Construction of a Boron Chain on a Single Metal by Dehydrocoupling of Borane Ligands
Christian Luz, Kai Oppel, Lukas Endres, Rian D. Dewhurst, Holger Braunschweig, Udo Radius
Abstract
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.