Crown Ether Complexes of Alkali-Metal Chlorides from SO₂

The structures of alkali-metal chloride SO₂ solvates (Li-Cs) in conjunction with 12-crown-4 or 1,2-disila-12-crown-4 show strong discrepancies, despite the structural similarity of the ligands. Both types of crown ethers form 1:1 complexes with LiCl to give [Li(1,2-disila-12-crown-4)(SO₂ Cl)] (1) and [Li(12-crown-4)Cl]⋅4 SO₂ (2). However, 1,2-disila-12-crown-4 proved unable to coordinate cations too large for the cavity diameter, for example, by the formation of sandwich-type complexes. As a result, 12-crown-4 reacts exclusively with the heavier alkali-metal chlorides NaCl, KCl and RbCl. Compounds [Na(12-crown-4)₂ ]Cl⋅4 SO₂ (3) and [M(12-crown-4)₂ (SO₂ )]Cl⋅4 SO₂ (4: M=K; 5: M=Rb) all showed S-coordination to the chloride ions through four SO₂ molecules. Compounds 4 and 5 additionally exhibit the first crystallographically confirmed non-bridging O,O'-coordination mode of SO₂ . Unexpectedly, the disila-crown ether supports the dissolution of RbCl and CsCl in the solvent and gives the homoleptic SO₂ -solvated alkali-metal chlorides [MCl⋅3 SO₂ ] (6: M=Rb; 7: M=Cs), which incorporate bridging μ-O,O'-coordinating moieties and the unprecedented side-on O,O'-coordination mode. All compounds were characterised by single-crystal X-ray diffraction. The crown ether complexes were additionally studied by using NMR spectroscopy, and the presence of SO₂ at ambient temperature was revealed by IR spectroscopy of the neat compounds.