The amide oxygen donor. Metal ion coordinating properties of the ligand nitrilotriacetamide. A thermodynamic and crystallographic study

Abstract

The metal ion coordinating properties of ntam (nitrilotriacetamide) are reported. The protonation constant (pK) for ntam is 2.6 in 0.1 M NaClO₄ at 25 °C. Formation constants (log K₁) in 0.1 M NaClO₄ at 25 °C, determined by ¹H NMR and UV-Vis spectroscopy are: Ca(II), 1.28; Mg(II), 0.4; La(III), 2.30; Pb(II), 3.69; Cd(II), 3.78; Ni(II), 2.38; Cu(II), 3.16. The measured log K₁ values for the ntam complexes are discussed in terms of the low basicity of the N-donor, as evidenced by the pK, and the effect of metal ion size on complex stability. The amide O-donors of ntam lead to the stabilization of complexes of large metal ions (Pb(II), Cd(II), La(III), Ca(II)) relative to log K₁ for the NH₃ complexes, while for small metal ions (Ni(II), Cu(II)) the amide O-donors lead to destabilization. This is discussed in terms of the role of chelate ring size in controlling metal ion size-based selectivity. The structures of [Pb(ntam)(NO₃)₂]₂ (1) and [Ca₂(ntam)₃(H₂O)₂](ClO₄)₄·3H₂O (2) are reported. For 1: triclinic, space group P, a = 7.4411(16), b = 9.0455(19), c = 11.625(3) Å, α = 69.976(4), β = 79.591(4), γ = 67.045(3)°, Z = 2, R = 0.0275. For 2: monoclinic, space group P2₁/c, a = 10.485(2), b = 11.414(2), c = 38.059(8) Å, β = 92.05(3)°, Z = 4, R = 0.0634. Structure 1 is dimeric with two Pb atoms linked by bridging O-donors from the two ntam ligands. The coordination sphere consists of one N-donor and 3 O-donors from the ntam ligand, two O-donors from nitrates, and one bridging O-donor. The variation in bond length suggests a stereochemically active lone pair of electrons on the Pb. Structure 2 consists of two Ca(II) ions held together by 3 bridging O-donors from ntam groups. One Ca is 9-coordinate with two ntam ligands present, plus one bridging O-donor from the other Ca(II) ntam complex. The other Ca is 8-coordinate, with a single coordinated ntam, plus two coordinated H₂O molecules, and two bridging O-donors from the other half of the complex. The role of M–OC bond angles in controlling selectivity for metal ions on the basis of their size is discussed.