Investigation of iron-based spin crossover complexes and cocrystals.

Spin Crossover (SCO) is the phenomenon in which transition metals with d4–d7 configurations can switch between the high and low spin states. The change in spin state can be promoted by external stimuli, such as a change in temperature, light irradiation, pressure, magnetic field, or guest molecules. Recently, cocrystallisation has been used to form new SCO materials, SCO-cocrystals. In this study, the synthesis of a neutral heteroleptic Fe(III) material 2,2-dihydroxyazobenzene and 5-X-2-8- quinolyliminomethylphenolate (X= F, Cl, Br, and I) via ligand exchange was attempted, although this was not synthetically achievable. However, four [Fe(qsal-X)2]BF4 structures, including one of which was novel, were successfully synthesised. F, Cl and Br analogues are known in literature, but the SCO behaviour of the F and Cl analogues were further studied. The iodine analogue is not known in literature and, using VT-SCXRD, its SCO behaviour was analysed, showing that this derivative undergoes a gradual SCO. Also in this study, we present ten novel cocrystals in which Fe2+ based [Fe(3-bpp)2](BF4)2 complexes were cocrystalised with monotopic coformers, nine of which are SCO-active, showing a variety of SCO behaviour. A series of lutidines (2,3-, 3,4-and 2,6-lutidine) were chosen as coformers, forming SCO active materials, two of which show gradual SCO while the 2,3-lutidine cocrystal shows an abrupt spin transition close to room temperature. Using 4-phenylpyridine as a coformer produced three different crystal forms A, B and C which resulted in abrupt, gradual and no SCO behaviour, respectively. Where 8-Methylquinoline was used as a coformer, the resulting complexes crystallised in two different forms in different spin states with reversible SCO behaviour. 3-Cyanopyridine and 4-styrylpyridine also formed SCO active cocrystals, resulting in irreversible gradual SCO.