Design of Multifunctionality in Fe(II) Spin-Crossover Complexes

Spin crossover (SCO) is a physical phenomenon where certain transition metal ions exhibit a change in their electronic configuration and magnetic state under the influence of external stimuli such as temperature, pressure, or light. SCO is commonly observed in complexes of the Fe(II) cation when a ligand field of intermediate strength is achieved, most commonly with six coordinating nitrogen atoms. This thesis explores two important ways of utilizing the SCO phenomenon. Firstly, SCO can be combined with electrical conductivity to produce multifunctional molecular materials. Secondly, Fe(II) SCO complexes can be designed in novel coordination environments, in addition to the typical N6 coordination. In Chapter 1, we present the fundamentals of SCO and organic conductors along with a brief overview of relevant literature. The understanding of these fundamentals allows us to design the desired SCO materials. In Chapter 2, we explain different synthetic methods, including various crystal growth techniques, to obtain fractional charge on TCNQ radical anions co-crystallized with SCO Fe(II) cationic complexes. We provide information on magnetic and photomagnetic measurements and special sample preparation requirements. Information on supplementary characterization methods is also presented. In Chapter 3, we show an effective way to incorporate TCNQ●– organic radicals into paramagnetic transition metal complexes. In such complexes, TCNQ●– radical anions (0 <  < 1) are arranged in stacks that provide conducting pathways. In Chapter 4, we expand this approach to using SCO moieties instead of simple paramagnetic cations. Our studies reveal that stacking distances of TCNQ●– can be affected by structural changes induced by the varying electronic configuration at the Fe(II) center. This coupling results in a synergy between SCO and electrical transport. In order to expand the design possibilities of SCO complexes, it is important to find alternative coordination environments that can provide the appropriate ligand field strength necessary for SCO. In Chapter 5, we demonstrate various heteroleptic Fe(II) complexes containing one N2 donating ligand and one tetradentate “capping” ligand for achieving SCO behavior. The tetradentate capping ligands include N4-donating ligands, leading to the typical N6 coordination environments, and N2S2-donating ligands, leading to N4S2 coordination. The combination of experimental design and investigations into hybrid Fe(II) SCO complexes with TCNQ radical anions, as presented in this thesis, advances the knowledge of spin-state switching in transition metal complexes and inches towards more applied studies of functionalized molecular materials. Designing SCO complexes with new coordination environments is appealing due to the possibility of discovering new families of compounds that might exhibit SCO, thus expanding the possibilities for the discovery of such complexes.