Aluminum(I)/Boron(III) Redox Reactions

Abstract

Reactions between B^III species and the novel nucleophilic cyclopentadienyl-stabilized Al^I reagent (1) result in a diversity of complexes bearing different Al/B oxidation states and coordination geometries. With the triarylborane B(C₆ F₅ )₃ , a simple Al^I →B^III adduct is formed. In contrast, a bulky aryldihaloborane undergoes oxidative addition with the formation of a covalent bora-alane species. With an N-heterocyclic carbene-stabilized amino(bromo)borenium ion, a redox reaction was observed, where the product is a borylene-alane B^I →Al^III complex. Additionally, reaction of 1 with BI₃ results in complete scrambling of all of the Al/B-bound substituents, and the formation of a cyclopentadienylboron(I)→AlI₃ complex. These latter reactions are the first examples of the reduction of a boron(III) compound to a borylene by a p-block reagent, and illustrate how subtle changes in the nature of the borane can result in highly divergent reaction outcomes.