Three-Coordinate Iron Complexes with Terminal Imido Ligands

Coordinatively unsaturated complexes are invoked as reactive intermediates in many catalytic processes, often formed via ligand dissociation from a saturated precursor. Metal centers can also be forced into a low coordination number by using a bulky ligand platform, and many three-coordinate transition metal complexes are known. This thesis describes a series of three-coordinate iron complexes with imide ligands, and explores their reactivity with hydrocarbons and other organic substrates. Chapter 2 details the synthesis and characterization of the first trigonal imidoiron(III) complex, as well as an unusual diiron-hexazene byproduct. The solvent dependence of the reaction outcome is discussed using Density-Functional Theory computations. The reactivity of the imidoiron(III) complex toward hydrocarbons is explored in Chapter 3 in a series of hydrogen-atom transfer (HAT) reactions. The HAT rate constants increase in more polar solvents or when using a pyridine additive, suggesting that the rate-limiting HAT step has some proton-transfer character. In Chapter 4, an iron-tetrazene complex is characterized as the product of the reaction between the imidoiron(III) complex and organoazide. Physical, spectroscopic, and computational techniques are used to elucidate the electronic structure of the tetraazametallacycle core. The combined experimental and theoretical data are most consistent with the description as a tetrazene radical coordinated to a high-spin iron(II) center. Chapter 5 explores the reactivity of imidoiron(III) complexes with CO, phosphines, and isocyanides. Transfer of the metal-coordinated nitrene fragment to these substrates is demonstrates, forming isocyanate, phosphinimine, and carbodiimide products. Significantly, both the imidoiron(III) complex and its iron(I) precursor catalyze the coupling of organoazides and isocyanides to form carbodiimides, which is a rare example of catalytic nitrene transfer from an isolable iron imide. Chapter 6 concludes this study by reporting imidoiron(III) complexes of a terphenyl-substituted diketiminate ligand. The new ligand is found to improve HAT rates by increasing the substrate binding pocket. The HAT rate for an alkylimidoiron(III) complex is found to be significantly faster than an arylimido complex, consistent with the greater basicity of the alkylimido ligand.