Size, Shape and Structure Dictate Magnetic Behavior of Metal Halide Clusters and Layered Metal–Organic Frameworks

Nanoscale materials display emergent electronic and magnetic properties that are highly correlated to their size and shape. The work in this dissertation describes efforts to synthesize nanoscale materials including molecular clusters and layered solids with exquisite control over size and shape. Particular emphasis is paid to studying the magnetic properties of these materials and how these properties relate to those of their bulk counterparts. Chapter 1 introduces basic concepts of magnetism including interactions between spin centers. The nature of these interactions dictates many magnetic properties and is essential in understanding the behavior of the materials contained within this dissertation. Furher emphasis is paid to the effects of quantum confinement on magnetic properties, and major advances in this field are provided. Finally, general strategies to purposefully control the size and shape of confined materials are highlighted along with illustrative examples. Chapter 2 describes the synthesis of atomically precise metal(II) halide clusters (M19X38, M = Fe, Co, Ni; X = Cl, Br) using the metal–organic framework Zr6O4(OH)4(bpydc)6 (bpydc2− = 2,2′-bipyridine-5,5′-dicarboxylate) as a template. Single-crystal X-ray diffraction techniques reveal that these clusters represent fragments excised from a single layer of the bulk, layered structure of the corresponding parent metal halide. Magnetometry and Mössbauer spectroscopy are then used the probe the magnetic behavior of these clusters. Remarkably, the intralayer ferromagnetic magnetic exchange pathways characteristic of the bulk materials are conserved in the clusters, leading to the isolation of high-spin magnetic ground states as well as superparamagnetism in the case of Fe19Cl38 clusters. While Chapter 2 focuses on the magnetic behavior of metal(II) halide clusters with predominantly ferromagnetic exchange coupling between spin centers, Chapter 3 focuses on the behavior of clusters with antiferromagnetic exchange coupling. In particular, the magnetic properties of a Mn19Br38 cluster are studied. In this cluster, spin centers are arranged on a triangular lattice. This topology combined with antiferromagnetic exchange correlations can be expected to lead to a geometrically frustrated magnetic ground state. Magnetometry as well as computational methods are used to probe the magnetic behavior of this cluster and confirm a highly frustrated ground state spin configuration. Chapter 4 focuses on the synthesis of a promising new metal–organic framework Cr(pz)2 (pz = pyrazine) that has exceptional magnetic properties. The framework is a layered material constructed of a square net of Cr(II) cations that are bridged by singly reduced pz radical anions. The framework is ferrimagnetic with an ordering temperature of 242 °C and a coercive field of 0.75 T at room temperature, extremely impressive parameters for a metal–organic system. Currently available synthetic methods, however, only yield a microcrystalline powder that is not amenable to in depth characterization techniques. This chapter therefore explores potential synthetic routes toward single-crystals or thin films of Cr(pz)2 using molecular precursors.

Advisor: Long, Jeffrey R