Tuning Electron-Transfer Reactivity of a Chromium(III)-Superoxo Complex Enabled by Calcium Ion and Other Redox-Inactive Metal Ions.

Calcium ion plays an indispensable role for water oxidation by oxygen-evolving complex (OEC) composed of a manganese-oxo cluster (Mn4CaO5) in Photosystem II. In this context, the effects of Ca2+ ion and other redox-inactive metal ions on the redox reactivity of high-valent metal-oxo and metal-peroxo complexes have been studied extensively. Among metal-oxygen intermediates involved in interconversion between H2O and O2, however, the effects of Ca2+ ion and other redox-inactive metal ions (Mn+) on the redox reactivity of metal-superoxo complexes have yet to be reported. Herein, we report that electron transfer (ET) from octamethylferrocene (Me8Fc) to a mononuclear nonheme Cr(III)-superoxo complex, [(Cl)(TMC)CrIII(O2)]+ (1), occurs in the presence of redox-inactive metal ions (Mn+ = Ca2+, Mg2+, Y3+, Al3+, and Sc3+); in the absence of the redox-inactive metal ions, ET from Me8Fc to 1 does not occur. The second-order rate constants (ket) of ET from Me8Fc to 1 in the presence of a redox-inactive metal ion increased with increasing concentration of Mn+ ([Mn+]), exhibiting a second-order dependence on [Mn+]: ket = kMCET[Mn+]2, where kMCET is the fourth-order rate constant of metal ion-coupled electron transfer (MCET). This means that two Mn+ ions are bound to the one-electron reduced species of 1. Such a binding of two Mn+ ions associated with the ET reduction of 1 resulted in a 92 mV positive shift of the one-electron reduction potential of 1 (Ered) with increasing log([Mn+]). The log kMCET values increased linearly with the increasing Lewis acidity of Mn+ (ΔE), which was determined from the g values of O2•--Mn+ complexes. The driving force dependence of log ket of MCET from ferrocene derivatives to 1 in the presence of Mn+ has been well-evaluated in light of the Marcus theory of electron transfer.

Remarkable Acid Catalysis in Proton-Coupled Electron-Transfer Reactions of a Chromium(III)-Superoxo Complex.

Much enhanced acid catalysis was observed in oxygen atom transfer (OAT) reactions by a mononuclear nonheme Cr(III)-superoxo complex, [(Cl)(TMC)CrIII(O2)]+ (1), in the presence of triflic acid. In the acid-catalyzed reactions, the reactivity of 1 in OAT of thioanisole was enhanced significantly, showing more than 104-fold acceleration in rate. Electron transfer (ET) from electron donors to 1 also occurred only in the presence of HOTf. The enhanced reactivity of 1 by HOTf was explained by proton-coupled electron transfer from electron donors, such as ferrocene, to 1 in light of the Marcus theory of ET. The present study reports for the first time the dramatic proton effect on the chemical properties of metal-superoxo species.

A Chromium(III)-Superoxo Complex as a Three-Electron Oxidant with a Large Tunneling Effect in Multi-Electron Oxidation of NADH Analogues.

Metal-superoxo species are involved in a variety of enzymatic oxidation reactions, and multi-electron oxidation of substrates is frequently observed in those enzymatic reactions. A CrIII -superoxo complex, [CrIII (O2 )(TMC)(Cl)]+ (1; TMC=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), is described that acts as a novel three-electron oxidant in the oxidation of dihydronicotinamide adenine dinucleotide (NADH) analogues. In the reactions of 1 with NADH analogues, a CrIV -oxo complex, [CrIV (O)(TMC)(Cl)]+ (2), is formed by a heterolytic O-O bond cleavage of a putative CrII -hydroperoxo complex, [CrII (OOH)(TMC)(Cl)], which is generated by hydride transfer from NADH analogues to 1. The comparison of the reactivity of NADH analogues with 1 and p-chloranil (Cl4 Q) indicates that oxidation of NADH analogues by 1 proceeds by proton-coupled electron transfer with a very large tunneling effect (for example, with a kinetic isotope effect of 470 at 233 K), followed by rapid electron transfer.

A high-spin alkylperoxo-iron(iii) complex with cis-anionic ligands: implications for the superoxide reductase mechanism.

The N3O macrocycle of the 12-TMCO ligand stabilizes a high spin (S = 5/2) [FeIII(12-TMCO)(OOtBu)Cl]+ (3-Cl) species in the reaction of [FeII(12-TMCO)(OTf)2] (1-(OTf)2) with tert-butylhydroperoxide (tBuOOH) in the presence of tetraethylammonium chloride (NEt4Cl) in acetonitrile at -20 °C. In the absence of NEt4Cl the oxo-iron(iv) complex 2 [FeIV(12-TMCO)(O)(CH3CN)]2+ is formed, which can be further converted to 3-Cl by adding NEt4Cl and tBuOOH. The role of the cis-chloride ligand in the stabilization of the FeIII-OOtBu moiety can be extended to other anions including the thiolate ligand relevant to the enzyme superoxide reductase (SOR). The present study underlines the importance of subtle electronic changes and secondary interactions in the stability of the biologically relevant metal-dioxygen intermediates. It also provides some rationale for the dramatically different outcomes of the chemistry of iron(iii)peroxy intermediates formed in the catalytic cycles of SOR (Fe-O cleavage) and cytochrome P450 (O-O bond lysis) in similar N4S coordination environments.

Acid-induced nitrite reduction of nonheme iron(ii)-nitrite: mimicking biological Fe-NiR reactions.

Nitrite reductase (NiR) catalyzes nitrite (NO2 -) to nitric oxide (NO) transformation in the presence of an acid (H+ ions/pH) and serves as a critical step in NO biosynthesis. In addition to the NiR enzyme, NO synthases (NOSs) participate in NO production. The chemistry involved in the catalytic reduction of NO2 -, in the presence of H+, generates NO with a H2O molecule utilizing two H+ + one electron from cytochromes and is believed to be affected by the pH. Here, to understand the effect of H+ ions on NO2 - reduction, we report the acid-induced NO2 - reduction chemistry of a nonheme FeII-nitrito complex, [(12TMC)FeII(NO2 -)]+ (FeII-NO2 -, 2), with variable amounts of H+. FeII-NO2 - upon reaction with one-equiv. of acid (H+) generates [(12TMC)Fe(NO)]2+, {FeNO}7 (3) with H2O2 rather than H2O. However, the amount of H2O2 decreases with increasing equivalents of H+ and entirely disappears when H+ reaches ≅ two-equiv. and shows H2O formation. Furthermore, we have spectroscopically characterized and followed the formation of H2O2 (H+ = one-equiv.) and H2O (H+ ≅ two-equiv.) and explained why bio-driven NiR reactions end with NO and H2O. Mechanistic investigations, using 15N-labeled-15NO2 - and 2H-labeled-CF3SO3D (D+ source), revealed that the N atom in the {Fe14/15NO}7 is derived from the NO2 - ligand and the H atom in H2O or H2O2 is derived from the H+ source, respectively.

Why intermolecular nitric oxide (NO) transfer? Exploring the factors and mechanistic aspects of NO transfer reaction.

Small molecule activation and their transfer reactions in biological or catalytic reactions are greatly influenced by the metal-centers and the ligand frameworks. Here, we report the metal-directed nitric oxide (NO) transfer chemistry in low-spin mononuclear {Co(NO)}8, [(12-TMC)CoIII(NO-)]2+ (1-CoNO, S = 0), and {Cr(NO)}5, ([(BPMEN)Cr(NO)(Cl)]+) (4-CrNO, S = 1/2) complexes. 1-CoNO transfers its bound NO moiety to a high-spin [(BPMEN)CrII(Cl2)] (2-Cr, S = 2) and generates 4-CrNOvia an associative pathway; however, we did not observe the reverse reaction, i.e., NO transfer from 4-CrNO to low-spin [(12-TMC)CoII]2+ (3-Co, S = 1/2). Spectral titration for NO transfer reaction between 1-CoNO and 2-Cr confirmed 1 : 1 reaction stoichiometry. The NO transfer rate was found to be independent of 2-Cr, suggesting the presence of an intermediate species, which was further supported experimentally and theoretically. The experimental and theoretical observations support the formation of µ-NO bridged intermediate species ({Cr-NO-Co}4+). Mechanistic investigations using 15N-labeled-15NO and tracking the 15N-atom established that the NO moiety in 4-CrNO is derived from 1-CoNO. Further, to investigate the factors deciding the NO transfer reactivity, we explored the NO transfer reaction between another high-spin CrII-complex, [(12-TMC)CrII(Cl)]+ (5-Cr, S = 2), and 1-CoNO, showing the generation of the low-spin [(12-TMC)Cr(NO)(Cl)]+ (6-CrNO, S = 1/2); however, again there was no opposite reaction, i.e., from Cr-center to Co-center. The above results advocate clearly that the NO transfer from Co-center generates thermally stable and low-spin and inert {Cr(NO)}5 complexes (4-CrNO & 6-CrNO) from high-spin and labile Cr-complexes (2-Cr & 5-Cr), suggesting a metal-directed NO transfer (cobalt to chromium, not chromium to cobalt). These results explicitly highlight that the NO transfer is strongly influenced by the labile/inert behavior of the metal-centers and/or thermal stability rather than the ligand architecture.

Acid-promoted hydride transfer from an NADH analogue to a Cr(iii)-superoxo complex via a proton-coupled hydrogen atom transfer.

The sequential transfer of an electron, a proton and an electron in a hydride transfer from dihydronicotinamide adenine dinucleotide (NADH) and its analogues has never been separated well. In addition, the effect of acids on hydride transfer from an NADH analogue to a metal-superoxo species has yet to be reported. We report herein the first example of an acid-promoted hydride transfer from an NADH analogue, 10-methyl-9,10-dihydroacridine (AcrH2), to a Cr(iii)-superoxo complex, [(TMC)CrIII(O2)]2+, in the presence of HOTf in MeCN at 233 K. The acid-promoted hydride transfer from AcrH2 to [(TMC)CrIII(O2)]2+ occurs via a proton-coupled hydrogen atom transfer from AcrH2 to [(TMC)CrIII(O2)]2+ to produce a radical cation (AcrH2˙+) with an inverse deuterium isotope effect (KIE) of 0.93(5). AcrH2˙+ decayed via a proton transfer from AcrH2˙+ to AcrH2 with a KIE of 2.0(1), followed by the reaction of 10-methylacridinyl radical (AcrH˙) with [(TMC)CrIII(H2O2)]3+ to produce a 10-methylacridinium ion (AcrH+) and [(TMC)CrIII]3+. This work provides valuable insights into the mechanism of hydride transfer of NADH analogues by metal-superoxo intermediates, such as the switchover of the reaction mechanism from a one-step to a separated multi-step pathway in the presence of an acid.

Oxygen atom transfer promoted nitrate to nitric oxide transformation: a step-wise reduction of nitrate → nitrite → nitric oxide.

Nitrate reductases (NRs) are molybdoenzymes that reduce nitrate (NO3 -) to nitrite (NO2 -) in both mammals and plants. In mammals, the salival microbes take part in the generation of the NO2 - from NO3 -, which further produces nitric oxide (NO) either in acid-induced NO2 - reduction or in the presence of nitrite reductases (NiRs). Here, we report a new approach of VCl3 (V3+ ion source) induced step-wise reduction of NO3 - in a CoII-nitrato complex, [(12-TMC)CoII(NO3 -)]+ (2,{CoII-NO3 -}), to a CoIII-nitrosyl complex, [(12-TMC)CoIII(NO)]2+ (4,{CoNO}8), bearing an N-tetramethylated cyclam (TMC) ligand. The VCl3 inspired reduction of NO3 - to NO is believed to occur in two consecutive oxygen atom transfer (OAT) reactions, i.e., OAT-1 = NO3 - → NO2 - (r1) and OAT-2 = NO2 - → NO (r2). In these OAT reactions, VCl3 functions as an O-atom abstracting species, and the reaction of 2 with VCl3 produces a CoIII-nitrosyl ({CoNO}8) with VV-Oxo ({VV[double bond, length as m-dash]O}3+) species, via a proposed CoII-nitrito (3, {CoII-NO2 -}) intermediate species. Further, in a separate experiment, we explored the reaction of isolated complex 3 with VCl3, which showed the generation of 4 with VV-Oxo, validating our proposed reaction sequences of OAT reactions. We ensured and characterized 3 using VCl3 as a limiting reagent, as the second-order rate constant of OAT-2 (k 2 /) is found to be ∼1420 times faster than that of the OAT-1 (k 2) reaction. Binding constant (K b) calculations also support our proposition of NO3 - to NO transformation in two successive OAT reactions, as K b(CoII-NO2 -) is higher than K b(CoII-NO3 -), hence the reaction moves in the forward direction (OAT-1). However, K b(CoII-NO2 -) is comparable to K b{CoNO}8 , and therefore sequenced the second OAT reaction (OAT-2). Mechanistic investigations of these reactions using 15N-labeled-15NO3 - and 15NO2 - revealed that the N-atom in the {CoNO}8 is derived from NO3 - ligand. This work highlights the first-ever report of VCl3 induced step-wise NO3 - reduction (NRs activity) followed by the OAT induced NO2 - reduction and then the generation of Co-nitrosyl species {CoNO}8.

Aromatic hydroxylation of anthracene derivatives by a chromium(iii)-superoxo complex via proton-coupled electron transfer.

The chemistry of metal-superoxo intermediates started being unveiled in oxidation reactions by enzymes and their synthetic model compounds. However, aromatic hydroxylation reactions by the metal-superoxo species are yet to be demonstrated. In this study, we report for the first time that the hydroxylation of aromatic compounds such as anthracene and its derivatives by a mononuclear nonheme Cr(iii)-superoxo complex, [(Cl)(TMC)CrIII(O2)]+ (1), occurs in the presence of triflic acid (HOTf) via the rate-determining proton-coupled electron transfer (PCET) from anthracene to 1, followed by a fast further oxidation to give anthraquinone. The rate constants of electron transfer from anthracene derivatives to 1 in the presence of HOTf are well analyzed in light of the Marcus theory of electron transfer.