Structural aspects of the protonation of small cages. Preparation of the new aza-cage 12,17-dimethyl-1,9,12,17-tetra-azabicyclo[7.5.5]nonadecane (L). Thermodynamic studies on solution equilibria. Crystal structures of [H2L][CoCl4] and [H2L1][CoCl4] salts

Abstract

The synthesis and characterization of the new aza-cage 12,17-dimethyl-1,9,12,17-tetra-azabicyclo-[7.5.5]nonadecane (L) are described. The basicity behaviour in both aqueous and water–DMSO (50 : 50, mol : mol) solutions has been investigated by potentiometric and microcalorimetric (25 °C, 0.15 mol dm^–3 NaCl) techniques. L behaves, in aqueous solution, as a fairly strong base (log K₁= 12.00; ΔH₁ ^⊖=–54.0 kJ mol^–1; TΔS₁ ^⊖= 14.5 kJ mol^–1) in the first protonation step and as a moderate base in the second step (log K₂= 7.86 ΔH₂ ^⊖=–44.8 kJ mol^–1; TΔS₂ ^⊖ca. 0). The enthalpic and entropic contributions are discussed in terms of molecular topology and translational entropy. The Cu^II ion is encapsulated by L, the electronic spectra being diagnostic of a square-planar structure. The molecular structures of the diprotonated salts [H₂L][CoCl₄] and [H₂L1][CoCl₄](L1 = 5,12,17-trimethyl-1,5,9,12,17-penta-azabicyclo[7.5.5]nonadecane) have been determined by single-crystal X-ray analysis. Crystals of [H₂L][CoCl₄] are orthorhombic, space group pbn2₁, with a= 9.311 (3)Å, b= 15.480(2)Å, c = 16.636(6)Å, and Z= 4. Final R values of 0.043 (R_w= 0.037) for 1 358 observed reflections. X-Ray analysis shows that the protonation occurs on the N–CH₃ groups. Each hydrogen atom of the NH⁺–CH₃ interacts with the two bridgehead nitrogen atoms. The compound [H₂L1][CoCl₄] crystallizes in a monoclinic unit cell (space group P2₁) with lattice constants a= 8.877(3)Å, b= 14.088(8)Å, c= 10.122(5)Å, β= 105.51 (4)°, and Z= 2. Final R values of 0.064 (R_w= 0.049) for 2 374 observed reflections. In this case the conformation of the diprotonated cage is different and the protonation probably occurs at one of the two methylated nitrogens and at the apical nitrogen atom.