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

Sequential Deoxygenation of CO2 and NO2- via Redox-Control of a Pyridinediimine Ligand with a Hemilabile Phosphine

The deoxygenation of environmental pollutants CO2 and NO2- to form value-added products is reported. CO2 reduction with subsequent CO release and NO2- conversion to NO are achieved via the starting complex Fe(PPhPDI)Cl2 (1). 1 contains the redox-active pyridinediimine (PDI) ligand with a hemilabile phosphine located in the secondary coordination sphere. 1 was reduced with SmI2 under a CO2 atmosphere to form the direduced monocarbonyl Fe(PPhPDI)(CO) (2). Subsequent CO release was achieved via oxidation of 2 using the NOx- source, NO2-. The resulting [Fe(PPhPDI)(NO)]+ (3) mononitrosyl iron complex (MNIC) is formed as the exclusive reduction product due to the hemilabile phosphine. 3 was investigated computationally to be characterized as {FeNO}7, an unusual intermediate-spin Fe(III) coupled to triplet NO- and a singly reduced PDI ligand.

Secondary Coordination Sphere Influences the Formation of Fe(III)-O or Fe(III)-OH in Nitrite Reduction: A Synthetic and Computational Study

The reduction of nitrite (NO₂^-) to generate nitric oxide (NO) is a significant area of research due to their roles in the global nitrogen cycle. Here, we describe various modifications of the tris(5-cyclohexyliminopyrrol-2-ylmethyl)amine H₃[N(pi^R)₃] ligand where the steric bulk and acidity of the secondary coordination sphere were explored in the non-heme iron system for nitrite reduction. The cyclohexyl and 2,4,6-trimethylphenyl variants of the ligand were used to probe the mechanism of nitrite reduction. While previously stoichiometric addition of nitrite to the iron(II)-species generated an iron(III)-oxo complex, changing the secondary coordination sphere to mesityl resulted in an iron(III)-hydroxo complex. Subsequent addition of an electron and two protons led to the release of water and regeneration of the starting iron(II) catalyst. This sequence mirrored the proposed mechanism of nitrite reduction in biological systems, where the distal histidine residue shuttles protons to the active site. Computational studies aimed at interrogating the dissimilar behavior of the cyclohexyl and mesityl ligand systems resulting in Fe(III)-oxo and Fe(III)-hydroxo complexes, respectively, shed light on the key role of H-bonds involving the secondary coordination sphere in the relative stability of these species.

Metal-Ligand Cooperative Transfer of Protons and Electrons

Realizing cooperativity between ligands and metal centers in the transfer of proton and electron equivalents has the potential to facilitate faster, selective, and novel transformations. Recent advances in the synthesis and application of ligands with these design features illustrate the value of this biomimetic strategy in synthetic chemistry.

NO Coupling by Nonclassical Dinuclear Dinitrosyliron Complexes to Form N2O Dictated by Hemilability

Selective coupling of NO by a nonclassical dinuclear dinitrosyliron complex (D-DNIC) to form N₂O is reported. The coupling is facilitated by the pyridinediimine (PDI) ligand scaffold, which enables the necessary denticity changes to produce mixed-valent, electron-deficient tethered DNICs. One-electron oxidation of the [{Fe(NO)₂}]₂^10/10 complex Fe₂(^PyrrPDI)(NO)₄ (4) results in NO coupling to form N₂O via the mixed-valent {[Fe(NO)₂]₂}^9/10 species, which possesses an electron-deficient four-coordinate {Fe(NO)₂}¹⁰ site, crucial in N-N bond formation. The hemilability of the PDI scaffold dictates the selectivity in N-N bond formation because stabilization of the five-coordinate {Fe(NO)₂}⁹ site in the mixed-valent [{Fe(NO)₂}]₂^9/10 species, [Fe₂(^Pyr2PDI)(NO)₄][PF₆] (6), does not result in an electron-deficient, four-coordinate {Fe(NO)₂}¹⁰ site, and hence no N-N coupling is observed.

A Redox-Active Tetrazine-Based Pincer Ligand for the Reduction of N-Oxyanions Using a Redox-Inert Metal

The reaction chemistry of the bis-tetrazinyl pyridine ligand (btzp) towards nitrogen oxyanions coordinated to zinc is studied in order to explore the reduction of the NO_x ^- substrates with a redox-active ligand in the absence of redox activity at the metal. Following syntheses and characterization of (btzp)ZnX₂ for X=Cl, NO₃ and NO₂ , featuring O-Zn linkage of both nitrogen oxyanions, it is shown that a silylating agent selectively delivers silyl substituents to tetrazine nitrogens, without reductive deoxygenation of NO_x ^-1 . A new synthesis of the highly hydrogenated H₄ btzp, containing two dihydrotetrazine reductants is described as is the synthesis and characterization of (H₄ btzp)ZnX₂ for X=Cl and NO₃ , both of which show considerable hydrogen bonding potential of the dihydrotetrazine ring NH groups. The (H₄ btzp)ZnCl₂ complex does not bind zinc in the pincer pocket, but instead H₄ btzp becomes a bridge between neighboring atoms through tetrazine nitrogen atoms, forming a polymeric chain. The reaction of AgNO₂ with (H₄ btzp)ZnCl₂ is shown to proceed with fast nitrite deoxygenation, yielding water and free NO. Half of the H₄ btzp reducing equivalents form Ag⁰ and thus the chloride ligand remains coordinated to the zinc metal center to yield (btzp)ZnCl₂ . To compare with AgNO₂ , experiments of (H₄ btzp)ZnCl₂ with NaNO₂ result in salt metathesis between chloride and nitrite, highlighting the importance of a redox-active cation in the reduction of nitrite to NO.

Graphite Conjugation of a Macrocyclic Cobalt Complex Enhances Nitrite Electroreduction to Ammonia

This work reports on the generation of a graphite-conjugated diimine macrocyclic Co catalyst (GCC-CoDIM) that is assembled at o-quinone edge defects on graphitic carbon electrodes. X-ray photoelectron spectroscopy and X-ray absorption spectroscopy confirm the existence of a new Co surface species with a coordination environment that is the same as that of the molecular analogue, [Co(DIM)Br₂]⁺. GCC-CoDIM selectively reduces nitrite to ammonium with quantitative Faradaic efficiency and at a rate that approaches enzymatic catalysis. Preliminary mechanistic investigations suggest that the increased rate is accompanied by a change in mechanism from the molecular analogue. These results provide a template for creating macrocycle-based electrocatalysts based on first-row transition metals conjugated to an extreme redox-active ligand.

Back donation, intramolecular electron transfer and N-O bond scission targeting nitrogen oxyanion reduction: how can a metal complex assist?

A density functional theory exploration studies a range of ancillary coordinated ligands accompanying nitrogen oxyanions with the goal of promoting back donation towards varied nitrogen oxidation states. Evaluation of a suite of Ru and Rh metal complexes reveals minimum back donation to the κ¹-nitrogen oxyanion ligand, even upon one-electron reduction. This reveals some surprising consequences of reduction, including redox activity at pyridine and nitrogen oxyanion dissociation. Bidentate nitrate was therefore considered, where ancillary ligands enforce geometries that maximize M-NO_x orbital overlap. This strategy is successful and leads to full electron transfer in several cases to form a pyramidal radical NO₃^2- ligand. The impact of ancillary ligand on degree of nitrate reduction is probed by comparing the powerful o-donor tris-carbene borate (TCB) to a milder donor, tris-pyrazolyl borate (Tp). This reveals that with the milder Tp donor, nitrate reduction is only seen upon addition of a Lewis base. Protonation of neutral and anionic (TCB)Ru(κ²-NO₃) at both terminal and internal oxygens reveals exergonic N-O bond cleavage for the reduced species, with one electron coming from Ru, yielding a Ru^III hydroxide product. Comparison of H⁺ to Na⁺ electrophile shows weaker progress towards N-O bond scission. Finally, calculations on (TCB)Fe(κ²-NO₃) and [(TCB)Fe(κ²-NO₃)]^- show that electron transfer to nitrate is possible even with an earth abundant 3d metal.

Biological and Bioinspired Inorganic N-N Bond-Forming Reactions

The metallobiochemistry underlying the formation of the inorganic N-N-bond-containing molecules nitrous oxide (N2O), dinitrogen (N2), and hydrazine (N2H4) is essential to the lifestyles of diverse organisms. Similar reactions hold promise as means to use N-based fuels as alternative carbon-free energy sources. This review discusses research efforts to understand the mechanisms underlying biological N-N bond formation in primary metabolism and how the associated reactions are tied to energy transduction and organismal survival. These efforts comprise studies of both natural and engineered metalloenzymes as well as synthetic model complexes.

Bis(imino)pyrazine-Supported Iron Complexes: Ligand-Based Redox Chemistry, Dearomatization, and Reversible C-C Bond Formation

Iron complexes supported by novel π-acidic bis(imino)pyrazine (P^PzDI) ligands can be functionalized at the nonligated nitrogen atom, and this has a marked effect on the redox properties of the resulting complexes. Dearomatization is observed in the presence of cobaltocene, which reversibly reduces the pyrazine core and not the imine functionality, as observed in the case of the pyridinediimine-ligated iron analogues. The resulting ligand-based radical is prone to dimerization through the formation of a long carbon-carbon bond, which can be subsequently cleaved under mild oxidative conditions.

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

Metalloenzymes catalyze many important reactions by managing the proton and electron flux at the enzyme active site. The motifs utilized to facilitate these transformations include hemilabile, redox-active, and so called proton responsive sites. Given the importance of incorporating and understanding these motifs in the area of coordination chemistry and catalysis, we highlight recent milestones in the field. Work incorporating the triad of hemilability, redox-activity, and proton responsivity into single ligand scaffolds will be described.

Five-Coordinate Low-Spin {FeNO}7 PNP Pincer Complexes

The synthesis and characterization of air-stable cationic mono nitrosonium Fe(I) PNP pincer complexes of the type [Fe(PNP)(NO)Cl]⁺ are described. These complexes are obtained via direct nitroslyation of [Fe(PNP)Cl₂] with nitric oxide at ambient pressure. On the basis of magnetic and EPR measurements as well as DFT calculations, these compounds were found to adopt a low-spin d⁷ configuration and feature a nearly linear bound NO ligand suggesting Fe^INO⁺ rather than Fe^IINO^• character. X-ray structures of all nitrosonium Fe(I) PNP complexes are presented. Preliminary investigations reveal that [Fe(PNP^NH- iPr)(NO)(Cl)]⁺ efficiently catalyzes the conversion of primary alcohols and aromatic and benzylic amines to yield mono N-alkylated amines in good isolated yields.

Cation-controlled catalysis with crown ether-containing transition metal complexes

This Feature Article reviews the structural motifs and catalytic applications of crown ether-containing catalysts and details the development of “pincer-crown ether” ligands for applications in controlled catalysis.