Reverse Catalase Reaction: Dioxygen Activation via Two-Electron Transfer from Hydroxide to Dioxygen Mediated By a Manganese(III) Salen Complex

Although atmospheric dioxygen is regarded as the most ideal oxidant, O₂ activation for use in oxygenation reactions intrinsically requires a costly sacrificial reductant. The present study investigated the use of aqueous alkaline solution for O₂ activation. A manganese­(III) salen complex, Mn^III(salen)­(Cl), in toluene reacts with aqueous KOH solution under aerobic conditions, which yields a di-μ-oxo dimanganese­(IV) salen complex, [Mn^IV(salen)]₂(μ-O)₂. The ¹⁸O isotope experiments show that ¹⁸O₂ is indeed activated to give [Mn^IV(salen)]₂(μ-¹⁸O)₂ via a peroxide intermediate. Interestingly, the ¹⁸OH^– ion in H₂¹⁸O was also incorporated to yield [Mn^IV(salen)]₂(μ-¹⁸O)₂, which implies that a peroxide species is also generated from ¹⁸OH^–. The addition of benzyl alcohol as a stoichiometric reductant selectively inhibits the ¹⁸O incorporation from ¹⁸OH^–, indicating that the reaction of Mn^III(salen)­(Cl) with OH^– supplies the electrons for O₂ reduction. The conversion of both O₂ and OH^– to a peroxide species is exactly the reverse of a catalase-like reaction, which has a great potential as the most efficient O₂ activation. Mechanistic investigations revealed that the reaction of Mn^III(salen)­(Cl) with OH^– generates a transient species with strong reducing ability, which effects the reduction of O₂ by means of a manganese­(II) intermediate

Drastic Redox Shift and Electronic Structural Changes of a Manganese(III)-Salen Oxidation Catalyst upon Reaction with Hydroxide and Cyanide Ion

Flexible redox properties of a metal complex are important for redox catalysis. The present study shows that the reaction of a manganese­(III) salen complex, which is a well-known oxidation catalyst, with hydroxide ion gives a transient manganese­(III) species with drastically lowered redox potential, where the redox difference is −1.21 V. The reaction with cyanide ion gives a stable manganese­(III) species with almost the same spectroscopic and redox properties, which was characterized as an anionic [Mn^III(salen)­(CN)₂]⁻ of low-spin <i>S</i> = 1 state, in contrast to the starting Mn^III(salen)­(OTf) having usual high-spin <i>S</i> = 2 manganese­(III). The present study has thus clarified that the drastic redox shift comes from an anionic six-coordinate [Mn^III(salen)­(X)₂]⁻ species where X is either OH^– or CN^–. Resonance Raman measurements show that the stretching band of the imino group shifts from 1620 to 1597 cm^–1 upon conversion from Mn^III(salen)­(OTf) to [Mn^III(salen)­(CN)₂]⁻, indicative of lowered CN double bond character for [Mn^III(salen)­(CN)₂]⁻. The observed deformation of a salen ligand is a clear indication of an increased electron population on the imino π*-orbital upon formation of low-spin manganese­(III). It was proposed that the electronic structure of [Mn^III(salen)­(CN)₂]⁻ may contain only limited contribution from valence tautomeric [Mn^IV(salen^– •)­(CN)₂]⁻, in which the imino group of a salen ligand is reduced by one-electron via intramolecular electron transfer from low-spin manganese­(III). The present study has clarified an unexpected new finding that a salen ligand works as a reservoir for negative charge to stabilize low-spin manganese­(III)

Unique Ligand-Radical Character of an Activated Cobalt Salen Catalyst That Is Generated by Aerobic Oxidation of a Cobalt(II) Salen Complex

The Co­(salen)­(X) complex, where salen is chiral <i>N</i>,<i>N</i>′-bis­(3,5-di-<i>tert</i>-butylsalicylidene)-1,2-cyclohexanediamine and X is an external axial ligand, has been widely utilized as a versatile catalyst. The Co­(salen)­(X) complex is a stable solid that has been conventionally described as a Co^III(salen)­(X) complex. Recent theoretical calculations raised a new proposal that the Co­(salen)­(H₂O)­(SbF₆) complex contains appreciable contribution from a Co^II(salen^•+) electronic structure (Kochem, A.; Kanso, H.; Baptiste, B.; Arora, H.; Philouze, C.; Jarjayes, O.; Vezin, H.; Luneau, D.; Orio, M.; Thomas, F. <i>Inorg. Chem.</i> <b>2012</b>, <i>51</i>, 10557–10571), while other theoretical calculations for Co­(salen)­(Cl) indicated a triplet Co^III(salen) electronic structure (Kemper, S.; Hrobárik, P.; Kaupp, M.; Schlörer, N. E. <i>J. Am. Chem. Soc.</i> <b>2009</b>, <i>131</i>, 4172–4173). However, there have been no experimental data to evaluate these theoretical proposals. We herein report key experimental data on the electronic structure of the Co­(salen)­(X) complex (X = CF₃SO₃^–, SbF₆^–, and <i>p</i>-MeC₆H₄SO₃^–). The X-ray crystallography shows that Co­(salen)­(OTf) has a square-planar N₂O₂ equatorial coordination sphere with OTf as an elongated external axial ligand. Magnetic susceptibility data indicate that Co­(salen)­(OTf) complexes belong to the <i>S</i> = 1 spin system. ¹H NMR measurements provide convincing evidence for the Co^II(salen^•+)­(X) character, which is estimated to be about 40% in addition to 60% Co^III(salen)­(X) character. The CH₂Cl₂ solution of Co­(salen)­(X) shows an intense near-IR absorption, which is assigned as overlapped transitions from a ligand-to-metal charge transfer in Co^III(salen)­(X) and a ligand-to-ligand charge transfer in Co^II(salen^•+)­(X). The present experimental study establishes that the electronic structure of Co­(salen)­(X) contains both Co^II(salen^•+)­(X) and Co^III(salen)­(X) character

Unique Ligand-Radical Character of an Activated Cobalt Salen Catalyst That Is Generated by Aerobic Oxidation of a Cobalt(II) Salen Complex

The Co­(salen)­(X) complex, where salen is chiral <i>N</i>,<i>N</i>′-bis­(3,5-di-<i>tert</i>-butylsalicylidene)-1,2-cyclohexanediamine and X is an external axial ligand, has been widely utilized as a versatile catalyst. The Co­(salen)­(X) complex is a stable solid that has been conventionally described as a Co^III(salen)­(X) complex. Recent theoretical calculations raised a new proposal that the Co­(salen)­(H₂O)­(SbF₆) complex contains appreciable contribution from a Co^II(salen^•+) electronic structure (Kochem, A.; Kanso, H.; Baptiste, B.; Arora, H.; Philouze, C.; Jarjayes, O.; Vezin, H.; Luneau, D.; Orio, M.; Thomas, F. <i>Inorg. Chem.</i> <b>2012</b>, <i>51</i>, 10557–10571), while other theoretical calculations for Co­(salen)­(Cl) indicated a triplet Co^III(salen) electronic structure (Kemper, S.; Hrobárik, P.; Kaupp, M.; Schlörer, N. E. <i>J. Am. Chem. Soc.</i> <b>2009</b>, <i>131</i>, 4172–4173). However, there have been no experimental data to evaluate these theoretical proposals. We herein report key experimental data on the electronic structure of the Co­(salen)­(X) complex (X = CF₃SO₃^–, SbF₆^–, and <i>p</i>-MeC₆H₄SO₃^–). The X-ray crystallography shows that Co­(salen)­(OTf) has a square-planar N₂O₂ equatorial coordination sphere with OTf as an elongated external axial ligand. Magnetic susceptibility data indicate that Co­(salen)­(OTf) complexes belong to the <i>S</i> = 1 spin system. ¹H NMR measurements provide convincing evidence for the Co^II(salen^•+)­(X) character, which is estimated to be about 40% in addition to 60% Co^III(salen)­(X) character. The CH₂Cl₂ solution of Co­(salen)­(X) shows an intense near-IR absorption, which is assigned as overlapped transitions from a ligand-to-metal charge transfer in Co^III(salen)­(X) and a ligand-to-ligand charge transfer in Co^II(salen^•+)­(X). The present experimental study establishes that the electronic structure of Co­(salen)­(X) contains both Co^II(salen^•+)­(X) and Co^III(salen)­(X) character

Manganese Porphyrin Catalyzed Cycloisomerization of Enynes

Cycloisomerization of 1,6-enynes to five- or six-membered ring systems is successfully carried out in the presence of a cationic manganese(III) catalyst. The use of a structurally rigid tetradentate porphyrin as the equatorial ligand and a weakly coordinating axial ligand is the key to bringing out the catalytic reactivity of manganese for the reaction. The axial ligand of the catalyst has a marked effect on the product and selectively aids the formation of five- or six-membered cyclic products

Manganese Porphyrin Catalyzed Cycloisomerization of Enynes

Cycloisomerization of 1,6-enynes to five- or six-membered ring systems is successfully carried out in the presence of a cationic manganese(III) catalyst. The use of a structurally rigid tetradentate porphyrin as the equatorial ligand and a weakly coordinating axial ligand is the key to bringing out the catalytic reactivity of manganese for the reaction. The axial ligand of the catalyst has a marked effect on the product and selectively aids the formation of five- or six-membered cyclic products

Manganese Porphyrin Catalyzed Cycloisomerization of Enynes

Cycloisomerization of 1,6-enynes to five- or six-membered ring systems is successfully carried out in the presence of a cationic manganese(III) catalyst. The use of a structurally rigid tetradentate porphyrin as the equatorial ligand and a weakly coordinating axial ligand is the key to bringing out the catalytic reactivity of manganese for the reaction. The axial ligand of the catalyst has a marked effect on the product and selectively aids the formation of five- or six-membered cyclic products

Cobalt(III) Porphyrin Catalyzed Aza-Diels–Alder Reaction

An efficient protocol for the aza-Diels–Alder reaction of electron-deficient 1,3-dienes with unactivated imines in the presence of a cationic cobalt(III) porphyrin complex was developed. The transformation proceeded smoothly to afford the desired piperidine scaffold within 2 h at ambient temperature. Highly chemoselective cycloaddition of imines with dienes in the presence of a variety of carbonyl compounds was also demonstrated

Synthesis of Quinolones by Nickel-Catalyzed Cycloaddition via Elimination of Nitrile

Substituted quinolones were efficiently synthesized via the nickel-catalyzed cycloaddition of <i>o</i>-cyanophenylbenzamide derivatives with alkynes. The reaction involves elimination of a nitrile group by cleavage of the two independent aryl–cyano and aryl–carbonyl C–C bonds of the amides

Cobalt(III) Porphyrin Catalyzed Aza-Diels–Alder Reaction

An efficient protocol for the aza-Diels–Alder reaction of electron-deficient 1,3-dienes with unactivated imines in the presence of a cationic cobalt(III) porphyrin complex was developed. The transformation proceeded smoothly to afford the desired piperidine scaffold within 2 h at ambient temperature. Highly chemoselective cycloaddition of imines with dienes in the presence of a variety of carbonyl compounds was also demonstrated