Activation of Molecular Hydrogen by a Singlet Carbene through Quantum Mechanical Tunneling

Abstract

Carbenes are among the few metal-free molecules that are able to activate molecular hydrogen. Whereas triplet carbenes have been shown to insert into H 2 through atwo-step mechanism that at low temperature is assisted by quantum mechanical tunneling (QMT), singlet carbenes insert in concerted reactions with considerable activation barriers,a nd are thus unreactive towards H 2 at cryogenic temperatures.Here we show that 1-azulenylcarbene with asinglet ground state readily inserts into H 2 ,a nd that QMT governs the insertion into both H 2 and D 2 .This is the first example that shows that QMT can also be important for singlet carbenes inserting into dihydrogen.The metal-free activation of molecular hydrogen is an emerging field in chemistry with al arge potential for applications,f rom organic synthesis to energy storage.O nly very few organic closed-shell molecules are able to break the strong H À Hb ond at room temperature,a mong which are (alkyl)(amino)carbenes [1] and frustrated Lewis acid-base pairs.[2] Whereas singlet (alkyl)(amino)carbenes such as 1 are able to split the H 2 molecule at room temperature, there is still asignificant activation barrier of 22-24 kcal mol À1