Liquid-Crystalline Zinc(II) and Iron(II) Alkyltriazoles One-Dimensional Coordination Polymers

Several series of unidimensional coordination polymers of formula [Zn­(C_<i>n</i>H_2<i>n</i>+1trz)₃]­(Cl)₂·<i>x</i>H₂O (<i>n</i> = 18, 16, 13, 11, 10, trz = 4-substituted-1,2,4-triazole), [Zn­(C₁₈H₃₇trz)₃]­(ptol)₂·<i>x</i>H₂O, [Fe­(C_<i>n</i>H_2<i>n</i>+1trz)₃]­(X)₂·<i>x</i>H₂O (<i>n</i> = 18, 16, 13, 10; X = Cl^– or ptol^–, where ptol^– = <i>p</i>-tolylsulfonate anion), and [Fe­(C₁₈H₃₇trz)₃]­(X)₂·<i>x</i>H₂O (X = C₈H₁₇PhSO₃^– and C₈H₁₇SO₃^–) are reported with their thermal, structural, and magnetic properties. Most of these materials exhibit thermotropic lamellar mesophases at temperatures as low as 410 K, as confirmed by textures observed by polarized optical microscopy. The corresponding phase diagrams deduced by differential scanning calorimetry are also reported. All iron-containing materials present a spin crossover phenomenon that occurs at temperatures ranging from 242 to 360 K, only slightly below the mesophase temperature domain, and remains complete and cooperative, even for the longer alkyl substituents. The use of stable diamagnetic Zn­(II) analogues proves to be very useful to characterize the comparatively less stable and less crystalline Fe­(II) analogues

Photoinduced Single-Molecule Magnet Properties in a Four-Coordinate Iron(II) Spin Crossover Complex

The four-coordinate Fe­(II) complex, PhB­(MesIm)₃Fe-NPPh₃ (<b>1</b>) has been previously reported to undergo a thermal spin-crossover (SCO) between high-spin (HS, <i>S</i> = 2) and low-spin (LS, <i>S</i> = 0) states. This complex is photoactive below 20 K, undergoing a photoinduced LS to HS spin state change, as determined by optical reflectivity and photomagnetic measurements. With continuous white light irradiation, <b>1</b> displays slow relaxation of the magnetization, i.e. single-molecule magnet (SMM) properties, at temperatures below 5 K. This complex provides a structural template for the design of new photoinduced mononuclear SMMs based on the SCO phenomenon

Steric and Electronic Control of the Spin State in Three-Fold Symmetric, Four-Coordinate Iron(II) Complexes

The three-fold symmetric, four-coordinate iron­(II) phosphoraminimato complexes PhB­(MesIm)₃Fe–NPRR′R″ (PRR′R″ = PMePh₂, PMe₂Ph, PMe₃, and PⁿPr₃) undergo a thermally induced <i>S</i> = 0 to <i>S</i> = 2 spin-crossover in fluid solution. Smaller phosphoraminimato ligands stabilize the low-spin state, and an excellent correlation is observed between the characteristic temperature of the spin-crossover (<i>T</i>_1/2) and the Tolman cone angle (θ). Complexes with <i>para</i>-substituted triaryl phosphoraminimato ligands (<i>p</i>-XC₆H₄)₃PN^– (X = H, Me and OMe) also undergo spin-crossover in solution. These isosteric phosphoraminimato ligands reveal that the low-spin state is stabilized by more strongly donating ligands. This control over the spin state provides important insights for modulating the magnetic properties of four-coordinate iron­(II) complexes

Thermochromic and Photoresponsive Cyanometalate Fe/Co Squares: Toward Control of the Electron Transfer Temperature

Two structurally related and photoresponsive cyanide-bridged Fe/Co square complexes, {Fe₂Co₂}, are reported: {[(Tp^Me)­Fe­(CN)₃]₂[Co­(bpy)₂]₂[(Tp^Me)­Fe­(CN)₃]₂}·12H₂O (<b>2</b>) and {[(Tp^Me)­Fe­(CN)₃]₂[Co­(bpy)₂]₂[BPh₄]₂}·6MeCN (<b>3</b>), where Tp^Me and bpy are hydridotris­(3-methylpyrazol-1-yl)­borate and 2,2′-bipyridine, respectively. Through electrochemical and spectroscopic studies, the Tp^Me ligand appears to be a moderate σ donor in comparison to others in the [NEt₄]­[(Tp^R)­Fe^III(CN)₃] series [where Tp^R = Tp, hydridotris­(pyrazol-1-yl)­borate; Tp^Me = hydridotris­(3-methylpyrazol-1-yl)­borate; pzTp = tetrakis­(pyrazol-1-yl)­borate; Tp* = hydridotris­(3,5-dimethylpyrazol-1-yl)­borate; Tp*^Me = hydridotris­(3,4,5-trimethylpyrazol-1-yl)­borate]. The spectroscopic, structural, and magnetic data of the {Fe₂Co₂} squares indicate that thermally-induced intramolecular electron transfer reversibly converts {Fe^II_LS(μ-CN)­Co^III_LS} pairs into {Fe^III_LS(μ-CN)­Co^II_HS} units near ca. 230 and 244 K (<i>T</i>_1/2) for <b>2</b> and <b>3</b>, respectively (LS: low spin; HS: high spin). These experimental results show that <b>2</b> and <b>3</b> display light-induced {Fe^III_LS(μ-CN)­Co^II_HS} metastable states that relax to thermodynamic {Fe^II_LS(μ-CN)­Co^III_LS} ones at ca. 90 K. Ancillary Tp^R ligand donor strength appears to be the dominant factor for tuning electron transfer properties in these {Fe₂Co₂} complexes