Boranophosphate salts as an excellent mimic of phosphate salts: Preparation, characterization, and properties

We report on the preparation of boranophosphate salts, BPi (2), and the exploration of their properties with a view to developing a new mimic of the parent phosphate. BPi salts were easily prepared in excellent yield in a one-pot two-step reaction from tris(trimethylsilyl) phosphite, and were characterized by X-ray crystallography and IR, ¹H and ³¹P NMR spectroscopy. We evaluated the acid/base character of BPi by determining its acidity constants. Likewise, we evaluated the stability of BPi at various pH values, and calculated the decomposition-rate constants at highly acidic pH. We also monitored the H-bonded clustering of BPi in organic solvents, including MeOH. Finally, we explored the chemical behavior of BPi with respect to various organic and inorganic reagents. BPi is stable under the following conditions: both basic and acidic pH (pH > 2), in the presence of amines, and in the presence of Mg²⁺ ions. However, a P-B bond cleavage is observed upon the reaction of BPi with carbodiimides or upon catalytic hydrogenation. The reducing nature of the BH₃ moiety is drastically decreased in BPi. Likewise, the nucleophilicity of BPi's oxygen atom is lower than in phosphate, Pi, salts. Based on its water solubility, acid-base character, and H-bonding properties, BPi appears as a perfect mimic of Pi and is an attractive alternative to the known phosphate isosters.