Carbodiphosphorane-Activated Distibene and Dibismuthene Dications

Low-valent antimony and bismuth have emerged as novel
platforms
for achieving reversible small-molecule activation at main-group metals.
Although various examples of oxidative addition reactions at monomeric
Sb(I) and Bi(I) have been reported, the chemistry of the heavy group
15 Sb(I)Sb(I)/Bi(I)Bi(I) double bonds toward small
molecules remains largely unexplored. In this study, we present a
straightforward synthesis of distibene and dibismuthene dications
coordinated with a neutral carbodiphosphorane (CDP) ligand. The nonbonding
interactions between the occupied p-orbital at the CDP ligand and
the π-bonding orbital of the SbSb/BiBi bonds
yield compounds with exceptionally small HOMO–LUMO gaps. In
addition, the reduction of steric hindrance compared to known neutral
derivatives stabilized with bulky aryl groups allows for better accessibility
of the double bonds. This high reactivity is demonstrated in the oxidative
addition of distibene to diphenyldisulfide as well as in [2+2] cycloadditions
to alkynes. Additionally, the SbSb bond reversibly adds to
2,3-dimethylbutadiene in a [4+2] cycloaddition reaction.