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SYNTHESIS, REACTIVITY, AND CATALYTIC BEHAVIOR OF IRON/ZINC-CONTAINING SPECIES INVOLVED IN OXIDATION OF HYDROCARBONS UNDER GIF-TYPE CONDITIONS. (R823377)
Citation:
Singh, B., J. R. Long, d. E. Fabrizi, D. Gatteschi, AND P. Stavropoulos. SYNTHESIS, REACTIVITY, AND CATALYTIC BEHAVIOR OF IRON/ZINC-CONTAINING SPECIES INVOLVED IN OXIDATION OF HYDROCARBONS UNDER GIF-TYPE CONDITIONS. (R823377). JOURNAL OF COLLOID AND INTERFACE SCIENCE. American Chemical Society, Washington, DC, 119(30):7030-7047, (1997).
Description:
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 [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 [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 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 of
treatment with H-2/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 [Fe-2.22(2)Zn-0.78(2)O(O2CCH3)(6)(py)(3)]. py
(1') and [Zn-2(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
nonpyridine solutions, Zn-containing species 5 and 6 rearrange to the crystallographically
characterized species [Zn(O2CCH3)(2)(PY)](n) (7) and [Zn-3(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 [Ph3P=N=PPh3]Fe-II(O2CCH3)(3)(py)] (9), which is
obtained from the reaction of 3 with [PNN[O2CCH3], and the water-coordinated iron in
[Fe-II(H2O)(4)(trans-py)(2)][O2CCH3](2) (10), which reveals an extensive two-dimensional
network of hydrogen-bonding interactions. The pyridine-free species
[Fe-3(II)(O2CCH3)(6)(OS(CD3)(2))(2)](n) (11) is isolable upon extensive incubation of 3 in
(CD3)(2)SO. 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-ore 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 Gif(IV)-type oxidation is discussed in light of these structural and functional
findings. (171 References)