Molecular Group 13 Metallaborates Derived from M-O-M Cleavage Promoted by BH₃

The reaction of metalloxanes [{^MeLM(OH)}₂(μ-O)] [M = Al (1), Ga (2); ^MeL = CH{CMe(NAr)}₂^-, Ar = 2,4,6-Me₃C₆H₂, Me = methyl] with an excess of BH₃·D (D = tetrahydrofuran (THF), SMe₂) affords annular metallaborate systems achieved through M-O-M cleavage. Compound 1 led exclusively to the formation of eight-membered ring systems [{^MeLAl(μ-O){B(OⁿBu)}(μ-O)}₂] (3) and [{^MeLAl(μ-O)(BH)(μ-O)}₂] (6), while for 2 the unprecedented six-membered ring systems [{(^MeLGa)₂(μ-O)}(μ-O)₂{B(OⁿBu)}] (4) and [(^MeLGa)(μ-O)₂{(BOⁿBu)₂(μ-O)}] (5) were observed. The use of BH₃·THF with 1 and 2 led to the concomitant THF ring-opening reaction, while with BH₃·SMe₂ in THF no such reaction was observed. The metallaborates [^MeLAl{OB(pinacol)}(OH)] (7) and [{^MeLGa(OB(pinacol))}₂(μ-O)] (8) were synthesized from pinacolborane and the corresponding metalloxane, providing structural evidence that supports the reaction pathways proposed for the formation of 3-6. Density functional theory studies were performed on 3-5 to assess the effect of the metal exchange between aluminum and gallium atoms on the energy of the general ring structures.