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Reactivities of Fe(IV) Complexes with Oxo, Hydroxo, and Alkylperoxo Ligands: An Experimental and Computational Study.

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Title: Reactivities of Fe(IV) Complexes with Oxo, Hydroxo, and Alkylperoxo Ligands: An Experimental and Computational Study.
Authors: Fiedler, Adam T.1, Quo Jr., Lawrence1 larryque@uma.edu
Source: Inorganic Chemistry. 12/7/2009, Vol. 48 Issue 23, p11038-11047. 10p.
Subjects: Iron compounds, Reactivity (Chemistry), Complex compounds, Hydroxides, Ligands (Chemistry), Dissociation (Chemistry), Electronic structure
Abstract: In a previous paper [Jensen et al. J. Am. Chem. Soc. 2005, 127, 10512], we reported the synthesis of Ihe turquoise-colored intermediate [FeIV(β-BPMCN)(OO'Bu)(OH)]2+ (Tq; BPMCN = N,N'-bis(2-pyridylmethyl)-N,N'-dimethyl- trans-1,2-diaminocyclohexane). The structure of Tq is unprecedented, as it represents the only synthetic example to dale of a nonheme FeIV complex with both alkylperoxo and hydroxide ligands. Given the significance of similar high-valent Fe intermediates in the mechanisms of oxygenase enzymes, we have explored the reactivity of Tq at -70 °C, a temperature at which it is stable, and found that it is capable of activating weak X-H bonds (X = C, O) with bond dissociation energies ⩽∼80 kcal/mol. The FeIV-OH unit of Tq, and not the alkylperoxo moiety, performs the initial H-atom abstraction. However at -45 °C, Tq decays at a rate that is independent of substrate identity and concentration, forming a species capable of oxidizing substrates with stronger C-H bonds. Parallel reactivity studies were also conducted with the related oxoiron(IV) complexes [FeIV(β-BPMCN)(O)(X)]2+ (3-X; X = pyridine or nitrile), thereby permitting a direct comparison of the reactivity of FeIV centers with oxo and hydroxide ligands. We found that the H-atom abstracting ability of the FeIV=O species greatly exceeds that of the FeIV-OH species, generally by greater than 100-fold. Examination of the electronic structures of Tq and 3-X with density functional theory (OFT) provides a rationale for their differing reactivities. [ABSTRACT FROM AUTHOR]
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Abstract:In a previous paper [Jensen et al. J. Am. Chem. Soc. 2005, 127, 10512], we reported the synthesis of Ihe turquoise-colored intermediate [FeIV(β-BPMCN)(OO'Bu)(OH)]2+ (Tq; BPMCN = N,N'-bis(2-pyridylmethyl)-N,N'-dimethyl- trans-1,2-diaminocyclohexane). The structure of Tq is unprecedented, as it represents the only synthetic example to dale of a nonheme FeIV complex with both alkylperoxo and hydroxide ligands. Given the significance of similar high-valent Fe intermediates in the mechanisms of oxygenase enzymes, we have explored the reactivity of Tq at -70 °C, a temperature at which it is stable, and found that it is capable of activating weak X-H bonds (X = C, O) with bond dissociation energies ⩽∼80 kcal/mol. The FeIV-OH unit of Tq, and not the alkylperoxo moiety, performs the initial H-atom abstraction. However at -45 °C, Tq decays at a rate that is independent of substrate identity and concentration, forming a species capable of oxidizing substrates with stronger C-H bonds. Parallel reactivity studies were also conducted with the related oxoiron(IV) complexes [FeIV(β-BPMCN)(O)(X)]2+ (3-X; X = pyridine or nitrile), thereby permitting a direct comparison of the reactivity of FeIV centers with oxo and hydroxide ligands. We found that the H-atom abstracting ability of the FeIV=O species greatly exceeds that of the FeIV-OH species, generally by greater than 100-fold. Examination of the electronic structures of Tq and 3-X with density functional theory (OFT) provides a rationale for their differing reactivities. [ABSTRACT FROM AUTHOR]
ISSN:00201669
DOI:10.1021/ic901391y