Monomeric Dinitrosyl Iron Complexes: Synthesis and Reactivity

Distortion of the [FeNO]2 Core in Flavodiiron Nitric Oxide Reductase Models Inhibits N-N Bond Formation and Promotes Formation of Unusual Dinitrosyl Iron Complexes: Implications for Catalysis and Reactivity

Flavodiiron nitric oxide reductases (FNORs) carry out the reduction of nitric oxide (NO) to nitrous oxide (N₂O), allowing infectious pathogens to mitigate toxic levels of NO generated in the human immune response. We previously reported the model complex [Fe₂(BPMP)(OPr)(NO)₂](OTf)₂ (1, OPr^- = propionate) that contains two coplanar NO ligands and that is capable of quantitative NO reduction to N₂O [White et al. J. Am. Chem. Soc. 2018, 140, 2562-2574]. Here we investigate, for the first time, how a distortion of the active site affects the ability of the diiron core to mediate N₂O formation. For this purpose, we prepared several analogues of 1 that contain two monodentate ligands in place of the bridging carboxylate, [Fe₂(BPMP)(X)₂(NO)₂]^3+/1+ (2-X; X = triflate, 1-methylimidazole, or methanol). Structural data of 2-X show that without the bridging carboxylate, the diiron core expands, leading to elongated (O)N-N(O) distances (from 2.80 Å in 1 to 3.00-3.96 Å in 2-X) and distorted (O)N-Fe-Fe-N(O) dihedral angles (from coplanarity (5.9°) in 1 to 52.9-85.1° in 2-X). Whereas 1 produces quantitative amounts of N₂O upon one-electron reduction, N₂O production is substantially impeded in 2-X, to an initial 5-10% N₂O yield. The main products after reduction are unprecedented hs-Fe^II/{Fe(NO)₂}^9/10 dinitrosyl iron complexes (DNICs). Even though mononuclear DNICs are stable and do not show N-N coupling (since it is a spin-forbidden process), the hs-Fe^II/{Fe(NO)₂}^9/10 DNICs obtained from 2-X show unexpected reactivity and produce up to quantitative N₂O yields after 2 h. The implications of these results for the active site structure of FNORs are 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.

N-O Bond Activation and Cleavage Reactions of the Nitrosyl-Bridged Complexes [M2Cp2(μ-PCy2)(μ-NO)(NO)2] (M = Mo, W)

The title complexes (1a,b) were prepared in two steps by first reacting the hydrides [M₂Cp₂(μ-H)(μ-PCy₂)(CO)₄] with [NO](BF₄) in the presence of Na₂CO₃ to give dinitrosyls [M₂Cp₂(μ-PCy₂)(CO)₂(NO)₂](BF₄), which were then fully decarbonylated upon reaction with NaNO₂ at 323 K. An isomer of the Mo₂ complex having a cisoid arrangement of the terminal ligands ( cis-1a) was prepared upon irradiation of toluene solutions of 1a with visible-UV light at 288 K. The structure of these trinitrosyl complexes was investigated using X-ray diffraction and density functional theory (DFT) calculations, these revealing a genuine pyramidalization of the bridging NO that might be associated in part to an increase of charge at the N atom and anticipated a weakening of the N-O bond upon reaction with bases or reducing reagents. Complexes 1a,b reacted with [FeCp₂](BF₄) to give first the radicals [M₂Cp₂(μ-PCy₂)(μ-NO)(NO)₂](BF₄) according to CV experiments, which then underwent H-abstraction to yield the nitroxyl-bridged complexes [M₂Cp₂(μ-PCy₂)(μ-κ¹:η²-HNO)(NO)₂](BF₄), alternatively prepared upon protonation with HBF₄·OEt₂. The novel coordination mode of the nitroxyl ligand in these products was thermodynamically favored over its tautomeric hydroximido form, according to DFT calculations, and similar nitrosomethane-bridged cations [M₂Cp₂(μ-PCy₂)( μ-κ¹:η²-MeNO)(NO)₂]⁺ were prepared by reacting 1a,b with CF₃SO₃Me or [Me₃O]BF₄. Complexes 1 reacted with M(Hg) (M = Zn, Na) in tetrahydrofuran to give the amido-bridged derivatives [M₂Cp₂(μ-PCy₂)(μ-NH₂)(NO)₂] with retention of stereochemistry, a transformation also induced by using mild O atom scavengers such as CO and phosphites in the presence of water. In the absence of water, phosphites accomplished a deoxygenation of the bridging NO of the Mo₂ complexes to yield the phosphoraniminato-bridged derivatives [Mo₂Cp₂(μ-PCy₂){μ-NP(OR)₃}(NO)₂] (R = Et, Ph), also with retention of stereochemistry.

Non-heme High-Spin {FeNO}6-8 Complexes: One Ligand Platform Can Do It All

Heme and non-heme iron-nitrosyl complexes are important intermediates in biology. While there are numerous examples of low-spin heme iron-nitrosyl complexes in different oxidation states, much less is known about high-spin (hs) non-heme iron-nitrosyls in oxidation states other than the formally ferrous NO adducts ({FeNO}⁷ in the Enemark-Feltham notation). In this study, we present a complete series of hs-{FeNO}^6-8 complexes using the TMG₃tren coligand. Redox transformations from the hs-{FeNO}⁷ complex [Fe(TMG₃tren)(NO)]²⁺ to its {FeNO}⁶ and {FeNO}⁸ analogs do not alter the coordination environment of the iron center, allowing for detailed comparisons between these species. Here, we present new MCD, NRVS, XANES/EXAFS, and Mössbauer data, demonstrating that these redox transformations are metal based, which allows us to access hs-Fe(II)-NO^-, Fe(III)-NO^-, and Fe(IV)-NO^- complexes. Vibrational data, analyzed by NCA, directly quantify changes in Fe-NO bonding along this series. Optical data allow for the identification of a "spectator" charge-transfer transition that, together with Mössbauer and XAS data, directly monitors the electronic changes of the Fe center. Using EXAFS, we are also able to provide structural data for all complexes. The magnetic properties of the complexes are further analyzed (from magnetic Mössbauer). The properties of our hs-{FeNO}^6-8 complexes are then contrasted to corresponding, low-spin iron-nitrosyl complexes where redox transformations are generally NO centered. The hs-{FeNO}⁸ complex can further be protonated by weak acids, and the product of this reaction is characterized. Taken together, these results provide unprecedented insight into the properties of biologically relevant non-heme iron-nitrosyl complexes in three relevant oxidation states.

E-H Bond Activation and Insertion Processes in the Reactions of the Unsaturated Hydride [W2Cp2(μ-H)(μ-PPh2)(NO)2]

The reactions of the title complex (1) with different p-block element (E) molecules was examined. Compound 1 reacted with BH₃·THF at room temperature to give the trihydride [W₂Cp₂(μ-H)H₂(μ-PPh₂)(NO)₂], which formally results from hydrogenation of 1, a reaction that actually does not take place when neat dihydrogen is used. Clean E-H bond oxidative addition, however, took place when 1 was reacted with HSnPh₃, to give the related dihydride stannyl derivative [W₂Cp₂(μ-H)H(μ-PPh₂)(NO)₂(SnPh₃)]. In contrast, the reaction of 1 with HSPh involved H₂ elimination to give the thiolate-bridged complex [W₂Cp₂(μ-SPh)(μ-PPh₂)(NO)₂], while that with (p-tol)C(O)H resulted in insertion of the aldehyde to yield the related alkoxide complex [W₂Cp₂{μ-OCH₂(p-tol)}(μ-PPh₂)(NO)₂]. Insertion also prevailed in the reactions of 1 with CN^tBu, which, however, involved the competitive formation of new C-H or N-H bonds, to give a mixture of formimidoyl and aminocarbyne derivatives, [W₂Cp₂(μ-κ¹:η²-HCN^tBu)(μ-PPh₂)(NO)₂] (W-W = 3.0177(2) Å) and [W₂Cp₂{μ-C(NH^tBu)}(μ-PPh₂)(NO)₂] (W-W = 2.9010(4) Å), respectively, even though the latter was thermodynamically preferred, according to density functional theory calculations. The former represents the first structurally characterized complex displaying a formimidoyl or iminoacyl ligand in the alkenyl-like μ-κ¹:η² coordination mode. The reaction of 1 with diazomethane proceeded with N₂ elimination and C-H coupling to yield the agostic methyl-bridged complex [W₂Cp₂(μ-κ¹:η²-CH₃)(μ-PPh₂)(NO)₂] (calculated W-W = 2.923 Å), whereas the reaction with N₂CH(SiMe₃) proceeded with insertion of the diazoalkane to give the corresponding hydrazonide complex [W₂Cp₂{μ-NH(NCHSiMe₃)}(μ-PPh₂)(NO)₂] (W-W = 2.8608(4) Å). The latter was converted under alkaline conditions to the methyldiazenide derivative [W₂Cp₂{μ-N(NMe)}(μ-PPh₂)(NO)₂] (W-W = 2.8730(2) Å), in a process involving hydrolysis of the C-Si bond coupled with a 1,3-H shift from N to C.

Synthesis, X-ray Structures, Electronic Properties, and O2/NO Reactivities of Thiol Dioxygenase Active-Site Models

Mononuclear non-heme iron complexes that serve as structural and functional mimics of the thiol dioxygenases (TDOs), cysteine dioxygenase (CDO) and cysteamine dioxygenase (ADO), have been prepared and characterized with crystallographic, spectroscopic, kinetic, and computational methods. The high-spin Fe(II) complexes feature the facially coordinating tris(4,5-diphenyl-1-methylimidazol-2-yl)phosphine (^Ph2TIP) ligand that replicates the three histidine (3His) triad of the TDO active sites. Further coordination with bidentate l-cysteine ethyl ester (CysOEt) or cysteamine (CysAm) anions yielded five-coordinate (5C) complexes that resemble the substrate-bound forms of CDO and ADO, respectively. Detailed electronic-structure descriptions of the [Fe(^Ph2TIP)(L_S,N)]BPh₄ complexes, where L_S,N = CysOEt (1) or CysAm (2), were generated through a combination of spectroscopic techniques [electronic absorption, magnetic circular dichroism (MCD)] and density functional theory (DFT). Complexes 1 and 2 decompose in the presence of O₂ to yield the corresponding sulfinic acid (RSO₂H) products, thereby emulating the reactivity of the TDO enzymes and related complexes. Rate constants and activation parameters for the dioxygenation reactions were measured and interpreted with the aid of DFT calculations for O₂-bound intermediates. Treatment of the TDO models with nitric oxide (NO)-a well-established surrogate of O₂-led to a mixture of high-spin and low-spin {FeNO}⁷ species at low temperature (-70 °C), as indicated by electron paramagnetic resonance (EPR) spectroscopy. At room temperature, these Fe/NO adducts convert to a common species with EPR and infrared (IR) features typical of cationic dinitrosyl iron complexes (DNICs). To complement these results, parallel spectroscopic, computational, and O₂/NO reactivity studies were carried out using previously reported TDO models that feature an anionic hydrotris(3-phenyl-5-methyl-pyrazolyl)borate (^Ph,MeTp^-) ligand. Though the O₂ reactivities of the ^Ph2TIP- and ^Ph,MeTp-based complexes are quite similar, the supporting ligand perturbs the energies of Fe 3d-based molecular orbitals and modulates Fe-S bond covalency, suggesting possible rationales for the presence of neutral 3His coordination in CDO and ADO.

Mild N-O Bond Cleavage Reactions of a Pyramidalized Nitrosyl Ligand Bridging a Dimolybdenum Center

Complex [Mo2Cp2(μ-PCy2)(μ-NO)(NO)2] (1) was prepared by reacting [Mo2Cp2(μ-H)(μ-PCy2)(CO)4] with 2 equiv of [NO]BF4 and then treating the resulting product [Mo2Cp2(μ-PCy2)(CO)2(NO)2](BF4) with NaNO2 at 323 K, and it was shown to display a bridging nitrosyl ligand with significant pyramidalization at the N atom, a circumstance related to an unusual behavior concerning degradation of the bridging nitrosyl. Indeed, complex 1 reacts with HBF4·OEt2 to give the nitroxyl-bridged derivative [Mo2Cp2(μ-PCy2)(μ-κ(1):η(2)-HNO)(NO)2](BF4), is reduced by Zn(Hg) in the presence of trace H2O to give the amido complex [Mo2Cp2(μ-PCy2)(μ-NH2)(NO)2], and reacts with excess P(OPh)3 to give the phosphoraniminato-bridged derivative [Mo2Cp2(μ-PCy2){μ-NP(OPh)3}(NO)2].