Open-Shell First-Row Transition Metal Complexes for C-H Functionalization: From Electronic Structure to Catalysis

Abstract

The modular electronic and steric properties of iron dipyrromethene complexes were explored to facilitate electronic structure investigations via intermediate isolation and catalyst design for controlled C–H amination processes. To this end, a sterically accessible tert-butyl substituted dipyrrin (tBuL = 1,9-di-tert-butyl-5-(2,6-dichlorophenyl)-dipyrromethene) scaffold allowed for identification of a terminal iron iminyl (tBuL)FeCl(•NC6H3-2,6-iPr2). Akin to the previously reported (ArL)FeCl(•NC6H4-4-tBu) (Ar = 2,4,6-Ph3-C6H2), this species is best described as a high-spin FeIII center antiferromagnetically coupled to an iminyl radical. These results establish the inherent preference of the dipyrrinato iron platforms for such a unique electronic construct which serves to render the iron iminyl species highly reactive and yet stabilize it to permit its direct isolation in the absence of a protective scaffold. Removal of the ortho aryl substituents, however, promotes formation of a di-iron bridging imido [(tBuL)FeCl]2(μ-NC6H3-3,5-(CF3)2) displaying two antiferromagnetically coupled high-spin ferric centers. This complex can successfully transfer the N-group into styrene as well as allylic and benzylic C–H bonds under stoichiometric and catalytic conditions. Both dissociation into an iron iminyl and an unprecedented direct hydrogen atom abstraction (HAA) at the dinuclear imide are viable routes for the C–H amination process, demonstrating the importance of an open-shell electronic configuration to promote reactivity at such otherwise inert dimeric motifs. With a broader understanding of the electronic features of the iron dipyrrin system, systematic tuning of ancillary anionic ligands enabled the development of a diastereoselective C–H amination synthesis for 2,5-disubstituted pyrrolidines from linear alkyl azides following a previously reported protocol in our group. A combination of experimental observations and theoretical investigations provided insight into the selectivity determining HAA transition state to recognize the steric clashes between the azide substrate and the catalyst that disfavor a trans arrangement of the 2,5-substituents. Building upon these interactions, among a series of alkoxide and phenoxide complexes surveyed, (AdL)Fe(OPh)(THF) was found to induce high diastereocontrol (> 20:1) for a range of 1-azido-1-aryl-hex-5-ene substrates. Attempts to expand the synthetic viability of these iron complexes led to the unexpected discovery of dipyrrinato iron tetrazenes as highly active catalysts for the intramolecular C–H amination transformation. 57Fe Mӧssbauer, kinetic and NMR studies corroborate the direct participation of these robust and rather sterically encumbered iron tetrazenes during catalytic turnover. Both electronic and steric effects were identified as contributors for the different reaction rates observed for a series of alkyl and aryl substituted tetrazene moieties. Overall, these systems manifest remarkable stability during catalysis, can operate at catalyst loadings as low as 0.01 mol%, and display enhanced tolerance to Lewis basic functionalities.