Valence-to-Core X-ray Emission Spectroscopy as a Probe of O-O Bond Activation in Cu O Complexes.

Valence-to-Core (VtC) X-ray emission spectroscopy (XES) was used to directly detect the presence of an O-O bond in a complex comprising the [Cu (μ-η :η -O )] core relative to its isomer with a cleaved O-O bond having a [Cu (μ-O) ] unit. The experimental studies are complemented by DFT calculations, which show that the unique VtC XES feature of the [Cu (μ-η :η -O )] core corresponds to the copper stabilized in-plane 2p π peroxo molecular orbital. These calculations illustrate the sensitivity of VtC XES for probing the extent of O-O bond activation in μ-η :η -O species and highlight the potential of this method for time-resolved studies of reaction mechanisms.

Revisiting the Synthesis and Nucleophilic Reactivity of an Anionic Copper Superoxide Complex.

The addition of 1 equiv of KO and Kryptofix222 (Krypt) in CHCN to a solution of LCu(CHCN) [L = N, N'-bis(2,6-diisopropylphenyl)-2,6-pyridinecarboxamide] in tetrahydrofuran at -80 °C yielded [K(Krypt)][LCuO], the enhanced stability of which enabled reexamination of its reactivity with 2-phenylpropionaldehyde (2-PPA). Mechanistic and product analysis studies revealed that [K(Krypt)][LCuO] reacts with wet 2-PPA to form [LCuOH], which then deprotonates 2-PPA to yield the copper(II) enolate complex [LCu(OC═C(Me)Ph)]. Acetophenone was observed upon workup of this complex or mixtures of KO and 2-PPA alone, in support of an alternative mechanism(s) to the one proposed previously involving an initial nucleophilic attack at the carbonyl group of 2-PPA.

Formally Copper(III)-Alkylperoxo Complexes as Models of Possible Intermediates in Monooxygenase Enzymes.

Reaction of [NBu][LCuOH] with excess ROOH (R = cumyl or tBu) yielded [NBu][LCuOOR], the reversible one-electron oxidation of which generated novel species with [CuOOR] cores (formally CuOOR), identified by spectroscopy and theory for the case R = cumyl. This species reacts with weak O-H bonds in TEMPO-H and 4-dimethylaminophenol (PhOH), the latter yielding LCu(OPh), which was also prepared independently. With the identification of [CuOOR] complexes, the first precedent for this core in enzymes is provided, with implications for copper monooxygenase mechanisms.

Determination of the Cu(III)-OH Bond Distance by Resonance Raman Spectroscopy Using a Normalized Version of Badger's Rule.

The stretching frequency, ν(Cu-O), of the [CuOH] core in the complexes LCuOH (L = N,N'-bis(2,6-diisopropyl-4-R-phenyl)pyridine-2,6-dicarboxamide, R = H or NO, or N,N'-bis(2,6-diisopropylphenyl)-1-methylpiperidine-2,6-dicarboxamide) was determined to be ∼630 cm by resonance Raman spectroscopy and verified by isotopic labeling. In efforts to use Badger's rule to estimate the bond distance corresponding to ν(Cu-O), a modified version of the rule was developed through use of stretching frequencies normalized by dividing by the appropriate reduced masses. The modified version was found to yield excellent fits of normalized frequencies to bond distances for >250 data points from theory and experiment for a variety of M-X and X-X bond distances in the range ∼1.

Copper-Oxygen Complexes Revisited: Structures, Spectroscopy, and Reactivity.

A longstanding research goal has been to understand the nature and role of copper-oxygen intermediates within copper-containing enzymes and abiological catalysts. Synthetic chemistry has played a pivotal role in highlighting the viability of proposed intermediates and expanding the library of known copper-oxygen cores. In addition to the number of new complexes that have been synthesized since the previous reviews on this topic in this journal (Mirica, L.