Crystal structure, conformational diversity and solution NMR spectroscopy of four tris(indenyl)lanthanoid(III) tetrahydrofuran adducts: [Ln(C 9H 7) 3 · THF] (Ln = La, Pr, Nd, Sm)

Abstract

Four homologous tris(indenyl)lanthanoid(III) adducts, [Ln(C 9H 7) 3THF](THF = tetrahydrofuran), with Ln = La ( 1), Pr ( 2), Nd ( 3) and Sm ( 4) were prepared in excellent yields, and six single crystals, 1a, 1b, 2, 3a, 3b and 4 ( 1 and 3 being at least dimorphic) have been subjected to detailed X-ray studies. The C 5-fragments of the C 9H 7 ligands are almost pentahapto-bonded, and either two or three C 9H 7 units adopt paddle-wheel-like orientations. The crystals 1a, 2 and 3a contain two molecules of different conformation, one molecule displaying one C 9H 7 benzo group in a transoid orientation relative to the THF ligand. A similar conformation, albeit with one cisoid benzo group, is realized in 1b. Three characteristic structural motifs, I ( 1a, 2, 3a), II ( 1b) and III ( 3b, 4), emerge from the present results, and appear to be consistent with earlier findings. Motif III is most remarkable in view of its hexagonal, chiral space group P6 3, owing to ‘trigonally disordered’ THF units. Solution 1H NMR spectroscopy indicates three rapidly equilibrating virtually equivalent C 9H 7 ligands, the 139La NMR spectrum of 1 and the MCD spectrum of 2 admitting again η 5-coordination. The long-pending suggestion of predominantly η 1-coordinated C 9H 7 ligands in dissolved molecules of 4 seems to be no longer justified.