Actinide-based single-molecule magnets: alone or in a group?

Abstract

In recent years, very significant progress has been made in single-molecule magnets (SMMs), and one of the major milestone works is the hysteresis blocking temperature beyond the boiling point of liquid nitrogen. As increasingly abundant experimental and theoretical cases are studied, our understanding of how to construct high-performance rare-earth SMMs is becoming clearer, while few good SMMs have been reported for actinides that possess stronger spin–orbital coupling than rare earths. Recently, attempts to replicate the successful strategy on rare-earth SMMs to actinides have proven to be a failure, or at least frustrating, with the main reason being the stronger covalent contribution in the actinide-ligand bonds. In this review, the progress on actinide SMMs is summarized to look back at how far we have come and try to find possible routes forward.

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Ming Liu

Ming Liu received his B.S. degree (2018) and M.S. degree (2021) from Harbin University of Science and Technology, and Nanjing Normal University, respectively, and now he is studying for a Ph.D. degree at University of Electronic Science and Technology of China. His current research focuses on the design and synthesis of low-dimensional molecular magnets.

Xiao-Han Peng

Xiao-Han Peng received her B.S. degree (2021) from Sichuan Normal University and now is studying for a M.S. degree at University of Electronic Science and Technology of China. Her current research focuses on the design and synthesis of low-dimensional molecular magnets.

Fu-Sheng Guo

Fu-Sheng Guo obtained his B.Sc. degree in 2009 and Ph.D. degree in 2014 under the supervision of Prof. Ming-Liang Tong at Sun Yat-Sen University. After a year working as a lecturer at Northwest University, PR China, He was awarded a two-year Marie Sklowdowska-Curie International Fellowship In 2015, and he continued as a research fellow in the research group of Prof. Richard Layfield. Now he is a full professor at University of Electronic Science and Technology of China. His current research interests are in the design, synthesis and properties of organometallic molecular magnets.

Ming-Liang Tong

Ming-Liang Tong was born in Hubei, China, in 1967. He received his BSc in 1989 from Central China Normal University (P.R. China), MSc in 1996 and Ph.D. in 1999 from Sun Yat-Sen University (P.R. China). Then he joined the faculty at Sun Yat-Sen University and was promoted to a Professor in 2004. He worked as a Japan Society for the Promotion of Science (JSPS) postdoctoral fellow at Kyoto University (Japan) from 2001 to 2003. His current interests focus upon the design and development of new synthetic methods towards molecular magnetic materials including single-molecule magnets, cryogenic molecular magnetic refrigerants, spin crossover, MRI contrast agents and multifunctional molecular materials.

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1. Introduction

The original discovery of [Mn₁₂] clusters showing magnetic behaviour of pure molecular origin at cryogenic temperatures akin to the bulk magnets (e.g., Nd₂Fe₁₄B and SmCo₅)¹ has driven the explosive development of the emerging materials with magnetic properties, namely single-molecule magnets (SMMs).^2–5 Given their largely promising applications in next-generation quantum processing, data storage, or molecular spintronics,^6–8 SMMs have aroused expected attention in recent years.

The requirements for high effective energy barrier (U_eff) and magnetic hysteresis blocking temperature (T_B) are two crucial keys to achieving the above attractive applications, which remain the unusually heaviest challenges in this area. T_B is closely going hand in hand with the U_eff determined by the uniaxial magnetic anisotropy and relaxing through higher excited states. To improve the value of U_eff, the efforts initially focused on the giant transition metal spin-systems providing massive magnetic centers with strong magnetic coupling, which, however, usually ensued with a severe decline in anisotropy itself, and was not very favourable for SMM behaviours.⁹

An alternative method to maximize the anisotropy, instead of the extreme pursuit of a larger number of spin ground states, came in the subsequent period. Lanthanide ions with large magnetic moments and high magnetic anisotropy, especially Tb(III), Dy(III), Ho(III), and Er(III), are very excellent spin carriers for the synthesis of SMMs.^10–13 After the mononuclear compound, [TbPc₂]⁻ (Pc = phthalocyanine dianion), with a high energy barrier of 230 cm⁻¹ was reported by Ishikawa et al.¹⁰ a large number of lanthanide-based complexes have been studied, and they are the research mainstay in the field of SMMs at present. Moreover, along with the continuous improvement of theoretical research, the understanding of how to construct lanthanide SMMs with large anisotropy barriers and blocking temperatures is becoming clearer, and several symmetry strategies based on an effective charge model, such as C_n (n ≥ 7), S₈/D_4d, C_5h/D_5h and S₁₂/D_6d are proposed or have been proved to be helpful.¹⁴

Among the lanthanide ions, Dy(III) is the most studied and brightest performer. Dysprosium possessing a 4f⁹ configuration is a Kramers ion and therefore ensures a bistable ground state regardless of the coordination environment. The oblate electron density of Dy(III) provides guidance to chemists on how to design molecular structures: creating a strong crystal field in the axial direction and minimizing the crystal field in the equatorial plane reduces the electron repulsion between the dysprosium ion and the ligand, allowing the ground state to stabilize at a higher M_J value. In terms of a dysprosium complex, although strict point symmetry is not essential for observing slow magnetic relaxation, the closer the coordination geometry is to ideal point symmetry, the better it behaves as a single-molecule magnet. The synthesis of low-coordinate complexes with perfect point symmetry is very challenging, mainly due to the large ionic radii and odd valence states of lanthanide ions. One of the most striking subsets of the SMM family is dysprosium metallocenes, in which the cyclopentadiene ligand generates a dominant axial crystal field. Recently, some of us reported a heteroligand dysprosium metallocene complex, [Cp^iPr5DyCp*][B(C₆F₅)₄] (Cp^iPr5 = pentaisopropylcyclopentadienyl; Cp* = pentamethylcyclopentadienyl), which has a more wider Cp–Dy–Cp angle and shorter Dy–Cp distances.¹¹ The strong axial crystal field results in a large U_eff of 1541 cm⁻¹ and a high blocking temperature of 80 K, making it the first SMM to show open magnetic hysteresis above the boiling point of liquid nitrogen.

Compared to lanthanide complexes, actinides appear as potential candidates for the construction of outstanding SMMs because of their significantly stronger spin–orbit coupling, which can result in greater magnetic anisotropy.¹⁵ The relatively more diffuse radial extension of 5f orbitals can lead to a greater degree of metal–ligand covalency, contributing to strong magnetic exchange between spin carriers in polymetallic complexes.¹⁶ It can be said that actinides combine the advantages of both lanthanides and transition metals. Furthermore, the accessible oxidation states in actinides ranging from +7 to +1 are potentially promising characteristics to design and synthesize various electronic structures.¹⁷ To date, however, the actinides have been far inferior to the lanthanides and transition metals in terms of SMMs, with only a small fraction exhibiting slow magnetic relaxation and mainly concentrated in uranium complexes. Actinide SMMs often exhibit remarkably small reversal barriers and no magnetic hysteresis.^18,19 Because of the rapid development and excellent performance of transition metals or lanthanides in high-performance SMMs in recent years, some similar successful strategies were attempted to replicate in actinides, but the results so far have been very frustrating, with actinides exhibiting complexities not encountered with transition metals and lanthanides. The main reason for such phenomena is generally traced to the covalent contribution in the actinide–ligand bonds, leading to the occurrence of rapid relaxation progress. Therefore, understanding the relaxation mechanism and exploring their rational strategies in actinide SMMs are highly critical to further progress, yet they remain a big challenge at the current stage.

In this review, we aim to present a survey of representative actinide SMMs to look back at how far we have come. Meanwhile, we pay attention to the influence of the ligand field and magnetic exchange of paramagnetic centres on the magnetic dynamics, trying to seek out a feasible strategy for top-performing An-SMMs. The survey consists of two parts, mononuclear actinide SMMs, and multinuclear An-SMMs with a magnetic exchange. Due to the radioactivity of the actinides, only a handful of laboratories are qualified to handle and study them, and most of them are concentrated in uranium. Transuranic elements have more restrictions because of their scarcity and instability, so there are even fewer cases of SMMs, and in the section of mononuclear actinide SMMs, transuranic elements are listed with uranium, classified by the unpaired electron number.

2. Mononuclear actinide-based SMMs

2.1 U(III) and Np(IV) with 3 unpaired electrons

The relatively stable and abundant U-238 isotope occupies the majority of the actinide SMMs. Uranium complexes reported so far show that the oxidation state can range from +1 to +6, and especially a Kramers ion, U(III) with 3 unpaired electrons, has a large total angular momentum, being potentially a good candidate for SMMs. The Long group reported the first actinide-based SMM, which is a neutral uranium(III) complex, [U(Ph₂BPz₂)₃] (1) (Pz = pyrazolyl) with quasi D_3h symmetry (Table 1).²⁰ In the structure, the six N atoms from three bidentate Ph₂BPz₂ ligands complete the coordination environment of uranium ions. The resultant field and symmetry effect caused the removal of the M_J substate degeneracy, producing slow magnetic relaxation with a U_eff = 20 cm⁻¹ and pre-exponential factor τ₀ = 1.0 × 10⁻⁷ s in zero applied field. Furthermore, despite the Kramers ground state, the obvious deviations from linearity in the ln(τ) vs. 1/T plot revealed that faster magnetic relaxations such as quantum tunnelling of the magnetisation (QTM) are still inevitable at low temperatures.

Table 1 Selected mononuclear actinide-based SMMs

Classification	Complex	Local symmetry	U eff (cm^−1)		H dc (Oe)	τ 0 (s)	Ref.
			Cal.	Exp.
a The dc field used in ac susceptibility measurements.
3 unpaired electrons	U(Ph2BPz2)3	D 3h	190	20	0	1.0 × 10^−7	21
	U(H2BPz2)3	D 3h	230	8	100	1.2 × 10^−6	22
	U(Bc^Me)3	C 3h	—	33	750	1.0 × 10^−7	24
	[K(18-crown-6)] [U(OSi(O^tBu)3)4]	—	—	18	0	2.6 × 10^−7	25
	[K(18-crown-6)] [U(N(SiMe3)2)4]	—	—	16	0	2.2 × 10^−8	25
	(BDI)U(ODipp)2	C 2v	—	15	500	3.4 × 10^−7	26
	[(Tp*2U)-CCPh]	C 2	—	7	1000	2.1 × 10^−6	27
	[(Tp*2U)(THF)]	C 2	—	9	500	4.3 × 10^−6	27
	[(Tp*2U)(MeCN)2]	C 2	—	8	1000	4.0 × 10^−6	27
	[U({SiMe2NPh}3-tacn)(OPPh3)]	C 3v	147	15	0	1.1 × 10^−7	29
	[U((SAr^Ad,Me)3mes)]	C 3v	—	10	2000	2.8 × 10^−6	30
	[UI3(THF)4]	C 2v	—	13	2000	6.0 × 10^−6	31
	[U(N(SiMe3)2)3]	C 3v	—	22	2000	1.5 × 10^−6	31
	[U(BIPM^TMS)(I)2(THF)]	Cs/C1	—	16	2000	2.8 × 10^−6	31
	[U(N(SiMe2tBu)2)3]	D 3h	—	15	600	3.1 × 10^−11	32
	[Li(DME)3] [U^III(COT′′)2]	C 2v	—	19	1000	4.6 × 10^−6	34
	[Np(η^8-COT)2]	D 8h	1400	29	300	1.1 × 10^−5	39
4 unpaired electrons	[(L^Ar)NpCl]	C 2v	—	—	1000	—	40
5 unpaired electrons	[PuTp3]	C 3h	332	18	100	2.9 × 10^−7	41
	[K(2.2.2-cryptand)] [(η^5-C5^iPr5)2U]	C 2v	—	14	0	1.1 × 10^−7	17
1 unpaired electron	[U(O)-(Tren^TIPS)]	C 3v	—	15	1000	2.6 × 10^−7	42

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Replacing the phenyl group with a hydrogen atom produced a closely related derivative [U(H₂BPz₂)₃] (2) (Fig. 1), which showed the same structure symmetry as that of complex 1 but a subtle elongation was observed along the direction of pseudo-C₃ axis.²² Compared with the isostructural complex 1, a much smaller energy barrier of 8 cm⁻¹ (τ₀ = 1.2 × 10⁻⁶ s) under the applied dc field was quantified for the analogue 2, which is likely attributed to the axial extension. Subsequently, the impact of magnetic dilution on the relaxation behaviour in 2 with different concentrations was further elucidated by Long and co-workers.²³ Dilution-induced SMM behaviour in the highest dilution (U : Y = 1 : 90) was presented, yielding a thermal-activated barrier U_eff = 16 cm⁻¹, two times larger than that of the previously concentrated sample. Intriguingly, computational studies of 1 and 2 revealed that the values of the first excited states are 190 and 230 cm⁻¹, respectively, which significantly exceed the U_eff determined from experiments.¹⁸

Fig. 1 The crystal structure of [U(H₂BPz₂)₃]. U atom is drawn in yellow, B in orange, N in blue, and C in grey; H atoms are omitted for clarity except for those bound to B atoms.

Donor atoms may play crucial roles in determining the magneto-dynamic characteristics of uranium-based complexes because of the discrepancies in the strength of the ligand field effect and uranium-ligand covalency. The long group compared the behaviour of SMM in two isostructural complexes, [U(Bp^Me)₃] (3a) (Bp^Me = dihydrobis(methylpyrazolyl)borate) and [U(Bc^Me)₃] (3b) (Bc^Me = dihydrobis(methylimidazolyl)borate), finding that the stronger ligand field of N-heterocyclic carbene in complex 3b gives rise to greater covalency and anisotropy, exhibiting slower relaxation than complex 3a.²⁴ The 12% magnetically dilute version of 3b yielded a large barrier of 33 cm⁻¹ with τ₀ = 1 × 10⁻⁷ under a field of 750 Oe, dominated by an Orbach relaxation in the high-temperature range.

In the same year, two novel pseudotetrahedral complexes, [K(18-crown-6)][U(N(SiMe₃)₂)₄] (4a) and [K(18-crown-6)][U(OSi(O^tBu)₃)₄] (4b) with the similar crystal structure but distinct ligands surroundings were reported by Mazzanti and co-workers.²⁵ Both demonstrated slow magnetic relaxation already at zero dc fields but exceedingly similar values of anisotropy barriers (16 cm⁻¹ for 4a and 18 cm⁻¹ for 4b). The insignificant differences were attributed to relatively slight variations in ligand field strength. Recently, another tetrahedral uranium(III) complex, (BDI)U(ODipp)₂ (5) (BDI = β-diketiminate, ODipp = 2,6-diisopropylphenolate), was reported by Arnold et al.²⁶ possessing a coordination environment between 4a and 4b. The complex showed obvious SMM behaviour and displayed well-defined peaks from 1.8 K to 4 K in the out-of-phase alternating current susceptibility (χ′′) under an applied 500 Oe field, with associated U_eff = 14.5(1) cm⁻¹ (τ₀ = 3.39(24) × 10⁻⁷ s), and the thermal barrier is similar to that of 4a and 4b.

Similarly, the insensitivity of dynamic magnetic performance to subtle changes in the ligand field is supported by a homologous series of mononuclear species, [Tp*₂UCCPh] (6a), [Tp*₂U(THF)](BPh₄) (6b) and [Tp*₂U(MeCN)₂](BPh₄) (6c), reported by the Shores group in 2019.²⁷ All complexes show field-induced SMM behaviours, with very small and similar energy barriers of 6.8 cm⁻¹ (τ₀ = 2.1 × 10⁻⁶ s), 9.0 cm⁻¹ (τ₀ = 4.3 × 10⁻⁶ s) and 8.4 cm⁻¹ (τ₀ = 4.0 × 10⁻⁶ s), respectively.

Generally, slow relaxation is common for U(III) complexes, however, a rare example, [U({SiMe₂NPh}₃-tacn)] (7a) (tacn = 1,4,7-triazacyclononane) with an equatorial charge distribution around the metal center, does not exhibit this kind of behaviour down to 1.7 K.^28,29 Then, the structure of complex 7a was modified by coordination of an axial OPPh₃ ligand, isolating complex [U({SiMe₂NPh}₃-tacn)(OPPh₃)] (7b) (Fig. 2). Upon the modification, the bond lengths of U–N(amido) and U–N(amino) are longer about 0.1 Å than those in 7a.²⁹ The U–O–P angle of 176.94(14)° is almost linear. Slow magnetic relaxation below 5 K was observed for complex 7b, with peaks in χ′ (in-phase alternating current susceptibility) and χ′′ even in zero applied fields. The energy barriers were obtained by fitting the relaxation time in zero and 2 kOe static field to the Arrhenius law, resulting in 15.2(4) cm⁻¹ and 17.4(2) cm⁻¹, respectively. Although the differences between these two complexes were intuitively explained by that the axial OPPh₃ could stabilize large M_J states, the authors also suggested the different behaviours cannot simply be attributed to differences in crystal field.

Fig. 2 The crystal structure of [U({SiMe₂NPh}₃-tacn)(OPPh₃)]. Color codes: U, yellow; Si, pale yellow; O, red; C, grey; N, blue; P, purple. H atoms are omitted for clarity.

Hard-donor ligands such as alkoxides, aryloxides, and amides have been largely introduced into uranium complexes, while examples of soft ligands involved are very rare. Recently, the Meyer group reported one uranium thiolate complex [U((SAr^Ad,Me)₃mes)] (8), with the ligands designed and synthesized through extremely tedious steps.³⁰ Complex 8 crystallizes in the space group of P3̄, and the uranium is located on the C₃ symmetry axis (Fig. 3). Compared to its aryloxide analogues, [U((OAr^Ad,Me)₃mes)] and [U((Oar^tBu,tBu)₃mes)], the U–S interaction is weaker, but a larger covalency contribution was found. An applied external field of 2 kOe allowed observation of the peaks of the out-of-phase components at the low-temperature range, resulting in a thermal barrier of U_eff = 9.7(7) cm⁻¹ with τ₀ = 2.8(8) × 10⁻⁶ s, as extracted from the fit of the relaxation times to the Arrhenius law below 3.6 K. A significant deviation from linearity in the ln(τ) vs. T⁻¹ plot can be observed, which might involve additional relaxation mechanisms, such as QTM, direct relaxation, and Raman processes.

Fig. 3 The crystal structure of [U((SAr^Ad,Me)₃mes)]. Color codes: U, yellow; S, pink; C, grey. H atoms are omitted for clarity.

The previous examples show that modulating the coordination environment by replacing substituents or replacing ligands with different crystal field strengths, does affect the SMM properties. However, the energy barriers or blocking temperatures do not change by orders of magnitude, for example, the energy barriers are always around 20 cm⁻¹ (Table 1). It is also noteworthy that the magnetic relaxation is closer to Raman rather than the Orbach process, therefore to some extent, it makes no sense to simply calculate and analyse the so-called energy barriers.

In 2013, Liddle and Slageren et al. reported in detail three completely unrelated U(III)-based complexes, [UI₃(THF)₄] (9a); [U(N(SiMe₃)₂)₃] (9b), and [U(BIPM^TMS)(I)₂(THF)] (9c) [BIPM^TMS = CH(PPh₂NSiMe₃)₂] with different ligands and symmetries (C_2v, C_3v, and C_s/C₁ respectively).³¹ All complexes 9a–c are SMMs, with relatively similar and small U_eff of 12.9, 22.1, and 16.2 cm⁻¹, respectively. Their magnetic bahaviours are comparable with other reported uranium(III) SMMs. This study demonstrates that the character of SMMs is intrinsic to uranium(III), but the ligand types or local symmetries have limited roles in the slow relaxation behaviour.

Subsequently, a bulkier amide ligand, N(SiMe₂tBu)₂⁻ (N**), was utilized by Mills and co-workers to prepare an actinide complex [U(N**)₃] (10) with a trigonal planar geometry.³² Compared to complex 9b, [U(N**)₃] is closer to the ideal trigonal symmetry. An energy barrier of 14.9 cm⁻¹ was obtained by the Arrhenius fit to the higher temperature ac data. The authors attribute the reason for the lower value compared to 9b to the selection of the temperature range. However, the relaxation time of 2.6 ms at 2 K is longer than that (0.3 ms) of 9b.

Cyclooctatetraenyl (COT) ligands have been very successful in the construction of lanthanide SMMs, with Er(III) analogues exhibiting high energy barriers and magnetic blocking temperatures.³³ To explore the influence of the sandwich-type ligand field on the magnetic relaxation of uranium, in 2015, the Murugesu group reported a U(III) sandwich complex, [Li(DME)₃][U(COT′′)₂] (11) (COT′′ = bis(trimethylsilyl)cyclooctatetraenyl dianion) (Fig. 4),³⁴ in which the U–C bond distances range from 2.726(0) to 2.755(1) A, and the Ring-U-Ring angle deviates from perfect linearity by 7.2°. Ac susceptibility measurements were performed on the uranium metallocene, showing that it is a field-induced SMM with a magnetic reversal barrier of 27 K and τ₀ = 4.6 × 10⁻⁶ s. Although its energy barrier is comparable to that of other uranium SMMs, it still lags far behind the Er analogues. The authors suggest that the covalent bonding between uranium and the COT′′ ligands could allow these mononuclear motifs to be ideal for the construction of high-performance multinuclear SMMs, even though a cyclooctatetraenyl ligand creating an equatorial crystal field does not suit well with an oblate U(III).

Fig. 4 The crystal structure of [U(COT′′)₂]⁻. Color codes: U, yellow; Si, light yellow; C, grey. H atoms are omitted for clarity.

In contrast to a 10 π-electron COT ligand possessing a large diameter, another aromatic ligand, cyclopentadiene with a compact size could provide a strong axial ligand field. Such ligands can stabilize the ground state of oblate dysprosium or terbium at higher M_J values, and a series of outstanding metallocene SMMs have been reported,^10–14 of which the most representative is [Cp^iPr5DyCp*]⁺,¹¹ the first one breaking through the nitrogen ceiling in hysteresis temperature. There are similarities between uranium(III) and dysprosium(III), for example, ionic radii, and their oblate shape of electron densities, so these drive some groups to replicate the successful strategy of dysprosium to uranium. In 2019, some of us reported the first base-free actinide bis-Cp sandwich complex, [(Cp^iPr5)₂U][B(C₆F₅)₄] (12a) (Fig. 5), which was isolated by using the super-electrophile of [(Et₃Si)₂(μ-H)][B(C₆F₅)₄] to abstract iodide from the precursor [(η⁵-Cp^iPr5)₂UI] (12b).³⁵ The two structures are very different, mainly in that the cations have shorter U–Cp bond lengths and wider Cp–U–Cp angles, which is due to the absence of coordination on the equatorial plane. The U–Cp distance in 12a is 2.472(3) Å, slightly longer than that (2.296(1) and 2.284(1) Å) in [Cp^iPr5DyCp*]⁺, and the Cp–U–Cp angle of 167.82(8)° is more approaching to linearity than that (162.507(1)°) in dysprosium cation. For both complexes, well-defined peaks in the ac susceptibility plot were only observed upon the application of optimized fields. The magnetic behaviours of these two complexes are quite similar, and there is no performance jump of several orders of magnitude like that between [Cp^iPr5DyCp*]⁺ and its precursor.¹¹ The absence of slow relaxation in zero applied fields is attributed to the non-negligible metal–ligand covalency and partial quenching of anisotropy resulting from strong ligand field splitting, which can lead to efficient quantum tunnelling of the magnetization. The temperature dependence of the relaxation time (τ) in the field fits the Raman expression well, indicating that Orbach relaxation processes commonly observed in dysprosium metallocene SMMs are not obvious in these uranium analogues. Therefore, the case of the actinides is far more complicated than that of the lanthanides, and the successful strategy of the lanthanides does not apply to the actinides, at least in unsymmetric uranocenium complexes.

Fig. 5 The crystal structure of [(η⁵-Cp^iPr5)₂U][B(C₆F₅)₄]. Color codes: U, yellow; F, green; B, deep green; C, grey. H atoms omitted for clarity.

After the uranocenium work, a perfectly linear uranium(II) metallocene, namely [(η⁵-Cp^iPr5)₂U] (13), was isolated by the reduction of [(η⁵-Cp^iPr5)₂UI] with potassium graphite.³⁶ In the uranocene, although the uranium(II) with a 5d³6f¹ electron configuration has four unpaired electrons and should be placed in the next section, we decided to discuss it here together because of its strong association with the above complex. No slow magnetic relaxation properties were observed in any applied fields, indicating that complex 13 is not an SMM at all. The reason for this is very complicated, in addition to the covalency effects arising from interactions of 5f orbitals with Cp, the Coulomb and exchange interaction between the 5f and 6d/7s could lead to additional splitting and mixing within the low-lying multiplets. Hence, it does not exhibit nice SMM properties, and is not even a SMM, although the non-Kramer U(II) is in a strict point symmetry.

A less bulky cyclopentadienyl ligand, Cp^iPr4 was used by Arnold et al. to isolate a neutral (Cp^iPr4)₂UI (14a), and a base-free metallocene, [(Cp^iPr4)₂U][B(C₆F₅)₄] (14b), extending the uranium metallocene family.³⁷ The major differences in the structural parameters are that the Cp–U–Cp angles in 14a and 14b are both smaller than those in 12a and 12b. One of the interesting points is that the Cp–U–Cp angle in 14a is about 10° smaller than that in 12a, but upon removing iodide, this parameter has a difference of approximately 25°. Similar to 12a and 12b, both complexes exhibit field-induced slow magnetic relaxation, and the process is dominated by a Raman relaxation mechanism. Remarkably, the field-induced relaxation time of 14b is longer than that of 12a, which is attributed to the flexibility of the Cp^iPr4 ligand, resulting in a smaller quenching of the orbital angular momentum.

Recently, Mills et al. reported one uranium(III) bis-Cp hypersilanide complex [U(Cp′′)₂{Si(SiMe₃)₃}] (15) (Cp′′ = {C₅H₃(SiMe₃)₂-1,3}) by the salt elimination reaction between [{M(Cp′′)₂(μ-I)}₂] with [K{Si(SiMe₃)₃}].³⁸ The mean U–Cp distance of 2.4726(2) Å, and the U–Si distance of 3.116(2) Å are comparable to those of other uranocenium and U–Si bond-containing complexes. The Cp–U–Cp is bent with an angle of 131.02(4)°, hence, the uranium center could be considered to be in a distorted pseudo-trigonal planar geometry. Ac susceptibility measurements on it show slow magnetic relaxation below 5 K, the authors suggested that the behaviour was due to Raman and quantum tunnelling of magnetisation processes rather than the Orbach process.

Next to uranium in the periodic table of elements is the artificially prepared transuranium element neptunium. Similar to uranium(III), neptunium(IV) possessing a 5f³ electron configuration, is a Kramer ion and is considered a promising candidate for the synthesis of outstanding SMMs. The first mononuclear transuranic complex showing magnetic memory effects was the homoleptic neptunocene, [Np(η⁸-COT)₂] (16) with an approximate D_8h symmetry (Fig. 6).³⁹ At 1.8 K, the magnetic hysteresis presents a butterfly shape, and it is open when the field is above 5 T. The magnetic moment is still far from saturation even at the maximum field of 14 T, indicating strong magnetic anisotropy. The temperature dependence of the out-of-phase susceptibility upon an applied field of 5 kOe shows clear peaks in the temperature range of 2–60 K. The Arrhenius fit is essentially linear at higher temperatures, resulting barrier of 28.5 cm⁻¹ with τ₀ = 1.1 × 10⁻⁵ s. Other processes are likely to contribute to the overall relaxation, including hyperfine interactions with the I = 5/2 nucleus of ²³⁵Np.

Fig. 6 The crystal structure of [Np(η⁸-COT)₂]. Colour code: Np, yellow-green; C, grey. H atoms are omitted for clarity.

2.2 Np(III) with 4 unpaired electrons

In lanthanide SMMs, one of the very important reasons why there are far fewer non-Kramer examples than Kramer ones is its stricter symmetry requirements. The situation is then more complicated for non-Kramer ions of actinides that possess a non-negligible covalent contribution to the metal–ligand bond. Hence, it is not surprising that such cases are very rare. The first and so far the only SMM based on Np(III) with a 5f⁴ electron configuration was reported by Arnold et al., namely [(L^Ar)NpCl] (17) (L^Ar = trans-calix[2]benzene[2]pyrrole).⁴⁰ Complex 17 shows the first instance of the bis(arene) sandwich structure in a transuranic complex. The Np-arene distance is 2.486(5) Å, and the arene sandwich angle of 174.20(4)° is quite close to linear. Two nitrogen atoms from the two pyrrole groups and one chloride ion complete the coordination environment of Np(III) on the equatorial plane, with an Np–N bond length of 2.447(2) Å and Np–Cl bond distance of 2.670(1) Å. Although the imaginary part of the ac magnetic susceptibility is very small, a significant increase at low temperatures could be attributed to the slow magnetic relaxation. No peaks in the ac susceptibility plot were observed above 2 K, indicating poor SMM behaviour.

2.3 Pu(III) and U(I) with 5 unpaired electrons

Irrespective of the ligands, transuranic plutonium SMMs are particularly rare, hence the presentation of slow relaxation behaviour in [PuTp₃] (18) is of significance.⁴¹ At high-temperature range, the magnetic relaxation time of the 5f⁵ complex in an applied field of 100 Oe could be fitted by an Arrhenius relation, resulting in a thermal-activated barrier of 18.3 cm⁻¹ (with τ₀ of 2.9 × 10⁻⁷ s). When the temperature is below 6 K, the plot of relaxation time starts to bend and tends to be horizontal, revealing a crossover to a tunnelling regime. Although, like other actinide SMMs, this rare 5f⁵ plutonium(III) complex does not possess a high energy barrier and magnetic blocking temperature, it expands the variety of the 5f block family.

Another actinide complex with 5 unpaired electrons was reported recently by Layfield and co-workers, namely, [K(2.2.2-cryptand)][U(η⁵-Cp^iPr5)₂] (19) (Fig. 7), which could be isolated by reducing [U(η⁵-Cp^iPr5)₂] or [IU(η⁵-Cp^iPr5)₂] with potassium graphite in the presence of 2.2.2-cryptand.¹⁷ The oxidation state of the uranium is +1, expanding the range of accessible oxidation states for actinides. Compared to the linear uranium(II) metallocene, the Cp–U–Cp angle of the uranium(I) complex is smaller, and U–Cp distances are longer due to the one-electron reduction. The electron configuration of the rare uranium(I) complex was confirmed to be 5f³(7s/6d_z²)¹(6d_x²−y²/6d_xy)¹ by theoretical studies and magnetic measurements. Remarkably, unlike its U(III) analogues, complexes 12 and 13, which possess field-induced slow magnetic relaxation behaviours, exhibit SMM properties in zero dc field. The temperature-dependent relaxation time could fit the Arrhenius equation well, resulting in an effective energy barrier of 14(1) cm⁻¹ with τ₀ = 1.1(3) × 10⁻⁷ s. The behaviour of magnetic hysteresis is similar to that of [Np(η⁸-COT)₂], presenting a narrow butterfly shape at 2 K.

Fig. 7 The crystal structure of [K(2.2.2-cryptand)][(η⁵-Cp^iPr5)₂U]. Color codes: U, yellow; K, purple; O, red; C, grey; N, blue. H atoms are omitted for clarity.

2.4 U(V) with 1 unpaired electron

Although compared to uranium(III) (5f³; J = 9/2), uranium(V) with the electron configuration of 5f¹ is less introduced in mononuclear SMMs, due to a relatively smaller total angular momentum J = 5/2. However, also being a Kramer ion, uranium(V) is still a potential candidate. The first mononuclear uranium(V) complex exhibiting SMM behaviours was reported by Liddle et al. in 2014, namely, [UO(Tren^TIPS)] (20; Tren^TIPS = [N(CH₂CH₂NSiⁱPr₃)₃]³⁻).⁴² The uranium(V) center in the terminal mono-oxo complex adopts a distorted trigonal bipyramidal geometry. The three nitrogen atoms from amide bond with uranium(V) have an average U–N bond distance of 2.290 Å. The U–N and U=O bonds, along the axis, have distances of 2.482(6) Å and 1.856(6) Å, respectively. Ac susceptibility measurements in zero static field imply that the complex does not show slow magnetic relaxation. Upon an applied field of 1 kOe, well-defined peaks were observed in the in-phase and out-of-phase plots. The anisotropy barrier of 14.9 cm⁻¹ (with τ₀ = 2.6 × 10⁻⁷ s) extracted from the fit to Arrhenius law is quite similar to the other monometallic 5f-block SMMs. A waist-restricted magnetic hysteresis was observed up to 2.4 K.

Thereafter, based on the same Tren^TIPS ligand, the same group replaced O²⁻ with N³⁻ to study the influence of varying crystal fields at the uranium(V) centre. A series of dinuclear contact ion pair complexes [{U(Tren^TIPS)(μ-N)( μ-M)}₂] (21a) (M = Li, Na, K, Rb, Cs), terminal nitride separated ion pair complexes [U(Tren^TIPS)(N)][M(12C4)₂] (21b), and the contact ion pair complexes [U(Tren^TIPS)(μ-N){M(15C5)}] (21c) were prepared.⁴³ All complexes, except 21Li, display SMM behaviours with U_eff of 14–28 cm⁻¹, but show no hysteresis.

3. Multinuclear An-SMMs with magnetic exchange

The study of SMMs began with multinuclear systems based on transition metals, and the magnetic exchange between metals has been one of the main focuses in the SMMs field. For actinide ions, the more diffuse radial extension of 5f orbitals could provide strong magnetic exchange between spin carriers, so the research on multinuclear actinide SMMs did not begin much later than mononuclear ones. Actually, actinide-containing complexes involved in magnetic exchange interactions have long been studied,¹⁹ but here we only briefly summarize those displaying SMMs features (Table 2), starting with the first multinuclear An-SMM possessing the 5f–5f exchange.

Table 2 Selected multinuclear actinide-based SMMs

Classification	Complex	Local symmetry	U eff (cm^−1)	H dc (Oe)	τ 0 (s)	Ref.
a The dc field used in ac susceptibility measurements.
5f–5f exchange	{Np^VIO2Cl2}{Np^VO2Cl(THF)3}2	D 5h/Oh	97.3	—	—	15
	[{U(BIPM^TMSH)-(I)}2(μ:η^6:η^6-C6H5CH3)]	C 4v	—	0	—	44
	[Cp3UO=U=O(THF)(Pacman)]	C 3v	19.0	10 000	3.3 × 10^−8	45
4f–5f exchange	[{UO2Dy(py)2(Pacman)}2]	D 5h	—	—	—	46
3d–5f exchange	[{UO2(salen)}2Mn(py)3]6	D 5h	98.7	0	3.0 × 10^−12	47
	[{[Mn(TPA)I][UO2(Mesaldien)][Mn(TPA)I]}I]	D 5h	56.3	0	5.0 × 10^−10	48
	[{Fe(TPA)Cl}{UO2(Mesaldien)}{Fe(TPA)Cl}]I	D 5h	37.5	0	3.4 × 10^−9	49
	[UFe2BPPAH]	D 5h	6.3	0	7.8 × 10^−6	49
	[UNi2BPPAH]	D 5h	19.0	0	2.4 × 10^−8	49
	[{[UCo2BPPAH][UO2-(Mesaldien)][Co(BPPA)(Py)]}]I	D 5h	21.2	1500	2.9 × 10^−9	50
	{[UO2-(salen)(py)][Mn(py)4](NO3)}n	D 5h	93.1	500	3.1 × 10^−11	51
	{[UO2(Mesaldien)][Mn(NO3)-(Py)2]}n	D 5h	84.8	0	6.2 × 10^−12	52
2p–5f exchange	[U(Tp^Me2)2(bipy)]	C 2	22.6	500	2.6 × 10^−7	55
	[{(SiMe2NPh)3-tacn}U^IV(η^2-N2Ph2˙)]	C 3v	12.2	500	4.7 × 10^−8	56
	[K(2.2.2-cryptand)][{((Me3Si)2N)3U(III)}2(μ-bpym)]	D 3h	8.9	1000	4.7 × 10^−8	57
	[{((Me3Si)2N)3U(IV)}2(μ-bpym)][BPh4]	D 3h	2.0	1000	6.4 × 10^−4	57

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3.1. 5f–5f exchange

Shortly after the first mononuclear uranium(III) SMM, Caciuffo and co-workers reported a trinuclear mixed-oxidation-state transuranic complex {Np^VIO₂Cl₂}{Np^VO₂Cl(THF)₃}₂ (22) (Fig. 8), the first multinuclear An-SMM displaying effective superexchange interactions between 5f ions.¹⁵ The two Np(V) ions in the neptunium triangle are in a distorted pentagonal bipyramidal symmetry, bridged by two Cl⁻ on the equatorial plan. Three oxygen atoms from THF molecules complete the equatorial coordination environment of each Np(V). One of two neptunyl oxygen atoms from each Np(V) coordinates into the equatorial plane of the Np(VI) ion. Two Cl anions complete the tetragonal bipyramidal geometry of Np(VI). The rise in the χT vs. T curve below 30 K indicates that the neptunium triangle is a ferromagnetically-coupled system. The calculations show that the coupling between Np(V) and Np(VI) is very strong with J = 7.5 cm⁻¹, but the coupling between the two Np(V) is very weak with J = 0.4 cm⁻¹. Well-defined peaks are observed in the ac susceptibility curves, and the relaxation time follows the Arrhenius law well with an effective energy barrier of 97.3 cm⁻¹. This study shows that multinuclear actinide clusters are capable of exhibiting nice SMM properties.

Fig. 8 The crystal structure of (Np^VIO₂Cl₂)[Np^VO₂Cl(THF)₃]₂. Color codes: Np, yellow-green; O, red; C, grey; Cl, yellow. H atoms are omitted for clarity.

An arene-bridged diuranium complex [{U(BIPM^TMSH)-(I)}₂(μ:η⁶:η⁶-C₆H₅CH₃)] (23) (BIPM^TMS = C(PPh₂NSiMe₃)₂ showing an inverted-sandwich structure was reported by the Liddle group.⁴⁴ Apart from the bridged arene, one carbanion and two nitrogen atoms from the BIPM^TMS ligand, and one halide complete the coordination geometry of each uranium. The U–C_arene bond lengths are in the range of 2.553(7)–2.616(7) Å, and the average C–C bond length in the toluene ring is 1.436(16) Å, slightly longer than that in free toluene. The none zero χT product at the lowest measurement temperature suggests that there are no strong interactions of antiferromagnetic exchange in the complex, and the authors noted that the magnetism cannot be described as a superexchange interaction between localized uranium ions. The magnetic hysteresis shows a butterfly shape at a low temperature of 1.8 K, and the zero coercivity at zero fields suggests the presence of QTM. Ac susceptibility measurements show that no out-of-phase signals were found in the zero applied field, but clear signals could be observed when it is in an applied field of 1000 Oe, indicating that complex 31 is a field-induced SMM. The energy barrier was not determined because too few ac peaks could be observed.

In 2016, Arnold and co-workers reported a U(IV)–uranyl(V) complex [Cp₃UO=U=O(THF)(H₂L)] (24; L = “Pacman” Schiff-based polypyrrolic macrocycle) formed by one electron transfer from Cp₃U to uranyl(VI) dication.⁴⁵ At room temperature, the χT product approaches the expected value for a U(IV)–U(V) pair, confirming that the reduction of the uranyl group occurs. Ac susceptibility measurements were performed to investigate the dynamic magnetic property of complex 24, and well-defined peaks in the out-of-phase component imply a slow relaxation of the magnetization below 4 K. The thermal activation barrier of 19 cm⁻¹ could be obtained by fitting to the relaxation time, and this value is quite similar with other mononuclear U(V) SMMs. Given that U(IV) is often non-magnetic in low symmetry geometries, and that no apparent sign of magnetic coupling between uranium ions, the authors attributed the SMM behaviour to the uranyl(V) group.

3.2 4f–5f exchange

The greater radial extension of the 5f orbitals could help actinides generate a significant exchange coupling, and lanthanides with high magnetic anisotropy and momentum, such as dysprosium, have been introduced into a large number of SMMs. To date, however, cases of lanthanide-actinide SMMs are still very rare. In 2013, Arnold et al. investigated the magnetic behaviour in a heterobimetallic complex [{UO₂Dy(py)₂(L)}₂] (25; L = “Pacman” Schiff-based polypyrrolic macrocycle).⁴⁶ In the structure, both the uranium(III) and the dysprosium(III) were in a distorted pentagonal bipyramidal geometry, and the two metals bridged through a uranyl oxo-group. The magnetic hysteresis of complex 25 showed a butterfly shape as a result of magnetic bistability. However, the authors noted that no superexchange interaction effects were visible in the magnetism, and the origin of the slow relaxation was ascribed to dysprosium(III).

3.3 3d–5f exchange

The first 3d–5f SMM, [{UO₂(salen)}₂Mn(py)₃]₆ (26) (salenH2 = N,N′-ethylenebis(salicylimine)), was reported by the Mazzanti group in 2012.⁴⁷ The wheel-shaped cluster containing UO₂⁺–Mn interactions was synthesized from the reaction of [UO₂(salen)(Py)][Cp*₂Co] with paramagnetic divalent manganese cation. In the structure, six vertices formed by Mn(II) ions are linked by six edges of UO₂⁺, assembling a hexagon, and the six Mn(II) ions coordinate with another six UO₂⁺ through the uranyl oxygen. The uranium centers are only bridged by the phenolate oxygens from the salen ligand (Fig. 9). The rise of χT between ∼60 and ∼30 K was attributed to a combination of ligand field and superexchange interactions between the uranium centers. The low value of χT observed at ∼2 K is due to the strong antiferromagnetic interactions in UO₂⁺–Mn and a weaker antiferromagnetic interaction between uranium centers. Clear opened hysteresis loops with non-zero coercive field could be observed below 4.5 K, but the magnetization was not saturated even at the maximum field of 7 T. Step-like changes at zero field, especially obvious at low temperatures, are due to quantum tunnelling of the magnetization. The clear peaks in the out-of-phase component indicated the occurrence of slow magnetic relaxation, and the relaxation barriers of 98.7 cm⁻¹ could be obtained by the fit to an Arrhenius relation. It should be noted that the structural complexity of this large cluster poses a great obstacle to specific magnetic analysis.

Fig. 9 Details of the cores in the ball-and-stick representation of [{UO₂(salen)}₂Mn(py)₃]₆. H atoms and co-crystallized solvent molecules are omitted for clarity. Color codes: U, yellow; Mn, pink; O, red; C, grey; N, blue. H atoms are omitted for clarity.

Simple models facilitate the analysis of magneto-structural relationships and help one to tune the SMM properties. Based on this, Mazzanti et al. presented two trimeric heterodimetallic complexes [{[M(TPA)I][UO₂(Mesaldien)][M(TPA)I]}I] (M = Mn (27a), Cd (27b) (Mesaldien = N,N′-(2-aminomethyl)diethylenebis(salicylidene imine, TPA = tris(2-pyridylmethyl)amine).⁴⁸ In the structures, the two terminal [M(TPA)I]⁺ are bridged by the central [UO₂(Mesaldien)]⁻ through the two oxo atoms. The mean U–O–Mn angle and the Mn–U–Mn angle are 169.7(1.7)° and 173.77(5)°, respectively, in the linear {Mn–O=U=O–M} core of complex 27a. Dc magnetic susceptibility measurements were performed on complexes 27a and 27b, and the diamagnetic Cd(II) in complex 27b could help to understand the spin–orbit and ligand field effects contribution from the uranium(V) centre. The fit to dc magnetic susceptibility yields J = +7.5 cm⁻¹, and this positive value indicates ferromagnetic coupling between uranium(V) and manganese(II) ions. Ac susceptibility measurements were performed to investigate the magnetization dynamics of complex 27a in a zero dc field, showing strong frequency-dependent peaks. An energy barrier of 56.3 cm⁻¹ with τ₀ = 5.02 × 10⁻¹⁰ s was obtained by fitting to the Arrhenius equation. The SMM behaviour of complex 27a can be attributed to the magnetic coupling between uranium(V) and manganese(II) ions and the large Ising-type anisotropy of the O=U=O unit. Slow magnetic relaxation could be observed for 27b under an applied dc field due to the single-ion anisotropy of uranium(V).

Soon afterwards, a series of closely related trimer [{Fe(TPA)Cl}{UO₂(Mesaldien)}{Fe(TPA)Cl}]I (28a) and [UM₂BPPAH] (Fe for 28b and Ni for 28c) (BPPAH = bis(2-picolyl)(2-hydroxybenzyl)amine) were reported,⁴⁹ showing similar linear (M–O=U=O–M) cores. The maxima in the χ′′_M(v) plot were observed at 2.1–5.7 K for 28a, at 1.8–5 K for 28b, and at 1.8–3.3 K for 28c in a zero applied field, with the anisotropy barriers of 37.5 cm⁻¹ (τ₀ = 3.40 × 10⁻⁹ s), 6.3 cm⁻¹ (τ₀ = 7.82 × 10⁻⁶ s) and 19.0 cm⁻¹ (τ₀ = 2.40 × 10⁻⁸ s), respectively. All the complexes are SMMs without the applied field, meaning that intramolecular magnetic coupling in U(V)-TM(II) (TM = transition metal) has a significant influence on the dynamic magnetic properties.

The same group then extended the UO₂-TM(II) family by preparing two cobalt-containing complexes, {Co(TPA)}{UO₂(Mesaldien)}I (29a) (Fig. 10 left) and [{[UCo₂BPPAH][UO₂-(Mesaldien)][Co(BPPA)(Py)]}]I (29b) (Fig. 10 right).⁵⁰ The isolation of the dinuclear or trinuclear complexes is controlled by the appropriate choice of the ligand binding with the cobalt ion. The trinuclear complex shows SMM behaviour with an energy barrier of 21.2 cm⁻¹ originating from the coupling between uranium(V) and cobalt ions, but it needs an applied field. In contrast, the dinuclear complex with a bent Co–O–U does not show frequency dependency due to no evidence of U(V)–Co(II) magnetic coupling.

Fig. 10 The crystal structures of {Co(TPA)}{UO₂(Mesaldien)}I (left) and [[UCo₂BPPAH] {[UO₂(Mesaldien)][Co(BPPA)(Py)]}]I (right). Color codes: U, yellow; Co, turquoise; O, red; C, grey; N, blue. H atoms are omitted for clarity.

Moreover, two U(V)-TM(II) coupling-based complexes [{[UO₂-(salen)(py)][Mn(py)₄](NO₃)}]_n (30)⁵¹ and {[UO₂(Mesaldien)][Mn(NO₃)-(Py)₂]}_n (31),⁵² showing single chain magnet behaviours were developed. Their energy barriers are 93.1 cm⁻¹ and 84.8 cm⁻¹, respectively. It should be emphasized that, unlike the previous U(V)-TM(II) SMMs, the hysteresis curves of these two complexes have nice rectangular shapes and show large coercive fields with no obvious drop in the zero fields.

3.4 2p–5f exchange

Almeida group developed a series of uranium(III) SMMs based on the ligand of Tp^Me2 (hydrotris(3,5-dimethylpyrazolyl)borate),^53–55 and one of them was a neutral complex [U(Tp^Me2)₂(bipy)] (32) containing a radical monoanionic bipyridine.⁵⁵ The faster decrease of χT below 100 K, and the lower moments than the corresponding non-radical analogue [U(Tp^Me2)₂(bipy)]I,⁵⁴ may indicate the antiferromagnetic interactions between uranium(III) and the radical ligand. In addition, [U(Tp^Me2)₂(bipy)]I (33) could not show obvious maxima in the ac susceptibility plots under zero applied fields. In contrast, clear peaks in both χ′ and χ′′ of ac susceptibility can be observed under zero and 500 Oe external fields, and effective barriers of 19.8 cm⁻¹ and 22.6 cm⁻¹ could be obtained, respectively, by a fit of the relaxation times to Arrhenius law. By excluding the influence of coordination geometry, this study implies that the coupling of the uranium ion to the radical ligand could remove QTM to some degree.

In general, the magnetic bistability of the ground state is required for slow magnetic relaxation, so tetravalent uranium with two single electrons is not considered a good spin carrier for the construction of SMMs as this non-Kramers ion exhibits an orbital singlet ground state at low temperatures. In 2015, Pereira and co-workers reported one mononuclear uranium(IV) complex with an zobenzene radical ligand [{(SiMe₂NPh)₃-tacn}U^IV(η²-N₂Ph₂˙)]⁵⁶ (34) (N₂Ph₂ = bipyridine radical) obtained by the addition of azobenzene to the trivalent uranium precursor 7a. Surprisingly, complex 34 showed field-induced slow magnetic relaxation in external dc fields in the range of 1–2.5 kOe, and a relaxation barrier with a value of 12.2 cm⁻¹ at 2.5 kOe could be obtained. The opening of the butterfly-shaped hysteresis loops could be apparently observed at 1.7 K but without a zero-field coercivity. This study provides the first SMM example based on the 5f² uranium(IV). The unusual magnetic properties for 34 were derived from the magnetic coupling between uranium(IV) center and the radical, which seems to switch the parity from non-Kramers to Kramers.

The redox activity of 2,2,-bipyrimidine (bpym) allows it to be a radical, and the double-pocket chelating features are often used as a bridging ligand in complexes. Recently, Mazzanti et al. reported a series of bpym-bridged diuranium complexes (Fig. 11), [{((Me₃Si)₂N)₃U(IV)}₂(μ-bpym)] (35a), [K(2.2.2-cryptand)][{((Me₃Si)₂N)₃U(III)}₂(μ-bpym)] (35b), [K(2.2.2-cryptand)]₂[{((Me₃Si)₂N)₃U(III)}₂(μ-bpym)] (35c), and [{((Me₃Si)₂N)₃U(IV)}₂(μ-bpym)][BPh₄] (35d),⁵⁷ showing analogous structures but different metrical parameters. Complexes 35b and 35d present radical-bridged U(III)–bpym˙⁻–U(III) and U(IV)–bpym˙⁻–U(IV) cores, respectively. Variable-temperature dc magnetic susceptibility measurements were performed for all four complexes, and non-strong ferromagnetic couplings were observed, but the weak interactions cannot be completely ruled out. Under a dc field of 2000 Oe, the maxima in the out-of-phase susceptibility component for 35b and 35d, were observed at low temperatures. The reversal barrier of 8.92(2) cm⁻¹ for 35b was determined with τ₀ = 8.87(23) × 10⁻⁷ s, while for 35d was a miniscule barrier of 1.95(3) cm⁻¹ with an unusually large τ₀ value of 6.38(10) × 10⁻⁴ s. In contrast, complex 35c containing two uranium(III) ions bridged by a diamagnetic bpym ligand, does not show slow magnetic relaxation behaviours. The differences between complex 35b and 35c could be assigned to the bridging radical, which enhances coupling and suppresses QTM.

Fig. 11 Crystal structure of the [{((Me₃Si)₂N)₃U}₂(μ-bpym)] core. Color codes: U, yellow; Si, light yellow; C, grey; N, blue. H atoms are omitted for clarity.

4. Challenges and outlook

As illustrated for the aforementioned mononuclear actinide SMMs, the majority of them exhibit very small energy barriers and little, if any, magnetic hysteresis. These so-called energy barriers should be treated with caution because they are often obtained in a relatively narrow temperature range and are accompanied by a relatively large τ₀, which means that the magnetic relaxation behaviours are complicated and closer to a Raman process or a mixture of multiple processes rather than a simple Orbach process. So far, the performances of actinide SMMs cannot be qualitatively improved by changing the ligand substituent, or the coordination atom, and recent attempts to replicate the strategies that have worked in lanthanides to actinides have yielded frustrating results. Moreover, very different from lanthanide SMMs, the calculated energy gaps of actinides are usually determined to be one or two even three orders of magnitude greater than the correspondingly measured U_eff values (Table 1), further illustrating its complexity.

Recently, Singh et al. suggested that the design of actinides SMMs with high anisotropies should consider not only the coordination geometry but also the nature of the ligand field interaction.⁵⁸ For a 5f³ ion with an oblate electron density, σ donor ligands are better than π donor ligands to increase the anisotropy in linear geometry. Then, extremely bulky N-heterocyclic carbenes might be helpful in the preparation of high-performance SMMs; however, considering the large ionic radii and high oxidation states of the actinides, this poses a great challenge for the synthesis. In 2022, the Rajaraman group studied a series of three-coordinate U(III) and Pu(III) SMMs by employing ab initio calculations.⁵⁹ The authors suggested that the blocking barriers of the two reported T-shaped complexes [U(NSiⁱPr₂)₂(I)] and [U(NHAr^iPr6)₂I] (Ar^iPr6 = 2,6-(2,4,6-iPr₃C₆H₂)₂C₆H₃) could exceed 900 cm⁻¹, and the pyramidal [Pu{N(SiMe₃)₂}₃] could yield an energy barrier of ∼1933 cm⁻¹. These three complexes are relatively easy to synthesize, so they could be quickly verified. Just this year, the Gagliardi group theoretically investigated the electronic structure and SMM behaviours of californium(III) complex Cf(dbm)₃(bpy) (bpy = 2,2′-bipyridine; dbm = dibenzoylmethanoate).⁶⁰ This study shows that the computed energy barrier of the 5f⁹ complex is higher than that of the Dy(III) analogue due to the stronger spin–orbit coupling and crystal field splitting. Then, a linear L–Cf(III)–L complex might show promising performance, however, the scarcity and instability limit experimental studies.

By comparison, the development of multinuclear actinide SMMs with magnetic exchange is better, and it seems to be a more promising route to prepare high-performance An-SMMs. However, a big challenge in this direction is that few theoretical calculations have been performed to study the magnetic configuration relations due to the increased complexity of multinuclear An-SMMs with magnetic exchange. Considering that some building blocks, such as Dy-D_6h,^61–63 Ln-TM₅,^64,65 and UO₂(salen) that exhibit high magnetic anisotropy have been developed, the combinations of lanthanides and actinides might be interesting targets to pursue. Scheme 1 shows two unknown species that might have fascinating SMM properties. The simple structures can help to understand the magneto-structural relationships, for example, by replacing magnetically anisotropic lanthanides with diamagnetic yttrium or isotropic gadolinium, facilitating the determination of magnetic exchange strength and the effect of each component on SMM properties.

Scheme 1 Two potentially excellent single-molecule magnets combined by UO₂(salen) and Dy-D_6h (left), and metallacrown (right, surrounding ligands are omitted for clarity).

Since the mononuclear [TbPc₂]⁻ SMM was reported, the field of lanthanide SMMs has developed rapidly, with the energy barrier increasing from the initial 230 cm⁻¹ to the latest 1631 cm⁻¹ and the hysteresis blocking temperature rising from the liquid helium region to 80 K. In contrast, the development of actinide SMMs has lagged much behind. However, as more examples of actinides are reported, more theoretical models are developed, and both contribute to each other, this direction will also flourish like the lanthanide SMMs, which of course requires a sincere collaboration between chemists and physicists.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

We thank the national high-level young talents program, the National Natural Science Foundation of China (grant no. 22131011 and 21821003), and the Pearl River Talent Plan of Guangdong (2017BT01C161).

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