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Reaction of an iron(IV) nitrido complex with cyclohexadienes: cycloaddition and hydrogen-atom abstraction.

ABSTRACT: The iron(IV) nitrido complex PhB(MesIm)3Fe?N reacts with 1,3-cyclohexadiene to yield the iron(II) pyrrolide complex PhB(MesIm)3Fe(?(5)-C4H4N) in high yield. The mechanism of product formation is proposed to involve sequential [4 + 1] cycloaddition and retro Diels-Alder reactions. Surprisingly, reaction with 1,4-cyclohexadiene yields the same iron-containing product, albeit in substantially lower yield. The proposed reaction mechanism, supported by electronic structure calculations, involves hydrogen-atom abstraction from 1,4-cyclohexadiene to provide the cyclohexadienyl radical. This radical is an intermediate in substrate isomerization to 1,3-cyclohexadiene, leading to formation of the pyrrolide product.

SUBMITTER: Lee WT

PROVIDER: S-EPMC4139179 | biostudies-literature | 2014 Aug

REPOSITORIES: biostudies-literature

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Reaction of an iron(IV) nitrido complex with cyclohexadienes: cycloaddition and hydrogen-atom abstraction.

Lee Wei-Tsung WT Juarez Ruth A RA Scepaniak Jeremiah J JJ Muñoz Salvador B SB Dickie Diane A DA Wang Haobin H Smith Jeremy M JM

Inorganic chemistry 20140728 16

The iron(IV) nitrido complex PhB(MesIm)3Fe≡N reacts with 1,3-cyclohexadiene to yield the iron(II) pyrrolide complex PhB(MesIm)3Fe(η(5)-C4H4N) in high yield. The mechanism of product formation is proposed to involve sequential [4 + 1] cycloaddition and retro Diels-Alder reactions. Surprisingly, reaction with 1,4-cyclohexadiene yields the same iron-containing product, albeit in substantially lower yield. The proposed reaction mechanism, supported by electronic structure calculations, involves hydr ...[more]

PMID: 25068927

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Project description:Iron(iv)-oxo species have been identified as the active intermediates in key enzymatic processes, and their catalytic properties are strongly affected by the equatorial and axial ligands bound to the metal, but details of these effects are still unresolved. In our aim to create better and more efficient oxidants of H-atom abstraction reactions, we have investigated a unique heteroleptic diiron phthalocyanine complex. We propose a novel intramolecular approach to determine the structural features that govern the catalytic activity of iron(iv)-oxo sites. Heteroleptic ?-nitrido diiron phthalocyanine complexes having an unsubstituted phthalocyanine (Pc1) and a phthalocyanine ligand substituted with electron-withdrawing alkylsulfonyl groups (PcSO2R) were prepared and characterized. A reaction with terminal oxidants gives two isomeric iron(iv)-oxo and iron(iii)-hydroperoxo species with abundances dependent on the equatorial ligand. Cryospray ionization mass spectrometry (CSI-MS) characterized both hydroperoxo and diiron oxo species in the presence of H2O2. When m-CPBA was used as the oxidant, the formation of diiron oxo species (PcSO2R)FeNFe(Pc1)[double bond, length as m-dash]O was also evidenced. Sufficient amounts of these transient species were trapped in the quadrupole region of the mass-spectrometer and underwent a CID-MS/MS fragmentation. Analyses of fragmentation patterns indicated a preferential formation of hydroperoxo and oxo moieties at more electron-rich iron sites of both heteroleptic ?-nitrido complexes. DFT calculations show that both isomers are close in energy. However, the analysis of the iron(iii)-hydroperoxo bond strength reveals major differences for the (Pc1)FeN(PcSO2R)FeIIIOOH system as compared to (PcSO2R)FeN(Pc1)FeIIIOOH system, and, hence binding of a terminal oxidant will be preferentially on more electron-rich sides. Subsequent kinetics studies showed that these oxidants are able to even oxidize methane to formic acid efficiently.

Project description:The factors that control the reactivities of aryl radicals toward hydrogen-atom donors were studied by using a dual-cell Fourier-transform ion cyclotron resonance mass spectrometer. Hydrogen-atom abstraction reaction efficiencies for two substrates, cyclohexane and isopropyl alcohol, were measured for 23 structurally different, positively charged aryl radicals, which included dehydrobenzenes, dehydronaphthalenes, dehydropyridines, and dehydro(iso)quinolines. A logarithmic correlation was found between the hydrogen-atom abstraction reaction efficiencies and the (calculated) vertical electron affinities (EA) of the aryl radicals. Transition state energies calculated for the reaction of three of the aryl radicals with isopropyl alcohol were found to correlate linearly with their (calculated) EAs. No correlation was found between the hydrogen-atom abstraction reaction efficiencies and the (calculated) enthalpy changes for the reactions. Measurement of the reaction efficiencies for the reactions of 15 different hydrogen-atom donors with two selected aryl radicals revealed a logarithmic correlation between the hydrogen-atom abstraction reaction efficiencies and the vertical ionization energies (IE) of the hydrogen-atom donors, but not the lowest homolytic X-H (X = heavy atom) bond dissociation energies of the hydrogen-atom donors. Examination of the hydrogen-atom abstraction reactions of 29 different aryl radicals and 18 different hydrogen-atom donors showed that the reaction efficiency increases (logarithmically) as the difference between the IE of the hydrogen-atom donor and the EA of the aryl radical decreases. This dependence is likely to result from the increasing polarization, and concomitant stabilization, of the transition state. Thus, the hydrogen-atom abstraction reaction efficiency for an aryl radical can be "tuned" by structural changes that influence either the vertical EA of the aryl radical or the vertical IE of the hydrogen atom donor.

Dataset Information

Reaction of an iron(IV) nitrido complex with cyclohexadienes: cycloaddition and hydrogen-atom abstraction.

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Reaction of an iron(IV) nitrido complex with cyclohexadienes: cycloaddition and hydrogen-atom abstraction.

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