Gels, xerogels and films of polynuclear iron(ii)–aminotriazole spin-crossover polymeric complexes

Abstract

The prominent polynuclear spin-crossover complex [Fe(NH 2 trz) 3 ](2ns) 2 (polymer) (NH 2 trz ¼ 4-amino-1,2,4-triazole, 2ns À ¼ 2-naphthalenesulfonate) as obtained from synthesis exists in a metastable state characterized by a columnar hexagonal packing (Col h) of polynuclear rigid-rod complexes, and shows considerable fluctuations in the spin-crossover temperature (between 22 and 41 C upon heating and between 14 and 25 C upon cooling). This structure is converted into a more stable columnar rectangular packing (Col r) by annealing this material at 250 C, which is accompanied by a shift of the spin-crossover towards higher temperatures ($52 C upon heating and $33 C upon cooling). Furthermore, solutions of [Fe(NH 2 trz) 3 ](2ns) 2 in dimethylformamide (DMF) can be converted into gels by increasing the concentration of [Fe(NH 2 trz) 3 ](2ns) 2 or by addition of toluene, while preserving the spincrossover behavior. Gels prepared by the latter method were converted into xerogels by CO 2 criticalpoint drying, or into transparent thermochromic films by slow evaporation of the solvent. For the xerogels an initial Col h packing is transformed into the more stable Col r packing by the same annealing procedure applied to the powders. On the other hand, the slow evaporation of solvent used for the manufacture of films allowed establishment of a Col r packing by annealing at lower temperatures (150 C). Polynuclear complexes of the type [Fe(Rtrz) 3 ] n X 2n (Rtrz ¼ 4-R-1,2,4-triazole) have attracted particular attention, not only due to their exceptional structure but also because their spincrossover temperature can be varied by applying differently substituted ligands and counter anions X À. 13-15 Thus, also materials with spin-crossover near room temperature can be fabricated. 16-18 Unfortunately, processing of those complexes is oen hampered due to lacking solubility or thermal stability (usually, decomposition takes place prior to melting). 19 One possibility to overcome the poor solubility is to attach long side chains at the ligands or counter anions. 20-22 However, introducing large Scheme 1 Schematic representation of the structure of the spincrossover complex [Fe(NH 2 trz) 3 ](2ns) 2 (NH 2 trz: 4-amino-1,2,4-triazole, 2ns À : 2-naphthalenesulfonate).

Preserving the spin transition properties of iron-triazole coordination polymers within silica-based nanocomposites

“…1b). Noticeably, the molar ATrz/Fe 2+ ratio used for this synthesis was increased to 6 : 1 in order to promote the expected compound, [Fe(ATrz) 3 ]SO 4 , 45,46 due to the strong complexing ability of SO 4 2À anions for Fe(II)…”

Co-Processing of [Fe(NH2trz)3](2ns)2 and UHMWPE into Materials Combining (Spin Crossover and High Mechanical Strength

“…Coordination polymers of iron (II) and 4-amino-1,2,4-triazole (NH 2 trz) ( Figure 1) of the generic formula [Fe(NH 2 trz) 3 ]X 2 , with X, a negatively charged counter ion, have attracted attention for their spin-crossover behavior [1][2][3][4][5][6][7][8][9][10][11][12]. Spin crossover is a rare phenomenon which designates a reversible change in spin state of a compound by an external trigger, for instance a change in temperature [13][14][15][16][17][18][19].…”

“…That transition from the low spin state to the high spin state is associated with a reversible color change, i.e., those compounds exhibit a thermochromic behavior [21,22]. In complexes of iron (II) and NH 2 trz the low spin state typically manifests in a pink-and the high spin state in a white color [1,10,11]. Notably, the spin crossover temperature markedly depends on the counter anion X [4,23].…”

A flexible polymer–nanoparticle hybrid material containing triazole-based Fe(ii) with spin crossover properties for magneto-optical applications

“…Over the past two decades, SCO materials were prepared as thin films, [13][14][15][16][17] liquid crystals, 18,19 and nanostructured objects. [20][21][22][23][24][25][26] Supramolecular gels have also been successfully obtained based on the alkyl functionalization of triazole ligands and/or counter-anions, [27][28][29][30][31][32][33] with few examples using hydrophilic ligands 34,35 or a polyol synthetic route. 36 The latter approach may be of particular interest as it should extend the possibility of obtaining gels from a broad diversity of SCO systems reported in the literature.…”