possibility of depositing (2) as thick films on Au(111), glass, and polymeric acetate by drop-casting as well as thermal sublimation, a key aspect for the development of future devices embedding these magnetic objects.

Field-induced slow magnetic relaxation in mixed valence di- and tri-nuclear CoII-CoIII complexes

Two novel mixed valence Co^II-Co^III complexes, namely [Co^IICo^III(L1)(ab)(mb)₂(H₂O)]·dmf (1) and [CoCo^II(L2)₄(H₂O)₄]·2H₂O (2) [H₂L1 = (E)-2-((1-hydroxybutan-2-ylimino)methyl)-6-methoxyphenol, ab = 2-amino-butan-1-ol anion, mb = p-methyl benzoate, H₂L2 = 3-((2-hydroxy-3-methoxy-benzylidene)-amino)-propionic acid, and dmf = N,N-dimethyl-formamide], were synthesized and characterized by single crystal X-ray diffraction and magnetic studies at low temperature. The structure determination reveals that both complexes belong to the monoclinic system with P21/c (1) and I2/a (2) space groups. Complex 1 is a dinuclear Co^IIICo^II compound with distorted octahedral cobalt centers showing different coordination environments. In 2, a bent trinuclear CoCo^II complex, the coordination environments around the two terminal Co^III sites are alike, whereas they are different in the central Co^II ion. Alternating current/direct current (ac/dc) magnetic studies revealed that both complexes show field-induced slow magnetic relaxation. The dc magnetic susceptibility and magnetization data were analyzed with the following Hamiltonianwhere D and E are the axial and rhombic zero-field splitting (zfs) parameters, respectively, and a good agreement between experimental and simulated results was found using the parameters g_⊥ = 2.585, g_∥ = 2.437, D = +98.1 cm^-1, E/D = 0.008 and F = 8.2× 10^-5 for 1 and g_⊥ = 2.580, g_∥ = 2.580, D = +55.4 cm^-1, and E/D = 0.000 for 2.

Synthesis, crystal structures, HF-EPR, and magnetic properties of six-coordinate transition metal (Co, Ni, and Cu) compounds with a 4-amino-1,2,4-triazole Schiff-base ligand

We have synthesized a series of transition metal compounds [M(L)₂(H₂O)₂] (M = Co (1), Ni (2), and Cu (3)) by using the 4-amino-1,2,4-triazole Schiff-base ligand via the hydrothermal methods. They are all mononuclear compounds with the octahedral geometry. Direct-current magnetic and HF-EPR measurements were combined to reveal the negative D values (-28.78 cm^-1, -10.79 cm^-1) of complexes 1 and 2, showing the easy-axis magnetic anisotropies of compounds 1 and 2. Applying a dc field of 800 Oe at 2.0 K, the slow magnetic relaxation effects were observed in compound 1, which is a remarkable feature of single-ion magnets.

Magneto-structural diversity of Co(ii) compounds with 1-benzylimidazole induced by linear pseudohalide coligands

The magneto-structural diversity of 1-benzylimidazole-containing cobalt(ii) compounds with linear pseudohalide ions (NCS⁻, NCO⁻, and N₃⁻) is explored.

Tuning magnetic anisotropy by the π-bonding features of the axial ligands and the electronic effects of gold(i) atoms in 2D {Co(L)2[Au(CN)2]2}n metal–organic frameworks with field-induced single-ion magnet behaviour

Au^I atoms play an important role in determining the anisotropy of Co^II nodes in 2D Au^I–Co^II field-induced SIMs.

Influence of the pyrazine substituent on the structure and magnetic properties of dicyanamide-bridged cobalt(ii) complexes

Substituted pyrazines were successfully used to prepare two new coordination polymers of formulas {[Co(dca)2(NH2pyz)2]·H2O}n (1) and [Co3(dca)6(HOpyz)5(H2O)2]n (2) [dca = dicyanamide, NH2pyz = 2-aminopyrazine and HOpyz = 2-hydroxypyrazine] whose structures were determined by single-crystal X-ray crystallography. The structure of 1 consists of a two-dimensional rhombus grid of cobalt(ii) ions where the dca ligand adopts the μ1,5 bridging mode with trans-positioned monodentate NH2pyz molecules completing the six-coordination around each metal ion. Compound 2 exhibits a stair-like two-dimensional structure where the intralayer connections are performed by the dca and HOpyz groups exhibiting μ1,5 and bis-monodentate coordination modes, respectively. The values of the cobalt-cobalt separation through the dca bridges are 8.2107(3) (1) and 8.4746(4) and 8.5249(4) Å (2) whereas the value through the hydroxypyrazine is 7.2052(6) Å (2). Solid-state direct-current magnetic susceptibility analyses in the temperature range of 1.9-300 K for 1 and 2 reveal the occurrence of magnetically isolated high-spin cobalt(ii) ions with a significant contribution to the magnetic moment (1 and 2), D = +95.4 (1) and +76.5 cm-1 (2) and the antiferromagnetically coupled pairs of cobalt(ii) centres through the bis-monodentate 2-hydroxypyrazine, J = -0.3 cm-1 (2). Both compounds exhibit frequency dependence of the out-of-phase alternating current (ac) magnetic susceptibility (χ''M) under non-zero applied dc fields, a feature which is characteristic of single-ion magnet behaviour (SIM). Q-band EPR studies on the polycrystalline samples of 1 and 2 at low temperatures confirm the positive sign of D and reveal the occurrence of a strong asymmetry in the g-tensors. Theoretical calculations by CASSCF/NEVPT2 support these results. An analysis of the dynamic behaviour of 1 and 2 suggests that the relaxation of the magnetization occurs in the ground state under applied fields through two Orbach processes possibly bound to low-lying vibrational modes in the high temperature range, and to the slowing down of the fast interconversion between the two contributions of the ground Kramers doublet at lower temperatures induced by the applied dc field.

Field-induced slow magnetic relaxation in two-dimensional and three-dimensional Co(ii) coordination polymers

Two coordination polymers formulated as [Co(1,4-bimb)_0.5(5-aip)(H₂O)]_n (1) and [Co(1,4-bib)_1.5(5-hip)(H₂O)]_n (2) (1,4-bimb = 1,4-bis(imidazol-1-ylmethyl)benzene, 5-aip = 5-aminoisophthalic acid, 1,4-bib = 1,4-bis(1-imidazolyl)benzene and 5-hip = 5-hydroxyisophthalic acid) have been prepared and structurally characterized. Complex 1 is a two-dimensional (2D) network where Co(ii) is six coordinate in a CoO₄N₂ coordination environment, while the structure of 2 consists of a three-dimensional (3D) framework built from mononuclear Co(ii) units with distorted octahedral geometry as nodes. Static magnetic studies show that first-order orbital angular momentum may play an important role in the magnetic properties of 1, whereas strong easy-axis anisotropy (D = -102 cm^-1) was observed in 2. Alternating current (ac) susceptibility measurements demonstrate that both the complexes display field-induced single ion magnet (SIM) behavior.

Slow Magnetic Relaxation Triggered by a Structural Phase Transition in Long-Chain-Alkylated Cobalt(II) Single-Ion Magnets

The behavior of single-ion magnets (SIMs) that reflects large distortions of their coordination environments caused by the packing of long alkyl chains for two Co(II) complexes of the type [Co(C _n-terpy)₂](BF₄)₂ (C _n-terpy = 4'-alkoxy-2,2':6',2″-terpyridine; n = 10 (1), 16 (2)) is reported. 1·2MeOH, which features a highly distorted octahedral high-spin Co(II) center, exhibits field-induced slow magnetic relaxation under an applied dc field of 1000 Oe. Further detailed analysis of the relaxation process indicated the prevalence of the Raman process at low temperature. Surprisingly, 2 shows a reverse spin transition (rST) and also exhibits remarkable field-induced SIM behavior, revealing the presence of magnetic anisotropy for this high-spin Co(II) species that is triggered by a structural phase transition. We present here the first examples of the coexistence of field-induced slow magnetic relaxation and rST associated with structural phase transitions involving long-alkyl-chain conformational changes from gauche to anti. These results indicate the prospect of inducing SIM properties in other distorted high-spin Co(II) species bearing long alkyl chains.

Field-induced slow magnetic relaxation in pseudooctahedral cobalt(ii) complexes with positive axial and large rhombic anisotropy

The preparation, X-ray crystal structure, spectroscopic and variable-temperature dc and ac magnetic properties of two six-coordinate cobalt(ii) complexes of formula [Co(bim)₄(tcm)₂] (1) and [Co(bmim)₄(tcm)₂] (2) are reported.

Syntheses, structures, and magnetic properties of three two-dimensional cobalt(ii) single-ion magnets with a CoIIN4X2 octahedral geometry

Three two-dimensional Co^II SIMs with (4,4) layer structures have been synthesized and characterized structurally and magnetically.

A Pseudo-Octahedral Cobalt(II) Complex with Bispyrazolylpyridine Ligands Acting as a Zero-Field Single-Molecule Magnet with Easy Axis Anisotropy

The homoleptic mononuclear compound [Co(bpp-COOMe)₂ ](ClO₄ )₂  (1) (bpp-COOMe=methyl 2,6-di(pyrazol-1-yl)pyridine-4-carboxylate) crystallizes in the monoclinic C2/c space group, and the cobalt(II) ion possesses a pseudo-octahedral environment given by the two mer-coordinated tridentate ligands. Direct-current magnetic data, single-crystal torque magnetometry, and EPR measurements disclosed the easy-axis nature of this cobalt(II) complex, which shows single-molecule magnet behavior when a static field is applied in alternating-current susceptibility measurements. Diamagnetic dilution in the zinc(II) analogue [Zn(bpp-COOMe)₂ ](ClO₄ )₂  (2) afforded the derivative [Zn_0.95 Co_0.05 (bpp-COOMe)₂ ](ClO₄ )₂  (3), which exhibits slow relaxation of magnetization even in zero field thanks to the reduction of dipolar interactions. Theoretical calculations confirmed the overall electronic structure and the magnetic scenario of the compound as drawn by experimental data, thus confirming the spin-phonon Raman relaxation mechanism, and a direct quantum tunneling in the ground state as the most plausible relaxation pathway in zero field.

The influence of pseudohalide ligands on the SIM behaviour of four-coordinate benzylimidazole-containing cobalt(ii) complexes

Three, mononuclear complexes of the formula [Co(bmim)2(SCN)2] (1), [Co(bmim)2(NCO)2] (2) and [Co(bmim)2(N3)2] (3) [bmim = 1-benzyl-2-methylimidazole] were prepared and structurally analyzed by single-crystal X-ray crystallography. The cobalt(ii) ions in 1-3 are tetrahedrally coordinated with two bmim molecules and two pseudohalide anions. The angular distortion parameter δ was calculated and the SHAPE program (based on the CShM concept) was used for 1-3 to estimate the angular distortion from an ideal tetrahedron. The molecules of 1-3 are effectively separated, and the values of the shortest distance of cobalt-cobalt are 8.442(6) and 6.774(8) Å for 1, 10.349(8) and 10.716(8) Å for 2 and 6.778(1) and 9.232(1) Å for 3. Direct current (dc) magnetic susceptibility measurements on the crushed crystals of 1-3 were carried out in the temperature range 1.9-295 K. The variable-temperature magnetic data of 1-3 mainly obey the zero-field splitting effect (D) of the 4A2 ground term of the tetrahedral cobalt(ii) complexes (2D being the energy gap between the |±1/2 and |±3/2 levels of the spin). The analysis of their magnetic data through the Hamiltonian H = D[S2z - S(S + 1)/3] + E(Sx2 - Sy2) + gβHS led to the following best-fit parameters: g = 2.29, D = -7.5 cm-1 and E/D = 0.106 (1), g = 2.28, D = + 6.3 cm-1 and E/D = 0.007 (2) and g = 2.36, D = + 6.7 cm-1 and E/D = 0.090 (3). The signs of D for 1-3 were confirmed by Q-band EPR spectra on powdered samples in the temperature range 4.0-20 K. Field-induced SIM behaviour was observed for 1-3 below 4.0 K, and the frequency-dependent maxima of χ''M were observed for 1 and only incipient signals of χ''M occurred for 2 and 3. The values of the exponential factor (τ0) and activation energy (Ea) for 1-3 which were obtained from the Arrhenius plot suggest a single relaxation process characteristic of an Orbach mechanism.

Single Ion Magnets from 3d to 5f: Developments and Strategies

Single-ion magnets (SIMs), exhibiting slow magnetization relaxation in the absence of the magnetic field, originate from their single spin-carrier centre. In pursuit of high-performance magnetic properties, such as high spin-reversal barrier and high blocking temperature, various metal centres were investigated to establish SIMs, including 3d and 5d transition metal ions, 4f lanthanide ions, and 5f actinide ions, which possess unique zero-field splitting and magnetic properties. Therefore, proper ligand field is of great importance to different types of metals. In the given great breakthroughs since the first SIM, [Pc₂ Tb]^- (Pc=dianion of phthalocyanine), was reported, strategies of ligand field design have emerged. In this review, the developments of SIMs with different metal centres are summarized, as well as the possible strategies.

ESR analyses of picket fence MnII and 6th ligand coordinated FeIII porphyrins (S = 5/2) and a CoII(hfac) complex (S = 3/2) with sizable ZFS parameters revisited: a full spin Hamiltonian approach and quantum chemical calculations

The conventional X-band ESR spectra are fully reanalyzed by a full spin Hamiltonian approach.

Metal complexes of a novel heterocyclic benzimidazole ligand formed by rearrangement-cyclization of the corresponding Schiff base. Electrosynthesis, structural characterization and antimicrobial activity

One-pot electrochemical synthesis of metal complexes containing a novel heterocyclic benzimidazole ligand is reported and characterized.

Field-induced slow magnetic relaxation of two 1-D compounds containing six-coordinated cobalt(ii) ions: influence of the coordination geometry

Two 1-D Co(ii) compounds with different coordination geometries of the same coordination sphere exhibited field-induced slow relaxation magnetization with different D values.

Magnetic anisotropy and relaxation behavior of six-coordinate tris(pivalato)-Co(ii) and -Ni(ii) complexes

Magnetic measurements, HFEPR and theoretical calculations have been used to study the magnetic anisotropy of the six-coordinate field-induced single ion magnet (NBu₄)[Co(piv)₃] and its Ni analogue.

One-dimensional cobalt(ii) coordination polymer featuring single-ion-magnet-type field-induced slow magnetic relaxation

Single-ion-magnet-type field-induced double magnetic relaxation was observed in a one-dimensional cobalt(ii) coordination polymer which shows easy-axis anisotropy with D = −33.9 cm⁻¹ and an energy barrier of U_eff = 38.8 K.

Two polymorphic Co(ii) field-induced single-ion magnets with enormous angular distortion from the ideal octahedron

A mononuclear complex [Co(neo)(PhCOO)₂] was prepared in two polymorphic forms, which both possess large magnetic anisotropy with different degrees of rhombicity. Furthermore, both polymorphs behave as field-induced single-ion magnets.

Field-induced slow relaxation of magnetization in a distorted octahedral mononuclear high-spin Co(ii) complex

By adopting the strategy of mixed rigid ligands, two novel mononuclear complexes with distorted octahedral geometries are obtained. SIM-type slow magnetic relaxation behavior is observed in Co(ii)-based complexes.

Cobalt(II) Ions Connecting [CoII4] Helicates into a 2-D Coordination Polymer Showing Slow Relaxation of the Magnetization

The reactions of cobalt(II) perchlorate with a diazine tetratopic helicand, H₄L, in the presence of sodium carbonate afford two coordination polymers constructed from tetranuclear anionic helicates as building blocks: _∞³[Co₄L₃Na₄(H₂O)₄]·4H₂O (1) and _∞²[Co₅L₃Na₂(H₂O)₉]·2.7H₂O·DMF (2). The tetranuclear triple-stranded helicates, {Co^II₄L₃}^4-, are connected in 1 by sodium(I) ions and in 2 by sodium(I) and cobalt(II) ions (H₄L results from the condensation reaction between 3-formylsalicylic acid and hydrazine). The crystal structures of the two compounds have been solved. In both compounds the anionic helicates interact with the assembling cations through the carboxylato oxygen atoms. Compound 2 features chains resulting from connecting the tetranuclear helicates through cobalt(II) ions. The analysis of the magnetic properties of compounds 1 and 2 evidenced a dominant antiferromagnetic coupling for 1, resulting in a diamagnetic ground state. In contrast, the magnetic behavior of 2 is dominated at low temperature by the Co^II ion which connects the antiferromagnetically coupled {Co^II₄} helical moieties. The ac magnetic measurements for 2 reveal the occurrence of slow relaxation of the magnetization that is due to the single, uncorrelated cobalt(II) ions, which are diluted in an essentially diamagnetic matrix of {Co^II₄} moieties (ΔE_eff = 26.7 ± 0.3 cm^-1 with τ₀ = (2.3 ± 0.2) × 10^-6 s).

A Cobalt Supramolecular Triple-Stranded Helicate-based Discrete Molecular Cage

We report a strategy to achieve a discrete cage molecule featuring a high level of structural hierarchy through a multiple-assembly process. A cobalt (Co) supramolecular triple-stranded helicate (Co-TSH)-based discrete molecular cage (1) is successfully synthesized and fully characterized. The solid-state structure of 1 shows that it is composed of six triple-stranded helicates interconnected by four linking cobalt species. This is an unusual example of a highly symmetric cage architecture resulting from the coordination-driven assembly of metallosupramolecular modules. The molecular cage 1 shows much higher CO2 uptake properties and selectivity compared with the separate supramolecular modules (Co-TSH, complex 2) and other molecular platforms.

A cyano-bridged coordination nanotube showing field-induced slow magnetic relaxation

Field-induced slow magnetic relaxation was observed in a cyano-bridged nanotube constructed from a pentagonal bipyramidal Co^II unit and a hexacyanocobaltate(iii) anion.

Two 2-D multifunctional cobalt(ii) compounds: field-induced single-ion magnetism and catalytic oxidation of benzylic C–H bonds

Two 2-D multifunctional cobalt(ii) compounds were reported with both field-induced single-ion magnetism and catalytic oxidation of benzylic C–H bonds.

Two-dimensional frameworks formed by pentagonal bipyramidal cobalt(ii) ions and hexacyanometallates: antiferromagnetic ordering, metamagnetism and slow magnetic relaxation

Two-dimensional frameworks constructed by pentagonal bipyramidal Co^II and [M(CN)₆]³⁻ units have been synthesized and characterized structurally and magnetically.

Evidence of field induced slow magnetic relaxation in cis-[Co(hfac)2(H2O)2] exhibiting tri-axial anisotropy with a negative axial component

We report a combined experimental characterization and theoretical modeling of the hexa-coordinated high-spin Co(ii) complex cis-[Co(hfac)₂(H₂O)₂] (I).