Double Reduction of Ph₃B to a Dianionic Borane With a Powerful Sr^I Intermediate

Reduction of [(BDI)SrI]₂ with KC₈ generated in situ a (BDI)Sr^I species which enabled the reduction of Ph₃B to the Ph₃B^2- dianion (BDI = HC[(Me)C═N-DIPeP]₂, DIPeP = 2,6-(Et₂CH)₂-phenyl). The product (BDI)Sr(BPh₃)Sr(BDI) features a Ph₃B^2- dianion that is bridging two Sr²⁺ cations by interaction with one of the Ph rings. This polarizes a large part of the electron density in the bridging Ph ring (NPA charge: -1.53) whereas the B is slightly positive (+0.44). The HOMO of the complex mainly consists of the π*-orbital in the bridging Ph ring overlapping with Sr d-orbitals. However, there is also 18% contribution of the B p-orbital. In solution, there is coalescence of ¹H NMR signals of all three Ph rings. Complex (BDI)Sr(BPh₃)Sr(BDI) can react as a 2e transfer reagent or as a B-nucleophile. It reacts with I₂ to Ph₃B and [(BDI)SrI]₂. Reaction with Ph₃B led to formation of a Gomberg-type dimer. The reaction with PhI is smooth and selectively gave (BDI)Sr(Ph₄B) and [(BDI)SrI]₂, likely through direct nucleophilic attack of an electron rich B atom at PhI. Isolation of a complex with the elusive Ph₃B^2- dianion demonstrates the very high reducing power of in situ generated (BDI)Sr^I species.