Synthesis, Characterization and Reactivity Control of Ni-Oxygen Adducts with Organic Substrates

The reactivity of mononuclear metal-O2 adducts, such as metal-superoxo and -peroxo species, has long fascinated researchers in many areas due to the significance of diverse biological and catalytic processes. To understand how the nature of the ligand influences reactivity patterns of the metal-O2 complexes, recently, a systematic study of the relationship between reactivity and ring size of ligand was undertaken for a series of metal-O2 complexes bearing N-tetramethylated macrocyclic chelates in biomimetic chemistry. In this study, the two ligands, CHDAP and Me3-TPADP, were designed and reactivity of Ni-O2 species bearing each ligand was investigated in part I and part II, respectively. For comparison of reactivity according to a steric effect, a set of nickel(III)-peroxo complexes bearing tetraazamacrocyclic ligands, [NiIII(CHDAP)(O2)]+ and [NiIII(TBDAP)(O2)]+, were prepared and fully characterized by various physicochemical methods. The different steric properties of the supporting ligands were confirmed by X-ray crystallography where the CHDPA ligand gives enough space around the Ni-O2 core compared to the TBDAP ligand. In the aldehyde deformylation reaction, the nucleophilic reactivity of the nicke(III)-peroxo complexes was highly dependent on the steric properties of the macrocyclic ligands, with the reactivity order of [NiIII(TBDAP)(O2)]+ < [NiIII(CHDAP)(O2)]+. This result provides fundamental insight into the mechanism of the structure (steric) – reactivity relationship of metal-peroxo intermediates. In part II, the Me3-TPADP ligand was synthesized, and the starting complex, [NiII(Me3-TPADP)(CH3CN)2]2+ (3), and Ni-O2 intermediate, [NiIII(Me3-TPADP)(O2)]+ (4), were prepared and successfully characterized by various methods. Also, the kinetic result of 4 was obtained with external organic substrates. ⓒ 2015 DGISTPart I. A Steric Effect on the Nucleophilic Reactivity of Nickel(III)-O2 Complex 1-- I. Introduction 2-- II. Experimental Section 7-- II-1. Materials and Instrumentation 7-- II-2. Synthesis of Pyridinophan Type Ligands 8-- II-2-a. Pyridine-2,6-dicarbaldehyde (L1) 8-- II-2-b. N,N’-(pyridine-2,6-diylbis(methylene))dicyclohexylamine (L2) 9-- II-2-c. 2,6-bis(chloromethyl)pyridine (L3) 9-- II-2-d. N,N’-di-cyclohexyl-2,11-diaza[3,3](2,6)pyridinophane (CHDAP) 9-- II-3. Generation of Ni Complexes 10-- II-3-a. [Ni(CHDAP)(NO3)]+ (1) 10-- II-3-b. [Ni(CHDAP)(O2)]+ (2) 10-- II-4. X-ray Crystallography 11-- II-5. Reactivity Studies 11-- III. Results and Discussion 13-- III-1. Synthesis and Characterization of CHDAP 13-- III-2. Preparation and Characterization of [NiII(CHDAP)(NO3)]+ (1) 15-- III-3. Characterization and Reactivity Studies of [NiIII(CHDAP)(O2)]+ (2) 19-- III-4. Comparison with Ni Complex bearing TBDAP Ligand 26-- IV. Conclusion 29-- V. References 30-- Part II. Synthesis, Characterization and Reactivity of a Mononuclear Nickel(III)-O2 Complex with Macrocyclic Ligand, Me3-TPADP 36-- I. Introduction 37-- II. Experimental Section 40-- II-1. Materials and Instrumentation 40-- II-2. Synthesis of Ligands 41-- II-2-a. 1,4,7-tris(p-tosylsulfonyl)-1,4,7-triazaheptane (L4) 41-- II-2-b. 3,6,9-tris(p-tosylsulfonyl)-3,6,9,15-tetraazbicyclo[9,3,1]pentadeca-1(15),11,13-triene (L5) 42-- II-2-c. 3,6,9,15-tetraazabocyclo(9,3,1)pentadeca-1(15),11,13-triane (L6) 42-- II-2-d. 3,6,9-trimethyl-3,6,9-triaza-1(2,6)-pyridinacyclodecaphane (Me3-TPADP) 42-- II-3. Generation of Ni Complexes 43-- II-3-a. [Ni(Me3-TPADP)(CH3CN)2] 2+ (3) 43-- II-3-b. [Ni(Me3-TPADP)(O2)]+ (4) 43-- II-4. X-ray Crystallography 44-- II-5. Reactivity Studies 44-- III. Results and Discussion 45-- III-1. Synthesis and Characterization of Me3-TPADP 45-- III-2. Preparation and Characterization of [NiII(Me3-TPADP)(CH3CN)2]2+ (3) 47-- III-3. Characterization and Reactivity Studies of [NiIII(Me3-TPADP)(O2)]+ (4) 51-- IV. Conclusion 56-- V. References 57생체 내 촉매 반응에 있어서 단일 금속-산소 종은 산화 환원 과정의 중요한 중간체 역할을 하기에 많이 연구되어 왔다. 리간드가 금속-산소 복합체의 반응성 패턴에 미치는 영향을 이해하기 위해, 최근, N-tetramethylated 거대 고리 킬레이트가 부착된 몇몇 금속-산소 복합체에서 N-리간드의 고리 크기와 반응성 사이의 관계에 대한 체계적인 공부가 진행되었다. 이번 연구에서, CHDAP 그리고 Me3-TPADP 리간드가 디자인되었고 각각의 리간드가 부착된 니켈-산소 종의 반응성은 part I 그리고 part II에서 각각 조사되었다. 구조적 영향에 따른 반응성의 비교를 위해 형성된 니켈(III)-peroxo 복합체, [NiIII(CHDAP)(O2)]+ (2) 그리고[NiIII(TBDAP)(O2)]+, 의 알데하이드 디포밀레이션 반응을 살펴 본 결과, 2 의 친핵성 반응은 거대고리 리간드의 입체적 성질에 크게 의존하며, 반응성은 [NiIII(TBDAP)(O2)]+< [NiIII(CHDAP)(O2)]+ 임을 알 수 있었다. 이 결과는 구조(입체) 의 방법을 통한 근본적인 통찰력을 준다-금속-peroxo 중간체의 반응성 관계. Part II에서는, Me3-TPADP 리간드를 합성하였고 [NiII(Me3-TPADP)(CH3CN)2]2+ (3) 와 [NiIII(Me3-TPADP)(O2)]+ (4)를 준비 및 특징화하였다. 또한, 4의 반응성 결과는 외부 기질을 사용하여 얻을 수 있었다. ⓒ 2015 DGISTMasterdCollectio

Oxygen Activation by Mononuclear Mn, Co, and Ni Centers in Biology and Synthetic Complexes

The active sites of metalloenzymes that catalyze O2-dependent reactions generally contain iron or copper ions. However, several enzymes are capable of activating O2 at manganese or nickel centers instead, and a handful of dioxygenases exhibit activity when substituted with cobalt. This minireview summarizes the catalytic properties of oxygenases and oxidases with mononuclear Mn, Co, or Ni active sites, including oxalate-degrading oxidases, catechol dioxygenases, and quercetin dioxygenase. In addition, recent developments in the O2 reactivity of synthetic Mn, Co, or Ni complexes are described, with an emphasis on the nature of reactive intermediates featuring superoxo-, peroxo-, or oxo-ligands. Collectively, the biochemical and synthetic studies discussed herein reveal the possibilities and limitations of O2 activation at these three “overlooked” metals

GENERATION, CHARACTERIZATION AND REACTIVITY OF COBALT DIAMOND CORE AND COBALT PEROXO COMPLEXES

The development of efficient and low-cost technologies that convert hydrocarbons, including methane, to liquid fuels through controlled functionalization of its inert C–H bond is a fundamental challenge. Also, the activation of carbon−hydrogen (C−H) bonds is the first step of functionalizing inert hydrocarbons. This transformation is a key step in many biological and synthetic processes. One representative example inspired by nature is the metalloenzyme called soluble methane monooxygenase (sMMO), a nonheme dinuclear iron-dependent enzyme that catalyzes the hydroxylation of the strong C–H bond of methane (bond dissociation energy BDE = 105 kcal/mol) using O2 as the oxidant. The catalytic cycle of sMMO has been extensively studied over decades, and features a highvalent bis-μ-oxo FeIV2(μ-O)2 “diamond core” intermediate called Q as the active oxidant for C–H bond activation. This research focuses on the study of an unprecedented highvalent CoIII,IV2(μ-O)2 complex supported by neutral tetradentate tris(2- pyridylmethyl)amine (TPA) ligand by one-electron oxidation of its CoIII2(μ-O)2 precursor. This new complex can activate C−H bonds 3−5 orders of magnitude faster than its iron and manganese counterparts, and represents the most reactive synthetic model for the sMMO enzymatic intermediate. This study expands the understanding of base metal complexes for C−H bond activation and serves as motivation to design C−H activation methods inspired by nature. Chapter 1 provides the introduction of C-H bond hydroxylation mechanism and the biological background that initially inspired this project. In Chapter 2, we reported the characterization of cobalt diamond core complexes supported by TPA and related ligands. In Chapter 3, we studied the reactivity of those complexes. In Chapter 4, we discovered that the open core species provides an excellent strategy to achieve substrate specificity and to be applied in the deaminative C(sp3)-N bond activation. Chapter 5 describes a proposed monomer [Co(III)(TPA)(O2)]+ species and its nucleophilic reactivity. Chapter 6 lays out the overall conclusions and points out a few future directions as the prospective scope of the entire project

Oxygen Activation and Radical Transformations in Heme Proteins and Metalloporphyrins

As a result of the adaptation of life to an aerobic environment, nature has evolved a panoply of metalloproteins for oxidative metabolism and protection against reactive oxygen species. Despite the diverse structures and functions of these proteins, they share common mechanistic grounds. An open-shell transition metal like iron or copper is employed to interact with O_2 and its derived intermediates such as hydrogen peroxide to afford a variety of metal–oxygen intermediates. These reactive intermediates, including metal-superoxo, -(hydro)peroxo, and high-valent metal–oxo species, are the basis for the various biological functions of O_2-utilizing metalloproteins. Collectively, these processes are called oxygen activation. Much of our understanding of the reactivity of these reactive intermediates has come from the study of heme-containing proteins and related metalloporphyrin compounds. These studies not only have deepened our understanding of various functions of heme proteins, such as O2 storage and transport, degradation of reactive oxygen species, redox signaling, and biological oxygenation, etc., but also have driven the development of bioinorganic chemistry and biomimetic catalysis. In this review, we survey the range of O_2 activation processes mediated by heme proteins and model compounds with a focus on recent progress in the characterization and reactivity of important iron–oxygen intermediates. Representative reactions initiated by these reactive intermediates as well as some context from prior decades will also be presented. We will discuss the fundamental mechanistic features of these transformations and delineate the underlying structural and electronic factors that contribute to the spectrum of reactivities that has been observed in nature as well as those that have been invented using these paradigms. Given the recent developments in biocatalysis for non-natural chemistries and the renaissance of radical chemistry in organic synthesis, we envision that new enzymatic and synthetic transformations will emerge based on the radical processes mediated by metalloproteins and their synthetic analogs

Overview of ligand versus metal centered redox reactions in tetraaza macrocyclic complexes of nickel with a focus on electron paramagnetic resonance studies

Copper(II) (3d9, S = 1/2) complexes are stable and widely investigated by electron paramagnetic resonance (EPR) spectroscopy. In contrast, isoelectronic nickel(I) is much less common and much less investigated. Nickel(I), however, is of biological interest as the active site of methyl coenzyme M reductase (MCR) contains a tetraaza macrocyclic ligand, F430, which coordinates NiI in its active form, MCRred1. As result, the redox behavior and spectroscopy of tetraaza macrocyclic complexes of nickel is of importance in biomimetic chemistry. Such efforts are complicated by the difficulty in generating NiI from their stable, NiII, precursors. Reduction of NiII macrocyclic complexes can afford NiI in certain cases, but in many other cases can lead instead to reduction of the macrocycle to generate an organic radical anion. Previous studies on the formation of tetraaza macrocyclic complexes of NiI are discussed in terms of the competition between metal-centered and ligand-centered reduction. EPR results are particularly important in making the distinction between these two reduction processes, as formation of NiI gives characteristic EPR spectra similar to those for CuII, while ligand-centered reduction gives narrow EPR spectra at g = 2.00, typical of organic radicals. Even if metal-centered reduction occurs, the geometry of the resulting NiI macrocyclic complex is highly variable and, as a result, the EPR spectral appearance is highly variable. In this case, the comparison is between the extremes of spectra typical for tetragonally distorted complexes (<img src="/img/revistas/jbchs/v21n7/a02img11.gif" align=absmiddle> ground state, which includes tetragonally distorted octahedral, square pyramidal and square planar geometries) and those for trigonal bipyramidal complexes (<img src="/img/revistas/jbchs/v21n7/a02img12.gif" align=absmiddle> ground state). Previous work on CuII was related to the situation for NiI. The different types of EPR spectra for such systems are specifically discussed using previously unpublished examples of several tetraaza macrocyclic complexes of nickel, including F430 and MCR itself

Bioinspired ligand designs for cobalt, iron and manganese complexes : understanding mono-iron hydrogenase (Hmd)

Mono-iron hydrogenase is one of three types of hydrogenases, catalyzing reversible hydride transfer to the substrate (methenyl-H₄MPT⁺) by heterolytically cleaving molecular hydrogen into a proton and a hydride. The key features of the enzyme’s active site include a pyridone moiety, an acyl unit and facial ligation of C [superscript acyl] N [superscript pyridone] S [superscript Cys] donors. Each feature was independently incorporated into a selected ligand system (Schiff-base N4, pincer and thianthrene scaffold, respectively), in efforts to i) develop possible bioinspired catalysts for H₂ activation using earth abundant metals (iron, cobalt, manganese) and ii) make synthetic models of the enzyme active site for deeper understanding of the architecture of the active site and catalytic mechanism. Synthetic routes for making pyridone-based Schiff-base N4 and NNS type ligands were explored: although not isolated, the ligands were spectroscopically detected. On the other hand, the simpler version—pyridine-based Schiff-base N4 ligands—afforded dinuclear cobalt complexes upon metalations with cobalt(II) precursors. Depending on the length of the diamine-linker as well as the substituents on the pyridine rings, either spontaneous O₂ activation or B–F activation was observed, yielding μ-peroxo dicobalt(III) complexes or μ-fluoride bridged dicobalt(II) complexes, respectively. In particular, two μ-fluoride bridged dicobalt complexes showed antiferromagnetic coupling between the two cobalt(II) centers. From the pincer ligands featuring the unique acyl moiety within C [superscript acyl] N [superscript pyridine] S [superscript thioehter] and C [superscript acyl] N [superscript pyridine] P [superscript Ph2] donor set, the (expected) meridional and an (unexpected) facial iron-acyl complexes were isolated, respectively. Upon deprotonation (pyridine→pyridinate de-aromatization), both complexes showed reactivity towards H₂ activation; no evidence for hydride-transfer was observed. For the facial ligation, thianthrene-scaffolded manganese system was examined as a more flexible version of the anthracene-scaffolded systems. Preliminary results of the kinetic studies support the correlation between the flexibility of the scaffold and the reactivity of the metal complex, without greatly altering the electronic environment of the metal center.Chemistr

Catalytic Applications of Pyridine-Containing Macrocyclic Complexes

The introduction of a pyridine moiety into the skeleton of a polyazamacrocyclic ligand affects both the thermodynamic properties and the coordination kinetics of the resulting metal complexes. These features have attracted great interest from the scientific community in recent years. The field of application of pyridine-containing macrocyclic ligands ranges from biology to supramolecular chemistry, encompassing MRI, molecular recognition, materials and catalysis. In this microreview we provide a perspective of the catalytic applications of metal complexes of pyridine-containing macrocycles, including an account of investigations from the authors' laboratories dealing with stereoselective C\u2013C and C\u2013O bond-forming reactions. The increased conformational rigidity imposed by the pyridine ring allowed for the isolation and characterisation of metal complexes in high oxidation states and the study of their relevance in oxidation reactions. On the other hand, the very different conformations accessible upon the metal coordination and the easily tuneable synthesis of the macrocyclic ligands have been exploited in stereoselective synthesis