Organische Chemie II - Physikalisch-Organische Chemie

Activation of Molecular Hydrogen by Arylcarbenes
E. Mendez-Vega, M. Maehara, A. H. Raut, J. Mieres-Perez, M. Tsuge, Y-P. Lee, W. Sander, Chem. Eur. J. (2018), Ahead of Print

The hydrogenation reactions of diphenylcarbene 1, fluorenylidene 2, and dibenzocycloheptadienylidene 3 were investigated in solid H₂ and D₂ matrixes and in H₂ and D₂ doped argon matrixes at cryogenic temps. The reactivity of the carbenes towards H₂ increases in the order 1<3<2. Whereas 1 is stable in solid H₂, 2 and 3 react fast under the same conditions via quantum chem. tunneling. In D₂ both 1 and 3 are stable, whereas 2 slowly reacts. The different reactivity of the three carbenes is rationalized in terms of differing carbene stabilization energies.

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NEW JACS Paper

Switching the Spin State of Pentafluoro-phenylnitrene: Isolation of a Singlet Arylnitrene Complex
J. Mieres-Perez, P. Costa, E. Mendez-Vega, R. Crespo-Otero, W. Sander, J. Am. Chem. Soc. (2018), Ahead of Print

The chemistry of arylnitrenes is dominated by their triplet ground states and excited open-shell singlet states. This results in radical-type reactions and unwanted rearrangements which diminish the use of arylnitrenes as intermediates in organic synthesis. While the closed-shell singlet states of arylnitrenes are expected to undergo useful chemical transformations (comparable to the closed shell singlet states of carbenes), these states are too high in energy to be chemically accessible. When triplet pentafluorophenylnitrene is interacting with the Lewis acid BF₃ under the conditions of matrix isolation, a Lewis acid-base complex consisting of the closed-shell singlet state of the nitrene and two molecules of BF₃ is formed. Although the closed shell singlet state of pentafluorophenylnitrene is calculated (CCSD(T)) to lie more than 25 kcal/mol above its triplet ground state, the reaction with BF₃ results in switching the spin state from triplet to singlet. The formation of the singlet complex was monitored by IR, UV-vis, and EPR spectroscopy. DFT, CCSD(T), and CASPT2 calculations confirm the experimental findings.

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NEW PAPER in Nature Communications

Light-controlled switching of the spin state of iron(III)
S. Shankar, M. Peters, K. Steinborn, B. Krahwinkel, F. D. Sönnichsen, D. Grote, W. Sander, T. Lohmiller, O. Rüdiger, R. Herges, Nature Comm. 9 (2018), Article number: 4750.

Controlled switching of the spin state of transition metal ions, particularly of Fe(II) and Fe(III), is a prerequisite to achieve selectivity, efficiency, and catalysis in a number of metalloenzymes. Here we report on an iron(III) porphyrin with a photochromic axial ligand which, upon irradiation with two different wavelengths reversibly switches its spin state between low-spin (S=1/2) and high-spin (S=5/2) in solution (DMSO-acetone, 2:598). The switching efficiency is 76% at room temperature. The system is neither oxygen nor water sensitive, and no fatigue was observed after more than 1000 switching cycles. Concomitant with the spin-flip is a change in redox potential by ~60mV. Besides serving as a simple model for the first step of the cytochrome P450 catalytic cycle, the spin switch can be used to switch the spin-lattice relaxation time T1 of the water protons by a factor of 15.

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NEW Angewandte PAPER

Imaging the Solvation of a One-Dimensional Solid on the Molecular Scale
K. Lucht, I. Trosien, W. Sander, K. Morgenstern, Angew. Chem. Int. Ed. (2018), Ahead of Print

We have observed the inversion of the solvation environment of a one-dimensional solid by low-temperature scanning tunneling microscopy. Adsorption of 3-methoxy-9-diazofluorene on Ag(111) yields highly oriented supramolecular chains, which are then exposed to water molecules. The annealing of dry and water-decorated chains results in diametrically opposed outcomes. While the former simply leads to an increase in chain length and number, the latter results in a complete loss of order and produces water clusters decorated with the organic molecule.

Read our paper in Angewandte Chemie

NEW 'Hot Paper' in CHEMISTRY A european journal

Reactions of Arylcarbenes with Lewis Acids
A. H. Raut, P. Costa, W. Sander, Chem. Eur. J. (2018), Ahead of Print

The reactions of the three triplet ground state arylcarbenes diphenylcarbene (1), fluorenylidene (2), and diben-zocycloheptadienylidene (3) with the Lewis acids H₂O, ICF₃,and BF₃ were studied under the conditions of matrix isolation. H₂O was selected as typical hydrogen bond donor, ICF₃ as halogen bond donor, and BF₃ as strong Lewis acid. H₂O forms hydrogen-bonded complexes of the singlet carbenes with 1 and 2, but not with 3. This is rationalized by the larger singlet–triplet gap of 3, which does not allow to stabilize the singlet state below the triplet state by hydrogen bonding.

With ICF₃, both 1 and 3 form halogen-bonded complexes of the singlet states of the carbenes. This indicates that halogen bonding stabilizes singlet carbenes more than hydrogen bonding. Carbene 2 reacts differently from 1 and 3 by forming an iodonium ylide, thus avoiding antiaromatic destabilization of the fluorenyl unit. With BF₃, all threecarbenes form zwitterionic Lewis acid/base complexes.

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NEW JOC PAPER

Reactions of Cyclopentadienylidenes with CF₃I: Electron Bond Donation versus Halogen Bond Donation of the Iodine Atom
S. Henkel, I. Trosien, J. Mieres-Perez, T. Lohmiller, A. Savitsky, E. Sanchez-Garcia, W. Sander, J. Org. Chem. 83 (2018), 7586-7592

The interaction of cyclopentadienylidene and tetrachlorocyclopentadienyli-dene with the halogen bond donor CF₃I has been studied by matrix isolation spectroscopy. The carbenes were produced by photolysis of the corresponding diazo compounds, matrix-isolated in argon doped with 1% CF₃I at 3 K. Bimolecular reactions between the carbenes and CF₃I were induced by annealing these matrices to 25–30 K to allow for the diffusion of trapped species. Instead of classical halogen-bonded complexes, these carbenes form complexes in which the iodine atom is shared between the carbene center and the CF₃ group. Photolysis of the complexes at 3 K yields radical pairs, which reversibly react back to the complexes when the matrices are warmed to 25–30 K.

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NEW JACS PAPER

Isolation of an Antiaromatic Singlet Cyclopentadienyl Zwitterion
P. Costa, I. Trosien, J. Mieres Perez, W. Sander, J. Am. Chem. Soc. 139 (2017), 13024-13030.

The reaction of triplet tetrachloro-cyclopentadienylidene with BF3 in rare gas matrices yields a zwitterion consisting of a cyclopentadienyl cation bearing the positive charge and a negatively charged BF3 unit. IR and UV-vis spectra as well as the absence of EPR signals demonstrate a singlet ground state of the zwitterion, and its calculated geometry and magnetic properties clearly reveal a strong antiaromatic character. The zwitterion is highly labile, and by visible or IR irradiation rearranges via a 1,2-fluorine migration from boron to carbon. Interaction with a second molecule of BF3 stabilizes the zwitterion and suppresses the fluorine migration, thus providing a convenient and efficient synthesis of an antiaromatic molecule under very mild conditions.

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NEW JACS PAPER

Is Magnetic Bistability of Carbenes a General Phenomenon? Isolation of Simple Aryl(trifluoromethyl)carbenes in Both their Singlet and Triplet States
Y. A. Tsegaw, P. E. Kadam, N. Tötsch, E. Sanchez-Garcia, W. Sander, J. Am. Chem. Soc. 139 (2017), 12310-12316.

p-Tolyl(trifluoromethyl) carbene and the related fluorenyl(trifluoromethyl)carbene were synthesized in solid argon and characterized by IR, UV-vis, and EPR spectroscopy as well as by quantum mechanical calculations. The carbenes can be generated in both their triplet and singlet states, and both states coexist under the conditions of matrix isolation. According to our calculations, the singlet and triplet states of these car-benes are energetically nearly degenerate in the gas phase. Warming of matrices containing pure triplet p-tolyl(trifluoromethyl) carbene from 3 to 25 K leads to an interconversion of up to 20% of the triplet into the singlet state. This interconversion is thermally irreversible, and cooling back to 3 K does not change the ratio of singlet to triplet. Irradiation at 365 nm results in a complete interconversion of singlet to triplet, whereas 450 nm irradiation produces again up to 20% of the singlet state. An alternative way to generate the singlet carbene is the reaction of the triplet with water molecules by annealing water-doped matrices at 25 K. This results in the irreversible formation of a hydrogen-bonded complex be-tween the singlet carbene and water. For fluorenyl(trifluoromethyl)carbene very similar results were obtained, only the yield of the singlet state is even higher. Magnetic bistability of carbenes seems to be a general phenomenon that only depends on the singlet-triplet gap rather than on the nature of the carbene.

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RUB Press Release

When a tiger escapes from an impenetrable cage

Quantum effects are rarely observed for heavy atoms. Now it’s been successful. RUB chemists have shown that carbon atoms can behave like particles and like waves. This double life has already been sufficiently described for light particles and electrons. However, researchers have only rarely been able to observe the wave-particle phenomenon for heavy atoms such as carbon.

Read the Press Release.

New Paper in Angewandte Chemie

The Cope Rearrangement of 1,5-Dimethylsemibullvalene-2(4)-₁: Experimental Evidence for Heavy-Atom Tunneling
T. Schleif, J. Mieres-Perez, S. Henkel, M. Ertelt, W. T. Borden, W. Sander, Angewandte Chemie, Int. Ed. 56, (2017), 10746-10749.

As an experimental test of the theoretical prediction that heavy-atom tunneling is involved in the degenerate Cope rearrangement of semibullvalenes at cryogenic temperatures, monodeuterated 1,5-dimethylsemibullvalene isotopomers were prepared and investigated by IR spectroscopy using the matrix isolation technique. As predicted, the less thermodynamically stable isotopomer rearranges at cryogenic temperatures in the dark to the more stable one, while broadband IR irradiation above 2000 cm^-1 results in an equilibration of the isotopomeric ratio. Since this reaction proceeds with a rate constant in the order of 10^-4 s^-1 despite an experimental barrier of Ea=4.8 kcal mol^-1 and with only a shallow temperature dependence, the results are interpreted in terms of heavy-atom tunneling.

Read our paper in Angew. Chem. Int. Ed.

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