Stable Lewis Base Adducts of Tetrahalodiboranes: Synthetic Methods and Structural Diversity

A series of 22 new bis(phosphine), bis(carbene), and bis(isonitrile) tetrahalodiborane adducts has been synthesized, either by direct adduct formation with highly sensitive B₂ X₄ precursors (X=Cl, Br, I) or by ligand exchange at stable B₂ X₄ (SMe₂ )₂ precursors (X=Cl, Br) with labile dimethylsulfide ligands. The isolated compounds have been fully characterized using NMR spectroscopy, elemental analysis, and, for 20 of these compounds, single-crystal X-ray diffraction, revealing an unexpected variation in the bonding motifs. In addition to the classical B₂ X₄ L₂ diborane(4) bis-adducts, certain more sterically demanding carbene ligands induce a halide displacement which led to the first halide-bridged monocationic diboron species, [B₂ X₃ L₂ ]A (A=BCl₄ , Br, I). Furthermore, low-temperature 1:1 reactions of B₂ Cl₄ with sterically demanding N-heterocyclic carbenes led to the formation of kinetically unstable mono-adducts, one of which was structurally characterized. A comparison of the NMR spectra and structural data of new and literature-known bis-adducts shows several trends pertaining to the nature of the halides and the stereoelectronic properties of the Lewis bases employed.