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 (PCy₃ )₂ ] and two dihydroboranes [DurBH₂ ] and [(Me₃ Si)₂ NBH₂ ] (Dur=2,3,5,6-Me₄ C₆ H) yielded bis(σ)-borane complexes fac-[M(CO)₃ (PCy₃ ){η² -(H₂ BR)}] (R=Dur; 1: M=Cr, 2: M=W; R=N(SiMe₃ )₂ ; 3: M=Cr, 4: M=W). In the case of molybdenum, we have isolated an arene complex (5) with [DurBH₂ ] in which the Dur group acts as a η⁶ -bound ligand, and with [(Me₃ Si)₂ NBH₂ ] a similar bis(σ)-borane complex was isolated, cis,trans-[Mo(CO)₂ (PCy₃ )₂ {η² -(H₂ BN(SiMe₃ )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.