Hemilabile Proton Relays and Redox Activity Lead to {FeNO} and Significant Rate Enhancements in NO 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(PDI)(CO) (3) and Fe(PDI)(CO) (4) display 40-fold enhancements in the initial rate of NO reduction, with respect to Fe(PDI)(CO) (7). Utilizing the proper sterics and p K of the pendant base(s) to introduce hemilability into our ligand scaffolds, we report unusual {FeNO} mononitrosyl iron complexes (MNICs) as intermediates in the NO reduction reaction. The {FeNO} species behave spectroscopically and computationally similar to {FeNO}, an unusual intermediate-spin Fe(III) coupled to triplet NO and a singly reduced PDI ligand. These {FeNO} MNICs facilitate enhancements in the initial rate.