The use of perchlorate ancillary ligands with relatively low coordination ability nicely illustrates a rich structural diversity in thiosemicarbazone copper(II) (TSC-Cu(II)) systems. Five compounds with the formulae {[CuL(OH2)][CuL(OClO3)]}n·nClO4·2nH2O (1), [{CuL(OClO3)}2] (2), [Cu(HL)(OClO3)2(OH2)] H2O (3), [Cu2L3](ClO4)·2H2O (4) and [Cu(HL)(NCS)](ClO4) (5) are studied, where HL = pyridine-2-carbaldehyde thiosemicarbazone. The crystal structures show the presence of polynuclear species in the compounds containing an anionic L− thiosemicarbazone ligand: 1D chains (1) and dinuclear (2 and 4) arrangements. Meanwhile, complexes with a neutral HL ligand give rise to mononuclear entities (3 and 5). The chemical and structural parallelism of these complexes with the analogous TSC–Cu(II)–nitrate system is discussed. The thiocyanate ligand in compound 5 comes from partial desulfurization of the thiosemicarbazone in acid medium and results in an incommensurate modulated structure. On the other hand, it is proposed that [Cu2L3]+ species present in 4 could play a possible role in desulfurization reactions. Experimental evidence and theoretical calculations support a proposed mechanism for the partial desulfurization of preformed TSC-Cu(II) entities in basic and physiological media to yield the [CuL(LCN)] (HLCN = pyridine-2-ylmethylene(hydrazine carbonitrile)) and [{CuL(SH)}2)] compounds.
