Molecular spin crossover phenomenon: recent achievements and prospects

Iron(II) Mononuclear Materials Containing Functionalised Dipyridylamino-Substituted Triazine Ligands: Structure, Magnetism and Spin Crossover

The spin crossover effect in iron(II) materials containing the di-2-pyridylamine functional group has been investigated for the new nitrile-functionalised ligand DTAC (2,2′,2″,2″′-((6-(di(pyridin-2-yl)amino)-1,3,5-triazine-2,4 diyl)bis(azanetriyl))tetra acetonitrile). This ligand has successfully been incorporated into a family of materials of the general formula trans-[Fe(DTAC)2(anion)2], wherein we have systematically varied the trans-nitrogen donor anion from NCS, NCSe, N(CN)2 (dca; dicyanamide) to NCBH3 – thus forming the four mononuclear materials trans-[Fe(DTAC)2(NCS)2]·6MeCN (1), trans-[Fe(DTAC)2(NCSe)2]·6MeCN (2), trans-[Fe(DTAC)2(N(CN)2)2] (3) and trans-[Fe(DTAC)2 (NCBH3)2]·3MeCN (4)). We find that the materials with a weaker crystal field strength anion remain high spin over all temperatures (1 and 2) whereas the materials containing stronger crystal field strength anions undergo a thermally induced spin crossover (3 and 4). Structural analysis revealed that the packing interactions in the solid state and the degree of solvation also play a large role in the observed magnetic behaviour. Indeed, aged or rapidly precipitated samples of 2 show a spin transition above room temperature.

Coherent transport through spin-crossover single molecules

Coherent quantum transport calculations were performed for high- and low-spin states of a mononuclear Fe(II) complex showing spin-crossover behavior using density functional theory methods combined with the non-equilibrium Green functions procedure. The high-spin state has a larger conductivity than the low-spin state; furthermore, it behaves as a spin filter, giving a β-polarized current.

Guest effect on nanopatterned spin-crossover thin films

Nanopatterned thin films of the metal-organic framework {Fe(bpac)[Pt(CN)4]} (bpac=bis(4-pyridyl)acetylene) are elaborated by the combination of a sequential assembly process and a lithographic method. Raman microspectroscopy is used to probe the temperature dependence of the spin state of the iron(II) ions in the films (40-90 nm in thickness), and reveals an incomplete but cooperative spin transition comparable to that of the bulk material. Adsorption/desorption of pyridine guest molecules is found to have a substantial influence on the spin-crossover properties of the thin layers. This interplay between host-guest and spin-crossover properties in thin films and nanopatterns demonstrates the potential ability of using this kind of material as a microsensor.

Tuning of the critical temperature in iron(II) spin-crossover materials based on bridging polycyanidometallates: pentacyanidonitrosylferrate(II) and hexacyanidoplatinate(IV)

The reactions of iron(II) sulfate, 4-amino-3,5-di-2-pyridyl-4H-1,2,4-triazole (abpt), and pentacyanidonitrosylferrate(II) or hexacyanidoplatinate(IV) resulted in the formation of one-dimensional iron(II) spin-crossover compounds [Fe(abpt)(2)(μ-Fe(CN)(5)(NO))](n) (1) and [Fe(abpt)(2)(μ-Pt(CN)(6))](n) (2) with the spin-transition critical temperature near or above room temperature accompanied by thermochromism. Furthermore, it has been proven that the critical temperature T(c) is influenced by the type of dianionic polycyanidometallate within the series of discussed systems, and it changes in the sequence of [Fe(CN)(5)(NO)](2-) < [Pt(CN)(6))](2-) < [Ni(CN)(4))](2-) ≈ [Pd(CN)(4))](2-) ≈ [Pt(CN)(4))](2-).

Spin transition sensors based on β-amino-acid 1,2,4-triazole derivative

A β-aminoacid ester was successfully derivatized to yield to 4H-1,2-4-triazol-4-yl-propionate (βAlatrz) which served as a neutral bidentate ligand in the 1D coordination polymer [Fe(βAlatrz)₃](CF₃SO₃)₂·0.5H₂O (1·0.5H₂O). The temperature dependence of the high-spin molar fraction derived from ⁵⁷Fe Mossbauer spectroscopy recorded on cooling below room temperature reveals an exceptionally abrupt single step transition between high-spin and low-spin states with a hysteresis loop of width 4 K (T_c^↑ = 232 K and T_c^↓ = 228 K) in agreement with magnetic susceptibility measurements. The material presents striking reversible thermochromism from white, at room temperature, to pink on quench cooling to liquid nitrogen, and acts as an alert towards temperature variations. The phase transition is of first order, as determined by differential scanning calorimetry, with transition temperatures matching the ones determined by SQUID and Mössbauer spectroscopy. The freshly prepared sample of 1·0.5H₂O, dried in air, was subjected to annealing at 390 K, and the obtained white compound [Fe(βAlatrz)₃](CF₃SO₃)₂ (1) was found to exhibit a similar spin transition curve however much temperature was increased by (T_c^↑ = 252 K and T_c^↓ = 248 K). The removal of lattice water molecules from 1·0.5H₂O is not accompanied by a change of the morphology and of the space group, and the chain character is preserved. However, an internal pressure effect stabilizing the low-spin state is evidenced.

Matrix-dependent cooperativity in spin crossover Fe(pyrazine)Pt(CN)4 nanoparticles

Anisotropic nanoparticles of the Fe(pyrazine)Pt(CN)(4) network were prepared embedded in various matrices that revealed to have a dramatic effect on the cooperative spin crossover phenomena. By a judicious choice of the nature of the matrix and the control of interparticle distances, a hysteresis of 15 K was achieved close to room temperature for such nano-objects.