Efficient aerobic oxidation of hydrocarbons promoted by high-spin nonheme Fe(II) complexes without any reductant
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.
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 1–3 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 1–3 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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