Dual Pathways in the Oxidation of an Osmium(III) Guanidine Complex. Formation of Osmium(VI) Nitrido and Osmium Nitrosyl Complex

The guanidine moiety of arginine is involved in the active sites of a variety of enzymes, such as nitric oxide synthase (NOS) and NiFe hydrogenase. In this paper we aim to investigate the effects of a metal center on the oxidation of guanidine, which should provide an interesting comparison with the biological aerobic oxidation of arginine catalyzed by NOS. We studied the oxidation of an osmium­(III) guanidine complex, <i>mer</i>-[Os­(<b>L</b>)­{N­(H)­C­(NH₂)₂}­(CN)₃]⁻, (<b>OsG</b>, <b>HL</b> = 2-(2-hydroxyphenyl)­benzoxazole) by <i>m</i>-chloroperbenzoic acid (<i>m</i>-CPBA), which is potentially an O atom transfer reagent, and by (NH₄)₂[Ce^IV(NO₃)₆], which is a one-electron oxidant. With <i>m</i>-CPBA, <i>mer</i>-[Os­(NO)­(<b>L</b>)­(CN)₃]⁻ (<i>mer</i>-<b>OsNO</b>) is the product, while with Ce^IV, <i>mer</i>-[Os^VI(N)­(<b>L</b>)­(CN)₃]⁻ (<i>mer</i>-<b>OsN</b>) is formed instead. The crystal structures of <i>mer</i>-<b>OsNO</b> and <i>mer</i>-<b>OsN</b> were determined by X-ray crystallography. The mechanisms for the oxidation of <b>OsG</b> by <i>m</i>-CPBA and Ce^IV are proposed

Dual Pathways in the Oxidation of an Osmium(III) Guanidine Complex. Formation of Osmium(VI) Nitrido and Osmium Nitrosyl Complex

The guanidine moiety of arginine is involved in the active sites of a variety of enzymes, such as nitric oxide synthase (NOS) and NiFe hydrogenase. In this paper we aim to investigate the effects of a metal center on the oxidation of guanidine, which should provide an interesting comparison with the biological aerobic oxidation of arginine catalyzed by NOS. We studied the oxidation of an osmium­(III) guanidine complex, <i>mer</i>-[Os­(<b>L</b>)­{N­(H)­C­(NH₂)₂}­(CN)₃]⁻, (<b>OsG</b>, <b>HL</b> = 2-(2-hydroxyphenyl)­benzoxazole) by <i>m</i>-chloroperbenzoic acid (<i>m</i>-CPBA), which is potentially an O atom transfer reagent, and by (NH₄)₂[Ce^IV(NO₃)₆], which is a one-electron oxidant. With <i>m</i>-CPBA, <i>mer</i>-[Os­(NO)­(<b>L</b>)­(CN)₃]⁻ (<i>mer</i>-<b>OsNO</b>) is the product, while with Ce^IV, <i>mer</i>-[Os^VI(N)­(<b>L</b>)­(CN)₃]⁻ (<i>mer</i>-<b>OsN</b>) is formed instead. The crystal structures of <i>mer</i>-<b>OsNO</b> and <i>mer</i>-<b>OsN</b> were determined by X-ray crystallography. The mechanisms for the oxidation of <b>OsG</b> by <i>m</i>-CPBA and Ce^IV are proposed

Dual Pathways in the Oxidation of an Osmium(III) Guanidine Complex. Formation of Osmium(VI) Nitrido and Osmium Nitrosyl Complex

The guanidine moiety of arginine is involved in the active sites of a variety of enzymes, such as nitric oxide synthase (NOS) and NiFe hydrogenase. In this paper we aim to investigate the effects of a metal center on the oxidation of guanidine, which should provide an interesting comparison with the biological aerobic oxidation of arginine catalyzed by NOS. We studied the oxidation of an osmium­(III) guanidine complex, <i>mer</i>-[Os­(<b>L</b>)­{N­(H)­C­(NH₂)₂}­(CN)₃]⁻, (<b>OsG</b>, <b>HL</b> = 2-(2-hydroxyphenyl)­benzoxazole) by <i>m</i>-chloroperbenzoic acid (<i>m</i>-CPBA), which is potentially an O atom transfer reagent, and by (NH₄)₂[Ce^IV(NO₃)₆], which is a one-electron oxidant. With <i>m</i>-CPBA, <i>mer</i>-[Os­(NO)­(<b>L</b>)­(CN)₃]⁻ (<i>mer</i>-<b>OsNO</b>) is the product, while with Ce^IV, <i>mer</i>-[Os^VI(N)­(<b>L</b>)­(CN)₃]⁻ (<i>mer</i>-<b>OsN</b>) is formed instead. The crystal structures of <i>mer</i>-<b>OsNO</b> and <i>mer</i>-<b>OsN</b> were determined by X-ray crystallography. The mechanisms for the oxidation of <b>OsG</b> by <i>m</i>-CPBA and Ce^IV are proposed

Proton-Coupled O‑Atom Transfer in the Oxidation of HSO₃^– by the Ruthenium Oxo Complex <i>trans</i>-[Ru^VI(TMC)(O)₂]²⁺ (TMC = 1,4,8,11-Tetramethyl-1,4,8,11-tetraazacyclotetradecane)

We have previously reported that the oxidation of SO₃^2– to SO₄^2– by a <i>trans</i>-dioxoruthenium­(VI) complex, [Ru^VI(TMC)­(O)₂)]²⁺ (<b>Ru</b>^<b>VI</b>; TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazcyclotetradecane) in aqueous solutions occurs via an O-atom transfer mechanism. In this work, we have reinvestigated the effects of the pH on the oxidation of S^IV by <b>Ru</b>^<b>VI</b> in more detail in order to obtain kinetic data for the HSO₃^– pathway. The HSO₃^– pathway exhibits a deuterium isotope effect of 17.4, which indicates that O–H bond breaking occurs in the rate-limiting step. Density functional theory calculations have been performed that suggest that the oxidation of HSO₃^– by <b>Ru</b>^<b>VI</b> may occur via a concerted or stepwise proton-coupled O-atom transfer mechanism

Aerobic Oxidation of an Osmium(III) <i>N</i>‑Hydroxyguanidine Complex To Give Nitric Oxide

The aerobic oxidation of the <i>N</i>-hydroxyguanidinum moiety of <i>N</i>-hydroxyarginine to NO is a key step in the biosynthesis of NO by the enzyme nitric oxide synthase (NOS). So far, there is no chemical system that can efficiently carry out similar aerobic oxidation to give NO. We report here the synthesis and X-ray crystal structure of an osmium­(III) <i>N</i>-hydroxyguanidine complex, <i>mer</i>-[Os^III{NHC­(NH₂)­(NHOH)}­(<b>L</b>)­(CN)₃]⁻ (<b>OsGOH</b>, <b>HL</b> = 2-(2-hydroxyphenyl)­benzoxazole), which to the best of our knowledge is the first example of a transition metal <i>N</i>-hydroxyguanidine complex. More significantly, this complex readily undergoes aerobic oxidation at ambient conditions to generate NO. The oxidation is pH-dependent; at pH 6.8, <i>fac</i>-[Os­(NO)­(<b>L</b>)­(CN)₃]⁻ is formed in which the NO produced is bound to the osmium center. On the other hand, at pH 12, aerobic oxidation of <b>OsGOH</b> results in the formation of the ureato complex [Os^III(NHCONH₂)­(<b>L</b>)­(CN)₃]^2– and free NO. Mechanisms for this aerobic oxidation at different pH values are proposed

Aerobic Oxidation of an Osmium(III) <i>N</i>‑Hydroxyguanidine Complex To Give Nitric Oxide

The aerobic oxidation of the <i>N</i>-hydroxyguanidinum moiety of <i>N</i>-hydroxyarginine to NO is a key step in the biosynthesis of NO by the enzyme nitric oxide synthase (NOS). So far, there is no chemical system that can efficiently carry out similar aerobic oxidation to give NO. We report here the synthesis and X-ray crystal structure of an osmium­(III) <i>N</i>-hydroxyguanidine complex, <i>mer</i>-[Os^III{NHC­(NH₂)­(NHOH)}­(<b>L</b>)­(CN)₃]⁻ (<b>OsGOH</b>, <b>HL</b> = 2-(2-hydroxyphenyl)­benzoxazole), which to the best of our knowledge is the first example of a transition metal <i>N</i>-hydroxyguanidine complex. More significantly, this complex readily undergoes aerobic oxidation at ambient conditions to generate NO. The oxidation is pH-dependent; at pH 6.8, <i>fac</i>-[Os­(NO)­(<b>L</b>)­(CN)₃]⁻ is formed in which the NO produced is bound to the osmium center. On the other hand, at pH 12, aerobic oxidation of <b>OsGOH</b> results in the formation of the ureato complex [Os^III(NHCONH₂)­(<b>L</b>)­(CN)₃]^2– and free NO. Mechanisms for this aerobic oxidation at different pH values are proposed

A Carbon Nitride/Fe Quaterpyridine Catalytic System for Photostimulated CO₂‑to-CO Conversion with Visible Light

Efficient and selective photostimulated CO₂-to-CO reduction by a photocatalytic system consisting of an iron-complex catalyst and a mesoporous graphitic carbon nitride (mpg-C₃N₄) redox photosensitizer is reported for the first time. Irradiation in the visible region (λ ≥ 400 nm) of an CH₃CN/triethanolamine (4:1, v/v) solution containing [Fe­(qpy)­(H₂O)₂]²⁺ (qpy = 2,2′:6′,2′′:6′′,2′′-quaterpyridine) and mpg-C₃N₄ resulted in CO evolution with 97% selectivity, a turnover number of 155, and an apparent quantum yield of ca. 4.2%. This hybrid catalytic system, comprising only earth abundant elements, opens new perspectives for solar fuels production using CO₂ as a renewable feedstock

Four-Electron Oxidation of Phenols to <i>p</i>‑Benzoquinone Imines by a (Salen)ruthenium(VI) Nitrido Complex

Proton-coupled electron-transfer reactions of phenols have received considerable attention because of their fundamental interest and their relevance to many biological processes. Here we describe a remarkable four-electron oxidation of phenols by a (salen)­ruthenium­(VI) complex in the presence of pyridine in CH₃OH to afford (salen)­ruthenium­(II) <i>p</i>-benzoquinone imine complexes. Mechanistic studies indicate that this reaction occurs in two phases. The first phase is proposed to be a two-electron transfer process that involves electrophilic attack by RuN at the phenol aromatic ring, followed by proton shift to generate a Ru­(IV) <i>p</i>-hydroxyanilido intermediate. In the second phase the intermediate undergoes intramolecular two-electron transfer, followed by rapid deprotonation to give the Ru­(II) <i>p</i>-benzoquinone imine product

Synthesis, Structures, and Photophysical Properties of Ruthenium(II) Quinolinolato Complexes

Reaction of [Ru^II(PR₃)₃Cl₂] with 2-methyl<b>-</b>8-quinolinolate (MeQ) in the presence of Et₃N in MeOH produced the neutral carbonyl hydrido complexes [Ru^II(MeQ)­(PR₃)₂(CO)­(H)] (R = Ph (<b>1</b>), MeC₆H₄ (<b>2</b>), MeOC₆H₄ (<b>3</b>)). An analogous reaction occurs between [Ru^II(PPh₃)₃Cl₂] and MeQH in ethanol to give [Ru^II(MeQ)­(PPh₃)₂(CO)­(CH₃)] (<b>4</b>). The carbonyl, hydride, and methyl ligands of these complexes are most likely derived from the decarbonylation of ROH. Reaction of [Ru^II(PPh₃)₃(CO)­(H)₂] with 5-substituted quinolinolato ligands (XQ, X = H, Cl, Ph) produced the neutral complexes [Ru^II(XQ)­(PPh₃)₂(CO)­(H)] (XQ = Q (<b>5</b>), ClQ (<b>6</b>), PhQ (<b>7</b>)). Treatment of <b>1</b> and <b>5</b>–<b>7</b> with excess KCN in MeOH following by metathesis with PPh₄Cl afforded PPh₄⁺ salts of the anionic carbonyl dicyano complexes [Ru^II(XQ)­(CO)­(CN)₂(PPh₃)]⁻ (XQ = MeQ (<b>8</b>), Q (<b>9</b>) ClQ (<b>10</b>), PhQ (<b>11</b>)). Under similar conditions, reaction of <b>1</b> with excess CyNC in the presence of NH₄PF₆ afforded [Ru^II(MeQ)­(CyNC)₂(CO)­(PPh₃)]⁺ (<b>12</b>). All complexes have been characterized by IR, ESI/MS, ¹H NMR and elemental analysis. The crystal structures of complexes <b>3</b>, <b>4</b>, <b>8</b>, and <b>12</b> have been determined by X-ray crystallography. The UV and emission spectra of these complexes have also been investigated. All complexes exhibit short-lived quinolinolate-based LC fluorescence in solution at room temperature and dual emissions derived from LC fluorescence and phosphorescence at 77 K glassy medium. These emissions are relatively insensitive to the nature of the ancillary ligands but are readily tunable by varying the substituents on the quinolinolato ligand

Synthesis, Structures, and Photophysical Properties of Ruthenium(II) Quinolinolato Complexes

Reaction of [Ru^II(PR₃)₃Cl₂] with 2-methyl<b>-</b>8-quinolinolate (MeQ) in the presence of Et₃N in MeOH produced the neutral carbonyl hydrido complexes [Ru^II(MeQ)­(PR₃)₂(CO)­(H)] (R = Ph (<b>1</b>), MeC₆H₄ (<b>2</b>), MeOC₆H₄ (<b>3</b>)). An analogous reaction occurs between [Ru^II(PPh₃)₃Cl₂] and MeQH in ethanol to give [Ru^II(MeQ)­(PPh₃)₂(CO)­(CH₃)] (<b>4</b>). The carbonyl, hydride, and methyl ligands of these complexes are most likely derived from the decarbonylation of ROH. Reaction of [Ru^II(PPh₃)₃(CO)­(H)₂] with 5-substituted quinolinolato ligands (XQ, X = H, Cl, Ph) produced the neutral complexes [Ru^II(XQ)­(PPh₃)₂(CO)­(H)] (XQ = Q (<b>5</b>), ClQ (<b>6</b>), PhQ (<b>7</b>)). Treatment of <b>1</b> and <b>5</b>–<b>7</b> with excess KCN in MeOH following by metathesis with PPh₄Cl afforded PPh₄⁺ salts of the anionic carbonyl dicyano complexes [Ru^II(XQ)­(CO)­(CN)₂(PPh₃)]⁻ (XQ = MeQ (<b>8</b>), Q (<b>9</b>) ClQ (<b>10</b>), PhQ (<b>11</b>)). Under similar conditions, reaction of <b>1</b> with excess CyNC in the presence of NH₄PF₆ afforded [Ru^II(MeQ)­(CyNC)₂(CO)­(PPh₃)]⁺ (<b>12</b>). All complexes have been characterized by IR, ESI/MS, ¹H NMR and elemental analysis. The crystal structures of complexes <b>3</b>, <b>4</b>, <b>8</b>, and <b>12</b> have been determined by X-ray crystallography. The UV and emission spectra of these complexes have also been investigated. All complexes exhibit short-lived quinolinolate-based LC fluorescence in solution at room temperature and dual emissions derived from LC fluorescence and phosphorescence at 77 K glassy medium. These emissions are relatively insensitive to the nature of the ancillary ligands but are readily tunable by varying the substituents on the quinolinolato ligand