Tris(indenyl)lanthanoid Complexes (Ln = La, Pr, Nd) Containing either (S)-(−)-Nicotine or Two Simpler Pyridine Bases

Abstract

Novel examples of the complex of the type [Ln(Ind)3·Nic] [Ind = indenyl; Nic = (S)-(−)-nicotine] with Ln = La (2), Pr (3), and Nd (4) have been synthesised, along with their “parent” system [LaCp3·Nic] (1). Complexes 2−4 were structurally, exhaustively characterised (XRS, 1H NMR spectroscopy). Attempts to prepare homologues of 3 with achiral N-bases, pyridine, α-picoline, β-picoline, 2,6-dimethylpyridine, 2,6-bis(tert-butyl)pyridine, 2,2′-bipyridine, and 4,4′-dimethyl-2,2′-bipyridine, were successful with only the two least bulky bases, pyridine and β-picoline, which afforded the new adducts 5 and 6, respectively. The chiral [Ln(Ind)3·Nic] adducts are virtually isostructural, although subtle but systematic conformational variations emerge from the crystal structure analyses. While the unit cells of 2−4 contain two epimeric diastereomers, those of 5 and 6 involve genuine optical antipodes. The chiral carbon atom of nicotine lies approximately twice as far from the metal centre than the chiral sulfur atom of the methyl tolyl sulfoxide (MTSO) ligand in the previously reported [Ln(Ind)3·MTSO] systems. As a result of this, the impact of the chiral MTSO ligand, both on the molecular structure and the 1H NMR spectra, is found to be notably stronger than that of (S)-(−)-nicotine. The comparatively weak f-f circular dichroism of 3 is also compared with that of its (S)-(−)-MTSO-containing congener.