Synthesis, crystal structure and ligational properties of a new macrotricyclic ligand

Abstract

The synthesis and characterization of the new macrotricycle 10,22,28,32-tetraoxa-1,4,7,13,16,18-hexaazatricyclo[17.5.5.5^7,13]tetratriacontane L is reported. Its basicity in aqueous solution has been investigated by potentiometric and spectroscopic (¹H, ¹³C NMR) techniques. The basicity constants have been determined (25 °C, I= 0.15 mol dm^–3) in two ionic media. Macrotricycle L behaves as a hexaprotic base: log K₁= 10.27(3), log K₂= 9.52(3), log K₃= 7.12(3), log K₄= 4.62(3), log K₅= 2.80(3), log K₆= 2.27(3) in NaClO₄ and log K₁= 10.09(3), log K₂= 9.27(3), log K₃= 6.54(3), log K₄= 4.88(3), log K₅= 3.75(3), log K₆= 3.07(3) in NaCl. The constants relative to the equilibrium Cl^–+ H_nLⁿ⁺⇌ H_nLCl^(n–1)+ have been measured: n= 5, log K= 2.48(3); n= 6, log K= 3.59(3). The crystal structure of L has been determined by single-crystal X-ray analysis. The compound crystallizes in a triclinic unit cell (space group P, Z= 2) with lattice constants a= 8.984(5), b= 12.122(2), c= 14.667(3)Å, α= 109.52(2), β= 105.47(3), γ= 95.24(3)°. Least-squares refinement converged at R= 0.088 (R′= 0.075) for 3198 unique reflections with I > 4σ(I). The structure shows that the molecule has a skewed cylindrical shape, the bases being formed by the two N₂O₂ macrocycles with the N(CH₂CH₂)₂ chains constituting the walls. The cylindrane L binds two Li⁺ ions and the resulting complex has been studied by ⁷Li NMR spectroscopy. Copper(II) and cadmium(II) complexes with L have been investigated in aqueous solution and their stability constants determined by potentiometric techniques. For both metals the following species have been found: [ML]²⁺, [MLH]³⁺, [ML(OH)]⁺ and [ML(OH)₂].