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Bernasconi, Leonardo,Baerends, Evert Jan American Chemical Society 2013 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.135 No.24
<P>Solvation effects on chemical reactivity are often rationalized using electrostatic considerations: the reduced stabilization of the transition state results in higher reaction barriers and lower reactivity in solution. We demonstrate that the effect of solvation on the relative energies of the frontier orbitals is equally important and may even reverse the trend expected from purely electrostatic arguments. We consider the H abstraction reaction from methane by quintet [EDTAH<SUB><I>n</I></SUB>·FeO]<SUP>(<I>n</I>−2)+</SUP>, (<I>n</I> = 0–4) complexes in the gas phase and in aqueous solution, which we examine using ab initio thermodynamic integration. The variation of the charge of the complex with the protonation of the EDTA ligand reveals that the free energy barrier in gas phase increases with the negative charge, varying from 16 kJ mol<SUP>–1</SUP> for [EDTAH<SUB>4</SUB>·FeO]<SUP>2+</SUP> to 57 kJ mol<SUP>–1</SUP> for [EDTAH<SUB><I>n</I></SUB>·FeO]<SUP>2–</SUP>. In aqueous solution, the barrier for the +2 complex (38 kJ mol<SUP>–1</SUP>) is higher than in gas phase, as predicted by purely electrostatic arguments. For the negative complexes, however, the barrier is lower than in gas phase (e.g., 45 kJ mol<SUP>–1</SUP> for the −2 complex). We explain this increase in reactivity in terms of a stabilization of the virtual 3σ* orbital of FeO<SUP>2+</SUP>, which acts as the dominant electron acceptor in the H-atom transfer from CH<SUB>4</SUB>. This stabilization originates from the dielectric screening caused by the reorientation of the water dipoles in the first solvation shell of the charged solute, which stabilizes the acceptor orbital energy for the −2 complex sufficiently to outweigh the unfavorable electrostatic destabilization of the transition-state relative to the reactants in solution.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2013/jacsat.2013.135.issue-24/ja311144d/production/images/medium/ja-2012-11144d_0010.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ja311144d'>ACS Electronic Supporting Info</A></P>
Bernasconi, Leonardo,Belanzoni, Paola,Baerends, Evert Jan Royal Society of Chemistry 2011 Physical chemistry chemical physics Vol.13 No.33
<P>We study the generation of a dinuclear Fe(<SMALL>IV</SMALL>)oxo species, [EDTAH·FeO·OFe·EDTAH]<SUP>2−</SUP>, in aqueous solution at room temperature using Density Functional Theory (DFT) and <I>Ab Initio</I> Molecular Dynamics (AIMD). This species has been postulated as an intermediate in the multi-step mechanism of autoxidation of Fe(<SMALL>II</SMALL>) to Fe(<SMALL>III</SMALL>) in the presence of atmospheric O<SUB>2</SUB> and EDTA ligand in water. We examine the formation of [EDTAH·FeO·OFe·EDTAH]<SUP>2−</SUP> by direct cleavage of O<SUB>2</SUB>, and the effects of solvation on the spin state and O–O cleavage barrier. We also study the reactivity of the resulting dinuclear Fe(<SMALL>IV</SMALL>)oxo system in CH<SUB>4</SUB> hydroxylation, and its tendency to decompose to mononuclear Fe(<SMALL>IV</SMALL>)oxo species. The presence of the solvent is shown to play a crucial role, determining important changes in all these processes compared to the gas phase. We show that, in water solution, [EDTAH·FeO·OFe·EDTAH]<SUP>2−</SUP> (as well as its precursor [EDTAH·Fe·O<SUB>2</SUB>·Fe·EDTAH]<SUP>2−</SUP>) exists as stable species in a <I>S</I> = 4 ground spin state when hydrogen-bonded to a single water molecule. Its structure comprises two facing Fe(<SMALL>IV</SMALL>)oxo groups, in an arrangement similar to the one evinced for the active centre of intermediate <B>Q</B> of soluble Methane Monooxygenase (sMMO). The inclusion of the water molecule in the complex decreases the overall symmetry of the system, and brings about important changes in the energy and spatial distribution of orbitals of the Fe(<SMALL>IV</SMALL>)oxo groups relative to the gas phase. In particular, the virtual 3σ* orbital of one of the Fe(<SMALL>IV</SMALL>)oxo groups experiences much reduced repulsive orbital interactions from ligand orbitals, and its consequent stabilisation dramatically enhances the electrophilic character of the complex, compared to the symmetrical non-hydrated species, and its ability to act as an acceptor of a H atom from the CH<SUB>4</SUB> substrate. The computed free energy barrier for H abstraction is 28.2 kJ mol<SUP>−1</SUP> (at the BLYP level of DFT), considerably below the gas phase value for monomeric [FeO·EDTAH]<SUP>−</SUP>, and much below the solution value for the prototype hydrated ferryl ion [FeO(H<SUB>2</SUB>O)<SUB>5</SUB>]<SUP>2+</SUP>.</P> <P>Graphic Abstract</P><P>The CH<SUB>4</SUB> hydroxylation activity of Fe(<SMALL>IV</SMALL>)oxo/EDTA catalysts obtained from direct activation of atmospheric O<SUB>2</SUB> is strongly enhanced by water solvation. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c1cp21244c'> </P>