A New Class of Lanthanide Complexes with Three Ligand Centered Radicals: NMR Evaluation of Ligand Field Energy Splitting and Magnetic Coupling

Abstract

Combination of three radical anionic Ph‐BIAN ligands (Ph‐BIAN=bis‐(phenylimino)‐acenaphthenequinone) with lanthanoid ions leads to a series of homoleptic, six‐coordinate complexes of the type Ln(Ph‐BIAN)3. Magnetic coupling data were measured by paramagnetic solution NMR spectroscopy. Combining 1H NMR with 2H NMR of partially deuterated compounds allowed a detailed study of the magnetic susceptibility anisotropies over a large temperature range. The observed chemical shifts were separated into ligand‐ and metal‐centered contributions by comparison with the Y analogue (diamagnetic at the metal). The metal‐centered contributions of the complexes with the paramagnetic ions could then be separated into pseudocontact and Fermi contact shifts. The latter is large within the Ph‐BIAN scaffold, which shows that magnetic coupling is significant between the lanthanide ion and the radical ligand. Pseudocontact shifts were further correlated to structural data obtained from X‐ray diffraction experiments. Ligand‐field parameters were determined by fitting the temperature dependence of the observed magnetic susceptibility anisotropies. The electronic structure determined by this approach shows, that the Er and Tm analogues are candidates for single molecule magnets (SMM). These results demonstrate the possibilities for the application of NMR spectroscopy in investigations of paramagnetic systems in general and single molecule magnets in particular.

NMR for Single Ion Magnets

“…Sugita and co-workers proposed an NMR-based method to determine the LF parameters from a series of isomorphically structured lanthanoid complexes that simultaneously reproduce the 1 H NMR paramagnetic shifts and the magnetic susceptibilities measured at various temperatures [81]. An NMR-only method for the LF parameters determination was proposed by the group of Enders [37,49,82,83]. The approach is based on the partial alignment effects, RDCs and RQCs, which depend on the paramagnetism (see paragraph 1.2 above and chapter 3 in ref.…”

Observability of Paramagnetic NMR Signals at over 10 000 ppm Chemical Shifts

“…All sources of information are therefore valuable, and there is a growing interest in using paramagnetic NMR data to constrain electron SH parameters. [28][29][30][31] The use of electronic structure theory in the analysis of paramagnetic NMR data remains a growing field of research, and the clarification of its fundamental aspects remains a focus of current investigations. [32][33][34][35][36][37][38][39][40][41][42] Paramagnetic relaxation enhancement (PRE) is a useful source of information about electron-nucleus distances and also provides estimates of electron relaxation rates.…”

A High‐Resolution View of the Coordination Environment in a Paramagnetic Metalloprotein from its Magnetic Properties

“…[17][18][19][20][21][22][23][24][25][26][27] Since PCS ultimately depend on the Dc tensor, they are being increasingly employed to evaluate magnetic properties and underlying ligand field parameters in the context of molecular magnetism. [28][29][30][31][32] In aparamagnetic metalloprotein, al arge number of signals can be observed, and thus provide av ery accurate and robust measure of the anisotropy of the magnetic susceptibility. [33] Thei mproved computational efficiency and quality of relativistic wavefunction-based QC methods [34] allow for the fast and reliable calculation of the magnetic susceptibility of metal complexes from first principles.…”

“…Since PCS ultimately depend on the Δ χ tensor, they are being increasingly employed to evaluate magnetic properties and underlying ligand field parameters in the context of molecular magnetism [28–32] . In a paramagnetic metalloprotein, a large number of signals can be observed, and thus provide a very accurate and robust measure of the anisotropy of the magnetic susceptibility [33] …”