Organometallic Single-Molecule Magnets - Organometallics (ACS

Review pubs.acs.org/Organometallics

Organometallic Single-Molecule Magnets Richard A. Layfield* School of Chemistry, The University of Manchester, Manchester M13 9PL, U.K. ABSTRACT: Single-molecule magnets (SMMs) display slow relaxation of the magnetization, purely of molecular origin, in the absence of an applied magnetic field. This review summarizes the important role played by organometallic chemistry in the recent development of SMMs. The broad applicability of organometallic synthesis has led to a series of organometallic SMMs containing transition metals, lanthanides, or actinides, with several examples accounting for some of the most fascinating low-temperature magnetism. The review has two main aims. The first aim is to provide organometallic chemists with an introduction to one of the most exciting areas of modern molecular magnetism and, in particular, to highlight how organometallic chemistry has allowed the field to evolve in new directions. The second aim is more of a clarion call: organometallic chemistry still has hugely underexploited potential in the development of single-molecule magnets, and it is reasonable to expect that different synthetic approaches will lead to new and unusual magnetic phenomena. By using this review as an entry point for studying the literature in more detail, hopefully more organometallic chemists will consider directing their synthetic repertoire toward the design and realization of new, and possibly improved, single-molecule magnets.

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INTRODUCTION For over 20 years, the magnetic memory properties of certain types of d- and f-block complexes have provided a rich source of intrigue for scientists with backgrounds ranging from synthetic coordination chemistry, through materials chemistry, to condensed matter physics and theoretical physics. The dand f-block complexes in question possess an ability to retain magnetization for relatively long periods of time in the absence of an applied magnetic f ield, invariably at very low temperatures, and they are known widely as single-molecule magnets (SMMs).1 SMMs have been classified according to various criteria, such as the block of the periodic table in which the paramagnetic metal ions reside, the number of metal centers in the molecular structure, and the types of ligands in the SMM.2 Other classifications are possible, although in collating and segregating different SMMs it is important to remain mindful of the fact that the different “types” have key properties in common, and that one type of SMM may yield insight into another. Notwithstanding the fundamental interest in SMMs, one of the main driving forces behind the rapid development of the field has been the applications that such materials may have as nanoscale magnetic devices.3 SMMs based on lanthanides have shown particular promise in this context, and several studies have described the use of terbium phthalocyanine complexes (see below) in molecular spintronics, which typically require individual SMM molecules to be attached to the surfaces of materials such as graphene and carbon nanotubes. The © 2014 American Chemical Society

remarkable properties of SMMs have been used to create ingenious devices capable of producing an electronic readout of magnetic properties, such as the nuclear spin, of individual molecules,4 thus going significantly beyond what can be achieved by conventional characterization of bulk magnetic properties. One of the crucial properties that unifies all SMMs is magnetic anisotropy. In general, all SMMs contain metal ions with anisotropic electronic structure; however the presence of anisotropy is still no guarantee of SMM properties, as will be highlighted in this review as appropriate.5 The physical significance of anisotropy in SMMs is that the magnetic moment of an individual molecule has a preferred orientation, which does not depend on an external magnetic field, leading to net magnetization in a bulk sample.6 If the orientation of the magnetic moment is reversedcrudely, analogous to flipping spin-up to spin-downthen the SMM properties are lost. In order to wipe the SMM properties in this way, a thermal energy barrier must, in principle, be surmounted: for the purposes of this review, the energy barrier is referred to as the anisotropy barrier, Ueff, in units of cm−1 (as opposed to units of K, which are also commonly used). The magnitude of the anisotropy barrier is one way of comparing the success of different SMMs, and the bigger the barrier, the more prominent the SMM properties at higher temperatures. Received: November 14, 2013 Published: January 9, 2014 1084

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In the vast majority of cases, the anisotropy barrier is determined from magnetic susceptibility measurements in a very small dynamic or alternating current (ac) magnetic field: this measurement can be performed either in zero static or direct current (dc) field or in an applied dc field, if appropriate. During such measurements, the in-phase (χ′) and the out-ofphase (χ″) components of the magnetic susceptibility are measured as a function of temperature and as a function of the ac frequency (ν). The χ″(ν) data then allow relaxation times, τ, to be determined from τ = 1/(2πν), where the value of ν is the peak maximum (see Figure 22, for example). The relaxation dynamics are characterized by a relaxation time τ at a particular temperature and frequency, and this allows the anisotropy barrier to be related to the relaxation time as τ = τ0 exp(Ueff/ kBT). The value of Ueff is then extracted from the linear section of the Arrhenius plot of ln τ vs 1/T, which describes a regime in which the relaxation is thermally activated. A second, and arguably more important, measure of the success of an SMM is whether or not the field dependence of the magnetization shows hysteresis. If a molecule is indeed “magnetic”, then, after being subjected to a reverse magnetic field (H) and subsequently returned to zero-field conditions, it will display nonzero magnetization (M). This phenomenon is also temperature dependent; however, other factors such as the field sweep rate determine the maximum temperature (the blocking temperature, TB) at which M(H) hysteresis is observed. It is possible to compare SMMs characterized using different sweep rates by defining the blocking temperature as the temperature at which the magnetic relaxation time is 100 s.1 Many SMMs show M(H) hysteresis; however, all current examples require liquid helium cooling, and one of the major challenges is to raise the blocking temperature to levels that will be more convenient for the development of device applications. Many more SMMs do not show any hysteresis at all: examples of both types of SMM are considered in the following sections. Organometallic SMMs. Relative to “classical” coordination chemistry, organometallic chemistry has played a minor role in the development of SMMs. Indeed, the first organometallic SMM was only reported in 2010.7 However, in recent years, organometallic chemistry and chemistry involving ligands that can be regarded as members of the extended organometallic family have produced many interesting developments. Some of these developments also account for notable “world records” in terms of magnetic hysteresis and anisotropy barriers. Purely for organizational purposes, this review is subdivided into sections dealing with organo-lanthanide SMMs, organo-actinide SMMs, and low-coordinate transition-metal SMMs. The last category represents an important emerging area within the wider SMM field and is an area where organometallic chemistry is likely to make further important contributions. Polymetallic Transition-Metal Cage SMMs. To put current research into SMMs in context, it is first necessary to pay homage to the pioneering work that initiated the field. The SMM phenomenon was discovered in the well-known manganese(III)/manganese(IV) cage complex [Mn12O12(OAc)16(H2O)4] (1) (Figure 1),8 and in the decade that followed the field was dominated by research that focused on polymetallic exchange-coupled cage complexes of high-spin, anisotropic 3d metal ions.9,10 Polymetallic SMM cage complexes based on the Jahn−Teller ion manganese(III) (high-spin d4) are ubiquitous, and indeed manganese still accounts for significant activity in the field.11 Cage complexes containing other transition-metal ions have

Figure 1. Molecular structure of [Mn12O12(OAc)16(H2O)4] (1). Color scheme: Mn(IV), green; Mn(III), brown; O, red; C, black.

also formed the basis of many other SMMs. Although polymetallic transition-metal cage complexes have played a pivotal role in the development of SMMs, no organometallic variants are currently known. The origins of slowly relaxing magnetization in polymetallic transition-metal complexes have been reviewed elsewhere,1,2,10 and only the key points will be summarized here. In polymetallic transition-metal SMMs, the anisotropy barrier can be expressed as Ueff = |D|S2 (for integer spin systems) and Ueff = |D|(S2 − 1/4) (for noninteger spins), in which D is the axial zero-field splitting (ZFS) parameter and S is the spin ground state of the molecule. The ZFS parameter D provides a measure of the anisotropy; it should be large and negative, and the ground-state spin S should be as large as possible. The projection of the total spin gives rise to [2S + 1] mS microstates, which are split in zero field, and where mS = ± S lies lowest when D < 0. Because of the anisotropy, the spin has a preferred orientation and mS = +S is the ground state, and so at low temperatures the molecule can be magnetized. For the magnetization to relax, it is necessary to flip the spins into mS = −S, which can be achieved by surmounting the thermal barrier in ΔmS = ± 1 steps until a thermal equilibrium is reached. In addition to the thermally activated relaxation mechanism, it is also possible for relaxation to occur via resonant quantum tunneling of the magnetization (QTM) between degenerate mS states occurs through the barrier, a phenomenon which can be observed experimentally as characteristic steps in the M(H) hysteresis loops.8 The first attempts to maximize the anisotropy barrier focused on maximizing S by designing systems with ferromagnetic exchange coupling. Using this approach, the largest anisotropy barrier and blocking temperature of Ueff = 62 cm−1 and TB ≈ 4.5 K, respectively, were reported for the dodecametallic phenolate-bridged cage [Mn6O2(sao)6(O2CPh)2(EtOH)4] ({Mn6}; saoH2 = 2-hydroxybenzaldehye oxime) (2) (Figure 2), which was determined to have a total spin of S = 12.12 Part of the appeal of the equations used to express Ueff in terms of D and S is their simplicity. However, although it is possible to synthesize polymetallic complexes with large S values, factors beyond the control of the synthetic chemist, such as inversion symmetry, can result in an overall lack of anisotropy.13 If this situation transpires, then the SMM properties are likely to be compromised. A recent theoretical study has even indicated that the key factor which influences 1085

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Figure 3. Qualitative illustration of magnetic relaxation pathways in Ln-SMMs, adapted from ref 17. The red dashed line represents QTM within the ground doublet, the green dashed line represents thermally assisted QTM via the first excited doublet, and the blue lines represent Orbach relaxation. Relaxation via the second excited doublet is also possible.17

the excited state can decay back down to both ground mJ sublevels, which is known as an Orbach process. Alternatively, upon reaching the first excited doublet, it is also possible for the SMM to relax via so-called thermally assisted QTM.17 The different mechanisms have their respective experimental signatures, particularly in relation to their temperature (in)dependence. However, more than one mechanism can occur within the same lanthanide center, even across a seemingly narrow range of physical conditions. These processes are complicated, and to understand them often requires insight from ab initio computational studies. Role of Ligand-Field Symmetry in Lanthanide SMMs. The pivotal result that established a genuinely new direction in a maturing field was the terbium(III) phthalocyanine (Pc) salt [NBu4][TbPc2] ([NBu4][3]), which produced anisotropy barriers of Ueff > 230 cm−1, depending on the experimental conditions.14a Complex 3 (Figure 4) is remarkable for several reasons.

Figure 2. Molecular structure of [Mn6O2(sao)6(O2CPh)2(EtOH)4] (2). From ref 12. Mn, red; O, green; N, blue.

Ueff is in fact the anisotropy and that the energy barrier may even be independent of S.5

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ORGANOMETALLIC LANTHANIDE SMMs Mechanistic Aspects. To design new SMMs with larger anisotropy barriers and more prominent hysteresis, a new approach, with greater emphasis on anisotropy, became necessary. In 2003, attention therefore switched to the lanthanide (Ln) elements,14 particularly terbium, dysprosium, holmium, and erbium,2a−g and it is in this branch of the SMM field that organometallic chemistry has had the greatest impact. The large magnetic moments in complexes of terbium−erbium stem from substantial orbital contributions, which are effectively unquenched relative to the free Ln3+ ion due to the weak influence of the ligand field on the energies of the corelike 4f orbitals.15 The magnetic properties of Ln-SMMs are typically confined to the electronic ground multiplet, which is composed of strongly coupled spin (S) and orbital (L) angular momenta, giving rise to a total angular momentum, J. By analogy to giant-spin transition-metal SMMs, the J value of the ground electronic state gives rise to [2J + 1] mJ microstates, which are perturbed by a small but significant ligand field effect (see below). For lanthanides with 4f orbitals that are more than half-filled, mJ = ±J should be the lowest in energy, and the anisotropy results in one mJ state being preferentially populated at low temperatures, with repopulation of the other mJ state being blocked by a thermal energy barrier, in a manner analogous to that described for polymetallic 3d cage SMMs. For the SMM properties to be switched off or, alternatively, for the magnetization to relax, the thermal energy barrier has to be surmounted. Unfortunately, this simple description of the way in which the magnetization in Ln-SMMs relaxes is not an accurate reflection of most experimental systems. First, the ground doublet does not necessarily take a well-defined value of mJ.16 Furthermore, the magnetization can relax through a range of mechanisms whose features depart significantly from the thermal overbarrier process described above.2h Indeed, one of the main problems that currently plagues Ln-SMMs is efficient QTM through the energy barrier within the ground doublet (Figure 3). If ground-state QTM can be blocked by the presence of strong axial anisotropy and a very small transverse component of the anisotropy, then thermally activated relaxation mechanisms become possible. One type of thermally activated relaxation mechanism involves absorption of sufficient energy via phonons to reach the first excited doublet, at which point

Figure 4. Structure of the complex anion [TbPc2]− (3). Color scheme: Tb, green; N, blue.

First, SMM behavior was observed in a complex containing only a single paramagnetic center. Second, the anisotropy barrier is much more substantial than anything observed previously in a transition-metal SMM. Third, the molecular origins of the SMM properties were clearly demonstrated and, indeed, were enhanced through magnetic dilution studies in a diamagnetic host lattice of [NBu4][YPc2], which provides a means of eliminating any cooperative intermolecular interactions. The original terbium phthalocyanine SMM spawned a large number of related complexes, often with even more impressive SMM properties.18 Different lanthanides can be readily inserted into the sandwich structure, and it is also possible to modify the ligand substituents and the redox state of the ligands themselves, by one- or two-electron oxidation to neutral [LnPc2] or to [LnPc2][X] (X = various anions), respectively. This approach allows changes in electronic structure to be investigated for the same (or very similar) molecular structures, and the impact of making such modifications can be substantial. At the time of writing, the current record anisotropy barrier for an SMM of any kind is held by the heteroleptic terbium(III) complex [PcTbPc′] (4), where Pc′ is a phthalocyanine ligand with eight OC6H4-p-tBu substituents, for which Ueff = 652 cm−1 was determined.19 1086

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COT)], with Ln = Tb (5), Dy (6), Ho (7), Er (8), Tm (9).24 Of these sandwich complexes, the Dy, Ho, and Er derivatives were classified as SMMs, and [(η5-Cp*)Er(η8-COT)] (8) (Figure 5) was found to show two thermally activated relaxation processes in zero applied field, with particularly large anisotropy barriers of Ueff = 137 and 224 cm−1 and with τ0 = 8.17 × 10−11 s and τ0 = 3.13 × 10−9 s, respectively. In designing 8, the aim was to use the π-electron density of the ligands to enforce strong axial anisotropy within the Er(III) cations (4I15/2 ground term). However, the solid-state structure of 8 revealed the presence of two unique molecules as a consequence of static crystallographic disorder within the COT ligand, and also showed that the COT and Cp* ligands tilt with respect to each other to generate an angle of 8°, which results in small distortions from ideal axial symmetry. As expected, the distance from the erbium ion to the COT centroid is considerably shorter than that to the Cp* centroid (1.66 vs 2.27 Å). The ac magnetic susceptibility measurements on 8 in zero applied field showed clear temperature- and frequencydependent behavior below 25 K, with the frequency dependence of the out-of-phase susceptibility showing two peaks for each measurement, indicating two relaxation processes. The two relaxation processes are thought to be consistent with the presence of two conformations of 8 in the crystal lattice. Complex 8 also showed magnetization versus field hysteresis loops up to 5 K (scan rate of 550 Oe/min), and the molecular origins of the hysteresis were confirmed by observing similar properties on magnetically dilute samples. A ligand field analysis (using idealized C∞v rather than Cs symmetry) of 8 suggested that the electronic ground state is mJ = ±15/2 and hence that the anisotropy is of the Ising type. This analysis also revealed an energy gap to the first excited state with mJ = ±13/2 of 190 cm−1, which is close to the average of the two experimentally determined anisotropy barriers in 8. A study of the angular dependence of the magnetic susceptibility in single crystals of 8 confirmed that both conformations do indeed possess an easy axis of magnetization and that it coincides with the expected orientation on the basis of molecular symmetry. Ab initio calculations on 8 were also performed, and the direction of the anisotropy axis is similar to the direction determined by angular-resolved magnetometry studies.25 The ac susceptibility measurements on 6 and 7 in zero field revealed that quantum tunneling of the magnetization is an efficient relaxation pathway. In the case of 6, studies on magnetically dilute samples showed clear frequency-dependent maxima in the χ″(T) plot, and application of a 100 Oe static field led to suppression of the QTM in 6, which enabled a small anisotropy barrier of Ueff = 25 cm−1, with τ0 = 7.1 × 10−8 s, to be determined. The QTM processes in 7 could also be suppressed by applying an external field of 6000 Oe to diluted samples, which led to two thermally activated relaxation processes being identified, although the relaxation barriers of Ueff = 23.5 and 17 cm−1, respectively, are much smaller than those in 8.24b In terms of molecular symmetry and structure, the closest parallels between the lanthanide bis(phthalocyanine) SMMs and their organometallic analogues are bis(COT) derivatives of the type [Ln(η8-COT)2]−, and in a very recent study, [Er(η8COT)2]− (10) was shown to be a particularly striking SMM.26 A crystallographic study of the ion-contacted species [(η8COT)Ln(μ:η8:η8-COT)K(18-c-6)] (10a) and of ion-separated [Ln(η8-COT)2][K(18-c-6)(thf)2] (10b) revealed that the COT ligands in 10a experience disorder, resulting in both staggered

The electronic structure of lanthanide ions is complicated, even in cases with “simple” molecular structures such as [LnPc2]n (n = 0, ±1). A series of experimental and theoretical studies has, however, begun to unravel the secrets of Ln-SMMs, and it has been shown that the symmetry and the electrostatic potential of the ligand field play an important role.20 The 4f orbitals have a strong angular dependence, and the anisotropic 4f electron density of a lanthanide(III) cation has a preferred orientation in any given complex. The electrostatic potential generated by the ligand donor atoms can either promote strong anisotropy or promote relatively weak anisotropy. These principles have been illustrated in terms of “oblate” and “prolate” electron density distributions in a recent review;21 it was shown that in complexes of terbium(III) and dysprosium(III) strong anisotropy can be achieved by using axial ligand fields, whereas strong anisotropy can be achieved for erbium with equatorial ligand fields. Ab initio calculations of the g tensors are often used to quantify the anisotropy, and in the case of dysprosium, which is by far the most widely studied 4f ion in this context, SMMs with high anisotropy barriers are often found to possess g tensors close to the Ising limit of gz = 20 with gx = gy = 0.20b In the case of [TbPc2]n, the molecular symmetry is close to D4d, and hence 4-fold axial symmetry has been adopted as one of the design tools for the synthesis of other SMMs, including organometallic versions. However, recent computational studies and single-crystal magnetic susceptibility measurements have shown that the principal molecular symmetry axis may not necessarily coincide with the orientation of the axis of magnetization, suggesting that caution must be exercised when using symmetry-based design approaches to SMMs.21,22 Monometallic Organolanthanide SMMs. Monometallic Ln-SMMs containing organic C-donor ligands are a recent innovation. Two ligands that have been key workhorses in organo-lanthanide chemistry for many years are cyclopentadienide ([Cp]−) and cyclooctatetraene dianion ([COT]2−), and the various synthetic routes to complexes of these ligands are well developed (Scheme 1).23 Particularly in the case of COT, the recognition that 4-fold symmetry in 3 can be beneficial for slow magnetic relaxation has resulted in this ligand being used to develop new SMMs. The first monometallic lanthanide COT complexes to show slow relaxation of the magnetization were [(η5-Cp*)Ln(η8Scheme 1. Common Synthetic Routes to Ln-SMMs Containing COT and/or Cp Ligandsa

a

X = halide, M = Li−K, CpR = Cp ligand with or without substituents, R = broad range of heteroatom-donor ligands. The lanthanide center may also be solvated, depending on the reaction conditions. The aggregation state, z, is variable and is determined largely by steric factors. Specific details of numbered compounds are provided in the text. 1087

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Figure 5. Molecular structure of one conformation of 8, and the corresponding χ″(ν) plot at various temperatures in the range 11−22 K in zero applied field. From ref 24a.

barriers in the range Ueff = 16−30 cm−1. The dynamic magnetic properties of the related ion-separated complex [Li(dme)3][Dy(η8-COT′′)2] ([Li(dme)3][12]) (Figure 7) are similar to those of 11.28

and eclipsed conformations forming, whereas those in 10b produce a complex with near-D8h symmetry (Figure 6).

Figure 6. Molecular structure of one conformation of 10 and the corresponding χ″(ν) values at various temperatures in the range 15 K (blue) to 22 K (red) in zero applied field. From ref 26.

The COT ligands in 10a produce a tilt angle of 2.8°, and the COT ligands in 10b are essentially parallel, yet despite the slight differences in their molecular structures, the magnetic properties of 10a and 10b are essentially identical. The SMM properties of [Er(η8-COT)2]− were designed by considering that the COT π system extends into the equatorial plane around the erbium ion, which enhances the anisotropy of the prolate electron density of the mJ = 15/2 state, ultimately stabilizing it and producing a substantial energy gap to the first excited mJ state. The anisotropy barrier in zero applied field is Ueff = 147(1) cm−1, with τ0 = [8.3(6)] × 10−8 s, and, significantly, M(H) hysteresis loops were observed up to 10 K; hence, the blocking temperature is one of the highest values yet reported. The complicated dynamic magnetism in Ln-SMMs was illustrated by the silyl-substituted complex [Dy(η8-COT′′)2{Li(thf)(dme)}] (11; COT′′ = 1,4-bis(trimethylsilyl)cyclooctatetraenide, dme = 1,2-dimethoxyethane).27 The Dy− C bond lengths in 11 lie in the range 2.6−2.7 Å and the η8COT′′ ligands are mutually staggered. Complex 11 was found to display multiple relaxation mechanisms despite the presence of only one lanthanide center and despite the fact that the lattice contains only one structural conformer of 11. In zero applied field, 11 is an SMM that relaxes via a thermally activated process down to 3.75 K, with Ueff = 12.5 cm−1 and τ0 = 6 × 10−6 s, and below 3.75 K prominent QTM is apparent. In a series of different applied fields, additional thermally activated relaxation processes can be observed, which produce anisotropy

Figure 7. Molecular structures of 12 (top) and 13 (bottom). From ref 28.

The triple-decker complex [{Dy(η8-COT′′)}2(μ,η8:η8COT)] (13) forms in the reaction of 11 with 0.5 equiv of CoCl2.28 The molecular structure of 13 reveals a Dy···Dy distance of 4.14 Å and Dy−COT(centroid) distances of 1.79 Å (terminal) and 2.07 Å (bridging). The terminal ligands are almost parallel (tilt angle of 1.86°), and the angle formed by the three COT centroid points is 177.4°. In a static applied field of 1000 Oe, the temperature dependence of the magnetic 1088

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lanthanide dimers of the general type [(η5-Cp)2Ln(μ-X)] (16) (Cp = cyclopentadienyl and substituted derivatives; X = various p-block donor atoms).30 Up to 2010, surprisingly few compounds fitting the general formula 16 had been subjected to magnetic susceptibility measurements, and no such compounds had been considered as SMMs. The first SMM of the type 16 was also the first organometallic SMM, [(η5Cp)2Dy(μ-bta)]2 (17) (Figure 9).7

susceptibility of 13 and its gadolinium analogue 14 revealed that the metal ions are coupled antiferromagnetically, and in the case of the gadolinium analogue a characteristically small coupling constant of J = −0.448(1) cm−1 was determined (−J formalism). The ac susceptibility measurements on 13 show particularly efficient QTM, and hence the anisotropy barrier is only 6 cm−1 in zero field. Application of an optimum static field of 600 Oe suppresses the QTM and allows three distinct thermally activated relaxation processes to be observed, with anisotropy barriers of Ueff = 16, 13, and 6 cm−1, with τ0 being on the order of 10−6 s. To gain insight into the anisotropy in 12 and 13, ab initio calculations were carried out, and the electronic structures of the unsubstituted model complexes [Dy(η8-COT)2]− (14) and [Dy2(η8-COT)3] (15) were also calculated for comparative purposes, with C8h symmetry at each Dy site. The impact of replacing the SiMe3 substituents with hydrogen atoms is striking: in 12 and 13, the anisotropy axes are tilted significantly relative to the principal molecular pseudosymmetry axis (Figure 8), but in 15 the anisotropy axis effectively coincides with the

Figure 9. Molecular structure of 17.

In 17, each dysprosium center can be regarded as occupying a nine-coordinate environment (on the basis of an η5-Cp ligand occupying three coordination sites), although on the basis of DFT calculations of Mayer bond orders the central nitrogen atoms interact with the dysprosium atoms only very weakly. The in-phase and the out-of-phase susceptibility in 17 in zero field indicate SMM behavior below 12 K. The plot of ln τ vs 1/ T revealed that the relaxation occurs via a thermally activated process down to about 5 K, where an abrupt change to relaxation via QTM is indicated by the relaxation time becoming temperature independent. The anisotropy barrier in zero field was determined to be a modest Ueff = 32.3 ± 1.7 cm−1 with τ0 = 4.5 × 10−7 s. In an applied field of 1000 Oe, the barrier increased to Ueff = 39.3 ± 0.6 cm−1. Within the context of single-molecule magnetism, there seems to be merit in measuring the magnetic properties of starting materials as well as measuring those of the target compounds. This point is illustrated by the SMM properties of [(η5-Cp)2Dy(μ-Cl)]n (n = 2, 18; n = ∞, 19) and [(η5Cp)2Dy(μ-Cl)(THF)]2 (20), which can all be used to synthesize other {Cp2Dy}-containing complexes via salt metathesis reactions.31 The centrosymmetric dimer [(η5Cp)2Dy(μ-Cl)]2 and the coordination polymer [(η5-Cp)2Dy(μ-Cl)]∞ cosublime in a ratio of 3:1, and both show distinct SMM behavior. In zero field, the dimer 18 shows maxima in the plot of χ″(T) below 10 K, and an anisotropy barrier of Ueff = 26.3 ± 0.8 cm−1 with τ0 = 1.4 × 10−6 s was determined. The SMM properties of the coordination polymer 19 are apparent at higher temperatures in the range 15−30 K, and the anisotropy barrier is Ueff = 67.8 ± 1.2 cm−1 with τ0 = 2.8 × 10−6 s (Figure 10). Although the crystal structures of 18 and 19 are very different, the differences in the molecular structures of the {Cp2DyCl} units in the dimer and the polymer are very slight, with the most prominent contrast being the Cl−Dy−Cl angles, which differ by only 10°. Thus, although the slow relaxation is of molecular origin, the contrasting dynamic magnetic behavior of these two species is evidently due to the arrangement of the molecules in the lattice and also the interactions between the dysprosium centers. Application of a static field of Hdc = 5000 Oe has very little effect on the slow relaxation properties of 19, which can be regarded as being

Figure 8. Plot of χ″(ν) at various temperatures in the range 2.5−9 K in an applied field of Hdc = 600 Oe and the ab initio calculated structure of 13 showing the orientation of the main magnetic axis in the ground Kramers doublets. Adapted from ref 28.

principal molecular axis and the two dysprosium ions. The ligand field parameters for 15 were also calculated, with the results showing that the equatorial component of the ligand field is in fact more substantial than the axial component. This situation should stabilize mJ = ±1/2; however, the actual ground state is probably produced from a mixing of several different states, although ultimately the state with mJ = ±15/2 is significantly destabilized. In the case of 13, the calculated g tensors for the ground state are gx = 10.67, gy = 10.63, and gz = 1.35, revealing easy-plane anisotropy with mJ = 1/2. The main magnetic axes in 13 are illustrated in Figure 8. Interestingly, on the basis of the easy-plane anisotropy in 14, efficient SMM properties for the erbium analogues were predicted and, indeed, subsequently found: viz., complexes 10a and 10b. Dimetallic Organolanthanide SMMs. Ln-SMMs containing two lanthanide ions form an extensive series, with some examples displaying large anisotropy barriers.29 The vast majority of dimetallic Ln-SMMs have been synthesized using classical coordination chemistry, and they share a number of structural and chemical features, such as lanthanides with coordination numbers of 7, 8, or 9, and also bridging oxygen donor ligands such as phenolates. Dimetallic Ln-SMMs are clearly an important subset of the larger family, and taking a basic dimetallic structural building block, it is possible to envisage a series of organometallic analogues based on one of the most ubiquitous structural types in organolanthanide chemistry: namely, heteroatom-bridged bis(cyclopentadienyl)1089

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Figure 10. Molecular structures of [Cp2Dy(μ-Cl)]2 (18) and [Cp2Dy(μ-Cl)]∞ (19) and the plot of χ″(T) for a cocrystallized sample of both polymorphs in zero applied field. From ref 31.

coordination sites, with no distinct molecular symmetry axes, and this is apparently amenable to blocking the reversal of the magnetization. In zero applied field, the temperature dependence of χ″ shows unsymmetrical peaks at various frequencies, implying that two overlapping relaxation processes occur in 21. Deconvolution of these peaks enabled an Arrhenius analysis, and hence the anisotropy barriers were determined to be Ueff ≈ 15 cm−1 for the low-temperature process, and Ueff = 65 cm−1 for the high-temperature process with τ0 = 1.04 × 10−7 s. Applying an optimized field of Hdc = 800 Oe increased the anisotropy barrier of the higher temperature process to Ueff = 85 cm−1 as a result of a reduced rate of QTM. In an effort to unravel which relaxation process corresponds to which dysprosium center, ab initio calculations were conducted. The eight-coordinate dysprosium was calculated to have ground doublet g values of gx = 0.0187, gy = 0.0298, and gz = 19.7236, whereas the seven-coordinate dysprosium has gx = 0.0615, gy = 0.137, and gz = 19.2638; hence, both metal centers have strongly axial magnetic moments that are oriented almost parallel to each other, but with a small angle of 4° between them (Figure 11). The calculated energy gap between the ground state and first excited state doublets in the seven-coordinate dysprosium is 94 cm−1, and is 231 cm−1 in the eight-coordinate dysprosium; hence, neither provides a close match to the higher of the two anisotropy barriers determined by experiment in zero field. The barrier of 85 cm−1 determined in an applied field of 800 Oe is closer to one of the two calculated barriers; however, caution must be exercised in assigning the magnetic blocking to one of the two dysprosium centers. One of the new possibilities brought to the SMM field by organometallic chemistry is the access to ligand fields based on softer donor atoms, such as the heavier elements in groups 15 and 16.33 Soft ligands can, in principle, introduce exchange coupling stronger than that which is achievable with harder donor ligands,33a,c and in the case of Ln-SMMs soft ligands could provide an alternative means of modifying single-ion anisotropy. However, the applications of soft p-block donor ligands in SMM studies are still not extensive.34 The centrosymmetric thiolate-bridged dimer [(η5-C5H4Me)2Dy(μSSiPh3)]2 (22) is the first sulfur-bridged SMM, and although 22

consistent with single-chain magnet behavior. In contrast, the applied field has the effect of making χ″(T) in 18 more prominent at lower temperatures, indicating suppression of the QTM. In the thf-solvated dimer 20, the SMM properties are reminiscent of those shown by 20; however, the anisotropy barrier is slightly higher at Ueff = 33.8 ± 0.5 cm−1, with τ0 = 4.0 × 10−7 s. Just as different polymorphs of the same material can give rise to different dynamic magnetic properties, so too can chemically distinct lanthanide ions within the same molecular framework, which further highlights the role of ligand field symmetry and single-ion effects in Ln-SMMs. These principles were illustrated in the hydride-bridged didysprosium SMM [Ln(Me 5 trenCH 2 )(μ-H) 3 Ln(Me 6 tren)][B{C 6 H 3 (CF 3 ) 2 } 4 ] 2 ([21][B{C6H3(CF3)2}4]2), for which two distinct thermally activated relaxations were observed in the ac susceptibility (Figure 11) (Me6tren = tris{2-(dimethylamino)ethylamine).32

Figure 11. Ab initio calculated structure of 21 showing the orientation of the main magnetic axis in the ground Kramers doublets. From ref 32.

The synthesis of [21][B{C6H3(CF3)2}4]2 was achieved by the consecutive reactions of Dy(CH2SiMe3)3 with [Et3NH][B{C6H3(CF3)2}4]2, Me6tren, and then dihydrogen. Although the “halves” of 21 contain seven- and eight-coordinate dysprosium, respectively, static site disorder throughout the lattice renders the crystal structure effectively centrosymmetric (P1̅ space group): hence, the atom labels of Dy(1) and Dy(1A). Both dysprosium centers in 21 occupy very low symmetry 1090

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Figure 12. Plot of χ″(T) at various ac frequencies in the range 0.1−1200 Hz in zero applied field and the ab initio calculated structure of 22 showing the orientation of the main magnetic axis in the ground state Kramers doublets. Adapted from ref 33.

is structurally similar to 18, its SMM properties are much more prominent at higher temperatures.35 In zero applied field, the out-of-phase susceptibility of 22 shows a strong temperature dependence below 40 K, with the maxima in χ″(T) shifting to lower temperature with decreasing ac frequency. The plot of ln τ versus 1/T revealed that the magnetization relaxes via a thermal mechanism down to about 20 K, which produces a substantial anisotropy barrier of Ueff = 133 ± 3.5 cm−1 with τ0 = 2.38 × 10−7 s. The metallocene-based Dy-SMMs 17−20 and 22 have been the subject of ab initio calculations, which were conducted to gain deeper insight into the orientation of the anisotropy and into the mechanism(s) of relaxation.35 A common feature of their electronic structure is that the main anisotropy axes are parallel (expected for Dy sites related by inversion symmetry) and almost perpendicular to the plane of the dysprosium ions and the bridging donor atoms (Figure 12), apparently for electrostatic reasons.20b In each case, the ground Kramers doublet shows significant axial character, as reflected in the calculated principal g values shown in Table 1. The dysprosium

Figure 13. Molecular structure of 24 and the hysteresis plots of magnetization (M) versus magnetic field (H) in the temperature range 2−7 K, using an average sweep rate of 2 mT/s. From ref 35.

Table 1. Principal g Values of the Ground State Kramers Doublets in 17−20 and 22 gx gy gz

17

18

19

20

22

0.0073 0.0884 19.0530

0.0004 0.0009 19.4090

0.0009 0.0015 19.3590

0.0224 0.0479 18.9208

0.0012 0.0019 19.3611

[(Cp*2Ln)(BPh4)], followed by a one-electron reduction with KC8.35 In 23 and 24, the lanthanide ions are exchangecoupled to the diffuse spin density of the μ-[bpym]•− ligand: the gadolinium analogue of 23 and 24 allowed an exchange coupling constant of J = −10 cm−1 to be determined (−2J formalism) between the Gd(III) and the ligand. The strong exchange coupling between the unpaired spin density on the ligand and the lanthanide ions is significant because it introduces a so-called exchange bias, which is capable of significantly enhancing the hysteresis. The origins of this effect can be understood by regarding the radical ligand as providing an additional, “internal” magnetic fieldakin to the application of an external dc fieldthat lifts the degeneracy of mJ pairs and significantly reduces the rate of QTM. In the case of 23, the relaxation is temperature dependent above 3 K and proceeds via an Orbach process, producing an anisotropy barrier of Ueff = 44(2) cm−1 with τ0 = [4(1)] × 10−8 s. The relaxation times measured for 24 were found to be strongly temperature dependent across the full range of the measurement, suggesting that an Orbach mechanism accounts for the relaxation at all temperatures and ac frequencies investigated. The resulting anisotropy barrier for 24 is Ueff = 87.8(3) cm−1 with τ0 = [1.03(4)] × 10−7 s. Although the anisotropy barriers determined for 23 and 24 are surpassed by many other examples, the particular significance of 24 stems from the

ions in 17−20 and 22 are also calculated to be weakly coupled via antiferromagnetic exchange. In the case of 22, the calculated energy gap between the ground state and first-excited state Kramers doublet is 113 cm−1, which is similar to the experimentally determined anisotropy barrier of Ueff = 133 cm−1, suggesting that the magnetization in 22 relaxes via an Orbach mechanism or thermally assisted QTM. The close correlation between measured and calculated energy gaps for 22 are consistent with the very small transverse components of the g tensors. For 17−20, the transverse components of the g tensors were generally found to be much more substantial, which can be invoked to describe the more prominent QTM in these systems, although the values in 18 do not fit this trend. Important examples of metallocene-based SMMs are the dimetallic systems [(Cp*2Ln)2(μ-bpym)][BPh4], where Ln = Tb is [23][BPh4], Ln = Dy is [24][BPh4], and [bpym]•− is the radical anion of bipyrimidyl (Figure 13), both of which were synthesized by the addition of bpym to 2 equiv of 1091

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hysteresis observed in the field dependence of the magnetization, which was observed up to 6.5 K, making it one of the highest blocking temperatures yet recorded. Although not organometallic compounds in the strictest sense, lanthanide amides can be regarded as members of the extended organometallic family. The dimetallic compounds [K(18-crown-6)(thf)2][Ln2{N(SiMe3)2}4(thf)2(μ,η2:η2-N2)], in which Ln = Gd (25), Tb (26) (Figure 14), Dy (27), Ho

substances,38 but, despite these issues, complexes of 5f elements still offer attractive prospects for developing SMMs. One feature of the electronic structure of the early actinides that lends itself to observing slow relaxation of the magnetization is the very strong spin−orbit coupling and substantial magnetic anisotropies associated with, for example, uranium in a range of formal oxidation states. Furthermore, the greater degree of covalent character in the actinide−ligand bonds in polymetallic systems (relative to analogous systems based on lanthanides) can enable stronger exchange coupling, which can generate larger magnetic moments and lead to greater energy separations between the ground and first excited sublevels within the spin− orbit coupled ground state. In addition to the organometallic An-SMMs discussed below, other examples based on uranium coordination chemistry are also known.39 The SMM properties of neptunocene, [Np(η8-C8H8)2] (31), were reported in 2011,40 some 40 years after the synthesis was originally published. The Np(IV) ion in 31 occupies a D8hsymmetric environment, and the 5f3 ion features a ground state of J = 9/2 with Jz = ±5/2 as the ground state doublet. The strong magnetic anisotropy in 31 was indicated by the magnetization not reaching saturation even in a field of 14 T at 1.8 K. In the high-field regime of the M(H) plot, hysteresis was observed (Figure 15).

Figure 14. Structure of [Tb2{N(SiMe3)2}4(thf)2(μ,η2:η2-N2)]− (26) and plot of magnetization versus field using a sweep rate of 0.9 mT/s. From ref 36b.

(28), Er (29), are intrinsically fascinating because the lanthanide centers are bridged by the unusual radical trianionic ligand [N2]3−, which forms as a result of consecutive reductions of dinitrogen by [Ln{N(SiMe3)2}3]/KC8.36 In the related yttrium salt [K(thf)6][Y2{N(SiMe3)2}4(thf)2(μ,η2:η2-N2)] ([K(thf)6][30]), the X-band EPR spectrum revealed that the unpaired spin density produces hyperfine coupling of 3.1 G with the 89Y (I = 1/2) centers, and 5.8 and 8.2 G with the bridging ligand 14N (I = 1) and 15N (I = 1/2) atoms, respectively.37 These observations indicated that the radical ligand could also enable much stronger exchange coupling between the rare earth ions and the bridging ligand, leading to an exchange bias and thus to an enhancement of the hysteresis. The terbium, dysprosium, and holmium versions 26−28 produced anisotropy barriers of Ueff = 227.0(4), 123, and 73(6) cm−1 in zero field, with τ0 = [8.2(1)] × 10−9, τ0 = 8 × 10−9, and τ0 = [3(3)] × 10−8 s, respectively. The erbium version generated an anisotropy barrier of Ueff = 36(1) cm−1 with τ0 = [4(2)] × 10−11 s only when a field of Hdc = 1000 Oe was applied. Although 26 and 27 possess substantial anisotropy barriers, their most eye-catching properties are their M(H) hysteresis loops, as result of the exchange bias. By defining the blocking temperature as the temperature at which the relaxation time (τ) is 100 s, 26 achieves an unprecedented blocking temperature of 13.9 K, and using the same definition, 27 has a blocking temperature of 6.7 K, with both systems showing much slower relaxation of the magnetization than any previous systems based on transition metals. Aside from the clear record-breaking properties of 26 and 27, an important general implication of these two SMMs is that they clearly illustrate the power of organometallic synthesis and how it can potentially be adapted to produce new physics. Organometallic chemistry of this type has considerable potential for wider exploitation in the SMM field.

Figure 15. Molecular structure of [Np(η8-C8H8)2] (31) (inset) and the field dependence of the magnetization at T = 1.8 K. From ref 40.

Below 60 K in an applied field of 5000 Oe, the χ″(T) plots for 31 are frequency dependent and show maxima at lower temperatures, which is thought to indicate weak dipolar exchange between the neptunium(IV) centers. The Arrhenius analysis of the ac susceptibility data revealed that multiple frequency-dependent relaxation modes occur in 31, and in a field of 3000 Oe the plot of ln τ vs 1/T is linear, which gives an anisotropy barrier of Ueff = 28.5 cm−1 with τ0 = 1.1 × 10−5 s. The relatively fast relaxation of 31 in fields of less than 2000 Oe was interpreted in terms of hyperfine interactions with the 237 Np nuclei, which possess I = 5/2 and can therefore open efficient relaxation pathways. At higher fields where hysteresis is observed, relaxation can occur either via a direct process involving the ground states with opposite values of Jz, or via an Orbach process. The multiple relaxation pathways observed in 31 are reminiscent of those observed in complexes 11−13. The SMM properties of complexes containing uranium(III) are apparently not strongly dependent on the symmetry of the ligand field or on the nature of the ligand donor atoms. This general principle was determined from a study of the dynamic magnetic properties of complexes 32−34, which contain

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ORGANOMETALLIC ACTINIDE SMMs SMMs based on the actinide (An) elements are rare. Transuranic SMMs are particularly uncommon, presumably for the obvious reasons associated with handling highly radioactive 1092

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uranium(III) in C2v-, C3v- and C1-symmetric environments, respectively (Figure 16).41

at temperatures that varied slightly with ac frequency, although an anisotropy barrier could not be determined. Although this review is devoted to organometallic SMMs, the section on actinide SMMs would seem incomplete without specific mention of the uranium(V)/manganese(II) macrocycle [{UO2(salen)}2Mn(py)3]6 (36), which not only has a stunning molecular structure (Figure 19) but also shows dynamic magnetic properties that are unprecedented for an An-SMM.43 Below 4 K, 36 displays open hysteresis loops, and at 2.25 K a coercive field of about 1.5 T was recorded. Furthermore, at 2.5 K clear steps in the hysteresis loops for 36 were observed, thus indicating resonant QTM. In zero dc field, the in-phase and the out-of-phase susceptibility reveal clear SMM properties, and the Arrhenius analysis produced an anisotropy barrier of Ueff ≈ 100 cm−1.

Figure 16. N″ = N(SiMe3)2.

In zero field, complexes 32−34 do not display any M(H) hysteresis, nor do the maxima in χ″ show any temperature dependence. In moderate applied fields of 500−3000 Oe, each complex shows maxima in the χ″(T) plot, which allowed anisotropy barriers of Ueff = 12.9 ± 0.5, 21.5 ± 2, and 16.3 ± 0.6 cm−1 to be determined for 32−34, respectively, with τ0 values of [6.4 ± 1.8] × 10−7, 1 × 10−11, and [2.9 ± 0.8] × 10−7 s, respectively (Figure 17). The linearity of the ln τ vs 1/T plots

■

LOW-COORDINATE TRANSITION-METAL SMMs In terms of new chemical approaches to SMMs, one of the most intriguing current trends is concerned with monometallic complexes of 3d transition metals.44 This approach combines two of the most important general observations from studies of the two established types of SMM: i.e., those based on polymetallic transition-metal complexes and those based on lanthanides. Thus, the primary focus is not on generating very large spins but on maximizing the single-ion anisotropy. Monometallic 3d metal complexes with large anisotropy have been known for many decades,6 but investigations of their dynamic magnetic properties have not been pursued until recently. Attention has begun to focus on 3d metals in lowcoordinate environments and/or conformationally well defined ligands with predictable coordination chemistry. Such ligands allow the targeted formation of complexes in which the symmetry of the coordination environment can be controlled and manipulated in order to produce d orbital configurations with large, unquenched orbital contributions to the magnetic moment, which provides the entry point for significant magnetic anisotropy and hence studies of their dynamic magnetism. The first example of field-induced slow relaxation in a monometallic transition-metal complex was a trigonal-pyramidal high-spin iron(II) (S = 2) complex of a mesityl-substituted tris(2-pyrrolylmethyl)amine, [(tpaMes)Fe]− (37) (Figure 20).45 A theoretical fit of the field dependence of the magnetization produced the desired large axial ZFS parameter of D = −39.6 cm−1, with a rhombic parameter of E = −0.4 cm−1 and with g = 2.21. Using a simple ligand field approach to the d orbital splittings in trigonal-pyramidal symmetry, the degenerate dxz and dyz orbitals are lowest in energy and contain three electrons, which accounts for the large D value, and hence the theoretical anisotropy barrier should be Ueff = |D|S2 = 158 cm−1. In zero applied field, 37 displayed no SMM properties; however, in an applied field of Hdc = 1500 Oe the maxima in χ″(ν) shifted to higher frequencies with increasing temperature, which led to a barrier of Ueff = 42 cm−1 with τ0 = 2 × 10−9 s. The fact that 37 does not show SMM properties in zero field indicates efficient QTM between pairs of ±MS states, which is probably enabled by the transverse component of the anisotropy. In terms of the molecular structure, a slight deviation from strict axial symmetry is reflected in the Fe− N(pyrrolyl) bond distances, which are in the range 2.008(3)− 2.041(2) Å. Application of a 1500 Oe field is evidently sufficient to lift the degeneracy of the ±MS pairs and reduce the rate of QTM and so to enable observation of a thermal relaxation

Figure 17. Plots of χ′(T) and χ″ (T) for 32−34 (left to right) in an applied field of Hdc = 2000 Oe, using ac frequencies in the range 1− 1488 Hz (increasing left to right). From ref 41.

suggested that the QTM was efficiently suppressed. Magnetic dilution studies of frozen THF solutions in applied fields showed that the slow relaxation behavior persists, although the anisotropy barriers decrease somewhat relative to the undiluted polycrystalline samples. Notably, fast QTM was also observed in the magnetically dilute samples in zero applied field; however, the absence of any significant intermolecular interactions and hyperfine coupling (238U has I = 0) leaves no obvious explanation for this phenomenon. In the arene-bridged diuranium complex [{U(BIPMtmsH)(I)}2(μ:η6:η6-C6H5CH3)] (35) (Figure 18), where BIPMtms = C(PPh2NSiMe3)2, M(H) hysteresis was observed at 1.8 K, although it was not possible to measure a coercive field.42 Fieldinduced slow relaxation of the magnetization in 35 was observed with Hdc = 1000 Oe, with χ″(T) just showing maxima

Figure 18. 1093

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Figure 19. Molecular structure of 36 (color scheme: U, green; Mn, purple; O, red) and the field dependence of the magnetization at T = 2.25 and 4 K (sweep rate of 4 mT/s). From ref 43.

metalation of the corresponding phenol by [Fe{N(SiMe3)2}2] in the case of 42 (Scheme 2).47 Crystallographic studies of 38− Scheme 2a

Figure 20. Molecular structure of 37 (color scheme: Fe, orange; N, blue) and simplified d orbital splitting diagram. From ref 45a.

process. Changing the mesityl substituents in 37 to other organic groups can produce changes in the dynamic magnetic properties of the corresponding iron(II) complexes, with infield anisotropy barriers up to 65 cm−1 being achievable.45b Two-Coordinate and Three-Coordinate 3d SMMs. Two-coordinate and three-coordinate transition-metal complexes have attracted considerable fundamental interest for many years.46 Two-coordinate complexes offer particular promise for development as SMMs, particularly in instances where the X−M−X bond angle is close to 180°. In linear twocoordinate complexes, the principal molecular symmetry axis is, to a first approximation, of the C∞ type, meaning that d-orbital degeneracy should occur and, with an appropriate d-electron count, the ensuing first-order orbital angular momentum should generate large magnetic anisotropy. This situation can be represented using a simple ligand-field splitting diagram, which shows that the lowest-lying degenerate d orbitals are dxy and dx2−y2 and that these orbitals are nonbonding. Using this model in the case of, for example, high-spin d6 configurations, the origin of the anisotropy becomes clear (Figure 21); however, factors such as mixing of the s and d orbitals can affect the ordering of the energy levels. The first two-coordinate compounds to be targeted as SMMs were the iron(II) complexes 38−43, which can be synthesized from reactions of the lithium salts of the appropriate bulky proligand with FeX2 in the case of 38−41 and 43 or by

a

Abbreviations: Dipp, 2,6-diisopropylphenyl; Mes, mesityl.

42 revealed that the iron(II) centers do indeed occupy linear environments, with crystallographically imposed X−Fe−X angles of 180°. In contrast, 43 has an N−Fe−N angle of 140.9(2)°. The Fe−X bond lengths vary significantly, with the shortest distance of 1.853(1) Å found in 38 and the longest distance of 2.045(5) Å found in 39. The dc magnetic susceptibility measurements confirmed the expected S = 2 configurations in each case; however, the corresponding g value for each complex determined from these experiments deviates significantly from the free-electron g value of 2.0023 (Table 2). For linear 38−42, the g values are higher than the freeelectron g value, whereas for bent 43 the g value is slightly less. The presence of significant magnetic anisotropy was confirmed in 38−41 and 43 by measuring the field dependence of the Table 2. Selected Parameters for 38−43a

38 39 40 41 42 43

X−Fe−X/ deg

Fe−X/Å

g

180 180 180 180 180 140.9(2)

1.853(1) 2.045(5) 1.893(1) 1.902(1) 1.947(1) 1.911(3)

2.44 2.53 2.23 2.13 2.06 1.96

Ueff/cm−1 (Hdc/ Oe)

ΔE/ cm−1 b

181 (500) 146 (500) 109 (1800) 104 (875) 43 (2500)

191 196 178 161 185 82

a

Data are taken from ref 47a. bEnergy separation (ΔE) between 1E ground state and the first-excited states (A1, 1A2).

Figure 21. Simple d orbital splitting diagram for linear, two-coordinate Fe(II). 1094

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the presence of the cryptand 222-crypt, to give [K(222crypt)][Fe{C(SiMe3)3}2] ([K(222-crypt)][44]) (Figure 23), in

magnetization, but surprisingly a similar measurement was not reported on 42, for reasons that remain unclear. None of the six two-coordinate iron(II) complexes displayed SMM properties in zero field, which was assigned to rapid QTM within the magnetic ground state between the degenerate +MS and −MS levels. However, application of small dc fields to 38−42 did induce slow relaxation of the magnetization, with temperaturedependent maxima being observed in χ″(ν) (Figure 22).

Figure 22. Out-of-phase susceptibility versus ac frequency for 38 at various temperatures in the range 2−17 K. From ref 47a.

Studying the field dependence of the relaxation time (τ) in 38−42 suggested that, in weaker applied fields, the QTM is efficiently suppressed and that the magnetization is reversed by an Orbach mechanism. At higher fields a rapid direct relaxation mechanism occurs again. In 38−42, the plot of ln τ vs 1/T is only linear over a short range of relatively high temperatures, providing further evidence of an Orbach relaxation process. At lower temperatures, significant curvature in the Arrhenius plot becomes apparent, which pointed to magnetization reversal via direct and Raman mechanisms. Modeling the temperature dependence of the relaxation time allowed the anisotropy barriers shown in Table 2 to be extracted, with the value of Ueff = 181 cm−1 (with τ0 = 10−11 s) for 38 being particularly substantial. A detailed theoretical analysis of the electronic structure in 38−43 showed that the lone pairs on the donor atoms lower the effective symmetry of the complexes, such that describing the molecular symmetry as D∞h is not strictly accurate (although complex 39 has no ligand-derived lone pairs).47a The key consequence of the reduced symmetry is to lift the degeneracy of the dxy and dx2−y2 orbitals and so to diminish the anisotropy, leading to more rapid relaxation of the magnetization. In the case of 43, the energetic separation between dxy and dx2−y2 was found to be significant and was attributed as the cause of the lack of slow relaxation behavior under all conditions studied. Ab initio calculations on 38−43 revealed separations (ΔE) between the 1E ground state and the firstexcited states (A1, 1A2) that could in principle correlate with the anisotropy barrier, assuming relaxation via an Orbach mechanism. Whereas the correlation is close for 38, in others the agreement is not as accurate, which suggests that other factors influence the spin reversal processes, such as molecular vibrations. The rapid QTM observed for 38−43 in zero applied field is due partially to mixing of the MS states within the ground state of the non-Kramers ion iron(II), whereas in a Kramers ion such as iron(I) the degree of MS mixing should be significantly diminished. This theoretical underpinning provided the inspiration for the one-electron reduction of 39 by KC8 in

Figure 23. Molecular structure of the iron(I) complex anion [Fe{C(SiMe3)3}2]− (44) and the energies of the 3d orbitals from ab initio calculations (top) and a plot of χ″(ν) at various temperatures in the range 9−29 K in zero applied field (lower). Taken from ref 48.

which the iron(I) S = 3/2 center enables a test of Kramers vs non-Kramers hypothesis.48,49 The complex anion 44 features an essentially linear coordination environment for iron(I), with an C−Fe−C angle of 179.2(2)°, although in contrast to 39 the substituents on the two trisyl ligands are mutually eclipsed (Figure 22). The 57Fe Mössbauer spectra of 39 and 44 at 295 K confirm the change in oxidation state, with isomer shift and quadrupole splitting of δ = 0.39(1) mm/s and ΔEQ = −1.08(1) mm/s for 39, respectively, and δ = 0.278(4) mm/s and ΔEQ = −2.520(7) mm/s for 44. A ligand-field analysis of 44 revealed that the d orbitals split according to Figure 23, with the weak overall splitting arising from the low-coordinate environment and the formal +1 oxidation state. The dz2 orbital was calculated to be the lowest energy orbital with significant d character because of extensive mixing with the 4s orbital. Thus, the anisotropy within the d7 configuration in 44 arises from the near-degeneracy of the dxy and dx2−y2 orbitals. For 44, the ac susceptibility measurements in zero field and the field dependence of the magnetization clearly demonstrate the SMM properties. The χ″(ν) plots feature maxima that are strongly temperature dependent in the range 9−29 K (Figure 24). The Arrhenius analysis showed that the variation of ln τ with 1/T is linear down to 20 K, and at lower temperatures the data deviate from linearity. At higher temperatures, the spin reversal in 44 is characterized by an Orbach mechanism, with a large anisotropy barrier of Ueff = 226(4) cm−1 with τ0 = [1.3(3)] × 10−9 s, which, at the time of writing, is the largest ever recorded for a nonlanthanide SMM. The Orbach relaxation process for 44 was probed by ab initio calculations, which revealed an energy 1095

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Figure 24. Molecular structure of 45 and the corresponding χ″(ν) values at various temperatures in the range 1.8−9 K (blue → red) in an applied field of Hdc = 600 Oe. From ref 50.

separation between ground state and first excited state MJ doublets of 210 cm−1, which is a close match to the experimental Ueff value. Hence, at higher temperatures the magnetization in 44 is reversed by a thermally activated process from the ground MJ = 7/2 levels via the first-excited MJ = 5/2 levels, and below 20 K the reduced temperature dependence of the relaxation time indicates the emergence of tunneling within the ground state. The M(H) hysteresis in 44 was observed below 4.5 K, although the loops all converge at M = 0 in zero field, meaning that remnant magnetization is absent and that tunneling processes are prominent in zero field at these temperatures. Conducting the ac susceptibility measurements on 44 in frozen 2-methyltetrahydrofuran solutions produced results similar to the solid-state measurements above 8 K. At a concentration of 4 mM, the relaxation times are approximately double those of the polycrystalline material, implying that intermolecular dipolar interactions can influence the magnetization dynamics, although slight structural distortions away from axial symmetry may also contribute. The Ni(I) N-heterocyclic carbene complex [Ni(6-Mes)2]+ (45) (Figure 24), which forms as the bromide salt [45][Br], only displays field-induced slow relaxation of the magnetization.50 The linear coordination environment in 45 is close to ideal, with a C−Ni−C angle of 179.27(13)°, and although the NHC donor atoms are not related by crystallographic symmetry the Ni−C bond lengths of 1.939(3) and 1.941(3) Å are essentially identical. The dc susceptibility measurements and the field dependence of the magnetization in 45 indicated the presence of significant magnetic anisotropy. Although the ac susceptibility in zero field did not reveal characteristic SMM features, application of a small field of Hdc = 600 Oe did result in frequency and temperature dependence of χ′ and χ″, which was the first time such an observation had been made on a monometallic nickel complex. The resulting Arrhenius analysis of the relaxation times yielded an anisotropy barrier of Ueff = 12 cm−1 with τ0 = 4.6 × 10−6 s. The origins of the unexpected fast QTM within the ground state in zero field are not immediately obvious, although examination of the crystal structure reveals that the bromide anion could bring two molecules of 45 into sufficiently close proximity (Ni···Ni distance of 10.3 Å) to allow intermolecular dipolar fields to influence the relaxation behavior. The cyclopentadienyliron(II)−aryl complex [(η5-C5iPr5)Fe(2,6-Dipp)] (46) (Figure 25) provides yet another intriguing example of a monometallic 3d complex showing field-induced slow relaxation of the magnetization.51 The molecular structure of 46 features an Fe−Cpcent distance of 1.93 Å, which is much longer than would be expected for a low-spin cyclo-

Figure 25.

pentadienyliron complex. The key structural feature that relates 46 to 45 and 44 is that the angle formed by the Cp centroid, the iron(II) center, and the aryl C donor atom is 177°. Modeling the static magnetic susceptibility of 46 duly showed that the Fe center has an S = 2 configuration with g = 2.29 and that significant axial anisotropy is present, as indicated by the large negative value of the axial ZFS parameter: i.e., D = −51.36 cm−1 (E/D = 0.006). SMM properties were not observed for 46 in zero field, but in various dc fields up to 3000 Oe slow relaxation was observed, with more than one process operating at Hdc = 1500 Oe. The Arrhenius analysis of the data for T = 10−13 K with Hdc = 750 Oe produced an anisotropy barrier of Ueff = 28 cm−1 with τ0 = 6.0 × 10−6 s, and the data for T = 12−15 K with Hdc = 2500 Oe gave Ueff = 100 cm−1 with τ0 = 7.8 × 10−6 s. The relaxation process with the lower anisotropy barrier is thought to involve tunneling within the ground doublet with MS = ±2, and the second process with the larger barrier is thought to involve an Orbach process via the MS = ± 1 pair. At lower temperatures, the relaxation in 46 is characterized by processes that shortcut the thermal barrier(s), which is due to the presence of a small but significant transverse component of the anisotropy (E = −0.32 cm−1). Three-coordinate, trigonal-planar iron(II) complexes of the type [LFeX2] can possess large axial zero-field splittings52 and are therefore candidates for SMM studies. These threecoordinate complexes can often be synthesized readily by the addition of L to a suitable Lewis acidic FeX2 precursor complex (X = various organometallic ligands), meaning that many examples with a range of ligands L can be obtained. One such complex, [(Cy3P)Fe{N(SiMe3)2}2] (47; Cy = cyclohexyl) (Figure 26), shows field-induced slow relaxation of the magnetization, with Ueff = 29 cm−1 and τ0 = 6 × 10−7 s in an optimum applied field of Hdc = 600 Oe.53 In the static field magnetic measurements, the anisotropy in 47 was apparent from the room-temperature value of χMT being significantly larger than the value expected for a spin-only iron(II) complex, and also from the nonsuperimposability of the reduced magnetization curves at low temperatures. From 1096

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bridging ligand in 48 are similar to those in the corresponding neutral precursor compound, indicating that the unpaired spin density is delocalized across the dmp2Nin ligand. The radical anion complex 49 contains two three-coordinate cobalt(II) centers in approximate trigonal-planar environments, and the unpaired spin density on the ligand is delocalized across the unsaturated portion of the ligand, between the two NAr donor groups (Ar = 2,6-Me2C6H3). In zero applied field, neither 48 nor 49 shows any response due to rapid QTM. In an applied field of 1200 Oe, 48 produces Ueff = 26 cm−1 and τ0 = 2.2 × 10−6 s, and 49 produces Ueff = 23 cm−1 and τ0 = 1 × 10−9 s. In the case of 49, increasing the applied field to 6000 Oe allows two relaxation processes to be observed. Examining the electronic structure of 48 and 49 with ab initio calculations furnished considerable insight into their dynamic magnetic properties. Treating the dmp2Nin ligand in 49 as an S = 1/2 spin revealed antiferromagnetic coupling between the cobalt(II) centers and the ligands, with Jex = −132.74 cm−1, and overall ferromagnetic coupling between the cobalt centers, with Jex = +14.51 cm−1. The ground-state g tensors calculated for 49 revealed strongly axial character (gz = 16.9) but also an appreciable transverse component (gx, gy = 0.3, 0.6), which explains the absence of SMM character in zero field and can also explain the field-induced slow relaxation. In 48, fitting of the experimental magnetization versus field data produced exchange couplings between cobalt and nindigo of Jex = −138 cm−1 and between the two cobalt(II) centers of Jex = −3.5 cm−1. The main components of the ground-state g tensors in 48 were found to be 0.08 and 0.09 (transverse) and 12.3 (axial), which should result in less efficient ground-state QTM and hence can be invoked to explain why 48 has a slightly higher anisotropy barrier than 49.

Figure 26. Structure of 47 and TD-DFT calculations of ground and first excited states, with spin density plots. The x axis coincides with the Fe−P bond, and the z axis is perpendicular to the trigonal plane.

these data, an axial ZFS parameter of D = −10.9 cm−1 was determined, implying a theoretical anisotropy barrier of Ueff ≈ 43.6 cm−1, which is greater than the measured anisotropy barrier owing to a reported rhombic contribution to the ZFS, which enables mixing of the wave functions for the various MS sublevels and hence leads to QTM. The case of the magnetic anisotropy in 47 is intriguing because the symmetry of the ligand field precludes any contributions to the magnetic moment arising from first-order spin−orbit coupling. The origins of the anisotropy in 47 were studied by time-dependent DFT, which revealed the presence of two relatively low-lying excited states at 2400 and 5600 cm−1 above the ground state. Since the ground-state β spin is predominantly iron 3dxz in character and the first and second excited states are predominantly 3dy2−z2 and 3dyz, respectively, the orbital contribution should originate from second-order spin−orbit coupling. The impact of using radical ligands on factors such as the anisotropy barrier and the blocking temperature in Ln-SMMs have been clearly demonstrated by 26 and 27. Although radical ligands such as nitronyl nitroxides have been used in molecular magnetism for many years,54 the use of other types of radical ligands in 3d SMMs is not widespread. The multiple redox states available to nindigo (Nin) ligands make them fascinating candidates for applications in SMMs. A recent investigation of the cobalt(II) complexes 48 and 49, which contain the bis(2,6dimethylphenyl)nindigo (dmp2Nin) ligand, showed that in instances where the nindigo ligand was present either as a radical monoanion (48) or as a radical trianion (49) fieldinduced slow relaxation of the magnetization can be observed (Figure 27).55 In 48, the cobalt(II) centers occupy distorted-tetrahedral coordination environments, and the bond lengths in the

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OUTLOOK Ligands that have formed part of the staple diet of synthetic organometallic chemists, in some cases for many years, are beginning to make an impact in single-molecule magnetism with d-block metals and f-block metals. In some instances, such as 26 and 44, the impact has been profound and has expanded the frontiers of the field. Indeed, the composition of SMMs such as 26 and 44 provide important clues as to how the field may evolve in the future. It is clear that, irrespective of the synthetic approach, rapid quantum tunneling of the magnetization is a major problem: the exchange bias introduced by radical ligands in polymetallic systems such as the diterbium SMM 26 efficiently reduces the rate of QTM and therefore there is merit in investing effort into developing new types of strongly exchange coupled Ln-SMMs, possibly based on ligands with soft donor atoms. The properties of the iron(I) SMM 44 also suggest that low-coordinate, high-symmetry environments should also be developed more extensively. However, such systems should not just focus on high-spin transition metals but also on low-coordinate, high-symmetry lanthanide coordination environments. The challenges associated with the synthesis of, for example, a linear two-coordinate lanthanide will be truly formidable, but the rewards in terms of SMM performance may well be worth the effort. It is important to emphasize that organometallic approaches to SMMs are not necessarily “better” than classical coordination chemistry, but the different electronic properties of many organometallic ligands, and the ways in which they can stabilize unusual coordination environments, have provided interesting new ways of testing established theories in molecular

Figure 27. Ar = 2,6-Me2C6H3. 1097

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Organometallics magnetism. It is also important to highlight that all the organometallic SMMs discussed in this review are sensitive to oxygen and moisture (unlike most SMMs synthesized using classical coordination chemistry), which only adds to the list of complications associated with the development of device applications. However, to end on a positive note, the key to developing the full potential of coordination chemistry for SMM synthesis will be the continued successful collaborations among physicists, theoreticians, and synthetic chemists from a broad range of traditional scientific backgrounds.

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ABBREVIATIONS

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REFERENCES

Review

SMM, single-molecule magnet; QTM, quantum tunneling of the magnetization; ZFS, zero-field splitting; Ueff, anisotropy barrier, or effective energy barrier to reversal of the magnetization; TB, magnetic blocking temperature

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AUTHOR INFORMATION

Corresponding Author

*E-mail for R.A.L.: Richard.Layfi[email protected]. Notes

The authors declare no competing financial interest. Biography

Richard A. Layfield graduated with a Master’s degree in Chemistry from the University of Leeds and subsequently obtained his Ph.D. at the University of Cambridge, working under the supervision of Prof. D. S. Wright on metal cyclopentadienide chemistry. After spells as a Junior Research Fellow and fixed-term Lecturer in Cambridge, he was appointed to a Lectureship in Inorganic Chemistry at The University of Manchester in 2007. In 2013, he was promoted to Reader in Inorganic Chemistry. His research interests encompass a range of topics in organometallic chemistry, with emphasis on applications in molecular magnetism, low-coordinate iron−NHC chemistry, and also s-block organometallics. He has been awarded the RSC Meldola Medal and Prize (2006), a Fellowship for Experienced Researchers by the Alexander von Humboldt Foundation (2010−2012), and the 2013 Sir Edward Frankland Fellowship of the RSC. He was also elected a Fellow of the Royal Society of Chemistry (FRSC) in 2013.

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ACKNOWLEDGMENTS The author gratefully acknowledges the contributions made to his research in molecular magnetism by his students and postdocs, past and present: Richard Grindell, Sara King, Dr. Thomas Pugh, Dr. Charlene A. Smith, Dr. Scott A. Sulway, and Dr. Daniel N. Woodruff. The value of the insight provided by our collaborators is incalculable, and sincere thanks are due to Prof. Reiner Anwander, Prof. Liviu Chibotaru, Nicholas Chilton, Prof. David Collison, Sonja König, Prof. Eric McInnes, Prof. Jun Okuda, Dr. Floriana Tuna, Dr. Liviu Ungur, Dr. Ajay Venugopal, Prof. Wolfgang Wernsdorfer, and Prof. Richard Winpenny. 1098

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NOTE ADDED IN PROOF Very recently, the cerium(III) complex [Ce(η8-COT)2]− was reported. This complex shows field-induced slow relaxation, and produces an anisotropy barrier of Ueff = 21 cm−1 in an applied field of Hdc = 400 Oe.56

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dx.doi.org/10.1021/om401107f | Organometallics 2014, 33, 1084−1099