Ab initio study on anomalous structures of anionic [(N-heterocycle)-CO] complexes.

Several unusual anionic complexes between carbon dioxide (CO) and N-heterocycles (NHCs) possessing a significantly positive adiabatic electron affinity over 0.7 eV were studied by density functional theory calculations (UB3LYP/6-311++g(d,p)). Unlike all previously reported [NHC-CO] anions with a coplanar structure that ensures full delocalization of the negative charge through extended π-conjugation, this new class of anionic [NHC-CO] complexes has a strongly non-coplanar geometry and no π-bond character between CO and NHC. Despite the fundamental differences in chemical bonding between all prior cases and the new class of [NHC-CO] complexes, we found that the CO moiety in the latter still has a large negative charge (∼0.4 e) and a strongly bent geometry (O-C-O angle of ∼140°) just like in the former. This seemingly anomalous case was explained by a simple model based on the torsional steric effect and the electron affinities of the constituent moieties.