Rationalization of Anomalous Pseudocontact Shifts and Their Solvent Dependence in a Series ofC₃-Symmetric Lanthanide Complexes

Abstract

Bleaney's long-standing theory of magnetic anisotropy has been employed with some success for many decades to explain paramagnetic NMR pseudocontact shifts, and has been the subject of many subsequent approximations. Here, we present a detailed experimental and theoretical investigation accounting for the anomalous solvent dependence of NMR shifts for a series of lanthanide(III) complexes, namely [LnL] (Ln = Eu, Tb, Dy, Ho, Er, Tm, and Yb; L: 1,4,7-tris[(6-carboxypyridin-2-yl)methyl]-1,4,7-triazacyclononane), taking into account the effect of subtle ligand flexibility on the electronic structure. We show that the anisotropy of the room temperature magnetic susceptibility tensor, which in turn affects the sign and magnitude of the pseudocontact chemical shift, is extremely sensitive to minimal structural changes in the first coordination sphere of L. We show that DFT structural optimizations do not give accurate structural models, as assessed by the experimental chemical shifts, and thus we determine a magnetostructural correlation and employ this to evaluate the accurate solution structure for each [LnL]. This approach allows us to explain the counterintuitive pseudocontact shift behavior, as well as a striking solvent dependence. These results have important consequences for the analysis and design of novel magnetic resonance shift and optical emission probes that are sensitive to the local solution environment and polarity.

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

“…In our case the disparities can clearly be attributed to the first assumption because the crystal field is found constant and lower than kT ( < r 2 >=52 cm −1 ; Table ). As depicted recently by Mason et al., crystal field changes can be rather related to differences in the orientation than to the degree of anisotropy of the magnetic susceptibility tensor . The use of a ligand leading to a tricapped trigonal prismatic more axially dissymmetric than our dpa ligand allowed Vonci et al .…”

“…As depicted recently by Mason et al., crystal field changes can be rather related to differences in the orientation than to the degree of anisotropy of the magnetic susceptibility tensor . The use of a ligand leading to a tricapped trigonal prismatic more axially dissymmetric than our dpa ligand allowed Vonci et al . to show that changes of a few degrees in the polar angles of the oxygen donor due to variations in the ionic radii or solvent effects lead to minimal variations of the coordination geometry but different orientation of the major component of the magnetic susceptibility tensor.…”

Derivation of Lanthanide Series Crystal Field Parameters From First Principles

“…It shows that Bleaney's , which parametrizes the entire magnetic anisotropy in a single parameter, is not clearly related to the “true” . It was recently pointed out by Vonci et al . that is very sensitive to small structural variations.…”

“…This value is four times smaller than the value of 250 cm À1 obtained in this work. However,i ts houldb en oted that B 2 0 is almostc onstant across the series and is the only CFP showingt his trend, which supports Bleaney's theory.I tshows that Bleaney's B 2 0 ,w hich parametrizes the entire magnetic anisotropy in as ingle parameter,isnot clearly related to the "true" B 2 0 .Itwas recently pointed out by Vonci et al [55] that B 2 0 is very sensitive to small structural variations.…”

Probing the Local Magnetic Structure of the [FeIII(Tp)(CN)3]− Building Block Via Solid‐State NMR Spectroscopy, Polarized Neutron Diffraction, and First‐Principle Calculations

“…The rationalization of these macroscopic magnetic properties is then commonlya ddressed by phenomenological approaches based on spin Hamiltonian models. Examples can be found for numerous kinds of paramagnetics ystems, including lanthanide-based compounds, [24][25][26] or solid-state materials [27] such as lithium ion batteries. However,the analysis of the EPR data relies on effective Hamiltonian (g-tensors and/ or zero-field splitting parameters).…”