Spectroscopic Investigation of a Metal-Metal-Bonded Fe6 Single-Molecule Magnet with an Isolated S = 19/2 Giant-Spin Ground State

Quantum Magnetism of the Iron Core in Ferritin Proteins-A Re-Evaluation of the Giant-Spin Model

The electron-electron, or zero-field interaction (ZFI) in the electron paramagnetic resonance (EPR) of high-spin transition ions in metalloproteins and coordination complexes, is commonly described by a simple spin Hamiltonian that is second-order in the spin S: H=D[Sz2-SS+1/3+E(Sx2-Sy2). Symmetry considerations, however, allow for fourth-order terms when S ≥ 2. In metalloprotein EPR studies, these terms have rarely been explored. Metal ions can cluster via non-metal bridges, as, for example, in iron-sulfur clusters, in which exchange interaction can result in higher system spin, and this would allow for sixth- and higher-order ZFI terms. For metalloproteins, these have thus far been completely ignored. Single-molecule magnets (SMMs) are multi-metal ion high spin complexes, in which the ZFI usually has a negative sign, thus affording a ground state level pair with maximal spin quantum number mS = ±S, giving rise to unusual magnetic properties at low temperatures. The description of EPR from SMMs is commonly cast in terms of the 'giant-spin model', which assumes a magnetically isolated system spin, and in which fourth-order, and recently, even sixth-order ZFI terms have been found to be required. A special version of the giant-spin model, adopted for scaling-up to system spins of order S ≈ 103-104, has been applied to the ubiquitous iron-storage protein ferritin, which has an internal core containing Fe3+ ions whose individual high spins couple in a way to create a superparamagnet at ambient temperature with very high system spin reminiscent to that of ferromagnetic nanoparticles. This scaled giant-spin model is critically evaluated; limitations and future possibilities are explicitly formulated.

High-Spin and Reactive Fe13 Cluster with Exposed Metal Sites

Atomically defined large metal clusters have applications in new reaction development and preparation of materials with tailored properties. Expanding the synthetic toolbox for reactive high nuclearity metal complexes, we report a new class of Fe clusters, Tp*4 W4 Fe13 S12 , displaying a Fe13 core with M-M bonds that has precedent only in main group and late metal chemistry. M13 clusters with closed shell electron configurations can show significant stability and have been classified as superatoms. In contrast, Tp*4 W4 Fe13 S12 displays a large spin ground state of S=13. This compound performs small molecule activations involving the transfer of up to 12 electrons resulting in significant cluster rearrangements.

Measuring Metal-Metal Communication in a Series of Ketimide-Bridged [Fe2]6+ Complexes

Reaction of Fe(acac)3 with 3 equiv of Li[N═C(R)Ph] (R = Ph, tBu) results in the formation of the [Fe2]6+ complexes, [Fe2(μ-N═C(R)Ph)2(N═C(R)Ph)4] (R = Ph, 1; tBu, 2), in low to moderate yields. Reaction of FeCl2 with 6 equiv of Li(N═C13H8) (HN═C13H8 = 9-fluorenone imine) results in the formation of [Li(THF)2]2[Fe(N═C13H8)4] (3) in good yield. Subsequent oxidation of 3 with ca. 0.8 equiv of I2 generates the [Fe2]6+ complex, [Fe2(μ-N═C13H8)2(N═C13H8)4] (4), along with free fluorenyl ketazine. Complexes 1, 2, and 4 were characterized by 1H NMR spectroscopy, X-ray crystallography, 57Fe Mössbauer spectroscopy, and SQUID magnetometry. The Fe-Fe distances in 1, 2, and 4 range from 2.803(7) to 2.925(1) Å, indicating that no direct Fe-Fe interaction is present in these complexes. The 57Fe Mössbauer spectra for complexes 1, 2, and 4 are all consistent with the presence of symmetry-equivalent high-spin Fe3+ centers. Finally, all three complexes exhibit a similar degree of antiferromagnetic coupling between the metal centers (J = -26 to -30 cm-1), as ascertained by SQUID magnetometry.

The Potential of Gadolinium Ascorbate Nanoparticles as a Safer Contrast Agent

There have been health concerns raised against the use of gadolinium (Gd)-based magnetic resonance imaging contrast agents. The primary observation is that Gd ions are prone to leaking into the bloodstream, causing nephrogenic systemic fibrosis as one of the side effects. In addition, such leakage of the ions inhibits easy clearance from the body. Herein we propose that Gd-ascorbate nanoparticles could be one of the safer choices as they are rather stable in aqueous dispersion and they do not get affected by Zn or Fe ions in the medium. The magnetic properties of the ions are preserved in the nanoparticles, and particles when sufficiently small may be amenable to renal clearance from the human body. Thus, when an aqueous solution of Gd-acetate and ascorbic acid was left to evolve with time, a Gd-ascorbate complex was formed that led to the formation of nanoparticles with time. The sizes of the nanoparticles increased with time, and when the particles were sufficiently large, they precipitated out of the medium. In addition, smaller nanoparticles were consistently present at all times of observations. UV-vis, photoluminescence and FTIR spectroscopy, mass spectrometry, and transmission electron microscopy analyses confirmed the formation of nanoparticles of Gd-ascorbate complex. In addition, magnetic measurements confirmed the high relaxivity of the nanoparticles as compared to the parent salt, indicating the effectiveness of the nanoparticles as contrast agents. Density functional theory-based calculations of the molecular complex-based nanoparticles accounted for the experimental observations.

Metal-Metal-Bonded Fe4 Clusters with Slow Magnetic Relaxation

Reaction of FeBr₂ with Li(N═C^tBu₂) (0.5 equiv) and Zn⁰ (2 equiv) results in the formation of the formally mixed-valent cluster [Fe₄Br₂(N═C^tBu₂)₄] (1) in moderate yield. The subsequent reaction of 1 with Na(N═C^tBu₂) results in formation of [Fe₄Br(N═C^tBu₂)₅] (2), also in moderate yield. Both 1 and 2 were characterized by zero-field ⁵⁷Fe Mössbauer spectroscopy, X-ray crystallography, and superconducting quantum interference device magnetometry. Their tetrahedral [Fe₄]⁶⁺ cores feature short Fe-Fe interactions (ca. 2.50 Å). Additionally, both 1 and 2 display S = 7 ground states at room temperature and slow magnetic relaxation with zero-field relaxation barriers of U_eff = 14.7(4) and 15.6(7) cm^-1, respectively. Moreover, AC magnetic susceptibility measurements were well modeled by assuming an Orbach relaxation process.

High-Spin Superatom Stabilized by Dual Subshell Filling

Quantum confinement in small symmetric clusters leads to the bunching of electronic states into closely packed shells, enabling the classification of clusters with well-defined valences as superatoms. Like atoms, superatomic clusters with filled shells exhibit enhanced electronic stability. Here, we show that octahedral transition-metal chalcogenide clusters can achieve filled shell electronic configurations when they have 100 valence electrons in 50 orbitals or 114 valence electrons in 57 orbitals. While these stable clusters are intrinsically diamagnetic, we use our understanding of their electronic structures to theoretically predict that a cluster with 107 valence electrons would uniquely combine high stability and high-spin magnetic moment, attained by filling a majority subshell of 57 electrons and a minority subshell of 50 electrons. We experimentally demonstrate this predicted stability, high-spin magnetic moment (S = 7/2), and fully delocalized electronic structure in a new cluster, [NEt₄]₅[Fe₆S₈(CN)₆]. This work presents the first computational and experimental demonstration of the importance of dual subshell filling in transition-metal chalcogenide clusters.

[Ni30S16(PEt3)11]: An Open-shell Nickel Sulfide Nanocluster with a “Metal-like” Core

Reaction of [Ni(1,5-cod)₂] (30 equiv.) with PEt₃ (46 equiv.) and S₈ (1.9 equiv.) in toluene, followed by heating at 115 °C for 16 h, results in the formation of the atomically precise nanocluster (APNC), [Ni₃₀S₁₆(PEt₃)₁₁] (1), in 14% isolated yield. Complex 1 represents the largest open-shell Ni APNC yet isolated. In the solid state, 1 features a compact “metal-like” core indicative of a high degree of Ni–Ni bonding. Additionally, SQUID magnetometry suggests that 1 possesses a manifold of closely-spaced electronic states near the HOMO–LUMO gap. In situ monitoring by ESI-MS and ³¹P{¹H} NMR spectroscopy reveal that 1 forms via the intermediacy of smaller APNCs, including [Ni₈S₅(PEt₃)₇] and [Ni₂₆S₁₄(PEt₃)₁₀] (2). The latter APNC was also characterized by X-ray crystallography and features a nearly identical core structure to that found in 1. This work demonstrates that large APNCs with a high degree of metal–metal bonding are isolable for nickel, and not just the noble metals.

The atomically-precise nanocluster, [Ni₃₀S₁₆(PEt₃)₁₁], features a compact “metal-like” core indicative of a high degree of Ni–Ni bonding, along with an open-shell ground state.

Insights into Molecular Magnetism in Metal-Metal Bonded Systems as Revealed by a Spectroscopic and Computational Analysis of Diiron Complexes

A pair of bimetallic compounds featuring Fe-Fe bonds, [Fe(ⁱPrNPPh₂)₃FeR] (R = PMe₃, ≡N^tBu), have been investigated using High-Frequency Electron Paramagnetic Resonance (HFEPR) as well as field- and temperature-dependent ⁵⁷Fe nuclear γ resonance (Mössbauer) spectroscopy. To gain insight into the local site electronic structure, we have concurrently studied a compound containing a single Fe(II) in a geometry analogous to that of one of the dimer sites. Our spectroscopic studies have allowed for the assessment of the electronic structure via the determination of the zero-field splitting and ⁵⁷Fe hyperfine parameters for the entire series. We also report on our efforts to correlate structure with physical properties in metal-metal bonded systems using ligand field theory guided by quantum chemical calculations. Through the insight gained in this study, we discuss strategies for the design of single-molecule magnets based on polymetallic compounds linked via direct metal-metal bonds.