Hydrophobic tail length in spin crossover active iron(ii) complexes predictably tunes T½ in solution and enables surface immobilisation

Abstract

The literature tetradentate acyclic Schiff base ligand HL^H-OH (from the 2 : 1 reaction of pyridine-2-carboxylaldehyde with 1,3-diamino-2-propanol) was alkylated using a range of 1-bromoalkanes varying in length, BrR (R = C₁₀H₂₁, C₁₆H₃₃, C₁₈H₃₇, C₂₀H₄₁ and C₂₂H₄₅) to yield five new ligands, L^H-OR. The synthesis, magnetic and Langmuir characteristics of the corresponding family of six neutral mononuclear Fe(II) complexes, the parent complex [Fe^II(HL^H-OH)(NCBH₃)₂] and the five alkylated ligand derivatives [Fe^II(L^H-OR)(NCBH₃)₂], is detailed. The solution spin crossover temperature, T_½, can be tuned by two methods: (a) increasing the solvent polarity index (P′) from 5 to 7 increases T_½ by 172 K, whereas (b) increasing the tail length from C₁₀ to C₂₂ decreases T_½ by 115 K. In both cases strong linear correlations are observed, so both of these, choice of solvent (R² = 0.99, albeit only for three solvents) and of tail length (R² = 0.98, for five complexes), are levers for the predictable tuning of the spin state in advance of synthesis. The compounds with longer tails, C₁₈, C₂₀ and C₂₂, showed a small window of thermal hysteresis (<5 K) in D₃-acetonitrile solution. Water solubility hampered the formation of Langmuir films of these complexes. But the use of a 1 : 1 mixture of stearic acid with [Fe^II(L^H-OC18)(NCBH₃)₂] facilitated the formation of a stable Langmuir film, a monolayer of which was transferred onto a quartz slide: multi-layering was not possible.