Aminoacid N-substituted 1,4,7-triazacyclononane and 1,4,7,10-tetraazacyclododecane Zn2+, Cd2+ and Cu2+ complexes. A preparative, potentiometric titration and NMR spectroscopic study

Abstract

The pK_as and Zn²⁺, Cd²⁺ and Cu²⁺ complexation constants (K) for 1,4,7-tris[(2″S)-acetamido-2″-(methyl-3″-phenylpropionate)]-1,4,7-triazacyclononane, 1, 1,4,7-tris[(2″S)-acetamido-2″-(1″-carboxy-3″-phenylpropane)]-1,4,7-triazacyclononane, H₃2, 1,4,7-tris[(2″S)-acetamido-2″-(methyl-3″-(1H-3-indolyl)propionate)]-1,4,7-triazacyclononane, 3, and 1,4,7,10-tetrakis[(2″S)-acetamido-2″-(methyl-3″-phenylpropionate)]-1,4,7,10-tetraazacyclododecane, 4, 1,4,7,10-tetrakis[(2″S)-acetamido-2″-(1″-carboxy-3″-phenylpropane)]-1,4,7,10-tetraazacyclododecane, H₄5, in 20 ∶ 80 v/v water–methanol solution are reported. The pK_as within the potentiometric detection range for H₃1³⁺ = 8.69 and 3.59, for H₆2³⁺ = 9.06, 6.13, 4.93 and 4.52, H₃3³⁺ = 8.79 and 3.67, H₄4⁴⁺ = 8.50, 5.62 and 3.77 and for H₈5⁴⁺ = 9.89, 7.06, 5.53, 5.46, 4.44 and 4.26 where each tertiary amine nitrogen is protonated. The complexes of 1: [Zn(1)]²⁺ ( 9.00), [Cd(1)]²⁺ (6.49), [Cd(H1)]³⁺ (4.54) and [Cu(1)]²⁺ (10.01) are characterized by the log(K/dm³ mol⁻¹) values shown in parentheses. Analogous complexes are formed by 3 and 4: [Zn(3)]²⁺ (10.19), [Cd(3)]²⁺ (8.54), [Cu(3)]²⁺ (10.77), [Zn(4)]²⁺ (11.41) [Cd(4)]²⁺ (9.16), [Cd(H4)]³⁺ (6.16) and [Cu(4)]²⁺ (11.71). The tricarboxylic acid H₃2 generates a greater variety of complexes as exemplified by: [Zn(2)⁻] (10.68) [Zn(H2)] (6.60) [Zn(H₂2)⁺] (5.15), [Cd(2)]⁻ (4.99), [Cd(H2)] (4.64), [Cd(H₂2)]⁺ (3.99), [Cd(H₃2)]²⁺ (3.55), [Cu(2)]⁻ (12.55) [Cu(H2)] (7.66), [Cu(H₂2)]⁺ (5.54) and [Cu₂(2)]⁴⁻ (3.23). The complexes of H₄5 were insufficiently soluble to study in this way. The ¹H and ¹³C NMR spectra of the ligands are consistent with formation of a predominant Zn²⁺ and Cd²⁺ Δ or Λ diastereomer. The preparations of the new pendant arm macrocycles H₃2, 3, 4 and H₄5 are reported.