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13 pages, 1947 KiB

Open AccessArticle

Preparations of trans- and cis-μ-1,2-Peroxodiiron(III) Complexes

by Yuji Kajita, Masaki Kubo, Hidekazu Arii, Shinya Ishikawa, Yamato Saito, Yuko Wasada-Tsutsui, Yasuhiro Funahashi, Tomohiro Ozawa and Hideki Masuda

Molecules 2024, 29(1), 205; https://doi.org/10.3390/molecules29010205 - 29 Dec 2023

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Abstract

The iron(II) complex with cis,cis-1,3,5-tris(benzylamino)cyclohexane (Bn₃CY) (1) has been synthesized and characterized, which reacted with dioxygen to form the peroxo complex 2 in acetone at −60 °C. On the basis of spectroscopic measurements for 2, it was [...] Read more.

The iron(II) complex with cis,cis-1,3,5-tris(benzylamino)cyclohexane (Bn₃CY) (1) has been synthesized and characterized, which reacted with dioxygen to form the peroxo complex 2 in acetone at −60 °C. On the basis of spectroscopic measurements for 2, it was confirmed that the peroxo complex 2 has a trans-μ-1,2 fashion. Additionally, the peroxo complex 2 was reacted with benzoate anion as a bridging agent to give a peroxo complex 3. The results of resonance Raman and ¹H-NMR studies supported that the peroxo complex 3 is a cis-μ-1,2-peroxodiiron(III) complex. These spectral features were interpreted by using DFT calculations. Full article

(This article belongs to the Special Issue Design Strategies for Metal Complexes that Activate Bio-Related Small Molecules)

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18 pages, 2847 KiB

Open AccessArticle

Coordinatively Unsaturated Nickel Nitroxyl Complex: Structure, Physicochemical Properties, and Reactivity toward Dioxygen

by Kiyoshi Fujisawa, Taisei Kataoka, Kohei Terashima, Haruka Kurihara, Felipe de Santis Gonçalves and Nicolai Lehnert

Molecules 2023, 28(17), 6206; https://doi.org/10.3390/molecules28176206 - 23 Aug 2023

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Abstract

For its important roles in biology, nitrogen monoxide (·NO) has become one of the most studied and fascinating molecules in chemistry. ·NO itself acts as a “noninnocent” or “redox active” ligand to transition metal ions to give metal–NO (M–NO) complexes. Because of this [...] Read more.

For its important roles in biology, nitrogen monoxide (·NO) has become one of the most studied and fascinating molecules in chemistry. ·NO itself acts as a “noninnocent” or “redox active” ligand to transition metal ions to give metal–NO (M–NO) complexes. Because of this uncertainty due to redox chemistry, the real description of the electronic structure of the M–NO unit requires extensive spectroscopic and theoretical studies. We previously reported the Ni–NO complex with a hindered N3 type ligand [Ni(NO)(L3)] (L3⁻ denotes hydrotris(3-tertiary butyl-5-isopropyl-1-pyrazolyl)borate anion), which contains a high-spin (hs) nickel(II) center and a coordinated ³NO⁻. This complex is very stable toward dioxygen due to steric protection of the nickel(II) center. Here, we report the dioxygen reactivity of a new Ni–NO complex, [Ni(NO)(I)(L1″)], with a less hindered N2 type bis(pyrazolyl)methane ligand, which creates a coordinatively unsaturated ligand environment about the nickel center. Here, L1″ denotes bis(3,5-diisopropyl-1-pyrazolyl)methane. This complex is also described as a hs-nickel(II) center with a bound ³NO⁻, based on spectroscopic and theoretical studies. Unexpectedly, the reaction of [Ni(NO)(I)(L1″)] with O₂ yielded [Ni(κ²-O₂N)(L1″)₂](I₃), with the oxidation of both ³NO⁻ and the I⁻ ion to yield NO₂⁻ and I₃⁻. Both complexes were characterized by X-ray crystallography, IR, and UV–Vis spectroscopy and theoretical calculations. Full article

(This article belongs to the Special Issue Design Strategies for Metal Complexes that Activate Bio-Related Small Molecules)

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12 pages, 2872 KiB

Open AccessArticle

Selective C–H Bond Cleavage with a High-Spin Fe^IV–Oxido Complex

by Chen Sun, Jennifer L. Jaimes, Alec H. Follmer, Joseph W. Ziller and Andrew S. Borovik

Molecules 2023, 28(12), 4755; https://doi.org/10.3390/molecules28124755 - 14 Jun 2023

Cited by 1 | Viewed by 1091

Abstract

Non-heme Fe monooxygenases activate C–H bonds using intermediates with high-spin Fe^IV–oxido centers. To mimic these sites, a new tripodal ligand [pop]³⁻ was prepared that contains three phosphoryl amido groups that are capable of stabilizing metal centers in high oxidation states. [...] Read more.

Non-heme Fe monooxygenases activate C–H bonds using intermediates with high-spin Fe^IV–oxido centers. To mimic these sites, a new tripodal ligand [pop]³⁻ was prepared that contains three phosphoryl amido groups that are capable of stabilizing metal centers in high oxidation states. The ligand was used to generate [Fe^IVpop(O)]⁻, a new Fe^IV–oxido complex with an S = 2 spin ground state. Spectroscopic measurements, which included low-temperature absorption and electron paramagnetic resonance spectroscopy, supported the assignment of a high-spin Fe^IV center. The complex showed reactivity with benzyl alcohol as the external substrate but not with related compounds (e.g., ethyl benzene and benzyl methyl ether), suggesting the possibility that hydrogen bonding interaction(s) between the substrate and [Fe^IVpop(O)]⁻ was necessary for reactivity. These results exemplify the potential role of the secondary coordination sphere in metal-mediated processes. Full article

(This article belongs to the Special Issue Design Strategies for Metal Complexes that Activate Bio-Related Small Molecules)

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12 pages, 1050 KiB

Open AccessArticle

Electrochemical Epoxidation Catalyzed by Manganese Salen Complex and Carbonate with Boron-Doped Diamond Electrode

by Pijush Kanti Roy, Keisuke Amanai, Ryosuke Shimizu, Masahito Kodera, Takuya Kurahashi, Kenji Kitayama and Yutaka Hitomi

Molecules 2023, 28(4), 1797; https://doi.org/10.3390/molecules28041797 - 14 Feb 2023

Cited by 1 | Viewed by 1966

Abstract

Epoxides are essential precursors for epoxy resins and other chemical products. In this study, we investigated whether electrochemically oxidizing carbonate ions could produce percarbonate to promote an epoxidation reaction in the presence of appropriate metal catalysts, although Tanaka and co-workers had already completed [...] Read more.

Epoxides are essential precursors for epoxy resins and other chemical products. In this study, we investigated whether electrochemically oxidizing carbonate ions could produce percarbonate to promote an epoxidation reaction in the presence of appropriate metal catalysts, although Tanaka and co-workers had already completed a separate study in which the electrochemical oxidation of chloride ions was used to produce hypochlorite ions for electrochemical epoxidation. We found that epoxides could be obtained from styrene derivatives in the presence of metal complexes, including manganese(III) and oxidovanadium(IV) porphyrin complexes and manganese salen complexes, using a boron-doped diamond as the anode. After considering various complexes as potential catalysts, we found that manganese salen complexes showed better performance in terms of epoxide yield. Furthermore, the substituent effect of the manganese salen complex was also investigated, and it was found that the highest epoxide yields were obtained when Jacobsen’s catalyst was used. Although there is still room for improving the yields, this study has shown that the in situ electrochemical generation of percarbonate ions is a promising method for the electrochemical epoxidation of alkenes. Full article

(This article belongs to the Special Issue Design Strategies for Metal Complexes that Activate Bio-Related Small Molecules)

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12 pages, 18165 KiB

Open AccessArticle

Catalytic Ammonia Synthesis Mediated by Molybdenum Complexes with PN³P Pincer Ligands: Influence of P/N Substituents and Molecular Mechanism

by Katja Bedbur, Nadja Stucke, Lina Liehrs, Jan Krahmer and Felix Tuczek

Molecules 2022, 27(22), 7843; https://doi.org/10.3390/molecules27227843 - 14 Nov 2022

Cited by 4 | Viewed by 1513

Abstract

Three molybdenum trihalogenido complexes supported by different PN³P pincer ligands were synthesized and investigated regarding their activity towards catalytic N₂-to-NH₃ conversion. The highest yields were obtained with the H-PN³P^tBu ligand. The corresponding Mo(V)-nitrido complex also [...] Read more.

Three molybdenum trihalogenido complexes supported by different PN³P pincer ligands were synthesized and investigated regarding their activity towards catalytic N₂-to-NH₃ conversion. The highest yields were obtained with the H-PN³P^tBu ligand. The corresponding Mo(V)-nitrido complex also shows good catalytic activity. Experiments regarding the formation of the analogous Mo(IV)-nitrido complex lead to the conclusion that the mechanism of catalytic ammonia formation mediated by the title systems does not involve N-N cleavage of a dinuclear Mo-dinitrogen complex, but follows the classic Chatt cycle. Full article

(This article belongs to the Special Issue Design Strategies for Metal Complexes that Activate Bio-Related Small Molecules)

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11 pages, 5115 KiB

Open AccessArticle

Direct Hydroxylation of Benzene with Hydrogen Peroxide Using Fe Complexes Encapsulated into Mesoporous Y-Type Zeolite

by Syuhei Yamaguchi, Yuito Ishida, Hitomu Koga and Hidenori Yahiro

Molecules 2022, 27(20), 6852; https://doi.org/10.3390/molecules27206852 - 13 Oct 2022

Cited by 1 | Viewed by 1198

Abstract

Mesoporous Y-type zeolite (MYZ) was prepared by an acid and base treatment of commercial Y-type zeolite (YZ). The mesopore volume of MYZ was six times higher than that of YZ. [Fe(terpy)₂]²⁺ complexes encapsulated into MYZ and YZ with different Fe [...] Read more.

Mesoporous Y-type zeolite (MYZ) was prepared by an acid and base treatment of commercial Y-type zeolite (YZ). The mesopore volume of MYZ was six times higher than that of YZ. [Fe(terpy)₂]²⁺ complexes encapsulated into MYZ and YZ with different Fe contents (Fe(X)L-MYZ and Fe(X)L-YZ; X is the amount of Fe) were prepared and characterized. The oxidation of benzene with H₂O₂ using Fe(X)L-MYZ and Fe(X)L-YZ catalysts was carried out; phenol was selectively produced with all Fe-containing zeolite catalysts. As a result, the oxidation activity of benzene increased with increasing iron complex content in the Fe(X)L-MYZ and Fe(X)L-YZ catalysts. The oxidation activity of benzene using Fe(X)L-MYZ catalyst was higher than that using Fe(X)L-YZ. Furthermore, adding mesopores increased the catalytic activity of the iron complex as the iron complex content increased. Full article

(This article belongs to the Special Issue Design Strategies for Metal Complexes that Activate Bio-Related Small Molecules)

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10 pages, 2560 KiB

Open AccessArticle

The Conversion of Superoxide to Hydroperoxide on Cobalt(III) Depends on the Structural and Electronic Properties of Azole-Based Chelating Ligands

by Toshiki Nishiura, Takehiro Ohta, Takashi Ogura, Jun Nakazawa, Masaya Okamura and Shiro Hikichi

Molecules 2022, 27(19), 6416; https://doi.org/10.3390/molecules27196416 - 28 Sep 2022

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Abstract

Conversion from superoxide (O₂⁻) to hydroperoxide (OOH⁻) on the metal center of oxygenases and oxidases is recognized to be a key step to generating an active species for substrate oxidation. In this study, reactivity of cobalt(III)-superoxido complexes supported [...] Read more.

Conversion from superoxide (O₂⁻) to hydroperoxide (OOH⁻) on the metal center of oxygenases and oxidases is recognized to be a key step to generating an active species for substrate oxidation. In this study, reactivity of cobalt(III)-superoxido complexes supported by facially-capping tridentate tris(3,5-dimethyl-4-X-pyrazolyl)hydroborate ([HB(pz^Me2,X)₃]⁻; Tp^Me2,X) and bidentate bis(1-methyl-imidazolyl)methylborate ([B(Im^N^-Me)₂Me(Y)]⁻; L^Y) ligands toward H-atom donating reagent (2-hydroxy-2-azaadamantane; AZADOL) has been explored. The oxygenation of the cobalt(II) precursors give the corresponding cobalt(III)-superoxido complexes, and the following reaction with AZADOL yield the hydroperoxido species as has been characterized by spectroscopy (UV-vis, resonance Raman, EPR). The reaction of the cobalt(III)-superoxido species and a reducing reagent ([Co^II(C₅H₅)₂]; cobaltocene) with proton (trifluoroacetic acid; TFA) also yields the corresponding cobalt(III)-hydroperoxido species. Kinetic analyses of the formation rates of the cobalt(III)-hydroperoxido complexes reveal that second-order rate constants depend on the structural and electronic properties of the cobalt-supporting chelating ligands. An electron-withdrawing ligand opposite to the superoxide accelerates the hydrogen atom transfer (HAT) reaction from AZADOL due to an increase in the electrophilicity of the superoxide ligand. Shielding the cobalt center by the alkyl group on the boron center of bis(imidazolyl)borate ligands hinders the approaching of AZADOL to the superoxide, although the steric effect is insignificant. Full article

(This article belongs to the Special Issue Design Strategies for Metal Complexes that Activate Bio-Related Small Molecules)

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15 pages, 2451 KiB

Open AccessArticle

The Steric Effect in Preparations of Vanadium(II)/(III) Dinitrogen Complexes of Triamidoamine Ligands Bearing Bulky Substituents

by Yoshiaki Kokubo, Itsuki Igarashi, Kenichi Nakao, Wataru Hachiya, Shinichi Kugimiya, Tomohiro Ozawa, Hideki Masuda and Yuji Kajita

Molecules 2022, 27(18), 5864; https://doi.org/10.3390/molecules27185864 - 09 Sep 2022

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Abstract

The reactions of newly designed lithiated triamidoamines Li₃L^R (R = iPr, Pen, and Cy₂) with VCl₃(THF)₃ under N₂ yielded dinitrogen–divanadium complexes with a μ-N₂ between vanadium atoms [{V(L^R)}₂ [...] Read more.

The reactions of newly designed lithiated triamidoamines Li₃L^R (R = iPr, Pen, and Cy₂) with VCl₃(THF)₃ under N₂ yielded dinitrogen–divanadium complexes with a μ-N₂ between vanadium atoms [{V(L^R)}₂(μ-N₂)] (R = iPr (1) and Pen (2)) for the former two, while not dinitrogen–divanadium complexes but a mononuclear vanadium complex with a vacant site, [V(L^Cy2)] (R = Cy₂ (3)), were obtained for the third ligand. The V–N_N2 and N–N distances were 1.7655(18) and 1.219(4) Å for 1 and 1.7935(14) and 1.226(3) Å for 2, respectively. The ν(¹⁴N–¹⁴N) stretching vibrations of 1 and 2, as measured using resonance Raman spectroscopy, were detected at 1436 and 1412 cm^–1, respectively. Complex 3 reacted with potassium metal in the presence of 18-crown-6-ether under N₂ to give a hetero-dinuclear vanadium complex with μ-N₂ between vanadium and potassium, [VK(L^Cy2)(μ-N₂)(18-crown-6)] (4). The N–N distance and ν(¹⁴N–¹⁴N) stretching for 4 were 1.152(3) Å and 1818 cm⁻¹, respectively, suggesting that 4 is more activated than complexes 1 and 2. The complexes 1, 2, 3, and 4 reacted with HOTf and K[C₁₀H₈] to give NH₃ and N₂H₄. The yields of NH₃ and N₂H₄ (per V atom) were 47 and 11% for 1, 38 and 16% for 2, 77 and 7% for 3, and 80 and 5% for 4, respectively, and 3 and 4, which have a ligand L^Cy2, showed higher reactivity than 1 and 2. Full article

(This article belongs to the Special Issue Design Strategies for Metal Complexes that Activate Bio-Related Small Molecules)

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10 pages, 1232 KiB

Open AccessArticle

Lewis Acid-Induced Dinitrogen Cleavage in an Anionic Side-on End-on Bound Dinitrogen Diniobium Hydride Complex

by Naofumi Suzuki, Yutaka Ishida and Hiroyuki Kawaguchi

Molecules 2022, 27(17), 5553; https://doi.org/10.3390/molecules27175553 - 29 Aug 2022

Viewed by 1330

Abstract

The side-on end-on dinitrogen hydride complex [{Na(dme)}₂{(O₃)Nb}₂(μ-η¹:η²-N₂)(μ-H)₂] (3-Na, [O₃]³⁻ = [(3,5-^tBu₂-2-O-C₆H₂)₃CH]³⁻) was [...] Read more.

The side-on end-on dinitrogen hydride complex [{Na(dme)}₂{(O₃)Nb}₂(μ-η¹:η²-N₂)(μ-H)₂] (3-Na, [O₃]³⁻ = [(3,5-^tBu₂-2-O-C₆H₂)₃CH]³⁻) was observed to undergo facile elimination of H₂ and cleavage of the N–N bond in the presence of 9-borabicyclo[3.3.1]nonane (9-BBN), AlMe₃, and ZnMe₂. Treatment of 3-Na with 9-BBN and ZnMe₂ afforded the nitride complex [{K(dme)₂}₂{(O₃)Nb}₂(μ-N)₂] (2-Na). The reaction of 3-Na with AlMe₃ afforded [{Na(dme)}₂{(O₃)AlMe}₂(NbMe₂)₂(μ-N)₂] (5). The nitride complex 2-Na was treated with 9-BBN and AlMe₃ to form [{Na(dme)}₂{(O₃)Nb}(μ-NH)(μ-NBC₈H₁₄){Nb(O₃C)}] (4) and 5, respectively. Complex 2-Na, 4, and 5 were structurally characterized. Full article

(This article belongs to the Special Issue Design Strategies for Metal Complexes that Activate Bio-Related Small Molecules)

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11 pages, 10968 KiB

Open AccessEditor’s ChoiceArticle

Synthesis and Reactivity of Manganese Complexes Bearing Anionic PNP- and PCP-Type Pincer Ligands toward Nitrogen Fixation

by Shogo Kuriyama, Shenglan Wei, Takeru Kato and Yoshiaki Nishibayashi

Molecules 2022, 27(7), 2373; https://doi.org/10.3390/molecules27072373 - 06 Apr 2022

Cited by 3 | Viewed by 2634

Abstract

A series of manganese complexes bearing an anionic pyrrole-based PNP-type pincer ligand and an anionic benzene-based PCP-type pincer ligand is synthesized and characterized. The reactivity of these complexes toward ammonia formation and silylamine formation from dinitrogen under mild conditions is evaluated to produce [...] Read more.

A series of manganese complexes bearing an anionic pyrrole-based PNP-type pincer ligand and an anionic benzene-based PCP-type pincer ligand is synthesized and characterized. The reactivity of these complexes toward ammonia formation and silylamine formation from dinitrogen under mild conditions is evaluated to produce only stoichiometric amounts of ammonia and silylamine, probably because the manganese pincer complexes are unstable under reducing conditions. Full article

(This article belongs to the Special Issue Design Strategies for Metal Complexes that Activate Bio-Related Small Molecules)

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Review

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19 pages, 6107 KiB

Open AccessReview

π–π Stacking Interaction of Metal Phenoxyl Radical Complexes

by Hiromi Oshita and Yuichi Shimazaki

Molecules 2022, 27(3), 1135; https://doi.org/10.3390/molecules27031135 - 08 Feb 2022

Cited by 4 | Viewed by 2293

Abstract

π–π stacking interaction is well-known to be one of the weak interactions. Its importance in the stabilization of protein structures and functionalization has been reported for various systems. We have focused on a single copper oxidase, galactose oxidase, which has the π–π stacking [...] Read more.

π–π stacking interaction is well-known to be one of the weak interactions. Its importance in the stabilization of protein structures and functionalization has been reported for various systems. We have focused on a single copper oxidase, galactose oxidase, which has the π–π stacking interaction of the alkylthio-substituted phenoxyl radical with the indole ring of the proximal tryptophan residue and catalyzes primary alcohol oxidation to give the corresponding aldehyde. This stacking interaction has been considered to stabilize the alkylthio-phenoxyl radical, but further details of the interaction are still unclear. In this review, we discuss the effect of the π–π stacking interaction of the alkylthio-substituted phenoxyl radical with an indole ring. Full article

(This article belongs to the Special Issue Design Strategies for Metal Complexes that Activate Bio-Related Small Molecules)

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