Paramagnetic NMR Analysis of Substituted Biscyclooctatetraene Lanthanide Complexes

Abstract

Cyclooctatetraene derivatives (COTR) with two carbocycles attached to the central ring have been used as dianionic ligands for the synthesis of double-decker complexes of the type [(COTR)2M]−. Two ligand derivatives were combined with diamagnetic Y3+ and with the five paramagnetic lanthanide ions from Tb3+ to Tm3+. The more complex substitution pattern in comparison to the parent ligand COT or the popular bis-trimethylsilyl derivative allows a sufficient number of signals to be obtained for a comprehensive paramagnetic NMR analysis. The anionic double-decker complexes gave well-resolved NMR spectra where almost all 1H and 13C NMR signals could be detected and assigned. With these data, it was possible to separate the two main contributions to the paramagnetic shift (pseudocontact and Fermi contact shifts, respectively) and to determine the magnetic anisotropy of the lanthanide ions in their ligand fields. We can easily obtain the sign and the magnitude of the anisotropy of the magnetic susceptibility, which is itself strongly related to the energy barrier for spin reversal in single-molecule magnets. Our results confirm that Bleaney factors are inadequate descriptors for magnetic anisotropy in these lanthanide complexes.

Lanthanide(III) Sandwich and Half‐Sandwich Complexes with Bulky Cyclooctatetraenyl Ligands: Synthesis, Structures, and Magnetic Properties

“…Meaningful NMR spectroscopic data could be obtained for 1 and in part for 3 , whereas the strongly paramagnetic nature of the Tb 3+ ion prevented a successful NMR investigation of 2 . This is in good agreement with a recent detailed study describing the paramagnetic NMR analysis of substituted bis(cyclooctatetraenyl) lanthanide complexes . As expected, the 1 H NMR signals of the COT′′ ligands in the Pr complex 1 experience significant line‐broadening and a considerable paramagnetic shift to a higher field .…”

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

“…A new series of lanthanide cyclooctatetraenide sandwich complexes was synthesized by the reduction of tdnCOT and subsequent reaction with anhydrous lanthanide trichlorides (Scheme ). This method was previously employed for similar systems . The five trivalent lanthanide ions from Tb 3+ to Tm 3+ were investigated because these are characterized by large magnetic moments and anisotropies.…”

“…Ishikawa et al combined paramagnetic NMR shifts and SQUID magnetization data in order to obtain LF parameters in phthalocyaninide (Pc)-based double- and triple-decker compounds. , This allowed for the prediction and validation of the SMM behavior for Pc 2 Tb, which had the highest anisotropy barrier to date and is one of the most studied SMM systems. In the last years, NMR has been applied in a growing number of studies for the investigation of SMMs and related compounds. − In this work, a purely NMR-based method for determination of the LF parameters in cyclooctatetraenide-based lanthanide double-decker complexes is described. This class of SMMs was first introduced by Long, Murugesu, and co-workers. − The methodology presented here is based on the fact that each of the 2 J + 1 substates is connected to a magnetic anisotropy.…”

“…Cyclic polydentate ligands like derivatives of Pc or COT have been utilized extensively for the synthesis of double-decker-type SMMs. In the case of monoanionic compounds of the type [Ln­(COT′) 2 ] − (COT′ = substituted cyclooctatetraenide), the Er 3+ and Dy 3+ compounds exhibit slow relaxation of magnetization. − In a recent study, we demonstrated the application of two new ligand derivatives, easily accessible by dimerization of dialkynes. , The axial magnetic susceptibility anisotropies (χ ax ) of the lanthanide complexes from Tb 3+ to Tm 3+ could be determined by a simultaneous analysis of the NMR spectra of the series. The analysis revealed, that, for the Dy 3+ , Er 3+ , and Tm 3+ ions, χ ax is positive; i.e., the magnetization along the magnetic main axis is larger than that in the perpendicular direction, which is a requirement for SMMs.…”

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

“…All observable 1 H NMR shifts were simultaneously fitted to an alternative expression of Equation (5), as described previously for substituted lanthanide‐cyclooctatetraene systems . For this purpose, the geometric factors G i are replaced by relative geometric factors G′ i (see the Supporting Information for details), which is fixed to unity for one nucleus chosen arbitrarily.…”