Bis(neopentylglycolato)diboron (Bnep) as a bidentate ligand and a reducing agent for early transition metal chlorides giving MCl(Bnep) complexes.

We found that bis(neopentylglycolato)diboron (Bnep) served as a bidentate ligand and a one-electron reducing agent for early transition metal chlorides to afford MCl(Bnep). Treatment of Bnep with MCl (M = Nb and Mo) produced MCl(Bnep) two successive reactions, coordination of Bnep to the metal center and one-electron reduction from M(V) to M(IV), while coordination of Bnep to MCl (M = Zr, Ti) was observed without reduction of the central metals. DFT studies for the reduction of NbCl by Bnep clarified the initial formation of seven-coordinated and Bnep-ligated Nb(V) species, NbCl(Bnep), and one chloride on niobium(V) moves to the Lewis acidic boron center to generate NbCl[(Bnep)Cl]. The chloride on the boron atom of NbCl[(Bnep)Cl] is trapped by the second Bnep to give [NbCl(Bnep)][ClBnep]. After the formation of [ClBnep] as an anionic sp-sp diboron adduct, one-electron reduction of the niobium(V) center produces NbCl(Bnep) along with [ClBnep]˙ as a plausible diboron species, whose decomposition affords ClBnep and Bnep. The reduction of metal halides in the presence of Bnep was exemplified by green LED irradiation of TiCl(Bnep), producing chloride-bridged titanium(III) species, (Bnep)TiCl(μ-Cl)TiCl(Bnep).