Disproportionation and ligand lability in low oxidation state boryl-tin chemistry

Boryltin compounds featuring the metal in the +1 or 0 oxidation states can be synthesized from the carbene-stabilized tin(II) bromide (boryl)Sn(NHC)Br (boryl = {B(NDippCH)2}; NHC = C{(NiPrCMe)2}) by the use of strong reducing agents. The formation of the mono-carbene stabilized distannyne and donor-free distannide systems (boryl)SnSn(IPrMe)(boryl) (2) and K2[Sn2(boryl)2], (3) using Mg(I) and K reducing agents mirrors related germanium chemistry. In contrast to their lighter congeners, however, systems of the type [Sn(boryl)]n are unstable with respect to disproportionation. Carbene abstraction from 2 using BPh3, and two-electron oxidation of 3 both result in the formation of a 2:1 mixture of the Sn(II) compound Sn(boryl)2, and the hexatin cluster, Sn6(boryl)4 (4). A viable mechanism for this rearrangement is shown by quantum chemical studies to involve a vinylidene intermediate (analogous to the isolable germanium compound, (boryl)2Ge=Ge), which undergoes facile atom transfer to generate Sn(boryl)2 and trinuclear [Sn3(boryl)2]. The latter then dimerizes to give the observed hexametallic product 4, with independent studies also showing that similar trigermanium species aggregate in analogous fashion.