Bis(neopentylglycolato)diboron (B2nep2) as a bidentate ligand and a reducing agent for early transition metal chlorides giving MCl4(B2nep2) complexes

Abstract

We found that bis(neopentylglycolato)diboron (B₂nep₂) served as a bidentate ligand and a one-electron reducing agent for early transition metal chlorides to afford MCl₄(B₂nep₂). Treatment of B₂nep₂ with MCl₅ (M = Nb and Mo) produced MCl₄(B₂nep₂) via two successive reactions, coordination of B₂nep₂ to the metal center and one-electron reduction from M(V) to M(IV), while coordination of B₂nep₂ to MCl₄ (M = Zr, Ti) was observed without reduction of the central metals. DFT studies for the reduction of NbCl₅ by B₂nep₂ clarified the initial formation of seven-coordinated and B₂nep₂-ligated Nb(V) species, NbCl₅(B₂nep₂), and one chloride on niobium(V) moves to the Lewis acidic boron center to generate NbCl₄[(B₂nep₂)Cl]. The chloride on the boron atom of NbCl₄[(B₂nep₂)Cl] is trapped by the second B₂nep₂ to give [NbCl₄(B₂nep₂)][ClB₂nep₂]. After the formation of [ClB₂nep₂]^– as an anionic sp²–sp³ diboron adduct, one-electron reduction of the niobium(V) center produces NbCl₄(B₂nep₂) along with [ClB₂nep₂]˙ as a plausible diboron species, whose decomposition affords ClBnep and B₂nep₂. The reduction of metal halides in the presence of B₂nep₂ was exemplified by green LED irradiation of TiCl₄(B₂nep₂), producing chloride-bridged titanium(III) species, (B₂nep₂)TiCl₂(μ-Cl)₂TiCl₂(B₂nep₂).