Reactivity Tuning by Ligand Effects: Gas-Phase Reactions of Fe(C6H6)O+ versus “Bare” FeO+

Vibrational spectroscopy of intermediates in benzene-to-pheno conversion by FeO+

Gas-phase FeO(+) can convert benzene to phenol under thermal conditions. Two key intermediates of this reaction are the [HO-Fe-C(6)H(5)](+) insertion intermediate and Fe(+)(C(6)H(5)OH) exit channel complex. These intermediates are selectively formed by reaction of laser ablated Fe(+) with specific organic precursors and are cooled in a supersonic expansion. Vibrational spectra of the sextet and quartet states of the intermediates in the O-H stretching region are measured by infrared multiphoton dissociation (IRMPD). For Fe(+)(C(6)H(5)OH), the O-H stretch is observed at 3598 cm(-1). Photodissociation primarily produces Fe(+) + C(6)H(5)OH; Fe(+)(C(6)H(4)) + H(2)O is also observed. IRMPD of [HO-Fe-C(6)H(5)](+) mainly produces FeOH(+) + C(6)H(5) and the O-H stretch spectrum consists of a peak at approximately 3700 cm(-1) with a shoulder at approximately 3670 cm(-1). Analysis of the experimental results is aided by comparison with hybrid density functional theory computed frequencies. Also, an improved potential energy surface for the FeO(+) + C(6)H(6) reaction is developed based on CBS-QB3 calculations for the reactants, intermediates, transition states, and products.

Probing bare high-valent transition oxo-metal complexes: an electrospray ionization Fourier transform ion cyclotron resonance study of reactive intermediates

Functional models of the Compound I intermediate of monooxygenase heme enzymes, namely [(TPFPP)(*+)Fe(IV)=O](+) and [(TPFPP)Mn(V)=O](+) (TPFPP = meso-tetrakis (pentafluorophenyl)porphyrinato dianion), are obtained as bare species by electrospray ionization from solutions of appropriate precursors and their reactivity is investigated in the gas-phase. By an alternative approach involving the reaction of a gaseous oxidant, the naked core of Compound I, [(PP-IX)(*+)Fe(IV)=O](+) (PP-IX = protoporphyrin IX dianion) has been produced as well. This achievement, unprecedented in studies run in solution, is now made possible working in the gas-phase. The long lifetime ensured by the dilute gas-phase allows to reveal both structural details and elementary steps of the catalytic activity of these high-valent oxo-metal intermediates. Depending on the features of the oxo-metal complex, ionic products are formed with neutral substrates involving: (i) addition, (ii) oxygen atom transfer, (iii) formal hydride transfer. In contrast, ionic products indicative of a net initial hydrogen atom transfer event are never observed. The reaction pathways of these ultimate catalytic intermediates void of any trans axial ligand, counterion, solvent or protein environment are thus elucidated.

Probing the Cytochrome P450-like Reactivity of High-Valent Oxo Iron Intermediates in the Gas Phase

Electrospray ionization in combination with Fourier transform ion cyclotron resonance spectrometry is used to prepare and characterize at a molecular level high-valent oxoiron intermediates formed in the reaction of [(TPFPP)Fe(III)]Cl (TPFPP= meso-tetrakis(pentafluorophenyl)porphinato dianion) (1-Cl) with H(2)O(2) in methanol. The intrinsic reactivity in the gas phase of the iron(IV) oxo porphyrin cation radical complex, [(TPFPP)(.+)Fe(IV)=O](+), has been probed toward selected substrates (S), chosen among naturally occurring and model compounds. Whereas CO and cyclohexane proved to be unreactive, olefins, sulfides, amines, and phosphites all undergo oxygen atom transfer in the gas phase yielding the reduction product 1 and/or an adduct ion ([1-S](+)). The reaction efficiencies show a qualitative correlation with the oxophilic character of the active site of S. A notable exception is nitric oxide, which displays a remarkably high reactivity, in line with the important role of NO reactions with iron porphyrin complexes. Furthermore, subsidiary information on the neat association reaction of 1 with selected ligands (L) has been obtained by a kinetic study showing that both the efficiency and the extent of ligation toward the naked ion 1 depend on the electron-donating ability of L.

Oxidative dealkylation of aromatic amines by "bare" FeO+ in the gas phase

The gas-phase oxidations of aniline, N-methylaniline, and N,N-dimethylaniline by FeO+ cation are examined by using mass spectrometric techniques. Although bare FeO+ is capable of hydroxylating aromatic C—H bonds, the fate of the oxidation of arylamines is determined by docking of the FeO+ unit at nitrogen. The major reactions of the metastable aniline/FeO+ complex are losses of molecular hydrogen, ammonia, and water, all involving at least one N-H proton. N-alkylation results in a complete shift of the course of the reaction. The unimolecular processes observed can be regarded as initial steps of an oxidative dealkylation of the amines mediated by FeO+. More detailed mechanistic insight is obtained by examining the C—H(D) bond activation of N-methyl-N-([D3]-methyl)aniline by bare and ligated FeO+ species. The gas-phase reactions of FeO+ with methylanilines show some similarities to the enzymatic dealkylation of amines by cytochrome P-450. The kinetic isotope effects observed experimentally suggest an electron transfer mechanism for the gas-phase reaction.Key words: mass spectrometry, gas-phase chemistry, iron, dealkylation, N,N-dimethylaniline.