Synthesis, Reactivity, and Catalytic Behavior of Iron/Zinc-Containing Species Involved in Oxidation of Hydrocarbons under Gif-Type Conditions

Abstract

The present study explores the nature and reactivity of iron- and zinc-containing species generated in hydrocarbon-oxidizing GifIV-type solutions (Fe catalyst/Zn/O2 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 [Fe3O(O2CCH3)6(py)3]·py (1) with zinc powder in CH3CN/CH3COOH or CH2Cl2/CH3COOH affords the trinuclear compound [Zn2FeII(O2CCH3)6(py)2] (2). Single-crystal X-ray analysis confirms that one monodentate 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/CH3COOH (10:1, 5:1, 2:1 v/v) yields [FeII(O2CCH3)2(py)4] (3), [FeII2(O2CCH3)4(py)3]n (4), and [Zn(O2CCH3)2(py)2] (5) depending on the isolation procedure employed. Compound 3 possesses a distorted octahedral geometry, featuring a C2 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 H2/Pd. Solutions of 3 and 4 in pyridine or py/CH3COOH react with pure dioxygen or air to eventually regenerate 1 in a concentration-dependent manner. Oxidation of 2 in py/CH3COOH with pure dioxygen or air yields [Fe2.22(2)Zn0.78(2)O(O2CCH3)6(py)3]·py (1‘) and [Zn2(O2CCH3)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 [Fe2ZnO(O2CCH3)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/CH3COOH solutions. In non-pyridine solutions, Zn-containing species 5 and 6 rearrange to the crystallographically characterized species [Zn(O2CCH3)2(py)]n (7) and [Zn3(O2CCH3)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 [Ph3PNPPh3][FeII(O2CCH3)3(py)] (9), which is obtained from the reaction of 3 with [PPN][O2CCH3], and the water-coordinated iron in [FeII(H2O)4(trans-py)2][O2CCH3]2 (10), which reveals an extensive two-dimensional network of hydrogen-bonding interactions. The pyridine-free species [FeII3(O2CCH3)6(OS(CD3)2)2]n (11) is isolable upon extensive incubation of 3 in (CD3)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/O2 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 (kH/kD = 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 GifIV-type oxidation is discussed in light of these structural and functional findings.