Solid-State and Solution Properties of Cationic Lanthanide Complexes of a New Neutral Heptadentate N4O3 Tripodal Ligand

Abstract

The synthesis of the potentially heptadentate ligand tris[6-((2-N,N-diethylcarbamoyl)pyridyl)methyl]amine, tpaam, containing three pyridinecarboxamide arms connected to a central nitrogen is described. Lanthanide complexes of this ligand are prepared and characterized. The crystallographic structure of the complexes of three lanthanide ions (La, Nd, Lu) is determined. The lanthanide(III) complexes of tpaam crystallize as monomeric species (in the presence of chloride or iodide counterions) in which the ligand tpaam acts as a N4O3 donor. The crystal structures presented here show that the Ln[bond]O and Ln[bond]N(pyridyl) distances in the complexes of tpaam are similar to those found for the tpaa complexes (H(3)tpaa = alpha,alpha',alpha' '-nitrilotri(6-methyl-2-pyridinecarboxylic acid) despite the difference in charge. A lengthening of the Ln[bond]N(apical) distance is observed in the tpaam complexes compared to the tpa (tris[(2-pyridyl)methyl]amine) complexes which is more marked for larger lanthanides than for smaller ones. The solution structures of the tpaam complexes were analyzed across the 4f series and compared to the solution structures of the lanthanide complexes of the tetradentate ligand tpa. Proton NMR studies are in agreement with the presence of C(3)(v) symmetric solution species for both ligands. For the larger lanthanides, the cation moves away from the apical nitrogen compared to the position occupied in tpa complexes, whereas for the smaller lanthanides, the metal ion is located in a similar position for the two ligands. Quite surprisingly, the formation constant of the Eu(tpaam)Cl(3) complex in D(2)O at 298 K (log beta(110) = 2.34(4)) is very similar to the one reported for Eu(tpa)Cl(3) (log beta(110) = 2.49(4) at 298 K in D(2)O) indicating that the addition of three amide groups to the ligand tpa does not lead to any increase in stability of the lanthanide complexes of tpaam compared to those of tpa.

Structural characterization of methanol substituted lanthanum halides

“…Alcohols came to the forefront based on a series of solubility experiments with MeOH in particular raising the most interest since LaX 3 readily dissolved in this solvent at room temperature. Structural reports of [LnX 3 (MeOH) x ] where the MeOH is directly bound to the Ln metal center have been limited to the Cl [40–46, 72, 73] derivatives only. [18, 19] Due to the void in structurally characterized ROH derivatives of LaX 3 , a study concerning the coordination and desolvation behavior of 1 with MeOH was undertaken.…”

“…The bound waters were successfully replaced by MeOH but an unusual salt structure was formed instead of the simple coordination species previously reported for ROH derivatives. [18, 19, 40–46, 55, 56, 72, 73] In 4 , the 9-coordinated La cation of the first molecule in the unit cell was modeled with 2.75 Br atoms and 5.25 MeOH solvent molecules for an overall +0.25 charge. One of the Br sites (Br7) was partially occupied by an oxygen atom from a MeOH ligand and was refined for the Br:O ratio at 0.75:0.25.…”

“…[18, 19] The majority of these compounds were found to be monomers, using pyridine n -oxide, polyamines, crown ethers, phosphine oxides, pyrazolyl borates, dimethoxyethane, acetonitrile, ROHs, and other simple organic solvents. Of the ROH derivatives bound to the Ln cation, only eight compounds were with MeOH [40–46, 72, 73] and four with ethanol (EtOH)[47–50]; however, these structures were isolated in the presence of other co-ligands such as crown ethers[44, 45, 47, 48] or polypyridyl[40–43, 49]. The one exception was the previously discussed [Ce(μ-Cl)Cl 2 (MeOH) 4 ] 2 .…”

“…[38–40, 83, 91–116] While these complexes ranged in nuclearity, the majority were monomers adopting a 2IS/1OS arrangement [(L) x LnI 2 ](I)[38, 40, 91–96, 106, 111, 117] (where L = neutral ligand or organic solvents such as: Pr i OH,[38] THF,[106, 117] and py[38, 106]) or the 3IS arrangement that utilizes combinations of bulky neutral ligands,[83, 95, 107, 110, 114, 118] THF,[98, 100, 102, 113] or Pr i OH [39] to fill the coordination sphere. However, no structure reports of the LaI 3 •MeOH derivatives have been previously reported.…”

Water Stability and Luminescence of Lanthanide Complexes of Tripodal Ligands Derived from 1,4,7‐Triazacyclononane: Pyridinecarboxamide versus Pyridinecarboxylate Donors

“…However, the presence of the amide groups in the heptadentate ligand tpaam do not result in high kinetic stability of its lanthanide complexes which form and dissociate rapidly in H 2 O solution [40]. In contrast, the additional presence of three pyridine N-donors in the ligand tpatcnam with respect to the notam ligand leads to a dramatic increase in the kinetic stability of the respective lanthanide complexes.…”