Electronic Structure/function Relationship in Metal Ligand Multiple Bonds for C-H Functionalization Chemistry

Abstract

The factors that enable intermolecular C–H amination by iron complexes supported by dipyrromethene ligands (RL, L=1,9-R2-5-mesityldipyrromethene R = 2,4,6-Ph3C6H2 (Ar), Ad) were investigated. Despite the diversity of characterized iron imide complexes (FeII, S = 0; FeIII, S = 1/2, 1, 3/2; FeIV, S = 0, 1; FeV, S = 1/2), group transfer reactivity by these molecules into unactivated C–H bonds is unique to the dipyrrin supported complex (ArL)FeCl(NC6H4tBu), which consists of a high-spin (S = 5/2) FeIII center antiferromagnetically coupled to an imido-centered radical (FeIII–•NR). However, the complex electronic structure complicates analysis of the salient features of the electronic structure that enable such unprecedented chemistry.

Therefore, a family of dipyrrin-supported iron imide complexes that do not possess N-radical character were synthesized. Extensive spectroscopy and magnetic characterization support a view of these complexes as the first crystallographically characterized S = 5/2 FeIII imide complexes (FeIII=NR). The unprecendented electronic structure supports the first examples of intermolecular C–H amination from isolated FeIII imides and indicates that the presence of a high-spin metal-ligand multiple bond is sufficient to engender the desired C–H functionalization chemistry.

The redox relationship between the FeIII–•NR and FeIII=NR electronic structure was exploited to prepare an FeIII (alkyl)iminyl complex, the direct analogue to the intermediate formed during the catalytic C–H amination chemistry reported previously by our group. X-ray absorption spectroscopy demonstrates a constant iron oxidation upon redox, suggesting N-atom valence chemistry at the transferrable imide moiety. Kinetic analysis implies that oxidation to the FeIII–•NR lowers the enthalpic barrier to C–H functionalization which contributes to a 1,000-fold increase in reactivity towards C–H bonds.

Finally, these studies lead to the development of a new generation of dipyrrin-supported iron catalysts for the intramolecular cyclization of alkyl azides into N-Boc-pyrrolidines that can function at 0.01 mol % catalyst loading – a decrease in loading by three-orders of magnitude as compared to our previous catalytic system. These catalysts manifest high tolerance to Lewis bases, and have enabled the cyclization of a new class of substrates featuring activation by a variety of heteroatom-containing functional groups.