Spin Crossover Star-Shaped Metallomesogens of Iron(II)

Three new types of spin crossover (SCO) metallomesogens of Fe^II based on symmetric tripod ligands and their magnetic and structural properties are reported here. These were obtained by condensation of tris­(2-aminoethyl)­amin (tren) with the aldehyde derived from 3-alkoxy-6-methylpyridine (<b>mpyN</b>, N (number of carbon atoms in <i>n</i>-alkyl chains) = 8, 18), 1-alkyl-1<i>H</i>-imidazole (<b>imN</b>, N = 4, 16, 18, 20, 22), or 1-alkyl-1<i>H</i>-benzimidazole (<b>bimN</b>, N = 6, 14, 16, 18, 20). A complex derived from 1-octadecyl-1<i>H</i>-naphtho­[2,3-<i>d</i>]­imidazole (<b>nim18</b>) retains the high spin state at any temperature. Single crystals of the short-chain complexes were investigated by a combination of X-ray crystallography, magnetic measurements and Mössbauer spectroscopy. Generally, in comparison with the short-chain complexes the long-chain complexes display more gradual SCO and undergo a phase transition crystal–liquid crystal that is reflected in their magnetic properties. Characterization by X-ray powder diffractometry and differential calorimetry reveal formation of a smectic mesophase upon melting

Magnetism and Molecular Nonlinear Optical Second-Order Response Meet in a Spin Crossover Complex

The quadratic hyperpolarizability of two inorganic Schiff base metal complexes which differ from each other by the nature of the central metal ion (Fe^II or Zn^II) is estimated using hyper-Rayleigh light-scattering (HRS) measurements. The investigated Fe^II microcrystals exhibit a thermal spin-crossover (SCO) from a diamagnetic to a paramagnetic state centered at <i>T</i>_1/2 = 233 K that can be reproduced by the HRS signal whose modest intensity is mainly due to their centrosymmetric packing structure. Diamagnetic Zn^II microcrystals even lead to much weaker (∼400 times) HRS intensities which are in addition temperature-independent. These observations allow us to ascribe the change in HRS of the Fe^II complex to two contributions, namely, the molecular SCO phenomenon and the crystal orientation with respect to the light polarization. A connection between the SCO and a nonlinear optical property has thus been demonstrated for the first time, with potential future applications in photonics

Strong Cooperative Spin Crossover in 2D and 3D Fe^II–M^I,II Hofmann-Like Coordination Polymers Based on 2‑Fluoropyrazine

Self-assembling iron­(II), 2-fluoropyrazine (Fpz), and [M^II(CN)₄]^2– (M^II = Ni, Pd, Pt) or [Au^I(CN)₂]⁻ building blocks have afforded a new series of two- (2D) and three-dimensional (3D) Hofmann-like spin crossover (SCO) coordination polymers with strong cooperative magnetic, calorimetric, and optical properties. The iron­(II) ions, lying on inversion centers, define elongated octahedrons equatorially surrounded by four equivalent centrosymmetric μ₄-[M^II(CN)₄]^2– groups. The axial positions are occupied by two terminal Fpz ligands affording significantly corrugated 2D layers {Fe­(Fpz)₂([M^II(CN)₄]}. The Pt and Pd derivatives undergo thermal- and light-induced SCO characterized by <i>T</i>_1/2 temperatures centered at 155.5 and 116 K and hysteresis loops 22 K wide, while the Ni derivative is high spin at all temperatures, even at pressures of 0.7 GPa. The great stability of the high-spin state in the Ni derivative has tentatively been ascribed to the tight packing of the layers, which contrasts with that of Pt and Pd derivatives in the high- and low-spin states. The synthesis and structure of the 3D frameworks formulated {Fe­(Fpz)­[Pt­(CN)₄]}·1/2H₂O and {Fe­(Fpz)­[Au­(CN)₂]₂}, where Fpz acts as bridging ligand, which is also discussed. The former is high spin at all temperatures, while the latter displays very strong cooperative SCO centered at 243 K accompanied by a hysteresis loop 42.5 K wide. The crystal structures and SCO properties are compared with those of related complexes derived from pyrazine, 3-fluoropyridine, and pyridine

K₂M^III₂(M^VIO₄)(PO₄)₂ (M^III = Fe, Sc; M^VI = Mo, W), Novel Members of the Lagbeinite-Related Family: Synthesis, Structure, and Magnetic Properties

The possibility of PO₄^3– for MoO₄^2– partial substitution in the langbeinite framework has been studied by exploration of the K–Fe­(Sc)–Mo­(W)–P–O systems using the high-temperature solution method. It was shown that 1/3PO₄^3– for MoO₄^2– substitution leads to formation of three novel compounds K₂Fe­(MoO₄)­(PO₄)₂, K₂Sc­(MoO₄)­(PO₄)₂, and K₂Sc­(WO₄)­(PO₄)₂ with slightly increased lattice parameters and significant distortion of the anion tetrahedra without structure changes. In contrast, the antiferromagnetic structure is modified by substitution in the low-temperature region. The structural peculiarities are discussed in light of bond-valence sums calculations

Homoleptic Iron(II) Complexes with the Ionogenic Ligand 6,6′-Bis(1<i>H</i>‑tetrazol-5-yl)-2,2′-bipyridine: Spin Crossover Behavior in a Singular 2D Spin Crossover Coordination Polymer

Deprotonation of the ionogenic tetradentate ligand 6,6′-bis­(1<i>H</i>-tetrazol-5-yl)-2,2′-bipyridine [H₂bipy­(ttr)₂] in the presence of Fe^II in solution has afforded an anionic mononuclear complex and a neutral two-dimensional coordination polymer formulated as, respectively, NEt₃H­{Fe­[bipy­(ttr)₂]­[Hbipy­(ttr)₂]}·3MeOH (<b>1</b>) and {Fe­[bipy­(ttr)₂]}<i>_n</i> (<b>2</b>). The anions [Hbipy­(ttr)₂]⁻ and [bipy­(ttr)₂]^2– embrace the Fe^II centers defining discrete molecular units <b>1</b> with the Fe^II ion lying in a distorted bisdisphenoid dodecahedron, a rare example of octacoordination in the coordination environment of this cation. The magnetic behavior of <b>1</b> shows that the Fe^II is high-spin, and its Mössbauer spectrum is characterized by a relatively large average quadrupole splitting, Δ<i>E</i>_Q = 3.42 mm s^–1. Compound <b>2</b> defines a strongly distorted octahedral environment for Fe^II in which one [bipy­(ttr)₂]⁻ anion coordinates the equatorial positions of the Fe^II center, while the axial positions are occupied by peripheral <i>N</i>-tetrazole atoms of two adjacent {Fe­[bipy­(ttr)₂]}⁰ moieties thereby generating an infinite double-layer sheet. Compound <b>2</b> undergoes an almost complete spin crossover transition between the high-spin and low-spin states centered at about 221 K characterized by an average variation of enthalpy and entropy Δ<i>H</i>^av = 8.27 kJ mol^–1, Δ<i>S</i>^av = 37.5 J K^–1 mol^–1, obtained from calorimetric DSC measurements. Photomagnetic measurements of <b>2</b> at 10 K show an almost complete light-induced spin state trapping (LIESST) effect which denotes occurrence of antiferromagnetic coupling between the excited high-spin species and <i>T</i>_LIESST = 52 K. The crystal structure of <b>2</b> has been investigated in detail at various temperatures and discussed