Search

Reaction of nitrogen monoxide with a copper(I) complex possessing a tridentate alkylamine ligand gives a Cu(I)–(·NO) adduct, which when exposed to dioxygen generates a peroxynitrite (O=NOO−)–Cu(II) species. This undergoes thermal transformation to produce a copper(II) nitrito (NO2 –) complex and 0.5 mol equiv O2. In the presence of a substituted phenol, the peroxynitrite complex effects oxidative coupling, whereas addition of chloride ion to dissociate the peroxynitrite moiety instead leads to phenol ortho nitration. Discussions include the structures (including electronic description) of the copper–nitrosyl and copper–peroxynitrite complexes and the formation of the latter, based on density functional theory calculations and accompanying spectroscopic data.

The uncatalyzed reactions of O 2 (S = 1) with organic substrates (S = 0) are thermodynamically favorable but kinetically slow because they are spin-forbidden and the one-electron reduction potential of O 2 is unfavorable. In nature, many of these important O 2 reactions are catalyzed by metalloenzymes. In the case of mononuclear non-heme iron enzymes, either Fe II or Fe III can play the catalytic role in these spin-forbidden reactions. Whereas the ferrous enzymes activate O 2 directly for reaction, the ferric enzymes activate the substrate for O 2 attack. The enzyme–substrate complex of the ferric intradiol dioxygenases exhibits a low-energy catecholate to Fe III charge transfer transition that provides a mechanism by which both the Fe center and the catecholic substrate are activated for the reaction with O 2 . In this Perspective, we evaluate how the coupling between this experimentally observed charge transfer and the change in geometry and ligand field of the oxidized metal center along the reaction coordinate can overcome the spin-forbidden nature of the O 2 reaction.

A discrete peroxynitrite−copper(II) complex, [(TMG3tren)CuII(−OON═O)]+ (3), has been generated in solution (ESI-MS, m/z = 565.15; tetragonal EPR) by reacting •NO(g) with superoxo complex [(TMG3tren)CuII(O2•−)]+ (2). Complex 3 undergoes a thermal transformation to give CuII−nitrite complex [(TMG3tren)CuII(−ONO)]+ (4) (X-ray) along with ca. 0.5 molar equiv dioxygen. A DFT calculation derived structure with cyclic bidentate k2-O,O′-OONO bound peroxynitrite moiety and dx2−y2 ground state is proposed. Experiments using 18O2 suggest that the adjacent peroxo oxygen atoms in 3 are derived from molecular oxygen. Further, 18O2 containing 3 undergoes O−O bond cleavage to form singly 18-O-labeled 4. The results suggest the viability of biological CuI/O2/(•NO) peroxynitrite formation and chemistry, that is, not coming from free superoxide plus •NO reaction.

Various mechanisms have been proposed for the initial O(2) attack in intradiol dioxygenases based on different electronic descriptions of the enzyme-substrate (ES) complex. We have examined the geometric and electronic structure of the high-spin ferric ES complex of protocatechuate 3,4-dioxygenase (3,4-PCD) with UV/visible absorption, circular dichroism (CD), magnetic CD (MCD), and variable-temperature variable-field (VTVH) MCD spectroscopies. The experimental data were coupled with DFT and INDO/S-CI calculations, and an experimentally calibrated bonding description was obtained. The broad absorption spectrum for the ES complex in the 6000-31000 cm(-1) region was resolved into at least five individual transitions, assigned as ligand-to-metal charge transfer (LMCT) from the protocatechuate (PCA) substrate and Tyr408. From our DFT calculations, all five LMCT transitions originate from the PCA and Tyr piop orbitals to the ferric dpi orbitals. The strong pi covalent donor interactions dominate the bonding in the ES complex. Using hypothetical Ga(3+)-catecholate/semiquinone complexes as references, 3,4-PCD-PCA was found to be best described as a highly covalent Fe(3+)-catecholate complex. The covalency is distributed unevenly among the four PCA valence orbitals, with the strongest interaction between the piop-sym and Fe dxz orbitals. This strong pi interaction, as reflected in the lowest energy PCA-to-Fe(3+) LMCT transition, is responsible for substrate activation for the O(2) reaction of intradiol dioxygenases. This involves a multi-electron-transfer (one beta and two alpha) mechanism, with Fe3+ acting as a buffer for the spin-forbidden two-electron redox process between PCA and O(2) in the formation of the peroxy-bridged ESO2 intermediate. The Fe ligand field overcomes the spin-forbidden nature of the triplet O(2) reaction, which potentially results in an intermediate spin state (S = 3/2) on the Fe(3+) center which is stabilized by a change in coordination along the reaction coordinate.