SYNTHESIS AND CHARACTERIZATION OF COMPOUNDS WITH BORON BORON DOUBLE-BONDS

The diborane(4) derivatives Mes2BB(Mes)Ph and Me2N(Ph)BB(Ph)NMe2 readily react with lithium powder in ether to give the doubly reduced salts [{(Et2O)Li}2MeS2BB(Mes)Ph] and [{(Et2O)Li}2Me2N(Ph)BB(Ph)NMe2]. Both species, which were isolated as red crystals, have been characterized by X-ray crystallography, B-11 NMR and electronic spectroscopy. The structural data reveal planar or almost planar B2C4 or B2C2N2 cores with B-B distances near 1.63 angstrom (cf. B-B = 1.71 angstrom in the neutral non-planar precursors) consistent with the formation of a pi-bond between the boron centers. Their B-11 NMR spectra display peaks that are shifted upfield relative to the starting compounds. In the case of the amido derivative, two B-N pi-bonds were cleaved in the reduction process. The electronic absorption spectrum of this compound in Et2O displays an intense absorption at 320 nm which may be attributed to the pi-pi* transition of the B-B pi-bond. This suggests that the strength of this bond is of the order of 40 kcal mol-1. 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.