On the structures of dinuclear symmetric lanthanide complexes and the selectivity towards heterodinuclear complexes based on molecular modeling

Abstract

Homo- and heterodinuclear lanthanide complexes are important platforms for studying lanthanide-lanthanide (Ln-Ln) interactions and how they affect their luminescent and magnetic properties. Thus, methods that can yield the selective formation of these complexes are relevant. In fact, dinuclear complexes of trivalent lanthanide ions with identical coordination sites [(H2O)3L2Ln(L)2Ln′L2(H2O)3] ≡ [LnLn′]s, with L = F5C6COO−, have been synthesized, including heteronuclear [EuTb]s complex. The origin of this selectivity, for complexes with Ln, Ln′ = Eu3+, Gd3+, and Tb3+, was then investigated by quantum chemical modeling. Of the semiempirical (Sparkle/SE) and density functional theory (DFT) based approaches, only the B3LYP functional reproduced the asymmetric coordination modes of the bridging ligands and also provided the smallest deviation from the crystallographic structure. Trends in the coordination distances consistent with the Eu3+ trivalent lanthanide ionic radii were observed. However, the Ln-Ln′ distances did not follow the trends of the ionic radii and had an important impact into the electronic energies of the complexes. The selectivity towards the heterodinuclear complexes was quantified by the Gibbs energy of the [LnLn]s + [Ln′Ln′]s ⇄ 2[LnLn′]s reaction. A combined DFT approach (B3LYP for structure and M06-2X for electronic energy) was successful in showing the formation of the [EuTb]s complex. However, it was predicted that the heterodinuclear complexes [EuGd]s and [GdTb]s should not be formed. The driving force for the selectivity towards the [EuTb]s complex was the reaction (electronic) energy, whereas the solvation and entropic contributions were the most relevant for the reaction involving the formation of [EuGd]s and [GdTb]s. These observations can provide guidelines for the rational design of new heterodinuclear lanthanide complexes with identical coordination sites.