Insight of the Metal–Ligand Interaction in f‐Element Complexes by Paramagnetic NMR Spectroscopy

Abstract

The magnetic properties of LnIII and AnIII complexes formed with dipicolinate ligands have been studied by NMR spectroscopy. To know precisely the geometries of these complexes, a crystallographic study by single‐crystal X‐ray diffraction (XRD) and extended X‐ray absorption fine structure (EXAFS) in solution was performed. Several methods to separate the paramagnetic shifts observed in the NMR spectra were applied to these complexes. Methods using a number of nuclei of the dipicolinate ligands revealed an abrupt change in the geometries of the complexes and a metal–ligand interaction in the middle of the lanthanide series. A study of the variation of the paramagnetic shifts with temperature demonstrated that higher‐order terms of the dipolar and contact contributions are required, especially for the lightest LnIII and almost all the studied AnIII. Bleaney's parameters <Sz>a and CaD relating to the contact and dipolar terms, respectively, were deduced from experimental data and compared with the results of ab initio calculations. Quite a good agreement was found for the temperature dependencies of <Sz>a and CaD . However, the CaD values obtained from cation magnetic anisotropy calculations showed some discrepancies with the values derived from Bleaney's equation defined for LnIII. Other parameters, such as the crystal field parameter and the hyperfine constants Fi obtained from the experimental data of the [An(ethyl‐dpa)3]3− complexes (ethyl‐dpa=4‐ethyl‐2,6‐dipicolinic acid), are at odds with the assumptions underlying Bleaney's theory.

Derivation of Lanthanide Series Crystal Field Parameters From First Principles

“…[10] The second series, denoted [Ln(dpa) 3 ] 3À ,c overst he whole period,t he environmenti sm ore prolate, and the complexes show close to three-fold symmetry,b ut not strictly. [28] As will appear from the calculations, the magnetization stays axial along the series with av ariation in the direction. Furthermore, the second-order parameter was recently deduced from paramagnetic NMR spectroscopy,a pplying Bleaney's theory.…”

“…This is a rare case in which the qualitative electrostatic model of Rinehart and Long applies almost perfectly . The second series, denoted [Ln(dpa) 3 ] 3− , covers the whole period, the environment is more prolate, and the complexes show close to three‐fold symmetry, but not strictly . As will appear from the calculations, the magnetization stays axial along the series with a variation in the direction.…”

“…As will appear from the calculations, the magnetization stays axial along the series with a variation in the direction. Furthermore, the second‐order parameter was recently deduced from paramagnetic NMR spectroscopy, applying Bleaney's theory …”

“…The [Ln(dpa) 3 ](C 3 H 5 N 2 ) 3 ⋅ 3H 2 O (denoted [Ln(dpa) 3 ] 3− ; Ln=Ce–Yb; dpa=2,6‐dipicolinic acid) series of compounds are also isostructural. The complexes have been structurally characterized by X‐ray diffraction, except praseodymium, and crystallize in the triclinic space group P 1 . The coordination sphere contains three DPA − that form a distorted tricapped trigonal prism (see Figure ).…”

“…Furthermore, the second-order parameter was recently deduced from paramagnetic NMR spectroscopy,a pplying Bleaney's theory. [28] The CFPs were deducedf rom ab initio calculations by using different methods and the results have been compared. The CFPs were determined from the energy matrix derived from either the basis sets of the orbitals (ab initio ligand field theory, AILFT) [29] or the basis set of the many-electron wave functions (irreducible technique operator,I TO).…”