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

One-Electron Oxidation of Electronically Diverse Manganese(III) and Nickel(II) Salen Complexes: Transition from Localized to Delocalized Mixed-Valence Ligand Radicals

Ligand radicals from salen complexes are unique mixed-valence compounds in which a phenoxyl radical is electronically linked to a remote phenolate via a neighboring redox-active metal ion, providing an opportunity to study electron transfer from a phenolate to a phenoxyl radical mediated by a redox-active metal ion as a bridge. We herein synthesize one-electron-oxidized products from electronically diverse manganese(III) salen complexes in which the locus of oxidation is shown to be ligand-centered, not metal-centered, affording manganese(III)–phenoxyl radical species. The key point in the present study is an unambiguous assignment of intervalence charge transfer bands by using nonsymmetrical salen complexes, which enables us to obtain otherwise inaccessible insight into the mixed-valence property. A d4 high-spin manganese(III) ion forms a Robin–Day class II mixed-valence system, in which electron transfer is occurring between the localized phenoxyl radical and the phenolate. This is in clear contrast to a d8 low-spin nickel(II) ion with the same salen ligand, which induces a delocalized radical (Robin–Day class III) over the two phenolate rings, as previously reported by others. The present findings point to a fascinating possibility that electron transfer could be drastically modulated by exchanging the metal ion that bridges the two redox centers

Chiral Distortion in a Mn^IV(salen)(N₃)₂ Derived from Jacobsen’s Catalyst as a Possible Conformation Model for Its Enantioselective Reactions

The MnIV(salen)(N3)2 complex (3) from Jacobsen’s catalyst is synthesized, and the X-ray crystal structures of 3 as well as the starting MnIII(salen)(N3)(CH3OH) complex (2) are determined in order to investigate the conformation of the high-valent MnIV(salen) molecule in comparison with that of MnIII(salen). The asymmetric unit of the crystal of 3 contains four complexes, all of which adopt a nonplanar stepped conformation effectively distorted by the chirality of the diimine bridge. The asymmetric unit of 2 also contains four complexes. Two of them show a stepped conformation of a lesser degree, but the other two adopt a bowl-shaped conformation. Comparison of the structural parameters shows that the Mn center in 3 is coordinated from both sides by two external axial N3 ligands with significantly shorter bond lengths, which could induce greater preference for the stepped conformation in 3. The CH3CN solution of 3 shows circular dichroism with a significantly strong band at 275 nm as compared to 2, suggesting that 3 may adopt a more chirally distorted conformation also in solution. The circular dichroism spectrum of 3 is slightly altered with isodichroic points from 298 to 253 K and shows no further change at temperatures lower than 253 K, suggesting that the solution of 3 contains an equilibrium between two conformers, where a low-energy conformer with more chiral distortion is predominantly favored even at room temperature. Complexes 2 and 3 are thoroughly characterized using various techniques including cyclic voltammetry, magnetic susceptibility, UV−vis, electron paramagnetic resonance, 1H NMR, infrared spectroscopy, and electrospray ionization mass spectrometry

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

Chiral Distortion in a Mn^IV(salen)(N₃)₂ Derived from Jacobsen’s Catalyst as a Possible Conformation Model for Its Enantioselective Reactions

The MnIV(salen)(N3)2 complex (3) from Jacobsen’s catalyst is synthesized, and the X-ray crystal structures of 3 as well as the starting MnIII(salen)(N3)(CH3OH) complex (2) are determined in order to investigate the conformation of the high-valent MnIV(salen) molecule in comparison with that of MnIII(salen). The asymmetric unit of the crystal of 3 contains four complexes, all of which adopt a nonplanar stepped conformation effectively distorted by the chirality of the diimine bridge. The asymmetric unit of 2 also contains four complexes. Two of them show a stepped conformation of a lesser degree, but the other two adopt a bowl-shaped conformation. Comparison of the structural parameters shows that the Mn center in 3 is coordinated from both sides by two external axial N3 ligands with significantly shorter bond lengths, which could induce greater preference for the stepped conformation in 3. The CH3CN solution of 3 shows circular dichroism with a significantly strong band at 275 nm as compared to 2, suggesting that 3 may adopt a more chirally distorted conformation also in solution. The circular dichroism spectrum of 3 is slightly altered with isodichroic points from 298 to 253 K and shows no further change at temperatures lower than 253 K, suggesting that the solution of 3 contains an equilibrium between two conformers, where a low-energy conformer with more chiral distortion is predominantly favored even at room temperature. Complexes 2 and 3 are thoroughly characterized using various techniques including cyclic voltammetry, magnetic susceptibility, UV−vis, electron paramagnetic resonance, 1H NMR, infrared spectroscopy, and electrospray ionization mass spectrometry

Chiral Distortion in a Mn^IV(salen)(N₃)₂ Derived from Jacobsen’s Catalyst as a Possible Conformation Model for Its Enantioselective Reactions

The MnIV(salen)(N3)2 complex (3) from Jacobsen’s catalyst is synthesized, and the X-ray crystal structures of 3 as well as the starting MnIII(salen)(N3)(CH3OH) complex (2) are determined in order to investigate the conformation of the high-valent MnIV(salen) molecule in comparison with that of MnIII(salen). The asymmetric unit of the crystal of 3 contains four complexes, all of which adopt a nonplanar stepped conformation effectively distorted by the chirality of the diimine bridge. The asymmetric unit of 2 also contains four complexes. Two of them show a stepped conformation of a lesser degree, but the other two adopt a bowl-shaped conformation. Comparison of the structural parameters shows that the Mn center in 3 is coordinated from both sides by two external axial N3 ligands with significantly shorter bond lengths, which could induce greater preference for the stepped conformation in 3. The CH3CN solution of 3 shows circular dichroism with a significantly strong band at 275 nm as compared to 2, suggesting that 3 may adopt a more chirally distorted conformation also in solution. The circular dichroism spectrum of 3 is slightly altered with isodichroic points from 298 to 253 K and shows no further change at temperatures lower than 253 K, suggesting that the solution of 3 contains an equilibrium between two conformers, where a low-energy conformer with more chiral distortion is predominantly favored even at room temperature. Complexes 2 and 3 are thoroughly characterized using various techniques including cyclic voltammetry, magnetic susceptibility, UV−vis, electron paramagnetic resonance, 1H NMR, infrared spectroscopy, and electrospray ionization mass spectrometry

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

Dehydrogenative Diels–Alder Reaction

The dehydrogenative cycloaddition of dieneynes, which possess a diene in the form of a styrene moiety and a dienophile in the form of an alkyne moiety, produces naphthalene derivatives when heated. It was found that a key requirement of this process is the presence of a silyl group attached to the alkyne moiety, which forces a dehydrogenation reaction to occur

Dehydrogenative Diels–Alder Reaction

The dehydrogenative cycloaddition of dieneynes, which possess a diene in the form of a styrene moiety and a dienophile in the form of an alkyne moiety, produces naphthalene derivatives when heated. It was found that a key requirement of this process is the presence of a silyl group attached to the alkyne moiety, which forces a dehydrogenation reaction to occur