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 <i>trans</i>-[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 <b>1</b>, R = Dur <b>3</b>; M = Mo: R = Mes <b>2</b>, R = Dur <b>4</b>). Three borane ligands are coordinated in <b>1</b><i>-</i><b>4</b> to the group 6 metal atom via five (σ-B-H) bonds. Reaction of <b>1</b> 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 <i>trans</i>-[W(IMes)(PR₃)(η¹-HBMes)(η²-H₂BMes)(H)] (R = Me <b>5</b>, R = Et <b>6</b>), in which the metal atom inserted into one of the remaining σ-B-H bonds of the borane ligands. Reaction of <b>1</b> with an additional equivalent borane BH₂Mes resulted in borane dehydrocoupling and formation of complex [W(IMes)(η⁴-BH₂Mes-BMes-BMes-BH₂Mes)] <b>7</b>, featuring a unique B₄ chain as a ligand. Reaction of <i>trans</i>-[W(IMes)₂Cl₄] with NaBH₄ also led to B-B coupling, and the metallaborane cluster [{W(IMes)(BH₄)}₂(B₅H₉)] <b>9</b> was formed, in which two tungsten atoms bridge a B₅ chain.