Bond Trading: Intramolecular Metal and Ligand Exchange within a NO/Ni/Co Complex

With the goal of generating hetero-redox levels on metals as well as on nitric oxide (NO), metallodithiolate (N2 S2 )CoIII (NO- ), N2 S2 = N,N- dibenzyl-3,7-diazanonane-1,9-dithiolate, is introduced as ligand to a well-characterized labile [Ni0 (NO)+ ] synthon. The reaction between [Ni0 (NO+ )] and [CoIII (NO- )] has led to a remarkable electronic and ligand redistribution to form a heterobimetallic dinitrosyl cobalt [(N2 S2 )NiII ∙Co(NO)2 ]+ complex with formal two electron oxidation state switches concomitant with the nickel extraction or transfer as NiII into the N2 S2 ligand binding site. To date, this is the first reported heterobimetallic cobalt dinitrosyl complex.

Coordinatively Unsaturated Nickel Nitroxyl Complex: Structure, Physicochemical Properties, and Reactivity toward Dioxygen

For its important roles in biology, nitrogen monoxide (·NO) has become one of the most studied and fascinating molecules in chemistry. ·NO itself acts as a "noninnocent" or "redox active" ligand to transition metal ions to give metal-NO (M-NO) complexes. Because of this uncertainty due to redox chemistry, the real description of the electronic structure of the M-NO unit requires extensive spectroscopic and theoretical studies. We previously reported the Ni-NO complex with a hindered N3 type ligand [Ni(NO)(L3)] (L3- denotes hydrotris(3-tertiary butyl-5-isopropyl-1-pyrazolyl)borate anion), which contains a high-spin (hs) nickel(II) center and a coordinated 3NO-. This complex is very stable toward dioxygen due to steric protection of the nickel(II) center. Here, we report the dioxygen reactivity of a new Ni-NO complex, [Ni(NO)(I)(L1″)], with a less hindered N2 type bis(pyrazolyl)methane ligand, which creates a coordinatively unsaturated ligand environment about the nickel center. Here, L1″ denotes bis(3,5-diisopropyl-1-pyrazolyl)methane. This complex is also described as a hs-nickel(II) center with a bound 3NO-, based on spectroscopic and theoretical studies. Unexpectedly, the reaction of [Ni(NO)(I)(L1″)] with O2 yielded [Ni(κ2-O2N)(L1″)2](I3), with the oxidation of both 3NO- and the I- ion to yield NO2- and I3-. Both complexes were characterized by X-ray crystallography, IR, and UV-Vis spectroscopy and theoretical calculations.

Promotion of S-nitrosation of cysteine by a {Co(NO)2}10 complex

S-Nitrosothiols (SNOs) serve as endogenous carriers and donors of NO within living cells, releasing nitrosonium ions (NO+), NO, or other nitroso derivatives. In this study, we present a bioinspired {Co(NO)2}10 complex 1 that achieved S-nitrosation towards Cys residues. The incorporation of a ferrocenyl group in 1 allowed for fine-tuning of the nitrosation reaction, taking advantage of the redox ability of Cys residues. Complex 1 was synthesized and characterized, demonstrating its NO translation reactivity. Furthermore, complex 1 successfully converted Cys into S-nitrosocysteine (Cys-SNO), as confirmed by UV-Vis, IR, and XAS spectroscopy. This study presents a promising approach for S-nitrosation of Cys residues for further exploration in the modification of Cys-containing peptides.

Nitric Oxide Oxygenation Reactions of Cobalt-Peroxo and Cobalt-Nitrosyl Complexes

Here, we report a comparative study of nitric oxide oxidation (NOO) reactions of Co^III-peroxo (Co^III-O₂^2-) and Co-nitrosyl ({CoNO}⁸) complexes bearing the same N₄-donor ligand (HMTETA) framework. In this regard, we prepared and characterized two new [(HMTETA)Co^III(O₂^2-)]⁺ (2, S = 2) and [(HMTETA)Co(NO)]²⁺ (3, S = 1) complexes from [(HMTETA)Co^II(CH₃CN)₂]²⁺ (1). Both complexes (2 and 3) are characterized by different spectroscopic measurements, including their DFT-optimized structures. Complex 2 produces Co^II-nitrato [(HMTETA)Co^II(NO₃^-)]⁺ (Co^II-NO₃^-, 4) complex in the presence of NO. In contrast, when 3 reacted with a superoxide (O₂^•-) anion, it generated Co^II-nitrito [(HMTETA)Co^II(NO₂^-)]⁺ (Co^II-NO₂^-, 5) with O₂ evolution. Experiments performed using ^18/16O-labeled superoxide (¹⁸O₂^•-/¹⁶O₂^•-) showed that O₂ originated from the O₂^•- anion. Both the NOO reactions are believed to proceed via a presumed peroxynitrite (PN) intermediate. Although we did not get direct spectral evidence for the proposed PN species, the mechanistic investigation using 2,4-di-tert-butylphenol indirectly suggests the formation of a PN intermediate. Furthermore, tracking the source of the N-atom in the above NOO reactions using ¹⁵N-labeled nitrogen (¹⁵NO) revealed N-atoms in 4 (Co^II-¹⁵NO₃^-) and 5 (Co^II-¹⁵NO₂^-) derived from the ¹⁵NO moiety.

Hydrogen-Bond-Assisted Adsorption of Nitric Oxide on Various Metal-Loaded ZSM-5 Zeolites

Understanding the characteristics of nitric oxide (NO) adsorption on metal-loaded zeolites is a prerequisite for developing efficient catalysts for NO abatement reactions. In this study, we probed the effect of the hydrogen bond that exists between adsorbed NO and Brønsted acid sites (BAS) in various metal-loaded ZSM-5 zeolites (M-ZSM-5, wherein M = Fe, Co, Ni, Cu, Zn, Pd, Ag, and Au) by using density functional theory calculations. The presence of a hydrogen bond has altered the NO adsorption energies significantly; appreciable stabilization via hydrogen bonding is noted for NO complexes of Zn, Fe, and Co, and reasonable stabilization is obtained for Ni and Cu complexes, whereas an anomalous effect of a hydrogen bond is identified in Ag, Pd, and Au species. Moderate weakening of the N-O bond in all NO-adsorbed complexes primarily due to a hydrogen bond has been realized in terms of Mayer bond order and quantum theory of atoms in molecules topological analyses; N-O bond activation follows the order Ag < Pd < Au < Ni < Cu < Co < Fe < Zn. We obtained a good correlation between hydrogen bond distance and molecular electrostatic potential at the O atom (V_O) of NO adsorbed on BAS-free M-ZSM-5; which suggests that V_O can be considered as a key descriptor to infer the strength of a hydrogen bond between the adsorbed NO and M-ZSM-5 with BAS. Finally, the energy decomposition analysis in combination with natural orbitals for chemical valence has provided the qualitative aspects of electron back-donation from the metal to the antibonding molecular orbital of NO; this back-donation is quite impressive in hydrogen-bond-assisted NO adsorption. We expect that the findings of this study will open up the possibility of the design of BAS-containing metal-loaded zeolites for the catalytic mitigation of NO.

What Is the Right Level of Activation of a High-Spin {FeNO}7 Complex to Enable Direct N-N Coupling? Mechanistic Insight into Flavodiiron NO Reductases

Flavodiiron nitric oxide reductases (FNORs), found in pathogenic bacteria, are capable of reducing nitric oxide (NO) to nitrous oxide (N₂O) to detoxify NO released by the human immune system. Previously, we reported the first FNOR model system that mediates direct NO reduction (Dong, H. T.; J. Am. Chem. Soc. 2018, 140, 13429-13440), but no intermediate of the reaction could be characterized. Here, we present a new set of model complexes that, depending on the ligand substitution, can either mediate direct NO reduction or stabilize a highly activated high-spin (hs) {FeNO}⁷ complex, the first intermediate of the reaction. The precursors, [{Fe^II(MPA-(RPhO)₂)}₂] (1, R = H and 2, R = ^tBu, Me), were prepared first and fully characterized. Complex 1 (without steric protection) directly reduces NO to N₂O almost quantitatively, which constitutes only the second example of this reaction in model systems. Contrarily, the reaction of sterically protected 2 with NO forms the stable mononitrosyl complex 3, which shows one of the lowest N-O stretching frequencies (1689 cm^-1) observed so far for a mononuclear hs-{FeNO}⁷ complex. This study confirms that an N-O stretch ≤1700 cm^-1 represents the appropriate level of activation of the FeNO unit to enable direct NO reduction. The higher activation level of these hs-{FeNO}⁷ complexes required for NO reduction compared to those formed in FNORs emphasizes the importance of hydrogen bonding residues in the active sites of FNORs to activate the bound NO ligands for direct N-N coupling and N₂O formation. The implications of these results for FNORs are further discussed.

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

An Integrated View of Nitrogen Oxyanion Deoxygenation in Solution Chemistry and Electrospray Ion Production

There has been an increasing interest in chemistry involving nitrogen oxyanions, largely due to the environmental hazards associated with increased concentrations of these anions leading to eutrophication and aquatic "dead zones". Herein, we report the synthesis and characterization of a suite of MNO_x complexes (M = Co, Zn: x = 2, 3). Reductive deoxygenation of cobalt bis(nitrite) complexes with bis(boryl)pyrazine is faster for cobalt than previously reported nickel, and pendant O-bound nitrito ligand is still readily deoxygenated, despite potential implication of an isonitrosyl primary product. Deoxygenation of zinc oxyanion complexes is also facile, despite zinc being unable to stabilize a nitrosyl ligand, with liberation of nitric oxide and nitrous oxide, indicating N-N bond formation. X-ray photoelectron spectroscopy is effective for discriminating the types of nitrogen in these molecules. ESI mass spectrometry of a suite of M(NO_x)_y (x = 2, 3 and y = 1, 2) shows that the primary form of ionization is loss of an oxyanion ligand, which can be alleviated via the addition of tetrabutylammonium (TBA) as a nonintuitive cation pair for the neutral oxyanion complexes. We have shown these complexes to be subject to deoxygenation, and there is evidence for nitrogen oxyanion reduction in several cases in the ESI plume. The attractive force between cation and neutral is explored experimentally and computationally and attributed to hydrogen bonding of the nitrogen oxyanion ligands with ammonium α-CH₂ protons. One example of ESI-induced reductive dimerization is mimicked by bulk solution synthesis, and that product is characterized by X-ray diffraction to contain two Co(NO)₂⁺ groups linked by a highly conjugated diazapolyene.

Exploring the Limits of Dative Boratrane Bonding: Iron as a Strong Lewis Base in Low-Valent Non-Heme Iron-Nitrosyl Complexes

We previously reported the synthesis and preliminary characterization of a unique series of low-spin (ls) {FeNO}8-10 complexes supported by an ambiphilic trisphosphineborane ligand, [Fe(TPB)(NO)]+/0/-. Herein, we use advanced spectroscopic techniques and density functional theory (DFT) calculations to extract detailed information as to how the bonding changes across the redox series. We find that, in spite of the highly reduced nature of these complexes, they feature an NO+ ligand throughout with strong Fe-NO π-backbonding and essentially closed-shell electronic structures of their FeNO units. This is enabled by an Fe-B interaction that is present throughout the series. In particular, the most reduced [Fe(TPB)(NO)]- complex, an example of a ls-{FeNO}10 species, features a true reverse dative Fe → B bond where the Fe center acts as a strong Lewis-base. Hence, this complex is in fact electronically similar to the ls-{FeNO}8 system, with two additional electrons "stored" on site in an Fe-B single bond. The outlier in this series is the ls-{FeNO}9 complex, due to spin polarization (quantified by pulse EPR spectroscopy), which weakens the Fe-NO bond. These data are further contextualized by comparison with a related N2 complex, [Fe(TPB)(N2)]-, which is a key intermediate in Fe(TPB)-catalyzed N2 fixation. Our present study finds that the Fe → B interaction is key for storing the electrons needed to achieve a highly reduced state in these systems, and highlights the pitfalls associated with using geometric parameters to try to evaluate reverse dative interactions, a finding with broader implications to the study of transition metal complexes with boratrane and related ligands.

The Fe2 (NO)2 Diamond Core: A Unique Structural Motif In Non-Heme Iron-NO Chemistry

Non-heme high-spin (hs) {FeNO}⁸ complexes have been proposed as important intermediates towards N₂ O formation in flavodiiron NO reductases (FNORs). Many hs-{FeNO}⁸ complexes disproportionate by forming dinitrosyl iron complexes (DNICs), but the mechanism of this reaction is not understood. While investigating this process, we isolated a new type of non-heme iron nitrosyl complex that is stabilized by an unexpected spin-state change. Upon reduction of the hs-{FeNO}⁷ complex, [Fe(TPA)(NO)(OTf)](OTf) (1), the N-O stretching band vanishes, but no sign of DNIC or N₂ O formation is observed. Instead, the dimer, [Fe₂ (TPA)₂ (NO)₂ ](OTf)₂ (2) could be isolated and structurally characterized. We propose that 2 is formed from dimerization of the hs-{FeNO}⁸ intermediate, followed by a spin state change of the iron centers to low-spin (ls), and speculate that 2 models intermediates in hs-{FeNO}⁸ complexes that precede the disproportionation reaction.

Structures and Properties of Dinitrosyl Iron and Cobalt Complexes Ligated by Bis(3,5-diisopropyl-1-pyrazolyl)methane

Two dinitrosyl iron and cobalt complexes [Fe(NO)2(L1”)](BF4) and [Co(NO)2(L1”)](BF4) are synthesized and characterized, supported by a less hindered bidentate nitrogen ligand bis(3,5-diisopropyl-1-pyrazolyl)methane (denoted as L1”), are surprisingly stable under argon atmosphere. X-ray structural analysis shows a distorted tetrahedral geometry. Spectroscopic and structural parameters of the dinitrosyl iron and cobalt complexes are consistent with the previous reported {Fe(NO)2}9 and {Co(NO)2}10. Two N–O and M–N(O) stretching frequencies and their magnetic properties are also consistent with the above electronic structural assignments. We explored the dioxygen reactivities of the obtained dinitrosyl complexes. Moreover, the related [FeCl2(L1”)], [Co(NO3)2(L1”)], and [Co(NO2)2(L1”)] complexes are also characterized in detail.

Substrate Redox Non-innocence Inducing Stepwise Oxidative Addition Reaction: Nitrosoarene C-N Bond Cleavage on Low-Coordinate Cobalt(0) Species

The reactions of nitrosoarenes with transition-metal species are fundamentally important for their relevance to metal-catalyzed transformations of organo-nitrogen compounds in organic synthesis and also the metabolization of nitroarenes and anilines in biology. In addition to the well-known reactivity of metal-mediated N-O bond activation and cleavage of nitrosoarenes, we present herein the first observation of a nitrosoarene C-N bond oxidative addition reaction upon the interaction of a three-coordinate cobalt(0) species [(IPr)Co(vtms)₂] with 2,4,6-tri( tert-butyl)-1-nitroso-benzene (Ar*NO). The reaction produces a cobalt nitrosyl aryl complex, [(IPr)Co(Ar*)(NO)] (1), with a bis(nitrosoarene)cobalt complex, [(IPr)Co(η²-ONAr)(κ¹- O-ONAr)] (2), as an intermediate. Spectroscopic characterizations, DFT calculations, and kinetic studies revealed that the redox non-innocence of nitrosoarene induces a stepwise pathway for the C-N bond oxidative addition reaction.