Low-Energy Electronic Transition in SiB Rings

Five- and six-membered rings containing Si–B bonds were synthesized by salt metathesis of magnesium disilanides with tetramethylpiperidinyldichloroborane (TMPBCl2). Materials were characterized by 1H, 11B, 13C, and 29Si NMR spectroscopy, as well as X-ray crystallography. Insights from crystallography facilitate conformational analysis of the cyclosilanes and elucidation of conformation-dependent optical properties. Crystallography also supports assignment of π character to the BN bond of the tetramethylpiperidinylborane fragment. The SiB rings have unique optical properties compared to all silicon rings. A 350 nm electronic transition only observed in SiB rings is assigned to a σ–π* transition on the basis of density functional theory calculations.