Phosphine-Stabilized Germylidenylpnictinidenes as Synthetic Equivalents of Heavier Nitrile and Isocyanide in Cycloaddition Reactions with Alkynes

The
reactions of chlorogermylene MsFluindtBu-GeCl 1, supported by a sterically
encumbered hydrindacene ligand MsFluindtBu, with NaPCO­(dioxane)2.5 and NaAsCO­(18-c-6) in the presence of trimethylphosphine afforded trimethylphosphine-stabilized
germylidenyl-phosphinidene 2 and -arsinidene 3, respectively. Structural and computational investigations reveal
that the Ge–E′ bond (E′ = P and As) features
a multiple-bond character. 2 and 3 exhibit
diverse reactivity toward trimethylsilylacetylene and 4-tetrabutylphenylacetylene.
Specifically, 2 underwent cycloadditions with both alkynes
affording the first six-membered aromatic phosphagermabenzen-1-ylidenes 4 and 5, respectively, through the heavier isocyanide
intermediate MsFluindtBu-PGe.
In contrast, 3 could serve as a synthetic equivalent
of heavier isocyanides and nitriles when treated with trimethylsilylacetylene
and 4-tetrabutylphenylacetylene yielding arsagermene 6 and arsolylgermylene 7, respectively. The reaction
mechanisms for the cycloadditions were investigated through density
functional theory calculations. The reactivity studies highlight the
potential of 2 and 3 in accessing heavy
main-group element-containing heterocycles.