Antiferromagnetic coupling between rare earth ions and semiquinones in a series of 1 ∶ 1 complexes

Abstract

We use the strategy of diamagnetic substitution for obtaining information on the crystal field effects in paramagnetic rare earth ions using the homologous series of compounds with the diamagnetic tropolonato ligand, Ln(Trp)(HBPz(3))(2), and the paramagnetic semiquinone ligand, Ln(DTBSQ)(HBPz(3))(2), (DTBSQ = 3,5-di-tert-butylsemiquinonato, Trp = tropolonate, HBPz(3)= hydrotrispyrazolylborate) for Ln = Sm(iii), Eu(iii), Gd(iii), Tb(iii), Dy(iii), Ho(iii), Er(iii) or Yb(iii). The X-ray crystal structure of a new form of tropolonate derivative is presented, which shows, as expected, a marked similarity with the structure of the semiquinonate derivative. The Ln(Trp)(HBPz(3))(2) derivatives were then used as a reference for the qualitative determination of crystal field effects in the exchange coupled semiquinone derivatives. Through magnetisation and susceptibility measurements this empirical diamagnetic substitution method evidenced for Er(iii), Tb(iii), Dy(iii) and Yb(iii) derivatives a dominating antiferromagnetic coupling. The increased antiferromagnetic contribution compared to other radical-rare earth metal complexes formed by nitronyl nitroxide ligands may be related to the increased donor strength of the semiquinone ligand.

Solvent‐Mediated Redox Transformations of Ytterbium Bis(indenyl)diazabutadiene Complexes

“…[18] Hatfield and co-workers have described a strong exchange coupling between the lanthanide ions and the phthalocyaninato radical in bis(phthalocyaninato)lanthanide sandwich compounds [(Pc -2 )Ln III (Pc -1 )] (Ln = Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu). The value of the roomtemperature magnetic moment for trivalent ytterbium antiferromagnetically coupled with an organic radical anion was calculated to be 3.46 μ B .…”

Application of the Hubbard Model to Cp*2Yb(bipy), a Model System for Strong Exchange Coupling in Lanthanide Systems

“…Direct overlap is also possible in Ln complexes containing stable radicals such as nitroxyl, yet these complexes display much weaker exchange coupling. [42][43][44][45][46][47] The HMM clearly explains why exchange coupling in 1 and 3 is strong while exchange coupling between Ln centers and ligands containing stable radicals such nitroxyl radicals is much weaker. [42][43][44][45][46][47] Trivalent lanthanide ions have large, negative reduction potentials, so strongly reducing ligands with similarly large, negative reductions potentials are needed to minimize U.…”

Decamethylytterbocene Complexes of Bipyridines and Diazabutadienes: Multiconfigurational Ground States and Open-Shell Singlet Formation

“…The heart of the controversy has been whether the reduced moment is caused by some type of antiferromagnetic coupling (mostly intramolecular, as shown by correlating solid-state magnetism and solution ] is unambiguously three, Yb(III), and the atom has an open-shell 4f 13 electron configuration, consistent with its physical properties. However, the valence of the ytterbium atom in Cp* 2 Yb(bipy) is neither two nor three; rather, it is in between these extreme values, i.e., it has an intermediate valence.…”