A four-coordinate cobalt(II) single-ion magnet with coercivity and a very high energy barrier

Impact of nuclearity and topology on the single molecule magnet behaviour of hexaazatrinaphtylene-based cobalt complexes

A hexaazatrinaphtylene-based transition metal complex that exhibits single molecule magnet behaviour is reported herein. This study reveals the influence of both nuclearity and topology on the magnetic properties of hexaazatrinaphtylene-based complexes.

Field influence on the slow magnetic relaxation of nickel-based single ion magnets

A mononuclear complex, [Ni(pydca)(dmpy)]·H₂O, exhibits a slow magnetic relaxation under an applied magnetic field with manifold relaxation channels. With an external field of 0.6 T, the low-frequency channel is the dominating relaxation path yielding a relaxation time of 322 ms at 2.0 K. At 1.2 T the relaxation time is as slow as 876 ms.

Diazine based ligand supported CoII3 and CoII4 coordination complexes: role of anions

Synthesis of triple helical CoII4 and double helical CoII3 from diazine-based ligands and their magnetic behavior rationalized by susceptibility measurements.

Ultra-broadband EPR spectroscopy in field and frequency domains

Electron paramagnetic resonance (EPR) is a powerful technique to investigate the electronic and magnetic properties of a wide range of materials.

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.

Slow relaxation of magnetization in a bis-mer-tridentate octahedral Co(ii) complex

A field-induced Co^II single-ion magnet is described. Supramolecular effects on magnetic anisotropy, and the relaxation mechanism are discussed.

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.

A new series of tetrahedral Co(ii) complexes [CoLX2] (X = NCS, Cl, Br, I) manifesting single-ion magnet features

The influence of ligand field strength on the magnetic anisotropy of a series of isostructural tetrahedral Co^II complexes has been investigated by using a combined experimental and theoretical approach.

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.

From Positive to Negative Zero-Field Splitting in a Series of Strongly Magnetically Anisotropic Mononuclear Metal Complexes

A series of mononuclear [M(hfa)₂(pic)₂] (Hhfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione; pic = 4-methylpyridine; M = Fe^II, Co^II, Ni^II, Zn^II) compounds were obtained and characterized. The structures of the complexes have been resolved by single-crystal X-ray diffraction, indicating that, apart from the zinc derivative, the complexes are in a trans configuration. Moreover, a dramatic lenghthening of the Fe-N distances was observed, whereas the nickel(II) complex is almost perfectly octahedral. The magnetic anisotropy of these complexes was thoroughly studied by direct-current (dc) magnetic measurements, high-field electron paramagnetic resonance, and infrared (IR) magnetospectroscopy: the iron(II) derivative exhibits an out-of-plane anisotropy (D_Fe = -7.28 cm^-1) with a high rhombicity, whereas the cobalt(II) and nickel(II) complexes show in-plane anisotropy (D_Co ∼ 92-95 cm^-1; D_Ni = 4.920 cm^-1). Ab initio calculations were performed to rationalize the evolution of the structure and identify the excited states governing the magnetic anisotropy along the series. For the iron(II) complex, an out-of-phase alternating-current (ac) magnetic susceptibility signal was observed using a 0.1 T dc field. For the cobalt(II) derivative, the ac magnetic susceptibility shows the presence of two field-dependent relaxation phenomena: at low field (500 Oe), the relaxation process is beyond single-ion behavior, whereas at high field (2000 Oe), the relaxation of magnetization implies several mechanisms including an Orbach process with U_eff = 25 K and quantum tunneling of magnetization. The observation by μ-SQUID magnetization measurements of hysteresis loops of up to 1 K confirmed the single-ion-magnet behavior of the cobalt(II) derivative.

Slow Spin Relaxation in Dioxocobaltate(II) Anions Embedded in the Lattice of Calcium Hydroxyapatite

Pure-phase cobalt-doped calcium hydroxyapatite ceramic samples with composition Ca₁₀(PO₄)₆[(CoO₂)_x(OH)_1-2x]₂, where x = 0-0.2, were synthesized by high-temperature solid-state reaction, and their crystal structures, vibrational spectra, and magnetic properties were studied. Co atoms are found to enter into the apatite trigonal channel formally substituting H atoms and forming bent dioxocobaltate(II) anions. The anion exhibits single-molecule-magnet (SMM) behavior: slow relaxation of magnetization below 8 K under a nonzero magnetic field with an energy barrier of 63 cm^-1. The barrier value does not depend on the concentration of Co ions, virtually coincides with the zero-field-splitting energy as determined from direct-current magnetization, and is very close to the value obtained earlier for cobalt-doped strontium hydroxyapatite. Moreover, the vibration frequencies of the dioxocobaltate(II) anion are found to be the same in calcium and strontium apatite matrixes. The very weak dependence of the SMM parameters on the matrix nature in combination with good chemical and thermal stabilities of the compounds provides wide opportunities to exploit the intrinsic properties of such a SMM-like anion.

Designing Single-Ion Magnets and Phosphorescent Materials with 1-Methylimidazole-5-carboxylate and Transition-Metal Ions

Detailed structural, magnetic, and photoluminescence (PL) characterization of four new compounds based on 1-methylimidazole-5-carboxylate (mimc) ligand and transition metal ions, namely [Ni(mimc)₂(H₂O)₄] (1), [Co(μ-mimc)₂]_n (2), {[Cu₂(μ-mimc)₄(H₂O)]·2H₂O}_n (3), and [Cd(μ-mimc)₂(H₂O)]_n (4) is reported. The structural diversity found in the family of compounds derives from the coordination versatility of the ligand, which coordinates as a terminal ligand to give a supramolecular network of monomeric entities in 1 or acts as a bridging linker to build isoreticular 2D coordination polymers (CPs) in 2-4. Magnetic direct-current (dc) susceptibility data have been measured for compounds 1-3 to analyze the exchange interactions among paramagnetic centers, which have been indeed supported by calculations based on broken symmetry (BS) and density functional theory (DFT) methodology. The temperature dependence of susceptibility and magnetization data of 2 are indicative of easy-plane anisotropy (D = +12.9 cm^-1, E = +0.5 cm^-1) that involves a bistable M_s = ±1/2 ground state. Alternating-current (ac) susceptibility curves exhibit field-induced single-ion magnet (SIM) behavior that occurs below 14 K, which is characterized by two spin relaxation processes of distinct nature: fast relaxation of single ions proceeding through multiple mechanisms (U_eff = 26 K) and a slow relaxation attributed to interactions along the polymeric crystal building. Exhaustive PL analysis of compound 4 in the solid state confirms low-temperature phosphorescent green emission consisting of radiative lifetimes in the range of 0.25-0.43 s, which explains the afterglow observed during about 1 s after the removal of the UV source. Time-dependent DFT and computational calculations to estimate phosphorescent vertical transitions have been also employed to provide an accurate description of the PL performance of this long-lasting phosphor.

Intra-molecular origin of the spin-phonon coupling in slow-relaxing molecular magnets

We perform a systematic investigation of the spin-phonon coupling leading to spin relaxation in the prototypical mononuclear single molecule magnet [(tpaPh)Fe]-. In particular we analyze in detail the nature of the most relevant vibrational modes giving rise to the relaxation. Our fully ab initio calculations, where the phonon modes are evaluated at the level of density functional theory and the spin-phonon coupling by mapping post-Hartree-Fock electronic structures onto an effective spin Hamiltonian, reveal that acoustic phonons are not active in the spin-phonon relaxation process of dilute SMMs crystals. Furthermore, we find that intra-molecular vibrational modes produce anisotropy tensor modulations orders of magnitude higher than those associated to rotations. In light of these results we are able to suggest new designing rules for spin-long-living SMMs which go beyond the tailoring of static molecular features but fully take into account dynamical features of the vibrational thermal bath evidencing those internal molecular distortions more relevant to the spin dynamics.

[MIII2MII3] n+ trigonal bipyramidal cages based on diamagnetic and paramagnetic metalloligands

A family of five [MIII2MII3] n+ trigonal bipyramidal cages (MIII = Fe, Cr and Al; MII = Co, Zn and Pd; n = 0 for 1-3 and n = 6 for 4-5) of formulae [Fe2Co3L6Cl6] (1), [Fe2Zn3L6Br6] (2), [Cr2Zn3L6Br6] (3), [Cr2Pd3L6(dppp)3](OTf)6 (4) and [Al2Pd3L6(dppp)3](OTf)6 (5) (where HL is 1-(4-pyridyl)butane-1,3-dione and dppp is 1,3-bis(diphenylphosphino)propane) are reported. Neutral cages 1-3 were synthesised using the tritopic [MIIIL3] metalloligand in combination with the salts CoIICl2 and ZnIIBr2, which both act as tetrahedral linkers. The assembly of the cis-protected [PdII(dppp)(OTf)2] with [MIIIL3] afforded the anionic cages 4-5 of general formula [MIII2PdII3](OTf)6. The metallic skeleton of all cages describes a trigonal bipyramid with the MIII ions occupying the two axial sites and the MII ions sitting in the three equatorial positions. Direct current (DC) magnetic susceptibility, magnetisation and heat capacity measurements on 1 reveal weak antiferromagnetic exchange between the FeIII and CoII ions. EPR spectroscopy demonstrates that the distortion imposed on the {MO6} coordination sphere of [MIIIL3] by complexation in the {MIII2MII3} supramolecules results in a small, but measurable, increase of the zero field splitting at MIII. Complete active space self-consistent field (CASSCF) calculations on the three unique CoII sites of 1 suggest DCo ≈ -14 cm-1 and E/D ≈ 0.1, consistent with the magnetothermal and spectroscopic data.

Recent progress in synchrotron-based frequency-domain Fourier-transform THz-EPR

We describe frequency-domain Fourier-transform THz-EPR as a method to assign spin-coupling parameters of high-spin (S>1/2) systems with very large zero-field splittings. The instrumental foundations of synchrotron-based FD-FT THz-EPR are presented, alongside with a discussion of frequency-domain EPR simulation routines. The capabilities of this approach is demonstrated for selected mono- and multinuclear HS systems. Finally, we discuss remaining challenges and give an outlook on the future prospects of the technique.

Impact of Halogenido Coligands on Magnetic Anisotropy in Seven-Coordinate Co(II) Complexes

The structural and magnetic features of a series of mononuclear seven-coordinate Co^II complexes with the general formula [Co(L)X₂], where L is a 15-membered pyridine-based macrocyclic ligand (3,12,18-triaza-6,9-dioxabicyclo[12.3.1]octadeca-1(18),14,16-triene) and X = Cl^- (1), Br^- (2), or I^- (3), were investigated experimentally and theoretically in order to reveal how the corresponding halogenido coligands in the apical positions of a distorted pentagonal-bipyramidal coordination polyhedron may affect the magnetic properties of the prepared compounds. The thorough analyses of the magnetic data revealed a large easy-plane type of the magnetic anisotropy (D > 0) for all three compounds, with the D-values increasing in the order 35 cm^-1 for 3 (I^-), 38 cm^-1 for 1 (Cl^-), and 41 cm^-1 for 2 (Br^-). Various theoretical methods like the Angular Overlap Model, density functional theory, CASSCF/CASPT2, CASSCF/NEVPT2 were utilized in order to understand the observed trend in magnetic anisotropy. The D-values correlated well with the Mayer bond order (decreasing in order Co-I > Co-Cl > Co-Br), which could be a consequence of two competing factors: (a) the ligand field splitting and (b) the covalence of the Co-X bond. All the complexes also behave as field-induced single-molecule magnets with the spin reversal barrier U_eff increasing in order 1 (Cl^-) < 2 (Br^-) < 3 (I^-); however, taking into account the easy-plane type of the magnetic anisotropy, the Raman relaxation process is most likely responsible for slow relaxation of the magnetization. The results of the work revealed that the previously suggested and fully accepted strategy employing heavier halogenido ligands in order to increase the magnetic anisotropy has some limitations in the case of pentagonal-bipyramidal Co^II complexes.

Spectroscopic and Computational Studies of Spin States of Iron(IV) Nitrido and Imido Complexes

High-oxidation-state metal complexes with multiply bonded ligands are of great interest for both their reactivity as well as their fundamental bonding properties. This paper reports a combined spectroscopic and theoretical investigation into the effect of the apical multiply bonded ligand on the spin-state preferences of threefold symmetric iron(IV) complexes with tris(carbene) donor ligands. Specifically, singlet (S = 0) nitrido [{PhB(Im^R)₃}FeN], R = ^tBu (1), Mes (mesityl, 2) and the related triplet (S = 1) imido complexes, [{PhB(Im^R)₃}Fe(NR')]⁺, R = Mes, R' = 1-adamantyl (3), ^tBu (4), were investigated by electronic absorption and Mössbauer effect spectroscopies. For comparison, two other Fe(IV) nitrido complexes, [(TIMEN^Ar)FeN]⁺ (TIMEN^Ar = tris[2-(3-aryl-imidazol-2-ylidene)ethyl]amine; Ar = Xyl (xylyl), Mes), were investigated by ⁵⁷Fe Mössbauer spectroscopy, including applied-field measurements. The paramagnetic imido complexes 3 and 4 were also studied by magnetic susceptibility measurements (for 3) and paramagnetic resonance spectroscopy: high-frequency and -field electron paramagnetic resonance (for 3 and 4) and frequency-domain Fourier-transform (FD-FT) terahertz electron paramagnetic resonance (for 3), which reveal their zero-field splitting parameters. Experimentally correlated theoretical studies comprising ligand-field theory and quantum chemical theory, the latter including both density functional theory and ab initio methods, reveal the key role played by the Fe 3d_z² (a₁) orbital in these systems: the nature of its interaction with the nitrido or imido ligand dictates the spin-state preference of the complex. The ability to tune the spin state through the energy and nature of a single orbital has general relevance to the factors controlling spin states in complexes with applicability as single molecule devices.

The role of anharmonic phonons in under-barrier spin relaxation of single molecule magnets

The use of single molecule magnets in mainstream electronics requires their magnetic moment to be stable over long times. One can achieve such a goal by designing compounds with spin-reversal barriers exceeding room temperature, namely with large uniaxial anisotropies. Such strategy, however, has been defeated by several recent experiments demonstrating under-barrier relaxation at high temperature, a behaviour today unexplained. Here we propose spin–phonon coupling to be responsible for such anomaly. With a combination of electronic structure theory and master equations we show that, in the presence of phonon dissipation, the relevant energy scale for the spin relaxation is given by the lower-lying phonon modes interacting with the local spins. These open a channel for spin reversal at energies lower than that set by the magnetic anisotropy, producing fast under-barrier spin relaxation. Our findings rationalize a significant body of experimental work and suggest a possible strategy for engineering room temperature single molecule magnets.

Single molecule magnets exhibit faster spin relaxation rates than expected from models based on tunnelling through the relaxation barrier. Here, the authors show, using first principles calculations, that anharmonic spin-phonon interactions may explain the under-barrier spin relaxation.

Magnetic Anisotropy and Field-induced Slow Relaxation of Magnetization in Tetracoordinate CoII Compound [Co(CH₃-im)₂Cl₂]

Static and dynamic magnetic properties of the tetracoordinate Co^II complex [Co(CH₃-im)₂Cl₂], (1, CH₃-im = N-methyl-imidazole), studied using thorough analyses of magnetometry, and High-Frequency and -Field EPR (HFEPR) measurements, are reported. The study was supported by ab initio complete active space self-consistent field (CASSCF) calculations. It has been revealed that 1 possesses a large magnetic anisotropy with a large rhombicity (magnetometry: D = −13.5 cm⁻¹, E/D = 0.33; HFEPR: D = −14.5(1) cm⁻¹, E/D = 0.16(1)). These experimental results agree well with the theoretical calculations (D = −11.2 cm⁻¹, E/D = 0.18). Furthermore, it has been revealed that 1 behaves as a field-induced single-ion magnet with a relatively large spin-reversal barrier (U_eff = 33.5 K). The influence of the Cl–Co–Cl angle on magnetic anisotropy parameters was evaluated using the CASSCF calculations.

Cobalt-Based Single-Ion Magnets on an Apatite Lattice: Toward Patterned Arrays for Magnetic Memories

Single-ion magnets (SIMs) that can maintain magnetization direction on an individual transition metal atom represent the smallest atomic-scale units for future magnetic data storage devices and molecular electronics. Here we present a robust extended inorganic solid hosting efficient SIM centers, as an alternative to molecular SIM crystals. We show that unique dioxocobaltate(II) ions, confined in the channels of strontium hydroxyapatite, exhibit classical SIM features with a large energy barrier for magnetization reversal (U_eff) of 51-59 cm^-1. The samples have been tuned such that a magnetization hysteresis opens below 8 K and U_eff increases by a factor of 4 and can be further enhanced to the highest values among 3d metal complexes of 275 cm^-1 when Ba is substituted for Sr. The SIM properties are preserved without any tendency toward spin ordering up to a high Co concentration. At a maximal Co content, a hypothetical regular hexagonal grid of SIMs with a 1 nm interspacing on the (001) crystal facet would allow a maximal magnetic recording density of 10⁵ Gb/cm².

Cobalt(II) chloride adducts with acetonitrile, propan-2-ol and tetrahydrofuran: considerations on nuclearity, reactivity and synthetic applications

High-spin cobalt(II) complexes are considered useful building blocks for the synthesis of single-molecule magnets (SMM) because of their intrinsic magnetic anisotropy. In this work, three new cobalt(II) chloride adducts with labile ligands have been synthesized from anhydrous CoCl2, to be subsequently employed as starting materials for heterobimetallic compounds. The products were characterized by elemental, spectroscopic (EPR and FT–IR) and single-crystal X-ray diffraction analyses.trans-Tetrakis(acetonitrile-κN)bis(tetrahydrofuran-κO)cobalt(II) bis[(acetonitrile-κN)trichloridocobaltate(II)], [Co(C2H3N)4(C4H8O)2][CoCl3(C2H3N)]2, (1), comprises mononuclear ions and contains both acetonitrile and tetrahydrofuran (thf) ligands, The coordination polymercatena-poly[[tetrakis(propan-2-ol-κO)cobalt(II)]-μ-chlorido-[dichloridocobalt(II)]-μ-chlorido], [Co2Cl4(C3H8O)4], (2′), was prepared by direct reaction between anhydrous CoCl2and propan-2-ol in an attempt to rationalize the formation of the CoCl2–alcohol adduct (2), probably CoCl2(HOiPr)m. The binuclear complex di-μ-chlorido-1:2κ4Cl:Cl-dichlorido-2κ2Cl-tetrakis(tetrahydrofuran-1κO)dicobalt(II), [Co2Cl4(C4H8O)4], (3), was obtained from (2) after recrystallization from tetrahydrofuran. All three products present cobalt(II) centres in both octahedral and tetrahedral environments, the former usually less distorted than the latter, regardless of the nature of the neutral ligand. Product (2′) is stabilized by an intramolecular hydrogen-bond network that appears to favour atransarrangement of the chloride ligands in the octahedral moiety; this differs from thecisdisposition found in (3). The expected easy displacement of the bound solvent molecules from the metal coordination sphere makes the three compounds good candidates for suitable starting materials in a number of synthetic applications.

End-to-end azido-pinned interlocking lanthanide squares

A self-assembled end-to-end azido-pinned interlocking lanthanide square displays a record energy barrier of 152(4) K among lanthanide azido-bridged SMMs in a zero dc field.

Heavy ligand atom induced large magnetic anisotropy in Mn(ii) complexes

In the search for single molecule magnets, metal ions are considered pivotal towards achieving large magnetic anisotropy barriers.

Determination of zero-field splitting in Co2+ halide complexes with magnetic and far-IR measurements

The ZFS parameters are determined by magnetic measurements and far-infrared magneto-transmission spectra for Co²⁺ complexes with halide substitutes.

A mixed-valence metallogrid [CoIII2CoII2] with an unusual electronic structure and single-ion-magnet characterization

Magnetically isolated weakly coordinated cobalt(ii) ions with an unusual electronic structure in a mixed-valence metallogrid show field-induced slow magnetization relaxation.

Three novel mononuclear Mn(iii)-based magnetic materials with square pyramidal versus octahedral geometries

Single crystal X-ray diffraction studies reveal that complexes 1 and 2 have square pyramidal geometry and 3 has octahedral geometry, which showed a dependence of negative anisotropy (D) values on the electronic, geometry and packing effects.

Synthesis, crystal structure and magnetic properties of [Co(bpcam)2]ClO4·dmso·H2O, [Co(bpcam)2]2[Co(NCS)4]·dmso·H2O and [Ni(bpcam)2]·H2O [Hbpcam = bis(2-pyrimidylcarbonyl)amide]

Synthesis, crystal structures, spectroscopy and magnetic properties of three new bpcam-containing complexes are presented here.

Probing the influence of molecular symmetry on the magnetic anisotropy of octahedral cobalt(ii) complexes

A series of field-induced cobalt(ii) SIMs exhibit varying axial zero-field splitting parameter D values from positive to negative with the increased distortion of the octahedral geometry.

On balancing the QTM and the direct relaxation processes in single-ion magnets – the importance of symmetry control

Two mononuclear trigonal-planar Co(ii) complexes with a similar coordination environment except for the symmetries were reported to exhibit distinct relaxation dynamics due to the effect of the QTM and direct process.

Dependence of magnetic coupling on ligands at the axial positions of NiII in phenoxido bridged dimers: experimental observations and DFT studies

Experimental and theoretical results reveal that the magnetic coupling in diphenoxido bridged dinuclear Ni^II compounds is strongly dependent on the axially coordinated non-bridging ligands.