When the dichloroiron(II) complex of the tetradentate bispidine ligand L=3,7-dimethyl-9-oxo-2,4-bis(2-pyridyl)-3,7-diazabicyclo[3.3.1]nonane-1,5-dicarboxylate methyl ester is oxidized with H(2)O(2), tBuOOH, or iodosylbenzene, the high-valent Fe=O complex efficiently oxidizes and halogenates cyclohexane. Kinetic D isotope effects and the preference for the abstraction of tertiary over secondary carbon-bound hydrogen atoms (quantified in the halogenation of adamantane) indicate that C-H activation is the rate-determining step. The efficiencies (yields in stoichiometric and turnover numbers in catalytic reactions), product ratios (alcohol vs. bromo- vs. chloroalkane), and kinetic isotope effects depend on the oxidant. These results suggest different pathways with different oxidants, and these may include iron(IV)- and iron(V)-oxo complexes as well as oxygen-based radicals.

Various DFT and ab initio methods, including B3LYP, HF, SORCI, and LF-density functional theory (DFT), are used to compute the structures, relative stabilities, spin density distributions, and spectroscopic properties (electronic and EPR) of the two possible isomers of the copper(II) complexes with derivatives of a rigid tetradentate bispidine ligand with two pyridine and two tertiary amine donors, and a chloride ion. The description of the bonding (covalency of the copper-ligand interactions) and the distribution of the unpaired electron strongly depend on the DFT functional used, specifically on the nonlocal DF correlation and the HF exchange. Various methods may be used to optimize the DFT method. Unfortunately, it appears that there is no general method for the accurate computation of copper(II) complexes, and the choice of method depends on the type of ligands and the structural type of the chromophore. Also, it appears that the choice of method strongly depends on the problem to be solved. LF-DFT and spectroscopically oriented CI methods (SORCI), provided a large enough reference space is chosen, yield accurate spectroscopic parameters; EDA may lead to a good understanding of relative stabilities; accurate spin density distributions are obtained by modification of the nuclear charge on copper; solvation models are needed for the accurate prediction of isomer distributions.

The interest in nonheme iron systems has lead to an increasingly detailed knowledge of the coordination geometries, electronic structures, and reaction mechanisms of oxygenases and halogenases (for example TauD and SyrB2), and this is due to numerous studies involving the biological processes, small molecule model systems, and quantumchemical model studies. [1][2][3][4][5][6][7] The thorough analysis of a variety of model systems has yielded a detailed understanding of the nature of the catalytically active high-valent iron oxo intermediates. [2,[4][5][6][7][8] Current focal points in experimental and theoretical modeling of the enzyme reactions are ambiguities in the degree of protonation, the oxidation and spin states of the catalytically active high-valent iron complex, and their relation to the reactivities (Fe IV vs. Fe V ; S = 1 versus S = 2; O versus OH versus OH 2 ).Important parameters for the characterization of the highvalent iron oxo complex and for the interpretation of its reactivity are 1) the reduction potential E red of the ferryl complex, [9][10][11] 2) the kinetic barrier for the electron-transfer process, 3) the basicity of the oxidized and reduced iron oxo species, [12] and 4) the energy gap between the various spin states, because computational studies indicate that the potential energy for oxidation reactions on the high-spin surface has lower barriers, whereas most low-molecularweight biomimetic Fe IV = O systems, in contrast to the enzymes, have an intermediate spin (S = 1) ground state. [6,[13][14][15] Herein, we report the electron-transfer properties of an Fe IV =O complex with the tetradentate bispidine ligand L (Scheme 1).[16] The coordination chemistry of ligand L has been thoroughly studied by computational (molecular mechanics, DFT) and experimental methods, and it was shown to be enforced by the rigid adamantane-type ligand backbone. A variety of X-ray crystal structures have been used to show that tetra-and pentadentate derivatives of L with different donor sets have an elastic coordination sphere as a consequence of the flat potential energy surface with various close-to-degenerate minima. The rigid and relatively large cavity of the bispidines is one reason for their high metal ion selectivity, and these ligands exhibit uncommon dependencies of the stability constants. [17,18] Their iron chemistry has been developed in the fields of alkane and alkene oxidation and biomimetic nonheme-halogenase reactivity. [19][20][21][22][23] Computational studies related to the bispidine-Fe IV = O species reveal a very small energy gap between the intermediate-spin (S = 1) and high-spin (S = 2) electronic configurations. [21][22][23] Furthermore, the reorganization energy of the electron transfer between the oxo Fe IV and the oxo Fe III species is expected to be small owing to the rigid ligand backbone, which is a possible reason for the exceptionally high reactivity. Therefore, we report herein the fundamental electron-transfer properties of [Fe IV =O(L)(NCMe)] 2+ (1). The high-valent comple...

Furan and its derivates are a potentially important, and little studied, class of volatile organic compounds of relevance to atmospheric chemistry. The emission of these reactive compounds has been attributed previously to biomass burning processes and biogenic sources. This paper investigates the natural abiotic formation of furans in soils, induced by the oxidation of organic matter by iron(III) and hydrogen peroxide. Several model compounds like catechol, substituted catechols, and phenols as well as different organic-rich soil samples were investigated for the release of furans. The measurements were performed with a purge and trap GC/MS system and the influence of hydrogen peroxide, reaction temperature, iron(III), pH, and reaction time on furan yield was determined. The optimal reaction turnover obtained with catechol was 2.33 microg of furan from 0.36 mg of carbon. Results presented in this paper show that a cleavage of catechol into a C2- and a C4-fragment occurs, in which the C4-fragment forms furan by integrating an oxygen atom stemming from H2O2. Furthermore, phenols could be transformed into catecholic structures under these Fenton-like conditions and also display the formation of furans. In conclusion, catalytic amounts of iron(III), the presence of hydrogen peroxide, and acidic conditions can be seen as the most important parameters required for an optimized furan formation.