Accessing Lanthanide Metallocene Two-Electron Reduction Chemistry Using 2,2′-Bipyridine

Abstract

The feasibility of using redox-active 2,2′-bipyridine (bipy) as a synthetically convenient electron carrier to enable lanthanide metallocenes to effect two-electron reduction chemistry has been examined. The Ln(III) precursor complexes, Cp*2LnCl(bipy), 1-Ln (Ln = Nd, Gd; Cp* = C5Me5), were readily synthesized in 75–81% yield in one pot from LnCl3, KCp*, and bipy and were identified by X-ray crystallography. Treatment of the 1-Ln compounds with K/KI (4.3 wt %) afforded in 74–77% yield the products, Cp*2Ln(bipy), 2-Ln, which contain (bipy)1– ligands based on their spectroscopic characteristics and X-ray crystal structures. The molecular structure of the previously reported 2-Eu was also determined, which showed structural features consistent with a neutral bipy ligand. Further reduction of 2-Nd or 2-Gd, or the previously reported 2-Sm, 2-Eu, and 2-Yb with K/KI (4.3 wt %) in the presence of 2.2.2-cryptand (crypt) afforded in 58–88% yield the salts, [K(crypt)][Cp*2Ln(bipy)], 3-Ln, which show metrical parameters identified by X-ray crystallography that are consistent with a (bipy)1– ligand for 3-Eu and 3-Yb and a (bipy)2– ligand for 3-Nd, 3-Sm, and 3-Gd. The two-electron reduction of azobenzene (PhN=NPh) to (PhN−NPh)2– using the 3-Ln complexes was examined as a proof of concept. The 1:1 reactions of 3-Nd, 3-Sm, and 3-Yb with azobenzene afforded in 36–79% yield the [K(crypt)][Cp*2Ln(N2Ph2)], 4-Ln (Ln = Nd, Sm, Yb) complexes, each of which were crystallographically characterized. In another example of two-electron reduction by 3-Ln, treatment of 3-Nd or 3-Sm with elemental sulfur afforded in 70–75% yield [K(crypt)][Cp*2Ln(S5)], 5-Ln (Ln = Nd, Sm), which were shown by crystallography to be rare examples of f-element organometallic complexes that contain the (S5)2– ligand.