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

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.