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 {CrIIIDyIII 6} 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 {CrIIIDyIII 6} reveals the pivotal role played by ferrotoroidic states in slowing down the magnetic relaxation, in spite of large calculated single-ion quantum tunneling rates.

The synthesis and magnetic and theoretical studies of three isostructural heterometallic [CoLn(μ-OH)(o-tol)(mdea)(NO)] (Ln = Dy (1), Tb (2), Ho (3)) "butterfly" complexes are reported (o-tol = o-toluate, (mdea) = doubly deprotonated N-methyldiethanolamine). The Co ions are diamagnetic in these complexes. Analysis of the dc magnetic susceptibility measurements reveal antiferromagnetic exchange coupling between the two Ln 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 U of 81.2 cm, while complexes 2 and 3 exhibit field induced SMM behavior, with a U value of 34.2 cm 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 Ln-Ln magnetic coupling and to develop an understanding of the role the diamagnetic Co ion plays in the magnetization relaxation. The calculations were able to rationalize the experimental relaxation data for all complexes and strongly suggest that the Co ion is integral to the observation of SMM behavior in these systems. Thus, we explored further the effect that the diamagnetic Co ions have on the magnetization blocking of 1. We did this by modeling a dinuclear {Dy} complex (1a), with the removal of the diamagnetic ions, and three complexes of the types {KDy} (1b), {ZnDy} (1c), and {TiDy} (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.

A family of halogen-substituted Schiff base iron(II) complexes, [Fe (qsal-X) ], (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 T values obtained, to date, for Fe N/O species. We have recently reported subtle symmetry breaking in [Fe (qsal-Cl) ] 2 with two spin transition steps occurring at 308 and 316 K. A photomagnetic study reveals almost full HS conversion of [Fe (qsal-I) ] 4 at low temperature (T(LIESST)=54 °K). The halogen substitution effects on the magnetic properties, as well as the crystal packing of the [Fe (qsal-X) ] 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, [Fe (qsal-X) ] , the two extra π-π and P4AE interactions found in [Fe (qsal-X) ] compounds, are believed to be accountable for the spin transitions occurring at ambient temperatures.

The first examples of metallacycles containing rare earth ions bridged by radicals are reported. The molecular triangles [Ln(hfac)(bptz)] (Ln = Dy, Y; 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 [Dy(hfac)(bptz)] reveal the presence of antiferromagnetic coupling between the Dy centers and the bptz ligands, with J = -6.62 cm.

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.

Three neutral bis(μ-chalcogenido)diiron(III) complexes, [{(N,N'-Pipiso)Fe(μ-E)}2] (Pipiso(-) = [(DipN)2C(cis-2,6-Me2NC5H8)](-), (Dip = C6H3Pr(I)2-2,6; E = O, S or Se) have been prepared by reactions of the iron(I) dimer [{(μ-N,N'-Pipiso)Fe}2] with O2, S8 or Se∞. Treating the μ-selenido compound [{(N,N'-Pipiso)Fe(μ-Se)}2] with O2 cleanly generated its μ-oxo counterpart, [{(N,N'-Pipiso)Fe(μ-O)}2]. X-ray crystallographic analyses of the compounds showed them to possess Fe2(μ-E)2 core structures with distorted square planar (E = O) or tetrahedral (E = S or Se) iron coordination geometries. Magnetic, (57)Fe Mössbauer spectroscopic and computational studies indicate medium to strong antiferromagnetic coupling between the two high-spin Fe(III) ions in all three compounds.

The synthesis of two new radical-bridged compounds [Co(bptz)(dbm)]·2toluene (1) and [Co(bptz)(dbm)]·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 for 1 and 2, respectively. DFT calculations further support the strong antiferromagnetic coupling between Co ions and bptz radicals and confirm S = 3 and S = 4 spin ground states for 1 and 2, respectively.

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 Co and Dy ions. Single-crystal X-ray diffraction studies revealed molecular formulae of [Co Dy (μ-OH) (μ -OMe) {O CC(CH ) } (tea) (H O) ]⋅4 H O (1) and [Co Dy (μ-F) (μ -OH) (o-tol) (mdea) ]⋅ 3 H O⋅EtOH⋅MeOH (2; tea =triply deprotonated triethanolamine; mdea =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, [Cr Dy (μ-F )(μ -OMe) (μ -OH) (O CPh) (mdea) ] (3), are discussed and compared with a structurally similar complex, [Cr Dy (μ -OH) (μ-N ) (mdea) (O CC(CH ) ) ] (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 (U ) of 39.0, 55.0 and 10.4 cm respectively. Complex 1, conversely, did not exhibit slow relaxation of magnetisation above 2 K. To probe the variance in the observed U 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 J as -1.6, 1.6 and 2.8 cm for complexes 1, 2 and 3, respectively, whereas the J interaction was estimated to be -1.8 cm 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 Co ions with paramagnetic Cr led to Cr ⋅⋅⋅Dy coupling, which resulted in quenching of QTM at low temperatures for complexes 3 and 4. The best example was found if both Cr 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 Dy SMMs studied herein.