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Single-molecule magnets (SMMs) have gained attention recently because of parallel/antiparallel bistability of magnetization to a given z direction of a magnetic field when that field is weakened or removed.[1] This bistability is due to an energy barrier to relaxation of the magnetization, which, at low temperatures, becomes significant with respect to ambient thermal energy and prevents loss of magnetic order. Molecules that show relaxation of the magnetization of purely molecular origin are thus described as SMMs.[2] In SMMs based on lanthanide ions the crucial magnetic anisotropy is the result of lifting the degeneracy of the ground states into new sublevels by some ligand-field potentials.Although mononuclear lanthanide SMMs were first generated by Ishikawa and co-workers using two macrocyclic ligands to sandwich the lanthanide ion in a double-decker fashion, [3] they can also be prepared by using a range of acyclic ligands such as polyoxometalates, [4] Schiff bases, [5] radicals, [6] and ketones. [7] There are several mononuclear lanthanide SMMs that also contain one or more 3d metal ions. [8] However, to the best of our knowledge, there are no examples of SMMs containing a single lanthanide ion bound within the cavity of just one organic macrocycle (rather than being sandwiched between two such macrocycles [3,9] or being part of a metallomacrocycle only, that is, no organic macrocycle [10] ). Such a system is highly desirable as it should enhance the nuclearity/geometry control, stability, tunability and solubility of the complex, all of which are important factors for potential future applications. Here we present the first example of such a SMM.The organic macrocycle system we selected for this purpose (Scheme 1) was inspired [11] by those reported by Nabeshima and co-workers [12] and MacLachlan and co-workers.[13]Macrocyclic ligands provide discrete, metal ion binding pockets and thus offer far more predictable cluster nuclearity and structure than acyclic analogues can. Here the [3+3] macrocycle provides three N 2 O 2 pockets for 3d metal ions and one central O 6 pocket for a lanthanide ion, so mixedmetal M 3 Ln tetrametallic macrocyclic complexes are a predictable outcome (in comparison, unpredictable aggregates of 4-12 metal ions were obtained with acyclic ligand analogues [14] ). Macrocycles usually also provide enhanced stability (macrocyclic effect), solubility (vary substituents on periphery, R, to modify solubility) and fine-tunability (vary the periphery, R and n, and choice of M and Ln, whilst retaining the M 3 Ln core) over acyclic analogues. Schiff base links were chosen due to the reversibility of bond formation (facilitates error correction) and our experience with such systems. [15] Initially the dialdehyde 1,4-diformyl-2,3-dihydroxybenzene (1) was combined with a range of aliphatic diamines in 6À , used in this work has R = H and n = 3, and was prepared and complexed in situ to generate the tetrametallic Zn 3 Dy complex 2 (M = Zn, Ln = Dy).

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The abrupt [HS-HS] ↔ localized [HS-LS] spin crossovers of a new triazole-based diiron(II) complex result in a record-equaling thermal hysteresis loop width for a dinuclear complex (ΔT = 22 K by SQUID magnetometer in "settle" mode) and show a remarkable scan rate dependence of only the cooling branch, as revealed by detailed magnetic, DSC, and Mössbauer studies.

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Materials capable of sensing volatile guests at room temperature by an easily monitored set of outputs are of great appeal for development as chemical sensors of small volatile organics and toxic gases. Herein the dinuclear iron(II) complex, [Fe (L) (CH CN) ](BF ) ⋅2 CH CN (1) [L=4-(4-methylphenyl)-3-(3-pyridazinyl)-5-pyridyl-4H-1,2,4-triazole], is shown to undergo reversible single-crystal-to-single-crystal (SCSC) transformations upon exposure to vapors of different guests: 1 (MeCN)⇌2 (EtOH)→3 (H O)⇌1 (MeCN). Whilst 1 and 2 remain dimetallic, SCSC to 3 involves conversion to a 1D polymeric chain (due to a change in L bridging mode), which, remarkably, can undergo SCSC de-polymerization, reforming dimetallic 1. Additionally, SC-XRD studies of two ordered transient forms, 1TF3 and 2TF3, confirm that guest exchange occurs by diffusion of the new guests into the non-porous lattices as the old guests leave. These reversible SCSC events also induce color and magnetic responses. Indeed dark red 1 is spin crossover active (T ↓ 356 K; T ↑ 369 K), whilst orange 2 and yellow 3 remain high spin.

Access to a new family of thioether-linked PSRT ligands, 4-substituted-3,5-bis{[(2-pyridylmethyl)sulfanyl]methyl}-4H-1,2,4-triazoles (analogues of the previously studied amino-linked PMRT ligands), has been established. Four such ligands have been prepared, PSPhT, PS(i)BuT, PS(t-Bu)PhT, and PS(Me)PhT, with R = Ph, (i)Bu, (t-Bu)Ph, and (Me)Ph, respectively. Three dinuclear colorless to pale green iron(II) complexes, [Fe(II)2(PSRT)2](BF4)4·solvent, featuring N4S2 donor sets, were prepared. Single-crystal structure determinations on [Fe(II)2(PSPhT)2](BF4)4·2MeCN·H2O, [Fe(II)2(PSPhT)2](BF4)4·2(1)/2MeCN·(1)/2H2O·THF, [Fe(II)2(PS(Me)PhT)2](BF4)4·2MeCN, and [Fe(II)2(PS(i)BuT)2](BF4)4·4MeCN reveal that all four are stabilized in the [HS-HS] state to 100 K and that both possible binding modes of the bis-terdentate ligands, cis- and trans-axial, are observed. Variable-temperature magnetic susceptibility studies of air-dried crystals (solvatomorphs of the single crystal samples) reveal the first examples of spin crossover (SCO) for a dinuclear iron(II) complex with N4S2 coordination. Specifically, [Fe(II)2(PSPhT)2](BF4)4·2(1)/2H2O undergoes a multistep but complete SCO from [HS-HS] to [LS-LS], whereas [Fe(II)2(PS(Me)PhT)2](BF4)4·1(1)/2MeCN·2H2O exhibits a half-SCO from [HS-HS] to [HS-LS]. In contrast, [Fe(II)2(PS(i)BuT)2](BF4)4·MeCN·H2O remains [HS-HS] down to 50 K. The reflectance spectrum of pale green [Fe(II)2(PSPhT)2](BF4)4·(1)/2CHCl3·2(1)/2H2O (solvatomorph A) reveals a trace of LS character (572 nm band (1)A1g → (1)T1g). Evans' (1)H NMR method and UV-vis spectroscopy studies revealed that on cooling dark green acetonitrile solutions of these complexes from 313 to 233 K, all three undergo SCO centered at or near room temperature. The tendency of the complexes to go LS in solution reflects the electronic impact of R on the σ-donor strength of the PSRT ligand, whereas the opposite trend in stabilization of the LS state is seen in the solid state, where crystal packing effects, of the R group and solvent content, dominate the SCO behavior.

Two isomeric pyrimidine-based Rdpt-type triazole ligands were made: 4-(4-methylphenyl)-3-(2-pyrimidyl)-5-phenyl-4 H-1,2,4-triazole (L) and 4-(4-methylphenyl)-3-(4-pyrimidyl)-5-phenyl-4 H-1,2,4-triazole (L). When reacted with [Fe(pyridine)(NCE)], where E = S, Se, or BH, two families of mononuclear iron(II) complexes are obtained, including six solvatomorphs, giving a total of 12 compounds: [Fe(L)(NCS)] (1), [Fe(L)(NCSe)] (2), 2·1.5HO, [Fe(L)(NCBH)]·2CHCl (3·2CHCl), 3 and 3·2HO, [Fe(L)(NCS)] (4), 4·HO, [Fe(L)(NCSe)] (5), 5·2CHOH, 5·1.5HO, and [Fe(L)(NCBH)]·2.5HO (6·2.5HO). Single-crystal X-ray diffraction reveals that the N-coordinated iron(II) centers in 1, 2, 3·2CHCl, 4, 5, and 5·2CHOH have two bidentate triazole ligands equatorially bound and two axial NCE co-ligands trans-coordinated. All structures are high spin (HS) at 100 K, except 3·2CHCl, which is low spin (LS). Solid-state magnetic measurements show that only 3·2CHCl ( T above 400 K) and 5·1.5HO ( T = 110 K) undergo spin crossover (SCO); the others remain HS at 300-50 K. When 3·2CHCl is heated at 400 K it desorbs CHCl becoming 3, which remains HS at 400-50 K. UV-Vis studies in CHCl, CHCl, (CH)CO, CHCN, and CHNO solutions for the BH analogues 3 and 6 led to a 6:1 ratio of L /Fe(II) being employed for the solution studies. These revealed SCO activity in all five solvents, with T values for the 2-pyrimidine complex (247-396 K) that were consistently higher than for the 4-pyrimidine complex (216-367 K), regardless of solvent choice, consistent with the 2-pyrimidine ring providing a stronger ligand field than the 4-pyrimidine ring. Strong correlations of solvent polarity index with the T values in those solvents are observed for each complex, enabling predictable T tuning by up to 150 K. While this correlation is tantalizing, here it may also be reflecting solvent-dependent speciation-so future tests of this concept should employ more stable complexes. Differences between solid-state (ligand field; crystal packing; solvent content) and solution (ligand field; solvation; speciation) effects on SCO are highlighted.