Magnetic Properties of Largest-Spin Single Molecule Magnets: Mn17 Complexes—A Density Functional Theory Approach

Solvent coligands fine-tuned the structures and magnetic properties of triple-bridged 1D azido-copper(ii) coordination polymers

The structures and magnetic properties of three triple-bridged 1D azido–Cu(ii) coordination polymers are well-regulated by altering the coordinated alkanol molecules.

Solvent-induced single-crystal-to-single-crystal transformation and tunable magnetic properties of 1D azido-Cu(ii) chains with a carboxylate bridge

The solvent effect leads to structural transformation and tunable magnetism of chain-like azido-copper coordination polymers.

Thermally Persistent High-Spin Ground States in Octahedral Iron Clusters

Chemical oxidation and reduction of the all-ferrous (^HL)₂Fe₆ in THF affords isostructural, coordinatively unsaturated clusters of the type [(^HL)₂Fe₆] ⁿ: [(^HL)₂Fe₆][BArF₂₄] (1, n = +1; where [BArF₂₄]^- = tetrakis[(3,5-trifluoromethyl)phenyl]borate), [Bu₄N][(^HL)₂Fe₆] (2a, n = -1), [P][(^HL)₂Fe₆] (2b, n = -1; where [P]⁺ = tributyl(1,3-dioxolan-2-ylmethyl)phosphonium), and [Bu₄N]₂[(^HL)₂Fe₆] (3, n = -2). Each member of the redox-transfer series was characterized by zero-field ⁵⁷Fe Mössbauer spectroscopy, near-infrared spectroscopy, single-crystal X-ray crystallography, and magnetometry. Redox-directed trends are observed when comparing the structural metrics within the [Fe₆] core. The metal octahedron [Fe₆] decreases marginally in volume as the molecular reduction state increases as gauged by the Fe-Fe_avg distance varying from 2.608(11) Å ( n = +1) to 2.573(3) ( n = -2). In contrast, the mean Fe-N distances and ∠Fe-N-Fe angles correlate linearly with the [Fe₆] oxidation level, or alternatively, the changes observed within the local Fe-N₄ coordination planes vary linearly with the aggregate spin ground state. In general, as the spin ground state ( S) increases, the Fe-N(H)_avg distances also increase. The structural metric perturbations within the [Fe₆] core and measured spin ground states were rationalized extending the previously proposed molecular orbital diagram derived for (^HL)₂Fe₆. Chemical reduction of the (^HL)₂Fe₆ cluster results in an abrupt increase in spin ground state from S = 6 for the all-ferrous cluster, to S = ¹⁹/₂ in the monoanionic 2b and S = 11 for the dianionic 3. The observation of asymmetric intervalence charge transfer bands in 3 provides further evidence of the fully delocalized ground state observed by ⁵⁷Fe Mössbauer spectroscopy for all species examined (1-3). For each of the clusters examined within the electron-transfer series, the observed spin ground states thermally persist to 300 K. In particular, the S = 11 in dianionic 3 and S = ¹⁹/₂ in the monoanionic 2b represent the highest spin ground states isolated up to room temperature known to date. The increase in spin ground state results from population of the antibonding orbital band comprised of the Fe-N σ* interactions. As such, the thermally persistent ground states arise from population of the resultant single spin manifolds in accordance with Hund's rules. The large spin ground states, indicative of strong ferromagnetic electronic alignment of the valence electrons, result from strong direct exchange electronic coupling mediated by Fe-Fe orbital overlap within the [Fe₆] cores, equivalent to a strong double exchange magnetic coupling B for 3 that was calculated to be 309 cm^-1.

A difunctional azido-cobalt(ii) coordination polymer exhibiting slow magnetic relaxation behaviour and high-energy characteristics with good thermostability and insensitivity

A novel one-dimensional azido-cobalt(ii) compound, [Co2(1-mbt)2(N3)4]n (1) (1-mbt = 1-((2-propyl-imidazol-1-yl)methyl)-benzo[1,2,3]triazole), was solvothermally synthesized. X-ray crystal structure analysis demonstrates that two crystallographically independent Co(ii) atoms in the asymmetrical unit of compound 1 exhibit a rectangular pyramid geometry. The 3D supermolecular network of 1 consists of well-isolated 1D metal chains in which the azido bridging ligands assume an unusual pattern of combination with the Co(ii) centre as the sign of [-EE-EO-EO-EO-]n. The various coordination modes of the azido anion are responsible for different magnetic exchanges between the adjacent Co(ii) ions. The end-to-end (EE) mode mediates the antiferromagnetic coupling, whereas the end-on (EO) manner contributes to the ferromagnetic interaction. Magneto-structural relationships are discussed with the aid of theoretical calculations which are employed to find the potential single-ion magnetic anisotropy and reproduce the observed magnetic coupling properly. Alternating current magnetic susceptibility measurements reveal that 1 features a typical behaviour of field-induced slow magnetic relaxation. Energetic characterization evidences that the resulting compound possesses satisfactory detonation properties, superior lack of sensitivity and thermostability owing to the high nitrogen content (N% = 40.10%) and a coherent intrachain configuration. The kinetic parameters of the exothermic processes for 1 are investigated by the Kissinger method and the Ozawa method. We note that 1 has potential application prospects as a new generation of environmentally friendly high-energy materials based on this nitrogen-rich and oxygen-free system. In addition, the compound is developed as a practical additive to promote the thermal decomposition of ammonium perchlorate (AP) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). The finding in this work highlights an example of effective development of advanced magneto-energetic materials.

Ferromagnetic ordering and slow magnetic relaxation observed in a triple-bridged azido-Cu(ii) chain compound with mixed carboxylate/ethanol linkers

A triple-bridged Cu(ii) chain compound with μ-1,1-(EO)azido, syn,syn-carboxylate, and μ2-ethanol linkers exhibits slow-relaxation and long-range magnetic ordering.

Coligand modifications fine-tuned the structure and magnetic properties of two triple-bridged azido-Cu(ii) chain compounds exhibiting ferromagnetic ordering and slow relaxation

Two triple-bridged azido-Cu(ii) compounds with different benzoates as coligands exhibit distinct magnetic properties.

A substituent effect of phenylacetic acid coligand perturbed structures and magnetic properties observed in two triple-bridged azido-Cu(ii) chain compounds with ferromagnetic ordering and slow magnetic relaxation

Two triple-bridged azido-Cu(ii) chains exhibit distinct magnetic behaviors.

Exchange Interactions on the Highest-Spin Reported Molecule: the Mixed-Valence Fe42 Complex

The finding of high-spin molecules that could behave as conventional magnets has been one of the main challenges in Molecular Magnetism. Here, the exchange interactions, present in the highest-spin molecule published in the literature, Fe₄₂, have been analysed using theoretical methods based on Density Functional Theory. The system with a total spin value S = 45 is formed by 42 iron centres containing 18 high-spin Fe^III ferromagnetically coupled and 24 diamagnetic low-spin Fe^II ions. The bridging ligands between the two paramagnetic centres are two cyanide ligands coordinated to the diamagnetic Fe^II cations. Calculations were performed using either small Fe₄ or Fe₃ models or the whole Fe₄₂ complex, showing the presence of two different ferromagnetic couplings between the paramagnetic Fe^III centres. Finally, Quantum Monte Carlo simulations for the whole system were carried out in order to compare the experimental and simulated magnetic susceptibility curves from the calculated exchange coupling constants with the experimental one. This comparison allows for the evaluation of the accuracy of different exchange-correlation functionals to reproduce such magnetic properties.

A Binuclear Copper(II) Complex with Diphenoxido-bridges: Synthesis, Characterisation, Crystal Structure and Predicting the Magnetic Coupling Constant

Reaction of copper(II) acetate with 1,1′-[(2,2-dimethylpropane-1,3-diyl)bis(nitrilomethylidyne)]-di-2-naphthol (H2L) gave the diphenoxido-bridged binuclear copper(II) complex [Cu2L2]. Its crystal structure shows a slightly distorted square pyramidal geometry around both of the copper(II) ions. In the symmetric Cu2O2 core, each of the phenoxido bridges occupies an equatorial position around one of the copper(II) centres. A broken symmetry (BS) computation based on density functional theory (DFT) evaluated the magnetic interaction in the complex using the (U)B3LYP level of theory in association with the basis sets LANL2DZ for the copper atoms and 6-31G** for the other atoms. The calculations showed ferromagnetic interaction between the two magnetic copper(II) centres with a magnetic coupling constant (J) of 1.16 cm−1.

Meta-Atom Behavior in Clusters Revealing Large Spin Ground States

The field of single molecule magnetism remains predicated on super- and double exchange mechanisms to engender large spin ground states. An alternative approach to achieving high-spin architectures involves synthesizing weak-field clusters featuring close M-M interactions to produce a single valence orbital manifold. Population of this orbital manifold in accordance with Hund's rules could potentially yield thermally persistent high-spin ground states under which the valence electrons remain coupled. We now demonstrate this effect with a reduced hexanuclear iron cluster that achieves an S = 19/2 (χ(M)T ≈ 53 cm(3) K/mol) ground state that persists to 300 K, representing the largest spin ground state persistent to room temperature reported to date. The reduced cluster displays single molecule magnet behavior manifest in both variable-temperature zero-field (57)Fe Mössbauer and magnetometry with a spin reversal barrier of 42.5(8) cm(-1) and a magnetic blocking temperature of 2.9 K (0.059 K/min).

A new cobalt(II)-containing coordination polymer composed of dinuclear metal units: synthesis, crystal structure, and magnetic properties with DFT calculations

A new Co(II)-based compound with a 3-phenyl-5-(pyridin-4-yl)-1,2,4-triazole (4-Hppt) and isophthalic acid (H2ip) mixed ligand, [Co(4-Hppt)2(ip)] (1), has been synthesized and structurally and magnetically characterized. The structure of compound 1 consists of one-dimensional coordination chains based on the Co2 units bridged by dual carboxylate groups, where the pyridyl nitrogen atoms in 4-Hppt ligands are located in the axial coordination sites of the octahedral Co(II) nodes. Magnetic investigation suggests that the magnetic coupling derived from the dinuclear Co(II) ions is a moderately antiferromagnetic exchange in the title compound. Moreover, DFT calculations have been performed on the compound to offer a qualitatively theoretical explanation of the magnetic behavior.

What controls the magnetic exchange interaction in mixed- and homo-valent Mn7 disc-like clusters? A theoretical perspective

Density functional theory (DFT) studies have been undertaken to compute the magnetic exchange and to probe the origin of the magnetic interactions in two hetero- and two homo-valent heptanuclear manganese disc-like clusters, of formula [Mn(II) 4 Mn(IV) 3 (tea)(teaH2 )3 (peolH)4 ] (1), [Mn(II) 4 Mn(III) 3 F3 (tea)(teaH)(teaH2 )2 (piv)4 (Hpiv)(chp)3 ] (2), [Mn(II) 7 (pppd)6 (tea)(OH)3 ] (3) and [Mn(II) 7 (paa)6 (OMe)6 ] (4) (teaH3 =triethanolamine, peolH4 =pentaerythritol, Hpiv=pivalic acid, Hchp=6-chloro-2-hydroxypyridine, pppd=1-phenyl-3-(2-pyridyl) propane-1,3-dione; paaH=N-(2-pyridinyl)acetoacetamide). DFT calculations yield J values, which reproduce the magnetic susceptibility data very well for all four complexes; these studies are also highlighting the likely ageing/stability problems in two of the complexes. It is found that the spin ground states, S, for complexes 1-4 are drastically different, varying from S=29/2 to S=1/2. These values are found to be controlled by the nature of the oxidation state of the metal ions and minor differences present in the structures. Extensive magneto-structural correlations are developed for the seven building unit dimers present in the complexes, with the correlations unlocking the reasons behind the differences in the magnetic properties observed. Independent of the oxidation state of the metal ions, the Mn-O-Mn/Mn-F-Mn angles are found to be the key parameters, which significantly influence the sign as well as the magnitude of the J values. The magneto-structural correlations developed here, have broad applicability and can be utilised to understand the magnetic properties of other Mn clusters.

All-round robustness of the Mn19 coordination cluster system: experimental validation of a theoretical prediction

Experimental and theoretical studies confirm that the maximum possible ground spin state of S_T = 83/2 for the Mn₁₉ aggregate is insensitive to replacement of its eight μ₃-N₃ ligands by μ₃-Cl, μ₃-Br, μ₃-OH or μ₃-OMe.

Role of dicarboxylate linkers in Mn(III)-salicylaldoximate based extended structures: synthesis, structures, and magnetic behavior

Four new oxo-centered Mn(III)-salicylaldoximate triangle-based extended complexes [Mn(III)6O2(salox)6(EtOH)4(phda)]n·(saloxH2)n·(2H2O)n (1), [Mn(III)6O2(salox)6(MeOH)5(5-I-isoph)]n·(3MeOH)n (2), [Mn(III)6O2(salox)6(MeOH)4(H2O) (5-N3-isoph)]n·(4MeOH)n (3) and [Mn(III)3NaO(salox)3(MeOH)4(5-NO2-isoph)]n·(MeOH)n (H2O)n (4) [salox=salicylaldoximate, phda=1,3-phenylenediacetate, isoph=isophthalate] have been synthesized under similar reaction conditions. Single crystal X-ray structures show that in 1, only one type of Mn6 cluster is arranged in 1D, whereas in 2 and 3 there are two types of clusters, differing in the way the triangle units are joined and assembled. In complex 4, however, the basic building structure is heteronuclear and based on Mn3 units extended in 2D. Susceptibility measurements (dc and ac) over a wide range of temperatures and fields show that the complexes 1, 2, and 3 behave as single molecule magnets (SMMs) with S=4 ground state, while 4 is dominantly antiferromagnetic with a ground spin state S=2. Density functional theory calculations have been performed on model complexes to provide a qualitative theoretical interpretation for their overall magnetic behavior.

A density functional theory approach to the magnetic properties of a coupled single-molecule magnet (Mn7)2 complex — An entangled qubit pair candidate

The exchange coupling constants of a Mn14 complex constituted by two weakly coupled Mn7 moieties were calculated using two different density functional theory (DFT) approaches: the Perdew–Burke–Ernzerhof (PBE) functional with a numerical basis set and the hybrid Becke, three-parameter Lee–Yang–Parr (B3LYP) functional employed with a Gaussian basis set. The sign and relative strength of the exchange coupling constants calculated with both methods were consistent; as expected, the values calculated with the PBE functional were slightly overestimated, as corroborated by comparison with the experimental magnetic susceptibility curve. Both methods gave a ground spin configuration of S = 3/2 for the Mn7 moiety, which was weakly antiferromagnetically coupled with the other Mn7 fragment, leading to an S = 0 ground spin configuration for the entire Mn14 complex.