Main Group Chemistry at the Interface with Molecular Magnetism

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Cite This: Chem. Rev. XXXX, XXX, XXX−XXX

Main Group Chemistry at the Interface with Molecular Magnetism Fu-Sheng Guo, Arun Kumar Bar, and Richard A. Layfield*

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Department of Chemistry, School of Life Sciences, University of Sussex, Brighton BN1 9QJ, United Kingdom ABSTRACT: Innovative synthetic coordination and, increasingly, organometallic chemistry are at the heart of advances in molecular magnetism. Smart ligand design is essential for implementing controlled modifications to the electronic structure and magnetic properties of transition metal and f-element compounds, and many important recent developments use nontraditional ligands based on low-coordinate main group elements to drive the field forward. This review charts progress in molecular magnetism from the perspective of ligands in which the donor atoms range from low-coordinate 2p elementsparticularly carbon but also boron and nitrogento the heavier p-block elements such as phosphorus, arsenic, antimony, and even bismuth. Emphasis is placed on the role played by novel main group ligands in addressing magnetic anisotropy of transition metal and f-element compounds, which underpins the development of single-molecule magnets (SMMs), a family of magnetic materials that can retain magnetization in the absence of a magnetic field below a blocking temperature. Nontraditional p-block donor atoms, with their relatively diffuse valence orbitals and more diverse bonding characteristics, also introduce scope for tuning the spin−orbit coupling properties and metal−ligand covalency in molecular magnets, which has implications in areas such as magnetic exchange coupling and spin crossover phenomena. The chemistry encompasses transition metals, lanthanides, and actinides and describes recently discovered molecular magnets that can be regarded, currently, as defining the state of the art. This review identifies that main group chemistry at the interface molecular magnetism is an area with huge potential to deliver new types of molecular magnets with previously unseen properties and applications.

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1. Introduction 2. Survey of Main Group Ligands in Molecular Magnetism 2.1. A Synthetic Chemist’s Guide to Transition Metal SMMs 2.1.1. Two-Coordinate 3d-SMMs with 2pDonor Ligands 2.1.2. Higher-Coordinate 3d-SMMs with 2pDonor Ligands 2.2. f-Block Molecular Magnets with 2p Donor Ligands 2.2.1. Lanthanide SMMs with B-, C-, and NDonor Ligands 2.2.2. Actinide SMMs with C- and N-Donor Ligands 2.3. Molecular Magnets with Heavier p-Block Donor Ligands 2.3.1. 3d Molecular Magnets with Heavy pBlock Donor Ligands 2.3.2. Lanthanide Molecular Magnets with Heavy p-Block Donor Ligands 3. Conclusion and Outlook Author Information Corresponding Author ORCID Author Contributions Notes Biographies Acknowledgments © XXXX American Chemical Society

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1. INTRODUCTION Molecular magnetism can be defined as the field of study concerned with the magnetic properties of discrete molecular and molecule-based materials.1,2 When considered in terms of fundamental structural units, such a description clearly distinguishes molecular magnets from atom-based inorganic magnets,3 such as magnetite and rare-earth magnets. The former type of material is based on coordination compounds or main group radicals, which typically possess subnanometer dimensions, whereas the latter material consists of magnetic domains with dimensions of tens or hundreds of nanometers.4 The different structural properties of the two types of material set the scene for establishing magnetic properties unique to each or, alternatively, for remarkably similar phenomena occurring in both despite their profoundly different chemical environments. When the two types of material show the same bulk magnetic property, the microscopic origins can be completely different. A striking illustration of this scenario relates to magnetic hysteresis, i.e., the ability of materials to retain magnetization for long periods of time in the absence of a magnetic field. Magnetic memory effects have been known in solid-state materials for several decades and form the basis of, for example, the function of a computer hard-disk drive.5

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Special Issue: Frontiers in Main Group Chemistry Received: February 18, 2019

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DOI: 10.1021/acs.chemrev.9b00103 Chem. Rev. XXXX, XXX, XXX−XXX

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example, the incorporation of nontraditional p-block elements as donor atoms? Expanding toward ligands with donor atoms such as boron, carbon, silicon, antimony, and tellurium introduces possibilities for studying novel chemical environments in the context of magnetism, e.g., coordination compounds in which the metal is bonded to a range of nonmetals, metalloids (semimetals), and even other metals. Such an approach could provide a unique means of varying the amount of covalency in the metal−ligand bonds and, hence, of modifying the electronic structure and magnetism. Using heavy 6p elements such as bismuth as a donor atom could allow strong ligand-based spin−orbit coupling to be transferred to a paramagnetic metal center, potentially providing a means of enhancing the magnetic anisotropy, which is of critical importance in, e.g., the design of SMMs. While the interface of molecular magnetism with modern main group chemistry as described above is not yet an established area, it is growing and has considerable potential for future development. This review will focus on recent developments in which nontraditional ligands, where the donor atoms are low-coordinate boron, carbon, or nitrogen, or where the donors are heavier p-block elements, have provided new lines of inquiry molecular magnetism that may otherwise not have been possible. This review is therefore focused on coordination and organometallic chemistry aspects of molecular magnetism and particularly single-molecule magnets (SMMs), where main group ligands are now making a considerable impact. Metal-free main group radicals are not included in this review since such materials have been in detail covered elsewhere.26,27 Since many excellent articles describing the physics of molecular magnetism are available, detailed explanations of phenomena such as single-molecule magnetism and spin crossover will not be provided herein: only the essential aspects will be mentioned.

Magnetic hysteresis can also be observed in types of coordination compounds known as single-molecule magnets (SMMs).6−13 Whereas strong interactions within and between magnetic domains in solid-state materials produce the hysteresis,14 the same property in an SMM can be explained in terms of the electronic structure of individual molecules, which is a crucial fundamental difference in light of the vastly different length scales involved. Magnetic hysteresis in a molecule is an exciting development, although expectations of revolutionary technology should be carefully managed: whereas magnetic hysteresis in solid-state materials is frequently observed above room temperature, hysteresis in an SMM has only recently surpassed the boiling point of liquid nitrogen, and even then in the form of an air-sensitive, kinetically stabilized organometallic compound.15 However, the properties of SMMs in bulk, polycrystalline formsand of other types of molecular magnets such as spin crossover (SCO) materials16,17 and magnetic refrigerants18combined with their molecular nature provide opportunities for developing innovative new materials with functionalities that may not be attainable with solid-state materials. Furthermore, the prospect of working at the level of single molecules has led to the creation of new research areas, notably molecular spintronics, which features, at its heart, a multidisciplinary combination of molecular magnetism, spintronics, molecular electronics, and quantum computing.19−21 In contrast to solid-state magnetism, progress in molecular magnetism and closely aligned fields is totally dependent on the synergic development of innovative synthetic chemistry and insightful physical models, which can form the basis of powerful magneto-structural correlations. The synthetic methods of coordination chemistry and main group chemistry set the day-to-day business of molecular magnetism apart from preparative solid-state magnetism. As in other branches of coordination chemistry, smart ligand design in molecular magnetism is the key to unlocking a desired property, and this is an area where modern main group chemistry has hugely untapped potential. The ligand types used to advance the frontiers of molecular magnetism are predominantly those rooted in Werner-type coordination chemistry, with well-established N- and O-donor ligands being ubiquitous. A very short, illustrative selection of popular ligand types includes: imines (Schiff bases), polyamines, pyrazolylborates, poly(pyridyls), β-diketonates, carboxylates, phenolates, phosphonates, silanolates, alkoxides, phosphinoxides, polyphosphines, cyano ligands, and halides.6−13 The careful choice of classical ligands has enabled many significant advances in molecular magnetism, including: the synthesis and characterization of Mn12Ac, the first molecule for which magnetization hysteresis was demonstrated,22 single-ion magnetism in terbium phthalocyanine complexes,23 and toroidal magnetism and a diamagnetic ground state in a dysprosium triangle with 15 unpaired electrons.24,25 The molecular approach to magnetism offers a distinct advantage in its ability to allow the stereoelectronic properties of a ligand to be fine-tuned according to requirements using established organic chemistry protocols, often leading to targeted improvements of a particular property. But what if molecular magnetism were to expand beyond the established comfort zone of traditional coordination chemistry toward the arena of modern main group chemistry? What if ligand synthesis in molecular magnetism were to focus on, for

2. SURVEY OF MAIN GROUP LIGANDS IN MOLECULAR MAGNETISM The major application of main group elements as nontraditional ligands in molecular magnetism is in single-molecule magnetismcurrently a very vibrant area of researchand this is reflected in the coverage of this review. Applications of main group ligands in other areas of molecular magnetism will, however, be described to highlight the potential for growth. The following sections are organized loosely into different pblock rows to highlight the specific features of each type of main group element. However, the chemistry and the physics of magnetism obviously transcend traditional periodic boundaries; hence, for practical purposes such a division is only implemented to structure the review. Similarly, transition metal molecular magnets will be described separately from those based on the lanthanides or actinides, the justification for which is the conceptual approach used to explain their electronic structure and phenomena such as SMM behavior. 2.1. A Synthetic Chemist’s Guide to Transition Metal SMMs

Single-molecule magnets retain magnetization in the absence of an applied magnetic field below a characteristic blocking temperature, TB, and they possess an energy barrier to reorientation of their magnetic moment, Ueff. A recurring theme in the successful design of high-performance SMMs irrespective of where the metal resides in the periodic tableis an axially symmetrical coordination geometry because this can lead to a strong axial or a strong equatorial crystal field. While B


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the ideal scenario is for the molecule to have strict axial point symmetry with a high-order principal rotation axis, this situation is rarely achieved. Fortunately, strict axial symmetry is not an indispensable condition, and deviations of bond lengths and angles from linearity or planarity do not preclude strong axial or equatorial crystal fields. Hence, SMM behavior is routinely observed in molecules with structures that distort from ideal geometries.28−30 In a conventional transition metal SMM, because spin−orbit coupling is subordinate to the ligand field, the orbital contribution to the magnetic moment is either partially or completely quenched, leading to the properties being described in terms of the total spin, S, which has 2S + 1 quantized projections, denoted as MS microstates. In systems where the spin−orbit coupling is far from quenched, such as a two-coordinate transition metal complex (see section 2.1.1) or a lanthanide complex (see section 2.2.1), the total angular momentum quantum number J is regarded as a more accurate descriptor, as are the ensuing 2J + 1 microstates, MJ. A distinction can be drawn between systems containing an odd number of unpaired electrons (Kramers ions) or an even number of unpaired electrons (non-Kramers ions). For the former, Kramers’ theorem posits that the MS states occur as a series of doublets, and the two components of each Kramers doublet (KD) are degenerate with equal but opposite projections of MS, meaning that the system has magnetic bistability.31 Non-Kramers ions can form the basis of an SMM, and the requirement for axial symmetry is more stringent. In a compound with an axially symmetrical coordination geometry and negative zero-field splitting, the maximum and minimum values of MSwhich lead to the largest magnetic moments are the ground KD, and the states with lower MS values are the excited KDs and have lower magnetic moments. Population of one component of the ground KD and not the other gives rise to SMM behavior, a situation that occurs in the presence of magnetic anisotropy. One of the ultimate aspirations in the design of an SMM is to obtain a system where the probability of a transition from the ground KD to the higher KDs is vanishingly low. Unfortunately, the probability of a transition from the ground state to another state is always finite, and often appreciable, the outcome of which is magnetic relaxation; i.e., the magnetization is reversed, an equilibrium population established, and the magnetic memory wiped. The process of magnetization reversal can consist of a series of different mechanistic steps (Figure 1), each of which produces a characteristic relaxation time at a given temperature. Many SMMs feature multiple mechanisms, often occurring simultaneously.32 Perhaps the most remarkable is quantum tunnelling of the magnetization (QTM), which is temperature-independent and occurs within the ground KD from +MS to −MS (or vice versa). A highly axial system will often have a very low probability of QTM, whereas a system with poor axiality will have QTM as a dominant process. High axial magnetic anisotropy is more likely to lead to Orbach relaxation, a thermally activated process that corresponds to the system moving through consecutive higher energy KDs with +MS (or −MS) followed by relaxation to the ground KD with the opposite projection. Often a system will not make it up to the highest KD before directly relaxing due to the poor axiality of the excited MS states. Alternatively, it is possible for the +MS component of an excited KD to be reached followed by a transition within the same KD to reach the component with −MS via thermally

Figure 1. Schematic representation of the possible relaxation processes in a hypothetical SMM consisting of four Kramers doublets. Dashed arrows represent simplified illustrations of common relaxation processes (QTM = quantum tunnelling of the magnetization, TAQTM = thermally assisted QTM).

assisted QTM, followed by relaxation to the ground −MS state. The final process to consider is Raman relaxation, in which inelastic scattering of a phonon promotes relaxation via real (first-order Raman) or virtual (second-order Raman) excited states. Quantum mechanically, the relaxation process(es) represent a mixing of the wave functions that combine to define an MS state. An axially symmetrical coordination geometry is ideal because it provides a series of microstates with well-defined projections of MS in the ground KD and in the excited KDs, i.e., very little mixing of wave functions, leading to dominant Orbach relaxation. Fundamentally, therefore, the efficient design of an SMM is an exercise in wave function engineering; however, the synthesis of real examples is often far from trivial. Experimentally, the efficiency of the theoretical blueprint is verified through two key performance metrics. First, the effective energy barrier to reversal of the magnetization (also referred to as the anisotropy barrier), Ueff, is essentially the energy required to flip a magnetic dipole from an “up” orientation to “down”. The parameter Ueff is normally determined from AC magnetic susceptibility measurements, in which the sample is placed in a susceptometer and subjected to small oscillating fields over a range of frequencies (ν), with commercial instruments typically operating in the range 0.1− 1500 Hz. During such measurements, a genuine SMM requires zero applied DC field to show slow relaxation of the magnetization since the system does not rely on an external stimulus to achieve magnetization. In contrast, some compounds do require the application of a DC field to slow relaxation, and such systems are more accurately said to show field-induced slow relaxation of the magnetization and are not SMMs in a strict sense. The AC magnetic susceptibility (χ) can be expressed as a sum of real (χ′) and imaginary (χ″) components, and relaxation times derived from the latter quantity are normally used to derive Ueff. Since the presence of magnetic anisotropy dictates that certain orientations of the magnetic moment are more stable than others, the molecule resists the reorientation change induced by the oscillating field, leading to a time lag of the magnetic moment. The time lag is related to the magnetization relaxation time (τ) at a given AC frequency and temperature. Measuring χ″ at different frequencies (ν) and temperatures allows the temperature dependence of the relaxation time to be plotted and modeled in terms of the various processes described above, which can give very detailed information on the dominant contributions C


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Chart 1. 3d-SMMs with Low-Coordinate C-Donor Ligands

to the mechanism. Several examples of such experiments will be illustrated in the following sections. In well-behaved cases, Ueff may correspond to the energy separation between the ground KD and one of the excited KDs, which provides insight into the details of the relaxation mechanism. One aim in SMM design is to produce systems with large separations between the KDs and for the system to undergo Orbach processes via the highest possible KD but not via QTM, which is a major challenge for synthetic chemists. The second SMM performance parameter is the magnetic blocking temperature, TB, which is perhaps most relevant to any technological application that relies on magnetic memory effects. TB still defies attempts at a strict definition despite the maturity of the field. The three popular ways of expressing TB are the temperature at which the relaxation time is 100 s; the maximum temperature at which magnetization versus field hysteresis loops remain open (which is strongly dependent on the sweep rate); and the temperature at which the field-cooled (FC) and zero-field-cooled (ZFC) magnetic susceptibilities diverge. While magnetic hysteresis is undoubtedly a key property of an SMM, the blocking temperature should not be considered in isolation but also alongside the magnetic remanence, i.e., the amount of residual magnetization in the absence of a field, and the coercive field, i.e., the field strength needed to drive the magnetization back to zero. Comparing the performance of different SMMs should therefore be done cautiously. 2.1.1. Two-Coordinate 3d-SMMs with 2p-Donor Ligands. Based on a simple consideration of the principles in the preceding section, the spin−orbit coupling in linear or near-linear two-coordinate transition metal complexes is not completely quenched. Hence, such compounds with anisotropic 3d configurations should have a high degree of magnetic axiality and be excellent candidates for observing SMM behavior. Indeed, considerable progress has been made with the synthesis, characterization, and understanding of SMMs in

which the metal is ligated primarily by low-coordinate carbonand nitrogen-donor ligands, and many examples containing transition metals and lanthanides are known. Commonly used ligands are σ-donors such as N-heterocyclic carbenes (NHCs), cyclic alkyl amino carbenes (CAACs), and bulky alkyl ligands such as [(Me3Si)3C]− (trisyl), as well as π-bonded ligands such as cyclopentadienyl (Cp) and cyclooctatetraenyl (COT). The ease with which the steric bulk of charge-neutral, monodentate NHC, and CAAC ligands can be varied has proven to be a valuable tool in the isolation of many two-coordinate 3d metal complexes, some of which show impressive slow magnetization relaxation properties that vary with the substituent pattern. Key examples are depicted in Chart 1. The homoleptic cobalt(I) complexes [Co(IMes)2]+ (1a), [Co(SIMes)2]+ (1b), and [Co(IAd)2]+ (1c) (IMes = 1,3dimesitylimidazol-2-ylidene, SIMes = 1,3-dimesitylimidazolin2-ylidene, IAd = 1,3-diadamantylimidazol-2-ylidene), which were synthesized as salts of noncoordinating tetra(aryl)borate anions by sodium extraction of a halide ligand from the threecoordinate precursors, have molecular structures with C−Co− C angles very close to linearity, i.e., 178.6(1)°, 178.4(1)°, and 180°, respectively, and remarkably uniform Co−C bond lengths in the range 1.936(2)−1.943(3) Å.33 The only major structural difference between the three compounds is the dihedral angle formed between the planes of the two NHC ligands, which is 39.55° and 35.02° for 1a and 1b, respectively, but 90° for 1c. Measuring the temperature dependence of the magnetic susceptibility for 1a and 1b revealed that the orbital contribution is unquenched since the values of χMT at 300 K (χM is the molar magnetic susceptibility) are 3.65 cm3 K mol−1 and 3.26 cm3 K mol−1, respectively, i.e., substantially greater than the expected value of 1.0 cm3 K mol−1 for a spin-only S = 1 system. In contrast, χMT at 300 K for 1c is only 1.94 cm3 K mol−1. Whereas χMT decreases gradually for 1a and 1c and the temperature is lowered, a much sharper decrease was observed for 1b below 8 K. The strong magnetic anisotropy of 1a−c was D


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SIPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene, and Ar = 2,6-dimesitylphenyl, the synthesis of which was accomplished by addition of ArN3 to an NHC-ligated cobalt(0) precursor.34 The Co−C distances in 2a−c are, at 1.953(6) Å, 1.949(4) Å/1.959(4) Å, and 1.971(5) Å, respectively, significantly longer than the CoN distances of 1.691(6) Å, 1.675(3) Å/1.677(3) Å, and 1.682(4) Å, respectively. The C−Co−N angles of 173.0(3)°, 177.5(2)°/ 179.3(2)°, and 175.7(2)° in 2a−c, respectively, are close to linearity, hence because cobalt(II) is a Kramers ion (d7, S = 3/ 2) a bistable magnetic ground state and, therefore, SMM behavior can be expected. Indeed, slow magnetic relaxation in zero applied DC field was observed for 2a−c, with anisotropy barriers of Ueff = 297, 288, and 413 cm−1, respectively. Fitting the relaxation time data for 2a and 2c revealed that Orbach and Raman processes are important, whereas for 2c a role for direct processes was also invoked to explain the properties. Furthermore, magnetic hysteresis with coercivity was measured for 2c up to 9.5 K using a fast field sweep rate of 700 Oe s−1 (Figure 3).

investigated theoretically at the CASPT2 level (complete active space second-order perturbation theory), which allowed the zero-field splitting parameters to be calculated for model complexes in which the methyl groups in 1a and 1b were replaced with hydrogen atoms (i.e., phenyl substituents). The D and E values of +33.4 cm−1 and −4.4 cm−1, respectively, are indicative of appreciable easy-plane anisotropy. The D value of −8.2 cm−1 determined for 1b indicates moderate easy-axis anisotropy, whereas for 1c weak anisotropy is implied by the small D-value of −0.11 cm−1. The D and E values calculated for 1a suggested that slow magnetic relaxation may be possible, and while no peaks could be observed in the frequency (ν) dependence of the out-ofphase AC magnetic susceptibility (χ″M), the application of an applied DC field of 2 kOe allowed peaks to be resolved in the temperature range 2−10 K (Figure 2). Analysis of the

Figure 2. Frequency dependence of the out-of-phase magnetic susceptibility for [1a][BPh4] in an applied field of 2 kOe. Reproduced with permission from ref 33. Copyright 2015 Royal Society of Chemistry.33

temperature dependence of the relaxation time (τ) yielded an anisotropy barrier of 21.3 cm−1 with τ0 = 6.6 × 10−6 s, with the high-temperature relaxation being dominated by an Orbach process and Raman processes and QTM becoming more prominent at lower temperatures. No slow magnetic relaxation was observed for 1b and 1c under any conditions. The contrasting behavior observed for 1b is thought to be connected to the degree of (un)saturation in the NHC ligand, which results in stronger d−π interactions and hence a lowering of the ligand field symmetry in 1b. In the case of 1c, a remarkably strong dependence of D on the NHC−NHC dihedral angle was found, with D becoming negligible above 50°, a result that can explain the dynamic magnetic properties of 1c. A potential role for secondary interaction between the NHC substituents and cobalt was also invoked. More generally, the study of 1a−c highlights the dramatic impact of ligand microstructure on the magnetism, which provides a potential means of modifying the properties in a controlled way. The synthetic design principles used to develop 1a−c were extended to the two-coordinate cobalt(II) NHC−imido complexes [(IPr)Co = NAr] (2a), [(CyIPr)Co = NAr] (2b), and [(SIPr)Co = NAr] (2c), where IPr = 1,3-bis(2,6diisopropylphenyl)imidazol-2-ylidene, CyIPr = 1,3-bis(2,6diisopropylphenyl)-4,5,6,7-tetrahydrobenzimidazol-2-ylidene,

Figure 3. Frequency dependence of χM ″ in zero DC field and magnetic hysteresis loops (sweep rate 700 Oe s−1) for 2c. Reproduced with permission from ref 34. Copyright 2017 American Chemical Society.34

The origins of the magnetic properties of 2a−c were explored using a range of theoretical methods. One analysis identified that the short cobalt−nitrogen bond is an important factor since a lengthening of the optimized bond length in a model complex substantially reduced both the energy separation between the ground and excited Kramers doublets and the gz tensor, which reflects a reduction in the axial magnetic anisotropy. However, further interrogation of the model complex revealed that an alternative and, potentially, more realistic interpretation of the electronic structure should consider three interacting components, i.e., [NHC]0[CoN]+[Ar]−. The core of the molecule is a strongly exchanged coupled cobalt−imido unit consisting of contribuE


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tions from two configurations, i.e., SCo = 3/2 with SN = 0 and SCo = 1 with SN = 1/2. The main magnetic axis in the ground KD is coincident with the [CoN]+ bond, and the inherent magnetic anisotropy in the [CoN]+ unit leads to appreciable separations between the ground and first-excited KDs, in line with the experimental anisotropy barriers. Furthermore, the two lowest KDs in the model complex can be defined by a specific MJ value, with the ground and first-excited KDs defined by MJ = ± 7/2 and ±5/2, respectively. Despite the dominance of the [CoN]+ unit, it was also noted that the differing electronic properties of the NHC ligands can also impact the magnetic properties. The two-coordinate NHC complex [Ni(6-Mes)2]+ (3) was isolated as the bromide salt by addition of 6-Mes to the threecoordinate precursor [Ni(6-Mes)(PPh3)(Br)] (6-Mes = 1,3bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidopyrimidin-2-ylidene).35 Complex 3 is a rare molecular nickel(I) species, and the linear C−Ni−C angle of 179.27(13)° and Ni− C distances of 1.939(3) and 1.941(3) Å combined with the Kramers nature of nickel(I) (d9) should be amenable to slow magnetic relaxation. The value of χMT at 300 K was determined to be 1.12 cm3 K mol−1, which is much larger than the theoretical value of 0.378 cm3 K mol−1 for a spin-only S = 1/2 system, indicating appreciable first-order orbital angular momentum. The magnetic anisotropy in 3 was further supported by field-dependent magnetization and reduced magnetization isotherms at low temperatures. Although no maxima were observed in the χM ″ (ν) data in zero DC field, field-induced slow relaxation was observed at 600 Oe, and fitting the high-temperature relaxation times yielded an anisotropy barrier of 12 cm−1 with τ0 = 4.6 × 10−6 s. The lack of zero-field slow relaxation in 3 despite the Kramers nature of nickel(I) was attributed to mixing between the ground and excited states, leading to efficient QTM. In the design of low-coordinate transition metal SMMs, cyclic alkyl-amino carbenes (CAACs) offer an interesting alternative to NHCs due to the greater steric bulk afforded by the quaternary carbon adjacent to the carbon donor atom.36 Furthermore, the much stronger σ-donor capacity of CAACs and their noninnocent character should also impact the magnetism. The three-coordinate metal(I) precursor compounds [(CAACR)2MCl], where R denotes the two substituents on the quaternary carbon adjacent to the carbene carbon, can be synthesized with M = Cr and R = Me (4a),37 M = Fe and R = Me (4b) or Et (4c), and M = Co and R = Et (4d). Each three-coordinate complex can be converted via halide abstraction with M[BArF] (M = Li or Na, BArF = [B(C6F5)4]−, or [B{C6H3(CF3)2}4]−) into salts of the cationic two-coordinate complexes [(CAACMe)2M]+ (M = Cr 5a, M = Fe 5b) and [(CAACEt)2M]+ (M = Fe 5c, M = Co 5d), and one-electron reduction of 5c and 5d produced the neutral complexes [(CAACEt)2M] (6a and 6b). The monometallic chromium(I) complexes 4a and 5a demonstrate one advantage of using CAAC ligands as opposed to NHCs. The distorted trigonal−planar complex 4a features Cr−C and Cr−Cl distances of ∼2.09 Å and 2.366(1) Å, respectively, and perhaps surprisingly, field-induced slow relaxation was observed, which is unusual for an S = 5/2 species, although the X-band EPR spectrum revealed some gtensor anisotropy, with gx = 1.47, gy = 1.40, and gz = 2.70, with | D| = 1.12 cm−1 and E/D = 0.07. An NBO analysis of the spin density on chromium in 4a revealed an appreciable degree of metal-to-ligand π-back-donation, which may play a role in the

slow magnetic relaxation. The two-coordinate complex 5a is isotropic and therefore not an SMM. The structurally similar three-coordinate iron complex 4b was found to possess appreciable spin−orbit coupling, as revealed by the χMT value of 2.93 cm3 K mol−1 at 300 K, which is larger than the predicted spin-only value of 1.875 cm3 K mol−1 for an S = 3/2 system; the analogous χMT value for 5b is 2.79 cm3 K mol−1. Easy-plane anisotropy of the iron(I) center in 4b was determined by simulation of the magnetic susceptibility and variable-temperature, variable-field magnetization data, which yielded g ≈ 2.5 and D = +20 cm−1. In contrast, easy-axis anisotropy was determined for 5b, which has g = 2.50 and D = −22.0 cm−1 (from magnetization measurements). No slow relaxation in zero DC field was observed for 4b; however, an applied DC field of 500 Oe allowed maxima in the temperature range 2.5−4.1 K to be ″ (ν) data, leading to an anisotropy barrier of observed in the χM Ueff = 22.4 cm−1 with τ0 = 7.0 × 10−8, as determined from a fit of the relaxation times above 3.6 K. For 5b, the application of a 3 kOe DC field allowed a limited number of peaks to be observed in the out-of-phase AC susceptibility, with the anisotropy barrier estimated to be no greater than 20 cm−1. A theoretical analysis of the weak slow relaxation in 5b concluded that although the C−Fe−C unit is perfectly linear the πbonding between the metal and the CAAC introduces a nonnegligible transverse component to the g-tensors, which appreciably reduces the magnetic axiality in the ground KD. Consistent with the theoretical study of 5b, the isoelectronic and isostructural complex [Mn(CAACMe)2] (7), which formally contains manganese(0), was also determined to have significant delocalization of spin density onto the carbon donor atoms.38 Ab initio calculations on 7 revealed a highly multiconfigurational S = 3/2 ground state, with the dominant contribution coming from six electrons in five manganese dorbitals and a single electron delocalized across the π-orbitals of both ligands. In effect, the ground state is generated by strong antiferromagnetic exchange interactions between an S = 2 manganese(I) center with an S = 1/2 bis(CAAC) radical ligand environment. From an experimental perspective, the strong coupling of the metal to the ligands in 7 was quantified with a coupling constant of J > −350 cm−1, with g(ligand) = 2.00 and g(Mn) = 2.10 also being determined from fits of the magnetic data. Furthermore, slight easy-plane anisotropy was determined through D = +1.4 cm−1. No AC magnetic susceptibility studies were described for 7, although the similarities in the electronic structure with 5b suggest that this may be worth exploring. Evidence for delocalization of spin density onto the CAAC ligands was also proposed for the formally cobalt(0) complex [Co(CAACEt)2] (6d), which features significantly shorter Co−C distances than those in the precursor complex [Co(CAACEt)2]+ (5d), although the large CAAC−CAAC torsional angles are thought to limit the extent to which delocalization can occur.39 Magnetic property measurements were not described for the two-coordinate cobalt−CAAC complexes, although again the similarities in the basic electronic and molecular structures of this d9 species are reminiscent of the nickel(I) complex 3. In addition to well-defined two-coordinate metal complexes, pseudo-two-coordinate species are also of interest in the context of molecular magnetism. In particular, structures containing a cyclopentadienyl ligand and one other ligand fit the description of pseudo-two-coordinate since η5-Cp coordination can be analyzed through the metal−Cpcent axis, where F


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Figure 4. Molecular structure of 10 (Co = purple, O = red, Si = cyan, C = gray; H atoms are omitted for clarity) and the d-orbital splitting and ground state electronic configuration and magnetic hysteresis loops studied at 1.8 K at field sweep rate = 32 Oe s−1 on neat 10 (orange) and the diluted version (purple). Reproduced with permission from ref 42. Copyright 2018 American Association for the Advancement of Science.42

“cent” refers to the centroid of the C5 ring. Although most Cpligated SMMs are based on lanthanides (see sections 2.2.1 and 2.3.2), a few examples are also known with transition metals. The carbene-ligated iron(I) “pogo-stick” complex [(η5-C5iPr5)Fe(IiPr2Me2)] (8, Chart 1) has a Cp−Fe−L angle of 166.6°, which, despite the substantial deviation from linearity, still shows field-induced slow relaxation.40 An S = 3/2 ground state was determined for 8 along with a large axial ZFS parameter of D = −33.4 cm−1, which implies that the ground Kramers doublet with MS = ±3/2 is occupied at low temperatures. The presence of a large and unquenched orbital contribution to the magnetic moment was indicated in the 57Fe Mössbauer spectrum of 8, which is broadened unsymmetrically at 80 K. No slow magnetic relaxation was observed in zero field; however, in a DC field of 1 kOe, maxima in the χ″M(T) data were observed up to 7 K at various frequencies in the range ν = 5−1500 Hz, which to led an anisotropy barrier of 63.6 cm−1 being determined, with τ0 = 1.8 × 10−10 s. A detailed theoretical analysis of 8 identified that π-anisotropy arising from the NHC donor carbon had a significant impact on the dorbital splitting; i.e., the degeneracy of the dxz and dyz orbitals is lifted. Furthermore, deviation of the axial geometry away from 180° results in the degeneracy of the dxy and dx2−y2 orbitals being lifted, and 3d−4s mixing results in the dz2 orbital being very similar in energy to dxy. The unusual neardegenerate orbital structure of 8 ultimately results in a nonaufbau ground state configuration with L = 3 and appreciable magnetic anisotropy, which explains the observed slow magnetic relaxation. The pogo-stick iron(II) complex [(η 5-C5iPr5)Fe(2,6C6H3iPr2)] (9, Chart 1) is isostructural to 8 and has an Fe− Cp distance of 1.93 Å with an Fe−C distance of 2.0427(9) Å to the aryl ligand.41 A high-spin S = 2 configuration was determined for the iron center in 9, and modeling the χMT(T) data yielded an axial ZFS parameter, D, of −51.36 cm−1, with E = −0.32 cm−1 and g = 2.29. The frequency dependence of the out-of-phase susceptibility for 9 revealed dominant fast QTM in zero DC field; however, the in-field measurements revealed the occurrence of two thermally activated processes. In the optimum DC fields of 750 Oe and 2.5 kOe, the anisotropy barriers were determined to be 28 and 100 cm−1, respectively, which are thought to correspond to phonon-initiated Orbach relaxation from the ground KD with MS = ±2 via the first excited KD with MS = ±1. It is interesting to compare the nonaufbau ground state of the d7 iron(I) center in carbene-ligated 8 with that determined for the d7 cobalt(II) center in the strictly linear bis(alkyl)

complex [Co{C(SiMe2ONaph)3}2] (10, Naph = 1-naphthyl) (Figure 4), where the substituted trisyl (i.e., derived from tris(trimethylsilyl)methyl) ligands are, in principle, exclusive σdonors.42 Compound 10 was synthesized in a classical salt metathesis reaction between cobalt(II) bromide and [KC(SiMe2ONaph)3], with an important feature being the use of an electron-withdrawing naphthoxy substituent, which overcomes the strongly reducing nature of the parent trisyl ligand [(Me3Si)3C]−, which has a tendency to reduce cobalt(II). A ligand field analysis of 10 revealed that two pairs of degenerate d-orbitalsdx2−y2 with dxy and dxz with dzyare each occupied with three electrons and a higher lying singly occupied dz2 orbital. The resulting4Φ ground term splits into four Kramers doublets due to spin−orbit coupling, with the magnetic ground state represented by MJ = ±9/2 and the first excited state MJ = ±7/2 calculated to lie at 476 cm−1. The computational model also produced the intriguing result that the σ-orbitals of the carbon donor atoms have a very minor effect on the energies of various states. Indeed, the complex derives only a small component of its stability from the Co−C bonds, which have a formal bond order of approximately 0.5; rather, appreciable intramolecular, interligand noncovalent interactions play an important role in stabilizing the molecule. The feeble ligand field strength in 10 is ultimately responsible for the observed d-electron configuration since the energy penalty incurred by promoting an electron from the dx2−y2/dxy degenerate pair in the aufbau configuration to the dxz/dyz pair to give the nonaufbau configuration is minor compared to the pairing energy. The use of variable-field far-infrared spectroscopy allowed the separation between the ground and first excited KDs to be measured as 450 cm−1, which is currently the largest Ueff value for a transition metal SMM. AC susceptibility measurements ″ (ν) data up to 70 K. Fitting on 10 revealed maxima in the χM the relaxation time data on the pure and magnetically dilute analogue (in a matrix of the isostructural zinc compound) invoked a role for direct transitions and revealed that QTM is not significant above 4 K, although some contribution by such processes was proposed. At higher temperatures, the nonlinear dependence of the relaxation time on temperature indicated that relaxation via Orbach processes still does not dominate. Finally, the relatively long relaxation times determined for 10 led to the observation of magnetic hysteresis in the form of waist-restricted loops, with a small coercive field of 180 Oe being measured at 1.8 K and a sweep rate of 32 Oe s−1 increasing to 600 Oe for the dilute analogue (Figure 4). G


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complex and therefore has a coordination number of greater than three. The tris(NHC)borate ligand scaffold has proven to be adept at stabilizing terminal nitride complexes of iron in a range of oxidation states, e.g., the C3v-symmetric iron(IV) nitride [PhB(MesIm)3FeN] (18).47 Addition of triphenylphosphine to 18 produces the phosphino−imide complex [PhB(MesIm)3Fe−NPPh3] (19), which is an extremely unusual example of a tetrahedral iron(II) spin crossover.48 The thermal spin crossover in 19 converts the high-spin S = 2 form into the low-spin S = 0 forms and occurs with a transition temperature of 81 K (Figure 5). The same effect is not

In contrast to 10, the isostructural and formally isoelectronic iron(I) complex anion [Fe{C(SiMe3)3}2]− (11), which was synthesized as the salt of [K(222-crypt)]+, has a ground state with L = 2.43 A ligand field analysis also revealed a different dorbital splitting for 11, with 3dz2 being the lowest-lying dorbital due to significant mixing with the iron 4s orbital, as opposed to being the highest-lying d-orbital as in 10. The value of χMT for 11 at 300 K is 3.39 cm3 K mol−1, which is significantly higher than the spin-only value for a system with S = 3/2, and the drop in this quantity at low temperatures indicated the onset of magnetic blocking. AC susceptibility measurements on 11 in zero DC field revealed the dominance of an Orbach mechanism above 20 K, with an associated anisotropy barrier of 226 cm−1, presumably due to excitation from a ground Kramers doublet with MJ = ±7/2 via the first excited doublet with MJ = ±5/2. QTM processes become prominent at lower temperatures. Furthermore, waistrestricted magnetic hysteresis loops were observed up to 6.5 K, using an average sweep rate of 50 Oe s−1. Solution-phase magnetic dilution experiments on 11 revealed that intermolecular dipolar exchange plays only a minor role in the magnetic relaxation processes at low temperatures. The observation of slow relaxation in 11 without the need for an applied field highlights the important role played by Kramers’ theorem in the design of SMMs; i.e., in noninteger spin systems (Kramers ions) the tunnelling rate should be minimized. In comparison, [Fe{C(SiMe3)3}2] (12), which contains the non-Kramers d6 ion Fe(II), is strictly linear but only shows field-induced slow relaxation, with an anisotropy barrier of 146 cm−1 determined in a DC field of 500 Oe.44 A detailed theoretical study of 12 and of the related twocoordinate iron(II) complexes [Fe{N(SiMe3)(Dipp)}2] (13), [Fe{N(H)Ar′}2] (14) (Ar′ = bis(2,4,6-triisopropylphenyl)phenyl), [Fe{N(H)Ar*} 2 ] (15) (Ar* = bis(2,4,6triisopropylphenyl)phenyl), and [Fe(OAr′)2] (16), each of which shows field-induced slow magnetic relaxation, emphasized the key role played by the ligand π-electrons and local pseudosymmetry in the Fe−X bonds.45 The resulting “bonding anisotropy” in 13−16 is responsible for increased splitting of the d-orbital structure and, hence, diminishing the magnetic anisotropy. A particularly interesting conclusion was that replacing the 2p donor atoms with 3p or heavier donor atoms would increase the spin−orbit coupling and suppress the vibronic coupling that drives the magnetic relaxation, thus identifying opportunities for the applications of heavier main group elements in the design of SMMs. The two-coordinate iron(I) amide [Fe{N(SiMe3)2}2]− as the salt of [K(18-crown-6)]+ (17a) or [K(2.2.2-crypt)]+ (17b) features strong axial magnetic anisotropy and shows well″ (ν), with associated defined peaks up to 22 K in the plots of χM anisotropy barriers of 43 and 64 cm−1 and with τ0 = 4.9 × 10−6 s and 8.9 × 10−6 s, respectively.46 The lower anisotropy barriers in 17a and 17b are consistent with the rationale used to explain the SMM properties of 13−16 and should be related to the electronic asymmetry of the local coordination environment, i.e., the π-electrons on the nitrogen donor atoms. The observation of slow magnetic relaxation in these iron(I) amido complexes in zero applied field contrasts to the field-induced behavior of the iron(II) complexes 13 and 15, which can be explained by invoking Kramers’ theorem. 2.1.2. Higher-Coordinate 3d-SMMs with 2p-Donor Ligands. For the purposes of the current discussion, a highcoordinate complex is defined relative to a low-coordinate

Figure 5. Temperature dependence of the χMT product in warming (red dots) and cooling modes (blue dots). Inset: molecular structure of [PhB(MesIm)3Fe−NPPh3] (19); for clarity, only the ipso carbon atoms of the phosphine phenyl substituents are shown, and only the ipso carbons of two mesityl substituents are shown. Reproduced with permission from ref 35. Copyright 2013 American Chemical Society.

observed for the version of 19 in which the mesityl substituents are replaced with tert-butyl. A variable-temperature singlecrystal X-ray diffraction study of 19 revealed, upon cooling from 150 to 30 K, a substantial shortening of the Fe−C distances by approximately 0.1 Å and of the Fe−N distances by 0.05 Å. In contrast, the P−N bond lengthened by 0.03 Å due to greater π-overlap. In terms of a qualitative MO description, the impact of the structural changes upon cooling was to destabilize the higher-lying pair of d-orbitals such that spinpairing effects become relatively small and the low-spin form becomes the ground state. Further studies on 19 showed significant changes in the optical reflectivity spectra consistent with SCO: the high-spin state was reformed upon irradiation with white light (0.4 mW cm −2 ) at 10 K. 49 These spectroscopic observations led to photoinduced SCO being observed in the temperature dependence of χMT, which converted greater than 70% of the low-spin form into the metastable high-spin form. Given the low temperatures at which light induces SCO to form the S = 2 form of 19, which contains anisotropic iron(II), it was subsequently possible to observe photoinduced slow relaxation of the magnetization in applied field of 1 kOe. Analysis of the AC susceptibility data yielded an anisotropy barrier estimated to be at least 15 cm−1. In an extension of the reactivity of 19, a strategy was developed for the targeted synthesis of NHC-capped bimetallic complexes, whereby advantage was taken of the two-electron H


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can be illustrated with the hypothetical two-coordinate complex [Dy(OPPh3)2]3+ (22), in which the metal occupies a strictly linear environment between the two triphenylphosphine oxide ligands (Figure 6).28,54−56

donor capacity of the nitride ligand. Thus, the heterobimetallic compound [PhB(MesIm)3Fe−NV(Mes)3] (20) can be accessed readily, with formal oxidation states of iron(II) and vanadium(V) being established by X-ray photoelectron spectroscopy (XPS).50 Although only field-induced slow relaxation with a small barrier of 6.6 cm−1 could be observed for 20, originating from the anisotropic high-spin iron(II) center, the synthetic method has a scope for expansion to incorporate other nitride-bridged heterobimetallic units, potentially with exchange coupling built into the design process. The manganese(IV) analogue of 18, i.e., [PhB(MesIm)3MnN] (21), which possesses a rare low-spin 3d3 configuration with S = 1/2, has an anisotropic ground-state doublet that gives rise to slow magnetic relaxation, albeit in a DC field of 200 Oe with a miniscule barrier of 3.5 cm−1 and an unusually large τ0 value of 5 × 10−6 s.51 A computational analysis of 21 established the prominent nature of QTM in the magnetic relaxation, with the inclusion of Raman processes being required to account for the temperature dependence of the relaxation time, but not Orbach or direct processes.

Figure 6. Structure of hypothetical [Dy(OPPh3)2]3+ (22, left) and the splitting of the eight KDs arising from the 6H15/2 ground term (right). The dashed red line corresponds to the main magnetic axes in each of the KDs, which are essentially colinear in 22. The numbers on the energy spectrum indicate the probability of a transition occurring. Reproduced with permission from ref 28. Copyright 2016 American Chemical Society.28

2.2. f-Block Molecular Magnets with 2p Donor Ligands

To a first approximation, designing SMMs is simpler with lanthanides than with transition metals because the lanthanide−ligand bonding is essentially electrostatic, and nuances such as π-anisotropy and back-donation are not important considerations. The weak overlap of lanthanide 4f orbitals with ligand orbitals means that the orbital angular momentum is essentially unquenched regardless of the ligand, leading to strong magnetic anisotropy and considerable potential for SMM properties. However, the predominance of ionic interactions and the large radii of Ln3+ cations mean that the synthesis of compounds with strict point symmetries is actually very challenging. As the ligands jostle for space around the cation, coordination geometries often distort away from “ideal”, which can have a dramatic impact upon the SMM properties. Furthermore, truly low-coordinate lanthanide environments analogous to those in 2 and 10 are hard to achieve, which is again a consequence of the large cation radius, which facilitates agostic interactions with the substituents on bulky ligands, raising the effective coordination number.52 Fortunately, strict point symmetry is not essential for observing slow magnetic relaxation in lanthanide compounds, and often an approximate or pseudosymmetry will suffice for the right lanthanide. The right lanthanide is usually dysprosium, the trivalent cation of which provides all the natural ingredients a magnetochemist could want to work with. The Dy3+ cation has a 4f9 configuration and is therefore a Kramers ion, ensuring a bistable ground state regardless of the coordination geometry. The strong angular dependence of the 4f orbitals results in the electron density of Dy3+ resembling a sphere flattened in the z-direction, meaning that Dy3+ is often referred to as an oblate spheroidal ion, a property that provides synthetic chemists with a guide on how to design the molecular structure of an SMM.53 The spin−orbit-coupled ground of the dysprosium term is 6H15/2, which has J = 15/2 and produces 16 MJ states in the form of eight KDs. The KD with MJ = ±15/2 will be stabilized as the magnetic ground state in the presence of an axial crystal field, and the nearer to linear the geometry, the nearer to colinear the magnetic axes in the ground and excited KDs will be. These fundamental properties explain why dysprosium SMMs are so prevalent and

In 22, the axial crystal field and the complete absence of an equatorial crystal field mean that QTM and other throughbarrier processes are highly improbable until the fifth KD at 1800 cm−1 is reached. The same principles have been used to design SMMs based on terbium. Although the 4f8 ion Tb3+ is also oblate and strongly anisotropic, it has the 7F6 ground term and is a non-Kramers ion; hence, the symmetry requirements are stricter, and terbium SMMs are less common. A complementary approach is required to design SMMs based on erbium because the 4f11 configuration of Er3+ has prolate spheroidal electron density (elongated in the z-direction), meaning that the anisotropy is enhanced by designing SMMs with strong equatorial crystal fields and weak (or, preferably, nonexistent) axial crystal fields. Examples for each of these lanthanides will be discussed in the following sections. 2.2.1. Lanthanide SMMs with B-, C-, and N-Donor Ligands. Given the importance of electrostatic bonding, ligands in lanthanide SMMs with carbon- and nitrogen-donor atoms tend to be anionic. This section focuses on lanthanide SMMs in which the metal is ligated solely by the lightest elements in Groups 13, 14, and 15, with SMMs containing heavier p-block donors in addition to carbon being reviewed in section 2.3. Synthetic routes to these compounds frequently draw on principles developed in classical studies of f-block sandwich compounds,57,58 hence salt metathesis reactions of alkali metal salts of the ligands with rare-earth halides are particularly popular due to their reliability and versatility. Although lanthanide compounds containing boron-donor ligands in the primary coordination environment are known,59 SMMs with B-donor ligands are very rare and essentially limited to the sandwich compounds [(η6-BRC5H5)Ln(η8C8H8)], in which Ln = Dy and R = H (23a), Me (23b) or NEt2 (23c) or Ln = Er and R = H (24a), Me (24b), or NEt2 (24c).60 In 23a−24c the boratabenzene ligand can be regarded as a ring-expanded version of cyclopentadienyl or as an anionic I


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Figure 7. Upper: molecular structures [(η6-BRC5H5)Er(η8-C8H8)] with R = H (24a), Me (24b), or NEt2 (24c), showing the tilt angle of the boratabenzene ligand with respect to the COT ligand. Lower: frequency dependence of the out-of-phase susceptibility in zero DC field for 24b and the temperature dependence of relaxation time (red dots) with the best fit (black line). Reproduced with permission from ref 60. Copyright 2016 Royal Society of Chemistry.60

differences in the SMM behavior. An ab initio theoretical study of the erbium boratabenzene complexes revealed that the dominant thermal relaxation pathway in 24a and 24b occurs via the second-excited KD, whereas in 24c, it is via the first excited KD; similarly, the calculated tunnelling gaps are larger for 24c, which explains the more prevalent QTM observed experimentally. The low-coordinate carbon-donor ligand to have had the biggest impact in single-molecule magnetism is undoubtedly cyclopentadienyl or, more accurately in the context of lanthanide chemistry, cyclopentadienide, [Cp]−.61,62 Dysprosium metallocenes are a large subset of the SMM family, with the very first organometallic SMM being the dimetallic benzotriazole-bridged compound [Cp2Dy(μ-bta)]2 (25), which has an anisotropy barrier of only 32 cm−1 and shows no opening of the hysteresis loops even at 1.8 K.63 As a proof of principle, however, the observation of slow magnetic relaxation in 25 set a precedent and led to the design of other dysprosium metallocene SMMs with improved properties, such as the isocarbonyl-ligated dimer [(Cp*)2Dy(μ-Fp)]2 (26, Fp = CpFe(CO)2), which has an impressive anisotropy barrier of 662 cm−1 and shows waist-restricted hysteresis loops up to 6.2 K.64 The experimental observations and theoretical analysis of SMMs such as 25 and 26, and on related compounds (see section 2.3.2), led to the development of a robust magnetostructural correlation for dysprosium metallocenes of the type [Cp2Dy(μ-X)]n (n = 1−3), in which a dominant axial crystal field generated by the [Cp]− ligands was identified as being responsible for the SMM properties (Figure 8). The equatorial X ligands in [Cp2Dy(μ-X)]n, which are known with a broad range of p-block donor atoms,61 were found to attenuate Ueff and to be responsible for the weak coercive fields in the magnetic hysteresis. Thus, removing the equatorial X ligands to generate a discrete dysprosium

version of benzene by virtue of the isoelectronic and isolobal analogy between {BR}− and {CR}, meaning that an axial crystal field can be anticipated. The erbium atoms in 24a−24c, which are isostructural to 23a−23c, are considerably closer to the COT centroid (1.674−1.679 Å) than to the boratabenzene centroids (2.245−2.257 Å), and slippage of the heteroaromatic ligand away from boron also occurs, which results in a tilting of the two ligands with respect to each other (Figure 7). The outof-phase susceptibility measurements on 24a and 24b as a function of frequency reflect qualitatively similar properties, with maxima observed at 15−24 K and 16−25 K, respectively, the position of which moves to higher temperatures on increasing the frequency. The anisotropy barriers determined for 24a and 24b in zero DC field are 259 and 300 cm−1, respectively, with τ0 = 5.3 × 10−12 s and 5.5 × 10−12 s. In contrast, the SMM properties of 24c are less prominent, with low-temperature QTM being more prevalent below 10 K but with thermal relaxation occurring in the 10−25 K region. In zero DC field, an anisotropy barrier of 174 cm−1 was determined (τ0 = 9.2 × 10−10 s), increasing to 219 cm−1 in a 2 kOe applied field (τ0 = 6.0 × 10−11 s). The contrasting dynamic magnetic properties of 24c are believed to be a consequence of how the NEt2 substituent affects the geometry of the boratabenzene ligand via πbonding, which results in the boron atom sitting 0.097 Å out of the mean C5 plane. Analogous measurements on 23a−23c revealed slow relaxation only in the presence of an applied field, and with very small barriers, highlighting that the COT/ boratabenzene crystal field environment is not well matched to the oblate ion Dy3+. While all three erbium compounds show waist-restricted hysteresis loops, those for 24a and 24b occur up to 8 and 6 K, whereas narrower loops were observed for 24c up to 5 K. Studying the hysteresis of 24a in a magnetically dilute form revealed similar properties to the nondilute form, confirming that the local geometry is responsible for the J


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2.284(1) Å, respectively, and the wide Cp−Dy−Cp angle of 162.507(1)°. The strong axial crystal field results in the principal magnetic axes in the first six KDs being oriented toward the center of the Cp ligands, hence they are essentially colinear and defined by a definite MJ value, although the relaxation in 33 probably proceeds via an Orbach process involving the fifth KD (Figure 10). The origins of the Orbach relaxation lie within the Cp ring vibrations, particularly out-ofplane deformations and tilting motion. The energies of the most important vibrations coincide with the separations between adjacent KDs, and the ensuing spin−phonon coupling drives the reversal of magnetization. The sensitivity of the anisotropy barrier and blocking temperature to simple changes in ligand substituents is illustrated by the homologous series of SMMs [(C5iPr4R)2Dy][B(C6F5)4], where R = H (34), Me (35), Et (36), and iPr (37) (Table 1).65 An examination of the metal−ligand distances and the Cp−Dy−Cp bending angle emphasizes the need to strike a balance between enforcing a geometry as close to perfectly axial as possible with bulky substituents (i.e., Cp−Dy−Cp = 180°) and imposing a crystal field that is sufficiently strong to produce a large splitting of the KDs. If the ligands are too bulky, although a near-linear geometry is found, the substituents seemingly push the ligands away from the metal, thus weakening the crystal field. If the ligand bulk is reduced, the ligands can approach the metal more closely, but now the geometry distorts further from linear. In comparing 34−37, the optimal scenario is found with 35 and R = Me, which produces Ueff = 1468 cm−1 and TB = 62 K (from the τ100 measurement). In comparing 33 to 35, the differences can be traced to the heteroleptic ligand environment, which consists of one small and one very bulky ligand: the small Cp* ligand allows close approach of both ligands, leading to a large splitting, and the CpiPr5 ligand enforces a near-linear geometry, which promotes relaxation via a higher-lying KD. The important role of the ligand substituents in 33 is further underscored when comparing its properties to those of [(Cpttt)2Dy][B(C6F5)4] ([38][B(C6F5)4], Cpttt = 1,2,4-tri-tert-butylcyclopentadienyl), which has a smaller barrier of 1277/1223 cm−1 and TB = 60 K.66,67 The C−H oscillators on the Cp ligands in 38 were found to play an important role in the relaxation mechanism,67,68 and the absence of hydrogen substituents in 33 forms part of the explanation for its superior properties. A study of the terbium analogue of 38, i.e., [(Cpttt)2Tb][B(C6F5)4] ([38Tb][B(C6F5)4]), revealed slow relaxation of the magnetization in zero applied DC field; however, open hysteresis loops were not observed above 2 K, highlighting

Figure 8. General design principle for a dysprosium metallocene SMM. The dashed orange line represents the easy axis of magnetization in the ground KD.

metallocene cation [(CpR)2Dy]+ was identified as a promising way of improving the SMM performance.64 Reacting [(η 5 -Cp*)Dy(η 5 -C 5 i Pr 5 )(BH 4 )] (27), 1 5 [(C5iPr4H)2DyI] (28), [(C5iPr4Me)2DyI] (29), [(C 5 i Pr 4 Et) 2 DyI] (30), [(C 5 i Pr 5 ) 2 DyI] (31), 6 5 or [(Cp t t t ) 2 DyCl] (32, Cp t t t = 1,2,4-tri(tert-butyl)cyclopentadienyl)66,67 with the superelectrophile [(Et3Si)2(μH)][B(C6F5)4] has provided access to six compounds of the type [(CpR)2Dy][B(C6F5)4] ([33−38][B(C6F5)4] via abstraction of the halide or borohydride ligand. The resulting cationic dysprosium metallocenes typically have very large anisotropy barriers and high blocking temperatures (Table 1), and in the Table 1. Properties of [(CpR)2Dy]+ SMMs 33 34 35 36 37 38

Cp−Dy/Å

Cp−Dy−Cp/°

Ueff/cm−1

TB/K

2.296(1), 2.284(1) 2.29(1) 2.298(5) 2.302(6) 2.340(7) 2.316(3)

162.507(1) 147.2(8) 156.6(3) 161.1(2) 162.1(7) 152.70(7)

1541 1285 1468 1380 1334 1277, 1223

80, 65 32 62 59 56 60

case of [(η 5 -Cp*)Dy(η 5 -C 5 i Pr 5 )][B(C 6 F 5 ) 4 ] ([33][B(C6F5)4]), the χM ″ (ν) data show maxima up to 130 K, leading to an unprecedentedly high Ueff of 1541 cm−1 (Figure 9).15 Magnetic hysteresis measurements on [33][B(C6F5)4] also produced a blocking temperature of 80 K using a sweep rate of 25 Oe s−1, making it the first SMM to function above the boiling point of liquid nitrogen and, therefore, the first hightemperature SMM. A detailed analysis of the electronic structure of 33 revealed that a large total splitting of the 6H15/2 multiplet occurs due to the synergic combination of two structural factors, i.e., the short Dy−Cp* and Dy−CpiPr5 distances of 2.296(1) Å and

Figure 9. Molecular structure of 33 and frequency dependence of the out-of-phase susceptibility, magnetic hysteresis at 2−75 K (sweep rate of 200 Oe s−1) and at 80 K (sweep rate of 25 Oe s−1) for [33][B(C6F5)4]. Reproduced with permission from ref 15. Copyright 2018 American Association for the Advancement of Science.15 K


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Figure 10. Left: easy axis of magnetization in the ground KD of 33. Right: relaxation pathways in 33; the most probable relaxation route is indicated by the blue arrows, and red arrows represent other possible transitions with lower probability, with the darker shading representing a greater probability. Reproduced with permission from ref 15. Copyright 2018 American Association for the Advancement of Science.15

SMMs, meaning that J is much greater than the subwavenumber exchange coupling constants typically observed in lanthanide compounds. The impact of suppressing QTM with a radical ligand, combined with the magnetic axiality of the metallocene unit, is particularly important for the hysteresis since the rapid drop in magnetization observed for most SMMs can be reduced significantly, up to the characteristic blocking temperature of the material. Below the blocking temperature, the coercivity can be huge and even on par with an inorganic magnetic material. These principles were demonstrated with [K(18crown-6)(thf)2][Ln2{N(SiMe3)2}4(thf)2(μ:η2:η2-N2)] with Ln = Tb ([K(18-crown-6)(thf)2][41]) and Ln = Dy ([K(18crown-6)(thf)2][42]), both of which contain a radical [N2]3− ligand synthesized by triple reduction of dinitrogen.73,74 The SMM properties of 41 and 42 include modest anisotropy barriers of 227 cm−1 (τ0 = 8.2 × 10−9 s) and 123 cm−1 (τ0 = 8 × 10−9 s), respectively. However, the hysteresis properties of 41 and 42 are notable due to the lack of quantum tunneling at zero field, leading to magnetic blocking with coercivity up to 8.3 and 14 K, respectively (sweep rates of 9 Oe s−1). A particularly striking example of what can be achieved with the charge-dense radical anion [N2]3− is shown in the hysteresis properties of [K(2.2.2-crypt)(thf)][{(C5Me4H)2Tb}2(μ:η2N2)], [K(2.2.2-crypt)(thf)][43], which displays open hysteresis loops up to 30 K (sweep rate 100 Oe s−1), a 100 s blocking temperature of TB = 20 K and, at 10 K, and a huge coercive field of 79 kOe (Figure 11).75 An indication of the strength of the metal−ligand exchange coupling in 41−43 was provided by the isostructural gadolinium analogues [K(18-crown-6)(thf) 2 ][Gd 2 {N(SiMe3)2}4(thf)2(μ:η2:η2-N2)] ([K(18-crown-6)(thf)2][44]) and [K(2.2.2-crypt)(thf)][{(C5Me4H)2Gd}2(μ:η2-N2)] ([K(2.2.2-crypt)(thf)][45]), for which isotropic exchange coupling constants of J = −27 and −20 cm−1, respectively, were determined (−2J formalism).75,76 However, a strongly coupled radical ligand alone is not sufficient to produce the desired improvement in the magnetic properties, and the symmetry/ geometry of the coordination environment must also be factored into the design. Emphasizing again that strict point symmetry is not a prerequisite for SMM properties to be observed, an asymmetric coordination environment can introduce electronic factors that promote mixing of the various MJ states within the spin−orbit-coupled ground state, leading

the need for stricter molecular symmetry in SMMs based on non-Kramers ions.69 The cationic dysprosium metallocenes 33−38 are an exceptional class of SMM. Their unusual properties are also highlighted by comparison to other dysprosium metallocene SMMs in which the metal is complexed by additional C- and N-donor ligands in equatorial positions, which are invariably detrimental to the anisotropy barrier and often, but not always, to the hysteresis. Indeed, the precursor compound 27 shows very poor SMM properties,15 with a very small barrier of 7 cm−1 and waist-restricted hysteresis loops due to the strong competing equatorial crystal field introduced by the borohydride ligand. Even when the additional ligands add very weak equatorial components to the crystal field, the impact on the slow magnetic relaxation is remarkable. For example, the soft tetraphenylborate ligand in [Cp*2Dy(μ-Ph2BPh2)] (39) is an SMM but with an anisotropy barrier of 349 cm−1 in a magnetically dilute form and waist-restricted hysteresis up to only 5.3 K.70 The [BPh4]− ligand in 39 can be displaced readily by ammonia in toluene to give [Cp*2Dy(NH3)2][BPh4] ([40][BPh4]), which shows an increased barrier of 546 cm−1 in zero DC field and waist-restricted hysteresis up to 5.2 K.71 The differing properties of 39 and 40 are thought to be related to the increase in the Cp−Dy−Cp angle from 134.0° to 140.2°, which reduces the equatorial component of the crystal field. The most notable exceptions to the general design criteria for lanthanide metallocene SMMs are the group of compounds in which the {Cp2Ln}+ units are bridged by radical ligands in equatorial coordination sites. Several compounds with the general composition [{(CpR)2Ln}n(μ-radical)] (n = 2, 3) are known in which the lanthanide is either terbium or dysprosium and the radical bridge is the exotic main group species [N2]3−, which has a spin of 1/2 or an unsaturated N-heterocycle, for which the spin can be greater than 1/2. The topic is the subject of a recent review,72 and only the main highlights from the field will be covered here. The role of the radical ligand is to engage in relatively strong direct exchange coupling with the lanthanide in a manner that allows the unpaired spin density on the ligand to act as an internal chemical equivalent of an external magnetic field, which addresses the problem of fast resonant QTM within the ground KD. The meaning of “strong” can be quantified by the exchange coupling constant, J, which reaches tens of wavenumbers for some radical-bridged L


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the equatorial plane, combined with the strength of the equatorial crystal field provided by the hard O-donor. The use of radical ligands with different, stable redox states, such as indigo, introduces possibilities for reversible switching of the magnetic properties. This idea was also illustrated with a series of lanthanide complexes of 2,3,5,6-tetra(2-pyridyl)pyrazine (tppz), which was shown by cyclic voltammetry to undergo a series of reversible redox events, and can be stabilized in dilanthanide complexes as the S = 1/2 forms [tppz]− and [tppz]3−.78 For the gadolinium compounds [(Cp*2Gd)2(μ-tppz)][BPh4] ([53][BPh4]) and [K(crypt222)][(Cp*2Gd)2(μ-tppz)] ([K(crypt-222)][54]), similar metal−ligand exchange coupling constants of J = −6.91 and −6.29 cm−1, respectively, were determined, indicating that the required exchange bias should occur in the anisotropic analogues [(Cp*2Ln)2(μ-tppz)][BPh4] (Ln = Tb, [55][BPh4]; Ln = Dy, [56][BPh4]) and [K(crypt-222)][(Cp*2Ln)2(μtppz)] (Ln = Tb, [K(crypt-222)][57]; Ln = Dy, [K(crypt222)][58]). In the case of 55 and 56, both compounds are SMMs, with zero-field anisotropy barriers of 5.1 cm−1 (τ0 = 6 × 10−6 s) and 35.9 cm−1 (τ0 = 2.1 × 10−7 s), respectively, with the relaxation mechanism in 55 possibly featuring different contributing factors and the relaxation in 56 occurring via a dominant Orbach process, which is consistent with suppression of the QTM. Compound 56 also shows open-hysteresis loops up to 3.25 K when using an average sweep rate of 30 Oe s−1. The lack of slow magnetic relaxation in compounds 57 and 58, which are bridged by the triply reduced S = 1/2 [tppz]3− ligand, is an interesting contrast to the SMMs with the singly bridged S = 1/2 [tppz]− ligand, a feature that was tentatively attributed to the different distribution of spin density across the ligand π* orbitals and the consequences for the symmetry of the coordination environment. The observation of toroidal magnetism in dysprosium triangles24,25 gives impetus to the synthesis and characterization of trimetallic molecular magnets with radical bridging ligand, such as hexaazatrinaphthylene (HAN).79−81 Radicalbridged trimetallic SMMs are much rarer than their dimetallic counterparts, with the most notable example being [{(Cp*)2Dy} 3(HAN)] (59), in which the trianion of hexaazatrinaphthylene adopts an S = 1/2 spin state (Figure 12).82 The gadolinium analogue [{(Cp*)2Gd}3(HAN)] (60) has an isotropic metal−ligand exchange coupling constant of J = −5.0 cm−1, implying exchange interactions at least as strong

Figure 11. Structure of [(C5Me4H)2Tb(thf)2(μ:η2-N2)]− (43) and the magnetic hysteresis loops for [K(2.2.2-crypt)(thf)][43] (sweep rate = 100 Oe s−1). Reproduced with permission from ref 82. Copyright 2017 Nature Publishing Group.75

to fast relaxation and poor hysteresis properties. A simple example of this effect is the THF-solvated analogue of 43, i.e., [K(2.2.2-crypt)(thf)][{(C 5 Me 4 H) 2 Tb(THF)} 2 (μ:η 2 -N 2 )] ([K(2.2.2-crypt)(thf)][46]), in which the additional ligand lowers the local site symmetry at terbium. Although the χMT(T) profiles for 43 and 46 are very similar, the hysteresis arising from 46 features loops only up to 15 K.75 A further illustration of the importance of symmetry is found in the indigo-bridged dilanthanide compounds [(Cp*2Ln)2(μind)] (Ln = Gd 47 or Dy 48), [K(thf)6][(Cp*2Ln)2(μ-ind)] (Ln = Gd 49 or Dy 50), and [{K(thf)3}2{Cp*2Ln}2(μ-ind)] (Ln = Gd 51 or Dy 52).77 Reduction of 47 and 48 with 1 equiv of KC8 gives 49 and 50, where the indigo ligand is present as the S = 1/2 trianionic form, whereas two-electron reduction gives the tetraanionic form and switches the spin of the indigo ligand back to a singlet state. Modeling the χMT(T) data for 49 yielded an S = 13/2 ground state and a relatively large exchange coupling constant of J = −11 cm−1 (−2J formalism). Given the similar temperature dependence of the susceptibility for 49 and 50, it is reasonable to assume similarly strong exchange coupling in the dysprosium version, yet despite this property the magnetic hysteresis loops are very narrow even at 1.8 K, which represents no improvement on the hysteresis measured for 48 and 52. Furthermore, the anisotropy barriers determined for 48 and 50 are very similar and, at 39 and 35 cm−1, very small. Evidently, the fact that no improvement in the SMM properties was observed is a reflection of the mixed O- and N-donor ligand environment in

Figure 12. Molecular structure of 59 and the corresponding magnetic hysteresis profile (sweep rate of 40 Oe s−1). Reproduced with permission from ref 82. Copyright 2017 Wiley-VCH.82 M


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as this in anisotropic 59. Maxima in the χ″M(ν) data were observed up to 8 K for 59, and fitting of the relaxation times to a model featuring QTM and Orbach relaxation produced Ueff = 51 cm−1 with τ0 = 1.2 × 10−8 s. Using a sweep rate of 40 Oe s−1, it was possible to observe hysteresis with coercivity up to 3.5 K. Although the slow magnetic relaxation properties of SMMs are influenced most strongly by the composition and symmetry of the primary coordination sphere, secondary coordination sphere effects can also play an important part. This concept is illustrated by the dilanthanide ion-contact compounds [{1,1′Fc(NSitBuMe2)2}Ln{K(toluene)}]2(μ:η6:η6-biphenyl)] (Fc = Fe(C5H4)2, Ln = Gd, Dy, Er = 61Ln) and their ion-separated analogues [K(18-crown-6)(THF)1.5][{1,1′-Fc(NSitBuMe2)2}Ln}]2(μ-biphenyl)] ([K(18-crown-6)(THF)1.5][62Dy]) (Figure 13), in which each lanthanide is ligated by two amido nitrogen atoms and a bridging tetra-reduced η6-biphenyl anion.83

Diamidoferrocenyl ligands such as those in 61Ln and 62Ln provide opportunities for developing “switchable” magnetic properties due to the facile nature of the Fe(II)/Fe(III) redox couple. This possibility was explored with the diferrous compounds [K(thf)5][{1,1′-Fc(NSitBuMe2)2}2Ln] (63Ln with Ln = Dy, Er), which can be oxidized by iodine to give the ferrous/ferric hybrids [{1,1′-Fc(NSitBuMe2)2}2Ln] (64Ln with Ln = Dy, Er).84 So-called “on−off” behavior was observed for the pair of tetrahedral dysprosium compounds, whereby 63Dy is an SMM in zero applied DC field with Ueff = 20.9 cm−1; however, 64Dy does not show any response in the frequency dependence of the χM ″ . The magnetic response can be turned on again in an applied DC field of 1000 Oe, leading to a barrier of 16.8 cm−1. Similar on−off behavior was observed for 63Er and 64Er in an applied DC field of 500 Oe. If an axial coordination geometry consisting of two cyclopentadienyl ligands is the ideal environment for observing SMM properties with the oblate ion Dy3+, an equatorial coordination environment should produce a similar effect in a complex based on the prolate 4f11 ion Er3+.53 An archetypal example of such an erbium SMM is the three-coordinate compound [Er{N(SiMe3)2}3] (65) (Figure 14), which adopts

Figure 14. Molecular structure of 65 and the frequency dependence of the out-of-phase susceptibility in zero DC field. Reproduced with permission from ref 85. Copyright 2014 American Chemical Society.85

Figure 13. Molecular structures of 61Dy (a) and 62Dy (b) and the ″ (ν) plots in zero DC field for 61Dy (c) and in an corresponding χM applied DC field of 900 Oe for 62Dy (d). Reproduced with permission from ref 83. Copyright 2015 American Chemical Society.83

a slightly pyramidalized, C3v geometry due to site disorder of the erbium ion.85 The out-of-phase susceptibility measurements on 65 show maxima at 4−15 K in zero applied field, leading to an anisotropic barrier of 85 cm−1 and waistrestricted hysteresis loops at 1.9 K. An analysis of the crystal field levels of Er3+ in 65 showed that the ground KD has MJ = ±15/2, with the seven higher-energy KDs occurring with decreasing MJ value, as expected for a prolate ion in an equatorial coordination environment. The related compound [Li(THF)4][ErCl{N(SiMe3)2}3] ([Li(THF)4][66]) consists of a C3v-symmetric erbium environment that also gives rise to the χ″M(ν) data peaks in zero DC field, albeit with a smaller anisotropy barrier of 46 cm−1 (τ0 = 1.07 × 10−7 s) and with Raman processes also playing a role in the relaxation.86 In light of the properties of SMMs such as 65 and 66, a coordination environment with two trans-axial ligands may not be the most obvious general choice for observing slow magnetic relaxation in compounds of Er3+. However, the out-

Antiferromagnetic exchange interactions occur between the gadolinium ions, with fits of the χMT(T) using a −2J spin Hamiltonian formalism being achieved using the parameters J = −0.642(6) cm−1, g = 2.01(7) for 61Gd and J = −0.664(6) cm−1, g = 2.03(7) for 62Gd. In contrast, whereas the exchange coupling in 61Dy is also antiferromagnetic, in 62Dy a ferromagnetic interaction was observed. This behavior was revealed by ab initio calculations to originate from the balance between the dipolar exchangewhich is similar in both compoundsand the superexchange via the bridging biphenyl ligand, which is much stronger in 62Dy. The weaker, antiferromagnetic exchange in 61Dy results in the SMM properties being dominated by single-ion effects, leading to a barrier of Ueff = 37 cm−1, with τ0 = 1.5 × 10−7 s in zero applied DC field. The stronger, ferromagnetic exchange in 62Dy switches the system from Kramers to non-Kramers character, hence the slow relaxation in zero field is dominated by QTM. N


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show χ″M(ν) maxima in the range 15−27 K, leading to an anisotropy barrier of 147 cm−1 with τ0 = 8.3 × 10−8 s, with the relaxation being dominated by an Orbach process. The AC susceptibility properties of ion-separated 74, in which the [Er(COT)2]− sandwich has approximate D8h symmetry, are very similar to those of its ion-contact analogue, suggesting that strict point symmetry does not play a dominant role in the dynamic magnetic properties. AC susceptibility measurements on magnetically dilute analogues of [K(18-crown-6)(THF)2][74] also produced similar outcomes; however, notable differences were observed in the magnetic hysteresis properties of the nondilute and dilute versions, with the nondilute [Er(COT)2]− anion displaying a magnetic avalanche effect, i.e., a substantial drop in the magnetization around zero field (Figure 15).90 The avalanche effect gradually diminished upon

of-phase magnetic susceptibility measurements on the heteroleptic bent sandwich complex [(η5-Cp*)Er(η8-COT)] (67) in zero DC field revealed the occurrence of two distinct thermally activated relaxation processes in the temperature range 10−20 K, the nature of which does not change upon magnetic dilution, indicating their single-ion origin.87 The anisotropy barriers determined for each thermally activated process are 137 and 224 cm−1, with τ0 = 3.13 × 10−9 s and 8.17 × 10−11 s, respectively. The reason for the two distinct Orbach processes in 67 was traced to the presence of two structural conformers in the crystalline form, which occur due to the COT ring being disordered over two sites. Magnetic blocking in 67 is sufficiently prominent to allow waist-restricted hysteresis loops to be observed up to 5 K when using a sweep rate of 9 Oe s−1. The 5% magnetically dilute version of 67 shows remanence of 0.87 Nβ, and at 1.8 K a coercive field of 1 kOe was measured; however, the lack of true axial symmetry is thought to be responsible for the substantial drop in magnetization at zero field. The isostructural compounds [Cp*Dy(COT)] (68) and [Cp*Ho(COT)] (69) also show slow relaxation in zero DC field, with the peaks in out-of-phase susceptibility being broad and only observed up to 10 and 5 K, respectively, indicating that QTM is very strong.88 Magnetic dilution and/or the application of a DC field effectively suppresses the QTM in 68 and 69, which allows one thermal relaxation process to be observed for 68 with an anisotropy barrier of 17.7 cm−1 and two thermal relaxation processes to be observed for 69, with very small anisotropy barriers of 5.1 and 2.5 cm−1. The thulium analogue [Cp*Tm(COT)] (70) does not show any slow relaxation of the magnetization even in an applied field because whereas the ground KD for this compound takes the maximum value of MJ, i.e., ± 6, the first-excited KD has MJ = 0. In contrast, the prolate electron density of Tm3+ in the related compounds [(Tp)Tm(COT)] (71) and [(Tp*)Tm(COT)] (72) (Tp = hydrotris(1pyrazolyl)borate, Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate) does give rise to field-induced slow relaxation, with 71 showing an anisotropic barrier of 111 cm−1 at 2 kOe and 72 showing an anisotropic barrier of 32 cm−1.89 Magnetic dilution allows zero-field slow relaxation to be observed for both 71 and 72. The improved properties of 71 relative to those of 72 were interpreted in terms of the Tm−N bonds, which are 0.089 Å longer in the bulkier Tp* complex; hence, the thulium center experiences a weaker crystal field. In addition, the greater distortion away from ideal C3v symmetry in 72 induces a greater degree of mixing of the MJ states, leading to more facile QTM. Further studies on COT-ligated SMMs led to the suggestion that the large effective radius of the ligand results in the creation of an equatorial and not an axial crystal field, despite the trans-axial disposition of two ηn-ligands. Therefore, the peculiar electronic structure of [COT]2− is particularly well matched to the requirements of erbium, and zero-field slow relaxation and magnetic hysteresis are much more prominent in the sandwich complexes [(η8-COT)Er(μ:η8:η8-COT)K(18crown-6)] (73) and [K(18-crown-6)(THF)2][Er(η8-COT)2] ([K(18-crown-6)(THF)2][74]) than in the heteroleptic compound 67.90−92 The molecular structure of 73 consists of an [Er(COT)2]− sandwich with one disordered COT ligand, such that staggered and eclipsed conformations both exist in the solid state. The COT−Er−COT unit is almost linear, with the mean planes of the two COT ligands forming a dihedral angle of 2.8°. AC susceptibility measurements on 73

Figure 15. Molecular structure of 73 and magnetic hysteresis loops (sweep rate 7.8 Oe s−1) for 69 at a dilution level of 1:85 in a diamagnetic matrix. Reproduced with permission from ref 90. Copyright 2013 American Chemical Society.90

increasing the dilution level in a matrix of the isostructural yttrium compound from 1:20 to 1:85, whereupon open hysteresis loops could be observed up to 10 K. The observations on [Er(COT)2]− point toward a significant role for intermolecular dipolar effects on the relaxation. A theoretical study of [Er(COT)2]− showed that the transverse components of the magnetic moment in the ground and first-excited KDs were negligible, such that relaxation is likely to proceed via the second-excited KD.91 In contrast, poor SMM properties were identified for the dysprosium analogue [K(18-crown-6)(THF)2][Dy(η8-COT)2], [K(18-crown-6)(THF)2][75], which is consistent with the dominant equatorial nature of the crystal field and its interaction with the oblate electron density of Dy3+.93 A simple chemical explanation for the slow relaxation and magnetization properties of [Er(COT)2]− stems from the ability of the [COT]2− ligand to provide an equatorial crystal field and stabilize the MJ = ±15/2 state of the prolate ion Er3+. The near-linear geometry of [Er(COT)2]− and its impact on the magnetic relaxation provided inspiration for synthesizing the multi-decker analogues [Er2(COT″)3] (76) (COT″ = 1,4bis(trimethylsilyl)cyclooctatetraenyl) and [K2(THF)4Er2(COT)4] (77).94,95 The structure of 76 features a near-linear Er−COT−Er angle of 175.65° and an erbium− erbium distance of 4.11 Å. The AC susceptibility measurements on 76 revealed maxima in the out-of-phase data from 14 to 26 K, which resulted in an anisotropy barrier of 231 cm−1 with τ0 = 5.7 × 10−10 s. The structure of 77 can be regarded as consisting of two [Er(COT)2]− sandwiches bridged by a potassium cation, which also results in a near-linear arrangeO


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QTM dominating at lower temperatures. Three relaxation processes were identified in 83, with barriers in the range 6.2− 16.7 cm−1. A computational study of 83 and 84 revealed that the main magnetic axes in the ground Kramers doublets deviate significantly from the molecular symmetry axis, which was attributed to the effect of the silyl substituents: calculations on the model system [Dy2(η8-COT)3] show that the magnetic axes are reoriented toward the ligand centroids, and an extension of the calculations to [Dy(η8-COT)2]− revealed a ground KD with dominant MJ = 1/2 character, providing further evidence of the easy-plane nature of the magnetization, i.e., that the COT ligand provides an equatorial crystal field. Thulium-containing SMMs are rare, which is partly a consequence of the non-Kramers nature of the 4f12 ion Tm3+. However, the versatility of the COT ligand and the importance of axial symmetry were illustrated by the fieldinduced slow relaxation in the thulium(III) compounds [Tm(η8-COT)I(THF)2] (85) and [K(18-crown-6][Tm(η8COT)2] ([K(18-crown-6][86]).100 In the case of 85, an anisotropy barrier of Ueff = 5.5 cm−1 with τ0 = 1.18 × 10−4 s was determined, with a role for Raman processes also identified and a relatively fast QTM rate of τQTM = 2.68 ms. The more symmetrical complex 86 showed two relaxation processes in the χM ″ (ν) data, one of which yielded a larger barrier of Ueff = 37 cm−1 with τ0 = 1.28 × 10−4 s, and the QTM rate in 86 is also, at 0.371 s, much slower than in 85. These results suggest that the non-Kramers nature of Tm3+ (akin to Tb3+) will require a strict high-symmetry environment for SMM properties to be observed. Regardless of the ligands, neodymium SMMs are also extremely rare, hence the observation of slow magnetic relaxation in [Li(DME)3][Nd(η8-COT″)2] ([Li(DME)3][87]) is significant. An anisotropy barrier of 14.6 cm−1 with τ0 = 5.5 × 10−5 s was determined for 87 in an applied field of 1 kOe, although no magnetic hysteresis was observed. The majority of lanthanide SMMs with low-coordinate carbon donor atoms contains π-bonded ligands, as typified by [Cp]− and [COT]2−. However, σ-bonded carbon-based ligands are potentially more charge dense and, therefore, able to provide a stronger crystal field, which provides an alternative strategy for influencing the dynamic magnetic properties. For instance, in the tricapped trigonal prismatic compounds [Ln{(MeIm)2BH2}3] (Ln = Tb 88, Dy 89; {(MeIm)2BH2} = dihydrobis(methylimidazolyl)borate), the carbene donors are thought to promote stronger anisotropy and slower relaxation than the nitrogen donors in the isostructural compounds with pyrazolyl N-donors, which leads to Orbach processes dominating the relaxation, as opposed to Raman processes.101 In an applied field of 1.5 kOe, the anisotropy barriers for 88 and 89 were determined to be 44.8 and 32.8 cm−1. A separate theoretical study of 88 and 89 reported, among other things, the Mulliken charges of carbon donor atoms, which revealed slight negative charges, as opposed to slight positive charges on the nitrogen donor atoms of the analogous pyrazolylborate complexes, which is consistent with the proposal that the NHC ligands provide a stronger crystal field.102 In the ion-separated compound [K(18-crown-6)(THF)2][Dy{C(PPh2)2(NSiMe3)2}2] ([K(18-crown-6)(THF)2][90]), the dysprosium center is six-coordinate and features a nearlinear (176.6°) DyC2 unit formulated as consisting of DyC double bonds, with lengths of 2.434(6) and 2.433(6) Å, and four additional imino N-donor groups. Peaks in the out-of-

ment with Er−K−Er and K−Er−K angles of 169.57° and 179.35°, respectively, and an erbium−erbium distance of 8.82 Å. Peaks in the out-of-phase susceptibility were observed in the temperature range 13−32 K, leading to an anisotropy barrier of 219 cm−1 with τ0 = 5.0 × 10−9 s. Both 76 and 77 show magnetic hysteresis in the solid state up to 12 K, and in frozen solution 76 shows hysteresis up to 14 K. Combined with the anisotropy barriers, the hysteresis in these multidecker compounds represents an improvement on the performance of 73 and 74: in the case of 76, the reason is thought to originate from strong antiferromagnetic exchange interactions between the metals, as mediated by the COT πp orbitals; in 77, the much greater distance between the metals diminishes the effects of magnetic exchange, and the improved properties are due to single-ion effects and a beneficial change to the crystal field arising from the geometry of the compound. The {Er(COT)}+ building block has been shown to demonstrate robust magnetic anisotropy in a series of compounds where the remaining coordination sphere of the metal is varied across a range of different ligands.96−98 Thus, [Er(η8-COT)I(THF)2] (78), [Er(η8-COT)I(MeCN)2] (79), [Er(η8-COT)I(py)2] (80), and [Er(η8-COT)Tp*] (81) all show slow magnetic relaxation in zero applied field, with remarkably similar features. The compounds used to derive this trendreferred to as metal−ligand pair anisotropy (MLPA)show peaks in χ″M(ν) due to high-temperature Orbach processes and low-temperature QTM, leading to anisotropy barriers of Ueff = 95.6, 102.9, 107.1, and 133.6 cm−1, with τ0 varying very little in the range 6.3−9.6 × 10−10 s and τQTM being on the order of 1.45−2.27 ms. A computational study revealed the main magnetic axis in the ground KD to be essentially perpendicular to the plane of the COT rings in 78− 81, a result consistent with previous observations on COTligated erbium SMMs. The ensuing equatorial crystal field results in the ground KDs in 78−81 having dominant (ca. 80%) MJ = ±15/2 character and a strongly admixed firstexcited KD mostly with MJ = ±13/2 character (ca. 50%). The corresponding energy gaps between ground and first-excited KDs match well with the experimental Ueff values, giving clear insight into the relaxation mechanisms, which feature contributions from Orbach and QTM processes. Despite the apparent mismatch between the electron density of Dy3+ and the crystal field provided by the [COT]2− ligand, the slow relaxation properties of [Dy(η8-COT″)2Li(THF)(DME)] (82, COT″ = 1,4-bis(trimethylsilyl)cyclooctatetraene, DME = 1,2-dimethoxyethane) are remarkably rich, with multiple processes being observed in zero DC field and a range of applied DC fields.99 The geometry of 82 is bent at dysprosium, with a COT−Dy−COT angle of 168.16°. In zero DC field, a thermally activated process with Ueff = 12.5 cm−1 and τ0 = 6 × 10−6 s was observed at 3.75−8 K, and at lower temperatures QTM occurs. Using DC fields of 100, 200, and 600 Oe resolves additional thermally activated processes with anisotropy barriers up to 30 cm−1, with the QTM being suppressed in the optimum field of 600 Oe. A comparative study of [Dy2(η8-COT″)3] (83) and its ion-separated relative [Li(thf)4][Dy(η8-COT″)2] ([Li(thf)4][84]) revealed the triple decker complex to show slow relaxation in an applied DC field of 600 Oe, whereas the double decker shows slow relaxation in zero DC field but requires an external field for the peaks in the out-of-phase susceptibility to be resolved. A small thermal barrier of 17 cm−1 was determined for 84, with τ0 being identical to that found in 82, for relaxation above 4.5 K, and P


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remnant magnetization at 2 K, with the hysteresis loops remaining open at 5 K. 2.2.2. Actinide SMMs with C- and N-Donor Ligands. The actinide SMM field is dominated by uranium particularly uranium(III)but SMMs with uranium in other oxidation states and a small number with other actinides are known.107 The attraction of actinides in the design of SMMs is the considerably stronger spin−orbit coupling experienced by the 5f electrons, which can in principle lead to greater magnetic anisotropy. However, the fundamental differences in the coordination chemistry of early actinides relative to that of the lanthanides typically manifest themselves in SMMs through the occurrence of very small anisotropy barriers and little, if any, magnetic hysteresis. The main problem is the greater degree of covalent character in the actinide−ligand bonds, which leads to extensive mixing of the various wave functions that define the low-lying energy spectrum arising from the freeion term. Therefore, aside from any issues relating to isotope decay, it seems unlikely that solutions to the world’s magnetic information storage conundrums will be provided by actinide SMMs. However, the merit in investigating the dynamic magnetic properties of actinide compounds derives from the fundamental knowledge of electronic structure that can be derived from such measurements. Furthermore, the development of insightful synthetic methods at the interface of actinide and main group chemistry provides a strategy that can ultimately be used to influence electronic structure in welldefined ways. One of the most striking actinide SMMs is neptunocene, [Np(η8-C8H8)2] (93), which contains a 5f3 Np(IV) ion in a D8h-symmetric environment. In fields up to 140 kOe, 93 shows narrow hysteresis loops at 1.8 K but without saturation, which indicates strong magnetic anisotropy (Figure 17).108

phase susceptibility versus frequency for 90 were observed at 22−41 K, with two thermal barriers being apparent (Figure 16).103 For each set of peaks, the higher-temperature relaxation

Figure 16. Molecular structure of 90 and the frequency dependence of the in-phase and out-of-phase magnetic susceptibilities of ([K(18crown-6)(THF)2][90]). Reproduced with permission from ref 103. Copyright 2016 Royal Society of Chemistry.103

is dominated by an Orbach process with barriers of 501 and 565 cm−1, respectively, and Raman processes occurring at lower temperatures. Hysteresis was observed up 10 K with a sweep rate of 35 Oe s−1. A theoretical study of 90 revealed that the main magnetic axes in the ground, first-excited, and second-excited KDs, which show essentially pure MJ = ±15/2, ±13/2, and ±11/2 character, respectively, are aligned with the DyC2 axis, whereas the magnetic axes in the third- and fourth-excited KDs are perpendicular to the DyC2 axis and strongly mixed, therefore representing crossing points for the two relaxation processes. The dysprosium and terbium metallocenophanes [Li(THF)4][LnFc3{Li(THF)}3] (Ln = Tb, Dy, 91Tb, and 91Dy) were synthesized by adding the lithium ferrocenyl compound [Li6Fc2(TMEDA)2] (TMEDA = tetramethylethylenediamine) to DyCl3.104,105 Compound 91Dy was then be converted into the trimetallic metallocenophane [Li2X][Dy3Fc6{Li(THF)}3] (92) by reacting 91Dy with DyX3 (X = Cl or I).106 The coordination environments of the lanthanides in 91 and 92 are trigonal prismatic and consist of strongly σ-donating ferrocenyl carbon atoms, with the Dy3 arrangement in 92 being strictly linear. The SMM properties of 91Dy were established with AC magnetic susceptibility and magnetization measurements, with the former revealing Orbach relaxation over a barrier of 110 cm−1 at higher temperatures, followed by a transition to a QTM regime at lower temperatures. The magnetization vs field hysteresis loops for 91Dy are waist-restricted up to 4.5 K when using a scan rate of 90 Oe s−1 but closed at higher temperatures. The magnetically dilute analogue of 91Dy showed a similar barrier of 108 cm−1, although the importance of intermolecular dipolar exchange in 91Dy was highlighted due to the much diminished contribution from QTM upon dilution. A larger barrier of 274 cm−1 was determined for 91Tb although hysteresis was only observed up to 3 K. Strong ferromagnetic coupling between the dysprosium centers in 92 led to large anisotropy barriers of 261 and 268 cm−1, reflecting the two distinct environments. The ferromagnetism in 92 is also thought to be responsible for the more pronounced

Figure 17. Molecular structure of neptunocene (93) and the magnetic hysteresis loops at 1.8 K. Reproduced with permission from ref 108. Copyright 2011 Wiley-VCH.108

It was also possible to observe peaks in the temperature dependence of the out-of-phase susceptibility when using an applied field of 5 kOe, with the peak maximum shifting to higher temperatures with increasing AC frequency. The resulting anisotropy barrier of 28 cm−1 (with τ0 = 1.1 × 10−5 s) is typical of an actinide SMM and reflects the occurrence of an Orbach process at higher temperatures, although several other relaxation events are likely to contribute to the overall picture, including hyperfine interactions with the I = 5/2 nucleus of 235Np. The substituted uranocenes [Li(DME)3][U(η8-COT″)2] ([Li(DME)3][94]) and [U(η8-COT″)2] (95) are isostructural to 93; however, the uranium−centroid Q


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reactivity for which there is no parallel with the 2p elements, have been underexploited in single-molecule magnetism. For example, the limited number of phosphine-ligated 3d-SMMs with the general formula [M{N(SiMe3)2}2(L)] (M = Fe, L = PCy3, 98; M = Co, L = THF, 99; M = Co, L = PCy, 100) show small, in-field anisotropy barriers of less than 100 cm−1.113,114 Aside from synthetic considerations, much of the motivation for using heavier p-block elements in molecular magnetism stems from the different physicochemical possibilities that become available, which allow magneto-structural correlations to be developed in new ways. For example, heavier p-block donors, with their more diffuse valence orbitals, introduce the potential for enhancing the covalent character of a metal−ligand bond in ways that could lead to stronger magnetic exchange coupling and, therefore, produce larger Ueff values in compounds with suitable anisotropy. The heaviest pblock elements also have extremely strong spin−orbit coupling which, under the correct conditions, could be used to increase the magnetic anisotropy of transition metal and lanthanide ions. This area has seen some notable developments, yet it is undoubtedly far from fully exploited. The highlight results in the following sections provide a taste of how modern main group chemistry could play a transformative role in molecular magnetism, particularly in the study of SMMs. 2.3.1. 3d Molecular Magnets with Heavy p-Block Donor Ligands. Detailed studies of magnetic exchange interactions involving transition metals and heavier p-block elements are remarkably uncommon, even in simple systems containing few metal ions. Since the exchange coupling in polymetallic cage compounds can impact the energy separation between the low-lying MS states, using heavier p-block elements potentially provides an alternative strategy for developing SMMs with larger anisotropy barriers. An illustration of the possibilities is provided by the structurally simple chromium(II) dimers [(η5-Cp)Cr{μ-E(SiMe3)2}]2 with E = P (101) or As (102), which were synthesized by reacting Li{E(SiMe3)2} with Cp2Cr.115 Both 101 and 102 contain chromium centers with S = 2 ground states, and evidence for the occurrence of very strong antiferromagnetic exchange was provided by the temperature dependence of χMT. At 300 K, χMT values of 0.60 cm3 K mol−1 and 1.24 cm3 K mol−1 were determined for 101 and 102, respectively, both of which are considerably lower than the value of 6.0 cm3 K mol−1 expected for two noninteracting Cr(II) ions. On lowering the temperature to 60 K, χMT for both compounds decreases rapidly, and at 2 K the susceptibility is almost zero. Modeling the susceptibility data with g = 2.0 led to the exchange coupling constants being determined as J = −166 cm−1 for 101 and −77.5 cm−1 for 102 (−2J formalism). In addition to the magnitude of the couplings, the strong variation of J with the pnictogen is also noteworthy. The same trend was also noticed in the isostructural compounds [(η5-Cp)Mn{μ-E(SiMe3)2}]2 with E = P (103) or As (104), but with smaller J-values of −13.5 cm−1 and −1.5 cm−1.116 The temperature dependence of χMT for 104 is particularly unusual and is thought to be indicative of a two-step SCO process involving the high-spin S = 5/2 and intermediate-spin S = 3/2 states of manganese(II) (Figure 19). Although small J-values are characteristic of manganese(II),117 this parameter can be much larger with appropriate heavy p-block ligands, as illustrated by the unusual cage compound [Mn5{N(SiMe3)2}{μ4-PSiiPr3}2{μ-P(H)SiiPr3}5] (105), which contains phosphide and phosphinidene (PR)

distance in the U(III) compound 94 is 0.07 Å longer than the analogous distance in 95, and the tilt angles of the COT ligands vary by 2.5°. 109 Since χ MT for uranium(IV) compounds often approaches zero at low temperatures due to the occupation of a singlet ground state,110 95 does not show slow magnetic relaxation. In contrast, field-induced (1 kOe) slow magnetic relaxation was observed for 94, with ″ (ν) observed at 1.8−6.5 K. The anisotropy maxima in χM barrier of 18 cm−1 determined from these data is again within the range normally found for uranium SMMs. The uranium(III) NHC compound [U{(MeIm)2BH2}3] (96) is isostructural with 88 and 89 and features uranium in a tricapped trigonal prismatic environment (Figure 18).101 At low temperatures, the lowest-lying KD arising from the 4I9/2 ground term of U(III) is populated, and field-induced slow relaxation was observed using a 750 Oe DC field.

Figure 18. Molecular structure of [U{(MeIm)2BH2}3] (96). Reproduced with permission from ref 101.101

The magnetic relaxation in 90 is characterized by a dominant Orbach process with Ueff ≈ 32 cm−1 but with some contribution from Raman processes and, at low temperatures, from direct processes. An EPR spectroscopic study revealed the presence of significant transverse anisotropy with g⊥ = 2.57 and g∥ = 1.03, which provides a rationale for the absence of peaks in the out-of-phase susceptibility in zero DC field. The stronger donor ability of NHC ligands in uranium chemistry relative to, for example, pyrazolylborate ligands provides one means of strengthening the crystal field and, therefore, of increasing Ueff. This design principle has also been illustrated through a theoretical study of the plutonium NHC complex [Pu{(HIm)3BH}3] (97), for which there is not yet an equivalent experimental system.111,112 2.3. Molecular Magnets with Heavier p-Block Donor Ligands

While a distinction between 2p elements and heavier p-block elements as donor atoms in SMMs can be made for the purpose of organizing this review, the temptation to install boundaries between the different rows of the Periodic Table in a practical sense should be resisted. Indeed, progress with one row of elements frequently informs developments with others, which is particularly true when considering synthetic aspects, which play a critical role in advancing molecular magnetism. However, synthetic considerations can also be very different and frequently more challengingwith the heavier p-block elements, especially when the chemistry necessitates the use of multiply bonded compounds to synthesize the ligands. This partly explains why 2p-donor ligands are ubiquitous in molecular magnetism, whereas heavier p-block elements are uncommon. Phosphorus-donor ligands, which often show R


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high-spin Mn(II) g-value of 2.00. The appreciable spin−orbit coupling in 106 is thought to occur simply by dint of the proximity of bismuth to manganese (3.2163(5) Å); i.e., an effect is observed despite the absence of a direct Mn−Bi bond: this points toward systems with such direct bonds as being important synthetic targets. To probe the idea that the covalent component of a metal− ligand bond is related to the extent of magnetic anisotropy transfer to the transition metal from the heavy p-block element, the series of iron(II) compounds [GeFeBr(L)] (107), [GeFeI(L)] (108), [SnFeBr(L)] (109), and [SnFeI(L)] (110) was synthesized.120 In 107−110, the iron(II) centers occupy similar, distorted trigonal bipyramidal EFeXN3 environments (E = Ge or Sn, X = Br or I) with Fe−Ge distances of 2.3764(3) and 2.35584(5) Å in 107 and 108, respectively, and Fe−Sn distances of 2.4828(5) and 2.47930(7) Å in 109 and 110, respectively, each of which is indicative of a direct Fe−E bond. The χMT values for 107−110 at 300 K reflect an S = 1 spin ground state for iron(II), and the more pronounced decrease in the susceptibility for 108 and 110 at lower temperatures was taken as an indication of greater anisotropy in the iodo-ligated compounds. The axial ZFS parameters D were determined to be −11.8(3), −15.5(4), −12.1(4), and −17.9(3) cm−1 for 107−110, respectively, broadly in agreement with the expected heavy-atom effect. Notably, however, a greater increase in the magnitude of D occurs on swapping bromide for iodide rather than germanium for tin. The variation in D was rationalized through a combination of Mössbauer and electronic spectroscopies and a theoretical study by considering the role of the axial halide and tetrel ligands and how they contribute to the ground and first-excited electronic states. Specifically, a dominant role for the π-donating ability of the halides was identified rather than the π-accepting ability of the tetrel elements, with the halides contributing significantly to the SOMOs (singly occupied molecular orbitals). Therefore, since the halides in 107−110 carry more spin density than the tetrel elements, they have a greater influence on the magnetic anisotropy. While the D-values determined for 107−110 represent the general trend of D increasing with the relative atomic mass of the halide, the reverse trend is also possible in circumstances where the metal contribution to the total spin−orbit coupling outweighs the ligand contribution.121 This phenomenon although relatively uncommonwas demonstrated in the intermediate spin, S = 1, cobalt(III) compounds [CoL(X)] with L = 2,2′-(2,2′-bipyridine-6,6′-diyl)bis(1,1-diphenylethanediolate) and X = Cl (111), Br (112), or I (113). The extent to which the square-pyramidal geometries in 111−113 are distorted, combined with the metal oxidation state and the electronic properties of the donor atoms, all contribute to the periodic variation in the large positive axial ZFS parameters, which were determined experimentally to be D = 35, 26, and 18 cm−1, respectively, and reproduced accurately by various computational methods. The impact of increasing the softness of the p-block donor atom, and its consequences for magnetic anisotropy applied in the design of SMMs, was illustrated by the series of tetrahedral cobalt(II) compounds [PPh4]2[Co(EPh)4] where E = O (114), S (115), or Se (116).122,123 The cobalt center in 115 adopts a geometry that is distorted from ideal tetrahedral toward D2d symmetry, one consequence of which is to place the fully occupied 3dx2−y2 orbital close in energy to singly

Figure 19. Molecular structure of 104 and the temperature dependence of the magnetic susceptibility in an applied field of 1 kOe. The red lines are the theoretical fit considering only the isotropic exchange between two high-spin Mn(II) centers. Reproduced with permission from ref 116. Copyright 2012 Royal Society of Chemistry.116

ligands.118 Studying the exchange in 105 using quantum chemical methods led to coupling constants of up to −220 cm−1 being predicted. To investigate the impact of heavy element spin−orbit coupling on the anisotropy of a transition metal, the heterobimetallic manganese(II)−bismuth(III) compound [LMnBi][SO3CF3]2 (106, LH3 = 1,1,1-tris[(3methoxysalicylideneamino)methyl]ethane) was synthesized, and its magnetic and EPR properties were studied.119 Compound 106 consists of a trigonal prismatic MnN3O3 environment connected to bismuth via three μ-phenolate ligands, resulting in a relatively short Mn···Bi interaction distance of 3.2163(5) Å (Figure 20). Bismuth, being the

Figure 20. Left: molecular structures of 106 (left) (Bi = purple, Mn = pink, S = yellow, F = green, O = red, N = blue, C = gray, H are omitted for clarity). Reproduced with permission from ref 119. Copyright 2016 Royal Society of Chemistry.119 Right 110 (I = purple, Sn = cyan, Fe = orange, P = plum, N = blue, C = gray, H are omitted for clarity). Reproduced with permission from ref 120. Copyright 2017 American Chemical Society.120

heaviest nonradioactive element, is the ideal candidate for observing the impact of spin−orbit coupling transfer on magnetic anisotropy. Typical values of the ZFS parameter D are very small for manganese(II); however, comparing the Dvalue of bismuth-free analogue of 106, which is 0.168 cm−1, with those of 0.70(2) and 0.74(2) cm−1 (determined by magnetometry and EPR spectroscopy, respectively), a clear enhancement in spin−orbit coupling occurs. This observation was corroborated by the g-value of 1.965(5) determined for 106, which is a small but significant deviation from the typical S


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Analysis of the solid-state electronic absorption spectra of 114−116 produced Racah B parameters of 763.3, 590, and 408 cm−1, respectively, which deviate substantially from the cobalt(II) free-ion value of 956 cm−1 and can be taken as indicators of greater covalency in the Co−E bonds with increasing softness. The AC susceptibility measurements on 114 revealed distinct behavior, with the slow relaxation in an applied field of 1.4 kOe thought to involve an Orbach process with an anisotropy barrier of 21 cm−1 and τ0 = 7 × 10−10 s. Both 114b and 115 show slow relaxation in zero DC field, although in the case of 114b an anistropy barrier of 34.0 cm−1 could only be determined using magnetic dilution, which implies that dipolar fields arising from intermolecular Co···Co interactions impact the dynamic magnetism. In contrast, undiluted 116 yielded an anisotropy barrier of 19 cm−1 with τ0 = 3 × 10−6 s. The limited variation in Ueff in light of the trend in D-values determined for 114−116 indicates that factors beyond a consideration of geometry and the apparent relaxation processes may play a role in determining the anisotropy and dynamic magnetic properties of these compounds. A comprehensive ab initio theoretical analysis of the [Co(EPh)4]2− anions in 114−116 considered the ligands as falling into two classes, one in which the oxygen atom in the PhO− ligands is considered as being sp2 hybridized and therefore an isotropic π-donor and another in which the PhE− ligands (E = S, Se) are not hybridized and are therefore anisotropic π-donors.124 In the case of the heavier S- and Sedonors, metal−ligand σ-interactions and an out-of-plane πinteraction are dominant, and the increased importance of πbonding on descending Group 16 combined with the molecular geometry explain the observed trend in the D-values. The polyimido−sulfonate-ligated cobalt(II) complex [Co{(NtBu)3SMe}2] (117) is structurally related to 114−116; however, the geometric constraints enforced by the main group poly(SN) ligand produce a highly distorted tetrahedral coordination environment, with one N−Co−N angle being as acute as 71.46(4)°.125 Compound 117 provides a rare example of a mononuclear cobalt SMM that shows slow magnetic relaxation in zero applied field, with maxima in the out-of-phase susceptibility observed at temperatures in the range 1.8−11.8 K, and the occurrence of two relaxation processes. Using an applied field of 1.5 kOe allowed an anisotropy barrier of 75 cm−1 with τ0 = 2.64 × 10−8 s to be determined, and magnetic hysteresis was also observed up to 2.6 K with a sweep rate of 13 Oe s−1. 2.3.2. Lanthanide Molecular Magnets with Heavy pBlock Donor Ligands. Lanthanide SMMs in which the metal is ligated either in an η-fashion or through formal σ-bonds by a heavy p-block element form a relatively small but rapidly growing family of compounds. Looking beyond magnetism, novel reactivity and beautiful structural properties have been developed in recent years, including the activation of elemental pnictogens and chalcogens and related inorganic substrates by low-valent lanthanides.126,127 The extension of this chemistry to lanthanides of relevance to single-molecule magnetism is an enticing prospect. In light of the progress made with carbon-based sandwich compounds, the introduction of isoelectronic and isolobal phosphorus atoms into the ligands to give, for example, phospholyl-ligated SMMs should become an important line of inquiry. The use of phospha-aromatic ligands introduces factors that could impact properties such as the anisotropy barrier and the blocking temperature. For instance, the longer,

occupied 3dxy, which creates a low-lying excited electronic state that generates large ZFS via second-order spin−orbit coupling. The value of χMT at 300 K for 115 is 3.11 cm3 K mol−1, which is considerably higher than the spin-only value for S = 3/2 and reflects appreciable spin−orbit coupling. A large D-value of −70 cm−1 was estimated for 115, which, along with the Kramers nature of cobalt(II), explains why this compound is an SMM in zero DC field. Peaks in the out-of-phase susceptibility were observed in the range 1.7−7.0 K, the appearance of which suggests relaxation via QTM below 2.5 K and, above this temperature, predominantly an Orbach process with Ueff = 21 cm−1 and τ0 = 1.0 × 10−7 s (Figure 21).

Figure 21. Molecular structure, d-orbital splitting with ground state electronic configuration, and frequency dependence of the out-ofphase AC magnetic susceptibility in zero applied DC field for 115. Reproduced with permission from ref 122. Copyright 2011 American Chemical Society.122

A second determination of the D-value for 115 yielded −64(1) cm−1, and using this species as a benchmark, the harder O-donor ligands in 114 were found to contribute to the much smaller D-value of −11.1(3) cm−1 for 116. The different D-value of −23.8(2) cm−1 found for the cobalt complex in [K][PPh4][Co(OPh)4] ([K][PPh4][114a]) highlights that ligand softness is not the only consideration, but that the geometric parameters are also critical. However, the softer selenolate ligands in 116 do lead to an increased D-value of −83(1) cm−1, which reflects stronger anisotropy and, in principle, a greater energy separation between MS = ±3/2 and MS = ±1/2, and therefore to a progressively larger Ueff value with increasing ligand softness on descending Group 16. T


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Figure 22. Molecular structure of 118 and magnetic hysteresis loops using a sweep rate of 200 Oe s−1. Reproduced with permission from ref 128. Copyright 2018 Royal Society of Chemistry.128

Figure 23. Molecular structure and frequency dependence of the out-of-phase susceptibility in zero applied field for 113. Reproduced with permission from ref 131. Copyright 2017 Royal Society of Chemistry.131

such as [Dy(η5-1,2,4-tBu3C3P2)2]+ (119) would be an interesting target to pursue.

weaker phosphorus−carbon bonds and the associated vibrational modes would affect the spin−phonon coupling properties identified as being importance for the magnetic relaxation in 33−38. To date, however, the sole example of an SMM with a phosphacyclopentadienyl ligand is the heteroleptic erbium sandwich complex [(η5-Dsp)Er(η8-COT)] (118), where Dsp = 3,4-dimethyl-2,5-bis(trimethylsilyl)phospholyl.128 The distances from erbium to the centroids of the phospholyl and COT ligands in 118 are 2.321 and 1.686 Å, respectively, with the former being markedly longer than the analogous distances of 2.268 Å to the Cp* ligand in 67 and 2.245−2.257 Å to the boratabenzene ligands in 24a−c. In contrast, the COT ligand in 118 is closer to erbium than in 24a−c and 67, which have Er−COT centroid distances of 1.727 Å and 1.674−1.679 Å; the Er−COT distance in 74 is also much greater, at 1.875 Å (Figure 22). An anisotropy barrier of 249 cm−1 was determined for 118 (τ0 = 1.6 × 10−11 s), which is larger than found in other COTligated erbium SMMs, presumably due to the closer proximity of the COT ligand in 118, thus providing the prolate Er3+ ion with a stronger equatorial crystal field. The role of the phosphorus atom is thought to be partly structural in nature since the longer bonds to the Dsp ligand enable close approach of the COT ligand. A blocking temperature of 9 K was determined for 118, which is almost double that of 67 but not quite on a par with 74, although for each compound a different sweep rate was used for the hysteresis measurement. Given the enhanced SMM properties of 118 relative to those of other COT-ligated erbium SMMs, and in light of the properties of the high-temperature SMM 33, a hitherto unknown species

Reflecting on the body of work in which the SMM properties of dysprosium metallocenes were discovered and developed into a predictive magneto-structural correlation,61,64 the isostructural pnictogen-bridged compounds [(η 5 CpMe)2Dy{μ-E(H)Mes}]3 with E = P (120),129 As (121),130 or Sb (122)131 (Figure 23), [Li(thf)4]2[{(η5-CpMe)2Dy(μ3EMes)}3Li] with E = P (123), As (124), and [{(η5CpMe)2Dy}3{(SbMes)Sb}] (125) played a pivotal role. Compounds 120 and 121 were synthesized by deprotonating an E−H bond in [(η5-CpMe)3Dy{E(H)2Mes}] with nBuLi, and 122 was obtained by deprotonation of MesSbH2 with [(η5CpMe)2Dy(nBu)] at temperatures below −40 °C.132 Compounds 120 and 121 can be deprotonated a second time by n BuLi to give 123 and 124, respectively, and 125 forms as a result of a dehydrocoupling reaction between 122 and MesSbH2 at room temperature. The {(η5-CpMe)2Dy} units in 120−125 are essentially isostructural, with the addition of the [E(H)Mes]− donor ligands creating an approximate C2vsymmetric local coordination environment. The Dy−E bond U


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performance is to be achieved at practically useful temperatures.

lengths increase in a manner consistent with the greater radii of the pnictogen elements on descending the group, which is 2.920(6)−2.946(6) Å, 2.984(2)−3.012(2) Å, and 3.118(2)− 3.195(2) Å, in 120−122, respectively. In this series, each compound displays slow magnetic relaxation in zero applied field, with the anisotropy barrier increasing uniformly from 210 cm−1 in 120 to 256 cm−1 in 121 and 345 cm−1 in 113. An increase in the barriers of 120 and 121 to 256 and 301 cm−1, respectively, was observed for magnetically dilute samples. Ab initio theoretical studies of 120−122 consistently demonstrated that the SMM properties arise from the {Cp2Dy}+ unit in which the cyclopentadienyl ligands generate the strong axial crystal field required by Dy3+. The pnictogen donor ligands therefore occupy equatorial positions and generate a crystal field which is detrimental to the magnetic axiality: these transverse components lead to mixing of the higher MJ states, ultimately moderating the height of the anisotropy barrier. Since the Dy−E bond lengths increase from phosphorus to antimony, the equatorial crystal field becomes progressively weaker on descending the group, which raises the barrier appreciably. While simple geometric factors play an important role, it is also possible that an enhancement in the anisotropy of the dysprosium center in 122 occurs due to its interaction with the relatively heavy 5p element antimony. Further support for the magneto-structural hypothesis is derived from 123 and 124, which again feature {Cp2Dy}+ units that are isostructural to those in 120−122, but now the more highly charged phosphinidene and arsinidene ligands are much closer to the metal center, as shown in the Dy−E distances of 2.785(2)−2.824(2) and 2.8515(6)−2.8908(7) Å in 123 and 124, respectively. The competing equatorial crystal field is therefore stronger than in 123 and 124, and the anisotropy barriers are, at 13 and 23 cm−1, very small. The stibinidene-bridged analogue could not be synthesized owing to a spontaneous dehydrocoupling reaction that occurs in which 122 reacts with MesSbH2, resulting in formation of the unusual Zintl-bridged {Dy3Sb4} compound 125, which is also an SMM with an anisotropy barrier of 272 cm−1. Although the Dy−Sb bond lengths in 122 and 125 are the same within error, the superior performance of 122 is noteworthy and can be explained in terms of the lower formal charges on the antimony donor atoms, which calculated using theoretical methods. The thiolate-bridged dimer [(CpMe)2Dy(μ-SSiPh3)]2 (126) and, in particular, the selenolate-bridged trimer [(η5-Cp′2Dy){μSeMes}]3 (127), which is isostructural to 121, also provide support for the magneto-structural correlation since their properties are very similar to those of 120−122. In the case of 126, an anisotropy barrier of 133 cm−1 was determined, and for 127 an anisotropy barrier of 252 cm−1 was found. If the impact of the equatorial donor atoms on the magnitude of Ueff in 120−127 is significant, their impact on the magnetic hysteresisparticularly in light of the properties of 33is dramatic. For each of 120−127, the hysteresis loops are characteristic of a conventional dysprosium SMM regardless of the nature of the coordination or organometallic chemistry; that is, waist-restricted hysteresis loops can only be observed up to about 5 K, and without appreciable coercivity. Very similar observations have been made on other dysprosium metallocene SMMs outside of the main group chemistry arena (to the extent that such a distinction is relevant),133−138 all of which further highlight the need to delete the equatorial crystal field completely if SMM

3. CONCLUSION AND OUTLOOK As the preceding sections demonstrate, the use of nontraditional main group ligands in molecular magnetism has led to many eye-catching results, some of which can be regarded as defining the state of the art. This is particularly true in the arena of single-molecule magnetism, where convenient metrics facilitate comparison of different systems. While main group chemistry applied in molecular magnetism is an innovative approach to a mature field, it is important to emphasize that it is complementary to traditional coordination chemistry. One set of synthetic tools is not necessarily better than another; there is a clear scope for synergy, which allows chemists, physicists, and materials scientists from experimental and theoretical backgrounds to learn from each other in a true spirit of collaboration. While the motivation for such cooperation may be the pursuit of “records” in molecular magnetism, the key outcome is the contribution to knowledge that allows the magnetic properties of molecular materials to be varied in a controlled way. There is no doubt that modern main group chemistry has a huge amount still to contribute to the development of molecular magnetism. As a specific projection for future research, silicon-donor ligands are well established in homogeneous catalysis in the form of silyls and silylenes, and their diverse structural and electronic properties suggest that they should also have considerable scope for incorporation into the design of SMMs in a manner reminiscent of alkyl and carbene ligands.139 Main group chemistry has demonstrated a remarkable ability to reinvent itself in ways that are genuinely paradigm-shifting. The biggest impact of this has undoubtedly been in the context of synthetic chemistry, with the concepts of main group elements as transition metals140 and frustrated Lewis pairs (FLPs)141 proving to be particularly influential. It has been noted that the reactivity of heavier p-block elements in the context of small-molecule activation often shows a strong resemblance to that of transition metals; here, a parallel can be drawn with the concept of anisotropy transfer recently described for transition metal compounds ligated with tin and bismuth, where spin−orbit coupling originating from the heavy p-block metal can influence properties such as the zerofield splitting.119 Viewed in this way, main group elements acting as transition metals may prove to be an analogy with broader scope than previously envisaged, and given the nature of spin−orbit coupling in 6p elements, main group elements acting as lanthanides may not be such an outlandish concept. More generally, the paucity of molecular magnets containing bonds between d- or f-block spin centers and heavy p-block metals suggests that main group chemistry has the potential to play a major role in the development of new chemistry and new physics. AUTHOR INFORMATION Corresponding Author

*E-mail: r.layfi[email protected]. ORCID

Arun Kumar Bar: 0000-0003-1261-327X Richard A. Layfield: 0000-0002-6020-0309 V


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Author Contributions

Free NdFeB Magnets by Grain Boundary Diffusion Process. Appl. Phys. Lett. 2018, 113, 152402. (5) Sprecher, B.; Kleijn, R.; Kramer, G. J. Recycling Potential of Neodymium: The Case of Computer Hard Disk Drives. Environ. Sci. Technol. 2014, 48, 9506−9513. (6) Liu, J.-L.; Chen, Y.-C.; Tong, M.-L. Symmetry Strategies for High Performance Lanthanide-Based Single-Molecule Magnets. Chem. Soc. Rev. 2018, 47, 2431−2453. (7) Woodruff, D. N.; Winpenny, R. E. P.; Layfield, R. A. Lanthanide Single-Molecule Magnets. Chem. Rev. 2013, 113, 5110−5148. (8) Zhu, Z.; Guo, M.; Li, X.-L.; Tang, J. Molecular Magnetism of Lanthanide: Advances and Perspectives. Coord. Chem. Rev. 2019, 378, 350−364. (9) Feng, M.; Tong, M.-L. Single Ion Magnets from 3d to 5f: Developments and Strategies. Chem. - Eur. J. 2018, 24, 7574−7594. (10) Gupta, S. K.; Murugavel, R. Enriching Lanthanide Single-Ion Magnetism through Symmetry and Axiality. Chem. Commun. 2018, 54, 3685−3696. (11) Lu, J.; Guo, M.; Tang, J. Recent Developments in Lanthanide Single-Molecule Magnets. Chem. - Asian J. 2017, 12, 2772−2779. (12) Pointillart, F.; Cador, O.; Guennic, B.; Ouahab, L. Uncommon Lanthanide Ions in Purely 4f Single Molecule Magnets. Coord. Chem. Rev. 2017, 346, 150−175. (13) Frost, J. M.; Harriman, K. L. M.; Murugesu, M. The Rise of 3-d Single-Ion Magnets in Molecular Magnetism: Towards Materials from Molecules? Chem. Sci. 2016, 7, 2470−2491. (14) Dumestre, F.; Chaudret, B.; Amiens, C.; Renaud, P.; Fejes, P. Superlattices of Iron Nanocubes Synthesized from Fe[N(SiMe3)2]2. Science 2004, 303, 821−823. (15) Guo, F.-S.; Day, B. M.; Chen, Y.-C.; Tong, M.-L.; Mansikkamäki, A.; Layfield, R. A. Magnetic Hysteresis up to 80 K in a Dysprosium Metallocene Single-Molecule Magnet. Science 2018, 362, 1400−1403. (16) Senthil Kumar, K.; Ruben, M. Emerging Trends in Spin Crossover (SCO) Based Functional Materials and Devices. Coord. Chem. Rev. 2017, 346, 176−205. (17) Hogue, R. W.; Singh, S.; Brooker, S. Spin Crossover in Discrete Polynuclear Iron(Ii) Complexes. Chem. Soc. Rev. 2018, 47, 7303− 7338. (18) Zheng, X.-Y.; Kong, X.-J.; Zheng, Z.; Long, L.-S.; Zheng, L.-S. High-Nuclearity Lanthanide-Containing Clusters as Potential Molecular Magnetic Coolers. Acc. Chem. Res. 2018, 51, 517−525. (19) Cornia, A.; Seneor, P. Spintronics: The Molecular Way. Nat. Mater. 2017, 16, 505−506. (20) Gaita-Ariño, A.; Luis, F.; Hill, S.; Coronado, E. Molecular Spins for Quantum Computation. Nat. Chem. 2019, 11, 301−309. (21) Escalera-Moreno, L.; Baldoví, J. J.; Gaita-Ariño, A.; Coronado, E. Spin States, Vibrations and Spin Relaxation in Molecular Nanomagnets and Spin Qubits: A Critical Perspective. Chem. Sci. 2018, 9, 3265−3275. (22) Bagai, R.; Christou, G. The Drosophila of Single-Molecule Magnetism: [Mn12O12(O2CR)16(H2O)4]. Chem. Soc. Rev. 2009, 38, 1011−1026. (23) Ishikawa, N.; Sugita, M.; Ishikawa, T.; Koshihara, S. Y.; Kaizu, Y. Lanthanide Double-Decker Complexes Functioning as Magnets at the Single-Molecular Level. J. Am. Chem. Soc. 2003, 125, 8694−8695. (24) Wernsdorfer, W.; Hewitt, I.; Chastanet, G.; Benelli, C.; Tang, J.; Madhu, N. T.; Powell, A. K.; Sessoli, R.; Anson, C. E. Dysprosium Triangles Showing Single-Molecule Magnet Behavior of Thermally Excited Spin States. Angew. Chem., Int. Ed. 2006, 45, 1729−1733. (25) Chibotaru, L. F.; Ungur, L.; Soncini, A. The Origin of Nonmagnetic Kramers Doublets in the Ground State of Dysprosium Triangles: Evidence for a Toroidal Magnetic Moment. Angew. Chem., Int. Ed. 2008, 47, 4126−4129. (26) Power, P. P. Persistent and Stable Radicals of the Heavier Main Group Elements and Related Species. Chem. Rev. 2003, 103, 789− 810.

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. Biographies 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, PR China. In 2015, he was awarded a two-year Marie Sklowdowska-Curie International Fellowship, and he is now continuing as a research fellow in the research group of Prof. Richard Layfield at Sussex. His current research interests are in the design, synthesis, and properties of organometallic molecular magnets. Arun Kumar Bar obtained his Ph.D. in coordination-driven supramolecular self-assembly from the Indian Institute of Science, Bangalore, India. He was then awarded a CEFIPRA postdoctoral fellowship in the group of Prof. Jean-Pascal Sutter, CNRS, LCC, Toulouse, France, where he worked on molecular magnets based on transition metal and lanthanide coordination complexes. He was then awarded a Royal Society Newton International Fellowship in the Layfield group, where he works on 3d/4f organometallic molecular magnets. In 2019 he will take up a position as Assistant Professor at the Indian Institute of Science Education and Research Thirupati. Richard A. Layfield graduated from the University of Leeds and subsequently obtained his Ph.D. at the University of Cambridge. After spells as a Junior Research Fellow and fixed-term Lecturer in Cambridge, he was Lecturer, Reader, and Professor at The University of Manchester from 2007 to 2018. He was appointed to a Chair in Inorganic Chemistry at the University of Sussex in 2018. His research interests encompass a range of topics in organometallic chemistry, with emphasis on applications in molecular magnetismparticularly dysprosium metallocene SMMs. His work has been recognized through numerous awards, including the Royal Society of Chemistry’s Meldola Medal and Sir Edward Frankland Fellowship, an Alexander von Humboldt Foundation Fellowship for Experienced Researchers, and a Rising Star Lectureship from the International Conference on Coordination Chemistry.

ACKNOWLEDGMENTS The authors thank the following organizations for financial support: the Engineering and Physical Sciences Research Council, The Royal Society, The Newton Fund, The Leverhulme Trust, and The Academy of Finland. We are grateful to the European Research Council, the EU Marie Sklowdowska-Curie Actions scheme, and the EU COST Action scheme: we thank the European Union for enabling the United Kingdom to participate in a range of scientific schemes that have led to new collaborations and to the free flow of ideas, expertise, and experiences. REFERENCES (1) Benelli, C.; Gatteschi, D. Introduction to Molecular Magnetism. In Introduction to Molecular Magnetism; John Wiley & Sons, Ltd., 2015; pp 1−23. (2) Gatteschi, D.; Sessoli, R.; Villain, J. Molecular Nanomagnets; Oxford University Press: Oxford, 2006. (3) Miller, J. S. Organic- and Molecule-Based Magnets. Mater. Today 2014, 17 (5), 224−235. (4) Salazar, D.; Martín-Cid, A.; Madugundo, R.; Barandiaran, J. M.; Hadjipanayis, G. C. Coercivity Enhancement in Heavy Rare EarthW


Chemical Reviews

Review

Dynamics in a Series of Two-Coordinate Iron(I) Complexes. Chem. Sci. 2013, 4, 125−138. (45) Atanasov, M.; Zadrozny, J. M.; Long, J. R.; Neese, F. A Theoretical Analysis of Chemical Bonding, Vibronic Coupling, and Magnetic Anisotropy in Linear Iron(Ii) Complexes with SingleMolecule Magnet Behavior. Chem. Sci. 2013, 4, 139−156. (46) Werncke, C. G.; Suturina, E.; Bunting, P. C.; Vendier, L.; Long, J. R.; Atanasov, M.; Neese, F.; Sabo-Etienne, S.; Bontemps, S. Homoleptic Two-Coordinate Silylamido Complexes of Chromium(I), Manganese(I), and Cobalt(I). Chem. - Eur. J. 2016, 22, 1668−1674. (47) Bucinsky, L.; Breza, M.; Lee, W.-T.; Hickey, A. K.; Dickie, D. A.; Nieto, I.; DeGayner, J. A.; Harris, T. D.; Meyer, K.; Krzystek, J.; et al. Spectroscopic and Computational Studies of Spin States of Iron(IV) Nitrido and Imido Complexes. Inorg. Chem. 2017, 56, 4751−4768. (48) Scepaniak, J. J.; Harris, T. D.; Vogel, C. S.; Meyer, K.; Smith, J. M. Spin Crossover in a Four-Coordinate Iron (II) Complex. J. Am. Chem. Soc. 2011, 133, 3824−3827. (49) Mathonière, C.; Lin, H.-J.; Siretanu, D.; Clérac, R.; Smith, J. M. Photoinduced Single-Molecule Magnet Properties in a FourCoordinate Iron(II) Spin Crossover Complex. J. Am. Chem. Soc. 2013, 135, 19083−19086. (50) Ding, M.; Rouzières, M.; Losovyj, Y.; Pink, M.; Clérac, R.; Smith, J. M. Partial Nitrogen Atom Transfer: A New Synthetic Tool to Design Single-Molecule Magnets. Inorg. Chem. 2015, 54, 9075− 9080. (51) Ding, M.; Cutsail, G. E., III; Aravena, D.; Amoza, M.; Rouzières, M.; Dechambenoit, P.; Losovyj, Y.; Pink, M.; Ruiz, E.; Clérac, R.; et al. A Low Spin Manganese(Iv) Nitride Single Molecule Magnet. Chem. Sci. 2016, 7, 6132−6140. (52) Chilton, N. F.; Goodwin, C. A. P.; Mills, D. P.; Winpenny, R. E. P. The First Near-Linear Bis(Amide) f-Block Complex: A Blueprint for a High Temperature Single Molecule Magnet. Chem. Commun. 2015, 51, 101−103. (53) Rinehart, J. D.; Long, J. R. Exploiting Single-Ion Anisotropy in the Design of f-Element Single-Molecule Magnets. Chem. Sci. 2011, 2, 2078−2085. (54) Liu, J.; Chen, Y.-C.; Liu, J.-L.; Vieru, V.; Ungur, L.; Jia, J.-H.; Chibotaru, L. F.; Lan, Y.; Wernsdorfer, W.; Gao, S.; et al. A Stable Pentagonal Bipyramidal Dy(III) Single-Ion Magnet with a Record Magnetization Reversal Barrier over 1000 K. J. Am. Chem. Soc. 2016, 138, 5441−5450. (55) Gupta, S. K.; Rajeshkumar, T.; Rajaraman, G.; Murugavel, R. An Air-Stable Dy(III) Single-Ion Magnet with High Anisotropy Barrier and Blocking Temperature. Chem. Sci. 2016, 7, 5181−5191. (56) Chen, Y.-C.; Liu, J.-L.; Ungur, L.; Liu, J.; Li, Q.-W.; Wang, L.F.; Ni, Z.-P.; Chibotaru, L. F.; Chen, X.-M.; Tong, M.-L. SymmetrySupported Magnetic Blocking at 20 K in Pentagonal Bipyramidal Dy(III) Single-Ion Magnets. J. Am. Chem. Soc. 2016, 138, 2829−2837. (57) Edelmann, F. T. Multiple-Decker Sandwich Complexes of fElements. New J. Chem. 2011, 35, 517−528. (58) Rausch, J.; Apostolidis, C.; Walter, O.; Lorenz, V.; Hrib, C. G.; Hilfert, L.; Kühling, M.; Busse, S.; Edelmann, F. T. One Ligand Fits All: Lanthanide and Actinide Sandwich Complexes Comprising the 1,4-Bis(Trimethylsilyl)Cyclooctatetraenyl (= COT″) Ligand. New J. Chem. 2015, 39, 7656−7666. (59) Saleh, L. M. A.; Birjkumar, K. H.; Protchenko, A. V.; Schwarz, A. D.; Aldridge, S.; Jones, C.; Kaltsoyannis, N.; Mountford, P. Group 3 and Lanthanide Boryl Compounds: Syntheses, Structures, and Bonding Analyses of Sc−B, Y−B, and Lu−B σ-Coordinated NHC Analogues. J. Am. Chem. Soc. 2011, 133, 3836−3839. (60) Meng, Y. S.; Wang, C. H.; Zhang, Y. Q.; Leng, X. B.; Wang, B. W.; Chen, Y. F.; Gao, S. (Boratabenzene)(Cyclooctatetraenyl) Lanthanide Complexes: A New Type of Organometallic Single-Ion Magnet. Inorg. Chem. Front. 2016, 3, 828−835. (61) Day, B. M.; Guo, F.-S.; Layfield, R. A. Cyclopentadienyl Ligands in Lanthanide Single-Molecule Magnets: One Ring to Rule Them All? Acc. Chem. Res. 2018, 51, 1880−1889.

(27) Leitch, A. A.; Yu, X.; Winter, S. M.; Secco, R. A.; Dube, P. A.; Oakley, R. T. Structure and Property Correlations in Heavy Atom Radical Conductors. J. Am. Chem. Soc. 2009, 131, 7112−7125. (28) Ungur, L.; Chibotaru, L. F. Strategies toward High-Temperature Lanthanide-Based Single-Molecule Magnets. Inorg. Chem. 2016, 55, 10043−10056. (29) Ungur, L.; Chibotaru, L. F. Magnetic Anisotropy in the Excited States of Low Symmetry Lanthanide Complexes. Phys. Chem. Chem. Phys. 2011, 13, 20086−20090. (30) Chilton, N. F. Design Criteria for High-Temperature SingleMolecule Magnets. Inorg. Chem. 2015, 54, 2097−2099. (31) Klein, M. J. On a Degeneracy Theorem of Kramers. Am. J. Phys. 1952, 20, 65−71. (32) Topping, C. V.; Blundell, S. J. A.C. Susceptibility as a Probe of Low-Frequency Magnetic Dynamics. J. Phys.: Condens. Matter 2019, 31, 13001. (33) Meng, Y.-S.; Mo, Z.; Wang, B.-W.; Zhang, Y.-Q.; Deng, L.; Gao, S. Observation of the Single-Ion Magnet Behavior of D8 Ions on Two-Coordinate Co(i)−NHC Complexes. Chem. Sci. 2015, 6, 7156− 7162. (34) Yao, X.-N.; Du, J.-Z.; Zhang, Y.-Q.; Leng, X.-B.; Yang, M.-W.; Jiang, S.-D.; Wang, Z.-X.; Ouyang, Z.-W.; Deng, L.; Wang, B.-W.; et al. Two-Coordinate Co(II) Imido Complexes as Outstanding Single-Molecule Magnets. J. Am. Chem. Soc. 2017, 139, 373−380. (35) Llobet, A.; Murphy, D. M.; López, I.; Murugesu, M.; Algarra, A. G.; Poulten, R. C.; Whittlesey, M. K.; Page, M. J.; Macgregor, S. A.; Carter, E.; et al. Synthesis, Electronic Structure, and Magnetism of [Ni(6-Mes)2]+: A Two-Coordinate Nickel(I) Complex Stabilized by Bulky N-Heterocyclic Carbenes. J. Am. Chem. Soc. 2013, 135, 13640− 13643. (36) Lavallo, V.; Canac, Y.; Präsang, C.; Donnadieu, B.; Bertrand, G. Stable Cyclic (Alkyl)(Amino)Carbenes as Rigid or Flexible, Bulky, Electron-Rich Ligands for Transition-Metal Catalysts: A Quaternary Carbon Atom Makes the Difference. Angew. Chem., Int. Ed. 2005, 44, 5705−5709. (37) Samuel, P. P.; Neufeld, R.; Chandra Mondal, K.; Roesky, H. W.; Herbst-Irmer, R.; Stalke, D.; Demeshko, S.; Meyer, F.; Rojisha, V. C.; De, S.; et al. Cr(i)Cl as Well as Cr+ Are Stabilised between Two Cyclic Alkyl Amino Carbenes. Chem. Sci. 2015, 6, 3148−3153. (38) Samuel, P. P.; Mondal, K. C.; Roesky, H. W.; Hermann, M.; Frenking, G.; Demeshko, S.; Meyer, F.; Stückl, A. C.; Christian, J. H.; Dalal, N. S.; et al. Synthesis and Characterization of a TwoCoordinate Manganese Complex and Its Reaction with Molecular Hydrogen at Room Temperature. Angew. Chem., Int. Ed. 2013, 52, 11817−11821. (39) Ung, G.; Rittle, J.; Soleilhavoup, M.; Bertrand, G.; Peters, J. C. Two-Coordinate Fe0 and Co0 Complexes Supported by Cyclic (Alkyl)(Amino)Carbenes. Angew. Chem., Int. Ed. 2014, 53, 8427− 8431. (40) Chakraborty, U.; Demeshko, S.; Meyer, F.; Rebreyend, C.; de Bruin, B.; Atanasov, M.; Neese, F.; Mühldorf, B.; Wolf, R. Electronic Structure and Magnetic Anisotropy of an Unsaturated Cyclopentadienyl Iron(I) Complex with 15 Valence Electrons. Angew. Chem., Int. Ed. 2017, 56, 7995−7999. (41) Lan, Y.; Powell, A. K.; Wolmershäuser, G.; Sitzmann, H.; Sun, Y.; Weismann, D. High-Spin Cyclopentadienyl Complexes: A SingleMolecule Magnet Based on the Aryl-Iron(II) Cyclopentadienyl Type. Chem. - Eur. J. 2011, 17, 4700−4704. (42) Bunting, P. C.; Atanasov, M.; Damgaard-Møller, E.; Perfetti, M.; Crassee, I.; Orlita, M.; Overgaard, J.; van Slageren, J.; Neese, F.; Long, J. R. A Linear Cobalt(II) Complex with Maximal Orbital Angular Momentum from a Non-Aufbau Ground State. Science (Washington, DC, U. S.) 2018, 362, No. eaat7319. (43) Zadrozny, J. M.; Xiao, D. J.; Atanasov, M.; Long, G. J.; Grandjean, F.; Neese, F.; Long, J. R. Magnetic Blocking in a Linear Iron(I) Complex. Nat. Chem. 2013, 5, 577−581. (44) Zadrozny, J. M.; Atanasov, M.; Bryan, A. M.; Lin, C. Y.; Rekken, B. D.; Power, P. P.; Neese, F.; Long, J. R. Slow Magnetization X


Chemical Reviews

Review

(62) Layfield, R. A. Organometallic Single-Molecule Magnets. Organometallics 2014, 33, 1084−1099. (63) Layfield, R. A.; McDouall, J. J. W.; Sulway, S. A.; Tuna, F.; Collison, D.; Winpenny, R. E. P. Influence of the N-Bridging Ligand on Magnetic Relaxation in an Organometallic Dysprosium SingleMolecule Magnet. Chem. - Eur. J. 2010, 16, 4442−4446. (64) Pugh, T.; Chilton, N. F.; Layfield, R. A. A Low-Symmetry Dysprosium Metallocene Single-Molecule Magnet with a High Anisotropy Barrier. Angew. Chem., Int. Ed. 2016, 55, 11082−11085. (65) Randall McClain, K.; Gould, C. A.; Chakarawet, K.; Teat, S. J.; Groshens, T. J.; Long, J. R.; Harvey, B. G. High-Temperature Magnetic Blocking and Magneto-Structural Correlations in a Series of Dysprosium(III) Metallocenium Single-Molecule Magnets. Chem. Sci. 2018, 9, 8492−8503. (66) Guo, F.-S.; Day, B. M.; Chen, Y.-C.; Tong, M.-L.; Mansikkamäki, A.; Layfield, R. A. A Dysprosium Metallocene Single-Molecule Magnet Functioning at the Axial Limit. Angew. Chem., Int. Ed. 2017, 56, 11445−11449. (67) Goodwin, C. A. P.; Ortu, F.; Reta, D.; Chilton, N. F.; Mills, D. P. Molecular Magnetic Hysteresis at 60 K in Dysprosocenium. Nature 2017, 548, 439−442. (68) Goodwin, C. A. P.; Reta, D.; Ortu, F.; Chilton, N. F.; Mills, D. P. Synthesis and Electronic Structures of Heavy Lanthanide Metallocenium Cations. J. Am. Chem. Soc. 2017, 139, 18714−18724. (69) Goodwin, C. A. P.; Reta, D.; Ortu, F.; Liu, J.; Chilton, N. F.; Mills, D. P. Terbocenium: Completing a Heavy Lanthanide Metallocenium Cation Family with an Alternative Anion Abstraction Strategy. Chem. Commun. 2018, 54, 9182−9185. (70) Demir, S.; Zadrozny, J. M.; Long, J. R. Large Spin-Relaxation Barriers for the Low-Symmetry Organolanthanide Complexes [Cp*2Ln(BPh4)] (Cp*= pentamethylcyclopentadienyl; Ln = Tb, Dy). Chem. - Eur. J. 2014, 20, 9524−9529. (71) Demir, S.; Boshart, M. D.; Corbey, J. F.; Woen, D. H.; Gonzalez, M. I.; Ziller, J. W.; Meihaus, K. R.; Long, J. R.; Evans, W. J. Slow Magnetic Relaxation in a Dysprosium Ammonia Metallocene Complex. Inorg. Chem. 2017, 56, 15049−15056. (72) Demir, S.; Jeon, I. R.; Long, J. R.; Harris, T. D. Radical LigandContaining Single-Molecule Magnets. Coord. Chem. Rev. 2015, 289− 290, 149−176. (73) Rinehart, J. D.; Fang, M.; Evans, W. J.; Long, J. R. Strong Exchange and Magnetic Blocking in N 2 3– Radical-Bridged Lanthanide Complexes. Nat. Chem. 2011, 3, 538−542. (74) Rinehart, J. D.; Fang, M.; Evans, W. J.; Long, J. R. A N23− Radical-Bridged Terbium Complex Exhibiting Magnetic Hysteresis at 14 K. J. Am. Chem. Soc. 2011, 133, 14236−14239. (75) Demir, S.; Gonzalez, M. I.; Darago, L. E.; Evans, W. J.; Long, J. R. Giant Coercivity and High Magnetic Blocking Temperatures for N23− Radical-Bridged Dilanthanide Complexes upon Ligand Dissociation. Nat. Commun. 2017, 8, 2144. (76) Rinehart, J. D.; Fang, M.; Evans, W. J.; Long, J. R. Strong Exchange and Magnetic Blocking in N23− Radical-Bridged Lanthanide Complexes. Nat. Chem. 2011, 3, 538−542. (77) Guo, F.-S.; Layfield, R. A. Strong Direct Exchange Coupling and Single-Molecule Magnetism in Indigo-Bridged Lanthanide Dimers. Chem. Commun. 2017, 53, 3130−3133. (78) Demir, S.; Nippe, M.; Gonzalez, M. I.; Long, J. R. Exchange Coupling and Magnetic Blocking in Dilanthanide Complexes Bridged by the Multi-Electron Redox-Active Ligand 2,3,5,6-Tetra(2-Pyridyl)Pyrazine. Chem. Sci. 2014, 5, 4701−4711. (79) Grindell, R.; Vieru, V.; Pugh, T.; Chibotaru, L. F.; Layfield, R. A. Magnetic Frustration in a Hexaazatrinaphthylene-Bridged Trimetallic Dysprosium Single-Molecule Magnet. Dalton Trans 2016, 45, 16556−16560. (80) Moilanen, J. O.; Chilton, N. F.; Day, B. M.; Pugh, T.; Layfield, R. A. Strong Exchange Coupling in a Trimetallic Radical-Bridged Cobalt(II)-Hexaazatrinaphthylene Complex. Angew. Chem., Int. Ed. 2016, 55, 5521−5525.

(81) Moilanen, J. O.; Day, B. M.; Pugh, T.; Layfield, R. A. OpenShell Doublet Character in a Hexaazatrinaphthylene Trianion Complex. Chem. Commun. 2015, 51, 11478. (82) Gould, C. A.; Darago, L. E.; Gonzalez, M. I.; Demir, S.; Long, J. R. A Trinuclear Radical-Bridged Lanthanide Single-Molecule Magnet. Angew. Chem., Int. Ed. 2017, 56, 10103−10107. (83) Huang, W.; Le Roy, J. J.; Khan, S. I.; Ungur, L.; Murugesu, M.; Diaconescu, P. L. Tetraanionic Biphenyl Lanthanide Complexes as Single-Molecule Magnets. Inorg. Chem. 2015, 54, 2374−2382. (84) Dickie, C. M.; Laughlin, A. L.; Wofford, J. D.; Bhuvanesh, N. S.; Nippe, M. Transition Metal Redox Switches for Reversible “on/off” and “Slow/Fast” Single-Molecule Magnet Behaviour in Dysprosium and Erbium Bis-Diamidoferrocene Complexes. Chem. Sci. 2017, 8, 8039−8049. (85) Zhang, P.; Zhang, L.; Wang, C.; Xue, S.; Lin, S.-Y.; Tang, J. Equatorially Coordinated Lanthanide Single Ion Magnets. J. Am. Chem. Soc. 2014, 136, 4484−4487. (86) Brown, A. J.; Pinkowicz, D.; Saber, M. R.; Dunbar, K. R. A Trigonal-Pyramidal Erbium(III) Single-Molecule Magnet. Angew. Chem., Int. Ed. 2015, 54, 5864−5868. (87) Jiang, S. Da; Wang, B. W.; Sun, H. L.; Wang, Z. M.; Gao, S. An Organometallic Single-Ion Magnet. J. Am. Chem. Soc. 2011, 133, 4730−4733. (88) Liu, S.-S.; Wang, Z.-M.; Jiang, S.-D.; Gao, S.; Wang, B.-W.; Zhou, L.-N. Series of Lanthanide Organometallic Single-Ion Magnets. Inorg. Chem. 2012, 51, 3079−3087. (89) Meng, Y.-S.; Qiao, Y.-S.; Zhang, Y.-Q.; Jiang, S.-D.; Meng, Z.S.; Wang, B.-W.; Wang, Z.-M.; Gao, S. Can Non-Kramers TmIII Mononuclear Molecules Be Single-Molecule Magnets (SMMs)? Chem. - Eur. J. 2016, 22, 4704−4708. (90) Meihaus, K. R.; Long, J. R. Magnetic Blocking at 10 K and a Dipolar-Mediated Avalanche in Salts of the Bis(η8-Cyclooctatetraenide) Complex [Er(COT)2]−. J. Am. Chem. Soc. 2013, 135, 17952− 17957. (91) Ungur, L.; Le Roy, J. J.; Korobkov, I.; Murugesu, M.; Chibotaru, L. F. Fine-Tuning the Local Symmetry to Attain Record Blocking Temperature and Magnetic Remanence in a Single-Ion Magnet. Angew. Chem., Int. Ed. 2014, 53, 4413−4417. (92) Harriman, K. L. M.; Murugesu, M. An Organolanthanide Building Block Approach to Single-Molecule Magnets. Acc. Chem. Res. 2016, 49, 1158−1167. (93) Gorelsky, S. I.; Korobkov, I.; Jeletic, M.; Le Roy, J. J.; Murugesu, M.; Ungur, L.; Chibotaru, L. F. An Organometallic Building Block Approach To Produce a Multidecker 4 f SingleMolecule Magnet. J. Am. Chem. Soc. 2013, 135, 3502−3510. (94) Le Roy, J. J.; Ungur, L.; Korobkov, I.; Chibotaru, L. F.; Murugesu, M. Coupling Strategies to Enhance Single-Molecule Magnet Properties of Erbium−Cyclooctatetraenyl Complexes. J. Am. Chem. Soc. 2014, 136, 8003−8010. (95) Le Roy, J. J.; Korobkov, I.; Murugesu, M. A Sandwich Complex with Axial Symmetry for Harnessing the Anisotropy in a Prolate Erbium(Iii) Ion. Chem. Commun. 2014, 50, 1602−1604. (96) Hilgar, J. D.; Bernbeck, M. G.; Flores, B. S.; Rinehart, J. D. Metal−ligand Pair Anisotropy in a Series of Mononuclear Er−COT Complexes. Chem. Sci. 2018, 9, 7204−7209. (97) Hilgar, J. D.; Bernbeck, M. G.; Rinehart, J. D. Million-Fold Relaxation Time Enhancement across a Series of PhosphinoSupported Erbium Single-Molecule Magnets. J. Am. Chem. Soc. 2019, 141, 1913−1917. (98) Hilgar, J. D.; Flores, B. S.; Rinehart, J. D. Ferromagnetic Coupling in a Chloride-Bridged Erbium Single-Molecule Magnet. Chem. Commun. 2017, 53, 7322−7324. (99) Jeletic, M.; Lin, P.-H.; Le Roy, J. J.; Korobkov, I.; Gorelsky, S. I.; Murugesu, M. An Organometallic Sandwich Lanthanide Single-Ion Magnet with an Unusual Multiple Relaxation Mechanism. J. Am. Chem. Soc. 2011, 133, 19286−19289. (100) Harriman, K. L. M.; Korobkov, I.; Murugesu, M. From a Piano Stool to a Sandwich: A Stepwise Route for Improving the Slow Y


Chemical Reviews

Review

Magnetic Relaxation Properties of Thulium. Organometallics 2017, 36, 4515−4518. (101) Meihaus, K. R.; Minasian, S. G.; Lukens, W. W.; Kozimor, S. A.; Shuh, D. K.; Tyliszczak, T.; Long, J. R. Influence of Pyrazolate vs N-Heterocyclic Carbene Ligands on the Slow Magnetic Relaxation of Homoleptic Trischelate Lanthanide(III) and Uranium(III) Complexes. J. Am. Chem. Soc. 2014, 136, 6056−6068. (102) Gupta, T.; Velmurugan, G.; Rajeshkumar, T.; Rajaraman, G. Role of Lanthanide-Ligand Bonding in the Magnetization Relaxation of Mononuclear Single-Ion Magnets: A Case Study on Pyrazole and Carbene Ligated LnIII (Ln = Tb, Dy, Ho, Er) Complexes. J. Chem. Sci. 2016, 128, 1615−1630. (103) Gregson, M.; Chilton, N. F.; Ariciu, A.-M.; Tuna, F.; Crowe, I. F.; Lewis, W.; Blake, A. J.; Collison, D.; McInnes, E. J. L.; Winpenny, R. E. P.; et al. A Monometallic Lanthanide Bis(Methanediide) Single Molecule Magnet with a Large Energy Barrier and Complex Spin Relaxation Behaviour. Chem. Sci. 2016, 7, 155−165. (104) Latendresse, T. P.; Bhuvanesh, N. S.; Nippe, M. Slow Magnetic Relaxation in a Lanthanide-[1]Metallocenophane Complex. J. Am. Chem. Soc. 2017, 139, 8058−8061. (105) Latendresse, T. P.; Vieru, V.; Wilkins, B. O.; Bhuvanesh, N. S.; Chibotaru, L. F.; Nippe, M. Magnetic Properties of a Terbium− [1]Ferrocenophane Complex: Analogies between Lanthanide− Ferrocenophane and Lanthanide−Bis-Phthalocyanine Complexes. Angew. Chem., Int. Ed. 2018, 57, 8164−8169. (106) Latendresse, T. P.; Bhuvanesh, N. S.; Nippe, M. Hard SingleMolecule Magnet Behavior by a Linear Trinuclear Lanthanide− [1]Metallocenophane Complex. J. Am. Chem. Soc. 2017, 139, 14877− 14880. (107) Liddle, S. T.; Slageren, J. van. Actinide Single-Molecule Magnets. In Lanthanides and Actinides in Molecular Magnetism; John Wiley & Sons, Ltd., 2015; pp 315−340. (108) Magnani, N.; Apostolidis, C.; Morgenstern, A.; Colineau, E.; Griveau, J. C.; Bolvin, H.; Walter, O.; Caciuffo, R. Magnetic Memory Effect in a Transuranic Mononuclear Complex. Angew. Chem., Int. Ed. 2011, 50, 1696−1698. (109) Le Roy, J. J.; Gorelsky, S. I.; Korobkov, I.; Murugesu, M. Slow Magnetic Relaxation in Uranium(III) and Neodymium(III) Cyclooctatetraenyl Complexes. Organometallics 2015, 34, 1415−1418. (110) Kindra, D. R.; Evans, W. J. Magnetic Susceptibility of Uranium Complexes. Chem. Rev. 2014, 114, 8865−8882. (111) Gaggioli, C. A.; Gagliardi, L. Theoretical Investigation of Plutonium-Based Single-Molecule Magnets. Inorg. Chem. 2018, 57, 8098−8105. (112) Magnani, N.; Colineau, E.; Griveau, J.-C.; Apostolidis, C.; Walter, O.; Caciuffo, R. A Plutonium-Based Single-Molecule Magnet. Chem. Commun. 2014, 50, 8171−8173. (113) Eichhöfer, A.; Lan, Y.; Mereacre, V.; Bodenstein, T.; Weigend, F. Slow Magnetic Relaxation in Trigonal-Planar Mononuclear Fe(II) and Co(II) Bis(Trimethylsilyl)Amido ComplexesA Comparative Study. Inorg. Chem. 2014, 53, 1962−1974. (114) Lin, P. H.; Smythe, N. C.; Gorelsky, S. I.; Maguire, S.; Henson, N. J.; Korobkov, I.; Scott, B. L.; Gordon, J. C.; Baker, R. T.; Murugesu, M. Importance of Out-of-State Spin-Orbit Coupling for Slow Magnetic Relaxation in Mononuclear FeII Complexes. J. Am. Chem. Soc. 2011, 133, 15806−15809. (115) Scheuermayer, S.; Tuna, F.; Pineda, E. M.; Bodensteiner, M.; Scheer, M.; Layfield, R. A. Transmetalation of Chromocene by Lithium-Amide, -Phosphide, and -Arsenide Nucleophiles. Inorg. Chem. 2013, 52, 3878. (116) Scheuermayer, S.; Tuna, F.; Bodensteiner, M.; Scheer, M.; Layfield, R. A. Spin Crossover in Phosphorus- and Arsenic-Bridged Cyclopentadienyl- Manganese(II) Dimers. Chem. Commun. 2012, 48, 8087. (117) Layfield, R. A. Manganese(II): The Black Sheep of the Organometallic Family. Chem. Soc. Rev. 2008, 37, 1098. (118) von Hänisch, C.; Weigend, F.; Clérac, R. Unique Manganese Phosphorus Complex with a Mn5P7 Core: Synthesis, Molecular

Structure, and Magnetic Properties. Inorg. Chem. 2008, 47, 1460− 1464. (119) Pearson, T. J.; Fataftah, M. S.; Freedman, D. E. Enhancement of Magnetic Anisotropy in a Mn-Bi Heterobimetallic Complex. Chem. Commun. 2016, 52, 11394−11397. (120) Coste, S. C.; Vlaisavljevich, B.; Freedman, D. E. Magnetic Anisotropy from Main-Group Elements: Halides versus Group 14 Elements. Inorg. Chem. 2017, 56, 8195−8202. (121) Brazzolotto, D.; Gennari, M.; Yu, S.; Pécaut, J.; Rouzières, M.; Clérac, R.; Orio, M.; Duboc, C. An Experimental and Theoretical Investigation on Pentacoordinated Cobalt(III) Complexes with an Intermediate S = 1 Spin State: How Halide Ligands Affect Their Magnetic Anisotropy. Chem. - Eur. J. 2016, 22, 925−933. (122) Zadrozny, J. M.; Long, J. R. Slow Magnetic Relaxation at Zero Field in the Tetrahedral Complex [Co(SPh)4]2−. J. Am. Chem. Soc. 2011, 133, 20732−20734. (123) Zadrozny, J. M.; Telser, J.; Long, J. R. Slow Magnetic Relaxation in the Tetrahedral Cobalt(II) Complexes [Co(EPh)4]2− (E = O, S, Se). Polyhedron 2013, 64, 209−217. (124) Suturina, E. A.; Maganas, D.; Bill, E.; Atanasov, M.; Neese, F. Magneto-Structural Correlations in a Series of Pseudotetrahedral [CoII(XR)4]2− Single Molecule Magnets: An Ab Initio Ligand Field Study. Inorg. Chem. 2015, 54, 9948−9961. (125) Carl, E.; Demeshko, S.; Meyer, F.; Stalke, D. Triimidosulfonates as Acute Bite-Angle Chelates: Slow Relaxation of the Magnetization in Zero Field and Hysteresis Loop of a CoII Complex. Chem. - Eur. J. 2015, 21, 10109−10115. (126) Schoo, C.; Bestgen, S.; Köppe, R.; Konchenko, S. N.; Roesky, P. W. Reactivity of Bulky Ln(Ii) Amidinates towards P4, As4, and As4S4. Chem. Commun. 2018, 54, 4770−4773. (127) Schoo, C.; Bestgen, S.; Egeberg, A.; Klementyeva, S.; Feldmann, C.; Konchenko, S. N.; Roesky, P. W. Samarium Polystibides Derived from Highly Activated Nanoscale Antimony. Angew. Chem., Int. Ed. 2018, 57, 5912−5916. (128) Chen, S.-M.; Xiong, J.; Zhang, Y.-Q.; Yuan, Q.; Wang, B.-W.; Gao, S. A Soft Phosphorus Atom to “Harden” an Erbium(Iii) SingleIon Magnet. Chem. Sci. 2018, 9, 7540−7545. (129) Pugh, T.; Tuna, F.; Ungur, L.; Collison, D.; McInnes, E. J. L.; Chibotaru, L. F.; Layfield, R. A. Influencing the Properties of Dysprosium Single-Molecule Magnets with Phosphorus Donor Ligands. Nat. Commun. 2015, 6, 8492. (130) Pugh, T.; Vieru, V.; Chibotaru, L. F.; Layfield, R. A. MagnetoStructural Correlations in Arsenic- and Selenium-Ligated Dysprosium Single-Molecule Magnets. Chem. Sci. 2016, 7, 2128−2137. (131) Pugh, T.; Chilton, N. F.; Layfield, R. A. Antimony-Ligated Dysprosium Single-Molecule Magnets as Catalysts for Stibine Dehydrocoupling. Chem. Sci. 2017, 8, 2073−2080. (132) Grindell, R.; Day, B. M.; Guo, F. S.; Pugh, T.; Layfield, R. A. Activation of C-H Bonds by Rare-Earth Metallocene-Butyl Complexes. Chem. Commun. 2017, 53, 9990−9993. (133) Burns, C. P.; Wilkins, B. O.; Dickie, C. M.; Latendresse, T. P.; Vernier, L.; Vignesh, K. R.; Bhuvanesh, N. S.; Nippe, M. A Comparative Study of Magnetization Dynamics in Dinuclear Dysprosium Complexes Featuring Bridging Chloride or Trifluoromethanesulfonate Ligands. Chem. Commun. 2017, 53, 8419−8422. (134) Burns, C. P.; Yang, X.; Wofford, J. D.; Bhuvanesh, N. S.; Hall, M. B.; Nippe, M. Structure and Magnetization Dynamics of Dy−Fe and Dy−Ru Bonded Complexes. Angew. Chem., Int. Ed. 2018, 57, 8144−8148. (135) Kilpatrick, A. F. R.; Guo, F. S.; Day, B. M.; Mansikkamäki, A.; Layfield, R. A.; Cloke, F. G. N. Single-Molecule Magnet Properties of a Monometallic Dysprosium Pentalene Complex. Chem. Commun. 2018, 54, 7085−7088. (136) Sulway, S. A.; Layfield, R. A.; Tuna, F.; Wernsdorfer, W.; Winpenny, R. E. P. Single-Molecule Magnetism in CyclopentadienylDysprosium Chlorides. Chem. Commun. 2012, 48, 1508−1510. (137) Day, B. M.; Guo, F.-S.; Giblin, S. R.; Sekiguchi, A.; Mansikkamäki, A.; Layfield, R. A. Rare-Earth Cyclobutadienyl Z


Chemical Reviews

Review

Sandwich Complexes: Synthesis, Structure and Dynamic Magnetic Properties. Chem. - Eur. J. 2018, 24, 16779−16782. (138) Harriman, K. L. M.; Le Roy, J. J.; Ungur, L.; Holmberg, R. J.; Korobkov, I.; Murugesu, M. Cycloheptatrienyl Trianion: An Elusive Bridge in the Search of Exchange Coupled Dinuclear Organolanthanide Single-Molecule Magnets. Chem. Sci. 2017, 8, 231−240. (139) Zhou, Y.-P.; Driess, M. Isolable Silylene Ligands Can Boost Efficiencies and Selectivities in Metal-Mediated Catalysis. Angew. Chem., Int. Ed. 2019, 58, 3715−3728. (140) Power, P. P. Main-Group Elements as Transition Metals. Nature 2010, 463, 171−177. (141) Stephan, D. W. Frustrated Lewis Pairs: From Concept to Catalysis. Acc. Chem. Res. 2015, 48, 306−316.

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