Pseudo-silapnictene and Bis(pseudo-silapnictene) – Hyperconjugatively Stabilized Heavier Analogues of Imine and Ethane-1,2-diimine

Abstract

The equilibrium geometry, bonding, and proton affinity of the pseudo-silapnictenes and the bis(pseudo-silapnictenes) are explored using computational quantum mechanical calculations carried out at M06/def2-TZVPP//BP86-D3(BJ)/def2-SVP level of theory. The pseudo-silapnictene 1E features a dicoordinated, monovalent group-15 center having two lone pairs of σ- and π-local symmetry. Both the lone pairs are stabilized by hyperconjugative donation to the silylene fragment. The extent of hyperconjugative stabilization of π-type lone pair is higher than that of σ-type lone pair and is comparable to classical Si−E π-bonds. Accordingly, the former interactions can be considered as pseudo-π-bonding interactions. The strength of the pseudo-π-bond reduces as the group-15 element changes from nitrogen to bismuth. Hence, 1E can be considered as heavier analogues of imine. Even though these lone pairs are stabilized by hyperconjugative interactions, they are very reactive as indicated by the very high values of first and second proton affinities. In the bis(pseudo-silapnictene) 2E, two Si−E pseudo-π-bonds formed by hyperconjugative interactions are delocalized over the E−Si−Si−E skeleton. Since the π-bonds are majorly localized on the group-15 element, the extent of delocalization is quite less than that of the classical π-delocalized system. Nevertheless, the bis(pseudo-silapnictene) can be considered as the heavier analogue of ethane-1,2-diimine.