Silylimidazolium Hexafluorophosphate Salts as Synthetic Precursors to N-Heterocyclic Carbene Pentafluorophosphorus Adducts.

N-Heterocyclic carbene pentafluorophosphorus (NHC-PF5) adducts are six-coordinate phosphorus(v) compounds with emerging applications but poor synthetic accessibility. We have developed a simple and high yielding protocol for synthesizing imidazolylidene NHC-PF5 adducts from silylimidazolium hexafluorophosphate salts. Using this methodology, we have prepared a series of NHC-PF5 adducts in high yields, including new NHC-PF5 building blocks amenable to subsequent synthetic diversification. We also demonstrate that a similar approach enables access to analogous, synthetically elusive NHC-BF3 and NHC-SbF5 adducts.

Dipolar Coupling as a Mechanism for Fine Control of Magnetic States in ErCOT-Alkyl Molecular Magnets.

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.

Intuitive Control of Low-Energy Magnetic Excitations via Directed Dipolar Interactions in a Series of Er(III)-Based Complexes.

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.

Size-Controlled Hapticity Switching in [Ln(C9 H9 )(C8 H8 )] Sandwiches.

Sandwich complexes of lanthanides have recently attracted a considerable amount of interest due to their applications as Single Molecule Magnet (SMM). Herein, a comprehensive series of heteroleptic lanthanide sandwich complexes ligated by the cyclononatetraenyl (Cnt) and the cyclooctatetraenyl (Cot) ligand [Ln(Cot)(Cnt)] (Ln=Tb, Dy, Er, Ho, Yb, and Lu) is reported. The coordination behavior of the Cnt ligand has been investigated along the series and shows different coordination patterns in the solid-state depending on the size of the corresponding lanthanide ion without altering its overall anisotropy. Besides the characterization in the solid state by single-crystal X-ray diffraction and in solution by 1 H NMR, static magnetic studies and ab initio computational studies were performed.

A method for extending AC susceptometry to long-timescale magnetic relaxation.

As the ability to generate magnetic anisotropy in molecular materials continues to hit new milestones, concerted effort has shifted towards understanding, and potentially controlling, the mechanisms of magnetic relaxation across a large time and temperature space. Slow magnetic relaxation in molecules is highly temperature-, field-, and environment-dependent with the relevant timescale easily traversing ten orders of magnitude for current single-molecule magnets (SMM). The prospect of synthetic control over the nature of (and transition probabilities between) magnetic states make unraveling the underlying mechanisms an important yet daunting challenge. Currently, instrumental considerations dictate that the characteristic relaxation time, τ, is determined by separate methods depending on the timescale of interest. Static and dynamic probe fields are used for long- and short-timescales, respectively. Each method captures a distinct, non-overlapping time range, and experimental differences lead to the possibility of fundamentally different meanings for τ being plotted and fitted globally as a function of temperature. Herein, we present a method to generate long-timescale waveforms with standard vibrating sample magnetometry (VSM) instrumentation, allowing extension of alternating current (AC) susceptometry to SMMs and other superparamagnets with arbitrarily long relaxation time. We fit these data to a generalized Debye model and present a comparison to results obtained from direct current (DC) magnetization decay.

Million-fold Relaxation Time Enhancement across a Series of Phosphino-Supported Erbium Single-Molecule Magnets.

Maintaining strong magnetic anisotropy in the presence of collective spin interactions has become a defining challenge in the advancement of single-molecule magnet (SMM) research. Herein we demonstrate effective decoupling of these often competing design goals in a series of new phosphino-supported SMMs containing the anisotropic [Er(COT)]+ (COT2- = cyclooctatetraene dianion) subunit. Across this series, a magnetic nuclearity increase from 1 to 2 and subsequent optimization of the relative local anisotropy axis orientation results in dramatic improvements to the long time scale behavior. Specifically, we observe a 6 orders of magnitude increase in relaxation time at 2 K and a consequent open magnetic hysteresis up to 6 K. This drastic scaling of the magnetic dynamics tracks monotonically with the introduction and approach to parallel of the angle between intramolecular anisotropy axes. These results illustrate the powerful implications of fully controlling direction and magnitude of anisotropy in the design of scalable SMMs.