Iron and Cobalt Complexes Supported by Weak Field Pyrrolic Scaffolds: Electronic Structure to Small Molecule Activation

Abstract

The importance of small molecule activation processes has grown as a potential chemical solution for current environmental problems as well as its applicability in the production of value-added pharmaceutics. Metal–ligand multiple bonds (MLMBs) have been investigated as key intermediates in a wide range of biological and synthetic small molecule activation processes. Understanding the electronic structure of MLMBs is critical to predicting their attendant reactivity. In particular, the unusual reactive MLMBs stems from attenuation of their multiple-bonded character as a consequence of their open-shell electronic configurations. As high valent metals or π-accepting ligands typically favor strong ligand fields to maximize MLMB character, the unique electronic structures and reactivity of metal nitride, hydroxo, and nitrosyl complexes afforded by a weak-field dipyrrin ligand scaffolds were explored to access new reactivity paradigms. As a primer to these themes, Chapter 1 introduces the electronic structure of metal nitride complexes and describes how attenuation of MLMB enables electrophilic N–atom transfer reactivity. These concepts are then extended to isolobal metal oxo and nitrosyl complexes. Transition metal nitride chemistry. In Chapter 2, I describe the photogeneration and characterization of the first open-shell terminal iron(IV) nitride (EmL)Fe(N). The photocrystallography, zero-field 57Fe Mössbauer spectroscopy, and computational analysis support a triplet ground state of the iron nitride complex. Notably, occupation of π*Fe–N attenuates Fe–N multiple bond character, enabling (i) primary C(sp3)–H amination, (ii) H2 cleavage, (iii) aromatic C–C cleavage, and (iv) photocatalytic N–atom transfer reactivity. These reactivity modes have not previously been observed for low-spin Fe(N) analogues. Chapter 3 expands the electrophilic N–atom transfer reactivity of (EmL)Fe(N) to catalytic aziridination of olefinic substrates, including unsubstituted ethylene, using Me3SiN3 as a nitrogen atom source. This synthetic strategy suppresses competing allylic C–H activation, and enables regio- and stereoselective aziridination of disubstituted internal alkenes. In Chapter 4, I examine the synthesis and electronic structure of terminal cobalt metallonitrene complex (EmL)Co(3N)(py). The subvalent metallonitrene character was characterized by crystallography, EPR spectroscopy, and computational analysis. This unusual electronic structure enables rare electrophilic N–atom transfer reactivity as well as cleavage of aromatic C–C and C=C π bonds. Redox non-innocence in weak-field complexes. In Chapter 5 I address changes in the transfer polarity of metal hydroxides as a function of metal oxidation state. A high-spin, low valent terminal iron hydroxo (EmL)FeII(OH) exhibits nucleophilic reactivity toward group 14 electrophiles and reversibly captures CO2 through pressure-swing modulation in both solution and the solid state. In contrast, a high-spin FeIII(OH) complex supported by the same ligand scaffold exhibits radical-type reactivity demonstrated in radical recombination processes with carbon-centered radicals. These results demonstrate the importance of metal-ligand covalency and relative electronegativities of metal and ligands in controlling group transfer polarity. In Chapter 6, I describe the synthesis and characterization of mono- and dinitrosyl iron complexes within the unified ligand scaffold (EmL). The electronic structure of a series of three- and four-coordinate {Fe(NO)}7, {Fe(NO)}8, {Fe(NO)2}9, and {Fe(NO)2}10 complexes were investigated using IR spectroscopy, EPR spectroscopy, zero-field 57Fe Mössbauer spectroscopy, magnetometry, X-ray absorption spectroscopy, crystallography, and broken-symmetry DFT calculations. In all cases, the Fe–NO motifs are best described as high-spin iron centers antiferromagnetically coupled to 3NO– spin manifolds. Within the unified ligand scaffold, the degree of N–O activation by symmetry, ligation, and ion-pairing were evaluated. These results expand the fundamental understanding of metal-nitrosyl bonding and provide a foundation for development in nitrosyl-based reactivity.