Enhancing the barrier height for magnetization reversal in 4d/4f RuIII2LnIII2 "butterfly" single molecule magnets (Ln = Gd, Dy) via targeted structural alterations.

A series of 4d-4f {RuIII2DyIII2} and {RuIII2GdIII2} 'butterfly' (rhombohedral) complexes have been synthesized and characterized and their magnetic properties investigated. Earlier, we have reported the first 4d/4f SMM - [RuIII2DyIII2(OMe)2(O2CPh)4(mdea)2(NO3)2] (1Dy) with a Ueff value of 10.7 cm-1. As the structural distortion around the DyIII centres and the RuIII⋯DyIII exchange interactions are key to enhancing the anisotropy, in this work we have synthesised three more {Ru2Dy2} butterfly complexes where structural alteration around the DyIII centres and alterations to the bridging groups are performed with an aim to improve the magnetic properties. The new complexes reported here are [Ru2Dy2(OMe)2(O2C(4-Me-Ph)4(mdea)2(MeOH)4], 2Dy, [Ru2Dy2(OMe)2(O2C(2-Cl,4,5-F-Ph)4(mdea)2(NO3)2], 3Dy, and an acac derivative [Ru2Dy2(OMe)2(acac)4(NO3)2(edea)2], 4Dy, where acac- = acetylacetonate, edea2- = N-ethyldiethanolamine dianion. Complex 2Dy describes alteration in the DyIII centers, while complexes 3Dy and 4Dy are aimed to alter the RuIII⋯DyIII exchange pathways. To ascertain the 4d-4f exchange, the Gd-analogues of 1Dy and 4Dy were synthesised [Ru2Gd2(OMe)2(O2CPh)4(mdea)2(NO3)2], 1Gd, [Ru2Gd2(OMe)2(acac)4(NO3)2(edea)2], 4Gd. Both ac and dc susceptibility studies were performed on all these complexes, and out-of-phase signals were observed for 3Dy in zero-field while 2Dy and 4Dy show out-of-phase signals in the presence of an applied field. Complex 3Dy reveals a barrier height Ueff of 45 K. To understand the difference in the magnetic dynamic behavior compared to our earlier reported {RuIII2DyIII2} analogue, detailed theoretical calculations based on ab initio CASSCF/RASSI-SO calculations have been performed. Calculations reveal that the JRu⋯Dy value varies from -1.8 cm-1 (4Dy) to -2.4 cm-1 (3Dy). These values are also affirmed by DFT calculations performed on the corresponding GdIII analogues. The origin of the largest barrier and observation of slow magnetic relaxation in 3Dy is routed back to the stronger single-ion anisotropy and stronger JRu⋯Dy exchange which quenches the QTM effects more efficiently. This study thus paves the way forward to tune local structure around the LnIII center and the exchange pathway to enhance the SMM characteristics in other {3d-4f}/{4d-4f} SMMs.

Hyperpolarization of Pyridyl Fentalogues by Signal Amplification By Reversible Exchange (SABRE).

Fentanyl, also known as 'jackpot', is a synthetic opiate that is 50-100 times more potent than morphine. Clandestine laboratories produce analogues of fentanyl, known as fentalogues to circumvent legislation regarding its production. Three pyridyl fentalogues were synthesized and then hyperpolarized by signal amplification by reversible exchange (SABRE) to appraise the forensic potential of the technique. A maximum enhancement of -168-fold at 1.4 T was recorded for the ortho pyridyl 1H nuclei. Studies of the activation parameters for the three fentalogues revealed that the ratio of ligand loss trans to hydride and hydride loss in the complex [Ir(IMes)(L)3(H)2]+ (IMes=1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene) ranged from 0.52 to 1.83. The fentalogue possessing the ratio closest to unity produced the largest enhancement subsequent to performing SABRE at earth's magnetic field. It was possible to hyperpolarize a pyridyl fentalogue selectively from a matrix that consisted largely of heroin (97 : 3 heroin:fentalogue) to validate the use of SABRE as a forensic tool.

New examples of triangular terbium(iii) and holmium(iii) and hexagonal dysprosium(iii) single molecule toroics.

The structural, magnetic and theoretical aspects are described for three triangular lanthanide complexes, [Tb(OH)(teaH2)3(paa)3]Cl2 (1), [Dy(OH)(teaH2)3(paa)3]Cl2 (2) and [Ho(OH)(teaH2)3(paa)3]Cl2 (3), and a hexanuclear wheel of formula [Dy(pdeaH)6(NO3)6] (4) [teaH3 = triethanolamine, paaH = N-(2-pyridyl)-acetoacetamide and pdeaH3 = 3-[bis(2-hydroxyethyl)amino]propan-1-ol]. Each complex displays single molecule toroidal behaviour as rationalised using high-level ab initio calculations. Complexes 2 and 3 are the first examples of mixed moment single molecule toroidal complexes featuring non-Kramers ions.

Oblate versus Prolate Electron Density of Lanthanide Ions: A Design Criterion for Engineering Toroidal Moments? A Case Study on {LnIII 6 } (Ln=Tb, Dy, Ho and Er) Wheels.

We report four new complexes based on a {LnIII 6 } wheel structure, three of which possess a net toroidal magnetic moment. The four examples consist of {TbIII 6 } and {HoIII 6 } wheels, which are rare examples of non DyIII based complexes possessing a toroidal magnetic ground state, and a {DyIII 6 } complex which improves its toroidal structure upon lowering the crystallographic symmetry from trigonal (R 3 ‾ ) to triclinic (P 1 ‾ ). Notably the toroidal moment is lost for the trigonal {ErIII 6 } analogue. This suggests the possibility of utilizing the popular concept of oblate and prolate electron density of the ground state MJ levels of lanthanide ions to engineer toroidal moments.

Rationalizing the sign and magnitude of the magnetic coupling and anisotropy in dinuclear manganese(iii) complexes.

We have synthesised twelve manganese(iii) dinuclear complexes, 1-12, in order to understand the origin of magnetic exchange (J) between the metal centres and the magnetic anisotropy (D) of each metal ion using a combined experimental and theoretical approach. All twelve complexes contain the same bridging ligand environment of one µ-oxo and two µ-carboxylato, that helped us to probe how the structural parameters, such as bond distance, bond angle and especially Jahn-Teller dihedral angle affect the magnetic behaviour. Among the twelve complexes, we found ferromagnetic coupling for five and antiferromagnetic coupling for seven. DFT computed the J and ab initio methods computed the D parameter, and are in general agreement with the experimentally determined values. The dihedral angle between the two Jahn-Teller axes of the constituent MnIII ions are found to play a key role in determining the sign of the magnetic coupling. Magneto-structural correlations are developed by varying the Mn-O distance and the Mn-O-Mn angle to understand how the magnetic coupling changes upon these structural changes. Among the developed correlations, the Mn-O distance is found to be the most sensitive parameter that switches the sign of the magnetic coupling from negative to positive. The single-ion zero-field splitting of the MnIII centres is found to be negative for complexes 1-11 and positive for complex 12. However, the zero-field splitting of the S = 4 state for the ferromagnetic coupled dimers is found to be positive, revealing a significant contribution from the exchange anisotropy - a parameter which has long been ignored as being too small to be effective.

Understanding the Mechanism of Magnetic Relaxation in Pentanuclear {MnIVMnIII2LnIII2} Single-Molecule Magnets.

A new family of heterometallic pentanuclear complexes of formulas [MnIVMnIII2LnIII2O2(benz)4(mdea)3(NO3)2(MeOH)] (Ln = Dy (1-Dy), Tb (2-Tb), Gd (3-Gd), Eu (4-Eu), Sm (5-Sm), Nd (6-Nd), Pr (7-Pr); benz(H) = benzoic acid; mdeaH2= N-methyldiethanolamine) and [MnIVMnIII2LnIII2O2(o-tol)4(mdea)3(NO3)2(MeOH)] (Ln = Gd (8-Gd), Eu (9-Eu); o-tol(H) = o-toluic acid) have been isolated and structurally, magnetically, and theoretically characterized. dc magnetic susceptibility measurements reveal dominant antiferromagnetic magnetic interactions for each complex, except for 2-Tb and 3-Gd, which reveal an upturn in the χMT product at low temperatures. The magnetic interactions between the spin centers in the Gd derivatives, 3-Gd and 8-Gd, which display markedly different χMT vs T profiles, were found to be due to the interactions of the GdIII-GdIII ions which change from ferromagnetic (3-Gd) to antiferromagnetic (8-Gd) due to structural differences. ac magnetic susceptibility measurements reveal a nonzero out-of-phase component for 1-Dy and 7-Pr, but no maxima were observed above 2 K (Hdc = 0 Oe), which suggests single-molecule magnet (SMM) behavior. Out-of-phase signals were observed for complexes 2-Tb, 4-Eu, 8-Gd, and 9-Eu, in the presence of a static dc field (Hdc = 2000, 3000 Oe). The anisotropic nature of the lanthanide ions in the benzoate series (1-Dy, 2-Tb, 5-Sm, 6-Nd, and 7-Pr) were thoroughly investigated using ab initio methods. CASSCF calculations predict that the origin of SMM behavior in 1-Dy and 7-Pr and the applied field SMM behavior in 2-Tb does not solely originate from the single-ion anisotropy of the lanthanide ions. To fully understand the relaxation mechanism, we have employed the Lines model to fit the susceptibility data using the POLY_ANISO program, which suggests that the zero-field SMM behavior observed in complexes 1-Dy and 7-Pr is due to weak MnIII/IV-LnIII and LnIII-LnIII couplings and an unfavorable LnIII/MnIII/MnIV anisotropy. In complexes 4-Eu, 8-Gd, and 9-Eu ab initio calculations indicate that the anisotropy of the MnIII ions solely gives rise to the possibility of SMM behavior. Complex 7-Pr is a Pr(III)-containing complex that displays zero-field SMM behavior, which is rare, and our study suggests the possibility of coupling weak SOC lanthanide metal ions to anisotropic transition-metal ions to derive SMM characteristics; however, enhancing the exchange coupling in {3d-4f} complexes is still a stubborn hurdle in harnessing new generation {3d-4f} SMMs.

Slow Magnetic Relaxation and Single-Molecule Toroidal Behaviour in a Family of Heptanuclear {CrIII LnIII6 } (Ln=Tb, Ho, Er) Complexes.

The synthesis, magnetic properties, and theoretical studies of three heterometallic {CrIII LnIII6 } (Ln=Tb, Ho, Er) complexes, each containing a metal topology consisting of two Ln3 triangles connected via a CrIII linker, are reported. The {CrTb6 } and {CrEr6 } analogues display slow relaxation of magnetization in a 3000 Oe static magnetic field. Single-crystal measurements reveal opening up of the hysteresis loop for {CrTb6 } and {CrHo6 } molecules at low temperatures. Ab initio calculations predict toroidal magnetic moments in the two Ln3 triangles, which are found to couple, stabilizing a con-rotating ferrotoroidal ground state in Tb and Ho examples and extend the possibility of observing toroidal behaviour in non DyIII complexes for the first time.

Ferrotoroidic ground state in a heterometallic {CrIIIDyIII6} complex displaying slow magnetic relaxation.

Toroidal quantum states are most promising for building quantum computing and information storage devices, as they are insensitive to homogeneous magnetic fields, but interact with charge and spin currents, allowing this moment to be manipulated purely by electrical means. Coupling molecular toroids into larger toroidal moments via ferrotoroidic interactions can be pivotal not only to enhance ground state toroidicity, but also to develop materials displaying ferrotoroidic ordered phases, which sustain linear magneto-electric coupling and multiferroic behavior. However, engineering ferrotoroidic coupling is known to be a challenging task. Here we have isolated a {CrIIIDyIII6} complex that exhibits the much sought-after ferrotoroidic ground state with an enhanced toroidal moment, solely arising from intramolecular dipolar interactions. Moreover, a theoretical analysis of the observed sub-Kelvin zero-field hysteretic spin dynamics of {CrIIIDyIII6} reveals the pivotal role played by ferrotoroidic states in slowing down the magnetic relaxation, in spite of large calculated single-ion quantum tunneling rates.

Coming full circle: constructing a [Gd6] wheel dimer by dimer and the importance of spin topology.

The syntheses, structures, magnetic and thermodynamic properties of three related triethanolamine-based GdIII complexes are described. The smallest, a dimer ([Gd2]), can be viewed as the subunit from which the two larger complexes, a linear tetramer ([Gd2]2) and a cyclic hexamer ([Gd2]3), are composed by further deprotonation of the triethanolamine ligand. In all cases, nearest neighbour magnetic ions are weakly correlated by antiferromagnetic isotropic exchange, whose strength does not change significantly from one complex to another; J ranging from -0.10 to -0.13 cm-1. Therefore, rather than the strength of the coupling, it is the spin topology that is the dominant factor in determining the differences between the physical properties - specifically, the nuclearity and the transition from open (dimer and tetramer) to cyclic (hexamer) boundary conditions. Indeed the hexanuclear wheel reaches the continuum limit of classical Heisenberg spin chains. In terms of the magnetocaloric properties, the smaller the nuclearity, the larger the magnetic entropy and adiabatic temperature changes.

Pentanuclear Lanthanide Mono-organophosphates: Synthesis, Structure, and Magnetism.

Research on rare-earth phosphates has recently received substantial interest because of their unique physical and chemical properties. In recent years, because of their low solubility, research interest has been built on developing methodologies to prepare nanostructures and grow single crystals of inorganic rare-earth phosphates. The chemistry of rare-earth organophosphates, however, is still at a latent stage. Contrary to the traditional hydrothermal route, we report rare examples of discrete pentanuclear lanthanide(III) organophosphate clusters assembled from a sterically encumbered monoester of phosphoric acid under mild reaction conditions. Single-crystal X-ray analysis revealed that all of the compounds possess a similar core structure, [Ln5(µ3-OH)(dipp)6(NO3)x(CH3OH)y(H2O)z]2+ [Ln = Nd (1), Sm (2), Eu (3), Gd (4), Tb (5), Dy (6), Ho (7), Er (8), Tm (9); dipp = 2,6-diisopropylphenylphosphate], where the anionic charge balance is maintained by the presence of chelating nitrate anions (in the case of 9, x = 0), protonated tmeda, or dipp2- ligands. The vacant coordination sites on the metal ions are satisfied by coordinated methanol or water molecules. The core structure of these clusters is built on a [Ln3(µ3-OH)(dipp)6] triangle where the phosphate ligands bridge to two further Ln(III) ions. The complexes display lanthanide contraction along the series, with Ln(III) ions displaying different coordination environments/geometries as we move along the series. All of the compounds have been characterized by both analytical and spectroscopic techniques. Magnetic studies revealed the presence of weak antiferromagnetic exchange through the bridging µ3-hydroxo moiety and organophosphate groups for the {GdIII5} analogue, with a significant magnetic entropy change (25.8 J kg-1 K-1, ΔH = 7 T). The anisotropic complexes reveal an absence of slow relaxation of magnetization, except for Nd (1), Dy (6), and Er (8), which show slow relaxation in an applied DC field.

Exploring the Influence of Diamagnetic Ions on the Mechanism of Magnetization Relaxation in {CoIII2LnIII2} (Ln = Dy, Tb, Ho) "Butterfly" Complexes.

The synthesis and magnetic and theoretical studies of three isostructural heterometallic [CoIII2LnIII2(µ3-OH)2(o-tol)4(mdea)2(NO3)2] (Ln = Dy (1), Tb (2), Ho (3)) "butterfly" complexes are reported (o-tol = o-toluate, (mdea)2- = doubly deprotonated N-methyldiethanolamine). The CoIII ions are diamagnetic in these complexes. Analysis of the dc magnetic susceptibility measurements reveal antiferromagnetic exchange coupling between the two LnIII ions for all three complexes. ac magnetic susceptibility measurements reveal single-molecule magnet (SMM) behavior for complex 1, in the absence of an external magnetic field, with an anisotropy barrier Ueff of 81.2 cm-1, while complexes 2 and 3 exhibit field induced SMM behavior, with a Ueff value of 34.2 cm-1 for 2. The barrier height for 3 could not be quantified. To understand the experimental observations, we performed DFT and ab initio CASSCF+RASSI-SO calculations to probe the single-ion properties and the nature and magnitude of the LnIII-LnIII magnetic coupling and to develop an understanding of the role the diamagnetic CoIII ion plays in the magnetization relaxation. The calculations were able to rationalize the experimental relaxation data for all complexes and strongly suggest that the CoIII ion is integral to the observation of SMM behavior in these systems. Thus, we explored further the effect that the diamagnetic CoIII ions have on the magnetization blocking of 1. We did this by modeling a dinuclear {DyIII2} complex (1a), with the removal of the diamagnetic ions, and three complexes of the types {KI2DyIII2} (1b), {ZnII2DyIII2} (1c), and {TiIV2DyIII2} (1d), each containing a different diamagnetic ion. We found that the presence of the diamagnetic ions results in larger negative charges on the bridging hydroxides (1b > 1c > 1 > 1d), in comparison to 1a (no diamagnetic ion), which reduces quantum tunneling of magnetization effects, allowing for more desirable SMM characteristics. The results indicate very strong dependence of diamagnetic ions in the magnetization blocking and the magnitude of the energy barriers. Here we propose a synthetic strategy to enhance the energy barrier in lanthanide-based SMMs by incorporating s- and d-block diamagnetic ions. The presented strategy is likely to have implications beyond the single-molecule magnets studied here.

What Controls the Magnetic Exchange and Anisotropy in a Family of Tetranuclear {Mn2IIMn2III} Single-Molecule Magnets?

Twelve heterovalent, tetranuclear manganese(II/III) planar diamond or "butterfly" complexes, 1-12, have been synthesized and structurally characterized, and their magnetic properties have been probed using experimental and theoretical techniques. The 12 structures are divided into two distinct "classes". Compounds 1-8 place the Mn(III), S = 2, ions in the body positions of the butterfly metallic core, while the Mn(II), S = 5/2, ions occupy the outer wing sites and are described as "Class 1". Compounds 9-12 display the reverse arrangement of ions and are described as "Class 2". Direct current susceptibility measurements for 1-12 reveal ground spin states ranging from S = 1 to S = 9, with each complex displaying unique magnetic exchange parameters (J). Alternating current susceptibility measurements found that that slow magnetic relaxation is observed for all complexes, except for 10 and 12, and display differing anisotropy barriers to magnetization reversal. First, we determined the magnitude of the magnetic exchange parameters for all complexes. Three exchange coupling constants (Jbb, Jwb, and Jww) were determined by DFT methods which are found to be in good agreement with the experimental fits. It was found that the orientation of the Jahn-Teller axes and the Mn-Mn distances play a pivotal role in determining the sign and strength of the Jbb parameter. Extensive magneto-structural correlations have been developed for the two classes of {MnII2MnIII2} butterfly complexes by varying the Mnb-O distance, Mnw-O distance, Mnb-O-Mnb angle (α), Mnb-O-Mnb-O dihedral angle (γ), and out-of-plane shift of the Mnw atoms (ß). For the magnetic anisotropy the DFT calculations yielded larger negative D value for complexes 2, 3, 4, and 6 compared to the other complexes. This is found to be correlated to the electron-donating/withdrawing substituents attached to the ligand moiety and suggests a possible way to fine tune the magnetic anisotropy in polynuclear Mn ion complexes.

Role of the Diamagnetic Zinc(II) Ion in Determining the Electronic Structure of Lanthanide Single-Ion Magnets.

Four complexes containing DyIII and PrIII ions and their LnIII -ZnII analogs have been synthesized in order to study the influence that a diamagnetic ZnII ion has on the electronic structure and hence, the magnetic properties of the DyIII and PrIII single ions. Single-crystal X-ray diffraction revealed the molecular structures as [DyIII (HL)2 (NO3 )3 ] (1), [PrIII (HL)2 (NO3 )3 ] (2), [ZnII DyIII (L)2 (CH3 CO2 )(NO3 )2 ] (3) and [ZnII2 PrIII (L)2 (CH3 CO2 )4 (NO3 )] (4) (where HL=2-methoxy-6-[(E)-phenyliminomethyl]phenol). The dc and ac magnetic data were collected for all four complexes. Compounds 1 and 3 display frequency dependent out-of-phase susceptibility signals (χM "), which is a characteristic signature for a single-molecule magnet (SMM). Although 1 and 3 are chemically similar, a fivefold increase in the anisotropic barrier (Ueff ) is observed experimentally for 3 (83 cm-1 ), compared to 1 (16 cm-1 ). To rationalize the larger anisotropic barrier (1 vs. 3), detailed ab initio calculations were performed. Although the ground state Kramer's doublet in both 1 and 3 are axial in nature (gzz =19.443 for 1 and 18.82 for 3), a significant difference in the energy gap (Ueff ) between the ground and first excited Kramer's doublet is calculated. This energy gap is governed by the electrostatic repulsion between the DyIII ion and the additional charge density found for the phenoxo bridging ligand in 3. This extra charge density was found to be a consequence of the presence of the diamagnetic ZnII ion present in the complex. To explore the influence of diamagnetic ions on the magnetic properties further, previously reported and structurally related Zn-DyIII complexes were analyzed. These structurally analogous complexes unambiguously suggest that the electrostatic repulsion is found to be maximal when the Zn-O-Dy-O dihedral angle is small, which is an ideal condition to maximize the anisotropic barrier in DyIII complexes.

Quenching the Quantum Tunneling of Magnetization in Heterometallic Octanuclear {TMIII4 DyIII4 } (TM=Co and Cr) Single-Molecule Magnets by Modification of the Bridging Ligands and Enhancing the Magnetic Exchange Coupling.

We report the synthesis, structural characterisation, magnetic properties and provide an ab initio analysis of the magnetic behaviour of two new heterometallic octanuclear coordination complexes containing CoIII and DyIII ions. Single-crystal X-ray diffraction studies revealed molecular formulae of [CoIII4 DyIII4 (µ-OH)4 (µ3 -OMe)4 {O2 CC(CH3 )3 }4 (tea)4 (H2 O)4 ]⋅4 H2 O (1) and [CoIII4 DyIII4 (µ-F)4 (µ3 -OH)4 (o-tol)8 (mdea)4 ]⋅ 3 H2 O⋅EtOH⋅MeOH (2; tea3- =triply deprotonated triethanolamine; mdea2- =doubly deprotonated N-methyldiethanolamine; o-tol=o-toluate), and both complexes display an identical metallic core topology. Furthermore, the theoretical, magnetic and SMM properties of the isostructural complex, [CrIII4 DyIII4 (µ-F4 )(µ3 -OMe)1.25 (µ3 -OH)2.75 (O2 CPh)8 (mdea)4 ] (3), are discussed and compared with a structurally similar complex, [CrIII4 DyIII4 (µ3 -OH)4 (µ-N3 )4 (mdea)4 (O2 CC(CH3 )3 )4 ] (4). DC and AC magnetic susceptibility data revealed single-molecule magnet (SMM) behaviour for 1-4. Each complex displays dynamic behaviour, highlighting the effect of ligand and transition metal ion replacement on SMM properties. Complexes 2, 3 and 4 exhibited slow magnetic relaxation with barrier heights (Ueff ) of 39.0, 55.0 and 10.4 cm-1 respectively. Complex 1, conversely, did not exhibit slow relaxation of magnetisation above 2 K. To probe the variance in the observed Ueff  values, calculations by using CASSCF, RASSI-SO and POLY_ANISO routine were performed on these complexes to estimate the nature of the magnetic coupling and elucidate the mechanism of magnetic relaxation. Calculations gave values of JDy-Dy as -1.6, 1.6 and 2.8 cm-1 for complexes 1, 2 and 3, respectively, whereas the JDy-Cr interaction was estimated to be -1.8 cm-1 for complex 3. The developed mechanism for magnetic relaxation revealed that replacement of the hydroxide ion by fluoride quenched the quantum tunnelling of magnetisation (QTM) significantly, and led to improved SMM properties for complex 2 compared with 1. However, the tunnelling of magnetisation at low-lying excited states was still operational for 2, which led to low-temperature QTM relaxation. Replacement of the diamagnetic CoIII ions with paramagnetic CrIII led to CrIII ⋅⋅⋅DyIII coupling, which resulted in quenching of QTM at low temperatures for complexes 3 and 4. The best example was found if both CrIII and fluoride were present, as seen for complex 3, for which both factors additively quenched QTM and led to the observation of highly coercive magnetic hysteresis loops above 2 K. Herein, we propose a synthetic strategy to quench the QTM effects in lanthanide-based SMMs. Our strategy differs from existing methods, in which parameters such as magnetic coupling are difficult to control, and it is likely to have implications beyond the DyIII SMMs studied herein.

Crystal structure of 2,4-di-tert-butyl-6-(hy-droxy-methyl)-phenol.

The title compound, C15H24O2, is an example of a phenol-based pendant-arm precursor. In the mol-ecule, the phenol hy-droxy group participates in an intra-molecular O-H⋯O hydrogen bond with the pendant alcohol group, forming an S(6) ring. This ring adopts a half-chair conformation. In the crystal, O-H⋯O hydrogen bonds connect mol-ecules related by the 31 screw axes, forming chains along the c axis. The C-C-O angles for the hy-droxy groups are different as a result of the type of hybridization for the C atoms that are involved in these angles. The C-C-O angle for the phenol hy-droxy group is 119.21 (13)°, while the angle within the pendant alcohol is 111.99 (13)°. The bond length involving the phenolic oxygen is 1.3820 (19) Å, which contrasts with that of the alcoholic oxygen which is 1.447 (2) Å. The former is conjugated with the aromatic ring and so leads to the observed shorter bond length.

What Controls the Sign and Magnitude of Magnetic Anisotropy in Tetrahedral Cobalt(II) Single-Ion Magnets?

A family of mononuclear tetrahedral cobalt(II) thiourea complexes, [Co(L1)4](NO3)2 (1) and [Co(Lx)4](ClO4)2 where x = 2 (2), 3 (3), 4 (4) (where L1 = thiourea, L2 = 1,3-dibutylthiourea, L3 = 1,3-phenylethylthiourea, and L4 = 1,1,3,3-tetramethylthiourea), has been synthesized using a rationally designed synthetic approach, with the aim of stabilizing an Ising-type magnetic anisotropy (-D). On the basis of direct-current, alternating-current, and hysteresis magnetic measurements and theoretical calculations, we have identified the factors that govern the sign and magnitude of D and ultimately the ability to design a single-ion magnet for a tetrahedral cobalt(II) ion. To better understand the magnetization relaxation dynamics, particularly for complexes 1 and 2, dilution experiments were performed using their diamagnetic analogues, which are characterized by single-crystal X-ray diffraction with the general molecular formulas of [Zn(L1)4](NO3)2 (5) and [Zn(L2)4](ClO4)2 (6). Interestingly, intermolecular interactions are shown to play a role in quenching the quantum tunneling of magnetization in zero field, as evidenced in the hysteresis loop of 1. Complex 2 exhibits the largest Ueff value of 62 cm-1 and reveals open hysteresis loops below 4 K. Furthermore, the influence of the hyperfine interaction on the magnetization relaxation dynamics is witnessed in the hysteresis loops, allowing us to determine the electron/nuclear spin S(Co) = 3/2/I(Co) = 7/2 hyperfine coupling constant of 550 MHz, a method ideally suited to determine the hyperfine coupling constant of highly anisotropic metal ions stabilized with large D value, which are otherwise hard to determine by conventional methods such as electron paramagnetic resonance.

Enhancing the magnetic blocking temperature and magnetic coercivity of {CrLn} single-molecule magnets via bridging ligand modification.

Replacing bridging benzoate ligands with 2-chloro-4,5-fluorobenzoate in a family of {CrLn} (Ln = Tb, Dy and Ho) single-molecule magnets result in significant improvements in magnetic relaxation time, magnetic hysteresis blocking temperature and magnetic coercivity.

Heteronuclear Ni(ii)-Ln(iii) (Ln = La, Pr, Tb, Dy) complexes: synthesis and single-molecule magnet behaviour.

The reaction of hydrated nickel(II) salts (chloride or nitrate) and hydrated lanthanide nitrate salts with the Schiff base ligand 2-methoxy-6-[(E)-phenyliminomethyl] phenol (HL) in methanol resulted in the isolation of three isostructural linear heterometallic trinuclear complexes and a heterometallic tetranuclear complex. The molecular structures of these complexes were determined via single crystal X-ray diffraction revealing molecular structures of formulae [Ni2La(L-)6](NO3)0.55(OH)0.45 (1), [Ni2Pr(L-)6](NO3)0.48(OH)0.52 (2), [Ni2Tb(L-)6](NO3)0.5(Cl)0.5 (3) and [Ni2Dy2(L-2(o-vanillin)2(CO3)2(NO3)2(MeOH)2] (4). Structural analysis for 1-3 reveals that the lanthanide ion is sandwiched between two Ni(II) ions and the Ni⋯Ln⋯Ni metallic core displays a linear arrangement, with an average ∠Ni⋯Ln⋯Ni bond angle of 179.7°. Analysis of 4 reveals the metal ions are arranged such that two Ni-Dy subunits are bridged by two carbonate ligands via the Dy sites. Direct current magnetic susceptibility measurements for complexes 1-4 reveal that the Ni(II) ions are coupled ferromagnetically with the Tb(III) (3) and Dy(III) (4) ions, and antiferromagnetically with the Pr(III) ion (2). For complex 1 a long range intramolecular ferromagnetic interaction is witnessed between the Ni(II) ions (Ni⋯Ni = 6.873(9) Å) via a closed shell La(III) ion. The magnetic data of 1 were fitted using the HDVV Hamiltonian revealing the following parameters; J = +0.46 cm(-1), g = 2.245, D = +4.91 cm(-1). Alternating current magnetic susceptibility measurements performed on complexes 2-4 revealed that 3 and 4 displayed frequency dependent χ''M signals (Hac = 3.5 Oe and Hdc = 0 Oe) which is a characteristic signature of a single-molecule magnet behaviour.

A Family of {Cr(III)2Ln(III)2} Butterfly Complexes: Effect of the Lanthanide Ion on the Single-Molecule Magnet Properties.

We report the synthesis of several heterometallic 3d-4f complexes which result from the replacement of the Dy(III) ions in the [Cr(III)2Dy(III)2(OMe)2(mdea)2(O2CPh)4(NO3)2] single-molecule magnet (SMM) by the trivalent Pr, Nd, Gd, Tb, Ho, and Er lanthanide ions. The parent {Cr2Dy(III)2} compound displayed an anisotropy barrier to magnetization reversal of 53 cm(-1), with magnetic hysteresis observed up to 3.5 K and with large coercive fields at low temperatures (2.7 T at 1.8 K). Magnetic studies for the new complexes revealed significantly different static and dynamic magnetic behavior in comparison to the parent {Cr(III)2Dy(III)2} complex. When Ln(III) = Pr, a complete loss of SMM behavior is found, but when Ln(III) = Nd or Er, frequency-dependent tails in the out-of-phase susceptibility at low temperatures are observed, indicative of slow magnetic relaxation, but with very small anisotropy barriers and fast relaxation times. When Ln(III) = Tb and Ho, SMM behavior is clearly revealed with anisotropy barriers of 44 and 36 cm(-1), respectively. Magnetic hysteresis is also observed up to 2.5 and 1.8 K (0.003 T/s) for the Tb and Ho complexes, respectively. A large loss of the magnetization is, however, observed at zero-field, and as a result, the large coercivity which is present in the {Cr2Dy2} example is lost. The {Cr2Tb2} and {Cr2Ho2} complexes are rare examples of Tb- and Ho-based SMMs which reveal both slow relaxation in the absence of a static dc field (ac susceptibility) and open hysteresis loops above 1.8 K.

Large Hexadecametallic {Mn(III) -Ln(III) } Wheels: Synthesis, Structural, Magnetic, and Theoretical Characterization.

The synthesis, gas sorption studies, magnetic properties, and theoretical studies of new molecular wheels of core type {Mn(III) 8 Ln(III) 8 } (Ln=Dy, Ho, Er, Y and Yb), using the ligand mdeaH2 , in the presence of ortho-toluic or benzoic acid are reported. From the seven wheels studied the {Mn8 Dy8 } and {Mn8 Y8 } analogues exhibit SMM behavior as determined from ac susceptibility experiments in a zero static magnetic field. From DFT calculations a S=16 ground state was determined for the {Mn8 Y8 } complex due to weak ferromagnetic Mn(III) -Mn(III) interactions. Ab initio CASSCF+RASSI-SO calculations on the {Mn8 Dy8 } wheel estimated the Mn(III) -Dy(III) exchange interaction as -0.1 cm(-1) . This weak exchange along with unfavorable single-ion anisotropy of Dy(III) /Mn(III) ions, however, led to the observation of SMM behavior with fast magnetic relaxation. The orientation of the g-anisotropy of the Dy(III) ions is found to be perpendicular to the plane of the wheel and this suggests the possibility of toroidal magnetic moments in the cluster. The {Mn8 Ln8 } clusters reported here are the largest heterometallic Mn(III) Ln(III) wheels and the largest {3d-4f} wheels to exhibit SMM behavior reported to date.