Hydrophobic tail length in spin crossover active iron(ii) complexes predictably tunes T-1/2 in solution and enables surface immobilisation
Year: 2020
Journal: Inorg. Chem. Front., Volume 7, MAY 21, page 2050–2059
Authors: Sundaresan, S; Kitchen, JA; Brooker, S
Organizations: University of Otago; MacDiarmid Institute for Advanced Materials and Nanotechnology
The literature tetradentate acyclic Schiff base ligand HLH-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, LH-OR. The synthesis, magnetic and Langmuir characteristics of the corresponding family of six neutral mononuclear Fe(ii) complexes, the parent complex [Fe-II(HLH-OH)(NCBH3)(2)] and the five alkylated ligand derivatives [Fe-II(LH-OR)(NCBH3)(2)], is detailed. The solution spin crossover temperature, T-1/2, can be tuned by two methods: (a) increasing the solvent polarity index (P ') from 5 to 7 increases T-1/2 by 172 K, whereas (b) increasing the tail length from C-10 to C-22 decreases T-1/2 by 115 K. In both cases strong linear correlations are observed, so both of these, choice of solvent (R-2 = 0.99, albeit only for three solvents) and of tail length (R-2 = 0.98, for five complexes), are levers for the predictable tuning of the spin state in advance of synthesis. The compounds with longer tails, C-18, C-20 and C-22, showed a small window of thermal hysteresis (<5 K) in D-3-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(LH-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.
