Spectroelectrochemical Evidence for the Nitrosyl Redox Siblings NO + , NO . , and NO − Coordinated to a Strongly Electron‐Accepting Fe II Porphyrin: DFT Calculations Suggest the Presence of High‐Spin States after Reduction of the Fe II –NO − Complex

Interplay and Competition Between Two Different Types of Redox-Active Ligands in Cobalt Complexes: How to Allocate the Electrons?

The field of molecular transition metal complexes with redox-active ligands is dominated by compounds with one or two units of the same redox-active ligand; complexes in which different redox-active ligands are bound to the same metal are uncommon. This work reports the first molecular coordination compounds in which redox-active bisguanidine or urea azine (biguanidine) ligands as well as oxolene ligands are bound to the same cobalt atom. The combination of two different redox-active ligands leads to mono- as well as unprecedented dinuclear cobalt complexes, being multiple (four or six) center redox systems with intriguing electronic structures, all exhibiting radical ligands. By changing the redox potential of the ligands through derivatisation, the electronic structure of the complexes could be altered in a rational way.

Azanone (HNO): generation, stabilization and detection

HNO (nitroxyl, azanone), joined the 'biologically relevant reactive nitrogen species' family in the 2000s. Azanone is impossible to store due to its high reactivity and inherent low stability. Consequently, its chemistry and effects are studied using donor compounds, which release this molecule in solution and in the gas phase upon stimulation. Researchers have also tried to stabilize this elusive species and its conjugate base by coordination to metal centers using several ligands, like metalloporphyrins and pincer ligands. Given HNO's high reactivity and short lifetime, several different strategies have been proposed for its detection in chemical and biological systems, such as colorimetric methods, EPR, HPLC, mass spectrometry, fluorescent probes, and electrochemical analysis. These approaches are described and critically compared. Finally, in the last ten years, several advances regarding the possibility of endogenous HNO generation were made; some of them are also revised in the present work.

Synthesis, electrochemical and spectroelectrochemical characterization of iron(III) tetraarylporphyrins containing four β,β′-butano and β,β′-benzo fused rings

Six iron(III) tetraarylporphyrins containing four [Formula: see text]-butano or [Formula: see text]-benzo fused rings were synthesized and characterized by electrochemistry and spectroelectrochemistry in nonaqueous media. The examined compounds are represented as butano(TpYPP)FeCl and benzo(TpYPP)FeCl, where TpYPP is a dianion of the meso-substituted porphyrin, Y is a CH[Formula: see text], H or Cl substituent on the para-position of the four meso-phenyl rings and butano and benzo are the [Formula: see text]-substituents on each of the four pyrrole rings of the compound. Up to three reductions are observed for each Fe(III) butano- and benzoporphyrin in CH[Formula: see text]Cl[Formula: see text] or pyridine containing 0.1 M TBAP, the first of which is assigned in each case to a metal-centered electron transfer. The second reduction is also metal-centered in CH[Formula: see text]Cl[Formula: see text] and leads to formation of an Fe(I) porphyrin, but it is porphyrin ring-centered and gives an Fe(II) porphyrin [Formula: see text]-anion radical reduction product when pyridine is used as the solvent. The effects of the solvent and type of fused ring system (butano or benzo) on the UV-vis spectra and electrochemical properties of the Fe(III) porphyrins are discussed and comparisons are made to both the structurally related non-[Formula: see text]-substituted iron porphyrins and earlier described butano- or benzotetraarylporphyrins containing Cu(II) or Co(II) central metal ions.

Lewis Acid Activation of the Ferrous Heme-NO Fragment toward the N-N Coupling Reaction with NO To Generate N2O

Bacterial NO reductase (bacNOR) enzymes utilize a heme/non-heme active site to couple two NO molecules to N₂O. We show that BF₃ coordination to the nitrosyl O-atom in (OEP)Fe(NO) activates it toward N-N bond formation with NO to generate N₂O. ¹⁵N-isotopic labeling reveals a reversible nitrosyl exchange reaction and follow-up N-O bond cleavage in the N₂O formation step. Other Lewis acids (B(C₆F₅)₃ and K⁺) also promote the NO coupling reaction with (OEP)Fe(NO). These results, complemented by DFT calculations, provide experimental support for the cis: b₃ pathway in bacNOR.

Selective octabromination of tetraarylporphyrins based on meso-substituent identity: Structural and electrochemical studies

Synthesis and isolation of selectively brominated tetraarylporphyrin derivatives is reported. Treatment with bromine of meso-5,10,15,20-tetrakis(3,4,5-trimethoxyphenyl)porphyrin (1) or meso-5,10,15,20-tetrakis(3,5-di-[Formula: see text]-butyl-4-hydroxyphenyl)porphinatocopper(II) (2-Cu) yields products octabrominated at the 2,6-positions of meso-aryl substituents [5,10,15,20-tetrakis(2,6-dibromo-3,4,5-trimethoxyphenyl)porphyrin, [Formula: see text]Br81] or macrocyclic [Formula: see text]-positions. The latter of these ([Formula: see text]-brominated) was identified as the oxoporphyrinogen 2,3,7,8,12,13,17,18-octabromo-5,10,15,20- tetrakis(3,5-di-[Formula: see text]-butyl-4-oxo-cyclohexa-2,5-dienylidene)porphyrinogen 3 obtained due to the adventitious oxidation and demetalation of 2-Cu, which could be alkylated at its macrocyclic nitrogen atoms yielding N[Formula: see text],N[Formula: see text],N[Formula: see text],N[Formula: see text]-tetrakis(4-bromobenzyl)-2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(3,5-di-[Formula: see text]-butyl-4-oxo-cyclo hexa-2,5-dienylidene)porphyrinogen 4. The former compound [Formula: see text]Br81 was complexed with Zn(II) ([Formula: see text]Br81-Zn) or Cu(II) ([Formula: see text]Br81-Cu) and could also be subjected to further bromination yielding 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(2,6-dibromo-3,4,5-trimethoxyphenyl)porphyrin, Br[Formula: see text]1. The effect on the electrochemical properties of the 2,6-bromophenyl-substituted porphyrin compounds over the more highly brominated products was assessed.

Reactions of HNO with metal porphyrins: underscoring the biological relevance of HNO

Azanone ((1)HNO, nitroxyl) shows interesting yet poorly understood chemical and biological effects. HNO has some overlapping properties with nitric oxide (NO), sharing its biological reactivity toward heme proteins, thiols, and oxygen. Despite this similarity, HNO and NO show significantly different pharmacological effects. The high reactivity of HNO means that studies must rely on the use of donor molecules such as trioxodinitrate (Angeli's salt). It has been suggested that azanone could be an intermediate in several reactions and that it may be an enzymatically produced signaling molecule. The inherent difficulty in detecting its presence unequivocally prevents evidence from yielding definite answers. On the other hand, metalloporphyrins are widely used as chemical models of heme proteins, providing us with invaluable tools for the study of the coordination chemistry of small molecules, like NO, CO, and O2. Studies with transition metal porphyrins have shown diverse mechanistic, kinetic, structural, and reactive aspects related to the formation of nitrosyl complexes. Porphyrins are also widely used in technical applications, especially when coupled to a surface, where they can be used as electrochemical gas sensors. Given their versatility, they have not escaped their role as key players in chemical studies involving HNO. This Account presents the research performed during the last 10 years in our group concerning azanone reactions with iron, manganese, and cobalt porphyrins. We begin by describing their HNO trapping capabilities, which result in formation of the corresponding nitrosyl complexes. Kinetic and mechanistic studies of these reactions show two alternative operating mechanisms: reaction of the metal center with HNO or with the donor. Moreover, we have also shown that azanone can be stabilized by coordination to iron porphyrins using electron-attracting substituents attached to the porphyrin ring, which balance the negatively charged NO¯. Second, we describe an electrochemical HNO sensing device based on the covalent attachment of a cobalt porphyrin to gold. A surface effect affects the redox potentials and allows discrimination between HNO and NO. The reaction with the former is fast, efficient, and selective, lacking spurious signals due to the presence of reactive nitrogen and oxygen species. The sensor is both biologically compatible and highly sensitive (nanomolar). This time-resolved detection allows kinetic analysis of reactions producing HNO. The sensor thus offers excellent opportunities to be used in experiments looking for HNO. As examples, we present studies concerning (a) HNO donation capabilities of new HNO donors as assessed by the sensor, (b) HNO detection as an intermediate in O atom abstraction to nitrite by phosphines, and (c) NO to HNO interconversion mediated by alcohols and thiols. Finally, we briefly discuss the key experiments required to demonstrate endogenous HNO formation to be done in the near future, involving the in vivo use of the HNO sensing device.

Synthesis, molecular structure, and spectroelectrochemistry of a nitrosyl iron porphyrin containing an unsymmetrical xanthene-linked porphyrin core

Synthetic nitrosyl porphyrins with meso-aryl substituents are potential models for the biologically-important NO-bound P460 heme cofactor. A five-coordinate iron nitrosyl tetraaryl-porphyrin (HTPPX-CO2H)Fe(NO) containing a xanthene-based meso substituent has been prepared. The crystal structure of this formally {FeNO}7 complex reveals an ordered axial and bent NO ligand (∠FeNO=142.5(6)Å) displaying an off-axis tilt of the nitrosyl N atom from the heme normal by 9.2°. Surprisingly, the porphyrin core does not display the expected asymmetry in FeN(por) distances frequently observed in iron nitrosyl porphyrins. The redox behavior as determined by cyclic voltammetry reveals, in contrast to most (por)Fe(NO) compounds, a fast NO dissociation after electrooxidation in CH2Cl2 to result in a net chemically-irreversible oxidation at Epa=+0.77V vs Ag/AgCl. IR spectroelectrochemistry reveals a recombination, on the spectroelectrochemistry time-scale, of the dissociated NO on oxidation with electrogenerated [(HTPPX-CO2H)Fe]+.

Nitric oxide as a non-innocent ligand in (bio-)inorganic complexes: spin and electron transfer in Fe(II)-NO bond

The nature of electron density transfer upon bond formation between NO ligand and Fe(II) center is analyzed on the basis of DFT calculation for two {Fe-NO}(7) complexes with entirely diverse geometric and electronic structures: Fe(II)P(NH3)NO (with bent Fe-N-O unit) and [Fe(II)(H2O)5(NO)](2+) (with linear Fe-N-O structure). Proper identification of an electronic status of the fragments, "prepared" to make a bond, was found necessary to get meaningful resolution of charge and spin transfer processes from a spin-resolved analysis of natural orbitals for chemical valence. The Fe(II)P(NH3)NO adduct (built of NO(0) (S=1/2) and Fe(II)P(NH3) (S=0) fragments) showed a strong π*-backdonation competing with spin transfer via a σ-donation, yielding significant red-shift of the NO stretching frequency. [Fe(II)(H2O)5(NO)](2+) (built of NO(0) (S=1/2) antiferromagnetically coupled to Fe(II)(H2O)5 (S=2) fragment) gave no noticeable charge or spin transfer between fragments; a slight blue-shift of the NO stretching frequency could be related to a residual π-donation due to weak π-bonding.

Spectroelectrochemical investigation of Bu4N[Fe(CO)3(NO)]: identification of a reversible EC-mechanism

Bu4N[Fe(CO)3(NO)] displays unique catalytic properties in electron-transfer catalysis such as in allylic substitutions, hydrosilylation, transesterifications, or carbene transfer chemistry. Herein we present a detailed spectroelectrochemical investigation of this complex that unravels an interesting electrochemical-chemical transformation in which two parts of [Fe(CO)3(NO)](-) are oxidized and undergo a disproportionation in the presence of CO to [Fe(CO)5] and [Fe(CO)2(NO)2]. Upon re-reduction the former two complexes regenerate [Fe(CO)3(NO)](-) to about 85%.

β-Nitro-substituted free-base, iron(III) and manganese(III) tetraarylporphyrins: synthesis, electrochemistry and effect of the NO2 substituent on spectra and redox potentials in non-aqueous media

Two free-base and four metal derivatives of substituted tetraarylporphyrins containing a nitro-substituent on the β-pyrrole position of the macrocycle were synthesized and characterized by UV-vis, FTIR, 1 H NMR and mass spectrometry as well as electrochemistry and spectroelectrochemistry in non-aqueous media. The porphyrins are represented as ( NO 2 TmPP ) M and ( NO 2 TdmPP ) M , where M = 2 H , Fe III Cl or Mn III Cl , m is a CH 3 group on the para-position of the four meso-phenyl rings of the tetraphenylporphyrin (TPP) and dm represents two OCH 3 substituents on the meta-positions of each phenyl ring of the TPP macrocycle. UV-visible spectra of the nitro-substituted porphyrins exhibit absorption bands which are red-shifted by 4–11 nm as compared to bands of the same substituted tetraarylporphyrins lacking a nitro substituent. Three or four reductions are observed for each iron and manganese nitroporphyrin, the first of which is metal-centered, leading to formation of an Fe ( II ) or Mn ( II ) complex. Further reduction at the metal center occurs for the iron porphyrins but this reaction proceeds via an Fe ( II ) π anion radical in the case of the two nitro-substituented derivatives. The β-nitro-substituted porphyrins are easier to reduce and harder to oxidize than the corresponding compounds lacking a nitro group. The effect of NO 2 substituent on reduction/oxidation potentials and the site of electron transfer was also discussed.

Detailed mechanism of the autoxidation of N-hydroxyurea catalyzed by a superoxide dismutase mimic Mn(III) porphyrin: formation of the nitrosylated Mn(II) porphyrin as an intermediate

The in vitro autoxidation of N-hydroxyurea (HU) is catalyzed by Mn(III)TTEG-2-PyP(5+), a synthetic water soluble Mn(III) porphyrin which is also a potent mimic of the enzyme superoxide dismutase. The detailed mechanism of the reaction is deduced from kinetic studies under basic conditions mostly based on data measured at pH = 11.7 but also including some pH-dependent observations in the pH range 9-13. The major intermediates were identified by UV-vis spectroscopy and electrospray ionization mass spectrometry. The reaction starts with a fast axial coordination of HU to the metal center of Mn(III)TTEG-2-PyP(5+), which is followed by a ligand-to-metal electron transfer to get Mn(II)TTEG-2-PyP(4+) and the free radical derived from HU (HU˙). Nitric oxide (NO) and nitroxyl (HNO) are minor intermediates. The major pathway for the formation of the most significant intermediate, the {MnNO} complex of Mn(II)TTEG-2-PyP(4+), is the reaction of Mn(II)TTEG-2-PyP(4+) with NO. We have confirmed that the autoxidation of the intermediates opens alternative reaction channels, and the process finally yields NO(2)(-) and the initial Mn(III)TTEG-2-PyP(5+). The photochemical release of NO from the {MnNO} intermediate was also studied. Kinetic simulations were performed to validate the deduced rate constants. The investigated reaction has medical implications: the accelerated production of NO and HNO from HU may be utilized for therapeutic purposes.

Manifestations of noninnocent ligand behavior

The potential of redox-active ligands to behave "noninnocently" in transition-metal coordination compounds is reflected with respect to various aspects and situations. These include the question of establishing "correct" oxidation states, the identification and characterization of differently charged radical ligands, the listing of structural and other consequences of ligand redox reactions, and the distinction between barrierless delocalized "resonance" cases M(n)/L(n) ↔ M(n+1)L(n-1) versus separated valence tautomer equilibrium situations M(n)/L(n) ⇌ M(n+1)L(n-1). Further ambivalence arises for dinuclear systems with radical bridge M(n)(μ-L(•))M(n) versus mixed-valent alternatives M(n+1)(μ-L(-))M(n), for noninnocent ligand-bridged coordination compounds of higher nuclearity such as (μ(3)-L)M(3), (μ(4)-L)M(4), (μ-L)(4)M(4), or coordination polymers. Conversely, the presence of more than one noninnocently behaving ligand at a single transition-metal site in situations such as L(n)-M-L(n-1) or L(•)-M-L(•) may give rise to corresponding ligand-to-ligand interaction phenomena (charge transfer, electron hopping, and spin-spin coupling) and to redox-induced electron transfer with counterintuitive oxidation-state changes. The relationships of noninnocent ligand behavior with excited-state descriptions and perspectives regarding material properties and single-electron or multielectron reactivity are also illustrated briefly.