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.

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.

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.

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.

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.

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.

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.

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.

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.

What controls the magnetic exchange interaction in mixed- and homo-valent Mn7 disc-like clusters? A theoretical perspective.

Density functional theory (DFT) studies have been undertaken to compute the magnetic exchange and to probe the origin of the magnetic interactions in two hetero- and two homo-valent heptanuclear manganese disc-like clusters, of formula [Mn(II) 4 Mn(IV) 3 (tea)(teaH2 )3 (peolH)4 ] (1), [Mn(II) 4 Mn(III) 3 F3 (tea)(teaH)(teaH2 )2 (piv)4 (Hpiv)(chp)3 ] (2), [Mn(II) 7 (pppd)6 (tea)(OH)3 ] (3) and [Mn(II) 7 (paa)6 (OMe)6 ] (4) (teaH3 =triethanolamine, peolH4 =pentaerythritol, Hpiv=pivalic acid, Hchp=6-chloro-2-hydroxypyridine, pppd=1-phenyl-3-(2-pyridyl) propane-1,3-dione; paaH=N-(2-pyridinyl)acetoacetamide). DFT calculations yield J values, which reproduce the magnetic susceptibility data very well for all four complexes; these studies are also highlighting the likely ageing/stability problems in two of the complexes. It is found that the spin ground states, S, for complexes 1-4 are drastically different, varying from S=29/2 to S=1/2. These values are found to be controlled by the nature of the oxidation state of the metal ions and minor differences present in the structures. Extensive magneto-structural correlations are developed for the seven building unit dimers present in the complexes, with the correlations unlocking the reasons behind the differences in the magnetic properties observed. Independent of the oxidation state of the metal ions, the Mn-O-Mn/Mn-F-Mn angles are found to be the key parameters, which significantly influence the sign as well as the magnitude of the J values. The magneto-structural correlations developed here, have broad applicability and can be utilised to understand the magnetic properties of other Mn clusters.

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.

Theoretical studies on polynuclear {Cu(II)5Gd(III)n} clusters (n = 4, 2): towards understanding their large magnetocaloric effect.

Density functional theory (DFT) studies on two polynuclear clusters, [Cu(II)5Gd(III)4O2(OMe)4(teaH)4(O2CC(CH3)3)2(NO3)4] (1) and [Cu5Gd2(OH)4(Br)2-(H2L)2(H3L)2(NO3)2(OH2)4] (2), have been carried out to probe the origin of the large magnetocaloric effect (MCE). The magnetic exchange interactions for 1 and 2 via multiple pathways are estimated using DFT calculations. While the calculated exchange parameters deviate from previous experimental estimates obtained by fitting the magnetic data, the DFT parameter set is found to offer a striking match to the magnetic data for both complexes, highlighting the problem of overparameterization. Magnetostructural correlations for {Cu-Gd} pairs have been developed where both the Cu-O-Gd angles and Cu-O-Gd-O dihedral angles are found to significantly influence the magnitude and sign of the exchange constants. The magnitude of the MCE has been examined as a function of the exchange interactions, and clues on how the effect can be enhanced are discussed.

Heterometallic 3d-4f single-molecule magnets: ligand and metal ion influences on the magnetic relaxation.

Six tetranuclear 3d­4f single-molecule magnet (SMM) complexes formed using N-n-butyldiethanolamine and N-methyldiethanolamine in conjunction with ortho- and para-substituted benzoic acid and hexafluoroacetoacetone ligands yield two families, both having a butterfly metallic core. The first consists of four complexes of type {Co2(III)Dy2(III)} and {Co2(III)Co(II)Dy(III)} using N-n-butyldiethanolamine with variation of the carboxylate ligand. The anisotropy barriers are 80 cm­1, (77 and 96 cm­1­two relaxation processes occur), 117 and 88 cm­1, respectively, each following a relaxation mechanism from a single DyIII ion. The second family consists of a {Co2(III)Dy2(III)} and a {Cr2(III)Dy2(III)} complex, from the ligand combination of N-methyldiethanolamine and hexafluoroacetylacetone. Both show SMM behavior, the Co(III) example displaying an anisotropy barrier of 23 cm­1. The Cr(III) complex displays a barrier of 28 cm­1, with longer relaxation times and open hysteresis loops, the latter of which is not seen in the Co(III) case. This is a consequence of strong Dy(III)­Cr(III) magnetic interactions, with the relaxation arising from the electronic structure of the whole complex and not from a single DyIII ion. The results suggest that the presence of strong exchange interactions lead to significantly longer relaxation times than in isostructural complexes where the exchange is weak. The study also suggests that electron-withdrawing groups on both bridging (carboxylate) and terminal (ß-diketonate) ligands enhance the anisotropy barrier.

Synthesis and structure of new lanthanoid carbonate "lanthaballs".

New insights into the synthesis of high-nuclearity polycarbonatolanthanoid complexes have been obtained from a detailed investigation of the preparative methods that initially yielded the so-called "lanthaballs" [Ln(13)(ccnm)(6)(CO(3))(14)(H(2)O)(6)(phen)(18)] Cl(3)(CO(3))·25H(2)O [α-1Ln; Ln = La, Ce, Pr; phen = 1,10-phenanthroline; ccnm = carbamoylcyanonitrosomethanide]. From this investigation, we have isolated a new pseudopolymorph of the cerium analogue of the lanthaball, [Ce(13)(ccnm)(6)(CO(3))(14)(H(2)O)(6)(phen)(18)]·C(l3)·CO(3) (ß-1Ce). This new pseudopolymorph arose from a preparation in which fixation of atmospheric carbon dioxide generated the carbonate, and the ccnm ligand was formed in situ by the nucleophilic addition of water to dicyanonitrosomethanide. From a reaction of cerium(III) nitrate, instead of the previously used chloride salt, with (Et4N)(ccnm), phen, and NaHCO(3) in aqueous methanol, the new complex Na[Ce(13)(ccnm)(6)(CO(3))(14)(H(2)O)(6)(phen)(18)](NO(3))(6)·20H(2)O (2Ce) crystallized. A variant of this reaction in which sodium carbonate was initially added to Ce(NO(3))(3), followed by phen and (Et(4)N)(ccnm), also gave 2Ce. However, an analogous preparation with (Me4N)(ccnm) gave a mixture of crystals of 2Ce and the coordination polymer [CeNa(ccnm)4(phen)3]·MeOH (3), which were manually separated. The use of cerium(III) acetate in place of cerium nitrate in the initial preparation did not give a high-nuclearity complex but a new coordination polymer, [Ce(ccnm)(OAc)(2)(phen)] (4). The first lanthaball to incorporate neodymium, namely, [Nd(13)(ccnm)(4)(CO(3))14(NO(3))(4)(H(2)O)(7)(phen)(15)](NO(3))(3)·10H(2)O (5Nd), was isolated from a preparation similar to that of the second method used for 2Ce, and its magnetic properties showed an antiferromagnetic interaction. The identity of all products was established by X-ray crystallography.

Nickel(II)-lanthanide(III) magnetic exchange coupling influencing single-molecule magnetic features in {Ni2 Ln2 } complexes.

Four isostructural [Ni2 Ln2 (CH3 CO2 )3 (HL)4 (H2 O)2 ](3+) (Ln(3+) =Dy (1), Tb (2), Ho (3) or Lu (4)) complexes and a dinuclear [NiGd(HL)2 (NO3 )3 ] (5) complex are reported (where HL=2-methoxy-6-[(E)-2'-hydroxymethyl-phenyliminomethyl]-phenolate). For compounds 1-3 and 5, the Ni(2+) ions are ferromagnetically coupled to the respective lanthanide ions. The ferromagnetic coupling in 1 suppresses the quantum tunnelling of magnetisation (QTM), resulting in a rare zero dc field Ni-Dy single-molecule magnet, with an anisotropy barrier Ueff of 19 K.

Synthesis and characterization of nickel(II) phosphonate complexes utilizing pyridonates and carboxylates as co-ligands.

The synthesis and structures of five new nickel complexes containing phosphonate ligands are reported. The compounds utilize pivalic acid (HPiv) and 6-chloro-2-pyridonate (Hchp) as co-ligands with the resulting complexes being of formulas [Ni10(chp)4(Hchp)4.5(O3P(t)Bu)3(Piv)5(HPiv)2(OH)6(H2O)4.5](HNEt3)·0.5MeCN·2.5H2O 1, [Ni12(chp)12(Hchp)2(PhPO3)2(Piv)5(HPiv)2(OH)2(H2O)6](F)·4.5MeCN·2H2O 2, [Ni10(chp)6(O3PCH2Ph)2(Piv)8(F)2(MeCN)4] 3, [Ni10(chp)6(O3PMe)2(Piv)8(F)2 (MeCN)4]·5MeCN·2H2O 4, and [Ni10(chp)6(O3PCH2Nap)2(Piv)8(F)2(MeCN)2(H2O)2] 5. The metallic core of compounds 1 and 2 display tetra- and hexa-capped trigonal prismatic arrangements, while the metallic and phosphorus core of 3, 4, and 5 display three face-sharing octahedra. Variable temperature direct current (dc) magnetic susceptibility measurements reveal dominant antiferromagnetic exchange interactions within each cluster, with diamagnetic spin ground states found.

Single-molecule magnetism in a family of {Co(III)2Dy(III)2} butterfly complexes: effects of ligand replacement on the dynamics of magnetic relaxation.

The synthesis and structural characterization of four related heterometallic complexes of formulas [Dy(III)2Co(III)2(OMe)2(teaH)2(O2CPh)4(MeOH)4](NO3)2·MeOH·H2O (1a) and [Dy(III)2Co(III)2(OMe)2(teaH)2(O2CPh)4(MeOH)2(NO3)2]·MeOH·H2O (1b), [Dy(III)2Co(III)2(OMe)2(dea)2(O2CPh)4(MeOH)4](NO3)2 (2), [Dy(III)2Co(III)2(OMe)2(mdea)2(O2CPh)4(NO3)2] (3), and [Dy(III)2Co(III)2(OMe)2(bdea)2(O2CPh)4(MeOH)4](NO3)2·0.5MeOH·H2O (4a) and [Dy(III)2Co(III)2(OMe)2(bdea)2(O2CPh)4(MeOH)2(NO3)2]·MeOH·1.5H2O (4b) are reported (teaH3 = triethanolamine, deaH2 = diethanolamine, mdeaH2 = N-methyldiethanolamine, and bdeaH2 = N-n-butyldiethanolamine). Compounds 1 (≡ 1a and 1b) and 4 (≡ 4a and 4b) both display two unique molecules within the same crystal and all compounds display a butterfly type core, with the Dy(III) ions occupying the central body positions and the diamagnetic Co(III) ions the outer wing-tip sites. Compounds 1-4 were investigated via direct current and alternating current magnetic susceptibility measurements, and it was found that each complex displayed single-molecule magnet (SMM) behavior. All four compounds display unique coordination and geometric environments around the Dy(III) ions and it was found that each displays a different anisotropy barrier. Ab initio calculations were performed on 1-4 and these determined the low lying electronic structure of each Dy(III) ion and the magnetic interactions for each cluster. It was found that there was a strong correlation between the calculated energy gap between the ground and first excited states of the single-ion ligand-field split Dy(III) levels and the experimentally observed anisotropy barrier. Furthermore, the transverse g factors found for the Dy(III) ions, defining the tunnelling rates within the ground Kramers doublets, are largest for 1, which agrees with the experimental observation of the shortest relaxation time in the high-temperature domain for this complex. The magnetic exchange between the Dy(III) ions revealed overall antiferromagnetic interactions for each compound, derived from the dominant dipolar exchange resulting in nonmagnetic ground states for 1-4. The diamagnetic ground states coupled with small tunneling gaps resulted in quantum tunneling time scales at zero field of between 0.1 and >1.5 s.

Synthesis, structure, and magnetism of a family of heterometallic {Cu2Ln7} and {Cu4Ln12} (Ln = Gd, Tb, and Dy) complexes: the Gd analogues exhibiting a large magnetocaloric effect.

The syntheses, structures, and magnetic properties of two heterometallic Cu(II)-Ln(III) (Ln(III) = Gd, Tb, and Dy) families, utilizing triethanolamine and carboxylate ligands, are reported. The first structural motif displays a nonanuclear {Cu(II)2Ln(III)7} metallic core, while the second reveals a hexadecanuclear {Cu(II)4Ln(III)12} core. The differing nuclearities of the two families stem from the choice of carboxylic acid used in the synthesis. Magnetic studies show that the most impressive features are displayed by the {Cu(II)2Gd(III)7} and {Cu(II)4Gd(III)12} complexes, which display a large magnetocaloric effect, with entropy changes -ΔSm = 34.6 and 33.0 J kg(-1) K(-1) at T = 2.7 and 2.9 K, respectively, for a 9 T applied field change. It is also found that the {Cu(II)4Dy(III)12} complex displays single-molecule magnet behavior, with an anisotropy barrier to magnetization reversal of 10.1 K.

Structure, magnetic behavior, and anisotropy of homoleptic trinuclear lanthanoid 8-quinolinolate complexes.

Three complexes of the form [Ln(III)3(OQ)9] (Ln = Gd, Tb, Dy; OQ = 8-quinolinolate) have been synthesized and their magnetic properties studied. The trinuclear complexes adopt V-shaped geometries with three bridging 8-quinolinolate oxygen atoms between the central and peripheral eight-coordinate metal atoms. The magnetic properties of these three complexes differ greatly. Variable-temperature direct-current (dc) magnetic susceptibility measurements reveal that the gadolinium and terbium complexes display weak antiferromagnetic nearest-neighbor magnetic exchange interactions. This was quantified in the isotropic gadolinium case with an exchangecoupling parameter of J = -0.068(2) cm(-1). The dysprosium compound displays weak ferromagnetic exchange. Variable-frequency and -temperature alternating-current magnetic susceptibility measurements on the anisotropic cases reveal that the dysprosium complex displays single-molecule-magnet behavior, in zero dc field, with two distinct relaxation modes of differing time scales within the same molecule. Analysis of the data revealed anisotropy barriers of Ueff = 92 and 48 K for the two processes. The terbium complex, on the other hand, displays no such behavior in zero dc field, but upon application of a static dc field, slow magnetic relaxation can be observed. Ab initio and electrostatic calculations were used in an attempt to explain the origin of the experimentally observed slow relaxation of the magnetization for the dysprosium complex.