Synthesis and Spectroscopic and Structural Characterization of the Novel Lithium Borylamide Salts trans-2, a Dimer, and the Ion Pair with a Linear Allene-like, -, Moiety

Article abstract of DOI:10.1021/ja00210a022

Treatment of the primary aminoborane H2NBMes2 (1) with n-BuLi in Et2O/hexane gives an almost quantitative yield of the novel dimeric lithium amide 2 (2).The addition of 1 equiv of Mes2BF to LiNHBMes2, generated in situ, gives the recently published diborylamine HN(BMes2)2 (3).The compound 3 reacts with 1 equiv of n-BuLi to give the salt ^+- (4) in quantitative yield.Details of the 11B NMR spectra and X-ray crystal structures of 1, 2, and 4 are reported.Crystal data α (λ = 0.71069 Angstroem)> at 130 K (293 K for 2) are as follows: 1, a = 8.602 (3) Angstroem, b = 9.637 (2) Angstroem, c = 10.248 (2) Angstroem, β = 109.38 (2) deg, Z = 2, monoclinic, space group P2₁, 1505 unique data, R(F) = 0.069; 2, a = 9.010 (4) Angstroem, b = 9.440 (5) Angstroem, c = 25.958 (12) Angstroem, β = 97.56 (4) deg, Z = 2, monoclinic, space group P2₁/n, 3859 unique data, R(F) = 0.063; 4, a = 12.699 (3) Angstroem, b = 12.889 (6) Angstroem, c = 14.371 (5) Angstroem, β = 91.87 (2) deg, Z = 2, monoclinic, space group P2₁, 5645 unique data, R(F) = 0.058.The structures of the compounds are notable for a number of reasons.For example, 1 is the first primary aminoborane, i.e.R2BNH2 (where R = alkyl or aryl group), to be structurally characterized.It possesses a short B-N distance of 1.375 (8) Angstroem.The X-ray crystal structure of 2 is the first for a lithium salt of a primary aminodiarylborane.The structure of the unique diborylamide anion ^- (4) reveals the first example of an essentially linear 176.2 (3) deg BNB system.The BN distances are 1.343 (5) and 1.348 (5) Angstroem, and the CBC planes have a dihedral angle of 88 deg.Thus, it is isoelectronic to and isostructural with the corresponding allene system and the borinium cations ⁺.