Lanthanide polyether complexation chemistry: the interaction of hydrated lanthanide(iii) nitrate salts with an acyclic 18-crown-6 analog, pentaethylene glycol

Abstract

The complexation reactions of 1 : 1 molar ratios of M(NO₃)₃·nH₂O (M = Y, La–Pr, Sm–Lu) and pentaethylene glycol (EO5) in 3 : 1 CH₃CN : CH₃OH were investigated. Crystalline complexes were isolated for all metals investigated and X-ray structural analyses performed. The M(NO₃)₃–EO5 complexes structurally characterized are remarkably similar to the corresponding 18-crown-6 complexes. Six structurally unique types of anhydrous complexes with the early- to mid-lanthanides (M = La–Nd, Sm–Dy) were found. For the largest metals studied, the twelve coordinate species [M(NO₃)₃(EO5)] (M = La, Ce) were isolated. A second form of the type [Ce(NO₃)₂(EO5)]₇[Ce(NO₃)₆][NO₃]₄ was isolated for M = Ce from the same reaction mixture as the above complex. Praseodymium through dysprosium form ten coordinate species which all have the same basic formula, [M(NO₃)₂(EO5)][NO₃], however, four structurally unique forms have been characterized with the major differences between each arising from the way in which the uncoordinated nitrate anion is hydrogen bonded to the glycol ligand, and the glycol conformation. The smallest lanthanides investigated, M = Ho–Lu, Y, form outer-sphere complexes [M(OH₂)₃(NO₃)₃]·EO5. Structural results suggest that the flexibility of the acyclic EO5 ligand allows for the formation of inner sphere complexes across most of the series, whereas the cyclic 18-crown-6 is too constrained to permit such complexation to the mid- to late-lanthanides.