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)₂Cl₄] (M = W, Mo) (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) with borates Li[BH₃R] (R = Mes, Dur; Mes = 2,4,6-Me₃C₆H₂ and Dur = 2,3,5,6-Me₄C₆H) afforded the complexes [M(IMes)(η²-H₂BR)₂(η¹-H₂BR)] (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 PMe₃ and PEt₃, 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)(PR₃)(η¹-HBMes)(η²-H₂BMes)(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 BH₂Mes resulted in borane dehydrocoupling and formation of complex [W(IMes)(η⁴-BH₂Mes-BMes-BMes-BH₂Mes)] 7, featuring a unique B₄ chain as a ligand. Reaction of trans-[W(IMes)₂Cl₄] with NaBH₄ also led to B-B coupling, and the metallaborane cluster [{W(IMes)(BH₄)}₂(B₅H₉)] 9 was formed, in which two tungsten atoms bridge a B₅ chain.

Christian Luz

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