Iron(II)-α-keto acid complexes of tridentate ligands on gold nanoparticles: the effect of ligand geometry and immobilization on their dioxygen-dependent reactivity.

Two mononuclear nonheme iron(II)-benzoylformate (BF) complexes [(6Me2-Me-BPA)Fe(BF)](ClO4) (1a) and [(6Me3-TPMM)Fe(BF)](ClO4) (1b) of tridentate nitrogen donor ligands, bis((6-methylpyridin-2-yl)methyl)(N-methyl)amine (6Me2-Me-BPA) and tris(2-(6-methyl)pyridyl)methoxymethane (6Me3-TPMM), were isolated and characterized. The structural characterization of iron(II)-chloro complexes indicates that the ligand 6Me2-Me-BPA binds to the iron(II) centre in a meridional fashion, whereas 6Me3-TPMM behaves as a facial ligand. Both the ligands were functionalized with terminal thiol for immobilization on gold nanoparticles (AuNPs), and the corresponding iron(II) complexes [(6Me2-BPASH)Fe(BF)(ClO4)]@C8Au (2a) and [(6Me3-TPMSH)Fe(BF)(ClO4)]@C8Au (2b) were prepared to probe the effect of immobilization on their ability to perform bioinspired oxidation reactions. All the complexes react with dioxygen to display the oxidative decarboxylation of the coordinated benzoylformate, but the complexes supported by 6Me3-TPMM and its thiol-appended ligand display faster reactivity compared to their analogues with the 6Me2-Me-BPA-derived ligands. In each case, an electrophilic iron-oxygen oxidant was intercepted as the active oxidant generated from dioxygen. The immobilized complexes (2a and 2b) display enhanced O2-dependent reactivity in oxygen-atom transfer reactions (OAT) and hydrogen-atom transfer (HAT) reactions compared to their homogeneous congeners (1a and 1b). Furthermore, the immobilized complex 2b displays catalytic OAT reactions. This study supports that the ligand geometry and immobilization on AuNPs influence the dioxygen-dependent reactivity of the complexes.

Nickel complexes of ligands derived from (o-hydroxyphenyl) dichalcogenide: delocalised redox states of nickel and o-chalcogenophenolate ligands.

Two monoanionic nickel complexes Bu4N[Ni(LSeO)2] (1) and Bu4N[Ni(LSO)2] (2) (H2LSeO = 3,5-di-tert-butyl-2-hydroxyselenophenol and H2LSO = 3,5-di-tert-butyl-2-hydroxythiophenol) were synthesised by reductive cleavage of the respective 2,2'-dichalcogenobis(4,6-di-tert-butylphenol) (H2LX-X; X = Se, S) with nickel(ii) salts. The crystal structures of 1 and 2 confirm the reductive X-X bond cleavage with the concomitant formation of the corresponding monoanionic square planar complex, where quinoidal distortions of the aromatic rings are observed. The monoanionic complexes (1 and 2) are paramagnetic (S = 1/2), exhibiting rhombic EPR signals, and the g anisotropies are well correlated with the spin-orbit coupling of chalcogenides. The spectral data indicate that the ligands H2LXO in 1 and 2 are redox non-innocent and stabilise the square planar S = 1/2 nickel complexes with a highly delocalised unpaired electron. DFT calculations further support the delocalised electronic structures of the nickel complexes.

Carbon-Nanotube-Appended PAMAM Dendrimers Bearing Iron(II) α-Keto Acid Complexes: Catalytic Non-Heme Oxygenase Models.

Poly(amidoamine) dendrimers grafted on carbon nanotubes have been appended with iron(II)-α-keto acid (benzoylformate) complex of polypyridyl ligand to design artificial non-heme oxygenase model. This nano-enzyme was applied for selective catalytic oxidation of organic molecules. Although the carbon nanotubes serve as a robust heterogeneous platform, the amine terminals of dendrimers provide catalysts binding sites and the amide bonds provide a necessary second coordination sphere similar to the enzymatic polypeptide chains. Such a hybrid design prevented the deactivation of the primary active sites leading to 8 times faster oxidative decarboxylation rates than those of its homogeneous analogue. An electrophilic iron(IV)-oxo intermediate has been intercepted, which catalyzes the selective oxidation of alcohols to aldehydes and incorporates single oxygen atoms into sulfides and olefins by using aerial oxygen with multiple turnover numbers. The catalyst was consecutively regenerated three times by mild chemical treatment and showed negligible loss of activity.

Oxidizing Ability of a Dioxygen-Activating Nonheme Iron(II)-Benzilate Complex Immobilized on Gold Nanoparticles.

An iron(II)-benzilate complex [(TPASH)FeII(benzilate)]ClO4@C8Au (2) (TPASH = 11-((6-((bis(pyridin-2-ylmethyl)amino)methyl)pyridin-2-yl)methoxy)undecane-1-thiol) immobilized on octanethiol stabilized gold nanoparticles (C8Au) of core diameter less than 5 nm has been prepared to evaluate its reactivity toward O2-dependent oxidations compared to a nonimmobilized complex [(TPA-O-Allyl)FeII(benzilate)]ClO4 (1a) (TPA-O-Allyl = N-((6-(allyloxymethyl)pyridin-2-yl)methyl)(pyridin-2-yl)- N-(pyridin-2-ylmethyl)methanamine). X-ray crystal structure of the nonimmobilized complex 1a reveals a six-coordinate iron(II) center in which the TPA-O-Allyl acts as a pentadentate ligand and the benzilate anion binds in monodentate fashion. Both the complexes (1a and 2) react with dioxygen under ambient conditions to form benzophenone as the sole product through decarboxylation of the coordinated benzilate. Interception studies reveal that a nucleophilic iron-oxygen intermediate is formed in the decarboxylation reaction. The oxidants from both the complexes are able to carry out oxo atom transfer reactions. The immobilized complex 2 not only performs faster decarboxylation but also exhibits enhanced reactivity in oxo atom transfer to sulfides. Importantly, the immobilized complex 2, unlike 1a, displays catalytic turnovers in sulfide oxidation. However, the complexes are not efficient to carry out cis-dihydroxylation of alkenes. Although the immobilized complex yields a slightly higher amount of cis-diol from 1-octene, restricted access of dioxygen and substrates at the coordinatively saturated metal centers of the complexes likely makes the resulting iron-oxygen species less active in oxygen atom transfer to alkenes. The results implicate that surface immobilized nonheme iron complexes containing accessible coordination sites would exhibit better reactivity in O2-dependent oxygenation reactions.

Covalent Grafting of BPin functions on Carbon Nanotubes and Chan-Lam-Evans Post-Functionalization.

The chemical functionalization of carbon nanotubes is often a prerequisite prior to their use in various applications. The covalent grafting of 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (BPin) functional groups directly on the surface of multi- and single-walled carbon nanotubes, activated by nucleophilic addition of nBuLi, was carried out. Thermogravimetric analysis (TGA) coupled with mass spectrometry, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ions mass spectrometry (ToF-SIMS) confirmed the efficiency of this methodology and proved the integrity and covalent grafting of the BPin functional groups. These groups were further reacted with various nucleophiles in the presence of a copper(II) source in the conditions of the aerobic Chan-Lam-Evans coupling. The resulting materials were characterized by TGA, XPS and ToF-SIMS. This route is efficient, reliable and among the scarce reactions that enable the direct grafting of heteroatoms at carbonaceous material surfaces.

Aliphatic C-H Bond Halogenation by Iron(II)-α-Keto Acid Complexes and O2: Functional Mimicking of Nonheme Iron Halogenases.

α-Ketoglutarate-dependent nonheme halogenases catalyze the halogenation of aliphatic C-H bonds in the biosynthesis pathway of many natural products. An iron(IV)-oxo-halo species has been established as the active oxidant in the halogenation reactions. With an objective to emulate the function of the nonheme halogenases, two iron(II)-α-keto acid complexes, [(phdpa)Fe(BF)Cl] (1) and [(1,4-tpbd)Fe2(BF)2Cl2] (2) (where phdpa = N,N-bis(2-pyridylmethyl)aniline, 1,4-tpbd = N,N, N',N'-tetrakis(2-pyridylmethyl)benzene-1,4-diamine, and BF = benzoylformate), have been prepared. The iron complexes are capable of carrying out the oxidative halogenation of aliphatic C-H bonds using O2 as the terminal oxidant. Although the complexes are not selective toward C-H bond halogenation, they are the only examples of nonheme iron(II)-α-keto acid complexes mimicking the activity of nonheme halogenases. The dinuclear complex (2) exhibits enhanced reactivity toward C-H bond halogenation/hydroxylation.

Aerobic alcohol oxidation and oxygen atom transfer reactions catalyzed by a nonheme iron(ii)-α-keto acid complex.

α-Ketoglutarate-dependent enzymes catalyze many important biological oxidation/oxygenation reactions. Iron(iv)-oxo intermediates have been established as key oxidants in these oxidation reactions. While most reported model iron(ii)-α-keto acid complexes exhibit stoichiometric reactivity, selective oxidation of substrates with dioxygen catalyzed by biomimetic iron(ii)-α-keto acid complexes remains unexplored. In this direction, we have investigated the ability of an iron(ii) complex [(TpPh,Me)FeII(BF)] (1) (TpPh,Me = hydrotris(3-phenyl-5-methylpyrazolyl)borate and BF = monoanionic benzoylformate) to catalyze the aerobic oxidation of organic substrates. An iron-oxo oxidant, intercepted in the reaction of 1 with O2, selectively oxidizes sulfides to sulfoxides, alkenes to epoxides, and alcohols to the corresponding carbonyl compounds. The oxidant from 1 is able to hydroxylate the benzylic carbon of phenylacetic acid to afford mandelic acid with the incorporation of one oxygen atom from O2 into the product. The iron(ii)-benzoylformate complex oxidatively converts phenoxyacetic acids to the corresponding phenols, thereby mimicking the function of iron(ii)-α-ketoglutarate-dependent 2,4-dichlorophenoxyacetate dioxygenase (TfdA). Furthermore, complex 1 exhibits catalytic aerobic oxidation of alcohols and oxygen atom transfer reactions with multiple turnovers.

Dioxygen activation and two consecutive oxidative decarboxylations of phenylpyruvate by nonheme iron(II) complexes: functional models of hydroxymandelate synthase (HMS) and CloR.

Two mononuclear iron(ii)-phenylpyruvate complexes of monoanionic facial N3 ligands are reported to react with dioxygen to undergo two consecutive oxidative decarboxylation steps via an iron-mandelate complex mimicking the function of HMS and CloR.

Catalytic and regiospecific extradiol cleavage of catechol by a biomimetic iron complex.

An iron(III)-catecholate complex of a facial tridentate ligand reacts with dioxygen in the presence of ammonium acetate-acetic acid buffer to cleave the aromatic C-C bond of 3,5-di-tert-butylcatechol regiospecifically resulting in the formation of an extradiol product with multiple turnovers.