Siegfried Schindler

Reactivity of Copper(I) Complexes Towards Dioxygen

Efforts to model the reactions of copper enzymes with dioxygen with low molecular weight complexes have been quite successful during recent years. Guided by Nature, intriguing model systems for the active sites of such proteins have been developed. In the course of these investigations, it has been discovered that a whole variety of copper−dioxygen adducts form when simple copper(I) complexes are oxidized with dioxygen. The course of these reactions depends on the temperature, the ligands present, and the solvent. The effects of these parameters (especially the effect of ligand modification) on the reactivity of copper(I) complexes towards dioxygen are discussed in detail in this review.

Aliphatic C-H bond oxidation of toluene using copper peroxo complexes that are stable at room temperature.

Dinuclear copper peroxo complexes obtained from mononuclear copper(I) complexes showed extremely high stabilities under ambient conditions in the solid state and could be heated above 100 degrees C without decomposition. The increased stability could be explained with regard to their molecular structures. Furthermore, the four complexes investigated showed a high potential for aliphatic C-H bond oxidations: for example, technical-grade toluene was oxidized to benzaldehyde in yields of up to 20%.

Hydroperoxidation of methane and other alkanes with H2O2 catalyzed by a dinuclear iron complex and an amino acid

Abstract The compound [Fe2(HPTB)(μ-OH)(NO3)2](NO3)2·CH3OH·2H2O ( 1 ) containing a dinuclear iron(III) complex in which HPTB=N,N,N′,N′-tetrakis(2-benzimidazolylmethyl)-2-hydroxo-1,3-diaminopropane catalyzes the oxidation of alkanes with hydrogen peroxide in acetonitrile solution at room temperature only if certain amino acids (pyrazine-2-carboxylic, pyrazine-2,3-dicarboxylic or picolinic acid) are added to the reaction mixture. Alkyl hydroperoxides are formed as main reaction products. The turnover numbers attain 140 for cyclohexane, 21 for ethane and four for methane oxidation. The oxidation proceeds non-stereoselectively and bond selectivity parameters are low which testifies the participation of hydroxyl radicals in alkane functionalization.

Reversible Binding of Dioxygen by a Copper(I) Complex with Tris(2-dimethylaminoethyl)amine (Me6tren) as a Ligand

A reversible reaction with dioxygen is observed for the copper(I) complex [Cu(Me6tren)]+ (Me6tren=tris(2-dimethylaminoethyl)amine) at low temperatures. This complex displays an interesting crystal structure (depicted) and the interaction with dioxygen in solution was investigated by low-temperature UV/Vis spectroscopic methods.

Spectroscopic and Computational Studies of an End-on Bound Superoxo-Cu(II) Complex: Geometric and Electronic Factors that Determine the Ground State

A variety of techniques including absorption, magnetic circular dichroism (MCD), variable-temperature, variable-field MCD (VTVH-MCD), and resonance Raman (rR) spectroscopies are combined with density functional theory (DFT) calculations to elucidate the electronic structure of the end-on (η(1)) bound superoxo-Cu(II) complex [TMG(3)trenCuO(2)](+) (where TMG(3)tren is 1,1,1-tris[2-[N(2)-(1,1,3,3-tetramethylguanidino)]ethyl]amine). The spectral features of [TMG(3)trenCuO(2)](+) are assigned, including the first definitive assignment of a superoxo intraligand transition in a metal-superoxo complex, and a detailed description of end-on superoxo-Cu(II) bonding is developed. The lack of overlap between the two magnetic orbitals of [TMG(3)trenCuO(2)](+) eliminates antiferromagnetic coupling between the copper(II) and the superoxide, while the significant superoxo π*(σ) character of the copper dz(2) orbital leads to its ferromagnetically coupled, triplet, ground state.

Reactivity towards dioxygen of a copper(I) complex of tris(2-benzylaminoethyl)amine

Abstract The reaction of dioxygen with the copper(I) Bz 3 tren complex (Bz 3 tren=tris(2-benzylaminoethyl)amine) complex has been investigated using low temperature stopped-flow techniques. The formation of a superoxo as well as a peroxo complex as intermediates was detected spectroscopically. The copper(II) complexes [Cu(Bz 3 tren)H 2 O](ClO 4 ) 2 and [Cu(Bz 3 tren)Cl]Cl were synthesized and structurally characterized. Both complexes react with dioxygen in solution and formation of benzaldehyde was observed.

Syntheses, Structures, and Properties of Copper(II) Complexes of Bis(2-pyridylmethyl) Derivatives of o-, m-, and p-Phenylenediamine and Aniline

Copper(II) complexes of the ligand 1,n-bis[bis(2-pyridylmethyl)amino]benzene with n = 2-4 (1,n-tpbd) and its mononuclear derivative bis(2-pyridylmethyl)aniline (phbpa) were synthesized and structurally characterized. Magnetic measurements and DFT calculations were performed on [CuCl2(phbpa)], [Cu2Cl4(1,3-tpbd)], [(Cu2Cl2(ClO4)(1,3-tpbd))Cl(Cu2Cl2(OH2)(1,3-tpbd))](ClO4)2, and [Cu2(OH2)2(S2O6)(1,3-tpbd)]S2O6, and the exchange-polarization mechanism was successfully demonstrated.

Aromatic Hydroxylation in a Copper Bis(imine) Complex Mediated by a μ‐η2:η2 Peroxo Dicopper Core: A Mechanistic Scenario

Detailed mechanistic studies on the ligand hydroxylation reaction mediated by a copper bis(imine) complex are presented. Starting from a structural analysis of the CuI complex and the CuII product with a hydroxylated ligand, the optical absorption and vibrational spectra of starting material and product are analyzed. Kinetic analysis of the ligand hydroxylation reaction shows that O2 binding is the rate-limiting step. The reaction proceeds much faster in methanol than in acetonitrile. Moreover, an inverse kinetic isotope effect (KIE) is evidenced for the reaction in acetonitrile, which is attributed to a sterically congested transition state leading to the peroxo adduct. In methanol, however, no KIE is observed. A DFT analysis of the oxygenation reaction mediated by the micro-eta2:eta2 peroxo core demonstrates that the major barrier after O2 binding corresponds to electrophilic attack on the arene ring. The relevant orbital interaction occurs between the sigma* orbital of the Cu2O2 unit and the HOMO of the ligand. On the basis of the activation energy for the rate-limiting step (18.3 kcal mol(-1)) this reaction is thermally allowed, in agreement with the experimental observation. The calculations also predict the presence of a stable dienone intermediate which, however, escaped experimental detection so far. Reasons for these findings are considered. The implications of the results for the mechanism of tyrosinase are discussed.

A new carbonate bridged dinuclear zinc complex with tripodal amine ligands

Abstract A zinc complex with the tripodal ligand bis[2-(2-pyridyl)ethyl]-(2-pyridyl)methylamine (pmap) reacts with carbon dioxide to form a new dinuclear carbonate bridged zinc complex, [μ-CO3{Zn(pmap)}2](ClO4)2. This complex was structurally characterized and shows a binding mode for carbonate, which so far has not been observed for zinc carbonate complexes.

Syntheses and Characterization of Copper(II) Complexes of the New LigandsN-[(2-Pyridyl)methyl]-2,2′-dipyridylamine andN-[Bis(2-pyridyl)methyl]-2-pyridylamine

The two new ligands N-[(2-pyridyl)methyl]-2,2′-dipyridylamine (L1) and N-[bis(2-pyridyl)methyl]-2-pyridylamine (L2) have been synthesized and the copper(II) complexes [Cu(L1)2(CH3OH)2](ClO4)2, [Cu(L1)2(H2O)2](ClO4)2, [Cu3(L1)2(DMF)2Cl6], [Cu(L1)Cl2]n, [Cu(L2)(Cl)(ClO4)], and [Cu(L2)Cl2] have been structurally characterised. The reactions of dioxygen with the copper(I) complexes with ligands L1 and L2 were investigated, but no copper−dioxygen intermediates could be detected spectrophotometrically.