Oxidative addition of 3,6-di-tert-butyl-o-benzoquinone and 4,6-di-tert-butyl-N-(2,6-di-iso-propylphenyl)-o-iminobenzoquinone to SnCl2

Addition of 3,6-di-tert-butyl-o-benzoquinone (3,6-DBBQ) to SnCl₂ in THF leads to the oxidation of Sn(II) to Sn(IV) with formation of catecholate complex (3,6-DBCat)SnCl₂ · 2THF (1), where 3,6-DBCat is 3,6-di-tert-butyl-catecholate dianion. The reaction of 4,6-di-tert-butyl-N-(2,6-di-iso-propylphenyl)-o-iminobenzoquinone (IBQ-Prⁱ) also proceeds on the oxidative-addition mechanism yielding bis-iminosemiquinonato species (ISQ-Prⁱ)₂SnCl₂(2), where ISQ-Prⁱ is anion-radical 4,6-di-tert-butyl-N-(2,6-di-iso-propylphenyl)-o-iminobenzosemiquinolate. The complexes have been characterized by IR, X-band EPR, ¹H NMR (for 1) spectroscopy and magnetochemistry (for 2). X-ray analysis data show the distorted octahedral environment of tin(IV) for both complexes. Complex 1 is diamagnetic (ground state S = 0), while 2 has triplet ground state (S = 1, biradical). Catecholate complex 1 is able to be a spin trap for different organic radicals.     

The course of oxidative addition reactions of haloalkynes and haloalkenes to dimethyl- and dialkynylaurate(I) anions [RAuR]

The reactions of halo-alkynes Cl-CCH, C-lCC-Cl or PhCC-I with solutions of Li⁺[MeAuMe]⁻ in diethylether containing Ph₃P do not give the expected oxidative addition products Me₂(RCC)Au(PPh₃) with R = H, Cl, Ph. A mixture of other complexes is obtained instead which are generated in secondary reactions involving reductive elimination of ethane and/or dialkyne. However, addition of the halo-alkene H(Cl)CCCl₂ to the same substrate solution affords trans-Me₂[trans-H(Cl)CC(Cl)]Au(PPh₃) in good yield. Its molecular structure with pseudo-C_s symmetry has been determined by the solution NMR spectra and a single-crystal X-ray diffraction study. The reaction of methyl iodide with solutions of Li⁺[RCCAuCCR]⁻ in diethylether containing PPh₃ give the quaternary salts Ph₃PMe⁺ [RCCAuCCR]⁻ as the main products and only small amounts of cis-Me₂(RCC)Au(PPh₃) complexes probably formed in a series of oxidative addition, reductive elimination, and substitution reactions. The structure of Ph₃PMe⁺ [PhCCAuCCPh]⁻ has been determined.     

Complexes of [Re(CO)3Cl] with different oxidation states of the polyfunctional bmtz/H2bmtz ligand system (bmtz=3,6-bis(2-pyrimidyl)-1,2,4,5-tetrazine)

Combining fac-[Re(CO)₃Cl] with components of the ligand redox system bmtz/bmtz⁻/H₂bmtz/H₂bmtz⁻ (bmtz=3,6-bis(2-pyrimidyl)-1,2,4,5-tetrazine) has led to the isolation of the complexes (H₂bmtz)Re(CO)₃Cl, (μ-H₂bmtz)[Re(CO)₃Cl]₂ and (μ-bmtz)[Re(CO)₃Cl]₂. Other species characterized were (bmtz)Re(CO)₃Cl (UV/Vis, IR), [(H₂bmtz)Re(CO)₃Cl]⁻ (UV/Vis, IR, EPR), {(μ-H₂bmtz)[Re(CO)₃Cl]₂}⁻ (UV/Vis, IR, EPR) and {(μ-bmtz)[Re(CO)₃Cl]₂}⁻ (UV/Vis, IR, X band and high-field EPR). The results confirm bmtz as very strong and H₂bmtz as moderate π acceptor ligand versus one or two chelate-bonded low-valent metal centers. Reactivity is observed in terms of oxidative proton and reductive chloride dissociation.     

Peroxynitrite detoxification by ferryl Mycobacterium leprae truncated hemoglobin O

During infection, Mycobacterium leprae is faced with the host macrophagic environment limiting the growth of the bacilli. However, (pseudo-)enzymatic detoxification systems, including truncated hemoglobin O (Ml-trHbO), could allow this mycobacterium to persist in vivo. Here, kinetics of peroxynitrite (ONOOH/ONOO⁻) detoxification by ferryl Ml-trHbO (Ml-trHbOFe(IV)O), obtained by treatment with H₂O₂, is reported. Values of the second-order rate constant for peroxynitrite detoxification by Ml-trHbOFe(IV)O (i.e., of Ml-trHbOFe(III) formation; k_on), at pH 7.2 and 22.0 °C, are 1.5 × 10⁴ M⁻¹ s⁻¹, and 2.2 × 10⁴ M⁻¹ s⁻¹, in the absence of and presence of physiological levels of CO₂ (∼1.2 × 10⁻³ M), respectively. Values of k_on increase on decreasing pH with a pK_a value of 6.7, this suggests that ONOOH reacts preferentially with Ml-trHbOFe(IV)O. In turn, peroxynitrite acts as an antioxidant of Ml-trHbOFe(IV)O, which could be responsible for the oxidative damage of the mycobacterium. As a whole, Ml-trHbO can undertake within the same cycle H₂O₂ and peroxynitrite detoxification.     

Manganese(III) and rhenium(II) complexes with bulky 4,6-di-tert-butyl-N-(2,6-di-iso-propylphenyl)-o-iminobenzoquinonato ligands via carbonyls of corresponding metals

Four-coordinate complex Mn^III(ISQ-Prⁱ)(AP-Prⁱ) (1), where ISQ-Prⁱ = 4,6-di-tert-butyl-N-(2,6-di-iso-propylphenyl)-o-iminobenzosemiquinonate anion-radical, AP-Prⁱ = 4,6-di-tert-butyl-N-(2,6-di-iso-propylphenyl)-o-amidophenolate dianion, has been prepared by the reaction of Mn₂(CO)₁₀ with free 4,6-di-tert-butyl-N-(2,6-di-iso-propylphenyl)-o-iminobenzoquinone (IBQ-Prⁱ) in the molar ratio 1:4 in toluene. In contrast to manganese, rhenium carbonyl reacts with o-iminobenzoquinone to form complex Re^II(ISQ-Prⁱ)₂(CO)₂ (2) with the retention of two carbonyls in coordination sphere of rhenium. The complexes have been characterized by IR, UV-Vis, and EPR spectroscopies. Molecular structures of compounds 1 and 2 have been determined by single-crystal X-ray crystallography. Compound 1 is centro-symmetric square-planar molecule with delocalized mixed valent state of AP-Prⁱ and ISQ-Prⁱ ligands. EPR spectrum of 1 in solid at 300-77 K is typical for manganese complexes with S = 3/2 state. The effective magnetic moment of 1 is 1.96 μ_B at temperature 5 K as it was established by variable-temperature magnetic susceptibility measurements. Six-coordinate octahedral complex 2 possesses an S = 1/2 ground state, which is attained via strong intramolecular antiferromagnetic interaction between t_2g orbital unpaired electron of the low spin Re^II ion and the unpaired electron on π-orbital of the radical ligand.     

Superoxide oxidase and reductase activity of cytochrome b559 in photosystem II

This study provides evidence for the superoxide oxidase and the superoxide reductase activity of cytochrome b₅₅₉ (cyt b₅₅₉) in PSII. It is reported that in Tris-treated PSII membranes upon illumination, both the intermediate potential (IP) and the reduced high potential (HP^red) forms of cyt b₅₅₉ exhibit superoxide scavenging activity and interconversion between IP and HP^red form. When Tris-treated PSII membranes were illuminated in the presence of spin trap EMPO, the formation of superoxide anion radical (O₂⁻) was observed, as confirmed by EPR spin-trapping spectroscopy. The observations that the addition of enzymatic (superoxide dismutase) and non-enzymatic (cytochrome c, α-tocopherol and Trolox) O₂⁻ scavengers prevented the light-induced conversion of IP ↔ HP^red cyt b₅₅₉ confirmed that IP and HP^red cyt b₅₅₉ are reduced and oxidized by O₂⁻, respectively. Redox changes in cyt b₅₅₉ by an exogenous source of O₂⁻ reconfirmed the superoxide oxidase and reductase activity of cyt b₅₅₉. Furthermore, the light-induced conversion of IP to HP^red form of cyt b₅₅₉ was completely inhibited at pH > 8 and by chemical modification of the imidazole ring of histidine residues using diethyl pyrocarbonate. We proposed that a change in the environment around the heme iron, induced by the protonation and deprotonation of His²² residue generates a favorable condition for the oxidation and reduction of O₂⁻, respectively.     

New four- and five-coordinated complexes of cobalt with sterically hindered o-iminobenzoquinone ligands: synthesis and structure

New square planar Co^II(ISQ-iPr)₂ (1), Co^II(ISQ-Me)₂ (2) and five-coordinate Co^III(ISQ-Me)₂Cl cobalt complexes (3) (ISQ-iPr=4,6-di-tert-butyl-N-(2,6-di-iso-propylphenyl)-o-iminobenzosemiquinonate radical-anion, ISQ-Me=4,6-di-tert-butyl-N-(2,6-di-methylphenyl)-o-iminobenzosemiquinonate radical-anion) have been prepared using different methods. Complexes 1-3 have been characterized by IR, UV-Vis, EPR spectroscopy. Molecular structures of 1 and 3 were determined using X-ray crystallography. It has been established from variable-temperature magnetic susceptibility measurements and X-band EPR spectroscopy that 1 and 2 possess an S=1/2 ground state. In the complex molecules the unpaired electrons of o-iminobenzosemiquinonate radical ligands (S_rad=1/2) are strongly coupled antiferromagnetically. The sterical hindrances of N-aryl in o-iminobenzosemiquinonate ligands prevent the formation of a hexacoordinate species.     

Oxidative decarboxylation of tris-(p-carboxyltetrathiaaryl)methyl radical EPR probes by peroxidases and related hemeproteins: Intermediate formation and characterization of the corresponding cations

Tris(p-carboxyltetrathiaaryl)methyl radicals (TAM) are good EPR probes for measurement of dioxygen concentration in biological systems and for EPR imaging. It has been previously reported that these radicals are efficiently oxidized by superoxide, O₂⁻, or alkylperoxyl radicals, ROO, and by liver microsomes via an oxidative decarboxylation mechanism leading to the corresponding quinone-methides (QM). This article shows that peroxidases, such as horseradish peroxidase (HRP), lactoperoxidase (LPO) and prostaglandin synthase (PGHS), and other hemeproteins, such as methemoglobin (metHb), metmyoglobin (metMb) and catalase, also efficiently catalyze the oxidation of TAM radicals to QM by H₂O₂ or alkylhydroperoxides. These reactions involve the intermediate formation of the corresponding cations TAM⁺ that have also been cleanly generated by K₂Ir(IV)Cl₆ and characterized by UV-Visible spectroscopy and mass spectrometry, and through their reactions with ascorbate or H₂O₂. Labelling experiments on HRP-catalyzed oxidation of TAM to QM using H₂¹⁸O or ¹⁸O₂ in the presence of glucose and glucose oxidase (GOX) showed that the oxygen atom incorporated into QM came both from O₂ and from H₂O. Mechanisms for these reactions in agreement with those data were proposed. Oxidative decarboxylation of TAM to QM is a new reaction catalyzed by peroxidases. Such reactions should be considered when using TAM as EPR oximetry probes invivo or in vitro in complex biological media.     

Radicals associated with the catalytic intermediates of bovine cytochrome c oxidase

Two radicals have been detected previously by electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopies in bovine cytochrome oxidase after reaction with hydrogen peroxide, but no correlation could be made with predicted levels of optically detectable intermediates (P_M, F and F) that are formed. This work has been extended by optical quantitation of intermediates in the EPR/ENDOR sample tubes, and by comparison with an analysis of intermediates formed by reaction with carbon monoxide in the presence of oxygen. The narrow radical, attributed previously to a porphyrin cation, is detectable at low levels even in untreated oxidase and increases with hydrogen peroxide treatments generally. It is presumed to arise from a side-reaction unrelated to the catalytic intermediates. The broad radical, attributed previously to a tryptophan radical, is observed only in samples with a significant level of F but when F is generated with hydrogen peroxide, is always accompanied by the narrow radical. When P_M is produced at high pH with CO/O₂, no EPR-detectable radicals are formed. Conversion of the CO/O₂-generated P_M into F when pH is lowered is accompanied by the appearance of a broad radical whose ENDOR spectrum corresponds to a tryptophan cation. Quantitation of its EPR intensity indicates that it is around 3% of the level of F determined optically. It is concluded that low pH causes a change of protonation pattern in P_M which induces partial electron redistribution and tryptophan cation radical formation in F. These protonation changes may mimic a key step of the proton translocation process.     

Reaction of N-hydroxyguanidine with the ferrous-oxy state of a heme peroxidase cavity mutant: A model for the reactions of nitric oxide synthase

Yeast cytochrome c peroxidase was used to construct a model for the reactions catalyzed by the second cycle of nitric oxide synthase. The R48A/W191F mutant introduced a binding site for N-hydroxyguanidine near the distal heme face and removed the redox active Trp-191 radical site. Both the R48A and R48A/W191F mutants catalyzed the H₂O₂ dependent conversion of N-hydroxyguanidine to N-nitrosoguanidine. It is proposed that these reactions proceed by direct one-electron oxidation of NHG by the Fe⁺⁴O center of either Compound I (Fe⁺⁴O, porph⁺) or Compound ES (Fe⁺⁴O, Trp⁺). R48A/W191F formed a Fe⁺²O₂ complex upon photolysis of Fe⁺²CO in the presence of O₂, and N-hydroxyguanidine was observed to react with this species to produce products, distinct from N-nitrosoguanidine, that gave a positive Griess reaction for nitrate + nitrite, a positive Berthelot reaction for urea, and no evidence for formation of NO. It is proposed that HNO and urea are produced in analogy with reactions of nitric oxide synthase in the pterin-free state.     

Spectroscopic and electrochemical properties of a new nitrosyl-complex of Ru(II): trans-[Ru(NO){(CH3CH2)2PCH2CH2P(CH2CH3)2}2Cl](PF6)2

A new ruthenium nitric oxide complex with the bidentate phosphine, 1,2-bis(diethylphosphino)ethane (depe), has been synthesized and characterized by UV-Vis, infrared, EPR, NMR, electrochemical techniques and X-ray structure determination. The electronic spectrum showed a typical band of dπ→pπ* charge-transfer (CT) transition, assigned to Ru(II)NO transition, and the vibrational spectrum exhibited a peak of nitrosyl ligand at (ν_NO=1851 cm⁻¹). A model structure for this complex has been proposed based on ¹H, ¹H{³¹P}, ³¹P{¹H}, ¹³C{¹H}, COSY ¹H¹H{³¹P}, J-Resolved, HSQC, HMBC, HSQC ¹H¹³C{³¹P} and ¹H¹³C HSQC/¹H¹H TOCSY spectral data, and confirmed by X-ray diffraction. The nitrosonium character for the NO ligand become evident through both electron paramagnetic resonance and X-ray data (angle RuNO=177.4(3)°). The reversible monoeletronic process at E_1/2=0.040 V versus SHE was assigned to the ligand NO⁺/NO redox couple. Under treatment with Cd(Hg) solutions containing the [Ru(NO)(depe)₂Cl](PF₆)₂ yields a signal in the EPR spectrum (g_⊥=1.99 and g_//=1.88) which fitted quite well with the simulated spectra of coordinated NO species.     

Evidence for a pathway that facilitates nitric oxide diffusion in the brain

Nitric oxide (NO) is a diffusible messenger that conveys information based on its concentration dynamics, which is dictated by the interplay between its synthesis, inactivation and diffusion. Here, we characterized NO diffusion in the rat brain in vivo. By direct sub-second measurement of NO, we determined the diffusion coefficient of NO in the rat brain cortex. The value of 2.2 × 10⁻⁵ cm²/s obtained in vivo was only 14% lower than that obtained in agarose gel (used to evaluate NO free diffusion). These results reinforce the view of NO as a fast diffusing messenger but, noticeably, the data indicates that neither NO diffusion through the brain extracellular space nor homogeneous diffusion in the tissue through brain cells can account for the similarity between NO free diffusion coefficient and that obtained in the brain. Overall, the results support that NO diffusion in brain tissue is heterogeneous, pointing to the existence of a pathway that facilitates NO diffusion, such as cell membranes and other hydrophobic structures.     

Palladium(II) amido complexes with an unsymmetrical PNP′ pincer-type coordination and a new (E,E)-tetradentate diphosphinoazine coordination mode

Cationic palladium(II) complexes [PdCl{PR₂CH₂C(Bu^t)NNC(Bu^t)CH₂PR₂}]Cl, where R = isopropyl, cyclohexyl or tert-butyl, were synthesized by the reactions of the corresponding diphosphinoazines with bis(acetonitrile)palladium(II) dichloride. When bis(benzonitrile)palladium(II) dichloride was used instead, in the molar ratio of 2:1 to the diphosphinoazine, a new complex was isolated with the isopropyl ligand showing a previously unknown (E,E) tetradentate coordination mode. Crystal and molecular structure was determined by X-ray diffraction. The solid complex was a racemate of two axially chiral enantiomers and the chirality was preserved in solution. Reactions of the cationic complexes with triethylamine gave complexes [PdCl{PR₂CHC(Bu^t)NNC(Bu^t)CH₂PR₂}], containing deprotonated diphosphinoazines in ene-hydrazone unsymmetrical pincer-like configuration. The complexes represent several of the still rare examples of Pd(II) amido bis(phosphine) complexes with a chlorine atom covalently bonded trans to the amide nitrogen.     

Co and Cu complexes of reduced Schiff bases: Generation of phenoxyl radical species

The three complexes [Co^IIIL¹Cl] (1), [Co^IIIL²]⁺·ClO₄⁻ (2⁺·ClO₄⁻), and [Cu^IIH₂L²]²⁺·2ClO₄⁻ (H₂3²⁺·2ClO₄⁻) [where H₂L¹ = N,N′-dimethyl-N,N′-bis(2-hydroxy-3,5-di-tert-butylbenzyl)ethylenediamine, H₂L² = N,N′-bis(2-pyridylmethyl)-N,N′-bis(2-hydroxy-3,5-di-tert-butylbenzyl)ethylenediamine] have been prepared. The bis-phenolate and bis-phenol complexes, 2⁺ and H₂3²⁺ respectively, have been characterized by X-ray diffraction, showing a metal ion within an elongated octahedral geometry. 1-2 exhibit in their cyclic voltammetry curves two anodic reversible waves attributed to the successive oxidation of the phenolates into phenoxyl radicals. The cobalt radical species (1)⁺, (2)²⁺, and (2)³⁺ have been characterized by combined UV-Vis and EPR spectroscopies. In the presence of one equivalent of base, one phenolic arm of H₂3²⁺ is deprotonated and coordinates the metal. The resulting complex (H3⁺) exhibits a single reversible redox wave at ca. 0.3 V. The electrochemically generated oxidized species is EPR silent and exhibits the typical features of a radical compound, with absorption bands at 411 and 650 nm. The fully deprotonated complex 3 is obtained by addition of two equivalents of nBu₄N⁺OH⁻ to H₂3²⁺. It exhibits a new redox wave at a lower potential (−0.16 V), in addition to the wave at ca. 0.3 V. We assigned the former to the one-electron oxidation of the uncoordinated phenolate into an unstable phenoxyl radical.     

Atmospheric pressure photoionization mass spectrometry of per-O-methylated oligosaccharides related to D-xylans

Three d-xylan type per-O-methylated trisaccharides with various types of linkages between the d-xylopyranose units were examined by atmospheric pressure photoionization (APPI) mass spectrometry in the positive ion mode. The most interesting feature of a thermospray mass spectrum using the APPI source with UV lamp switched off, is the exclusive production of [M+Na]⁺ adduct ions. [M+Na]⁺ cationized ions are the most abundant species in the case of APPI mass spectrometry. The second ionization process has no analogy in the case of substances studied using APPI to date. This aspect involves the addition of a water molecule to the molecular ion of a per-O-methylated saccharide, giving rise to [M+H₂O]⁺ adduct ions. The [M+H₂O]⁺ species are readily detected at m/z 544, and are clearly visible for all three isomers studied. The MS/MS spectrum of [M+Na]⁺ ions contains a base peak at m/z 375, produced by a Y-type cleavage of the trisaccharide, along with a hydrogen rearrangement on the terminal interglycosidically linkage glycosidic oxygen atom. The [M+H₂O]⁺ species fragment largely give rise to ions at m/z 175, 143 and, as a result, the m/z 111 ion is unique to nonreducing terminal units.     

ortho-Carom-N bond fusion in aniline associated with electrophilic chlorination reactions at ruthenium(III) coordinated acetylacetonates

In an unusual reaction of [Ru^III(acac)₂(CH₃CN)₂](ClO₄) ([1], acac = acetylacetonate) and aniline (Ph-NH₂), resulted in the formation of ortho-semidine due to dimerisation of aniline via oxidative ortho-C_arom-N bond formation reaction. This oxidation reaction is associated with stepwise chlorination of coordinated acac ligands at the γ-carbon atom resulting in the formation of [Ru^III(acac)₂L⁻] [2a], [Ru^III(Cl-acac)(acac)L⁻] [2b], [Ru^III(acac)(Cl-acac)L⁻] [2c] and [Ru^III(Cl-acac)₂L⁻] [2d] (L⁻ = N-phenyl-ortho-semiquinonediimine) complexes, respectively. These have been characterized by ¹H NMR, UV-Vis-NIR, ESI-MS and cyclic voltammetry studies. Single crystal X-ray structures of 2c and 2d are reported. Crystallographic structural bond parameters of 2c and 2d revealed bond length equalization of C-C, C-O and M-O bonds. It has been shown that perchlorate () counter anion, present in the starting ruthenium complex, acts as the oxidizing agent in bringing about oxidation of Ph-NH₂ to ortho-semidine. The chloronium ions, produced in situ, chlorinate the coordinated acac ligands at the γ-carbon atom. Such electrophilic substitution of coordinated acac ligands indicates that the Ru-acac metallacycles in the reference compounds are aromatic. The complexes showed an intense and featureless band centered near 520 nm, and a structured band near 275 nm. These displayed one reversible cathodic response in the range, −1.1 to −0.8 V and one reversible anodic response between 0.4 and 0.6 V versus the Saturated Calomel reference Electrode, SCE. The response at the anodic potential is due to oxidation of the coordinated ligand L, while the reversible response at cathodic potential is due to reduction of the metal center.     

A new type of electrochemical oxidation of alcohols mediated with a ruthenium-dioxolene-amine complex in neutral water

Electrochemical oxidation of [Ru^II(terpy)(sq)(NH₃)]⁺ in neutral water (pH 8.0) at +0.8 V (versus SCE) generated [Ru^II(terpy)(q)(NH₂)]²⁺ and/or [Ru^III(terpy)(sq)(NH₂)]²⁺ (terpy = 2,2′:6′,2′′-terpyridine, sq = 3,5-di-tert-butyl-1,2-semiquinonate, q = 3,5-di-tert-butyl-1,2-benzoquinone), which played roles in hydrogen abstraction and one-electron acceptor in the catalytic oxidation of methanol, ethanol, and 2-propanol affording formaldehyde, acetoaldehyde, and acetone, respectively, under the electrolysis conditions.     

Regioselective addition of bifunctional oximehydroxamic acid by the hydroxamic group to Pt(IV)-coordinated nitriles

The metal-mediated coupling between the nitriles RCN in the platinum(IV) complexes trans-[PtCl₄(RCN)₂] (RMe, Et, CH₂Ph, Ph), cis/trans-[PtCl₄(MeCN)(Me₂SO)] and the newly synthesized bifunctional oximehydroxamic acid, viz. N,2-dihydroxy-5-(1-hydroxyiminoethyl)benzamide, proceeds smoothly in CH₂Cl₂ at 40-45 °C to accomplish the new metallaligands HNC(R)ONHC(O)C₆H₃(2-OH)(5-C(Me)NOH) with pendant oxime functionalities due to the regioselective addition of the reagent via its hydroxamic groups. The obtained iminoligands exist in hydroxamic/hydroximic tautomeric equilibrium in solution. The structures of the isolated compounds are based on elemental analyses (C, H, N), IR, 1D ¹H, ¹³C{¹H}, and 2D NMR correlation experiments, i.e. ¹H,¹³C-COSY, ¹H,¹³C long range COSY, ¹H,¹⁵N-COSY, and ¹H,¹⁵N long range COSY.