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.