Bis(neopentylglycolato)diboron (B 2 nep 2 ) as a bidentate ligand and a reducing agent for early transition metal chlorides giving MCl 4 (B 2 nep 2 ) complexes.

We found that bis(neopentylglycolato)diboron (B 2 nep 2 ) served as a bidentate ligand and a one-electron reducing agent for early transition metal chlorides to afford MCl 4 (B 2 nep 2 ). Treatment of B 2 nep 2 with MCl 5 (M = Nb and Mo) produced MCl 4 (B 2 nep 2 ) via two successive reactions, coordination of B 2 nep 2 to the metal center and one-electron reduction from M(V) to M(IV), while coordination of B 2 nep 2 to MCl 4 (M = Zr, Ti) was observed without reduction of the central metals. DFT studies for the reduction of NbCl 5 by B 2 nep 2 clarified the initial formation of seven-coordinated and B 2 nep 2 -ligated Nb(V) species, NbCl 5 (B 2 nep 2 ), and one chloride on niobium(V) moves to the Lewis acidic boron center to generate NbCl 4 [(B 2 nep 2 )Cl]. The chloride on the boron atom of NbCl 4 [(B 2 nep 2 )Cl] is trapped by the second B 2 nep 2 to give [NbCl 4 (B 2 nep 2 )][ClB 2 nep 2 ]. After the formation of [ClB 2 nep 2 ] - as an anionic sp 2 -sp 3 diboron adduct, one-electron reduction of the niobium(V) center produces NbCl 4 (B 2 nep 2 ) along with [ClB 2 nep 2 ]˙ as a plausible diboron species, whose decomposition affords ClBnep and B 2 nep 2 . The reduction of metal halides in the presence of B 2 nep 2 was exemplified by green LED irradiation of TiCl 4 (B 2 nep 2 ), producing chloride-bridged titanium(III) species, (B 2 nep 2 )TiCl 2 (μ-Cl) 2 TiCl 2 (B 2 nep 2 ).