Magneto-Structural Correlations in a Series of Pseudotetrahedral [CoII(XR)4]2– Single Molecule Magnets: An ab Initio Ligand Field Study

The Structural and Magnetic Properties of FeII and CoII Complexes with 2-(furan-2-yl)-5-pyridin-2-yl-1,3,4-oxadiazole

Two novel coordination compounds containing heterocyclic bidentate N,N-donor ligand 2-(furan-2-yl)-5-(pyridin-2-yl)-1,3,4-oxadiazole (fpo) were synthesized. A general formula for compounds originating from perchlorates of iron, cobalt, and fpo can be written as: [M(fpo)₂(H₂O)₂](ClO₄)₂ (M = Fe(II) for (1) Co(II) for (2)). The characterization of compounds was performed by general physico-chemical methods-elemental analysis (EA), Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR) in case of organics, and single crystal X-ray diffraction (sXRD). Moreover, magneto-chemical properties were studied employing measurements in static field (DC) for 1 and X-band EPR (Electron paramagnetic resonance), direct current (DC), and alternating current (AC) magnetic measurements in case of 2. The analysis of DC magnetic properties revealed a high spin arrangement in 1, significant rhombicity for both complexes, and large magnetic anisotropy in 2 (D = -21.2 cm^-1). Moreover, 2 showed field-induced slow relaxation of the magnetization (U_eff = 65.3 K). EPR spectroscopy and ab initio calculations (CASSCF/NEVPT2) confirmed the presence of easy axis anisotropy and the importance of the second coordination sphere.

Entrapment of a Pseudo-Tetrahedral CoII Center by Thioether Sulfur Bound {Co2 (μ-L)} Fragments: Synthesis, Field-Induced Single-Ion Magnetism and Catechol Oxidase Mimicking Activity

Simultaneous incorporation of both Co^II and Co^III ions within a new thioether S-bearing phenol-based ligand system, H₃ L (2,6-bis-[{2-(2-hydroxyethylthio)ethylimino}methyl]-4-methylphenol) formed [Co₅ ] aggregates [Co^II Co^III ₄ L₂ (μ-OH)₂ (μ_1,3 -O₂ CCH₃ )₂ ](ClO₄ )₄ ⋅H₂ O (1) and [Co^II Co^III ₄ L₂ (μ-OH)₂ (μ_1,3 -O₂ CC₂ H₅ )₂ ](ClO₄ )₄ ⋅H₂ O (2). The magnetic studies revealed axial zero-field splitting (ZFS) parameter, D/hc=-23.6 and -24.3 cm^-1 , and E/D=0.03 and 0.00, respectively for 1 and 2. Dynamic magnetic data confirmed the complexes as SIMs with U_eff /k_B =30 K (1) and 33 K (2), and τ₀ =9.1×10^-8  s (1), and 4.3×10^-8  s (2). The larger atomic radius of S compared to N gave rise to less variation in the distortion of tetrahedral geometry around central Co^II centers, thus affecting the D and U_eff /k_B values. Theoretical studies also support the experimental findings and reveal the origin of the anisotropy parameters. In solutions, both 1 and 2 which produce {Co^III ₂ (μ-L)} units, display solvent-dependent catechol oxidation behavior toward 3,5-di-tert-butylcatechol in air. The presence of an adjacent Co^III ion tends to assist the electron transfer from the substrate to the metal ion center, enhancing the catalytic oxidation rate.

Correlating magnetic anisotropy with the subtle coordination geometry variation of a series of cobalt(ii)-sulfonamide complexes

Systematic substitution on the N-(pyridine-2-ylmethyl)-sulfonamide ligand leads to the subtle variation of the CoN4 coordination geometry in a series of cobalt(ii) complexes sharing the common formula of Co[R1(C6N2H5)R2]2, where R1 = H, R2 = 4-tert-butylphenylsulfonyl (tBuphs) 1, R2 = 5-(dimethylamino)naphthalen-1-ylsulfonyl (DNps) 2, R2 = mesitylsulfonyl (Ms) 3, R2 = tosyl (Tos) 4, and R2 = naphthalen-1-ylsulfonyl (Nps) 5; R1 = Me, R2 = tBuphs 6. Magnetic studies show that the axial zero-field splitting parameter (D) is subtlely correlated with the coordination geometric variation subjected to the peripheral substituted groups. Specifically, the distortion from the ideal tetrahedral geometry (Td symmetry) to the seesaw geometry (D2d symmetry) increases uniaxial magnetic anisotropy. The degree of distortion measured by the continuous symmetry measure (CSM) shows that a narrow interval of CSM (6-7), which corresponds to 14-15 degree deviation from the standard tetrahedron, is ideal for maximising the D value in this coordination geometry, while the direction of the D tensor is less sensitive to such a structural variation.

Chemical Kinetics Approves the Occurrence of C (3Pj) Reaction with H2O

Although both atomic carbon and water are omnipresent in human life, there is a debate about the possibility of carbon reaction with water. Some low-temperature spectroscopic investigations have rejected the reaction, whereas some room-temperature experiments and theoretical studies have accepted the possibility of the reaction by reporting rate coefficients ranging from 10⁵ to 10⁹ L mol^-1 s^-1. This study provides new lines of evidence about the reaction through exploration of the reaction mechanism using the CCSD(T) method and solving the corresponding master equation by following two main approaches. According to the results, the rate coefficient of the reaction is significantly influenced by the tunneling and hindered rotation effects, in addition to the selected total angular momentum (J). Furthermore, the total rate coefficient of the reaction increases dramatically (from 10⁷ to 10¹¹ L mol^-1 s^-1) with the rise of temperature from 100 to 4000 K, while the total rate coefficient is insensitive to pressure (0.1-10 atm). Despite some differences between the results of the two approaches, the rate coefficients of both methods are consistent with the previously reported rate coefficients. Also, in agreement with the previous studies, the major products are ²HOC + ²H and ²HCO + ²H. In general, the findings approve the occurrence of the title reaction and indicate that the mentioned conflict is due to the sensitivity of the reaction to the investigated temperature and J level. The sensitivity does not permit low-temperature spectroscopic studies to detect any products and varies the measured and calculated rate coefficients.

Main Group Chemistry at the Interface with Molecular Magnetism

Innovative synthetic coordination and, increasingly, organometallic chemistry are at the heart of advances in molecular magnetism. Smart ligand design is essential for implementing controlled modifications to the electronic structure and magnetic properties of transition metal and f-element compounds, and many important recent developments use nontraditional ligands based on low-coordinate main group elements to drive the field forward. This review charts progress in molecular magnetism from the perspective of ligands in which the donor atoms range from low-coordinate 2p elements-particularly carbon but also boron and nitrogen-to the heavier p-block elements such as phosphorus, arsenic, antimony, and even bismuth. Emphasis is placed on the role played by novel main group ligands in addressing magnetic anisotropy of transition metal and f-element compounds, which underpins the development of single-molecule magnets (SMMs), a family of magnetic materials that can retain magnetization in the absence of a magnetic field below a blocking temperature. Nontraditional p-block donor atoms, with their relatively diffuse valence orbitals and more diverse bonding characteristics, also introduce scope for tuning the spin-orbit coupling properties and metal-ligand covalency in molecular magnets, which has implications in areas such as magnetic exchange coupling and spin crossover phenomena. The chemistry encompasses transition metals, lanthanides, and actinides and describes recently discovered molecular magnets that can be regarded, currently, as defining the state of the art. This review identifies that main group chemistry at the interface molecular magnetism is an area with huge potential to deliver new types of molecular magnets with previously unseen properties and applications.

Zero-Field Splitting Parameters from Four-Component Relativistic Methods

We report an approach for determination of zero-field splitting parameters from four-component relativistic calculations. Our approach involves neither perturbative treatment of spin-orbit interaction nor truncation of the spin-orbit coupled states. We make use of a multi-state implementation of relativistic complete active space perturbation theory (CASPT2), partially contracted N-electron valence perturbation theory (NEVPT2), and multi-reference configuration interaction theory (MRCI), all with the fully internally contracted ansatz. A mapping is performed from the Dirac Hamiltonian to the pseudospin Hamiltonian, using correlated energies and the magnetic moment matrix elements of the reference wave functions. Direct spin-spin coupling is naturally included through the full 2-electron Breit interaction. Benchmark calculations on chalcogen diatomics and pseudotetrahedral cobalt(II) complexes show accuracy comparable to the commonly used state-interaction with spin-orbit (SI-SO) approach, while tests on a uranium(III) single-ion magnet suggest that for actinide complexes the strengths of our approach through the more robust treatment of spin-orbit effects and the avoidance of state truncation are of greater importance.

Magnetic Cationic Copper(II) Chains and a Mononuclear Cobalt(II) Complex Containing [Ln(hfac)4]- Blocks as Counterions

Four new heterospin compounds with molecular formula {[Cu₂(hfac)₃(TlTrzNIT)₂][Ln(hfac)₄]} _n·C₇H₁₆ (Ln^III = Gd (1), Tb (2), or Dy (3)) and [Co(hfac)(TlTrzNIT)₂][Dy(hfac)₄] (4), where hfac is hexafluoroacetylacetonato and TlTrzNIT is the nitronylnitroxide radical 1-( m-tolyl)-1 H-1,2,3-triazole-4-(4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide), were obtained. All structures were determined by single-crystal X-ray diffraction. In compounds 1-3, the TlTrzNIT radical is bridge-coordinated to copper(II) ions, leading to positively charged copper(II)-radical chains containing [Ln(hfac)₄]^- as counterions. In compound 4, the cobalt(II) ion is coordinated to two TlTrzNIT radicals and one hfac ligand in bidentate mode leading to a mononuclear cationic complex that contains [Dy(hfac)₄]^- as counterion. Magnetic measurements of all complexes were performed. Magnetic data were fit considering the contributions of the copper(II)-radical chain and a paramagnetic gadolinium(III) ion for 1. The sign and magnitude of the magnetic coupling constants extracted from the fit were confirmed by density functional theory calculations. The obtained spin topology shows an alternated ferro-antiferromagnetic chain. Field-induced single molecule magnet behavior was observed for the Dy derivatives 3 and 4, in agreement with CASSCF calculations performed for the latter system.

Chemistry and Quantum Mechanics in 2019: Give Us Insight and Numbers

This Perspective revisits Charles Coulson’s famous statement from 1959 “give us insight not numbers” in which he pointed out that accurate computations and chemical understanding often do not go hand in hand. We argue that today, accurate wave function based first-principle calculations can be performed on large molecular systems, while tools are available to interpret the results of these calculations in chemical language. This leads us to modify Coulson’s statement to “give us insight and numbers”. Examples from organic, inorganic, organometallic and surface chemistry as well as molecular magnetism illustrate the points made.

Magnetic Properties of Co(II) Complexes with Polyhedral Carborane Ligands

In this work we present a computational analysis of a new family of magnetic Co(II) single-ion complexes with large magnetic anisotropy based on icosahedral and octahedral carborane ligands. In particular, we extend our previous computational work on mononuclear Co(II) complexes with 1,2-(HS)₂-1,2-C₂B₁₀H₁₀ and 9,12-(HS)₂-1,2-C₂B₁₀H₁₀ icosahedral o-carborane ligands to a larger set of complexes where the Co(II) ion is doubly chelated by those ligands and by other two positional isomers belonging to the 1,2-dicarba- closo-dodecaborane family. We also describe Co(II) complexes with octahedral ligands derived from 1,2-dicarba- closo-hexaborane and study the effects of replacing a thiol group by a hydroxy group in both polyhedral geometries, as well as the influence of the position of the carbon atoms. On analysis of the results for a total of 20 complexes, our results show that carborane-based Co(II) single-ion compounds present a distorted-tetrahedral geometry, high-spin ground states, and high values for the magnetic anisotropy parameters. We point out which of these would be suitable candidates to be synthesized and used as molecular magnets.

Field-induced slow relaxation of magnetization in the S = 3/2 octahedral complexes trans-[Co{(OPPh2)(EPPh2)N}2(dmf)2], E = S, Se: effects of Co–Se vs. Co–S coordination

Magnetometry studies on octahedral trans-[Co{(OPPh₂)(EPPh₂)N}₂(dmf)₂], E = S, Se, complexes.

Counter-complementarity control of the weak exchange interaction in a bent {Ni(ii)3} complex with a μ-phenoxide-μ-carboxylate double bridge

We report a combined computational and experimental study of a weak exchange interaction in a bent {Ni(ii)₃} complex controlled by counter-complementarity.

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.

Trapping of a Pseudotetrahedral CoIIO4 Core in Mixed-Valence Mixed-Geometry [Co5] Coordination Aggregates: Synthetic Marvel, Structures, and Magnetism

A systematic one-step one-pot multicomponent reaction of Co(ClO₄)₂·6H₂O, H₃L (2,6-bis((2-(2-hydroxyethylamino)ethylimino)methyl)-4-methylphenol), and readily available carboxylate salts (RCO₂Na; R = CH₃, C₂H₅) resulted in the two structurally novel coordination aggregates [Co^IICo^III₄L₂(μ_1,3-O₂CCH₃)₂(μ-OH)₂](ClO₄)₄·4H₂O (1) and [Co^IICo^III₄L₂(μ_1,3-O₂CC₂H₅)₂(μ-OH)(μ-OMe)](ClO₄)₄·5H₂O (2). At room temperature, reactions of H₃L in MeOH with cobalt(II) perchlorate salts led to coassembly of initially formed ligand-bound {Co^II₂} fragments following aerial oxidation of metal centers and bridging by in situ generated hydroxido/alkoxido groups and added carboxylate anions. Available alkoxido arms of the initially formed {L(μ_1,3-O₂CCH₃)(μ-OH/OMe)Co₂}⁺ fragments were utilized to trap a central Co^II ion during the formation of [Co₅] aggregates. In the solid state, both complexes have been characterized by X-ray crystallography, variable-temperature magnetic measurements, and theoretical studies. Both 1 and 2 show field-induced slow magnetic relaxation that arises from the single pseudo- T _d Co^II ion present. The structural distortion leads to an easy-axis magnetic anisotropy ( D = -31.31 cm^-1 for 1 and -21.88 cm^-1 for 2) and a small but non-negligible transverse component ( E/ D = 0.11 for 1 and 0.08 for 2). The theoretical studies also reveal how the O-Co-O bond angles and the interplanar angles control D and E values in 1 and 2. The presence of two diamagnetic {Co₂(μ-L)} hosts controls the distortion of the central {CoO₄} unit, highlighting a strategy to control single-ion magnetic anisotropy by trapping single ions within a diamagnetic coordination environment.

Pseudotetrahedral cobalt(ii) complexes with PNP-ligands showing uniaxial magnetic anisotropy

Two mononuclear pseudotetrahedral cobalt(ii) complexes with the formula Co(PNP)X2, where X = Cl (1) or X = SCN (2) and PNP = bis(2-(diphenylphosphaneyl)-4-methylphenyl)amine, have been synthesised. Magnetic and high-frequency/field electron paramagnetic resonance (HF-EPR) spectroscopy and ab initio calculation studies reveal that both complexes show uniaxial magnetic anisotropy.

Determination of d-Orbital Populations in a Cobalt(II) Single-Molecule Magnet Using Single-Crystal X-ray Diffraction

The tetrahedral cobalt(II) compound (Ph₄P)₂[Co(SPh)₄] was the first mononuclear transition-metal complex shown to exhibit slow relaxation of the magnetization in zero external magnetic field. Because the relative populations of the d orbitals play a vital role in dictating the magnitude of the magnetic anisotropy, the magnetic behavior of this complex is directly related to its electronic structure, yet the exact role of the soft, thiophenolate ligands in influencing the d-electron configuration has previously only been investigated via theoretical methods. To provide detailed experimental insight into the effect of this ligand field, the electron density distribution in this compound was determined from low-temperature, single-crystal X-ray diffraction data and subsequent multipole modeling. Topological analysis of the electron density indicates significant covalent contributions to the cobalt-sulfur bonds. The derived d-orbital populations further reveal a fully occupied d _z² orbital, minor d _xz orbital population, and nearly equal population of the d _xy, d _{x²- y²}, and d _yz orbitals. Notably, we find that an electrostatic interaction between Co(II) and one hydrogen atom from a thiophenolate group in the xz plane increases the energy of the d _{x²- y²} orbital, leading to the nearly equal population with d _xy and strong magnetic anisotropy.

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.

Modelling spin Hamiltonian parameters of molecular nanomagnets

Molecular nanomagnets encompass a wide range of coordination complexes possessing several potential applications. A formidable challenge in realizing these potential applications lies in controlling the magnetic properties of these clusters. Microscopic spin Hamiltonian (SH) parameters describe the magnetic properties of these clusters, and viable ways to control these SH parameters are highly desirable. Computational tools play a proactive role in this area, where SH parameters such as isotropic exchange interaction (J), anisotropic exchange interaction (Jx, Jy, Jz), double exchange interaction (B), zero-field splitting parameters (D, E) and g-tensors can be computed reliably using X-ray structures. In this feature article, we have attempted to provide a holistic view of the modelling of these SH parameters of molecular magnets. The determination of J includes various class of molecules, from di- and polynuclear Mn complexes to the {3d-Gd}, {Gd-Gd} and {Gd-2p} class of complexes. The estimation of anisotropic exchange coupling includes the exchange between an isotropic metal ion and an orbitally degenerate 3d/4d/5d metal ion. The double-exchange section contains some illustrative examples of mixed valance systems, and the section on the estimation of zfs parameters covers some mononuclear transition metal complexes possessing very large axial zfs parameters. The section on the computation of g-anisotropy exclusively covers studies on mononuclear Dy(III) and Er(III) single-ion magnets. The examples depicted in this article clearly illustrate that computational tools not only aid in interpreting and rationalizing the observed magnetic properties but possess the potential to predict new generation MNMs.

Structural diversity and magnetic properties of six cobalt coordination polymers based on 2,2'-phosphinico-dibenzoate ligand

Six novel Co(ii) coordination polymers, namely, [Co₁₀L₆(OH)₂(H₂O)₉]·10.5H₂O (1), [Co₃L₂(3-abpt)₂]·4H₂O (2), [Co₃L₂(4-azpy)₂(H₂O)₂(EtOH)] (3), [Co₃L₂(4,4'-bipy)₂(H₂O)₂(MeCN)] (4), [Co₃L₂(4,4'-bipy)₂] (5), and [Co₅L₂(OH)₂(ina)₂(H₂O)₂] (6) (H₃L = 2,2'-phosphinico-dibenzoic acid, 3-abpt = 4-amino-3,5-bis(3-pyridyl)-1,2,4-triazole, 4-azpy = 4,4'-azobispyridine, 4,4'-bipy = 4,4'-bipyridine, Hina = isonicotinic acid), have been hydrothermally synthesized and their magnetic properties have been characterized. The L^3- anion displays six types of coordination modes in the compounds. Compound 1 exhibits a novel 1D ladder-like structure, which consists of non-centrosymmetric Co₁₀ units. Compounds 2-4 comprise 2D networks assembled from Co₃L₂ chains and N-heterocyclic linkers. Compound 5 comprises a 3D framework built from six neighboring parallel Co₃L₂ ladders bridged by 4,4'-bipy linkers. Compound 6 features a 3D framework that exhibits pcu topology with the Schläfli symbol of (4¹²·6³) using a pentanuclear [Co₅(OH)₂]⁸⁺ cluster as the node. Variable-temperature magnetic susceptibility studies indicate that the six coordination polymers exhibit remarkable magnetic behavior such as spin-canted antiferromagnetism and spin glass, which were found to coexist in compound 6.

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.

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.

Pseudo-Contact NMR Shifts over the Paramagnetic Metalloprotein CoMMP-12 from First Principles

Long-range pseudo-contact NMR shifts (PCSs) provide important restraints for the structure refinement of proteins when a paramagnetic metal center is present, either naturally or introduced artificially. Here we show that ab initio quantum-chemical methods and a modern version of the Kurland-McGarvey approach for paramagnetic NMR (pNMR) shifts in the presence of zero-field splitting (ZFS) together provide accurate predictions of all PCSs in a metalloprotein (high-spin cobalt-substituted MMP-12 as a test case). Computations of 314 ¹³ C PCSs using g- and ZFS tensors based on multi-reference methods provide a reliable bridge between EPR-parameter- and susceptibility-based pNMR formalisms. Due to the high sensitivity of PCSs to even small structural differences, local structures based either on X-ray diffraction or on various DFT optimizations could be evaluated critically by comparing computed and experimental PCSs. Many DFT functionals provide insufficiently accurate structures. We also found the available 1RMZ PDB X-ray structure to exhibit deficiencies related to binding of a hydroxamate inhibitor. This has led to a newly refined PDB structure for MMP-12 (5LAB) that provides a more accurate coordination arrangement and PCSs.

Magnetic Anisotropy of Tetrahedral CoII Single-Ion Magnets: Solid-State Effects

This study reports the static and dynamic magnetic characterization of two mononuclear tetrahedral Co^II complexes, [Co{ⁱPr₂P(E)NP(E)ⁱPr₂}₂], where E = S (CoS₄) and Se (CoSe₄), which behave as single-ion magnets (SIMs). Low-temperature (15 K) single-crystal X-ray diffraction studies point out that the two complexes exhibit similar structural features in their first coordination sphere, but a disordered peripheral ⁱPr group is observed only in CoS₄. Although the latter complex crystallizes in an axial space group, the observed structural disorder leads to larger transverse magnetic anisotropy for the majority of the molecules compared to CoSe₄, as confirmed by electron paramagnetic resonance spectroscopy. Static magnetic characterization indicates that both CoS₄ and CoSe₄ show easy-axis anisotropy, with comparable D values (∼-30 cm^-1). Moreover, alternating-current susceptibility measurements on these Co^II complexes, magnetically diluted in their isostructural Zn^II analogues, highlight the role of dipolar magnetic coupling in the mechanism of magnetization reversal. In addition, our findings suggest that, despite their similar anisotropic features, CoS₄ and CoSe₄ relax magnetically via different processes. This work provides experimental evidence that solid-state effects may affect the magnetic behavior of SIMs.

Transformation of the coordination complex [Co(C3S5)2]2- from a molecular magnet to a potential qubit

We employ ac susceptibility as a probe of small changes of transverse zero-field splitting, revealing that these subtle changes transform [Co(C₃S₅)₂]^2– from a molecular magnet to a candidate qubit.

Mononuclear transition metal complexes demonstrate significant potential in the divergent applications of spintronics and quantum information processing. The facile tunability of these complexes enables structure function correlations for a plethora of relevant magnetic quantities. We present a series of pseudotetrahedral [Co(C₃S₅)₂]^2– complexes with varying deviations from D_2d symmetry to investigate the influence of structural distortions on spin relaxation dynamics and qubit viability, as tuned by the variable transverse magnetic anisotropy, E. To overcome the traditional challenge of measuring E in species where D ≫ E, we employed a different approach of harnessing ac magnetic susceptibility to probe the emergence of quantum tunneling of magnetization as a proxy for E. Across the range of values for E in the series, we observe magnetic hysteresis for the smallest value of E. The hysteresis disappears with increasing E, concomitant with the appearance of an observable, low frequency (L-band) electron paramagnetic resonance (EPR) signal, indicating the potential to controllably shift the molecule's utilization from classical to quantum information processing applications. The development of design principles for molecular magnet information processing requires separate design principles for classical versus quantum regimes. Here we show for the first time how subtle structural changes can switch the utility of a complex between these two types of applications.

Enhancing the Magnetic Anisotropy of Linear Cr(II) Chain Compounds Using Heavy Metal Substitutions

Magnetic properties of the series of three linear, trimetallic chain compounds Cr2Cr(dpa)4Cl2, 1, Mo2Cr(dpa)4Cl2, 2, and W2Cr(dpa)4Cl2, 3 (dpa = 2,2'-dipyridylamido), have been studied using variable-temperature dc and ac magnetometry and high-frequency EPR spectroscopy. All three compounds possess an S = 2 electronic ground state arising from the terminal Cr(2+) ion, which exhibits slow magnetic relaxation under an applied magnetic field, as evidenced by ac magnetic susceptibility and magnetization measurements. The slow relaxation stems from the existence of an easy-axis magnetic anisotropy, which is bolstered by the axial symmetry of the compounds and has been quantified through rigorous high-frequency EPR measurements. The magnitude of D in these compounds increases when heavier ions are substituted into the trimetallic chain; thus D = -1.640, -2.187, and -3.617 cm(-1) for Cr2Cr(dpa)4Cl2, Mo2Cr(dpa)4Cl2, and W2Cr(dpa)4Cl2, respectively. Additionally, the D value measured for W2Cr(dpa)4Cl2 is the largest yet reported for a high-spin Cr(2+) system. While earlier studies have demonstrated that ligands containing heavy atoms can enhance magnetic anisotropy, this is the first report of this phenomenon using heavy metal atoms as "ligands".

Tetracoordinate Co(ii) complexes containing bathocuproine and single molecule magnetism

Chlorido complex [Co(bcp)Cl₂] exhibits a field-induced SMM behaviour, while in the bromide and iodido analogues only weak antiferromagnetic interactions are present.

Polymeric cobalt(ii) thiolato complexes – syntheses, structures and properties of 1∞[Co(SMes)2] and 1∞[Co(SPh)2NH3]

Structural and magnetic characterization of the first examples of polymeric cobalt chalcogenolato complexes namely 1∞[Co(SMes)₂] and 1∞[Co(SPh)₂NH₃].