Equilibrium constants for homolysis of metal-metal-bonded organometallic dimers in cyclohexane solution: reaction of the (MeCp)Mo(CO)3 radical with the nitroxide radical trap TMIO

Tenhaeff, Steve C.; Covert, Katharine J.; Castellani, Michael P.; Grunkemeier, John; Kunz, Craig; Weakley, Timothy J. R.; Koenig, T.; Tyler, David R.

doi:10.1021/om00036a046

Cited by 35 publications

(23 citation statements)

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“…Under thermal activation at constant temperature, the residual [(CO) 5 Mn(R F )] concentration was monitored by 19 F NMR spectroscopy by using hexafluorobenzene as an internal standard and in the presence of a large amount of a radical trapping agent (T in Scheme ) that was sufficient to ensure saturation kinetics—reaction rate limited by the bond cleavage step and a first order decay, −d[(CO) 5 Mn(R F )]/dt= k a [(CO) 5 Mn(R F )]. The same protocol was previously employed for the measurement of activation barriers for other radical‐generating reactions, such as the homolytic splitting of metal–metal bonds, the Co III −R cleavage in vitamin B12 and related model compounds, or the alkyl halide activation by metal catalysts in atom transfer radical polymerization systems . For the metal–metal and metal–carbon bond breaking processes, in which the reverse reaction is presumed to have a very small activation barrier, the kinetic activation enthalpy was considered as a good upper‐limit approximation of the thermodynamic bond dissociation enthalpy (BDE).…”

Section: Resultsmentioning

confidence: 99%

Fluoroalkyl Radical Generation by Homolytic Bond Dissociation in Pentacarbonylmanganese Derivatives

Morales‐Cerrada

Fliedel

Daran

et al. 2018

Chemistry A European J

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Thermal decarbonylation of the acyl compounds [Mn(CO)5(CORF)] (RF = CF3, 1; CHF2, 2; CH2CF3, 3; CF2CH3, 4) yields the corresponding alkyl derivatives [Mn(CO)5(RF)] (RF = CF3, 5; CHF2, 6; CH2CF3, 7; CF2CH3, 8). Compounds 3, 4, 7 and 8 have not been previously reported. All compounds, except 8 which could not be obtained in a pure state, were fully characterized by analytical and spectroscopic methods, as well as by single-crystal X-ray diffraction for 1, 2, 4, 5, 6 and 7. The solution IR characterization in the CO stretching region, with the assistance of DFT calculations, has allowed the assignment of several weak bands to vibrations of the [Mn(12CO)4(eq-13CO)(RF)] and [Mn(12CO)4(ax-13CO)(RF)] isotopomers and a ranking of the RF donor power in the order CF3 < CHF2 < CH2CF3 ~ CF2CH3. The homolytic Mn-RF bond cleavage in [Mn(CO)5(RF)], carried out at various temperatures for compounds 5, 6 and 7 under saturation conditions, with trapping of the generated RF radicals by tris(trimethylsilyl)silane (TTMSS, 10 equiv), has yielded activation parameters ΔH‡ and ΔS‡ that are believed to represent close estimates of the homolytic bond dissociation thermodynamic parameters. The values (ΔH‡ = 53.8 ± 3.5 (5), 46.3±1.6 (6), 50.6±0.8 (7) kcal/mol; ΔS‡ = 66.0±9.5 (5), 55.8±4.7 (6), 65.4±2.2 (7) cal mol-1 K-1) are in close agreement with the results of a recent DFT study (J. Organomet. Chem. 2018, 864, 12-18).

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Section: Resultsmentioning

confidence: 99%

Fluoroalkyl Radical Generation by Homolytic Bond Dissociation in Pentacarbonylmanganese Derivatives

Morales‐Cerrada

Fliedel

Daran

et al. 2018

Chemistry A European J

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“…Addition of an aliquot of the Co(II)bl solution to a deoxygenated pBR322 solution results in efficient DNA cleavage, which is effectively inhibited by TEMPO (Fig. ), a metal‐centered radical trap . In addition, photolysis of MeCbl under anaerobic conditions, which furnishes Co(II)bl causes DNA strand scission that is prevented by the presence of TEMPO (Fig.…”

Section: Resultsmentioning

confidence: 99%

Visible Light‐Induced Radical Mediated DNA Damage

McCue

Moreau²,

Shell

2018

Photochem & Photobiology

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Light-responsive compounds have been used to manipulate biological systems with spatial and temporal control of the event of interest. Illumination of alkylcobalamins with green light (>500 nm) produces carbon-centered radicals, which have been demonstrated to effectively cause DNA damage. Molecules that cause DNA and RNA strand scission are useful for studying polynucleotide structure and the binding of small molecules and proteins to polynucleotides. Most molecules that cause DNA damage in a light-dependent manner require high energy, short wavelength ultraviolet light, which is readily absorbed by nucleotide bases causing damage to the polynucleotides. Therefore, using alkylcobalamins is advantageous for causing strand scission of polynucleotides, because they are activated by light wavelengths that are not absorbed by nucleotide bases. Green-light illumination of methylcobalamin effectively causes DNA strand scission based on gel mobility assays. This cleavage is due to the generation of carbon-centered radicals based on the results of a radical trapping study. In addition, synthesis of an alkylcobalamin with a DNA binding moiety, spermine, improves DNA cleavage efficacy by an order of magnitude in comparison with methylcobalamin.

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“…The M–H − and M–M − bond dissociation enthalpies have been measured for the series CpM(CO) 3 H and [CpM(CO) 3 ] 2 (M = Cr, Mo, W). Although the exact values for the bond strengths of interest (especially the M–M bonds) − remain a subject of continued debate, it appears that the reaction is actually the most favorable for CpW(CO) 3 H. The W–W bond is substantially stronger than the corresponding Cr–Cr bond. Thus, dinuclear reductive elimination of H 2 may actually be the most thermodynamically favored for the third-row transition metals!…”

Section: Discussionmentioning

confidence: 99%

Metal/Ligand Proton Tautomerism Facilitates Dinuclear H₂Reductive Elimination

Kuo

Goldberg

2020

J. Am. Chem. Soc.

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Using the doubly protic bis-pyrazole-pyridine ligand (N(NNH)2), we have synthesized an octahedral IrIII–H [HIr(κ3-N(NNH)(NN–))(CO)( t BuPy)]+ ([1-MH]+) from an IrI starting material. This hydride was generated by adding sufficient electron density to the metal center such that it became the thermodynamically preferred site of protonation. It was observed via UV–vis spectroscopy that [1-MH]+ establishes a [ t BuPy] dependent equilibrium with a ligand protonated square-planar IrI [Ir(N(NNH)2)(CO)]+ ([2-LH]+). This example of metal/ligand proton tautomerism is unusual in that the position of the equilibrium can be controlled by the concentration of exogeneous ligand (i.e., t BuPy). This equilibrium was shown to be key to the reactivity of the IrIII–H; 2 equiv of [1-MH]+ release H2, converting to the IrII dimer [[Ir(N(NN–)(NNH))(CO)( t BuPy)]2]2+ ([7]2+) under mild conditions (observable at room temperature). Mechanistic evidence is presented to support that this dinuclear reductive elimination occurs by tautomerization of the metal hydride [1-MH]+ to a ligand protonated species [1-LH]+, from which ligand dissociation is facile, generating [2-LH]+. Subsequent reaction of [2-LH]+ with [1-MH]+ allows for production of H2 and the IrII dimer [7]2+. The tautomerization between the metal-hydride and the ligand protonated species provides a low energy pathway for ligand dissociation, opening the needed coordination site. The ability to control the interconversion between a metal-hydride and a ligand-protonated congener using an exogeneous ligand introduces a new strategy for catalyst design with proton responsive ligands.

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Equilibrium constants for homolysis of metal-metal-bonded organometallic dimers in cyclohexane solution: reaction of the (MeCp)Mo(CO)3 radical with the nitroxide radical trap TMIO

Cited by 35 publications

References 0 publications

Fluoroalkyl Radical Generation by Homolytic Bond Dissociation in Pentacarbonylmanganese Derivatives

Fluoroalkyl Radical Generation by Homolytic Bond Dissociation in Pentacarbonylmanganese Derivatives

Visible Light‐Induced Radical Mediated DNA Damage

Metal/Ligand Proton Tautomerism Facilitates Dinuclear H₂Reductive Elimination

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Equilibrium constants for homolysis of metal-metal-bonded organometallic dimers in cyclohexane solution: reaction of the (MeCp)Mo(CO)3 radical with the nitroxide radical trap TMIO

Cited by 35 publications

References 0 publications

Fluoroalkyl Radical Generation by Homolytic Bond Dissociation in Pentacarbonylmanganese Derivatives

Fluoroalkyl Radical Generation by Homolytic Bond Dissociation in Pentacarbonylmanganese Derivatives

Visible Light‐Induced Radical Mediated DNA Damage

Metal/Ligand Proton Tautomerism Facilitates Dinuclear H2Reductive Elimination

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Metal/Ligand Proton Tautomerism Facilitates Dinuclear H₂Reductive Elimination