Abstract:Catalytic oxygenation of t-butylphenols has been examined with the cobalt(III) complexes with N,N′-disalicylideneethylenediamine (H2salen), K[Co(salen)CO3], Na[Co(salen)(N3)2], K[Co(salen)(CN)2], [Co(salen)(py)2]PF6, and [Co(salen)(NH3)2]PF6. The CO3-complex showed a high catalytic activity while the other complexes a low or no catalytic activity. Based on electronic and ESR spectral investigations, it has been shown that the reaction is initiated by the direct oxidation of t-butylphenols by the CO3-complex th… Show more
“…( N , N ‘-bis(salicylidene)ethylenediaminato)cobalt(II), Co(salen), 1 , , is classified as an oxygen carrier and has been used as a catalyst for the preparative oxygenation of phenols, − indoles, , flavonols, amines, , anilines, , and other substrates , with both dioxygen or hydroperoxides as the oxygen source. Some authors compare these oxygen transfer reactions to oxygenation by enzymes of mono-, dioxygenase, or peroxidase type, and in some cases, indeed biomimetic activity of the model complex was found. ,,,, …”
Redox and ligand exchange reactions of the oxygenation catalyst (N,N '-bis(salicylidene)ethylenediaminato)cobalt(II), Co(salen), and its one-electron oxidation product, Co(salen)(+), are investigated in DMF, pyridine, and mixtures of these solvents. Electron transfers and solvent exchange reactions involving three neutral Co(II) and three cationic Co(III) complexes with different axially bound solvent molecules (two DMF, one DMF and one pyridine, or two pyridine molecules) form a three-rung ladder scheme. All formal potentials E(0) and equilibrium constants K in this scheme are determined from electrochemical or spectrophotometric experiments or the construction of thermodynamic cycles. The latter are also used to prove consistency of the results. Values for the E(0) and K are discussed in terms of the Co coordination geometry, solvent effects on the potentials, the thermodynamics of cross reactions, and the distribution of Co(II) and Co(III) species as a function of the solvent composition. Some peculiarities found in the oxygenation of flavonols and indoles are explained.
“…( N , N ‘-bis(salicylidene)ethylenediaminato)cobalt(II), Co(salen), 1 , , is classified as an oxygen carrier and has been used as a catalyst for the preparative oxygenation of phenols, − indoles, , flavonols, amines, , anilines, , and other substrates , with both dioxygen or hydroperoxides as the oxygen source. Some authors compare these oxygen transfer reactions to oxygenation by enzymes of mono-, dioxygenase, or peroxidase type, and in some cases, indeed biomimetic activity of the model complex was found. ,,,, …”
Redox and ligand exchange reactions of the oxygenation catalyst (N,N '-bis(salicylidene)ethylenediaminato)cobalt(II), Co(salen), and its one-electron oxidation product, Co(salen)(+), are investigated in DMF, pyridine, and mixtures of these solvents. Electron transfers and solvent exchange reactions involving three neutral Co(II) and three cationic Co(III) complexes with different axially bound solvent molecules (two DMF, one DMF and one pyridine, or two pyridine molecules) form a three-rung ladder scheme. All formal potentials E(0) and equilibrium constants K in this scheme are determined from electrochemical or spectrophotometric experiments or the construction of thermodynamic cycles. The latter are also used to prove consistency of the results. Values for the E(0) and K are discussed in terms of the Co coordination geometry, solvent effects on the potentials, the thermodynamics of cross reactions, and the distribution of Co(II) and Co(III) species as a function of the solvent composition. Some peculiarities found in the oxygenation of flavonols and indoles are explained.
“…1) [8,9] is a tetradentate, planar Schiff's base complex which is classified as an oxygen carrier [10]. It has been used as a catalyst for the preparative oxygenation of phenols [11][12][13], indoles [14,15], flavonols [16], amines [17,18], anilines [19,20] and other substrates [11,21] with both dioxygen or hydroperoxides as the oxygen source. It is most likely that the ROOH formed initially reacts with the cobalt catalyst to form the intermediate of cobalt alkylperoxy [22].…”
Co(salen)-type [where salen = di-(salicylal)-ethylenediimine] complexes were shown to be efficient catalysts in the oxidation of 2-phenylpropane (cumene) by dioxygen primarily to 2-phenyl-2-propanol (cumyl alcohol), 2-phenylpropene (a-methylstyrene), and 1-phenylethanone (acetophenone) applying 1 H NMR spectroscopy and gas chromatography-mass spectrometry (GC-MS). The effect of substitution on the ligand was also monitored in both oxygen-absorption and the catalytic reaction. Based on these results, the trend observed for the production of a-methylstyrene and cumyl alcohol were parallel to dioxygen uptake by the catalyst in neat cumene, while acetophenone productions obeyed a non-linear trend. The best selectivity for the reaction in terms of acetophenone production was observed for the complex with the least oxygen-absorption feature. The intermediate of the reaction, LCo(III)-OOcumyl (where L = salen) complex, was synthesized and characterized by IR, 1 H NMR spectroscopy as well as elemental analysis, and its reactivity in the present catalytic reaction was also studied. A series of experiments were performed to propose a mechanism for the reaction on the basis of the product distributions in the reaction mixture.
“…This compound had been obtained previously, but without a detailed structure analysis given. 3 The tetradentate N2O2 Schiff-base ligand (salen) was prepared by refluxing a solution of 1 mmol of salicylaldehyde and 1 mmol of ethylenediamine in 10 mL of methanol for 1 h. Na[Co(salen)(N3)2] was synthesized by adding 2 mmol of Co(NO3)2·6H2O in 20 mL of a methanol:acetone (1:1 v/v) mixture to the ligand solution prepared above. The resulting solution was stirred for 1 h. Then a 2 mL aqueous solution of NaN3 (8 mmol) was added slowly under stirring at room temperature, and stirring was continued for 5 min.…”
Cobalt(III) Schiff-base complexes have been used as catalysts for oxidation reactions, 1 and as model substances for cobalamin (B12) coenzymes. 2 We report herein on the synthesis and crystal structure of a cobalt(III) salen azido complex, viz. Na[Co(salen)(N3)2] (Fig. 1). This compound had been obtained previously, but without a detailed structure analysis given. 3 The tetradentate N2O2 Schiff-base ligand (salen) was prepared by refluxing a solution of 1 mmol of salicylaldehyde and 1 mmol of ethylenediamine in 10 mL of methanol for 1 h. Na[Co(salen)(N3)2] was synthesized by adding 2 mmol of Co(NO3)2·6H2O in 20 mL of a methanol:acetone (1:1 v/v) mixture to the ligand solution prepared above. The resulting solution was stirred for 1 h. Then a 2 mL aqueous solution of NaN3 (8 mmol) was added slowly under stirring at room temperature, and stirring was continued for 5 min. The resulting dark-brown solution was kept undisturbed at room temperature. Under these conditions Co(II) was apparently oxidized to Co(III). Brown single crystals of the title compound were obtained by slow evaporation.Intensity data of a plate-like crystal with dimensions of 0.35 × 0.10 × 0.10 mm 3 were collected on a SMART APEX 1 diffractometer (graphite monochromator, Mo Kα radiation) at 295 K. H atoms were placed geometrically (d(C-H) = 0.93 Å for aromatic C atoms; d(C-H) = 0.97 Å for methylen C atoms) and were refined with Uiso(H) = 1.2Ueq(C). Details of the data collection and refinement parameters are listed in Table 1. The final atomic coordinates and selected interatomic distances are gathered in Tables 2 and 3, respectively.The asymmetric unit of Na[Co(salen)(N3)2] contains one cobalt(III) cation, two azido ligands, one salen ligand, and one Na counter cation. In this complex the cobalt(III) ion exhibits a distorted octahedral coordination by two N atoms and two O atoms from the tetradentate salen ligand, and by terminal N atoms from two different azido anions (Fig. 2).The Na counter cation displays a distorted 5-coordination by three N atoms of three azido bridges, and by two O atoms of the salen ligand (Fig. 3)
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