Controlling Reactivity-Real-Space Imaging of a Surface Metal Carbene.

Metal carbenes are key intermediates in a plethora of homogeneous and heterogeneous catalytic processes. However, despite their importance to heterogeneous catalysis, the influence of surface attachment on carbene reactivity has not yet been explored. Here, we reveal the interactions of fluorenylidene (FY), an archetypical aromatic carbene of extreme reactivity, with a Ag(111) surface.

Conformational Spin Switching and Spin-Selective Hydrogenation of a Magnetically Bistable Carbene.

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.

Imaging the Solvation of a One-Dimensional Solid on the Molecular Scale.

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.

Reactions of Cyclopentadienylidenes with CFI: Electron Bond Donation versus Halogen Bond Donation of the Iodine Atom.

The interaction of cyclopentadienylidene and tetrachlorocyclopentadienylidene with the halogen bond donor CFI 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% CFI at 3 K. Bimolecular reactions between the carbenes and CFI were induced by annealing these matrices to 25-30 K to allow for the diffusion of trapped species.

Isolation of an Antiaromatic Singlet Cyclopentadienyl Zwitterion.

The reaction of triplet tetrachlorocyclopentadienylidene with BF in rare gas matrices yields a zwitterion consisting of a cyclopentadienyl cation bearing a positive charge and a negatively charged BF 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.

Light and Temperature Control of the Spin State of Bis(p-methoxyphenyl)carbene: A Magnetically Bistable Carbene.

Bis(p-methoxyphenyl)carbene is the first carbene that at cryogenic temperatures can be isolated in both its lowest energy singlet and triplet states. At 3 K, both states coexist indefinitely under these conditions. The carbene is investigated in argon matrices by IR, UV-vis, and X-band EPR spectroscopy and in MTHF glasses by W-band EPR and Q-band ENDOR spectroscopy.

The fluorenyl cation.

The fluorenyl cation is a textbook example for a 4π antiaromatic cation. However, contrasting results have been published on how the annelated benzene rings compensate the destabilizing effect of the 4π antiaromatic five-membered ring in its core. Whereas previous attempts to synthesize this cation in superacidic media resulted in undefined polymeric material only, we herein report that it can be generated and isolated in amorphous water ice at temperatures below 30 K by photolysis of diazofluorene.