The semicarbazone of 8-hydroxyquinoline-2-carbaldehyde can be easily synthesized and is an effective tetradentate ligand for the coordination of rare-earth(III) ions. Investigations with yttrium(III) and lanthanum(III) in solution and in the solid state show, that the small yttrium ion can form 2 : 2 (1 : 1 stoichiometry) and 2 : 1 ligand to metal complexes (X-ray structures: [LY(NO3)(DMF)2]2Cl2·2DMF and [LL′Y]·3MeOH·Et2O). With the larger lanthanum(III) ion only a well defined 1 : 1 complex (X-ray structure: [LLa(NO3)(MeOH)2]2(NO3)2) can be observed but probably 2 : 1 complexes are also formed. The X-ray structure analyses of [(L-H)MCl3]·MeOH (M = Er, Ho) and Na[(μ-NO3){LEu(NO3)2}2]·2DMF show different coordination modes of the ligand. It can coordinate in its deprotonated but also in the protonated form.
![Graphical abstract: 2-[(8-Hydroxyquinolinyl)methylene]hydrazinecarboxamide: expanding the coordination sphere of 8-hydroxyquinoline for coordination of rare-earth metal(iii) ions](http://hg.y866.cn/compound/lib/scimg/usr/1/B507621H.jpg)
