Mechanochemicals synthesis and modification of spin crossover materials

Spin crossover (SCO) is a phenomenon of some first-row transition metal complexes in which magnetic, optical, and structural properties change as a result of external stimuli. The change in

properties is induced by the switching between high-spin (HS) and low-spin (LS) states. Prediction of the presence of SCO-activity is difficult. Therefore, discovery of novel SCO materials requires synthesis of a wide variety of potential SCO-active candidates. Traditional SCO synthesis is done by standard solution-state techniques, which are time consuming and have no guarantee of

obtaining a SCO-active material at the end. The pursuit of novel SCO-active materials is therefore extremely slow. Mechanochemistry is an alternative synthetic technique which has recently begun to be applied to a variety of different fields, driven by the extremely short reaction times alternative reaction products and the ever-increasing importance of green chemistry. The short reaction times make mechanochemistry an ideal solution to synthesis of novel SCO materials. This study explores the application of mechanochemical synthesis to SCO research for the first

time, investigating the viability of the synthetic route for obtaining SCO materials (chapter 2). The known SCO complexes Fe(phen)2(NCS)2, [Fe(Htrz)3](BF4)2, [Fe(atrz)3]SO4, Fe(4-phpy)2[Ni(CN)4] and Fe(pz)[Au(CN)2]2 were successfully synthesised using mechanochemical techniques. In general, SCO was more gradual, and shifted to a slightly lower temperature with a decrease in hysteresis,

as seen in Fe(phen)2(NCS)2, Fe(4-phpy)2[Ni(CN)4] and Fe(pz)[Au(CN2)2. However, the SCO properties of mechanochemical and solution-state synthesised [Fe(atrz)3]SO4 were

indistinguishable. Further, mechanochemical synthesis of [Fe(Htrz)3](BF4)2 yielded a polymorphic mixture.

Additional investigation of the differences in properties between mechanochemical and solution-state synthetic products was undertaken in Chapter 3 and showed the effects of mechanochemical synthesis are not straightforward. Mechanochemical synthesis of both [Fe(atrz)3]SO4 and Fe(pz)[Au(CN)2]2 yielded particle sizes smaller than solution-state synthesis, but the transition temperature and SCO properties of only Fe(pz)[Au(CN)2]2 were effected. Mechanochemical synthesis of [Fe(atrz)3](BF4)2 yielded a previously observed but uncharacterised

polymorph. Comparison between manual grinding and ball-milling for different time durations yielded the same product with relatively insignificant differences in SCO properties.

Further application of mechanochemical techniques such as solid-state metathesis, were investigated as alternative synthetic routes within mechanochemical synthesis, Chapter 4, and mwas used to successfully exchange chloride anions for both bromide and iodide in the complex [Fe(atrz)3]Cl2. The anion exchange was undertaken by post-synthetic grinding of [Fe(atrz)3]Cl2 and

NaX (where X = Br and I). Further exchange attempts were undertaken using NaBF4, NaSCN and NaReO4 yielded products which underwent partial exchange.

A screening protocol was devised, tested, and optimised for the identification of both thermochromic and non-thermochromic SCO-active materials prepared by mechanochemistry, Chapter 5. The different analytical techniques used for routine analysis of SCO materials were assessed for their speed, cost and the information they can provide, in an attempt to address the change in the rate-limiting step that occurs through the rapid synthesis of materials using mechanochemistry.