The Properties of the [Mn12O12(O2CR)16(H2O)4] Single-Molecule Magnets in Truly Axial Symmetry: [Mn12O12(O2CCH2Br)16(H2O)4]·4CH2Cl2

Engineering Clock Transitions in Molecular Lanthanide Complexes

Molecular lanthanide (Ln) complexes are promising candidates for the development of next-generation quantum technologies. High-symmetry structures incorporating integer spin Ln ions can give rise to well-isolated crystal field quasi-doublet ground states, i.e., quantum two-level systems that may serve as the basis for magnetic qubits. Recent work has shown that symmetry lowering of the coordination environment around the Ln ion can produce an avoided crossing or clock transition within the ground doublet, leading to significantly enhanced coherence. Here, we employ single-crystal high-frequency electron paramagnetic resonance spectroscopy and high-level ab initio calculations to carry out a detailed investigation of the nine-coordinate complexes, [HoIIIL1L2], where L1 = 1,4,7,10-tetrakis(2-pyridylmethyl)-1,4,7,10-tetraaza-cyclododecane and L2 = F- (1) or [MeCN]0 (2). The pseudo-4-fold symmetry imposed by the neutral organic ligand scaffold (L1) and the apical anionic fluoride ion generates a strong axial anisotropy with an mJ = ±8 ground-state quasi-doublet in 1, where mJ denotes the projection of the J = 8 spin-orbital moment onto the ∼C4 axis. Meanwhile, off-diagonal crystal field interactions give rise to a giant 116.4 ± 1.0 GHz clock transition within this doublet. We then demonstrate targeted crystal field engineering of the clock transition by replacing F- with neutral MeCN (2), resulting in an increase in the clock transition frequency by a factor of 2.2. The experimental results are in broad agreement with quantum chemical calculations. This tunability is highly desirable because decoherence caused by second-order sensitivity to magnetic noise scales inversely with the clock transition frequency.

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

The metal-metal-bonded molecule [Bu₄N][(^HL)₂Fe₆(dmf)₂] (Fe₆) was previously shown to possess a thermally isolated spin S = ¹⁹/₂ ground state and found to exhibit slow magnetization relaxation below a blocking temperature of ∼5 K [J. Am. Chem. Soc. 2015, 137, 13949-13956]. Here, we present a comprehensive spectroscopic investigation of this unique single-molecule magnet (SMM), combining ultrawideband field-swept high-field electron paramagnetic resonance (EPR) with frequency-domain Fourier-transform terahertz EPR to accurately quantify the spin Hamiltonian parameters of Fe₆. Of particular importance is the near absence of a 4th-order axial zero-field splitting term, which is known to arise because of quantum mechanical mixing of spin states on account of the relatively weak spin-spin (superexchange) interactions in traditional polynuclear SMMs such as the celebrated Mn₁₂-acetate. The combined high-resolution measurements on both powder samples and an oriented single crystal provide a quantitative measure of the isolated nature of the spin ground state in the Fe₆ molecule, as well as additional microscopic insights into factors that govern the quantum tunneling of its magnetization. This work suggests strategies for improving the performance of polynuclear SMMs featuring direct metal-metal bonds and strong ferromagnetic spin-spin (exchange) interactions.

Exploring the High-Temperature Frontier in Molecular Nanomagnets: From Lanthanides to Actinides

Molecular nanomagnets based on mononuclear metal complexes, also known as single-ion magnets (SIMs), are crossing challenging boundaries in molecular magnetism. From an experimental point of view, this class of magnetic molecules has expanded from lanthanoid complexes to both d-transition metal and actinoid complexes. From a theoretical point of view, more and more improved models have been developed, and we are now able not only to calculate the electronic structure of these systems on the basis of their molecular structures but also to unveil the role of vibrations in the magnetic relaxation processes, at least for lanthanoid and d-transition metal SIMs. This knowledge has allowed us to optimize the behavior of dysprosocenium-based SIMs until reaching magnetic hysteresis above liquid-nitrogen temperature. In this contribution, we offer a brief perspective of the progress of theoretical modeling in this field. We start by reviewing the developed methodologies to investigate the electronic structures of these systems and then move on focus to the open problem of understanding and optimizing the vibrationally induced spin relaxation, especially in uranium-based molecular nanomagnets. Finally, we discuss the differences in the design strategies for 4f and 5f SIMs, including an analysis of the metallocenium family.

Rare "Janus"-faced single-molecule magnet exhibiting intramolecular ferromagnetic interactions

The unusual ferromagnetically coupled compound was prepared by the use of Me₃SiN₃ with the metal ions being exclusively bridged by end-on N₃^–. Th cationic molecule is a rare example of a 3d-metal cluster exhibiting a “Janus”-faced SMM behavior for the dried and wet forms.

A rare disk-like single-molecule magnet (SMM) exclusively bridged by end-on azides with a spin ground state of S = 14 was prepared by the reaction of a divalent Fe^II precursor with Me₃SiN₃ under basic conditions. AC magnetic susceptibility studies revealed unusual, “Janus”-faced SMM behavior for the dried and pristine forms of the compound attributed to solvation/de-solvation effects of the coordinated MeCN ligands which leads to alterations in the crystal field and symmetry of the metal ions. DFT calculations confirmed the ferromagnetic nature of the interactions between the Fe^II spin carriers with the zero-field splitting parameters D = –0.2323 cm^–1 and E/D = 0.027. The results have important implications for the future study of single-molecule magnets incorporating volatile solvent molecules in the first coordination sphere of the metal ions and their effect on the relaxation dynamics.

Syntheses, structures, and magnetic properties of three two-dimensional cobalt(ii) single-ion magnets with a CoIIN4X2 octahedral geometry

Three two-dimensional Co^II SIMs with (4,4) layer structures have been synthesized and characterized structurally and magnetically.

Porous substrates as platforms for the nanostructuring of molecular magnets

This article highlights recent advances in the newly emerging field on the nanostructuration of molecular magnets using porous substrates.

Transition Metal Single-Molecule Magnets: A {Mn31} Nanosized Cluster with a Large Energy Barrier of ∼60 K and Magnetic Hysteresis at ∼5 K

The first {Mn₃₁} cluster (1) has been prepared from carboxylate ions and the chelating/bridging ligand α-methyl-2-pyridine-methanol. Compound 1 possesses a unique nanosized structural topology with one of the largest energy barriers reported to-date for high-nuclearity 3d-metal clusters. Single-crystal magnetic hysteresis studies reveal the presence of hysteresis loops below 5 K, one of the highest temperatures below which molecular hysteresis has been observed for 3d-based SMMs.

Three-Dimensional (3-D) Ferromagnetic Network of Mn12 Single-Molecule Magnets: Subtle Environmental Effects and Switching to Antiferromagnetic

A new member of the Mn₁₂ family of single-molecule magnets (SMMs) has been prepared and found to be the first of this family to give a 3-D ferromagnetic network. [Mn₁₂O₁₂(O₂CC₆H₄-p-F)₁₆(H₂O)₄] (2) was prepared by carboxylate substitution on the acetate derivative with p-F-benzoic acid and crystallizes as 2·8MeCN in space group I4̅2m with extensive formation of intermolecular C-H···F hydrogen-bonding. The latter leads to a combination of ferromagnetic (F) and antiferromagnetic (AF) interactions and an overall F network that gives a χ_MT value at low T that is abnormally high for an S = 10 ground state. 2·8MeCN undergoes solvent loss under vacuum to 2, with a decrease in unit-cell volume of 17%, primarily due to a 13% decrease in the c-axis. The χ_MT vs T plot for 2 indicates a switch to a net AF network. Exposure to air causes hydration to 2·3H₂O, a concomitant increase in unit cell volume, and a switch back to a F network. The same conversion of 2·8MeCN to 2·3H₂O can also be accomplished in one step rather than two steps, by leaving crystals of the former exposed to air at ambient temperature and pressure for 10 days, giving the same magnetic plots. Interestingly, the desolvation/solvation processes cause Jahn-Teller isomerism to occur, but the ratio of the faster-relaxing isomer to the normal slowly relaxing one does not change monotonically. Single-crystal micro-SQUID studies on 2·8MeCN show the expected magnetization hysteresis loops for a SMM and a small exchange-bias from the intermolecular interactions that is unexpectedly AF. Since the micro-SQUID study only identifies interactions along the easy-axis (z-axis) of the crystal, this is readily rationalized as due to the J_z components of the intermolecular interactions in 2·8MeCN being net AF. The combined results offer useful insights into the degree of sensitivity of the magnetic properties to small environmental perturbations.

Syntheses, crystal structures, and magnetic properties of a family of heterometallic octanuclear [Cu6Ln2] (Ln = Dy(iii), Tb(iii), Ho(iii), Er(iii), and Gd(iii)) complexes

Five novel [Cu₆Ln₂] complexes were synthesized, wherein Ln^III ions are seven-coordinate and coordinated by one triethanolamine and three di-2-pyridyl ketone ligands, without coordinated solvent molecules and small anions.

Structural diversity in Ni(II) cluster chemistry: Ni5, Ni6, and {NiNa2}n complexes bearing the Schiff-base ligand N-naphthalidene-2-amino-5-chlorobenzoic acid

The employment of the fluorescent bridging and chelating ligand N-naphthalidene-2-amino-5-chlorobenzoic acid (nacbH2) in Ni(II) cluster chemistry has led to a series of pentanuclear and hexanuclear compounds with different structural motifs, magnetic and optical properties, as well as an interesting 1-D coordination polymer. Synthetic parameters such as the inorganic anion present in the NiX2 starting materials (X = ClO4(-) or Cl(-)), the reaction solvent and the nature of the organic base employed for the deprotonation of nacbH2 were proved to be structure-directing components. Undoubtedly, the reported results demonstrate the rich coordination chemistry of nacbH2 in the presence of Ni(II) metal ions and the ability of this chelate to adopt a variety of different modes, thus fostering the formation of high-nuclearity molecules with many physical properties.

Structural anisotropy of cyanido-bridged {CoII9WV6} single-molecule magnets induced by bidentate ligands: towards the rational enhancement of an energy barrier

{CoII9[W^V(CN)₈]₆} clusters capped by odd and even number of bidentate ligands reveal the improved slow magnetic relaxation due to the significant structural anisotropy.

Effects of electron donating/withdrawing groups in the 5-substituted-2-hydroxybenzaldehyde on the synthesis of neutral cubanes with a NiII4O4core: synthesis, crystal structures and magnetic properties

This work reports synthesis, crystal structures and magnetic properties of two tetranuclear cubane-like nickel(ii) complexes of formula [Ni₄(CH₃O)₄(L)₄(CH₃OH)₄].

Heterometallic 3d–4f single-molecule magnets

This perspective gives an overview of what researchers want to achieve by preparing 3d–4f single-molecule magnets, the most significant results obtained so far and the challenges still ahead of us.

Nanoscopic molecular magnets

This review highlights fundamental concepts and synthetic strategies of SMMs and selected examples of 3d, 4f, 5f and mixed 3d–4f, 4d–5d and 3d–5f based SMMs are discussed.

Influence of the coordination environment on slow magnetic relaxation and photoluminescence behavior in two mononuclear dysprosium(iii) based single molecule magnets

Two luminescent mononuclear Dy(iii) based complexes with SMM behaviour have been reported. The SMM behaviours differ due to the slight variation of local symmetry around the Dy(iii) centers and photoluminescence study showed quantum yields of 0.98 and 1.44%.

What controls the magnetic exchange interaction in mixed- and homo-valent Mn7 disc-like clusters? A theoretical perspective

Density functional theory (DFT) studies have been undertaken to compute the magnetic exchange and to probe the origin of the magnetic interactions in two hetero- and two homo-valent heptanuclear manganese disc-like clusters, of formula [Mn(II) 4 Mn(IV) 3 (tea)(teaH2 )3 (peolH)4 ] (1), [Mn(II) 4 Mn(III) 3 F3 (tea)(teaH)(teaH2 )2 (piv)4 (Hpiv)(chp)3 ] (2), [Mn(II) 7 (pppd)6 (tea)(OH)3 ] (3) and [Mn(II) 7 (paa)6 (OMe)6 ] (4) (teaH3 =triethanolamine, peolH4 =pentaerythritol, Hpiv=pivalic acid, Hchp=6-chloro-2-hydroxypyridine, pppd=1-phenyl-3-(2-pyridyl) propane-1,3-dione; paaH=N-(2-pyridinyl)acetoacetamide). DFT calculations yield J values, which reproduce the magnetic susceptibility data very well for all four complexes; these studies are also highlighting the likely ageing/stability problems in two of the complexes. It is found that the spin ground states, S, for complexes 1-4 are drastically different, varying from S=29/2 to S=1/2. These values are found to be controlled by the nature of the oxidation state of the metal ions and minor differences present in the structures. Extensive magneto-structural correlations are developed for the seven building unit dimers present in the complexes, with the correlations unlocking the reasons behind the differences in the magnetic properties observed. Independent of the oxidation state of the metal ions, the Mn-O-Mn/Mn-F-Mn angles are found to be the key parameters, which significantly influence the sign as well as the magnitude of the J values. The magneto-structural correlations developed here, have broad applicability and can be utilised to understand the magnetic properties of other Mn clusters.