Brigitte Schwederski

Coordination compounds of pteridine, alloxazine and flavin ligands: structures and properties

Abstract The pteridine and (iso)alloxazine π systems are the underlying heterocyclic structures of coenzymatic constituents (pterins, lumazines, flavins) of oxidoreductase enzymes. These molecules are potential ligands for metal centers which, in the absence of additional coordination sites, bind predominantly through O(4) and N(5) to form five-membered, redox-active α -iminoketo chelate rings. Recent results from spectroscopic and electrochemical studies and structural data are summarized in this article.

C4 Cumulene and the Corresponding Air‐Stable Radical Cation and Dication

A neutral C4 cumulene 1 that includes a cyclic alkyl(amino) carbene (cAAC), its air-stable radical cation 1(·+) , and dication 1(2+) have been synthesized. The redox property of 1(·+) was studied by cyclic voltammetry. EPR and theoretical calculations show that the unpaired electron in 1(·+) is mainly delocalized over the central C4 backbone. The commercially available CBr4 is utilized as a source of dicarbon in the cumulene synthesis.

Isolation of Neutral Mononuclear Copper Complexes Stabilized by Two Cyclic (Alkyl)(amino)carbenes

Two (cAAC)2Cu complexes, featuring a two-coordinate copper atom in the formal oxidation state zero, were prepared by reducing (Et2-cAAC)2Cu(+)I(-) with metallic sodium in THF, and by a one-pot synthesis using Me2-cAAC, Cu(II)Cl2, and KC8 in toluene in a molar ratio of 2:1:2, respectively. Both complexes are highly air and moisture sensitive but can be stored in the solid state for a month at room temperature. DFT calculations showed that in these complexes the copper center has a d(10) electronic configuration and the unpaired electron is delocalized over two carbene carbon atoms. This was further confirmed by the EPR spectra, which exhibit multiple hyperfine lines due to the coupling of the unpaired electron with (63,65)Cu isotopes, (14)N, and (1)H nuclei.

Enhanced hydroxyl radical production by dihydroxybenzene-driven Fenton reactions: implications for wood biodegradation.

Brown rot fungi degrade wood, in initial stages, mainly through hydroxyl radicals (•OH) produced by Fenton reactions. These Fenton reactions can be promoted by dihydroxybenzenes (DHBs), which can chelate and reduce Fe(III), increasing the reactivity for different substrates. This mechanism allows the extensive degradation of carbohydrates and the oxidation of lignin during wood biodegradation by brown rot fungi. To understand the enhanced reactivity in these systems, kinetics experiments were carried out, measuring •OH formation by the spin-trapping technique of electron paramagnetic resonance spectroscopy. As models of the fungal DHBs, 1,2-dihydroxybenzene (catechol), 2,3-dihydroxybenzoic acid and 3,4-dihydroxybenzoic acid were utilized as well as 1,2-dihydroxy-3,5-benzenedisulfonate as a non-Fe(III)-reducing substance for comparison. Higher amounts and maintained concentrations of •OH were observed in the driven Fenton reactions versus the unmodified Fenton process. A linear correlation between the logarithms of complex stability constants and the •OH production was observed, suggesting participation of such complexes in the radical production.

Rhenium(I) coordinated lumazine and pterin derivatives: structure and spectroelectrochemistry of reversibly reducible (6-ATML)Re(CO)3Cl (6-ATML=6-acetyl-1,3,7-trimethyllumazine)

Abstract A number of complexes between substituted lumazines (2,4-dioxo-(1H,3H)pteridines) or pterins (2-amino-4-oxo-(3H)pteridines) and Re(CO)3Cl have been synthesized and characterized electrochemically and spectroscopically. The structure of the (6-ATML)Re(CO)3Cl derivative (6-ATML=6-acetyl-1,3,7-trimethyllumazine) could be determined in [fac-(6-ATML)Re(CO)3Cl]·3C6H6. The rhenium(I) center coordinates in a rather symmetrical fashion through the O4–N5 α-carbonylimino chelate site of 6-ATML; the potentially available acetyl function is not involved in the metal coordination. The acetyl acceptor substituent facilitates the reversible one-electron reduction of this compound to a persistent anion radical complex [(6-ATML)Re(CO)3Cl] −, which is accompanied by variable low-frequency shifts of the six carbonyl stretching bands and by the appearance of a broad EPR signal.

Stabilization of a cobalt-cobalt bond by two cyclic alkyl amino carbenes.

(Me2-cAAC:)2Co2 (2, where Me2-cAAC: = cyclic alkyl amino carbene, :C(CH2)(CMe2)2N-2,6-iPr2C6H3)) was synthesized via the reduction of precursor (Me2-cAAC:Co(II)(μ-Cl)Cl)2 (1) with KC8. 2 contains two cobalt atoms in the formal oxidation state zero. Magnetic measurement revealed that 2 has a singlet spin ground state S = 0. The cyclic voltammogram of 2 exhibits both one-electron oxidation and reduction, indicating the possible synthesis of stable species containing 2(•-) and 2(•+) ions. The latter was synthesized via reduction of 1 with required equivalents of KC8 and characterized as [(Me2-cAAC:)2Co2](•+)OTf(-) (2(•+)OTf(-)). Electron paramagnetic resonance spectroscopy of 2(•+) reveals the coupling of the electron spin with 2 equiv (59)Co isotopes, leading to a (Co(0.5))2 state. The experimental Co1-Co2 bond distances are 2.6550(6) and 2.4610(6) Å for 2 and 2(•+)OTf(-), respectively. Theoretical investigation revealed that both 2 and 2(•+)OTf(-) possess a Co-Co bond with an average value of 2.585 Å. A slight increase of the Co-Co bond length in 2 is more likely to be caused by the strong π-accepting property of cAAC. 2(•+) is only 0.8 kcal/mol higher in energy than the energy minimum. The shortening of the Co-Co bond of 2(•+) is caused by intermolecular interactions.

Cooperation of metals with electroactive ligands of biochemical relevance: Beyond metalloporphyrins*

In addition to the widely studied biometal complexes of tetrapyrrole ligands such as hemes (Fe), cobalamins (Co), and factor F430 (Ni), there are other, more recently established systems in which transition metals and redox-active cofactors such as pterins, flavins, quinones, or phenoxyl radicals cooperate in electron transfer and substrate activation. The cases of the molybdenum or tungsten containing oxotransferases involving pyranopterin as essential ligand and the copper-dependent quinoproteins such as amine oxidases will be discussed. The structural and functional description of these systems will be complemented by results from model studies.

Tri(supersilyl)dialanyl (tBu3Si)3Al2• and Tetra(supersilyl)cyclotrialanyl (tBu3Si)4Al3• − New Stable Radicals of a Group 13 Element from Thermolysis of (tBu3Si)4Al2

The thermolysis of tetra(supersilyl)dialane R*2Al−AlR*2 (R* = SitBu3 = Supersilyl) in heptane or cyclohexane at 50 °C leads under Al−Al dissociation reversibly to monoalanyl radicals [R*2Al]•, which could be trapped by hydrogen or iodine (formation of R*2AlH, R*2AlI). Simultaneously, SiAl dissociation and elimination of supersilyl radicals tBu3Si• leads irreversibly and slowly to radicals [R*2Al−AlR*]•, the existence of which could be established by ESR spectroscopy. The structure was clarified by ab initio calculations (nearly planar Si2Al−AlSi and linear Al−Al−Si skeleton; short Al−Al distance). By thermolyzing R*2Al−AlR*2 at 100 °C, the radical [R*4Al3]• (ESR spectroscopically detected) and the tetrahedro-tetraalane R*4Al4 (NMR spectroscopically seen) are formed via radicals [R*3Al2]•; the supersilyl radicals tBu3Si•, formed at the same time, are stabilized by dimerization to superdisilane R*−R* and by taking up hydrogen, giving supersilane R*−H. According to X-ray structure analysis, the Al atoms in [R*4Al3]• are located at the corners of a triangle; one Al atom is connected with two groups R*, the remaining two Al atoms each bind one substituent R* in different ways.

Electrochemistry, electron spin resonance, optical spectroscope, and reactivity of organometallic platinum(II) complexes containing strongly π-accepting aromatic α-diimine ligands

Abstract Low-lying metal-to-ligand charge transfer (MLCT) transitions of the new complexes LPtPh 2 (L = 3,3′-bipyridazine (bpdz), 4,4′-bipyrimidine (bpm), 2,2′-bipyrazine (bpz) and 1,4,7,10-tetraazaphenanthrene (tap)) have been studied by emission and solvent-dependent absorption spectroscope. The complex (bpm) PtPh 2 is strongly solvatochromic, and exhibits five absorption bands in toluene solution, but does not show luminescence. The other complexes emit visible light on excitation in the solid or in acetonitrile solution (bpdz complex). The one- and two-electron-reduced states were generated electrochemically; the ESR spectra of the paramagnetic intermediates, including [(bpy)PtPh 2 ] −· , are consistent with the formulation Pt II /(L −· ). The rate of oxidative addition of iodomethane to Ph 2 Pt(L) correlates with the basicity of L.

First crystal structure determination and high-frequency EPR study of an organoarsanecopper radical complex

Abstract The red radical complex {(μ-bptz)[Cu(AsPh3)2]2}(BF4) 1, bptz=3,6-bis(2-pyridyl)-1,2,4,5-tetrazine, was obtained as a stable species and characterized by X-ray crystallography, spectroelectrochemistry and EPR at 9.5 and 285 GHz. Comparison with the previously reported {(μ-bptz)[Cu(PPh3)2]2}(BF4) 2 reveals longer Cu-element bonds by about 0.1 A but otherwise a similar “organic sandwich” structure involving intramolecular π(phenyl)–π(tetrazine)–π(phenyl) interactions, a 3+1 coordination at the copper(I) centers, and averaged tetrazine intraring bond distances. Reversible oxidation to a blue dication with an intense MLCT band at 650 nm occurs at −0.24 V vs. Fc+/o. EPR studies show the effect of the higher spin–orbit coupling constant of the As vs. P atoms through slightly larger g anisotropy as determined through high frequency measurements.