Solid-State and Solution Structure of Lanthanide Complexes of a New Nonadentate Tripodal Ligand Containing Phenanthroline Binding Units

Abstract

The new nonadentate tripodal ligand trenphen {tris[(1,10-phenanthroline-2-carboxamido)-ethyl]amine} has been synthesized by condensation of tren [tris(2-aminoethyl)amine] with an excess of 1,10-phenanthroline-2-carboxylic acyl chloride. The ligand trenphen and its lanthanide complexes (Sm, Nd, Eu, Tb, and Lu) have been structurally characterized by single-crystal X-ray diffractometry. Crystals of trenphen·H₂O·CH₃CN, 1, are monoclinic, space group P2(1)/n, a = 14.9923(8) Å, b = 17.4451(10) Å, c = 17.1880(10) Å, β = 114.8290(10)°, V = 4079.9(4) ų, Z = 4. The solid-state crystal structures of the isostructural [Ln(trenphen)](OTf)₃·yH₂O·xEt₂O·zCH₃CN (OTf = CF₃SO₃) (Ln = Nd, y = 0.5, x = 1, z = 3 (2); Ln = Sm, y = 0.5, x = 1, z = 3 (3); Ln = Eu, y = 0.5, z = 3 (4); Ln = Tb, y = 0.5, x = 1, z = 1.5 (5); Ln = Lu, y = 0.5, x = 1, z = 1.5 (6)) (trigonal, P-3, Z = 2) show that the covalent tripod trenphen undergoes a rearrangement in the presence of lanthanide ions yielding three tridentate binding units which encapsulate the nine-coordinated lanthanide ion with a slightly distorted, tricapped, trigonal prismatic coordination geometry. The correlation observed between the decrease of Ln−N distances and the metal ionic radius indicates that trenphen, although containing rigid bidentate phenanthroline units, is sufficiently flexible to self-organize without steric constraints around lanthanide ions of different size. Solution-state NMR studies show that complexes 2−6 exist in acetonitrile solution as discrete rigid C₃-symmetric species retaining the triple-helical structure observed in the solid state. NMR and ES-MS titration show the formation of bimetallic and trimetallic species in the presence of an excess of metal, whereas mononuclear bistrenphen complexes are obtained in the presence of an excess of ligand.