Hemilabile Proton Relays and Redox Activity Lead to {FeNO} ^x and Significant Rate Enhancements in NO₂^- Reduction

Incorporation of the triad of redox activity, hemilability, and proton responsivity into a single ligand scaffold is reported. Due to this triad, the complexes Fe(^PyrrPDI)(CO)₂ (3) and Fe(^MorPDI)(CO)₂ (4) display 40-fold enhancements in the initial rate of NO₂^- reduction, with respect to Fe(^MeOPDI)(CO)₂ (7). Utilizing the proper sterics and p K_a of the pendant base(s) to introduce hemilability into our ligand scaffolds, we report unusual {FeNO} ^x mononitrosyl iron complexes (MNICs) as intermediates in the NO₂^- reduction reaction. The {FeNO} ^x species behave spectroscopically and computationally similar to {FeNO}⁷, an unusual intermediate-spin Fe(III) coupled to triplet NO^- and a singly reduced PDI ligand. These {FeNO} ^x MNICs facilitate enhancements in the initial rate.