Elsevier

Inorganica Chimica Acta

Volume 378, Issue 1, 30 November 2011, Pages 19-23
Inorganica Chimica Acta

Efficient aerobic oxidation of hydrocarbons promoted by high-spin nonheme Fe(II) complexes without any reductant

https://doi.org/10.1016/j.ica.2011.08.004Get rights and content

Abstract

Fe(II)–tris(2-pyridylmethyl)amine complexes, Fe(II)–tpa, having different co-existing anions, [Fe(tpa)(MeCN)2](ClO4)2 (1), [Fe(tpa)(MeCN)2](CF3SO3)2 (2) and [Fe(tpa)Cl2] (3), were prepared. Effective magnetic moments (evaluated by the Evans method) revealed that while 13 in acetone and 3 in acetonitrile (MeCN) have a high-spin Fe(II) ion at 298 K, the Fe(II) ions of 1 and 2 are in the low-spin state in MeCN. The aerobic oxidation of 13 was monitored by UV–Vis spectral changes in acetone or MeCN under air at 298 K. Only the high-spin Fe(II)–tpa complexes were oxidized with rate constants of kobs = 0.1–1.3 h−1, while 1 and 2 were stable in MeCN. The aerobic oxidation of 1 or 2 in acetone was greatly accelerated in the presence of pure, peroxide-free cyclohexene (1000 equiv.) and yielded a large amount of oxidized products; 2-cyclohexe-1-ol (A) and 2-cyclohexene-1-one (K) (A + K: 23 940% yield based on Fe; A/K = 0.3), and cyclohexene oxide (810%). Besides cyclohexene, aerobic oxidation of norbornene, cyclooctene, ethylbenzene, and cumene proceeded in the presence of 1 in acetone at 348 K without any reductant. Essential factors in the reaction are high-spin Fe(II) ion and labile coordination sites, both of which are required to generate Fe(II)-superoxo species as active species for the H-atom abstraction of hydrocarbons.

Graphical abstract

Aerobic oxidation of alkenes was promoted efficiently by a high-spin Fe(II) complex supported by tris(2-pyridylmethyl)amine in acetone without any reductant. The reactions proceed presumably through hydrogen atom abstraction by an Fe(III)-superoxo species.

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Highlights

► Aerobic hydrocarbon oxidation without reductants. ► Dioxygen activation by high-spin Fe(II) complex. ► H-atom abstraction by Fe(III)-superoxide species.

Introduction

The catalytic oxidation of hydrocarbons such as alkanes, alkenes and aromatic compounds to oxygen-containing materials is one of the most important chemical transformations in industrial chemistry [1]. For economic and environmental reasons, the use of dioxygen as sole oxidant is highly desirable [2]. Since it reacts slowly with hydrocarbons owing to its triplet ground state, appropriate activation of dioxygen is required for efficient oxidation of hydrocarbons using dioxygen. In nature, such dioxygen activation is performed by iron- and/or copper-dependent enzymes such as cytochrome P450 [3], methane monooxygenase [4], tyrosinase [5], dopamine β-monooxygenase [6] and various types of mononuclear nonheme iron dioxygenases [7], where dioxygen is reductively activated via the Cu(I) or Fe(II) ion(s). It is noteworthy that a high-spin Fe(II) center activates dioxygen in heme and nonheme iron-dependent enzymes [8] and model complexes [9], [10], [11].

Previously, we reported that low spin Fe(III)–tpa catecholate complexes (tpa: tris(2-pyridylmethyl)amine, Fig. 1) have a lower dioxygen reactivity than the high-spin species, even though the former has a much higher Fe(II)-semiquinoate character [12]. This result motivated us to examine the spin-state dependency on the dioxygen activation ability of simple Fe(II)–tpa complexes. Diebold and Hagen reported that the spin state of the Fe(II)–tpa complex is different depending on its counter anions [13]. Therefore, we prepared three Fe(II)–tpa complexes; [Fe(tpa)(MeCN)2](ClO4)2 (1), [Fe(tpa)(MeCN)2](CF3SO3)2 (2) and [Fe(tpa)Cl2] (3), and examined their spin states and dioxygen activation ability. We also examined the aerobic oxidation of hydrocarbons in the presence of the Fe(II)–tpa complexes.

Section snippets

Characterizations of Fe(II)–tpa complexes

The effective magnetic moments of complexes 13 were measured in acetonitrile (MeCN) and acetone by the Evans NMR method [14]. The μeff values, listed in Table 1, indicate that 1 and 2 possess mainly a low spin Fe(II) ion in MeCN, whereas 3 is in the high-spin state in MeCN. The spin states of Fe(II)–tpa type complexes in MeCN have been widely investigated [13], [15], [16], [17]. The present results in MeCN are well consistent with the reported data [13], [15], [16]. In contrast, all the Fe(II)

Conclusions

We found that aerobic oxidation of alkenes was efficiently promoted by the Fe(II)–tpa complex, when the Fe center was in the high-spin state. It is noteworthy that the aerobic oxidation proceeded without any reductants, since most of the aerobic oxidation systems catalyzed by heme or nonheme iron complexes require stoichiometric co-reductants such as aldehydes and Zn/AcOH [50]. We propose that an Fe(III)-superoxo species is a key intermediate in the efficient aerobic oxidation of alkenes. The

Materials

The tpa ligand was synthesized according to published procedures [12]. All other reagents and dehydrated solvents were purchased from commercial sources. All dehydrated solvents were deoxygenated by N2 bubbling prior to use. Treatment of the tpa ligand with Fe(ClO4)2, Fe(OTf)2 and FeCl2 in MeCN gave the corresponding complexes 13 as crystalline solids in high yield.

  • [Fe(tpa)(MeCN)2](ClO4)2 (1): ESI-MS calcd. (found): m/z 193.6 (193.5) ([FeII(tpa)(MeCN)]2+).

  • [Fe(tpa)(MeCN)2](OTf)2 (2): ESI-MS

Acknowledgments

This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas (No. 19028033, “Chemistry of Concerto Catalysis”) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. S.F. thanks the JSPS Research Fellowships for Young Scientists.

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