Elevating the Performance of Heterometallic 3d/4f SMMs: The Role of Diamagnetic CoIII and ZnII Ions in Magnetization Dynamics.

Recent advances in the synthesis of 3d/4f Single-Molecule Magnets (SMMs) have revealed the effective role of incorporating diamagnetic CoIII or ZnII ions to enhance the magnetic properties of LnIII ions. This concept highlights notable examples of CoIII/LnIII and ZnII/LnIII SMMs documented in the recent literature, illustrating how the selection of various peripheral and/or bridging ligands can modulate the magnetic anisotropy of 4f metal ions, thereby increasing their energy barriers.

Slow Magnetization Relaxation in a Family of Triangular {CoIII 2 LnIII } Clusters: The Effect of Diamagnetic CoIII Ions on the LnIII Magnetic Dynamics.

The first use of the Schiff base chelate N-naphthalidene-o-aminophenol (naphH2 ) in Co/Ln chemistry has afforded a family of isostructural [CoIII 2 LnIII (OMe)2 (naph)2 (O2 CMe)3 (MeOH)2 ] (Ln=Tb, Dy and Er) complexes, revealing a rare {CoIII 2 Ln(µ3 -OMe)}8+ triangular core composed of two diamagnetic CoIII ions and a 4f-ion with slightly distorted square antiprismatic geometry. Alternating current (ac) magnetic susceptibility studies revealed that {Co2 Dy}, and its magnetic diluted analogue {Co2 Dy0.05 Y0.95 }, behave as mononuclear single-molecule magnets (SMMs) with similar energy barriers for the magnetization reversal, Ueff , of ~85-90 K. SMM properties were also detected for {Co2 Er}, with the compound exhibiting a Ueff of 18.7 K under an applied magnetic field of 800 Oe. To interpret the experimental magnetic results, ab initio CASSCF/RASSI-SO and DFT calculations were performed as a means of exploring the single-ion characteristics of LnIII ions and comprehend the role of the diamagnetic CoIII ions in the magnetization relaxation of the three heterometallic compounds.

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.

Lanthanide Triangles Supported by Radical Bridging Ligands.

The first examples of metallacycles containing rare earth ions bridged by radicals are reported. The molecular triangles [Ln3(hfac)6(bptz•-)3] (Ln = DyIII, YIII; hfac = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; bptz = 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine) consist of lanthanide ions bridged by bptz radical anion (bptz•-) ligands. Magnetic susceptibility measurements and CASSCF calculations performed on [Dy3(hfac)6(bptz•-)3] reveal the presence of antiferromagnetic coupling between the DyIII centers and the bptz•- ligands, with J = -6.62 cm-1.

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.

Putting a New Spin on Supramolecular Metallacycles: Co3 Triangle and Co4 Square Bearing Tetrazine-Based Radicals as Bridges.

The synthesis of two new radical-bridged compounds [Co3(bptz)3(dbm)3]·2toluene (1) and [Co4(bptz)4(dbm)4]·4MeCN (2) (bptz = 3,6-bis(pyridyl)-1,2,4,5-tetrazine; dbm = 1,3-diphenyl-1,3-propanedionate) is reported. The presence of the ligand-centered radical has been confirmed by X-ray crystallography and SQUID magnetometry. These complexes are the first metallacycles bearing nitrogen heterocyclic radicals as bridges. Magnetic studies reveal strong antiferromagnetic metal···radical coupling with coupling constants of J = -67.5 and -66.8 cm-1 for 1 and 2, respectively. DFT calculations further support the strong antiferromagnetic coupling between CoII ions and bptz radicals and confirm S = 3 and S = 4 spin ground states for 1 and 2, respectively.

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.

Halogen Substitution Effects on N2 O Schiff Base Ligands in Unprecedented Abrupt FeII Spin Crossover Complexes.

A family of halogen-substituted Schiff base iron(II) complexes, [FeII (qsal-X)2 ], (qsal-X=5-X-N-(8-quinolyl)salicylaldimines)) in which X=F (1), Cl (2), Br (3) or I (4) has been investigated in detail. Compound 1 shows a temperature invariant high spin state, whereas the others all show abrupt spin transitions, at or above room temperature, namely, 295 K (X=I) up to 342 K (X=Br), these being some of the highest T1/2 values obtained, to date, for FeII N/O species. We have recently reported subtle symmetry breaking in [FeII (qsal-Cl)2 ] 2 with two spin transition steps occurring at 308 and 316 K. A photomagnetic study reveals almost full HS conversion of [FeII (qsal-I)2 ] 4 at low temperature (T(LIESST)=54 °K). The halogen substitution effects on the magnetic properties, as well as the crystal packing of the [FeII (qsal-X)2 ] compounds and theoretical calculations, are discussed in depth, giving important knowledge for the design of new spin crossover materials. In comparison to the well known iron(III) analogues, [FeIII (qsal-X)2 ]+ , the two extra π-π and P4AE interactions found in [FeII (qsal-X)2 ] compounds, are believed to be accountable for the spin transitions occurring at ambient temperatures.

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.

Influence of the Ligand Field on the Slow Relaxation of Magnetization of Unsymmetrical Monomeric Lanthanide Complexes: Synthesis and Theoretical Studies.

A series of monomeric lanthanide Schiff base complexes with the molecular formulas [Ce(HL)3(NO3)3] (1) and [Ln(HL)2(NO3)3], where LnIII = Tb (2), Ho (3), Er (4), and Lu (5), were isolated and characterized by single-crystal X-ray diffraction (XRD). Single-crystal XRD reveals that, except for 1, all complexes possess two crystallographically distinct molecules within the unit cell. Both of these crystallographically distinct molecules possess the same molecular formula, but the orientation of the coordinating ligand distinctly differs from those in complexes 2-5. Alternating-current magnetic susceptibility measurement reveals that complexes 1-3 exhibit slow relaxation of magnetization in the presence of an optimum external magnetic field. In contrast to 1-3, complex 4 shows a blockade of magnetization in the absence of an external magnetic field, a signature characteristic of a single-ion magnet (SIM). The distinct magnetic behavior observed in 4 compared to other complexes is correlated to the suitable ligand field around a prolate ErIII ion. Although the ligand field stabilizes an easy axis of anisotropy, quantum tunnelling of magnetization (QTM) is still predominant in 4 because of the low symmetry of the complex. The combination of low symmetry and an unsuitable ligand-field environment in complexes 1-3 triggers faster magnetization relaxation; hence, these complexes exhibit field-induced SIM behavior. In order to understand the electronic structures of complexes 1-4 and the distinct magnetic behavior observed, ab initio calculations were performed. Using the crystal structure of the complexes, magnetic susceptibility data were computed for all of the complexes. The computed susceptibility and magnetization are in good agreement with the experimental magnetic data [χMT(T) and M(H)] and this offers confidence on the reliability of the extracted parameters. A tentative mechanism of magnetization relaxation observed in these complexes is also discussed in detail.

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.

Strong Ferromagnetic Exchange Coupling Mediated by a Bridging Tetrazine Radical in a Dinuclear Nickel Complex.

The radical bridged compound [(Ni(TPMA))2-µ-bmtz•-](BF4)3·3CH3CN (bmtz = 3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine, TPMA = tris(2-pyridylmethyl)amine) exhibits strong ferromagnetic exchange between the S = 1 NiII centers and the bridging S = 1/2 bmtz radical with J = 96 ± 5 cm-1 (-2JNi-radSNiSrad). DFT calculations support the existence of strong ferromagnetic exchange.

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 combined theoretical and experimental approach to determine the right choice of co-ligand to impart spin crossover in Fe(II) complexes based on 1,3,4-oxadiazole ligands.

We present the synthesis of two new novel tetradentate ligands based on 1,3,4-oxadiazole, 2-(2-pyridyl)-5-[N,N-bis(2-pyridylmethyl)aminomethyl]-1,3,4-oxadiazole (LTetraPy-ODA) and 2-(2-phenyl)-5-[N,N-bis(2-pyridylmethyl)aminomethyl]-1,3,4-oxadiazole (LTetraPh-ODA). The ligands were used to prepare six mononuclear complexes [FeII(LTetraPy-ODA)(NCE)] (C1-C3) and [FeII(LTetraPh-ODA)(NCE)] (C4-C6) where E = S, Se or BH3. In addition, the ligand LTetraPy-ODA was employed in the synthesis of a new di-nuclear complex [FeII2(LTetraPh)](ClO4)4·1 CH3NO2·1.5 H2O (C7). Characterization of all complexes was carried out using single-crystal X-ray crystallography, elemental analysis, and infrared spectroscopy. Magnetic susceptibility measurements, performed in the temperature range of 2-300 K using a SQUID magnetometer, revealed spin crossover behaviour exclusively in the mononuclear complexes C3 and C6, in which two monodentate NCBH3- co-ligands coordinate. The presence of the lattice solvent was found to be crucial to the spin transition property, with complex C3 exhibiting a switching temperature (T1/2) of approximately 165 K and C6 approximately 194 K. The other four mononuclear complexes C1, C2, C4, C5, as well as the dinuclear complex C7 are locked in the high spin state over the measured temperature range. Density Functional Theory (DFT) calculations were performed on complexes C1-C6 to rationalise the observed magnetic behaviour, demonstrating the significant effect of the NCS-, NCSe- and NCBH3- co-ligands ligands on the spin-crossover behaviour of the [FeII(L)(NCE)] complexes.

Efficient synthetic route to heterobimetallic trinuclear complexes [Ln-Mn-Ln] and their single molecule magnetic properties.

A series of mononuclear lanthanoidate complexes isolated as [Bu4N][Ln(QCl4)] 1Ln (QCl = 5-chloro-8-quinolinolate; Ln = Eu, Gd, Tb, Dy, Ho, and Er) have been prepared, characterised, and used as facile precursors to obtain a series of new heterobimetallic complexes as crystalline materials. Reaction of 1Ln with manganese nitrate forms [Ln2Mn(QCl)8] (2Ln, where Ln = Tb, Dy, Er and Yb) which have been structurally characterised in the cases of 2Tb and 2Yb. The heteroleptic trinuclear complex [Dy3(QCl)8Cl(OH2)], 3, has also been obtained. Compounds 1Dy, 1Tb, and 1Er display slow relaxation of magnetisation below 10K, particularly for the prolate Er3+ ion. These results also suggest that the positive effects of the change from mononuclear to trinuclear lanthanoid complexes enhance their single molecule magnetic (SMM) behaviour, as evidenced by the well resolved frequency dependent AC out-of-phase susceptibility maxima seen in the 2Ln systems, that have been analysed quantitatively. The synthesis used here provides a promising strategy in obtaining heterobimetallic complexes with quinolinolate ligands and also constructing efficient heterobimetallic SMMs.

Strategies to Design Single-Molecule Toroics Using Triangular {Ln3} n Motifs.

In this mini-review, we highlight the research advanced in the field of single-molecule toroics (SMTs) with a specific focus on the triangular Ln3-based SMTs. SMTs are molecules with a toroidal magnetic state and are insensitive to homogeneous magnetic fields but cooperate with charge and spin currents. The rapid growth in the area of SMTs witnessed in recent years is correlated not only to the interest to understand the fundamental physics of these molecules but also to the intriguing potential applications proposed, as the SMTs have several advantages compared to other classes of molecules such as single-molecule magnets (SMMs). The important chemico-structural strategy in SMT chemistry is to choose and design ligand and bridging species that will help to attain toroidal behavior. Considering this primarily, all the Dy3 SMTs reported so far are summarized, showing how utilizing different peripheral ligands influences the toroidal nature beyond the role of the symmetry of the molecule and stronger dipolar interactions. Likewise, linking Dy3 toroidal units through 3d ions with suitable peripheral/bridging ligands enhances the toroidal magnetic moment and leads to fascinating physics of ferrotoroidal/antiferrotoridal behavior. Further, we have also summarized the recently reported non-Dy triangular SMTs.

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.

Quinoxaline radical-bridged transition metal complexes with very strong antiferromagnetic coupling.

A new family of radical-bridged compounds, (Cp*2Co)[M2Cl4(dpq)] (M = Fe (1), Co (2), Zn (3)), (dpq = 2,3-di(2-pyridyl)-quinoxaline) is reported. Magnetic studies, DFT and ab initio calculations reveal strong antiferromagnetic metal-radical interactions with coupling constants of J = -213.1 and -218.8 cm-1 for 1 and 2, respectively.