Anions featuring an aluminium–silicon core with alumanyl silanide and aluminata-silene characteristics

Molecular species containing multiple bonds to aluminium have long been challenging synthetic targets. Despite recent landmark discoveries in this area, heterodinuclear Al–E multiple bonds (where E is a group-14 element) have remained rare and limited to highly polarized π-interactions (Al=E ↔ ⁺Al–E^–). Here we report the isolation of three alumanyl silanide anions that feature an Al–Si core stabilized by bulky substituents and a Si–Na interaction. Single-crystal X-ray diffraction studies, spectroscopic analysis and density functional theory calculations show that the Al–Si interaction possesses partial double bond character. Preliminary reactivity studies support this description of the compounds through two resonance structures: one that displays a predominant nucleophilic character of the sodium-coordinated silicon centre in the Al–Si core, as shown by silanide-like reactivity towards halosilane electrophiles and the CH-insertion of phenylacetylene. Moreover, we report an alumanyl silanide with a sequestered sodium cation. Cleavage of the Si–Na bond by [2.2.2]cryptand increases the double bond character of the Al–Si core to produce an anion with high aluminata-silene (^–Al=Si) character. [Figure not available: see fulltext.]