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-((LNH)-N-Mes)(2)-C2H4)[OTs](2) (1(2+) 2[OTs](-); L-Mes = 1,3-dimesityl imidazolin-2-ylidene, OTs = p-toluenesulfonate) with lithium borohydride yields the boronium dihydride cation (1,2-((LN)-N-Mes)(2)-C2H4)BH2[OTs] (2(+) [OTs](-)). The boronium cation 2(+) [OTs](-) reacts with elemental sulfur to give the thioxoborane salt (1,2-((LN)-N-Mes)(2)-C2H4)BS[OTs] (3(+) [OTs](-)). The hitherto unknown compounds 1(2+) 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.