Critical Factors in Determining the Heterolytic versus Homolytic Bond Cleavage of Terminal Oxidants by Iron(III) Porphyrin Complexes

Heterolytic versus homolytic cleavage of the metal-bound terminal oxidant is the key for determining the nature of reactive intermediates in metalloenzymes and metal catalyzed oxygenation reactions. Here, we study the bond cleavage process of hypochlorite by iron­(III) porphyrin complexes having 4-methoxy-2,6-dimethylphenyl (1), 2,4,6-trimethylphenyl (2), 4-fluoro-2,6-dimethylphenyl (3), 2-chloro-6-methylphenyl (4), 2,6-dichlorophenyl (5), and 2,4,6-trichlorophenyl (6) groups at the meso position. Oxoiron­(IV) porphyrin π-cation radical complexes (CompI) are characterized from the reactions of 1–4 with tetra-n-butylammonium hypochlorite (TBA-OCl) in dichloromethane at −80 °C, while oxoiron­(IV) porphyrin complexes (CompII) are characterized for 5 and 6 under the same conditions. For all of 1–6, we find the formation of an epoxidation product in good yields from the catalytic reactions with TBA-OCl, suggesting heterolytic cleavages of the O–Cl bonds. CompI of 5 and 6 are reduced to the corresponding CompII by both chloride and hypochlorite, while CompI of 1–4 are not. The reduction reactions with hypochlorite are much faster than those with chloride. These results provide a mechanism where the O–Cl bond of the iron-bound hypochlorite is cleaved heterolytically to form CompI for all of 1–6, but the subsequent reduction reaction with remaining hypochlorite affords CompII for 5 and 6. The E(OCl•/OCl–) value is the boundary to discriminate the identity of the final product: CompI or CompII. Thermodynamic analysis based on the redox potential is successfully applied for explaining the bond cleavage processes of the hypochlorite, hydroperoxide, and tert-butyl peroxide complexes

¹³C and ¹⁵N NMR Studies of Iron-Bound Cyanides of Heme Proteins and Related Model Complexes: Sensitive Probe for Detecting Hydrogen-bonding Interactions at the Proximal and Distal Sides

Studies of the 13C and 15N NMR paramagnetic shifts of the iron-bound cyanides in the ferric cyanide forms of various heme proteins containing the proximal histidine and related model complexes are reported. The paramagnetic shifts of the 13C and 15N NMR signals of the iron-bound cyanide are not significantly affected by the substitution of the porphyrin side chains. On the other hand, the paramagnetic shifts of both the 13C and 15N NMR signals decrease with an increase in the donor effect of the proximal ligand, and the 13C NMR signal is more sensitive to a modification of the donor effect of the proximal ligand than the 15N NMR signal. With the tilt of the iron−imidazole bond, the paramagnetic shift of the 13C NMR signal increases, whereas that of the 15N NMR signal decreases. The hydrogen-bonding interaction of the iron-bound cyanide with a solvent decreases the paramagnetic shift of both 13C and 15N NMR signals, and the effect is more pronounced for the 15N NMR signal. Data on the 13C and 15N NMR signals of iron-bound cyanide for various heme proteins are also reported and analyzed in detail. Substantial differences in the 13C and 15N NMR shifts for the heme proteins can be explained on the basis of the results for the model complexes and structures around the heme in the heme proteins. The findings herein show that the paramagnetic shift of the 13C NMR signal of the iron-bound cyanide is a good probe to estimate the donor effect of the proximal imidazole and that the ratio of 15N/13C NMR shifts allows the hydrogen-bonding interaction on the distal side to be estimated

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

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

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

Participation of Electron Transfer Process in Rate-Limiting Step of Aromatic Hydroxylation Reactions by Compound I Models of Heme Enzymes

Hydroxylation reactions of aromatic rings are key reactions in various biological and chemical processes. In spite of their significance, no consensus mechanism has been established. Here we performed Marcus plot analysis for aromatic hydroxylation reactions with oxoiron­(IV) porphyrin π-cation radical complexes (compound I). Although many recent studies support the mechanism involving direct electrophilic attack of compound I, the slopes of the Marcus plots indicate a significant contribution of an electron transfer process in the rate-limiting step, leading us to propose a new reaction mechanism in which the electron transfer process between an aromatic compound and compound I is in equilibrium in a solvent cage and coupled with the subsequent bond formation process