A Switch from Mechanistic Competition Mediated by a Combination of Temperature and Concentration Effects in the Oxidation Reaction of [FeII (N4Py/TPA)](OTf)2.

The formation of [(N4Py)FeIV =O]2+ species was accomplished by the reaction of [FeII (N4Py)]2+ with 20 equivalents of tBuO2 H (TBHP, 70 % in H2 O). The temperature, [FeII (N4Py)]2+ -concentration and H2 O-concentration in anhydrous TBHP (5.5 m in decane) dependences of its yields and rates were analyzed to indicate that the proton migration from [(N4Py)FeII -HOOtBu]2+ to [(N4Py)FeII -OO⊕ HtBu]2+ is the rate-determining step followed by rapid heterolytic O-O bond cleavage of FeII -OO⊕ HtBu to FeIV =O complex. The formation of [(TPA)FeIV =O]2+ is thus revealed to be greatly enhanced by the similar oxidation of [FeII (TPA)]2+ (40 mm) with 10 equivalents of tBuO2 H at -45 °C. These results demonstrate the heterolytic O-O bond cleavage of FeII -alkylperoxo species to form FeIV =O originating from the direct reaction of iron(II) complexes/TBHP. The observation of concentration and temperature effects leads to the hypothesis that O-O bond homolysis is a kinetic control pathway and O-O bond heterolysis is a thermodynamic control pathway.

Linear oligoarylamines: electrochemical, EPR, and computational studies of their oxidative states.

A series of straight-chain oligoarylamines were synthesized and examined by electrochemical, spectroelectrochemical, electron paramagnetic resonance techniques, and density functional theory (DFT) calculation. Depending on their electrochemical characteristics, these oligoarylamines were classified into two groups: one containing an odd number and the other an even number of redox centers. In the systems with odd redox centers (N1, N3, and N5), each oxidation was associated with the loss of one electron. As for the systems with even redox centers (N2, N4, and N6), oxidation occurred by taking N2 as a unit. Absorption spectra of linear oligoarylamines at various oxidative states were obtained to investigate their charge transfer behaviors. Moreover, DFT-computed isotropic hyperfine coupling constants and spin density were in accordance with the EPR experiment, and gave a close examination of oligoarylamines at charged states.

Dual-channel-mediated spin coupling for one-electron-oxidized cobalt(II)-saddled porphyrin.

Saddle-shaped Co(II)[OET(p-R)PP] (R = CF3, H, CH3) can be readily oxidized with Cl2, Br2, and I2 to the corresponding one-electron-oxidation product Co[OET(p-R)PP]X (X = Cl, Br, I) with the clear character of a ring cation radical. With the series of (1)H and (13)C NMR spectra of these related complexes, both the axial ligand and peripheral substituent of the ring macrocycle are proven to act as a dual channel to tune spin coupling between low-spin Co(II) and a porphyrin π-cation radical. Density functional theory calculations have shown that the antiferromagnetic coupling between spins residing in d(z)(2) and a(2u) are expected to exist as the ground state. The paramagnetic properties are attributed to an a(1u)-type ferromagnetic excited triplet state.

A mechanistic study of nitrite reduction on iron(ii) complexes of methylated N-confused porphyrins.

Proton delivery to the prosthetic group is a crucial step to sustain the activity of nitrite reductase. An iron N-confused porphyrin (NCP) complex, which is capable of relaying protons from the outer pyrrolic nitrogen (Nout-H) of the inverted pyrrole ring to the axial coordinated ligand, has been demonstrated to facilitate facile nitrite reduction. Time-dependent FTIR studies on the reaction between [FeII(HCTPPMe)Br] (1) and a nitrite anion revealed a two-step process involving conversion of the starting complex 1 to an {Fe(NO)}7 intermediate, [Fe(CTPPMe)(NO)] (5), before the detection of [Fe(CTPPCH2)(NO)] (3), an {Fe(NO)}6 end product. Moreover, spectroscopic data confirm that Nout-H on the NCP core is indispensable to the proceeding of the nitrite reduction reaction. Mass spectra have detected the coordination of a nitrite to the iron center while DFT theoretical calculations suggest that subsequent intramolecular proton transfer to a nitro group to form [Fe(CTPPMe)(HNO2)] (6a) evokes a homolytic N-OH bond fission on axial nitrous acid due to an enhanced π-back-bonding to produce an {Fe(NO)}7 intermediate and to release a hydroxyl radical. The subsequent oxidation of an {Fe(NO)}7 intermediate by the hydroxyl radical gave the final product, {Fe(NO)}6 [Fe(CTPPCH2)(NO)] (3). This study illustrates a proton assisted small molecule activation on the iron N-confused porphyrin coordination sphere and provides complemental insights into the mechanism of enzymatic nitrite reduction reactions.

Electron transfer and binding affinities in an electrochemically controlled ligand transfer system containing zinc porphyrin and a meso-phenylenediamine substituent.

Investigations on the transfer of the ligand, imidazole (HIm), between two covalently linked redox centres--zinc porphyrin and phenylenediamine (PD)--and the influence of the length of the linker are reported. Since the binding affinity of the ligand with zinc porphyrin is different from that of the ligand with the phenylenediamine moiety, the transfer of the ligand could be electrochemically controlled by adjusting the oxidation potentials. Changes in cyclic voltammograms and absorption spectra of the complexes revealed the site of ligand binding in the various oxidation states of the modified zinc porphyrins. Binding constants of the modified zinc porphyrins in various oxidation states were also determined by photometric titration with the ligand and digital simulations. Evidence for the delocalization of the electron from the zinc porphyrin to the phenylenediamine moiety and the influence of the delocalization on them were obtained from EPR studies.

Metal-free arylation of benzene and pyridine promoted by amino-linked nitrogen heterocyclic carbenes.

An amino-linked nitrogen heterocyclic carbene (amino-NHC), 1-tBu, has been shown to mediate carbon-carbon coupling through the direct C-H functionalization of benzene and pyridine in the absence of a metal catalyst. Using EPR, the first spectroscopic evidence corroborating the single electron transfer mechanism for the metal-free carbon-carbon coupling manifold, as reported by others, is introduced.