The importance of the counter-cation in reductive rare-earth metal chemistry: 18-crown-6 instead of 2,2,2-cryptand allows isolation of [YII(NR2)3]1− and ynediolate and enediolate complexes from CO reactions

Abstract

The use of 18-crown-6 (18-c-6) in place of 2.2.2-cryptand (crypt) in rare earth amide reduction reactions involving potassium has proven to be crucial in the synthesis of Ln(II) complexes and isolation of their CO reduction products. The faster speed of crystallization with 18-c-6 appears to be important. Previous studies have shown that reduction of the trivalent amide complexes Ln(NR₂)₃ (R = SiMe₃) with potassium in the presence of 2.2.2-cryptand (crypt) forms the divalent [K(crypt)][Ln^II(NR₂)₃] complexes for Ln = Gd, Tb, Dy, and Tm. However, for Ho and Er, the [Ln(NR₂)₃]¹⁻ anions were only isolable with [Rb(crypt)]¹⁺ counter-cations and isolation of the [Y^II(NR₂)₃]¹⁻ anion was not possible under any of these conditions. We now report that by changing the potassium chelator from crypt to 18-crown-6 (18-c-6), the [Ln(NR₂)₃]¹⁻ anions can be isolated not only for Ln = Gd, Tb, Dy, and Tm, but also for Ho, Er, and Y. Specifically, these anions are isolated as salts of a 1 : 2 potassium : crown sandwich cation, [K(18-c-6)₂]¹⁺, i.e. [K(18-c-6)₂][Ln(NR₂)₃]. The [K(18-c-6)₂]¹⁺ counter-cation was superior not only in the synthesis, but it also allowed the isolation of crystallographically-characterizable products from reactions of CO with the [Ln(NR₂)₃]¹⁻ anions that were not obtainable from the [K(crypt)]¹⁺ analogs. Reaction of CO with [K(18-c-6)₂][Ln(NR₂)₃], generated in situ, yielded crystals of the ynediolate products, {[(R₂N)₃Ln]₂(μ-OCCO)}²⁻, which crystallized with counter-cations possessing 2 : 3 potassium : crown ratios, i.e.{[K₂(18-c-6)₃]}²⁺, for Gd, Dy, Ho. In contrast, reaction of CO with a solution of isolated [K(18-c-6)₂][Gd(NR₂)₃], produced crystals of an enediolate complex isolated with a counter-cation with a 2 : 2 potassium : crown ratio namely [K(18-c-6)]₂²⁺ in the complex [K(18-c-6)]₂{[(R₂N)₂Gd₂(μ-OCHCHO)₂]}.