Synthesis and reactivity of a parent phosphathioethynolato-borane and a boraarsaketene

We present the synthesis and isolation of the first main-group phosphathioethynolates, [LBH₂(SCP)] (L = SIMes, CAAC^Me; SIMes = 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene, CAAC^Me = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene), in which phosphathioethynolate coordination occurs exclusively through the sulfur atom, giving rise to a phosphaalkyne-type structural motif. This is reflected in the reactivity of the SIMes derivative towards organic azides and [(η²-C₂H₄)₂CoCp] (Cp = C₅H₅), which mirrors the behavior of 1-adamantylphosphaalkyne, yielding triazaphospholes and a mixed cyclopentadienyl-(1,3-diphosphete) sandwich complex, respectively. Deviations from typical phosphaalkyne reactivity are observed in reactions with boron-containing heterocycles such as pentaphenylborole (PPB) and a carboranyl-substituted 9-borafluorene, which yield an unprecedented bicyclic structure and a zwitterionic spiro compound, respectively. Furthermore, we report the synthesis of the first arsaketenylborane, [(SIMes)BH₂(AsCO)], which, although too unstable for isolation, generates an arsinidene <i>in situ</i>, which can be trapped by coordination to a PPB C[double bond, length as m-dash]C double bond.