The P-cluster is a unique iron-sulfur center that likely functions as a dynamic electron (e−) relay site between the Fe-protein and the catalytic FeMo-cofactor in nitrogenase. The P-cluster has been shown to undergo large conformational changes upon 2-e− oxidation which entail the coordination of two of the Fe centers to a Ser side chain and a backbone amide nitrogen, respectively. Yet, how and if this 2-e− oxidized state (POX) is involved in catalysis by nitrogenase is not well established. Here, we present the crystal structures of reduced and oxidized MoFe-protein (MoFeP) from Gluconacetobacter diazotrophicus (Gd), which natively possesses an Ala residue in the position of the Ser ligand to the P-cluster. While reduced Gd-MoFeP is structurally identical to previously characterized counterparts around the FeMo-cofactor, oxidized Gd-MoFeP features an unusual Tyr coordination to its P-cluster along with ligation by a backbone amide nitrogen. EPR analysis of the oxidized Gd-MoFeP P-cluster confirmed that it is a 2-e− oxidized, integer-spin species. Importantly, we have found that the sequence positions corresponding to the Ser and Tyr ligands are almost completely covariant among Group I nitrogenases. These findings strongly support the possibility that the POX state is functionally relevant in nitrogenase catalysis and that a hard, O-based anionic ligand serves to stabilize this state in a switchable fashion.

High-valent nonheme Fe IV −oxido species are key intermediates in biological oxidation, and their properties are proposed to be influenced by the unique microenvironments present in protein active sites. Microenvironments are regulated by noncovalent interactions, such as hydrogen bonds (H-bonds) and electrostatic interactions; however, there is little quantitative information about how these interactions affect crucial properties of high valent metal−oxido complexes. To address this knowledge gap, we introduced a series of Fe IV −oxido complexes that have the same S = 2 spin ground state as those found in nature and then systematically probed the effects of noncovalent interactions on their electronic, structural, and vibrational properties. The key design feature that provides access to these complexes is the new tripodal ligand [poat] 3− , which contains phosphinic amido groups. An important structural aspect of [Fe IV poat(O)] − is the inclusion of an auxiliary site capable of binding a Lewis acid (LA II ); we used this unique feature to further modulate the electrostatic environment around the Fe−oxido unit. Experimentally, studies confirmed that H-bonds and LA II s can interact directly with the oxido ligand in Fe IV −oxido complexes, which weakens the FeO bond and has an impact on the electronic structure. We found that relatively large vibrational changes in the Fe−oxido unit correlate with small structural changes that could be difficult to measure, especially within a protein active site. Our work demonstrates the important role of noncovalent interactions on the properties of metal complexes, and that these interactions need to be considered when developing effective oxidants.

Oxidation of the nucleophilic nitride, (salen)Mn≡N (1) with stoichiometric [Ar N][X] initiated a nitride coupling reaction to N , a major step toward catalytic ammonia oxidation (salen=N,N'-bis(salicylidene)-ethylenediamine dianion; Ar=p-bromophenyl; X=[SbCl ] or [B(C F ) ] ). N production was confirmed by mass spectral analysis of the isotopomer, 1- N, and the gas quantified. The metal products of oxidation were the reduced Mn dimers, [(salen)MnCl] (2) or [(salen)Mn(OEt )] [B(C F ) ] (3) for X=[SbCl ] or [B(C F ) ] , respectively. The mechanism of nitride coupling was probed to distinguish a nitridyl from a nucleophilic/electrophilic coupling sequence. During these studies, a rare mixed-valent Mn /Mn bridging nitride, [(salen)Mn (μ-N)Mn (salen)][B(C F ) ] (4), was isolated, and its oxidation-state assignment was confirmed by X-ray diffraction (XRD) studies, perpendicular and parallel-mode EPR and UV/Vis/NIR spectroscopies, as well as superconducting quantum interference device (SQUID) magnetometry. We found that 4 could subsequently be oxidized to 3. Furthermore, in view of generating a catalytic system, 2 can be re-oxidized to 1 in the presence of NH and NaOCl closing a pseudo-catalytic "synthetic" cycle. Together, the reduction of 1→2 followed by oxidation of 2→1 yield a genuine synthetic cycle for NH oxidation, paving the way to the development of a fully catalytic system by using abundant metal catalysis.

An efficient iodination reaction of electron-deficient heterocycles is described. The reaction utilizes KO(t)Bu as an initiator and likely proceeds by a radical anion propagation mechanism. This new methodology is particularly effective for functionalization of building blocks for electron transport materials. Its utility is demonstrated with the synthesis of a new perylenediimide-thiazole non-fullerene acceptor capable of delivering a power conversion efficiency of 4.5% in a bulk-heterojunction organic solar cell.

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.