Isolation of a Three-Coordinate Boron Cation with a Boron-Sulfur Double Bond

The reaction of the bulky bis(imidazolin‐2‐iminato) ligand precursor (1,2‐(LMesNH)2‐C2H4)[OTs]2 (12+ 2[OTs]−; LMes=1,3‐dimesityl imidazolin‐2‐ylidene, OTs=p‐toluenesulfonate) with lithium borohydride yields the boronium dihydride cation (1,2‐(LMesN)2‐C2H4)BH2[OTs] (2+ [OTs]−). The boronium cation 2+ [OTs]− reacts with elemental sulfur to give the thioxoborane salt (1,2‐(LMesN)2‐C2H4)BS[OTs] (3+ [OTs]−). The hitherto unknown compounds 12+ 2[OTs]−, 2+ [OTs]−, and 3+ [OTs]− were fully characterized by spectroscopic methods and single‐crystal X‐ray diffraction. Moreover, DFT calculations were carried out to elucidate the bonding situation in 2+ and 3+. The theoretical, as well as crystallographic studies reveal that 3+ is the first example for a stable cationic complex of three‐coordinate boron that bears a BS double bond. A healthy relationship: The first three‐coordinate boron cation with a boron–sulfur double bond has the shortest BS distance reported for a molecular complex. In the calculated LUMO, the contribution from the boron center confirms that the compound is a boron‐centered cation. The HOMO−1 reveals the π interaction which is the cause of the close contact between B and S.