Heteroditopic cryptands of tunable cavity size: imposition of distorted geometry onto Copper(II) and Nickel(II) and molecular recognition of water molecules

Abstract

Complexation behavior of a previously described heteroditopic cryptand L¹ and a newly synthesized cryptand L² toward transition metal ions is described. The cryptands readily accept a transition metal ion like Cu(II) or Ni(II) at the N₄ end of the cavity forming mononuclear cryptates. The Cu(II)-cryptate of L¹ further accepts a water molecule to form [Cu(L¹)(OH₂)]·(picrate)₂·(H₂O), i.e., C₄₅H₅₃N₁₁O₁₉Cu. This molecule crystallizes in the monoclinic space group C2/c with a = 34.199(8) Å, b = 12.286(5) Å, c = 23.289(7) Å, β = 93.12(4)°, Z = 8. The O atom of the water molecule is strongly bonded to the Cu(II) ion, while its H atoms are H-bonded to the nearest benzene rings. The UV-vis spectral and magnetic studies of the complexes are consistent with the mononuclear nature of the cryptates. Cryptand L¹ undergoes diprotonation upon treatment with a mineral acid like HCl or HClO₄. This diprotonated cryptand is found to be a good host for two molecules of water inside the cavity. The diperchlorate salt, [H₂L¹(H₂O)_1.5]·(ClO₄)₂·(H₂O)_0.75, i.e., C₃₃H_51.5Cl₂N₅O_13.25, crystallizes in the monoclinic space group P2₁/c with a = 12.204(1) Å, b = 11.465(1) Å, c = 28.537(3) Å, β = 91.98(2)°, Z = 4. The cryptand has an ellipsoidal cavity with both the bridgehead N atoms adopting an endo-endo conformation in the solid state. The water O atoms O(1w) and O(2w) lie in the pseudo 3-fold axis joining the two bridgehead N atoms. Occupancy of O(2w) is found to be 0.5 while that of O(1w) is unity. The water molecules are held inside the cavity through H-bonding with an ice-like structure. The FAB-mass spectral data and the bond distances involving the water oxygens suggest that both the water molecules are tightly held inside the cavity. The present cryptate is the first X-ray crystallographically characterized complex with two water molecules inside the cavity of a cryptand.