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A Mononuclear Nonheme Iron(IV)-Oxo Complex of a Substituted N4Py Ligand : Effect of Ligand Field on Oxygen Atom Transfer and C-H Bond Cleavage Reactivity

Singh, Reena LU ; Ganguly, Gaurab ; Malinkin, Sergey O. LU ; Demeshko, Serhiy ; Meyer, Franc ; Nordlander, Ebbe LU and Paine, Tapan Kanti (2019) In Inorganic Chemistry 58(3). p.1862-1876
Abstract

A mononuclear iron(II) complex [FeII(N4PyMe2)(OTf)](OTf)(1), supported by a new pentadentate ligand, bis(6-methylpyridin-2-yl)-N,N-bis((pyridin-2-yl)methyl)methanamine (N4PyMe2), has been isolated and characterized. Introduction of methyl groups in the 6-position of two pyridine rings makes the N4PyMe2 a weaker field ligand compared to the parent N4Py ligand. Complex 1 is high-spin in the solid state and converts to [FeII(N4PyMe2)(CH3CN)](OTf)2 (1a) in acetonitrile solution. The iron(II) complex in acetonitrile displays temperature-dependent spin-crossover behavior over a wide range of temperature. In its reaction with m-CPBA or oxone in acetonitrile... (More)

A mononuclear iron(II) complex [FeII(N4PyMe2)(OTf)](OTf)(1), supported by a new pentadentate ligand, bis(6-methylpyridin-2-yl)-N,N-bis((pyridin-2-yl)methyl)methanamine (N4PyMe2), has been isolated and characterized. Introduction of methyl groups in the 6-position of two pyridine rings makes the N4PyMe2 a weaker field ligand compared to the parent N4Py ligand. Complex 1 is high-spin in the solid state and converts to [FeII(N4PyMe2)(CH3CN)](OTf)2 (1a) in acetonitrile solution. The iron(II) complex in acetonitrile displays temperature-dependent spin-crossover behavior over a wide range of temperature. In its reaction with m-CPBA or oxone in acetonitrile at -10 °C, the iron(II) complex converts to an iron(IV)-oxo species, [FeIV(O)(N4PyMe2)]2+ (2). Complex 2 exhibits the Mössbauer parameters δ = 0.05 mm/s and ΔEQ = 0.62 mm/s, typical of N-ligated S = 1 iron(IV)-oxo species. The iron(IV)-oxo complex has a half-life of only 14 min at 25 °C and is reactive toward oxygen-atom-transfer and hydrogen-atom-transfer (HAT) reactions. Compared to the parent complex [FeIV(O)(N4Py)]2+, 2 is more reactive in oxidizing thioanisole and oxygenates the C-H bonds of aliphatic substrates including that of cyclohexane. The enhanced reactivity of 2 toward cyclohexane results from the involvement of the S = 2 transition state in the HAT pathway and a lower triplet-quintet splitting compared to [FeIV(O)(N4Py)]2+, as supported by DFT calculations. The second-order rate constants for HAT by 2 is well correlated with the C-H bond dissociation energies of aliphatic substrates. Surprisingly, the slope of this correlation is different from that of [FeIV(O)(N4Py)]2+, and 2 is more reactive only in the case of strong C-H bonds (>86 kcal/mol), but less reactive in the case of weaker C-H bonds. Using oxone as the oxidant, the iron(II) complex displays catalytic oxidations of substrates with low activity but with good selectivity.

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Contribution to journal
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published
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in
Inorganic Chemistry
volume
58
issue
3
pages
1862 - 1876
publisher
The American Chemical Society (ACS)
external identifiers
  • scopus:85060245763
  • pmid:30644733
ISSN
0020-1669
DOI
10.1021/acs.inorgchem.8b02577
language
English
LU publication?
yes
id
9911d050-d671-4067-9eb7-5888b2eaa06b
date added to LUP
2019-02-01 12:29:31
date last changed
2024-06-12 06:51:15
@article{9911d050-d671-4067-9eb7-5888b2eaa06b,
  abstract     = {{<p>A mononuclear iron(II) complex [Fe<sup>II</sup>(N4Py<sup>Me2</sup>)(OTf)](OTf)(1), supported by a new pentadentate ligand, bis(6-methylpyridin-2-yl)-N,N-bis((pyridin-2-yl)methyl)methanamine (N4Py<sup>Me2</sup>), has been isolated and characterized. Introduction of methyl groups in the 6-position of two pyridine rings makes the N4Py<sup>Me2</sup> a weaker field ligand compared to the parent N4Py ligand. Complex 1 is high-spin in the solid state and converts to [Fe<sup>II</sup>(N4Py<sup>Me2</sup>)(CH<sub>3</sub>CN)](OTf)<sub>2</sub> (1a) in acetonitrile solution. The iron(II) complex in acetonitrile displays temperature-dependent spin-crossover behavior over a wide range of temperature. In its reaction with m-CPBA or oxone in acetonitrile at -10 °C, the iron(II) complex converts to an iron(IV)-oxo species, [Fe<sup>IV</sup>(O)(N4Py<sup>Me2</sup>)]<sup>2+</sup> (2). Complex 2 exhibits the Mössbauer parameters δ = 0.05 mm/s and ΔE<sub>Q</sub> = 0.62 mm/s, typical of N-ligated S = 1 iron(IV)-oxo species. The iron(IV)-oxo complex has a half-life of only 14 min at 25 °C and is reactive toward oxygen-atom-transfer and hydrogen-atom-transfer (HAT) reactions. Compared to the parent complex [Fe<sup>IV</sup>(O)(N4Py)]<sup>2+</sup>, 2 is more reactive in oxidizing thioanisole and oxygenates the C-H bonds of aliphatic substrates including that of cyclohexane. The enhanced reactivity of 2 toward cyclohexane results from the involvement of the S = 2 transition state in the HAT pathway and a lower triplet-quintet splitting compared to [Fe<sup>IV</sup>(O)(N4Py)]<sup>2+</sup>, as supported by DFT calculations. The second-order rate constants for HAT by 2 is well correlated with the C-H bond dissociation energies of aliphatic substrates. Surprisingly, the slope of this correlation is different from that of [Fe<sup>IV</sup>(O)(N4Py)]<sup>2+</sup>, and 2 is more reactive only in the case of strong C-H bonds (&gt;86 kcal/mol), but less reactive in the case of weaker C-H bonds. Using oxone as the oxidant, the iron(II) complex displays catalytic oxidations of substrates with low activity but with good selectivity.</p>}},
  author       = {{Singh, Reena and Ganguly, Gaurab and Malinkin, Sergey O. and Demeshko, Serhiy and Meyer, Franc and Nordlander, Ebbe and Paine, Tapan Kanti}},
  issn         = {{0020-1669}},
  language     = {{eng}},
  month        = {{01}},
  number       = {{3}},
  pages        = {{1862--1876}},
  publisher    = {{The American Chemical Society (ACS)}},
  series       = {{Inorganic Chemistry}},
  title        = {{A Mononuclear Nonheme Iron(IV)-Oxo Complex of a Substituted N4Py Ligand : Effect of Ligand Field on Oxygen Atom Transfer and C-H Bond Cleavage Reactivity}},
  url          = {{http://dx.doi.org/10.1021/acs.inorgchem.8b02577}},
  doi          = {{10.1021/acs.inorgchem.8b02577}},
  volume       = {{58}},
  year         = {{2019}},
}