Strontium Tris(cyclopentadienyl) Complexes: Isostructural f0/d0 Analogues of Divalent Lanthanides

Abstract

Isostructural, diamagnetic diluents are commonly used in magnetic investigations to eliminate interactions between neighboring electron spins. For divalent lanthanides, these diluents are currently limited to closed-shell ytterbium(II) complexes; ytterbium(II), however, is a poor size-match for the large early- and mid-lanthanides, precluding access to certain ligand frameworks. To aid in the study of lanthanide(II) complexes with unconventional electronic structures and magnetic properties, three strontium tris-cyclopentadienyl complexes, [K(2.2.2-cryptand)][SrCp^R₃], with common substituted cyclopentadienyl ligands, Cp^R = C₅Me₄H (Cp^tet), C₅H₄(SiMe₃) (Cp′), and C₅H₃(SiMe₃)₂ (Cp″) were synthesized, characterized, and examined as diamagnetic diluents. En route to their synthesis, two new Sr metallocenes, [SrCp^R₂] Cp^R = Cp^tet, Cp′, were also prepared. [K(2.2.2-cryptand)][SrCp^tet₃] was used to dilute the new complex [K(2.2.2-cryptand)][Eu^IICp^tet₃], and the EPR spectra of the diluted material shows narrower linewidths than those of pure [K(2.2.2-cryptand)][Eu^IICp^tet₃], suggesting an increase in the spin–spin relaxation time that enables determination of the europium hyperfine coupling constant.