Construction of a Boron Chain on a Single Metal by Dehydrocoupling of Borane Ligands.

Borane coordination, B-H borane bond activation, and borane catenation via metal-mediated dehydrocoupling to form electron-precise B-B bonds are reported. The reaction of trans -[M(IMes) 2 Cl 4 ] (M = W, Mo) (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) with borates Li[BH 3 R] (R = Mes, Dur; Mes = 2,4,6-Me 3 C 6 H 2 and Dur = 2,3,5,6-Me 4 C 6 H) afforded the complexes [M(IMes)(η 2 -H 2 BR) 2 (η 1 -H 2 BR)] (M = W: R = Mes 1 , R = Dur 3 ; M = Mo: R = Mes 2 , R = Dur 4 ). Three borane ligands are coordinated in 1 - 4 to the group 6 metal atom via five (σ-B-H) bonds. Reaction of 1 with the phosphines PMe 3 and PEt 3 , respectively, led to the elimination of one of the borane ligands and afforded the hydrido (σ-B-H)-boryl bis(σ-B-H)-borane complexes trans -[W(IMes)(PR 3 )(η 1 -HBMes)(η 2 -H 2 BMes)(H)] (R = Me 5 , R = Et 6 ), in which the metal atom inserted into one of the remaining σ-B-H bonds of the borane ligands. Reaction of 1 with an additional equivalent borane BH 2 Mes resulted in borane dehydrocoupling and formation of complex [W(IMes)(η 4 -BH 2 Mes-BMes-BMes-BH 2 Mes)] 7 , featuring a unique B 4 chain as a ligand. Reaction of trans -[W(IMes) 2 Cl 4 ] with NaBH 4 also led to B-B coupling, and the metallaborane cluster [{W(IMes)(BH 4 )} 2 (B 5 H 9 )] 9 was formed, in which two tungsten atoms bridge a B 5 chain.