Crystal structures of a MACROPA-bis-alkyne precursor and its lanthanum complex: structural consequences of the functionalization

Bis-picolinic acid-functionalized diaza-18-crown-6 (MACROPA) is among the most effective chelators for lanthanide and actinide ions. Although the synthesis of a MACROPA derivative with a 4-propargyloxy functionalized pyridine suitable for click chemistry has been previously reported, no structural information is available. In this work, the crystal structures of the key intermediate methyl 6-(chloromethyl)-4-(prop-2-yn-1-yloxy)picolinate and of the La(III) complex of the final ligand, La(MACROPA-bis-alkyne), are reported and discussed. The modified picolinate derivative adopts an essentially planar conformation and packs in a herringbone-like arrangement sustained by weak C–H⋅⋅⋅N and C–H⋅⋅⋅O interactions. The crystals are non-merohedral twins and the structure was refined against HKLF 5 twin data [twin fraction 0.382(2)]. A short Cl⋅⋅⋅O contact is also observed, whereas no significant π − π stacking interactions are present. In La(MACROPA-bis-alkyne), the La(III) ion is 11-coordinate and is bound to the ten donor atoms of the decadentate ligand and to a hydroxido ligand, which balances the charge. Continuous shape measures indicate a distorted augmented sphenocorona (J87), with values almost identical to those of the parent La(MACROPA) complex. The macrocyclic framework retains an overall conformation comparable to that of the parent La(MACROPA) complex, while disorder of the propargyloxy substituents [occupancies 0.557(6):0.443(6)] indicates a degree of conformational flexibility. The crystal packing is governed by an extended hydrogen-bonding network involving the hydroxido ligand and lattice water molecules, which generates supramolecular layers further connected by weak C–H⋅⋅⋅O interactions. The structural comparison demonstrates that introduction of the propargyloxy groups does not significantly affect the coordination environment around the La(III) centre.