Modular Giant-Spin Hexairon Clusters as Meta-Atom Building Blocks

Abstract

Exploration of new synthetic space is required to meet the demands for polyfunctional materials and device miniaturization that drive modern materials chemistry. Molecule-based materials present an alternative to conventional atom-based solids, with advantages in synthetic tunability as well as the potential to exhibit combinations of properties difficult to achieve in conventional materials, as in photo-switchable magnets and magnetic insulators, which may have applications in future devices. Molecular metal clusters as building blocks for materials have the additional advantage of stability in multiple oxidation states, as well as the potential to exhibit meta-atom properties such as giant-spin behavior. However, most metal clusters have geometries that are difficult to control due to their ill-defined syntheses and exhibit only weak metal-metal interactions, causing them to behave as collections of isolated metal atoms without exhibiting collective meta-atom behavior. As such, most clusters cannot provide access to the desired synthetic space of solid-state lattices composed of tunable meta-atom building blocks. The Betley lab recently prepared a series of octahedral hexairon clusters (HL)2Fe6 templated by a polynucleating o-phenylene diamine–based ligand. Due to their short metal-metal contacts, these clusters exhibit strong intramolecular magnetic exchange and large spin ground states ranging from S = 6 to S = 11 that are thermally isolated from excited states at room temperature, allowing them to behave as true giant spins. In this work, we develop synthetic tools for manipulating the physical properties of these clusters to allow them to serve as modular giant-spin building blocks. We then explore the incorporation of these building blocks into molecular magnetic solids. In Chapter 2, we develop a method for altering the spin ground states of these clusters while maintaining their giant-spin character. We postulate that coordination of ligands to the open axial coordination sites may result in an increase in the total cluster spin ground state as a result of a change in the local spin state of the iron centers from intermediate spin to high spin, as occurs in mononuclear porphyrinoids with similar square-planar nitrogen-based coordination environments. This hypothesis is tested by synthesis of clusters of the form (FL)2Fe6(Lax)n. Magnetometry studies reveal increased spin ground states of S = 8 for n = 2 and S = 10 for n = 4, and structural, spectroscopic, and theoretical studies are employed to rationalize this behavior. The stability of these giant-spin clusters in a wide range of oxidation states is one of their primary advantages as building blocks for molecule-based materials, as the cluster reduction potential relative to a reaction partner will determine the band gap of the resulting solid. In Chapter 3, we develop a method for tuning the cluster reduction potentials by substituting the o-phenylene diamine–based supporting ligands with a range of electron-donating and electron-withdrawing substituents. Electrochemical studies reveal a potential shift of 600 mV across the series in intervals from 30 to 150 mV, while structural, magnetometry, and spectroscopic studies indicate that the giant-spin ground states of the clusters are maintained. Studies of the magnetic interactions between molecules that exhibit slow magnetic relaxation have the potential to reveal new elements of their physics and potential applications. In Chapter 4, we employ the large shift in cluster redox potential as a function of ligand substitution to prepare cluster-cluster ion pairs in which both components have the potential to undergo slow magnetic relaxation. Magnetometry studies suggest that the magnetic relaxation profiles in these materials differ from those of the isolated cationic and anionic clusters. Additionally, we take advantage of the very high crystallinity of these ion-pair materials to study their magnetic properties at the atomic scale by means of polarized neutron measurements, which reveal that strong intramolecular exchange dominates the contributions to the atomic susceptibility tensors, a unique signature of true giant-spin clusters. Having established tools to control the magnetic and electronic properties of these giant-spin clusters, in Chapter 5 we develop methods to incorporate them into molecule-based magnetic solids. Reaction of cationic clusters with the fulleride anion leads to a series of meta-atom magnets with strong intermolecular exchange arising from close cluster-fulleride contacts. The dimensionality of the magnetic exchange pathways can be tuned by the coordination of axial solvents to the clusters, and magnetometry studies reveal that the magnetic ordering temperatures vary from 2 to 21 K as a function of the dimensionality of the exchange pathway. Additionally, appreciable remanence and coercivity are observed due to the large spin ground states and magnetic anisotropies of these giant-spin clusters.