C-C Coupling of Carbene Molecules on a Metal Surface in the Presence of Water

A novel surface-confined C-C coupling reactioninvolvingtwo carbene molecules and a water molecule was studied by scanningtunneling microscopy in real space. Carbene fluorenylidene was generatedfrom diazofluorene in the presence of water on a silver surface. Whilein the absence of water, fluorenylidene covalently binds to the surfaceto form a surface metal carbene, and water can effectively competewith the silver surface in reacting with the carbene. Water moleculesin direct contact with fluorenylidene protonate the carbene to formthe fluorenyl cation before the carbene can bind to the surface. Incontrast, the surface metal carbene does not react with water. Thefluorenyl cation is highly electrophilic and draws electrons fromthe metal surface to generate the fluorenyl radical which is mobileon the surface at cryogenic temperatures. The final step in this reactionsequence is the reaction of the radical with a remaining fluorenylidenemolecule or with diazofluorene to produce the C-C couplingproduct. Both a water molecule and the metal surface are essentialfor the consecutive proton and electron transfer followed by C-Ccoupling. This C-C coupling reaction is unprecedented in solutionchemistry.