Synthesis, Characterization, and Reactivity of Ferrous and Ferric Oxo/Peroxo Pivalate Complexes in Relation to Gif-Type Oxygenation of Substrates

Abstract

This study examines structural features and aspects of reactivity of Gif-type reagents, which depend on O2/Zn to mediate oxidation of hydrocarbons. The reagents investigated derive from the use of iron complexes with the anion of the weak carboxylic acid Me3CCO2H (pivalic acid (PivH)) in pyridine/PivH. In these solutions, the known compound [Fe3O(O2CCMe3)6(py)3] is reduced by Zn to generate yellow-green [FeII(O2CCMe3)2(py)4], which readily reverts to [Fe3O(O2CCMe3)6(py)3], and eventually to [Fe3O(O2CCMe3)6(py)3]+, upon exposure to dioxygen. All three species are equally well suited to mediate Gif-like oxygenation of substrates supported by O2/Zn. [FeIII3O(O2CCMe3)6(L)3]+ (L = H2O, py) is converted by H2O2 to afford the hexairon(III) peroxo compounds [Fe6(O2)(O)2(O2CCMe3)12(L)2] (L = Me3CCO2H, py), which feature a [Fe6(eta 2-mu 4-O2)(mu 3-O)2] core previously documented in the closely related [Fe6(O2)(O)2(O2CPh)12(H2O)2]. A similar peroxo species, [Fe6(O2)(O)2(O2CCMe3)2(O2CCF3)10(H2O)2], is obtained upon replacing all pivalate ligands by trifluoroacetate groups with the exception of those pivalates that bridge between the two [Fe3O(O2CCF3)5(H2O)]2+ units. The structure of the [Fe6(O2)(O)2] core in these peroxo species is found to range from a recliner to a butterfly-type conformation. Reduction of [Fe6(O2)(O)2(O2CCMe3)12(HO2CCMe3)2] with NaBH4 generates [Na2Fe4(O)2(O2CCMe3)10(L)(L')] (L = CH3CN, L' = Me2CO; L = L' = Me3CCO2H), which feature a [Na2Fe4(O)2] core possessing a bent butterfly conformation of the [Fe4(O)2] unit. Oxidation of the same peroxo complex by CeIV or NOBF4 regenerates the oxo-bridged [Fe3O(O2CCMe3)6(solv)3]+ (solv = EtOH, H2O, thf). Employment of the sterically encumbered 2-Me-5-Etpyridine provides the tetrairon compound [Fe4(O)2(O2CCMe3)8(2-Me-5-Etpy)2], which can be readily transformed upon treatment with H2O2 to the asymmetric peroxo complex [Fe6(O2)(O)2(O2CCMe3)12(2-Me-5-Etpy)2]. The peroxo-containing complexes oxidize both cis-stilbene and adamantane in either benzene or py/PivH, but only under forceful conditions and at very low yields. The low reactivity and high selectivity (tert/sec = 8) obtained in the oxidation of adamantane suggests that the present type of peroxo species is not directly involved in catalytic Gif-type oxygenations of adamantane.

Iron(III)‐Pivalate‐Based Complexes with Tetranuclear {Fe4(μ3‐O)2}8+ Cores and N‐Donor Ligands: Formation of Cluster and Polymeric Architectures

“…The two body ions bridged by two μ 3 -oxides have an Fe1···Fe3 separation of 2.909(1) Å. Similar distances may be found in {Fe 4 O 2 }-core-based complexes even if they do not have an additional bridge between the body atoms; see, for example, in other reported works [18,[26][27] ( Table 1 Each iron atom is six-coordinate and has a distorted octahedral coordination sphere. The Fe1, Fe3, and Fe4 ions are all O 6 -coordinated: the Fe1 atom is coordinated by two μ 3 -oxo atoms and four oxygen atoms from pivalate ligands (two bridging and one chelating); the Fe3 atom is bonded to two μ 3 -oxo atoms and four oxygen atoms from bridging pivalates; the Fe4 atom is ligated by a μ 3 -oxygen atom, three oxygen atoms from bridging pivalates, and two oxygen atoms from a chelating carboxylate.…”

Synthesis and Characterization of Iron(III) Phosphonate Cage Complexes

“…[13] 1: [Fe 3 O(O 2 CMe) 6 (11) 3 , Z = 4, T = 100(2) K, 1 = 1.503 g cm À1 , F(000) = 3408, m(Mo Ka ) = 1.294 mm À1 . Data were collected on Bruker SMART CCD diffractometer (Mo Ka , l = 0.71069 ).…”

“…Peroxides have been found previously in reactions of iron carboxylate triangles, [10] and have shown interesting reactivity in Gif type oxidations. [11] The two trigonal prisms are linked through single Fe III sites, which adopt a similar structural role to the linking sites in 3-5. The phosphonates bridging to these sites adopt the 3.111 mode, and the coordination geometry of the Fe sites is completed by four molecules of H 2 O in each case.…”

First triangular carboxylate cluster with the Fe(II)Fe(II)Fe(II) metal core

“…1). In contrary to the known [Fe 3 O(OOCR) 6 L 3 ] þ species with the Fe(III) atoms [3][4][5][6], the Fe 3 O fragment in complex I is non-planar (the O atom protrudes from the Fe 3 plane at 0.226 A). The potassium cation interacts with the triangle iron anion via the KÁ Á ÁO(OOCR) contacts (2.748(4)-3.059(4) A).…”