![]() ![]() 16 Although slight structural differences modify their reactivity, the ligands illustrated in Figure 1 share a common characteristic, that is, they stabilize high valent metal centers exceptionally well. 15 Employing another related dianionic 16-membered tetra-NHC (Figure 1, right), D. M. could isolate and crystallize an iron(IV) oxo complex, usually considered a highly reactive intermediate. ![]() Utilizing a different, yet related 18-membered tetra-NHC (Figure 1, left), F. ![]() Structurally related macrocyclic tetra-NHC ligands by F. Although such Fe III- O-Fe III compounds are generally regarded inactive, 14 the respective cCCCC complex catalytically oxidizes 1,4-hydroquinone employing O 2. 11, 13 When exposed to air, a μ 2-oxo diiron(III) compound is formed. Hence, it is the ideal oxidant candidate, which motivated us to study the reactivity of Fe II cCCCC complex with O 2. 12 In contrast, O 2 from air is far less problematic, environmentally benign, readily accessible and exhibits superior atom-efficiency. 8b, 10 While it is generally considered environmentally friendly, as water is formed as the only by-product, 11 risks are associated when working with higher concentrations. 10 Both iron and rhenium containing catalysts utilize H 2O 2 as oxidant. 8 This unique ligand induces unprecedented activity in olefin epoxidation catalysis of the respective Fe III complex (TOF up to 180 000 h −1 without additives TOF up to 415 000 h −1 in presence of strong Lewis acids), 8b, 9 considerably exceeding that of the optimized homogeneous benchmark system methyltrioxorhenium (MTO, TOF up to 40 000 h −1). Recently, we reported a heme-analogous iron(II) complex bearing a methylene-bridged 16-membered macrocyclic tetra-NHC ligand (cCCCC Figure 1, center) and its iron(III) derivative. Bottom right: formation of 4+ by activation of O 2 at −35 ☌ and re-release of O 2 at r.t. Reaction conditions: a: 75 ☌, 18 h, (MeCN) b: r.t., 2 h, (MeCN) c: r.t., 30 min, (MeCN) (Supporting Information for experimental details). Synthesis of cobalt tetra-NHC 1 activation of O 2 from air resulting in formation of 2 via superoxo intermediate 4 e nd–on reaction of μ 2-peroxo complex 2 with HOAc resulting in formation of 3 and H 2O 2. For this purpose, a suitable ligand capable of stabilizing high valent species occurring during this process is vital. 7 Ideally, O 2 activation should proceed at room temperature to provide more applicable and sustainable conditions. 7 Although those results suggest a catalytic application to make use of air, the low stability of the peroxo species at room temperature requires cooling. Further heating to room temperature results in the re-release of O 2 to form the initial Co II complex (Scheme 1, bottom right). 7 Subsequently, it converts to a dicobalt(III) μ 2-peroxo compound when increasing the temperature to −35 ☌. reported a Co II complex bearing the bio-inspired pyridine triazacyclononane (PyTACN) ligand, which forms a Co III superoxo species upon reaction with O 2 at −80 ☌. 4 Starting in the 1970s with coboglobin, a cobalt analogue of hemoglobin, 5 major progress has been made in this research field. 1a, 1b, 2a- 2c, 3Īlthough of lower biological relevance in comparison to copper and iron, the study of bio-inspired cobalt complexes towards O 2 activation is of interest due to the higher thermodynamic and kinetic stability of Co III dioxygen adducts formed upon reaction of their corresponding Co II compounds with O 2. 1, 2 The remarkable efficiency of these compounds led to the development of several bio-inspired complexes, which have been applied in oxidation catalysis and studied regarding their ability to activate dioxygen, preferably from air at ambient conditions. 1 Active intermediates primarily include superoxo, (hydro)peroxo and oxo species. The activation of molecular oxygen by iron- and copper- containing metalloproteins plays a vital role in biological processes, comprising oxygen transport and enzymatic oxidation reactions, amongst others. ![]()
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