Orthophosphate and Sulfate Utilization for C–E (E = P, S) Bond Formation via Trichlorosilyl Phosphide and Sulfide Anions

Reduction of phosphoric acid (H3PO4) or tetra-n-butylammonium bisulfate ([TBA]­[HSO4]) with trichlorosilane leads to the formation of the bis­(trichlorosilyl)­phosphide ([P­(SiCl3)2]−, 1) and trichlorosilylsulfide ([Cl3SiS]−, 2) anions, respectively. Balanced equations for the formation of the TBA salts of anions 1 and 2 were formulated based on the identification of hexachlorodisiloxane and hydrogen gas as byproducts arising from these reductive processes: i) [H2PO4]− + 10HSiCl3 → 1 + 4O­(SiCl3)2 + 6H2 for P and ii) [HSO4]− + 9HSiCl3 → 2 + 4O­(SiCl3)2 + 5H2 for S. Hydrogen gas was identified by its subsequent use to hydrogenate an alkene ((−)-terpinen-4-ol) using Crabtree’s catalyst ([(COD)­Ir­(py)­(PCy3)]­[PF6], COD = 1,5-cyclooctadiene, py = pyridine, Cy = cyclohexyl). Phosphide 1 was generated in situ by the reaction of phosphoric acid and trichlorosilane and used to convert an alkyl chloride (1-chlorooctane) to the corresponding primary phosphine, which was isolated in 41% yield. Anion 1 was also prepared from [TBA]­[H2PO4] and isolated in 62% yield on a gram scale. Treatment of [TBA]1 with an excess of benzyl chloride leads to the formation of tetrabenzylphosphonium chloride, which was isolated in 61% yield. Sulfide 2 was used as a thionation reagent, converting benzophenone to thiobenzophenone in 62% yield. It also converted benzyl bromide to benzyl mercaptan in 55% yield. The TBA salt of trimetaphosphate ([TBA]3[P3O9]·2H2O), also a precursor to anion 1, was found to react with either trichlorosilane or silicon­(IV) chloride to provide bis­(trimetaphosphate)­silicate, [TBA]2[Si­(P3O9)2], characterized by NMR spectroscopy, X-ray crystallography, and elemental analysis. Trichlorosilane reduction of [TBA]2[Si­(P3O9)2] also provided anion 1. The electronic structures of 1 and 2 were investigated using a suite of theoretical methods; the computational studies suggest that the trichlorosilyl ligand is a good π-acceptor and forms σ-bonds with a high degree of s character.