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

Reduction of phosphoric acid (H 3 PO 4 ) or tetra- n -butylammonium bisulfate ([TBA][HSO 4 ]) with trichlorosilane leads to the formation of the bis(trichlorosilyl)phosphide ([P(SiCl 3 ) 2 ] −, 1 ) and trichlorosilylsulfide ([Cl 3 SiS] −, 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 ) [H 2 PO 4 ] − + 10HSiCl 3 → 1 + 4O(SiCl 3 ) 2 + 6H 2 for P and ii ) [HSO 4 ] − + 9HSiCl 3 → 2 + 4O(SiCl 3 ) 2 + 5H 2 for S. Hydrogen gas was identified by its subsequent use to hydrogenate an alkene ((−)-terpinen-4-ol) using Crabtree’s catalyst ([(COD)Ir(py)(PCy 3 )][PF 6 ], 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][H 2 PO 4 ] 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 [P 3 O 9 ]·2H 2 O), 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(P 3 O 9 ) 2 ], characterized by NMR spectroscopy, X-ray crystallography, and elemental analysis. Trichlorosilane reduction of [TBA] 2 [Si(P 3 O 9 ) 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.