Metal complexes composed of redox-active pyridinediimine (PDI) ligands are capable of forming ligand-centered radicals. In this Forum article, we demonstrate that integration of these types of redox-active sites with bioinspired secondary coordination sphere motifs produce direduced complexes, where the reduction potential of the ligand-based redox sites is uncoupled from the secondary coordination sphere. The utility of such ligand design was explored by encapsulating redox-inactive Lewis acidic cations via installation of a pendant benzo-15-crown-5 in the secondary coordination sphere of a series of Fe(PDI) complexes. Fe(PDI)(CO) was shown to encapsulate the redox-inactive alkali ion, Na, causing only modest (31 mV) anodic shifts in the ligand-based redox-active sites. By uncoupling the Lewis acidic sites from the ligand-based redox sites, the pendant redox-inactive ion, Na, can entice the corresponding counterion, NO, for reduction to NO. The subsequent initial rate analysis reveals an acceleration in anion reduction, confirming this hypothesis.

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 the enhancements in the initial rate.

A set of distorted square planar Cu(I) complexes were synthesized and characterized utilizing the sterically encumbering pyridinediimine ligand, (iPr)PDI (where (iPr)PDI = 2,6-(2,6-(i)Pr2C6H3N=CMe)2C5H3N). The oxidation state of the Cu center(s) were elucidated to be Cu(I) with a neutral PDI ligand system based on structural, spectroscopic, and computational data.