Uncoupled Redox-Inactive Lewis Acids in the Secondary Coordination Sphere Entice Ligand-Based Nitrite Reduction

Abstract

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.

First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands

“…312,314 Ion coordination, to the crown ether moiety resulted in an anodic shift of about 50 mV (Figure 12). 312,314 Figure 13. Electronic structure of the four-and five-coordinate iron PDI dinitrogen complexes.…”

Harnessing the active site triad: merging hemilability, proton responsivity, and ligand-based redox-activity

“…In this context, the redox innocent ion can be thought of as a fourth motif in ligand design and incorporating redox innocent ions into ligand scaffolds is an area of intense investigation. [67][68][69][70][71][72][73][74][75][76] However, ligand scaffolds containing redox innocent ions in conjunction with the triad (i.e. the tetrad) achieved by the OEC have not been reported to date.…”

Use of Crown Ether Functions as Secondary Coordination Spheres for the Manipulation of Ligand–Metal Intramolecular Electron Transfer in Copper–Guanidine Complexes

“…Herein we report on copper complexes with a redox‐active bisguanidine ligand with attached crown ether function, allowing to control the electronic structure of the metal complexes by encapsulation of a metal. Crown ether functions were used in the past to vary the redox potential of transition metal complexes, for example, of ferrocene, [37] cobalt Schiff base complexes, [38] or iron complexes of pyridine‐diimines [39, 40] . The example in Figure 2 a shows a ferrocene with a crown ether group attached to one of the cyclopentadienyl rings [36] .…”

“…Crown ether functions were used in the past to vary the redox potential of transition metal complexes, for example, of ferrocene, [37] cobalt Schiff base complexes, [38] or iron complexes of pyridine-diimines. [39,40] The example in Figure 2a showsaferrocene with ac rown ether group attachedt oone of the cyclopentadienyl rings. [36] The encapsulation of Na + leads to an anodic shift of~60 mV of the Fe III /Fe II reduction potential.…”