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 pKa 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.