Phenoxyl Radical Complexes Research Articles

Crystal structure of a phenoxyl radical complex relevant to the metal site of the galactose oxidase enzyme: A facile one-pot synthesis, evidence for hydrogen atom transfer and DNA cleavage via self-activation.

Copper complexes [Cu(L1H)ClO4] (1) and [Cu(L2)NO3] (2), which are relevant to the metal site of the galactose oxidase enzyme, were synthesized and characterized by different spectroscopic methods. L1H2 and L2H2 [where L1H2 stands for 2,2'-((1E,1'E)(2,2'-(pyridine-2,6-diyl)bis(2-phenylhydrazin-2-yl-1-ylidene))bis(methanylylidene))diphenol and L2H2 stands for 6,6'-((1E,1'E)-(2,2'-(pyridine-2,6-diyl)bis(2-phenylhydrazin-2-yl-1-ylidene))bis(methanylylidene))bis(2,4-di-tert-butylphenol), H stands for dissociable proton] are pentadentate ligands. These ligands provide pyridyl N, two imine N, and two non-innocent phenoxyl and phenolato O donors, forming complex 1 as a non-radical complex, while complex 2 is a phenoxyl radical complex. The molecular structures of complexes 1 and 2 were authenticated by X-ray crystallography. Benzyl alcohol oxidation was investigated, and the conversion of 9,10-dihydroanthracene to anthracene was examined to scrutinize the H-atom abstraction reaction. Nuclease activity with complexes 1 and 2 was investigated by self-activated plasmid DNA (pBR322) cleavage. Non-innocent properties of the ligand-containing phenolato function were investigated by DFT calculations.

Redox Chemistry of Pt(II) Complex with Non-Innocent NHC Bis(Phenolate) Pincer Ligand: Electrochemical, Spectroscopic, and Computational Aspects

A Pt(II) complex bearing chelating tridentate bis-aryloxide tetrahydropyrimidinium-based N-heterocyclic carbene (NHC) was synthesized and characterized by using different techniques. Both cyclic voltammetry and differential pulse voltammetry were used to study the electrochemical properties of the complex, revealing two reversible one-electron oxidation processes. The chemical generation and isolation of one-electron-oxidized species were performed oxidizing the initial complex by means of AgBF4. A combination of spectroscopic (UV-Vis/NIR- and EPR-) and theoretical (density functional theory (DFT)) studies suggests the formation of a Pt(II)-phenoxyl radical complex. The latter open-shell derivative was structurally characterized by means of X-ray diffraction analysis. Finally, the neutral platinum complex was tested as a mediator in the process of electrocatalytic oxidation of 2-(methylamino)ethanol (MEA).

Facile synthesis of benzoxazole derivatives by a multi-component reaction catalysed by copper complexes capable of generating phenoxyl radical complex

Cu(ii) complexes were designed, synthesized, and characterized. The molecular structures of the complexes were determined by X-ray crystallography. Catalytic activity studies on these complexes afforded benzoxazole derivatives via a radical pathway.

Non-innocent redox behavior of CuII-p-dimethylaminophenolate complexes: formation and characterization of the CuI-phenoxyl radical species.

Cu complexes with p-dimethylaminophenolate ligands were synthesized by the reaction of CuII ions with the ligands under inert gas atmosphere and characterized. The complexes showed a valence state change from CuII-phenolate to CuI-phenoxyl radical on loss of the coordinated solvent. The CuI-phenoxyl radical species showed the characteristic properties and reactivities.

Oxidative cyclization and synthesis of benzoxazole derivatives and hydrolytic phosphatase activity studies on dinuclear diphenoxo-bridged zinc(II)complexes

Diphenoxo bridged dinuclear zinc complexes, [Zn2(Phimp)2(Cl)2] (1) (PhimpH = (E)-2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)phenol), [Zn2(Me-Phimp)2(Cl)2] (2) (Me-PhimpH = (E)-4-methyl-2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)phenol), [Zn2(OMe-Phimp)2(Cl)2] (3) (OMe-PhimpH = (E)-4-methoxy-2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)phenol), were synthesized and spectroscopically characterized. The molecular structure of 2 was determined using X-ray crystallography. DFT and TD-DFT calculations were performed to optimize the molecular geometry, interpret the spectroscopic results and investigate the contribution of the ligands to the redox properties of the complexes. Phenoxyl radical complexes were generated in solution via chemical oxidation using ceric ammonium nitrate (CAN) and the redox properties were examined through cyclic voltammetric measurements. The hydrolysis of para-nitrophenylphosphate (PNPP) by all the dinuclear zinc complexes was investigated to mimic phosphatase activity. Catalytic oxidative cyclization by these complexes and synthesis of benzoxazole derivatives was investigated.

Formation of Ni(II)-phenoxyl radical complexes by O2: a mechanistic insight into the reaction of Ni(II)-phenol complexes with O2.

A reaction of Ni(ClO4)2·6H2O with a tripodal ligand having two di(tert-butyl)phenol moieties, H2tbuL, and 1 equivalent of triethylamine in CH2Cl2/CH3OH (1 : 1, v/v) under N2 gave a NiII-(phenol)(phenolate) complex, [Ni(HtbuL)(CH3OH)2]ClO4. The formation of the NiII-phenoxyl radical complex by O2 was observed in the reaction of this complex in the solid state. On the other hand, the NiII-phenoxyl radical complex [Ni(Me2NL)(CH3OH)2]ClO4 was obtained by the reaction of H2Me2NL having a p-(dimethylamino)phenol moiety with Ni(ClO4)2·6H2O in a similar procedure under O2, through the oxidation of the NiII-(phenol)(phenolate) complex. However, a direct redox reaction of the NiII ion could not be detected in the phenoxyl radical formation. The results of the reaction kinetics, XAS and X-ray structure analyses suggested that the O2 oxidation from the NiII-(phenol)(phenolate) complex to the NiII-phenoxyl radical complex occurs via the proton transfer-electron transfer (PT-ET) type mechanism of the phenol moiety weakly coordinated to the nickel ion.

Recent Advances in One-Electron-Oxidized CuII -Diphenoxide Complexes as Models of Galactose Oxidase: Importance of the Structural Flexibility in the Active Site.

The phenoxyl radical plays important roles in biological systems as cofactors in some metalloenzymes, such as galactose oxidase (GO) catalyzing oxidation of primary alcohols to give the corresponding aldehydes. Many metal(II)-phenoxyl radical complexes have hitherto been studied for understanding the detailed properties and reactivities of GO, and thus the nature of GO has gradually become clearer. However, the effects of the subtle geometric and electronic structural changes at the active site of GO, especially the structural change in the catalytic cycle and the effect of the second coordination sphere, have not been fully discussed yet. In this Review, we focus on further details of the model studies of GO and discuss the importance of the structural change at the active site of GO.

Aerobic oxidation of alcohol by model complexes relevant to metal site galactose oxidase: role of copper(I) intermediate, evidence for the generation of end-on copper(II)–OOH species and catalytic promiscuity for oxidation of benzyl alcohol, catechol and o-aminophenol

Tridentate ligands having meridional NNO donor centres were designed and synthesized mimicking the copper coordination in the metal site of galactose oxidase enzyme. Mononuclear copper complexes [Cu(L1)Cl] (1) (L1H = (E)-2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)phenol), [Cu(L2)Cl] (2) (L2H = (E)-4-methyl-2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)phenol), [Cu(L3)Cl] (3) (L3H = (E)-1-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)naphthalen-2-ol), [Cu(L4)Cl] (4) (L4H = (E)-2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)phenol), [Cu(L5)Cl] (5) (L5H = (E)-2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)phenol), and [Cu(L6)Cl] (6) (L6H = (E)-2,4-di-tert-butyl-6-(((pyridin-2-ylmethyl)imino)methyl)phenol) were synthesized and characterized. Molecular structure of complex 3 was determined by single crystal X-ray crystallography. Phenoxyl radical complexes were generated in solution via chemical oxidation using ceric ammonium nitrate (CAN), and the radical complexes were characterized by UV–Vis–NIR spectrophotometer. DFT calculations were performed at B3LYP level to optimize the ground-state molecular geometry of the complexes. To understand the electronic properties and absorption spectra of the complexes, TD-DFT calculations were executed for phenoxyl radical complexes considering triplet as well as singlet spin states. Alcohol oxidation was examined utilizing complexes 1–6 as catalyst, and importance of stabilization of Cu(I) intermediate was scrutinized and generation of Cu(II)–OOH was examined. Catalytic promiscuity for catechol oxidase and phenoxazinone synthase activity by complexes (1–5) was investigated. Theoretical calculations and ESI–MS spectral studies were performed during oxidation chemistry of benzyl alcohol, catechol and o-amino phenol to support the proposed mechanism.

Distorted copper(ii) radicals with sterically hindered salens: electronic structure and aerobic oxidation of alcohols.

The sterically hindered salen ligands featuring biphenyl and tetramethyl putrescine linkers were synthesized and chelated to copper. The resulting complexes CuLbp,tBu, CuLbp,OMe, CuLpu,tBu and CuLpu,OMe were structurally characterized, showing a significanty tetrahedrally distorted metal center. The complexes show two reversible oxidation waves in the range 0.2 to 0.8 V vs. Fc+/Fc. A further reduction wave is detected in the range -1.4 to -1.7 V vs. Fc+/Fc. It is reversible for CuLbp,tBu and CuLbp,OMe and assigned to the CuII/CuI redox couple. One-electron oxidation of CuLbp,OMe, CuLpu,tBu and CuLpu,OMe was performed chemically and electrochemically. It is accompanied by a quenching of the EPR resonances. Phenoxyl radical formation was established by X-Ray diffraction on the cations [CuLbp,OMe]+ and [CuLpu,OMe]+, whereby the coordination sphere is elongated upon oxidation with quinoidal distributions of bond distances. The cations exhibit a NIR band of moderate intensity in their optical spectrum, supporting their classification as class II mixed-valent radical species according to the Robin Day classification. The proposed electronic structures are supported by DFT calculations. The cations [CuLbp,OMe]+, [CuLpu,tBu]+ and [CuLpu,OMe]+ were active towards aerobic oxidation of the unactivated alcohol 2-phenylethanol, with TON numbers up to 58 within 3 h.

Formation of the CuII -Phenoxyl Radical by Reaction of O2 with a CuII -Phenolate Complex via the CuI -Phenoxyl Radical.

Reaction of Cu(ClO4 )2 ⋅6 H2 O with a tripodal 2N2O ligand, H2 Me2 NL, having a p-(dimethylamino)phenol moiety, in CH2 Cl2 /MeOH (1:1 v/v) under basic conditions under an inert gas atmosphere gave [Cu(Me2 NL)(H2 O)] (1). The same reaction carried out under aerobic conditions gave [Cu(Me2 NL)(MeOH)]ClO4 (2), which could be obtained also from the isolated complex 1 by reaction with O2 in CH2 Cl2 /MeOH. The X-ray crystal structures of 1 and 2 revealed similar square-pyramidal structures, but 2 showed the (dimethylamino)phenoxyl radical features. Complex 1 exhibits characteristic CuII EPR signals of the d ground state in CH2 Cl2 /MeOH at 77 K, whereas 2 is EPR-silent. The EPR and X-ray absorption fine structure (XAFS) results suggest that 2 is assigned to the CuII -(dimethylamino)phenoxyl radical. However, complex 1 showed different features in the absence of MeOH. The EPR spectrum of the CH2 Cl2 solution of 1 exhibits distortion from the d ground state and a temperature-dependent equilibrium between the CuII -(dimethylamino)phenolate and the CuI -(dimethylamino)phenoxyl radical. From these results, CuII -phenoxyl radical complex 2 is concluded to be formed by the reaction of 1 with O2 via the CuI -phenoxyl radical species.

Redox processes in the solution of Ni(II) complex with salen type ligand and in the polymer films

The investigations of oxidation processes of (±)-trans-N,N′-bis(3,5-di-methylsalicylidene)-1,2-cyclohexanediaminenickel(II)) [Ni(salcn(2Me))] and ligands with Me substituents attached in o- (H2salcn(Me)) or o- and p-positions (H2salcn(2Me)) in CH2Cl2 were performed by CV, exhaustive electrolysis, UV VIS NIR and IR methods. H2salcn(Me) and H2salcn(2Me) are irreversibly oxidized to phenoxyl radicals and bis-phenoxyl radicals. The hydrogen bonds stabilize both forms of the oxidized ligands. [Ni(salcn(2Me))] is oxidized in three steps. The overall process involves ligand and partially nickel ion. In the I step, [Ni(salcn(2Me))] is oxidized to phenoxyl radical complex, which is well stabilized by the nickel ion. In the II step the oxidation is realized by forming bis-phenoxyl radicals and phenoxonium cations complexes.The films derived from salen complexes deposited on the Pt electrode surface (poly[Ni(salcn)], poly[Ni(salcn(Me))], poly[Ni(salcn(Bu))]) were obtained as a result of anodic electropolymerization processes. On the basis of FTIR ATR spectroscopy and CV experiments, the relationship of the polymer films structures, charge transport and the potential range of the applied electrosynthesis was shown.

Characterization of Group 10-Metal-p-Substituted Phenoxyl Radical Complexes with Schiff Base Ligands.

Methylthiophenoxyl radical plays an important role as the active form of galactose oxidase (GO), which catalyzes oxidation of a primary alcohol to the corresponding aldehyde. Although many metal(II)-phenoxyl radical species have been reported, only a few studies have been reported on the properties of methylthiophenoxyl radical-metal complexes. We have prepared the group 10 metal (Ni, Pd and Pt) complexes of a salen-type ligand with a methylthio group at para-position of the two phenolate moieties and characterized them by X-ray crystal structure analyses and various spectroscopic methods in order to understand the role of the methylthio moiety in phenoxyl radical metal complexes. The corresponding p-methoxy substituted derivatives have been also characterized for comparison. All the one-electron oxidized group 10 metal methylthiophenolate complexes have a relatively localized radical site on one of the two phenolate moieties in comparison to the one-electron oxidized complexes of p-methoxy derivatives and exhibit different properties dependent on the central metal ions.

Tuning the locus of oxidation in Cu-diamido-diphenoxo complexes: From Cu(III) to Cu(II)-phenoxyl radical

While N2O2 tetraanionic ligands containing a strong N-amidate σ-donor are generally assumed to stabilise metal high valence states, we herein have shown that, in dianionic Cu(II)-diamido-diphenoxo complexes, H-bonding and electronic effects on the phenolate groups may modulate the electronic structure of their oxidised species from Cu(III) to Cu(II)-phenoxyl radical complexes; and so in the negative potential range. We observe that electron-poor phenolate complexes 22− and 32− oxidise to Cu(III) species, whereas electron rich phenolate complex 12− oxidises to a Cu(II)-phenoxyl radical. Our DFT results suggest that π-electron-rich phenolate rings in 12− are responsible for an increase of the HOMO orbital energy, bringing the HOMO-SOMO gap small enough to favour a ligand-based oxidation process. Further DFT-calculations have also shown that upon changing the o,p-phenol substituent from electron-widthdrawing groups (NO2) to electron-donating ones (OMe), the favoured oxidised state switches from Cu(III) to Cu(II)-radical. These results emphasize the use of the versatile diamido-diphenoxo backbone asa promising way to novel GO-chemical models, as well as molecular switches.

Non‐Innocent Property of Tridentate Ligand in Novel Cobalt Complex : Crystal Structure and Evidences for Cobalt(II) Phenoxyl Radical Complex Formation

AbstractA phenoxyl radical complex [CoII(L)(L+)]NO3 (where LH=(E)‐2,4‐di‐tert‐butyl‐6‐((2‐phenyl‐2‐(pyridin‐2‐yl)hydrazono)methyl)phenol and H=dissociable proton) was synthesized at room temperature in aerobic condition and was characterized. Crystal structure, UV‐Vis‐NIR and 1H‐NMR spectra, magnetic properties and electrochemical studies authenticated the presence of Co(II) phenoxyl radical complex.

X-ray structure of a Ni(II)-tri-phenoxyl radical complex.

The diimino-diphenolato neutral square-planar Ni(ii) complex, NiL2, is readily oxidised with 2 equiv. of Ag[SbF6], to produce an unprecedented octahedral Ni(ii) tris(phenoxyl) radical complex, [Ni(L˙)3][SbF6]2. This study reveals, for the first time, the X-ray structure of a metal-tri-phenoxyl radical complex.

Copper(II) mediated phenol ring nitration by nitrogen dioxide.

Cu(II) complexes of N2O2 type ligands, L(1)H2 and L(2)H2 [L(1)H2 = 6,6'-(((pyridin-2-ylmethyl)azanediyl)bis(methylene))bis(2,4-di-tert-butylphenol); L(2)H2 = 2,4-di-tert-butyl-6-(((3-(tert-butyl)-2-hydroxy-5-methylbenzyl)(pyridin-2-yl-methyl)amino)methyl)phenol], have been synthesized. Addition of nitrogen dioxide (NO2) in THF solutions of the complexes resulted in the nitration at the 4-position of a coordinated equatorial phenolate ring of the ligand frameworks. This nitration did not occur at the phenol ring which is axially coordinated to the metal center. Spectroscopic evidence suggests that the reaction proceeds through a phenoxyl radical complex formation.

Characterization of one-electron oxidized copper(ii)-salophen-type complexes; effects of electronic and geometrical structures on reactivities

One-electron oxidized salophen-type complexes, [Cu(salophen)](+) (H2salophen = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-diaminobenzene), and its methoxy derivatives, [Cu(MeO-salophen)](+) and [Cu(salophen-(MeO)2)](+) (H2MeO-salophen = N,N'-bis(3-tert-butyl-5-methoxysalicylidene)-1,2-diaminobenzene, H2salophen-(MeO)2 = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-diamino-4,5-dimethoxybenzene), have been synthesized and structurally characterized, and their reactivities have been investigated. The solid state structures of the one-electron oxidized forms of these complexes suggested that [Cu(salophen)](+) and [Cu(MeO-salophen)](+) can be assigned to relatively localized Cu(ii)-phenoxyl radical complexes, while [Cu(salophen-(MeO)2)](+) is the diiminobenzene radical complex. On the other hand, [Cu(salophen)](+) in solution showed a different electronic structure from that of the solid sample, the radical electron being delocalized over the whole π-conjugated ligand. The reaction of these oxidized complexes with benzyl alcohol has been investigated in the presence of a large excess of substrate, which revealed the difference in the kinetic behavior between the complexes. The mechanisms of the oxidation have been discussed on the basis of the electronic and geometrical structures and the reaction kinetics.

Polyfluorinated Pd(II)-3,5-di-tert-butylsalicylaldimenes complexes: Synthesis, structure, spectroscopy, redox behaviors and catalytic activity

A series of new polyfluorinated palladium(II) complexes (7–12) of N-polyfluorophenyl-3,5-di-tert-butylsalicylaldimines (1–6) have been synthesized. They were characterized by analytical, spectroscopic (UV/Vis, IR, 1H NMR, and ESR), electrochemical methods and their chemical oxidation and hydrogenation catalytic activity were studied. The X-ray crystal structure analysis of bis[N-(3,5-di-tert-butylsalicylidene)-F5Ph]Pd(II) (12) revealed a slightly distorted square-planar trans-PdN2O2 geometry around the palladium center. The UV/Vis and EPR results indicate that chemical oxidation of 7–10 by Ce(IV) in CHCl3 generates relatively stable Pd(II)–phenoxyl radical complexes (g=2.0044–2.0062). The results of chemical and electrochemical oxidation of 1–12, as well as the catalytic activity of 7–10 complexes in the hydrogenation of PhNO2 were presented.

Formation of a Cu(II)–phenoxyl radical complex from a Cu(II)–phenolate complex: A new model for galactose oxidase

Abstract A new N3O type tetradentate ligand (HL) [2,4-di-tert-butyl-6-((4-(2-(6,8-di-tert-butyl-2H-benzo[e][1,3]oxazin-3(4H)-yl)ethyl)piperazin-1-yl)methyl)phenol] and its complex [CuL(CH3OH)]ClO4(CH3OH) have been synthesised. The complex has been crystallographically and spectroscopically characterised. It has been noticed that in the presence of acetonitrile the Cu(II) centers in the complex undergo reduction with a concomitant oxidation of the ligand. EPR and UV–Vis spectroscopic studies of the complex in acetonitrile indicate that this disproportionation proceeds via the formation of a Cu(II)–phenoxyl intermediate. The Cu(II)–phenoxyl complex is found to be stable in methanol in the presence of base. The oxidised products of the ligand are isolated and characterized. The paramagnetic centers in the Cu(II)–phenoxyl complexes were found to be weakly ferromagnetically coupled.

Synthesis and characterization of chromium(III) complexes derived from tridentate ligands: Generation of phenoxyl radical and catalytic oxidation of olefins

Abstract The mononuclear chromium complexes [Cr(Phimp) 2 ](ClO 4 ) ( 1 ), [Cr(N–Phimp) 2 ](ClO 4 ) ( 2 ), and [Cr(Me–Phimp) 2 ](ClO 4 ) ( 3 ) (where PhimpH = (2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)phenol), N–PhimpH = (2-((2-phenyl-2-(pyridin-2-yl)hydrazono)methyl)napthalen-1-ol), Me–PhimpH = (2-(1-(2-phenyl-2-(pyridine-2-yl)hydrazono)ethyl)phenol) where H stands for dissociable proton) were synthesized and characterized spectroscopically. Electrochemical studies were investigated for the stabilization of Cr(III) complexes and electrochemically generation of phenoxyl radical complexes. Phenoxyl radical complexes were also generated by ceric ammonium nitrate (CAN) and characterized by UV–visible spectra. The complexes were evaluated for their activity as catalysts for the epoxidation of olefins. The chromium complexes catalyzed epoxidation of olefins, viz. styrene, cis -cyclooctene and trans -stilbene with hydrogen peroxide (H 2 O 2 ) as terminal oxidant, to give corresponding epoxides and oxidative products at ambient temperature.