Self-Assembly of Hydrogen-Bonded Fibrous FeII Triazole Complexes and Their Spin Crossover Characteristics in Organic Media

The lipophilic linear FeII triazole complexes [FeII(L)3]Cl2 (L = 1–5) were synthesized using ligands 1–5 containing amide bonds between alkyl chains and 1,2,4-triazole ligands with various spacer methylene length. When the amido and ether linkages are introduced in the alkyl chain moiety, the iron complexes are dissolved in chloroform, [FeII(1)3]Cl2 forms a pale purple jelly-like phase. The purple color is accompanied by a structured absorption around 540 nm, characteristic of iron (II) in the low spin (LS) state. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) of the jelly-like phase confirm the formation of networks of fibrous nano assemblies with widths of 10–30 nm. The observed widths are larger than the molecular lengths of the triazole ligands. The pale purple jelly-like phase turned into a pale-yellow solution by heating above ca 310 K, indicating the formation of high spin (HS) state complexes. The complexes show irreversible spin crossover in the solid state, characterized by SQUID. Interestingly, an abrupt spin crossover is observed in solution reversibly with some thermal hysteresis. UV-vis spectra also showed reversible spin crossover phenomena dependent on the spacer length between the amide group and the Fe(II) triazole complexes. IR spectra of these complexes in chloroform show the formation of hydrogen bonding from amide groups, which enhanced alkyl-chain packing in the coordination polymers. The freeze-dried iron triazole complexes form lamellar structures, which indicates the alkyl chains extending radially from the octahedral triazole complex moiety are oriented in a lamellar packing due to the presence of flexible ether linkages in alkyl chains, which allowed decoupling the alignment of the dodecyloxy alkyl chains from the spacer methylenes connected to the Fe(II) triazole complexes. Introducing amide bondages to the lipophilic one-dimensional coordination systems stabilizes the low-spin state by hydrogen bond networks. It provides hysteresis in the spin crossover in solution, ascribed to the recombination of hydrogen bonds during the temperature change between the heating and cooling sides. Combining hydrogen bonds and lipophilic one-dimensional complexes provides a valuable means to enhance their stability and control physical properties in solution.