Magnetically bistable carbenes and nitrenes
Conformational Spin Switching
Conformational Spin Switching and Spin-Selective Hydrogenation of a Magnetically Bistable Carbene
I. Trosien, E. Medez-Vega, T. Thomanek, W. Sander, Angew. Chem. Int. Ed. 58 (2019), 15162.
The control of the spin states of molecules opens the path to tuning selectivity in chemical reactions and to developing novel magnetically switchable materials. 3-Methoxy-9-fluorenylidene is a carbene that is generated in cryogenic matrices both in its lowest energy singlet and triplet states, and the ratio of these states can be shifted by selective irradiation. The interconversion of the nearly degenerate spin states is induced by a conformational change of the methoxy group: switching the methoxy group into the “up” position results in the singlet state and switching into the “down” position in the triplet state. The spin control via a remote functional group makes this carbene unique for the study of spin-specific reactions, which is demonstrated for the hydrogenation reaction. Spin switching by switching the conformation of a remote functional group is a novel phenomenon with potential applications in the design of functional materials.
Read the Paper in Angew. Chem.
Switching the Spin State of Pentafluorophenylnitrene
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. 140 (2018), 17271-17277.
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.
Magnetically Bistable Carbenes
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.