Mason, Jarad ADev, Vidhya Meenakshi2026-06-0920262026-06-052026Dev, Vidhya Meenakshi. 2026. Spinning Iron into Refrigerants. Doctoral Dissertation, Harvard University Graduate School of Arts and Sciences.32702390https://dash.harvard.edu/handle/1/42740406Spin-crossover (SCO) complexes exhibit a change from a low-spin (LS) to high-spin (HS) state upon the application of an external stimulus such as temperature, pressure, a magnetic field, or light. Due to changes in the volume and entropy upon the transition, SCO materials have recently garnered interest for barocaloric applications, wherein pressure changes are used to induce the spin transition and allow heat transport with the surroundings. Generally, the transition behavior and subsequent barocaloric properties can vary significantly depending on the nature of the ligand field. Specifically, small changes in the primary-coordination sphere through ligand derivatization, or even in the secondary-coordination sphere through intermolecular interactions, can greatly shift the transition temperature and behavior. Due to the high sensitivity to many factors at play, however, it is difficult to controllably shift the transition properties synthetically, let alone predict the effect on the barocaloric metrics which are far less established for SCO materials. Hence, the work presented in this dissertation describes multiple synthetic efforts to establish structure-property relationships in iron(II) spin-crossover complexes towards controllably tuning transition behavior and barocaloric properties. Chapter One motivates our current cooling predicament and describes the fundamentals of barocaloric cooling. The second half of the chapter describes critical concepts for spin-crossover in iron(II) materials, ending with an introduction to the iron(II) bis-hydrotris triazole complex (Fe[HB(tz)3]2) that is the synthetic prototype for chapters two and three. In Chapter Two, primary-coordination sphere effects, induced through ligand derivatization, are investigated for Fe[HB(tz)3]2. Synthetic combinations of ligand derivatives are used to make homoleptic and heteroleptic complexes, which are analyzed through magnetometry and single-crystal X-ray diffraction data. Notably, heteroleptic combinations can allow for transition temperatures between those of their respective homoleptic derivatives. Chapter Three investigates secondary-coordination sphere effects via the co-crystallization of Fe[HB(tz)3]2 with solid-state hydrogen bond donors (coformers). This chapter covers the large degree of transition tuning available through simple coformer substitution. Additionally, an isostructural series of benzoic-acid based co-crystals is analyzed in-depth to determine structure-property relationships and establish co-crystallization as a very promising method for SCO manipulation. Finally, the effects of coformers on the barocaloric properties, especially in comparison to the original parent complex, are described. Chapter Four expands beyond SCO transitions alone through the incorporation of alkyl-chain substituents on the ligand scaffold, which can undergo their own pressure-responsive chain-melting transitions. Specifically, this chapter will describe the interplay between SCO and chain-melting, showing that the combination of multiple transitions can lead to unexpected behavior and enhanced thermal properties. Relations between the alkyl chain length and the spin transition are established. The chapter ends with a brief discussion on thermal conductivity changes across the combined SCO / chain-melting transition.application/pdfenBarocaloricIronSpin-CrossoverThermalTransitionChemistryInorganic chemistryThermodynamicsThesis or Dissertation2026-06-090009-0005-1673-1654