Comparative Structural Studies of Iodide Complexes of Uranium(III) and Lanthanide(III) with Hexadentate Tetrapodal Neutral N-Donor Ligands

The syntheses, the solution structures, and the crystal structures of the two new tetrapodal N-donor ligands N,N,N‘,N‘-tetrakis(2-pyrazylmethyl)-1,3-trimethylenediamine (tpztn), 1, and N,N,N‘,N‘-tetrakis(2-pyrazylmethyl)-trans-1,2-cyclohexanediamine (tpzcn), 2, are described. Two different geometric isomers of the cation [La(tpztn)I2]+ were
isolated in which the ligand adopts two different conformations leading to strong differences in the metal−ligand
bond distances. The crystal structure of isostructural complexes of La, U, Ce, and Nd were determined by X-ray
diffraction studies for the ligands tpztn and tpzcn. In both series of complexes the two methylpyrazyl arms and the
diamine spacer (trimethylene or cyclohexane) around each aliphatic nitrogen adopt the same helical configuration.
The complexes crystallize as a racemic mixture of Λ,Λ and Δ,Δ enantiomers with distorted square antiprism
geometries. In these complexes the M−Npyrazine distances show a decrease from La to Ce and from La to Nd which
corresponds well to the decrease in ionic radius as expected in a purely ionic bonding model. Conversely the
mean value of the U−Npyrazine distances is shorter (0.043(3) Å for tpztn and 0.054(11) Å for tpzcn) than the mean
value of the La−Npyrazine distances. These differences are significantly larger than the decrease expected from the
variation of the ionic radii and can be interpreted in terms of a stronger M−N interaction for U(III). Previously
reported extraction studies have shown that while the tripod tris[(2-pyrazyl)methyl]amine (tpza) containing three
pyrazyl nitrogens extracts An(III) preferentially to Ln(III), tpztn and tpzcn display no selectivity despite the presence
of four pyrazyl groups connected to a different spacer. The structural studies described here show that despite the
lack of selectivity observed in the extraction conditions, the arrangement of pyrazyl nitrogens in the tetrapodal
architectures of tpztn and tpzcn allows for metal−ligand interaction similar to that observed for tpza.