Synthesis, structure, and unprecedented solubility of lipophilic borate salts

The multi-gram scale preparation of halide free lipophilic borate salts from inexpensive precursor compounds 3,3′,5,5′-tetra-tert-butyl-2,2′-biphenol and tetrahydridoboranate salts is reported. The so-called “bortebate” anion is more stable against water and bases than its aluminum analog “altebate”, but bortebate formation is significantly slower. Bortebate salts are highly soluble in hydrocarbon solvents, e.g. >35 mmol/L lithium bortebate in pentane at 20 °C. Quantitative salt metathesis reactions between sodium bortebate and halide salts can be easily achieved by precipitation of the sodium halide in methylene chloride.

Salts of a new borate anion have been prepared in a multi-gram scale. The anion’s eight tert-butyl groups characterize it as a molecular “wax ball” with an extreme solubility of its salts in hydrocarbons. Its S4 symmetry results in high melting points and a distinct tendency towards crystallization of the so-called “bortebate” salts. The figure of a single-crystal X-ray diffraction study displays tetraphenylphosphonium bortebate.Figure optionsDownload as PowerPoint slideResearch highlights
► Slow reaction of fourfold tert  -butylated 2,2′-dihydroxybiphenyl with BH4-.
► Highly lipophilic borate ester anion.
► Thermostable borate salts with high melting points.
► Unstable towards acidic water, metastable in basic medium.
► Unprecedented 35 mmol/L of lithium salt in pentane at 22 °C.