Synthesis, Reactivity, and Catalytic Behavior of Iron/zinc-containing Species Involved in Oxidation of Hydrocarbons under Gif-type Conditions


The present study explores the nature and reactivity of iron- and zinc- containing species generated in hydrocarbon-oxidizing Gif(IV)-type solutions (Fe catalyst/Zn/O 2 in pyridine/acetic acid (10:1 v/v). The ultimate goal of this investigation is to unravel the role of metal sites in mediating dioxygen-dependent C-H activation, which in the case of Gif chemistry demonstrates an enhanced selectivity for the ketonization of secondary carbons. Reaction of [Fe 3O(O 2CCH 3) 6(py) 3]·py (1) with zinc powder in CH 3CN/CH 3COOH or CH 2Cl 2/CH 3COOH affords the trinuclear compound [Zn 2Fe(II)(O 2CCH 3) 6(py) 2] (2). Single-crystal X-ray analysis confirms that one monedentate and two bidentate acetate groups bridge adjacent pairs of metals with the iron atom occupying a centrosymmetric position. The analogous reduction of 1 in py/CH 3COOH (10:1.5:1,2:1 v/v) yields [Fe(II)(O 2CCH 3) 2(py) 4] (3), [Fe(II) 2(O 2CCH 3) 4(py) 3](n) (4), and [Zn(O 2CCH 3) 2(py) 2] (5) depending on the isolation procedure employed. Compound 3 possesses a distorted octahedral geometry, featuring a C 2 axis bisecting the equatorial, pyridine-occupied plane, whereas the two acetate groups reside along the perpendicular axis. Compound 4 is a one-dimensional solid constructed by asymmetric diferrous units. Two bidentate and one monodentate acetate groups bridge the two iron sites, with the monodentate bridge also acting as a chelator to one ferrous center. The two iron centers exhibit weak antiferromagnetic coupling. Compounds 3 and 4 are also accessible from the reduction of 1 with iron powder or treatment with H 2/Pd. Solutions of 3 and 4 in pyridine or py/CH 3COOH react with pure dioxygen or air to eventually regenerate 1 in a concentration-dependent manner. Oxidation of 2 in py/CH 3COOH with pure dioxygen or air yields [Fe(2.22(2)(Zn(0.78(2))O(O 2CCH 3) 6(PY) 3]·py (1') and [Zn 2(O 2CCH 3) 4(py) 2] (6). Compound 1' is isostructural to 1, exhibiting rhombohedral symmetry at 223 K. The filtrate of the reduction of 1 with zinc in neat pyridine, when exposed to dioxygen, affords dichroic red-green crystals of monoclinic [Fe 2ZnO(O 2CCH 3) 6(py) 3]·py (1'). Species 1' yields products identical with those provided by 1 under reducing conditions. Compounds 2-6 are related by pyridine-dependent equilibria, as demonstrated by mutual interconversions and electronic absorption data in pyridine and py/CH 3COOH solutions. In non-pyridine solutions, Zn-containing species 5 and 6 rearrange to the crystallographically characterized species [Zn- (O 2CCH 3) 2(py)](n) (7) and [Zn 3(O 2CCH 3) 6(py) 2] (8). Compound 7 is a one-dimensional solid featuring a chain of Zn sites linked by a bidentate acetate group while additionally coordinated by a chelating acetate. Compound 8 is isostructural to 2. Further perturbations of the described structures are apparent in ionic iron-containing species, such as the pseudo-seven- coordinate iron in [Ph 3P=N=PPh 3][Fe(II)(O 2CCH 3) 3(py)] (9), which is obtained from the reaction of 3 with [PPN] [O 2CCH 3], and the water- coordinated iron in [Fe(II)(H 2O) 4(trans-py) 2][O 2CCH 3] 2 (10), which reveals an extensive two-dimensional network of hydrogen-bonding interactions. The pyridine-free species [Fe(II) 3(O 2- CCH 3) 6(OS(CD 3) 2) 2](n) (11) is isolable upon extensive incubation of 3 in (CD 3) 2SO. Compound 11 exhibits a remarkable one-dimensional structure, featuring four different types of acetate groups. Catalytic oxidations of adamantane, isopentane, benzene, toluene, cis-stilbene, and pyridine mediated by the system 1 (or 2-4)/Zn/O 2 in py/AcOH (10:1) afford product profiles which are not fully compatible with the reported outcome of analogous oxidations by hydroxyl radicals or biologically relevant high-valent iron- oxo species alone. The intermolecular deuterium kinetic isotope effect for the oxidation of adamantane to adamantanone is small (k(H)/k(D) = 2.01(12)) by comparison to values obtained for oxidation of hydrocarbons by biological oxygenases. Employment of hydrogen peroxide, t-BuOOH, or peracetic acid as potential oxo donors does not provide viable shunt pathways in the catalytic oxygenation of adamantane. The nature of active oxidant in Git(IV)-type oxidation is discussed in light of these structural and functional findings.



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