Theoretical Study on the Effect
of Annelation and
Carbonylation on the Electronic and Ligand Properties of <i>N</i>‑Heterocyclic Silylenes and Germylenes: Carbene Comparisons
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Abstract
Quantum
chemical calculations have been carried out to investigate
the effect of annelation and carbonylation on the electronic and ligand
properties of <i>N</i>-heterocyclic silylenes and germylenes.
The thermodynamic stability of these ligands has been found to increase
with annelation, while the reverse is true for carbonylation. This
is in sharp contrast to N-heterocyclic carbenes (NHCs) where annelation
leads to a decrease in their thermodynamic stabilities. Compared to
nonannelated derivatives, annelated and carbonylated ones are found
to be weaker σ donors but better π acceptors. The effect
of carbonylation is more pronounced than annelation toward increasing
the π acidity of these ligands. Carbonylation at the α-position
with respect to the N atom attached to the Si/Ge center has been found
to be the most effective way of enhancing the π acidity of these
ligands. The computed natural charges reveal that electrophilicity
increases upon both annelation and carbonylation. The calculated values
of <sup>31</sup>P NMR chemical shifts of corresponding phosphinidene
adducts of these ligands have been found to correlate well with the
π acidity of these Si/Ge centers