Reactivity comparison of high-valent iron(iv)-oxo complexes bearing N-tetramethylated cyclam ligands with different ring size

Hong, Seung-Woo; So, Hee; Yoon, Hai Jeon; Cho, Kyung‐Bin; Lee, Yong Min; Fukuzumi, Shunichi; Nam, Wonwoo

doi:10.1039/c3dt50750e

Cited by 65 publications

(73 citation statements)

References 36 publications

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“…DFT calculations reproduce the higher HAT reactivity of 2, which is attributed to the greater oxyl character acquired by the oxo atom of the 5 2 fragment in 5 TS. The reactivity data presented herein comparing 2 and 3, together with the recent results of Mandal et al 65 and Nam et al on Fe(13-TMC) complexes, 60,76 serve to emphasize what a gold mine of information is provided by even the subset of oxoiron(IV) complexes supported by cyclam ligands and demonstrate that our understanding of the reactivity of nonheme oxoiron(IV) complexes remains incomplete and justifies continued investigation.…”

Section: ■ Conclusionmentioning

confidence: 65%

Oxoiron(IV) Complex of the Ethylene-Bridged Dialkylcyclam Ligand Me₂EBC

et al. 2015

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We report herein the first example of an oxoiron(IV) complex of an ethylene-bridged dialkylcyclam ligand, [Fe(IV)(O)(Me2EBC)(NCMe)](2+) (2; Me2EBC = 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane). Complex 2 has been characterized by UV-vis, (1)H NMR, resonance Raman, Mössbauer, and X-ray absorption spectroscopy as well as electrospray ionization mass spectrometry, and its properties have been compared with those of the closely related [Fe(IV)(O)(TMC)(NCMe)](2+) (3; TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), the intensively studied prototypical oxoiron(IV) complex of the macrocyclic tetramethylcyclam ligand. Me2EBC has an N4 donor set nearly identical with that of TMC but possesses an ethylene bridge in place of the 1- and 8-methyl groups of TMC. As a consequence, Me2EBC is forced to deviate from the trans-I configuration typically found for Fe(IV)(O)(TMC) complexes and instead adopts a folded cis-V stereochemistry that requires the MeCN ligand to coordinate cis to the Fe(IV)═O unit in 2 rather than in the trans arrangement found in 3. However, switching from the trans geometry of 3 to the cis geometry of 2 did not significantly affect their ground-state electronic structures, although a decrease in ν(Fe═O) was observed for 2. Remarkably, despite having comparable Fe(IV/III) reduction potentials, 2 was found to be significantly more reactive than 3 in both oxygen-atom-transfer (OAT) and hydrogen-atom-transfer (HAT) reactions. A careful analysis of density functional theory calculations on the HAT reactivity of 2 and 3 revealed the root cause to be the higher oxyl character of 2, leading to a stronger O---H bond specifically in the quintet transition state.

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Section: ■ Conclusionmentioning

confidence: 65%

Oxoiron(IV) Complex of the Ethylene-Bridged Dialkylcyclam Ligand Me₂EBC

et al. 2015

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“…The observed isotopic shift of Δ ν =34 cm −1 upon 18 O-substitution matches with the calculated value of Δ ν =34 cm −1 , as expected for a diatomic Co–O oscillator. It is of interest to note that the Co–O stretching frequency of 770 cm −1 for 2 is lower than the Fe–O stretching frequencies of an Fe IV =O complex bearing the same supporting ligand, [(13-TMC)Fe IV (O)] 2+ (that is, ν (Fe–O)=833 cm −1 )34, and other mononuclear non-haem Fe IV =O complexes (that is, ν (Fe–O)=820–850 cm −1 )35. Notably, the observed differences are larger than expected from differences in the reduced masses ( μ ) of Fe–O ( μ =12.44) and Co–O ( μ =12.58) alone, suggesting that the Co–O bond order of 2 is lower than that of the non-haem Fe IV =O complexes, including [(13-TMC)Fe IV (O)] 2+ (see below for EXAFS and computational results and discussions).…”

Section: Resultsmentioning

confidence: 99%

Synthesis and reactivity of a mononuclear non-haem cobalt(IV)-oxo complex

et al. 2017

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Terminal cobalt(IV)–oxo (CoIV–O) species have been implicated as key intermediates in various cobalt-mediated oxidation reactions. Herein we report the photocatalytic generation of a mononuclear non-haem [(13-TMC)CoIV(O)]2+ (2) by irradiating [CoII(13-TMC)(CF3SO3)]+ (1) in the presence of [RuII(bpy)3]2+, Na2S2O8, and water as an oxygen source. The intermediate 2 was also obtained by reacting 1 with an artificial oxidant (that is, iodosylbenzene) and characterized by various spectroscopic techniques. In particular, the resonance Raman spectrum of 2 reveals a diatomic Co–O vibration band at 770 cm−1, which provides the conclusive evidence for the presence of a terminal Co–O bond. In reactivity studies, 2 was shown to be a competent oxidant in an intermetal oxygen atom transfer, C–H bond activation and olefin epoxidation reactions. The present results lend strong credence to the intermediacy of CoIV–O species in cobalt-catalysed oxidation of organic substrates as well as in the catalytic oxidation of water that evolves molecular oxygen.

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“…14,15, 16, 17,18,19 These enable an important step toward the elucidation of geometric and electronic properties contributing to NHFe function: the spectroscopic and quantum-chemical elucidation of the frontier molecular orbitals (FMOs) and their contributions to reactivity in model S = 1 and S = 2 Fe IV =O systems. 20,21,22,23,24,25,26,27,28 For the S = 1 Fe IV =O species, the pair of singly-occupied d π* FMOs, resulting from the strong antibonding interaction between oxo p x,y and Fe d xz/yz orbitals, were shown to define the π channel for H-atom abstraction (HAA) which requires for reactivity the perpendicular orientation of the substrate C—H bond with respect to the Fe—oxo bond.…”

Section: Introductionmentioning

confidence: 99%

Excited state potential energy surfaces and their interactions in Fe^IVO active sites

2014

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The non-heme ferryl active sites are of significant interest for their application in biomedical and green catalysis. These sites have been shown to have an S = 1 or S = 2 ground spin state; the latter is functional in biology. Low-temperature magnetic circular dichroism (LT MCD) spectroscopy probes the nature of the excited states in these species including ligand-field (LF) states that are otherwise difficult to study by other spectroscopies. In particular, the temperature dependences of MCD features enable their unambiguous assignment and thus determination of the low-lying excited states in two prototypical S = 1 and S = 2 NHFeIV=O complexes. Furthermore, some MCD bands exhibit vibronic structures that allow mapping of excited-state interactions and their effects on the potential energy surfaces (PESs). For the S = 2 species, there is also an unusual spectral feature in both near-infrared absorption and MCD spectra - Fano antiresonance (dip in Abs) and Fano resonance (sharp peak in MCD) that indicates the weak spin-orbit coupling of an S = 1 state with the S = 2 LF state. These experimental data are correlated with quantum-chemical calculations that are further extended to analyze the low-lying electronic states and the evolution of their multiconfigurational characters along the Fe—O PESs. These investigations show that the lowest-energy states develop oxyl FeIII character at distances that are relevant to the transition state (TS) for H-atom abstraction and define the frontier molecular orbitals that participate in the reactivity of S = 1 vs. S = 2 non-heme FeIV=O active sites. The S = 1 species has only one available channel that requires the C—H bond of a substrate to approach perpendicular to the Fe—oxo bond (the π channel). In contrast, there are three channels (one σ and two π) available for the S = 2 non-heme FeIV=O system allowing C—H substrate approach both along and perpendicular to the Fe—oxo bond that have important implications for enzymatic selectivity.

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Reactivity comparison of high-valent iron(iv)-oxo complexes bearing N-tetramethylated cyclam ligands with different ring size

Cited by 65 publications

References 36 publications

Oxoiron(IV) Complex of the Ethylene-Bridged Dialkylcyclam Ligand Me₂EBC

Oxoiron(IV) Complex of the Ethylene-Bridged Dialkylcyclam Ligand Me₂EBC

Synthesis and reactivity of a mononuclear non-haem cobalt(IV)-oxo complex

Excited state potential energy surfaces and their interactions in Fe^IVO active sites

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Reactivity comparison of high-valent iron(iv)-oxo complexes bearing N-tetramethylated cyclam ligands with different ring size

Cited by 65 publications

References 36 publications

Oxoiron(IV) Complex of the Ethylene-Bridged Dialkylcyclam Ligand Me2EBC

Oxoiron(IV) Complex of the Ethylene-Bridged Dialkylcyclam Ligand Me2EBC

Synthesis and reactivity of a mononuclear non-haem cobalt(IV)-oxo complex

Excited state potential energy surfaces and their interactions in FeIVO active sites

Contact Info

Product

Resources

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Oxoiron(IV) Complex of the Ethylene-Bridged Dialkylcyclam Ligand Me₂EBC

Oxoiron(IV) Complex of the Ethylene-Bridged Dialkylcyclam Ligand Me₂EBC

Excited state potential energy surfaces and their interactions in Fe^IVO active sites