Ligand-Field Energy Splitting in Lanthanide-Based Single-Molecule Magnets by NMR Spectroscopy

Abstract

A method for the experimental determination of ligand-field (LF) energy splitting in mononuclear lanthanide complexes, based purely on variable-temperature NMR spectroscopy, was developed. The application of this method in an isostructural series of anionic lanthanide bis(cyclooctatetraenide) double-decker compounds bearing large rigid substituents is demonstrated. Using the three-nuclei plot approach devised by Reilley, the isostructurality of the compound series and the identical orientation of the magnetic main axis of all Ln ions in the series Tb to Tm are demonstrated. Measurement of the H NMR spectra of partially deuterated analogues of the complexes permitted determination of the axial magnetic susceptibility anisotropies χ for all five ions in the temperature range from 185 to 335 K. For this purpose, analysis of the hyperfine shifts was combined with structural models derived from density functional theory calculations. In a final step, the temperature dependence of the χ values was used for determination of the three axial LF parameters, adapting a method employed previously for phthalocyanine-based systems. The temperature dependence dictated by the LF parameters determined by this NMR-based approach is compared to the results of recently published ab initio calculations of the system, indicating reasonable agreement of both methods. For all ions except Dy, the NMR method determines the same m ground state as the calculations and the order and energies of the excited states match well. However, the sign of the magnetic anisotropy of the dysprosium complex in the temperature range evaluated here is not correctly predicted by the published calculations but can be described accurately by the NMR approach. This shows that our experimental method for determination of the LF parameters is an ideal complementation to other theoretical and experimental methods.

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

“…In 1970, Kurland and McGarvey showed more complex behavior deriving the general formula of the dipolar contribution in terms of magnetic susceptibility . Later, McGarvey performed a theoretical study to determine the amplitude of T − n terms for several lanthanide ions . It has been established that although the temperature dependence is not exactly T −2 , an accuracy of about 10–20 % can be obtained at room temperature.…”

“…This is in agreement with the recent results of Hiller et al. who reported that the magnetic anisotropy χ as (which is another description of the dipolar contribution, see the Quantum chemistry calculations section) of Ho III complexes deviates from T −2 behavior . The greatest τ 1 and τ 2 values are observed for the lightest cations (Ce to Eu) with a significant T −2 contact term up to 47 % for Sm III and Nd III and a large T −3 dipolar contribution up to 40 % for Ce III .…”

“…[23] Later,M cGarvey performed at heoretical study to determine the amplitude of T Àn terms for severall anthanide ions. [26] It has been established that althought he temperature dependencei sn ot exactly T À2 ,a na ccuracy of about 10-20 %c an be obtained at room temperature. This discrepancy may be overcome by adding a T À3 term to the dipolarterm.…”

A Trigonal Prismatic Cobalt(II) Complex as a Single Molecule Magnet with a Reduced Contribution from Quantum Tunneling

“…Magnetometry in alternating current magnetic fields ( ac ‐magnetometry) is the most popular method in SMM research, which directly accesses the magnetization relaxation time; other techniques of choice are dc‐magnetometry,, far‐infrared spectroscopy, magnetic circular dichroism, NMR spectroscopy, and electron paramagnetic resonance (EPR) spectroscopy ,. The latter is rarely used for studying SMMs.…”

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

“…For the determinationo ft he ligand field parameters the approach recently applied in substituted bis-cyclooctatetraenyl lanthanide double decker compounds was adapted. [30] By diagonalization of the energy matrices,t he contributions of the various m J states to the wave functions y i and their relative energiesw ere obtained. Subsequently,t he magnetic susceptibility tensor components were calculated using Equation (9).…”