- Aromaticity and sterics control whether a cationic olefin radical is resistant to disproportionation
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We elucidate why some electron rich-olefins such as tetrathiafulvalene (TTF) or paraquat (1,1'-dimethyl-4,4'-bipyridinylidene) form persistent radical cations, whereas others such as the dimer ofN,N'-dimethyl benzimidazolin-2-ylidene (benzNHC) do not. Specifically, three heterodimers derived from cyclic (alkyl) (amino) carbenes (CAAC) withN,N'-dimethyl imidazolin-2-ylidene (NHC),N,N'-dimethyl imidazolidin-2-ylidene (saNHC) andN-methyl benzothiazolin-2-ylidene (btNHC) are reported. Whereas the olefin radical cations with the NHC and btNHC are isolable, the NHC compound with a saturated backbone (saNHC) disproportionates instead to the biscation and olefin. Furthermore, the electrochemical properties of the electron-rich olefins derived from the dimerization of the saNHC and btNHC were assessed. Based on the experiments, we propose a general computational method to model the electrochemical potentials and disproportionation equilibrium. This method, which achieves an accuracy of 0.07 V (0.06 V with calibration) in reference to the experimental values, allows for the first time to rationalize and predict the (in)stability of olefin radical cations towards disproportionation. The combined results reveal that the stability of heterodimeric olefin radical cations towards disproportionation is mostly due to aromaticity. In contrast, homodimeric radical cations are in principle isolable, if lacking steric bulk in the 2,2' positions of the heterocyclic monomers. Rigid tethers increase accordingly the stability of homodimeric radical cations, whereas the electronic effects of substituents seem much less important for the disproportionation equilibrium.
- Messelberger, Julian,Grünwald, Annette,Goodner, Stephen J.,Zeilinger, Florian,Pinter, Piermaria,Miehlich, Matthias E.,Heinemann, Frank W.,Hansmann, Max M.,Munz, Dominik
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- Telluroureas and Derived Transition Metal Complexes: the Crystal and Molecular Structure of Et>>
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Reaction of the electron-rich olefin 2N(Et)C>=>2 with tellurium affords the thermally or photochemically labile Et-cyclo2N(Et)C>=Te which reacts with (M = Cr, Mo, or W) or to yield 2N>Et>> or cis-2N>Et>>; the compound 2N>Et>> is readily detellurated to yield the carbenemetal complex 2N>Et>>; the molecule of the title compound lies on a crystallographic mirror plane with the carbene ligand perpendicular to this plane and Te-Cr = 2.765(4), Te-Ccarb = 2.12(2) Angstroem, and angle Cr-Te-Ccarb = 96.1(5) deg.
- Lappert, Michael F.,Martin, Tony R.
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- Steric stabilization of nucleophilic carbenes
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Two tert-butyl groups at the nitrogen atoms of the cyclic carbene 2, which is accessible from 1 by reduction with potassium, provide sufficient stability to enable it to be stored indefinitely under exclusion of air and moisture. In contrast, sterically less shielded carbenes of type 2 (R = Me, Et, iPr) dimerize slowly at room temperature to olefins 3.
- Denk,Thadani,Hatano,Lough
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- Metal-free reduction of the greenhouse gas sulfur hexafluoride, formation of SF5 containing ion pairs and the application in fluorinations
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A protocol for the fast and selective two-electron reduction of the potent greenhouse gas sulfur hexafluoride (SF6) by organic electron donors at ambient temperature has been developed. The reaction yields solid ion pairs consisting of donor di
- Rueping, Magnus,Nikolaienko, Pavlo,Lebedev, Yury,Adams, Alina
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p. 2571 - 2575
(2017/07/17)
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