Crown-ether coordination compounds of zirconium, hafnium and scandium using metal nanoparticles

Abstract

Crown-ether coordination compounds of zirconium, hafnium and scandium were rarely reported in the past. Conventional syntheses via Lewis-acid-base reactions or metathesis reactions suffer from the high Lewis acidity and high oxophilicity of Zr⁴⁺, Hf⁴⁺ and Sc³⁺. Therefore, a redox approach using nanoparticles of zerovalent zirconium, hafnium, and scandium is suggested here. Zr(0), Hf(0), Sc(0) nanoparticles, 2-8 nm in size, are prepared by reduction of ZrCl₄, HfCl₄ and ScCl₃ with sodium naphthalenide in a liquid-phase synthesis. The as-prepared Zr(0), Hf(0), Sc(0) nanoparticles are reacted in ionic liquids with 12-crown-4 (12c4), 15-crown-5 (15c5) and 18-crown-6 (18c6) exhibiting ring-opening diameters of 140 to 300 pm. As a result, four new crown-ether coordination complexes are obtained and characterized by single-crystal structure X-ray analysis. The structural variety ranges from the layered structure [AlCl₂(12c4)][NaAl₂Cl₈] (1) over the chiral chain-type compounds [ZrCl₂(15c5)][Na₂Al₄Cl₁₆] (2) and [HfCl₂(15c5)][Na₂Al₄Cl₁₆] (3) to [ScCl₂(18c6)][AlCl₄] (4) with isolated ions. All compounds crystallize in space groups without inversion symmetry. Exemplarily, this was confirmed via second-harmonic generation (SHG) measurements of [ZrCl₂(15c5)][Na₂Al₄Cl₁₆] and [ScCl₂(18c6)][AlCl₄], whereof the latter shows a strong SHG signal (comparable to KH₂PO₄/KDP) and appears to be phase matchable.