Despite their ubiquitous presence, formation of the C-N bond has been challenging from a synthetic point of view. Typical methodologies for C-N bond formation include reductive amination, nucleophilic substitutions, and related processes that all require pre-installed directing and functional groups. These transiently installed groups generally compromise the atom-economy of the reaction and lead to considerable amounts of waste. A much more attractive approach towards C-N bond formation is the direct amination of unactivated C-H bonds, a method that relies on catalysts capable of transferring and inserting nitrenes. Many different nitrene sources have been utilized including iminoiodanes, 10 chloroamines, hydroxylamines, anthranils, 13 nitrosoarenes 14 and others. Despite their high reactivity, these nitrene sources are complicated to prepare and are poorly atom-economical as they produce considerable waste products upon generating the reactive nitrene. Using organic azides as nitrene precursor mitigates these issues, as only dinitrogen is formed as benign side product. However, organic azides are by far the least reactive of these nitrene precursors, which adds to the challenge of catalyst development. In pioneering work, Betley et al. described the use of an iron dipyrrin complex for the intramolecular C-H amination with organic azides to form 5-membered N-heterocycles, a class of compounds relevant to the pharmaceutical industry. Based on this breakthrough, several other catalysts were developed by using sophisticated ligand design and involving iron, cobalt, nickel, ruthenium 39,40 and palladium. Currently, iron is the most attractive metal for this transformation with high turnover numbers (TONs) up to 7600, and with additional benefits such as low costs, low toxicity and high Earth-abundance. Despite the popularity of iron for this transformation, remarkably little is known about the mechanism of C-H amination with azides. In parts, mechanistic studies have been complicated by the fact that a stoichiometric amount of Boc2O is needed in the catalytic reaction to prevent product inhibition. Only a few cobalt and nickel complexes, which do not require Boc2O as additive, were studied mechanistically by both experimental and computational methods.