Ab initio study on anomalous structures of anionic [(N-heterocycle)-CO₂]^- complexes

Area of Science:

• Computational Chemistry
• Physical Chemistry
• Materials Science

Background:

• Anionic complexes of carbon dioxide (CO2) and N-heterocycles (NHCs) typically feature planar structures facilitating charge delocalization.
• Previous studies established a correlation between coplanar geometry, extended π-conjugation, and charge distribution in [NHC-CO2]- anions.

Purpose of the Study:

• To investigate the structural and electronic properties of a new class of anionic [NHC-CO2]- complexes.
• To understand the factors governing the bonding and geometry in these unusual complexes, particularly when N-heterocycles possess high electron affinity.

Main Methods:

• Density functional theory (DFT) calculations were employed using the UB3LYP/6-311++g(d,p) level of theory.
• The study focused on complexes where N-heterocycles exhibited adiabatic electron affinities exceeding 0.7 eV.

Main Results:

• A novel class of anionic [NHC-CO2]- complexes with strongly non-coplanar geometries was identified.
• These complexes lack π-bond character between the CO2 and NHC moieties, differing from previously reported structures.
• Despite the structural and bonding differences, the CO2 moiety retained significant negative charge (~0.4 e) and a bent O-C-O angle (~140°).

Conclusions:

• The anomalous charge distribution and bent geometry in the non-coplanar [NHC-CO2]- complexes can be explained by a model considering torsional steric effects and the electron affinities of the components.
• This finding challenges previous assumptions about the structural requirements for charge delocalization in such anionic complexes.