SYNTHESIS, REACTIVITY, AND CATALYTIC BEHAVIOR OF IRON ZINC-CONTAININGSPECIES 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 so lutions Fe catalyst/Zn/O-2 in pyridine/acetic acid (10:1 v/v). The ult imate 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 ketonizatio n of secondary carbons. Reaction of [Fe3O(O2CCH3)(6)(PY)(3)]. Py (1) w ith zinc powder in CH3CN/CH3COOH or CH2Cl2/CH3COOH affords the trinucl ear compound [Zn2FeII(O2CCH3)(6)(py)(2)] (2). Single-crystal X-ray ana lysis confirms that one monodentate and two bidentate acetate groups b ridge adjacent pairs of metals with the iron atom occupying a centrosy mmetric position. The analogous reduction of 1 in py/CH3COOH (10:1, 5: 1, 2:1 v/v) yields [Fe-II(O2CCH3)(2)(py)(4)] (3), [Fe-2(II)(O2CCH3)(4) (py)(3)](n) (4)1 and [Zn(O2CCH3)(2)(py)(2)] (5) depending on the isola tion procedure employed. Compound 3 possesses a distorted octahedral g eometry, featuring a C-2 axis bisecting the equatorial, pyridine-occup ied plane, whereas the two acetate groups reside along the perpendicul ar axis. Compound 4 is a one-dimensional solid constructed by asymmetr ic diferrous units. Two bidentate and one monodentate acetate groups b ridge the two iron sites, with the monodentate bridge also acting as a chelator to one ferrous center. The two iron centers exhibit weak ant iferromagnetic coupling. Compounds 3 and 4 are also accessible from th e reduction of 1 with iron powder of treatment with H-2/Pd. Solutions of 3 and 4 in pyridine or py/CH3COOH react with pure dioxygen or air t o eventually regenerate 1 in a concentration-dependent manner. Oxidati on of 2 in py/CH3COOH with pure dioxygen or air yields [Fe-2.22(2)Zn-0 .78(2)O(O2CCH3)(6)(py)(3)]. py (1') and [Zn-2(O2CCH3)(4)(PY)(2)] (6) C ompound 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 monoc linic [Fe2ZnO(O2CCH3)(6)(py)(3)]. py (1 ''). Species 1 '' yields produ cts identical with those provided by 1 under reducing conditions. Comp ounds 2-6 are related by pyridine-dependent equilibria, as demonstrate d by mutual interconversions and electronic absorption data in pyridin e and py/CH3COOH solutions. In nonpyridine solutions, Zn-containing sp ecies 5 and 6 rearrange to the crystallographically characterized spec ies [Zn(O2CCH3)(2)(PY)](n) (7) and [Zn-3(O2CCH3)(6)(PY)(2)] (8) Compou nd 7 is a one-dimensional solid featuring a chain of Zn sites linked b y a bidentate acetate group while additionally coordinated by a chelat ing acetate. Compound 8 is isostructural to 2. Further perturbations o f the described structures are apparent in ionic iron-containing speci es, such as the pseudo-seven-coordinate iron in [Ph3P=N=PPh3]Fe-II(O2C CH3)(3)(py)] (9), which is obtained from the reaction of 3 with [PNN[O 2CCH3], and the water-coordinated iron in [Fe-II(H2O)(4)(trans-py)(2)] [O2CCH3](2) (10), which reveals an extensive two-dimensional network o f hydrogen-bonding interactions. The pyridine-free species [Fe-3(II)(O 2CCH3)(6)(OS(CD3)(2))(2)](n) (11) is isolable upon extensive incubatio n of 3 in (CD3)(2)SO. Compound 11 exhibits a remarkable one-dimensiona l structure, featuring four different types of acetate groups. Catalyt ic oxidations of adamantane, isopentane, benzene, toluene, cis-stilben e, and pyridine mediated by the system 1 (or 2-4)/Zn/O-2 in py/AcOH (1 0:1) afford product profiles which are not fully compatible with the r eported outcome of analogous oxidations by hydroxyl radicals or biolog ically relevant high-valent iron-ore species alone. The intermolecular deuterium kinetic isotope effect for the oxidation of adamantane to a damantanone is small (k(H)/k(D) = 2.01(12)) by comparison to values ob tained for oxidation of hydrocarbons by biological oxygenases. Employm ent of hydrogen peroxide, t-BuOOH, or peracetic acid as potential oxo donors does not provide viable shunt pathways in the catalytic oxygena tion of adamantane. The nature of active oxidant in Gif(IV)-type oxida tion is discussed in light of these structural and functional findings .