A systematic study of halide-template effects in the assembly of lanthanide hydroxide cluster complexes with histidine

Abstract

A series of polynuclear lanthanide (Ln) complexes were prepared by the hydrolysis of lanthanide ions using L-histidine as a supporting ligand. Halide ions (X⁻ = Cl⁻, Br⁻, and I⁻) have been found to play a major role in templating the assembly of pentadecanuclear complexes for Nd(III) and Gd(III) with the definitive evidence of μ₅-X⁻ in the cluster core formed by five corner-shared cubane units of [Ln₄(μ₃-OH)₄]⁸⁺. For the heavier and smaller Er(III), analogous pentadecanuclear complex templates by μ₅-Cl⁻ and μ₅-Br⁻ were also isolated, but an otherwise identical reaction in the presence of added I⁻ afforded a dodecanuclear complex with a core of four corner-shared cubanes. Two I⁻ guests are found, one on each side of the wheel-like structure and in close contact with the μ₃-OH groups. For the medium size Gd(III), a dodecanuclear complex with two I⁻ can also be formed by changing the metal–ligand ratio. The halide-template effects are analyzed, and the production of different cluster complexes can be rationalized by invoking the match of the physical size of the lanthanide and halide ions. The magnetic properties of dodecanuclear 6 were investigated for magnetic cooling applications, and it showed a magnetic entropy change of 30.95 J kg⁻¹ K⁻¹ at 2 K for 7 T.