Autoxidation of substituted phenols catalyzed by cobalt Schiff base complexes in supercritical carbon dioxide

Stoichiometric Formation of an Oxoiron(IV) Complex by a Soluble Methane Monooxygenase Type Activation of O2 at an Iron(II)-Cyclam Center

In soluble methane monooxygenase enzymes (sMMO), dioxygen (O₂) is activated at a diiron(II) center to form an oxodiiron(IV) intermediate Q that performs the challenging oxidation of methane to methanol. An analogous mechanism of O₂ activation at mono- or dinuclear iron centers is rare in the synthetic chemistry. Herein, we report a mononuclear non-heme iron(II)-cyclam complex, 1-trans, that activates O₂ to form the corresponding iron(IV)-oxo complex, 2-trans, via a mechanism reminiscent of the O₂ activation process in sMMO. The conversion of 1-trans to 2-trans proceeds via the intermediate formation of an iron(III)-superoxide species 3, which could be trapped and spectroscopically characterized at -50 °C. Surprisingly, 3 is a stronger oxygen atom transfer (OAT) agent than 2-trans; 3 performs OAT to 1-trans or PPh₃ to yield 2-trans quantitatively. Furthermore, 2-trans oxidizes the aromatic C-H bonds of 2,6-di-tert-butylphenol, which, together with the strong OAT ability of 3, represents new domains of oxoiron(IV) and superoxoiron(III) reactivities.

Deactivation of Co-Schiff base catalysts in the oxidation of para-substituted lignin models for the production of benzoquinones

Those features which enhance the reactivity of Co-Schiff base oxidation catalysts can also contribute to their demise.

A Nonheme Thiolate-Ligated Cobalt Superoxo Complex: Synthesis and Spectroscopic Characterization, Computational Studies, and Hydrogen Atom Abstraction Reactivity

The synthesis and characterization of a Co(II) dithiolato complex Co(Me₃TACN)(S₂SiMe₂) (1) are reported. Reaction of 1 with O₂ generates a rare thiolate-ligated cobalt-superoxo species Co(O₂)(Me₃TACN)(S₂SiMe₂) (2) that was characterized spectroscopically and structurally by resonance Raman, EPR, and X-ray absorption spectroscopies as well as density functional theory. Metal-superoxo species are proposed to S-oxygenate metal-bound thiolate donors in nonheme thiol dioxygenases, but 2 does not lead to S-oxygenation of the intramolecular thiolate donors and does not react with exogenous sulfur donors. However, complex 2 is capable of oxidizing the O-H bonds of 2,2,6,6-tetramethylpiperidin-1-ol derivatives via H atom abstraction. Complementary proton-coupled electron-transfer reactivity is seen for 2 with separated proton/reductant pairs. The reactivity studies indicate that 2 can abstract H atoms from weak X-H bonds with bond dissociation free energy (BDFE) ≤ 70 kcal mol^-1. DFT calculations predict that the putative Co(OOH) product has an O-H BDFE = 67 kcal mol^-1, which matches the observed pattern of reactivity seen for 2. These data provide new information regarding the selectivity of S-oxygenation versus H atom abstraction in thiolate-ligated nonheme metalloenzymes that react with O₂.

Highly selective aerobic oxidation of alkyl arenes and alcohols: cobalt supported on natural hydroxyapatite nanocrystals

Cobalt was successfully immobilized on natural hydroxyapatite nanocrystals which was obtained from cow bones (Co–NHAp). NHAp as the support enhanced both catalytic activity and selectivity in aerobic oxidation of alkyl arenes and alcohols.

Catalytic oxidation of biorefinery lignin to value-added chemicals to support sustainable biofuel production

Transforming plant biomass to biofuel is one of the few solutions that can truly sustain mankind's long-term needs for liquid transportation fuel with minimized environmental impact. However, despite decades of effort, commercial development of biomass-to-biofuel conversion processes is still not an economically viable proposition. Identifying value-added co-products along with the production of biofuel provides a key solution to overcoming this economic barrier. Lignin is the second most abundant component next to cellulose in almost all plant biomass; the emerging biomass refinery industry will inevitably generate an enormous amount of lignin. Development of selective biorefinery lignin-to-bioproducts conversion processes will play a pivotal role in significantly improving the economic feasibility and sustainability of biofuel production from renewable biomass. The urgency and importance of this endeavor has been increasingly recognized in the last few years. This paper reviews state-of-the-art oxidative lignin depolymerization chemistries employed in the papermaking process and oxidative catalysts that can be applied to biorefinery lignin to produce platform chemicals including phenolic compounds, dicarboxylic acids, and quinones in high selectivity and yield. The potential synergies of integrating new catalysts with commercial delignification chemistries are discussed. We hope the information will build on the existing body of knowledge to provide new insights towards developing practical and commercially viable lignin conversion technologies, enabling sustainable biofuel production from lignocellulosic biomass to be competitive with fossil fuel.

Lessons from isolable nickel(I) precursor complexes for small molecule activation

Small-molecule activation by transition metals is essential to numerous organic transformations, both biological and industrial. Creating useful metal-mediated activation systems often depends on stabilizing the metal with uncommon low oxidation states and low coordination numbers. This provides a redox-active metal center with vacant coordination sites well suited for interacting with small molecules. Monovalent nickel species, with their d(9) electronic configuration, are moderately strong one-electron reducing agents that are synthetically attractive if they can be isolated. They represent suitable reagents for closing the knowledge gap in nickel-mediated activation of small molecules. Recently, the first strikingly stable dinuclear β-diketiminate nickel(I) precursor complexes were synthesized, proving to be suitable promoters for small-molecule binding and activation. They have led to many unprecedented nickel complexes bearing activated small molecules in different reduction stages. In this Account, we describe selected achievements in the activation of nitrous oxide (N(2)O), O(2), the heavier chalcogens (S, Se, and Te), and white phosphorus (P(4)) through this β-diketiminatonickel(I) precursor species. We emphasize the reductive activation of O(2), owing to its promise in oxidation processes. The one-electron-reduced O(2) activation product, that is, the corresponding β-diketiminato-supported Ni-O(2) complex, is a genuine superoxonickel(II) complex, representing an important intermediate in the early stages of O(2) activation. It selectively acts as an oxygen-atom transfer agent, hydrogen-atom scavenger, or both towards exogenous organic substrates to yield oxidation products. The one-electron reduction of the superoxonickel(II) moiety was examined by using elemental potassium, β-diketiminatozinc(II) chloride, and β-diketiminatoiron(I) complexes, affording the first heterobimetallic complexes featuring a [NiO(2)M] subunit (M is K, Zn, or Fe). Through density functional theory (DFT) calculations, the geometric and electronic structures of these complexes were established and their distinctive reactivity, including the unprecedented monooxygenase-like activity of a bis(μ-oxo)nickel-iron complex, was studied. The studies have further led to other heterobimetallic complexes containing a [NiO(2)M] core, which are useful for understanding the influence of the heterometal on structure-reactivity relationships. The activation of N(2)O led directly to the hydrogen-atom abstraction product bis(μ-hydroxo)nickel(II) species and prevented isolation of any intermediate. In contrast, the activation of elemental S, Se, and Te with the same nickel(I) reagent furnished activation products with superchalcogenido E(2)(-) (E is S, Se, or Te) and dichalcogenido E(2)(2-) ligand in different activation stages. The isolable supersulfidonickel(II) subunit may serve as a versatile building block for the synthesis of heterobimetallic disulfidonickel(II) complexes with a [NiS(2)M] core. In the case of white phosphorus, the P(4) molecule has been coordinated to the nickel(I) center of dinuclear β-diketiminatonickel(I) precursor complexes; however, the whole P(4) subunit is a weaker electron acceptor than the dichalcogen ligands E(2), thus remaining unreduced. This P(4) binding mode is rare and could open new doors for subsequent functionalization of P(4). Our advances in understanding how these small molecules are bound to a nickel(I) center and are activated for further transformation offer promise for designing new catalysts. These nickel-containing complexes offer exceptional potential for nickel-mediated transformations of organic molecules and as model compounds for mimicking active sites of nickel-containing metalloenzymes.

1-(2-Chlorobenzylidene)-2-(2,4-dinitrophenyl)hydrazine

In the title compound, C₁₃H₉ClN₄O₄, there are two crystallographically independent mol­ecules in the asymmetric unit, which have very similar conformations. The C=N—N angles in each independent mol­ecule are 115.0 (2) and 116.6 (2)°, which are significantly smaller than the ideal value of 120° expected for sp²-hybridized N atoms. This is probably a consequence of repulsion between the nitro­gen lone pairs and the adjacent N—N bonds. Two bifurcated intra­molecular N—H⋯O hydrogen bonds help to establish the mol­ecular conformation and consolidate the crystal packing.

N'-(Furan-2-ylmethyl-ene)-2-hydroxy-benzohydrazide

In the title mol-ecule, C(12)H(10)N(2)O(3), the aromatic and furan rings form a dihedral angle of 8.89 (1)° and an intra-molecular N-H⋯O hydrogen bond occurs. In the crystal structure, inter-molecular O-H⋯O hydrogen bonds link the mol-ecules into zigzag chains running along the c axis.

2-Hydroxy-N′-[(1E,2E)-3-phenylprop-2-enylidene]benzohydrazide

In mol­ecule of the title compound, C₁₆H₁₄N₂O₂, the two aromatic rings form a dihedral angle of 6.93 (3)° and an intramolecular N—H⋯O hydrogen bond occurs. In the crystal structure, inter­molecular O—H⋯O hydrogen bonds link the mol­ecules into zigzag chains running in the [10] direction.

(E)-4-Chloro-N'-[1-(4-hydroxy-phenyl)-ethylidene]benzohydrazide

The mol-ecule of the title compound, C(15)H(13)ClN(2)O(2), displays a trans configuration with respect to the C=N double bond. The dihedral angle between the two benzene rings is 15.1 (3)°. A strong intra-molecular O-H⋯N hydrogen bond is observed. In the crystal structure, mol-ecules are linked through inter-molecular N-H⋯O hydrogen bonds, forming chains running along [101].

(E)-4-Chloro-N'-(4-hydroxy-benzyl-idene)-benzohydrazide

The mol­ecule of the title compound, C₁₄H₁₁ClN₂O₂, displays a trans configuration with respect to the C=N double bond. The dihedral angle between the two benzene rings is 12.8 (3)°. In the crystal structure, mol­ecules are linked through inter­molecular O—H⋯O and N—H⋯O hydrogen bonds and C—H⋯π inter­actions, forming a three-dimensional network.

4-Chloro-N'-(2-hydr-oxy-1-naphthyl-idene)benzohydrazide

The mol-ecule of the title compound, C(18)H(13)ClN(2)O(2), displays a trans configuration with respect to the C=N double bond. The dihedral angle between the benzene and naphthyl ring systems is 6.0 (2)°. An O-H⋯N hydrogen bond is observed in the mol-ecular structure. In the crystal structure, mol-ecules are linked through inter-molecular N-H⋯O hydrogen bonds and π-π stacking inter-actions [centroid-centroid distance = 3.603 (2) Å], forming chains running along the b axis.

4-Chloro-N'-(5-chloro-2-hydroxy-benzyl-idene)benzohydrazide

The mol-ecule of the title compound, C(14)H(10)Cl(2)N(2)O(2), displays a trans configuration with respect to the C=N double bond and has an intramolecular O-H⋯N hydrogen bond. The dihedral angle between the two benzene rings is 1.4 (2)°. In the crystal structure, mol-ecules are linked through inter-molecular N-H⋯O hydrogen bonds, forming chains running along the a direction.

Structural characterization and electrochemical properties of the 3,3'-5,5'-tetra-tert-butyl-4,4'-diphenoquinone

Crystals of the 3,3'-5,5'-tetra-tert-butyl-4,4'-diphenoquinone (TTBDQ) in the reaction mixture DCM/MeOH (1:1, v/v) were obtained as a result of CC coupling reaction of the sterically hindered phenol (2,6-di-tert-butylphenol, DTBP) using the binuclear Co(II) complexes. The oxidation product (TTBDQ), C(28)H(40)O(2), crystallizes in the space group P1 with one-half molecule in the asymmetric unit and the other half generated by an inversion centre. The diphenoquinone moiety is planar within +/-0.016(3)A. The crystal structure is stabilized by intramolecular C-H...O hydrogen bonds. The spectroscopic and electrochemical properties of the TTBDQ also have been studied.

2-(1H-1,2,3-Benzotriazol-1-yl)-N′-cyclopentylideneacetohydrazide

The title compound, C₁₃H₁₅N₅O, was synthesized by the reaction of 2-(1H-1,2,3-benzotriazol-1-yl)acetohydrazide with cyclo­penta­none. In the cyclopentane ring, two C atoms and their attached H atoms are disordered over two positions; the site occupancy factors are ca 0.63 and 0.37. In the crystal structure, mol­ecules are linked into infinite chains directed along the b axis by N—H⋯O hydrogen bonds. In addition, there are weak C—H⋯O and C—H⋯N hydrogen bonds, as well as C—H⋯π-ring inter­actions in the structure.