A Cubic Tinkertoy-like Heterometallic Cluster with a Record Magnetocaloric Effect.

Clusters showing a giant magnetocaloric effect (MCE) are of interest as molecular coolants for magnetic refrigeration. Herein, we report two heterometallic clusters, denoted as Gd152Ni14@Cl24 and Sm152Ni8, just to highlight their inorganic core motifs, obtained by ligand-controlled co-hydrolysis of Ni2+ and Ln3+ (Ln = Gd, Sm) in the presence of N-(2-hydroxyethyl)iminodiacetic acid (H2HEIDA). Both clusters display fascinating cubic Tinkertoy-like structures, with the core motifs being built of multiple metallic shells of Platonic and Archimedean polyhedra. The isothermal magnetic entropy change─a direct measurement of MCE─was determined to be 52.65 J·kg-1·K-1 at 2.5 K and 7.0 T for the Gd-containing cluster; this value is the highest known for any molecular clusters so far reported.

A tetravalent praseodymium complex with field-induced slow magnetic relaxation.

Herein the synthesis, structural characterization, and magnetic properties of a Pr(IV) complex [Pr(OSiPh3)4(L)] (1, L = 4,4'-dimethoxy-2,2'-bipyridine) are reported. The stability of the Pr(IV) complex significantly enhanced with the use of the bidentate ligand L. Slow magnetic relaxation was observed at low temperatures, indicating that the complex may be the first single-ion magnet with a tetravalent lanthanide ion being the magnetic center.

Substituent Positioning Effects on the Magnetic Properties of Sandwich-Type Erbium(III) Complexes with Bis(trimethylsilyl)-Substituted Cyclooctatetraenyl Ligands.

Lanthanide complexes with judiciously designed ligands have been extensively studied for their potential applications as single-molecule magnets. With the influence of ligands on their magnetic properties generally established, recent research has unearthed certain effects inherent to site differentiation due to the different types and varying numbers of substituents on the same ligand platform. Using two new sandwich-type Er(III) complexes with cyclooctatetraenyl (COT) ligands featuring two differently positioned trimethylsilyl (TMS) substituents, namely, [Li(DME)Er(COT1,5-TMS2)2]n (Er1) and [Na(DME)3][Er(COT1,3-TMS2)2] (Er2) [COT1,3-TMS2 and COT1,5-TMS2 donate 1,3- and 1,5-bis(trimethylsilyl)-substituted cyclooctatetraenyl ligands, respectively; DME = 1,2-dimethoxyethane], and with reference to previously reported [Li(DME)3][Er(COT1,4-TMS2)2] (A) and [K(DME)2][Er(COT1,4-TMS2)2] (B), any possible substituent position effects have been explored for the first time. The rearrangement of the TMS substituents from the starting COT1,4-TMS2 to COT1,3-TMS2 and COT1,5-TMS2, by way of formal migration of the TMS group, was thermally induced in the case of Er1, while for the formation of Er2, the use of Na+ in the placement of its Li+ and K+ congeners is essential. Both Er1 and Er2 display single-molecule magnetic behaviors with energy barriers of 170(3) and 172(6) K, respectively. Magnetic hysteresis loops, butterfly-shaped for Er1 and wide open for Er2, were observed up to 12 K for Er1 and 13 K for Er2. Studies of magnetic dynamics reveal the different pathways for relaxation of magnetization below 10 K, mainly by the Raman process for Er1 and by quantum tunneling of magnetization for Er2, leading to the order of magnitude difference in magnetic relaxation times and sharply different magnetic hysteresis loops.

Electron Paramagnetic Resonance Spectra of Pentagonal Bipyramidal Gadolinium Complexes.

Gadolinium is a special case in spectroscopy because of the near isotropic nature of the 4f7 configuration of the +3 oxidation state. Gd3+ complexes have been studied in several symmetries to understand the underlying mechanisms of the ground state splitting. The abundance of information in Gd3+ spectra can be used as a probe for properties of the other rare earth ions in the same complexes. In this work, the zero-field splitting (ZFS) of a series of Gd3+ pentagonal bipyramidal complexes of the form [GdX1X2(Leq)5]n+ [n = 1, X = axial ligands: Cl-, -OtBu, -OArF5 or n = 3, X = tBuPO(NHiPr)2, Leq = equatorial ligand: Py, THF or H2O] with near fivefold symmetry axes along X1-Gd-X2 was investigated. The ZFS parameters were determined by fitting of room-temperature continuous wave electron paramagnetic resonance (EPR) spectra (at X-, K-, and Q-band) to a spin Hamiltonian incorporating extended Stevens operators compatible with C5 symmetry. Examination of the acquired parameters led to the conclusion that the ZFS is dominated by the B20 term and that the magnitude of B20 is almost entirely dependent on, and inversely proportional to, the donor strength of the axial ligands. Surveying the continuous shape measure and the X1-Gd-X2 angle of the complexes showed that there is some correlation between the proximity of each complex to D5h symmetry and the magnitude of the B65 parameter, but that the deformation of the X1-Gd-X2 angle is more significant than other distortions. Finally, the magnitude of B20 was found to be inversely proportional to the thermal barrier for the reversal of the magnetic moment (Ueff) of the corresponding isostructural Dy3+ complexes.

Determinative Effect of Axial Linearity on Single-Molecule Magnet Performance in Dinuclear Dysprosium Complexes.

Two dichloride-bridged dinuclear dysprosium(III) complexes based on salen ligands, namely, [Dy(L1 )(µ-Cl)(thf)]2 (1; H2 L1 =N,N'-bis(3,5-di-tert-butylsalicylidene)phenylenediamine) and [Dy2 (L2 )2 (µ-Cl)2 (thf)2 ]2 (2; H2 L2 =N,N'-bis(3,5-di-tert-butylsalicylidene)ethylenediamine) are reported. These two complexes have two short Dy-O(PhO) bonds that exhibit angles of ∼90° for 1 and ∼143° for 2, leading to clear slow relaxation of the magnetization for 2 and not for 1. Compound 2 has a near-identical core to the recently reported compound [Dy2 (L3 )2 (µ-Cl)2 (thf)2 ] (3; H2 L3 =N-(2-pyridylmethyl)-N,N-bis(2'-hydroxy-3',5'-di-tert-butylbenzyl)amine). The only substantial difference is the relative angle of the two O(PhO) -Dy-O(PhO) vectors, which is collinear in 2 owing to inversion symmetry and ∼68° in 3 due to a molecular C2 axis. It is shown that this subtle structural difference leads to large differences in the dipolar ground states, giving rise to open magnetic hysteresis for 3 and not for 2.

Ligand Fluorination to Mitigate the Raman Relaxation of DyIII Single-Molecule Magnets: A Combined Terahertz, Far-IR and Vibronic Barrier Model Study.

The fast Raman relaxation process via a virtual energy level has become a puzzle for how to chemically engineer single-molecule magnets (SMMs) with better performance. Here, we use the trifluoromethyl group to systematically substitute the methyl groups in the axial position of the parent bis-butoxide pentapyridyl dysprosium(III) SMM. The resulting complexes-[Dy(OLA )2 py5 ][BPh4 ] (LA =CH(CF3 )2 - 1, CH2 CF3 - 2, CMe2 CF3 - 3)-show progressively enhanced TB hys (@100 Oe s-1 ) from 17 K (for 3), 20 K (for 2) to 23 K (for 1). By experimentally identifying the varied under barrier relaxation energy in the 5-500 cm-1 regime, we are able to identify that the C-F bond related vibration energy of the axial ligand ranging from 200 to 350 cm-1 is the key variant for this improvement. Thus, this finding not only reveals a correlation between the structure and the Raman process but also provides a paradigm for how to apply the vibronic barrier model to analyze multi-phonon relaxation processes in lanthanide SMMs.

Studies of the Temperature Dependence of the Structure and Magnetism of a Hexagonal-Bipyramidal Dysprosium(III) Single-Molecule Magnet.

The hexagonal-bipyramidal lanthanide(III) complex [Dy(OtBu)Cl(18-C-6)][BPh4] (1; 18-C-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane ether) displays an energy barrier for magnetization reversal (Ueff) of ca. 1000 K in a zero direct-current field. Temperature-dependent X-ray diffraction studies of 1 down to 30 K reveal bending of the Cl-Ln-OtBu angle at low temperature. Using ab initio calculations, we show that significant bending of the O-Dy-Cl angle upon cooling from 273 to 100 K leads to a ca. 10% decrease in the energy of the excited electronic states. A thorough exploration of the temperature and field dependencies of the magnetic relaxation rate reveals that magnetic relaxation is dictated by five mechanisms in different regimes: Orbach, Raman-I, quantum tunnelling of magnetization, and Raman-II, in addition to the observation of a phonon bottleneck effect.

A Cost-Effective Semi-Ab Initio Approach to Model Relaxation in Rare-Earth Single-Molecule Magnets.

We discuss a cost-effective approach to understand magnetic relaxation in the new generation of rare-earth single-molecule magnets. It combines ab initio calculations of the crystal field parameters, of the magneto-elastic coupling with local modes, and of the phonon density of states with fitting of only three microscopic parameters. Although much less demanding than a fully ab initio approach, the method gives important physical insights into the origin of the observed relaxation. By applying it to high-anisotropy compounds with very different relaxation, we demonstrate the power of the approach and pinpoint ingredients for improving the performance of single-molecule magnets.

A Local D4h Symmetric Dysprosium(III) Single-Molecule Magnet with an Energy Barrier Exceeding 2000†K*.

Three six-coordinate DyIII single-molecule magnets (SMMs) [Dy(Ot Bu)2 (L)4 ]+ with local D4h symmetry are obtained by optimizing the equatorial ligands. One of the compounds with L=4-phenylpyridine shows an energy barrier (Ueff ) of 2075(11) K, which is the third largest Ueff , and the first Ueff >2000 K for SMMs with axial-type symmetry so far. Ab initio analysis indicates that the exceptional uniaxial magnetic anisotropy is deeply related to the axially compressed octahedral geometry. This work provides a new insight into the local D4h symmetry for high-performance SMMs.

Total Synthesis of Prostratin, a Bioactive Tigliane Diterpenoid: Access to Multi-Stereocenter Cyclohexanes from a Phenol.

Tiglianes such as prostratin and related diterpenoids are biologically significant natural molecules and long-standing targets for organic synthesis community. Due to the complex polycyclic scaffolds, high oxygenation level, and dense functional groups and stereocenters, their de novo chemical syntheses still face formidable challenges despite extensive efforts in the past 40 years. This account details the development of a modular and concise synthesis of prostratin, a potent anti-HIV and anticancer agent. The key approach in this synthesis involved a sequence of oxidative dearomatization and sequential stereoselective installation of peripheral groups to rapidly build the contiguously substituted cyclohexane C-ring. Inspired by Wender's work, an acid- and solvent-controlled stereodivergent formation of cyclopropane D-ring was developed. Mechanistic investigations by computational methods revealed that the competition between intra- and intermolecular hydrogen bonding led to different conformations, thus favoring different protonation processes. The designed and unexpected chemistry along this campaign reflected the uniqueness of the natural structures and should be amenable to future chemical syntheses of related complex polycyclic molecules.

Understanding a pentagonal-bipyramidal holmium(iii) complex with a record energy barrier for magnetisation reversal.

We report herein three pentagonal-bipyramidal holmium(iii) complexes sharing the formula [Ho(L)2(py)5][BPh4], where L is the alkoxide ligand. For L = (OSi(CH3)3)-, a record energy barrier for magnetization reversal (Ueff = 715(6) K) was revealed, which is due to the pure state transition of mJ = |±8〉→mJ = |±7〉→mJ = |±6〉.

A Study of Magnetic Relaxation in Dysprosium(III) Single-Molecule Magnets.

Although the development of single-molecule magnets (SMMs) is rapid, there are only two families of high energy barrier (Ueff ) dysprosium(III) SMMs known so far: the cyclopentadienyl (Cp) family with a sandwich structure and the pentagonal-bipyramidal (PB) family with D5h symmetry. These high-barrier SMMs, which usually possess Ueff >500 cm-1 allow the separate study of the four magnetic relaxation paths, namely, direct, quantum tunnelling, Raman and Orbach processes, in detail. Whereas the first family is chemically more challenging to modify the Cp rings, it is shown herein that the latter family, with the common formulae [DyX1 X2 (Leq )5 ]+ , such as X1 /X2 =- OCMe3 , - OSiMe3 , - OPh, Cl- or Br- ; Leq =THF/pyridine/4-methylpyridine, can be readily fine-tuned with a range of axial and equatorial ligands by simple substitution reactions. This allows unambiguous confirmation that the Ueff mainly depends on the identity of X1 and X2 , rather than on Leq . More importantly, the fitted parameters are barrier dependent. If X1 is an O donor and X2 is a halide, 500

Exchange-Biasing in a Dinuclear Dysprosium(III) Single-Molecule Magnet with a Large Energy Barrier for Magnetisation Reversal.

A dichlorido-bridged dinuclear dysprosium(III) single-molecule magnet [Dy2 L2 (µ-Cl)2 (thf)2 ] has been made by using a diamine-bis(phenolate) ligand, H2 L. Magnetic studies show an energy barrier for magnetisation reversal (Ueff ) around 1000 K. An exchange-biasing effect is clearly seen in magnetic hysteresis with steps up to 3 K. Ab initio calculations exclude the possibility of a pure dipolar origin of this effect leading to the conclusion that super-exchange through the chloride bridging ligands is important.

Equatorial coordination optimization for enhanced axiality of mononuclear Dy(iii) single-molecule magnets.

We present a controlled synthetic route to optimize the equatorial coordination environment of three Dy(iii) borohydride complexes: Dy(BH4)3(THF)3 (1), [Dy(BH4)2(THF)5][BPh4] (2) and [Dy(BH4)2(18-C-6)][Na(THF)2(18-C-6)][BPh4]2 (3) (THF = tetrahydrofuran, BPh4- = tetraphenyl borate, 18-C-6 = 18-crown-6-ether), which have the same axial coordination environment, while different equatorial sites. Alteration of the coordination environment on the equatorial sites leads to a significant change in their magnetic properties. In the absence of the dc field, complex 1 with three THF molecules and one BH4- ligand in the equatorial plane shows no single-molecule magnet (SMM) behaviour, complex 2 having five THF molecules at equatorial sites displays small tails of out-of-phase (χ'') signals, and complex 3 containing one 18-C-6 with six O atoms in the equatorial plane exhibits χ'' signals at higher temperatures. These results show the optimization of the equatorial coordination environment of Dy(iii) mononuclear single-molecule magnets.

Air-Stable Hexagonal Bipyramidal Dysprosium(III) Single-Ion Magnets with Nearly Perfect D6h Local Symmetry.

Eight-coordinated DyIII centres with D6h symmetry are expected to act as high-performance single-molecule magnets (SMMs) due to the simultaneous fulfilment of magnetic axiality and a high coordination number (a requisite for air stability). But the experimental realization is challenging due to the requirement of six coordinating atoms in the equatorial plane of the hexagonal bipyramid; this is usually too crowded for the central DyIII ion. Here a hexaaza macrocyclic Schiff base ligand and finetuned axial alkoxide/phenol-type ligands are used to show that a family of hexagonal bipyramidal DyIII complexes can be isolated. Among them, three complexes possess nearly perfect D6h local symmetry. The highest effective magnetic reversal barrier is found at 1338(3) K and an open hysteresis temperature of 6 K at the field sweeping rate of 1.2 mT s-1 ; this represents a new record for D6h SMMs.

An imido ligand significantly enhances the effective energy barrier of dysprosium(iii) single-molecule magnets.

We report herein an imido ligand 1,3-bis(2,6-diisopropylphenyl) imidazolin-2-imine (ImDippNH) that can form a very short Dy-N bond (2.12 Å) with the dysprosium(iii) ion, which leads to a much larger effective energy barrier for magnetisation reversal (803 K) compared to the analogous alkoxide ligand (53 K). Moreover, we predict that a linear two-coordinate [Dy(ImDippN)2]+ complex may have an effective energy barrier larger than 4000 K.

A dichlorido-bridged dinuclear Dy(iii) single-molecule magnet with an effective energy barrier larger than 600 K.

We report a dichlorido-bridged dinuclear dysprosium(iii) single-molecule magnet [Dy(Cy2N)2(µ-Cl)(THF)]2 which shows an effective energy barrier for magnetization reversal (Ueff) of ca. 623 K. This is by far the largest Ueff barrier for any chlorido-bridged lanthanide single-molecule magnet. We observe two relaxation processes with near-identical temperature dependencies, one of which disappears upon magnetic dilution. We suspect that these two processes are the isolated and coupled relaxation processes.

Studies of hysteresis and quantum tunnelling of the magnetisation in dysprosium(iii) single molecule magnets.

We report magnetic hysteresis studies of three Dy(iii) single-molecule magnets (SMMs). The three compounds are [Dy(tBuO)Cl(THF)5][BPh4] (1), [K(18-crown-6-ether)(THF)2][Dy(BIPM)2] (2, BIPM = C{PPh2NSiMe3}2), and [Dy(Cpttt)2][B(C6F5)4] (3), chosen as they have large energy barriers to magnetisation reversal of 665, 565, and 1223 cm-1, respectively. There are zero-field steps in the hysteresis loops of all three compounds, that remain in magnetically dilute samples and in samples that are isotopically enriched with 164Dy, which has no nuclear spin. These results demonstrate that neither dipolar fields nor nuclear hyperfine coupling are solely responsible for the quantum tunnelling of magnetisation at zero field. Analysing their vibrational modes, we find that the modes that most impact the first coordination sphere occur at the lowest energies for 1, at intermediate energies for 2 and at higher energies for 3, in correlation with their coercive fields. Therefore, we suggest that the efficiency of quantum tunnelling of magnetisation is related to molecular flexibility.

Field- and temperature-dependent quantum tunnelling of the magnetisation in a large barrier single-molecule magnet.

Understanding quantum tunnelling of the magnetisation (QTM) in single-molecule magnets (SMMs) is crucial for improving performance and achieving molecule-based information storage above liquid nitrogen temperatures. Here, through a field- and temperature-dependent study of the magnetisation dynamics of [Dy(tBuO)Cl(THF)5][BPh4]·2THF, we elucidate the different relaxation processes: field-independent Orbach and Raman mechanisms dominate at high temperatures, a single-phonon direct process dominates at low temperatures and fields >1 kOe, and a field- and temperature-dependent QTM process operates near zero field. Accounting for the exponential temperature dependence of the phonon collision rate in the QTM process, we model the magnetisation dynamics over 11 orders of magnitude and find a QTM tunnelling gap on the order of 10-4 to 10-5 cm-1. We show that removal of Dy nuclear spins does not suppress QTM, and argue that while internal dipolar fields and hyperfine coupling support QTM, it is the dynamic crystal field that drives efficient QTM.

Metallacrowns as Templates for Diabolo-like {LnCu8 } Complexes with Nearly Perfect Square Antiprismatic Geometry.

A series of diabolo-like nonanuclear {LnIII CuII8 } (Ln=Tb, Dy, Ho, Er, Tm, Yb, and Y) clusters were prepared in which the LnIII ion is capped by two 8-MC-4 metallacrown ligands to form a nearly ideal square antiprismatic (SAP) coordination geometry with D4d symmetry. Despite the lack of crystallographic symmetry, these molecules engender the lanthanide ions with highly axial mJ states. The axial/equatorial nature of the crystal field in environments close to ideal SAP geometry is very subtle and influenced by the nature of the ligand lone pairs. Slow magnetic relaxation behaviour was observed for the DyIII , ErIII , TmIII , and YbIII analogues, and the obtained effective energy barriers are not consistent with excitations on the LnIII ion, suggesting a more nuanced situation.