Investigation of the “bent sandwich-like” divalent lanthanide hydro-tris(pyrazolyl)borates Ln(TpiPr2)2 (Ln = Sm, Eu, Tm, Yb)

Abstract

The series of homoleptic lanthanide(II) “bent sandwich-like” hydro-tris(pyrazolyl)borate complexes Ln(Tp^iPr₂)₂ (Ln = Sm (1), Eu (2), Tm (3), Yb (4); Tp^iPr₂ = hydro-tris(3,5-diisopropylpyrazolyl)borate) has been completed by the synthesis of the hitherto unknown europium and ytterbium derivatives 2 and 4. Both compounds were prepared in high yields by treatment of LnI₂(THF)₂ (Ln = Eu, Yb) with 2 equiv. of KTp^iPr₂ in a THF solution. Although the molecules are sterically highly congested, an X-ray diffraction study of bright red 4 revealed a similar bent B–Yb–B arrangement (151.1° and 153.9°, two independent molecules) as in the previously investigated Sm(II) and Tm(II) complexes 1 and 3. An initial reactivity study showed a very different behavior with acetonitrile. While 2 and 4 proved to be unreactive toward acetonitrile, the more strongly reducing Sm(II) complex 1 yielded two new products. The major product was the dark green-black acetonitrile solvate Sm^II(Tp^iPr₂)₂·CH₃CN (5), while the second product, the colorless (Tp^iPr₂)Sm^III(3,5-ⁱPr₂pz)₂(NCCH₃) (6) with two 3,5-diisopropyl-pyrazolate ligands, resulted from oxidation of samarium to the trivalent state and degradation of a Tp^iPr₂ ligand. Disappointingly, from the most reducing Tm(II) complex 3 only the ligand fragmentation product pyrazabole, [HB(3,5-ⁱPr₂pz)₂]₂ (7), could be isolated and the fate of the Tm containing by-product(s) remains unknown. The new compounds 4–6 were structurally authenticated through single-crystal X-ray diffraction. The europium compound 2 shows an extremely bright yellow emission in solution, which can be observed also at daylight excitation, as well as in the solid state. The high intensity is even remarkable when compared to other Eu(II) containing materials. The photoluminescence was investigated with the conclusion that the rigidity of this complex is responsible for these outstanding luminescence properties.