Double Reduction of Ph3B to a Dianionic Borane With a Powerful SrI Intermediate.

Reduction of [(BDI)SrI]2 with KC8 generated in situ a (BDI)SrI species which enabled the reduction of Ph3B to the Ph3B2- dianion (BDI = HC[(Me)C═N-DIPeP]2, DIPeP = 2,6-(Et2CH)2-phenyl). The product (BDI)Sr(BPh3)Sr(BDI) features a Ph3B2- 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 Ph3B2- dianion demonstrates the very high reducing power of in situ generated (BDI)SrI species.