First Bis(σ)-borane Complexes of Group 6 Transition Metals: Experimental and Theoretical Studies.

A series of bis(σ)-borane complexes of Group 6 transition metals were prepared by direct dihydroborane coordination to the metal center. Reaction of [M(CO) ₃ (PCy ₃ ) ₂ ] and two dihydroboranes [DurBH ₂ ] and [(Me ₃ Si) ₂ NBH ₂ ] (Dur=2,3,5,6-Me ₄ C ₆ H) yielded bis(σ)-borane complexes fac-[M(CO) ₃ (PCy ₃ ){η ² -(H ₂ BR)}] (R=Dur; 1: M=Cr, 2: M=W; R=N(SiMe ₃ ) ₂ ; 3: M=Cr, 4: M=W). In the case of molybdenum, we have isolated an arene complex (5) with [DurBH ₂ ] in which the Dur group acts as a η ⁶ -bound ligand, and with [(Me ₃ Si) ₂ NBH ₂ ] a similar bis(σ)-borane complex was isolated, cis,trans-[Mo(CO) ₂ (PCy ₃ ) ₂ {η ² -(H ₂ BN(SiMe ₃ ) ₂ }] (6), with a different pattern of auxiliary ligands. The complexes were investigated by multinuclear NMR spectroscopy, mass spectrometry, X-ray diffraction analysis, and computational methods. Quantum theory of atoms in molecules (QTAIM) calculations demonstrated that the borane complexes may be described as pure bis(σ)-borane complexes rather than elongated or stretched examples given that the calculations do not show the presence of a ring-critical point (RCP) at the ring formed by the interactions of the B-H with metal center.
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