Stabilization of superoxocopper(II) and oxoiron(IV) complexes using bulky bis- and tris(2-pyridylmethyl)amine ligand

• Main Author: Quek, Sebastian Yongsen
• Other Authors: Jason England
• Format: Theses and Dissertations
• Language: English
• Published: 2019
• Subjects: Science::Chemistry
• Online Access: https://hdl.handle.net/10356/86178; http://hdl.handle.net/10220/49273
• Institution: Nanyang Technological University

Highly reactive ‘end-on’ superoxocopper(II) and oxoiron(IV) intermediates are recognized as active oxidants responsible for substrate C-H bond hydroxylation in a wide range of non-heme metalloenzymes. Mimicking such reactivity in synthetic systems would provide a major improvement over the current de-facto methods used in oxidation chemistry, where direct CH remains inefficient and a niche concern. This goal requires a greater understanding of the properties of the aforementioned oxidants and how they can be tailored. As part of a greater effort to achieve these ends, mononuclear ‘end-on’ superoxocopper(II) and intermediate-spin (S = 1) oxoiron(IV) supported by new ligands, Ar3TPA and Ar2RBPA (Me or Bn), have been prepared and their reactivity characterized. From a survey of the literature, it can be inferred that the ligands bearing significant steric bulk are able to stabilize mononuclear ‘end-on’ superoxocopper(II) complexes and provide highspin oxoiron(IV) species by enforcing a trigonal bipyramidal geometry at the iron centre. Unfortunately, the use of bulky ligands was accompanied, in both cases, by poor intermolecular reactivity toward substrates. It is tempting to assign such observation to steric effects, but the influence of the different donor properties of ligands used has not been accounted for. In an effort to address this glaring omission and obtain sterically stabilized superoxocopper(II) and oxoiron(IV) that retain the reactivity properties of non-bulky systems, we developed tris- and bis(2-pyridylmethyl)amine ligands that incorporate bulky aryl substituents at the 5th position of the pyridyl donors. 2 Coordination of Ar3TPA to first-row transition M2+ ions revealed that with weak field co-ligands, in the solution state, five coordinate geometries are preferred (Chapter 2). In contrast, X-ray crystallographic studies show that these complexes display either octahedral (Fe and Mn) or trigonal bipyramidal (Fe, Co and Cu) geometries, depending on the metal and the co-ligand involved. Moreover, comparison of the electrochemical properties of these complexes with the unsubstituted parent TPA complexes indicate that the bulky aryl substituents do not weaken the donor strength of the ligand, which suggests that their complexes should display similar inherent reactivity. Similar conclusions were reached from electrochemical studies of an analogous series of Ar2RBPA complexes (Chapter 3). In the case of the Ar2RBPA ligands, only 6-coordinate geometries, in which the ligand coordinated in a ‘mer’ fashion, were observed. Crucially, unlike the parent BPA ligand, no evidence was found for formation of bischelate metal(II) complexes, [MII(Ar2RBPA)2]2+. Reaction of [CuI(Ar3TPA)(NCMe)]+ with O2 yielded mononuclear ‘end-on’ superoxocopper(II) complexes (Chapter 4). Remarkably, this reaction was found to be reversible, there was no ‘dimerization’ to yield a peroxo-bridged dicopper(II) complex at any temperature, and evidence for formation of the superoxocopper(II) moiety could be observed even at room temperature. Furthermore, the superoxocopper(II) complexes possess good reactivity with a range of 2,6-di-tert-buyl-para-subsituted-phenols, despite it being slower than observed for less hindered TPA systems. In-depth mechanistic studies were performed and, the activation parameters and relationship with oxidation potential of the phenols, suggest the possibility that their comparatively poor reactivity may be contributed, in part, by the difference in donor properties of the ligand, rather than steric factors alone. Regardless, this is the first example of a stable mononuclear ‘end-on’ superoxocoppr(II) complex that displays significant substrate reactivity. 3 In contrast, whereas oxygenation of [CuI(tpb2MeBPA)(NCMe)]+ still yielded only superoxocopper(II) complexes (i.e., no dimerization was observed), they were only stable at temperatures below -85C. Furthermore, the UV-vis spectra of these superoxocopper(II) complexes displayed smaller extinctions coefficients compared with published systems. This, combined with the high sensitivity of oxygenation to the solvent medium and issues regarding reproducibility, raises the possibility of incomplete reaction. Oxidation of the [FeII(tpb3TPA)]2+ starting complex yielded [FeIV(O)(tpb3TPA)]2+ (Chapter 5) that, on the basis of Mössbauer and UV-vis spectroscopic studies, is assigned to be of intermediate-spin (S = 1). This spin-state is indicative of an octahedral geometry and, thus, there is insufficient inter-pyridine repulsion in the tpb3TPA ligand to enforce trigonal bipyramidial geometry. Various anions were coordinated in the cis-labile site of the oxoiron(IV) complex and their stability is discussed. Unfortunately, these weak field anions did not elicit a change in spin-state. Although the targeted high-spin state was not achieved, the complex [FeIV(O)(tpb3TPA)(NCMe)]2+ showed good stability at room temperature and reactivity towards a wide range of substrates, including those with strong C-H bonds (e.g., cyclohexane). The linear correlation of the logarithm of experimentally determined second order rate constants versus C-H bond dissociation energy, and the large KIEs observed are all consistent with H-atom transfer reactivity involving tunneling.