Extended isomerism in heteronuclear metal–organic frameworks: synthetic strategies and crystal structures of lanthanide–cobalt–oxydiacetate systems

Abstract

Herein, we present the synthesis and crystal structure of a series of heteronuclear metal–organic frameworks consisting of cobalt(II) and lanthanide(III) ions or yttrium(III) ions that are connected by oxydiacetato (oda²⁻) as a ligand. Our investigation led to the discovery of 16 new compounds that were categorized into two distinct structural types. Bigger lanthanide ions (La to Ho) and Y favor the formation of a neutral, highly porous hexagonal structure (space group P6/mcc). This assembly gives rise to nano-sized channels accommodating a significant number of crystallization water molecules (12–14 per formula unit). Conversely, smaller lanthanide ions (Sm to Lu) and Y favor the formation of an extended anionic cubic network neutralized by Co(II) hexaaqua ions (space group Fdc). Both structures are assembled from the same building blocks, [Ln(oda)₃]³⁻ and Co(II), connected by either a syn–anti or an anti–anti carboxylate bridge. Both the hexagonal and cubic compounds show the same general formula [Ln₂Co₃(oda)₆(H₂O)₆]·nH₂O with different numbers of crystallization water molecules. Although the formation of different frameworks from the same combination of ligands and metal ions has already been reported, an exceptionally extended phenomenon is observed in this case, encompassing a total of seven isomers from Sm to Ho and Y. The synthesis of these compounds can be achieved either through a direct reaction at room temperature or by a hydrothermal reaction, and this has a marked influence on the obtained crystal structure. Our findings offer valuable data to discuss the rationale concerning the main chemical aspects of the synthetic strategies that can lead to the isolation of a unique pure phase, especially in systems where two structures are feasible.