How to Make the σ<sup>0</sup>π<sup>2</sup> Singlet the Ground State of Carbenes
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Abstract
Successful
strategies have previously been developed to stabilize
the σ<sup>2</sup>π<sup>0</sup> singlet states of carbenes,
relative to σ<sup>1</sup>π<sup>1</sup> triplet states.
However, little or no attention has been paid to the stabilization
of the σ<sup>0</sup>π<sup>2</sup> singlet states. We present
two simple strategies to stabilize the σ<sup>0</sup>π<sup>2</sup> singlet states of carbenes, relative to both the σ<sup>2</sup>π<sup>0</sup> singlet and σ<sup>1</sup>π<sup>1</sup> triplet states. These strategies consist of destabilization
of the carbene σ orbital by two, adjacent, sp<sup>2</sup> 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 σ<sup>0</sup>π<sup>6</sup> singlet ground states. Differences between
the calculated geometries and binding energies of the Xe complexes
of the σ<sup>0</sup>π<sup>6</sup> singlet ground state
of 2,5-diazacyclopentadienylidene (<b>5</b>) and the σ<sup>2</sup>π<sup>0</sup> singlet states of CH<sub>2</sub> and CF<sub>2</sub> are discussed