Abstract
Spin crossover (SCO)-active iron(II)-based materials are of great academic interest, and of relevance in fields such as molecular materials and nanoscience, thanks to their versatility as molecular switches. However, chemists still lack the ability to reliably predict the occurrence of SCO. Presented in this thesis is the design of 1,2,4-triazole Rdpt-type ligands [Rdpt ligands are 4-substituted-3,5-di(2-pyridinyl)-1,2,4-triazoles] for the purpose of accessing SCO-active dinuclear and mononuclear iron(II) systems, as well as the development of a method to reliably predict the effect that ligand modification has on the spin state of the resulting iron(II) complex.
Chapter 1 provides an introduction to the SCO phenomenon, various methods of monitoring SCO in the solid state and in solution, and the potential of SCO-active iron-based systems as chemical sensors. Then a summary of some literature examples of SCO-active Rdpt-based iron(II) systems is given. Finally, the target Rdpt-type 1,2,4-triazole ligands are introduced.
In Chapter 2 the synthesis of the new unsymmetrical and potentially ditopic pyridazine-substituted ligand Lpydzpy and the first SCO-active Rdpt-based dinuclear iron(II) complex [FeII2(Lpydzpy)2(CH3CN)4](BF4)4·2CH3CN are described. This dark red dinuclear complex undergoes a set of reversible single-crystal-to-single-crystal (SCSC) transformations upon exposure to different volatile guests, generating the orange dinuclear [FeII2(Lpydzpy)2(EtOH)4](BF4)4 or the yellow 1D polymeric {[FeII2(Lpydzpy)2(H2O)4](BF4)4}∞. Each SCSC event induces a change in color, powder X-ray diffraction pattern and magnetic response.
In Chapter 3 the synthesis of a new family of ten solvatomorphs and polymorphs of mononuclear [FeII(Lpydzpy)2(NCE)2] (E = S and Se) complexes is described, six of which are SCO-active. The guest sensing abilities in the solid state of [FeII(Lpydzpy)2(NCS)2]·4MeCN and [FeII(Lpydzpy)2(NCSe)2]·4MeCN are investigated using optical microscopy, powder X-ray diffraction, thermogravimetric analysis and magnetic measurements showing that both complexes reversibly produce different optical, crystallographic and magnetic outputs depending on the type and content of guest in the lattice.
In Chapters 4-7 five unsymmetrical monotopic azine-substituted Rdpt-type LR ligands (R = pyridine, pyridazine, 4-pyrimidine, pyrazine and 2-pyrimidine) are prepared and complexed.
In Chapter 4 a set of six mononuclear solvatomorphs of [FeII(Lpyridine)2(NCE)2] (E = S, Se and BH3) complexes is described, including the first structure determination of such a E = BH3 complex. Differences in the SCO-behavior between the solid state and solution-phase for the NCBH3 complex are discussed.
In Chapter 5 the new L2pyrimidine and L4pyrimidine ligands are employed to synthesize a set of ten mononuclear [FeII(Lnpyrimidine)2(NCE)2] (E = S, Se and BH3) complexes and solvatomorphs. Differences between the solid state (HS) and solution-phase (SCO-active) magnetic responses of the two NCBH3 complexes are discussed.
In Chapter 6 the new Lpyridazine ligand and the four analogous LR ligands are used to synthesize a set of [Fe(LR)3](BF4)2 complexes which are characterized in the solid state and solution-phase.
In Chapter 7 the five LR ligands are characterized by 15N NMR spectroscopy, and DFT calculations are used to better understand their σ-donor and π-acceptor character. Five SCO-active [FeII(LR)2(NCBH3)2] complexes are studied in the solution-phase. The relationships between the magnetic behavior of the complexes and the choice of azine-substituent in the ligands are discussed.