Synthesis and characterization of sterically and electronically tuned ligands toward magnetic control of iron and cobalt complexes

Abstract

Presented within this dissertation are the syntheses and characterizations of iron and cobalt complexes featuring ligands designed to tune the magnetic properties. Two key magnetic phenomena are of interest: spin crossover and single-molecule magnetism. Both of these topics are known to be significantly influenced by subtle changes in coordination and inter- and intramolecular interactions. The overarching goal is to understand how the magnetic properties of the metal center can be controlled via electronic and steric modifications. In Chapter 1, I offer a brief introduction into the background and motivation of the works presented in this dissertation in the realm of spin crossover and single-molecule magnetism. The first section of this chapter is focused on spin crossover and how host:guest interactions can be exploited to alter the magnetic behavior of first-row transition metals. Examples of Fe(II) complexes that display anion-dependent spin state behaviors in both the solid-state and in solution are discussed. Functionalized tripodal Schiff-base ligands are placed into context as an extension of previous research into tripodal ligands for use as metal-based anion-receptors and tripodal spin crossover complexes. The second section of Chapter 1 gives a brief introduction into single-molecule magnetism. An examination of mononuclear Co(II) complexes displaying slow magnetic relaxation and application of acetylide-bridged metal centers to enhance magnetic communication are also given. In Chapter 2, I discuss the preparation and characterizations of a Fe(II) complex coordinated by the alcohol functionalized hexadentate tripodal iminopyridine L6-OH with varying anions. Solid-state magnetic susceptibility measurements of [FeL6-OH]X2 (X = OTf-, Br-, I-, or BPh4-) reveal an anion-dependence on the magnetic behavior. Magnetostructural correlations indicate that stronger hydrogen-bonding interactions are achieved with larger anions, which are better able to undergo bifurcated interactions with the hydroxyl groups from two of the arms. Removal of the tether between the ligand arms leads to the formation of [Fe(L2)2](OTf)2, a bis(tridentate) complex that remains high spin at all temperatures. Variable temperature magnetic measurements in d3-methanol reveal that the high spin state of [FeL6-OH]2+ persists regardless of the anion down to 183 K. In Chapter 3, attempts towards synthesizing the heteroarmed tris(imine) [FeL556]2+ and analogous bis(imine)-mono(amine) [FeL556-NH]2+ complexes are discussed. Several routes are attempted to synthesize the tris-iminopyridine species including selective deprotonation of tris(2-aminoethyl)amine*3 HCl, in situ complex formation via metal-templated self-assembly, and use of presynthesized ligands. Analyses of the reaction mixtures by mass spectrometry suggest that mixtures of products are formed regardless of the method. An anion and solvent dependence leads to preferential formation of the low-spin species [FeL5-ONHtBu]2+, while using solvents such as acetonitrile and ethanol lead to increased production of the desired [FeL556]2+. To test if anion-dependent magnetic behavior can be observed with this ligand type, the comparable complex [FeL556-NH]2+ was synthesized and characterized. Variable temperature solution measurements in d3-acetonitirile suggest that host:guest interactions in solution induce a stabilization of the low-spin state for [FeL556-NH]2+ as indicated by a decrease in susceptibility at lower temperatures for the Cl- salt. In Chapter 4, the preparation, structural, and magnetic characterizations for a family of Fe(II) complexes of tripodal ligands based on L5-ONHtBu are presented. The series of ligands aim to tune the ligand field by selectively reducing imines to amines, producing the ligands L5-(NH)x (x = 1 - 3, number of amines). In the solid state, the three Fe(II) complexes formed are high spin, but significant differences in the structural distortion of both the coordination environment of the Fe(II) center as well as the anion-binding pocket of the amides are noted. In solution, the complexes [FeL5-(NH)3]2+ and [FeL5-NH]2+ are high spin between 183 and 308 K in d6-acetone but interestingly, [FeL5-(NH)2]2+ undergoes a spin-state change with decreasing temperature. Variable temperature studies in d6-acetone and anion titrations in d3-acetonitrile at room temperature monitored by Evans' method of [FeL5-(NH)2]2+ show host:guest interactions stabilize the high spin state. These studies suggest a viable method of ligand tuning for spin-state control by host:guest interactions. In Chapter 5, I discuss the structural and magnetic properties of [Co5-ONHtBu]X2 (X = Cl-, Br-, I-, and ClO4-). These hexadentate Co(II) complexes vary only in the charge-balancing anion, but marked differences in their magnetic properties are observed. Investigation of the magnetic anisotropy of the various salts reveal that the chloride salt possesses the most axial anisotropy, which manifests as an exhibition of slow magnetic relaxation under application of an external field. To my knowledge this is the first example of anion-binding influencing the magnetic anisotropy and 'turning on' single-molecule magnet-like behavior. Lastly, Chapter 6 describes the syntheses and magnetic properties of a series of mono-and dinuclear Fe(III) complexes bridged by ethynylmesitylene ligands. Inclusion of steric bulk onto the bridging-aryl ligand is predicted to increase orbital overlap between the singly-occupied molecular orbital of the metal center and the π-system of the aryl linker. The addition of methyl groups to the aryl ring cements the desired equatorial ligand orientation with respect to the π-system. This leads to an increase in ferromagnetic coupling between the metal centers.