Crystalline-Statecis-to-transTransformation of a Two-Dimensional Spin-Crossover System

Single crystal-to-single crystal transformation - from two distinct to three distinct spin crossover centers in 2D coordination polymer [Fe(bbtr)3](CF3SO3)2

1,4-Di(1,2,3-triazol-1-yl)butane (bbtr) forms a two-dimensional (2D) coordination polymer (1) in a reaction with iron(II) triflate. In the crystal lattice there are two crystallographically unique iron(II) ions surrounded octahedrally by a 1,2,3-triazole ring coordinated through nitrogen atoms N3. Single crystal X-ray diffraction studies revealed that spin crossover for each crystallographically independent iron(II) ion proceeds at a different temperature (T_1/2(Fe1) = 201 K; T_1/2(Fe2) = 216 K), while the magnetic measurements showed that there is one step, complete thermally induced spin crossover (T_1/2 = 205 K). Complex 1 undergoes, with time, single crystal-to-single crystal transformation (SCSC) to the converted system (1c) from the R3̄ to the P6₃ space group, accompanied by significant changes in the lattice parameter c (a shortening of approximately one-third) and consequently unit cell volume. Structural transformation is associated with rebuilding of the polymeric layer as well as the anion network, which is reflected in the results of Mössbauer studies. In the polymorphic system (1c) there are three crystallographically independent iron(II) ions. The temperature dependence results for magnetic susceptibility indicated complete, one-step spin crossover very similar to that of 1; however, single-crystal X-ray diffraction studies of 1c revealed that spin crossover for each crystallographically independent iron(II) ion occurs in a different manner, revealing three elementary stages (T_1/2(Fe1) = 200 K; T_1/2(Fe2) = 212 K, T_1/2(Fe3) = 214 K).

Spin crossover crystalline materials engineered via single-crystal-to-single-crystal transformations

This highlight illustrates the latest crystalline materials engineered via SCSC transformations, with emphasis on the onset and progress of spin crossover in a crystal control.

Cooperative Large-Hysteresis Spin-Crossover Transition in the Iron(II) Triazolate [Fe(ta)2] Metal-Organic Framework

The metal-organic framework [Fe(ta)₂] (Hta = 1H-1,2,3-triazole) containing Fe(II) ions and 1,2,3-triazolate ligands shows a reversible phase transition while retaining the cubic crystal symmetry and space group Fd3m (no. 227). The phase transition between room temperature (RT-[Fe(ta)₂]; a = 16.6315(2) Å, V = 4600.39(8) ų) and high temperature (HT-[Fe(ta)₂]; a = 17.7566(4) Å, V = 5598.6(1) ų) phases occurs at a temperature above 290 °C, whereas the phase transition between HT- and RT-[Fe(ta)₂] starts at a temperature below 210 °C. Both [Fe(ta)₂] polymorphs have identical bond topologies, but they differ by a large increase of the unit cell's volume of 22% for HT-[Fe(ta)₂]. The compounds are characterized by powder X-ray diffraction, differential scanning calorimetry, and thermogravimetric analyses. Additionally, Mössbauer spectroscopy, magnetic studies, and the electronic structure of both phases are discussed in detail with respect to the spin-crossover transition from the low-spin (RT-[Fe(ta)₂]) to the high-spin phase (HT-[Fe(ta)₂]).

Tuning the Spin States of Two Apical Iron(II) Ions in a Pentanuclear Iron(II) Cluster Helicate through the Choice of Anions

Spin-crossover clusters with iron(II) high nuclearity are rare. By using 3,5-bis(pyridin-2-yl)-1,2,4-triazole (bptH) as the ligand with two bidentate chelating sites, we successfully obtained three pentanuclear iron(II) cluster helicate compounds [{FeII(μ-bpt)3}2FeII3(μ3-O)][FeII2(μ-Br)(μ-bpt)(NCS)4(H2O)]·2H2O·C2H5OH (1), [{FeII(μ-bpt)3}2FeII3(μ3-O)][FeII2(μ-bpt)2(NCS)4] (2), and [{FeII(μ-bpt)3}2FeII3(μ3-O)]I2·2C2H5OH (3). Research on single-crystal structure and magnetism has indicated that tuning the spin state of two iron(II) ions in axial direction is successfully realized by regulating the different counter anions: one apical [FeII(bpt)3]− unit exhibits spin-crossover behavior while the other [FeII(bpt)3]− unit remains in low spin state in 1, both apical ions are of high spin states in 2, and are of low spin states in 3.

Reversible crystal-to-crystal transformation from a trinuclear cluster to a 1D chain and the corresponding spin crossover (SCO) behaviour change

Reversible crystal-to-crystal transformation between a linear trinuclear Fe(ii) complex and a 1D chain, and the SCO behaviour change are reported.

Molecular isomerism induced Fe(ii) spin state difference based on the tautomerization of the 4(5)-methylimidazole group

Two iron(ii) structural isomers with only one methyl position difference in their ligands exhibit different spin states due to the competition of the electronic effect and steric crowding.

A ladder-type iron(ii) coordination polymer with enhanced spin-crossover behavior

A ladder-type 1D coordination polymer with enhanced spin-crossover behaviour is reported. It showed an abrupt spin transition above room temperature with a 3 K hysteresis (T_1/2↓ = 335 K andT_1/2↑ = 338 K).

High-temperature spin crossover in two solvent-free coordination polymers with unusual high thermal stability

Two solvent-free two-dimensional (2D) coordination polymers, (2)∞[Fe(ptim)2] (1) and (2)∞[Fe(ptpy)2] (2) (Hptim = 2-(5-(4-(1H-imidazol-1-yl)phenyl)-4H-1,2,4-triazol-3-yl)pyridine; Hptpy = 2-(5-(4-(pyridin-3-yl)phenyl)-4H-1,2,4-triazol-3-yl)pyridine), have been successfully prepared by solvothermal reactions. Their iron atoms are bridged by the corresponding multidentate anionic ligands into dense neutral structures. The magnetic data reveal that complexes 1 and 2 are rare examples exhibiting reversible one-step high-temperature spin crossover behaviors with spin transition temperatures of 419 and 416 K, respectively. Moreover, these structures also display remarkable thermal stability up to 714 K (for 1) and 690 K (for 2), which are confirmed by thermogravimetric and variable-temperature powder X-ray diffraction analyses.

Peculiar phenomena of structural transformations triggered from a nickel coordination polymer

Four multi-dimensional nickel-based coordination polymers have been prepared, and their solvent-mediated and solid-state structural transformations have been observed and studied systematically.

Enhanced spin-crossover behavior mediated by supramolecular cooperative interactions

Three one-dimensional (1D) hetereobimetallic coordination polymers [Fe(II)(L)2(AgCN)2]·Solv (L = bpt(-), 1; L = Mebpt(-), Solv = 1.75EtOH, 2; L = bpzt(-), 3) with in situ generated AgCN species were synthesized by solvothermal reactions of Fe(II) salt, K[Ag(CN)2], and the corresponding ligands [bptH = 3,5-bis(pyridin-2-yl)-1,2,4-triazole, MebptH = 3-(3-methyl-2-pyridyl)-5-(2-pyridyl)-1,2,4-triazole, and bpztH = 3,5-bis(pyrazin-2-yl)-1,2,4-triazole]. They were further characterized by X-ray crystallography, magnetic and photomagnetic measurements, and differential scanning calorimetry. Single-crystal X-ray analyses show that they are isostructural with 1D zigzag chain structures with rhombus {Fe2Ag2} units, in which the substituted bpt(-) ligand connects the Fe(II) ion and AgCN species in a cis bridging mode. Then the zigzag chains are packed into three-dimensional supramolecular structures by π···π interactions. Most importantly, weak Ag···N interactions (2.750 Å at 150 K) between the π-stacked neighboring chains present in complex 3. Magnetic susceptibility measurements exhibit that complex 1 displays characteristic paramagnetic behavior in the temperature range investigated. Complex 2 undergoes a gradual spin-crossover (SCO) with critical temperatures T(1/2)↓ = 232 K and T(1/2)↑ = 235 K, whereas 3 exhibits an abrupt SCO with critical temperatures T(1/2)↓ = 286 K and T(1/2)↑ = 292 K. The magnetostructural relationships suggest that the magnetic behaviors can be modulated from paramagnetic behavior to abrupt and hysteretic SCO near room temperature through adjustment of the electronic substituent effect and intermolecular interactions.

Spin-crossover behavior in two new supramolecular isomers

Two spin-crossover (SCO) supramolecular isomers formulated as [Fe(Mebpt){Au(CN)2}]n·xH2O (1, x = 0, and 2·H2O, x = n, MebptH = 3-(5-methyl-2-pyridyl)-5-(2-pyridyl)-1,2,4-triazole) have been successfully isolated and characterized by single-crystal X-ray crystallography, thermogravimetric analysis, differential scanning calorimetry, variable-temperature powder X-ray diffraction, and magnetic measurements. Structural analysis reveals that 1 is a two-dimensional coordination layer and 2·H2O is a one-dimensional coordination ladder structure, in which the Mebpt(-) ligands are coordinated in trans and cis bridging mode in 1 and 2·H2O, respectively. Dramatically, their SCO properties are further influenced by the octahedral coordination configuration of Fe(II). In 1, only the Fe(II) centers in trans configuration exhibit spin transition (Tc = 303 K), while in 2·H2O and the dehydrated 2, gradual two-step SCO (Tc1 = 235 K, Tc2 = 313 K) and one-step SCO behaviors (Tc = 315 K) occur, respectively.

Cyanide-bridged bimetallic 3D Hoffman-like coordination polymers with tunable magnetic behaviour

Two isostructural cyanide-bridged bimetallic coordination polymers with a 3D doubly interpenetrated Hoffman-like network exhibit complete two-step spin-crossover behaviours.

Structures, photoluminescence and photocatalytic properties of two novel metal–organic frameworks based on tetrazole derivatives

Two novel, efficient metal organic framework photocatalysts based on silver salts and tetrazolate derivatives have been successfully prepared and characterized.

A chiral spin crossover metal–organic framework

A rare chiral spin crossover metal–organic framework, [Fe^II(mptpy)₂]·solv, has been solvothermally synthesized through spontaneous resolution.

Accessibility and selective stabilization of the principal spin states of iron by pyridyl versus phenolic ketimines: modulation of the 6A1 ↔ 2T2 ground-state transformation of the [FeN4O2]+ chromophore

Several potentially tridentate pyridyl and phenolic Schiff bases (apRen and HhapRen, respectively) were derived from the condensation reactions of 2-acetylpyridine (ap) and 2'-hydroxyacetophenone (Hhap), respectively, with N-R-ethylenediamine (RNHCH(2)CH(2)NH(2), Ren; R = H, Me or Et) and complexed in situ with iron(II) or iron(III), as dictated by the nature of the ligand donor set, to generate the six-coordinate iron compounds [Fe(II)(apRen)(2)]X(2) (R = H, Me; X(-) = ClO(4)(-), BPh(4)(-), PF(6)(-)) and [Fe(III)(hapRen)(2)]X (R = Me, Et; X(-) = ClO(4)(-), BPh(4)(-)). Single-crystal X-ray analyses of [Fe(II)(apRen)(2)](ClO(4))(2) (R = H, Me) revealed a pseudo-octahedral geometry about the ferrous ion with the Fe(II)-N bond distances (1.896-2.041 Å) pointing to the (1)A(1) (d(π)(6)) ground state; the existence of this spin state was corroborated by magnetic susceptibility measurements and Mössbauer spectroscopy. In contrast, the X-ray structure of the phenolate complex [Fe(III)(hapMen)(2)]ClO(4), determined at 100 K, demonstrated stabilization of the ferric state; the compression of the coordinate bonds at the metal center is in accord with the (2)T(2) (d(π)(5)) ground state. Magnetic susceptibility measurements along with EPR and Mössbauer spectroscopic techniques have shown that the iron(III) complexes are spin-crossover (SCO) materials. The spin transition within the [Fe(III)N(4)O(2)](+) chromophore was modulated with alkyl substituents to afford two-step and one-step (6)A(1) ↔ (2)T(2) transformations in [Fe(III)(hapMen)(2)]ClO(4) and [Fe(III)(hapEen)(2)]ClO(4), respectively. Previously, none of the X-salRen- and X-sal(2)trien-based ferric spin-crossover compounds exhibited a stepwise transition. The optical spectra of the LS iron(II) and SCO iron(III) complexes display intense d(π) → p(π)* and p(π) → d(π) CT visible absorptions, respectively, which account for the spectacular color differences. All the complexes are redox-active; as expected, the one-electron oxidative process in the divalent compounds occurs at higher redox potentials than does the reverse process in the trivalent compounds. The cyclic voltammograms of the latter compounds reveal irreversible electrochemical generation of the phenoxyl radical. Finally, the H(2)salen-type quadridentate ketimine H(2)hapen complexed with an equivalent amount of iron(III) to afford the μ-oxo-monobridged dinuclear complex [{Fe(III)(hapen)}(2)(μ-O)] exhibiting a distorted square-pyramidal geometry at the metal centers and considerable antiferromagnetic coupling of spins (J ≈ -99 cm(-1)).