Synthesis and characterization of compounds with boron-boron double bonds

The diborane(4) derivatives Mes 2 BB(Mes)Ph and Me 2 N(Ph)BB(Ph)NMe 2 readily react with lithium powder in ether to give the doubly reduced salts [{(Et 2 O)Li} 2 Mes 2 BB(Mes)Ph] and [{(Et 2 O)Li} 2 Me 2 N(Ph)BB(Ph)NMe 2 ]. Both species, which were isolated as red crystals, have been characterized by X-ray crystallography, 11 B NMR and electronic spectroscopy. The structural data reveal planar or almost planar B 2 C 4 or B 2 C 2 N 2 cores with B-B distances near 1.63 A (cf. BB=1.71 A in the neutral non-planar precursors) consistent with the formation of a π-bond between the boron centers. Their 11 B NMR spectra display peaks that are shifted upfield relative to the starting compounds. In the case of the amido derivative, two B-N π-bonds were cleaved in the reduction process. The electronic absorption spectrum of this compound in Et 2 O displays an intense absorption at 320 nm which may be attributed to the π-π* transition of the BB π-bond. This suggests that the strength of this bond is of the order of 40 kcal mol − . The compounds, which may be considered as boron analogues of the corresponding alkenes, represent the first structurally characterized acyclic species that possess formal BB double bonds.