Activation of Molecular Hydrogen by Arylcarbenes

The hydrogenation reactions of diphenylcarbene 1, fluorenylidene 2, and dibenzocycloheptadienylidene 3 were investigated in solid H₂ and D₂ matrices and in H₂- and D₂-doped argon matrices at cryogenic temperatures. 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 chemical 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.