N-aromatic heterocycle adducts of bulky [1,2,4-(Me3C)3C5H2]2Sm: synthesis, structure and solution analysis

Abstract

The reactions of the bulky samarocene [1,2,4-(Me₃C)₃C₅H₂]₂Sm (1) with several N-aromatic heterocycles, namely, pyridine (3), picoline (4), 4-tert-butyl-pyridine (5), isoquinoline (6) and quinoline (7), are reported. All reactions proceed smoothly in pentane in good yield and crystals of all adducts have been obtained in moderate to good yield and analyzed by X-ray crystallography. The X-ray crystal structures are in agreement with Sm(II) coordination adducts in which no electrons are transferred to the N-aromatic heterocycle. Magnetic data were recorded for 1 and for all adducts, 3–7, and reinforce the electronic structure of Sm(II), f⁶ simple coordination adducts. ¹H NMR at different temperatures and visible spectroscopy in toluene show that fast exchange of the N-aromatic heterocyclic ligand (L) occurs in solution according to a ML = M + L equilibrium in which the equilibrium constant is dependent on the bulkiness of L. This fast exchange is stopped below a coalescence temperature T_c, at which the three tert-butyl groups of the cyclopentadienyl are differentiated. The associated rotational barriers have been calculated. Cyclic voltammetry of 1 in THF showed a quasi-reversible wave at a reducing potential of −2.10 V versus the ferricinium/ferrocene couple for the Sm(III)–Sm(II) couple and leads to the conclusion that the non-reduction of L lies in the sterics of 1.