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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.

Read our paper in JACS


Paolo Costa won the Dr.-Heinrich-Kost-Preis of the Ruhr University for his PhD thesis


Paolo studied the interplay between solvents and reactive intermediates on the molecular scale. He observed that a diphenyl carbene, matrix isolated in solid argon at cryogenic temperatures changes its spin state from triplet to singlet because of the interaction between a single methanol or water molecule by forming hydrogen bonds. He found the same behaviour for Fluorenyliden and water in argon matrix. Using water matrices the corresponding carbo cations are formed via protonation. Paolo's PhD thesis was supervised by Prof. W. Sander.

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