Activation of Sulfur Dioxide by a Phosphazene Base: Synthesis and Solid-State Structure of a Phosphazenium SO2 Adduct.

The superbasic phosphazene {(Et2N)3P = N}3P = NtBu (EtP4) activates sulfur dioxide (SO2) to form the neutral adduct [EtP4·SO2], which was isolated as a crystalline solid and structurally characterized by single-crystal X-ray diffraction. The solid-state structure confirms the coordination of SO2 to the phosphazene nitrogen atom and represents the first crystallographically characterized SO2-phosphazene adduct. Upon the addition of water, ethanol, or trifluoroethanol, the adduct undergoes selective transformation to give the corresponding phosphazenium sulfite salts [EtP4H]+[HSO3]-, [EtP4H]+[H3CCH2OSO2]-, and [EtP4H]+[F3CCH2OSO2]-, all of which were obtained as crystalline solids and fully characterized. In contrast, no reactivity was observed with the weak C-H acid fluoroform (HCF3) and pentafluoroethane (HC2F5). These findings establish [EtP4·SO2] as a rare, bench-stable SO2 adduct with defined reactivity and potential utility as a metal-free SO2 transfer reagent.