Eberhard Bothe

An octahedral coordination complex of iron(VI)

The hexavalent state, considered to be the highest oxidation level accessible for iron, has previously been found only in the tetrahedral ferrate dianion, FeO42–. We report the photochemical synthesis of another Fe(VI) compound, an octahedrally coordinated dication bearing a terminal nitrido ligand. Mössbauer and x-ray absorption spectra, supported by density functional theory, are consistent with the octahedral structure having an FeN triple bond of 1.57 angstroms and a singlet \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{d}_{xy}^{2}\) \end{document} ground electronic configuration. The compound is stable at 77 kelvin and yields a high-spin Fe(III) species upon warming.

Neutral Bis(α-iminopyridine)metal Complexes of the First-Row Transition Ions (Cr, Mn, Fe, Co, Ni, Zn) and Their Monocationic Analogues: Mixed Valency Involving a Redox Noninnocent Ligand System

A series of bis(alpha-iminopyridine)metal complexes featuring the first-row transition ions (Cr, Mn, Fe, Co, Ni, and Zn) is presented. It is shown that these ligands are redox noninnocent and their paramagnetic pi radical monoanionic forms can exist in coordination complexes. Based on spectroscopic and structural characterizations, the neutral complexes are best described as possessing a divalent metal center and two monoanionic pi radicals of the alpha-iminopyridine. The neutral M(L*)2 compounds undergo ligand-centered, one-electron oxidations generating a second series, [(L(x))2M(THF)][B(ArF)4] [where L(x) represents either the neutral alpha-iminopyridine (L)0 and/or its reduced pi radical anion (L*)-]. The cationic series comprise mostly mixed-valent complexes, wherein the two ligands have formally different redox states, (L)0 and (L*)-, and the two ligands may be electronically linked by the bridging metal atom. Experimentally, the cationic Fe and Co complexes exhibit Robin-Day Class III behavior (fully delocalized), whereas the cationic Zn, Cr, and Mn complexes belong to Class I (localized) as shown by X-ray crystallography and UV-vis spectroscopy. The delocalization versus localization of the ligand radical is determined only by the nature of the metal linker. The cationic nickel complex is exceptional in this series in that it does not exhibit any ligand mixed valency. Instead, its electronic structure is consistent with two neutral ligands (L)0 and a monovalent metal center or [(L)2Ni(THF)][B(ArF)4]. Finally, an unusual spin equilibrium for Fe(II), between high spin and intermediate spin (S(Fe) = 2 <--> S(Fe) = 1), is described for the complex [(L*)(L)Fe(THF)][B(ArF)4], which consequently is characterized by the overall spin equilibrium (S(tot) = 3/2 <--> S(tot) = 1/2). The two different spin states for Fe(II) have been characterized using variable temperature X-ray crystallography, EPR spectroscopy, zero-field and applied-field Mössbauer spectroscopy, and magnetic susceptibility measurements. Complementary DFT studies of all the complexes have been performed, and the calculations support the proposed electronic structures.

Yields of Radiation-induced Main Chain Scission of Poly U in Aqueous Solution: Strand Break Formation Via Base Radicals

The G values for single-strand breaks G(ssb) in polyuridylic acid (poly U) have been measured by low-angle laser light scattering in aqueous solutions under various conditions (e.g. in the presence of N2O, Ar and t-butanol). In N2O-saturated solutions at room temperature and pH 5.6, the G(ssb) is 2.3. The efficiency of ssb formation was found to be 41 per cent for OH radicals, 19 per cent for H atoms and congruent to zero for e-aq. On the basis of 20 per cent and less than 5 per cent attack on the sugar moiety by OH radicals and H atoms, respectively, the large G(ssb) values obtained cannot be explained solely as resulting from radicals produced by reaction of OH radicals and H atoms on the sugar moiety. It is therefore proposed that base radicals produced by the reaction of OH radicals or H atoms with the uracil moiety can also lead to chain break formation in poly U via radical transfer to the sugar moiety.

HO2 ELIMINATION FROM α-HYDROXYALKYLPEROXYL RADICALS IN AQUEOUS SOLUTION

Abstract— In aqueous solutions α‐hydroxyalkylperoxyl radicals undergo a spontaneous and a base catalysed HO2 elimination. From kinetic deuterium isotope effects, temperature dependence, and the influence of solvent polarity it was concluded that the spontaneous reaction occurs via an HO2 elimination followed by the dissociation of the latter into H+ and O2‐. The rate constant of the spontaneous HO2 elimination increases with increasing methyl substitution in α‐position (k(CH2(OH)O2) < 10s‐1k(CH3CH(OH)O2) = 52s‐1k((CH3)2C(OH)O2) = 665 s‐1). The OH‐ catalysed reaction is somewhat below diffusion controlled. The mixture of peroxyl radicals derived from polyhydric alcohols eliminate HO2 at two different rates. Possible reasons for this behaviour are discussed. The mixture of the six peroxyl radicals derived from d‐glucose are observed to eliminate HO2 with at least three different rates. The fastest rate is attributed to the HO2 elimination from the peroxyl radical at C‐l (k > 7000s‐1). Because of the HO2 eliminations the peroxyl radicals derived from d‐glucose do not undergo a chain reaction in contrast to peroxyl radicals not containing an α‐OH group. In competition with the first order elimination reactions the α‐hydroxylalkylperoxyl radicals undergo a bimolecular decay. These reactions are briefly discussed.

Phenoxyl-copper(II) complexes: models for the active site of galactose oxidase

Abstract The reaction of the macrocycles 1,4,7-tris (3,5-di-tert-butyl-2-hydroxy-benzyl)-1,4,7-triazacyclononane, L1H3, or 1,4,7-tris(3-tert-butyl-5-methoxy-2-hydroxy-benzyl)-1,4,7-triazacyclononane, L2H3, with Cu(ClO4)2·6H2O in methanol (in the presence of Et3N) affords the green complexes [CuII(L1H)] (1), [CuII(L2H)]·CH3OH (2) and (in the presence of HClO4) [CuII(L1H2)](ClO4) (3) and [CuII(L2H2)] (ClO4) (4). The CuII ions in these complexes are five-coordinate (square-base pyramidal), and each contains a dangling, uncoordinated pendent arm (phenol). Complexes 1 and 2 contain two equatorially coordinated phenolato ligands, whereas in 3 and 4 one of these is protonated, affording a coordinated phenol. Electrochemically, these complexes can be oxidized by one electron, generating the phenoxyl-copper(II) species [CuII(L1H)]+ ·, [Cu(L2H)]+ ·, [CuII(L1H2)]2+ ·, and [CuII(L2H2)]2+ ·, all of which are EPR-silent. These species are excellent models for the active form of the enzyme galactose oxidase (GO). Their spectroscopic features (UV-VIS, resonance Raman) are very similar to those reported for GO and unambiguously show that the complexes are phenoxyl-copper(II) rather than phenolato-copper(III) species.

Hydroxyl Radical-induced Strand Break Formation of Poly(U) in the Presence of Oxygen: Comparison of the Rates as Determined by Conductivity, e.s.r. and Rapid-mix Experiments with a Thiol

The rate of OH radical-induced strand break formation of single-stranded poly(U) in N2O/O2-saturated aqueous solution was studied by measuring the time-dependence of the electrical conductivity following pulse radiolysis. The first half-life of the total conductivity increase depends slightly on pH and the molecular weight and on the dose per pulse. The activation parameters for strand break formation were found to be EA = 52 kJ mol-1 and A = 5 X 10(8) s-1. Similar first half-lives were observed when the decay of peroxyl radicals of poly(U) was measured by e.s.r. under various conditions. This indicates that poly(U)-peroxyl radicals are involved in the rate-determining step of strand break formation. After pulse radiolysis, strand break formation can be inhibited by the addition of dithiothreitol (DTT) in a rapid-mix apparatus. It is postulated that peroxyl radicals of poly(U) react with DTT by formation of hydroperoxides, thereby preventing strand breakage.

SINGLE‐ and DOUBLE‐STRAND BREAK FORMATION IN DOUBLE‐STRANDED DNA UPON NANOSECOND LASER‐INDUCED PHOTOIONIZATION

Abstract— Double‐stranded (ds) calf thymus DNA (0.4 mM), excited by 20 ns laser pulses at 248 nm, was studied in deoxygenated aqueous solution at room temperature and pH 6.7 in the presence of a sodium salt (10 mM). The quantum yields for the formation of hydrated electrons (Φe.), single‐strand breaks (Φssb) and double‐strand breaks (Φdsb) were determined for various laser pulse intensities (I1). Φc. and Φssb increase linearly with increasing IL; however, Φssb has a tendency to reach saturation at high IL (Φ5 times 100 Wcm−2). The ratio Phi;ssh/Φc‐. representing the number of ssb per radical cation, is about 0.08 at IL 5 times 106 Wcm−2. For comparison, the number of ssb per OH radical reacting with dsDNA is 0.22. On going from argon to N2O saturation, Φssb and Φdsb become larger by factors of 5 and10–15, respectively. This enhancement is produced by attack on DNA bases by OH radicals generated by N2O‐scavenging of the photoelectrons. While Φssb is essentially independent of the dose (Etot), Φdsb, depends linearly on Etot in both argon‐ and N2O‐saturated solutions. The linear dependence of Φdsb implies a square dependence of the number of dsb on Etot. This portion of dsb formation is explained by the occurrence of two random ssb, generated within a critical distance (h) in opposite strands. For both argon‐ and N2O‐saturated solutions h was found to be of the order of40–70 phosphoric acid diester bonds. On addition of electron scavengers such as 2‐chloroethanoI (or N2O plus t‐butanol), Φdsb is similar to that in neat, argon‐saturated solutions. Thus, hydrated electrons are not involved in the chemical pathway leading to laser‐pulse‐induced dsb of DNA.

Pulse radiolysis in model studies toward radiation processing

Abstract Using the pulse radiolysis technique, the OH-radical-induced reactions of poly(vinyl alcohol) PVAL, poly(acrylic acid) PAA, poly(methacrylic acid) PMA, and hyaluronic acid have been investigated in dilute aqueous solution. The reactions of the free-radical intermediates were followed by UV-spectroscopy and low-angle laser light-scattering; the scission of the charged polymers was also monitored by conductometry. For more detailed product studies, model systems such as 2,4-dihydroxypentane (for PVAL) and 2,4-dimethyl glutaric acid (for PAA) was also investigated. With PVA, OH-radicals react predominantly by abstraction of an H-atom in α-position to the hydroxyl group (70%). The observed bimolecular decay rate constant of the PVAL-radicals decreases with time. This has been interpreted as being due to an initially fast decay of proximate radicals and a decrease of the probability of such encounters with time. Intramolecular crosslinking (loop formation) predominates at high doses per pulse. In the presence of O2, peroxyl radicals are formed which in the case of the α-hydroxyperoxyl radicals can eliminate HO2-radicals in competition with bimolecular decay processes which lead to a fragmentation of the polymer. In PAA, radicals both in α-position (characterized by an absorption near 300 nm) and in β-position to the carboxylate groups are formed in an approximately 1:2 ratio. The lifetime of the radicals increases with increasing electrolytic dissociation of the polymer. The β-radicals undergo a slow (intra- as well as intermolecular) H-abstraction yielding α-radicals, in competition to crosslinking and scission reactions. In PMA only β-radicals are formed. Their fragmentation has been followed by conductometry. In hyaluronic acid, considerable fragmeentation is observed even in the absence of oxygen which, in fact, has some protective effect against this process. Thus free-radical attack on this important biopolymer makes it especially vulnerable with respect to a reduction of its viscosity, and in rheumatic diseases this effect may be the reason for their painfulnes.

An Electron‐Transfer Series of High‐Valent Chromium Complexes with Redox Non‐Innocent, Non‐Heme Ligands

An investigation of the electronic interplay between ligand radical(s) and a high-valent metal center in the three-member electron-transfer series shown in the picture reveals that, upon oxidation and removal of both ligand radicals, the chromium center becomes reduced from Cr{sup IV} to Cr{sup III} with concomitant formation of an imidyl radical (NR{center_dot}){sup -}.

Cobalt(II)/(III) Complexes Containingo-Iminothiobenzosemiquinonato(1−) ando-Iminobenzosemiquinonato(1−) π-Radical Ligands

The coordination chemistry of 6-amino-2,4-di-tert-butylthiophenol (H[LSAP]) and of 2-anilino-4,6-di-tert-butylphenol (H[LOAP]) with cobalt(II) ions was investigated in the presence and absence of dioxygen. It was shown that both compounds were redox-non-innocent ligands, existing in different protonation and oxidation levels as: (i) o-aminophenolates(1−) [(LXAP)1−], (ii) o-imidophenolates(2−) [(LXIP)2−], and (iii) one-electron oxidized forms o-iminobenzosemiquinonates(1−) [(LXISQ)1−], which are π-radicals (X = S, O for the sulfur or oxygen derivative). The following complexes were synthesized: [CoII(LSAP)2]2 (1), [CoII(LSISQ)2]2·0.5n-hexane (2·0.5n-hexane), [CoIIII(LSISQ)2] (3), [CoIIII(LOISQ)2] (4), and [CoIIICl(LOISQ)2] (5). The compounds 2·0.5n-hexane, 4, and 5 were structurally characterized by X-ray crystallography. It was shown that the different protonation and oxidation levels of the ligands could readily be identified by X-ray crystallography. Complex 2 could be reversibly oxidized and reduced electrochemically, yielding a monocation and a monoanion, respectively. It was shown that these processes were metal-centered ([CoIII(LSISQ)2]+, [CoII(LSISQ)2], and [CoI(LSISQ)2]−). The electronic structures were elucidated by UV/Vis and EPR spectroscopy and magnetochemistry. Complexes 3, 4, and 5 were shown to be singlet diradicals. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)