Deprotonation of Group 14 Metal Amide Complexes Bearing Ditopic Carbanionic N‑Heterocyclic Carbene Ligands. Constitutional Isomerism and Dynamic Behavior

The reactivity of
the lithiated N-heterocyclic carbene [:C­[N­(2,6-iPr2C6H3)]2(CH)­CLi]∞ with two-coordinate homoleptic
group 14 amides E­[N­(SiMe3)2]2 (E
= Sn, Pb) is described. Solutions of these mixtures readily result
in the formal loss of 1 equiv of bis­(trimethylsilyl)­amine, HN­(SiMe3)2, to afford novel metallacycles. Reactions involving
Sn­[N­(SiMe3)2]2 initially give rise
to the anionic complex [{:C­[N­(2,6-iPr2C6H3)]2C­(CH)}­Sn­{N­(SiMe3)2}2]− (1), which eventually affords the novel distannane [{(THF)2Li:C­[N­(2,6-iPr2C6H3)]2(CH)­C}­Sn­{μ-κ1:κ1-N­(SiMe3)­(SiMe2CH2)}­Sn­{C­(CH)­[N­(2,6-iPr2C6H3)]2C:}­{N­(SiMe3)2}] (2), in
which one of the trimethylsilyl substituents has been deprotonated
and bridges the Sn–Sn bond, affording a five-membered metallacycle.
By contrast, reactions involving Pb­[N­(SiMe3)2]2 give rise to a species that may be considered a constitutional
isomer of 2, [{(THF)2Li:C­[N­(2,6-iPr2C6H3)]2(CH)­C}­Pb­{N­(SiMe3)2}­{C­(CH)­[N­(2,6-iPr2C6H3)]2C:}­Pb­{κ2-N­(SiMe3)­(SiMe2CH2)}] (3), featuring a base-stabilized plumbylene
species. The dynamic behavior of 3 in solution suggests
stereochemical inversion at the plumbylene moiety, Pb­{κ2-N­(SiMe3)­(SiMe2CH2)}, a process
which can be retarded at low temperatures, allowing for the observation
of the two inequivalent proton environments of the methylene bridge.
This process is believed to involve rapid intermolecular transfer
of the plumbylene fragment between the ditopic carbanionic carbene
ligands of the plumbyl core.