Synthesis and characterization of racemic mixture and meso isomers of bis(trans-2-aminocyclohexyl)aminepentaacetic acid and the stabilities of their Gd(III) complexes

The synthesis and characterization of two multidentate ligands, the racemic mixture and meso isomers of bis(trans-2-aminocyclohexyl)aminepentaacetic acid, are described. Equilibrium constants for their Gd(III) complexes were determined by direct potentiometry. The formation constants (K(ML) = [ML]/[M][L]) of Gd(III)-Cycy(racemic) in 0.10 M KCl at 25.0 °C is 10^20.71; that for the meso isomer is 10^20.42. The crystal structure of (1S,2S,1'S,2'S)bis(trans-2-aminocyclohexyl)amine-N,N,N',N'',N''-pentaacetic acid, penta-tert-butyl ester (C₄₂H₇₅N₃O₁₀), is reported. This compound crystallizes in the triclinic system with space group P1 with cell parameters a = 10.805(2) Å, b = 11.382(2) Å, c = 20.999(4) Å, α = 91.41(3)°, β = 98.23(3)°, γ = 113.88(3)°, V = 2327.8(8) ų, Z = 2, D(x) = 1.116 g mL^-1. The results are compared to the crystal structures of the gadolinium complexes and the predictions derived from molecular mechanics.