Heterocyclic carbenes of diverse flexibility: A theoretical insight

A systematic density functional investigation has been carried out on the structure, stability and reactivity of heterocyclic carbenes of diverse flexibility, i.e., carbenes with different modes of binding specially normal and remote mode of binding. Ligating properties of these carbenes have been assessed by virtue of their HOMO energies and verified further by inspection of the symmetric CO frequencies of their respective palladium carbonyl complexes. All the remote carbenes were found to have higher σ-donating abilities compared to their normal analogs. N-heterocyclic carbenes 1 and 5 are found to be electrophilic in nature while the remote carbene 3 and P-heterocyclic carbene 6 are found to be nucleophilic. Quantum theory of atoms in molecules (QTAIM) reveals significant covalent character in the Ccarbene–Pd bonds.

Density functional calculations predict that remote carbenes are better electron donor than their normal analogs. The bonds formed by these carbenes with transition metal complexes are found to have significant covalent character.Figure optionsDownload as PowerPoint slideResearch highlights
► Remote carbenes are more basic than their normal analogs.
► Remote carbenes are softer than their normal analogs.
► N-heterocyclic carbenes 1 and 5 are found to be electrophilic in nature.
► Remote carbene 3 and P-heterocyclic carbene 6 are found to be nucleophilic.