Reactivity of Two-Electron-Reduced Boron Formazanate Compounds with Electrophiles: Facile N-H/N-C Bond Homolysis Due to the Formation of Stable Ligand Radicals.

The reactivity of a boron complex with a redox-active formazanate ligand, LBPh2 [L = PhNNC( p-tol)NNPh], was studied. Two-electron reduction of this main-group complex generates the stable, nucleophilic dianion [LBPh2]2-, which reacts with the electrophiles BnBr and H2O to form products that derive from ligand benzylation and protonation, respectively. The resulting complexes are anionic boron analogues of leucoverdazyls. N-C and N-H bond homolysis of these compounds was studied by exchange NMR spectroscopy and kinetic experiments. The weak N-C and N-H bonds in these systems derive from the stability of the resulting borataverdazyl radical, in which the unpaired electron is delocalized over the four N atoms in the ligand backbone. We thus demonstrate the ability of this system to take up two electrons and an electrophile (E+ = Bn+, H+) in a process that takes place on the organic ligand. In addition, we show that the [2e-/E+] stored on the ligand can be converted to E• radicals, reactivity that has implications in energy storage applications such as hydrogen evolution.