How to Make the σ⁰π² Singlet the Ground State of Carbenes

Abstract

Successful strategies have previously been developed to stabilize the σ²π⁰ singlet states of carbenes, relative to σ¹π¹ triplet states. However, little or no attention has been paid to the stabilization of the σ⁰π² singlet states. We present two simple strategies to stabilize the σ⁰π² singlet states of carbenes, relative to both the σ²π⁰ singlet and σ¹π¹ triplet states. These strategies consist of destabilization of the carbene σ orbital by two, adjacent, sp² nitrogen lone pairs of electrons and stabilization of the carbene 2p−π orbital by incorporating it into a five-membered ring, containing two double bonds, or into a six-membered ring, containing two double bonds and a sixth atom that has a low-lying empty π orbital. B3LYP, CASPT2, and CCSD­(T) calculations have been performed in order to assess the success of these strategies in creating derivatives of cyclopenta-2,4-dienylidene and cyclohexa-2,5-dienylidene with σ⁰π⁶ singlet ground states. Differences between the calculated geometries and binding energies of the Xe complexes of the σ⁰π⁶ singlet ground state of 2,5-diazacyclopentadienylidene (<b>5</b>) and the σ²π⁰ singlet states of CH₂ and CF₂ are discussed