Elucidating Magnetic Exchange and Anisotropy in Weakly Coupled MnIII Dimers

Tuning of Cr-Cr Magnetic Exchange through Chalcogenide Linkers in Cr2 Molecular Dimers

A set of three Cr-dimer compounds, Cr₂Q₂(en)₄X₂ (Q: S, Se; X: Br, Cl; en: ethylenediamine), with monoatomic chalcogenide bridges have been synthesized via a single-step solvothermal route. Chalcogenide linkers mediate magnetic exchange between Cr³⁺ centers, while bidentate ethylenediamine ligands complete the distorted octahedral coordination of Cr centers. Unlike the compounds previously reported, none of the chalcogenide atoms are connected to extra ligands. Magnetic susceptibility studies indicate antiferromagnetic coupling between Cr³⁺ centers, which are moderate in Cr₂Se₂(en)₄X₂ and stronger in Cr₂S₂(en)₄Cl₂. Fitting the magnetic data requires a biquadratic exchange term. High-frequency EPR spectra showing characteristic signals due to coupled S = 1 spin states could be interpreted in terms of the "giant spin" Hamiltonian. A fourth compound, Cr₂Se₈(en)₄, has a single diatomic Se bridge connecting the two Cr³⁺ centers and shows weak ferromagnetic exchange interactions. This work demonstrates the tunability in strength and type of exchange interactions between metal centers by manipulating the interatomic distances and number of bridging chalcogenide linkers.

Long-Range Ferromagnetic Exchange Interactions Mediated by Mn-CeIV-Mn Superexchange Involving Empty 4f Orbitals

Reactions involving reductive aggregation of MnO₄^- in methanol in the presence of Ce^IV and an excess of carboxylic acid have led to the synthesis of structurally related Ce/Mn clusters, [Ce₃Mn₅O₈(OMe)(O₂CBu^t)₁₃(MeOH)] (1) and [Ce₂Mn₃O₅(O₂CPh)₉(MeOH)₃] (2), containing at least one {Mn₂Ce₂O₄} cubane unit. The cores of both clusters contain Mn_x units separated by three (1) or two (2) Ce^IV ions. Fits of variable-temperature, solid-state dc and ac magnetic susceptibility data reveal dominant ferromagnetic interactions within 1 and 2, resulting in the maximum S = ¹⁷/₂ and S = 5 ground state spins, respectively, and thus suggesting significant ferromagnetic (F) interactions between the Mn_x units that are ≥6 Å apart and separated by four intervening bonds through diamagnetic Ce^IV. Fits of magnetic susceptibility data also revealed unusual long-range F interactions, and this finding was further supported by high-field EPR measurements and simulations. Density functional theory calculations and a Wannier function analysis confirm long-range interactions and indicate a Mn-Ce-Mn superexchange pathway via Mn-d/Ce-f orbital overlap/hybridization.

Dinuclear manganese(iii) complexes with bioinspired coordination and variable linkers showing weak exchange effects: a synthetic, structural, spectroscopic and computation study

High-resolution HFEPR indicates weak exchange interactions between Mn^III ions in agreement with DFT calculations.

Tuning of Exchange Coupling and Switchable Magnetization Dynamics by Displacing the Bridging Ligands Observed in Two Dimeric Manganese(III) Compounds

Two Mn(III)-based dimers, [Mn₂(bpad)₂(CH₃O)₄]_n (1) and [Mn₂(bpad)₂(pa)₂]_n·2H₂O (2) (Hbpad = N³-benzoylpyridine-2-carboxamidrazone, H₂pa = phthalic acid), have been assembled from a tridentate Schiff-base chelator and various anionic coligands. Noteworthily, compound 1 could be identified as a reaction precursor to transform to 2 in the presence of phthalic acid, resulting in a rarely structural conversion process in which the bridges between intradimer Mn(III) ions alter from methanol oxygen atom with μ₂-O mode in 1 (Mn Mn distance of 3.046 Å) to syn-anti carboxylate in 2 (Mn Mn distance of 4.043 Å), while the Mn(III) centers retain hexa-coordinated geometries with independently distorted octahedrons in two compounds. The dc magnetic determinations reveal that ferromagnetic coupling between two metal centers with J = 1.31 cm⁻¹ exists in 1, whereas 2 displays weak antiferromagnetic interactions with the coupling constant J of −0.56 cm⁻¹. Frequency-dependent ac susceptibilities in the absence of dc field for 1 suggest slow relaxation of the magnetization with an energy barrier of 13.9 K, signifying that 1 features single-molecule magnet (SMM) behavior. This work presents a rational strategy to fine-tune the magnetic interactions and further magnetization dynamics of the Mn(III)-containing dinuclear units through small structural variations driven by the ingenious chemistry.

HFEPR and Computational Studies on the Electronic Structure of a High-Spin Oxidoiron(IV) Complex in Solution

Nonheme iron enzymes perform diverse and important functions in biochemistry. The active form of these enzymes comprises the ferryl, oxidoiron(IV), [FeO](2+) unit. In enzymes, this unit is in the high-spin, quintet, S = 2, ground state, while many synthetic model compounds exist in the spin triplet, S = 1, ground state. Recently, however, Que and co-workers reported an oxidoiron(IV) complex with a quintet ground state, [FeO(TMG3tren)](OTf)2, where TMG3tren = 1,1,1-tris{2-[N2-(1,1,3,3-tetramethylguanidino)]ethyl}amine and OTf = CF3SO3(-). The trigonal geometry imposed by this ligand, as opposed to the tetragonal geometry of earlier model complexes, favors the high-spin ground state. Although [FeO(TMG3tren)](2+) has been earlier probed by magnetic circular dichroism (MCD) and Mössbauer spectroscopies, the technique of high-frequency and -field electron paramagnetic resonance (HFEPR) is superior for describing the electronic structure of the iron(IV) center because of its ability to establish directly the spin-Hamiltonian parameters of high-spin metal centers with high precision. Herein we describe HFEPR studies on [FeO(TMG3tren)](OTf)2 generated in situ and confirm the S = 2 ground state with the following parameters: D = +4.940(5) cm(-1), E = 0.000(5), B4(0) = -14(1) × 10(-4) cm(-1), g⊥ = 2.006(2), and g∥ = 2.03(2). Extraction of a fourth-order spin-Hamiltonian parameter is unusual for HFEPR and impossible by other techniques. These experimental results are combined with state-of-the-art computational studies along with previous structural and spectroscopic results to provide a complete picture of the electronic structure of this biomimetic complex. Specifically, the calculations reproduce well the spin-Hamiltonian parameters of the complex, provide a satisfying geometrical picture of the S = 2 oxidoiron(IV) moiety, and demonstrate that the TMG3tren is an "innocent" ligand.