Six-Coordinated CoII Single-Molecule Magnets: Synthetic Strategy, Structure and Magnetic Properties

The pursuit of molecule-based magnetic memory materials contributes significantly to high-density information storage research in the frame of the ongoing information technologies revolution. Remarkable progress has been achieved in both transition metal (TM) and lanthanide based single-molecule magnets (SMMs). Notably, six-coordinated CoII SMMs hold particular research significance owing to the economic and abundant nature of 3d TM ions compared to lanthanide ions, the substantial spin-orbit coupling of CoII ions, the potential for precise control over coordination geometry, and the air-stability of coordination-saturated structures. In this review, we will summarize the progress made in six-coordinated CoII SMMs, organized by their coordination geometry and molecular structure similarity. Valuable insights, principles, and new mechanism gleaned from this research and remaining issues that need to be addressed will also be discussed to guide future optimization.

Control of the geometry and anisotropy driven by the combination of steric and anion coordination effects in CoII complexes with N6-tripodal ligands: the impact of the size of the ligand on the magnetization relaxation time

Four mononuclear CoII complexes of formula [Co(L)(SCN)2(CH3OH)0.5(H2O)0.5]·1.5H2O·0.75CH3OH (1), [Co(L1)Cl2]·H2O·2CH3CN (2), [Co(L1)(SCN)2]·1.5H2O·CH3OH (3) and [Co(L1)]ClO4·2CH3OH (4) were prepared from the N6-tripodal Schiff base ligands (S)P[N(Me)NC(H)2-Q]3 (L) and (S)P[N(Me)NC(H)1-ISOQ]3 (L1), where Q and ISOQ represent quinolyl and isoquinolyl moieties, respectively. In 1, the L ligand does not coordinate to the CoII ion in a tripodal manner but using a new N,N,S tridentate mode, which is due to the fact that the N6-tripodal coordination promotes a strong steric hindrance between the quinolyl moieties. However, L1 can coordinate to the CoII ions either in a tripodal manner using CoII salts with poorly coordinating anions to give 4 or in a bisbidentate fashion using CoII salt-containing medium to strongly coordinating anions to afford 2 and 3. In the case of L1, there is no steric hindrance between ISOQ moieties after coordination to the CoII ion. The CoII ion exhibits a distorted octahedral geometry for compounds 1-3, with the anions in cis positions for the former and in trans positions for the two latter compounds. Compound 4 shows an intermediate geometry between an octahedral and trigonal prism but closer to the latter one. DC magnetic properties, HFEPR and FIRMS measurements and ab initio calculations demonstrate that distorted octahedral complexes 1-3 exhibit easy-plane magnetic anisotropy (D > 0), whereas compound 4 shows large easy-axis magnetic anisotropy (D < 0). Comparative analysis of the magneto-structural data underlines the important role that is played not only by the coordination geometry but also the electronic effects in determining the anisotropy of the CoII ions. Compounds 2-3 show a field-induced slow relaxation of magnetization. Despite its large easy-axis magnetic anisotropy, compound 4 does not show significant slow relaxation (SMR) above 2 K under zero applied magnetic fields, but its magnetic dilution with ZnII triggers SMR at zero field. Finally, it is worth remarking that compounds 2-4 show smaller relaxation times than the analogous complexes with the tripodal ligand bearing in its arms pyridine instead of isoquinoline moieties, which is most likely due to the increase of the molecular size in the former one.

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.

Tetracoordinate Co(II) complexes with semi-coordination as stable single-ion magnets for deposition on graphene

We present a theoretical and experimental study of two tetracoordinate Co(II)-based complexes with semi-coordination interactions, i.e., non-covalent interactions involving the central atom. We argue that such interactions enhance the thermal and structural stability of the compounds, making them appropriate for deposition on substrates, as demonstrated by their successful deposition on graphene. DC magnetometry and high-frequency electron spin resonance (HF-ESR) experiments revealed an axial magnetic anisotropy and weak intermolecular antiferromagnetic coupling in both compounds, supported by theoretical predictions from complete active space self-consistent field calculations complemented by N-electron valence state second-order perturbation theory (CASSCF-NEVPT2), and broken-symmetry density functional theory (BS-DFT). AC magnetometry demonstrated that the compounds are field-induced single-ion magnets (SIMs) at applied static magnetic fields, with slow relaxation of magnetization governed by a combination of quantum tunneling, Orbach, and direct relaxation mechanisms. The structural stability under ambient conditions and after deposition was confirmed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Theoretical modeling by DFT of different configurations of these systems on graphene revealed n-type doping of graphene originating from electron transfer from the deposited molecules, confirmed by electrical transport measurements and Raman spectroscopy.

Alkyl Chains Modulated Magnetization Dynamics of Mononuclear Trigonal Prismatic CoII Complexes

Four benzeneboron-capped mononuclear CoII complexes with different alkyl substitutions on the fourth position of phenylboronic acid were obtained. The CoII ions are all wrapped by the pocket-like ligands and located in trigonal prismatic coordination geometries. Alternating-current magnetic susceptibility measurements reveal that they show different magnetization dynamics, such as distinct relaxation rates at the same temperature, the faster QTM rates for the ethyl and propyl substituted complexes, as well as different relaxation processes. Magneto-structural correlation study reveals that the various deviations of coordination geometry of CoII ion, diverse crystal packings and possible different vibration modes of substituents caused by modifying alkyl chains are the key factors affecting the magnetization dynamics. This work demonstrates that the alkyl chains even locating far away from the metal center can have a large impact on the magnetic behavior of the CoII complex with a very rigid coordination geometry, offering a new perspective towards transition metal based single-molecule magnets.

Trigonally Distorted Hexacoordinate Co(II) Single-Ion Magnets

By simple reactions involving various cobalt(II) carboxylates (acetate and in situ prepared pivalate and 4-hydroxybenzoate salts) and neocuproine (neo), we were able to prepare three different carboxylate complexes with the general formula [Co(neo)(RCOO)₂] (R = –CH₃ for 1, (CH₃)₃C– for 2, and 4OH-C₄H₆– for 3). The [Co(neo)(RCOO)₂] molecules in the crystal structures of 1–3 adopt a rather distorted coordination environment, with the largest trigonal distortion observed for 1, whereas 2 and 3 are similarly distorted from ideal octahedral geometry. The combined theoretical and experimental investigations of magnetic properties revealed that the spin Hamiltonian formalism was not a valid approach and the L-S Hamiltonian had to be used to reveal very large magnetic anisotropies for 1–3. The measurements of AC susceptibility showed that all three compounds exhibited slow-relaxation of magnetization in a weak external static magnetic field, and thus can be classified as field-induced single-ion magnets. It is noteworthy that 1 also exhibits a weak AC signal in a zero-external magnetic field.

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...

Large easy-axis magnetic anisotropy in a series of trigonal prismatic mononuclear cobalt (II) complexes with zero-field hidden single-molecule magnet behaviour: The important role of the distortion of the coordination sphere and intermolecular interactions on the slow relaxation

The complexes [Co(L)]X·S (X = CoCl42- , S = CH3CN (1); X = ZnCl42- , S = CH3OH (2)), [Co(L)]X2·S (X = ClO4-, S = 2CH3OH (3) and X =...

Modulation of Magnetic Anisotropy and Exchange Interaction in Phenoxide-Bridged Dinuclear Co(II) Complexes

We report a new class of four dimeric Co(II) complexes [Co₂(bbpen)(X)₂] (H₂bbpen = N,N'-bis(2-hydroxybenzyl)-N,N'-bis(2-methylpyridyl)ethylenediamine) [X^- = SCN (1), Cl (2), Br (3), and I (4)] with different coordination geometry of two Co(II) centers (trigonal-prismatic and pseudo-tetrahedral) and their magnetic study. Interestingly, the two Co(II) centers show two different types of magnetic anisotropy. State of the art ab initio CASSCF analysis reveals that the six-coordinate or the trigonal-prismatic Co(II) center possesses a consistently large negative axial zero-field splitting (negative D) parameter (∼-60 cm^-1), while the four-coordinate or the pseudo-tetrahedral Co(II) center exhibits a range of D values from +13 to -23 cm^-1. Ab initio calculations employing the lines model were used to estimate the magnetic exchange as both the Co(II) centers possess significant magnetic anisotropy. All the complexes display rare ferromagnetic interaction, and the strength of this interaction decreases as the ligand field on the pseudo-tetrahedral Co(II) center decreases from SCN^- > Cl^- > Br^- > I^-.

Zero-field slow relaxation of magnetization in cobalt(ii) single-ion magnets: suppression of quantum tunneling of magnetization by tailoring the intermolecular magnetic coupling

The correlation between magnetic relaxation dynamics and the alignment of single-ion magnets (SIMs) in a crystal was investigated using four analogous cobalt(ii) complexes with unique hydrogen-bond networks. The hydrogen-bonding interactions in the crystals resulted in a relatively short intermolecular Co⋯Co distance, which led to non-zero intermolecular magnetic coupling. All the complexes with a Co⋯Co distance shorter than 6.5 Å exhibited zero-field slow magnetic relaxation as weak magnetic interactions split the ground ±Ms levels and suppressed quantum tunneling of magnetization (QTM). In particular, antiferromagnetically coupled one-dimensional chain SIM networks effectively suppressed QTM when the two intrachain Co⋯Co distances were non-equivalent. However, when the two distances in a chain were equivalent and each molecular symmetry axis aligned parallell within the chain, QTM suppression was insufficient because magnetic coupling from the adjacent molecules was virtually cancelled. Partial substitution of the CoII ion with the diamagnetic ZnII ion up to 33% for this complex resulted in complete QTM suppression in the absence of an external field. These results show that the manipulation of intermolecular distances and alignments is effective for suppressing undesired QTM events in SIMs.

Role of Coordination Number and Geometry in Controlling the Magnetic Anisotropy in FeII , CoII , and NiII Single-Ion Magnets

Since the last decade, the focus in the area of single-molecule magnets (SMMs) has been shifting constructively towards the development of single-ion magnets (SIMs) based on transition metals and lanthanides. Although ground-breaking results have been witnessed for Dy^III -based SIMs, significant results have also been obtained for some mononuclear transition metal SIMs. Among others, studies based on Co^II ion are very prominent as they often exhibit high magnetic anisotropy or zero-field splitting parameters and offer a large barrier height for magnetisation reversal. Although Co^II possibly holds the record for having the largest number of zero-field SIMs known for any transition metal ion, controlling the magnetic anisotropy in these systems are is still a challenge. In addition to the modern spectroscopic techniques, theoretical studies, especially ab initio CASSCF/NEVPT2 approaches, have been used to uncover the electronic structure of various Co^II SIMs. In this article, with some selected examples, the aim is to showcase how varying the coordination number from two to eight, and the geometry around the Co^II centre alters the magnetic anisotropy. This offers some design principles for the experimentalists to target new generation SIMs based on the Co^II ion. Additionally, some important Fe^II /Fe^III and Ni^II complexes exhibiting large magnetic anisotropy and SIM properties are also discussed.

Paramagnetic Organocobalt Capsule Revealing Xenon Host-Guest Chemistry

We investigated Xe binding in a previously reported paramagnetic metal-organic tetrahedral capsule, [Co4L6]4-, where L2- = 4,4'-bis[(2-pyridinylmethylene)amino][1,1'-biphenyl]-2,2'-disulfonate. The Xe-inclusion complex, [XeCo4L6]4-, was confirmed by 1H NMR spectroscopy to be the dominant species in aqueous solution saturated with Xe gas. The measured Xe dissociation rate in [XeCo4L6]4-, koff = 4.45(5) × 102 s-1, was at least 40 times greater than that in the analogous [XeFe4L6]4- complex, highlighting the capability of metal-ligand interactions to tune the capsule size and guest permeability. The rapid exchange of 129Xe nuclei in [XeCo4L6]4- produced significant hyperpolarized 129Xe chemical exchange saturation transfer (hyper-CEST) NMR signal at 298 K, detected at a concentration of [XeCo4L6]4- as low as 100 pM, with presaturation at -89 ppm, which was referenced to solvated 129Xe in H2O. The saturation offset was highly temperature-dependent with a slope of -0.41(3) ppm/K, which is attributed to hyperfine interactions between the encapsulated 129Xe nucleus and electron spins on the four CoII centers. As such, [XeCo4L6]4- represents the first example of a paramagnetic hyper-CEST (paraHYPERCEST) sensor. Remarkably, the hyper-CEST 129Xe NMR resonance for [XeCo4L6]4- (δ = -89 ppm) was shifted 105 ppm upfield from the diamagnetic analogue [XeFe4L6]4- (δ = +16 ppm). The Xe inclusion complex was further characterized in the crystal structure of (C(NH2)3)4[Xe0.7Co4L6]·75 H2O (1). Hydrogen bonding between capsule-linker sulfonate groups and exogenous guanidinium cations, (C(NH2)3)+, stabilized capsule-capsule interactions in the solid state and also assisted in trapping a Xe atom (∼42 Å3) in the large (135 Å3) cavity of 1. Magnetic susceptibility measurements confirmed the presence of four noninteracting, magnetically anisotropic high-spin CoII centers in 1. Furthermore, [Co4L6]4- was found to be stable toward aggregation and oxidation, and the CEST performance of [XeCo4L6]4- was unaffected by biological macromolecules in H2O. These results recommend metal-organic capsules for fundamental investigations of Xe host-guest chemistry as well as applications with highly sensitive 129Xe-based sensors.

Incorporating Trigonal-Prismatic Cobalt(II) Blocks into an Exchange-Coupled [Co2Cu] System

Taking advantage of a rigid tetradentate ligand of bis(pyrazoly)(3-pyrazolypyridinyl)methane (PyPz₃) and the [Cu^II(opba)]^2- unit [opba^4- = o-phenylenebis(oxamato)], the trinuclear complex [{Co^II(PyPz₃)}₂Cu^II(opba)][ClO₄]₂·5MeCN·MeOH (1) was constructed, in which the Co^II centers adopt a trigonal-prismatic geometry, while considerable intramolecular magnetic coupling was successfully introduced through the oxamido bridges, representing another very first example of single-molecule magnets marrying both selected coordination geometry and magnetic exchanges.

Bulky Schiff-base ligand supported Co(ii) single-ion magnets with zero-field slow magnetic relaxation

Two mononuclear Co(ii) complexes with tetrahedral coordination geometry have been constructed from different bulky Schiff-base ligands. Both complexes exhibit slow magnetic relaxation without a static field and their relaxation behaviors can be tuned by ligand substitution. Clear magnetic hysteresis loops were observed for both complexes at 2 K.

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.

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.

Long magnetic relaxation time of tetracoordinate Co2+ in imidazo[1,5-a]pyridinium-based (C13H12N3)2[CoCl4] hybrid salt and [Co(C13H12N3)Cl3] molecular complex

The novel organic-inorganic hybrid salt [L]₂[CoCl₄] (1) and molecular complex [CoLCl₃] (2), where L⁺ is 2-methyl-3-(pyridin-2-yl)imidazo[1,5-a]pyridinium cation, feature simple {CoCl₄} and {CoCl₃N} tetrahedral environments of negligible (1) and a slightly higher distortion (2) that are responsible for rather low positive magnetic anisotropy of Co^II ion with D/hc = 12.1(6) (1) and 19.4(15) cm^-1 (2). Both compounds exhibit field-induced slow magnetic relaxation with three relaxation channels [low- (LF), intermediate- and high-frequency (HF) modes] that is frequency and field dependent. With the increased DC field, the peaks referring to the LF relaxation path are moved to lower frequencies so that the applied DC field causes prolongation of the relaxation time. The opposite is true for the HF relaxation branch: the peak is moved to higher frequencies. Considering the simplicity of the coordination environment and moderate magnetic anisotropy of the metal ion in 1 and 2, the compounds are unique with respect to the remarkably long relaxation time for a given applied DC field and temperature: τ_LF = 0.54(4) s at B_DC = 1.0 T and T = 2.0 K for 1, and τ_LF = 1.8(2) s at B_DC = 1.2 T and T = 1.9 K for 2.

Coordination [CoII2] and [CoIIZnII] Helicates Showing Slow Magnetic Relaxation

The slow magnetic relaxation of Co^II ions in the elusive intermediate geometry between the trigonal prism and antiprism has been studied on the new [Co₂L₃]⁴⁺ and [CoZnL₃]⁴⁺ coordination helicates [L is a bis(pyrazolylpyridine) ligand]. Solution paramagnetic ¹H NMR and solid-state magnetization measurements unveil single-molecule-magnet behavior with small axial anisotropy, as predicted previously.

Anionic guest-dependent slow magnetic relaxation in Co(ii) tripodal iminopyridine complexes

We report the syntheses and magnetic property characterizations of four mononuclear cobalt(ii) complex salts featuring a tripodal iminopyridine ligand with external anion receptor groups, [CoL5-ONHtBu]X2 (X = Cl (1), Br (2), I (3) and ClO4 (4)). While all four salts exhibit anion binding through pendant amide moieties, only in the case of 1 is field-induced slow relaxation of magnetisation observed, whereas in the other salts this phenomenon is absent at the limits of our instrumentation. The effect of chloride inducing a seventh Co-N interaction and concomitant structural distortion is hypothesized as the origin of the observed dynamic magnetic properties observed in 1. Ab initio computational studies carried out on a 7-coordinate Co(ii) model species survey the complex interplay of coordination number and trigonal twisting on the sign and magnitude of the axial anisotropy parameter (D), and identify structural features whose distortions can trigger large switches in the sign and magnitude of magnetic anisotropy.

A Trigonal Prismatic Cobalt(II) Complex as a Single Molecule Magnet with a Reduced Contribution from Quantum Tunneling

Herein, we report a new trigonal prismatic cobalt(II) complex that behaves as a single molecule magnet. The obtained zero-field splitting, which is also directly accessed by THz-EPR spectroscopy (-102.5 cm^-1 ), results in a large magnetization reversal barrier U of 205 cm^-1 . Its effective value, however, is much lower (101 cm^-1 ), even though there is practically no contribution from quantum tunneling to magnetization relaxation.

Hydrogen-bonding interactions and magnetic relaxation dynamics in tetracoordinated cobalt(ii) single-ion magnets

Zero field slow magnetic relaxation was observed in two cobalt(ii) complexes with 1-D chain hydrogen-bonded structures.

Synthesis and Characterization of Cobalt(II) N,N'-Diphenylazodioxide Complexes

Removal of chloride from CoCl₂ with TlPF₆ in acetonitrile, followed by addition of excess nitrosobenzene, yielded the eight-coordinate cobalt(II) complex salt [Co{Ph(O)NN(O)Ph}₄](PF₆)₂, shown by single-crystal X-ray analysis to have a distorted tetragonal geometry. The analogous treatment of the bipyridyl complex Co(bpy)Cl₂ yielded the mixed-ligand cobalt(II) complex salt [Co(bpy){Ph(O)NN(O)Ph}₂](PF₆)₂, whose single-crystal X-ray structure displays a trigonal prismatic geometry, similar to that of the iron(II) cation in the previously known complex salt [Fe{Ph(O)NN(O)Ph}₃](FeCl₄)₂. The use of TlPF₆ to generate solvated metal complex cations from chloride salts or chlorido complexes, followed by the addition of nitrosobenzene, is shown to be a useful synthetic strategy for the preparation of azodioxide complex cations with the noncoordinating, diamagnetic PF₆ ^- counteranion. Coordination number appears to be more important than d electron count in determining the geometry and metal-ligand bond distances of diphenylazodioxide complexes.

Field-induced single-molecule magnet behavior in ideal trigonal antiprismatic cobalt(ii) complexes: precise geometrical control by a hydrogen-bonded rigid metalloligand

A new cobalt(ii) complex bearing a pair of cobalt(iii) tris-chelate complexes as metalloligands was prepared. The CoII ion possesses an ideal trigonal antiprismatic geometry because of the intermolecular hydrogen-bonds between the metalloligands via counter anions. This complex exhibits slow magnetic relaxation under a dc field reminiscent of a single-molecule magnet behavior.

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.

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.

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.

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.