Synthesis and characterization of the aza-cage 4-benzyl-10,15-dimethyl-1,4,7,10,15-pentaazabicyclo[5.5.5]heptadecane (L). Its proton transfer properties and lithium complex. The crystal structure of the monoprotonated salt [HL][BPh4]

Abstract

The synthesis and characterization of the small, new benzyl aza-cage 4-benzyl-10,15-dimethyl-1,4,7,10,15-pentaazabicyclo[5.5.5]heptadecane (L) are reported. The basicity constants have been determined in aqueous solution by potentiometry (25 °C, [NaCl] 0.15 mol dm^–3). L behaves as a biprotic base: log K₁= 11.8, log K₂= 8.3. The crystal structure of the monoprotonated salt [HL][BPh₄] has been determined by single-crystal X-ray analysis. The compound crystallizes in an orthorhombic unit cell (space group Pc2₁n, Z= 4) with lattice constants a= 11.400(4), b= 16.893(3), c= 20.537(6)Å. Least-squares refinement converged at R= 0.078 (R_w= 0.056) for 1290 unique reflections with I > 3σ(I). The structure of [HL]⁺ shows the five nitrogens atoms in an endo configuration, indicating that the proton is inside the cage cavity. The cage L encapsulate Li⁺ and the complex formation equilibrium for [LiL]⁺ has been investigated by potentiometry [log K= 3.0(1)] and ⁷Li NMR techniques.