The haloboration of tri-tert-butylazadiboriridine NB2R3: Ring opening versus NB3 cluster formation

The azadiboriridine [-BR-NR-BR-] (1; R = tBu) is bromoborated at the B-B bo nd by alkyldibromoboranes R'BBr2 to give the products Br-BR-NR=BR-BR'-Br (8 a-g: R' = Me, Bu, tBu, Bzl, CH2CHEt2, CH2Cy, CH2(4-C(6)H(4)tBu)). Two isome rs of each of the products 8a-g are formed and attributed to a cis/trans is omerism at the BN double bond; the isomerization is followed thermodynamica lly and kinetically by NMR methods with 8a-d. The analogous chloroboration of 1 with BCl3 yields Cl-BR-NR=BR-BCl2 (8h), which at ambient temperature u ndergoes a degenerate exchange of the ligands Cl and BCl2 along the B-N-B s keleton. At room temperature, the isomer Cl-BR-NR=BCl-BR-Cl (8h') is slowly formed by an irreversible exchange of R and Cl along the B-B bond of 8 h. Different from BCl3, the chloroborane BH2Cl is simply added to the B-B bond of 1 under formation of the azanido-tetraborane NB3R3H2Cl (2b). The chloro borane BHCl2 gives a mixture of 8h' and 2b upon addition to 1, apparently a ccording to a preceding dismutation into BCl3 and BH2Cl. The configuration at the B3 atom of the nido-clusters NB3R3H2X (X = H, Cl) is discussed on th e basis of the cor responding model molecules NB3Me3H2X, whose structure an d NMR signals are computed by the B3LYP method. The boranes 8b-g can be deb rominated with Li in the presence of tmen on applying ultrasound. The produ cts are found to be the B-borylated azadiboriridines [-BR-NR-B(BRR')-] (9b- g). The 2-borylazadiboriridines NB3H4 (9h) and NB3Me4 (9i) were found as lo cal minima on the energy hyperface by the B3LYP method, but minima for stru ctural isomers with lower energy were also found; the tetrahedral clusters NB3R4 give high-energy minima with triplet ground states. Computations of t he B-11 NMR shifts of 9h and 9i support the proposed structures of 9 b-g.