The Inherent Single‐Molecule Magnet Character of Trivalent Uranium

Abstract

Single-molecule magnets (SMMs) are molecules that exhibit slow relaxation of their magnetization of purely molecular origin. [1] This is because SMMs possess high-spin ground states in which spin-orbit coupling results in zero-field splitting of the (2S + 1)-fold degenerate ground multiplet, which in turn creates a thermal barrier to relaxation of the magnetization that gives rise to magnetic bistability. SMMs form part of a larger family of molecular nanomagnets which are the subject of intense research because of their potential applications in fields as diverse as molecule-based information storage, [1] quantum information processing, [2] spintronics, [3] and magnetocaloric refrigeration. [4] SMMs are often based on transition-metal clusters, but significant attention has recently focused on complexes of single and multiple lanthanoid ions, because the crystal-field splitting of the lowest Russell-Saunders multiplet engenders large magnetic anisotropies. [5][6][7][8][9][10][11] These anisotropies are responsible for high relaxation barriers and therefore slow magnetic relaxation. However, well isolated high-spin ground states are difficult to achieve within polynuclear lanthanoid SMMs because the valence 4f orbitals have limited radial extension and are usually energetically incompatible with ligand orbitals. These inherent 4f orbital properties give predominantly ionic interactions and results in weak magnetic exchange coupling with neighboring spin centers, with very few exceptions. [7,12] In principle, actinoids, and in particular uranium, possess properties that render these ions ideal candidates from which to construct SMMs. This is because, compared to the lanthanoids, uranium exhibits enhanced crystal field splitting, [13,14] as well as increased covalency, the latter enabling significant spin couplings in polynuclear systems, [15] and therefore both stronger magnetic exchange and anisotropies can be envisaged. This premise was realized recently with reports of several closely related single-ion pyrazolylborate uranium(III) complexes which were shown to display SMM behavior. [14,[16][17][18][19][20] In addition, two neptunium complexes were shown to display slow relaxation of the magnetization. [15,21] The fact that all uranium(III) SMMs reported to date are closely related to each other raises the question as to how sensitive SMM behavior in trivalent uranium is to the composition of the coordination sphere and its symmetry. SMM behavior in all published examples is much more pronounced in an external field than in zero field, which suggests that quantum tunneling of the magnetization plays a significant role in shortening the relaxation times. However, in principle, low-symmetry crystal field components cannot induce tunneling of the magnetization, because uranium(III) is a Kramers half-integer angular momentum ion. Also the nuclear spin I = 0 of 238 U cannot induce tunneling of the magnetization. The answer to the question of what causes efficient quantum tunneling will have serious implications for ...

Molecular single-ion magnets based on lanthanides and actinides: Design considerations and new advances in the context of quantum technologies

“…Five-coordinate trigonal bipyramidal compounds containing both Er and Dy ions have been synthesized: (26) or Dy (27)). Both exhibit slow magnetisation dynamics, although much weaker than the three-coordinated Er compound (25) 28), notable for having a chloride anion placed in the axial position (Fig.…”

“…Indeed, some examples of 5f-SIM systems based on U 3+ (5f 3 , J = 9/2) [24,27,28,30,[118][119][120][121][122][123], and U 5+ (5f 1 , J = 5/2) [25,26] ions have been reported in the last few years. Both are Kramers ions (S = half integer) with large values of angular momentum, J, which are guaranteed to possess a doubly-degenerate mJ ground state-an important requirement in SIM research.…”

“…Single-ion magnets (SIMs) are simply a subclass of SMMs wherein the electronic spin term originates from a single magnetic centre [7,8]. In SIMs, the ion bearing the spin is most commonly a transition metal [9][10][11][12][13], lanthanoid [14][15][16][17][18][19][20][21][22], or actinoid [23][24][25][26][27][28][29][30]. The term SIM is somewhat of a misnomer: all SIMs are molecular in nature since the ligand field is a vital prerequisite for their slow relaxation.…”

Radical ligand-containing single-molecule magnets

“…Additionally, in contrast to lanthanides, actinides provide a wide range of accessible oxidation states [320]. However, only a few actinide complexes have been shown to exhibit slow magnetic relaxation, with nearly all of these being uranium complexes [343][344][345][346][347][348][349][350][351]. Nevertheless, despite the difficulties associated with handling of the more radioactive transuranic elements, neptunium and plutonium-based singlemolecule magnets have also been reported [352][353][354].…”

Magnetic and Photo-Physical Properties of Lanthanide Dinuclear Complexes Involving the 4,5-Bis(2-Pyridyl-N-Oxidemethylthio)-4′,5′-Dicarboxylic Acid-Tetrathiafulvalene-, Dimethyl Ester Ligand

“…Lanthanide ions indeed have several advantages, such as strong single-ion anisotropy and large magnetic moment [12,13]. Recently, actinides have started to contribute to the development of new SMMs due to their strong magnetic anisotropy and larger exchange interactions coming from the more extended nature of the 5f orbitals [14][15][16][17][18][19][20].…”