Jeffrey D. Rinehart

The low‐valent 15e ⁻ monovalent chromium complex, Cp*Cr(CNAr ^Dipp2 ) ₂ ( 2 ) (CNAr ^Dipp2 = 2,6‐(2,6‐( ⁱ Pr) ₂ C ₆ H ₃ ) ₂ C ₆ H ₃ ), was isolated and serves as an isolobal analogue of the unstable, monomeric CpCr(CO) ₂ fragment. Complex 2 was found to have an unusual intermediate spin ground state ( S = 3/2), which enables reversible N ₂ binding to afford the 17e ⁻ low‐spin ( S = 1/2) complex, Cp*Cr(N ₂ )(CNAr ^Dipp2 ) ₂ ( 2‐N ₂ ), which is a rare example of an N ₂ ‐induced spin state change. Complex 2 demonstrates distinct redox activity, which allowed for a stepwise investigation of its N ₂ binding ability as a function of its redox state. While 1e ^– reduction of 2 or 2‐N ₂ under an N ₂ atmosphere suppresses N ₂ dissociation to give a saturated dinitrogen‐bound monoanionic species, KCp*CrN ₂ (CNAr ^Dipp2 ) ₂ ( 4 ), further reduction leads to complete N ₂ dissociation to afford the dianionic unsaturated metalloradical, K ₂ Cp*Cr(CNAr ^Dipp2 ) ₂ ( 5 ). This demonstrates how N ₂ binding at metal centers is affected by subtle changes to electronic structure, a concept of critical importance to catalytic N ₂ reduction.

The low‐valent 15e ⁻ monovalent chromium complex, Cp*Cr(CNAr ^Dipp2 ) ₂ ( 2 ) (CNAr ^Dipp2 = 2,6‐(2,6‐( ⁱ Pr) ₂ C ₆ H ₃ ) ₂ C ₆ H ₃ ), was isolated and serves as an isolobal analogue of the unstable, monomeric CpCr(CO) ₂ fragment. Complex 2 was found to have an unusual intermediate spin ground state ( S = 3/2), which enables reversible N ₂ binding to afford the 17e ⁻ low‐spin ( S = 1/2) complex, Cp*Cr(N ₂ )(CNAr ^Dipp2 ) ₂ ( 2‐N ₂ ), which is a rare example of an N ₂ ‐induced spin state change. Complex 2 demonstrates distinct redox activity, which allowed for a stepwise investigation of its N ₂ binding ability as a function of its redox state. While 1e ^– reduction of 2 or 2‐N ₂ under an N ₂ atmosphere suppresses N ₂ dissociation to give a saturated dinitrogen‐bound monoanionic species, KCp*CrN ₂ (CNAr ^Dipp2 ) ₂ ( 4 ), further reduction leads to complete N ₂ dissociation to afford the dianionic unsaturated metalloradical, K ₂ Cp*Cr(CNAr ^Dipp2 ) ₂ ( 5 ). This demonstrates how N ₂ binding at metal centers is affected by subtle changes to electronic structure, a concept of critical importance to catalytic N ₂ reduction.

A unique low valent 15 e- Cr(I) species, Cp*Cr(CNArDipp2)2 (2) (Cp* = Me5C5, CNArDipp2 = 2,6-(2,6-(iPr)2C6H3)2C6H3) was isolated under Ar atmosphere. It’s intermediate spin ground state (S = 3/2) was determined via Evans’s method with 1H NMR spectroscopy, and electron paramagnetic resonance (EPR) spectroscopy. N2 reversibly binds 2 to afford a 17 e- low spin (S = 1/2) complex, Cp*CrN2(CNArDipp2)2 (2-N2), This dynamic N2 coordination was characterized based upon their distinct magnetic properties via collaborative analysis of Evans’s method and SQUID magnetometry. One electron reduction of 2 or 2-N2 under N2 atmosphere effectively suppress the N2 elimination to give a saturated monoanionic species, KCp*CrN2(CNArDipp2)2 (4). Compound 2 can also be further reduced with 2 equiv. of KC8 to generate a dianionic unsaturated metalloradical, K2Cp*Cr(CNArDipp2)2 (5), The exceedingly red shifted vCN stretching bands suggest highly activated C-N triple bond of the bulky terphenyl isocyanide ligands.

The spin relaxation of an Er³⁺ tetranuclear single-molecule magnet, [Er(hdcCOT)I]₄, (hdcCOT = hexahydrodicyclopentacyclooctatetraenide dianion), is modeled as a near-tetrahedral arrangement of Ising-type spins. Combining evidence from single-crystal X-ray diffraction, magnetometry, and computational techniques, the slow spin relaxation is interpreted as a consequence of symmetry restrictions imposed on quantum tunneling within the cluster core. The union of spin and spatial symmetries describe a ground state spin-spin coupled manifold wherein 16 eigenvectors generate the 3D quantum spin-space described by the vertices of a rhombic dodecahedron. Analysis of the experimental findings in this context reveals a correlation between the magnetic transitions and edges connecting cubic and octahedral subsets of the eigenspace convex hull. Additionally, the model is shown to map to a theoretically proposed quantum Cayley network, indicating an underexplored synergy between mathematical descriptions of molecular spin interactions and quantum computing configuration spaces.

The design of molecular magnets has progressed greatly by taking advantage of the ability to impart successive perturbations and control vibronic transitions in 4fⁿ systems through the careful manipulation of the crystal field. Herein, we control the orientation and rigidity of two dinuclear ErCOT-based molecular magnets: the inversion-symmetric bridged [ErCOT(μ-Me)(THF)]₂ (2) and the nearly linear Li[(ErCOT)₂(μ-Me)₃] (3). The conserved anisotropy of the ErCOT synthetic unit facilitates the direction of the arrangement of its magnetic anisotropy for the purposes of generating controlled internal magnetic fields, improving control of the energetics and transition probabilities of the electronic angular momentum states with exchange biasing via dipolar coupling. This control is evidenced through the introduction of a second thermal barrier to relaxation operant at low temperatures that is twice as large in 3 as in 2. This barrier acts to suppress through-barrier relaxation by protecting the ground state from interacting with stray local fields while operating at an energy scale an order of magnitude smaller than the crystal field term. These properties are highlighted when contrasted against the mononuclear structure ErCOT(Bn)(THF)₂ (1), in which quantum tunneling of the magnetization processes dominate, as demonstrated by magnetometry and ab initio computational methods. Furthermore, far-infrared magnetospectroscopy measurements reveal that the increased rigidity imparted by successive removal of solvent ligands when adding bridging methyl groups, along with the increased excited state purity, severely limits local spin-vibrational interactions that facilitate magnetic relaxation, manifesting as longer relaxation times in 3 relative to those in 2 as temperature is increased.

We present a wide-ranging interrogation of the border between single-molecule and solid-state magnetism through a study of erbium-based Ising-type magnetic compounds with a fixed magnetic unit, using three different charge-balancing cations as the means to modulate the crystal packing environment. Properties rooted in the isolated spin Hamiltonian remain fixed, yet careful observation of the dynamics reveals the breakdown of this approximation in a number of interesting ways. First, differences in crystal packing lead to a striking 3 orders of magnitude suppression in magnetic relaxation rates, indicating a rich interplay between intermolecular interactions governed by the anisotropic Ising lattice stabilization and localized slow magnetic relaxation driven by the spin-forbidden nature of quantum tunneling of the f-electron-based magnetization. By means of diverse and rigorous physical methods, including temperature-dependent X-ray crystallography, field, temperature, and time-dependent magnetometry, and the application of a new magnetization fitting technique to quantify the magnetic susceptibility peakshape, we are able to construct a more nuanced view of the role nonzero-dimensional interactions can play in what are predominantly considered zero-dimensional magnetic materials. Specifically, we use low field susceptibility and virgin-curve analysis to isolate metamagnetic spin-flip transitions in each system with a field strength corresponding to the expected strength of the internal dipole–dipole lattice. This behavior is vital to a complete interpretation of the dynamics and is likely common for systems with such high anisotropy. This collective interactivity opens a new realm of possibility for molecular magnetic materials, where their unprecedented localized anisotropy is the determining factor in building higher dimensionality.

Magnetization is a common measurable for characterizing bulk, nanoscale, and molecular materials, which can be quantified to high precision as a function of an applied external field. These data provide detailed information about a material's electronic structure, phase purity, and impurities, though interpreting this data can be challenging due to many contributing factors. In sub-single-domain particles of a magnetic material, an inherently time-dependent rotation of the entire particle spin becomes possible. This phenomenon, known as superparamagnetism (SPM), simultaneously represents a very early size-dependent property to be considered, while being one of the least explored in the current quantum materials era. This discrepancy is, at least in part, due to the need for models with less built-in complexity that can facilitate the generation of comparative data. In this work, we map an extensive dataset of variable-size SPM Fe₃O₄ (magnetite) to an intrinsic statistical model for their field-dependence. By constraining the SPM behavior to a probabilistic model, the data are apportioned to several decorrelated sources. From this, there is strong evidence that standard measures such as saturation magnetization, M_S, are poor comparative parameters, being dependent on experimental knowledge and measurement of the magnetic mass. In contrast, parameters of the intrinsic probability distribution, such as the maximum susceptibility, χ_max, are far better suited to describe the SPM behavior itself and do not propagate unknown magnetic mass error. By confining the data fitting to intrinsic variables of the model distribution, scaling parameters, and linear contributions, we find greater value in magnetic data, ultimately aiding potential synthesis diagnostics and prediction of new properties and functionality.

The synthesis of iron oxide nanoparticles with control over size and shape has long been an area of research, with iron oleate being arguably the most successful precursor. Issues with reproducibility and versatility in iron oleate-based syntheses remain, however, in large part due to the mutable nature of its structure and stoichiometry. In this work, we characterize two new forms of iron oleate precursor that can be isolated in large quantities, show long-term stability, and have well-defined stoichiometry, leading to reproducible and predictable reactivity. Synthesis with these precursors is shown to produce iron oxide nanoparticles in a tunable size range of 4-16 nm with low size dispersity and properties consistent with magnetite in the superparamagnetic size regime.

Dipolar coupling is rarely invoked as a driving force for slow relaxation dynamics in lanthanide-based single-molecule magnets, though it is often the strongest mechanism available for mediating inter-ion magnetic interactions in such species. Indeed, for multinuclear lanthanide complexes, the magnitude and anisotropy of the dipolar interaction can be considerable given their ability to form highly directional, high-moment ground states. Herein, we present a mono-, di-, and trinuclear erbium-based single-molecule magnet sequence, ([Er-TiPS₂COT]⁺)_n (n = 1-3), wherein a drastic reduction in the allowedness of magnetic relaxation pathways is rationalized within the framework of the dipole-dipole interactions between angular momentum quanta. The resulting design principles for multinuclear molecular magnetism arising from intramolecular dipolar coupling interactions between highly anisotropic magnetic states present a nuanced justification of the relaxation dynamics in complex manifolds of individual quantized transitions. Experimental evidence for the validity of this model is provided by coupling the relaxation dynamics to an AC magnetic field across an unprecedented frequency range for molecular magnetism (10³-10^-5 Hz). The combination of slow dynamics and multiple, low-energy transitions leads to a number of noteworthy phenomena, including a lanthanide single-molecule magnet with three well-defined relaxation processes observable at a single temperature.