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 = N^tBu (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.