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%.
The spectroscopic properties and the electronic structure of the only nitrous oxide complex existing in isolated form, [Ru(NH(3))(5)(N(2)O)]X(2) (1, X = Br(-), BF(4)(-)), are investigated in detail in comparison to the nitric oxide precursor, [Ru(NH(3))(5)(NO)]X(3) (2). IR and Raman spectra of 1 and of the corresponding (15)NNO labeled complex are presented and assigned with the help of normal coordinate analysis (NCA) and density functional (DFT) calculations. This allows for the identification of the Ru-N(2)O stretch at approximately 300 cm(-)(1) and for the unambiguous definition of the binding mode of the N(2)O ligand as N-terminal. Obtained force constants are 17.3, 9.6, and 1.4 mdyn/A for N-N, N-O, and Ru-N(2)O, respectively. The Ru(II)-N(2)O bond is dominated by pi back-donation, which, however, is weak compared to the NO complex. This bond is further weakened by Coulomb repulsion between the fully occupied t(2g) shell of Ru(II) and the HOMO of N(2)O. Hence, nitrous oxide is an extremely weak ligand to Ru(II). Calculated free energies and formation constants for [Ru(NH(3))(5)(L)](2+) (L = NNO, N(2), OH(2)) are in good agreement with experiment. The observed intense absorption at 238 nm of 1 is assigned to the t(2g) --> pi(*) charge transfer transition. These data are compared in detail to the spectroscopic and electronic structural properties of NO complex 2. Finally, the transition metal centered reaction of nitrous oxide to N(2) and H(2)O is investigated. Nitrous oxide is activated by back-donation. Initial protonation leads to a weakening of the N-O bond and triggers electron transfer from the metal to the NN-OH ligand through the pi system. The implications of this mechanism for biological nitrous oxide reduction are discussed.
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.
IR and Raman spectra of [Ru(NH3)5(N2O)]Br2 and [Ru(NH3)5(N2O)](BF4)2 and of the corresponding 15 NNO labeled complex are presented and assigned with the help of normal coordinate analysis (NCA) and density functional (DFT) calculations. The results unambiguously show that the N 2 O ligand is end-on terminally bonded via the N atom to Ru II . Nitrous oxide is found to act as an extremely weak ligand to Ru II . The results are compared in detail with spectroscopic and electronic properties of the NO complex [Ru(NH3)5(NO)]Br3. -(PAULAT, F.; KUSCHEL, T.; NAETHER, C.; PRANEETH, V. K. K.; SANDER, O.; LEHNERT*, N.; Inorg.
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